Abstract:
With the objective of reducing the coupling capacitance of a pattern crossing section in a simple structure thereby to produce a high-quality tomographic image based on a larger value of the coil, a magnetic resonance signal receiving coil includes a pair of first and second conductor patterns each having a partial conductor pattern set which branches into three partial conductor patterns at the pattern crossing section. Each confronting pair of the first and second partial conductor patterns cross each other by being insulated from each other at the pattern crossing section. The partial conductor patterns of the first and second conductor patterns have their one ends beyond the crossing section each connected together to other ends of the second and first conductor patterns by arcuate conductors. The conductor patterns have their open ends connected to the signal outlets by conductor bars.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a coil for an MRI apparatus which forms at least two loops, and particularly to a coil for an MRI apparatus which is capable of enhancing the coupling characteristics of the reception coil by having a reduced coupling capacitance at the crossing section of the loops. 
     An MRI apparatus have been designed to implement the imaging process by detecting with a reception coil a magnetic signal which is created by the nuclear magnetic resonance. FIG. 7 is a diagram showing a developed view of a conventional saddle-type reception coil. In FIG. 7, a coil  101  forms a pair of loop coils  201  and  202  on the right and left, and the loop coils  201  and  202  are connected in series. The loop coils  201  and  202  have conductor patterns  105  and  106  which form loop conductor patterns  107  and a pattern crossing section  111 . Disposed between the conductor pattern  106  and conductor pattern  107  is a resonance capacitor C 1 , which is connected to a cable section  103  for leading out a signal received by the coil  101 . A balance/unbalance converting circuit such as an impedance matching circuit and balun is provided between the resonance capacitor C 1  and the cable section  103 . 
     The conductor patterns  105  and  106  cross each other at the pattern crossing section  111 . FIG. 8 is a diagram showing the detailed structure of the pattern crossing section  111 . In FIG. 8, the conductor patterns  105  and  106  cross each other by being interposed by a glass-epoxy substrate  121  which is an insulator. The conductor patterns  105  and  106  cross each other at right angles in order to reduce their magnetic coupling. 
     Based on this structure, there exists at the pattern crossing section  111  a coupling capacitance C, which is expressed in terms of the crossing area S of the conductor patterns  105  and  106 , the thickness d of the glass-epoxy substrate  121 , and the dielectric constant ε of the glass-epoxy substrate  121  as in the following formula (1). 
     
       
           C=εS/d   (1) 
       
     
     The conductor patterns  105  and  106  have a width D, and the formula (1) is reformed as in the following formula (2). 
     
       
           C=ε· ( D×D )/ d   (2) 
       
     
     The conductor patterns  105  and  106  have their width D set large in order to reduce the resistance component of the coil. Consequently, the crossing area S is large. The glass-epoxy substrate  121  has its thickness d set small due to the limited layout space and cost of the coil  101 . On this account, the coupling capacitance C of the pattern crossing section  111  is nonnegligible with respect to the resonance capacitor C 1 . 
     FIG. 9 is a diagram showing an equivalent circuit of the coil  101 . This equivalent circuit forms a parallel resonance circuit. The impedance characteristic of this equivalent circuit is represented by a resonance curve which has a large impedance value at the resonant frequency fc as shown in FIG.  10 . Generally, a coil has its Q value expressed in terms of the inductance L of the coil, the resistance component r of the coil, and the resonant frequency ω as in the following formula (3). 
     
       
           Q=ωL/r=fc/Δf   (3) 
       
     
     By setting a 3-dB band width Δf of the peak value on the resonance curve of FIG. 10, the Q value is evaluated by the formula (3). The resonant frequency fc relates to ω as ω=2πfc, and the S/N factor (signal to noise ratio), which is a crucial parameter indicative of the quality of the tomographic image produced by the MRI apparatus, relates to the Q value as in the following formula (4). 
     
