Abstract
Acute aortic conditions include, but are not limited to, aortic rupture, aortic dissection, intramural hematoma, and penetrating aortic ulcer. Prompt diagnosis of these conditions is essential for managing these conditions. Because these conditions often have similar symptoms, namely, chest and abdominal pain, the imaging characteristics are key to prompt and accurate diagnosis.
The original version of this chapter was revised. An erratum to this chapter can be found at DOI 10.1007/978-3-319-65397-6_27
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Introduction
Acute aortic conditions include, but are not limited to, aortic rupture, aortic dissection, intramural hematoma, and penetrating aortic ulcer. Prompt diagnosis of these conditions is essential for managing these conditions. Because these conditions often have similar symptoms, namely, chest and abdominal pain, the imaging characteristics are key to prompt and accurate diagnosis.
Abdominal Aortic Aneurysm and Aortic Rupture
Abdominal aortic aneurysm (AAA) is seen in 5–10% of elderly male smokers. Most AAAs are true aneurysms and involve all three layers of the aortic wall. The two most common etiologies of AAA are degenerative and inflammatory (Tables 1.1 and 1.2).
The most significant complication of AAA is aortic rupture. The mortality rate for ruptured AAA is 50%; thus, an accurate diagnosis is essential for prompt surgical intervention. The risk of rupture is proportional to the maximum cross-sectional diameter, with 1%/year risk for aneurysms measuring 5–5.9 cm. The risk of rupture increases up to 20%/year for an aneurysm measuring greater than 7 cm in diameter. Although AAAs are less common in females (M:F = 4:1), they are more likely to rupture when compared to males.
Ultrasound is the most commonly used imaging modality to screen for AAA and has been shown to reduce mortality. The imaging criteria to diagnose AAA include aortic caliber of more than 3 cm and an aortic caliber of more than 1.5 times the expected diameter of the abdominal aorta (Fig. 1.1). The aortic caliber is measured perpendicular to the long axis of the aorta, from outer wall to outer wall. Although ultrasound is highly sensitive in making the diagnosis of abdominal aortic aneurysm, it is not as reliable as CT in diagnosing aortic rupture. However, the demonstration of normal caliber of abdominal aorta by ultrasound makes aortic rupture an unlikely possibility.
Most aortic aneurysms rupture involves the middle third of the aneurysm, through the posterolateral wall and into the retroperitoneum (Fig. 1.2a). However, intraperitoneal rupture and rupture into the bowel (usually the duodenum) and very rarely into the IVC may occur (Fig. 1.2b, c).
Risk Factors for Aortic Rupture
Progressive aneurysmal dilatation of the aorta with increased wall tension is directly related to the risk of rupture (Fig. 1.3). The decreased proportion of thrombus-to-lumen ratio is also thought to play a part, as a larger thrombus better protects against rupture by providing protection against the high aortic pressures [1]. In addition, discontinuity in aortic wall calcification is associated with an increased risk of rupture [2].
Imaging
The imaging modality of choice is a contrast-enhanced multidetector CT (MDCT). The CT can demonstrate an AAA with surrounding retroperitoneal hemorrhage into psoas compartment, pararenal space, and perirenal space. A contrast-enhanced CT provides additional information about the aortic size, presence or absence of active extravasation, and anatomic relationships (Table 1.3). A hyperdense crescent sign and draped aorta sign are indicators of contained aortic leak or impending rupture. Focal discontinuity of intimal calcification is also a secondary sign of aortic rupture.
Hyperdense Crescent Sign
Draped Aorta Sign
Draped aorta sign indicates a contained aortic rupture and shows posterior aortic wall not identifiable as a separate structure and draping over the adjacent vertebral bodies (Fig. 1.4b, c). If rupture should occur, the most common sign of aneurysmal rupture is a retroperitoneal hematoma adjacent to the aneurysm.
Tangential Calcium Sign
The intimal calcification in the aorta points away from the circumference of the aneurysm (Fig. 1.4d).
Mycotic Aneurysm
One of the less frequent etiologies of AAA is mycotic aneurysm , which constitutes 1–3% of aortic aneurysms. However, mycotic aneurysm is known to more commonly involve aorta than any other artery. Staphylococcus and Streptococcus species are the most common pathogens of mycotic aneurysm. The cases of mycotic aneurysm due to Salmonella species are more common in East Asia and demonstrate an early tendency to rupture.
