A single-shot shimming sequence using low-power RF noise pulses for localized in vivo NMR spectroscopy

Abstract

The authors describe a technique for spatially localized nuclear magnetic resonance (NMR) in vivo clinical spectroscopic examinations. It concerns the optimization of the magnetic field homogeneity ('shimming'). A localizing shimming pulse sequence designed specifically for use with the 'ISIS' spectroscopic localization pulse sequence is presented. The shimming sequence employs reduced bandwidth RF noise pulses to randomize unwanted signals from outer regions of the sample. By reducing the bandwidth of the noise pulses substantial RF power reduction is achieved (e.g. a factor of five), without impairment of localization performance. Identical gradient amplitudes and timings are used for both the shimming and data acquisition sequences, therefore the effects of induced eddy currents can be evaluated and minimized during the shimming process. Experimental data demonstrating the use of the method in vivo are presented.