Patent Publication Number: US-2022236351-A1

Title: Flexible radio frequency coil for magnetic resonance imaging

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. Application No. 15/930,237, which was filed on May 12, 2020. 
    
    
     BACKGROUND 
     Technical Field: This application generally concerns radio frequency (RF) coils for magnetic resonance imaging (MRI). 
     Background: MRI is an imaging modality that uses magnetic fields and RF energy to create images of the interior of an object (e.g., a human patient) without using X-rays or other ionizing radiation. MRI scanners include a main magnet, which produces a static magnetic field (the B 0  field) that polarizes an object. Also, as pulses of RF energy are transmitted through the object, the RF pulses may produce other magnetic fields (B 1  fields). MRI scanners can use B 0  shimming coils to improve the static magnetic field homogeneity and use B 1  shimming techniques to enhance the RF transmit field&#39;s uniformity. 
     SUMMARY 
     Some embodiments of a radio-frequency coil for magnetic resonance imaging comprise three or more electrical conductors that form an RF coil element. Each of the three or more electrical conductors extends along at least a respective part of a length of the radio-frequency coil element, and, along the length of the RF coil element, the three or more electrical conductors are separated from each other by respective distances and by one or more dielectric materials. Furthermore, in some embodiments, along at least part of a length of the radio-frequency coil element, the three or more electrical conductors are locally parallel to each other. And, in some embodiments, the three or more electrical conductors are arranged such that a capacitance is generated between any two of the three or more electrical conductors. Adjusting any electrical conductor&#39;s length (e.g., if one end of the electrical conductor is not connected) or adjusting the distance between any two electrical conductors changes the distributed capacitance and allows the RF coil element to be tuned to a desired resonant frequency. 
     Some embodiments of an RF coil comprise three or more transmission lines that form an RF coil element. Each of the three or more transmission lines extends along at least part of a length of the RF coil element, and, along the part of the length of the RF coil element, the three or more transmission lines are separated from each other by respective distances and by one or more dielectric materials. 
     Some embodiments of a radio-frequency circuit comprise three or more electrical conductors that form an RF coil element and one or more dielectric materials. Along at least part of a length of the RF coil element, the three or more electrical conductors are separated from each other by the one or more dielectric materials. Also, the length of the RF coil element and the one or more dielectric materials are configured such that the RF coil element is configured to generate a magnetic field for B 0  shimming or to generate a magnetic field for B 1  shimming. 
     Some embodiments of a radio-frequency circuit comprise two or more electrical conductors that form a coil, wherein, along a length of the coil, the two or more electrical conductors are separated from each other by respective distances, wherein, across at least some of the respective distances, the two or more electrical conductors are separated from each other by one or more dielectric materials, and wherein the length of the coil, the respective distances, and the one or more dielectric materials are configured such that the coil is configured to generate a magnetic field for B0 shimming or to generate a magnetic field for B1 shimming. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  illustrates an example embodiment of a medical-imaging system. 
         FIG. 1B  illustrates a cutaway view of an example embodiment of the MRI device in  FIG. 1A  along the line A-A. 
         FIG. 2A  illustrates an example embodiment of a radio-frequency (“RF”) coil. 
         FIG. 2B  illustrates a cross-sectional view of the RF coil of  FIG. 2A  taken from the line A-A. 
         FIG. 2C  illustrates one area of overlapping capacitance in the RF coil of  FIG. 2A . 
         FIG. 2D  illustrates another example embodiment of the connections between the members of the RF coil of  FIG. 2A . 
         FIG. 3A  illustrates an example embodiment of an RF coil. 
         FIG. 3B  illustrates a cross-sectional view of the RF coil of  FIG. 3A  taken from the line A-A. 
         FIG. 3C  illustrates an example embodiment of the circuitry of an RF coil. 
         FIG. 4A  illustrates an example embodiment of an RF coil. 
         FIG. 4B  illustrates a cross-sectional view of the RF coil of  FIG. 4A  taken from the line A-A. 
         FIG. 4C  illustrates the cross-sectional view of another embodiment of the RF coil of  FIG. 4A  taken from the line A-A. 
         FIG. 5A  illustrates an example embodiment of an RF coil. 
         FIG. 5B  illustrates a cross-sectional view of the RF coil of  FIG. 5A  taken from the line A-A. 
         FIG. 6  illustrates an example embodiment of an RF coil. 
         FIG. 7A  illustrates an example embodiment of an RF coil. 
         FIG. 7B  illustrates a cross-sectional view of the RF coil of  FIG. 7A  taken from the line A-A. 
         FIG. 8A  illustrates an example embodiment of an RF coil. 
         FIG. 8B  illustrates a cross-sectional view of the RF coil of  FIG. 8A  taken from the line A-A. 
         FIG. 9A  illustrates an example embodiment of an RF coil. 
         FIG. 9B  illustrates a cross-sectional view of the RF coil of  FIG. 9A  taken from the line A-A. 
         FIG. 9C  illustrates the cross-sectional view of another embodiment of the RF coil of  FIG. 9A  taken from the line A-A. 
         FIG. 9D  illustrates the cross-sectional view of another embodiment of the RF coil of  FIG. 9A  taken from the line A-A. 
         FIG. 10A  illustrates an example embodiment of an RF coil. 
         FIG. 10B  illustrates a cross-sectional view of the RF coil of  FIG. 10A  taken from the line A-A. 
         FIG. 10C  illustrates a cross-sectional view of the RF coil of  FIG. 10A  taken from the line B-B. 
         FIG. 11A  illustrates an example embodiment of an RF coil. 
         FIG. 11B  illustrates a cross-sectional view of the RF coil of  FIG. 11A  taken from the line A-A. 
         FIG. 11C  illustrates a cross-sectional view of the RF coil of  FIG. 11A  taken from the line B-B. 
         FIG. 12A  illustrates an example embodiment of an RF coil. 
         FIG. 12B  illustrates a cross-sectional view of the RF coil of  FIG. 12A  taken from the line A-A. 
         FIG. 12C  illustrates a cross-sectional view of the RF coil of  FIG. 12A  taken from the line B-B. 
         FIG. 13A  illustrates an example embodiment of an RF coil. 
         FIG. 13B  illustrates a cross-sectional view of the RF coil of  FIG. 13A  taken from the line A-A. 
         FIG. 13C  illustrates a cross-sectional view of the RF coil of  FIG. 13A  taken from the line B-B. 
         FIGS. 14A-I  illustrate example embodiments of RF-coil shapes. 
         FIGS. 15A-H  illustrate example embodiments of RF-coil arrays. 
         FIG. 16A  illustrates an example embodiment of an RF coil. 
         FIG. 16B  illustrates the capacitances between the electrical conductors in  FIG. 16A . 
         FIG. 17A  illustrates an example embodiment of an RF coil. 
         FIG. 17B  illustrates the capacitances between the electrical conductors in  FIG. 17A . 
         FIG. 18A  illustrates an example embodiment of an RF coil. 
         FIG. 18B  illustrates an example embodiment of an RF coil. 
         FIG. 19A  illustrates an example embodiment of an RF coil. 
         FIG. 19B  illustrates an example embodiment of an RF coil. 
         FIG. 20  illustrates an example embodiment of an RF coil. 
         FIG. 21A  illustrates an example embodiment of an RF coil. 
         FIG. 21B  illustrates an example embodiment of matching and decoupling circuits. 
         FIG. 22A  illustrates an example embodiment of an RF coil. 
         FIG. 22B  illustrates an example embodiment of matching and decoupling circuits. 
         FIG. 23  illustrates an example embodiment of an RF coil. 
         FIG. 24  illustrates an example embodiment of an RF coil. 
         FIG. 25  illustrates an example embodiment of an RF coil. 
         FIGS. 26A-H  illustrate example embodiments of cross-sectional views of RF coils. 
     
