Atherosclerotic Disease and Atypical Dissection

On contrast-enhanced CT, IMH has the appearance of a nonenhancing low attenuation rim or crescent, which can be difficult to distinguish from atherosclerotic intraluminal thrombus (Figure 5.5c-f). Three imaging characteristics are useful to differentiate between IMH and intraluminal thrombus:

First, the false lumen of acute IMH shows high attenuation on unenhanced CT; intraluminal thrombus does not. However, this imaging criterion becomes unreliable in subacute and chronic intramural hematoma because the attenuation of IMH decreases over time and the initial high attenuation of the aortic wall is commonly absent in the subacute and chronic stage. In a study by Yamada et al.14, high attenuation of the aortic wall on unenhanced CT was observed in all eight patients evaluated in the acute stage of intramural hematoma but in none of the four patients evaluated while in the subacute stage14.

Second, most aortic calcifications are intima calcifications. Thus, calcifications within a thrombus detached from the aortic wall suggest IMH (Figure 5.5b) and calcifications at the outside of a thrombus suggest intraluminal thrombus (Figure 5.6). However, differentiation between atherosclerotic intraluminal thrombus and intramural hematoma on the basis of aortic calcification alone can be problematic because chronic intraluminal thrombus can calcify over time and mimic detached intima calcification (Figure 5.7). Furthermore, intima calcifications may not be present, particularly in the younger age group19. In the acute setting, this diagnostic dilemma can be solved by review of the unenhanced CT images, which demonstrate the high attenuation of acute intramural hematoma (Figure 5.5a-b) or the low attenuation of intraluminal thrombus (Figure 5.6a).

Intramural Hematoma

Figure 5.5. CT examination of type B intramural hematoma (IMH): (a) unenhanced axial image shows higher attenuation of acute intramural hematoma (white arrow), compared to low attenuation of the perfused true lumen of the aorta (black arrow); (b) unenhanced axial image inferior to (a) shows displaced intima calcification (white arrow); (c) axial contrast-enhanced image of the aortic arch shows low attenuation of the intramural hematoma (black arrow), compared to the contrast filled true lumen of the aorta; a focal contrast outpouching of the true lumen (white arrow) suggests intimal tear or ulceration; (d-e) axial contrast enhanced images at the same level as (a) and (b) demonstrate intramural hematoma of the descending thoracic aorta (black arrow); (f) maximum intensity projection reformatted image nicely demonstrates the extent of the intramural thrombus (black arrows), however, does not add additional diagnostic information.

Figure 5.5. CT examination of type B intramural hematoma (IMH): (a) unenhanced axial image shows higher attenuation of acute intramural hematoma (white arrow), compared to low attenuation of the perfused true lumen of the aorta (black arrow); (b) unenhanced axial image inferior to (a) shows displaced intima calcification (white arrow); (c) axial contrast-enhanced image of the aortic arch shows low attenuation of the intramural hematoma (black arrow), compared to the contrast filled true lumen of the aorta; a focal contrast outpouching of the true lumen (white arrow) suggests intimal tear or ulceration; (d-e) axial contrast enhanced images at the same level as (a) and (b) demonstrate intramural hematoma of the descending thoracic aorta (black arrow); (f) maximum intensity projection reformatted image nicely demonstrates the extent of the intramural thrombus (black arrows), however, does not add additional diagnostic information.

Aortic Intraluminal Thrombus

Figure 5.6. Axial CT images of atherosclerotic intraluminal thrombus in abdominal aortic aneurysm demonstrate intima calcification at the outside of the thrombus (white arrow): (a) un-enhanced image shows low attenuation of the intraluminal thrombus (black arrowhead), which cannot be distinguished from the perfused lumen of the aorta; (b) contrast enhanced image delineates the intraluminal thrombus (black arrowhead), which shows low attenuation compared to the contrast enhanced lumen of the aorta. This example of intraluminal thrombus is relatively smoothly demarcated and could be confused with IMH on the contrast enhanced scan; however, low attenuation on the unenhanced scan excludes acute IMH.

Lastly, IMH is usually smoothly marginated and crescent shaped (Figure 5.5), as opposed to the often jagged appearance of intraluminal thrombus (Figure 5.8). However, this imaging criterion is not always helpful, as demon-

Figure 5.7. Axial CT images of calcified intraluminal thrombus simulating detached intima calcification (black arrow): (a) unenhanced image; (b) contrast enhanced image; note isoattenu-ation of intraluminal thrombus compared to the perfused lumen of the aorta on the unenhanced image.

Figure 5.7. Axial CT images of calcified intraluminal thrombus simulating detached intima calcification (black arrow): (a) unenhanced image; (b) contrast enhanced image; note isoattenu-ation of intraluminal thrombus compared to the perfused lumen of the aorta on the unenhanced image.

Figure 5.8. Axial contrast-enhanced CT image of atherosclerotic intraluminal thrombus in a different patient demonstrates the more typical jagged appearance.

strated by the relatively smooth appearance of intraluminal thrombus in an abdominal aneurysm in the example in Figure 5.6.

Was this article helpful?

0 0

Post a comment