Advances in heterostructured scintillators: toward a new generation of detectors for TOF-PET.

Objective.Time-of-flight-positron emission tomography would highly benefit from a coincidence time resolution (CTR) below 100 ps: improvement in image quality and patient workflow, and reduction of delivered dose are among them. This achievement proved to be quite challenging, and many approaches have been proposed and are being investigated for this scope. One of the most recent consists in combining different materials with complementary properties (e.g. high stopping power for 511 keVγ-ray and fast timing) in a so-calledheterostructure,metascintillatorormetapixel. By exploiting a mechanism of energy sharing between the two materials, it is possible to obtain a fraction of fast events which significantly improves the overall time resolution of the system.Approach.In this work, we present the progress on this innovative technology. After a simulation study using the Geant4 toolkit, aimed at understanding the optimal configuration in terms of energy sharing, we assembled four heterostructures with alternating plates of BGO and EJ232 plastic scintillator. We fabricated heterostructures of two different sizes (3 × 3 × 3 mm3and  3 × 3 × 15 mm3), each made up of plates with two different thicknesses of plastic plates. We compared the timing of these pixels with a standard bulk BGO crystal and a structure made of only BGO plates (layeredBGO).Main results.CTR values of 239 ± 12 ps and 197 ± 10 ps FWHM were obtained for the 15 mm long heterostructures with 100µm and 200µm thick EJ232 plates (both with 100µm thick BGO plates), compared to 271 ± 14 ps and 303 ± 15 ps CTR for bulk and layered BGO, respectively.Significance.Significant improvements in timing compared to standard bulk BGO were obtained for all the configurations tested. Moreover, for the long pixels, depth of interaction (DOI) collimated measurements were also performed, allowing to validate a simple model describing light transport inside the heterostructure.

Improving depth-of-interaction resolution in pixellated PET detectors using neural networks.

Parallax error is a common issue in high-resolution preclinical positron emission tomography (PET) scanners as well as in clinical scanners that have a long axial field of view (FOV), which increases estimation uncertainty of the annihilation position and therefore degrades the spatial resolution. A way to address this issue is depth-of-interaction (DOI) estimation. In this work we propose two machine learning-based algorithms, a dense and a convolutional neural network (NN), as well as a multiple linear regression (MLR)-based method to estimate DOI in depolished PET detector arrays with single-sided readout. The algorithms were tested on an 8× 8 array of 1.53× 1.53× 15 mm3 crystals and a 4× 4 array of 3.1× 3.1× 15 mm3 crystals, both made of Ce:LYSO scintillators and coupled to a 4× 4 array of 3× 3 mm3 silicon photomultipliers (SiPMs). Using the conventional linear DOI estimation method resulted in an average DOI resolution of 3.76 mm and 3.51 mm FWHM for the 8× 8 and the 4× 4 arrays, respectively. Application of MLR outperformed the conventional method with average DOI resolutions of 3.25 mm and 3.33 mm FWHM, respectively. Using the machine learning approaches further improved the DOI resolution, to an average DOI resolution of 2.99 mm and 3.14 mm FWHM, respectively, and additionally improved the uniformity of the DOI resolution in both arrays. Lastly, preliminary results obtained by using only a section of the crystal array for training showed that the NN-based methods could be used to reduce the number of calibration steps required for each detector array.

Experimental time resolution limits of modern SiPMs and TOF-PET detectors exploring different scintillators and Cherenkov emission.

