Abstract:
A lower cost MRI device with a portable table detachable from the gantry is provided along with a lower cost table equipped with an easy-to-use and easy-to-install RF coil system able to perform high-resolution head and neck imaging and high-speed whole body imaging in both head-first and feet-first modes. This magnetic resonance imaging device has a portable table detachable from the gantry, and the portable table has a docking connector that can be connected to the gantry and a ceiling panel on which an RF coil can be installed to receive nuclear magnetic resonance signals generated by the examinee and to irradiate the examinee with a high-frequency magnetic field. The RF coil is divided into two or more RF coil units, the ceiling panel has a plurality of ceiling panel connectors enabling each RF coil unit to be connected, and each RF coil unit has a connector. Each RF coil unit is arranged in overlapping fashion on the ceiling plate and the ceiling panel connectors and coil connectors are connected without having to establish an electrical connection between RF coil units.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a magnetic resonance imaging device (MRI device), and relates to a radiofrequency coil (RF coil) used on a portable table. 
       BACKGROUND ART 
       [0002]    An MRI device is a device which arranges a subject in a uniform static magnetic field space in a shield room, applies a gradient magnetic field and an exciting high frequency magnetic field, receives a magnetic resonance signal generated by using nuclear magnetic resonance with a high frequency coil (RF reception coil), and images an object to be examined. A range in which simultaneous imaging can be performed is limited to the static magnetic field space at maximum, and a range from which a high image quality can be obtained is limited to a sensitivity range of the RF reception coil. 
         [0003]    The performances demanded for the RF reception coil include an S/N ratio which determines whether image quality is good or not, the sensitivity range which determines a field of view of an image, and a parallel imaging performance required for higher-speed imaging. The parallel imaging is a method in which a reception coil formed by a plurality of coil elements is used and simultaneous signal measurement is performed, so that an imaging time is reduced. A plurality of rectangular or circular loop coil elements are arranged in a certain direction (a body width direction or a body axis direction), and a phase encoding direction is selected in that direction. The parallel imaging is a technique which can reduce the imaging time to 1/(the number of the coil elements arranged in the phase encoding direction) if the arrangement of the coil elements is the most appropriate. Moreover, by arranging a plurality of loop coil elements two-dimensionally (for example, in the body width direction and in the body axis direction), a reception coil can be realized which can perform higher-speed imaging and has the sensitivity area in a wide range such as the whole body. 
         [0004]    The RF reception coil which receives a nuclear magnetic resonance signal can be applied as an RF transmission coil, when an amplifier with a low-output impedance is connected instead of an amplifier for signal detection and amplification with a low-input impedance. Moreover, when a transmission/reception switching circuit is used between the reception amplifier, the transmission amplifier, and the coil, the coil can be used as a coil which can perform both transmission and reception. In the following description, the RF reception coil is described as an RF coil because it can be used not only for reception but also for transmission. 
         [0005]    In a case where the wide range such as the whole body is an object to be examined, a technique for imaging the whole body by moving a table table top is used. In this case, seamless imaging can be performed by using a plurality of loop coil elements arranged two dimensionally as described before as an RF coil having a wide sensitivity range. As the RF coil having the wide sensitivity range, an integrated RF coil unit permanently installed on the table top or incorporated in the table top is known. In such an integrated RF coil unit, a plurality of coil elements forming one or more channels are arranged in one housing case in the most appropriate manner to minimize electromagnetic coupling, and a required element is selected, switched, and used. 
         [0006]    However, since the integrated RF coil is heavy, there is a problem that a lot of labor is required for detachment. Therefore, in Japanese Patent Application Laid-Open No. 2006-14823 (Patent Literature 1), the coil is taken apart, so that the size and the weight thereof are reduced and the coil can be easily moved from a stand to a place for the coil. More specifically, Patent Literature 1 discloses an RF coil used for receiving a high frequency wave. The RF coil includes a plurality of coil units two or more of which are connected along a predetermined direction and which can be detached. Each of the coil units includes at least one type of first coil element having a shape corresponding to various types of imaged regions, defining one channel, and receiving a high frequency wave from the various types of imaged regions; a portion of at least one type of second coil element having a shape corresponding various types of imaged regions, defining one channel, and, when another coil unit is connected to that coil unit, forming the second coil element for receiving a high frequency wave from the various types of imaged regions together with a remaining portion of the second coil element included in the other coil unit; and a joint portion for joining that coil unit along the predetermined direction. Especially, Patent Literature 1 is characteristic in that the coil units are arranged to be connected in straight portions thereof for making the structure of a connection member of the coil units as simple as possible. 
       CITATION LIST 
     Patent Literature 
       [0007]    PTL 1: Japanese Patent Application Laid-Open No. 2006-14823 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0008]    In the MRI device, it is necessary that a subject has to be placed in a narrow tunnel bore for a long time. 
         [0009]    Therefore, a subject having claustrophobia cannot enter therein or a subject may feel fear while the subject placed on a table top is moved into the bore from the head side (Head-First mode) in some cases. For relieving the feeling of fear, a treatment in which the table top is moved such that the subject enters into the bore from the foot side (Feet-First mode) may be performed. Therefore, it is desirable the both Head-First mode imaging and Feet-First mode imaging can be performed. 
