Abstract:
A probe head for nuclear magnetic resonance measurements is disclosed in which at least a first kind of nuclei with a first, higher resonance frequency and a second kind of nuclei with a second, lower resonance frequency are excited within a magnetic field. The probe head comprises a first input/output terminal for the higher resonance frequency and a second input/output terminal for the lower resonance frequency. A measuring coil cooperates with a sample. The measuring coil has a first terminal end and a second terminal end. The first terminal end is coupled to the first input/output terminal and the second terminal end is coupled to the second input/output terminal. A stop circuit tuned to signals of the higher resonance frequency is arranged between the second terminal end and the second input/output terminal. The stop circuit, further, comprises a line having a length equalling a quarter wave length of the higher resonance frequency. The first line is arranged in series with the measuring coil.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention is related to the field of nuclear magnetic resonance (NMR). More specifically, the invention is related to probe heads or sample heads as are used for conducting NMR measurements in which at least two distinct kinds of nuclei are excited, for example with one kind of nuclei being observed while the other kind of nuclei is saturated.  
       BACKGROUND OF THE INVENTION  
       [0002]     U.S. Pat. specification No. 5,229,724 in  FIG. 1A  discloses a probe head for NMR measurements in which at least a first kind of nuclei, namely protons ( 1 H) with a first, higher resonance frequency and a second kind of nuclei, for example  15 N or  31 P (X) with a second, lower resonance frequency are excited within a magnetic field. The probe head comprises a first input/output (I/O) terminal for feeding a signal of the  1 H resonance frequency so as to excite  1 H nuclei and to receive, resp., a resonance signal emitted by the  1 H nuclei. A second I/O terminal is also provided for feeding a signal of the X resonance frequency so as to excite X nuclei and to receive, resp., a resonance signal emitted by the X nuclei. A measuring coil within the probe head cooperates with a sample. It may surround the sample or be applied to a surface thereof. The measuring coil has a first terminal end and a second terminal end. The first terminal end is coupled to the  1 H I/O terminal and the second terminal end is coupled to the X I/O terminal. A stop circuit is tuned to signals of the  1 H resonance frequency and is arranged between the second terminal end and the X I/O terminal. The stop circuit comprises a coaxial line having a length equalling a quarter wave length λ H /4 of the  1 H resonance frequency.  
         [0003]     This prior art, hence, utilizes a λ H /4 line on the X side of the measuring coil to act as a  1 H stop, with one end of the λ H /4 line connected to the X side of the measuring coil and the other end being open. The X side also connects to the X I/O terminal. The λ H /4 line is, therefore, arranged transversely thereto.  
         [0004]     This prior art probe head, therefore, has a first disadvantage that the measuring coil is operated non-symmetrically. The X end of the measuring coil to which the λ H /4 line is connected, is namely “cold” for the  1 H frequency because the λ H /4 line acts as a short. In contrast, the other end of the measuring coil that is connected to the  1 H I/O terminal is “hot” for the  1 H frequency. This non-symmetry results in inhomogeneities of the high frequency magnetic field within the measuring coil.  
         [0005]     A second disadvantage of this prior art probe head consists in that a capacitor is provided directly at the “cold” end of the measuring coil. This capacitor is, hence, directly exposed to the temperature of the measuring coil which may vary within broad ranges when the sample is brought to varying measuring temperatures by means of an appropriate variable temperature control unit. At high temperatures, however, the breakdown voltage or, speaking in more general terms, the rating, in particular the power rating of capacitors goes down. On the other hand, in the field of NMR it is always desired to make measurements at radio frequency power levels being as high as possible. For example, when measurements are made in the area of 0.5 kW, this power level corresponds to a peak voltage of 5 kV at a measuring frequency of 800 MHz or, via the gyromagnetic ratio of the particular kind of nuclei involved, to a magnetic field amplitude of between 100 and 200 kHz.  
