Analysis of on- and off-resonance magnetization transfer techniques

Three methods of performing magnetization transfer (MT) MR imaging are analyzed: (a) off-resonance continuous wave, (b) off-resonance shaped pulses, and (c) on-resonance binomial pulses. With two-pool Bloch-model simulations, signal levels from "MT active" spin systems were calculated, with reference to direct saturation of "MT inactive" systems, allowing calculation of contrast due to MT. Simulations demonstrate several trends with variation of excitation amplitude and offset frequency for the off-resonance methods and with variation of excitation amplitude and pulse shape "order" for binomial pulses. The simulations show that nominally optimized versions of each of these approaches provide essentially equivalent contrast at a given level of applied MT power, contrary to previous claims. Experiments with an MT-inactive phantom, with a whole-body system, show results with off-resonance pulses to be in good agreement with simulations, whereas binomial-pulse experiments show anomalously large direct saturation.