The hida scan shows if your gall bladder is functioning normally. Mine wasn't, so it came out. When they did an autopsy on it (lol), they found that it was chronically inflamed. I believe my mother's GB was undersized/deformed (which actually didn't show up on her ultrasound). You can also have pre-stone material in your gall bladder, known as "sludge", that might not be detected on the ultrasound, but which can keep your gall bladder from emptying properly. And a lady I worked with here said that her ultrasound was inconclusive, but her hida scan showed it wasn't functioning at all, and when they took it out, it was completely filled with stones. There had been so many stones packed in there, they couldn't see them with an ultrsound because there was no space around them to highlight them!