Patent Publication Number: US-2023133077-A1

Title: Radio-frequency coil assemblies of a magnetic resonance system and assembling methods thereof

Description:
BACKGROUND 
     The field of the disclosure relates generally to a magnetic resonance (MR) system, and more particularly, to radio frequency (RF) coil assemblies for an MR system. 
     Magnetic resonance imaging (MRI) has proven useful in diagnosis of many diseases. MRI provides detailed images of soft tissues, abnormal tissues such as tumors, and other structures, which cannot be readily imaged by other imaging modalities, such as computed tomography (CT). Further, MRI operates without exposing patients to ionizing radiation experienced in modalities such as CT and x-rays. 
     In MRI, a radio-frequency (RF) coil assembly is used to detect MR signals emitted from a subject. As such, the RF coil assembly is desirable to be light-weighted and flexible to conform to the anatomy of the subject for comfort and image quality. Known RF coil assemblies is disadvantaged in some aspects and improvements are desired. 
     BRIEF DESCRIPTION 
     In one aspect, a radio frequency (RF) coil assembly for a magnetic resonance (MR) system is provided. The RF coil assembly includes an RF coil array and a substrate assembly. The RF coil array includes one or more RF coils each RF coil including a coil loop that includes a wire conductor, the wire conductor formed into the coil loop. The substrate assembly includes a first substrate layer and a second substrate layer, wherein the first substrate layer is coupled with the second substrate layer at seams without a separately-provided fastening mechanism, wherein the RF coil array is positioned between the first substrate layer and the second substrate layer. 
     In another aspect, a method of assembling an RF coil assembly of a medical imaging system is provided. The method includes positioning one or more coil loops on a first substrate layer, wherein each coil loop includes a wire conductor, the wire conductor formed into the coil loop. The method also includes positioning a second substrate layer over the one or more coil loops. The method further includes forming a substrate assembly that includes the first substrate layer and the second substrate layer by coupling the first substrate layer with the second substrate layer at seams without a separately-provided fastening mechanism. 
     In one more aspect, an RF coil assembly for a medical imaging system is provided. The RF coil assembly includes an RF coil array and a substrate array. The RF coil array includes one or more RF coils each RF coil including a coil loop that includes a wire conductor, the wire conductor formed into the coil loop. The substrate assembly includes a first substrate layer and a second substrate layer, wherein the first substrate layer is coupled with the second substrate layer at seams without a separately-provided fastening mechanism, wherein the RF coil array is positioned between the first substrate layer and the second substrate layer. 
    
    
     
       DRAWINGS 
       Non-limiting and non-exhaustive embodiments are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various drawings unless otherwise specified. 
         FIG.  1    is a block diagram of a magnetic resonance (MR) system. 
         FIG.  2    is a block diagram of an exemplary radio frequency (RF) coil. 
         FIG.  3 A  is a schematic diagram of an exemplary RF coil shown in  FIG.  2   . 
         FIG.  3 B  is a schematic diagram of another exemplary RF coil shown in  FIG.  2   . 
         FIG.  3 C  is a cross-sectional view of an exemplary distributed capacitance coil loop of the RF coils shown in  FIGS.  3 A and  3 B . 
         FIG.  4 A  is a schematic diagram of one more exemplary RF coil shown in  FIG.  2   . 
         FIG.  4 B  is a schematic diagram of one more exemplary RF coil shown in  FIG.  2   . 
         FIG.  4 C  is a cross-sectional view of an exemplary wire conductor used in the coil loop of the RF coils shown in  FIGS.  4 A and  4 B   
         FIG.  5 A  is a top perspective view of an exemplary RF coil assembly. 
         FIG.  5 B  is a bottom perspective view of the RF coil assembly shown in  FIG.  5 A . 
         FIG.  5 C  is an exploded view of the RF coil assembly shown in  FIG.  5 A . 
         FIG.  5 D  is a block diagram of the RF coil assembly shown in  FIG.  5 A . 
         FIG.  6    is a cross-sectional view of a known RF coil assembly. 
         FIG.  7 A  is a top perspective view of the interior of the RF coil assembly shown in  FIG.  5 A . 
         FIG.  7 B  is a top view of the RF coil assembly shown in  FIG.  7 A . 
         FIG.  7 C  is a bottom view of the RF coil assembly shown in  FIG.  7 A . 
         FIG.  8 A  shows the RF coil assembly shown in  FIG.  7 A  with the top substrate layer removed. 
         FIG.  8 B  shows the bottom substrate layer of the RF coil assembly shown in  FIG.  8 A . 
         FIGS.  9 A- 9 C  are schematic diagrams illustrating an exemplary method of assembling RF coil assemblies shown in  FIGS.  2 - 5 D and  7 A- 8 B . 
         FIG.  9 A  shows that a coil array is positioned over a substrate layer. 
         FIG.  9 B  shows that another substrate layer is to be placed over the assembly shown in  FIG.  9 A . 
         FIG.  9 C  shows an RF coil assembly after welding of the components shown in  FIG.  9 B . 
         FIG.  10 A  is a top view of another exemplary RF coil assembly. 
         FIG.  10 B  is a perspective view of the RF coil assembly shown in  FIG.  10 A . 
         FIG.  10 C  is a side view of the RF coil assembly shown in  FIG.  10 A . 
         FIG.  11 A  is a top view of one more exemplary embodiment of RF coil assembly. 
         FIG.  11 B  is a schematic diagram of the RF coil assembly shown in  FIG.  11 A  with a top substrate layer removed. 
         FIG.  11 C  is a perspective view of the RF coil assembly shown in  FIG.  11 A  when being placed on a subject. 
         FIG.  12 A  is a top view of one more exemplary embodiment of an RF coil assembly. 
         FIG.  12 B  is a perspective view of the RF coil assembly shown in  FIG.  12 A  with a flap in an opened position. 
         FIG.  12 C  is a perspective view of the RF coil assembly shown in  FIG.  12 A  when being placed on a subject. 
         FIG.  12 D  is a perspective view of the RF coil assembly shown in  FIG.  12 C  with a flap in an opened position. 
         FIG.  12 E  is a side view of the RF coil assembly shown in  FIG.  12 D . 
         FIG.  13 A  is a top view of one more exemplary embodiment of an RF coil assembly. 
         FIG.  13 B  is a perspective view of the RF coil assembly shown in  FIG.  13 A  with a flap in an opened position. 
         FIG.  13 C  is a perspective view of the RF coil assembly shown in  FIG.  13 A  when being placed on a subject. 
         FIG.  13 D  is a perspective view of the RF coil assembly shown in  FIG.  13 C  with a flap in an opened position. 
         FIG.  13 E  is a schematic diagram of the RF coil assembly shown in  FIG.  13 D . 
         FIG.  14 A  is a perspective view of one more exemplary embodiment of an RF coil assembly. 
         FIG.  14 B  is a perspective view of the RF coil assembly shown in  FIG.  14 A  with a flap in an opened position. 
         FIG.  14 C  is another perspective view of the RF coil assembly shown in  FIG.  14 B . 
         FIG.  15 A  is a perspective view of one more exemplary embodiment of an RF coil assembly. 
         FIG.  15 B  is a perspective view of the RF coil assembly shown in  FIG.  15 A  with a flap in an opened position. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosure includes radio frequency (RF) coil assemblies for use in magnetic resonance (MR) systems in scanning a subject. As used herein, a subject is a human, an animal, a phantom, or any object scanned by a medical imaging system. MR imaging is described as an example only. The assemblies, systems, and methods described herein may be used for MR spectroscopy. MR systems are described as an example only. The RF coil assemblies and method of assembling RF coil assemblies described herein may be used for medical imaging systems other than MR systems, such as positron emission tomography (PET)-MR systems. Method aspects of assembling and using the RF coil assemblies will be in part apparent and in part explicitly discussed in the following description. 
     In MR imaging (MRI), a subject is placed in a magnet. When the subject is in the magnetic field generated by the magnet, magnetic moments of nuclei, such as protons, attempt to align with the magnetic field but precess about the magnetic field in a random order at the nuclei&#39;s Larmor frequency. The magnetic field of the magnet is referred to as B0 and extends in the longitudinal or z direction. In acquiring an MRI image, a magnetic field (referred to as an excitation field B1), which is in the x-y plane and near the Larmor frequency, is generated by a RF coil and may be used to rotate, or “tip,” the net magnetic moment Mz of the nuclei from the z direction to the transverse or x-y plane. A signal, which is referred to as an MR signal, is emitted by the nuclei, after the excitation signal B1 is terminated. To use the MR signals to generate an image of a subject, magnetic field gradient pulses (Gx, Gy, and Gz) are used. The gradient pulses are used to scan through the k-space, the space of spatial frequencies or inverse of distances. A Fourier relationship exists between the acquired MR signals and an image of the subject, and therefore the image of the subject can be derived by reconstructing the MR signals. 
       FIG.  1    illustrates a schematic diagram of an exemplary MR system  10 . In the exemplary embodiment, MR system  10  includes a workstation  12  having a display  14  and a keyboard  16 . Workstation  12  includes a processor  18 , such as a commercially available programmable machine running a commercially available operating system. workstation  12  provides an operator interface that allows scan prescriptions to be entered into MR system  10 . Workstation  12  is coupled to a pulse sequence server  20 , a data acquisition server  22 , a data processing server  24 , and a data store server  26 . Workstation  12  and each server  20 ,  22 ,  24 , and  26  communicate with each other. 
     In the exemplary embodiment, pulse sequence server  20  responds to instructions downloaded from workstation  12  to operate a gradient system  28  and an RF system  30 . The instructions are used to produce gradient and RF waveforms in MR pulse sequences. An RF coil assembly  38  and a gradient coil assembly  32  are used to perform the prescribed MR pulse sequence. RF coil assembly  38  may be a whole body RF coil. RF coil assembly  38  may also be a local RF coil assembly  38  that may be placed in proximity to the anatomy to be imaged, or a coil array that includes a plurality of coils. 
     In the exemplary embodiment, gradient waveforms used to perform the prescribed scan are produced and applied to gradient system  28 , which excites gradient coils in gradient coil assembly  32  to produce the magnetic field gradients G x , G y , and G z  used for position-encoding MR signals. Gradient coil assembly  32  forms part of a magnet assembly  34  that also includes a polarizing magnet  36  and RF coil assembly  38 . 
     In the exemplary embodiment, RF system  30  includes an RF transmitter for producing RF pulses used in MR pulse sequences. The RF transmitter is responsive to the scan prescription and direction from pulse sequence server  20  to produce RF pulses of a desired frequency, phase, and pulse amplitude waveform. The generated RF pulses may be applied to RF coil assembly  38  by RF system  30 . Responsive MR signals detected by RF coil assembly  38  are received by RF system  30 , amplified, demodulated, filtered, and digitized under direction of commands produced by pulse sequence server  20 . RF coil assembly  38  is described as a transmit and receive coil such that RF coil assembly  38  transmits RF pulses and detects MR signals. In one embodiment, MR system  10  may include a transmit RF coil that transmits RF pulses and a separate receive coil that detects MR signals. A transmission channel of RF system  30  may be connected to a RF transmit coil and a receiver channel may be connected to a separate RF receive coil. Often, the transmission channel is connected to the whole body RF coil assembly  38  and each receiver section is connected to a separate local RF coil. 
     In the exemplary embodiment, RF system  30  also includes one or more RF receiver channels. Each RF receiver channel includes an RF amplifier that amplifies the MR signal received by RF coil assembly  38  to which the channel is connected, and a detector that detects and digitizes the I and Q quadrature components of the received MR signal. The magnitude of the received MR signal may then be determined as the square root of the sum of the squares of the I and Q components as in Eq. (1) below: 
         M =√{square root over ( I   2   +Q   2 )}  (1);
 
