Qualitative and Quantitative Computed Tomography and Computed Tomography‐Myelography Findings for Differentiating Thoracolumbar Intervertebral Disc Extrusion From Protrusion in Dogs

Computed tomography (CT) and computed tomography-myelography (CT-myelography) are commonly used to evaluate intervertebral disc herniation in dogs. Qualitative CT and CT-myelography imaging criteria used to differentiate intervertebral disc extrusion (IVDE) from disc protrusion (IVDP) are often influenced by observer experience and subjective interpretation. Therefore, this prospective observational study aimed to assess qualitative imaging features, as well as quantitative morphometric indices, on CT and CT-myelography to provide more objective, measurable parameters for differentiating disc herniation type in 36 dogs with surgically confirmed thoracolumbar intervertebral disc disease. Qualitative characteristics, including extension of herniated material beyond the intervertebral disc space and lateralization of the lesion on both imaging modalities, were more frequently associated with IVDE. In contrast, midline disc herniation on both imaging modalities and degenerative bridging spondyloarthropathy on CT were more commonly observed in IVDP. The ratio of the maximum cross-sectional area of the herniated disc material to the area of the vertebral canal at the same level (area index) demonstrated moderate discriminatory ability on both imaging modalities, with high sensitivity, moderate accuracy, but lower specificity, especially on CT, reducing its reliability as a standalone classifier. The ratio of the total volume of herniated material to the total canal volume at the corresponding vertebral level (volume index) demonstrated excellent discriminatory ability with high sensitivity, specificity, and accuracy on both CT and CT-myelography. Overall, the V-index showed the highest diagnostic performance for differentiating disc herniation types, whereas the A-index was limited by lower specificity, with qualitative imaging features providing complementary information.