NMR measurement method

There is disclosed an NMR measurement method and NMR apparatus in which the temperature of the NMR detection coil or the RF irradiation coil hardly varies if pulsed RF power is applied to the coil during NMR measurements. The apparatus includes the detection coil or the RF irradiation coil, a first RF power application means for applying RF power of a frequency necessary for measurement of NMR signals, a second RF power application means for applying RF power of a frequency not affecting the measurement of NMR signals, and a control means for controlling the two power application means such that the sum of the RF power applied to the coil from the first application means and the RF power applied to the coil from the second application means is kept almost constant.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an NMR measurement method and NMR apparatus and, more particularly, to an NMR measurement method and NMR apparatus using a detector assembly that is cooled to a cryogenic temperature by low-temperature helium gas to thereby enhance the sensitivity with which NMR signals are detected.

2. Description of Related Art

In an NMR apparatus, a strong static magnetic field is applied to a sample to induce a precessional motion of the magnetic moment of each atomic nucleus having a nuclear spin within the sample about the direction of the static field. Under this condition, an RF magnetic field is applied perpendicularly to the direction of the static field to induce a precessional motion of the magnetic moment of the atomic nucleus. Then, an NMR signal released when the precessional motion of the magnetic moment of the atomic nucleus returns to ground state from an excited state is observed as an RF magnetic field having a frequency intrinsic to the sample.

Usually, NMR signals are quite feeble and so attempts have been made to increase the detection sensitivity of the NMR apparatus. See Japanese Patent Laid-Open No. H10-307175, Japanese Patent Laid-Open No. H10-332801, and Japanese Patent Laid-Open No. 2001-153938. In particular, an NMR probe having a built-in detector is fitted with piping for circulating low-temperature gas. Thermal noise in the NMR apparatus is reduced by cryogenically cooling the detector, thus achieving gains in sensitivity.

The positional relation between the prior art NMR probe and a superconducting magnet producing a static magnetic field is shown inFIG. 1, where the superconducting magnet is indicated by A. A main coil B of superconducting wire is wound inside the superconducting magnet A. Normally, the main coil B is placed in an adiabatic vessel (not shown) capable of holding liquid helium or the like therein and is cooled to a cryogenic temperature. A nuclear magnetic resonance (NMR) probe C is made up of a jaw-like base portion placed outside the magnet and a cylindrical portion inserted inside the magnet. The superconducting magnet A is provided with a cylindrical hole D extending along the center axis of the magnet. The cylindrical portion is usually inserted into the hole D in the upward direction from its lower opening.

An example of structure of the prior art NMR probe is shown inFIG. 2. This example of structure is a low-temperature probe, known as a cooling probe. A probe container8is connected with a cryogenic cooling system14by a transfer line9. The inside of each of the probe containers8and cooling system14is evacuated for thermal insulation from the outside. A detector assembly1consisting of a detector coil and a tuning-and-matching circuit is placed in the probe container8. The detector assembly1is in thermal contact with a heat exchanger2and can be cooled. A heater100is mounted near the detector assembly1to control the temperature of the detector assembly1.

The detector assembly1detects a nuclear magnetic resonance and produces an output signal. This signal is applied to a head amplifier3via a cable6and amplified. The output signal from the head amplifier3is sent to a spectrometer (not shown) via a cable7. The head amplifier3is in thermal contact with a heat exchanger4and can be cooled. A heater5is mounted close to the head amplifier3to provide temperature control of the head amplifier3.

The detector assembly1has a structure that permits a sample to be entered from outside of the probe container8. Since this structure is not associated with the cooling, it is not shown.

The cryogenic cooling system14has a first cooling stage20and a second cooling stage22. A cryocooler19, such as a Gifford-McMahon cryocooler, is mounted in the cooling system14. Heat exchangers21and23are mounted in the first cooling stage20and second cooling stage22, respectively. Furthermore, heat exchangers24and25are mounted in pipes15and16, respectively. Pipes17and18for supplying a working gas are connected with the cryocooler19. The transfer line9has pipes10,11,12, and13therein. The pipes10–11are connected with the heat exchanger2, whereas the pipes12–13are connected with the heat exchanger4.

