Abstract:
With the objective of providing a switching apparatus capable of reducing a geometrical size while a radiation loss is being suppressed, the switching apparatus that switches a state of an RF coil of an MRI between an effective state and an ineffective state, comprises a coaxial cable which has a length equal to ¼ of a wavelength corresponding to an operating frequency and in which an inner conductor and an outer conductor are connected to a coil body of the RF coil so as to interpose a capacitor therebetween, a diode connected to the inner conductor and outer conductor of the coaxial cable, and a capacitor corresponding to a lumped constant element connected to the inner conductor and outer conductor of the coaxial cable.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of Japanese Application No. 2005-124733 filed Apr. 22, 2005. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to a switching apparatus, an RF coil, and a magnetic resonance imaging apparatus. 
   Since a plurality of RF coils (such as a body coil and a head coil) might be used in a magnetic resonance imaging apparatus, there is a need to avoid coupling among these RF coils. Therefore, there might be provided a switching apparatus for eliminating the characteristic as a resonant circuit by shifting a resonant frequency of a resonant loop or cutting the loop. Incidentally, the switching apparatus is generally identified by names such as “Dynamic disabling switch”, a detuner, etc. 
   Since the frequency of a signal of the magnetic resonance imaging apparatus is relatively high, a radiation loss of energy occurs even in the switching apparatus. Therefore, a patent document 1 discloses a technique wherein a switching apparatus (dynamic disabling switch) is configured by a distributed constant circuit and a coaxial cable low in radiation loss of energy is used as a distributed constant element to thereby suppress the radiation loss. 
   [Patent Document 1] Japanese Unexamined Patent Publication No. 2004-358259 
   If, however, the switching apparatus is configured by the distributed constant circuit, then the geometrical size of the switching apparatus is affected by required circuit characteristics. On the other hand, the size of the RF coil may preferably be set in accordance with the size of a subject or a region intended for imaging of the subject. The size of the switching apparatus based on the circuit characteristics, and the size of the RF coil based on the imaging object do not necessarily coincide with each other. Since the switching apparatus is disposed in the RF coil, there is a need to allow one size or the like to coincide with the other size or the like. If the size of the RF coil is increased in accordance with the size of the switching apparatus where the switching apparatus is larger than the RF coil, then it exerts an influence on the mountability of the coil onto the subject. If the coaxial cable of the distributed constant circuit is waved in accordance with the size of the RF coil, for example, then a manufacturing process becomes complex. 
   SUMMARY OF THE INVENTION 
   Therefore, an object of the present invention is to provide a switching apparatus capable of reducing a geometrical size while a radiation loss is being suppressed, an RF coil and a magnetic resonance imaging apparatus. 
   There is provided a switching apparatus according to a first aspect of the present invention, which switches a state of an RF coil of a magnetic resonance imaging apparatus between a state in which a resonant circuit with a predetermined operating frequency as a resonant frequency is configured, and a state in which a resonant circuit or a non-resonant circuit having another resonant frequency is configured, comprising a coaxial cable having a length equal to an odd multiple of ¼ of a wavelength corresponding to the operating frequency, said coaxial cable including one end side at which an inner conductor is connected to one side of a coil electrostatic capacitive element contained in the RF coil and an outer conductor is connected to the other side of the coil electrostatic capacitive element, and the other end side from which a bias is applied; a diode connected to the inner conductor and the outer conductor at the other end of the coaxial cable; and a lumped constant element connected to the inner conductor and the outer conductor at the other end of the coaxial cable, and having a predetermined electrostatic capacitance. 
   Preferably, the electrostatic capacitance of the lumped constant element is set in such a manner that when a bias is applied to the other end of the coaxial cable in a direction in which no current flows through the diode, the resonant circuit with the operating frequency as the resonant frequency is constituted of the coil electrostatic capacitive element, the coaxial cable and the lumped constant element. 
   Preferably, the lumped constant element is a ceramic capacitor. 
   There is provided a switching apparatus according to a second aspect of the present invention, which switches a state of an RF coil of a magnetic resonance imaging apparatus between a state in which a resonant circuit with a predetermined operating frequency as a resonant frequency is configured, and a state in which a resonant circuit or a non-resonant circuit having another resonant frequency is configured, the switching apparatus comprising a coaxial cable including one end side at which an inner conductor is connected to one side of a coil electrostatic capacitive element contained in the RF coil and an outer conductor is connected to the other side of the coil electrostatic capacitive element, and the other end side from which a bias is applied; a diode connected to the inner conductor and the outer conductor at the other end of the coaxial cable; and a lumped constant element connected to the inner conductor and the outer conductor at the other end of the coaxial cable, the lumped constant element having such an inductance that when a bias is applied to the other end of the coaxial cable in a direction in which no current flows through the diode, the combined impedance thereof with the coaxial cable becomes equivalent to the impedance of the coaxial cable whose other end is short-circuited with a length equal to an odd multiple of ¼ of a wavelength corresponding to the operating frequency. 
   Preferably, the length of the coaxial cable is set in such a manner that when a bias is applied to the other end of the coaxial cable in a direction in which a current flows through the diode, the resonant circuit with the operating frequency as the resonant frequency is constituted of the coaxial cable and the coil electrostatic capacitive element. 
   Preferably, the switching apparatus includes a first inductance element connected in series with the inner conductor at the other end side of the coaxial cable than the lumped constant element, and a second inductance element connected in series with the outer conductor at the other end side of the coaxial cable than the lumped constant element. A bias is applied to the coaxial cable via the first and second inductance elements. 
