System and method for annular shielded two-wire feeder cable trap

Abstract

The present invention is entitled systems and methods for annular shielded two-wire feeder cable traps. The present invention provides various methods and systems for flexible, lightweight and low-cost radio frequency (RF) traps used in magnetic resonance imaging (MRI) systems. In one example, a radio frequency (RF) trap assembly for use in a magnetic resonance imaging (MRI) system is provided, the RF trap assembly comprising: a shielded two-wire feeder conductor assembly having a plurality of annular portions, each of the plurality of annular portions being in tangential contact with a shielded cable; and at least one support structure for substantially maintaining the shape of the plurality of annular portions, the support structure surrounding a portion of the shielded cable, wherein the shielded two-wire feeder conductor assembly is tuned to a frequency suitable for increasing the impedance of the shielded cable.

Description

Technical Field

[0001] Embodiments of the subject matter disclosed herein relate to magnetic resonance imaging (MRI), and more particularly, to MRI radio frequency (RF) cable traps. Background Art

[0002] In magnetic resonance imaging (MRI), a subject is placed in a magnet. The subject is a person (alive or dead), an animal (alive or dead), or a part of a person or an animal. When the subject is in a magnetic field generated by a magnet, the magnetic moments of the nuclei (such as protons) attempt to align with the magnetic field, but are processed around the magnetic field in a random order at the Larmor frequency of the nuclei. The magnetic field of the magnet is called B0 and is in the longitudinal or z direction. In acquiring MRI images, a magnetic field in the xy plane and close to the Larmor frequency (called the excitation field B1) is generated by a radio frequency (RF) coil and can be used to rotate or "tilt" the net magnetic moment Mz of the nucleus from the z direction to the transverse or xy plane. After the excitation signal B1 terminates, a signal called an MR signal is emitted by the nucleus. In order to use the MR signal to generate an image of the subject, magnetic field gradient pulses (Gx, Gy, and Gz) are used. Gradient pulses are used to scan through k-space, the space or reciprocal of the spatial frequency. There is a Fourier relationship between the acquired MR signals and the image of the subject, and thus the image of the subject can be derived by reconstructing the MR signals.

[0003] RF coils are used to transmit RF excitation signals in MRI systems and receive MR signals emitted by the imaged subject. Coil crossover cables, such as coaxial cables, can be used to transmit signals between the RF coils and other aspects of the processing system. Coaxial cables are designed to prevent the system from picking up external RF signals present in the environment. Coaxial cables are characterized by a surrounding shield or ground conductor separated from the current-carrying center conductor by a dielectric material. The surrounding ground conductor acts as a shield that minimizes the pickup of extraneous frequencies by the center conductor of the cable.

[0004] Despite the use of coaxial cable, there are still various problems with its use. For example, the shielded conductor of the coaxial cable itself tends to carry extraneous induced currents such as from TV transmissions, stray harmonics from gradient pulse oscillators and clock circuits in nearby equipment, etc. The induced current is often called "skin current" because it flows outside the shielded conductor. Stray RF currents tend to flow out of the hole and into other circuits, such as amplifiers, analog-to-digital converters, receivers, and reconstruction processors to cause errors in the resulting image.

[0005] Balanced / unbalanced ("balun") circuits are used as a means for reducing or "trapping" noise and / or stray RF currents generated by induced currents in coaxial cables. Prior art baluns consist of LC frequency filters for each cable located in a copper shielded box. The balun is tuned to the frequency of interest, such as by tuning capacitors. In the art, the terms "balun" and "RF trap" are often used interchangeably. A cable RF trap has essentially the same function, but can be placed anywhere on the cable. Its role is to reduce cable radiation.

[0006] A balun can be used to connect a balanced transmission line (such as a twisted pair - consisting of two conductors with two surfaces) to an unbalanced transmission line (a coaxial cable - also consisting of two conductors, but with three surfaces, each capable of sustaining independent high frequency currents). The balun forces the signal from the balanced transmission line to travel on two (of the three) surfaces of the unbalanced transmission line. This is achieved by creating a high impedance on the third surface that realizes the interface between the two transmission lines at the ends of the transmission line.

