title: Internal fiducial markers and susceptibility effects in MRI-simulation and measurement of spatial accuracy.
abstract: BACKGROUND
  It is well-known that magnetic resonance imaging (MRI) is preferable to computed tomography (CT) in radiotherapy target delineation. To benefit from this, there are two options available: transferring the MRI delineated target volume to the planning CT or performing the treatment planning directly on the MRI study. A precondition for excluding the CT study is the possibility to define internal structures visible on both the planning MRI and on the images used to position the patient at treatment. In prostate cancer radiotherapy, internal gold markers are commonly used, and they are visible on CT, MRI, x-ray, and portal images. The depiction of the markers in MRI are, however, dependent on their shape and orientation relative the main magnetic field because of susceptibility effects. In the present work, these effects are investigated and quantified using both simulations and phantom measurements.
  
  
  METHODS
  Software that simulated the magnetic field distortions around user defined geometries of variable susceptibilities was constructed. These magnetic field perturbation maps were then reconstructed to images that were evaluated. The simulation software was validated through phantom measurements of four commercially available gold markers of different shapes and one in-house gold marker.
  
  
  RESULTS
  Both simulations and phantom measurements revealed small position deviations of the imaged marker positions relative the actual marker positions (<1 mm).
  
  
  CONCLUSIONS
  Cylindrical gold markers can be used as internal fiducial markers in MRI.
authors.affiliation: Radiation Physics Section, Department of Radiation Sciences, Umeå University, Umeå, Sweden. joakim.jonsson@radfys.umu.se
authors.forename: Joakim H
authors.initials: JH
authors.lastname: Jonsson
authors.forename: Anders
authors.initials: A
authors.lastname: Garpebring
authors.forename: Magnus G
authors.initials: MG
authors.lastname: Karlsson
authors.forename: Tufve
authors.initials: T
authors.lastname: Nyholm
mesh_terms: D016477:Artifacts; D057918:Fiducial Markers; D006046:Gold; D006801:Humans; D060526:Magnetic Fields; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D009068:Movement; D056888:Patient Positioning; D019047:Phantoms, Imaging; D011467:Prostate / Q000000981:diagnostic imaging / Q000473:pathology*; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology* / Q000532:radiotherapy; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D061089:Radiotherapy, Image-Guided / Q000379:methods; D014057:Tomography, X-Ray Computed / Q000379:methods
chemical_list: D006046:Gold