Abstract:
A two-channel magnetic resonance tomography system is provided with a regulation circuit for an amplification system in order to be able to take into account different load situations of the MRI system in a flexible and efficient manner. It is thus possible to improve the MRI measurements greatly if the MRI system is set to the respective load situation beforehand by an idle state measurement. The adaptation may optionally also be carried out during the MRI measurement. Therefore, a multiplicity of completely different load situations may be taken into account in an optimized manner by the regulation circuit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present patent document is a §371 nationalization of PCT Application Serial Number PCT/EP2012/067840, filed Sep. 12, 2012, designating the United States, which is hereby incorporated by reference, and this patent document also claims the benefit of DE 10 2011 084 072.9, filed on Oct. 6, 2011, which is also hereby incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The embodiments relate to methods for setting an amplification system for a two-channel magnetic resonance imaging system, and to an apparatus for driving a two-channel magnetic resonance imaging system. 
       BACKGROUND 
       [0003]    Magnetic resonance imaging (MRI) is an imaging method that is used primarily in medical diagnostics for representing the structure and function of the tissues and organs in the body. MRI is based physically on the principles of the nuclear spin resonance and is therefore also designated as nuclear spin tomography. 
         [0004]    MRI may be used to generate slice images of the human (or animal) body that allow an assessment of the organs and of many pathological organ changes. Magnetic resonance imaging is based on strong magnetic fields and alternating electromagnetic fields in the radio-frequency range that resonantly excite specific atomic nuclei (e.g., the hydrogen nuclei/protons) in the body, which then induce electrical signals in the receiver circuit. No burdensome X-ray radiation or other ionizing radiation is generated or used in the device. Different relaxation times of different types of tissue are an essential basis for the image contrast. In addition, the different content of hydrogen atoms in different tissues (e.g., muscle, bone) also contributes to the image contrast. 
       SUMMARY AND DESCRIPTION 
       [0005]    The scope of the present invention is defined solely by the appended claims and is not affected to any degree by the statements within this summary. The present embodiments may obviate one or more of the drawbacks or limitations in the related art. 
         [0006]    The approach proposed here relates, in particular, to voltage control or a device for voltage control for a radio-frequency power amplifier (RFPA) system such as is used in MRI. 
         [0007]    The object of an embodiment is to improve MRI technology and to be able to optimize the MRI technology flexibly, in particular, toward different load situations (e.g., patients, organs, positions). 
         [0008]    In order to achieve the object, a method for setting an amplification system for a two-channel magnetic resonance imaging system is specified, wherein the amplification system is adapted by a control circuit depending on a load situation. 
         [0009]    Consequently, an RF excitation signal may be stabilized and a flexible optimization to different load situations may be achieved. 
         [0010]    In certain embodiments, the amplification system includes a dedicated amplifier for each channel of the magnetic resonance imaging system. 
         [0011]    The approach proposed here makes it possible to calibrate the two-channel magnetic resonance imaging system specifically to the respective load situation and thus to use a setting that is expedient for the load situation. This approach leads to a significant improvement in the measurement results and increases the flexibility of the two-channel magnetic resonance imaging system also with regard to body coils that may be used. 
         [0012]    It is thus also possible to fulfill specific predefined stipulations (fast settling time while complying with a predefined maximum, exact or suitable amplification) even with regard to totally different load situations. 
         [0013]    In one development, the control circuit has a feedback of the output signal of the amplification system, in particular, to the input of the amplification system. 
         [0014]    The feedback may be used for setting controllers (control elements) in a feedforward branch of the control circuit. 
         [0015]    In another development, a delay of the output signal is carried out during the feedback. 
         [0016]    In particular, a delay element may be provided for this purpose. 
         [0017]    In particular, in one development, with the aid of the amplification system, the load situation is determined and at least one controller of the two-channel magnetic resonance imaging system is set depending on the load situation. 
         [0018]    In particular, the type of load situation or at least one parameter of the load situation may be determined (e.g. estimated) with the aid of a measurement, e.g. an open-loop measurement (measurement without closed-loop control). 
         [0019]    In one development, moreover, the at least one controller includes a load-dependent feedforward controller for setting a suitable (in particular, maximum or exact) amplification of the amplification system. 
