Depth-Encoding Based on Optical Photon Time-of-Flight in Prism-PET with Tapered Scintillator Crystals

1138 Introduction: Depth of interaction (DOI) estimation has been shown to be correlated with optical photon time-of-flight (TOF) in TOF-DOI PET modules that rely on intercrystal light sharing with single-ended readout. Consequently, depth-encoding has been used to improve TOF resolution in such detector modules by deconvolving the effects of depth-dependent optical photon transport on coincidence timing. In this work, we demonstrate a strong correlation between optical photon TOF and DOI in a 4-to-1 coupled Prism-PET module that has unilaterally tapered scintillator crystals. Methods: We acquired depth-collimated data on a 4-to-1 coupled Prism-PET module. A 3 MBq Na-22 point source (1 mm active diameter) was placed in a lead cylinder with a 1 mm diameter pinhole positioned between a depth-aligned Prism-PET module and a single reference LYSO crystal (0.9 x 0.9 x 20 mm) coupled to a 3 x 3 mm silicon photomultiplier (SiPM) pixel. The Prism-PET module consisted of a 16 x 16 array of 1.4 x 1.4 x 20 mm LYSO crystals coupled 4-to-1 to 3 x 3 mm SiPM pixels on one end and to a prismatoid light guide array on the opposite end. All crystals were tapered off down to 1.2 x 1.2 mm starting 5 mm from the crystal-readout interface in order to minimize light leak at this interface (Figure 1). Light leak at the crystal-readout interface is random, whereas light sharing within the prismatoid light guides in Prism-PET has been experimentally shown to be deterministic. We acquired depth-collimated data at 19 different depths (1-19 mm in steps of 1 mm) in the Prism-PET module. Only coincidence events between the Prism-PET module and reference crystal were used for analysis in order to reject Compton. Photopeak filtering was performed on a per-crystal basis with a 15% energy window. Energy weighted average method was used for energy-based DOI estimation (wE). Timing-based DOI (wTOF) was estimated 2 different ways: (1) taking the timestamp from the pixel coupled to the primary crystal and subtracting it from the first timestamp of one of the nearest-neighbor pixels coupled to the same prismatoid light guide (wTOF = tn1 - tp where tn1 is the first nearest-neighbor timestamp and tp is the primary timestamp), and (2) taking the average of the primary timestamps subtracted from all 3 nearest-neighbor timestamps coupled to the same prismatoid (wTOF = ((tn1 - tp) + (tn2 - tp) + (tn3 - tp)) / 3). Linear regression analysis was used to determine the correlation between energy-weighted DOI and both TOF-weighted DOI parameters. Results: We observed a strong correlation between wTOF and wE across all depths using both methods, with a stronger correlation for the 3 timestamp method (R2 = 0.53) compared to the single timestamp method (R2 = 0.31, Figure 2). The estimated DOI resolution of the Prism-PET module based on each method was 2.22 mm full-width at half-maximum (FWHM) for the energy-weighted method, 7.38 mm FWHM for the TOF-weighted method with 1 timestamp, and 5.38 mm FWHM for the TOF-weighted method with 3 timestamps (Figure 3). Conclusions: We’ve experimentally demonstrated a strong correlation between energy-weighted DOI and timing-based DOI using nearest-neighbor timestamps in Prism-PET when minimizing light leak at the crystal-readout interface. Our tapered crystal design is a practical method to improve DOI and timing performance with Prism-PET.Figure 1. Schematic (left) and fabricated (right) cross-sectional views of a 4-to-1 coupled Prism-PET detector module with crystal tapering on 2 sides in each crystal.Figure 2. Correlation between energy-based DOI (x-axis, wE) and timing-based DOI (y-axis, wTOF) in the depth-aligned Prism-PET module (red = 1 timestamp, blue = 3 timestamps).Figure 3. DOI estimation histograms at 5 different ground truth depths (2, 6, 10, 14 and 18 mm) using standard energy-weighted average method (left), optical photon TOF weighting based on the first neighbor timestamp (middle) and optical photon TOF weighting using the average of all 3 nearest neighbor timestamps (right).