Abstract:
A multiple resonance sample coil for a magic angle spinning nuclear magnetic resonance probe is comprised of a solenoid coil that resonates at low frequencies and a resonator that resonates at high frequencies. The ends of the low frequency solenoid coil are electrically connected to the high frequency resonator to eliminate arcing and allow the solenoid coil to extend the full width of the resonator. In some embodiments, the high frequency resonator is constructed from the outermost turns of the solenoid coil in the form of a birdcage resonator. In another embodiment, the solenoid coil is electrically connected to one turn of the resonator and the other turn is used as part of a trap to shunt the resonator at low frequencies.

Description:
BACKGROUND 
     Nuclear magnetic resonance (NMR) is a physical phenomenon involving quantum mechanical magnetic properties of atomic nuclei in the presence of an applied, external magnetic field. NMR phenomena can be observed with an NMR spectrometer and used to study molecular physics, crystalline and non-crystalline materials. In particular, nuclear spin phenomena can be used to generate a spectrum comprised of a pattern of lines representing the various nuclear spins and spin interactions. 
     In order to perform an NMR experiment, a sample is placed in the external or B 0  magnetic field to create a net magnetization in the sample. A radio-frequency (RF) field or B 1  field is then applied to the sample to rotate the net magnetization in a pulse sequence. Sample coils that surround the sample not only create the B 1  field for the pulse sequence, but also detect the NMR signal from the sample. 
     Single or multiple sample coil combinations can be used. The set of coils must be configured so that, for each nucleus to be observed, a resonance frequency similar to the Larmor frequency of the nucleus is created. Single coils may formed exclusively from wire (a mainly inductive element, which can be used, for example, for broad banded applications) or as a combination of inductive and capacitive elements that form a resonator at a given frequency. Since the presence of the sample affects the resonant frequency of the coils, the resonances have to be tuned for the specific sample being studied in order to achieve the highest signal-to-noise ratios. Another requirement of a sample coil is that the B 1  field produced by the coil must be homogeneous over the volume of the sample. If the B 1  field is not constant, the magnetization will be rotated by a distribution of rotation angles and the resulting spectra will be distorted. 
     NMR experiments can be performed on both liquid and solid samples. Spatial proximity and/or a chemical bond between two atoms can give rise to interactions between the nuclei of the atoms. In general, these interactions are orientation dependent. In an NMR experiment involving a liquid sample, Brownian motion of the molecules and atoms causes an averaging of anisotropic interactions. In such cases, these interactions can be neglected on the time-scale of the NMR experiment. However, in solid samples, for example crystals, powders and molecular aggregates, the anisotropic interactions between nuclei have a substantial influence on the behavior of a system of nuclear spins. In particular, in solid materials, the great number of interactions produces very broad and featureless NMR result lines. However, the interactions are time-dependent and can be averaged by physically spinning the sample (at high rotation speeds up to 80 kHz) at an inclination of the so-called magic angle (54.74°) with respect to the direction of the external B 0  magnetic field. The averaging causes the normally broad lines become narrower, increasing the resolution for better identification and analysis of the spectrum. 
     To perform a magic angle spinning (MAS) nuclear magnetic resonance experiment, a sample is typically packed into a rotor that fits inside the sample coil and is rotated at high speed by an air turbine. The rotor is held in place by air bearings. The entire structure is then inserted into the bore of a high strength magnet. This design places stringent considerations on the sample coil size and location. 
     Due to the very restricted space between the air bearings and the high strength B 1  fields and thus high power requirements, a number of coil designs are used to provide “optimal” performance. With the “best” filling factor in this configuration, a solenoid coil was the coil of choice for some time. In the last decade experiments on biosol id samples have been performed with the drawback of lossy (usually salty) samples that absorb energy and heat the sample while destroying the biomass inside. Several different coils have been developed including a “cross coil” version with some success. These two coil systems consists of two separate coils, one high frequency resonator with a reduced E-field (the E-field causes heating) and one highly efficient solenoid coil for the lower frequencies. 
     To make matters more complicated, many present day experiments require NMR probes with sample coils tuned to several different frequencies so that B 1  energy at these frequencies can be applied simultaneously to the sample or at least applied sequentially without removing the sample from the magnet bore. For example, a typical triple resonant probe might have sample coils tuned to the Larmor frequencies of  13 C,  15 N and  1 H atoms. At a B 0  field strength of 18.8 Tesla, these Larmor frequencies correspond to 200, 80, and 800 MHz, respectively. Due to the large difference in Larmor resonant frequency between the  15 N and  1 H atoms, a two coil “cross coil” structure is generally used to separate the frequencies. Isolation of the three NMR signals generated during the NMR experiment is achieved using different approaches, including rejection traps, geometrically decoupled coils or transmission lines that pass different wavelengths. 
