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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.
Usage in magnetic resonance spectrometry
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. The probe and the sample or the patient get slightly magnetized when brought into the strong magnetic field and create additional inhomogeneous fields. The process of correcting for these inhomogeneities is called shimming the magnet, shimming the probe or shimming the sample depending on what is supposed to be responsible for the remaining inhomogeneity.
A field homogeneity to the order of 1ppm over a volume of several liters is needed in a MRI scanner. High-resolution NMR spectroscopy even demands a field homogeneity better than 1ppb in a volume of a few milliliters.
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 measure volume root mean square (VRMS) values of the field for the different (mostly concentric) volumes in the middle of the scanner.
- Weston A. Anderson, Electrical Current Shims for Correcting Magnetic Fields; Rev. Sci. Instrum. 32, 241 (1961)
- F. Roméo, D. I. Hoult, Magnet field profiling: analysis and correcting coil design; Magnetic Resonance in Medicine 1(1), 44-65 (1984)
- Gerald A. Pearson, SHIMMING AN NMR MAGNET