Ischemic stroke caused by intracranial occlusion is a major cause of mortality worldwide¹. Over the last two decades, significant progress in the use of thrombolytic agents for the treatment of acute stroke has significantly improved the outcome for stroke patients^2,3. However, it is known that intravenous recombinant tissue plasminogen activator (tPA), when needed, is efficacious only if administered less than 3 hours after symptom onset, i.e. in the early stage of hyperacute ischemic stroke^3,4. Diffusion weighted MR imaging has shown to give good sensitivity to probe hyperacute cerebral ischemia⁵, provided that patients have access to an MRI scanner at this very early stage. In previous work⁶ we have demonstrated fast 3D MRI in vivo in the human brain at ultra-low magnetic field and we believe that practical implementation of ultra-low field MRI scanners could provide clinically relevant images in robust portable devices. We have also previously shown that MR Fingerprinting techniques can be applied at ultra-low magnetic field⁷. Here, we show distinct T₁ contrast at ultra-low magnetic field during the hyperacute and acute phase of brain ischemia in rat brains in vivo. MR fingerprinting was performed in vivo in two male Wistar rats in the previously described ultra-low field scanner⁶. A custom-made Tx/Rx rat head NMR probe⁸ was used that provides both high homogeneity and sensitivity. For each rat, permanent ischemia was induced by surgically occluding the middle cerebral artery (MCA). Balanced steady state free precession (b-SSFP) was used for imaging, with Cartesian acquisition and random sampling of 40% of k-space with a variable density Gaussian pattern⁹. The sequence was set with matrix size=128×35×11, corresponding voxel size = (1.7×2.2×4.2) mm³, and number of average NA=3. The minimum TR was 47.8 ms with 9091 Hz bandwidth. The total acquisition time was 18.2 min. A flip angle/TR trajectory of length N=20 was generated using an optimization method previously described⁷ (Fig. 1). The optimization scheme used here has FA ranging from 1 to 180°, and TR from 47.8ms to 191 ms. After each experiment, the animals were sacrificed and their brain extracted for staining with 2,3,5-Triphenyltetrazolium chloride (TTC).The MRF acquisitions were reconstructed by best match to a pre-computed dictionary, and the resultant images reveal five different types of quantitative maps: Proton density, T₁ (ms), T₂ (ms), off-resonance (Hz) and B₁ homogeneity (% of total field) (Fig. 2). T₁ images at three different time points following occlusion (t1=20min, t2=3h, and t3=24h) are shown for the two occluded rats (Fig. 3a). In rat 1 and 2, T₁ increase of about 25% can be seen at t=20 min, up to 75% at t=3h, and more than 100% at t=24h (red arrows). TTC staining of the extracted and then sliced animal brains reveals large infarcts in the same regions where T₁ changes were observed (Fig. 3b) in both animals. The infarcted hemisphere also shows significant herniation resulting from what we assume is cytotoxic edema.We have demonstrated that quantitative T₁ maps in ischemic animals at ultra-low magnetic field reveal changes in the hyperacute phase of ischemic stroke, noticeable at t=20 min after occlusion, and clearly observable at t=3 h. We believe that these observed changes in T₁ result from cytotoxic edema in the damaged region. Optimized MR fingerprinting techniques enable the generation of relevant contrast unique to the ultra-low field regime. Further work will aim at investigating T₁ contrast more frequently in the hyperacute phase of ischemic stroke, in particular at t<3h, as well as providing quantitative maps at high magnetic field strength for comparison purpose. This preliminary work shows that monitoring of ischemic stroke at ultra-low magnetic field is possible at a very early stage. We believe that this application for ultra-low field MRI could lead to disruptive technology where safe and portable mobile ultra-low field scanners sited in an ambulance are able to diagnose early ischemia and have potential to change the outcome for the patient as a pre-hospital imaging tool.