Abstract:
An NMR probe comprises a resonator formed of two separate conducting loops disposed on opposite sides of the sample, where one loop is driven and the other floats in a preferred operating mode. Slotted shields are disposed coaxially within said loops and outside the sample with slots aligned with gaps between the loops.

Description:
FIELD OF THE INVENTION 
     The invention is in the field of NMR and relates particularly to RF probe structures. 
     BACKGROUND OF THE INVENTION 
     The central components of a modern high resolution liquid sample NMR probe include, at least one resonator for coupling RF (resonant) radiation to (at least one) resonating aggregation of nuclear spins of a sample. The sample is typically of elongate extension along an axis coincidental with the direction of the static polarizing magnetic field B 0 . The resonator imposes on the sample an RF magnetic field (B 1 ) transverse to B 0 . The achievable homogeneity of B 0  is spatially limited and the practicality of coupling RF power to the resonator through finite leads motivate the use of an RF shielding structure interposed between the leads of the resonator and the sample. The RF shield structure ideally limits RF coupling to the resonant spins located within a prescribed axial region of B 0  homogeneity. In particular excitation of sample portions outside the desired region of carefully shimmed B 0  homogeneity due to irradiation from the leads is a parasitic effect to be minimized by the shields. 
     The RF coil and shielding is subject to the deleterious effects of eddy currents arising from rapidly switched independent magnetic fields, such as magnetic gradient fields. Eddy currents induced in the coil and shields produce transient magnetic fields in opposition to the switched field inducing the eddy current. Inasmuch as these parasitic fields are particularly close to the sample, the B 0  field homogeneity is degraded and the parasitic fields, with undesirable persistence, also degrade the relative timing of steps associated with a given NMR experiment. 
     In order to control/reduce eddy currents in RF coil and shield structures operating at room temperature, the prior art has incorporated slots into these conductors. See, for example, U.S. Pat. Nos. 6,008,650; 5,192,911; and WO 92/17799 all commonly assigned with the present invention. To similar effect, see U.S. Pat. No. 4,875,013. It should be appreciated that the eddy current problem is many times more deleterious with the RF resonator at cryogenic temperatures than that experienced with the RF resonator at room temperature. For the purpose of this work cryogenic temperature shall be understood to include temperatures substantially below ambient. Recent advances in NMR include very high Q probes operating at rather low temperature. Under such conditions, eddy currents effects are enhanced and their consequent deleterious effects require more rigorous suppression. 
     SUMMARY OF THE INVENTION 
     As employed herein, the RF resonator of saddle coil geometry is completely divided between longitudinally adjacent inductive members to provide two electrically separate loops disposed on opposite facing surfaces enclosing the sample space. Each loop, defining a window to the sample encompasses slightly less then 2π (around the loop) to accommodate the two leads of each loop. The leads from both loops are disposed longitudinally in the same direction away from the central region (windows) of the loops. In the preferred embodiment the coil is excited by application of RF power to one loop with mutual induction coupling to symmetrically excite the opposite loop. 
     The shields comprise cylindrical conduction each slotted to provide azimuthal shield portions approaching π in angular extent. These shield cylinders are coaxial with the RF coil with the inner axial extent preferably aligned with corresponding outer edges of the RF coil windows. The separate gaps between the loops of the RF coil are preferably aligned with the slots of the shields to provide transverse windows where double resonance experiments are contemplated. 
     The subject matter of the present application is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in conjunction with accompanying drawings wherein like reference characters refer to like elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an NMR system incorporating the invention. 
     FIG. 2 a  shows a planar mapping of an RF coil of prior art. 
     FIG. 2 b  shows a planar mapping of RF shields for the prior art of FIG. 2 a.    
     FIG. 3 a  shows a planar mapping of the RF coil of the invention. 
     FIG. 3 b  shows the planar mapping of the RF shields for the RF coils of FIG. 3 a.    
     FIG. 3 c  illustrates the equivalent circuit for the preferred mode of operation. 
     FIG. 3 d  shows a prior art cryogenic RF coil and shields for a comparative test purpose. 
     FIG. 4 is a perspective view of the embodiment of FIGS. 3 a  and  3   b.    
     FIG. 5 shows eddy current responsiveness performance of the invention. 
     FIG. 6 illustrates another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 represents the context of the invention represented by schematicised NMR instrument. An acquisition/control processor  110  communicates with an RF source  112 , modulator  114  and RF receiver  116 , including analog-to-digital convertor  118  and a further digital processor  120 . The modulated RF power irradiates an object/sample  123  in a magnetic field  121  through a probe  122  and response of the sample/object is intercepted by probe  122  communicating with receiver  116 . The response typically takes the form of a transient time domain waveform or free induction decay. This transient waveform is sampled at regular intervals and the samples are digitized in adc  118 . The digitized time domain waveform is then subject to further processing in processor  120 . The nature of such processing may include averaging the time domain waveform over a number similar such waveforms and transformation of the averaged time domain waveform to the frequency domain yields a spectral distribution function directed to output device  124 . Alternatively, this procedure may be repeated with variation of some other parameter, and the transformation(s) from the data set may take on any of a number of identities for display or further analysis. 
