Medical Imaging 2001: Physics of Medical Imaging

Comparative study of Pbl2 and Hgl2 as direct detector materials for high-resolution x-ray image sensors

Robert A. Street, Marcelo Mulato, Michael M. Schieber, et al.

Show abstract

X-ray imaging properties are reported for HgI₂ and PbI₂, as candidate materials for future direct detection x- ray image sensors, including the first results from screen- printed HgI₂ arrays. The leakage current of PbI₂ is reduced by using new deposition conditions, but is still larger than HgI₂. Both HgI₂ and PbI₂ have high spatial resolution but new data shows that the residual image spreading of PbI₂ is not due to k-edge fluorescence and its possible origin is discussed. HgI₂ has substantially higher sensitivity than PbI₂ at comparable bias voltages, and we discuss the various loss mechanisms. Unlike PbI₂, HgI₂ shows a substantial spatially non-uniform response that is believed to originate from the large grain size, which is comparable to the pixel size. We obtain zero spatial frequency DQE values of 0.7 - 0.8 with PbI(subscript 24/ under low energy exposure conditions. A model for signal generation in terms of the semiconducting properties of the materials is presented.

Evaluation of direct detection and indirect detection active matrix flat-panel imagers (AMFPIs) for digital mammography

Kyung-Wook Jee, Larry E. Antonuk, Youcef El-Mohri, et al.

Show abstract

Initial results from an examination of the performance of small-area, high-spatial-resolution, active matrix, flat- panel imager prototypes, under conditions relevant to mammographic imaging, are reported. These prototypes are based on two 512 X 512 pixel designs: (1) a 75 micrometers pitch indirect-detection array incorporating a continuous photodiode surface operated with a 34 mg/cm² Gd₂O₂S:Tb phosphor screen and with a 150 micrometers thick structured CsI(Tl) scintillator on a fiber-optic plate (which was designed to provide high spatial resolution at the expense of light output); and (2) two 100 micrometers pitch direct-detection arrays with approximately 47 micrometers and approximately 100 micrometers thick PbI₂ photoconductive coatings.

Comparison of the image quality produced by two examination stands for thoracic imaging equipped with a CsI/a-Si flat-panel detector respective to a transparent imaging plate with both-side reading

Christoph Hoeschen, Steffen Reissberg, Olaf Effenberger M.D., et al.

Show abstract

The image quality produced by two examination stands designed for thoracic imaging was evaluated. One of the two stands was using a CsI/a-Si flat panel detector (FD) as the imaging system. The other system was using an imaging plate with a transparent support which is read out from both-sides (tIP). 75 images of an anthropomorphic thorax phantom were produced with both examination stands and on a screen-film-system. Different structures - representing pathological alterations - have been superimposed to the phantom. An ROC analysis of these images has been done. The structures simulating different pathology have been evaluated with respect to the frequency components of their X-ray intensity pattern. This was done by using the method presented at the SPIE 'Medical Imaging' conferences 1999 and 2000 for investigating the normal anatomy. The simulated structures represent different pathological structures sufficiently. The frequency range of their X-ray intensity pattern is in the same order as that found for anatomical structures in the lung. The performed ROC-analysis shows very promising results for both detectors. For both detectors there will probably be a potential to reduce the X-ray dose per exposure without loosing relevant diagnostic quality. This potential is slightly higher for the FD corresponding to its slightly higher DQE at lower frequencies.

Alternate pixel architectures for large-area medical imaging

Karim S. Karim, Arokia Nathan, John A. Rowlands

Show abstract

Amorphous silicon active matrix flat-panel imagers have gained considerable significant in digital diagnostic medical imaging applications in view of their large area readout capability. The pixel, forming the fundamental unit of the active matrix, consists of a detector and readout circuit. The most widely used architecture is a passive pixel sensor (PPS) where the pixel consists of a detector and an a-Si:H thin-film transistor readout switch. While the PPS has the advantage of being compact and amenable towards high-resolution imaging, reading the low PPS output signal require external circuitry such as column charge amplifiers. More importantly, these amplifiers add a large noise component to the PPS that reduces the minimum readable sensor input signal. This work presents an alternate pixel architecture that can perform on-pixel input signal amplification, i.e. an active pixel sensor (APS). Two operating modes of the APS, voltage output (V-APS) and current output (C-APS) are introduced but the focus is on C- APS. Theoretical calculations indicating the feasibility of the C-APS for low-noise, real time imaging applications (e.g. fluoroscopy) are presented. Specifically, signal gain, dynamic range, readout rate and noise of the C-APS are examined. Lastly, initial experimental results of C-APS linearity and gain are presented in addition to a discussion on APS threshold voltage stability.

Temporal artifacts in flat dynamic x-ray detectors

Michael Overdick, Torsten Solf, Hans-Aloys Wischmann

Show abstract

Flat X-ray detectors based on CsI:Tl scintillators and amorphous silicon photodiodes are known to exhibit temporal artefacts (ghost images) which decay over time. Previously, these temporal artefacts have been attributed mainly to residual signals from the amorphous silicon photodiodes. More detailed experiments presented here show that a second class of effects, the so-called gain effects, also contributes significantly to the observed temporal artefacts. Both the residual signals and the photodiode gain effect have been characterized under various exposure conditions in the study presented here. The results of the experiments quantitatively show the decay of the temporal artefacts. Additionally, the influence of the detector's reset light on both effects in the photodiode has been studied in detail. The data from the measurements is interpreted based on a simple trapping model which suggests a strong link between the photodiode residual signals and the photodiode gain effect. For the residual signal effect a possible correction scheme is described. Furthermore, the relevance of the remaining temporal artefacts for the applications is briefly discussed for both the photodiode residual signals and the photodiode gain effect.

Factors influencing image quality in digital radiographic systems

Richard L. Van Metter, John Yorkston

Show abstract

The image quality of digital radiography systems is influenced by the interplay of many sources of image degradation. These include technology-dependent sources of image blur and noise. This paper systematically investigates the relative influence of the most common sources of image degradation for the entire digital radiography image chain including the detector, image processing and display using well established methods based on linear systems theory. Image quality is quantified in terms of NEQ and DQE. Baseline data are taken from experimental blur and noise measurements from several digital radiography systems, state-of-the-art image processing algorithms and current displays. A generalized theoretical model is exercised to demonstrate the relative importance of the intrinsic noise sources for both direct- and indirect-conversion digital radiography technologies. Since clinical imaging requires a complete system, including a detector, image processing and display, the model is extended to predict the role of image processing and display. This demonstrates the importance of the choice of display parameters such as those that control grayscale rendering, equalization and edge restoration. In summary, the performance of digital radiography systems depends on the interplay of many sources of image degradation from capture to display. Their effects are shown to be interdependent.

Effect of depth-dependent modulation transfer function and K-fluorescence reabsorption on the detective quantum efficiency of indirect-conversion flat-panel x-ray imaging systems using CsI

John A. Rowlands, Winston Guangda Ji, Wei Zhao

Show abstract

A theory of the imaging behavior of structured phosphor layers is developed based on linear analysis and Carlo model modeling and tested by comparison with experiments. The experiments include: the evaluation of Swank factor; modulation transfer function (MTF) measured as a function of spatial frequency f on CsI phosphor layers of different thicknesses and optical properties (e.g. presence or absence of reflective backing layer). These measurements on structured screens were used to establish phosphor parameters (e.g. absorption and scattering lengths). In addition, MTF(f), Wiener noise power spectra [NPS(f)] and detective quantum efficiency [DQE(f)] measurements were obtained from the literature for x-ray image intensifiers and indirect conversion flat panel detectors constructed using CsI layers. These results were compared with the theory using the previously established phosphor parameters. The effects of K-fluorescence and depth dependence of the MTF on DQE (i.e. Lubberts' effect) were investigated.

Method for determining the two-dimensional presampling modulation transfer function in digital radiography

Magnus Bath, Patrik Sund, Lars Gunnar Mansson

Show abstract

In digital radiography, radial asymmetry may be present. The use of a one-dimensional representation of the resolution properties can therefore be questioned. Although measurements are often done in two orthogonal directions, there may be a need for a more detailed description. A method of measuring the two-dimensional presampling modulation transfer function (MTF) has therefore been developed. A finely sampled 'disk spread function' is obtained by imaging an aperture mask, consisting of N2 circular holes arranged in an NxN manner in an opaque material, in such a way that each hole is positioned at a different phase relative to the sampling coordinates of the detector system. This spread function is resampled, extrapolated, Fourier transformed, and finally corrected for the finite hole size in order to obtain the presampling MTF. The method was tested on a computed radiography (CR) system through measurements with a prototype mask, consisting of 100 holes of radius 0.2 mm drilled in a lead alloy. The results were compared with measurements using the slit method, and were found to be consistent. Problems associated with the method, e.g. errors due to incorrect alignment of the holes in the aperture mask with the beam, and limitations due to the finite hole size, are discussed.

DQE of digital x-ray imaging systems: a challenge for standardization

Dietmar Hoeschen

Show abstract

In July of 2000, a new Working Group (WG33) in IEC/SC62B met for the first time to develop a new IEC standard for the measurement of the detective quantum efficiency of digital x-ray imaging detectors. Experts from meanwhile nine countries are involved. Because of the manifold systems on the market intended for different applications, it was decided to limit the scope of the standard to static systems to be used in general radiography. Dynamic systems (fluoroscopy), systems for mammography and dental systems are excluded but may be treated in additional standards later on.

Methodology to measure fundamental performance parameters of x-ray detectors

Falko Busse, Walter Ruetten, Hans-Aloys Wischmann, et al.

Show abstract

To judge the potential benefit of a new x-ray detector technology and to be able to compare different technologies, some standard performance measurements must be defined. In addition to technology-related parameters which may influence weight, shape, image distortions and readout speed, there are fundamental performance parameters which directly influence the achievable image quality and dose efficiency of x-ray detectors. A standardization activity for detective quantum efficiency (DQE) for static detectors is already in progress. In this paper we present a methodology for noise power spectrum (NPS), low frequency drop (LFD) and signal to electronic noise ratio (SENR), and the influence of these parameters on DQE. The individual measurement methods are described in detail with their theoretical background and experimental procedure. Corresponding technical phantoms have been developed. The design of the measurement methods and technical phantoms is tuned so that only minimum requirements are placed on the detector properties. The measurement methods can therefore be applied to both static and dynamic x-ray systems. Measurement results from flat panel imagers and II/TV systems are presented.

Megalopinakophobia: its symptoms and cures

Harrison H. Barrett, Kyle J. Myers, Brandon D. Gallas, et al.

Show abstract

This paper addresses issues in the calculation of a detectability measure for the ideal linear (Hotelling) observer performing a detection task on a digital radiograph. The main computational problem is that the inverse of a very large covariance matrix is required. The conventional approach is to assume some form of stationarity and argue that the matrix is diagonalized by discrete Fourier transformation, but there are many reasons why this assumption is unrealistic. After a brief review of the underlying mathematics, we present several practical algorithms for computing the detectability and some hints as to when each is applicable. The main conclusion is that large matrices should not be feared.

Detective quantum efficiency of fluoroscopic systems: the case for a spatial-temporal approach (or, does the ideal observer have infinite patience?)

Ian A. Cunningham, T. Moschandreou, Varja Subotic

Show abstract

The detective quantum efficiency (DQE) of an imaging system describes how well the ideal observer performs for a specific imaging detection task. While it can be calculated from measured image data and used to quantify system performance, it is rarely used for assessing fluoroscopic systems. This is primarily due to the effects of system lag. Lag results in a temporal averaging of image signals that reduces noise. As a consequence, the measured DQE of fluoroscopic systems having lag will be erroneously high relative to systems having less lag. This effect can be substantial, resulting in measures of the DQE that can be 15-40% greater than the 'lag-free' DQE. The description of a spatial-temporal NPS and DQE is presented as a means of accommodating system lag. The spatial-temporal NPS has units mm2 s and the spatial-temporal DQE is unitless. Using this generalized interpretation, the (conventional) spatial NPS and DQE are described as sections of the spatial-temporal NPS and DQE along the zero temporal-frequency axis. Calculation of spatial-temporal metrics requires determining an effective temporal aperture related to the temporal MTF. It is shown, both theoretically and experimentally, that the spatial component of the spatial-temporal DQE of a system operating in a fluoroscopic mode is the same as the conventional DQE of the same system operating in a radiographic mode under quantum-noise limited conditions.

Sensor gain and noise requirements for fluoroscopic applications

Tatsuya Yamazaki, Laurence Court, Toshio Kameshima

Show abstract

The low x-ray exposures used in fluoroscopic applications (0.1 - 10mR at the sensor surface) mean that the requirements for sensor gain and noise are particularly strict. The achievable DQE is determined by a number of factors, including the sensor quantum efficiency, x-ray absorption Swank factor, secondary quanta conversion efficiency, internal gain (e.g. the number of electrons collected per visible photon produced in the phosphor), and additive noise. The influence of these factors is examined for three direct detection x-ray sensors (PbI2, a-Se and GaAs), and one indirect detector sensor (CsI). Although the characteristics of these sensors are very different, it is demonstrated that all are appropriate for use in fluoroscopic applications as a replacement for current image intensifier based systems.

