Abstract:
An RF coil assembly for an NMR instrument includes a cylindrical shield disposed concentrically about a central axis and a set of linear conductors disposed concentrically about the central axis and radially inward from the shield. Assembly further includes an annular member positioned to span the gap between the shield and linear conductors. The annular member includes an insulating substrate, a conductive layer over the substrate, and a series of non-conducting gaps through the conductive layer. One or more lumped element capacitors are connected across one or more of the gaps to tune to resonance.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to nuclear magnetic resonance (NMR) apparatus. More specifically, this invention relates to radio frequency (RF) coils useful with such apparatus for transmitting and/or receiving RF signals. 
     In the past, the NMR phenomenon has been utilized by structural chemists to study, in vitro, the molecular structure of organic molecules. Typically, NMR spectrometers utilized for this purpose were designed to accommodate relatively small samples of the substance to be studied. More recently, however, NMR has been developed into an imaging modality utilized to obtain images of anatomical features of live human subjects, for example. Such images depicting parameters associated with nuclear spins (typically hydrogen protons associated with water in tissue) may be of medical diagnostic value in determining the state of health of tissue in the region examined. NMR techniques have also been extended to in vivo spectroscopy of such elements as phosphorus and carbon, for example, providing researchers with the tools, for the first time, to study chemical processes in a living organism. The use of NMR to produce images and spectroscopic studies of the human body has necessitated the use of specifically designed system components, such as the magnet, gradient and RF coils. 
     By way of background, the nuclear magnetic resonance phenomenon occurs in atomic nuclei having an odd number of protons or neutrons. Due to the spin of the protons and neutrons, each such nucleus exhibits a magnetic moment, such that, when a sample composed of such nuclei is placed in a static, homogeneous magnetic field, B 0 , a greater number of nuclear magnetic moments align with the field to produce a net macroscopic magnetization M in the direction of the field. Under the influence of the magnetic field B 0 , the aligned magnetic moments precess about the axis of the field at a frequency which is dependent on the strength of the applied magnetic field and on the characteristics of the nuclei. The angular precession frequency, ω, also referred to as the Larmor frequency, is given by the Larmor equation ω=γB in which γ is the gyromagnetic ratio (which is constant for each NMR isotope) and wherein B is the magnetic field (B 0  plus other fields) acting upon the nuclear spins. It will be thus apparent that the resonant frequency is dependent on the strength of the magnetic field in which the sample is positioned. 
     The orientation of magnetization M, normally directed along the magnetic field B 0 , may be perturbed by the application of magnetic fields oscillating at or near the Larmor frequency. Typically, such magnetic fields designated B 1  are applied orthogonal to the direction of the B 0  field by means of RF pulses through a coil connected to an RF transmitting apparatus. Under the influence of RF excitation, magnetization M rotates about the direction of the B 1  field. In NMR studies, it is typically desired to apply RF pulses of sufficient magnitude and duration to rotate magnetization M into a plane perpendicular to the direction of the B 0  field. This plane is commonly referred to as the transverse plane. Upon cessation of the RF excitation, the nuclear moments rotated into the transverse plane precess around the direction of the static field. The vector sum of the spins forms a precessing bulk magnetization which can be sensed by an RF coil. The signals sensed by the RF coil, termed NMR signals, are characteristic of the magnetic field and of the particular chemical environment in which the nuclei are situated. In NMR imaging applications, the NMR signals are observed in the presence of magnetic-field gradients which are utilized to encode spatial information into the signals. This information is later used to reconstruct images of the object studied in a manner well-known to those skilled in the art. 
     In design of RF coils for use in whole-body NMR studies, it is advantageous to limit radiative RF magnetic field loss by containing the field within the imaging volume, to the extent possible. Additionally, it is known that RF coils employ capacitance appropriate for the given RF coil inductance to attain the desired resonant frequency. In this regard, it is desirable to permit adjustment of the capacitance while expending minimum time and effort in the task. U.S. Pat. No. 4,751,464 (the “&#39;464 patent”) to Bridges and U.S. Pat. No. 4,746,866 (the “&#39;866 patent”) to Röschmann recognize these design goals, but are limited in meeting them. 
