This PDF file contains the front matter associated with SPIE Proceedings Volume 9539, including the Title Page, Copyright information, Table of Contents, Authors, and Conference Committee listing.

There is considerable interest in the development of photoacoustic endoscopy probes (PAE) for applications in foetal medicine, interventional surgery and gastroenterology. However, most previous PAE probes employ a combination of mechanical scanning and piezoelectric transducers at the distal end which can be technically complex and pose challenges in achieving the required level of miniaturisation and acoustic performance. To overcome these limitations, we present two novel all-optical forward-viewing endoscopic probes that use coherent fibre bundles to address a Fabry-Perot polymer film ultrasound sensor.

Opto-acoustic imaging (OAI) shows particular promise for in-vivo biomedical diagnostics. Its applications include cardiovascular, gastrointestinal and urogenital systems imaging. Opto-acoustic endoscopy (OAE) allows the imaging of body parts through cavities permitting entry. The critical parameter is the physical size of the device, allowing compatibility with current technology, while governing flexibility of the distal end of the endoscope based on the needs of the sensor. Polymer optical fibre (POF) presents a novel approach for endoscopic applications and has been positively discussed and compared in existing publications. A great advantage can be obtained for endoscopy due to a small size and array potential to provide discrete imaging speed improvements. Optical fibre exhibits numerous advantages over conventional piezo-electric transducers, such as immunity from electromagnetic interference and a higher resolution at small sizes. Furthermore, micro structured polymer optical fibres offer over 12 times the sensitivity of silica fibre. We present a polymer fibre Bragg grating ultrasound detector with a core diameter of 125 microns. We discuss the ultrasonic signals received and draw conclusions on the opportunities and challenges of applying this technology in biomedical applications.

Many biological applications require a simultaneous observation of different anatomical features. However, unless potentially harmful staining of the specimens is employed, individual microscopy techniques do generally not provide multi-contrast capabilities. We present a hybrid microscope integrating optoacoustic microscopy and multiphoton microscopy, including second-harmonic generation, into a single device. This combined multiphoton and optoacoustic microscope (MPOM) offers visualization of a broad range of structures by employing different contrast mechanisms and at the same time enables pure label-free imaging of biological systems. We investigate the relative performance of the two microscopy modalities and demonstrate their multi-contrast abilities through the label-free imaging of a zebrafish larva ex vivo, simultaneously visualizing muscles and pigments. This hybrid microscopy application bears great potential for developmental biology studies, enabling more comprehensive information to be obtained from biological specimens without the necessity of staining.

The characterization of the degradation of surgical sutures (~500 μm diameter) up to ~9 mm in tissue phantoms and up to ~3 mm depth in euthanized mice, and its potential application in in vivo animals is demonstrated using a custom dark-field photo-acoustic microscope (PAM). By using a simple theoretical approach and modelling the characteristics of our ultrasound transducer, both theoretical and experimental observations are in good agreement. The implications of this work for industrial applications are discussed by comparing the measurements with an optical microscope and with a developed algorithm on tissue simulating phantoms and with ex vivo measurements using PAM.

Photoacoustic (PA) or optoacoustic (OA) imaging combines the high (blood) contrast to light with the high-resolution of ultrasound. The method can visualize vascularization deep inside tissue. Of late there is interest in PA imaging of synovial joints which are expected to be associated with increased vascularization in the event of rheumatoid arthritis (RA). We here describe our approach in investigating the application of the PA technique in arthritis. We are developing a CT-geometry version PA finger imager, intended for early clinical assessment of the method. The imager uses two curved array ultrasound detectors each with 64 elements with central frequencies 1.5 and 7.5 MHz respectively, stacked above each other. Both cover approximately 180 degrees of the circle. Illumination is provided with a multiple of optical fiber bundles coupled to a laser-OPO system. Ultrasound imaging is also possible with the system, since the curved arrays are each provided with 12 or 8 ultrasound pulsers. We have investigated systematically imaging of finger vasculature in healthy volunteers using an earlier laboratory prototype. In this paper we present finger imaging results of a patient diagnosed with rheumatoid arthritis.