    
    
     DESCRIPTION 
     The following paragraphs describe certain explanatory embodiments. Other embodiments may include alternatives, equivalents, and modifications. Additionally, the explanatory embodiments may include several novel features, and a particular feature may not be essential to some embodiments of the devices, systems, and methods that are described herein. 
       FIG. 1A  illustrates an example embodiment of a medical-imaging system  10 . The medical-imaging system  10  includes at least one magnetic-resonance-imaging (“MRI”) device  100 ; one or more image-generation devices  110 , each of which is a specially-configured computing device (e.g., a specially-configured desktop computer, a specially-configured laptop computer, a specially-configured server); and a display device  120 . 
     The MRI device  100  is configured to acquire scan data by scanning a region (e.g., area, volume, slice) of an object (e.g., a patient) using magnetic resonance imaging. The one or more image-generation devices  110  obtain scan data from the MRI device  100  and generate an image of the region of the object based on the scan data. After the one or more image-generation devices  110  generate the image, the one or more image-generation devices  110  send the image to the display device  120 , which displays the image. 
       FIG. 1B  illustrates a cutaway view of an example embodiment of the MRI device  100  along the line A-A in  FIG. 1A . The MRI device  100  houses a main magnet  102  that generates a static magnetic field (B 0  magnetic field). The main magnet  102  has a hollow, cylindrical shape. The MRI device  100  also includes gradient coils  103  and may include one or more RF whole-body coils  104 , and the MRI device  100  may house the gradient coils  103  and the RF whole-body coils  104  (e.g., on the inner perimeter of the main magnet  102 ). And, in addition to the RF whole-body coils  104  included in the MRI device  100 , one or more RF coils  130  (e.g., phased array coils, surface coils) may be contained in another coil-holding device  105  or housing, such as a blanket, a cover, or a shield that is placed on a patient. The MRI device  100  or another specially-configured computing device can act as a control device of the RF coils  130 . 
     RF coils include RF transmit coils, RF receive coils, and RF transmit-receive coils. An RF transmit coil generates an RF pulse that produces a B 1  magnetic field that is perpendicular to the B 0  magnetic field, which rotates the net magnetization away from an alignment with the B 0  magnetic field, resulting in a transverse precessing magnetization. The RF transmit coils may be configured to oscillate the B 1  magnetic field at a Larmor Frequency ω r , resulting in a precessing magnetization that creates a transverse magnetic field. The Larmor Frequency ω, depends on the mass of the precessing system (e.g., the atomic nuclei of the materials that compose a scanned object) and on the strength of the B 0  magnetic field. 
     An RF receive coil detects the precessing magnetization caused by the B 1  magnetic field via electromagnetic induction that produces an induced electromotive force (EMF). The detected induced EMF may be used as the scan data, or the scan data may be otherwise based on the detected induced EMF. Also, an RF transmit-receive coil combines the functions of an RF transmit coil and an RF receive coil. 
     RF receive coils, RF transmit coils, and RF transmit-receive coils are resonant circuits (e.g., LC resonant circuits) that include tuned electrical components. A tuned RF coil has a capacitance (C) (and sometimes an inductance (L) as well as a capacitance) that is configured such that the resonance frequency of the RF coil matches a desired frequency (e.g., the frequency of the nuclear magnetic resonance of the spins in the materials that compose a patient&#39;s tissue). At the resonance frequency, a small external perturbation caused by the precessing magnetization produces a large response from the RF coil. 
       FIG. 2A  illustrates an example embodiment of an RF coil. The RF coil  230  includes three dielectric-wrapped conductors  231 A-C (e.g., jacketed conductors) that compose an RF coil element. Each dielectric-wrapped conductor  231 A-C includes a respective electrical conductor (e.g., transmission line). Thus, the RF coil  230  includes three electrical conductors  232 A-C (collectively, “electrical conductors  232 ”) that are arranged such that locally they are in parallel (or such that the distance between the two closest parts of two electrical conductors is constant or approximately constant over the lengths of the electrical conductors  232 ) and such that they form a closed RF loop. For example, the electrical conductors  232  may be copper wires, one or more copper layers on top or bottom of a flexible PCB substrate, one or more copper layers sandwiched between thin layers of polyimide or dielectric material, copper traces, or other conducting materials. In this embodiment and in the other embodiments that are described herein, at least some of the gauges (cross-sectional diameters) of the electrical conductors  232  may be the same, or the gauges may all be different from each other. Also, an electrical conductor  232  may be composed of multiple smaller conductors (e.g., multiple intertwined wires). 
     In this embodiment, each of the electrical conductors  232  is wrapped in (e.g., jacketed by, surrounded by) a respective dielectric material  234 A-C (collectively, “dielectric materials  234 ”). At least some of the first dielectric material  234 A, the second dielectric material  234 B, and the third dielectric material  234 C may be the same dielectric material, or they may all be different from each other.  FIG. 2B  illustrates a cross-sectional view of the RF coil  230  of  FIG. 2A  taken from the line A-A. 
     The ends of the electrical conductors  232  are coupled (e.g., connected) to one or more of the following: a B 0  shimming circuit  241 , a B 1  shimming circuit  242 , matching and decoupling circuits  243 , and at least one capacitor  244 . This example embodiment of an RF coil  230  has only a single breakpoint capacitor  244 . In this embodiment, the first electrical conductor  232 A has (i) a first end that is connected to the capacitor  244  and to the matching and decoupling circuits  243  and (ii) a second end that is connected to the B 1  shimming circuit  242 . The second electrical conductor  232 B has (i) a first end that is connected to the B 0  shimming circuit  241  and (ii) a second end that is connected to the B 0  shimming circuit  241 . And the third electrical conductor  232 C has (i) a first end that is connected to the B 1  shimming circuit  242  and (ii) a second end that is connected to the capacitor  244  and to the matching and decoupling circuits  243 . 
     Although, over the length of the RF coil  230 , the dielectric materials  234  maintain some electrical separation of the electrical conductors  232 , the electrical conductors  232  have overlapping capacitances. For example,  FIG. 2C  illustrates one area of overlapping capacitance in the RF coil  230  of  FIG. 2A . Because the first electrical conductor  232 A is separated from the second electrical conductor  232 B by a dielectric material and overlaps the second electrical conductor  232 B along the length of the RF coil  230  (e.g., the circumference of the RF coil  230 , the perimeter of the RF coil  230 ), the first electrical conductor  232 A and the second electrical conductor  232 B produce an overlapping capacitance in an overlap area  239 . The overlapping capacitances between the electrical conductors  232  can act as one or more additional breakpoint capacitors along the length of the RF coil  230 . 
     If wire A with a radius r a  and wire B with a radius r b  are parallel and are separated by a distance D, the capacitance C between wire A and wire B can be described by 
     
       
         
           
             
               
                 
