Abstract:
An ultra low output impedance RF power amplifier for driving a multiple transmit coil Magnetic Resonance Imaging (MRI) system is described, comprising an output matching network with high power MOSFET operatively coupled to at least one transmit coil in the MRI system for a desired output power and impedance. This invention also describes a method for achieving decoupling using the RF power amplifier to drive at least one transmit coil.

Description:
BACKGROUND 
       [0001]    The present invention relates to magnetic resonance systems, and more particularly to RF power amplifiers for parallel excitation in magnetic resonance systems. 
         [0002]    There has been very active development in the field of magnetic resonance (MR) where parallel RF transmission with multiple transmit elements is used to benefit various applications by improving spin excitation. In high field MR, inhomogeneity in the RF magnetic field caused by wave propagation and dielectric effect in particular, may be reduced by optimizing the amplitude and phase of the driving currents when conducting multi-port excitation on birdcage coils or transmit arrays consisted of individual coil elements. It is possible to further reduce the RF magnetic field in homogeneity effect by independently controlling the RF waveforms of individual transmit channels and leveraging the capacity of full fledged parallel RF transmission in accelerating multidimensional excitation and managing power deposition. 
         [0003]    Recent developments have provided support for direct validation of full-fledged parallel RF transmission principles. However, the development of efficient parallel transmit coil arrays remains a significant challenge. Coupling between the transmit coil elements is one of the key challenges to transmit coil array construction and use. Many approaches have been proposed to address the issue of inter coil coupling. One of the approaches defines, for parallel RF reception, a preamplifier decoupling scheme. This scheme reduces the input impedance of a preamplifier to nearly zero, thereby maximizing the input impedance seen by the corresponding receive coil at its output port and causing blockage of coupled current in the coil. However, for parallel transmit, practicing an analogous scheme is ineffective due to the typical RF amplifiers&#39; 50Ω impedance seen by the coils in effect. 
         [0004]    Many decoupling methods have been proposed to address the inter-coil coupling problem. One category of methods introduces partial geometric overlap of coils to annul the mutual inductance between them. Such methods are effective for nearest neighbor elements only, and tend to impose stringent constraints on the geometry and placement of the individual coils. Another category of methods employs a capacitive or inductive decoupling bridge or a multi-port network, at the cost of increased RF loss and increased complexity of the decoupling circuits and tuning efforts. A third category of methods suppresses the coupling-induced currents with high source impedance, by, for example, integrating RF power MOSFET&#39;s with the rungs of a TEM coil or driving nonresonant loop-shaped coils directly. In these examples, a MOSFET is configured to function approximately as a current source, and thus to yield high impedance at the driving ports. However, the series resonant element in this method also acts as a severely mismatched load to the MOSFET, which may significantly degrades its maximum available output power. A fourth category of methods applies active decoupling. Such methods calibrate coupling between element coils first and then introduce proper correlations, realized either by analog circuits or a digital vector modulation array, between the driving voltages of each element to cancel the coupling components in the currents. 
         [0005]    Therefore, there is a need for a system and method for development of a decoupling method that supports parallel transmit applications and facilitates transmit performance optimization by eliminating constraints on array geometry. 
       BRIEF DESCRIPTION 
       [0006]    Multiple transmit chains are employed in parallel RF transmission for setting up the currents in an array of transmit coils in a coordinated way. The currents in turn induce transmit magnetic field in the subject. However the current in each of the element coils is typically subject to corruption due to inter-coil coupling. The coupling-induced corruptive components are much influenced by the source impedance seen by the element coils. To overcome the disadvantages as above, embodiments of the invention improve the inter-element isolation of transmit array with the aid of matching networks on the RF power amplifiers and/or the element coils. 
         [0007]    In a first aspect, a Radio Frequency (RF) power amplifier for driving a multiple RF transmit coil Magnetic Resonance Imaging (MRI) system is provided. The RF amplifier includes an output matching network with high power MOSFET operatively coupled to at least one transmit coil in the MRI system for a desired output power and impedance. 
         [0008]    In another aspect, an ultra-low output impedance Radio Frequency (RF) power amplifier for transmit coil decoupling in parallel excitation is provided. The ultra-low output impedance Radio Frequency (RF) amplifier includes an output matching network and a high power MOSFET operatively coupled to the input of the output matching network to provide desired impedance for the transmit coil decoupling and to maximize available power. 
         [0009]    In yet another aspect, a method for achieving decoupling using radio-frequency (RF) power amplifier to drive at least one transmit coil is provided. The method includes, transforming a drain source resistive component of a MOSFET to a desired output impedance using an output matching network, transforming an input impedance of at least one transmit coil to a load impedance, and matching of the load impedance to an optimum load of the MOSFET for a desired output power. 
     
