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Emergency · 2 min read

Pulmonary embolism on CT: the filling defect

How a pulmonary embolus looks on CT pulmonary angiography, how acute and chronic clot differ, and why a poor study should say what it could not assess.

Key points

• On CT pulmonary angiography an embolus is a filling defect: grey clot inside an artery that should be bright with contrast.
• The clot has a sharp edge against the contrast around it.
• An acute embolus that only partly fills the artery is often central, or meets the wall at an acute angle.
• Chronic emboli look different: eccentric, meeting the wall at obtuse angles, with webs or flaps, and occluded arteries smaller than their neighbours.
• On a poor study, the report should name which arteries cannot be assessed.

What you are looking at

CT pulmonary angiography is timed so that the pulmonary arteries fill with iodinated contrast. On a good study every artery in the lung is bright, and that is exactly what makes a clot visible: an embolus does not take up contrast, so it shows as a filling defect, a grey shape inside a vessel that should be white.

In the image above, follow the right lower lobe pulmonary artery from the hilum outwards. One branch is not uniformly bright: part of its lumen is grey, outlined in gold. Notice how sharp the edge is where the grey clot meets the contrast around it. That sharp interface with the contrast is one of the diagnostic criteria for an embolus.

Acute clot

An acute embolus that only partly fills the artery is often central, sitting in the middle of the lumen with contrast flowing around it. When it lies against the wall, it usually meets the wall at an acute angle, like a bead pushed to one side. An artery completely blocked by acute clot may even look larger than its normal neighbours, because the clot expands it.

Chronic clot

Old emboli look different. They tend to be eccentric, lying flat against the wall and meeting it at obtuse angles. Some leave thin webs or flaps across the lumen. An artery chronically occluded by organised thrombus is often smaller than its neighbours rather than larger. Recognising these patterns matters, because the same scan can show both.

Reading a difficult study

Not every study is perfect. Missed or false emboli come from the patient (for example breathing motion), from the technique (for example weak contrast enhancement), from anatomy, or from other disease in the lung. When the images are not good enough to judge some arteries, the honest report names which arteries cannot be assessed, instead of calling the whole study negative.

How to practise

Scroll through the arteries in order, from the main pulmonary artery to each lobe and segment, and ask the same question at every branch: is this bright all the way across? A grey shape with a sharp edge is the clue.

Check yourself

On CT pulmonary angiography, which finding favours an acute embolus?

1. A. A central filling defect with contrast around it
2. B. An eccentric defect meeting the wall at an obtuse angle
3. C. A thin web across the artery
4. D. An occluded artery smaller than its neighbours

Show the answer

Sources

1. Wittram C, Maher MM, Yoo AJ, Kalra MK, Shepard JA, McLoud TC. CT angiography of pulmonary embolism: diagnostic criteria and causes of misdiagnosis. RadioGraphics 2004;24(5):1219-1238. doi:10.1148/rg.245045008 · PMID 15371604