Patent Publication Number: US-2022229129-A1

Title: Cross inductor/capacitor to simplify mri coil element decoupling

Description:
TECHNICAL FIELD 
     The present disclosure relates to the technical field of magnetic resonance (MR) systems, in particular to a coil unit decoupling device and a magnetic resonance system. 
     BACKGROUND 
     In an MR system, especially a low-field MR system, coupling between coil units is very important. For a low-field MR system, because of the high Q-factors of coil units, coupling between coil units far away from each other must not be ignored, either. 
     In order to realize decoupling between coil units, a number of solutions have been proposed in the prior art. The most common decoupling method is to use overlapping to offset the magnetic fields in the positive and negative directions. If overlapping decoupling does not work, inductance decoupling or capacitance decoupling is used. Another method is to use a crossover capacitance to realize strong decoupling between up to 3˜4 coil units. Recently, a method of using an end-ring for decoupling between coil units has also been proposed. Complex adjustments are required for all these decoupling methods during the manufacturing process. In addition, an additional signal-to-noise ratio (SNR) loss will be caused because the wire lengths of coil units and the inductance loss are increased in these decoupling methods. 
       FIG. 1  is a schematic diagram of four coil units distributed on a cylindrical surface, wherein overlapping decoupling can be realized between adjacent coil units, for example, between coil units  11  and  12 , between coil units  11  and  14 , between coil units  12  and  13 , and between coil units  13  and  14 , but it is difficult to realize decoupling between coil units  11  and  13  and between coil units  12  and  14  because the coil units  11  and  13  are not adjacent to each other and the coil units  12  and  14  are not adjacent to each other. 
     SUMMARY 
     In view of this, aspects of the present disclosure provide coil unit decoupling devices to lower the complexity in decoupling coil units in an MR system. 
     Aspects of the present disclosure further provide MR systems to lower the complexity in decoupling the coil units in the MR systems. 
     The technical solution of the aspects of the present disclosure is realized in this way: 
     A coil unit decoupling device comprises a first phase shift circuit, a second phase shift circuit and a first crossover element, and the first crossover element is a capacitor or inductor, wherein 
     a first connecting end of the first phase shift circuit is connected with a first port of a first coil unit; 
     a second connecting end of the first phase shift circuit is connected with a first connecting end of the first crossover element; 
     a first connecting end of the second phase shift circuit is connected with a first port of a second coil unit; 
     a second connecting end of the second phase shift circuit is connected with a second connecting end of the first crossover element; 
     the first phase shift circuit enables the first coil unit to be matched and enables the first coil unit to have a phase shift of 180° between a matched state and a non-matched state, the second phase shift circuit enables the second coil unit to be matched and enables the second coil unit to have a phase shift of 180° between a matched state and a non-matched state, the first coil unit and the second coil unit are located in a magnetic resonance system, the first port is any port on a self-contained loop of the first coil unit, and the second port is any port on a self-contained loop of the second coil unit. 
     The first phase shift circuit comprises a first capacitor and a first inductor group, and the first inductor group comprises one inductor or multiple inductors connected in series, wherein 
     a first connecting end of the first capacitor is connected with the first port of the first coil unit and a first connecting end of the first inductor group, and a second connecting end of the first capacitor is grounded, wherein the first connecting end of the first inductor group is the connecting end of a first inductor in the first inductor group for an external connection, and a second connecting end of the first inductor group is the connecting end of a last inductor in the first inductor group for an external connection; 
     and/or, the second phase shift circuit comprises a second capacitor and a second inductor group, wherein 
     a first connecting end of the second capacitor is connected with the first port of the second coil unit and a first connecting end of the second inductor group, and a second connecting end of the second capacitor is grounded, wherein the first connecting end of the second inductor group is the connecting end of a first inductor in the second inductor group for an external connection, and a second connecting end of the second inductor group is the connecting end of a last inductor in the second inductor group for an external connection; 
     the first connecting end of the first crossover element is connected with any connecting end of any inductor in the first inductor group, and the second connecting end of the first crossover element is connected with any connecting end of any inductor in the second inductor group. 
     The device further comprises at least one crossover element, the at least one crossover element being a capacitor or inductor, wherein a first connecting end of each crossover element of the at least one crossover element is connected with any connecting end of any inductor in the first inductor group, and a second connecting end of each crossover element of the at least one crossover element is connected with any connecting end of any inductor in the second inductor group. 
     The device further comprises a second crossover element, wherein the second crossover element is a capacitor or inductor, and 
     a first connecting end of the second crossover element is connected with the first connecting end of the first inductor group, and a second connecting end of the second crossover element is connected with the first connecting end of the second inductor group. 
     The first phase shift circuit further comprises a third capacitor, and/or the second phase shift circuit further comprises a fourth capacitor, wherein 
     a first connecting end of the third capacitor is connected with the second connecting end of the first inductor group, and a second connecting end of the third capacitor is grounded; 
     a first connecting end of the fourth capacitor is connected with the second connecting end of the second inductor group, and a second connecting end of the fourth capacitor is grounded. 
     The device further comprises first radio-frequency (RF) traps and/or second RF traps, wherein 
     the first RF traps are connected between the second connecting end of the first inductor group and the first connecting end of the first crossover element; 
     the second RF traps are connected between the second connecting end of the second inductor group and the second connecting end of the first crossover element. 
     The first phase shift circuit comprises a first capacitor and a first inductor, wherein 
     a first connecting end of the first capacitor is connected with the first port of the first coil unit and a first connecting end of the first inductor, a second connecting end of the first capacitor is grounded, and a second connecting end of the first inductor is connected with the first connecting end of the first crossover element; 