The operation of this apparatus is next described. The working gas (helium gas) is supplied from an external compressor (not shown) via the pipes17and18to operate the cryocooler19. Besides, a refrigerant consisting of helium gas is supplied from the pipe16and passed through the heat exchanger24. Then, the refrigerant is cooled by the heat exchanger21in the first cooling stage20. Furthermore, the refrigerant passes through the heat exchanger25and reaches the heat exchanger23in the second cooling stage22, where the helium gas is cooled further. At this time, the temperature of the gas is 10 K.

The cooled helium gas is supplied into the heat exchanger2in the pipe10within the transfer line9, thus cooling the detector assembly1. The temperature of the gas immediately prior to entering the heat exchanger2is 15 K. The temperature of the gas just leaving the heat exchanger2is 23 K. This temperature rise is caused by reception of the heat from the detector assembly1and by heating by means of the heater100operating to control the temperature of the detector assembly1.

Because the detector coil and tuning-and-matching circuit received in the detector assembly1are cooled, the Q value improves and the thermal noise decreases. Consequently, the sensitivity is improved. The helium gas returns to the cooling system14through the pipe11and precools the helium gas on the outward route by the heat exchanger25. The gas is increased to a temperature of 40 K and then supplied to the heat exchanger4via the pipe12. The gas cools the head amplifier3and improves the noise factor (NF) of the head amplifier3. Consequently, the output signal from the detector assembly1can be transferred to the spectrometer (not shown) via the cable7without deteriorating the signal-to-noise ratio (S/N).

The head amplifier3is maintained at an appropriate temperature by the heater5. The temperature of the gas immediately prior to entering the heat exchanger4is 40 K. The temperature of the gas just leaving the heat exchanger4is 90 K. This temperature rise is caused by reception of heat from the head amplifier3and by being heated by the heater5operating to control the temperature of the head amplifier3.

The helium gas returns to the cooling system14via the pipe13within the transfer line9and precools the helium gas on the outward route by means of the heat exchanger24. Then, the gas passes through the pipe15and returns into the external compressor (not shown). In this way, the gas is circulated.

The structure of an NMR detector assembly positioned at the front end of the prior art NMR probe is shown inFIG. 3. This assembly has a vacuum-insulated container8in which a heat exchanger2(cryocooler) is supported by pillars101. The detector assembly made up of an NMR detection coil33and a tuning-and-matching circuit36is in thermal contact with the heat exchanger2(cryocooler) via a cooling stage34and made stationary. The NMR detection coil33is wound along the outer periphery of a cylindrical bobbin (not shown). The center of detection of the detection coil33is set at a position where the magnetic field homogeneity is maximal within the external static magnetic field applied from a superconducting magnet (not shown).

The NMR signal detected by the NMR detection coil33is pulled out via a lead35and sent to the external spectrometer (not shown) through the tuning-and-matching circuit36, cable6, head amplifier3, and cable7.

The pipes10and11for injecting and discharging low-temperature refrigerant, such as low-temperature helium gas, are connected with the heat exchanger2(cryocooler). The cooling stage34is in thermal contact with the heat exchanger2(cryocooler) and has a thermometer26and the heater100for regulating the temperature. The cooling stage34is appropriately heated by the heater100while detecting the temperature of the stage34. A pipe31for a gas for varying the sample temperature extends along the center axis of the detection coil33. This gas is blown in the upward direction through the gas pipe31.

A sample tube40is inserted in the downward direction further into the gas pipe31for varying the sample temperature and positioned coaxially with the gas pipe31such that the center of the sample40is coincident with the center of detection of the detection coil33.

In this configuration, the low-temperature refrigerant, such as low-temperature helium gas, is injected from the outside into the heat exchanger2(cryocooler) via the pipe10, thus cooling the NMR detection coil33and tuning-and-matching circuit36. This improves the Q value of the detection coil33and reduces thermal noise in the coil33and tuning-and-matching circuit36. In consequence, the sensitivity of the NMR apparatus is improved. At the same time, a temperature-controlled gas is injected from below into the gas pipe31for varying the sample temperature, in order to maintain the sample tube40at an appropriate temperature.

The structure of an NMR detection coil assembly using a saddle coil is shown inFIGS. 4(a)–4(d). This is one example of the prior art NMR detection coil33.FIG. 4(a) is a perspective view of the detection coil assembly, showing the manner in which the coil assembly has been completed.FIG. 4(b) shows the components.FIG. 4(c) is a vertical cross section of the coil assembly.FIG. 4(d) is an expanded view of the coil foil.

