MR/PET or PET/MRI: does it matter?

After the very successful clinical introduction of combined PET/CT imaging a decade ago, a hardware combination of PET and MR is following suit. Today, three different approaches towards integrated PET/MR have been proposed: (1) a triple-modality system with a 3T MRI and a time-of-flight PET/CT installed in adjacent rooms, (2) a tandem system with a 3T MRI and a time-of-flight PET/CT in a co-planar installation with a joint patient handling system, and (3) a fully-integrated system with a whole-body PET system mounted inside a 3T MRI system. This special issue of MAGMA brings together contributions from key experts in the field of PET/MR, PET/CT and CT. The various papers share the author's perspectives on the state-of-the-art PET/MR imaging with any of the three approaches mentioned above. In addition to several reviews discussing advantages and challenges of combining PET and MRI for clinical diagnostics, first clinical data are also presented. We expect this special issue to nurture future improvements in hardware, clinical protocols, and efficient post-processing strategies to further assess the diagnostic value of combined PET/MR imaging. It remains to be seen whether a so-called "killer application" for PET/MRI will surface. In that case PET/MR is likely to excel in pre-clinical and selected research applications for now. This special issue helps the readers to stay on track of this exciting development.