A herniated disc appears on CT as a herniated disc beyond the posterior edge of the vertebral body and reaching the spinal canal, which may or may not compress the nerves, but all herniated discs can compress the dural sac. At the posterior edge of the vertebral body it is upward in the shape of a half-moon. If it is a herniated disc on a CT film, it will be a half-moon shape with the posterior edge of the vertebral body going down. Therefore, if the posterior edge of the vertebral body is seen on the CT film in a downward half-moon shape and the density is lower than the bony density, a herniated disc can be diagnosed. It is also possible to see whether the dural sac and nerve roots are displaced by compression and to indirectly diagnose disc herniation.