InFIG. 4(a), coil foil37assumes a form as shown inFIG. 4(d) and is wound on the outermost side of a cylindrical detector assembly. The coil foil37is formed by stamping metal foil. This coil foil37is provided with two rectangular windows. A narrow cutout extends downward from the center of the bottom side of each window to the outer bottom side of the coil foil37. That is, the narrow cutout extends along the central vertical axis of each window.

This coil foil37is wound into a cylindrical form. As a result, a saddle coil having several portions is formed. That is, a cylindrical annular portion is formed in an upper portion. Two vertical band portions extending axially of the cylinder and having upper ends connected with the annular portion are formed in an intermediate portion. Four winged portions consisting of two opposite pairs of arc-shaped portions are formed in a lower portion.

A coil bobbin32made of a cylindrical dielectric is placed immediately inside of the coil foil37. This coil foil37is held on the outer surface of the coil bobbin32. Thus, the shape of the detection coil is maintained.

A cylindrical conductor38in the form of a cylindrical band is placed inside the annular portion made of the coil foil37. Another cylindrical conductor39also in the form of a cylindrical band is placed inside the winged portions of the coil foil37. The cylindrical conductor38and the annular portion of the coil foil37are opposite to each other with the coil bobbin32therebetween. Also, the cylindrical conductor39and the winged portions of the coil foil37are opposite to each other with the coil bobbin32therebetween.

The cylindrical winged portions of the coil foil37, the coil bobbin32made of the cylindrical dielectric, and the cylindrical conductor39made of the cylindrical band together form first and second capacitors. The annular portion of the coil foil37and the two vertical band portions together form an inductor. In this way, an LC resonator capable of resonating with radio-frequency signals is formed.

The sample tube40holding a sample therein is inserted to the inside of the cylindrical conductors38and39along the center axis of the cylindrical detector assembly.

RF magnetic fields are produced in the windows of the coil foil37vertically to the plane of the paper. The cylindrical conductors38and39act as shields against the produced RF fields. The ranges irradiated with the RF fields are so limited that the RF magnetic fields apply only to a desired region of the inserted sample.

The prior art low-temperature cooled NMR probe has one problem. In the NMR instrument, during measurement of an NMR signal, pulsed RF power is applied to the NMR detection coil within the probe to excite nuclear spins within the sample. When the applied RF power flows as an RF current on the surface of the NMR detection coil, the current is converted into heat by the electrical resistance intrinsic to the material of the NMR detection coil. This increases the temperature of the detection coil itself.

FIG. 5is a schematic diagram showing the RF power applied to the NMR detection coil and variations in the temperature of the coil. The top portion of the figure shows one example of the RF power applied to the NMR detection coil. In this example, six RF pulses of various magnitudes are applied to the detection coil for a period on the order of tens of milliseconds. After a lapse of a given time from the application, an NMR signal, known as a free induction decay (FID) signal, is detected for a period of about 0.5 second from the application of the RF pulse sequence.

The bottom portion of the figure shows variations in the temperature of the NMR detection coil during this time interval. The NMR detection coil is cooled to a low temperature of about 25 K by the cryocooler through which low-temperature helium gas is circulated. However, the cooling capability is limited. Furthermore, there is a thermal resistance between the cryocooler and the detection coil. Therefore, if pulsed RF power is applied to the detection coil, the coil shows an electrical resistance against the RF current and thus produces heat. Consequently, the low temperature can no longer be maintained. The temperature rises close to 30 K in a short time.

When the temperature increases, the electrical resistance of the metal material forming the NMR detection coil increases. This varies the Q value of the coil. Concomitantly, the matching condition is varied. As a result, RF magnetic fields of desired intensity cannot be produced within the detection coil. Nuclear spins within the sample cannot be excited normally.

This problem produces an adverse effect during reception of an NMR signal, as well as during excitation of nuclear spins. That is, during reception of an NMR signal, it is impossible to maintain the temperature of the NMR detection coil constant and so neither the Q value of the coil nor the matching condition is kept constant. Hence, normal NMR signals cannot be obtained.

