I had three HIDA scans, so I'm an old hand! LOL. You'll have an IV line put in (it's the same process as having blood drawn, only they leave a plastic needle, if you will, in your arm. You will get an injection of some mildly radioactive stuff and they will put you on the narrow little table and start taking pictures. When the radioactive stuff is properly concentrated in your GB, they will give you another injection, this time of a chemical which causes your GB to empty (this is where my tests differed from Cathy's--she got something to eat instead; it's my understanding that eating something is less common than being injected with something). Then they will continue to take pictures as they watch that radioactive marker leave (or not leave) your GB. It will probably take a few days for you to hear back from your doctor's office with the results.
You may feel nauseated when they give you the chemical injection; my mother and I did. You may also cramp a bit. It wasn't as bad for me as a normal GB attack, but it was uncomfortable, especially since laying down always made my symptoms worse and you can't get off the table while they're scanning you (it took 45 minutes to an hour). I have heard, however, that it's rare for people to get so sick that they actually vomit, but as some people are more prone to it than others, be sure to tell the tech if you think you're about to.