CT Angiography (CTA) of the Chest, Abdomen, and Pelvis with IV Contrast
For suspected acute aortic dissection, order a CTA of the chest, abdomen, and pelvis with IV contrast—this is the preferred initial imaging modality given its wide availability, speed, diagnostic accuracy (98-100% sensitivity and specificity), and ability to visualize the entire extent of dissection including branch vessel involvement. 1, 2, 3
Why CTA Chest, Abdomen, and Pelvis?
The evidence strongly supports extending imaging beyond just the chest because:
- 98.6% of complicated type B dissections extend below the diaphragm into the abdominal aorta and visceral vessels 1
- A single contrast injection captures the entire aortic tree from root to iliac bifurcation in one breath-hold 4
- This allows assessment of visceral malperfusion (kidneys, mesentery), which directly impacts mortality and surgical planning 1
- Branch vessel involvement and organ perfusion status are critical for determining immediate management 3
Essential Technical Protocol Elements
Your CTA order should specify:
1. Arterial Phase Timing
- Timed to coincide with peak arterial enhancement
- This is what differentiates CTA from standard contrast CT 5
2. Thin-Section Acquisition
- Submillimeter, isotropic voxels for multiplanar reformatting 1
- Enables accurate measurement perpendicular to vessel long axis
3. 3-D Rendering Capability
- Required element that distinguishes CTA from routine contrast CT 5
- Facilitates visualization of intimal flap, entry tears, and branch vessel anatomy
4. Consider ECG-Gating for Ascending Aorta
- Reduces cardiac motion artifacts in the aortic root 1, 2
- Ensures reproducible measurements in the same cardiac phase
- One study showed 5-10% diameter variation between systole and diastole 1
- Particularly important for type A dissections involving the ascending aorta 2, 3
Special Consideration: Add Noncontrast Phase for Intramural Hematoma (IMH)
If IMH is suspected clinically, request dual-phase protocol (noncontrast + contrast):
- Noncontrast images show the hyperattenuating crescent of IMH (>45 HU) 1, 4
- This crescent is often masked on contrast-enhanced images alone 1
- One study of 306 patients showed significantly improved sensitivity, specificity, and accuracy with dual-phase versus single-phase CTA for IMH diagnosis 1
Alternative: Dual-energy CT (DECT) can generate virtual noncontrast images, eliminating need for separate noncontrast acquisition while maintaining diagnostic confidence 1
When to Limit to Chest Only
CTA chest alone is acceptable only if:
- Serial imaging has documented dissection limited to thorax 1
- This is a follow-up study, not initial diagnosis 1
For initial evaluation, one retrospective study of 1,143 patients found no acute dissections limited to the abdominal aorta that would have been missed on chest-only imaging 6. However, this contradicts the guideline evidence showing 98.6% extension below the diaphragm in complicated cases 1, so err on the side of complete imaging for initial diagnosis.
Critical Diagnostic Information CTA Provides
The study must assess and report:
For Type A (Ascending) Dissection:
- Aortic valve involvement and aortic regurgitation 1
- Coronary artery involvement 1
- Pericardial effusion/hemopericardium 3
- Mediastinal hematoma 3
For Type B (Descending) Dissection:
- Visceral organ perfusion and malperfusion 1
- Branch vessel involvement (celiac, SMA, renal arteries) 4, 3
- Extension into iliac vessels 1
For All Dissections:
- Location and extent of intimal flap (visible ~70% of time) 1
- Entry tear site 3
- True versus false lumen differentiation 4
- Thrombus in false lumen 5
Alternative Imaging Modalities
MRA (chest, abdomen, pelvis with contrast) is a reasonable alternative if:
- Patient has contrast allergy or severe renal dysfunction 5, 7, 3
- Sensitivity and specificity approach 100% for dissection 4
- Major limitation: Longer acquisition time, which is problematic in unstable patients 3
TEE (transesophageal echocardiography) is reasonable if:
- CT unavailable or patient too unstable to transport 7, 3
- Limitation: Cannot visualize abdominal aorta or assess malperfusion 7
- Has "blind spot" in distal ascending aorta due to trachea/bronchus interposition 2
Common Pitfalls to Avoid
Don't order "CT chest with contrast" instead of CTA—this lacks arterial timing, thin sections, and 3-D rendering 5
Don't skip abdominal/pelvic imaging on initial evaluation—you'll miss visceral malperfusion and full dissection extent 1
Don't rely on chest x-ray to exclude dissection—it should never delay definitive imaging in high-risk patients 7
Don't forget to specify ECG-gating if available, especially for ascending aorta evaluation 1, 2
If initial CTA is negative but clinical suspicion remains high, order a second imaging study (different modality preferred) 7