A generalized QUCESOP method with evaluating CEST peak overlap

The overlapping peaks of the target chemical exchange saturation transfer (CEST) solutes and other unknown CEST solutes affect the quantification results and accuracy of the chemical exchange parameters-the fractional concentration, $f_b$ , exchange rate, $k_b$ , and transverse relaxation rate, $R_{2b}$ -for the target solutes. However, to date, no method has been established for assessing the overlapping peaks. This study aimed to develop a method for quantifying the $f_b$ , $k_b$ , and $R_{2b}$ values of a specific CEST solute, as well as assessing the overlap between the CEST peaks of the specific solute(s) and other unknown solutes. A simplified $R_{1\rho }$ model was proposed, assuming linear approximation of the other solutes' contributions to $R_{1\rho }$ . A CEST data acquisition scheme was applied with various saturation offsets and saturation powers. In addition to fitting the $f_b$ , $k_b$ , and $R_{2b}$ values of the specific solute, the overlapping condition was evaluated based on the root mean square error (RMSE) between the trajectories of the acquired and synthesized data. Single-solute and multi-solute phantoms with various phosphocreatine (PCr) concentrations and pH values were used to calculate the $f_b$ and $k_b$ of PCr and the corresponding RMSE. The feasibility of RMSE for evaluating the overlapping condition, and the accurate fitting of $f_b$ and $k_b$ in weak overlapping conditions, were verified. Furthermore, the method was employed to quantify the nuclear Overhauser effect signal in rat brains and the PCr signal in rat skeletal muscles, providing results that were consistent with those reported in previous studies. In summary, the proposed approach can be applied to evaluate the overlapping condition of CEST peaks and quantify the $f_b$ , $k_b$ , and $R_{2b}$ values of specific solutes, if the weak overlapping condition is satisfied.