Figure 3b. Matched ventilation-perfusion scan defects. No areas of increased opacity were seen at chest radiography, and pulmonary angiography showed no pulmonary embolism. (a) Left posterior oblique (LPO) and posterior images from a perfusion scan show multiple areas of diminished perfusion that are especially prominent in the lung bases. Note the irregular lung margins; these are not artifactual but represent diminished perfusion. (b) Images from a xenon-133 ventilation scan demonstrate matched ventilation-perfusion defects in all three zones of both lungs. The equilibrium image was obtained over a 2-minute period at the end of the 4-minute equilibration period. Note the relatively sharp lung margins (especially at the bases) compared with the perfusion scan (cf a). The posterior image was obtained over a 45-second period that ended 90 seconds after the start of washout. Procurement of the left posterior oblique (LPO) image ended 180 seconds after the start of washout. The focal areas of increased activity represent abnormal ventilation (slow washout). Matched basilar perfusion and ventilation abnormalities are particularly well visualized on left posterior oblique images.