[0007] However, prior art baluns are problematic for a number of reasons. For example, the balun is inserted in line with each coaxial cable that interconnects the balun. As the number of RF channels increases, the number of coaxial connections becomes labor intensive. Prior art baluns are also space consuming. In MRI systems, there are severe space limitations, creating a need to reduce the size and volume associated with the balun. Similarly, prior art baluns are expensive due to the use of dedicated non-magnetic tuning capacitors, and can be difficult and time consuming to manufacture. Such cable traps are problematic for a number of reasons, including: they are bulky; they are difficult to tune, and it is difficult to check the tuning; and it is often difficult to find the optimal location for the trap. Summary of the invention

[0008] In one embodiment, an RF trap for blocking stray signals on a shielded RF cable having an outer shield conductor and at least one inner conductor for carrying an RF (radio frequency) signal comprises: a conductive system that is inductively coupled to the shield conductor of the shielded RF cable, the conductive system comprising two parallel conductors, wherein at least one of the conductors is non-adjacent; and a capacitive coupling between the conductive system and the shield conductor, the capacitive coupling being sized and the conductive system being shaped such that the capacitive coupling and the inductive coupling define an LC circuit having a resonant frequency at the frequency of the RF signal carried on the inner conductor. The RF trap comprises a support structure for substantially maintaining the shape of the conductive system and which can be sized to surround the shielded RF cable, the shape of the conductive system comprising a plurality of loops wrapped around the support structure.

[0009] It should be understood that the above brief description is provided to introduce in a simplified form selected concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is solely defined by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The present disclosure will be better understood by reading the following description of non-limiting embodiments with reference to the accompanying drawings, in which:

[0011] Figure 1 is a schematic diagram of an exemplary magnetic resonance imaging (MRI) system.

[0012] Figure 2A-2D An embodiment of the present disclosure is shown.

[0013] Figure 3 An embodiment of the present disclosure is shown.

[0014] Figure 4 An illustration of the tuning of an embodiment of an RF trap.

[0015] Figure 5 is a cross section of an RF trap shown with a magnetic field.

[0016] Figure 6 is a cross section of the conductive system of the FR trap of the present disclosure.

[0017] Figure 7 Multiple consecutive and / or contiguous RF traps along a linear shielded RF cable are shown.

[0018] Figure 8 An embodiment of the present disclosure is shown. DETAILED DESCRIPTION

[0019] The following description relates to various embodiments of a radio frequency (RF) trap in an MRI system. Specifically, systems and methods for a low-cost, flexible, and lightweight RF trap are provided that effectively attenuate stray RF currents and noise in RF transmission cables. The RF trap can be used with any cable associated with an MRI system, and can be used with coaxial cables. Typically, coaxial cables used in MRI systems are composed of a braided tubular conductor that can be separated from a current-carrying center conductor by a dielectric material. The tubular outer conductor can be grounded to act as a shield that minimizes the pickup of extraneous frequencies by the center conductor. However, when using coaxial cable, coupling problems may still exist at high frequencies associated with magnetic resonance imaging and spectroscopy. Specifically, the braid or shield of the coaxial cable itself can carry extraneous induced currents or "skin currents." Such stray RF currents tend to flow out of the cable holes and into other circuits in the magnetic resonance device.

[0020] The RF trap of the present disclosure includes a conductive system that can be inductively coupled to a shielded conductor of a shielded RF cable. The conductive system can include two parallel conductors, wherein one or both of the conductors are cut or otherwise not adjacent. The conductive system can be capacitively coupled to the shielded conductor of the shielded RF cable, such that the capacitive coupling and the inductive coupling can define an LC circuit that can have a resonant frequency at or near the frequency of an RF signal carried on the inner conductor of the shielded RF cable.

[0021] Each RF trap may include a support structure that may substantially maintain the annular shape of the conductive system. The support structure may be torus-shaped, wherein each loop of the conductive system passes through the center of the torus and completely surrounds the outer periphery of the support structure. The center of the torus (including the conductive system positioned therein) may be sized to slidably fit over the shielded RF cable so that the conductive system may capacitively couple to the shielded conductor of the shielded RF cable.

