Virtual Medical Centre

Cardiac MRI (Magnetic Resonance Imaging)

• What is cardiac MRI?
• What is MRI?
• How does the MRI scanner work?
• What will the MRI examination involve?
• Contraindications and risks of cardiac MRI
• The benefits of cardiac MRI

 What is MRI?

MRI is a safe and painless means of looking inside the body without using radiation. Detailed pictures of the body can be produced by using a large magnet, radio waves, and a computer, often providing information that could not be obtained from other investigations (e.g. x-rays or ultrasound).

How does the MRI scanner work?

The body is made up of small particles called atoms. Hydrogen atoms, a major component of water, make up 95% of your body. Normally, the hydrogen atoms within your body spin around at random. However, when a patient lies within the bore (the central tunnel) of a large, strong magnet, the same hydrogen atoms naturally line up and all spin in the same direction. When a radio wave is passed through the body, it causes the hydrogen atoms to give off signals. The scanner detects these signals and, with the aid of a computer, they are reconstructed into images of the body.

Until recently, clear pictures of the beating heart could not be obtained due to the heart's constant movement. To overcome this, cardiac MRI uses ECG gating. This allows the scanner to coordinate the pictures with the movement of the heart.

What will the MRI examination involve?

Patients are asked to change into a medical gown. As the magnetic field will damage credit cards, mobile phones and any electronic equipment, these must be left outside the scan room, along with all jewellery and clothing with metal (e.g. zips or jean rivets). The machine will make loud knocking noises while scanning, so patients are often provided with head phones which act as ear protectors and allow them to listen to music during the procedure. The patient lies down on a sliding 'table' and ECG electrodes are attached, before they are moved into the scanner.

The scan can last from 30 minutes to an hour. Patients may be asked to hold their breath for 10-20 seconds during some imaging sequences. The patient should remain motionless in order to produce a clear picture. Throughout the examination, radiographers are able to communicate with the patient via intercom to ensure that the patient is informed and comfortable.

Contraindications and risks of cardiac MRI

There are several contraindications to MRI. In particular, patients who have implanted medical devices (e.g. pacemakers or defibrillators, cochlear implants, cerebral aneurysm clips), or who may have iron fragments in their eyes, are not suitable for MRI investigation. Orthopaedic pins, mediastinal clips, coronary stents, and the majority of artificial heart valves are safe to scan.

There are no known risks from undergoing MRI during pregnancy. However, as with any medical investigation, a patient who suspects that she is pregnant should seek advice on the risks versus benefits of undergoing the investigation.

Around 4% of patients will suffer from claustrophobia to a degree that will not allow them to tolerate a MRI scan. This can be reduced to < 2% by sedating with oral or IV benzodiazepines.

There is also the risk of patients being injured if they forget to remove pieces of metal from their person or clothing. If sedation is required, there are associated risks of overmedication. If a contrast dye is used, there is a small risk of an allergic reaction, although this risk is very low.

The benefits of cardiac MRI

The MRI scan is a painless and safe scan that produces clear images of the body, from any angle. The pictures are clearer than those obtained by most other techniques, including both echo and SPECT scanning. It uses no radiation and therefore eliminates the substantial x-ray doses of cardiac CT scans.

Cardiac MRI is useful in a wide variety of cardiac conditions. It can demonstrate the structure and function of cardiac chambers, and help quantify the flow of blood through the cardiac valves. It is particularly valuable for assessing abnormal heart muscle (cardiomyopathy) and the damage done by previous heart attacks.