Patent Publication Number: US-2022231656-A1

Title: Coil unit decoupling apparatus and magnetic resonance system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of the filing date of China patent application no. CN 202011473453.8, filed on Dec. 15, 2020, the contents of which are incorporated herein by reference in their entirety. 
     TECHNICAL FIELD 
     The present disclosure relates to magnetic resonance (MR) systems and, in particular, to a coil unit decoupling apparatus and a magnetic resonance system. 
     BACKGROUND 
     In a magnetic resonance (MR) system, particularly in a low-field MR system, the coupling between coil units is a very important issue. For a low-field MR system, the coupling between coil units that are far away from each other cannot be ignored due to a high Q factor of the coil units. 
     To implement decoupling between any coil units, many solutions have been proposed. The most common decoupling technique is to cancel out magnetic fields in the positive and negative directions in an overlapping manner. If overlapping-based decoupling is not feasible, inductive decoupling or capacitive decoupling is used. Another method is to implement strong decoupling of up to three or four coil units using cross capacitors. Recently, a method for decoupling coil units using end rings has also been proposed. However, each of these decoupling methods require complex adjustments during the manufacturing process. In addition, because a wire length of coil units and the loss of inductors are increased in these methods, additional signal-to-noise ratio losses are also caused. 
       FIG. 1  is a schematic diagram of eight typical coil units distributed on the surface of a cylinder, in which there are a total of two layers: a top layer and a bottom layer, and each layer has four coil units, i.e., coil units  11  to  14  at the top layer, and coil units  15  to  18  at the bottom layer. Taking the coil unit  11  as an example, the coil unit  11  can be easily decoupled from the coil units  12 ,  14 , and  15  by means of coil overlapping. However, since the coil unit  11  is not adjacent to the coil units  13 ,  16 ,  17 , and  18 , it is difficult to decouple from the coil units  13 ,  16 ,  17 , and  18 . Therefore, four decoupling circuits will be used to decouple the coil unit  11  from all other coil units. To completely decouple all the coil units from each other, a total of 8*4/2=16 decoupling circuits are required. 
       FIG. 2  shows a way of decoupling between the eight coil units shown in  FIG. 1 , in which y indicates that decoupling (in addition to overlapping) may be performed in each case, and x indicates that decoupling (in addition to overlapping) must be performed in each case. For example, overlapping-based decoupling y is used between the coil unit  11  and the coil units  12 ,  14 , and  15 , and circuit-based decoupling x needs to be used between the coil unit  11  and the coil units  13 ,  16 ,  17 , and  18 . 
     Still another method is to move coil units at one layer horizontally by half a coil width, that is, rotate the coil units horizontally by 45 degrees. As shown in  FIG. 3 , the coil units  11  to  14  at the top layer in  FIG. 1  are rotated horizontally by 45 degrees, to implement overlapping-based decoupling between the coil units  11  and  15  and between the coil units  11  and  16 . In this way, the number of decoupling circuits may be reduced to 12, as shown in  FIG. 4 . However, the symmetry between the coil units at the top layer and the bottom layer is broken. As a result, parallel imaging quality is reduced. In addition, it is very difficult to decouple  12  coil units. 
     SUMMARY 
     In view of this, the embodiments of the present disclosure provide a coil unit decoupling apparatus and an MR system to reduce the complexity of decoupling coil units in an MR system. 
     The technical solutions of the embodiments of the present disclosure are implemented as follows. 
     A coil unit decoupling apparatus is provided. The apparatus is connected to a first coil unit and a second coil unit in a magnetic resonance system, and is configured to separate, by using a distribution characteristic of a spatial quadrature field between the first coil unit and the second coil unit, a Helmholtz signal and an anti-Helmholtz signal from signals received from the first coil unit and the second coil unit, so as to implement decoupling between the first coil unit and the second coil unit. 
     The apparatus includes: a first phase-shift circuit, a second phase-shift circuit, and a combiner, wherein: 
     a first connection terminal of the first phase-shift circuit is connected to a first port of the first coil unit; 
     a second connection terminal of the first phase-shift circuit is connected to a first connection terminal of the combiner; 
     a second connection terminal of the combiner is connected to a first connection terminal of the second phase-shift circuit; 
     a second connection terminal of the second phase-shift circuit is connected to a first port of the second coil unit; and 
     the sum of phase differences between the first phase-shift circuit, the combiner, and the second phase-shift circuit is 180 degrees or −180 degrees. 
     The first phase-shift circuit includes: a first capacitor, a second capacitor, a first inductor, and a third capacitor, wherein: 
     a first connection terminal of the first capacitor is connected to the first port of the first coil unit; 
     a second connection terminal of the first capacitor is connected to a first connection terminal of the second capacitor and a first connection terminal of the first inductor; 
     a second connection terminal of the first inductor is connected to a first connection terminal of the third capacitor; 
     a second connection terminal of the third capacitor is grounded; and 
     a second connection terminal of the second capacitor is connected to the first connection terminal of the combiner. 
     The first phase-shift circuit further includes: a fourth capacitor, a fifth capacitor, and a sixth capacitor, wherein: 
     the fourth capacitor is connected in parallel across the first capacitor, the fifth capacitor is connected in parallel across the second capacitor, and the sixth capacitor is connected in parallel across the third capacitor; 
     alternatively, the first phase-shift circuit further includes: a fourth capacitor, a fifth capacitor, a sixth capacitor, and a seventh capacitor, wherein: 
