Figure 4. Figures 4, 5. (4) Mural thrombus. CT scan shows an atheromatous thrombus with an irregular internal border in the thoracic descending aorta (arrow) and motion artifact in the ascending aorta. A thrombosed aortic dissection usually demonstrates a smooth internal border. (5) Focal periaortic soft-tissue mass. (a) CT scan shows idiopathic periaortic fibrosis (arrows). (b) CT scan shows periaortic lymphoma as a focal rounded mass at the aortic border (arrows). These periaortic masses have an irregular external border, whereas intramural hematoma appears as smooth, crescentic thickening of the aortic wall (see Fig 6).