     Referring to the &#39;464 patent, a cavity resonator comprises a plurality of transmission lines aligned parallel to a common axis and magnetically coupled to produce an essentially uniform dipole magnetic field perpendicular to the axis. The transmission lines comprise two conductor lines, the first conductors being arranged inwardly of a second conductor common to all lines. The common conductor is concentrically arranged about the axis of the resonator, and constitutes a cylindrically-shaped shield to help contain the magnetic field within the cavity. However, the shield is open at its two ends, permitting undesirable radiative loss of magnetic field through the open ends. 
     The transmission lines of the &#39;464 patent include capacitance for tuning to resonance. However, distributed capacitance is used, which is best understood by knowing what it is not—lumped element capacitance. Lumped element capacitance is provided by discrete capacitors. Distributed capacitance does not use discrete capacitors; rather its capacitance is inherent to design structure, meaning the capacitance is distributed across various parts of a structure which together provide electrical capacitance. In the &#39;464 patent, those “parts” providing distributed capacitance consist of the ends of the first and second conductors which are bent to form a pair of plates having a dielectric disposed therebetween. 
     Impedance matching to resonance is well known to those skilled in the art, and yet to achieve the desired resonant frequency, one requires a precise amount of capacitance. As a matter of manufacturing practicality, this is a difficult requirement to meet when using distributed capacitance. To understand why, it is necessary to consider the nature of distributed and lumped capacitances. Lumped capacitance is provided by one or more discrete capacitors having a precise quantity of capacitance (within a selectable tolerance range). The nature or purpose of discrete capacitors consists, solely, of providing capacitance. 
     In contrast, the nature of distributed capacitance is multifunctional, as the structure providing distributed capacitance also serves as mechanical support. For example in the &#39;464 patent, shield  12  and conductors  18  serve: 1) a mechanical support function, with its own design requirements; and 2) a distributed capacitance function, also having specific design limitations. It is difficult to manufacture structures serving such dual purposes, one of which is the provision of a precise quantity of capacitance. This is one of the inherent problems to the distributed capacitance offering of the &#39;464 patent. 
     Another problem is that the distributed capacitance of the &#39;464 patent is not adjustable. Variable capacitance is useful in a number of circumstances. For example, if the quantity of capacitance provided is incorrect for resonant operation at a desired frequency, then adjustment is desired. Similarly, if over time the resonant frequency changed due to structural changes in the resonator, then one would want to adjust the capacitance. Nevertheless, the distributed capacitance provided by the &#39;464 patent is fixed, and often incorrectly so due to manufacturing complexities mentioned above. 
     The &#39;866 patent attempts to solve the latter problem by providing variable distributed capacitance. Specifically, the &#39;866 patent includes an outer conductor in which there is arranged a dielectric. Two inner conductors are arranged, in the dielectric. One or more of the inner conductors are displaceable, thereby forming a variable distributed capacitance between the inner displaceable conductor(s) and the outer conductor. The system of conductors is tuned or readjusted by displacement of one or more of the inner conductors. 
     While the &#39;866 patent offers variable capacitance, it too has shortcomings. Specifically, in order to establish a desired capacitance for resonant operation, one needs to know where to move the inner conductor to create such capacitance. Additionally, even if the inner conductor is moved into the desired position, it needs to be held in place to maintain the proper capacitance for resonant operation. As a practical matter, this may be a difficult achievement because the resonator vibrates during operation, causing undesirable relative motion between the inner and outer conductors, thereby changing the distributed capacitance from that which is required for resonant operation. 
