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FAST AND SAFE: A computed tomography (CT) scan has lower radiation, is completed within seconds and provides three- dimensional information.

Taking a good look at the heart

There are a number of non-invasive direct tests which allow direct visualisation of the heart arteries
30/07/2016 - 05:50

IT is not uncommon for those who experience shortness of breath or chest tightness to consider getting their heart checked. This is especially so if they have risk factors for heart disease such as high cholesterol, high blood pressure, diabetes mellitus, a smoking habit or a family history of heart disease.

Based on data from the American Heart Association, about half of all men and about two thirds of all women who die suddenly from a heart attack do not have prior symptoms. Hence, it is not uncommon to find that there are those with multiple risk factors who are considered high risk for heart disease and yet have no symptoms.

Common tests used to assess the heart include the electrocardiogram (electrical recording of the heart), echocardiogram (ultrasound of the heart), treadmill testing and stress nuclear scans. These are non-invasive indirect tests which do not allow visualisation of the heart arteries but provide doctors with an indirect assessment of the likelihood of the presence of significant blockage of the heart arteries (coronary artery disease or CAD).

In a publication by Patel in the New England Journal of Medicine in 2010, for every 100 patients in the USA who had an abnormal non-invasive indirect stress test and underwent an invasive coronary angiogram (a test which involves the insertion of plastic tubes into the heart arteries and taking X-ray images), only about 40 per cent were found to have significant CAD. Hence, in contrast to the high 70 to 90 per cent accuracy reported in publications from high volume centres with extensive experience, in the real world, the accuracy of non-invasive indirect tests can vary significantly as it is very dependent on the experience and skill of the staff performing and reporting the tests.

Over the last decade, there have been significant advances in the development of non-invasive direct tests which allow direct visualisation of the heart arteries. These tests include computed tomography (CT) scan of the heart arteries (also called coronary CT angiogram or CCTA) and magnetic resonance imaging of the heart arteries (also called coronary magnetic resonance angiography or CMRA). The other test that allows direct visualisation of the heart arteries is invasive coronary angiogram (CAG).

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Coronary CT angiogram

Nature of test: A CCTA or CT scan of the heart arteries involves the injection of an iodine-based contrast agent through the arm vein to visualise the heart arteries using X-rays from a CT scanner. The scan is completed within seconds and it is an outpatient procedure. The recorded images are reconstructed to three-dimensional heart artery images.

Basis of test: X-ray photographic snapshots of heart arteries taken when a contrast agent fills the arterial lumen enable the viewer to differentiate between the lumen and wall of the heart artery.

Understanding results: The CCTA images allow direct visualisation of the heart arteries and provide highly accurate information on the degree of narrowing, the distribution of the cholesterol deposits and the presence of calcification. The images also allow the differentiation of "soft" or "hard" plaques at the site of narrowing.

Best for patient: It is the most accurate non-invasive test for those who are suspected to have significant CAD. The quality of the scan, the radiation exposure, the accuracy and degree of detail in the report varies from centre to centre. Having a scan done at a highly experienced centre means low radiation, high accuracy, and detailed reports.

Practical implications: Studies show that a negative CCTA scan predicts no heart attacks for the next two years. If you are asked to undergo a CAG of the heart, you should consider a CCTA instead. Compared to CAG, the CT scan has lower radiation, is of lower cost, is safer, can be performed as an outpatient procedure, is completed within seconds and provides three-dimensional information.

Coronary CT calcium score

Nature of test: Coronary CT calcium score or CT scan to detect the presence of calcium deposits in the wall of heart arteries is used to assess the likelihood of narrowing of heart arteries. It is an outpatient scan that is completed within seconds. Unlike CCTA, the heart artery cannot be seen.

Basis of test: The calcium is a surrogate for plaque deposits in the heart artery. Hence, a higher calcium score means more calcium deposits which suggest more extensive plaque deposits in heart arteries.

Understanding results: A high calcium score means more extensive disease of heart arteries. It does not mean that there is significant CAD. The segments with the highest amount of calcium deposits may not be the segment with the most severe blockage. A low calcium score does not mean that there is definitely no significant CAD as cholesterol deposits often have no calcium deposits.

