Coronary arteries are the vessels that carry oxygen-rich blood to the heart muscles. Due to the accumulation of fat and plaque under their inner layer, calcification of arteries can occur. Eventually, the plaque build-up can narrow the inside of the coronary artery and limit the flow of blood to the heart muscle, resulting in coronary artery disease (CAD) or a myocardial infarction (heart attack).
Currently, there are many invasive and non-invasive methods to evaluate coronary artery blockage and to prognosticate the risk of CAD visualising the lumen of coronary arteries. The invasive method is the conventional coronary angiography. With technological advancements, we can evaluate coronary artery blockage by coronary angiography with computed tomography (CT) scan. There is also an indirect non-invasive preliminary method of assessing risk of coronary artery disease in CT scan by assessing calcified plaque burden, called coronary artery calcium (CAC) score.
A special CT scan called a CAC Scoring CT is used to measure the amount of calcium, giving a patient their CAC score. It is performed with a cardiac CT scanner with ECG gating without injecting any medicine or without a needle prick. Currently, guidelines from around the world endorse measurement of CAC score to improve clinical risk prediction in appropriately selected asymptomatic individuals. The CT calcium score is supposed to estimate the patient's risk of having a heart attack or stroke in the next 5-10 years. Being fast, painless, easy and non-invasive, Coronary Artery Calcium Scoring CTs are recommended by doctors for patients at low to medium risk - including those with a personal history of high cholesterol, diabetes, hypertension, obesity, or a family history of CAD. Thus, many patients entirely rely on these test scores to determine their risk of coronary heart disease.
Although a higher calcium score signifies a higher risk of significant CAD, as observed in many studies, it does not always hold true. Through a CAC score only calcified plaques in coronary arteries can be assessed and not the extent of underlying coronary artery blockage.
The next advanced test is non-invasive CT coronary angiography, which is performed with a high-end cardiac CT scanner with ECG gating. A CT coronary angiogram is an imaging test to view your coronary arteries and commonly indicated in population with low to moderate probability of CAD. The pre-requisite to perform this test is normal kidney function (normal serum creatinine level) and fasting for 3 hours. Iodinated medicine is injected through a peripheral venous access and then CT angiography scan is performed in an especially capable CT scanner. Apart from providing the CAC score, a CT coronary angiogram provides the extent of luminal stenosis or the underlying blockage in a patient. The risk of significant CAD is determined according to the extent of stenosis.
The question is whether to use CT CAC score alone to assess the risk of CAD or to directly perform a CT coronary angiography to get complete information of the coronary artery stenosis. A recent study conducted by the authors of this article, gives insight into whether your calcium score can be used independently to determine your risk of coronary artery disease (CAD). The research was focussed on individuals of Indian origin. Overall, 17% patients in the sample population had severe stenosis or complete occlusion. A significant directional correlation between the calcium score and the extent of stenosis was observed. Of patients with extensive calcium score (above 400), 59% had moderate to severe stenosis compared to only 1.2% amongst the patients who had a calcium score of zero.
However, it was observed that even amongst patients with minimal calcium score (less than 10), 7% had moderate to severe stenosis. Amongst patients with mild calcium score (10 to 100), 13% had moderate to severe stenosis and another 1% had total occlusion. As a corollary, it was observed that 8% of the patients with extensive calcium score had minimal stenosis and 33% had only mild stenosis. Therefore, relying completely on calcium score to determine risk of CAD may not be advisable.
Stenosis is not significantly lower in the asymptomatic population (no chest pain), as compared to the symptomatic (suffering from chest pain). 16% of the asymptomatic population had occlusion or severe stenosis compared to 20% of the symptomatic population. As observed, while 61% of symptomatic patients with extensive calcium score had Severe stenosis, even for asymptomatic patients with extensive calcium score, severe stenosis percentage was quite high at 44%. Also, while 98.9% of the asymptomatic patients with zero calcium score have no stenosis, 1.1% had severe stenosis. This is not significantly different from 1.3% of symptomatic patients with zero calcium score who have severe stenosis. Further, 5.7% of asymptomatic patients with extremely low calcium score had moderate to severe stenosis. It may therefore not be entirely safe to depend on calcium score to determine zero risk of CAD even for asymptomatic patients with zero calcium score.
The quantitative grading of the stenosis of coronary arteries has been carried out based on Society for Cardiovascular Computed Tomography (SCCT) recommendations. Minimal: plaque with < 25% stenosis. Mild: 25-49%, Moderate: 50-69%, Severe: 70-99%, Occluded: 100
Calcium Score Range: Zero - Up to 0; Minimal – up to 10; Mild – up to 100; Moderate – up to 400; Extensive – above 400.
In view of increasing prevalence of CAD in younger population due to multiple factors, some form of screening test for CAD may be worthwhile for high-risk individuals. Cardiac CT is one of the most commonly available, proven and useful diagnostic tools for screening CAD. It is common practice to use CT coronary calcium score as a basic test for risk stratification of CAD. However, data indicates that often independently it is not a very accurate method to rule out significant CAD. Given that CAD can be life threatening, it would be prudent to do a more detailed investigation through a non-invasive CT coronary angiogram, instead of CT Coronary calcium scoring alone.
A-D show CT angiography image a 43 -year-old gentleman, who came in for a general check-up but calcium score was found to be 127.3 (E) and diagnosed with chronic total occlusion (100% stenosis) of right coronary artery (B, D), normal left coronary artery (A, C)
(The research was conducted on a sample population of 691 patients who were either referred for a medical investigation or those who chose to do a general check-up, and may not be entirely representative of the country’s population at large)
(Dr. Himanshu S Choudhury is a senior radiologist with over 12 years of experience. He is presently working as a Lead, Cardiothoracic imaging at Sir HN Reliance Foundation Hospital, Mumbai)
(Aarna Sanghai is a grade 12 student at Dhirubhai Ambani International School, Mumbai with a keen interest in biology and biotechnology)
