Factors influencing timing resolution in a commercial LSO PET camera

Abstract

Abstract--The CPS Accel is a commercial PET camera based on a block detector with 64 LSO scintillator crystals (each 6.75 x 6.75 x 25 mm) read out with 4 photomultiplier tubes. The excellent timing resolution of LSO suggests that this camera might be used for time-of-flight (TOF) PET, thereby reducing the statistical noise significantly. Although the Accel achieves 3 ns coincidence resolution (a factor of two better than BGObased PET cameras), its timing resolution is nearly an order of magnitude worse than that demonstrated with individual LSO crystals. This paper quantifies the effect on the timing of each component in the Accel timing chain to identify which components most limit the camera's timing resolution. The components in the timing chain are: the scintillator crystal, the photomultiplier tube (PMT), the constant fraction discriminator (CFD), and the time to digital converter (TDC). To measure the contribution of each component, we construct a single crystal test system with high-performance versions of these components. This system achieves 221 ps fwhm coincidence timing resolution, which is used as a baseline measurement. One of the high-performance components is replaced by a production component, the coincidence timing resolution is remeasured, and the difference between measurements is the contribution of that (production) component. We find that the contributions of the TDC, CFD, PMT, and scintillator are 2000 ps, 1354 ps, 422 ps, and 326 ps fwhm respectively, and that the overall timing resolution scales like the square root of the amount of scintillation light detected by the PMT. Based on these measurements we predict that the limit for the coincidence timing resolution in a practical, commercial, LSO-based PET camera is 528 ps fwhm.

Time based readout of a silicon photomultiplier (SiPM) for time of flight positron emission tomography (TOF-PET)

“…An exhaustive investigation of these effects is found in [22]- [26]. Already Section III addressed this question treating the issue with Monte Carlo simulation.…”

Multi-CFD Timing Estimators for PET Block Detectors

“…Differences in the propagation times between the 4 PMTs affect the leading edge of the summed signal and thus affect the timing resolution. Previous work [9] has shown this contribution to be 250 ps to 500 ps fwhm. In conventional PET systems with timing resolutions on the order of nanoseconds this is insignificant, but in time-offlight (TOF) PET systems with sub-nanosecond timing M. Ullisch is with Lawrence Berkeley National Laboratory, Berkeley, CA 94720 USA (e-mail: marcus@ullisch.de).…”

“…As shown in [9] a scintillator array has a significantly worse timing resolution than a single crystal, caused by a lower light-output of the crystal array. Thus the Table 1: Timing Resolution of the Accel Detector Module with the conventional single-CFD timing estimator and multi-CFD timing estimator using different propagation time correction techniques for the part-to-part variation of the propagation time in the PMTs.…”

Development of a Combined microPET®-MR System

“…The signal attenuation associated with the long fiber optic bundles will decrease spatial resolution slightly and increase the proportion of scattered photons detected. Though we have not yet measured the timing resolution of the detectors with long fiber bundles, we anticipate that the timing resolution will be degraded due to light loss (11). That will result in an increase in the random fraction compared to the standard microPET Focus120 system.…”