Acute aortic emergencies are life-threatening conditions that may require urgent surgical or interventional management. Imaging plays an important role in the diagnosis and planning of the management, and timely intervention helps in reducing mortality and morbidity.
Acute aortic syndrome (AAS) is a spectrum of life-threatening aortic diseases consisting of aortic dissection (AD), intramural haematoma (IMH) and penetrating atherosclerotic ulcer (PAU).
Multidetector computed tomography (MDCT), transoesophageal echocardiography and magnetic resonance imaging are the imaging modalities which could diagnose AASs. However, computed tomography (CT) is the most important investigation performed in acute emergency situations, owing to its accessibility, rapid acquisition and high sensitivity and specificity. Apart from diagnosing, the purpose of imaging is to locate the exact site, extension of disease, complications and associated findings which may affect the therapeutic management.
Triple-rule-out (TRO) CT angiography may be employed for evaluation of acute chest pain, as it examines the thoracic aorta, and pulmonary and coronary arteries in a single study. It is a useful investigation in low and moderate risk acute coronary syndrome patients; however, performing TRO is technically challenging. The protocol needs to be customised according to the patient profile to achieve the best results. There are studies which did not show any improved clinical outcome of TRO, as compared with dedicated pulmonary and coronary angiographies. Also, a higher radiation exposure and large contrast medium doses are two major disadvantages of TRO.
As the risk and clinical profile of patients with AD and IMH are similar, unenhanced CT is recommended, prior to a contrast study, in patients suspected with AAS and blunt chest trauma. Unenhanced CT acquisition plays a pivotal role in the diagnosis of IMH, as a contrast study alone is not sufficient to make a confident diagnosis. High attenuation contrast decreases the conspicuity to visualise hyperdense IMH. Intimal calcifications and acute haemorrhages, such as haemothorax and mediastinal haematomas are also easily interpreted on unenhanced CT scans.
The recommended scan protocol includes an unenhanced CT scan, followed by a dual phase contrast study, covering the lung apices to the inguinal regions. A total of 100 mL of non-ionic contrast medium containing a high iodine concentration (350 mg/dL) is injected at a flow rate of 4.5 mL/s using an 18-gauge catheter in the right antecubital vein, to reduce artefact from the left brachiocephalic vein. Image acquisition is started using the bolus track technique after placing the region of interest (ROI) on the ascending aorta with the trigger at 200 HU. Automatic acquisition may fail if the ROI is placed over a thrombosed vessel; hence, the technician should be prepared to manually start the study. Electrocardiogram- (ECG-) gated CT is advised when scanning the ascending aorta and arch to reduce motion-related artefacts to produce superior diagnostic-quality images. ECG-based tube current modulation and adjusting the peak voltage according to the patient’s morphology are two of the most effective methods to reduce radiation exposure. In addition, a slow regular heart rate and prospective ECG-gated acquisition could further bring down the dose.
Aortic dissection is the longitudinal splitting of the aortic media when circulating blood enters through a tear in the intima under haemodynamic force, resulting in the formation of true and false lumens. The incidence ranges from 2.6 to 3.5 cases per 100,000 persons per year with a male predominance.
On unenhanced CT, intraluminal displacement of intimal calcifications can occasionally be seen, suggesting AD (
Axial unenhanced computed tomography scan of thorax demonstrating displaced intimal calcification (arrow), suggesting aortic dissection.
Contrast-enhanced axial computed tomography scan demonstrating a displaced intimal flap (arrow), separating the true and false lumens in a patient with a Stanford Type A aortic dissection.
(a) Contrast-enhanced axial computed tomography (CT) scan showing intimal calcifications around the true lumen (thick arrow) and linear scattered hypodense areas within the false lumen termed the Cobweb sign (thin arrow), representing the remnants of media. The calibre of the true lumen (T) appears smaller than the false (F). (b) Axial contrast-enhanced CT scan showing a beak sign (arrow) which is a wedge of haematoma at the distal end of false lumen forming an acute angle with the vessel wall.
Stanford type B dissection with extension into the superior mesenteric artery. Contrast-enhanced computed tomography showing lesser enhancement in the false lumen (*) as compared to true lumen.