                   
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     The parameters of the RF coil  230 , the first electrical conductor  232 A, the second electrical conductor  232 B, and the dielectric materials  234  can be selected to tune the RF coil  230  by configuring the overlapping capacitances between the electrical conductors  232 . Examples of parameters include the size (e.g., diameter) of the RF coil  230 , the shape of the RF coil  230 , the distances between the electrical conductors  232 , the materials that compose the electrical conductors  232 , the cross-sectional diameters (gauges) of the electrical conductors  232 , the lengths of the electrical conductors  232 , the lengths of the overlap of the electrical conductors  232 , the materials that compose the dielectric materials  234 , the thicknesses of the dielectric materials  234 , and the capacitance of the capacitor  244 . 
     A switching circuit (e.g., a switching circuit in a control device) can enable or otherwise activate the B 0  shimming circuit  241 , the B 1  shimming circuit  242 , and the matching and decoupling circuits  243 . When active (e.g., when supplying a current or a voltage to the RF coil  230 ), the B 0  shimming circuit  241  causes the RF coil  230  to produce a magnetic field that compensates for variations in the B 0  magnetic field. When active, the B 1  shimming circuit  242  causes the RF coil  230  to perform a B 1  shim. Furthermore, some embodiments of the RF coil  230  and the other RF coils that are described herein do not include the B 0  shimming circuit  241 , do not include the B 1  shimming circuit  242 , or do not include either the B 0  shimming circuit  241  or the B 1  shimming circuit  242 . 
     Also, when active, some embodiments of the matching and decoupling circuits  243  cause the RF coil to act as one or more of the following: an RF transmit coil, an RF receive coil, and an RF transmit-receive coil. Moreover, some embodiments of the RF coil  230  and the other RF coils that are described herein are configured to operate as only a transmit coil, and some embodiments of the RF coil  230  and the other RF coils that are described herein are configured to operate as only a receive coil. And the matching and decoupling circuits  243  may be used for impedance matching or noise matching the RF coil  230  to other circuitry (e.g., an amplifier, such as a Low Noise Amplifier). Additionally, in an array of RF coils  230  (e.g., the arrays shown in  FIGS. 15A-15H ), the matching and decoupling circuits  243  decouple (e.g., reduce the coupling caused by mutual inductance) the RF coils  230  in the array. 
       FIG. 2D  illustrates another example embodiment of the connections between the members of the RF coil of  FIG. 2A . In this embodiment, one side of the capacitor  244  is connected to both a first electrical conductor  232 A and a second electrical conductor  232 B, and the other side of the capacitor  244  is connected to only a third electrical conductor  232 C. 
     Also, RF coils may have other configurations, as shown by the following embodiments. 
       FIG. 3A  illustrates an example embodiment of an RF coil, and  FIG. 3B  illustrates a cross-sectional view of the RF coil of  FIG. 3A  taken from the line A-A. Although the RF coil  330  forms a loop (like the RF coil  230  in  FIG. 2A ),  FIG. 3A  does not show the half of the RF coil  330  that is opposite to the capacitor  344 . 
     This embodiment is similar to the embodiment shown in  FIG. 2A , but the dielectric-wrapped conductors  331 A-C are twisted together or otherwise intertwined (e.g., in the form of a triple helix). Thus, the cross-sectional view of the RF coil  330  is different. In some other embodiments of the RF coil in  FIG. 3A , the dielectric-wrapped conductors  331 A-C are not intertwined, but have a cross section that is similar or identical to the cross section in  FIG. 3B . 
     The intertwining of the dielectric-wrapped conductors  331 A-C is also a parameter that can be adjusted to tune the RF coil  330 . The dielectric-wrapped conductors  331 A-C include respective electrical conductors  332 A-C and a respective dielectric material  334 A-C. At least some of the respective dielectric materials  334 A-C may be the same, or they may all be different from each other. Although the electrical conductors  332 A-C are intertwined, the distance between the two closest parts of any two of the electrical conductors  332 A-C may be constant or nearly constant over the lengths of the electrical conductors  332 A-C. 
     The ends of the electrical conductors  332 A-C are connected to one or more of the following: a B 0  shimming circuit  341 , a B 1  shimming circuit  342 , matching and decoupling circuits  343 , and a capacitor  344 . In this embodiment, the first electrical conductor  332 A has (i) a first end that is connected to the capacitor  344  and to the matching and decoupling circuits  343  and (ii) a second end that is connected to the B 1  shimming circuit  342 . The second electrical conductor  332 B has (i) a first end that is connected to the B 1  shimming circuit  342  and (ii) a second end that is connected to the capacitor  344  and to the matching and decoupling circuits  343 . And the third electrical conductor  332 C has (i) a first end that is connected to the B 0  shimming circuit  341  and (ii) a second end that is connected to the B 0  shimming circuit  341 . 
     Also, other embodiments of the RF coil  330  may have different configurations of the capacitor  344  and the matching and decoupling circuits  343 , which are shown in box  349 .  FIG. 3C  illustrates an example embodiment of the circuitry of an RF coil. This circuitry shows another embodiment of the circuitry that is shown in the box  349 . This embodiment includes decoupling or tuning circuits  345  that are separate from a matching network  346 , which includes matching circuitry. Also, this embodiment includes two capacitors  344 A-B. Furthermore, this embodiment of the circuitry in the box  349  may be used with other embodiments of the RF coil  330  (e.g., the other embodiments that are described herein). 
     Additionally, in this embodiment and the other embodiments that are described herein, the B 0  shimming circuit  341 , the B 1  shimming circuit  342 , and the matching and decoupling circuits  343  are coupled to a control device (e.g., a MRI device, a specially-configured computing device), which controls their operation. 
       FIG. 4A  illustrates an example embodiment of an RF coil, and  FIG. 4B  illustrates a cross-sectional view of the RF coil of  FIG. 4A  taken from the line A-A. The RF coil  430  includes two dielectric-wrapped conductors  431 B-C and another electrical conductor  432 A that is not wrapped in a dielectric. The two dielectric-wrapped conductors  431 B-C and the other electrical conductor  432 A form an RF coil element. Also, each dielectric-wrapped conductor  431 B-C includes a respective electrical conductor  432 B-C and a respective dielectric material  434 B-C. The respective dielectric materials  434 B-C may be the same dielectric material, or they may be different from each other. Thus, the RF coil  430  includes three electrical conductors  432 A-C that form a closed RF loop. 
     One of the electrical conductors  432 B has an unconnected end  433 B (e.g., a floating end). The length of the electrical conductor  432 B that has the unconnected end  433 B can be adjusted, and this adjustment can be used to tune the RF coil  430 . 
     The other ends of the electrical conductors  432 A-C are connected to one or more of the following: a B 0  shimming circuit  441 , a B 1  shimming circuit  442 , matching and decoupling circuits  443 , and a capacitor  444 . In this embodiment, the first electrical conductor  432 A has (i) a first end that is connected to the B 1  shimming circuit  442  and (ii) a second end that is connected to the capacitor  444  and to the matching and decoupling circuits  443 . The second electrical conductor  432 B has (i) a first end that is connected to the capacitor  444  and to the matching and decoupling circuits  443  and (ii) an unconnected end  433 B. And the third electrical conductor  432 C has (i) a first end that is connected to the B 0  shimming circuit  441  and (ii) a second end that is connected to the B 0  shimming circuit  441  and to the B 1  shimming circuit  442 . 
       FIG. 4C  illustrates the cross-sectional view of another embodiment of the RF coil of  FIG. 4A  taken from the line A-A. In this embodiment, the electrical conductor  432  that is not wrapped in a dielectric is positioned such that it is equidistant or approximately equidistant to the other two electrical conductors  432 . 
       FIG. 5A  illustrates an example embodiment of an RF coil, and  FIG. 5B  illustrates a cross-sectional view of the RF coil of  FIG. 5A  taken from the line A-A. The RF coil  530  includes three dielectric-wrapped conductors: a first dielectric-wrapped conductor  531 A, a second dielectric-wrapped conductor  531 B, and a third dielectric-wrapped conductor  531 C (collectively, the “dielectric-wrapped conductors  531 ”) that compose an RF coil element. The dielectric-wrapped conductors  531  are arranged in a twin axial (twinax) configuration. Thus, the first dielectric-wrapped conductor  531 A includes an electrical conductor  532 A (e.g., formed in a braided weave) that surrounds the second dielectric-wrapped conductor  531 B and the third dielectric-wrapped conductor  531 C. Also, the second dielectric-wrapped conductor  531 B and the third dielectric-wrapped conductor  531 C include respective electrical conductors  532 B-C that are each surrounded by a respective dielectric material  534 B-C. At least some of the first dielectric material  534 A, the second dielectric material  534 B, and the third dielectric material  534 C may be the same dielectric material, or they may all be different from each other. Therefore, the RF coil  530  includes three electrical conductors  532 A-C that form a closed RF loop. 
