Correlation between Bone Mineral Density Measured by Dual-Energy X-Ray Absorptiometry and Hounsfield Units Measured by Diagnostic CT in Lumbar Spine

Abstract

Objective: Use of quantitative computed tomography (CT) to evaluate bone mineral density was suggested in the 1970s. Despite its reliability and accuracy, technical shortcomings restricted its usage, and dual-energy X-ray absorptiometry (DXA) became the gold standard evaluation method. Advances in CT technology have reduced its previous limitations, and CT evaluation of bone quality may now be applicable in clinical practice. The aim of this study was to determine if the Hounsfield unit (HU) values obtained from CT correlate with patient age and bone mineral density.

Methods: A total of 128 female patients who underwent lumbar CT for back pain were enrolled in the study. Their mean age was 66.4 years. Among them, 70 patients also underwent DXA. The patients were stratified by decade of life, forming five age groups. Lumbar vertebrae L1-4 were analyzed. The HU value of each vertebra was determined by averaging three measurements of the vertebra's trabecular portion, as shown in consecutive axial CT images. The HU values were compared between age groups, and correlations of HU value with bone mineral density and T-scores were determined.

Results: The HU values consistently decreased with increasing age with significant differences between age groups (p<0.001). There were significant positive correlations (p<0.001) of HU value with bone mineral density and T-score.

Conclusion: The trabecular area HU value consistently decreases with age. Based on the strong positive correlation between HU value and bone mineral density, CT-based HU values might be useful in detecting bone mineral diseases, such as osteoporosis.

Keywords: Bone mineral density; Computed tomography; Hounsfield unit; T-score.

Fig. 1

Computed tomography (CT) scans of lumbar vertebra L3 illustrating the method of determining Hounsfield unit (HU) values by using a picture archiving and communication system (PACS). From a reconstructed sagittal image, we select three axial planes of interest : slice (A) is taken just inferior to the superior endplate, slice (B) is from the middle of the vertebral body, and slice (C) is taken just superior to the inferior endplate. The PACS program automatically calculates the mean HU value of the regions of interest which are marked with ellipses in the figure. The average of HU values from three axial cuts, which is 89 HU in this case, was used for the analysis.

Fig. 2

Mean Hounsfield unit values among five decadal age groups show consistent decreases as age increases. ^*Groups which showed significant difference (p<0.05).

Fig. 3

Scatter plots showing correlations between Hounsfield unit values obtained from CT and T-scores obtained from dual-energy X-ray absorptiometry for lumbar vertebrae L1-L4. All had showed significant correlation coefficients (p<0.001).

Fig. 4

Scatter plots showing correlations between Hounsfield unit values obtained from CT scans and bone marrow density obtained from dual-energy X-ray absorptiometry for lumbar vertebrae L1-4. All had significant correlation coefficients (p<0.001).