Abstract:
A magnetic resonance imaging apparatus includes a bore configured to accommodate a subject therein, an RF coil positioned about the bore, and an RF shield positioned about the RF coil. The RF coil includes a first portion positioned adjacent a lower surface side of the bore and spaced a distance from the RF shield that is larger than a distance between a second portion of the RF coil and an upper surface side of the bore.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of Japanese Patent Application No. 2010-169901 filed Jul. 29, 2010, which is hereby incorporated by reference in its entirety. 
         [0002]    The embodiments described herein relate to a magnetic resonance imaging apparatus having an RF coil. 
         [0003]    An RF coil for sending transmission pulses is installed within a magnetic resonance imaging apparatus. The diameter of the RF coil is related to the size of a bore into which a subject is carried. Therefore, the value of the RF coil diameter is very important. The bore can be made large by making the RF coil diameter large, thus making it possible to diminish the sense of oppression of a subject when carried into the bore. However, if the RF coil diameter is made large, it is required to increase the electric power to be supplied to the RF coil, thus giving rise to the problem that electric power consumption increases. Moreover, since an RF shield is disposed around the RF coil, if the RF coil diameter is made large, the spacing between the RF coil and the RF shield becomes narrower. The RF shield acts to cancel a magnetic field generated by the RF coil, and the narrower the spacing between the RF coil and the RF shield, the more remarkable the action of the RF shield. Thus, there arises the problem that the narrower the spacing between the RF coil and the RF shield, the larger the electric power to be supplied to the RF coil, resulting in a further increase of electric power consumption. 
         [0004]    The use of an elliptic RF coil has been proposed as a method for solving the above problem in, for example, Japanese Unexamined Patent Publication No. Hei 7 (1995)-222729. 
         [0005]    However, since the RF coil described in Japanese Unexamined Patent Publication No. Hei 7 (1995)-222729 is elliptic, the coil diameter in the minor axis direction of the ellipse cannot be made large. Accordingly, there is the problem that the bore cannot be made large in the minor axis direction of the ellipse and that therefore a subject who has been carried into the bore is apt to have a sense of oppression. 
       BREIF DESCRIPTION OF THE INVENTION 
       [0006]    The embodiments described herein provide a magnetic resonance imaging apparatus including: a bore for accommodating a subject; an RF coil disposed around the bore; and an RF shield disposed around the RF coil, the RF coil being constructed such that a portion of the RF coil disposed on a lower surface side of the bore is spaced more distant from the RF shield than a portion of the RF coil disposed on an upper surface side of the bore. 
         [0007]    By constructing the RF coil as above it is possible to decrease electric power consumption of the RF coil while ensuring a required size of the bore. 
         [0008]    Further objects and advantages of the embodiments described herein will be apparent from the following description of embodiments of the invention as illustrated in the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a perspective view showing a magnetic resonance imaging apparatus. 
           [0010]      FIG. 2  is a diagram explaining a positional relation between a birdcage coil  22  and an RF shield  23 . 
           [0011]      FIG. 3  is a diagram showing a state in which the RF shield  23  has been displaced backward with respect to the birdcage coil  22 . 
           [0012]      FIG. 4(   a ) and  FIG. 4(   b ) are diagrams showing the birdcage coil  22 . 
           [0013]      FIG. 5(   a ) and  FIG. 5(   b ) are diagrams explanatory of the shape of the birdcage coil  22  and that of the RF shield  23 . 
           [0014]      FIG. 6(   a ) and  FIG. 6(   b ) are diagrams explanatory of the shape of an upper half Da of a ring D 1  and that of a lower half Db of the ring D 1 . 
           [0015]      FIG. 7(   a ) and  FIG. 7(   b ) are diagrams for explaining an effect obtained by the birdcage coil  22  used in the embodiment. 
           [0016]      FIG. 8  is a graph showing conditions for simulation of an impedance distribution of the birdcage coil  22 . 
           [0017]      FIG. 9  is a diagram showing the result of the simulation. 
