Which test is considered most appropriate for identifying a perfusion defect in the lungs when pulmonary embolism is suspected?

• A. CT Angiography
• B. Chest X-ray
• C. VQ Scan
• D. MRI

Answer: (C)

Understanding how blood and air move through the lungs helps explain why this test is best for spotting a perfusion defect from a suspected pulmonary embolism. A V/Q scan evaluates both ventilation (air reaching the airspaces) and perfusion (blood flowing through the lung tissue). In the case of an embolism, a clot blocks blood flow to portions of the lung, creating perfusion defects. If ventilation in those same regions is normal or relatively preserved, you get a ventilation–perfusion mismatch, which is a classic sign of an embolic process. This direct mapping of perfusion tends to be the most sensitive way to identify perfusion abnormalities caused by emboli, especially in patients where contrast CT isn’t ideal.

Chest X-ray often appears normal or non-specific and doesn’t reliably reveal perfusion problems. CT angiography is superb for visualizing clots directly and is widely used, but it answers the question of clot presence rather than mapping perfusion defects. MRI can image pulmonary vessels but is not the standard rapid test for acute PE due to practicality and availability.