Comparative Performance in Vitro and in Vivo of Lunar DPX and Hologic QDR-1000 Dual Energy X-ray Absorptiometers

The measured absolute bone mineral density values of spine and femur and their precision were compared on two dual energy X-ray absorptiometers, the Lunar DPX and the Hologic QDR-1000. There were systematic differences between the two systems, the Lunar DPX always giving a higher bone density value. The ratio of the mean Lunar DPX/Hologic QDR-1000 bone density measurements obtained in vivo was 1.13 g/cm2 for spine (L2-L4) and 1.20-1.43 g/cm2 for femur measurements. For both systems, short- and long-term precision (coefficient of variation) was below 0.5% for spine in vitro measurements and below 1.5% for neck of femur in vitro measurements. Precision of in vivo measurements was less good, especially for femur measurements. The coefficient of variation of spine measurements was dependent on bone density and deteriorated with decreased bone mineral density and increased body thickness.