GATE Optical Simulation of U-Shaped DOI Detectors for Optimal Crystal Surface Roughness

For brain dedicated positron emission tomography (PET), depth-of-interaction (DOI) information is essential to minimize the parallax error at the periphery of the PET field-of-view, where the human cortex is located. In this work, therefore, we propose a new U-shaped light sharing crystal design that has a tapered geometry to achieve good DOI resolution in the single-ended readout scheme. We investigated the optimal crystal surface roughness of the new DOI detector using GATE optical simulation in terms of DOI resolution. In addition, the DOI resolution of the proposed design was compared with previous U-shaped detectors, the crosshair light sharing (CLS) detector, and the Prism detector. The proposed crystal geometry had dimensions of 3.0×6.2×15 mm
³
, in which the central part has a 0.2 mm slit with a 12 mm in length, thereby yielding a 3 mm optical window at the crystal top region. The bottom of the crystal was optically coupled to two SiPM channels (each with sensitive area of 3×3 mm
²
) by one-to-one coupling. The emission spectrum of the LYSO crystal and the photon detection efficiency of the SiPM were considered in the optical simulation. To evaluate DOI resolution, collimated 511 keV radiations were irradiated into different crystal depths (2.5, 5.0, 7.5, 10.0, 12.5, 13.5, and 14.5 mm). Three different surface roughness of 0.5º (mechanical polishing), 10º (roughened), and 26º (as-cut) were investigated. The proposed detector with the as-cut surface finish provided the best DOI resolution (1.5±0.1 mm), which outperformed those (as-cut) of the CLS detector (1.8±0.2 mm) and Prism detector (1.9±0.2 mm). By the time of the conference, the GATE optical simulation results will be compared with experimental results for validation of the simulation as well as for investigation of the effect on timing resolution.