An improved contrast agent for magnetic resonance imaging comprises particles to each of which is coupled a multiplicity of chelating agents containing paramagnetic ions. In the improved agent, the position of the ion is offset from the surface of the particle so as to improve the relaxivity imparted by the contrast agent. A tether offsetting the chelate from the surface of the particle may optionally contain cleavage sites permitting more facile excretion of the chelated paramagnetic ion.