CT pulmonary angiography

CT pulmonary angiography (CTPA) is a CT scan timed precisely to photograph the lung's arteries while they are brightly filled with contrast dye. Iodine-based contrast is injected into an arm vein, the scanner waits the few seconds it takes for that dye to reach the pulmonary arteries, and then images the chest. The result is a vivid map of the blood vessels carrying blood from the heart to the lungs.

Its main job is to find a pulmonary embolism — a blood clot lodged in those arteries. On a well-timed scan the contrast-filled arteries glow white, and a clot shows up as a dark filling defect, a plug where the dye cannot flow. CTPA can also reveal how large the clot burden is, whether the right side of the heart is strained, and often offers an alternative explanation for the symptoms if no clot is found.

It has become the first-line test for suspected pulmonary embolism because it is fast, widely available and directly visualises the clot. The trade-offs are radiation exposure and the iodinated contrast, which must be used cautiously in kidney impairment or contrast allergy. In those situations, or in pregnancy, a ventilation–perfusion scan may be chosen instead, and the decision to scan at all is usually guided by clinical probability and a D-dimer blood test.