Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2000-01-20
2002-05-28
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S322000, C324S309000
Reexamination Certificate
active
06396272
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a method for operating a magnetic resonance apparatus having a gradient tube at which at least one gradient coil, in which current flows during operation, is arranged, and at which a number of elements are arranged for generating a force as needed that acts on the gradient tube to counteract oscillations of the tube.
2. Description of the Prior Art and Related Application
With a magnetic resonance apparatus, tomograms of an examination subject, usually a patient, can be obtained through specific body planes. This occurs with the use of electromagnetic fields. In order to enable spatial resolution of the signal obtained in the presence of an applied magnetostatic basic field and an exciting radio frequency field, a gradient field is produced with a number of gradient coils. Gradually, three different gradient coils are utilized that produce fields in the x, y, z directions with respect to the gradient tube. Due to the flow of current in these coils, Lorentz forces occur that act on the gradient tube and cause it to oscillate due to their time curve. These mechanical oscillations in turn cause the air around the gradient tube to exhibit fluctuations in air pressure. These oscillations are the cause for a considerable development of noise during the operation of the magnetic resonance apparatus. Noise peaks far above 100 dB occur. In order to oppose these oscillations and, consequently, to dampen the noise, it is known, for example from German OS 44 32 747, to generate opposing forces with piezoelectric elements that are arranged at the gradient tube and to thus cancel the oscillations excited by Lorentz forces. The arrangement of the piezoelectric elements disclosed therein, however, ensues essentially in the region of the coil conductors. The described arrangement is non-selective in view of the actually generated oscillations; a targeted noise damping is consequently not possible.
In order to achieve noticeably improved noise damping, German Patent Application 198 29 296 corresponding to pending U.S. application Ser. No. 09/343,848, filed Jun. 30, 1999 (“Magnetic Resonance Apparatus,” Dietz et al.), discloses exciting one or more natural oscillation modes of the gradient tube with the elements arranged at the gradient tube, while opposing the oscillations of the gradient tube produced by the Lorentz forces. It has been shown that each oscillation of the gradient tube is a superimposition of a number of natural oscillation modes, i.e. each oscillation can be reduced to specific natural oscillation modes. The natural oscillation modes can supply different contributions to the actual tube oscillation; the elements, however, allow specific natural oscillation modes to be intentionally and specifically excited, so as to oppose the respective natural oscillation mode components of the tube oscillations, and thereby eliminating them. A considerable reduction of the noise can be achieved as a result. Change in the oscillatory behavior of the gradient tube can occur, however, during the operation of the magnetic resonance apparatus or during a longer operating time thereof. These can be reversible or irreversible modifications of the initial conditions. A rigid drive spectrum of the elements, i.e. a force on the gradient tube generated by the elements that is always constant, is non-specific in view of the changes which may occur, and can no longer adequately compensate these.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method of the type initially described that enables compensation of vibratory changes as may occur during the operation of a magnetic resonance apparatus.
This object is inventively achieved in a method for operating a magnetic resonance apparatus having a gradient tube at which at least one gradient coil is mounted that has current-flowing therein during operation, and at which a number of elements are attached for generating a force as needed that acts on the gradient tube to counteract oscillations (vibrations), wherein one or more natural oscillation modes of the gradient tube are excited with the elements, to oppose the oscillations of the gradient tube produced by Lorentz forces that are generated as a consequence of the flow of current through the gradient coils, and wherein frequency-dependent drive signals are used for the excitation of the elements, the amplitudes and/or the phases of the drive signals for modifying the force generated by the elements and acting on the gradient tube having varied for compensation of a change of the oscillatory behavior of the gradient tube, dependent on at least one measured value representing a criterion for the change of the oscillatory behavior.
In the inventive method, at least one measured value is identified, which serves as a criterion for the change of the oscillatory behavior, i.e. the modified oscillatory behavior is directly or indirectly acquired with the measured value. Dependent on this measured value, a modification of the drive signals of the force-generating elements subsequently ensues, i.e. the generated force is varied and set dependent on the oscillatory change, so that this oscillatory change can be largely compensated. The variation ensues by modifying the amplitude and/or the phase of the force, these being available as variable quantities, but it is primarily variation of the amplitude that is suitable for compensation. The phase of the drive is usually very stable, since the force of the force-generating elements must always oppose the Lorentz forces and this is only possible in a well-defined phase relationship; due to a time-delayed response behavior of the force-generating elements relative to the control signal, nevertheless some potential oscillatory changes resulting therefrom can be compensated by phase variation. Changes in the oscillatory behavior of the tube can be determined directly from the gradient tube. For example, as a result of the flow of current through the gradient coils, the tube may be heated. This change is reversible, i.e. the oscillatory behavior also changes correspondingly when the tube cools. In addition, changes can be produced, for example, by aging of the tube, for example, resulting in a change of the modulus of elasticity, caused, for example, by the cyclical heating and cooling that repeatedly occur during prolonged use. The attenuation of the gradient tube also may change. Further, the influence of the force of the force-generating elements on the tube can change, for example due to fatigue of the material with local modification of the material properties; the force-generating elements themselves also can age, so that the force that is generated decreases despite the same drive. The modifications of the oscillatory behavior resulting in one or more of these sources can be advantageously compensated with the inventive method.
When the modification can be attributed exclusively to a change in a force-generating element (for example, partial or complete failure), then this change should likewise be compensated at only this element. (Replacement or adaptation of the drive of this one element to the required force) Respective sensors can be disposed at each force-generating element which, when the force-generating element is being driven, serve as a force-measuring sensor whose signal is used only for the controlling drive of the one element at which it is desired. When the force-generating element is not being driven, the sensor measures excursions generated by the other force-generating elements relative to a previously identified reference value. An arbitrary sensor (excursion, acceleration, expansion) can be used for this purpose.
Further features relate to all force-generating elements of a group in common:
It has proven expedient when the change of the drive signals ensues on the basis of a change of at least one drive curve stored in a control unit, the drive curve representing frequency dependent values for the force to be exerted by the
Arz Winfried
Dietz Peter
Roeckelein Rudolf
Fetzner Tiffany A.
Lefkowitz Edward
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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