     the fourth capacitor is connected in parallel across the first capacitor, the fifth capacitor is connected in parallel across the second capacitor, and the sixth capacitor and the seventh capacitor are respectively connected in parallel across the third capacitor. 
     The second phase-shift circuit includes: an eighth capacitor, a second inductor, and a ninth capacitor, wherein: 
     a first connection terminal of the eighth capacitor is connected to the second connection terminal of the combiner and a first connection terminal of the second inductor; 
     a second connection terminal of the eighth capacitor is grounded; 
     a second connection terminal of the second inductor is connected to a first connection terminal of the ninth capacitor and the first port of the second coil unit; and 
     a second connection terminal of the ninth capacitor is grounded. 
     The second phase-shift circuit further includes: a tenth capacitor and an eleventh capacitor, wherein: 
     the tenth capacitor is connected in parallel across the eighth capacitor, and the eleventh capacitor is connected in parallel across the ninth capacitor. 
     The combiner includes: a twelfth capacitor, a third inductor, a fourth inductor, and a thirteenth capacitor, wherein: 
     a first connection terminal of the twelfth capacitor is connected to the second connection terminal of the first phase-shift circuit and a first connection terminal of the fourth inductor; 
     a second connection terminal of the twelfth capacitor is connected to a first connection terminal of the third inductor; 
     a second connection terminal of the third inductor is connected to a first connection terminal of the thirteenth capacitor and the first connection terminal of the second phase-shift circuit; and 
     a second connection terminal of the thirteenth capacitor is connected to a second connection terminal of the fourth inductor. 
     A phase difference between the second connection terminal of the twelfth capacitor and the first port of the first coil unit is a first phase difference, a phase difference between the second connection terminal of the twelfth capacitor and the first port of the second coil unit is a second phase difference, and the difference between the first phase difference and the second phase difference is 180 degrees or −180 degrees. 
     The attenuation between the second connection terminal of the twelfth capacitor and the first port of the first coil unit is equal to the attenuation between the second connection terminal of the twelfth capacitor and the first port of the second coil unit. 
     The first coil unit and the second coil unit satisfy the condition of being symmetrical with respect to a plane. 
     A port in the apparatus that is used for outputting an anti-Helmholtz signal is connected to a fourteenth capacitor to eliminate coupling with another anti-Helmholtz signal. 
     A magnetic resonance system is provided, which includes a coil unit decoupling apparatus as described in any one of the above embodiments. 
     In the embodiments of the present disclosure, the first coil unit and the second coil unit in the MR system are connected to the coil unit decoupling apparatus. The apparatus is configured to separate, by using a distribution characteristic of a spatial quadrature field between the first coil unit and the second coil unit, a Helmholtz signal and an anti-Helmholtz signal from signals received from the first coil unit and the second coil unit, so as to implement decoupling between the first coil unit and the second coil unit, thereby reducing the complexity of decoupling the coil units. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
       The above and other features and advantages of the present disclosure will be more apparent to those of ordinary skill in the art from the detailed description of preferred embodiments of the present disclosure with reference to the accompanying drawings, in which: 
         FIG. 1  is an example schematic diagram of eight typical coil units distributed on the surface of a cylinder; 
         FIG. 2  is a schematic diagram of an existing way of decoupling between the eight coil units shown in  FIG. 1 ; 
         FIG. 3  is a schematic diagram of an existing way of overlapping-based decoupling between the coil units  1  and  5  and between the coil units  1  and  6  by horizontally rotating the coil units  1  to  4  in the first row in  FIG. 1  by 45 degrees; 
         FIG. 4  is a schematic diagram of an existing way of decoupling between the eight coil units shown in  FIG. 3 ; 
         FIG. 5  is a schematic diagram of an example coil unit decoupling apparatus according to an embodiment of the present disclosure; 
         FIG. 6  is a schematic structural diagram of an example coil unit decoupling apparatus according to an embodiment of the present disclosure; 
         FIG. 7  is a structural diagram of an example coil unit decoupling apparatus according to another embodiment of the present disclosure; 
         FIG. 8  is a structural diagram of an example coil unit decoupling apparatus according to still another embodiment of the present disclosure; 
         FIG. 9  is a schematic diagram of an example of four coil units distributed on the surface of a cylinder in an application example of the present disclosure; 
         FIG. 10  is a schematic diagram of simulation by connecting a coil unit decoupling apparatus between coil units  91  and  93  shown in  FIG. 9 ; 
         FIG. 11  is a schematic diagram of an example of port matching and decoupling effect achieved by performing, using simulation software, field simulation on the coil units shown in  FIG. 9  to which a coil unit decoupling apparatus according to an embodiment of the present disclosure has been applied; 
         FIG. 12  is a schematic diagram of a decoupling effect (unit: dB) achieved by performing field simulation on the coil units shown in  FIG. 9  to which a coil unit decoupling apparatus according to an embodiment of the present disclosure has been applied; 
         FIG. 13  is a diagram of a structure for simulation of decoupling the eight coil units, as shown in  FIG. 1 , in an MR system by using a coil unit decoupling apparatus according to an embodiment of the present disclosure; 
         FIG. 14  is a schematic diagram of a simulation result of the structure shown in  FIG. 13 ; 
       and 
         FIG. 15  is a diagram of an example application of a coil unit decoupling apparatus according to an embodiment of the present disclosure, which is connected between two coil units. 
     
