Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
1999-12-27
2001-11-06
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S318000, C324S309000
Reexamination Certificate
active
06313634
ABSTRACT:
This application claims Paris convention priority of German patent application 199 01 331.4 filed Jan. 15, 1999, the complete disclosure of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
The invention concerns a device and a method to homogenize in a working volume of a magnetic resonance apparatus the magnetic field created by its main magnet. The working volume is surrounded by a multitude of ferromagnetic platelets whose number, magnetization strengths and fixed positions are chosen such that in combination they essentially cancel field inhomogeneities of the main magnet across the working volume.
Such a device and such a method are known for example from EP 0 272 411 B1.
In contrast to magnet shimming with so-called shim coils which are fed with correction currents, recently in particular in magnets of imaging nuclear magnetic resonance (MRI) the so-called “passive shimming” has more and more become dominating where the actively controllable shim coils are replaced by passive, ferromagnetic or also by permanent magnetic small platelets being placed around the working volume at predetermined locations. In common superconductive tomography magnets with room temperature bore, rod-shaped support structures are inserted into axial guiding mechanisms at the wall of the room temperature bore. In each support structure, at predetermined positions, a pre-calculated number of ferromagnetic “shim platelets” is stacked and fixed.
Whereas for example in superconducting high resolution NMR magnets shim coil sets are still in use in order to homogenize the field at the sample location, in the field of nuclear spin tomography (MRI) in most cases the above mentioned method of passive shimming is used which requires more calculation effort but is less expensive and is on the long run more convenient since it has to be performed only once and needs later on no current or power supplies.
However, even when rather small individual shim platelets are used, the field correction is in a certain sense “digitized” since at predetermined positions only platelets with fixed magnetic moment can be added or removed. Moreover, this procedure requires access to the working volume itself or at least reconstructions in the immediate surroundings of the working volume. Therefore passive shimming can fully profit from its advantages only if the field homogenizing can really be performed once and for all, if need be in a few iteration steps. In cases where the field distribution across the working volume changes with time, e.g. already by introduction of a measuring object, by the time-dependent influence of ferromagnetic objects in the neighborhood or also by a change of the main coil field profile during operation, in particular of permanent magnets or resistive magnets with pole pieces, readjustment at a later time can be desirable.
In EP 0 823 641 A1 it is suggested to use passive shim platelets at predetermined locations as well as shim coils in order to limit on the one hand the number of required shim positions and on the other hand the required shim currents. This requires both, installation of a conventional shim system with coils and support structures for passive shims.
U.S. Pat. No. 4,682,111 discloses a pole piece magnet, each pole piece comprising a concave portion surrounded by a toroidal portion. To further improve field uniformity, a plurality of hydraulically adjustable, ferromagnetic projections extend from the surface of each pole piece. The number of these projections is fixed and they are moved by complex spring mechanisms integrated into the ferromagnetic pole pieces comprising pressurized channels through the pole piece material of the electromagnet.
There is a need for a homogenizing device and a homogenizing method of the kind mentioned initially, making use of passive shim platelets but which allow in a second step a fine adjustment of the field homogenizing by means of these platelets without requiring a re-distribution or a reconstruction of the support structure.
SUMMARY OF THE INVENTION
The object is solved by a device to homogenize in a working volume of a magnetic resonance apparatus the magnetic field created by its main magnet, the device surrounding the working volume and comprising a multitude of ferromagnetic platelets (shims) mounted in stacks of pre-determined numbers of platelets at fixed positions into one or more support structures removably attached to the main magnet wherein the fixed positions, numbers of platelets at these positions and magnetization strengths of the platelets are chosen such that in combination they essentially cancel field inhomogeneities of the main magnet across the working volume, and for an actual fine correction at least some of the shim stacks are mounted movably about their fixed positions with small amplitudes along at least one direction.
With respect to a method using such a device, the object is solved by a method to homogenize a magnetic field created by a main magnet in a working volume of a magnetic resonance apparatus, by means of a device surrounding the working volume and comprising a multitude of ferromagnetic platelets (shims) mounted in stacks of pre-determined numbers at fixed positions into one or more support structures removably attached to the main magnet wherein the fixed positions, numbers of platelets at these positions and magnetization strengths of the platelets are chosen such that in combination they essentially cancel field inhomogeneities of the main magnet across the working volume, and for an actual fine correction at least some of the shim stacks are mounted movably about their fixed positions with small amplitudes along at least one direction, the method comprising the following steps:
a) a field distribution across the working volume is measured;
b) by means of this field distribution fixed positions, numbers of shims at these positions and their magnetization strengths are calculated such that in combination they largely cancel field inhomogeneities of the main magnet across the working volume;
c) the shims are transferred to the stacks in the calculated positions and rigidly fixed with respect to the working volume in stacks in one or more removable support structures;
d) the field distribution across the working volume is measured again;
e) at least for some of the shim stacks, small displacements along at least one direction from their position as calculated in step b) are determined which further reduce the remaining field inhomogeneities determined in step d);
f) as a fine correction, at least some of the shim stacks—in their mounted state—are displaced from their positions calculated in step b) by the small amplitudes determined in step e).
The object is further achieved by a method to homogenize a magnetic field created by a main magnet in a working volume of a magnetic resonance apparatus, by means of a device surrounding the working volume and comprising a multitude of ferromagnetic platelets (shims) mounted in stacks of pre-determined numbers at fixed positions into one or more support structures removably attached to the main magnet wherein the fixed positions, numbers of platelets at these positions and magnetization strengths of the platelets are chosen such that in combination they essentially cancel field inhomogeneities of the main magnet across the working volume, and for an actual fine correction at least some of the shim stacks are mounted movably about their fixed positions with small amplitudes along at least one direction, the method comprising the following steps:
a) the field distribution across the working volume is measured;
b) by means of this field distribution numbers, magnetization strengths and fixed positions of shim stacks are calculated such that in combination they largely cancel field inhomogeneities of the main magnet across the working volume;
c) the shims are transferred to the calculated positions and rigidly fixed with respect to the working volume in stacks in one or more support structures;
d) for a fine correction and based on one or mor
Bruker Analytik GmbH
Patidar Jay
Shrivastav Brij B.
Vincent Paul
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