Influence of fat on bone measurements with dual-energy absorptiometry

In order to investigate the influence of fat on bone in dual-energy absorptiometry measurements, we evaluated a special phantom on the three scanners: Lunar DP3, Lunar DPX and Hologic QDR-1000. The phantom employed hydroxyapatite blocks of various thicknesses to simulate bone, water to simulate muscle and lucite to simulate fat. The lucite plates were arranged in one and two layers in three different configurations: over the whole measurement area, over the hydroxyapatite blocks only and at both sides of the hydroxyapatite blocks. For all scanners, no influence of fat could be demonstrated if it was homogeneously distributed over the whole measurement area. However, changes in area bone-density were observed if fat was distributed inhomogeneously over the measurement area. Fat over only the bone area reduced the measured bone values by 0.051 g/cm2 per cm fat layer. Fat over only the soft-tissue area increased the measured bone values by the same amount. These results apply to the Lunar DPX scanner. The results for the Lunar DP-3 scanner are similar; those for the Hologic QDR-1000 show a slightly smaller fat dependence of 0.044 g/cm2 per cm fat layer. The fat influences are not dependent on the amount of bone and only minimally on the soft-tissue thickness. A change of 50% in the fat content of the bone marrow will change the measured area bone-density of an averaged sized vertebra by 5-6% depending on scanner model. Inhomogeneous fat distribution in soft tissue, resulting in a difference of 2 cm fat layer between soft-tissue area and bone area, will influence the measured area bone-density by 9-10%.