Verification of concepts for DOI determination in a three-layer small animal PET

Improved spatial resolution without sacrificing sensitivity is one of the most challenging development goals for small animal PET scanners. The 3-layer configuration that we proposed utilizes relative offsets of half a crystal pitch in x- and y-directions, and pulse shape discrimination to obtain depth of interaction (DOI). Three layers of crystals with a dimension 1.5times1.5times7.0 mm³ were composed of a L_0.2GSO (Lu_0.4Gd_1.6SiO₄:Ce) crystal layer and a L_0.9GSO (Lu_1.6Gd_0.4SiO₄) crystal layer aligned with each other, and a L_0.9GSO crystal layer offset of at half a crystal pitch in x- and y-directions. The L_0.2GSO crystal layer was attached to a Hamamatsu H9500 flat-panel PMT. The devised small animal PET scanner has a diameter of 84 mm with one detector ring, and can be upgraded to two detector rings. GEANT4 Monte-Carlo simulation was used to estimate sensitivities of ~12 % and ~20%, respectively, at the center of one and two PMT ring system. We present flood images with peak-to-valley ratios of about 5~6 obtained using ²²Na and layer identification capability of ~95% with pulse shape analysis using a 100 mega-sample per second (MSPS) analog-to-digital converter (ADC), and verified the basic concepts of the multi-layer small animal PET.