           [0018]      FIG. 10  is a diagram showing an example of a ring D 1  having another shape. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    An embodiment of the invention will be described below, but the invention is not limited to the following embodiment. 
         [0020]      FIG. 1  is a perspective view showing a magnetic resonance imaging apparatus. 
         [0021]    The magnetic resonance imaging apparatus (hereinafter referred to as “MRI apparatus,” MRI: Magnetic Resonance Imaging) indicated at  100  has a magnetic field generator  2  and a table  3 . 
         [0022]    The magnetic field generator  2  is provided with a bore  21  for accommodating a subject. Within the magnetic field generator  2 , a birdcage coil  22  for transmission of RF pulses and reception of magnetic resonance signals from the subject and an RF shield  23  for decreasing RF power radiated outside the MRI apparatus  100  are installed. 
         [0023]      FIG. 2  is a diagram explaining a positional relation between the birdcage coil  22  and the RF shield  23  and  FIG. 3  is a diagram showing a state in which the RF shield  23  has been displaced backward with respect to the birdcage coil  22 . 
         [0024]    The magnetic field generator  2  has a coil support  220  for supporting the birdcage coil  22 . The coil support  220  is cylindrical, and inside the coil support  220  is mounted a cradle support base  221  for supporting a cradle  31  (see  FIG. 1 ) within the bore  21 . The space surrounded by the coil support  220  and the cradle support base  221  forms the bore  21  for accommodating the subject. The birdcage coil  22  is provided on an outer surface  220   a  of the coil support  220 . The RF shield  23  is disposed around the birdcage coil  22 . 
         [0025]      FIG. 4(   a ) is a perspective view of the birdcage coil  22  and  FIG. 4(   b ) is a diagram showing which positions legs L 1  to L 16  assume when the birdcage coil  22  is seen from a front side. 
         [0026]    The birdcage coil  22  has two rings D 1 , D 2  and n number of legs Li (i=1 to n) for connecting the two rings D 1  and  2  with each other. In this embodiment, n is set at 16. Therefore, the birdcage coil  22  has sixteen legs L 1  to L 16 . 
         [0027]    The birdcage coil  22  has loop circuits Ci,j. Each loop circuit Ci,j is formed using adjacent legs Li, Lj and the rings D 1 , D 2 . For example, a loop circuit C 1 , 2  is formed using two adjacent legs L 1 , L 2  and the two rings D 1 , D 2 , and a loop circuit C 16 ,  1  is formed using two legs L 16 , L 1  and the two rings D 1 , D 2 . In  FIG. 4(   a ), a route of the loop circuit C 1 ,  2  and that of the loop circuit C 16 ,  1  are shown schematically using dot-dash lines. 
         [0028]    Next, a description will be given below about the shape of the birdcage coil  22  and that of the RF shield  23 . 
         [0029]      FIG. 5(   a ) is a view of the birdcage coil  22  and the RF shield  22  as shown in  FIG. 2  as seen in a z direction and  FIG. 5(   b ) is a sectional view taken on line A-A in  FIG. 5(   a ). 
         [0030]    As shown in  FIG. 5(   a ), the RF shield  23  has a circular shape when seen in the z direction. More specifically, the RF shield has a circular shape of radius r 1  centered on a reference axis A. 
         [0031]    On the other hand, the ring D 1  of the birdcage coil  22  is constructed so that an upper half Da (a portion positioned on an upper surface  21   a  side of the bore  21 ) of the ring D 1  and a lower half Db (a portion positioned on a lower surface  21   b  side of the bore  21 ) of the ring D 1  provide an asymmetric shape. 
         [0032]      FIG. 6(   a ) is a diagram explanatory of the shape of the upper half Da of the ring D 1  and  FIG. 6(   b ) is a diagram explanatory of the shape of the lower half Db of the ring Dl. In  FIG. 6(   a ), the upper half Da of the ring D 1  is indicated with a solid line and the lower half Db of the ring D 1  is indicated with a broken line. On the other hand, in  FIG. 6(   b ), the upper half Da of the ring D 1  is indicated with a broken line and the lower half Db of the ring D 1  is indicated with a solid line. 
