Patent Publication Number: US-10761158-B2

Title: Radio frequency coil, magnetic resonance imaging device using same, and method for adjusting multi-channel radio frequency coil

Description:
TECHNICAL FIELD 
     The present invention relates to a magnetic resonance imaging (MRI) apparatus, and more particularly, it relates to a radio frequency coil (RF coil) for applying an RF magnetic field and for detecting a nuclear magnetic resonance signal. 
     BACKGROUND ART 
     An MRI apparatus utilizes a nuclear magnetic resonance phenomenon to create an image from any cross section traversing a subject. Specifically, such an MRI apparatus applies an RF magnetic field to a subject placed in a spatially homogeneous magnetic field (static magnetic field), causing nuclear magnetic resonance to occur, detects nuclear magnetic resonance signals being generated, and performs an imaging process on the signals thus detected, whereby a tomographic image is acquired. 
     A device that applies the RF magnetic field to the subject, and detects the nuclear magnetic resonance signals generated from the subject, is referred to as a radio frequency coil (hereinafter, referred to as an RF coil). The RF coil is provided with a loop unit (coil loop) for performing application and detection of the RF magnetic field. The smaller is the coil loop, the higher is the sensitivity of the coil, though its sensitivity region becomes narrower. On the other hand, when the coil loop is made larger, the sensitivity region can be expanded. Accordingly, in the RF coil, there are tradeoffs between the sensitivity level and the size of the sensitivity region. In addition, since the nuclear magnetic resonance signals are rotating-magnetic-field signals that are generated in the direction vertical to the static magnetic field, it is preferable to place the RF coil in the orientation allowing application and detection of the magnetic field to be vertical to the static magnetic field. 
     As discussed above, the sensitivity of the RF coil is enhanced with reduction of the size thereof, whereas the sensitivity region becomes narrower. As a technique to solve this problem, there is disclosed a multichannel array coil comprising RF coils small in diameter with high sensitivity, arranged in arrays (see Non Patent Document 1, for example). Since the multichannel array coil is high in sensitivity and provided with a wide sensitivity region, allowing acquisition of an image with a high SNR (Signal to Noise Ratio), currently, it serves as a dominating receiver RF coil. In the following description, each of the RF coils constituting the multichannel array coil will be referred to as a subcoil. 
     In general, when the RF coils having the same resonance characteristics are placed adjacent to each other, they interfere with each other due to magnetic coupling. Since performance of the RF coil is deteriorated due to the magnetic coupling, it is imperative to remove the magnetic coupling between the subcoils in the multichannel array coil. The Non Patent Document 1 discloses that the adjacent subcoils are placed in a manner that a part of the coil loop overlaps on another coil loop, thereby minimizing the magnetic coupling. Furthermore, a low-input preamplifier, an inductor, and a capacitor are used, so as to render a part of the coil loop to be high impedance, and interference from anything other than the subcoils can be reduced. 
     The Patent Document 1 and the Patent Document 2 disclose techniques where a decoupling means is provided in a multichannel coil, whereby magnetic coupling between the subcoils constituting the multichannel coil is reduced. 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1 
     
         
         Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2013-530019
 
Patent Document 2
 
         Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2009-534161 
       
    
     Non Patent Document 
     Non Patent Document 1 
     
         
         Roemer P B, et al., “The NMR phased array”, Journal of Magnetic Resonance, USA, 1990, 16, p. 192-225 
       
