Abstract:
An integrated quadrature splitter-combiner and balun comprising: a transmission line balun operably connected to a first port and a second port; a first capacitor operably connected across the transmission line balun; a second capacitor operably connected to the first port and said balun; and a third capacitor operably connected to the second port and the balun. The second capacitor, the third capacitor, and the transmission line balun combine to form an RF splitter-combiner to split a balanced RF signal received at the first port into a first and second unbalanced quadrature RF signal transmitted at the second port and combines the first and second unbalanced quadrature RF signals received at the second port into the balanced RF signal transmitted at the first port.

Description:
BACKGROUND OF INVENTION 
     The field of the invention is radio frequency (RF) signal transmission, transmission lines and balanced to unbalanced impedance transformation and simultaneously to split a single balanced input RF signal into a first and second quadrature unbalanced RF output signals. Similarly, the combination of a first and second unbalanced quadrature RF input signals may be combined into a single RF output signal. It will be appreciated, however, that the invention is also amenable to other like applications. 
     Magnetic resonance imaging (MRI) tomography is a known technique for acquiring images of the inside of the body of a living examination subject. To this end, magnetic gradient fields and an RF field, which are generated by gradient and RF coils respectively, are superimposed on a static magnetic field. The gradient fields that influence the examination subject are characterized by a magnetic flux density that varies over time, which may be utilized for imaging techniques. In certain MR systems, birdcage RF coils are employed, which require quadrature excitation and reception. Such quadrature excitation and reception is commonly achieved with a 90 (ninety) degree splitter-combiner. Additionally, to reduce system noise and cable currents, a balanced to unbalanced (balun) transformer is commonly employed. 
     It is well known in the art that a typical RF power transmission requires some form of RF power amplifier and transmission line. The interfacing of RF componentry usually requires the amplification, combination, and splitting of RF signals. The combination is usually performed by a splitter-combiner, which may further require the use of a balun or transformer. The balun performs a balanced-to-unbalanced (balun) transformation. 
     Commonly in the art, a power amplifier circuit must be cascaded with balun impedance transformers to match the impedance of the amplifier. Thus, the prior art requires a power amplifier cascaded with the balun impedance transformers to enable RF power to be split, amplified and then recombined at a higher power level. 
     Utilizing separate components for each function adds size cost and weight to existing MRI systems. What is needed in the art is a quadrature splitter-combiner integrated with a balun transformer. 
     SUMMARY OF INVENTION 
     The above discussed and other drawbacks and deficiencies are overcome or alleviated by an integrated quadrature splitter-combiner and balun comprising: a transmission line balun operably connected to a first port and a second port; a first capacitor operably connected across the transmission line balun; a second capacitor operably connected to the first port and said balun; and a third capacitor operably connected to the second port and the balun. The second capacitor, the third capacitor, and the transmission line balun combine to form an RF splitter-combiner to split a balanced RF signal received at the first port into a first and second unbalanced quadrature RF signal transmitted at the second port and combines the first and second unbalanced quadrature RF signals received at the second port into the balanced RF signal transmitted at the first port. 
     Also disclosed herein an exemplary embodiment is an integrated quadrature splitter and balun comprising: a transmission line balun operably connected to a first port and a second port; a first capacitor operably connected across the transmission line balun; a second capacitor operably connected to the first port and the balun; and a third capacitor operably connected to the second port and the balun. The second capacitor, the third capacitor, and the transmission line balun combine to form an RF splitter to split an balanced RF signal received at the first port into a first and second unbalanced quadrature RF signals transmitted at the second port. 
     Further, disclosed herein another exemplary embodiment is an integrated quadrature combiner and balun comprising: a transmission line balun operably connected to a first port and a second port; a first capacitor operably connected across the transmission line balun; a second capacitor operably connected to the first port and the balun; and a third capacitor operably connected to the second port and the balun. The second capacitor, the third capacitor, and the transmission line balun combine to form an RF combiner to combine an unbalanced quadrature RF signal received at the second port into a balanced RF signal transmitted at the first port. 
