What is a ventilation-perfusion scan?

Radiological test: A ventilation-perfusion (v/q) scan is a specialized radiological lung test comparing mismatches between any part of your lung that still exchanges oxygen (affected in COPD e.g.) and any part that is not receiving blood flow. This is done through radio-tracers. A mismatch means there is a clot in that part of the lung hindering blood flow. Diseases & surgery of the lung complicate interpretation.

Pulmonary embolism: The v/q scan is a nuclear medicine study that is used to evaluate for possible pulmonary embolism or to evaluate resolution of previously known pulmonary embolism. It involves imaging perfusion of the lungs by the blood and imaging ventilation of the lungs by air. A tiny amount of radiation is put into the blood to allow this, and a tiny amount into the air.