Cardiac MRI

Cardiac magnetic resonance imaging (cardiac MRI, or CMR) is the gold-standard imaging test for assessing the heart muscle. It uses powerful magnets and radio waves — no x-rays, no radiation — to produce detailed, three-dimensional images of the heart and surrounding structures.

What does cardiac MRI show?

Cardiac MRI is uniquely good at characterising the heart muscle itself. In a single study it can assess:

Pumping function — accurate measurement of left and right ventricular volumes and ejection fraction.
Heart muscle disease (cardiomyopathy) — distinguishes between different types of cardiomyopathy by the pattern of scar and thickening.
Scar from previous heart attack — late-gadolinium enhancement reveals areas of irreversible damage.
Inflammation — myocarditis and other inflammatory conditions show characteristic patterns.
Infiltrative disease — amyloidosis, sarcoidosis, iron overload and other rare but important diseases have distinctive MRI features.
Valve disease — accurate quantification of regurgitant volumes when echo findings are equivocal.
Congenital heart disease — detailed anatomy of complex congenital conditions.
The aorta and pulmonary arteries — for aneurysms, dissections and pulmonary hypertension.
Stress perfusion — pharmacological stress combined with MRI imaging to look for coronary disease, an alternative to stress echocardiography or nuclear scans.

When is cardiac MRI used?

Cardiac MRI is particularly useful when:

Echocardiography raises a question that needs clarifying.
A cardiomyopathy is suspected.
The diagnosis of myocarditis is being considered.
Right ventricular size and function need accurate quantification (for example in tricuspid regurgitation or congenital heart disease).
Quantification of mitral or aortic regurgitation needs to go beyond echocardiography.
A non-invasive stress test is needed and stress echocardiography or CT are not the right choice.

What can I expect?

You will be asked to lie on the scanner bed, which then slides into the bore of the MRI machine. The scan takes around 45 to 60 minutes. The scanner is loud, so headphones with music are provided. You can communicate with the radiographer throughout.

In most cardiac MRI scans a contrast agent (gadolinium) is injected through a small cannula in the arm part-way through the scan; this highlights any areas of scar or inflammation. You will be asked to hold your breath for short periods (10–15 seconds) while certain images are taken.

There is no radiation. Most patients can drive themselves home immediately after the scan.

Are there any risks?

Cardiac MRI is very safe. The main considerations are:

Implants and devices — most modern pacemakers and defibrillators are MRI-conditional and can be safely scanned, but each device must be checked first. Some older devices and certain implants are not compatible.
Claustrophobia — the scanner is an enclosed tube. Most patients tolerate the scan well; if you are very claustrophobic, mild sedation or an open scanner can sometimes be arranged.
Gadolinium contrast — generally very safe; rarely causes allergic reactions; not given to patients with severe kidney disease.