PURPOSE: To evaluate the low attenuation of mosaic pattern in pulmonary embolism, as observed on HRCT, and to correlate the findings with the pathologic features of resected lung. MATERIALS AND METHODS: Using permanent embolic materials, pulmonary embolism was induced in eight Yorkshire pigs. Pre-and post-embolic pulmonary angiography was performed and after 6 weeks, the incidence and pattern of parenchymal change in low attenuation (mosaic pattern), as seen on HRCT, was evaluated. The animals were then sacrified and contact radiography of the lung was performed. Thirty-eight segments of pathology were taken from the area in which the presence of embolism had been suggested. Pathologic and HRCT findings were then correlated. RESULTS: On HRCT, low attenuation was seen in 23 of 36 segments (64%) and showed variable patterns : crescent peripheral hyperlucency (61%, n=14), heterogeneous mottled hyperlucency (17%, n=4), lobular hyperlucency (13%, n=3), and homogeneous segmental hyperlucency (9%, n=2). Parenchymal low attenuation was seen on HRCT in 10 of 11 segments (91%) in which large segmental arterial occlusion occurred, and in 3 of 16 segments (19%) in which there was small segmental arterial occlusion. Abnormal pathologic findings were pulmonary congestion, dilatation of pulmonary arteries, interlobular septal thickening, and thrombus formation. Among the 38 pathologic segmental specimens, 29 were from the area in which HRCT findings were positive, and in which pulmonary embolism subsequently occurred. In only four of nine segments (44%) in the area in which HRCT fingings were negative was pulmonary embolism subsequently. CONCLUSION: HRCT findings of pulmonary embolism at six weeks after embolization showed variable patterns of low attenuation, diminished diameter of pulmonary arteries, and normal diameter of bronchi. In cases with large segmental arterial occlusion, the finding of low attenuation was more common ; this may be due to reduced blood flow to the embolic area, in combination with bronchiolar spasm. For the early diagnosis of pulmonary embolism, these findings may be useful.