     The ends of the electrical conductors  532 A-C are connected to one or more of the following: a B 0  shimming circuit  541 , a B 1  shimming circuit  542 , matching and decoupling circuits  543 , and a capacitor  544 . In this embodiment, the first electrical conductor  532 A has (i) a first end that is connected to the B 0  shimming circuit  541  and (ii) a second end that is connected to the B 0  shimming circuit  541 . The second electrical conductor  532 B has (i) a first end that is connected to the B 1  shimming circuit  542  and (ii) a second end that is connected to the capacitor  544  and to the matching and decoupling circuits  543 . And the third electrical conductor  532 C has (i) a first end that is connected to the capacitor  544  and to the matching and decoupling circuits  543  and (ii) a second end that is connected to the B 1  shimming circuit  542 . 
       FIG. 6  illustrates an example embodiment of an RF coil. Like  FIG. 5A , the RF coil  630  includes three dielectric-wrapped conductors  631 -a first dielectric-wrapped conductor  631 A, a second dielectric-wrapped conductor  631 B, and a third dielectric-wrapped conductor  631 C-that are arranged in a twin axial configuration and that compose an RF coil element. Thus, the first dielectric-wrapped conductor  631 A includes an electrical conductor  632 A that surrounds the second dielectric-wrapped conductor  631 B and the third dielectric-wrapped conductor  631 C. Also, the second dielectric-wrapped conductor  631 B and the third dielectric-wrapped conductor  631 C include respective electrical conductors  632 B-C. However, two of the electrical conductors  632 B-C have respective unconnected ends  633 B-C. 
     Each of the other ends of the electrical conductors  632 A-C are connected to one or more of the following: a B 0  shimming circuit  641 , a B 1  shimming circuit  642 , matching and decoupling circuits  643 , and a capacitor  644 . In this embodiment, the first electrical conductor  632 A has (i) a first end that is connected to the B 0  shimming circuit  641  and (ii) a second end that is connected to the B 0  shimming circuit  641 , to the B 1  shimming circuit  642 , to the capacitor  644 , and to the matching and decoupling circuits  643 . The second electrical conductor  632 B has (i) a first end that is connected to the B 1  shimming circuit  642  and (ii) an unconnected end  633 B. And the third electrical conductor  632 C has (i) a first end that is connected to the capacitor  644  and to the matching and decoupling circuits  643  and (ii) an unconnected end  633 C. 
       FIG. 7A  illustrates an example embodiment of an RF coil, and  FIG. 7B  illustrates a cross-sectional view of the RF coil of  FIG. 7A  taken from the line A-A. The RF coil  730  includes three dielectric-wrapped conductors: a first dielectric-wrapped conductor  731 A, a second dielectric-wrapped conductor  731 B, and a third dielectric-wrapped conductor  731 C (collectively, the “dielectric-wrapped conductors  731 ”). The dielectric-wrapped conductors  731  are arranged in a triaxial configuration and compose an RF coil element. Thus, the first dielectric-wrapped conductor  731 A includes a first electrical conductor  732 A that surrounds the second dielectric-wrapped conductor  731 B and the third dielectric-wrapped conductor  731 C, and the second dielectric-wrapped conductor  731 B includes a second electrical conductor  732 B that surrounds the third dielectric-wrapped conductor  731 C. Accordingly, the RF coil  730  includes three electrical conductors  732 A-C that form a closed RF loop, and the three electrical conductors  732 A-C are each immediately surrounded by a respective dielectric material  734 A-C. At least some of the first dielectric material  734 A, the second dielectric material  734 B, and the third dielectric material  734 C may be the same dielectric material, or they may all be different from each other. 
     In this embodiment, the second electrical conductor  732 B has an unconnected end  733 B, and the third electrical conductor  732 C has an unconnected end  733 C. Each of the other ends of the electrical conductors  732 A-C are connected to one or more of the following: a B 0  shimming circuit  741 , a B 1  shimming circuit  742 , matching and decoupling circuits  743 , and a capacitor  744 . In this embodiment, the first electrical conductor  732 A has (i) a first end that is connected to the B 0  shimming circuit  741 , to the capacitor  744 , and to the matching and decoupling circuits  743  and (ii) a second end that is connected to the B 0  shimming circuit  741  and to the B 1  shimming circuit  742 . The second electrical conductor  732 B has (i) a first end that is connected to the capacitor  744  and to the matching and decoupling circuits  743  and (ii) an unconnected end  733 B. And the third electrical conductor  732 C has (i) a first end that is connected to the B 1  shimming circuit  742  and (ii) an unconnected end  733 C. 
       FIG. 8A  illustrates an example embodiment of an RF coil, and  FIG. 8B  illustrates a cross-sectional view of the RF coil of  FIG. 8A  taken from the line A-A. The RF coil  830  includes three dielectric-wrapped conductors that compose an RF coil element: a first dielectric-wrapped conductor  831 A, a second dielectric-wrapped conductor  831 B, and a third dielectric-wrapped conductor  831 C (collectively, the “dielectric-wrapped conductors  831 ”). The second dielectric-wrapped conductor  831 B and the third dielectric-wrapped conductor  831 C are arranged in a coaxial configuration. The first dielectric-wrapped conductor  831 A includes a first electrical conductor  832 A, and the second dielectric-wrapped conductor  831 B includes a second electrical conductor  832 B that surrounds the third dielectric-wrapped conductor  831 C. The third dielectric-wrapped conductor  831 C also includes a third electrical conductor  832 C. Accordingly, the RF coil  830  includes three electrical conductors  832 A-C, and the three electrical conductors  832 A-C are each immediately surrounded by a respective dielectric material  834 A-C. At least some of the first dielectric material  834 A, the second dielectric material  834 B, and the third dielectric material  834 C may be the same dielectric material, or they may all be different from each other. 
     In this embodiment, the first electrical conductor  832 A has an unconnected end  833 A, and the third electrical conductor  832 C has an unconnected end  833 C. Each of the other ends of the electrical conductors  832 A-C are connected to one of more of the following: a B 0  shimming circuit  841 , a B 1  shimming circuit  842 , matching and decoupling circuits  843 , and a capacitor  844 . In this embodiment, the first electrical conductor  832 A has (i) a first end that is connected to the capacitor  844  and to the matching and decoupling circuits  843  and (ii) an unconnected end  833 A. The second electrical conductor  832 B has (i) a first end that is connected to the B 0  shimming circuit  841 , to the capacitor  844 , and to the matching and decoupling circuits  843  and (ii) a second end that is connected to the B 0  shimming circuit  841  and to the B 1  shimming circuit  842 . And the third electrical conductor  832 C has (i) a first end that is connected to the B 1  shimming circuit  842  and (ii) an unconnected end  833 C. 
     Some embodiments of RF coils include more than three electrical conductors. For example,  FIG. 9A  illustrates an example embodiment of an RF coil, and  FIG. 9B  illustrates a cross-sectional view of the RF coil of  FIG. 9A  taken from the line A-A. The RF coil  930  includes four dielectric-wrapped conductors that compose an RF coil element: a first dielectric-wrapped conductor  931 A, a second dielectric-wrapped conductor  931 B, a third dielectric-wrapped conductor  931 C, and a fourth dielectric-wrapped conductor  931 D (collectively the “dielectric-wrapped conductors  931 ”). The first dielectric-wrapped conductor  931 A includes a first electrical conductor  932 A, the second dielectric-wrapped conductor  931 B includes a second electrical conductor  932 B, the third dielectric-wrapped conductor  931 C includes a third electrical conductor  932 C, and the fourth dielectric-wrapped conductor  931 D includes a fourth electrical conductor  932 D. Accordingly, the RF coil  930  includes four electrical conductors  932 A-D, and the four electrical conductors  932 A-D are each surrounded by a respective dielectric material  934 A-D. At least some of the first dielectric material  934 A, the second dielectric material  934 B, the third dielectric material  934 C, and the fourth dielectric material  934 D may be the same dielectric material, or they may all be different from each other. 
     In this embodiment, an end of the second electrical conductor  932 B is connected to an end of the fourth electrical conductor  932 D. The other ends of the electrical conductors  932 A-D are connected to one or more of the following: a B 0  shimming circuit  941 , a B 1  shimming circuit  942 , matching and decoupling circuits  943 , and a capacitor  944 . In this embodiment, the first electrical conductor  932 A has (i) a first end that is connected to the B 1  shimming circuit  942  and (ii) a second end that is connected to the capacitor  944  and to the matching and decoupling circuits  943 . The second electrical conductor  932 B has (i) a first end that is connected to a second end of the fourth electrical conductor  932 D and (ii) a second end that is connected to the B 0  shimming circuit  941 . The third electrical conductor  932 C has (i) a first end that is connected to the capacitor  944  and to the matching and decoupling circuits  943  and (ii) a second end that is connected to the B 1  shimming circuit  942 . And the fourth electrical conductor  932 D has (i) a first end that is connected to the B 0  shimming circuit  941  and (ii) a second end that is connected to a first end of the second electrical conductor  932 B. 