Performance of a real-time selenium-based x-ray detector for fluoroscopy

Martin Choquette, Yves Demers, Ziad Shukri, et al.

Show abstract

As amorphous selenium based flat panel detectors gain more interest for direct, real-time x-ray imaging, we report in this paper the performance of such a detector by ANRAD Corporation. This new detector is based on a 1536 X 1536 array of amorphous silicon TFT pixels coupled with a 1000 micrometers selenium converter biased at 10 V/micrometers . Each 150 micrometers X 150 micrometers pixel is made of a thin film transistor, a storage capacitor and a collecting electrode having a geometrical fill factor of 77% and an effective fill factor of nearby 100%.

Flat x-ray image intensifer system for real-time fluoroscopy

Hiroshi Onihashi, Hiroshi Aida, Kiyumi No, et al.

Show abstract

We have developed new X-ray image intensifier (Flat II) system using a large-area electron multiplier for real-time fluoroscopy. This paper describes the development of prototype Flat II that has been carried out recently. The heart of new prototype is the multiplier with a smaller channel pitch (0.2 mm) located adjacent to a photo-cathode. We performed reduction of the channel pitch of multiplier, and the optimization of the geometrical arrangement- a photo-cathode, a multiplier and an output phosphor screen in order to improve the performance of Flat II. Moreover, we discuss the spatial aliasing associated with a pitch pattern of multiplier and CCD camera. Finally, physical characteristics such as modulation transfer function (MTF), etc. are discussed.

Development of a real-time digital radiography system using a scintillator-type flat-panel detector

Shigeyuki Ikeda, Katsumi Suzuki, Ken Ishikawa, et al.

Show abstract

In order to study the advantage and remaining problems of FPD (flat panel detector) for clinical use by the real-time DR (digital radiography) system, we developed a prototype system using a scintillator type FPD and which was compared with previous I.I.-CCD type real-time DR. We replaced the X- ray detector of DR-2000X from I.I.-4M (4 million pixels)-CCD camera to the scintillator type dynamic FPD(7' X 9', 127 micrometers ), which can take both radiographic and fluoroscopic images. We obtained the images of head and stomach phantoms, and discussed about the image quality with medical doctors.

Investigation of imaging performance of amorphous selenium flat-panel detectors for digital mammography

Wei Zhao, W. G. Ji, John A. Rowlands, et al.

Show abstract

Our work is to investigate and understand the factors affecting the imaging performance of amorphous selenium (a-Se) flat-panel detectors for digital mammography. Both theoretical and experimental methods were developed to investigate the spatial frequency dependent detective quantum efficiency [DQE(f)] of a-Se flat-panel detectors for digital mammography. Since the k-edge of a-Se is 12.66 keV and within the energy range of a mammographic spectrum, a cascaded linear system model was developed which takes into account the effect of k-fluorescence on the modulation transfer function (MTF), noise power spectrum (NPS) and DQE(f) of the detector. This model was used to understand the performance of a prototype detector with 85 mm pixel size. The presampling MTF, NPS and DQE(f) of the prototype were measured, and compared to the theoretical calculation by the model. The calculation showed that k-fluorescence reduces the MTF by 15% at the Nyquist frequency (f_NY) of the prototype detector, and the NPS at f_NY was reduced to 82% of that at zero spatial frequency. Because of the decrease in both MTF and NPS at high spatial frequencies, k-fluorescence only has a small degradation effect on DQE(f) for mammography. The measurement of presampling MTF of the prototype detector revealed an additional source of blurring, which was attributed to the blocking layer at the interface between a-Se and the active matrix. This introduced high frequency drop in both presampling MTF and NPS, and reduced aliasing in the NPS. As a result, the DQE(f) of the prototype detector at f_NY approaches 50% of that at zero spatial frequency.

Breast cancer diagnosis using scattered x rays?

Robert A. Lewis, Keith D. Rogers, Christopher J. Hall, et al.

Show abstract

We have performed small angle X-ray scattering measurements at the Synchrotron Radiation Source at Daresbury to make an initial assessment of the diagnostic information obtainable from various breast tissues. These experiments have indicated that high quality interpretable diffraction data can be rapidly produced from breast core cut biopsy specimens. We have demonstrated a remarkable and systematic difference between the X-ray scattering from normal, benign and malignant breast tissue collagen. Our findings indicate that it may be possible to use molecular structure characteristics of breast tissue as novel markers of disease progression.

Lesion detection in digital mammograms

Arthur E. Burgess, Francine L. Jacobson, Philip F. Judy

Show abstract

Detection of mass lesions in mammograms is essentially limited by image variation due to normal patient structure, which has an average power-spectrum of the form `1/f³'. Image noise plays little role in limiting mass detection. The contrast-detail (CD) diagram for lesion detection in mammographic structure is novel, for both human and model observers. Contrast thresholds increase with increasing signal size for signals larger than about 1 mm, with CD slopes of about 0.3 for humans and 0.4 for model observers. Similar results were obtained in search experiments. The work was done using hybrid images, with of tumor masses (extracted from specimen radiographs) added to digitized mammographic backgrounds. We have been able to explain the results using a number of observer models. These results demonstrate that CD diagrams based on image noise-limited detection with simple phantoms are not useful for evaluation of mass detection in mammograms--so more realistic approaches are necessary in order to model mammographic imaging systems for optimization.

What does DQE say about lesion detectability in digital radiography?

Angel R. Pineda, Harrison H. Barrett

Show abstract

Currently, there is significant interest in quantifying the performance of digital radiography systems. Digital radiography systems can be thought of as continuous linear shift-invariant systems followed by sampling. This view, along with the large number of pixels used for flat-panel systems, has motivated much of the work which attempts to extend figures of merit developed for analog systems, in particular, the detective quantum efficiency (DQE) and the noise equivalent quanta (NEQ). A more general approach looks at the system as a continuous-to-discrete mapping and evaluates the signal-to-noise ratio (SNR) completely from the discrete data. In this paper, we study the effect of presampling blur on these figures of merit. We find that even in this idealized model the DQE/NEQ formulations do not accurately track the behavior of the fully digital SNR. Therefore, DQE/NEQ cannot be viewed as indicators of the signal-known-exactly/background-known-exactly (SKE/BKE) detectability. In order to make design decisions by optimizing detectability one must work with the fully digital definition of detectability.

Modeling the x-ray energy characteristics of DQE for full-field digital mammography

J. Eric Tkaczyk, James W. LeBlanc, Robert L. Nevin, et al.

Show abstract

The modulation transfer function and detective quantum efficiency are modeled for a Full Field Digital Mammography detector constructed with a CsI scintillator deposited on an amorphous silicon active matrix array. The model is evaluated against experimental measurements using different exposure levels, x-ray tube voltages, target composition and beam filtrations as well as varying thicknesses and compositions of filtration materials placed in the path between the tube and detector. Available x-ray tube emission spectrum models were evaluated by comparison against the measured transmission through aluminum. The observed variation of DQE at zero spatial frequency among different target/filter conditions, acrylic filtration thicknesses and kVp is well characterized by a x-ray model. This variation is largely accounted for by just two effects -- the attenuation of x-rays through the detector enclosure and the stopping power of x-rays in the CsI layer. Additional considerations such as the Lubberts effect were included in the analysis in order to match the measured DQE(k) as a function of spatial frequency, k. The pixel aperture and light channeling through the scintillator shape the MTF which acts favorably to avoid aliasing due to digital sampling.

Se-based flat-panel detector for screening mammography

Brad T. Polischuk, Serge Savard, Vincent Loustaneau, et al.

Show abstract

A selenium-based flat-panel direct converter detector suitable for digital mammography was developed. The detector is based on a TFT-array with a resolution of 2816 X 2048 pixels, and a pixel pitch of 85 micrometers . Although the geometric fill factor for each pixel is around 70% , the effective fill factor for the detector is closer to 88% due to internal electric field shaping within the selenium layer. A selenium multilayer p-i-n structure of 200 micrometers was deposited onto the array by selectively doping the regions near each contact to produce unipolar conducting blocking layers. This structure absorbs more than 95% of a typical mammography beam.

New needle-crystalline CR detector

Paul J. R. Leblans, Luc Struye, Peter Willems

Show abstract

The storage phosphor RbBr:Tl⁺ can be grown in needles via vacuum deposition. Thanks to reduced lateral light diffusion thick needle screens still offer acceptable resolution. Due to its low intrinsic X-ray absorption, however, a RbBr:Tl⁺ needle screen does not lead to a better absorption/resolution compromise than a BaFBr_1-xI_x:Eu²⁺ powder screen. CsBr:Eu²⁺ does combine high specific X-ray absorption and the possibility of needle growth. Its blue emission, peaking at 440 nm and near IR stimulation band, with maximum at 685 nm, make it well suited for use in CR systems. Sensitivity and sharpness of a 500 (mu) thick CsBr:Eu²⁺ needle screen were measured in a flying-spot scanner. The number of photostimulated light quanta per absorbed X-ray quantum is higher than for BaFBr_1-xI_x:Eu²⁺. At 70 kVp and 0.5 mm Cu filtration, equal sharpness is obtained for 85% vs. 46% X-ray absorption in BaFBr_1-xI_x:Eu²⁺ screens. DQE was measured at 2.5 (mu) Gy, 70 kVp, and 0.5 mm Cu filtration for a CsBr:Eu²⁺ needle screen in a flying-spot scanner. Up to 3 lp/mm, DQE was 2 times higher than for state-of-the-art CR systems and equal to the DQE claimed for flat panel DR systems, based on a-Si photodiodes combined with a CsI:Tl scintillator layer.

High-resolution CMOS imaging detector

Thorsten Graeve, Gene P. Weckler

Show abstract

A 2.5 cm by 5 cm, 512 by 1024 pixel CMOS photodiode array detector designed specifically for digital radiography will be described. All necessary scanning and readout circuitry is integrated within the detector. The small pixel spacing of 48 micrometers allows the imager to easily achieve the 10 lp/mm resolution required for the targeted interventional mammography application. Direct coupling to the scintillator and a pixel fill factor of more than 80% leads to high DQE over a large range of exposure values. The detector exhibits very low dark current of about 30 pA/cm² at room temperature, which allows for low-noise operation and long integration times. Read noise of less than 200 electrons rms and a saturation level of 2.8 X 10⁶ electrons combine for a large dynamic range greater than 80 dB. The conversion gain of the detector is 0.5 (mu) V/electron. Combined with a Gd₂O²S scintillator, the imager achieves up to 25% MTF at 10 lp/mm and a DC detective quantum efficiency of 50%. The detector design is optimized for x-ray energies between 10 kV and 50 kV, but can be retrofitted with different scintillator screens to cover a large range of imaging applications up to 160 kV.

Performance of a flat-panel cardiac detector

Paul R. Granfors, Douglas Albagli, J. Eric Tkaczyk, et al.

Show abstract

We report the results of performance measurements for an amorphous silicon flat panel detector used in a cardiovascular imaging system. The detector contains 1024 x 1024 elements on a 0.2 mm pitch for an active image area of about 20.5 x 20.5 cm². The system allows imaging at fluoroscopic and dynamic cardiac record exposure levels at rates of up to 30 Hz. We measured MTF, NPS, DQE, contrast ratio, response uniformity, resolution uniformity, and lag. Measurements were made on 28 production detectors. The MTF was greater than 0.2 at 2.5 cycles/mm. Contrast ratio was several hundred, indicating negligible long range scatter (veiling glare) within the detector. The DQE of the detector was measured at exposures typical of fluoroscopic imaging, dynamic cardiac record imaging, and digital subtraction angiography (DSA). The DQE was at least 0.65, 0.54, and 0.34 at 0, 1, and 2 cycles/mm, respectively, for all of these exposure levels. The response of the detector varied by less than 12% across its surface. The MTF, measured at nine positions over the surface of the detector, was found to have a maximum difference among positions of less than 0.05 at both 1 and 2 cycles/mm. First frame lag was less than 5%.

Fast imaging of a 41-cm amorphous silicon flat-panel detector for radiographic applications

Kenneth S. Kump

Show abstract

A production 41cm square digital flat panel detector constructed of amorphous silicon with a cesium iodide scintillator was characterized for its applicability to 'fast' radiographic imaging. Representative fast imaging clinical applications include tomosynthesis and dual energy utilizing two exposures. The detector supports a read time of 125msec at a radiographic dynamic range of 7mR, giving a frame rate of 7.5 acquisitions per second. Detector performance measures of lag, detector sensitivity, and DQE were evaluated as a function of acquisition rate and exposure. The first frame lag is approximately 2% of the stimulus and decays to less than 0.5% for all subsequent frames. In a sequence at constant exposure, the lag builds up as the signal level grows to 5%, reaching a steady state after approximately 15 frames. The DQE measurement is affected by lag and must be corrected. The GE radiographic detector has outstanding dynamic performance and gives all indications that it will meet the fast imaging application requirements.

