Shim (magnetism)

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Free induction decay (short: FID, from the English “Free Induction Decay”) of the nuclear magnetic resonance of a poorly homogenized apparatus.
Free induction decay of the nuclear magnetic resonance of a well homogenized apparatus.

A shim (eng "shim". Wedge shim device) is a device for the homogenization of magnetic fields . The term comes from English and originally means washer or wedge. The term “electrical current shims” for current-carrying coils, which have assumed the same function, was derived from the purely mechanical means with which the position and parallelism of the pole shoes of an electromagnet was once set. They achieve this by generating magnetic fields that are significantly weaker than the field to be homogenized, with which they are superimposed .

The auxiliary coils are designed in such a way that their fields in the sample can be described with simple spherical surface functions , since interference fields, the cause of which are spatially remote from the sample, also (approximately) have the shape of such spherical surface functions of low order in them.

With shimming, the inhomogeneities of the magnetic field are eliminated by appropriately attaching additional coils (active shimming) or magnetized iron parts (passive). The correction fields generated in this way superimpose the field of the main coil and homogenize it.

application

Shims are used for the correct operation of a magnetic resonance tomograph , but primarily with significantly more powerful devices for scientific research.

Individual evidence

  1. ^ Weston A. Anderson: Electrical Current Shims for Correcting Magnetic Fields . In: Review of Scientific Instruments / NS , Vol. 32 (1961), pp. 241-250, ISSN  0034-6748 .
  2. X25 Undulator Magnet Shimming ( Memento from May 25, 2013 in the Internet Archive ) Comparison of magnetic fields before and after homogenization, status: March 12, 2006

literature

  • Gerald A. Pearson, Shimming At NMR Magnet . Pp. 1-16.
  • Pierre-Marie Robitaille, Lawrence J. Berliner (Ed.): Ultra high field magnetic resonance imaging (Biological Magnetic Resonance; Vol. 26). Springer, New York 2006, ISBN 0-387-34231-1 .