title: PET/MR brain imaging: evaluation of clinical UTE-based attenuation correction.
abstract: One of the greatest challenges in PET/MR imaging is that of accurate MR-based attenuation correction (AC) of the acquired PET data, which must be solved if the PET/MR modality is to reach its full potential. The aim of this study was to investigate the performance of Siemens' most recent version (VB20P) of MR-based AC of head PET data, by comparing it to CT-based AC.
  
  
  METHODS
  (18)F-FDG PET data from seven lymphoma and twelve lung cancer patients examined with a Biograph mMR PET/MR system were reconstructed with both CT-based and MR-based AC, avoiding sources of error arising when comparing PET data from different systems. The resulting images were compared quantitatively by measuring changes in mean SUV in ten different brain regions in both hemispheres, as well as the brainstem. In addition, the attenuation maps (μ maps) were compared regarding volume and localization of cranial bone.
  
  
  RESULTS
  The UTE μ maps clearly overestimate the amount of bone in the neck, while slightly underestimating the amount of bone in the cranium, and the localization of bone in the cranial region also differ from the CT μ maps. In air/tissue interfaces in the sinuses and ears, the MRAC method struggles to correctly classify the different tissues. The misclassification of tissue is most likely caused by a combination of artefacts and the insufficiency of the UTE method to accurately separate bone. Quantitatively, this results in a combination of overestimation (0.5-3.6 %) and underestimation (2.7-5.2 %) of PET activity throughout the brain, depending on the proximity to the inaccurate regions.
  
  
  CONCLUSIONS
  Our results indicate that the performance of the UTE method as implemented in VB20P is close to the theoretical maximum of such an MRAC method in the brain, while it does not perform satisfactorily in the neck or face/nasal area. Further improvement of the UTE MRAC or other available methods for more accurate segmentation of bone should be incorporated.
authors.affiliation: Department of Circulation and Medical Imaging, Norwegian University of Science and Technology (NTNU), 7489, Trondheim, Norway, lbaasheim@gmail.com.
authors.forename: Lars Birger
authors.initials: LB
authors.lastname: Aasheim
authors.forename: Anna
authors.initials: A
authors.lastname: Karlberg
authors.forename: Pål Erik
authors.initials: PE
authors.lastname: Goa
authors.forename: Asta
authors.initials: A
authors.lastname: Håberg
authors.forename: Sveinung
authors.initials: S
authors.lastname: Sørhaug
authors.forename: Unn-Merete
authors.initials: UM
authors.lastname: Fagerli
authors.forename: Live
authors.initials: L
authors.lastname: Eikenes
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000465:Algorithms; D001921:Brain / Q000000981:diagnostic imaging*; D005260:Female; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D008875:Middle Aged; D009929:Organ Size; D049268:Positron-Emission Tomography; D012886:Skull / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D013997:Time Factors; D014057:Tomography, X-Ray Computed