     What is needed is an apparatus which reduces radiative losses of the RF magnetic field, and which provides lumped element capacitance for resonant operation. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention includes an RF coil assembly for an NMR instrument. The RF coil assembly includes a cylindrical shield disposed concentrically about a central axis and a set of linear conductors disposed concentrically about the central axis and radially inward from the shield. The assembly further includes an annular member positioned to span a gap between the shield and linear conductors, and adapted to receive an end of each of the linear conductors. The annular member has an insulating substrate, a conductive layer over the substrate, and a series of non-conducting gaps through the conductive layer corresponding to respective linear conductors. One or more lumped element capacitors are connected across one or more of the gaps to tune to resonance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an RF coil assembly which incorporates the present invention; 
     FIG. 2 is a close-up view of a portion of the assembly of FIG. 1; 
     FIG. 3 is an end view of one of the annular members; 
     FIG. 4 is a partially exploded view of rods in between a pair of annular members; 
     FIG. 5 is an end view of an end cap; 
     FIG. 6 is a simplified electrical schematic of the low-pass pi circuit of FIG. 7 modified to show transmission line connection; and 
     FIG. 7 is a simplified electrical schematic of a low-pass pi circuit, representative of the assemblies conductive segments. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1 and 2, an RF coil assembly  10  includes inner  12  and outer  18  cylinders, concentrically arranged with one another about central axis  11 . The length of inner cylinder  12  extends beyond the ends of outer cylinder  18 , producing a lip  14  at the front and back end of assembly  10  (i.e., the left and right ends of assembly  10 , as respectively viewed in FIG.  1 ). An annular cavity in between inner  12  and outer  18  cylinders is capped off with a pair of annular members  24  at the front and back end of assembly  10 . Each annular member  24  includes a bolt circle with sixteen equally-spaced apertures  42  (see FIG. 3) aligned to receive the ends of sixteen conductive rods  28  that extend between annular members  24 . 
     Outer cylinder  18  comprises an inner tubular member (not shown) with an external shield  20  attached to the outer circumferential surface thereof. The inner tubular member is made from insulating material, like fiberglass or molded polyurethane, using conventional production techniques. Shield  20  is made from a thin conductive sheath of flexible circuit board material, such as RT duroid manufactured by the Rogers Corp. of Phoenix, Ariz. 
     RT duroid provides an insulating substrate with a conductive layer on one surface of the substrate. The substrate thickness is 0.005 inches (0.127 mm), and slots  22  are etched in axial fashion through the conductive layer to produce sixteen conductive panels, collectively representing shield  20 . The RT duroid material is attached to the inner tubular member using any conventional technique, such as the application of adhesive in between the insulating substrate and the outer circumferential surface of the inner tubular member. 
     The conductive layer of shield  20  establishes an electric shield and, effectively, a ground plane for coil assembly  10 . The slots  22  in shield  20  prevent the flow of gradient-induced eddy currents. 
     Inner cylinder  12  comprises a tubular member made from insulating material, such as fiberglass or molded polyurethane, using any conventional manufacturing technique. Each lip  14  has an outer circumferential surface  16 , which may be used to mount a grounding strip (discussed below). 
     Referring to FIGS. 1-4, annular member  24  is sized to fit snugly in the annulus in between inner  12  and outer  18  cylinders, supporting cylinders  12  and  18  in coaxial arrangement. The outer circumferential edge of annular member  24  is attached to outer cylinder  18  using sixteen conductive tabs  26 , one per conductive panel of shield  20 . Conductive tabs  26  comprise any commercially available conducting adhesive strip. The inner circumferential edge of annular member  24  may be attached to inner cylinder  12  using an adhesive, or the like. 
     Annular member  24  comprises a rigid circuit board material, such as RT duroid with an insulating substrate thickness of 0.065 inches (1.651 mm), and a conductive layer on the outer side thereof. As best shown in FIG. 3, sixteen apertures  42  penetrate annular member  24 , equally spaced around its circumference, to support the ends of sixteen conductive rods  28 . Circuitry sites  30  are located on an easily accessible outer surface of annular member  24 , and surround each aperture  42 . 
     Conventional etching techniques are used to create the circuitry sites  30 . Specifically, a non-conducting gap  38  is etched through the conductive layer for each circuitry site  30 , so each non-conducting gap  38  encloses the perimeter of a conductive island  36  remaining around each aperture  42 . Etching also creates a pair of diametrically opposed non-conducting slots  34  through the conductive layer of annular member  24 , thereby forming two crescent-shaped regions in the conductive layer, as best shown in FIGS. 3 and 4. Alternatively, slots  34  may be omitted, or more slots  34  may be used to further divide the conductive layer into more than two regions. 