     In the embodiment shown in  FIG. 9B , the dielectric-wrapped conductors  931  are locally parallel. Also, the dielectric-wrapped conductors  931  are arranged such that, in the cross-sectional view, they are aligned in a row. 
       FIG. 9C  illustrates the cross-sectional view of another embodiment of the RF coil of  FIG. 9A  taken from the line A-A. In this embodiment, the four dielectric-wrapped conductors  931  are arranged so that the first electrical conductor  932 A and the fourth electrical conductor  932 D are closer to each other than they are in  FIG. 9B . Also, in the cross-sectional view, the dielectric-wrapped conductors  931  are arranged such that they form a diamond shape. 
     And the four dielectric-wrapped conductors  931 A-D may have other arrangements. For example,  FIG. 9D  illustrates the cross-sectional view of another embodiment of the RF coil of  FIG. 9A  taken from the line A-A. In the cross-sectional view, the dielectric-wrapped conductors  931  are arranged such that the second dielectric-wrapped conductor  931 B, the third dielectric-wrapped conductor  931 C, and the fourth dielectric-wrapped conductor  931 D form a triangular shape. Also, the second dielectric-wrapped conductor  931 B is closest to the first dielectric-wrapped conductor  931 A. Thus, in the cross-sectional view, the dielectric-wrapped conductors  931  are arranged such that they form an approximate   shape, where the first dielectric-wrapped conductor  931 A forms the base of the   shape and where the second dielectric-wrapped conductor  931 B, the third dielectric-wrapped conductor  931 C, and the fourth dielectric-wrapped conductor  931 D form the triangle of the   shape. 
       FIG. 10A  illustrates an example embodiment of an RF coil,  FIG. 10B  illustrates a cross-sectional view of the RF coil of  FIG. 10A  taken from the line A-A, and  FIG. 10C  illustrates a cross-sectional view of the RF coil of  FIG. 10A  taken from the line B-B. The RF coil  1030  includes three dielectric-wrapped conductors that compose an RF coil: a first dielectric-wrapped conductor  1031 A, a second dielectric-wrapped conductor  1031 B, and a third dielectric-wrapped conductor  1031 C. The first dielectric-wrapped conductor  1031 A includes a first electrical conductor  1032 A, the second dielectric-wrapped conductor  1031 B includes a second electrical conductor  1032 B, and the third dielectric-wrapped conductor  1031 C includes a third electrical conductor  1032 C. Accordingly, the RF coil  1030  includes three electrical conductors  1032 A-C, and the three electrical conductors  1032 A-C are each surrounded by a respective dielectric material  1034 A-C. At least some of the first dielectric material  1034 A, the second dielectric material  1034 B, and the third dielectric material  1034 C may be the same dielectric material, or they may all be different from each other. 
     In this embodiment, the second electrical conductor  1032 B and the third electrical conductor  1032 C each do not extend around the entire length (e.g., circumference, perimeter) of the RF coil  1030  and do not overlap each other. Instead, in this embodiment, the second electrical conductor  1032 B and the third electrical conductor  1032 C each extends around approximately half of the length of the RF coil  1030 . However, in some embodiments, the second electrical conductor  1032 B and the third electrical conductor  1032 C are not symmetrical, one of the second electrical conductor  1032 B and the third electrical conductor  1032 C extends around more than half of the length of the RF coil  1030 , or at least one of the second electrical conductor  1032 B and the third electrical conductor  1032 C extends around substantially less than half of the length of the RF coil  1030 . The lengths of the second electrical conductor  1032 B and the third electrical conductor  1032 C-and thus the lengths of their overlaps with the first electrical conductor  1032 A-can be adjusted, and this adjustment can be used to tune the RF coil  1030 . 
     Also, the second electrical conductor  1032 B has an unconnected end  1033 B and an end that connects to matching and decoupling circuits  1043  and a capacitor  1044 , and the third electrical conductor  1032 C has an unconnected end  1033 C and an end that connects to the matching and decoupling circuits  1043  and the capacitor  1044 . The first electrical conductor  1032 A has two ends that connect to a B 0  shimming circuit  1041 . Furthermore, some embodiments include a B 1  shimming circuit in addition to or in alternative to the B 0  shimming circuit  1041 . 
       FIG. 11A  illustrates an example embodiment of an RF coil,  FIG. 11B  illustrates a cross-sectional view of the RF coil of  FIG. 11A  taken from the line A-A, and  FIG. 11C  illustrates a cross-sectional view of the RF coil of  FIG. 11A  taken from the line B-B. The RF coil  1130  includes three dielectric-wrapped conductors: a first dielectric-wrapped conductor  1131 A, a second dielectric-wrapped conductor  1131 B, and a third dielectric-wrapped conductor  1131 C. The first dielectric-wrapped conductor  1131 A includes a first electrical conductor  1132 A, the second dielectric-wrapped conductor  1131 B includes a second electrical conductor  1132 B, and the third dielectric-wrapped conductor  1131 C includes a third electrical conductor  1132 C. Accordingly, the RF coil  1130  includes three electrical conductors  1132 A-C, and the three electrical conductors  1132 A-C are each surrounded by a respective dielectric material  1134 A-C. At least some of the first dielectric material  1134 A, the second dielectric material  1134 B, and the third dielectric material  1134 C may be the same dielectric material, or they may all be different from each other. 
     In this embodiment, the first dielectric-wrapped conductor  1131 A and the second dielectric-wrapped conductor  1131 B are arranged coaxially, and the first dielectric-wrapped conductor  1131 A and the third dielectric-wrapped conductor  1131 C are arranged coaxially. Also, in this example, the second electrical conductor  1132 B and the third electrical conductor  1132 C each extends around only part of the length of the RF coil  1130 , and, inside the first dielectric-wrapped conductor  1131 A, the second electrical conductor  1132 B and the third electrical conductor  1132 C do not overlap each other. The second electrical conductor  1132 B has an unconnected end  1133 B and an end that connects to matching and decoupling circuits  1143  and a capacitor  1144 , and the third electrical conductor  1132 C has an unconnected end  1133 C and an end that connects to the matching and decoupling circuits  1143  and the capacitor  1144 . The first electrical conductor  1132 A has ends that connect to a B 0  shimming circuit  1141 . Also, some embodiments include a B 1  shimming circuit in addition to or in alternative to the B 0  shimming circuit  1141 . 
       FIG. 12A  illustrates an example embodiment of an RF coil,  FIG. 12B  illustrates a cross-sectional view of the RF coil of  FIG. 12A  taken from the line A-A, and  FIG. 12C  illustrates a cross-sectional view of the RF coil of  FIG. 12A  taken from the line B-B. The RF coil  1230  includes four dielectric-wrapped conductors: a first dielectric-wrapped conductor  1231 A, a second dielectric-wrapped conductor  1231 B, a third dielectric-wrapped conductor  1231 C, and a fourth dielectric-wrapped conductor  1231 D. The first dielectric-wrapped conductor  1231 A and the second dielectric-wrapped conductor  1231 B are arranged coaxially, and the third dielectric-wrapped conductor  1231 C and the fourth dielectric-wrapped conductor  1231 D are arranged coaxially. The first dielectric-wrapped conductor  1231 A includes a first electrical conductor  1232 A, the second dielectric-wrapped conductor  1231 B includes a second electrical conductor  1232 B, the third dielectric-wrapped conductor  1231 C includes a third electrical conductor  1232 C, and the fourth dielectric-wrapped conductor  1231 D includes a fourth electrical conductor  1232 D. The electrical conductors  1232 A-D are each jacketed in a respective dielectric material  1234 A-C. At least some of the first dielectric material  1234 A, the second dielectric material  1234 B, the third dielectric material  1234 C, and the fourth dielectric material  1234 D may be the same dielectric material, or they may all be different from each other. 
     Also, in this embodiment, the first electrical conductor  1232 A and the second electrical conductor  1232 B extend around approximately half of the length of the RF coil  1230 , and the third electrical conductor  1232 C and the fourth electrical conductor  1232 D extend around approximately another half of the length of the RF coil  1230 . Neither the first electrical conductor  1232 A nor the second electrical conductor  1232 B overlaps either of the third electrical conductor  1232 C and the fourth electrical conductor  1232 D along the length of the RF coil  1230 . Furthermore, neither the third electrical conductor  1232 C nor the fourth electrical conductor  1232 D overlaps either of the first electrical conductor  1232 A and the second electrical conductor  1232 B along the length of the RF coil  1230 . 
     Each electrical conductor  1232 A-D has a respective unconnected end  1233 A-D. Also, the first electrical conductor  1232 A has an end that connects to a first capacitor  1244 A, and the second electrical conductor  1232 B has an end that connects to matching and decoupling circuits  1243  and to a second capacitor  1244 B. The third electrical conductor  1232 C has an end that connects to the first capacitor  1244 A, and the fourth electrical conductor  1232 D has an end that connects to the matching and decoupling circuits  1243  and to the second capacitor  1244 B. Additionally, depending on the embodiment, the capacitance of the first capacitor  1244 A may be the same as the capacitance of the second capacitor  1244 B, or the capacitance of the first capacitor  1244 A may be different from the capacitance of the second capacitor  1244 B. 
