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

Abstract

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 keV γ -ray and fast timing) in a so-called heterostructure, metascintillator or metapixel. 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 mm3 and 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 (layered BGO). Main results. CTR values of 239 ± 12 ps and 197 ± 10 ps FWHM were obtained for the 15 mm long heterostructures with 100 µm and 200 µm thick EJ232 plates (both with 100 µm 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.

Timing performance of lead halide perovskite nanoscintillators embedded in a polystyrene matrix

“…21 On the other hand, EJ232 was chosen as an ultra-fast plastic scintillator: it shows similar properties to the BC422 plastic scintillator (effective decay-time of ≈1.7 ns, rise time of ≈35 ps, intrinsic light yield of about 10 ph keV −1 21 ) but it can be produced in very thin layers with a better surface state. For the aforementioned reasons, EJ232 is one of the favorite materials in preliminary studies 27,28 about heterostructures.…”

“…The detector time resolution (DTR) – also known as single time resolution (STR), as opposed to coincidence time resolution (CTR) which takes into account a pair of detectors – was measured using a novel experimental setup with low energy X-ray irradiation developed purposely for the characterization of low-stopping power scintillators. 32,33 The setup includes a Hamamatsu pulsed X-ray tube, excited by a picosecond diode laser whose external trigger is used as the start signal for the DTR measurement. The stop signal instead is given by the output of the SiPM and is used for collecting the scintillation light emitted by the sample following the X-ray excitation.…”

“…21,31 The coincidence time resolution (CTR) is already an established quantity usually measured using higher energy sources, like 22 Na emitting 511 keV g-rays. Due to low detection efficiency, the standard characterization procedure is not possible for these samples, therefore we used a novel experimental setup 32,33 to measure the time resolution of scintillators upon low energy (about 10 keV) X-ray irradiation.…”

“…BC422 and EJ232 whose decay time is about 1.5 ns) have been published in the last few years. [26][27][28] Lead halide perovskites embedded in polystyrene constitute an alternative candidate as a fast performing material for heterostructures with potentially better time resolution than BC422 or EJ232 as a result of faster scintillation decay kinetics and higher light yield (about 24 ph keV À129 instead of 8.4 ph keV À130 ).…”

Image Reconstruction Analysis for Positron Emission Tomography with Heterostructured Scintillators

“…Both works mention that the layers of the heterostructure should be smaller than the recoil range of the electrons to allow energy sharing between the two materials. In BGO, the average range of the primary electrons generated by 511 keV gamma rays is below 100 µm, whereas for LYSO it is slightly above 100 µm [24], [25].…”