         [0020]    The amplification of the amplification system may be set or controlled by the load-dependent feedforward controller. This makes it possible to provide, in particular, that a predefined maximum amplification is not exceeded. 
         [0021]    Furthermore, in one development, the at least one controller includes a further load-dependent controller having four Single Input Single Output (SISO) proportional-integral (PI) controllers. 
         [0022]    In particular, the two load-dependent controllers are connected in parallel with one another. What may be achieved with the aid of the four SISO PI controllers (two controllers for each channel of the two-channel magnetic resonance imaging system) is that the settling time is accelerated and the lag of an MRI recording is thus improved. 
         [0023]    In the context of an additional development, the signals are decoupled upstream of the amplification system. 
         [0024]    In particular, a decoupling component may be connected upstream of the amplification system. By way of example, with the aid of the decoupling component it is provided that a static portion in the signal is reduced or suppressed and/or that the values are scaled to a predefined extent. 
         [0025]    In a next development, the load situation is determined on the basis of at least one of the following parameters: (1) the size of the patient; (2) the weight of the patient; (3) a region or organ to be examined; (4) a position of the examination table; (5) a position of the body coil for carrying out the MRI measurement; and (6) a position of the patient in relation to the examination table. 
         [0026]    In one configuration, the setting of the amplification system is carried out during an open-loop measurement. 
         [0027]    An open-loop measurement (also designated as open-loop operation) may concern a measurement that serves for setting the amplification system or the control circuit. This may be effected depending on the load situation in such a way that e.g. a patient occupies a predefined position in the MRI system and measurements are carried out before the actual MRI examinations in order to set the MRI system in an optimized manner for the imminent measurement. 
         [0028]    In an alternative embodiment, an MRI measurement with the patient is carried out after the open-loop measurement. 
         [0029]    In a next configuration, the control circuit is adapted during the MRI measurement. 
         [0030]    Even during the MRI measurements, the control circuit may be adapted and the quality of the measurements obtained may thus be improved. 
         [0031]    In one configuration, moreover, the load situation is determined by the open-loop measurement. 
         [0032]    The load situation may also be (concomitantly) determined depending on parameters that are determined during the open-loop measuring. In other words, parameters that are determined during the open-loop measurement may also be used in order to determine the load situation and thus, depending on the load situation, to adapt the control circuit or the amplification system via the control circuit. 
         [0033]    The abovementioned object is also achieved by an apparatus for driving a two-channel magnetic resonance imaging system including an amplification system, wherein the amplification system is adaptable by a control circuit depending on a load situation. 
         [0034]    For this purpose, the control circuit may be embodied as a processor unit and/or an at least partly hardwired or logical circuit arrangement designed, for example, in such a way that the method as described herein may be carried out. In this case, it is possible to use any type of processor or computer with correspondingly required peripherals (memory, input/output interfaces, input-output devices, etc.). 
         [0035]    The above explanations concerning the method correspondingly apply to the apparatus. The apparatus may be embodied in one component or in a distributed manner in a plurality of components. 
         [0036]    In one development, the control circuit has a feedback of the output signal of the amplification system, in particular, to the input of the amplification system. 
         [0037]    In one development, moreover, at least one controller is provided in a feedforward branch of the control circuit, with the aid of which at least one controller an amplification of the amplification system is settable and with the aid of which at least one controller a settling time of the amplification system is settable. 
         [0038]    The abovementioned object is also achieved by a two-channel magnetic resonance imaging system including one of the apparatuses described here. 
         [0039]    The solution presented here furthermore includes a computer program product that is directly loadable into a memory of a digital computer, including program code parts suitable for carrying out acts of the method described here. 
         [0040]    Furthermore, the abovementioned problem is solved by a computer-readable storage medium, e.g. of an arbitrary memory, including instructions (e.g. in the form of program code) that are executable by a computer are suitable to the effect that the computer carries out acts of the method described here. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0041]      FIG. 1  depicts an embodiment of a schematic block diagram for a RFPA system with closed-loop control for use in a two-channel MRI system. 
           [0042]      FIG. 2  depicts an alternative embodiment of a schematic block diagram of a two-channel MRI system. 
       