     and the phase of the received MR signal may also be determined as in Eq. (2) below: 
     
       
         
           
             
               
                 
                   φ 
                   = 
                   
                     
                       
                         tan 
                         
                           - 
                           1 
                         
                       
                       ( 
                       
                         Q 
                         I 
                       
                       ) 
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     In the exemplary embodiment, the digitized MR signal samples produced by RF system  30  are received by data acquisition server  22 . Data acquisition server  22  may operate in response to instructions downloaded from workstation  12  to receive real-time MR data and provide buffer storage such that no data is lost by data overrun. In some scans, data acquisition server  22  does little more than pass the acquired MR data to data processing server  24 . In scans that need information derived from acquired MR data to control further performance of the scan, however, data acquisition server  22  is programmed to produce the needed information and convey it to pulse sequence server  20 . For example, during prescans, MR data is acquired and used to calibrate the pulse sequence performed by pulse sequence server  20 . Also, navigator signals may be acquired during a scan and used to adjust the operating parameters of RF system  30  or gradient system  28 , or to control the view order in which k-space is sampled. 
     In the exemplary embodiment, data processing server  24  receives MR data from data acquisition server  22  and processes it in accordance with instructions downloaded from workstation  12 . Such processing may include, for example, Fourier transformation of raw k-space MR data to produce two or three-dimensional images, the application of filters to a reconstructed image, the performance of a backprojection image reconstruction of acquired MR data, the generation of functional MR images, and the calculation of motion or flow images. 
     In the exemplary embodiment, images reconstructed by data processing server  24  are conveyed back to, and stored at, workstation  12 . In some embodiments, real-time images are stored in a database memory cache (not shown in  FIG.  1   ), from which they may be output to operator display  14  or a display  46  that is located near magnet assembly  34  for use by attending physicians. Batch mode images or selected real time images may be stored in a host database on disc storage  48  or on a cloud. When such images have been reconstructed and transferred to storage, data processing server  24  notifies data store server  26 . workstation  12  may be used by an operator to archive the images, produce films, or send the images via a network to other facilities. 
     During a scan, RF coil array interfacing cables (not shown) may be used to transmit signals between RF coil assembly  38  and other aspects of MR system  10  (e.g., data acquisition server  22  and pulse sequence server  20 ), for example to control the RF coils and/or to receive signals from the RF coils. As described above, the RF coil assembly  38  may be a transmit coil that transmits RF excitation signals, or a receive coil that receives the MR signals emitted by the subject. In an example, the transmit and receive coils are a single mechanical and electrical structure or array of structures, with transmit/receive mode switchable by auxiliary circuitry. In other examples, a transmit coil and a receive coil may be independent structures that are physically coupled to each other via the RF system  30 . For enhanced image quality, however, a receive coil is desirable to be mechanically and electrically isolated from the transmit coil. In such cases, the receive coil in the receive mode is electromagnetically coupled to and resonant with an RF “echo” that is stimulated by the transmit coil. On the other hand, during a transmit mode, the receive coil is electromagnetically decoupled from and therefore not resonant with the transmit coil, during transmission of the RF signal. Such decoupling averts a potential problem of noise produced within the auxiliary circuitry when the receive coil couples to the full power of the RF signal. Additional details regarding the uncoupling of the receive RF coil will be described below. 
     A traditional receive coil for MR includes several conductive intervals joined between themselves by capacitors. By adjusting the capacitors&#39; values, the impedance of the RF coil may be brought to its minimal value, usually characterized by a low resistance. At a resonant frequency, stored magnetic and electric energy alternate periodically. Each conductive interval, due to its length and width, possesses a certain self-capacitance, where electric energy is periodically stored as static electricity. The distribution of this electricity takes place over the entire conductive interval length in the order of 5-15 cm, causing similar range electric dipole field. In proximity of a large dielectric load, self-capacitance of the intervals change, resulting in detuning of the coil. In case of a lossy dielectric, dipole electric field causes Joule dissipation characterized by an increase overall resistance observed by the coil. 
     Traditional RF coils may include acid etched copper traces or loops on printed circuit boards (PCBs) with lumped electronic components (e.g., capacitors, inductors, baluns, and resisters), matching circuitry, decoupling circuitry, and pre-amplifiers. Such a configuration is typically bulky, heavy, and rigid, and requires relatively strict placement of the coils relative to each other in an array to prevent coupling interactions among coil elements that may degrade image quality. As such, traditional RF coils and RF coil arrays lack flexibility and hence may not conform to subject anatomy, degrading imaging quality and subject comfort. 
     The RF coil used in the RF coil assemblies described herein includes a coil loop formed by wire conductors. In the case of two RF coils overlapping, the coupling electronics portion that couples with the coil loop of the RF coil has high blocking or source impedance, thereby minimizing mutual inductance coupling. Thin cross-sections of the wire conductor in the RF coil reduces the parasitic capacitance at the cross-overs or overlaps, and reduces other coupling such as electric field coupling and eddy current, in comparison to two traditional trace-based loops. The combination of high blocking impedance and thin cross-sections of the RF coil loop allows flexible placement of multiple coils into one RF coil assemblies over a finite area, while coupling between the RF coils is minimized and critical overlap between two loops is not required. Wire conductors also add flexibility to the coils, allowing the coil assembly to conform with a curved anatomy of a subject. 
     Turning now to  FIG.  2   , a schematic view of an RF coil  202  that includes a coil loop  201  coupled to a controller unit  210  via a coupling electronics portion  203  and a coil-interfacing cable  212  is shown. In one example, the RF coil may be a surface receive coil, which may be single- or multi-channeled. RF coil  202  may operate at one or more frequencies in MR system  10 . Coil-interfacing cable  212  may be a coil-interfacing cable extending between coupling electronics portion  203  and an interfacing connector of an RF coil array or an RF coil array interfacing cable extending between the interfacing connector of the RF coil array and other components of MR system  10  such as RF system  30  (see  FIG.  5 D  described later). 
     Two coil loops can couple magnetically and electrically. One form of coupling is mutual inductance where signal and noise are transferred from one coil loop to another. The mutual inductance may be reduced by overlapping the coil loops. The mutual inductance may also be reduced by using a high blocking impedance in the coupling electronics portion. The blocking impedance R block  seen by the coil loop in general depends on the resistance R of the coil loop, matching characteristic impedance Z 0  of the transmission line, and input impedance of the LNA (a linear amplifier or preamplifier) R lna  and may be approximated as: 
     
       
         
           
             
               R 
               block 
             
             = 
             
               
                 
                   
                     Z 
                     0 
                   
                   ⁢ 
                   R 
                 
                 
                   R 
                   lna 
                 
               
               . 
             