    
     
       DRAWINGS 
         [0010]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0011]      FIG. 1  is an equivalent circuit model of coupled transmit coils driven by two independent RF power amplifiers. 
           [0012]      FIG. 2  illustrates the equivalent circuit model of MOSFET. 
           [0013]      FIG. 3  illustrates an output-matching network in the ultra-low output impedance RF power amplifier in one embodiment of the present invention. 
           [0014]      FIG. 4  shows an embodiment of the transmit coils driven independently by two ultra low output impedance RF power amplifiers of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Referring to  FIG. 1 , it shows an equivalent circuit model of two identical coils  103 ,  104  driven by two independent RF power amplifiers  101 ,  102 . Each power amplifier is modeled as a voltage source with a source resistance r s . The impedance of each coil without matching components is r+jx, and the inductive coupling between these two coils is captured by mutual inductance M. Representing a common configuration, the L-shaped matching network on each coil, which is consisted of a capacitor and an inductor, not only transforms the low impedance of the series resonant coil into a desired value (normally  50 Q), but also amplifies the input current by x/r times. When both coils  103 ,  104  are driven by their corresponding amplifiers  101 ,  102 , the current I running in coil  103  is consisted of two components, the desired one I (S)  that is due to the controlling voltage V 1  and the undesired (corruptive) one I (M)  that is due to V 2 : 
         [0000]    
       
         
           
             
               
                 
                   { 
                   
                     
                       
                         
                           
                             I 
                             
                               ( 
                               S 
                               ) 
                             
                           
                           = 
                           
                             
                               - 
                               
                                 jxV 
                                 1 
                               
                             
                             
                               
                                 rr 
                                 S 
                               
                               + 
                               
                                 
                                   ω 
                                   2 
                                 
                                  
                                 
                                   M 
                                   2 
                                 
                                  
                                 
                                   
                                     r 
                                     S 
                                     2 
                                   
                                   / 
                                   
                                     ( 
                                     
                                       
                                         rr 
                                         S 
                                       
                                       + 
                                       
                                         x 
                                         2 
                                       
                                     
                                     ) 
                                   
                                 
                               
                               + 
                               
                                 x 
                                 2 
                               
                             
                           
                         
                       
                     
                     
                       
                         
                           
                             
                               I 
                               
                                 ( 
                                 M 
                                 ) 
                               
                             
                             = 
                             
                               
                                 
                                   - 
                                   
                                     jxV 
                                     2 
                                   
                                 
                                 
                                   
                                     rr 
                                     S 
                                   
                                   + 
                                   
                                     
                                       ω 
                                       2 
                                     
                                      
                                     
                                       M 
                                       2 
                                     
                                      
                                     
                                       
                                         r 
                                         S 
                                         2 
                                       
                                       / 
                                       
                                         ( 
                                         
                                           
                                             rr 
                                             S 
                                           
                                           + 
                                           
                                             x 
                                             2 
                                           
                                         
                                         ) 
                                       
                                     
                                   
                                   + 
                                   
                                     x 
                                     2 
                                   
                                 
                               
                               · 
                               
                                 
                                   jω 
                                    
                                   
                                       
                                   
                                    
                                   M 
                                 
                                 
                                   r 
                                   + 
                                   
                                     
                                       x 
                                       2 
                                     
                                     / 
                                     
                                       r 
                                       s 
                                     
                                   
                                 
                               
                             
                           
                           , 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   1 
                   ] 
                 
               
             
           
         
       
     
         [0000]    where ω is the Larmor frequency. The severity of current corruption caused by the coupling effect could be represented by the ratio of I (M)  to I (S) : 
         [0000]    
       
         
           
             
               
                 
                   
                     
                        
                       
                         I 
                         
                           ( 
                           M 
                           ) 
                         
                       
                        
                     
                     
                        
                       
                         I 
                         
                           ( 
                           S 
                           ) 
                         
                       
                        
                     
                   
                   = 
                   
                     
                       
                         ω 
                          
                         
                             
                         
                          
                         M 
                       
                       
                         r 
                         + 
                         
                           
                             x 
                             2 
                           
                           / 
                           
                             r 
                             s 
                           
                         
                       
                     
                      
                     
                       
                          
                         
                           
                             V 
                             2 
                           
                           
                             V 
                             1 
                           
                         
                          
                       
                       . 
                     
