Imaging modalities: CT and MRI
Multidetector computed tomography (MDCT) is the method of choice for emergency diagnosis of aortic dissection due to its high scanning speed. The use of ECG gating is critical as it minimizes pulsation artifacts at the aortic root level.
Technical nuance: the contrast agent should be injected into the right arm. If injected through the upper left extremity, its high density in the left brachiocephalic vein will produce streak artifacts (streak artefact) at the level of the arch, hindering evaluation.
Magnetic resonance imaging (MRI) is not used in urgent settings, but is indispensable for follow-up.
- Time-resolved 3D MR angiography excellently detects false lumen thrombosis without ionizing radiation.
- 4D flow MRI (free-breathing volumetric cine scanning) enables comprehensive 3D hemodynamic analysis and assists in surgical planning.
- The SSFP (Steady-State Free-Precession) sequence provides ideal contrast resolution for visualizing the intimal flap.
For surgical planning, volume rendering (VR) techniques are preferred over maximum intensity projection (MIP) because VR better demonstrates the variability of luminal enhancement.
CT semiotics: true and false lumens
On unenhanced (native) CT, inward displacement of intimal calcifications may be observed. Following contrast administration, the main sign of dissection is the intimal flap (intimal flap), a linear hypodense structure dividing the blood flows.
Correct identification of the lumens is vital for surgeons:
- True lumen (true lumen): typically has a smaller diameter, is located more medially, and shows more intense contrast enhancement. It continues seamlessly into the non-dissected part of the aorta.
- False lumen (false lumen): often spirals along the aorta. It may demonstrate the "cobweb sign" (cobweb sign).
Aortic branches may be perfused from different lumens. For example, the right renal artery may arise from the false lumen, whereas the celiac trunk and superior mesenteric artery arise from the true lumen.
Rare form: intimointimal intussusception (intimointimal intussusception). In circular dissection, the intima inverts in a "windsock" manner. On CT, the true lumen appears tightly wrapped by the false lumen (the inner lumen is always the true one).
Intramural hematoma (IMH)
Intramural hematoma (intramural haematoma, IMH) is a "dissection without intimal tear", resulting from hemorrhage of the vasa vasorum into the media.
On unenhanced CT, IMH appears as a symmetric or asymmetric crescentic wall thickening that is hyperdense relative to the lumen. On post-contrast scans, the hematoma shows no enhancement.
How to differentiate IMH from a thrombosed false lumen?
- IMH maintains a constant circular relationship to the wall and does not compress the true lumen.
- The false lumen follows a spiral course and frequently compresses the true lumen.
MRI is the only method capable of determining the age of the hematoma based on hemoglobin degradation products. In the acute phase (up to 7 days), the T1WI signal is intermediate, and in the subacute phase, it is high (methemoglobin). T2WI helps differentiate a fresh hematoma (high signal) from a chronic mural thrombus (low signal).
Penetrating atherosclerotic ulcer (PAU)
Forms when a large atheromatous plaque erodes, destroying the internal elastic lamina and allowing blood flow to reach the media of the aorta. Most commonly localized in the descending thoracic aorta.
On CT, it is visualized as a crater-like outpouching with jagged (irregular) margins, filled with contrast. Unlike a regular plaque, the ulcer extends beyond the intima and is almost always accompanied by a localized intramural hematoma.
PAU is a dangerous precursor to complications. Rupture through the adventitia forms a pseudoaneurysm, while propagation through the media causes localized dissection.
Traumatic aortic injury (TAI)
Occurs in blunt chest trauma, most commonly in motor vehicle collisions. Up to 90% of all ruptures occur at the aortic isthmus (isthmus), the transition zone between the mobile arch and the fixed (by the ligamentum arteriosum) descending part.
Main mechanisms of injury:
- Deceleration (sudden braking) at speeds over 50 km/h (affects the isthmus).
- Torsion (twisting) during a fall from a height (affects the ascending aorta).
- "Osseous pinch" — compression of the aorta between the sternum and the spine.
Approximately 80–90% of victims die at the scene. In survivors, the injury ranges from an intimal tear to pseudoaneurysm formation. Plain chest radiography lacks specific signs (mediastinal widening is often caused by venous hemorrhage), making MDCT the primary diagnostic tool.
Briefly: answers to topical questions
Which classifications are used for the clinical categorization of aortic dissection by localization and extent?
The Stanford, DeBakey, and Crawford systems are used to classify aortic dissection by location and extent.
- Stanford classification is the most commonly used, determining prognosis and management. Type A involves the ascending aorta, whereas Type B is localized distal to the left subclavian artery.
- DeBakey classification: Type I involves both the ascending and descending aorta, Type II involves only the ascending aorta and arch.
- Crawford classification describes the extent of aortic aneurysms.
What indirect radiographic signs of traumatic aortic rupture can be detected on a plain chest radiograph?
A series of indirect signs of traumatic aortic injury can be detected on a supine plain chest radiograph.
- Widening of the superior mediastinum — the ratio of mediastinal width to chest width is approximately 0.3.
- Widening of the right paratracheal stripe — an additional sign of pathology.
- Tracheal deviation — displacement of the structure to the right of the midline.
- Alteration of the aortic arch contour (aortic knuckle) — the contour appears enlarged and partially obscured by mediastinal hematoma.
Distinction between thrombosed false lumen and hematoma: The false lumen is "spiral" (follows a spiral course), whereas the intramural hematoma is a "layer" (maintains a constant circular relationship to the wall and does not compress the true lumen).
Class and exam questions
Why is a non-contrast CT scan necessary when dissection is suspected?
The unenhanced phase is essential for detecting acute intramural hematoma, which appears as a hyperdense "crescent", as well as for assessing inward displacement of intimal calcifications.
How to differentiate an aneurysm with a thrombus from a dissection with a thrombosed false lumen on CT?
The false lumen has a smooth inner contour and a spiral course. The thrombus in an aneurysm is fixed (does not follow a spiral course) and is characterized by an irregular inner contour with peripheral calcification.
What else to study on this topic
- Features of 4D flow MRI in evaluating dissection
- Specifics of MRI staging of intramural hematoma by age
- Mechanism of the "osseous pinch" in blunt chest trauma
- Intimointimal intussusception ("windsock" sign)
- Choosing between MIP and VR algorithms for surgical planning
Master the entire topic in the "Diagnostic Radiology" course
This is a summary. The course covers everything tested in classes and exams.
- Comprehensive study guide on "Thoracic Aortic Dissection" and 2,199 more subject pages
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 10 video lessons on the subject
Other topics in the «Chest Radiology» section
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.