Nuclear ventilation / perfusion scans are often used to diagnose a PE. A normal perfusion scan usually excludes a PE but can be found in a minority of individuals with a PE. Perfusion defects are non-specific; about one third of those with defects have a PE. The probability that a perfusion defect is a PE increases with the size, shape and number of defects as well as the presence of a normal ventilation scan. Mismatched perfusion defects (normal ventilation scan), which are segmental in size or larger are “high probability” defects and are associated with an approximately 80% prevalence of PE. Three or more mismatched defects are associated with a prevalence of approximately 90%. Individuals should be treated if presenting with a positive ventilation/perfusion scan and high clinical suspicion of a PE.