When attempting to treat stroke, it pays to know the nature of the beast. Acute stroke consists of an evolving infarction — an area of brain tissue that is damaged beyond repair — surrounded by an 'ischaemic penumbra', which, although threatened, can potentially be rescued by restoring blood flow to the affected area. But brain 'reperfusion' with thrombolytic agents carries with it the risk of haemorraghic complications, and is effective only in those cases in which there is a penumbra to save. So, how to tell when to use it? A study published in the Annals of Neurology now shows how a new version of X-ray computed tomography (CT) can be used in acute stroke patients to decide whether reperfusion will be effective, and also to predict the final infarct size and expected clinical progression.

Stroke diagnosis has relied on the use of CT for more than 30 years, but this study uses a modern variant, perfusion CT, that can generate brain images at a much faster rate. Perfusion CT can be used to monitor the passage of injected CT dye through the brain, and therefore to create maps of cerebral blood volume and cerebral blood flow — measures that correlate with the size of the infarct and the affected penumbra.

The technique was applied to 22 adult stroke patients at the time of emergency-room admission, and was found to be highly predictive of the cerebral infarct size measured three days later by magnetic resonance (MR) imaging, which is the accepted best method at present for detecting the area of long-term damage. Furthermore, the initial size of the combined cerebral infarct and penumbra defined by the admission perfusion CT correlated better with various standard assessments of clinical status than did the delayed MR measurements. Interestingly, perfusion CT also seems to allow the identification of tissue within the penumbra that can be salvaged by reperfusion, and so can be used to guide the choice of therapy.

The main hurdle for the development of drugs for stroke is the difficulty of organizing large-scale clinical trials. Importantly, these new findings might pave the way for the use of perfusion CT in monitoring the effects of neuroprotective drugs in individual patients, therefore allowing small Phase II efficacy trials.