           
         
       
     
     When a relatively-high blocking impedance R block  is used, 
     
       
         
           
             
               
                 X 
                 L 
               
               R 
             
             ≤ 
             
               
                 Z 
                 0 
               
               
                 R 
                 lna 
               
             
           
         
       
     
     and the induced current from one coil loop to another is minimized, where X L =ω 0 L is the reactance of the coil loop at the resonance frequency of the coil loop. 
     Other coupling such as coupling through electric field and eddy current may be minimized by reducing the cross-section of the wire conductor in coil loop  201 . 
     The coupling electronics portion  203  may be coupled to coil loop  201  of RF coil  202 . Herein, coupling electronics portion  203  may include a decoupling circuit  204 , impedance inverter circuit  206 , and a pre-amplifier  208 . Decoupling circuit  204  may effectively decouple the RF coil during a transmit operation. Typically, RF coil  202  in the receive mode may be positioned adjacent a body of a subject being imaged by MR system  10  in order to receive echoes of the RF signal transmitted during the transmit mode. If RF coil  202  is not used for transmission, RF coil  202  is decoupled from the RF transmit coil such as the RF body coil while the RF transmit coil is transmitting the RF signal. The decoupling of the receive coil from the transmit coil may be achieved using resonance circuits and PIN diodes, microelectromechanical systems (MEMS) switches, or another type of switching circuitry. Herein, the switching circuitry may activate detuning circuits operatively connected to RF coil  202 . 
     The impedance inverter circuit  206  may form an impedance matching network between RF coil  202  and pre-amplifier  208 . Impedance inverter circuit  206  is configured to transform a coil impedance of RF coil  202  into an optimal source of impedance for pre-amplifier  208 . The impedance inverter circuit  206  may include an impedance matching network and an input balun. Pre-amplifier  208  receives MR signals from corresponding RF coil  202  and amplifies the received MR signals. In one example, the pre-amplifier may have a low input impedance that is configured to accommodate a relatively high blocking or source impedance. Additional details regarding the RF coil and associated coupling electronics portion will be explained in more detail below with respect to  FIGS.  3 A,  3 B,  4 A, and  4 B . coupling electronics portion  203  may be packaged in a small PCB with a surface area of approximately 2 cm 2  or smaller. The PCB may be protected with a conformal coating or an encapsulating resin. 
     The coil-interfacing cable  212 , such as a RF coil array interfacing cable, may be used to transmit signals between the RF coils and other components of MR system  10 . The RF coil array interfacing cables may be disposed within the bore or imaging space of MR system  10  and subjected to electro-magnetic fields produced and used by MR system  10 . In MR systems, coil-interfacing cables, such as coil-interfacing cable  212 , may support transmitter-driven common-mode currents, which may in turn create field distortions and/or unpredictable heating of components. Typically, common-mode currents are blocked by using baluns. Baluns or common-mode traps provide high common-mode impedances, which in turn reduces the effect of transmitter-driven currents. 
     Thus, coil-interfacing cable  212  may include one or more baluns. In traditional coil-interfacing cables, baluns are positioned with a relatively high density, as high dissipation/voltages may develop if the balun density is too low or if baluns are positioned at an inappropriate location. However, this dense placement may adversely affect flexibility, cost, and performance. As such, the one or more baluns in the coil-interfacing cable may be continuous baluns to ensure no high currents or standing waves, independent of positioning. The continuous baluns may be distributed, flutter, and/or butterfly baluns. 
       FIG.  3 A  is a schematic of an exemplary RF coil  202  having segmented conductors formed in accordance with an embodiment. RF coil  202  is a non-limiting example of RF coil  202  shown in  FIG.  2    and as such includes coil loop  201  and coupling electronics portion  203 . The coupling electronics portion allows the RF coil to transmit and/or receive RF signals when driven by RF system  30  (shown in  FIG.  1   ). In the illustrated embodiment, RF coil  202  includes a first conductor  300  and a second conductor  302 . first and second conductors  300 ,  302  may be segmented such that the conductors form an open circuit (e.g., form a monopole). The segments of conductors  300 ,  302  may have different lengths. The length of first and second conductors  300 ,  302  may be varied to achieve a select distributed capacitance, and accordingly, a select resonance frequency. 
     The first conductor  300  includes a first segment  304  and a second segment  306 . First segment  304  includes a driven end  312  at an interface terminating to coupling electronics portion  203 , which will be described in more detail below. First segment  304  also includes a floating end  314  that is detached from a reference ground, thereby maintaining a floating state. Second segment  306  includes a driven end  316  at the interface terminating to the coupling electronics portion and a floating end  318  that is detached from a reference ground. 
     The second conductor  302  includes a first segment  308  and a second segment  310 . First segment  308  includes a driven end  320  at the interface. First segment  308  also includes a floating end  322  that is detached from a reference ground, thereby maintaining a floating state. Second segment  310  includes a driven end  324  at the interface, and a floating end  326  that is detached from a reference ground. Driven end  324  may terminate at the interface such that end  324  is only coupled to the first conductor through the distributed capacitance. The capacitors shown around the loop between the conductors represent the capacitance between the wire conductors. 
     Distributed capacitance (DCAP), as used herein, represents a capacitance exhibited between conductors that grows evenly and uniformly along the length of the conductors and is void of discrete or lumped capacitive components and discrete or lumped inductive components. In the examples herein, the capacitance may grow in a uniform manner along the length of first and second conductors  300 ,  302 . For example, first conductor  300  exhibits a distributed capacitance that grows based on the length of first and second segments  304 ,  306 . Second conductor  302  exhibits a distributed capacitance that grows based on the length of first and second segments  308 ,  310 . First segments  304 ,  308  may have a different length than second segments  306 ,  310 . The relative difference in length between first segments  304 ,  308  and second segments  306 ,  310  may be used to produce an effective LC circuit have a resonance frequency at the desired center frequency. For example, by varying the length of first segments  304 ,  308  relative to the lengths of second segments  306 ,  310 , an integrated distributed capacitance may be varied. 
     In the illustrated embodiment, the first and second wire conductors  300 ,  302  are shaped into a coil loop that terminates to an interface. But in other embodiments, other shapes are possible. For example, the coil loop may be a polygon, shaped to conform the contours of a surface (e.g., housing), and/or the like. The coil loop defines a conductive pathway along the first and second conductors. The first and second conductors are void of any discrete or lumped capacitive or inductive elements along an entire length of the conductive pathway. The coil loop may also include loops of varying gauge of stranded or solid conductor wire, loops of varying diameters with varying lengths of the first and second conductors  300 ,  302 , and/or loops of varying spacing between the first and second conductors. For example, each of the first and second conductors may have no cuts or gaps (no segmented conductors) or one or more cuts or gaps (segmented conductors) at various locations along the conductive pathway. 
     A dielectric material  303  encapsulates and separates first and second conductors  300 ,  302 . Dielectric material  303  may be selectively chosen to achieve a select distributive capacitance. Dielectric material  303  may be based on a desired permittivity E to vary the effective capacitance of the coil loop. For example, the dielectric material  303  may be air, rubber, plastic, or any other dielectric material. In one example, the dielectric material may be polytetrafluoroethylene (pTFE). For example, dielectric material  303  may be an insulating material surrounding the parallel conductive elements of first and second conductors  300 ,  302 . Alternatively, first and second conductors  300 ,  302  may be twisted upon one another to form a twisted pair cable. As another example, dielectric material  303  may be a plastic material. First and second conductors  300 ,  302  may form a coaxial structure in which plastic dielectric material  303  separates the first and second conductors. As another example, the first and second conductors may be configured as planar strips. 
