Quasi–steady‐state CEST (QUASS CEST) solution improves the accuracy of CEST quantification: QUASS CEST MRI‐based omega plot analysis

Purpose CEST MRI omega plot quantifies the labile proton fraction ratio ( f r ) and exchange rate ( k sw ), yet it assumes long RF saturation time (Ts) and relaxation delay (Td). Our study aimed to test if a quasi–steady‐state (QUASS) CEST analysis that accounts for the effect of finite Ts and Td could improve the accuracy of CEST MRI quantification. Methods We modeled the MRI signal evolution using a typical CEST EPI sequence. The signal relaxes toward its thermal equilibrium following the bulk water relaxation rate during Td, and then toward its CEST steady state following the spin‐lock relaxation rate during Ts from which the QUASS CEST effect is derived. Both f r and k sw were solved from simulated conventional apparent CEST and QUASS CEST MRI. We also performed MRI experiments from a Cr‐gel phantom under serially varied Ts and Td times from 1.5 to 7.5 s. Results Simulation showed that, although k sw could be slightly overestimated (3%‐15%) for the range of Ts and Td, f r could be substantially underestimated by as much as 67%. In contrast, the QUASS solution provided accurate k sw and f r determination within 2%. The CEST MRI experiments confirmed that the QUASS solution enabled robust quantification of k sw and f r , superior over the omega plot analysis based on the conventional apparent CEST MRI measurements. Conclusions The QUASS CEST MRI algorithm corrects the effect of finite Ts and Td times on CEST measurements, thereby allowing robust and accurate CEST quantification.