[Basic study for the measurement of the bone mineral density of the calcaneus by using the radiographic contrast]

We performed a fundamental study for the measurement of the bone mineral density (BMD) of the calcaneus by using the radiographic contrast (RC). The RC of the bone radiography is proportional to the product of the thickness of the bone H(b), the gradient of the image detector system and the difference of the linear attenuation coefficient between the bone and the soft tissue. Therefore, the radiographic contrast per unit bone thickness (RC/H(b)) can be used as a good measure of the BMD. Though the linearity of RC/H(b) is maintained for x-rays with the monochromatic energy spectrum, the energy spectrum of the x-rays employed in clinical examinations is continuous. The relationship between RC/H(b) value and BMD value was examined by two kinds of added filter (Al with 5 mm and 15 mm thickness) and two kinds of bone phantoms (the phantom with 1 cm and 4 cm thickness, which is made by a bone equivalent material). The experimental results obtained from the 1 cm bone phantom indicated an approximately linear relationship. However, the experimental results obtained from the 4 cm bone phantom indicated a non-linear relationship due to the beam hardening. Therefore, the estimation of the calcaneus BMD value in clinical practice is performed by using the relationship between RC/H(b) and BMD values obtained from experimental results for the bone phantom with 4 cm thickness.