     However, the conventional two coil approach has significant problems with uniformity of the B 1  field inside of the sample coil. More specifically, the high frequency and low frequency coils are not connected together so that a potential difference develops between the ends of the solenoid coil and the Helmholtz coils which can cause arcing. Experiments on solids need high B 1  fields for long time intervals which also increases the chance of arcing in these applications. Therefore, in order to reduce arcing between the coils either the B 1  field strength must be limited and/or a significant space must be left between the ends of solenoid coil and the high frequency resonator. Since the overall size of the coil structure is limited by other factors, the result is that the length of the solenoid coil is reduced.  FIG. 1  is a graph of the B 1  field strength inside of a conventional sample coil configuration. The horizontal axis is the position inside of the coils measured from one in millimeters with the center of the coils occurring at 6.8 mm. The vertical axis indicates the B 1  field strength normalized to the field strength at the center of the coils. The graph represented by the filled diamonds is the B 1  field strength at the  1 H frequency; the graph represented by the hollow squares is the B 1  field strength at the  13 C frequency and the graph represented by the hollow triangles is the B 1  field strength at the  15 N frequency. As can be seen from the graphs, the field strength at the  1 H frequency is relatively flat over the range of 3.3 mm to 10.3 mm. However the  13 C and  15 N field strengths fall off rapidly away from the center position of the coil system due to the restricted length of the coil. In general, a variation of the B 1  field strength of more than ten percent is not tolerable for the reasons discussed above. Therefore, as shown in the figure, the usable area of the coil system extends only from 5.8 mm to 7.8 mm or a total of 2 mm. This usable area severely restricts the sample size. 
     Therefore, there is a need for an improved multiple resonant coil design. 
     SUMMARY 
     In accordance with the principles of the invention, the resonator and the solenoid coil are combined such that, at the ends, both use the same conductive material, i.e. are electrically connected together. This eliminates arcing between the coils and allows the solenoid coil to extend the full width between the high frequency resonator turns. 
     In one embodiment, a single solenoid coil is used to electrically create both a solenoid coil for low frequency B 1  fields and a high frequency resonator in the form of a “birdcage” resonator for the high frequency B 1  field. A plurality of discrete capacitors are connected across the outermost turns of the solenoid coil. The capacitors are connected around the outermost turns of the solenoid coil so that, at high frequency, the low impedance of the capacitors creates the birdcage resonator from sections of the outermost solenoid coils and the capacitors. At low frequency, the high impedance of the capacitors allows the solenoid coil to generate the low frequency B 1  field. 
     In a second embodiment, the solenoid coil is center tapped and split into two sections which are wound in opposing directions. As in the first embodiment, a plurality of capacitors are connected across the outermost turns of the solenoid coil. The capacitors are connected around the outermost turns of the solenoid coil so that, at high frequency, the low impedance of the capacitors creates Helmholtz coils from sections of the outermost solenoid coils. At low frequency, a trap connected across the ends of the solenoid coil shorts the ends of the coil together to effectively remove the capacitors from the low frequency circuit. With the ends of the solenoid coil connected together, the coil sections are connected in parallel, but because the sections are wound in opposing directions, the B 1  field is in the same direction in both sections. The low frequency circuits are connected to the center tap of the solenoid coil. 
     In a third embodiment, similar to the previous embodiment, a center-tapped two-section solenoid coil is used for low frequencies. Instead of capacitors, a high frequency resonator with two turns is used for high frequencies. The solenoid is connected across one turn of the resonator. A capacitor connected across the resonator together with the other turn of the resonator forms the trap of the previous embodiment. As with the previous embodiment, the low frequency circuits are connected to the center tap of the solenoid coil. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a graph of B 1  field strength inside of the sample coil in a conventional sample coil system. 
         FIG. 2  is an electrical circuit schematic in accordance with a first embodiment of the invention. 
         FIG. 3  is a perspective drawing of the physical components of the circuit shown in  FIG. 2  mounted on a convention ceramic base. 
         FIG. 4  is an electrical circuit schematic in accordance with a second embodiment of the invention. 
         FIG. 5  is a perspective drawing of the physical components of the circuit shown in  FIG. 4  mounted on a convention ceramic base. 
         FIG. 6  is a plan view of the underside of the ceramic base shown in  FIG. 5  illustrating a trap inductor and capacitor. 
         FIG. 7  is an electrical circuit schematic in accordance with a third embodiment of the invention. 