Comprehensive and automated image quality performance measurement of computed radiography systems

Xiaohui Wang, Richard L. Van Metter, David H. Foos, et al.

Show abstract

A method is developed that enables comprehensive and automatic image quality testing of storage phosphor-based computed radiography (CR) systems. The measurements made with this method are both quantitative and objective, and are consistent with the guidelines set forth by the American Association of Physics in Medicine Task Group 10. The method is implemented as a toolkit that consists of a test phantom, a procedure for making controlled x-ray exposures, and software that automatically analyzes phantom and flat-field images then compares the measured results against a set of tolerances. By using this new method, acceptance testing and periodic image quality control testing of CR systems can be performed quickly and efficiently. A complete CR test that includes measurements for three cassette sizes can be finished in about one hour.

Comparison of VRX CT scanners geometries

Frank A. DiBianca, Roman Melnyk, Christopher N. Duckworth, et al.

Show abstract

A technique called Variable-Resolution X-ray (VRX) detection greatly increases the spatial resolution in computed tomography (CT) and digital radiography (DR) as the field size decreases. The technique is based on a principle called `projective compression' that allows both the resolution element and the sampling distance of a CT detector to scale with the subject or field size. For very large (40 - 50 cm) field sizes, resolution exceeding 2 cy/mm is possible and for very small fields, microscopy is attainable with resolution exceeding 100 cy/mm. This paper compares the benefits obtainable with two different VRX detector geometries: the single-arm geometry and the dual-arm geometry. The analysis is based on Monte Carlo simulations and direct calculations. The results of this study indicate that the dual-arm system appears to have more advantages than the single-arm technique.

Monte-Carlo-based simulation tool to model the physics and geometry of electron beam computed tomography

Chris H. Cagnon, Michael F. McNitt-Gray, John J. DeMarco

Show abstract

The CT image is a representation of the patient's anatomy as measured in terms of such physical characteristics as density, electron density, and atomic number. The process of sampling the patient's molecular composition with the x-ray beam is subject to varied physical effects that degrade the ability of the CT image data set to accurately represent the tissues of the body. To assess the impact of patient and scanner related characteristics on the final CT image a Monte Carlo based modeling has been developed that can simulate such effects as scatter and beam hardening on the reconstructed CT image. By selectively studying the effects of these variables, the model can be used as a design tool for improving the diagnostic capabilities of a CT scanner and /or correcting or eliminating unwanted sources of variation in the CT image. Initial simulations model the unique geometry of Electron Beam CT with a clinical goal of correcting for scanner and patient related physical factors that may cause variations in the assessment of Coronary Artery Calcium.

Novel reconstruction algorithm for multiphasic cardiac imaging using multislice helical CT

Erdogan Cesmeli, Peter M. Edic, Maria Iatrou, et al.

Show abstract

Cardiac imaging is still a challenge to CT reconstruction algorithms due to the dynamic nature of the heart. We have developed a new reconstruction technique, called the Flexible Algorithm, which achieves high temporal resolution while it is robust to heart-rate variations. The Flexible Algorithm, first, retrospectively tags helical CT views with corresponding cardiac phases obtained from associated EKG. Next, it determines a set of views for each slice, a stack of which covers the entire heart. Subsequently, the algorithm selects an optimum subset of views to achieve the highest temporal resolution for the desired cardiac phase. Finally, it spatiotemporally filters the views in the selected subsets to reconstruct slices. We tested the performance of our algorithm using both a dynamic analytical phantom and clinical data. Preliminary results indicate that the Flexible Algorithm obtains improved spatiotemporal resolution for a large range of heart rates and variations than standard algorithms do. By providing improved image quality at any desired cardiac phase, and robustness to heart rate variations, the Flexible Algorithm enables cardiac applications in CT, including those that benefit from multiphase information.

Cone-beam volume CT mammographic imaging: feasibility study

Biao Chen, Ruola Ning

Show abstract

X-ray projection mammography, using a film/screen combination or digital techniques, has proven to be the most effective imaging modality for early detection of breast cancer currently available. However, the inherent superimposition of structures makes small carcinoma (a few millimeters in size) difficult to detect in the occultation case or in dense breasts, resulting in a high false positive biopsy rate. The cone-beam x-ray projection based volume imaging using flat panel detectors (FPDs) makes it possible to obtain three-dimensional breast images. This may benefit diagnosis of the structure and pattern of the lesion while eliminating hard compression of the breast. This paper presents a novel cone-beam volume CT mammographic imaging protocol based on the above techniques. Through computer simulation, the key issues of the system and imaging techniques, including the x-ray imaging geometry and corresponding reconstruction algorithms, x-ray characteristics of breast tissues, x-ray setting techniques, the absorbed dose estimation and the quantitative effect of x-ray scattering on image quality, are addressed. The preliminary simulation results support the proposed cone-beam volume CT mammographic imaging modality in respect to feasibility and practicability for mammography. The absorbed dose level is comparable to that of current two-view mammography and would not be a prominent problem for this imaging protocol. Compared to traditional mammography, the proposed imaging protocol with isotropic spatial resolution will potentially provide significantly better low contrast detectability of breast tumors and more accurate location of breast lesions.

Four-dimensional x-ray attenuation model of the human heart and the coronary vasculature for assessment of CT system capability

Peter M. Edic, Maria Iatrou, Harvey E. Cline, et al.

Show abstract

With the introduction of helical, multi-detector computed tomography (CT) scanners having sub-second scanning speeds, clinicians are currently investigating the role of CT in cardiac imaging. In this paper, we describe a four-dimensional (4D) x-ray attenuation model of a human heart and the use of this model to assess the capabilities of both hardware and software algorithms for cardiac imaging. We developed a model of the human thorax, composed of several analytical structures, and a model of the human heart, constructed from several elliptical surfaces. A model for each coronary vessel consists of a torus placed at a suitable location on the heart's surface. The motion of the heart during the cardiac cycle was implemented by applying transformational operators to each surface composing the heart. We used the 4D model of the heart to generate forward projection data, which then became input into a model of a CT imaging system. The use of the model to predict image quality is demonstrated by varying both the reconstruction algorithm (sector-based, half-scan) and CT system parameters (gantry speed, spatial resolution). The mathematical model of the human heart, while having limitations, provides a means to rapidly evaluate new reconstruction algorithms and CT system designs for cardiac imaging.

Large-area two-dimensional detector system for real-time three-dimensional CT (4D CT)

Yasuo Saito, Hiroshi Aradate, Hiroaki Miyazaki, et al.

Show abstract

Real-time 3D CT is a high-speed cone-beam CT (4D-CT) with good low-contrast detectability. In a single rotation, a voxel data set with higher spatial resolution over a wide z- axis range can be obtained. Using continuous rotation, temporarily continuous voxel data sets and 3D dynamic images can be acquired. We have developed a large area 2D detector for 4D-CT, and we have got the first 4D-CT test images.

Development and characterization of a dual-energy subtraction imaging system for chest radiography based on CsI:Tl amorphous silicon flat-panel technology

John M. Sabol, Gopal B. Avinash, Francois Nicolas, et al.

Show abstract

Dual-energy subtraction imaging increases the sensitivity and specificity of pulmonary nodule detection in chest radiography by reducing the contrast of overlying bone structures. Recent development of a fast, high-efficiency detector enables dual-energy imaging to be integrated into the traditional workflow. We have modified a GE Revolution^TM XQ/i chest imaging system to construct a dual-energy imaging prototype system. Here we describe the operating characteristics of this prototype and evaluate image quality. Empirical results show that the dual-energy CNR is maximized if the dose is approximately equal for both high and low energy exposures. Given the high detector DQE, and allocation of dose between the two views, we can acquire dual-energy PA and conventional lateral images with total dose equivalent to a conventional two-view film chest exam. Calculations have shown that the dual-exposure technique has superior CNR and tissue cancellation than single-exposure CR systems. Clinical images obtained on a prototype dual-energy imaging system show excellent tissue contrast cancellation, low noise, and modest motion artefacts. In summary, a prototype dual-energy system has been constructed which enables rapid, dual-exposure imaging of the chest using a commercially available high-efficiency, flat-panel x-ray detector. The quality of the clinical images generated with this prototype exceeds that of CR techniques and demonstrates the potential for improved detection and characterization of lung disease through dual-energy imaging.

Micrometer resolution imaging using unmonochromatized synchrotron x rays: phantom, human breast tissue, and live animal imaging studies

Hee-Joung Kim, Haijo Jung, Jin-O Hong, et al.

Show abstract

A new 5C1 beam line was recently built on Pohang Light Source (PLS) in Korea for possible medical applications. An imaging system on the beam line consisted of silicon wafer attenuators, tungsten slit, CdWO₄ scintillator, CCD camera, and personal computer. Both spatial resolution test pattern and glass capillary filled with air bubbles were imaged to evaluate resolution and phase-contrast effects for synchrotron imaging system, respectively. Mammographic accreditation phantom, cancerous human breast specimens, and fish and nude mouse were also imaged. Both spatial resolution and image quality of the synchrotron imaging system were compared with those of conventional mammography system. The images of resolution test pattern showed a few tens of micrometer spatial resolution on synchrotron imaging system. The images of capillary filled with air bubbles revealed phase contrast effect. Mammographic accreditation phantom, human breast specimen, and animal images with synchrotron imaging system showed much higher resolution with great details of object and higher enhanced contrast compared to those with conventional mammography system. We successfully implemented and tested a micrometer resolution imaging system on PLS 5C1 beam line. Using a simple and inexpensive synchrotron imaging system on PLS 5C1 beam line, micrometer resolution images for resolution test pattern, mammographic accreditation phantom, human breast specimens, and live animals were acquired and visually evaluated.

Adaptive view synthesis for aliasing artifact reduction

Jiang Hsieh, Christopher L. Slack, Sandeep Dutta, et al.

Show abstract

In recent years, the scan speed of computed tomography (CT) has increased significantly. Not long ago, the state-of-the- art CT was only capable of completing a single scan in 1.0 s per gantry rotation. Nowadays, 0.5 s per revolution is nearly an industry standard. Faster scan speeds demand faster sampling of the projections to combat aliasing artifacts, and higher x-ray tube output to ensure sufficient x-ray photon flux delivered to the scan. These demands often exceed the technological capability of these components. In this paper we performed a detailed analysis on the characteristics of the view aliasing artifact. Based on our analysis and clinical observations, we propose an adaptive view synthesis (AVS) scheme that effectively reduces the demands on the data acquisition system. Detailed performance comparison between the full view sampling and the adaptive view synthesis are performed through computer simulations and phantom experiments. Our analysis indicates that AVS is adequate for routine clinical applications.

Effect of scatter on image quality in synchrotron radiation mammography

Raphael Moeckli, Francis R. Verdun, Stefan Fiedler, et al.

Show abstract

The display of low-contrast structures and fine microcalcifications is essential for the early diagnosis of breast cancer. In order to achieve a high image quality level with a minimum amount of radiation delivered to the patient, the use of different spectra (Mo or Rh anode and filters) was introduced. The European Synchrotron Radiation Facility is able to produce a monochromatic beam with a high photon flux. It is thus a powerful tool to study the effect of beam energy on image quality and dose in mammography. Our image quality assessment is based on the calculation of the size of the smallest microcalcification detectable on a radiograph, derived from the statistical decision theory. The mean glandular dose is simultaneously measured. Compared with conventional mammography units, the monochromaticity of synchrotron beams improves contrast and the use of a slit instead of an anti-scatter grid leads to a higher primary beam transmission. The relative contribution of these two effects on image quality and dose is discussed.

Flat-panel detector-based cone-beam volume CT breast imaging: preliminary phantom study

Ruola Ning, Biao Chen, David L. Conover, et al.

Show abstract

The clinical goal of breast imaging is to detect tumor masses when they are as small as possible, preferably less than 10 mm in diameter. Conventional film-screen mammography is the most effective tool for the early detection of breast cancer currently available. However, conventional mammography has relatively low sensitivity to detect small breast cancers (under several millimeters). Specificity and the positive predictive value of mammography remain limited owing to an overlap in the appearance of benign and malignant lesions, and surrounding structure. The limitations accompanying conventional mammography is to be addressed by incorporating a cone beam volume CT reconstruction technique with a recently developed flat panel detector. A computer simulation study has been performed to prove the feasibility of developing a flat panel detector-based cone beam volume CT breast imaging (FPD-CBVCTBI) technique. In this study, a preliminary phantom experiment is conducted to verify the findings in the computer simulation using a prototype flat panel detector-based cone beam volume CT scanner. The results indicate that the FPD-CBVCTBI technique effectively removes structure overlap and significantly improves the detectability of small breast tumors. This suggests that FPD-CBVCTBI is a potentially powerful breast-imaging tool.

