Imaging modalities and capabilities
The choice of diagnostic modality depends on the clinical situation and the goals of the examination:
- Computed tomography (CT): The modality of choice for diagnosing acute conditions. High spatial resolution and ECG-gating (ECG-gated protocols) allow for a detailed evaluation of dissections. A non-contrast scan is a mandatory initial step.
- Magnetic resonance imaging (MRI): Superior to CT in assessing the thrombosed false lumen and identifying peripheral intramural hematomas. Blood-suppression sequences (BBFSE) and steady-state free precession (SSFP) are highly specific for blood detection. The development of 4D flow MRI allows for the assessment of rupture risk.
- Transesophageal echocardiography (TEE): Indispensable in urgent bedside situations, especially when traumatic aortic injury is suspected. However, CT angiography is preferred for surgical planning.
Signs of impending rupture
The primary goal of imaging is to search for predictors of an acute catastrophic event. A direct marker of rupture is the extravasation of contrast medium beyond the vessel wall, often combined with an extensive periaortic hematoma. If no overt rupture is present, indirect signs are evaluated:
- Presence of a periaortic or mediastinal hematoma appearing as a mass-like lesion around the vessel.
- Hemorrhagic pleural effusion (attenuation of 20–40 HU on unenhanced CT).
- Appearance of a right-sided effusion or rapid increase in aortic diameter.
- Discontinuity of wall calcification on a non-contrast study.
In acute traumatic injury, there are two absolute indications for immediate surgery ("red flags"): the formation of a circumferential lesion (involving >270 degrees of the circumference) and pseudocoarctation syndrome, in which a pseudoaneurysm sac compresses the aorta externally, narrowing its true lumen.
Visceral malperfusion in dissection
In 25–50% of cases, acute dissection is complicated by end-organ ischemia due to partial or complete occlusion of aortic branches. Two mechanisms of obstruction are distinguished:
- Dynamic: The intimo-medial flap bulges into the true lumen, obstructing blood flow to the vessels. On CT, the true lumen appears thread-like (thread-like).
- Static: The dissection extends into the aortic branch itself without a re-entry point (re-entry point). Pressure or thrombus in the false lumen of the branch causes its stenosis.
Malperfusion can lead to renal ischemia (seen on CT as decreased parenchymal contrast enhancement) and hepatomesenteric ischemia. In the latter, the venous phase reveals a "shock bowel" pattern with bowel wall thickening and mesenteric fat stranding.
Planning endovascular treatment (TEVAR)
Successful stent-graft deployment requires a thorough anatomical evaluation based on CT angiography. Key parameters include:
- Proximal and distal necks: A segment of unaffected aorta at least 15 mm long from the ostia of major branches is required for a secure seal. If the left subclavian artery needs to be covered, preliminary revascularization may be necessary.
- Graft sizing: The graft diameter should exceed the aortic diameter (oversizing) by 10–20% for secure seating.
- Vascular access: The caliber, tortuosity (tortuosity), and patency of the iliac and femoral arteries are assessed for delivery system passage.
- Paraplegia risk: When the graft covers spinal cord-supplying arteries, the risk of ischemia is about 2%. Cerebrospinal fluid drainage serves as a preventive measure.
Briefly: answers to topical questions
What are the features of MRI in diagnosing acute aortic conditions?
MRI is an accurate method for assessing acute aortic syndromes, detecting false lumen thrombosis, and identifying signs of instability.
- Blood recognition — BBFSE T1 and T2 sequences are highly specific for detecting blood.
- Signs of instability — high signal intensity from effusion, periaortic tissues, or intra-aneurysmal thrombus, as well as periadventitial contrast enhancement, indicate hemorrhage or acute inflammation.
- Intramural hematoma (IMH) — the method is used to detect peripheral hematoma.
- Risk assessment — 4D flow MRI (Four-dimensional flow MRI) is a promising tool for assessing the risk of aneurysm rupture and dissection.
Rapid MRI protocols surpass TEE in planning surgical and endovascular treatment.
What CT signs are characteristic of hepatomesenteric ischemia in malperfusion?
Hepatomesenteric malperfusion is characterized by a number of specific CT signs of bowel distress and hypoperfusion.
- Bowel distress — manifests as edematous infiltration of mesenteric fat and dilation of GI loops.
- Bowel hypoperfusion (shock bowel) — venous phase reveals direct signs: wall thickening and decreased contrast enhancement.
- Static branch obstruction — is reliably confirmed if the intimal flap extends into the vessel and a thrombus is visualized in the false lumen.
Evaluation of the venous phase of scanning is critical for identifying direct signs of injury.
Two mechanisms of branch occlusion in dissection: Static — Specific branch is dissected and thrombosed (thrombus is stationary). Dynamic — Dflap exerts pressure (moves and bulges from the true lumen).
Class and exam questions
Why is non-contrast CT mandatory when examining the aorta?
The unenhanced phase is essential for identifying discontinuity of wall calcifications (an indirect sign of impending rupture) and assessing effusion attenuation. An attenuation of 20–40 HU reliably indicates the hemorrhagic nature of the effusion, pointing to instability.
What is pseudocoarctation syndrome in aortic trauma?
This is a life-threatening complication where the resulting pseudoaneurysm sac compresses the aorta distal to the site of injury. This leads to localized narrowing of the true lumen, resembling congenital coarctation, and requires emergency intervention.
How is renal malperfusion assessed in aortic dissection on CT?
The main sign is absent or decreased contrast enhancement of the parenchyma of one of the kidneys. Evaluation in the venous or delayed phase is mandatory, as perfusion from the false lumen can be significantly delayed.
What else to study on this topic
- Aneurysms of the aortic sinuses: clinical presentation of fistula rupture into heart chambers
- Mycotic (infectious) pseudoaneurysms: CT signs of gas formation
- Inflammatory abdominal aortic aneurysms and ureteral involvement
- Differential diagnosis of thrombotic masses using MRI
- Assessment of vascular access routes prior to stenting
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