         [0010]    As a method for enabling both Head-First mode imaging and Feet-First mode imaging by using an integrated RF coil permanently installed on the table top, which is integrated with the portable table, or an RF coil incorporated in the table top, it is necessary to slide the coil position or change the orientation of the coil. However, there is a problem of a large worker&#39;s burden because the coil is large in size and heavy. 
         [0011]    As a technique for overcoming the heavy weight, a technique of dividing the RF coil in Patent Literature 1 is effective. However, the RF coil disclosed in Patent Literature 1, in which a plurality of detachable coil units are provided and are jointed to one another at joint portions, needs an operation for joining the joint portions to one another in addition to an operation for placing the coil when the RF coil is installed. Thus, there is a problem of operability. 
         [0012]    It is an object of the present invention to provide a table provided with an RF coil system which enables high-resolution imaging of a head and a neck and high-speed imaging of a whole body both in Head-First mode and Feet-First mode in an MRI device having a portable table which can be attached to and detached from a gantry, and which is good in installability and operability, and an MRI device having that table at a relatively low cost. 
       Solution to Problem 
       [0013]    As an aspect for solving the above problems, the present invention provides a magnetic resonance imaging device in which an RF coil thereof includes: a plurality of coil elements which receive a high frequency signal from a subject on different channels, respectively; feed points connected to the coil elements to detect the high frequency signal received by the coil elements, respectively; and a plurality of housings accommodating the coil elements and the feed points, the RF coil is divided into a plurality of units each including the coil element, the feed point, and the housing by dividing portions of the housings, coil elements of adjacent units overlap each other at least partly, and the dividing portion of the housing is provided between the coil elements of the adjacent units. 
       ADVANTAGEOUS EFFECTS OF INVENTION 
       [0014]    The present invention realizes a portable table which can be attached to and detached from a gantry, enables high-resolution imaging of a head and a neck and high-speed imaging of a whole body both in Head-First mode and Feet-First mode, and is provided with an RF coil system with good installability and operability, and an MRI device including that portable table at relatively low cost. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0015]      FIG. 1  is an exemplary structure diagram of an MRI device according to the present invention. 
           [0016]      FIG. 2  shows a first example of the present invention, and shows a portable table and an RF coil installed thereon in a case of Head First mode and a case of Feet First mode. 
           [0017]      FIG. 3  shows the first example of the present invention, and is an appearance view of the portable table and an overlapped portion of RF coil units installed on a table top of the portable table, when seen from the side. 
           [0018]      FIG. 4  is a view showing the inside of the portable table, the RF coil units installed thereon, and the table top in the first example of the present invention. 
           [0019]      FIG. 5  is an appearance view of the portable table and four types of RF coil units installed thereon in the first example of the present invention, when seen from the side. 
           [0020]      FIG. 6  is an appearance view of a table top of a portable table and RF coil units installed thereon or therein in a conventional technique. 
           [0021]      FIG. 7  shows a second example of the present invention, and shows the inside of a portable table, RF coil units installed on a table top of the portable table, and the table top. 
           [0022]      FIG. 8  shows a third example of the present invention, and is a diagram showing a portable table and an exemplary arrangement of RF coil units installed on a table top of the portable table. 
           [0023]      FIG. 9  is a diagram showing an example of division of the RF coil units in the third example of the present invention. 
           [0024]      FIG. 10  is a cross-sectional view showing a connector in the table top of the portable table, a connector of the RF coil unit installed thereon, and a mechanism for coil identification according to the present invention, when seen from the side. 
           [0025]      FIG. 11  shows a fourth example of the present invention, and is a diagram showing that a portion of RF coil units arranged in a matrix has been detached and instead an RF coil exclusive for a region is installed in that space. 
           [0026]      FIG. 12  shows the fourth example of the present invention, and is a diagram showing that a portion of the RF coil units arranged in a matrix has been detached and instead an RF coil exclusive for a local region is installed in that space. 
           [0027]      FIG. 13  is a diagram showing one of a plurality of elements in the RF coil unit and a signal detection portion and a trap circuit therefore. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0028]    Embodiments of the present invention will be described below, referring to  FIGS. 1 to 13 . In the following description, components having the same functions and structures are labeled with the same reference signs and redundant description is omitted. 
       Example 1 
       [0029]    The best mode of the present invention is described below as a first example of the present invention, referring to  FIGS. 1 to 6  and  FIG. 13 . 
         [0030]      FIG. 1  is a diagram showing an entire magnetic resonance imaging device of the present invention, and is an exemplary structure diagram of an MRI device having a portable table  400  which can be attached to and detached from a gantry  100 . As shown in  FIG. 1 , the gantry  100  of the MRI device in the present invention includes a magnet  101  which generates a static magnetic field, a gradient magnetic field generation coil  102  which generates a gradient magnetic field, and a high frequency transmitter coil  103  which generates a high frequency magnetic field. 
