JP2010190591A - Nmr probe and design method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an NMR probe which can detect two-mode resonance frequencies by one terminal and a design method of the same. <P>SOLUTION: Two different cylindrical inductances are placed concentrically and coaxially, and the values of capacitance coupling and magnetic coupling between the inductances are selected, thereby achieving resonance at two frequencies. In that case, the cylindrical inductances are two Helmholtz type sample coils having different diameters or lengths. The magnitude of the magnetic coupling between both coils is selected by differentiating the directions of the generation axes of high-frequency magnetic fields generated by both coils by a predetermined angle. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an NMR probe that can be tuned to resonance frequencies of a plurality of nuclides and a design method thereof, and more particularly to an NMR probe that can be tuned to close resonance frequencies and a design method thereof.

  The NMR device applies a strong magnetic field to the sample, causes precession about the static magnetic field direction to occur in the magnetic moment of the nucleus with the nuclear spin in the sample, and is orthogonal to the static magnetic field direction. A high frequency magnetic field is applied to excite the precession of the nuclear magnetic moment, and then the NMR signal emitted when the precession of the nuclear magnetic moment returns from the excited state to the ground state is applied to the sample. It is a device that observes as a unique high frequency magnetic field.

An NMR apparatus including a conventional NMR probe is shown in FIG. In NMR apparatus of FIG. 1, a nuclide to be observed, it is assumed that the combination of the ¹ H nuclei and ¹⁹ F nuclei and ¹ H nucleus and ¹³ C nuclei is the same case of a combination of other nuclides.

  This NMR apparatus includes a magnet 2 for applying a substantially uniform static magnetic field to a sample tube 1 for storing a sample, a room temperature shim 3 for correcting non-uniformity of a static magnetic field by the magnet 2 at room temperature, a sample tube 1 and an NMR probe. 4 and a temperature variable (VT) device 5 that changes the temperature of the sample.

  The NMR probe 4 has a single or a plurality of sample coils, and can detect an NMR signal from a sample containing at least two nuclides. The NMR probe 4 will be described in detail later.

  The NMR apparatus also displays a magnetic field correction apparatus 6 that controls the correction of the static magnetic field by the room temperature shim 3, a computer 7 that controls the entire NMR apparatus, the operating state of the NMR apparatus, and the observed NMR signals. Display 8 such as an LCD, duplexer 9 for switching the timing of signals sent to and received from NMR probe 4, amplifier 10 for amplifying NMR signals sent from NMR probe 4 through duplexer 9, and amplification by amplifier 10 A demodulation detector 11 for demodulating and detecting the NMR signal, and an analog-to-digital converter (ADC) 12 for performing analog-to-digital conversion on the signal from the demodulation detector 11.

  Furthermore, this NMR apparatus amplifies the power of the first power amplifier 13 that amplifies the signal of the first frequency f1 sent to the NMR probe 4 via the duplexer 9, and the signal of the second frequency f2. Second power amplifier 15, third power amplifier 16 that similarly amplifies the signal of third frequency f 3, first power amplifier 13, second power amplifier 15, and third power amplifier 16 And an oscillator 14 for supplying signals of the first, second, and third frequencies f1, f2, and f3, respectively.

Here, the first and second frequencies f1 and f2 correspond to resonance frequencies of a combination of ¹ H nucleus and ¹⁹ F nucleus, or ¹ H nucleus and ¹³ C nucleus. The oscillator 14 generates signals of the first and second frequencies f1 and f2 based on the observation signal sent from the demodulation detector 11. Third frequency f3 corresponds to NMR signals of ² D nucleus used in the third species, e.g. NMR lock other than the radionuclide.

  FIG. 2 shows an NMR detector having a single mode resonant circuit with one main coil and an RF circuit associated therewith. Ct1 and Ct2 are tuning capacitors. Tuning is set to the desired NMR frequency by adjusting either tuning capacitor Ct1 or Ct2. If the RF impedance at that time is assumed to be a 50Ω system system, it is matched to the system impedance by a matching capacitor Cm1.

  The main coil is a simple Helmholtz coil. A guard ring indicated by a cross is an electrode of a metal plate, and an insulator (usually a glass bobbin) is sandwiched between arcs (also called wings) on the lower side of the main coil, which is also made of a metal plate. Configure. This capacitor has a structure wired in parallel with the main coil.