    
     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     With the configuration of multichannel, even though a high signal strength is obtained, noise may be increased simultaneously. Noise of the RF coil is mainly thermal noise of the subject, and this is Gaussian noise. Upon creating a composite image, noise is increased along with increasing the number of channels being combined. Therefore, in the multichannel coil, in some cases, the SNR of the image may not be improved, even though the signal strength gains with the increase of the number of channels. 
     Furthermore, in the array coil, multiple RF coils are placed covering the subject, and there may occur noise correlation due to electromagnetic coupling between the RF coils via the subject. The stronger is the noise correlation, the more increases the noise of the composite image. 
     However, the conventional multichannel techniques aim at achieving an arrangement that is optimum for avoiding loss of sensitivity at each channel, and optimization (minimization) of noise has not been addressed. In addition, magnetic coupling removal for implementing the multichannel coil intends to reduce the magnetic coupling as much as possible, but complete removal thereof is not the goal. Therefore, with the combination of the subcoils placed at positions where the magnetic coupling occurs intensively, there should remain considerable magnetic coupling, and signal strength in the multichannel coil may be lowered. 
     There is an image reconstruction method for minimizing noise by using noise correlation, but in the case where the noise correlation is not obtained correctly or the SNR is low, this method may turn to reduction of the SNR instead. 
     The present invention has been made in view of the situations as described above, and the present invention is directed to a multichannel array coil of an MRI apparatus, achieving both a wide sensitivity region and high sensitivity at deep levels, without complex configurations, and further reducing noise, whereby providing a high quality image. 
     Means for Solving the Problems 
     The present invention provides an RF coil (array coil) for an MRI apparatus, the RF coil having multiple subcoils and being placed and adjusted in a manner that an electric field generated in a subject by current passing through the subcoils may operate to reduce mutual noise between the subcoils. Specifically, at least two subcoils among the multiple subcoils are coupled electromagnetically, so that a part of the current passing through a loop coil unit of one subcoil is made to pass through the loop coil unit of the other subcoil, in the form of sub-current, and circuit components of each subcoil and an amount of electromagnetic coupling between the subcoils are adjusted so that the current passing through the loop coil unit of one subcoil is tuned to the sub-current passing through the other subcoil, and the current flows in the direction opposite to the direction of the sub-current. 
     Advantage of the Invention 
     According to the present invention, noise is reduced in the array coil of the MRI apparatus, whereby a high quality image can be obtained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1( a )  and  FIG. 1( b )  are external views of an MRI apparatus; 
         FIG. 2  is a block diagram showing a schematic configuration of the MRI apparatus; 
         FIG. 3  illustrates a connection between a transmit RF coil and a receiver RF coil in the MRI apparatus according to an embodiment; 
         FIG. 4( a )  illustrates a configuration of a birdcage RF coil that is used as the transmit RF coil, and  FIG. 4( b )  illustrates one example of a transmit-receive magnetic coupling prevention circuit of the transmit RF coil; 
         FIG. 5( a )  illustrates an embodiment of an array coil used as the receiver RF coil, and  FIGS. 5( b ) and 5( c )  illustrate examples of the transmit-receive magnetic coupling prevention circuit of the receiver RF coil; 
         FIGS. 6( a ) to 6( c )  each illustrates actions and operations of the array coil, according to the present embodiment and a conventional example; 
         FIGS. 7( a ) to 7( c )  each illustrates a layout of the array coil in the MRI apparatus of horizontal magnetic field system; 
         FIGS. 8( a ) to 8( c )  each illustrates a layout of the array coil in the MRI apparatus of vertical magnetic field system; 
         FIG. 9  schematically illustrates the array coil according to a first embodiment; 
         FIG. 10( a )  illustrates an equivalent circuit of the array coil according to the first embodiment, FIG.  10 ( b ) illustrates the equivalent circuit looked at a low-impedance circuit side of the first subcoil side, and  FIG. 10( c )  illustrates the equivalent circuit looked at the low-impedance circuit side of the second subcoil side; 
         FIGS. 11( a ) and 11( b )  illustrate operations of a general parallel resonant circuit; 
         FIG. 12( a )  shows a result of imaging by a conventional array coil (comparative example),  FIG. 12( b )  shows a result of imaging by the array coil (example 1) according to the first embodiment, and  FIG. 12( c )  is a graph showing sensitivity profiles of the embodiment and the comparative example; 
         FIG. 13  is a graph showing a relationship between a current ratio of sub-current and noise correlations; 
         FIG. 14( a )  illustrates an equivalent circuit of the array coil according to a second embodiment,  FIG. 14( b )  illustrates the equivalent circuit looked at a low-impedance circuit side of the first subcoil side, and  FIG. 14( c )  illustrates the equivalent circuit looked at the low-impedance circuit side of the second subcoil side; 
         FIG. 15  illustrates a modification example of an electromagnetic coupler; 
         FIG. 16  is a schematic view showing another modification example of the electromagnetic coupler; 
         FIG. 17  is a schematic view showing a modification example of a loop coil unit; 
         FIG. 18  schematically illustrates the array coil according to a third embodiment; 
         FIG. 19  illustrates a modification example of the array coil according to the third embodiment; and 
         FIG. 20  illustrates another modification example of the array coil according to the third embodiment. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     There will now be described embodiments of an MRI apparatus and an RF coil according to the present invention, with reference to the accompanying drawings. Hereinafter, in all the figures illustrating the embodiments of the present invention, components with an identical function are labeled with the same reference numeral, and they will not be redundantly explained. 
     &lt;Embodiments of the MRI Apparatus&gt; 
     There will be described the embodiments of the MRI apparatus to which the present invention is applied. The MRI apparatus of the present embodiments features that a specific multichannel RF coil is employed as a receiver RF coil. 
     [Configuration of the Apparatus] 
     With reference to  FIG. 1 , there will now be described an overall configuration of the MRI apparatus of the present embodiment.  FIG. 1  is an external view of the MRI apparatus of the present embodiment.  FIG. 1( a )  illustrates the MRI apparatus  100  of a horizontal magnetic field system that uses a tunnel-type magnet for generating a static magnetic field by a solenoid coil.  FIG. 1( b )  illustrates the MRI apparatus  101  of a hamburger-type (open-type) vertical magnetic field system in which the magnets  111  are separated vertically so as to enhance a sense of openness. Each of these MRI apparatuses  100  and  101  are provided with a table  102  for placing an examinee (subject)  103  thereon. The subject  103  placed on the table is put in the space for examination where homogeneous magnetic field (static magnetic field) is generated by magnet  110  or  111 . The magnet  110  or  111  constitutes a static magnetic field former that forms the static magnetic field. 
     The present embodiments are applicable to any of the MRI apparatus  100  of the horizontal magnetic field system and the MRI apparatus  101  of the vertical magnetic field system. The types of the MRI apparatus as shown in  FIG. 1  are just examples, and any publicly-known various MRI apparatuses may be used for the present invention, regardless of modes or types of the apparatus. In the description below, as a coordinate system commonly used in any of the horizontal magnetic field system and the vertical magnetic field system, the coordinate system  090  will be employed, where a direction of the static magnetic field is indicated as z-direction, and two directions orthogonal thereto are indicated as x-direction and y-direction, respectively. In the following, this is similarly applied to all the figures in association with the present specification. 
     Next, a schematic configuration of the MRI apparatus  100  will be described, taking the MRI apparatus of the horizontal magnetic field system as an example.  FIG. 2  is a block diagram showing the schematic configuration of the MRI apparatus. As illustrated, the MRI apparatus  100  is provided with the magnet  110  of the horizontal magnetic field system, a gradient magnetic field coil  131 , a transmit RF coil  151 , a receiver RF coil  161 , a gradient magnetic field power source  132 , a shim coil  121 , a shim power source  122 , an RF magnetic field generator  152 , a receiver  162 , a magnetic coupling prevention circuit driver  180 , a computer (PC)  170 , a sequencer  140 , and a monitor  171 . The reference numeral  102  represents the table for placing the subject  103  thereon. 
     The gradient magnetic field coil  131  is connected to the gradient magnetic field power source  132 , and generates a gradient magnetic field. The gradient magnetic field coil  131  and the gradient magnetic field power source  132  constitute a gradient magnetic field former that forms the gradient magnetic field. The shim coil  121  is connected to the shim power source  122 , and controls a degree of homogeneity in the magnetic field. The transmit RF coil  151  is connected to an RF magnetic field generator  152 , and applies (transmits) an RF magnetic field to the subject  103 . The receiver RF coil  161  is connected to the receiver  162 , and receives nuclear magnetic resonance signals from the subject  103 . The magnetic coupling prevention circuit driver  180  is connected to a magnetic coupling prevention circuit (described below). The magnetic coupling prevention circuit being connected to each of the transmit RF coil  151  and the receiver RF coil  161 , is a circuit for preventing magnetic coupling between the transmit RF coil  151  and the receiver RF coil  161 . 
     The sequencer  140  sends commands, respectively to the gradient magnetic field power source  132 , to the RF magnetic field generator  152 , and to the magnetic coupling prevention circuit driver, so as to operate those units. The commands are transmitted according to instructions from the computer (PC)  170 . Further according to the instructions from the computer (PC)  170 , a magnetic resonance frequency is set as a reference of detection at the receiver  162 . By way of example, according to the command from the sequencer  140 , the RF magnetic field is applied to the subject  103  via the transmit RF coil  151 . By applying the RF magnetic field, nuclear magnetic resonance signals are generated from the subject  103 . Then, the receiver RF coil  161  detects thus generated nuclear magnetic resonance signals, and detection is performed in the receiver  162 . 
     The computer (PC)  170  performs control on the overall operations and various signal processing in the MRI apparatus  100 . For example, the computer receives signals detected by the receiver  162  via an A/D converter, and performs signal processing such as image reconstruction (functions of an image reconstructor). Results of the processing above are displayed on the monitor  171 . Detected signals and measuring conditions are stored in a storage medium as necessary. Furthermore, the computer allows the sequencer  140  to send out commands so that each unit operates at a timing and strength programmed in advance. In addition, if it is necessary to adjust the degree of homogeneity in the static magnetic field, the sequencer  140  sends commands to the shim power source  122 , allowing the shim coil  121  to adjust the degree of homogeneity in the magnetic field. 
     [Overviews of the Transmit RF Coil and the Receiver RF Coil] 
     As described above, the MRI apparatus of the present embodiment employs two types of RF coil; the transmit RF coil  151  and the receiver RF coil  161 . One RF coil may serve as both the transmit RF coil  151  and the receiver RF coil  161 , or independent RF coils may be used respectively. Details of the RF coils will be described in the following, taking as an example that the transmit RF coil  151  and the receiver RF coil  161  are independent RF coils, and the transmit RF coil  151  is a birdcage-like shape RF coil (birdcage RF coil), and the receiver RF coil  161  is a multichannel array coil comprising multiple subcoils. 
     Firstly, with reference to  FIG. 3 , there will be described an arrangement of the birdcage RF coil  300  used as the transmit RF coil  151  and the array coil  400  used as the receiver RF coil  161 , together with describing a connection mode of the birdcage RF coil  300 , the array coil  400 , the RF magnetic field generator  152 , the receiver  162 , and the magnetic coupling prevention circuit driver  180 . 
     As illustrated, the birdcage RF coil  300  has an approximately cylindrical outer shape (including an elliptic cylinder or a polygonal column), and this approximate cylinder is placed to be coaxial with the central axis of the magnet  110  (the axis in the Z-direction). The subject  103  is placed inside the birdcage RF coil  300 . The array coil  400  is arranged in proximity to the subject  103  within the birdcage RF coil  300 . As described above, the birdcage RF coil  300  is connected to the RF magnetic field generator  152 . The array coil  400  is connected to the receiver  162 . 
     In addition, the birdcage RF coil  300  is provided with a magnetic coupling prevention circuit  210  for preventing magnetic coupling with the array coil  400 , and the array coil  400  is provided with a magnetic coupling prevention circuit  220  for preventing magnetic coupling with the birdcage RF coil  300 . They are referred to as transmit-receive magnetic coupling prevention circuits. Those transmit-receive magnetic coupling prevention circuits enable transmission of the RF magnetic field and reception of the nuclear magnetic resonance signals, without magnetic coupling in the arrangement as described above. 
     [Transmit RF Coil] 
     Next, with reference to  FIG. 4 , the birdcage RF coil  300  used as the transmit RF coil  151  of the present embodiment will be described. In the birdcage RF coil  300  according to the present embodiment, a resonance frequency is tuned to the resonance frequency (magnetic resonance frequency) of an element targeted for excitation, and the RF magnetic field at this magnetic resonance frequency is applied. In the present embodiment, the resonance frequency is tuned to the magnetic resonance frequency of hydrogen nucleus, allowing hydrogen nucleus to be excited. Hereinafter, the magnetic resonance frequency of the RF magnetic field to be applied is indicated as f0. 
