Abstract:
A probe head is used for nuclear resonance measurements during which two different kinds of nuclei are excited by means of radio frequency irradiation in a constant magnetic field. The probe head is provided with a pick-up coil receiving a sample under investigation. The pick-up coil is connected to a first input for feeding a signal of higher frequency for exciting a first kind of nuclei and/or for receiving a resonance signal emitted by the first kind of nuclei. The pick-up coil, further, is connected to a second input for feeding a signal of a lower frequency for exciting a second kind of nuclei and/or for receiving a resonance signal emitted by the second kind of nuclei. The pick-up coil, moreover, is connected to a radio frequency line the electrical length of which corresponds to an integer multiple of a quarter of the wave length of the higher frequency. A series capacitor is interconnected between the radio frequency line and the pick-up coil.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of magnetic resonance, in particular nuclear magnetic resonance measurements. More specifically, the invention is related to nuclear magnetic resonance measurements wherein at least two different kinds of nuclei are excited by means of radio frequency irradiation within a constant magnetic field. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a probe head for nuclear magnetic resonance measurements comprising a pick-up coil cooperating with a sample under investigation. The pick-up coil is connected to a first input and to a second input. The first input is used for feeding a signal of a higher radio frequency (f H ) to the pick-up coil for exciting a first kind of nuclei, for example protons (H) and/or for receiving from the pick-up coil a resonance signal emitted by the nuclei of the first kind of nuclei (H). The second input is used for feeding a signal of a lower radio frequency (f X ) to the pick-up coil for exciting a second kind of nuclei (X) and/or for receiving from the pick-up coil a resonance signal emitted by the nuclei of the second kind of nuclei (X). A radio frequency line is also connected to the pick-up coil. The radio frequency line has an electrical length corresponding to an integer multiple of a quarter wave length (λ/4) of the higher radio frequency (f H ). 
     A probe head of the afore-mentioned kind is disclosed in U.S. Pat. No. 5,229,724 (Zeiger) assigned to the applicant of the present application. 
     Probe heads of the kind of interest in the present context are used for conducting nuclear double resonance experiments. In such experiments a first kind of nuclei, mostly protons ( 1 H) or fluorine ( 19 F) is excited and/or observed, whereas a second kind of nuclei is simultaneously excited and/or observed, for example certain isotopes of nitrogen ( 15 N) or phosphor ( 31 P) or carbon ( 13 C) or silicon ( 29 Si) or aluminum (27Al). In the art of magnetic resonance the first kind of nuclei is usually designated as “H” whereas the second kind of nuclei is identified as “X”. 
     In modern high field nuclear magnetic resonance spectrometers, the exciting frequency for protons ( 1 H) is, for example, 800 MHz. In that case, the field strength of the constant magnetic field is about 18.8 Tesla. As is well-known in the art, the resonance frequency of nuclei and the magnetic field strength of the constant magnetic field are interrelated by a proportionality factor called the gyromagnetic ratio having an individual value for each kind of nuclei. 
     In the case specified before, i.e. always related to the same field strength of the magnetic field of 18.8 Tesla, the resonance frequency for the above-mentioned isotopes of nitrogen ( 15 N) is 81 MHz, of phosphor ( 31 P) is 324 MHz, of carbon ( 13 C) is 201 MHz, of silicon ( 29 Si) is 159 MHz and of aluminum ( 27 Al) is 208 MHz roughly. 
     The probe head according to U.S. Pat. No. 5,229,724 mentioned above is designed for a proton resonance frequency of 400 MHz. With a proton resonance frequency of 400 MHz, the resonance frequency of e.g. nitrogen ( 15 N) is only about 40.5 MHz. Therefore, the prior art probe head is designed for a frequency range of between 40 and 400 MHz. 
     The above-mentioned double resonance experiments are preferably conducted such that measurements are taken at X-frequency and decoupling is effected at H-frequency. 
     Probe heads for such experiments are designed such that the electric probe head network comprising the pick-up coil as well as the radio frequency line are optimized in their equivalent electrical circuit for the higher radio frequency, as seen from the first input and for the lower radio frequency, respectively, as viewed from the second input. 