   Preferably, the switching apparatus further includes a shield body for the switching apparatus, which is comprised of a non-magnetic material having conductivity and covers the lumped constant element. 
   Preferably, the switching apparatus further includes a first electrostatic capacitive element connected to the inner conductor and connected to a reference potential at the other end side of the coaxial cable than the lumped constant element, and a second electrostatic capacitive element connected to the outer conductor and connected to the reference potential at the other end side of the coaxial cable than the lumped constant element. 
   Preferably, the operating frequency is 64 MHz or more. 
   There is provided an RF coil suitable for use in a magnetic resonance imaging apparatus, according to a third aspect of the present invention, comprising a coil conductor; a coil electrostatic capacitive element connected to the coil conductor; and a switching apparatus which switches a state of a circuit including the coil conductor and the coil electrostatic capacitive element between a state in which a resonant circuit with a predetermined operating frequency as a resonant frequency is configured, and a state in which a resonant circuit or a non-resonant circuit having another resonant frequency is configured, wherein the switching apparatus includes a coaxial cable having a length equal to an odd multiple of ¼ of a wavelength corresponding to the operating frequency, the coaxial cable including one end side at which an inner conductor is connected to one side of the coil electrostatic capacitive element and an outer conductor is connected to the other side of the coil electrostatic capacitive element, and the other end side from which a bias is applied, a diode connected to the inner conductor and the outer conductor at the other end of the coaxial cable, and a lumped constant element connected to the inner conductor and the outer conductor at the other end of the coaxial cable, and having a predetermined electrostatic capacitance. 
   There is provided an RF coil suitable for use in a magnetic resonance imaging apparatus, according to a fourth aspect of the present invention, comprising a coil conductor; a coil electrostatic capacitive element connected to the coil conductor; and a switching apparatus which switches a state of a circuit including the coil conductor and the coil electrostatic capacitive element between a state in which a resonant circuit with a predetermined operating frequency as a resonant frequency is configured, and a state in which a resonant circuit or a non-resonant circuit having another resonant frequency is configured, wherein the switching apparatus includes a coaxial cable including one end side at which an inner conductor is connected to one side of a coil electrostatic capacitive element contained in the RF coil and an outer conductor is connected to the other side of the coil electrostatic capacitive element, and the other end side from which a bias is applied, a diode connected to the inner conductor and the outer conductor at the other end of the coaxial cable, and a lumped constant element connected to the inner conductor and the outer conductor at the other end of the coaxial cable, the lumped constant element having such an inductance that when a bias is applied to the other end of the coaxial cable in a direction in which no current flows through the diode, the combined impedance thereof with the coaxial cable becomes equivalent to the impedance of the coaxial cable whose other end is short-circuited with a length equal to an odd multiple of ¼ of a wavelength corresponding to the operating frequency. 
   Preferably, the RF coil further includes a coil shield body which covers the coil conductor, and the coaxial cable has the one end disposed inside the coil shield body and the other end disposed outside the coil shield body, and the lumped constant element is disposed outside the coil shield body. 
   Preferably, the switching apparatus further includes a switching apparatus shield body which covers the lumped constant element outside the coil shield body, and the switching apparatus shield body is fixed to the coil shield body by fixing means having conductivity. 
   There is provided a magnetic resonance imaging apparatus according to a fifth aspect of the present invention, comprising static magnetic field forming means which forms a static magnetic field; tilt magnetic field forming means which forms a tilt magnetic field; high-frequency magnetic field forming means which forms a high-frequency magnetic field at a subject lying within the static magnetic field; magnetic resonance signal receiving means which receives a magnetic resonance signal from the subject; image forming means which forms an image of the subject on the basis of the magnetic resonance signal received by the magnetic resonance signal receiving means; bias input means which applies a bias to an RF coil included in at least any one of the high-frequency magnetic filed forming means and the magnetic resonance signal receiving means; and control means which controls the operation of the bias input means, wherein the RF coil includes a coil conductor, a coil electrostatic capacitive element connected to the coil conductor, and a switching apparatus which switches a state of a circuit including the coil conductor and the coil electrostatic capacitive element between a state in which a resonant circuit with a predetermined operating frequency as a resonant frequency is configured, and a state in which a resonant circuit or a non-resonant circuit having another resonant frequency is configured, and wherein the switching apparatus includes a coaxial cable having a length equal to an odd multiple of ¼ of a wavelength corresponding to the operating frequency, the coaxial cable including one end side at which an inner conductor is connected to one side of the coil electrostatic capacitive element and an outer conductor is connected to the other side of the coil electrostatic capacitive element, and the other end side to which the bias input means is connected, a diode connected to the inner conductor and the outer conductor at the other end of the coaxial cable, and a lumped constant element connected to the inner conductor and the outer conductor at the other end of the coaxial cable, and having a predetermined electrostatic capacitance. 
