Inaccuracy in magnetic resonance image (MRI)-based attenuation correction (MR-AC) leads to errors in quantification and the misinterpretation of lesions in brain PET/MRI studies. To resolve this problem, we propose an improved ultrashort echo time (UTE) MR-AC method that is based on a multiphase level-set algorithm with B 0 inhomogeneity correction. We also assessed the feasibility of this level-set-based MR-AC method (MR-AC level ), compared with CT-AC and MR-AC provided by the manufacturer of

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... the PET/MRI scanner (MR-AC mMR ). Methods: Ten healthy volunteers and 20 Parkinson's disease patients underwent [ 18 F]FDG and [ 18 F]FP-CIT PET scans, respectively, using both PET/MRI and PET/CT scanners. The levelset-based segmentation algorithm automatically delimited air, bone, and soft tissue from the UTE MR images. For the comparison, MR-AC maps were coregistered to reference CT. PET sinogram data obtained from PET/CT studies were then reconstructed using the CT-AC, MR-AC mMR and MR-AC level maps. The accuracies of SUV, SUVr and specific to nonspecific binding ratios (BR) obtained using MR-AC level and MR-AC mMR were compared with CT-AC using ROI-and voxel-based analyses. Results : There was remarkable improvement in the segmentation of air cavities and bones and the quantitative accuracy of PET measurement using the level set. While the striatal and cerebellar activities in [ 18 F]FP-CIT PET and frontal activity in [ 18 F]FDG PET were significantly underestimated by the MR-AC mMR , the MR-AC level provided almost equivalent PET images to the CT-AC. PET quantification error was reduced by a factor of three using MR-AC level (SUV error < 10% in MR-AC level and < 30% in MR-AC mMR [version VB18P], and < 5% in MR-AC level by on November 7, 2016. For personal use only. jnm.snmjournals.org Downloaded from 2 and < 15% in MR-AC mMR [VB20P]). Conclusion: The results of this study indicate that our new multiphase level-set-based MR-AC method improves the quantitative accuracy of brain PET in PET/MRI studies.