       FIG. 13A  illustrates an example embodiment of an RF coil,  FIG. 13B  illustrates a cross-sectional view of the RF coil of  FIG. 13A  taken from the line A-A, and  FIG. 13C  illustrates a cross-sectional view of the RF coil of  FIG. 13A  taken from the line B-B. The RF coil  1330  includes four dielectric-wrapped conductors: a first dielectric-wrapped conductor  1331 A, a second dielectric-wrapped conductor  1331 B, a third dielectric-wrapped conductor  1331 C, and a fourth dielectric-wrapped conductor  1331 D. The first dielectric-wrapped conductor  1331 A includes a first electrical conductor  1332 A, the second dielectric-wrapped conductor  1331 B includes a second electrical conductor  1332 B, the third dielectric-wrapped conductor  1331 C includes a third electrical conductor  1332 C, and the fourth dielectric-wrapped conductor  1331 D includes a fourth electrical conductor  1332 D. The electrical conductors  1332 A-D are jacketed in respective dielectric materials  1334 A-D. At least some of the first dielectric material  1334 A, the second dielectric material  1334 B, the third dielectric material  1334 C, and the fourth dielectric material  1334 D may be the same dielectric material, or they may all be different from each other. 
     In this embodiment, the first electrical conductor  1332 A and the second electrical conductor  1332 B extend around approximately half of the length of the RF coil  1330 , and the third electrical conductor  1332 C and the fourth electrical conductor  1332 D extend around approximately another half of the length of the RF coil  1330 . Neither the first electrical conductor  1332 A nor the second electrical conductor  1332 B overlaps either of the third electrical conductor  1332 C and the fourth electrical conductor  1332 D along the length of the RF coil  1330 . Furthermore, neither the third electrical conductor  1332 C nor the fourth electrical conductor  1332 D overlaps either of the first electrical conductor  1332 A and the second electrical conductor  1332 B along the length of the RF coil  1330 . However, in some embodiments, the first electrical conductor  1332 A and the third electrical conductor  1332 C are not symmetrical, the second electrical conductor  13328  and the fourth electrical conductor  1332 D are not symmetrical, or at least one of the electrical conductors  1332 A-D extends around substantially less than half of the length of the RF coil  1330 . 
     Each electrical conductor  1332 A-D has a respective unconnected end  1333 A-D. Also, the first electrical conductor  1332 A has an end that connects to a first capacitor  1344 A and to matching and decoupling circuits  1343 , and the second electrical conductor  1332 B has an end that connects to a second capacitor  1344 B. The third electrical conductor  1332 C has an end that connects to the first capacitor  1344 A and to the matching and decoupling circuits  1343 , and the fourth electrical conductor  1332 D has an end that connects to the second capacitor  1344 B. Additionally, depending on the embodiment, the capacitance of the first capacitor  1344 A may be the same as the capacitance of the second capacitor  1344 B, or the capacitance of the first capacitor  1344 A may be different from the capacitance of the second capacitor  1344 B. 
     Some embodiments of the RF coils have shapes that are different from the shapes in  FIGS. 2A, 3A, 4A, 5A, 6, 7A, 8A, 9A, 10A, 11A, 12A, and 13A . For example,  FIGS. 14A-I  illustrate example embodiments of the shapes of RF coils  1430 . The shape of an RF coil, as well as its other parameters, can be selected and configured for a particular application (e.g., an RF transmit coil, and RF receive coil, and RF transmit-receive coil). 
     Also, multiple RF coils may be arranged in an RF-coil array.  FIGS. 15A-H  illustrate example embodiments of RF-coil arrays. As shown by the example embodiment in  FIG. 15A , some RF-coil arrays include RF coils  1530  that do not overlap. And, as shown by  FIGS. 15B-H , some RF-coil arrays include RF coils  1530  that overlap in varying amounts and arrangements. The shimming circuits, the matching and decoupling circuits, and the control methods that are used to operate the RF-coil arrays can be configured for different applications. For example, the shimming circuits, the matching and decoupling circuits, and the control methods may be configured based on one or more of the following: the parameters of an MRI device, the scanning pattern, the anatomy of the patient or object to be scanned, the material (e.g., tissue) to be scanned, and the positions and orientations of the RF coils  1530  in the RF-coil arrays (including the positions and orientations when the RF-coil arrays are placed around the object (e.g., patient) being scanned). 
       FIG. 16A  illustrates an example embodiment of an RF coil, and  FIG. 16B  illustrates the capacitances between the electrical conductors in  FIG. 16A . The RF coil  1630  includes three dielectric-wrapped conductors: a first dielectric-wrapped conductor  1631 A, a second dielectric-wrapped conductor  1631 B, and a third dielectric-wrapped conductor  1631 C. The first dielectric-wrapped conductor  1631 A includes a first electrical conductor  1632 A, the second dielectric-wrapped conductor  1631 B includes a second electrical conductor  1632 B, and the third dielectric-wrapped conductor  1631 C includes a third electrical conductor  1632 C. One end of a capacitor  1644  is connected to both the first electrical conductor  1632 A and the second electrical conductor  1632 B, and the other end of the capacitor  1644  is connected to the third electrical conductor  1632 C. 
     In  FIG. 16A , the first electrical conductor  1632 A, the second electrical conductor  1632 B, and the third electrical conductor  1632 C each include a respective unconnected end  1633 A-C. However, in some embodiments, at least one of the unconnected ends  1633 A-C is connected to one or more other circuits (e.g., matching and decoupling circuits, a B 0  shimming circuit, a B 1  shimming circuit). Likewise, at least one of the ends of the first electrical conductor  1632 A, the second electrical conductor  1632 B, and the third electrical conductor  1632 C that are connected to the capacitor  1644  may also be connected to one or more other circuits. 
     The distributed capacitance between the first electrical conductor  1632 A and the third electrical conductor  1632 C is C AC . Also, the distributed capacitance between the second electrical conductor  1632 B and the third electrical conductor  1632 C is C BC . Thus, the total distributed capacitance C T  is C T =C AC +C BC . Because two electrical conductors are connected to one end of the capacitor  1644 , and because one electrical conductor is connected to another end of the capacitor  1644 , this embodiment of an RF coil  1630  may have a greater tuning range or a higher Q value than some other embodiments. This can also be applied to RF coils that include more than three electrical conductors, such as the embodiment of an RF coil in  FIG. 17A . 
       FIG. 17A  illustrates an example embodiment of an RF coil, and  FIG. 17B  illustrates the capacitances between the electrical conductors in  FIG. 17A . The RF coil  1730  includes four dielectric-wrapped conductors: a first dielectric-wrapped conductor  1731 A, a second dielectric-wrapped conductor  1731 B, a third dielectric-wrapped conductor  1731 C, and a fourth dielectric-wrapped conductor  1731 D. The first dielectric-wrapped conductor  1731 A includes a first electrical conductor  1732 A, the second dielectric-wrapped conductor  1731 B includes a second electrical conductor  1732 B, the third dielectric-wrapped conductor  1731 C includes a third electrical conductor  1732 C, and the fourth dielectric-wrapped conductor  1731 D includes a fourth electrical conductor  1732 D. Also, each of the electrical conductors  1732 A-D has a respective unconnected end  1733 A-D. 
     One end of a capacitor  1744  is connected to both the first electrical conductor  1732 A and the second electrical conductor  1732 B, and the other end of the capacitor  1744  is connected to both the third electrical conductor  1732 C and the fourth electrical conductor  1732 D. 
     Although in  FIG. 17A  each of the electrical conductors  1732 A-D has a respective unconnected end  1733 A-D, in some embodiments, at least one of the unconnected ends  1733 A-D is connected to one or more other circuits (e.g., matching and decoupling circuits, a B 0  shimming circuit, a B 1  shimming circuit). Likewise, at least one of the ends of the first electrical conductor  1732 A, the second electrical conductor  1732 B, the third electrical conductor  1732 C, and the fourth electrical conductor  1732 D that are connected to the capacitor  1744  may also be connected to one or more other circuits. 
     The distributed capacitance between the first electrical conductor  1732 A and the third electrical conductor  1732 C is C AC . And the distributed capacitance between the first electrical conductor  1732 A and the fourth electrical conductor  1732 D is C AD . Also, the distributed capacitance between the second electrical conductor  1732 B and the third electrical conductor  1732 C is C BC . And the distributed capacitance between the second electrical conductor  1732 B and the fourth electrical conductor  1732 D is C BD . Thus, the total distributed capacitance C T  is C T =C AC +C AD +C BC +C BD . 
     Additionally, Table 1 shows some parameters from example embodiments of RF coils. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Example 
                 1 
                 2 
                 3 
                 4 
                 5 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 RF coil diameter (cm) 
                 11.0 
                 11.0 
                 11.0 
                 11.0 
                 11.0 
               
