Patent Application
20100253454
1. Field of the Invention
The invention relates to a coil arrangement or inductive component in the form of a balun for magnetic resonance imaging tomography, and in particular to medical instruments for examination of human and animal bodies.
2. Description of the Relevant Art
Magnetic resonance imaging tomography is an imaging method which is based on the physical phenomenon of magnetic spin resonance. Magnetic resonance imaging is also designated as magnetic resonance tomography, shortened to MRT. An object to be examined is subjected to a strong magnetic field. Owing to this, the spins of nuclei of individual atoms, which previously were statistically distributed, are aligned. Owing to an outer excitation with high-frequency energy, a measurable oscillation is excited. Now in order to make possible a spatial localization, magnetic fields are generated along three spatial axes in the magnetic field by means of gradient coils. Transmission coils are provided for emitting the high-frequency excitation energy. Reception of the excited oscillations is effected by receiving coils. Frequently transmission coils and receiving coils are combined with each other. In the following these various types of coils are also designated as HF coils, because they serve for coupling-in or coupling-out high-frequency signals.
With the non-invasive MRT imaging method, sectional images along desired axes through a human or animal body can be recorded.
Various circuits are known for coupling-on the HF coils to an evaluating circuit of a magnetic resonance imaging tomograph. Owing to their function, these are frequently also designated as baluns. The purpose of these circuits usually consists of adapting a symmetrical HF coil to an unsymmetrical line, and simultaneously of suppressing sheath currents at the resonance frequency of the device. A circuit of this kind is disclosed, for example, in U.S. Pat. No. 5,371,466. The basic circuit is illustrated in FIG. 2A of U.S. Pat. No. 5,371,466. A coil 60 built up of a coaxial cable serves to adapt the symmetrical coil 24 to an unsymmetrical HF receiver. Furthermore, a capacitor 62 is connected in parallel with the coil to form with the coil itself a resonance circuit which is tuned to the resonance frequency of the HF coil 24. This achieves that a signal received by the coil 24 can be transmitted exclusively as a difference signal via the coaxial cable. A sheath current that propagates preferably along the screen of the coaxial cable to a receiver is suppressed by the resonance circuit consisting of the coil 68 and the capacitor 62. The technical design of a device of this kind is illustrated for example in FIG. 5A of U.S. Pat. No. 5,371,466. A coil 102 built-up of a coaxial cable is connected between the input and output terminals of the arrangement. Located on the underside of the arrangement, as illustrated in FIG. 5B of U.S. Pat. No. 5,371,466, and parallel to the coil is a fixed capacitor 106 together with a parallel trimming capacitor 104 for fine adjustment of the resonance frequency of the resonance circuit. A disadvantage of this arrangement is the large requirement of space, and also the difficulty of performing adjustment which is effected first in rough steps by inserting a suitable capacitor 106, and then by fine adjustment by rotating a control knob on a trimming capacitor 104. Furthermore it is known that trimming capacitors of this kind become de-adjusted during the course of time. Accordingly, a rotation of the capacitor from its set position must be prevented, for example with screw-locking lacquer.
The embodiments described herein address some of the disadvantages of prior art systems by introducing a circuit arrangement which permits a simplification of mechanical construction, a reduction of size and manufacturing costs, and also a simplification of adjustment, whilst retaining or improving the electrical properties. Additionally, a coil arrangement has been developed that can be manufactured to be of smaller size and at more favorable cost, thus permitting easier adjustment.
In accordance with an embodiment, these objects may be achieved with an inductive component in the form of a balun for use in magnetic resonance imaging tomographs, including:
a winding formed by a coaxial line to have an inductance;
a holder for accommodating the winding; and
at least one means for changing a geometry of the inductive component for adjusting the inductance of the inductive component.
In the following the invention will be described by way of example, without limitation of the general inventive concept, on examples of embodiment and with reference to the drawings.
While the invention may be susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. The drawings may not be to scale. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but to the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.
