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 PbF2 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 mm3 sized PbF2 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 mm3 (2 × 2 × 3 mm3) sized PbF2 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.
Highlights
Positron emission tomography (PET) is presently the most sensitive molecular imaging modality, allowing to image the metabolic process in a patient and to detected diseases
In the case of high frequency (HF) electronics, we have shown that corrections based on the slope of the signal will outperform corrections based on the amplitude or charge, demonstrated in figure 10 for two-single photon avalance diode (SPAD) events
The Cherenkov photon yield largely depends on the choice of surface treatment, while longer length is of secondary importance
Summary
Positron emission tomography (PET) is presently the most sensitive molecular imaging modality, allowing to image the metabolic process in a patient and to detected diseases. The Cherenkov photon yield is largely dropping when approaching lower particle energies such as it is the case in prompt gamma imaging for range verification in hadron therapy (ArinoEstrada et al 2019) and TOF-PET. For the latter the detection of few prompt photons on top of scintillation can largely improve the timing performance (Gundacker et al 2016a, Kratochwil et al 2020a) and is one way toward the achievement of the 10 ps time resolution target (Lecoq et al 2020). Beside aiming for the best possible timing performance, Cherenkov photons can be utilized to enable more affordable TOF-PET scanners by optimizing sensitivity and cost (Brunner and Schaart 2017, Kratochwil et al 2020b), since crystals with lower melting temperature and cheaper raw materials like BGO or PbF2 can be used
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