       FIG. 4( a )  is a block diagram illustrating a configuration of the birdcage RF coil  300  according to the present embodiments. As illustrated, the birdcage RF coil  300  of the present embodiment comprises multiple linear conductors  301 , end conductors  302  for connecting the ends of each of the linear conductors  301 , and capacitors  303  inserted in the end conductors  302 . 
     In addition, the birdcage RF coil  300  is provided with two input ports  311  and  312 . It is configured such that transmitted signals, 90 degrees out of phase with each other, are inputted into the first input port  311  and into the second input port  312 , respectively, allowing the RF magnetic field to be efficiently applied to the subject  103 . 
     In the birdcage RF coil  300  of the present embodiment, the transmit-receive magnetic coupling prevention circuits  210  for preventing magnetic coupling with the receiver RF coil  161  (array coil  400 ) are inserted, respectively in series with the linear conductors  301  of the birdcage RF coil  300 . 
     As shown in  FIG. 4( b ) , for example, the transmit-receive magnetic coupling prevention circuit  210  may comprise a PIN diode  211  inserted in series with the linear conductor  301 , and control signal lines  212  are connected respectively to both ends of the PIN diode. The control signal lines  212  are connected to the magnetic coupling prevention circuit driver  180 . Preferably, a choke coil (not illustrated) may be inserted into the control signal line  212 , so as to avoid mixture of high frequencies. 
     The PIN diode  211  possesses properties that it normally indicates high resistance (OFF), and when a value of direct current flowing in the forward direction of the PIN diode  211  becomes a certain amount or more, it mostly indicates conductive (ON). In the present embodiment, such properties are utilized to control ON/OFF of the PIN diode  211 , according to the direct current outputted from the magnetic coupling prevention circuit driver  180 . In other words, when RF signals are transmitted, control current is made to flow to render the PIN diode  211  to be conductive via the control signal lines  212 , whereby the birdcage RF coil  300  functions as the transmit RF coil  151 . On the other hand, when nuclear magnetic resonance signals are received, the control current is suspended, whereby the birdcage RF coil  300  becomes high impedance, and changed to an open state. 
     As thus described, in the present embodiment, controlling of the direct current (control current) from the magnetic coupling prevention circuit driver  180  allows the birdcage RF coil  300  to function as the transmit RF coil  151  upon transmitting RF signals, whereas upon receiving nuclear magnetic resonance signals, the birdcage RF coil is changed to the open state, thereby removing magnetic coupling with the array coil  400  that serves as the receiver RF coil  161 . 
     [Receiver RF Coil] 
     Next, with reference to  FIG. 5 , there will be described the array coil  400  used as the receiver RF coil  161  of the present embodiment. For ease of explanation, an array coil where two loop-shaped RF coils (surface coils) are placed side by side will be described as an example. However, any multichannel RF coil with an arrangement of multiple subcoils is applicable as the receiver RF coil of the present embodiment, and the number of subcoils is not limited to a particular number. 
     As shown in  FIG. 5( a ) , the array coil  400  of the present embodiment comprises two subcoils  410 . The two subcoils  410  constituting the array coil  400  are referred to as a first subcoil  410 A and the second subcoil  410 B, respectively. If there is no need to particularly identify each of the subcoils  410  as components of the array coil  400 , the alphabet attached to the end of the reference numerals may be omitted (the same shall apply hereafter). The first subcoil  410 A and the second subcoil  410 B are surface coils, each having a loop structured on an approximate plane, and there is provided a means (an electromagnetic coupler)  450  therebetween for coupling the two subcoils, electrically, magnetically, or electromagnetically. Functions and details of the electromagnetic coupler  450  will be described later. 
     Each of the two subcoils  410 A and  410 B is adjusted to be able to receive nuclear magnetic resonance signals from elements that can be excited by the birdcage RF coil  300 , and each subcoil functions as one channel. Signals received by the first subcoils  410 A and  410 B respectively are transferred to the receiver  162 . 
     Since the configurations of the two subcoils  410 A and  410 B are identical, only the configuration of the first subcoil  410 A will be described below as a representative example. The first subcoil  410 A comprises a loop coil unit  420  (a first loop coil unit  420 A) for detecting nuclear magnetic resonance signals (RF magnetic field), a low-input impedance signal processing circuit  430  (a first low-input impedance signal processing circuit  430 A), and an electromagnetic coupling adjuster  441  (a first electromagnetic coupling adjuster  441 A) for connecting the loop coil unit  420  with the low-input impedance signal processing circuit  430 , and the first subcoil  410 A is connected to the receiver  162  via the low-input impedance signal processing circuit  430 . The electromagnetic coupling adjuster  441  may comprise at least either of a capacitor and an inductor. 
     A loop section of the first loop coil unit  420 A (the first loop  421 A) is formed of conductor. Then, the first loop coil unit  420 A is provided with a capacitor  424 A that is inserted in series with an inductor component of the first loop  421 A. This inductor component and the capacitor  424 A constitute a parallel resonant circuit. In order to distinguish the capacitor  424 A from other capacitors, it is referred to as a parallel capacitor  424  (a first parallel capacitor  424 A). 
     In addition, a capacitor  422 A for adjusting a resonance frequency and the transmit-receive magnetic coupling prevention circuit  220  are inserted in series with the first loop  421 A. In order to distinguish the capacitor  422 A from other capacitors, it is referred to as a series capacitor  422  (a first series capacitor  422 A). Here, there will be described an example where two first series capacitors ( 422 A) are provided, but any number at least one may be applicable as the number of the first series capacitors. 
     As described so far, the first subcoil  410 A is provided with, as circuit components for adjustment, the first electromagnetic coupling adjuster  441 A, the first series capacitors  422 A inserted in series with the inductor component of the first loop  421 A, and the first parallel capacitor  424 A that is inserted in series with the inductor component, rendering the first loop coil unit  420 A to serve as the parallel resonant circuit. Similarly, the second subcoil  410 B is provided with, as circuit components for adjustment, a second electromagnetic coupling adjuster  441 B, a second series capacitors  422 B inserted in series with the inductor component of a second loop  421 B, and a second parallel capacitor  424 B that is inserted in series with the inductor component, rendering the second loop coil unit  420 B to serve as the parallel resonant circuit. 
     One terminal of the low-input impedance signal processing circuit  430 , provided on the loop coil unit  420  side, is connected to one end of the parallel capacitor  424  of the loop coil unit  420  via the electromagnetic coupling adjuster  441 . The other terminal of the low-input impedance signal processing circuit  430 , provided on the loop coil unit  420  side, is connected directly to the other end of the parallel capacitor  424  of the loop coil unit  420 . 
     The transmit-receive magnetic coupling prevention circuit  220  removes magnetic coupling with the birdcage RF coil  300  being the transmit RF coil  151 . By way of example, as shown in  FIG. 5( b ) , the transmit-receive magnetic coupling prevention circuit  220  may comprise a capacitor  423  that is inserted in series with the conductor constituting the loop  421 , the PIN diode  221  connected in parallel with the capacitor  423 , and the inductor  222 . Control signal lines  223  may be connected to both ends of the PIN diode  221 . Furthermore, the control signal lines  223  may be connected to the magnetic coupling prevention circuit driver  180 . Preferably, a choke coil (not illustrated) may be inserted into the control signal line  223 , so as to avoid mixture of high frequencies. The inductor  222  and the capacitor  423  are adjusted so that they resonate in parallel at a frequency of the received nuclear magnetic resonance signals. 
     Generally, the parallel resonant circuit has characteristics of becoming high impedance (high resistance) at the resonance frequency. Therefore, when the current passes through the PIN diode  221 , the PIN diode  221  is turned on, and the capacitor  423  of the loop  421  becomes resonant in parallel with the inductor  222  at the frequency of the received nuclear magnetic resonance signals, resulting in that the capacitor  423  becomes high impedance. Therefore, at the frequency of thus received nuclear magnetic resonance signals, a part of the loop coil unit  420  becomes high impedance, turned to be in an open state, and then, the subcoil  410  having the loop coil unit  420  is also turned to be in the open state. 
     As thus described, current passes through the PIN diode  221  and it is turned on, whereby the magnetic coupling between each of the subcoils  410 A and  410 B, and the birdcage RF coil  300  is removed. Accordingly, the magnetic coupling is also removed between the array coil  400  comprising the subcoils  410  each functioning as a coil element, and the birdcage RF coil  300 . 
       FIG. 5( a )  illustrates as an example, one transmit-receive magnetic coupling prevention circuit  220  is inserted into the subcoil  410 , but the number of the transmit-receive magnetic coupling prevention circuits  220  to be inserted into the subcoil  410  is not limited to one. Two or more transmit-receive magnetic coupling prevention circuits may be inserted into each of the loops  421 . By inserting a plurality of the transmit-receive magnetic coupling prevention circuits, the magnetic coupling between the transmit RF coil  151  and the receiver RF coil  161  can be lowered to a sufficient level. 
     The configuration of the transmit-receive magnetic coupling prevention circuit  220  is not limited to the above-mentioned configuration. As illustrated by a modification example of the transmit-receive magnetic coupling prevention circuit  220   m  in  FIG. 5( c ) , a cross diode  221   m  may be employed instead of the PIN diode  221 . With this configuration, when large signals pass through the conductor constituting the loop  421 , the cross diode  221   m  is turned on, and the capacitor  423  of the loop  421  and the inductor  222  realize parallel resonance at the frequency of the received nuclear magnetic resonance signals, and then high impedance occurs. In this case, the magnetic coupling prevention circuit driver  180  is not necessary. 
     In the array coil  400  of the present embodiment, values of the inductance and capacitance of the circuit components for adjustment included in each of the subcoils  410 A and  410 B, and values of the inductance and capacitance provided by the electromagnetic coupler  450  are adjusted, whereby each of the subcoils  410 A and  410 B is adjusted so that they can receive the nuclear magnetic resonance signals, and the electric fields generated between the subcoils intensify each other, according to the current passing through each coil upon receipt of signals by the subcoils. 
     Specifically, the first subcoil  410 A is arranged in a manner that, upon receipt of signals, the first current (induced current) based on the signals detected by first subcoil  410 A passes therethrough, and a part of the first current (the first sub-current) passes through the second subcoil  410 B, in the opposite direction of the current in the first subcoil, thereby intentionally guiding the current to flow in the other coil loop unit. Simultaneously, the second subcoil is arranged in a manner that, upon receipt of signals, the second current (induced current) based on the signals detected by the second subcoil  410 B passes therethrough, and a part of the second current (the second sub-current) passes through the first subcoil  410 A, in the opposite direction of the current in the second subcoil, thereby intentionally guiding the current to flow in the other coil loop unit. 
     Specific examples of the adjustment method and electromagnetic coupling means will be described in the following embodiments of the RF coil. There will now be described that when induced current passes through one subcoil, a part of the induced current passing through the subcoil also passes through the other subcoil in the form of sub-current, and thereby noise is reduced. 
     Noise generated from each channel of the receiver RF coil in the MRI apparatus is mainly thermal noise, caused by the subject, and it is Gauss type noise detected by electric field coupling between the coils and subject. This type of noise is increased according to the number of channels, when the channels are combined in reconstructing an image. Furthermore, in the array coil, multiple subcoils are placed adjacent to each other, covering the subject, and thus electric field distributions of the subcoils are likely to be analogous to each other. Accordingly, correlation may occur between the detected signals (noise signals). If this correlation (noise correlation) is strong, noise is intensified upon combining images, and an SNR of the composite image is lowered due to the thus intensified noise. Therefore, it is preferable that the noise correlation should be lower. The noise correlation Ψ between the RF coils is obtained according to the following formulas 1 and 2: 
                     R   ij     =     σ   ⁢       ∫   V             ⁢             E   -&gt;     i     ⁡     (     x   ,   y   ,   z     )       ·         E   -&gt;     j   *     ⁡     (     x   ,   y   ,   z     )         ⁢   dV                 (   1   )                 Ψ   ij     =       R   ij           R   ii   *     ⁢     R   jj                   (   2   )               
where σ is electric conductivity of the subject, V is volume of the subject, Ei is the electric field (complex number) generated by the i-th coil, Ej is the electric field (complex number) generated by the j-th coil. E* represents complex conjugate of E. In other words, the magnitude of noise correlation is determined by volume integral of an inner product of the electric fields that are generated, respectively by the coils within the subject. By reducing the inner product Rij of the electric fields, the noise correlation can be controlled to be low, thereby preventing deterioration of the SNR due to the noise correlation.
 
