Unknown symptom onset time is a common contraindication for thrombolysis of hyperacute ischaemic stroke. MRI may identify patients within the 4.5-hour thrombolysis treatment window, but it is unclear which parameter is most accurate. We compared the ability of hemispheric differences in quantitative T2 (qT2), ADC, and signal intensities of DWI, T2-weighted and T2-weighted FLAIR images at distinguishing between patients scanned within and beyond 4.5-hours. qT2 correlated significantly with time from onset (r = .491, p =.003) and had the highest and only significant AUC (0.77, p = .007). These data point to qT2 as a stroke timer.