Communications: radio wave antennas – Antennas – High frequency type loops
Reexamination Certificate
1999-09-28
2001-11-13
Wong, Don (Department: 2821)
Communications: radio wave antennas
Antennas
High frequency type loops
C343S867000, C324S318000
Reexamination Certificate
active
06317091
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to an apparatus for inductively coupling a nuclear magnetic resonance signal into a reception antenna having a resonant coil arrangement that has an imaging area.
The invention is also directed to a medical intervention instrument that is connected to such an apparatus for inductively coupling a nuclear magnetic resonance signal into a reception antenna.
2. Description of the Prior Art
An apparatus of the above type is disclosed in U.S. Pat. No. 4,680,549. It is stated therein that a nuclear magnetic resonance signal from a limited area of an examination subject to be imaged can be noticeably enhanced when a separate set of coils that are closed to form a resonant circuit is arranged in immediate proximity to the area. The separate set of coils has no electrical connection to the rest of the apparatus. The precessing nuclear magnetization then induces a current in the resonant coil arrangement, this current in turn inducing an additional signal in a reception antenna that is significantly greater than the signal which is directly induced in the reception antenna by the precessing nuclear magnetization itself. The signal-to-noise ratio in the area of the image, which corresponds to the area covered by the induction coil, is noticeably improved. A detuning means in the form of two diodes connected with opposite polarities is connected to the resonant reception coil arrangement so that the excitation field is not locally modified when transmitting with a separate transmission antenna, i.e. when exciting the nuclear magnetic spins.
The improvement of the signal-to-noise ratio of the inductively coupled, signal-intensifying resonant circuit and of the resonant coil arrangement is dependent on the quality (Q) of the coil arrangement and on the angle of the imaging area of the resonant coil arrangement relative to the basic magnetic field direction. When the two reside parallel to one another, no improvement derives. This is disadvantageous when, for example, a resonant coil arrangement is to be attached to a surface of the subject to be imaged, if the surface only allows an alignment of the imaging area of the resonant coil arrangement parallel to the basic magnetic field.
On the other hand, circularly polarizing antenna arrangements are known that have two sub-antenna arrangements arranged perpendicularly to one another. Such arrangements thus have antenna characteristics perpendicular to one another that must in turn be aligned perpendicularly to the basic magnetic field direction. For example, a circularly polarizing antenna arrangement disclosed in U.S. Pat. No. 5,602,557 has two sub-antenna arrangements arranged perpendicularly to one another, with each sub-antenna arrangement comprising two saddle-shaped antenna conductors arranged opposite one another. The imaging area is located between the saddle-shaped antenna conductors.
U.S. Pat. No. 5,198,768 discloses an antenna array having dipole and quadrupole antennas.
German OS 34 29 386, corresponding to U.S. Pat. No. 4,932,411, discloses a small antenna for a nuclear magnetic resonance tomography apparatus that can be directly introduced into body organs, for example into the brain, into the liver or into the kidney, via channels. It is attached to the end of a thin plastic carrier.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an apparatus for inductively coupling a nuclear magnetic resonance signal, wherein the signal-intensifying effect is independent of the alignment of the apparatus in the basic magnetic field.
Another object of the present invention is to provide an interventional instrument that can be well-localized, independently of its alignment, using nuclear magnetic resonance technology.
The first object is achieved in an apparatus wherein three resonant coil arrangements are provided (in contrast to the conventional single resonant coil arrangement), these being decoupled from one another and their imaging areas being perpendicular relative to one another and at least partially overlapping. As a result an intensified signal is coupled into the reception antenna from the apparatus independently of the alignment of the apparatus. Ideally, this signal is even higher than that of a resonant single-coil arrangement inductively coupled perpendicular to the basic magnetic field, because the circular polarization of the nuclear magnetization is now always utilized in the inventive apparatus.
In the simplest case, a triplet of resonant dipole antennas or toroidal coils can be employed.
In an embodiment the coils are arranged around a common center and their symmetry axes are perpendicular to one another. The decoupling of the coil arrangements from one another thus achieved exclusively by means of the geometrical arrangement.
When it is intended to apply an arrangement for inductively coupling a nuclear magnetic resonance signal into a reception antenna on a surface of a subject to be imaged, one would expect, if only resonant dipole coils were used, a poor coupling to the subject by the dipole coils standing off from the surface of the subject, and thus only a slight signal amplification would be expected. To avoid this problem, in a further embodiment the arrangement for inductive coupling of a nuclear magnetic resonance signal has at least one resonant quadrupole coil, for example in the form of a butterfly coil or figure-eight coil. Due to the rapidly decaying quadrupole field of the butterfly coil with increasing spacing, however, no noticeable signal boost would be achieved at a greater distance from the reception antenna. A resonant dipole coil therefor is arranged perpendicularly to the resonant quadrupole coil and coupled thereto. The magnetic quadrupole field of the butterfly coil then proceeds through the further resonant dipole coil and induces a current therein, and the magnetic field of this further resonant dipole coil then produces the inductive coupling of the quadrupole coil to the reception antenna.
There is often a desire in magnetic resonance tomography-guided interventions to display, in a tomogram, the position of an interventional instrument, for example a biopsy arrangement, an endoscope or a pointer for designating a location at the examination subject. It is often advantageous when endoscopic nuclear magnetic resonance images can be prepared directly on site with the interventional instrument. In any case, it be important that this display is independent of the alignment of the interventional instrument.
The aforementioned second object is achieved by arranging apparatus for inductively coupling a nuclear magnetic resonance signal at an invasive part of a medical interventional instrument, this apparatus having three orthogonal imaging areas that at least partially overlap.
The apparatus for inductively coupling a nuclear magnetic resonance signal can be to a catheter or to an interventional instrument and thus allows tracking (localization and navigation) of the instrument in the body using nuclear magnetic resonance technology. Either blood flowing through the apparatus or a specimen with protons, for example paramagnetically doped water, introduced into the marking coils can serve as visible marking of the location in the application with a catheter. Tracking thus can be accomplished using a transmission power that is reduced in comparison to that required for imaging, because the excitation field emitted by a transmission antenna is intensified by the resonant coil arrangements. Since the reduced transmission power is inadequate for exciting the examination subject to nuclear resonance, only the signal of the marking coils inductively coupled into the reception coil appears as the reception signal. The position of the marking coils can be derived from three reception signals registered in orthogonal magnetic gradient fields and, for example, can be entered into a previously registered image. The signal of the marking coils, however, also can be utilized for imaging the area surr
Chen Shih-Chao
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
Wong Don
LandOfFree
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