  FIG. 3 is a standard example when a two-mode circuit is formed in one main coil. In general, a separation circuit is provided in the external circuit, and a circuit for obtaining the f2 frequency is attached to the tip of the separation circuit. As the separation circuit, an LC parallel resonance circuit, a wavelength resonance circuit, or a resonator is used. Either Cd1 or Cd2 is a f2 frequency tuning capacitor, and Cd3 is an impedance matching capacitor.

Generally speaking, the combination of ¹ H nucleus and ¹⁹ F nucleus often has only one tuning matching circuit, and conversely, the combination of ¹ H nucleus and ¹³ C nucleus has only one tuning circuit. There are a case where two are prepared and switched, and a case where a separation circuit is provided and two tuning matching circuits are provided.

Japanese Patent No. 3914735 Japanese Patent No. 4156646

  When a two-mode frequency resonance circuit of f1 and f2 is formed in one main coil as shown in FIG. 3, the circuit efficiency decreases due to the addition of the separation circuit, and it is desired to increase the detection sensitivity as much as possible. The original purpose cannot be met. Since the two are in a trade-off relationship, even if the optimum configuration is used, conventionally, the operation efficiency is at most about 50 to 60%.

  As can be seen from FIG. 3, one terminal is added to realize f1 and f2. Therefore, a high-frequency transmission line from the NMR spectrometer is added there, and the equipment becomes large.

  As another method for obtaining f1 and f2, there is a method realized by arranging another sample coil inside or outside the main coil and forming an RF circuit in the coil in the same manner. In this case, in the prior art, the outer coil is smaller than the inner coil diameter of ~ 6 mm when expressed in terms of several mm, for example, both coil diameters, so that both sample coils do not interfere with each other and their respective efficiency decreases. It was common sense to arrange a large space such as 11 mm in diameter.

  In this example, it is assumed that the sample tube to be measured has a diameter of 5 mm. Therefore, in the above example, the detection efficiency of the nuclide of the frequency formed by the circuit of the outer coil is formed by the circuit of the inner coil. It must be prepared that this method has the disadvantage that the detection efficiency of the frequency nuclide is reduced to about 6/11 (56%) or less.

  As described above, since the reason for the trade-off is inherent, there is a demand for a one-terminal two-mode detector having a simple two-mode detection function and a mutual efficiency of about 70 to 80% or more. There was a strong demand for the disclosure of ideas for its realization.

  An object of the present invention is to provide a method for designing an NMR probe capable of detecting a two-mode resonance frequency with one terminal in view of the above points.

In order to achieve this object, a method for designing an NMR probe according to the present invention includes:
It is characterized in that two different cylindrical inductances are arranged concentrically and are resonated at two different frequencies by selecting values of capacitance coupling and magnetic coupling between the inductances.

  Further, the two cylindrical inductances are two Helmholtz type sample coils having different diameters or lengths.

  The Helmholtz-type sample coil is characterized in that it is LC-resonated at a desired frequency by being disposed opposite to a metal ring electrode with a cylindrical insulator bobbin interposed therebetween.

  The two Helmholtz type sample coils are characterized in that the magnitude of the magnetic coupling between the two coils is selected by changing the direction of the generation axis of the high-frequency magnetic field generated as a result of resonance by a predetermined angle φ. It is said.

  The predetermined angle φ is 0 <φ ≦ 20 °.

  The two Helmholtz-type sample coils are associated with the main coil and the subcoil, of which the coil end on the main coil side is connected to the high frequency input / output means, and the LC resonance circuit on the subcoil side is not connected to the external circuit. It is characterized by that.

  Further, the size of the capacitive coupling is selected by changing the area of the facing portion of the Helmholtz type sample coil facing the ring electrode.

  In addition, in order to operate a resonant coil complex in which the Helmholtz type sample coil and the ring electrode are opposed to each other with the cylindrical insulator bobbin sandwiched between them, an impedance is set for each resonant frequency of the resonant coil complex. It is characterized in that switching means for switching the capacitive element for matching is provided.

  The NMR probe according to the present invention is designed by the design method according to claim 1.

According to the NMR probe design method of the present invention,
Since two different cylindrical inductances are arranged concentrically on the same axis and the values of the capacitance coupling and the magnetic coupling between the inductances are adjusted to resonate at two different frequencies.
It has become possible to provide an NMR probe capable of detecting a two-mode resonance frequency with one terminal.