Type A chronic dissection. Contrast-enhanced axial (a) and coronal (b) computed tomography scan showing an ascending aortic aneurysm with chronic dissection. A thin linear low attenuation discontinuous dissection flap (arrow) is seen within the aneurysm.
Aortic dissection with a thrombosed false lumen could mimic an aortic aneurysm with intraluminal thrombus. Intimal calcifications are useful in differentiating between them, as they are located in the centre in AD and at periphery in aortic aneurysm with thrombus. The dissection flap in AD has a smooth contour, whereas mural thrombus usually has an irregular internal surface.
Intramural haematoma is the aortic wall thickening, caused by acute haemorrhage contained within the media. It was widely believed that it occurs owing to spontaneous rupture of the vasa vasorum with an intact intima. However, with recent advances in imaging, intimo-medial tears have been detected, suggesting that IMH has one entry tear with no exit, whereas AD has entry and exit intimo-medial tears into the aortic lumen. Intramural haematoma constitutes 5% – 15% of all cases of AAS.
Crescentic aortic wall thickening of increased attenuation is the most vital finding on unenhanced CT scan images (
Stanford type A intramural haematoma (IMH): (a) Unenhanced axial computed tomography (CT) scan showing hyperdense crescentic aortic wall thickening involving the ascending aorta (arrow); (b) Stanford type A IMH. Unenhanced coronal CT scan showing crescentic hyperdense aortic wall thickening (*) compressing the lumen.
Type A intramural haematoma. Contrast-enhanced axial (a) and coronal (b) computed tomography scan showing a non-enhancing hypodense crescent-shaped area (arrow) compressing the normal contrast-enhanced aortic lumen.
Coarctation of the aorta with a type A intramural haematoma (IMH) and aortic dissection. (a) Unenhanced axial computed tomography (CT) scan showing circumferential hyperdense thickening of the ascending aorta (arrow); (b) Contrast-enhanced axial CT showing crescentic non-enhancing hypodense area (thin arrow) representing IMH with haemopericardium (*) and concentric left ventricular hypertrophy (thick arrow); (c) Contrast-enhanced CT scan showing the displaced intimal flap (arrow) and haemopericardium (*). (d) Sudden narrowing of the aorta (arrow) distal to origin of the left subclavian artery on sagittal contrast-enhanced images. Multiple collaterals (thick arrow) along the anterior chest wall.
Management of IMH is similar to AD, where surgery is advised for type A lesions and type B IMHs are managed conservatively.
Penetrating atherosclerotic ulcer is the ulceration of atheromatous plaque, resulting in disruption of the internal elastic lamina and aortic media with haematoma formation. Penetrating atherosclerotic ulcer typically occurs in the elderly with a background of advanced atherosclerosis, in contrast to AD which is seen in younger patients. The incidence of PAU in AAS ranges between 2.3% and 7.6%.
Extensive intimal calcifications are the frequent finding on unenhanced CT. A contrast-filled outpouching extending beyond the aortic wall is diagnostic of PAU (
Penetrating atherosclerotic ulcer. Contrast-filled outpouching seen beyond the aortic wall involving the arch of the aorta (arrow).
Ruptured penetrating atherosclerotic ulcer (PAU) with contained haematoma. Contrast-enhanced computed tomography scan of thorax showing a ruptured PAU along the posterior aortic wall (white arrow) with adjacent intramural haematoma (black arrow) appearing as hyperdense aortic wall thickening and a contained haematoma in left pleural space (*). Findings were confirmed at surgery.
Medical management is the preferred treatment option for type B lesions, as in AD, with follow-up imaging to observe progression. Surgery is the treatment of choice for patients with aortic rupture, persistent pain, haemodynamic instability and risk of embolisation. The disease prognosis remains poor as surgical intervention is difficult owing to extensive atherosclerotic changes in the remaining aorta.
A dedicated CT protocol comprising an initial unenhanced study followed by post-contrast imaging should be immediately performed in patients suspected with AAS. Knowledge about the imaging findings of these interrelated conditions can facilitate prompt diagnosis.
Dr Hasta Kushte for her help in preparation of manuscript.
The author declares that he has no financial or personal relationship(s) which may have inappropriately influenced him in writing this article.