MRI Shim Coils

Shim Coils

 

Most modern MRI techniques (e.g. EPI, chemical shift imaging) require magnetic fields homogeneous to less than 3.5 parts per million (ppm) over the imaging volume. The raw field produced by a superconducting magnet is approximately 1000 ppm or worse, thus the magnetic field has to be corrected or shimmed. Usually this is accomplished by a combination of current loops (active or dynamic shims) and ferromagnetic material (passive or fixed shims). Gradient coils are used to provide a first-order shim. The patient distorts the magnetic field when put into the scanner and so an active shim correction must be made before scanning. The operator will perform active shimming to improve the homogeneity on an individual patient basis