Comparison of Detector Intrinsic Spatial Resolution Characteristics for Sensor on the Entrance Surface and Conventional Readout Designs

We report on a high resolution, monolithic crystal PET detector design concept that provides depth of interaction (DOI) positioning within the crystal. Our design utilizes a novel sensor on the entrance surface (SES) approach combined with a maximum likelihood positioning algorithm. We compare the intrinsic spatial resolution characteristics (i.e., X, Y and Z) using our SES design versus conventional placement of the photo-sensors on the rear surface of the crystal. The sensors can be any two-dimensional array of solid state readout devices (e.g., silicon photomultipliers (SiPM) or avalanche photodiodes (APD)). SiPMs are a new type of solid-state photodetector with Geiger mode operation that can provide signal gain similar to a photomltipiler tube (PMT). Utilizing a multi-step simulation process, we determined the intrinsic spatial resolution characteristics for a variety of detector configurations. The SES design was evaluated via simulation for three different two-dimensional array sizes: 8 × 8 with 5.8 × 5.8 mm
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pads; 12 × 12 with 3.8 × 3.8 mm
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pads; and 16 × 16 with 2.8 × 2.8 mm
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pads. To reduce the number of signal channels row-column summing readout was used for the 12 × 12 and 16 × 16 channel array devices. The crystal was modeled as a 15 mm monolithic slab of a lutetium-based scintillator with the large area surface varying from 48.8 × 48.8 mm
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up to 49.6 × 49.6 mm
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depending upon the dimensions of the two-dimensional photo-sensor array. The intrinsic spatial resolution for the 8 × 8 array is 0.88 mm FWHM in X and Y, and 1.83 mm FWHM in Z (i.e., DOI). Comparing the results versus using a conventional design with the photo-sensors on the backside of the crystal, an average improvement of ~24% in X and Y and 20% in Z is achieved. The X, Y intrinsic spatial resolution improved to 0.67 mm and 0.64 mm FWHM for the 12 × 12 and 16 × 16 arrays using row-column readout. Using the 12 × 12 and 16 × 16 arrays also led to a slight improvement in the DOI positioning accuracy.