Magnetic Resonance Imaging (MRI) has brought about substantial changes in the diagnostic algorithms formulated in almost all aspects of Central Nervous System (CNS) pathology. This has occurred in part secondary to MRI's greater sensitivity to changes in the water content of the tissue examined, and therefore, in demonstrating all categories of CNS pathology. However, in the particular case of ischaemic cerebral vascular disease, MRI initially had a more limited impact, due mainly to the large preceding experience gained in the treatment of this type of pathology using CT, and therefore the resulting clinical reliance on the CT semeiology (3, 4). This situation has recently undergone profound changes, both with regard to the technical progress made in MRI as well as to the newer treatments that are being introduced, in particular thrombolysis (2, 5, 7, 12-15). The diagnostic questions a clinician poses with regard to the patient with ischaemic cerebrovascular disease are mainly concerned with the site, extent and time of the ictus, the clinical severity of the ischaemic lesion, the potential reversibility of the damage and the presence or absence of reperfusion. Because it is now universally recognized that early surgery can potentially improve or even reverse otherwise serious events such as cerebral ischemia, the necessity to respond in the very first minutes and hours following the onset of the neurological deficit has become all the more pressing (1, 6, 8-11, 16). The responses to such important considerations often surpass the capabilities of traditional neuroradiological imaging techniques to aid in this emergent situation; in the past, CT and MRI only very rarely provided direct diagnostic information during the first six hours from an ischaemic cerebral event. This is predictable given that even from a pathoanatomical point of view " identification of an ischaemic lesion on human or animal brains cannot be detected by macroscopic examinations less than 24-72 hours from the event" (Davis, 1997). It therefore follows that, in order to obtain useful diagnostic neuroimaging information in the hyperacute phase, other aspects of ischaemic disease including morphological, macroscopic, submacroscopic, and pathophysiological parameters must be explored. The rational use of MRI in a way that enhances its intrinsic potential, as well as taking advantages of recent progress in acquisition techniques (e.g., perfusion, diffusion and spectroscopy) have begun to provide answers to the clinical questions posed during the hyperacute phase of cerebral ischaemia. The goal of this chapter is to explore the chronology of the pathophysiological events of acute/hyperacute cerebral ischaemia and consider how these alterations can be investigated using the newer advancements of MRI.
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