Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
1999-06-30
2002-03-05
Williams, Hezron (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S322000
Reexamination Certificate
active
06353319
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a magnetic resonance apparatus of the type having a gradient tube at which at least one gradient coil, in which current flows in operation, is arranged and at which a number of elements for generating a force acting on the gradient tube as needed are arranged, wherein the position of the elements is selected dependent on at least one mode of characteristic vibration of the gradient tube such that this mode of characteristic vibration can be excited upon operation of the elements.
2. Description of the Prior Art
In a magnetic resonance apparatus tomograms of the subject to be examined, usually a patient, are produced through specific body planes. This occurs using electromagnetic fields. In order to enable a spatial resolution of the signal obtained in the presence of a magnetostatic basic field and an exciting radio frequency field, a gradient field is generated with a number of gradient coils. Three different gradient coils are usually utilized, generating fields in the x-y-z directions with respect to the gradient tube. Lorentz forces occur due to the flowing current, these forces acting on the gradient tube and exciting the tube to vibrate as a result of their time curve. These mechanical vibrations in turn excite the air around the gradient tube to produce fluctuations of air pressure. These fluctuations are the cause for a considerable creation of noise during the operation of a magnetic resonance apparatus, whereby noise spikes far above 100 dB occur. In order to oppose these vibrations and, consequently damp the noise, German OS 44 32 747, for example, discloses that forces be generated with piezoelectric elements that are arranged at the gradient tube, these forces opposing the Lorentz forces and thus reducing the vibrations excited by Lorentz forces. The disclosed arrangement of the piezoelectric elements, however, ensues essentially in the region of the coil conductors. The described arrangement is non-selective in view of the vibrations actually generated; a targeted damping of noise, consequently, is not possible.
The article by J. Qiu, J. Tani, “Vibration Control of a Cylindrical Shell Used in MRI Equipment” in Smart Mater. Struct. 4, 1995, A 75-A 81, provides a theoretical approach with respect to the arrangement of piezoelectric elements. This, however, is based on boundary conditions that are not established in practice, for which reason it leads to results that are not practically unusable. Further, Japanese Application 08-257 008 A discloses the possibility of employing piezoelectric elements for reducing noise.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a magnetic resonance apparatus wherein an effective noise damping is realized.
For solving this problem, in a magnetic resonance apparatus of the type initially described, elements are inventively provided serving for the excitation of X and/or Y vibration modes which are distributed in the middle and over the circumference of the gradient tube with respect to the length of the gradient tube and/or elements serving for the excitation of Z vibration modes are distributed along the length and the circumference of the gradient tube.
The inventive arrangement of the elements is based on the fact that each vibration of the gradient tube is a superimposition of a number of vibration modes, i.e. each vibration can be reduced to specific component vibration modes. The vibration modes can make different contributions to the actual tube vibration. In the inventive apparatus, the arrangement of the elements makes it advantageously possible to intentionally and definitively excite at least one vibration mode that opposes respective component of the vibration mode of the tube vibrations and eliminates them. As a result, the tube vibration can be effectively opposed, leading to a damping thereof, and thus a damping of the generated noise as well.
The basic magnetic field proceeds along the cylinder (tube) axis; and the high currents flow within cylinder shells, for which reason the arising Lorentz forces are radially directed. The spatial distribution of the Lorentz forces is approximately symmetrical in the longitudinal direction of the tube toward the middle of the length of the tube. Given X and Y gradients, forces with opposed phase are exerted on locations lying opposite one another in the circumference direction. This means that this Lorentz force only excites characteristic vibration modes that exhibit corresponding symmetry properties. These are only those modes with uneven mode parameters, i.e. with an uneven number of equaiphase antinodes, for which reason the elements are inventively placed dependent at least on one vibration mode having uneven mode parameters. Since the elements can be inventively placed dependent on circumferential and/or longitudinal vibration modes, which is due to a desire that no significant radial vibrations within the tube occur in the acoustically relevant frequency range, it is particularly modes with uneven circumferential parameter and uneven longitudinal parameter that are relevant. The element placement can be such that only circumferential vibration modes can be generated, since it has proven that each of the characteristic modes under consideration is composed of circumferential and longitudinal vibrations, and, due to the symmetry established by the Lorentz force of the X-Y gradient, it is particularly characteristic forms having circumferential vibrations with uneven mode numbers and longitudinal vibrations that are symmetrical toward the middle of the tube that are excited. Since the suppression of only one component, i.e. either of the circumferential vibration or the longitudinal vibration, leads to the elimination or damping of the entire vibration mode and since the circumferential vibration modes can be more easily defined and separated, it suffices to select the position of the elements only on the basis of these circumferential vibration modes and to generate only such circumferential vibration modes for noise elimination. As a result of the symmetry properties of the relevant vibration modes that have already been described, the elements for generating the X and/or Y vibration modes are arranged in the middle and distributed over the circumference with respect to the length of the gradient tube. The elements thus need not be applied over the entire length of the gradient tube; on the contrary, an arrangement that is selected centrally and only at specific tube angle positions leads, due to symmetry, to an adequate damping by itself. A further advantage is that all even-numbered vibration modes exhibit an vibration node in the middle, so that a central arrangement assures that no undesired vibrations are excited during the “elimination”; these would in turn have a disadvantageous effect. Since the greatest unstiffened expanse in the longitudinal direction usually occur in the middle of the tube, a central positioning relative to the tube is also the most effective. As already described, it suffices for the elimination of the vibrations caused by Lorentz forces to excite only uneven circumferential vibration modes. To this end, the placement of the elements for exciting circumferential vibration modes can be selected with a mode number m=1, m=3 and, if necessary, m=5. Higher-numbered modes are not excited at the given operating frequencies, or are only excited to a negligible extent; their discrete elimination is not absolutely required.
The Z gradient, by contrast, is largely anti-symmetrical in the longitudinal direction toward the tube center. Only longitudinal vibrations that exhibit the same anti-symmetry are thus excited, i.e. points that are equidistant from the tube center in different directions vibrate with the same amplitude but with opposite phase. Consequently, only even-numbered modes are relevant here, for which reason these elements, dependent on the Z vibration modes (longitudinal vibrations), are placed to excit
Arz Winfried
Dietz Peter
Roeckelein Rudolf
Fetzner Tiffany A.
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
Williams Hezron
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