Improvement of a PET Detector Performance by Setting Reflectors in Parallel With PMT Face

A whole-body positron emission tomography scanner must be equipped with many detectors so that they are often composed of low-cost photomultiplier tubes (PMTs), that is, large anode-type PMTs. The general detector structure has a scintillation crystal element array coupled on an array of the large anode PMTs, and for crystal element identification, the method is needed to get sufficient spread of scintillation light for distribution among distant PMT anodes. Besides the common method of using a light guide, some methods have been proposed for better element identification performance. In this paper, we introduce a new method, in which the scintillation light spread is promoted not only by removing reflectors between crystal elements but also by restricting light exit to the PMTs by placing additional reflectors at the bottom of the crystal element array. Because the additional reflectors are parallel to the PMT surface, we call them parallel reflectors. We verified our method with a detector consisting of the 2.45 mm × 5 mm × 15 mm Lu
_2x
Gd
_2(1-x)
SiO
₅
:Ce crystal elements and two dualphotocathode PMTs. Each photocathode was 8 mm × 18 mm in size. We set a 9 × 10 crystal element array on the two PMTs and tried to identify the elements by the 2 × 2 PMT signals. Detector performance was evaluated with 137Cs point sources (662-keV gamma rays). The results showed that despite a significant decrease in light output at the boundary of the two PMTs, the method made crystal element identification possible. We measured energy resolutions of 13.1% and 17.8% for the elements on the PMT photocathode area and the PMT boundary, respectively. Our method is applicable to the depth-of-interaction detector consisting of multilayer crystal element arrays. By inserting parallel reflectors between the layers, we would be able to control the path of scintillation light originating in the upper layer. The effect was first examined in a basic study with two layers of a 2×5 crystal element array. Then, the array of each layer was increased to 9×9 for performance evaluation. It was difficult to identify all 162 crystal elements with four PMT signals; however, results suggest the possibility of scintillation light path control in each layer separately by the parallel reflectors and the potential for better performance by more precise adjustment of parallel reflectors.