     With reference to  FIG. 6 , there will be described the electric fields that are generated when induced current passes through each of the first subcoil  410 A and the second subcoil  410 B in the array coil  400  of the present embodiment. In this figure, as one example, the electric field in the y-axis direction is shown, but the electric fields in other axes may also be considered in the same manner. 
       FIG. 6( b )  schematically illustrates an electric field distribution that is generated in the subject  103 , when clockwise current passes through the subcoils  470 A and  470 B in the array coil that is provided with a conventional magnetic coupling elimination means. A fine dot pattern indicates a positive electric field, and a coarse dot pattern indicates a negative electric field (the same is applied to  FIG. 6( a ) ). In the conventional array coil, when the clockwise current passes through the subcoil  470 A, the electric field distribution (Det: ch1) becomes positive on the right side of the subcoil  470 A in the figure, whereas in the left side of the figure, the electric field distribution becomes negative. When the clockwise current passes through the subcoil  470 B (Det: ch2), the electric field distribution becomes positive on the right side of the subcoil  470 B in the figure, whereas in the left side of the figure, the electric field distribution becomes negative. Upon receiving signals by the array coil, the electric field distributions according to those two channels are generated simultaneously. At this time, when the inner product of the electric fields generated in each of the coils is obtained, as shown in the lower row of  FIG. 6( b ) , the electric field becomes negative between the two subcoils  470 A and  470 B, whereas it becomes positive on both sides. 
     On the other hand, in the array coil according to the present embodiment, as shown in  FIG. 6( a ) , when clockwise current passes through the subcoil  410 A, the electric field distribution in the (Det: ch1) is generated by a part of the current (sub-current) passing though the subcoil  410 B, in addition to the electric field generated by the induced current (the first current) passing through the subcoil  410 A. In  FIG. 6( a ) , a region surrounded by a narrow rectangular indicates the electric field based on this sub-current. Similarly, when the clockwise current passes through the subcoil  410 B, the electric field distribution in the (Det: ch2) includes the electric field according to the induced current passing through the subcoil  410 B and the electric field according to the sub-current (a part of the induced current) passing through the subcoil  410 A. Consequently, as shown in the lower row of  FIG. 6( a ) , the inner product of the electric fields becomes negative between the two subcoils  410 A and  410 B, whereas it becomes positive on both sides, but smaller relative to  FIG. 6( b ) . 
     In other words, in the array coil of the present embodiment, a part of the current (sub-current) passing through one subcoil flows in the opposite direction in the other subcoil. Therefore, the electric fields generated between the subcoils have the same sign and thus they intensify each other, whereas the electric fields generated in the regions other than the region above have different signs and thus they weaken each other. With this configuration, the noise correlation between the coils is reduced, and the SNR of the image obtained by combining channels may be improved. 
     When a part of the current passing through one subcoil flows into the other subcoil, the current flows in mutually opposite directions, and thus the current flows in so-called figure-of-eight path. However, the current path is not limited to this shape. By way of example, a part of the figure-of-eight path may be cut off. It is only required to establish a current configuration where the electric fields generated in a living body by the current passing through the subcoils, intensify each other between the subcoils. This configuration enhances flexibility in design, and sensitivity can be improved in the area of interest. 
     Adjustment for generating the part of current (sub-current) in the other subcoil may be performed either in a symmetrical manner or in an unsymmetrical manner. In other words, following adjustments are also applicable; when current flows according to the signals detected by the first subcoil, a part of the current (sub-current) flows into the second subcoil. On the other hand, when current flows according to the signals detected by the second subcoil, no current may pass through the first subcoil, or the amount of current passing through the first subcoil may be different from the amount of the sub-current passing through the second subcoil.  FIG. 5  illustrates an example of the array coil comprising two subcoils, but it is further possible to configure such that the array coil comprises three channels or more, and following adjustments are also applicable; at least one subcoil, and two subcoils placed on both sides thereof are provided, and when current passes through the one subcoil, a part of the current passes through the subcoils on both sides, or the part of the current passes through either one of the subcoils on both sides. 
     Next,  FIG. 7  illustrates a layout example of the array coil  400  of the MRI apparatus according to the present embodiment.  FIG. 7( a )  illustrates an example that the array coil  400  comprising two subcoils  410 A and  410 B is arranged on a plane approximately vertical to the direction of the static magnetic field (Z direction), and the subcoils are arranged in the Y-direction. Furthermore as shown in  FIG. 7( b ) , the array coil  400  may be arranged on a plane at an angle approximately parallel to the direction of static magnetic field. Alternatively, as shown in  FIG. 7( c ) , the first subcoil  410 A may be arranged on the plane vertical to the static magnetic field, and the second subcoil  410 B may be arranged on the plane parallel to the static magnetic field. In addition, even when the array coil is placed on the plane as illustrated in  FIG. 7( a )  or  7 ( b ), the array of the subcoils may be oriented in the X-direction. Such modification of the layout angle enables detection or generation of rotating magnetic field that cannot be provided by the subcoil  410  alone, allowing acquisition of magnetic resonance signals with high sensitivity in the area of interest. 
     In the examples as shown in  FIGS. 7( a ) to 7( c ) , with regard to the distance between the subcoils (the centers of the loop units) and the body axis of the subject  103 , it is preferable that the distance from the two subcoils  410 A and  410 B should be almost equal, in the viewpoint of reduction of noise correlation. As shown in  FIG. 7( b ) , when the subject  103  has an approximately symmetrical shape with respect to the x-axis, it is preferable that the subcoils should also be arranged symmetrically with respect to the X-axis. With this configuration, the subject-form dependence of the coil properties may be reduced. It should be noted that the symmetry axis is not limited to the X-axis. Any axis (direction) having a property of symmetry with respect to the subject may be applicable. 
     A configuration of the MRI apparatus of the present embodiment has been described so far. An imaging method using the MRI apparatus of the present embodiment is similar to operations of conventional MRI apparatuses. For example, according to a pulse sequence selected by the imaging method, RF magnetic field pulses are applied from the transmit RF coil  151  (e.g., the birdcage RF coil  300 ) to the subject  103  that is placed in the static magnetic field space generated by the magnetostatic field magnet  110 , together with applying gradient magnetic field pulses from the gradient magnetic field coil  131 . During the operation of the transmit RF coil  151 , the transmit-receive magnetic coupling prevention circuit  220  is open in the receiver RF coil  161 , to remove magnetic coupling with the receiver RF coil  161 . After a lapse of a predetermined period from the time the RF magnetic field pulses are applied, the receiver RF coil  161  (multichannel coil: array coil  400 ) arranged in proximity to the subject  103  receives nuclear magnetic resonance signals that are generated from nuclei of elements constituting living tissues of the subject  103 . During the receiving operation, the transmit-receive magnetic coupling prevention circuit  210  becomes open, so as to remove the magnetic coupling between the transmit RF coil  151  and the receiver RF coil  161 . 
     The computer (signal processor)  170  processes MR signals received respectively by the subcoils of the receiver RF coil  161 , and if the imaging method thereof is a high-speed imaging method, such as employing a parallel imaging, for example, the computer creates an image of the subject according to the imaging reconstruction method that follows an algorithm of the parallel imaging. It is also possible to obtain an image by subjecting the signals obtained from respective channels to MAC (multi-array coil) synthesis. In creating the image, sensitivity distribution information of each of the subcoils is utilized as appropriate. 
     According to the MRI apparatus of the present embodiment, a multichannel coil having been adjusted in a particular manner is used as the receiver RF coil, whereby noise correlation between the subcoils is reduced, and a high quality image can be obtained. 
     In other words, the MRI apparatus of the present embodiment comprises the static magnetic field former configured to form a static magnetic field, the gradient magnetic field former configured to form a gradient magnetic field, the transmit RF coil configured to irradiate a subject placed in the static magnetic field, with an RF magnetic field, the receiver RF coil configured to detect nuclear magnetic resonance signals from the subject, and the signal processor configured to process the nuclear magnetic resonance signals detected by the receiver RF coil. 
     The receiver RF coil comprises a first subsoil having a first loop coil unit made of a conductor, being capable of receiving nuclear magnetic resonance signals from the subject, a second subsoil having a second loop coil unit made of a conductor, being capable of receiving nuclear magnetic resonance signals from the subject, and an electromagnetic coupler placed between the first subcoil and the second subcoil, for coupling the first subcoil and the second subcoil electromagnetically, wherein adjustments are made in a manner that a part of the first current passing through the first loop coil unit according to signals detected by the first loop coil unit, is made to flow into the second loop coil unit in the form of the first sub-current, and an electric field generated in the subject by the first current and an electric field generated in the subject by the first sub-current intensify each other in the space between the first loop coil unit and the second loop coil unit. 
     Specifically, at least one of the first subcoil and the second subcoil further comprises a magnetic coupling adjuster connecting the loop coil unit of the subcoil with a low-impedance signal processing circuit that is connected to the subcoil, wherein the magnetic coupling adjuster further comprises at least either of a capacitor and an inductor as an adjusting circuit component, and the loop coil unit comprises a series capacitor that is inserted in series with an inductor component thereof, and a parallel capacitor inserted in series with the inductor component to render the loop coil unit to serve as a parallel resonant circuit, and adjustment of at least one of the subcoils is performed by adjusting values of the adjusting circuit component, the series capacitor, and the parallel capacitor. 
     The array coil  400  of the present embodiment, which is arranged and adjusted as described above, is tuned to the magnetic resonance frequency f0. Upon signal reception, signals detected by the first loop coil unit  420 A pass through the second subcoil  410 B, thus forming current that flows in a figure-of-eight path, and the electric fields generated between the loops intensify each other. With this configuration, noise correlation between the first subcoil  410 A and the second subcoil  410 B is lowered, and accordingly, the SNR of a composite image can be increased. 
     As discussed so far, according to the array coil  400  of the present embodiment, low noise can be achieved in the form of multichannel. In addition, adjustments of the arrangement and values of the circuit components enable implementation of such multichannel and low noise. Therefore, this may prevent increase in complexity of the configuration. Furthermore, by using this array coil  400  as the receiver RF coil  161 , the MRI apparatus of the present embodiment is allowed to obtain a high-quality image targeted to a wide region. 
     The MRI apparatus of the present invention features in using the receiver RF coil, particularly adjusted, and the configuration other than this feature may be modified variously. By way of example, in the present embodiment some of the components that are shown in  FIG. 2  may be excluded, or any component not shown in  FIG. 2  may be added. Furthermore, in the aforementioned embodiment, descriptions have been made for the MRI apparatus of the horizontal magnetic field system, but the present embodiment may also be applicable to the MRI apparatus of the vertical magnetic field system. 
       FIG. 8  illustrates an arrangement example where the multichannel array coil is employed in the MRI apparatus  101  of the vertical magnetic field system.  FIG. 8( a )  illustrates an example where two subcoils  410  constituting the array coil  400  are arranged on a plane approximately vertical to the direction of the static magnetic field, and  FIG. 8( b )  illustrates an example where one of the subcoils is placed on the plane vertical to the direction of the static magnetic field, and the other subcoil is placed on a plane parallel to the direction of the static magnetic field.  FIG. 8( c )  illustrates an example where a multichannel array coil  800  using multiple surface coils  810  is placed. In the MRI apparatus of the vertical magnetic field system, when the RF coil is placed on the XY plane, a positional relation of the RF coil to the magnetic field direction is vertical. Usually, this relationship makes it hard to efficiently acquire revolving field signals generated on the XY plane, and a ratio of noise is likely to be larger. Therefore conventionally, array coils have not been employed, because the ratio of noise increase has been still large, even though the coil placed on the XY plane is configured to have multichannel. However, noise can be reduced according to the present embodiment, and thus also in the MRI apparatus of the vertical magnetic field system, flexibility in designing the array coil can be enhanced, allowing the sensitivity to be improved. Such enhanced flexibility enables simplification of the array coil, and accordingly, it is further possible to design a lightweight array coil. This may take the load off both an operator and an examinee. 
     &lt;Embodiments of RF Coil&gt; 
     Embodiments of the RF coil according to the present invention will now be described. The RF coil of the present invention is a multichannel RF coil that is used as the receiver RF coil of the MRI apparatus, and it is provided with multiple subcoils. Each of the multiple subcoils is capable of receiving nuclear magnetic resonance signals generated in the MRI apparatus, and those subcoils are adjusted so that electric fields generated within the subject by the current passing through each of the subcoils intensify each other between adjacent subcoils. There will be described embodiments of a specific configuration of the coil and an adjusting method for implementing such adjustments as described above. 
     The array coil ( FIG. 5 ) used as the receiver RF coil as described in the foregoing embodiment of the MRI apparatus is one of the embodiments of the RF coil according to the present invention. Operations ( FIG. 6 ) and modification examples as described in the foregoing embodiment may be commonly applied to each of the following embodiments described below. 
     First Embodiment 
     The RF coil of the present embodiment has a configuration similar to the configuration of the array coil  400  as shown in  FIG. 5 , comprising two subcoils; the first subcoil  410 A and the second subcoil  410 B, and further comprising an electromagnetic coupler  450  between the subcoils  410 A and  410 B. The RF coil is adjusted in a manner that a part of the current (sub-current) detected by the first subcoil  410 A passes through the second subcoil  410 B upon receipt of signals by the first subcoil  410 A, and simultaneously, a part of the current (sub-current) detected by the second subcoil  410 B passes through the first subcoil  410 A upon receipt of signals by the second subcoil  410 B. The amount of the sub-current can be adjusted by circuit components provided for the adjustment, being inserted in or connected with the subcoil. The sub-current preferably corresponds to 5% to 30% of the current that is detected by the first subcoil or the second subcoil. With this configuration, the electric fields generated within the subject between the subcoils intensify each other, whereas the electric fields in an area other than this region are weakened, and accordingly, noise correlation can be reduced effectively. 
     The first subcoil  410 A and the second subcoil  410 B may be placed approximately on the same plane. Here, “approximately on the same plane” indicates a relationship that adjacent coil elements are placed approximately on the same plane, and a region other than the coil elements (end portions other than the coil elements) may be bent or curved. It is also possible that the first subcoil  410 A and the second subcoil  410 B are placed in a manner that the coil elements partially overlap one on another. 
     With reference to  FIG. 9 , the RF coil of the present embodiment will be described in detail. In  FIG. 9 , the vertical direction on the figure represents X-axis direction, the horizontal direction on the figure represents Y-axis direction, and the direction orthogonal to the paper plane represents Z-axis direction. Also in the present embodiment, if there is no need to particularly identify components of each of the subcoils  410 , where the subcoils  410  constitute the array coil  400 , the alphabets attached to the end of the reference numerals may be omitted. 
     As shown in  FIG. 9 , in the present embodiment, there will be described an example where the first subcoil  410 A and the second subcoil  410 B are placed in a manner that a plane formed by the loop  421  of the loop coil unit  420  in each subcoil becomes relatively close to a plane that is vertical to the magnetic field direction (Z axis direction). It is to be noted that the loop  421  of the loop coil unit  420  may have any shape, such as the rectangle as illustrated, a polygonal, and a circular shape (including an oval shape). 
     In the present embodiment, the electromagnetic coupler  450  between the two subcoils  410 A and  410 B includes an inductor  451 A on the first subcoil  410 A side and an inductor  451 B on the second subcoil  410 B side, and the two subcoils  410 A and  410 B are coupled according to magnetic coupling. 
     The array coil of the present embodiment employs low-input impedance signal amplifiers  431 A and  431 B, respectively as the first low-impedance signal processing circuit  430 A and the second low-impedance signal processing circuit  430 B. By using the low-input impedance signal amplifier  431 , signals detected by the loop coil unit  420  can be amplified immediately, and thus data with less noise can be acquired. A magnitude of input impedance of the low-input impedance signal amplifier  431  is not limited, but it may be approximately equal to or less than 2Ω. It should be noted that the low-impedance signal processing circuit  430  is not restricted to the low-impedance signal amplifier  431 . 
     The series capacitors  422 , the transmit-receive magnetic coupling prevention circuit  220  (including the capacitor  423 ), and the parallel capacitor  424 , which are inserted in each loop coil unit  420 , are configured the same as those in the array coil illustrated in  FIG. 5 . Also in the present embodiment, three capacitors (such as  422  and  424 ) are inserted in the loop  421  of the loop coil unit  420 , but the number of the capacitors is not limited to this example. It is sufficient if at least one capacitor is inserted. 
     Further in the embodiment as shown in  FIG. 9 , an inductor (referred to as an adjustment inductor, for the purpose of distinguishing from other inductors) is used as the electromagnetic coupling adjuster  441 , but the inductor is not necessarily used as the electromagnetic coupling adjuster  441 . Generally, the parallel capacitor  424  is connected to the electromagnetic coupling adjuster  441  via a conductor. Since the conductor contains an inductor component, even without additional inductor, the parallel capacitor  424 , the electromagnetic coupling adjuster  441 , and the inductor component contained in the conductor connecting therebetween may form a parallel resonant circuit. If a resonance frequency of this parallel resonant circuit can be adjusted according to any method, a capacitor may also serve as the electromagnetic coupling adjuster  441 . Alternatively, a parallel circuit of the capacitor and the inductor may also serve as the electromagnetic coupling adjuster  441 . For ease of explanation, there shall be no inductor component contained in the conductor for connecting the parallel capacitor  424  with the electromagnetic coupling adjuster  441 , in the following description. 
     Next, each of the circuit components in the array coil  400  of the present embodiment will be described. Assuming that the transmit RF coil  151  is constantly in the open state, removal of the magnetic coupling between the transmit RF coil  151  and the receiver RF coil  161  shall not be described in this example. 
     The first subcoil  410 A and the second subcoil  410 B of the array coil  400  according to the present embodiment implement the aforementioned functions, by adjusting values of the inductor  451 A and the inductor  451 B constituting the electromagnetic coupler  450 , the first electromagnetic coupling adjuster  441 A, the second electromagnetic coupling adjuster  441 B, the first series capacitors  422 A, the second series capacitors  422 B, the first parallel capacitor  424 A, and the second parallel capacitor  424 B. 
     Adjustment of each of those circuit components will be described with reference to equivalent circuits of the array coil  400  as illustrated in  FIG. 10 .  FIG. 10( a )  illustrates the equivalent circuit  600  of the array coil  400  according to the present embodiment. The number value in the hundreds digit of the reference numeral representing each component as shown in  FIG. 9  is altered from “4” to “6”, to indicate the corresponding component in  FIG. 10 . By way of example, the inductor is  641  in  FIG. 10  corresponds to the inductor  441  in  FIG. 9 , and the series capacitor  622  in  FIG. 10  corresponds to the capacitor  422  in  FIG. 9 . The capacitor  622 A corresponds to a combination of two of the first series capacitors  422 A, and the capacitor  622 B corresponds to a combination of two of the second series capacitors  422 B. The inductor  621 A indicates the inductor component of the first loop  421 A, and the inductor  621 B indicates the inductor component of the second loop  421 B. Furthermore, values of the inductors and the capacitors being the components, are represented by the reference symbols L and C, respectively, each with a subscript, shown in proximity to the respective components. 
     The value Z11 of the impedance  632 A corresponds to a value of the input impedance of the low-input impedance signal amplifier  431 A used as the first low-impedance signal processing circuit  430 A. The value Z21 of the impedance  632 B corresponds to a value of the input impedance of the low-input impedance signal amplifier  431 B used as the second low-impedance signal processing circuit  430 B. Those impedance values Z11 and Z21 are sufficiently low, and therefore, they shall be considered as 0Ω (short circuits) in the following. 
       FIGS. 10( b ) and 10( c )  illustrate the equivalent circuits showing the state that the first subcoil and the second subcoil as shown in  FIG. 10( a )  are coupled via the electromagnetic coupler  450 . The equivalent circuit  601  as shown in  FIG. 10( b )  indicates the equivalent circuit of the first subcoil  410 A (the first resonator), excluding the low-impedance signal processing circuit  430 A ( 631 A), in the state where the current of the first loop coil unit  420 A passes through the second loop coil unit  420 B, when looked at the first low-impedance signal processing circuit  430 A ( 631 A). The equivalent circuit  602  as shown in  FIG. 10( c )  indicates the equivalent circuit of the second subcoil  410 B (the second resonator), excluding the low-impedance signal processing circuit  430 B ( 631 B), in the state where the current of the second loop coil unit  420 B passes through the first loop coil unit  420 A, when looked at the second low-impedance signal processing circuit  430 B ( 631 B). 
     In those equivalent circuits  601  and  602 , the inductor  451 A of the electromagnetic coupler  450  for the first loop coil unit  420 A, and the inductor  451 B of the electromagnetic coupler  450  for the second loop coil unit  420 B are collectively referred to as a mutual inductance  650 , and M represents a value of the mutual inductance  650 . The magnitude of the mutual inductance  650  is expressed by the following formula 3.
 