Temperature variations occurring in a quite short time as described above cannot be controlled by the method consisting of correcting the temperature, using the thermometer26and heater100mounted on the cooling stage34, for the following reasons. There exists a thermal resistance between the NMR detection coil33generating heat and the thermometer26. Each of the coil33and thermometer26has a thermal capacity. As a result, the response time of the temperature control between the coil33and thermometer26has a relatively large time constant. Accordingly, if the temperature detected by the thermometer26is controlled to be constant, it is not assured that the temperature of the detection coil33itself is kept constant. The similar problem will also occur at the provided RF irradiation coils.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present invention to provide an NMR measurement method and NMR apparatus for performing an NMR measurement in such a way that if pulsed RF power is applied to the NMR detection coil or the RF irradiation coil, the temperature of the coils hardly varies.

An NMR measurement method according to the present invention for achieving the above-described object consists of applying RF power of a frequency not affecting measurement of NMR signals to the detection coil or the RF irradiation coil complementarily.

In one feature of the present invention, the detection coil or the RF irradiation coil has a resonance mode which is different from the resonance mode at the measurement frequency for an NMR signal and which does not affect the measurement of the NMR signal.

In another feature of the present invention, the aforementioned frequency which does not affect the measurement of the NMR signal can resonate in the same resonance mode as the measurement frequency for the NMR signal and is shifted from the measurement frequency for the NMR signal by a given frequency.

In a further feature of the present invention, the total amount of RF power applied to the detection coil or the RF irradiation coil is controlled so as not to substantially differ irrespective of whether the RF power of the frequency necessary for the measurement of the NMR signal is being applied or not.

In an additional feature of the present invention, the RF power having the frequency not affecting the measurement of the NMR signal is applied to the detection coil or the RF irradiation coil. An increase in the temperature of the detection coil or the RF irradiation coil is detected from the strength of the reflected RF power.

In still another feature of the present invention, the detection coil or the RF irradiation coil has a resonance mode which is different from the resonance mode at the measurement frequency for the NMR signal and which does not affect the measurement of the NMR signal.

In yet another feature of the present invention, the aforementioned frequency not affecting the measurement of the NMR signal can resonate in the same resonance mode as the measurement frequency for the NMR signal and is shifted from the measurement frequency for the NMR signal by a given frequency.

In a still additional feature of the present invention, the RF power of the frequency not affecting the measurement of the NMR signal can be so adjusted that the ratio of the RF power reflected from the detection coil or the RF irradiation coil to the RF power applied to the detection coil or the RF irradiation coil is minimized.

An NMR apparatus according to the present invention comprises: a detection coil or the RF irradiation coil; first RF power application means for applying RF power having a frequency necessary for measurement of an NMR signal to the detection coil or the RF irradiation coil; and second RF power application means for applying RF power having a frequency not affecting the measurement of the NMR signal to the detection coil or the RF irradiation coil.

In one feature of the present invention, the detection coil or the RF irradiation coil has a resonance mode which is different from the resonance mode at the measurement frequency for the NMR signal and which does not affect the measurement of the NMR signal.

In another feature of the present invention, the aforementioned frequency which does not affect the measurement of the NMR signal can resonate in the same resonance mode as the measurement frequency for the NMR signal and is shifted from the measurement frequency for the NMR signal by a given frequency.

In a further feature of the present invention, there is further provided control means for controlling the two RF power application means such that the sum of the RF power applied to the detection coil or the RF irradiation coil from the first RF power application means and the RF power applied to the detection coil or the RF irradiation coil from the second RF power application means is kept almost constant.

In an additional feature of the present invention, there is provided a power meter for detecting the ratio of the RF power reflected from the detection coil or the RF irradiation coil to the RF power applied to the detection coil or the RF irradiation coil from the second RF power application means.

In a still additional feature of the present invention, the RF power applied from the second RF power application means can be adjusted based on the value of the power meter such that the reflected RF power is minimized.

Other objects and features of the invention will appear in the course of the description thereof, which follows.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention are hereinafter described with reference to the accompanying drawings.FIGS. 6(a) and6(b) show an NMR apparatus according to one embodiment of the present invention.FIG. 6(a) is a circuit diagram, andFIG. 6(b) shows the relation between the NMR probe and the spectrometer.

The apparatus has an NMR detection coil33. Where the coil33is of the saddle-type as shown inFIGS. 4(a)–4(d), a cylindrical winged portion of coil foil37, a coil bobbin32made of a cylindrical dielectric, and a cylindrical conductor39in the form of a cylindrical band form two capacitors43and44as shown inFIG. 6(a). A cylindrical annular portion of the coil foil37and two vertical band portions form two inductors41and42, thus forming an LC resonator capable of resonating with RF signals.

