Performance evaluation for 120 four-layer DOI block detectors of the jPET-D4

Abstract

The jPET-D4 is a brain positron emission tomography (PET) scanner that we have developed to meet user demands for high sensitivity and high spatial resolution. For this scanner, we developed a four-layer depth-of-interaction (DOI) detector. The four-layer DOI detector is a key component for the jPET-D4, its performance has great influence on the overall system performance. Previously, we reported the original technique for encoding four-layer DOI. Here, we introduce the final design of the jPET-D4 detector and present the results of an investigation on uniformity in performance of the detector. The performance evaluation was done over the 120 DOI crystal blocks for the detectors, which are to be assembled into the jPET-D4 scanner. We also introduce the crystal assembly method, which is simple enough, even though each DOI crystal block is composed of 1,024 crystal elements. The jPET-D4 detector consists of four layers of 16 x 16 Gd(2)SiO(5) (GSO) crystals and a 256-channel flat-panel position-sensitive photomultiplier tube (256ch FP-PMT). To identify scintillated crystals in the four-layer DOI detector, we use pulse shape discrimination and position discrimination on the two-dimensional (2D) position histogram. For pulse shape discrimination, two kinds of GSO crystals that show different scintillation decay time constants are used in the upper two and lower two layers, respectively. Proper reflector arrangement in the crystal block then allows the scintillated crystals to be identified in these two-layer groupings with two 2D position histograms. We produced the 120 DOI crystal blocks for the jPET-D4 system, and measured their characteristics such as the accuracy of pulse shape discrimination, energy resolution, and the pulse height of the full energy peak. The results show a satisfactory and uniform performance of the four-layer DOI crystal blocks; for example, misidentification rate in each GSO layer is <5% based on pulse shape discrimination, the averaged energy resolutions for the central four crystals of the first (farthest from the FP-PMT), second, third, and 4th layers are 15.7 +/- 1.0, 15.8 +/- 0.6, 17.7 +/- 1.2, and 17.3 +/- 1.4%, respectively, and variation in pulse height of the full energy peak among the four layers is <5% on average.

Timing resolution improvement using DOI information in a four-layer scintillation detector for TOF-PET

“…Averaged pulse heights of photoelectric events for the first, second, and third layers were, respectively, 46%, 48%, and 55% relative to that of the fourth layer. In our real four-layer GSO detectors for jPET-D4, this problem has been mitigated by optimizations of several factors such as crystal surface finishing, reflectors, optical grease and adhesive, and jigs for assemblage, so that we have achieved 82% pulse height of the first layer relative to that of the fourth layer [22,39]. In any case, however, the least number of photons could be obtained from the first layer having the highest count rate.…”

“…The four-layer encoding was achieved using a combination of Anger-logic and pulse shape discrimination; the upper and lower halves were discriminated by the reflector structure, and each half was a phoswich detector composed of two kinds of GSO crystals doped at the Ce concentrations of 0.5 mol% (decay time, 60 ns) and 1.5 mol% [21,22]. Recently, a new reflector structure was invented and the four-layer encoding was achieved without the pulse shape discrimination [23,24].…”

Development of a gamma-ray imager using a large area monolithic 4 × 4 MPPC array for a future PET scanner

“…A number of dark counts is an outstanding problem for a scintillation detector utilizing MPPCs as discussed. Waveform sampling in the GHz range will not only enable us to eliminate the contamination of the dark counts but also enable pulse shape discrimination, which is applicable for depth-of-interaction PET detectors with fast scintillators e.g., [16,17]. Although conventional devices such as flash ADCs have been capable of realizing fast waveform digitizers, the rising cost Counts Energy (keV) 0 200 400 600 800 1000 and power consumption is inevitably a major concern for units involving high channel densities like PET scanners.…”

Proposed helmet PET geometries with add-on detectors for high sensitivity brain imaging

“…Therefore, it is important to use depth-of-interaction (DOI)-PET detectors, which can maintain high spatial resolution even at the peripheral region in the field-of-view (FOV) without loss of sensitivity. As a DOI-PET detector, we have developed the 4-layer DOI detector (Murayama et al 1998, Tsuda et al 2004, Inadama et al 2007. In addition, we have developed the X'tal cube PET detector and proven that it had isotropic spatial resolution regardless of the angle of incidence (Yamaya et al 2011, Yazaki et al 2012, Yoshida et al 2013.…”