Influence of misalignment of a scintillator array and a multi-anode PMT for 4-layer DOI PET detector

We are developing a depth of interaction (DOI) detector having a crystal block composed of four layers of a scintillation crystal array. By removing reflectors between crystal elements for every two lines and shifting reflector patterns depending on layers, responses of all crystal elements in the four layers can be mapped to a 2-dimentional (20) position histogram after applying conventional Anger-type calculation. Because this method projects four times more number of crystal responses onto the position histogram than conventional non-DOI detectors do. Slight difference of light distribution on a multi-anode photomultiplier tube (MA-PMT) discriminates each crystal responses. In particular, location of the crystal responses in the position histogram is sensitive to spatial relationship between the crystal block and the MA-PMT at the edge side of the responses. In this paper, to set up the crystal block appropriate position, we developed an optical camera-based system for positioning of the crystal block. Using this system, we studied the influence of positioning accuracy of the crystal block on the crystal responses in the 20 position histogram. We used a MA-PMT with a 49 × 49 mm² effective area in 52 × 52 mm² opening window and anode interval of 6 mm. We prepared two types of crystal blocks, a non-DOI crystal block and the four-layer DOI crystal block, and obtained 20 position histograms with shifting each crystal block from a corner of the MA-PMT to the center. The crystal blocks were composed of 2.8 x 2.8 x 7.5 mm³ GSO crystal elements. We found that all responses of the non-DOI detector can be discriminated, even it was on the corner of the MA-PMT, however, responses of the DOI detector are not able to be discriminated on the edge sides of the 2D position histogram e