Magnetic Field Stability and Radiofrequency Coil Efficiency

The temporal variation of the metabolite signal, especially during long examinations, strongly impairs the quality of the resulting spectra. Beside the usual patient motion, the reason for this instability can be found in the drift of the main magnetic field, which particularly affects high-field systems due to the higher technical demands for these magnets. The observable magnet drift over a long spectroscopy experiment can be within the order of the linewidth that can be achieved in 1H-MRS. Spectral distortions due to temporal signal instabilities can be efficiently compensated for by the use of a correction algorithm on the basis of phase and frequency post-correction of the time-resolved raw data or by using additional signals from interleaved acquisition with a navigator scan [38].

A further hurdle for clinical spectroscopy at high-field is the difficulty in building high-sensitivity, high-homogeneity volume coils. Since RF penetration into tissues becomes more demanding [39]. The problems become striking for large structures, where achieving satisfactory field homogeneity is difficult. An inadequate RF coil may sacrifice the SNR gained with the greater field strength. To address these problems, some technical strategies have been proposed leading to a second generation of head coils for 3.0 T and 4.0 T systems [39-41]. Further improvements can be expected with the development of new multi-element coil arrays, low temperature coils and new concepts of coil designs for volume resonators.

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