The patient is a 65-year-old male with a history of right femoral thrombosis and a recent cardiac arrest who is currently anticoagulated on heparin. He is referred for evaluation of a possible acute pulmonary embolism.

Perfusion study: Multiple large segmental defects are seen in the mid and inferior left lung fields as well as the right mid lung field.

Ventilation study: The defects in tracer activity seen on the perfusion images appear mismatched.

Multiple bilateral mismatched segmental defects are identified. Under PIOPED criteria, this study is consistent with a high probability for an acute pulmonary embolism. The differential diagnosis includes chronic pulmonary embolism.