Sub-3 mm, near-200 ps TOF/DOI-PET imaging with monolithic scintillator detectors in a 70 cm diameter tomographic setup

Abstract

Recently, a monolithic scintillator detector for time-of-flight (TOF)/depth-of-interaction (DOI) positron emission tomography (PET) was developed. It has a detector spatial resolution of ∼1.7 mm full-width-at-half-maximum (FWHM), a coincidence resolving time (CRT) of ∼215 ps FWHM, and ∼4.7 mm FWHM DOI resolution. Here, we demonstrate, for the first time, the imaging performance of this detector in a 70 cm diameter PET geometry. We built a tomographic setup representative of a whole-body clinical scanner, comprising two coaxially rotating arms, each carrying a detector module, and a central, rotating phantom table. The fully automated setup sequentially acquires all possible lines of response (LORs) of a complete detector ring, using a step-and-shoot acquisition approach. The modules contained 2 × 2 detectors, each detector consisting of a 32 mm × 32 mm × 22 mm LYSO crystal and a digital silicon photomultiplier (dSiPM) array. The system spatial resolution was assessed using a Na-22 point source at different radial distances in the field-of-view (FOV). Using 2D filtered back projection (2D FBP, non-TOF), tangential and radial spatial resolutions of ∼2.9 mm FWHM were obtained at the center of the FOV. The use of DOI information resulted in almost uniform spatial resolution throughout the FOV up to a radial distance of 25 cm, where the radial and tangential resolution are ∼3.3 mm FWHM and ∼4.7 mm FWHM, respectively, whereas without DOI the resolution deteriorates to ∼9 mm FWHM. Additional measurements were performed with a Na-22 filled Derenzo-like phantom at different locations within the FOV. Images reconstructed with a TOF maximum-likelihood expectation-maximization (TOF ML-EM) algorithm show that the system is able to clearly resolve 3 mm diameter hot rods up to 25 cm radial distance. The excellent and uniform spatial resolution, combined with an energy resolution of 10.2% FWHM and a CRT of ∼212 ps FWHM, indicates a great potential for monolithic scintillators as practical high-performance detectors in TOF/DOI-PET systems.