       
           S/N∝ ( Q )  (4) 
       
     
     As described above, the resistance component r increases with the increase of the coupling capacitance C, which results in a decreased Q value as suggested by the formula (3). The smaller Q value of the coil deteriorates the S/N factor as suggested by the formula (4), which results in a degraded quality of tomographic image. Namely, an increase of coupling capacitance C of the pattern crossing section  111  reduces the Q value of the coil  101 , which gives rise to a problem of a degraded quality of tomographic image. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the present invention to provide a coil for an MRI apparatus which is designed to reduce the coupling capacitance C of the pattern crossing section  111  based on a simple structure so that the Q value of the coil  101  does not decrease, thereby producing a high-quality tomographic image. 
     In order to achieve the above objective, the coil for an MRI apparatus according to the first aspect resides in a coil for an MRI apparatus which forms a plurality of loops and has an insulated crossing section, and is characterized by including a first conductor pattern which forms a first loop and has its one end at the crossing section branching into a set of a prescribed number of first partial conductor patterns, and a second conductor pattern which forms a second loop and has its one end at the crossing section branching into a set of the prescribed number of second partial conductor patterns, and is further characterized in that each confronting pair of the first and second partial conductor pattern sets cross each other by being insulated from each other at the crossing section, and the adjacent first partial conductor patterns and adjacent second partial conductor patterns have their ends beyond the crossing section each connected together to other ends of the second conductor pattern and first conductor pattern by conductors which are spaced out from the second partial conductor patterns and first partial conductor patterns, respectively, by a prescribed distance or more. 
     The coil for an MRI apparatus according to the first aspect is designed to reduce the coupling capacitance of the crossing section based on the structure in which each confronting pair of the first and second partial conductor patterns each formed in a prescribed number of branches cross each other by being insulated from each other at the crossing section, and the first and second partial conductor patterns each have their ends beyond the crossing section connected together by conductors which are spaced out from the second and first partial conductor patterns by a prescribed distance or more. 
     The coil for an MRI apparatus according to the second aspect resides in a coil for an MRI apparatus which forms a plurality of loops and has an insulated crossing section, and is characterized by including a first conductor pattern which forms a first loop and has its one end at the crossing section branching into first partial conductor patterns of two in number, and a second conductor pattern which forms a second loop and has its one end at the crossing section branching into second partial conductor patterns of two in number, and is further characterized in that each confronting pair of the first and second partial conductor patterns cross each other by being insulated from each other at the crossing section, and the first partial conductor patterns and second partial conductor patterns have their ends beyond the crossing section each connected together to other ends of the second conductor pattern and first conductor pattern by conductors which are spaced out from the second partial conductor patterns and first partial conductor patterns, respectively, by a prescribed distance or more. 
     The coil for an MRI apparatus according to the second aspect is designed to reduce the coupling capacitance of the crossing section based on the structure in which each confronting pair of the first and second partial conductor patterns each formed in two branches cross each other by being insulated from each other at the crossing section, and the first and second partial conductor patterns each have their ends beyond the crossing section connected together to another end of the second and first conductor patterns by conductors which are spaced out from the second and first partial conductor patterns by a prescribed distance or more. 
