Method for making a permanent magnet particularly for mri scanners and a permanent magnet, particularly for mri scanners made by said method

Method for making magnets particularly to be used in MRI scanners, which magnets are three-dimensional and have a tubular wall made of magnetized material, with a longitudinal axis and with a cross-section with respect to said longitudinal axis, which cross-section has a closed or open annular shape,

• the tube-like wall being composed of individual elements made of magnetized material having a longitudinal extension parallel to the axis of the tubular shape and they are arranged one after the other;
• all the adjacent elements made of magnetized material or all of adjacent elements except for two subsequent elements of magnetized material being provided one in contact to the other along at least a peripheral surface or a linear edge thereof having a longitudinal extension parallel to the axis of the tubular shape;
• the magnetization of each element made of magnetized material having a predetermined direction in the plane perpendicular to the longitudinal axis of the tubular wall and said directions being defined such to generate a uniform static magnetic field having a predetermined direction which field passes through the cavity delimited by the tube-like wall,
• said method comprising the steps of:
• a) determining the modulus and direction of the magnetization vector of invidividual elements made of magnetized material by calculating for each magnetized element the modulus and direction of the magnetization vector necessary for generating a static magnetic field having a predetermined strength and a predetermined direction and which field passes through the cavity delimited by the tube-like wall on the basis of a theoretical two-dimensional model of the magnet which corresponds to a cross-section thereof;
• b) calculating changes of the static magnetic field into the tube-like wall without modifying magnetization vectors of individual magnetized elements in the condition of the two-dimensional model extended to a three-dimensional model obtained by projecting the two-dimensional model, that is the structure of the magnet in the section plane, along an axis perpendicular to the said section plane and coinciding with the longitudinal axis of the tubular wall such that said tubular wall in the direction of said axis has a predetermined extension greater than zero and smaller than infinity;
• c) modifying the direction of magnetization vectors of at least some of the magnetized elements in the section plane perpendicular to the longitudinal axis of the tube-like wall such to minimize differences of the characteristics of the static magnetic field determined on the basis of the three-dimensional model with respect to the characteristics obtained on the basis of the two-dimensional model;
• d) providing magnetized elements having geometrical shapes corresponding to the ones of the three-dimensional model and having a magnetization with a modulus and direction corresponding to the ones set at step c);
• and
• e) assembling the magnet by mounting individual magnetized elements in the relative position provided by the three-dimensional model.