Schematic representation of the ischemic penumbra. Glucose utilization (anaerobic glycolysis) transiently increases at a CBF value below about 40% of normal, which corresponds to the beginning of acidosis with lactate accumulation. At a CBF value below about 20% of normal, a breakdown of energy state begins with reduced adenosine triphosphate (ATP), resulting in the condition of ischemic core. The penumbra is defined as an area in which metabolism is impaired due to reduced CBF, but the cellular polarization is still maintained without diffusion- weighted imaging lesions on magnetic resonance imaging. Therefore, an acidosis/diffusion- weighted imaging mismatch represents penumbra.