     In the prior art probe head, a very broad frequency band may be swept on the X-side, i.e. on the lower radio frequency side, in contrast to still older prior art probe heads which had only been optimized for a very narrow frequency band on the X-side. In the probe head mentioned at the outset, one has attempted an optimization of between 40 MHz for  15 N up to 162 MHz for  31 P. The X-frequencies, therefore, amount to between one tenth and one half of the H-frequency being 400 MHz. 
     In the prior art probe head, the radio frequency line coupled to the pick-up coil is configured as a λ/2 line (related to the H-frequency of e.g. 400 MHz). At about one half of the λ/2 line, there is a switchable bridge so that the radio frequency line may be operated with its entire length (λ/2) when the bridge is open and at half length (λ/4) when the bridge is closed. 
     In the first case of the λ/2 line one has, therefore, an equivalent electrical circuit as viewed from the X-side in which the pick-up coil is terminated by a capacitance at its terminal end facing away from the X input (the second input). This results in a high pass characteristic in which the pick-up coil has a low resistive load for current of high frequency whereas, corresponding to the size of the capacitor, a lower threshold frequency has to be taken into account. 
     If, however, the bridge is closed and in the electrical equivalent circuit (again viewed from the X-side), the pick-up coil is terminated at its output by an inductivity so that in this situation the pick-up coil has a low resistive load for current of low frequency and, depending on the size of the inductivity, an upper threshold frequency may be determined. 
     Moreover, with the prior art probe head the electrical bridge must remain open for high X-frequencies (e.g.  31 P) whereas it is closed for lower X-frequencies (e.g.  15 N). 
     A disadvantage of the prior art probe head, when operated as a λ/4 line (as well as with other prior art probe heads utilizing a λ/4 line) the circuitry of the pick-up coil is asymmetric because the value of the radio frequency voltage at one terminal end of the pick-up coil is very high, whereas it is very low at the opposite end of the pick-up coil. This is highly disadvantageous in particular for high measuring frequencies. 
     Still another disadvantage of the prior art probe head, again when operated as a λ/4 line, is that the filling factor of the entire inductivity is not optimized because only a portion of the resonant circuit inductivity is filled with sample volume. This portion of the inductivity of the resonant circuit configured by the λ/4 line will be larger, the higher the X-frequency is. For that reason measurements at high X-frequencies are difficult so that supplemental tricks must be used, for example another inductivity switched in parallel to the pick-up coil. 
     Further probe heads of the kind of interest in the present context are disclosed in an article by Jiang, “An Efficient Double-Tuned  13 C/ 1 H Probe Circuit for CP/MAS NMR and Its Importance in Linewidths”, JOURNAL OF MAGNETIC RESONANCE, 71, 1987, pages 485-494, in U.S. Pat. No. 4,633,181 and in published PCT patent application WO 92/17792. 
     It is, therefore, an object underlying the present invention to improve a probe head of the kind mentioned at the outset such that improved measurements may be made at X-frequencies having up to one half the value of the H-frequency. Still another object is to enable improved measurements on very lossy samples which normally negatively affect the resonance frequency and the quality of the resonant circuit. 
     SUMMARY OF THE INVENTION 
     These and other objects underlying the invention are solved by a probe head as specified above which, additionally, has a series capacitor being interconnected between the radio frequency line and the pick-up coil. The radio frequency line is preferably a λ/4 line. 
     The object underlying the invention is thus entirely solved. 
     By providing a series capacitor between the radio frequency line and the pick-up coil one can, as compared with prior art probe head circuits utilizing short-circuited λ/4 lines, avoids the portion of the inductivity within that λ/4 line by setting a “push-pull”-operation at the pick-up coil in which operation the radio frequency voltage at opposite ends of the pick-up coil has opposite polarity. As compared with prior art probe head circuits utilizing a λ/2 line, the load capacity at the end of the open λ/2 line is mostly avoided. Seen as a whole, the provision of a series capacitor makes it possible to utilize the probe head for X-frequencies up to the order of one half the H-frequency. 
