Patent ID: 7332912

Claim:
A method for correcting inhomogeneity of a static magnetic field generated by a magnetic structure of a machine for acquiring nuclear magnetic resonance images, the method comprising: a) defining a surface enclosing a spatial volume permeated by the magnetic field of the magnetic structure; b) describing said surface by a system of three-dimensional coordinates; c) measuring a value of components of the magnetic field along at least one direction of said system of three-dimensional coordinates; d) describing the components of the magnetic field along at least one direction by a series expansion based on an orthogonal set of functions f n truncated at a predetermined finite maximum order; e) for at least one component of the magnetic field, defining an inhomogeneity vector whose components comprise coefficients of the series expansion of step (d), said vector describing a trend of the corresponding component of the magnetic field; f) from the measured value of at least one component of the magnetic field, obtaining numerical values of the components of the inhomogeneity vector; g) defining a multi-dimensional grid that defines a number of positioning points P, each having a predetermined position with respect to the spatial volume in which the magnetic field inhomogeneity is to be corrected; h) defining an effect vector d p that describes an effect of a component of a magnetic field of a dipole D p positioned in any of the positioning points P on the corresponding component of the magnetic field in the spatial volume, wherein components of the effect vector d p comprise coefficients of a series expansion that describes said component of the magnetic field generated by said dipole based on a set of orthogonal functions f n truncated at an order no less than the order of the series describing the magnetic field in the spatial volume; i) defining a linear combination of the effect vectors of each of said dipoles D p describing a combined effect on at least one component of the magnetic field in the spatial volume generated by a set of the dipoles each lying in a different position on the grid and each provided with a magnetic moment; j) determining a number, position and magnetic moment of compensation magnetic dipoles that minimize a norm of a difference between said linear combination of effect vectors of said dipoles and the inhomogeneity vector of the corresponding component of the magnetic field in the spatial volume; k) positioning said compensation dipoles on the grid in corresponding positions; wherein a function is minimized such that a total magnetic moment of said compensation dipoles for compensating the magnetic field inhomogeneity is minimized and said norm of the difference between the inhomogeneity vector of each component of the magnetic field and the linear combination of effect vectors corresponding to said component of the magnetic field of said compensation dipoles is minimized.