     The coupling electronics portion  203  is operably and communicatively coupled to RF system  30  to allow RF coil  202  to transmit and/or receive RF signals. In the illustrated embodiment, coupling electronics portion  203  includes a signal interface  358  configured to transmit and receive the RF signals. Signal interface  358  may transmit and receive the RF signals via a cable. The cable may be a 3-conductor triaxial cable having a center conductor, an inner shield, and an outer shield. The center conductor is connected to the RF signal and pre-amp control (RF), the inner shield is connected to ground (GND), and the outer shield is connected to the multi-control bias (diode decoupling control) (MC_BIAS). A 10V power connection may be carried on the same conductor as the RF signal. 
     As explained above with respect to  FIG.  2   , coupling electronics portion  203  includes a decoupling circuit, impedance inverter circuit, and pre-amplifier. As illustrated in  FIG.  3 A , the decoupling circuit includes a decoupling diode  360 . Decoupling diode  360  may be provided with voltage from MC_BIAS, for example, in order to turn decoupling diode  360  on. When on, decoupling diode  360  causes conductor  300  to short with conductor  302 , thus causing the coil be off-resonance and hence decouple the coil during a transmit operation, for example. 
     The impedance inverter circuit includes a plurality of inductors, including first inductor  370   a , second inductor  370   b , and third inductor  370   c ; a plurality of capacitors, including first capacitor  372   a , a second capacitor  372   b , a third capacitor  372   c , and a fourth capacitor  372   d ; and a diode  374 . The impedance inverter circuit includes matching circuitry and an input balun. As shown, the input balun is a lattice balun that includes first inductor  370   a , second inductor  370   b , first capacitor  372   a , and second capacitor  372   b . In one example, diode  374  limits the direction of current flow to block RF receive signals from proceeding into decoupling bias branch (MC_BIAS). 
     The pre-amplifier  362  may be a low input impedance pre-amplifier that is optimized for high source impedance by the impedance matching circuitry. The pre-amplifier may have a low noise reflection coefficient, γ, and a low noise resistance, Rn. In one example, the pre-amplifier may have a source reflection coefficient of γ substantially equal to 0.0 and a normalized noise resistance of Rn substantially equal to 0.0 in addition to the low noise figure. However, γ values substantially equal to or less than 0.1 and Rn values substantially equal to or less than 0.2 are also contemplated. With the pre-amplifier having the appropriate γ and Rn values, the pre-amplifier provides a blocking impedance for RF coil  202  while also providing a large noise circle in the context of a Smith Chart. As such, current in RF coil  202  is minimized, the pre-amplifier is effectively noise matched with RF coil  202  output impedance. Having a large noise circle, the pre-amplifier yields an effective signal to noise ratio (SNR) over a variety of RF coil impedances while producing a high blocking impedance to RF coil  202 . 
     In some examples, pre-amplifier  362  may include an impedance transformer that includes a capacitor and an inductor. The impedance transformer may be configured to alter the impedance of the pre-amplifier to effectively cancel out a reactance of the pre-amplifier, such as capacitance caused by a parasitic capacitance effect. Parasitic capacitance effects can be caused by, for example, a PCB layout of the pre-amplifier or by a gate of the pre-amplifier. Further, such reactance can often increase as the frequency increases. Advantageously, however, configuring the impedance transformer of the pre-amplifier to cancel, or at least minimize, reactance maintains a high impedance (i.e. a blocking impedance) to RF coil  202  and an effective SNR without having a substantial impact on the noise figure of the pre-amplifier. The lattice balun described above may be a non-limiting example of an impedance transformer. 
     In examples, the pre-amplifier described herein may a low input pre-amplifier. For example, in some embodiments, a “relatively low” input impedance of the preamplifier is less than approximately 5 ohms at resonance frequency. The coil impedance of RF coil  202  may have any value, which may be dependent on coil loading, coil size, field strength, and/or the like. Examples of the coil impedance of RF coil  202  include, but are not limited to, between approximately 2 ohms and approximately 10 ohms at 1.5 T magnetic field strength, and/or the like. The impedance inverter circuitry is configured to transform the coil impedance of RF coil  202  into a relatively high source impedance. For example, in some embodiments, a “relatively high” source impedance is at least approximately 100 ohms and may be greater than 150 ohms. 
     The impedance transformer may also provide a blocking impedance to RF coil  202 . Transformation of the coil impedance of RF coil  202  to a relative high source impedance may enable the impedance transformer to provide a higher blocking impedance to RF coil  202 . Exemplary values for such higher blocking impedances include, for example, a blocking impedance of at least 500 ohms, and at least 1000 ohms. 
       FIG.  3 B  is a schematic of another exemplary RF coil  202  and coupling electronics portion  203  according to another embodiment. The RF coil of  FIG.  3 B  is a non-limiting example of RF coil  202  and coupling electronics shown  FIG.  2   , and as such includes a coil loop  201  and coupling electronics portion  203 . RF coil  202  includes a first conductor  1300  in parallel with a second conductor  1302 . Different from RF coil  202  shown in  FIG.  3 A  that includes segmented conductors  300 ,  302 , at least one of first and second conductors  1300 ,  1302  are elongated and continuous. 
     In the illustrated embodiment, first and second conductors  1300 ,  1302  are uninterrupted and continuous along an entire length of the coil loop. The coil loop may also include loops of varying gauge of stranded or solid conductor wire, loops of varying diameters with varying lengths of first and second conductors  1300 ,  1302 , and/or loops of varying spacing between the first and second conductors. 
     The first and second conductors  1300 ,  1302  have a distributed capacitance along the length of the coil loop (e.g., along the length of first and second conductors  1300 ,  1302 ). First and second conductors  1300 ,  1302  exhibit a substantially equal and uniform capacitance along the entire length of the coil loop. In the examples herein, the capacitance may grow in a uniform manner along the length of first and second conductors  1300 ,  1302 . At least one of first and second conductors  1300 ,  1302  are elongated and continuous. In the illustrated embodiment, both first and second conductors  1300 ,  1302  are elongated and continuous. But in other embodiments, only one of first or second conductors  1300 ,  1302  may be elongated and continuous. First and second conductors  1300 ,  1302  form continuous distributed capacitors. The capacitance grows at a substantially constant rate along the length of conductors  1300 ,  1302 . In the illustrated embodiment, first and second conductors  1300 ,  1302  form elongated continuous conductors that exhibits DCAP along the length of first and second conductors  1300 ,  1302 . First and second conductors  1300 ,  1302  are void of any discrete capacitive and inductive components along the entire length of the continuous conductors between terminating ends of first and second conductors  1300 ,  1302 . For example, first and second conductors  1300 ,  1302  do not include any discrete capacitors, or any inductors along the length of the coil loop. 
       FIG.  3 C  shows a cross-sectional view of an exemplary coil loop  201 . Coil loop  201  includes first wire conductor  392  and second wire conductor  394  surrounded by and encapsulated in dielectric material  303 . Wire conductors  392 ,  394  may be conductors  300 ,  302 ,  1300 ,  1302  described above. Each wire conductor may have a suitable cross-sectional shape, herein a circular cross-sectional shape. However, other cross-sectional shapes for the wire conductors are possible, such as elliptical, cylindrical, rectangular, triangular, or hexagonal. The wire conductors may be separated by a suitable distance, and the distance separating the conductors as well as the diameters of the wire conductors may be selected to achieve a desired capacitance. Further, each of first wire conductor  392  and second wire conductor  394  may be a multi-strand wire conductor, which has a plurality of strands  395 , such as a seven conductor stranded wire (e.g., having seven stranded wires), but solid conductors may also be used instead of stranded wire. Stranded wire may provide more flexibility relative to solid conductors, at least in some examples. 
     As appreciated by  FIGS.  3 A and  3 B , the two parallel conductors including the coil loop of an RF coil may each be continuous conductors, as illustrated in  FIG.  3 B , or one or both of the conductors may be non-continuous, as illustrated in  FIG.  3 A . For example, both conductors shown in  FIG.  3 A  may include cuts, resulting in each conductor having two segments. The resulting space between conductor segments may be filled with the dielectric material that encapsulates and surrounds the conductors. The two cuts may be positioned at different locations, e.g., one cut at 135° and the other cut at 225° (relative to where the coil loop interfaces with the coupling electronics). By including discontinuous conductors, the resonance frequency of the coil may be adjusted relative to a coil that includes continuous conductors. In an example, an RF coil that includes two continuous parallel conductors encapsulated and separated by a dielectric, the resonance frequency may be a smaller, first resonance frequency. If that RF coil instead includes one discontinuous conductor (e.g., where one of the conductors is cut and filled with the dielectric material) and one continuous conductor, with all other parameters (e.g., conductor wire gauge, loop diameter, spacing between conductors, dielectric material) being the same, the resonance frequency of the RF coil may be a larger, second resonance frequency. In this way, parameters of the coil loop, including conductor wire gauge, loop diameter, spacing between conductors, dielectric material selection and/or thickness, and conductor segment number and lengths, may be adjusted to tune the RF coil to a desired resonance frequency. 
