Shim - Wikipedia, the free encyclopedia: "Nuclear magnetic resonance / Magnetic resonance imaging
In NMR or MRI, 'shimming' is used prior to the operation of the magnet to eliminate inhomogeneitites in its field.
Initially magnetic field inside a MR scanner is far from being homogeneous. It could be even 100 times worse with respect to its homogeneity than an 'ideal' field of the scanner. This is a result of the production tolerances and magnetic field of the 'environment' - iron constructions in walls and floor of the examination room gets magnetized and disturb field of the scanner.
There are two types of shimming: active, and passive. The active shimming is done using coils with adjustable current. The passive shimming involves pieces of steel with good magnetic qualities. The steel is placed in the neighbourhood of the permanent of superconducting magnet. It gets magnetized and produces its own magnetic field. Additional magnetic field (produced by coils or steel) adds to the magnetic field of the magnet in such a way that total field is getting more homogeneous."
Thursday, May 10, 2007
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