     the second phase shift circuit comprises a second capacitor and a second inductor, wherein 
     a first connecting end of the second capacitor is connected with the first port of the second coil unit and a first connecting end of the second inductor, a second connecting end of the second capacitor is grounded, and a second connecting end of the second inductor is connected with the second connecting end of the first crossover element. 
     The first phase shift circuit comprises a first capacitor, a third inductor and a fourth inductor, wherein 
     a first connecting end of the first capacitor is connected with the first port of the first coil unit and a first connecting end of the third inductor, a second connecting end of the first capacitor is grounded, and a second connecting end of the third inductor is connected with the first connecting end of the first crossover element and a first connecting end of the fourth inductor; 
     the second phase shift circuit comprises a second capacitor, a fifth inductor and a sixth inductor, wherein 
     a first connecting end of the second capacitor is connected with the first port of the second coil unit and a first connecting end of the fifth inductor, a second connecting end of the second capacitor is grounded, and a second connecting end of the fifth inductor is connected with the second connecting end of the first crossover element and a first connecting end of the sixth inductor. 
     The first phase shift circuit comprises a first capacitor, a seventh inductor and an eighth inductor, wherein 
     a first connecting end of the first capacitor is connected with the first port of the first coil unit and a first connecting end of the seventh inductor, a second connecting end of the first capacitor is grounded, and a second connecting end of the seventh inductor is connected with the first connecting end of the first crossover element and a first connecting end of the eighth inductor; 
     the second phase shift circuit comprises a second capacitor and a ninth inductor, wherein 
     a first connecting end of the second capacitor is connected with the first port of the second coil unit and a first connecting end of the ninth inductor, a second connecting end of the second capacitor is grounded, and a second connecting end of the ninth inductor is connected with the second connecting end of the first crossover element. 
     The first phase shift circuit comprises a first capacitor, a seventh inductor and an eighth inductor, wherein 
     a first connecting end of the first capacitor is connected with the first port of the first coil unit and a first connecting end of the seventh inductor, a second connecting end of the first capacitor is grounded, and a second connecting end of the seventh inductor is connected with the first connecting end of the first crossover element and a first connecting end of the eighth inductor; 
     the second phase shift circuit comprises a second capacitor and a ninth inductor, wherein 
     a first connecting end of the second capacitor is connected with the first port of the second coil unit, a first connecting end of the ninth inductor and a second connecting end of the first crossover element, and a second connecting end of the second capacitor is grounded. 
     An MR system comprises the above-mentioned coil unit decoupling device. 
     In aspects of the present disclosure, the first coil unit is connected with the first phase shift circuit, the second coil unit is connected with the second phase shift circuit, the first crossover capacitor or inductor is connected between the first phase shift circuit and the second phase shift circuit, and reactance coupling and/or impedance coupling between the first coil unit and the second coil unit is offset by the first crossover capacitor or inductor to realize decoupling between the first coil unit and the second coil unit. Thus, decoupling between the coil units is realized, and the complexity in decoupling the coil units is lowered. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred aspects of the present disclosure will be described in detail below by referring to the drawings so that those skilled in the art can have a clearer idea of the above-mentioned and other characteristics and advantages of the present disclosure. 
         FIG. 1  is a schematic diagram of typical four coil units distributed on a cylindrical surface; 
         FIG. 2  shows the structure of a coil unit decoupling device provided by one aspect of the present disclosure; 
         FIG. 3  shows the structure of a coil unit decoupling device provided by another aspect of the present disclosure; 
         FIG. 4  shows the structure of a coil unit decoupling device provided by a still another aspect of the present disclosure; 
         FIG. 5  compares the port matching effects obtained after field simulations are performed for a coil unit in an MR system by use of simulation software, wherein the coil unit decoupling device provided by the present disclosure is applied and is not applied, respectively; 
         FIG. 6  shows the structure of a coil unit decoupling device provided by one aspect of the present disclosure, wherein an RF trap is added; 
         FIG. 7  shows the structure of a coil unit decoupling device provided by another aspect of the present disclosure, wherein an RF trap is added; 
         FIG. 8  shows the circuit of the application of the coil unit decoupling device provided by the present disclosure to the coil units shown in  FIG. 1 ; 
         FIG. 9  shows the port matching effect and the decoupling effect obtained after field simulations are performed for the circuit shown in  FIG. 8  by use of simulation software; 
         FIG. 10  compares the reflection coefficients of signals on different simulation ports of the circuit in  FIG. 8  before and after mistuning of the coil unit  11 ; 
         FIG. 11  shows the structure of a coil unit decoupling device provided by yet another aspect of the present disclosure; 
         FIG. 12  shows the circuit obtained after a crossover element (capacitor or inductor) is added between the first connecting ends of the inductors in the phase shift circuit of every two coil units on the basis of the circuit shown in  FIG. 8 ; 
         FIG. 13  shows the port matching effect and the decoupling effect obtained after field simulations are performed for the circuit shown in  FIG. 12  by use of simulation software; 
         FIG. 14  shows the structure of a coil unit decoupling device provided by another aspect of the present disclosure; 
         FIG. 15  shows the circuit obtained after the inductor in the phase shift circuit of each coil unit is replaced by two inductors connected in series and the connection point between the phase shift circuit and the crossover element is located on the connection line between the two inductors on the basis of the circuit shown in  FIG. 8 ; 
         FIG. 16  shows the port matching effect and the decoupling effect obtained after field simulations are performed for the circuit shown in  FIG. 15  by use of simulation software; 
         FIG. 17  shows the structure of a coil unit decoupling device provided by yet another aspect of the present disclosure; 
         FIG. 18  shows the structure of a coil unit decoupling device provided by still another aspect of the present disclosure; and 
         FIG. 19  shows the port matching effect and the decoupling effect obtained after field simulations are performed in the case that the phase of the simulation ports on the circuit shown in  FIG. 8  is −45°. 
     
