Shimming

What is shimming and why is it needed?
Even following the most rigorous manufacturing tolerances, the homogeneity of an MR magnet arriving fresh from the factory will likely be two orders of magnitude away from ideal specifications levels. Once the magnet is sited in the imaging suite, its field will be further distorted by the presence of metal in pipes, wires, ducts, and structural beams in the immediate environment.  Fringe fields of nearby scanners may also affect the field of the newly installed magnet. 

Shimming is the process by which the main magnetic field (Bo) is made more homogenous.
Shimming may be passive, active, or both. In passive shimming small pieces of sheet metal or ferromagnetic pellets are affixed at various locations within the scanner bore. In active shimming, currents are directed through specialized coils to further improve homogeneity.  Each of these shimming techniques will be described in detail in the next two questions. 

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References
      Flasche M, Fischer D. White Paper: Magnet homogeneity and shimming. Siemens Healthcare GmbH, 2017.   
      Keller P.  Technologies for precision field mapping. MetroLab Instruments (www.metrolab.com), Geneva, 2006.
     Wendt M. Second order shimming of high field magnets. Siemens Medical Systems.  A sales brochure but contains good information on active shims and spherical harmonics. 
Related Questions
     An MR salesman claims his scanner has better "homogeneity" than that of his competitor.  What is he talking about? 
     How is passive shimming performed?
     How does active shimming work?