It sounds like your doctor suspects a disc problem. A discogram (also known as discography) can tell if the disc is really the source of your pain. CTs and MRIs show the anatomy and may show a bulging disc but can't really confirm that the disc is the problem.
Using a special X-ray to guide the doctor, discogram involves putting a needle into the disc along with a contrast dye. Identifying if the disc is the problem (and which one)helps the surgeon plan the correct surgery. You may even avoid surgery if the discogram disproves the disc theory.
Discograms are done when the patient is in extreme pain and a candidate for surgery. The test itself can be somewhat painful and may make symptoms worse. But it's the only test that can show for sure that the disc is the source of your pain.