Abstract
We
review the hemodynamic, metabolic and cellular parameters affected
during early ischemia and their changes as a function of approximate
cerebral blood flow (CBF) thresholds. These parameters underlie
the current practical definition of an ischemic penumbra, namely
metabolically affected but still viable brain tissue. Such tissue is at
risk of infarction under continuing conditions of reduced CBF,
but can be rescued through timely intervention.(How fast is that? Seconds? Minutes? Hours? With what?) This definition will be
useful in clinical diagnosis only if imaging techniques exist that can
rapidly, and with sufficient accuracy, visualize the existence of a
mismatch between such a metabolically affected area and regions that
have suffered cell depolarization. Unfortunately, clinical data show
that defining the outer boundary of the penumbra based solely on
perfusion-related thresholds may not be sufficiently accurate. Also,
thresholds for CBF and cerebral blood volume (CBV) differ for
white and gray matter and evolve with time for both inner and outer
penumbral boundaries. As such, practical penumbral imaging would involve
parameters in which the physiology is immediately displayed in a manner
independent of baseline CBF or CBF threshold, namely pH, oxygen extraction fraction (OEF), diffusion constant and mean transit time (MTT). Suitable imaging technologies will need to meet this requirement in a 10–20 min exam.
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