Page 374 - Biomedical Engineering and Design Handbook Volume 2, Applications

352 DIAGNOSTIC EQUIPMENT DESIGN

the range of x-ray fluence incident on the sensor surface in a given mammogram can be 400:1 or more
(Johns and Yaffe, 1987; Nishikawa et al., 1987). Thus the sensitivity and resolution must be maintained
over an appreciable dynamic range.
Film-Based Imaging. Most mammography is currently performed using screen-film systems; that is,
systems that employ image receptors consisting of film placed in direct contact with an x-ray-to-light
converting screen. The screen-film combination, housed in a light-tight cassette, is placed below the
breast, with a rotating anode x-ray tube placed 60 to 65 cm above the cassette. A major technical chal-
lenge in screen-film mammography arises from the fact that the film is used both as an acquisition and
display medium. That means that system parameters such as film speed, detection and light conversion
efficiency of the screen, and x-ray spectrum must be chosen not only to maximize the efficiency of the
image acquisition process, but also to result in the optimum film darkening to maximize the visual con-
trast between structures when the film is viewed on a light box. Furthermore, as with other film-based
imaging procedures, there is an inescapable trade-off between image contrast and dynamic range. One
motivation for the development of digital detectors to replace screen-film receptors in mammography
is the desire to increase dynamic range without a concomitant loss of image contrast.
Digital Mammography
Current Technologies for Image Acquisition. The principal motivations for the development of
electronic receptors to replace film-based receptors are (1) the limited dynamic range of x-ray inten-
sities over which the sensitivity of film-based systems is appreciable, (2) the inherent noise properties
of film due to the random distribution of the grains on the film substrate (Nishikawa and Yaffe, 1985),
and (3) the desire to decouple image acquisition and image display, which are inherently coupled in
film-based imaging. The desire to overcome these limitations, combined with the advantages inherent
in having images in digital form (e.g., computer-aided diagnosis, enhancement through image pro-
cessing, easier image storage, remote transmission of images, and the possibility for more advanced
acquisition approaches such as tomosynthesis, computed tomography, and dual energy imaging), has
led to the development of several technical solutions to digital mammographic image acquisition
during the past two decades. Leading large area technologies for full-field digital mammography
(FFDM) currently include: (1) CsI(Tl) converters optically coupled to arrays of amorphous silicon
(a-Si) photodiodes that are read out using a-Si thin-film transistor (TFT) arrays, (2) amorphous
selenium (a-Se) converters electrically coupled to a-Si TFT arrays, and (3) storage phosphor plates
(typically europium-doped barium fluorobromide) that retain a metastable latent image during x-ray
exposure that can be de-excited and quantified using a raster-scanned laser beam. Technologies (1)
and (2) are examples of indirect and direct flat-panel technologies, respectively. Indirect flat-panel
detectors use an intermediate stage to convert x-rays to visible photons after which a photodetector
generates charge. In direct conversion detectors, electron-hole pairs are generated in the x-ray
absorber itself without the generation of visible photons. Technology (3) is often called computed radi-
ography (CR). In each of the above digital mammography detectors, the detector area is equal to the
imaged area. An alternative is to use a narrow, slot-shaped detector that spans the imaged area in one
dimension (the chest wall-to-nipple direction), and is scanned from left to right during image acqui-
sition. The x-ray beam, collimated to a similar slot shape, is scanned in synchrony with the detector.
Commercial FFDM systems using a CsI-fiber optic taper-CCD combination have been developed
(Tesic et al., 1999), and more recently systems employing gas-filled detectors have been introduced
(Thunberg et al., 2004). With the full area detector technologies, x-ray tube target materials (molyb-
denum, rhodium, or tungsten) and external filtration (molybdenum or rhodium) combinations similar
to those used in screen-film mammography are employed. For the scanned devices, a tungsten target
is typically used, and the tube voltage is often somewhat higher than that used with Mo or Rh targets.
Figure 12.2 compares analog (screen-film) and digital cranial-caudal images of a spiculated mass
(1.2-cm invasive ductal carcinoma) located in the upper inner quadrant of the right breast. The mass is
seen best in the posterior aspect of the breast on the MLO view. As of this writing, five manufacturers
have received approval from the U.S. Food and Drug Administration (FDA) to market full-field digital
mammography (FFDM) systems in the United States (some of these manufacturers have more than one
FDA-approved FFDM system), including indirect flat-panel systems, direct flat-panel systems, one