Abstract:
ABSTRACT OF THE DISCLOSURE An array coil including at least three conductive elements arranged at predetermined intervals, each of the conductive elements being in the form of a loop, and a plurality of switches that enable the conductive elements to be connected together according to a plurality of connecting patterns.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a division of U.S. Ser. No. 11/808,121 filed Jun. 6, 2007, which is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2006-158808 filed Jun. 7, 2006, the entire contents of both of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an array coil suitably used as the receiving RF coil of a magnetic resonance imaging apparatus. The present invention also relates to a magnetic resonance imaging apparatus provided with the array coil. 
         [0004]    2. Description of the Related Art 
         [0005]    Array coils are in wide use as receiving radio frequency (RF) coils of magnetic resonance imaging apparatuses (MRI apparatuses). An array coil is made by arranging a plurality of element coils. 
         [0006]    Known array coils include a type provided with a large number of element coils and enabling a wide imaging region. When this type of array coil is put to use, the field of view (FOV) may be narrower than the sensitivity region of the entire array coil. In this case, the element coils are partially made effective in such a manner that the size of the actual sensitivity region corresponds to the size of the FOV. 
         [0007]    Jpn. Pat. Appln. KOKAI Publication No. 4-212329 discloses a magnetic resonance imaging apparatus related to the above technology. In the magnetic resonance imaging apparatus of the publication, only the coil assemblies that are used for imaging are selected and made effective. 
         [0008]    Where the sensitivity region required in accordance with the FOV (the sensitivity region will be hereinafter referred to as the “required sensitivity region”) is attained by part of the element coils, the element coils that are made effective are selected from all element coils of the array coil. Then, each of the required sensitivity regions has to overlap each of the sensitivity regions of the selected element coils (which will be hereinafter referred to as “individual sensitivity regions”). Depending upon the positional relationships between the FOV and each element coil, therefore, the number of element coils that have to be selected may be larger than the minimal number of element coils required for forming the actual sensitivity region having the same size as the FOV. Since, in this case, the sensitivity region is larger than the FOV, aliasing artifacts may occur. 
         [0009]    As a method for reducing the number of selected element coils to a value approximately equal to the above-mentioned minimal number, it is thought to employ element coils whose width is decreased with respect to the arrangement direction of the element coils. It is also thought to widen the overlap portion between the adjacent element coils. However, the former method has problems in that the sensitivity is degraded in regions away from the array coil. Likewise, the latter method has problems in that the interference between the element coils is so intense that the SN ratio is degraded. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    Under the circumstances, there has been a demand for providing a high degree of freedom when the sensitivity region is determined or adjusted, without relying on the conventional methods described above. 
         [0011]    According to a first aspect of the present invention, there is provided an array coil comprising: at least three conductive elements arranged at predetermined intervals, each of the conductive elements being in the form of a loop; and a plurality of switches that enable the conductive elements to be connected together according to a plurality of connecting patterns. 
         [0012]    According to a second aspect of the present invention, there is provided an array coil comprising: n conductive elements, wherein one element coil is formed by arranging m of the n conductive elements, m being an integer greater than one, and n being an integer greater than m; and a unit configured to connect the conductive elements according to one of a plurality of connecting patterns, each of the connecting patterns being determined such that at least one element coil is formed. 
         [0013]    According to a third aspect of the present invention, there is provided a magnetic resonance imaging apparatus comprising: an array coil including (i) at least three conductive elements arranged at predetermined intervals, each of the conductive elements being in the form of a loop; and (ii) a plurality of switches that enable the conductive elements to be connected together according to a plurality of connecting patterns; a pattern selecting unit configured to select one of the connecting patterns; a control unit configured to control the switches such that the conductive elements are connected together according to a connecting pattern selected by the pattern selecting unit; and a reconstruction unit configured to reconstruct an image of a subject on the basis of signals which a conductive element group outputs in response to magnetic resonance signals radiating from the subject, the conductive element group being constituted by those conductive elements which are connected together according to the connecting pattern selected by the pattern selecting unit. 
         [0014]    According to a fourth aspect of the present invention, there is provided a magnetic resonance imaging apparatus comprising: an array coil including (i) coil groups in each of which element coils for receiving the magnetic resonance signals are arranged while being shifted from each other, and (2) an effective coil group-providing unit configured to selectively make coil groups effective; a coil group selecting unit configured to select one coil group of the coil groups on the basis of positional relationships between an imaging region and the coil groups; a control unit configured to control the effective coil group-providing unit such that the coil group selected by the coil group selecting unit is made effective; and a reconstruction unit configured to reconstruct an image of a subject on the basis of signals which at least one element coil outputs in response to magnetic resonance signals radiating from the subject, said at least one element coil being included among the element coils of the coil groups made effective by the effective coil group-providing unit. 
         [0015]    According to a fifth aspect of the present invention, there is provided a magnetic resonance imaging apparatus comprising: an array coil including (i) n conductive elements, wherein one element coil is formed by arranging m of the n conductive elements, m being an integer greater than one, and n being an integer greater than m, and a connecting unit configured to connect the conductive elements according to one of a plurality of connecting patterns, each of the connecting patterns being determined such that at least one element coil is formed; and a reconstruction unit configured to reconstruct an image of a subject on the basis of signals which at least one element coil outputs in response to magnetic resonance signals radiating from the subject, said at least one element coil being included among the element coils of coil groups formed by connecting the m conductive elements. 
         [0016]    Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0017]    The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. 
           [0018]      FIG. 1  shows a configuration of a magnetic resonance imaging apparatus (MRI apparatus) according to an embodiment of the present invention. 