     Disclosed herein in yet another exemplary embodiment is an imaging system comprising: a imaging system with quadrature RF coils; an integrated quadrature splitter or combiner and balun comprising: a transmission line balun operably connected to a first port and a second port; a first capacitor operably connected across the transmission line balun; a second capacitor operably connected to the first port and said balun; and a third capacitor operably connected to the second port and the balun. The second capacitor, the third capacitor, and the transmission line balun combine to form an RF splitter or combiner to either split a balanced RF signal received at the first port into a first and second unbalanced quadrature RF signals transmitted at the second port or to combine the first and second unbalanced quadrature, RF signals received at the second port into the balanced RF signal transmitted at the first port. 
     Also disclosed herein is a magnetic resonance imaging system comprising: a magnetic resonance imaging system with quadrature RF coils; an integrated quadrature splitter-combiner and balun comprising: a transmission line balun operably connected to a first port and a second port; a first capacitor operably connected across the transmission line balun; a second capacitor operably connected to the first port and the balun; and a third capacitor operably connected to the second port and the balun. The second capacitor, the third capacitor, and the transmission line balun combine to form an RF splitter or combiner to either split a balanced RF signal received at the first port into a first and second unbalanced quadrature RF signals transmitted at the second port and to combine the first and second unbalanced quadrature RF signals received at the second port into the balanced RF signal transmitted at the first port. 
     In another exemplary embodiment, disclosed herein is a method of splitting a balanced RF signal into unbalanced quadrature RF signals comprising: receiving a balanced RF signal at first port of an integrated quadrature splitter and balun, the integrated quadrature splitter and balun comprising: a transmission line balun operably connected to a first port and a second port; a first capacitor operably connected across the transmission line balun; a second capacitor operably connected to the first port and the balun; and a third capacitor operably connected to the second port and the balun. The second capacitor, said third capacitor, and said transmission line balun combine to form an RF splitter. The method also includes generating a first and second unbalanced quadrature RF signals transmitted at the second port. 
     In yet another exemplary embodiment, there is disclosed herein a method of combining unbalanced quadrature RF signals into a balanced RF signal comprising: receiving a first and a second unbalanced quadrature RF signals at second port of an integrated quadrature combiner and balun, the integrated quadrature combiner and balun comprising: a transmission line balun operably connected to a first port and a second port; a first capacitor operably connected across the transmission line balun; a second capacitor operably connected to the first port and the balun; and a third capacitor operably connected to the second port and the balun. The second capacitor, the third capacitor, and the transmission line balun combine to form an RF combiner. The method also includes generating a balanced RF signal transmitted at the first port. 
    
    
     The above discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings. 
     BRIEF DESCRIPTION OF DRAWINGS 
     Referring to the exemplary drawings wherein like elements are numbered alike in the several Figures: 
     FIG. 1 depicts an exemplary MRI system; 
     FIG. 2 depicts an existing quadrature splitter-combiner circuit configuration employing discrete components; 
     FIG. 3 depicts a quadrature splitter-combiner configuration employing a transmission line filter section; 
     FIG. 4 depicts a balun impedance matching transformer configured with a transmission line; 
     FIG. 5 depicts an integrated quadrature splitter-combiner and balun in accordance with an exemplary embodiment; and 
     FIG. 6 depicts an integrated quadrature splitter-combiner and balun in accordance with an exemplary embodiment. 
    
    
     DETAILED DESCRIPTION 
     Disclosed herein is a quadrature splitter-combiner with balun. Also disclosed herein is a magnetic resonance imaging system (MRI) incorporating the abovementioned quadrature splitter-combiner with balun with quadrature RF coils. 
     Referring to FIG. 1, there is shown the major components of an exemplary MRI system, within which an exemplary embodiment may be implemented. The operation of the system is controlled from an operator console  100 , which includes a keyboard and control panel  102  and a display  104 . The console  100  communicates through a link  116  with a separate computer system  107  that enables an operator to control the production and display of images on the screen  104 . The computer system  107  includes a number of modules, which communicate with each other through a backplane. These include an image processor module  106 , a CPU module  108  and a memory module  113 , known in the art as a frame buffer for storing image data arrays. The computer system  107  is linked to storage media  111  and  112 , depicted as disk storage and a tape drive respectively for storage of image data and programs, and it communicates with a separate system control  122  through a high speed serial link  115 . 
