Quadrature surface coil construction for magnetic resonance imaging apparatus.

0001This invention relates to a quadrature surface coil system for detecting magnetic fields induced in a body being examined in a magnetic resonant imaging system.

0002Quadrature surface coil systems for use in magnetic resonance imaging apparatuses are known. For example, reference is made to U.S. Patent 4,816,765, incorporated by reference herein which discloses a quadrature coil system which is used in a magnetic resonance imaging apparatus. The detection coil is one referred to as a surface coil in that it is placed adjacent a surface of a body being examined by the magnetic resonance imaging system apparatus. As disclosed in the aforementioned patent, the detection coil comprises a stack of two coils with locally mutually perpendicular related B fields. A first coil is formed as a butterfly coil having a B field extending parallel to the plane of the coil and the second coil is formed of a single flat coil having a B field extending transversely the plane of the coil. The coils are arranged in a fixed mutual orientatien so that for each of the coils reciprocally applied fluxes are compensated for.

0003The present invention is an improvement of that quadrature coil system. Such quadrature coil systems as disclosed in the aforementioned U.S. patent have the disadvantage in that the coils must be typically arranged planar. The reason for the planar arrangement is that should the coils be bent or otherwise contoured, typically to the shape of the surface to be examined, for example, the spinal column of a human being in the neck-head area, bending of the coil causes the butterfly and single flat coil to be misaligned. The resulting signals become distorted and do not reproduced faithfully the desired magnetic fields produced by the body under examination. The flat coils while satisfactory for some applications where the body under examination is planar and therefore equidistant from the stacked coils, in other instances, is not as satisfactory when the body under examination is curved and non-planar. As a result, the magnetic fields induced by the body and to be detected by the quadrature coil system are spaced different distances from different portions of the coil causing distortions in the signal.

0004The present inventors recognize a need for a detection quadrature surface detection coil for use with a non-planar body under examination and which overcomes the drawbacks of the stack coil system described above. One problem with producing a coil system for a curved body is that any bends in the coils out of the planar region tend to distort the coil and make the coil non-symmetrical. Further, it is extremely difficult to axially align the magnetic field responsive region of the butterfly coil to the single flat coil once the coils are bent. If the bends are slightly off center, i.e., non-symmetrical, then the two different coil systems, the flat coil and the butterfly coil, tend to respond in a manner producing a distored unsatisfactory signal. To manufacture bent coils which are exactly physically aligned to respond to aligned magnetic fields is not generally practical. To obtain an optimum signal, the stack of coils should be contoured to the object being examined aligned, and matched.

0005In a magnetic resonance imaging examination system an apparatus according to the present invention comprises a surface coil magnetic field gradient detection construction comprising an outer conductor forming a first coil and a transverse inner conductor connected at its ends to the outer conductor at spaced points along the outer conductor for dividing the outer conductor into mirror image second coils. The construction includes means for tuning the first and second coils to a given radio frequency such that the first and second coils generate corresponding first and second signals manifesting orthogonal magnetic fields in the body under examination.

0006A feature of the invention includes signal isolation means coupled to the first and second coils for isolating the first and second signals. By way of example, such isolation means may include 180° phase shift means coupled to the outer conductor at the junction with one end of the transverse inner conductor. This phase shift means places the transverse inner conductor at virtual ground so that the outer conductor corresponds to a single turn coil while the inner conductor with the outer conductor forms a pair of coils corresponding to a butterfly coil, both coils being aligned regardless their bent state because they comprise a common coil structure. <ul id="ul0001" list-style="none"><li>Figure 1 is a schematic diagram of a magnetic resonance imaging apparatus including a coil system in accordance with one embodiment of the present invention;</li><li>Figure 2 is a fragmented sectional side elevational view of a quadrature surface coil detection construction in accordance with one embodiment of the present invention in use for examining the spinal cord in the neck region of a person;</li><li>Figure 3 is a sectional elevation view transverse to the view of Figure 2 through the neck region of a person;</li><li>Figure 4 is a schematic diagram of a quadrature surface coil circuit system used with the embodiment of Figures 2 and 3;</li><li>Figure 5 is a schematic diagram of a portion of the coil construction of Figure 4 illustrating an alternate embodiment for the center conductor of the detection coil construction;</li><li>Figure 6 is a schematic diagram of a circuit employed with a coil system quadrature coil construction in accordance with a second embodiment of the present invention; and</li><li>Figures 7a, 7b and 7c are diagrams illustrating generally the configuration of alternate coil geometries employed with the embodiments of Figures 4 and Figure 6.</li></ul>

