Comparison of acrylic polymer adhesive tapes and silicone optical grease in light sharing detectors for positron emission tomography

Abstract

Optical coupling is an important factor in detector design as it improves optical photon transmission by mitigating internal reflections at light-sharing boundaries. In this work we compare optical coupling materials, namely double-sided acrylic polymer tapes and silicone optical grease (SiG), in the context of positron emission tomography. Four double-sided tapes from 3 M of varying thicknesses (0.229 mm-1.016 mm) and adhesive materials ('100MP', 'A100', and 'GPA') were characterized with spectrophotometer measurements as well as photopeak amplitude and energy resolution measurements using lutetium-yttrium oxy-orthosilicate (LYSO) coupled to photomultiplier tubes (PMT) or silicon photomultipliers (SiPMs). Transmission spectra from the spectrophotometer showed over 80% transmission for all tapes at 420 nm and above, with 89.6% and 88.8% transmission for the 0.508 mm and 1.016 mm thick GPA tapes, respectively, at 420 nm. Measurements with single-pixel LYSO-PMT and 4 × 4 array (one-to-one coupled) LYSO-SiPM setups determined that SiG had the greatest photopeak amplitude, with tapes showing 2.1%-14.8% reduction in photopeak amplitude with respect to SiG. Energy resolution changed by less than 4% on a relative basis between tapes and SiG with PMT measurements, however for the SiPM array measurements the energy resolution improved from 15.6% ± 2.7% full-width at half-maximum to 11.4% ± 1.2% for SiG and 1 mm GPA respectively. Data acquired with dual-layer offset LYSO arrays (light sharing detector designs) demonstrated that a detector coupled with 1 mm thick GPA tape produced equivalent detector flood histograms to those from a design coupled with SiG and a 1 mm thick glass lightguide. No significant degradation in photopeak amplitude and energy resolution was observed over five months of measurements, indicating the tapes maintain their coupling integrity over several months. Though minimal photopeak amplitude degradation compared to SiG occurs, double-sided tapes are convenient alternatives for optical coupling materials since they diffuse light intrinsically, acting as a light guide, offer mechanical support and durability, are easily applied and removed from scintillators/photodetectors, and are relatively inexpensive and readily available.

Feasibility Study of a Time-of-Flight Brain Positron Emission Tomography Employing Individual Channel Readout Electronics

“…(a) (b) The MPPC and LYSO arrays were coupled using a silicone adhesive (3145 RTV, The Dow Chemical Company, Midland, MI, USA) with a refractive index of 1.49 at 430 nm to match the refractive index between optical boundaries (MPPC: 1.55 at 450 nm, LYSO: 1.82 at 420 nm) taking Snell's law into account. In addition, this optical coupling material was used to improve the collection efficiency of scintillation light and to maintain its adhesion and optical properties for a long time [26,27].…”

Towards a second-generation PET/MR insert with enhanced timing and count rate performance

“…Two LYSO:Ce array sizes were constructed due to differences in photodetector array size and active area size: 3 × 3 crystal arrays for SiPM-B and MPPC arrays, and 4 × 4 crystal arrays for SiPM-C arrays. Optical coupling was done using 3 M 1.016 mm VHB doublesided acrylic tape (Van Elburg et al 2018). Figure 3.…”