         [0031]    The portable table  400  which can be attached to and detached from the gantry  100  by docking connectors  501  and  502  includes a table top  401  which can move in z-axis direction on a frame  402 , and wired composite cable wiring  403 . On or in the table top  401 , an RF coil  300  is provided. A subject  104  is placed on the RF coil  300 , and is arranged in the magnet  101  and the gradient magnetic field generation coil  102  by movement of the table top in z-axis direction. The RF coil  300  is provided with a coil connector  300   c.  The coil connector  300   c  is connected to a table top connector  410  connected to a composite cable in the table top  401 , when the RF coil is placed. 
         [0032]    In general, the gradient magnetic field generation coil  102  is formed by three-axis gradient magnetic field coils of which three axes are perpendicular to one another. Moreover, a sequencer  110  sends a command to a gradient magnetic field power source  107  and a high frequency magnetic field generator  106  to generate a gradient magnetic field and an RF pulse from the gradient magnetic field coil  102  and the high frequency transmitter coil  103 , respectively. The high frequency magnetic field generator  106  is usually formed by an RF pulse generator and a power amplifier which amplifies the RF pulse. The amplified RF pulse output is applied to the subject  104  via the high frequency transmitter coil  103  at a timing in accordance with the command from the sequencer  110 . 
         [0033]    The gradient magnetic field power source  107  also applies a gradient magnetic field to the subject  104  from the respective three-axis gradient magnetic field coils at a timing in accordance with the command from the sequencer  110 . A magnetic resonance signal generated from the subject  104  is received by the RF coil  300 , is amplified in a preamplifier (not shown here) in the RF coil, and is then transmitted to a receiver  105  via the coil connector  300   c,  the table top connector  410 , the cable wiring  403 , and the docking connectors  501  and  502 . In the receiver  105 , appropriate signal processing is performed, A/D conversion (sampling) is performed, and then signal detection is performed. The detected signal is sent to a calculation portion  111 , and is subjected to sampling processing there. Then, digital signal processing such as image reconstruction is performed and the result is displayed on a display portion  109 . 
         [0034]    Digital data or a measurement condition can be stored in a storage medium  108  as needed. The sequencer  110  performs controls in accordance with the measurement condition stored in the storage medium  108  and/or a command received from the calculation portion  111  so that respective devices operate at programmed timings with programmed intensities. Among the programs, those that describe timings of application of the RF pulse, application of the gradient magnetic field, and reception of a nuclear magnetic resonance signal and intensities of the RF pulse and the gradient magnetic field are especially referred to as an imaging sequence. 
         [0035]    A circuit in the RF coil  300  controlled by the imaging sequence is described by using  FIG. 13 .  FIG. 13  illustrates only one of a plurality of channels, and shows a coil element  300   a  provided in the RF coil (used as a reception coil here), a signal detection portion  300   b,  a trap circuit  300   e,  a reception cable  300   d,  the coil connector  300   c,  the table top connector  410  and the docking connector  501  provided in the table top  401 , the docking connector  502  provided in the gantry  100 , the receiver  105 , and an A/D signal processor  105   a  which is in charge of one channel among A/D signal processors respectively provided for a plurality of channels in the receiver  105 . 
         [0036]    In the signal detection portion  300   b,  a matching circuit and a preamplifier  701  are provided, and a power supply cable  702  for receiving power supply is connected to the preamplifier  701 . The trap circuit  300   e  is formed by a capacitor  703 , an inductor  704 , and a diode  705 . At both ends of the diode  705 , a current supply cable  706  for supplying a diode driving current is connected. On the other hand, a table top control portion  200  has functions of controlling movement of the table top of the portable table and identifying the type of the coil connected to the table top, and is also provided with a current supply source  200   a  and a preamplifier driving power source  200   b.    
         [0037]    The current supply source  200   a  and the preamplifier driving power source  200   b  are connected to the current supply cable  706  and the power supply cable  702 , respectively, and, as needed, are assembled into one cable together with the reception cable  300   d  via a relay board within the receiver  105  such that that cable is connected as the composite cable wiring  403  to the diode  705  and the preamplifier  701  via the coil connector  300   c,  the table top connector  410 , and the docking connectors  501  and  502 . The preamplifier  701  operates by the preamplifier driving power source  200   b  and receives a nuclear magnetic resonance signal in accordance with the reception timing of the imaging sequence. 
         [0038]    The reception cable  300   d  amplifies the nuclear magnetic resonance signal received by the coil element  300   a  with the preamplifier  701  in the signal detection portion  300   b  and transmits it to the A/D signal processor  105   a  via the coil connector  300   c,  the table top connector  410 , and the docking connectors  501  and  502 . In the trap circuit  300   e,  the diode  705  is driven by the current supply source  200   a  such that, when the diode is turned ON, a resonance circuit formed by the capacitor  701  and the inductor  704  resonates in series, the impedance seen from both ends of the capacitor  703  toward the coil element  300   a  side becomes large, and the resonance frequency of the coil element  300   a  largely shifts from the resonance frequency of the nuclear magnetic resonance signal. 
