Acute aortic dissection (AAD) is the most common emergency affecting t
he aorta. Noninvasive imaging allows prompt and reliable diagnosis of
AAD and has largely supplanted aortography. However, atypical imaging
features and diagnostic pitfalls can delay lifesaving therapy. An inti
mal flap is the characteristic feature of AAD. If there is flow within
both lumina, typical imaging features are probably present. If the fa
lse lumen is thrombosed or there is no intimal tear to permit flow thr
ough the false lumen, a distinct intimal flap may not be present. Seco
ndary signs of AAD include an intramural or periaortic acute thrombus,
which manifests as a high-attenuation cuff or crescent on unenhanced
computed tomographic scans. Other conditions that can reduce the consp
icuity of the intimal flap include atypical configurations of the flap
, such as seen with short dissections or with multiple false channels,
in which case the flaps are complex. Finally, aortic anomalies may ca
use confusion.