Beyond k-space: spectral localization using higher order gradients.

Chemical shift imaging (CSI) often suffers from the inconvenient shape of its spatial response function (SRF), which affects both localization and signal-to-noise ratio. Replacing the magnetic field gradients for phase encoding by higher order magnetic fields allows a better adjustment of the SRF to the structures in the sample. We combined this principle with the SLOOP (spectral localization with optimal pointspread function) technique to simultaneously obtain spectra from several arbitrarily shaped compartments within a sample. Linear combinations of the fields of the shim coils are used to generate the pulsed fields for phase encoding. Their shapes are matched to the given sample geometry by numerical optimization. Using this method, spectra from a phantom were obtained that show a higher signal-to-noise ratio and a strongly reduced contamination compared to an equivalent CSI experiment. Copyright 1999 Academic Press.