Techniques for determining a nuclear magnetic resonance relaxation time and/or a nuclear magnetic resonance spectrum of a probe

Abstract

A system and method for determining a nuclear magnetic resonance relaxation time of a probe includes polarizing first nuclei and second nuclei by applying a longitudinal static magnetic field to the probe, exchanging the polarizations of the first nuclei and the second nuclei by irradiating a swap sequence of transverse magnetic field pulses, transversely magnetizing the second nuclei by irradiating at least one excitation pulse and measuring the resulting magnetization signal of the second nuclei, and determining the nuclear magnetic resonance relaxation time of the second nuclei based on the measured magnetization signal of the second nuclei.

Claims

1. A method of determining a nuclear magnetic resonance relaxation time of a probe and/or a nuclear magnetic resonance spectrum of a probe, wherein the probe comprises first nuclei with a first gyromagnetic ratio and second nuclei with a second gyromagnetic ratio, and the first gyromagnetic ratio is larger than the second gyromagnetic ratio, the method comprising: a first step of polarizing the first nuclei and the second nuclei by applying a longitudinal static magnetic field B.sub.0 to the probe; a second step of exchanging the polarizations of the first nuclei and the second nuclei by irradiating at least one swap sequence of transverse magnetic field pulses; a third step of transversely magnetizing the second nuclei by irradiating at least one excitation pulse and measuring the resulting magnetization signal; a fourth step of determining the nuclear magnetic resonance relaxation time and/or the nuclear magnetic resonance spectrum based on the measured magnetization signal, wherein a temporal length of the at least one swap sequence is substantially identical to an inverse of the longitudinal spin-spin interaction strength between the first nuclei and the second nuclei.

2. The method according to claim 1, wherein by irradiating the at least one swap sequence of transverse magnetic field pulses in the second step the nuclear spins of the first nuclei and the nuclear spins of the second nuclei become realigned in the direction of the longitudinal static magnetic field B.sub.0 immediately after the second step is carried out and before the third step is carried out.

3. The method according to claim 1, wherein the irradiating of the at least one swap sequence of transverse magnetic field pulses in the second step comprises irradiating a first sequence of transverse magnetic field pulses to excite the first nuclei and irradiating a second sequence of transverse magnetic field pulses to excite the second nuclei, wherein pulse frequencies of the transverse magnetic field pulses of the first sequence correspond to the Larmor frequency of the first nuclei in the longitudinal static magnetic field B.sub.0, and pulse frequencies of the transverse magnetic field pulses of the second sequence correspond to the Larmor frequency of the second nuclei in the longitudinal static magnetic field B.sub.0.

4. The method according to claim 3, wherein the transverse magnetic field pulses of the first sequence and the transverse magnetic field pulses of the second sequence are irradiated simultaneously and synchronously, and/or the temporal length of the first sequence of transverse magnetic field pulses is the same as the temporal length of the second sequence of transverse magnetic field pulses.

5. The method according to claim 3, wherein transverse magnetic field pulse of the first sequence corresponds to a transverse magnetic field pulse of the second sequence in terms of a pulse-center timing, a magnetic field pulse rotation angle and/or a magnetic field pulse oscillation direction.

6. A non-transitory computer readable medium comprising instructions which, when the program is executed by a computer, cause the computer to carry out the steps of the method according to claim 1.

