# 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.

![Non-contrast cardiac CT showing coronary artery calcification with Agatston score calculation](images/cardiac-ct-cac-scoring.png)

## 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

| CAD-RADS | Stenosis Severity | Management |
|----------|-------------------|------------|
| 0 | No stenosis (0%) | No further testing |
| 1 | Minimal (1-24%) | No further testing |
| 2 | Mild (25-49%) | Preventive therapy |
| 3 | Moderate (50-69%) | Consider functional testing |
| 4A | Severe (70-99%), 1-2 vessels | Consider ICA or functional testing |
| 4B | LM >50% or 3-vessel >70% | Consider ICA |
| 5 | Total occlusion (100%) | Correlate clinically |
| N | Non-diagnostic | Additional testing needed |

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).

![CCTA maximum intensity projection showing severe stenosis of the proximal LAD](images/cardiac-ct-ccta-lad-stenosis.png)

## 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.

![CCTA with CT-FFR color map showing hemodynamically significant stenosis in the mid-LAD](images/cardiac-ct-ffr-map.png)

## 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.