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.
Highlights
Positron emission tomography (PET) is a well-established in vivo molecular imaging technique, both in research and in clinical practice
The recent GE Healthcare Signa TOF-positron emission tomography (PET)/magnetic resonance imaging (MRI) scanner, which is based on analog silicon photomultipliers (SiPMs), has an energy resolution of 10.3% (Levin et al 2016)
For the Philips Vereos TOF-PET/computed tomography (CT) scanner, which is based on arrays of 4 mm × 4 mm LYSO:Ce crystals coupled to the same digital photon counter (DPC) sensors as used in this work, an energy resolution of 11.1% FWHM has been reported (Miller et al 2015)
Summary
Positron emission tomography (PET) is a well-established in vivo molecular imaging technique, both in research and in clinical practice. The main challenge in the context of clinical PET is to obtain a precise estimation of the gamma-ray position of interaction, combined with high sensitivity, an excellent coincidence resolving time (CRT), and good energy resolution. An important innovation that could be introduced in clinical scanners is the capability of estimating the depth of interaction (DOI) of gamma rays inside the detectors, without compromising on other performance parameters. This additional information would allow scanners to achieve a higher and more homogeneous spatial resolution throughout the field of view (FOV) (Surti et al 2013, Thoen et al 2013). Benefits of DOI estimation have been experimentally demonstrated only in preclinical PET scanners or demonstrators (González et al 2016, Lee et al 2017), whereas there are no results available for PET scanners or demonstrators having ring dimensions compatible with clinical applications
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