Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

individual images from each camera are stitched together with the use of software toproduce a seamless, high-resolution digital mammographic image.Full-field mammography systems (18 X 24 cm) are available that utilizeTFT/flat panel detector technology. Currently, indirect detection (CsI scintillator)flat panel array systems are available with 100- X 100-J..lm detector elements (FN =5 cycles/mm); however, flat panels (indirect and direct) with smaller detector elementsare being developed. Flat panel mammography devices are high-resolutionversions (thinner screens and smaller detector elements) of standard flat panel systems,which were described previously.Another technology that is used for full-field digital mammography is the slotscansystem (see Fig. 11-11). The slot-scan system uses a long, narrow array ofCCOchips. Because the largest CCO arrays are only 6- to 8-cm square, full-field mammographyusing a single CCO is not possible. However, the slot-scan systemrequires a detector array with dimensions of about 4 mm X 18 cm, and several long,narrow CCO chips can meet this need. The breast remains stationary and undercompression while the x-ray beam is scanned across it. At any time during theacquisition sequence, only a narrow slot of tissue is being exposed to the highly collimatedx-ray beam. Consequently, little scattered radiation is detected due to thenarrow beam geometry. The geometric efficiency of the slot-scan system is quite lowcompared with that of full-field systems, because only the x-rays that pass throughthe narrow collimation into the slot are used to make the image. To acquire imagesin clinically realistic time frames (1 to 2 seconds), the x-ray tube in this system usesa tungsten anode and is operated at higher peak kilovoltage (kVp) than in conventionalmammography (about 45 kV). Tungsten is a more efficient anode materialthan molybdenum, and x-ray emission is more efficient at higher kVp. These twofactors help overcome the heat-loading issues that can be a problem with slot-scangeometries.slot scan (+V) \• •charge transfer(-V)FIGURE 11-12. The readout logic of a time delay and integration (TOI) system,which is used for slot-scan digital mammography, is illustrated. The breast remainsstationary under compression during image formation. A through C: The x-raybeam is collimated by a slot (as shown), and the slot is scanned across the breast.While the x-ray beam is scanned from left to right at velocity +V, the CCO is read outin the opposite direction at velocity -v. The x-ray shadow of a small structure in thebreast (dark circle) projects onto the same position of the signal train on the CCOcamera as the signal train moves across the chip. Because of the synchrony betweenthe slot scan and the CCO readout, the image of fine structures in the breast is preservedwith high resolution.