Sequential MR signal change of the thrombus in the false lumen of thrombosed aortic dissection.

The evolution of thrombus in the false lumen was investigated in 14 patients with thrombosed aortic dissection, by reviewing the findings acquired at a total of 21 magnetic resonance imaging (MRI) examinations performed between 2 and 146 days after the onset. On electrocardiographic gated 1.5-Tesla MRI, T1-(TR/TE = 860 +/- 190 ms/17-40 ms) and T2-(TR/TE = 1620 +/- 240 ms/70-80 ms) weighted spin echo and gradient echo images were obtained, and the signal intensity of the thrombus on these images was evaluated independently by two observers. The density of the thrombus was also evaluated using computed tomography (CT) images obtained at a total of 54 examinations. On both T1- and T2-weighted images, the thrombus showed signal iso- or hypointensity compared to that of skeletal muscle during the first several days after the onset and, thereafter, showed signal intensity similar to that of fat tissue. It is suggested that the low signal intensity of the thrombus observed during the initial period after the onset was caused by the presence of deoxyhemoglobin and the high intensity observed thereafter was caused by methemoglobin. Focal discrepancy of the signal intensities within two parts of the lumen on spin echo images was observed in 7 patients, and a low-intensity layer on the surface of the thrombus inside the false lumen was observed on gradient echo images in 5 of these 7 patients. This characteristic MR signal change of the thrombus in the false lumen of thrombosed aortic dissection provides useful information concerning the age of the thrombus and in the differential diagnosis of the thrombus from a mural thrombus of aortic aneurysm.