James A. Hanson

Bilateral Measurement of Femoral Bone Mineral Density

Both femora were measured on 61 normal adults using dual X-ray absorptiometry (DXA). In a subset of 31 subjects, each femur was scanned once using the conventional leg-positioning device supplied with the densitometer, and once using a new positioning device and software that allowed both legs to be measured simultaneously. In another subgroup (n = 30), subjects were measured three times using the new dual-femur approach to better assess precision error. The data were analyzed for differences owing to the different positioning devices and for differences between right and left sides. The correlation between results with the old and new positioners was high (r > 0.99, standard error of the estimate [SEE] = 0.01-0.02 g/cm(2)). There was no significant difference in the average bone mineral density (BMD) values between the old and new positioner. The precision errors for each femur alone with the dual-femur approach were similar to those reported for the single-femur scans (1 to 2%), but the precision errors for the combined femora were reduced by 30% as expected. The correlation between right and left sides was high (r = 0.94-0.96), and the SEE in predicting one side from the other was moderate for total, trochanteric, and femoral neck BMD (0.05, 0. 05, and 0.06 g/cm(2), respectively). These SEE equate to about 0.5 standard deviation in terms of T-score. Differences in many individual cases between the right and left sides were significantly greater than the precision error. The new dual-femur software and leg positioner allows rapid measurement and analysis of both femora, thereby eliminating the uncertainty between sides.

DEXA measurement of spine density in the lateral projection. I: Methodology.

SummaryBone mineral content and bone mineral density (BMC in g and BMD in g/cm2) were measured using dualenergy X-ray absorptiometry (DEXA). DEXA scans in the lateral decubitus position required about 12 minutes for the L2−L4 sequence at 0.75 mA (dose 5 mrem) and 4 minutes at 4.75 mA (7 mrem). The former scans were done with the Lunar DPX densitometer and the latter with the Lunar DPX-L One test of the algorithms used for measurement is the equality of BMC in both AP and lateral projections. BMC in the lateral projection averaged about 1% lower than in the AP projection in phantoms and for L2+L3 in 8 subjects, but the difference was not significant. Additional tests were done on the effects of tissue thickness and position from the tabletop. There was little or no influence of tissue thickness from 18 to 30 cm on BMD results, but there was a small influence of thickness below 18 cm (0.01 g/cm2;P=0.01) and of distance from the tabletop at extremes of positioning (0.02 g/cm2;P=0.06). The precisionin vivo was similar for both 4- and 12-minute scans; the standard deviation of repeat measurements was about 0.02 g/cm2, which was about 2% relative to the mean BMD for a region within the vertebral body. The latter region included half the BMC of the body, or 24% of the entire vertebra. Results of 4-minute scans on the DPX and 12-minute scans on the DPX-L in 9 subjucts were highly correlated (r=0.98;P<0.001).