Bismuth germanate coupled to near ultraviolet silicon photomultipliers for time-of-flight PET

Abstract

Bismuth germanate (BGO) was a very attractive scintillator in early-generation positron emission tomography (PET) scanners. However, the major disadvantages of BGO are lower light yield and longer rise and decay time compared to currently popular scintillators such as LSO and LYSO. This results in poorer coincidence timing resolution and it has generally been assumed that BGO is not a suitable scintillator for time-of-flight (TOF) PET applications. However, when a 511-keV photon interacts in a scintillator, a number of Cerenkov photons are produced promptly by energetic electrons released by photoelectric or Compton interactions. If these prompt photons can be captured, they could provide a better timing trigger for PET. Since BGO has a high refractive index (increasing the Cerenkov light yield) and excellent optical transparency down to 320 nm (Cerenkov light yield is higher at shorter wavelengths), we hypothesized that the coincidence timing resolution of BGO can be significantly improved by efficient detection of the Cerenkov photons. However, since the number of Cerenkov photons is far less than the number of scintillation photons, and they are more abundant in the UV and blue part of the spectrum, photosensors need to have high UV/blue sensitivity, fast temporal response, and very low noise in order to trigger on the faint Cerenkov signal. In this respect, NUV-HD silicon photomultipliers (SiPMs) (FBK, Trento, Italy) are an excellent fit for our approach. In this study, coincidence events were measured using BGO crystals coupled with NUV-HD SiPMs. The existence and influence of Cerenkov photons on the timing measurements were studied using different configurations to exploit the directionality of the Cerenkov emissions. Coincidence resolving time values (FWHM) of ~270 ps from 2 × 3 × 2 mm3 BGO crystals and ~560 ps from 3 × 3 × 20 mm3 BGO crystals were obtained. To our knowledge, these are the best coincidence resolving time values reported for BGO to date. With these values, BGO can be considered as a relevant scintillator for TOF PET scanners, especially if photodetectors with even better near UV/blue response can be developed to further improve the efficiency of Cerenkov light detection.

Total-Body PET: Maximizing Sensitivity to Create New Opportunities for Clinical Research and Patient Care

“…Prospects for cost reduction will focus on decreasing component costs (especially the scintillation crystals, which are a dominant component) and defining the axial extent needed for specific applications. For example, recent demonstrations that bismuth germanate scintillators may be able to support time-of-flight PET (59,60) offer one pathway to lowercost systems without sacrificing sensitivity. As indicated above, opportunities also exist to further improve the effective sensitivity, primarily through better timing resolution.…”

Evaluation of a clinical TOF-PET detector design that achieves ⩽100 ps coincidence time resolution

“…Recently, researchers have also been investigating the potential for improving CTR by exploiting prompt emissions such as hot intra-band luminescence, Cherenkov yield from the primary photoelectron, or other methods that exploit sub-ps optical phenomena [Lecoq et al 2014, Gundacker et al 2016b, Dolenec 2013, Kwon et al 2016, Brunner et al 2017, Tao et al 2016, Tao et al 2017]. The ultimate goal of efforts to develop TOF-PET detectors with the ability to localize 511 keV photon interactions along LORs with an accuracy that approaches the spatial resolution limits dictated by positron range, 511 keV photon accolinearity, and detector element width [Levin et al 1999].…”

In situ beta-gamma separation algorithm for cost-effective assessment of radioactive waste resources

“…All photon sources were assumed pure gamma‐emitters, so the beta ray normally released during the radioactive decay process was ignored to evaluate the response of the scintillator to the photons. To emulate the real detection environment, relevant parameters were carefully selected . The elemental composition and mass density of the BGO scintillator were Bi 4 Ge 3 O 12 and 7.13 g/cm 3 , respectively, and those of the plastic scintillator were H:C = 1:1.103 and 1.05 g/cm 3 , respectively.…”

“…In previous research, a method of beta–gamma‐ray separation was carried out using the sensitivity difference between plastic and cadmium tungstate (CdWO 4 ) scintillators, but the energy resolution of the cadmium tungstate was poor (14.6% at 662 keV) and the decay time was too long (12.7 μs). Bismuth germanate (Bi 4 Ge 3 O 12 , BGO) was used in this study for these reasons and because it is a candidate material which has been used to detect gamma rays in medical applications . One of the issues in previous study was the energy dependence of the sensitivity for each scintillator.…”