No MRI's at all. Also, sometimes they don't want to do ultrasound directly over the implant.
A lot of times the same information can be acquired in a different way. For example, last Christmas as a substitute for an L-spine MRI to find out why I suddenly had severe back pain they did 1) a CT with contract, 2) a bone scan, 3) some kind of nuclear medicine where they tag your blood cells with radioactive particles and 4) some other kind of scan (don't remember what it was called). It took a lot longer than just doing an MRI, but supposedly accomplished the same thing. I would imagine, but don't know for sure, that there would also be alternatives to an MRI for your condition.
I would check with your GI doc to see what tests they would be able to run in absence of an MRI. You can let your GI know that the MRI can cause the leads to heat up to dangerous levels (I know for the SCS it heats the wires to 104F) that can put you at risk for burns, so it will not be possible to do any more MRI's. I would *very strongly* recommend that you get an MRI of your GI tract where the Chron's is located prior to the surgery. It is always good to have the most updated films possible for the future.
Don't just rely on the PM or NS to explain it to you. It really is important to find out from the doc who will actually be treating your condition b/c different docs are comfortable with different tests. If your GI won't treat you unless he can do MRI's, that would be important to know ahead of time so you can opt out of the surgery or find a new GI.
I know it's a lot to to through, but you really will make it. The MRI is part of the reason why I suggested maybe you consider an RF procedure first, but only you know what's right for you. The other thing I didn't find out until near my surgery date was that the batteries don't last forever. Depending on usage, you will have to have surgery to replace them approx. every 7-10 years. Not the worst thing, but still something to consider.
wishing you the best,