Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2002-04-10
2004-03-23
Gutierrez, Diego (Department: 2859)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S322000
Reexamination Certificate
active
06710597
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to the use of a switch element for switching in an electrical line system of a magnetic resonance tomography apparatus, and to a magnetic resonance tomography apparatus of the type having a radio-frequency coil, particularly a radio-frequency reception coil, and with a switch element for influencing the coil activity.
2. Description of the Prior Art
Magnetic resonance tomography systems of the above type are know, as are magnetic resonance tomography systems having a number of radio-frequency transmission coils and a number of radio-frequency reception coils with at least one switch element for multiplexing control signals or for demultiplexing reception signals.
In a magnetic resonance tomography apparatus, radio-frequency pulses are emitted into the examination region of a subject or of a patient under examination in the presence of a uniform, time-constant basic magnetic field. During this transmission phase, a radio-frequency transmission coil is fed by a power amplifier. The radio-frequency transmission coil transmits at the Larmor frequency of the hydrogen nuclei that is typical for the basic field. As a consequence of the application of the radio-frequency pulse, an echo signal that can be received by a radio-frequency reception coil acting as antenna arises at a later point in time when the radio-frequency transmission pulse has already been deactivated. A raw data matrix is compiled from a number of radio-frequency echo signals that have been acquired with successive variation of an applied, gradient magnetic field. An image of the region under examination in the patient or subject is generated from this raw data matrix by Fourier transformation.
The radio-frequency transformation coil and the radio-frequency reception coil can be the same radio-frequency coil. In this case, the supply line of the radio-frequency coil must be switched back and forth between the power amplifier and a pre-amplifier serving the purpose of amplifying incoming echo signals, given each change between the transmission phase and the reception phase.
A number of radio-frequency reception coils can be present, for example a permanently installed reception coil, a local coil fashioned as a volume coil and/or a local coil fashioned as a surface coil. A single electronic reception channel can be present for these reception coils, so that a demultiplexing of the reception signals generated by the various reception coils is necessary.
In the case of the radio-frequency reception coil be separate from the radio-frequency transmission coil, it is necessary to detune the radio-frequency reception coil during the transmission phase, since the radio-frequency reception coil is likewise tuned to the Larmor frequency during the reception phase, because the radio-frequency reception coil would otherwise pick up power from the radiated radio-frequency field with high efficiency. This must be avoided in order to protect the patient. A known radio-frequency reception coil therefore has a detuning circuit. A PIN diode is integrated therein that activates the detuning circuit in the through-connected state, so that the radio-frequency reception coil is deactivated, i.e. detuned. The radio-frequency reception coil then can no longer receive.
The PIN diode in the detuning circuit acts as a fast, active switch element. A PIN diode, however, can act as a good switch only with low ON resistance when it is integrated for this purpose in the existing resonant circuit of the detuning circuit. In the through-connected condition of the PIN diode, the resonant circuit of the detuning circuit, which is tuned to the resonant frequency of the magnetic resonance tomograph (Larmor frequency), acts as a blocking circuit that minimizes the current in the radio-frequency reception coil acting as antenna in the transmission phase.
Conventional resonant detuning circuits of this type have the disadvantage that all of the voltage up to 500 V induced by the radio-frequency reception coil drops across the detuning circuit, so that an extremely high current flows in the blocking circuit during the transmission phase. This current—if only because of the magnetic field connected therewith—results in disturbances in the image acquisition in the nuclear magnetic resonance tomography apparatus.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a switch element for employment in a magnetic resonance tomography apparatus that can be utilized in the frequency range of radio-frequency fields and that satisfies the switching times required in nuclear magnetic resonance tomography, and with which, in particular, the aforementioned disadvantages can be avoided or minimized.
A further object of the invention is to provide a magnetic resonance tomography apparatus having such a switch.
The object directed to a suitable switch element is inventively achieved by employing an electrostatic relay in an electrical line system of a magnetic resonance tomography apparatus.
The invention is based on the perception that an electrostatic relay is well suited for utilization in the presence of a strong magnetic field (constant magnetic basic field). An electrostatic relay operates largely free of current, so that interaction with the magnetic basic field does not occur. An electrostatic relay can be constructed without employing a coil whose magnetic field would be influenced by the basic field.
In contrast to a diode or a tube, that function as purely electronic switch elements, a relay is an electromechanical switch that can be electrically actuated but wherein the actual interrupt event occurs mechanically. The relay producing a completely galvanic separation. A relay can be implemented with an especially low ON resistance and with high dielectric strength in the radio-frequency range. Moreover, extremely short switching times can be achieved with the electrostatic relay. A PIN diode is a switch element that is voltage-stable at high frequencies and that would also be adequately fast for many purposes, however, this can only be utilized in a magnetic resonance apparatus by accepting the disadvantages that were set forth above.
An electrostatic relay can be utilized with particular advantages for employment in the uniform basic field of a magnetic resonance tomography apparatus preferably is arranged in the region of this basic field.
In a preferred version of the inventive method, a micro-mechanical relay is employed as relay, this being preferably at least partially manufactured with a material erosion technique, for example by etching, from a substrate, particularly a silicon substrate. Such a micro-mechanical relay can, for example, have a structural height in the range of 0.5 nm through approximately 1 mm that lies in the range of the thickness of typical silicon wafers. Moreover, the advantage is achieved that a micro-mechanical electrostatic relay can implement especially fast switching events, since the masses to be moved and, thus, the forces of inertia that occur are small. The distance over which the mass must be moved also is small.
German OS 32 07 920, corresponding to U.S. Pat. No. 4,480,162, discloses a micro-mechanical electrostatic relay for telephone systems that is suitable for the inventive employment. The inventively employed electrostatic relay has, in particular, a carrier part composed of a material having an electrically insulating effect that forms a carrier member and a structural unit with the carrier member and has an armature secured in articulated fashion to the carrier that carries at least one electrical contact. The teachings of German OS 32 07 920 and U.S. Pat. No. 4,480,162 are incorporated herein by reference.
A micro-mechanical electrostatic relay that can be employed for the purpose of the invention is disclosed in German PS 42 05 029. This relay has at least one armature that is elastically pivotably connected to a carrier at one side, the free end thereof carrying at least one contact piece and having at lea
Reykowski Arne
Wang Jianmin
Gutierrez Diego
Schiff & Hardin LLP
Shrivastav Brij B.
Siemens Aktiengesellschaft
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