Published online by Cambridge University Press: 26 August 2009
The brain responds dynamically to transient episodes of ischemic insult. Standard brain imaging techniques, computed tomography (CT) and conventional magnetic resonance imaging (MRI), are insensitive to dynamic and regionally varying neural parenchymal responses to tissue ischemia. In contrast, the novel MRI techniques of perfusion and diffusion imaging permit visualization of these critical tissue processes, and have afforded new insights into the physiopathology of human cerebral ischemia. In addition, clinical studies have demonstrated that magnetic resonance imaging is of substantial clinical utility in patients with transient ischemic attacks (TIAs).
The current conventional definition of transient ischemic attack is neurologic symptoms due to focal cerebral ischemia that resolve completely within 24 hours (Special Report from the National Institute of Neurological Disorders and Stroke, 1990). Defects of this definition, already apparent in the CT era, have been demonstrated even more pointedly by MRI investigations. We will consequently propose a new definition of TIA, informed by MRI findings, at the conclusion of this chapter. Nonetheless, all MRI studies to be reviewed in this chapter have employed this conventional definition.
Recent studies in animal models and in human patients with conventionally defined TIAs have identified three distinct tissue patterns on MRI, reflecting three somewhat dissimilar ischemic episodes that can underlie clinically similar TIAs. A very brief or low-intensity period of focal ischemia may disrupt synaptic transmission and produce transient neurologic deficits without causing early cytotoxic edema or permanent tissue injury.