By G. E. Btown
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Both single-channel APDs (up to about 15 mm in size) and arrays of smaller APDs are now available. An 8 ϫ 8 array of APD elements 29 511 keV Photon Detectors FIGURE 17. PET detectors with depthencoding capability. A: Detector is similar to the standard block detector but is made of two layers of scintillators that have different decay times. An interaction can be assigned to the top or bottom layer, depending on the decay time of the pulse that it generates. This provides one-level (top or bottom) depth of interaction information.
B. ) Accidental coincidences When positron annihilation occurs, the two 511-keV photons are emitted simultaneously. Therefore, the detectors should ideally respond simultaneously. Because of the finite time resolution of the detectors, as discussed earlier, signals must be accepted if they occur within a certain finite time interval or timing window. Because of the finite width of the timing window, it is possibile that two unrelated single annihilation photons can be detected and registered as a valid coincidence.
The relationship that relates which elements in this matrix (r,) record data from radioactivity in the object at location (x,y) is given by: r ϭ x cos ϩ y sin (24) This 2-D matrix s(r,) is known as a sinogram because a point source at a location (x,y) traces a sinusoidal path in the matrix as given by Equation 24. The mapping of detector pairs into a sinogram is shown in Figure 27. In practice, as Figure 27 demonstrates, the data in each row do not come from a single angular view but rather from two adjacent angles that are interleaved together.