Quantifying the fractional concentrations and exchange rates of small‐linewidth CEST agents using the QUCESOP method under multi‐solute conditions in MRI signals

Purpose To develop a novel method for quantifying the fractional concentration ( f b ) and the exchange rate ( k b ) of a specific small‐linewidth chemical exchange saturation transfer (CEST) solute in the presence of other unknown CEST solutes. Theory and Methods A simplified R 1ρ model was proposed assuming a small linewidth of the specific solute and a linear approximation of the other solutes’ contribution to R 1ρ . Two modes of CEST data acquisition, using various saturation offsets and various saturation powers, were used. The f b and k b of the specific solute could be fitted using the proposed model. In MRI experiments, using either single‐solute or multi‐solute phantoms with various creatine concentrations and pHs, the f b and k b values of creatine were calculated for each phantom; the f b and k b values of phosphocreatine in rats’ skeletal muscles were also evaluated. Results The fitted f b value of creatine from the phantoms were in excellent agreement. The fitted k b value of creatine from the phantoms coincides with that from the literature, as do the f b and k b values of phosphocreatine in skeletal muscles. Conclusion The proposed approach enables us to quantify the f b and k b values of a specific small‐linewidth solute in the presence of other unknown solutes.