A first tuning-and-matching circuit for matching to a first RF frequency RF1necessary for measurement of NMR signals and a second tuning-and-matching circuit for matching to a second RF frequency RF2that do not affect the measurement of NMR signals and hence is unnecessary for the measurement are connected with the NMR detection coil33. The tuning and matching to the frequency RF1are done using two variable capacitors46and47. The tuning and matching to the frequency RF2are performed using two variable capacitors45and48.

The NMR detection coil33is grounded at point a because a resonance mode (indicated by arrows51and52) at the frequency RF2is prepared for the NMR detection coil33, as well as the resonance mode (indicated by arrows49and50) at the measurement frequency RF1for measurement of NMR signals. The arrows49and50indicate the senses of RF currents when they maximize. Similarly, the arrows51and52indicate the senses of RF currents when they maximize. The resonance mode at the frequency RF2is different from the resonance mode at the measurement frequency RF1for NMR signals and does not affect the measurement.

A spectrometer55and an NMR probe54are connected by RF cables56and57that carry the RF1and RF2, respectively. A control means is incorporated in the spectrometer55to control two power application means of RF1and RF2such that the sum of the RF power applied to the NMR detection coil from the power application means of RF1and the RF power applied to the coil from the power application means of RF2is kept constant.

In this configuration, where the NMR detection coil is resonating with the frequency of RF1, if the RF currents are maximized, the RF currents flowing through the two vertical band portions of the NMR detection coil33are opposite in sense as indicated by the arrows49and50. Therefore, an RF magnetic field is produced inside the detection coil33, permitting NMR measurements. At this time, parts of the RF currents are converted into heat due to electrical resistance. This increases the temperature of the detection coil33.

On the other hand, where the NMR detection coil is resonating with the frequency of RF2, if the RF currents are maximized, the RF currents flowing through the two vertical band portions of the detection coil33are the same in sense as indicated by the arrows51and52. Therefore, no RF magnetic field is set up in the coil33. Hence, NMR measurements cannot be performed. However, this case is the same as the case of RF1only in that the RF currents are partly converted into heat due to electrical resistance. Again, the temperature of the NMR detection coil33rises.

FIGS. 7(a),7(b), and7(c) schematically show the electric power of RF1applied to the NMR detection coil, and the electric power of RF2.FIG. 7(d) shows the temperature variations of the coil.FIG. 7(a) indicates an example of the electric power of RF1applied to the NMR detection coil. In this example, six RF pulses having different magnitudes are applied to the coil for a period on the order of tens of milliseconds. An NMR signal, known as a FID signal, is detected after a lapse of a given time from the application of the RF pulse sequence. The FID signal is detected for about 0.5 second after the application of the RF pulse sequence.

FIG. 7(b) shows an example of electric power of RF2applied to the NMR detection coil together with the power of RF1. In this example, convexly protruding, pulsed power of RF1is applied to the NMR detection coil. At the same time, electric power corresponding to the pulsed power of RF1is subtracted from the power of RF2during a period corresponding to the pulse width of RF1, and power of this reduced RF2is applied to the detection coil.

Specifically, electric power of RF2that does not affect measurement of NMR signals and thus is unnecessary for the measurement is applied to the detection coil complementarily during the period in which electric power of RF1necessary for the measurement is not being applied to the coil. The total amount of RF power applied to the coil is so controlled that it does not substantially vary irrespective of whether the power of RF1necessary for the measurement is being applied or not.

In this way, the RF power output of RF2is made to have the concave portion such that the RF power intensity during the time when RF1is being applied is equal to the RF power intensity during the time when RF1is not being applied.

Consequently, the power of RF2acts complementarily as a dummy of the power of RF1. Therefore, as shown at the third stage ofFIG. 7(c) as viewed from above, the sum of the power of RF1and the power of RF2does not vary irrespective of whether the pulsed power of RF1is being applied or not.

As a result, the amount of RF power converted into heat due to the electrical resistance of the NMR detection coil does not vary irrespective of whether the pulsed power of RF1is being applied to the detection coil or not. The temperature of the detection coil33is kept constant as shown in the bottom ofFIG. 7(d).

