Wave transmission lines and networks – Transmission line inductive or radiation interference...
Reexamination Certificate
2002-09-13
2003-12-30
Gutierrez, Diego (Department: 2862)
Wave transmission lines and networks
Transmission line inductive or radiation interference...
C333S176000, C333S206000, C324S318000, C324S322000
Reexamination Certificate
active
06670863
ABSTRACT:
BACKGROUND
The invention relates to a resonance trap for suppressing electromagnetic coupling phenomena for at least one line for conducting electric currents or electromagnetic waves, notably for RF alternating currents in an MR apparatus, which resonance trap includes a conductor which extends along a part of the length of the line.
In the context of a typical magnetic resonance imaging method the magnetic moment of the protons is oriented in one spatial direction by means of a strong, steady magnetic field of, for example, 1.5 Tesla. Using brief electromagnetic RF pulses, the individual protons are excited to precession and subsequently become oriented in conformity with the external, strong magnetic field again. In particular the excitation and relaxation times and the frequencies of the precessional motions are dependent on the tissue and in the context of the measurement they provide, in conjunction with a position code of the excitation, information concerning the situation in space of various tissues. The position code utilizes position-dependent frequencies and phases of the precessional excitation and enables information on the location of the relevant emission to be derived via Fourier transformation of the measured MR signal.
In order to achieve a high image quality it makes sense to utilize several characteristics so as to distinguish the types of tissue being examined. The speed at which the magnetic fields in the MR apparatus can be varied represent forms a limiting factor in achieving a high image quality within an acceptable examination time. Therefore, it is continuously being attempted to develop coil systems which enable high magnetic field strength transients to be obtained in conjunction with amplifiers and voltage sources.
In order to generate the magnetic field strength gradients desired for the position code, it is common practice to use different coil systems in three mutually perpendicular spatial directions. Two Helmholz coils which face one another are usually arranged in the direction of the longitudinal axis of the body, that is, the axis which is usually referred to as the z axis. In the direction perpendicular to the longitudinal axis of the body there are usually provided spiral-like planar coils which are arranged opposite one another so as to enclose the examination volume in a cylindrical fashion. Along the longitudinal axis of the body two or more of such spiral-like coils are often provided for a spatial direction extending perpendicularly to the longitudinal axis of the body. The two spatial directions which are oriented perpendicularly to the longitudinal axis of the body are usually referred to as the x direction and the y direction for which respective, separate coil systems of the latter kind are used.
The coils used for the precessional excitation of the protons usually are situated in the examination room enclosed by the other coils. For ease of operation the walls of the examination room are often provided with connections for the RF coils in different locations, thus enabling the coils to be arranged on the patient to be examined in conformity with the requirements of the relevant examination. The lines leading to the individual connections, however, may readily be subject to electromagnetic coupling to the RF magnetic fields of the RF coils, so that electric currents and voltages could be generated in the lines and shields. On the one hand, the voltages, currents and electromagnetic fields thus arising falsify the measuring results while on the other hand the induced voltages and currents may reach an order of magnitude such that they become a hazard for the patient to be examined.
In order to avoid at least the voltages and currents which are hazardous to the patient to be examined, it is already known to wrap a conductor around the line to the RF coils. The inductance thus formed is customarily connected in series with a capacitor which is coupled back to the line, the resonance frequency of the resonant circuit thus obtained being tuned to the frequency of the MR apparatus. The line present in the coil of the resonant circuit is thus shielded by means of the excited resonant circuit and the RF signal of the line remains unaffected. It is a drawback, however, that a stray field of the resonant circuit arises, thus necessitating the use of an additional shield for this arrangement. A further drawback in respect of the manufacture and maintenance of this device, moreover, resides in the necessary connection between the capacitor of the resonant circuit and the actual line for the RF signal. These components are connected to one another so as to be quasi inseparable, so that in the case of a defect the complete line, including the shields, must be replaced. The alternative in the form of a modular construction would necessitate a multitude of detachable connections which on the one hand would raise the manufacturing costs to an unacceptable extent while on the other hand the number of vulnerable would strongly increase to the detriment of the availability.
For the shielding of the RF conductor the U.S. Pat. No. 5,742,165 already teaches to enclose the line by means of a cylindrical conductor over a length which corresponds to one quarter of the wavelength of the electromagnetic radiation of the MR apparatus, the relevant wavelength being the wavelength present in a dielectric between the cylindrical shield and the line. At one end the cylindrical shield is short-circuited directly to the line to be shielded whereas at the other axial end it is connected thereto via a capacitor. Because of the connection via the capacitor, the electrically effective length of the cylindrical shield (also referred to as the electrical length hereinafter) is significantly reduced relative to the actual length.
This intricate arrangement has the drawback that a direct connection exists between the RF line and the individual shields, so that in the case of failure of one of the components the entire line with all shields must be replaced at high costs.
SUMMARY
Considering the drawbacks and problems of the present state of the art, it is an object of the invention to provide a resonance trap for a line for conducting electric currents which reliably shields RF electromagnetic radiation and enables a modular construction of the resonance trap on the line.
This object is achieved in accordance with the invention by means of a resonance trap of the kind set forth in which at least one inner conductor extends along the line over a part of its length, that at least one outer conductor extends along the inner conductor, that the inner conductor is arranged at a distance from the line which is smaller than that at which the outer conductor is arranged, and that the outer conductor is arranged so as to cover the inner conductor at least partly relative to the line.
The inner conductor can then extend parallel to the line. Arrangement of the outer conductor parallel to the inner conductor ensures a structurally sensible distance between the conductors and between the conductors and the line. The line and the inner as well as the outer conductor should be situated in such a manner that their principal dimension extends at least partly along a common straight connecting line, thus ensuring high-quality shielding.
An advantage of the resonance trap in accordance with the invention resides in the fact that it is no longer necessary to connect the resonance trap conductively to the line. Because of the dissociation of the resonance trap from the lead, for the first time a modular construction can be realized in which the line and the resonance trap constitute separate components. There is no longer a need for plug-type connections between the line and the resonance trap. The construction of the line and the resonance trap as separate components at the same time enhances the robustness of these two components. The modular construction enables a variety of possibilities for standardization, so that substantial cost savings can be achieved. Finally, the resonance trap
Holle Eerke
Jacobs Cornelis Wilhelmus
Rietjens Math
Van Helvoort Marinus Johannes Adrianus Maria
Gutierrez Diego
Lundin, Esq. Thomas M.
Vargas Dixomara
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