Hi there...
I went through a similar situation with my dad, who was in the end stage of multiple myeloma. He couldn't lay still because he was in a great amount of pain due to the tumors in his bone marrow, and bone damage.
Working with the doctor who had ordered the MRI and the MRI Tech, we finally were able to have him assigned a nurse who worked at the MRI facility. She administered high doeses of sedatives (not just valium, but a combination of things) via IV which were prescribed beforehand by his doctor. This put him into a sort of "twilight sleep".
You may want to talk with her doctor to see if he would be willing to do something like that.
Hang in there!! Hope this helps...