What is the difference between a pulmonary computed tomography (CT) angiogram and a coronary computed tomography (CT) angiogram?

Pulmonary CT Angiogram vs Coronary CT Angiogram: Key Differences

A pulmonary CT angiogram (CTPA) is specifically timed and protocoled to visualize the pulmonary arteries for detecting pulmonary embolism, while a coronary CT angiogram requires ECG-gating to synchronize with the cardiac cycle for detailed visualization of the coronary arteries and assessment of coronary artery disease. 1, 2

Primary Anatomic Target

• CTPA targets the pulmonary circulation with contrast timing optimized to capture peak arterial enhancement specifically in the pulmonary arteries, scanning from the top of the aortic arch to the diaphragm 3, 1
• Coronary CTA targets the coronary arteries and requires visualization of these small, tortuous vessels that are highly susceptible to cardiac motion 3

Technical Acquisition Differences

ECG Gating Requirements

• Coronary CTA mandates ECG synchronization to minimize cardiac motion artifacts and allow clear visualization of coronary arteries, with image acquisition timed to specific phases of the cardiac cycle 3, 2
• CTPA does not require ECG gating in standard protocols, though newer high-pitch dual-source techniques with ECG triggering can reduce motion artifacts 3, 2

Contrast Timing Protocol

• CTPA uses high injection rates (5.0-7.5 mL/s) with scan durations <1 second to achieve homogenous opacification of pulmonary arteries, typically using low-kilovoltage (80-100 kV) scanning 3, 4
• Coronary CTA requires precise timing to capture coronary arterial enhancement, often with beta-blocker administration to reduce heart rate below 65 beats per minute if no contraindication exists 3

Image Reconstruction

• CTPA uses thin-section acquisition (typically 1mm collimation) to visualize pulmonary vessels down to subsegmental branches, with multiplanar reformations focused on pulmonary vascular assessment 3, 4
• Coronary CTA requires dedicated multiplanar reformation of coronary arteries with 3D renderings as a mandatory element, per ACR definitions 3

Clinical Indications

Pulmonary CTA

• Primary indication is suspected acute pulmonary embolism, serving as the first-line diagnostic imaging modality with 96-99% sensitivity and specificity 3, 4
• Evaluates pulmonary arterial pathology including chronic thromboembolic disease, pulmonary artery stenosis, and branch pulmonary artery abnormalities 3
• Provides prognostic information including RV/LV ratio, pulmonary artery diameter, and septal deviation that predict short-term mortality 3, 4

Coronary CTA

• Primary indication is evaluation of coronary artery disease, including assessment for stenosis, anomalous coronary arteries, and atherosclerotic plaque 3, 2
• Useful for detecting coronary artery anomalies, which occur with increased incidence in congenital heart disease populations, particularly to assess interarterial or aortic intramural course 3
• Can assess coronary artery fistulas or malformations and screen for potential stenosis, usually without sedation due to high spatial and temporal resolution 3

Radiation Dose Considerations

• CTPA typically uses lower radiation doses with low-kilovoltage techniques (80-100 kV) as standard protocol 3, 4
• Coronary CTA traditionally delivers higher radiation doses due to ECG synchronization requirements, though newer techniques including prospective triggering, adaptive statistical iterative reconstruction, and high-pitch spiral acquisition have substantially reduced this 3, 2

Critical Ordering Pitfalls

• Never order "CT chest with contrast" when evaluating for PE—the study must be specifically protocoled as CTPA with appropriate timing and injection rates 4
• Coronary CTA cannot be adequately performed without ECG gating—a standard chest CTA will show cardiac structures but with motion artifacts that prevent detailed coronary artery evaluation 2
• CTPA requires all three CTA elements per ACR definition: (1) timing for peak vascular enhancement, (2) multiplanar reconstructions, and (3) 3D renderings 3, 4

Incidental Findings

• CTPA can reveal cardiac pathology including myocardial disease, pericardial disease, valvular abnormalities, and intracardiac abnormalities, though these are not the primary focus 5
• Coronary CTA provides information about cardiac chamber size, function, and morphology in addition to coronary artery assessment 3, 2