Precise and efficient guidance of medical devices is of paramount importance for many minimally invasive procedures. These procedures include fetal interventions, tumor biopsies and treatments, central venous catheterisations and peripheral nerve blocks. Ultrasound imaging is commonly used for guidance, but it often provides insufficient contrast with which to identify soft tissue structures such as vessels, tumors, and nerves. In this study, a hybrid interventional imaging system that combines ultrasound imaging and multispectral photoacoustic imaging for guiding minimally invasive procedures was developed and characterized. The system provides both structural information from ultrasound imaging and molecular information from multispectral photoacoustic imaging. It uses a commercial linear-array ultrasound imaging probe as the ultrasound receiver, with a multimode optical fiber embedded in a needle to deliver pulsed excitation light to tissue. Co-registration of ultrasound and photoacoustic images is achieved with the use of the same ultrasound receiver for both modalities. Using tissue ex vivo, the system successfully discriminated deep-located fat tissue from the surrounding muscle tissue. The measured photoacoustic spectrum of the fat tissue had good agreement with the lipid spectrum in literature.

A virtual intraoperative surgical photoacoustic microscopy at 1064 nm wavelength (VISPAM) system was designed and fabricated by integrating a commercial type surgical microscope and laser scanning photoacoustic microscopy (PAM) with a 1064 nm pulsed laser. Based on simple augmented reality device, VISPAM could simultaneously provide 2D depth-resolved photoacoustic and magnified microscope images of surgery regions on the same vision of surgeon via an eyepiece of the microscope. The invisible 1064 nm laser removed the interruption of surgical sight due to visible laser scanning of previous report, and decreased the danger of tissue damage caused by over irradiated laser. In addition, to approach the real practical surgery application, a needle-type transducer was utilized without a water bath for PA signal coupling. In order to verify our system’s performance, we conducted needle guiding as ex vivo phantom study and needle guiding and injection of carbon particles mixtures into a melanoma tumor region as in vivo study. We expect that VISPAM can be essential tool of brain and ophthalmic microsurgery.

Optoacoustic (photoacoustic) imaging has a high potential for imaging melanin-rich structures in skin and the microvasculature of the dermis due to the natural chromophores (de)oxyhemoglobin, and melanin. The vascular network in human dermis comprises a large network of arterioles, capillaries, and venules, ranging from 5 μm to more than 100 μm in diameter. The frequency spectrum of the microcirculatory network in human skin is intrinsically broadband, due to the large variety in size of absorbers. In our group we have developed raster-scan optoacoustic mesoscopy (RSOM) that applies a 100 MHz transducer with ultra-wide bandwidth in raster-scan mode achieving lateral resolution of 18 μm. In this study, we applied high frequency RSOM to imaging human skin in a healthy volunteer. We analyzed the frequency spectrum of anatomical structures with respect to depth and show that frequencies >60 MHz contain valuable information of structures in the epidermis and the microvasculature of the papillary dermis. We illustrate that RSOM is capable of visualizing the fine vascular network at and beneath the epidermal-dermal junction, revealing the vascular fingerprint of glabrous skin, as well as the larger venules deeper inside the dermis. We evaluate the ability of the RSOM system in measuring epidermal thickness in both hairy and glabrous skin. Finally, we showcase the capability of RSOM in visualizing benign nevi that will potentially help in imaging the penetration depth of melanoma.

In a wide range of clinical procedures, accurate placement of medical devices such as needles and catheters is critical to optimize patient outcomes. Ultrasound imaging is often used to guide minimally invasive procedures, as it can provide real-time visualization of patient anatomy and medical devices. However, this modality can provide low image contrast for soft tissues, and poor visualization of medical devices that are steeply angled with respect to the incoming ultrasound beams. Photoacoustic sensors can provide information about the spatial distributions of tissue chromophores that could be valuable for guiding minimally invasive procedures. In this study, a system for guiding minimally invasive procedures using photoacoustic sensing was developed. This system included a miniature photoacoustic probe with three optical fibers: one with a bare end for photoacoustic excitation of tissue, a second for photoacoustic excitation of an optically absorbing coating at the distal end to transmit ultrasound, and a third with a Fabry-Perot cavity at the distal end for receiving ultrasound. The position of the photoacoustic probe was determined with ultrasonic tracking, which involved transmitting pulses from a linear-array ultrasound imaging probe at the tissue surface, and receiving them with the fiber-optic ultrasound receiver in the photoacoustic probe. The axial resolution of photoacoustic sensing was better than 70 μm, and the tracking accuracy was better than 1 mm in both axial and lateral dimensions. By translating the photoacoustic probe, depth scans were obtained from different spatial positions, and two-dimensional images were reconstructed using a frequency-domain algorithm.

