Overview of basic components

The magnet generates the magnetic field which polarizes the nuclear magnetic moments. Its strength determines the degree of polarization and thus enters the sensitivity of the measurement. Furthermore, it determines the measurement frequency as well as 

The starting point for the calculation of magnetic field distributions for static fields, field gradients, and if fields is the Biot-Savart law. It determines the magnetic field B at a distance r from the wire element dl created by a current of strength / 

By integration, the field at the centre of a current loop of radius a can be obtained. Two such current loops spaced a apart in parallel and concentric configuration form the Helmholtz coil, which exhibits a uniform field about the centre. 

The shim coils are constructed based on an expansion of the magnetic field inhomogeneity in terms of spherical harmonics. The fields produced by the coils are orthogonal and can be adjusted independent of each other. Following (2.1.18) the shim fields Bs m are characterized by the associated Legendre polynomials P m (cos 6) and the expansion 

The fields with m = 0 are independent of the angle p. They are called zonal fields. The others are called tesseral fields. Some shim coils for production of tesseral and zonal fields are sketched in Fig. 2.3.2 [Krel, Mori], Room temperature shims may provide correction fields up to = 5, while cryoshims frequently provide corrections only up to n = 1. In this way homogeneities of 2 ppm can be reached over 30 cm and 5 ppm over 50 cm diameter volumes. 

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