It is a figure which shows the conventional NMR apparatus. It is a figure which shows an example of the conventional NMR probe. It is a figure which shows another example of the conventional NMR probe. It is a figure which shows one Example of the NMR probe concerning this invention. It is a figure which shows the three-dimensional assembly drawing of the NMR probe concerning this invention. It is a figure which shows the three-dimensional assembly drawing of the NMR probe concerning this invention. It is a figure which shows the name of each part of the NMR probe concerning this invention. It is a figure which shows the equivalent circuit of the NMR probe concerning this invention. It is a figure which shows the cross-section of the NMR probe concerning this invention. It is a figure which shows the equivalent circuit of the NMR probe concerning this invention. It is a figure which shows the electromagnetic analysis result of the NMR probe concerning this invention. It is a figure which shows the electromagnetic analysis result of the NMR probe concerning this invention. It is a figure which shows the electromagnetic property of the NMR probe concerning this invention. It is a figure which shows the electromagnetic property of the NMR probe concerning this invention. It is a figure which shows another Example of the NMR probe concerning this invention. It is a figure which shows another Example of the NMR probe concerning this invention. It is a figure which shows another Example of the NMR probe concerning this invention. It is a figure which shows another Example of the NMR probe concerning this invention. It is an electromagnetic analysis result of another Example of the NMR probe concerning this invention.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 4 shows an embodiment of the detection coil of the NMR probe according to the present invention. The parts not shown are the same as those in FIG. 2 or FIG. 3 of the prior art. In this detection coil, the inner coil (coil 1) and the outer coil (coil 2) are arranged substantially concentrically with a glass bobbin interposed therebetween, and the axes of the high-frequency magnetic field generated by the resonance are shifted by a very small angle φ. There is a feature in that. Due to the deviation of the angle φ, although the two coils have substantially the same resonance frequency, a slight deviation occurs in the resonance frequency.

  FIG. 5 shows a three-dimensional assembly drawing of the bobbin constituting the detection unit. The glass bobbin has a triple structure including an innermost bobbin 1, a bobbin 2 that concentrically surrounds the outside of the bobbin 1, and a bobbin 3 that concentrically surrounds the outside of the bobbin 2.

  FIG. 6 shows a three-dimensional relationship between the bobbin and the coil. A coil 1 is attached to the outer peripheral surface of the bobbin 1, and a ring electrode 1 is attached to the inner peripheral surface. A coil 2 is attached to the outer peripheral surface of the bobbin 2. A ring electrode 2 is attached to the outer peripheral surface of the bobbin 3.

  If such a structure is named M, FIG. 10 represents the circuit configuration of the structure M with an equivalent circuit. FIG. 7 shows names of the main coil and the subcoil constituting the structure M. As shown in FIG. 8, in the structure M, the main coil has a coil end connected to the high-frequency input / output means. On the other hand, the subcoil performs high-frequency input / output by snuggling up to the main coil, and the coil end of the subcoil itself is released and is not connected to any high-frequency input / output terminal.

  Basically, as shown in the cross-sectional view of FIG. 9, a metal ring 1 is placed on the innermost side, a cylindrical glass bobbin for an inner coil just outside it, and a metal inner coil just outside it. A cylindrical glass bobbin for the outer coil just outside it, a metal outer coil just outside it, a cylindrical glass bobbin for ring 2 just outside it, and a metal ring 2 on the outermost side Has been.

  If C1 and C2 in the structure M of FIG. 8 are described based on the names shown in FIGS. 7, 9, and 10, C1 is composed of the arc part a and the area of the arc part A of coil1, bobbin1 and ring1. C2 is a capacitor constituted by the regions of arc part a and arc part A of coil 2 and bobbin 3 and ring 2.

  The structure M is defined as a structure that generates a two-mode resonance including a transformer coupling and an electrostatic coupling composed of two coils. The structure M has substantially the same concentric axis and a high-frequency magnetic field generating axis with a substantially close diameter. This is realized by two coils arranged slightly shifted (angle φ ≠ 0).

  The operation of this embodiment is shown in FIG. The circuit configuration used for electromagnetic analysis is shown on the lower side of FIG. Of these, the portion surrounded by the right line is a sample coil portion according to the present invention, and the portion surrounded by the left line is a portion added due to the necessity for measuring the pass characteristic of the present invention. Therefore, the circuit portion surrounded by the left line is not an essential element of the present invention.