    
    
     Reference numerals in the accompanying drawings are as follows: 
     
       
         
           
               
               
             
               
                   
               
               
                 Reference numeral 
                 Meaning 
               
               
                   
               
             
            
               
                 11-18 
                 Coil unit 
               
               
                 50 
                 Coil unit decoupling apparatus according 
               
               
                   
                 to embodiments of the present disclosure 
               
               
                 100 
                 First coil unit 
               
               
                 200 
                 Second coil unit 
               
               
                 51 
                 First phase-shift circuit 
               
               
                 52 
                 Second phase-shift circuit 
               
               
                 53 
                 Combiner 
               
               
                 101 
                 First port of first coil unit 
               
               
                 201 
                 First port of second coil unit 
               
               
                 511 
                 First capacitor 
               
               
                 512 
                 Second capacitor 
               
               
                 513 
                 First inductor 
               
               
                 514 
                 Third capacitor 
               
               
                 515 
                 Fourth capacitor 
               
               
                 516 
                 Fifth capacitor 
               
               
                 517 
                 Sixth capacitor 
               
               
                 518 
                 Seventh capacitor 
               
               
                 521 
                 Eighth capacitor 
               
               
                 522 
                 Second inductor 
               
               
                 523 
                 Ninth capacitor 
               
               
                 524 
                 Tenth capacitor 
               
               
                 525 
                 Eleventh capacitor 
               
               
                 531 
                 Twelfth capacitor 
               
               
                 532 
                 Third inductor 
               
               
                 533 
                 Fourth inductor 
               
               
                 534 
                 Thirteenth capacitor 
               
               
                 91~94 
                 Coil unit 
               
               
                 901 
                 First simulation port 
               
               
                 902 
                 Second simulation port 
               
               
                 151 
                 First phase-shift circuit 
               
               
                 152 
                 Second phase-shift circuit 
               
               
                 153 
                 Combiner 
               
               
                   
               
            
           
         
       
     
