Radiofrequency map of an NMR coil by imaging

We propose a new imaging method to obtain a map of the radiofrequency (RF) field amplitude over a sample. The sequence contains three RF pulses (alpha, 2 alpha, and alpha) and produces two images by a classical spin echo and a stimulated echo. A third image is computed and gives the distribution of the flip angle alpha, and so the RF amplitude, over the sample. The accuracy of the flip angle determination is verified on an homogeneous sample and results show a good correlation between experimental and theoretical flip angles in the range of 50 degrees to 130 degrees. Experiments with a surface coil and a resonator show the method is available in an inhomogeneous RF field. Images obtained on the calf of a volunteer confirms the independence of the computed RF distribution from proton density, T1, or T2 contrast.