1. A method for automatically locating and segmenting ischemic brain penumbra during an exam of a brain by a magnetic resonance (MR) imaging system and evaluating a volume of penumbra mismatch, which comprises the steps of: a) performing MR diffusion-weighted imaging (DWI) of the brain for acquiring b0 and b1000 native diffusion images of slices of the brain and creating an apparent diffusion coefficient (ADC) map of each of the slices of the brain; b) performing MR perfusion-weighted imaging (PWI) of the brain for acquiring native perfusion images of the slices of the brain, wherein for each slice, several of the native perfusion images are successively acquired during successive acquisition periods; c) for each of the slices of the brain, segmenting the b0 native diffusion image of the slice of the brain to create a contour mask, the contour mask being a mask of a contour of a parenchyma appearing in the b0 native diffusion image of the slice of the brain; d) for each of the slices of the brain, creating a necrosis and cerebrospinal fluid mask, hereafter NC mask, from the apparent diffusion coefficient (ADC) map of the slice of the brain; e) fusing the contour mask and the NC mask of a same slice of the brain; f) for each of the slices of the brain, aligning all the native perfusion images acquired for the slice with the NC mask obtained for the slice so that a head axis in the native perfusion images becomes aligned with a head axis in the NC mask and then fusing the NC mask with each native perfusion image for obtaining an improved perfusion image; h) randomly selecting N perfusion voxels whose positions are outside of the necrosis, wherein N is smaller or equal to a total amount M of the perfusion voxels acquired during one acquisition period for all the slices, and using improved perfusion images for classifying each randomly selected perfusion voxel in a class of voxels among K classes of voxels, wherein a random selection of the perfusion voxels includes increasing a distance between a randomly selected voxel and the necrosis from first randomly selecting perfusion voxels to a last randomly selecting perfusion voxel; i) creating a map of the brain that represents the K classes; j) creating a final image of the brain by grouping in the map of the brain the classes in a predefined number F of families, wherein the family whose voxels represent a largest volume corresponds to a healthy parenchyma, and remaining families which have at least one voxel that is connected directly or indirectly to the necrosis correspond to the penumbra; and k) calculating a volume of the penumbra mismatch from the final image of the brain, wherein the volume of the penumbra mismatch equals to a sum of volumes of the voxels belonging to each family corresponding to the penumbra.