Magnetic resonance imaging (MRI) has revolutionised many areas of medicine but it is only recently that MRI has been able to assess the heart. This is because the heart contracts with every heart beat and also moves within the chest during breathing which makes it harder to image with MRI than other organs in the body. Cardiac MRI (CMR) uses powerful magnets and radiowaves to image the heart. It does not use what is called ionising radiation (which includes X-Rays, nuclear imaging tests and CT scanning). Since there are no known long term hazards from CMR this makes it a very useful test. Also the resolution of the images from CMR is high which means it can provide extremely detailed information about the heart. Most people are able to have a CMR. As it requires lying down inside a tube for 30 minutes some people who are claustrophobic may not be able to tolerate the scan although modern MRI scanners have a larger hole inside (these are called wide bore scanners). Since powerful magnets are used it is not possible to scan patients who have certain metallic devices. Patients with certain types of metal clips in the brain cannot have a scan. Most patients with pacemakers cannot have a CMR Scan (although this is now changing and this is possible for some patients). However, many metal implants are perfectly safe. These include all heart stents and valves and all metal knee and hip replacements. In most cases a contrast agent will be given in the middle of the scan which will be injected into a vein.
CMR has many uses and the exact scan that is performed varies depending on your symptoms and the results of your other tests. CMR is the best test for looking at how well the heart chambers pump. It can also look for scarring in the heart and it is the only test that can look directly for scarring. The commonest cause of scarring in the heart is a heart attack but other diseases of the heart muscle (cardiomyopathy) cause certain patterns of heart scarring which CMR can detect. It can also look at the heart valves although an echocardiogram would usually be performed first). It can look for swelling of the heart (called oedema) which can be useful if there is inflammation of the heart (such as in myocarditis).
One very common use for CMR is to assess if the heart arteries are narrowed or blocked. This is a stress CMR. Medication is given through a vein to exercise or stress the heart for 3-4 minutes. After this contrast or dye in injected and images of how the dye enters into the heart muscle are taken (this is called perfusion of the heart). An abnormal CMR perfusion scan can be used to assess if the heart arteries are narrow and to try to determine which heart arteries are affected.