PET/MR Imaging: A Critical Appraisal

Despite considerable excitement about the potential of PET/MR imaging for the detection, staging, and functional characterization of cancer, this new technology is evolving significantly more slowly than PET/CT. This slower evolution is due partly to ongoing technologic challenges (e.g., accurate attenuation correction of PET images) but also to the complex logistics of combining a whole-body PET scan with whole-body or organ-specific MR imaging. Most PET/MR imaging research published so far has focused on cancer staging and restaging in patients undergoing ¹⁸F-FDG PET/CT as the standard of care. These studies have demonstrated the feasibility of clinical ¹⁸F-FDG PET/MR imaging but so far have not shown substantial improvements in staging. This situation may not be unexpected in view of the fact that MR imaging has not replaced CT for staging of the malignancies for which ¹⁸F-FDG PET/CT is most commonly used. Given the widespread concerns about rising health care costs in general and the costs of advanced imaging techniques in particular, establishing ¹⁸F-FDG PET/MR imaging for whole-body cancer staging may be challenging because it requires more expensive equipment and longer acquisition times than ¹⁸F-FDG PET/CT. An alternative approach to developing clinical PET/MR imaging is to study how stand-alone, organ-specific MR imaging can be improved by PET/MR imaging. Unfortunately, however, ¹⁸F-FDG PET has significant limitations for the tumors that are most commonly studied with MR imaging (brain, liver, pancreatic, and prostate tumors). However, this situation may change with the development of new radiopharmaceuticals, such as prostate-specific membrane or gastrin-releasing peptide receptor ligands for the imaging of prostate cancer. In conclusion, PET/MR imaging has many potential advantages over PET/CT (lower radiation exposure, higher soft-tissue contrast, and multiparametric imaging). Realizing this potential in clinics likely will require new radiopharmaceuticals and applications other than whole-body cancer staging.