         FIG. 8  is a perspective drawing of the physical components of the circuit shown in  FIG. 7  mounted on a convention ceramic base. 
         FIG. 9  is a is a graph of B 1  field strength inside of the sample coil in the embodiments shown in  FIGS. 7 and 8 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  shows a circuit schematic of a circuit that is useful for double resonance applications that require both a  1 H frequency signal and a lower frequency resonance signal, such as  15 N resonance signal, which signal is indicated as “X” in  FIG. 2 .  FIG. 3  shows a perspective view of an exemplary arrangement of the respective physical circuit parts for the circuit shown in  FIG. 2 . In these figures L sample  is the solenoid coil. C 1 , C 2 , C 3  and C 4  are identical capacitors, illustrated in  FIG. 3  as chip capacitors, which are connected across the solenoid coil L sample . The angle between the capacitor wires at the point where they connect to solenoid coil outermost turns can be optimized for homogeneity or strength or a combination of both. As an example, the wires could be connected around the outermost turn at points  208 ,  206 ,  220  and  210  at 60°, 120°, 180° and 300°, respectively, but other angles can also be used. Similarly, the capacitors are connected to the opposing outermost coil of L SAMPLE  at points  212 ,  218 ,  216  and  214 . In addition, although four capacitors are shown in the figure, more than four connections can be used for improved homogeneity. 
     At the  1 H frequency the capacitors form low impedance paths across the solenoid coil L SAMPLE  so that only portions of the outmost turns of the solenoid coil are part of the  1 H circuit. At the  1 H frequency, the capacitors (and the connecting wires) and the portions of the outmost turns of the solenoid coil form a bird cage resonator. The high inductance of the solenoid coil effectively “stops” the  1 H frequency so that the solenoid coil is not part of the  1 H circuit. 
     Therefore, the  1 H circuit is formed from portions of the outermost turns of the sample coil L sample , capacitors C 1 , C 2 , C 3  and C 4  and capacitors C TH , C H  and C MH . The values of capacitors C 1 , C 2 , C 3  and C 4  are chosen so that the self-resonance frequency of the circuit including the sample coil portions plus the capacitors is high enough to tune the circuit to the  1 H frequency, but low enough to maximize the  1 H channel efficiency. Capacitor C H  is a fixed value capacitor used to adjust the  1 H frequency for matching. Capacitor C TH  is an adjustable  1 H tuning trimmer capacitor and capacitor C MH  is an adjustable matching trimmer capacitor. The 1H frequency is taken from the terminal  1 H. 
     At the low frequency, the capacitors C 1 , C 2 , C 3  and C 4  are effectively open and the low frequency circuit consists of the entire inductance of the sample coil L sample , and capacitors C TX , C X  and C MX . Capacitor C X  is a fixed value capacitor used to adjust the X frequency. Capacitor C TX  is an adjustable X frequency tuning trimmer capacitor and capacitor C MX  is an adjustable matching trimmer capacitor. The X frequency is taken from the terminal X. 
     The low X frequency circuit is isolated from the  1 H frequency circuit by traps  200  and  202 . Each trap consists of a small half-turn inductor L TRAPH  connected in parallel with a capacitor C TRAPH . This trap circuit resonates at the  1 H frequency and isolates the X-channel circuit. 
       FIG. 3  shows an illustrative arrangement of the circuit components shown in  FIG. 2  on a conventional ceramic base  300  of the type used in magic-angle spinning NMR experiments. The base  300  has four feedthroughs  302 - 308 . The end  310  of sample coil L SAMPLE  is connected to feedthrough  308  by lead  312  and to feedthrough  306  by lead  314 . Similarly, end  316  of sample coil L SAMPLE  is connected to feedthrough  302  by lead  318  and to feedthrough  304  by lead  320 . The remainder of the circuit components (not shown in  FIG. 3 ) are connected to the underside of the feedthroughs  302 - 308 . For example, capacitors C H  and C MH  and the  1 H terminal might be connected to terminal  308  and trap  202 , capacitors C TX , and C MX  and the X frequency terminal might be connected to feedthrough  306 . Similarly, capacitor C TH  can be connected to terminal  302  and trap  200  and capacitor C X  can be connected to terminal  304 . 
     Since both modes (the  1 H bird cage mode and the low frequency solenoid mode) share the last turn together, the solenoid coil L SAMPLE  can be made considerably longer than is possible with the conventional two-coil system and is limited by the physical constraints introduced by the magic angle spinning system. The circuit shown in  FIGS. 2 and 3  is effective for high and low B 1  field frequencies such as  1 H and  15 N. However, the capacitors C 1 , C 2 , C 3  and C 4  lower the self resonance of the coil L SAMPLE  at low frequencies so that self resonance at an intermediate frequency, such as the  13 C frequency, cannot be obtained with reasonable efficiency. 
