Dissecting aneurysm, (thoracic saggital MRI). The dissecting aneurysm, or aortic dissection, is a longitudinal tear of the aorta wall. This tear can be worsened, as here, by a hematoma, (dissecting or intramural hematoma), separating the tunica intima, (internal layer), of the adventitia, (external layer of the arterial wall). This hematoma leads to medianecrosis, progressive destruction of the media, (intermediary layer of the wall), and then to the dissection.