Abstract:
A near field apparatus comprises a Radio Frequency (RF) coil including at least one of a shorting bridge from a first point to a second point along an electrical path of the RF coil and/or a discontinuity in the electrical path of the RF coil.

Description:
TECHNICAL FIELD 
       [0001]    The present description relates, in general, to Radio Frequency (RF) coils and, more specifically, to RF coils with new techniques to provide frequency shifting and/or impedance matching. 
       BACKGROUND OF THE INVENTION 
       [0002]    For wireless communication, far field transmission and reception of electromagnetic waves is familiar, even to consumers. Applications utilizing near field energy also exist and are becoming increasingly common. 
         [0003]    For instance, Radio Frequency Identification (RFID) applications usually employ near field transmitters and detectors. Structures used to transmit and receive near field energy are referred to as Radio Frequency (RF) coils (as opposed to antennas), and include structures such as loop coils, spiral coils, and dipole-like wires, etc. Thus, whereas the term “antenna” refers to a far field radiating structure, RF coils are understood to be for near field applications. 
         [0004]    An outgrowth of RFID is Near Field Communications (NFC), which is an extension of the ISO 14443 standard. An NFC device typically includes an interface of a contactless smartcard and a reader and can communicate with smartcards, readers, and other NFC devices. Examples of NFC devices include phones and other handheld devices for applications like wireless payment. NFC communications, at least in the United States, use the ISM band at 13.56 MHz. 
         [0005]    Another near field application is Magnetic Resonance Imagine (MRI). MRI machines often include RF coils to produce and/or detect the magnetic field energy that is used to image organs and structures within the body. MRI devices typically operate at about 42.58 MHz/Tesla. Therefore, operating frequency bands will be in the 63.87 MHz band and in the 127.74 MHz band for 1.5 Tesla and 3 Tesla magnetic field systems, respectively. 
         [0006]      FIG. 13  is an illustration of conventional near field application  1300 . Near field application  1300  includes RF coil  1301 , which is an N-turn coil (where N is a positive integer), matching or tuning circuit  1302 , transmission line  1303 , and RF circuitry (e.g., a transceiver)  1305 . RF circuitry  1305  receives and transmits RF signal  1304  over transmission line  1303  to circuit  1302  and RF coil  1301 . Conventional RF coils typically employ an external LC circuit as a matching or tuning circuit. Furthermore, conventional RF coils use a continuous conductor so that there is an unbroken current path between the + and − coil terminals. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    Various embodiments of the invention include RF coils that have a shorting bridge and/or a current path discontinuity. For example, in one embodiment, an RF coil has a current path discontinuity (or gap) therein. The current path discontinuity adds some amount of capacitance to the RF coil, thereby affecting the frequency of operation, as well as the impedance of the coil. 
         [0008]    In another embodiment, an RF coil includes a shorting bridge from one point on the coil to another. The shorting bridge adds some amount of inductance to the coil and affects operating frequency and impedance. Another example RF coil includes both a current path discontinuity and a shorting bridge. Embodiments of the invention include RF coils with one or more current path discontinuities and/or one or more shorting bridges thereby providing desired matching impedance as well as operation in a desired frequency band. 
         [0009]    The use of current path discontinuities and/or shorting bridges can enable an RF coil designer to create an RF coil that has an impedance that is already matched to a transmission line. Thus, some embodiments can eliminate external LC matching circuits such as circuit  1302  of  FIG. 13 . Furthermore, a current path discontinuity and/or a shorting bridge can be used to shift the operating frequency of a given coil, even shifting the operating frequency downward, thereby getting a longer wavelength out of a smaller sized coil. By correct placement of the shorting bridge and/or current path discontinuity, the overall volume of the RF coil can be significantly reduced. Moreover, some embodiments provide for multi-band operations. 
