SiPM signal readout for inter-crystal scatter event identification in PET detectors

Abstract

In positron emission tomography (PET) with pixelated detectors, a significant number of annihilation photons interact with scintillation crystals through single or multiple Compton scattering events. When these partial energy depositions occur across multiple crystal elements, we call them inter-crystal scatter (ICS) events. ICS events lead to incorrect localization of the annihilation photons, thereby degrading the PET image contrast, spatial resolution, and lesion detectability. The accurate identification of ICS events is the first essential step to improve the quality of PET images by rejecting ICS events or recovering ICS events without losing PET sensitivity. In this study, we propose a novel silicon photomultiplier (SiPM) readout method to identify ICS events in one-to-one coupled PET detectors with a reduced number of data acquisition channels. For concept verification, we assembled a PET detector that consists of a 16-channel SiPM array and 4 4 lutetium oxyorthosilicate (LSO) array with a 3.2 mm crystal pitch. The proposed SiPM readout scheme serializes the 16 SiPM anode signals into four pulse train outputs encoded with four increasing time-delays in steps of 250 ns intervals. A Sum signal of the 16 SiPM anodes provides the timing information for time-of-flight measurement and a trigger signal for coincidence detection. A time-over-threshold (TOT) method was applied for obtaining the energy information followed by a subsequent TOT-to-energy calibration. We successfully identified the ICS events and determined their interacted positions and deposited energies by analyzing the digital pulses from the four pulse train output channels. The occurrence rate of ICS events was 10.85% for the 4 × 4 PET detector module with 3.2 mm-pitch LSO crystals. The PET detector yielded an energy resolution of 10.9 0.6% and coincidence timing resolution of 285 12 ps FWHM. We expect that the proposed method can be a useful solution for alleviating the readout burden of SiPM-based PET scanners with ICS event identification capability.

Silicon photomultiplier signal readout and multiplexing techniques for positron emission tomography: a review

“…The use of coaxial cables is not a practical solution for time delays because of their bulkiness. Therefore, delay chips are used in the subsequent investigations based on the delay line multiplexing methods [ 81 , 84 , 150 ].

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“…The digital delay encoding technique was also useful in the development of a PET detector with ICS event identification capability [ 84 ]. The proposed method serializes 16 one-to-one coupled SiPM anode signals into 4 digital pulse sequences, in which time delays in 250-ns increment were introduced (Fig.…”

“…a Concept illustration. b Representative signals from the pulse train outputs: (1) PE absorption event (2) scatter/escape event (3) ICS event, and (4) ICS/escape event (Reprint from [ 84 ] with permission; © 2020 Institute of Physics and Engineering in Medicine) …”

“…This method also allows for the 3D measurement of each position of high energy photon interaction (Compton scattering or photo-electric absorption) within a pixelated crystal array, enabling the inter-crystal scatter (ICS) event identification and recovery [81,82]. The ICS event recovery increases PET system sensitivity and improves image quality [83,84]. However, the lateral side-readout configuration requires a substantially increased number of photosensors and thus suffers from a high readout complexity and manufacturing costs.…”

A simulation study of 1D U-Net-based inter-crystal scatter event recovery of PET detectors

“…The ICS phenomenon and its influence on PET performance have been thoroughly studied by several research groups (Zeraatkar et al 2010, Kim and McDaniel 2017, Zhang et al 2019, Park and Lee 2020. Their works demonstrate that the ICS events occupy a significant proportion of the total events and obviously decrease the spatial resolution and the SNR of the reconstructed image.…”