by Dr Daniel Sado, clinical research fellow in cardiac MRI, and Dr Anna Herrey, consultant in cardiology and cardiac MRI, both from the Heart Hospital in London
In the last 30 years, the number of patients diagnosed with a cardiomyopathy has constantly increased. This is partly due to improvements in the way we take images of the heart.
Echocardiography or echo is an ultrasound scan and the main method used by cardiologists to look at the structure and function of the heart.
But if doctors need to know more about the composition of the heart muscle, for example if there is scarring or fat, or if they are interested in the structures surrounding the heart, such as the blood vessels or the lining of the heart, a different test called magnetic resonance imaging (MRI) is used.
In this article we will explain how cardiac MRI (often called CMR) fits into cardiomyopathy diagnosis and what a patient might expect from the scan.
Introduction to MRI
An MRI scanner is a powerful magnet. The technology used to obtain images of the body uses the fact that the human body is predominantly made up of water molecules.
During an MRI scan, the water molecules in the area of the body being studied are provided at intervals with energy by the scanner. When this energy is then lost from the water molecules, it can be measured.
The early research in MRI showed us that different body tissues lose this energy at differing rates. This knowledge allows the computer software to produce a detailed picture of all of the tissues in the area being studied. One of the best features of MRI is that it does not require ionising radiation to work and so it conveys no radiation exposure to patients or staff.
The scan – what to expect
Before the scan is undertaken, the patient is asked to fill in a safety form. This is because the magnet can be unsafe for patients with pacemakers, internal defibrillators (ICDs) and metal clips in the brain. Most joint replacements, heart valves, stents or fillings in teeth are safe to scan.
A small intravenous drip will be inserted into a vein in the arm of the patient. A contrast agent called gadolinium is given via the drip during the scan.
The agent allows doctors to see any scar tissue in the heart, something that cannot be seen on echocardiography. Gadolinium contrast is safe as long as the patient does not have poor kidney function.
The scanner is an open ended tunnel. About one in ten patients find it claustrophobic but mostly this can be overcome. The scan usually lasts for around 30-40 minutes. Many images are taken, each usually taking around eight seconds to acquire. During these seconds patients are asked to hold their breath to keep the heart in the same position and so optimising the picture quality.
What information can we get from a CMR scan?
In the first part of the scan doctors get images that allow a basic assessment of the organs and blood vessels in the chest and upper abdomen. These images also show the orientation of the heart in the chest, allowing doctors to customise the scan according to the individual patient. Then a series of moving “cine” images of the heart are taken in different planes so doctors can assess the three-dimensional structure in detail.
These images show the heart size, weight and pump function. The next set of images depends upon the reason for doing the scan. In some patients, a drug that will “stress” the heart is administered, in effect mimicking exercise. This allows assessment of the blood supply to the heart muscle.
Following this, the contrast agent is given. This shows if there are any blood clots present in the heart. A few minutes later any areas of scarring become apparent. The scar shows up as white and normal myocardium (heart muscle) as black.
CMR in hypertrophic cardiomyopathy (HCM)
HCM is characterised by thickening of the muscular wall of the left ventricle. Most commonly the thickening is in the septal wall. However, sometimes it can be in other areas, particularly the tip (apex) of the heart.
The apex is sometimes difficult to see using echocardiography. The cine images from CMR show the apex of the heart in great detail allowing assessment of whether there is thickening of the muscle or any complications resulting from it.
Some patients with HCM develop obstruction to blood getting out of the heart which can cause them symptoms. This can be treated in different ways.
To decide what is likely to work best, it is important to have high quality images showing the precise anatomy of the heart. Often the images from an echocardiogram are more than adequate for this, but in some patients CMR is used to see detail.
Lastly, the imaging of scar in HCM is important. Research has shown that patients with a lot of scar are more likely to develop worsening pump function.
This information will become even more important if specific therapies are developed to prevent this from occurring.
There are some rare conditions that can appear very similar to HCM in the way they show themselves because they also cause thickening of the heart muscle. It is important to distinguish these from HCM as the treatment is often different.
In older patients one cause of this is amyloidosis. This is a condition where proteins are laid down in the heart muscle, making it appear thicker on imaging. The scar pattern it causes on CMR is unique and very different to HCM. A finding of this unusual scar pattern will prompt referral to an amyloidosis expert for further assessment.
Another rare HCM mimic is a genetic condition called Anderson Fabry disease. Many centres routinely perform blood tests on patients with HCM to ensure they do not have this problem, but on CMR it can cause a type of scar pattern that we do not often see in HCM otherwise. This finding would ensure the patient was tested for the disease.
CMR in dilated cardiomyopathy (DCM)
DCM is characterised by enlargement and loss of pump function of the heart’s main pumping chambers, the right and left ventricles.
There are many potential causes of this. It can be an inherited condition or can result from a large heart attack or other disease processes.
If the cause of DCM is not clear, then the scar pattern can help us to make the diagnosis. For example, heart attacks cause a certain type of scar pattern, whereas myocarditis (inflammation of the heart muscle that can lead to DCM) often causes a different pattern.
CMA in arrhythmogenic right ventricular cardiomyopathy (ARVC)
In ARVC, fat replaces heart muscle tissue in the right, and sometimes left, ventricle. The right ventricle is more difficult to study using an echo. It has a complicated shape and the muscle is thinner and more difficult to see. So the challenge is most often to make the diagnosis.
World experts in ARVC have recently produced guidelines for cardiologists to help them make a diagnosis using ECG, 24 hour ECG monitoring, echocardiography and sometimes CMR.
CMR will show us the size and function of the right ventricle and whether there are areas where the contraction is abnormal. It can sometimes show areas of fat in the heart. In the later stages of the disease, patients can develop scar tissue in the right and left ventricle which is seen on CMR.
Left ventricular noncompaction (LVNC)
This disease is characterised by the presence of abnormal left ventricular architecture with a “spongy” muscle layer and deep recesses. This occurs mostly towards the apex and is sometimes difficult to see on echocardiography but clear on CMR.
The future
Manufacturers and researchers continue to strive to improve CMR. Improvements will make the scan times shorter and make the image quality even better. We will also find new applications for CMR.
Conclusions
CMR is a safe and useful tool in assessing patients with cardiomyopathy and their families. The information it provides is complimentary to echocardiography.
CMR provides us with high quality moving images of the heart allowing precise assessment of its size and function, but most importantly, it tells us about the composition of the heart muscle tissue, especially the presence of scar tissue.
The information from the scan can be helpful in making a diagnosis and deciding on future therapy.
Dedicated to Damian McLoughlin, who died age 20 of cardiomyopathy. This site is supported by Damian's family and friends.
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