Lesion detectability in digital radiography

Robert M. Gagne, Jonathan S. Boswell, Kyle J. Myers, et al.

Show abstract

The usefulness of Fourier-based measures of imaging performance has come into question for the evaluation of digital imaging systems. Figures of merit such as detective quantum efficiency are relevant for linear, shift-invariant systems with stationary noise. However, no digital imaging system is shift invariant, and realistic images do not satisfy the stationarity condition. Our methods for task- based evaluation of imaging systems, based on lesion detectability, do not require such assumptions. We have computed the performance of Hotelling and nonprewhitening matched-filter observers for the task of lesion detection in digital radiography.

Tailoring the x-ray beam energy spectrum to enhance image quality of new radiography contrast agents based on Gd or other lanthanides

F. Avraham Dilmanian, Hanns-Joachim Weinmann, Zhong Zhong, et al.

Show abstract

Gadovist, a 1.0-molar Gd contrast agent from Schering AG, Berlin, Germany, in use in clinical MRI in Europe, was evaluated as a radiography contrast agent. In a collaboration with Brookhaven National Laboratory (BNL), Schering AG is developing several such lanthanide-based contrast agents, while BNL evaluates them using different x-ray beam energy spectra. These energy spectra include a 'truly' monochromatic beam (0.2 keV energy bandwidth) from the National Synchrotron Light Source (NSLS), BNL, tuned above the Gd K-edge, and x-ray-tube beams from different kVp settings and beam filtrations. Radiographs of rabbits' kidneys were obtained with Gadovist at the NSLS. Furthermore, a clinical radiography system was used for imaging rabbits' kidneys comparing Gadovist and Conray, an iodinated contrast agent. The study, using 74 kVp and standard Al beam filter for Conray and 66 kVp and an additional 1.5 mm Cu beam filter for Gadovist, produced comparable images for Gadovist and Conray; the injection volumes were the same, while the radiation absorbed dose for Gadovist was slightly smaller. A bent-crystal silicon monochromator operating in the Laue diffraction mode was developed and tested with a conventional x-ray tube beam; it narrows the energy spectrum to about 4 keV around the anode tungsten's K' line. Preliminary beam-flux results indicate that the method could be implemented in clinical CT if x-ray tubes with approximately twice higher output become available.

Elimination of volume anisotropy in tomosynthesis through biorthogonal merging of reconstructions produced from contiguous projections

Richard L. Webber, Alexander L. Berestov, Jeffrey W. Duryea

Show abstract

This investigation tests the hypothesis that volume anisotropy intrinsic to tomosynthetic reconstructions can be minimized through integration of contiguously sampled orthogonal projections. The basic idea involves the acquisition of data from two orthogonal projection geometries using total disparity angles of 90 degree(s) for each projection series. This allows projections at the angular extremes of one series to correspond to projections produced at opposite angular extremes of the series orthogonal to the first.

Design and image quality results from volumetric CT with a flat-panel imager

William Ross, Samit Basu, Peter M. Edic, et al.

Show abstract

Preliminary MTF and LCD results obtained on several volumetric computed tomography (VCT) systems, employing amorphous flat panel technology, are presented. Constructed around 20-cm x 20-cm, 200-mm pitch amorphous silicon x-ray detectors, the prototypes use standard vascular or CT x-ray sources. Data were obtained from closed-gantry, benchtop and C-arm-based topologies, over a full 360 degrees of rotation about the target object. The field of view of the devices is approximately 15 cm, with a magnification of 1.25-1.5, providing isotropic resolution at isocenter of 133-160 mm. Acquisitions have been reconstructed using the FDK algorithm, modified by motion corrections also developed by GE. Image quality data were obtained using both industry standard and custom resolution phantoms as targets. Scanner output is compared on a projection and reconstruction basis against analogous output from a dedicated simulation package, also developed at GE. Measured MTF performance is indicative of a significant advance in isotropic image resolution over commercially available systems. LCD results have been obtained, using industry standard phantoms, spanning a contrast range of 0.3-1%. Both MTF and LCD measurements agree with simulated data.

Application of digital tomosynthesis to hand radiography for arthritis assessment

Jeffrey W. Duryea, James T. Dobbins III

Show abstract

Arthritis is a painful condition with enormous societal impact. Arthritis damages the articular cartilage between adjacent bones in a joint, which is seen radiographically as narrowing of the joint space width (JSW). JSW is an important arthritis outcome measure however a single radiographic image is a 2D projection of a 3D structure and diseased areas can be obscured. To quantify the JSW in three dimensions we have applied digital tomosynthesis imaging to hand radiography. A tomosynthesis algorithm, developed for use in chest radiography, was modified to provide reconstructed slices through the bones that formed joints of the hand. The methodology was tested using simulated radiographs of dry-bone specimens from 3 hand skeletons. Estimates to the JSW in 3D were made from the reconstructed slices. The algorithm produced tomographic slices through the bones of the joint with minimal loss of spatial resolution. We discovered that hand radiography is ideally suited for tomosynthesis imaging due to the small amount of scatter and lack of truncation artifacts. We have demonstrated the utility of digital tomosynthesis for use in quantifying JSW for arthritis assessment. The method shows promise for improving the assessment of disease progression.

Optimization of matrix inverse tomosynthesis

Devon J. Godfrey, Richard J. Warp, James T. Dobbins III

Show abstract

Digital tomosynthesis is a method for reconstructing arbitrary planes in an object from a series of projection radiographs, acquired with limited angle tube movement. Conventional 'shift and add' tomosynthesis suffers from the presence of blurring artifacts, created by objects located outside of each reconstructed plane. Matrix inversion tomosynthesis (MITS) uses known geometry, and a set of coupled linear algebra equations to solve for the blurring function in each reconstructed plane, enabling removal of the unwanted out-of-plane blur artifacts. For this paper, both MITS and conventional tomosynthesis reconstructions were generated for a simulated impulse located at varying distance from the detector, and also an anthropomorphic chest phantom. Exploration of the effects of total angular tube movement, number of projection radiographs acquired, and number of planes reconstructed via matrix inversion tomosynthesis, on residual out-of-plane blur ensued. We conclude that optimization of image acquisition and plane reconstruction parameters can improve slice image quality. In all examined scenarios, the MITS algorithm outperforms conventional tomosynthesis in removing out-of-plane blur.

Image reconstruction of optical tomographic data from a highly scattering medium

Junwon Lee, Donald W. Wilson, Harrison H. Barrett, et al.

Show abstract

Recently new technologies for detecting biomolecules have been developed and are opening a new era of medical imaging. Chemiluminescence and fluorescence are emerging as promising tools for these tasks. These molecules emit optical photons that can be observed outside the body. Unfortunately, they are heavily scattered and absorbed in biological tissue. This is an obstacle for determining a way of mapping an original source distribution. In order to overcome this obstacle, we suggest a new concept, Optical Emission Computed Tomography and test its feasibility with computer simulation.

Volumetric cone-beam CT system based on a 41x41 cm2 flat-panel imager

David A. Jaffray, Jeffrey H. Siewerdsen

Show abstract

Cone-beam computed tomography (CBCT) based upon large-area flat-panel imager (FPI) technology is a flexible and adaptable technology that offers large field-of-view (FOV), high spatial resolution, and soft-tissue imaging. The imaging performance of FPI-based cone-beam CT has been evaluated on a computer-controlled bench-top system using an early prototype FPI with a small FOV (20.5 X 20.5 cm²). These investigations demonstrate the potential of this exciting technology. In this report, imaging performance is evaluated using a production grade large-area FPI (41 X 41 cm²) for which the manufacturer has achieved a significant reduction in additive noise. This reduction in additive noise results in a substantial improvement in detective quantum efficiency (DQE) at low exposures. The spatial resolution over the increased FOV of the cone-beam CT system is evaluated by imaging a fine steel wire placed at various locations within the volume of reconstruction. The measured modulation transfer function (MTF) of the system demonstrates spatial frequency pass beyond 1 mm-1 (10% modulation) with a slight degradation at points off the source plane. In addition to investigations of imaging performance, progress has also been made in the integration of this technology with a medical linear accelerator for on-line image-guided radiation therapy. Unlike the bench-top system, this implementation must contend with significant geometric non-idealities caused by gravity-induced flex of the x-ray tube and FPI support assemblies. A method of characterizing and correcting these non-idealities has been developed. Images of an anthropomorphic head phantom qualitatively demonstrate the excellent spatial resolution and large FOV achievable with the cone-beam approach in the clinical implementation.

Simultaneous assessment of human brain functional hemodynamics by magnetic resonance and near-infrared imaging

Vladislav Yu. Toronov, Andrew Webb, Jee Hyun Choi, et al.

Show abstract

Near-infrared spectroscopy is a relatively new imaging method, which can provide important information on concentrations of oxy-and deoxy-hemoglobin in cortical areas of the brain. We discuss the advantages of the integration of magnetic resonance and optical imaging techniques and present the results of our experimental study on the comparison of optical and fMRI signals obtained simultaneously on humans during functional activity and at rest. In all subjects we found a good collocation of the brain activity centers revealed by both methods. We also found a high temporal correlation between the BOLD signal (fMRI) and the deoxy-hemoglobin concentration (near-infrared spectroscopy) in the subjects who exhibited low fluctuations in superficial head tissues. The contamination of optical signals by superficial tissue layers urges applying algorithms of three-dimensional optical tomography.

Monochromatic imaging with a conventional source using polycapillary x-ray optics

Francisca R. Sugiro, Carolyn A. MacDonald

Show abstract

Monochromatic parallel beam imaging produces high subject contrast, high resolution, and low patient dose. Polycapillary collimating optics can be used to create a beam of sufficient intensity for monochromatization from a conventional source. Monochromatization is achieved by diffraction from a single crystal. Contrast, resolution, and intensity measurements were performed with both high and low angular acceptance crystals. Testing was first done at 8 keV with an intense copper rotating anode, then preliminary 17.5 keV measurements were made with a low power molybdenum source. At 8 keV, contrast enhancement was a factor of 5 relative to the polychromatic case, in good agreement with theoretical values. At 17.5 keV, monochromatic subject contrast is a factor of 2 times greater than the conventional polychromatic contrast. An additional factor of two increase in contrast is expected from the removal of scatter obtained from using the air gap which is allowable from the parallel beam. The measured angular resolution after the crystal was 0.6 mrad for a silicon crystal. The use of polycapillary collimating optics allowed monochromatic imaging measurements using a conventional rotating anode source and computed radiography plate in 300 mAs.

Copper crystal lens for medical imaging: first results

Dante Eduardo Roa, Robert K. Smither

Show abstract

A copper crystal lens designed to focus gamma ray energies of 100 to 200 keV has been assembled at Argonne National Laboratory. In particular, the lens has been optimized to focus the 140.6 keV gamma rays from technetium-99 m typically used in radioactive tracers. This new approach to medical imaging relies on crystal diffraction to focus incoming gamma rays in a manner similar to a simple convex lens focusing visible light. The lens is envisioned to be part of an array of lenses that can be used as a complementary technique to gamma cameras for localized scans of suspected tumor regions in the body. In addition, a 2- lens array can be used to scan a woman's breast in search of tumors with no discomfort to the patient. The incoming gamma rays are diffracted by a set of 828 copper crystal cubes arranged in 13 concentric rings, which focus the gamma rays into a very small area on a well-shielded NaI detector. Experiments performance with technetium-99 m and cobalt 57 radioactive sources indicate that a 6-lens array should be capable of detecting sources with (mu) Ci strength.

New approach for analyzing magnetic resonance elastography images

Ingolf Sack, Gerd Buntkowsky, Johannes Bernarding, et al.

Show abstract

A new method for evaluating Magnetic Resonance Elastography (MRE) wave images is introduced, which consists of both local frequency estimation (LFE) and simulation of wave patterns by a coupled harmonic oscillator (CHO) approach. It is shown that i) LFE performs improved reconstruction by use of Gauss filters and ii) CHO calculations can help to refine the resulting wave speed or elasticity map by taking local attenuation, reflection, and diffraction into account. The performance of new LFE and CHO calculations is demonstrated by MRE experiments on a gel phantom as well as by simulations of shear waves in a brain phantom, such as for potential in-vivo MRE-experiments.

Physics of medical imaging with crystal diffraction lenses

Robert K. Smither, Dante Eduardo Roa

Show abstract

A new type of medical imaging system based on crystal diffraction is being developed at the Advanced Photon Source at Argonne National Laboratory. It is designed to image very small amounts of radioactivity in the human body. The system has very fine spatial resolution (1-3 mm) and very high sensitivity. By combining two or more lenses, one can generate a 3-D image of the cancer. Micro-Curie sources can be detected with relative ease. These features make the system very useful to confirm or reject possible sites for a cancer in the human body following a full body scan. It could also have considerable use as a method of checking for breast cancer.

40 x 30 cm flat-panel imager for angiography, R&F, and cone-beam CT applications

Richard E. Colbeth, Sarah J. Boyce, Robert Fong, et al.

