Power Optimization of In-vivo Chemical Exchange Saturation Transfer (CEST) contrast for pH weighted Imaging

Chemical exchange saturation transfer (CEST) imaging provides a unique contrast through the exchange between solute protons and bulk water protons 1. It can detect tissue acidosis and elevated protein content in acute ischemia and animal tumor models 2,3. Conventionally, a continuous wave (CW) RF pulse is applied at the solute proton frequency and the saturation is transferred to the bulk water. The obtained MT-type contrast depends on the exchange rate and chemical shift of the labile solute protons as well as experimental parameters such as the duration and strength of the irradiation pulse 4. The in vivo endogenous CEST effect, also called the amide proton transfer (APT) contrast 2,3, is only a few percent. Therefore, it is necessary to maximize the CEST sensitivity by optimizing the irradiation pulses. It has been reported previously that the CEST effect in poly-l-lysine solution is irradiation power dependent, and there are significant direct saturation effects on bulk water (spillover effects) 5, 6 when the pulse is stronger than the optimal power. The in-vivo CEST process is further complicated due to the presence of conventional MT effects. We model the CEST process using an empirical formula considering both the saturation transfer and spillover effects at low to intermediate power. The model is shown to be able to correctly describe the effects of irradiation on endogenous CEST process and allowed us to achieve a significant improvement of the contrast when using pH-weighted imaging in an acute ischemia animal model.