     The coil for an MRI apparatus according to the third is characterized in that the first and second partial conductor patterns cross each other at right angles. 
     The coil for an MRI apparatus according to the third aspect is designed to reduce the crossing area thereby to reduce the coupling capacitance of the crossing section based on the structure in which the first and second partial conductor patterns cross each other at right angles. 
     The coil for an MRI apparatus according to the fourth aspect is characterized in that the partial conductor patterns have a virtually equal width. 
     The coil for an MRI apparatus according to the fourth is designed to reduce the crossing area thereby to reduce the coupling capacitance of the crossing section based on the structure in which the partial conductor patterns have a virtually equal width. 
     Therefore, the coil for an MRI apparatus according to the first aspect is designed such that each confronting pair of the first and second partial conductor patterns each formed in a prescribed number of branches cross each other by being insulated from each other at the crossing section, and the first and second partial conductor patterns each have their ends beyond the crossing section connected together by conductors which are spaced out from the second and first partial conductor patterns by a prescribed distance or more, so that the coupling capacitance of crossing section decreases, whereby the coil can have a large Q value to produce a high-quality MRI tomographic image based on a high S/N factor. 
     The coil for an MRI apparatus according to the second aspect is designed such that each confronting pair of the first and second partial conductor patterns each formed in two branches cross each other by being insulated from each other at the crossing section, and the first and second partial conductor patterns each have their ends beyond the crossing section connected together to other ends of the second and first conductor patterns by conductors which are spaced out from the second and first partial conductor patterns by a prescribed distance or more, so that the coupling capacitance of the crossing section decreases, whereby the coil can have a large Q value to produce a high-quality MRI tomographic image based on a high S/N factor. 
     The coil for an MRI apparatus according to the third aspect is designed such that the first and second partial conductor patterns cross each other at right angles at the crossing section, so that the crossing area is decreased to reduce the coupling capacitance, whereby the coil can have a larger Q value to produce a high-quality MRI tomographic image based on a high S/N factor. 
     The coil for an MRI apparatus according to the fourth aspect is designed such that the partial conductor patterns have a virtually equal width at the crossing section, so that the crossing area is decreased to reduce the coupling capacitance, whereby the coil can have a larger Q value to produce a high-quality MRI tomographic image based on a high S/N factor. 
     Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing the overall arrangement of the MRI apparatus which uses the coil based on Embodiment 1 of this invention. 
     FIG. 2 is a diagram showing the structure of the coil of Embodiment 1 of this invention. 
     FIG. 3 is a development diagram showing the structure of the coil shown in FIG.  2 . 
     FIG. 4 is an enlarged perspective view of the pattern crossing section shown in FIG.  3 . 
     FIG. 5 is a development diagram showing the structure of the coil based on Embodiment 2 of this invention. 
     FIG. 6 is an enlarged perspective view of the pattern crossing section shown in FIG.  5 . 
     FIG. 7 is a development diagram showing the structure of the conventional coil. 
     FIG. 8 is an enlarged perspective view of the pattern crossing section shown in FIG.  7 . 
     FIG. 9 is a diagram showing an equivalent circuit of the coil shown in FIG.  7 . 
     FIG. 10 is diagram showing the resonant characteristics of the coil. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Preferable embodiments of a coil for an MRI apparatus based on this invention will be explained in detail with reference to the attached drawings. 
     (Embodiment 1) 
     Embodiment 1 of this invention will be explained. 
