An MRI machine consists of a large circular magnet with an open horizontal tube running through the center of the magnet. Patients undergoing an MRI exam will lie flat on a table that will move the patient into the center of the magnet. In patients undergoing a cardiac MRI, the technician will often place a set of special cardiac coils over the chest. These coils act as “antennas” to pick up a signal that is emitted from the heart during the examination. In addition, several ECG leads will be placed on the skin of the chest to monitor the heart rhythm. Depending on the type of MRI model, the patient may enter either head or foot first into the magnet. The average cardiac MRI examination will take one hour to complete. During this time, the patient will be instructed to undergo a series of pictures that require breath holds of roughly ten seconds (in order to minimize respiratory motion). Some additional pictures will be obtained while lying still and breathing freely. A significant number of cardiac MRI examinations will require the use of an intravenous contrast agent called gadolinium to help visualize certain changes in the heart muscle. Gadolinium is an FDA-approved contrast agent that is also used in a variety of different MRI examinations. If this information is required, gadolinium will be injected into a peripheral vein by a nurse during the examination.
A significant amount of complicated technology and physics are required to understand how an MRI machine is able to produce such detailed pictures. In short, MRI scanners use high magnetic fields, as well as pulses of radio waves to produce signals from a patient’s hydrogen atoms. These signals are captured by the MRI scanner and the computer system recreates the signal and converts the information into an image. These images can be viewed on a work station at the hospital or printed out on film.
Is MRI safe?
An MRI study utilizes radio waves to acquire pictures and therefore NO ionizing radiation is required (as opposed to a CT scan, cardiac catherization or X-ray which does require the use of ionizing radiation). As a result, MRI is a very safe test and no long term ill effects have been reported. Claustrophobia may be problematic in about 2% of patients but often a mild anxiolytic (prescribed by your doctor) prior to the test can prevent this from occurring. Patients with pacemakers, implantable cardiac defibrillators (ICD) or retained pacemaker leads cannot undergo an MRI examination. However, metallic implants such as hip prostheses, prosthetic heart valves, coronary stents and sternal sutures are not a problem.
MRI in Hypertrophic Cardiomyopathy
As MRI can provide tomographic high resolution pictures of the heart, it has recently become an important new test well suited for the assessment of the size and extent of left ventricular hypertrophy in HCM. In fact, recent studies have shown that a cardiac MRI may be better than an echocardiogram to reliably detect hypertrophy in areas such as the left ventricular anterolateral wall and apex. As a result, in some patients an echocardiogram may not be sufficient to confidently exclude a diagnosis of HCM and in that situation a cardiac MRI may be recommended.
In addition, because of its high spatial resolution a cardiac MRI may also be performed to define the precise extent of wall thickening. This information is crucial since the degree of left ventricular wall thickening that a patient has may change therapeutic recommendations. Cardiac MRI can also show if left ventricular outflow tract obstruction due to systolic anterior motion of the mitral leaflet (SAM) is present. In addition, for those patients who are going to undergo an invasive procedure such as alcohol septal ablation or surgical septal myectomy, a cardiac MRI can precisely define the cardiac anatomy prior to and following the procedure.
Finally, one area of current investigation involves the use of the intravenous contrast agent gadolinium. When gadolinium is injected into a vein, it is quickly taken up in areas of the heart where scarring or fibrosis may be present. After waiting approximately 10-20 minutes after the injection, more pictures are performed and areas where gadolinium is taken up in the heart will appear very bright relative to the dark areas of normal heart muscle. In the future, the amount of scarring detected by gadolinium may help determine which patients are more likely to experience arrhythmias or heart failure symptoms.
The role of MRI in evaluating HCM
The role of MRI has advanced greatly in the past few years. The key difference between echocardiograms and MRI are the ability to better visualize the edges of the heart walls and the advantage of being able to see in the walls of the heart to pick up areas of scar or fibrosis.
Here are some images obtained with MRI - Images courtesy of Dr. Marty Maron, TUFTS Medical Center Boston.