Empirical Functions for Conversion of Femur Areal and Volumetric Bone Mineral Density

Abstract

Bone mineral density (BMD) measured by dual energy X-ray absorptiometry (DXA) is areal and in the unit of g/cm², while BMD measured by quantitative computed tomography (QCT) is volumetric and in the unit of g/cm³. There is often a need to convert them to each other, but a simple conversion method is not available. The objective of this study was to establish empirical functions for the conversion. QCT of left femur from 67 subjects were acquired from a local clinical centre. For each subject, volumetric BMD was extracted from QCT using QCT Pro; the corresponding areal BMD was measured by CTXA-Hip. Both QCT Pro and CTXA-Hip are commercial software. The paired volumetric and areal BMD datasets were randomly split into two groups, and used respectively in construction and validation of empirical functions. Correlation between volumetric and areal BMD was 0.9073 (p < 0.0001) without considering femoral neck width (FNW), and 0.9970 (p < 0.0001) with the consideration of FNW. In the validations, the best agreement between predicted and measured volumetric BMD was R² = 0.9796, SSE = 0.0074, CV = 2.7%; the best agreement between predicted and measured areal BMD was R² = 0.9713, SSE = 0.0072, CV = 2.8%. Femur size represented by FNW had substantial effect on correlation between areal and volumetric BMD. With the consideration of FNW, areal and volumetric BMD can be converted to each other using the empirical functions constructed in this study.

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