This test is performed with a contrast agent that is injected into the spinal canal with a technique called a lumbar puncture. The patient is placed on a table that can tilt and is tilted so that the contrast medium can run up and down the spinal canal. A CT scan of the spine is performed at the same time and images are then generated that can indicate whether there is an obstruction of the nerves of the spine. This procedure has mostly been replaced by MRI scans. It is still performed in cases where patients cannot undergo an MRI scan due to certain metal implants in the body and for other specific indications.