MRI WITH DIXON-TYPE WATER/FAT SEPARATION WITH ESTIMATION OF THE MAIN MAGNETIC FIELD VARIATIONS

摘要

The invention relates to a method of MR imaging of at least two chemical species having different MR spectra. It is an object of the invention to provide a Dixon water/fat separation technique that avoids swaps of water and fat signals in the reconstructed MR images due to large gradients of the main magnetic field B0. The method of the invention comprises the steps of: a) generating echo signals at different echo times by subjecting an object (10) positioned in a main magnetic field to a multi-echo imaging sequence of RF pulses and switched magnetic field gradients; b) acquiring the echo signals; c) correcting the acquired echo signals by virtual shimming, which comprises:—reconstructing single echo images from the echo signals;—selecting a number of voxel positions;—fitting a given mathematical function, which reproduces the spatial variation of the main magnetic field, to the echo time-dependent phase evolution of the single echo image values at the selected voxel positions; demodulating the single echo images according to the spatial variation of the main magnetic field as reproduced by the mathematical function; and d) reconstructing a MR image, wherein signal contributions of the at least two chemical species are separated on the basis of the demodulated single echo images. Moreover, the invention relates to a MR device and to a computer program to be nm on a MR device.

主权项

1. A method of magnetic resonance (MR) imaging of at least two chemical species having different MR spectra, the method comprising:
a) generating echo signals at different echo times by subjecting an object positioned in a main magnetic field to an imaging sequence of radio frequency (RF) pulses and switched magnetic field gradients; b) acquiring the echo signals; c) correcting the acquired echo signals by virtual shimming, which comprises:
reconstructing single echo images from the echo signals;selecting a number of voxel positions wherein the criterion for the selection of the voxel positions is that single echo images contain only signal contributions from one of the chemical species at respective voxel positions;fitting a given mathematical function, which reproduces the spatial variation of the main magnetic field, to an echo time-dependent phase evolution of the single echo image values at the selected voxel positions;demodulating the single echo images according to the spatial variation of the main magnetic field as reproduced by the mathematical function; and d) reconstructing a MR image, wherein signal contributions of the at least two chemical species are separated on the basis of the demodulated single echo images.