Chapter 2.1 Coronary artery calcium imaging

Martin J. Willemink

Teaching points

• The presence or absence of coronary artery calcium in asymptomatic individuals is a robust measure of subclinical atherosclerosis, allowing for risk reclassification for the primary prevention of atherosclerotic cardiovascular events.

• The optimal calcium scoring acquisition protocol is a prospectively electrocardiogram-triggered non-contrast-enhanced CT scan.

• The Agatston score is the most validated coronary artery calcium quantification method.

• The Coronary Artery Calcium Data and Reporting System (CAC-DRS) is a standardized method to communicate coronary artery calcium findings on non-contrast CT scans.

• Coronary artery calcium scoring is recommended in asymptomatic adults at intermediate cardiovascular risk. In select cases of adults with low-to-intermediate risk coronary artery calcium may be used to further evaluate risk.

Coronary artery calcium has shown to be a strong predictor of the long-term risk for atherosclerotic cardiovascular disease in asymptomatic individuals. Coronary artery calcium can be quantified with a non-contrast CT scan. Coronary artery calcium can be used to reclassify asymptomatic individuals at intermediate cardiovascular risk. The absence of coronary artery calcium on a CT scan is a strong marker for identification of individuals at very low 10-year risk. Elevated coronary artery calcium scores indicate that the individual is likely to benefit from treatment.

Acquisition protocol

Patient preparation

• There is no need for beta-blockers, nitroglycerin, or other medications. Calcium CT scans are performed without iodinated contrast agents.

• Patients do not need to fast before the exam.

• Patients should be able to lie still and hold their breath for 3–5 seconds.

Image acquisition

• Prospectively electrocardiogram (ECG)-triggered non-contrast CT is the most optimal protocol due to the relatively low radiation exposure.

• Recommended tube potential is 120 kVp, while tube current can be adjusted based on patient size.

• Reducing the tube potential to 100 kVp results in a substantial radiation dose reduction. However, low-voltage acquisition is not routinely performed because it changes the calcium scores. This can be partially corrected by adjustment of the Hounsfield unit (HU) threshold.

• The effective radiation dose should be approximately 1 mSv, or lower.

• Reconstructed slice thickness should be 2.5 or 3.0 mm, depending on the CT manufacturer.

• Iterative reconstruction algorithms are currently not recommended for calcium imaging because this method results in lower calcium scores.

• In general, convolution kernels with medium sharpness are used for calcium scoring, to compromise between noise and blooming of calcifications. The selection of convolutional kernel depends on the vendor.

• Besides dedicated ECG-triggered CT scans, the calcium burden can be semi-quantified on non-gated chest CT exams. Visual interpretation (as discussed later) may be used to quantify coronary calcium on these exams, but Agatston scores cannot be calculated on non-gated chest CT.

See also Chapter 1.5.

Calcium quantification

Agatston score

• The Agatston score is the most validated quantification method for coronary calcium.

• A calcification within the coronary arteries is defined as a ≥1 mm² area with a CT density of ≥130 HU.

• The Agatston score per calcification is calculated by multiplying the area of each calcification by a density weighting factor related to the maximum attenuation within the calcification:

• 130–199 HU: factor 1;

• 200–299 HU: factor 2;

• 300–399 HU: factor 3;

• ≥400 HU: factor 4.

• The total Agatston score is calculated by taking the weighted sum of each calcification.

• A basic interpretation of the absolute Agatston score with regard to coronary plaque burden:

• Agatston score = 0, no identifiable disease;

• Agatston score = 1–99, mild disease;

• Agatston score = 100–399, moderate disease;

• Agatston score ≥400, severe disease.

Volume and mass scores

• Besides the Agatston score, coronary calcium can be quantified as volume and mass (Table 2.1.1).

• The calcium volume (mm³) is calculated by multiplying the number of voxels within calcified plaques with an attenuation of >130 HU by the volume of each voxel.

• Calcium mass (mg) is calculated by multiplying the mean attenuation of each calcified plaque by the plaque volume. A correction factor based on the attenuation of water is used to calculate the mass.

• Volume and mass have better reproducibility than the Agatston score; however, they are less validated for cardiovascular outcomes. Volume and mass are thus rarely used in the clinical setting.

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