Technologies to Improve PCD Performance
Besides proper selection of detector parameters, a variety of technological advances have been proposed to improve PCD-equipped CT systems. These technologies range from being simple to complex, and may have significant trade-offs that offset their adoption.
Charge sharing causes large degradations in the ERF. A very powerful method for recovering lost performance is the integration of charge across multiple pixels (41, 42). Charge summing could provide drastic improvements in image quality, but is also complex to implement. In addition, charge summing effectively locks multiple pixels when a photon arrives, so that coincident events that arrive in adjacent pixels may be inappropriately summed. For this reason, the dead time will increase by a factor of 4 or 9 and the pixel size may need to be reduced.
Charge summing is technically complex and many PCDs lack this feature, although some newer designs have included some type of charge summing (10, 43). A variation is anti-coincidence logic,
FIGURE 17.6 Three examples of dynamic bowtie filters. The fan beam of the x-ray is shown as a gray triangle, with x-ray source being at the vertex of the triangle. Attenuating material is shown in black, with an alternate dynamic configuration provided in dotted line. (Left) A translating attenuator. When the attenuator translates into or out of the page, the detector changes shape. (Middle) A two-wedge attenuator. The wedges can individually move to accommodate smaller or larger patients. (Right) A piecewise-1 inear attenuator. Each wedge can individually translate, and the sum of the wedges is a customizable piecewise-linear function with as many control points as there are wedges.
where coincident events are detected after digitization. This can incorporate spectral bin information and hence serve as a digital form of charge sharing correction (44,45).
Dynamic Bowtie Filters
The “bowtie” filter is a pre-patient attenuating filter that shapes the flux distribution of the beam. The bowtie filter is so named because of its shape, which is thin for rays directed at isocenter but thick for rays directed at the periphery. Conventional bowtie filters are fixed, but dynamic filters could adjust their shape to reduce flux before significant pileup appears on the detector.
Several bowtie filters have been described in the literature (46-50). Figure 17.6 provides three examples. One class of dynamic bowties consists of multiple elements which move to adjust the incident flux on a region-by-region basis. Simpler designs have also been described in the patent literature (51, 52) but may lack the flexibility to deal with pileup because they only present one or two degrees of freedom. A multiple-aperture-device has also been proposed for its rapid switching capabilities but also presents a limited number of degrees of freedom unless more than two devices are used (53). The inclusion of dynamic bowties together with PCDs is expected to decrease peak flux and improve image quality (48, 54).