     The system control  122  includes a set of modules connected together by a backplane  118 . These include a CPU module  119  and a pulse generator module  121 , which connects to the operator console  100  through a serial link  125 . It is through this link  125  that the system control  122  receives commands from the operator that indicate the scan sequence that is to be performed. As will be described in more detail below, the operator enters parameters, which indicate the prescribed scan. From these parameters, a pulse sequence is calculated and downloaded to the pulse generator module  121 . 
     The pulse generator module  121  operates the system components to carry out the desired scan sequence. It produces data, which indicates the timing, strength and shape of the RF pulses that are to be produced, and the timing of and length of the data acquisition window. The pulse generator module  121  connects to a set of gradient amplifiers  127 , to indicate the timing and shape of the gradient pulses to be produced during the scan. The pulse generator module  121  also receives patient data from a physiological acquisition controller  129  that receives signals from a number of different sensors connected to the patient, such as ECG signals from electrodes or respiratory signals from a bellows. Finally, the pulse generator module  121  connects to a scan room interface circuit  133 , 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  133  that a patient positioning system  134  receives commands to move the patient to the desired position for the scan. 
     The gradient waveforms produced by the pulse generator module  121  are applied to a gradient amplifier  127  comprised of G x , G y , and G z  amplifiers. Each gradient amplifier  127  excites a corresponding gradient coil in an assembly generally designated  139  to produce the magnetic field gradients used for position encoding acquired signals. The gradient coil assembly  139  forms part of a magnet assembly  141 , which includes a polarizing magnet  140  and a whole-body RF coil  152 . A transceiver module  150  in the system control  122  produces pulses, which are amplified by an RF amplifier  151  and coupled to the RF coil  152  by a transmit/receive switch  154  and quadrature splitter-combiner. The resulting signals radiated by the excited nuclei in the patient may be sensed by the same RF coil  152  and coupled through the transmit/receive switch  154  to a preamplifier  153 . The amplified MR signals are demodulated, filtered, and digitized in the receiver section of the transceiver  150 . The transmit/receive switch  154  is controlled by a signal from the pulse generator module  121  to electrically connect the RF amplifier  151  to the RF coil  152  during a transmit mode and to connect the preamplifier  153  during a receive mode. The transmit/receive switch  154  also enables a separate RF coil  152  (for example, a head coil or surface coil) to be used in either the transmit mode or receive mode. A decoupling method is used to switch on/off a large body coil during transmit/receive respectively. Other options include using another transmit/receive coil, such as a miniature body coil for the head, and leaving the system body coil in an off state during the entire scanning with the head coil. In an exemplary embodiment, the combiner disclosed herein may be employed to split the transmit RF signal and combine the receive signal(s) to and from the various coils. 
     The MR signals picked up by the RF coil  152  are digitized by the transceiver module  150  and transferred to a memory module  160  in the system control  122 . When the scan is completed and an entire array of data has been acquired in the memory module  160 , an array processor  161  operates to Fourier transform the data into an array of image data. This image data is conveyed through the serial link  115  to the computer system  107  where it is stored in a storage medium  111  or  112  such as disk memory or tape drive. The storage medium  111  and  112  could be various storage methodologies, such as disk, static memory, solid state, removable media, and the like, as well as combinations including at least one of the foregoing. In response to commands received from the operator console  100 , this image data may be archived on the tape drive, or it may be further processed by the image processor  106 , and conveyed to the operator console  100  and presented on the display  104 . 
     Referring still to FIG. 1 the NMR signal produced by the subject is picked up by the receiver coil  152  and applied through the preamplifier  153  to the input of a transceiver  150 . The received signal is at or around the Larmor frequency of a hydrogen atom, and this high frequency signal is down converted in a two-step process, which first mixes the NMR signal with a carrier signal and then mixes the resulting difference signal with a reference signal. The down converted NMR signal is applied to the input of an analog-to-digital (A/D) converter, which samples and digitizes the analog signal and applies it to a digital detector and signal processor. The resulting stream of values of the received signal are output through backplane  118  to the memory module  160  and array processor  161  where they are employed to reconstruct an image. 
