Abstract
Lutetium oxyorthosilicate (LSO) or lutetium yttrium oxyorthosilicate (LYSO) are the scintillator materials most widely used today in PET detectors due to their convenient physical properties for the detection of 511 keV annihilation photons. Natural lutetium contains 2.6% of 176Lu which decays beta to excited states of 176Hf producing a constant background signal. Although previous works have studied the background activity from LSO/LYSO, the shape of the spectrum, resulting from β-particle and γ radiation self-detection, has not been fully explained. The present work examines the contribution of the different β-particle and γ-ray interactions to provide a fuller comprehension of this background spectrum and to explain the differences observed when using crystals of different sizes. To this purpose we have shifted the continuous β-particle energy spectrum of 176Lu from zero to the corresponding energy value for all combinations of the isomeric transitions of 176Hf (γ-rays/internal conversion). The area of each shifted β-spectrum was normalized to reflect the probability of occurrence. To account for the probability of the γ-rays escaping from the crystal, Monte Carlo simulations using PENELOPE were performed in which point-like sources of monoenergetic photons were generated, inside LYSO square base prisms (all 1 cm thick) of different sizes: 1.0 cm to 5.74 cm. The analytic distributions were convolved using a varying Gaussian function to account for the measured energy resolution. The calculated spectra were compared to those obtained experimentally using monolithic crystals of the same dimensions coupled to SiPM arrays. Our results are in very good agreement with the experiment, and even explain the differences observed due to crystal size. This work may prove useful to calibrate and assess detector performance, and to measure energy resolution at different energy values.
Highlights
To date, most clinical and preclinical positron emission imaging systems, combine scintillation crystals coupled to position-sensitive photodetectors like photomultiplier tubes or silicon photomultipliers (SiPM)
Notice that the peak corresponding to all three γ-rays detected in the crystal has a higher intensity for the large prism compared to the small cube because of its larger intrinsic detection efficiency due to the crystal dimensions; this effect is evident in the final energy spectra explaining the differences in the relative peak intensities
As it can be seen, the calculated energy spectra are able to reproduce the structure observed in both crystal sizes, in good agreement with the experimental data. This agreement is despite of the limitations of this work, namely: (a) We have only considered the energy deposited by β-particles, secondary electrons produced from γ-rays interactions and conversion electrons, neglecting the contribution of scattered photons and low energy X-rays escaping the crystal, (b) Our calculations do not consider the light transport within LYSO, nor the type of reflector material, which do have an impact through light attenuation in the crystal lattice and light absorption/reflection on the crystal surfaces on the amount of light reaching the photodetector
Summary
Most clinical and preclinical positron emission imaging systems, combine scintillation crystals coupled to position-sensitive photodetectors like photomultiplier tubes or silicon photomultipliers (SiPM). Natural lutetium contains about 2.6% of 176Lu which decays by beta emission with mean and maximum β-particle energy of 182 keV and 593 keV, respectively, to excited states of 176Hf with a half-life of 3.76 × 1010 years producing a constant background signal, which can be removed by means of coincidence detection. This intrinsic radioactivity may have an impact when imaging low levels of activity, especially when using wide energy windows[6,7] and even more when designing detectors for single photon imaging when background emissions tally with those used in SPECT scanners that have LSO/LYSO scintillators[8]. The present work aims at providing a more detailed explanation of the structure of the LSO/LYSO scintillation intrinsic radioactivity energy spectrum due to the β and γ radiation from 176Lu present in natural lutetium and to explain the differences observed when using crystals of different dimensions
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