Development of a depth of interaction detector for γ-rays

A depth of interaction (DOI) detector for γ-rays has been developed for a high-resolution positron emission tomography (PET) system dedicated to small animal studies. The DOI detector consists of a double-layer array of lutetium oxyorthosilicate (LSO) crystals and a position-sensitive photomultiplier tube (PS-PMT), Hamamatsu R7600-C12, where the element size of the LSO array is 1.8×1.8×10 mm. In the LSO crystal block, the upper LSO array is placed on the lower LSO array with a shift of half the element pitch in both X and Y directions. Since the centroid of the output light distribution from each crystal element locates at a different position on the PS-PMT, the crystal element can be identified on the image plane derived from the centroid calculation. The performance of the DOI detector was measured in terms of its application to PET. Each crystal element is clearly separated on the position map so that a single bit of DOI information can be derived, as well as the position information. The timing resolution for the upper array was 0.98 ns in FWHM, and was 0.83 ns in FWHM for the lower array. The spatial resolution in coincidence was measured with a pair of DOI detectors at different incident angles of γ-rays. The averaged FWHM values were 1.4 mm at 0° and 3.4 mm at 30°, while those of the detector pair without DOI were 1.5 mm at 0° and 4.5 mm at 30°. It is confirmed that the DOI detector provides better spatial resolution for the oblique incidence angle of γ-rays, which results in improved PET resolution in the peripheral field of view (FOV). The experimental results show that the new DOI detector is applicable for high-resolution PET systems.