CT Angiography (CTA) of the Abdomen: Protocol and Considerations
CTA without and with IV contrast is the preferred protocol for abdominal CT angiography, providing optimal visualization of vascular structures while detecting active bleeding as slow as 0.3 mL/min. 1
Protocol Components
Acquisition Technique
- Multiphasic imaging is essential for abdominal CTA, typically including:
- Non-contrast phase (true unenhanced images) to identify baseline high-attenuation materials that could mimic bleeding or contrast extravasation 1
- Arterial phase with properly timed IV contrast administration to visualize arterial structures 2
- Portal venous phase (when needed) to evaluate venous structures and parenchymal enhancement 2
Contrast Administration
- IV contrast timing is critical for optimal arterial opacification 2
- Bolus tracking or test bolus technique ensures proper scan timing relative to contrast transit 2
- No oral contrast should be used as it may obscure vascular findings and decrease sensitivity for detecting active bleeding 3
- Low contrast volume protocols (as low as 30 mL) may be considered for patients with renal insufficiency when using low kVp techniques 4
Technical Parameters
- Thin collimation (≤1 mm) enables high-quality multiplanar reformations 1
- Fast acquisition with modern CT scanners allows for sub-second imaging of the abdomen and pelvis 4
- 3D reconstructions are required elements of CTA (differentiating it from standard CT with contrast) 1
Clinical Applications
Gastrointestinal Bleeding
- CTA can detect bleeding rates as slow as 0.3 mL/min (more sensitive than conventional angiography which detects at 0.5 mL/min) 1
- Particularly valuable when endoscopy is not feasible due to large volume bleeding or clinical instability 1
- The Society of Abdominal Radiology Gastrointestinal Bleeding Disease-Focused Panel recommends non-contrast followed by contrast-enhanced phases 1
Vascular Pathology
- Aneurysm evaluation: Provides accurate measurements and 3D visualization of aneurysm morphology, thrombus, and branch vessel involvement 1, 5
- Arterial stenosis: Highly sensitive for detecting renovascular disease and mesenteric ischemia 2, 5
- Trauma: Rapid assessment of vascular injury in abdominal trauma 5
Retroperitoneal Bleeding
- CTA provides exact location of hematoma and can identify active extravasation 1
- Sensitivity for detecting active bleeding is approximately 59.5% with positive predictive value of 62.9-71.0% compared to transcatheter arterial embolization 1
Advantages Over Other Modalities
- Compared to conventional angiography: Less invasive, better sensitivity for detecting slow bleeding, and provides comprehensive evaluation of non-vascular structures 1
- Compared to standard CT with IV contrast: Better vascular detail, required 3D reconstructions, and optimized contrast timing 1
- Compared to MRA: Faster acquisition time, making it more suitable for acute bleeding scenarios 1
Common Pitfalls and Limitations
- Intermittent bleeding may result in false-negative studies 1
- Contrast-induced nephropathy risk in patients with renal insufficiency 4
- Radiation exposure is a consideration, particularly in younger patients 1
- Intraluminal medications can appear hyperdense and mimic active bleeding or obscure true pathology 3
Special Considerations
- Renal insufficiency: Consider low kVp (80 kVp) technique with reduced contrast volume 4
- Dual-energy CT can generate virtual non-contrast images from contrast-enhanced data, potentially reducing radiation exposure, but this remains user/site-specific 1
- Timing of acquisition is critical - early performance (within first 5 hours of presentation) is associated with significantly higher visualization of extravasation in bleeding cases 1