         [0039]    On the other hand, when the diode is turned OFF, the trap circuit is not established and the resonance frequency of the coil element  300   a  is coincident with that of the nuclear magnetic resonance signal, so that the coil element  300   a  operates as the RF coil. With the above-described structure, the table top control portion  200  controls the current supply source  200   a  and the preamplifier driving power source  200   b  in accordance with the command from the sequencer  110 . Especially in a case where the RF coil  300  is used as the reception coil, the table top control portion  200  controls turning ON and OFF of the diode  705  in the trap circuit  300   c  in synchronization with the application timing of the RF pulse and the reception timing of the nuclear magnetic resonance signal that are in accordance with the imaging sequence for preventing electromagnetic coupling with the transmitter coil  103 . 
         [0040]    For turning an operation for receiving a nuclear magnetic resonance signal from a coil element located outside a desired imaging range OFF, the power source  200   a  for the diode  705  in the trap circuit  300   c  may be turned ON or the preamplifier power source  200   b  may be turned OFF. Although only a case where the RF coil  300  is the reception coil is described here, in a case where the RF coil  300  is an irradiation coil, this can be applied to the irradiation coil when a transmission/reception switching circuit is inserted between the signal detection portion  300   b  and  300   a,  the current supply cable  706  is connected to the transmission/reception switching circuit, and the table top control portion  200  controls the transmission/reception switching circuit. In this case, the trap circuit  300   e  is not required. 
         [0041]      FIG. 2  are appearance views of the MRI device having the portable table  400  which can be attached to and detached from the gantry  100 , one of which is a view showing coil installation in a case of Head First mode where the table top  401  moves such that a subject (not shown) enters into the gantry  100  from the head side, and the other of which is a view showing coil installation in a case of Feet First mode where the table top  401  moves such that a subject (not shown) enters into the gantry  100  from the feet side. 
         [0042]    The portable table  400  and the gantry  100  are attached to and detached from each other by connection and separation of the docking connectors  501  and  502 . Considering the attachability/detachability and the material cost of the portable table  400  and a wiring process of the cable wiring  403  within the table, it is desirable that the docking connector  501  be provided at only one position on a certain surface of the table top  401  or the frame  402 . 
         [0043]    Therefore, in the RF coil  300 , each of two or more RF coil units (divided into a first RF coil unit  301 , a second RF coil unit  302 , a third RF coil unit  303 , and a fourth RF coil unit  304  here) has an independent coil connector (not shown here), is connected to the table top connector (not shown here) on the table top  401 , and is placed on the table top  401 . Please note that the first RF coil unit  301  is an RF coil unit having sensitivity at the head portion of the subject. In a case of Head First mode, the first RF coil unit  301  is arranged on the table top  401  on the side closer to the gantry  101 . In a case of Feet First mode, the first RF coil unit  301  is arranged on the table top  401  on the side farther from the gantry  101 . Thus, it is possible to provide the docking connector  501  provided in the portable table, at only one position on a certain surface of the table top  401  or the frame  402 . 
         [0044]      FIG. 3  is a diagram showing the RF coil  300  and the table top  401  of the present invention.  FIG. 3  is a side view in a case where the RF coil  300  is divided into a plurality of RF coil units (the first RF coil unit  301 , the second RF coil unit  302 , and the third RF coil unit  303 ) by dividing portions of the housings (positions shown with  312  and  323 ) as described above, and those are placed on the table top  401 . The respective RF coil units are installed such that end portions overlap each other as shown as overlapped portions  312  and  323 . 
         [0045]    The reason why the end portions are arranged to overlap each other is described here. When the distance between two coil elements is relatively large, electromagnetic coupling between the two coil elements can be reduced to a level of practically no problem. However, in a case where the loop dimension of the coil element is large relative to the distance between the two coil elements, magnetic coupling between the coil elements can be removed by appropriately overlapping the adjacent two coil elements (in about 10% of the area). 
         [0046]    As shown in  FIG. 3 , by overlapping the end portions of the adjacent coil units, the coil elements in the adjacent coil units can always overlap each other in the constant area, so that magnetic coupling between the coil elements can be removed. 
         [0047]    The third RF coil unit  303  is placed at an approximately center portion (i.e., in a portion close to the waist of a subject) in the longitudinal direction of the table top  401 , the second RF coil unit  302  is placed next to the third RF coil unit  303 , and the first RF coil unit  301  is placed next to the second RF coil unit  302 . While the adjacent RF coil units are arranged to overlap each other in the end portions thereof, the overlapped portions  323  and  312  are formed so that the end portion of the RF coil unit closer to the center portion is arranged on the lower side. 
         [0048]    Thus, the first RF coil unit  301  (RF coil for the head portion), that is the most frequently attached and detached, is formed such that it is always arranged on the upper side in the overlapped portion. The second RF coil unit  302 , that is attached and detached relatively less frequently, is formed such that the end portion is arranged on the lower side in the overlapped portion  312  and is arranged on the upper side in the overlapped portion  323 . 
         [0049]    The RF coil unit  303  (RF coil arranged at a position around the waist portion of a subject and used for imaging from the abdomen to the thighs) is arranged at the most closest position to the center portion of the table top  401 , and is formed such that the end portion thereof is always on the lower side as in the overlapped portion  323 . The third RF coil unit  303  is an RF coil unit for a portion around the waist, and the position of the third RF coil unit  303  is considered to be almost the same both in a case of Head-First mode and a case of Feet-First mode. Therefore, the third RF coil unit  303  is characteristic that it is the least frequently attached and detached. Moreover, the RF coil unit  303  may be fixed and connected to the table top in some cases, instead of being attachable/detachable. In addition, the third RF coil unit  303  is formed such that the end portion thereof which is to overlap the fourth RF coil unit  304  shown in  FIG. 2  or the second RF coil unit  302  in Feet-First mode (i.e., the farther end portion from the gantry) is also arranged on the lower side. Furthermore, the respective RF coil units are arranged with no electric connection therebetween. 