Solid state photodetectors like silicon photomultipliers (SiPMs) are playing an important role in several fields of medical imaging, life sciences and high energy physics. They are able to sense optical photons with a single photon detection time precision below 100 ps, making them ideal candidates to read the photons generated by fast scintillators in time of flight positron emission tomography (TOF-PET). By implementing novel high-frequency readout electronics, it is possible to perform a completely new evaluation of the best timing performance achievable with state-of-the-art analog-SiPMs and scintillation materials. The intrinsic SiPM single photon time resolution (SPTR) was measured with Ketek, HPK, FBK, SensL and Broadcom devices. Also, the best achieved coincidence time resolution (CTR) for these devices was measured with LSO:Ce:Ca of [Formula: see text] mm3 and [Formula: see text] mm3 size crystals. The intrinsic SPTR for all devices ranges between 70 ps and 135 ps FWHM when illuminating the entire [Formula: see text] mm2 or [Formula: see text] mm2 area. The obtained CTR with LSO:Ce:Ca of [Formula: see text] mm3 size ranges between 58 ps and 76 ps FWHM for the SiPMs evaluated. Bismuth Germanate (BGO), read out with state of-the-art NUV-HD SiPMs from FBK, achieved a CTR of 158 [Formula: see text] ps and 277 [Formula: see text] ps FWHM for [Formula: see text] mm3 and [Formula: see text] mm3 crystals, respectively. Other BGO geometries yielded 167 [Formula: see text] 3 ps FWHM for [Formula: see text] mm3 and 235 [Formula: see text] 5 ps FWHM for [Formula: see text] mm3 also coupled with Meltmount (n = 1.582) and wrapped in Teflon. Additionally, the average number of Cherenkov photons produced by BGO in each 511 keV event was measured to be 17 [Formula: see text] 3 photons. Based on this measurement, we predict the limits of BGO for ultrafast timing in TOF-PET with Monte Carlo simulations. Plastic scintillators (BC422, BC418), BaF2, GAGG:Ce codoped with Mg and CsI:undoped were also tested for TOF performance. Indeed, BC422 can achieve a CTR of 35 [Formula: see text] 2 ps FWHM using only Compton interactions in the detector with a maximum deposited energy of 340 keV. BaF2 with its fast cross-luminescence enables a CTR of 51 [Formula: see text] 5 ps FWHM when coupled to VUV-HD SiPMs from FBK, with only ∼22% photon detection efficiency (PDE). We summarize the measured CTR of the various scintillators and discuss their intrinsic timing performance.

High-frequency SiPM readout advances measured coincidence time resolution limits in TOF-PET.

Scintillator based radiation detectors readout by SiPMs successively break records in their reached time resolution. Nevertheless, new challenges in time of flight positron emission tomography (TOF-PET) and high energy physics are setting unmatched goals in the 10 ps range. Recently it was shown that high frequency (HF) readout of SiPMs significantly improves the measured single photon time resolution (SPTR), allowing to evaluate the intrinsic performance of large area devices; e.g. FBK NUV-HD SiPMs of [Formula: see text] mm2 area and 40 [Formula: see text]m single photon avalanche diode (SPAD) size achieve 90 ps FWHM. In TOF-PET such readout allows to lower the leading edge detection threshold, so that the fastest photons produced in the crystal can be utilized. This is of utmost importance if a high SPTR and prompt Cherenkov light generated by the hot-recoil electron upon 511 keV photo-absorption should improve timing. This paper shows that high-frequency bipolar transistor readout of state-of-the-art SiPMs coupled to high-performance scintillators can substantially improve the best achievable coincidence time resolution (CTR) in TOF-PET. In this context a CTR of 158 [Formula: see text] 3 ps FWHM with [Formula: see text] mm3 BGO crystals coupled to FBK SiPMs is achieved. This faint Cherenkov signal is as well present in standard LSO scintillators, which together with low SPTR values (<90 ps FWHM) improves the CTR of [Formula: see text] mm3 LSO:Ce:Ca coupled to FBK NUV-HD [Formula: see text] mm2 with 25 [Formula: see text]m SPAD size to 61 [Formula: see text] 2 ps FWHM using HF-electronics, as compared to 73 [Formula: see text] 2 ps when readout by the NINO front-end ASIC. When coupling the LSO:Ce:Ca crystals to FBK NUV-HD SiPMs of [Formula: see text] mm2 and 40 [Formula: see text]m SPAD size, using HF-electronics, a CTR of even 58 [Formula: see text] 3 ps for [Formula: see text] mm3 and 98 [Formula: see text] 3 ps for [Formula: see text] mm3 is achieved. This new experimental data will allow to further discuss the timing limits in scintillator-based detectors.