PURPOSE: To determine the magnetic resonance imaging (MRI) findings in patients with traumatic thoracic aortic injury and to assess the usefulness of MRI for the diagnosis of aortic injury. MATERIALS AND METHODS: Between May 1990 and June 2000, sixteen patients with blunt thoracic aortic injury underwent MRI. The findings were evaluated with regard to the type of aortic injury, aortic circumference, the size, direction and shape of the pseudoaneurysm, the intimal flap, and pseudocoarctation. Six patients underwent follow-up MRI, and any changes in the findings were assessed. RESULTS: MRI indicated that traumatic thoracic aortic injury comprised localized pseudoaneurysm in 15 patients and extensive aortic dissection in one. The aortic circumference was partially involved in all cases. Pseudoaneurysms were located at the aortic isthmus in 16 cases and the descending thoracic aorta in one. Two patients each had two lesions: two pseudoaneurysms in one, and aortic dissection and pseudoaneurysm in the other. The mean diameter and length of the pseudoaneurysms was 2.8+/-0.8 cm (mean+/-SD) and 3.3+/-1.0 cm (mean+/-SD), respectively. Their direction was anteromedial or anterolateral in 15 cases and posterolateral in two. All were saccular shaped. An intimal flap was present in seven cases and pseudocoarctation was demonstrated in ten. Follow-up MRI revealed changes in the size of a pseudoaneurysm or the length of an aortic dissection. CONCLUSION: The most common finding demonstrated by MRI in patients with traumatic thoracic aortic injury was an anteromedially-directed saccular pseudoaneurysm in the aortic isthmus. This modality was considered useful for evaluation of the entire aorta in cases of multiple pseudoaneurysms or aortic dissection.