Abstract:
A multiple-frequency RF trap and an MRI system including a multiple-frequency RF trap. The multiple-frequency RF trap being tuned to block RF energy at two frequencies.

Description:
FIELD OF THE INVENTION 
     This disclosure relates generally to an RF trap and an MRI system that is tuned to block RF energy at two different frequency bands. 
     BACKGROUND OF THE INVENTION 
     Magnetic resonance imaging (MRI) is a medical imaging modality that can create images of the inside of a human body without using x-rays or other ionizing radiation. MRI uses a powerful magnet to create a strong, uniform, static magnetic field. When a human body, or part of a human body, is placed in the main magnetic field, the nuclear spins that are associated with the hydrogen nuclei in tissue or fat become polarized. This means that the magnetic moments that are associated with these spins become preferentially aligned along the direction of the main magnetic field, resulting in a small net tissue magnetization along that axis. An MRI system also comprises components called gradient coils that produce smaller amplitude, spatially varying magnetic fields when a current is applied to them. Typically, gradient coils are designed to produce a magnetic field component that is aligned along the z-axis and that varies linearly in amplitude with position along one of the x, y, or z-axes. The effect of a gradient coil is to create a small ramp on the magnetic field strength and, in turn, on the resonant frequency of the nuclear spins along a single axis. Three gradient coils with orthogonal axes are used to “spatially encode” the MRI signal by creating a signature resonance frequency at each location in the body. Typically a radio frequency (RF) body coil is used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei. The RF body coil is used to add energy to the nuclear spins in a controlled fashion. As the nuclear spins then relax back to their rest energy state, they give up energy in the form of an RF signal. The RF signal is detected by one or more RF receive coils and is transformed into an image using a computer and known reconstruction algorithms. 
     In order to work most effectively, it is important that the RF receive coils are isolated from electrical noise and stray currents. The RF receive coils typically pass the RF signal to a processor in the MRI system by way of one or more coaxial cables. Even though the coaxial cables typically include a layer of conductive shielding, it is possible for currents to be induced on the outer conductive shielding during transmit and receive phases. These induced currents distort the original transmit or receive fields and need to be minimized for optimal imaging. In addition to degrading the image quality, having excessive RF current on the coaxial cables can lead to overheating within the RF receive coils. Since the RF receive coils are typically placed very close to the patient, overly high temperatures can also lead to patient discomfort. A typical technique used to eliminate stray or induced currents on the conductive shielding of the coaxial cables involves creating a high impedance by placing multiple RF traps along the conductive shielding of the coaxial cables. 
     In a conventional MRI system, each RF trap is typically tuned to a single frequency. For example, in a 3T MRI system, each RF trap is tuned so that it creates a high impedance at the resonance frequency of H (hydrogen), which is around 128 MHz. However, recent developments have shown that a double-tuned RF coil could be useful for creating images at more than one resonant frequency. For example, some of the double-tuned RF coils are used to obtain RF signals from both hydrogen and C13 (carbon 13). In order to eliminate the problems associated with excess RF current on the coaxial cable, it is necessary to have RF traps to eliminate excess current at the resonant frequency of H and at the resonant frequency of C13. For a 3T system, this equates to a resonant frequency of approximately 128 MHz for H and approximately 31 MHz for C13. Using conventional designs, RF traps tuned to 128 MHz and separate RF traps tuned to 31 MHz would be needed for the coaxial cables of a 3T MRI system using a double-tuned RF coil. However, modern MRI systems are very tightly packaged, particularly in the region surrounding the RF coil and associated coaxial cables. It is clear that simply increasing the number of RF traps will lead to wasting unnecessary space. Also, since there is a desire both to keep the patient bore as large as possible for patient comfort and to have the smallest possible magnet to minimize the cost of the MRI system, it is clearly undesirable to add additional space-consuming RF traps to existing designs. Therefore, in order to address these problems as well as others, there is a need for an RE trap that is tuned for multiple resonant frequencies. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The above-mentioned shortcomings, disadvantages and problems are addressed herein which will be understood by reading and understanding the following specification. 
