A patient undergoes CT pulmonary angiography (CTPA) for suspected pulmonary embolism. Which of the following represents the optimal scanning window for CTPA?

• A Arterial phase: 10 seconds after injection, with bolus tracking in the aorta
• B Portal venous phase: 70 seconds after injection
• C Pulmonary arterial phase: 4–5 seconds delay after the trigger HU threshold (~100–150 HU) is reached in the main pulmonary artery using bolus tracking ✓
• D Fixed delay: 60 seconds after injection regardless of cardiac output

Explanation

CTPA is optimised by bolus-tracking in the main pulmonary artery or right ventricular outflow tract. When the attenuation reaches a threshold (~100–150 HU) in the pulmonary artery, scanning is initiated after a 4–5 second delay to allow the contrast bolus to fill the pulmonary arteries. This accounts for variability in cardiac output and ensures optimal pulmonary arterial opacification. Aortic bolus tracking (arterial phase) would be appropriate for aortic CT, not CTPA. Fixed delays are less reliable as cardiac output varies. Portal venous phase is appropriate for hepatic parenchymal imaging.

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

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