   There is provided a magnetic resonance imaging apparatus according to a sixth aspect of the present invention, comprising static magnetic field forming means which forms a static magnetic field; tilt magnetic field forming means which forms a tilt magnetic field; high-frequency magnetic field forming means which forms a high-frequency magnetic field at a subject lying within the static magnetic field; magnetic resonance signal receiving means which receives a magnetic resonance signal from the subject; image forming means which forms an image of the subject on the basis of the magnetic resonance signal received by the magnetic resonance signal receiving means; bias input means which applies a bias to an RF coil contained in at least any one of the high-frequency magnetic filed forming means and the magnetic resonance signal receiving means; and control means which controls the operation of the bias input means, wherein the RF coil includes a coil conductor, a coil electrostatic capacitive element connected to the coil conductor, and a switching apparatus which switches a state of a circuit including the coil conductor and the coil electrostatic capacitive element between a state in which a resonant circuit with a predetermined operating frequency as a resonant frequency is configured, and a state in which a resonant circuit or a non-resonant circuit having another resonant frequency is configured, and wherein the switching apparatus includes a coaxial cable including one end side at which an inner conductor is connected to one side of a coil electrostatic capacitive element contained in the RF coil and an outer conductor is connected to the other side of the coil electrostatic capacitive element, and the other end side to which the bias input means is connected, a diode connected to the inner conductor and the outer conductor at the other end of the coaxial cable, and a lumped constant element connected to the inner conductor and the outer conductor at the other end of the coaxial cable, the lumped constant element having such an inductance that when a bias is applied to the other end of the coaxial cable in a direction in which no current flows through the diode, the combined impedance thereof with the coaxial cable becomes equivalent to the impedance of the coaxial cable whose other end is short-circuited with a length equal to an odd multiple of ¼ of a wavelength corresponding to the operating frequency. 
   Preferably, the control means controls the bias input means in such a manner that the direction of the bias applied to the coaxial cable is a reverse direction upon formation of the high-frequency magnetic field by the high-frequency magnetic field forming means and reception of the magnetic resonance signal by the magnetic resonance signal receiving means. 
   According to the switching apparatus of the present invention, a geometrical size can be reduced while a radiation loss is being suppressed. 
   Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram showing a configuration of a magnetic resonance imaging apparatus according to a first embodiment of the present invention. 
       FIG. 2  is a schematic perspective view illustrating a configuration of an RF coil of the magnetic resonance imaging apparatus shown in  FIG. 1 . 
       FIGS. 3   a  and  3   b  are circuit diagrams depicting a configuration of a conventional switching apparatus. 
       FIGS. 4   a  and  4   b  are diagrams showing electric characteristics of a coaxial cable. 
       FIGS. 5   a  and  5   b  are diagrams showing an equivalent circuit of the switching apparatus of  FIG. 3  while being in operation. 
       FIG. 6  is a circuit diagram illustrating a configuration of a switching apparatus provided in the RF coil shown in  FIG. 2 . 
       FIG. 7  is a diagram showing a method for packaging the RF coil shown in  FIG. 2 . 
       FIG. 8  is a sectional view taken along an ABCD plane of  FIG. 7 . 
       FIGS. 9   a  and  9   b  are circuit diagrams illustrating a configuration of a conventional switching apparatus. 
       FIGS. 10   a  and  10   b  are diagrams showing an equivalent circuit of the switching apparatus of  FIG. 9  while being in operation. 
       FIG. 11  is a circuit diagram illustrating a configuration of a switching apparatus employed in a magnetic resonance imaging apparatus according to a second embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   First Embodiment 
     FIG. 1  is a block diagram of a magnetic resonance imaging apparatus (MRI)  10  according to a first embodiment of the present invention. The MRI  10  includes an operator console  12  including a keyboard and control panel  14 , and a display  16 . The operator console  12  is in communication with an independent computer system  20  via a link  18 . Thus, an operator is able to control the creation and display of an image on the screen  16 . 
   The computer system  20  includes a plurality of modules caused to communicate with one another through a backplane. For instance, the computer system  20  includes an image processor module  24 , a CPU module  26 , and a memory module  28  for storing an image data array therein, which might be called “frame buffer” in the present specification. The computer system  20  links a disk storage unit  30  and a tape driving device  32  to each other to facilitate storage of image data and programs. The computer system  20  communicates with an independent system controller  34  via a high-speed serial link  36 . 
   The system controller  34  includes a plurality of modules electrically connected using a back plane (not shown). For instance, the system controller  34  includes a CPU module  40  and a pulse generator module  42  electrically connected to the operator console  12  using a serial link  44 . The link  44  makes it easy to transmit/receive commands between the operator console  12  and the system controller  34 . Thus, the operator is able to input a scan sequence that the operator tries to allow the MRI system  10  to execute. 
   The pulse generator modulator  42  operates a system component so as to execute a desired scan sequence and generates data for giving instructions as to the timing for an RF pulse to be generated and its intensity and shape, and the timing for a data acquisition window and its length. The pulse generator module  42  allows an electrical connection to a tilt amplifier system  46  and provides the tilt amplifier system  46  with a signal indicating the timing and shape of a tilt magnetic field pulse to be generated during scanning. Further, the pulse generator module  42  is configured so as to receive patient data from a physiological acquisition controller  48 . The physiological acquisition controller  48  is configured so as to receive inputs such as ECG signals (however not limited thereto) obtained from electrodes mounted to a patient, which inputs indicate patient&#39;s physiological states and are sent from a plurality of sensors. The pulse generator module  42  is electrically connected to a scan chamber interface circuit  50 . The scan chamber interface circuit  50  is configured so as to receive signals sent from various sensors, which represent patient&#39;s states and a magnet system. Further, the scan chamber interface circuit  50  is configured so as to transmit command signals such as a command signal (however not limited thereto) for moving the patient to a desired position, etc. to a patient positioning system  52 . 