         [0050]    As described above, since the RF coil unit  301  for the head portion is considered to be the most frequently attached and detached, the shape of the stepped portion of the dividing portion of the RF coil unit for the head portion has a projection on the farther side from the table top so that the overlapping portion of the RF coil unit for the head portion is arranged at the uppermost level. On the other hand, in the RF coil for the portion around the waist, the shape of the stepped portion in the dividing portion projects on the closer side to the table top so that the overlapping portion of that RF coil unit is located at the bottom. The shape of the dividing portion may be stepped as shown in  FIG. 3  or inclined. In a case where it is stepped, the coil unit can be stably fixed. Also, by modifying the projection of the stepped portion as described above, the coil unit which is more frequently attached and detached can be arranged at an upper level, thereby allowing easy attachment and detachment. 
         [0051]      FIG. 4  shows an internal structure of  FIG. 3 . The first RF coil unit  301 , the second RF coil unit  302 , and the third RF coil unit  303  have a first coil element  301   a,  a second coil element  302   a,  a third coil element  303   a;  a first feed point  301   b,  a second feed point  302   b,  and a third feed point  303   b;  and a first coil connector  301   c,  a second coil connector  302   c,  and a third coil connector  303   c  in housings  301   d,    302   d,  and  303   d,  respectively, which are connected in a high-frequency manner. 
         [0052]    The coil elements in the adjacent units ( 301   a  and  302   a,  and  302   a  and  303   a  in  FIG. 4 ) overlap each other at least partly. The respective housings  301   d  to  303   d  are boxes each accommodating the coil element ( 301   a  to  303   a ) and the feed point ( 301   b  to  303   b ) and separate the RF coil at respective dividing portions ( 312  and  323 ), as shown in the drawing. The housing dividing portions ( 312  and  323 ) are provided between the coil elements of the adjacent units, that is, are provided so as not to cut the coil elements and so as to avoid the coil elements. The feed point is a circuit which is connected to each coil element to detect a high frequency signal received by that coil element. Moreover, the dividing portion is a portion of the housing which overlaps the housing of the adjacent unit. By the dividing portions, the RF coil  300  is divided into a plurality of units. 
         [0053]    The table top  401  includes a first table top connector  411 , a second table top connector  412 , and a third table top connector  413 . The respective table top connectors are connected to the docking connector  501  via the cable wiring  403 . When the respective divided RF coil units are installed on the table top, the coil connector  303   c  is connected to the table top connector  413 , the coil connector  302   c  is connected to the table top connector  412 , and the coil connector  301   c  is connected to the table top connector  411 . 
         [0054]    For example, in a case where the RF coil  300  is a reception coil, a magnetic resonance signal is received by the coil elements  301   a,    302   a,  and  303   a  and is detected by the feed points  301   b,    302   b,  and  303   b.  The detected signals are transmitted to the docking connector  501  through the respective coil connectors  301   c,    302   c,  and  303   c  and the table top connectors  411 ,  412 , and  413 , and further through the cable wiring  403 . In  FIG. 4 , in the overlapped portions  312  and  323 , the coil elements  301   a  and  302   a  spatially overlap each other and the coil elements  302   a  and  303   a  spatially overlap each other. The respective RF coil units are arranged to overlap each other by the optimum amount for minimizing magnetic coupling between the adjacent coil elements, and a mechanism for electrically connecting the respective RF coil units (connection portion) is not required. Further, the coil elements are folded back such that the physical distance between a subject (not shown) on the RF coil unit  300  and each coil element is as close as possible in a portion other than the overlapped portion, thereby a high-sensitivity RF coil is realized. 
         [0055]    Each coil element  301   a  to  303   a  includes one or more coil elements  300  shown in  FIG. 13 . In a case where there are a plurality of coil elements  300  in one coil element ( 301   a,  for example), the respective coil elements  300  overlap each other to form the coil element  301   a.  A state where the respective coil elements  300  overlap each other is described. The coil elements  300  are arranged to be laid along the inside of the housing of the unit, and one of the coil elements  300  is raised only at an intersection of the coil elements  300 . Inside the overlapped portion, in a portion other than the intersection of the coils, each coil element  300  is arranged to be laid along the inside of the housing. 
         [0056]    On the other hand, in a case where portions of the coil elements of different units overlap each other, the coil elements overlap via the housing. Therefore, in the portion where the coil elements overlap (portions  312  and  323  in  FIG. 4 ), inside portions where two coils overlap each other, other than the intersection, overlap each other with a width in a vertical direction (thickness direction), as shown in  FIG. 4 . Since the coil elements  301   a  and  302   a  are installed by being laid along the inside of the housings thereof, respectively, two coils are arranged away from each other in the vertical direction by at least two times the thickness of the material (e.g., plastic) forming the housing. In a case where the housing is made of plastic material having a width of about 1 cm, the two coils are away from each other by about 2 to 3 cm. In a case where the material is thicker, the two coils may be away from each other by 2 cm or more. 
