Just to recap, we have been looking at how we diagnose coronary artery disease (blocked arteries). So far we have looked at the role of the history and physical examination, blood testing, electrocardiogram, echocardiogram, stress tests, and coronary angiography.

We noted that coronary angiography is the gold standard for detecting the presence of blocked arteries and that the other tests can give an idea of whether a blockage is present, but does not give actual visualisation of the coronary arteries.

As a reminder, a coronary angiogram is a specialised X-ray where small tubes (catheters) are passed through the arteries in the leg or arm directly to the heart and an iodine-based dye is injected through these catheters directly into the coronary arteries (the blood vessels that bring blood to the heart). The test usually requires at least 12 hours in the cardiac catheterisation laboratory and sometimes an overnight stay for observation. It involves exposure to radiation.

Coronary angiography is not without risk and is expensive, so the cardiologist has to have a very strong index of suspicion or evidence that blockage actually exists to justify sending a patient for a coronary angiogram. This is where cardiac computed tomography (CCT) and cardiac magnetic resonance imaging (CMRI) come in, as they allow direct visualisation of the coronary arteries without the invasive access and subsequent risks associated with a traditional coronary angiogram.

Over the past two decades, CMRI (1993) and CCT (1995) have become indispensable tools for cardiac imaging because of their ability to generate extraordinarily vivid images of the beating heart and blood vessels in a strictly non-invasive manner. These imaging modalities were developed as a way to provide direct visualisation of the coronary arteries and detailed examination of the heart without the small, but potentially serious risks associated with invasive testing such as cardiac catheterisation.

So which patients would the cardiologist recommend for cardiac MRI or cardiac CT?

• If stress testing does not give a diagnosis in spite of persistent symptoms;

• If there is strong clinical suspicion which conflicts with findings on stress testing;

• Coronary artery disease risk assessment, in case of coronary artery calcium scoring.

Differences between cardiac MRI and CT

Five characteristic differences between cardiac MRI and CT that patients need to be aware of are:

1.

Radiation and safety: Cardiac MRI utilises magnetic fields in combination with radio waves and powerful computers to generate images. Cardiac CT, on the other hand, relies on shooting X-rays from multiple angles to produce a full three-dimensional computer model of the body and internal organs. The radiation involved with CT may result in an increased lifetime risk of cancer, especially in people who’ve had repeated CT scans. Cardiac MRI is radiation-free.

2.

Need for intravenous (IV) contrast: An intravenous injection of contrast media is absolutely necessary for cardiac CT to define the blood-tissue interface and tell the difference between different kinds of tissues. While with cardiac MRI, IV contrast isn’t needed. There are some occasions when a doctor might need to use contrast during a cardiac MRI to highlight conditions that would otherwise be undetectable.

3.

Contrast side effects: There are two types of contrast used: iodine-based (for cardiac CT) and gadolinium-based (for cardiac MRI). The iodine-based agents have the potential to cause “contrast-induced nephropathy” in some patients. That is a condition that can cause reduction in kidney function for patients with pre-existing kidney problems. Gadolinium-based contrast agents, on the other hand, are much safer for the kidneys, but have the potential to cause a condition known as “nephrogenic systemic fibrosis” in those with reduced kidney function. Ultimately, iodine-based agents can be removed during haemodialysis, if needed, whereas the fate of gadolinium-based agents is less clear.

4.

Heart rate control: To produce blur-free images of the beating heart, both CT and MRI require a fairly regular heart rate. Ideally that is 60 beats per minute with a CT scan. While detrimental to cardiac CT, rapid heart rates are usually not much of an issue for cardiac MRI as long as the pulse remains regular.

5.

Patient-scanner compatibility: Because cardiac MRI uses strong magnets, a very thorough screening for ferromagnetic objects on or within the patient’s body needs to be conducted before any patient is deemed safe to undergo cardiac MRI. Fortunately, patients with implanted ferromagnetic devices can safely undergo cardiac CT, provided they have good kidney function and are not severely allergic to iodinated contrast media.

Dr Claudine Lewis is an adult cardiologist and medical director at Heart Smart Centre in Montego Bay. She is also a cardiologist at the Cornwall Regional Hospital and an associate lecturer with the University of The West Indies. Questions may be sent to questions@heartsmartcentre.com and for additional information call 684-9989 or visit the website www.heartsmartcentre.com