           [0019]      FIG. 2  is a circuit diagram of an array coil used in the first embodiment. 
           [0020]      FIG. 3  schematically shows how the element coils shown in  FIG. 2  are arranged. 
           [0021]      FIG. 4  is a flowchart illustrating the processing the main controller shown in  FIG. 1  performs. 
           [0022]      FIG. 5  shows an example of a position where on a top table the array coil shown in  FIG. 2  is provided. 
           [0023]      FIG. 6  shows an example of a first setting table used for selecting an effective channel in the first mode. 
           [0024]      FIG. 7  shows an example of a second setting table used for selecting an effective channel in the second mode. 
           [0025]      FIG. 8  is a circuit diagram of an array coil used in the second embodiment. 
           [0026]      FIG. 9  is a circuit diagram illustrating a detailed configuration of the switch circuit shown in  FIG. 8 . 
           [0027]      FIG. 10  illustrates how the array coil of the second embodiment operates. 
           [0028]      FIG. 11  shows an example of a configuration of a two-dimensional array coil. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]    An embodiment of the invention will be described with reference to the accompanying drawings. 
         [0030]      FIG. 1  is a block diagram illustrating the configuration of a magnetic resonance imaging apparatus (MRI apparatus) according to the embodiment. The MRI apparatus of  FIG. 1  comprises a static field magnet  1 , a gradient coil  2 , a gradient power supply  3 , a bed  4 , a bed controller  5 , RF coil units  6   a,    6   b  and  6   c,  a transmitter  7 , a selecting circuit  8 , a receiver  9  and a computer system  10 . 
         [0031]    The static field magnet  1  is a hollow cylindrical member, and generates a uniform static magnetic field. The static field magnet  1  is, for example, a permanent magnet or a superconducting magnet. 
         [0032]    The gradient coil  2  is also a hollow cylindrical member located inside the static field magnet  1 . The gradient coil  2  is formed of three coils corresponding to three axes X, Y and Z perpendicular to each other. In the gradient coil  2 , the three coils are individually supplied with a current from a gradient power supply  3 , thereby generating gradient magnetic fields having their magnetic field intensities varied along the X, Y and Z axes. Assume here that the Z-axis direction corresponds to, for example, the magnetization direction of the static magnetic field. The gradient magnetic fields along the X, Y and Z axes correspond to, for example, a slice-selecting gradient magnetic field Gs, a phase-encoding gradient magnetic field Ge and a readout gradient magnetic field Gr, respectively. The slice-selecting gradient magnetic field Gs is used to determine an arbitrary imaging section. The phase-encoding gradient magnetic field Ge is used to change the phase of a magnetic resonance signal in accordance with its spatial position. The readout gradient magnetic field Gr is used to change the frequency of a magnetic resonance signal in accordance with its spatial position. 
         [0033]    A subject  100  placed on a top table  4   a  of the bed  4  is inserted into the cavity (imaging space) of the gradient coil  2  along with the bed  4 . The top table  4   a  of the bed  4  is longitudinally and vertically driven by the bed controller  5 . Normally, the bed  4  is positioned with its longitudinal direction set parallel to the axis of the static field magnet  1 . 
         [0034]    RF coil unit  6   a  is made by arranging one or a plurality of coils in a cylindrical case. RF coil unit  6   a  is located inside the gradient coil  2 , and is used to generate a high-frequency magnetic field upon receiving a high-frequency pulse signal from the transmitter  7 . 
         [0035]    RF coil units  6   b  and  6   c  are mounted on the top table  4   a,  built in the top table  4   a,  or attached to the subject  100 . When imaging is performed, they are inserted into the imaging space along with the subject  100 . Array coils are used as RF coil units  6   b  and  6   c.  Each of RF coil units  6   b  and  6   c  includes a plurality of element coils. The element coils of RF coil units  6   b  and  6   c  receive magnetic resonance signals radiating from the subject  100 . Output signals from each element coil are supplied to the selecting circuit  8 . The RF coil units for signal reception are not limited to the RF coil units  6   b  and  6   c  described above, and various types of RF coil units may be used for signal reception. In addition, the RF coil units are not limited to two in number. A single RF coil unit may be used; alternatively, three or more RF coil units may be used. 
         [0036]    The transmitter  7  transmits a high-frequency pulse signal corresponding to the Larmor frequency to RF coil unit  6   a.    
         [0037]    The selecting circuit  8  selects signals from a large number of magnetic resonance signals output from RF coil units  6   a  and  6   c.  The selecting circuit  8  supplies the selected magnetic resonance signals to the receiver  9 . The computer system  10  designates which channel should be selected at a given time. 
         [0038]    The receiver  9  includes processing systems corresponding to a plurality of channels, and each processing system includes an amplifier unit, a phase detection unit and an analog/digital converter unit. Magnetic resonance signals selected by the selecting circuit  8  are supplied to the plural-channel processing systems. The amplifier unit amplifies magnetic resonance signals. The phase detection unit detects the phase of a magnetic resonance signal output from the amplifier unit. The analog/digital converter unit converts a signal output from the phase detection unit to a digital signal. The receiver  9  outputs digital signals produced from each processing system. 
         [0039]    The computer system  10  includes an interface  11 , a data collection unit  12 , a reconstruction unit  13 , a memory  14 , a display unit  15 , an input unit  16  and a main controller  17 . 
         [0040]    The interface  11  is connected to the gradient power supply  3 , bed controller  5 , RF coil unit  6   b,  transmitter  7 , receiver  9 , selecting circuit  8 , etc. The interface  11  permits signals to be exchanged between the structural components described above and the computer system  10 . 