   A tilt waveform generated from the pulse generator module  42  is inputted to the tilt amplifier system  46  including a GX amplifier  54 , a GY amplifier  56  and a GZ amplifier  58 . The amplifiers  54 ,  56  and  58  respectively excite corresponding tilt coils (gradient field forming means) in a tilt coil section  60  to generate a plurality of magnetic field tilts used to position-encode collected signals. The MRI  10  includes a magnet section (static magnetic field forming means)  62  including a deflecting magnet  64  and a whole-body RF coil  66  therein. The magnet section  62  forms a static magnetic field thereinside. The magnitude of the static magnetic field ranges from  3 T to  7 T. An operating frequency of the RF coil  66  is, for example, 64 MHz or more, specifically, 130 MHz and 300 MHz. 
   A transmitter-receiver module  70  positioned within the system controller  34  generates a plurality of electrical pulses amplified by an RF amplifier  72  upon its use. The RF amplifier  72  is electrically connected to the RF coil  66  by using a transmit/receive switch  74 . A signal obtained by emission based on excited atomic nuclei in the patient is detected by the RF coil  66 , followed by being transmitted to a preamplifier  76  through the transmit/receive switch  74 . Next, the so-amplified NMR (Nuclear Magnetic Resonance) signal is demodulated, filter-processed and digitized within a receiver section of the transmitter-receiver module  70 . The transmit/receive switch  74  is controlled by a signal supplied from the pulse generator module  42  to electrically connect the RF amplifier  72  to the RF coil  66  in a transmission mode and electrically connect the preamplifier  76  and the same coil to each other in a reception mode. Further, the transmit/receive switch  74  makes it possible to use the single RF coil (e.g., surface coil) even in both transmission and reception modes. 
   The NMR signal received by the RF coil  66  is digitized by the transmitter-receiver module  70 , which in turn is transferred to a memory module  78  lying in the system controller  34 . Upon completion of scanning, arrays of untreated k space data are collected within the memory module  78 . The untreated k space data are relocated so as to reach independent k space data arrays for reconstructing respective cardiac phase images. The respective k space data arrays are inputted to an array processor  80  configured so as to Fourier-transform data into arrays of image data. The image data are transmitted via the serial link  36  to the computer system  20  from which the data are stored in the disk storage unit  30 . The image data can be archived onto the tape driving device  32  in accordance with a command received from the operator console  12 . Alternatively, the image data are further processed by the image processor  24 , which in turn are sent to the operator console  12 , where the image data can also be displayed on the display  16 . 
     FIG. 2  is a schematic diagram showing a structure of a coil body  100  included in the RF coil  66 . The coil body  100  is configured as a so-called birdcage type coil and includes two conductive end loops (coil conductors)  102  and a plurality of conductors  104  cylindrically arranged around their central axes or patient bores  106  so as to electrically connect between the end loops  102 . The end loops  102  and the conductors  104  define a cylindrical imaging volume. The coil body  100  is provided with a plurality of capacitors (electrostatic capacitive elements for coil) respectively connected in series with the end loops  102 . The capacitors  108  are respectively disposed among the plural conductors  104 . Further, the MRI system  10  includes at least one switching apparatus  110  electrically connected to the coil body  100 . 
   A conventional switching apparatus will first be explained with reference to  FIGS. 3 through 5 , and the switching apparatus  110  according to the present embodiment will be described. 
     FIG. 3(   a ) is a circuit diagram showing a configuration of the conventional switching apparatus  110 ′, and  FIG. 3(   b ) is a circuit diagram showing an equivalent circuit of the conventional switching apparatus  110 ′, respectively. 
   The switching apparatus  110 ′ is provided with semirigid coaxial cables  122 ′. The coaxial cable  122 ′ includes an inner conductor  124 ′, an insulator  126 ′ that surrounds the inner conductor  124 ′, and an outer conductor  128 ′ that surrounds the insulator  126 ′. The inner conductor  124 ′ and the outer conductor  128 ′ are respectively connected to an end loop  102  at one end side (the left side of the sheet) of the coaxial cable  122 ′ with a capacitor  108  interposed therebetween. A bias input device  156  is connected to the other end side (the left side of the sheet) of the inner conductor  124 ′, and a bias is applied to the other end side of the coaxial cable  122 ′. 
   The inner conductor  124 ′ and the outer conductor  128 ′ are connected by a diode  138 ′ in the midstream of the coaxial cables  122 ′. When the diode  138 ′ is forward-biased, a current flows from the inner conductor  124 ′ to the outer conductor  128 ′. When the diode  138 ′ is reverse-biased, no current flows between the inner conductor  124 ′ and the outer conductor  128 ′. 
   In the coaxial cables  122 ′, a length  146 ′ from the end thereof on the capacitor  108  side to a portion thereof connected to the diode  138 ′ is set to ¼ of a wavelength λ corresponding to an operating frequency of the RF coil  66  at transmission. A length  140 ′ from the end thereof on the capacitor  108  side to the end of the other is set to ¼ to ½ of the wavelength λ. 
   Upon transmission, a forward bias is applied to the diode  138 ′. At this time, the coaxial cable  122 ′ indicates electrical characteristics of a coaxial cable having a short-circuited end such as shown in  FIG. 4(   a ). In  FIG. 4(   a ), the horizontal axis indicates the ratio of the length of the coaxial cable  122 ′ to the wavelength λ, and the vertical axis indicates impedance. 
   On the other hand, since the length  146 ′ of the coaxial cable  122 ′ is λ/4, the impedance of the coaxial cable  122 ′ becomes infinite as shown in  FIG. 4(   a ). Thus, as shown in  FIG. 5(   a ), the switching apparatus  110 ′ and the capacitor  108  become equivalent to the capacitor  108 . That is, a current flows through the capacitor  108  so that the operation of the coil body  100  becomes effective. 
   Upon reception, a reverse bias is applied to the diode  138 ′. At this time, the coaxial cable  122 ′ indicates electrical characteristics of a coaxial cable having an open end such as shown in  FIG. 4(   b ). 
   On the other hand, since the length  140 ′ of the coaxial cables  122 ′ is set to λ/4 to λ/2. As shown in  FIG. 4(   b ), the impedance of the coaxial cable  122 ′ becomes positive. That is, it functions as inductance. Thus, if the length  140 ′ of the coaxial cables  122 ′ is set as expressed by the following equation when the capacitance of the capacitor  108  is C and the inductance of the coaxial cable  122 ′ is L:
 
