Abstract:
A RF transmit coil decoupling circuit in a parallel drive configuration comprises a power amplifier and an output matching network to shim the B 1  field in response to inhomogeneities therein. The separate coil segments in a transmit array are effectively decoupled from each other despite inherent mutual inductance between coil segments by the decoupling circuit. The output matching network forms a high impedance block seen by the coil elements, while it provides a load line impedance at the output of the power amplifier. The transmission line transformer in the output matching network suppresses higher order harmonics for clearer RF wave forms. It also dampen the Q factor of the output matching network allowing stable operation with a series tuned coil element.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on provisional application 60/798,872 filed May 9, 2006 entitled “Magnetic Resonance Imaging RF Coil Drive Circuit with Transmission Line Transformer” and claims the benefit thereof. 
     
     BACKGROUND OF THE INVENTION 
       [0002]    The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to systems and methods for decoupling coil elements in an RF transmit coil array used to apply radio frequency magnetic fields to the subject being examined. 
         [0003]    When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B 0 ) applied along the z axis of a Cartesian coordinate system, the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B 1 ) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment M t . A NMR signal is emitted by the excited spins after the excitation signal B 1  is terminated, this signal may be received and processed to form an image or produce a spectrum. 
         [0004]    When utilizing these signals to produce images, magnetic field gradients (G x , G y  and G z ) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. 
         [0005]    Radio frequency antennas, or coils are used to produce the excitation field B 1  and other RF magnetic fields in the subject being examined. Such coils are also used to receive the very weak NMR signals that are produced in the subject. Such coils may be so-called “whole body” coils that are large enough to produce a uniform magnetic field for a human subject or, they can be much smaller “local” coils that are designed for specific clinical applications such as head imaging, knee imaging, wrist imaging, etc. Local coils may be either volume coils or surface coils. 
         [0006]    The quality of an MR image depends on the homogeneity of the applied excitation field B 1 . The tipping of the aligned moment M z  into the transverse plane should be as uniform as possible throughout the region of interest (ROI) and this in turn depends on the homogeneity of the excitation field B 1  throughout the ROI. MRI systems include a whole-body RF coil that is specifically designed to produce such a homogeneous field throughout the ROI. 
         [0007]    The most common whole body coil found in commercial MRI systems is the so-called “birdcage” coil. A birdcage coil has a pair of circular end rings which are bridged by a plurality (typically 8 to 24) of equi-spaced longitudinal straight segments. Traditionally, RF volume coils such as birdcage coils have been used to generate both transmission (Tx) and reception (Rx) B 1  fields. In a primary mode, currents in the straight segments are sinusoidally distributed which results in good B 1  field uniformity across the axis of the coil. Birdcage coils are the structure of choice in horizontal field MRI systems because they produce a homogeneous magnetic field in the bore of the magnet and they have a high SNR which enables them to pick up the small NMR signals emanating from the examined subject. That is, these coils have a periodic, resonant structure with multiple resonant modes. While birdcage coils and its variants generate uniform B 1  fields at low B 0  field strengths (1.5T and below), their performance is compromised at higher B 0  field strengths (3T and above). 
         [0008]    The homogeneity of the B 1  field is affected by the presence of the subject being imaged due to the different electrical characteristics of various tissue types found in the human body. Thus, even though the RF coil is shimmed by adjusting capacitors distributed throughout its structure to produce a homogeneous RF field, this homogeneity is lost when a subject is placed in the scanner. This problem is magnified as the polarizing field B 0  is increased and the resulting Larmor frequency increases due to the increased coil-sample interactions at higher frequencies. 
         [0009]    Recognizing this problem in MRI systems with higher B 0  field strengths, techniques have been developed to shim the B 1  field after the subject is placed in the scanner. As described in U.S. Pat. No. 6,982,554 issued on Jan. 3, 2006 and entitled “System And Method For Operating Transmit Or Transmit/Receive Elements In An MR System”, the phase and current magnitude applied to the individual segments in a birdcage RF coil can be separately controlled to shim the RF field it produces. As described in published US Pat. Application No. 2005/0231203, field on Mar. 18, 2005 and entitled “Method And Magnetic Resonance System For Homogenizing the B 1  Field”, an iterative method may be used to measure RF field homogeneity after the subject is in place and make adjustments in the RF field until it reaches an acceptable homogeneous level. 