         [0057]    Next, two embodiments of the present invention are described, using  FIGS. 5 and 6 . In  FIG. 5 , a plurality of RF coils (units  302  to  304 ) are provided in addition to a head-portion RF coil (unit  301 ). In  FIG. 6 , other than the head-portion RF coil (unit  301 ), one RF coil (unit  305 ) is provided. In the embodiment of  FIG. 5 , the RF coil is divided into at least three units (four units in  FIG. 5 ), the position of the center unit ( 303  in  FIG. 5 , the center unit is located around the waist of the subject) is fixed, and the positions of both the adjacent units can be exchanged. 
         [0058]    In  FIG. 6 , other than the head-portion unit  301 , one unit  305  is provided. The unit  301  can be arranged on both sides of the unit  305  while the unit  305  is fixed. In other words, imaging in Head-First mode and imaging in Feet-First mode can be easily realized. More specifically, the first RF coil unit  301  and the fifth RF coil unit  305  are installed on the table top  401 . The first RF coil unit is an exclusive RF coil generally having the sensitivity range from the head portion to the cervical vertebra of a subject (not shown), while the fifth RF coil unit  305  is a general-purpose RF coil having the sensitivity range from under the neck to the toe of the subject (not shown). For performing both Head-First imaging and Feet-First imaging with this configuration, the first RF coil unit  301  is moved to the other side of the fifth RF coil unit  305  on the table top  401 . 
         [0059]    On the other hand,  FIG. 5  is a diagram of a plurality of RF coil units into which a permanent RF coil is divided and which can be attached/detached, where the third RF coil unit  303  is arranged around the center portion of the table top  401 , the second RF coil unit  302  and the fourth RF coil unit  304  are arranged on both sides of the third RF coil unit  303 , and the first RF coil unit  301  is arranged next to the second RF coil unit  302 . End portions of the adjacent RF coil units overlap each other so that the end portion of the RF coil unit closer to the center portion of the table top  401  is located on the lower side. As described above, by dividing the RF coil into a number of RF coil units and arranging the RF coil units so that one of them is located at the center of them on the center portion of the table top  401  and the other RF coil units are arranged on the table top in appropriate sizes, the portable table and the RF coil system can be realized which correspond to both Head-First imaging and Feet-First imaging. 
         [0060]    As compared with the arrangement of  FIG. 6 , the arrangement of  FIG. 5  can reduce the entire length of the table top  401  while the same sensitivity range is kept, and therefore can reduce the installation space of the device. 
       Example 2 
       [0061]    A second example of the present invention is described below, referring to  FIG. 1  and  FIG. 7 . In this example, an example of division and installation of RF coil units is described which enables the RF coil units to be arranged not only in the longitudinal direction of the table top  401  but also in the lateral direction (body width direction). 
         [0062]      FIG. 7  is a structure diagram showing a third RF coil unit  303 , a second RF coil unit  302 ′, and a sixth RF coil unit  309  in the lateral direction (body width direction) of a table top  401  of a portable table in an MRI device having the portable table  400  which can be attached to and detached from a gantry  100 , and shows an exemplary internal structure. In the RF coil unit  300  of the MRI device of  FIG. 1 ,  FIG. 4  shows an example in which division of the RF coil unit is made in the body axis direction. This example is different from that example in that division is made in the lateral (body width) direction of the table top. 
         [0063]    For portions having the same functions as the structures labeled with the same reference signs in  FIG. 1  which have been already described, the description is omitted. Moreover, because the second RF coil unit  302 ′ has the same shape as the aforementioned second RF coil unit  302  but is different in the orientation, the same reference numeral is assigned and “ ′ ” is additionally assigned. 
         [0064]    In  FIG. 7 , an example is shown in which end portions of adjacent RF coil units in the lateral (body width) direction of the table top  401  overlap each other as in portions  323 ′ and  392  circled with broken line. The end portion of the third RF coil unit  303  which is closer to the center portion of the table top  401  is arranged on the lower side and the end portion of the second RF coil unit  302 ′ which is farther from the center portion of the table top  401  in the body width direction is arranged on the upper side, so that the overlapped portion  323 ′ is formed. Moreover, in a case where the sixth RF coil unit  309  is arranged to be farther away from the center portion of the table top  401  in the body width direction, the overlapped portion  392  is formed in such a manner that the end portion of the second RF coil unit  302 ′ which is closer to the center portion of the table top is arranged on the lower side and the end portion of the sixth RF coil unit  309  is arranged on the upper side. 
         [0065]    Inside the RF coil unit, the third RF coil unit  303 , the second RF coil unit  302 ′, and the sixth RF coil unit  309  include the third coil element  303   a,  the second coil element  302 ′ a,  and the sixth coil element  309   a;  the third feed point  303   b,  the second feed point  302 ′ b,  and the sixth feed point  309   b;  and the third coil connector  303   c,  the second coil connector  302 ′ c,  and the sixth coil connector  309   c,  respectively, which are connected in a high-frequency manner. 