[0022] The support structure of the RF trap may include an insulating material and may include, for example, polytetrafluoroethylene (PTFE). For example, the support structure may be solid or may be an inflatable structure. In either case, the support structure may generally take the form of a "ring", thereby allowing a ring or circle-like structure of the conductive system to be maintained around the perimeter of the shielded RF cable. Similarly, the support structure may be incorporated into the conductive system so as to provide sufficient rigidity to maintain the ring or circle-like structure without having a different support structure.

[0023] The diameter of the RF trap may be from about 2 cm to about 5 cm, and may preferably be less than about 3 cm in diameter. Similarly, the support structure may be from about 1 cm to about 4 cm at its thickest portion, and may preferably be less than about 2 cm in thickness. The number of ring-like or circular structures in each RF trap may vary depending on the impedance and tuning requirements and the size of the conductive system. Each RF trap may include more than two but less than twenty rings, and may preferably include fourteen rings.

[0024] Figure 1 A schematic diagram of an exemplary MRI system 10 is shown. In an exemplary embodiment, the MRI system 10 includes a workstation 12 having a display 14 and a keyboard 16. The workstation 12 includes a processor 18, such as a commercially available programmable machine running a commercially available operating system. The workstation 12 provides an operator interface that allows a scan prescription to be entered into the MRI system 10. The workstation 12 is coupled to a pulse sequence server 20, a data acquisition server 22, a data processing server 24, and a data storage server 26. The workstation 12 and each of the servers 20, 22, 24, and 26 communicate with each other.

[0025] In an exemplary embodiment, the pulse sequence server 20 responds to instructions downloaded from the workstation 12 to operate the gradient system 28 and the radio frequency ("RF") system 30. The instructions are used to generate gradient waveforms and RF waveforms in the MR pulse sequence. The RF coil 38 and the gradient coil assembly 32 are used to execute the prescribed MR pulse sequence. The RF coil 38 is shown as a whole body RF coil. The RF coil 38 can also be a local coil that can be placed near the anatomical structure to be imaged, or a coil array including multiple coils.

[0026] In the exemplary embodiment, a gradient waveform for performing a prescription scan is generated and applied to a gradient system 28, which excites the gradient coils in a gradient coil assembly 32 to generate a magnetic field gradient G for position encoding of the MR signals. x , G y and G z The gradient coil assembly 32 forms part of a magnet assembly 34 , which also includes a polarizing magnet 36 and an RF coil 38 .

[0027] In an exemplary embodiment, the RF system 30 includes an RF transmitter for generating RF pulses used in an MR pulse sequence. The RF transmitter responds to the scan prescription and instructions from the pulse sequence server 20 to generate RF pulses with a desired frequency, phase, and pulse amplitude waveform. The generated RF pulses can be applied to the RF coil 38 by the RF system 30. The responsive MR signals detected by the RF coil 38 are received by the RF system 30 and amplified, demodulated, filtered, and digitized under the instructions of the command generated by the pulse sequence server 20. The RF coil 38 is described as a transmitter and receiver coil, so that the RF coil 38 transmits RF pulses and detects MR signals. In some embodiments, the MRI system 10 may include a transmitter RF coil that transmits RF pulses and a separate receiver coil that detects MR signals. The transmission channel of the RF system 30 can be connected to the RF transmission coil, and the receiver channel can be connected to the separate RF receiver coil. Typically, the transmission channel is connected to the entire body RF coil 38, and each receiver segment is connected to a separate local RF coil.

[0028] In an exemplary embodiment, the RF system 30 also includes one or more RF receiver channels. Each RF receiver channel includes an RF amplifier that amplifies the MR signals received by the RF coil 38 to which the channel is connected, and a detector that detects and digitizes the I quadrature component and the Q quadrature component of the received MR signal. The magnitude of the received MR signal can then be determined as the square root of the sum of the squares of the I component and the Q component, as shown in the following equation (1):

[0029]

[0030] And the phase of the received MR signal can also be determined as shown in the following equation (2):

[0031]

[0032] In some embodiments, the pulse sequence server 20 also optionally receives subject data from a physiological acquisition controller 40. The controller 40 receives physiological signals from sensors connected to the subject, such as electrocardiogram ("ECG") signals from electrodes, or respiration signals from a respiratory monitoring device such as bellows. The physiological signals are typically used by the pulse sequence server 20 to synchronize or "gate" the performance of the scan with the heartbeat or respiration of the subject.

