High field magnetic resonance offers new opportunities because of its high SNR and better spectral resolution for MRI and MRS. However, new problems also emerge at high field. As the field strength increases, the wavelength in the tissue becomes shorter and comparable with the body dimensions. This perturbs the field and also causes standing waves within the patient as a result of the impedance mismatching at the tissue interfaces. Due to the complexity of the boundary conditions and the solution of Maxwell's equations, an exact analytical calculation for a loaded RF resonator has not been possible. In this paper, we present a birdcage coil simulation study based on a 3D finite element method (FEM) model for the characterization of the field within the tissue. First, the accuracy of the FEM solutions is validated by the 2D analytical solutions at 64 and 223 MHz. In these solutions, the frequency dependence of the conductivity and permittivity is also taken into account. Then, a more realistic 3D model is studied. The results are compared with the experimental measurements. It is shown that the 3D model makes it possible to explore the effects of the end rings in the presence of a tissue sample inside the coil.

A method of fast nuclear magnetic resonance (NMR) imaging which uses only square wave gradient shapes and also collects data on Cartesian coordinates in a way similar to the blipped echo planar imaging method (BEPI) is described. The method is a heuristic attempt at finding an optimal data collection strategy for NMR imaging. Its advantages and disadvantages compared to other methods are carefully analyzed. Clinical size anatomical (head and body) images are shown. It is found that the interecho phase-angle discrepancies that are caused bv the B(0) inhomogeneity and by the gradient wave form distortion can largely be eliminated by using the half Fourier method with full Fourier phase map correction, but one phase map is required for each echo. GRASS/FLASH spoiling techniques are incorporated into the method in order to allow multiecho speed increases for scans which would run under 10 s using a single-echo method. The technique can be applied directly to GRASS/FLASH itself, as is demonstrated by a two-echo GRASS scan. The fastest image was 1.9 s.

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