A method for filtering image data having at least three types of distinct regions from a selected type of region, each of the at least three types of distinct regions being representative of one of three different substances, and comprising a plurality of unitary image elements, each unitary image element of the plurality of unitary image elements being characterized by a value, the method comprising: providing said image data; selecting a portion of said image data comprising at least a corresponding portion of at least two of said at least three types of regions, said at least two types of regions comprising the selected type of region; determining at least one distribution for at least one of said at least two types of regions in said selected portion, wherein said determining at least one distribution comprises determining a distribution of the values characterizing a representative part of the unitary image elements corresponding to said selected portion; determining at least one function for associating another value to each unitary image element of said selected type of region in said selected portion, the another value characterizing a unitary image element of another type of region of the at least two types of regions such that said at least one distribution of said selected type of regions in said selected portion is transposed into a corresponding distribution of said another type of region, wherein said determining at least one function comprises determining an initial approximating function for approximating the corresponding distribution of said selected type of region, and determining a transposition function adapted for transposing the initial approximating function into the corresponding distribution of said another type of region, and applying said at least one function to each unitary image element of said selected type of region of said selected portion to thereby provide filtered image data. providing said image data; selecting a portion of said image data comprising at least a corresponding portion of at least two of said at least three types of regions, said at least two types of regions comprising the selected type of region; determining at least one distribution for at least one of said at least two types of regions in said selected portion, wherein said determining at least one distribution comprises determining a distribution of the values characterizing a representative part of the unitary image elements corresponding to said selected portion; determining at least one function for associating another value to each unitary image element of said selected type of region in said selected portion, the another value characterizing a unitary image element of another type of region of the at least two types of regions such that said at least one distribution of said selected type of regions in said selected portion is transposed into a corresponding distribution of said another type of region, wherein said determining at least one function comprises determining an initial approximating function for approximating the corresponding distribution of said selected type of region, and determining a transposition function adapted for transposing the initial approximating function into the corresponding distribution of said another type of region, and applying said at least one function to each unitary image element of said selected type of region of said selected portion to thereby provide filtered image data. <EOS>