The origin and reduction of spurious extrahepatic counts observed in⁹⁰Y non-TOF PET imaging post radioembolization

Abstract

Our literature survey revealed a physical effect unknown to the nuclear medicine community, i.e. internal bremsstrahlung emission, and also the existence of long energy resolution tails in crystal scintillation. None of these effects has ever been modelled in PET Monte Carlo (MC) simulations. This study investigates whether these two effects could be at the origin of two unexplained observations in Y imaging by PET: the increasing tails in the radial profile of true coincidences, and the presence of spurious extrahepatic counts post radioembolization in non-TOF PET and their absence in TOF PET. These spurious extrahepatic counts hamper the microsphere delivery check in liver radioembolization. An acquisition of aP vial was performed on a GSO PET system. This is the ideal setup to study the impact of bremsstrahlung x-rays on the true coincidence rate when no positron emission and no crystal radioactivity are present. A MC simulation of the acquisition was performed using Gate-Geant4. MC simulations of non-TOF PET and TOF-PET imaging of a synthetic Y human liver radioembolization phantom were also performed. Internal bremsstrahlung and long energy resolution tails inclusion in MC simulations quantitatively predict the increasing tails in the radial profile. In addition, internal bremsstrahlung explains the discrepancy previously observed in bremsstrahlung SPECT between the measure of theY bremsstrahlung spectrum and its simulation with Gate-Geant4. However the spurious extrahepatic counts in non-TOF PET mainly result from the failure of conventional random correction methods in such low count rate studies and poor robustness versus emission-transmission inconsistency. A novel proposed random correction method succeeds in cleaning the spurious extrahepatic counts in non-TOF PET. Two physical effects not considered up to now in nuclear medicine were identified to be at the origin of the unusual Y true coincidences radial profile. TOF reconstruction removing of the spurious extrahepatic counts was theoretically explained by a better robustness against emission-transmission inconsistency. A novel random correction method was proposed to overcome the issue in non-TOF PET. Further studies are needed to assess the novel random correction method robustness.

Y-90 SPECT ML image reconstruction with a new model for tissue-dependent bremsstrahlung production using CT information: a proof-of-concept study

“…While the yield of positrons used in Y-90 PET is independent of the tissue media, the yield of bremsstrahlung photons used in SPECT is tissue dependent. The Y-90 bremsstrahlung yield consists of 2 components: internal bremsstrahlung (IB) and external bremsstrahlung (EB) (Cengiz and Almaz (2004), Venkataramaiah et al (1980) and Walrand et al (2018)). The bremsstrahlung energy spectra corresponding to both components extend from zero to the beta endpoint energy, 2.3 MeV for Y-90.…”

Yttrium-90 TOF-PET-Based EUD Predicts Response Post Liver Radioembolizations Using Recommended Manufacturer FDG Reconstruction Parameters

“…On the other hand, due to the constant random rate generated by the natural LYSO radioactivity, a reduction of the specific activity cannot be counterbalanced by increasing the acquisition time and by summing slices (24)(25)(26)(27). Indeed, below 18Gy the imaging count rate mainly originates from the crystal radioactivity.…”

“…The natural crystal radioactivity generates a constant 1400 randoms/sec in the whole field of view (FOV), i.e., after the rejection of the randoms located outside the ring diameter by the TOF information ( 13 ). Note that in TOF-PET the randoms rate contribution is reduced by the square of the ratio between the target diameter D and the FOV diameter D FOV , as shown by the generalized signal to noise ratio (SNR) developed by M. Conti ( 26 ) taking into account the TOF information:…”

“…However, due to the constant random rate generated by the natural radioactivity of lutetium based crystals ( 24 ), lower specific activity in the target of interest cannot fully be counterbalanced by increasing the acquisition time or by summing several slices. Low count rate acquisition can result in reconstruction bias ( 25 ), extra-hepatic artefactual activity ( 26 ) and reduced signal to noise ratio (SNR) ( 27 ).…”

“…The modelled liver specific activity was about 1/2 to 1/8 fold that reached in typical liver radioembolization. However, due to the constant randoms rate generated by the natural radioactivity of lutetium based crystals, a lower specific activity in the target of interest cannot fully be counterbalanced by increasing the acquisition time or by summing several slices (24)(25)(26)(27).…”