Bone mineral density measured by dual photon and dual energy X-ray absorptiometry: a problem of accuracy

Accuracy and precision of bone mineral density and bone mineral content in excised rat humeri using fan beam dual-energy X-ray absorptiometry

The aim of this study was the evaluation of fan beam dual-energy X-ray absorptiometry (DXA) for measuring bone mineral density (BMD) and bone mineral content (BMC) of isolated rat humeri. Defleshed rat humeri from male Lewis rats were examined with a Hologic QDR 4500 A (Hologic, Inc., Bedford, MA) high-resolution densitometer both in water and 0.9% saline solution. The small animal scan protocol with the regional high-resolution mode was used. BMC measured by DXA was compared with bone dry weight, ash weight, and bone calcium content. Furthermore, DXA BMD and BMC precision were determined. We also evaluated the effect of salinity of the water bath in which the bones were measured. Correlations (r(2)) of BMC, as determined by DXA with dry weight, ash weight, and bone calcium content, were 0.978, 0.988, and 0.890, respectively. DXA overestimated ash weight by 5%-9%. Precision errors for BMC (BMD) were 0.90% (0.76%) without and 1.3 (0.86) with repositioning. Changes in the salinity of the water bath had a significant influence on the DXA results: At the 0.9% physiological level, BMC (-4.4%) and area (-4.1%), but not BMD, values were significantly lower (p < 0.005) compared with measurements in tap water. Fan beam DXA is a highly accurate and precise technique for measuring BMC and BMD in excised small animal bones. A physiological saline concentration in the water bath had a significant impact on BMC and area, but not on BMD, and should therefore be strictly controlled to avoid an underestimation of BMC.

Clinical significance of heterogeneity of vertebral mineral density

Important variations in Z-score per vertebra, which is a common expression of bone mineral density (BMD), are sometimes observed. The present study evaluates the clinical significance of this heterogeneity. Normal and osteoporotic subjects were defined by using strict criteria. For every scan, the minimal Z-score (the vertebra with the lowest Z-score) and the delta Z (highest Z-score--lowest Z-score) was calculated. Of the investigated subjects, 30% presented a delta Z > or = 1. No significant correlation could be found between delta Z and age, BMD, height and weight. There was no difference in delta Z between scans of good, average or poor quality. Osteoporotic subjects had significantly lowered BMD values, whether evaluated through Z-scores for the L2-L4 site (P < 0.001; t = 3.71) or by minimal Z-score (P < 0.001; t = 3.97). Reproducibility calculated for the L2-L4 site on phantoms as well as on patients was excellent (C.V. < 1%). When reproducibility was calculated on each vertebra in vitro or in vivo, an increase in variability was observed. These data show that marked heterogeneity in BMD per vertebra is not infrequent. In some subjects low BMD may be measured at certain vertebrae but not at the total site. Our data suggest that in those cases the lowest BMD should be considered. In follow-up studies however, the BMD should be calculated on the L2-L4 segment, since a loss of precision is observed when only one vertebra is measured.