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

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 similar to 1.7 mm full-width-at-half-maximum (FWHM), a coincidence resolving time (CRT) of similar to 215 ps FWHM, and similar to 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 stepand-shoot acquisition approach. The modules contained 2 x 2 detectors, each detector consisting of a 32 mm x 32 mm x 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 similar to 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 similar to 3.3 mm FWHM and similar to 4.7 mm FWHM, respectively, whereas without DOI the resolution deteriorates to similar 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 similar to 212 ps FWHM, indicates a great potential for monolithic scintillators as practical high-performance detectors in TOF/DOI-PET systems.