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Magnetic Resonance Angiogram

Magnetic Resonance Angiogram

Definition
Purpose
Description
Preparation
Aftercare
Risks
Normal results
Abnormal results
Morbidity and mortality rates

Definition

A magnetic resonance angiogram uses the equipment and technology of magnetic resonance imaging (MRI) to assess the arterial system in the body. Unlike other radiologic techniques (x rays, CT scans), magnetic resonance imaging does not involve radiation. Instead, MRI employs a combination of magnetic fields and radio waves to generate images. The magnets cause hydrogen atoms in the subject’s body to line up in a particular way; the radio waves then bounce off of these aligned hydrogen atoms. This signal is captured and recorded by a computer, which uses the information to create a two- or three-dimensional image of the tissue being studied. In order to be able to adequately image the arteries during a magnetic resonance angiogram, radioactive contrast is injected in the patient. This circulates throughout the arterial system, and “lights up” the arterial system. In this way, the outline of the arteries can be visualized, and any blockages, bulges, leaks, or other abnormalities can be evaluated.

Purpose

A magnetic resonance angiogram can be performed to assess a variety of conditions involving the arterial system throughout the body, including to

  • Diagnose and monitor aneurysms
  • Evaluate the extent of atherosclerosis in the coronary arteries, the carotid arteries in the neck, or in the major leg veins; may be useful prior to surgery to remove atherosclerotic plaques or to place a bypass stent
  • Evaluate and monitor arteriovenous malformations, such as in the brain
  • Diagnose aortic dissection
  • Evaluate the arterial system that supplies the kidneys prior to kidney transplantation
  • Assess the coronary arteries prior to bypass surgery
  • Map out the arteries that supply a tumor prior to surgery to remove that tumor
  • Evaluate any area of narrowing (stenosis) throughout the arterial system

Description

Prior to starting the scanner for an MRA, radioactive contrast is injected through an IV in the patient’s arm. The classic MRI unit consists of an examination

table on which the patient lies, and a doughnut-shaped scanner into which the table slides. During the course of the MRA, which may take between thirty minutes and two hours, the patient must lie very still, and may at times be asked to hold his or her breath. Some people are bothered by the sounds that the MRI scanner makes, which include a variety of tapping, bumping, and fan sounds. Although no one is in the room with the patient, the patient can usually communicate with the MRI technician through a two-way sound system installed within the MRI unit.

Preparation

Because the strong magnetic field employed in MRA can interact with anything else that contains metal, it is crucial that the patient remove any jewelry, including from any piercings, prior to undergoing MRA. Other personal objects that should be removed include hearing aids, dentures, eyeglasses, hairpins. Pockets should be emptied of any metal-containing items, including coins, credit cards, Patients should inform the radiologist about any potentially metal-containing objects or medical devices that they have, such as tattoed eyeliner, a pacemaker, implanted defibrillator, aneurysm clips, cochlear implant, artificial limb, bone pin, medication patch, artificial heart valve, stent, infusion pump, or intrauterine device. People who have occupations in which they work frequently with metal should also inform the radiologist of this fact. In some cases, the MRA cannot be performed due to the presence of metal that cannot be removed and would be unsafe to expose to the magnetic fields of an MRI scanner. Sometimes, an x-ray will be ordered prior to an MRA in order to verify that there is not other metal in the body that would prelude performing the test.

KEY TERMS

Aneurysm— A weakness in the wall of an artery which can cause an area of outpouching or bulging. This weakness can break, resulting in massive bleeding.

Aortic dissection— A situation in which a tear in the interior lining of the wall of the aorta causes bleeding between the layers of that major artery.

Arteriovenous malformation— An anomaly present since birth in which the arteries and veins in a particular part of the body are caught up in a complex tangle, and in which there is an abnormal pattern of blood flowing from the arteries directly into the veins.

Atherosclerosis— A condition in which the major arteries throughout the body become obstructed by fatty plaques, causing narrowing, obstruction of blood flow, and ultimately hardening and stiffening of the arterial walls.

Some patients with a strong history of anxiety or claustrophobia find it difficult to be enclosed in the doughnut-shaped MRI machine. There are some open machines available that may cause less anxiety. Sometimes, a sedative can be used to help the patient relax during the MRA.

Women who are pregnant or who think they may be pregnant are advised against undergoing MRA. Women who are breastfeeding and who require MRA should feed their baby with formula for two days following the procedure, and should pump and discard their breast milk, since it will be contaminated with the radioactive dye.

Most MRI units have an upper limit of weight that they can hold. Patients over 300 pounds may not be able to undergo MRI, or may need to seek an open MRI unit for their study.

Aftercare

There is no aftercare necessary following an MRA. The patient can return to a normal diet and normal activities.

Risks

An MRA poses very little risk to the patient. Rarely, a patient may have an allergy to the radioactive contrast utilized.

Normal results

Normal results of an MRA would reveal normal arterial architecture, with fully patent arteries throughout the arterial tree. No narrowing, blockages, reduced blood flow, or outpouchings of the arterial walls are visualized in a normal MRA.

Abnormal results

An MRA is abnormal if there is reduced blood flow through any part of the arterial tree. This may result in being unable to visualize an area of an artery, due to an obstruction which prevents any blood flow (and therefore any dye) from reaching that part of the arterial system. Stenosis of an artery will cause the channel of dye to appear of smaller caliber than normal. An abnormal collection of dye may accumulate in an aneurysm pocket. An aortic dissection would reveal leakage of dye between the tissue planes of the aorta. Abnormal flow of dye into the venous system may indicate the presence of an arteriovenous malformation.

Morbidity and mortality rates

Under rare circumstances, patients may exhibit signs of allergy to the tracer.

Resources

BOOKS

Grainger, R. G., et al. Grainger & Allison’s Diagnostic Radiology: A Textbook of Medical Imaging. 4th ed. Philadelphia: Saunders, 2001.

Mettler, F. A. Essentials of Radiology, 2nd ed. Philadelphia: Saunders, 2005.

Rosalyn Carson-DeWitt, MD

Magnetic resonance angiography seeMagnetic resonance imaging

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