A combined time-of-flight and depth-of-interaction detector for total-body positron emission tomography

Abstract

Purpose: In support of a project to build a total-body PET scanner with an axial field-of-view of 2 m, the authors are developing simple, cost-effective block detectors with combined time-of-flight (TOF) and depth-of-interaction (DOI) capabilities.

Methods: This work focuses on investigating the potential of phosphor-coated crystals with conventional PMT-based block detector readout to provide DOI information while preserving timing resolution. The authors explored a variety of phosphor-coating configurations with single crystals and crystal arrays. Several pulse shape discrimination techniques were investigated, including decay time, delayed charge integration (DCI), and average signal shapes.

Results: Pulse shape discrimination based on DCI provided the lowest DOI positioning error: 2 mm DOI positioning error was obtained with single phosphor-coated crystals while 3-3.5 mm DOI error was measured with the block detector module. Minimal timing resolution degradation was observed with single phosphor-coated crystals compared to uncoated crystals, and a timing resolution of 442 ps was obtained with phosphor-coated crystals in the block detector compared to 404 ps without phosphor coating. Flood maps showed a slight degradation in crystal resolvability with phosphor-coated crystals; however, all crystals could be resolved. Energy resolution was degraded by 3%-7% with phosphor-coated crystals compared to uncoated crystals.

Conclusions: These results demonstrate the feasibility of obtaining TOF-DOI capabilities with simple block detector readout using phosphor-coated crystals.

FIG. 1.

The phosphor coating absorbs a fraction of the scintillation photons depending on the interaction depth. Delayed photon emission from the phosphor results in depth-dependent signal shape changes.

FIG. 2.

Illustration of phosphor coating configurations considered in this study. Shaded areas near the top of the crystal denote phosphor coating.

FIG. 3.

Phosphor-coated quadrants used in the block detector. Array size is not uniform due to increased crystal pitch caused by phosphor coating.

FIG. 4.

Experimental setup for testing single-coated crystals in side-on configuration. For head-on measurements, the coated crystals were rotated 90° to face the 3 × 3 × 10 mm³ reference crystal.

FIG. 5.

Decay time and DCI pulse shape discrimination.

FIG. 6.

Histograms of (a) decay time, (b) DCI values, and (c) the average signal shape obtained with each side-on irradiation position for the two sides stepped coating configuration.

FIG. 7.

Classification distributions using DCI pulse shape discrimination with the two sides stepped coating configuration. The 18 mm irradiation position (i.e., 2 mm from top of the crystal) displayed the highest classification sensitivity: at this depth, 81% of the events were correctly classified for the data shown. The correct DOI bin for each plot is indicated by the red outline.

FIG. 8.

(a) DOI positioning error computed at each side-on irradiation position using DCI pulse shape discrimination. All coating configurations generally showed lowest DOI positioning error at the 18 or 2 mm irradiation positions. (b) Average DOI positioning error for all coating configurations and pulse shape discrimination methods. DCI discrimination provides superior DOI encoding for all coating configurations, <2 mm with two sides coated.

FIG. 9.

(a) Energy spectra obtained at each side-on irradiation position with the two sides stepped coating configuration. (b) Changes in photopeak position vs DOI for all coating configurations. The photopeak positions are normalized according the photopeak position of the uncoated crystal at 2 mm.

FIG. 10.

(a) Correlation of light collection with DCI. The expected correlation based on side-on trends for photopeak position vs DOI and mean DCI vs DOI is given by the red line. (b) Corrected energy data based on the side-on correlation of energy and DCI. (c) Corrected energy using the nonlinear correlation method. Only a subset of the data is shown for clarity.

FIG. 11.

Uncorrected energy spectra (solid) and DOI corrected photopeak (dashed) using the DCI-based energy depth dispersion correction.

FIG. 12.

(a) Timing resolution obtained with side-on irradiation for all coating configurations. All crystals showed similar timing resolution at 2 mm (closest to PMT face) due to minimal absorption of scintillation light by the phosphor coating. (b) Trends of timing pick-offs (mean time in timing spectra) vs DOI. All values are offset to the respective value at 2 mm. Coated crystals show a larger end-to-end difference in timing pick-offs due to strong dependence of light collection on DOI.

FIG. 13.

Correlation of leading edge timing pick-offs with (a) decay time, (b) DCI, and (c) energy using the two sides stepped coating configuration. Only a subset of the data is shown for clarity.

FIG. 14.

Timing resolution for all coating configurations and depth-dispersion correction method. Energy-based correction provides the optimal timing resolution for all configurations.

FIG. 15.

Flood maps obtained with (a) original uncoated module, (b) module after applying phosphor coatings to each quadrant. The orientation of the flood maps matches Fig. 3.

FIG. 16.

Timing resolution for each coated quadrant using each timing pick-off method. Error bars represent ±one standard deviation of all crystals in the quadrant.