title: Towards quantitative PET/MRI: a review of MR-based attenuation correction techniques.
abstract: BACKGROUND
  Positron emission tomography (PET) is a fully quantitative technology for imaging metabolic pathways and dynamic processes in vivo. Attenuation correction of raw PET data is a prerequisite for quantification and is typically based on separate transmission measurements. In PET/CT attenuation correction, however, is performed routinely based on the available CT transmission data.
  
  
  OBJECTIVE
  Recently, combined PET/magnetic resonance (MR) has been proposed as a viable alternative to PET/CT. Current concepts of PET/MRI do not include CT-like transmission sources and, therefore, alternative methods of PET attenuation correction must be found. This article reviews existing approaches to MR-based attenuation correction (MR-AC). Most groups have proposed MR-AC algorithms for brain PET studies and more recently also for torso PET/MR imaging. Most MR-AC strategies require the use of complementary MR and transmission images, or morphology templates generated from transmission images. We review and discuss these algorithms and point out challenges for using MR-AC in clinical routine.
  
  
  CONCLUSIONS
  MR-AC is work-in-progress with potentially promising results from a template-based approach applicable to both brain and torso imaging. While efforts are ongoing in making clinically viable MR-AC fully automatic, further studies are required to realize the potential benefits of MR-based motion compensation and partial volume correction of the PET data.
authors.affiliation: Max Planck Institute for Biological Cybernetics, Spemannstrasse 38, 72076, Tuebingen, Germany. matthias.hofmann@tuebingen.mpg.de
authors.forename: Matthias
authors.initials: M
authors.lastname: Hofmann
authors.forename: Bernd
authors.initials: B
authors.lastname: Pichler
authors.forename: Bernhard
authors.initials: B
authors.lastname: Schölkopf
authors.forename: Thomas
authors.initials: T
authors.lastname: Beyer
mesh_terms: D000818:Animals; D001921:Brain / Q000000981:diagnostic imaging; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D049268:Positron-Emission Tomography / Q000379:methods*; D014057:Tomography, X-Ray Computed; D051598:Whole Body Imaging