         [0006]     A third disadvantage of this prior art probe head consists in that the λ H /4 line in its orientation transverse to the X signal line results in a construction with a considerable radial dimension. For so-called “wide bore” applications this may be acceptable because there is enough space available within the bore of the cryostat of a superconducting magnet. However, for other applications with a narrow bore, large radial dimensions of a probe head are prohibitive.  
         [0007]     In a scientific article of Martin, R. et al. entitled “Design of a triple resonance magic angle spinning probe for high field solid state nuclear magnetic resonance” in Review of Scientific Instruments, Vol. 74, page 3045 (2003) a probe head is disclosed (see  FIG. 1 ( a )) in which a coaxial line, arranged within the X signal line, is directly connected to the lower frequency X side of the measuring coil. However, the  1 H stop in this prior art probe head is also configured as a further coaxial line being likewise directly connected to the X side of the measuring coil and transversely to the X signal line. This further coaxial line likewise transforms its open free end as a  1 H short or node, resp., having impedance zero (“cold”) to the X side of the measuring coil. In contrast, on the opposite  1 H side of the coil, the impedance for  1 H is infinite (“hot”). The measuring coil is, therefore also operated asymmetrically.  
         [0008]     A similar arrangement is described in a scientific article of Cross, V. R. et al. in J. Am. Chem. Soc., Vol. 98, page 1031 (1976).  
         [0009]     It is, therefore, an object, underlying the invention to improve a probe head of the type specified at the outset such that the afore-mentioned disadvantages are overcome. It is a further object underlying the invention to provide a probe head in which the measuring coil can be operated symmetrically. According to another object, the probe head shall be insensitive to high variations in temperature, in particular when variable temperature control units are utilized. Still one more object consists in that the probe head shall have small radial dimensions.  
       SUMMARY OF THE INVENTION  
       [0010]     The afore-mentioned object is achieved by a probe head for nuclear magnetic resonance measurements in which at least a first kind of nuclei with a first, higher resonance frequency and a second kind of nuclei with a second, lower resonance frequency are excited within a magnetic field, the probe head comprising: 
        a) a first input/output (I/O) terminal for feeding a signal of the first, higher resonance frequency for exciting nuclei of the first kind of nuclei and for receiving, resp., a resonance signal emitted by the nuclei of the first kind of nuclei;     b) a second I/O terminal for feeding a signal of the second, lower resonance frequency for exciting nuclei of the second kind of nuclei and for receiving, resp., a resonance signal emitted by the nuclei of the second kind of nuclei;     c) a measuring coil cooperating with a sample, the measuring coil having a first terminal end and a second terminal end, the first terminal end being coupled to the first I/O terminal and the second terminal end being coupled to the second I/O terminal;     d) a first stop circuit tuned to signals of the higher resonance frequency of the first kind of nuclei, the first stop circuit being arranged between the second terminal end and the second I/O terminal, the first stop circuit, further, comprising a first line having a length equalling a quarter wave length of the higher resonance frequency of the first kind of nuclei, the first line being arranged in series with the measuring coil.        
 
         [0015]     The object underlying the invention is thus entirely solved.  
         [0016]     The invention makes it possible to locate the  1 H stop as close as possible to the measuring coil, for example very close to a MAS rotor (Magic Angle Spinning). Although at that location the  1 H stop is exposed to highly varying temperatures in the event of experiments involving changing the sample temperature by means of a variable temperature control unit, this is far less critical for e.g. coaxial lines as compared to discrete components, in particular capacitors. The arrangement, thus, becomes frequency-stable and may be exposed to high loads. In applications with a MAS rotor, where the angle must be adapted to be adjusted, the invention provides for lower mechanical loads during adjustment as compared to the prior art, in particular in the area of the soldering joints. This holds true also for high  1 H measuring frequencies of e.g. 900 MHz.  
         [0017]     Moreover, the invention allows to design the probe head with small radial dimensions, because the components are also physically arranged in series.  
         [0018]     Last but not least the probe head according to the invention may be operated symmetrically.  