Show abstract

Preliminary results are presented from the PaxScan 4030A; a 40x30cm, 2048 x 1536 landscape, flat panel imager, with 194um pixel pitch. This imager builds on our experience with the PaxScan 2520, a 127um real-time flat panel detector capable of both high-resolution radiography and low dose fluoroscopy. While the PS2520 has been applied in C-arms, neuroangiography, cardiac imaging and small area radiographic units, the larger active area of the PaxScan 4030A addresses the broader applications of angiography, general R&F and cone-beam CT. The PaxScan 4030A has the same electrical and software interfaces as the PS2520; however, a number of innovations have been incorporated into the 4030A to increase its versatility. The most obvious change is that the data interface between the receptor and command processor has been reduced to one very flexible and thin fiber-optic cable. A second new feature for the 4030A is the use of split datalines. Split datalines facilitate scanning the two halves of the array in parallel, cutting the readout time in half and increasing the time window for pulsed x-ray delivery to 15ms at 30fps. In addition, split datalines result in lower noise, which, coupled with the larger signal of the 194um pixels, enables high quality imaging at lower fluoroscopy doses rates.

Development and evaluation of a portable amorphous silicon flat-panel x-ray detector

Minoru Watanabe, Chiori Mochizuki, Toshio Kameshima, et al.

Show abstract

The design, development and evaluation of a portable x-ray detector are described. The completed detector has a pixel pitch of 100 micrometers , an active imaging area of 22.5 x 27.5 cm² (9 x 11 inch²), package outer dimensions of 32.5 x 32.5 cm², a thickness of only 20 mm, and a weight of around 2.8 kg. A number of significant advances in the design and production processes were needed to produce such a compact detector with such a small pixel pitch, while maintaining the image quality achieved a current detector (CXDI-22) which has a 160 mm pixel pitch. These include the development of a low power readout IC, advances in detector packaging design, concentrating on lightweight and strong components, and redesign of the pixel structure to improve the fill-factor. A comparison is made of the imaging characteristics of this new detector with the CXDI-22 detector, and it is shown that the new detector demonstrates improved CTF, and NEQ. The new detector is also shown to demonstrate superior performance in a contrast-detail phantom evaluation. This new detector should be useful for limb and joint examinations as it offers high spatial resolution, combined with the same freedom in positioning provided by conventional screen-film cassettes.

Detective quantum efficiency of an x-ray image intensifier chain as a benchmark for amorphous silicon flat-panal detectors

Andreas Koch, Jean-Marie Macherel, Thibaut Wirth, et al.

Show abstract

Amorphous silicon flat panel x-ray detectors (A-Si FXD) are expected eventually to replace traditional x-ray image intensifier systems (XRII) in medical radiography in the long term. The advantages of FXD's are their large detection area, no distortion, no sensitivity to magnetic fields, low weight and compactness. However, they do not provide the high sensitivity of specific optimized systems based on image intensifiers, which approach the sensitivity of single x-ray photon counting in an appropriate configuration whereas the noise equivalent number of photons for an a-Si imager is typically several photons at medical energies. That is, the detective quantum efficiency of an XRII at low dose is expected to be higher.

Dose reduction using photon-counting x-ray imaging

Tom Francke, Mathias Eklund, Leif Ericsson, et al.

Show abstract

Radiation dose to the patient, contrast and position resolution are important quality factors in medical X-ray imaging. A novel technique for digital X-ray imaging has been developed, using photon counting with high signal-to-noise ratios for single X-ray photons. The novel technique allows a significant dose reduction over screen-film systems, as well as a high contrast and good position resolution. The novel technique is based on photon counting gaseous detectors. A high signal-to-noise ratio for single X-ray photons allows virtually noise-free counting of photons. Low noise together with a high contrast in the image allows a significant dose reduction compared to film-based systems. The new technique makes photon counting X-ray imaging possible, only limited by quantum fluctuations.

Filtered gain calibration and its effect on frequency-dependent DQE and image quality in Se-based general radiography and full-field mammographic digital imaging

Denny L. Y. Lee, Brian G. Rodricks, Michael G. Hoffberg, et al.

Show abstract

This paper will describe details of and results for a frequency-dependent filtered gain calibration technique that optimizes DQE, yet does not reduce MTF performance which is important to both systems.

Deposition of thick films of polycrystalline mercuric iodide x-ray detectors

Haim Hermon, Michael M. Schieber, Asaf Zuck, et al.

Show abstract

We report x-ray imaging results on polycrystalline HgI₂ detector used for direct x-ray imaging. Due to its good electrical properties and high stopping power for x-rays and gamma rays, the material is a good candidate for many applications in medical imaging. The deposition of the HgI₂ thick films is made by hot wall physical vapor deposition, (PVD) method and some of the structural features are described here. The x-ray response and some dark current data measured on some recently prepared detectors are reported. Some results obtained with poly-HgI₂ thin film deposited on an amorphous-Si TFT's imaging array 4' X 4' performed at the Ginzton Technology Center is reported for the first time. Also phantom images received with a similar deposited poly-HgI₂ thin film deposited on an amorphous- Si TFT's imaging array 2' X 2' performed at Xerox-PARC Research Center is also given here. The status of HgI₂ technology will be discussed.

Experimental evaluation of a novel CdZnTe flat-panel x-ray detector for digital radiography and fluoroscopy

Satoshi Tokuda, Susumu Adachi, Toshiyuki Sato, et al.

Show abstract

We made a prototype flat-panel X-ray detector with a polycrystalline CdZnTe film, and evaluated its imaging performance with respect to leakage current, X-ray sensitivity, MTF, DQE and image lag. The detector incorporates a novel hybrid technique in which zinc-doped CdTe is pre-deposited onto a ceramic substrate and then connected to a TFT circuit substrate. We carefully selected the material for the sensor substrate in order to avoid both incident x-ray attenuation in the substrate and micro-cracks in CdZnTe film. The film thickness was approximately 300 micrometers . The imaging area is composed of 512 X 384 pixels, with a pixel pitch of 150 micrometers .

CdZnTe detector as an x-ray spectrometer

Satoshi Miyajima, Hideaki Sakuragi, Masao Matsumoto

Show abstract

In this study, we evaluated a CdZnTe (CZT) detector as an x-ray spectrometer. The output of the semiconductor detector is basically distorted due to the processes of energy deposition, charge generation and pulse processing. In addition, the distortion due to the charge transport process must be taken into account when using a CZT detector. First, we calculated response functions of the CZT detector using the EGS4. In the code, the Hecht equation was utilized to deal with the trapping of charge carriers during charge transport. The parameters in the equation, mean free path of charge carriers, were determined to make the calculated response functions the same as the measured response to mono-energetic gamma-rays. Secondly, we corrected x-ray spectra using the calculated response functions. The stripping method was employed in the correction procedure. The results indicate that a CZT detector is useful in x-ray spectrometry with the proper corrections.

New optimized hemispherical rf coil design for breast imaging

Haiying Liu, Hellmut Merkle, Michael Garwood

Show abstract

Optimal specialty RF coils have always been desirable for their high signal to noise ratio in today's magnetic resonance imaging applications, such as breast imaging. A technique based on the least square field error minimization for designing a hemispherical RF coil that yields an optimal field homogeneity is presented.

How does magnification affect image quality and patient dose in digital subtraction angiography?

Nikolaos A. Gkanatsios, Walter Huda, Keith R. Peters M.D.

Show abstract

Digital subtraction angiography images were obtained of a phantom containing 1 mm diameter vessels. The iodine concentrations ranged from 5 to 50 mg/cc, which permitted the detection threshold iodine concentration to be determined. The source to image receptor distance was 105 cm, and image magnification was varied between 1.15 and 2.0. One experiment was performed at an input exposure of 1 (mu) Gy per frame, and a second experiment was performed at 4 (mu) Gy per frame.

New technique for high-resolution absolute conductivity imaging using magnetic-resonance electrical impedance tomography (MR-EIT)

Ozlem Birgul, B. Murat Eyuboglu, Y. Ziya Ider

Show abstract

A novel MR-EIT imaging modality has been developed to reconstruct high-resolution conductivity images with true conductivity value. In this new technique, electrical impedance tomography (EIT) and magnetic resonance imaging (MRI) techniques are simultaneously used. Peripheral voltages are measured using EIT and magnetic flux density measurements are determined using MRI. The image reconstruction algorithm used is an iterative one, based on minimizing the difference between two current density distributions calculated from voltage and magnetic flux density measurements separately. The performance of the proposed method and the suggested reconstruction algorithm are tested on simulated data. A finite element model with 1089 nodes and 2048 triangular elements is used to generate the simulated potential and magnetic field measurements. A 16 electrode opposite drive EIT strategy is adopted. The spatial resolution is space independent and limited by either the finite element size or half the MR resolution. The worst of the two defines the spatial resolution. The rms error in reconstructed conductivity for a concentric inhomogeneity is calculated as 5.35% and this error increases to 13.22% when 10% uniformly distributed random noise is added to potential and magnetic flux density measurements. The performance of the algorithm for more complex models will also be presented.

Development of an automated method for evaluation of sharpness of digital radiographs using edge method

Ikuo Kawashita, Kouji Maeda, Hidetaka Arimura, et al.

Show abstract

The presampling modulation transfer function (MTF) can be determined by the edge spread function in which the sampling interval is narrower than the pixel-to-pixel interval from slight angled edge image. It is important that the precision of the presampling MTF depend on the precision of the edge angle. In this study, we have developed the automated method, which includes a precise edge angle determination process for the measurement of the presampling MTF.

Separability of three-dimensional geometric sensitivity correction in triple-headed gamma camera systems

Yves D'Asseler, Stefaan Vandenberghe, Michel Koole, et al.

Show abstract

Gamma camera PET (Positron Emission Tomography) offers a low-cost alternative for dedicated PET scanners. However, sensitivity and count rate capabilities of dual-headed gamma cameras with PET capabilities are still limited compared to full-ring dedicated PET scanners. To improve the geometric sensitivity of these systems, triple-headed gamma camera PET has been proposed. As is the case for dual-headed PET, the sensitivity of these devices varies with the position within the field of view (FOV) of the camera. This variation should be corrected for when reconstructing the images. In earlier work, we calculated the two-dimensional sensitivity variation for any triple-headed configuration. This can be used to correct the data if the acquisition is done using axial filters, which effectively limit the axial angle of incidence of the photons, comparable to 2D dedicated PET. More recently, these results were extended to a fully 3D calculation of the geometric sensitivity variation. In this work, the results of these calculations are compared to the standard approach to correct for 3D geometric sensitivity variation. Current implementations of triple-headed gamma camera PET use two independent corrections to account for three-dimensional sensitivity variations: one in the transaxial direction and one in the axial direction. This approach implicitly assumes that the actual variation is separable in two independent components. We recently derived a theoretical expression for the 3D sensitivity variation, and in this work we investigate the separability of our result. To investigate the separability of the sensitivity variations, an axial and transaxial profile through the calculated variation was taken, and these two were multiplied, thus creating a separable function. If the variation were perfectly separable, this function would be identical to the calculated variation. As a measure of separability, we calculated the percentual deviation of the separable function to the original variation. We investigated the separability for several camera configurations and rotation radii. We found that, for all configurations, the variation is not separable , and becomes less separable as the rotation radius tends to smaller values. This indicates that in this case, our sensitivity correction will give better results than the separable correction now applied.

Analytical approximations to the Hotelling trace for digital x-ray detectors

Eric Clarkson, Angel R. Pineda, Harrison H. Barrett

Show abstract

The Hotelling trace is the signal-to-noise ratio for the ideal linear observer in a detection task. We provide an analytical approximation for this figure of merit when the signal is known exactly and the background is generated by a stationary random process, and the imaging system is an ideal digital x-ray detector. This approximation is based on assuming that the detector is infinite in extent. We test this approximation for finite-size detectors by comparing it to exact calculations using matrix inversion of the data covariance matrix. After verifying the validity of the approximation under a variety of circumstances, we use it to generate plots of the Hotelling trace as a function of pairs of parameters of the system, the signal and the background.

Investigation of parameters concerning the modulation transfer function in digital radiography

Magnus Bath, Patrik Sund, Lars Gunnar Mansson

Show abstract

The effects of the design of a radiographic system on the modulation transfer function (MTF) are studied with a specially developed computer program. The program simulates a digital radiographic system by using three parameters: sampling distance, sampling aperture, and the spread of the signal in the detector due to the interaction processes of the incoming photons. The signal spread is approximated by Gaussian distributions. The influence of the three parameters is studied on the presampling MTF and on the two extreme cases of the digital MTF: the maximum MTF and the minimum MTF. From theoretical data on the interaction processes, the resolution properties of an amorphous selenium flat-panel detector are simulated. The program is also used to simulate a measurement of the presampling MTF with the slit method, and the effect of the slit width on the measured presampling MTF is examined.

Measurement of the noise power spectrum in digital x-ray detectors

Richard Aufrichtig, Yu Su, Yu Cheng, et al.