The NMR detection coil33is heated by the power of RF2instead of using the temperature-controlling heater100. Therefore, the coil heats itself in response to the power of RF2. As a consequence, the response time is much shorter than in the case where the detection coil33is heated by the temperature-controlling heater100. As a result, an NMR apparatus can be provided in which the temperature of the NMR detection coil hardly varies if pulsed power of RF1is applied to the coil during NMR measurements.

In the embodiment ofFIG. 6(b), the spectrometer55and NMR probe54are connected by the two RF cables56and57that carry RF1and RF2, respectively. This configuration may be replaced by the structure shown inFIG. 8(b). That is, only a port tuned and matched to the frequency of RF1is used. This port is so constructed that it can also be tuned and matched to the frequency RF2. In this case, the input port for RF2and variable capacitors45and48are unnecessary. Electric power of frequency RF1necessary for NMR measurement and power of frequency RF2necessary for the NMR measurement can be applied using only one RF cable56. In this modified embodiment, RF1and RF2resonate in different resonant modes in the NMR detection coil33in the same way as in the case ofFIGS. 6(a) and6(b).

In another modified embodiment in which only the input port for RF1is used and the input port for RF2and variable capacitors45,48are omitted, only the resonance mode at RF1of the NMR detection coil33may be utilized as shown inFIG. 9(a), where the arrows49and50indicate the resonance mode, i.e., the senses of RF currents when they maximize.

Measurement of the vicinities of the resonance mode at RF1of the NMR detection coil33by the use of a network analyzer has produced reflection characteristics as shown inFIG. 9(b). In this figure, the frequency is plotted on the horizontal axis, while the reflection power is on the vertical axis. In normal NMR measurements, the RF1is tuned to a frequency f1at which the reflection power is lowest. Since the frequency range in which the reflection power is low is considerably wide, the detection coil33can be sufficiently resonated at a frequency f2shifted from the f, by a frequency Δf in the same resonance mode as the resonance mode at RF1. The arrows49and50indicate the senses of RF currents when they maximize. At the frequency f2, the NMR measurements are not affected.

Accordingly, temperature control of the NMR detection coil33can be provided as shown inFIG. 7(d) by preparing the second frequency f2which is shifted from the measurement frequency f, for NMR by frequency differences Δf, i.e., f2is close to f1, and at which NMR measurements are not affected. In addition to the f1, using the f2as RF power playing the same role as the RF2inFIG. 7(b), and using only the input port for RF1and resonance mode at RF1.

In this modified embodiment, f1and f2resonate in the same RF1resonance mode (indicated by the arrows49and50indicating the senses of RF currents when the currents maximize) in the NMR detection coil33. In this respect, this modified embodiment is fundamentally different from the modified embodiment ofFIGS. 8(a) and8(b).

FIG. 10shows another NMR apparatus according to the present invention. This apparatus has an NMR probe54and a spectrometer55which are connected by RF cables56and57carrying RF1and RF2, respectively.

A power meter58is inserted in the RF cable57that carries the RF2. RF input power A applied to the NMR probe54and RF power B reflected from the NMR probe54are measured. The power of RF2is adjusted.

The RF1and RF2are adjusted in the manner described now. RF power of about 5 W, for example, is first applied at the frequency of RF2. Tuning and matching are performed to make zero the reflected power. In this way, the RF power of 5 W is constantly applied to the NMR detection coil. The temperature of the detection coil is in equilibrium at some temperature T. Then, RF power of about 1 mW, for example, which seems to hardly affect the temperature of the detection coil, is applied at the frequency RF1. Tuning and matching are done to make zero the reflected power.

Then, the method already described in connection withFIGS. 7(a)–7(d) is implemented. That is, the sum of the power of RF1and the power of RF2is kept constant. RF power is output such that the total amount of heat produced from the detection coil and the surroundings is maintained constant. However, after a lapse of some time, if an adjustment error occurs, the temperature of the NMR detection coil rises.

If the temperature of the NMR detection coil rises, the electrical resistance of the coil increases and the Q value decreases, then the RF2deviates from the matching condition and the reflected power of RF2increases. Accordingly, the ratio of the RF power A applied to the NMR probe54to the RF power B reflected from the probe54is detected by the power meter58. If the ratio B/A increases, it is judged that the temperature of the detection coil has increased. The amplitude of the output power of RF2is readjusted to minimize the ratio B/A.

