Patent Application
20060290448
Not applicable.
Not applicable
The present invention relates generally to radio frequency traps and, more particularly, to a tunable resonant cable trap suitable for use with magnetic resonance imaging equipment.
This section of this document is intended to introduce various aspects of art that may be related to various aspects of the present invention described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the present invention. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art.
Electrical conductors used for transmitting signals susceptible to external electromagnetic noise often employ a center conductor surrounded by a conductive shield. The shield is typically grounded to prevent external electric fields from influencing the signal on the central conductor. A common “coaxial cable” shielded conductor, used for radio-frequency (RF) signals, employs a braided or solid shield surrounding a central multi-strand or solid conductor separated from the shield by an insulator of predetermined diameter and dielectric properties. The shield is surrounded, in turn, by a second insulator that protects the shield from damage or electrical contact with other conductors.
In applications where there are intense external electrical/magnetic fields, for example, in magnetic resonance imaging (MRI), significant current may be induced in the shield, causing failure of the shielding effect and possibly damage to the shield and its adjacent insulation from heating. One method of reducing shield current employs an S-trap in which the coaxial cable is wound in a first direction and then optionally a second direction about a cylindrical form to produce a self-inductance among the coils of each winding set. A capacitance is connected in parallel with the inductance (by attaching leads of a capacitor to the shield at separated points in each winding) providing parallel resonant circuits tuned to the particular frequency of the offending external radio frequency field. The resonance provides the shield with a high impedance at the frequency of the interference, resisting current flow at this frequency, while the counter-winding reduces inductive coupling of the trap to the noise.
Another technique for constructing a cable trap involves winding the cable shield to increase its inductance and connecting a capacitor in parallel to the winding to resonate with this inductance. Commonly, the windings are encased in a conducting cylinder that is broken around its circumference to allow the capacitors to be attached. These breaks, however, reduce the shielding effectiveness of the enclosure, and the exposed capacitors provide a potential site for coupling.
Yet another technique, referred to as a floating shield current trap, inductively couples the shield to an inductive member and associated capacitors. No ohmic connection exists between the shield and the trap. In such traps, it is sometimes difficult to achieve enough impedance through the magnetic coupling to provide an effective trap. The effectiveness of this floating shield current trap requires that it be closely tuned to the expected frequency of the shield current.
When such traps are used with MRI equipment, the predominant shield currents will be equal to the Larmor frequency of precessing hydrogen protons within the magnetic field of the MRI machine. The Larmor frequency depends on the strength of the magnet and varies among manufacturers for a given magnet size (e.g. 1.5 Tesla) and for different magnet sizes among a single manufacturer.
It would be desirable for shield current trap to tunable to the specific frequencies of a variety of systems without the open capacitors or poor magnetic coupling evident in the techniques described above.
The present inventors have recognized that a tunable resonant cable trap may be constructed using overlapping members with conductive surfaces coupled in a parallel with a coil defined in the shield of a coaxial cable. The resonant frequency of the cable trap may be varied by varying the degree of overlap between the members. The first and second members may be cylindrical threaded members that may be tuned by rotating one of the members with respect to the other to adjust the amount of overlap.
One aspect of the present invention is seen in a resonant cable trap for use with a shielded cable having an outer shield surrounding at least one inner conductor. The resonant cable trap includes first and second members and a coil defined in the outer shield. The first member has a first conductive surface coupled to the shield. The second member has a second conductive surface coupled to the shield and is disposed to overlap at least a portion of the first member. The first and second conductive surfaces define a capacitor. The capacitor is coupled to the shield in parallel with the coil.
Another aspect of the present invention is seen a method for tuning the resonant cable trap. The method includes adjusting the amount of overlap between the first and second members to tune the resonant frequency of the resonant cable trap.
These and other objects, advantages and aspects of the invention will become apparent from the following description. The particular objects and advantages described herein may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made, therefore, to the claims herein for interpreting the scope of the invention.
The invention will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and:
While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.
One or more specific embodiments of the present invention will be described below. It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. Nothing in this application is considered critical or essential to the present invention unless explicitly indicated as being “critical” or “essential.”
Referring now to the drawings wherein like reference numbers correspond to similar components throughout the several views and, specifically, referring to
Referring to
The material and construction of the outer cylindrical member 40 and inner cylindrical member 45 may vary. In one embodiment, the outer cylindrical member 40 is constructed from a dielectric material (e.g., Teflon®) with a conductive plating (e.g., copper) formed on its outer surface. The inner cylindrical member 45 may be formed of an entirely conductive material (e.g., copper) or a dielectric material with a conductive plating.
The capacitance of the resonant cable trap 10 is affected by factors such as the type and thickness of the materials used for the cylindrical members 40, 45, the diameter of the cylindrical members 40, 45, and the amount of overlap between the cylindrical members 40, 45. In the embodiment of
In some embodiments, a jam nut 60 (not shown in
Still referring to
where L represents the inductance formed by the coil 65 in the coaxial cable 15, and C represents the capacitance of the overlapping cylindrical members 40, 45.
The value of L is determined by the geometry of the coil 65 (e.g., number of turns, turn radius, etc.). The value of C may be varied by changing the amount of overlap between the cylindrical members 40, 45. Hence, the resonant frequency of the resonant cable trap 10 may be tuned to accommodate various applications with differing signal frequencies. For example, a typical MRI machine may have expected radio frequency interference (i.e., Larmor frequency) at an approximate frequency of 64 MHz. However, the application of the resonant cable trap 10 is not limited to any particular frequency range.
In the illustration of
Referring to
Collectively, the plating patterns on the body member 46 and face member 47 form a noise shield. Unshielded, the coil 65 formed in the coaxial cable 15 may act as an antenna for high frequency noise, which could hinder or defeat the noise-reducing purpose of the resonant cable trap 10. The plating patterns compensate for this effect by shielding the coil 65 from all directions. The plating provided by the ring 52 which covers the gap 51 cooperates with plating on the inner surface of the face member 48 to shield the coil from noise entering the annular region 62 from a direction intersecting the face member 48. Noise could still enter the annular region 62 at an extreme angle which bypasses the ring 52 and passes through the face member 48 without hitting the plating on the inner surface, but the magnitude of such a noise component is virtually negligible.
Turning now to
As seen in
Referring now to
In the embodiment of
The resonant cable trap 10 is not limited to the cylindrically shaped overlapping members 40, 45 illustrated. The circular cross section is useful in the embodiment of
The various embodiments described herein provide a resonant cable trap 10 that may be readily tuned to adjust its resonant frequency to match the frequency of expected or measured interference of its intended application. Hence, a particular configuration of the resonant cable trap 10 may be used in a variety of applications (e.g., varying manufacturers or magnet sizes).
The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.
