Abstract:
A nuclear magnetic resonance (NMR) probe circuit is used with a sample coil tuned to a primary frequency f 1 . The circuit is arranged to have a plurality of points of electric field minima at the f 1  frequency. One or more additional frequencies may be coupled to the circuit at these points, without interaction with f 1 . The probe circuit also uses an impedance coupled between two of the minima points that affects the frequency response at the additional frequency or frequencies, without affecting the frequency response at f 1 . The impedance may be made adjustable to allow tuning of the relative frequency resonances.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates generally to the field of nuclear magnetic resonance (NMR) spectroscopy and, more particularly, to an NMR probe circuit having multiple simultaneous resonant frequencies.  
       BACKGROUND OF THE INVENTION  
       [0002]     In the field of NMR spectroscopy, a sample is surrounded by an NMR probe that consists of a radio frequency (RF) coil tuned to generate a field at a desired excitation frequency and receive a return NMR signal. More complex probes will generate multiple frequencies so as to excite the nuclei of more than one different element in the sample (e.g., hydrogen nuclei  1 H (proton) and fluorine nuclei  19 F). These “double resonance” probes (in the case of a probe generating two separate frequencies) and “triple resonance” probes (in the case of a probe generating three separate frequencies) have been used for many years, with varying degrees of success. One of the problems faced by multiple resonance probes arises when trying to adjust the response at one frequency without disturbing that of another.  
         [0003]     In systems having a single sample coil, it is necessary to generate each of the desired resonant frequencies and apply them to the coil, and some form of frequency isolation is incorporated into the circuits themselves. Transmission line resonators have been used to produce high Q resonances with high power handling for NMR probes, particularly at high Larmor frequencies such as  1 H and  19 F. These resonators have nodes at which the electric field is at a minimum, and at these locations circuitry for lower nuclei resonances can be added without affecting the high frequency resonances. This allows a single sample coil to be used to excite an NMR sample at several isolated frequencies, as opposed to using double orthogonal coils to prevent mutual coupling between the resonances. A single sample coil has the advantage of improved sensitivity with higher filling factor and better power handling without inter-coil arcing. However, use of a single sample coil also has the disadvantage of efficiency tradeoffs between the high frequency and low frequency channels.  
         [0004]     In existing systems, the resonant frequency of the sample coil determines the trade off between the efficiency of the high frequency (such as  1 H) and low frequency (such as  13 C or  15 N) channels. Increasing the self resonance of the sample coil, either by reducing its inductance or capacitance, improves  1 H efficiency while degrading the efficiency of a lower frequency. Decreasing the self-resonance of the sample coil has the opposite effect.  
       SUMMARY OF THE INVENTION  
       [0005]     In accordance with the present invention, a nuclear magnetic resonance probe has an inductive sample coil that is resonant at a frequency f 1 . A resonator circuit is electrically connected to the sample coil, and has a plurality of points of electric field minima for an RF signal at the frequency f 1 . Thus, if f 1  is a relatively high frequency, such as that used for exciting hydrogen ( 1 H) nuclei, the sample coil may be resonant at f 1 , and the resonator circuit may be arranged so as to provide accessible minima points for f 1 . Such a resonator circuit may, for example, make use of transmission lines, such as quarter-wave or half-wave transmission lines, to create the desired minima points. An input port may be connected to one of the minima points of the resonator circuit so as to allow the connection of an electrical signal at a second frequency, f 2 . At this point of insertion of the f 2  signal, there is no interaction with the f 1  signal, since it is an electrical iso-point for f 1 . The probe also includes an impedance located between two of the minima that affects the frequency response of f 2 , but has no effect on f 1 .  
         [0006]     The impedance located between the minima points may be adjustable, allowing the frequency response of f 2  to be adjusted. This may be an adjustable capacitor or an adjustable inductor, or some combination of capacitors and/or inductors with one or more adjustable components. Whether or not it is adjustable, the impedance may include a parallel combination of at least one capacitor and one inductor, or a series combination of at least one capacitor and one inductor, each of which will have a different effect on the frequency response of f 2 , while still having no effect on the frequency response of f 1 .  
         [0007]     The impedance that is connected between two minima may be electrically in parallel with the sample coil, and the resonator circuit may be balanced such that an electric field minimum for f 1  is located at the center of the sample coil. The invention also provides for the introduction of more that one additional resonant frequency to the probe circuit and, like f 2 , an additional frequency may be introduced at a minima point for f 1 . Thus, while the sample coil may be tuned to f 1  (possible a high frequency, such as  1 H), two additional frequencies may be added to create a triple resonance probe. In such a case, the input port mentioned above may be used to introduce a first of these additional frequencies to the resonator circuit at a minima point, and a second input port may also be used to introduce another signal at a different frequency, f 3 . With an impedance connected between two of the minima points, the impedance would have an effect on the resonant frequency of both f 2  and f 3 , while not disturbing f 1 . If the impedance is adjustable, it can be used to simultaneously change the frequency response at both f 2  and f 3 , without affecting f 1 . 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The above and further advantages of the invention may be better understood by referring to the following description in conjunction with the accompanying drawings in which:  
         [0009]      FIG. 1  is a schematic view of a probe circuit according to the present invention;  
         [0010]      FIG. 2  is a schematic view of a probe circuit like that shown in  FIG. 1 , but showing specific transmission line and adjustable capacitor components; and  
         [0011]      FIG. 3  is a schematic view of a probe circuit like that of  FIG. 2 , but using an adjustable inductor rather than an adjustable capacitor. 