         [0010]    Trimming the B 1  field by adjusting the currents in the separate coil segments to match the dielectric loading of the subject works well in principle but is difficult in practice to achieve. Unfortunately, because of the mutual inductance between RF coil segments, a change in the RF current in one segment to trim the RF field in a desired manner has the undesirable result of also changing the RF currents in adjacent RF coil segments through mutual inductance. Referring to  FIG. 3 , a single RF coil segment  2  is shown schematically to include an inductance  3 , resistance  4  and capacitance  5 . The inductance  3 , being a function of conductor length, capacitance  5  is accordingly selected to tune the segment  2  to a frequency near the Larmor frequency of the scanner. The RF coil segment  2  is connected across the output of a current control circuit  6  that is driven by an input signal  7  to produce a current in the RF coil segment  2  that contributes to a homogeneous B 1  field. The problem is that an additional current I ind  is also flowing through the RF coil segment  2  due to signals indicated at  8  induced by the currents flowing in adjacent RF coil segments. Because the output impedance Z 0  of the current control circuit  6  is necessarily low in order to efficiently drive large currents into the RF coil segment  2 , the induced currents I M  can be substantial, which can result in substantial artifacts in a resulting image. 
         [0011]    With multiple driving loops tuned at single image frequency, as in phased array coil, the current on each loop is a superposition of the driven current and currents induced by neighboring transmitters due to electromagnetic induction. The B 1  field generated by each transmitting element is, therefore, due to the resultant current on the element. When transmitting elements are strongly coupled to each other as in Birdcage coils or transverse electromagnetic (TEM) coils, independent control of B 1  fields on each transmitting element becomes very difficult. 
         [0012]    The coupling through the mutual impedance in two interacting elements can be minimized either by canceling the mutual impedance or reducing the current in the coil itself. Mutual inductance is typically cancelled by either partial overlap of two adjacent coils (transformer type decoupling) or by using decoupling capacitors. Although the mutual inductance can be driven to zero between two coils, those configurations are restricted to decoupling only the nearest neighbor. 
         [0013]    Capacitive ladder networks can be used to decouple not only the nearest element but also distant neighbors by cancelling decaying mutual inductances as the distance between the coil elements increases. However, this technique is not robust because it requires a perfectly symmetrical arrangement of identical array elements and is sensitive to the placement of a sample and its electrical properties. 
         [0014]    Alternatively, particularly in a receive array, the preamplifier decoupling network is used to isolate the coil elements by creating a large impedance block at the terminals of each receive element, thereby, suppressing currents driven by the spin induced electromotive force (emf). The simultaneous input matching network enables the formation of a high-impedance block with an optimum noise match for the minimum noise figure at the input terminal of preamplifier. Such a matching network is based on the fact that optimum noise matching is not generally the same as conjugate impedance matching at the preamplifier input terminal. This decoupling technique allows flexible receiver array design for the optimum noise correlation among array elements. Furthermore, the mismatched impedance seen by a coil element is known to reduce receiver ringing, thereby, improving transient response of a receiver coil. However, this technique is unusable at the output terminal of a power device in a transmit phased array because of the suppression of driven current used to generate desired B 1  fields. 
         [0015]    A Cartesian feedback approach in two interacting transmit elements has been proposed to suppress the induced current using the negative feedback. The feedback efficacy, however, depends heavily on the large open loop gain of the transmitter based on the small bandwidth of the feedback filter. As such, this approach limits the RF excitation bandwidth in some special applications. Furthermore, the inherent narrow bandwidth and system complexity of this approach have rendered it impractical for implementation at this time. 
         [0016]    Therefore, it would be desirable to have a system and method for decoupling elements in an RF coil array and independently controlling the current flowing through elements in an RF coil. 
       SUMMARY OF THE INVENTION 
       [0017]    The present invention overcomes the aforementioned drawbacks by providing a decoupling circuit for a RF coil that includes power amplifier (e.g. power MOSFET) and matching networks that provide inter-channel isolation in terms of the driven current amplitude. The circuit may be used with a whole body RF coil on an MRI system or a local coil with an MRI system to transmit a uniform B 1  magnetic field. 
