Gradient shimming with spectrum optimisation.

Abstract

Shimming, i.e. homogenising the unavoidable distortion of the static magnetic field B0 in NMR spectroscopy, is still an annoying, time consuming task. Although compared with conventional manual or computerised search methods gradient shimming initiated a new era in terms of operation and efficiency, there remain aspects that inhibit fully automated shimming with a result of guaranteed quality. The major reason for this limitation is that the judgement of the quality of the B0 homogeneity takes place in the spatial domain, although the actual objects of interest are the lines in the spectral domain. In this work, this restriction is removed by the introduction of a new framework for gradient shimming that enables to directly aim at the spectrum quality. Based on the mapped B0 field shimming is simulated and spectra are calculated for the virtual residual inhomogeneity. Using a suitable criterion to judge the spectrum quality an optimisation is performed, thus providing the predicted optimum spectrum and the corresponding residual B0 field. This target field is then aimed at during the real, iterative shimming procedure. For the widely applicable case of optimising the shape of a single line a powerful quality criterion was developed using an envelope of the calculated lineshape spectrum. The whole procedure is demonstrated for adjusting the on-axis shim functions based on one-dimensional field map data and for both on- and off-axis shimming using three-dimensional data. The results are verified with 1H NMR spectra acquired on standard NMR test samples.