What are the differences between pulmonary perfusion scan and pulmonary ventilation scan?

Lung V-Q scan: Ventilation scanning involves the patient breathing in a radiotracer while perfusion scanning involves injecting the radiotracer through an iv. The procedure is done usually to evaluate the presence of absence of a pulmonary embolism (usually a blood clot) and involves comparing the radiotracer's presence in the alveoli vs. Its presence in the lung circulation.

Blood vs air: The perfusion scan images blood perfusion of the lungs. The ventilation scan is used to image ventilation by air of the lungs. Generally, both are performed together to see how well lung perfusion (blood) matches lung ventilation (air).