0007A magnetic resonance imaging apparatus as shown in Fig. 1 comprises a magnet system 2 for generating a steady, uniform magnetic field, a magnet system 4 for generating magnetic gradient fields, and power supply sources 6 and 8 for the magnet system 2 and the magnet system 4, respectively. A transmit rf coil 10 generates a radio frequency (rf) magnetic alternating field and is connected to an amplifier 12 for an rf source. A surface coil system 13 detects magnetic resonance signals generated in an object to be examined by an rf transmission field. For reading purposes, the system 13 is connected to a signal amplifier 14. The signal amplifier 14 is connected to a phase-sensitive rectifier 16 which is connected to a central control 18. The central control 18 controls a modulator 20 which applies the modulated signal to amplifier 12, the supply source 8 for the gradient coils, and a monitor 22 for display. An rf oscillator 24 controls the modulator 20 as well as the phase-sensitive rectifier 16 which processes the measurement signals. Cooling can be provided by a cooling device 26 comprising cooling ducts 27. A cooling device of this kind may be constructed as a water cooling system for resistance coils or as a liquid helium drawer system for super-conducting coils. The transmitter coil 10, arranged within the magnet systems 2 and 4, encloses a measurement space 28 which offers sufficient room so as to accommodate patients in medical diagnostic apparatus. A steady magnetic field, gradient fields for position selection of slices to be imaged, and a spatially uniform rf alternating field are generated in the measurement space 28. The measurement space 28 is shielded against interference fields by a Faraday cage 29.

0008Detection coil systems composed of surface coils are not normally used as transmitter coils for generating magnetic resonance signals. The detection and transmitting functions used for other types of coils is less suitable for these coils, although such dual use of surface coils is not precluded for specific measurements in practice.

0009In Figure 2, surface coil system 13 comprises quadrature coil arrangement 30 encased in a magnetic field transparent support 32. The support 32, for example, may be molded thermoplastic or thermoset foam material and the like. The coil system 13 may be placed in grooves in the support. The support 32 has a contoured surface 34. Surface 34 is contoured in complex curves so as to accommodate the neck region, for example, of a patient 35. The patient 35 has a spinal column 36 which is approximately parallel with the curvature of surface 34. The coil arrangement 30 is also curved so as to be approximately equally spaced from the spinal column 36. In Figure 3, the coil arrangement 30 includes conductors 37, 38 and 39 which are approximately equidistant from the spinal column 36. By arranging the coil arrangement 30 conductors 37, 38 and 39, and other conductors, not shown in Figures 2 and 3, substantially equidistant from the spinal column 36 magnetic fields of substantially uniform intensity are sensed by the coil arrangement 30. That is, the column 36 is at the magnetic iscocenter of arrangement. However, the complex curved arrangement of the curved configuration of the coil arrangement 30 would not be possible for an optimum response detection signal in prior art surface coils, due to the misalignment of the different stacked coils of such prior art systems.

0010The reason that the coil arrangement 30 of the present invention does not have the misalignment problem of the prior art coil system is that the coils forming the so-called butterfly arrangement and the single coil arrangement are formed by the same coil conductors. The conductors mechanically forming the coils are the same but are electrically coupled so as to sense orthogonal magnetic fields. The coils sense orthogonal B_x and B_y fields independently of each other and create currents induced by those magnetic fields 90° out of phase with respect to each other in a common coil structure.

0011In Figure 4, coil arrangement 30 comprises an outer series coil 42 and two parallel inner coils 43 and 44. The series coil 42 comprises a conductor 37 coupled to conductor 39 through two capacitors 45' and 45 and intermediate conductor 48. Conductor 39 is coupled to junction 49 via capacitor 50, conductor 51, pin diode 52, conductor 53 and capacitor 54. Conductor 37 is coupled to junction 49 via capacitor 50', conductor 51', pin diode 52' and conductor 53'. The primed and unprimed numbers correspond to identical components. The anode of diode 52 is connected to conductor 51 and the anode of diode 52' is connected to conductor 51'. The conductors 37, 48, 39, 51, 53, and the intervening capacitors and pin diodes form the outer series coil 42. The capacitors tune the series coil 42 to a given radio frequency of the system of Figure 1. Junction 49 is coupled to conductor 48 by a transverse coupling network comprising conductor 38, fixed capacitor 60 and adjustable capacitor 61. Capacitor 60 is essentially the same in value as capacitors 50 and 50'. The capacitor 61 tunes the radio frequency resonance of coils 43 and 44 to the same resonance frequency of coil 42. Conductor 38 and capacitors 60 and 61 divide coil 42 into the two matched coils 43 and 44. Coils 43 and 44 are mirror images of one another.