Best for patient: It is a safe test which predicts likelihood of coronary artery disease.

Practical implications: This was a popular test in the 1990s and early 2000s but increasingly, more physicians are choosing CCTA as it allows visualisation of heart arteries. Nevertheless, it remains a safe option.

Coronary magnetic resonance angiography (CMRA)

Nature of test: A CMRA records images of the heart arteries using a magnetic resonance imaging (MRI) scanner. The scan takes minutes, is an outpatient procedure and does not result in any ionising radiation exposure. The recorded images are reconstructed to two-dimensional heart artery images. This scan cannot be done on routine MRI scanners and can only be done with specialised MRI scanners. Some scanners do not require any contrast media during the CMRA.

Basis of test: The CMRA detects flow in the heart arteries and the recorded data is converted to images of the heart artery lumen.

Understanding results: Unlike CCTA, it cannot provide visualisation of the wall and hence is less accurate than the CCTA.

Best for patient: It is the only non-invasive test enabling visualisation of heart arteries which has no ionising radiation risk and does not require any contrast injection.

Practical implications: For the young and for women of child bearing age who are suspected of having underlying heart artery disease, this is a safe alternative option. The main downside is that there are very few centres which have the training and skills to perform CMRA.

Invasive coronary angiogram (CAG)

Nature of test: A CAG involves the administration of local anaesthesia at the wrist or groin followed by the insertion of plastic tubes (catheters) of about 2mm diameter into the arteries and manipulation of the catheters to the opening of the heart arteries. This is followed by injection of iodine containing contrast media into the heart arteries and X-ray "snapshots" of the heart arteries.

Understanding results: Like CCTA , CAG allows accurate direct visualisation of the heart arteries but unlike CCTA, it only allows two-dimensional visualisation of the lumen. Among the three tests which allow visualisation of heart arteries, CAG has the best resolution. However, as the vessel wall cannot be visualised in CAG, assessment of the degree of narrowing may be limited by the inability to know what the true arterial diameter is when there is diffuse disease.

Best for patient: If there is strong evidence that there is severe CAD, such as following a heart attack, CAG will usually be performed. If there is no history of heart attack but there is strong possibility of heart artery disease based on assessment and testing, CAG can be considered to provide confirmatory evidence of the disease and to assist the doctor make a decision on treatment options.

Practical implications: CAG carries real risk, including major stroke, heart attack, tear of major arteries and even death. There is less than 1 per cent risk of major complications. There are however several prospective brain MRI studies that have shown that CAG is associated with 5-22 per cent risk of silent strokes. In the study by Patel in the New England Journal of Medicine on CAG in the USA, almost close to two thirds of the patients who underwent CAG in the USA did not have significant coronary artery disease. Hence, CAG could have been avoided and a non-invasive test such as CCTA will provide a very good alternative. In addition, the CAG cost and radiation dose can be several times that of CCTA.

Knowledge is power

Medical advances have grown by leaps and bounds. For example, CCTA a decade ago took about 15 seconds with a radiation dose higher than that of CAG. Fast forward to today, the newest CT scanners can perform a CCTA within a few seconds and with a radiation dose that is about one tenth of that of CAG. These advances have been so rapid that even physicians have been left behind in the knowledge race. While physicians send their patients for mammogram routinely without batting an eyelid, there are still physicians who do not realise the radiation dose from a CCTA using the latest scanners is comparable to a routine mammogram. Your health is your personal responsibility. Empowering yourself with knowledge will help you make better informed decisions.

  • This is the final of two parts on Best Test For The Heart
  • Dr Lim is medical director at the Singapore Heart, Stroke & Cancer Centre. He is a professor at Fudan University, Shanghai and vice-president of both the World Chinese Doctors Association and the World Federation of Chinese Cardiovascular Physicians. He is also founding editor of Heart Asia, a journal of the British Medical Journal Publishing Group

This series is brought to you by Heart, Stroke and Cancer Centre. It is produced on alternate Saturdays.