Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2003-01-09
2004-06-08
Shrivastav, Brij B. (Department: 2859)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S322000
Reexamination Certificate
active
06747455
ABSTRACT:
This application claims Paris Convention priority of DE 102 03 279.3 filed Jan. 29, 2002 the complete disclosure of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
The invention concerns a method for influencing the homogeneous static magnetic field B
0
in the direction of a z axis in a nuclear magnetic resonance (NMR) apparatus having disturbances caused by superconducting components of radio frequency (RF) coils for receiving NMR signals of a sample in a measuring volume of the NMR apparatus, wherein the superconducting components of the RF coils project past the RF active region of the sample in the z direction, and wherein the disturbances effect the z component of the B
0
field in the RF active region of the sample. The invention also concerns an NMR (nuclear magnetic resonance) resonator with at least one RF (radio frequency) coil for emitting and/or receiving RF signals at one or more desired resonance frequencies into and/or from a measuring sample in an investigational volume of an NMR apparatus, disposed about a coordinate origin (x,y,z=0), having means for generating a homogeneous magnetic field B
0
in the direction of a z axis, wherein superconducting conductor structures of the NMR resonator are disposed at a radial (x,y) separation from the measuring sample, and wherein the superconducting conductor structures project past the RF active part of the measuring sample in the z direction.
A corresponding device is disclosed in the document DE 101 50 131.5-33 (cited ref. [6]), which does not constitute prior art with respect to the present invention.
NMR is a very distinctive analysis method which is, however, not very sensitive. According to prior art, it is possible to considerably improve the S/N (signal to noise) ratio through use of cooled and in particular superconducting radio frequency coils (see reference [1]). The main problem involved in the use of superconductors in NMR receiving systems (RF receiving coil) is their static magnetization. It can, if not controlled, cause field disturbances within the sample of such a strength that the line width becomes unsuitably large. A number of methods to minimize this undesired magnetization have been published: [2], [3], [4]. The described methods are, however, complicated and have further disadvantages which are described below. In particular, with coils according to [1], substantial disturbing fields are produced in response to subsequent transverse magnetic fields.
There are two classes of coil arrangements (described in [5] and [6]) which, in addition to other advantages, are immune to such disturbances and are therefore superior to the above-mentioned coil types [1], also when methods [2,3] are used.
Despite their advantages, if the coils of [5] or [6] are not uniformly magnetized, they lose their favorable properties.
As shown in detail in [2], global magnetization of a type II superconductor results from induced currents which flow in closed paths within the superconductor. These are determined by the history of the superconductor and, as long as the external conditions do not change, remain constant for a practically unlimited time due to the zero resistance of the superconductor. These currents generate a magnetic field outside of the superconductor which can produce strong undesired field disturbances in the sample volume.
Disturbances in the spectra due to inhomogeneities of the static magnetic field caused by superconducting RF coils have been conventionally minimized using the following strategies:
A. Minimization of Disturbances in the Spectra According to Prior Art through
A1 minimization of the maximum possible size of magnetization (through subdivision of the coil into sufficiently narrow strips [1], [5]).
A2 preventing the generation of any possible residual magnetization.
In all conventional methods, the superconductor is always cooled in the magnetic field so that the undisturbed form of the magnetic field lines is always frozen therein during the superconducting transition of the type II superconductor. This minimizes the production of field lines which differ from the original homogeneous B
0
field dependence, and therefore also minimizes disturbances of the homogeneity of the magnetic field. The patent document [4] even recommends carrying out this cooling process as slowly as possible so that the B
0
field is frozen as uniformly as possible and without disturbances.
A3 Post-treatment of the superconducting coil with a sequence of decreasing transverse magnetic fields for “demagnetization” [2], [3] (a current structure with closely adjacent opposite current regions is thereby induced such that the sum of the individual magnetic field contributions cancels to a good approximation).
All previous methods are based on a common effort of minimizing or completely eliminating the effective magnetic susceptibility and therefore the magnetization of the superconducting coil(s) and the external magnetic fields produced thereby to minimize the magnetic fields which they generate in the sample.
The final aim of all methods and devices discussed herein is actually to eliminate the NMR disturbances in the spectra produced by the superconducting (SC) coils. We will see that this is not necessarily equivalent to minimization or elimination of the effective susceptibility or minimization/elimination of the additional fields produced by the SC coils. This subtle but very substantial difference in the goals has been completely ignored in prior art [2], [3], [4] and all previous approaches therefore concentrated on the elimination or reduction of all additional fields produced by the SC coils. If this difference is analyzed precisely, completely different solutions of this central object become possible, namely elimination or significant reduction of NMR disturbances in the spectra.
It is therefore the object of the present invention to transfer radio frequency coils with homogeneous transverse saturation magnetization, i.e. coils in accordance with [5] and a subset of the coils according to [6], from any magnetization state into a state in which the NMR relevant field disturbances are essentially eliminated.
SUMMARY OF THE INVENTION
This object is achieved in accordance with the inventive method in a surprisingly simple and effective fashion in that the superconducting components of the RF coils are exposed to an additional magnetic field which is sufficiently strong that, during application of this additional magnetic field, all superconducting structures in the superconducting components of the RF coils disposed in the vicinity of the RF active region of the sample are magnetized to a maximum possible extent, wherein their magnetization along the z axis and transverse with respect to B
0
assumes a value which is substantially constant and different from zero.
This object is also achieved in accordance with the invention in a device having superconducting conductor structures which are magnetized transverse to the B
0
direction after application of an additional magnetic field with maximum magnetization of all superconducting conductor structures, the magnetization along the z axis having a substantially constant value different than zero.
Many of the coils having superconductors which extend sufficiently beyond the RF region ([5], [6]) can be freed from magnetic disturbances using the present invention in a simple and rapid fashion.
In contrast to the conventional methods [2], [3], [4], the present invention eliminates NMR disturbances through homogenisation, in the simplest form through maximizing the magnetization of the superconductor and associated maximizing the magnetic disturbing fields in the sample. This fact would appear at first glance to be completely contradictory and is incompatible wi
Bruker Biospinag
Shrivastav Brij B.
Vincent Paul
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