What is a cardiac MRI?

Lindsay Urbinelli, MD

What is a cardiac MRI?
MRI stands for magnetic resonance imaging. An MRI machine uses a powerful magnet, radio waves, and a computer to acquire 3 dimensional images of the heart. Using these images, we can examine the structure and function of the heart and blood vessels in great detail.

It is painless! It uses no radiation!

Why would a cardiac MRI be needed?

A cardiac MRI takes clear pictures of the heart and blood vessels in the body. Sometimes echocardiography (ultrasound) cannot provide clear or detailed enough images. MRI can also provide detailed measurements of certain heart parameters that are used to guide treatment, such as determining whether a catheterization or surgery is needed.

Common cardiac conditions in which a cardiac MRI is useful:

Vascular rings and aortic arch abnormalities

Evaluation of the branch pulmonary arteries

Evaluation of cardiac function and scar

Follow up of repair congenital heart disease (such as tetralogy of Fallot) to determine whether a valve replacement is needed

Cardiomyopathies

Any time clearer images are needed than can be obtained with echo (as patients get older, seeing structures on echo becomes harder)

Example of images obtained from echo versus cardiac MRI of the aortic arch for coarctation (narrowing) of the aorta:

Can you have an MRI if you have metal in your chest?

Yes! Except in rare circumstances. It is safe to have an MRI with hardware in your chest such as metal clips, sternal wires, artificial valves or ASD or VSD devices. However, while safe, the metal may cause artifact (blurry images) which can make interpretation of the MRI difficult. Your cardiologist will decide ahead of time whether the MRI can still be done. Sometimes a very brief MRI scan is done to determine the extent of the artifact and if too much, the scan is not completed. In the past patients who have a pacemaker or ICD (internal cardiac defibrillator) have not been permitted to undergo MRI because it can damage the device.  Now there are devices and leads (the pacemaker wire) that are “MRI Conditional”. This means that the device can be programmed to a special function or mode during the MRI to permit the study. The device is programmed back to its usual settings after the MRI procedure. This only applies to transvenous devices, so if you or your child have leads (pacemaker or ICD wires) on the outside of the heart then unfortunately they cannot have an MRI. If an MRI is recommended please speak to your device specialist, Dr. LeGras, to find out if it is possible to undergo an MRI. Typically, an MRI cannot be done in people who have vagal nerve stimulators, cochlear implants, and implanted pumps.

Details of the cardiac MRI procedure:

A cardiac MRI takes longer than other types of MRIs because MRI is affected by motion. As we cannot keep the heart still, there is a special technique called gating that allows us to control for the heart beating. However, this lengthens the MRI time. In addition, motion from breathing affects image quality, so holding your breath periodically throughout the MRI is necessary. Most cardiac MRIs take between 30 minutes to 3 hours (typically 1-2 hours).

Before the MRI, you will receive instructions on eating and drinking if sedation is required. If your child will be receiving sedation, they cannot have any food or liquid in their stomach. As the MRI machine is a large magnet, be sure to remove all metal (i.e. hair bands that have metal, jewelry) and wear comfortable clothes that do not contain metal.  

During the MRI, your child will be lying on a bed in a large machine in the shape of tunnel. Nothing will touch or hurt you during the scan. Straps or pillows may be used to help your child stay in the correct position. You will be given ear plugs and headphones so you can listen to music. When the pictures are being taken, the machine makes a loud knocking noise. During this time, you will have to hold perfectly still. Babies and young child will receive sedation during an MRI as they will not be able to follow the breathing instructions and cannot be expected to stay still for 1-3 hours. Some older children and young adults may also require sedation if they are claustrophobic.  If sedation is needed, an anesthesiologist will be providing the sedation.

For most cardiac MRIs, it is necessary that contrast (called gadolinium) be given which is a clear fluid that shows up on the pictures and allows for better image quality as well as the ability to look for certain things in the heart such as scar tissue. The contrast needs to be given through an IV so it is likely your child will have an IV placed.

After the MRI:

After the MRI, your child will have the IV removed, be woken up from anesthesia if sedation was given, and can go home the same day. It takes several hours after the MRI is complete for the cardiologist to process the MRI to get all the information. The results of the MRI are typically available by the following day and your cardiologist will be in touch with you with the results in the coming days or at your next appointment.