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Figure 7. Spine spondylosis producing artifactually elevated spine bone density (1.317 g/cm2, T-score+0.98). The hip measurement in this patient was normal (1.000 g/cm2, T-score 0.00).

show only moderate correlation with bone density measured at proximal skeletal sites, though this is similar to the ability of peripheral x-ray-based measurements to predict central bone density. The accuracy of QUS can be indirectly validated through its ability to identify patients at risk of fracture. To date, this has only been reported for a small number of the available instruments, but available data show that fracture prediction with calcaneal QUS is comparable to that obtained with x-ray-based techniques.

Precision

Precision (also referred to as reproducibility) is the ability of a system to obtain the same results in repeated measurements of the same individual. A technique must have good precision if serial measurements are to be used in following an individual. Greater precision makes it possible to detect smaller changes in a subject. Current methodologies typically demonstrate precision errors that are larger than annual changes in bone density. Thus, in an individual patient, it may be difficult to determine whether a small change in the bone mass measurement reflects precision error or true change (Table 2).

DEXA reproducibility is influenced by instrument-, operator- and subject-dependent factors. These last two tend to be much more important than the instrument itself, and patient positioning is the single most important determinant. Reproducibility is optimized through a systemic process that includes careful quality control of the instrument, scanning technique and analysis (Table 3). Hip measurements are less reproducible than those of the spine, in large part due to the difficulty in obtaining consistent positioning. Reproducibility is further compromised when examining smaller regions of interest. Femoral neck precision is much worse

Figure 8. Spine compression fracture causing apparent improvement in bone density. The first scan (top) gives L1-L4 bone density 0.470 g/cm2. The follow-up examination (bottom) shows an apparent increase in bone density to 0.550 g/cm2. In fact, all of the change is confined to L1 (0.512 to 0.808 g/cm2) which is sclerotic on the image. After excluding L1 from the analysis, the L2-L4 spine shows stable bone density (0.460 to 0.476 g/cm2).

Figure 8. Spine compression fracture causing apparent improvement in bone density. The first scan (top) gives L1-L4 bone density 0.470 g/cm2. The follow-up examination (bottom) shows an apparent increase in bone density to 0.550 g/cm2. In fact, all of the change is confined to L1 (0.512 to 0.808 g/cm2) which is sclerotic on the image. After excluding L1 from the analysis, the L2-L4 spine shows stable bone density (0.460 to 0.476 g/cm2).

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