Method of magnet shimming

Details Method of magnet shimming Method of magnet shimming

1995-12-14

1997-04-01

Arana, Louis M.

Electricity: measuring and testing

Particle precession resonance

Spectrometer components

324319, G01V 300

Patent

active

056170297

ABSTRACT:
A method is described which corrects magnetic field inhomogeneities in a magnetic resonance (MR) imaging system that takes into account the effects of a subject on the magnetic fields. A magnetic resonance sequence is applied to the subject to extract MR response signals from the subject indicating the magnetic field inhomogeneity over space. The changes of magnetic field inhomogeneity with a change in radial variable .rho. (or r, in a cylindrical coordinate system) are calculated. A derivative with respect to a radial variable is calculated of a generalized 3D infinite series polynomial, such as a Legendre polynomial expressed in spherical coordinates. These inhomogeneity changes are then fit to 3D polynomial derivative to result in coefficients [dI.sub.c ]. Similarly coefficients of a derivative shim coil calibration matrix [dM] are determined for each shim coil modeling the effect of each shim coil is on the magnetic field within the imaging volume. An inverse derivative matrix [dM] is calculated to result in the inverse derivative matrix [dM.sup.-1 ]. The inverse derivative matrix [dM.sup.-1 ] is multiplied by the coefficients [dI.sub.c ] to determine current coefficients [C] which define currents to be passed through each shim coil to correct the magnetic field inhomogeneities.

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