TOF-PET imaging with sub-3 mm resolution and 215 ps coincidence resolving time using digital SiPM based monolithic scintillator detectors in a 70 cm diameter tomographic setup

Objectives Positron emission tomography (PET) detectors with better detection efficiency, spatial resolution, depth-of-interaction (DOI) capability, and time-of-flight (TOF) resolution are needed to reduce PET scanning times while improving image quality and quantitative accuracy. We recently presented a monolithic scintillator detector with single-sided digital silicon photomultiplier (dSiPM) readout that fulfills each of these requirements [1]. The purpose of this work is to demonstrate the imaging performance of this fully digital detector in a 70-cm diameter PET setup representative of a clinical scanner.

Methods A setup was built that allows three-dimensional (3D) tomographic imaging using only two detector modules. Each module is placed on a rotating arm moving coaxially around a central, rotating sample table. All possible lines of response (LORs) of a PET ring of arbitrary diameter and length can be acquired in a fully automated, step-and-shoot acquisition approach. The detector modules studied in this work each contain four monolithic LYSO:Ce crystals with dimensions of 32 mm x 32 mm x 22 mm (Crystal Photonics). The 32 mm x 32 mm back surface of the crystal was optically coupled to a 64-pixel dSiPM array (Philips Digital Photon Counting, DPC-3200-22-44). Complete tomographic acquisitions of a Na-22 point source and a Na-22 filled Derenzo phantom (rod diameters varying between 2.5 mm and 7 mm) were performed with the source or phantom placed at different radial distances within the field-of-view (FOV).

Results Using filtered back projection (FBP) for the reconstruction of the point source images, the radial and tangential system spatial resolution were determined to be ~2.9 mm FWHM at the center of the FOV. At a radial distance of 15 cm, the measured radial resolution equals ~3.2 mm FWHM if DOI correction is applied, compared to ~5.5 mm FWHM without DOI correction. Derenzo phantom images, reconstructed using a maximum-likelihood expectation-maximization (ML-EM) algorithm, feature clearly distinguishable 3 mm rods at all radial distances up to 20 cm. The energy resolution and coincidence resolving time (CRT) were measured to be 10.2% FWHM and 215 ps FWHM, respectively. Full results will be presented at the conference.

Conclusions Monolithic LYSO:Ce detectors with single-sided dSiPM readout enable TOF-PET imaging with sub-3 mm spatial resolution, 215 ps FWHM time resolution, 10.2% FWHM energy resolution, and effective DOI correction, in a 70 cm diameter detector ring. The results obtained in this work illustrate the excellent potential of monolithic scintillator detectors as a practical high-performance detector for clinical whole-body TOF-PET applications. ACKNOWLEDGEMENTS This work was supported in part by EU FP7 project SUBLIMA, Grant Agreement No 241711 (www.sublima-pet-mr.eu). REFERENCES [1] G. Borghi, V. Tabacchini, D.R. Schaart, “Experimental Characterization of a Practical Monolithic Scintillator Detector for Clinical TOF-PET Scanners Based on a 32 mm x 32 mm x 22 mm LYSO:Ce Crystal and a DPC Array,” 2014 IEEE NSS-MIC, Seattle, WA, Nov 8-15, 2014.