What Is a CAC Scan and How Does It Work?

Introduction

A coronary artery calcium scan detects calcified plaque in the arteries that supply blood to your heart. The test uses computed tomography without contrast dye to create cross-sectional images of the heart. Software then quantifies how much calcium is present in each coronary artery, producing a single number called the Agatston score.

The scan matters because calcium deposits serve as a marker of atherosclerosis. They correlate with total plaque burden. A higher score indicates more extensive disease and higher future risk of heart attack. Unlike blood tests or risk calculators that estimate probability, CAC scanning directly visualizes disease that has already developed.

This article explains the technology behind CAC scanning, how the Agatston scoring method works, what to expect during the test, and how radiation exposure compares to other imaging. Subsequent articles address how to interpret your results, how CAC compares to other cardiac tests, and how to obtain a scan if you believe it could benefit you.

What exactly does a coronary artery calcium scan measure?

A CAC scan measures calcified plaque deposits within the walls of your coronary arteries. Calcium accumulates in atherosclerotic lesions as part of the body’s healing response to vascular injury and inflammation. The test does not measure blood flow, soft plaque, or vessel narrowing. It simply quantifies the amount and density of calcium present.

The presence of coronary calcium confirms that atherosclerosis exists. The MESA study demonstrated that coronary calcium predicts heart attacks across all major ethnic groups, independent of traditional risk factors (Detrano, 2008). A score of zero suggests minimal to no calcified plaque, though it does not exclude the presence of non-calcified or soft plaque entirely.

Higher calcium scores correlate with greater total plaque burden, though the relationship is not perfectly linear. Someone with a score of 400 has substantially more atherosclerosis than someone with a score of 40, but two people with similar scores may have different plaque compositions and distributions. The CAC Consortium analysis of over 66,000 patients confirmed that higher scores predict cardiovascular mortality in a dose-dependent fashion (Grandhi, 2020).

How does the CT technology detect and quantify calcium?

Non-contrast CT uses X-rays to create detailed cross-sectional images. Calcium appears bright white because it absorbs X-rays more efficiently than soft tissue. The scanner captures images during a single breath-hold, timed to the heart’s electrical activity so that motion blur is minimized.

Modern CT scanners acquire images in a few seconds. ECG-gating synchronizes image acquisition to the resting phase of the cardiac cycle, when the heart is relatively still. Photon-counting CT technology represents the newest advance, offering improved spatial resolution and potentially lower radiation doses compared to conventional detector systems (Si-Mohamed et al., 2022).

The scanner identifies regions where X-ray attenuation exceeds 130 Hounsfield Units, the threshold that distinguishes calcified plaque from other tissue. Each calcified lesion is then measured for area and density. These measurements feed into the scoring algorithms that produce the final result.

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What is the Agatston scoring method and why is it the standard?

Arthur Agatston developed the original calcium scoring method in the late 1980s using electron beam CT. His approach multiplies the area of each calcified lesion by a density factor based on the peak Hounsfield Unit value within that lesion. The products are summed across all coronary arteries to produce a total score.

The density factor ranges from 1 to 4, increasing with calcium density. A score of 100 might represent many small, less dense lesions or fewer larger, denser ones. This weighting system means the Agatston score reflects both the extent and intensity of calcification. The method became the standard because most validation studies used it, making new results comparable to existing literature.

Despite its widespread adoption, the Agatston method has limitations. The 130 Hounsfield Unit threshold was chosen somewhat arbitrarily. Scores can vary by 10-20% between scans due to technical factors like heart rate variability and scanner differences. Still, the extensive validation across large cohorts established its predictive value firmly enough that alternatives have not displaced it (LaMonte, 2005).

What’s the difference between a calcium score and a calcium volume score?

The Agatston score incorporates both lesion area and calcium density. The volume score measures only the total volume of calcified plaque above the detection threshold, ignoring density differences. Both approaches aim to quantify coronary calcium, but they weight different aspects of the measurement.

Volume scoring may be more reproducible for serial testing because it does not depend on density factors that can vary with scanner settings and patient heart rate. However, most clinical decision-making and risk prediction models are based on Agatston scores, limiting the practical utility of volume measurements in everyday care.

Some reports include both metrics, but physicians typically use the Agatston score for clinical decisions. The percentile rankings and risk categories that inform treatment recommendations derive from studies using the Agatston method. For most patients, understanding the Agatston score is sufficient.

How long does the scan take and what’s the patient experience?

The actual imaging takes less than 30 seconds. Patients lie on a table that slides into the CT scanner, which resembles a large doughnut. A technician provides breath-hold instructions. Three to five breath-holds of about ten seconds each are typically required. Total time in the imaging suite is usually 10-15 minutes, including positioning and setup.

No contrast dye is injected. No intravenous line is placed. Patients do not need to fast beforehand. There is no sedation. The scan itself is painless. People with claustrophobia may feel anxious in the scanner, but the open design of most CT systems and the brief duration make this manageable for most.

Results are usually available within a few days, though some centers provide same-day reports. The interpreting radiologist or cardiologist identifies calcified lesions in each coronary artery and calculates the total Agatston score. The report may include individual vessel scores, a total score, and a percentile ranking.

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What radiation exposure does a CAC scan involve compared to other imaging?

A typical CAC scan delivers 1-3 millisieverts of radiation. For comparison, a screening mammogram delivers approximately 0.4 millisieverts, a chest X-ray about 0.1 millisieverts, and a CT angiogram of the coronary arteries approximately 3-10 millisieverts. Background radiation exposure from natural sources averages about 3 millisieverts per year in the United States.

The radiation dose from a single CAC scan is considered low. The theoretical cancer risk from this level of exposure is very small and must be weighed against the potential benefit of identifying subclinical atherosclerosis. For most adults in the target screening population (ages 40-75), the cardiovascular risk information gained likely outweighs the minimal radiation exposure.

Newer scanner technologies continue to reduce radiation doses while maintaining image quality (Skoog et al., 2023). Photon-counting detectors and iterative reconstruction algorithms allow diagnostic-quality images at lower doses than older systems required.

Can you get a CAC score from a regular chest CT, or does it require a dedicated protocol?

A dedicated CAC protocol uses ECG-gating to minimize cardiac motion and standardized acquisition parameters to ensure accurate scoring. This differs from a routine chest CT, which may not be timed to the cardiac cycle and uses settings optimized for lung or mediastinal evaluation rather than coronary calcium quantification.

Some facilities can calculate an approximate CAC score from non-gated chest CT scans, sometimes called “opportunistic” calcium scoring. These estimates correlate reasonably well with gated scores but are less precise. They may be useful for patients who have already undergone chest CT for other reasons, such as lung cancer screening.

For primary screening, a dedicated gated CAC protocol remains the standard. The precision matters because treatment decisions often depend on whether a score falls above or below specific thresholds. If you are obtaining a CAC scan specifically for cardiovascular risk assessment, confirm that the facility uses a dedicated cardiac protocol with ECG-gating.

Conclusion

CAC scanning provides a direct measurement of calcified atherosclerotic plaque in the coronary arteries. The Agatston method has been validated across millions of patients and predicts cardiovascular events better than traditional risk factors alone. The test is quick, painless, requires no contrast, and involves modest radiation exposure.

Understanding what the scan measures establishes the foundation for interpreting results and making informed decisions. The next article explains how to interpret your CAC score, including what different score ranges mean and why percentile rankings matter. For information on how CAC compares to other cardiovascular tests, see CAC vs Other Cardiac Tests.