Show abstract

The noise power spectrum, NPS, is a key imaging property of a detector and one of the principle quantities needed to compute the detective quantum efficiency. NPS is measured by computing the Fourier transform of flat field images. Different measurement methods are investigated and evaluated with images obtained from an amorphous silicon flat panel x-ray imaging detector. First, the influence of fixed pattern structures is minimized by appropriate background corrections. For a given data set the effect of using different types of windowing functions is studied. Also different window sizes and amounts of overlap between windows are evaluated and compared to theoretical predictions. Results indicate that measurement error is minimized when applying overlapping Hanning windows on the raw data. Finally it is shown that radial averaging is a useful method of reducing the two-dimensional noise power spectrum to one dimension.

Measurement of myocardial T1 relaxation times in humans with inversion recovery EPI

Napapong Pongnapang, Geoffrey D. Clarke, Bahadir Ozus, et al.

Show abstract

Changes in myocardial longitudinal relaxation time (T1) are important for evaluating myocardial blood flow with first-pass contrast-enhanced MRI. Relaxation dynamics for inversion recovery echo planar imaging (IR-EPI) are less complex than partial flip angle, field echo techniques. The goal of this project is to develop and evaluate a robust method for measuring T1 in intact, beating human hearts. IR-EPI was performed on eight asymptomatic volunteers using a 1.5 Tesla MRI system. Imaging parameters were: FOV equals 42.5 cm, phase sampling ratio equals 0.609-0.781, 64 X 112 matrix, TE_eff equals 47.8 ms., at least six inversion times, TI, ranging from 72 to 1400 ms, one shot. Four short axis slices were obtained for each TI. Signal intensities were measured for four ROI myocardial segments in each slice and plotted versus TI. T1 measurements were validated using a phantom containing gadolinium contrast at various dilutions. Linear interpolation was used following logarithmic transformation to calculate myocardial T1 based on the determination of the null point. The mean global myocardial T1 value was 811 ms+/- 49.8 ms (mean +/- SD). We conclude that myocardial T1 measurements, specific to each patient, can be measured with good accuracy and reproducibility using IR-EPI.

3D distance-weighted Wiener filter for Poisson noise reduction in sinogram space for SPECT imaging

Hongbing Lu, Jui-His Cheng, Guoping Han, et al.

Show abstract

A three-dimensional (3D) distance-weighted Wiener filtering, which takes the characteristics of Poisson noise into account as well as the frequency-distance relationship of projection data, is described and evaluated. The task of spatial filtering on Poisson noise can be greatly simplified without the estimation of noise-power spectrum by first applying the Anscombe transformation to the projection data, which converts Poisson distributed noise into Gaussian distributed one with constant variance. By extending the stationary-phase condition and frequency-distance relationship (derived from the noise-free 2D sinogram) into 3D situation, we obtain a weighting function in frequency domain which is only dependent on the distance of an interested point source from the object center. Since the Anscombe transformation only changes the distribution of projection data, not the pixel location, a distance-variant weighting window for the Anscombe transformed data is derived and incorporated into the Wiener filter. Considering the regions with higher signal-to-noise ratio (SNR) receive greater weight in the estimation of signal-power spectrum, the proposed filter optimizes the data used to estimate the power spectrum of observed data and thus produces a better spatial resolution. Simulation and experimental results show improved noise reduction, especially in the peripheral regions, as compared with conventional filtering methods.

Optically coupled digital radiography: sources of inefficiency

Robert M. Gagne, Philip W. Quinn, Liying Chen, et al.

Show abstract

Imaging systems comprised of a small detector which `looks' at a large area of a light emitting phosphor with a lens coupling system are commercially available. These imaging systems can be quite inefficient depending on the characteristics of the optical coupling. We have implemented an optically coupled imaging system in our laboratory for use as a test bed. The system includes a Kodak MR-2190 screen and a digital detector, i.e., a CCD camera. The laboratory system has provided the ability to investigate when a system of this configuration starts to develop a `secondary quantum sink' as a result of poor optical coupling. Experimental measurements have been made of the large area gray-scale transfer, resolution and noise properties of the imaging system at two different values of optical demagnification factor and three levels of x-ray exposure. The values of the detective quantum efficiency calculated from these measurements demonstrate a significant increase in absolute value and bandwidth when the demagnification decreases by a factor of approximately two.

Effect of shift invariance and stationarity assumptions on simple detection tasks: spatial and spatial frequency domains

Robert M. Gagne, Kyle J. Myers, Philip W. Quinn

Show abstract

A lens-coupled digital radiography system was used to investigate the impact of shift invariance and stationarity assumptions on task-based measures of performance. Experimental data are available on the imaging system's large area gray-scale transfer (sensitivity), resolution (modulation transfer function) and noise (variance map, autocovariance function, and noise power spectrum) properties in different regions of the field of view. These data were obtained for two different values of optical demagnification factor and three levels of x-ray exposure. Evidence of shift variant and non-stationary behavior is seen in the position dependence of the sensitivity, resolution and noise properties of the imaging system. Estimates of observer performance in terms of signal-to- noise-ratio were obtained from one algorithmic observer (DC- suppressed matched filter) and from one analytic observer (pre-whitening matched filter) for the task of detecting a known object (CDMAM phantom) on a flat background. For the special case of a large object at the center of the field- of-view, estimates of observer performance were essentially the same whether based on spatial or spatial frequency domain measures.

High-resolution in-vivo micro-CT scanner for small animals

Alexander Sasov

Show abstract

Small laboratory animals (mice and rats) are widely used in development of drags and treatments. To recognize the internal changes in the very early stage inside the animal body, Skyscan starts development on high-resolution micro-CT scanner for in-vivo 3D-imaging. Initial changes in the bone structure can be found as features in the size range of 10 microns. By this reason a voxel size for reconstructed cross sections has been chosen as < 10 microns. Because of full animal may be up to 8 cm in diameter the reconstructed cross section format selected as 8000 X 8000-pixels (float- point). A 2D detection system with new multi-beam geometry produce dataset for reconstruction of hundreds cross- sections after one scan. Object illuminated by microfocus sealed X-ray source with 5 microns spot size. Continuously variable energy in the range of 20 - 100 kV and energy filters allows estimate material composition like in DEXA systems. Direct streaming of the projection data to the disk reduce irradiation dose to the animal under scanning. Software package can create realistic 3D-images from the set of reconstructed cross sections and calculate internal morphological parameters.

Comparison of fan-beam, cone-beam, and spiral scan reconstruction in x-ray micro-CT

Alexander Sasov

Show abstract

We developed and tested reconstruction software packages for different algorithms: fan-beam, cone-beam (Feldkamp) and spiral (helical) scans. All algorithms were applied to different simulations as well as to the real datasets from the commercial micro-CT instruments. From the results of testing a number of strong and weak points at different approaches was found. Several examples from the different application areas (bone microstructure, industrial applications) show typical reconstruction artifacts with different algorithms.

Contrast enhancement measurement using polycapillary x-ray optics at 20 to 40 keV

Padiyar Cari, - Suparmi, Walter M. Gibson, et al.

Show abstract

Polycapillary optics are shaped arrays of tiny hollow tubes through which x rays are guided by total external reflection at grazing incidence. Optics could be used prepatient, to shape the beam, or post patient, as scatter rejection grids. Significant resolution and contrast enhancement have been previously demonstrated at mammographic and lower energies for post patient optics. Measurements were performed to investigate the application of polycapillary post patient optics at higher energies. Measurement of contrast enhancement was performed using a small tapered optic. This tapered optic was designed to be placed into a multiple taper jig to create a large area device. The transmission of the taper was 71% at 20 keV and close to 30% at 40 keV. The scatter transmission of the taper, measured using a 6 cm thick polyethylene phantom, was about 1% at 40 keV. Because of the removal of scatter radiation, the measured contrast enhancement for the optic was a nearly factor of three at 40 keV.

Absorption dose reduction by tube current modulation in CT system

Takayuki Nagaoka, Osamu Miyazaki

Show abstract

We developed the `Adaptive mA control system' that can modulate the tube current according to a patient's shape at the time of helical CT measurement. The purpose of our system is consistent with the improvement in image quality and the dose reduction. The patient shape is obtained by single direction scanogram before scanner rotation. And the controller of our system builds a 3D water equivalent model about patient's body. Therefore, our system can always control tube current during scans. The image artifact from high-density organ is reduced by the tube current control without rising mAs (milli-ampere second). From the results of the simulation with phantom's Voxel data, we have confirmed that our method reduces absorption dose about 12% (average) in lung area.

Development of curved planar reconstruction image using virtual endoscopy

Hiroki Taniguchi, Yoshihiro Goto, Hiromitsu Hayashi, et al.

Show abstract

Multi Planar Reconstruction (MPR) is important for diagnosis because of having all information as CT images. But it is difficult to reconstruct the MPR image along a median centerline of tubular organs (i.e., Curved Planar Reconstruction: CPR), since it isn't so accurate that the median centerline of tubular organs have been plotted manually in spite of requiring the accuracy. Therefore we have developed novel technique for simple, accurate and automatic computation based on viewing points of virtual endoscopy (Cruising Eye View: CEV) which is approximately along the median centerline of the tubular organs (i.e., CEV-CPR).

Dual-energy x-ray absorptiometry using 2D digital radiography detector: application to bone densitometry

Jean-Marc Dinten, Christine Robert-Coutant, Michel Darboux

Show abstract

Dual Energy X-Rays Absorptiometry (DXA) is commonly used to separate soft tissues and bone contributions in radiographs. This decomposition leads to bone mineral density (BMD) measurement. Most clinical systems use pencil or fan collimated X-Rays beam with mono detectors or linear arrays. On these systems BMD is computed from bi-dimensional (2D) images obtained by scanning. Our objective is to take advantage of the newly available flat panels detectors and to propose a DXA approach without scanning, based on the use of cone beam X-Rays associated with a 2D detector. This approach yields bone densitometry systems with an equal X and Y resolution, a fast acquisition and a reduced risk of patient motion.Scatter in this case becomes an important issue. While scattering is insignificant on collimated systems, its level and geometrical structure may severely alter BMD measurement on cone beam systems. In our presentation an original DXA method taking into account scattering is proposed. This new approach leads to accurate BMD values.In order to evaluate the accuracy of our new approach, a phantom representative of the spine regions tissue composition (bone, fat , muscle) has been designed. The comparison between the expected theoretical and the reconstructed BMD values validates the accuracy of our method. Results on anthropomorphic spine and hip regions are also presented.

Evaluation of dual-energy radiography with a digital x-ray detector

Regis Guillemaud, Christine Robert-Coutant, Michel Darboux, et al.

Show abstract

Dual-energy imaging has been proposed as a method for producing material-specific images, thus permitting separate examination of bone and soft-tissue structures. Interesting clinical results, particularly for chest, have been presented usually for screen-film or phosphor plate detectors and with single exposure. The purpose of the paper is to investigate double exposure dual-energy with a digital X-Ray detector. The study is performed with a CCD-based large field digital X-Ray detector (Paladio detector, Apelem) installed on a remote table. Dual exposure is feasible on this detector with little registration problem because we have a very short delay (< 0.5 s) between two acquisitions. For each examination, two radiographs are acquired at two different high and low energies and with adapted X-ray tube filtrations. X-ray generator energy voltages and filtrations are optimized in order to obtain thin energy peak spectra with good spectral separation (50 keV), much better than with single exposure systems. Tissue decomposition images are estimated from both acquisitions. The decomposition process is helped by the nice spectral separation. Scatter correction, applied to the raw dual-exposure acquisitions, provides an improvement of tissue decomposition. Results are shown for a chest phantom.

Performance of a small-field digital detector for soft x-ray imaging

Allen Seifert, Michael J. Flynn, Manjul Shah, et al.

Show abstract

X-ray imaging detectors capable of very high resolution for a small field of view are important for x-ray micro-tomography, small specimen radiography, and certain x-ray scattering experiments. We have investigated the performance of small field detectors using scintillation phosphors coupled to a scientific CCD detector. The specific detector designs considered had fields of 8-12 mm that were used to record x-ray energies of 8-20 keV. The purpose of this work is to report the resolution (MTF) of designs that employed different optical coupling methods and different scintillation phosphor materials. For one detector system with a thin Gd₂O₂S phosphor a resolution of 48 lp/mm (presampled MTF = 0.10) was measured with pixels of 10.54 microns (Nyquist = 47.44) and a field of view of 12.14 mm x 13.09 mm.

Practical x-ray scatter measurements for volume CT detector design

Timothy R. Fox, David T. Nisius, Hiroshi Aradate, et al.

Show abstract

To help design a volume CT scanner, we measured x-ray scatter through large irradiated volumes, with and without detector collimator. An x-ray tube (125 to 150 kV) with an adjustable diaphragm irradiates volumes 25 to 200 mm thick. The scattering objects are water cylinders (approximate diameters 200, 300, and 500 mm). Complementary apertures (between the object and the detector collimator, along a line from the source) select `scatter' or `direct' detector signals. A direct-defining hole in a lead plate mounts over a pilot hole in a thin plastic sheet. With the lead plate removed, a scatter-defining plug fits into the pilot hole to block the same solid angle.