         [0019]     In a preferred embodiment of the invention, the first line has a first and a second end, the second end being arranged opposite the measuring coil and being connected to a wave trap circuit generating a constrained oscillation node of the signal of the higher resonance frequency at the second end.  
         [0020]     This measure has the advantage that the “hot” point is located at the opposite end of the stop and, hence, that the symmetrical excitation of the measuring coil may be guaranteed.  
         [0021]     In another embodiment of the invention, a second line is arranged between the first terminal end of the measuring coil and the first I/O terminal, the second line having a length equalling one half wavelength (λ H /2) of the higher resonance frequency of the first kind of nuclei ( 1 H).  
         [0022]     This measure, known per se, has the advantage that the measuring coil may be excited effectively.  
         [0023]     In that case it is particularly preferred when the second line is configured as a coaxial line having an inner conductor and an outer conductor, the inner conductor and the outer conductor being connected with each other at a middle of the second line via a capacitance.  
         [0024]     This measure has the advantage that also the  1 H side contributes to the symmetrical excitation of the measuring coil.  
         [0025]     This advantage becomes still more apparent when the outer conductor of the first line has two ends and is connected to ground at the two ends.  
         [0026]     In another embodiment of the invention the second line is configured as a coaxial line having an inner conductor and an outer conductor, the inner conductor and the outer conductor being connected with each other at a middle of the second line via a capacitance and a second stop circuit, the second stop circuit being tuned to signals of the lower resonance frequency of the second kind of nuclei (X).  
         [0027]     This measure, too enables an improved symmetry for the excitation of the measuring coil, wherein it is likewise true in this case that a still better effect is achieved, when the outer conductor of the second line has two ends and is connected to ground at the two ends.  
         [0028]     In still another embodiment of the invention a third I/O terminal is provided for feeding a signal of a third, still lower resonance frequency, as compared to the second resonance frequency, for exciting nuclei of a third kind of nuclei (Y) and for receiving, resp., a resonance signal emitted by the nuclei of the third kind of nuclei (Y).  
         [0029]     This measure has the advantage that also so-called triple resonance experiments may be conducted.  
         [0030]     Insofar, it is preferred when the third I/O terminal is coupled to a terminal end of the first stop circuit adjacent the second I/O terminal.  
         [0031]     This measure has the advantage that the first stop circuit as such needs no modification.  
         [0032]     In a preferred modification of this embodiment, the third I/O terminal is coupled to a terminal of the first stop circuit adjacent the second I/O terminal via a third stop circuit tuned to signals of the lower resonance frequency of the second kind of nuclei (X).  
         [0033]     This measure has the advantage that the circuit for the third kind of nuclei (Y) is effectively decoupled against the invasion of signals from the second kind of nuclei (X).  
         [0034]     In an alternate embodiment of the invention, the first line is configured as a portion of a third line, the third line having a length equalling a half wave length (λ x /2) of the lower resonance frequency of the second kind of nuclei (X), the third I/O terminal being coupled to a first point of an inner conductor of the third line, the first point being at a distance from the measuring coil equalling one quarter wave length (λ x /4) of the lower resonance frequency of the second kind of nuclei (X), the second I/O terminal being coupled to a second point of the inner conductor of the third line, the second point being at a distance from the measuring coil equalling one quarter wavelength (λ H /4) of the higher resonance frequency of the first kind of nuclei ( 1 H).  
         [0035]     This measure has the advantage that a probe head design with very small radial dimensions becomes possible, because the second and the third I/O terminal are located at different axial positions of the probe head.  
         [0036]     For reasons of symmetry it is also preferred in this case when the outer conductor of the third line has two ends and is connected to ground at the two ends.  
         [0037]     If, in the context of the present application fractions of wavelengths are mentioned, this is to be understood to also include technically feasible multiples of these fractions.  
         [0038]     Further advantages of the invention will become apparent form the subsequent description of preferred embodiments and from the enclosed drawing.  