 M=k √{square root over (L 11   L   21 )}  (3)
 
where k is a magnetic coupling coefficient, representing a value that indicates a ratio of coupling between the inductor  451 A and the inductor  451 B. The magnetic coupling coefficient k is a value between or equal to 0 and 1. L11 indicates the magnitude of the inductor  451 A of the first subcoil  410 A, and L21 indicates the magnitude of the inductor  451 B. In other words, the mutual inductance M is determined by the magnitude of the inductors, and the magnetic coupling coefficient. Variation of the distance between the inductors  451 B and  451 A enables adjustment of the amount of electromagnetic coupling. Therefore, controlling the magnitude of the inductors and the distance between the subcoils allows adjustment of the amount of the sub-current (a part of the current detected by one subcoil and passing through the other subcoil).
 
     Depending on the arrangement, there may also be mutual inductance according to the inductor component of the first loop coil unit  420 A and the inductor component of the second loop coil unit  420 B. In the example here, for ease of explanation, all those inductor components shall be included within the mutual inductance between the inductor  451 A and the inductor  451 B. 
     Next, there will be described adjustment of the circuit components in the array coil  400  having the aforementioned circuit configuration. A resonance frequency of the series resonant circuit only comprising the parallel capacitor  424 A, the adjustment inductor  441 A, and the first low-impedance signal processing circuit  430 A, is represented by f10. Similarly, a resonance frequency of the series resonant circuit only comprising the parallel capacitor  424 B, the adjustment inductor  441 B, and the second low-impedance signal processing circuit  430 B, is indicated as f20. Further as shown in  FIG. 10( b ) , upon receiving signals, the resonance frequency of the resonator of the first subcoil  410 A (the first resonator) is indicated as f11, when looked at the first low-impedance signal processing circuit  430 A ( 631 A). As shown in  FIG. 10( c ) , the resonance frequency of the resonator of the second subcoil  410 B (the second resonator) is indicated as f21, when looked at the second low-impedance signal processing circuit  430 B ( 631 B). In addition, the frequency of the nuclear magnetic resonance signal to be detected (the nuclear magnetic resonance frequency) is indicated as f0. 
     Each of the circuit components of the array coil  400  according to the present embodiment is adjusted so that each of the aforementioned resonance frequencies satisfies the following formulas 4 to 8. 
     
       
         
           
             
               
                 
                   
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     When each of the circuit components is adjusted according to the formulas 5, 7, both ends of the capacitor  424 A of the first subcoil  410 A do not become highly resistant upon receiving signals. Therefore, this facilitates flowing of the current from the second subcoil  410 B into the first subcoil  410 A. Similarly, when each of the circuit components is adjusted according to the formulas 6 and 8, both ends of the capacitor  424 B of the second subcoil  410 B do not become highly resistant upon receiving signals. Therefore, this facilitates flowing of the current from the first subcoil  410 A into the second subcoil  410 B. 
     As shown in  FIG. 10( b ) , according to the adjustment above, upon receiving signals, the first resonator of the first subcoil  410 A becomes the circuit  601  where the inductor component (L10) of the first loop  421 A and the inductor component (L20) of the second loop  421 B allow current to flow via the inductors  451 A and  451 B of the magnetic coupler  450 . In  FIG. 10( b ) , the value of the inductor  626 A and the value of the inductor  626 B are equal to the values obtained by subtracting the value M of the mutual inductance, respectively from the values L11 and L21 of the inductors  651 A (inductor  451 A) and  651 B ( 451 B). 
     At this time, as shown in  FIG. 10( b ) , when each of the circuit components is adjusted so that magnetic coupling (a function of the magnetic coupler  450 ) allows for example, circulating current I 1  in a clockwise direction to pass through the first loop  421 A, and circulating current I 2  in a counterclockwise direction to pass through the second loop  421 B, resulting in that a current path having a figure-of-eight shape is formed effectively by the loop  421 A and the loop  421 B, and current passes therethrough. In addition, by adjusting each of the circuit components according to the formula 4, the coils are tuned to the magnetic resonance signals, and nuclear magnetic resonance signals can be detected. 
     Similarly, in the circuit  602  as shown in  FIG. 10( c ) , upon receiving signals, the second resonator of the second subcoil  410 B becomes the circuit  602  where the inductor component (L20) of the second loop  421 B and the inductor component (L10) of the first loop  421 A allow current to flow via the inductors  451 A and  451 B of the magnetic coupler  450 . 
     At this time, as shown in  FIG. 10( c ) , when each of the circuit components is adjusted so that magnetic coupling allows, for example, circulating current I 4  in a counterclockwise direction to pass through the first loop  421 A, and circulating current I 3  in a clockwise direction to pass through the second loop  421 B, resulting in that a current path having a figure-of-eight shape is formed effectively by the loop  421 A and the loop  421 B, and current passes therethrough. In addition, by adjusting each of the circuit components according to the formula 4, the coils are tuned to the magnetic resonance signals, and nuclear magnetic resonance signals can be detected. 
     According to the adjustment as described above, each of the subcoils  410  is allowed to receive the nuclear magnetic resonance signals from the subject as a detection target. Forming of the current path as described above may generate the electric fields as shown in  FIG. 6( a ) , whereby noise correlation is reduced. Furthermore, in any of the cases above, a part of the current in one coil is made to flow in the other coil, and thus it is possible to provide various sensitivity distributions for a region to be imaged. Therefore, this allows those coils to function as a multichannel coil. 
     Adjustment Example 1 
     There will now be specifically described a procedure for adjusting each of the circuit components according to the present embodiment. In the present example, description will be made, taking an example where the array coil  400  is adjusted to resonate at 64 MHz (f0=64 MHz), corresponding to the magnetic resonance frequency of hydrogen nucleus in the static magnetic field strength 1.5 T (tesla). 
     As for the shape and the size of the subcoil, in an example, the shape of the loop is rectangle, and the diameter of each of the loops  421 A and  421 B is set to be 100 mm, respectively for the first loop coil unit  420 A and the second loop coil unit  420 B. In addition, the distance between the centers of the two subcoils  410  is set to be 140 mm. 
     As shown in  FIG. 10( b ) , when the first subcoil  410 A is electromagnetically coupled with the second subcoil  410 B, adjustments are made so that current in the clockwise direction passed through the first loop  421 A and current in the counterclockwise direction passes through the second loop  421 B, so as to form the current path having the figure-of-eight shape effectively by the loop  421 A and the loop  421 B. Specifically, it is adjusted in a manner that 10% of the current passing through the first loop  421 A is made to pass through the second loop  421 B. 
     Simultaneously, as shown in  FIG. 10( c ) , when the second subcoil  410 B is electromagnetically coupled with the first subcoil  410 A, adjustments are made so that current in the clockwise direction passes through the second loop  421 B and current in the counterclockwise direction passes through the first loop  421 A, so as to form the current path having the figure-of-eight shape effectively by the loop  421 A and the loop  421 B. Specifically, it is adjusted in a manner that 10% of the current passing through the second loop  421 B is made to pass through the first loop  421 A. 
     For this purpose, firstly, each of the circuit components of the second subcoil  410 B is adjusted. Values of the capacitance C21 of the series capacitor  622 B and the capacitance C24 of the parallel capacitor  624 B are adjusted. Here, those values are adjusted so that the equivalent circuit  602  as shown in  FIG. 10( c )  could resonate at 64 MHz, and impedance across the series circuit of the adjustment inductor  641 B and the parallel capacitor  624 B should become 50Ω. 
     Simultaneously, the value L22 of the adjustment inductor  641 B and the value C24 of the parallel capacitor  624 B are adjusted so that the formula 8 is satisfied. 
     At this time, in order to make the current flow upon coupling as described above, the values of L22 and C24 are determined so that the parallel resonant circuit (referred to as L22C24 resonant circuit) made up of the adjustment inductor  641 B and the parallel capacitor  624 B, could operate as a capacitor. Those values are adjusted on the basis of a principle of the parallel resonant circuit characteristic. The principle of the parallel resonant circuit characteristic will be described later. In the present embodiment, those values are adjusted so that the resonance frequency (f10) of the L22C24 resonant circuit could be a value smaller than f0. By way of example, 51 MHz is used as the value smaller than f0, which is 20% lower. 
     Next, each of the circuit components in the first subcoil  410 A is adjusted. It is assumed that at this stage, the circuit components in the second subcoil  410 B have already been adjusted as described above. 
     In this example, the value C11 of the series capacitor  622 A and the value C14 of the parallel capacitor  624 A are adjusted so that the equivalent circuit  601  as shown in  FIG. 10( b )  could resonate at 64 MHz, and the impedance at the both ends of the series circuit of the inductor  641 A and the parallel capacitor  624 A (C14) should become 50Ω. 
     Simultaneously, the value L12 of the adjustment inductor  641 A and the value C14 of the parallel capacitor  624 A are adjusted in a manner that the formula 7 is satisfied, so that the second subcoil  410 B could be electromagnetically coupled with the first subcoil  410 A. At this time, in order to make the current flow upon coupling as aforementioned, the values of L12 and C14 are determined so that the parallel resonant circuit (L12C14 resonant circuit) made up of the adjustment inductor  641 A and the parallel capacitor  624 A could operate as a capacitor. Those values are adjusted on the basis of a principle of the parallel resonant circuit characteristic. The principle of the parallel resonant circuit characteristic will be described later. In the present embodiment, those values are adjusted so that the resonance frequency (f20) of the L12C14 resonant circuit could become a value smaller than f0. By way of example, 51 MHz is used as the value smaller than f0, which is 20% lower. 
     In order to achieve a current ratio of 10%, a coupling amount of electromagnetic coupling is adjusted by varying the distance between the inductors  451 B and  451 A of the magnetic coupler  450 . In this example, the values of both the inductors  451 B and  451 A are set to 50 nH. 
     Such adjustments of the first subcoil  410 A and the second subcoil  410 B may be repeated several times as required. 
     When 51 MHz is used as the value smaller than f0, the values of the parameters obtained according to the above adjustments, may be as the following, for example; C11=20.2 pF, C14=225 pF, C21=20.2 pF, C24=225 pF, L12=26 nH, and L22=26 nH. 
     With the adjustments as described above, the array coil  400  of the present embodiment resonates at the frequency of the nuclear magnetic resonance, and receives nuclear magnetic resonance signals. In addition, two subcoils  410 A and  410 B form the current flow having the figure-of-eight shape. In other words, as shown in the upper part and the middle part of  FIG. 6( a ) , the electric fields generated between the subcoils are adjusted so that they intensify each other. Noise correlation is obtained by the inner product of those electric fields. Therefore, when the inner product is obtained in each region, as shown in the lower part of  FIG. 6( a ) , in the present embodiment, the region between the coils indicates a large negative value relative to the other regions. In other words, since the noise correlation is obtained by the inner product, it indicates a negative value with respect to the conventional method as shown in  FIG. 6( b ) . Consequently, this means that the noise correlation becomes smaller than the conventional method. 
     Simultaneously in the present embodiment, since the value of the resonance frequency of the L12C14 resonant circuit and that of the L22C24 resonant circuit are lower than f0 by 20%, the L12C14 resonant circuit and the L22C24 resonant circuit are allowed to operate like capacitors, and the first subcoil  410 A and the second subcoil  410 B operate being tuned to each other. Therefore, the sensitivity region is enlarged with higher sensitivity, and consequently, the SNR of the composite image can be improved. 
     [Principle of the Parallel Resonant Circuit Characteristic] 
     Characteristic of the parallel resonant circuit used in the aforementioned example 1 will be described.  FIG. 11( a )  and  FIG. 11( b )  illustrate an operation of the parallel resonant circuit. 
     As shown in  FIG. 11( a ) , in the parallel resonant circuit  500 , the inductor  502 (L) and the capacitor  501 (C) are connected in parallel. The impedance Z at the both ends of the parallel resonant circuit  500  is expressed by the following formula 9, where the frequency of the voltage applied to the parallel resonant circuit  500  is f, and an angular frequency is ω (ω=2πf). 
     
