Optimizing PET DOI Resolution With Crystal Coating and Length

Abstract

In the last few years, one of the main research programs for PET systems has been the improvement of spatial resolution using Depth Of Interaction (DOI) information. In the context of developing a new PET system dedicated to small animals with axially oriented LYSO crystals and DOI capability, we have investigated the influence of the crystal coating and its length on the DOI resolution. The proposed PET system is composed of four detection modules arranged around the animal. Each module consists in 768 LYSO crystals read at both ends by multichannel plate photodetectors. The particular geometry combined with an inner diameter of 61.2 mm, lead to high detection efficiency close to the system solid angle. The LYSO crystal is chosen for its light yield of 33 ph/keV and its attenuation length of 11.2 mm at 511 keV. To obtain a transverse spatial resolution of 1 mm, the section of the crystal was fixed to 1.5 mm. To achieve a DOI resolution close to 1 mm, measurements have been performed on different LYSO crystal coatings with a length ranging from 25 mm to 35 mm. Each crystal is positioned on an translation stage and read out at both ends by H3164-10 Hamamatsu PMTs. The DOI information is then derived every 0.5 mm along the crystal extent. The use of an electronic collimation leads to a 22 Na source beam size of (1 58 0 04) mm reaching the crystal. The optimized coating in terms of packing fraction and DOI resolution is found to be a mixture made with 30% TiO 2 powder in a PMMA binder. With this appropriate coating, an average DOI resolution of (0 82 0 13) mm can be achieved with a 25 mm crystal length using a 20% photopeak energy window. The resolution degrades to (1 39 0 16) mm when a wide-open energy window is used. Those values are corrected for the source beam size. Using this experimental proposal, a matrix of LYSO crystals has been built reaching a packing fraction of 93%. In this study, we demonstrate that using a 1 5 1 5 25 mm 3 LYSO crystal, a DOI resolution of less than a millimeter can be achieved while keeping a high packing fraction for a system detection efficiency close to 15%.

Effects of reflector and crystal surface on the performance of a depth-encoding PET detector with dual-ended readout

“…Dual-ended readout of a scintillation crystals or arrays is one popular depth-encoding method which measures DOI by using the energy ratio of the two photodetector signals and can achieve very good DOI resolution. [16][17][18][19][20] For dual-ended readout depth-encoding detectors, the proper compromise needs to be found between flood histogram quality, DOI resolution, and energy resolution since a strong gradient in light collection efficiency with depth (GLCED) is required to obtain good DOI resolution, which implies a significant and depth-dependent loss of scintillation light and therefore a degradation in the quality of the flood histogram and energy resolution. The GLCED is mainly governed by crystal surface treatment, intercrystal reflector, and crystal size.…”

Characteristics study of an ends-read edge-on DOI detector for positron breast imaging

“…Compared with that (9.3 mm for 10-cm-long crystal) reported in [8] and that (6-12 mm for 6.9-cm-long crystal) in [9], the estimated errors are approximately consistent. When the crystal length was reduced to 2.5 cm, better precisions of 5.5 and 4.9 mm were estimated and were obviously larger than that of 3-mm reported in [6] and 1.39-mm in [13]. This implies that the model in equation (3.1) is apt to be applied in evaluation of practical crystal length exceeding 10-cm, but not suitable for shorter crystal length, for it may cause overestimation.…”

Improving the Time Resolution of TOF-PET Detectors by Double-Sided Readout

“…DSR of the crystals allows utilizing the contrast between the two photosensors as a measure of the DOI [19]: (1) where and are the light intensities recorded by the two photosensors. In order to determine the relation between the DOI and (or any other DOI-dependent parameter), extensive calibration measurements are commonly performed.…”