Dear Alison:
I can only speak about my personal experience. Since 2009, I have had a total of 7 MRIs - all of them with contrast. I have not had any adverse reaction to the contrast. I also had a CT scan with contrast for my Cyberknife radiosurgery. As far as I understand, contrast is not recommended if you have kidney problems or are allergic to the contrast. They do not routinely give you a blood test here in the US to check for this specific allergy. They do ask you about any allergies/kidney problems on the questionnaire before the MRI scan though. It is also recommended to drink plenty of water after the scan to flush out the contrast from your system. But like I said, I never had any reaction whatsoever. You are correct, that ANs show up on scans even without contrast; however, for radiosurgery, every millimeter matters and contrast makes the tumor nice and shiny on the scans which allows for more precise contouring (again, this is purely my understanding). If you are really worried about the contrast, I would discuss it with your doctor in the context of your specific medical history/allergies, etc.
I cannot comment on the headframe. I had Cyberknife which does not involve a headframe. A mesh mask is used instead and it is not directly attached to your head.
I hope this helps.