In this method, the temperature rise of the NMR detection coil can be detected from the Q value of the detection coil rather than from the thermometer. Therefore, the temperature rise can be detected directly without being affected by thermal resistance and thermal capacity existing between the detection coil and thermometer.

This method can be applied to any NMR apparatus using the second frequency not affecting NMR measurements, such as the NMR apparatus shown inFIGS. 6(a) and6(b),FIGS. 8(a) and8(b), andFIGS. 9(a) and9(b).

In this way, if NMR measurements are performed for a long time, the total amount of heat generated from the NMR detection coil and its surroundings can be controlled to be constant at all times.

FIGS. 11(a)–11(d) show a modified embodiment illustrating the power of RF1applied to the NMR detection coil, the power of RF2, and temperature variations of the detection coil.FIG. 11(a) indicates one example of electric power of RF1applied to the detection coil. In this example, six RF pulses of different magnitudes are applied to the detection coil for a period on the order of tens of milliseconds. After a lapse of a given time from the application of the RF pulse sequence, an NMR signal (FID signal) is detected for a period of about 0.5 second from the application.

FIG. 11(b) shows one example of the electric power of RF2applied to the NMR detection coil together with the power of RF1. In this example, pulsed power of RF1is applied to the detection coil. At the same time, the power of RF2of intensity weaker than that of RF1is subtracted from the steady value of the power of RF2, and this differential power is applied to the detection coil for a period longer than the period corresponding to the pulse width of the pulsed power of RF1.

More specifically, at the frequency RF1, a short and strong RF power is applied. On the other hand, at the frequency RF2, a long and short RF power is subtracted from the steady value of the RF2, and the differential power is applied. As a result, shown inFIG. 11(d), the temperature of the detection coil rises temporarily but the temperature of the detection coil is controlled to be constant during a short period (e.g., less than 0.1 millisecond) that does not affect NMR signals.

Consequently, the maximum power at the frequency RF2can be set much lower than the maximum power at the frequency RF1. The thermal load on the cryocooler that cools the NMR detection coil can be alleviated.

FIGS. 12(a)–12(c) show another NMR apparatus according to the present invention.FIG. 12(a) is an expanded view of the NMR detection coil.FIG. 12(b) is a cross-sectional view of the coil.FIG. 12(c) is a circuit diagram.

The NMR detection coil, indicated by numeral60, consists of coil wire wound around a coil bobbin61. In this embodiment, a saddle coil fabricated by winding a wire is used instead of the saddle coil (FIGS. 4(a)–4(d)) fabricated from foil. The NMR measurement method according to the present invention can be applied to this NMR detection coil in the same way as in the above-described embodiments.

FIGS. 13(a) and13(b) show a further NMR apparatus according to the present invention.FIG. 13(a) is an expanded view of the NMR detection coil, andFIG. 13(c) is a circuit diagram.

The NMR detection coil, indicated by numeral33, is an Alderman-Grant coil (resonator coil) instead of the saddle coil shown inFIGS. 4(a)–4(d). This resonator coil is fabricated by placing H-shaped coil foil68on the inside of a dielectric bobbin65and placing cylindrical conductors66and67on the outside of the bobbin65. The NMR measurement method according to the present invention can also be applied to this NMR detection coil in the same way as in the above-described embodiments.

InFIG. 13(a), the cylindrical conductors66and67are grounded for the following reason. A resonance mode (indicated by the arrows49and50) at the measurement frequency RF1for NMR signals is prepared for the NMR detection coil33. The arrows49and50indicate the senses of RF currents when they maximize. In addition, a second resonance mode (indicated by the arrows51and52) at frequency RF2that does not affect the measurement of NMR signals is prepared for the detection coil33, the second resonance mode being different from the first-mentioned mode at RF1. The arrows51and52indicate the senses of RF currents when they maximize.

FIG. 14shows still another NMR apparatus according to the present invention. Where this NMR detection coil, indicated by reference numeral33, is a saddle coil as shown inFIGS. 4(a)–4(d), a cylindrical winged portion of coil foil37, a coil bobbin32made of a cylindrical dielectric, and a cylindrical conductor39in the form of a cylindrical band form two capacitors43and44as shown in ofFIG. 6(a). The cylindrical annular portion of the coil foil37and the two vertical band portions form two inductors41and42. Consequently, an LC resonator capable of resonating with RF signals is formed.

