Abstract:
An NMR resonant structure is formed of axial conductors ( 54   a,    54   b,    54   c , and  54   d ) and end members ( 50,51 ), supporting said conductors to form a coil structure ( 8 ) of desired electrical topology wherein the end members combine the function of RF interconnects between selected axial conductors (inductors) with an axial constraint on RF field prevailing outside the axial bounds of the end members, and if so desired, comprise a selected capacitance  61  for the resonant structure.

Description:
BACKGROUND OF THE INVENTION 
     This work relates to nuclear magnetic resonance (NMR) apparatus and particularly to the RF coupling to nuclei of the sample under study. This is the function of a module widely termed the NMR probe, which controls the distribution of the RF field within a sensitive region. A sample within the sensitive region is closely coupled to the RF radiation generated in the sensitive region for spin excitation and subsequently emitted with the de-excitation of sample nuclei. The heart of the probe is the RF coil and the salient property of such coil in typical use is the degree of homogeneity of the RF field achieved over the interior of the sensitive volume defined by the coil. It is also desired to constrain the RF field distribution for the coil to a limited region within the sensitive volume because the spatial variation of the polarizing magnetic field is not eliminated outside of the sensitive volume. 
     Limiting the spatial distribution of the RF field of the NMR coil is the subject of a number of prior art works. Of particular interest for this purpose are U.S. Pat. Nos. 6,008,650 and 5,192,911, both commonly assigned herewith. In general these works describe shielding implemented to protect the sensitive volume of the resonance apparatus from RF influence external thereto, or, to reduce the RF field of the coil outside of the sensitive volume. For example, it is desired to shield that portion of the sample extending beyond the sensitive volume from irradiation arising from the coil or the coil leads. This does not protect that same portion of the sample (disposed in a possibly slightly different polarizing field) from irradiation due to the RF field distributed predominantly, but not completely in that sensitive volume. In general these shielding arrangements include a conducting member, typically of cylindrical form, coaxial with the sample axis and axially displaced from the central region of the RF coil. Such shielding predominantly attenuates the radial components of the RF field in the axial region beyond the shield structure. 
     Another approach to the problem of undesired excitation of sample outside the sensitive volume is based upon physically limiting the sample volume to coincide with the axial extent of the probe coil. In order to avoid axial discontinuity in magnetic susceptibility, the prior art utilized plugs, susceptibility matched to the sample, and inserted into the sample vessel to confine the sample to the desired region, coincident with the probe coil. This is disclosed by U.S. Pat. No. 4,549,136 to Zens, commonly assigned herewith. 
     RF cavity resonators are known for NMR apparatus and such resonators effectively contain the internal RF field and therefore shield the sample volume from RF influence external to the cavity. A representative example of such NMR cavity resonator is disclosed in U.S. Pat. No. 4,607,224, commonly assigned herewith. Although the cavity structure provides both axial and radial constraint to the field distribution, the ability is lost to impose an independent RF field on the sample from outside of such resonator. This is a necessary tool for spin decoupling and for a number of multiple resonance techniques. 
     SUMMARY OF THE INVENTION 
     A novel RF resonator structure features an RF resonator (or “coil”) that includes axial constraint to the RF dipole field generated in the sensitive volume defined by the coil. This constraint serves to shield those portions of the physical sample located outside the sensitive volume. This is the usual case for liquid samples disposed in a long tube. This axial constraint also serves as a conducting member of the coil forming the azimuthal interconnects between the axial inductive members of the coil. The novel coil is an “open” structure; that is, a second, coaxial coil, outside of the “open” coil, may be arranged to independently irradiate a sample tube disposed on the common axis of the two resonators. 
     The field confining/interconnect structure is mechanically an end member of a coil support or former and has yet another benefit in providing a platform for realizing a lumped capacitance and supporting an adjustable (vernier) capacitance in close proximity to the coil itself. The field confining structure is preferably a composite of conducting surface-dielectric-conducting surface to provide a selected value of capacitance in parallel with the coil structure. One end member of a coil preferably includes a third capacitive member comprising another conductor, axially displaced from the outward facing conducting surface, axially outside the sensitive volume of the coil and disposed for precise axial translation to provide a capacitive vernier. In a preferred arrangement, the RF excitation source is inductively coupled to the coil through a loop proximate the axial inductive members of the coil. 
     Following the general structure for an RF coil comprising end members in combination with inductive members, the interconnect function of one or both of the end members can be exploited to yield a variety of types of RF resonators. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a concise illustration of the context of the novel probe coil. 