    
    
     Description of reference numerals in the drawings: 
     REFERENCE NUMERAL MEANING 
     
         
           11 - 14  Coil units 
           20  Coil unit decoupling device provided by the present disclosure 
           100  First coil unit 
           200  Second coil unit 
           21  First phase shift circuit 
           22  Second phase shift circuit 
           120  First crossover element 
           121  Second crossover element 
           101  First port of first coil unit 
           201  First port of second coil unit 
           211  First capacitor 
           212  First inductor 
           2121  Third inductor 
           2122  Fourth inductor 
           2123  Seventh inductor 
           2124  Eighth inductor 
           213  Third capacitor 
           214 ,  215  First RF trap 
           221  Second capacitor 
           222  Second inductor 
           2221  Fifth inductor 
           2222  Sixth inductor 
           2223  Ninth inductor 
           223  Fourth capacitor 
           224 ,  225  Second RF trap 
       
    
     DETAILED DESCRIPTION 
     To make clearer the objectives, technical solutions, and advantages of the present disclosure, aspects are used below to further describe the present disclosure. 
       FIG. 2  shows the structure of a coil unit decoupling device  20  provided by one aspect of the present disclosure. The device  20  mainly comprises a first phase shift circuit  21 , a second phase shift circuit  22  and a first crossover element  120 , and the first crossover element  120  is a capacitor or inductor, wherein 
     a first connecting end of the first phase shift circuit  21  is connected with a first port  101  of a first coil unit; 
     a second connecting end of the first phase shift circuit  21  is connected with a first connecting end of the first crossover element  120 ; 
     a first connecting end of the second phase shift circuit  22  is connected with a first port  201  of a second coil unit; 
     a second connecting end of the second phase shift circuit  22  is connected with a second connecting end of the first crossover element  120 ; 
     wherein, the first phase shift circuit  21  enables the first coil unit to be matched and enables the first coil unit to have a phase shift of 180° between a matched state and a non-matched state; the second phase shift circuit  22  enables the second coil unit to be matched and enables the second coil unit to have a phase shift of 180° between a matched state and a non-matched state, the first coil unit and the second coil unit are located in an MR system, the first port  101  is any port on the self-contained loop of the first coil unit, and the second port  201  is any port on the self-contained loop of the second coil unit. 
     A plurality of ports for connecting capacitors are available on the self-contained loop of each coil unit in the MR system. Any port on the self-contained loop of the first coil unit can serve as the first port  101 , and any port on the self-contained loop of the second coil unit can serve as the second port  201 . 
     In the above-mentioned aspect, the first coil unit is connected with the first phase shift circuit, the second coil unit is connected with the second phase shift circuit, the first crossover capacitor or inductor is connected between the first phase shift circuit and the second phase shift circuit, and reactance coupling and/or impedance coupling between the first coil unit and the second coil unit is offset by the first crossover capacitor or inductor to realize decoupling between the first coil unit and the second coil unit. Thus, decoupling between the coil units is realized, and the complexity in decoupling the coil units is lowered. 
     The specific implementation of the coil unit decoupling device  20  may be as follows: 
     The first phase shift circuit  21  comprises a first capacitor  211  and a first inductor  212 , wherein 
     a first connecting end of the first capacitor  211  is connected with a first port  101  of a first coil unit  100  and a first connecting end of the first inductor  212 , a second connecting end of the first capacitor  211  is grounded, a second connecting end of the first inductor  212  is connected with a first connecting end of a first crossover element  120 , and the second connecting end of the first inductor  212  is also connected with a subsequent circuit of the first coil unit  100 , for example, the input end of a front-end low-noise amplifier of the first coil unit  100 , through a signal line; 
     and/or, the second phase shift circuit  22  comprises a second capacitor  221  and a second inductor  222 , wherein 
     a first connecting end of the second capacitor  221  is connected with the first port  201  of the second coil unit  200  and a first connecting end of the second inductor  222 , a second connecting end of the second capacitor  221  is grounded, and a second connecting end of the second inductor  222  is connected with the second connecting end of the first crossover element  120 . The second connecting end of the second inductor  222  is also connected with a subsequent circuit of the second coil unit  200 , for example, the input end of a front-end low-noise amplifier of the second coil unit  200 , through a signal line. 