       FIGS.  4 A and  4 B  shows more exemplary RF coils  202 .  FIG.  4 C  is a cross-sectional view of a wire conductor  452  used in coil loop  201  of RF coils  202 . Different from coil loops  201  shown in  FIGS.  3 A- 3 C  that include first conductor  300 ,  1300  and second conductor  302 ,  1302  and two driven ends at each end of the conductors, coil loops  201  shown in  FIGS.  4 A- 4 B  includes one single wire conductor  452  and one driven end  462 ,  466  at each end of wire conductor  452 . Coil loop  201  may form into one turn  470  ( FIG.  4 A ) or a plurality of turns  470  ( FIG.  4 B ). The resistance of coil loop  201  increases approximately by number of turns  470 , and the loop loss increases approximately by square root of the number of turns  470 , while the body loss increases approximately by the number of turns  470 . As a result, the SNR of coil loop  201  is increased approximately by the square root of the number of turns. In other words, multiple turns are used to increase the ratio of the body loss over the loop loss, compared with a single turn coil loop. Coil loop  201  forms into a shape of a circle, and may form into other shapes such as a polygon, oval, or irregular shapes. Coil loop  201  defines a conductive pathway along wire conductor  452 . Wire conductor  452  is shown as uninterrupted and continuous along an entire length of the coil loop. The coil loop may also include loops of varying gauge of stranded or solid conductor wire, loops of varying diameters with varying lengths of conductors  452 . For example, conductor  452  may have no cuts or gaps (no segmented conductors) or one or more cuts or gaps (segmented conductors) at various locations along the conductive pathway. One or more capacitors  472  may be placed at the cuts, gaps, or at the end of the coil loop. The capacitance of capacitors  472  may be variable. 
       FIG.  4 C  shows a cross-sectional view of wire conductor  452 . In the exemplary embodiment, conductor  452  has a suitable cross-sectional shape, such as circular, elliptical, rectangular, triangular, or other shapes that enable conductor  452  functions as described herein. Insulating material  403  surrounds conductors  452 . Dielectric material  403  may be rubber, plastic, or any other dielectric material. conductor  452  includes one or a plurality of strands  395 . For example, conductor  452  is a single-strand wire conductor. Alternatively, conductor  452  is a multi-strand wire conductor having a plurality of strands  395 , where an individual strand  395  may be surrounded by insulating material or not surrounded by insulating material. Individual strands  395  may be twisted upon each other or may be parallel to each other, along the length of strand  395 . In one example, wire conductor  452  includes 19 strands that are 36 AWG each for an overall thickness of 24 AWG, and the cross section of wire conductor  452  has a diameter of 0.025 inches (0.06 cm). A coil loop  201  including multi-strand conductors  452  has a higher penetration depth and a higher SNR than a coil loop  201  of the same diameter that includes distributed capacitance wire conductors  300 ,  302 ,  1300 ,  1302 . Therefore, the size of coil loop  201  may be reduced by including multi-strand wire conductors  452  instead of distributed capacitance wire conductors  300 ,  302 ,  1300 ,  1302  for the same penetration depth, and consequently an increase number of RF coils  202  may be included in a coil array. 
     Referring back to  FIGS.  2 ,  3 A,  3 B,  4 A, and  4 B , coil loop  201  is coupled to coupling electronics portion  203 . Coupling electronics portion  203  may be the same coupling electronics described above with respect to  FIGS.  2 ,  3 A,  3 B,  4 A , and  4 B, and hence like reference numbers are given to like components and further description is dispensed with. 
     The RF coils  202  presented above with respect to  FIGS.  2 ,  3 A,  3 B,  4 A, and  4 B  may be used in order to receive MR signals during an MR imaging session. As such, the RF coils of  FIGS.  2 ,  3 A,  3 B,  4 A, and  4 B  are configured to be coupled to downstream components of MR system  10 . RF coils  202  of  FIGS.  2 ,  3 A,  3 B,  4 A, and  4 B  may be present in an array of RF coils having various configurations. 
       FIGS.  5 A- 5 D  show an exemplary RF coil assembly  500  that includes RF coils  202  described above. RF coil assembly  500  may be a local RF coil assembly  38  of system  10  (see  FIG.  1   ).  FIG.  5 A  is a top perspective view of RF coil assembly  500 .  FIG.  5 B  is a bottom perspective view of RF coil assembly  500 .  FIG.  5 C  is an exploded view of RF coil assembly  500 .  FIG.  5 D  is a block diagram of RF coil assembly  500 . 
     In the exemplary embodiment, RF coil assembly  500  further includes an RF coil array interfacing cable  504  extending from a coil interfacing connector  506  of the RF coil array  514 . RF coil array interfacing cable  504  may be used to connect the RF coil assembly  500  to other components of the MR system  10  such as the RF system  30  through a coil array interfacing connector  507 . The RF coil array interfacing cable  504  may include a plurality of baluns  508  or contiguous/continuous distributed baluns (not shown). 
     In the exemplary embodiment, RF coil assembly  500  further includes an outer enclosure  510 , RF coils  202  ( FIG.  5 C ), and a substrate assembly  512 . RF coils  202  may form into an RF coil array  514 . Each RF coils  202  may include an RF coil loop  201 . RF coil loop  201  includes wire conductor  516 . Wire conductor  516  may be wire conductors  300 ,  302 ,  1300 ,  1302 ,  452  described above. Wire conductor  516  forms into coil loop  201 . RF coil  202  may also include coupling electronics portion  203 . RF coil array  514  is coupled to substrate assembly  512  of flexible fabric material. Sandwiching the RF coil array  514  and substrate assembly  512  is an inner enclosure  511  including a first layer  556  and second layer  558 . The material of the inner enclosure may be NOMEX® or other suitable material that provides padding, spacing, and/or flame-retardant properties. An outer enclosure including a first layer  560  and a second layer  562  sandwiches RF coil array  514 , substrate assembly  512 , and inner enclosure  511 . The material of first layer  560  of the outer enclosure  510  may be fabricated from a biocompatible material that is cleanable, thus enabling use of the RF coil array in clinical contexts. Second layer  562   o  of the outer enclosure  510  may be fabricated from the deformable material. In this way, the RF coils may be positioned on a top surface of the subject. The RF coils may flex and deform as needed to accommodate patient anatomy. In another embodiment, first layer  556  and second layer  558  of inner enclosure  511  may be eliminated from RF coil assembly  500 , such that RF coil array  514  and substrate assembly  512  are sandwiched between a first layer  560  and second layer  562  of padded or deformable material of outer enclosure  510 . 
     In the exemplary embodiment, circular coil loop  201  is depicted as an example only. Coil loop  201  may in other shapes, such as oval, irregularly curved, or rectangular, that enable coil loop  201  to function as described herein. In one example, coil loop  201  is fabricated from a flexible 1.3 millimeter (mm) diameter conductor optimized for zero reactance at 127.73 MHz, the resonance frequency of a 3 T MR system. RF coils  202  may be designed for an MR system  10  having a different field strength, such as 1.5 T. Because wire conductor  300 ,  302 ,  1300 ,  1302 ,  452  of coil loop  201  is flexible, the shape of coil loop  201  may change and be deformed to conform to a curved anatomy of the subject, such as deforming from being circular to other shapes such as oval, elliptical, or irregular shapes like Pringles® chips. A coil-interfacing cable  212  ( FIG.  5 D ) is connected to and extends from each coupling electronic PCB or coupling electronics portion  203  to coil interfacing connector  506 . Coil interfacing connector  506  further couples to other components of MR system  10  such as RF system  30  through RF coil array interfacing cable  504  (see  FIGS.  5 A and  5 B ). For example, coil interfacing connector  506  is coupled to coil array interfacing connector  507  and coil array interfacing connector  507  is plugged in a coil interface (not shown) when RF coil assembly  500  is in use, coupling RF coil assembly  500  to the rest of the MR system  10 , such as RF system  30 . 
