Reliability of Hounsfield Units as a Measure of Bone Density: Applications in the Treatment of Thoracolumbar Fractures

Purpose: The aim of this study was to evaluate the reliability of Hounsfield units (HU) for bone density measurement in spinal trauma. HU have already been demonstrated to be reliable for this purpose in degenerative spine disease. Methods: This study is a retrospective cross-sectional analysis considering CT scanner model, slice thickness and HU in native and contrast CT scans. Linear regression, bivariate correlation and ROC analyses were performed to analyze the relationship between qCT values and HU. The inter-rater reliability of HU measurements was assessed using the intra-class correlation coefficient. Results: 344 patient datasets were analyzed. CT scanner model and slice thickness were found to have no significant impact on HU. The inter-rater reliability for HU measurements from native CT scans was ICC (3, 1) =0.932 (CI 95 0.919 – 0.943, p<0.001). The formula qCT=0.8·HU without CM +5 can be used to predict qCT values from native CT scan HU. ROC analysis revealed maximum sensitivity and specificity of 0.91 and 0.93, respectively, with a cut-off value of 93 HU to predict osteoporosis.For contrast CT scans we determined an ICC (3,1) of 0.889 (CI 95 0.801 – 0.948, p<0.001) and a formula of qCT=0.6·HU with CM -4. The maximum sensitivity and specificity calculated using HU in the presence of contrast medium were 0.83 and 0.89 with cut-off of values of 122 and 125 HU, respectively, depending on the observer. Conclusions: HU are reliable for measurement of bone quality in spinal trauma, especially in native CT scans. HU measurements from contrast CT scans should be used more cautiously for bone quality estimation.