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

Abstract

Summary

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.

Introduction

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.

Objective

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

Methods

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.

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.

Conclusions

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

Appendix

The TH QCT-derived aBMD may be converted to an equivalent TH DXA T score using the following relationships to facilitate application with WHO guidelines:

$${\text{DXA}}_{{{\text{TH\_}}T_{{{\text{scores}}}} }} = {\text{QCT}}_{{{\text{DXA\_TH\_}}T_{{{\text{scores}}}} }} \times 1.15 + 0.43\,\,\,\,\,...$$

(2)

or

$${\text{DXA}}_{{{\text{TH\_}}T_{{{\text{scores}}}} }} = {\text{QCT}}_{{{\text{TH\_aBMD}}}} \times 9.40 - 7.48\,\,\,\,\,\,\,\,...$$

(3)

Where ${\text{QCT}}_{{\text{DXA\_TH\_}}T_{{\text{scores}}} }$ is the NHANES III QCT_DXA T score for the TH; ${\text{QCT}}_{{\text{TH\_aBMD}}}$ is the QCT-derived aBMD for the TH and ${\text{DXA}}_{{\text{TH\_}}T_{{\text{scores}}} }$ is the equivalent DXA T score for the TH that may be applied to WHO T scores (for TH).

Similar relationships may also be derived for the FN:

$${\text{DXA}}_{{{\text{FN\_}}T_{{{\text{scores}}}} }} = {\text{QCT}}_{{{\text{DXA\_FN\_}}T_{{{\text{scores}}}} }} \times 1.04 + 0.18\,\,\,\,\,\, \ldots $$

(4)

or

$${\text{DXA}}_{{{\text{FN\_}}T_{{{\text{scores}}}} }} = {\text{QCT}}_{{{\text{FN\_aBMD}}}} \times 10.59 - 7.76\,\,\,\,\,\,\,\, \ldots $$

(5)

inter-trochanter (IT):

$${\text{DXA}}_{{{\text{IT\_}}T_{{{\text{scores}}}} }} = {\text{QCT}}_{{{\text{DXA\_IT\_}}T_{{{\text{scores}}}} }} \times 1.20 + 0.46\,\,\,\,\,\, \ldots $$

(6)

or

$${\text{DXA}}_{{{\text{IT\_}}T_{{{\text{scores}}}} }} = {\text{QCT}}_{{{\text{IT\_aBMD}}}} \times 7.79 - 7.17\,\,\,\,\,\,\,\, \ldots $$

(7)

and trochanter (TR):

$${\text{DXA}}_{{\text{TR\_}}T_{{\text{scores}}} } = {\text{QCT}}_{{\text{DXA\_TR\_}}T_{{\text{scores}}} } \times 1.09 + 0.26\quad \ldots $$

(8)

or

$${\text{DXA}}_{{\text{TR\_}}T_{{\text{scores}}} } = {\text{QCT}}_{{\text{TR\_aBMD}}} \times 10.97 - 6.51\quad \ldots $$

(9)