     Coupling electronics portion  203  may include a decoupling circuit, impedance inverter circuit, and a pre-amplifier. The decoupling circuit may effectively decouple an RF coil during a transmit operation. The impedance inverter circuit may form an impedance matching network between an RF coil and the pre-amplifier. The impedance inverter circuit is configured to transform a coil impedance of a RF coil into an optimal source impedance for the pre-amplifier. The impedance inverter circuit may include an impedance matching network and an input balun. The pre-amplifier receives MR signals from a RF coil and amplifies the received MR signals. In one example, the pre-amplifier may have a low input impedance that is configured to accommodate a relatively high blocking or source impedance. The coupling electronics portion  203  may be packaged in a small PCB, for example having an area of approximately 2 cm 2  or smaller. The PCB may be protected with pad or padding material, a conformal coating, or an encapsulating resin. 
     Control circuitry  520  ( FIG.  5 D ) is the MC_BIAS for switching RF coils between receive and decoupled modes. Elements of control circuitry  520  are incorporated in both coupling electronics portion  203  and coil interfacing connector  506 . 
     In the exemplary embodiment, RF coil assembly  500  is flexible without increased stress on coil loops  201  and other components of RF coil assembly  500 . As used herein, an RF coil assembly is flexible when the RF coil assembly may be flexed or bent to change the shape of the RF coil assembly. The RF coils  202  described above are configured to maintain the performance as RF coil when the RF coils are flexed or bent. 
     As described above, in MR, signals are acquired by an RF coil. Therefore, an RF coil plays a major role in image quality, such as sign-to-noise ratio (SNR) or image distortion, of images acquired by an MR system. RF coils are desirable to be flexible such that RF coils conform to and are proximate to the anatomy of the subject. On the other hand, the shape and relative positions among components of an RF coil should be maintained to ensure consistency of image quality. 
     In some known RF coil assemblies, coil loops are coupled to a substrate layer through stiches or other attachment mechanisms, When coil loops are attached to substrate through attachment mechanism, coil loops  201  are under stress at the attachment point, causing coil loop  201  to break and reducing the life of RF coil assemblies. Stiches themselves are also under stress in holding the coil loops together, especially when the coil loops are flexed or bent. As a result, stiches will also break and repair and/or maintenance is needed. Further, attaching through attachment mechanisms like stiches is labor intensive. For example, to stich coil loops to a substrate layer, a special industrial sewing machine is needed to apply stitches around coil loops at circumference or part of circumference of a coil loop. Areas around openings, windows, or gaps are challenging for a sewing machine to maneuver. Operating the sewing machine and setting up the sewing machine demand skills of the operator. Extra care is needed to make sure coil loops or coupling electronics are not damaged during the stitching process. 
     In other known RF coil assemblies, coil loops are positioned in grooves formed in a substrate layer.  FIG.  6    shows a cross-sectional view of a known RF coil assembly  600 , where coil loops are positioned in grooves formed in a layer  620 . RF coil assembly  600  includes a first, outer layer  610 . Outer layer  610  is fabricated from one or more sheets of a flexible fabric material. Outer layer  610  may have a first thickness  615 . In one example, first thickness  615  may be 1.5 cm or less. RF coil assembly  600  includes a second, inner layer  620 . Inner layer  620  is fabricated from a compressible material such as memory foam and may have a second thickness  625 . Second thickness  625  may be greater than first thickness  615  and may be 5 cm. Inner layer  620  has a plurality of annular grooves each configured to accommodate an RF coil. Inner layer  620  includes a first annular groove  650 . First annular groove  650  accommodates first coil loop  652 . For example, first annular groove  650  may be a cut, indentation or groove formed in inner layer  620  that is sized to fit first coil loop  652 . When first coil loop  652  is positioned in first annular groove  650 , the material including inner layer  620  may surround first coil loop  652 , thereby embedding the loop portion of first coil loop  652  in the second inner layer. Inner layer  620  also includes a second annular groove  655  (accommodating second coil loop  657 ), a third annular groove  660  (accommodating third coil loop  662 ), a fourth annular groove  665  (accommodating fourth coil loop  667 ), and a fifth annular groove  670  (accommodating fifth coil loop  672 ). Coil loops  652 ,  657 ,  662 ,  667 ,  672  are collectively referred to as coil loops  651 . While not shown in  FIG.  6   , a plurality of rectangular grooves may be present in inner layer  620 , each adjacent a respective annular groove. The rectangular grooves may accommodate the coupling electronics portion of each RF coil. 
     Each annular groove (and hence each RF coil) may be present at a top portion of inner layer  620 , and thus a top surface of each RF coil may not be covered by the material of inner layer  620 . However, outer layer  610  may cover the top surface of each RF coil. Each of outer layer  610  and inner layer  620  may be compressible, allowing the RF coils embedded therein to conform to a shape of the subject positioned on the RF coil array. 
     In RF coil assembly  600 , layers  610 ,  620  needs to be compressible such that grooves or indentation may be formed. The thickness of the layers  610 ,  620  also needs to be at least thicker than coil loop  651  and/or coupling electronics portion. Further, in order to ensure coil loops  651  are embedded in grooves, inner layer  620  is typically not flexible and does allow movement of coil loops  651 . In addition, to avoid coil loop  651  and coupling electronics portion from being dislodged from grooves or indentation, layers  610 ,  620  may need to be attached to one another through attachment mechanisms such as adhesive, which will deteriorate and become ineffective. Because of the thickness of layers  610 ,  620 , RF coil assembly  600  is typically placed under a subject, limiting applications of RF coil assembly  600 . 
     In contrast, in RF coil assembly  500 , substrate assemblies are welded together, where substrate assemblies are coupled without a separately-provided fastening mechanism such as attachment mechanisms, e.g. stiches or adhesives, thereby avoiding problems associated with a separately-provided fastening mechanism. In addition, the welded RF coil assemblies and methods described herein provide space for RF coils to reposition and flex without increased stress to the RF coils and fastening mechanism when placing the RF coil assemblies to the subject, thereby increasing image quality of acquired images by increasing conformity of the RF coil with the anatomy of the subject and increasing usable life of the RF coil assembly. 
       FIGS.  7 A- 8 B  show RF coil assembly  500  with outer enclosure  510  and inner enclosure  511  removed and without showing wires and electronics coupled to coupling electronics portions  203 .  FIG.  7 A  is a perspective view of RF coil assembly  500 .  FIG.  7 B  is a top view of RF coil assembly  500 .  FIG.  7 C  is a bottom view of RF coil assembly  500 .  FIG.  8 A  shows RF coil assembly  500  shown in  FIGS.  7 A- 7 C  with a top substrate layer  513 - t  removed.  FIG.  8 B  shows a bottom substrate layer  513 - b  without coil loops  201  or coupling electronics portion  203 . 
     In the exemplary embodiment, RF coil assembly  500  includes RF coil array  514  and substrate assembly  512 . RF coil array  514  includes one or more RF coils  202 . RF coils may be arranged in an array. RF coil  202  includes coil loop  201  formed by wire conductors  516 . Wire conductor may be wire conductor  516 . RF coil  202  may further include coupling electronics portion  203 . Coupling electronics portion  203  is electrically connected to coil loop  201  at ends  517  of wire conductor  516 . 
     In the exemplary embodiment, substrate assembly  512  includes a first substrate layer  513  and a second substrate layer  513 . First or second substrate layer  513  may be top or bottom substrate layer  513 - t ,  513 - b . First and second substrate layers  513  are coupled with one another without a separately-provided fastening mechanism, such as attachment mechanism like stitching or adhesive. Substrate assembly  512  is welded, where substrate layers  513  are coupled to one another through welding. For example, substrate assembly  512  is an RF welded substrate assembly, where substrate layers  513  are coupled together through RF welding. Substrate layers  513  are fabricated from RF weldable material such as thermoplastics (e.g. polyvinylchloride and polyurethanes), thermoplastics laminated or coated fabrics, or compound rubber. For example, substrate layers  513  is fabricated from a flexible fabric material, such as DARTEX® material. In RF welding, polar molecules are melted by heat due to movements of polar molecules under RF electric fields, thereby bonding to one another. As such, coupling formed by RF welding is secure. Other welding methods may be used to weld substrate layers  513  to one another. When other welding methods are used, substrate layers  513  are fabricated from material suitable for those welding methods. Similarly, because layers  513  are bonded through fused molecules from heat or solvent during welding, coupling through welding is secure. 
