The first column shows the B1 transmit field of a phantom and of a volunteer’s brain. The second column shows the influence of the B1 inhomogeneity on the CEST contrast under saturation with CP-Mode. The third column shows the mitigation of the B1 inhomogeneity influence through saturation with the use of MIMOSA.
Chemical exchange saturation transfer (CEST) MRI provides information about metabolites and proteins that have protons that exchange with the water pool. Selective saturation of the exchangeable protons results in attenuation of the water signal. The CEST contrast of specific metabolites can be acquired through the quantification of the water saturation spectrum.
A large number of metabolites, proteins, and peptides have exchangeable protons that can generate a CEST contrast. In brain measurements, amide proton transfer (APT) CEST and relayed Nuclear Overhauser Effect (rNOE) are often exploited. APT-CEST is able to detect endogenous mobile proteins and peptides in tissue. Additionally, it was reported that APT-CEST can provide information about ischemic and tumor tissue. The rNOE contrast is connected to the mobile macromolecular components that are composed of aliphatic and olefinic molecules. This contrast provides information about tissue cellularity and protein mobility, which is of potential interest for glioblastoma imaging. Another CEST contrast is guanidyl-CEST, which is sensitive to protons of the NH2 group, which can be bound to different metabolites. One example of such a metabolite is creatine (Cr). Cr-CEST can provide information on pH value in acute stroke in brain as well as in muscle tissue.
CEST MRI largely benefits from the increasing availability of ultra-high field (UHF) MRI systems (B0 ≥ 7 Tesla). However, the use of UHF systems also comes with specific challenges, since the inhomogeneity of the main magnetic field (B0) and the transmit field (B1+) increase. Another major factor limiting the clinical application of chemical exchange saturation transfer (CEST) imaging is the long acquisition time required for whole brain CEST MRI. Thus, CEST studies are often limited to 2D acquisitions with a small number of acquired slices.
We focus on overcoming these limiting factors through the application of new saturation methods termed Multiple Interleaved Mode Saturation (MIMOSA)  together with new sampling patterns (Snapshot CEST in cooperation with M. Zaiss ) (see Fig. 1). Current application focus is on the rNOE/APT contrast in head imaging as well as on metabolic imaging in muscle (Cr and urea). Another area of interest is the application of CEST to imaging of inflammation of the synovia in knees.
 Liebert A, Zaiss M, Gumbrecht R, Tkotz K, Linz P, Schmitt B, Laun FB, Doerfler A, Uder M and Nagel AM.
Multiple interleaved mode saturation (MIMOSA) for B1+ inhomogeneity mitigation in chemical exchange saturation transfer.
Magn Reson Med. 2019;82:693-705.
 Zaiss M, Ehses P, Scheffler K.
Snapshot-CEST: Optimizing spiral-centric-reordered gradient echo acquisition for fast and robust 3D CEST MRI at 9.4 T.
NMR Biomed. 2018;31(4):e3879.