     Disclosed herein is an integrated quadrature splitter-combiner with balun. Also disclosed herein is a magnetic resonance imaging system (MRI) incorporating the abovementioned quadrature splitter-combiner with balun for quadrature RF coils  152 . In an exemplary embodiment, a splitter-combiner configured/implemented in a selected form is then integrated with a balun transformer (implemented and employed as a ground notch filter). The integrated configuration of an exemplary embodiment replaces two existing separate elements while reducing insertion loss and increasing signal to noise ratio (SNR). 
     Turning now to FIG. 2, a circuit for an existing quadrature splitter-combiner  200  configuration employing discrete components is depicted. The circuit depicts two pi filters, phase shifters,  202  and  204  shunted together with two capacitors  206  and  208 . The configuration as depicted is bi-directional, operating as a splitter of a single ended ground referenced signal applied at port  201  to two quadrature signals at ports  207  and  209  respectively in one direction and yet may also be a combiner of two quadrature signals applied at ports  207  and  209  respectively into a single signal output from port  203  in the other direction. 
     Turning now to FIG. 3 another quadrature splitter-combiner  210  is depicted. 
     Here the quadrature splitter-combiner  210  is realized by employing two 45-degree (also called ⅛ wavelength) sections of coaxial transmission line  212  and  214  shunted together by capacitors  216  and  218 . It will be appreciated that if the phase of a 50 ohm transmission line  212 , and  214  is equal to about 45 degrees at a selected frequency and the reactance of the shunt capacitors  216  and  218  is about 50 ohms at the selected frequency, then all ports  211 ,  213 ,  217 ,  219  of the splitter-combiner  210  exhibit an impedance of 50 ohms at the selected frequency and power of the input signal applied at input port  211  will be split equally into two signals from the two output ports  217  and  219  having a phase of 0 and 90 degrees respectively. Similarly, as the circuit is symmetric, therefore it will be appreciated that it may also operate as a combiner, e.g., combining two signals that are out of phase by 90 degrees into one applied at ports  217  and  219  respectively into a single signal transmitted from port  213 . 
     Turning now to FIG. 4, a balanced to unbalanced (balun) transformer hereinafter balun  220  is depicted. A balun  220  receives an balanced RF input, e.g., referenced to ground, and creates a unbalanced output, single ended. Additionally, the balun  220  may utilized a notch filter applied in the shields of the transmission lines, inhibiting transmission of undesired frequencies. Moreover, the balun  220  may be configured as a filter to pass or attenuate only selected frequencies. In addition, a balun  220  may function equally well in reverse. That is, they can accept unbalanced first and second input signals and combine them into a balanced output signal referenced to a ground. 
     There are several different types of baluns  220  and splitter-combiner configurations, including stripline, microstrip, transformer types, twisted pair, and transmission line or coaxial cable types, and the like, as well as combinations including at least one of the foregoing. In an exemplary embodiment, a coaxial cable balun  220  comprises a coaxial cable transmission line  222  and  224  that have an inner conductors  226  and  228  respectively, and outer conductors  230  and  232  respectively, which may be a metallic sheath, which encases the inner conductors  226  and  228  respectively. Typically, balanced input RF signals are coupled to inner conductors  226  and  228 . Unbalanced output signals are generated on conductors  240  and  242 . The balanced signals on conductors  240  and  242  may optionally be coupled to amplifiers to obtain power amplification. 
