dye allergies

The contrast given for a CT scan and for an IVP is entirely different than the contrast given for an MRI scan.

If a patient is allergic to shellfish, one doesn't know if the pt is allergic to the iodine in the shellfish, or to something else. We'd ALWAYS premedicate with Prednisone and Benadryl as a precaution.

Yes, using the "new" nonionic, iso-osmolar contrast agents has GREATLY reduced the amount of reactions we see, even though they (the contrast agents) STILL CONTAIN IODINE. Patients may still have any number of allergic responses to the contrast, we just see a greatly decreased percentage of reactions. Do not let use of these new agents fool you into thinking NO reaction, or "just" a mild one, can happen with their use. There is still potential for anaphylactic responses to them, and I've seen it happen.

The MRI contrast contains no iodine and, while allergic reactions are more rare, they can still happen. Do an internet search for stats and more info.

The bottom line is comparing risk with benefit. Does the risk of the exam out-weigh the benefit of having the exam? Example: if a patient reports he had an anaphylactic response to the "old" contrast years ago, and an IVP is ordered for him to r/o stones, his PCP and the Radiologist (and he) have some choices: one, premedicate well (at least 12 hr pre-exam with Predisone, Benadryl and possibly H2 blocker like Cimetidine) and risk giving the "new" nonionic contrast, and hope he does well (most pts do, but with the history of anphylactic response I'd still be VERY cautious!!); two, change the order to a non-contrast spiral CT of the abdomen, specifically looking for stones. There may be other options, too, although not as diagnostic and possible cost-prohibitive. Risk VS benefit.

Well, there I go, getting wordy again. Hope that answers your question. :)