Computed Tomography

Helical computed tomography (CT) is an highly accurate and most frequently selected imaging technique (sensitivity and specificity of nearly 100%) for the diagnosis of AAD22-26,48,49. It has also been proven to be useful to identify atypical forms of AAD, such as intramural hematoma and atherosclerotic ulcers50,51 .

It has important advantages. It is fast and noninvasive, easy to perform, almost always well-accepted by the patient, and not operator-dependent52. It allows three-dimensional visualization of the entire aorta and its branches with a single breath-hold and a single bolus injection (Figures 11.1 and 11.2), and it provides the detection of life-threatening complications such as side branch involvement and fluid extravasation into pericardium, pleural space, and/or mediastinum (Figure 11.3)53,54. However, it requires the use of an intravenous contrast agent and presents limitations related to the diagnosis of aortic insufficiency, tear localization, as well as detections of intimal tears and subtle aortic dissection15-19,23,55. Pitfalls and artefacts should also be taken into account during the interpretations of CT. Cardiac motion artefacts are more

Figure 11.1. Postprocessing of native CT images is required for the evaluation of aortic dissection. Multiplanar reformation (MPR) represents the best reconstruction modality to demonstrate the course of the intimal flap throughout the thoracic aorta ((a), sagittal view), whereas the maximum intensity projection (MIP) does not seem to be able to clearly visualize the intimal flap and does not differentiate the true and the false lumen ((b), sagittal view).

Figure 11.1. Postprocessing of native CT images is required for the evaluation of aortic dissection. Multiplanar reformation (MPR) represents the best reconstruction modality to demonstrate the course of the intimal flap throughout the thoracic aorta ((a), sagittal view), whereas the maximum intensity projection (MIP) does not seem to be able to clearly visualize the intimal flap and does not differentiate the true and the false lumen ((b), sagittal view).

evident at the level of the proximal ascending aorta56-58. They consist of localized duplication and pseudo-thickening of the aortic wall and are located mainly in the left antero-lateral and right postero-lateral positions and limited to only few contiguous scans. They can be distinguished from intimal flaps

Aortic Archn Scan
Figure 11.2. On unenhanced CT scans, the diagnosis of aortic dissection can be indicated by the identification of intimal calcifications displaced toward the aortic lumen, such as in this image (axial scan) at the level of the aortic arch (arrows).

Figure 11.3. Unenhanced CT images are important to identify complications of aortic dissection, such as pericardial effusion (arrows, axial scan), that can be slightly hyperdense, indicating the presence of hemopericardium.

because of their position and shape and by careful analysis of contiguous images. Moreover, the use of cardiac gating can help reducing the cardiac motion artefacts59'60. Cardiac gating is limited by reduced volume coverage, limiting the study to the thoracic aorta alone, which is not sufficient when an AAD is suspected. The streak artefacts are hypodense straight lines (Figures 11.4 and 11.5) extending beyond the confines of the aorta and are visible in only a few axial sections caused by the aliasing of markedly attenuating materials,

Coronary Artery Calcium Scoring Study

Figure 11.4. On the arterial phase, the diagnosis of an aortic dissection is based on the identification of the intimal flap that is visible as a thin hypodense band within the opacified aortic lumen, such as in this example (axial scan) of Stanford type A aortic dissection, involving the origin of the ascending aorta (arrow). Pericardial effusion can also be clearly appreciated.

Figure 11.4. On the arterial phase, the diagnosis of an aortic dissection is based on the identification of the intimal flap that is visible as a thin hypodense band within the opacified aortic lumen, such as in this example (axial scan) of Stanford type A aortic dissection, involving the origin of the ascending aorta (arrow). Pericardial effusion can also be clearly appreciated.

Figure 11.5. The relationship between the intimal flap and the supraaortic vessels can be clearly delineated by MPR reconstruction (coronal and sagittal view). In this example of type A dissection, the origin of the brachiocephalic trunk is involved by the intimal flap (arrow), whereas the left carotid and subclavian arteries are not involved.

Figure 11.5. The relationship between the intimal flap and the supraaortic vessels can be clearly delineated by MPR reconstruction (coronal and sagittal view). In this example of type A dissection, the origin of the brachiocephalic trunk is involved by the intimal flap (arrow), whereas the left carotid and subclavian arteries are not involved.

cardiac motion, and sharp contrast interfaces, such as the concentration of contrast material within the left brachio-cephalic venous trunk when the bolus is injected into the left arm61.

CT is also a useful imaging tool for serial follow-up of patients with AAD, being capable of detecting any increase in aortic diameter, development of false or true lumen aneurysms (Figures 11.6-11.10), organ malperfusion, and leakages at anastomoses or stent sites (Figures 11.10-11.14)62. Furthermore,

Figure 11.6. The false lumen can be identified through the "beak sign", represented by an acute angle between the dissection flap and the outer wall of the aorta at the level of the false lumen (arrow, axial scan).
Cobweb Sign Aortic Dissection

Figure 11.7. Another sign that may allow the identification of the false lumen is the "cobweb sign"; the cobwebs appear as fine, hypodense linear areas within the false lumen, attached to the aortic wall (arrows, axial scan). The intimal tears are also clearly delineated within the ascending aorta (arrowheads).

Figure 11.7. Another sign that may allow the identification of the false lumen is the "cobweb sign"; the cobwebs appear as fine, hypodense linear areas within the false lumen, attached to the aortic wall (arrows, axial scan). The intimal tears are also clearly delineated within the ascending aorta (arrowheads).

it may monitor healing or progression of intramural hematoma to a saccular or fusiform aneurysm (Figure 11.13)63. Finally, helical CT scanning is very useful in identifying static or dynamic obstruction of abdominal branch vessels associated with increased mortality and morbidity rates in AAD patients (Figure 11.11)64,65.

Displaced Calcifications Dissection
Figure 11.8. The false lumen can be completely thrombosed. In this case, the presence of displaced intimal calcifications (arrow, axial scan) can help differentiating the dissection from an aortic aneurysm or partial thrombosis.

Figure 11.9. Aortic rupture can occur after dissection. In this example, a dissected and dilated ascending aorta can be clearly appreciated. The opacified lumen appears irregular and highly dilated, indicating an impeding rupture. The intimal flap can be clearly visualized (arrow). Moreover, pericardial and pleural effusion (arrowhead) are appreciated as complications of the dissection.

Figure 11.9. Aortic rupture can occur after dissection. In this example, a dissected and dilated ascending aorta can be clearly appreciated. The opacified lumen appears irregular and highly dilated, indicating an impeding rupture. The intimal flap can be clearly visualized (arrow). Moreover, pericardial and pleural effusion (arrowhead) are appreciated as complications of the dissection.

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