Transmission line transformer

Abstract: A birdcage knee coil operated in quadrature in an NMR imaging system includes a pair of like butterfly coils each formed from a strip transmission line formed on a common dielectric substrate. The pair inputs the quadrature signals created by the birdcage coil to a combiner output circuit. Each butterfly coil of the pair comprises a pair of wings of like area connected by a common transmission line so that currents induced by a magnetic field on the ground conductors of one wing oppose and cancel the currents induced by that field on the ground conductors of the other wing. The transmission line forming each butterfly coil has opposing ground conductors of the same transverse width with a center conductor equally spaced between the ground conductors and a transverse width of about one tenth that of the ground conductors. The pair of coils are arranged symmetrically relative to the birdcage coil to provide an electrically balanced system.

Description: The invention relates to a transmission line transformer comprising a transmission line having a ground conductor, a dielectric member and a signal conductor, the signal conductor being spaced from the ground conductor by the dielectric member. The invention also relates to a quadrature RF coil for use in an NMR apparatus, said RF coil having a central axis, said coil comprising first and second transformers of the type referred to. A transmission line transformer of the type referred to for use in NMR (Nuclear Magnetic Resonance) systems is disclosed in DE-A-38 17 613. In NMR systems, antennas are frequently used to receive RF signals generated in a static magnetic Bo field. Certain of these antennas are referred to as surface coils and they take a variety of shapes and configurations. Another type of such antenna is a birdcage coil comprising a pair of spaced rings coupled by a set of parallel linear rods. The coil includes spaced capacitors for tuning the coil to a given RF frequency, typically about 64 MHz. Such coils may further include means for operating the coils linearly or in quadrature, the latter arrangement increasing the signal-to-noise ratio in a known manner. An NMR system usually comprises a magnet which creates the Bo field, the magnet comprising a circular bore which receives the body of a patient being examined. The bore is usually of limited dimensions for receiving a patient table with the patient lying on the table. Typically the bore is about 50-60 cm in diameter, leaving about 30-35 cm above the table for a surface or birdcage coil for examining a particular part of the body. The birdcage coil may be used locally to examine a persons head, for example. The head is inside the coil while in the Bo field and coaxial cables connect the local coil to the imaging system outside the field. Such cables in some cases may be connected to a balun before connection to instrumentation. As disclosed in DE-A-38 17 613, the balun is used to prevent RF currents that may be induced on the cable ground shield in the RF field generated by a body in response to the system-generated RF flux. Such currents need be prevented because they degrade the efficiency of the local coil and because only a small amount of power is available from body tissue for sensing by the local coil. Typically all of this power would go toward producing currents in the local coil. These currents are amplified and sent to the rest of the NMR system. Any currents induced in the cable shield represent a loss of power and must be eliminated. In the known construction of document DE-A-3 817 613, the transformer comprises a coaxial cable having a length of a quarter wavelength and mounted adjacent to the antenna (surface coil or birdcage coil). This is not entirely satisfactory because a considerable length of cable is present in the vicinity of the antenna. Such a construction is usually not practical due to the limited amount of space available within the bore as discussed above. Additionally, forming the coaxial cable into a balun is also not practical in certain implementations because the cables limit the balun diameter to too large a dimension for use in the limited space. The invention is carried out in accordance with the appended claims. The transformer in accordance with the invention is characterized in that the dielectric member is a planar sheet material, said ground conductor being formed into first and second parts, the signal conductor being centrally disposed in the sheet material between opposing broad surfaces of the material, a first ground conductor part being disposed on one of said broad surfaces and a second ground conductor part being disposed on the other opposing broad surface, the first and second ground conductor parts having the same transverse width, the transmission line being formed into a first planar coil for receiving a radio frequency signal at an input thereto and a second planar coil coplanar with said first coil for outputting said signal at an output therefrom, a conductor of one coil corresponding to and being in common with a conductor of the other coil, said coils being oriented, positioned and dimensioned relative to one another so that in the presence of a magnetic field inducing a current in each coil ground conductor the current induced in a ground conductor of the first coil opposes the current