Depth of interaction (DOI) determination in three-layer small animal PET

Improved spatial resolution without sacrificing sensitivity is one of the most challenging developmental goals for small animal PET scanners. The 3-layer configuration that we propose here utilizes relative offsets of half a crystal pitch in x, y directions, and pulse shape discrimination to obtain DOI. We present recent progress in developing the 3-layer PET scanner with LGSO crystals and H9500 PMTs. 3-layers of crystals (of each crystal 1.5x1.5x7.0 mm3) were composed of a L0.2GSO crystal layer and a L0.9GSO crystal layer aligned with each other, and a L0.9GSO crystal layer offset at half a crystal pitch in x, y directions. The L0.2GSO crystal layer was attached to a Hamamatsu H9500. The devised small animal PET scanner has a diameter of 84 mm. We built a charge division circuit consisted of resistor matrix. Struck positions by photons were obtained with the 4 signals from the circuit. To study the layer identification capability of two crystal layers with a relative offset by half a crystal, we assembled two crystal layers with a 7x7 L0.9GSO crystal block and a 6x6 L0.9GSO crystal and obtained flood images at the center and the corner of the H9500 PMT. The crystal identification between the aligned L0.2GSO and L0.9GSO crystal layers was obtained using different pulse characteristics from these two crystals. The flood image of the 2-layer crystal blocks at the center and the corner of the H9500 PMT shows the clear separation of individual crystals and no distortion of image. The ratios of peaks to valleys of horizontal and vertical projection diagrams were about 5 ∼ 6(center) and 3 ∼ 4(corner). Energy resolutions were 14.4% (6x6 crystals) and 18.1% (7x7 crystals). We also achieved the crystal identification efficiencies of ∼99%. Based on these results, we are confident that a 3-layer animal PET scanner with high resolution and sensitivity can be built