       FIG. 3  shows the first phase shift circuit  21  comprising a first capacitor  211  and a first inductor  212 , and the second phase shift circuit  22  comprising a second capacitor  221  and a second inductor  222 . Wherein,  300  and  400  represent coaxial cables, respectively. 
     In practical applications, tuning is first required in the production of coils, that is, the frequency of each coil unit is adjusted to the MR frequency. When a coil unit is tuned, it is necessary to first disconnect the loops of all other coil units, and then adjust the capacitances of the capacitors on the loop of the coil unit until the frequency of the coil unit reaches the MR frequency. As shown in  FIG. 3 , there are only two coil units. When the first coil unit  100  is tuned, it is necessary to disconnect the loop of the second coil unit  200 , for example, disconnect one of the capacitors C 21 , C 22  and C 23  on the loop of the second coil unit  200 , and then adjust the capacitances of the capacitors C 11 , C 12  and C 13  on the loop of the first coil unit  100  until the frequency of the first coil unit  100  reaches the MR frequency. 
     For the device shown in  FIG. 3 , the values of the first capacitor  211 , the first inductor  212 , the second capacitor  221 , the second inductor  222  and the first crossover element  120  are determined in the following way: 
     When the first coil unit  100  and the second coil unit  200  are electrified, the values of the first capacitor  211 , the first inductor  212 , the second capacitor  221 , the second inductor  222  and the first crossover element  120  are continuously adjusted, and the phase shift of the first connecting end of the first crossover element  120  between the current state and a non-matched state and the phase shift of the second connecting end of the first crossover element  120  between the current state and a non-matched state are respectively measured for each group of values through simulations. When the two phase shifts are both 180°, the adjustment of the values of the above-mentioned elements is stopped, and it is determined that the first coil unit  100  and the second coil unit  200  are both in a matched state. 
     After the first coil unit  100  and the second coil unit  200  are both in a matched state, the value of the first crossover element  120  is adjusted (the values of the first capacitor  211 , the first inductor  212 , the second capacitor  221 , and the second inductor  222  may need to be fine-tuned during this process) until optimal decoupling between the first coil unit  100  and the second coil unit  200  is achieved. At this time, the values of the first capacitor  211 , the first inductor  212 , the second capacitor  221 , the second inductor  222  and the first crossover element  120  are the desired values. 
     The standard impedance of an RF circuit is usually 50 ohms. When the impedances of the first connecting end and second connecting end of the first crossover element  120  reach 50 ohms during adjustments of the values of the first capacitor  211 , the first inductor  212 , the second capacitor  221 , the second inductor  222  and the first crossover element  120 , it indicates that the first coil unit  100  and the second coil unit  200  are in a matched state. 
     In practical applications, when more than two coil units, for example, four coil units as shown in  FIG. 1 , or even more coil units, for example, eight coil units, are distributed on a cylindrical surface in an MR system, a crossover element (like the first crossover element) needs to be connected between the phase shift circuits of every two coil units and each coil unit needs to be connected with a phase shift circuit (like the first phase shift circuit or the second phase shift circuit shown in  FIG. 2 or 3 ), respectively. In this case, when the values of the capacitors, inductors and crossover elements are determined, coil units are opened first in pairs, then in triples, then in quadruples, and so on, and the values of the capacitors, inductors and crossover elements are continuously adjusted until optimal decoupling between every two coil units is achieved. The larger the number of coil units is, the more difficult it is to make each adjustment satisfy or approximately satisfy the condition: the phase shift of each coil unit between a matched state and a non-matched state is 180°. The following optimization solution is given for this case: 
     As shown in  FIG. 4 , the first phase shift circuit  21  further comprises a third capacitor  213 , and/or the second phase shift circuit  22  further comprises a fourth capacitor  223 , wherein 
     a first connecting end of the third capacitor  213  is connected with the first connecting end of the first crossover element  120 , and a second connecting end of the third capacitor  213  is grounded; 
     a first connecting end of the fourth capacitor  223  is connected with the second connecting end of the first crossover element  120 , and a second connecting end of the fourth capacitor  223  is grounded. 
