Estimation of bone mineral density from the digital image of the calcanium bone

Osteoporosis is more commonly seen in routine clinical practices, and its affects post-menopausal women, and elderly of both sexes. In India, it is more prevalent due to the mal-nutritional status, vitamin D deficiency, and lack of physical activity of majority of the general population. Bone strength is determined by its quality and its density. Bone mineral density (BMD) can be measured non-invasively by several techniques. Radiographic absorptiometry (RA) is one of the oldest methods of measurement of BMD from the conventional radiograph exposed simultaneously with a calibration phantom. Dual energy x-ray absorptiometry (DXA) is the `gold' standard for measuring BMD (g cm
^-2
). The aim of this pilot study was to estimate BMD (Est-BMD) of the calcaneous from the radiograph with good accuracy using RA technique. A total number of 19 Indian women, aged above 20 years were included in the study. Each woman, BMD of the right proximal femur was measured by DXA; also, a standard digital x-ray of right proximal femur exposed along with a small size step-wedge bone phantom was obtained. From the image, the mean grey level (MGL) values were measured in each defined region of interest (ROI) as well as in each step of the phantom. The measured MGL values of all the steps as well as their corresponding known BMD (g cm
^-3
) values were used to plot the calibration curve. It was found that there was a statistically significant (p=0.001) correlation (r
²
=0.93) between the mean MGL values and their corresponding known bone phantom BMD values. In the obtained linear regression equation, the measured mean MGL on various ROI's of the calcaneous were substituted separately and their corresponding BMD (g cm
^-2
) values were estimated. WHO's diagnostic criteria was used for the disease. In all women (n=19), Est-BMD at centre ROI of the calcaneous was correlated significantly (p=0.05) with femur neck BMD, N-BMD (r
²
=0.19), total hip BMD, T-BMD (r
²
=0.24), and shaft BMD, S-BMD (r
²
=0.24) of the proximal femur measured by DXA. In osteoporotic women group, the percentage decreases in DXA-BMD of different region of interest (ROI)'s of the proximal femur were ranged from 26.9% to 46.7%, when compared to normal healthy women, and it was high in T-BMD. On the other hand, in osteoporotic women, the percentage decrease in Est-BMD of the calcaneous in different ROI's were ranged from 7.5% to 13.6% when compared to normal women, and it was high in the centre ROI of the bone. It requires a calibration study using either European spine phantom or Hologic's spine phantom to test the accuracy of the method. Also, it requires further trials with more number of patients with and without osteoporotic fractures to test its predictability of future fracture risk.