If someone has been referred for an MRI scan then it is because that is the best test for whatever the referring physician believes might be wrong with them. Ultrasound is a different imaging technique with its own very different strengths and weaknesses from MRI. In particular, an ultrasound scan is much cheaper (and the machines are more widely available) than an MRI scan. If ultrasound was the best test for that person, then they would have been referred for an ultrasound scan.

Indeed NSF is a dangerous, yet rare, side effect of gadolinium based contrast agents. However, as it says in the paper referenced (http://www.scribd.com/doc/55205280/Nephr...Systemic-1), more than 95% of cases were in people with severe kidney failure. Awareness of the risks related to gadolinium based contrast agents has increased and some agents have a greater associated risk than others. Like any other medical procedure, the clinician must weigh up the risk against the benefit - the risk being almost negligible if the patient has no known kidney problems. A bit like crossing the road - it has a level of danger, but the benefit outweighs the risk and we take certain precautions.

Certain things cannot be diagnosed on an MRI scan without the use of contrast agents - for example measuring the blood flow to you heart in a perfusion scan. They are not used to simply make life easier but to improve the accuracy of the diagnosis.

As far as I am aware, the contrast agent is prescribed by a doctor (radiologists are qualified doctors) even if they are administered by radiographers (who are the highly qualified staff who perform the scan).

As I said before, you should discuss your concerns with a qualified person - either the doctor that referred you or ask to speak to one at the MRI unit.