The influence of tissue depth and composition on the performance of the Lunar dual-energy X-ray absorptiometer whole-body scanning mode

To assess the performance of the Lunar dual-energy X-ray absorptiometer (DPX) whole-body scanning mode (software version 3.1), a simple phantom was devised. This had known but variable amounts of lard to represent fat, and water to represent fat-free tissue. The bone mineral content of the phantom remained constant and consisted of the Lunar spine phantom to represent the spine/trunk and a slab of aluminium to represent the head. This phantom was scanned using the whole-body mode by the Lunar DPX to determine bone mineral content (BMC), area, bone mineral density (BMD), and body composition in terms of fat and fat-free tissue. The determinations of total tissue mass were very accurate and precise but fat tissue mass was slightly overestimated (103% of calculated value) and lean tissue mass slightly underestimated (98% of calculated value). The spine and head sections of the phantom were scanned individually using the spine mode and hence by addition the total BMC, area, and BMD of the whole phantom were determined. These results were compared to the bone mineral results obtained using the whole-body mode. The whole-body mode gave higher values for BMC and area than spine-mode measurements (at depths above 10 cm). This discrepancy increased with increasing depths of water (140% and 170% respectively of spine-mode value at soft-tissue depths of 22 cm and 15% fat). The values for BMD of the whole phantom tended to decrease with increasing depths of water and were lower than spine-mode values at tissue depths above 12cm. Precision was excellent for all measurements but deteriorated with increasing depths of soft tissue. Although these results cannot be extrapolated quantitatively to the in vivo situation, they demonstrate that bone mineral measurements are affected by the depth and composition of the subject and will be least accurate for obese subjects.