What is a Computed Tomography (CT) angiogram?

What is a CT Angiogram?

A CT angiogram (CTA) is a specialized computed tomography scan that uses thin-section CT acquisition timed to coincide with peak arterial or venous contrast enhancement, with the resultant volumetric dataset interpreted using primary transverse reconstructions, multiplanar reformations, and 3D renderings—all three elements are essential and distinguish it from standard contrast-enhanced CT. 1

Technical Definition and Essential Components

The American College of Radiology formally defines CTA as requiring three mandatory procedural elements 1:

• Timing: Scan acquisition precisely timed to coincide with peak arterial (or venous) enhancement of the target vessels 1
• Reconstructions/Reformats: Multiplanar reformations in addition to primary transverse (axial) reconstructions 1
• 3D Renderings: Three-dimensional visualization is a required element that distinguishes CTA from standard contrast CT 1

This definition corresponds to the Centers for Medicare & Medicaid Services definitions applied to Current Procedural Terminology codes, making 3D rendering the key distinguishing feature from standard contrast-enhanced CT. 1

How CTA Works

CTA utilizes volumetric helical (spiral) CT data acquisition performed rapidly while high levels of circulating contrast material are present in the vascular system 2:

• Contrast injection: Iodinated contrast medium is injected intravenously, typically using bolus tracking over target vessels or empiric scan delays (15-20 seconds for arterial phase) 1, 3
• Thin-section acquisition: Modern multidetector CT scanners acquire thin slices (often submillimeter) to provide high spatial resolution 1, 3
• Volumetric coverage: The entire region of interest is scanned during a single breath-hold, typically 5-30 seconds depending on the area 2, 3

Clinical Applications by Anatomic Region

Thoracic Applications

CTA chest is specifically tailored to evaluate central vessels, including the aorta, pulmonary arteries, and subclavian/axillary vessels. 1

• Pulmonary embolism: Multidetector CT pulmonary angiography (CTPA) is now the primary imaging modality for evaluating suspected acute PE 1
• Thoracic aortic aneurysm: CTA has high sensitivity for detecting aneurysms, thrombus, and dissection, with delayed phase useful for diagnosing aortic wall thickening in infectious or inflammatory aortitis 1
• Aortic dissection: CTA provides rapid, accurate diagnosis of acute aortic pathology 1, 4

Cardiac Applications

Coronary CTA (CCTA) requires ECG-gating to synchronize image acquisition with the cardiac cycle, minimizing motion artifacts for detailed coronary artery visualization. 1, 5, 6

• Coronary artery disease: The European Society of Cardiology recommends CCTA as a first-line test for patients with low to moderate (15-50%) pre-test likelihood of obstructive CAD, with negative predictive value >95% 6
• Coronary anomalies: High-resolution assessment of anomalous coronary artery origin and course, especially interarterial or aortic intramural courses for surgical planning 1
• Congenital heart disease: ECG-gated cardiac CT provides valuable information about intracardiac anatomy and great vessels, including postoperative complications, with short acquisition times reducing sedation needs 1

A critical distinction: gated heart CT uses ECG synchronization to eliminate cardiac motion artifacts, while regular CTA chest is performed without ECG gating and shows cardiac structures with motion artifacts that limit detailed coronary evaluation. 5

Abdominal Applications

CTA of the abdomen has become the preferred imaging test for various aortic and visceral vascular conditions 4:

• Abdominal aortic aneurysm: Standard imaging for pre-intervention planning and evaluating acute rupture, dissection, or traumatic injury 4
• Renal artery stenosis: Optimized protocols achieve 90-98% sensitivity and specificity, with negative predictive values >95% 7
• Mesenteric ischemia: Evaluation of bowel ischemia and active inflammatory bowel disease 8
• Hepatic applications: Preoperative planning for hepatic resection, liver transplantation evaluation, and portal vein patency assessment 8

Key Advantages Over Other Imaging Modalities

CTA provides several advantages that have made it a dominant vascular imaging technique 1, 2, 4:

• Rapid acquisition: Entire examinations completed in seconds to minutes, crucial for unstable patients 1, 4
• High spatial resolution: Superior visualization of small and tortuous vessels compared to MR angiography 1, 3
• Wide availability: More accessible than MRI in most emergency and outpatient settings 4
• Extravascular information: Simultaneously evaluates surrounding structures (mediastinum, airway, soft tissues, bone) 1
• Alternative to MRI: Useful in patients with contraindications to MRI or metallic implants 1

Important Limitations and Caveats

Radiation Exposure

Patient exposure to ionizing radiation is a significant consideration, though modern techniques have substantially reduced doses 1, 6:

• Variable pitch and low-kilovolt techniques (80 kV) permit low-millisievert radiation dosing in many patients 1
• Prospective ECG triggering, adaptive statistical iterative reconstruction, and high-pitch spiral acquisition further reduce cardiac CTA radiation 1
• However, these dose-reduction techniques may not be appropriate for all patients (obese, arrhythmic, high heart rate) 1

Contrast-Related Issues

• Iodinated contrast medium is required, with potential for allergic reactions and contrast-induced nephropathy, though adverse effects occur less often with low-osmolality agents 1
• Esophageal varices are a relative contraindication for certain applications 1

Technical Limitations

Heavy coronary calcification can limit accurate stenosis assessment and plaque characterization on coronary CTA 6:

• High heart rates (>65 bpm), arrhythmias, and obesity adversely affect image quality, particularly for cardiac applications 1, 6
• Streak artifact from metallic implants can degrade images 1
• CTA is limited in evaluating the interatrial septum and membranous ventricular septum, and cannot assess flow patterns and turbulence like echocardiography 1, 9

Diagnostic Pitfalls

The increased sensitivity of modern CT has raised concerns about overdiagnosis of pulmonary embolism, with substantial increases in anticoagulation complications 1:

• Studies are needed to determine which small emboli benefit from treatment versus those that can go untreated 1
• Patient selection using clinical judgment and tools like the HEART score is critical, as routine noninvasive cardiac imaging has historically low yield in truly low-risk patients 1

Relationship to Other Cardiac Imaging

CTA and echocardiography are complementary rather than competitive modalities 9:

• Echocardiography: The American College of Radiology recommends transthoracic echocardiography as one of the first procedures in cardiac evaluation, providing real-time functional assessment without radiation 9
• CTA excels at: Detailed coronary artery evaluation, assessment of stenosis and plaque characterization, bypass graft evaluation, and visualization when echocardiographic windows are poor 9
• Echocardiography excels at: Real-time valve function assessment, wall motion abnormalities, and hemodynamic measurements 9

Practical Imaging Protocols

Coronary CTA Specific Considerations

Beta-blockers may be administered to decrease heart rate below 65 bpm if no contraindication exists, improving image quality on certain scanner types 1:

• Oral or intravenous beta-blockers optimize coronary artery visualization by reducing cardiac motion 1
• However, newer high-pitch spiral acquisition techniques may eliminate this requirement in some patients 1

Contrast Injection Strategies

Bolus tracking or test bolus injections should be employed to match the plateau phase of contrast enhancement precisely 7, 3:

• Automated bolus triggering techniques optimize timing for target vessel enhancement 7
• Saline flush following contrast improves contrast agent utilization 7