[0033] In an exemplary embodiment, the pulse sequence server 20 is also connected to a scan room interface circuit 42, which receives signals from sensors associated with the conditions of the subject and the magnet system. Through the scan room interface circuit 42, a subject positioning system 44 receives commands to move the subject to a desired position before and / or during scanning.

[0034] In an exemplary embodiment, the digitized MR signal samples generated by the RF system 30 are received by the data acquisition server 22. The data acquisition server 22 can operate in response to instructions downloaded from the workstation 12 to receive real-time MR data and provide buffer storage so that no data is lost due to data overload. In some scans, the data acquisition server 22 only passes the acquired MR data to the data processing server 24. However, in scans where information derived from the acquired MR data is needed to control further execution of the scan, the data acquisition server 22 is programmed to generate the required information and transmit it to the pulse sequence server 20. For example, during a pre-scan, MR data is acquired and used to calibrate the pulse sequence executed by the pulse sequence server 20. In addition, navigator signals can be acquired during a scan and used to adjust operating parameters of the RF system 30 or gradient system 28, or to control the order of views in which k-space is sampled.

[0035] In an exemplary embodiment, the data processing server 24 receives MR data from the data acquisition server 22 and processes the MR data according to instructions downloaded from the workstation 12. Such processing may include, for example, Fourier transforming the raw k-space MR data to produce a two-dimensional or three-dimensional image, applying filters to reconstructed images, performing back-projection image reconstruction on the acquired MR data, generating functional MR images, and calculating motion or flow images.

[0036] In an exemplary embodiment, the image reconstructed by the data processing server 24 is transmitted back to the workstation 12 where the image is stored. In some embodiments, the real-time image is stored in a database memory cache ( Figure 1 ), the real-time image can be output from the database memory cache to the operator display 14 or a display located near the magnet assembly 34 ( Figure 1 The images are stored in a disk storage device 48 or in a host database on the cloud. The data processing server 24 notifies the data storage server 26 on the workstation 12 when such images have been reconstructed and transferred to the storage device. The operator can use the workstation 12 to archive the images, make movies, or send the images to other facilities via the network.

[0037] Now go to Figure 2A, the RF trap or balun 200 comprises a spatially distributed inductor, which in a preferred embodiment is a conductor system 210 comprising a set of wire loops wrapped around a toroidal support structure 220 such that each loop of the conductor system 210 is spaced apart on both the outside and inside of the support structure 220. In a preferred embodiment, the support structure comprises polytetrafluoroethylene (PTFE) and is a solid structure. The size of the hole in the center of the support structure 220 can be designed to allow the RF trap to slidably fit onto a shielded RF cable, such as a coaxial cable used in an MRI system. For example, the use of PTFE can allow the support structure 220 to be semi-rigid and partially deform, thereby providing a friction fit between the wire loops of the conductor system 210 and the coaxial cable.

[0038] like Figure 2B As further shown in FIG. 2 , the shielded conductors of the shielded RF cable 230 can be capacitively coupled to the conductive system 210 (shown without the support structure 220). The support structure used with the present disclosure may include any known insulator material that can provide support for the conductive system 210 and can take the form of a solid or inflatable structure, such as a structure including, for example, a bladder. The support structure may also be included in or incorporated into the wire used in the conductive system 210 so that the wire is stiff enough to maintain the shape of any loops during handling, installation, and use of the RF trap 200. Although Figure 2A A uniformly rounded torus-shaped support structure is shown in FIG, but the support structure may take any shape and may be, for example, rectangular or rhombic in its cross section.