7. A method of determining a nuclear magnetic resonance relaxation time of a probe and/or a nuclear magnetic resonance spectrum of a probe, wherein the probe comprises first nuclei with a first gyromagnetic ratio and second nuclei with a second gyromagnetic ratio, and the first gyromagnetic ratio is larger than the second gyromagnetic ratio, the method comprising: a first step of polarizing the first nuclei and the second nuclei by applying a longitudinal static magnetic field B.sub.0 to the probe; a second step of exchanging the polarizations of the first nuclei and the second nuclei by irradiating at least one swap sequence of transverse magnetic field pulses; a third step of transversely magnetizing the second nuclei by irradiating at least one excitation pulse and measuring the resulting magnetization signal; and a fourth step of determining the nuclear magnetic resonance relaxation time and/or the nuclear magnetic resonance spectrum based on the measured magnetization signal; wherein the irradiating of the at least one swap sequence of transverse magnetic field pulses, the irradiating of the first sequence of transverse magnetic field pulses and/or the irradiating of the second sequence of transverse magnetic field pulses in the second step comprises: irradiating a first transverse magnetic field pulse, wherein the first transverse magnetic field pulse is a (π.sub.x/2)-pulse; irradiating a second transverse magnetic field pulse with a predetermined time delay t.sub.D after irradiating the first transverse magnetic field pulse, wherein the second transverse magnetic field pulse is a π.sub.x-pulse; irradiating a third transverse magnetic field pulse with the predetermined time delay t.sub.D after irradiating the second transverse magnetic field pulse, wherein the third transverse magnetic field pulse is a π.sub.x-pulse; irradiating a fourth transverse magnetic field pulse with no time delay and immediately after irradiating the third transverse magnetic field pulse, wherein the fourth transverse magnetic field pulse is a (π.sub.−x/2)-pulse; irradiating a fifth transverse magnetic field pulse with no time delay and immediately after irradiating the fourth transverse magnetic field pulse, wherein the fifth transverse magnetic field pulse is a (π.sub.y/2)-pulse; irradiating a sixth transverse magnetic field pulse with the predetermined time delay t.sub.D after irradiating the fifth transverse magnetic field pulse, wherein the sixth transverse magnetic field pulse is a π.sub.x-pulse; and irradiating a seventh transverse magnetic field pulse with the predetermined time delay t.sub.D after irradiating the sixth transverse magnetic field pulse, wherein the seventh transverse magnetic field pulse is a (π.sub.−y/2)-pulse.

8. The method according to claim 7, wherein the predetermined time delay is t.sub.D=1/(4 J.sub.HN), wherein J.sub.HN is the longitudinal spin-spin interaction strength between the first nuclei and the second nuclei.

9. The method according to claim 7, wherein at least the second step and the third step form a scan sequence that is repeatedly carried out before the fourth step is carried out in order to achieve an improved signal-to-noise ratio for the determination of the nuclear magnetic resonance relaxation time and/or the nuclear magnetic resonance spectrum in the fourth step, wherein the time delay between carrying out two scan sequences corresponds to a thermal equilibration time T.sub.eq of the first nuclei.

10. A method of determining a nuclear magnetic resonance relaxation time of a probe and/or a nuclear magnetic resonance spectrum of a probe, wherein the probe comprises first nuclei with a first gyromagnetic ratio and second nuclei with a second gyromagnetic ratio, and the first gyromagnetic ratio is larger than the second gyromagnetic ratio, the method comprising: a first step of polarizing the first nuclei and the second nuclei by applying a longitudinal static magnetic field B.sub.0 to the probe; a second step of exchanging the polarizations of the first nuclei and the second nuclei by irradiating at least one swap sequence of transverse magnetic field pulses; a third step of transversely magnetizing the second nuclei by irradiating at least one excitation pulse and measuring the resulting magnetization signal; and a fourth step of determining the nuclear magnetic resonance relaxation time and/or the nuclear magnetic resonance spectrum based on the measured magnetization signal; wherein the probe also comprises third nuclei with a third gyromagnetic ratio, and the exchanging of the polarization of the first nuclei and the second nuclei by irradiating at least one swap sequence of transverse magnetic field pulses in the second step comprises: exchanging of the polarization of the first nuclei and the third nuclei by irradiating a first swap sequence of transverse magnetic field pulses; exchanging of the polarization of the third nuclei and the second nuclei by irradiating a second swap sequence of transverse magnetic field pulses; and exchanging of the polarization of the first nuclei and the third nuclei by irradiating a third swap sequence of transverse magnetic field pulses.

11. The method according to claim 10, wherein: a longitudinal spin-spin interaction strength J.sub.HN between the first nuclei and the second nuclei is smaller than a longitudinal spin-spin interaction strength J.sub.HC between the first nuclei and the third nuclei; and/or a longitudinal spin-spin interaction strength J.sub.HN between the first nuclei and the second nuclei is smaller than a longitudinal spin-spin interaction strength J.sub.CN between the second nuclei and the third nuclei.