         [0010]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
           [0012]      FIG. 1  is an illustration of an exemplary RF coil adapted according to one embodiment of the invention; 
           [0013]      FIG. 2  is an illustration of an exemplary RF coil, adapted according to one embodiment of the invention; 
           [0014]      FIG. 3  is an illustration of an exemplary RF coil, adapted according to one embodiment of the invention; 
           [0015]      FIG. 4  shows the frequency response of a prototype built according to the embodiment of  FIG. 3 ; 
           [0016]      FIG. 5  is an illustration of an exemplary RF coil adapted according to one embodiment of the invention; 
           [0017]      FIG. 6  is an illustration of a frequency response of a working prototype of the embodiment of  FIG. 5 ; 
           [0018]      FIG. 7  is an illustration of an exemplary method adapted according to one embodiment of the invention; 
           [0019]      FIG. 8  is an illustration of an exemplary RF coils adapted according to embodiments of the invention; 
           [0020]      FIG. 9  is an illustration of exemplary coil designs that can be used in various embodiments; 
           [0021]      FIG. 10 , is an illustration of exemplary coils adapted according to embodiments of the invention; 
           [0022]      FIG. 11  is an illustration of an exemplary RF coil array adapted according to one embodiment of the invention; 
           [0023]      FIG. 12  is an illustration of an exemplary RF coil array adapted according to one embodiment of the invention; and 
           [0024]      FIG. 13  is an illustration of a conventional near field application. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]      FIG. 1  is an illustration of exemplary RF coil  100  adapted according to one embodiment of the invention. RF coil  100  includes shorting bridge  101  and gap  102 , which together affect the frequency performance of RF coil  100  as well as provide impedance matching at input/output port  103 . 
         [0026]    The scope of embodiments is not limited to the double-layer spiral type coil shown in  FIG. 1 . Embodiments of the invention can be adapted for use in any shape or type of RF coil, such as coils that include any number of layers, any number of turns, any sense of rotation in the turns, etc. For instance,  FIG. 2  is an illustration of exemplary RF coil  200 , adapted according to one embodiment of the invention. RF coil  200  is a helical type coil that includes shorting bridge  201  and gap  202  to provide impedance matching at input/output port  203  and frequency shifting. 
         [0027]    Nor is the scope of embodiments limited to coils that have both a gap and a shorting bridge, as some embodiments include either a gap or a shorting bridge. In fact some embodiments include two or more shorting bridges and/or gaps. The number of gaps and shorting bridges can be adapted for any given RF coil application. 
         [0028]    RF coils adapted according to embodiments of the invention can be used in any application that transmits or detects energy in the near field. For example, embodiments of the invention can be used in RFID tags and readers, NFC cards and readers, MRI devices, and the like. 
         [0029]      FIG. 3  is an illustration of exemplary RF coil  300 , adapted according to one embodiment of the invention. RF coil  300  is topologically similar to RF coil  100  ( FIG. 1 ) in that it is a double-layer spiral coil. RF coil  300  is disposed upon Printed Circuit Board (PCB)  304  with one layer on a top surface of PCB  304  (as shown in view  310 ) and another layer on a bottom surface of PCB  304  (as shown in view  320 ). Via  305  connects the two layers by providing a conductive path through PCB  304 . RF coil  300  includes input/output port  303 , thereby allowing RF coil  300  to be coupled to supporting components, such as a transmission line (not shown), an RF transceiver (not shown), amplifiers, driving circuits, and/or the like. View  310  shows gap  302 , which is a discontinuity in the current path of RF coil  300 . View  320  shows shorting bridge  301 , which connects two adjacent turns of the current path. 
         [0030]    A working prototype of the embodiment of  FIG. 3  has been built on a PCB where the overall physical dimensions of the form factor (including PCB) is 80×55 mm, which is a typical RFID smart card size. The operating frequency of the prototype is 13.56 MHz, such that the prototype coil can be used in an RFID tag or reader.  FIG. 4  shows the frequency response of the prototype compared to a frequency response of a similar coil that does not have a gap or shorting bridge. Curve  401  shows the frequency response of the prototype (employing a gap and shorting bridge, as shown in  FIG. 3 ). Curve  402  shows the frequency response of a similar coil that does not include a gap or shorting bridge. As can be seen, the prototype according to an embodiment of the invention experiences a frequency shift and split due to the addition of the gap and shorting bridge. Thus, the addition of the gap and shorting bridge provides performance in a lower frequency range (as well as in another higher frequency range) compared to the other design that does not have a gap or shorting bridge. 
         [0031]    As shown in  FIG. 4 , the embodiment of  FIG. 3  provides performance in a lower frequency band than a coil without a gap or a short, but with no increase in coil size. It is estimated that the coil of  FIG. 3  occupies seventy percent less surface area than a coil of the same shape as that of the coil of  FIG. 3 , lacking a gap or a shorting bridge, and configured to operate in the 13.56 MHz frequency band. Thus, one advantage of some embodiments is that coils can be miniaturized. Furthermore, the frequency split can provide for multi-band performance in some embodiments. 
