Patent ID: 7605589

Claim:
A method for homogenising a static magnetic field with a magnetic field distribution B 0 (r) for nuclear magnetic resonance spectroscopy by adjusting the currents C i through the shim coils, thus creating spatial field distributions C i ·S i (r), where r stands for one, two, or three of the spatial dimensions x, y, and z, and said magnetic field distribution B 0 (r) has only a field component along z, in a working volume of a magnetic resonance apparatus with one or more radio frequency (=RF) coils ( 5 ) for inducing RF pulses and receiving RF signals within a working volume, said RF coils having a spatial sensitivity distribution of magnitudes B 1 k (r), and with shim coils ( 6 ) for homogenising the magnetic field within the working volume, said shim coils ( 6 ) being characterised by their magnetic field distributions per unit current S i (r) and having components only along z, the method including the following steps: (a) Mapping the magnetic field distribution B 0 (r) of the main magnetic field, (b) calculating a simulated spectrum I s (f) based on the sum of the magnetic field distribution B 0 (r) and the additional field distributions C i ·S i (r) generated by the shim coils ( 6 ), and on the sensitivity distributions B 1 k (r) of the RF coils( 5 ), (c) optimising a quality criterion derived from the simulated spectrum I s (f) by using an optimisation procedure within a search range with the shim currents C i as a set of parameters, whereby for each new set of parameter values step (b) has to be repeated, the quality criteria being the smoothness of spectral line, and/or the symmetry of a spectral line, and/or the splitting between neighbouring lines, and/or the height of an envelope Env(f) for spectral lines, and/or the full-width-at-half-maximum of the envelope Env(f) and/or the remaining difference between the envelope Env(f) and the spectrum I s (f), and (d) realising the found optimum of the quality criterion of step (c) by generating the associated target field distribution B 0 T (r).