Study of optical reflectors for a 100ps coincidence time resolution TOF-PET detector design

Abstract

Positron Emission Tomography (PET) reconstructed image signal-to-noise ratio (SNR) can be improved by including the 511 keV photon pair coincidence time-of-flight (TOF) information. The degree of SNR improvement from this TOF capability depends on the coincidence time resolution (CTR) of the PET system, which is essentially the variation in photon arrival time differences over all coincident photon pairs detected for a point positron source placed at the system center. The CTR is determined by several factors including the intrinsic properties of the scintillation crystals and photodetectors, crystal-tophotodetector coupling configurations, reflective materials, and the electronic readout configuration scheme. The goal of the present work is to build a novel TOF-PET system with 100 picoseconds (ps) CTR, which provides an additional factor of 1.5-2.0 improvement in reconstructed image SNR compared to state-of-the-art TOF-PET systems which achieve 225-400 ps CTR. A critical parameter to understand is the optical reflector's influence on scintillation light collection and transit time variations to the photodetector. To study the effects of the reflector covering the scintillation crystal element on CTR, we have tested the performance of four different reflector materials: Enhanced Specular Reflector (ESR)coupled with air or optical grease to the scintillator; Teflon tape; BaSO 4 paint alone or mixed with epoxy; and TiO 2 paint. For the experimental set-up, we made use of 3×3×10 mm 3 fast-LGSO:Ce scintillation crystal elements coupled to an array of silicon photomultipliers (SiPMs) using a novel 'side-readout' configuration that has proven to have lower variations in scintillation light collection efficiency and transit time to the photodetector. Results: show CTR values of 102.0±0.8, 100.2±1.2, 97.3±1.8 and 95.0±1.0 ps full-width-half-maximum (FWHM) with non-calibrated energy resolutions of 10.2±1.8, 9.9±1.2, 7.9±1.2, and 8.6±1.7% FWHM for the Teflon, ESR (without grease), BaSO 4 (without epoxy) and TiO 2 paint treatments, respectively.

Advances in Detector Instrumentation for PET

“…Different inorganic scintillator materials have been used in PET detectors; Supplemental Table 1 (supplemental materials are available at http://jnm.snmjournals.org) summarizes the most commonly used inorganic scintillators and their properties for the purpose of indirect radiation detection. To promote good spatial resolution, energy resolution, and coincidence time resolution (CTR) performance, important aspects to consider in addition to high light yield, Z eff , and density, and short rise and decay times, are the reflector applied to the scintillator crystal surfaces (to constrain the generated light photons within the crystal), the crystal surface condition (e.g., polished or rough), and the scintillator-photodetector coupling medium (9).…”

“…Nevertheless, designs attaching the photodetector to the entrance surface (Fig. 3D) (9), at the side surfaces (Fig. 3E) (9), or to both top and bottom surfaces (for 511-keV photon depth of interaction [DOI] positioning) (Fig.…”

“…3D) (9), at the side surfaces (Fig. 3E) (9), or to both top and bottom surfaces (for 511-keV photon depth of interaction [DOI] positioning) (Fig. 3F) (14), or even covering the 4 lateral edges of a monolithic crystal (Fig.…”

“…Regarding the monolithic crystal-based detector design, an additional challenge is the truncation of scintillation light at the edges, which degrades the spatial, timing, and energy resolutions in these regions (7). Several approaches to mitigating this effect have been tested, such as different crystal surface treatments (e.g., covering the edges with an optical absorber) or novel photodetector arrangements (9,11). Moreover, monolithic detectors require 2-dimensional calibration procedures to estimate the 511-keV photon interaction position within the scintillator.…”

Time Resolution Studies of Thallium Based Cherenkov Semiconductors

“…To simplify simulations, the correlated noise of the SiPM (cross talk, after pulsing) is not included and, to provide identical conditions between measurements and simulations, only events with one-triggered SPAD (one detected Cherenkov photon) are considered. The tuning of the parameters is required to correctly reproduce the coincidence time spectra since the surface state has a strong impact on the time resolution [31][32][33]. The lateral surfaces have a different roughness with respect to the front and back; therefore, two different parameters are considered: σ α 1 for the lateral faces and σ α 2 for the back and front.…”