Figure 2. (A) Relative cerebral blood volume map derived from gadolinium bolus perfusion imaging. The focal area of low-intensity signal apparent in the left parietal lobe (arrow) reflects a low cerebral blood volume and thus hypoperfusion of this region. (B) Diffusion-weighted image of the same subject demonstrates a corresponding high-intensity signal indicating that ischemic damage has already occurred and that this tissue is unlikely to respond to thrombolytic treatment.

Data from each perfusion variable are easily viewed on a "map" on which values in different voxels can be compared and areas of relative perfusion change identified (Fig. 2). When compared to PET and Xenon-CT,[9,21] perfusion imaging is a highly sensitive technique for detecting cerebral hypoperfusion. Attempts to define a perfusion threshold at which ischemic tissue becomes irreversible, however, have been largely unsuccessful despite evidence from animal studies that such a threshold may exist.[10]

In addition to defining an area of hypoperfusion, perfusion imaging may also help define whether reperfusion, either spontaneous or after thrombolysis, has occurred. Both rCBV and CBF increase to normal or elevated levels with reperfusion,[16] reflecting restoration of blood flow. Perfusion imaging may be able to monitor treatment outcomes, assessing whether reperfusion has occurred in response to thrombolysis.