MRI and gastric bypass?

It depends on how strong the MRI machine is (there are different levels of magnetism). The technician will screen you - with lots of questions- before the test and if your staples are not compatible with the level of magnetism they will not do it. So if your doctor feels you need it, go on to the appointment, but disclose all of your history to the technician.

An MRI is simply a "scan" of a portion of your body to take an internal picture. It should not be effected by a Gastric bypass unless they are requesting you drink an abnormal amount of something. I recently had to have an abdominal MRI and my dr. simply requested that I not have to drink anything. It creates a different type of pix, but it still works. Have them discuss this prior to the procedure.

some of the metals used in surgeries are MRI friendly.. ask at the radiology center where u will get the MRI

have a balanced diet and donate blood

I don't think your doctor would suggest having one if he thought it would be a problem.