         FIG. 2   a  shows the basic aspect of the novel coil. 
         FIG. 2   b  shows the basic structure with further structural enhancement. 
         FIG. 2   c  illustrates a complex end member forming a fixed capacitance. 
         FIG. 2   d  shows an adjustable capacitance implemented with coaxial threaded shafts. 
         FIG. 2   e  is preferred arrangement using a magnetic coupling loop. 
         FIG. 2   f  is an effective circuit for the coil of  FIGS. 2   a,b    
         FIG. 3  compares axial distribution of signal intensity for a conventional coil and the axially constrained coil of this work. 
         FIG. 4  is a birdcage coil exhibiting the present axially constrained feature. 
         FIG. 5   a  represents an end member for implementing a pair of coaxial coils. 
         FIG. 5   b  is a section through the end member of  FIG. 5   a.    
         FIG. 6  is a 2 conductor (inductor) embodiment of the saddle coil of  FIGS. 2   a,b.    
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The context of the invention is conceptually illustrated in  FIG. 1  (the same label appears in the several figures for the same object.) An NMR probe  9  is disposed within the bore  11  of a superconducting magnet  10 . A sample for analysis is presented in a sample vessel (not shown) inserted in the probe  9 . The probe  9  inductively couples to the nuclear spins of the sample for excitation through at least a first RF excitation channel  12  and separately for signal acquisition through a receiver channel  14 . Excitation and receive functions often share a common probe coil for non-concurrent operation through correllator  13 , but multiple coils are frequently employed to serve different functions, such as spin decoupling, field-frequency lock, and the like. The receive channel ordinarily includes a preamplifier and RF demodulator, phase detector, analog-to-digital conversion (ADC) and various signal processing apparatus together with a digital processor  15  to effect averaging, Fourier transformation, storage and the like. In like manner, processor  15  controls both the excitation channel  12  and receive channel  14 . More recently, some of these functions are consolidated in a direct digital receiver, but these variations are not critical to the understanding or operation of the present probe. 
     The essential component of the probe  9  is one, or more resonant structures, e.g., coils, for coupling nuclear spins of sample molecules to excitation channel  12  and receive channel  14 . The basic essentials of the present axially constrained RF coil  8  are shown in  FIG. 2   a . Facing conducting planar end members  50  and  51  are each composed of segments  50   a  and  50   b , and  51   a  and  51   b , respectively. The segments have a common boundary, (shown here as the diameter of a disk shaped end member, but not limited to such geometry). Each pair of segments (of each end member) are displaced across the common boundary to form slots  52  and  52 ′ which define the corresponding segment and provide for electrical isolation therebetween. Apertures  53  and  53 ′ are formed in respective end members and these are in axial alignment to receive an NMR sample vessel. Slots  52  and  52 ′ are arranged in angular offset: Here, the angular offset is shown as 90o for a simple case. Paraxial conductors  54   a ,  54   b ,  54   c  and  54   d  are shown for a simple embodiment. The paraxial conductors  54   a ,  54   b ,  54   c , and  54   d  are of such length to extend well beyond the bound of the sensitive volume. Preferably, the total length is about 3 times the axial extent of the coil itself with the coil increment centered. This arrangement removes, to a large extent, a discontinuity in axial dependence of gross magnetic susceptibility. See U.S. Pat. No. 4,517,516 to Hill and Zens, commonly assigned. 
     A first adjacent pair of such conductors,  54   a  and  54   b , is arranged such that at one end, the adjacent ends of conductors  54   a  and  54   b  are commonly connected to the same segment  50   a , while at the other end of the adjacent pair, the two conductors connect to separate segments  51   a  and  51   b , forming one loop of the coil. The second pair of paraxial conductors,  54   c  and  54   d , are similarly connected to segments  51   b  and  50   a  and  50   b . The electrical topology of this arrangement is immediately recognizable as that of a saddle coil with the added feature that each end member offers significant shielding to restrict undesired axial extension of the RF field as it is exists beyond those end members  50  and  51 . For simplicity, a single turn saddle coil is a convenient model for illustration. A saddle coil of more than one turn is a straightforward extension of this discussion. 