     Through the device shown in  FIG. 4 , the values of the third capacitor  213  and/or the fourth capacitor  223  can further be adjusted to optimize the decoupling between the first coil unit  100  and the second coil unit  200 . 
       FIG. 5  compares the port matching effects obtained after field simulations are performed for a coil unit in an MR system by use of simulation software, where the coil unit decoupling device provided by the present disclosure is applied and is not applied, respectively. Wherein, m 3  represents the reflection parameter of signals on a simulation port (namely, a port connecting a crossover element and a phase shift circuit) on the coil unit, and the reflection parameter is denoted as S(1,1). The left graph shows the simulation when the coil unit decoupling device provided by the present disclosure is not applied, namely, the coil unit is in a non-matched state, and the right graph shows the simulation when the coil unit decoupling device provided by the present disclosure is applied and the coil unit is in a matched state. S(1,1)=0.730/−179.394 in the left graph and S(1,1)=0.018/−91.372 in the right graph, wherein the value before “/” represents an amplitude and the value after “/” represents a phase. The MR frequency in the above-mentioned simulations is 80 MHz. It can be seen from the reflection coefficient of signals on the simulation port that the smaller the amplitude is, the better the matching effect is. 
     In the aspects of the present disclosure, the first coil unit and the second coil unit may be symmetric with respect to a plane. 
     In practical applications, decoupling between all coil units can be realized simply by applying the coil unit decoupling device provided by the present disclosure to every two coil units. It can be seen that the complexity is greatly lowered. 
     In practical applications, the distance between the phase shift circuit connected with each coil unit and the crossover element may be long, and a cable is usually connected between the phase shift circuit and the crossover element in an MR system. When the cable is long, the grounding of the cable may have an antenna effect. To eliminate the antenna effect, an RF trap may be connected between the phase shift circuit and the crossover element. In addition, for some reasons, a coil unit may not be grounded before a front-end amplifier, and in this case, an RF trap also needs to be added before the front-end amplifier.  FIG. 6  shows that an RF trap  214  is added between the first inductor  212  and a front-end amplifier in the coil unit decoupling device  20  and an RF trap  224  is added between the second inductor  222  and a front-end amplifier. 
     In addition, if the distance between coil units is long, an RF trap can also be connected to the cable connecting the first crossover element  120 . As shown in  FIG. 7 , an RF trap  215  is connected between the first crossover element  120  and the first inductor  212 , an RF trap  225  is connected between the first crossover element  120  and the second inductor  222 , wherein the first inductor  212  and the second inductor  222  may be connected with the crossover element  120  by use of short cables or coaxial cables, and  401  and  402  shown in  FIG. 7  are coaxial cables. 
       FIG. 8  shows the circuit of the application of the coil unit decoupling device provided by the present disclosure to the coil units shown in  FIG. 1 . Wherein, the coil units  11  and  13  are symmetric with respect to a central vertical cross-section of the cylinder, and the coil unit  12  and  14  are symmetric with respect to a central vertical cross-section of the cylinder. The self-contained loop of each coil unit has four ports for connecting capacitors, one port is used for connecting the coil unit decoupling device provided by the present disclosure, and the other three ports are used for connecting capacitors. As shown in  FIG. 8 , the ports  3 ,  7 ,  11  and  15  are used for connecting the decoupling device  20  provided by the present disclosure, and the other three ports on the self-contained loop of each coil unit are used for connecting capacitors. Wherein, a crossover element is respectively connected between the phase shift circuits (namely, between coil units  11  and  12 , between coil units  11  and  13 , between coil units  11  and  14 , between coil units  12  and  13 , between coil units  12  and  14  and between coil units  13  and  14 ) of every two coil units, and the crossover elements shown in  FIG. 8  are inductors or capacitors. After the coil unit decoupling device provided by the present disclosure is applied to the coil units  11 ,  13 ,  12  and  14 , the capacitances of the capacitors on the loops of the coil units  11 - 14  are adjusted until the frequency of the coil units is the MR frequency (80 MHz in the example). In  FIG. 8 , the port, connected with a crossover element, on each coil unit is a simulation port, and therefore there are four simulation ports, namely, there are ports  801 ,  802 ,  803  and  804  as shown in  FIG. 8 . 
       FIG. 9  shows the port matching effect and the decoupling effect obtained after field simulations are performed for the circuit shown in  FIG. 8  by use of simulation software. 