  Referring to the analysis diagram by the network analyzer shown on the upper side of FIG. 11, in the circuit based on this configuration, in the reflection characteristic measurement from port 1 to port 1, 470.64 MHz (−31.98 dB) and 500.16 MHz (− It was shown that there is a resonance point at 31.85 dB). Further, in the pass characteristic measurement from the port 1 to the port 2, it was shown that there is an isolation point of 488.05 MHz (−85.11 dB) between the two resonance points.

The upper side of FIG. 12 is an enlarged view of the vicinity of 500 MHz (resonance frequency of f1, ¹ H nucleus) in FIG. 11, and simultaneously, the Q value at the resonance position and the resonance voltage at both ends of the coil when 30 W RF power is injected. The value of is shown. The lower side of FIG. 12 is an enlarged view of the vicinity of 470 MHz (resonant frequency of f2, ¹⁹ F nucleus) in FIG. 11. At the same time, the Q value at the resonance position when 30 W RF power is injected and the both ends of the coil This shows the value of the resonance voltage.

  From FIG. 12, at 500 MHz, the Q value was 206.3, and the resonance voltage at both ends of the coil was 1089V. At 470 MHz, the Q value was 389.8, and the resonance voltage at both ends of the coil was 1145V.

  Normally, assuming that the efficiency is 100% when the operation is performed by setting the f1 individually with only the main coil, when 30 W of RF power is injected under the same conditions, the generated voltage at that time is ˜1400V. Therefore, when the induced voltage shown in FIG. 12 is determined from this value, in the case of the present invention, it exceeds .about.70% in the case of 100% efficiency, and the theoretical efficiency in the case where two modes are put in one sample coil is 50. % (50% power max), considering that it is ˜70% in terms of voltage, in the present invention, it was found that the efficiency was increased to a value almost close to the theoretical value.

  The electromagnetic elements related to the coupling between the coils include a coupling capacitor C and a coupling constant K of the transformer. Since these have characteristics as shown in FIGS. 13 and 14, by adopting the optimum value, The present invention is realized.

FIG. 13 shows resonance characteristics when the capacitance C (coupling capacitor between the coils) is changed by changing the thickness of the glass bobbin. In the figure, the horizontal axis represents the value (pF) of the coupling capacitor C, the left vertical axis represents the resonance frequency (MHz) of each of the two coils, and the right vertical axis represents the resonance frequency difference (MHz) between the two coils. Looking at the resonance characteristics from the figure, both f1 and f2 tend to decrease in frequency as the value of the coupling capacitor C increases. Further, it was found that the gap between f1 and f2 shows a minimum value when the coupling capacitor C is near 2 pF. From these results, it was found that 1.5 pF is optimal for the coupling capacitor C when f1 is assigned to ¹ H nucleus (= 500 MHz) and f2 is assigned to ¹⁹ F nucleus (= 470 MHz).

FIG. 14 shows the resonance characteristics when the coupling constant K of the transformer is changed by changing the angle φ of the two coils. In the figure, the horizontal axis represents the value of the coupling constant K, the left vertical axis represents the resonance frequency (MHz) of each of the two coils, and the right vertical axis represents the resonance frequency difference (MHz) between the two coils. The coupling constant K is a parameter representing the strength of inductive coupling between the coils, and is a constant represented by the equation: μ = K√ (L ₁ × L ₂ ) where the induced inductance is μ. In the case of tight coupling such as a transformer, it is known that K≈1.

Looking at the resonance characteristics from the figure, f1 shows a minimum value when the value of the coupling constant K is 0.18 to 0.19, and f2 is when the value of the coupling constant K is 0.18 to 0.19. Shows the maximum value. It was also found that the gap between f1 and f2 showed a minimum value when the value of the coupling constant K was 0.18 to 0.19. From these results, it was found that the coupling constant K is optimally 0.24 when f1 is assigned to ¹ H nucleus (= 500 MHz) and f2 is assigned to ¹⁹ F nucleus (= 470 MHz).

  Judging from FIG. 14, there are two optimum values of K, a higher one (K = 0.24) and a lower one (K = 0.13) across 0.18 to 0.19. However, the reason why the higher K value is selected in this embodiment is that when K = 0.13 side, the performance is close to the limit value and the design margin is poor, whereas K = 0. In the case of the 24 side, since the performance is not a limit value, there is a design margin, and a large degree of freedom can be taken in the design.