         [0032]    The embodiment of  FIG. 3  is shown on a PCB substrate (i.e., PCB  304 ). However, the scope of embodiments includes any of a variety of substrates, such as flexible PCB or films used in RFID tags, and even includes no substrate at all for some coils. 
         [0033]      FIG. 5  is an illustration of exemplary RF coil  500  adapted according to one embodiment of the invention. RF coil  500  is a helical type coil that has eighteen turns above the input/output port  503  and eighteen turns below the input/output port. The dimensions are about 115 mm long with 60 mm in coil diameter. RF coil  500  includes shorting bridge  501  and gap  502 . 
         [0034]    RF coil  500  can find use in an MRI application. For instance, a body part such as a knee or a wrist, can be put inside coil  500 , which transmits and detects near field energy. RF coil  500  can be scaled up or scaled down to cover other body parts, such as legs, arms, and torsos. Scaling can be accomplished by making the diameter of coil  500  either larger or smaller, such that as the diameter increases, the number of turns of coil  500  can be reduced without changing the frequency response. 
         [0035]      FIG. 6  is an illustration of a frequency response of a working prototype of the embodiment of  FIG. 5 . A coil with the same dimensions as coil  500  ( FIG. 5 ), but without a gap or shorting bridge, has a frequency response shown by curve  602 . By contrast, coil  500  has a frequency response shown by curve  601 . Curve  601  shows that the addition of gap  502  and shorting bridge  501  in coil  500  splits and shifts the bands of operation, providing bands of operation at 11.74 MHz and 34.27 MHz. 
         [0036]    It is estimated that coil  500  occupies ninety percent less volume than a similarly shaped coil, without gap  502  and shorting bridge  501 , configured to operate at 12 MHz. Thus, space savings is an advantage of some embodiments of the invention. 
         [0037]      FIG. 7  is an illustration of exemplary method  700  adapted according to one embodiment of the invention. Method  700  may be performed, for example, by a group of designers creating an RF coil according to embodiments herein. 
         [0038]    In action  701 , a shape, an operating frequency, and an impedance of the RF coil are discerned. Often, but not always, the shape, the impedance, and the operating frequency are dictated by the particular application for which the RF coil will be used. In such instances, discerning the shape, impedance, and operating frequency includes becoming familiar with the constraints imposed by the near field application that will use the RF coil. For instance, MRI machines typically use helical coils operating at predefined frequencies, and matching impedances of the coils are chosen to correspond to transmission lines in the supporting circuitry. 
         [0039]    In action  702 , the placement of one or more of a current path discontinuity and a shorting bridge are determined by performing one or more actions. For instance, some embodiments include making one or more computer simulations to arrive at a satisfactory placement of the gap and/or short. The first simulation can be based on a best guess, and subsequent iterations can modify the starting simulated model until desired behavior and properties are achieved. A best guess often includes placing a shorting bridge on one half of a simulation model and placing a current path gap on the other half of the model in about the same place to achieve somewhat symmetrical placement of the shorting bridge and gap. 
         [0040]    Yet another approach employs building and testing prototypes. The first prototype can be based on a best guess, whereas subsequent prototypes modify the original design iteratively until a desired model is achieved. Other techniques combine simulation modeling and prototypes. However, the scope of embodiments is not limited to any particular technique for determining placements of current path discontinuities and/or shorting bridges. 
         [0041]    In action  703  the RF coil is manufactured with the current path discontinuity and/or the shorting bridge configured according to the determined placement. Any appropriate manufacturing process is within the scope of embodiments. In action  704 , the RF coil is disposed in a near field application, such as an MRI device, an RFID tag or reader, an NFC tag or reader, and/or the like. In one example, the RF coil is coupled to a transmission line (e.g., a coaxial line) and the line is coupled to an RF transceiver. The transceiver is further coupled to one or more processors providing signal transmission control and received signal processing/display. Various embodiments of the invention can be disposed in near field applications in the same manner as conventional RF coils (though with different matching/tuning circuits or no matching/tuning circuits). Furthermore, RF coils according to various embodiments of the present invention can be expected to perform similarly to prior art RF coils of the same operating frequency. 
         [0042]    Method  700  is not limited to placement of both a gap and a shorting bridge. Embodiments may place a gap and/or a shorting bridge and may even place more than one of a gap and/or a shorting bridge (e.g., two gaps and three shorting bridges or two shorting bridges and no gaps). Generally, a gap adds capacitance to the RF coil while a shorting bridge adds inductance. Embodiments that include both a gap and a shorting bridge, in effect, add an LC component to the RF coil. This is in contrast to other RF coils that have external LC components and/or components loaded within the coil structure that include capacitors and inductors. Accordingly, one advantage of some embodiments is the elimination of those LC components and, correspondingly, reduced numbers of parts and manufacturing costs. 
