Time of flight positron emission tomography towards 100ps resolution with L(Y)SO: an experimental and theoretical analysis

Abstract

Scintillation crystals have a wide range of applications in detectors for high energy and medical physics. They are recquired to have not only good energy resolution, but also excellent time resolution. In medical applications, L(Y)SO crystals are commonly used for time of flight positron emission tomography (TOF-PET). This study aims at determining the experimental and theoretical limits of timing using L(Y)SO based scintillators coupled to silicon photomultipliers (SiPMs). Measurements are based on the time-over-threshold method in a coincidence setup utilizing the ultra-fast amplifier-discriminator NINO and a fast oscilloscope. Using a 2 × 2 × 3 mm 3 LSO:Ce codoped 0.4% Ca crystal coupled to a commercially available SiPM (Hamamatsu S10931-050P MPPC), we achieve a coincidence time resolution (CTR) of 108±5ps FWHM measured at E=511keV. We determine the influence of the data acquisition system to 27±2ps FWHM and thus negligible as compared to the CTR. This shows that L(Y)SO scintillators coupled to SiPM photodetectors are capable of achieving very good time resolution close to the desired 100ps FWHM for TOF-PET systems. To fully understand the measured values, we developed a simulation tool in MATLAB that incorporates the timing properties of the photodetector, the scintillation properties of the crystal and the light transfer within the crystal simulated by SLITRANI. The simulations are compared with measured data in order to determine their predictive power. Finally we use this model to discuss the influence of several important parameters on the time resolution like scintillation rise-and fall time and light yield, as well as single photon time resolution (SPTR) and the detection efficiency of the SiPM. In addition we find the influence of photon travel time spread in the crystal not negligible on the CTR, even for the used 2 × 2 × 3 mm 3 geometry.

Analytical calculation of the lower bound on timing resolution for PET scintillation detectors comprising high-aspect-ratio crystal elements

“…Figure 9(a) shows the calculated statistical limit on CTR as a function of the normalized input parameters. For LYSO:Ce, improving the scintillation rise time has little influence on achievable timing resolution, as has also been reported in [Gundacker et al 2013a, Seifert et al 2012a]. With an experimentally measured rise time of 72 ± 3 ps [Seifert et al 2012b], this material has extremely fast early luminescence processes that do not dominate the contribution to timing uncertainty.…”

“…Measured CTRs approaching or below 100 ps FWHM have been reported for LaBr 3 :Ce and LSO:Ce,Ca(0.4%) crystals ≤5 mm in length and small cross-section ~3×3 mm 2 [Schaart et al 2010, Wiener et al 2010, Gundacker et al 2013a]. However, crystal lengths of 20 mm or greater are necessary for an appropriate level of sensitivity for detecting annihilation photons in whole-body PET.…”

Enhancing Light Extraction of Inorganic Scintillators Using Photonic Crystals

“…The CTR can be measured using two coincident branches (see [72] for a detailed explanation). On one branch, the sample crystal is placed facing a silicon photo-multiplier (SiPM), which is then readout by a NINO chip, a fast and low noise discriminator developed for the ALICE experiment at CERN [73].…”

A comparative study of the time performance between NINO and FlexToT ASICs

“…In this work we have used short LSO:Ce,Ca scintillators with a fast decay time of 30 ns [6,7] wrapped in Teflon tape and coupled to the S13360-3050CS SiPM from Hamamatsu, a low afterpulse and low crosstalk device with cell sizes of 50 × 50 µm 2 and effective photosensitive area of 3 × 3 mm 2 . Additionally, as a fast reference SiPM for the CTR measurements, we have utilized a NUV SiPM from the group at FBK (Fondazione Bruno Kessler) [7,8] with cell sizes of 25 × 25 µm 2 .…”