What is a cardiac MRI?
Magnetic resonance imaging, or MRI, uses radiowaves and a strong magnetic field to produce clear images of internal organs and tissues without the need for radiation. The MRI machine is actually a large magnet that surrounds the patient. Cardiac MRI (sometimes referred to as CMR) generates images of the heart and coronary arteries. It is mainly used to check if your heart is getting enough blood and oxygen or to see if there is any damage to the heart muscle. MRIs may also diagnose heart defects that occur at birth, heart tumors, an enlarged heart (cardiomyopathy), and blockages in the carotid arteries in the neck (that may lead to a stroke).
What is a cardiac MRA?
A cardiac MRA, or magnetic resonance angiography, is a noninvasive version of the angiogram. For the traditional cardiac catheterization and angiogram, a catheter is threaded into the groin and fed into the heart. A contrast dye is then injected and X-rays are taken of the heart. With cardiac MRA, there is no radiation or catheter, only the injection of contrast dye. It uses computer analysis of the images to generate a clear three-dimensional picture of the heart. Cardiac MRA is considered experimental and is not widely available. It is unlikely to replace the tried and tested gold standard of traditional cardiac catheterization, but it may help some patients avoid unnecessary invasive testing. The contrast dye used in cardiac MRI is also less likely to trigger an allergic reaction or damage the kidneys compared with the dyes used in cardiac catheterization.
What is a chemical stress cardiac MRI?
Some heart problems only show up when the heart is working hard (or stressed). Stress tests use exercise or chemicals that mimic the effect of exercise on the heart. Exercise stress is not feasible with cardiac MRI because you have to lie still in the machine throughout the test. Studies show that cardiac MRI chemical stress tests are accurate and they may be more widely used in the future.1 Currently, this test is not done very often because it is still very new and there are some safety issues that do not exist with other types of chemical stress tests. The doctor is not in the room with you during the test, and it may not be immediately obvious if you experience problems because you are hidden inside the machine.2
Who might have a cardiac MRI or MRA?
Cardiac MRI may be used in women with symptoms suggestive of heart disease such as chest pain or shortness of breath, or those at risk for heart disease who had a previous abnormal test such as an ECG or chest X-ray. Cardiac MRA is not routinely used because it is still considered experimental. Both cardiac MRI and MRA are relatively new and not widely available.
Who should not have a cardiac MRI or MRA?
If you are pregnant, you should not have a cardiac MRI or MRA. Very obese patients (330 lbs or heavier) may not fit in the MRI scanner. If you have an implanted metal device or other metallic material inside your body, you should not have any type of MRI. These objects include (but are not limited to): pacemakers, implantable cardioverter defibrillators (ICD), intrauterine devices (IUD), artificial joints, titanium implants in the mouth, or inner ear implants.
Some cardiac patients with artificial heart valves, stents, or those who have undergone open-heart surgery should not have cardiac MRI or MRA. You should speak with your healthcare provider about whether it is all right for you to have one.
Cardiac MRI – Cardiac MRI Procedure
How do I prepare for a cardiac MRI or MRA?
You should remove all metal objects (e.g., jewelry, belts, underwire bras) before a cardiac MRI or MRA. The preparation will vary depending on the type of test you are having; for some, there is no special preparation. If you are having a chemical stress MRI, a cardiac MRA, or you have asked for a sedative, you may be asked not to eat or drink anything for up to 8 hours before the test. If you have diabetes, you should discuss dietary concerns with your healthcare provider to control your blood sugar levels. Talk to your doctor about any medications or dietary supplements you are taking in case they affect the accuracy of the test.
What does a cardiac MRI or MRA entail?
You will wear a hospital gown; you may wear clothing such as a T-shirt and panties underneath, but you cannot wear a bra. You will lie on a bed that slides into the machine, which is a large tube. Some people experience claustrophobia inside the MRI machine because it is a closed space. For some tests, you are strapped down on the bed. You can request to be given a mild sedative if you think you might experience claustrophobia; otherwise, you will be awake for the entire test. If you are prone to claustrophobia, you may ask your healthcare provider if there are any “open” MRI scanners available in your area.
There will be several imaging periods, lasting from 1 to 15 minutes each, during which you must lie very still. At some points, you will be asked to hold your breath.
