Published Online
on February 7, 2008

A more recent version of this article appeared on March 1, 2008

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Submitted on July 27, 2007
Accepted on August 14, 2007

How Reliable Is Perfusion MR in Acute Stroke?. Validation and Determination of the Penumbra Threshold Against Quantitative PET

Masashi Takasawa MD, PhD; P. Simon Jones MSc; Joseph V. Guadagno PhD, MRCP; Soren Christensen MSc; Tim D. Fryer PhD; Sally Harding PhD; Jonathan H. Gillard BSc, MD, FRCR; Guy B. Williams PhD; Franklin I. Aigbirhio PhD; Elizabeth A. Warburton DM, MRCP; Leif stergaard MD, MSc, DMSc; and Jean-Claude Baron MD, FRCP, FMedSci^*

From the Stroke Research Group (M.T., P.S.J., J.V.G., F.I.A., E.A.W., J.-C.B.) and the Wolfson Brain Imaging Centre (T.D.F., S.H., J.H.G., G.B.W., F.I.A., J.-C.B.), Department of Clinical Neurosciences, University of Cambridge, Cambridge, England; the Department of Radiology (S.C.), University of Melbourne, Royal Melbourne Hospital, Melbourne, Australia; and the Department of Neuroradiology (L..), Danish National Research Foundations Center of Functionally Integrative Neuroscience, Aarhus University Hospital, Aarhus, Denmark.

^* To whom correspondence should be addressed. E-mail: jcb54{at}cam.ac.uk.

Background and Purpose—Perfusion magnetic resonance imaging (pMR) is increasingly used in acute stroke, but its physiologic significance is still debated. A reasonably good correlation between pMR and positron emission tomography (PET) has been reported in normal subjects and chronic cerebrovascular disease, but corresponding validation in acute stroke is still lacking.

Methods—We compared the cerebral blood flow (CBF), cerebral blood volume, and mean transit time (MTT) maps generated by pMR (deconvolution method) and PET (¹⁵O steady-state method) in 5 patients studied back-to-back with the 2 modalities at a mean of 16 hours (range, 7 to 21 hours) after stroke onset. We also determined the penumbra thresholds for pMR-derived MTT, time to peak (TTP), and Tmax against the previously validated probabilistic PET penumbra thresholds.

Results—In all patients, the PET and pMR relative distribution images were remarkably similar, especially for CBF and MTT. Within-patient correlations between pMR and PET were strong for absolute CBF (average r²=0.45) and good for MTT (r²=0.35) but less robust for cerebral blood volume (r²=0.24). However, pMR overestimated absolute CBF and underestimated MTT, with substantial variability in individual slopes. Removing individual differences by normalization to the mean resulted in much stronger between-patient correlations. Penumbra thresholds of 6, 4.8, and 5.5 seconds were obtained for MTT delay, TTP delay, and Tmax, respectively.

Conclusions—Although derived from a small sample studied relatively late after stroke onset, our data show that pMR tends to overestimate absolute CBF and underestimate MTT, but the relative distribution of the perfusion variables was remarkably similar between pMR and PET. pMR appears sufficiently reliable for clinical purposes and affords reliable detection of the penumbra from normalized time-based thresholds.

Key words: acute stroke • cerebral blood flow • imaging • magnetic resonance • positron emission tomography • penumbra • perfusion

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