High resolution mapping of highly nonlinear RF magnetic fields

For certain applications it is desirable to know quantitatively the precise field distribution produced by a circuit or coil external to the phantom or body. This is useful for characterization and calibration of the coil. A variety of methods have been developed which do this for uniform fields and tip angles less than 90 degrees which is typical in most MRI applications. In this setup, the external coil is used to both excite and receive the NMR signal. Due to the proximity of the external circuit, the transverse magnetic field it produces falls off quite rapidly and is highly nonlinear. In order to accurately characterize the field, images with very fine resolution must be made in order to avoid cancellation of the signal within each voxel due to a wide range of tip angles over that region. In this-paper, high resolution images are acquired and the magnetic transmit/receive coil are extracted, magnitude of the transverse magnetic field in each voxel is found by determining the tip angle in that voxel using a stimulated echo technique. Since the RF pulsewidth and gyromagnetic ratio are already known, solving for the magnetic field amplitude from the tip angle is easily achieved. Initial results shown in this paper indicate that the method works for tip angles well beyond 360 degrees