An edge‐readout, multilayer detector for positron emission tomography

Abstract

Purpose We present a novel gamma-ray-detector design based on total internal reflection (TIR) of scintillation photons within a crystal that addresses many limitations of traditional PET detectors. Our approach has appealing features, including sub-millimeter lateral resolution, DOI positioning from layer thickness, and excellent energy resolution. The design places light sensors on the edges of a stack of scintillator slabs separated by small air gaps, and exploits the phenomenon that more than 80% of scintillation light emitted during a gamma-ray event reaches the edges of a thin crystal with polished faces due to TIR. Gamma-ray stopping power is achieved by stacking multiple layers, and DOI is determined by which layer the gamma ray interacts in. Method The concept of edge readouts of a thin slab was verified by Monte-Carlo simulation of scintillation-light transport. An LYSO crystal of dimensions 50.8 mm × 50.8 mm × 3.0 mm was modeled with 5 rectangular SiPMs placed along each edge face. The mean-detector-response functions (MDRFs) were calculated by simulating signals from 511 keV gamma-ray interactions in a grid of locations. Simulations were carried out to study the influence of choice of scintillator material and dimensions, gamma-ray photon energies, introduction of laser or mechanically-induced optical barriers (LIOBs, MIOBs), and refractive indices of optical-coupling media and SiPM windows. We also analyzed timing performance including influence of gamma-ray interaction position and presence of optical barriers. We also modeled and built a prototype detector, a 27.4 mm × 27.4 mm × 3.0 mm CsI(Tl) crystal with 4 SiPMs per edge in order to experimentally validate the results predicted by the simulations. The prototype detector used CsI(Tl) crystals from Proteus outfitted with 16 Hamamatsu model S13360-6050PE MPPCs read out by an AiT-16-channel readout. The MDRFs were measured by scanning the detector with a collimated beam of 662-keV photons from a 137Cs source. The spatial resolution was experimentally determined by imaging a tungsten slit that created a beam of 0.44 mm (FWHM) width normal to the detector surface. The energy resolution was evaluated by analyzing list-mode data from flood illumination by the 137Cs source. Result We find that in a block-detector-sized LYSO layer read out by 5 SiPMs per edge, illuminated by 511 keV photons, the average resolution is 1.49 mm (FWHM). With the introduction of optical barriers, average spatial resolution improves to 0.56 mm (FWHM). The DOI resolution is the layer thickness of 3.0 mm. We also find that optical-coupling media and SiPM-window materials have an impact on spatial resolution. The timing simulation with LYSO crystal yields a coincidence resolving time (CRT) of 200 – 400 ps, which is slightly position-dependent. And the introduction of optical barriers has minimum influence. The prototype CsI(Tl) detector, with a smaller area and fewer SiPMs, was measured to have central-area spatial resolutions of 0.70 mm and 0.39 mm without and with optical ba...

Characterization of a High-Aspect Ratio Detector With Lateral Sides Readout for Compton PET

“…We made it possible to collect the scintillation light using photosensors located at the lateral sides of the crystal using custom arrays of 1 × 16 SiPMs. The aspect ratio of our design is 17.2, compared to prior experimental works following a similar methodology of 4.8 [7] or 9.1 [6]. This very high-aspect ratio could cause some performance deterioration of the detector block due to the very long travel paths of the scintillation photons.…”

“…One approach of this detector concept is to stack multiple layers with the minimum separation between layers or even no separation in one detector module. In particular, Li et al [6] developed a prototype detector based on CsI(Tl) with layers of dimensions 27.4 × 27.4 × 3 mm 3 (aspect ratio of 9.1). A similar concept was carried out based on LYSO crystals [7] but with smaller layer dimensions of 13.34 × 13.34 × 2.75 mm 3 (aspect ratio of 4.8).…”

“…We envisaged the use of very high-aspect ratio crystal configurations based on LYSO [8]. We have selected a crystal design with entrance and exit faces of the gamma-ray radiation of conventional dimensions of 51.5 × 51.5 mm 2 and a thickness of only 3 mm, resulting in a high-aspect ratio of 17.2, which is significantly larger than previous experimental works [6], [7]. The dimensions of our detector make them suitable for both whole or total-body PET, but also to organ-dedicated PET systems [9].…”

Experimental Validation of a Gamma Detector With a Novel Light-Guide-PMT Geometry to Reduce Dead Edge Effects

“…Subsequently a trigger calibration and a position calibration were applied. To this end, a beam collimator was used as is conventional in gamma detector calibration with monolithic scintillators [4], [9], [14], [15], [27], [35], [36]. A disk with 50 MBq 99m Tc inside was put in a lead-made beam collimator with a Ø1.1 × 25 mm 3 hole to obtain a perpendicular gamma photon beam on the detector.…”

Fast gamma-ray interaction-position estimation using k-d tree search

“…The prototype detector presented in Li et al (2018) was also used for experimental measurements; the scintillation crystal was a slab of CsI(Tl) of size 27.4 mm × 27.4 mm × 3 mm. 16 Hamamatsu S13360-6050PE SiPMs (active area is 6 mm × 6 mm) were attached to the scintillator crystal to read from edges (4 SiPM sensors on each edge).…”