title: A strategy for multimodal deformable image registration to integrate PET/MR into radiotherapy treatment planning.
abstract: BACKGROUND
  Combined positron emission tomography (PET)/magnetic resonance imaging (MRI) is highly promising for biologically individualized radiotherapy (RT). Hence, the purpose of this work was to develop an accurate and robust registration strategy to integrate combined PET/MR data into RT treatment planning. Material and methods. Eight patient datasets consisting of an FDG PET/computed tomography (CT) and a subsequently acquired PET/MR of the head and neck (HN) region were available. Registration strategies were developed based on CT and MR data only, whereas the PET components were fused with the resulting deformation field. Following a rigid registration, deformable registration was performed with a transform parametrized by B-splines. Three different optimization metrics were investigated: global mutual information (GMI), GMI combined with a bending energy penalty (BEP) for regularization (GMI+ BEP) and localized mutual information with BEP (LMI+ BEP). Different quantitative registration quality measures were developed, including volumetric overlap and mean distance measures for structures segmented on CT and MR as well as anatomical landmark distances. Moreover, the local registration quality in the tumor region was assessed by the normalized cross correlation (NCC) of the two PET datasets.
  
  
  RESULTS
  LMI+ BEP yielded the most robust and accurate registration results. For GMI, GMI+ BEP and LMI+ BEP, mean landmark distances (standard deviations) were 23.9 mm (15.5 mm), 4.8 mm (4.0 mm) and 3.0 mm (1.0 mm), and mean NCC values (standard deviations) were 0.29 (0.29), 0.84 (0.14) and 0.88 (0.06), respectively.
  
  
  CONCLUSIONS
  Accurate and robust multimodal deformable image registration of CT and MR in the HN region can be performed using a B-spline parametrized transform and LMI+ BEP as optimization metric. With this strategy, biologically individualized RT based on combined PET/MRI in terms of dose painting is possible.
authors.affiliation: Section for Biomedical Physics, Department of Radiation Oncology, University Hospital Tübingen , Tübingen , Germany.
authors.forename: Sara
authors.initials: S
authors.lastname: Leibfarth
authors.forename: David
authors.initials: D
authors.lastname: Mönnich
authors.forename: Stefan
authors.initials: S
authors.lastname: Welz
authors.forename: Christine
authors.initials: C
authors.lastname: Siegel
authors.forename: Nina
authors.initials: N
authors.lastname: Schwenzer
authors.forename: Holger
authors.initials: H
authors.lastname: Schmidt
authors.forename: Daniel
authors.initials: D
authors.lastname: Zips
authors.forename: Daniela
authors.initials: D
authors.lastname: Thorwarth
mesh_terms: D000465:Algorithms; D019788:Fluorodeoxyglucose F18; D006258:Head and Neck Neoplasms / Q000175:diagnosis* / Q000532:radiotherapy; D006801:Humans; D008279:Magnetic Resonance Imaging; D064847:Multimodal Imaging; D049268:Positron-Emission Tomography; D011379:Prognosis; D019275:Radiopharmaceuticals; D011880:Radiotherapy Planning, Computer-Assisted; D061089:Radiotherapy, Image-Guided; D014057:Tomography, X-Ray Computed
chemical_list: D019275:Radiopharmaceuticals; D019788:Fluorodeoxyglucose F18