I have friends and know of others who had "normal" scans, but actually had gall bladder disease. In fact, my doctor had a patient who was in great difficulty with her gall bladder, the tests came back normal, he knew it was gall bladder, and got a surgeon who would remove it for her. Voila! The gall bladder was diseased, and she was cured.
I've heard it's best to have it done when you're having an attack. Kitt would know better.