Abstract
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Objectives The aim of our study was to optimize reconstruction parameters for PET/MRI post-therapy Y-90 imaging using PET/CT as a standard.
Methods In a continuing study after selective internal radiation therapy (SIRT) with Y-90 microspheres, patients were imaged on a 4R, 40 slice PET/CT system (Siemens mCT). The low mA, non-diagnostic CT images were used for attenuation correction and localization of the Y-90 microspheres in the PET/CT studies. The acquisition time was 15 min. Immediately after PET/CT study, in an adjacent room a PET/MRI study was done on a Siemens mMR system. The acquisition time was 20 min, and attenuation correction was done using the scanner standard 4 segment MR based attenuation map. In both studies, the Na-22 isotope was selected, because neither system has Y-90 in their database. The data were reconstructed using the vendor’s reconstruction software and were transferred to a common platform where the PET/MRI and PET/CT Y-90 microspheres liver distributions were visually compared.
Results PET images for the PET/CT system were reconstructed according to QUEST recommendations, 2 iterations 21 subsets (2i21s) and 5mm Gaussian smoothing. PET images from the PET/MRI system were reconstructed using different number of iteration steps and different Gaussian smoothing. However, the number of subsets was fixed to 21. In a visual comparison we concluded that the PET images from PET/MRI system, which has the best qualitative match with corresponding PET images from PET/CT system are also the ones reconstructed with 2i21s and 5mm Gaussian smooth, although other combinations also were close match, like 3i21s and 8mm Gaussian smooth.
Conclusions In liver SIRT with Y-90 microspheres, the final administrated Y-90 distribution can be adequately imaged by either PET/CT or PET/MRI state-of-the-art system. The distributions of Y-90 microspheres obtained from PET/MRI and PET/CT in post-therapy studies were very similar.