         [0066]    The table top  401  includes a fourth table top connector  412 ′ and a fifth table top connector  419 , in addition to the aforementioned third table top connector  419 . The respective table top connectors are connected to the docking connector  501  via the cable wiring  403 . When the respective divided RF coil units are installed on the table top, the third coil connector  303   c  is connected to the third table top connector  413 , the second coil connector  302 ′ c  is connected to the fourth table top connector  412 ′, and the sixth coil connector  309   c  is connected to the fifth table top connector  419 . 
         [0067]    Similarly to Example 1, in a case where the RF coil  300  is a reception coil, the coil elements  303   a,    302 ′ a,  and  309   a  receive a magnetic resonance signal. The magnetic resonance signal is detected by the feed points  303   b,    302 ′ b,  and  309   b.  The detected signals are transmitted to the docking connector  501  through the coil connectors  303   c,    302 ′ c,  and  309   c  and the table top connectors  413 ,  412 ′, and  419 , and further through the cable wiring  403 . In  FIG. 7 , in the overlapped portions  332 ′ and  329 , the coil elements  303   a  and  302 ′ a  spatially overlap each other and the coil elements  302 ′ a  and  309   a  spatially overlap each other. In this example, the respective RF coil units are designed to overlap each other by the optimum amount for minimizing magnetic coupling between the adjacent coil elements, and a mechanism for electrically connecting the respective RF coil units (connection portion) is not required. Moreover, the RF coil unit  300  is folded back such that the physical distance between the subject (not shown) on the RF coil unit  300  and each coil element is as close as possible in a portion other than the overlapped portion, thereby a highly sensitive RF coil is realized. 
       Example 3 
       [0068]    A third example of the present invention is described below, referring to  FIGS. 1 ,  8 ,  9 , and  10 . In this example, an example of installation in which the divided RF coil units are arranged in a matrix on the table top  401 , an exemplary arrangement of table top connectors provided in the table top  401 , and an identification structure for installing the respective coil units at correct installation positions are described. 
         [0069]    Example 1 is characteristic in that the adjacent RF coil units in the body axis direction overlap each other such that the end portion of the RF coil unit closer to the center portion of the table top  401  is arranged on the lower side and the end portion of the RF coil unit farther from the center portion of the table top  401  is arranged on the upper side, and Example 2 is characteristic in that the RF coil units adjacent in the body width direction overlap each other in the same manner as that in Example 1. This example shown in the upper diagram of  FIG. 8  is an example in which the arranging direction of the RF coil units is expanded in both the body axis direction ( 302 - 303 - 302 ) and the body width direction ( 302 ′- 303 - 302 ′), and a diagram is shown in which seventh RF coil units  310  are arranged on the outside of the RF coil units  302  and  302 ′ in the body width direction of the table top. 
         [0070]    Moreover, the lower diagram in  FIG. 8  shows an example in which an eighth RF coil unit  311  having a further different shape is arranged on the outside of the 
         [0071]    RF coil unit  310  in the body width direction. As shown in the lower diagram of  FIG. 8 , also in the body axis direction, the RF coil unit  301  or  304  can be arranged further outside. 
         [0072]      FIG. 9  shows the unit shape when the respective RF coil units in the arrangement are divided. In any exemplary arrangement, the RF coil unit  303  which is the closest to the center portion of the table top  401  is arranged at the center, the end portions of the adjacent RF coil units overlap each other, the end portion of the RF coil unit closer to the center portion of the table top  401  is arranged on the lower side, the end portion of the RF coil unit farther from the center portion of the table top  401  is arranged on the upper side, and the RF coil units are arranged in a two-dimensional matrix with no electrical connection therebetween. 
         [0073]      FIG. 10  shows a cross-section of the table top  401 , the RF coil unit  300  arranged thereon, and the divided (first to third) RF coil units  301 ,  302 , and  303 , when seen from the side. In a case where the RF coil units are arranged two-dimensionally as in this example, it is likely that the table top connectors and the coil connectors are wrongly connected to each other, respectively, if connection relies only on matching of the overlapping shapes of the end portions of the respective RF coil units. Therefore, concave/convex portions  362   c,    363   c,    462   c,  and  463   c  for preventing that are shown. The concave/convex portion  462   c  or  463   c  is formed in the installation surface of the table top very close to the table top connector, and the convex/concave portion  362   c  or  363   c  is formed in the installation surface of the bottom surface of each RF coil unit very close to the coil connector. Connection of the table top connector and the coil connector to each other can be established only when the concave/convex shapes match each other. In place of the concave/convex portions, marks such as colors, figures, or numerals may be applied on each RF coil unit to be connected and the table top at positions very close to the coil connector and the table top connector, so that correct combination can be determined without electrical connection. 
         [0074]    The concave/convex portions for preventing wrong connection between the table top connectors and the coil connectors of the respective units or the marks such as colors, figures, or numerals can be applied to the embodiments described in Examples 1, 2, and 4. 
         [0075]    When the portable table is connected to the gantry, an identification command may be issued from the table control portion  200  to data in a coil ID memory provided in the coil connector of each RF coil unit to make an access and, if the ID is not coincident, a user may be notified that the coil units are wrongly arranged. 
