MRI-based attenuation correction for PET/MRI using multiphase level-set method

Abstract

Inaccuracy in MR image-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 proposed an improved ultrashort echo time MR-AC method that was based on a multiphase level-set algorithm with main magnetic field (B0) inhomogeneity correction. We also assessed the feasibility of this levelset- based MR-AC method (MR-AClevel), compared with CT-AC and MR-AC provided by the manufacturer of the PET/MRI scanner (MRACmMR). Methods: Ten healthy volunteers and 20 Parkinson disease patients underwent 18F-FDG and 18F-fluorinated-N-3-fluoropropyl-2- β-carboxymethoxy-3-β-(4-iodophenyl)nortropane (18F-FP-CIT) PET scans, respectively, using both PET/MRI and PET/CT scanners. The level-set-based segmentation algorithm automatically delimited air, bone, and soft tissue from the ultrashort echo time 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-ACmMR, and MR-AClevel maps. The accuracies of SUV, SUVr (SUV and its ratio to the cerebellum), and specific-to-nonspecific binding ratios obtained using MR-AClevel and MR-ACmMR were compared with CT-AC using region-of-interest- 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. Although the striatal and cerebellar activities in 18F-FP-CIT PET and frontal activity in 18F-FDG PET were significantly underestimated by the MR-ACmMR, the MR-AClevel provided PET images almost equivalent to the CT-AC images. PET quantification error was reduced by a factor of 3 using MR-AClevel (SUV error < 10% in MR-AClevel and < 30% in MR-ACmMR [version VB18P], and < 5% in MR-AClevel and < 15% in MR-ACmMR [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.