The typical imaging features of mycotic aneurysm (Fig. 1.4e) include rapidly increasing caliber of a saccular aortic aneurysm with wall irregularity, periaortic edema and soft tissue mass, and the presence of gas. Periaortic soft tissue stranding and soft tissue mass are the most common features seen on imaging of mycotic aneurysm. Calcifications and thrombus are uncommon in a mycotic aneurysm. The lack of calcification in the aortic wall is due to the nonatherosclerotic origin of the aneurysm.
Traumatic Aortic Transection
Traumatic aortic transection is usually caused by rapid deceleration injury, resulting from shearing forces. It involves a tear in all layers of the aortic wall and usually occurs in the aortic arch, most commonly between the origin of left subclavian artery and ligamentum arteriosum. CT is the imaging modality of choice, and findings include periaortic hematomas, mediastinal hematoma, pseudoaneurysm, and change in aortic diameter. Transection of aorta has irregular margins with acute angles relative to the aorta (Fig. 1.5).
Aortic Dissection
Aortic dissection is the most common acute presentation involving the aorta [3]. It usually originates with a tear in the intima, which causes high-pressure blood to enter and dissect the aortic wall (Table 1.4) (Fig. 1.6). Based on the period from onset of symptoms to clinical presentation, aortic dissection is classified as: hyperacute (symptom onset to 24 h), acute (2–7 days), subacute (8–30 days), and chronic (>30 days).
The most commonly used classification for aortic dissection is the Stanford classification system.
-
1.
Type A aortic dissection : Regardless of origin and extent of dissection, a Type A aortic dissection involves the ascending aorta (Fig. 1.7) [4]. The potential for complications with Type A dissection necessitates urgent surgical intervention [4]. The complications include dissection into the pericardium resulting in cardiac tamponade, dissection into the coronary arteries resulting in occlusion, and aortic insufficiency with involvement of the valve [4].
-
2.
Type B aortic dissection : The aortic dissection originates past the left subclavian artery [5]. Unlike Type A dissection, the Type B dissections are usually medically treated.
Imaging
The goals of imaging in aortic dissection include identification of:
-
1.
Site of intimal tear site
-
2.
Extent of dissection (for classification)
-
3.
Cardiac involvement (pericardial, myocardial, and valvular)
-
4.
Aortic rupture
-
5.
Major branch-vessel involvement
The imaging modality of choice to evaluate aortic dissection is MDCT. It allows accurate assessment of the extent of the disease, including the origin of the dissection, involvement of the visceral branches, and presence of a false lumen [3, 4]. The most characteristic findings of aortic dissection include an intimal flap and two distinct lumens. Secondary findings include intimal displacement of calcified wall, delayed enhancement of false lumen, pericardial or mediastinal hematoma, and ischemia or infarction of distal organs supplied by the false lumen [4].
True Versus False Lumen
Once the recognition of an aortic dissection is made, it is important to distinguish between the true and false lumen for treatment purposes, especially endovascular repair. Lepage et al. evaluated signs to distinguish between the true and false lumen and determined two consistent signs: beak sign and larger cross-sectional area of false lumen as the best indicators. The beak sign is present in the false lumen and consists of an acute angle between the dissection flap and the aortic wall [5]. The larger caliber lumen is generally the false lumen and is most commonly present anteriorly, to the right side in the ascending aorta (Figs. 1.7, 1.8, and 1.9). In the descending thoracic aorta, the false lumen is most often seen posteriorly and to the left. Cobwebs are seen in the false lumen while aortic wall calcifications are usually seen around true lumen. The true lumen may show systolic expansion and diastolic collapse during the cardiac cycle.
Intramural Hematoma
Intramural hematoma is a hematoma that has dissected through the media without an originating intimal tear (Figs. 1.10 and 1.11). The intramural hematoma may represent hemorrhage of the vasa vasorum (nutrient vessels for the vessel wall) that has dissected through the media [6]. It can be seen in hypertensive and can also be seen after blunt trauma. It can progress to rupture of the aortic wall or aortic dissection.
Unlike mural thrombus, intramural hematoma is deep to the intimal calcification and does not demonstrate the continuous flow seen with aortic dissection. Intramural hematoma can be diagnosed on CT, transesophageal echocardiography, and MRI. Since there is no intimal disruption, it cannot be diagnosed on conventional aortography. Aneurysmal dilatation, focal contrast enhancement, and intramural thickness >16 mm on CT scan indicate poor prognosis. The treatment of intramural hematoma is similar to aortic dissection.
Endoleak
Endoleak is extravasation of blood out of the endovascular stent but within the aneurysmal sac. The aneurysmal sac may enlarge and may ultimately rupture over time, especially in the setting of hypertension. CT scan is the imaging modality of choice in the detection of endoleak (Fig. 1.12).
