Shim coil current adjustment with NMR MRI magnetic field inhomogeneity determination - applying sequence of RF and bipolar gradient pulses, Fourier transforming resonance, repeating with various directions of gradient pulse and producing fitted polynomial corresp. to field spherical harmonic functions.

摘要

The magnetic field in an investigation vol. is mathematically represented as spherical harmonic functions and shim coils produce a field in the investigation vol. corresp. to a defined coefficient of the spherical harmonic functions. A gradient echo sequence contg. a high frequency pulse (RF) and a bipolar gradient pulse (G) in a defined direction is used and the resulting spin resonance signal (s) detected and Fourier transformed to determine the phase curve of the spin in the defined direction. This is repeated for different directions of the bipolar gradient pulse and the phase curves analysed using a fit method. The coefficients of the spherical harmonic functions describing the field distribution are derived from the resulting polynomial coefficients and the associated shim coil currents determined. ADVANTAGE - Enables shimming of magnetic field of nuclear spin resonance equipment with very high degree of homogeneity achieved in short time.