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than the total hip region, and Ward's triangle (an area within the femoral neck that contains little trabecular bone) is so variable that it is of little clinical value.

Precision can be stated as either standard deviation (SD) or percent coefficient of variation (%CV, defined as 100 x SD/mean). Although precision error is commonly stated as a %CV, several reports indicate that error is independent of bone mass and will therefore be underestimated in the lower (osteoporotic) range. This suggests that it may be preferable to express precision error as the absolute SD. The smallest change that must be present before one can conclude (with 95% confidence) that the change is not related to measurement error is 2.8 x SD (or 2.8 x %CV).

The best sources of short-term precision data come from large multicentre studies that have used replicated patient measurements. Vendors frequently cite in vivo

Figure 9. Spine artifact from a barium swallow performed one week earlier. Barium can be seen throughout the colon (upper) and had cleared out after a month (lower). Barium caused bone density to be underestimated (L2-L4 1.246 g/cm2 with T-score +0.39) versus the repeat scan (1.541 g/cm2 with T-score +2.84).

Figure 9. Spine artifact from a barium swallow performed one week earlier. Barium can be seen throughout the colon (upper) and had cleared out after a month (lower). Barium caused bone density to be underestimated (L2-L4 1.246 g/cm2 with T-score +0.39) versus the repeat scan (1.541 g/cm2 with T-score +2.84).

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