The effects of inter-crystal scattering events on the performance of PET detectors

Abstract

The probability of inter-crystal scattering (ICS) events for 511 keV gamma rays in all current scintillation crystals is high and the ICS events degrade the spatial resolution of PET scanners. In this work, Monte Carlo simulations were performed to study the effects of ICS events on the sensitivity and spatial resolution of PET detectors. LaBr3, LYSO, and PWO that represent scintillation crystals of low, medium and high density, respectively, were used. For a point source placed in the middle of two scintillation detectors of 50 × 50 × 20 mm3 and a lower energy threshold (LET) of 350 keV, the probabilities that at least one gamma ray undergoes ICS are 94%, 84% and 76% for LaBr3, LYSO, and PWO, respectively. The ICS events still provide useful spatial information. The full width at half maximum (FWHM), the full width at tenth maximum (FWTM) and the mean absolute error (MAE) of the curve of the mispositioning of a point source caused by ICS events are 0.45, 3.0 and 0.9 mm if the most popular PET scintillator LYSO is used. The MAE is smaller than the spatial resolution of most current PET scanners. The effect of ICS increases as the detector LET increases, scintillator density decreases, and crystal size decreases. The intrinsic spatial resolutions of a pair of LYSO detectors were calculated using curves of the coincidence counts between one column of the crystals in the two detectors and the sum of the coincidence counts between two opposite crystals of the columns in the two detectors that are in line with the point source changing with the source positions. The latter method removes almost all of ICS events. The FWHM (FWTM) intrinsic spatial resolutions obtained by the two methods are 0.40 (2.0) mm and 0.33 (0.8) mm if the crystal size is 0.5 mm, and are 0.8 (3.0) and 0.68 (1.5) mm if the crystal size is 1.0 mm. ICS events have much bigger contributions to the FWTM rather than the FWHM of the intrinsic spatial resolution of PET detectors. The spatial resolution of a PET scanner can still be improved by decreasing the crystal size to as small as 0.5 mm.

Experimental evaluation of convolutional neural network-based inter-crystal scattering recovery for high-resolution PET detectors

“…The higher significance of improvement in FWTM resolution than FWHM resolution aligns with previous studies reporting a high contribution of ICS to FWTM (Schmall et al 2016) and a large impact of ICS recovery on FWTM (Zhang et al 2019). One plausible explanation for these results is that events with large scatter angles and ICS of obliquely incident photons would largely contribute to the peripheral region of the point spread.…”

Towards quantitative small-animal imaging on hybrid PET/CT and PET/MRI systems

“…Intercrystal scattering is another source of scattered photons, which depends upon the crystal density and detector size. As recently demonstrated by Zhang et al, intercrystal scattering becomes significant by increasing the detector size and scintillator density [148].…”

Physical and imaging performance of SIAT aPET under different energy windows and timing windows

“…For the detector of SIAT aPET, although the flood histogram of low energy events (such as 250–350 keV) was worse than that of high energy events (such as 450–550 keV), in the flood histogram of low energy events, all but the corner crystals can still be resolved. Meanwhile, since the probability of photoelectric effect of 511 keV gamma‐rays in the LYSO crystal is less than 40%, there is significant probability that one of the two coincidence gamma‐rays undergoes inter‐crystal scattering in the LYSO array 40,41 . The events with large energy depositions are likely to include more inter‐crystal scattering events, which deteriorate the spatial resolution of a PET scanner.…”

“…Meanwhile, since the probability of photoelectric effect of 511 keV gamma-rays in the LYSO crystal is less than 40%, there is significant probability that one of the two coincidence gamma-rays undergoes inter-crystal scattering in the LYSO array. 40,41 The events with large energy depositions are likely to include more intercrystal scattering events, which deteriorate the spatial resolution of a PET scanner. The combination of the above two effects affects the spatial resolution that can be obtained with different EWs.…”