Identification Of True And False Lumen

Identification of true and false lumen is an important prerequisite for percutaneous treatment of dissection. In most cases of type B dissection, the true lumen is easily identified by its continuity with the nondissected ascending aorta. However, in type A dissection and in cases where the aorta is incompletely imaged, identification of true versus false lumen may not be straightforward. The following imaging criteria are helpful to distinguish between true and false lumen33-35.

• The beak sign: This is defined as acute angle between the dissection flap and the outer aortic wall, and indicates the location of the false lumen (Figure 5.14). This sign has been reported to be 100% accurate for identification of the false lumen in both acute and chronic aortic dissection33.

• Cobwebs: These are thin, linear low attenuation filling defects attached to the aortic wall, and are only seen in the false lumen (Figure 5.15), as shown by LePage et al.33. In this study, cobwebs were 100% specific for identification of the false lumen. Unfortunately, cobwebs were observed

Figure 5.10. Contrast-enhanced CT examination of type B classic dissection, multiplanar reformatted images: (a) sagittal reformatted image of the descending aorta shows dissection flap (black arrow) starting just distal to the left subclavian artery (white arrow) (the false lumen is indicated by the thin black arrow; note dilation of the descending aorta and lower attenuation of the false lumen due to slow flow and early image acquisition); (b) sagittal reformatted image of the descending aorta shows tear of the dissection flap (black arrow); (c) axial reformatted image shows same tear of the dissection flap (the end of the dissection flap is indicated by the thin black arrow; status postsurgery to the ascending aorta (large black arrow), as indicated by surgical clips; trace left pleural effusion (white arrowhead); (d) axial oblique reformatted image shows origin of celiac artery (black arrow) from true lumen; (e) axial oblique reformatted image shows origin of right renal artery (black arrow) from true and left renal artery (thin black arrow) from false lumen (note again lower attenuation of the false lumen and delayed enhancement of the left kidney (arrowhead) due to slow flow in the false lumen and early image acquisition; incidental low attenuation hepatic lesions are also noted).

Figure 5.10. Contrast-enhanced CT examination of type B classic dissection, multiplanar reformatted images: (a) sagittal reformatted image of the descending aorta shows dissection flap (black arrow) starting just distal to the left subclavian artery (white arrow) (the false lumen is indicated by the thin black arrow; note dilation of the descending aorta and lower attenuation of the false lumen due to slow flow and early image acquisition); (b) sagittal reformatted image of the descending aorta shows tear of the dissection flap (black arrow); (c) axial reformatted image shows same tear of the dissection flap (the end of the dissection flap is indicated by the thin black arrow; status postsurgery to the ascending aorta (large black arrow), as indicated by surgical clips; trace left pleural effusion (white arrowhead); (d) axial oblique reformatted image shows origin of celiac artery (black arrow) from true lumen; (e) axial oblique reformatted image shows origin of right renal artery (black arrow) from true and left renal artery (thin black arrow) from false lumen (note again lower attenuation of the false lumen and delayed enhancement of the left kidney (arrowhead) due to slow flow in the false lumen and early image acquisition; incidental low attenuation hepatic lesions are also noted).

Figure 5.11. Axial CT images of the descending aorta show classic dissection and displaced intima calcifications within the dissection flap (black arrow): (a) unenhanced image; (b) contrast-enhanced image (the false lumen is indicated by the white arrowhead; note again the lower attenuation of the false lumen on the contrast-enhanced image due to slow flow and early image acquisition; the false lumen is larger than the true lumen).

Figure 5.11. Axial CT images of the descending aorta show classic dissection and displaced intima calcifications within the dissection flap (black arrow): (a) unenhanced image; (b) contrast-enhanced image (the false lumen is indicated by the white arrowhead; note again the lower attenuation of the false lumen on the contrast-enhanced image due to slow flow and early image acquisition; the false lumen is larger than the true lumen).

in only 9% of the scans reviewed, limiting the usefulness of this sign due to its low prevalence.

• Wrapping: If one lumen wraps around the other at the level of the aortic arch, the inner lumen is always the true and the outer lumen is always the false lumen33.

• Lumen size: The false lumen is usually larger than the true lumen in classic dissection (Figures 5.10 and 5.11). In the study of LePage et al.33, at one quarter of the distance along the dissected length of the aorta, the larger lumen was the false lumen in 85% of the acute cases and 83% of

Figure 5.12. Axial contrast-enhanced CT image of complex dissection flap with two false lumens visualized. The arrow indicates the true lumen.

Figure 5.13. Multiplanar reformatted contrast-enhanced CT images of type A dissection: (a) demonstrates a localized dissection flap in the ascending aorta (black arrows); the brachio-cephalic artery (black arrowhead) is not involved; the white arrowhead indicates the descending thoracic aorta; (b) shows the origin of the left coronary artery, which is calcified but not involved by the dissection; there is a large tear in the dissection flap (black arrowhead).

Figure 5.13. Multiplanar reformatted contrast-enhanced CT images of type A dissection: (a) demonstrates a localized dissection flap in the ascending aorta (black arrows); the brachio-cephalic artery (black arrowhead) is not involved; the white arrowhead indicates the descending thoracic aorta; (b) shows the origin of the left coronary artery, which is calcified but not involved by the dissection; there is a large tear in the dissection flap (black arrowhead).

the chronic cases. At one half of the distance along the dissected length of the aorta, the larger lumen was the false lumen in 94% of acute cases and in 96% of chronic cases.

• Outer wall calcification: This indicates the true lumen in acute dissection (Figure 5.16). However, this sign is unreliable in chronic dissection, where calcification of the outer wall of the false lumen is occasionally observed (Figure 5.17). Outer wall calcification has been shown to be 60% sensitive and 100% specific for identification of the true lumen in acute aortic dissection. On the other hand, outer wall calcification of the false lumen has been observed in 17% of chronic cases33.

• Thrombosis: A thrombosed lumen is usually the false lumen. This sign, however, is of limited value in patients with aneurysm, where intraluminal thrombus may be present in the true lumen as well. In the study of LePage et al., thrombosis in the false lumen was observed in 46% of cases with acute dissection and 83% of cases with chronic dissection. On the other hand, intraluminal thrombus in the true lumen was observed in 6% of the cases with acute dissection and 4% of the cases with chronic dissection.

• Direction: Blood flow direction is from true to false lumen at the level of intimal tears, with the ends of the flaps pointing toward the false lumen in acute dissection. LePage et al. observed the dissection flap curved toward the false lumen at one quarter of the distance along the dissected length of the aorta in 56%, flat dissection flap in 38%, and the dissection flap curved toward the true lumen in 6% of cases with acute dissection. For chronic dissection, the dissection flap was flat in 75% and curved toward the false lumen in 25% of cases. In a study of Kapoor et al., the free edges of the dissection flap were pointing toward the false lumen in all five patients with acute aortic dissection in which an intimal tear was visualized35.

On most exams, attenuation of the false lumen will be lower than that of the true lumen, due to early image acquisition (Figures 5.10,5.11,5.17). However, attenuation of the lumina is not a reliable sign to distinguish between true and false lumen. Although flow is faster in the true lumen than in the false lumen, attenuation of the false lumen may be less, equal or higher than that of the true lumen, dependent on timing of the image acquisition36.

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Responses

  • FERRUCCIO
    What are true and false lumens?
    6 months ago
  • Tecla
    How to differentiate true from false lumen dissection?
    5 months ago
  • fastolph
    What is heterogeneous calcification of a false lumen?
    2 months ago

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