In these systems, the detector is in the form of a multilinear array of dels and the image is acquired by scanning the detector assembly across the breast in synchrony with a fan beam of x rays. This design limits the required number of dels, while allowing high spatial resolution (small detector aperture and pitch) to be achieved. This is an important cost factor when expensive detector technologies are employed. Another advantage is the inherent high efficiency rejection of scattered radiation afforded by slot-beam systems, where the detector can be collimated to match the pre-patient fan beam. This can be accomplished without the need for interspace material in the beam.
One disadvantage of scanning systems is the longer overall time to acquire the image. Although this may preclude some dynamic studies, it does not result in blurring because each portion of the image is acquired in a very short time. If there is significant motion in the breast, then a misregistration artifact is more likely to occur. Another disadvantage is that because most of the x-ray beam is removed by the fan-beam collimation, there is inefficient use of x-ray tube heat loading. This requires that the x-ray tube used in this application be designed with an increased heat capacity. To mitigate against both of these effects, the number of rows in the detector (and the width of the x-ray slot beam) can be increased. The system design then involves a tradeoff between the shorter imaging time and improved use of heat loading afforded by a wide slot detector system and the improved scatter rejection and less critical mechanical alignment available with a narrower slot.
Scanned slot systems have been built using the phosphor/fiberoptic taper/CCD concept described in the article of Full-area digital mammography detector systems. We have used such a design to construct a digital mammography system. The detector consists of a strip of CsI(Ti) phosphor material, coupled to three fiberoptic tapers which are abutted with mitre joints at their input surfaces as shown in Figure 1(a). Their taper ratio of 1.58:1 provides demagnification with acceptable light-collection efficiency for this application while creating a space between the tapers at the output to accommodate the outer non-active regions of 3 CCD arrays, which are bonded directly on the tapers. The detector slot is approximately 3.5 mm wide.
Fig. 1 Scanned slot digital mammography detector based on Csl:Ti coupled to a TDI CCD via a)demagnifying fiber-optic taper, b) straight coupling(Fischer system)
In scanning systems it is useful to acquire the image in time-delay-integration (TDI)mode, in which the x-ray beam is activated continuously during the image scan and charge collected in pixels of the CCDs is shifted down CCD columns at a rate equal to but in the opposite direction of the motion of the x-ray beam and detector assembly across the breast. The collected charge packets remain essentially stationary with respect to a given projection path of the x rays through the breast and the charge is integrated in the CCD column to form the resultant signal.
When the charge packet has reached the final element of the CCD, it is read out on a transfer register and digitized. The CCD array can be cooled using thermoelectric devices to reduce noise and increase the dynamic range of the image receptor as necessary. In such systems the spatial resolution is dependent on accurate alignment of the signal-gathering columns of the detector with the scan direction and on correct synchronization with charge clocking in the CCDs with the scan motion. Fischer Imaging Inc. is currently evaluating a scanning system at several clinical sites (Figure 1(b)). The Fischer system differs from the one described above in that it employs a non-tapered fiber-optic coupling and a wider slot (～14 mm). In addition, the inherent del is 25 μm, allowing a limited-area, high-resolution “spot”mode. For normal resolution, the signals from adjacent dels are summed to provide an effective 50 μm del.