CEST-imaging: A new contrast in MR-mammography by means of chemical exchange saturation transfer

Abstract

Chemical exchange saturation transfer (CEST) is a technique in H MR imaging (MRI) that enables visualization of chemical exchange processes between protons bound to solutes and surrounding bulk water molecules [1]. To induce a CEST contrast, the off-resonant solute protons are labeled by a saturation radiofrequency (RF) pulse and the label is then transferred to bulk water by chemical exchange. The magnitude of the subsequent reduction of bulk water signal depends consequently on the dynamics of chemical exchange as well as on the ratio of exchangeable solute protons to bulk water protons. The rate constant of chemical exchange (k) is influenced by the pH value and the temperature within the exchange environment. If the latter two parameters can be assumed to be distributed homogeneously in tissue, the CEST effect will be a surrogate marker of the concentration of a certain solute molecule in tissue. In order for a solute molecule to be considered suitable as an endogenous CEST agent, it must carry labile protons that exchange with bulk water at exchange rates that fulfill the condition k <Dw; where Dw is the resonance offset of the solute protons to the water protons in [s]. The most prominent endogenous CEST contrast is amide proton transfer (APT) imaging [2]. It relies on the exchange between amide protons from the backbone of small intracellular mobile proteins or peptides and bulk water. APT imaging has successfully been applied to imaging of ischemic areas in stroke [3], brain tumors [4] and also radiation necrosis [5] due to its strong dependence on tissue pH value. However, of course, aside from its pH sensitivity, APT contrast