[0039] In a preferred embodiment of the present disclosure, at least one of the two parallel wires in the conductor system 210 is non-adjacent, with its end adjacent to its lead. Figure 2C As shown, each of the parallel conductors of the conductor system 210 may be non-adjacent, with the ends of each parallel conductor adjacent to its lead. In such an embodiment, the present disclosure may be tuned to approximately 128 MHz. Similarly, as Figure 2D As shown, the parallel conductors may be formed from a single conductor that may be crimped, twisted, or otherwise manipulated to provide two parallel conductors over a majority of the conductive system. In such an embodiment, the present disclosure may be tuned to approximately 64 MHz.

[0040] The number of wire loops in the conductor system 210 may be, for example, between eight (8) and twenty (20). Generally, more turns are desired. The number of turns may be limited by, for example, the cross-section of the shielded twinax feeder and the inner diameter of the toroidal coil. The number of loops may be determined by the overall size of the wire used, the size of the shielded RF cable, and / or the size and shape of the support structure. In a preferred embodiment, the wire used in the loops may lie flat against the support structure or otherwise perpendicular to the shielded RF cable, with the wires placed side by side along the inner loop of the support structure (e.g., Figure 2A ). In embodiments of the present disclosure, the loops of the wire may abut one another and may be spaced apart from one another by up to at least 0.5 mm, for example along an inner ring of the support structure.

[0041] In embodiments of the present disclosure, the conductor may be twisted or turned at at least one location, and each loop may include, for example, at least one twist or turn. Variations in the number of twists, turns, and spacing between loops of the conductor may allow for different tunings of the RF trap. Similarly, as discussed above, the shape and material type of the support structure may also play a role in the tuning and capacitance of the RF trap. The RF trap of the present disclosure is preferably a tunable capacitor by which the RF trap can be tuned to match a frequency range.

[0042] In an embodiment of the present disclosure, the conductor system of the RF trap may include a shielded two-wire feeder conductor (two parallel conductors within a dielectric sheath) wrapped around an elliptical annular support structure comprising a low dielectric loss material (such as PTFE). For a shielded RF cable, the conductor shape may be defined by the following equation:

[0043] x c (t) = cos(t)(r0+acos(nt))

[0044] y c (t) = sin(t)(r0+acos(nt))

[0045] z c (t) = bsin(nt)

[0046] Where n represents the number of turns of the shielded two-wire feed line around the support structure and r0,a,b represents the geometry of the support structure.

[0047] To account for the direction of any twist of the conductor system, a directional parameter σ = ± 1 may be used. Thus, the equation for the first conductor may be:

[0048] x 1σ (t)=r0cos(σt)+acos(σt)cos(nt)+r1sin(σt)

[0049] y 1σ(t)=r0sin(σt)+asin(σt)cos(nt)-r1cos(σt)

[0050] z 1σ (t) = bsin(nt)

[0051] And the equation for the second wire may be:

[0052] x 2σ (t)=r0cos(σt)+acos(σt)cos(nt)-r1sin(σt)

[0053] y 2σ (t)=r0sin(σt)+asin(σt)cos(nt)+r1cos(σt)

[0054] z 2σ (t) = bsin(nt)

[0055] where n represents the number of turns in the conductor and r0,a,b represents the geometry of the support structure, and where:

[0056] Table 1

[0057]

[0058] corresponds to a regular ellipse) and the order of the following function.

[0059]

[0060] Therefore, the center line is defined as:

[0061] x c (t) = cos(t)(r0+aψ s (cos(nt)))

[0062] y c (t) = sin(t)(r0+aψ s (cos(nt)))

[0063] z c (t) = bψ s (sin(nt))

[0064] And the wire is defined as:

[0065] x (1,2)σ (t)=r0cos(σt)+acos(σt)ψ s (cos(nt))±r1sin(σt)

[0066]

[0067] z (1,2)σ (t) = bψ s (sin(nt)).

[0068] In the embodiments of the present disclosure, Figure 8 As shown, a super-elliptical toroidal coil of order s=3 may be used with an RF trap and may include a pair of wires 720 .

