Diagnosis: Main right pulmonary artery embolus and thrombosed type B aortic dissection

Unenhanced (left) and contrast-enhanced (right) axial CT of the chest shows crescentic high attenuation in the descending thoracic aorta on unenhanced CT that does not enhance following intravenous contrast material (yellow arrows). Pulmonary embolus is present in the right main pulmonary artery (red arrows).

Discussion

Overview of aortic dissection (pulmonary embolism is case 31)

CT is the modality of choice for evaluating aortic dissection. Most institutions perform a low-dose unenhanced CT followed by a contrast-enhanced CT of the chest.
Although aortic dissection occurs most commonly in older hypertensive patients, risk factors in younger patients include hypertension, connective tissue diseases (e.g., Marfans, Ehlers–Danlos), bicuspid aortic valve, aortic coarctation, pregnancy, and cocaine use.
Approximately 60% of aortic dissections involve the ascending aorta (type A) and 40% involve the descending aorta (type B).
Type A dissection is usually accompanied by substernal chest pain at onset, while type B dissection is usually accompanied by interscapular back pain at onset. Pain symptoms can migrate as aortic dissection progresses, and can abate as the dissection stops.
Type A dissection is generally managed surgically, and type B dissection is generally managed medically. In-hospital mortality is 26% and 11% for type A and type B dissection, respectively.

Clinical synopsis

The patient was admitted to the hospital where echocardiography showed no evidence of right heart strain despite the large burden of pulmonary embolus. A decision was made not to anticoagulate the patient because of the aortic dissection. An IVC filter was placed because of the continued presence of DVT in both lower extremities. The aortic dissection was managed medically with a beta-blocker for blood pressure control. The patient became asymptomatic and was discharged to rehabilitation 5 days later.

Self-assessment

What CT findings may be observed on unenhanced CT when aortic dissection is present?	The positive intimal flap sign, intramural hematoma, and displaced intimal calcification are findings that may be present on unenhanced CT.
Can aortic dissection be excluded on the basis of an unenhanced CT?	No. Contrast-enhanced CT must be performed to exclude aortic dissection.
How is aortic dissection classified using either the Stanford or DeBakey classification system?	Stanford type A dissection is any dissection involving the ascending aorta proximal to the innominate artery regardless of whether it is confined to the ascending aorta or extends beyond it. The DeBakey classification subdivides this group to those with involvement beyond the ascending aorta (type I) and those confined to the ascending aorta (type II). Stanford type B and DeBakey type III describe aortic dissection confined to the aortic arch and/or descending aorta with no involvement proximal to the innominate artery.
How is the false aortic lumen differentiated from the true aortic lumen?	The most reliable method is to show continuity of the true lumen with the uninvolved aorta. In the involved portion, the false lumen is usually larger and often shows the beak sign (acute angle formed between the intimal flap and the aortic wall). If the dissection is circumferential, the true lumen is always the inner lumen.
What complications can occur with aortic dissection?	Aortic dissection may rupture into the pericardial sac, mediastinum, pleural cavity, or retroperitoneum. The dissection can occlude arterial branches leading to end-organ ischemia or infarct (e.g., stroke, myocardial infarction, renal infarction, etc.). Complete thrombosis of the false or true lumen can occur.

Spectrum of aortic dissection

The spectrum of aortic dissection ranges from conspicuous intimal flap to subtle thickening of the aortic wall. Dissection may involve both the ascending and descending aorta, or be confined to either. Extent of dissection may be limited to several centimeters within a single segment, or involve the entire aorta and propagate distally into the common iliac arteries. Associated complications of aortic dissection may be absent, or may include aortic rupture, thrombosis, and/or end-organ ischemia/infarction.

Stanford type A / DeBakey type I aortic dissection

Approximately 60% of aortic dissections involve the ascending aorta. Because ascending aortic involvement is associated with higher mortality when treated medically, dissection involving this segment is generally managed surgically. Axial CT angiogram demonstrates an obvious dissection flap involving the ascending aorta with beak sign in the false lumen (arrow) where the flap meets the aortic wall. The true lumen is the inner lumen. In the descending aorta the false lumen is unperfused and larger than the true lumen (red arrow).

Stanford type A / DeBakey type I aortic dissection

The false lumen of an aortic dissection can thrombose, which may diminish conspicuity of the pathognomonic feature of dissection, the intimal flap. When this occurs, unenhanced CT will reveal smooth, crescentic high attenuation. Following administration of intravenous contrast material, the false lumen will not perfuse. Unenhanced CT (top image) shows crescentic high attenuation intramural hematoma in the ascending aorta (yellow arrow) and the intimal flap sign in the descending aorta (red arrow). Following intravenous contrast material, no significant enhancement is seen in the thrombosed ascending false lumen (yellow arrow). There is some enhancement of the false lumen in the descending aorta indicating slow flow as compared to the true lumen (red arrow).