Feedback Control of the Nuclear Magnetization State: Modeling and Control Design

Magnetic resonance images are obtained by observing the fluctuations in nuclear magnetization produced by sequences of RF pulses and applied magnetic field gradients. The design of a pulse sequence is based on the expected response of the nuclear magnetization. Currently, all pulse sequence parameters are loaded into the pulse programmer before the start of the sequence and remain unchanged until the completion of the sequence. A fundamentally different approach is considered in this paper, whereby the sequence parameters are adjusted between successive RF excitation pulses. In this manner, the nuclear magnetization is regulated to a desired state by using measurements of the magnetization to adjust the amplitude and duration of the RF pulses. Feedback control of the nuclear magnetization state may be advantageous in certain types of MR experiments. For example, those requiring a repeated spin-echo or gated MR image acquisition. As a first step in developing this approach a simple scheme is presented for regulating the angle between the bulk magnetization and the axis of the applied static magnetic field. The behavior of the closed-loop system is explored using computer simulations.