         [0041]    The data collection unit  12  collects digital signals output from the receiver  9 . The data collection unit  12  stores the collected digital signals (i.e., magnetic resonance signal data) in the memory  14 . 
         [0042]    The reconstruction unit  13  performs post-processing such as Fourier transform on the magnetic resonance signal data stored in the memory  14 , thereby acquiring spectrum data or image data corresponding to a desired nuclear spin in the subject  100 . 
         [0043]    The memory  14  stores the magnetic resonance signal data and spectrum data or image data of each subject. 
         [0044]    The display unit  15  displays various information items, such as spectrum data, image data, etc., under the control of the controller  17 . The display unit  15  may be a liquid crystal display, for example. 
         [0045]    The input unit  16  receives various instructions or information items input by an operator. The input unit  16  may be a pointing device (such as a mouse or a track ball), a selective device (such as a mode switch), or an input device (such as a keyboard). 
         [0046]    The main controller  17  includes a CPU, a memory, etc., and controls the entire MRI apparatus of the embodiment. In addition to the functions for realizing known operations of the MRI apparatus, the main controller  17  has two functions. One of the two functions is to automatically select an operation mode of RF coil unit  6   b,  and the other is to automatically select an element coil that should be made effective, from among the element coils of RF coil unit  6   b.    
         [0047]    The configuration of the MRI apparatus of the embodiment has been described. The embodiment is featured by the array coils used as RF coil units  6   b  and  6   c.  Therefore, a detailed description will be given of the array coils used as RF coil units  6   b  and  6   c.    
       FIRST EMBODIMENT 
       [0048]      FIG. 2  shows a circuit configuration of an array coil  200  used in the first embodiment. 
         [0049]    The array coil  200  comprises two coil groups G 1  and G 2 . Coil group G 1  includes four element coils  21 - 1 ,  21 - 2 ,  21 - 3  and  21 - 4 , and coil group G 2  includes four element coils  22 - 1 ,  22 - 2 ,  22 - 3  and  22 - 4 . 
         [0050]    In addition, the array coil  200  comprises: capacitors  23 - 1 ,  23 - 2 ,  23 - 3  and  23 - 4 ; matching circuits  24 - 1 ,  24 - 2 ,  24 - 3  and  24 - 4 ; capacitors  25 - 1 ,  25 - 2 ,  25 - 3  and  25 - 4 ; coils  26 - 1 ,  26 - 2 ,  26 - 3  and  26 - 4 ; PIN diodes  27 - 1 ,  27 - 2 ,  27 - 3 ,  27 - 4 ,  28 - 1 ,  28 - 2 ,  28 - 3  and  28 - 4 ; choke coils  29  and  30 ; capacitors  31 - 1 ,  31 - 2 ,  31 - 3  and  31 - 4 ; matching circuits  32 - 1 ,  32 - 2 ,  32 - 3  and  32 - 4 ; capacitors  33 - 1 ,  33 - 2 ,  33 - 3  and  33 - 4 ; coils  34 - 1 ,  34 - 2 ,  34 - 3  and  34 - 4 ; PIN diodes  35 - 1 ,  35 - 2 ,  35 - 3 ,  35 - 4 ,  36 - 1 ,  36 - 2 ,  36 - 3  and  36 - 4 ; choke coils  37  and  38 ; and preamplifier  39 - 1 ,  39 - 2 ,  39 - 3  and  39 - 4 . 
         [0051]    Among capacitors  23 - 1  to  23 - 4 , matching circuits  24 - 1  to  24 - 4 , capacitors  25 - 1  to  25 - 4 , coils  26 - 1  to  26 - 4  and PIN diodes  27 - 1  to  27 - 4  and  28 - 1  to  28 - 4 , those elements denoted by suffix “-1” are provided for element coil  21 - 1 , those elements denoted by suffix “-2” are provided for element coil  21 - 2 , those elements denoted by suffix “-3” are provided for element coil  21 - 3 , and those elements denoted by suffix “-4” are provided for element coil  21 - 4 . Among capacitors  31 - 1  to  31 - 4 , matching circuits  32 - 1  to  32 - 4 , capacitors  33 - 1  to  33 - 4 , coils  34 - 1  to  34 - 4  and PIN diodes  35 - 1  to  35 - 4  and  36 - 1  to  36 - 4 , those elements denoted by suffix “-1” are provided for element coil  22 - 1 , those elements denoted by suffix “-2” are provided for element coil  22 - 2 , those elements denoted by suffix “-3” are provided for element coil  22 - 3 , and those elements denoted by suffix “-4” are provided for element coil  22 - 4 . 
         [0052]    Element coil  21 - 1  receives a magnetic resonance signal. The magnetic resonance signal received by element coil  21 - 1  is supplied to preamplifier  39 - 1  after passing through matching circuit  24 - 1  and capacitor  25 - 1 . Matching circuit  24 - 1  performs impedance matching between element coil  21 - 1  and preamplifier  39 - 1 . Capacitor  25 - 1  serves to remove DC components from the signals supplied to preamplifier  39 - 1 . 