     Lumped element circuitry is mounted to the annular member  24  and bridges each gap  38 . More specifically, to compensate for the inductance of each rod  28 , one or more lumped element capacitors  40  are attached across each gap  38 , as required for resonant operation of coil assembly  10  as is well known in the art. Similarly, one or more lumped element capacitors  40  are attached across each slot  34 , having appropriately selected combined capacitance (e.g., above 1000 picofarads/slot) to function as an open circuit at low frequencies and a short circuit at high frequencies (e.g., above 100 Megahertz). As such, use of lumped element capacitors across slots  34  interrupts direct current and low frequency continuity, but maintains radio frequency continuity. 
     Capacitors  40  may comprise any commercially available lumped element capacitor, though those known in the art as surface mounted or porcelain chip capacitors and commercially available from American Technical Ceramics, Inc. of Huntington Station, N.Y. are preferred. Such capacitors are preferred because they have a pair of short flat plates for connection (as opposed to wire leads) minimizing device inductance, as desired. Lumped element capacitors  40  form capacitive bridges over their respective gaps  38 . Specifically, one capacitor plate is connected to its respective conductive island  36  and the other plate is connected to the conductive layer of annular member  24 . In similar manner, one or more lumped element capacitors are connected to form capacitive bridges over slots  34 . 
     Referring to FIG. 4, the front and back annular members  24  (left and right members  24 , as viewed) are virtually identical, each having sixteen apertures  42  in opposing axial alignment for receiving rods  28  therebetween, sixteen corresponding circuitry sites  30 , and two slots  34 . The front annular member  24  differs in that two of its circuitry sites  30 , located ninety degrees apart, are modified for connection to coaxial transmission lines  32 , as best shown in FIG.  1 . Specifically, each of the two circuitry sites  30  modified for coaxial transmission line connection include a pair of coaxial non-conducting gaps  38 , with a band of the conductive layer remaining therebetween. 
     Conductive rods  28  are hollow tubes made from copper, though a variety of commercially available rigid conductive materials may be used. Additionally, conductive rods  28  may be solid, if desired. The ends of conductive rods  28  are soldered in place in their respective apertures  42  to make electrical contact with their respective conductive islands  36 . 
     With conductive rods  28  and lumped element capacitors  40  in place, RF coil assembly  10  provides sixteen separate conductive segments, each consisting of the well known low-pass pi circuit  54  represented in FIG.  7 . In this Figure, inductance L 1 , capacitances C 1 , and ground respectively symbolize rod  28 , lumped element capacitors  40 , and shield  20 . The low-pass pi circuits  54  are conventionally tuned to identical frequencies, producing an array of identical coupled resonators, disposed concentrically about central axis  11 . As is known, such an array of resonators has formal properties resembling those of the so-called bird-cage resonator. Accordingly, the usual perturbation theory of bird-cage resonators applies, and symmetry correction in the perturbation limit can be applied to simplify tuning. Additionally, since tuning is accomplished by applying appropriate capacitance in the form of lumped element capacitors  40 , the proper frequencies are easily obtained and adjusted. 
     The capacitance value C 1  for lumped element capacitor(s)  40  is selected in accordance with established principles governing the operation of bird-cage resonators. Specifically, it is known that bird-cage resonators have several resonant modes, of which only one (the “useful mode”) produces a uniform magnetic field of value in magnetic resonance imaging. The frequency of the useful mode bears a calculable relationship to the frequency of an individual isolated LC circuit (e.g., low-pass pi circuit  54 ). Formulas for the required calculations are well known to those skilled in the art as disclosed in “Mutual Inductance in the Bird-Cage Resonator” by James Tropp in  Journal of Magnetic Resonance  126, 9-17 (1997). 
