CT Abdomen and Pelvis Without Contrast: Aortic Visualization
CT of the abdomen and pelvis without contrast can detect aortic diameter changes, identify aortic wall calcifications, and visualize the aortic contour, but it has significant limitations for comprehensive aortic evaluation and should not be used as the primary diagnostic tool when aortic pathology is suspected. 1
What Noncontrast CT Can Show
Aortic Diameter and Size:
- Noncontrast CT reliably measures the outer-to-outer aortic diameter, with sensitivity of 83-89% and specificity of 98-99% for detecting abdominal aortic aneurysms (AAA) 1
- The study can detect aneurysmal dilation of the aorta and track changes in diameter over time 1
- Measurements should be performed perpendicular to the long axis of the aorta using multiplanar reformations, especially if the aorta is tortuous 2
Aortic Wall Characteristics:
- Aortic wall calcifications are clearly visible and may provide additional diagnostic information 1
- Intramural hematoma (IMH) appears as a hyperattenuating crescent within the aortic wall (>45 HU), which is a critical finding in acute aortic syndrome 1, 3
- Displaced calcified intimal flaps can suggest aortic dissection 4
Associated Complications:
- Mediastinal or pericardial hemorrhage that could indicate aortic rupture 1
- Retroperitoneal hematoma in cases of AAA rupture 5
- Extension of pathology to thoracic and iliac arteries 1
Critical Limitations of Noncontrast CT
Cannot Adequately Assess:
- The aortic lumen and its patency 6
- Dissection flaps (though displaced calcified intima may provide clues) 4
- Thrombus characteristics and extent 2
- Branch vessel involvement or compromise 2
- True versus false lumen differentiation in dissection 1
- Malperfusion syndromes or end-organ ischemia 1
Diagnostic Inadequacy:
- Noncontrast CT "has little use in the follow-up of thoracic aortic aneurysm or dissection" beyond detecting diameter changes 1
- It lacks the arterial-phase bolus timing, standard thin-section acquisition, and 3D renderings that are critical for surgical planning 1
- Cannot replace contrast-enhanced CTA, which provides near 100% sensitivity and specificity for aortic pathology 1, 2
When Noncontrast CT May Be Appropriate
Limited Clinical Scenarios:
- AAA screening in obese patients with poor sonographic windows, where it outperforms ultrasound 1
- Patients with severe renal insufficiency or contrast allergy when MRA is unavailable 5
- Suspected renal calculus workup where incidental aortic findings may be detected 5
- Serial surveillance when prior studies have established that no dissection, thrombus, or branch vessel involvement exists 1
Dual-Phase Protocols:
- For known intramural hematoma, noncontrast images combined with contrast-enhanced images improve diagnostic accuracy, as IMH is often masked on contrast-enhanced images alone 1
- Dual-energy CT can generate virtual noncontrast images, potentially reducing radiation while maintaining diagnostic capability 1
Clinical Algorithm
If aortic pathology is suspected:
- Do NOT order noncontrast CT as the primary study 2, 6
- Order CTA abdomen/pelvis with IV contrast (including run-off to iliofemoral vessels) for suspected AAA 2
- Order CTA chest/abdomen/pelvis with IV contrast with ECG-gating for suspected thoracic aortic pathology 6
- Reserve noncontrast CT only for patients with absolute contraindications to contrast AND when MRA is unavailable 3, 5
Common Pitfalls to Avoid:
- Do not delay definitive imaging (CTA) to obtain noncontrast CT first in symptomatic patients—this delays diagnosis and treatment 2
- Do not assume noncontrast CT is adequate for surgical planning—vascular surgeons require detailed information about branch vessels, thrombus, and anatomic relationships that only contrast-enhanced studies provide 2, 7
- Do not miss subtle findings on noncontrast CT (displaced calcified intima, intraluminal high-density areas, aneurysmal dilation) that should prompt immediate CTA 4