Dynamics of magnetization under stimulus-induced rotary saturation sequence

We studied stimulus-induced rotary-saturation preparation (which enables measurement of oscillating magnetic fields using MRI) and derived an analytical solution of the Bloch equation to understand magnetization dynamics mathematically and comprehensively and to conduct simulations without sequential-calculation techniques such as the Runge-Kutta method. We formulated the dynamics using the Bloch equation, introducing an additional rotating frame and some approximations to make it into a homogeneous differential equation. Moreover, we found that there are two modes depending on the target oscillating magnetic field. To confirm the validity of the solution, we experimentally investigated its characteristics and performed curve fitting using the analytical model. Considering the constraints on the frame, the analytical solution was found to agree with experimental data. The experimental data indicate that it is necessary to design robust sequences compensating B₀ or B_1lock spatial inhomogeneity to improve measurements. Therefore, experimenters should consider the dynamics of magnetization with RF pulses to rewind the spin phase for accurate measurements.