               
                 No. of Conductors 
                 2 
                 2 
                 3 (2 of 3 to 
                 3 (1 of 3 to 
                 2 
               
               
                   
                   
                   
                 1 of 3)* 
                 1 of 3)** 
                   
               
               
                 Conductor Cross-sec. 
                 0.36 
                 0.51 
                 2 × 0.51 
                 2 × 0.51 
                 Outer: 1.85 
               
               
                 Diameter (mm)*** 
                   
                   
                 1 × 0.40 
                 1 × 0.40 
                 Inner: 0.50 
               
               
                 Conductor type 
                 Twisted 
                 Twisted 
                 Twisted 
                 Twisted 
                 Coaxial 
               
               
                   
               
               
                 *One side of a capacitor is connected to two of the three conductors, and another side of the capacitor is connected to only one of the three conductors (e.g., as shown in FIG. 2D). 
               
               
                 **One side of a capacitor is connected to only one of the three conductors and another side of the capacitor is connected to only one of the three conductors (e.g. as shown in FIG. 2A). 
               
               
                 ***Some of the conductors are composed of multiple smaller conductors, and the cross-sectional diameter is the diameter of the combined multiple smaller conductors. 
               
            
           
         
       
     
     As noted above, the lengths and overlaps of the electrical conductors can be adjusted to tune the RF coil, for example as shown in  FIGS. 18A, 18B, 19A, and 19B . 
       FIG. 18A  illustrates an example embodiment of an RF coil. The RF coil  1830  includes three dielectric-wrapped conductors: a first dielectric-wrapped conductor  1831 A, a second dielectric-wrapped conductor  1831 B, and a third dielectric-wrapped conductor  1831 C. The first dielectric-wrapped conductor  1831 A includes a first electrical conductor  1832 A, the second dielectric-wrapped conductor  1831 B includes a second electrical conductor  1832 B, and the third dielectric-wrapped conductor  1831 C includes a third electrical conductor  1832 C. One end of a capacitor  1844  is connected to both the first electrical conductor  1832 A and the second electrical conductor  1832 B, and the other end of the capacitor  1844  is connected to the third electrical conductor  1832 C. Also, each of the electrical conductors  1832 A-C has a respective unconnected end  1833 A-C. 
     In this embodiment, the electrical conductors  1832 A-C extend around less of the length (e.g., circumference, perimeter) of the RF coil  1830  than the electrical conductors  1632 A-C in  FIG. 16A . Moreover, along the length of the RF coil  1830 , the electrical conductors  1832 A-C overlap each other less than the electrical conductors  1632 A-C in  FIG. 16A  overlap each other. 
     Although in  FIG. 18A  each of the electrical conductors  1832 A-C has a respective unconnected end  1833 A-C, in some embodiments, at least one of the unconnected ends  1833 A-C is connected to one or more other circuits (e.g., matching and decoupling circuits, a B 0  shimming circuit, a B 1  shimming circuit). Likewise, at least one of the ends of the first electrical conductor  1832 A, the second electrical conductor  1832 B, and the third electrical conductor  1832 C that are connected to the capacitor  1844  may also be connected to one or more other circuits. 
       FIG. 18B  illustrates an example embodiment of an RF coil. The RF coil  1830  includes three dielectric-wrapped conductors: a first dielectric-wrapped conductor  1831 A, a second dielectric-wrapped conductor  1831 B, and a third dielectric-wrapped conductor  1831 C. The first dielectric-wrapped conductor  1831 A includes a first electrical conductor  1832 A, the second dielectric-wrapped conductor  1831 B includes a second electrical conductor  1832 B, and the third dielectric-wrapped conductor  1831 C includes a third electrical conductor  1832 C. 
     The second electrical conductor  1832 B has an unconnected end  1833 B. The other ends of the electrical conductors  1832 A-C are connected to one or more of the following: a B 0  shimming circuit  1841 , a B 1  shimming circuit  1842 , matching and decoupling circuits  1843 , and a capacitor  1844 . In this embodiment, the first electrical conductor  1832 A has (i) a first end that is connected to the B 0  shimming circuit  1841  and (ii) a second end that is connected to the B 0  shimming circuit  1841  and to the B 1  shimming circuit  1842 . The second electrical conductor  1832 B has (i) a first end that is connected to the capacitor  1844  and to the matching and decoupling circuits  1843  and (ii) the unconnected end  1833 B. The third electrical conductor  1832 C has (i) a first end that is connected to the B 1  shimming circuit  1842  and (ii) a second end that is connected to the capacitor  1844  and to the matching and decoupling circuits  1843 . 
     In this embodiment, the second electrical conductor  1832 B, which has the unconnected end  1833 B, extends around less of the length of the RF coil  1830  than the electrical conductor  432  in  FIG. 4A  that has the unconnected end  433 . 
       FIG. 19A  illustrates an example embodiment of an RF coil. The RF coil  1930  includes three dielectric-wrapped conductors: a first dielectric-wrapped conductor  1931 A, a second dielectric-wrapped conductor  1931 B, and a third dielectric-wrapped conductor  1931 C. The first dielectric-wrapped conductor  1931 A includes a first electrical conductor  1932 A, the second dielectric-wrapped conductor  1931 B includes a second electrical conductor  1932 B, and the third dielectric-wrapped conductor  1931 C includes a third electrical conductor  1932 C. 
     The second electrical conductor  1932 B and the third electrical conductor  1932 C each include a respective unconnected end  1933 B-C. Also, a respective end of each of the second electrical conductor  1932 B and the third electrical conductor  1932 C is connected to a capacitor  1944 . And the ends of the first electrical conductor  1932 A are connected to one or more other circuits (e.g., matching and decoupling circuits, a B 0  shimming circuit, a B 1  shimming circuit) (not shown in  FIG. 19A ). Likewise, the ends of the second electrical conductor  1932 B and the third electrical conductor  1932 C that are connected to the capacitor  1944  may also be connected to one or more other circuits. 
     In contrast to the RF coil  1030  in  FIG. 10A , in this embodiment the second electrical conductor  1932 B and the third electrical conductor  1932 C are not symmetrical or approximately symmetrical. Instead, the second electrical conductor  1932 B is substantially shorter than the third electrical conductor  1932 C, and the third electrical conductor  1932 C extends around more than half of the length of the RF coil  1930 . Also, along the length of the RF coil  1930 , the third electrical conductor  1932 C has a greater overlap with the first electrical conductor  1932 A than the overlap between the second electrical conductor  1932 B and the first electrical conductor  1932 A. 
       FIG. 19B  illustrates an example embodiment of an RF coil. The RF coil  1930  includes four dielectric-wrapped conductors: a first dielectric-wrapped conductor  1931 A, a second dielectric-wrapped conductor  1931 B, a third dielectric-wrapped conductor  1931 C, and a fourth dielectric-wrapped conductor  1931 D. The first dielectric-wrapped conductor  1931 A includes a first electrical conductor  1932 A, the second dielectric-wrapped conductor  1931 B includes a second electrical conductor  1932 B, the third dielectric-wrapped conductor  1931 C includes a third electrical conductor  1932 C, and the fourth dielectric-wrapped conductor  1931 D includes a fourth electrical conductor  1932 D. 
     Each of the electrical conductors  1932 A-D has a respective unconnected end  1933 A-D. A respective end of each of the first electrical conductor  1932 A and the third electrical conductor  1932 C is connected to a first capacitor  1944 A. And a respective end of each of the second electrical conductor  1932 B and the fourth electrical conductor  1932 D is connected to a second capacitor  1944 B. Furthermore, at least one of the ends of the first electrical conductor  1932 A and the third electrical conductor  1932 C that are connected to the first capacitor  1944 A may also be connected to one or more other circuits (e.g., matching and decoupling circuits, a B 0  shimming circuit, a B 1  shimming circuit) (not shown in  FIG. 19A ). Additionally, at least one of the ends of the second electrical conductor  1932 B and the fourth electrical conductor  1932 D that are connected to the second capacitor  1944 B may also be connected to one or more other circuits. 
     In contrast to the RF coil  1330  in  FIG. 13A , in  FIG. 19B  the first electrical conductor  1931 A and the third electrical conductor  1931 C are not symmetrical or approximately symmetrical, and the second electrical conductor  1931 B and the fourth electrical conductor  1931 D are not symmetrical or approximately symmetrical. Also, along the length of the RF coil  1930 , the overlap between the first electrical conductor  1932 A and the second electrical conductor  1932 B is greater than the overlap between the third electrical conductor  1932 C and the fourth electrical conductor  1932 D. 
       FIG. 20  illustrates an example embodiment of an RF coil. The RF coil  2030  includes three dielectric-wrapped conductors: a first dielectric-wrapped conductor  2031 A, a second dielectric-wrapped conductor  2031 B, and a third dielectric-wrapped conductor  2031 C. The first dielectric-wrapped conductor  2031 A includes a first electrical conductor  2032 A, the second dielectric-wrapped conductor  2031 B includes a second electrical conductor  2032 B, and the third dielectric-wrapped conductor  2031 C includes a third electrical conductor  2032 C. The ends of the electrical conductors  2032 A-C are connected to one or more of the following: a B 0  shimming circuit  2041 , a B 1  shimming circuit  2042 , a transmit/receive circuit  2048 , and a capacitor  2044 . The transmit/receive circuit  2048 , which may also implement matching and decoupling circuits, includes circuitry that supplies a voltage or a current that causes the RF coil  2030  to operate as one of the following: a transmit coil, a receive coil, or a transmit-receive coil. Also, a switching circuit  2049  activates and deactivates the B 0  shimming circuit  2041 , the B 1  shimming circuit  2042 , and the transmit/receive circuit  2048 . 