                   
                 
               
               
                 
                   [ 
                   2 
                   ] 
                 
               
             
           
         
       
     
         [0016]    For given r, x, M and ω, the minimum of the ratio is achieved by minimizing r s , which suggests that minimizing source impedance will improve isolation. 
         [0017]    In practical solid-state RF power amplifiers, metallic oxide semiconductor field effect transistors (MOSFET) are commonly used to drive the RF power to a desired level. When it operates in the saturation region of its DC characteristic, a MOSFET behaves approximately as a voltage-controlled current source.  FIG. 2  illustrates the equivalent circuit model in this case, in which the drain-source resistance R DS    206  is typically of a very high value. When a MOSFET operates in a linear mode (Class A or AB) with a fixed DC drain-source voltage  205 , its maximum output power without distortion critically depends on its load impedance. The maximum rated power can only be achieved when the load impedance equals an optimum value, which is normally provided by the manufacturer. 
         [0018]    To maximize inter-coil or inter-element isolation by taking advantage the low source-impedance idea above and to simultaneously maximize the available output power, a new amplifier output stage design has been developed. As used herein the terms coils and elements are used interchangeably and refer to the transmit array coils in the imaging system. Referring to  FIG. 3 , an output-matching network  308  for MOSFET  311  is introduced. In an embodiment of the invention, the matching network applies an inductor L  312  in parallel to the output of MOSFET  311  to resonate its drain-source capacitance Coss  207 . Then a T-shaped network consisted of two capacitors C  310 ,  314  and an inductor L  313 , which are chosen to be series resonant at the working frequency, further transforms the drain-source resistance R DS    206  into 
         [0000]    
       
         
           
             
               
                 
                   
                     Z 
                     OUT 
                   
                   = 
                   
                     
                       1 
                       
                         
                           ω 
                           2 
                         
                          
                         
                           C 
                           2 
                         
                          
                         
                           R 
                           DS 
                         
                       
                     
                     . 
                   
                 
               
               
                 
                   [ 
                   3 
                   ] 
                 
               
             
           
         
       
     
         [0019]    The R DS  can be evaluated by measuring the differential of the drain source voltage V DS  to the current I DS  in the saturation region of the MOSFET&#39;s DC characteristics, with the gate voltage fixed to a value that could bias the I DS  to a given value at a given V DS . Because of the high resistance presented by R DS    206 , the output impedance Z OUT    315  can thus be made very low as it is primarily determined by the series resonant circuit, which is nearly a short circuit at the resonant frequency. As analogous to the receive case, when Z OUT    315  is close to zero, the input-matching network  319  at the coil side  303  acts as a parallel resonant circuit and the corruptive current component due to inter-coil coupling sees a large impedance and will thus be substantially suppressed. In one embodiment, the output impedance is about 10 ohms or less. In a further embodiment, the output impedance is about 5 ohms or less. In the meanwhile, the same output-matching network  308  for the MOSFET  311  transforms the input impedance of the coil, normally matched to 50 Ω, into the load impedance  309   
         [0000]    
       
         
           
             
               
                 
                   
                     Z 
                     L 
                   
                   = 
                   
                     
                       1 
                       
                         
                           50 
                            
                           
                             ω 
                             2 
                           
                            
                           
                             C 
                             2 
                           
                         
                         - 
                         
                           j 
                           
                             ω 
                              
                             
                                 
                             
                              
                             
                               L 
                               1 
                             
                           
                         
                       
                     
                     . 
                   
