Advances and Shortfalls in MRI Evaluation of Osteoporosis

See also the article by Jones et al in this issue.

Dr Won Bae is an associate professor in the musculoskeletal imaging section in the Department of Radiology at the University of California, San Diego. His research interests include quantitative and imaging evaluation of musculoskeletal diseases, including low back pain and knee osteoarthritis. He has publications in the areas of ultrashort echo time (UTE) imaging, UTE T2* quantification, perfusion imaging, and artificial intelligence.

Osteoporosis is a major risk factor for bone fracture. Bone mineral density (BMD) is often evaluated with dual-energy x-ray absorptiometry (DXA) to determine the fracture risk (1), but DXA alone may be insufficient to evaluate bone quality or different compartments such as cortical and trabecular bone. CT, including multidetector CT and high-resolution peripheral quantitative CT (QCT), provide detailed information about bone microstructure (2). Furthermore, recent advancements in MRI, using ultrashort echo time (UTE) or zero echo time (3) sequences to acquire rapidly decaying MRI signal from in the bone, has enabled direct MRI evaluation of bone without the use of ionizing radiation.

In this issue of Radiology, Jones et al (4) report on a solid-state MRI technique (5) that uses hydrogen 1 UTE and phosphorous 31 zero echo time acquisitions at 3 T to determine bone water content, mineralization, and morphologic characteristics in postmenopausal women. The MRI measures were compared between women who had not undergone treatment and who had osteoporosis and an age-matched control group without osteoporosis, and correlations with BMD values from DXA and peripheral QCT were assessed. The authors found higher total and pore water content and lower bone mineralization and cortical thickness in women with osteoporosis. Additionally, against BMD values obtained from DXA and peripheral QCT, water contents showed a negative correlation, whereas mineralization and cortical thickness showed a positive correlation.

This in vivo study has clinical implications for a wider use of MRI in assessing osteoporosis by providing measures of bone water and mineralization. A minor inconvenience is that it requires the use of calibration phantoms at imaging, which can be cumbersome for acquisition and analysis. The findings of the study (4) agree with those of past studies (6) that used UTE-based techniques such as bicomponent UTE T2* analysis to determine free and bound water in the cortical bone, which correlated with porosity at micro-CT and mechanical strength. Conventional high-spatial-resolution MRI techniques have also been used and are aimed at acquiring images that depict trabecular microstructure for finite element analysis (7). The study by Chang et al (7) showed that postmenopausal women with fragility fracture had lower elastic modulus in the proximal femur compared with control participants without fracture but with similar BMD. Studies such as these collectively suggest that multifaceted MRI approaches may provide information unavailable by using DXA alone.

Despite recent advances, challenges to MRI bone evaluation remain. Scientifically, relative roles of bone “quantity” versus “quality” in bone fracture is still being actively investigated (8). This is leading to an ever-increasing number of imaging targets (eg, collagen cross-linking, composition, and cellular activity), particularly for the bone “quality” biomarker where MRI has the greatest potential to make an advancement. MRI-specific issues include nonstandard hardware (eg, coil) and methods, high cost, long imaging time, and complex postprocessing. This leads to smaller-scale studies that are difficult to generalize to a large population, as was possible for DXA (9).

The study by Jones et al (4) demonstrates that evaluation of bone at MRI is feasible for clinical translation, albeit with many shortcomings. Improvements are needed in many areas, including collaboration with clinicians and basic researchers to better define imaging targets and corresponding MRI biomarkers, hardware to more quickly acquire images with higher signal-to-noise ratio, approaches for facilitating introduction of MRI methods, and larger-scale studies.