Magnetic Field Homogeneity Adjustment for Magnetic Resonance Imaging Equipment

Imaging based on magnetic resonance (MR) methods needs a source of the highest homogeneous magnetic field in the whole volume of the investigated object. The goal of this paper is to demonstrate three methods of correcting coils (shim coils) current calculations and homogeneity adjustment on a whole-body MR imager, based on three methods for static magnetic field measurements. Measurements are carried out using: magnetometer based on the Hall effect, nuclear MR point magnetometer, and gradient-echo imaging method applied to circular test tubes. All three measurements measure the magnetic field on a circle (12 measuring points) and one point in the center. A set of linear equations, determination of a target function, and optimization computations are procedures that provide optimal values of currents for correcting coils. Results of all three methods are compared and discussed. The proposed methods are suitable for periodic testing and correction of magnet inhomogeneities for MRI magnets.