0012The coils 43 and 44 are in parallel and act similarly as a butterfly coil such as disclosed, for example, in the aforementioned U.S. Patent 4,816,765. The series coil 42 functions similarly as the single flat coil 32 disclosed in the aforementioned patent. A phase shift network 65 is coupled to conductors 53 and 53' via adjustable respective impedance matching capacitors 66 and 66'. Network 65 provides a 180° phase shift of the signal on conductor 53 as compared to the signal on conductor 53'. Because capacitors 54 and 54' are matched, the junction 49 is at virtual ground to those signals on conductors 53 and 53' produced by the coil 42 in response to a given magnetic field orientation. The current in coil 42 is indicated by arrows c. Because the current flowing in directions of arrows c is shifted 180° by phase shift network 65, the relative potential produced by that current at junction 49 and conductor 48 remains at virtual ground. Because the currents at junction 49 and conductor 48 are at virtual ground no current flows for that induced signal through conductor 38 and capacitors 60 and 61.

0013However, to ensure that the potentials at junction 49 and conductor 48 are at virtual ground, it is important that the coils 43 and 44 are symmetrical with respect to the sensed magnetic fields. The coils 43 and 44 are in parallel such that the coil 44 when exposed to a magnetic field othogonal to the magnetic field to which the coil 42 is responsive exhibits a current in the direction of arrows <u style="single">a</u>. Coil 43 exhibits a current flowing in direction of the arrows b in the presence of that same orthogonal magnetic field. As a result, the two currents, <u style="single">a</u> and b flow in the same direction through common conductor 38 of coils 43 and 44. As observed by the arrows b and c at conductors 37 and 39, respectively, those currents flow in opposite directions in coil 42. The diodes 52 and 52' permit those currents in the direction of arrows <u style="single">a</u> and b to flow into the junction 49.

0014It is important that the currents induced in coils 43 and 44 be as close as possible in value. To achieve that end, the coils 43 and 44 need to be as symmetrical as possible and need to have a resonance frequency that is as closely matched as possible. For this purpose, a pair of ganged coupled capacitors 70 are connected with their junction connected to junction 49 and their extreme terminals connected to conductors 53 and 53'. The ganged capacitors serve to match the resonance frequency of the coils 43 and 44 and, in effect, provide signal isolation of the current induced in coil 42 from the currents induced in coils 43 and 44 by the sensed respective orthogonal magnetic fields. The ganged capacitors 70 preferably may be a differential capacitor coupled to junction 49 and conductors 53 and 53'. By tuning each of the coils 43 and 44 to the same rf frequency the current at junction 49 and conductor 48 in combination with the phase shift network 65 is as close to virtual ground as possible.

0015Tuning capacitor 71 tunes the coil 42 to the desired resonance frequency. Ganged series connected capacitors 72 and capacitors 66 and 60' provide impedance matching between the phase shift network 65 and coils 42, 43 and 44. The capacitors 66 and 60' as well as the ganged capacitor 62 are adjustable. A radio frequency connector 73 has its center conductor connected to conductor 38 through variable impedance matching capacitor 74 and to junction 49 through variable impedance matching capacitor 75. Capacitors 74 and 75 are ganged. The phase shift network 65 is connected to rf connector 76.

0016The two parallel mode coils 43 and 44 respond to a parallel mode flux field B_x. The series coil 42 responds to a magnetic field B_y orthogonal to field B_x. The currents induced by the parallel mode field B_x in directions of arrows <u style="single">a</u> and b are 90° out of phase with the current induced by the orthogonal field B_y which current flows in the direction of arrow c. Ideally, when the B_y flux is present, no current flows between junction 49 and conductor 48. At this time, no current appears at the rf connector 73. However, during this period, a current appears at rf connector 76 representing the current in the series coil 42. A current that flows on conductor 38 in response to the presence of the B_x flux is 90° out of phase with this current. This current appears at rf connector 73. Connectors 73 and 76 are coupled to a 90° phase shift network 80. Network 80 combines the out of phase signals appearing at connectors 73 and 76 and places those currents in phase at rf connector 81. The connector 81 is coupled to a magnetic resonance imaging system which displays an image corresponding to the signals appearing at connector 81.

