Submitted to: Journal of Bone and Mineral Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/10/1998
Publication Date: N/A
Citation: N/A Interpretive Summary: This is the first definitive comparison, involving both children and adults, of the new fan-beam absorptiometer (DXA) with the older pencil-beam instrument. The newer fan-beam devices take only about 3 min to provide bone mineral and body composition data. It is important to know whether they provide the same information as the older machines. We used both machines to measure the amounts of bone mineral, fat, and lean tissue mass in a group of 47 subjects that included 33 children. The results were related, but not directly interchangeable. In general, the newer instrument provided higher estimates for smaller and younger subjects. We developed equations to allow an investigator or clinician to adjust for these differences between the two DXA machines.
Technical Abstract: Pencil-beam dual energy X-ray absorptiometers (DXA) are being replaced with instruments that rely solely on fan-beam technology. However, information has been lacking regarding the translation of bone mineral and body composition data between the two devices. We have compared total body scans using pencil-beam (Hologic QDR-2000W) and fan-beam (QDR-4500A) instruments for 33 children (ages: 3-18 y) and 14 adults. Bone mineral content (BMC), bone mineral density (BMD), fat, lean and body fatness (%fat) values were highly correlated (r2=0.984-0.998) between the two DXA instruments. The mean differences between the two sets of measurements were: deltaBMC=7.5 +/- 73.6 g, deltaBMD=0.0074 +/- 0.0252 g/cm2, deltalean=1.05 +/- 1.8 kg, deltafat=-0.77 +/- 1.7 kg, and delta%fat=-0.94% +/- 2.5%. The BMC and BMD values were not statistically different, whereas the differences for the body composition values were significant (p<0.02 - 0.005). Furthermore, the edifferences between scans varied as a function of the mass of each compartment. Regression equations are provided for conversion between pencil-beam and fan-beam values. To test the performance of these equations for a second group (23 subjects), predicted values were compared with the measured data obtained using the fan-beam instrument. The mean differences were -1.0% to 1.4%, except for the body fat, where the difference was 6.4%. For cross-sectional studies, the two DXA technologies can be considered equivalent after using the translational equations provided. For longitudinal studies in which small changes in the individual are to be detected, we recommend that the same DXA instrument be used whenever possible.