Characterization and readout of MADPET-II detector modules: validation of a unique design concept for high resolution small animal PET

MADPET-II is a novel high-resolution, high-sensitivity three-dimensional lutetium oxyorthosilicate avalanche photodiode small animal PET scanner with a unique detector design and readout that creates new possibilities for data processing and analysis. The scanner consists of a ring of dual-layer detector modules (O71 mm), each containing a 4×8 array of 2×2×6mm³ (front) and 2×2×8 mm³ (back) LSO crystals that are each optically isolated and coupled one-to-one to a monolithic 4×8 APD array. Signals from each channel are individually processed using fully-integrated 16-channel, low noise, charge sensitive preamplifiers and custom integrated electronics that include a four channel shaping amplifier, peak detector and non-delay line CFD. Analog-to-digital converters and time-to-digital converters digitise the pulse height and time-stamp each event, and data are stored exclusively in list mode format. Two 32-channel detector blocks were tested in a coincidence and in a dual-layer configuration. Coincidences were sorted post-acquisition in software, allowing for maximum flexibility in the data processing. Monte Carlo simulations have calculated the system spatial resolution to be 1.1 mm FWHM. Average energy resolution for a single detector block ranged from 20.2% FWHM to 23.9% FWHM. System pulse height linearity was measured, and all channels responded with R²>0.9988 (Pearson correlation coefficient). Intrinsic uniformity for each detector block ranged from 2.0% to 4.8% (400 keV threshold). Overall system timing resolution was measured to be 10.2 ns FWHM, with individual LORs having time resolutions as low as 4.5 ns FWHM.