Chemical Exchange Saturation Transfer MRI Optimal Continuous Wave RF Irradiation Parameters for Glycogen (glycoCEST) Detection

Chemical exchange saturation transfer (CEST) enables detection of molecules such as glycogen, whose concentrations are too low to impact the signal intensity of standard MR imaging. Detection of these molecules is achieved by selectively saturating a molecule of interest and by measuring the reduction in water signal due to saturation transfer. CEST effects are dependent on parameters such as CEST agent concentration, pH, temperature, relaxation rate, magnetic field strength as well as on experimental parameters such as repetition time, RF irradiation amplitude, and the imaging readout scheme. Measurement of molecules with exchangeable protons that resonate very close to water, e.g., hydroxyl groups in glycogen, is challenging especially at lower magnetic field strengths, mainly due to the effect of direct water saturation. Therefore, optimal RF irradiation parameters that maximize the CEST signal and reduce the competing factors are important for better quantification of glycogen-weighted CEST effects. In this study, analytical solution of the Bloch-McConnell equations was used to find optimal continuous wave RF irradiation parameters for detection of glycogen. In vivo tests were performed on a human calf muscle at different saturation powers to validate the optimal saturation parameters determined via simulation. The selected parameters were applied in vitro to CEST measurements in a phantom with varying glycogen concentrations and also in vivo in a human calf muscle. Our results show the possibility of detecting glycogen using CEST MRI at 3 T. It is further shown that the glycoCEST signal can be maximized by optimizing the RF pulse irradiation parameters (duration and power) and that different glycogen concentrations can be identified when applying the optimized saturation pulse.