Abstract:
A radio frequency (RF) coil assembly for imaging a subject volume using a very high field Magnetic Resonance Imaging (MRI) system operable at substantially high frequencies includes a plurality of conductors arranged cylindrically and disposed about a patient bore of the MRI system, a plurality of capacitive elements disposed between and connecting respective ends of the conductors, the plurality of conductors and plurality of capacitive elements forming a high band pass birdcage configuration, and a plurality of dynamic disabling switches, each dynamic disabling switch electrically coupled in parallel with a respective capacitive element to form a parallel resonant circuit.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a Magnetic Resonance Imaging (MRI) system. More particularly, this invention relates to radio frequency (RF) coils used in MRI systems for transmitting and/or receiving RF signals. 
     MRI scanners, which are used in various fields such as medical diagnostics, typically use a computer to create images based on the operation of a magnet, a gradient coil assembly, and at least one radiofrequency coil. The magnet creates a uniform main magnetic field that makes nuclei, such as hydrogen atomic nuclei, responsive to radiofrequency excitation. The gradient coil assembly imposes a series of pulsed, spatial magnetic fields upon the main magnetic field to give each point in the imaging volume a spatial identity corresponding to its unique set of magnetic fields during the imaging pulse sequence. The radiofrequency coil(s) creates an excitation frequency pulse that temporarily creates an oscillating transverse magnetization that is detected by the radiofrequency coil and used by the computer to create the image. 
     Generally, very high field strength is characterized as greater than 2 Tesla (2T). Higher magnetic field strength imposes challenges on the RF coil, such as balancing inductance and capacitance at relatively higher frequencies, i.e. greater than 64 MegaHertz (MHz). At very high magnetic fields, and therefore very high Larmor frequencies, standard birdcage coils with moderately narrow rung copper strips have relatively high inductance requiring very low capacitor values in order to resonate the coil. This is problematic for a number of reasons. First, high currents through small value capacitors will have very high voltage potential across them which can result in a local stray electric field that dissipates RF power in the form of heat in an imaging subject. 
     There are two types of electric fields associated with MRI. The first is due to time-varying B1 magnetic field present within the imaging subject and the second type is due to electric charges on the capacitors in the RF coil structure. When a NMR system is operating at a relatively high frequency range, for example above 100 MHz, significant radiation loss may occur. The increased radiation loss in high frequency ranges results in an increase in RF power used to generate the excitation and a resultant decrease in the signal-to-noise (SNR) of the signals received. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one aspect, a radio frequency (RF) coil assembly for imaging a subject volume using a very high field Magnetic Resonance Imaging (MRI) system operable at substantially high frequencies is provided. The MRI system includes a plurality of conductors arranged cylindrically and disposed about a patient bore of the MRI system, a plurality of capacitive elements disposed between and connecting respective ends of the conductors, the plurality of conductors and plurality of capacitive elements forming a high band pass birdcage configuration, and a plurality of dynamic disabling switches, each dynamic disabling switch electrically coupled in parallel with a respective capacitive element to form a parallel resonant circuit. 
     In another aspect, a magnetic resonance imaging (MRI) system is provided. The MRI system includes a radio frequency (RF) coil assembly for imaging a subject volume using substantially high frequencies. The RF coil includes a plurality of conductors arranged cylindrically and disposed about a patient bore of the MRI system, a plurality of capacitive elements disposed between and connecting respective ends of the conductors, the plurality of conductors and plurality of capacitive elements forming a high band pass birdcage configuration, and a plurality of dynamic disabling switches, each dynamic disabling switch electrically coupled in parallel with a respective capacitive element to form a parallel resonant circuit. 
     In a further aspect, a TEM resonator is provided. The TEM resonator includes a plurality of conductors arranged cylindrically and disposed about a patient bore, a plurality of capacitive elements disposed between and connecting respective ends of the conductors, the plurality of conductors and plurality of capacitive elements forming TEM resonator configuration, and a plurality of dynamic disabling switches, each dynamic disabling switch electrically coupled in parallel with a respective capacitive element to form a parallel resonant circuit. 
