This paper discusses the criteria underlying the design of an innovative X-ray active pixel sensor in CMOS
technology. This X-ray detector is used in a Full Field-of-view Digital Mammography (FFDM) camera. The
CMOS imager is a three-side buttable 29mm x 119mm, 48 μm active pixel CMOS sensor in 0.18 μm
technology. The 1<sup>st</sup> silicon FFDM devices were fabricated at the end of June, 2007. The device suffers a
common failure mode of high current and currently is in failure analysis at Bioptics foundry. Current target for
revision A1 tape out is at the end of August, 2007.
While full-field digital mammography (FFDM) technology is gaining clinical acceptance, the overwhelming majority (96%) of the installed base of mammography systems are conventional film-screen (FSM) systems. A high performance, and economical digital cassette based product to conveniently upgrade FSM systems to FFDM would accelerate the adoption of FFDM, and make the clinical and technical advantages of FFDM available to a larger population of women. The planned FFDM cassette is based on our commercial Digital Radiography (DR) cassette for 10 cm x 10 cm field-of-view spot imaging and specimen radiography, utilizing a 150 micron columnar CsI(Tl) scintillator and 48 micron active-pixel CMOS sensor modules. Unlike a Computer Radiography (CR) cassette, which requires an external digitizer, our DR cassette transfers acquired images to a display workstation within approximately 5 seconds of exposure, greatly enhancing patient flow. We will present the physical performance of our prototype system against other FFDM systems in clinical use today, using established objective criteria such as the Modulation Transfer Function (MTF), Detective Quantum Efficiency (DQE), and subjective criteria, such as a contrast-detail (CD-MAM) observer performance study. Driven by the strong demand from the computer industry, CMOS technology is one of the lowest cost, and the most readily accessible technologies available for FFDM today. Recent popular use of CMOS imagers in high-end consumer cameras have also resulted in significant advances in the imaging performance of CMOS sensors against rivaling CCD sensors. This study promises to take advantage of these unique features to develop the first CMOS based FFDM upgrade cassette.
Physical characteristics necessary to calculate the Detective Quantum Efficiency of a prototype imaging detector based on a 4 x 2 array of tiled CMOS sensors designed for small-field-digital-mammography (SFDM; 10 cm x 10 cm active area) are presented. Objective quantities such as modulation transfer function (MTF), noise power spectrum (NPS) and detective quantum efficiency (DQE) have been evaluated. The X-ray photon fluence per X-ray exposure was determined using Half-Value-Layer (HVL) techniques. At an X-ray beam characterized by 28 kVp, Mo-anode, a Mo filter of 0.025 mm and beam hardening by 4.5 cm Lucite, the detector is practically linear with x-ray exposure at least up to 40.7 mR. At an exposure of 40.7 mR and close to zero spatial frequency the DQE is in the vicinity of 60 to 70 %.
We have developed a CMOS-based x-ray imaging detector in the same form factor of a standard film cassette (18 cm × 24 cm) for Small Field-of-view Digital Mammography (SFDM) applications. This SFDM cassette is based on our three-side buttable, 25 mm × 50 mm, 48μm active-pixel CMOS sensor modules and utilizes a 150μm columnar CsI(Tl) scintillator. For imaging up to 100 mm × 100 mm field-of-view, a number of CMOS sensor modules need to be tiled and electronically synchronized together. By using fiber-optic communication, acquired images from the SFDM cassette can be transferred, processed and displayed on a review station within approximately 5 seconds of exposure, greatly enhancing patient flow. We present the physical performance of this CMOS-based SFDM cassette, using established objective criteria such as the Modulation Transfer Function (MTF), Detective Quantum Efficiency (DQE), and more subjective criteria, by evaluating images from a phantom study and the clinical studies of our collaborators. Driven by the strong demand from the computer industry, CMOS technology is one of the lowest cost, and the most readily accessible technologies available for digital mammography today. Recent popular use of CMOS imagers in high-end consumer cameras have also resulted in significant advances in the imaging performance of CMOS sensors against rivaling CCD sensors. The SFDM cassette can be employed in various mammography applications, including spot imaging, stereotactic biopsy imaging, core biopsy and surgical biopsy specimen radiography. This study demonstrates that all the image quality requirements for demanding mammography applications can be addressed with CMOS technology.