The key interpretive principle is comparison, not either image alone. Ventilation shows where inhaled tracer reaches the lungs, while perfusion shows tracer distribution through pulmonary vessels. If an area remains ventilated but has reduced perfusion, the resulting mismatch highlights a regional air-flow and blood-flow difference that can support clinical assessment for pulmonary embolism.
The two tracers provide complementary functional information. A radiolabeled gas or aerosol maps the inhaled component, whereas an intravenously delivered radiotracer maps pulmonary blood flow. The gamma camera records each distribution so clinicians can compare the paired images rather than relying on a single tracer pattern. This separation is central to recognizing regional mismatches.
Because the two distributions represent different physiological steps, their relationship carries more information than either pattern by itself. A region with reduced perfusion despite preserved ventilation produces the characteristic mismatch considered during assessment for pulmonary embolism. The comparison therefore links a blood-flow abnormality to the local ventilation pattern rather than describing tracer location alone.
During the examination, ventilation images are obtained while the patient inhales a radiolabeled gas or aerosol. Perfusion imaging uses a radiotracer delivered through an intravenous route. A gamma camera records both tracer distributions, and clinicians compare the resulting ventilation and perfusion patterns. This sequence produces paired functional images for interpretation.
V/Q scanning may be considered when clinicians need functional information about pulmonary air delivery and blood flow, but other imaging options are limited or unsuitable. Its value in that setting comes from the paired tracer patterns, which can reveal a regional mismatch without depending on a single type of information. The result supports assessment of suspected pulmonary embolism.
Although suspected pulmonary embolism is the primary clinical use, the scan can also help evaluate selected lung disorders. In those cases, clinicians can examine how ventilation and perfusion are distributed across the lungs and whether the patterns correspond. The study therefore provides functional context for disorders affecting pulmonary air delivery, pulmonary blood flow, or their relationship.