Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2002-10-29
2004-08-17
Gutierrez, Diego (Department: 2859)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
Reexamination Certificate
active
06777938
ABSTRACT:
This application claims Paris Contention priority of DE 101 55 997.6 filed Nov. 15, 2001 the complete disclosure of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
The invention concerns an NMR (nuclear magnetic resonance) magnet coil system, comprising superconducting conductor structures, with an inductance L
0
for generating a homogeneous magnetic field B
0
in a measuring volume, wherein the magnet coil system is short-circuited by at least one superconducting switch through which an operating current I
0
flows in the persistent mode, and wherein one or more further superconducting switches are provided, each between two points of the winding of the magnet coil system which, during operation, separately superconductingly short-circuit one or more disjoint partial regions of the magnet coil system with the inductances L
1
, L
2
, . . . , L
n−1
which generate magnetic field contributions B
1
, B
2
, . . . , B
n−1
to the homogeneous magnetic field B
0
in the measuring volume.
An arrangement of this type is known e.g. from DE 199 32 412 C1.
Superconducting magnets are used in various fields of application including, in particular, magnetic resonance methods, wherein one distinguishes between imaging methods (magnetic resonance imaging=MRI) and spectroscopic methods. To obtain good spatial or spectral resolution with such methods, the magnetic field in the measuring volume must have high homogeneity of generally less than 1 ppm.
These investigations require increasingly higher magnetic field strengths. Many magnetic resonance apparatus are therefore equipped with superconducting magnet coil systems which can operate in the superconductingly short-circuited persistent mode for very long time periods without necessitating recharging which would interrupt the measuring process (and often cause considerable disturbances due to possibly required re-adjustments).
However, even superconductors below the transition temperature are not, in reality, completely free from residual electrical resistance. Such resistances can vary by orders of magnitude and are due to unpredictable, generally not retraceable, and often manufacture-related variations in the several windings of an NMR magnet coil system.
These residual resistances in the windings of the magnet coil system cause a drift in the operational current I
0
which changes the strength of the homogeneous magnetic field B
0
and thereby causes a shift in the resonance frequency set in the NMR arrangement. Such drifts can typically be of the order of magnitude of 100 Hz/h for magnet coil systems with a resonance frequency of more than 300 MHz.
One approach to solve or at least reduce the drift problem in NMR spectrometers is to provide a separate drift compensation coil. NMR spectrometers of this type have a so-called z
0
coil as part of a superconducting shim means which, in addition to its actual shim function, can also be used for compensation of the above-described magnetic field drifts and for exact adjustment of the resonance frequency of the NMR arrangement. Since z
0
coils of this type can only compensate for very small drifts, the above-described drift problem cannot be solved or can only be solved to an insufficient degree in superconducting magnet systems having a coil winding with a somewhat larger residual resistance.
The above-cited DE 199 32 412 C1 describes an NMR magnet coil system comprising a means for compensating external magnetic field disturbances. Since the magnetic field drift is caused by a residual resistance in the superconducting coil windings of the magnet system itself, this known arrangement cannot compensate for such a drift.
In contrast thereto, it is the underlying purpose of the invention to further develop an NMR magnet coil system having the above-mentioned features such that a magnetic field drift produced by a residual resistance in a winding of the superconducting conductor structures of the coil system is compensated for, at least to a considerable extent, with little technical effort, without separate drift compensation coil and, if possible, also for existing coil systems.
SUMMARY OF THE INVENTION
This object is achieved in accordance with the invention in a surprisingly simple and effective fashion through:
α
=
&LeftBracketingBar;
L
0
∑
j
=
1
n
(
L
-
1
)
jn
B
j
B
0
&RightBracketingBar;
≤
0.8
with 1≦j≦n
wherein B
n
is the magnetic field contribution to the homogeneous magnetic field B
0
of the residual region of the magnet coil system, reduced by the separately superconductingly short-circuited partial regions, and having the inductance L
n
, with (L
−1
)
jn
being the entry of the jth line and nth column of the inverse of the overall inductance matrix of the magnet coil system, wherein L
0
is the total magnetic inductance (sum of all contributions to the inductance matrix).
Highly effective compensation of magnetic field drifts due to erratically occurring residual resistances in regions of the superconducting conductor structures of an NMR magnet coil system of this type is thereby possible with means which are technically easy to implement and without using an additional drift compensation coil. The invention can also be realized in existing superconducting NMR magnet coil systems, since no additional space is required.
The “trick” of the invention is essentially that, through separate superconducting short-circuiting of suitable partial regions of the magnet coil system with respect to the superconductingly short-circuited residual region, the current drift and therefore also the magnetic field drift can be substantially compensated for. To obtain usable results, the above-described condition for the value &agr; must be maintained, since &agr; corresponds to the ratio between the magnetic field drift with short-circuited partial regions and the magnetic field drift without short-circuited partial regions.
The invention can be realized in a relatively simple fashion through installation of one or more additional superconducting switches or through already existing superconducting switches in the apparatus. During charging of the NMR magnet system, the additional switches are also heated. As soon as the system passes over into the operational mode, the switches are short-circuited to effect compensation of the drift.
One embodiment of the inventive magnet arrangement is particularly preferred, wherein exactly one further superconducting switch is provided between two points P
1
and Q
1
of the winding of the magnet coil system which, during operation, separately superconductingly short-circuits a partial region of the magnet coil system with the inductance L
1
which generates in the measuring volume a magnetic field contribution B
1
to the homogeneous magnetic field B
0
, wherein:
α
=
&LeftBracketingBar;
L
0
L
1
L
2
-
L
12
2
(
B
2
B
0
L
1
-
B
1
B
0
L
12
)
&RightBracketingBar;
≤
0.8
wherein B
2
is the magnetic field contribution to the homogeneous magnetic field B
0
of the residual region of the magnet coil system, which is reduced by the separately superconductingly short-circuited partial region and having the inductance L
2
and mutual inductance L
12
relative to the separate superconducting short-circuited partial region.
This simple arrangement provides merely one additional switch for short-circuiting a partial region with respect to the residual region of the magnet coil system. NMR high field magnet systems are generally built from coaxially nested winding sections. If there is an increased residual resistance in one of these sections, separate superconducting short-circuiting of one or more suitable sections (in addition to the superconducting short-circuit of the entire arrangement during persistent mode) can compensate for the magnetic field drift in a simple and effective fashion such that demanding and usually very expensive exchange of the defective coil section with another (which could also prove to be defective) is eliminated.
As mentioned above,
Amann Andreas
Bovier Pierre-Alain
Frantz Wolfgang
Roth Gerhard
Schauwecker Robert
Bruker Biospin GmbH
Gutierrez Diego
Vargas Dixomara
Vincent Paul
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