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Purpose/Introduction: Ultra-high field (UHF) MRI in combination with specific saturation modules is a promising strategy to isolate CEST effects, e.g. of glutamate, or sugars. For in vitro optimization of sequence parameters, realistic T1 and T2 relaxation times are crucial [1]. Relaxation is often adjusted using agar, which was suspected previously to yield a CEST effect [2]. The CEST effect of agar is here characterized at various field strengths, concentrations and pH values. To avoid false positive results, the agar CEST effect must be taken into account when optimizing for CEST effects in realistic model solutions. Subjects and Methods: Multiple agar model solutions were investigated under different conditions (pH 6–8, T = 25, 37 C) at a Siemens whole body MR systems (B0 = 9.4T and 3T) and 14T (Bruker). Different CEST saturation modules (B1 = 4 to 7lT, 3 to 5 pulses, CW for B0 = 14T; Gauss, Spinlock (SL), matched adiabatic SL[3]) were applied, before a 3D-gradient-echo readout (TE = 1.98 ms, TR = 3.74 ms). Post processing included B1 [4] and B0 correction based on WASABI[5] before evaluation of Z-spectrum asymmetry (MTRasym [6]) at saturation offset Dx. Results: The CEST effect of agar was detectable at all field strengths, increasing from\1% MTRasym at B0 = 3T to 4% at 14T (Fig. 1). The effect of increased agar concentration (2% to 3%) seems to be counterbalanced by increased spillover due to shorter T2, resulting in an almost unchanged MTRasym at 9.4T (Fig. 2a). MTRasym values were found to be anti-correlated to pH (Fig. 2b). The observed CEST effect of agar was pronounced for relatively short (0.5 to 0.8 s), but strong saturation (B1 = 5–6 lT), indicating a faster exchange process (Fig. 3). Discussion/Conclusion: The agar CEST effect contributes to MTRasym with up to 2% at 9.4T, the behavior hints to faster hydroxyl exchange processes of -OH groups apparent in agar. CEST effects of agar were smaller, but still not negligible at lower field strengths (Fig. 1). Thus, agar is not a neutral baseline in model solutions for CEST characterization, and agar CEST effects could be mistaken for false positive CEST effects of metabolites studied in vitro. The agar CEST effect size depends on both, saturation and solution parameters, but can now be removed to enable realistic in vitro optimization of CESTMRI.
