Objective: To assess intra-and inter-site soft tissue variability by dual energy X-ray absorptiometry (DXA). Design: Cross-sectional trial. Setting: Three medical research institutions. Subjects: Five humans (in vivo) and four phantoms (in vitro), con®gured from two whole body phantoms with arti®cial skeletons and thickness overlays. Interventions: Duplicate total-body DXA scans were performed on all subjects at each institution within a 15 d period.Results: All intra-site coef®cients of variation (CV) were`0.5% for total tissue mass, but in vitro and in vivo Cvs were 7.2% and 2.3% for fat mass (FM) and 2.5% and 0.9% for lean mass (LM), respectively. Several totalbody and regional FM and LM measurements were signi®cantly different between sites (P`0.05), with percent differences between sites ranging from 2.6±13.3% for FM and from 1.6±13.6% for LM. Site 2 was consistently lower for FM and Site 3 was consistently lower for LM. Conclusions: These results stress the need for both rigorous and standardized cross-calibration procedures for soft tissue measurement by DXA.

Qualitative ultrasound (QUS) is a portable, safe and relatively inexpensive technique used to obtain information on bone mineral quality in adults and children. QUS measures bone stiffness index (SI) through the incorporation of speed of sound (SOS) and broadband ultrasound attenuation (BUA). QUS technology may prove to be extremely useful in field research where more than one machine is used over different periods of time. 13 adults (27.6+/-4.6 years old) were recruited to determine the internal stability of two Lunar Achilles+ QUS machines (Lunar1, Lunar2), as well as the repeatability in bone stiffness measures between the two machines over time. Triplicate measurements of the calcaneus were taken within the same day (n = 258) and at 1 week (n = 120), 6 months (n = 54) and 1 year (n = 18) apart to determine the time-dependent repeatability. Using paired t-tests and separate mixed effects models, there were no differences reported in SI, SOS or BUA values within one machine, or between two machines over these short- and long-term time-frames. These results indicate that QUS machines are internally consistent and different machines may be used over time to provide reliable measurements of changes in bone quality.

The delayed-gamma neutron activation facility at Brookhaven National Laboratory was originally calibrated using an anthropomorphic hollow phantom filled with solutions containing predetermined amounts of Ca. However, 99% of the total Ca in the human body is not homogeneously distributed but contained within the skeleton. Recently, an artificial skeleton was designed, constructed, and placed in a bottle phantom to better represent the Ca distribution in the human body. Neutron activation measurements of an anthropomorphic and a bottle (with no skeleton) phantom demonstrate that the difference in size and shape between the two phantoms changes the total body calcium results by less than 1%. To test the artificial skeleton, two small polyethylene jerry-can phantoms were made, one with a femur from a cadaver and one with an artificial bone in exactly the same geometry. The femur was ashed following the neutron activation measurements for chemical analysis of Ca. Results indicate that the artificial bone closely simulates the real bone in neutron activation analysis and provides accurate calibration for Ca measurements. Therefore, the calibration of the delayed-gamma neutron activation system is now based on the new bottle phantom containing an artificial skeleton. This change has improved the accuracy of measurement for total body calcium. Also, the simple geometry of this phantom and the artificial skeleton allows us to simulate the neutron activation process using a Monte Carlo code, which enables us to calibrate the system for human subjects larger and smaller than the phantoms used as standards.

A system in vitro consisting of a femur from a cadaver and soft-tissue equivalent material was used to test the agreement between several techniques for measuring bone mineral. Calcium values measured by delayed gamma neutron activation (DGNA) and bone mineral content (BMC) by Lunar, Hologic and Norland dual-energy X-ray absorptiometers (DXA) were compared with calcium and ash content determined by direct chemical analysis. To assess the effect of soft-tissue thickness on measurements of bone mineral, we had three phantom configurations ranging from 15.0 to 26.0 cm in thickness, achieved by using soft-tissue equivalent overlays. Chemical analysis of the femur gave calcium and ash content values of 61.83 g +/- 0.51 g and 154.120 +/- 0.004 g, respectively. Calcium measured by DGNA did not differ from the ashed amount of calcium at any of the phantom configurations. The BMC measured by DXA was significantly higher, by 3-5%, than the amount determined by chemical analysis for the Lunar densitometer and significantly lower, by 3-6%, for the Norland densitometer (p<0.001-0.024), but only 1% lower (not significant) for the Hologic densitometer. DXA instruments showed a decreasing trend in BMC as the thickness increased from 20.5 to 26.0 cm (p<0.05). However, within the entire thickness range (15.0-26.0 cm), the overall influence of thickness on BMC by DXA was very small. These findings offer insight into the differences in these currently available methods for bone mineral measurement and challenge the comparability of different methods.