                 
               
               
                 
                   [ 
                   4 
                   ] 
                 
               
             
           
         
       
     
         [0020]    Generally, the optimum load can be expressed as 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       Z 
                       OL 
                     
                     = 
                     
                       1 
                       
                         
                           1 
                           
                             R 
                             OL 
                           
                         
                         - 
                         
                           jω 
                            
                           
                               
                           
                            
                           
                             C 
                             OSS 
                           
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   [ 
                   5 
                   ] 
                 
               
             
           
         
       
     
         [0000]    in which R OL  represents the load resistance that enables the MOSFET to output highest power. By setting L  313  and C  310 ,  314  to satisfy 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       R 
                       OL 
                     
                     = 
                     
                       
                         1 
                         
                           50 
                            
                           
                             ω 
                             2 
                           
                            
                           
                             C 
                             2 
                           
                         
                       
                       = 
                       
                         50 
                          
                         
                           ω 
                           2 
                         
                          
                         
                           L 
                           2 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   [ 
                   6 
                   ] 
                 
               
             
           
         
       
     
         [0000]    and resonating C OSS    207  with L  312 , the load impedance Z L    309  is matched to the optimum value specified for the MOSFET  311 , and thus ensures that highest output power can be achieved. In a non-limiting example, the output power is at least about 500 W. This design allows flexible placement of the RF power amplifier&#39;s power stage. For off-coil placement, a coaxial cable with nλ/2 length  316  may be used to connect a coil  303  with its corresponding amplifier that is some distance away. Because of the current amplification effect at the matching network on each coil, the current in the cable can be much lower than that in the coil, which facilitates management of cable loss. 
         [0021]    Referring to  FIG. 4 , an exemplary embodiment of the transmit coils driven independently by two ultra low output impedance RF power amplifiers of the present invention is shown. With this example, we illustrate the choice of components and the tuning of the matching networks. Two ultra-low output impedance RF power amplifiers  401 ,  402  are developed to work at 128 MHz. Each amplifier is consisted of three amplification stages, and the final stage is built with a high power MOSFET based on the scheme shown in  FIG. 3 . Based on power efficiency and linearity considerations, the device is set to operate in Class AB and the bias current is set to 200 mA. The bias voltage is applied in pulse mode, as triggered by an external gating signal. The drain voltage is set to 150V and the R OL  of the MOSFET at this voltage is about 25 Ω. According to Eqs. 6, C  310 ,  314  is selected to be 35 pF and L  313  is adjusted to resonate with it at 128 MHz. 
         [0022]    A phased array consisting of two 8×8 cm 2  surface coils  403 ,  404  is employed to evaluate the decoupling performance of the two amplifiers  401 ,  402 . The array is placed on a saline phantom  441  (1.33 g/L NaCl, 0.66 g/L CuSO4) that is with a 30 cm length, 20 cm width and 20 cm height. The separation between coils  403 ,  404  and the distance from each of them to phantom  441  is adjustable. Initially coils  403 ,  404  are placed 1 cm above the phantom  441  with a 3 cm inter-element separation. Half wavelength cables are used to connect the prototype amplifiers  401 ,  402  to element coils  403 ,  404  respectively. For the bench tests in this embodiment, three ferrite rings  409 ,  405  associated with each cable are used to block the common mode current. 
         [0023]    The current running in the elements  403 ,  404  are respectively monitored through two sensing coils  435 ,  438 . The sensing coil  438  for  403  employed a butterfly structure, which has two 1 cm diameter loop placed across the conductor that is farthest away from coil  435 . With this configuration the electromotive forces (EMF) induced in the two loops of the sensing coil  438  by the current of element  403  are in-phase and thus enhanced, while those by the current of element  404  are approximately anti-phase and thus neutralized. Consequently, compared to element  403 , the contribution of element  404  to sensing coil  438  is reduced to a negligible level in this embodiment of the invention. Similarly, another sensing coil  435  is constructed to detect the current in element  404  only. 
         [0024]    The two elements  403 ,  404  with half wavelength cables are first tuned and matched to 50 Ω independently. Then coil  404  is driven by a network analyzer while coil  403  is terminated with a short connecter. The current induced in coil  403  is sensed through the S 21  measurement of its sensing coil  438 , and the matching inductor  318  is adjusted until the induced current is lowest. To determine the value of L  312  to compensate for the output capacitance of high power MOSFET  311 , each element coil  403 ,  404  is driven by its corresponding amplifier. Both amplifiers  401 ,  402  are simultaneously gated on with 3 ms pulses and 1% duty circle, and power amplifier  402  is excited by the network analyzer to output 1 W power. Then the L  312  of power amplifier  401  is tuned to make the induced current in coil  403  lowest. Similar strategies are applied to tune coil  404  and power amplifier  402 . 
         [0025]    While only certain features of the invention have been illustrated and described herein, the embodiments described are exemplary and non-limiting as many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.