This thesis is an investigation of the access provided to the vertebral canal in dogs by the hemilaminectomy and mini-hemilaminectomy surgical techniques using computed tomography (CT), and the completeness of evacuating extruded material in dogs with intervertebral disc (IVD) extrusion using magnetic resonance imaging (MRI). Hemilaminectomy and mini-hemilaminectomy were performed on opposite sides of the spine at T11-T12, T13-L1, and L2-L3 in 10 cadavers. Measurements of the vertebral canal height, defect height, and any dorsal and ventral remnants of the vertebral arch were obtained by CT. A covariate analysis was used to compare measurements with the surgical technique, surgical site, and side of the vertebral column. Defect height was greater with hemilaminectomy due to a smaller dorsal lamina remnant. There was no statistical difference in the height of the ventral remnant, or with surgical site. Nineteen prospectively recruited dogs with suspected IVD extrusion were randomly assigned to hemilaminectomy (10 dogs) or mini-hemilaminectomy (9 dogs) groups. Intervertebral disc extrusion was identified pre-operatively with MRI and later confirmed surgically, and immediate post-operative MRI was performed at the surgical site. The volume of extruded IVD material pre- and post-operative was calculated from transverse T2 images. Although residual IVD material was present in post-operative images from all dogs in the hemilaminectomy group and only 4 in the mini-hemilaminectomy group, there was no statistically significant difference between the proportionate volumes of material removed by either technique. The median residual volume with hemilaminectomy was 13.6% (confidence interval: 7.8 – 23.6%), and with mini-hemilaminectomy was 7.7% (4.3 – 13.8%). The results of this study confirm that the difference in the defect height between techniques is related to the removal of the articular processes creating a larger defect along the dorsal vertebral canal, while no difference in access to the ventral canal was observed. No effect of vertebral site was detected suggesting neither procedure provides an advantage over the other due to location of the lesion along the thoracolumbar spine. Residual extruded IVD material occurs with both techniques; while no statistical difference was noted, a larger population should be examined. / Ontario Veterinary College Pet Trust