         [0018]    The present invention is a decoupling circuit for individual coil elements or segments of an RF coil that enables each coil element or segment to be accurately driven without interference from currents induced from any neighboring coil segments or elements. An output matching network of the decoupling circuit improves the driven power efficiency and its capability and at the same time presents a high impedance to currents induced into the coil segment from other coil segments. To do so, the matching network utilizes the principle that power matching for a chosen power device is not generally the same as conjugate impedance matching. 
         [0019]    Another aspect of the invention is to construct a transmission line transformer that dampens the Q factor of the output matching network and improves the system bandwidth. In addition, the frequency response of the output matching network suppresses the higher order harmonics generated by the driving amplifier allowing clear RF waveforms. 
         [0020]    Yet another aspect of the invention is a drive circuit for an input matching network that provides a high unloaded Q factor at the drain creating a high output drain impedance with a shunt inductor. This high output impedance differs from the load line impedance allowing a simultaneous output matching network for a chosen power device. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a block diagram of an MRI system which employs the present invention; 
           [0022]      FIG. 2  is a block diagram of the RF system that forms part of the MRI system of  FIG. 1 ; 
           [0023]      FIG. 3  is an electrical schematic diagram of one RF coil segment that is separately driven as taught by the prior art; 
           [0024]      FIG. 4  a block diagram of a parallel RF transmit coil drive circuit and RF coil assembly which form part of the RF system of  FIG. 2 ; 
           [0025]      FIG. 5  is an electrical schematic diagram of a single RF transmit decoupling network integrated with a tuned coil segment in the associated drive circuit of  FIG. 4  in accordance with the present invention; and 
           [0026]      FIG. 6  is a detailed schematic of an output matching network in accordance with the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0027]    Referring particularly to  FIG. 1 , the preferred embodiment of the invention is employed in an MRI system. The MRI system includes a workstation  10  having a display  12  and a keyboard  14 . The workstation  10  includes a processor  16  which is a commercially available programmable machine running a commercially available operating system. The workstation  10  provides the operator interface which enables scan prescriptions to be entered into the MRI system. 
         [0028]    The workstation  10  is coupled to four servers: a pulse sequence server  18 ; a data acquisition server  20 ; a data processing server  22 , and a data store server  23 . In the preferred embodiment the data store server  23  is performed by the workstation processor  16  and associated disc drive interface circuitry. The remaining three servers  18 ,  20  and  22  are performed by separate processors mounted in a single enclosure and interconnected using a 64-bit backplane bus. The pulse sequence server  18  employs a commercially available microprocessor and a commercially available quad communication controller. The data acquisition server  20  and data processing server  22  both employ the same commercially available microprocessor and the data processing server  22  further includes one or more array processors based on commercially available parallel vector processors. 
         [0029]    The workstation  10  and each processor for the servers  18 ,  20  and  22  are connected to a serial communications network. This serial network conveys data that is downloaded to the servers  18 ,  20  and  22  from the workstation  10  and it conveys tag data that is communicated between the servers and between the workstation and the servers. In addition, a high speed data link is provided between the data processing server  22  and the workstation  10  in order to convey image data to the data store server  23 . 
         [0030]    The pulse sequence server  18  functions in response to program elements downloaded from the workstation  10  to operate a gradient system  24  and an RF system  26 . Gradient waveforms necessary to perform the prescribed scan are produced and applied to the gradient system  24  which excites gradient coils in an assembly  28  to produce the magnetic field gradients G x , G y  and G z  used for position encoding NMR signals. The gradient coil assembly  28  forms part of a magnet assembly  30  which includes a polarizing magnet  32  and a whole-body RF coil  34  made in accordance with the teachings of the present invention. 
         [0031]    RF excitation waveforms are applied to the RF coil  34  by the RF system  26  to perform the prescribed magnetic resonance pulse sequence. Responsive NMR signals detected by the RF coil  34  are received by the RF system  26 , amplified, demodulated, filtered and digitized under direction of commands produced by the pulse sequence server  18 . The RF system  26  includes an RF transmitter for producing a wide variety of RF pulses used in MR pulse sequences. The RF transmitter is responsive to the scan prescription and direction from the pulse sequence server  18  to produce RF pulses of the desired frequency, phase and pulse amplitude waveform. The generated RF pulses may be applied to the whole body RF coil  34  or to one or more local coils or coil arrays. 
