Investigation of analog charge multiplexing schemes for SiPM based PET block detectors

Reducing the number of output channels in pixelated positron emission tomography (PET) detectors is an effective way to minimize cost and complexity while minimizing the impact on detector performance. This paper compares the system performance of two multiplexing schemes by using both simulation and experimental studies, with respect to spatial, time and energy resolutions. Simulations were performed using the SPICE environment to investigate differences in resulting flood histograms and rising edge slopes. Experiments were performed using lutetium-yttrium oxyorthosilicate (LYSO) crystals coupled to a SensL ArraySL-4 silicon photomultiplier (SiPM) connected to interchangeable circuit boards containing the two multiplexing schemes of interest. Three crystal configurations were tested: a single crystal element (3 x 3 x 20 mm(3)), 2 x 2 array (crystal pitch: 3 x 3 x 20 mm(3)) and 6 x 6 array (crystal pitch: 2.1 x 2.1 x 20 mm(3)). Good agreement was found between the simulations and experiment results. It is found that the capacitive multiplexing is able to achieve an improved time resolution of good uniformity (average of 1.11 +/- 0.01 ns and 1.90 +/- 0.03 ns for the arrays, respectively) and crystal separation, compared to the resistive multiplexing (average of 1.95 +/- 0.03 ns and 3.33 +/- 0.10 ns). On the other hand, the resistive multiplexing demonstrates slightly improved energy resolution (11 +/- 0.1% and 22 +/- 0.6%, compared to 12 +/- 0.1% and 24 +/- 0.4% for the capacitive array), which is believed to be caused by the RC circuit formed between the splitting capacitors and the input impedance of amplifiers. The relevancy of this work to the PET block detector design using SiPM arrays is also discussed, including light sharing, edge compression and gain variation among SiPM pixels.