Characterization of a 64 Channel PET Detector Using Photodiodes for Crystal Identification*

element detector module and still yield a practical design. In this paper we report results of measurements made on a 64 element detector module made with LSO scintillator crystal. We present performance results for a prototype PET detector module consisting of 64 LSO scintillator crystals (3x3x20 mm) coupled on one end to a single photomultiplier tube and on the opposite end to a 64 pixel array of 3 mm square silicon photodiodes (typical pixel parameters are 5 pF capacitance, 300 pA dark current, and 73% quantum efficiency at 415 nm). The photomultiplier tube (PMT) provides an accurate timing pulse and energy threshold for all crystals in the module, the silicon photodiodes (PD) identify the crystal of interaction, the sum (PD+PMT) provides a total energy signal, and the PD/(PD+PMT) ratio determines the depth of interaction. With 32 of the channels instrumented, the detector module correctly identifies the crystal of interaction (where “correct” includes the adjacent 4 crystals) 79±4% of the time with high detection efficiency. The timing resolution for a single LSO detector module is 750 ps fwhm, while its pulse height resolution at 511 keV is 24±3% fwhm. The depth of interaction (DOI) measurement resolution is 8±1 mm fwhm. We prefer this ratio based technique to determine the depth of interaction because: (a) it is less sensitive to patient Compton scatter than approaches that use a single photodetector [3]; (b) it obtains better DOI resolution than approaches that use physically segmented crystals [4-6]; and (c) it has a smaller deadtime-surface area product than approaches that use large scintillator plates [7]. 1" Square Photomultiplier Tube 30 mm Array of 64 Photodiodes 64 Scintillator Crystals 3 mm square 1"