     FIG. 1 is a diagram showing the overall arrangement of an MRI apparatus. In FIG. 1, this MRI apparatus has a magnet section  100  and a table section  200 . Placed at the center of the magnet section  100  are a subject body  102  and a coil  101 . The coil  101  is connected to the main body of the magnet section  100  through a cable section  103  and connector  104 . The subject body  102  is placed to lie inside the coil  101 . 
     FIG. 2 is a diagram showing the detailed structure of the coil  101  shown in FIG.  1 . FIG.  2 ( a ) is a diagram showing the external view of the coil  101 , and FIG.  2 ( b ) is a diagram showing the structure of the coil  101 . As shown in FIG.  2 ( a ), the coil  101  which is covered with cloth is laid on a case  108  of plastics. The case  108  is connected with the cable section  103  for the connection to the main body of the magnet section  100 , and the cable section  103  has the connection at its end with the connector  104 . 
     The coil  101  is a saddle-type reception coil as shown in FIG.  2 ( b ), which has the conventional formation of a pair of loop coils  201  and  202  which confront each other at the right and left positions. The conventional loop coils  201  and  202  are conductor patterns  107  formed on a printed wiring board. The coil  101  has its loop coils  201  and  202  crossing each other at the pattern crossing section  111  shown in FIG.  7 . 
     FIG. 3 is a development diagram of a coil  10  which is derived from the coil  101  and based on Embodiment 1 of this invention. FIG. 4 is an enlarged perspective view of the pattern crossing section shown in FIG.  3 . 
     In FIG. 3, the loop coils  1  and  2  are connected in series and arranged to cross each other by being insulated at the pattern crossing section  11 . The loop coils  1  and  2  have conductor patterns  7   a  and  7   b  which form loops. The conductor pattern  7   b  has at the pattern crossing section  11  a partial conductor pattern set  21  of three branches of an equal width to include partial conductor patterns  22  through  24 . The conductor pattern  7   a  has at the pattern crossing section  11  a partial conductor pattern set  25  of three branches of an equal width to include partial conductor patterns  26  through  28 . Disposed between the conductor pattern  7   a  and the partial conductor pattern set  25  is a resonance capacitor C 1 , which is connected to a cable section  103  for leading out a signal received by the coil  10 . A balance/unbalance converting circuit such as an impedance matching circuit and balun is provided between the resonance capacitor C 1  and the cable section  103 . 
     The partial conductor patterns  22  to  24  cross the partial conductor patterns  26  to  28  at right angles at respective pattern crossing points  29  through  31 . The partial conductor patterns  22  to  24  have their ends reaching to outlets  40  through  42  which are formed in a glass-epoxy substrate  60  shown in FIG. 4, and they are connected together between  22  and  23  and between  23  and  24  by arcuate conductors  53  and  54 , respectively. The outlet  40  is connected to a connection terminal  47  which is formed at another end of the conductor pattern  7   a  by a conductor bar  33  by being spaced out from the glass-epoxy substrate  60  by a prescribed distance. 
     Similarly, the partial conductor patterns  26  to  28  have their ends reaching to outlets  43  through  45  which are formed in the glass-epoxy substrate  60 , and they are connected together between  26  and  27  and between  27  and  28  by arcuate conductors  51  and  52 , respectively. The outlet  43  is connected to a connection terminal  46  which is formed at another end of the conductor pattern  7   b  by a conductor bar  32  by being spaced out from the glass-epoxy substrate  60  by the prescribed distance. 
     As shown in FIG. 4, the partial conductor pattern sets  21  and  25  are printed on the top and rear surfaces, respectively, of the glass-epoxy substrate  60 . For the conductor patterns  7   a  and  7   b  having a width of D, the partial conductor patterns  22  to  24  and  26  to  28  have width D/3. The partial conductor patterns  22  to  24  and the partial conductor patterns  26  to  28  cross each other at right angles at the pattern crossing points  29  to  31 . Accordingly, each of the pattern crossing points  29  to  31  has its crossing area S expressed by the following formula (5). 
     