     DETAILED DESCRIPTION 
     In order to make the objectives, technical solutions, and advantages of the present disclosure more apparent, the present disclosure will be described in further detail by way of embodiments hereinafter. 
       FIG. 5  is a schematic diagram of a coil unit decoupling apparatus  50  according to an embodiment of the present disclosure. The apparatus  50  is respectively connected to a first coil unit  100  and a second coil unit  200  in an MR system, and is configured to separate, by using a distribution characteristic of a spatial quadrature field between the first coil unit  100  and the second coil unit  200 , a Helmholtz signal and an anti-Helmholtz signal from signals received from the first coil unit  100  and the second coil unit  200 , so as to implement decoupling between the first coil unit  100  and the second coil unit  200 . 
     In the foregoing embodiment, the first coil unit and the second coil unit in the MR system are connected to the coil unit decoupling apparatus. The coil unit decoupling apparatus is configured to separate, by using a distribution characteristic of a spatial quadrature field between the first coil unit and the second coil unit, a Helmholtz signal and an anti-Helmholtz signal from signals received from the first coil unit and the second coil unit, so as to implement decoupling between the first coil unit and the second coil unit, thereby reducing the complexity of decoupling the coil units. 
       FIG. 6  is a schematic structural diagram of a coil unit decoupling apparatus  50  according to an embodiment of the present disclosure. The coil unit decoupling apparatus  50  mainly includes: a first phase-shift circuit  51 , a second phase-shift circuit  52 , and a combiner  53 . 
     I). A first connection terminal of the first phase-shift circuit  51  is connected to a first port  101  of a first coil unit in an MR system; and 
     a second connection terminal of the first phase-shift circuit  51  is connected to a first connection terminal of the combiner  53 , 
     wherein the first port of the first coil unit is any port of the first coil unit. Generally, there are a plurality of ports for connecting capacitors in each loop of coil units, and any one of the ports may be selected as the first port  101 . 
     II). A second connection terminal of the combiner  53  is connected to a first connection terminal of the second phase-shift circuit  52 . 
     III). A second connection terminal of the second phase-shift circuit  52  is connected to a first port  201  of a second coil unit in the MR system, 
     wherein the first port of the second coil unit is any port of the second coil unit. Generally, there are a plurality of ports for connecting capacitors in each loop of coil units, and any one of the ports may be selected as the first port  201 . 
     The first coil unit and the second coil unit satisfy the condition of being symmetrical with respect to a plane. The sum of phase differences between the first phase-shift circuit  51 , the combiner  53 , and the second phase-shift circuit  52  is 180 degrees or −180 degrees. 
     In the foregoing embodiment, the decoupling apparatus with a phase difference of 180 degrees or −180 degrees is connected between the first coil unit and the second coil unit in the MR system, so as to implement decoupling between the first coil unit and the second coil unit easily, thereby reducing the decoupling complexity. 
     In actual application, the coil unit decoupling apparatus provided in the embodiment of the present disclosure needs to be connected only between every two coil units in the MR system that are symmetrical with respect to a plane. For example, in a coil unit structure shown in  FIG. 1 , a coil unit decoupling apparatus needs to be connected only between the coil units  1  and  3 ,  2  and  4 ,  5  and  7 , as well as  6  and  8 , respectively, in which case a total of only four decoupling apparatuses are required. It can be seen that the complexity is greatly reduced. 
       FIG. 7  is a schematic structural diagram of a coil unit decoupling apparatus  50  according to another embodiment of the present disclosure. The coil unit decoupling apparatus  50  mainly includes: a first phase-shift circuit  51 , a second phase-shift circuit  52 , and a combiner  53 . 
     I). The first phase-shift circuit  51  includes: a first capacitor  511 , a second capacitor  512 , a first inductor  513 , and a third capacitor  514 , wherein: 
     a first connection terminal of the first capacitor  511  is connected to a first port  101  of a first coil unit  100  in an MR system; 
     a second connection terminal of the first capacitor  511  is connected to a first connection terminal of the second capacitor  512  and a first connection terminal of the first inductor  513 ; 
     a second connection terminal of the first inductor  513  is connected to a first connection terminal of the third capacitor  514 ; 
