A Method for Imaging the Ischemic Penumbra with MRI using IVIM

BACKGROUND AND PURPOSE: In acute ischemic stroke, the amount of “local” CBF distal to the occlusion, i.e. all blood flow within a region whether supplied antegrade or delayed and dispersed through the collateral network, may contain valuable information regarding infarct growth rate and treatment response. DSC CBF using a local arterial input function (AIF) is one method of quantifying local CBF (local-qCBF) and correlates with collaterals. 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. The purpose of this work was to compare IVIM local-qCBF against DSC local-qCBF in the ischemic penumbra, compare measurement of perfusion-diffusion mismatch (PWI/DWI), and examine if local-qCBF may improve prediction of final infarct.

MATERIALS AND METHODS: Eight experiments in a pre-clinical canine model of middle cerebral artery occlusion were performed; native collateral circulation was quantified via x-ray DSA 30 minutes post-occlusion, and collateralization was subsequently enhanced in a subset of experiments with simultaneous pressor and vasodilator. IVIM and DSC MRI were acquired 2.5hr post-occlusion. IVIM was post-processed 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 2.5hrs and 4hrs post-occlusion.

RESULTS: 2.5hr post-occlusion, IVIM local-qCBF in the non-infarcted ipsilateral territory strongly correlated with DSC local-qCBF (slope=1.00, R²=0.69, Lin’s CCC=0.71). Correlation was weaker between IVIM local-qCBF and DSC standard-qCBF (R²=0.13). DSC localqCBF and IVIM local-qCBF in the non-infarcted 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 (R²=0.60).

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

ABBREVIATIONS: AIF: arterial input function, IVIM: intravoxel incoherent motion, qCBF: quantitative cerebral blood flow, WTT: water transport time, MCAO: middle cerebral artery occlusion, MD: mean diffusivity