     Alternate electrical topology for the RF coil is obtained through appropriate design of one or both of the end members  50 ,  51 . For example, if one substitutes an unslotted planar conductor for end member  50  of  FIG. 2   a , the conducting (inductive) members  54   a  and  54   b  are commonly connected at that end member of the coil and the resulting structure support two RF current loops on opposite sides of the sample axis: a first loop comprises conducting members  54   a  and  54   b , in parallel with a second loop comprising conducting members  54   c  and  54   d.    
       FIG. 2   b  illustrates additional structure of this form of preferred embodiment. (For clarity of presentation, all labels from  FIG. 2   a  are not repeated in  FIG. 2   b .) Non-conductive paraxial rods  60  are added radially outside the paraxial conductors  54   a ,  54   b ,  54   c , and  54   d , which are shown here with shading to aid the eye. Rods  60  provide structural stability and support for an adjustable capacitor plate  62  (more particularly shown in  FIG. 2   d ) which capacitively couples to end member  51 . It is a further preference that end member  51  has a composite structure as shown in  FIG. 2   c , that is, an additional one or more planar conducting members  57 ,  57 ′ displaced by dielectric layer (or empty space)  58  to constitute selected fixed capacitance(s) and form a conducting shield surface of end plate  51 . This structure may be formed from discrete components, such as for example, standard printed circuit board or copper sheet and dielectric of thin planar sapphire or a simple gap. In accord with circuit requirements, the capacitance(s) may be implemented at either or both end members.  FIG. 2   c  represents a further extension of the structure of end member  51  to support lumped capacitance for the resonant probe coil. The capacitances of  FIG. 2   c  are implemented with the outward facing conductor  57  (facing outside the sensitive volume of coil  8 ) having a segmented construction defined by slot  56 . With the orientation of slots  56  and  52  orthogonal the respective segmented conducting faces of the capacitor-end member  61  form two parallel pairs of series capacitors. The effective circuit of  FIG. 2   f  is completed with the dotted connection supplied by end plate  50 . Other apertures  59 , one of which is so labeled, are provided for various purposes as discussed below. It is apparent that endplate(s)  50  and/or  51  serve multiple purposes of RF field confinement, coil member interconnects, and implementation of a capacitive element. One of the slots  59  is provided for admittance of a feed from an RF source as shown in  FIG. 2   e . Preferably, in a preferred arrangement, an inductive coupling loop is employed between the RF source and the coil for excitation to minimize inductive losses from leads. The inductive loop leads are in close mutual proximity and thus tend to provide a cancellation of distributed parasitic inductive effect. 
       FIG. 2   d  shows one mechanical tuning linkage for precise displacement of an adjustable (vernier) capacitor plate  88 . This linkage is enabled through a differential threaded coaxial shaft arrangement as shown in simplified form in  FIG. 2   d . An immobile nut  80  is fixed relative to a strut  82  that is part of the probe support and enclosure structure. Threaded outer shaft  84  is rotated externally for capacitance adjustment and, as a consequence, translates axially in the thread T 1  of the immobile nut  80 . Outer shaft  84  has a threaded bore T 2  mating with inner threaded shaft  83  which in turn translates axially toward immobile nut  80  if T 1  and T 2  have the same sense of pitch. Through a cantilever linkage  86 , the adjustable capacitor plate  88  travels in a precise drive reduced relationship (depending upon the relative pitches T 1  and T 2 ) to rotation of outer shaft  84 . The coil  8  is conveniently referred to  FIG. 2   a , to which there are added stops  90  and  91  and an additional set of insulating rods  92  and  94 . The insulating rods  92  and  94  differ from insulating rods  60  in that they are fixed in respect of adjustable plate  88  and are slidably supported through appropriate apertures  55  in end members  50  and  51 . 
     The adjustable plate  88  comprises a segmented construction similar to the end member segmentation. The relative orientation of the plate  88  segments with the facing segments of the proximate end member correspond to simple series connection of the vernier capacitance for congruent relative orientation and parallel connection for the orthogonal relative orientation. 
     Drive reduction through differential threaded shafts is known. Differential threaded coaxial shafts achieve excellent drive reduction without the need for plural shafts and conventional gearing and concurrently, the desired rotary to translatory conversion. Such structure is ideal for the necessarily compact structure of an NMR probe. A known class of NMR probe for magic angle spinning, sold under the designation Varian Chemagnetics Double Resonance HXMAS and Triple Resonance HXYMAS, employ similar drive reduction linkage for precise adjustment of the spinning axis inclination. 
