The title of this talk is ambitious in that the idea that PET/MRI will replace PET/CT is likely an unattainable given the prevalence of the two modalities and their relative availability. But there are many circumstances where PET/MRI is currently superior to PET/CT, predominantly in tumors where MR imaging is crucial in treatment planning. This talk will review a number of the indications and uses where PET/MRI has already developed into an important imaging tool.

The two most frequently applications currently are in head and neck cancers as well as pelvic malignancies. The main reason for this is that MR imaging is critical in staging and not only is it more convenient for patients to have both studies performed as the same time, but simultaneous imaging allows easier co-localization of imaging findings. In the United States, PET/MRI of intracranial malignancies is limited with the absence of approved radiotracers. Prostate cancer PET/MRI is thriving with the development and available of numerous radiotracers targeting the Prostate Specific Membrane Antigen.

At the same time, it is important to remember that there are two significant limitations of PET/MRI compared to PET/CT. First is the difficultly that PET/MRI has in imaging small pulmonary nodules. Many cancers metastasize to the lungs, and therefore imaging small pulmonary nodules is critical for the accurate staging of patients. Second is the issues related to attenuation correction. The two commercially available PET/MRI systems do not include bone in their attenuation correction algorithms and so accurate quantification can be a significant issue.

Finally, there are circumstances where PET/MRI will be preferred due to the lower dose associated with the removal of the CT component of PET/CT - most obviously is in pediatric applications where many tumors may be cured and patients will live long enough to develop secondary cancers. This will become a more common application in the near future as more dedicated pediatric hospitals install PET/MRI systems.

Overall the future of oncologic PET/MRI is bright, and its clinical adoption remains strong at sites that have installations. Every day new applications and uses are being developed and translated into the clinic.