title: 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 level-set-based MR-AC method (MR-AC(level)), compared with CT-AC and MR-AC provided by the manufacturer of the PET/MRI scanner (MR-AC(mMR)).
  
  
  METHODS
  Ten healthy volunteers and 20 Parkinson disease patients underwent(18)F-FDG and(18)F-fluorinated-N-3-fluoropropyl-2-β-carboxymethoxy-3-β-(4-iodophenyl)nortropane ((18)F-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-AC(mMR), and MR-AC(level) maps. The accuracies of SUV, SUVr (SUV and its ratio to the cerebellum), and specific-to-nonspecific binding ratios obtained using MR-AC(level) and MR-AC(mMR) 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 (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 PET images almost equivalent to the CT-AC images. PET quantification error was reduced by a factor of 3 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) and < 15% in MR-AC(mMR) [VB20P]).
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Nuclear Medicine, Seoul National University College of Medicine, Seoul, Korea Department of Biomedical Sciences, Seoul National University College of Medicine, Seoul, Korea.
authors.forename: Hyun Joon
authors.initials: HJ
authors.lastname: An
authors.affiliation: Department of Nuclear Medicine, Seoul National University College of Medicine, Seoul, Korea Department of Brain and Cognitive Sciences, Seoul National University College of Natural Sciences, Seoul, Korea.
authors.forename: Seongho
authors.initials: S
authors.lastname: Seo
authors.affiliation: Department of Nuclear Medicine, Seoul National University College of Medicine, Seoul, Korea.
authors.forename: Hyejin
authors.initials: H
authors.lastname: Kang
authors.affiliation: Department of Nuclear Medicine, Seoul National University College of Medicine, Seoul, Korea.
authors.forename: Hongyoon
authors.initials: H
authors.lastname: Choi
authors.affiliation: Department of Nuclear Medicine, Seoul National University College of Medicine, Seoul, Korea Institute of Radiation Medicine, Medical Research Center, Seoul National University College of Medicine, Seoul, Korea.
authors.forename: Gi Jeong
authors.initials: GJ
authors.lastname: Cheon
authors.affiliation: Department of Neurology, Seoul National University College of Natural Sciences, Seoul, Korea.
authors.forename: Han-Joon
authors.initials: HJ
authors.lastname: Kim
authors.affiliation: Department of Nuclear Medicine, Seoul National University College of Medicine, Seoul, Korea Institute of Radiation Medicine, Medical Research Center, Seoul National University College of Medicine, Seoul, Korea Department of Molecular Medicine and Biopharmaceutical Sciences, Graduate School of Convergence Science and Technology, Seoul National University, Suwon, Korea.
authors.forename: Dong Soo
authors.initials: DS
authors.lastname: Lee
authors.affiliation: Department of Radiology, Seoul National University College of Medicine, Seoul, Korea; and.
authors.forename: In Chan
authors.initials: IC
authors.lastname: Song
authors.affiliation: Department of Nuclear Medicine, Seoul National University College of Medicine, Seoul, Korea Department of Nuclear Medicine, Seoul Metropolitan Government Boramae Medical Center, Seoul, Korea jaes@snu.ac.kr yk3181@snu.ac.kr.
authors.forename: Yu Kyeong
authors.initials: YK
authors.lastname: Kim
authors.affiliation: Department of Nuclear Medicine, Seoul National University College of Medicine, Seoul, Korea Department of Biomedical Sciences, Seoul National University College of Medicine, Seoul, Korea Department of Brain and Cognitive Sciences, Seoul National University College of Natural Sciences, Seoul, Korea Institute of Radiation Medicine, Medical Research Center, Seoul National University College of Medicine, Seoul, Korea jaes@snu.ac.kr yk3181@snu.ac.kr.
authors.forename: Jae Sung
authors.initials: JS
authors.lastname: Lee
mesh_terms: D000368:Aged; D000465:Algorithms; D001842:Bone and Bones / Q000000981:diagnostic imaging; D004574:Electromagnetic Fields; D005260:Female; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D008875:Middle Aged; D064847:Multimodal Imaging / Q000379:methods; D010300:Parkinson Disease / Q000000981:diagnostic imaging; D049268:Positron-Emission Tomography / Q000379:methods*; D019275:Radiopharmaceuticals; D014326:Tropanes
chemical_list: D019275:Radiopharmaceuticals; D014326:Tropanes; C087552:2-carbomethoxy-8-(3-fluoropropyl)-3-(4-iodophenyl)tropane
keywords: PET/MRI; attenuation correction; brain PET; level-set segmentation