     It should be appreciated that the example illustrated in  FIGS. 2   a–f  represents a simple example of a coil consisting of an RF current dipole supported by inductive members disposed on opposite sides of a sample. A greater number coil turns, is available by suitable extension of the number and arrangement of segments. One common and here, preferred form of saddle coil is shown in  FIGS. 2   a ,  2   b ,  2   c , and  2   f ; that is, parallel connection of facing current loops to provide a transverse RF magnetic field and transverse to the axis of the sample tube and polarizing field. An NMR probe featuring the above described resonant coil has been built for operation at 700 MHz. The interconnect function of the end member is easily altered to provide serial connection of the loops if such structure is desired. Moreover, other forms of resonator (as described below) may advantageously use the versatile end member structure of this work to support the desired electrical topology. The geometry of the interconnects is not limited to arcuate segments, but is readily adaptable to different RF field configurations. It is a matter of design choice whether both end members implement a desired capacitance. It is a desirable result of segmented construction that the influence of eddy currents is reduced. 
     As a result of the structure of the fixed and variable capacitance described above, these reactive elements are placed in very close proximity to the coil. Parasitic reactive elements (attributable to leads) are thereby reduced. The structure of  FIGS. 2   a  and  2   b  contrasts with structure such as the well-known Alderman-Grant resonator and its derivatives. These, as well as the present structure, exhibit a minimal inductance for the LC resonant device. However, the Alderman Grant type resonators derive their capacitive reactance from distributed capacitance furnished by axial extension of the slotted tube. In certain applications, such as magic angle spinning, extended axial structure conflicts with the spatial limitations of the superconducting magnet bore. Moreover, contemporary magic angle spinner apparatus employs a pair of gas bearings displaced along the spinning axis. An Alderman-Grant resonator of conventional form, commonly employed in such arrangement, requires substantial physical length (along the spinning axis) to supply the necessary capacitance. This, again conflicts with spatial limitations of the magnet bore because the bore axis and spinning axis are at an angle of about 54°. The present structure presents a reduced spatial extension in comparison with the conventional Alderman-Grant resonator. 
       FIG. 3  is a comparison of axial distribution of signal intensity for the axial-field constrained coil (curve  75 ) with a conventional coil (curve  77 ) of the same axial and radial dimensions. This data was acquired using a standard sample tube containing a water droplet of about 1 mm axial extent. The droplet was advanced along a graduated scale by selected displacement to acquire each datum. Observe that both curves reach null signal value at identical axial coordinates as would be expected for coils of the same overall dimensions. There is a similar flat response of signal as a function of axial displacement within the sensitive volume for both coils, but the axial extent of the flat region is greater for the present coil construction, and distinction is also found in the rate of signal attenuation outside the sensitive volume, where the axially constrained coil exhibits a steeper fall-off of signal. This behavior reflects the comparison of the apertured end member of the present coil compared to the conventional coil that is fully open, e.g., unshielded. The particular example plotted in  FIG. 3  features an aperture of 6 mm and an outer diameter of the coil of 12 mm, forming a nominal 75% shield compared with the 0% shield of the conventional coil which exhibits essentially transparent axial bounds. While it appears that a smaller aperture relative to coil diameter will yield a better measure of axial confinement of the RF field such condition similarly implies a smaller filling factor and therefore, lower signal amplitude. 
     The multi-use end members of the present work may be employed to implement birdcage type resonators as shown in  FIG. 4 . For clarity of presentation, a 4 rung birdcage resonator is shown, but it is understood that the number of rungs is not a limitation. Slotted end members  270  and  271  are disposed with respective slots in parallel orientation. That is, the segments of respective end members are congruently oriented, in contrast with the saddle coil of  FIG. 2   a . For example, inductive member  54   a  is in electrical contact with segments  260  and  262  at corresponding ends of the conductive (inductive) member  54   a . As shown, adjacent inductive members  54   b  and  54   d  are coupled to inductive member  54   a  through chip capacitors  254  and  255 . Each inductive member is directly connected to a unique segment at the respective end member. As an alternative to chip capacitors, an end member may be designed to implement the coupling capacitances through a composite construction of first and second segmented conductive surfaces sandwiching a dielectric with rotation of the segment defining slots to establish the desired capacitances. The inner facing segments of each end member remain in one-to-one association, together with the corresponding paraxial conductor to form the well-known birdcage network. 
     In all of the examples discussed, the conducting surfaces of end member construction typically exhibited segmented areas bounded and defined by slots. In this manner, eddy current effects are minimized when the active probe is immersed in a polarizing field of the magnet  10 . 