       FIG. 21  illustrates an example embodiment of an RF coil. The RF coil  2130  includes three dielectric-wrapped conductors: a first dielectric-wrapped conductor  2131 A, a second dielectric-wrapped conductor  2131 B, and a third dielectric-wrapped conductor  2131 C. The first dielectric-wrapped conductor  2131 A includes a first electrical conductor  2132 A, the second dielectric-wrapped conductor  2131 B includes a second electrical conductor  2132 B, and the third dielectric-wrapped conductor  2131 C includes a third electrical conductor  2132 C. Each end of the electrical conductors  2132 A-C is connected to one of the following: a B 0  shimming circuit  2141 , a B 1  shimming circuit  2142 , a first capacitor  2144 A, and a second capacitor  2144 B. In this embodiment, the matching and decoupling circuits  2143  are connected to the RF coil  2130  via two capacitors  2144 A-B, one for each connection to the RF coil  2130 . This embodiment of the connection between the RF coil  2130  and the matching and decoupling circuits  2143  can be used with other RF coils, for example the RF coils in  FIGS. 3A, 4A, 5A, 6, 7A, 8A, 9A, 10A, 11A, 12A, 13A, 16A, 17A, 18A, and 18B . 
       FIG. 21B  illustrates an example embodiment of matching and decoupling circuits. In this embodiment, the matching and decoupling circuits  2143  include the first capacitor  2144 A and the second capacitor  2144 B. Some embodiments of the matching and decoupling circuits  2143  include only one of the first capacitor  2144 A and the second capacitor  2144 B. And at least one of the first capacitor  2144 A and the second capacitor  2144 B may be tuning capacitors. 
       FIG. 22  illustrates an example embodiment of an RF coil. The RF coil  2230  includes three dielectric-wrapped conductors: a first dielectric-wrapped conductor  2231 A, a second dielectric-wrapped conductor  2231 B, and a third dielectric-wrapped conductor  2231 C. The first dielectric-wrapped conductor  2231 A includes a first electrical conductor  2232 A, the second dielectric-wrapped conductor  2231 B includes a second electrical conductor  2232 B, and the third dielectric-wrapped conductor  2231 C includes a third electrical conductor  2232 C. Each end of the electrical conductors  2232 A-C is connected to one of the following: a B 0  shimming circuit  2241 , a B 1  shimming circuit  2242 , matching and decoupling circuits  2243 , and a capacitor  2244 . In this embodiment, one connection of the matching and decoupling circuits  2243  to the RF coil  2230  includes the capacitor  2244  between the matching and decoupling circuits  2243  and the RF coil  2230 . This embodiment of the connection between the RF coil  2230  and the matching and decoupling circuits  2243  can be used with other RF coils, for example the RF coils in  FIGS. 3A, 4A, 5A, 6, 7A, 8A, 9A, 10A, 11A, 12A, 13A, 16A, 17A, 18A, and 18B . 
       FIG. 21B  illustrates an example embodiment of matching and decoupling circuits. In this embodiment, the matching and decoupling circuits  2243  include the capacitor  2244 . Also, the capacitor  2244  may be a tuning capacitor. 
       FIG. 23  illustrates an example embodiment of an RF coil. The RF coil  2330  includes three electrical conductors: a first electrical conductor  2332 A, a second electrical conductor  2332 B, and a third electrical conductor  2332 C. The second electrical conductor  2332 B is a member of a first dielectric-wrapped conductor  2331 B, and the third electrical conductor  2332 C is a member of a second dielectric-wrapped conductor  2331 C. The ends of the electrical conductors  2332 A-C are connected to one or more of the following: a B 0  shimming circuit  2341 , a B 1  shimming circuit  2342 , matching and decoupling circuits  2343 , and a capacitor  2344 . Also, unlike the embodiment of an RF coil in  FIG. 4A , this embodiment does not include an unconnected end. 
       FIG. 24  illustrates an example embodiment of an RF coil. The RF coil  2430  includes three dielectric-wrapped conductors: a first dielectric-wrapped conductor  2431 A, a second dielectric-wrapped conductor  2431 B, and a third dielectric-wrapped conductor  2431 C. The dielectric-wrapped conductors  2431 A-C are arranged in a triaxial configuration. Thus, the first dielectric-wrapped conductor  2431 A includes a first electrical conductor  2432 A that surrounds the second dielectric-wrapped conductor  2431 B and the third dielectric-wrapped conductor  2431 C, and the second dielectric-wrapped conductor  2431 B includes a second electrical conductor  2432 B that surrounds the third dielectric-wrapped conductor  2431 C. Accordingly, the three electrical conductors  2432 A-C are each immediately surrounded by a respective dielectric material. The ends of the electrical conductors  2432 A-C are connected to one or more of the following: a B 0  shimming circuit  2441 , a B 1  shimming circuit  2442 , matching and decoupling circuits  2443 , and a capacitor  2444 . Unlike the embodiment of an RF coil in  FIG. 7A , this embodiment does not include an unconnected end. 
       FIG. 25  illustrates an example embodiment of an RF coil. The RF coil  2530  includes three dielectric-wrapped conductors: a first dielectric-wrapped conductor  2531 A, a second dielectric-wrapped conductor  2531 B, and a third dielectric-wrapped conductor  2531 C. The second dielectric-wrapped conductor  2531 B and the third dielectric-wrapped conductor  2531 C are arranged in a coaxial configuration. The first dielectric-wrapped conductor  2531 A includes a first electrical conductor  2532 A, and the second dielectric-wrapped conductor  2531 B includes a second electrical conductor  2532 B that surrounds the third dielectric-wrapped conductor  2531 C. The third dielectric-wrapped conductor  2531 C includes a third electrical conductor  2532 C. The ends of the electrical conductors  2532 A-C are each connected to one of more of the following: a B 0  shimming circuit  2541 , a B 1  shimming circuit  2542 , matching and decoupling circuits  2543 , and a capacitor  2544 . Unlike the embodiment of an RF coil in  FIG. 8A , this embodiment does not include an unconnected end. 
       FIGS. 26A-H  illustrate example embodiments of cross-sectional views of RF coils. As illustrated by  FIGS. 26A-H , multiple electrical conductors may be enclosed in the same body of dielectric material, which may help to maintain the distances between the electrical conductors at desired distances. 
       FIG. 26A  illustrates a cross-sectional view of a group of dielectric-wrapped conductors  2631  that includes three electrical conductors  2632 A-C that are surrounded by a dielectric material  2634 . In this embodiment, in the cross-sectional view, the three electrical conductors  2632 A-C form the points of an equilateral or approximate equilateral triangle. 
       FIG. 26B  illustrates a cross-sectional view of a group of dielectric-wrapped conductors  2631  that includes three electrical conductors  2632 A-C that are surrounded by a dielectric material  2634 . In this embodiment, in the cross-sectional view, the three electrical conductors  2632 A-C are aligned in a row in which a second electrical conductor  2632 B is closer to a first electrical conductor  2632 A than to a third electrical conductor  2632 C. And, in the cross-sectional view, the dielectric material  2634  forms an approximate stadium shape (i.e., discorectangle, obround). 
       FIG. 26C  illustrates a cross-sectional view of a group of dielectric-wrapped conductors  2631  that includes three electrical conductors  2632 A-C that are surrounded by a dielectric material  2634 . In this embodiment, in the cross-sectional view, the three electrical conductors  2632 A-C form the points of an isosceles or an approximate isosceles triangle. 
       FIG. 26D  illustrates a cross-sectional view of a group of dielectric-wrapped conductors  2631  that includes two electrical conductors  2632 A-B that are surrounded by a dielectric material  2634 . In this embodiment, in the cross-sectional view, the dielectric material  2634  forms an approximate stadium shape. 
       FIG. 26E  illustrates a cross-sectional view of a group of dielectric-wrapped conductors  2631  that includes four electrical conductors  2632 A-D that are surrounded by a dielectric material  2634 . In this embodiment, in the cross-sectional view, the four electrical conductors  2632 A-D are aligned in a row in which the four electrical conductors  2632 A-D are equally or approximately equally spaced. And, in the cross-sectional view, the dielectric material  2634  forms an approximate stadium shape. 
       FIG. 26F  illustrates a cross-sectional view of a group of dielectric-wrapped conductors  2631  that includes four electrical conductors  2632 A-D that are surrounded by a dielectric material  2634 . In this embodiment, in the cross-sectional view, the four electrical conductors  2632 A-D form the points of a diamond shape. And, in the cross-sectional view, the dielectric material  2634  forms an approximate diamond shape. 
       FIG. 26G  illustrates a cross-sectional view of a group of dielectric-wrapped conductors  2631  that includes four electrical conductors  2632 A-D that are surrounded by a dielectric material  2634 . In this embodiment, in the cross-sectional view, the four electrical conductors  2632 A-D are arranged such that they form an approximate   shape, where three of the electrical conductors  2632 B-C form the points of a triangle. And, in the cross-sectional view, the dielectric material  2634  forms an approximate   shape. 
       FIG. 26H  illustrates a cross-sectional view of a group of dielectric-wrapped conductors  2631  that includes four electrical conductors  2632 A-D that are surrounded by a dielectric material  2634 . In this embodiment, in the cross-sectional view, three of the electrical conductors  2632 A-C form the points of a diamond shape, and one of the electrical conductors  2632 D is located between two of the electrical conductors  2632 B-C on the base of the triangle. Also, in the cross-sectional view, the dielectric material  2634  forms an approximate triangle shape. 
     As used herein, the conjunction “or” generally refers to an inclusive “or,” though “or” may refer to an exclusive “or” if expressly indicated or if the context indicates that the “or” must be an exclusive “or.”