The reciprocal linear QUEST analysis method facilitates the measurements of chemical exchange rates with CEST MRI

Magnetic resonance imaging (MRI) contrast media that are detected via chemical exchange saturation transfer (CEST) often require an accurate estimation of their chemical exchange rate, k_ex. A variety of analysis methods have been proposed to estimate k_ex, including the nonlinear QUEST analysis method that evaluates the CEST amplitude as a function of saturation time. We have derived a linear version of QUEST, termed the Reciprocal Linear QUEST (RL-QUEST) method. Our simulations and experimental results show that RL-QUEST performs as well as QUEST, while providing a more simplistic fitting procedure. Although CEST results should be acquired with saturation power that has a nutation rate that is faster than k_ex of the CEST agent, an exact determination of the saturation power is not required to accurately estimate k_ex with RL-QUEST. This new analysis method requires a determination of the CEST agent's concentration, which is straightforward for the analysis of CEST agents in chemical solutions, but may be a limitation during in vivo CEST MRI studies. Based on the results of this study and previous studies, we provide recommendations for the linear analysis method that should be employed for each type of CEST MRI study.