Shim coils, room temperature

Shimming The process of adjusting current flowing through the room temperature shim coils in order to achieve optimal magnetic field homogeneity prior to the acquisition of NMR data. The process may be performed manually or under automation. 

The gradient coils are inserted inside the room temperature shim coils in the magnet (Fig. 2.3.1). Depending on the type of spectrometer, they can be part of the magnet assembly or part of the removable probe assembly. The former are typical for medical... 

Shim coils Coils built into NMR magnets designed to improve the homogeneity of the magnetic field experienced by the sample. Two types of shims are used cryoshims and room temperature shims. Normal shimming involves the use of the room temperature shims. 

The fields with m = 0 are independent of the angle 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. 

The whole NMR imaging sensor system usually consists of a magnet, a shim system mounted inside the room-temperature bore of the magnet, a gradient system mounted inside the shim system and the rf coil mounted inside the gradient system. In the case of a saddle coil or a birdcage resonator, open access can be realized from the bottom to the top of the entire system with the coil diameter. 

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