Key facts

CT is the investigation of choice in acute stroke due to its availability and speed
However, MRI is most sensitive
CT scan in suspected stroke patient is used to rule out haemorrhagic stroke/other CNS pathology e.g. tumour
Once haemorrhagic stroke r/o, and if clinical indicators suggest ischaemic stroke, thrombolysis treatment can be initiated

CT scan in stroke

Immediate (within minutes): Clot may be seen (esp. in MCA territory) but otherwise no other signs
Hyperacute phase (0-24 hours): Clot may be seen, Loss of grey-white matter differentiation, cortical hypodensity (especially to areas with poor collateral supplies such as the insular ribbon)
Acute (24hrs-1week): Marked hypodensity and swelling; swelling can lead to mass effect
Subacute (1-3 weeks): Some small (petechial) haemorrhages (not haemorrhagic transformation; this phenomenon is known as CT fogging), brain matter can look near normal as swelling reduces

Highly sensitive
Within minutes of occlusion, diffusion weighed imaging (DWI) can identify ischaemic core
High DWI and low ADC (Apparent diffusion co-efficient) can persist for up to a week
T2 signal is raised especially on FLAIR in acute phase
After a week ADC value starts to rise
If brain parenchymal enhancement persists >12 weeks à consider underlying lesion

An emerging tool
- accurate diagnosis of ischaemic stroke
- allows identification of penumbra (salvageable brain with reperfusion)

To identify clot (thrombus) for thrombolysis or clot retrieval
To assess carotid and vertebral arteries for stroke aetiology and/or assess for endovascular access

Detailed discussion on this topic can be found here Radiopaedia-imaging in ischaemic stroke