     Wherein, the port decoupling effect described in terms of dB is shown in the upper graph of  FIG. 9 . dB(S(A,B)) represents the coupling described in terms of dB between the simulation port A and the simulation port B, and A and B may be any of ports  801 - 804 . For example, dB(S(2,1)) represents the coupling described in terms of dB between the ports  802  and  801 . 
     Wherein, dB(S(2,1))=−27.357, dB(S(3,1))=−11.191, dB(S(3,2))=−27.365, dB(S(4,1))=−27.458, dB(S(4,2))=−11.195, and dB(S(4,3))=−27.368. 
     It can be seen that the coupling between the ports is all below −10 dB. 
     The two lower graphs of  FIG. 9  show the port matching effects described in terms of amplitudes and phases. S(A,A) represents the reflection coefficient of signals on the simulation port A, described in terms of phases and amplitudes, and A may be any of ports  801 - 804 . For example, S(1,1) represents the reflection coefficient of signals on the port  801 , described in terms of phases and amplitudes. 
     Wherein, S(1,1)=0.016/13.563, S(2,2)=0.025/−53.298, S(3,3)=0.016/−2.472 and S(4,4)=0.016/10.280. Wherein, the value before “/” represents an amplitude and the value after “/” represents a phase. 
     It can be seen from the reflection coefficient of signals on a port that the smaller the amplitude, the better the matching effect. 
     In addition, experiments show that the coil unit decoupling device provided by the present disclosure has another advantage: If a plurality of coil units exist in an MR system and the coil unit decoupling device provided by the present disclosure is applied to the plurality of coil units, decoupling between other coil units is hardly influenced when one coil unit is mistuned.  FIG. 10  compares the reflection coefficients of signals on different simulation ports of the circuit shown in  FIG. 8  before and after mistuning of the coil unit  11 . Wherein, the upper left graph and the lower left graph show the results before mistuning of the coil unit  11 , and the upper right graph and the lower right graph show the results after mistuning of the coil unit  11 . 
     Wherein, before mistuning of the coil unit  11 : S(1,1)=0.016/13.563, S(2,2)=0.025/−53.298, S(3,3)=0.016/−2.472, S(4,4)=0.016/10.280; dB(S(1,1))=−35.926, dB(S(2,2))=−32.028, dB(S(3,3))=−36.136, dB(S(4,4))=−35.791; 
     after mistuning of the coil unit  11 : dB(S(1,1))=0.008, dB(S(2,2))=−32.355, dB(S(3,3))=−23.132, dB(S(4,4))=−37.821. 
     It can be seen that the reflection coefficients of signals on the simulation ports of other coil units are still low and the matching effect is still good after the coil unit  11  is mistuned. 
     This also indicates that after a coil unit or some coil units in an MR system are mistuned, the coil unit decoupling device  20  provided by the present disclosure can still be used to decouple other coil units in the MR system. 
     For the circuit shown in  FIG. 8 , although the coupling between each pair of ports is below −10 dB, impedance coupling still exists between the coil units. Thus, the decoupling effect is not ideal, yet. The decoupling effect can be further improved by adding crossover elements (capacitors or inductors). As shown in  FIG. 11 , a second crossover element  121  is connected between the first connecting end of the first inductor  212  and the first connecting end of the second inductor  222 . 
     On the basis of the circuit shown in  FIG. 8 , a crossover element (capacitor or inductor) is added between first connecting ends of the inductors in the phase shift circuit of every two coil units, and then the circuit shown in  FIG. 12  is obtained. 
       FIG. 13  shows the port matching effect and the decoupling effect obtained after field simulations are performed for the circuit in shown  FIG. 12  by use of simulation software. Wherein, 
     dB(S(3,2))=−42.727, dB(S(4,1))=−45.471, dB(S(4,3))=−48.862, dB(S(2,1))=−42.712, dB(S(3,1))=−38.888, dB(S(4,2))=−38.836; 
     S(1,1)=0.007/−130.010, S(2,2)=0.021/−93.435, S(3,3)=0.014/−108.701, S(4,4)=0.013/−130.673. 
     It can be seen that dB(S(3,1)) and dB(S(4,2)) are further optimized to below −30 dB, and that dB(S(3,2)), dB(S(4,1)), dB(S(4,3)) and dB(S(2,1)) are further optimized to below −40 dB provided that each port still remains matched. 
     In an alternative aspect, the first inductor  212  may be replaced by a first inductor group consisting of a plurality of inductors connected in series, and/or the second inductor  222  may also be replaced by a second inductor group consisting of a plurality of inductors connected in series; in addition, according to the goal of achieving the optimal decoupling effect, for the first crossover element  120 , a connecting end may be selected from the connecting ends of the plurality of inductors in the first inductor group to connect the first connecting end of the first crossover element  120 , and a connecting end may be selected from the connecting ends of the plurality of inductors in the second inductor group to connect the second connecting end of the first crossover element  120 . 
