Impact of Contrast Administration and Gantry Reconstruction Angle on Hounsfield Unit for Assessing Underlying Bone Quality

Introduction: Hounsfield Units (HU) may better predict biomechanical complications of instrumented fusion than conventional measures of bone quality. Currently, measurement utilizes multiple axial slices; however, the effect of gantry plan on HU measures is unclear. This study aims to address the influence of reconstruction plane and contrast administration on measured HU.

Methods: Patients without prior lumbar spine surgery, tumor or infection were queried and sorted into three groups: 1) 150 with thin slice (0.75mm) non-contrast lumbar CT, 2) 100 with non-contrast and contrast-enhanced lumbar CT, and 3) 100 with non-contrast lumbar CT and lumbar CT myelograms. Non-contrast HU measures in group 1 were compared for axial, sagittal, and coronal reconstructions. HU measures for groups 2 and 3 were all obtained as axial slice averages. Measures between groups were compared using Wilcoxon signed-rank with significance defined as p< 0.05. Linear regression was also used to determine the degree of correlation between HU measures on non-contrast axial CT slices and HU measures on other sequences.

Results: Mean age was 58.8±15.9 for group 1 (54.0% male), 65.5±15.9 for group 2 (61.4% male), and 65.2±14.2yr (53.5% male). Group 1 showed significant differences across measurement modalities with the exception of axial versus coronal average measures (p=0.257) and sagittal average vs mid-sagittal measures (p=0.726). There were significant differences in average axial HU for each vertebral body between contrast and noncontrast lumbar CT and between CT myelography and non-contrast CT. Linear regression analysis demonstrated extremely high correlation between measures for all methodologies (r² 0.850-0.986; all p< 0.001), and between contrast and non-contrast enhanced studies (r²=0.870; p< 0.001). Measurements on CT myelography were significantly correlated with those on non-contrast studies, though to a lesser degree (r²=0.745; p< 0.001).

Conclusion : HU measures on noncontrast CT appear similar across multiple different planes on CT, suggesting that a simpler methodology such as single-slice measurement on mid-sagittal reconstruction may allow for more rapid assessment of underlying bone quality. Consistent with prior work, contrast-enhanced CT sequences appear to reliably mirror underlying bone quality, though CT myelogram measures may vary in a more unpredictable way that precludes their use being used to interpret underlying bone quality.