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Dr Brett Lorraine
Dr Charles Lott
Date last modified: August 19, 2016
Magnetic resonance imaging (MRI) uses a high-strength magnet and radio waves to scan the body and produce images or pictures. MRI does not use ionising radiation, which is required for many other types of imaging, and is not known to have any long-term harmful effects. Cardiac MRI is imaging relating to the heart and how it is working.
Most people are suited to MRI scanning, although there are some restrictions due to the strength of the magnet and its possible effects on devices or implants, such as pacemakers. The MRI machine is shaped like a small tunnel, and the bed on which you lie will move you inside the tunnel where the images are being taken. Occasionally, MRI scanning cannot be carried out if you are uncomfortable in enclosed spaces (claustrophobia).
Some cardiac MRIs might involve the injection of a contrast medium (gadolinium: see topic in InsideRadiology) during the scan. The contrast medium highlights the blood vessels on the images to clearly show their structure and how they are working, and can also assist in providing information related to tissue blood supply, inflammation and scarring.
There are several different types of cardiac MRI scans including:
Cardiac viability (perfusion and delay) MRI – This involves the injection of a contrast medium into a vein during the scan. The contrast medium highlights the heart muscle in areas receiving a good blood supply. Areas receiving less blood do not highlight as well as the good areas, which can be an indicator of ischaemic heart disease (undersupply of blood and oxygen to the heart).
After a short delay, a second scan is carried out. In this part of the scan, the contrast medium highlights the parts of the heart muscle that are scarred, usually from a previous heart attack. The images from the MRI show how extensive and how thick the scarring is.
This enables the radiologist (specialist doctor) to interpret the images and provide your doctor with information about how much of the heart is scarred. Significantly scarred muscle is termed ‘non-viable’, meaning it will not recover even if blood flow is restored.
Heart tissue that is not scarred, but potentially undersupplied with blood and oxygen, is termed ‘viable’. Surgical procedures to increase blood supply can be targeted to the parts of the heart muscle most likely to benefit. Understanding the degree of scarring avoids unnecessary surgery to restore blood supply to parts of the heart that have been irreversibly damaged.
RV and LV function MRI – The right ventricle (RV) and left ventricle (LV) are the two lower chambers of the heart that receive blood from the two upper chambers of the heart and pump it into the arteries by contraction or tightening of the chamber walls.
A RV and LV function MRI scan will examine the way in which your heart works as a pump, moving blood through the lungs and around the body through the arteries. A short movie of the moving heart will be created from the images produced by the MRI scan, so that the way in which the heart muscle moves can be clearly seen and assessed. Calculations are carried out to measure the volume of the heart chambers, and work out how much blood is being pumped with each heartbeat. Both the RV and LV are assessed at the same time. An injection of contrast medium might occasionally be needed for this type of scan if there are additional heart problems requiring assessment.
Stress perfusion MRI (with contrast and adenosine) – This examines the blood flow (perfusion) to the heart both at rest and under stress. The stress stage of the test is carried out during the injection of a medication called adenosine. This drug has an effect on the heart that simulates or copies physical exercise and, combined with contrast medium that has been injected, can show parts of the heart muscle not receiving an adequate blood supply. The ‘rest’ component of the test is usually carried out afterwards, without adenosine, and is used for direct comparison with the stress images.
Structural assessment MRI – An MRI heart structure scan shows both the anatomy (the structure) of the heart and how it connects to the rest of the body. The heart has four chambers. Blood is pumped through the chambers with the help of four heart valves that open and close so that the blood flows in one direction. This type of scan is often carried out in the setting of a history of congenital heart disease and provides your radiologist with an overview of cardiac anatomy, and the relationships of the various cardiac chambers and major blood vessels. It might also be indicated in the assessment of cardiac masses (tumours). This examination can occasionally involve an injection of contrast medium.
MRI angiogram – aorta and great vessels – An MRI of the aorta and great vessels examines the main artery leaving the heart (aorta) and its branches supplying blood to the head and arms (also known as the ‘great vessels’). The examination will show the size of the aorta, its wall and any associated diseases.
Angiogram is the medical term used when imaging of blood vessels is carried out. Narrowing of the major vessels to the head and neck can be evaluated. Blood flow through the aorta can also be measured. An MRI angiogram might involve an injection of contrast medium.
Your doctor (usually your heart specialist) will refer you for a cardiac MRI if they suspect there are problems with how your heart is working. The type of cardiac MRI scan requested will depend on what type of problem is suspected.
You will be asked to complete a questionnaire before the MRI scan to ensure it is safe for you to enter the MRI machine and be exposed to the magnet.
If you have a history of kidney disease, your doctor might wish to do a blood test before the scan to ensure that the contrast medium (gadolinium) can be safely given, if required (see Gadolinium Contrast Medium for important information for those with impaired kidney function: InsideRadiology topic).
No other preparation is required, except for the cardiac stress perfusion MRI where you will be asked to avoid caffeine for 24–48 hours before the test. Caffeine interferes with the action of adenosine (see Stress Perfusion MRI above), which is used to simulate the stress part of this MRI scan. Types of caffeine include tea, coffee, herbal teas, Milo, and even decaffeinated coffee and soft drinks, such as cola. You might also be requested to fast for 6 hours.
You can wear your normal clothing to the examination, but you might need to remove some clothing before the scan. You will be offered a hospital gown instead. This is to eliminate any metallic objects that might interfere with the magnetic field inside the scanning room, and to allow easy access for electrical leads that will be placed on your chest to monitor your heart beat.
