A myelogram is an older, invasive test that is still used in certain cases to examine the spinal canal and spinal cord. Before the existence of CT and MRI, a myelogram was the best test to determine the cause of pressure on the spinal cord or spinal nerves. It is still used in patients who have metal plates and screws in their spine, which prevents them from undergoing MRI, and can make a CT scan difficult.
Following the application of a topical anesthetic, a small sample of spinal fluid is removed by lumbar puncture. A contrast dye is injected into the spinal sac that mixes with the spinal fluid so that it will show up on x-rays. Anything that is pushing into the nerves will show up as an indentation pushing into the spinal sac. This indentation may indicate a herniated disc, lesions, tumors, or injury to the spinal nerve roots. The myelogram may be combined with a CT scan to get a better cross-section view of the spine. This takes 45 to 60 minutes to complete. Following the test, some patients may experience discomfort and/or a headache, caused by removal of the spinal fluid.