Actively shielded magnet system with Z2 shim

Details Actively shielded magnet system with Z2 shim Actively shielded magnet system with Z2 shim

2000-08-23

2001-07-24

Barrera, Ramon M. (Department: 2832)

Electricity: magnetically operated switches, magnets, and electr

Magnets and electromagnets

Magnet structure or material

C324S320000

Reexamination Certificate

active

06265960

ABSTRACT:

This application claims Paris Convention priority of DE 199 40 694.4 filed Aug. 27, 1999 the complete disclosure of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
The invention concerns a superconducting magnet system for magnetic resonance spectrometers, wherein the magnet system comprises an actively shielded superconducting magnet for generating a homogeneous magnetic field H
0
along a z-axis in a working volume, arranged about z=0, comprising a radially inner and a radially outer coil system, wherein the two coil systems carry approximately identical currents and have approximately identical opposite dipole moments, and wherein a shim system is provided comprising a superconducting shim coil system for correcting magnetic field inhomogeneities in the working volume, the z component of which is proportional to z
2
.
An actively shielded superconducting magnet system of this kind, comprising a Z
2
shim is contained e.g. in the NMR Magnet System 500 SB UltraShield™ distributed by the company Bruker Magnetics and shown in the company leaflet dated May 15, 1999.
An actively shielded magnet system without Z
2
shim is e.g. known from EP 0 144 171 A1.
To achieve a good resolution of the resonance lines in the spectrum by means of magnetic resonance spectroscopic methods, the magnetic field must have a good homogeneity in the sample volume. The basic homogeneity of the superconducting magnet can be optimized with the geometric arrangement of the field-generating magnet coils. With demanding applications, such as high-resolution nuclear magnetic resonance spectroscopy, the basic homogeneity of the magnet is, in most cases, insufficient, since e.g. deviations from the design occur due to production tolerances. To compensate residual inhomogeneities of the magnet, the magnet system comprises autonomous superconducting coils which can compensate field inhomogeneities with a particular geometric symmetry in the sample volume, so-called shims.
An example of such a shim is the Z
2
shim which generates a field with a strength proportional to z
2
along the magnet axis z. A problem with known Z
2
shim means according to prior art is the fact that with large magnets, the shim must be wound within the magnet coils since it would be too weak at the outside. This reduces the space for magnet windings which makes the magnet more expensive and increases the stray field with the consequence that stray field shielding must be stronger for magnet systems with active stray field compensation.
Optimization of the Z
2
shim can be achieved by reducing the space required for the Z
2
shim coils which are wound radially within the magnet windings. This is possible without reducing the shim efficiency only by distributing the shim coils onto several radii. This gives rise to new problems, in particular in connection with the coupling of the shim with fluctuating external magnetic fields.
In contrast thereto, it is the underlying purpose of the present invention to optimize a magnet system of the above-described kind with as simple as possible means such that the above-mentioned problems do not occur.
SUMMARY OF THE INVENTION
This object is achieved in accordance with the invention in that the shim coil system comprises a radially inner shim coil set which is substantially inductively decoupled from the magnet system and mainly generates a magnetic field in the working volume whose z component varies, as &Dgr;H
0
+c
2
·z
2
with c
2
=const., and a radially outer shim coil set which is also substantially inductively decoupled from the magnet system and generates a homogeneous magnetic field −&Dgr;H
0
in the working volume. A positive side effect thereby is the reduction of the susceptibility to disturbances of the magnet itself.
FIG. 2
schematically shows a conventional Z
2
shim according to prior art. The &Dgr;H
0
contribution of the coil S
1
′ which provides the main contribution to the shim strength c
2
is compensated for with a coil pair S
2
′ (&Dgr;H
0
: homogeneous field part of the coil per ampere=field gradient H
0
of order zero generated per ampere). With complete &Dgr;H
0
compensation, the coil pair S
2
′ will normally couple more strongly with the magnet than S
1
′. This coupling is eliminated by a coil pair S
3
′ which strongly reduces the shim efficiency c
2
.
With the present invention, however, the shim is divided into a shim coil set S
i
, decoupled from the magnet, comprising partial coils S
1
and S
2
for generating the gradient field C
2
*z
2
+&Dgr;H
0
and shim coil set S
a
, also decoupled from the magnet, comprising partial coil S
3
, which eliminates the impurity &Dgr;H
0
. Since the H
2
efficiency of a coil with increasing coil radius R with a given length L decreases more than its H
0
efficiency, the gradient-generating shim coil set S
i
must lie on a smaller radius than the shim coil set S
a
which suppresses &Dgr;H
0
.
The advantage of this new shim system consists in that with suppression of &Dgr;H
0
by a radially outer shim coil set, the shim strength c
2
of the radially inner shim coil set is only slightly weakened. This necessitates fewer windings in the radially inner shim coil set and the magnet gets more compact if the radially inner shim coil set is at a location where otherwise magnet windings would be wound.
The advantage of separate decoupling of the inner and outer shim coil sets from the magnet consists in that in this manner, an embodiment is possible wherein each of the two shim coil sets is superconductingly short-circuited individually without the possibility that the individual shim coil sets can be charged e.g. by a magnetic drift, a quench or during spontaneous opening of the main switch of the magnet. Furthermore, in this fashion, also the entire shim is decoupled from the magnet which has the advantage that induced currents in the magnet are reduced during charging of the shim.
In a preferred embodiment of the magnet system in accordance with the invention, both the radially inner shim coil set and the radially outer shim coil set can be individually superconductingly short-circuited. The fact that the inner and outer shim coil set can be separately short-circuited has i.a. advantages for the manufacture of the magnet system, since bridging of the radius difference between the radially inner and outer shim coil set does not necessarily have to be superconducting. It is furthermore possible to operate the two shim portions with different currents.
A decisive advantage of separately short-circuiting the radially inner and outer shim coil sets shows when external field fluctuations occur. To prevent field inhomogeneities in the form of &Dgr;I
2
*c
2
*z
2
it is now sufficient to decouple only the inner shim coil set for the disturbance (i.e. to dimension it so that an external disturbance does not induce a current &Dgr;I
2
) since the outer shim coil set generates essentially a homogeneous field if the external disturbance induces a current therein. It is possible to arrange the outer shim coil set additionally such that its reaction to the external field fluctuation can be used for dampening the homogeneous part of the field disturbance in the working volume.
In a particularly preferred embodiment of the inventive magnet system, the magnet system is part of an apparatus for high-resolution magnetic resonance spectroscopy. The advantages of the present invention pay off in particular in systems such as high-resolution magnetic resonance spectrometers since these magnet systems have anyway a larger volume than others due to the high homogeneity requirements, and thus increased compactness of the Z
2
shim is particularly effective.
A further development of this embodiment is advantageous, wherein the magnetic resonance apparatus comprises a means for field-locking the magnetic field generated in the working volume, wherein the radially inner and the radially outer shim coil sets are largely decoupled from the coils of the device for field-locking. An acti

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