title: Magnetic Resonance-Based Automatic Air Segmentation for Generation of Synthetic Computed Tomography Scans in the Head Region.
abstract: OBJECTIVE
  To incorporate a novel imaging sequence for robust air and tissue segmentation using ultrashort echo time (UTE) phase images and to implement an innovative synthetic CT (synCT) solution as a first step toward MR-only radiation therapy treatment planning for brain cancer.
  
  
  METHODS
  Ten brain cancer patients were scanned with a UTE/Dixon sequence and other clinical sequences on a 1.0 T open magnet with simulation capabilities. Bone-enhanced images were generated from a weighted combination of water/fat maps derived from Dixon images and inverted UTE images. Automated air segmentation was performed using unwrapped UTE phase maps. Segmentation accuracy was assessed by calculating segmentation errors (true-positive rate, false-positive rate, and Dice similarity indices using CT simulation (CT-SIM) as ground truth. The synCTs were generated using a voxel-based, weighted summation method incorporating T2, fluid attenuated inversion recovery (FLAIR), UTE1, and bone-enhanced images. Mean absolute error (MAE) characterized Hounsfield unit (HU) differences between synCT and CT-SIM. A dosimetry study was conducted, and differences were quantified using γ-analysis and dose-volume histogram analysis.
  
  
  RESULTS
  On average, true-positive rate and false-positive rate for the CT and MR-derived air masks were 80.8% ± 5.5% and 25.7% ± 6.9%, respectively. Dice similarity indices values were 0.78 ± 0.04 (range, 0.70-0.83). Full field of view MAE between synCT and CT-SIM was 147.5 ± 8.3 HU (range, 138.3-166.2 HU), with the largest errors occurring at bone-air interfaces (MAE 422.5 ± 33.4 HU for bone and 294.53 ± 90.56 HU for air). Gamma analysis revealed pass rates of 99.4% ± 0.04%, with acceptable treatment plan quality for the cohort.
  
  
  CONCLUSIONS
  A hybrid MRI phase/magnitude UTE image processing technique was introduced that significantly improved bone and air contrast in MRI. Segmented air masks and bone-enhanced images were integrated into our synCT pipeline for brain, and results agreed well with clinical CTs, thereby supporting MR-only radiation therapy treatment planning in the brain.
authors.affiliation: Department of Radiation Oncology, Henry Ford Health Systems, Detroit, Michigan.
authors.forename: Weili
authors.initials: W
authors.lastname: Zheng
authors.affiliation: Department of Radiation Oncology, Henry Ford Health Systems, Detroit, Michigan.
authors.forename: Joshua P
authors.initials: JP
authors.lastname: Kim
authors.affiliation: Philips Healthcare, Cleveland, Ohio.
authors.forename: Mo
authors.initials: M
authors.lastname: Kadbi
authors.affiliation: Department of Radiation Oncology, Henry Ford Health Systems, Detroit, Michigan.
authors.forename: Benjamin
authors.initials: B
authors.lastname: Movsas
authors.affiliation: Department of Radiation Oncology, Henry Ford Health Systems, Detroit, Michigan.
authors.forename: Indrin J
authors.initials: IJ
authors.lastname: Chetty
authors.affiliation: Department of Radiation Oncology, Henry Ford Health Systems, Detroit, Michigan. Electronic address: churst2@hfhs.org.
authors.forename: Carri K
authors.initials: CK
authors.lastname: Glide-Hurst
mesh_terms: D000388:Air; D001932:Brain Neoplasms / Q000175:diagnosis* / Q000000981:diagnostic imaging; D005188:False Negative Reactions; D005189:False Positive Reactions; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008875:Middle Aged; D064847:Multimodal Imaging / Q000379:methods*; D012886:Skull / Q000000981:diagnostic imaging*; D014057:Tomography, X-Ray Computed / Q000379:methods*