title: Shine-Through in PET/MR Imaging: Effects of the Magnetic Field on Positron Range and Subsequent Image Artifacts.
abstract: Simultaneous PET/MR imaging is an emerging hybrid modality for clinical and preclinical imaging. The static magnetic field of the MR imaging device affects the trajectory of the positrons emitted by the PET radioisotopes. This effect translates into an improvement of the spatial resolution in transaxial images. However, because of the elongation of the positron range distribution along the magnetic field, the axial resolution worsens and shine-through artifacts may appear. These artifacts can lead to misinterpretation and overstaging. The aim of this work was to study the relevance of this effect.
  
  
  METHODS
  Measurements were performed in a 3-tesla PET/MR scanner. A 1-cm(2) piece of paper, soaked with a radioisotope and placed in air, was scanned, and the magnitude of the shine-through was quantified from the PET images for various radioisotopes. Additionally, PET/MR and PET/CT images of the lungs and the larynx with trachea of a deceased swine were obtained after injecting a mixture of NiSO4 and (68)Ga to simulate hot tumor lesions.
  
  
  RESULTS
  For the radioactive paper, shine-through artifacts appeared in the location of the acrylic glass backplane, located 3 cm from the source in the axial direction. The ratio between the activity of the shine-through and the activity reconstructed in the original location ranged from 0.9 ((18)F) to 5.7 ((68)Ga). For the larynx-with-trachea images, the magnitude of the artifacts depended on the organ orientation with respect to the magnetic field. The shine-through activity could reach 46% of the reconstructed activity (larynx lesion). The lesion within the trachea produced 2 artifacts, symmetrically aligned with the magnetic field and characterized by artifact-to-lesion volume-of-interest ratios ranging from 21% to 30%.
  
  
  CONCLUSIONS
  In simultaneous PET/MR imaging, the effect of the magnetic field on positrons may cause severe artifacts in the PET image when the lesions are close to air cavities and high-energy radioisotopes are used. For accurate staging and interpretation, this effect needs to be recognized and adequate compensation techniques should be developed.
authors.affiliation: Werner Siemens Imaging Center, Department of Preclinical Imaging and Radiopharmacy, Eberhard Karls University, Tübingen, Germany armin.kolb@med.uni-tuebingen.de.
authors.forename: Armin
authors.initials: A
authors.lastname: Kolb
authors.affiliation: Department of Radiology, Eberhard Karls University, Tübingen, Germany.
authors.forename: Alexander W
authors.initials: AW
authors.lastname: Sauter
authors.affiliation: Siemens Medical Solutions, Knoxville, Tennessee; and.
authors.forename: Lars
authors.initials: L
authors.lastname: Eriksson
authors.affiliation: Molecular Imaging Program at Stanford University, Stanford, California.
authors.forename: Arne
authors.initials: A
authors.lastname: Vandenbrouke
authors.affiliation: Werner Siemens Imaging Center, Department of Preclinical Imaging and Radiopharmacy, Eberhard Karls University, Tübingen, Germany.
authors.forename: Chih C
authors.initials: CC
authors.lastname: Liu
authors.affiliation: Molecular Imaging Program at Stanford University, Stanford, California.
authors.forename: Craig
authors.initials: C
authors.lastname: Levin
authors.affiliation: Werner Siemens Imaging Center, Department of Preclinical Imaging and Radiopharmacy, Eberhard Karls University, Tübingen, Germany.
authors.forename: Bernd J
authors.initials: BJ
authors.lastname: Pichler
authors.affiliation: Werner Siemens Imaging Center, Department of Preclinical Imaging and Radiopharmacy, Eberhard Karls University, Tübingen, Germany.
authors.forename: Magdalena
authors.initials: M
authors.lastname: Rafecas
mesh_terms: D000818:Animals; D016477:Artifacts; D005710:Gallium Radioisotopes / Q000737:chemistry; D007091:Image Processing, Computer-Assisted; D007830:Larynx / Q000000981:diagnostic imaging / Q000473:pathology; D008168:Lung / Q000000981:diagnostic imaging / Q000473:pathology; D060526:Magnetic Fields; D008279:Magnetic Resonance Imaging; D064847:Multimodal Imaging; D015282:Octreotide / Q000031:analogs & derivatives; D009942:Organometallic Compounds; D019047:Phantoms, Imaging; D049268:Positron-Emission Tomography; D011868:Radioisotopes; D013552:Swine; D014132:Trachea / Q000000981:diagnostic imaging / Q000473:pathology
chemical_list: C499142:Ga(III)-DOTATOC; D005710:Gallium Radioisotopes; D009942:Organometallic Compounds; D011868:Radioisotopes; D015282:Octreotide
keywords: PET/MRI; artifact; high-energy positrons; positron-range; shine-through