     FIG. 2 a  is representative of (room temperature) prior art slotted resonator structures  12  and FIG. 2 b  similarly depicts the shielding structure  14  interposed between a sample and the resonator. These figures are planar mappings of the cylindrical forms in use wherein the resonator and shield are understood to exhibit radii a and b respectively where a&gt;b. Arrows within FIG. 2 a  indicate the instantaneous RF current direction in an embodiment where RF power is applied between terminal portions  16  and  17 . Windows  18  and  19  define the two current loops of the coil. Arrows on the several conductor portions suggest the instantaneous current direction. RF magnetic flux (B 1 ) is ideally distributed uniformly through the area of these oppositely facing windows. Slots  20   a  and  20   b  (comprising half slots  20   b ″ and  20   b ′) serve to define electrical structure of the resonator structure  12 . Prior art has employed RF shields taking the form of cylindrical shell conductors, axially flush with the inner edge of the RF window and extending axially outward to shield portions of the sample (distal in relation to the windows) from unwanted excitation. These shields support eddy currents due to rapidly switched gradient fields. The shield structure  14  may be briefly described as a pair of slotted rings symmetrically aligned with windows  18  and  19  and with slots  21   a  and  21   b  (comprising  21   b ′ and  21   b ″) similarly aligned with resonator slots  20   a  and  20   b . The shield structure is typically electrically floating and serves to shield the leads  16  and  17  from the sample and to shield the sample portion remote from the window regions from excitation. This limits sample excitation to the axial region projected from the windows  38  and  39 , a region of homogeneity of both the polarizing field and the RF field. As thus described, the prior art may be more closely identified with U.S. Pat. No. 6,008,650 as representative prior art. 
     Turning now to FIGS. 3 a  and  3   b  there is shown the preferred embodiment of the invention wherein the resonator  32  comprises two completely distinct current loops surrounding windows  38  and  39 . One said loop is excited from leads  36  and  37  and the other loop is excited wholly through mutual inductive coupling to the first loop. Balance and electrical symmetry of the two loops is preserved through maintenance of geometrical symmetry: that is, the phantom (floating) leads  34  and  35  of the inductively coupled loop present similar capacity contribution to loop surrounding window  39  as do the leads  36  and  37  to the driven loop surrounding window  38 . In addition to the electrical symmetry thus served, the geometrical symmetry aids in avoiding magnetic inhomogeneties in the polarizing field. 
     The shield structure  34  presents slot  41   a  and slot  41   b  (comprising half slots  41   b ′ and  41   b ″). Further, the capacitance furnished to each loop is adjusted by indented regions  42 . It should be recognized that the slots  41   a  and  41   b  in the shield  34 , together with corresponding slots  40   a  and  40   b  in the resonator form another pair of windows open to the sample. (To preserve nomenclature, the word “slot” is here synonymous with “gap”.) These slot windows are oriented orthogonal to the prevailing azimuthal orientation of the windows  38  and  39 . These slot windows are convenient for double resonance experiments involving a second coaxial resonator disposed externally to resonator  32 . The second resonator illuminates the sample through the aperture formed by the gaps between the loops of resonator  32 , wherein two independent resonance conditions are concurrently available with orthogonal directions for the corresponding RF magnetic fields. 
     FIG. 4 is a perspective view of the resonator and shields of FIGS. 3 a  and  3   b . Corresponding portions of the perspective figure bear the same labels as the planar mappings of FIGS. 3 a  and  3   b.    
     One quantitative measure of the efficacy of the invention is the recovery time after a sharp magnetic impulse such as provided by a rapidly switched magnetic gradient field. FIG. 5 compares the response of the present invention with a prior art resonator intended for a cryogenic environment. This prior art is similar to FIG. 2 a  adapted for cryogenic operation, for example in a Varian “Chili™ type NMR probe as further shown in FIG. 3 d  for performance comparison with FIG. 3 a . The two resonator structures (present invention and prior art), constructed from identical materials are characterized by similar dimensions. It is apparent that the FIG. 3 d  device is a two terminal slotted resonator with unslotted shields. In contrast the inventive device features full separated current loops, slotted shields and only a single loop is driven (mode a of Table 1 below). For this test, the two resonators were each subject to a magnetic field pulse about 1 ms in width supplied by a surrounding gradient coil of about 30 (gauss/cm). The measured time for recovery to 90% of full NMR amplitude for the resonator of this invention (curve  60 ) was (about) 250 μsec compared to about 2.5 ms for the prior art (curve  62 ). It is apparent that the example represents an improved recovery time of about a factor of 10 compared to the representative prior art. 
     Table 1 is a tabular summary illustrating the several modalities supported by the slotted/gap resonator structure of FIG. 2 b  through connection of the terminals  34 , 35  of loop  39 , and terminals  36  and  37  of loop  38 . 
     
