Densitometers offer visible proof of osteoporosis risk

January 17, 2006

New technology now allows bone densitometers to make a more visible contribution to evaluating risk of osteoporosis. Scans that once generated only quantitative measurements can be postprocessed into low-resolution images of the spine. This capability is designed to help physicians spot deformities in vertebral bodies that might otherwise go unnoticed.

New technology now allows bone densitometers to make a more visible contribution to evaluating risk of osteoporosis. Scans that once generated only quantitative measurements can be postprocessed into low-resolution images of the spine. This capability is designed to help physicians spot deformities in vertebral bodies that might otherwise go unnoticed.

"Because so many vertebral fractures are not appreciated clinically, having the ability to detect these fractures easily, quickly, and at low radiation exposure provides one-stop shopping," said Dr. Neil Binkley, an associate professor of geriatrics and endocrinology at the University of Wisconsin, Madison.

Hologic's Discovery bone densitometer offers an imaging component, called instant vertebral assessment (IVA), as part of its radiologic vertebral assessment. IVA, introduced as a feature on Discovery in 2003, was upgraded last year to IVA HD (high definition).

GE Lunar offers a similar imaging feature on its bone densitometers. The technology, called dual-energy vertebral assessment, is built into its Prodigy Advance DXA. The soft-tissue subtraction technique yields high-contrast images of the thoracic and lumbar spine as part of a tool Lunar has dubbed vertebral fracture assessment (VFA).

The use of VFA and IVA is buoyed by a CPT code for vertebral fracture assessment (76077), which became effective about a year ago. Obtaining reimbursement can still be an uphill battle, however. Third-party payers in some geographic areas of the U.S. are paying for such assessments, while others are not.

The International Society for Clinical Densitometry is trying to homogenize coverage, according to Binkley, who serves as president-elect of the nonprofit organization.

The ISCD has identified indications for using the new technology. The ability to visualize bone mineral density data is a handy adjunct to traditional measurements, but it should not be used in every case, Binkley said.

"We need to guard against the indication for VFA being the presence of a spine," he said.

Two key indicators include significant loss in height and the use of corticosteroids, which increase the risk of fracture. A visual assessment of the vertebrae adds a new dimension in assessing these patients.

In some cases, the deformities apparent on VFA and IVA may clearly indicate the presence of fractures. These patients may be in the greatest need of aggressive care, according to Dr. Bradford Richmond, a musculoskeletal radiologist who is also director of bone densitometry at the Cleveland Clinic Osteoporosis and Metabolic Bone Disease Center.

"In reality, the more vertebral fractures you have, the more likely you are to continue fracturing, not just in the spine but elsewhere," he said.

The staff at the Cleveland bone disease center operates 10 BMD systems from both Hologic and GE Lunar. They perform about 16,000 exams on the devices annually.

Richmond estimates that about 45% of BMD exams in the U.S. are currently performed by radiologists. Nonradiologists stand to benefit the most from VFA and IVA. Whereas radiologists have ready access to radiography, family practitioners, endocrinologists, and rheumatologists may have to refer patients suspected of osteoporosis to radiologists for spinal imaging.

As a result, BMD visualizations might be construed as a threat to referrals to radiologists, but Binkley contends they are not. The low-resolution images obtained using a bone densitometer lack the clarity needed to diagnose fractures.

"VFA is not a replacement for spine radiography," he said. "I look at it as a low-radiation exposure tool that enhances patient care."

Just how much it enhances may vary. The image quality of bone densitometers can be influenced by the make of the machine and its power output, which can range from 30 to 300 mR, Richmond noted.

And just like conventional radiographs, these images may be affected by patient-specific issues. Obese patients present special challenges to low-power machines. Ribs and bowel gas can complicate interpretations.

The utility of VFA and IVA is also limited by field-of-view, with coverage restricted to the T7 to L4 vertebrae. While most vertebral fractures do occur in this area, users must keep in mind that fractures can also occur outside the restricted region.

As screening tools that impose no additional radiation burden to the original exam, these components provide an attractive side benefit that may soon grow in importance. Possibly by the end of this year, the World Health Organization may raise the bar regarding osteoporosis assessment, adding clinical risk factors for fracture along with BMD measurements.

"When that (WHO document) comes out, we are expecting treatment thresholds to be based on absolute fracture risk," Binkley said. "Two very powerful risk factors are age and prior fracture."

At a given BMD measurement, a 50-year-old patient would have a much lower absolute fracture risk than an 80-year-old, Binkley said. The equation changes markedly, however, if the 50-year-old is found to have vertebral fractures.

The presence of fractures, however, goes only so far in identifying patients at risk for osteoporosis. Patient history is critically important. Physicians have to consider if trauma, whether recent or long past, is responsible for findings that crop up on a visual spine assessment.

Richmond asks patients about their participation in violent sports, such as football or wrestling, while in high school or college. Binkley, who also sees patients at the VA Hospital in Madison, asks about military history. Paratroopers, for example, may show evidence of vertebral fracture but not have osteoporosis. Their injuries, suffered decades earlier, may have nothing to do with bone loss.

Having a readily available means for spotting fractures, however, is a great convenience and a boon to characterizing osteoporosis risk.

"With this you can find people who have borderline low-bone density and clinically unappreciated vertebral compression fractures that put them at a higher fracture risk and thus should be treated," Binkley said.

Richmond emphasizes that VFA and IVA are not diagnostic for any specific abnormality. Conventional radiographs are essential to adequately evaluate patients with suspected vertebral fractures. CT or MR may be necessary to differentiate between compression fractures and other disease conditions. As screening tools, however, VFA and IVA stand out as major enhancements in bone densitometry.

"Other than increasing the speed of the scan, this is probably the most significant thing manufacturers have done in the last few years for clinicians and for the clinical care of patients," Richmond said.