BONE-MINERAL AND BODY-COMPOSITION MEASUREMENTS - CROSS-CALIBRATION OFPENCIL-BEAM AND FAN-BEAM DUAL-ENERGY X-RAY ABSORPTIOMETERS

Pencil-beam dual-energy X-ray absorptiometers (DXA) are being replaced with instruments that rely solely on fan-beam technology. However, in formation has been lacking regarding the translation of bone mineral a nd body composition data between the two devices. We have compared tot al body scans using pencil-beam (Hologic QDR-2000W) and fan-beam (Holo gic QDR-4500A) instruments for 33 children (ages 3-18 years) and 14 ad ults. Bone mineral content (BMC), bone mineral density (BMD), fat, lea n, and body fatness (%fat) values were highly correlated (r(2) = 0.984 -0.998) between the two DXA instruments. The mean differences between the paired measurements were: Delta BMC = 7.5 +/- 73.6 g, Delta BMD = 0.0074 +/- 0.0252 g/cm(2), Delta lean = 1.05 +/- 1.8 kg, Delta fat = - 0.77 +/- 1.7 kg, and Delta%fat = -0.94% +/- 2.5%. The BMC and BMD valu es,were not statistically different, whereas the differences for the b ody composition values were significant (p < 0.02-0.005). Regression e quations are provided for conversion of bone and body composition data between pencil-beam and fan-beam values for the whole body. To test t he performance of these equations for a second group (23 subjects), pr edicted values were compared with the measured data obtained using the fan-beam instrument. The mean differences were -1.0% to 1.4%, except for body fat mass, where the difference was 6.4%. For cross-sectional studies, the two DXA technologies can be considered equivalent after u sing the translational equations provided. For longitudinal studies in which small changes in body composition for the individual are to be detected, we recommend that the same DXA instrument be used whenever p ossible. For example, transition from a pencil-beam to a fan-beam inst rument could, in extreme cases, result in differences as large as 19% for the estimate of body fat mass.