     In combination with appropriately designed end members, the inductive members have been shown to implement saddle coil ( FIGS. 2   a,b ) and birdcage ( FIG. 4 ) resonators. It should be apparent that the interconnect function of the end member may be arranged to implement electrically serial RF loops. A single pair of paraxial conductors (inductors) may be interconnected to form an Alderman-Grant type resonator. 
     Although the figures and discussion imply coincidence of the outer coil diameter with the outer dimensions of an axial shield/end member, this is not required. As suggested above, there is a trade-off in the balance of filling factor (large aperture relative to radial coil extent) with axial confinement (small aperture relative to the radial coil dimension). To reflect this consideration, in another embodiment the paraxial conductors (inductive members) are distributed on radii approaching the outer dimensions of the sample vessel while the axial shield exhibits rather larger radial dimensions than those of the coil. 
     As a practical matter, an NMR probe employs a plurality of resonating means for coupling to a plurality of different nuclear spins present in the sample molecules under analysis. Coaxially disposed coils to serve this purpose may employ the structure described as either a (radially) inner coil, a (radially) outer coil, or both coils may share the novel structure.  FIG. 5   a  is one surface of an end member  51 ″ having an inner coil pair of segments  153  and  153  separated by slots  152  and  152 ′. Outer coil segments  151  and  151 ′ are similarly separated by slots  154  and  154 ′ and circular slot  155  isolates the inner and outer coil end members. Aperture  53  accommodates a sample tube. The two coils are ordinarily required to resonant at rather widely separated frequencies. The fixed capacitances for the two coils are, in the first instance limited by their respective geometrical areas. Further gross relative capacitance may be obtained, if required, by implementing appropriate capacitance at one or the other end member for one or the other coil. Additional gross variation may be obtained by forming disparate dielectric material(s) between the opposite surfaces of an end member for respective co-axial coils as shown in  FIG. 5   b . As there shown (by way of example), separate monolithic dielectric portions  160  and  162  are disposed in alignment with the inner segments  153  and  153 ′ to supply the desired capacitance for the circuit of the inner coil and another dielectric  162  selected for realizing the desired capacitance for the outer coil. Another approach to realizing a desired lumped capacitance for one of the coaxial coils comprises alternating sheets of dielectric  162  and (commonly connected) conducting surfaces (not shown) aligned with segments e.g.,  151  and  151 ′ to supply the desired capacitance for the outer coil circuit. Conducting surfaces  151  and  251  are external faces of the lumped capacitance. Independent adjustable capacitance for either inner or outer capacitance is available through a pair of independent coaxial adjustment linkages implemented as shown in  FIG. 2   d . Apertures for support of the paraxial conductors are labeled  170  for the outer coil and  172  for the inner coil. 
     In a preferred embodiment, each of the paraxial conductors  54   a ,  54   b ,  54   c , and  54   d  are doubled by disposing each such conductor as an electrically paralleled pair of conductors in slight azimuthal displacement. The RF homogeneity is found to be improved as determined by the “810o/90o” method (see Vaugn, J. B. Jr, Spectroscopy, v.10, p. 36 (1995) and the inductance is slightly reduced allowing somewhat higher frequency performance. 
     In another embodiment, present work is extended to a 2-inductor resonator as shown in  FIG. 6 . The separation of each of the paraxial conductor pairs (first pair  54   a  with  54   d , and second pair  54   c  with  54   b ) is reduced to produce respective azimuthally distributed paraxial conductors (inductors)  54 ′ad and  54 ′cb in combination with end members  50  and  51 . The conductors  54 ′ad and  54 ′cb are slotted at one end (slots  63 ) where slots  63  coincide with end member slot  52 ′ to limit eddy current propogation. The capacitance required for the resonant structure is supported by the end member  51  in the manner of the  FIG. 2   c  where an such end member is configured to receive the azimuthally distributed paraxial conductors. 
     The aperture and coil cross section referenced herein should not be understood as limited to a particular shape. Although circular cross section sample vessels and circular cross section coils are widely employed for NMR measurements, elliptical and rectangular shapes present certain advantages for static samples. See U.S. Pat. No. 6,917,201, commonly assigned. Nor should the paraxial conductors be interpreted as limited to any particular cross sectional shape. 
     Although this invention has been described with reference to particular embodiments and examples, other modifications and variations will occur to those skilled in the art in view of the above teachings. It should be understood that, within the scope of the appended claims, this invention may be practiced otherwise than as specifically described.