What is a magnetic resonance angiogram?
A magnetic resonance angiogram (MRA) is a diagnostic test that uses a powerful magnet and radio waves to create detailed images of the arteries and surrounding tissues. The test does not require entering the body (it is noninvasive) and does not involve exposure to radiation.
MRA can determine the location and severity of artery narrowing or blockages caused by:
- PAD in the legs
- Kidney artery disease
- Carotid (neck) artery disease
- Aortic disease
MRA images of the body
Who might have an MR angiogram?
MRA is used to provide a closer look at the arteries in women who have been diagnosed with PAD using another test (such as an Ankle-Brachial Index). MRA provides more detailed information than a duplex ultrasound, but is not as detailed as contrast angiography, the gold-standard test for examining artery disease.
MRA is most often used in women who are considering a procedure to restore blood flow to arteries that have been narrowed by PAD, such as angioplasty and stents or lower extremity bypass surgery. The detailed images produced by MRA allow doctors to determine which procedure may be best for you, develop a plan to treat the blockages, and monitor you after the treatment.
MRA can also diagnose and monitor aortic aneurysms. In rare cases it may be used to diagnose pulmonary embolism, a blood clot that formed in the deep veins of the legs (DVT) and traveled to the lungs.
How does MRA work?
Magnetic resonance imaging (MRI) uses radio waves and a strong magnetic field to produce clear images of internal organs and tissues without the need for radiation. MRA combines an MRI scanner with an injection of contrast dye that highlights the arteries and blood flow through them. A computer can combine these images into a 3-dimensional representation of the arteries and surrounding tissues.
How do I prepare for the test?
You should wear loose-fitting clothes without metal fasteners, and you may be asked to change into a hospital gown before the test. If you are having an MRA that includes the arteries in your abdomen, you may be asked not to eat or drink before the test. Unless you are told otherwise, take your medications as usual before the test.
Metallic objects can interfere with the magnetic field of the MRI scanner, so before you enter the scanner room you should remove all metal from your clothing and body. You will need to remove:
- Jewelry, watches, pins, hairpins, metal zippers
- ATM and credit cards
- Hearing aids, dentures, eyeglasses
- Pens, pocket knives
If you have an implanted pacemaker or defibrillator, a cochlear (inner ear) implant, or have had a metal clip implanted in your brain to treat an aneurysm, you should not be scanned and should not enter the MRI exam room. In some centers, women with these devices can still be scanned after taking special precautions.
You should also should inform the technician if you have any of the following in or on your body because they may limit the use of a magnetic field:
- Stent implanted in the arteries
- Intrauterine device (IUD)
- Artificial heart valves
- Artificial limb or metallic joint prosthesis
- Implanted nerve stimulators or drug infusion ports
- Tooth fillings, braces, metal pins, screws, plates, or surgical staples
- Metallic-based tattoos
You will usually be provided with a questionnaire that lists all potentially dangerous objects.
Tell your doctor if you are pregnant. Although pregnant women can usually undergo an MRA safely, the dye used during the test may pose a risk to the fetus (especially early in the pregnancy), and you should discuss the risks and benefits with your doctor before the test.
The dye used during an MRA does not pass into breast milk in large quantities and is probably safe for use in breastfeeding mothers.1 If you are concerned, talk to your doctor about the risks and benefits of the test. She or he may recommend you save up breast milk before the test and discard all milk produced for 24 hours after the test.
What happens during the test?
You will lie on your back on a narrow, movable table with your feet toward the entrance of the tunnel of the MRI scanner. Pillows and straps may be used to help you maintain proper body position during the test.
The technician will slide the table into the scanner using an automatic control. You will be alone in the exam room during the scan; however, the technician will be able to see you through a window from an adjacent room and hear and speak with you through a two-way intercom.
An MRI scanner
Inside the scanner you will hear loud humming or thumping noises as the machine takes each picture. Earplugs are usually provided. You may also experience a slightly warm feeling in the area of your body that is being scanned. This is normal, but tell the technician if it feels uncomfortable.
A contrast dye will be injected through an intravenous (IV) line in your hand or arm, usually after taking an initial set of images. You may experience coolness and a flushed sensation for a minute. After the dye injection, more images are taken.
It is very important that you lie completely still and hold your breath if instructed while each scan is being done because movement can blur the image. You can relax between each set of images, but try not to move too much. If you have a serious problem with being in narrow or closed spaces and cannot hold still, you may be given a mild sedative to help you relax. “Open” MRI scanners that are not as enclosed may also be available at some centers.
The test usually takes about 45 minutes to an hour.
