Perfusion-Weighted Imaging/Diffusion-Weighted Imaging Mismatch on MRI Can Now Be Used to Select Patients for Recombinant Tissue Plasminogen Activator Beyond 3 Hours
Section Editors: Geoffrey A. Donnan MD, FRACP Stephen M. Davis MD, FRACP
“It’s now or never. I was caught in a dead end street. A look (into your eyes) can heal me. And after this moment, you gave me something (back). What I really need.”
— Primal Fear, “Nuclear Fire,” 2001
Of course, the “dead end street” refers to computed tomography (CT), “the healing look” to reading a stroke MRI, and being “given something that one really needs” to thrombolytic therapy.
What do we need to establish that a new methodology is ready to be put to use instead of an older one? We need to show that it is at least as good, if not better, than the old modality with regard to safety, feasibility, cost efficiency, and diagnostic and prognostic power. Over the last years, a growing number of reports on the use of multiparametric MRI protocols including diffusion-weighted imaging (DWI) and perfusion-weighted imaging (PWI) for guiding treatment in acute stroke patients have been published.1
MRI is safe in acute stroke patients; side effects such as allergic reactions to contrast agent and x-ray load virtually do not exist. Feasibility of stroke MRI is estimated between 75% and 95%,2–4 granted, somewhat lower than that of CT. The feasibility of stroke MRI depends in part on how patient instability, and therefore safety concerns, is defined in different centers. When compared with noncontrast CT alone, the cost of stroke MRI is higher; as soon as CT angiography and perfusion CT are added to the CT protocol, the difference in cost is marginal. Again, we admit that cost effectiveness of stroke MRI has not been proven yet.
However, it has been shown beyond doubt that the diagnostic accuracy of stroke MRI for ischemic stroke is significantly higher than that of CT, and for intracerebral hemorrhage, it is equally as good as CT.5,6 On the other hand, time to treatment is a very strong prognostic variable within the first 90 to 180 minutes after stroke onset.7 All recombinant tissue plasminogen activator trials with a time window exceeding 3 hours were negative, suggesting that thereafter, patient selection may be more important than time in combination with an insensitive diagnostic tool.
Several open controlled studies used the PWI/DWI mismatch concept to extend the therapeutic time window for thrombolytic therapy and showed that selected patients profit from treatment.8,9 Another series showed that patients who are treated on the basis of stroke MRI criteria within 3 to 6 hours fare at least as well as those being treated on the basis of CT within 3 hours.10 Finally, the recently presented Desmoteplase In Acute Ischemic Stroke phase II trial illustrated 3 important things.11 First, it demonstrated that thrombolysis beyond 3 hours works if an appropriate tool (ie, stroke MRI) for patient selection is applied. Second, reperfusion on stroke MRI paralleled clinical outcome, showing that stroke MRI may be used as a surrogate parameter for outcome. Third, the therapeutic effect on clinical and MRI outcomes did not depend on time to treatment, illustrating that patient selection may be more important than time. To prevent time loss where it really counts, CT should be the primary diagnostic tool within 3 hours if a center is not able to provide stroke MRI as fast as CT. In keeping with the times, stroke MRI can and should be applied to guide stroke therapy within institutional protocols outside the 3-hour time window if inclusion in a randomized controlled trial is not possible.
“Who all need it—who. Who all need it—you. Who all need it, who all need it (yes you do). You all breathe it, we all need it. Are you ready for a good time? Are you ready?”
— Taken from AC/DC, “The Razors Edge”, 1990
Yes, we are ready for stroke MRI!
- Received October 21, 2004.
- Accepted October 22, 2004.
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