     In an embodiment, a multiple-frequency RF trap for one or more shielded cables includes a first cylindrical member disposed around the one or more shielded cables. The first cylindrical member includes a first conductive cylinder and a first capacitive component electrically connected to the first conductive cylinder. The first cylindrical member is electrically connected to a cable shield surrounding the one or more shielded cables. The multiple-frequency RF trap includes a second cylindrical member disposed around the first cylindrical member. The second cylindrical member includes a second conductive cylinder and a second capacitive component electrically connected to the second conductive cylinder. The second cylindrical member is electrically connected to the cable shield. The multiple-frequency RF trap also includes an outer shield surrounding the second conductive cylinder. The outer shield is electrically connected to the cable shield. 
     In an embodiment, a multiple-frequency RF trap for an MRI system includes a first former with a first conductive path in the shape of a first discontinuous figure-eight. The first former is adapted to receive one or more cables wrapped in a first figure-eight pattern. The multiple-frequency RF trap includes a first capacitive component electrically connected to the first former, where the first capacitive component completes a first LC circuit with the first former. The first LC circuit is tuned to a first RF frequency. The multiple-frequency RF trap includes a second former with a second conductive path in the shape of a second discontinuous figure-eight, where the second former is adapted to receive one or more cables wrapped in a second figure-eight pattern. The second former is positioned at a generally perpendicular angle to the first former. The multiple-frequency RF trap also includes a second capacitive component electrically connected to the second former, where the second capacitive component completes a second LC circuit with the second former. The second LC circuit is tuned to a second RF frequency. 
     In another embodiment, an MRI system includes a superconducting main coil configured to generate a B0 field, an RF body coil disposed inside the superconducting main coil, and at least one cable connected to the RF body coil. The MRI system also includes a multiple-frequency RF trap affixed to the at least one cable. The multiple-frequency RF trap is tuned to block RF energy at two discrete frequency bands. 
     Various other features, objects, and advantages of the invention will be made apparent to those skilled in the art from the accompanying drawings and detailed description thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a magnetic resonance imaging (MRI) system in accordance with an embodiment; 
         FIG. 2  is a schematic representation of a multiple-frequency RF trap in accordance with an embodiment; 
         FIG. 3  is a schematic cross-sectional view of a multiple-frequency RF trap; 
         FIG. 4  is a schematic representation of perspective views of a first cylindrical member and a second cylindrical member in accordance with an embodiment; 
         FIG. 5  is a schematic representation of a multiple-frequency RF trap in accordance with an embodiment; 
         FIG. 6  is a schematic representation of a conductive path in accordance with an embodiment; 
         FIG. 7  is a is a schematic representation of a side-view of a multiple-frequency RF trap in accordance with an embodiment; and 
         FIG. 8  is a schematic representation of a figure-eight pattern that may be used to wrap one or more cables around a multiple-frequency RF trap in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments that may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments, and it is to be understood that other embodiments may be utilized and that logical, mechanical, electrical and other changes may be made without departing from the scope of the embodiments. The following detailed description is, therefore, not to be taken as limiting the scope of the invention. 
       FIG. 1  is a schematic block diagram of an exemplary magnetic resonance imaging (MRI) system in accordance with an embodiment. The operation of MRI system  10  is controlled from an operator console  12  that includes a keyboard or other input device  13 , a control panel  14 , and a display  16 . The console  12  communicates through a link  18  with a computer system  20  and provides an interface for an operator to prescribe MRI scans, display resultant images, perform image processing on the images, and archive data and images. The computer system  20  includes a number of modules that communicate with each other through electrical and/or data connections, for example, such as are provided by using a backplane  20   a . Data connections may be direct wired links or may be fiber optic connections or wireless communication links or the like. The modules of the computer system  20  include an image processor module  22 , a CPU module  24  and a memory module  26  which may include a frame buffer for storing image data arrays. In an alternative embodiment, the image processor module  22  may be replaced by image processing functionality on the CPU module  24 . The computer system  20  is linked to archival media devices, permanent or back-up memory storage or a network. Computer system  20  may also communicate with a separate system control computer  32  through a link  34 . The input device  13  can include a mouse, joystick, keyboard, track ball, touch activated screen, light wand, voice control, or any similar or equivalent input device, and may be used for interactive geometry prescription. 
