Residency · Residency · Diagnostic Radiology

Cardiac CT: Coronary Artery Calcium Scoring and CT Angiography

Introduction

Cardiac CT has emerged as a powerful non-invasive tool for evaluating coronary artery disease. Coronary artery calcium (CAC) scoring provides risk stratification for asymptomatic patients, while coronary CT angiography (CCTA) offers high-resolution anatomic assessment of coronary stenosis. Both techniques require ECG synchronization and understanding of cardiac-specific acquisition parameters.

Coronary Artery Calcium Scoring

Technique

CAC scoring is performed as a non-contrast, ECG-gated CT acquisition using prospective ECG triggering at 70-80% of the R-R interval (mid-to-late diastole), with a slice thickness of 2.5-3 mm. Calcium is defined as a lesion with attenuation greater than 130 Hounsfield units (HU) across at least 3 contiguous pixels (1 mm squared). The study requires no IV contrast, no beta-blockers, and no breath-hold coaching in most protocols, resulting in a very low radiation dose of approximately 1-3 mSv.

Agatston Score

The Agatston method is the standard scoring system. The score is calculated by multiplying the area of each calcified plaque by a density-weighting factor (1 for 130-199 HU, 2 for 200-299 HU, 3 for 300-399 HU, 4 for 400+ HU), with the total summed across all coronary arteries. A score of 0 indicates no identifiable atherosclerotic plaque and very low risk; 1-99 reflects mild plaque burden with low risk; 100-399 indicates moderate plaque burden with moderate risk; and 400 or higher signifies extensive plaque burden with high risk. A CAC score of 0 confers a negative predictive value exceeding 99% for significant coronary events over 5 years. Results should be reported with age-, sex-, and ethnicity-based percentile rankings.

Clinical Applications

CAC scoring is used for risk stratification in asymptomatic intermediate-risk patients (10-20% ten-year ASCVD risk) and aids in decision-making for statin initiation. It is not useful in symptomatic patients, where CCTA or stress testing is preferred. Key limitations include the inability to detect non-calcified (soft) plaque and the inability to quantify stenosis.

Coronary CT Angiography (CCTA)

Patient Preparation

Optimal CCTA requires heart rate control with a target of less than 65 bpm, typically achieved with oral or IV metoprolol administered pre-scan. Sublingual nitroglycerin (0.4-0.8 mg) is given 3-5 minutes before the scan to dilate coronary arteries, though it is contraindicated in severe aortic stenosis, recent PDE-5 inhibitor use, and hypotension. IV access with an 18-gauge antecubital catheter supports the high flow rate contrast injection (5-7 mL/s), and breath-hold training with the patient before acquisition is important.

Acquisition Technique

Retrospective ECG gating acquires data throughout the cardiac cycle, resulting in a higher dose but allowing functional assessment. Prospective ECG triggering acquires data during a narrow window (typically 70-80% R-R), delivering a lower dose and is preferred when possible. Tube voltage is set at 100-120 kVp (80 kVp for thin patients to reduce dose), with a contrast volume of 60-100 mL of high-concentration iodinated contrast. Bolus tracking or test bolus technique ensures optimal coronary opacification, with a target coronary enhancement of greater than 300 HU.

Normal Coronary Anatomy

The left main coronary artery (LMCA) arises from the left coronary cusp and bifurcates into the LAD and LCx. The left anterior descending (LAD) courses in the anterior interventricular groove and gives off diagonal and septal branches. The left circumflex (LCx) courses in the left atrioventricular groove and gives off obtuse marginal branches. The right coronary artery (RCA) arises from the right coronary cusp, courses in the right atrioventricular groove, and gives off the posterior descending artery (PDA) in right-dominant circulation (85%). Dominance is determined by which artery gives rise to the PDA.

Interpretation and Reporting

The CAD-RADS reporting system (Coronary Artery Disease Reporting and Data System) standardizes communication. CAD-RADS 0 indicates no stenosis (0%); CAD-RADS 1 indicates minimal stenosis (1-24%); CAD-RADS 2 indicates mild stenosis (25-49%); CAD-RADS 3 indicates moderate stenosis (50-69%); CAD-RADS 4A indicates severe stenosis (70-99%) in one or two vessels; CAD-RADS 4B indicates left main stenosis greater than 50% or three-vessel stenosis greater than 70%; CAD-RADS 5 indicates total occlusion (100%); and CAD-RADS N designates a non-diagnostic study. Modifiers include S (stent), G (graft), and V (vulnerability features such as positive remodeling, low-attenuation plaque, napkin-ring sign, and spotty calcification).

Plaque Characterization

Plaque Types on CCTA

Calcified plaque is high attenuation (>130 HU), stable but indicative of atherosclerosis. Non-calcified (soft) plaque is low attenuation (<30 HU) and more concerning for vulnerability. Mixed plaque contains both calcified and non-calcified components. High-risk plaque features include positive remodeling (remodeling index >1.1), a low-attenuation plaque core (<30 HU), the napkin-ring sign (a thin rim of high attenuation surrounding a low-attenuation core), and spotty calcification.

Functional Assessment

CT Fractional Flow Reserve (CT-FFR)

CT-FFR applies computational fluid dynamics to CCTA datasets to estimate the hemodynamic significance of stenosis. An FFR less than or equal to 0.80 indicates a hemodynamically significant stenosis. This technique reduces unnecessary invasive catheterization and is performed off-site or with on-site software from standard CCTA acquisitions.

CT Perfusion

Dynamic or static stress CT perfusion identifies myocardial ischemia. Combined with CCTA, it provides both anatomic and functional information in a single modality. A stress agent (regadenoson or adenosine) is administered before acquisition.

Key Clinical Pearls

A CAC score of zero provides a very high negative predictive value for coronary events and can guide shared decision-making about statin therapy. Heart rate control to less than 65 bpm is critical for diagnostic CCTA image quality. The CAD-RADS system standardizes reporting and guides downstream management. CT-FFR adds functional significance to anatomic stenosis, reducing unnecessary invasive angiography.

References

1. CAD-RADS 2.0: Coronary Artery Disease Reporting and Data System. Radiology. 2022;305(1):209-221.
2. 2019 SCCT Guidelines for Coronary CT Angiography. J Cardiovasc Comput Tomogr. 2019;13(3):161-186.
3. Coronary Artery Calcium Scoring: Current Status and Future Directions. Radiographics. 2021;41(3):655-673.
4. CT-Derived Fractional Flow Reserve: Clinical Evidence and Future Directions. Eur Heart J. 2020;41(37):3504-3517.

Read this lecture as Markdown