         [0033]    As shown in  FIG. 6(   a ), the upper half Da of the ring D 1  has an upper half shape (semi-circular shape) of a circle of radius ra centered on a reference axis A. On the other hand, as shown in  FIG. 6(   b ), the lower half Db of the ring D 1  has a lower half shape (semi-elliptic shape) of an ellipse having a major axis length of 2ra (twice the ra) and a minor axis length of rb (&lt;ra). Therefore, the lower half Db of the ring D 1  is spaced more distant from the RF shield  23  than the upper half Da. In this embodiment, the upper half Da of the ring D 1  is spaced Δra from the RF shield  23 , but the lower half Db of the ring D 1  is spaced a maximum of Δrb (=Δra+Δx) from the RF shield  23 . Thus, the lower half Db of the ring D 1  is further spaced a maximum of Δx from the RF shield  23 . 
         [0034]    Although the ring D 1  is illustrated in  FIGS. 6(   a ) and  6 ( b ), the other ring D 2  (shown in  FIG. 4(   a ) also has the same shape as the ring D 1 . 
         [0035]    Therefore, as shown in  FIG. 5(   b ), a lower half (the portion positioned on the lower surface  21   b  side of the bore  21 )  22   b  of the birdcage coil  22  is further spaced a maximum of Δx from the RF shield  23  with respect to an upper half (the portion positioned on the upper surface  21   a  side of the bore  21 )  22   a  of the birdcage coil  22 . By thus constructing the birdcage coil  22  there is obtained an effect that the electric power consumption of the birdcage coil  22  can be reduced while making the subject-accommodating bore as wide as possible. The reason why this effect is obtained will be explained below with reference to  FIGS. 7(   a ) and  7 ( b ). 
         [0036]      FIG. 7(   a ) is a diagram showing the RF shield  23  and the birdcage coil  22  having a circular ring DC and  FIG. 7(   b ) is a diagram showing the RF shield  23  and the birdcage coil  22  having the ring D 1 . 
         [0037]    In the case of a birdcage coil  22 ′ shown in  FIG. 7(   a ), the larger the coil diameter ra, the wider can be the bore  21 , so that the sense of oppression which the subject has within the bore  21  can be diminished. However, there is the problem that the larger the coil diameter ra, the smaller the magnetic field generated at the coil center. Moreover, as the coil diameter ra is made larger, the spacing Δra between the birdcage coil  22 ′ and the RF shield  23  becomes narrower. The RF shield  23  acts to cancel the magnetic field generated by the birdcage coil, and the narrower the spacing Δra, the more remarkable the action. Therefore, in order to prevent the magnetic field generated at the coil center from becoming small, it is necessary to supply a larger electric power to the birdcage coil  22 ′. As a result, there arises the problem that the electric power consumption increases. 
         [0038]    On the other hand, in  FIG. 7(   b ), the lower half  22   b  of the birdcage coil  22  is constructed so as to be further spaced a maximum of Δx from the RF shield  23  with respect to the upper half  22   a  of the birdcage coil  22 . Therefore, by an amount of Δx, the action of the magnetic field being cancelled by the RF shield  23  can be diminished and hence it is possible to decrease the electric power consumption of the birdcage coil  22 . 
