Choosing and interpreting emergency CT — a clinical imaging viva

Vignette 1 — acute stroke. The non-contrast head CT is the correct first scan: its purpose is to exclude haemorrhage, a contraindication to thrombolysis. Read it for the early ischaemic signs — the hyperdense MCA sign (a bright middle cerebral artery from acute thrombus), loss of the insular ribbon, obscuration of the lentiform nucleus, and cortical sulcal effacement with subtle hypodensity. Quantify the burden with ASPECTS: ten regions of the MCA territory (M1 to M6 cortex, caudate, lentiform, internal capsule, insula), scored ten for normal down to zero for a fully infarcted territory.^[3] Within the first 90 minutes the scan may be normal, and that is expected. The non-contrast CT does not exclude ischaemia — it excludes haemorrhage. Proceed to CT angiography to find a large-vessel occlusion eligible for thrombectomy, and CT perfusion to define the core and the penumbra if the window is extended.

Vignette 2 — thunderclap headache. The CT is normal, but the onset was nine hours ago. CT sensitivity for subarachnoid haemorrhage is 100 percent within six hours (121 of 121, 95 per cent CI 97 to 100) and falls with time (92.9 per cent overall).^[4] At nine hours a normal CT does not reliably exclude it — the next investigation is a lumbar puncture to detect xanthochromia. The scan is read for blood in the basal cisterns, the Sylvian and the interhemispheric fissures. Within six hours, in the right patient, a normal CT may exclude the diagnosis without a lumbar puncture; after six hours it may not.

Vignette 3 — CT pulmonary angiogram. The finding is a filling defect within an opacified pulmonary artery — a pulmonary embolism. PIOPED II found multidetector CTA 83 percent sensitive and 96 percent specific (90 percent sensitive combined with venous-phase imaging), with high predictive value when concordant with clinical probability — and a need for further testing when discordant.^[6] Read the scan additionally for right ventricular strain — RV dilatation, septal bowing, reflux of contrast into the inferior vena cava — which predicts 30-day mortality and drives disposition toward a higher level of monitoring.