Evaluating plastic scintillator performance as a substitute of LYSO in SiPM based animal PET scanners: A GEANT4 simulation analysis

TL;DR

The paper assesses whether plastic scintillators can substitute LYSO in SiPM-based animal PET scanners by performing GEANT4 simulations of LYSO and two plastics (BC404 and Ooty) coupled to a SiPM pad. It quantifies CRT and gamma-ray interaction efficiency, finding that LYSO yields CRT ≈ 174 ps, while plastics require ~4× longer bars to achieve comparable 511 keV photopeak efficiency, resulting in CRT ≈ 300 ps. To match the imaging performance of LYSO, a plastic bar of about 20 mm can provide similar resolution (≈4.5 cm FWHM) albeit with reduced detection efficiency, whereas longer bars (≈40 mm) recover timing at the cost of efficiency. The results suggest plastics, particularly BC404 and Ooty, could enable cost-effective, scalable animal-PET modules, with future MOBY phantom studies planned to optimize bar geometries and gantry designs.

Abstract

A systematic study is conducted to understand the coincident resolving time (CRT) for a pair of Lutetium-yttrium oxyorthosilicate (LYSO) and the plastic scintillation detector bars under the Geant4 framework. Crystals are coupled to a silicon photomultiplier single pad wafer with an appropriate optical coupling for signal readout. Pad reads the light photons undergoing optoelectronic conversion at the wafer site and generates electrical pulses with a bi-exponential shape. These signals are used to determine the trigger time stamp of back-to-back gamma-rays emitted from a point source, enabling the evaluation of CRT performance at different plastic scintillator lengths. For the LYSO detector, the simulation yields the CRT response of 174 ps, which aligns to the literature reported value for the dimensions 2 mm x 2 mm x 10 mm. To identify the plastic scintillator dimensions with an integrated gamma-ray detection efficiency comparable to LYSO's photopeak efficiency at 511 keV gamma photons, various bar lengths of commercial plastic BC404 and TIFR Ooty's in-house developed plastic material are attempted in Geant4. Consequently, for both the plastic scintillators, the equivalent length (for the same cross-sectional area) was found to be 4 times that of LYSO crystal length at a threshold of 25 keV. CRT value determined for this dimension is found to be $\approx$ 300 ps for both the plastic medium. It suggests that, if an animal preclinical PET scanner is developed with plastic bars, the minimum achievable image resolution (FWHM) of $\approx$ 4.5 cm can be expected for a pair of detection elements.

Figures (7)

• Figure 1: Assembly of scintillation bar with SiPM pad, constructed in Geant4. It is coupled to pad with a optical grease for a smooth light transmission. All the other surface is wrapped with a reflective epoxy resin layer of thickness 50 $\mu$m, shown as a wireframe. The surface roughness is also taken into account with optical properties mentioned in Table \ref{['optical_table']}. For the clarity of reader, different geometrical dimensions are mentioned in the legend.
• Figure 2: Left : Collection of SiPM signal output, each is generated from the fastest 300 light photons reaches to the SiPM pad. Timestamp of each signal is calculated as soon as it crosses the trigger level. Right : Evolution of coincidence resolving time (CRT) as a function of trigger threshold of SiPM signal.
• Figure 3: Flowchart of SiPM signal calculation used to determine the CRT value of two bars.
• Figure 4: Left : Arrival time distribution of various light photons under photopeak events of a LYSO bar. Right : The graph shows the variation in the width of the j$^{th}$ photon arrival time and the accumulated width, calculated for a full energy $\gamma$-ray event.
• Figure 5: Contribution to the time broadening by the fastest collected photon at different bar lengths.