PET Performance Evaluation of a Pre-Clinical SiPM-Based MR-Compatible PET Scanner

Abstract

We have carried out a PET performance evaluation a silicon photo-multiplier (SiPM) based PET scanner designed for fully simultaneous pre-clinical PET/MR studies. The PET scanner has an inner diameter of 20 cm with an LYSO crystal size of 1.3 by 1.3 by 10 mm. The axial PET field of view (FOV) is 30.2 mm. The PET detector modules, which incorporate SiPMs, have been designed to be MR-compatible allowing them to be located directly within a Philips Achieva 3T MR scanner. The spatial resolution of the system measured using a point source in a non-active background, is just under 2.3 mm full width at half maximum (FWHM) in the transaxial direction when single slice rebinning (SSRB) and 2D filtered back-projection (FBP) is used for reconstruction, and 1.3 mm FWHM when resolution modeling is employed. The system sensitivity is 0.6% for a point source at the center of the FOV. The true coincidence count rate shows no sign of saturating at 30 MBq, at which point the randoms fraction is 8.2%, and the scatter fraction for a rat sized object is approximately 23%. Artifact-free images of phantoms have been obtained using FBP and iterative reconstructions. The performance is currently limited because only Manuscript one of three axial ring positions is populated with detectors, and due to limitations of the first-generation detector readout ASIC used in the system. The performance of the system as described is sufficient for simultaneous PET-MR imaging of rat-sized animals and large organs within the mouse. This is demonstrated with dynamic PET and MR data acquired simultaneously from a mouse injected with a dual-labeled PET/MR probe. Index Terms-MagneticResonance Compatible PET, PET-MR, PET-MRI, positron emission tomography, silicon photo-multiplier.

Comments on the NEMA NU 4-2008 Standard on Performance Measurement of Small Animal Positron Emission Tomographs

“…To evaluate the spatial resolution, the NEMA standard mandates the use of point source scans which are reconstructed using filtered backprojection. However, basically all modern PET scanners instead use an iterative maximum likelihood expectation maximization (MLEM) algorithm for reconstruction [5,6,7,8,9,10,4,11,12,13,14,15], so a scanner's spatial resolution using filtered backprojection is not necessarily indicative of its spatial resolution for applications. While the mandated filtered backprojection is intended to benchmark the detector performance alone, we will demonstrate in the following that it disadvantages certain scanner geometries.…”

“…Line of responses (LORs) perpendicular to the detectors front face are detected with the highest resolution, while tilted LORs have a parallax error in the detected position, which increases with more tilt of the LORs relative to the detector's front faces as illustrated in Figure 1. In principle, this effect can be reduced by detectors which are able to determine the depth of interaction (DOI) of the gamma interaction, but in practice most PET system don't employ detectors with DOI determination [5,6,4,11,12,14,16]. Additionally, PET rings have gaps between the detector, where no LORs are detected at all.…”

“…However, most modern scanners store their data in listmode format and often only implement sinogram support to conduct the NEMA measurements. To our knowledge, all NEMA NU-8 measurements published in in the last 5 years had to convert their native listmode files to sinograms after the measurements [5,6,7,8,9,10,4,11,12,13,14,15]. Spinks et al [8] even mentions that the calculation of scatter fractions were omitted due to missing sinogram support, so this performance evaluation did apparently only use listmode data for the data analysis.…”

PET iterative reconstruction incorporating an efficient positron range correction method

“…The proposed method was evaluated using simulated data generated using STIR software [16] of acquisitions of the HYPERImage preclinical PET/MR scanner (diameter: 20.8 cm, length: 3.1 cm) [17].…”

Development of a data acquisition architecture with distributed synchronization for a Positron Emission Tomography system with integrated front-end.

“…Silicon photomultipliers are a more recent development [210] and, now that their cost has gone down, are progressively replacing APDs in PET applications [211] as they retain most of their advantages while overcoming disadvantages such as reduced time resolution and complexity of the readout circuitry due to reduced gain [212]. Thus, most new scanners use either PMT [213,214] or SiPM [215,216] as photodetectors, although PET systems based on APDs are still being developed [217]. Table 4.3 summarizes and compares the most important properties of all three photodetectors.…”