Algorithm enables automated CT bone mineral density analysis

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In osteoporotic patients, a fracture of the proximal femur is among the worst consequences of osteoporosis. It increases mortality risk, is a major cause of disability, and substantially reduces the quality of life. A multinational team of researchers has developed an algorithm that enables compartment-specific quantitative CT analysis of the proximal femur, which potentially can be used to measure bone mineral density (BMD) and investigate the influence of osteoporosis medications.

“Quantitative CT-derived BMD of the proximal femur may have an advantage over dual-energy x-ray absorptiometry (DXA)-derived BMD because it can be used to measure the trabecular compartment in selected volumes of interest (VOIs),” wrote the authors of a study published in this month’s issue of Radiology. “Trabecular bone is known to be metabolically more active and thus to display larger changes during the evolution and treatment of osteoporosis.”

Scientists from the departments of radiology at the University of California, San Francisco and colleagues in Germany and Austria sought to prospectively evaluate an automated VOI-fitting algorithm for quantitative CT of proximal femur specimens, to correlate BMD with biomechanically determined bone strength, and to compare the correlation with DXA measurements.

Their study cohort consisted of 178 left femur specimens harvested from formalin-fixed cadavers over a period of four years. The donors had a mean age of 79.4 years at time of death, and the femurs were harvested from 91 women and 87 men.

DXA measurements were obtained by using a GE Healthcare Lunar Prodigy scanner and evaluation of the data were performed using GE’s Lunar Prodigy Encore software by a radiologist with 3 years of experience. Cross-sectional images of the femora were acquired using a Siemens Medical Solutions Sensation 16 (detector) system. A high-spatial resolution-reconstruction algorithm was then applied.

Coronal anterior (left) and coronal posterior (right) quantitative CT depictions of head (sphere), neck (cylinder), and trochanter (irregular) VOIs in femur specimens. Image and caption courtesy of the Radiological Society of North America.  

The mean BMD of each VOI was calculated by the scientists by converting the pixel attenuation in Hounsfield units into BMD values of milligrams per cubic centimeter using a calibration phantom for densities of water-like and bone-like parts of the phantom. Failure load was assessed by using a side-impact test in which a lateral fall on the greater trochanter was simulated.

The researchers found that trabecular boned BMD and bone mineral content (BMC) measured with quantitative CT can be used to predict biomechanical strength. They noted that strong relationships between these parameters and femur failure load were observed, particularly in the femur head. The findings also demonstrated that the variation in BMD pixel values within the VOIs also facilitated the prediction of failure load.

“Quantitative CT can be used to measure bone compartments individually, whereas DXA enables a purely integral projectional bone measurement, including that of cortical and trabecular bone,” the authors observed.

They noted that compared with quantitative CT, DXA exposes patients to less radiation and does have lower costs. However, the procedure has limitations including soft-tissue errors and the lack of capability for differentiating trabecular bone from cortical bone. In addition, DXA-derived BMD values for patients with and without prevalent and incident femur fractures have been shown to overlap.

“With our method, as compared with other algorithms used to generate VOIs in the proximal femur for BMD measurement, the image itself is not altered by interpolating pixel attenuation, which can increase precision errors,” they wrote. “Instead our algorithm fits the VOIs to the binary masks of the images. Other potential applications of this approach are trabecular bone structure analysis and finite element modeling.”