Quantitative bone mineral analysis using computed tomography with the new calibration phantom

There are several problems in quantitative bone mineral analysis using computed tomography. Recently, CaCO3 phantom (B-MAS) has been widely used to measure the bone mineral content, although B-MAS has two problems. Firstly, the characteristics of X-ray absorption of CaCO3 are different from those of hydroxyapatite which is the main component of bone tissue. Secondly, in small patients, the phantom does not fit the body well due to its large (300 mm) width. This discrepancy between patient and phantom leads to inexact CT-number measurement of the most lateral chambers on both sides. To solve these problems, a new calibration phantom containing known concentrations of hydroxyapatite (0, 50, 100, 150, 200 mm/cm3) as a standard substance was developed, and the width of the phantom was reduced from 300 mm to 200 mm. Clinically, quantitative computed tomography (QCT) of the third lumbar vertebral body was performed with this new phantom in 178 cases and compared with those of the B-MAS phantom. The correlations between the concentration of the standard substance and CT-number were significantly higher in the new phantom than in the B-MAS. Based on these results, we consider the new calibration phantom useful for bone-mineral measurement by QCT. (author)