Usually not bad
The radiologist cleanses the skin, numbs the skin and tissue deep to it.
Then a long needle is put through the numb area, between the bones of the spine to puncture the spinal sac (spinal tap). Spinal fluid is removed and replaced with dye. The needle is removed and x-rays taken. Then a ct is done. For most people, the discomfort is mild to minimal, and they can go home the same day.