The invention comprises a coil 10 of variable length, and thus of variable inductance. A change of inductance is effected by shortening or stretching the coil by means of a mechanical adjusting element, for example an adjusting screw 34. By adapting the geometry of the coil in this way and correspondingly changing the inductance, a simple adjustment of the resonance frequency of the resonance circuit consisting of the coil 13 and the capacitor 16 connected in parallel with this is possible. In order to achieve as large as possible a suppression of sheath currents, it is of importance that the parallel resonance circuit including the coil 13 and the parallel capacitor 16 be of high quality. However, this also makes necessary a particularly exact adjustment of the resonance frequency. The coils known from prior art already exhibit considerable variations of inductance owing to mechanical tolerances of fabrication. A substantial improvement can be achieved here by the coil being built into a suitable holder. Thus, its geometry cannot be changed inadvertently during assembly. Because here the inductance is already exactly fixed, a determination of suitably matching capacities during adjustment of the resonance frequency can be frequently dispensed with. Thus, usually capacities having narrow tolerances and previously calculated values can be connected in parallel with the coil. Now in order to dispense with a trimming capacitor, i.e. an adjustable capacitor during fine adjustment, the inductance of the coil is adjusted. As the coil is used in an inner magnetic field of magnetic resonance imaging tomographs, it must not contain any ferromagnetic materials. An adjustment by altering a core introduced into the coil is thus not possible. In one embodiment, the adjustment is effected by deforming the coil. For this, in particular, its length is changed. In a typical case of application, complete adjustment can be effected by using capacitors 16 of narrow tolerance, which typically still have a tolerance of 1%, and by altering the inductance by a similar order of magnitude, namely about 1%. For this, as a first approximation, a change of length of an order of magnitude of only 1% is necessary. A change of length of this kind can be achieved with components that are firmly mounted on printed circuits.
In an especially advantageous embodiment of the invention, a holder is provided for the coil, which holder has a first fastening block 30 and also a second fastening block 31. The first fastening block fixes a part of the coil, by holding preferably a first turn, whilst the second fastening block fixes another part of the coil, by holding preferably a further turn, so that at least one non-fixed turn is located between the first fastening block and the second fastening block. Preferably a plurality of non-fixed turns are located between the two fastening blocks. Adjustment of the inductance is effected by changing the distance between the fastening blocks. If the fastening blocks 30 and 31 are moved close to each other, then the coil is shortened, whilst an increase of the distance between the fastening blocks stretches the coil. Accordingly, the spacing between the turns, or the entire length of the coil, changes. When the coil is shortened, its inductance increases, whilst it is reduced during stretching. As the adjusting element, preferably a screw is provided with which the distance between the first and the second fastening block can be adjusted. For use in magnetic resonance imaging tomographs this screw must be of a non-ferromagnetic material. Here, in particular, a screw of a synthetic material, and more preferably a screw comprising a glass-fiber-reinforced synthetic material can be chosen in order to achieve a high stability and long-time constancy of a set value.
In order to achieve a long-time stability of the arrangement, the coaxial line 20 is preferably designed to be a semi-rigid line. Of course, a conventional coaxial cable also can be used.
In another embodiment, the individual turns of the coaxial line 20 are designed to have a reduced tilt angle 25. Typically, an angle at which a turn of a coil is inclined to the center axis of the coil, here designated as angle of tilt, is determined by the thickness of the coaxial line from which the coil has been formed, provided that there is no space between adjacent turns. According to an embodiment, the angle of tilt of the turns with respect to the center axis of the coil can be reduced further, so that an oblique winding results. Furthermore, according to an embodiment an adjustability of the angle of tilt can be utilized for fine adjustment of the inductance. For this, the length of the coil can remain unchanged. In another embodiment, holder elements are mounted laterally of the coil, which holder elements are urged against each other for adjustment of the inductance, so that the tilt angle of the individual turns changes.
In another embodiment the winding is embedded in an at least slightly elastic mass, such as silicon rubber for example. Furthermore, a means is provided, for example a screw, for shortening or stretching the mass. It is of special advantage when a threaded element for engagement with the screw is provided in at least at one place of the mass. Furthermore, it is of advantage when another additional rigid element is provided, against which the head of the screw abuts. However, this can be also a turn of the coaxial line 20.
Further modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein are to be taken as examples of embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the invention may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Changes may be made in the elements described herein without departing from the spirit and scope of the invention as described in the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102008041465.4 | Aug 2008 | DE | national |
This application claims priority from pending German Patent Application No. 10 2008 041 465.4 filed on Aug. 22, 2008 and is a Continuation of U.S. patent application Ser. No. 12/542,491 filed Aug. 17, 2009.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12542491 | Aug 2009 | US |
| Child | 12752679 | US |
