A modified rotating frame experiment based on a fourier series window function. Application to in vivo spatially localized NMR spectroscopy

A fourier series window approach to the rotating frame experiment is employed in conjunction with surface coils to accomplish one-dimensional spatial localization of phosphorus-containing metabolites in phantoms and the in vivo rat brain. The rotating frame experiment delineates sample regions on the basis of a B₁ field gradient across the sample which for the experiments described in this paper are provided by a surface coil. In the usual rotating frame experiment, a suite of spectra (termed a metabolite map) is obtained by incrementing the evolution pulse duration with the spectra displayed as a function of the nutational frequency about the B₁ field. The total time of the experiment depends on the desired degree of spatial resolution, and substantial time savings can be obtained when the desired result is spectral information from a single or a few selected regions (slices). The approach taken here is to utilize a Fourier series window function representation of the nutational frequencies over the region of interest. This procedure allows convenient selection of the localized region, thus avoiding Fourier transformation in the second (mapping) dimension. For the case of a single region of interest, the spectral parameters can be adjusted to maximize the number of data files in which the evolution pulse nutates the magnetizations at the center of the slice through a multiple of 180°. Data files not contributing significant signal to a selected slice can be omitted to further minimize the experimental accumulation time.