     The series capacitor may be dimensioned very small, e.g. between 3 and 6 pF. When doing so, higher X-frequencies may be utilized as compared with the prior art. In particular, the upper threshold value according to which the X-frequency may, at a maximum, have half the value of the H-frequency (of. U.S. Pat. No. 5,229,724), is no more existent. 
     As already mentioned, it is preferred within the scope of the present invention to dimension the series capacitor such that a radio frequency voltage of the lower radio frequency has opposite polarity at terminal ends of the pick-up coil. 
     The “push-pull”-operation of the pick-up coil being a consequence of a so-dimensioned series capacitor results in a lower mistuning of the resonant circuit and in a lower reduction of the quality factor, in particular when lossy samples are investigated. The relatively large capacity of the series capacitor (as compared to the high H-frequency) affects the operation on the H-side only marginally. These effects may, further, be mostly compensated for by slightly adjusting the tuning for the H-frequency. This may, for example, be effected by varying the line length of the λ/4 line accordingly. 
     In a further preferred embodiment of the invention, the series capacitor is adapted to be bridged. 
     This feature has the advantage that the probe head measuring range may be broadened towards lower X-frequencies by simply bridging the capacitor. 
     The invention may be utilized advantageously in high field nuclear magnetic resonance spectrometers having a constant magnetic field with a field strength in excess of 9 Tesla. If, for example, the magnetic field has a field strength of 18.8 Tesla, the resonance frequency of protons is about 800 MHz, as already mentioned, and the resonance frequency of phosphor ( 31 P) is about 324 MHz. It is, therefore, in the order of one half of the proton resonance frequency. 
     In another preferred embodiment of the invention, the constant magnetic field has a strength of 11.8 Tesla. In that case, measurements may be effected on the H-side while de-coupling phosphor ( 31 P) at about 202 MHz, whereas the measuring frequency for silicon ( 29 Si) is about 99 MHz and for aluminum ( 27 Al) is about 132 MHz. 
     Still another distinct advantage may be obtained when the probe head of the present invention is used for so-called MAS-measurements in which the sample is rotated under the so-called “magic” angle of 54° 44′ with respect to the direction of the constant magnetic field. 
     Further advantages of the invention will become apparent from the description and the enclosed drawing. 
     It goes without saying that the features mentioned above and those that will be recited hereinafter may not only be used in the particularly given combination but also in other combinations without leaving the scope of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention are shown in the annexed drawing and will be described in further detail in the subsequent description. 
     FIG. 1 shows a circuit diagram of a first embodiment of a probe head according to the invention; 
     FIGS. 2A and 2B show an equivalent electrical circuit as well as a voltage graph for a probe head of the prior art; 
     FIGS. 3 a  and  3 B show depictions, similar to those of FIGS. 2A and 2B, however, for the probe head of FIG. 1; 
     FIG. 4 shows a circuit diagram of a second embodiment of a probe head according to the invention; and 
     FIG. 5 shows a circuit diagram of a third embodiment of a probe head according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Nuclear magnetic resonance probe heads for carrying out double resonance experiments are well-known in the prior art. An extensive description of such probe heads may, for example, be found in U.S. Pat. No. 5,229,724, the contents of which is incorporated herein by way of reference. 
     In FIG. 1, reference numeral  10  as a whole designates a probe head for nuclear magnetic resonance measurements and adapted to carry out double resonance experiments. The kind of nuclei to be observed is normally the one with the lower frequency ( 15 N,  31 P,  13 C,  29 Si or  27 Al), whereas the other kind of nuclei at a higher frequency ( 1 H or  31 P) is decoupled simultaneously. The measuring frequency is hereinafter designated by “X” and the decoupling frequency by “H”. 
     Probe head  10  has a first input  12  for the decoupling frequency and a second input  14  for the measuring frequency. The pick-up coil having an inductivity L M  is designated by reference numeral  16 . Pick-up coil  16  encloses sample  18 . R and S are the terminal ends of the pick-up coil  16 . 
     From input  12  a tunable capacitor  20  is directed to a coupling loop  22  being arranged at a λ/4 line  24 . The designation “λ” is related to the wave length of the decoupling signal H. The length of λ/4 line  24  may be adjusted by means of a shorting plunger  26 , as indicated by an arrow T H . Line  24  has a capacitance per unit length, as indicated at  25 . With usual lines  24 , the capacitance  25  per unit length is in the order of 0.5 pF/cm line length. 
