Mri with dixon-type water/fact separation and prior knowledge about inhomogeneity of the main magnetic field

Method of MR imaging of at least two chemical species having different MR spectra, the method comprising the steps of:

• a1) accessing prior knowledge about the spatial variation of the main magnetic field B0 in the examination volume gained (i) from the design parameters of a main magnet coil or (ii) from measurements of the main magnetic field B0,
• a2) generating two or more echo signals by subjecting a body placed in the examination volume of a MR device to an imaging sequence of RF pulses and switched magnetic field gradients;
• b1) acquiring the echo signals;
• b2) based on the prior knowledge perform a distortion correction of the echo signals to compensate for misregistration arising from both imperfections of the main magnetic field and the magnetic field gradients,
• c) separating signal contributions of the at least two chemical species to the distortion corrected echo signals on the basis of a multi-peak spectral model for at least one of the chemical species and the prior knowledge about the spatial variation of the main magnetic field B0 in the examination volume;
• and
• d) reconstructing a MR image from the signal contributions of at least one of the chemical species.