         [0039]    Since the lower half  22   b  of the birdcage coil  22  and the upper half  22   a  of the birdcage coil  22  are asymmetric in shape, there sometimes is a case where the uniformity of the magnetic field is disordered. Once the uniformity of the magnetic field is disordered, a bad influence is exerted on the image quality. Therefore, it is desirable that the magnetic field be as uniform as possible. The magnetic field can be made as uniform as possible by adjusting the impedance distribution of the birdcage coil  22  so that the impedance of the upper half  22   a  of the birdcage coil  22  becomes high, while the impedance of the lower half  22   b  of the birdcage coil  22  becomes low. For example, in connection with the loop circuits Ci,j (see  FIG. 4(   b )) of the birdcage coil  22 , the above impedance distribution can be achieved by making the loop circuits C 1 , 2  to C 8 , 9  high in impedance and the loop circuits C 9 , 10  to C 16 , 1  low in impedance. By so adjusting the impedance distribution of the birdcage coil  22  it is possible to enhance the uniformity of the magnetic field. Next, simulation has been conducted to verify that the uniformity of the magnetic field can be enhanced by adjusting the impedance distribution. The following description is provided about conditions and result of the simulation. 
         [0040]      FIG. 8  is a graph showing conditions for simulation of the impedance distribution of the birdcage coil  22 . 
         [0041]    In the graph of  FIG. 8 , the loop circuits Ci,j of the birdcage coil  22  are plotted along the axis of abscissa, while the impedances of the loop circuits Ci,j are plotted along the axis of ordinate. As shown in the graph of  FIG. 8 , the loop circuits C 1 , to C 8 , 9  are set high in impedance, while the loop circuits C 9 , 10  to C 16 , 1  are set low in impedance. Particularly, in the simulation conducted this time, the impedances are set so that the loop circuits C 4 , 5  and C 5 , 6  located at the highest position are the highest in impedance and the loop circuits C 12 ,  13  and C 13 ,  14  located at the lowest position are the lowest in impedance. 
         [0042]    In  FIG. 7(   b ), the spacings Δra and Δrb between the birdcage coil  22  and the RF shield  23  are set so as to satisfy the relationship of Δra :Δrb=1: 2. 
         [0043]      FIG. 9  is a diagram showing the result of the simulation. 
         [0044]    In the graph of  FIG. 9 , the position of the bore  21  in AP direction is plotted along the axis of abscissa, while the strength of a magnetic field B 1  generated by the birdcage coil is plotted along the axis of ordinate. A solid curve A 1  in the graph indicates the strength of a magnetic field B 1  generated by the conventional birdcage coil  22 ′ shown in  FIG. 7(   a ), while a dot-dash line curve A 2  in the graph indicates the strength of a magnetic field B 1  generated by the birdcage coil  22  according to this embodiment which has the impedance distribution shown in  FIG. 8 . Comparison between both curves A 1  and A 2  shows that there are obtained almost the same magnetic field strengths. Thus, it is seen that the magnetic field strength distribution can be made sufficiently uniform by adjusting like  FIG. 8  the impedance distribution of the birdcage coil  22  used in this embodiment. The impedance distribution of the birdcage coil  22  can be adjusted for example by adjusting the capacitance and inductance of the birdcage coil  22 . 
         [0045]    In the birdcage coil  22  according to this embodiment, the upper half Da of the ring D 1  has a semi-circular shape (see  FIG. 6(   a ) and the lower half Db of the ring D 1  has a semi-elliptic shape (see  FIG. 6(   b )). However, the shape of the ring D 1  is not limited to the shape shown in  FIGS. 6(   a ) and  6 ( b ). It may be another shape. Reference will be made below to an example of a ring D 1  having another shape. 
         [0046]      FIG. 10  is a diagram showing an example of a ring D 1  having another shape. 
         [0047]    An upper half Da of the ring D 1  has a semi-circular shape like the ring D 1  shown in  FIGS. 6(   a ) and  6 ( b ). However, unlike the ring D 1  shown in  FIGS. 6(   a ) and  6 ( b ), a lower half Db&#39; of the ring D 1  has a rectilinearly extending portion P. Thus, various changes may be made as to the ring shape of the birdcage coil  22 . 
         [0048]    Although in the above embodiment there is shown an example of using the birdcage coil  22  as the RF coil, the RF coil used in the invention may be an RF coil other than the birdcage coil. 
         [0049]    Many widely different embodiments of the invention may be configured without departing from the spirit and the scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.