         [0039]     It goes without saying that the features of the invention mentioned above and those that will be explained hereinafter may not only be used in the particular given combination but also in other combinations ore alone without leaving the scope of the present invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0040]      FIG. 1  shows an electrical network of e probe head according to the prior art;  
         [0041]      FIG. 2  shows an electrical network of a first embodiment of a probe head according to the invention;  
         [0042]      FIG. 3  shows an electrical network of a second embodiment of a probe head according to the invention;  
         [0043]      FIG. 4  shows an electrical network of a third embodiment of a probe head according to the invention;  
         [0044]      FIG. 5A  shows a portion of an electrical network of a fourth embodiment of a probe head according to the invention;  
         [0045]      FIG. 5B  shows a diagram depicting a complex resistance of a line utilized in the probe head of  FIG. 5A ; and  
         [0046]      FIG. 6  shows a portion of an electrical network of a fifth embodiment of a probe head according to the invention on a somewhat enlarged scale. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0047]     In  FIG. 1 , reference numeral  10  indicates a probe head according to the prior art. The probe head is depicted as a network or equivalent circuit diagram with its characterizing electrical components.  
         [0048]     For exciting and/or receiving signals from a first kind of nuclei, for example for the excitation of protons ( 1 H) a first input/output (I/O) terminal  11  is provided, whereas a second I/O terminal  12  serves for exciting and/or receiving signals from a second kind of nuclei, for example from isotopes of nitrogen ( 15 N), or of phosphor ( 31 P), generally referred to as “X” nuclei. High frequency signals are fed to or received from I/O terminals  11  and  12 , the frequency of which depends on a prevailing static magnetic field B. In the context of the present application the wave length of the signal having the resonance frequency of protons ( 1 H) at the prevailing magnetic field B is designated as X.  
         [0049]     If, in the context of the present application fractions of wavelengths are mentioned, like, for example, λ H /2, this is to be understood to also include technically feasible multiples of these fractions like 3λ H /2 or the like.  
         [0050]     As shown in  FIG. 1 , magnetic field B acts on a measuring coil  13  and on a sample  14  arranged therein. Sample  14  may have the shape of a glass vial with a liquid or a solid chemical substance to be investigated contained therein. Sample  14 , however, may also be a biological sample or, in the case of nuclear spin tomography, also referred to as magnetic resonance imaging (MRI), may be a body member or an entire body of a living creature. Measuring coil  13  may also be configured as a surface coil which is placed onto a surface of an object under investigation.  
         [0051]     In the context of the present invention emphasis is on so-called “Magic Angle Spinning” (MAS) experiments. In such experiments the measuring coil  13  as well as the sample  14  are contained within a rotating system having an axis which is inclined relative to the direction of the static magnetic field by the so-called “magic angle” and which rotates about that inclined axis. Details of “Magic angle spinning” experiments and apparatuses are well known to the person of ordinary skill in the art of magnetic resonance and need no further explanation in this application.  
         [0052]     In  FIG. 1  the first I/O terminal  11  of the  1 H side is connected to a capacitive voltage divider  15 ,  16 ,  17 , namely, on the one hand, via a tuneable or otherwise adjustable matching capacity  15  to ground and, on the other hand, via a capacitance  16  to a point from which a tuneable trimming capacitance  17  is also switched to ground. The point is further connected to an inner conductor of a coaxial line  18  having a length of λ H /2. At a middle point the inner conductor may be connected to the outer conductor of line  18  via a switch  19 , as disclosed in further detail in U.S. Pat. specification No. 5,229,724 of the same applicant, discussed at the outset of this application and the disclosure of which is incorporated herein by way of reference. The opposite end of line  18  is connected to the end of measuring coil  13  being the right hand end in the depiction of  FIG. 1 .  
         [0053]     Second I/O terminal  12  of the X side is connected to ground via a tuneable matching inductance  21  and also via a tuneable trimming capacitance  22  to a so-called  1 H stop  23 . The  1 H stop  23  is a parallel resonance circuit having an inductance  24  as well as a capacitance  25 , acting together as a stop circuit for the  1 H frequency. The other end of the  1 H stop  23  is connected to the left hand end of measuring coil  13 .  
