I had a MUGA scan done last February. I don't know if this is what they do with ALL MUGA scans, but for mine I got a little bit of a surprise when I showed up and they informed me that I would have the IV inserted into my juggular vein. That surprised me. It was a bit freaky to think about them putting an IV into my neck while I was awake (I'd had one when I was in surgery before, but I wasn't awake when they put it in), and it wasn't comfortable, but they numbed the area so it wasn't painful. Like Terry said, the worst part was laying still for so long on that hard narrow table. And I had to have my arms above my head, which became tiresome. At one point I had drifted off to sleep, and accidentally pulled my arm under the x-ray machine, which brought the people in really quick to correct it. I don't know if it added to my time in there or not, but they didn't like it.
Essentailly, what I was told, was that they were going to take my own blood out of me, tag it with radiation (or something), then send it back in and track it as it moved through the heart to get a better picture of what kind of blood flow I was getting. Once they start putting it back in, it's a very long and boring process. Like I said, I fell asleep.
Hope that helped some.