Assessment of photodiodes as a light detector for PET scanners

Current PET systems based on pixelated scintillator arrays coupled to photomultiplier tubes suffer from pile-up and electronics dead time at high count rates. With a pixelated readout, i.e. one-to-one coupling of a scintillator crystal to a photo detector, these effects can be strongly reduced. Recent developments of high light output scintillators like LYSO and LaBr₃ in combination with very low noise amplifiers based on modern CMOS processes make it possible to use high quantum efficiency blue-sensitive PIN photodiodes as a light detector. To explore the potential of this approach, a model of the signal detection chain was implemented. It comprises the scintillation light pulse and its quantum noise, optical coupling, charge conversion in the diode, noise sources of the integrating amplifier, shaper circuits for the energy and timing channel, and the discriminator for the timing channel. The model is verified using off-the-shelf PIN photodiodes and a dedicated CMOS preamplifier excited by a picosecond laser as well as scintillator pulses. The model predicts that with high light output scintillators, high quantum efficiency photodiodes and optimized preamplifiers, a pixelated PET readout with very good energy resolution and sufficient timing resolution can be realized. To complement the study, an APD-based readout is also considered and the related signal to noise issues are discussed