First and second year — the floor first, then every step
Judge the contrast bolus before you judge the arteries, then work centrally to peripherally, then look at the right heart.
Orient first
A CTPA is only interpretable if the pulmonary arteries are properly opacified. Assess the bolus FIRST — a poorly-timed study is non-diagnostic, and reporting it as negative is the error this entry exists to prevent.
An acute embolus is a filling defect that is CENTRAL in the lumen or makes an ACUTE angle with the wall, often with a rim of contrast around it. A chronic one is eccentric, makes an OBTUSE angle, may be calcified, and may have webs.
Right heart strain is what turns a positive study into a high-risk one, and it is assessed on the same images.
Acquire the study
Check opacification of the main pulmonary artery. If it is poorly opacified, or dense contrast in the SVC causes streak artefact across the right main artery, say the study is limited.
Look for respiratory motion, particularly at the lung bases, which blurs segmental vessels.
Use a narrow window (a "PE window") — widening the window makes low-attenuation clot far more conspicuous against opacified blood.
Scroll on thin slices; MIPs help follow vessels but can hide small defects.
The manoeuvre
Follow the pulmonary arteries systematically: main, right and left, then lobar, then segmental, one lobe at a time. A named order stops you skipping a territory.
For each filling defect note whether it is occlusive or non-occlusive and its most proximal level.
Assess RIGHT HEART STRAIN: the RV:LV short-axis diameter ratio on axial images, interventricular septal bowing toward the left, reflux of contrast into the IVC and hepatic veins.
Look at the lungs for a wedge-shaped peripheral opacity (infarct), and for effusions.
Look for the alternative diagnosis that explains the presentation — pneumonia, dissection, pneumothorax, malignancy. A CTPA that finds no PE should still answer the clinical question.
Check the venous structures included in the scan for thrombus.
What confirms it
A central intraluminal filling defect, or one making an acute angle with the wall, in an adequately opacified artery, on more than one slice and in more than one plane.
What licenses you to exclude it
An adequately opacified, motion-free CTPA with no filling defect excludes clinically significant pulmonary embolism.
⚠️ A POORLY OPACIFIED OR MOTION-DEGRADED STUDY EXCLUDES NOTHING. Report it as non-diagnostic for PE, ideally naming the level at which it stops being interpretable, rather than as negative.
The classic misread
Reporting a technically inadequate study as negative.
Mistaking a mucus-filled bronchus, a lymph node or a partial volume artefact for clot — track the structure on adjacent slices.
Missing chronic thromboembolic disease by calling eccentric mural material "no acute PE" and stopping.
Not reporting right heart strain on a positive study.
Reference values
Each value carries the caveat that keeps it from being misused. Normal limits and diagnostic criteria are kept apart on purpose: a disease cut-off read as a normal range is the more dangerous mistake.
Normal limits
Pulmonary arteries · main pulmonary artery diameter at the bifurcationup to 29 mm
The MPA-to-ascending-aorta ratio above 1 is the more robust sign in younger patients.
CT
Diagnostic criteria
Pulmonary embolism burden · Qanadli and Mastora scores — how to report them
named clot-burden scores: Qanadli is a 40-point segmental-obstruction index; Mastora is a more granular 150-point alternative — they are not interchangeable
Clot-burden scores correlate imperfectly with clinical severity — RV/LV ratio and the clinical pathway outrank a score nobody in the receiving team uses. If you quote one, name which.
CT
Right ventricle (strain signs) · CT signs of RV strain in acute PE — how to report them
a named constellation: RV/LV ratio above 1, septal bowing toward the LV, IVC contrast reflux, and a dilated MPA — no single sign is sufficient
Chronic PH and an acute PE can look identical on a single CTPA — the clinical timeline and prior imaging decide. The RV/LV-ratio entry carries the measurement technique. Versioned criterion — verify against the current edition before clinical use.
CT
Pulmonary arteries · Main pulmonary artery to ascending aorta ratio
a ratio under 1.0 is the conventional CT screening figure for pulmonary hypertension discussion
Axial CT at the PA bifurcation: widest main-PA diameter perpendicular to its long axis, divided by the ascending-aorta diameter on the same slice.
A ratio ≥1 supports, it does not diagnose, pulmonary hypertension — echo and right-heart catheterisation remain the reference. The absolute MPA diameter and the ratio can disagree in a dilated aorta.
CT
See it on real cases
Direct links to Radiopaedia — the reference article and worked cases with their images. Each opens on Radiopaedia.