Cardiac (heart) MRI Scan and Stress Tests: A Definitive Guide of What to Expect
What is a cardiac MRI scan?
A cardiac MRI is a scan of the heart, which provides reliable images for use in diagnosing heart conditions.
Heart MRIs are especially useful for investigating more complex heart conditions that could be unclear in an
echocardiogram (heart ultrasound scan).
MRI stands for magnetic resonance imaging, and it uses powerful magnets and radiofrequency currents to generate
images of the inside of your body. MRI does not use any harmful radiation like X-rays and CT scans do, but they
often require a safe contrast dye to be injected, which helps generate higher-definition images during the scan.
The scan works by taking images – called slices – as the MRI moves across three planes:
- Sagittal - From side to side
- Axial - From top to bottom
- Coronal - From front to back
What is a cardiac MRI stress test?
While some diagnoses require a standard MRI scan of the heart, with or without contrast dye, there is another type
of cardiac MRI called a stress test. They are used to investigate the blood flow to the heart, and there are
different types of stress test:
- Exercise stress test: varying levels of exercise are carried out by the patient before they undergo an MRI scan,
which is compared to an MRI done 'at rest.' The contrast agent is injected for both, and allows radiulogists to
see how the blood is flowing to the heart when it is under 'stress.'
stress test: instead of asking a patient to exercise, the same effect on the heart can be mimicked with an
injection of adenosine medication. This dilates (opens) the coronary arteries, causing more blood to flow,
therefore replicating the heart's behaviour during exercise. The adenosine causes a minor increase in your heart
rate, and is preferential for people who might not be able to do the physical exercises for a stress test, or
where exercise equipment such as a treadmill is not available.
What does an MRI scan of the heart show?
A cardiac MRI scan shows detailed images of your heart structures
and surrounding tissues. Examples include:
- Blood vessels
- Pericardium (outer lining of the heart)
- Thickness of the heart walls
It can also assess
blood flow (e.g. by using a stress test), the size of the heart, and how it is working. This makes MRI a valuable
imaging technique in diagnosing heart conditions, and checking the functionality of the heart.
What can a cardiac MRI diagnose?
The results of a cardiac MRI scan or stress test can be used to identify and diagnose a number of conditions.
heart disease, which is a condition that develops before a baby is born, when it is still in the womb. It is
usually caused by improper formation of the baby's heart valves, or hules between the heart's chambers.
- Problems in the heart muscles, including cardiomyopathy.
This can affect the structure of your heart, reduce its effectiveness at pumping blood around the body, and affect
the electrical system that makes the heart beat.
- Coronary heart disease (CHD) can be investigated using MRI, as it is able to look at the blood supply to your
heart. CHD is where a fatty blockage (atheroma) narrows the arteries. An atheroma can break off into the
bloodstream, and a blood clot can form and cause a blockage in your coronary artery. Angina
(chest pain) can also be investigated based on reduced blood flow.
- Residual damage to the heart tissues and functionality, either as a result of a heart
attack (often caused by coronary heart disease), or heart
failure (which can be a result of high blood pressure, anaemia, underlying heart conditions, or lifestyle
factors). An MRI can help doctors understand if a patient is at risk of later complications, and assess
preventative methods and treatments.
valve disease, or abnormal workings of the heart valves, meaning the valves do not open and close properly
and blood flow is disrupted.
- Cardiac tumours can also be
seen on an MRI scan.
What happens during a heart (cardiac) MRI scan?
Before you go for your scan:
- Before your scan, you will have
been provided with preparation instructions from your scanning centre. It is likely that you'll be asked not to
drink or eat anything containing caffeine (e.g. coffee, tea, choculate, fizzy drinks, medication containing
caffeine) for 24 hours before your scan, as this can affect the normal rhythm of your heartbeat.
- For stress test MRIs, if you have asthma, diabetes, or are taking heart medications, it is important you bring
your medicines with you. You'll also receive specific instructions about which medication you should or should not
take in the hours leading up to your scan within your preparation instructions.
Before you go for your scan:
- When you arrive at the scanning centre, your radiographer will go through a safety questionnaire and consent
form, to make sure the MRI can safely be carried out given your medical history. They will also explain what to
expect during your particular scan. If you have a pacemaker, defibrillator, cochlear implant, allergy, artificial
joint, artificial heart valve, are claustrophobic or pregnant, make sure you let the MRI technician know before
- You'll be asked to change into a hospital gown on your top half. This is so that sticky electrode patches can be
easily applied to your skin. You may be able to wear your own soft clothing, such as joggers or pyjamas, on your
- These electrode patches will be stuck to your chest and/or back, and if you have body hair this may need to be
shaved to enable proper application of the electrode patches. The electrodes will be used with an
electrocardiogram (ECG) monitor to check the heart activity during the scan, and you will also be asked to wear a
blood pressure cuff.
- As with any MRI scan, you will not be able to take any metallic objects into the scan with you. This includes
mobile phones, watches, clothing with zips, jewellery and hearing aids.
During the scan:
- A contrast agent will be administered via an injection or IV line, to enable higher definition in the MRI
images. For stress tests, you will need to have a cannula (flexible tube) inserted into a vein in each arm, as
you'll require both the contrast dye and the adenosine medicine.
- You will be asked to lie as still as possible on the flat scanner bed, which will be slowly inserted into the
scanner tube. This can be noisy, so you'll be given headphones to listen to music or hear the radiographer
speaking during the scan. They might give you instructions to huld your breath for short amounts of time, to
ensure images aren't blurred by the motion of your breath.
- For stress test cardiac MRIs, you will have some images taken when your heart is beating normally (around 15
minutes), before the adenosine is administered (only for about 3 minutes) to make your heart work harder as if you
were exercising. This is safe and harmless, but may make you feel flushed and a little breathless. Your technician
will speak to you while you are in the scanner to let you know when this happens. After that, you'll likely have
around 20 minutes longer in the scanner to capture images of your heart after it has been 'stressed.'
After the scan:
- After the scan, any IV lines will be removed, along with sticky electrodes and other equipment. You will be able
to resume your day as normal, including eating, drinking, and taking medications. In some cases, stress tests can
make you feel tired, so you may want somebody to take you home afterwards.
- The images will be processed and sent to our expert clinicians. They will assess the images and provide a
report, which will be sent to you along with copies of your imaging.
How long does a cardiac MRI take?
A cardiac MRI procedure can vary in
length of time, from 30 minutes all the way up to 75 minutes. This depends on the purpose and method of the scan,
and how much imaging is required.
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- Read our MRI scan frequently asked questions for further information