The magnet essentially comprises at least three pairs of coaxial coils in order to increase the number of available parameters, to reduce the non-uniformity B.sub.d, to provide an examination volume with a double access consisting of a longitudinal access along the axis of revolution Oz and of a transverse access parallel to the plane of symmetry, to minimize the overall size as well as the dissipated electric power. The nine principal characteristic parameters of the coils are chosen so as to optimize the uniformity of the magnetic field within the examination volume.