What happens after the test?
When the test is complete, the intravenous (IV) line will be removed from your arm and the images will be sent to a radiologist to be analyzed. Your doctor will discuss the results with you.
You can resume your normal activities immediately after the exam. If you have received a medication to help you relax you will need to rest until the effect wears off, and you should avoid driving. Your doctor may give you other special instructions.
Be sure to drink plenty of fluids to help flush the contrast dye from your system. In most cases, nursing mothers can resume breastfeeding after the exam because the mild contrast dye used during an MRA probably does not harm the baby. If you are concerned about the effects of the dye on the baby, talk to your doctor about the risks. She or he may recommend you wait for 24 hours before resuming breastfeeding, and discard all milk produced during that period.
What are the risks of the test?
MRA tests are safe and usually painless. There have been no reports of harmful effects from the magnetic field or radio waves. Unlike regular X-rays and other types of angiogram tests, MRA does not involve exposure to ionizing radiation.
The dye used in MRA tests is made with the element gadolinium, instead of with iodine like the dye used for CT angiograms and contrast angiograms. Gadolinium dye is generally safer than iodine dyes, and less likely to cause kidney damage or an allergic reaction. If you are given medication to help you relax, there is a slight risk of excessive sedation, but your vital signs will be carefully monitored.
How accurate is an MR angiogram?
MRA is an accurate and reliable test for diagnosing and investigating PAD. One large analysis that included 1090 patients (28% were women) found that MRA was 94% accurate at diagnosing significant artery narrowing caused by PAD.
How does the MR angiogram compare to other imaging tests?
Click here for a quick comparison of the MR angiogram with other imaging tests used to diagnose and evaluate PAD.
First Imaging Test for PAD
As an initial imaging test for diagnosing the extent and severity of PAD, MRA is an alternative to the more commonly used duplex ultrasound. MRA is at least as accurate as ultrasound for diagnosing PAD. One study of 352 patients (32% were women) found that patients who underwent MRA as the initial imaging test were 42% less likely to need additional tests compared with patients who had a duplex ultrasound. This was because doctors were more confident using MRA results to make treatment decisions. However, many hospitals still prefer ultrasound as the first imaging test because it is fast and relatively inexpensive.
MRA Compared with Other Angiogram Tests
For women considering a procedure to treat PAD, the gold-standard test to look at the structure of the arteries has long been a contrast angiogram. MRA produces images that are less detailed than a contrast angiogram, but they are accurate enough to diagnose PAD and plan treatments, and the test does have several advantages. Unlike a contrast angiogram, MRA does not require entering the body. The contrast dye used during MRA (called gadolinium) does not contain iodine and is less likely to cause an allergic reaction or kidney damage than the dye used in other angiogram tests (such as a CT angiogram or contrast angiogram).
Another less-invasive option for women who are considering a procedure to treat PAD is a CT angiogram. MRA has been more studied and is more widely available than CT angiography. Both tests are more detailed than a duplex ultrasound test, but they are also more expensive and may take longer to perform.
Although MRA produces images that are slightly less detailed than a CT angiogram, the tests are equally accurate for diagnosing PAD and planning treatments. MRA has the advantage of using a milder dye that is less likely to cause an allergic reaction or kidney damage, and MRA avoids the radiation exposure that occurs during CT.
One disadvantage of MRA is that because of the strong magnetic field used to produce the images, women with implanted pacemakers, defibrillators, or surgical aneurysm clips cannot be scanned safely. Metal stents from previous treatment for artery disease disrupt the MRA images, so doctors cannot reliably tell whether the artery has become re-blocked inside the stent. In addition, MRA cannot reliably detect the buildup of calcium that hardens and blocks the arteries and often occurs in women with diabetes. For these women, a CT angiogram may be the preferred test. MRA also takes longer to perform than a CT angiogram.
In most cases, an MRA provides the same information as a CT angiogram or a contrast angiogram, and the choice of test depends on which are available at your hospital, what your doctor is most comfortable with, and your personal preferences.
What do the results mean?
The images produced by an MRA test allow doctors to detect narrowing or blockages in your blood vessels caused by PAD. In healthy arteries, the dye will fill the arteries completely, causing them to appear white with uniform borders on the final image. Dark areas where the dye flows in a thinner stream through the artery or stops flowing all together indicate locations where PAD is interfering with blood flow.
If you have an MRA to diagnose an aortic aneurysm, doctors will look for places where the dye has entered the wall of the aorta, or filled up a balloon-like bulging out of the artery wall.
An MR angiogram of the aorta and renal arteries.