Clinical Positron Emission Tomography/ Magnetic Resonance Imaging Applications Gustav K. von Schulthess, MD, PhD, MD hon., Felix Pierre Kuhn, MD, MS, Philipp Kaufmann, MD, and Patrick Veit-Haibach, MD Although clinical positron emission tomography (PET)/computed tomography (CT) applications were obvious and have completely replaced PET in oncology, clinical applications of PET/ magnetic resonance (MR) are currently not clearly defined. This is due to the lack of clinical data, which is mainly because PET/MR technology is not clinically mature at this point. Open issues are technical and concern ease of obtaining PET attenuation correction maps, dealing with, for example, MR surface coil metal in the PET field-of-view and appropriate workflows leading to a cost-effective examination. All issues can be circumvented by using a shuttle- connected PET/CT-MR system, but the penalty is that simultaneous PET and MR imaging are not possible and potential motion between examinations may occur. Clinically, some systems installed worldwide start to have a reasonable bulk of clinical data. Preliminary results suggest that in oncology, PET/MR may have advantages over PET/CT in head and neck imaging. In liver imaging, more PET-positive lesions are seen on MR than on CT, but that does not mean that PET/MR is superior to PET/CT. Possibly in some settings where a contrast-enhanced PET/CT is needed to be diagnostic, PET/MR can be done without contrast media. Although PET/CT has virtually no role in brain imaging, this may be an important domain for PET/MR, particularly in dementia imaging. The role of PET/MR in the heart is as yet undefined, and much research will have to be done to elucidate this role. At this point, it is also not clear where the simultaneity afforded by a fully integrated PET/MR is really needed. Sequential data acquisition even on separate systems and consecutive software image fusion may well be appropriate. With the increasing installed base of systems, clinical data will be forthcoming and define more clearly where there is clinical value in PET/MR at an affordable price. Semin Nucl Med 43:3-10 © 2013 Elsevier Inc. All rights reserved. P ositron emission tomography (PET)/magnetic resonance (MR) has been used in experimental setups for almost 20 years. 1 The long time it has taken until this imaging modality has started to enter the clinical arena has to do with technical and practical hurdles that the clinical applications of PET/MR are facing. Technically, implementation of PET/MR faces 3 major problems. - First, the photomultiplier-based PET scanners cur- rently used do not work within or near the magnetic environment of an MR scanner, - Second, metallic objects such as surface coils used to get best MR image quality interfere with the gamma rays from PET, causing unwanted attenuation, and - Third, MR data, unlike those acquired using computed tomography (CT), are not readily usable for attenuation correction (AC). 2-4 This limits quantification of PET data, which is particularly problematic when using PET in therapy response monitoring. Clinically, PET/MR does not arrive into a “blue ocean,” but into an environment where PET/CT has proven its clinical capabilities in thousands of publications and millions of clin- ical examinations worldwide. 5-8 Because of the relative tech- nical ease of producing clinically effective PET/CT scanners, the focus shifted away from PET/MR, and in 2001, effective PET/CT scanners were introduced for clinical use. The inte- gration of PET and CT has resulted in much data that give us insights into the need for clinical integrated imaging. The data have also helped identify the strengths and weaknesses of PET/CT, the latter being hoped to be partly overcome by PET/MR. In contrast, the technical challenges in implement- ing PET/MR clearly demonstrate its major shortcomings. In- tegrated imaging with PET/CT is highly synergistic: the sum Department of Medical Radiology, University Hospital, CH-8091 Zurich, Switzerland. Address reprint requests to Gustav K. von Schulthess, Department of Medical Radiology, University Hospital, CH-8091, Zurich, Switzer- land. E-mail: gustav.vonschulthess@usz.ch 3 0001-2998/13/$-see front matter © 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1053/j.semnuclmed.2012.08.005