     a second connection terminal of the third capacitor  514  is grounded; and 
     a second connection terminal of the second capacitor  512  is connected to a first connection terminal of the combiner  53 . 
     II). The second phase-shift circuit  52  includes: an eighth capacitor  521 , a second inductor  522 , and a ninth capacitor  523 , wherein: 
     a first connection terminal of the eighth capacitor  521  is connected to a second connection terminal of the combiner  53  and a first connection terminal of the second inductor  522 ; 
     a second connection terminal of the eighth capacitor  521  is grounded; 
     a second connection terminal of the second inductor  522  is connected to a first connection terminal of the ninth capacitor  523  and a first port  201  of a second coil unit in the MR system; and 
     a second connection terminal of the ninth capacitor  523  is grounded. 
     III). The combiner  53  includes: a twelfth capacitor  531 , a third inductor  532 , a fourth inductor  533 , and a thirteenth capacitor  534 , wherein: 
     a first connection terminal of the twelfth capacitor  531  is connected to a second connection terminal of the first phase-shift circuit and a first connection terminal of the fourth inductor  533 ; 
     a second connection terminal of the twelfth capacitor  531  is connected to a first connection terminal of the third inductor  532 ; 
     a second connection terminal of the third inductor  532  is connected to a first connection terminal of the thirteenth capacitor  534  and a first connection terminal of the second phase-shift circuit; and 
     a second connection terminal of the thirteenth capacitor  534  is connected to a second connection terminal of the fourth inductor  533 . 
     Assuming that a phase difference between the second connection terminal of the twelfth capacitor  531  and the first port  101  of the first coil unit is a first phase difference, and a phase difference between the second connection terminal of the twelfth capacitor  531  and the first port  201  of the second coil unit is a second phase difference, the difference between the first phase difference and the second phase difference is 180 degrees or −180 degrees. The setting of a capacitance value of each capacitor and an inductance value of each inductor in the coil unit decoupling apparatus  50  only needs to satisfy the condition that the difference between the first phase difference and the second phase difference is 180 degrees or −180 degrees. 
     In addition, in actual application, it can be further considered that the setting of a capacitance value of each capacitor and an inductance value of each inductor in the coil unit decoupling apparatus  50  satisfies the condition that the attenuation between the second connection terminal of the twelfth capacitor  531  and the first port  101  of the first coil unit is equal to the attenuation between the second connection terminal of the twelfth capacitor  531  and the first port  201  of the second coil unit, where the attenuation is generally 3 decibels (dB). 
       FIG. 8  is a schematic structural diagram of a coil unit decoupling apparatus  50  according to another embodiment of the present disclosure. The apparatus has the following elements added therein in comparison with the apparatus shown in  FIG. 7 . 
     I). The first phase-shift circuit  51  has added therein: a fourth capacitor  515 , a fifth capacitor  516 , and a sixth capacitor  517 , wherein: 
     the fourth capacitor  515  is connected in parallel across the first capacitor  511 , the fifth capacitor  516  is connected in parallel across the second capacitor  512 , and the sixth capacitor  517  is connected in parallel across the third capacitor  514 . 
     In actual application, the first phase-shift circuit  51  may further include: a seventh capacitor  518 , which is connected in parallel across the third capacitor  514 . 
     II). The second phase-shift circuit  52  has added therein: a tenth capacitor  524  and an eleventh capacitor  525 , wherein: 
     the tenth capacitor  524  is connected in parallel across the eighth capacitor  521 , and the eleventh capacitor  525  is connected in parallel across the ninth capacitor  523 . 
     Application examples of the present disclosure are provided as follows. 
       FIG. 9  shows four coil units in an MR system that are distributed on the surface of a cylinder, in which coil units  91  and  93  are symmetrical with respect to a central vertical section of the cylinder, and coil units  92  and  94  are symmetrical with respect to the central vertical section of the cylinder. Therefore, it is impossible to implement overlapping-based decoupling between  91  and  93  and between  92  and  94 . The coil unit decoupling apparatus provided in the embodiment of the present disclosure is connected between  91  and  93  and between  92  and  94 . 