induced in the corresponding ground conductor of the second coil. A quadrature RF coil in accordance with the invention is characterized in that the first and second transformers are secured to the quadrature RF coil with the first coils of each transformer arranged symmetrical with the second coils relative to the said central axis. These and other aspects will be apparent from the embodiments described hereinafter with reference to the drawings. FIGURE 1 is an isometric view of a transformer construction according to an embodiment of the present invention for use with an NMR birdcage coil system operated in quadrature; FIGURE 2 is a plan view of the transformer construction of Figure 1 illustrating a pair of transformers each coupled to a different coil of the birdcage system; FIGURE 3 is a schematic circuit diagram of the circuit shown in Figure 2 coupled to the birdcage coil system of Figure 1; FIGURE 4 is a more detailed plan view of a representative one of the transformers of Figure 2; FIGURE 5 is a sectional view taken along lines 5-5 of Figure 4; FIGURE 6 is a sectional view taken along lines 6-6 of Figure 4; FIGURE 7 is a sectional view taken along lines 7-7 of Figure 4; and FIGURE 8 is the equivalent circuit of a representative transformer shown in Figure 4. In Figure 1, antenna system 10 comprises a birdcage antenna 12 and output circuit 14. Birdcage antenna 12 comprises a pair of spaced parallel equally dimensioned and equal inductance circular conductors 3 and 4. The conductors 3 and 4 are connected by parallel rods 6. The rods include capacitances 7 for rough tuning the coil to its operating frequency, typically 64 MHz. A variable capacitance 8 is in rod 6' and a variable capacitance 9 is in rod 6''. These capacitances tune the coils to which they are associated to the same frequency. PIN diodes and associated circuitry (not shown) electrically divide the rods into orthogonal coils for responding to magnetic fields oriented 90° to one another. Capacitance 8 is associated with one of these coils and capacitance 9 is associated with the other coil. A housing 11 partially shown forms the antenna into an enclosed cylinder which supports the output circuit 14. The antenna 12 has a longitudinal axis 16. System 10 is for use in an NMR imaging system (not show) particularly for examining the knee of a patient lying on a table positioned within a magnet which generates a Bo field. The Bo field is parallel to the longitudinal axis 16. RF pulses are generated by means not shown which produce rf magnetic response pulses in the knee under examination. The rf response pulses are detected by the coils of antenna 12 in quadrature which supplies the detected signals to circuit 14 via coaxial cables 13 and 15. The two coils are physically oriented 90° relative to each other about axis 16. Cable 13 is connected to one of the two coils and cable 15 is connected to the other for receiving the detected 90° out of phase rf signals. Circuit 14 amplifies and phase combines the signals on cables 13 and 15. The combined signals are supplied to utilization equipment (not shown) outside the Bo field via output coaxial cable 18. Cable 18 is situated within the rf field generated by the knee coil in response to the tissue generated rf field, and therefore, exhibits induced currents on its ground conductor. These currents represent a loss and detrimentally affect the operation of the birdcage coil system unless otherwise precluded. In Figure 2, circuit 14 includes a first transmission line transformer balun 20 and a second balun 22 which is identical to balun 20. Balun 20 receives an input signal from one of the antenna coils via cable 15 and preamplifier 24. Balun 22 receives an input signal from the other of the antenna coils via cable 13 and preamplifier 24'. Parts in the different figures with a primed reference numeral are identical to parts with the same reference numeral unprimed. The outputs of baluns 20 and 22 are applied to combiner circuit 26 which combines the 90° out of phase signals and applies the combined signals to output cable 18. Figure 3 shows in block diagram form the circuit 14 of Figure 2. Prior art systems mentioned in the introductory portion above in contrast provide a balun at the output of the combiner circuit which is unsatisfactory in many implementations, notwithstanding that such a balun is formed from a coiled coaxial output cable and suffers the drawback of being relatively large. The baluns 20 and 22, in contrast to coiled coaxial cables and their attendant drawbacks, are formed from planar transmission lines, are electrically isolated from one another, take up little room in the region next to the antenna and provide isolation closer to the input cables 13 and 15 than prior art systems, thus exposing less cabling to the detrimental effects of the rf field. Representative balun 20 is shown in more detail in Figures 4-7. Balun 20 comprises a planar transmission line 28 formed into a butterfly coil comprising two wings 30 and 32. The transmission line 28 forming the wings 30 and 32 is continuous having an identical cross-section throughout. In Figure 5, representative cross-section of transmission line 28 comprises planar parallel outer ground conductors 34 and 36 and planar central signal conductor 38 supported on a planar dielectric sheet 40. Sheet 40 may be phenolic or any other dielectric