Measurement of intrinsic rise times for various L(Y)SO and LuAG scintillators with a general study of prompt photons to achieve 10 ps in TOF-PET

Abstract

The coincidence time resolution (CTR) of scintillator based detectors commonly used in positron emission tomography is well known to be dependent on the scintillation decay time (τd) and the number of photons detected (n'), i.e. CTR proportional variant √τd/n'. However, it is still an open question to what extent the scintillation rise time (τr) and other fast or prompt photons, e.g. Cherenkov photons, at the beginning of the scintillation process influence the CTR. This paper presents measurements of the scintillation emission rate for different LSO type crystals, i.e. LSO:Ce, LYSO:Ce, LSO:Ce codoped Ca and LGSO:Ce. For the various LSO-type samples measured we find an average value of 70 ps for the scintillation rise time, although some crystals like LSO:Ce codoped Ca seem to have a much faster rise time in the order of 20 ps. Additional measurements for LuAG:Ce and LuAG:Pr show a rise time of 535 ps and 251 ps, respectively. For these crystals, prompt photons (Cherenkov) can be observed at the beginning of the scintillation event. Furthermore a significantly lower rise time value is observed when codoping with calcium. To quantitatively investigate the influence of the rise time to the time resolution we measured the CTR with the same L(Y)SO samples and compared the values to Monte Carlo simulations. Using the measured relative light yields, rise- and decay times of the scintillators we are able to quantitatively understand the measured CTRs in our simulations. Although the rise time is important to fully explain the CTR variation for the different samples tested we determined its influence on the CTR to be in the order of a few percent only. This result is surprising because, if only photonstatistics of the scintillation process is considered, the CTR would be proportional to the square root of the rise time. The unexpected small rise time influence on the CTR can be explained by the convolution of the scintillation rate with the single photon time resolution (SPTR) of the photodetector and the photon travel spread (PTS) in the crystal. The timing benefits of prompt photons at the beginning of the scintillation process (Cherenkov etc) are further studied, which leads to the conclusion that the scintillation rise time, SPTR and PTS have to be lowered simultaneously to fully profit from these fast photons in order to improve the CTR significantly.

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

“…Laboratory results already have demonstrated timing of under 100 ps under idealized conditions (56), and as sensors, scintillators, and detector designs continue to improve, we can anticipate that the timing performance of scanners will continue to get better. Looking even farther into the future, it may be possible to exploit prompt mechanisms, instead of relying on the relatively slow production of scintillation light, to generate signals in detectors that could obviate image reconstruction and provide event coordinates directly (57,58). This possibility, however, would require a timing resolution on the order of 20 ps to achieve 3-mm spatial resolution.…”

BGO as a hybrid scintillator / Cherenkov radiator for cost-effective time-of-flight PET

“…This digital silicon photomultiplier (dSiPM) array allows triggering on the first detected photon , Gundacker et al 2016b. Additionally, the DPC array provides a validation cycle that can be used to reject unwanted triggers resulting from dark counts.…”

“…Specifically, an undoped LuAG crystal with a size of 3 mm × 3 mm × 8 mm was used. It was reported that pure LuAG shows mainly Cherenkov emission upon excitation of 511 keV annihilation photons and thus is a good candidate to estimate the IRF including the photon travel time spread within the crystal (Lecoq et al 2010, Brunner 2014, Gundacker et al 2016b. The crystal was black-taped and coupled to the SPAD with optical grease (BC-630) and to the stop-pixel with Cargille Meltmount 1.528, in the same way as the BGO crystals under test (see figure 3).…”

“…Before fitting, this function is convolved with the Gaussian shaped IRF as described in Seifert et al (2012) and Gundacker et al (2016b).…”

Luminescence rise time in self-activated PbWO4 and Ce-doped Gd3Al2Ga3O12 scintillation crystals

“…Moreover, free carrier capture centers might additionally delay the population of luminescent centers. The rise time of scintillation development might be exploited for substantial shifting the time resolution to picosecond and sub-picosecond domain [10], as seeked for in many future applications. One of the novel approaches to substantially improve the time resolution is splitting the detection into the readout of the moment of irradiation interaction with the scintillator to ensure high time resolution and the readout of the amplitude of the scintillator response carrying information on the deposited energy of ionizing radiation (see [11] and references therein).…”