    
    
     DETAILED DESCRIPTION 
       [0043]    An MIMO (Multiple Input Multiple Output) system denotes a system having a plurality of inputs and a plurality of outputs or having a plurality of input variables and a plurality of output variables. The term multi-variable system may be used as well. Systems having exactly one input variable and one output variable are designated as SISO (Single Input Single Output) system. 
         [0044]    A RFPA system includes a power amplifier in a radio-frequency range, e.g., in a high-frequency range. One radio-frequency signal (also designated as RF signal) is amplified per channel, wherein the RF signal has a specific amplitude and phase. 
         [0045]    A two-channel RFPA system is provided below. From the standpoint of open-loop or closed-loop control, a two-channel RFPA system corresponds to a 4×4 MIMO system, including two channels each for amplitude and phase of the settable RF signal. 
         [0046]      FIG. 1  depicts a schematic block diagram of an RFPA system with closed-loop control for use in a two-channel MRI system. 
         [0047]    The desired or reference values s i  where i=1 . . . 4 are applied to a load-dependent feedforward controller  102 . Furthermore delayed output values d, (i=1 . . . 4) are subtracted from the desired values and the result is fed in the form of values: 
         [0000]        g   i   =S   i   −d   i  where  i= 1 . . . 4 
         [0000]    to a load-dependent controller  101  having four SISO PI controllers, one for each channel i. 
         [0048]    A PI controller, also designated as a proportional-integral controller, includes the portions of a proportional element and an integral-action element. 
         [0049]    Output values f, of the controller  102  are combined with output values w i  of the controller  101  to form values k i , e.g.: 
         [0000]        k   i   =w   i   +f   i  where  i= 1 . . . 4, 
         [0000]    and fed to a decoupling component  103 . By way of example, with the aid of the decoupling component  103  it is provided that a static portion in the signal k i  is reduced or suppressed and/or that the values k i  are scaled to a predefined extent. 
         [0050]    The decoupling component  103  decouples with respect to an RFPA system  104 . In other words, as a result of the use of the decoupling component upstream of the RFPA system, this corresponds functionally to four SISO systems decoupled from one another. 
         [0051]    At the output of the decoupling system, values p i  are correspondingly provided to the RFPA system  104 . 
         [0052]    The decoupling component  103  decouples the inputs and outputs of the RFPA system. For example, each signal of the four input signals corresponds to one of the four output signals of the RFPA system, and the couplings originally present between the input and output signals are thus reduced or (largely) eliminated. The 4×4 RFPA system is thus subdivided into four 1×1 subsystems. The scaling is effected, for example, in such a way that each subsystem of the four 1×1 subsystems has the same DC voltage amplification. 
         [0053]    The decoupling component 103 is determined depending on the load situation. This is effected by introducing an act successively upon the temporary switching-off of the closed-loop control to the desired voltage signals s 1 , s 2 , s 3  and s 4  and determining the output voltages u 1 , u 2 , u 3  and u 4 . The DC voltage amplification (a 4×4 matrix) of the RFPA system may be determined from the act responses. In the simplest case, the decoupling component  103  is the inverse of the DC voltage amplification matrix. Alternatively, the inverse of the DC voltage amplification matrix may also be multiplied by four input scaling factors. 
         [0054]    With the aid of the load situation or the experimentally determined DC voltage amplification matrix of the RFPA system  104 , corresponding information  106  is communicated to a look-up table  105 , with the aid of which the information  106  is converted into an associated setting for the controllers  101  and  102 . This may take place in the context of an open-loop measurement and/or while an examination is being carried out. 
         [0055]    The RFPA system  104  provides output values u i  (e.g. in the form of output voltages), which are also converted into the delayed output values d i  by a delay element  107 . 
         [0056]    Consequently, the RFPA system  104  is driven efficiently, and by the (feedback) information  106 , it is provided that the RFPA system is operated in an optimized manner for the respective load situation. 
         [0057]    The controller  101  is used in this case to reduce or to eliminate control errors in the settled state. 
         [0058]    The controller  102  is used to accelerate the settling time and thus to improve a lag during the MRI recording. 
         [0059]    The decoupling component  103  includes, for example, a static decoupling and scaling matrix, which serves to reduce (or at least proportionally), avoid crosstalk between the four channels. 
