Magnetic resonance angiography (MRA) represents a minimally invasive alternative to traditional methods of angiography for imaging the blood vessels of the human body. It is viewed upon as a desirable screening method for those patients suspected of having vascular disease of the main arteries of the body including the carotid arteries in the neck, the aorta in the chest and abdomen, and the vessels supplying blood to the arms and legs. Medicare is now recognizing the value of MRA as a minimally invasive approach to evaluating the vasculature; and is allowing reimbursement for the majority of the procedures that we do. It is not viewed upon as a replacement for catheter angiography; but in some patients it can help them avoid the more invasive catheter angiography.
Indications for MRA would include evaluation of the carotid arteries in patients who suffer from neurologic diseases such as transient ischemic attacks, where disease in the carotid arteries is suspected. MRA has been recognized as a valuable method to evaluate the thoracic and abdominal aorta for aneurysm and dissection. We have utilized MRA in the evaluation of the renal arteries supplying blood to the kidneys in patients suspected of having renovascular hypertension. Occasionally we have evaluated the vascular anatomy of the legs in patients who cannot tolerate catheter angiography for one reason or another, such as those patients who are allergic to iodine contrast or who are taking anticoagulants such as Coumadin and cannot be taken off this medication.
The magnet we use for our MRA studies is a 0.5 tesla superconducting GE Contour magnet. It is not an “open magnet”, but with its 60 centimeter bore diameter and indirect illumination, most patients do well when undergoing studies in it. Patients also wear headphones to listen to their favorite music during the exam. However an occasional patient is claustrophobic and we can treat them with appropriate medication to allow them to proceed with the examination as comfortably as possible. The magnet also has a 330 pound weight limit, so most patients can be imaged within it.
A typical MRA examination involves a series of preliminary localizing scans of short duration to localize the anatomy of interest in three dimensions in the patient’s body. For studies of the blood vessels in the brain, or in the legs we use sequences that do not require the use of the contrast agent, gadolinium. For studies of the carotid arteries and the aorta, gadolinium is quite useful. To perform a gadolinium-enhanced study, after the localizing scans are done, the patient is brought out of the scanner and an intravenous line is started (this is what we mean by “minimally invasive”). The patient is then returned to the magnet bore, and a small dose of the gadolinium is injected to time the arrival of the agent in the area of interest. This is followed by a larger, diagnostic dose of gadolinium. Once the study is complete, the patient is free to leave. The average scan time is usually less than one hour. The “raw data” is then transmitted to a computer in the radiology department; where it is reconstructed into a 3-dimensional image for analysis by a board-certified radiologist.
Gadolinium, the contrast used in MRA, has been shown to be safe and effective in the vast majority of patients who have received it. It is injected in much smaller doses than iodine contrast used for catheter angiography. It has been shown to have little harmful effect on kidney function, even patients with marginal renal function. If you have questions regarding gadolinium, you may ask the technologist present or the radiologist, who is on site in the building and only a phone call away.
As mentioned before, MRA is an excellent screening exam for patients suspected of having vascular disease. It does not completely replace catheter angiography; for most surgeons will request a catheter angiogram prior to performing surgery. But, if MRA can demonstrate normal or near normal vessels, it can help the patient avoid the more invasive and costly catheter angiogram.
Authored by: Robert Allen, M.D.