The upper left-hand image is an abnormal perfusion scan in the right posterior oblique (RPO) position. All of the other images are ventilation scans in the RPO position; the ventilation is normal. This "mismatch" between abnormal perfusion and normal ventilation of the right lung indicates a high probability of acute pulmonary embolism.
By permission of the publisher. From Kucher N, Goldhaber S, Sewell J, et al. In Bone's Atlas of Pulmonary and Critical Care Medicine . Edited by J Crapo. Philadelphia, Current Medicine, 2005.