0017In Figure 5, an alternate embodiment is illustrated for conductor 38. Conductors 83 and 85 are coupled via a pair of parallel conductors 86 and 86'. Conductor 83 is connected to capacitor 60 and conductor 85 is connected to capacitor 61. The parallel conductors 86 and 86' are identical in length and are spaced apart a distance d symmetrical relative to a line through conductors 83 and 85. The further apart that the conductors 86 and 86' are spaced, i.e., as d increases, the closer the parallel mode flux B_x, Figure 4, will be sensed relative to the physical position of conductors 86, 86'. That is, in Figure 2, the distance d' becomes smaller as distance d, Figure 5, increases. Therefore, depending on the distance d' for a given patient, a surface coil system 13 can be provided which is closely matched to the patient's configuration. The closer the distance between the spinal column 36 and the coil arrangement 30, the more sensitive the system to the magnetic flux B_x of the coil arrangement and the more optimum the induced currents in the coils 43 and 44.

0018As indicated, it is important that the coils 43 and 44 be matched. This implies that these coils are identically mirror image symmetrical arrangements. The rectangular arrangement of Figure 4 is only by way of example. In Figures 7a, 7b and 7c the two coils may have any arrangement wherein the outer conductor may be circular, elliptical or odd shaped as shown in the figures. The important thing is that both coils 43 and 44 are mirror images of each other and can be tuned to the same frequency.

0019In Figure 6, parts identical to Figure 4 have the same reference numerals. In Figure 6, a first impedance matching field effect transistor Q1 is coupled with its gate connected to conductor 38, its drain 83 coupled through a resistance to a voltage source and its source 84 coupled to ground through a biasing network 85. The source 83 is coupled to rf connector 73 via a DC blocking capacitor 86. The junctions between the cathode of diode 52' and capacitor 54' is coupled to one of ganged capacitors 70 and to the gate of field effect transistor Q2 on one side of capacitor 71. The cathode of diode 52 is connected to the other of ganged capacitors 70, to the other side of adjustable capacitor 71 and to the gate of field effect transistor Q3. The source 86 of transistor Q2 is coupled to a positive voltage through a resistor network and the drain 88 is connected to ground via a biasing network 89. The source 87 is also coupled through a DC blocking capacitor 90 to one side of phase shift network 65, which it will be recalled, provides 180° phase shift.

0020The source 91 of field effect transistor Q3 is coupled to a positive voltage source via a resistor and to the phase shift network 65 through a DC blocking capacitor 92. The drain 93 is coupled to ground through a bias network 94. The source 91 is coupled to phase shift network 65 through a DC blocking capacitor 92. Thus, the signals appearing across capacitors 90 and 92 represents the series signal produced by coil 42. The capacitors 70 adjusts the orthogonality of coils 43 and 44, i.e., make mirror image of one another, as discussed above. Capacitor 71 also performs similarly as described above in connection with the embodiment of Figure 4. The transistors Q1, Q2 and Q3 provide high impedance isolation between the phase shift networks 65 and 80 to the coil system. The transistors Q1, Q2 and Q3 replace the impedance matching networks of the embodiment of Figure 4.

0021The conductors 37, 38, 39 and so on are all made preferably of identical conductive tubing of the same inductance. Of cource, the conductors may have other configurations in accordance with a given implementation. The parallel mode flux B_x induces a current in each of the coils 43 and 44 and this current is sensed by transistor Q1 and applied to the phase shift network 80 via rf connector 73. The orthogonal flux B_y is sensed by the series coil 42 and is applied to the phase shift network 80 via the phase shift network 65 and transistors Q2 and Q3. Transistors Q2 and Q3 and their biasing networks 89 and 94, respectively, are matched so as to provide a signal at junction 49 for the coil 42 current that is at virtual ground potential.

0022As a result, a single coil system comprising an outer conductor and a transverse inner conductor connected to the single outer conductor may be bent to any desired non-planar configuration that is relatively symmetrical relative to the central conductor 38 to conform to a given curved configuration. The capacitors 61, 70 and 71 adjust the resonance of the separate coils so as to match and adapt for the non-symmetry that may be caused by the actual mechanical bending of the coils to the particular non-planar configuration. The capacitor 61 which may be buried within the support 32, Figure 2, may be made accessible by an aperture (not shown) in the support 32. Regardless the bend configuration of the coils, misalignment of the butterfly coil, i.e., the two parallel coils, to the series coil, e.g., coil 42, is not a problem because these coils comprise the same conductors. It is a simple matter to tune the different coils via the associated tuning coils to the desired frequency. It should be understood that the term conductor as employed in the claims includes reactive components such as capacitors and inductors.