ventilation–perfusion scan

A ventilation–perfusion (V/Q) scan is a nuclear-medicine test that compares two maps of the lung side by side: where air goes (ventilation) and where blood goes (perfusion). The patient breathes in a small amount of harmless radioactive gas or aerosol to map airflow, and is injected with tiny radioactive particles to map blood flow. A gamma camera then images where each tracer ends up.

The logic is elegant. In healthy lung, air and blood reach the same places, so the two maps match. A pulmonary embolism blocks blood flow to a lung region while air still reaches it normally, creating a tell-tale “mismatch” — a zone that is ventilated but not perfused. Several such mismatched segments make embolism likely, while matched defects point to other lung problems.

V/Q scanning predates CTPA and is now used mainly when CT contrast should be avoided — for instance in significant kidney impairment, contrast allergy, or to limit radiation to breast tissue in younger or pregnant patients. Its main limitation is that pre-existing lung disease (such as COPD or prior pneumonia) muddies the maps and yields indeterminate results, which is one reason CTPA usually comes first when it is feasible.