         [0032]    The RF system  26  also includes one or more RF receiver channels. Each RF receiver channel includes an RF amplifier that amplifies the NMR signal received by the RF coil to which it is connected and a quadrature detector which detects and digitizes the I and Q quadrature components of the received NMR signal. The magnitude of the received NMR signal may thus be determined at any sampled point by the square root of the sum of the squares of the I and Q components: 
         [0000]        M= √{square root over (I 2   +Q   2 )}  Eqn. 1; 
         [0000]    and the phase of the received NMR signal may also be determined: 
         [0000]      φ=tan −1   Q/I   Eqn. 2. 
         [0033]    The pulse sequence server  18  also optionally receives patient data from a physiological acquisition controller  36 . The controller  36  receives signals from a number of different sensors connected to the patient, such as ECG signals from electrodes or respiratory signals from a bellows. Such signals are typically used by the pulse sequence server  18  to synchronize, or “gate”, the performance of the scan with the subject&#39;s respiration or heart beat. 
         [0034]    The pulse sequence server  18  also connects to a scan room interface circuit  38  which receives signals from various sensors associated with the condition of the patient and the magnet system. It is also through the scan room interface circuit  38  that a patient positioning system  40  receives commands to move the patient to desired positions during the scan. 
         [0035]    It should be apparent that the pulse sequence server  18  performs real-time control of MRI system elements during a scan. As a result, it is necessary that its hardware elements be operated with program instructions that are executed in a timely manner by run-time programs. The description components for a scan prescription are downloaded from the workstation  10  in the form of objects. The pulse sequence server  18  contains programs which receive these objects and converts them to objects that are employed by the run-time programs. 
         [0036]    The digitized NMR signal samples produced by the RF system  26  are received by the data acquisition server  20 . The data acquisition server  20  operates in response to description components downloaded from the workstation  10  to receive the real-time NMR data and provide buffer storage such that no data is lost by data overrun. In some scans the data acquisition server  20  does little more than pass the acquired NMR data to the data processor server  22 . However, in scans which require information derived from acquired NMR data to control the further performance of the scan, the data acquisition server  20  is programmed to produce such information and convey it to the pulse sequence server  18 . For example, during prescans NMR data is acquired and used to calibrate the pulse sequence performed by the pulse sequence server  18 . Also, navigator signals may be acquired during a scan and used to adjust RF or gradient system operating parameters or to control the view order in which k-space is sampled. And, the data acquisition server  20  may be employed to process NMR signals used to detect the arrival of contrast agent in an MRA scan. In all these examples the data acquisition server  20  acquires NMR data and processes it in real-time to produce information which is used to control the scan. 
         [0037]    The data processing server  22  receives NMR data from the data acquisition server  20  and processes it in accordance with description components downloaded from the workstation  10 . Such processing may include, for example: Fourier transformation of raw k-space NMR data to produce two or three-dimensional images; the application of filters to a reconstructed image; the performance of a backprojection image reconstruction of acquired NMR data; the calculation of functional MR images; the calculation of motion or flow images, etc. 
         [0038]    Images reconstructed by the data processing server  22  are conveyed back to the workstation  10  where they are stored. Real-time images are stored in a data base memory cache (not shown) from which they may be output to operator display  12  or a display  42  which is located near the magnet assembly  30  for use by attending physicians. Batch mode images or selected real time images are stored in a host database on disc storage  44 . When such images have been reconstructed and transferred to storage, the data processing server  22  notifies the data store server  23  on the workstation  10 . The workstation  10  may be used by an operator to archive the images, produce films, or send the images via a network to other facilities. 
         [0039]    Referring particularly to  FIG. 2 , the RF system  26  includes a transmitter that produces a prescribed RF excitation field. The base, or carrier, frequency of this RF excitation field is produced under control of a frequency synthesizer  200  which receives a set of digital signals from the pulse sequence server  18 . These digital signals indicate the frequency and phase of the RF carrier signal produced at an output  201 . The RF carrier is applied to a modulator and up converter  202  where its amplitude is modulated in response to a signal R(t) also received from the pulse sequence server  18 . The signal R(t) defines the envelope of the RF excitation pulse to be produced and is produced by sequentially reading out a series of stored digital values. These stored digital values may, be changed to enable any desired RF pulse envelope to be produced. 