     From the terminal end of λ/4 line  24  facing away from shorting plunger  26 , a series capacitor  28  having a capacity C S  leads to terminal end S of pick-up coil  16 . The opposite terminal end R of pick-up coil  16  is connected to a wave trap  30  consisting of a capacitor  32  and a parallel wire bridge  34  of predetermined inductivity. Wave trap  30  protects the X-side by rejecting decoupling signals H. 
     In a modified embodiment series capacitor  28  may be bridged by means of a switch  29 . However, in that case one has to take special precautions because then series capacitor  28  is then radio frequency wise “hot” against ground. 
     Wave trap  30  leads to a tunable capacitor  36  having a capacity C Tx , the opposite side of which being in turn connected with second input  14 . Second input  14 , further, is connected to ground via a tunable inductor  38  of inductivity L Mx . 
     Probe head  10  shown in FIG. 1, therefore, distinguishes from the prior art probe head according to U.S. Pat. No. 5,229,724 (FIG. 4) in that line  24  is a λ/4 line and a series capacitor  28  is provided between line  24  and pick-up coil  16 . The capacity C S  of series capacitor  28  is significantly lower as compared with equivalent capacity  49  shown in FIG. 2C of U.S. Pat. No. 5,229,724 because equivalent capacity  49  is highly predetermined by the capacity of the line section of the λ/2 line being in the order of about 0.5 pF/cm. Due to the low value of C S , significantly higher X-frequencies may be reached as compared with that of prior art. 
     FIG. 2A shows an equivalent electrical circuit of a conventional probe head having no such series capacitor  28 . The equivalent electrical circuit of FIG. 2A, when viewed from the X-side, shows an equivalent inductance  44  behind terminal end S of pick-up coil  16 . 
     Considering now in FIG. 2B a voltage graph  48  over pick-up coil  16 , i.e. between terminal ends R and S, one can clearly see the asymmetrical character of the equivalent electrical circuit of FIG.  2 A. The radio frequency voltage values U R1  and U S1  at the terminal ends of pick-up coil  16  have the same polarity and the higher voltage U R1  appears at terminal end R. 
     If, in FIG. 3A, we now compare the corresponding equivalent electrical circuit of a probe head  10  according to the invention as shown in FIG. 1, one can see an equivalent capacity  46  behind terminal end S of pick-up coil  16 , as viewed from the X-side. 
     A corresponding voltage graph  50  is depicted in FIG.  3 B. One can clearly see that we now have a symmetrical system. The values of radio frequency voltage U R2  and U S2  at the terminal ends R and S now have opposite polarity and the absolute voltage values are lower. 
     As compared with the prior art, the series capacitor  28 , therefore, increases the upper frequency threshold on the X-side while maintaining the same range of variation of tunable capacitor  36 , however, it also shifts the lower frequency threshold towards higher frequencies. 
     Turning now to the selection of the capacitance value of series capacitor  28  which may be 6 pF for a H-frequency of 400 MHz, one has to take into account that a tuning of the H-side must be possible, for example by setting the length of line  24  somewhat differently. 
     For a comparative consideration on the X-side one may start from some electrical characteristics, namely inductivity L M  of pick-up coil  16 , measuring frequency f x , a preselected capacitance value C Tx  of adjustable capacitor  36  and its minimum value, respectively. 
     If, as compared with the prior art, one maintains the inductivity L M  of pick-up coil  16  as well as operating frequency f X , the voltage amplitude at terminal end R of pick-up coil  16  according to the invention is smaller as compared with the prior art (FIG. 3B) due to the prevailing push-pull operation. Accordingly, the tunable capacitor  36  has a lower voltage load. Moreover, the de-tuning of the resonance circuit for different sample materials, i.e. for sample materials with different losses, is lower. Finally, the radio frequency losses are lower, too, in particular for sample materials having high radio frequency losses anyway. 