When inside the machine you will hear loud clanging noises, which are a normal part of the MRI procedure. You can ask for ear plugs before the test starts, or you may be given headphones to wear. You will still be able to hear the technician’s instructions. Some testing facilities use an intercom through which you can talk to the technician if necessary, and he or she will periodically make sure you are comfortable. In other locations, the technician cannot hear you and you are given a handheld “panic” button to press if you are uncomfortable and want to get out of the machine. A contrast dye may be injected into your arm though an IV line depending on the type of test your healthcare provider has ordered. If a contrast dye is used, you may feel a warm sensation throughout the body. A cardiac MRI or MRA can last from 45 minutes to one and a half hours.
What happens after a cardiac MRI or MRA?
After either test, you can leave immediately with no side effects. If you choose to take a sedative (because of claustrophobia), you will be monitored after the test until the effects of the sedative have passed, and a friend or relative will need to drive you home. If you had a chemical stress test, you may experience some minor side effects from the medication including nausea, heart palpitations, numbness in the arms or legs, flushing, chest pain, or headaches. The test results are recorded on film and a radiology report is sent to your healthcare provider. What does a negative (normal) test indicate?
Because cardiac MRI testing is relatively new, there are few studies that have followed people over time to see how they fare after a negative or positive test. So far, it seems that women with a negative (normal) cardiac MRI have a similar low risk of having a heart attack or dying from heart disease as women with a normal echo or nuclear test.3
Cardiac MRI – Results & Risks
What does a positive (abnormal) test indicate?
Results so far suggest that women with a positive (abnormal) test have a higher risk of suffering a heart attack or dying from heart disease in the next few years than women with a normal test.1 Your healthcare provider will discuss the tests results with you and may prescribe further tests or treatments.
How accurate is cardiac MRI or MRA?
Cardiac MRI is very accurate if it is done well. The images produced with cardiac MRI are much clearer than those for other noninvasive imaging tests such as echocardiography or a nuclear scan. So far, cardiac MRI appears to work equally well in men and women.
What are the risks of cardiac MRI or MRA?
People with tattoos or permanent makeup may feel some mild discomfort or a burning feeling on their skin during a cardiac MRI caused by the metallic components of the inks. Large or dark tattoos around the scanned area may also cause false shadows to appear on the film produced from the test. MRIs are usually discouraged for up to 6 months after stent placement because the magnetic field could move the stent. If you have had bypass surgery, the surgical clips may distort the MRI image. The contrast dye used in cardiac MRI is less likely to trigger an allergic reaction or damage the kidneys compared with the dyes used in cardiac catheterization. You should still tell your healthcare provider if you have had a reaction to X-ray dye, shellfish, or iodine in the past.
What are the limitations of cardiac MRI or MRA?
These techniques are still considered experimental and are not widely used. Some arteries in the heart cannot be seen with cardiac MRA.4 To get the best pictures of the heart, you have to hold your breath and lie very still; this can be too hard for some people and, in these cases, the images of the heart may not be good enough to use. During other types of noninvasive testing such as a stress echo, you are hooked up to an ECG so your doctor can monitor your heart’s activity. This is difficult to do with cardiac MRI because the magnetic field affects the ECG. There are some safety issues with chemical stress cardiac MRI testing compared with other types of chemical stress tests. The doctor is not in the room with you during the test, and it may not be immediately obvious if you experience problems because you are hidden inside the machine.
References
1. Hundley WG, Morgan TM, Neagle CM, Hamilton CA, Rerkpattanapipat P, Link KM. Magnetic resonance imaging determination of cardiac prognosis. Circulation. Oct 29 2002;106(18):2328-2333.
2. Wahl A, Paetsch I, Gollesch A, et al. Safety and feasibility of high-dose dobutamine-atropine stress cardiovascular magnetic resonance for diagnosis of myocardial ischaemia: experience in 1000 consecutive cases. Eur Heart J. Jul 2004;25(14):1230-1236.
3. Hundley WG, Hamilton CA, Thomas MS, et al. Utility of fast cine magnetic resonance imaging and display for the detection of myocardial ischemia in patients not well suited for second harmonic stress echocardiography. Circulation. Oct 19 1999;100(16):1697-1702.
4. Yang TP, Pohost GM. Magnetic Resonance Coronary Angiography. Am Heart Hosp J. 2003;I:141-148,163.