       
         
           
             
               
                 
                   
                     1 
                     Z 
                   
                   = 
                   
                     
                       j 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       ω 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       C 
                     
                     + 
                     
                       1 
                       
                         j 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         ω 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         L 
                       
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     As shown in  FIG. 11( b ) , the impedance Z varies depending on the frequency f being applied, and resonance occurs at a predetermined frequency (referred to as the resonance frequency fR). In other words, the impedance Z at the both ends of the parallel resonant circuit  500  is maximized at the frequency fR. 
     The impedance Z is expressed by the formula 10 at a frequency lower than the resonance frequency fR of the parallel resonant circuit  500  (f&lt;fR), and the parallel resonant circuit  500  operates as an inductive reactance (inductor). 
                   Z   =       1   -       (     f   /     f   R       )     2         j   ⁢           ⁢   2   ⁢   π   ⁢           ⁢   fL               (   10   )               
Here, the value L′ of virtual inductance of the parallel resonant circuit  500  is expressed by the formula 11.
 
     
       
         
           
             
               
                 
                   
                     L 
                     ′ 
                   
                   = 
                   
                     L 
                     
                       1 
                       - 
                       
                         
                           ( 
                           
                             f 
                             / 
                             
                               f 
                               R 
                             
                           
                           ) 
                         
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
     On the other hand, the impedance Z is expressed by the formula 12 at a frequency higher than the resonance frequency fR of the parallel resonant circuit  500  (f&gt;fR), and the parallel resonant circuit  500  operates as a capacitive reactance (capacitor). 
     
       
         
           
             
               
                 
                   Z 
                   = 
                   
                     j 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                     ⁢ 
                     π 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     fC 
                     ⁢ 
                     
                       
                         
                           
                             ( 
                             
                               f 
                               / 
                               
                                 f 
                                 R 
                               
                             
                             ) 
                           
                           2 
                         
                         - 
                         1 
                       
                       
                         
                           ( 
                           
                             f 
                             / 
                             
                               f 
                               R 
                             
                           
                           ) 
                         
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
           
         
       
     
     Here, the value C′ of apparent capacitance of the parallel resonant circuit  500  is expressed by the formula 13. 
     
       
         
           
             
               
                 
                   
                     C 
                     ′ 
                   
                   = 
                   
                     
                       
                         
                           
                             ( 
                             
                               f 
                               / 
                               
                                 f 
                                 R 
                               
                             
                             ) 
                           
                           2 
                         
                         - 
                         1 
                       
                       
                         
                           ( 
                           
                             f 
                             / 
                             
                               f 
                               R 
                             
                           
                           ) 
                         
                         2 
                       
                     
                     ⁢ 
                     C 
                   
                 
               
               
                 
                   ( 
                   13 
                   ) 
                 
               
             
           
         
       
     
     As described above, the parallel resonant circuit  500  operates differently in accordance with the frequency f of the applied voltage, with a boundary of its resonance frequency fR. In the aforementioned example of the adjustments, by using such characteristic of the parallel resonant circuit  500 , the value of the resonance frequency, the values of L12 and C14, and the values of L22 and C24 are determined, so that the L12C14 resonant circuit and the L22C24 resonant circuit could operate as the capacitor. 
     Result of Example 1 
       FIG. 12  illustrates simulation results of imaging a water phantom by the MRI apparatus (1.5 tesla, horizontal magnetic field system), using the array coil (the number of subcoils=2) adjusted as described above.  FIG. 12( a )  shows a result of imaging as a comparative example where a conventional array coil (the number of subcoils=2, and a magnetic coupling remover was provided between the subcoils), and  FIG. 12( b )  is a result of imaging according to the present embodiment. Noise correlations were 0.15 and 0.01, respectively. 
     In addition,  FIG. 12( c )  illustrates a sensitivity profile on the Z-axis after sensitivities of the two subcoils are combined. The solid line indicates the profile of the present embodiment and the broken line indicates the profile of the conventional RF coil (comparative example) where the magnetic coupling has been removed. In the present embodiment, the resonance frequency of the parallel resonant circuit (L22C24 resonant circuit) made up of the adjustment inductor  641 B (L22) and the parallel capacitor  624 B (C24) was adjusted to 51 MHz, which was smaller value than f0. 
     As seen from those results, according to the array coil  400  of the present embodiment configuring a current distribution having a shape like figure-of-eight by electromagnetic coupling, the noise correlation was reduced and the SNR of the resultant composite image was improved. 
     In the present embodiment, both the L12C14 resonant circuit and the L22C24 resonant circuit were set to be the value lower than f0 by 20%, but the value is not limited to this value. It is only required to generate current having the figure-of-eight shape, so as to achieve adjustments for reducing the noise correlation. In addition, the resonance frequency of the L12C14 resonant circuit may be different from the resonance frequency of the L22C24 resonant circuit. Such difference in resonance frequencies may allow deriving of an optimum SNR that is suitable for the shape of the coils. Further in the aforementioned embodiment, 51 MHz was used as a value lower than f0 (64 MHz) for the adjustments of the resonance frequency of the L22C24 resonant circuit, but any other values may be applicable as the resonance frequency of the L22C24 resonant circuit. 
     Adjustment of this value allows adjustment of a phase difference between the first current passing through the first loop coil unit  420 A, and the second current passing through the second loop coil unit  420 B. When the current phase difference θ becomes π/2&lt;θ&lt;3π/2, current having the figure-of-eight shape can be generated. Basically, it is desirable that the phase difference should be π. 
     In the present embodiment, the amount of the sub-current (the current ratio) was adjusted to 10%. However, it is essential only that the amount of the sub-current should allow the noise correlation to change to any value enabling enhancement of the SNR in a targeted imaging area, and thus the amount of the sub-current may be adjusted appropriately, in accordance with the distance between coils or other conditions.  FIG. 13  is a graph showing the relationship between the noise correlation and the current. The solid line indicates the relationship when the distance between coils (distance between the centers of the loop sections of the subcoils) is 140 mm (Example 1), and the dotted line indicates the relationship when the distance is 110 mm. In the case where the distance between coils is 140 mm, the noise correlation becomes 0.2 according to the conventional method, where the current ratio is adjusted to be 0%. On the other hand, when the current ratio is 10%, the noise correlation becomes zero. In the case where the distance between coils is 110 mm, when the current ratio is 0%, the noise correlation becomes 0.4. On the other hand, by increasing the current ratio, the noise correlation is reduced, and when the current ratio is 20%, the noise correlation becomes zero. As thus described, with the variation of the current ratio in accordance with the position and arrangement of the coils, a desired noise correlation can be achieved. Basically, it is desirable that the amount of the sub-current passing through the other subcoil should fall into the range from 5 to 30% of the current that is detected by one subcoil. In addition, the amount of the first sub-current (sub-current passing through the subcoil  410 A) may be equal to or different from that of the second sub-current (sub-current passing through the subcoil  410 B). Such difference may allow deriving of an optimum SNR that is suitable for the shape of the coils. Furthermore, since the noise correlation may vary depending on the shape and the size of the subject and the arrangement of coils, it is preferable to define the position and arrangement of coils and to adjust the values, considering the shape of the subject to some extent. 
     The RF coil (array coil) of the present embodiment is adjusted in a manner that a part of the first current passing through the first loop coil unit according to the signals detected by the first loop coil, flows into the second loop coil unit as the first sub-current, and an electric field generated by the first current within the subject, and an electric field generated by the first sub-current within the subject intensify each other in the space between the first loop coil unit and the second loop coil unit. Simultaneously, the RF coil is also adjusted in a manner that a part of the second current passing through the second loop coil unit according to the signals detected by the second loop coil, flows into the first loop coil unit as the second sub-current, and an electric field generated by the second current within the subject, and an electric field generated by the second sub-current within the subject intensify each other in the space between the first loop coil unit and the second loop coil unit. 
     According to the present embodiment, noise correlation between the subcoils can be reduced. In addition, the first subcoil  410 A and the second subcoil  410 B can operate, being tuned to each other, whereby the sensitivity region is enlarged, allowing acquisition of high sensitivity. Consequently, the SNR of the composite image can be improved. 
     Second Embodiment 
     In the first embodiment and the example 1 thereof, there has been described that, upon receiving signals, the first sub-current being a part of the current of the signals detected by the first subcoil  410 A as one of the two subcoils  410 , passes through the second subcoil  410 B, and the second sub-current being a part of the current of the signals detected by the second subcoil  410 B passes through the first subcoil  410 A. 
     In the present embodiment, adjustments are made in a manner that the current detected by the second subcoil  410 B is prevented from flowing into the first subcoil  410 A. In the present embodiment, the configuration of the array coil  400  is the same as that of the first embodiment, but an adjustment method is different in adjusting values of constituent circuit components (the adjustment inductor  441 , the series capacitors  422 , and the parallel capacitor  424 ). 
     There will now be described the method for adjusting values of the circuit components according to the present embodiment, using the equivalent circuit  610  of the array coil  400  as shown in  FIG. 14 . In the following description,  FIG. 6  and  FIG. 9  are quoted as appropriate. In  FIG. 14 , the capacitors and values thereof, the inductors and values thereof, and the resonance frequency of each circuit are represented by the same reference numerals as those of the equivalent circuit  600  of the first embodiment as shown in  FIG. 10 . 
     In order to achieve both high sensitivity and multichannel, the circuit components constituting the array coil  400  are adjusted so that the following formulas 14 to 17 are satisfied. 
     
       
         
           
             
               
                 
                   
                     f 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     10 
                   
                   = 
                   
                     
                       f 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       11 
                     
                     = 
                     
                       
                         f 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         21 
                       
                       = 
                       
                         f 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         0 
                       
                     
                   
                 
               
               
                 
                   ( 
                   14 
                   ) 
                 
               
             
             
               
                 
                   
                     f 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     20 
                   
                   ≠ 
                   
                     f 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     0 
                   
                 
               
               
                 
                   ( 
                   15 
                   ) 
                 
               
             
             
               
                 
                   
                     1 
                     
                       2 
                       ⁢ 
                       π 
                       ⁢ 
                       
                         
                           
                             L 
                             12 
                           
                           ⁢ 
                           
                             C 
                             14 
                           
                         
                       
                     
                   
                   = 
                   
                     f 
                     0 
                   
                 
               
               
                 
                   ( 
                   16 
                   ) 
                 
               
             
             
               
                 
                   
                     1 
                     
                       2 
                       ⁢ 
                       π 
                       ⁢ 
                       
                         
                           
                             L 
                             22 
                           
                           ⁢ 
                           
                             C 
                             24 
                           
                         
                       
                     
                   
                   ≠ 
                   
                     f 
                     0 
                   
                 
               
               
                 
                   ( 
                   17 
                   ) 
                 
               
             
           
         
       
     
