Abstract
Chemical exchange (CE) sensitive MRI contrast acquired with an off-resonance irradiation pulse is affected by other relaxation mechanisms, such as longitudinal and transverse relaxations. In particular, for intermediate CEs, the effect of transverse relaxation often dominates CE contrast. Since water relaxation rates can change significantly in many pathological conditions or during physiological challenge, it is crucial to separate these relaxation effects in order to obtain pure CE contrast. Here we proposed a novel acquisition scheme in which a toggling inversion pulse is applied prior to the off-resonance irradiation. By combined acquisition of irradiation images with and without an inversion pulse at both the labile proton frequency and the reference frequency, longitudinal and transverse relaxation contributions are cancelled, and the quantification of CE parameters, such as the exchange rate and the labile proton concentration, can be simplified. Furthermore, the CE-mediated relaxation rate can be readily determined with a relatively short irradiation pulse and without approaching the steady state, therefore, reducing the limitations on hardware and specific absorption rate requirements. The signal characteristics of the proposed method are evaluated by numerical simulations and phantom experiments.