     In the exemplary embodiment, substrate assembly  512  includes channels  515  ( FIG.  8 B ) when first substrate layer  513  is bonded with second substrate layer  513 . Wire conductor  516  is positioned in channel  515 . Channel  515  has a width  519  sized to receive wire conductor  516  therein as well as to provide space for movement, repositioning and flexing of wire conductor  516 , thereby reducing stress on coil loops  201 , increasing usable life of the RF coil assembly  500 , and increasing conformity of RF coil assembly with anatomy of the subject. An exemplary width of channel  515  is in the range of 6-8 mm. At ends  521  of channel  515 , a gap  518  is formed or defined. Gap  518  may be positioned at other locations of substrate layer  513 . Gap  518  is sized to receive coupling electronics portion  203  therein. 
     In operation, RF coil array  514  is positioned between first and second substrate layers  513 . Wire conductors  516  are in channels  515  and may move, reposition, and flex at various dimensions. For example, wire conductors  516  may shift in channel  515 . Wire conductors may flex, bend, or rotate with substrate assembly to conform with subject&#39;s anatomy. As a result, wire conductors  516  are not under stress from attachment mechanisms, increasing the usable life of RF coil assembly  500 . Further, channels  515  and/or gaps  518  may be sealed such that substrate assembly  512  is water-proof to prevent liquid from entering from exterior of substrate assembly  512  into electrical components of RF coil assembly such as coupling electronics portion  203 , ensuring performance of RF coil assembly. 
       FIGS.  9 A- 9 C  illustrate an exemplary method  900  of assembling RF coil assemblies. RF coil assemblies may be RF coil assemblies  500  described above. In the exemplary embodiment, an RF coil array is positioned  902  on a substrate layer ( FIG.  9 A ). The RF coil array  514  may include RF coils  202  that have wire conductors  516 . In some embodiments, RF coil  202  also includes coupling electronics portion  203  electrically connected to wire conductor  516 . In other embodiments, RF coil  202  does not include a coupling electronics portion. Alternatively, in positioning  902  an RF coil array, coupling electronics portions  203  are not electrically connected with wire conductor  516  and not placed on substrate layer  513 , and ends of wire conductor  516  include a gap  518  for connection with coupling electronics portion  203  later in the process. Method  900  further includes positioning  904  ( FIG.  9 B ) another substrate layer  513 - 2  over RF coil array  514  and first substrate layer  513 - 1 . Substrate layer  513 - 1 ,  513 - 2  may include apertures  905  corresponding to gaps  518  positioned at ends of the wire conductors such that coupling electronics portions  203  and/or coil-interfacing cable (not shown) connected to and extending from coupling electronic portions  203  or wire conductors  516  may pass through apertures  905 . The depicted embodiment shows apertures  905  are on substrate layer  513 - 2 . Apertures  905  may be on either one of substrate layers  513  or on both of substrate layers  513 . 
     In the exemplary embodiment, method  900  further includes coupling  906  ( FIG.  9 C ) the first substrate layer with the second substrate layer by welding the first substrate layer with the second substrate layer. Welding may be RF welding. Welding may be other dielectric welding or dielectric sealing, such as microwave welding. Other welding methods may be used such as ultrasonic welding. Welding as used herein refers to coupling materials with the aid of heat or solvent. Comparing to sewing which may take hours or more, RF welding takes seconds, drastically increasing manufacturing speed. After welding, substrate layers  513  are coupled to one another at the welded seams  907 . Gaps between segments of seams  907 , such as gap  518 , may be sealed such that RF coil assembly  500  is water-proof For examples, sealing may be applied at apertures  905  where coupling electronics portion  203  and/or coil-interfacing cable exiting from substrate layers. Welded coupling itself is secure and water proof because molecules are fused together during welding at seams  907 . Referring back to  FIGS.  5 A- 5 C , other layers of RF coil assembly  500  may also be welded using the same welding method as in welding substrate layers  513 . For example, first layer  560  and second layer  562  of outer enclosure  510  may be RF welded together ( FIGS.  5 A and  5 B ) at seams  522 . Layers  556 ,  558  of inner enclosure  511  may also be coupled by RF welding (not shown). The same welding method is used to simplify the manufacturing process where the same machinery is used. 
       FIGS.  10 A- 10 C  shows another exemplary welded RF coil assembly  500 . Compared to RF coil assembly  500  shown in  FIGS.  5 A and  5 B , RF coil assembly  500  further includes welded apertures  1002 .  FIG.  10 A  is a top or bottom view of RF coil assembly  500 .  FIG.  10 B  is a perspective view of RF coil assembly  500 .  FIG.  10 C  is a side view of RF coil assembly  500 . Outer enclosure  510  may not be breathable and may be uncomfortable for the subject, causing the subject to move, after being placed on the subject for an extended period of time, such as one hour. Motion of a subject deteriorates image quality of acquired MR images. Apertures  1002  allow body heat and perspiration to dissipate from the subject, increasing comfort to the subject. In addition, apertures  1002  provide access to anatomy of the subject for interventional treatments such as biopsy, surgery, or therapy. Apertures  1002  are located within coil loop  201  (see  FIG.  8 A ) such that apertures  1002  do not intersect coil loop  201 , coupling electronics portion  203 , or a coil-interfacing cable. In other words, the area defined by aperture  1002  is within an area defined or encircled by coil loop  201 , or aperture  1002  is enclosed or surrounded by coil loop  201 . Apertures  1002  being positioned within a coil loop  201  are advantageous because the anatomy that apertures  1002  provide access to is the same anatomy being imaged by coil loop  201 , thereby allowing MR images to be used as guidance for interventional treatments. Apertures  1002  may be constructed before welding, where areas marked for apertures  1002  are removed and edges along apertures  1002  are welded together. Alternatively, apertures  1002  may be constructed after welding, where locations marked for apertures  1002  are welded and then areas surrounded by seams formed by welding are removed. 
       FIGS.  11 A- 11 C  show one more exemplary embodiment of RF coil assembly  500 .  FIG.  11 A  is a top view of RF coil assembly  500 .  FIG.  11 B  is schematic diagram of RF coil assembly  500  with a top substrate layer removed to show coil loops  201 .  FIG.  11 C  is a side perspective view of RF coil assembly  500  when being placed on a subject  1102 . Similar to RF coil assembly  500  shown in  FIGS.  10 A- 10 C , RF coil assembly  500  includes one or more welded apertures  1002 . Apertures  1002  are positioned within RF coil loops  201 . Within RF coil loop  201 , a plurality of apertures  1002  may be included within circumferences defined by RF coil loop  201  and its neighboring RF coil loop  201 - n.    
     In operation, during interventional treatments such as biopsy or surgery, apertures  1002  provide access to anatomy of subject  1102 , such as an upper torso  1104  of subject  1102 , including breasts, the areas below the armpit, and/or the upper chest area. 
     In the depicted embodiment, the coil loop  201  defines a circumference having a diameter of approximately 7 cm. Smaller-sized coil loops are advantageous than larger-sized coil loops because RF coil assembly  500  having smaller-sized coil loops conforms better to contours of the subject and allows to more coil loops than RF coil assembly  500  having larger-sized coil loops, thereby providing faster image acquisition and higher signal-to-noise (SNR) ratio in images. When sizes of the coil loops reduce, space is limited to weld apertures  1002  through RF coil assembly  500 .  FIGS.  12 A- 14 B  show embodiments of RF coil assembly  500 - f  that includes one or more flaps to provide access to anatomies of subject  1102  during interventional treatments. 
       FIGS.  12 A- 12 E  show an exemplary embodiment of RF coil assembly  500 - f .  FIG.  12 A  is top view of RF coil assembly  500 - f .  FIG.  12 B  is a perspective view of RF coil assembly  500 - f  when a flap  1202  is lifted up.  FIG.  12 C  is a perspective view of RF coil assembly  500 - f  when being placed on subject  1102 .  FIG.  12 D  is a perspective view of RF coil assembly  500 - f  with flap  1202  lifted when being placed on subject  1102 .  FIG.  12 E  is side view of RF coil assembly  500 - f  with flap  1202  lifted when being placed on subject  1102 . Different from RF coil assemblies shown in  FIGS.  5 A- 5 D and  7 A- 11 C , RF coil assembly  500 - f  includes flap  1202 . Flap  1202  includes a portion of first substrate layer  513 - 1  ( FIG.  12 C ), a portion of second substrate layer  513 - 2  ( FIG.  12 D ), and at least one RF coil  202  of RF coil array  514  (see  FIG.  5 C ). RF coils  202  are positioned between first substrate layer  513 - 1  and second substrate layer  513 - 2 . Flap  1202  includes a coupled side  1204 - c  and uncoupled sides  1204 - u . First substrate layer  513 - 1  and second substrate layer  513 - 2  are welded together along uncoupled sides  1204 - u , e.g., through RF welding. RF coils  202  of flap  1202  are positioned within coupled side  1204 - c  and uncoupled sides  1204 - u . Flap  1202  is coupled to the remaining  1206  of RF coil assembly  500 - f  along coupled side  1204 - c . A seam  1210  of remaining  1206 , where flap  1202  is decoupled from remaining  1206 , may be sealed through welding, e.g., RF welding. Flap  1202  is positionable between a closed position ( FIGS.  12 A and  12 C ) and an opened position ( FIGS.  12 B,  12 D, and  12 E ). In the depicted embodiment, RF coil assembly  500 - f  includes two flaps  1202 - 1 ,  1202 - 2  and two RF coil arrays  514 - 1 ,  514 - 2 , RF coil array  514 - 1  for imaging one part of subject  1102  such as a left breast and its surrounding anatomy and RF coil array  514 - 2  for imaging another part of subject  1102  such as a right breast and its surrounding anatomy. Flap  1202 - 1  includes RF coil array  514 - 1 . Flap  1202 - 2  includes RF coil array  514 - 2 . In some embodiments, RF coil assembly  500 - f  includes any other number of flaps, such as one, three, or more. 