     
    
     DETAILED DESCRIPTION  
       [0012]     Shown in  FIG. 1  is a block diagram depicting the general architecture of a probe circuit according to the present invention. A conventional sample coil  10  is in close proximity to a sample location, and provides the NMR excitation signal as well as detects the sample response. Connected to opposite sides, respectively, of the sample coil are impedances Z 1  and Z 3 , and Z 2  and Z 4 . These impedances are connected in series with the sample coil to produce electric field maxima on either end of the coil, and mimina at the center of the coil. This symmetry provides a balanced magnetic field maximum in the center of the sample coil to excite the sample.  
         [0013]     With appropriate selection of circuit elements or resonators, electric field minima at a primary frequency can also be created at points between the impedances Z 1  and Z 3  and between the impedances Z 2  and Z 4 . These points are labeled in  FIG. 1  as point A and point B, respectively. For the primary frequency in question, points A and B represent electrical iso-points, or points of equi-phase and equi-potential for those frequencies. These points can therefore be used to connect additional channels to the probe circuit to allow excitation of the sample at those other frequencies via sample coil  10 .  
         [0014]     In practice, Z 1  and Z 2  may be realized using quarter wavelength (or odd multiples of it) transmission lines relative to the primary frequency. Z 3  and Z 4  may also be realized using quarter wavelength (or odd multiples of it) transmission lines with open-ended terminations at the ends opposite points A and B, as well as by using half wavelength (or multiples of it) transmission lines relative to the primary frequency, with ground terminations at the ends opposite point A and B. Of course, all of these impedances may also be formed using other types of components that are electrically equivalent to the transmission lines.  
         [0015]     Since points A and B represent electric field minima for the primary resonant frequency of the coil, the potential difference between point A and point B is zero for that frequency. In the present invention an additional impedance Z 5  is located between these points to affect the response at the frequencies of the additional channels. Since this additional impedance has no effect on the response for the primary frequency, it allows modification of the response for the secondary frequencies without adversely affecting the primary frequency. Z 5  may be a capacitor or inductor, or any combination of circuit components, and may include tuning elements such as a trimmer, depending on the desired response of the additional channels.  
         [0016]     Shown in  FIG. 2  is an example of the present invention in which an extra high frequency signal is added to the probe circuit. This circuit may be for a triple tuned NMR probe that excites, for example, the nuclei  1 H,  13 C and  15 N. In such a circuit, the  1 H nucleus has a Larmor frequency that is approximately four times higher than that for  13 C, and approximately ten times higher than that for  15 N.  
         [0017]     As shown in the figure, the sample coil is connected in series with quarter wavelength resonators  20  and  24 , each of which is tuned to the  1 H frequency. These resonators are connected, respectively, to quarter wavelength resonators  22  and  26 . Resonators  22  and  26  are also tuned to the  1 H frequency and have open-ended terminations. In practice, some of the resonator lengths may be absorbed by the sample coil and must be compensated for by adjusting the physical lengths of the lines. With the arrangement shown in  FIG. 2 , the transmission lines create a standing wave at the  1 H frequency that has electrical field maxima at the top end of resonators  20  and  24  and at the bottom end of resonators  22  and  26 . Corresponding electric field minima are created at the center of the sample coil  10  and at the electric field nodal points A and B located, respectively, between resonators  20  and  22  and between resonators  24  and  26 . These nodal points allow the addition of the  13 C and  15 N channels without affecting the  1 H resonance. For example, the  13 C frequency could be inserted at point A, while the  15 N frequency is inserted at point B.  
         [0018]     The symmetry of the sample coil and resonators  20 ,  22 ,  24 ,  26  generates a balanced magnetic field maximum in the center of the sample coil where the sample is excited. The resonant frequency of the sample coil determines a tradeoff between the efficiency of the high frequency (in this case,  1 H) and the low frequency (in this case,  13 C and  15 N) channels. Increasing the self-resonance of the sample coil, by reducing its inductance or capacitance, improves the  1 H efficiency, while degrading the  13 C and  15 N efficiency. Decreasing the self-resonance of the sample coil would have the opposite effect.  
