Patent ID: 7348775

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 ), a simulation process of the spectrum I S (f) comprising: i. choice of a “virtual” NMR sample substance and a “virtual” experiment, ii. incorporation of hardware properties of RF coils ( 5 ) and/or gradient coils ( 4 ), used in the planned NMR experiment, in particular of the sensitivity distributions B 1 k (r) of the RF coils ( 5 ), in the calculation, iii incorporation of the actual sample volume into the calculation; (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; and (d) realising the found optimum of the quality criterion of step (c) by generating the associated target field distribution B 0 T (r).