MRI permits non-invasive and serial study of acute stroke pathophysiology. In addition to brain structure, MRI can define tissue viability (cytotoxic oedema seen on diffusion-weighted imaging, DWI), brain perfusion (bolus-tracking perfusion imaging, PI), vascular integrity (MR angiography, MRA) and tissue metabolism (1H MR spectroscopy, MRS). In acute middle cerebral artery (MCA) occlusion, evolution of cerebral damage has been defined with these techniques. The volume of hypoperfused tissue on PI initially exceeds the DWI lesion, and, over time, the DWI lesion expands to finally incorporate the majority of the PI lesion. The region of tissue with normal DWI but abnormal PI is thought to correspond to the "ischaemic penumbra", the region where hypoperfusion causes electrical failure of neurones with progression to infarction over time due to adverse metabolic and neurochemical events. The fate of the penumbra may be determined by treatment – e.g. it is salvaged by thrombolysis – and it is this penumbral region that is vulnerable to hyperglycaemia-related lactic acidosis.