         [0060]    In order to enable efficient MRI measurements for different load situations and thus to improve the performance and efficiency of the MRI system, it is possible to provide different feedforward control amplification matrices in the controller  102  and, if appropriate, correspondingly matching settings for the controller  101  (parameterizations of the PI controllers), which are selected depending on the information  106 , which is in turn determined depending on the respective load situations. In this regard, the information  106  may serve to address an entry in the look-up table  105  that include a corresponding setting of the controllers  101  and  102 . This setting is thereupon adopted for the controllers  101 ,  102 . 
         [0061]    Depending on the load of the RFPA system  104 , it is thus possible to choose a suitable amplification by the look-up table  105 . It is thus possible to achieve the required fast settling time depending on the respective specific load situation and at the same time to provide a good quality of the recordings. 
         [0062]    In particular, the RFPA system may be set such that, e.g., the following predefined stipulations are fulfilled: (1) For a step response, a settling time (for reaching a tolerance band amounting to e.g. 5% around a target value) is less than 10 microseconds (without steady-state error portion). (2) A predefined maximum voltage at the output of the amplifier is not exceeded. (3) The predefined stipulations (1) and (2) are complied with for different load situations. In this case, the load situations correspond, e.g., to different patients, to their organs to be examined, and the different positions during the MRI examination. 
         [0063]    The load situation may depend on different factors. By way of example, the size of the patient may be estimated in relation to a region to be examined. Such an estimation may take account of at least one of the following parameters: (1) the weight of the patient, (2) the region to be examined, e.g., depending on a position of an examination table and/or on a position of the patient on the examination table (e.g., lying on the back/stomach, lying on the right/left side), and (3) an open-loop measurement. 
         [0064]    During the open-loop measurement, an actual load-dependent system behavior may be measured and the parameters of the control loop may be adjusted in such a way that an optimized dynamic system response is achieved. 
         [0065]    The actual examination (MRI measurement) of the patient is effected after said open-loop measurement. 
         [0066]      FIG. 2  shows a schematic diagram of a two-channel MRI system. 
         [0067]    A desired value sw 1  is applied to a propagation time component  201  and to an adder element  203 , and the result of the propagation time component  201  is passed to an adder element  207 . The result of the adder element  207  is passed via a switch  218  to a loop filter  205  to the adder element  203 . The output of the adder element  203 , that is to say the addition of the desired value sw 1  to the output value of the loop filter  205 , is fed to a decoupling component  209 . 
         [0068]    A desired value sw 2  is applied to a propagation time component  202  and to an adder element  204 , and the result of the propagation time component  202  is passed to an adder element  208 . The result of the adder element  208  is passed via a switch  219  to a loop filter  205  to the adder element  204 . The output of the adder element  204 , that is to say the addition of the desired value sw 2  to the output value of the loop filter  206 , is likewise fed to the decoupling component  209 . 
         [0069]    Consequently, by the decoupling component  209 , a modified (desired) value sw 1 ′ arises from the desired value sw 1  and a modified (desired) value sw 2 ′ correspondingly arises from the desired value sw 2 . 
         [0070]    The value sw 1 ′ is fed via a digital-to-analog converter  210  and an amplifier  212  to a processing unit  214 . The value sw 2 ′ is correspondingly fed via a digital-to-analog converter  211  and an amplifier  213  to the processing unit  214 . 
         [0071]    The processing unit  214  communicates incoming waves a 1  and a 2  via a cable of length l BC  to a body coil  217 . 
         [0072]    The body coil  217  supplies incoming waves b 1  and b 2  or voltage values U BC1  and U BC2  to the processing unit  214 . 
         [0073]    The voltage U BC1  is fed by the processing unit  214  via an analog-to-digital converter  215  as a digital measured actual value to the adder element  207  and is subtracted from the output value of the propagation time component  201 . The voltage U BC2  is correspondingly fed by the processing unit  214  via an analog-to-digital converter  216  as a digital measured actual value to the adder element  208  and is subtracted from the output value of the propagation time component  202 . 
         [0074]    The desired values in  FIG. 2  are complex signals including a real part and an imaginary part. 
         [0075]    The modified desired values sw 1 ′ and sw 2 ′ arise from the values shown in  FIG. 1  as follows: 
         [0000]        sw   1   ′=p   1   +i*p   2 , 
         [0000]        sw   2   ′=p   3   +i*p   4 . 
         [0076]    The output voltages U BC1  and U BC2  (voltages across the body coil  217 ) determined arise from the values illustrated in  FIG. 1  as follows: 
         [0000]        u   BC1   =u   1   +i*u   2 , 
         [0000]        u   BC2   =u   3   +i*u   4 . 
         [0077]    The relationship between input and output variables may be described by a complex 2×2 matrix K (coupling matrix): 
         [0000]    
       
