Comparison of Position-Sensitive versus Discrete Avalanche Photodiodes in a Continuous Crystal Gamma Camera

Position-sensitive avalanche photodiodes (PSAPDs) have been proposed as an alternative to photomultiplier tubes (PMTs) for nuclear medicine applications. These solid-state devices offer high gain and quantum efficiency, and are capable of decoding positions with a high multiplexing ratio. Furthermore, PSAPDs do not require light sharing among multiple devices to allow spatial decoding. It is unclear however if this constitutes an advantage over discrete avalanche photodiodes (APDs). To address this question, we have developed Monte Carlo simulations to evaluate the relative merit of each technology in a simple, continuous-crystal gamma camera. The simulated camera was composed of either a 4times4 array of 8times8 mm² PSAPDs each with 4-channel readout or a 8times8 array of 4times4 mm² discrete APDs. These configurations, requiring 64 channels each, were used to read the scintillation light from a 6 mm thick continuous CsI:Tl crystal covering the entire photodiode array. The performance of the simulated cameras was evaluated in terms of spatial resolution and uniformity for ^99mTc and ¹²⁵I radionuclide energies. Intrinsic spatial resolutions of 0.9 and 1.0 mm were obtained for the PSAPD- and APD-based cameras respectively for ^99mTc, and corresponding values of 1.3 and 1.2 mm FWHM for ¹²⁵I. Although both PSAPDs and discrete APDs can achieve intrinsic spatial resolution close to one millimeter for the radionuclides studied, PSAPDs seem to provide a better uniformity compared to discrete APDs. This improved performance is due to the ability of a single PSAPD to decode positions. This task requires light sharing among multiple devices in APDs, a condition poorly fulfilled for events occurring close to the surface of the photodiodes.