Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Patent
1992-08-07
1994-07-19
Arana, Louis
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
324320, G01V 300
Patent
active
053312821
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a magnetic field generating assembly and to apparatus for performing a nuclear magnetic resonance (NMR) experiment using such an assembly.
In the field of nuclear magnetic resonance, it is necessary to generate within a working volume a substantially homogenous magnetic field to enable the NMR characteristics of molecules within the working volume to be examined. Conventionally, this has been achieved by making use of a coil assembly at the centre of which an approximately spherical region having a homogeneous magnetic field is generated. Such an assembly is described in U.S. Pat. No. 4,656,423. This has proved satisfactory in the case of NMR imaging of the human body and also NMR spectroscopy. However, there is a need to be able to perform NMR experiments on relatively large bodies, for example polymer composites such as aircraft parts and the like and the cost of manufacturing a magnet of a conventional form into which the body can be placed is so large as to be prohibitive.
Samoilenko et al JETP Lett. Vol. 47, No 7, 10 Apr. 1988, describe how large field gradients within conventional high resolution NMR magnets can provide sufficient field gradient to facilitate a selected region of the order of 0.1 mm with the application of an rf pulse only. However, again the working region is confined within the magnet.
Recently, attempts have been made to change the location of the homogenous, working volume and in one such attempt described in U.S. Pat. No. 4,701,736 a magnet is described in which the spherical homogeneous region is projected outside the volume of the magnet. A number of nested, counter running coils are used. This suffers from the disadvantages firstly that more power (in a resistive system) or more superconducting material (in a superconducting system) is required to produce a given field strength in the working volume; and secondly the working volume is spherical which is not particularly suitable for certain applications.
A second assembly for generating a substantially homogeneous region projected from the assembly is described in EP-A-0186998 which comprises a number of coil sets with their axes substantially parallel and spaced apart and this requires an undesirable amount of superconductor (in the case of superconducting magnets) or power (in the case of resistive magnets) to achieve a given field strength.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, a magnetic field generating assembly comprises at least two magnetic field generators for generating magnetic fields having substantially coaxial portions extending in the same sense, the generators being arranged such that in use in a working region external to the generators the resultant magnetic field exhibits a gradient extending in a first direction, and wherein at least the second order variations in the fields in directions transverse to the first direction within the region are substantially balanced such that an NMR experiment can be performed on a sample within the working region.
In this assembly, it is possible to arrange for two or more magnetic field generators to generate substantially coaxial fields in the same sense and arrange for the second order variations in the field in directions transverse to the one direction to balance in the working region, thereby generating a relatively large working region spaced from the generators.
Although the magnetic field generators could comprise permanent magnets, preferably the generators comprise substantially coaxial, electrical coils, most preferably the coils generating fields extending in the first direction. If superconducting coils are used, these could be fabricated from the recently developed high temperature superconducting materials.
As will be explained below, it can be shown that the second (and fourth) radial derivatives of the magnetic fields exhibit changes in sign depending upon the radius of the coil. Consequently, by suitable choice of coil radius it is possible to cancel at l
REFERENCES:
patent: 4656423 (1987-04-01), Sugimoto
patent: 4701736 (1987-10-01), McDougall et al.
patent: 4721914 (1988-01-01), Fukushima et al.
patent: 4875485 (1989-10-01), Matsutani
patent: 5138326 (1992-08-01), Edwards et al.
Hanley Peter
Hawkes Robert C.
McDougall Ian L.
Arana Louis
Oxford Instruments (UK) Limited
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