A roadmap for sole Cherenkov radiators with SiPMs in TOF-PET

Abstract

Time of flight positron emission tomography can strongly benefit from a very accurate time estimator given by Cherenkov radiation, which is produced upon a 511 keV positron-electron annihilation gamma interaction in heavy inorganic scintillators. While time resolution in the order of 30 ps full width at half maximum (FWHM) has been reported using MCP-PMTs and black painted Cherenkov radiators, such solutions have several disadvantages, like high cost and low detection efficiency of nowadays available MCP-PMTs. On the other hand, silicon photomultipliers (SiPMs) are not limited by those obstacles and provide high photon detection efficiency with a decent time response. Timing performance of PbF 2 crystals of various lengths and surface conditions coupled to SiPMs was evaluated against a reference detector with an optimized test setup using high-frequency readout and novel time walk correction, with special attention on the intrinsic limits for one detected Cherenkov photon only. The average number of detected Cherenkov photons largely depends on the crystal surface state, resulting in a tradeoff between low photon time spread, thus good timing performance, and sensitivity. An intrinsic Cherenkov photon yield of 16.5 ± 3.3 was calculated for 2 × 2 × 3 mm 3 sized PbF 2 crystals upon 511 keV γ-deposition. After time walk correction based on the slew rate of the signal, assuming two identical detector arms in coincidence, and using all events, a time resolution of 215 ps FWHM (142 ps FWHM) was obtained for 2 × 2 × 20 mm 3 (2 × 2 × 3 mm 3 ) sized PbF 2 crystals, compared to 261 ps (190 ps) without correction. Selecting on one detected photon only, a single photon coincidence time resolution of 113 ps FWHM for black painted and 166 ps for Teflon wrapped crystals was measured for 3 mm length, compared to 145 ps (black) and 263 ps (Teflon) for 20 mm length.

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

“…Recent efforts to detect fast prompt photons, such as Cherenkov photons, have pushed the limits of timing performance in SiPM-based TOF PET detectors [178,179,[182][183][184][185][186]. When estimating the interaction time of gamma rays based on the prompt photons, the following two points should be considered [184]: (1) SiPM and subsequent readout electronics 19 Bootstrapping methods for SiPM capacitance compensation: a passive method using a balun transformer and b active method using a bootstrapping amplifier should have a single-photon timing resolution that is as low as possible and a − 3 dB bandwidth with > 1 GHz sampling rate [178,187], respectively; (2) Low noise level should be sufficiently low to avoid false triggers caused by the threshold equivalent to a few photon level [188].…”

Time Resolution Studies of Thallium Based Cherenkov Semiconductors

“…One of the 3 × 3 mm 2 faces is coupled to a SiPM using Cargille Meltmount with n = 1.58 and cutoff at 300 nm. The remaining surfaces are Teflonwrapped or black-painted using a spray with a refractive index n = 1.5 [16], to emphasize the crystal properties. The transmission of the crystals is measured without wrapping, using a PerkinElmer LAMBDA spectrophotometer.…”

“…The CTR of the described setup is defined as the full width at half maximum (FWHM) of the distribution of the time-stamps, and it represents the time resolution of the coincidence between the Cherenkov radiator-based detector and the reference detector. To reproduce the CTR assuming two identical detectors in coincidence, the value is multiplied by 2 √ , and the resolution of the reference crystal (CTR reference = 61 ± 3 ps from [16]) is subtracted as:…”

“…Similar behavior is presented in [13] when events with lower amplitude are removed from the dataset. To model the shape, a Crystal Ball fit function is used, which consists of a Gaussian convoluted with an exponential function [16]. The CTR values obtained after correction for the contribution of the reference detector according to Eq.…”

“…CTR values as low as 30 ps FWHM have been reported using microchannel plate photomultipliers (MCP-PMTs) with embedded pure Cherenkov radiators of 5 mm thickness [14]. When using SiPMs and crystals with PET-sized geometry instead, the timing performance deteriorates significantly [15,16]. Without optimized readout conditions, time resolution values between 300 and 400 ps FWHM have been measured in a proofof-concept study conducted with TlBr and TlCl crystals [13].…”