Optimizing the parameters of continuous RF pulses for CEST MRI by numerical solution to the Bloch-McConnell equations

Chemical exchange saturation transfer (CEST) is a new contrast in magnetic resonance imaging (MRI) that isolates molecule biomarkers through chemical shift. CEST MRI contrast depends on radio frequency (RF) power and RF pulse type. Two approaches to saturate contrast agent (CA) protons have been used: continuous wave CEST (CW-CEST) and pulsed CEST. To find the optimal RF pulse, numerical solution of Bloch-McConnell equations (BME) may be used. The purpose of this work is to compare the effects of different CW-CEST. In this paper, Rectangular, Gaussian and Fermi pulses are investigated as CW RF pulses. To compare the efficiency of pulses their resultant Flip angles (FA) are assumed equal. Efficiency of CW-CEST is investigated by two parameters, CEST ratio and specific absorption ratio (SAR). Our results suggest that it is possible to maximize CEST ratio by optimizing parameters of Rectangular (amplitude about 5.7 μT), Gaussian (Ã about 0.7s) and Fermi (avalue about 0.3s). Results are verified by empirical formulation of CEST ratio.