What is a CT (Computed Tomography) angiograph?

What is a CT Angiography?

CT angiography (CTA) is a specialized computed tomography imaging technique that uses thin-section CT acquisition precisely timed to coincide with peak arterial or venous enhancement from intravenous contrast, with interpretation requiring primary transverse reconstructions, multiplanar reformations, and mandatory 3-D renderings to visualize blood vessels throughout the body. 1, 2

Technical Definition and Requirements

CTA differs fundamentally from standard CT with contrast in three essential elements 1:

• Precise timing to capture peak vascular enhancement (not just general contrast administration) 1, 2
• Multiplanar reconstructions and reformations to visualize vessels in optimal planes 1, 2
• 3-D rendering as a mandatory component (this distinguishes CTA from standard contrast CT, which may use timing and reconstructions but does not require 3-D visualization) 1

How CTA Works

The technique involves thin-section CT acquisition (typically 1mm collimation) that creates a volumetric dataset 1, 3:

• Contrast injection is administered intravenously with specific timing protocols—either empiric scan delays of 15-20 seconds or bolus tracking over target vessels 1
• Rapid scanning captures the entire arterial system during peak enhancement, with modern multidetector CT scanners completing acquisition in seconds 1, 4
• Image processing generates multiplanar reformatted images, maximum-intensity projections, and 3-D renderings to create a comprehensive vascular "road map" 2

Clinical Applications

CTA has become the primary imaging modality for comprehensive vascular assessment across multiple territories 1, 4:

• Thoracic aorta evaluation: CTA is the primary modality for comprehensive assessment of the entire aorta and proximal branch vessels, readily identifying acute conditions like aortic dissection, intramural hematoma, or penetrating ulcer 1
• Peripheral arterial disease: CTA with runoff demonstrates 90-100% sensitivity and specificity for detecting arterial stenoses greater than 50% diameter, providing visualization from the aorta through tibial vessels to pedal arteries 2, 5
• Cerebrovascular disease: CTA measurements of carotid stenosis show high correlation with digital subtraction angiography, with 3D reconstructions permitting more complete assessment of eccentric lesions 1
• Coronary artery disease: Cardiac CTA using 64-slice technique provides 93-97% sensitivity and 80-90% specificity for detecting ischemic heart disease, with very high negative predictive value for obstructive coronary disease 1
• Pulmonary embolism: CT pulmonary angiography (CTPA) serves as first-line diagnostic imaging with 96-99% sensitivity and specificity 3

Key Advantages Over Other Imaging

CTA offers several critical benefits that have made it the reference standard for many vascular conditions 1, 2, 4:

• Comprehensive visualization: Shows both luminal and extraluminal pathology, including vessel wall characteristics, collateral vessels, and arteries distal to occlusions 2
• Rapid acquisition: Scan times measured in seconds, with less susceptibility to motion artifact than MRA 2
• Wide availability: More accessible than MRI and better tolerated in patients with pacemakers, defibrillators, or claustrophobia 2
• Accurate measurements: Provides precise vessel diameter measurements using double oblique method for treatment planning 1
• Bleeding detection: Can detect active bleeding at rates as low as 0.3 mL/min 2

Important Limitations and Pitfalls

Several factors can significantly limit CTA diagnostic accuracy 1, 2:

• Heavy calcification: Creates blooming artifacts that interfere with stenosis assessment, particularly problematic in complex calcified plaques and tibial arteries 1, 2
• High-risk populations: Patients over 80 years, diabetics, and dialysis patients are at increased risk for nondiagnostic studies due to extensive calcification 2
• Metallic artifacts: Stents, clips, or coils cause streak artifacts reducing diagnostic accuracy 2
• Contrast requirements: Requires iodinated contrast with associated nephropathy risk (though not a concern in hemodialysis patients) 1
• Radiation exposure: Involves ionizing radiation, though newer techniques have substantially reduced doses 1, 2

Specialized CTA Protocols

Different vascular territories require specific protocols 1, 3:

• Coronary CTA: Mandates ECG synchronization to minimize cardiac motion artifacts, often requiring beta-blocker administration to reduce heart rate below 65 bpm 3, 6
• Pulmonary CTA: Uses high injection rates (5.0-7.5 mL/s) with low-kilovoltage scanning (80-100 kV) but does not require ECG gating 3
• Thoracic outlet CTA: Requires imaging in both neutral and stressed (abducted) arm positions with contralateral contrast injection to minimize streak artifact 1
• CTA with runoff: Uses multistation acquisition to evaluate the entire lower extremity arterial system from aorta to pedal arteries in a single examination 2

Critical Ordering Considerations

Proper study ordering is essential for diagnostic quality 3:

• Never order "CT chest with contrast" when evaluating for pulmonary embolism—the study must be specifically protocoled as CTPA with appropriate timing and injection rates 3
• Coronary evaluation requires specifically ordered cardiac CTA with ECG gating—standard chest CTA will show cardiac structures but with motion artifacts preventing detailed coronary assessment 3, 6
• Consultation with radiology at the time of ordering can optimize protocols for the specific clinical question 1