Multiplanar whole-body MR imaging outclasses bone scans

Whole-body MRI, not the traditional bone scan, should be considered the test of choice for staging skeletal metastatic disease, according to Prof. Stephen Eustace, a professor of musculoskeletal radiology at Cappagh National Orthopaedic and Mater Misericordiae Hospitals in Dublin.

A head-to-toe MRI examination is more sensitive than scintigraphy for detecting bony metastases, and it provides additional diagnostic information, he said during a presentation at the European Society for MR in Medicine and Biology meeting in Warsaw.

"Scintigraphy is a good time-honored technique, but it has deficiencies," he said. "MRI is a better technique than the bone scan, and it should replace the bone scan for this role."

Scintigraphy's main failing is its indirect nature of visualization, Eustace said. Patients are injected with a radiopharmaceutical that provides a map of osteoblastic activity. But metastases with only minimal osteoblastic response, such as highly active lytic deposits, may be missed using this method. MRI offers a way to visualize all lesions directly with good spatial and contrast resolution, and without using ionizing radiation.

Strong evidence of MRI's superior sensitivity in detecting bone metastases has been available for over a decade. Researchers held back from recommending that scintigraphy surveys be discontinued, however, until head-to-toe MRI examinations became clinically viable.

Medical practices that switch to MR-based skeletal surveys now have two different approaches from which to choose. While Philips Medical Systems has pioneered whole-body MRI with a body coil and moving tabletop, Siemens Medical Solutions is promoting surface coil scanning. Each approach has its own advantages and disadvantages, Eustace said. His own team now has access to both systems.

T1- and T2-weighted MRI demonstrates sclerotic metastases as hypointense lesions. Lytic metastases-or metastases that will become lytic-tend to be hyperintense on T2-weighted scans. Whole-body MRI can also identify the bone marrow disease multiple myeloma. Affected marrow tends to be hypointense on T1-weighted scans and becomes relatively hyperintense on fat-suppressed images, with signal characteristics similar to muscle.

"Multiple myeloma is a forgotten disease, and for some reason we have continued to stage these patients with a skeletal survey," Eustace said. "Skeletal surveys should be left in the past. The technique that should be employed is one that allows us to directly visualize the marrow, and that is whole-body MRI."

MRI may also be used to monitor treatment response in myeloma patients by tracking changes to the marrow. The role of PET in imaging myeloma patients is being investigated as well, though previous studies have suggested that FDG uptake in affected marrow is unpredictable.

"PET doesn't compete with whole-body MRI in myeloma, but studies are in progress comparing the two," he said.

Eustace is similarly unconvinced that PET can rival whole-body MRI in staging skeletal metastases. Both techniques have high sensitivity but lack specificity. The high cost of FDG makes PET expensive, results can depend on patient preparation, and radiation doses (to patients and staff) must be considered. Sclerotic lesions, common to prostate and breast cancer, are poorly detected using FDG-PET but can be identified on MRI with T1-weighted sequences. PET excels at detecting high-activity lytic deposits, but it struggles to distinguish metastatic activity from "rebound" marrow activity in patients on chemotherapy.

PET/CT is currently the best option for staging soft-tissue metastatic disease, though whole-body MRI is showing promise in this area. The emergence of diffusion-weighted techniques, which produce a PET-like map of the molecular movement of water, may tilt the balance further in MRI's favor. STIR and T1-weighted sequences may still be preferred for imaging bony metastatic deposits.

"The reality is that sclerotic metastases do not have increased diffusion and will be missed using this technique," he said.

Whole-body MRI may also gain specificity from the addition of novel contrast agents. Researchers are investigating the tagging of glucose to gadolinium-based agents, to concentrate MRI contrast in areas of high metabolic activity. Studies are also under way in the use of MRI agents tagged with monoclonal antibodies. This will require the development nanosized carriers that are sufficiently small to permeate into the extravascular space but can carry enough contrast to generate a detectable signal. Liposomes and membrane-bound nanoparticles have been suggested as possible options.

"The next frontier in this exciting technology will be the development of targeted contrast agents," Eustace said.