material used in strip transmission lines. Conductors 34, 36 and 38 are copper foils typically used in printed circuits and in planar transmission lines. Conductors 34, 36 and 38 are of the same thickness, e.g., 2 mils. Conductor 38 has a transverse width from left to right in the figure of about one tenth the transverse width of conductors 34 and 36 which have the same width. By way of example, conductor 38 has a width of about .040 inch and conductors 34 and 36 have a width of about 0.40 inch. Conductors 34, 36 and 38 may be formed by etching each conductor from a copper clad dielectric layer. The layers are then bonded into a single layer structure as shown. Construction of similar multilayer boards is known. In Figure 4, wings 30 and 32 enclose respective areas A and B. Areas A and B are identical and symmetrical relative to axis 42. Axis 42 is the transverse axis of symmetry of the antenna 12 (Figure 1). That is, axis 42 is normal to axis 16 and lies in a plane which divides the antenna into two symmetrical halves. Wing 30 lies in one half and wing 32 lies in the other half. The line 28 in forming the wings follows a tortuous path which creates a central region 44 between the two wings. The amplifier 24 is on dielectric 40 in region 44. The amplifier 24 may be an FET transistor, for example. Wing 30 comprises a set of linear transmission lines 48, 50, 52 and 54 at right angles to each other forming a rectangular coil. Wing 30 further includes a leg 56 which forms a terminal for conductors 34, 36 and 38. Transmission line 54 includes a bent extension 58 which directly connects to wing 32. Leg 56 is sufficiently spaced from the extension 58 an amount to provide electrical isolation therebetween. For example, the spacing between leg 56 and extension 58 may be about 100 mils and is not critical. A terminal 60 is formed in this space for connecting the amplifier 24 output terminal to the center conductor 38 of the leg 56 transmission line. For this purpose, conductor 38 extends beyond conductors 34 and 36 somewhat. Extension 58 connects to transmission line 62 of wing 32. Wing 32 comprises lines 62, 64, 66 and 68 connected at right angles as shown. Line 68 terminates at terminal leg 70 which is closely spaced to extension 58 similar to the spacing with leg 56. The center conductor 38 of leg 70 extends beyond ground conductors 34 and 36 so as to be connected to terminal 72. Terminal 72 is connected to combiner circuit by conductor 74 on one side of dielectric 40. The output of amplifier 24 is connected to terminal 60 by wire 76. The input of amplifier 24 is connected to cable 15 by a connector (not shown). The ground conductors 34 and 36 of terminal leg 56 are connected by terminal 78 and the ground conductors of leg 70 are connected by terminal 80. In Figure 2, a variable isolation capacitor 82 is connected to ground conductor 34 of legs 56 and 70. In Figure 6, a representative terminal 78 connecting the ground conductors 34 and 36 comprises a plated through hole having a metal plated conductor 84 which electrically connects conductors 34 and 36. The center conductor is not in the region of terminal 78. In this way the capacitor 82 is coupled at opposing ends to both conductors 34 and 36 at the ends of the two wings 30 and 32. This produces with the coils formed by the wings the circuit of Figure 8. The capacitor further increases the impedance of the balun which otherwise would require an increased undesirable balun dimension to create an equivalent impedance. In Figure 7, a representative terminal 72 for connection to the center conductor 38 comprises a plated through hole having a metal plated conductor 86 which connects the conductor 38 with the conductor 74 on the external surface of the dielectric 40. Terminal 60 is similar except it forms a convenient terminal for receiving the wire connected to the amplifier 24 output. In the alternative, terminal 60 may be similar to terminal 72. A printed conductor connects the amplifier 24 to conductor 34 for completing the ground circuit to the amplifier. The entire structure forming circuit 14 comprising the dielectric 40 and its associated circuitry described above including connectors for the coaxial input and output cables forms a compact printed circuit construction. The circuit 14 is conveniently secured to the support 11 of the antenna housing in the position and orientation shown. Because the circuit 14 construction is relatively flat and compact it can be mounted centrally of the antenna 12 so that it protrudes above the rods a minimum distance. Also the antenna is open at both ends for receiving the leg of a patient therethrough. In the alternative, the antenna could also be used as a head coil. In operation, the system 10 of Figure 1 is placed on a patient table within a Bo field. The magnetic rf pulses sensed by the antenna have flux lines normal to the plane of the coils formed by wings 30 and 32. In Figure 2, for example, assume an rf magnetic pulse field is perpendicular to the drawing sheet coming toward the reader. The field in wing 30 creates currents on conductors 34 and 36 in a given direction. The field in wing 32 creates currents on the conductors 34 and 36 of the same magnitude but in opposing directions, cancelling one another. These currents flow on the outer surface of the ground conductors. The signal and associated ground