1221 Impact of Contrast Administration and Gantry Reconstruction Angle on Hounsfield Unit for Assessing Underlying Bone Quality

INTRODUCTION: Hounsfield Units (HU) on computed tomography may better predict biomechanical complications of instrumented fusion. Currently, HU measurement relies on the review of multiple axial slices; however, it is unclear if measurements on sagittal or coronal reconstructions offer similar results. METHODS: Patient with no prior history of spine surgery who underwent lumbar spine imaging were retrospectively identified. Three groups were generated: 1) patients with non-contrast lumbar CT (n=150), 2) patients with non-contrast and contrast-enhanced lumbar CT (n=100), and 3) patients with non-contrast lumbar CT and lumbar CT myelograms (n=100). RESULTS: Mean age was 58.8±15.9yr for group 1 (54.0% male), 65.5±15.9yr for group 2 (61.4% male), and 65.2±14.2yr (53.5% male). Comparison of HU values in group 1 showed significant differences across measurement modalities with the exception of axial verus coronal average value (p=0.970), and axial average versus mid-sagittal value (p=0.004). there were significant differences in average axial HU for each vertebral body between contrast and noncontrast lumbar CT and between CT myelogram and noncontrast CT. Linear regression extremely high correlation between measures for all methodologies (r 2 0.850-0.986; all p<0.001), and between contrast and non-contrast enhanced studies (r 2 =0.870; p<0.001). Measures on CT myelogram were significantly correlated with those on non-contrast studies, though to a lesser degree (r 2 =0.745; p<0.001). CONCLUSIONS: HU measures on noncontrast CT appear similar across multiple measurement methodologies, suggesting a simpler methodology such as single-slice measurement on mid-sagittal reconstruction may allow for more rapid assessment of underlying bone quality. Measures on contrast-enhanced CT sequences may also provide a valid measure of underlying bone quality, though CT myelogram measures may vary in a way that precludes them from being used to interpret underlying bone quality.