Measurement of whole-brain and gray matter atrophy in multiple sclerosis: assessment with MR imaging by Storelli, L, Rocca, M, Pagani, E, Van Hecke, W, Horsfield, M, De Stefano, N, Rovira, A, Sastre-Garriga, J, Palace, J, Sima, D, Smeets, D, Filippi, M, Magnims Study Group