現代の化学と応用

概要

Gradient Magnetic Field of MR Scanners

Zeljko D. Vujovic*

The topic of this paper is parts of modern MR devices, in which the magnet windings are located. MR scanner magnets are made of four types of electromagnetic windings: The main magnet, made of superconducting material, creates a variable magnetic field; X coil, made of resistive material, creates a variable magnetic field, horizontally, from left to right, across scanning tube; Y coil creates a varying magnetic field, vertically, from bottom to top; Z coil creates varying magnetic field, longitudinally, from head to toe, within scanning tube. Superconductors, which create the main magnetic field, should be cooled by liquid helium and liquid nitrogen. Main magnets made of superconductors should use cryostat, with cooling vessels with liquid helium and liquid nitrogen, thermal insulation, and other protective elements of the magnet system. The types of magnets that exist in the basic configurations of MR scanners are analyzed. Scanners in the form of a closed cylindrical cavity create their own, magnetic, fields by passing a current through the solenoid, which is held at the temperature of the superconductor. The superconductors used exclusively are Niobium-Titanium (NbTi), Niobium-Tin (Nb₃Sn), Vanadium-Gallium (V₃Ga), and Magnesium-Diboride (MgB₂). Only magnesium diboride is a high temperature superconductor, with a critical temperature of Tc=39°K.

The three remaining superconductors are low temperatures. New high-temperature superconductors have been discovered, as well as room-temperature superconductors. Newly discovered superconducting materials are not used in MR scanners. The magnet structure of the MR scanner is complex. The resonant frequency changes at each point of the field in a controlled manner. The windings of the main magnet made of superconducting material in the form of microbial fibers are built into the copper core. The nonlinear gradient field is created by windings of conductive material. It is added to the main magnetic field. Thus, the resulting magnetic field is obtained.