       
           S=D/ 3× D/ 3= D×D/ 9  (5) 
       
     
     Substituting the crossing area S to the formula (1) gives the following formula (6). 
     
       
           C=ε· ( D×D/d )/9  (6) 
       
     
     Due to the parallel connection of the three pattern crossing points  29  to  31 , the total coupling capacitance Ct is expressed by the following formula (6). 
       Ct=ε ( D×D/d )/3  (7) 
     Consequently, the coupling capacitance Ct at the pattern crossing section  11  based on Embodiment 1 decreases to ⅓ of the coupling capacitance C of the pattern crossing section  111  shown in FIG.  7 . 
     Although the foregoing Embodiment 1 is designed to have partial conductor patterns of three branches at the pattern crossing section  11 , the number of branches is not confined to this case, but further reduction of coupling capacitance is obviously possible based on an increased number of branches. Although the partial conductor patterns  22  to  24  and the partial conductor patterns  26  to  28  cross each other at right angles at the pattern crossing points  29  to  31 , their orthogonal crossing is not compulsory. However, orthogonal crossing is preferable so that the crossing area is small. Although the partial conductor patterns  22  to  24  and  26  to  28  have an equal width, they may have different widths. However, an equal width is preferable so that the total crossing area is minimized. 
     Embodiment 1 has a reduced crossing area at the pattern crossing section  11  thereby to reduce the coupling capacitance significantly, whereby the coil  10  can have a large Q value and thus suppress the decay of an MRI tomographic image. 
     (Embodiment 2) 
     Next, Embodiment 2 of this invention will be explained. In contrast to the foregoing Embodiment 1 in which the partial conductor patterns  22  to  24  and  26  to  28  have their open ends connected by using the arcuate conductors  51  to  54 , Embodiment 2 is designed to join the ends of partial conductor patterns and connect the joining portions to the conductor patterns  7   a  and  7   b  with conductor bars. 
     FIG. 5 is a development diagram showing the structure of the coil based on Embodiment 2 of this invention. FIG. 6 is an enlarged perspective view of the pattern crossing section shown in FIG.  5 . The coil  20  of Embodiment 2 differs in the structure of pattern crossing section  70  from the pattern crossing section  11  of Embodiment 1, and the remaining portions are identical to Embodiment 1. 
     In FIG. 5, the loop coils  91  and  92  are connected in series and arranged to cross each other by being insulated at the pattern crossing section  70 . The loop coils  91  and  92  have conductor patterns  7   a  and  7   b  which form loops. The conductor pattern  7   b  has at the pattern crossing section  70  a partial conductor pattern set  71  of two branches of an equal width to include partial conductor patterns  73  and  74 , which join again at their ends. The conductor pattern  7   a  has at the pattern crossing section  70  a partial conductor pattern set  72  of two branches of the equal width to include partial conductor patterns  75  and  76 , which join again at their ends. 
     Disposed between the conductor pattern  7   a  and the partial conductor pattern set  72  is a resonance capacitor C 1 , which is connected to a cable section  103  for leading out the signal received by the coil  20 . A balance/unbalance converting circuit such as an impedance matching circuit and balun is provided between the resonance capacitor C 1  and the cable section  103 . 
     The partial conductor patterns  73  and  74  cross the partial conductor patterns  75  and  76  at right angles at respective pattern crossing points  85  and  86 . The partial conductor patterns  73  and  74  have their ends reaching and connecting to an outlet  77  which is formed between the partial conductor patterns  75  and  76 . The outlet  77  is connected to a connection terminal  80  which is formed at another end of the conductor pattern  7   a  by a conductor bar  82  by being spaced out from the glass-epoxy substrate  61  by a prescribed distance. 
     Similarly, the partial conductor patterns  75  and  76  cross the partial conductor patterns  73  and  74  at right angles at respective pattern crossing points  85  and  86 . The partial conductor patterns  75  and  76  have their ends reaching and connecting to an outlet  78  which is formed between the partial conductor patterns  73  and  74 . The outlet  78  is connected to a connection terminal  79  which is formed at another end of the conductor pattern  7   b  by a conductor bar  81  by being spaced out from the glass-epoxy substrate  61  by the prescribed distance. 
     As shown in FIG. 6, the partial conductor pattern sets  71  and  72  are printed on the top and rear surfaces, respectively, of the glass-epoxy substrate  61  via the glass-epoxy substrate. For the conductor patterns  7   a  and  7   b  having a width of D, the partial conductor patterns  73  to  76  have a width D/2. The partial conductor patterns  73  and  74  and the partial conductor patterns  75  and  76  cross each other at right angles at the pattern crossing points  85  and  86 . 
     Accordingly, each of the pattern crossing points  85  and  86  has its crossing area S expressed by the following formula (8). 
     
       
           S=D/ 2× D/ 2= D×D/ 4  (8) 
       
     
     Substituting the crossing area S to the formula (1) gives the following formula (9). 
     
       
           C=ε· ( D×D/d )/4  (9) 
       
     
     Due to the parallel connection of the two pattern crossing points  85  and  86 , the total coupling capacitance Cu is expressed by the following formula (10). 
     
       
           Cu=ε ( D×D/d )/2  (10) 
       
     
     Consequently, the coupling capacitance Cu at the pattern crossing section  70  based on Embodiment 2 decreases to ½ of the coupling capacitance C of the pattern crossing section  111  shown in FIG.  7 . 
     Although, in the foregoing Embodiment 2, the partial conductor patterns  73  and  74  and the partial conductor patterns  75  and  76  cross each other at right angles at the pattern crossing points  85  and  86 , their orthogonal crossing is not compulsory. However, orthogonal crossing is preferable so that the crossing area is small. Although the partial conductor patterns  73  to  76  have an equal width, they may have different widths instead. However, an equal width is preferable so that the total crossing area is minimized. 
     Embodiment 2 has a reduced crossing area at the pattern crossing section  70  based on a simple structure thereby to reduce the coupling capacitance significantly, whereby the coil  20  can have a large Q value and thus suppress the decay of an MRI tomographic image. Although Embodiments 1 and 2 are designed to connect the outlets  40 ,  43 ,  77  and  78  to other ends  47 ,  46 ,  80  and  79  by using the conductor bars, conductor patterns may be formed in place of the conductor bars. 
     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.