       FIG. 4  shows a circuit schematic of a second embodiment of the invention that is useful for triple resonance applications. The most frequently used application requires a  1 H frequency signal, a  13 C frequency signal and a  15 N resonance signal, but the circuit can be used for other triple resonance combinations of single frequency nuclei.  FIGS. 5 and 6  shows a perspective view and a bottom view of an exemplary arrangement of the respective physical circuit parts. In these figures L sample  is a center tapped solenoid coil in which the two coil sections are wound in opposing directions. 
     For the 1H circuit, the capacitors C 1 , C 2 , C 3  and C 4  and the sample coil L SAMPLE  are connected, and function, in the same manner as described with respect to  FIG. 2 . Therefore, the  1 H circuit is formed from portions of the outermost turns of the sample coil L sample , capacitors C 1 , C 2 , C 3  and C 4  and capacitors C TH , C N  and C MH  and operates as described with respect to  FIG. 2 . 
     The center tapped solenoid is operated as two parallel coils from the center tap to the outside ends. As such, the both sides of the coil are on the same or similar potential which eliminates the effects of the capacitors C 1 , C 2 , C 3  and C 4  on the self resonant frequency of the coil L SAMPLE  at the lower frequencies. Since the two outer leads of the tapped solenoid must be connected for parallel operation this would generate a short at the  1 H frequency. The inductor L TRAPHS  and capacitor C TRAPHS  thus form a  1 H trap connected across the sample coil L SAMPLE  so that the ends of the sample coil are effectively shorted together for frequencies in the  13 C and  15 N channels but are not shorted at the  1 H frequency. With the ends of the solenoid coil connected together, the coil sections are connected in parallel, but because the sections are wound in opposing directions, the B 1  field is in the same direction in both sections. The low frequency circuits are connected to the center tap of the solenoid coil so that the self resonance frequency of the sample coil for the  13 C and  15 N channels is determined by the capacitor C CS  and the full inductance of the sample coil L SAMPLE  and is chosen to be close to the  13 C frequency. The low frequency circuits are isolated from the  1 H frequency circuit by traps  400  and  402 . Each trap consists of a small half-turn inductor L TRAPH  connected in parallel with a capacitor C TRAPH . The trap circuits resonate at the  1 H frequency and isolate the  13 C and  15 N channel circuits. 
     The  13 C frequency circuit consists of the sample coil L sample , and capacitors C TC , and C MC . Capacitor C TC  is an adjustable  13 C frequency tuning trimmer capacitor and capacitor C MN  is an adjustable matching trimmer capacitor. The  13 C frequency is taken from the terminal  13 C. 
     The  15 N frequency circuit consists of the sample coil L sample , and capacitors C TN , and C MN . Capacitor C TN  is an adjustable  15 N frequency tuning trimmer capacitor and capacitor C MN  is an adjustable matching trimmer capacitor. The  15 N frequency is taken from the terminal  15 N. Inductor L N  provides a ground path for the  15 N channel. However, since it is in parallel with the  13 C frequency tuning trimmer capacitor C TC  it also affects the  13 C tuning and efficiency. Therefore, the value of inductor L N  must be chosen with consideration of the tuning and efficiency of both the  13 C and  15 N channels. 
       FIG. 5  shows an illustrative arrangement of the physical circuit components shown in  FIG. 4  on a conventional ceramic base  500  and  FIG. 6  shows a bottom view of the base  500 . The base  500  has four feedthroughs  502 - 508 . The end  510  of sample coil L SAMPLE  is connected to feedthrough  508  by lead  512 . Similarly, end  514  of sample coil L SAMPLE  is connected to feedthrough  502  by lead  516 . The center tap  518  of coil L SAMPLE  is connected to feed through  506  by lead  520 . The capacitor C CS  is shown as two chip capacitors connected together, but may be a single capacitor. The remainder of the circuit components (not shown in  FIG. 5 ) are connected to the underside of the feedthroughs  502 - 508 . For example, capacitors C H  and C MH  and the  1 H terminal might be connected to terminal  508  and trap  402 , capacitors C TN , and C MN  and the  15 N frequency terminal might be connected to feedthrough  506 . Similarly, capacitor C TH  can be connected to terminal  504  and trap  400 , capacitors C MC  and C TC  and inductor L N  can be connected to terminal  502 .  FIG. 6  shows the underside of base  500  illustrating the trap formed by inductor L TRAPHS  and capacitor C TRAPHS . 