     In still a further aspect, a method for operating a RF coil in a very high field Magnetic Resonance Imaging (MRI) system operable at substantially high frequencies is provided. The method includes arranging a plurality of conductors cylindrically around a patient bore of the MRI system, connecting a plurality of capacitive elements between respective ends of the conductors, the plurality of conductors and the plurality of capacitive elements forming a high band pass birdcage configuration, connecting a plurality of dynamic disabling switches in parallel with a respective capacitive element to form a parallel resonant circuit, each dynamic disabling switch including a diode, and connecting a switching bias to a second end of said dynamic disabling switch, the switching bias configured to forward bias and reverse bias said diode. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block schematic diagram of a Magnetic Resonance Imaging (MRI) system. 
     FIG. 2 is an exemplary radio frequency (RF) coil that can be used with the MRI system shown in FIG.  1 . 
     FIG. 3 is a first exemplary dynamic disabling switch that can be used with the RF coil shown in FIG.  2 . 
     FIG. 4 is portion of the dynamic disabling switch shown in FIG.  3 . 
     FIG. 5 a  is an exemplary simplified electrical schematic of the switch shown in FIG.  3 . 
     FIG. 5 b  is an exemplary simplified electrical schematic of the switch shown in FIG.  3 . 
     FIG. 6 is an operationally equivalent schematic of the switch shown in FIG.  3 . 
     FIG. 7 is an operationally equivalent schematic of the switch shown in FIG.  3 . 
     FIG. 8 is the dynamic disabling switch shown in FIG. 3 with an open end. 
     FIG. 9 is a graphical representation of the electrical characteristics of the switch shown in FIG.  8 . 
     FIG. 10 is the dynamic disabling switch shown in FIG. 3 with a shorted end. 
     FIG. 11 is a graphical representation of the electrical characteristics of the switch shown in FIG.  10 . 
     FIG. 12 is a second exemplary dynamic disabling switch that can be used with the RF coil shown in FIG.  2 . 
     FIG. 13 is portion of the dynamic disabling switch shown in FIG.  12 . 
     FIG. 14 is portion of the dynamic disabling switch shown in FIG.  12 . 
     FIG. 15 a  is an exemplary simplified electrical schematic of the switch shown in FIG.  12 . 
     FIG. 15 b  is an exemplary simplified electrical schematic of the switch shown in FIG.  12 . 
     FIG. 16 is an operationally equivalent schematic of the switch shown in FIG.  12 . 
     FIG. 17 is an operationally equivalent schematic of the switch shown in FIG.  12 . 
     FIG. 18 is an exemplary birdcage coil including at least one dynamic disabling switch as illustrated in FIG.  3  and FIG.  12 . 
     FIG. 19 is cross-sectional view of the birdcage coil including at least one dynamic disabling switch in FIG.  18 . 
     FIG. 20 is an exemplary TEM resonator including at least one dynamic disabling switch as illustrated in FIG.  3  and FIG.  12 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural said elements or steps, unless such exclusion is explicitly recited. Furthermore, references to “one embodiment” of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. 
     FIG. 1 is a block diagram of an embodiment of a magnetic resonance imaging (MRI) system  10  in which the herein described systems and methods are implemented. MRI  10  includes an operator console  12  which includes a keyboard and control panel  14  and a display  16 . Operator console  12  communicates through a link  18  with a separate computer system  20  thereby enabling an operator to control the production and display of images on screen  16 . Computer system  20  includes a plurality of modules  22  which communicate with each other through a backplane. In the exemplary embodiment, modules  22  include an image processor module  24 , a CPU module  26  and a memory module  28 , also referred to herein as a frame buffer for storing image data arrays. Computer system  20  is linked to a disk storage  30  and a tape drive  32  to facilitate storing image data and programs. Computer system  20  is communicates with a separate system control  34  through a high speed serial link  36 . 
     System control  34  includes a plurality of modules  38  electrically coupled using a backplane (not shown). In the exemplary embodiment, modules  38  include a CPU module  40  and a pulse generator module  42  that is electrically coupled to operator console  12  using a serial link  44 . Link  44  facilitates transmitting and receiving commands between operator console  12  and system command  34  thereby allowing the operator to input a scan sequence that MRI system  10  is to perform. Pulse generator module  42  operates the system components to carry out the desired scan sequence, and generates data which indicative of the timing, strength and shape of the RF pulses which are to be produced, and the timing of and length of a data acquisition window. Pulse generator module  42  is electrically coupled to a gradient amplifier system  46  and provides gradient amplifier system  46  with a signal indicative of the timing and shape of the gradient pulses to be produced during the scan. Pulse generator module  42  is also configured to receive patient data from a physiological acquisition controller  48 . In the exemplary embodiment, physiological acquisition controller  48  is configured to receive inputs from a plurality of sensors indicative of a patients physiological condition such as, but not limited to, ECG signals from electrodes attached to the patient. Pulse generator module  42  is electrically coupled to a scan room interface circuit  50  which is configured to receive signals from various sensors indicative of the patient condition and the magnet system. Scan room interface circuit  50  is also configured to transmit command signals such as, but not limited to, a command signal to move the patient to a desired position, to a patient positioning system  52 . 
