Coronary Artery Calcium (CAC) scoring is a non-invasive medical procedure that uses a computed tomography (CT) scan to measure the amount of calcified plaque present in the coronary arteries. These calcium deposits are a direct indicator of atherosclerosis, which is the underlying cause of coronary artery disease. To ensure the highest quality images for an accurate assessment, patients are instructed to follow specific preparation guidelines. This preparation includes avoiding strenuous activity or exercise before the test. The primary goal of this strict instruction is to keep the heart rate as low and steady as possible during the brief scanning period.
The Basics of Calcium Scoring
The CAC scan relies on specialized imaging technology to capture rapid, high-resolution images of the heart. The heart is a constantly moving organ, making it a challenging subject for still photography. To overcome this challenge, the procedure uses a technique known as electrocardiogram (ECG) gating.
ECG gating synchronizes the timing of the CT scanner’s image acquisition with the electrical activity of the heart. The scanner is programmed to capture images specifically during the quiescent phase of the heart’s cycle, which is known as diastole. This is the brief period when the heart muscle is relaxed and relatively still, occurring between contractions.
Any blurring caused by cardiac movement can compromise the ability to correctly identify and measure the small calcium deposits. Precise timing and minimal motion are necessary for the software to accurately quantify the calcified plaque, which determines the final Agatston score.
Exercise and Heart Rate Dynamics
Exercise immediately increases the heart rate (tachycardia) and also increases the force of myocardial contraction. These physiological changes directly interfere with the imaging requirements of the CAC scan, working against the scanner’s need for a slow, steady, and quiet heart.
The optimal conditions for a successful CAC scan require the heart to be beating at a slow, resting rate, ideally below 60 beats per minute. When the heart rate is elevated due to recent exercise, the quiescent phase of diastole becomes significantly shorter. A faster heart rate means the heart spends less time in the relaxed, still phase, leaving a smaller window for the CT scanner to capture a clear image.
Beyond heart rate, physical activity also causes coronary vasodilation, which is the widening of the coronary blood vessels. This increased blood flow and vessel diameter can contribute to the overall motion of the arteries being scanned. This compounded movement makes it more difficult for the ECG-gated system to synchronize the image capture precisely, leading to a high probability of image degradation.
Protecting Scan Accuracy
The increased speed and motion of the heart and coronary arteries cause motion artifacts in the resulting CT images. These artifacts appear as image blurring, making it nearly impossible for the scoring software to accurately define the edges of any calcified plaque.
A blurry scan prevents the computer from precisely measuring the size and density of the calcium deposits. This can lead to a mis-scoring of the patient’s coronary artery calcium, potentially resulting in an artificially high or low Agatston score. An inaccurate score may require the patient to undergo a repeat scan or could lead to incorrect clinical interpretations of their cardiovascular risk.
To mitigate an elevated heart rate, technicians sometimes administer medication, such as beta-blockers, to slow the heart down to an acceptable rate before the scan can proceed. However, avoiding all strenuous activity before the test is the simplest and safest way for the patient to ensure the best possible image quality.