CTA Should Be Performed Both Without and With Contrast
In this 62-year-old man with severe hypertension, chest pain, and elevated troponin concerning for acute aortic syndrome (AAS), the CTA chest should be performed both without and with IV contrast to optimize diagnostic accuracy, particularly for detecting intramural hematoma (IMH).
Why Both Phases Are Critical
The ACR Appropriateness Criteria explicitly addresses this scenario 1. While CTA with contrast alone can diagnose most acute aortic syndromes, adding a noncontrast phase significantly improves detection of IMH—a life-threatening condition that can be missed or misdiagnosed on contrast-only imaging.
The Evidence for Dual-Phase Imaging
A key study demonstrated that interrater agreement for diagnosing IMH increased from κ = 0.65 with contrast-only CTA to κ = 0.92 when radiologists also had access to noncontrast images 1. This represents a substantial improvement in diagnostic reliability—the difference between moderate and near-perfect agreement.
What Each Phase Provides
Noncontrast Phase Identifies:
- Displaced aortic calcifications (94% sensitive for AAS)
- Intramural hematoma as hyperdense aortic wall thickening (71% sensitive)
- Mediastinal, pericardial, or pleural hemorrhage
- Baseline attenuation values to distinguish true IMH from contrast enhancement
Contrast Phase (CTA) Identifies:
- Intimal flaps in classic dissection (82-100% sensitivity, 100% specificity) 1
- Branch vessel involvement and malperfusion
- Entry and reentry sites
- Penetrating atherosclerotic ulcers
- Focal intimal disruptions (hazard ratio 24.43 for adverse outcomes) 1
Clinical Context Matters
Your patient's presentation—severe hypertension with chest pain and elevated troponin—is classic for AAS. The elevated troponin could reflect:
- Coronary malperfusion from dissection
- Type 2 myocardial injury from hypertensive stress
- Actual acute coronary syndrome
This triad demands ruling out aortic dissection before considering cardiac catheterization, as catheterization in undiagnosed dissection can be catastrophic.
Important Caveats
Virtual Unenhanced Images Are Not Adequate
While dual-energy CT can generate "virtual" noncontrast images, their use remains controversial and should not replace true noncontrast acquisition when AAS is suspected 1. The stakes are too high for diagnostic uncertainty.
Contrast-Only May Be Acceptable If:
- Time is absolutely critical and the patient is unstable
- IMH is considered very unlikely based on clinical presentation
- Immediate surgical intervention is planned regardless
However, given that acquiring noncontrast images adds only 30-60 seconds to the examination, the risk-benefit strongly favors the dual-phase protocol 2.
Practical Protocol
- Noncontrast chest CT first (aortic arch through diaphragm)
- CTA with IV contrast (same coverage, arterial phase timing)
- Total radiation can be kept reasonable (5-9 mSv with modern techniques) 3
- Contrast volume: 95-130 mL is standard and safe unless severe renal dysfunction present 4, 5
The Bottom Line
The modest additional time and radiation exposure of adding noncontrast imaging is justified by the substantial improvement in IMH detection—a diagnosis that fundamentally changes management and carries high mortality if missed. In suspected AAS, diagnostic certainty outweighs minimizing radiation, especially when modern protocols keep doses reasonable 1.