Intracerebral blood immediately appears as an area of high density on CT, but thereafter decreases so that haemorrhagic lesions will eventually appear either isodense or hypodense and thus be indistinguishable from an infarct.31 Smaller haemorrhages may become isodense within days, although usually this process takes weeks. CT in the hyperacute stage of an ischaemic stroke is often normal although there may be subtle changes which are easily overlooked by the inexperienced observer. Infarcts are most easily seen on CT after a few days or in the chronic phase, when they may become markedly hypodense and well defined, although up to 50% of patients with a clinically definite stroke never have an appropriate lesion identified on CT.32 To help radiologists in their interpretation, we recommend that the date and time of symptom onset are always recorded on the CT request form. Although early CT will reliably identify intracerebral haemorrhage, the distinction between a primary intracerebral haemorrhage (PICH) and haemorrhagic transformation of an infarct (HTI) is unreliable and difficult. The frequency and clinical relevance of HTI is uncertain and radiologically ranges from small petechial haemorrhages to frank haematoma, which may or may not be accompanied by clinical deterioration. HTI can occur very early33 and the only definitive way of diagnosing HTI is to have an earlier scan excluding haemorrhage, so we recommend scanning as early as possible, ideally at the time of initial assessment.
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