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.