They initially try and locate which vertebral body is physically causing the pain. With the aid of evidence from the MRI and some palpation they make their best determination. The anesthetic nerve blocks serve as diagnostic. The anesthetic wears off quickly, so if immediately the pain is gone, voila. Then it is up to the steroid and sometimes the steroid cannot sufficiently reduce the inflammation.
Usually they like to do at least one block before proceeding to radiofrequency. But if the steroid works for several months, the block can be repeated.
Sometimes they surmise incorrectly, sometimes it is difficult to locate the nerve on fluoroscopy and they can miss the nerve.