CEST: from basic principles to applications, challenges and opportunities.

Chemical Exchange Saturation Transfer (CEST) offers a new type of contrast for MRI that is molecule specific. In this approach, a slowly exchanging NMR active nucleus, typically a proton, possessing a chemical shift distinct from water is selectively saturated and the saturated spin is transferred to the bulk water via chemical exchange. Many molecules can act as CEST agents, both naturally occurring endogenous molecules and new types of exogenous agents. A large variety of molecules have been demonstrated as potential agents, including small diamagnetic molecules, complexes of paramagnetic ions, endogenous macromolecules, dendrimers and liposomes. In this review we described the basic principles of the CEST experiment, with emphasis on the similarity to earlier saturation transfer experiments described in the literature. Interest in quantitative CEST has also resulted in the development of new exchange-sensitive detection schemes. Some emerging clinical applications of CEST are described and the challenges and opportunities associated with translation of these methods to the clinical environment are discussed.