Stroke is the second most frequent causes of mortality in the word after cardiovascular disease and the most frequent cause of disability. The majority of strokes are ischemic (87%). Ischemic stroke may benefit from efficient therapy if the delay from stroke onset not exceeds 4H30 (intra-venous thrombolysis). In case of irreversible cerebral lesion, the long term functional recovery is largely dependent from the ability of the brain to reorganize. Conventional magnetic resonance imaging represents the most sensitive tool to diagnose stroke at the acute phase. However, up to now, no imaging tool is available to determine the time of stroke onset. Moreover, no imaging tool is available to precisely quantify the functional and structural plasticity mechanisms occurring after stroke. The main objective of the present study is to test the accuracy of sodium MRI to determine the time of stroke onset. The secondary objectives of the study are to determine the ability of non-conventional MRI techniques to quantify the degree of tissue damage and brain reorganization after stroke. Thirty patients will be included during the first 24 hours after stroke onset. Multimodal MRI (including perfusion MRI, sodium MRI, resting-state functional MRI, high resolution anatomical MRI) and clinical evaluation will be carried on at the inclusion, after 24 hours, at 3 months and at 12 months.

Inclusion Criteria:

- Patient having presented a cerebral infarct beforehand confirmed by the brain imaging

• Patient having presented a cerebral infarct the hour of the beginning of which is known with certainty and dating less than 24 hours
• Patient having presented a cerebral infarct the area(extent) of which is superior to 1,5 ml

Exclusion Criteria:

• Patients presenting the usual contraindications to the MRI (pacemaker, agitation, metallic brightness, claustrophobia etc.)
• Patients presenting a risk of not compliance to the examination: disorders(confusions) of the elementary understanding, the confusion, the involuntary movements, the bad tolerance of the prolonged dorsal decubitus