         [0054]     In order to achieve a maximum efficiency of the stop action of  1 H stop, the latter must be positioned as close as possible to measuring coil  13 , i.e. as close as possible to a MAS rotor, for example. However, it is then exposed to highly varying temperatures in the case of variable temperature controlled experiments. It is then no more frequency-stable and may not be exposed to high loads. At high temperatures the risk of damaging capacitance  25  due to an electrical breakdown is also significant. In the application of a MAS rotor, where the inclination angle of the rotor must be adapted to be adjusted, mechanical stresses occur during such adjustment, in particular in the area of soldering joints. While these problems may be more or less sufficiently held under control at measuring frequencies in the conventional 400 MHz range, they present substantial difficulties at frequencies in the 900 MHz range as used today.  
         [0055]     In the embodiments of the invention depicted in  FIGS. 2 through 6  like elements are designated with the corresponding reference numerals from  FIG. 1 . In case of modified elements, a letter, characterizing the particular embodiment, is added to the respective reference numeral.  
         [0056]      FIG. 2  shows a probe head  10   a.    
         [0057]     Probe head  10   a  distinguishes from probe head  10  by the particular design of its  1 H stop  23   a . Instead of a parallel resonance circuit  24 ,  25 , the embodiment of  FIG. 2  uses a preferably coaxial line  31  having a length of λ H /4. At a middle  32  of its outer conductor, line  31  is connected to ground. A lower end  33  of its outer conductor is connected to the inner conductor of line  31  via a series circuit of an inductance  34  and a tuneable capacitance  35  acting as a wave trap circuit.  
         [0058]     Line  31 , for example, has a capacity of between 3 and 4 pF. The lower end of the λ H /4 line  31  is a “cold” point for  1 H. The “cold” point, further, is a constrained oscillation node due to the provision of the wave trap circuit  34 ,  35 .  
         [0059]     Due to this circuitry measuring coil  13  is operated symmetrically for  1 H in the first place, i.e. measuring coil  13  is “hot” on both sides and is operated in a push-pull mode. Thereby it is possible to make the distance between the upper end of line  31  and the lower end of measuring coil  13  much smaller as compared to prior art probe heads. The symmetry is also shown in  FIG. 2  in a diagram  36  depicting the voltage U HC  and the current I HC  over the length l s  of measuring coil  13 .  
         [0060]     Due to the serial connection between line  31  and measuring coil  13  a set up of probe head  10   a  with very small radial dimensions is possible.  
         [0061]      FIG. 3  shows a probe head  10   b.    
         [0062]     Probe head  10   b  on the one hand distinguishes from probe head  10   a  of  FIG. 2  in that in the middle  40  of line  18   b  being λ H /2 long, i.e. at a length of λ H /4, a capacity  41  is switched between the inner conductor and the outer conductor. Capacitance  41  is, for example, of the order of magnitude of 50 pF which approximately corresponds the size of trimming capacity  22 .  
         [0063]     On the other hand, line  18   b  is connected to ground at the two ends  42  and  43  of its outer conductor. Likewise, λ H /4 line  31   b  of stop circuit  23   b  is also connected to ground at the two ends  33  and  45  of its outer conductor.  
         [0064]     Due to these measures measuring coil  13  is operated symmetrically also for X nuclei. A first diagram  44  in  FIG. 3  depicts the X voltage U XR  at the right hand side of measuring coil  13  along line  18   b . A corresponding second diagram  46  for the left hand side depicts voltage U XL  along line  31   b . One can see from diagrams  44  and  46  that voltages U XR  and U XL  have the same value U XR0  and U XL0  at the respective upper end of lines  18   b  and  31   b , resp., evidencing that measuring coil  13  is operated symmetrically.  
