Studies of a Next-Generation Silicon-Photomultiplier–Based Time-of-Flight PET/CT System

Abstract

This article presents system performance studies for the Discovery MI PET/CT system, a new time-of-flight system based on silicon photomultipliers. System performance and clinical imaging were compared between this next-generation system and other commercially available PET/CT and PET/MR systems, as well as between different reconstruction algorithms. Spatial resolution, sensitivity, noise-equivalent counting rate, scatter fraction, counting rate accuracy, and image quality were characterized with the National Electrical Manufacturers Association NU-2 2012 standards. Energy resolution and coincidence time resolution were measured. Tests were conducted independently on two Discovery MI scanners installed at Stanford University and Uppsala University, and the results were averaged. Back-to-back patient scans were also performed between the Discovery MI, Discovery 690 PET/CT, and SIGNA PET/MR systems. Clinical images were reconstructed using both ordered-subset expectation maximization and Q.Clear (block-sequential regularized expectation maximization with point-spread function modeling) and were examined qualitatively. The averaged full widths at half maximum (FWHMs) of the radial/tangential/axial spatial resolution reconstructed with filtered backprojection at 1, 10, and 20 cm from the system center were, respectively, 4.10/4.19/4.48 mm, 5.47/4.49/6.01 mm, and 7.53/4.90/6.10 mm. The averaged sensitivity was 13.7 cps/kBq at the center of the field of view. The averaged peak noise-equivalent counting rate was 193.4 kcps at 21.9 kBq/mL, with a scatter fraction of 40.6%. The averaged contrast recovery coefficients for the image-quality phantom were 53.7, 64.0, 73.1, 82.7, 86.8, and 90.7 for the 10-, 13-, 17-, 22-, 28-, and 37-mm-diameter spheres, respectively. The average photopeak energy resolution was 9.40% FWHM, and the average coincidence time resolution was 375.4 ps FWHM. Clinical image comparisons between the PET/CT systems demonstrated the high quality of the Discovery MI. Comparisons between the Discovery MI and SIGNA showed a similar spatial resolution and overall imaging performance. Lastly, the results indicated significantly enhanced image quality and contrast-to-noise performance for Q.Clear, compared with ordered-subset expectation maximization. Excellent performance was achieved with the Discovery MI, including 375 ps FWHM coincidence time resolution and sensitivity of 14 cps/kBq. Comparisons between reconstruction algorithms and other multimodal silicon photomultiplier and non-silicon photomultiplier PET detector system designs indicated that performance can be substantially enhanced with this next-generation system.

18F-FDG silicon photomultiplier PET/CT: A pilot study comparing semi-quantitative measurements with standard PET/CT

“…The DMI PET/CT scanner uses small lutetium-based scintillator crystal arrays combined with a silicon photomultiplier (SiPM) bloc design, leading to a high NEMA sensitivity [15]. The SiPMs in DMI PET/CT enable direct conversion of photons into a digital signal that eliminates the signal loss and noise, making the scanner more efficient.…”

Evaluation of a clinical TOF-PET detector design that achieves ⩽100 ps coincidence time resolution

“…Current state-of-the-art commercial PET systems have ~315–530 picoseconds (ps) full-width-at-half-maximum (FWHM) timing performance, constraining annihilation events to lie somewhere within a ~5–8 cm region along system detector response lines (LORs) [Jakoby et al 2011, Miller et al 2015, Grant et al 2016, Hsu et al 2017]. The reconstructed image SNR improvement this localization provides can be estimated according to the relationship between assumed patient diameter (D), the speed of light (c), and the CTR of system (dt) in Equation 1 [Conti 2008].…”

Advances in clinical molecular imaging instrumentation

“…The need for compact, MR-compatible components has stimulated the fast adoption of solidstate read-out systems, initially APDs, but more recently SiPMs. The superior coincidence timing has resulted in a better TOF timing resolution than was previously achievable, with the result that similar PET designs are being adopted for the latest PET/CT instruments [15,70,71]. Prior to the introduction of SiPM-based PET, the coincidence timing resolution in commercial systems was in the order of 600 ps, with a signal-to-noise gain of around 1.8 for a 30 cm object.…”