         [0053]    Capacitor  23 - 1  is inserted in element coil  21 - 1 . One end of coil  26 - 1  is connected to the cathode of PIN diode  27 - 1 . Coil  26 - 1  and PIN diode  27 - 1  are connected in parallel with capacitor  23 - 1 . One end of choke coil  29  is connected to the connection node between coil  26 - 1  and PIN diode  27 - 1 . The cathode of PIN diode  28 - 1  is connected to the connection node between matching circuit  24 - 1  and capacitor  25 - 1 . The anode of PIN diode  28 - 1  is grounded and is also connected to one end of choke coil  30 . A first control signal from the computer system  10  is applied between the other ends of choke coils  29  and  30 . The signal line connected to choke coil  30  is grounded. In addition, the computer system  10  controls the potential of the signal line connected to choke coil  29  to be positive or negative, thereby supplying positive bias or negative bias to array coil  200  as the first control signal. 
         [0054]    As can be understood from  FIG. 2 , the circuits related to elements  21 - 2  to  21 - 4  are similar in configuration to the above-mentioned circuit related to element coil  21 - 1 . Capacitors  25 - 2 ,  25 - 3  and  25 - 4  are connected to preamplifier  39 - 2 ,  39 - 3  and  39 - 4 , respectively. That is, magnetic resonance signals received by element coils  21 - 2  to  21 - 4  are supplied to preamplifier  39 - 2  to  39 - 4 , respectively. Choke coil  29  is connected to the connection node between coil  26 - 2  and PIN diode  27 - 2 , the connection node between coil  26 - 3  and PIN diode  27 - 3 , and the connection node between coil  26 - 4  and PIN diode  27 - 4 . The anodes of PIN diodes  28 - 2 ,  28 - 3  and  28 - 4  are grounded, and are also connected to choke coil  30 . 
         [0055]    The circuit related to element coil  22 - 1  (which is formed by capacitor  31 - 1 , matching circuit  32 - 1 , capacitor  33 - 1 , coil  34 - 1  and PIN diode  35 - 1  and  36 - 1 ) is similar in configuration to the circuit related to element coil  21 - 1 . Likewise, the circuits related to element coils  22 - 2  to  22 - 4  are similar in configuration to the circuit related to element coil  22 - 1 . Capacitors  33 - 1  to  33 - 4  are connected to preamplifier  39 - 1  to  39 - 4 , respectively. Magnetic resonance signals received by element coils  22 - 2  to  22 - 4  are supplied to preamplifiers  39 - 2  to  39 - 4 , respectively. One end of choke coil  37  is connected to the connection node between coil  34 - 1  and PIN diode  35 - 1 , the connection node between coil  34 - 2  and PIN diode  35 - 2 , the connection node between coil  34 - 3  and PIN diode  35 - 3 , and the connection node between coil  34 - 4  and PIN diode  35 - 4 . The anodes of PIN diodes  36 - 1  to  36 - 4  are grounded, and are also connected to one end of choke coil  38 . 
         [0056]    The computer system  10  applies a second control signal between the other ends of choke coils  37  and  38 . The signal line connected to choke coil  38  is grounded. In addition, the computer system  10  controls the potential of the signal line connected to choke coil  37  to be positive or negative, thereby supplying positive bias or negative bias to array coil  200  as the second control signal. 
         [0057]    As can be seen from the above, capacitors  25 - 1  and  33 - 1  are connected to the input terminal of preamplifier  39 - 1 . The length of the transmission line between connection point C 1  (where preamplifier  39 - 1 , capacitor  25 - 1  and capacitor  33 - 1  are connected together) and connection point C 2  (where capacitor  25 - 1  and diode  28 - 1  are connected together) should desirably be λ/4+(λ/2)×r (r being an integer). Also, the length of the transmission line between connection point C 1  and connection point C 3  (where capacitor  33 - 1  and diode  36 - 1  are connected together) should desirably be λ/4+λ/2)×r. This holds true for the input terminals of preamplifiers  39 - 2  to  39 - 4 . Symbol λ denotes a wavelength of a magnetic resonance signal. 
         [0058]    Preamplifiers  39 - 1  to  39 - 4  amplify signals supplied to their input terminals and output the amplified signals. The outputs of preamplifiers  39 - 1  to  39 - 4  are supplied to the selecting circuit  8  as magnetic resonance signals of first channel (ch 1 ) to fourth channel (ch 4 ). 
         [0059]      FIG. 3  schematically shows how the element coils  21 - 1  to  21 - 4  and  22 - 1  to  22 - 4  are arranged. The upper portion of  FIG. 3  is a plan view, and the lower portion thereof is a side view. In the plan view, element coils  22 - 1  to  22 - 4  are indicated by broken lines so as to clearly show the positional differences between element coils  21 - 1  to  21 - 4  and element coils  22 - 1  to  22 - 4 . 
         [0060]    Element coils  21 - 1  to  21 - 4  are arranged in a first direction at predetermined intervals P 1 . The end portions of the adjacent ones of element coils  21 - 1  to  21 - 4  overlap each other. Likewise, element coils  22 - 1  to  22 - 4  are arranged in the first direction at predetermined intervals P 1 . The end portions of the adjacent ones of element coils  22 - 1  to  22 - 4  overlap each other. With this arrangement, coil groups G 1  and G 2  are formed. Coil groups G 1  and G 2  are arranged along the second direction that is perpendicular to the first direction. With respect to the third direction that is perpendicular to both the first and second directions, coil groups G 1  and G 2  correspond in position. With respect to the first direction, the position of coil group G 1  and the position of coil group G 2  differ from each other. Coil groups G 1  and G 2  are shifted from each other by one half of interval P 1 . With this structure, element coils  21 - 1  to  21 - 4  and element coils  22 - 1  to  22 - 4  do not face each other. It should be noted that the arrangement directions and positions of element coils  21 - 1  to  21 - 4  and  22 - 1  to  22 - 4  do not have to strictly satisfy the above-mentioned conditions. They may be arranged in a different way from that mentioned above. 
