V/Q Scan
First Step:
Track the lung periphery on the perfusion images.
If this is continuous and conforms to the normal expected contour, then there is no PE, regardless of any other findings in the study.
If the perfusion images are not normal, then ventilation images and radiologic evaluation come into play.
Interpretation
1: If perfusion images show two or more large defects, or their equivalent, with normal or better ventilation (V) and normal appearance or smaller abnormality at chest radiography (CXR), then there is high probability of PE.
2: Ventilation abnormalities with normal perfusion and normal or near normal chest radiographs represent a reverse mismatch and are a classic appearance of airway disease such as chronic obstructive pulmonary disease (COPD).
Pulmonary embolism present (high probability)
two or more large mismatched segmental perfusion defects or the arithmetic equivalent of moderate and/or large defects
Nondiagnostic (low or intermediate probability)
all other findings not falling into the pulmonary embolism present or absent categories
Pulmonary embolism absent (normal or very low probability)
Figure 3. Schematic guide to interpretation of V/Q scans. If perfusion (Q) images are normal, then there is no PE.
Scenario 1: If perfusion images show two or more large defects, or their equivalent, with normal or better ventilation (V) and normal appearance or smaller abnormality at chest radiography (CXR), then there is high probability of PE.
Scenario 2: Ventilation abnormalities with normal perfusion and normal or near-normal chest radiographs represent a reverse mismatch and are a classic appearance of airway disease such as chronic obstructive pulmonary disease (COPD).
Scenarios 3 and 4: A defect that is matched on perfusion and ventilation images may or may not have a corresponding radiographic abnormality and is usually not a PE. This can be segmental or lobar (eg, in pneumonia) (scenario 3) or have round or smooth margins in cases of tumors or pleural effusions (scenario 4).
Derenoncourt P. Published Online: October 22, 2021
https://doi.org/10.1148/rg.2021210060
Triple Match
If there is a matched V/Q defect with a corresponding radiologic abnormality, then it is a “triple match” and is typical of pneumonia, lung mass, or pleural effusion.
Triple matches are a problem because sometimes PEs are complicated by pulmonary infarction. Because PEs are more likely to occur in the lower lobes (because of increased blood flow), triple matches in the lower lobes are more likely to be due to PE than triple matches in the upper lobes.
Ventilation-Perfusion Scan - A Primer for Practicing Radiologists.pdf