currents, however, flow on the central conductor and inner surfaces of the outer ground conductors, respectively, and are not affected by the external fields nor by the fields created by these signal currents in accordance with known theory for baluns. The capacitor 82 in combination with the balun 20 across which the capacitor is connected provides high isolation impedance for current induced on the balun ground conductors outer surfaces. For example, this impedance may be about 12K ohms which provides an acceptable minimum isolation impedance. The resulting balun structure effectively stops the flow of induced currents along the output cable ground shield. While the invention has been described in connection with a quadrature antenna system it is equally useful with a linear antenna system. Further the balun of the present invention may be used with non-NMR antennas, e.g., dipole antennas. It should be noted that it is important that the ground conductors 34 and 36 be of the same transverse width. If the butterfly ground conductor were formed on only one side of a dielectric then the signal currents are not confined entirely to the surface of the ground conductor near, i.e., facing, the center conductor. In this case, the desired signal "sees" the high impedance balun, which in effect prevents both the desired signal currents along with the undesired unbalanced ground current. This was surprising to the present inventor. While a particular butterfly configuration has been described others may be used. For example, the space for the amplifiers may be reduced or enlarged or the number and shape of the turns changed in accordance with a given implementation.

Claims: A transmission line transformer comprising a transmission line (28) having a ground conductor (34,36), a dielectric member (40) and a signal conductor, the signal conductor (38) being spaced from the ground conductor by the dielectric member, characterized in that the dielectric member (40) is a planar sheet material said ground conductor being formed into first and second parts, the signal conductor (38) being centrally disposed in the sheet material between opposing broad surfaces of the material, a first ground conductor part (34) being disposed on one of said broad surfaces and a second ground conductor part (36) being disposed on the other opposing broad surface, the first and second ground conductor parts having the same transverse width, the transmission line (28) being formed into a first planar coil (30) for receiving a radio frequency signal at an input (56) thereto and a second planar coil (32) coplanar with said first coil for outputting said signal at an output (78,80) therefrom, a conductor (34,36,38) of one coil (30) corresponding to and being in common with a conductor (34,36,38) of the other coil (32), said coils being oriented, positioned and dimensioned relative to one another so that in the presence of a magnetic field inducing a current in each coil ground conductor (34,36), the current induced in a ground conductor of the first coil (30) opposes the current induced in the corresponding ground conductor of the second coil (32). A transformer as claimed in Claim 1, characterized in that it further includes capacitance means (82) coupled to at least one of the ground conductors (34,36) at said input (56) and output (70) for isolating the ground conductor relative to radio frequency currents between said input and output. A transformer as claimed in Claim 1 or 2, characterized in that said coils (30,32) are arranged so that a current flowing in said transmission line (28) flows through one coil in a clockwise direction and counterclockwise through the other coil relative to an axis normal to the plane of the coils and through the center of each coil. A transformer as claimed in Claim 1, 2 or 3, characterized in that each coil (30,32) substantially encloses a given area (A,B) of about the same magnitude. A transformer as claimed in any one of the preceding Claims, characterized in that the transmission line (28) has first and second terminating ends (56,70), each end corresponding to a different coil (30,32) and spaced from the transmission line forming portion of that coil, said transformer further including connection means (78) ohmically connecting the first and second ground conductors (34,36) at the first end (56) and connection means (80) ohmically connecting the first and second ground conductors at the second end (70). A transformer as claimed in any one of the preceding Claims, characterized in that the coils (30,32) each comprise one turn, the signal conductor (38) and the ground conductors (34,36) of the turn of one coil (30) being about the same dimensions of the corresponding conductors of the turn of the other coil (32). A quadrature RF coil (12) for use in an NMR apparatus, said RF coil having a central axis (42), said coil comprising first and second transformers (20,22) as claimed in any one of the preceding Claims, characterized in that the first and second transformers (20,22) are secured to the quadrature RF coil (12) with the first coils (30) of each transformer arranged symmetrical with the second coils (32) relative to the said central axis (42).

Inventors: BEZJAK GREG WILLIAM, BEZJAK, GREG WILLIAM
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1991
Main classification: H03H7
IPC codes: H03H7, H01Q9, A61B5