         [0040]    The magnitude of the RF excitation pulse produced at output  205  is attenuated by an exciter attenuator circuit  206  which receives a digital command from the pulse sequence server  18 . The attenuated RF excitation pulses are applied to an RF coil drive circuit  155  that, as will be described in detail below, drives the RF coil  34  through a transmit/receive switch  152 . For a more detailed description of this transmitter section reference is made to U.S. Pat. No. 4,952,877 which is incorporated herein by reference. 
         [0041]    Referring still to  FIG. 2  the NMR signal produced by the subject is picked up by the RF coil  34  and applied through the transmit/receive switch  152  to a preamplifier and attenuator  207 . The preamplifier and attenuator  207  amplify the NMR signal by an amount determined by a digital attenuation signal received from the pulse sequence server  18 . The received NMR signal is at or around the Larmor frequency, and this high frequency signal is down converted in a two step process by a down converter  208  which first mixes the NMR signal with the carrier signal on line  201  and then mixes the resulting difference signal with a reference signal on line  204 . The down converted NMR signal is applied to the input of an analog-to-digital (A/D) converter  209  which samples and digitizes the analog signal and applies it to a digital detector and signal processor  210  which produces 16-bit in-phase (I) values and 16-bit quadrature (Q) values corresponding to the received signal. The resulting stream of digitized I and Q values of the received signal are output to the data acquisition server  20 . The reference signal as well as the sampling signal applied to the A/D converter  209  are produced by a reference frequency generator  203 . For a more detailed description of the receiver, reference is made to U.S. Pat. No. 4,992,736 which is incorporated herein by reference. 
         [0042]    The transmit/receive switch  152  is used when the RF coil  34  is employed to both produce the uniform B 1  field and receive the resulting NMR signals. It is operated by the pulse sequence server  18  to switch to the transmitter during parts of the pulse sequence in which RF fields are to be produced, and to switch connection to the receiver when NMR data is to be acquired. The RF transmit signal is coupled to the RF coil assembly  34  that will be described in detail below. 
         [0043]    To overcome the above-described drawbacks associated with Tx B 1  inhomogeneity, individual and independent amplitude and phase control of the B 1  field generated by each array element in a transmit coil can be used to enable correction of deterministic inhomogeneous patterns. However, to achieve such independent elemental B 1  field in the presence of large mutual coupling B 1  fields generated by neighboring array elements can be difficult. That is, the currents induced by the B 1  fields of neighboring elements have to be suppressed while allowing a controlled current to be driven in a transmitting element. Traditional decoupling techniques, such as partial overlap or a capacitive decoupling network, are unsuitable. That is, as with Rx arrays, decoupling is achieved only with nearest neighboring elements, which allows the B 1  fields caused by distant neighbors to couple into the transmit element. While microstrip transmitting elements achieve decoupling, the decoupling comes at the cost of a rapidly attenuating B 1  field towards the center of the imaging volume. 
         [0044]    The present invention recognizes that an active rung topology can be employed as a power MOSFET decoupling network. This approach uses an active device as a controlled RF current source directly connected to a series-tuned current element carrying induced voltages. Due to its unidirectional property, the induced RF energy from other transmitters is suppressed in its reverse direction like an active isolator. This technique for manipulating induced RF energy with an active device is promising because it allows the flexibility of a transmit array design regardless of induced voltage. However, the active-rung technique is limited by the output impedance of a chosen power MOSFET because the output impedance becomes a part of induced current path. 
         [0045]    While an active rung topology can be employed to utilize a power MOSFET, it comes at the cost of reduced drive efficiency and provides only a limited decoupling performance that depends on the output impedance of the chosen power device. As will be described, the present invention provides an output decoupling network and an input matching topology for a chosen power MOSFET that provides increased decoupling between neighboring elements while simultaneously increasing the drive efficiency compared to the active-rung design. 
         [0046]    Referring particularly to  FIG. 4 , the RF coil drive circuit  155  includes a control circuit portion  157  that includes a power splitter  157   a,  variable attenuators  157   b,  variable phase shifters  157   c,  and driving stage amplifiers  157   d  that are connected to a decoupling network  168 . The circuit  157  produces eight output signals during the transmit mode that are phase shifted (0°, 45°, 90°, 135°, 180°, 225°, 270°, 315°), delivered to the decoupling network  168 , and provided to drive the respective eight segments in the coil assembly  34 . In addition, the magnitude and phase of each of these output signals can be adjusted for B 1  field shimming purposes in response to commands received at input  160 . 