     The system control computer  32  includes a set of modules in communication with each other via electrical and/or data connections  32   a . Data connections  32   a  may be direct wired links, or may be fiber optic connections or wireless communication links or the like. In alternative embodiments, the modules of computer system  20  and system control computer  32  may be implemented on the same computer system or a plurality of computer systems. The modules of system control computer  32  include a CPU module  36  and a pulse generator module  38  that connects to the operator console  12  through a communications link  40 . The pulse generator module  38  may alternatively be integrated into the scanner equipment (e.g., resonance assembly  52 ). It is through link  40  that the system control computer  32  receives commands from the operator to indicate the scan sequence that is to be performed. The pulse generator module  38  operates the system components that play out (i.e., perform) the desired pulse sequence by sending instructions, commands and/or requests describing the timing, strength and shape of the RF pulses and pulse sequences to be produced and the timing and length of the data acquisition window. The pulse generator module  38  connects to a gradient amplifier system  42  and produces data called gradient waveforms that control the timing and shape of the gradient pulses that are to be used during the scan. The pulse generator module  38  may also receive patient data from a physiological acquisition controller  44  that receives signals from a number of different sensors connected to the patient, such as ECG signals from electrodes attached to the patient. The pulse generator module  38  connects to a scan room interface circuit  46  that receives signals from various sensors associated with the condition of the patient and the magnet system. It is also through the scan room interface circuit  46  that a patient positioning system  48  receives commands to move the patient table to the desired position for the scan. 
     The gradient waveforms produced by the pulse generator module  38  are applied to gradient amplifier system  42  which is comprised of G x , G y  and G z  amplifiers. Each gradient amplifier excites a corresponding physical gradient coil in a gradient coil assembly  50  generally designated to produce the magnetic field gradient pulses used for spatially encoding acquired signals. The gradient coil assembly  50  forms part of a resonance assembly  52  that includes a polarizing superconducting magnet. Resonance assembly  52  may include an RF body coil  56 , surface or local RF coils  76  or both. The coils  56 ,  76  of the RF coil assembly may be configured for both transmitting and receiving, for transmit-only, or for receive-only. The surface coils  76  can be an array of RF surface coils in which each coil (or coil element) separately detects the MRI signals. Such RF surface coil arrays are well-known in the art. 
     A patient or imaging subject  70  may be positioned within a cylindrical patient imaging volume  72  of the resonance assembly  52 . A transceiver module  58  in the system control computer  32  produces pulses that are amplified by an RF amplifier  60  and coupled to the RF coils  56 ,  76  by a transmit/receive switch  62 . The resulting signals emitted by the excited nuclei in the patient may be sensed by either of the RF coils  56 ,  76  and coupled through the transmit/receive switch  62  to a preamplifier  64 . The amplified MR signals are demodulated, filtered and digitized in the receiver section of the transceiver  58 . The transmit/receive switch  62  is controlled by a signal from the pulse generator module  38  to electrically connect the RF amplifier  60  to the RF body coil  56  during the transmit mode and to connect the preamplifier  64  to the RF body coil  56  during the receive mode. The transmit/receive switch  62  can also enable a separate RF coil (for example, a parallel or surface coil  76 ) to be used in either the transmit or receive mode. 
     The MR signals sensed by the RF body coil  56  are digitized by the transceiver module  58  and transferred to a memory module  66  in the system control computer  32 . Typically, frames of data corresponding to MR signals are stored temporarily in the memory module  66  until they are subsequently transformed to create images. An array processor  68  uses a known transformation method, most commonly a Fourier transform, to create images from the MR signals. These images are communicated through the link  34  to the computer system  20  where it is stored in memory. In response to commands received from the operator console  12 , this image data may be archived in long-term storage or it may be further processed by the image processor  22  and conveyed to the operator console  12  and presented on display  16 . 
     As mentioned above, an RF body coil  56  may be used in a transmit mode to transmit RF excitation signals and a surface coil or coils (e.g., an array of surface coils)  76  may be used in a receive mode to detect the signals emitted by the subject. A decoupling circuit (or circuits) is provided to decouple, or disable, the surface coil or coils during the transmit mode when the RF body coil  56  is transmitting RF excitation signals. In an embodiment where a plurality (or array) of surface coils is used, a decoupling circuit may be provided for each surface coil. 
     Referring to  FIG. 2 , a schematic representation of a sectional view of a multiple-frequency RF trap in accordance with an embodiment is shown. The multiple-frequency RF trap  100  includes a first cylindrical member  104 , a second cylindrical member  106 , and an outer shield  108 . 