     The gradient waveforms produced by pulse generator module  42  are input to gradient amplifier system  46  that includes a G x  amplifier  54 , a G y  amplifier  56 , and a G z  amplifier  58 . Amplifiers  54 ,  56 , and  58  each excite a corresponding gradient coil in gradient coil assembly  60  to generate a plurality of magnetic field gradients used for position encoding acquired signals. In the exemplary embodiment, gradient coil assembly  60  includes a magnet assembly  62  that includes a polarizing magnet  64  and a whole-body RF coil  66 . 
     In use, a transceiver module  70  positioned in system control  34  generates a plurality of electrical pulses which are amplified by an RF amplifier  72  that is electrically coupled to RF coil  66  using a transmit/receive switch  74 . The resulting signals radiated by the excited nuclei in the patient are sensed by RF coil  66  and transmitted to a preamplifier  76  through transmit/receive switch  74 . The amplified NMR (nuclear magnetic resonance) signals are then demodulated, filtered, and digitized in a receiver section of transceiver  70 . Transmit/receive switch  74  is controlled by a signal from pulse generator module  42  to electrically connect RF amplifier  72  to coil  66  during the transmit mode and to connect preamplifier  76  during the receive mode. Transmit/receive switch  74  also enables a separate RF coil (for example, a surface coil) to be used in either the transmit or receive mode. 
     The NMR signals received by RF coil  66  are digitized by transceiver module  70  and transferred to a memory module  78  in system control  34 . When the scan is completed and an array of raw k-space data has been acquired in the memory module  78 . The raw k-space data is rearranged into separate k-space data arrays for each cardiac phase image to be reconstructed, and each of these is input to an array processor  80  configured to Fourier transform the data into an array of image data. This image data is transmitted through serial link  36  to computer system  20  where it is stored in disk memory  30 . In response to commands received from operator console  12 , this image data may be archived on tape drive  32 , or it may be further processed by image processor  24  and transmitted to operator console  12  and presented on display  16 . 
     FIG. 2 is schematic illustration of an exemplary RF coil  100  that can be used with MRI system  10  shown in FIG.  1 . RF coil  100  includes two conductive end loops  102  and a plurality of conductors  104  electrically coupled to end loops  102  and arranged substantially cylindrically around a central axis or patient bore  106  to form a coil structure commonly referred to as a “birdcage”. End loops  102  and conductors  104  define a substantially cylindrical imaging volume into which a subject to be examined is subjected to a RF field generated by RF coil  100 . RF coil  100  also includes a plurality of capacitors  108  such as, but not limited to, low inductance end ring capacitors serially coupled to conductors  104  and configured to electrically interconnect connect connectors  104  at each respective end of conductor  104 . MRI system  10  also includes at least one dynamic disabling switch  110  electrically to RF coil  100 . 
     FIG. 3 is an exemplary dynamic disabling switch  120  that can be used with RF coil  100  (shown in FIG. 2) or RF coil  66  (shown in FIG.  10 ). FIG. 4 is a portion of switch  120  shown in FIG.  3 . In one embodiment, switch  120  is fabricated using semi-rigid coaxial cable  122 . In another embodiment, switch  120  is fabricated using lumped elements as shown in FIG. 5 a . Coaxial cable  122  includes a center conductor  124  fabricated from a metallic material, a middle portion  126  surrounding center conductor  124 , and an outer conductor  128 , fabricated from a metallic material, surrounding middle portion  126 . In another embodiment, switch  120  can be fabricated using lumped elements. In another embodiment, switch  120  is fabricated using lumped elements as shown in FIG. 5 a . In the exemplary embodiment, middle portion  126  is an insulator fabricated from a material such as, bot not limited to, Polytetrafluoroethylene (PTFE), i.e. Teflon. In another exemplary embodiment, middle portion  126  is fabricated from any suitable material that includes low loss characteristics such that a signal loss is reduced in RF coil  100 . 
