Magnetic Resonance Myocardial Perfusion Imaging
Published in The Business Times on 17 December 2017
By Senior Consultant Cardiologist, Dr Wong Siong Sung
What is magnetic resonance myocardial perfusion imaging?
Magnetic resonance myocardial perfusion imaging (stress CMR) is a non-invasive cardiac imaging scan to evaluate the blood flow (perfusion) to heart muscles (myocardium) using medication such as adenosine.
What is the purpose of doing this test?
Stress CMR is frequently used to evaluate patients who have symptoms of chest pain or breathlessness in order to assess for ischemia in the heart muscle (a condition where the heart muscle is starved of oxygen and other nutrients which are required to keep tissue alive due to inadequate blood supply as a result of blocked or narrowed arteries). In addition, it is also used to evaluate for the physiologic significance of the coronary artery lesions as identified by CT coronary angiography or invasive coronary angiography in order to determine whether patients should be treated medically or require revascularization with coronary artery stenting or coronary artery bypass surgery. Besides, myocardial infarction (heart muscle damage) due to underlying coronary artery disease can also be identified.
According to ACCF/AHA/ASE/ASNC/HFSA/HRS/SCAI/SCCT/SCMR/STS 2013 Multimodality Appropriate Use Criteria for the Detection and Risk Assessment of Stable Ischemic Heart Disease, stress CMR can be considered in patient who is symptomatic and for patient who is unable to exercise with uninterpretable ECG. It is appropriate to use stress CMR to evaluate patients who have newly diagnosed systolic or diastolic heart failure, ventricular arrhythmias, abnormal resting ECG suggestive of coronary artery disease, abnormal exercise ECG test, obstructive coronary artery disease or coronary artery stenosis of uncertain significance on CT coronary angiography study or invasive coronary angiography.
Stress CMR is also indicated in patients with new or worsening symptoms following a prior normal/abnormal exercise ECG test; normal prior stress imaging study; non-obstructive coronary artery disease or obstructive coronary artery disease on CT coronary angiography/ invasive coronary angiography and abnormal coronary artery calcium score of >100.
Stress CMR may also be considered in asymptomatic patient with high global coronary artery disease risk; ≥ 2 years after percutaneous coronary intervention (PCI); ≥ 5 years after coronary artery bypass grafting (CABG); prior left main coronary stent; prior incomplete revascularization and patients with poor functional capacity (<4 METs) undergo intermediate risk surgery or vascular surgery.
In addition, stress CMR may also be considered for patients who are asymptomatic or have stable symptoms with the following tests performed ≥2 years ago - abnormal prior stress imaging, obstructive coronary artery disease on prior coronary angiography and also patients with intermediate to high global coronary artery disease risk with normal prior stress imaging scan or non-obstructive coronary artery disease on angiogram (invasive or non-invasive) or normal prior exercise ECG test. Asymptomatic patients with elevated prior coronary calcium Agatston score (≥100) and high global coronary artery disease risk or patient with prior coronary calcium Agatston score (>400) may also be considered for the test.
How is the preparation for this test?
Patients should abstain from caffeinated products (eg coffee, tea, espresso, soft drinks, cola, chocolate, cocoa, ovaltine, mountain dew, monster energy and redbull) for at least 12 hours prior to the test. Even decaf coffee contains small amount of caffeine. Caffeine, a methylxanthine alkaloid derivative, is a competitive inhibitor of the adenosine receptor due to its similar molecular structure to adenosine. As adenosine is used as stress agent for the heart muscle, the consumption of caffeinated products will make the test less accurate. Do inform your doctor if you have kidney impairment, implants or metal in your body and claustrophobia (fear of being enclosed in a small space).
How long does it take to complete this test?
It will generally take up 30-45 minutes. The scan time varies depending on patient’s heart rhythm and if patient can follow breathing instruction.
How does it compare to other imaging modalities?
Stress perfusion CMR is generally a very safe test. It is more accurate than treadmill exercise test alone in the detection of ischemic heart disease (coronary artery disease). As the heart is being stressed pharmacologically using adenosine, patients do not need to perform any exercise. This is ideal test for patients who are not able to do exercise due to various reasons.
According to CE-MARC study, stress CMR is shown to have better sensitivity and specificity (86.5%/83.4%) as compared to SPECT (cardiac nuclear imaging) (66.5%/82.6%) in regard to the detection of significant coronary artery disease. It also demonstrates higher negative predictive value (90.5%) than SPECT (79.1%). Nonetheless, over the years, the technique in the field of nuclear cardiology has improved substantially. Existing cardiac imaging modalities complement one another due to the unique design and technology of each imaging modality that targets on certain aspect of cardiac assessment for the patients. One of the major advantages of the stress perfusion CMR is that there is no ionising radiation being used in CMR perfusion. Therefore, there is no concern for the exposure to radiation.
Is there any other information that we can obtain from the stress perfusion CMR?
Yes. Stress perfusion CMR can also demonstrate perfusion defect due to coronary microvascular disease (small vessel disease) where patients do not have evidence of significant epicardial coronary artery disease (large vessel disease). Coronary microvascular disease has been shown to be of great prognostic value lately. Apart from the protocol used for stress perfusion, MRI can be used to detect abnormalities of the valves and congenital heart disease, for example, atrial or ventricular septal defect. In addition, MRI can also be used to assess for the presence of myocarditis (inflammation of the heart muscle), cardiomyopathy (heart muscle disease), for example, infiltrative cardiomyopathies like amyloidosis, sarcoidosis and iron overload in heart and liver. If indicated, coronary MR angiography can be performed to assess the coronary arteries origin and coronary artery stenosis. At times, significant abnormalities outside the heart can be incidentally picked up eg lung tumor etc.