Hi and welcome to the board. Sorry you are in such pain . With respect, I have to disagree with Terra. I dont think you should be doing any exercises without your doctor's approval first. We dont know what's going on with you and to offer you to do exercise might worsen your condition right now.
If you are able to exercise at some point, there are some great SI exercises that can offer relief.
The SI joint is a complex and tight joint if you have not seen it. There isn't much between the bones and there are nerves and fibers running between them. If it becomes inflammed.. it can cause all sorts of pain and get referred into the back, into the buttocks and legs and feet. You know.. you are having really bad sciatica it sounds like.
I am guessing the bone scan is looking to see that joint more clearly to see if you have any arthritis in there, bone deterioration, spurs, etc which might try and explain your pain.
Hopefully the test will show them something that will help give you some relief. Dont give up and hang in there.. they seem to be trying to help you. Hang in there. Tammy