Scalable, Time-of-Flight and Depth-of-Interaction Detector Units for High-Resolution PET Systems

Abstract

Resolving the depth of interaction (DOI) of a γ-photon in the scintillator is necessary to correct for parallax errors in organ-dedicated and large-scale time-of-flight positron emission tomography (TOF-PET) scanners or enable the precise recovery of Compton-scattered γ-photons. Doubling the number of readout channels and moving towards more complex detector designs are methods to encode DOI, often associated with high costs. We propose a DOI-capable TOF-PET detector unit concept confining light-sharing to two detector channels, where the high benefit lies in scalability and the prospect of Compton recovery between adjacent units. We evaluate these scalable, DOI-capable unit concepts, realizing DOI encoding between two LYSO:Ce,Ca crystals (3x3x20 mm 3 ; Taiwan Applied Crystals) one-to-one coupled to two Broadcom AFBR-S4N33C013 silicon-photomultipliers (SiPMs) read out with the TOFPET2 ASIC. The best-performing unit employing a triangular reflector sheet and optical glue between the two crystals and mounted on two FBK NUV-MT SiPMs results in a DOI resolution of about 3 mm (RMSE) based on the energy ratio digitized by the two channels while maintaining a coincidence time resolution (CTR) of 226 ps (FWHM) with TOFPET2 ASIC readout, applying a linear DOI correction. Using HF readout, the CTR of the proposed detector unit was improved to 141 ps (FWHM). I. INTRODUCTION While currently applied for mid-to late-stage disease diagnostics, organ-dedicated positron emission tomography (PET) is envisioned to become a future diagnosis tool to detect, classify and treat early-stage neurological and inflammatory diseases such as Alzheimer's and Parkinson's among the population. Before undergoing a PET scan, patients are injected with a tracer sensitive to β-amyloid aggregations in case of Alzheimer's [1], [2]. Upon decay, these tracers emit positrons, which recombine with electrons in the surrounding tissue, releasing two 511-keV γ-photons back to back. These are commonly stopped and digitized by detector blocks, which enclose the patient and consist of dense scintillation material, a photosensor and customized readout electronics [3]. For a fast, reliable and -most of allsafe screening process, a high signal-to-noise ratio (SNR) and a high sensitivity are required to reduce scan times and administered dosage for patients as much as possible [4]-[7].

16-channel SiPM high-frequency readout with time-over-threshold discrimination for ultrafast time-of-flight applications

“…Apart from reducing the power consumption [ 15 ], sophisticated shielding techniques may come into play to reduce the interference between components. First shielding attempts have been demonstrated using caskets [ 15 , 16 , 46 ]. Final 3D-integration could be facilitated by omitting the balun transformer as proposed in [ 15 ].…”

“…It is expected to realize TOF resolutions close to the limits of photosensor and scintillator measured with the oscilloscope. A combination of this 16-channel HF readout with previous studies on TOF- and depth-of-interaction-capable detector units is as well planned [ 46 ]. Overall, it can be concluded that the presented electronic front end does not affect the timing performance of the back-end electronics used in single- and multi-channel experiments and is hence a promising readout approach to be connected to ultrafast, precise back-end electronics.…”