     The embodiment shown in  FIG. 2  includes a cable shield  107  surrounding a plurality of coaxial cables  110 . The cable shield  107  comprises a cylindrical structure formed from a conductive material such as copper. The cable shield  107  functions to keep magnetic fields from inducing current in the plurality of coaxial cables  110 . The first cylindrical member  104  comprises a cylindrical structure formed from a conductive material such as copper and a first capacitive component  120  (shown in  FIG. 4 ). The first cylindrical member  104  is disposed around the cable shield  107 . A first insulating layer  112  separates the cable shield  107  from the first cylindrical member  104 . According to an embodiment the first insulating layer  112  may comprise air or another material that is not electrically conductive. The first cylindrical member  104  is electrically connected to the cable shield through a first conductive end piece  113  and a second conductive end piece  115 . Other embodiments may use additional methods of electrically connecting the first cylindrical member  104  to the cable shield  107 . The first cylindrical member  104  will be discussed in detail hereinafter. 
     The second cylindrical member  106  comprises a cylinder formed of a conductive material such as copper and a second capacitive component  126  (shown in  FIG. 4 ). A second insulating layer  114  is disposed between the first cylindrical member  104  and the second cylindrical member  106 . The second insulating layer  114  may comprise air or another material that is not electrically conductive. The second cylindrical member  106  is also electrically connected to the cable shield  107  through the first conductive end piece  113  and the second conductive end piece  115 . Other embodiments may use additional methods of electrically connecting the second cylindrical member  106  to the cable shield  107 . The second cylindrical member  106  will be discussed in detail hereinafter. 
     The outer shield  108  is disposed outside of the second cylindrical member  106 . According to an embodiment, the outer shield  108  is separated from the second cylindrical member  106  by a third insulating layer  116 . The third insulating layer  116  may comprise air or another electrically insulating material. The outer shield  108  is electrically connected to the cable shield  107  by the first conductive end piece  113  and the second conductive end piece  115 . 
       FIG. 3  is a schematic cross-sectional view of the multiple-frequency RF trap  100  from  FIG. 2  along line A-A′. Common reference numbers have been used to indicate structures that are identical between  FIG. 2  and  FIG. 3 .  FIG. 3  shows how the first cylindrical member  104 , the second cylindrical member  106 , and the outer shield  108  are concentrically disposed around the plurality of coaxial cables  110 . 
     Referring to  FIG. 4 , schematic representations of perspective views of a first cylindrical member and a second cylindrical member are shown in accordance with an embodiment. Common reference numbers will be used to identify elements that are identical to those shown in  FIG. 2  and  FIG. 3 . The first cylindrical member  104  may comprise a first conductive cylinder  118  and a first capacitive component  120 . The first capacitive component  120  may include a pair of rings. According to the embodiment shown in  FIG. 3 , a first ring  121  is located at a first end of the first conductive cylinder  118  and a second ring  122  is located at a second end. Each ring comprises a plurality of capacitors  123 . For instance, each ring may comprise 6 individual capacitors  123  in accordance with an embodiment. The capacitive values of the individual capacitors  123  in the first capacitive component  120  are selected so that a first circuit formed by the cable shield  107  (shown in  FIG. 2 ) and the first cylindrical member  104  achieves resonance at a desired frequency. The resonant frequency depends on both the element used to generate the MRI image as well as the strength of the magnetic field. The tuning of a circuit formed by a conductive cylinder, a capacitive component and a cable shield to a single frequency is well-known by those skilled in the art and will therefore not be discussed in detail. It should be appreciated by those skilled in the art that it may be necessary to take any other capacitive components of the RF trap into consideration when selecting the values of the capacitive component as will be discussed hereinafter. According to an exemplary embodiment, the first capacitive component  120  may have a capacitance selected so that the first circuit resonates at approximately 128 MHz, which corresponds to the resonant frequency of hydrogen in a 3T magnetic field. 