Five types of endoleak are:
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Type I: Incomplete seal between graft and vessel wall in (a) proximal or (b) distal anchors.
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Type II: Most common type. Retrograde blood flow into the sac from aortic branch vessels.
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Type III: Relatively uncommon type. Due to separation or tear of graft material.
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Type IV: Due to porosity of stent-graft material and have no CT findings.
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Type V: Continued expansion of the aneurysm sac without radiographic identification of a leak.
Penetrating Ulcer
Penetrating ulcer is characterized by atherosclerotic ulceration that has penetrated through the elastic lamina and formed a hematoma in the media. On CT scan, it is seen as an ulcer with focal hematoma and adjacent arterial wall thickening (Fig. 1.13) [7, 8]. Unlike penetrating ulcer, an atherosclerotic plaque with ulceration does not extend beyond the intima and is not associated with intramural hematoma.
Penetrating ulcer and aortic dissection are characterized by disruption of the intima, while aortic rupture is characterized by disruption of the aortic wall.
CT is the key diagnostic modality in the emergency room evaluation of acute aortic syndromes and allows different pathologies to be diagnosed for proper triage as well as treatment (Table 1.5).
Teaching Points
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Most common cause of abdominal aortic aneurysm are degenerative and inflammatory.
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The imaging criteria to diagnose AAA include aortic caliber of more than 3 cm and an aortic caliber of more than 1.5 times the expected diameter of the abdominal aorta.
-
There is increased risk of rupture with increasing caliber of the aneurysm and reduced thrombus-to-lumen ratio.
-
Hyperdense crescent sign and draped aorta sign are indicators of contained aortic leak or impending rupture.
-
The most common imaging features of mycotic aneurysm are periaortic soft tissue stranding and soft tissue mass.
-
Type A aortic dissection involves the ascending aorta and is surgically managed.
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Type B aortic dissection originates past the left subclavian artery and is usually medically managed.
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Beak sign and larger cross-sectional area of lumen are indicators of false lumen.
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Intramural hematoma represents hemorrhage of the vasa vasorum and is not associated with intimal discontinuity (unlike penetrating ulcer).
Questions
-
1.
What is the most common cause of abdominal aortic aneurysm?
-
(a)
Inflammatory
-
(b)
Marfan’s syndrome
-
(c)
Degenerative
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(d)
Traumatic
-
(a)
Answer: C
-
2.
Which of the following is the imaging criteria to diagnose abdominal aortic aneurysm?
-
(a)
Caliber >1.5 times the expected
-
(b)
Active extravasation
-
(c)
Hyperdense crescent sign
-
(d)
Draped Aorta Sign
-
(a)
Answer: A
-
3.
What is the most common predisposing factor to aortic dissection?
-
(a)
Drug abuse
-
(b)
Coarctation
-
(c)
Trauma
-
(d)
Hypertension
-
(a)
Answer: D
-
4.
Which of the following is an indicator of false lumen?
-
(a)
Acute angle between the dissection flap and aortic wall
-
(b)
Smaller lumen
-
(c)
Posterior location in the ascending aorta
-
(d)
Aortic wall calcifications around the lumen
-
(a)
Answer: A
-
5.
Aortic wall irregularity and surrounding edema are the imaging features of which of the following aortic pathology ?
-
(a)
Aortic rupture
-
(b)
Mycotic aneurysm
-
(c)
Aortic dissection
-
(d)
Intramural hematoma
-
(a)
Answer: B
-
6.
CTA of an elderly man with acute chest pain in depicted on Fig. 1.14. Which of the following is a possible cause of the patient’s chest pain?
-
(a)
Hematoma
-
(b)
Aneurysm
-
(c)
Endoleak
-
(d)
Penetrating ulcer
-
(a)
Answer: D
-
7.
Which of the following is the CT (Fig. 1.15) diagnosis of patient involved in motor vehicle collision ?
-
(a)
Transection
-
(b)
Endoleak
-
(c)
Hematoma
-
(d)
Aneurysm
-
(a)
Answer: A
-
8.
What is the finding depicted on angiography (Fig. 1.16) in a patient with chest pain?
-
(a)
Aneurysm
-
(b)
Endoleak
-
(c)
Aortic pseudoaneurysm
-
(d)
Hematoma
-
(a)
Answer: C
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Chun, J., Mansouri, M., Singh, A. (2018). Imaging of Acute Aortic Conditions. In: Singh, A. (eds) Emergency Radiology. Springer, Cham. https://doi.org/10.1007/978-3-319-65397-6_1
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