Concurrent images of the distribution of blood flow (perfusion scan) and the distribution of alveolar ventilation (ventilation scan) are obtained following the inhalation of a radioactive gas and the IV injection of labeled albumin.

A properly performed perfusion scan, which includes at least six projections, is highly sensitive for the detection of small defects. However, this high sensitivity is not matched by a high specificity for the diagnosis of pulmonary embolism. Various conditions can cause mismatched defects on ventilation-perfusion scanning including emphysema, tuberculosis, previous irradiation etc.

For the reasons given above, it is helpful to have a baseline CXR to evaluate the patient for the underlying presence of COPD, or other conditions, which could complicate the interpretation of lung scan and lead to a false positive diagnosis.

A normal perfusion scan virtually excluded the diagnosis of pulmonary embolus.
A high probability lung scan indicated a high likelihood of emboli, particularly in patients with a high pretest probability. The cardinal sign of pulmonary embolism is an underperfused part of the lungs on perfusion scanning. Typically the defect is segmental, while the ventilation scan remains normal, the so-called ‘mismatched perfusion defect'.
Low probability scan when there is matching perfusion and ventilation defects.
Indeterminate scan

Low probabilty and indeterminate scans do not rule out the possibility of PE. If there is a clinical suspicion for PE further studies should be done.

Ventilation scan on left and perfusion scan on right show matching V/Q abnormalities.

Almost absence of perfusion of right lung in the perfusion scan with normal ventilation. Patient had normal CXR.