       FIG. 14  shows the structure of a coil unit decoupling device wherein the first inductor  212  is replaced by two inductors: inductor  2121  and inductor  2122 , and the second inductor  222  is replaced by two inductors: inductor  2221  and inductor  2222 , and in  FIG. 14 , the first connecting end of the first crossover element  120  is connected with a second connecting end of the inductor  2121  and the second connecting end of the first crossover element  120  is connected with a second connecting end of the inductor  2221 . 
     On the basis of the circuit shown in  FIG. 8 , an inductor on the phase shift circuit of each coil unit is replaced by two inductors connected in series, the connection point between the phase shift circuit and the crossover element is located on the connection line between the two inductors, and then the circuit shown in  FIG. 15  is obtained. 
       FIG. 16  shows the port matching effect and the decoupling effect obtained after field simulations are performed for the circuit shown in  FIG. 15  by use of simulation software. Wherein, 
     dB(S(3,1))=−37.248, dB(S(4,2))=−37.198, dB(S(2,1))=−50.387, dB(S(3,2))=−66.903, dB(S(4,3))=−69.122, dB(S(4,1))=−54.746; 
     S(1,1)=0.014/−66.578, S(3,3)=0.017/−64.275, S(4,4)=0.012/−51.724, S(2,2)=0.042/−84.246. 
     It can be seen that each port has high matching and decoupling performances. 
       FIG. 17  shows another alternative solution. Wherein, the first inductor  212  on the first phase shift circuit  21  is replaced by two inductors: the inductor  2123  and the inductor  2124 , and only one inductor  2223  is still adopted on the second phase shift circuit  22 . In  FIG. 17 , the first connecting end of the first crossover element  120  is connected with the second connecting end of the inductor  2121  and the second connecting end of the first crossover element  120  is connected with the second connecting end of the inductor  2223 . 
       FIG. 18  shows a still another alternative solution. Like that shown in  FIG. 17 , the first inductor  212  on the first phase shift circuit  21  is replaced by two inductors: the inductor  2123  and the inductor  2124 , and only one inductor  2223  is still adopted on the second phase shift circuit  22 . Different from  FIG. 17 , in  FIG. 18 , the first connecting end of the first crossover element  120  is connected with the second connecting end of the inductor  2121 , and the second connecting end of the first crossover element  120  is connected with the first connecting end of the inductor  2223 . 
     In addition, in an alternative aspect, when the first inductor  212  is replaced by a first inductor group consisting of a plurality of inductors connected in series and the second inductor  222  is replaced by a second inductor group consisting of a plurality of inductors connected in series, a plurality of (for example, more than two) crossover elements can be adopted to achieve an optimal decoupling effect. Wherein, according to the goal of achieving the optimal decoupling effect, the first connecting end of each crossover element can be connected with any connecting end of any inductor in the first inductor group, and the second connecting end of each crossover element can be connected with any connecting end of any inductor in the second inductor group. 
     The coil unit decoupling device provided by the present disclosure must be placed before the front-end low-noise amplifier of a coil unit. The phase can be adjusted according to the actual requirements so that the coil unit and the front-end low-noise amplifier behind the coil unit can be decoupled and the coil unit and the amplifier can still work normally. For example, if the noise matching angle of the front-end low-noise amplifier is 45°, the phase of the port (namely, the port connected with the crossover element on the phase shift circuit of the coil unit, or the simulation port shown in  FIG. 8 ) into which the coil unit is looked can be adjusted to −135° by use of the phase shift circuit.  FIG. 19  shows the port matching effect and the decoupling effect obtained after field simulations are performed in the case that the phase of the simulation ports on the circuit shown in  FIG. 8  is −135°. Wherein, 
     S(1,1)=0.007/46.405, S(2,2)=0.006/34.683, S(4,4)=0.006/24.920, S(2,2)=0.006/35.532; 
     dB(S(2,1))=−47.728, dB(S(3,1))=−40.638, dB(S(4,1))=−51.412, dB(S(3,2))=−47.853, dB(S(4,2))=−40.727, dB(S(4,3))=−48.319. 
     After the aspects of the present disclosure are adopted, crossover elements will cause an additional SNR loss. Table 1 lists the SNR losses when the coil unit decoupling device provided by aspects of the present disclosure is applied to the coil units shown in  FIG. 1  and one, two or more than three crossover inductors with a Q-factor of 80 are adopted for capacitance coupling, inductance coupling and impedance coupling. It can be seen that the SNR losses are respectively 0.072 dB (1.6%) and 0.081 dB (1.8%) when one crossover inductor is adopted for decoupling, and the SNR losses respectively increase to 0.122 dB (2.85%) and 0.087 dB (2.02%) when two or more crossover inductors are adopted. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 SNR loss when 
                 SNR loss when 
                 SNR loss when 
                 SNR loss when 
               
