Abstract
Chemical exchange saturation transfer (CEST) enables indirect detection of small metabolites in tissue by MR imaging. To optimize and interpret creatine‐CEST imaging we characterized the dependence of the exchange‐rate constant ksw of creatine guanidinium protons in aqueous creatine solutions as a function of pH and temperature T in vitro. Model solutions in the low pH range (pH = 5–6.4) were measured by means of water‐exchange (WEX)‐filtered 1H NMR spectroscopy on a 3 T whole‐body MR tomograph. An extension of the Arrhenius equation with effective base‐catalyzed Arrhenius parameters yielded a general expression for ksw(pH, T). The defining parameters were identified as the effective base‐catalyzed rate constant kb,eff(298.15 K) = (3.009 ± 0.16) × 109 Hz l/mol and the effective activation energy EA,b,eff = (32.27 ± 7.43) kJ/mol at a buffer concentration of cbuffer = (1/15) M. As expected, a strong dependence of ksw on temperature was observed. The extrapolation of the exchange‐rate constant to in vivo conditions (pH = 7.1, T = 37 °C) led to the value of the exchange‐rate constant ksw = 1499 Hz. With the explicit function ksw(pH, T) available, absolute‐pH CEST imaging could be realized and experimentally verified in vitro. By means of our calibration method it is possible to adjust the guanidinium proton exchange‐rate constant ksw to any desired value by preparing creatine model solutions with a specific pH and temperature.
