Accurate and convenient evaluation method of magnetic field

An evaluation method for the magnetic field employed in an MRI (magnetic resonance imaging) system is described. In the past, the magnetic-field distribution was approximated by a linear combination of spherical harmonics, and the coefficients were used for the evaluation of the field homogeneity. The accuracy of the evaluation method presented is demonstrated by comparisons between field distributions computed from the coefficients and measurements. The results give an estimate for the number of coefficients needed for a good approximation. The contour map computed in this way conveniently given the field distribution from measurements at only a few points. The image distortion estimated from the field distribution is demonstrated to be a valuable index for evaluating the specification of the magnetic circuit in an MRI system.>