Timing Performance Study of New Fast PMTs With LYSO for Time-of-Flight PET

Most efforts at developing time-of-flight (TOF) positron emission tomography (PET) instruments have focused on improving their time resolution, which is affected by many factors. The aim of this study was to evaluate the performance of two new fast photomultiplier tubes (PMT) (Hamamatsu R9800 and R11194) for use as a TOF-PET detector, and to investigate the correlation of time resolution with time discrimination methods, the properties of the PMTs, the locations of the scintillation crystal on the PMT, and the size of the scintillation crystal. The PMTs evaluated in this study have fast rise time and short time jitter. The performance evaluation of the PMTs was conducted using a LYSO scintillation crystal. A ²²Na source was placed between the testing detector and a reference detector (with a time resolution of 200 ps), and data were acquired by using the coincidence trigger of the two detectors. To determine the optimal time discrimination method, time resolutions were obtained using a leading-edge discriminator (LED) with various thresholds and a constant-fraction discriminator (CFD) with various CFD delays. The effects on time resolution of PMT properties, the crystal position on the PMT, and the size of the crystal were observed. Based on the comparison of time resolutions obtained by various time discrimination setups, the optimal setup was determined to be CFD with 1.0 ns CFD delay. When these PMTs were coupled with 4 mm × 4 mm ×10 mm LYSO crystal and CFD with 1.0 ns delay was used for time discrimination, the average time resolution (FWHM) for 7 R9800 was 200.5 ps and for 7 R11194 was 227.8 ps. The average energy resolutions were 11.1% and 11.8%, respectively. Time resolutions at the edge of the PMT were degraded due to light collection loss and worse transit time jitter. Time resolutions were inversely proportional to the square root of the numbers of photoelectrons per pulse. The numbers of photoelectrons increased proportionately with the blue sensitivity of individual PMTs. Time resolutions have a distinct dependence as a function of the length and cross-section of the crystal. The new fast PMT coupled with LYSO crystals allow for the improvement of timing performance in TOF-PET detectors. The results of this study will be of value as a guideline for optimizing the TOF-PET detector design using the fast PMTs.

I. Introduction

Recently there has been a growing interest in the time-of-flight (TOF) positron emission tomography (PET) scanner, because the incorporation of the TOF technique into PET (current time resolution of ) has demonstrated a significant improvement in image quality [1], [2]. The TOF information, which is the difference in the arrival times of the two annihilation gamma-rays, provides localization of the annihilation position on the line-of-response (LOR) [3]–[5]. This allows the use of a segment-of-response (SOR) instead of LOR in image reconstruction, and reduces the propagation of noise along the LOR in the reconstructed images. The reduction of noise can be equated with an increase in sensitivity [6]. Furthermore, the additional TOF information enables PET images to achieve high signal-to-noise ratio (SNR) and high contrast, or short scan times [1], [4], [7].