ω o= 2 πf =1/( CL ) 1/2 ,
         the switching apparatus  110 ′ and the capacitor  108  become equivalent to a parallel resonant circuit as shown in  FIG. 5(   b ). That is, the capacitor  108  is brought to a state made non-conductive to the end loop  102 , and the coil body  100  becomes ineffective.       

   In the switching apparatus  110  according to the present embodiment, its shortening can be attained by substituting a portion (portion obtained by subtracting the length  146 ′ from the length  140 ′) of the coaxial cables  122 ′ from the connected position of the diode  138 ′ to the applied side of the bias with a capacitor corresponding to a lumped constant element equivalent to the portion. 
     FIG. 6  is a circuit diagram showing a configuration of the switching apparatus  110 . The switching apparatus  110  is provided with a semirigid coaxial cable  122 . The coaxial cable  122  includes an inner conductor  124  made of a metal material, an insulator  126  that covers the inner conductor  124 , an outer conductor  128  made of a metal material, which covers the insulator  126 , and an unillustrated outer sheath that covers the outer conductor  128 . Incidentally, the outer conductor  128  is formed of copper, for example. 
   In the coaxial cable  122 , its overall length  146  is set to ¼ of a wavelength λ corresponding to the operating frequency of the RF coil  66  at transmission. When the operating frequency is 300 MHz, for example, the overall length is about 20 cm, whereas when the operating frequency is 130 MHz, the overall length is about 50 cm. At one end side (the left side of the sheet) of the coaxial cable  122 , the inner conductor  124  and the outer conductor  128  are respectively connected to an end loop  102  with a capacitor  108  interposed therebetween. 
   The switching apparatus  110  includes a diode  138 , a capacitor (lumped constant element)  148 , inductors  158  and  160 , and a shield case (shield body for the switching apparatus)  162  that shields these elements, all of which are connected to the other end side (bias input side) of the coaxial cable  122 . 
   The diode  138  is connected to the inner conductor  124  and the outer conductor  128 . When the diode  138  is forward-biased, a current flows from the inner conductor  124  to the outer conductor  128 . When the diode  138  is reverse-biased, no current flows between the inner conductor  124  and the outer conductor  128 . Incidentally, although  FIG. 6  illustrates the case in which the diode  138  is disposed in such a manner that the direction from the inner conductor  124  to the outer conductor  128  is set to the forward direction, the diode  138  may be provided in the direction opposite to the direction shown in  FIG. 6 . 
   The capacitor  148  is connected to the inner conductor  124  and the outer conductor  128  in shunt with the diode  138 . The capacitor  148  is made up of a capacitor corresponding to a lumped constant element. The capacitor  148  is constituted of, for example, a ceramic capacitor or a mica capacitor. 
   The capacitor  148  is of a capacitor equivalent to a portion (portion obtained by subtracting the length  146 ′ from the length  140 ′) of the coaxial cables  122 ′ of the switching apparatus  110 ′ shown in  FIG. 3(   a ), which extends from the connected position of the diode  138 ′ to the applied side of the bias. Thus, the electrostatic capacitance of the capacitor  148  is set in such a manner that when the diode  138  is reverse-biased, a resonant circuit with an operating frequency at transmission as a resonant frequency is configured of the capacitor  108 , the coaxial cable  122  and the capacitor  148 . 
   Described specifically, the above is expressed as follows. Assuming that the characteristic impedance of the coaxial cable is Zo and the electrostatic capacitance of the capacitor  148  is C 2 , the equivalent reactance of the λ/4-wavelength coaxial cable  122  and the capacitor  148  is expressed in the following equation: 
   
