Feedback control of the nuclear magnetization state: experimental results

In a previous paper (Proc. Soc. Magn. Reson. Med., San Francisco, CA, USA, 1991), the authors investigated feedback control of the nuclear magnetization state. This strategy facilitates the optimization of the pulse parameters during the imaging sequence. In this work, results from a series of experiments used to test the theoretical predictions are presented. Two groups of experiments were performed using a commercial imager and a small sample of distilled water. In the first group of experiments the control objective is to regulate the magnetization orientation to a constant reference angle. The results show that a closed-loop system produces the desired orientation. In contrast, an open-loop input results in an appreciable error between desired and measured orientation. A second group of experiments shows that a closed-loop system can force the magnetization orientation to track a desired trajectory. In both groups, the closed-loop experimental and simulation results are in excellent agreement. The application of feedback control to MRI and spectroscopy is discussed.