     Switch  120  includes an opening  130 , that includes a width  132 , and extends from a outer conductor outer surface  134  to an inner conductor outer surface  136 . More specifically, a portion of middle portion  126  is removed thus exposing inner conductor  124 . A diode  138  is positioned in opening  130  and electrically coupled to inner conductor  124  and outer conductor  128 . Switch  120  includes a first length  140  that extends from a first end  142  of switch  120  to a second end  144  of switch  120  and a second length  146  that extends from first end  142  to first opening end  148 . Diode  138  is electrically coupled such that when diode  138  is forward biased, current flows from outer conductor  128  to inner conductor  124 . Alternatively, when diode  138  is reverse biased, current does not flow between inner conductor  124  and outer conductor  128 . 
     FIG. 5 a  is simplified electrical schematic of dynamic disabling switch  120  shown in FIG.  3 . FIG. 5 b  is another simplified electrical schematic of dynamic disabling switch  120  shown in FIG. 3 using lumped circuit elements instead of the coaxial cable. FIG. 6 is schematic illustration of an operationally equivalent circuit depicting diode  138  in a forward biased state. FIG. 7 is schematic illustration of an operationally equivalent circuit depicting diode  138  in a reverse biased state. 
     In use, switch  120  is electrically coupled in parallel with at least one end ring capacitor  108  by connecting inner conductor  124  to a first side  150  of capacitor  108  and connecting outer conductor  128  to a second side  152  of capacitor  108 . A portion of outer conductor is removed to form opening  130 . Diode  138  is then electrically coupled between inner conductor  124  and outer conductor  128 . As shown in FIG. 6, when diode  138  is forward biased, switch  120  and capacitor  108  are equivalent to capacitor  108 . As shown in FIG. 7, when diode  138  is reverse biased, switch  120  can be modeled as an inductor  154  in parallel with capacitor  108 . Accordingly, length  146  is selected such that length  146  is approximately equivalent to one-quarter wavelength (λ/4) of the working frequency of RF coil  100 . Length  140  is selected by modeling switch  120  as inductor  154 . The size of inductor  154  is selected such that when inductor  154 , i.e. switch  120 , is electrically coupled in parallel with capacitor  108 , the combination of capacitor  108  and inductor  154  form a resonant circuit. More specifically, the circuit&#39;s natural frequency ω 0  makes the imaginary part of the complex impedance equal to zero. 
     When ω 0  is defined as an operational angular frequency of RF coil  100 , then inductor  154  can be defined in accordance with:          ω   0     =       2      π                 f     =     1     CL                                
     where, 
     C is cpacitor  108 , and 
     L is inductor  154 . 
     For example, if f is set equal to 298 MHz, then length  140  can be selected between approximately one-quarter wavelength and approximately one-half wavelength of the operational angular frequency of RF coil  100 . 
     FIG. 8 illustrates dynamic disabling switch  120  with an open end. FIG. 9 illustrates the electrical characteristics of switch  120  shown in FIG.  8 . FIG. 10 illustrates a dynamic disabling switch  120  with a shorted end. FIG. 11 illustrates the electrical characteristics of switch  120  shown in FIG.  10 . During operation, when diode  138  is forward biased, dynamic disabling switch  120  is ideally equivalent to an open circuit at a predetermined frequency of operation, i.e. infinite impedance, as shown in FIG. 11, such that current flows through capacitor  108  thus enabling operation of RF coil  100 . To disable RF coil  100 , diode  138  is reverse-biased. When diode  138  is reverse biased, capacitor  108  and dynamic disabling switch  120  function as a parallel resonant circuit about the operational frequency of RF coil  100  as shown in FIGS. 9 and 10. The impedance of the parallel resonant circuit is high enough to stop the current flow through capacitor  108 , thus disabling RF coil  100 . In the exemplary embodiment, a plurality of dynamic disabling switches  120  are electrically coupled to RF coil  108  to disable RF coil  100  when dynamic disabling switches  120  are reverse biased. A switching bias  156  is applied to dynamic disabling circuit  120  through an inductor  158 , inductor  160  functions as a lowpass filter. In use, inductors  158  and  160  include an inductance capable of disabling the RF signal path in the operational frequency of RF coil  100  such as, but not limited to, greater than 500 nH at 298 Mhz. 