         
           
             
               ( 
               
                 
                   
                     
                       u 
                       
                         BC 
                          
                         
                             
                         
                          
                         1 
                       
                     
                   
                 
                 
                   
                     
                       u 
                       
                         BC 
                          
                         
                             
                         
                          
                         2 
                       
                     
                   
                 
               
               ) 
             
             = 
             
               
                 K 
                 _ 
               
               · 
               
                 ( 
                 
                   
                     
                       
                         sw 
                         1 
                         ′ 
                       
                     
                   
                   
                     
                       
                         sw 
                         2 
                         ′ 
                       
                     
                   
                 
                 ) 
               
             
           
         
       
     
         [0078]    The matrix K depends on the load situation of the body coil  217  and on output reflection coefficients r Q1  and r Q2  of the power amplifier. The matrix K may be determined in the context of the open-loop measurement (adaptation) before the actual examination (recording). 
         [0079]    The open-loop measurement may be effected by determining a scattering matrix of the body coil used and determining the matrix K with the aid of estimated output reflection coefficients of the power amplifier. This approach is suitable in particular because the scattering matrix is also required and therefore predetermined for the specific absorption rate (SAR) monitoring. Therefore, only a small additional outlay with regard to calculation and communication is required. 
         [0080]    The open-loop measurement may also be effected by directly determining the matrix K. In this case, the actual properties of the power amplifier may be taken into account. An inverse of the matrix K for the static decoupling and scaling may be used (also cf. the decoupling component  103  in  FIG. 1 ) to enable optimized dynamic measurements of the MRI system. 
         [0081]    The explanations below also apply in particular to closed-loop control based on voltages u BC  across the body coil. This corresponds to an exemplary controlled variable. Alternatively or additionally, e.g. the incoming waves may also be used as a controlled variable. 
         [0082]    Provided that no further coupling takes place in the processing unit  214  itself and the latter itself is (virtually) free of inherent reflection, the incoming waves arise as: 
         [0000]    
       
         
           
             
               
                 
                   
                     ( 
                     
                       
                         
                           
                             a 
                             1 
                           
                         
                       
                       
                         
                           
                             a 
                             2 
                           
                         
                       
                     
                     ) 
                   
                   = 
                     
                    
                   
                     
                       S 
                       _ 
                     
                     · 
                     
                       ( 
                       
                         
                           
                             
                               sw 
                               1 
                               ′ 
                             
                           
                         
                         
                           
                             
                               sw 
                               2 
                               ′ 
                             
                           
                         
                       
                       ) 
                     
                   
                 
               
             
             
               
                 
                   = 
                     
                    
                   
                     
                       1 
                       term 
                     
                     · 
                     
                       ( 
                       
                         
                           
                             
                               
                                 τ 
                                 1 
                               
                                
                               
                                 ( 
                                 
                                   1 
                                   - 
                                   
                                     
                                       s 
                                       22 
                                     
                                      
                                     
                                       r 
                                       
                                         Q 
                                          
                                         
                                             
                                         
                                          
                                         2 
                                       
                                     
                                   
                                 
                                 ) 
                               
                             
                           
                           
                             
                               
                                 τ 
                                 2 
                               
                                
                               
                                 s 
                                 12 
                               
                                
                               
                                 r 
                                 
                                   Q 
                                    
                                   
                                       
                                   
                                    
                                   1 
                                 
                               
                             
                           
                         
                         
                           
                             
                               
                                 τ 
                                 1 
                               
                                
                               
                                 s 
                                 21 
                               
                                
                               
                                 r 
                                 
                                   Q 
                                    
                                   
                                       
                                   
                                    
                                   2 
                                 
                               
                             
                           
                           
                             
                               
                                 τ 
                                 2 
                               
                                
                               
                                 ( 
                                 
                                   1 
                                   - 
                                   
                                     
                                       s 
                                       11 
                                     
                                      
                                     
                                       r 
                                       
                                         Q 
                                          
                                         
                                             
                                         
                                          
                                         1 
                                       
                                     
                                   
                                 
                                 ) 
                               
                             
                           
                         
                       
                       ) 
                     
                     · 
                     
                       ( 
                       
                         
                           
                             
                               sw 
                               1 
                               ′ 
                             
                           
                         
                         
                           
                             
                               sw 
                               2 
                               ′ 
                             
                           
                         
                       
                       ) 
                     
                   
                 
               
             
           
         
       
     
         [0000]    where: 
         [0000]      term=1− r   Q1   s   11   −r   Q2   s   22   +r   Q1   r   Q2 ·det( S   BC ),
 
         [0000]    wherein τ I  (i=1,2) denotes the respective path transmission between the desired variables sw i ′ and a calibration plane of the scattering matrix of the body coil  217  relative to the calibration plane and the (nonlinear) reflection factors r Qi  of the power amplifiers (likewise transformed into the calibration plane) 
         [0000]    
       
         
           
             
               
                 S 
                 _ 
               
               BC 
             
             = 
             
               
                 ( 
                 
                   
                     
                       
                         s 
                         11 
                       
                     
                     
                       
                         s 
                         12 
                       
                     
                   
                   
                     
                       
                         s 
                         21 
                       
                     
                     
                       
                         s 
                         22 
                       
                     
                   
                 
                 ) 
               
               . 
             
