Cherenkov Radiation–Based Coincidence Time Resolution Measurements in BGO Scintillators

Abstract

Bismuth germanate oxide (BGO) scintillators can be re-introduced in time-of-flight positron emission tomography (TOF-PET) by exploiting the Cherenkov luminescence emitted as a result from 511 keV interactions. Accessing the timing information from the relatively few emitted Cherenkov photons is now possible due to the recent improvements in enhanced near-ultraviolet high-density (NUV-HD) silicon photomultiplier (SiPM) technology, fast and low noise readout electronics, and the development of efficient data post-processing methods. In this work, we aim to develop a scalable detector element able to achieve excellent coincidence time resolution (CTR) required for TOF-PET using BGO scintillator elements of various lengths. The proposed detector element is optically coupled to 3.14 × 3.14 mm2 NUV-sensitive SiPMs mounted on a custom design circuit board. In particular, we have evaluated the CTR performance of BGO crystal elements of dimensions 3 × 3 × 3 mm3, 3 × 3 × 5 mm3, 3 × 3 × 10 mm3, and 3 × 3 × 15 mm3, with chemically etched surfaces and wrapped in Teflon tape. To achieve excellent CTR performance, we apply state-of-the-art post-processing methods during data analysis. Best values of 156 ± 6 ps, 188 ± 5 ps, 228 ± 8 ps, and 297 ± 8 ps CTR FWHM have been achieved for the 3, 5, 10, and 15 mm length BGO crystals, respectively. These values improve to 105 ± 6 ps, 127 ± 8 ps, 133 ± 4 ps, and 189 ± 8 ps CTR FWHM, when only considering the Cherenkov component of the timing signal, which is extracted by considering the events with the fastest rise time (20% of the total data). The accurate classification of the events based on their rise time is possible; thanks to the implementation of a dual threshold approach that sets the lower threshold below one light photon equivalent level and the upper one above the signal amplitude of a single photon avalanche diode (SPAD).

Image Reconstruction Analysis for Positron Emission Tomography with Heterostructured Scintillators

“…However, here, the variety of timing resolutions obtained from the different combinations of detected events is much wider than in cases investigated previously [27], [43]. Efthimiou et al [27], [43], studying Cherenkov-based detectors [44]- [48], reported that the complexity of the reconstruction with multiple (25) kernels slowed down their convergence. In this work, with only three Gaussian kernels, the Contrast Recovery Coefficient (CRC) converged slightly faster than the single and slower TOF kernel used for the BGO.…”

Advances in Detector Instrumentation for PET

“…Although most modern PET systems are built using lutetiumbased scintillation crystals (e.g., lutetium-yttrium oxyorthosilicate [LYSO]) because of their high light yield and excellent timing properties, there has been a recent resurgence of studies that explore BGO, especially its achievable timing properties using prompt luminescence emissions (10). As the field is moving toward scanners with a long axial field of view, BGO is especially attractive because of its significantly decreased production cost and increased Z eff compared with lutetium-based crystals.…”

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

“…BGO crystals with dimensions of 2 × 2 × 3 mm also yielded a remarkable CTR value of 158 ± 3 ps. In other investigations with 3 × 3 × 3 mm BGO crystals [ 179 , 183 , 184 ], the capacitance compensated high-frequency SiPM readout electronics for measuring fast Cherenkov photons achieved timing resolution of 105–200 ps.…”

“…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].…”

“…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]. Gundacker et al [178] implemented a high-frequency readout electronics that utilize passive capacitance compensation and RF amplifiers to take full advantage of the SiPM with low single-photon timing resolution.…”