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Introduction: Establishing a timeline of acute ischemic stroke (AIS) onset is critically important for selecting time-sensitive treatments. However, there is currently no accepted imaging method for reliably dating stroke onset. T1 Rho MR is an investigative imaging modality for quantifying the spin lattice relaxation time constant in a rotating frame. T1 Rho is sensitive to chemical exchange, and in acute stroke, has been shown in animal models to increase linearly in the first 6 hours after onset of cerebral infarction, making it possible to accurately date onset.Hypothesis: We hypothesize that a rapid non-contrast T1 Rho MR sequence can be used to detect and quantify AIS in humans.Methods: AIS patients (n=11) underwent 3.0T brain MR scans in the late acute to subacute period after onset (mean 56 hours). The infarct volume was identified on DWI and infarct regions of interest (ROIs) were mapped onto the contralateral side to serve as within-subject controls. Infarct and control ROIs were co-registered with T1 Rho maps and compared using a Student t-test.Results: T1 Rho infarct volume correlated well to DWI volume and mean T1 Rho intensity values in the infarct, were significantly higher compared to controls (p<0.004, mean difference 37ms, std 19.3). When inter-individual T1rho values were plotted against time from stroke onset, we found that they remained stable.Conclusion: T1 Rho MR is feasible, and can detect and quantify AIS in humans. Further, T1 Rho values in the infarct bed were stable in the late acute to early subacute time period, consistent with animal studies that show T1rho values to undergo predictable change only in the first 6 hours after infarct onset. Further studies are needed to determine if T1rho in humans has a predicable rate of change in the early acute phase, that can one day be used as a quantifiable surrogate of stroke onset and thus inform stroke therapy when time of onset is unknown.