In this paper we investigate the possibilities to use a pulsed laser diode based setup to achieve the photoacoustic signal amplification via the Grueneisen relaxation effect. It is shown that the system is capable of producing the required multiple pulses burst with pulse widths of 12 ns and pulse inter-delays down to approximately 135 ns. With additional fluence considerations we expect no improvement from this technique for photoacoustic tomography setting, while our laser diode based setup is a highly promising compact alternative for Grueneisen relaxation related studies in photoacoustic microscopy.

Optoacoustic (OA) imaging is a rising biomedical technique that has attracted much interest over the last 15 years. This technique permits to visualize the internal soft tissues in depth by using short laser pulses, able to generate ultrasonic signals in a large frequency range. It combines the high contrast of optical imaging with the high resolution of ultrasound systems. The OA signals detected from the whole surface of the body serve to reconstruct in detail the image of the internal tissues, where the absorbed optical energy distribution outlines the regions of interest. In fact, the use of contrast agents could improve the detection of growing anomalies in soft tissues, such as carcinomas. This work proposes the use of double-walled carbon nanotubes (DWCNTs) as a potential nontoxic biodegradable contrast agent applicable in OA to reveal the presence of malignant in-depth tissues in near infrared (NIR) wavelength range (0.75–1.4 μm), where the biological tissues are fairly transparent to optical radiation. A dual-wavelength (870 and 905 nm) OA system is presented, based on arrays of high power diode lasers (HPDLs) that generate ultrasound signals from a DWCNT solution embedded within a biological phantom. The OA signals generated by DWCNTs are compared with those obtained using black ink, considered to be a very good absorber at these wavelengths. The experiments prove that DWCNTs are a potential contrast agent for optoacoustic spectroscopy (OAS).

Tomographic photoacoustic imaging (PAT) allows to overcome the anisotropic image resolution of conventional reflection mode imaging. In order to achieve high-resolution, tomographic images, precise information on the position of each detector element is required. PAT systems that acquire signals from rotating linear transducer arrays come with inevitable transducer misalignments. Up to now, transducer orientation (x/y-tilt) and radial distance uncertainty were measured experimentally or have not been considered. Uncalibrated, these systems suffer from characteristic artifacts yielding misinterpretations of anatomic structures. Herein, we derive the artifact mathematically and investigate an analytical calibration method that enables the calculation and compensation of important transducer positioning parameters: the rotational radius and in-plane tilt. We studied the approach theoretically and evaluated the performance of the developed algorithm both on numerical and experimental data. A PAT system based on a 5-MHz linear transducer array, a multichannel electronics platform with channel data access, a NIR-emitting laser system and a rotating samples is used to demonstrate the benefit of the transducer calibration method providing isotropic resolution of 160 μm.

Reflection artifacts caused by acoustic reflectors is an important problem in reflection-mode photoacoustic imaging. The light absorbed by skin and superficial optical absorbers may produce high photoacoustic signals, which traverse into the tissue and get reflected from structures having different acoustic impedance. These reflected photoacoustic signals, when reconstructed may appear in the region of interest, which causes complications in interpreting the images. We propose a novel method to identify and reduce reflection artifacts in photoacoustic images by making use of PhotoAcoustic-guided Focused UltraSound [PAFUSion]. Our method ultrasonically mimics the photoacoustic image formation process and thus delivers a clinically feasible way to reduce reflection artifacts. Simulation and phantom measurement results are presented to demonstrate the validity and impact of this method. Results show that PAFUSion technique can identify and differentiate reflection signals from the signals of interest and thus foresees good potential for improving photoacoustic imaging of deep tissue.

