Comparison of QCT-derived and DXA-derived areal bone mineral density and T scores

Two-dimensional areal bone mineral density (aBMD) of the proximal femur measured by three-dimensional quantitative computed tomography (QCT) in 91 elderly women was compared to dual-energy X-ray absorptiometry (DXA) aBMD results measured in the same patients. The measurements were highly correlated, though QCT aBMD values were marginally lower in absolute units. Transformation of the QCT aBMD values to T score values using National Health and Nutrition Examination Survey (NHANES) DXA-derived reference data improved agreement and clinical utility.

World Health Organization guidelines promulgate aBMD (g cm(-2)) measurement of the proximal femur for the diagnosis of bone fragility. In recent years, there has been increasing interest in QCT to facilitate understanding of three-dimensional bone structure and strength.

To assist in comparison of QCT-derived data with DXA aBMD results, a technique for deriving aBMD from QCT measurements has been developed.

To test the validity of the QCT method, 91 elderly females were scanned on both DXA and CT scanners. QCT-derived DXA equivalent aBMD (QCT(DXA) aBMD) was calculated using CTXA Hip software (Mindways Software Inc., Austin, TX, USA) and compared to DXA-derived aBMD results.

Test retest analysis indicated lower root mean square (RMS) errors for CTXA; F test between CTXA and DXA was significantly different at femoral neck (FN) and trochanter (TR) (p < 0.05). QCT underestimates DXA values by 0.02 +/- 0.05 g cm(-2) (total hip, TH), 0.01 +/- 0.04 g cm(-2) (FN), 0.03 +/- 0.07 g cm(-2) (inter-trochanter, IT), and 0.02 +/- 0.05 g cm(-2) (TR). The RMS errors (standard error of estimate) between QCT and DXA T scores for TH, FN, IT, and TR were 0.36, 0.40, 0.39, and 0.49, respectively.

This study shows that results from QCT aBMD appropriately adjusted can be evaluated against NHANES reference data to diagnose osteoporosis.