title: Quantification of image distortions on the Utrecht interstitial CT/MR brachytherapy applicator at 3T MRI.
abstract: OBJECTIVE
  To quantify distortions on MR images of the Utrecht interstitial CT/MR applicator at a field strength of 3T using an MRI-only method.
  
  
  METHODS
  An MR-compatible phantom suspending the applicator in water was built and imaged on a Philips Ingenia 3T MRI scanner. A map of the magnetic field (B0) was calculated from multiecho images and used to quantify the field inhomogeneity. The expected displacements of the applicator could be quantified using the measured field inhomogeneity and sequence bandwidth. Additionally, two scans were acquired using opposing readout gradients. These scans were rigidly matched and their displacement was compared with the expected displacements from the B0 map. These same methods were applied in 4 patients. By rigid matching of the scans acquired with opposing readout direction the applicator displacement due to image distortion from B0 inhomogeneity as well as patient movement and organ deformation was determined.
  
  
  RESULTS
  According to the B0 map, the displacement on the intrauterine device of the plastic brachytherapy applicator was <0.4 mm for both the phantom and patients. Displacements obtained by the opposing readout method were ≤0.8 mm for each patient with a mean ± SD over the patients of 0.3 ± 0.1 mm.
  
  
  CONCLUSIONS
  The results of our study indicate that the B0 method agrees with the opposing readout method. Displacements caused by magnetic field inhomogeneity on 3T MRI were small compared with displacements due to patient movement and organ deformation.
authors.affiliation: Department of Radiation Oncology, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands. Electronic address: l.e.vanheerden@amc.uva.nl.
authors.forename: L E
authors.initials: LE
authors.lastname: van Heerden
authors.affiliation: Department of Radiation Oncology, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands; Department of Radiology, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands.
authors.forename: O J
authors.initials: OJ
authors.lastname: Gurney-Champion
authors.affiliation: Department of Radiation Oncology, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands.
authors.forename: Z
authors.initials: Z
authors.lastname: van Kesteren
authors.affiliation: Department of Radiation Oncology, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands.
authors.forename: A C
authors.initials: AC
authors.lastname: Houweling
authors.affiliation: Department of Radiation Oncology, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands.
authors.forename: C
authors.initials: C
authors.lastname: Koedooder
authors.affiliation: Department of Radiation Oncology, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands.
authors.forename: C R N
authors.initials: CR
authors.lastname: Rasch
authors.affiliation: Department of Radiation Oncology, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands.
authors.forename: B R
authors.initials: BR
authors.lastname: Pieters
authors.affiliation: Department of Radiation Oncology, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands.
authors.forename: A
authors.initials: A
authors.lastname: Bel
mesh_terms: D001918:Brachytherapy / Q000295:instrumentation; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging; D019047:Phantoms, Imaging; D061089:Radiotherapy, Image-Guided; D014057:Tomography, X-Ray Computed; D035501:Uncertainty; D002583:Uterine Cervical Neoplasms / Q000532:radiotherapy*
keywords: Applicator reconstruction; Brachytherapy; MRI; Uncertainties; Uterine cervical neoplasm