Non iterative shimming in magnetic resonance imaging in the presence of high LIPID levels

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 Δt1-Δtn, said evolution delays measured relative to a reference image corresponding to evolution delay Δt0=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.