       FIG. 10  is a schematic diagram of a simulation by connecting a coil unit decoupling apparatus between the coil units  91  and  93 , in which  101  denotes a first port of the coil unit  91 , and  201  denotes a first port of the coil unit  93 .  901  denotes a first simulation port, and  902  denotes a second simulation port, that is,  901  is connected to a first connection terminal of a third inductor  532 , and  902  is connected to a second connection terminal of a fourth inductor  533 . Here,  901  is set to be in an anti-Helmholtz (hereinafter referred to as AH) mode, that is,  901  inputs or outputs an AH signal.  902  is set to be in a Helmholtz (hereinafter referred to as H) mode, that is,  902  inputs or outputs an H signal. 
     Similarly, the coil unit decoupling apparatus connected between the coil units  92  and  94  is also provided with two simulation ports, which are respectively set to be a third simulation port and a fourth simulation port, where the third simulation port is connected to the first connection terminal of the third inductor  532  in the decoupling apparatus, and the fourth simulation port is connected to the second connection terminal of the fourth inductor  533  in the decoupling apparatus. The third simulation port is set to be in the AH mode, and the fourth simulation port is set to be in the H mode. 
       FIG. 11  is a schematic diagram of a port matching and decoupling effect achieved by performing, using simulation software, field simulation on the coil units shown in  FIG. 9  to which a coil unit decoupling apparatus according to an embodiment of the present disclosure has been applied. After the coil unit decoupling apparatus is respectively connected between the coil units  91  and  93  and between  92  and  94 , capacitors in a loop of coil units  91  to  94  are adjusted to maintain the frequency of the coil units at 80 MHz. In  FIG. 11 , m 1  denotes a reflection parameter of an AH signal on a first simulation port, m 2  denotes a reflection parameter of an H signal on a fourth simulation port, and m 5  denotes a reflection parameter of an AH signal (used for describing a decoupling effect of a third simulation port and the first simulation port) input from the third simulation port and coupled to the first simulation port for output, where the amplitudes and phases of the signals corresponding to m 1 , m 2 , and m 5  are denoted by S(1,1), S(4,4), and S(1,3), respectively. As shown in  FIG. 11 , S(1,1)=0.013/−147.022, S(4,4)=0.012/−114.217, and S(1,3)=0.211/65.901, where 0.013, 0.012, and 0.211 respectively denote the amplitudes, and −147.022, −114.217, and 65.901 respectively denote the phases. 
       FIG. 12  is a schematic diagram of a decoupling effect (unit: dB) achieved by performing field simulation on the coil units shown in  FIG. 9  to which a coil unit decoupling apparatus according to an embodiment of the present disclosure has been applied, in which m 3  denotes the degree of coupling between a second simulation port and a first simulation port, m 4  denotes the degree of coupling between a third simulation port and the first simulation port, and m 6  denotes the degree of coupling between a fourth simulation port and the first simulation port. 
     It can be seen from  FIG. 11  and  FIG. 12  that: 
     I). Owing to symmetry, a good decoupling effect is achieved between signals in the H mode and between signals in the H mode and AH mode. For example, S(4,4)=0.012/−114.217 in  FIG. 11  corresponds to the degree of coupling between the signals in the H mode, and dB(S(1,4))=−71.244 and dB(S(1,2))=−59.207 in  FIG. 12  correspond to the degrees of coupling between the signals in the H mode and AH mode. It can be seen that the degrees of coupling all have relatively small values. 
     II). A decoupling effect between the signals in the AH mode and AH mode is poor, such as S(1,3)=0.211/65.901 in  FIG. 11  and dB(S(1,3))=−13.508 in  FIG. 12 , both of which are larger values. 
     In actual application, the coupling between the signals in the AH mode and AH mode can be further reduced by adjusting an overlap between adjacent coil units. 
       FIG. 13  is a diagram of a structure for simulation of decoupling the eight coil units, as shown in  FIG. 1 , in a magnetic resonance (MR) system by using a coil unit decoupling apparatus according to an embodiment of the present disclosure. This structure uses a 4× mixed mode to separate H signals from AH signals. 
     There are four ports in each loop of coil units, and one of the ports is used to connect the coil unit decoupling apparatus provided in the embodiment of the present disclosure. As shown in  FIG. 13 , ports  3 ,  7 ,  11 ,  15 ,  21 ,  25 , and  29  are separately used to connect the decoupling apparatus  50  provided in the embodiment of the present disclosure. The other three ports in each loop of coil units are connected to capacitors. As shown in  FIG. 13 , c 1  to c 12  correspond to capacitors in a loop of four coil units at the bottom layer, and c 13  to c 24  correspond to capacitors in a loop of four coil units at the top layer. c 1  to c 24  are adjusted so that all H signals are tuned to a magnetic resonance (MR) frequency. All the signals have a slight frequency offset due to the Hall effect. 