         [0043]    Furthermore, while method  700  is shown as a series of discrete steps, the scope of embodiments is not so limited. Embodiments can add, omit, modify, and/or rearrange the actions of method  700 . For instance, action  702  can be modified to include manual mathematical calculation of the coil properties. 
         [0044]      FIG. 8  is an illustration of exemplary RF coils  810 ,  820 , and  830  adapted according to embodiments of the invention.  FIG. 8  illustrates that shorting bridges, such as bridges  811 ,  821 , and  831 , can be configured in a variety of different ways. For instance, shorting bridge  821  links points in two adjacent turns of RF coil  820 . Shorting bridge  811  links two points in non-adjacent turns. Shorting bridge  831  links a top half of RF coil  830  to a bottom half. The placement and length will generally affect the inductance provided by a shorting bridge. A shorting bridge can be placed anywhere in an RF coil to link two non-adjacent points if it provides desired behavior. Furthermore, the width and placement of a gap affects its capacitance, and embodiments of the invention can include a gap of any of a variety of widths or placements to provide desired behavior. 
         [0045]    Moreover, the RF coils themselves can conform to any of a variety of designs. For example, in a spiral coil with an upper and lower half, the two halves can have the same or different rotation sense, and the number of turns in the upper and lower halves can be the same or different.  FIG. 9  is an illustration of exemplary coil designs that can be used in various embodiments. For instance, coil  910  is commonly called a “butterfly” coil, and coil  920  can be referred to as a “flower” coil. Any of the coils in  FIG. 9  can be used alone, with a gap and/or a shorting bridge, in various embodiments. Furthermore, the coils of  FIG. 9  can be combined in three-dimensional (3D) structures, for instance, with coil  910  on top and coil  920  below it, all connected in a current path. Gaps and/or shorting bridges can be distributed among the parts of the 3D structure. 
         [0046]    Further, in many embodiments, the connections of the two ends of the current paths can be inside or outside of a structure, as in  FIG. 10 , which illustrates exemplary coils  1010  and  1020  adapted according to embodiments of the invention. In RF coil  1010 , the ends of the current path are connected inside, whereas in RF coil  1020 , the ends of the current path are connected outside. Such distinction does not typically affect performance of an RF coil, though it can impact usability of the RF coil as it is applied to a near field application. For instance, for an MRI coil that fits around a body part, a designer may prefer to connect to two ends of the current path outside of the coil, since the body part will fit inside. Furthermore, input/output ports are often connected to the current path at the place where the two ends of the current path are connected, and it can sometimes be more convenient to place a port outside, rather than inside, a 3D structure. 
         [0047]      FIG. 11  is an illustration of exemplary RF coil array  1100  adapted according to one embodiment of the invention. For a given array, such as array  1100 , a gap and/or shorting bridge can be placed in a single coil only. Alternatively, each coil in the array may include a gap and/or a short. Still further, gaps and/or shorting bridges can be distributed in any desirable way amongst the coils in the array. For instance, in array  1100 , coil  1101  includes a gap and a shorting bridge, but coils  1102  and  1103  do not include a gap or a shorting bridge. An example MRI system can use a coil array, such as array  1100 , to cover the whole body instead of using one large loop. 
         [0048]      FIG. 12  is an illustration of exemplary RF coil array  1200  adapted according to one embodiment of the invention. Array  1200  is an array of spiral type RF coils, and the principle is the same as that of array  1100 . Array  1200  includes a gap in RF coil  1202  and a shorting bridge in RF coil  1201 . While not shown here, different types of coils (e.g., helical and spiral) can be combined in any of a variety of arrays. 
         [0049]    Various embodiments of the invention offer advantages over the prior art. As mentioned above, embodiments provide a way to produce near field radiation in a given frequency band with a smaller coil than prior art embodiments provide. Furthermore, some embodiments provide a technique for adding inductance and/or capacitance to a coil other than by an external LC circuit or direct loading the capacitor or inductor within the coil. 
         [0050]    Moreover, it should be noted that embodiments of the invention that employ a current path gap are new and revolutionary. Conventional thinking holds that RF coils should have continuous current paths, and embodiments of the invention directly contradict this long-held belief. 
         [0051]    Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.