Diagram of the contrast mechanism for three different types of CEST contrast particles. (a) Traditional LIPOCEST agents, with shift agent enclosed in the liposomal lumen, and chemical exchange occurring between the interior and exterior water. (b) Two-hop LIPOCEST agents, with CEST agent (not shift agent) enclosed in the liposomal lumen, and proton exchange occurring between agent and interior water, then interior water and exterior water. (c) CEST coated particles, where the exchange occurs entirely on the periphery of the particle.