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Loop configuration 
                   
                 jumpers 
                 driven terminals 
               
               
                   
               
             
             
               
                 a1) Loop 38 driven, loop 39 floats 
                 RF dipole field 
                 N/A 
                 36 + 37 
               
               
                 or,  
               
               
                 a2) loop 39 driven, loop 38 floats 
                   
                 N/A 
                 34 + 35 
               
               
                 b) loops in series/same helicity 
                 RF dipole field 
                 34→36 
                 37 + 35 
               
               
                 c) loops in series/opposite helicity 
                 RF gradient field 
                 34→37 
                 35 + 36 
               
               
                 d) loops in parallel/same helicity 
                 RF dipole field 
                 34→37, 35→36 
                 jumpers 
               
               
                 e) loops in parallel/opposite helicity 
                 RF gradient field 
                 36→34, 37→35 
                 jumpers 
               
               
                   
               
             
          
         
       
     
     In modes a), terminals  36  and  37  are driven while terminals  34  and  35  float (or vice versa). An homogeneous RF dipole field is excited with axis principally through the (large) windows  38  and  39 . In the mode b, terminal  36  (or  37 ) is excited together with terminal  34  (or  35 ) to produce an RF resonance at a lower frequency than the a) modes corresponding to the larger inductance of the series combination of the conductors. 
     Another operational possibility (d in table 1) is established by exciting both loops in phase by driving the two loops in parallel such that adjacent inductive axial members of the opposite facing loops support instantaneous RF currents in the same sense while circulating on the respective loops to produce the principal resonance. [Capacitive coupling between loops across the gaps  40   a  and  40   b  (at the opposite end from the terminals) supports a circulating RF current component around gaps  40   a    40   b  to produce another resonance usually positioned at much higher frequency than the main resonance.] This choice is illustrated in FIG. 3 a  by the dotted lines representing jumpers  44 . This operation (and any jumpering between terminals) restores (to some extent) a path for eddy currents that was removed by the major slots  40   a  and  40   b  separating the resonator into two distinct loops. It has been found that this operational approach conveniently facilitates maintenance of a deuterium lock where attenuated eddy current effects are tolerable. 
     In another operational mode, the corresponding terminals of the respective loops may be connected serially in opposite helicity, e.g., adjacent axial inductive members of the loops support opposite sense of instantaneous RF currents. Similarly, parallel combination of the loops  38  and  39  in opposite helicity may be selected. These operational modes produce a radial RF magnetic field gradient, of interest in certain specialized experiments. 
     It should be apparent that the several operational modes represented in table 1 would not support resonant operation at the same frequency. For example direct coupling of the coil loops in series will not exhibit the same lumped inductance as where the same loops are connected in parallel. The connection of the opposite facing loops in the same helicity will not present the same mutual inductive coupling as where the same loops are connected in opposite helicity. 
     Turning to FIG. 6, there is shown another embodiment of the invention particularly suited to cryogenic NMR probe structure wherein the RF coil  32  and shields  34  are disposed in a vacuum space at cryogenic temperature. The inner surface of the vacuum enclosure is the outer (radial) surface of a tubular structure  52 , which supports a pair of RF inner shields  54 . These inner shields  54  preferably comprise about one mil Cu, e.g., large compared to the skin depth at operational resonance frequencies. These shield members  54 , centered on window  38  and  39  are axially separated by a distance in the range of 100% to 200% of the axial window dimensions. The length of shields  54  is not critical. These inner shields serve to further reduce excitation of unwanted resonances from the sample region axially displaced from the window region. For the cryogenic case, these unslotted shield members  54  are at an intermediate temperature, typically, close to ambient because a relatively poor conductivity for these shields is desirable to better reduce the attendant eddy currents. Alternately, these innermost shields might be implemented from a relatively poorly conductor, e.g., an alloy, or these shields might be extremely thin. 
     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.