The fundamental importance of the concept of the ischemic penumbra is the recognition that ischemic processes may be reversible. Although founded on the concept of critical changes of blood flow, the ischemic penumbra can also be described in molecular terms. A molecular delineation of the ischemic core employs analyses of appropriate proteins, many of which have a short half-life and hence rapid reductions in concentration. Multitracer PET imaging with 15O allows generation of quantitative brain maps for CBF, CMRO2, OEF, cerebral blood volume (CBV), and regional cerebral metabolic rate of glucose. Evolving MRI techniques are useful for assessment of penumbral tissue in acute stroke. DWI is increasingly available in the setting of acute stroke, and for rapid acquisition it is performed using echoplanar magnetic resonance imaging methods. The blood oxygen level dependent (BOLD) technique has been used to differentiate perfused and non-perfused tissues during experimental ischemia in cats.

Stroke is a leading cause of death in Western countries, with a case mortality rate of 24% within the first month, higher than most forms of cancer. Echoplanar magnetic resonance imaging (EPI) enables rapid, non-invasive imaging and analysis of cerebral pathophysiology in acute stroke. Various functional imaging methods have been used in the evaluation of acute stroke. Positron emission tomography (PET) enables imaging and measurement of both perfusion and metabolism. At present, diffusion-weighted imaging (DWI) is the most used of the EPI parameters in clinical practice. DWI is both highly sensitive and specific for acute stroke. Magnetic resonance spectroscopy (MRS) allows the non-invasive assessment of metabolic changes during stroke. The addition of chemical shift imaging (CSI) to a multimodal acute stroke MRI examination may confer additional accuracy to the identification of at-risk ischemic tissue. EPI with DWI, PWI and MRS have now moved from the research arenainto routine clinical use.