MR/PET systems offer specific advantages compared with PET/CT and are expected to carve a role in oncologic imaging as well as cardiovascular and neurologic diseases, according to a review article published in the August issue of American Journal of Roentgenology.

Matthias K. Werner, MD, from the department of radiology at Eberhard-Karls-University Tübingen in Tübingen, Germany, and colleagues reviewed the advantages and applications of MR/PET.

The authors explained the two modalities complement each other and compensate for each other’s shortcomings to produce data about the patient’s disease. Specifically, PET displays metabolic and receptor information, but its resolution is notoriously low. MR provides a high-resolution partner.

In fact, Werner and colleagues wrote that MR outperforms CT in several areas. Its provides higher soft-tissue contrast, which translates into advantages for imaging malignant disease such as brain tumors, head and neck cancer, melanoma and prostate and cervical malignancies. Other potential applications include breast and liver tumors.

Another critical advantage provided by MR/PET relates to MR’s capability to detect changes that do not create large anatomic alterations. For example, bone marrow metastases in the spinal column can be detected early via MRI because of displaced bone marrow. In contrast, detection via CT hinges on destruction and sclerosis, which lag behind and may not be detected.

The authors explained MR detects parameters such as tumor vascularization and perfusion which may help further characterize malignancies. This allows physicians to evaluate the effects of treatments such as chemoembolization and selective internal radiation therapy.

Diffusion-weighted imaging may play a role in the assessment of targeted therapies that do not readily fit into the conventional Response Evaluation Criteria in Solid Tumors approach.

Finally, the combination of MR and PET provides information about the tumor’s microenvironment, and PET tracers may provide data about tumor marker or oncogene expression in the future. “MR/PET has the potential to define a patient-specific tumor in detail and could provide sufficient information to individualize tumor therapy and ensure exact monitoring of therapy effects,” wrote Werner and colleagues.

The authors outlined other possible applications, specifically early diagnosis of neurodegenerative disorders and dementia and assessment of cardiovascular disease. The hybrid modality may be helpful in repeat imaging of pediatric patients with malignancies, they continued, as it omits ionizing radiation.

Despite the solid list of advantages and applications, MR/PET is not without drawbacks. “[One] specific deficit will prevent CT from becoming obsolete anytime soon.” MR is less reliable than CT in the detection of small pulmonary lesions, and CT also holds an advantage in it capability to assess bone metastases. Finally, some patients, including those with pacemakers and implanted defibrillators, are not candidates for MR.

Peering into the future
Werner and colleagues concluded with an outline of areas for future research. Studies to examine multiparametric MRI with PET to evaluate therapeutic response are needed, they wrote. “It is crucial to develop the half of MR/PET relying on radiotracers, and this is where a lot of future efforts are needed to provide specific, reliable, and sensitive compounds for interesting new applications, especially in oncologic and neurologic imaging.”