  FIG. 15 shows another embodiment of the detection coil of the NMR probe according to the present invention. This embodiment is characterized in that the axes of the high-frequency magnetic field generated by resonance are shifted from each other by a larger angle φ as compared with the first embodiment. A suitable angle φ is ± 20 ° or less.

  FIG. 16 shows another embodiment of the detection coil of the NMR probe according to the present invention. In the present embodiment, unlike the first embodiment, it is not a method of shifting the axis of the high-frequency magnetic field generated by resonance by the angle φ, but a method in which the lengths in the vertical axis direction are different, so This realizes double resonance.

^When f1 and f2 are separated from each other, such as a combination of ¹ H nucleus and ¹³ C nucleus, instead of a combination of ¹ H nucleus and ¹⁹ F nucleus, an inductance L of the coil as shown in FIGS. The double resonance to the two frequencies f1 and f2 can also be realized by the method of different values.

FIG. 17 shows an example in which LF such as ¹³ C nucleus is assigned to f1, and HF such as ¹ H nucleus is assigned to f2, and the inductance L value of the coil is set so that L1 >> L2. FIG. 18 shows an example in which HF such as ¹ H nucleus is assigned to f1, and LF such as ¹³ C nucleus is assigned to f2, and the inductance L value of the coil is set so that L2 >> L1.

  Judging from the example of the analysis diagram by the network analyzer shown in FIG. 19, the example of the circuit based on this configuration shows that there are resonance points at 125.71 MHz (Q to 65) and 500 MHz (Q to 269), respectively. It was. In addition, the values of the resonance voltage at both ends of the coil when 30 W RF power was injected were analyzed as 1387V and 1651V, respectively.

The above data was obtained by re-adjusting the matching capacitor for ¹ H and ¹³ C nuclei, respectively. Therefore, a modification including a switching device for switching the matching capacitor between the case of ¹ H nucleus and the case of ¹³ C nucleus, and a modification having a function of remotely sensing it are also within the scope of the present invention. Needless to say.

  Can be widely used in NMR apparatus.

1: Sample tube, 2: Magnet, 3: Room temperature shim, 4: NMR probe, 5: Temperature variable device, 6: Magnetic field correction device, 7: Computer, 8: Display, 9: Duplexer, 10: Amplifier, 11: Demodulation detector, 12: analog-digital converter, 13: first power amplifier, 14: oscillator, 15: second power amplifier, 16: third power amplifier

Claims (9)

An NMR characterized in that two different cylindrical inductances are arranged concentrically and resonated at two different frequencies by selecting the value of capacitive coupling and magnetic coupling between the inductances. Probe design method. 2. The NMR probe design method according to claim 1, wherein the two cylindrical inductances are two Helmholtz type sample coils having different diameters or lengths. 3. The NMR probe design according to claim 2, wherein the Helmholtz-type sample coil is arranged to face a metal ring electrode across a cylindrical insulator bobbin so as to resonate at a desired frequency. Method. The two Helmholtz type sample coils are characterized in that the magnitude of the magnetic coupling between the two coils is selected by changing the direction of the generation axis of the high-frequency magnetic field generated as a result of resonance by a predetermined angle φ. A method for designing an NMR probe according to claim 2. The NMR probe design method according to claim 4, wherein the predetermined angle φ is 0 <φ ≦ 20 °. The two Helmholtz type sample coils are associated with the main coil and the subcoil, of which the coil end on the main coil side is connected to the high frequency input / output means, and the LC resonance circuit on the subcoil side is not connected to the external circuit. The method for designing an NMR probe according to claim 2, wherein: 4. The method of designing an NMR probe according to claim 3, wherein the magnitude of the capacitive coupling is selected by changing the area of the facing portion of the Helmholtz type sample coil facing the ring electrode. In order to operate the resonant coil complex in which the Helmholtz type sample coil and the ring electrode are opposed to each other with the cylindrical insulator bobbin sandwiched between them, impedance matching is performed for each resonant frequency of the resonant coil complex. 4. The method for designing an NMR probe according to claim 3, further comprising switching means for switching a capacitive element to be taken. An NMR probe designed by the designing method according to claim 1.

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