Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2000-09-18
2002-12-31
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
Reexamination Certificate
active
06501275
ABSTRACT:
This application claims Paris Convention priority of DE 199 47 539.3 filed Oct. 2, 1999 the complete disclosure of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
The invention concerns a gradient coil arrangement for a magnetic resonance apparatus comprising a main field magnet system having a pipe-shaped opening for receiving the object to be examined, a gradient coil system located in said opening, and a pipe-shaped shielding cylinder surrounding the gradient coil system, being surrounded by the main field magnet system, and having a large electrical conductive value. The electrical conductive value is defined as the product between the electric conductivity and the wall thickness of the cylinder.
An arrangement of this type is disclosed in DE 39 00 725 A1. The main field magnet system described therein comprises a superconducting cylindrically-symmetric main field coil cooled to the boiling temperature of liquid helium. It is installed in a helium tank containing liquid helium consisting in general of non-magnetic stainless steel, and has the shape of a hollow cylinder. The tank is surrounded by at least one cooled radiation shield which consists in general of sheet metal having a large electric conductivity, e.g. aluminium, but can also consist of non-magnetic steel with poor electric conductivity. The helium tank and also the radiation shields are installed in the outer vacuum case - a metallic, evacuated outer shell having a pipe-shaped axial opening. The outer vacuum case forms, together with the installed radiation shields and tanks, the cryostat of the magnet system. The superconducting main field coil has an optimized geometric design such that the main field coil generates a homogeneous magnetic field B
z
in an approximately spherical examination volume about the geometric center of the opening in the direction of the axis of the room temperature pipe (z axis) which is suitable for magnetic resonance examinations. The diameter of the examination volume is generally approximately half the size of the diameter of the room temperature pipe. The relative deviations of the magnetic field from its average value are frequently only a few ppm (parts per million) in the examination volume, e.g. <10 ppm. The pipe-shaped gradient system is located in the room temperature bore of the cryostat which also surrounds the examination volume. The gradient system contains, in general, three gradient coils to which temporally varying electric currents can be fed for generating corresponding temporally varying magnetic gradient fields, dB
z
/dz, dB
z
/dy and dB
z
/dx in the examination volume. X and y are thereby perpendicular to each other and to the z axis. The electric currents are generally switched on and off in the gradient coils to thereby switch the associated gradient coils on and off within a few 100 microseconds. The gradient coils are generally rigidly mechanically connected to a supporting structure, e.g. a supporting pipe.
Gradient coils of older design generate magnetic stray fields in the area of the metallic radiation shields of the cryostat of the main field magnet. Subsequently, during switching of the currents in the gradient coils, temporally varying eddy currents are induced in the metallic structures whose magnetic fields are superimposed on the magnetic field of the gradients as undesired temporal and spatial disturbances. Gradient coils of newer design, e.g. according to U.S. Pat. No. 5,323,135 are actively shielded and only generate very small stray fields in the area of the main field magnet and its metal structure to considerably reduce this problem. With these coils, one radially inner gradient coil is basically surrounded by an encasing radially outer shielding coil through which the same current flows and whose conductor paths are disposed such that the entire field of this arrangement theoretically or approximately vanishes in the volume radially out side of the shielding coil.
When calculating the optimum geometric course of the conductor paths of an active shielding coil, one generally tries to approximate the path of the electric current density in a layered “ideal” shielding. A theoretically ideal shielding in this sense is an infinitely long shielding cylinder with infinitely large electric conductivity which surrounds the gradient coil at a certain radial separation. During charging of a gradient coil, an electric current density distribution is induced in this shielding cylinder which completely compensates for the magnetic field of the gradient coil in the entire volume outside of the shielding cylinder, i.e. in the area of the cryostat and the main field coil. The same current density distribution is also given in a shielding cylinder having finite conductivity, if the gradient coil is operated with alternating current for the limiting case of infinitely large frequencies. To obtain ideal shielding, the shielding cylinder must not be a circular cylinder. Any structure is suitable which divides the volume into two completely separate half-volumes, an inner and an outer half-volume, wherein the gradient coil is located in the inner half-volume. There are no fields present in the outer half-volume. Examples thereof are elliptic shielding cylinders or also dented pipes. Since the axial length of the gradient coils is limited, such an “ideal” shielding cylinder does not have to be infinitely long but only slightly longer than the gradient coil. The publication Sh. Shvartsman, R. Brown, H. Fujita, M. Morich, L. Petropoulos, J. Willig, “A New Supershielding Method Applied to the Design of Gradient Coils”, Proceedings ISMRM 1999, Philadelphia, US describes gradient coils with which an ideally functioning shielding cylinder can have a finite, relatively small length.
The infinitely large electric conductivity can indeed only be realized with superconducting materials which must currently, even for the case of high temperature superconductors, be cooled to temperatures of less than 100 K and, as described e.g. in DE 39 00 725 A1 , are mounted to or in the helium tank. In the case of high temperature superconductors, such a shielding cylinder can also be mounted on a radiation shield of the cryostat.
As mentioned above, one attempts, with actively shielded gradient coils, to approximate an ideal shielding cylinder or an inductively generated current density path through a conductor extending in windings and carrying the same current as the gradient coil. This conductor path is thereby generally produced by grooves milled or cut into a cylindrical copper pipe. This produces a very good, however, due to mechanical tolerances, nevertheless imperfect shielding effect, which is comparable to that of an ideal shielding cylinder. In addition to DE 39 00 725 A1, numerous other publications describe a shielding cylinder with large finite or infinitely large electrical conductive value, e.g. PCT application WO 99/28757 or U.S. Pat. No. 4,881,035. The shielding cylinder is always a component of the cryostat, mainly since shielding cylinders having the desired high conductive values are particularly easy to realize at the low temperatures within a cryostat or since it must be mechanically decoupled from the gradient coil system itself for fundamental reasons (e.g. WO 99/28757).
Switching of currents in the gradient coils in the strong magnetic field of the main field magnet creates Lorentz forces acting on the electric conductors of the gradient coil and on the radiation shields in the cryostat of the main field magnet through which eddy currents flow. The sum of the Lorentz forces acting on all electric conductors of a gradient coil can be, depending on the detailed geometric design of the gradient coil, translational or rotational. Often, the translational and rotational forces acting on the supporting structure of the gradient coil vanish due to their symmetry. In this case, only “internal forces” occur which neither displace nor turn but deform the supporting structure.
Translational and rotational forces acting on the gradient system a
Bruker Biospin GmbH
Vargas Dixomara
Vincent Paul
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