Enabling cost-effective TOF-PET by exploiting the Cherenkov emission in BGO

Objectives: The use of time-of-flight (TOF) information in positron emission tomography (PET) is known to improve image quality. Clinical TOF-PET systems with excellent coincidence resolving times (CRT) of 325 - 400 ps FWHM have recently been developed, based on silicon photomultipliers (SiPMs) and L(Y)SO:Ce crystals. BGO is considerably less expensive than L(Y)SO:Ce, while having a higher attenuation coefficient and photoelectric fraction. However, BGO is generally considered an inferior TOF-PET scintillator. In recent years, TOF-PET detectors based on the Cherenkov effect have been proposed, motivated by the prompt character of this emission. Unfortunately, the low Cherenkov photon yield in the order of ~10 photons per event implies poor energy resolution - a severe disadvantage in clinical PET. Here, we study the feasibility of combining event timing based on Cherenkov emission with energy discrimination based on scintillation in BGO, as a potential solution for cost-effective TOF-PET.

Methods: The rise-time of the BGO luminescent emission upon excitation by 511 keV photons was measured using a setup for time-correlated single-photon counting (TCSPC) based on a Philips digital photon counter (DPC) array. Furthermore, coincidence timing measurements were performed with pairs of BGO crystals of equal dimensions that were optically coupled to DPC arrays and positioned face-to-face with a ²²Na source in between them. All crystals had a cross section of 3 mm x 3 mm, while 5 different crystal lengths were investigated, viz. 3 mm, 5 mm, 8 mm, 12 mm, and 20 mm.

Results: Non-Gaussian coincidence spectra with a FWHM of 200 ps were obtained with the 27 mm³ BGO cubes, while FWHM values down to 330 ps were achieved with the 20 mm long crystals. These unprecedented time resolutions are congruent with the results obtained in the TCSPC measurements, which reveal a significant fast component with a rise time of about 120 ps and a FWHM of about 160 ps, preceding slower luminescent components. The fast component cannot only be explained by the scintillation characteristics found for BGO in literature, but agrees with the expected response due to Cherenkov emission.

Conclusion: The results presented in this work indicate that it may be feasible to use BGO as a hybrid scintillator / Cherenkov-radiator for TOF-PET, utilizing the Cherenkov effect for fast timing while maintaining the possibility of energy discrimination based on scintillation. To the best of our knowledge, the timing results presented here are the best reported for BGO to date. Research Support: This work was partly funded by European Union's Horizon 2020 Framework Programme, Marie Skłodowska-Curie action 659317 (PALADIN). See also: fasttiming.weebly.com