Simultaneous optimization of power and duration of radio-frequency pulse in PARACEST MRI.

Chemical exchange saturation transfer (CEST) MRI is increasingly used to probe mobile proteins and microenvironment properties, and shows great promise for tumor and stroke diagnosis. The CEST effect is complex and depends not only on the CEST agent concentration, exchange rates, the characteristic of the magnetization transfer (MT), and the relaxation properties of the tissue, but also varies with the experimental conditions such as radio-frequency (RF) pulse power and duration. The RF pulse is one of the most important factors that promote the CEST effect for biological properties such as pH, temperature and protein content, especially for contrast agents with intermediate to fast exchange rates. The CEST effect is susceptible to the RF duration and power. The present study aims at determining the optimal power and the corresponding optimal duration (that maximize the CEST effect) using an off-resonance scheme through a new definition of the CEST effect. This definition is formulated by solving the Bloch-McConnell equation through the R1ρ method (based on the eigenspace solution) for both of the MT and CEST effects as well as their interactions. The proposed formulations of the optimal RF pulse power and duration are the first formulations in which the MT effect is considered. The extracted optimal RF pulse duration and power are compared with those of the MTR asymmetry model in two- and three-pool systems, using synthetic data that are similar to the muscle tissue. To validate them further, the formulations are compared with the empirical formulation of the CEST effect and other findings of the previous researches. By extending our formulations, the optimal power and the corresponding optimal duration (in the biological systems with many chemical exchange sites) can be determined.