ShimIt
Software Shimming of NMR and MRI Magnets
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The locking (see program LockIt) keeps the magnetic field drift from broadening the spectral resonances during the data acquisition. The second major source of bad spectral lineshapes is an inhomogeneous magnetic field during the data acquisition. The times when electro-magnets were adjusted with metal wedges ("shims") are over, but the homogeneity adjustment continues to be called "shimming".

In contrast to common wisdom, shimming requires neither special hardware (field mapping equipment, gradient probes, etc.), human experts (read "spectroscopists"), nor "artificial intelligence" software. A simple iterative lineshape moment minimization can be mathematical proven to lead to the optimal lineshape as measured by the chosen lineshape moment. Yes, the iterative optimization of the signal height ("lock level") is bound to get stuck in a local optimum corresponding to a bad lineshape. But this simply means that the signal height is not appropriate for anything but for a slight touch-up of a nearly optimal shim setting. The lineshape moment criterion (when properly implemented) does not have such a problem.

ShimIt implements the algorithm and can be used to shim magnets from scratch. See the ShimIt Manual or download its PDF version for details. Please contact Mail: dunkel@ScienceSoft.net if interested in obtaining a copy of ShimIt.

ShimIt Advantages

Reliable, automated shimming technology

  • It has been mathematically proven that this shimming algorithm can't encounter local optima.
  • Reliably shims large numbers of non-orthogonal shims from scratch. 
  • Reliably distinguishes between shimming and instrumental problems.

Model independent shimming

  • Works with every spectrometer (which has computer controlled shims).
  • Works with every probe (cryo-probes, nano-probes, solid-state probes, etc.).
  • Can shim arbitrary shim sets and the shims don't have to be orthogonal.
  • No lock circuit or lock solvent is necessary.

Pure software solution

  • No hardware modifications required.
  • Easy to distribute and install (one floppy disk).
  • Full control over acquisition parameters and shimming conditions.

Improvements over current automated shimming technology

  • Allows shimming of new magnets and probes.
  • Shims room temperature shims from scratch (~ 1h).
  • Accurately adjusts higher-order and non-spinning shims.
  • Produces an excellent baseline and a narrow signal base.

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