     Referring to FIG. 6, an electrical schematic is shown for the two circuitry sites  30  modified for connection to coaxial transmission lines  32 . In this Figure, inductance L 1 . capacitance C 1 , and ground respectively represent rod  28 , lumped element capacitor(s)  40  (on the back annular member  24 ), and shield  20 . Capacitances C 2  and C 3  symbolize lumped element capacitors  40  located on the front annular member  24 . Specifically, lumped element capacitor(s)  40  representing capacitance C 2  have one plate connected to conductive island  36  which is soldered to rod  28 , and the other plate connected to the band of conductive layer between the pair of coaxial gaps  38 . Lumped element capacitor(s)  40  representing capacitance C 3  have one plate connected to the inter-gap conductive layer, and the other plate connected to the conductive layer outside of the outermost gap  38 , which is connected to shield  20 . The inner cable of coaxial transmission line  32  is connected to the shared node between capacitances C 2  and C 3 , while the outer cable is connected to ground. Any one of a number of well known driver circuits may be used to produce RF driver signals through coaxial transmission lines  32 . 
     Once capacitance value C 1  is chosen, the following relationships hold for capacitance values C 2  and C 3 : 
     
       
           C   1 =( C   2 * C   3 )/( C   2 + C   3 );  Eqn. 1 
       
     
     and 
     
       
         ω* C   3 =(1/50* R )  Eqn. 2 
       
     
     Where ω is 2 πv and v is the resonant frequency of the useful mode (i.e., the Larmor frequency), and R is the equivalent series resistance of the resonator when operating in pure excitation of the useful mode. This assumes operation in an RF system with 50 ohm characteristic input impedance. 
     As previously mentioned, one or both of the outer circumferential surfaces  16  (see FIG. 1) may be used to mount a grounding strip, such as a strip of conductive material adhered to surface  16 . The grounding strip would be conventionally connected to one or more portions of annular member  24  in electrical communication with shield  20 . The purpose of the grounding strip(s) is to provide an electrostatic shield between a patient within the resonant volume, and the high impedance points of the low-pass pi circuits, where their respective inductances and capacitances are joined. 
     Referring to FIG. 5, an end cap  44  is depicted that may be used in lieu of the back end annular member  24 . In this case, the back end lip  14  (see FIG. 1) would preferably not be used, permitting flush engagement of the end cap  44  against inner cylinder  12  and outer cylinder  18 . End cap  44  functions as an RF mirror, limiting signal leakage through the back end of RF coil assembly  10  by closing the opening that would otherwise exist with use of the back end annular member  24 . 
     End cap  44  comprises a disc of rigid circuit board material, such as RT duroid with an insulating substrate of desired thickness, and a conductive layer on the outer side thereof. Apertures  42  and circuitry sites  30  are provided in end cap  44 , as they were for annular member  24 . In addition, sixteen conductive sectors  46  are formed by etching radial slots  48  through the conductive surface of end cap  44 . The radial slots  48  intersect with a centrally located circular slot  52  through the conductive layer. Sixteen lumped element capacitors  40  bridge across circular slot  52 , making respective connection between the sixteen conductive sectors  46  and a common conductive surface  50 . These lumped element capacitors  40  are appropriately selected to interrupt DC and low frequency continuity, while maintaining radio frequency continuity. 
     End cap  44  is connected to the back end of RF coil assembly  10  using any conventional technique. For example, a non-conductive plug located on the back side of end cap  44  may be inserted into the annular cavity between inner  12  and outer  18  cylinders, and attached with adhesive. Additionally, each conductive sector  46  is coupled with a conductive tab  26  to a respective conductive panel of shield  20 . 
     The apparatus of the present invention permits tuning the RF coil assembly to resonance with ease and efficiency. This is accomplished by using lumped element, as opposed to distributed, capacitance to tune the assembly. Additionally, certain portions of the assembly permit easy access to sites for mounting the lumped element capacitors. These mounting portions are made from an insulating substrate having a conductive layer with a pattern of non-conducting gaps formed through the conductive layer. 
     Additionally, the apparatus of the present invention substantially reduces radiative loss of RF magnetic field. This is accomplished by using a shield fitting around the irradiated volume. Moreover, an end cap may be used to further limit such losses. 
     The foregoing descriptions of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying current knowledge, readily modify and/or adapt the embodiments for various applications without departing from the generic concept. Therefore, such adaptations and modifications should, and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. 
     It is to be understood that the phraseology or terminology employed herein is for the purpose of description, and not of limitation. Accordingly, the invention is intended to embrace all such alternatives, modifications, equivalents, and variations as fall within the spirit and broad scope of the appended claims.