You will be positioned on the scanner bed by a radiographer, who is specially trained to carry out MRI scanning. The leads to monitor your heartbeat will then be placed on your chest. If an injection of contrast medium (gadolinium) is required, a small needle will be placed in a vein in your arm. A special set of detectors encased in plastic will be rested on your chest like a blanket. This works in conjunction with the main magnet to receive the radio wave signals that produce the images.
Once ready, the bed will move inside the MRI machine, which is like going into a short tunnel.
Once you are comfortable and positioned, the radiographer will return to the control console, leaving you in the MRI machine. From here, the radiographer will control the scanner to instruct the machine which part of the body to examine, and which views will best investigate your particular condition. You will be able to communicate with the radiographer at all times. If at any time you feel anxious or uncomfortable about being in the tunnel (claustrophobia), you can talk to the radiographer who might decide to take you out of the scanner.
You will be aware of humming and knocking noises going on around you, which indicates that the scanner is running. It is normal to feel a little warm during the scan. You will be asked to hold your breath for short periods from time to time during the scan, so that you remain still to help produce the best images possible.
The MRI machine can be noisy, so you will be provided with headphones, and you can listen to music (you are welcome to bring your own CD) and speak with the radiographer. You will also be given a squeeze ball to hold in your hand during the scan. Squeezing the ball will make the radiographer aware that you wish to speak. A microphone is located within the MRI machine.
A radiologist (specialist doctor), who will supervise the procedure, might require injection of contrast medium (gadolinium) during the scan. The contrast medium can assist in identifying abnormalities within the heart muscle, and to highlight the blood vessels. If contrast medium is to be used, the injection is given while you are inside the scanner, using the small needle that may have been placed in your arm at the beginning of the study. The injection is given through an extension tube connected to the needle.
The gadolinium is very safe, but as with all medications, allergic reactions can occur (see Gadolinium Contrast Medium (MRI Contrast Agents)). The hospital radiology department or radiology practice where you are having the scan is equipped to deal with this on the rare occasions that it arises.
If you are having a cardiac stress perfusion MRI, injection of both contrast and adenosine will be given during the scan. Many people experience flushing of the face during the adenosine injection, whereas others might feel a discomfort around the jaw and tightness in the chest. These effects are short-lived and usually end soon after the injection is given.
The ‘rest’ component of the scan is carried out at the end of the study and requires a further contrast injection, but no adenosine.
Usually there are no after effects. You will be free to continue the day you have planned once the scan is complete.
The examination uses very different technology to a normal X-ray, and does take more time to carry out. Depending on the problem being investigated, the scan time can vary from 20 to 45 minutes. Particularly complex heart conditions can require up to an hour of scanning.
Once you have completed the pre-scan questionnaire and have been assessed as safe to enter the MRI machine, there are no significant risks from the MRI machine itself.
Most people are suited to this examination, although there are some restrictions due to the strength of the magnet and its possible effects on devices or implants, such as pacemakers.
Occasionally, scanning cannot be carried out if you are upset by the enclosed space of the magnet (claustrophobia). An alternative test might then be recommended by your doctor.
For children where structural assessment of the heart is required, a general anaesthetic might be required to ensure the child keeps very still, so that accurate images can be obtained. This will have to be done in a hospital.
If a contrast medium (gadolinium) injection is required for the scan, there is a very small risk of an allergic reaction.
Recently, a condition called nephrogenic systemic fibrosis (NSF) has been identified as a rare but significant side effect of contrast injection. This complication is more likely to occur in those people with very poor kidney function, including people who are already on dialysis (a process that filters the blood of patients whose kidneys are not functioning properly using a kidney machine). This rare but serious reaction takes weeks to months to develop. For important information on NSF, see the information item on Gadolinium Contrast Medium (MRI Contrast Agents).
Adenosine (used in cardiac stress perfusion MRI)
Adenosine given during the stress part of the test has a very short duration of action, and there are usually no lasting effects. It can have more serious immediate short-term side effects, including:
These side effects occur in approximately 1 in 1500 cases. If they do occur, they are likely to be either during or immediately after the scan. The risk of death is approximately 1 in 10,000.
MRI scans avoid the need for exposure to potentially harmful radiation (X-rays). This is of particular benefit to younger patients, and those who will require repeat scans through their life to monitor their condition.
MRI scans have an advantage over X-rays in their ability to show clear images of the soft tissues of the body, and the scan can be specifically tailored to show complex anatomy (areas of the body). The scan is unique in its ability to calculate blood flow through the blood vessels. Blurring of the image due to movement of the heart and blood vessels can be overcome by scanning in time with the heartbeat. MRI is considered to be the most effective test for measuring the pumping of blood through the heart.
Additional information for cardiac viability MRI – The major benefit of the study is to show which regions of the heart have been injured, usually as a result of a heart attack, and the thickness of heart muscle involved. Based on this information, decisions can be made as to the suitability for procedures, such as bypass surgery, and the likely future benefits.
Additional information for stress perfusion MRI – The stress perfusion MRI can provide important information regarding whether or not there are parts of the heart muscle with inadequate blood supply. This can be used to guide your treatment. The study shows the effect of stress on the heart and the way in which this affects blood supply.
The examination is a highly specialised test, and is not available at all hospitals or radiology practices. Your doctor is likely to be able to refer you to the hospital or radiology practice nearest to you offering these scans.
The time that it takes your doctor to receive a written report on the test or procedure you have had will vary, depending on:
The private practice, clinic or hospital where you are having your test or procedure will be able to tell you when your doctor is likely to have the written report.
It is important that you discuss the results with the doctor who referred you, either in person or on the telephone, so that they can explain what the results mean for you.