     Continuing with FIG. 4, in an exemplary embodiment, each of the transmission lines  222  and  224  is formed from coaxial cable segments. Each segment has a length, L, which is approximately lambda/8 where lambda is the wavelength at the center frequency of the pass band of a selected frequency of interest. The length, L, could also be greater than or less than lambda/8 to act as an unequal power splitter-combiner. Continuing with FIG. 4, the coaxial cable transmission lines  222  and  224  are connected to ensure conductivity is maintained between outer conductors  230  and  232  along the entire length of the transmission lines  222  and  224 . In an exemplary embodiment the outer conductors  230  and  232  are soldered together along their respective lengths. The connected transmission lines  222  and  224  are thereafter wound in a coil, helix, and the like to form a pair of air core inductors  234  and  236 . A capacitor  238  is added shunting the ends of the inductors  234  and  236  to formulate a filter. It will be appreciated that the size of the inductors  234 ,  236  and the value of the capacitance for capacitor  238  may be selected to formulate a notch filter at a selected frequency. In an exemplary embodiment, the inductors  234  and  236  and capacitor  238  are selected so that the filter formulated exhibits a notch center frequency equivalent to the operating frequency designed for the splitter-combiner. The balun  220  is implemented as a notch filter in the outer conductors  230  and  232  of the transmission lines and configured to eliminate any currents on the shields of the transmission line. It will be appreciated and is well understood that elimination of currents on the transmission line enhances safety and prevents noise from passing through to other system components. It will be appreciated that the frequency of the balun and the combiner splitter are equal in this embodiment. However, in other implementations different frequencies could be employed. Such a configuration may be beneficial for image rejection and the like in transceiver applications. 
     Turning now to FIG. 5 a schematic depiction of an integrated quadrature splitter-combiner and balun  250  in accordance with another exemplary embodiment is depicted. In an exemplary embodiment, the quadrature splitter-combiner  210  formulated with a transmission line and balun  220  are integrated. In this embodiment, the quadrature splitter-combiner and balun  250  are realized by employing the transmission line balun  220  described above shunted together by capacitors  252  and  254 . A balanced input signal applied at port  256  results in quadrature unbalanced output signals that are generated on conductors at ports  260  and  262 . These output signals are isolated from the input and are 90 degrees out of phase with each other. Once again, the balanced signals on ports  256  and  258  may be coupled to amplifiers to obtain power amplification. Earlier designs for a splitter-combiner and balun have been separate units, which are then cascaded with each other. Because the present integrated quadrature splitter-combiner and balun  250  performs both functions in a single stage, it occupies significantly less space than a cascaded arrangement. 
     Similar to the embodiments above, if the phase of a 50 ohm transmission lines e.g.,  222 , and  224  (FIG. 4) is equal to about 45 degrees and the reactance of the shunt capacitors  252  and  254  is about 50 ohms, then all ports  256 ,  258 ,  260 , and  262  of the combiner/splitter and balun  250  exhibit an impedance of 50 ohms and power will be split equally into the two ports  260  and  262  having a phase of 0 and 90 degrees respectively. Similarly, it will further be appreciated that the circuit is symmetric and bi-directional, therefore it may also operate as a combiner, e.g., adding two signals that are out of phase by 90 degrees into one. 
     Turning now to FIG.  6  and continuing with FIGS. 3,  4 , and  5 , a quadrature splitter-combiner and balun  250  in accordance with another exemplary embodiment is depicted. In this embodiment, the quadrature splitter-combiner and balun  250  may be implemented utilizing two 45° (degree) (at the frequency of the desired application) coaxial cables transmission lines  222  and  224  soldered together to achieve a 0° and 90° power split. The 45° coax cable transmission lines  222  and  224  are soldered together along their length to ensure a common ground throughout the length of the combined cable. The shielded double coax cable is then covered in plastic heat shrink tubing or equivalent jacket insulation and wound into a coil to air core inductors  234  and  236  of the balun  220  (ground filter or cable trap). The balun  220  is designed to filter out any signal or noise at a selected frequency. The balun  220  is resonated by placing a capacitor  238  across the grounds on either side of the cable inductance. Placing capacitors  252  and  254  respectively of impedance of 50 ohms at the selected frequency at the ends, the combined system becomes a quadrature power splitter-combiner and balun at a selected frequency. In an exemplary embodiment, selected frequencies of 42.57, 63.86, and 127.72 megahertz respectively, have been utilized for MRI applications at 1.0 T (Tesla), 1.5 T, and 3.0 T. 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another.