Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Patent
1988-05-04
1989-08-29
Levy, Stewart J.
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
324320, G01N 2406
Patent
active
048620858
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a method for regulating the magnetic field delivered by a resistive magnet for eliminating any drifting of this field due more particularly to the temperature variations caused mainly by the Joule effect dissipation of the magnet itself; the invention also relates to a resistive magnet system using this method and, by way of application, an installation for forming images by nuclear magnetic resonance (NMR) incorporating such a system.
It is known that NMR image forming installations, among others, require a large sized magnet capable of generating a uniform magnetic field, called: basic field, in a given region in space. Typically, it is necessary to generate a field of 0.15 to 0.5 teslas with a homogeneity of 1 to 10 parts per million (ppm) in a sphere of a diameter of 50 cm at least.
Among the different types of magnets currently used for creating such fields, there may be mentioned the resistive magnets formed of one or more coils arranged, very often without a ferromagnetic shrouding, so as to obtain the required homogeneity. The stabilization of the operating point of such a magnet requires a considerable time for it is impossible to determine the value of the field in the center of the "sphere of homogeneity" because this volume is reserved for the patient and means for measuring the field cannot be installed therein and especially because, during operation, the magnetic field is disturbed by the subsidiary fields generated by the gradient coils of the NMR image forming installation.
The stability and uniformity of the magnetic field are then only really acquired when the temperature of the magnet has become completely stabilized, which does not exclude subsequent drifting, which is difficult to master. Because of all these difficulties, it has never been a question up to now of considering the value of the basic field as a parameter capable of being chosen by the operator in NMR image formation.
The purpose of the invention is to overcome all the disadvantages of the prior technique mentioned above.
SUMMARY OF THE INVENTION
To this end, the invention provides a method of regulating the magnetic field delivered by a main resistive magnet cooled by a fluid flow and creating a field of required homogeneity in a given region in space, which consists:
in electrically supplying an auxiliary magnet and the main magnet from an adjustable common DC source,
in placing said auxiliary magnet in thermal contact with said fluid so that said auxiliary magnet is substantially at the temperature of said main magnet,
in measuring a reference magnetic field representative of the main field and created by said auxiliary magnet, and
in adjusting said DC source common to the two magnets so as to stabilize said reference field at a chosen value.
Preferably, the two magnets are connected in series and the value of the current which flows therethrough is varied.
According to another important feature of the invention, the reference magnetic field is measured by using the nuclear magnetic resonance phenomenon on a sample of chosen atoms, placed in a zone of said reference magnetic field where this latter has sufficient homogeneity for using said phenomenon.
"NMR" probes containing said sample of chosen atoms are known and proposed commercially. It is desirable for the homogeneity of the reference magnetic field to be large (comparable to that of the field of the main magnet) throughout the zone occupied by the sample so that the resultant NMR phenomenon concerns the largest number possible of nuclei of the sample and gives rise to a detectable phenomenon, the conditions of magnetic resonance being as is known directly related to the value of the magnetic field and to that of the frequency of excitation of the nuclei.
The invention also relates to a resistive magnet system, more particularly for generating a field having the required homogeneity in a given region in space, of the type comprising a main resistive magnet thermally coupled to a cooling fluid flow circuit
REFERENCES:
J. Phys. E; Sci. Instrum., vol. 11, 1978, No. 9, "Clamping of Bitter Coils by a Fibreglass Cylindrical Housing", K. Trojnar, B. Drys, and M. Luczak.
Revue de Physique Appliquee (France), No. 3, Mars 1979, p. 517, "Asservissement d'un champ magnetique statique, Dans la gamme 0,1 a 2 T, par R.M.N. impulsionelle; optimalisation des performances", J. Alizon, J. P. Dagois, H. Robert.
Science, vol. 122, "Magnets and Magnetic Field Measurements", A. L. Bloom, M. E. Packard.
IEEE Transactions on Magnetics, vol. Mag-17, No. 5, Sep. 1981, "Polyhelix Magnets", H.-J. Schneider-Muntau.
Aubert Guy
Kaplan Daniel
Fess Laurence G.
Levy Stewart J.
Thomson-CGR
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