Direct-conversion flat-panel detector for region-of-interest angiography

Arundhuti Ganguly, Stephen Rudin, Daniel R. Bednarek, et al.

Show abstract

Minimally invasive image-guided interventions require very high image resolution and quality, specifically over regions-of-interest (ROI) crucial to the procedure. An ROI high quality image allows limited patient radiation deposition while permitting rapid frame transfer rates. Considering current developments in direct conversion Flat Panel Detectors (FPD), advantages of such an imager for ROI angiography were investigated. The performance of an amorphous-selenium based FPD was simulated to evaluate improvements in MTF and DQE under various angiographic imaging conditions. The detector envisioned incorporates the smallest pixel size of 70 mm, reported to date, and a photoconductor thickness of 1000 mm to permit angiography. The MTF of the FPD is calculated to be 60% at the Nyquist frequency of 7.1 lp/mm compared to 6% for a previously reported CsI(Tl)-based ROI CCD camera. The DQE(0) of the FPD at 0.7 mR and 70 kVp is 74% while for the CCD camera is 70%. At 7.1 lp/mm, the FPD's DQE is 26% while for the CCD camera it is 12%. Images of an undeployed stent with 70 mm pixel mammography FPD prototype, compare favorably with images acquired with the CCD camera. Thus a practical direct flat-panel ROI detector with both improved performance and physical size is proposed.

Comparison of a-Si:H CsI flat-panel digital imaging systems with a CCD-based system, CR systems, and conventional screen-film systems: a contrast-detail phantom study

John X. Rong, Chris C. Shaw, Dennis A. Johnston, et al.

Show abstract

Low-contrast detail detectability was evaluated and compared for a flat-panel digital chest system, a computed radiography (CR) system and a conventional screen/film (SF) system. Images of a contrast-detail phantom were acquired using these three systems under identical conditions. Additional images were acquired at varied exposures to study the potential for reduction of patient exposure using the flat-panel system. The results demonstrated that in chest imaging, the flat-panel system performed significantly better than the CR and the SF systems while the latter two performed about the same. Alternatively, an exposure reduction of at least 50% is possible if the same performance is maintained. For mammographic imaging, detectability for microcalcifications ((mu) Cs) was evaluated and compared for a flat-panel based full-field digital mammography (FFDM) system, a charge-coupled device (CCD) -based small-field system, a high resolution CR system and a conventional SF system. Images of simulated calcifications of three size ranges were acquired and evaluated by readers for detectability of the (mu) Cs. A Receiver Operating Characteristics (ROC) analysis was also performed to compare the overall detection accuracy for these four mammographic imaging systems. Our results show that in both the detectability analysis and the ROC analysis, the flat-panel systems performed the best followed by the screen/film system. The CCD based system showed better detection accuracy compared to the CR system in the ROC analysis. However, there was no significant difference between the CCD and the CR systems in the detectability analysis.

Comparison of a-Si:H/Csl flat-panel digital imaging systems with CR-and CCD-based systems: image quality measurements

Xinming Liu, Chris C. Shaw, Xiujiang Rong, et al.

Show abstract

The amorphous silicon/cesium iodide (a-Si:H/CsI:Tl) flat-panel imaging systems have recently become commercially available for both chest and mammographic imaging applications. This new detector technology is considered to be a significant improvement over CR techniques. In this work, we measured the image properties for two commercial flat-panel systems and compared them with those measured for CR and CCD based imaging systems. Image quality measurements related to detector properties such as linearity, MTF, NPS and DQE are presented and compared at selected chest and mammographic imaging techniques. Factors and issues related to these measurements are discussed. For chest imaging, the flat-panel system was found to have slightly lower MTFs but significantly higher DQEs than the CR system. For mammographic imaging, the CCD-based system was found to have the highest MTF, followed in order by the flat-panel and CR systems. The flat-panel system was found to have the highest DQEs, followed in the order by the CCD-based and CR systems. The DQEs of the flat-panel systems were found to increase with exposure while those of the CR systems decrease slightly with the exposure in both chest and mammographic imaging. The DQEs of the CCD-based system were found to vary little for exposures ranging from 1 to 30 mR.

DQE of direct and indirect digital radiography systems

Ehsan Samei, Michael J. Flynn, Harrell G. Chotas, et al.

Show abstract

Current flat-panel detectors either directly convert x-ray energy to electronic charge or use indirect conversion with an intermediate optical process. The purpose of this work was to compare direct and indirect detectors in terms of their modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE). Measurements were made on three flat-panel detectors, Philips Digital Diagnost, GE Revolution XQ/i, and Hologic Direct-Ray DR1000 using the IEC-defined RQA5 (approximately 75 kVp, 21 mm Al) and RQA9 (approximately 120 kVp, 40 mm Al) radiographic techniques. The presampled MTF of the systems was measured using an edge method (Samei et al., Med Phys 25:102, 1998). The NPS of the systems was determined for a range of exposure levels by 2D Fourier analysis of uniformly exposed radiographs (Flynn and Samei, Med Phys 26:1612, 1999). The ideal signal-to-noise ratio per exposure for each system was estimated using a semi-empirical x-ray model. The DQE, reported only at the RQA5 technique, was assessed from the measured MTF, NPS, exposure, and the ideal signal-to-noise ratio. For the direct system, the MTF was found to be significantly higher than that for the indirect systems and very close to an ideal function associated with the detector pixel size. The NPS for the direct system was found to be constant in relation to frequency. The DQE results reflected expected differences based on the absorption efficiency of the different detector materials. Using RQA5 and 0.3 mR exposure, the measured DQE values at spatial frequencies of 0.15 mm-1 and 2.5 mm-1 were 64% and 14% for the XQ/i system and 35% and 19% for DR-1000. Using RQA5 and the averages at all exposures, the corresponding values were 58% and 13% for the XQ/i system and 36% and 19% for DR-1000.

Comparison of the imaging physics performance of a prototype flat-panel detector with a 400-speed screen-film system

Walter Huda, Kent M. Ogden, Marsha L. Roskopf, et al.

Show abstract

The performance of a digital radiography system that included a prototype flat panel detector (StingRay) was compared with a 400 speed screen-film system. The flat panel detector consisted of a 500 micrometers thick CsI scintillator with an image matrix size of 3k². The limiting spatial resolution of screen-film (approximately 4 line pairs/mm) was superior to that of the flat panel detector (approximately 2.5 line pairs/mm). The digital detector had an excellent linearity response (r² equals 0.997), a dynamic range of 20,000:1, and saturated at a radiation exposure of 60 mR.

Electronic noise analysis of a 127-um pixel TFT/photodiode array

Richard L. Weisfield, N. Robert Bennett

Show abstract

In this paper we examine origins of electronic noise in a 127-micron pixel thin film transistor (TFT)/photodiode image sensor array. The imaging array is a 1536 data line by 1920 gate line amorphous Silicon sensor array connected to low noise charge amplifiers and 14-bit electronics. We measure the contributions of A/D converters, charge amplifiers, data-line resistance and capacitance, and pixel switching to the overall electronic noise of 1040 e^- per pixel. Noise power spectra are evaluated for each dark offset image.

Environment influence on PSPL-based digital dental radiology systems

Eduardo Tavares Costa, Jorge Andre G. Albuquerque, Francisco H. Neto, et al.

Show abstract

Photo-stimulable phosphor luminescence technology (PSPL) has been used in Digora (Soredex, Finland) and Denoptix (CEDH Gendex, Italy) digital dental radiology imaging systems. PSPL plates store X-ray energy during exposition, being later processed by a laser reader and digitizer. Afterward they are erased and re-used. The large band of energy absorption provides PSPL systems with an extensive dynamic scale but at the same time a high sensibility to the incoming noise of environmental radiations. We have measured environment influences (electromagnetic radiation) for Digora and Denoptix plates after X-ray exposure and before digital processing. We have first compared the processing of PSPL plates 'in dark' against 'in light' environments. In another experiment, the exposed plates were also processed after being positioned 10 cm away from a 17 inches video monitor screen and to its laterals for 5, 10, 15, 20, 25 and 30 minutes (plates protected against light). The acquired images were used to calculate the noise power spectra (NPS) in each case. We have noticed that there was an increase in the noise spectra energy of 'in light' processing compared to 'in dark' processing. There was also an increment in the NPS energy when the images were processed after the exposition of the plates to the radiation emanated from video monitor.

Evaluation of PSPL plate erasing time of a digital dental radiology system

Jorge Andre G. Albuquerque, Eduardo Tavares Costa, Francisco H. Neto, et al.

Show abstract

Denoptix (CEDH Gendex, USA) dental imaging system uses photo-stimulable phosphor luminescence (PSPL) plates to store energy during X-ray exposure, being later processed by a laser reader and digitizer. Afterwards the plate is erased and re-used. The cleaning process described by the manufacturer consists of exposing the PSPL plates to negatoscope light for 5 minutes. Proper light intensity and exact erasing time must be considered in order to guarantee good quality procedures in its re-utilization. X-ray exposed plates were submitted to four negatoscopes with different measured light intensities for several periods of light exposure, until the Denoptix system was unable to process the latent image in the plates, and we considered then that the plates were cleaned. We have found the relationships between erasing time, exposed dose and negatoscope light intensity. We have also measured the relative plate image fading with negatoscope light exposure time. We have concluded that a Poisson process governs plate erasing. Considering clinical situations, we have shown that it was possible to largely reduce erasing time and increase plate re-utilization. The exponential decay of image data also suggested a still smaller erasing time, representative of a partial cleaning status assuming that residual noise presence in the erased plate is clinically acceptable.

Megavoltage radiation dose response of digital computed radiography

Zhiheng Wang, Qingrong J. Wu, Claudio H. Sibata

Show abstract

The response of a digital computed radiography system to the megavoltage therapeutic radiation beams was invested. A narrow slit of radiation beam was used to test the line spread function of the system. The effects of various facts such as cassette, beam energy, radiation dose, scanning orientation and timing on the line spread function were investigated. The calibration curves were established to calibrate the image intensity to the megavoltage radiation dose. The calibration curves were applied to measure the beam profiles of the radiation fields with various wedges.

Method using a Monte Carlo simulation database for optimizing x-ray fluoroscopic conditions

Hironori Ueki, Kenichi Okajima

Show abstract

To improve image quality (IQ) and reduce dose in x-ray fluoroscopy, we have developed a new method for optimizing x-ray conditions such as x-ray tube voltage, tube current, and gain of the detector. This method uses a Monte Carlo (MC)-simulation database for analyzing the relations between IQ, x-ray dose, and x-ray conditions. The optimization consists of three steps. First, a permissible dose limit for each object thickness is preset. Then, the MC database is used to calculate the IQ of x-ray projections under all the available conditions that satisfy this presetting. Finally, the optimum conditions are determined as the ones that provide the highest IQ. The MC database contains projections of an estimation phantom simulated under emissions of single-energy photons with various energies. By composing these single-energy projections according to the bremsstrahlung energy distributions, the IQs under any x-ray conditions can be calculated in a very short time. These calculations show that the optimum conditions are determined by the relation between quantum noise and scattering. Moreover, the heat-capacity limit of the x-ray tube can also determine the optimum conditions. It is concluded that the developed optimization method can reduce the time and cost of designing x-ray fluoroscopic systems.

Performance of cone-beam CT using a flat-panel imager

Masahiro Endo, Takanori Tsunoo, Kazumasa Satoh, et al.

Show abstract

An active matrix flat-panel imager (FPI) is a good candidate for the 2-dimensional detector of cone beam CT (CBCT), because it has a wider dynamic range and less geometrical distortion than video-fluoroscopic system so far employed. However the performance of FPI-based CBCT has not been sufficiently examined yet. The aim of this work is to examine the performance of CBCT using a FPI with several phantoms. An X-ray tube, a phantom and a FPI were aligned on an experimental table. The FPI was PaxScan2520 provided by Varian Medical Systems. It has an active area of approximately 180x240mm and the pixel size is 127 micrometer. CsI is used as a scintillator. The phantom was rotated with 1-degree steps while 360 projection frames (1408x1888 active pixels each frame) were collected. 2x2 pixels were combined into a single pixel to reduce noise. 512x512x512 voxels were reconstructed with the Feldkamp method. The comparison was made between reconstructed images with or without scatter rejecting grid. The uniformity and linearity of reconstruction value was drastically improved with the grid. Scatter rejection using a thin-vane collimator was also examined, and it showed more effective than the grid.

Cone-beam volumetric CT imagers: simulations for rapid prototype system design and prediciton of image quality

Deborah J. Walter, Baorui Ren, Armin H. Pfoh, et al.

