Introduction
This topic covers neuroimaging of acute ischaemic stroke: which modality to use at each stage, which early and late signs to recognise, how imaging selects patients for reperfusion therapy, and how infarcts are distinguished from other causes of a focal deficit.
Clinical and imaging goals
Ischaemic stroke is a sudden focal neurological deficit caused by occlusion of a cerebral artery, leading to infarction of the territory it supplies. In the acute phase imaging has three jobs: exclude intracranial haemorrhage, identify the arterial occlusion, and estimate how much tissue is already irreversibly damaged (core) versus salvageable (penumbra). Non-contrast CT is the first examination in most centres because it is fast and widely available.
- Haemorrhage must be excluded before any thrombolytic agent is given.
- CT angiography locates the occlusion and decides on mechanical thrombectomy.
- Perfusion imaging separates core from penumbra when the treatment window is extended.
Non-contrast CT: early signs
Early ischaemic changes are subtle and often absent in the first hours. A normal CT does not exclude infarction, so the clinical picture must always be taken into account.
- Hyperdense middle cerebral artery sign: a bright thrombus within the M1 segment.
- Loss of grey-white differentiation in the insular cortex, basal ganglia and adjacent cortex, the earliest parenchymal sign.
- Obscuration of the lentiform nucleus and effacement of the insular ribbon.
- Cortical sulcal effacement and, later, a hypodense area with mass effect.
- ASPECTS: a 10-point score applied to the middle cerebral artery territory at the basal ganglia and supraganglionic levels. One point is subtracted for each region with early ischaemic change; a low score predicts a larger infarct and poorer outcome.