Detectors for digital mammography

Introduction Mammography is the most technically demanding radiographic modality, requiring high spatial resolution, excellent low contrast discrimination, and wide dynamic range. The development of dedicated mammography systems [1] in the mid-1960s and the refinements in film-screen technology over its long evolution [2,3] were of critical importance in establishing the benefits of mammography in reducing breast cancer mortality [4–10]. The use of film-screen technology over 30 years ensured excellent spatial resolution under optimal conditions. The high spatial resolution requirement was thought to be essential for imaging small and subtle calcifications as small as 100–200μm, in particular for visualizing its morphology. In spite of the excellent imaging characteristics of film-screen technology under optimal exposure and film development conditions, intrinsically images are more susceptible to artifacts. Small deviations from optimal exposure and processing conditions can have profound effects on mammographic image quality, such as its ability to provide a balanced image over regions of the breast that vary in radiographic density. The well-documented weaknesses of film-screen technology [11–13] include limited dynamic range, limited tolerance to exposure conditions, complexity and instabilities due to the chemical processing of film, and the lack of ability to digitally communicate, store, and enhance the images. Digital mammography, a term coined for electronic image capture of x-rays transmitted through the breast, is a concept that was formed about 30 years ago as a means of circumventing some of the major limitations of film-screen technology. An electronic imaging detector that replaces film-screen for image capture would minimize the possibility of suboptimally exposed mammograms, which can potentially conceal subtle soft tissue lesions and microcalcifications. Digital mammography can provide wide dynamic range, wide exposure latitude, and the ability to communicate, store, and digitally manipulate images. The concept of computer-aided detection and diagnosis (CADe and CADx) [14,15] was envisioned even before digital mammography became a reality, and the idea of obtaining mammograms using electronic image capture was certainly very attractive as an enabling technology for further development and proliferation of computer-aided techniques. Convenient digital storage and communication of mammographic images was another potential advantage that fueled the early development of digital mammography. The ensemble of all these positive attributes served as the overall motivation for seeking replacement of film-screen with digital mammography, in the belief that the benefits provided by digital mammography would improve detection, diagnosis, and management of breast cancer. In the early days of investigations into the potential of digital mammography, particularly those pertaining to digital communication, image enhancement, and computer-aided techniques, digitization of filmmammogramswas used as a proxy because of the unavailability of digital mammography detectors. Digitization was accomplished by flying spot scanners, video cameras, and eventually high-resolution digital scanners. This early experience with digitized mammograms was important, because it provided an early experience of digital enhancement techniques, understanding the need for digital communication and storage, and most importantly it acted as a platform for the development and evaluation of CADe/CADx. Creation of databases with digitized mammograms for teaching and research that were readily accessible through the internet [16] was also particularly important in improving our understanding of image display (monitor) requirements and the tools needed by the user for digital image manipulation. Several studies demonstrated techniques for enhancing digitized mammograms, and others reported on the “equivalence” of digitized mammograms to the original film mammograms. Digitized mammograms added another dimension to film-screen mammography, and considering some of the initial challenges in the development of digital mammography detectors, it appeared that replacing film-screen mammography was a nearly impossible task. One of the beliefs during the developmental stages of digital mammography was that the detectors designed for digital mammography should replicate many of the desirable characteristics of film-screen technology while circumventing