     FIG. 12 is another exemplary dynamic disabling switch  220  that can be used with RF coil  66  (shown in FIG. 1) or RF coil  100  (shown in FIG.  2 ). FIG. 13 is a portion of switch  220  shown in FIG.  12 . FIG. 14 is another portion of switch  220  shown in FIG.  12 . Switch  220  is fabricated using semi-rigid coaxial cable  222  that includes a center conductor  224  fabricated from a metallic material, a middle portion  226  surrounding center conductor  224 , and an outer conductor  228 , fabricated from a metallic material, surrounding middle portion  226 . In the exemplary embodiment, middle portion  226  is an insulator fabricated from a material such as, bot not limited to, Polytetrafluoroethylene (PTFE), i.e. Teflon. In another exemplary embodiment, middle portion  226  is fabricated from any suitable material that includes low loss characteristics such that a signal loss is reduced in RF coil  100 . 
     Switch  220  includes an opening  230 , that includes a width  232 , and extends from a outer conductor outer surface  234  to an inner conductor outer surface  236 . More specifically, a portion of middle portion  226  is removed thus exposing inner conductor  224 . A diode  238  is positioned in opening  230  and then electrically coupled to inner conductor  224  and outer conductor  228 . Switch  220  includes a first length  240  that extends from a first end  242  of switch  220  to a second end  244  of switch  220  and a second length  246  that extends from first end  242  to first opening end. Diode  238  is electrically coupled such that when diode  238  is forward biased, current flows from outer conductor  228  to inner conductor  224 . Alternatively, when diode  238  is reverse biased, current does not flow between inner conductor  224  and outer conductor  228 . 
     FIG. 15 a  is simplified electrically schematic of dynamic disabling switch  220  shown in FIG.  12 . FIG. 15 b  is another simplified electrical schematic of dynamic disabling switch  220  shown in FIG. 12 using lumped circuit elements instead of the coaxial cable. FIG. 16 is schematic illustration of an operationally equivalent circuit depicting diode  238  in a forward biased state. FIG. 17 is schematic illustration of an operationally equivalent circuit depicting diode  238  in a reverse biased state. 
     In use, at least one end of switch  220  is electrically coupled in parallel with at least one end ring capacitor  208  by connecting inner conductor  224  to a first side  250  of capacitor  208  and connecting outer conductor  228  to a second side  252  of capacitor  208 . A portion of outer conductor is removed to form opening  230 . Diode  238  is then electrically coupled between inner conductor  224  and outer conductor  228 . As shown in FIG. 16, when diode  238  is reverse biased, switch  220  and capacitor  208  are equivalent to capacitor  208 . As shown in FIG. 17, when diode  238  is forward biased, switch  220  can be modeled as an inductor  254  in parallel with capacitor  208 . Accordingly, the total length  240  from first end  242  to second end  244  is approximately equivalent to a quarter wavelength about the working frequency of RF coil  100 . A capacitor  209  is electrically coupled to second end  244  between inner conductor  224  and outer conductor  228  such that an impedance between inner conductor  224  and outer conductor  228  is relatively low, i.e. less than approximately 0.5 ohms for the working frequency of RF coil  100  such that switch  220  approximates a short-ended coaxial cable. In the exemplary embodiment, switch  220  has a capacitance greater than approximately 1000 picoFarad (pF) when RF coil  100  is operating at approximately 100 Mz. Capacitor  209  facilitates cutting a DC path between inner conductor  224  and outer conductor  228  such that a DC switching bias through  260  and inductor  261  works on diode  238 . A length  240  and such that inductor  254  forms a parallel resonant circuit with capacitor forward-biased, i.e. an ideal short circuit). 
     When ω 0  is defined as an operational angular frequency of RF coil  100 , then inductor  254  can be defined in accordance with:          ω   0     =       2      π                 f     =     1     CL                                
     where, 
     C is cpacitor  208 , and 
     L is inductor  254 . 
     For example, if f is set equal to 298 MHz, then length  240  can be selected between approximately onequarter wavelength and approximately one-quarter wavelength of the operational angular frequency of RF coil  100 . 
