What is MR Angiography?

Magnetic resonance imaging (MRI) is a method of producing extremely detailed pictures of body tissues and organs without the need for x-rays. The electromagnetic energy that is released when exposing a patient to radio waves in a strong magnetic field is measured and analyzed by a computer, which forms two- or three-dimensional images that may be viewed on a TV monitor.

MR angiography (MRA) is an MRI study of the blood vessels. It utilizes MRI technology to detect, diagnose and aid the treatment of heart disorders, stroke, and blood vessel diseases. MRA provides detailed images of blood vessels without using any contrast material. The procedure is painless, and the magnetic field is not known to cause tissue damage of any kind.
 

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What are some common uses of the MRA procedure?

Many patients with arterial disease now have it treated in the radiology department rather than undergoing surgery in an operating room. MRA is a very useful way of finding problems with blood vessels (blockages, blow-outs or abnormal connections) and determining how to best to treat those problems.

The carotid arteries in the neck that conduct blood to the brain are a common site of atherosclerosis, which may severely narrow or block off an artery, reducing blood flow to the brain and even causing a stroke. If an ultrasound study shows that such disease is present, many surgeons now will do the necessary operation after confirmation by MRA, dispensing with the need for catheter angiography.
 
MRA has found wide use in checking patients for diseased arteries in the head, so that only those with positive findings will need to have a more invasive catheter study.

MRA also is used to detect disease in the aorta and in blood vessels supplying the kidneys, lungs and legs.

Patients with a family history of arterial aneurysm (a ballooning out of a segment of the vessel wall) can be screened by MRA to see if they have a similar disorder that has not produced symptoms. If an aneurysm is found, it may be treated either by surgery or endovascular therapy, potentially avoiding serious or fatal bleeding due to rupture of the aneurysm.
 

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How should I prepare for the procedure?

The magnetic field used for MRA will pull on any iron-containing object in the body, such as a heart pacemaker, intrauterine device, vascular access port, metal plate, or pins, screws or staples. You will be given a questionnaire to answer regarding these issues. The radiologist or technologist should know about any such item and also whether you have ever had a bullet in your body, whether you ever worked with metals, or if you have had a joint replacement. If there is any question, an x-ray can be taken to detect metal objects. The radiologist also should know if you have fillings in your teeth, which could distort images of the facial region or brain. Braces make it harder to properly adjust the MRI unit. You will be asked to remove hairpins, jewelry, eyeglasses, hearing aids, and any dental work that can be taken out. Some wigs contain metal and must be removed. Red or blue dyes used in tattoos and permanent eyeliner may contain metallic iron, but this is rarely a problem. You should report any drug allergies to the radiologist or technologist, and should mention if there is any possibility that you might be pregnant.

You can eat normally before the exam (unless told differently), but a young child should not eat or drink for about four hours if they will receive a sedative. The rules vary at different MRI facilities, so be sure to check with your medical center about eating and drinking before the exam. Medications may be taken as usual. Some patients will feel uncomfortably confined (claustrophobic) when enclosed in an MRI unit. If necessary, you will be given a sedative to help put you at ease, though probably less than one in every 20 patients will need this. You will wear a lightweight medical gown for the exam.

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What does the MRI equipment look like?

The traditional MRI unit is a large tube surrounded by a circular magnet, in which the patient lies without moving for several seconds at a time. The patient is placed on a wheeled bed that is moved into the magnet. In recent years patient-friendly units have been designed, and examination in such units is becoming increasingly available. These machines are both shorter and wider than a conventional MRI unit, and do not fully enclose the patient. Some of the newer C-shaped units are even open on all sides, and so are very attractive to patients who tend toward claustrophobia. A drawback is that image quality is not as consistently good.
 

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How does the procedure work?

Exposing the patient to radio waves in a strong magnetic field generates data that are used by a computer to create images of tissue slices. The magnetic field lines up atomic particles in the tissues called protons, which are then spun by a beam of radio waves and produce signals that are picked up by a receiver in the scanner. It is these signals that are processed by the computer to produce images. The resulting images are very sharp and detailed, and so are able to detect tiny changes from the normal pattern that are caused by disease or injury. Special settings are used to image various structures, uch as arteries in the case of MRA.
 

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How is the procedure performed?