     
       
         
             
           
             
               
                 
                   X 
                   = 
                     
                   ⁢ 
                   
                     
                       Zo 
                       2 
                     
                     / 
                     
                       ( 
                       
                         
                           - 
                           j 
                         
                         × 
                         
                           1 
                           / 
                           
                             ( 
                             
                               ω 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 oC 
                                 2 
                               
                             
                             ) 
                           
                         
                       
                       ) 
                     
                   
                 
               
             
             
               
                 
                   = 
                     
                   ⁢ 
                   
                     jω 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       oC 
                       2 
                     
                     ⁢ 
                     
                       Zo 
                       2 
                     
                   
                 
               
             
           
         
       
     
   
   That is, the inductance L of the λ/4-wavelength coaxial cable  122  and the capacitor  148  is expressed as follows:
 
 L=C   2   Zo   2  
 
   Thus, when the operating frequency is coo and the electrostatic capacitance of the capacitor  108  is C 1 , C 2  may be set such that the following equation is established:
 
ω o= 1/( C   1   L ) 1/2 =1/( C   1   C   2   Zo   2 ) 1/2  
 
   The inductor  158  is connected in series with the outer conductor  124  on the bias input side than the capacitor  148 . The inductor  160  is connected in series with the inner conductor  122  on the bias input side than the capacitor  148 . The inductors  158  and  160  respectively function as low-pass filters for preventing the mixing of noise due to the application of a bias. The inductances of the inductors  158  and  160  are respectively 600 nH where the operating frequency of the RF coil  66  is 300 MHz, for example. 
   The shield case  162  is constituted of a non-magnetic material having conductivity. The shield case  162  is made up of a copper foil plate, for example. The shield case  162  is configured in such a shape as to be capable of shielding the diode  138 , the capacitor  148  and the inductors  158  and  160 . For instance, the shield case  162  is shaped in the form of a box capable of accommodating these elements therein. The shield case  162  is connected to a reference potential. 
   On the outer side of the shield case  162 , there are provided a capacitor  164  whose one end side is connected to the bias input side of the inductor  158  and whose other end side is connected to the shield case  162 , and a capacitor  166  whose one end side is connected to the bias input side of the inductor  160  and whose other end side is connected to the shield case  162 . Each of the capacitors  164  and  166  functions as a filter which causes a bias&#39;s high frequency component to escape to the reference potential and prevents the mixing of noise due to the bias. 
   The input of the bias to the switching apparatus  110  is performed by a bias input device  156 . The bias input device  156  is connected to, for example, the inner conductor  124  and applies a forward or reverse bias at the diode  138 . The operation of the bias input device  156  is controlled by the system controller  34 . 
   The system controller  34  controls the bias input means in such a manner that, for example, the forward bias is applied upon transmission to render the RF coil  66  effective and the reverse bias is applied upon reception to make the RF coil  66  ineffective. Thus, coupling at the time that a high-frequency magnetic field is formed at a subject by the RF coil  66  used as, for example, a body coil, and a magnetic resonance signal from the subject is received by a receive-only coil constituted of a phased array coil or the like, is suppressed. 
     FIG. 7  is a perspective view showing a method for packaging the coil body  100  and the switching apparatus  110  employed in the RF coil  66 . 
   The RF coil  66  is provided with a cylindrical RF shield (coil shield body)  310  which includes the coil body  100  therein and shields it. The RF shield  310  is formed of a conductor and connected to the reference potential. The coaxial cable  122  of the switching apparatus  110  is connected to the coil body  100  inside the RF shield  310  and extends in parallel with each conductor  104  inside the RF shield  310 . Thereafter, the coaxial cable  122  extends outside the RF shield  310  and extends to the shield case  162 . Incidentally, the coaxial cable  122  extends from the inside of the RF shield  310  to its outside at a virtual ground point  308 . The virtual ground point  308  is located substantially midway between the end loops  102  and  102  and corresponds to such a position that the potential substantially reaches the reference potential. 
     FIG. 8  is a sectional view taken along a plane ABCD shown in  FIG. 7 . The coaxial cable  122  is fixed to the RF shield  310  by, for example, soldering its outer sheath. By, for example, soldering the shield case  162  to the RF shield  310 , the shield case  162  is fixed to the RF shield  310  and electrically connected thereto, and connected to the reference potential. Incidentally, solder functions as fixing means having conductivity. 
