Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2002-03-29
2003-12-23
Lefkowitz, Edward (Department: 2859)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
Reexamination Certificate
active
06667619
ABSTRACT:
This application claims Paris Convention priority of DE 101 16 623.0 filed Apr. 3, 2001, the complete disclosure of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
The invention concerns a magnetic resonance (MR) apparatus comprising a main field magnet system with a tubular opening for receiving the object to be examined, and at least one mechanically oscillatory tubular component which is disposed in this opening, in particular a gradient coil system, and at least one cylindrical element of electrically conducting material which is surrounded by the main field magnet system and which is rigidly mechanically connected to at least one of the mechanically oscillatory components or which forms an integral mechanical part therewith.
An arrangement of this type is known from WO 00/25146 A1.
An MR apparatus for medical purposes is used i.e. for producing cross-sectional images of a body. Towards this end, a strong, constant and uniform magnetic field is generated in an investigational volume of the MR apparatus. A gradient field is superposed on the uniform field to determine the location to be recorded. An RF field then excites the atoms of the sample substance located in the investigational volume and the spin resonance signal which is produced during relaxation of the excited atoms is used to reconstruct an image of the cross-section which was defined by the gradient field. The constant uniform field is generated by a main field magnet which may be superconducting. The main field magnet has, together with the associated covering, a tubular opening in which the investigational volume is disposed. The diameter of this tube is determined by the size of the patient to be examined and therefore has a predetermined minimum value, e.g. 90 cm.
The gradient system for producing the magnetic gradient field in the investigational volume is disposed within the above-mentioned tube and about the investigational volume. The gradient system comprises gradient coils for generating a gradient field. Gradients are generated for each of the three coordinate directions x, y and z. During the measurement, current pulses are fed to the gradient coil which produces structure-borne noise in the gradient system due to the Lorentz forces acting on the gradient coils in the strong background field of the main field magnet. This structure-borne noise is irradiated from the surfaces of the gradient system in the form of acoustical noise. To prevent such irradiation, a preferred embodiment of WO 00/25146 A1 discloses a gradient system surrounded by an evacuated capsule which is connected, in a vacuum tight manner, to a tubular access to the working volume and to the tube of the main field magnet on the side of the gradient system facing away from the working volume. The tube therefore forms part of the capsule.
The above-mentioned feeding of the gradient coils with current pulses can also produce stray magnetic fields of a greater or lesser strength radially outside of the gradient system, i.e. in the region of the main field magnet. These stray fields can induce eddy currents in the electrically conducting parts of the MR apparatus which are disposed in the vicinity of the gradient coils, in particular the metallic parts provided for producing the main field. Examples thereof are the tubular part of the covering of the coil system, any thermal shielding (for a superconducting coil system) provided within the covering, and the magnet field coil itself. The magnetic fields produced by the eddy currents generate considerable noise since the parts which conduct the eddy currents are also subjected to Lorentz forces in the background field of the main field magnet which produce structure-borne noise and transfer it to the capsule.
To counteract this, attempts have been made to shield or compensate the magnetic fields which are produced outside of the investigational volume. A shielding coil has been provided which surrounds the gradient coil to compensate for the stray fields. This shielding coil is intended to actively shield the space outside of the investigational volume through generation of a compensating field. However, complete shielding is not possible with a shielding coil since the shielding coil is formed of discrete windings and deviations cannot be avoided due to production tolerances such that magnetic flux can escape outwardly through the windings.
To improve active shielding, WO 00/25146 proposes providing the gradient system with an additional eddy current shielding consisting of an electrically conducting, substantially closed cylindrical element. The gradient coil system and the shielding coil are disposed within the cylindrical element to form a mechanically rigid unit therewith. In consequence thereof, the parasitic fields of the gradient coils build up and decay slowly when current pulses are input. For this reason, only weak eddy currents, and resulting mechanical oscillations and structure-borne noise, are excited in the electrically conducting parts of the main field magnet. The function of the arrangement improves, the larger the product p of the electric conductivity, the wall thickness d and the radius R of the cylindrical element. In one embodiment, the product p=80000 A mN. This embodiment of WO 00/25146 advantageously effects an additional reduction in noise over and above that already given through the conventional use of an evacuated capsule which completely surrounds the gradient system.
One substantial disadvantage of the use of an evacuated capsule is that the production of a capsule of this type is relatively difficult, since a gradient system has numerous supply lines such as current cables and lines for cooling water which must be guided through the capsule in a vacuum-tight fashion and in such a manner that no structure-borne noise is transmitted at the vacuum-tight feed-throughs via these lines from the mechanically oscillating gradient system to the capsule.
It is therefore the object of the invention to further develop an MR apparatus of the above-mentioned type with as simple technical means as possible such that the noise of oscillatory components can be effectively reduced to make an MR recording more convenient for the patient.
SUMMARY OF THE INVENTION
This object is achieved in accordance with the invention in a surprisingly simple and technically straightforward manner in that the sum of the products p of the electrical conductivities &sgr;, the cylinder radii R and the wall thicknesses d of the cylindrical element per mechanically oscillatory component is between 1 Am/V and 10,000 Am/V, preferably between 10 Am/V and 1000 Am/V, and particularly preferably between 20 Am/V and 500 Am/V.
A cylindrical element which meets these requirements has relatively poor electrical conductivity. In general, mechanical oscillations in the background field of the main field magnet induce eddy currents in a cylindrical element of a greater or lesser strength having the frequency of the mechanical oscillation which fundamentally damp the original mechanical oscillation in accordance with Lenz's Law. This can be achieved in a practically useful fashion only if the cylindrical element meets the cited requirements. Since the cylindrical element is rigidly mechanically connected to an oscillatory component, in particular the gradient system, the mechanical oscillations of the oscillatory component, in particular of the gradient system, are damped.
Contrary to prior art described in WO 00/25146, vibration of the cylindrical element of the inventive MR apparatus is optimally damped, whereas in WO 00/25146 the rise-time or the decay-time of parasitic magnetic fields of the gradient system in the region of the main field magnet is as slow as possible. It is not possible to achieve both aims with the same arrangement. In e.g. patent application U.S. Ser. No. 09/663,789 filed Sep. 18, 2000, a value for the product p of at least 20,000 Am/V, preferably 40,000 Am/V is stated for the shielding cylinder. For the shielding cylinder of WO 00/25146, a value p of
Schmidt Hartmut
Westphal Michael
Bruker BioSpin GmbH
Lefkowitz Edward
Vargas Dixomara
Vincent Paul
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