         [0019]     The tradeoff discussed above is minimized in the present invention by the use of an impedance component between the nodal points A and B. In the example shown in  FIG. 2 , the impedance is provided by adjustable capacitor  28 . Since the points A and B are electric field minima for the  1 H frequency, the potential across the capacitor  28  for the  1 H frequency is zero. Thus, the addition of this impedance has no effect on resonant frequency of the probe circuit at the  1 H frequency, and the efficiency of  1 H frequency signal remains high. However, for the lower frequency nuclei  13 C and  15 N, the use of the capacitor  28  in parallel with the sample coil gives the circuit a lower resonant frequency. That is, the resonant frequency appears lower to the  13 C and  15 N frequencies, despite the fact that the sample coil is unchanged. Thus, the use of the impedance  28  increases the efficiency at the lower frequencies, without detrimentally affecting the efficiency at the  1 H frequency.  
         [0020]     In the embodiment of  FIG. 2 , a tunable capacitor is used. One advantage of using a tunable impedance is that is makes it convenient to find the appropriate value to maximize the  13 C efficiency. Increasing the capacitance provided by capacitor  28  lowers the self-resonance of the sample coil/capacitor combination, thereby improving the efficiencies at both the  13 C and  15 N frequencies. However,  13 C has a higher Larmor frequency than  15 N, and continuing to increase the capacitance value of capacitor  28  would eventually make the resonant frequency of the sample coil/capacitor combination lower than the Larmor frequency of  13 C. At this point, the efficiency of  13 C begins to degrade (although the efficiency of  15 N would continue to increase until its own Larmor frequency was reached).  
         [0021]     A tunable impedance between the nodes A and B of the  FIG. 2  embodiment allows for the relative adjustment of both low frequency channels simultaneously. An example of when this might be useful is when a sample with high dielectric constant is inserted within the sample coil and lowers the resonant frequency of the sample coil too much. The adjusting of the additional impedance could then be used to increase the frequencies of both resonances, which is particularly useful if individual tuning elements of those resonances were limited in range. If, for example, the impedance element is a tunable capacitor like that shown in the figure, the capacitance value of capacitor  28  could be lowered to increase the frequencies of the low frequency channels.  
         [0022]      FIG. 3  shows an alternative embodiment of the invention in which an adjustable inductor  38  is used as an impedance between two nodal points A and B. This example depicts a probe circuit for which the efficiency of a low frequency channel is to be maximized. In  FIG. 3 , each of the components  30 ,  32 ,  34 ,  36  is a quarter wavelength transmission line tuned to the wavelength of the low frequency channel. This, again, produces nodal points at (A and B, respectively) between resonators  30  and  32  and between resonators  34  and  36 . A high frequency channel that is not an odd multiple of the low frequency can be inserted at either of the points A and B without affecting the low frequency resonance or efficiency. In order to increase the frequency of the high frequency channel, the inductor  38  can be adjusted accordingly.  
         [0023]     It is also possible to use an impedance between the nodal points A and B that is made up of a combination of components. Referring again to  FIG. 1 , the impedance Z 5  could be, for example, a parallel combination of a capacitor and an inductor. Assuming that the quarter wavelength resonators Z 1 , Z 2 , Z 3  and Z 4  are tuned to the high frequency resonance, e.g., that of  1 H, the nodal points A and B will be electrical iso-points at the  1 H frequency. Two lower frequencies, such as  15 N and  13 C could be input at  1 H iso-points, and could be affected by the Z 5  impedance. For example, if the self-resonance tuning of Z 5  was to a frequency between that of  13 C and  15 N, Z 5  would appear as a capacitor to the  13 C channel, and as an inductor to the  15 N channel. This would have the effect of reducing the resonant frequency of the  13 C channel, while increasing the resonant frequency of the  15 N channel. In essence, to any channel that is below the self-resonance frequency of Z 5 , it will appear as an inductor, and to any channel that is above the self-resonance frequency of Z 5 , it will appear as a capacitor.  
         [0024]     In another example, the resonance Z 5  could be a series combination of an inductor and a capacitor. In such an arrangement, any channel having a frequency lower than the self-resonance frequency of Z 5  will see Z 5  as a capacitor, and any channel having a frequency above the self-resonance frequency of Z 5  will see Z 5  as an inductor. Thus, in the case of two additional frequencies being  13 C and  15 N, if Z 5  has a self-resonance frequency between  13 C and  15 N, it will increase the resonant frequency of the  13 C channel while reducing the resonant frequency of the  15 N channel.  
         [0025]     A variety of other impedance networks could be used a Z 5 , including a series of trap together with a number of capacitors and inductors. Other such arrangements are presumed to be within the scope of the invention. Indeed, those skilled in the art will recognize other types of impedance arrangements that may be connected to electrical iso-points for one particular frequency, so as to affect the other channel frequencies, while having no effect on the particular frequency.  
         [0026]     While the invention has been shown and described with reference to preferred embodiments thereof, it will be recognized by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.