High-resolution NMR chemical-shift imaging with reconstruction by the chirp z-transform

A study of a novel nuclear magnetic resonance (NMR) chemical-shift image reconstruction method with a high chemical-shift resolution achieved by the chirp z-transform (CZT) is presented. Phase encoding is used for the spatial coordinates x and y, and the frequency coordinate is reserved especially for the chemical shift. The Fourier transform (FT) image reconstruction algorithm, which forms the basis of the new CZT image reconstruction method, is introduced. The novel method, using the CZT instead of the FT to evaluate the chemical-shift spectrum at a much higher resolution, is studied. The chemical-shift resolutions, achieved by the FT and the CZT, are studied theoretically from the aspect of the peak height and the peak width of chemical-shift spectra. The chemical-shift spectra calculated at a selected point in the image plane, and the chemical shift-images reconstructed by this method, are shown for a simple phantom containing ethanol and methanol at different locations. The results obtained by this method and by the FT method are compared and discussed. The experimental results have shown that a chemical-shift as small as 39 Hz, relative to the proton resonance frequency of 21.34 MHz, can be resolved successfully by this method without improvements in magnetic field homogeneity