Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2000-03-07
2002-10-08
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
Reexamination Certificate
active
06462547
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a magnetic resonance tomography device, of the type having a basic field magnet and a gradient coil system, formed by gradient coils and a carrier structure.
2. Description of the Prior Art
Magnetic resonance tomography is a known modality for the acquisition of images of the inside of a body of the living patient. For this purpose, dynamic magnetic fields with a linear gradient are superimposed on the static basic magnetic field in three spacial directions in magnetic resonance tomography devices. Currents flow in the gradient coils, whose amplitudes reach several 100 A and which are subject to frequent and rapid changes of the current direction with rise rates and fall rates of several 100 kA/s. These currents are controlled by pulse sequences, which are provided by a control system, and cause oscillations that lead to the noise due to Lorentz forces, given a basic magnetic field that is on the order of magnitude of 1 Tesla.
A series of measures have been proposed in order to reduce the noise of the gradient coil system. For example, the rigidity of the gradient coil system has been increased and/or the gradient coils have been acoustically damped or insulated and/or the fastening of the gradient coil system has been modified. For example, the U.S. Pat. No. 5,698,980 describes fastening a tube-shaped gradient coil system at its dominant natural oscillation node at the inner cynical surface of the housing of the basic field magnet. Such modified fastening, however, does not achieve a significant noise reduction, since the gradient coil system is the most rigid element of the entire device.
Further developments in the field of the magnetic resonance tomography for shortening measuring times and improving imaging properties are associated with ever faster pulse sequences. These sequences employ even higher current amplitudes and faster current rise rates and current fall rates in the gradient coils. Such larger gradient coil currents, due to increasing Lorentz forces, lead to ever greater amounts of noise without counter-measures. Such faster pulse sequences cause ever more rapid and frequent changes of the current direction in the gradient coils. As a result, the dominant spectral portions of the gradient coil currents shift to higher frequencies. The oscillation excitation of the gradient coil system is maximal and the produced noise is extremely great when one of these portions has the same frequency as a natural frequency of the gradient coil system. Such an excitation with ever faster pulse sequences becomes more likely.
For example, an increase of the rigidity can be one response to larger gradient coil currents and faster pulse sequences. Merely an increase of the natural frequencies by the factor of approximately 1.4 can be achieved by doubling the rigidity. The increase of the rigidity is technically and economically limited, since the current gradient coil system is already an extremely rigid element.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a gradient coil system that inhibits the process of the noise development and therefore reduces the occurring noise.
The object is inventively achieved in a gradient coil system wherein the natural oscillation modes of the gradient coil system (or at least noise-producing natural oscillation modes) and the oscillation-exciting Lorentz forces, which result from the gradient coil currents and from the static basic magnetic field, are optimally orthogonally oriented relative to one another for noise reduction. As used herein, the phrase “optimally orthogonally oriented” means that the natural oscillation modes which contribute to noise production are oriented, relative to the Lorentz forces, as close to “ideal” orthogonality as is practical, given the structural and operational limitations of the device. The term “optimally orthogonally oriented” thus means the substantially orthogonal relationship which results in the maximum amount of noise reduction, given the aforementioned practical limitations. More specifically, in the inventive magnetic resonance device the natural oscillation modes of the gradient coil systems which contribute to noise production are optimally orthogonally oriented relative to the Lorentz forces such that a scalar product of a natural oscillation mode and the Lorentz force for each point of the gradient coil system, with a subsequent summation over all points of the gradient coil system, results in a value that is as close to zero as possible for each natural oscillation mode of the gradient coil system which contributes to noise production to any significant degree.
Only the noise-producing natural oscillation modes are taken into consideration in one embodiment of the invention. This has the advantage that the inventive fashioning of the gradient coil system is simpler due to a few free parameters given remaining high efficiency. The noise-relevant natural oscillation forms are particularly the ones that exhibit a high spatial conformity with the Lorentz force distribution.
In another embodiment, the natural oscillation modes are fixed by weight distribution and/or rigidity distribution of the carrier structure. Except the gradient coils, the term “carrier structure” means all other elements of the gradient coil system, which, together with the gradient coils, determine the natural oscillation modes. Therefore, central carrier elements, casting material, cooling means, shim means and possibly high frequency transmission means and high frequency reception means normally belong to the carrier structure.
In a further embodiment, in the gradient coil system weights are introduced into the carrier structure and/or components are introduced into the carrier structure that alter the elastic rigidity thereof. Particularly advantageous is that the layout of the gradient coil system, which is primarily directed to the generation of a magnetic field with a linear gradient, is only insignificantly changed. Given compound-filled (potted) gradient coil systems, weights are introduced at the locations at which casting material is otherwise present, for example. In contrast to the introduction of acoustic insulating material, for example, the inventive fashioning of the gradient coil system does not cause an increase in volume of the gradient coil system. This is particularly advantageous for magnetic resonance tomography devices with a superconductive basic field magnet and a cylindrical patient opening.
In another embodiment, rings that are similar to the tube cross section are firmly connected to the gradient coil system at at least one of the ends given a tube-shaped gradient coil system. Thereby, the inventive fashioning is achieved in an extremely simple manner. The basic concept of a currently employed tube-shaped gradient coil system is hardly changed. Even comparatively large weights are thus introduced into the gradient coil system without space problems.
In a further embodiment, rings that are similar to the tube cross section are connected to the gradient coil system at at least one of the ends via an intermediate layer made of elastic material, given a tube-shaped gradient coils system. Apart from the inventive fashioning of the gradient coil system, this has the particular advantage that oscillation energy is withdrawn from the oscillating system and is converted into heat energy. The damping is thereby increased and the oscillation amplitude is lowered, and as a result the noise is further reduced.
In another advantageous embodiment, the rings are composed of material of high-density. Heavy weights are thereby realized given small dimensions of the rings. This means a high noise-reducing effect given a small additional volume.
In an embodiment employing the elastic material, the elastic material has a hysteresis loop in the force-deformation-diagram; the area bounded by the hysteresis loop is so large that the elastic material absorbs much oscillation energy of the gradient coil syste
Dietz Peter
Heid Oliver
Kimmlingen Ralph
Fetzner Tiffany A.
Lefkowitz Edward
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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