         [0061]    A description will now be given of an operation of the array coil  200  having the above structure. 
         [0062]    When positive bias is received as the first control signal, PIN diodes  27 - 1  to  27 - 4  and  28 - 1  to  28 - 4  are applied with reverse bias and are therefore in the OFF state. As a result, element coils  21 - 1  to  21 - 4  can receive magnetic resonance signals. 
         [0063]    When negative bias is received as the first control signal, PIN diodes  27 - 1  to  27 - 4  and  28 - 1  to  28 - 4  are applied with forward bias and are therefore in the ON state. As a result, element coils  21 - 1  to  21 - 4  cannot receive magnetic resonance signals. 
         [0064]    When positive bias is received as the second control signal, PIN diodes  35 - 1  to  35 - 4  and  36 - 1  to  36 - 4  are applied with reverse bias and are therefore in the OFF state. As a result, element coils  22 - 1  to  22 - 4  can receive magnetic resonance signals. 
         [0065]    When negative bias is received as the second control signal, PIN diodes  35 - 1  to  35 - 4  and  36 - 1  to  36 - 4  are applied with forward bias and are therefore in the ON state. As a result, element coils  22 - 1  to  22 - 4  cannot receive magnetic resonance signals. 
         [0066]    As can be seen from the above, when positive bias is input as the first control signal and negative bias is input as the second control signal, coil group G 1  is made effective. Conversely, when negative bias is input as the first control signal and positive bias is input as the second control signal, coil group G 2  is made effective. Only the magnetic resonance signals received by the element coils contained in the effective coil group are supplied to preamplifiers  39 - 1  to  39 - 4 . Let us assume that the selecting circuit  8  selects all element coils of each of the coil groups G 1  and G 2 . In this case, the position of the actual sensitivity region provided when coil group G 1  is made effective and the position of the actual sensitivity region provided when coil group G 2  is made effective are shifted from each other by P 1 / 2 . This means that the position of the actual sensitivity region can be varied by a distance shorter than the distance at which the element coils in one coil group are arranged. 
         [0067]    The user can select a mode between the first mode (in which coil group G 1  is made effective) and the second mode (in which coil group G 2  is made effective). Moreover, the user can select a channel that should be made effective, from among the four channels through which magnetic resonance signals from preamplifiers  39 - 1  to  39 - 4  are supplied. In this case, the main controller  17  makes coil group G 1  effective when the user designates the first mode by use of the input unit  16 , and makes coil group G 2  effective when the user designates the second mode. In addition, the main controller  17  controls the selecting circuit  8  to select magnetic resonance signals of the effective channel which the user designates by use of the input unit  16 . It should be noted, however, that the main controller  17  can select a mode and an effective channel automatically, as will be described below. 
         [0068]      FIG. 4  is a flowchart illustrating the processing the main controller  17  performs. 
         [0069]    The coordinates referred to in the description given below are those of a one-dimensional coordinate system in which one end of the top table  4   a  is used as a reference point and which extends in the Z-axis direction. 
         [0070]    In step Sa 1 , the main controller  17  determines end coordinates A 0 . The end coordinates A 0  are coordinates at which element coil  21 - 1  is located, as shown in  FIG. 5 . The end coordinates A 0  may be entered by the user; alternatively, it may be determined based on the position where the array coil  200  is arranged on the top table  4   a.  The position where the array coil  200  is arranged on the top table  4   a  may be detected by a sensor provided on the top table  4   a;  alternatively, it may be detected based on signals element coils  21 - 1  to  21 - 4  and  22 - 1  to  22 - 4  receive. 
         [0071]    In step Sa 2 , the main controller  17  determines center coordinates C 0  of the FOV and width k of the FOV as measured in the Z-axis direction. The FOV is determined in a known way on the basis of the imaging conditions the user designates. 
         [0072]    In step Sa 3 , the main controller  17  determines the minimal value of j that satisfies the formula (1) below and substitutes the minimal value for variable j 1 . 
         [0000]        C 0+ k/ 2 ≦A 0+ P 1/2× j    (1) 
         [0073]    The variable j 1 , thus determined, is the number of the block which is one of the first to ninth blocks B 1 -B 9  shown in  FIG. 5 , which contains the coordinates of the FOV at least partially, and which is located at the same coordinates as that end of the FOV farther from the reference point (the end will be hereinafter referred to as “farther end”). In the example shown in  FIG. 5 , variable j 1  is determined as “7.” This variable j 1  shows that the seventh block B 7  is a block which contains the coordinates of the FOV at least partially (the range indicated by the coordinates will be referred to as “FOV range”) and which is located at the same coordinates as the farther end. It should be noted that the first to ninth blocks (B 1  to B 9 ) are determined by partitioning the region in units of P 1 / 2  from the end of element coil  21 - 1 . 
         [0074]    In step Sa 4 , the main controller  17  determines the maximal value of j that satisfies the formula (2) below and substitutes the maximal value for variable j 2 . 
         [0000]        C 0− k/ 2≧ A 0+ P 1/2× j    (2) 
         [0075]    The variable j 2 , thus determined, is the number of a block which is completely outside the FOV range, and which is located closer to the reference point than the “closer end” of the FOV range. The “closer end” being the end that is closer to the reference point than the “farther end” described above. In the example shown in  FIG. 5 , variable j 2  is determined as “1”, which means that only the first block B 1  is outside the FOV range. 
