Shim (magnetism)
Encyclopedia
A shim is a device used to adjust the homogeneity of a magnetic field
. Originally, shims have their name from the purely mechanical shims that are used to adjust position and parallelity of the pole faces of an electromagnet. Coils that are used to adjust the homogeneity of a magnetic field by changing the current flowing through it were called "electrical current shims" because of their similar function.
In NMR
and MRI
, shimming is used prior to the operation of the magnet to eliminate inhomogeneities in its field.
Initially the magnetic field inside an NMR spectrometer or MRI scanner will be far from homogeneous. It could be up to 100 times worse with respect to its homogeneity than an "ideal" field of the device. This is a result of the production tolerances and magnetic field of the "environment" - iron constructions in walls and floor of the examination room get magnetized and disturb the field of the scanner.
There are two types of shimming: active and passive.
Active shimming is done using coils with adjustable current.
Passive shimming involves pieces of steel with good magnetic qualities. The steel pieces are placed near the permanent or superconducting magnet. They get magnetized and produce their own magnetic field.
In both cases, the additional magnetic fields (produced by coils or steel) add to the overall magnetic field of the superconducting magnet in such a way that the total field becomes more homogeneous.
There are different ways to define inhomogeneity of a magnetic field in the center of the MR spectrometer. Right now, for medical MR scanners, the industry standard is to mention VRMS values of the field for the different (mostly concentric) volumes in the middle of the scanner.
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
. Originally, shims have their name from the purely mechanical shims that are used to adjust position and parallelity of the pole faces of an electromagnet. Coils that are used to adjust the homogeneity of a magnetic field by changing the current flowing through it were called "electrical current shims" because of their similar function.
Usage in magnetic resonance spectrometry
Nuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...
and MRI
Magnetic resonance imaging
Magnetic resonance imaging , nuclear magnetic resonance imaging , or magnetic resonance tomography is a medical imaging technique used in radiology to visualize detailed internal structures...
, shimming is used prior to the operation of the magnet to eliminate inhomogeneities in its field.
Initially the magnetic field inside an NMR spectrometer or MRI scanner will be far from homogeneous. It could be up to 100 times worse with respect to its homogeneity than an "ideal" field of the device. This is a result of the production tolerances and magnetic field of the "environment" - iron constructions in walls and floor of the examination room get magnetized and disturb the field of the scanner.
There are two types of shimming: active and passive.
Active shimming is done using coils with adjustable current.
Passive shimming involves pieces of steel with good magnetic qualities. The steel pieces are placed near the permanent or superconducting magnet. They get magnetized and produce their own magnetic field.
In both cases, the additional magnetic fields (produced by coils or steel) add to the overall magnetic field of the superconducting magnet in such a way that the total field becomes more homogeneous.
There are different ways to define inhomogeneity of a magnetic field in the center of the MR spectrometer. Right now, for medical MR scanners, the industry standard is to mention VRMS values of the field for the different (mostly concentric) volumes in the middle of the scanner.
Further reading
- Gerald A. Pearson, SHIMMING AN NMR MAGNET, http://nmr.chem.uiowa.edu/manuals/Shimming-GAP-NMR-magnet.pdf