       FIG. 7  shows a circuit schematic of a third embodiment of the present invention which is a simplified version of the circuit illustrated in  FIG. 4 . This circuit can also be used for triple resonance applications, such as  1 H,  13 C and  15 N resonances. Similar to the previous embodiments, this circuit uses both a solenoid coil and a two-turn high frequency resonator. The turns  704  and  706  in a Helmholtz configuration are only interrupted once by capacitors rather than twice as in the previous embodiments. In this configuration, the inductance of the two parallel turns  704  and  706  forms the inductance L HS . Since the outer ends of the center tapped solenoid are now connected by wire only (capacitors C 2  and C 1  have been effectively removed), the  1 H trap consists of the inductance L TRAPHS ; capacitor C TRAPS  is no longer needed. If the connection and turn direction are properly chosen, almost one turn can be gained since in this case, the solenoid uses the last coil from the outermost coil (in contrast to the embodiments discussed above where the  1 H resonator used the outermost turns from the solenoid). 
     In the circuit shown in  FIG. 7 , the capacitance values of capacitors C 1 , C 2 , C 3  and C 4  shown in  FIG. 4  have been combined into the capacitance value of capacitor C HS  shown in  FIG. 7 . Inductor L HS  and capacitor C HS  form a  1 H frequency coil resonator. The  1 H frequency coil resonator is connected across the sample coil L SAMPLE  so that the ends of the sample coil are effectively shorted together for frequencies in the  13 C and  15 N channels but are not shorted at the  1 H frequency. 
     The sample coil L SAMPLE  is a center tapped sample coil in which the two coil sections are wound in opposing directions which functions in the same manner as discussed above. The ends of coil L SAMPLE  are connected to the ends of resonator turn  704 . The capacitor C CS  is connected to the center tap of sample coil L SAMPLE  and the center of inductor L HS . Capacitor C CS  and the sample coil L SAMPLE  determine the self-resonance frequency of the sample coil for the  13 C and  15 N channels. This self-resonance frequency is usually set to be close to the  13 C resonance frequency. The remaining circuit components have the same functions as the corresponding components shown in  FIG. 4 . 
       FIG. 8  physical circuit components shown in  FIG. 7  on a conventional ceramic base  800 . The base  800  has four feedthroughs  802 - 808 . The end  810  of sample coil L SAMPLE  and the end  812  of the inductor L HS  are connected to feedthrough  808  by flange  818 . Similarly, end  814  of sample coil L SAMPLE  and the end  816  of the inductor L HS  are connected to feedthrough  804  by a similar flange (not shown in  FIG. 8 ). The center tap of sample coil L SAMPLE  is connected to a feedthrough (both not shown in  FIG. 8 ). The capacitor C HS  is shown as three chip capacitors connected together, but may be a single capacitor and is connected to the ends of inductor L HS . The inductor L HS  has a center tap connected to feedthrough  802 . The remainder of the circuit components (not shown in  FIG. 8 ) are connected to the underside of the feedthroughs  802 - 808 . For example, capacitors C H  and C MH  and the  1 H terminal are connected to terminal  808  and trap  702 , capacitors C TN , and C MN  and the  15 N frequency terminal might be connected to feedthrough that is connected to the center tap of sample coil L SAMPLE . Similarly, capacitor C TH  can be connected to terminal  804  and trap  700 , capacitors C MC  and C TC  and inductor L N  can be connected to terminal  802 . 
       FIG. 9  is a graph on the B 1  field strength inside of the coil embodiment shown in  FIGS. 7 and 8 . The horizontal axis is the position inside of the coils measured from one in millimeters with the center of the coils occurring at 6.8 mm. The vertical axis indicates the B 1  field strength normalized to the field strength at the center of the coils. The graph represented by the filled diamonds is the B 1  field strength at the  1 H frequency; the graph represented by the hollow squares is the B 1  field strength at the  13 C frequency and the graph represented by the hollow triangles is the B 1  field strength at the  15 N frequency. As can be seen from the graphs, the field strength at the  1 H frequency is relatively flat over the range of 4.3 mm to 9.3 mm. Therefore, as shown in the figure, the usable area of the coil system extends from 4.3 mm to 9.3 mm or a total of 5 mm which is approximately two and one half times the usable area of a conventional coil. 
     Although the inventive configurations have been shown for use with magic angle spinning NMR probes, the inventive design can also be used with static probes that do not use magic angle spinning. In addition, the number, form and position of the connecting wires can vary dependent on application, available space and performance. 
     While the invention has been shown and described with reference to a number of embodiments thereof, it will be recognized by those skilled in the art that various changes in form and detail may be made herein without departing from the spirit and scope of the invention as defined by the appended claims.