Flood Histogram Quality of Prism-PET

1504 Introduction: Spatial performance of single-ended readout depth-encoding PET modules with multicrystal scintillator arrays that are n-to-1 coupled to readout pixels relies on the ability to identify the crystal where each gamma ray is absorbed. Energy weighted average method is the most popular method for performing crystal identification in such detector modules. We’ve developed a metric that characterizes the flood histogram quality of single-ended readout depth-encoding modules based on the degree of crystal separation for crystals belonging to the same readout pixels. We compare the flood histogram quality between 4-to-1 coupled modules with a standard uniform glass light guide and our newly developed Prism-PET light guide. Methods: Crystal identification in single-ended readout depth-encoding PET modules with multicrystal scintillator arrays and n-to-1 crystal coupling to readout pixels depends on how well-separated the crystal contours are for all crystals coupled to the same readout pixel. In the case of a 4-to-1 coupled detector module, the crystal separation quality for a single crystal is calculated as k1 = (D12D13D14)1/3/(σx1 + σy1 + σx2 + σy3 + (σx4 +σy4)1/2) where Dij is the distance between crystals i and j, and σxi and σyi are the standard deviations of crystal ’s centroiding contour in the x- and y-directions, respectively. The total flood histogram quality, k, of a detector module is calculated as the mean of the individual crystal separation quality metrics across all m crystals: k = (Σki)/m for i=1, 2, ⋯, m. We calculated the flood histogram quality of two different modules. The first module consisted of a 16 x 16 array of 1.4 x 1.4 x 20 mm3 LYSO crystals with 0.2 mm gaps coupled 4-to-1 to an 8 x 8 silicon photomultiplier (SiPM) array with 3.2 x 3.2 mm2 pixels on one end and with a 1 mm thick uniform glass light guide coupled to the opposite end. The second module had the same LYSO and SiPM arrays as the first module, but we swapped out the uniform glass light guide with a segmented Prism-PET light guide with 4-to-1 crystal-to-prism coupling. Barium sulfate (BaSO4) was used to fill the intercrystal spaces and act as a diffuse reflector in the crystal arrays and light guides. All crystals were fully polished and the modules were wrapped in black tape. Flood histogram data was acquired using a 3 MBq Na-22 source. Energy weighted average method was used to localize each gamma ray event to its respective scintillator crystal. A gaussian mixture model (GMM) was used to determine the mean and standard deviation of each crystal contour based on energy weighted average localization. The GMM parameters were used to calculate the flood histogram quality metrics for each tested module. We also acquired flood data at 5 different depths (2, 6, 10, 14 and 18 mm) in both modules using lead collimation with a 1 mm pinhole to determine how flood histogram crystal contour positioning varies with respect to interaction depth. Results: Flood histogram quality was calculated to be k = 1.31 for the uniform glass light guide module and k = 1.85 for the Prism-PET module, representing a 34% improvement when using Prism-PET. The depth-dependent contours were visually diverging from the center of the pixel and more spread out from each other in the Prism-PET module compared with the uniform glass light guide module, demonstrating how higher flood histogram quality is achieved. Conclusions: We’ve explored the improved crystal identification capabilities of Prism-PET using our newly developed flood histogram quality metric and depth-specific crystal contouring. Acknowledgments: We would like to thank the NIH for funding this project (R21 EB024849) and our colleagues at PETsys Electronics SA. Figure 1. (a) Fabricated 4-to-1 coupled PET modules with a uniform light guide (top) and Prism-PET (bottom). (b) Depth-dependent flood histogram contours of a typical center pixel in both modules. (c) Flood histogram crystal identification contours of one center, one corner and two edge pixels in the uniform glass light guide and Prism-PET modules.