MR Imaging Offers New Way of Evaluating Bone Loss

A new study from the University of Pennsylvania demonstrates the ability of ultrashort echo time (UTE) MR imaging to quantify bone water, paving the way for a new, noninvasive approach to assessing bone loss.

Researcher Felix W. Wehrli, Ph.D., a professor of radiologic science, biochemistry and biophysics, said the team utilized customized radial pulse sequences to measure the volume of bone water contained in the pores of the Haversian system. The team was able to evaluate bone quality by looking at the cortical bones in the tibias of pre- and postmenopausal women and patients with end-stage kidney disease.

The study appears in the September issue of Radiology.

"It's a new idea because bone water, in all likelihood, is a very good surrogate for porosity," said Dr. Wehrli. "We can get information indirectly because the pores don't contain air, they contain water in some form. By quantifying the fraction of water in the bone, we get an indirect measure of porosity."

Previous studies have shown that increased bone porosity contributes significantly to the age-related decrease of bone mechanical strength. "What these investigators did in a clever way was say, 'Even though we can't see the tiny little pores, we can add up the total amount of water in the cortical bone and that will give us an indication of the space between them, and that will be a new way to look at strength,'" said David A. Rubin, M.D., an associate professor of radiology at the Mallinckrodt Institute of Radiology, Washington University in St. Louis. Dr. Rubin chairs the musculoskeletal subcommittee of the RSNA Scientific Program Committee.

Denser Cortical Bone a Challenge

The makeup of the denser cortical bone posed a challenge for the team as they worked to utilize the UTE MR signals, said Dr. Wehrli. "The cortical bone is solid bone, versus spongy bone immersed in the marrow," he explained. "With virtually all pulse sequences in MR imaging, cortical bone appears black—there's no signal in it because bone water has properties that are distinctly different from the water in soft tissues."

The team eventually perfected a methodology that Dr. Wehrli said he believes is reproducible. "It requires specialized pulse sequences that allow detection a very short time after the signal was excited," he said. "That is the crux. We have not invented this technique, we have improved it. I think we were the first to develop a quantitative method with the idea of measuring the pore volume fraction."

(a–c) Transverse ultrashort echo-time MR images of human specimens of the tibial midshaft from four donors. Specimens of left and right tibia are shown, except in (a) where the specimens represent two different donors (57-year-old female specimen and 64-year-old male specimen). Circular structure superior to specimens is from reference sample. Imaging time was 4 minutes and voxel size was 0.2 x 0.2 x 5 mm. (Radiology 2008;248:824–833) © RSNA, 2008. All rights reserved. Printed with permission.

Bone water was measured at the tibial mid-shaft in healthy pre- and postmenopausal women between 34 and 69 years old and in patients with renal osteodystrophy on maintenance hemodialysis. Results among patient groups were clearly different and correlated strongly with their medical risk for bone weakness, said Dr. Wehrli.

Comparing the patient groups to a healthy reference group, researchers found that postmenopausal women had 65 percent more water than did younger women. Bone water amounts in patients with renal osteodystrophy exceeded those in postmenopausal women by 43 percent and premenopausal women by 135 percent.

Peripheral quantitative CT and dual energy X-ray absorptiometry tests showed bone mineral density decreased relative to bone water increase; however, bone density differences among the groups were much smaller than bone water differences.

Dr. Wehrli said researchers were startled by the clarity of the UTE images and the obvious differences among patient groups. "The parameter we measured was a much better differentiator of these groups than bone density, which is what people ordinarily measure," he said. "We did not really know how sensitive the method could be. That was a very pleasant surprise."

Dr. Rubin noted: "It's an interesting technique that shows us detail in structures we knew about but couldn't see. What's interesting is the magnitude of difference in the subject groups was much higher with their method of measuring water than it was with the method of measuring the bone itself, implying maybe this is a technique that will be more sensitive to smaller changes or subtle differences in cortical bone."

Future Studies to Focus on Femoral Neck Fractures

The University of Pennsylvania team is quick to admit the limitations of the small study, which included just five premenopausal and five postmenopausal women and six patients on dialysis. "We have to be a little humble here because the number of subjects is very small," said Dr. Wehrli. "This definitely requires more work on a larger cohort of patients."

While data in this trial were gathered from patients' tibias, the group wants to concentrate in the future on a bone that poses a greater threat to patients at risk for osteoporosis—the femoral neck, a common fracture site involving cortical bone. "The femoral neck is the most common traumatic fracture site for this population," said Dr. Wehrli. "A significant percentage of older women and men with hip fracture actually die within a year post-fracture. Obviously, one goal would be to identify these people who are at risk of fracturing early, so that appropriate interventions can be devised."

The team is also interested in learning more about prevention of bone weakness. Said Dr. Wehrli: "Another interesting question is 'Is porosity modifiable?' Is it reversible, for example, using drug intervention such as treatment with bisphosphonates or parathyroid hormone? The ability to quantify the effectiveness of drug therapies requires precision of measurement and repeatability—ultimately you want to be able to bring these patients back after six, 12 or 24 months to determine whether an intervention has been effective."

The study results are exciting because they provide a unique view of a part of the body scientists haven't previously been able to examine, said Dr. Rubin. "We're able to accurately measure, something we've not been able to measure, inside the body," he said. "And it's doable on a clinical MR scanner with this new ultrashort TE. It shows profound differences among groups as we would predict and higher differences than we can measure utilizing standard methods for measuring the bone integrity."