         [0076]    Moreover, when there is a unit which is not connected, that may be recognized and a user may be notified of that. Such a user notification of wrong connection or the like can be also applied to the embodiments of Examples 1, 2, and 4 in the same manner. 
         [0077]    As a method for notifying a user of wrong connection or the like,  FIG. 11 ,  FIG. 12  or the like may be displayed on the display device  109  so that a wrongly connected portion or a portion not connected is displayed in a different color as warning. Also in the embodiments of Examples 1, 2, and 4, the wrongly connected unit or the unit not connected is displayed in a different color together with a diagram showing how the coil units are divided as in  FIG. 11  or  12 . 
       Example 4 
       [0078]    A fourth example of the present invention is described, referring to  FIGS. 11 and 12 . In this example, an example of division and installation of the RF coil units arranged in a matrix on the table top  401  and an example of effectively using the space on the table top  401  are described. In the structure in which the permanent RF coil  300  installed on the table top can be partly attached and detached, in a space for the RF coil unit which has been partly detached, another region exclusive RF coil can be installed instead. For example,  FIG. 11  shows an example in which the first RF coil unit  301 , the second RF coil unit  302 , the seventh RF coil units  310 , and the eighth RF coil units  311  have been removed from the state of the lower diagram of  FIG. 8  and instead a first region exclusive RF coil unit  321  is installed. For example, when an RF coil exclusive for mammography is used as the RF coil unit  321  as in a case of mammo MRI, a higher quality image can be obtained than in a case of using the permanent RF coil installed on the table top.  FIG. 12  shows an example in which the fourth RF coil unit  304 , the sixth RF coil unit  309 , and the eighth RF coil units  311  have been removed from the state of the lower diagram of  FIG. 8  and instead a second region exclusive RF coil unit  322  and a third region exclusive RF coil unit  323  are installed. For example, when an RF coil for upper arm imaging is installed as the RF coil unit  322  or an RF coil for knee imaging is installed as the RF coil unit  323 , a desired region of interest can be imaged by means of a local RF coil with higher sensitivity. Although the above examples describe cases of mammo MRI, upper arm imaging, and knee imaging, the present invention is not limited thereto. According to the present invention, a part unit of the permanent RF coil units co-existing can be always arranged closer to the center portion of the table top than the region exclusive RF coil, and, in a case where the end portions of the RF coil units are overlapped to reduce electromagnetic coupling, the permanent RF coil unit can be arranged on the lower side. Therefore, it is enough that only the minimum required RF coil unit is removed. Furthermore, when electromagnetic coupling is reduced by control of the trap circuit  300   c  in the RF coil by the table control portion  200  shown in  FIG. 13  in accordance with a pulse sequence, the permanent RF coil which can be partly attached and detached and the region exclusive RF coil can be simultaneously used in combination on a relatively small table top. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           100  Gantry 
           101  Magnet 
           102  Gradient magnet field generation coil 
           103  High frequency transmitter coil 
           104  Subject 
           105  Receiver 
           105   a  A/D signal processor 
           106  High frequency magnetic field generator 
           107  Gradient magnetic field power source 
           108  Storage medium 
           109  Display device 
           110  Sequencer 
           111  Calculation portion 
           200  Table top control portion 
           200   a  Current supply source 
           200   b  Preamplifier driving power source 
           300  RF coil 
           300   a  Coil element 
           300   b  Signal detection portion 
           300   c  Coil connector 
           300   d  Reception cable 
           300   e  Trap circuit 
           301  to  311  RF coil unit 
           301  First RF coil unit 
           301   a  First coil element 
           301   b  First feed point 
           301   c  First coil connector 
           302  Second RF coil unit 
           302 ′ Second RF coil unit 
           302   a  Second coil element 
           302 ′ a  Second coil element 
           302   b  Second feed point 
           302 ′ b  Second feed point 
           302   c  Second coil connector 
           302 ′ c  Second coil connector 
           303  Third RF coil unit 
           303   a  Third coil element 
           303   b  Third feed point 
           303   c  Third coil connector 
           304  Fourth RF coil unit 
           305  Fifth RF coil unit 
           309  Sixth RF coil unit 
           309   a  Sixth coil element 
           309   b  Sixth feed point 
           309   c  Sixth coil connector 
           310  Seventh RF coil unit 
           311  Eighth RF coil unit 
           321  First region exclusive RF coil unit 
           322  Second region exclusive RF coil unit 
           323  Third region exclusive RF coil unit 
           312 ,  323 ,  323 ′,  392  Overlapped portion 
           362   c,    363   c,    462   c,    463   c  Concave/convex portion 
           400  Portable table 
           401  Table top 
           402  Frame 
           403  Composite cable wiring 
           410  Table top connector 
           411  First table top connector 
           412  Second table top connector 
           413  Third table top connector 
           412 ′ Fourth table top connector 
           419  Fifth table top connector 
           501 ,  502  Docking connector 
           701  Preamplifier 
           702  Power supply cable 
           703  Capacitor 
           704  Inductor 
           705  Diode 
           706  Current supply cable