           
         
       
     
         [0083]    Consequently, the outgoing waves arise from the incoming waves via the scattering matrix of the body coil  217 : 
         [0000]    
       
         
           
             
               ( 
               
                 
                   
                     
                       b 
                       1 
                     
                   
                 
                 
                   
                     
                       b 
                       2 
                     
                   
                 
               
               ) 
             
             = 
             
               
                 
                   S 
                   _ 
                 
                 BC 
               
               · 
               
                 ( 
                 
                   
                     
                       
                         a 
                         1 
                       
                     
                   
                   
                     
                       
                         a 
                         2 
                       
                     
                   
                 
                 ) 
               
             
           
         
       
     
         [0000]    with a transmission phase: 
         [0000]    
       
         
           
             
               ϕ 
               BC 
             
             = 
             
               2 
                
               π 
                
               
                 
                   l 
                   BC 
                 
                 λ 
               
             
           
         
       
     
         [0000]    which results in: 
         [0000]    
       
         
           
             
               ( 
               
                 
                   
                     
                       u 
                       
                         BC 
                          
                         
                             
                         
                          
                         1 
                       
                     
                   
                 
                 
                   
                     
                       u 
                       
                         BC 
                          
                         
                             
                         
                          
                         2 
                       
                     
                   
                 
               
               ) 
             
             = 
             
               
                 
                    
                   
                     - 
                     
                       jϕ 
                       BC 
                     
                   
                 
                 · 
                 
                   ( 
                   
                     
                       
                         
                           a 
                           1 
                         
                       
                     
                     
                       
                         
                           a 
                           2 
                         
                       
                     
                   
                   ) 
                 
               
               + 
               
                 
                    
                   
                     + 
                     
                       jϕ 
                       BC 
                     
                   
                 
                 · 
                 
                   ( 
                   
                     
                       
                         
                           b 
                           1 
                         
                       
                     
                     
                       
                         
                           b 
                           2 
                         
                       
                     
                   
                   ) 
                 
               
             
           
         
       
     
         [0000]    and thus in: 
         [0000]    
       
         
           
             
               ( 
               
                 
                   
                     
                       u 
                       
                         BC 
                          
                         
                             
                         
                          
                         1 
                       
                     
                   
                 
                 
                   
                     
                       u 
                       
                         BC 
                          
                         
                             
                         
                          
                         2 
                       
                     
                   
                 
               
               ) 
             
             = 
             
               
                 
                   
                     
                       ( 
                       
                         
                           
                              
                             
                               - 
                               
                                 jϕ 
                                 BC 
                               
                             
                           
                           · 
                           
                             E 
                             _ 
                           
                         
                         + 
                         
                           
                              
                             
                               + 
                               
                                 jϕ 
                                 BC 
                               
                             
                           
                           · 
                           
                             
                               S 
                               _ 
                             
                             BC 
                           
                         
                       
                       ) 
                     
                     · 
                     
                       S 
                       _ 
                     
                   
                    
                 
                 
                   K 
                   _ 
                 
               
               · 
               
                 
                   ( 
                   
                     
                       
                         
                           sw 
                           1 
                           ′ 
                         
                       
                     
                     
                       
                         
                           sw 
                           2 
                           ′ 
                         
                       
                     
                   
                   ) 
                 
                 . 
               
             
           
         
       
     
         [0084]    A decoupling matrix D may provide that during the open-loop measurement (e.g., during open-loop operation) the following relationship holds true: 
         [0000]    
       
         
           
             
               ( 
               
                 
                   
                     
                       u 
                       
                         BC 
                          
                         
                             
                         
                          
                         1 
                       
                     
                   
                 
                 
                   
                     
                       u 
                       
                         BC 
                          
                         
                             
                         
                          
                         2 
                       
                     
                   
                 
               
               ) 
             
             = 
             
               
                 
                   K 
                   _ 
                 
                 · 
                 
                   D 
                   _ 
                 
                 · 
                 
                   ( 
                   
                     
                       
                         
                           sw 
                           1 
                         
                       
                     
                     
                       
                         
                           sw 
                           1 
                         
                       
                     
                   
                   ) 
                 
               
                
               
                 = 
                 ! 
               
                
               
                 
                   ( 
                   
                     
                       
                         
                           sw 
                           1 
                         
                       
                     
                     
                       
                         
                           sw 
                           1 
                         
                       
                     
                   
                   ) 
                 
                 . 
               
