[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Radiology](https://durgical.com/en/radiology/) › [Chest](https://durgical.com/en/radiology/areas/chest/) › CT pulmonary angiography for pulmonary embolism # CT pulmonary angiography for pulmonary embolism - Finding A CT scan taken while contrast fills the lung arteries. A clot shows as a dark gap inside the bright, contrast-filled vessel. Updated Nov 13, 2026 ## Key points - Embolus = filling defect in a contrast-filled pulmonary artery. - Probability first, D-dimer if low or intermediate, CTPA if likely or D-dimer raised. - Look for right ventricular strain on the same scan: RV wider than LV, septal bowing, IVC reflux. - Unstable: echocardiography at the bedside, not CT. ## What it is CT pulmonary angiography (CTPA) is a chest CT timed so that the injected iodinated contrast fills the pulmonary arteries at their brightest. A pulmonary embolus is a filling defect: a darker, non-enhancing shape inside the bright artery, from the main pulmonary artery out to the segmental and subsegmental branches. CTPA is the test of choice when pulmonary embolism is likely or the D-dimer is raised, because it confirms or excludes the clot and often shows another cause of the symptoms. The same scan also shows strain on the right ventricle, which helps decide how ill the patient is. It is not the first step in every breathless patient: clinical probability and, when appropriate, a D-dimer come first. ## What it looks like - Acute embolus: a central filling defect with a thin rim of contrast around it, seen as a ring in cross-section and as parallel lines along the vessel. - Acute clot often sits in the middle of the vessel, meets the wall at an acute angle and may widen the artery; a total cut-off of contrast is also possible. - Saddle embolus: a clot straddling the bifurcation of the main pulmonary artery. - Chronic clot: eccentric and stuck to the wall, obtuse angles, webs or bands, narrowed vessels, sometimes calcified. - Right ventricular strain: right ventricle wider than the left, septum bowing toward the left ventricle, contrast reflux into the IVC and hepatic veins. - Lung: a peripheral, wedge-shaped opacity based on the pleura suggests infarction; a small effusion is common. - On a plain chest X-ray, usually normal or nonspecific; classic but uncommon signs are a peripheral wedge-shaped opacity (Hampton hump) and a focal area of reduced vascular markings (Westermark sign). The X-ray mainly rules out other causes. ## Why it looks that way - The clot does not take up contrast, so it stays grey against the bright blood around it. - A sudden large clot load raises pressure in the pulmonary circulation, and the thin-walled right ventricle dilates and pushes the septum to the left. - Lung tissue beyond a blocked branch may infarct because its bronchial supply is not enough, giving a pleural-based wedge. ## Differential and pitfalls - Poor contrast timing makes the arteries too faint to judge; the report should say whether the study was diagnostic. - Mixing artefact: unopacified blood from the IVC during a deep breath can mimic a clot; it is ill-defined and not seen on every slice. - Breathing motion, partial-volume effect and lymph nodes around the hila can look like a defect; follow the vessel on consecutive slices. - Pulmonary veins mistaken for arteries: arteries run with the bronchi, veins run between segments to the left atrium. - An isolated small subsegmental defect may be a true small clot or an artefact; its clinical importance is debated. ## Next step - Unstable patient (shock, hypotension): do not send to the scanner; bedside echocardiography for right ventricular dilatation and call a senior at once. - Pulmonary embolism unlikely by a clinical score: a D-dimer first (age-adjusted after 50); a normal result excludes it without CT. - PE likely or D-dimer raised: CTPA, after checking kidney function and any previous contrast reaction. - Contrast contraindicated or pregnancy with a normal chest X-ray: discuss a V/Q scan; leg symptoms: compression ultrasound of the legs first. - Clot found: read the right ventricle on the same scan and stratify risk; non-diagnostic study: speak with the radiologist about repeating it or another test. ## Test yourself Think of the answer, then tap the question to see it. ## What does an acute pulmonary embolus look like on CTPA? A filling defect inside the contrast-filled artery, often central with a thin rim of contrast around it, sometimes widening the vessel or cutting off contrast completely. ## Which CTPA findings suggest right ventricular strain? A right ventricle wider than the left, the interventricular septum bowing toward the left ventricle, and contrast refluxing into the IVC and hepatic veins. ## When can pulmonary embolism be excluded without CTPA? When clinical probability is low or PE is unlikely by a score and the D-dimer is normal (using an age-adjusted cut-off after 50). ## How is a chronic clot told apart from an acute one on CT? Chronic clot is eccentric and adherent to the wall with obtuse angles, webs or bands and narrowed vessels; acute clot is central, with acute angles, and may widen the artery. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/radiology/ct-pulmonary-angiography-for-pulmonary-embolism/) ## Elsewhere in DurgiPedia - [Signs of deep vein thrombosis: Homans and the Wells score](https://durgical.com/en/clinical-exam/signs-of-deep-vein-thrombosis-homans-and-the-wells-score/) - [Chest pain: ischemic, pericardial, pleural and other patterns](https://durgical.com/en/clinical-exam/chest-pain-ischemic-pericardial-pleural-and-other-patterns/) ## Connections ## Related 8 - [CT: windows, Hounsfield units and contrast phases](https://durgical.com/en/radiology/ct-windows-hounsfield-units-and-contrast-phases/) - [Reading a chest CT step by step](https://durgical.com/en/radiology/reading-a-chest-ct-step-by-step/) - [Compression ultrasound for deep vein thrombosis](https://durgical.com/en/radiology/compression-ultrasound-for-deep-vein-thrombosis/) - [Focused cardiac ultrasound: effusion, function and the IVC](https://durgical.com/en/radiology/focused-cardiac-ultrasound-effusion-function-and-the-ivc/) - [V/Q, bone and thyroid scans: when they are used](https://durgical.com/en/radiology/v-q-bone-and-thyroid-scans-when-they-are-used/) - [Iodinated contrast: reactions and the kidney](https://durgical.com/en/radiology/iodinated-contrast-reactions-and-the-kidney/) - [Imaging in pregnancy: what is safe](https://durgical.com/en/radiology/imaging-in-pregnancy-what-is-safe/) - [Which chest study to order: X-ray, CT, CT angiography or ultrasound](https://durgical.com/en/radiology/which-chest-study-to-order-x-ray-ct-ct-angiography-or-ultrasound/) ## Related conditions 2 - [Pulmonary embolism](https://durgical.com/en/conditions/pulmonary-embolism/) - [Deep vein thrombosis](https://durgical.com/en/conditions/deep-vein-thrombosis/) [← All of radiology](https://durgical.com/en/radiology/) Educational content; it does not replace a radiologist's report and is not a clinical instruction. Each study is ordered on the patient's assessment and your institution's protocols. Sources: [Acute Pulmonary Embolism](https://www.ncbi.nlm.nih.gov/books/NBK560551/) (StatPearls (NCBI Bookshelf), 2024); [D-Dimer Test](https://www.ncbi.nlm.nih.gov/books/NBK431064/) (StatPearls (NCBI Bookshelf), 2025). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/radiology/ct-pulmonary-angiography-for-pulmonary-embolism/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/radiology/ct-pulmonary-angiography-for-pulmonary-embolism/