     During operation, when diode  238  is reverse biased, dynamic disabling switch  220  is ideally equivalent to an open circuit, i.e. infinite impedance, as shown in FIG. 16, such that current flows through capacitor  208  thus enabling operation of RF coil  100 . To disable RF coil  100 , diode  238  is forward biased. When diode  238  is forward biased, capacitor  208  and dynamic disabling switch  220  function as a parallel resonant circuit about the operational frequency of RF coil  100 . The impedance of the parallel resonant circuit is high enough to stop the current flow through capacitor  208 , thus disabling RF coil  100 . In the exemplary embodiment, a plurality of dynamic disabling switches  220  are electrically coupled to RF coil  100  to disable RF coil  100  when dynamic disabling switches  220  are forward biased. A switching bias  256  is applied to dynamic disabling circuit  220  through an inductor  260  and inductor  261 . In use, inductors  260  and  261  include an inductance capable of disabling the RF signal path in the operational frequency of RF coil  100  such as, but not limited to, greater than 500 nH at 298 Mhz. 
     FIG. 18 illustrates at least one of dynamic disabling switch  120  or dynamic disabling switch  220  implemented into birdcage coil  300  that includes approximately sixteen elements  302 . FIG. 19 shows the cross section view of planes A, B, C, and D shown in FIG.  18 . In the exemplary embodiment, the dynamic disabling switch is electrically coupled in parallel to at least one end ring capacitor of birdcage coil  100  as described previously herein. An outer shield (not shown) of the coaxial cable of dynamic disabling switch  120  or  220  is electrically coupled to element  302  between end ring  304  and end ring  306  at a midpoint  308  located approximately halfway between end ring  304  and end ring  306  where an electric potential is ideally ground point, i.e. a virtual ground. The coaxial cable of dynamic disabling switch  120  or  220  is bent approximately 90 degrees toward an RF shield  310  and is taken out of RF shield  310  through RF shield  310 . The coaxial cable of dynamic disabling switch  120  or  220  is bent again by 90 degree toward RF shield  310  and electrically attached to RF shield  310 . In the exemplary embodiment, dynamic disabling switch  120  or  220  can be electrically coupled to birdcage  310  without any effect on electric characteristic of birdcage coil  300 . The cabling of the switching bias for dynamic disabling switch  120  or  220  can be accomplished without any special care about degradation of the electric characteristic of birdcage coil  300  because the opposite end of the coaxial cable is positioned outside RF shield  310 . 
     In the exemplary embodiment, the dynamic disabling circuit is fabricated using a coaxial cable such that it is difficult for large valued capacitor to be put in dynamic disabling switch  120  or  220  circuit for DC cut high-pass filtering. A quantity of dynamic disabling switches are installed on birdcage  300  based on previous electrical analysis of birdcage coil  300 . For example, in the exemplary embodiment, sixteen dynamic disabling switches are used in birdcage coil having thirty-two elements operating at approximately 7T. 
     FIG. 20 illustrates a TEM resonator  400  including a plurality of dynamic disabling switches  402 , such as switch  120  or switch  220 . Each dynamic disabling switch  402  includes a first end  404  electrically coupled around an end ring capacitor  406 . Since dynamic disabling switch  402  is already outside of an RF shield  408  after first end  404  is connected, an outer shield of the coaxial cable of dynamic disabling switch  402  can be electrically connected to any position on RF shield  408 . 
     In one embodiment, switch  402  includes a length of approximately 250 mm for switch  120  and approximately 160 mm for switch  220  when used for 7T proton imaging. For 3T proton imaging, a length of switches  120  and  220  are approximately 2.3 times longer than used for 7T proton imaging, i.e. approximately 575 mm for switch  120  and approximately 370 mm for switch  220 . 
     The dynamic disabling switches described herein can be used for high frequency applications and facilitate using a receive only coil including a phased array coil. Therefore, enabling a higher SNR imaging in high frequency system. Dynamic disabling switch circuits and those implementation methods into volume RF coils like birdcage coil or TEM resonator for high field MRI system to switch the volume coil between in enable mode and in disable mode. The invention makes receive only coil available in high field MRI system. The radiation loss by attaching this kind of switch circuit to the RF coil, which is typically more severe in the higher frequency, is reduced by this invention owing to employing semi-rigid coaxial cable (distribution circuit) as the basis of the design. The invention is formed typically by around quarter wavelength of semi-rigid coaxial cable about the desired frequency. The length of this switching circuit becomes practical in high field RF coil like 3T proton body coil or 7T proton head coil etc. The invention is applicable in the higher field by adding additional one or more half wavelength. The invention is also available in lower field by implementation into the RF coil in zigzag way. 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.