     It should be noted that each the frequencies is defined in the same manner as the first embodiment, and they are deigned as the following. 
     f0: frequency of the nuclear magnetic resonance 
     f10: resonance frequency of the L12C14 resonant circuit 
     f20: resonance frequency of the L22C24 resonant circuit 
     f11: resonance frequency of the first subcoil (the first resonator) 
     f21: resonance frequency of the second subcoil (the second resonator) 
     By adjusting the circuit components according to the formula 17, upon receiving signals, both ends of the capacitor  424 B of the second subcoil  410 B are prevented from being highly resistant, allowing the second subcoil  410 B to be magnetically coupled with the first subcoil  410 A. 
       FIG. 14( b )  illustrates the equivalent circuit  611  of the first subcoil  410 A, in the state that the first loop coil unit  420 A and the second loop coil unit  420 B are magnetically coupled according to the aforementioned adjustment. As illustrated in  FIG. 14( b ) , upon receiving signals, the first loop coil unit  420 A and the second loop coil unit  420 B are magnetically coupled, and according to the signals detected by the first loop coil unit  420 A, current I 1  in a clockwise direction passes through the first loop coil unit  420 A, and simultaneously, a part of the current I 2  passes through the second loop coil unit  420 B in a counterclockwise direction. 
     Furthermore, each of the circuit components is adjusted according to the formula 16, whereby both ends of the capacitor  424 A of the first subcoil  410 A become highly resistant, upon receiving signals, and thus the second subcoil  410 B operates independently. 
       FIG. 14( c )  illustrates the equivalent circuit  612  of the second resonator of the second subcoil  410 B, when the second loop coil unit  420 B operates independently according to the aforementioned adjustments. As illustrated in  FIG. 14( c ) , upon receiving signals, the second resonator of the second subcoil  410 B corresponds to the circuit  612  of the second loop coil unit  420 B alone. Therefore, the current I 3  that flows according to the second loop coil unit  420 B never passes through the first loop coil unit  420 A. 
     Then, each of the circuit components is adjusted according to the formula 14, whereby the resonance frequency f11 of the first resonator and the resonance frequency f22 of the second resonator, upon receiving signals, become equal to the nuclear magnetic resonance frequency f0. This allows the subcoil  410 A and the subcoil  410 B to detect the nuclear magnetic resonance signals. 
     Adjustment Example 2 
     There will be described an example where each of the circuit components is adjusted, setting the magnetic resonance frequency f0 to 64 MHz that corresponds to the nuclear magnetic resonance frequency of hydrogen in the static magnetic field strength of 1.5 T (tesla), in the array coil according to the second embodiment. It is to be noted that the shape and the size of the coil are the same as those of example 1 in the first embodiment. 
     Firstly, as in the case of the first embodiment (example 1), each of the circuit components is adjusted so that the equivalent circuits  611  and  612  as shown in  FIGS. 14( b ) and 14( c )  respectively, resonate at 64 MHz, and the impedance at the both ends of the series circuit of the inductor  641 A (L12) and the parallel capacitor  624 A (C14) becomes 50Ω. Then, a value of the adjustment inductor  641  and a value of the parallel capacitor  624  are adjusted on the basis of the principle of the parallel resonant circuit characteristic, so that the aforementioned formulas 14 to 17 should be satisfied and the current upon coupling should flow in a desired manner. 
     Simultaneously, in order to achieve the current ratio of 10%, distance between the inductors  451 B and  451 A of the magnetic coupler  450  is made to vary, thereby adjusting the amount of electromagnetic coupling. It should be noted that the values of the inductors  451 B and  451 A are both 50 nH. 
     In the case of the present example 2, any of the first subcoil  410 A and the second subcoil  410 B may be adjusted first. 
     When the value of 52 MHz, smaller than f0, is used as f20, parameters after the adjustments are made as described above may be for example; C11=20.2 pF, C14=225 pF, C21=21 pF, C24=231 pF, L12=13.6 nH, L22=26 nH. 
     With the adjustments as described above, the array coil  400  of the present example may resonate at the nuclear magnetic resonance frequency, and the coil is allowed to receive nuclear magnetic resonance signals. 
     In the present example, the value of 52 MHz is used as the resonance frequency f20 of the L22C24 resonant circuit, when each of the circuit components is adjusted, but it is not limited to this example. Furthermore, a magnitude of difference between the resonant frequency f20 of the resonant circuit and f0 is not limited either, but it is desirable that the difference between the resonant frequency f20 of the L22C24 resonant circuit and f0 should be 10% or more. Furthermore, the current ratio of the sub-current is not limited to 10%, and as described in  FIG. 13 , the current ratio may be adjusted appropriately within the range from 5 to 30%, for instance, in accordance with the distance between the coils. 
     Further in the present example, there has been described that adjustments are made so that the current detected by the second coil is prevented from flowing into the first subcoil  410 A, but it may be adjusted vice versa. In other words, the adjustments may also be made so that the current detected by the first subcoil  410 A is prevented from flowing into the second subcoil  410 B. 
     As described above, in the array coil of the present embodiment, the second subcoil  410 B operates independently, whereas the first subcoil  410 A is formed in a manner that a part of the current of signals detected by the first subcoil  410 A is made to flow into the second subcoil  410 B in the opposite direction. In other words, as shown in  FIG. 6( c ) , the electric fields generated between the subcoils are adjusted so that they intensify each other. Noise correlation is obtained by the inner product of those electric fields. Therefore, when the inner product is obtained in each region, the region between the coils indicates a large negative value, relative to the other regions in the present embodiment as shown in the lower part of  FIG. 6( c ) . In other words, the noise correlation is obtained by an integral of those values, and thus the noise correlation indicates a lower value with respect to the conventional method. Consequently, the noise correlation becomes smaller, and thereby improving the SNR of the composite image. 
     In addition, sensitivity distributions in the imaging areas in the subcoils are different from each other. Therefore, the array coil  400  of the present embodiment is allowed to operate as an array coil with a high sensitivity without reducing the number of channels. This configuration achieves an array coil with coils that provide a wide sensitivity region, even in a multichannel structure, and therefore, the array coil  400  of the present embodiment can implement all of the features, i.e., multichannel, wide sensitivity region, and low noise. In addition, since this configuration can be implemented by the arrangement of the subcoils  410  and adjustment of values of the circuit components, the structure thereof will never be complicated. 
     The first and second embodiments of the RF coil according to the present invention, and the examples thereof have been described so far. However, the RF coil of each embodiment is not limited to those in the figures and the aforementioned descriptions, and it may be modified variously. Representative modification examples will be described in the following. 
     &lt;Modification Example of the Electromagnetic Coupler&gt; 
     In the first embodiment, the electromagnetic coupler  450  comprises the inductor  451 A and the inductor  451 B, and the amount of coupling between the two subcoils is adjusted according to a positional relationship between those inductors. In the modification example, a magnetic coupling means may be employed as the electromagnetic coupler  450 , instead of the pair of the inductors. 
       FIG. 15  shows a configuration of the array coil  400  according to the modification example. As illustrated, this array coil  400  is arranged in a manner that the first loop coil unit  420 A is magnetically coupled with the second loop coil unit  420 B, and the amount of coupling is adjusted. In order to adjust the amount of coupling according to the magnetic coupling means, a coupling inductor  451  (not illustrated in  FIG. 15 ) may be arranged as a part of each of the loops  421 , for example, and the magnetic coupling may be adjusted by using this coupling inductor. Alternatively, the inductor  451  may be installed on one subcoil only, out of the two subcoils. 
     Such coupling as described above may reduce the component count of the array coil, thereby holding down the cost in producing the array coil. In addition, since a loss generated in the inductors may also be reduced, the sensitivity is enhanced. 
     Furthermore, as shown in  FIG. 16 , for example, the two subcoils may be electrically coupled via a shared capacitor  452 . The amount of coupling may be adjusted with the adjustment of the distance between coils and the value of the capacitor  452 . 
     Such coupling as described above may reduce the component count of the array coil, thereby holding down the cost in producing the array coil. Furthermore, this configuration enables adjustments that are difficult with the use of the inductors. 
     Also in this modification example, as in the case of the first embodiment, the circuit components are adjusted so that each of the subcoils is allowed to receive nuclear magnetic resonance signals, and in addition, the circuit components are adjusted in a manner that a part of the current detected by one subcoil flows into the other subcoil in the form of sub-current, flowing in the opposite direction. 
     &lt;Modification Example of Each Subcoil in Terms of its Shape and Others&gt; 
     In the first and the second embodiments, there are described the cases where the first loop  421 A and the second loop  421 B of the same size and of the same shape are used. However, those loops may be different in shape and/or in size. Using the loops  421 , different in shape and size may increase flexibility in arrangement patterns. Reduction of constraints in shape and/or in size of the loops  421  may facilitate adjustment of the magnitude of current that flows from one loop to the other loop, thereby reducing noise. 
     Subcoils in various shapes may be combined, thereby achieving an optimum coil that is suitable for the subject  103 . There is a possibility that a current flow from one subcoil into the other subcoil increases mutual interference between the coils and accordingly, sensitivity may be lowered, but such desensitization can be controlled by using the subcoils in different sizes. 
     In the first and the second embodiments, there have been described examples where the shape of the loop  421  of each subcoil  410  is a rectangle on an approximate plane, or a single loop having a circular shape. However, the shape of the loop  421  is not limited to those examples as described above. Any shape is applicable as far as an equivalent circuit of the loop is comparable to the equivalent circuit  600 . 
     By way of example, each of the first loop constituting the first subcoil and the second loop constituting the second subcoil may have a butterfly shape, placed on approximately the same plane.  FIG. 17  illustrates an example of the array coil (butterfly array coil)  700  provided with the butterfly loops. 
     This array coil  700  is provided with the first subcoil  710 A and the second subcoil  710 B, each having the butterfly shape. The loop coil unit  720 A constituting the first subcoil  710 A has two loop sections  721 A and  725 A, and the loop coil unit  720 B constituting the second subcoil  710 B has two loop sections  721 B and  725 B. The parallel capacitors  724 A and  724 B are respectively inserted into one of the two loop sections, i.e., the loop sections  721 A and  721 B in the figure, and both ends of each of those loop sections are connected to the low-impedance signal processing circuit  730  in parallel. The series capacitors  722 A and  722 B, functioning as adjustment components, are inserted respectively into the loop sections  725 A and  725 B. Any number at least one may be applicable as the number of the capacitors, and it is not limited to the example as illustrated. 
     In addition, in the embodiment as illustrated, a coupler  750  for electromagnetically coupling the two subcoils  710 A and  710 B may be provided for coupling the loop section  721 A with the loop section  721 B. As in the case of either the first embodiment or the second embodiment, the electromagnetic coupler  750  may be the inductor  451  as shown in  FIG. 9 , or any other magnetic coupling means (as shown in  FIG. 15  or  FIG. 16 , for example) may be applicable. 
     Also in this modification example, adjustments of the circuit components, distance of the electromagnetic coupler, and the like, may allow a part (e.g., 5% to 30%) of the current passing through the first subcoil  710 A to flow into the second subcoil  710 B, and the direction of the part of the current flowing into the second subcoil  710 B should be opposite to the direction of the current passing through the first subcoil  710 A. 
     The aforementioned adjustments may be made so that the formulas 4 to 8 or the formulas 14 to 17 are satisfied, as in the case of the first embodiment or the second embodiment. It should be noted that the loop shape and the mutual inductance of the array coil  700  are different from those of the array coil  400 , and thus, values of the parallel capacitor, the series capacitor, and the adjustment inductor may be adjusted accordingly. 
     Similar to the array coil  400 , the butterfly array coil  700  can be represented by the equivalent circuit  600  or  610  as shown in  FIG. 10  or  FIG. 14 , and the operating principles are identical to those of the array coil  400 . In other words, the loops  721 A and  725 A of the first subcoil  710 A operate, being coupled with the loops  721 B and  725 B of the second subcoil  710 B. When each of the circuit components are adjusted so that the formulas 4 to 8 are satisfied, a part of the current passing through the first subcoil  710 A flows into the second subcoil  710 B, and a part of the current passing through the second subcoil  810 B flows into the first subcoil  710 A. When each of the circuit components is adjusted so that the formulas 14 to 17 are satisfied, a part of the current passing through the first subcoil  710 A flows into the second subcoil  710 B, but the second subcoil  710 B operates independently. Alternatively, a part of the current passing through the second subcoil  710 B flows into the first subcoil  710 A, but the first subcoil  710 A operates independently. 
     Also in the butterfly array coil  700  of the modification example, according to the adjustments as described above, each subcoil  710  is provided with sensitivity against nuclear magnetic resonance signals from the detection target. The loop  721 A of the first subcoil  710 A allows the sub-current to pass through the loop  721 B of the second subcoil  710 B via the electromagnetic coupler  750 , and thus the electric fields generated between the coils intensify each other, whereby noise correlation is reduced and the SNR is enhanced. 
     In addition, since the butterfly array coil  700  has the loops  721 ,  725  in the butterfly shape, it is possible to detect magnetic resonance signals not only from the surface of the subject  103 , but also from a region in the depth direction with high sensitivity. 
     In the array coil  700  as shown in  FIG. 17 , loops having the same shape and size are used respectively for the first loop coil unit  720 A and the second loop coil unit  720 B. However, those loops may be different in shape and/or in size. Using the loops different in shape and size respectively for the first subcoil  710 A and the second subcoil  710 B may increase flexibility in arrangement patterns. In addition, this may facilitate adjustment of the magnitude of magnetic coupling. 
     In addition, also in the modification example, any of the applicable modification examples of the first embodiment and the second embodiment may be appropriately combined and employed. 
     Third Embodiment 
     In the first and the second embodiments, there have been described the cases where two subcoils are combined to constitute the array coil. In the present embodiment, three or more subcoils are combined to constitute the array coil. With this configuration, both multiple channels and low noise can be implemented. In addition, this configuration achieves more enhanced or enlarged sensitivity. 
     The MRI apparatus of the present embodiment has basically the same configuration as the MRI apparatus  100  of the first embodiment. There will now be described the present embodiment, focusing on the points that are different from the first embodiment. 
       FIG. 18  illustrates the array coil  800  according to the present embodiment. As illustrated, the array coil  800  of the present embodiment comprises a first subcoil  810 A, a second subcoil  810 B, and a third subcoil  810 C. Those subcoils  810 A,  810 B, and  810 C are arranged in this order, and adjacent subcoils  810  are placed, being connected via electromagnetic couplers  850 . Each of the first, the second, and the third subcoils  810  is connected to the receiver  162  via the low-impedance signal processing circuit  830 , and those subcoils constitute one receive channel. 
     The configurations of each of the first subcoil  810 A and the third subcoil  810 C are the same as the first subcoil  410 A of the first embodiment ( FIG. 9 ). The second subcoil  810 B is the same as the second subcoil  410 B of the first embodiment. Capacitors and inductors of each subcoil are adjusted so that the following formulas 18 to 21 are satisfied, as in the case of the first embodiment. 
     