         [0065]     A third diagram  47  depicts the X voltage U XC  and the X current I XC  along the length l s  of measuring coil  13 . Diagram  47  shows, for example, oppositely equal end values U XCL  and U XCR , typical for a push-pull operation.  
         [0066]      FIG. 4  shows a probe head  10   c.    
         [0067]     Probe head  10   c  is special insofar as it allows to feed or receive, resp., a further signal for a further kind of nuclei, referred to in the art as “Y”, via a third I/O terminal  50 . The third measuring frequency is required for triple resonance experiments.  
         [0068]     Third I/O terminal  50  one the one hand is connected with ground via a tuneable matching inductance  51 . On the other hand, it is connected to a further stop circuit  53  via a tuneable trimming capacitance  52 . Stop circuit  53  is configured as a parallel resonance circuit comprising an inductance  54  and a capacitance  55 . It is connected to the end of stop circuit  23   c  adjacent second I/O terminal  12 . Stop circuit  53  stops X frequencies and, therefore, determines the X frequency within a relatively narrow band of e.g. 1%. The Y frequency, in contrast, may be varied within relatively broad ranges, provided it is lower than the X frequency.  
         [0069]      FIG. 5A  shows just a portion of a probe head  10   d , namely the portion on the right hand side of measuring coil  13 . The left hand portion of probe head  10   d  may be configured according to any of the embodiments shown in  FIGS. 2 through 4 .  
         [0070]     Probe head  10   d  is special insofar as it has a capacitance  61  coupled to a middle  60  of the inner conductor of λ H /2 line  18   d . Capacitance  61  connects to one end of a stop circuit  62 , the other end of which being connected to the outer conductor of line  18   d . Stop circuit  62  is configured as a parallel resonance circuit comprising a capacitance  63  and an inductance  64 . Stop circuit  62  stops the X frequency.  
         [0071]      FIG. 5B  shows a diagram  65  depicting the complex resistance Z of line  18   d  over the frequency f. One can see that the resistance Z has a minimum at the lowermost frequency Y, and has a maximum at the intermediate frequency X. Line  18   d  is capacitive for both frequencies Y and X.  
         [0072]      FIG. 6 , finally, shows a portion of a probe head  10   e , namely the portion on the left hand side of measuring coil  13 , on a somewhat enlarged scale. The right hand portion of probe head  10   e  may be configured according to any of the other embodiments of the present invention.  
         [0073]     In probe head  10   e  the coupling of the third frequency Y is made somewhat different as compared to probe head  10   c  of  FIG. 4 . Probe head  10   e  utilizes a line  70  of λ x /2 length on the left hand side of measuring coil  13 .  
         [0074]     A first point  71  on the middle of the inner conductor of line  70 , i.e. at λ x /4, connects to a network, consisting of a trimming capacitance  54   e  and a matching inductance  55   e  which, in turn, connects to the third I/O terminal  50  for the Y frequency. Matching inductance  55   e  connects to ground. The first point  71  on the inner conductor, being the point at which the Y frequency is coupled in, therefore, lies on an oscillation node for the X frequency, i.e. on zero potential.  
         [0075]     At a distance of λ H /4 from the upper end  45   e  of line  70 , as shown in  FIG. 6 , there is positioned a second point  72  on the inner conductor. By doing so, the λ H /4 line of the present invention is integrated into the λ H /2 line  70 . Second point  72  on the inner conductor, on the one hand, connects to the network consisting of trimming capacity  22   e  and matching inductance  21   e  which, in turn, connects to second I/O terminal  12  for the X frequency. On the other hand, second point  72  on the inner conductor connects to the wave trap circuit consisting of capacitance  35   e  and inductance  34   e , constraining potential zero for the  1 H frequency at the second point  72  on the inner conductor. Line  70  is coupled to ground on both ends of its outer conductor.  
         [0076]     The arrangement of  FIG. 6  allows to locate terminals  12  and  50  at distinct axial positions of probe head  10   e , thus enabling a radially narrower design thereof.