Ischemic Penumbra: MRI or PET
“ … the outcome of incomplete cerebral ischemia is of particular interest. … one hopes it will become possible to conduct treatment and to evaluate prognosis in the acute stroke patient by reproducible repeatable measurement in man.”
Astrup et al1
Why is the penumbra so important? In the 1970s it gave us the first insight into the prolonged nature of tissue survival after stroke. In the 1990s its importance as a target for therapy was confirmed with the introduction of tPA as the first treatment aimed at rapid reperfusion and salvage of critically hypoperfused brain.
The protagonists have nicely positioned the current status of in vivo measurement of the penumbra. On one hand we have PET, which has provided such important biological insights (particularly quantitative) into penumbral characteristics such as blood flow and metabolism, while on the other we have the pragmatic advantages of MRI. PET and MRI provide complementary information about a phenomenon that can be defined in more than one way. Both techniques have contributed enormously to our current understanding of tissue viability.
We are, therefore, of the view that there is no gold standard for human penumbral measurement. Indeed, both PET and MRI concepts continue to evolve. For example, as outlined by Heiss, we have moved from the original 15O techniques through to 11C flumazenil and 18FMISO. Using MRI, the mismatch concept (hypoperfusion volume > DWI lesion core) has been modified with the recognition that a portion of hypoperfused brain reflects benign oligemia and that the DWI core may be partially salvageable. Other incremental refinements, including measurement of biochemical thresholds with spectroscopy, are likely to occur. Even imaging of a “molecular penumbra,” with measurement of gene markers of protein expression, cannot be discounted.2
Measurement of the penumbra is being utilized in proof of concept stroke trials. Further, patient selection based on penumbral presence in clinical trials is being explored. Hence, the introduction of new therapies is likely to catalyze improvements in penumbral imaging and measurement, even more quickly than before.