         [0076]    In step Sa 5 , the main controller  17  confirms whether the value of “j 1 −j 2 ” is odd. The value of “j 1 −j 2 ” corresponds to the number of blocks at least part of which is located inside the FOV range. When the value of “j 1 −j 2 ” is even, the FOV can be covered by using element coils, which are ½ of the value of “j 1 −j 2 .” In this case, the main controller  17  advances from step Sa 5  to step Sa 6  so as to confirm whether variable j 1  is odd. Let us assume that one of element coils  21 - 1  to  21 - 4  of coil group G 1  and one of element coils  22 - 1  to  22 - 4  of coil group G 2  are both located at the same coordinate position as the farther end of the FOV. In this case, when variable j 1  is even, the former element coil includes a smaller portion that is projected out of the FOV range than the latter element coil does. When variable j 1  is odd, the latter element coil includes a smaller portion that is projected out of the FOV range than the former element coil does. Unless variable j 1  is odd, the main controller  17  advances from step Sa 6  to step Sa 9 . If variable j 1  is odd, the main controller  17  advances from step Sa 6  to step Sa 11 . 
         [0077]    Assuming that the value of “j 1 −j 2 ” is odd, the main controller  17  advances from step Sa 5  to step Sa 7 . In step Sa 7 , the main controller  17  compares amount L 1  (in which the block located at the same coordinate position as the farther end is outside the FOV range) with amount L 2  (in which the block located at the same coordinate position as the closer end is outside the FOV range). L 1  and L 2  are calculated by the following formulas (3) and (4): 
         [0000]        L 1=( A 0+ P 1/2× j 1)−( C 0+ k/ 2)   (3) 
         [0000]        L 2=( C 0− k/ 2)−( A 0+ P 1/2× j 2)   (4) 
         [0078]    When L 1  is smaller than L 2 , it is desirable to use an element coil including a small area that is outside the FOV range, from among the element coils located at the same coordinate position as the farther end. Thus, the main controller  17  advances from step Sa 7  to step Sa 6  so as to execute the processing described above. 
         [0079]    When L 1  is not smaller than L 2 , it is desirable to use an element coil including a small area that is outside the FOV range, from among the element coils located at the same coordinate position as the closer end. Thus, the main controller  17  advances from step Sa 7  to step Sa 8  so as to confirm whether variable j 1  is odd. If variable j 1  is odd, the main controller  17  advances from step Sa 8  to step Sa 9 . If variable j 1  is not odd, the main controller  17  advances from step Sa 8  to step Sa 11 . 
         [0080]    Where the control advances to step Sa 9  from either step Sa 6  or step Sa 8 , the main controller  17  selects the first mode. And in step Sa 10 , the main controller  17  selects an effective channel on the basis of the first setting table shown in  FIG. 6 . 
         [0081]    Where the control advances to step Sa 11  from either step Sa 6  or step Sa 8 , the main controller  17  selects the second mode. And in step Sa 10 , the main controller  17  selects an effective channel on the basis of the second setting table shown in  FIG. 7 . 
         [0082]    In the example shown in  FIG. 5 , j 1  is “7” and j 2  is “1”. Since the value of “j 1 −j 2 ” is even and j 1  is odd, the main controller  17  selects the second mode. On the basis of the second setting table shown in  FIG. 7 , channels ch 1 -ch 3  are selected as effective channels. As a result, element coils  22 - 1  to  22 - 3  are made effective. If the first mode is selected in the example shown in  FIG. 5 , the FOV range cannot be covered unless element coils  21 - 1  to  21 - 4  are made effective. As can be seen from this, it is obvious that the selection described above is proper. 
         [0083]    As described above, the present embodiment automatically selects a coil group and element coils in consideration of the positional relationships between each element coil and the FOV and in such a manner that the portion of a selected element coil which is outside the FOV is minimal. Thanks to this feature, magnetic resonance signals can be acquired, with the sensitivity kept at the lowest possible level with respect to regions other than the FOV. 
       SECOND EMBODIMENT 
       [0084]      FIG. 8  shows an array coil  300  used in the second embodiment. 
         [0085]    This array coil  300  contains conductive elements  41 - 49 , switch circuits  50 - 59 , matching circuits  60 - 63 , and preamplifiers  64 - 67 . 
         [0086]    The conductive elements  41 - 49 , each of which is in the form of a loop, are arranged in a line and at the constant intervals. The adjacent conductive elements overlap each other. 
         [0087]    Each of the switch circuits  51 - 58  is located between the adjacent ones of the conductive elements  41 - 49 . Switch circuits  50  and  59  are provided for conductive elements  41  and  49 , respectively. Based on the first and second control signals, switch circuits  51 - 58  connect the two adjacent conductive elements to each other or disconnect them from each other. The first control signal is supplied to switch circuits  50 ,  52 ,  54 ,  56  and  58  and the second control signal is supplied to switch circuits  51 ,  53 ,  55 ,  57  and  59 . 
         [0088]    Matching circuits  60 ,  61 ,  62  and  63  are provided for conductive elements  42 ,  44 ,  46  and  48 , respectively. The matching circuits  60 - 63  perform impedance matching between the element coils (which are formed in the manner described later) and the preamplifiers  64 - 67  by means of the conductive elements  42 ,  44 ,  46  and  48  and the conductive element adjacent thereto. 
         [0089]    The preamplifiers  64 - 67  receive output signals of the matching circuits  60 - 63  and amplify the signals. The outputs of the preamplifiers  64 - 67  are supplied to the selecting circuit  8  as magnetic resonance signals of the first to fourth channels ch 1  to ch 4 . 
