Abnormal myelogram is characterised by changes in the size and location of the subarachnoid contrast medium columns and the width and opacity of the spinal cord. Myelographic lesions can be grouped into the following patterns: extradural, intraduralextramedullary, intramedullary swelling, and intramedullarry opacification. Either extradural or intramedullary patterns may result from disc protrusion.. Disc protrusion typically causes an extradural lesion characterised by thinning and dorsal deviation of the ventral subarachnoid contrast medium column (lateral radiograph) and compensatory widening of the cord (ventrodorsal radiograph). Computed tomography (CT) and CT myelography are useful when conventional myelography does not clearly demonstrate a suspected extradural lesion caused by disc protrusion. Because the contrast resolution of CT is superior to that of conventional radiography, extradural compressive lesions caused by lesions other than disc protrusion (e.g. ligamentum hypertrophy, hematoma, tumor) can be identified. Cord swelling and intervertebral foraminal changes are accurately diagnosed with CT myelogrphy, especially when there is minimal subarachnoid distention. CT findings provide prognosis information about paraspinal soft-tissue structures and the cord parenchyma that cannot be obtained with conventional radiography.