Searching for salvageable brain: the detection of ischemic penumbra using various imaging modalities?

Various imaging modalities are used to identify the ischemic penumbra in patients with acute ischemic stroke. Although single-photon emission computed tomography (CT), perfusion-weighted magnetic resonance imaging (MRI), and perfusion CT are used to evaluate residual cerebral blood flow in the ischemic area, validation of the perfusion threshold for the penumbra has not been established. The ischemic core corresponds generally to areas with early ischemic changes, such as parenchymal hypoattenuation on CT, restricted apparent diffusion coefficient on diffusion-weighted MRI, or very low cerebral blood volume on perfusion CT. A mismatch of these abnormalities is thought to correspond to the ischemic penumbra and has been used in clinical trials. Although new ligands, such as (18)F-fluoromisonidazole, that bind to hypoxic but viable ischemic tissue are currently available, positron emission tomography has yet to be developed as a clinical tool.