     In operation, at a closed position, flap  1202  may be coupled to remaining  1206  through a fastener  1208  such as a hook-and-loop fastener. Fastener  1208  may be welded such as RF welded to substrate assembly  512 . Fastener  1208  may be coupled to substrate assembly  512  through other mechanism such as adhesive. At an opened position, flap  1202  is lifted up and may be folded back, providing access to anatomy of subject  1102 . Because flap  1202  may be lifted and folded back, RF coil assembly  500 - f  provides full access to anatomy covered by RF coil array  514 - 1 ,  514 - 2 . 
       FIGS.  13 A- 13 E  show another exemplary embodiment of RF coil assembly  500 - f .  FIG.  13 A  is a top view of RF coil assembly  500 - f .  FIG.  13 B  is a perspective view of RF coil assembly  500 - f  with one of flaps  1202  is lifted.  FIGS.  13 C- 13 E  show RF coil assembly  500 - f  being placed on subject when flaps are at closed positions ( FIG.  13 C ), and when a flap  1202  is at an opened position ( FIGS.  13 D and  13 E ).  FIG.  13 D  is a perspective view.  FIG.  13 E  is a side view. Different from RF coil assembly  500 - f  shown in  FIGS.  12 A- 12 E , where flap  1202  includes an entire RF coil array  514  (see  FIG.  5 C ), flap  1202 - s  of RF coil assembly  500 - f  does not include the entire RF coil array  514 . In the depicted embodiment, flap  1202 - s  includes one RF coil  202  of RF coil array  514 . In some embodiments, flap  1202 - s  includes more than one RF coil  202  of RF coil array  514 . That is, one RF coil array  514  includes two or more flaps  1202 . Flaps  1202  may overlap with one another. In some embodiments, flaps  1202  do not overlap with one another, or some flaps  1202  overlap and some do not. Each flap  1202 - s  includes a coupled side  1204 - c  and uncoupled sides  1204 - u . Each flap  1202 - s  is positionable at a closed position ( FIG.  13 A  and  FIG.  13 C ) and at an opened position ( FIGS.  13 B,  13 D, and  13 E ). Flap  1202 - s  may be individually lifted and folded back, providing access to anatomy covered by RF coil(s) included in flap  1202 - s . Compared to RF coil assembly  500 - f  shown in  FIGS.  12 A- 12 E , opening and closing flaps  1202 - s  have a reduced effect on the positions of RF coils because a reduced number of RF coils are moved and the moved RF coils are moved at a reduced distance. RF coil assembly  500 - f  having flap  1202 - s  also provides increased accuracy in interventional treatments. Individual images of individual RF coil(s) included in flap  1202 - s  may be used to identify the location of a lesion. If a lesion only appears in an image of one RF coil  202  or images of a few RF coils  202 , lesion would be located directly beneath those RF coil(s)  202 . 
     RF coil assemblies  500  shown in  FIGS.  11 A- 13 E  are configured as a breast coil assembly, which is configured to image at least a portion of an upper torso of subject  1102 . Compared to conventional breast coil assembly, which is limited to imaging subject  1102  at a prone position, breast RF coil assembly  500  is configures to image subject  1102  at a supine position, as well as at a prone position. Breast imaging at a supine position is advantageous over imaging at a prone position. Interventional treatments typically are performed when the subject is at a supine position, providing a greater access to the tissue than performing the treatments when the subject is at a prone position, which provides access at the sides of the subject and increase risks of damages to the subject when the lesion such as tumor is positioned away from the sides. Therefore, breast imaging at a supine position provides matching images and anatomy during treatments, allowing guidance to treatments with increased accuracy. Further, conventional breast RF coils are limited to imaging breasts of the subject. In contrast, breast RF coil assembly  500  allows imaging areas other than breasts, such as the areas below the armpit and upper chest area such as level III axillary lymph nodes and supraclavicular lymph nodes. Although supine breast RF coil assembly may be more sensitive to motion such as breathing motion because supine breast RF coil assembly moves with the chest during breathing, the motion effects are largely reduced with increased number of RF coils in the RF coil assembly  500 . For example, conventional breast RF coil assembly takes minutes to image the breast. During that period of time, breast moves and patient may also become uncomfortable and shift around. In contrast, supine RF coil assembly may include a large number of RF coils such as 60 RF coils and takes seconds to image the breast areas, drastically reducing motion effects. 
       FIGS.  14 A - FIG.  15 B  show exemplary embodiments of RF coil assembly  500 - f  configured as a pelvic RF coil assembly. A pelvic RF coil assembly is configured to image at least a portion of a pelvis  1402  of subject  1102 . Pelvic RF coil assembly  500 - f  conforms to pelvic contour of subject  1102 . 
       FIGS.  14 A- 14 C  show an exemplary embodiment of pelvic RF coil assembly  500 - f    FIG.  14 A  is a perspective view of RF coil assembly  500 - f  when flap  1202  is at a closed position.  FIGS.  14 B and  14 C  are perspective views of RF coil assembly  500 - f  when flap  1202  is at an opened position.  FIG.  14 B  is a perspective view when viewing from a lower torso of subject  1102 .  FIG.  14 B  is a perspective view when viewing from an upper torso of subject  1102 . 
     In the exemplary embodiment, RF coil assembly  500 - f  includes a torso portion  1404  configured to cover a lower torso of subject  1102  and a crotch portion  1406  configured to cover a crotch area of subject  1102 . Torso portion  1404  may include an RF coil array  514  (see  FIG.  5 C ). Crotch portion  1406  includes RF coil array  514 . Crotch portion  1406  is configured as a flap  1202 . Flap  1202  is positionable at a closed position ( FIG.  14 A  or an opened position ( FIGS.  14 B and  14 C ). Flap  1202  includes the entire RF coil array of crotch portion  1406 . Flap  1202  is coupled to torso portion  1404  at a coupled side  1204 - c  (not shown). At a closed positioned, flap  1202  is coupled to torso portion  1404  at an uncoupled side  1204 - u.    
     In operation, during imaging, flap  1202  is at a closed portion with flap  1202  coupled to torso portion  1404  at coupled side  1204 - c . During interventional treatments, to access anatomy of subject  1102 , such as prostate, flap  1202  is decoupled from torso portion  1404  at uncoupled side  1204 - u.    
       FIGS.  15 A- 15 B  are perspective views of another embodiment of pelvic RF coil assembly  500 - f , viewing from the lower torso of subject  1102 . FIG.  FIG.  15 A  shows RF coil assembly  500 - f  when flap  1202 - s  is at a closed position.  FIG.  15 B  shows RF coil assembly  500 - f  when flap  1202 - s  is at an opened position. Different from RF coil assembly  500 - f  shown in  FIGS.  14 A- 14 C , RF coil assembly  500 - f  includes flap  1202 - s  that includes one RF coil or a few RF coils  202  (see  FIG.  5 C ), instead of an entire RF coil array  514  (see  FIG.  5 C ) of crotch portion  1406 . 
     In the depicted embodiment, RF coil assembly  500 - f  includes one flap  1202 - s . RF coil assembly  500 - f  may include two or more flaps  1202 - s , where flaps  1202 - s  may or may not overlap with one another. Alternatively, some flaps  1202 - s  overlap and some flaps  1202  do not. Like RF coil assembly  500 - f  shown in  FIGS.  13 A- 13 E , RF coil assembly  500 - f  reduces position changes of RF coils  202  and increases location accuracy during interventional treatments. 
     At least one technical effect of the systems and methods described herein includes (a) an RF coil assembly that provides flexibility of coil loops without stressing coil loops; (b) an RF coil assembly that provides secure coupling of coil loops as well as permitting movement of the coil loops; (c) assembling an RF coil assembly through welding; (d) a simplified and quick manufacturing process of an RF coil assembly; (e) an RF coil assembly that provides access to interventional treatments; and (f) an RF coil assembly that includes a flap for access to anatomy when the flap is at an opened position. 
     Exemplary embodiments of assemblies, systems, and methods of RF coil assemblies are described above in detail. The systems and methods are not limited to the specific embodiments described herein but, rather, components of the systems and/or operations of the methods may be utilized independently and separately from other components and/or operations described herein. Further, the described components and/or operations may also be defined in, or used in combination with, other systems, methods, and/or devices, and are not limited to practice with only the systems described herein. 
     Although specific features of various embodiments of the invention may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of the invention, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing. 
     This written description uses examples to disclose the invention, including the best mode, and to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.