Radio-frequency magnetic field mapping in two dimensions

This work demonstrates a technique to map the radiofrequency (RF) magnetic field produced by MR coils in two dimensions. The technique involves a modified rotating frame zeugmatography experiment. The pulse sequence is based on a gradient-echo experiment with frequency and phase-encoding gradients and is initiated by a nonselective RF pulse. The RF pulse length and the phase-encoding gradient strength are systematically incremented in successive experiments. Three-dimensional (3D) Fourier transformation of the echoes obtained in this manner produces a 3D data matrix in which one of the dimensions corresponds to the RF field strength while the other two dimensions correspond to the spatial axes defined by the applied gradients. Pulse length variation causes the signal to be amplitude-modulated, and the frequency of the modulation corresponds to the strength of the RF field. The method described has been employed to characterize the RF field profiles of surface coils and other coil configurations. This technique provides a straightforward approach for experimental determination of the RF field homogeneity in a plane. This should prove more convenient than point-by-point measurement techniques and can be used in the design and evaluation of RF coils