A tuning-and-matching circuit for high frequency RF1necessary for measurement of NMR signals and a tuning-and-matching circuit for high frequency RF2that does not affect the measurement of NMR signals and thus are unnecessary for the measurement are connected with the NMR detection coil33. Tuning and matching at the frequency RF1are done using two variable capacitors46and47. Tuning and matching at the frequency RF2are performed using two variable capacitors45and48.

In this embodiment, a respective one end of coils73and70is connected with each of the two inductors41and42. The other end of the coil73is grounded via a variable capacitor74. The other end of the coil70is grounded via a capacitor71. A third RF wave RF3. that induces resonance with deuterium nuclei (2D) is injected into the junction of the coil73and variable capacitor74via a matching variable capacitor75.

In this embodiment, RF1is the RF wave for observation of hydrogen nuclei (1H). On the other hand, RF2corresponds to a dummy RF wave used to control the temperature of the NMR detection coil. RF3. corresponds to the RF wave for locking used to compensate for drift in the static magnetic field in the NMR apparatus. A locking resonator circuit for resonating RF3. is composed of capacitor71, coil70, NMR detection coil33, coil73, and variable capacitors74,75.

Also, in this embodiment, pulsed power of RF1protruding convexly is applied to the NMR detection coil. At the same time, power corresponding to the pulsed power of RF1is subtracted from the power of RF2, and this reduced power of RF2is applied to the detection coil during a period corresponding to the pulse width of the RF1pulse.

In this way, the RF output power of the frequency RF2is made to have a convex portion. As a result, the RF power intensity during the period in which RF1is being applied is made equal to the RF power intensity during the period in which it is not.

Thus, the power of RF2acts complementarily as a dummy of the power of RF1. The sum of the power of RF1and the power of RF2is constant irrespective of whether pulsed power of RF1is being applied or not. As a result, the amount of RF power converted into heat by the electrical resistance of the NMR detection coil is constant regardless of whether the pulsed power of RF1is being applied to the detection coil or not. The temperature of the detection coil33is kept constant.

In this embodiment, RF3. for locking is applied to the NMR detection coil, in addition to RF1and RF2. RF1is at a power level of tens of watts to more than one hundred watts. In contrast, RF3. is only at a power level on the order of milliwatts and hardly affects the temperature of the NMR detection coil. Accordingly, it is not necessary to control the RF2so as to complement the period in which the RF3. is interrupted. RF3. can be neglected.

In the description of the above embodiments, the NMR apparatus uses only one wave as the RF wave for observing an NMR signal. Obviously, the invention can be applied to an NMR apparatus using two RF waves HF and LF to observe an NMR signal or an NMR apparatus providing an RF irradiation system.

In particular, where the invention is applied to an NMR apparatus using two RF waves HF and LF for observing an NMR signal, the applied power of RF2may be recessed at the timings of pulsed power of HF and pulsed power of LF.

Furthermore, where the invention is applied to an NMR apparatus providing an RF irradiation system, the power of RF2may also be applied to an RF irradiation coil of the probe in the same manner.

Furthermore, the invention can be applied to a multiple-tuning NMR apparatus using three or more RF waves for observing an NMR signal. The applied power of RF2may be recessed at the timings of the application of each RF pulse.

As described so far, according to the NMR measurement method of the present invention, RF power of a frequency not affecting measurement of NMR signals is applied to the detection coil complementarily. The average value of the sum of RF powers applied to the detection coil is kept almost constant irrespective of whether the RF power of the frequency necessary for the measurements of NMR signals is applied or not. Therefore, an NMR measurement method can be offered in which the temperature of the detection coil hardly varies if pulsed RF power is applied to the NMR detection coil during NMR measurements.

Furthermore, the NMR apparatus of the present invention comprises a detection coil, first RF power application means for applying RF power of a frequency necessary for measurements of NMR signals to the detection coil, second RF power application means for applying RF power of a frequency not affecting the measurements of NMR signals to the detection coil, and control means for controlling the two RF power application means to maintain almost constant the average value of the sum of the RF power applied to the coil from the first RF power application means and the RF power applied to the coil from the second RF power application means. Therefore, an NMR apparatus can be offered in which the temperature of the detection coil hardly varies if pulsed RF power is applied to the detection coil during NMR measurements.

Having thus described our invention with the detail and particularity required by the Patent Laws, what is desired protected by Letters Patent is set forth in the following claims.