Performance Characterization of a Novel Thin Position-Sensitive Avalanche Photodiode for 1 mm Resolution Positron Emission Tomography

We are developing dedicated breast and small animal positron emission tomography (PET) systems using scintillation detectors comprising 1times1times3 mm³ LSO crystals coupled to novel, extremely thin position-sensitive avalanche photodiodes (PSAPD). The detectors are placed in a novel configuration such that PSAPDs read the relatively large side faces of the crystals, with normally incident 511 keV photons entering parallel to the PSAPD surface. This configuration facilitates 1 mm³ spatial resolution, directly measured photon interaction depth in 2 cm thick LSO, and >90% scintillation light collection efficiency. For this design, extremely thin (<300 mum) PSAPDs are required to achieve high crystal packing fraction for high intrinsic detection efficiency. The standard PSAPD (8times8 mm² active area) is packaged on a ceramic substrate, which is not compact enough for the desired detector configuration. The new PSAPD is packaged on a polyimid (Kapton) "flex" circuit with ~250 mum total thickness. This paper investigates whether the required modification to the PSAPD chip, packaging, and manufacturing processes lead to degraded performance for the thin device compared to the standard PSAPD. Electronically collimated coincidence measurements yielded an average intrinsic spatial resolution of 1.1 plusmn 0.1 mm FWHM, energy resolution of 10.8 plusmn 0.6% at 511 keV, and coincidence time resolution of 2.1 plusmn 0.2 ns FWHM for the standard PSAPD. The corresponding values measured for the thin PSAPD were very similar, 1.1 plusmn 0.1 mm FWHM, 10.9 plusmn 0.7%, and 2.0 plusmn 0.3 ns, respectively. In summary, the new thin PSAPD allows us to attain the design specifications for the proposed ultra-high resolution, high sensitivity PET systems and performs comparable to the standard device.