“…The intensity of a voxel located at the spatial position k (k = 1,..., N, where N is the number of voxels) can be represented as 38 (4) where o k is the observed intensity, t k is the true intensity, G k is the diagonal matrix representing the gain field, and noise (k) is the noise. In multi-spectral case, the focus of the current studies, all the above variables, exceptG k , are vectors.…”