A Method for Imaging the Ischemic Penumbra with MRI Using Intravoxel Incoherent Motion

BACKGROUND AND PURPOSE: In acute ischemic stroke, the amount of “local” CBF distal to the occlusion, ie, all blood flow, whether supplied antegrade or delayed and dispersed through the collateral network, may contain valuable information regarding infarct growth rate and treatment response. DSC processed with a local arterial input function (AIF) is one method of measuring local CBF (local-qCBF) and has been shown to correlate with collateral supply. Similarly, intravoxel incoherent motion MRI (IVIM) is “local,” with excitation and readout in the same plane, and a potential alternative way to measure local-qCBF. This work compares IVIM local-qCBF against DSC local-qCBF in the ischemic penumbra, compares the measurement of perfusion-diffusion mismatch (PWI/DWI), and examines if local-qCBF may improve prediction of the final infarct.

MATERIALS AND METHODS: Eight experiments in a preclinical canine model of middle cerebral artery occlusion were performed. Native collateral circulation was quantified via x-ray DSA 30 minutes postocclusion, and collateral supply was subsequently enhanced in a subset of experiments with simultaneous pressor and vasodilator. IVIM, DSC, and DWI MRI were acquired 2.5 hours postocclusion. IVIM was postprocessed to return local-qCBF from fD*, water transport time (WTT) from D*, diffusion from D, and the PWI/DWI mismatch. These were compared with DSC parameters processed first with a standard global-AIF and then with a local-AIF. These DSC parameters included time-to-maximum, local MTT, standard-qCBF, local-qCBF, and PWI/DWI mismatch. Infarct volume was measured with DWI at 2.5 hours and 4 hours postocclusion.

RESULTS: Two and one-half hours postocclusion, IVIM local-qCBF in the noninfarcted ipsilateral territory correlated strongly with DSC local-qCBF (slope = 1.00, R²= 0.69, LinCCC = 0.77). Correlation was weaker between IVIM local-qCBF and DSC standard-qCBF (p = 0.38, R² = 0.13). DSC local-qCBF and IVIM local-qCBF in the noninfarcted ipsilateral territory both returned strong prediction of final infarct volume (R² = 0.78, R² = 0.61, respectively). DSC standard-qCBF was a weaker predictor (R² = 0.12). The hypoperfused lesion from DSC local-qCBF and from IVIM local-qCBF both predicted final infarct volume with good sensitivity and correlation (slope = 2.08, R² = 0.67, slope = 2.50, R² = 0.68, respectively). The IVIM PWI/DWI ratio was correlated with infarct growth (R² = 0.70), and WTT correlated with DSC MTT (slope = 0.82, R² = 0.60).

CONCLUSIONS: Noncontrast IVIM measurement of local-qCBF and PWI/DWI mismatch may include collateral circulation and improve prediction of infarct growth.