Comparisons between a pencil beam and two fan beam dual energy X-ray absorptiometers used for measuring total body bone and soft tissue.

A pencil beam Hologic QDR 1000W scanner (1000), a fan beam QDR 4500A scanner (4500) and a fan beam Lunar Expert scanner (Expert) were compared for bone mineral and body composition measurement accuracy. Phantoms were scanned with each instrument to assess magnification effects and to compare calibrations for bone mineral and fat proportion. 41 volunteers were scanned with both the 1000 and the 4500, and 21 patients with both the 4500 and the Expert. The height of a bone within the body affected the measured bone mineral content (BMC) and, to a lesser extent, the bone mineral density (BMD). There were differences in calibration against recognized standards for fat proportion between the three instruments. The 1000 underestimated low fat proportions and the 4500 underestimated high fat proportions. Fat results for the Expert were closer to nominal values. Comparisons on volunteers showed that measured mean total body BMD was 4% higher and BMC was 7% higher with the 1000 compared with the 4500; some regional differences were greater. Mean values of per cent fat were equal, but the total and regional regression coefficients were well above unity. Mean BMD was 3% higher and mean BMC was 10% higher with the Expert compared with the 4500, but most regression coefficients for these comparisons were less than unity. Mean values of per cent fat were equal, but regression coefficients were above unity. Errors due to magnification are acceptable. Differences between the instruments are appreciable, but can be accommodated by cross-calibration.