A myelogram is an older test that is still used to examine the spinal canal and spinal cord. A special dye is placed into the spinal sac that shows up on X-rays to indicate any abnormalities. Before there were CT and MRI scans, the myelogram was the best test to determine the cause of pressure on the spinal cord or spinal nerves. Today the myelogram is used only for very special purposes, such as for complicated revision spine surgeries. It is rarely the first test used if your doctor suspects you have a herniated disc.
The dye used during a myelogram outlines the spinal cord and nerve roots. This helps your doctor determine if there are any unusual indentations or abnormal shapes in the spinal cord. Anything that is pushing into the nerves shows up as an indentation into the spinal sac. This indentation could be from a herniated or bulging disc, a tumor, or an injury to the spinal nerve roots. The myelogram is useful for patients who have metal plates and screws in their spine, which prevents them from having either a CT or MRI scan.
The doctor must perform a spinal tap to inject dye into the spinal sac. The dye mixes with the spinal fluid so that it will show up on X-rays. You will be asked to lie on a tilting table while multiple X-rays are taken to show the flow of the dye through the spine. The myelogram is usually combined with a CT scan to get a better view of the spine in cross section and to check the health of the bones and nerves.
A myelogram does not show the soft tissues. It shows only the bones and the spinal fluid where the dye has mixed with the fluid.
Because the myelogram requires a spinal tap, there are more risks associated with it than most other tests. This is one reason that doctors prefer to use "noninvasive" tests first, such as the MRI and CT scan. The risks associated with a spinal tap include meningitis (infection of the spinal fluid), spinal headache, and allergic reaction to the dye. There is also a very small chance that the needle will cause bleeding around the spinal sac. The myelogram requires X-rays, which use radiation. Large doses of radiation can increase the risk of cancer. The vast majority of patients who have X-rays taken will never get enough radiation to worry about cancer. Only patients who have large numbers of X-rays-hundreds-over many years need to be concerned.