Patent ID: 7609060

Claim:
A method for mapping the inhomogeneity of a magnetic field within an object in an MRI system, said object containing a first and a second material comprising the steps of: a. acquiring a plurality of images with said MRI system at a corresponding plurality of evolution times, b. calculating the phase contribution to the plurality of images due to the second material, c. removing said contribution from each of said plurality of images to obtain a phase map reflective of the inhomogeneity of said magnetic field, d. unwrapping aliasing present in the phase maps of images having relatively longer evolution times by reference to phase maps of images having relatively shorter evolution times, and e. generating a single map of magnetic field inhomogeneity from the unwrapped results, wherein said plurality of images comprises a first image set of at least three images separated by a predetermined evolution time interval, Δt, and a second image set of n≧3 images each having evolution times respectively corresponding to ascending evolution delays Δt 1 -Δt n , said evolution delays measured relative to a reference image corresponding to evolution delay Δt 0 =0, wherein the predetermined evolution time interval Δt is a function of the chemical shift between said first and second material, and Δt is defined by the equation Δt=½Δv where Δv is the chemical shift between said first and second material in Hertz, wherein the step of calculating the phase contribution to the plurality of images due to the second material comprises calculating a Dixon image from the images in said first image set, and wherein the step of unwrapping aliasing is performed on images in the second image set.