To determine the segment of the crystal in which an annihilation photon is detected, the signals from a four-PMT array are combined as follows:where PMTA, PMTB, and so forth are the signals from different PMTs. It will be recognized that these are essentially identical to Equations 13-1 and 13-2 for position localization for an Anger camera, except that only four PMTs are used here. The X and Y signals then are used to determine the subelement of the array in which the annihilation photon was detected. Figure 18-15 shows the image obtained from uniform irradiation of a block detector. The image is not uniform. Rather, the calculated locations for recorded events are clustered in small localized areas corresponding to the individual detector elements. There is a small amount of overlap, but the individual elements are clearly resolved. Although the array pattern is nonlinear, the separation is sufficiently clear to allow each (x,y) location in the image to be assigned to a specific detector element in the array, for example, by using a look-up table. The major advantage of the block detector is that it enables many detector elements (e.g., 8 × 8 = 64) to be decoded using only four PMTs. This dramatically lowers the cost per detector element while providing high spatial resolution. Typical block detectors are made from 20- to 30-mm-thick BGO, LSO, or LYSO scintillator crystals, with 4- to 6-mm-wide sub-elements.