title: Toward magnetic resonance-only simulation: segmentation of bone in MR for radiation therapy verification of the head.
abstract: OBJECTIVE
  To develop a practical method to localize bones in magnetic resonance (MR) images, to create "computed tomography-like" MR images (ctMRI) that could be used for radiation therapy verification, and to generate MR-based digitally reconstructed radiographs (DRR).
  
  
  METHODS
  Using T1-weighted MR images, an air mask was derived from the manual contouring of all airways within the head and neck region using axial images at 6 anatomic levels. Compact bone, spongy bone, and soft tissue masks were then automatically generated using the statistical data derived from MR intensities and the air mask. ctMRI were then generated by mapping the MR intensities of the voxels within these masks into the CT number ranges of corresponding tissues. MR-based DRRs created from ctMRI were quantitatively evaluated using the co-registered MR and CT head images of 20 stereotactic radiosurgery patients. Ten anatomical points, positioned on the skull segmented using a threshold of 300 HU, in CT and ctMRI, were used to determine the differences in distance between MR-based DRRs and CT-based DRRs, and to evaluate the geometric accuracy of ctMRI and MR-based DRRs.
  
  
  RESULTS
  The bony structures were identified on ctMRI and were visible in the MR-based DRRs. From the 20 patient cases, the mean geometric difference and standard deviation between the 10 anatomical points on MR-based and CT-based DRRs was -0.05 ± 0.85 mm, respectively. This included uncertainty in image fusion. The maximum distance difference was 1.88 mm.
  
  
  CONCLUSIONS
  A practical method was developed to segment bone from MR images. The ctMRI created can be used for radiation treatment verification when MR-only simulation is performed. MR-based DRRs can be used in place of CT-based DRRs.
authors.affiliation: Department of Medical Physics, Odette Cancer Centre, Sunnybrook Health Science Center, Toronto, ON, Canada.
authors.forename: Huan
authors.initials: H
authors.lastname: Yu
authors.affiliation: Department of Medical Biophysics, University of Toronto, Toronto, ON, Canada; Medical Imaging, Sunnybrook Health Science Center, Toronto, ON, Canada; Department of Medical Imaging, University of Toronto, Toronto, ON, Canada.
authors.forename: Curtis
authors.initials: C
authors.lastname: Caldwell
authors.affiliation: Department of Radiation Oncology, Odette Cancer Centre, Sunnybrook Health Science Center, Toronto, ON, Canada; Department of Radiation Oncology, University of Toronto, Toronto, ON, Canada.
authors.forename: Judith
authors.initials: J
authors.lastname: Balogh
authors.affiliation: Department of Medical Physics, Odette Cancer Centre, Sunnybrook Health Science Center, Toronto, ON, Canada; Department of Radiation Oncology, University of Toronto, Toronto, ON, Canada. Electronic address: kathy.mah@sunnybrook.ca.
authors.forename: Katherine
authors.initials: K
authors.lastname: Mah
mesh_terms: D000388:Air; D059925:Anatomic Landmarks / Q000033:anatomy & histology* / Q000000981:diagnostic imaging; D003238:Connective Tissue / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D004424:Ear Canal / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D006257:Head / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D006801:Humans; D007090:Image Interpretation, Computer-Assisted; D008279:Magnetic Resonance Imaging / Q000379:methods*; D009055:Mouth / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D009305:Nasopharynx / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D009333:Neck / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D009666:Nose / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D010256:Paranasal Sinuses / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D012886:Skull / Q000033:anatomy & histology* / Q000000981:diagnostic imaging; D014057:Tomography, X-Ray Computed / Q000379:methods