             
           
         
       
     
         [0000]    which yields as a condition: 
         [0000]        D=K   −1 . 
         [0085]    If the control loops of the systems decoupled are closed, then both operate independently of one another. This holds true, in particular, until the amplitudes become high enough that the nonlinear output reflection factors differ from the small-signal value on which the decoupling is based. 
         [0086]    Under specific boundary conditions, it may happen that the matrix K becomes non-invertible, or the inversion becomes at least numerically unstable. This is manifested in a determinant of the matrix K whose value is zero or at least very close to the value zero. 
         [0087]    From a physical viewpoint, the voltage u BC2  in this case differs from the voltage u BC1  only in a single complex factor independently of the combination of the two exciting signals. Such a case may be prevented in practice, since otherwise the system would become unstable upon the least change in the scattering matrix during operation or upon a change in the output reflection factor as a result of strong modulation. 
         [0088]    With the aid of the closed representation of the decoupling matrix: 
         [0000]    
       
         
           
             
               D 
               _ 
             
             = 
             
               
                 1 
                 
                   
                      
                     
                       
                         - 
                         2 
                       
                        
                       
                         jϕ 
                         BC 
                       
                     
                   
                   + 
                   
                     s 
                     11 
                   
                   + 
                   
                     s 
                     22 
                   
                   + 
                   
                     
                        
                       
                         
                           - 
                           2 
                         
                          
                         
                           jϕ 
                           BC 
                         
                       
                     
                      
                     
                       det 
                        
                       
                         ( 
                         
                           
                             S 
                             _ 
                           
                           BC 
                         
                         ) 
                       
                     
                   
                 
               
               · 
               
                 ( 
                 
                   
                     
                       
                         z 
                         11 
                       
                     
                     
                       
                         z 
                         12 
                       
                     
                   
                   
                     
                       
                         z 
                         21 
                       
                     
                     
                       
                         z 
                         22 
                       
                     
                   
                 
                 ) 
               
             
           
         
       
       
         
           where 
         
       
       
         
           
             
               
                 z 
                 11 
               
               = 
               
                 
                   
                      
                     
                       - 
                       
                         jϕ 
                         BC 
                       
                     
                   
                   + 
                   
                     
                        
                       
                         jϕ 
                         BC 
                       
                     
                      
                     
                       s 
                       22 
                     
                   
                   - 
                   
                     
                       r 
                       
                         Q 
                          
                         
                             
                         
                          
                         1 
                       
                     
                      
                     
                       [ 
                       
                         
                           
                              
                             
                               - 
                               
                                 jϕ 
                                 BC 
                               
                             
                           
                            
                           
                             s 
                             11 
                           
                         
                         + 
                         
                           
                              
                             
                               jϕ 
                               BC 
                             
                           
                            
                           
                             det 
                              
                             
                               ( 
                               
                                 
                                   S 
                                   _ 
                                 
                                 BC 
                               
                               ) 
                             
                           
                         
                       
                       ] 
                     
                   
                 
                 
                   τ 
                   1 
                 
               
             
             ; 
           
         
       
       
         
           
             
               
                 z 
                 12 
               
               = 
               
                 
                   - 
                   
                     
                       s 
                       12 
                     
                      
                     
                       ( 
                       
                         
                           
                             r 
                             
                               Q 
                                
                               
                                   
                               
                                
                               1 
                             
                           
                            
                           
                              
                             
                               - 
                               
                                 jϕ 
                                 BC 
                               
                             
                           
                         
                         + 
                         
                            
                           
                             jϕ 
                             BC 
                           
                         
                       
                       ) 
                     
                   
                 
                 
                   τ 
                   1 
                 
               
             
             ; 
           
         
       
       
         
           
             
               z 
               21 
             
             = 
             
               
                 - 
                 
                   
                     s 
                     21 
                   
                    
                   
                     ( 
                     
                       
                         
                           r 
                           
                             Q 
                              
                             
                                 
                             
                              
                             2 
                           
                         
                          
                         
                            
                           
                             - 
                             
                               jϕ 
                               BC 
                             
                           
                         
                       
                       + 
                       
                          
                         
                           jϕ 
                           BC 
                         
                       
                     
                     ) 
                   
                 
               
               
                 τ 
                 2 
               
             
           
         
       
       
         
           and 
         
       
       
         
           
             
               z 
               21 
             
             = 
             
               
                 
                    
                   
                     - 
                     
                       jϕ 
                       BC 
                     
                   
                 
                 + 
                 
                   
                      
                     
                       jϕ 
                       BC 
                     
                   
                    
                   
                     s 
                     11 
                   
                 
                 - 
                 
                   
                     r 
                     
                       Q 
                        
                       
                           
                       
                        
                       2 
                     
                   
                    
                   
                     [ 
                     
                       
                         
                            
                           
                             - 
                             
                               jϕ 
                               BC 
                             
                           
                         
                          
                         