         [0047]    The signals provided to the decoupling network  168  are coupled to eight separate channels in a voltage controlled current source  162  mounted on a transmit coil, as will be described, through coaxial cables. As will be described in detail below, the eight current sources  162  produce eight driven currents at the outputs  164  that drive the respective eight segments in the RF coil assembly  34  at the commanded frequency, phase, and magnitude. However, rather than the output  164  of current sources  162  being directly connected to the RF coil segments in the assembly  34 , the outputs  164  drive respective coil elements through output matching networks  166 . As will be described in detail below, each channel in the output matching network  166  provides a low input impedance (load line impedance) at the outputs  164  to extract the maximum power out of the current source  162 . The output matching networks  166  also provides efficient coupling of the commanded RF current to its associated coil segment in the coil assembly  34 , and it provides a very high impedance to any induced current flow in its associated coil segment. 
         [0048]    The RF coil assembly  34  can take many forms. For example, the RF coil assembly  34  may be a traditional transverse electromagnetic (TEM), a modified TEM coil, such as described in U.S. patent application Ser. No. 11/726,643, or even a traditional birdcage coil or surface coils arranged arbitrarily. In any case, rather than driving this coil assembly with a single signal or a pair of quadrature signals as is commonly done, each of the eight segments or rungs is separately driven to provide more control over the RF field that is produced. 
         [0049]    Referring particularly to  FIG. 5 , a schematic of a portion  162 ,  166  of the drive circuit  155  forming a single-ended power MOSFET decoupling network  168  in accordance with the present invention is illustrated. The MOSFET decoupling network  168  includes a variety of sub-systems. For example, the MOSFET decoupling network  168  includes an input matching network  170 , a power MOSFET  172 , and an output matching network  174 . In this regard, the decoupling network  168  can be divided into a variety of sections along lines a/a′, A/A′, b/b′, and B/B′. As is well know, the MOSFET  172  includes a drain terminal, D, a gate terminal, G, and a source terminal, S. 
         [0050]    The decoupling network  168  acts as a simultaneous output matching network of a power MOSFET  172 , which suppresses induced current by forming a high output impedance seen by an induced voltage, V ind , and sets up the load-line impedance at the drain terminal, D, of the MOSFET  172 . It should be noted that the term “matching” does not refer to conjugate matching. Rather, it refers to the load line matching for a chosen power device in view of driven current source. Such a matching network is accomplished when the output capacitance of the MOSFET  172 , C oss , is not significantly high and, therefore, it can produce a high output impedance block with a shunt inductance  178  at the drain terminal, D, with the aid of the input matching network  170 . This allows a high output reflection coefficient at A-A′ Accordingly, the high output reflection coefficient allows a high impedance block at B-B′ seen by the current element. As will be described in detail with respect to  FIG. 6 , the output matching network  174  also accommodates higher-order harmonic suppression for clear RF wave forms due to its band-pass response at the imaging frequency. 
         [0051]    The load resistance at B-B′, R TEM,res    175 , is a resistance of a series tuned coil element, representing distributed loss along that element In view of the driven current, the output matching network  174  sees the load impedance at b-b′, Z bb′,i ˜R TEM,res  when a shunt reactance  176  at b-b′, X om,sh,  is much greater than R TEM,res,  and the matching network  174  transforms R TEM,res  to R L,opt  at A-A′. A shunt inductor  178 , X d,sh,  is connected to the drain, D, of the MOSFET  172  to cancel out the output capacitance of the MOSFET  172 , creating a high drain impedance, Z AA′,o , under the control of the input matching network  170 . An induced voltage  179 , V ind , at the element terminal depends on the geometry of the array, and results in the induced current flow, I ind , along the loop formed by the coil element  175  and the output terminal impedance at B-B′, Z BB′,o . In case X om,sh,  is in parallel resonance with the output impedance at b-b′, Z bb′,o,  then Z BB′,o  is proportional to the square of the Q factor at b-b′, Q bb′,o , as given by: 
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         [0052]    where Q bb′,o  is determined by the output matching network  174  and the drain impedance, Z AA′,o . 
         [0053]    The amount of induced current suppression may be defined by the ratio of the induced current, I ind , to the driven current, I driv , at B-B′ with the same amount of driven and induced voltages at B-B′ as shown in Eqn. 4. In case Z BB′,o  is much greater than R TEM,res , Z BB′,o  dominates the induced current suppression, as given by: 
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                                 + 
                                 
