Four-layer DOI detector with a multi pixel APD by light sharing method for small animal PET

Recently, avalanche photodiodes (APDs) are used as photo detectors in positron emission tomography (PET) because they have many advantages over photomultipliers (PMTs) typically used in PET detectors. We have been developing a PET detector with a multi pixel APD for a small animal PET scanner. Previously, we succeeded in the identification of four-layer depth of interaction (DOI) with a position sensitive PMT coupled to the backside of a crystal array by only an optimized reflector arrangement. We adopt this method for the APD detector to replace the position sensitive PMT. While APDs have many advantages as photo detectors, the detector performance with them is affected by electrical noise because of their low internal gain. Since scintillation lights are shared among many pixels by the presented method, weaker signals in pixels far from the interacting crystals are strongly affected by the noise. To evaluate the possible use of the four-layer DOI detector with APDs using the light sharing method, we constructed a prototype DOI detector and tested its performance. The prototype detector consisted of four layers of a 6 x 6 array with Lu_2(1-x) Y_2x SiO₅ (LYSO) (Lu: 98 %, Y: 2 %) (Proteus Inc., U. S. A.) crystals, and a pixelized APD with a 4x8 pixel array (S8550, Hamamatsu Photonics K.K. ). The size of each crystal element was 1.46 mm x 1.46 mm x 4.5 mm and all element surfaces were chemically etched. The experimental results indicated that the four-layer DOI detector with the multi pixel APD can be used as a small animal PET detector.