1. A denoising method for an MRI, the denoising method comprising: acquiring a plurality of MRI's for a same site using different image-acquisition protocols, such that the MRI's have different contrast levels; determining pixels in each of the plurality of MRI's corresponding to a same tissue by using the plurality of MRI's; and calculating a new luminance value for the pixels by using luminance values of the pixels that are determined to be belonging to the same tissue in said determining; wherein said determining comprises: calculating a similarity level between a first pixel and a second pixel from the plurality of MRI's by using a difference in luminance value of the first pixel and the second pixel in each of the plurality of MRI's, wherein the first and second pixels are from a single MRI; and determining whether or not the first pixel and the second pixel belong to a same tissue by comparing the similarity level with a preset threshold, and wherein said calculating the new luminance value comprises determining the new luminance value for the first pixel and the second pixel from a weighted average of luminance values of the first pixel and the second pixel based on the similarity level between the first pixel and the second pixel.