                                   Q 
                                   
                                     
                                       bb 
                                       ′ 
                                     
                                     , 
                                     o 
                                   
                                   2 
                                 
                               
                               ) 
                             
                           
                            
                           
                             [ 
                             dB 
                             ] 
                           
                         
                         . 
                       
                     
                   
                 
               
               
                 
                   Eqn 
                   . 
                   
                       
                   
                    
                   4 
                 
               
             
           
         
       
     
         [0054]    As addressed above, inter-channel coupling due to mutual impedance can be reduced by suppressing the induced current I ind  on each current element in the coil array of  FIG. 4 . As in preamplifier decoupling techniques, if the output reflection coefficient at the drain terminal, D, approaches unity, a high impedance block seen by the induced voltage  179  is accomplished by the output matching network  174  at each element terminal B-B′. 
         [0055]    Referring now to  FIG. 6 , the matching network  174  includes a transmission line transformer (TLT)  180 , and a parallel pi-tank circuit  182  formed by an inductor L p    184  and a capacitor C p    186 . Additionally, the matching network includes a series variable inductor L s    187  configured to cancel input reactance at α-aα′, Z αα′,i . 
         [0056]    The output matching network  174  can be implemented on a printed circuit board including the TLT  180  formed by a 50 Ohm, λ ε /10 long, semi-rigid coaxial conductor and having mounted thereon the capacitive and inductive components required to form the tank  182 . As will be described in detail below, in accordance with one embodiment, the TLT  180  is formed by a coaxial cable having a center conductor  188  and a surrounding shielding  190 . The TLT  180  is curled to reduce the outer shield current. For example, the TLT  180  may be formed by a length of 50 ohm coaxial cable curled to have a 1.3-cm diameter and be 1.9-cm long. 
         [0057]    Referring to  FIGS. 4 and 6 , the output  164  of the current source  162  connects to the center conductor  188  of the TLT  180  and the same center conductor  188  is connected to ground at the output end  194 . The shielding  190  included on the TLT  180  is connected to ground at the input end  192  of and the same shield  190  is connected to ground through the capacitor C p    186  at the output end  194 . 
         [0058]    The cable has a length that is λ ε /10, which is one-tenth of the cable wavelength of the Larmor frequency at which the system operates (i.e., the RF wavelength within the dielectric of the coaxial cable, which is, generally, shorter than the wavelength in free space). However, it is contemplated that the cable length may vary some to have a minimum insertion loss of the TLT for example, λ/8. As will be described, the capacitor C p    186  is chosen to resonate with the reactance observed at b-b′ such that the minimum insertion loss of the TLT  180  is achieved. 
         [0059]    The impedance of the series tuned coil element  175  is small and need to be scaled up to the load line resistance at A-A′ in  FIGS. 5 . The above mentioned TLT structure has a finite impedance scaling factor. The impedance scaling factor of the TLT  180 , |n eff | 2 , is assessed by measuring self and terminal impedances through Eqn. 5. Where n eff  is the terminal current ratio of I ββ′,o  to I αα′,i  and jωL c  and jωL o , are self impedances of the center and shield conductors, respectively and Z αα′,i  and Z ββ′o , are terminal impedances measured at α-α′ and β-β′, respectively in  FIGS. 6 : 
         [0000]    
       
         
           
             
               
                 
                   
                     
                        