                           s 
                           22 
                         
                       
                       + 
                       
                         
                            
                           
                             jϕ 
                             BC 
                           
                         
                          
                         
                           det 
                            
                           
                             ( 
                             
                               
                                 S 
                                 _ 
                               
                               BC 
                             
                             ) 
                           
                         
                       
                     
                     ] 
                   
                 
               
               
                 τ 
                 2 
               
             
           
         
       
     
         [0000]    it can be discerned that the common denominator term must not become zero and that this condition is independent of the output reflection factors and is dependent only on the scattering matrix of the body coil and the electrical length l BC  with respect to the voltage plane of the body coil. 
         [0089]    Accordingly, the following parameters may be known for determining the decoupling matrix D: (1) the scattering matrix of the body coil relative to the calibration plane (this may be measured for each load situation); (2) the small-signal output reflection factor of the power amplifiers transformed into the calibration plane (this may be measured once, for example); (3) the length l BC  between the calibration plane and the reference plane of the voltages of the body coil (this involves a structural predefined specification, for example, which may be correspondingly adopted). 
         [0090]    The efficiency of the open-loop measurement may additionally be increased by the parameters being adapted or estimated taking account of (1) the frequency, (2) the waveform, and/or (3) the magnitude of at least one subsequent pulse (or signal). 
         [0091]    As already explained, the actual examination (MRI measurement) of the patient may be effected after the open-loop measurement. In this case, the MRI measurement may also be subject to temporal fluctuations that may not have been taken into account or compensated for during the preceding calibration (open-loop measurement). However, it is possible for changes in the system parameters also to be detected during the MRI measurement, by virtue of said system parameters being compared with the input and output variables of the RFPA system in the open-loop measurement. On the basis of such additional information during the MRI measurement itself, it is possible to adaptively track (set) the parameters. By way of example, a phase shift that occurs on an amplifier channel via the connected antenna may be compensated for by the feedback loop and the feedforward controller. By such (e.g. continuous) adaptation, the dynamic behavior of the control loop, even during the MRI measurement, may be constantly improved. 
         [0092]    Consequently, the approach presented here enables the flexible and dynamic setting and tracking (e.g. of an amplification) of the RFPA system, to be precise depending on the actual load situation. In this case, the load situation may be dependent, in particular, on the weight of a patient, the position of the examination table, the position of the patient on the examination table, the organ to be examined, the previous measurement data and other parameters obtained in the course of the MRI measurement(s). 
         [0093]    A list of reference signs used within the above-described embodiments are provided in the table below. 
         [0000]    
       
         
               
               
             
           
               
                   
               
             
             
               
                 101 
                 Load-dependent controller (including four SISO PI controllers) 
               
               
                 102 
                 Load-dependent feedforward controller 
               
               
                 103 
                 Decoupling component 
               
               
                 104 
                 RFPA system 
               
               
                 105 
                 Look-up table 
               
               
                 106 
                 (Load-situation-dependent) information 
               
               
                 107 
                 Delay element 
               
               
                 201 
                 Propagation time component 
               
               
                 202 
                 Propagation time component 
               
               
                 203 
                 Adder element 
               
               
                 204 
                 Adder element 
               
               
                 205 
                 Loop filter 
               
               
                 206 
                 Loop filter 
               
               
                 207 
                 Adder element 
               
               
                 208 
                 Adder element 
               
               
                 209 
                 Decoupling component 
               
               
                 210 
                 Digital-to-analog converter 
               
               
                 211 
                 Digital-to-analog converter 
               
               
                 212 
                 Amplifier 
               
               
                 213 
                 Amplifier 
               
               
                 214 
                 Processing unit 
               
               
                 215 
                 Analog-to-digital converter 
               
               
                 216 
                 Analog-to-digital converter 
               
               
                 217 
                 Body coil 
               
               
                 218 
                 Switch 
               
               
                 219 
                 Switch 
               
               
                   
               
             
          
         
       
     
         [0094]    It is to be understood that the elements and features recited in the appended claims may be combined in different ways to produce new claims that likewise fall within the scope of the present invention. Thus, whereas the dependent claims appended below depend from only a single independent or dependent claim, it is to be understood that the dependent claims may, alternatively, be made to depend in the alternative from any preceding or following claim, whether independent or dependent, and that such new combinations are to be understood as forming a part of the present specification. 
         [0095]    While the present invention has been described above by reference to various embodiments, it may be understood that many changes and modifications may be made to the described embodiments. It is therefore intended that the foregoing description be regarded as illustrative rather than limiting, and that it be understood that all equivalents and/or combinations of embodiments are intended to be included in this description.