Melding Modalities: Emerging Clinical Indications of PET/MR

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Source: FDG-PETMR.jpg - FDG-PET/MR
Axial software-fused FDG-PET/MR image of 72-year-old man with history of head and neck squamous cell carcinoma shows avid FDG uptake in enlarged right jugulodigastric lymph node (arrow) due to malignancy.

Capitalizing on the benefits of hybrid imaging, PET/MR holds much promise for in oncology, musculoskeletal and cardiovascular imaging. In many ways, however, the cart was put before the horse. Rapid FDA approval in 2011 led to clinical adoption of the scanners without substantive scientific evidence of its effectiveness in imaging specific conditions and diseases and advantages over other modalities. So how is PET/MR helping patients in every day practice?

A complementary relationship

While research is still playing catch up, it is evident that PET/MR are a well-matched pair. “They’re complementary,” says Drew Torigian, MD, MA, of the University of Pennsylvania in Philadelphia. 

Though more studies are needed to prove the modality’s effectiveness over others for specific indications, evolving research is beginning to show PET/MR’s strengths. “At the moment, all PET/MRI research groups are gathering data which indicate that PET/MRI is clearly superior to PET/CT,” says Karsten Beiderwellen, MD, of University Hospital Essen in Germany.

Beiderwellen and colleagues conducted a study, published in the November 2013 issue of the European Journal of Radiology, assessing PET/MR’s ability to characterize liver lesions. Results revealed that the modality showcased significantly higher scores for lesion conspicuity and better performance for diagnostic confidence than PET/CT. MR’s high soft-tissue contrast makes it an ideal candidate for improved lesion depiction, explains Beiderwellen.

PET/MR scanners are especially helpful in more complex oncologic cases. “There are many attractive things to do with the complementary parts of the dual modalities,” Torigian says. “For example, cancers in the head and neck involve complicated anatomy, for which MRI is often more helpful than CT.”

Registering MRI to a PET/CT in the neck, for example, is difficult given the complex anatomy and degrees of freedom between PET and MRI acquired sequentially, says Georges El Fakhri, PhD, professor of radiology at Harvard Medical School and director of the Massachusetts General Hospital PET Core in Boston. “This is an area where pairing PET and MRI together makes perfect sense. The modality also is beneficial for lung or liver lesions that are constantly moving. If you acquire an MRI scan at the same time as PET, you can correct for motion by freezing the tumors in one position without losing any sensitivity and without any radiation dose overhead.”

Beyond imaging intricate anatomy, PET/MR also is advantageous in oncologic clinical applications because of its reduced radiation exposure. “This is particularly appealing for pediatric patients as well as adolescents,” says Beiderwellen. This sensitive demographic is spared radiation exposure and multiple sedations, a clear benefit for those more vulnerable to further medical complications.

Despite PET/MR’s emerging applications for oncology, it will not replace PET/CT, says El Fakhri. “I don’t think we’re going to see a difference in the imaging of melanomas with PET/MRI,” he says. “Lung cancer and lymphoma do extremely well with PET/CT. PET/MR won’t completely supersede the previous technology, but it will perform better in some areas. The technology is better suited for the pediatric population, as it reduces the number of sedations needed to perform the PET and MRI while halving the radiation dose.”

Musculoskeletal mapping & matters of the heart

In addition to cancer, MR also is superior to CT for imaging of the brain, spinal cord, small organs, testes and prostate gland. It is expected to a play a role in the assessment of non-neoplastic musculoskeletal disorders including infection, back pain, and bone marrow disorders.

“MR offers excellent soft-tissue contrast for the structural assessment of bone marrow, muscles, tendons, ligaments, cartilaginous structures and fat,” says Torigian.

As such, [18F]-fluorodeoxyglucose (FDG) PET/MR holds great promise for diagnostic evaluation of musculoskeletal disorders. “PET/MRI is synergistic for the detection and characterization of complications of the diabetic foot such as Charcot neuroarthropathy, osteomyelitis, and soft-tissue infection,” Torigian says. “It’s also complementary for the evaluation of various arthritides, musculoskeletal tumors, and for treatment monitoring.”

Torigian predicts that the modality also may be useful for improved evaluation of meniscal tears, cartilage disorders, and synovitis.

Cardiology is another area where clinical applications of PET/MR are becoming more relevant, as El Fakhri and colleagues noted in a report in the August 2013 issue of Current Cardiovascular Imaging Report, saying the technology shows promise in gathering detailed information about cardiac conditions.

“PET/MRI enables investigation of cardiovascular disease, including complementary assessment of wall motion abnormalities, identification of regional and segmental hypokinesis, wall thinning, scar tissue and correlation between microcirculation with delayed contrast-enhanced MRI and tissue characterization,” wrote El Fakhri et al.

Increased axial coverage of patients is another asset of PET/MR. With an effective field of 25 centimeters, compared to typically 15 to 20 centimeters with PET/CT, the modality yields a substantial increase in imaging sensitivity for all PET studies. Typically, time is an issue with MRI because it takes up to an hour to perform whole-body MR examinations. However, this is not a challenge for cardiology, explains El Fakhri, because only the heart is being imaged.

The decreased radiation exposure combined with the higher soft-tissue contrast also makes PET/MR an advantageous system to use in hepatic and musculoskeletal oncology studies.

“PET/MR will likely have a strong niche for cardiovascular applications,” adds Torigian.

As demonstrated by the myriad PET/MR clinical applications and uses in the oncologic, musculoskeletal, and cardiovascular realms, the future of the modality seems bright. However, certain challenges must be addressed and overcome before widespread adoption can really take hold.

One of the most obvious obstacles of PET/MR is cost. With a price tag up to $5 million per scanner, the technology must go through rigorous justification. 

Lack of reimbursement is another large obstacle as PET/MR exams are not currently reimbursed in the U.S. Torigian frames the situation in relative terms, explaining, “PET imaging with FDG took over 20 years to gain Medicare reimbursement.”

Radiologists and nuclear medicine physicians who can interpret both types of images are needed, as well as trained technologists who are well informed about safety considerations relevant to both PET and MRI. This is doable, but complex, according to Torigian. Furthermore, patients with relative or absolute contraindications to MRI such as pacemakers, certain metal or electronic implants, and claustrophobia may not be able to be examined with MR.

Clinical indications and scanning protocols for PET/MR are currently being defined. Implementing scanners clinically across the U.S. will take some time, says Torigian, but research is starting to increase.

“All of these challenges are surmountable if research reflects benefits to patients,” says El Fakhri.