title: Characterization of the onboard imaging unit for the first clinical magnetic resonance image guided radiation therapy system.
abstract: OBJECTIVE
  To characterize the performance of the onboard imaging unit for the first clinical magnetic resonance image guided radiation therapy (MR-IGRT) system.
  
  
  METHODS
  The imaging performance characterization included four components: ACR (the American College of Radiology) phantom test, spatial integrity, coil signal to noise ratio (SNR) and uniformity, and magnetic field homogeneity. The ACR phantom test was performed in accordance with the ACR phantom test guidance. The spatial integrity test was evaluated using a 40.8 × 40.8 × 40.8 cm(3) spatial integrity phantom. MR and computed tomography (CT) images of the phantom were acquired and coregistered. Objects were identified around the surfaces of 20 and 35 cm diameters of spherical volume (DSVs) on both the MR and CT images. Geometric distortion was quantified using deviation in object location between the MR and CT images. The coil SNR test was performed according to the national electrical manufacturers association (NEMA) standards MS-1 and MS-9. The magnetic field homogeneity test was measured using field camera and spectral peak methods.
  
  
  RESULTS
  For the ACR tests, the slice position error was less than 0.10 cm, the slice thickness error was less than 0.05 cm, the resolved high-contrast spatial resolution was 0.09 cm, the resolved low-contrast spokes were more than 25, the image intensity uniformity was above 93%, and the percentage ghosting was less than 0.22%. All were within the ACR recommended specifications. The maximum geometric distortions within the 20 and 35 cm DSVs were 0.10 and 0.18 cm for high spatial resolution three-dimensional images and 0.08 and 0.20 cm for high temporal resolution two dimensional cine images based on the distance-to-phantom-center method. The average SNR was 12.0 for the body coil, 42.9 for the combined torso coil, and 44.0 for the combined head and neck coil. Magnetic field homogeneities at gantry angles of 0°, 30°, 60°, 90°, and 120° were 23.55, 20.43, 18.76, 19.11, and 22.22 ppm, respectively, using the field camera method over the 45 cm DSV.
  
  
  CONCLUSIONS
  The onboard imaging unit of the first commercial MR-IGRT system meets ACR, NEMA, and vendor specifications.
authors.affiliation: Department of Radiation Oncology, Washington University School of Medicine, St. Louis, Missouri 63110 and Department of Radiation Oncology, Mayo Clinic in Arizona, Phoenix, Arizona 85054.
authors.forename: Yanle
authors.initials: Y
authors.lastname: Hu
authors.affiliation: Department of Radiation Oncology, Washington University School of Medicine, St. Louis, Missouri 63110.
authors.forename: Leith
authors.initials: L
authors.lastname: Rankine
authors.affiliation: Department of Radiation Oncology, Washington University School of Medicine, St. Louis, Missouri 63110.
authors.forename: Olga L
authors.initials: OL
authors.lastname: Green
authors.affiliation: Department of Radiation Oncology, Washington University School of Medicine, St. Louis, Missouri 63110.
authors.forename: Rojano
authors.initials: R
authors.lastname: Kashani
authors.affiliation: Department of Radiation Oncology, Washington University School of Medicine, St. Louis, Missouri 63110.
authors.forename: H Harold
authors.initials: HH
authors.lastname: Li
authors.affiliation: Department of Radiation Oncology, Washington University School of Medicine, St. Louis, Missouri 63110.
authors.forename: Hua
authors.initials: H
authors.lastname: Li
authors.affiliation: ViewRay, Inc., Oakwood Village, Ohio 44146.
authors.forename: Roger
authors.initials: R
authors.lastname: Nana
authors.affiliation: Department of Radiation Oncology, Washington University School of Medicine, St. Louis, Missouri 63110.
authors.forename: Vivian
authors.initials: V
authors.lastname: Rodriguez
authors.affiliation: Department of Radiation Oncology, Washington University School of Medicine, St. Louis, Missouri 63110.
authors.forename: Lakshmi
authors.initials: L
authors.lastname: Santanam
authors.affiliation: ViewRay, Inc., Oakwood Village, Ohio 44146.
authors.forename: Shmaryu
authors.initials: S
authors.lastname: Shvartsman
authors.affiliation: ViewRay, Inc., Oakwood Village, Ohio 44146.
authors.forename: James
authors.initials: J
authors.lastname: Victoria
authors.affiliation: Department of Radiation Oncology, Washington University School of Medicine, St. Louis, Missouri 63110.
authors.forename: H Omar
authors.initials: HO
authors.lastname: Wooten
authors.affiliation: ViewRay, Inc., Oakwood Village, Ohio 44146.
authors.forename: James F
authors.initials: JF
authors.lastname: Dempsey
authors.affiliation: Department of Radiation Oncology, Washington University School of Medicine, St. Louis, Missouri 63110.
authors.forename: Sasa
authors.initials: S
authors.lastname: Mutic
mesh_terms: D006801:Humans; D060526:Magnetic Fields; D008279:Magnetic Resonance Imaging / Q000379:methods*; D019047:Phantoms, Imaging; D011786:Quality Control; D061089:Radiotherapy, Image-Guided / Q000379:methods*; D059629:Signal-To-Noise Ratio