   According to the above embodiment, the distributed constant element configured of the λ/4-wavelength coaxial cable  122  is connected to the capacitor  108  of the RF coil  66 . Upon transmission, the coaxial cable  122  is short-circuited via the diode  138  to make the RF coil  66  effective, whereas upon reception, the coaxial cable  122  is not short-circuited. It is therefore possible to change the characteristic of the RF coil  66  from its characteristic at transmission upon reception. Since the impedance at reception is adjusted by the capacitor  148  corresponding to the lumped constant element, the switching apparatus  110  can be reduced or scaled down as compared with the case where the impedance at reception is adjusted by the distributed constant element. 
   Described specifically, since the electrostatic capacitance of the capacitor  148  is set such that the resonant circuit is configured where no coaxial cable  122  is short-circuited, the characteristic of the RF coil  66  as the resonant circuit can be lost upon reception. 
   Since the bias is applied to the diode  138  or the like through the inductors  158  and  160 , the mixing of noise is prevented. Owing to the provision of the shield case  162 , the emission of noise from the capacitor  148  or the like and its energy loss are suppressed. Allowing the bias&#39;s high-frequency component to escape through the capacitors  164  and  166  prevents the mixing of the noise. With the combination thereof with the inductors  158  and  160  in particular, the high frequency component is blocked by the inductors  158  and  160 . Further, the emission of noise can efficiently be prevented by causing the blocked high frequency component to escape. 
   The RF coil  66  is provided with the RF shield  310  and the coaxial cable  122  is connected to the coil body  100  within the RF shield  310 . Further, the other end thereof is disposed outside the RF shield  310 , and the diode  138 , the capacitor  148 , etc. are mounted to the coaxial cable  122 . Therefore, the mounting of the diode  138  and the like can be rendered easy, and wiring routing can be carried out without concern for the influence of stray components on the coil body  100 . 
   Second Embodiment 
   An overall configuration of an MRI according to a second embodiment is similar to the first embodiment shown in  FIGS. 1 and 2 . Incidentally, constituents similar to the first embodiment are given reference numerals similar to those employed in the first embodiment below and their description will therefore be omitted. The MRI according to the second embodiment is different from the first embodiment in terms of the configuration of a switching apparatus. 
   A conventional switching apparatus will first be explained by referring to  FIGS. 9 and 10 , and a switching apparatus  210  according to the present embodiment will be described. 
     FIG. 9(   a ) is a circuit diagram showing a configuration of the conventional switching apparatus  210 ′, and  FIG. 9(   b ) is a circuit diagram showing an equivalent circuit of the conventional switching apparatus  210 ′, respectively. 
   The switching apparatus  210 ′ is provided with semirigid coaxial cables  222 ′. The coaxial cable  222 ′ includes an inner conductor  224 ′, an insulator  226 ′ that surrounds the inner conductor  224 ′, and an outer conductor  228 ′ that surrounds the insulator  226 ′. The inner conductor  224 ′ and the outer conductor  228 ′ are respectively connected to an end loop  102  at one end side (the left side of the sheet) of the coaxial cable  222 ′ with a capacitor  108  interposed therebetween. A bias input device  156  is connected to the other end side (the left side of the sheet) of the inner conductor  224 ′, and a bias is applied to the other end side of the coaxial cable  222 ′. A length  246 ′ from an end on the capacitor  108  side, of the coaxial cable  222 ′ to an end on the bias input side, of the coaxial cable  222 ′ is set to ¼ of a wavelength λ. 
   The inner conductor  224 ′ and the outer conductor  228 ′ are connected by a diode  238 ′ in the midstream of the coaxial cables  222 ′. When the diode  238 ′ is forward-biased, a current flows from the inner conductor  224 ′ to the outer conductor  228 ′. When the diode  238 ′ is reverse-biased, no current flows between the inner conductor  224 ′ and the outer conductor  228 ′. 
   A capacitor  209 ′ connected to the inner conductor  224 ′ and the outer conductor  228 ′ is provided at the end on the bias input side, of the coaxial cable  222 ′. The impedance of the capacitor  209 ′ is set so as to be relatively small. 
   Upon transmission, a reverse bias is applied to the diode  238 ′. Since the impedance of the capacitor  209 ′ is set so as to be relatively small at this time, the coaxial cable  222 ′ indicates electrical characteristics that approximate the electrical characteristics of the coaxial cable having the short-circuited end such as shown in  FIG. 4(   a ). 
   On the other hand, since the length  246 ′ of the coaxial cable  222 ′ is λ/4, the impedance of the coaxial cable  222 ′ becomes infinite as shown in  FIG. 4(   a ). Thus, as shown in  FIG. 10(   a ), the switching apparatus  210  and the capacitor  108  become equivalent to the capacitor  108 . That is, a current flows through the capacitor  108  so that the operation of the coil body  100  becomes effective. 
   Upon reception, a forward bias is applied to the diode  238 ′. At this time, the coaxial cable  222 ′ indicates the electrical characteristics of the coaxial cable having the short-circuited end such as shown in  FIG. 4(   a ). 
   On the other hand, since the length  240 ′ of the coaxial cable  222 ′ is set to 0 to λ/4. As shown in  FIG. 4(   a ), the impedance of the coaxial cable  222 ′ becomes positive. That is, it functions as inductance. Thus, if the length  240 ′ of the coaxial cable  222 ′ is set as given by the following equation when the capacitance of the capacitor  108  is C and the inductance of the portion corresponding to the length  240 ′ of the coaxial cable  222 ′ is L:
 
ω0=2 πf= 1/( CL ) 1/2 ,
         the switching apparatus  210  and the capacitor  108  become equivalent to a parallel resonant circuit as shown in  FIG. 10(   b ). That is, the capacitor  108  is brought to a state made non-conductive to the end loop  102 , and the coil body  100  becomes ineffective.       

