Quantitative Computed Tomography

The main advantage of quantitative computed tomography (QCT) is that of actually measuring the volume (and, consequently, mineral density) of the studied bone. Normally, QCT instruments—endowed with a specific software and calibration phantom—can measure BMD only at the level of lumbar spine. More recently, specific peripheral QCT (pQCT) instruments have been developed to measure BMD at appendicular sites (forearm and femur). Both QCT and pQCT can measure cortical and trabecular bone density separately. The limits are however in the much higher X-ray dose with respect to DXA. Moreover, in the evaluation of vertebral BMD

(mainly trabecular bone), the precision can be reduced because trabecular thickness can be lower than the diameter of the "pixel." In evaluating long bone metaphyses, particularly the radius, the measurement of trabecular bone is influenced by cortical bone thickness, a source of error (38). In order to obtain precise measurements in children, a special pediatric phantom must be available for instrument calibration. Moreover—a crucial aspect in longitudinal studies—the used phantom must always be the same, as different phantoms give significantly different results.

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