Reporting of bone mineral density values as T-scores or Z-scores: comment on the article by Sowers et al.
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Bone Density↗
Biomedical subjects
S Prevrhal
Publications and source records attributed to S Prevrhal.
[Quantitative computer tomography].
Quantitative computed tomography (QCT) can determine the true volumetric bone density of trabecular and cortical bone separately and at any skeletal site. QCT, because of its sensitivity to changes in bone status, is widely accepted as the superior method for the axial skeleton because of the high responsiveness of spinal trabecular bone to osteoporotic changes. The precision and accuracy of QCT at this site are somewhat lower than the respective values of other densitometric techniques. Nevertheless, because QCT measures a higher rate of bone loss at early premenopausal age, it allows better estimation of risk of vertebral fracture and smaller time intervals between follow-up measurements. The clinical acceptance of QCT is constrained by limited access to CT scanners for bone densitometry, the higher degree of operator dependence and the inability of QCT to measure the femur. New developments currently in scientific trial show that using volumetric CT can increase precision of QCT at the spine and allow highly accurate, precise and meaningful measurements at the femur.
Female↗
Accuracy limits for the determination of cortical width and density: the influence of object size and CT imaging parameters.
In this study we analysed the accuracy of computed tomography (CT) measurements in assessing cortical bone. We determined the dependency of thickness and density measurements on the true width and density of the cortex and on the spatial resolution in the CT images using two optimized segmentation methods. As a secondary goal, we assessed the ability of CT to reflect small changes in cortical thickness. Two different bone-mimicking phantoms with varying cortical thickness were scanned with single-slice CT on a Somatom Plus 4 scanner. Images were reconstructed with both a standard and a high-resolution convolution kernel. Two special operator-independent segmentation methods were used to automatically detect the edges of the cortical shell. We measured cortical thickness and density and compared the phantom measurements with theoretical computations by simulating a cross-sectional shape of the cortical shell. Based on the simulations, we calculated CT's power to detect small changes in cortical thickness. Simulations and phantom measurements were in very good agreement. Cortical thickness could be measured with an error of less than 10% if the true thickness was larger than 0.9 (0.7) mm for the standard (high-resolution) kernel which is close to the full width at half maximum (FWHM) of the point spread functions for these kernels and our scanner. Density measurements yielded errors of less than 10% for true cortical thickness values above two to three times the FWHM corresponding to 2.5 (2) mm in our case. The simulations showed that a 10% change in cortical width would not be detected with satisfying probability in bones with a cortical shell thinner than 1.2 mm. An accurate determination of the cortical thickness is limited to bones with a thickness higher than the FWHM of the scanner's point spread function. Therefore, the use of a high-resolution reconstruction kernel is crucial. Cortical bone mineral density can only be measured accurately in bones two to three times thicker than this number. In thinner bones, the measured density becomes dependent on the thickness. Changes in cortical thickness can only be assessed if the change is rather large or if the measured bone has sufficient thickness. Therefore, assessing density or thickness of the vertebral shell by CT should be treated with caution.
Bone Density↗