Paramagnetic chemical exchange saturation transfer agents and their perspectives for application in magnetic resonance imaging

Magnetic resonance imaging (MRI) has emerged as very important tool in biomedical research and is an essential diagnostic method in clinical radiology today. Lately, chemical exchange saturation transfer (CEST) has become a very attractive alternative to the classical MRI methodologies. CEST uses a unique operating mechanism to generate contrast and possesses great potential for functional imaging investigations, especially in combination with diamagnetic and paramagnetic (dia- and paraCEST, respectively) contrast agents. However, CEST is governed by a combination of several parameters that together influence the overall intensity of observed CEST effect. The understanding of the physics of CEST has advanced significantly to provide a reliable assessment on contribution of individual parameters important for generation of a CEST signal. Nevertheless, there seem to be a missing link between the above mentioned theory and its practical application, especially in the development of new probes. This review article provides background information on CEST and paraCEST, analyzing the importance of the main physical parameters, such as exchange rate, saturation power and time, or paramagnetic shift and relaxation times. We describe the different types of paramagnetic complexes based on lanthanide or transition metal ions, and discuss their properties in the context of potential CEST application.