Coincidence energy spectra due to the intrinsic radioactivity of LYSO scintillation crystals

Abstract

Background: Lutetium oxyorthosilicate or lutetium yttrium oxyorthosilicate (LYSO) scintillation crystals used in most current PET scanner detectors contain 176 Lu, which decays by beta emission to excited states of 176 Hf accompanied by the emission of prompt gamma rays or internal conversion electrons. This intrinsic radioactivity can be self-detected in singles mode as a constant background signal that has an energy spectrum whose structure has been explained previously. In this work, we studied the energy spectrum due to the intrinsic radioactivity of LYSO scintillation crystals of two opposing detectors working in coincidence mode. The investigation included experimental data, Monte Carlo simulations and an analytical model. Results: The structure of the energy spectrum was completely understood and is the result of the self-detection of beta particles from 176 Lu in one crystal and the detection of one or more prompt gamma rays detected in coincidence by the opposing crystal. The most probable coincidence detection involves the gamma rays of 202 and 307 keV, which result in two narrow photopeaks, superimposed on a continuous energy distribution due to the beta particle energy deposition. The relative intensities of the gamma ray peaks depend on crystal size and detector separation distance, as is explained by the analytical model and verified through the Monte Carlo simulations and experiments. Conclusions: The analytical model used in this work accurately explains the general features of the coincidence energy spectrum due to the presence of 176 Lu in the scintillation crystals, as observed experimentally and with Monte Carlo simulations. This work will be useful to those research studies aimed at using the intrinsic radioactivity of LYSO crystals for transmission scans and detector calibration in coincidence mode.

Energy spectra due to the intrinsic radiation of LYSO/LSO scintillators for a wide range of crystal sizes

“…The fact that this model reproduces the main features of the background energy spectra, without the inclusion of optical processes of the scintillation light, indicates that the radiation decay and detection physics are the dominating processes over other factors (e.g., properties of the crystal wrapping material, scintillation light absorption, photon detection efficiency, and linearity of the photodetector) which need to be taken into consideration if a more realistic energy spectrum is required. More recently, this model has been extended to understand the energy spectra for a pair of LYSO crystals working in coincidence mode due to the intrinsic radioactivity, with predictions in good agreement with Monte Carlo simulations and experiments 18 …”

“…More recently, this model has been extended to understand the energy spectra for a pair of LYSO crystals working in coincidence mode due to the intrinsic radioactivity, with predictions in good agreement with Monte Carlo simulations and experiments. 18 Monte Carlo simulations can also be used to obtain the energy spectra for a particular crystal size, as shown in the work by Enríquez-Mier-y-Terán et al, 17 who used Geant4 Application for Tomographic Emission (GATE) v8.1. 19 Monte Carlo simulations produce more accurate energy spectra since Compton scattered photons escaping from the crystal, depositing a fraction of the gamma ray energy, are not considered in the analytical model.…”

Design Study of a Novel Positron Emission Tomography System for Plant Imaging

“…The radioactive decay of the fraction of 176 Lu in LYSO is not considered in the simulation since the intrinsic radioactivity poorly affects the evaluations of camera performances, mainly due to the small number of crystals we used for the scanner architecture. In fact, the count rate derived from intrinsic radioactivity in a 176 Lu-based crystal is approximately 300Bq/ml (Enr-quez-Mier-y Terán et al, 2020 ). Considering the total crystal volume for CONC system (438 ml) and for MINI systems (from 18.7 ml to 28.8 ml), the total intrinsic radioactivity can be estimated at most 8 kBq for MINI systems and 131 kBq for CONC system.…”

Discrete Convolution-Based Energy Spectrum Configuring Method for the Analysis of the Intrinsic Radiation of 176Lu

“…The results derived from the calculation were the weight coefficients that minimize the root-mean square error (RMSE) of the simulated and experimental spectrum within specific conditions. Thereby, based on Equation (7), the IRL spectrum of the simulation was generated by the sum of the multiplication of the optimized weight coefficients and the corresponding base spectrum. As can be seen in Figure 6, the simulation spectrum was generated in good agreement with the experimental spectrum.…”

“…Although the effects of IRL on medical imaging have been studied, they have not been a major concern in common in-vivo situations because the activity is negligibly low compared to the radiopharmaceuticals and the nuclear medicine scanners have background rejection capabilities, such as coincidence-timing windows and energy windows [7][8][9]. However, when used in preclinical PET, in-beam PET, the PET/SPECT system, and celltrafficking studies under the aforementioned special conditions, these effects become 2 of 14 problematic.…”