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 T ₁ and T ₂ MRI relaxation times (qT ₁ and qT ₂) 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 qT ₁ and qT ₂ within 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 qT ₁ and qT ₂ relaxation times and the location and volume of abnormal qT ₁ and qT ₂ voxels within the lesion. Uncertainties associated with onset time estimates of qT ₁ and qT ₂ MRI 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 qT ₁ and qT ₂ lesion volumes, termed 'V ^overlap' (± 25 min) or by quantifying hemispheric differences in qT ₂ relaxation times only (± 28 min). Overall, qT ₂ derived parameters outperform those from qT ₁. 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.