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

ABSTRACT

PURPOSE: To develop a novel method for quantifying the fractional concentration (fb ) and the exchange rate (kb ) of a specific small-linewidth chemical exchange saturation transfer (CEST) solute in the presence of other unknown CEST solutes. THEORY AND METHODS: A simplified R1ρ model was proposed assuming a small linewidth of the specific solute and a linear approximation of the other solutes' contribution to R1ρ . Two modes of CEST data acquisition, using various saturation offsets and various saturation powers, were used. The fb and kb 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 fb and kb values of creatine were calculated for each phantom; the fb and kb values of phosphocreatine in rats' skeletal muscles were also evaluated. RESULTS: The fitted fb value of creatine from the phantoms were in excellent agreement. The fitted kb value of creatine from the phantoms coincides with that from the literature, as do the fb and kb values of phosphocreatine in skeletal muscles. CONCLUSION: The proposed approach enables us to quantify the fb and kb values of a specific small-linewidth solute in the presence of other unknown solutes.