CMOS-based digital imaging detector for mammography applications

In recent years, digital mammography (DM) has been introduced as an alternative to conventional x-ray film-screen mammography (FSM) systems. Conventional FSM can deliver superb spatial resolution of 15–20 line pairs per millimeter (lp/mm) with good contrast, ¹ but the imaging ability of FSM technology for low-contrast masses is limited. Moreover, film is nonlinear and has limited dynamic range, film development conditions influence the image quality, film development is labor-intensive, and there is no direct method for digitally archiving the images. ² Recent studies of DM and FSM have shown that DM is comparable to FSM in terms of detecting breast cancer. ^3–5 In DM systems, a digital x-ray receptor and a computer replace conventional film for capturing and displaying images.

A number of studies indicate that DM can provide benefits such as superior sensitivity to low-contrast masses and use of lower radiation dose, and it may lead to fewer patient recalls than FSM. ⁶ A digital cassette-based product that could be used to conveniently upgrade FSM systems, while offering high performance and economy, would accelerate the adoption of DM, and make the clinical and technical advantages of DM available to a larger population of women.

Our commercial small field-of-view DM (SFDM) cassette for10cm × 10cm field-of-view (FOV) spot imaging and specimen radiography provides a digital upgrade path for a range of mammography applications. ⁷

The SFDM cassette is composed of eight CMOS sensor modules, each 50mm × 25mm, with 48μm square pixels. The sensors are arranged in a 2 × 4 matrix to provide an imaging area of 100mm × 100mm (see Figure 1). Sensors are three-side buttable, with approximately 100μm of dead space around the edges, which is interpolated out in software. ⁷ On top of the fiber-optic faceplate covering the sensors, we used a 150μm-thick, columnar, thallium-doped, cesium-iodide scintillator to convert x-rays into visible photons. Each of the eight sensors are digitized in parallel with 14bits of depth, at a rate of 1.5Mpixels/s and the data is transferred to a display workstation through an optical fiber or camera-link communications.

Due to its extremely low power consumption and thin profile, CMOS is the only digital technology that can be easily packaged in the form of a standard mammography film cassette without special cooling and mechanical retrofits, permitting the development of a low-cost upgrade path from FSM to DM. The only other commercially available removable cassette upgrade is a storage phosphor based (CR) cassette, which requires an external digitizer and has lower performance than CMOS technology. Amorphous silicon (a-Si) or selenium (a-Se) flat panel technologies are expensive alternatives and require bulky mechanical retrofits. The other alternative, CCD (charge-coupled device) technology, requires bulky reduction optics and special cooling (because of its higher power consumption) and is therefore not offered in cassette form for larger than 5cm × 5cm FOV.

We studied the physical performance of our CMOS based SFDM system using established objective criteria such as the modulation transfer function (MTF) and detective quantum efficiency (DQE). Our MTF of 15% at 10 lp/mm Nyquist frequency and DQE of 65% at 0 lp/mm and 30mR exposure are comparable to other full field DM systems in clinical use today, including a-Si and a-Se flat panel based systems. Tests with the acrylic phantom used for Mammography Quality Standards Act inspections show that our small-field CMOS-based DM cassette may offer a better performance than a screen-film system and comparable performance to two high-resolution commercial full-field DM systems ⁷

In diagnostic spot mammography, x-ray images of a small section of the breast are acquired, in order to take a more careful look at an abnormality detected during screening mammography. The 10cm ×  10cm active area of the SFDM CMOS cassette is ideal for this application (see Figure 2).

Another spot imaging application ideal for the SFDM cassette is pre-operative needle localization. In order to accurately position a hook-wire needle inside a suspected area before surgery, users must acquire two orthogonal x-ray images of the breast. The quick availability of images from the SFDM cassette makes the needle localization procedure more convenient and comfortable for the patient and the radiologist.

In vacuum-assisted breast core and surgical biopsy procedures, small pieces of suspect breast tissue are extracted and sent to pathology for a definitive diagnosis. The SFDM cassette provides an ideal platform to provide a real-time radiograph of the tissue sample to the surgeon in the operating room.

We are in the process of developing an 18cm × 24cm full-field digital mammography system based on the design of our small field sensor.