A magnetic resonance imaging protocol for stroke onset time estimation in permanent cerebral ischemia

Abstract

MRI provides a sensitive and specific imaging tool to detect acute ischemic stroke by means of a reduced diffusion coefficient of brain water. In a rat model of ischemic stroke, differences in quantitative T1and T2MRI relaxation times (qT1and qT2) between the ischemic lesion (delineated by low diffusion) and the contralateral non-ischemic hemisphere increase with time from stroke onset. The time dependency of MRI relaxation time differences is heuristically described by a linear function and thus provides a simple estimate of stroke onset time. Additionally, the volumes of abnormal qT1and qT2within the ischemic lesion increase linearly with time providing a complementary method for stroke timing. A (semi)automated computer routine based on the quantified diffusion coefficient is presented to delineate acute ischemic stroke tissue in rat ischemia. This routine also determines hemispheric differences in qT1and qT2relaxation times and the location and volume of abnormal qT1and qT2voxels within the lesion. Uncertainties associated with onset time estimates of qT1and qT2MRI data vary from ± 25 min to ± 47 min for the first 5 hours of stroke. The most accurate onset time estimates can be obtained by quantifying the volume of overlapping abnormal qT1and qT2lesion volumes, termed ’Voverlap’ (± 25 min) or by quantifying hemispheric differences in qT2relaxation times only (± 28 min). Overall, qT2derived parameters outperform those from qT1. The current MRI protocol is tested in the hyperacute phase of a permanent focal ischemia model, which may not be applicable to transient focal brain ischemia.