title: Hybrid ZTE/Dixon MR-based attenuation correction for quantitative uptake estimation of pelvic lesions in PET/MRI.
abstract: OBJECTIVE
  This study introduces a new hybrid ZTE/Dixon MR-based attenuation correction (MRAC) method including bone density estimation for PET/MRI and quantifies the effects of bone attenuation on metastatic lesion uptake in the pelvis.
  
  
  METHODS
  Six patients with pelvic lesions were scanned using fluorodeoxyglucose (18F-FDG) in an integrated time-of-flight (TOF) PET/MRI system. For PET attenuation correction, MR imaging consisted of two-point Dixon and zero echo-time (ZTE) pulse sequences. A continuous-value fat and water pseudoCT was generated from a two-point Dixon MRI. Bone was segmented from the ZTE images and converted to Hounsfield units (HU) using a continuous two-segment piecewise linear model based on ZTE MRI intensity. The HU values were converted to linear attenuation coefficients (LAC) using a bilinear model. The bone voxels of the Dixon-based pseudoCT were replaced by the ZTE-derived bone to produce the hybrid ZTE/Dixon pseudoCT. The three different AC maps (Dixon, hybrid ZTE/Dixon, CTAC) were used to reconstruct PET images using a TOF-ordered subset expectation maximization algorithm with a point-spread function model. Metastatic lesions were separated into two classes, bone lesions and soft tissue lesions, and analyzed. The MRAC methods were compared using a root-mean-squared error (RMSE), where the registered CTAC was taken as ground truth.
  
  
  RESULTS
  The RMSE of the maximum standardized uptake values (SUVmax) is 11.02% and 7.79% for bone (N = 6) and soft tissue lesions (N = 8), respectively, using Dixon MRAC. The RMSE of SUVmax for these lesions is significantly reduced to 3.28% and 3.94% when using the new hybrid ZTE/Dixon MRAC. Additionally, the RMSE for PET SUVs across the entire pelvis and all patients are 8.76% and 4.18%, for the Dixon and hybrid ZTE/Dixon MRAC methods, respectively.
  
  
  CONCLUSIONS
  A hybrid ZTE/Dixon MRAC method was developed and applied to pelvic regions in an integrated TOF PET/MRI, demonstrating improved MRAC. This new method included bone density estimation, through which PET quantification is improved.
authors.affiliation: Department of Radiology and Biomedical Imaging, University of California San Francisco, 1700 4th St, San Francisco, CA 94158, USA.
authors.forename: Andrew P
authors.initials: AP
authors.lastname: Leynes
authors.affiliation: Department of Radiology and Biomedical Imaging, University of California San Francisco, 1700 4th St, San Francisco, CA 94158, USA.
authors.forename: Jaewon
authors.initials: J
authors.lastname: Yang
authors.affiliation: GE Global Research, Plot #122, Export Promotion Industrial Park, Phase 2, Hoodi Village, Whitefield Road, Bangalore, 560066, India.
authors.forename: Dattesh D
authors.initials: DD
authors.lastname: Shanbhag
authors.affiliation: GE Global Research, Plot #122, Export Promotion Industrial Park, Phase 2, Hoodi Village, Whitefield Road, Bangalore, 560066, India.
authors.forename: Sandeep S
authors.initials: SS
authors.lastname: Kaushik
authors.affiliation: Department of Radiology and Biomedical Imaging, University of California San Francisco, 1700 4th St, San Francisco, CA 94158, USA.|UC Berkeley-UCSF Graduate Program in Bioengineering, University of California, Berkeley, CA 94158, USA.|University of California, 1700 4th St, San Francisco, CA 94158, USA.
authors.forename: Youngho
authors.initials: Y
authors.lastname: Seo
authors.affiliation: Department of Radiology and Biomedical Imaging, University of California San Francisco, 1700 4th St, San Francisco, CA 94158, USA.
authors.forename: Thomas A
authors.initials: TA
authors.lastname: Hope
authors.affiliation: GE Global Research, Freisinger Landstrasse 50, 85748 Garching bei München, Germany.
authors.forename: Florian
authors.initials: F
authors.lastname: Wiesinger
authors.affiliation: Department of Radiology and Biomedical Imaging, University of California San Francisco, 1700 4th St, San Francisco, CA 94158, USA.|UC Berkeley-UCSF Graduate Program in Bioengineering, University of California, Berkeley, CA 94158, USA.|University of California, 1700 4th St, San Francisco, CA 94158, USA.
authors.forename: Peder E Z
authors.initials: PE
authors.lastname: Larson
mesh_terms: D015519:Bone Density; D001859:Bone Neoplasms / Q000000981:diagnostic imaging / Q000556:secondary; D001842:Bone and Bones / Q000000981:diagnostic imaging; D005260:Female; D019788:Fluorodeoxyglucose F18; D006801:Humans; D021621:Imaging, Three-Dimensional / Q000379:methods; D016014:Linear Models; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D008875:Middle Aged; D064847:Multimodal Imaging / Q000379:methods*; D010386:Pelvic Neoplasms / Q000000981:diagnostic imaging* / Q000473:pathology; D049268:Positron-Emission Tomography / Q000379:methods*; D019275:Radiopharmaceuticals; D012983:Soft Tissue Neoplasms / Q000000981:diagnostic imaging / Q000556:secondary
chemical_list: D019275:Radiopharmaceuticals; D019788:Fluorodeoxyglucose F18
keywords: 
  MRAC
  ; Dixon MRI; TOF-PET/MRI; pelvis; zero echo-time (ZTE) MRI