US 6,634,088 B1Granted

Method and apparatus for shimming a magnet to control a three-dimensional field

AssigneeFonar Corporation

InventorTerry Morrone

Life of this patent

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Abstract

A magnetic field produced by a magnet is measured and modeled as a sum of additive components, each having a different symmetry with respect to the three-dimensional field. The sum of all the additive components completely describes the measured magnetic field, without approximation due to truncation of terms. The degree of non-homogeneity in the measured field is determined by examining the additive components individually. A shim is designed to correspond to a magnitude change of an additive component in order to improve the homogeneity of the magnetic field, such as in an MRI application. The shim is placed in a predetermined location against a metal plate that is connected to or placed near a pole of the magnet, and a plastic plate is attached to the metal plate to hold the shim in place. Grooves are machined into corresponding symmetrical plate locations to complement the shim effect.

Claims26

Dependent claims

A method for passively shimming a magnet to control a three-dimensional field, comprising:

measuring a magnetic field produced within a predetermined volume by the magnet, at selected points within the volume;
modeling the measured magnetic field in the form of a finite plurality of additive components, wherein a sum of all of the plurality of additive components completely describes the measured magnetic field, without approximation due to truncation of terms;
detecting the degree of homogeneity in the measured field within the predetermined volume by examining measured magnetic field values at ones of the selected points corresponding to individual ones of the plurality of additive components;
determining which additive component should be modified in order to change the homogeneity detected in the measured field;
and
coupling a metal element that provides the determined additive component modification to the measured magnetic field when coupled to the magnet;
wherein the additive components are gi such that for the field f ⁢ ⁢ ( x , y , z ) = ∑ i = 1 8 ⁢ ⁢ g i , 8g1=f(x, y, z)+f(−x, y, z)+f(x, −y, z)+f(−x, −y, z)+f(x, y, −z)+f(−x, y, −z)+f(x, −y, −z)+f(−x, −y, −z);
8g2=f(x, y, z)+f(−x, y, z)+f(x, −y, z)+f(−x, −y, z)−f(x, y, −z)−f(−x, y, −z)−f(x, −y, −z)−f(−x, −y, −z);
8g3=f(x, y, z)+f(−x, y, z)−f(x, −y, z)−f(−x, −y, z)+f(x, y, −z)+f(−x, y, −z)−f(x, −y, −z)−f(−x, −y, −z);
8g4=f(x, y, z)+f(−x, y, z)−f(x, −y, z)−f(−x, −y, z)−f(x, y, −z)−f(−x, y, −z)+f(x, −y, −z)+f(−x, −y, −z);
8g5=f(x, y, z)−f(−x, y, z)+f(x, −y, z)−f(−x, −y, z)+f(x, y, −z)−f(−x, y, −z)+f(x, −y, −z)−f(−x, −y, −z);
8g6=f(x, y, z)−f(−x, y, z)+f(x, −y, z)−f(−x, −y, z)−f(x, y, −z)+f(−x, y, −z)−f(x, −y, −z)+f(−x, −y, −z);
8g7=f(x, y, z)−f(−x, y, z)−f(x, −y, z)+f(−x, −y, z)+f(x, y, −z)−f(−x, y, −z)−f(x, −y, −z)+f(−x, −y, −z);
and
8g8=f(x, y, z)−f(−x, y, z)−f(x, −y, z)+f(−x, −y, z)−f(x, y, −z)+f(−x, y, −z)+f(x, −y, −z)−f(−x, −y, −z);
wherein x, y, and z are coordinates in a three-dimensional coordinate system, and the summation of the additive components completely describes the measured field f(x, y, z) without truncating terms.

13 more elements

An apparatus for changing a homogeneity of a three-dimensional measured magnetic field produced by a magnet having a pole, comprising:

a first plate for placement near enough to the pole of the magnet so as to have an effect on the measured magnetic field;
a shim for placement near enough to the first plate so as to have an effect on the measured magnetic field;
and
a second plate for attachment to the first plate such that the shim is held in place in a predetermined location between the first plate and the second plate;
wherein the shim corresponds to a magnitude change of an additive component of an equation describing the measured magnetic field, in order to change the homogeneity of the measured magnetic field;
wherein the equation consists of a finite plurality of additive components, the sum of which completely describes the measured magnetic field, without approximation due to truncation of terms, such that f ⁢ ⁢ ( x , y , z ) = ∑ i = 1 8 ⁢ ⁢ g i ,
and the additive component is one of gi, i={1,2,3,4,5,6,7,8}, wherein
8g1=f(x, y, z)+f(−x, y, z)+f(x, −y, z)+f(−x, −y, z)+f(x, y, −z)+f(−x, y, −z)+f(x, −y, −z)+f(−x, −y, −z);
8g2=f(x, y, z)+f(−x, y, z)+f(x, −y, z)+f(−x, −y, z)−f(x, y, −z)−f(−x, y, −z)−f(x, −y, −z)−f(−x, −y, −z);
8g3=f(x, y, z)+f(−x, y, z)−f(x, −y, z)−f(−x, −y, z)+f(x, y, −z)+f(−x, y, −z)−f(x, −y, −z)−f(−x, −y, −z);
8g4=f(x, y, z)+f(−x, y, z)−f(x, −y, z)−f(−x, −y, z)−f(x, y, −z)−f(−x, y, −z)+f(x, −y, −z)+f(−x, −y, −z);
8g5=f(x, y, z)−f(−x, y, z)+f(x, −y, z)−f(−x, −y, z)+f(x, y, −z)−f(−x, y, −z)+f(x, −y, −z)−f(−x, −y, −z);
8g6=f(x, y, z)−f(−x, y, z)+f(x, −y, z)−f(−x, −y, z)−f(x, y, −z)+f(−x, y, −z)−f(x, −y, −z)+f(−x, −y, −z);
8g7=f(x, y, z)−f(−x, y, z)−f(x, −y, z)+f(−x, −y, z)+f(x, y, −z)−f(−x, y, −z)−f(x, −y, −z)+f(−x, −y, −z);
and
8g8=f(x, y, z)−f(−x, y, z)−f(x, −y, z)+f(−x, −y, z)−f(x, y, −z)+f(−x, y, −z)+f(x, −y, −z)−f(−x, −y, −z);
wherein the additive component has a particular symmetry with respect to a three-dimensional coordinate system oriented with respect to a location of the pole.

14 more elements

Dependencies are inferred from the claim text.