     For the eight coil units, there are eight simulation ports, in which four simulation ports (simulation ports T 2 , T 4 , T 6 , and T 8  in  FIG. 12 ) are in the H mode, and four simulation ports (simulation ports T 1 , T 3 , T 5 , and T 7  in  FIG. 12 ) are in the AH mode. 
       FIG. 14  shows a simulation result of the structure shown in  FIG. 13 . It can be seen that there is strong coupling only between the simulation ports T 1  and T 7  and between T 3  and T 5  in the AH mode, and the degree of coupling between signals in other modes is lower than 12 dB. 
     To further reduce the coupling between signals in the AH mode and AH mode, additional cross capacitors c 25  and c 26  may be added to the structure. In other words, in the coil unit decoupling apparatus  50  provided in the embodiment of the present disclosure, a port for outputting an AH signal may be connected to a fourteenth capacitor to eliminate coupling with other AH signals. 
     Because AH signals have no signal strength at the center of the entire coil structure, the signal-to-noise ratio at the center of the coil structure is only provided by H signals. Even when there is a need to take complex measures to resolve the problem of coupling between signals in the AH mode and AH mode, the signal-to-noise ratio at the center of the coil structure is not affected, although a signal-to-noise ratio loss may be caused. 
       FIG. 15  presents a diagram of an application of a coil unit decoupling apparatus according to an embodiment of the present disclosure, which is connected between two coil units.  151  denotes a first phase-shift circuit,  152  denotes a second phase-shift circuit, and  153  denotes a combiner. 
     The coil unit decoupling apparatus provided in the embodiment of the present disclosure may be placed in front of a preamplifier of the coil units, which will additionally cause a signal-to-noise ratio loss. Generally, the signal-to-noise ratio loss is approximately 0.1 dB to 0.2 dB, which is equivalent to a decrease of 1% to 2% in signal-to-noise ratio. It has been found through tests that, after the use of the coil unit decoupling apparatus provided in the embodiment of the present disclosure, a signal-to-noise ratio loss can be limited to 3.1 dB. Theoretically, a signal-to-noise ratio loss within 3 dB is considered as zero loss. Therefore, it may be considered that, after the use of the coil unit decoupling apparatus provided in the embodiment of the present disclosure, the additional signal-to-noise ratio loss is only 0.1 dB, which is equivalent to a signal-to-noise ratio loss of 1%. It can be seen that, after the use of the coil unit decoupling apparatus provided in the embodiment of the present disclosure, little signal-to-noise ratio loss is caused. 
     In addition, it should be noted that the standard impedance of a radio frequency circuit is generally 50 Ohms. After the coil unit decoupling apparatus provided in the embodiment of the present disclosure is connected to the coil units, capacitors in a loop of coil units may be first adjusted, so that the impedance of ports in the H mode is tuned to 50 Ohms. At this moment, the impedance of ports in the AH mode may deviate from 50 Ohms. Then, the capacitors are adjusted again to tune the impedance of the ports in the AH mode to 50 Ohms. 
     The embodiments of the present disclosure further provide an MR system, which includes a coil unit decoupling apparatus  50  as described in any one of the above embodiments. 
     The beneficial technical effects of the embodiments of the present disclosure are as follows: 
     I. The coil unit decoupling apparatus provided in the embodiments of the present disclosure has a simple structure, is easily implemented, and has a strong decoupling effect. The coil units may be adjusted systematically without difficulty. For multiple layers of coil units in the magnetic resonance system that are distributed on the surface of the cylinder, the coil units at each layer may have the same layout and may be arranged in the same manner, to achieve the optimal performance of magnetic resonance imaging. 
     II. There is no need to use a complex copper structure, and the signal-to-noise ratio at the center of the coil structure can be maintained at the optimal value. 
     III. The coil unit decoupling apparatus provided in the embodiments of the present disclosure is particularly applicable to a low-field system in which a Q factor is high and decoupling is hard to implement. 
     The above description is merely illustrative of the preferred embodiments of the present disclosure and is not intended to limit the present disclosure. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present disclosure shall fall within the scope of protection of the present disclosure.