         [0090]      FIG. 9  is a circuit diagram showing a detailed configuration of each of the switch circuits  50 - 59 . Since the switch circuits  50 - 59  have the same circuit configuration,  FIG. 9  shows the configuration of only one of them. Although the conductive elements connected or disconnected by the switch circuits  50 - 59  are different, they will be referred to here as the first and second conductive elements  91  and  92 , for the sake of simplicity. In other words, the first conductive element  91  collectively represents the conductive elements  41 - 48  connected or disconnected by the switch circuits  51 - 58 . Likewise, the second conductive element  92  collectively represents the conductive elements  42 - 49  connected or disconnected by the switch circuits  51 - 58 . In switch circuit  50 , conductive element  41  corresponds to the second conductive element  92 , but no conductive element corresponds to the first conductive element  91 . In switch circuit  59 , conductive element  49  corresponds to the first conductive element  91 , but no conductive element corresponds to the second conductive element  92 . 
         [0091]    As shown in  FIG. 9 , switch circuits  50 - 59  contain capacitors  71 - 74 , coils  75 - 78 , PIN diodes  79 - 82 , and choke coils  83 - 86 , respectively. 
         [0092]    Capacitor  71  is inserted in the first conductive element  91 . One end of coil  75  and the cathode of PIN diode  79  are connected to capacitor  71 . Coil  75  and PIN diode  79  are in parallel with capacitor  71 . 
         [0093]    Capacitor  72  is inserted in the second conductive element  92 . One end of coil  76  and the cathode of PIN diode  80  are connected to capacitor  72 . Coil  76  and PIN diode  80  are in parallel with capacitor  72 . 
         [0094]    Capacitors  73  and  74  are inserted between the first conductive element  91  and the second conductive element  92 . Capacitor  71  is located between the node at which capacitor  73  is connected to the first conductive element  91  and the node at which capacitor  74  is connected to the first conductive element  91 . Likewise, capacitor  72  is located between the node at which capacitor  73  is connected to the second conductive element  92  and the node at which capacitor  74  is connected to the second conductive element  92 . One end of coil  77  and the cathode of PIN diode  81  are connected together. Coil  77  and PIN diode  81  are connected in parallel with capacitor  73 . One end of coil  78  and the cathode of PIN diode  82  are connected together. Coil  78  and PIN diode  82  are connected in parallel with capacitor  74 . 
         [0095]    When one of the PIN diodes is ON, a closed loop is formed by (i) a coil connected to the cathode of the PIN diode and (ii) a capacitor connected in parallel with the PIN diode and the coil. The inductance of the coil and the capacitance of the capacitor are determined in such a manner that the closed loop resonates at the frequency of magnetic resonance signals. 
         [0096]    One end of choke coil  83  is connected to the node at which capacitor  73  is connected to the second conductive element  92 . One end of choke coil  84  is connected to the node at which capacitor  74  is connected to the first conductive element  91 . One end of choke coil  85  is connected to the node at which capacitor  74  is connected to the second conductive element  92 . One end of choke coil  86  is connected to the node at which capacitor  73  is connected to the first conductive element  91 . Other end of choke coil  83  is connected to other end of choke coil  84 . Other end of choke coil  85  is connected to other end of choke coil  86 . 
         [0097]    The computer system  10  applies a control signal between the node between the choke coils  83 ,  84  and the node between the choke coils  85 ,  86 . The computer system  10  grounds the signal line connected to choke coils  85  and  86 . In addition, the computer system  10  controls the potential of the signal line connected to choke coils  83  and  84  to be positive or negative, thereby supplying positive bias or negative bias to array coil  300  as the control signal. This control signal serves as a first control signal in switch circuits  50 ,  52 ,  54 ,  56  and  58 , and serves as a second control signal in switch circuits  51 ,  53 ,  55 ,  57  and  59 . 
         [0098]    A description will now be given as to how the array coil having the above configuration operates. 
         [0099]    The computer system  10  outputs a positive bias as one of the first and second control signals, and outputs a negative bias as the other control signal. To be more specific, in the first mode, the main controller  17  controls the interface  11  in such a manner that a positive bias is output as the first control signal and a negative bias is output as the second control signal. In the second mode, the main controller  17  controls the interface  11  in such a manner that a negative bias is output as the first control signal and a positive bias is output as the second control signal. 
         [0100]    When a positive bias is input as the first or second control signal, PIN diodes  79  and  80  are applied with a forward bias, and PIN diodes  81  and  82  are applied with a reverse bias. As a result, PIN diodes  79  and  80  are turned on, and PIN diodes  81  and  82  are turned off. Since signals can flow along paths Pc and Pd shown in  FIG. 9 , the first and second conductive elements  91  and  92  are connected to each other. 
         [0101]    On the other hand, when a negative bias is input as the first or second control signal, PIN diodes  79  and  80  are applied with a reverse bias, and PIN diodes  81  and  82  are applied with a forward bias. As a result, PIN diodes  79  and  80  are turned off, and PIN diodes  81  and  82  are turned on. Since signals can flow along paths Pa and Pb shown in  FIG. 9 , the first and second conductive elements  91  and  92  are disconnected from each other. 
