Evaluation of a new set of calibration standards for the measurement of fat content via DPA and DXA.

A simulation study was performed to evaluate a new set of calibration standards for estimating the fat content of the body via dual-photon absorptiometry (DPA) and dual-energy x-ray absorptiometry (DXA). The standards, proposed by Nord and Payne [presented at the 2nd meeting of The Bath Conference on Bone Mineral Measurement (1990)] consist of stearic acid (100% fat) and 0.6% NaCl in water (100% lean). They were compared with other standards consisting of average composition adipose/muscle tissues and fatty adipose/lean muscle tissues. Source and detector properties of a Gd-153 DPA system and three commercial DXA systems were modeled. For each system and calibration set, rms errors in the calculated fat contents of simulated tissues having fat mass percentages that ranged from about 4%-44% and thicknesses that ranged from 5-20 cm were determined. Beam hardening errors for the systems were evaluated as was a calibration technique employed by one of the manufacturers to correct for such errors. In general, the smallest rms errors (2% or less when the calibration standards and tissues were of equal thickness) were obtained with the average adipose/muscle standards. Equivalent results were obtained with standards consisting of stearic acid and 0.8% NaCl. The latter is a higher salt content than proposed by Nord and Payne and results from differences in the x-ray attenuation coefficients that were employed in calculating the fat equivalence of water. Other, more convenient standards, such as lucite and water may be employed by using appropriate fat equivalences (approximately 69% for lucite and approximately 10% for water). Beam hardening errors for the DXA systems are considerable, and the simulated correction technique was shown to be effective.