       
         
           
             
               
                 
                   
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     Definitions of respective frequencies are the same as those of the first embodiment as the following: 
     f0: nuclear magnetic resonance frequency 
     f10: resonance frequency of the L12C14 resonant circuit in the first subcoil; 
     f20: resonance frequency of the L22C24 resonant circuit in the second subcoil; 
     f30: resonance frequency of the L32C34 resonant circuit in the third subcoil; 
     f11: resonance frequency of the first subcoil (the first resonator) 
     f21: resonance frequency of the second subcoil (the second resonator) 
     f31: resonance frequency of the third subcoil (the third resonator) 
     The resonance frequency of the third subcoil indicates the resonance frequency of the third subcoil  810 C (the third resonator) excluding the low-impedance signal processing circuit  830 , looked at the low-impedance signal processing circuit  830  of the third subcoil  810 C. The L32C34 resonant circuit is a parallel resonant circuit of the parallel capacitor  824 C inserted into the loop coil unit  820 C of the third subcoil  810 , and the magnetic coupling adjuster  841 C (inductor  841 C), and the value of the parallel capacitor  824 C is represented by C34, and the value of the inductor  841 C is represented by L32. 
     With those adjustments as described above, the array coil  800  of the present embodiment is adjusted so that a part of the current of the signals detected by the first subcoil  810 A and a part of the current of the signals detected by the second subcoil  810 B respectively are allowed to flow into each other, and a part of the current of the signals detected by the third subcoil  810 C and a part of the current of the signals detected by the second subcoil  810 B are allowed to flow into each other. In addition, each of the subcoils is also adjusted to resonate at the nuclear magnetic resonance frequency f0. 
     For example, the magnetic resonance frequency f0 is set to be the nuclear magnetic resonance frequency of 64 MHz of hydrogen in the static magnetic field strength 1.5 T (tesla). Then, the circuit components of the first subcoil  810 A, the circuit components of the second subcoil  810 B, and the circuit components of the third subcoil  810 C are adjusted so that the L12C14 resonant circuit, the L22C24 resonant circuit, and the L32C34 resonant circuit should not resonate at the nuclear magnetic resonance frequency. In other words, the adjustments are made so that those resonant circuits may not become highly resistant, upon receiving signals at this nuclear magnetic frequency. 
     The loop coil unit  820 A of the first subcoil  810 A allows a part of the signals detected by the subcoil  810 A to flow into the loop coil unit  820 B of the second subcoil  810 B, according to the loop coil unit  820 B of the second subcoil  810 B and the electromagnetic coupler  850 , upon receiving signals. This is because each of the circuit components of the second subcoil  810 B is adjusted according to the aforementioned formula 20, thereby preventing high resistance upon receiving signals. 
     On the other hand, the first loop coil unit  820 A is hardly magnetically coupled with the loop coil unit  820 C of the third subcoil  810 C, nor vice versa. This is because both are separated in some distance away, and coupling is dominant between the adjacent coils. 
     Similarly, the loop coil unit  820 C of the third subcoil  810 C allows a part of the signals detected by the subcoil  810 C to flow into the loop coil unit  820 B of the second subcoil  810 B, according to the loop coil unit  820 B of the second subcoil  810 B and the electromagnetic coupler  850 , upon receiving signals. This is because each of the circuit components of the second subcoil  810 B is adjusted according to the aforementioned formula 20, preventing high resistance upon receiving signals. 
     In addition, as for the second subcoil  810 B, a part of the signal detected by the subcoil  810 B upon receiving signals is allowed to flow into the loop coil unit  820 C of the third subcoil  810 C and into the loop coil unit  820 A of the first subcoil  810 A, respectively according to the two electromagnetic couplers  850  provided in the loop coil unit  820 B. This is because the circuit components in the first subcoil  810 A and the third subcoil  810 C are adjusted respectively according to the formulas 19 and 21, thereby preventing high resistance upon receiving signals. 
     With the configuration above, the array coil  800  of the present embodiment forms a current path having the figure-of-eight shape in the first loop coil unit  820 A and in the second loop coil unit  820 B, upon receiving signals by the first subcoil  810 A. In addition, upon receiving signals by the third subcoil  810 C, a current path having the figure-of-eight shape is formed in the third loop coil unit  820 C and in the second loop coil unit  820 B. As thus described, the subcoils  810  send and receive the current mutually between the adjacent subcoils, so as to form the current paths each having the figure-of-eight shape, and intensify each other the electric fields generated between the coils. Therefore, noise correlation is reduced and the SNR is enhanced. In addition, each of the subcoils  810  resonates at the nuclear magnetic resonance frequency of a detection target. Therefore, each of the subcoils  810  is allowed to acquire nuclear magnetic resonance signals of the detection target. 
     As described so far, according to the receiver RF coil of the present embodiment, a wide sensitivity region and low noise can be achieved, as in the case of the other embodiments. In particular, increasing the number of subcoils may ensure much larger sensitivity region. 
     &lt;Modification Example of the Third Embodiment&gt; 
     The array coil of the present embodiment is not limited to the example as shown in  FIG. 18 , and it may be modified variously. 
     [Modification Example of the Magnetic Coupling] 
     In the present embodiment, each of the circuit components is adjusted in a manner that, upon receiving signals, the first subcoil  810 A and the second subcoil  810 B mutually send and receive current, and the third subcoil  810 C and the second subcoil  810 B mutually send and receive current. 
     However, the pattern of the magnetic coupling is not limited to this configuration. For example, the circuit components may be adjusted so that each of the first subcoil  810 A and the third subcoil  810 C is magnetically coupled with the second subcoil  810 B, and the second subcoil  810 B is made independent, without any coupling. 
     [Modification Example of the Number and Arrangement of the Subcoils] 
     In the present embodiment, there has been described the example where the three subcoils  810 A,  810 B, and  810 C are combined. However, the number of the subcoils  810  is not limited to this number. It is further possible to use four or more subcoils  810  (e.g.,  FIG. 8( c ) ). Increasing the number of subcoils  810  may provide sensitivity in a much wider area. 
     When the number of the subcoils (the number of channels) is increased, the array coil of the example as shown in  FIG. 18  may be extended in the longitudinal direction. Alternatively, like the multichannel coil  900  as shown in  FIG. 19 , for example, the layout direction of the subcoils  910  may be varied. In this case, the subcoil  910 A and the subcoil  910 C positioned on the ends may be or may not be coupled via the electromagnetic coupler  950 . In addition, as a pattern of the magnetic coupling, it may be arranged one-directionally or bi-directionally in the array. 
     Furthermore, in the present embodiment, the coils coupled via the electromagnetic coupler  950  are adjacent to each other, but the configuration is not limited to this example. Another subcoil may be inserted between some of the subcoils, or between all of the subcoils. With this configuration, density of the coil arrangement becomes higher, thereby raising the sensitivity. In addition, flexibility in designing the sensitivity region can be enhanced. 
       FIG. 20  illustrates an example of the multichannel coil where a third subcoil is placed between two subcoils being electromagnetically coupled, and the third subcoil is not electromagnetically coupled therewith. 
     In the multichannel coil  400 - 2  as illustrated, two subcoils  410 A and  410 B are coupled via the electromagnetic coupler  450 , constituting the array coil, and each of the circuit components is adjusted similar to the array coil  400  of the first embodiment or the second embodiment. 
     The third subcoil  410 C is placed between the loop  421 A of the first subcoil  410 A and the loop  421 B of the second subcoil  410 B, and the third subcoil is not magnetically coupled with those two subcoils. Upon receiving signals by the first subcoil  410 A or upon receiving signals by the second subcoil  410 B in the multichannel coil  400 - 2 , that is, upon receiving signals by at least either one of the subcoils, there occurs a flow of current in the shape of figure-of-eight, and this is similar to the array coil  400  of the first embodiment or the second embodiment. 
     As described so far, the two loops  421 A and  421 B being electromagnetically coupled are not necessarily adjacent to each other, and arranging the third sub coil therebetween may enhance a density of the RF coil arrangement. This allows improvement of the sensitivity. 
     In addition, the modification examples as described in the first and the second embodiments may be applicable to the present embodiment. By way of example, the shape of the loop coil unit in each subcoil may be a shape other than the rectangle, and the size and shape may be different as to each subcoil. The electromagnetic coupler  450 , the magnetic coupling adjuster  441  and others, may also be configured variously. 
     DESCRIPTION OF SYMBOLS 
       090 : coordinate system,  100 : MRI apparatus,  101 : MRI apparatus,  102 : table,  103 : subject,  110 : magnet,  111 : magnet,  121 : shim coil,  122 : shim power source,  131 : gradient magnetic field coil,  132 : gradient magnetic field power source,  140 : sequencer,  151 : transmit RF coil,  152 : RF magnetic field generator,  161 : receiver RF coil,  162 : receiver,  170 : computer,  171 : monitor,  180 : magnetic coupling prevention circuit driver,  210 : transmit-receive magnetic coupling prevention circuit,  211 : PIN diode,  212 : control signal line,  220 : transmit-receive magnetic coupling prevention circuit,  220   m : transmit-receive magnetic coupling prevention circuit,  221 : PIN diode,  221   m : cross diode,  222 : inductor,  223 : control signal line,  300 : birdcage RF coil,  301 : linear conductor,  302 : end conductor,  303 : capacitor,  311 : input port,  312 : input port,  400 : array coil,  400 - 1 : array coil,  410 : subcoil,  420 : loop coil unit,  421 : loop,  422 : series capacitor,  423 : capacitor,  424 : parallel capacitor,  430 : low-impedance signal processing circuit,  431 : low-input impedance signal amplifier,  441 : magnetic coupling adjuster,  450 : electromagnetic coupler,  451 : coupling inductor,  462 : coil part,  463 : coil part,  500 : parallel resonant circuit,  501 : capacitor,  502 : inductor,  600 : equivalent circuit,  601 : equivalent circuit,  602 : equivalent circuit,  610 : equivalent circuit,  611 : equivalent circuit,  612 : equivalent circuit,  621 : inductor,  622 : series capacitor,  624 : parallel capacitor,  626 : inductor,  627 : inductor,  632 : impedance,  641 : adjustment inductor,  700 : butterfly array coil,  710 : subcoil,  720 : loop coil unit,  721 : loop,  725 : loop,  730 : low-impedance signal processing circuit,  750 : electromagnetic coupler,  800 : array coil,  810 : subcoil,  820 : loop coil unit,  830 : low-impedance signal processing circuit,  841 : adjustment inductor,  850 : electromagnetic coupler,  900 : array coil,  910 : subcoil,  920 : loop coil unit,  921 : loop,  930 : low-impedance signal processing circuit,  941 : magnetic coupling adjuster,  950 : electromagnetic coupler