Whole-body MR imaging outclasses bone scans

Whole-body MRI should now be regarded as the test of choice for staging skeletal metastatic disease, and not the traditional bone scan. A head-to-toe MRI exam is more sensitive than scintigraphy for detecting bony metastases, and provides additional diagnostic information, according to Dr. Stephen Eustace, a professor of musculoskeletal radiology at Cappagh National Orthopaedic and Mater Misericordiae Hospitals in Dublin.

Scintigraphy's main failing is the indirect nature of its visualization, Eustace told delegates at the annual meeting of the European Society for MR in Medicine and Biology held in Warsaw. 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, 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 exams became clinically viable.

Practices switching to MR-based skeletal surveys now have two approaches to choose from. While Philips Medical Systems has pioneered whole-body MRI with a body coil and a 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 MR images demonstrate 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 those of 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 a technique 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, he said.

PET/CT is currently the best option for staging soft-tissue metastatic disease, though whole-body MRI is also 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. But sclerotic metastases do not have increased diffusion and will be missed using this technique, Eustace said.