A method for determining a signature index of an observed tissue comprises the step of providing a generic attenuation model of a motion-probing gradient pulse MRI attenuated signal S(b), and providing a reference model parameter vector (pR(i)) corresponding to a reference state of the tissue. On the basis of the evolution of the determined partial differential sensitivities dSi(b) of the model attenuated signal S(b) to each model parameter p(i) at the neutral state attenuated signal SN(b) versus b values, a discrete and narrow size set of key b is built and a series of MRI images of the observed tissue are acquired by using the key b values. Then, for each voxel a signature index (sADC(V), Sdist(V), SCdist(V), Snl(V), SI(V)) of the voxel V is determined as a scalar function depending on a distance between the voxel signal pattern acquired at the key b values and the signal pattern of the reference state of the tissue at the same key b values. An apparatus is configured for implementing such a method.
