Abstract
In the context of improving the detector performance of time-of-flight positron emission tomography (TOF-PET), the combination of charge induction readout and prompt Cherenkov photon production in semiconductor materials can lead to an outstanding detector performance in energy, timing, and spatial resolution. Energy resolutions as good as 1.2% at 662 keV and 5% at 122 keV are reported for pixel thallium bromide (TlBr) detectors. The high refractive index of Tl-based materials, between 2.3 and 2.6, leads to a high Cherenkov photon generation yield but can also challenge photon extraction, potentially affecting the time performance. In this work, the timing properties of TlBr and thallium chloride (TlCl) crystals of different geometries are measured using an optimized test setup with high-frequency readout electronics. A coincidence time resolution (CTR) value of 167 ± 6 ps FWHM is achieved using a 3 × 3 × 3 mm3black-painted TlBr crystal. In order to assess potential improvements, a Geant4-based simulation tool kit is developed and validated against experimental measurements. The simulation tool kit is used to predict the contributions limiting the time resolution regarding the crystal and photodetector properties, highlighting the potential of such materials. Finally, paths to further improve the detector performance in TOF-PET are discussed.
Highlights
Image quality in positron emission tomography (PET) is primarily determined by the detector performance, and it can be largely improved by better localizing the positron–electron annihilation point
We evaluate the time resolution of thallium bromide (TlBr) and thallium chloride (TlCl) semiconductor materials coupled to analog silicon photomultipliers (SiPMs) from Fondazione Bruno Kessler (FBK) [17] in an optimized test bench [18]
The coincidence time resolution (CTR) values obtained after correction for the contribution of the reference detector according to Eq (1) are CTRBlack painting = 204 ± 7 ps and CTRTeflon = 269 ± 9 ps, assuming two identical TlBr crystals in coincidence
Summary
Image quality in positron emission tomography (PET) is primarily determined by the detector performance, and it can be largely improved by better localizing the positron–electron annihilation point. Two other crucial parameters are the coincidence time resolution (CTR) between detector pairs and the energy resolution of the detectors, in order to identify possible scatterings of the detected gamma and discard or correct for it [1]. Inorganic scintillators such as cerium-doped lutetium yttrium orthosilicate (L(Y)SO:Ce), barium fluoride (BaF2), or bismuth germanate (BGO) coupled to silicon photomultipliers (SiPMs) can achieve CTR values below 100 ps [2–4]; their energy resolution is limited to approximately 10% for LYSO [5] and 18% for BGO [6]. Values between 1 and 2% at 662 keV and 5% at 122 keV are reported for pixelated TlBr
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