Shimming defects

Figure 3.46. Lineshape defects that arise from inappropriate settings of various shims. These effects have been exaggerated for the purpose of illustration.

A portion of the magnet bore tube space is occupied by a sleeve of insulated wires called a room temperature (RT) shim set. The RT shim set should never be removed from a magnet s bore tube unless it is defective. On the other hand, the NMR probe fits inside the RT shim set in the bore and can be more or less freely exchanged with other NMR probes, depending on need. Both the RT shim set and the NMR probe are inserted into the NMR magnet s bore tube from the bottom. In most cases there will be an upper magnet bore assembly (also called the upper bore tube) through which the spinner/tube assembly will travel as the spinner/tube assembly is raised and lowered pneumatically. 

To provide an indication of progress when shimming, one requires a suitable indicator of field homogeneity. Essentially, there are three schemes that are in widespread use, all of which have their various advantages and disadvantages (1) the lock level, (2) the shape of the FID and (3) the shape of the NMR resonance. The ultimate measure of homogeneity is the NMR resonance itself, since defects apparent in the spectrum can often be related... 

J.H. Shim, T.S. Moon, M.J. Yun, Y.C. Jeon, C.M. Jeong, D.W Cho, J.B. Huh. Stimulation of healing within a rabbit calvarial defect by a PCL/PLGA scaffold blended with TCP using solid freeform fabrication technology. / Mater Sci Mater Med 23,2993-3002,2012. 

Shim H and Harris ZL (2003) Genetic defects in copper metabolism. Journal of Nutrition 133 1527S-1531S. 

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