[0069] An RF trap having parameters such as those in Table 1 may be tuned to approximately 127.5 MHz, as Figure 4 , and can demonstrate the magnetic field generated by the current from the shielded RF cable, such as Figure 5 As shown. In an embodiment of the present disclosure, the RF trap can generate a magnetic field having at least 100A / m within the boundaries of the trap. The mutual electromagnetic coupling between the resonant circuit of the present disclosure and the shielded RF cable generates a high impedance on the shielded RF cable at the resonant frequency, which is used to block the RF current on the cable shield. Although the shielded RF cable has been described herein as a coaxial cable, other RF cable types (such as triaxial cable) can also be captured by the RF trap.

[0070] like Figure 6 As shown, the conductor system 210 of the present disclosure may have two wires 610 enclosed in a dielectric jacket 615, which may contain, for example, PTFE. Each of the wires 610 may also include a set of discrete wires 620. In an embodiment of the present disclosure, the height of the conductor system 210 may preferably be about 1 mm, and the width may be about 1.5 mm to about 2 mm, and the diameter of each of the wires 610 may be about 0.36 mm. Each of the wires 610 may be vertically centered within the dielectric jacket 615, and may be horizontally centered in each half of the dielectric jacket 615 of the conductor system 210.

[0071] In an embodiment of the present disclosure, each conductor 610 may include a subset of seven conductors 620. Each conductor in the conductors 620 may be composed of a conductive metal (such as copper) and may preferably be parallel to each other. In an embodiment of the present disclosure, one or more of the conductors 620 may be twisted around each other to form at least one twisted pair. The housing 625 of the conductor system 210 may include the same material as the dielectric jacket 615 or may include a supporting structural material. As will be appreciated by those skilled in the art, the housing 625 may be coated with a relatively hard but flexible material, such as a thermoplastic, for example to provide self-supporting rigidity to the conductor system 210 when in, for example, an annular form.

[0072] Two or more RF traps can be used together in series to increase the impedance along the shielded RF cable. For example, with each RF trap tuned to approximately the same frequency, a single RF trap can increase the impedance by about 84 ohms, two RF traps in series can increase the impedance by about 167 ohms, and a plurality of about seven RF traps can increase the impedance by about 550 ohms. Figure 7 As shown, a plurality of RF traps 200 may reside in series on a portion of a shielded RF cable 230 and may preferably be spaced about 1 cm from one another. Alternatively, each RF trap 200 may be spaced a distance equal to the thickness 710 of the RF trap. In an embodiment of the present disclosure, a plurality of ten RF traps having a thickness 710 of about 1 cm and spaced about 1 cm from one another may provide an impedance increase of about 780 ohms. Although spacing is preferred, if any of the plurality of RF traps may be in contact with one another, the impedance characteristics of a series of RF traps will not substantially change. In practice, maintaining spacing between RF traps may be impractical or impossible within the confines of an MRI machine.

[0073] like Figure 7 As shown, an RF trap may be placed on a linear portion of a shielded RF cable. In embodiments of the present disclosure, the shielded RF cable may be curved. The curvature of the shielded RF cable allows for greater impedance for both a single RF trap and multiple RF traps in series. For example, a single RF trap on a curved shielded RF cable may increase impedance by approximately 87 ohms, where two RF traps placed in series along the same curvature of the cable may increase impedance by approximately 193 ohms (again assuming that each RF trap is tuned to approximately the same frequency). Similarly, a series of ten RF traps along the curvature may increase impedance by approximately 880 ohms, compared to an increase of approximately 780 ohms.

[0074] Generally speaking, when RF traps with the same characteristics are added in series with each other, the impedance increases almost proportionally. In embodiments of the present disclosure, multiple RF traps may not be tuned to the same frequency. For example, RF traps tuned to 64 MHz and 128 MHz may alternatively be placed in series to produce various impedance results. As will be appreciated by those skilled in the art, 64 MHz is the resonant frequency of a 1.5 T MRI system, and 128 MHz is the resonant frequency of a 3 T MRI system. By using a subset of each type of RF trap, for example, a cable having such an array of RF traps may be compatible with both system use cases.