         [0102]    As shown in  FIG. 10 , in the second mode, switch circuits  51 ,  53 ,  55  and  57  connect the adjacent conductive element to each other, and switch circuits  52 ,  54 ,  56  and  58  disconnect the adjacent conductive elements from each other. That is, conductive elements  41  and  42  are connected together, conductive elements  43  and  44  are connected together, conductive elements  45  and  46  are connected together, and conductive elements  47  and  48  are connected together. The magnetic resonance signal received by conductive elements  41  and  42 , that received by conductive elements  43  and  44 , that received by conductive elements  45  and  46 , and that received by conductive elements  47  and  48 , are supplied to the selecting circuit  8  after passing through matching circuits  60 ,  61 ,  62  and  63  and preamplifiers  64 ,  65 ,  66  and  67 , respectively. As can be seen from this, conductive elements  41  and  42  serve as the element coil of the first channel, conductive elements  45  and  46  serve as the element coil of the second channel, conductive elements  43  and  44  serve as the element coil of the third channel, and conductive elements  47  and  48  serve as the element coil of the fourth channel. 
         [0103]    As shown in  FIG. 10 , in the first mode, switch circuits  51 ,  53 ,  55  and  57  disconnect the adjacent conductive element from each other, and switch circuits  52 ,  54 ,  56  and  58  connect the adjacent conductive elements from each other. That is, conductive elements  42  and  43  are connected together, conductive elements  44  and  45  are connected together, conductive elements  46  and  47  are connected together, and conductive elements  48  and  49  are connected together. The magnetic resonance signal received by conductive elements  42  and  43 , that received by conductive elements  44  and  45 , that received by conductive elements  46  and  47 , and that received by conductive elements  48  and  49 , are supplied to the selecting circuit  8  after passing through matching circuits  60 ,  61 ,  62  and  63  and preamplifiers  64 ,  65 ,  66  and  67 , respectively. As can be seen from this, conductive elements  42  and  43  serve as the element coil of the first channel, conductive elements  44  and  45  serve as the element coil of the second channel, conductive elements  46  and  47  serve as the element coil of the third channel, and conductive elements  48  and  49  serve as the element coil of the fourth channel. 
         [0104]    The intervals at which the element coils are arranged in the arrangement direction thereof (i.e., in the direction in which conductive elements  41 - 49  are arranged) are the same irrespective of the control signal being supplied (the first or second control signal), as indicated by P 2  in  FIG. 10 . The element coils of the same channel are shifted by one half of P 2  (=P 2 / 2 ) between the time when the first control signal is input and the time when the second control signal is input. That is, the position of the actual sensitivity region can be varied by a distance shorter than the intervals at which the element coils are arranged. 
         [0105]    The user can select either the first mode or the second mode. In addition, the user can also select which channel should be made effective, from among the four channels through which magnetic resonance signals are supplied. In this case, the main controller  17  selects the mode which the user designates by operating the input unit  16 . Further, the main controller  17  controls the selecting circuit  8  to select magnetic resonance signals of the effective channel which the user designates by use of the input unit  16 . It should be noted, however, that the main controller  17  can select a mode and an effective channel automatically, as in the first embodiment, though “P 2 ” is used in place of “P 1 ” in formulas (1)-(4) in the second embodiment. 
         [0106]    The first and second embodiments described above can be modified in various ways, as will be described below. 
         [0107]    In the first embodiment, the number of element coils included in each of coil groups G 1  and G 2  may be an arbitrary number greater than one. 
         [0108]    In the first embodiment, the number of coil groups may be three or more. Assuming that the number of coil groups is p, the distance by which the two element coils of the same coil group are shifted from each other should desirably be 1/p. However, this is not an indispensable requirement to the present invention, and the distance by which the coil groups should be shifted can be determined in an arbitral manner. 
         [0109]    In the first embodiment, each coil group may be a combination of element coils arranged in two dimensions. 
         [0110]    In the second embodiment, the number of channels may be an arbitrary number greater than one. 
         [0111]    In the second embodiment, each element coil may be made of three or more conductive elements that are connected together. 
         [0112]    In the second embodiment, the conductive elements  41  and  49  located at the ends do not have to be disconnected. In other words, switch circuits  50  and  59  can be omitted, if so desired. 
         [0113]    A two-dimensional array coil can be provided by arranging two or more sets of an internal structure of the array coils  200  and  300  shown in connection with the first and second embodiments.  FIG. 11  shows the configuration of a two-dimensional array coil  400  obtained by arranging three sets of the internal structure of the array coil  300  of the second embodiment. In  FIG. 11 , the elements corresponding to those shown in  FIG. 8  are represented by reference numerals including the suffixes “-1”, “-2” and “-3”. The elements denoted by the same suffix (“-1”, “-2” or “-3”) belong to the same group. 
         [0114]    In the two-dimensional array coil  400 , the direction in which the element coils of one group are arranged is the same as the direction in which the element coils of another group are arranged. To be more specific, the arrangement direction of element coils  41 - 1 ,  42 - 1 ,  43 - 1 , . . . the arrangement direction of element coils  41 - 2 ,  42 - 2 ,  43 - 2 , . . . and the arrangement direction of element coils  41 - 3 ,  42 - 3 ,  43 - 3 , . . . are the same. In general, this direction is the body axis direction of the subject  100  (i.e., the Z axis direction). 
         [0115]    The direction in which the element coils of different groups are arranged is perpendicular to the direction in which the element coils of the same group are arranged. To be more specific, the arrangement direction of element coils  41 - 1 ,  41 - 2 ,  41 - 3  is perpendicular to the arrangement direction of coils  41 - 3 ,  42 - 3 ,  43 - 3 , . . . In general, this direction is the X axis direction. 
         [0116]    With respect to each of the groups, a mode and an effective channel may be selected in the same manner as in the second embodiment. In normal use, however, it is desirable that the mode and effective channel selected for one group be the same as the mode and effective channel selected for another group. 
         [0117]    Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.