     Still referring to  FIG. 4 , the second cylindrical member  106  comprises a second conductive cylinder  124  and a second capacitive component  126 . The second cylindrical member  106  includes a first cylindrical section  128  and a second cylindrical section  130  according to an embodiment. The first cylindrical section  128  may be separated from the second cylindrical section  130  by the second capacitive component  126  as shown in  FIG. 3 . The second capacitive component  126  may comprise a plurality of individual capacitors as shown in  FIG. 3 . The capacitance of the second capacitive component  126  is selected so that a circuit comprising the cable shield  107  (shown in  FIG. 2 ), the second conductive cylinder  124 , and the second capacitive component  126  resonates at the desired frequency. According to an embodiment, the second capacitive component  126  may be selected with a capacitance so that the circuit resonates at 31 MHz, which corresponds to the resonant frequency of C13 in a 3T magnetic field. It may be necessary to take any other capacitive components of the RF trap into consideration when selecting the value of the second capacitive component  126 . For example, when determining the appropriate value of the second capacitive component  126 , it may be necessary to factor in the effects of the first capacitive component  120  and the first circuit on the second circuit. Likewise, when determining the appropriate value of the first capacitive component  120 , it may be necessary to factor in the effects of the second capacitive component  126  and the second circuit. According to one embodiment, RF circuit modeling software may be used to predict the approximate values of the first capacitive component  120  and the second capacitive component  126 . The exact values of the first capacitive component  120  and the second capacitive component  126  may be empirically refined once a working model of the RF trap has been constructed. It should be appreciated by those skilled in the art that other well-known techniques of selecting values of the first capacitive component  120  and the second capacitive component  126  may be used in accordance with other embodiments. Both the first capacitive component  120  and the second capacitive component  126  may comprise different designs according to other embodiments. The positioning and dimensions of the first capacitive component and the second capacitive component are not critical. However, the first capacitive component  120  must have a capacitance that tunes the first circuit to resonate at a first desired frequency and the second capacitive component  126  must have a capacitance that tunes the second circuit to resonate at a second desired frequency that is distinct from the first desired frequency. 
       FIG. 5  is a schematic representation of a top view of a multiple-frequency RF trap  200  in accordance with an embodiment. The multiple-frequency RF trap  200  includes a first former  202 , a second former  204 , a first capacitive component  206 , and a second capacitive component  208 . The first former  202  includes a first spool  210  and a second spool  212 . The first spool  210  includes a first electrically conductive portion  214  and the second spool  212  includes a second electrically conductive portion  216 . The spools ( 210 ,  212 ) will be discussed further hereinafter. The first electrically conductive portion  214  is electrically connected to the second electrically conductive portion  216  by a first conductive strip  218 . A second conductive strip  220  connects the first electrically conductive portion  214  to the second electrically conductive portion  216 . The second conductive strip  220  is electrically interrupted by the first capacitive component  206 . 
     The first electrically conductive portion  214 , the second electrically conductive portion  216 , the first conductive strip  218 , and the second conductive strip  220  together form a first conductive path  222 . The first conductive path  222  is in the form of a discontinuous figure-eight. Details of a discontinuous figure-eight will be discussed hereinafter. The combination of the first conductive path  222  and the first capacitive component  206  forms a first LC circuit. The value of the first capacitive component  206  is selected so that the first LC circuit resonates at a first RF frequency. The value of the first capacitive component  206  may be selected using conventional modeling software as is well-known by those skilled in the art. 
     The second former  204  of the multiple-frequency RF trap  200  shown in  FIG. 5  includes a third spool  250  and a fourth spool  252 . The third spool  250  includes a third electrically conductive portion  254  and the fourth spool  252  includes a fourth electrically conductive portion  256 . The third electrically conductive portion  254  is connected to the fourth electrically conductive portion  256  by a third conductive strip  258  and a fourth conductive strip  260 . The fourth conductive strip  260  is electrically interrupted by the second capacitive component  208 . The combination of the third electrically conductive portion  254 , the fourth electrically conductive portion  256 , the third conductive strip  258 , and the fourth conductive strip  260  collectively form a second conductive path  262 . The second conductive path  262  is in the form of a second discontinuous figure-eight. Details of a discontinuous figure-eight will be discussed hereinafter. 
       FIG. 6  shows a schematic representation of a conductive path  270  that is in the form of an exemplary discontinuous figure-eight. Starting at a first end  272 , the conductive path  270  forms a first curve  274  in a counter-clockwise direction and then forms a second curve  276  in a clockwise direction until it reaches a second end  278 . The first end  272  and the second end  278  are separated by a first distance  280 . Those skilled in the art will appreciate that the conductive path  270  is an exemplary conductive path and that the conductive paths may vary according to other embodiments. For example, according to an embodiment, the first curve  274  may loop in a clockwise direction and the second curve  276  may loop in a counter-clockwise direction. Additionally, the first distance  280  between the first end  272  and the second end  278  may vary in accordance with other embodiments. 