[0075] As used herein, the elements or steps listed in the singular and beginning with the word "one" or "a kind of" should be understood as not excluding a plurality of the elements or steps, unless such exclusion is explicitly stated. In addition, the reference to "an embodiment" of the present disclosure is not intended to be interpreted as excluding the existence of additional embodiments that also include the described features. In addition, unless explicitly stated to the contrary, the embodiments of "including", "comprising" or "having" the elements or multiple elements with a specific characteristic may include additional such elements without the characteristic. The terms "including" and "in..." are used as the concise language equivalents of the corresponding terms "including" and "wherein". In addition, the terms "first", "second" and "third" etc. are only used as marks, and are not intended to impose numerical requirements or specific positional order on their objects.

[0076] This written description uses examples to disclose the disclosure, including the best mode, and also to enable a person of ordinary skill in the relevant art to practice the disclosure, including making and using any devices or systems and performing any included methods. The patentable scope of the disclosure is defined by the claims, and may include other examples that occur to a person of ordinary skill in the art. Such other examples are intended to fall within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with minor differences from the literal language of the claims.

Claims

1. An RF trap for blocking stray signals on a shielded RF cable having an outer shield conductor and at least one inner conductor for carrying an RF signal, the RF trap comprising: a conductive system inductively coupled to the shielded conductor of the shielded RF cable, the conductive system comprising two parallel conductors, wherein at least one of the conductors is non-adjacent; a capacitive coupling between the conductive system and the shield conductor, the capacitive coupling being sized and the conductive system being shaped such that the capacitive coupling and the inductive coupling define an LC circuit having a resonant frequency at the frequency of the RF signal carried on the inner conductor; and A support structure for maintaining the shape of the conductive system and sized to surround the shielded RF cable, the shape of the conductive system comprising a plurality of loops wrapped around the support structure.

2. The RF trap of claim 1, wherein the support structure is torus-shaped.

3. The RF trap of claim 1, wherein the support structure comprises a material containing polytetrafluoroethylene. The RF trap of claim 1 , wherein the conductive system comprises the support structure. The RF trap of claim 1 , wherein each of the conductors is non-contiguous.

6. The RF trap of claim 1, wherein the impedance of the LC circuit reaches a maximum at 128 MHz.

7. The RF trap of claim 1, wherein the conductive system comprises at least 20 loops.

8. A magnetic resonance imaging system, comprising: a magnet system configured to generate a polarized magnetic field around at least a portion of a subject disposed in the magnetic resonance imaging system; a magnetic gradient system, the magnetic gradient system comprising a plurality of magnetic gradient coils configured to apply at least one magnetic gradient field to the polarizing magnetic field; a radio frequency system configured to apply an RF field to the subject and receive magnetic resonance signals from the subject using a coil array; one or more RF traps, the one or more RF traps comprising a conductive system that is inductively coupled to a shielded conductor of a shielded RF cable, each of the conductive systems comprising two parallel conductors, wherein at least one of the conductors is non-adjacent; a capacitive coupling between the conductive system and the shield conductor, the capacitive coupling being sized and the conductive system being in a toroidal shape such that the capacitive coupling and the inductive coupling define an LC circuit having a resonant frequency at the frequency of the RF signal carried on the inner conductor; and a support structure for maintaining the annular shaped conductive system and sized to surround the shielded RF cable; Wherein each of the plurality of RF traps are non-adjacently located to one another along the shielded RF cable.

9. The system of claim 8, wherein the impedance of the LC circuit is 780 ohms.

10. The system of claim 8, wherein the support structure is in the shape of a torus, ellipse, or superellipse having an order greater than 2.

11. The system of claim 8, wherein the support structure comprises a material comprising polytetrafluoroethylene.

12. The system of claim 8, wherein each of the conductors of at least one RF trap of the plurality of RF traps is non-contiguous.

13. The system of claim 8, wherein the LC circuit of at least one of the plurality of RF traps is tuned to 64 MHz, and wherein the LC circuit of at least one of the plurality of RF traps is tuned to 128 MHz.

14. The system of claim 8, wherein the support structure of at least one RF trap of the plurality of RF traps has a diameter of less than 3 cm.

15. The system of claim 8, wherein the conduction system of at least one RF trap of the plurality of RF traps comprises more than two but less than fourteen rings.

Images