               
                   
                 one crossover 
                 two crossover 
                 more than three 
                 one crossover 
               
               
                   
                 inductor is 
                 inductors are 
                 crossover inductors 
                 element is 
               
               
                   
                 adopted for 
                 adopted for 
                 are adopted 
                 adopted for 
               
               
                   
                 capacitance 
                 capacitance, 
                 for capacitance, 
                 capacitance, 
               
               
                   
                 and inductance 
                 inductance and 
                 inductance and 
                 inductance and 
               
               
                   
                 decoupling 
                 impedance decoupling 
                 impedance decoupling 
                 impedance decoupling 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Coil unit 1 
                 0.071 
                 0.122 
                 0.087 
                 0.081 
               
               
                 Coil unit 2 
                 0.072 
                 0.122 
                 0.086 
                 0.081 
               
               
                 Coil unit 3 
                 0.071 
                 0.122 
                 0.087 
                 0.081 
               
               
                 Coil unit 4 
                 0.072 
                 0.122 
                 0.087 
                 0.081 
               
               
                   
               
            
           
         
       
     
     Aspects of the present disclosure further provide an MR system and the system comprises the above-mentioned coil unit decoupling device  20 . 
     The advantageous technical effects of aspects of the present disclosure are as follows: 
     1. The coil unit decoupling device provided aspects of the present disclosure is structurally simple, is easy to implement and has a strong decoupling effect. Coil units can systematically be adjusted, without any difficulty. 
     2. No complex copper structure is required and the SRN at the center of coil structure can remain an optimal value. 
     3. The coil unit decoupling device provided aspects of the present disclosure is especially applicable to a low-field system where the Q-factor is high and it is difficult to realize decoupling. 
     4. The coil unit decoupling device provided aspects of the present disclosure can still achieve a good decoupling effect when one or more coil units are mistuned. 
     The above-mentioned aspects are only preferred aspects of the present disclosure and are not intended to restrict the present disclosure. Modifications, equivalent replacements, and improvements made without departing the spirit and principle of the present disclosure should all fall within the scope of protection of the present disclosure.