If CT pulmonary angiography is contraindicated in suspected pulmonary embolism, which imaging test is commonly used?

• A. Echocardiography.
• B. V/Q scan.
• C. D-dimer.
• D. CT angiography.

Answer: (B)

When CT pulmonary angiography cannot be used, the ventilation-perfusion scan is commonly chosen to evaluate for a pulmonary embolism. This test separately assesses ventilation (airflow) and perfusion (blood flow) in the lungs. Pulmonary embolism often causes a mismatch—areas that are well ventilated but poorly perfused—creating a perfusion defect with preserved ventilation. A high-probability V/Q result supports PE in the right clinical context, and a normal scan can effectively rule out PE for many patients. If the result is non-diagnostic, additional testing may be needed.

Echocardiography can reveal signs of strain on the heart from a large PE but it does not visualize the clot itself and isn’t diagnostic for PE. D-dimer is a blood test used to screen for clotting but does not provide imaging evidence. CT angiography is the definitive imaging test when not contraindicated, so it isn’t used when it’s specifically contraindicated, which is why the V/Q scan is the preferred alternative in those cases.