Show abstract

A framework for rapid and reliable design of Volumetric Computed Tomography (VCT) systems is presented. This work uses detailed system simulation tools to model standard and anthropomorphic phantoms in order to simulate the CT image and choose optimal system specifications. CT systems using small-pitch, 2-D flat area detectors, initially developed for x-ray projection imaging, have been proposed to implement Volume CT for clinical applications. Such systems offer many advantages, but there are also many trade-offs not fully understood that affect image quality. Although many of these effects have been studied in the literature for traditional CT applications, there are unique interactions for very high-resolution flat-panel detectors that are proposed for volumetric CT. To demonstrate the process we describe an example that optimizes the parameters to achieve high detectability for thin slices. The VCT system was modeled over a range of operating parameters, including: tube voltage, tube current, tube focal spot size, detector cell size, number of views, and scintillator thickness. The response surface, which captures the effects of system components on image quality, was calculated. Optimal and robust designs can be achieved by determining an operating point from the response equations, given the constraints. We verify the system design with images from standard and low contrast phantoms. Eventually this design tool could be used, in conjunction with clinical researchers, to specify VCT scanner designs, optimize imaging protocols, and quantify image accuracy and repeatability.

X-ray flat-panel imager (FPI)-based cone-beam volume CT (CBVCT) under a circle-plus-two-arc data acquisition orbit

Xiangyang Tang, Ruola Ning, Rongfeng Yu, et al.

Show abstract

The potential of cone beam volume CT (CBVCT) to improve the data acquisition efficiency for volume tomographic imaging is well recognized. A novel x-ray FPI based CBVCT prototype and its preliminary performance evaluation are presented in this paper. To meet the data sufficiency condition, the CBVCT prototype employs a circle-plus-two-arc orbit accomplished by a tiltable circular gantry. A cone beam filtered back-projection (CB-FBP) algorithm is derived for this data acquisition orbit, which employs a window function in the Radon domain to exclude the redundancy between the Radon information obtained from the circular cone beam (CB) data and that from the arc CB data. The number of projection images along the circular sub-orbit and each arc sub-orbit is 512 and 43, respectively. The reconstruction exactness of the prototype x-ray FPI based CBVCT system is evaluated using a disc phantom in which seven acrylic discs are stacked at fixed intervals. Images reconstructed with this algorithm show that both the contrast and geometric distortion existing in the disc phantom images reconstructed by the Feldkamp algorithm are substantially reduced. Meanwhile, the imaging performance of the prototype, such as modulation transfer function (MTF) and low contrast resolution, are quantitatively evaluated in detail through corresponding phantom studies. Furthermore, the capability of the prototype to reconstruct an ROI within a longitudinally unbounded object is verified. The results obtained from this preliminary performance evaluation encourage an expectation of medical applications of the x-ray FPI based CBVCT under the circle-plus-two-arc data acquisition, particularly the application in image-guided interventional procedures and radiotherapy where the movement of a patient table is to be avoided.

Field-of-view-dependent helical pitch in multislice CT

Michael D. Silver, Katsuyuki Taguchi, Kyung S. Han

Show abstract

A goal of multi-scale CT (MSCT) is to decrease the scan time needed to image a volume of the patient. Detectors with multiple rows of sensors enable higher helical pitches and thus shorter scan times. One of the difficulties in devising an efficient reconstruction method with good image quality and good dose utilization is that each image pixel is irradiated by the cone-beam for a different range of gantry orientations. We derive a new half-scan weighting scheme for a helical, cone-beam backprojection algorithm based on the virtual fan angle. The virtual fan angle, in turn, determines the gantry view range such that image quality is maintained by allowing only valid ray-sums while using the available dose. This restricts the virtual fan angle to be at least the true geometric fan angle but less than 180 degree(s). The result is a computational efficient and dose efficient reconstruction algorithm with a continuous range of field-of-view dependent helical pitches. A 43% higher helical pitch is possible for the smallest field-of-view compared with the largest field-of-view, using the parameters of a commercial MSCT-scanner. Dose efficiency is compared among the new method and standard half scan and full scan approaches.

Antialiasing weighting function for helical CT

Patrick J. La Riviere, Xiaochuan Pan

Show abstract

Longitudinal aliasing effects plague most volumes reconstructed from single-slice helical computed tomography data, and its presence can degrade resolution and distort image structures. We have recently developed a Fourier-based approach to longitudinal interpolation in helical CT that can, under certain conditions, essentially eliminate this longitudinal aliasing by exploiting a generalization of the sampling theorem whose conditions are satisfied by the interlaced pairs of direct and complementary longitudinal samples. However, the algorithm is computationally intensive and cannot be pipelined. In the present work, we address this shortcoming by deriving a spatial-domain, projection- data weighting function that approximates the application of the Fourier-based approach, and preserves its aliasing suppression properties to some degree, while allowing for a pipelined implementation. We call the approach 180AA, for anti-aliasing.

Z-axis high-contrast resolution for multi- vs. single-slice CT

Kerry T. Krugh, Dianna D. Cody, Stephen K. Thompson

Show abstract

One aspect of image quality that should be considered when migrating scan protocols from single-slice helical CT (SSCT) to multi-slice CT (MSCT) is z-axis high-contrast resolution. The aim of this study was to compare z-axis high-contrast resolution of MSCT with that of a similarly designed SSCT for various combinations of slice thickness and pitch. A point response phantom was used to acquire slice sensitivity profiles (SSPs) and calculate z-axis resolution (10% of MTF). Additionally, a resolution pattern phantom was used to subjectively evaluate limiting z-axis resolution. Both analytical and subjective results revealed that the SSCT had higher z-axis resolution than the MSCT for nominal 5 mm slice thickness (at comparable pitches). However, for nominal 10 mm slice thickness the MSCT demonstrated z-axis resolution that was comparable or superior to SSCT. Additionally, notable differences in the shape of the SSP for axial scans were observed on the MSCT unit due to the presence of detector septa. Several conclusions can be drawn from these results including: (1) z-axis resolution is not necessarily 'better' for MSCT and should be evaluated prior to transferring scan protocols from single- to multi-slice CT, and (2) a simple acrylic plate resolution phantom may be used to quickly and effectively evaluate z-axis resolution.

Coherent-scatter computed tomography applying a fan-beam geometry

Stefan M. Schneider, Jens-Peter Schlomka, Geoffrey L. Harding

Show abstract

Recently, fan-beam geometry based Coherent Scatter Computed Tomography (CSCT) was proposed. Coherently scattered X-rays are used in order to reconstruct the spatial distribution of the material dependent structure function, allowing for superior tissue discrimination. In the present paper, the proposed 'third generation' CT geometry and acquisition scenario is simulated, taking into account effects like quantum noise and spectral smearing. A modified iterative algebraic reconstruction algorithm is used for reconstructing the structure function distribution from a number of two-dimensional projections acquired at different viewing angles. The visibility of small objects is investigated for several dose values. In addition to simulation studies, first results of an experimental fan-beam CSCT set-up are shown and discussed. Experiment and simulations indicate the feasibility of CSCT. Future investigations will deepen our understanding of the possibilities and limitations of this new imaging modality.

Compensation for MR geometric image distortion caused by the implantable drug infusion pump

Haiying Liu, John Kucharczyk, Charles L. Truwit

Show abstract

Implantable devices often cause serious MR image artifacts in the vicinity of their implanted locations as the result of susceptibility-induced image distortion, and the resulting artifacts may render the MR images useless for diagnosis. To reduce the MR artifacts in the stage of image acquisition, we have implemented a scheme by which the field inhomogeneity is compensated in a spin echo imaging sequence by introducing a readout-like gradient pulse along the slice-select gradient direction temporally concurrent with the read-out gradient (TE equals 20 msec). It can be shown that this slice direction compensation gradient pulse during the read-out period compensates the field inhomogeneity during the image data collection period.

Laser microbeam CT scanning of dosimetry gels

Marek J. Maryanski, Manisha K. Ranade

Show abstract

A novel design of an optical tomographic scanner is described that can be used for 3D mapping of optical attenuation coefficient within translucent cylindrical objects with spatial resolution on the order of 100 microns. Our scanner design utilizes the cylindrical geometry of the imaged object to obtain the desired paths of the scanning light rays. A rotating mirror and a photodetector are placed at two opposite foci of the translucent cylinder that acts as a cylindrical lens. A He-Ne laser beam passes first through a focusing lens and then is reflected by the rotating mirror, so as to scan the interior of the cylinder with focused and parallel paraxial rays that are subsequently collected by the photodetector to produce the projection data, as the cylinder rotates in small angle increments between projections. Filtered backprojection is then used to reconstruct planar distributions of optical attenuation coefficient in the cylinder. Multiplanar scans are used to obtain a complete 3D tomographic reconstruction. Among other applications, the scanner can be used in radiation therapy dosimetry and quality assurance for mapping 3D radiation dose distributions in various types of tissue-equivalent gel phantoms that change their optical attenuation coefficients in proportion to the absorbed radiation dose.

Investigation into the influence of x-ray scatter on the imaging performance of an x-ray flat-panel imager-based cone-beam volume CT

Xiangyang Tang, Ruola Ning, Rongfeng Yu, et al.

Show abstract

The advent of the x-ray flat panel imager (FPI) is making the study of cone beam volume CT (CBVCT) more competitive. Motivated by recent encouraging developments in CBVCT, this paper investigates the influence of x-ray scatter on the imaging performance of an x-ray FPI based CBVCT prototype. The prototype employs a circle-plus-two-arc orbit to meet the data sufficiency condition, and can reconstruct a region of interest within a longitudinally unbounded object using a cone beam filtered back-projection algorithm derived for the data acquisition orbit. First, the humanoid phantom is used to investigate the temporal variation of both scatter intensity and scatter to primary ratio (SPR) in the projection images acquired for CB reconstruction. Second, a 160 mm cylindrical water phantom consisting of four 16 mm rods made up of Acrylic, Polyethelene, Polycarborate and Polystrene respectively is utilized to evaluate the variation of interference caused by x-ray scatter (cupping effect) and signal to noise ratio vs. SPR in projection images. Third, a disc phantom consisting of seven acrylic discs stacked at even intervals is employed to evaluate the influence of x-ray scatter on reconstruction accuracy and the improvement of CBVCT image quality with recourse to an anti-scatter grid. Finally, the alleviation of the cupping effect in the presence of a beam-shaping (bow-tie) attenuator is assessed . The quantitative investigation shows that the influence of x-ray scatter on the SNR and CT number accuracy is a crucial problem to be addressed for the application of x-ray CBVCT.

Method of evaluating interdetector scattering in block detectors for positron emission tomography

M. Esmail Mesbahi

Show abstract

To evaluate the performance and localization capabilities of a BGO block detector used in Positron Emission Tomography, the effects of inter-crystal scatter must be assessed and quantified. Since the positioning accuracy of a block detector and the algorithms used for identifying the crystal of interaction is of major concern in designing PET systems, the spatial response of an eight by four block detector made up of a 5.6 mm wide, 12.9 mm high and 30 mm thick individual detector crystals to a collimated line source of 511 keV annihilation photons was examined. The response of each crystal showed a spread around the average positioning values and distributions from adjacent crystals overlapped. This leads to possible errors in the event assignment. Furthermore the inter-detector scattering which exists does not allow distinction to be made between the crosstalk that occurs between two crystals or more than two neighboring crystals.

Multiple-reader studies, digital mammography, computer-aided diagnosis, and the Holy Grail of imaging physics: I

Robert F. Wagner, Sergey V. Beiden, Gregory Campbell

Show abstract

There are multiple sources of variability in clinical studies of imaging systems. The variation of the reader `mindset' establishes the need for ROC analysis to control for that fundamental variable. The demonstration of the range of reader skills in mammography shows the need for a multivariate approach to ROC analysis. The multiple-reader, multiple-case (MRMC) ROC experimental paradigm addresses this need and several practical solutions to the problem of analysis of MRMC data have been developed. We review the application of these methods to an important clinical comparison of digital and conventional mammography.

Multiple-reader studies, digital mammography, computer-aided diagnosis, and the Holy Grail of imaging physics: II

Sergey V. Beiden, Robert F. Wagner, Gregory Campbell, et al.

Show abstract

The metaphor of the Holy Grail is used here to refer to the classic and elusive problem in medical imaging of predicting the ranking of the clinical performance of competing imaging modalities from the ranking obtained from physical laboratory measurements and signal-detection analysis, or from simple phantom studies. We show how the use of the multiple-reader, multiple-case (MRMC) ROC paradigm and new analytical techniques allows this masking effect to be quantified in terms of components-of-variance models. Moreover, we demonstrate how the components of variance associated with reader variability may be reduced when readers have the benefit of computer-assist reading aids. The remaining variability will be due to the case components, and these reflect the contribution of the technology without the masking effect of the reader. This suggests that prediction of clinical ranking of imaging systems in terms of physical measurements may become a much more tractable task in a world that includes MRMC ROC analysis of performance of radiologists with the advantage of computer-assisted reading.

Medical Imaging 2001: Physics of Medical Imaging

Volume Details

Table of Contents

Table of Contents