Intracranial haemorrhages account for approximately 20% of all cerebral ictuses related to vascular disease, with a higher prevalence of intraaxial localizations (15%) as compared to those principally within the subarachnoid space (5%) (1, 6, 9). Although MRI is highly specific with regard to haemorrhagic lesions, CT remains the examination of choice, especially in emergencies, in part due to the difficulty in managing acutely ill patients within the MR scanning unit. Any physician who acquired an MRI unit in its early stages of clinical application will remember the sense of confusion experienced with the first acute intracerebral haematoma patient, as initially diagnosed on MRI. In such cases, the lesion MR signal was "ambiguous", nearly devoid of the specific characterizations seen on CT in patients with haemorrage. In the early stages haemorrhage could appear quite similar to an ischaemic ictus, being differentiated when possible by the differing topography and morphology. This impact conditioned the attitude of neuroradiologists against the use of MRI in acute haemorrhagic pathology for some years, and this instinctive wariness has only partly been reconditioned by a clear working knowledge of the evolution of the MR signal of parenchymal haemorrhagic collections (13, 15-18). In this chapter, we will analyse the information that can be deduced from intracranial haemorrhages using MRI, making a distinction between intraaxial and subarachnoid forms due to the difference in the anatomopathological and biochemical environment in which the phenomena that influence the haemorrhagic MRI signal occur.
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