     If, on the other hand side, one maintains the preselected capacitance value C TX  of tunable capacitor  36  as well as the corresponding measuring frequency f X , one gets a higher inductivity of pick-up coil  16 , as compared with the prior art, and, hence, the possibility of using a push-pull operation. With the same pick-up coil length a higher winding density is possible which, in turn, enables the use of higher excitation magnetic field strengths B 1  and yields a better signal-to-noise ratio during the recording of nuclear signals. Finally, a greater pick-up coil length and/or a higher winding density yields a higher homogeneity within pick-up coil  16 . 
     If, finally, one maintains the inductivity L M  of pick-up coil  16  and a minimum capacitance value C TX  of the tunable capacitor, one obtains a higher resonance frequency on the X-side as compared with the prior art. If the capacitance value C S  of series capacitor  28  is selected cleverly, measurements are possible in which the measuring frequency f X  may be higher than half the decoupling frequency f H . 
     FIG. 4 shows a further embodiment of a probe head  10  according to the invention. Probe head  10  comprises a first input  62  on the H-side as well as a second input  64  on the X-side. Inputs  62 ,  64  are coupled to lines  66  and  67 , respectively. 
     As one can take from FIG. 4, we have an inductive coupling on the X-side and a capacitive coupling on the H-side. 
     For a measuring frequency f X =323.9 MHz and a decoupling frequency f H =800.1 MHz, a pick-up coil  16  was designed with several windings of a wire having 0.5 mm diameter. The pick-up coil length was 6 mm and the inner pick-up coil diameter was 2.8 mm. The capacitance value C S  of series capacitor  28  was 3.4 pF. The capacitive voltage divider C 1 /C 2  behind line  66  was configured with capacitors having capacitance values C 1 =0.3 pF and C 2  =1.5 through 25 pF. The capacitance value C TX  of tunable capacitor  36  was between 1 and 10 pF. 
     At point W shown in FIG. 4 at the output of line  67 , a capacitor C TR  of 2.7 pF was installed and, together with its feeding wires (which in the depiction of FIG. 4 are shown as inductors on the left hand side and the right hand side of this capacitor) was tuned to 800 MHz. By doing so, signal portions H at frequency f H  at second input X coming from first input  62  were attenuated. 
     In a practical test with probe head  60  of FIG. 4, a magic angle spinning (MAS) probe head having a 2.5 mm rotor was used. The probe head was located in a small bore magnet at a field strength of 18.79 Tesla. The measuring frequency f X  was 323.85 MHz for isotope  31 P, whereas protons were decoupled at f H =800.1 MHz. 
     FIG. 5 shows a third embodiment of a probe head  70  according to the invention having inputs  72  (H) and  74  (X). On the H-side, the decoupling signal H is fed via three line sections  76 ,  78  and  80 . There exists an inductive coupling between the first line sections  76  and  78 , whereas line sections  78  and  80  are interconnected by means of a shorting plunger  82 . 
     The third line section  80  at the same time is the inner conductor of a λ/4 line  84 . 
     Inputs  72 ,  74  together with their associated lines and line sections are altogether located within a conductive tube  86  enclosing the afore-mentioned elements and being grounded at its outside. 
     In probe head  70  there is an inductive coupling on the H-side via line sections  76 ,  78  and  80 . There is also an inductive coupling on the X-side via an inductivity L MX . 
     In probe head  70  pick-up coil  16  consists of thirteen windings of a wire of 0.5 mm diameter. The pick-up coil length is 9.6 mm and the inner pick-up coil diameter is 4.4 mm. 
     In that case the series capacitor  28  has a capacitance value C S  of 5.5 pF. The value C 1  of adjustable capacitor  20  at the H-input is between 1 and 10 pF. The value C 2  of capacitor  32  within wave trap  30  is 25 pF. The capacitance C TX  of tunable capacitor  36  can be varied between 1.5 and 52 pF. 
     MAS measurements utilizing rotors of 4 mm diameter were also conducted with probe head  70  of FIG.  5 . The field strength of magnetic field B was 11.74 Tesla in a wide bore magnet. The measuring frequency f X  was 99 and 132 MHz for isotopes  29 Si and  27 Al, respectively. Decoupling took place on the frequency of isotope  31 P namely f H =202.4 MHz.