   In the switching apparatus  210  according to the present embodiment, its shortening can be attained by substituting a portion (portion obtained by subtracting the length  246 ′ from the length  240 ′) of the coaxial cables  222 ′ from the connected position of the diode  138 ′ to the applied side of the bias with an inductor corresponding to a lumped constant element equivalent to the portion. 
     FIG. 11  is a circuit diagram showing a configuration of the switching apparatus  210 . The switching apparatus  210  has a configuration substantially similar to the switching apparatus  110  according to the first embodiment shown in  FIG. 6 . 
   That is, the switching apparatus  210  is provided with such a coaxial cable  222  that on the one end side thereof, an inner conductor  224  is connected to one side of a capacitor  108  and an outer conductor  228  is connected to the other side of the capacitor  108 , and a bias is applied to the other end thereof. The switching apparatus  210  is provided, on the bias input side of a coaxial cable  222 , with a diode  238  connected to the inner conductor  224  and the outer conductor  228 . The switching apparatus  210  is provided with a capacitor  209 ′ connected to the inner conductor  224  and the outer conductor  228  on the bias input side of the coaxial cable  222 . The switching apparatus  210  is provided, on the bias input side of the coaxial cable  222 , with an inductor  258  connected in series with the inner conductor  224 , and provided, on the bias input side thereof, with an inductor  260  connected in series with the outer conductor  228 . The switching apparatus  210  is equipped with a shield case  262 , which is formed of a non-magnetic material having conductivity and covers the capacitor  209 ′ and the like. A capacitor  264  connected to the inner conductor  224  on the bias input side than the inductor  258  and connected to the shield case  262 , and a capacitor  266  connected to the outer conductor  228  on the bias input side than the inductor  260  and connected to the shield case  262  are provided therein. 
   However, the switching apparatus  210  is provided with an inductor  248 . The switching apparatus  210  is different from the switching apparatus  110  in terms of the length  246  of the coaxial cable  222  and the electrostatic capacitance of the capacitor  209 ′ and also different therefrom even in terms of the direction of a bias where the RF coil  66  is made effective or ineffective. 
   The capacitor  209 ′ is equivalent to the capacitor  209 ′ employed in the conventional switching apparatus  210 ′ shown in  FIGS. 9(   a ) and  9 ( b ) and is connected to the inner conductor  224  and the outer conductor  228  of the coaxial cable  222 . The capacitor  209 ′ is used to short-circuit the end on the bias input side, of the coaxial cable  222  with respect to a high frequency component. Its electrostatic capacitance is set relatively high (set low in reactance) as before. 
   The inductor  248  is connected in series with the capacitor  209 ′. In other words, the inductor  248  is connected to the inner conductor  224  and the outer conductor  228  at the end on the bias input side, of the coaxial cable  222  so as to be parallel with the diode  238 . The inductor  248  may preferably be a non-magnetic inductor in terms of an improvement in SN ratio. Incidentally, the inductor  248  may be any of a winding inductor, a laminated inductor and a thin-film inductor. The inductor  248  may be either a lead wire type or a chip inductor. A suitable material may be used for a core, or an air core may be used. 
   When a forward bias is applied to the diode  222 , the RF coil is rendered ineffective as shown in  FIG. 10(   b ) in the switching apparatus  210 . That is, the capacitor  108  and the coaxial cable  222  constitute a resonant circuit with its operating frequency as a resonant frequency. Thus, the length  240  of the coaxial cable  222  is set in a manner similar to the length  240 ′ of the coaxial cable  222 ′ shown in  FIG. 9 . 
   On the other hand, when a reverse bias is applied to the diode  222 , the RF coil  66  is made effective as shown in  FIG. 10(   a ) in the switching apparatus  210 . That is, the combined impedance of the coaxial cable  222  and the inductor  248  becomes equal to the impedance where a short-circuited end of a coaxial cable having a length equal to an odd multiple of λ/4, that is, it becomes infinite. Thus, the inductance of the inductor  248  is set in such a manner that the impedance of the coaxial cable  222  used as a distributed constant element whose terminal is short-circuited by the capacitor  209 ′ becomes infinite. 
   Incidentally, the mounting or packaging of the switching apparatus  210  may be carried out by a method similar to the packaging method of the switching apparatus  110  shown in  FIGS. 7 and 8 . According to the above embodiment, an advantageous effect similar to the first embodiment is obtained. 
   The present invention is not limited to the above embodiments. The present invention may be implemented in various forms. 
   The RF coil is not limited to the body coil or the birdcage type coil. One that performs at least any of the formation of a high-frequency magnetic field and the reception of a magnetic resonance signal may be used. For instance, a head coil or a surface coil may be used. One that performs only any one of the transmission and reception may be used, or one that performs both of them may be used. 
   The switching apparatus is not limited to such a type that the RF coil is disabled or made ineffective. The resonant frequency of the RF coil may simply be shifted. In this case, in the first embodiment, the electrostatic capacitance of the capacitor  148  may suitably be set in such a manner that the resonant frequency of the RF coil is shifted to a desired value. In the second embodiment, the length  240  of the coaxial cable  222  may suitably by set in such a manner that the resonant frequency of the RF coil is shifted to a desired value. 
   In the switching apparatus, whether the impedance of the switching apparatus should be increased than the other of the transmission and reception may suitably be set upon any one of the transmission and reception. For example, the switching apparatus is not limited to one wherein the RF coil is made effective upon transmission and the RF coil is made ineffective upon reception. The switching apparatus may be one wherein the RF coil is made ineffective upon transmission and the RF coil is made effective upon reception. 
   The length of the coaxial cable employed in the first embodiment is not limited to λ/4. The length thereof may be an odd multiple of λ/4. 
   Many widely different embodiments of the invention may be configured without departing from the spirit and the scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.