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-to-photodetector 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; BaSO₄paint alone or mixed with epoxy; and TiO₂paint. For the experimental set-up, we made use of 3 × 3 × 10 mm³fast-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), BaSO₄(without epoxy) and TiO₂paint treatments, respectively.

Keywords: coincidence time resolution; optical reflector; positron emission tomography; scintillation detector; silicon photomultipliers; time-of-flight.

Figure 1.

Sketches and photos of: (top) conventional end-readout configuration comprising a single scintillation crystal element coupled end-on to a single SiPM of the linear array, and (bottom) Side-readout configuration where the scintillation crystal element is side-coupled to the linear array of SiPMs.

Figure 2.

Photograph of the two detector elements used in the experiments. The RF amplifier readout board was connected to the SiPMs and 3×3×10 mm³ LGSO crystals were side-coupled to the SiPMs by means of optical grease. The red and blue arrows point to the input SiPM and RF amplifier bias and, to the energy (cable not connected for the clarity of the photo) and timing outputs, respectively.

Figure 3.

Sketch of (a) base line correction showing two signals with offset, and (b) LED technique applied for the time pickoff; dashes lines represent different threshold values.

Figure 4.

Measured CTR (FWHM) for 10 mm length LGSO:Ce crystals versus applied LEDTh for different SiPM bias voltage when (a) end-coupled or (b) side coupled to the SiPM array.

Figure 5.

(a) Measured CTR FWHM versus applied SiPMs bias voltage for the ESR reflector case when coupled to the crystal by means of optical grease (OG) (blue line) or air (orange line). (b) Measured photopeak gain versus applied SiPMs bias voltage for the ESR case when coupled to the crystal by means of optical grease (dark and light blue) or air (dark and light orange).

Figure 6.

(a) Measured CTR FWHM versus applied SiPMs bias voltage for the BaSO4 reflector case when directly coupled to the crystal (green line) or when mixed with an Epoxy (red line). (b) Measured photopeak gain for each coincidence detector versus applied SiPMs bias voltage for the BaSO4 reflector case when directly coupled to the crystal (dark and light green) or when mixed with an epoxy (dark and light red).

Figure 7.

(a) Measured CTR at FWHM versus applied SiPMs bias voltage for each one of the four studied reflector conditions applied to the 10 mm LGSO:Ce crystal surfaces. (b) Measured photopeak gain for each detector (top, detector 1; bottom, detector 2) in the coincidence experiment versus applied SiPMs bias voltage for each reflective material.

Figure 8.

Schematic of concept for (a) one ‘edge-on’ detector layer, and (b) the 8-layer sub-module and X-layer full module design that achieves ~100 ps FWHM CTR.