The patient is placed on a special table and positioned inside the opening of the MRI unit. A typical exam consists of two to six imaging sequences, each taking two to 15 minutes. Each sequence provides a specific image orientation and a specified degree of image clarity or contrast. Depending on the type of exam being done, the total time needed can range from 10 to 60 minutes, not counting the time needed to change clothing, have an IV put in and answer questions. When contrast material is needed, a substance called gadolinium is given by IV injection during one of the imaging sequences. It highlights blood vessels, making them stand out from surrounding tissues.

The radiologist and technologist leave the examining room during the actual imaging process, but the patient can communicate with them at any time using an intercom. Some centers permit a friend to stay nearby, or a parent if a child is being examined. When the exam is completed you will be asked to wait to make sure that more images are not needed.

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What will I experience during the MRA procedure?

The technologist will make you as comfortable as possible, but at times the receiver or the magnet may be within a few inches of your face. For those who become very uncomfortable when enclosed in a small space, a mild sedative is nearly always effective. With certain sequences, you may notice a warm feeling in the area being studied. This is normal, but do not hesitate to report it if it bothers you. If you receive a contrast material injection there may be some local coolness at the IV site. The loud tapping, buzzing or knocking noises that are heard during the exam can be quite loud. This may disturb some patients; all patients are provided with earplugs to reduce these sounds and protect your hearing

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Who interprets the results and how do I get them?

A radiologist experienced in MRI will analyze the results and send a report to your physician, along with an interpretation of the findings. Your physician in turn will discuss the MRA findings with you. Some centers now send diagnostic reports and images over the Internet, speeding up the process.

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What are the benefits vs. risks?

Benefits

• Detailed images of blood vessels and blood flow are obtained without having to insert a catheter directly into the area of interest, so that there is no risk of damaging an artery, or, in the case of the brain, causing a stroke.
• The procedure itself is shorter than after a traditional catheter angiogram. As well, there is no recovery time with the procedure, unlike the 4 to 6 hours needed for catheter angiography.
• MRA is less costly than catheter angiography.
• There is no exposure to x-rays during an MRI study.
• Contrast material may be injected to produce high-detail MR angiograms. The risk of an allergic reaction from MRA contrast is extremely low and kidney damage does not occur. This is different from catheter angiography or CT angiography, which make use of iodine-based contrast material, which has a much higher (though still small – less than 1 in 1,000) chance of some form of allergic reaction. In patients with kidney failure or poor kidney function, the iodine-containing contrast agents can worsen kidney function. Even without using contrast material, MRA can provide high-quality images of many blood vessels, making it very useful for patients prone to allergic reactions.
• As with catheter-based angiography or CT angiography, it may be possible to defer or cancel proposed surgery after getting the results of an MRA study. If surgery remains necessary based on the MRA, it can potentially be performed more accurately.

Risks 

• There are no definite side effects from any type of MRI study, including MR angiography. Claustrophobia may be a problem, however. When it is severe and not relieved by giving a sedative, an alternative imaging method may have to be tried. If a metal implant is present but goes undetected, it may be affected by the strong magnetic field to which the patient is exposed. In addition, if the implant is close to the artery being examined it may be hard to get high-quality images.
• MRI generally is avoided during the first three months of pregnancy. Ultrasound is preferred at this time unless the woman might have a very serious condition that is best detected by MRA. If there are any effects of MRI on the fetus, none have been found as yet. The general rule for MRI and other diagnostic studies in pregnancy is that they should be avoided unless there is substantial risk from missing the correct diagnosis because the procedure is not done. Women who are breast-feeding should inform the radiologist and ask how to proceed. They may pump breast milk before the exam for use until the gadolinium contrast material has cleared from the body.

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What are the limitations of MR Angiography?

MRA does not image calcium, X-ray and CT angiography do see calcium on the images.
 
The procedure has to be avoided in any patient having a pacemaker, some types of aneurysm clip (older types contain large amounts of iron, which is magnetic), some types of implanted neurostimulator, metallic ear implant, or metallic object within the eyeball. It should also be avoided if there is a bullet fragment near or in the brain, heart or spinal cord. It may also be avoided if the patient has a port for delivering insulin or chemotherapy. For patients who are very claustrophobic, adequate nursing or medical staff must be on hand to monitor sedation.

The high-detail artery capability of MRA images does not yet match that obtained by x-ray catheter angiography. In particular, MRI of very small vessels may not be adequate for diagnosis and treatment planning, and regular catheter angiography will be needed. Occasionally it may be difficult to separate images of arteries from veins by MRA.

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