| 摘要 |
A method for homogenizing a static magnetic field with a magnetic field distribution B0(r) for nuclear magnetic resonance spectroscopy by adjusting the currents Ci through the shim coils, thus creating spatial field distributions Ci.Si(r), where r stands for one, two, or three of the spatial dimensions x, y, and z, and said magnetic field distribution B0(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 B1k(r), and with shim coils (6) for homogenizing the magnetic field within the working volume, said shim coils (6) being characterized by their magnetic field distributions per unit current Si(r) and having components only along z, includes the following steps: (a) Mapping the magnetic field distribution B0(r) of the main magnetic field, (b) calculating a simulated spectrum IS(f) based on the sum of the magnetic field distribution B0(r) and the additional field distributions Ci.Si(r) generated by the shim coils (6), and on the sensitivity distributions B1k(r) of the RF coils(5), (c) optimising a quality criterion derived from the simulated spectrum IS(f) by using an optimisation procedure within a search range with the shim currents Ci as a set of parameters, whereby for each new set of parameter values step (b) has to be repeated, (d) realising the found optimum of the quality criterion of step (c) by generating the associated target field distribution B0T(r). In this way a direct one-to-one link is obtained between a set of shim currents and the associated NMR spectrum The quality of the desired NMR spectra can be improved with this method.
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