       FIG. 7  is a schematic representation of a side-view of the multiple-frequency RF trap  200  from  FIG. 5 . Common reference numbers will be used to identify components that are identical between  FIG. 5  and  FIG. 7 .  FIG. 7  shows three of the 4 spools that are components of the multiple-frequency RF trap  200 . The second spool  212 , the third spool  250 , and the fourth spool  252  are all clearly visible in  FIG. 7 . The first spool  210  (shown in  FIG. 4 ) is not visible in  FIG. 6  because it is obscured by the second spool  212 . The second spool  212  includes a top plate  280 , a bottom plate  282 , and an inner cylinder  284 . The inner cylinder  284 , the top plate  280 , and the bottom plate  282  may also include conductive portions to help couple the one or more cables to the first conductive path  222 . The conductive portions of the inner cylinder  284 , the top plate  280 , and the bottom plate  282  may also be disposed in a discontinuous figure-eight pattern. Collectively, the inner cylinder  284 , the top plate  280 , and the bottom plate  282  form a channel adapted to receive one or more cables wrapped around the inner cylinder  284 . The first spool  210  (shown in  FIG. 5 ), the third spool  250 , and the fourth spool  252  may all be structurally similar to the second spool  212  according to an embodiment. According to other embodiments, a first former and a second former may each be comprised of components other than those shown in  FIG. 5 . For example, a first former may be comprised of a single integral component according to an embodiment. The first former and the second former need to be able to at least partially receive one or more cables and include a conductive path in the shape of a discontinuous figure-eight. 
     Referring back to  FIG. 5 , an exemplary embodiment includes the first former  202  disposed at a generally perpendicular angle to the second former  204 . For purposes of this disclosure the term “generally perpendicular angle” is defined to include an angle between a first line connecting the centers of the first discontinuous figure-eight to a second line connecting the centers of a second discontinuous figure-eight. In other words, a first line (not shown) connecting the center of the first spool  210  to the center of the second spool  212  would cross a second line (not shown) connecting the center of the third spool  250  to the center of the fourth spool  252  at a generally perpendicular angle. It is important that the first former  202  is at a generally perpendicular angle to the second former  204  in order to minimize coupling between the first former  202  and the second former  204 . However it should be noted that the first former  202  and the second former  204  are both disposed in generally the same plane according to the embodiment shown in  FIG. 5 . That is, the first former  202  and the second former  204  both form loops of their respective discontinuous figure-eights within substantially the same plane. 
     However, according to other embodiments, a first former and a second former may be disposed in planes that are generally perpendicular to each other. For example, if a first former is disposed in an x-y-plane, a second former may be disposed in either the x-z-plane or the y-z-plane. According to an embodiment, the first former and a first capacitive element form a first LC circuit tuned to a first frequency, and the second former and a second capacitive element form a second LC circuit tuned to a second frequency. By positioning the first former in a plane that is perpendicular to the plane of the second former, it is possible to minimize the electromagnetic coupling between the first LC circuit and the second LC circuit. 
       FIG. 8  is a schematic representation of an exemplary figure-eight pattern. One or more cables may be wrapped around a multiple-frequency RF trap in a figure-eight pattern, such as that shown in  FIG. 8 , in accordance with an embodiment. For the purposes of this disclosure, the term “figure-eight pattern” is defined to include a pattern where a section of wire or cable is wrapped to form a clockwise loop adjacent to a counter-clockwise loop. The positions of each of four spools, such as those shown in  FIG. 5 , are schematically represented by Roman Numerals on  FIG. 8 . Starting at point  290 , one or more cables start around the third spool in a counter-clockwise direction. The one or more cables wrap around the fourth spool in a clockwise direction before wrapping back around the third spool in a counterclockwise direction. The one or more cables then wrap around the first spool in a counterclockwise direction and cross over to wrap around the second spool in a clockwise direction. The one or more cables then wrap partially around the first spool in a counterclockwise direction in order to reach point  292 . According to this exemplary pattern, the first and second spools are components of a first former and the third and fourth spools are components of a second former. The one or more cables may be wrapped around the multiple-frequency RF trap in other patterns in accordance with other embodiments. However, it is important that the one or more cables wrap are wrapped in a figure-eight pattern around each of the formers. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be 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 insubstantial differences from the literal language of the claims.