Abstract :
Comparative analysis of dual energy x-ray absorptiometry (DXA) and quantitative computed tomography (QCT) with phantoms and rat bones showed that both are necessary to adequately quantitate ovariectomy induced bone changes, in vivo. Precision and accuracy analysis of a Hologic QDR 1000/W and a Stratec 960 QCT showed that the latter is a highly precise, modestly accurate device that is more suited to the analysis of small bones. Specifically, smaller error was achieved by threshold optimization for the QCT in the analysis of phantoms 1.04–9.6 mm in diameter, with precision comparable to or better than DXA. Additionally, the QCT measured significant differences between groups in proximal tibiae when DXA could not; and measured a larger difference between sham and ovariectomized controls which was suitable for analysis of the dose dependent effects of a pharmacological test compound, raloxifene. This may reflect the ability of the QCT to measure volumetric mineral density (VMD, mg/cc), compared to two dimensional analyses by DXA. However, QCT was not able to analyze vertebrae in vivo, a site of considerable clinical interest. Therefore, DXA is useful to analyze the axial skeleton in vivo, while the appendicular skeleton is better analyzed in vivo by QCT. By both techniques, a similar dose response was observed for raloxifene, with ED50= 0.3 mg/kg for both axial and appendicular skeleton. Comparative analysis of rat L1-4 by the Hologic QDR 1000/W and Lunar DPXL showed 12–14% higher BMD and BMC values for the Hologic, which is the opposite relationship between these instruments to that observed clinically for this site.