                       
                         n 
                         eff 
                       
                        
                     
                     2 
                   
                   = 
                   
                     
                        
                       
                         
                           Z 
                           
                             
                               α 
                                
                               
                                   
                               
                                
                               
                                 α 
                                 ′ 
                               
                             
                             , 
                             i 
                           
                         
                         
                           Z 
                           
                             
                               β 
                                
                               
                                   
                               
                                
                               
                                 β 
                                 ′ 
                               
                             
                             , 
                             i 
                           
                         
                       
                        
                     
                     = 
                     
                       
                          
                         
                           
                             
                               Z 
                               
                                 
                                   α 
                                    
                                   
                                       
                                   
                                    
                                   
                                     α 
                                     ′ 
                                   
                                 
                                 , 
                                 i 
                               
                             
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                                  
                                 
                                   
                                     Z 
                                     
                                       
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                                          
                                         
                                             
                                         
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                                           β 
                                           ′ 
                                         
                                       
                                       , 
                                       i 
                                     
                                   
                                   = 
                                   
                                     Z 
                                     L 
                                   
                                 
                               
                                
                               
                                 
                                   - 
                                   j 
                                 
                                  
                                 
                                     
                                 
                                  
                                 ω 
                                  
                                 
                                     
                                 
                                  
                                 
                                   L 
                                   c 
                                 
                               
                             
                           
                           
                             
                               Z 
                               
                                 
                                   β 
                                    
                                   
                                       
                                   
                                    
                                   
                                     β 
                                     ′ 
                                   
                                 
                                 , 
                                 o 
                               
                             
                              
                             
                               
                                  
                                 
                                   
                                       
                                   
                                   
                                     
                                       Z 
                                       
                                         
                                           β 
                                            
                                           
                                               
                                           
                                            
                                           
                                             β 
                                             ′ 
                                           
                                         
                                         , 
                                         i 
                                       
                                     
                                     = 
                                     
                                       Z 
                                       L 
                                     
                                   
                                 
                               
                                
                               
                                 
                                   - 
                                   j 
                                 
                                  
                                 
                                     
                                 
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                                 ω 
                                  
                                 
                                     
                                 
                                  
                                 
                                   L 
                                   o 
                                 
                               
                             
                           
                         
                          
                       
                       . 
                     
                   
                 
               
               
                 
                   Eqn 
                   . 
                   
                       
                   
                    
                   5 
                 
               
             
           
         
       
     
         [0060]    The inductance L p  in  FIGS. 6  is chosen to have the load line resistance at A-A′ in  FIGS. 5  with the determined capacitance C p . The tank circuit  182  suppresses the higher order harmonics as a low pass filter (LPF) circuit. 
         [0061]    In view of the output impedance at b-b′ Z bb′,o  is dominated by the off resonance, X Lp -X Cp , because the output impedance at β-β′, Z ββ′,o , is large enough compared to the C p    186  with the high Z AA′,o . The amount of the off resonance determines the Q bb′,o , and, thereby the Z BB′,o , with a shunt inductor, X om,sh,    176  in  FIG. 5 . 
         [0062]    This high output impedance block, Z BB′,o , keeps the induced current, I ind , at a substantially low level, such that it does not interfere with the driven current needed to produce the desired RF magnetic field. The induced currents, I ind , caused by the inherent mutual inductance between the RF coil segments is thus reduced to an insignificant amount. 
         [0063]    Furthermore, the frequency response of the output matching network  174  is designed to suppress higher order harmonics generated by the driving amplifier and, thereby, allow clear RF waveforms. Specifically, the frequency response of the output matching network  174  is designed for minimum insertion loss at the Larmor frequency. As will be described, the frequency response of the network substantially suppresses second and third harmonics, for example by the factor of 29.3 and 33.2 dB, respectively due to the insertion loss of the output matching network  174 . 
         [0064]    Even with a high power rating device the above-described invention may be applied if the chosen power device provides a high unloaded Q factor at the drain. 
         [0065]    The present invention has been described in terms of the various embodiments, and it should be appreciated that many equivalents, alternatives, variations, and modifications, aside from those expressly stated, are possible and within the scope of the invention. Therefore, the invention should not be limited to a particular described embodiment.