title: Visibility and artifacts of gold fiducial markers used for image guided radiation therapy of pancreatic cancer on MRI.
abstract: OBJECTIVE
  In radiation therapy of pancreatic cancer, tumor alignment prior to each treatment fraction is improved when intratumoral gold fiducial markers (from here onwards: markers), which are visible on computed tomography (CT) and cone beam CT, are used. Visibility of these markers on magnetic resonance imaging (MRI) might improve image registration between CT and magnetic resonance (MR) images for tumor delineation purposes. However, concomitant image artifacts induced by markers are undesirable. The extent of visibility and artifact size depend on MRI-sequence parameters. The authors' goal was to determine for various markers their potential to be visible and to generate artifacts, using measures that are independent of the MRI-sequence parameters.
  
  
  METHODS
  The authors selected ten different markers suitable for endoscopic placement in the pancreas and placed them into a phantom. The markers varied in diameter (0.28-0.6 mm), shape, and iron content (0%-0.5%). For each marker, the authors calculated T2 (∗)-maps and ΔB0-maps using MRI measurements. A decrease in relaxation time T2 (∗) can cause signal voids, associated with visibility, while a change in the magnetic field B0 can cause signal shifts, which are associated with artifacts. These shifts inhibit accurate tumor delineation. As a measure for potential visibility, the authors used the volume of low T2 (∗), i.e., the volume for which T2 (∗) differed from the background by >15 ms. As a measure for potential artifacts, the authors used the volume for which |ΔB0| > 9.4 × 10(-8) T (4 Hz). To test whether there is a correlation between visibility and artifact size, the authors calculated the Spearman's correlation coefficient (Rs) between the volume of low T2 (∗) and the volume of high |ΔB0|. The authors compared the maps with images obtained using a clinical MR-sequence. Finally, for the best visible marker as well as the marker that showed the smallest artifact, the authors compared the phantom data with in vivo MR-images in four pancreatic cancer patients.
  
  
  RESULTS
  The authors found a strong correlation (Rs = 1.00, p < 0.01) between the volume of low T2 (∗) and the volume with high |ΔB0|. Visibility in clinical MR-images increased with lower T2 (∗). Signal shift artifacts became worse for markers with high |ΔB0|. The marker that was best visible in the phantom, a folded marker with 0.5% iron content, was also visible in vivo, but showed artifacts on diffusion weighted images. The marker with the smallest artifact in the phantom, a small, stretched, ironless marker, was indiscernible on in vivo MR-images.
  
  
  CONCLUSIONS
  Changes in T2 (∗) and ΔB0 are sequence-independent measures for potential visibility and artifact size, respectively. Improved visibility of markers correlates strongly to signal shift artifacts; therefore, marker choice will depend on the clinical purpose. When visibility of the markers is most important, markers that contain iron are optimal, preferably in a folded configuration. For artifact sensitive imaging, small ironless markers are best, preferably in a stretched configuration.
authors.affiliation: Department of Radiation Oncology, Academic Medical Center, University of Amsterdam, Meibergdreef 9, 1105 AZ, Amsterdam, The Netherlands and Department of Radiology, Academic Medical Center, University of Amsterdam, Meibergdreef 9, 1105 AZ, Amsterdam, The Netherlands.
authors.forename: Oliver J
authors.initials: OJ
authors.lastname: Gurney-Champion
authors.affiliation: Department of Radiation Oncology, Academic Medical Center, University of Amsterdam, Meibergdreef 9, 1105 AZ, Amsterdam, The Netherlands.
authors.forename: Eelco
authors.initials: E
authors.lastname: Lens
authors.affiliation: Department of Radiation Oncology, Academic Medical Center, University of Amsterdam, Meibergdreef 9, 1105 AZ, Amsterdam, The Netherlands.
authors.forename: Astrid
authors.initials: A
authors.lastname: van der Horst
authors.affiliation: Department of Radiation Oncology, Academic Medical Center, University of Amsterdam, Meibergdreef 9, 1105 AZ, Amsterdam, The Netherlands.
authors.forename: Antonetta C
authors.initials: AC
authors.lastname: Houweling
authors.affiliation: Department of Medical Oncology, Academic Medical Center, University of Amsterdam, Meibergdreef 9, 1105 AZ, Amsterdam, The Netherlands and Laboratory for Experimental Oncology and Radiobiology, Academic Medical Center, University of Amsterdam, Meibergdreef 9, 1105 AZ, Amsterdam, The Netherlands.
authors.forename: Remy
authors.initials: R
authors.lastname: Klaassen
authors.affiliation: Department of Gastroenterology & Hepatology, Academic Medical Center, University of Amsterdam, Meibergdreef 9, 1105 AZ, Amsterdam, The Netherlands.
authors.forename: Jeanin E
authors.initials: JE
authors.lastname: van Hooft
authors.affiliation: Department of Radiology, Academic Medical Center, University of Amsterdam, Meibergdreef 9, 1105 AZ, Amsterdam, The Netherlands.
authors.forename: Jaap
authors.initials: J
authors.lastname: Stoker
authors.affiliation: Department of Radiation Oncology, Academic Medical Center, University of Amsterdam, Meibergdreef 9, 1105 AZ, Amsterdam, The Netherlands.
authors.forename: Geertjan
authors.initials: G
authors.lastname: van Tienhoven
authors.affiliation: Department of Radiology, Academic Medical Center, University of Amsterdam, Meibergdreef 9, 1105 AZ, Amsterdam, The Netherlands.
authors.forename: Aart J
authors.initials: AJ
authors.lastname: Nederveen
authors.affiliation: Department of Radiation Oncology, Academic Medical Center, University of Amsterdam, Meibergdreef 9, 1105 AZ, Amsterdam, The Netherlands.
authors.forename: Arjan
authors.initials: A
authors.lastname: Bel
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D016477:Artifacts; D054893:Cone-Beam Computed Tomography / Q000295:instrumentation / Q000379:methods; D005260:Female; D057918:Fiducial Markers; D006046:Gold; D006801:Humans; D007501:Iron; D008279:Magnetic Resonance Imaging / Q000295:instrumentation / Q000379:methods*; D008297:Male; D008875:Middle Aged; D010179:Pancreas / Q000473:pathology; D010190:Pancreatic Neoplasms / Q000473:pathology* / Q000532:radiotherapy*; D019047:Phantoms, Imaging; D061089:Radiotherapy, Image-Guided / Q000295:instrumentation / Q000379:methods*
chemical_list: D006046:Gold; D007501:Iron