We present the concept, the setup and a preliminary experiment using optical ultrasound detection with a CCD camera combined with focused line excitation for photoacoustic microscopy. The line scanning optical-acoustical resolution photoacoustic microscope (LS-OAR-PAM) with optical ultrasound detection is capable of real-time B-scan imaging providing acoustical resolution within the individual B-scans and optical out of plane resolution up to a depth limited by optical diffusion. A 3D image is composed of reconstructed B-scan images recorded while scanning the excitation line along the sample surface. Proof of concept is shown by imaging a phantom containing black human hairs and carbon fibers. The obtained C-scan image clearly shows the different resolution in the two perpendicular directions, namely diffraction limited by optical focusing in scan direction and acoustically limited in direction parallel to line orientation by the properties of acoustic wave propagation.

We have developed an epi-illumination raster-scan optoacoustic mesoscopy system (RSOM), the new system is capable of imaging model organisms, and vasculature. The newly developed system is based on a custom designed; spherically focused detector with a Characterization of the system shows an isotropic lateral resolution of 18 μm, and an axial resolution of 4 μm. The scan times are on the order of 8 minutes for a field of view of 10×10 mm². The achieved resolution is slightly degraded up to a depth of 5 mm. After characterizing the system we showcase it’s performance on a zebrafish ex vivo, and an excised mouse ear. Additionally, to improve the visibility of small structures we have reconstructed the high frequencies, and the low frequencies separately, and at the end overplayed the two reconstructions using different colors, this way the high frequencies are not masked by the low frequencies which have a higher signal to noise ratio.

In optoacoustic imaging, the high optical contrast between different tissue types is combined with the high resolution and low scattering of ultrasound. Using adapted reconstruction algorithms, images of the distribution of light absorption in tissue can be obtained. Such as in any emerging modality, there is limited experience regarding the interpretation of optoacoustic images. For this reason, we developed a flexible hardware platform combining ultrasound imaging with optoacoustics. The system is based on the software processing of channel data and different types of reconstruction algorithms are implemented. It combines optoacoustic imaging based on linear arrays for detection with plane wave compounding ultrasound. Our system further includes a custom made probe based on a 7,5 MHz array, custom made fibre bundles for targeted light delivery and an acoustic coupling pad. The system was characterized on phantoms and first in-vivo datasets from subcutaneous vasculature were acquired.

MultiSpectral Optoacoustic Tomography (MSOT) is a fast developing imaging modality, combining the high resolution and penetration depth of ultrasound with the excellent contrast from optical imaging of tissue. Absorption and scattering of the near infrared excitation light modulates the spectral profile of light as it propagates deep into biological tissue, meaning the images obtained provide only qualitative insight into the distribution of tissue chromophores. The goal of this work is to accurately recover the spectral profile of excitation light by modelling light fluence in the data reconstruction, to enable quantitative imaging. We worked with a commercial small animal MSOT scanner and developed our light fluence correction for its' cylindrical geometry. Optoacoustic image reconstruction pinpoints the sources of acoustic waves detected by the transducers and returns the initial pressure amplitude at these points. This pressure is the product of the dimensionless Grüneisen parameter, the absorption coefficient and the light fluence. Under the condition of constant Grüneisen parameter and well modelled light fluence, there is a linear relationship between the initial pressure amplitude measured in the optoacoustic image and the absorption coefficient. We were able to reproduce this linear relationship in different physical regions of an agarose gel phantom containing targets of known optical absorption coefficient, demonstrating that our light fluence model was working. We also demonstrate promising results of light fluence correction effects on in vivo data.

Photoacoustic imaging can provide high resolution images of tissue structure, pathology and function. As these images can be obtained at multiple wavelengths, quantitatively accurate, spatially resolved, estimates for chromophore concentration, for example, may be obtainable. Such a capability would find a wide range of clinical and pre-clinical applications. However, despite a growing body of theoretical papers on how this might be achieved, there is a noticeable lack of studies providing validated evidence that it can be achieved experimentally, either in vitro or in vivo. Well-defined, versatile and stable phantom materials are essential to assess the accuracy, robustness and applicability of multispectral Quantitative Photoacoustic Imaging (qPAI) algorithms in experimental scenarios. This study assesses the potential of polyvinyl chloride plastisol (PVCP) as a phantom material for qPAI, building on previous work that focused on using PVCP for quality control. Parameters that might be controlled or tuned to assess the performance of qPAI algorithms were studied: broadband acoustic properties, multiwavelength optical properties with added absorbers and scatterers, and photoacoustic efficiency.

The optical and acoustic properties of PVCP can be tuned to be broadly representative of soft tissue. The Grüneisen parameter is larger than expected in tissue, which is an advantage as it increases the signal-to-noise ratio of the photoacoustic measurements. Interestingly, when the absorption was altered by adding absorbers, the absorption spectra measured using high peak power nanosecond-pulsed sources (typical in photoacoustics) were repeatably different from the ones measured using the low power source in the spectrophotometer, indicative of photochemical reactions taking place.

Model-based optoacoustic reconstruction can incorporate the shape of transducers. However, the accompanying memory cost will hinder it for high resolution performance. The propose method provides over an order of magnitude reduction in inversion time in experiments. Additionally, it also suits for the analysis of inversion stability.

A method based on a modified inverse Radon transform is presented to correct photoacoustic and laser-ultrasound section images taken with a cylindrical integrating detector for heterogeneity in the speed of sound. Data for the correction are obtained with a laser ultrasound transmission method, providing two-dimensional maps of the speed of sound distribution. Photoacoustic, laser ultrasound and speed of sound tomography are combined in a single setup using illumination of the sample and several external absorbers with short pulses from a single laser source. The performance of the corrected reconstruction is compared in a simulation and a phantom experiment with a reconstruction using an average, constant sound speed. In this comparison, the reconstruction using the modified inverse Radon transform shows a small but distinguishable improvement.

Photo-acoustic tomography is a hybrid imaging modality that combines the advantages of optical as well as ultrasound imaging techniques to produce images with high resolution and good contrast at high penetration depths. Choice of reconstruction algorithm as well as experimental and computational parameters plays a major role in governing the accuracy of a tomographic technique. Therefore error estimates with the variation of these parameters have extreme importance. Due to the finite support, that photo-acoustic source has, the pressure signals are not band-limited, but in practice, our detection system is. Hence the reconstructed image from ideal, noiseless band-limited forward data (for future references we will call this band-limited reconstruction) is the best approximation that we have for the unknown object. In the present study, we report the error that arises in the universal back-projection (UBP) based photo-acoustic reconstruction for planer detection geometry due to sampling and filtering of forward data (pressure signals).Computational validation of the error estimates have been carried out for synthetic phantoms. Validation with noisy forward data has also been carried out, to study the effect of noise on the error estimates derived in our work. Although here we have derived the estimates for planar detection geometry, the derivations for spherical and cylindrical geometries follow accordingly.

In optoacoustic tomography, images representing the light absorption distribution are reconstructed from the measured acoustic pressure waves at several locations around the imaged sample. Most reconstruction algorithms typically yield negative absorption values due to modelling inaccuracies and imperfect measurement conditions. Those negative optical absorption values have no physical meaning and their presence hinders image quantification and interpretation of biological information. We investigate herein the performance of optimization methods that impose non-negative constraints in model-based optoacoustic inversion. Specifically, we analyze the effects of the non-negative restrictions on image quality and accuracy as compared to the unconstrained approach. An efficient algorithm based on the projected quasi-Newton scheme and the limitedmemory Broyden-Fletcher-Goldfarb-Shannon method is used for the non-negative constrained inversion. We showcase that imposing non-negative constraints in model-based reconstruction leads to a quality increase in cross-sectional tomographic optoacoustic images.

Multispectral optoacoustic tomography offers unprecedented capabilities in biological research and newly-developed systems prompt the clinical translation of this modality. By exciting tissues at multiple optical wavelengths, the distribution of spectrally-distinctive absorbers can be resolved with high resolution in deep tissues, thus enabling reading important biological parameters such as blood oxygenation or the biodistribution of photo-absorbing agents. Multispectral three-dimensional optoacoustic imaging generally comes at the expense of slow acquisition times, which limits the dynamic imaging capabilities of this modality. Recently, the feasibility of multispectral three-dimensional imaging in real time (five dimensional imaging) has been showcased. Two different illumination strategies can be used for this purpose. The first approach is based on tuning the wavelength of the laser on a per-pulse basis, which enables acquisition of large multispectral datasets on a very short time. The second approach is based on properly synchronizing the light beams from two (or more) laser sources. The performances of these two approaches are compared and discussed herein based on experiments with mice and human volunteers.