12. The method according to claim 10, wherein the first nuclei are atomic nuclei of .sup.1H isotopes and/or the second nuclei are atomic nuclei of .sup.15N isotopes or .sup.13C isotopes.

13. The method according to claim 10, wherein: the nuclear magnetic resonance relaxation time is a longitudinal relaxation time of the second nuclei or a transverse relaxation time of the second nuclei; and/or the nuclear magnetic resonance spectrum is a one-dimensional nuclear magnetic resonance spectrum of the second nuclei; and/or multiple excitation pulses are irradiated in the third step, wherein the multiple excitation pulses correspond to a saturation-recovery pulse sequence, an inverse-recovery pulse sequence and/or a Carr-Purcell-Meiboom-Gil pulse sequence.

14. An apparatus configured to determine a nuclear magnetic resonance relaxation time of a probe and/or a nuclear magnetic resonance spectrum of a probe, wherein the probe comprises first nuclei with a first gyromagnetic ratio and second nuclei with a second gyromagnetic ratio, and the first gyromagnetic ratio is larger than the second gyromagnetic ratio, wherein the apparatus comprises: at least one static magnetic field generating unit configured to polarize the first nuclei and the second nuclei by applying a longitudinal static magnetic field B.sub.0 to the probe; at least one electromagnetic pulse generating unit configured to exchange the polarizations of the first nuclei and the second nuclei by irradiating at least one swap sequence of transverse magnetic field pulses, and to transversely magnetize the second nuclei by irradiating at least one excitation pulse of transverse magnetic field pulses; at least one signal receiving unit configured to measure a magnetization signal resulting from the at least one excitation pulse; and at least one electronic control and evaluation unit configured to determine the nuclear magnetic resonance relaxation time and/or the nuclear magnetic resonance spectrum based on the measured magnetization signal, wherein a temporal length of the at least one swap sequence is substantially identical to an inverse of the longitudinal spin-spin interaction strength between the first nuclei and the second nuclei.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Exemplary embodiments of the invention are illustrated in the drawings and will be described with reference to FIGS. 1 to 5.

(2) In the figures:

(3) FIG. 1 shows a schematic diagram of an embodiment of the apparatus,

(4) FIG. 2 shows a schematic flow diagram of an embodiment of the method,

(5) FIG. 3 shows an embodiment of a swap pulse sequence,

(6) FIG. 4a shows a saturation-recovery pulse sequence as an embodiment of an excitation pulse sequence,

(7) FIG. 4b shows an inverse-recovery pulse sequence as an embodiment of an excitation pulse sequence,

(8) FIG. 4c shows a CPMG pulse sequence as an embodiment of an excitation pulse sequence,

(9) FIG. 5a shows the molecular structure of ethylphthalimidomalonate-2-.sup.13C-.sup.15N,

(10) FIG. 5b shows a nuclear magnetic resonance spectrum determined based on a magnetization signal without prior exchange of the polarizations,

(11) FIG. 5c shows a nuclear magnetic resonance spectrum determined based on a magnetization signal with prior exchange of the polarizations,

DETAILED DESCRIPTION OF EMBODIMENTS

(12) FIG. 1 shows a schematic diagram of an embodiment of the apparatus. The apparatus comprises one static magnetic field generating unit STATIC configured to polarize the first nuclei H and the second nuclei N by applying a longitudinal static magnetic field B.sub.0 to the probe P.

(13) The apparatus further comprises one electromagnetic pulse generating unit PULSE configured to exchange the polarizations of the first nuclei H and the second nuclei N by irradiating a swap sequence SWAP of transverse magnetic field pulses. The electromagnetic pulse generating unit PULSE is also configured to transversely magnetize the second nuclei N by irradiating an excitation pulse sequence EXC of transverse magnetic field pulses.

(14) The apparatus further comprises one signal receiving unit DECT configured to measure a magnetization signal FID of the second nuclei N resulting from irradiating the excitation pulse sequence EXC.

(15) The apparatus further comprises one electronic control and evaluation unit CONTROL configured to determine a nuclear magnetic resonance relaxation time and a nuclear magnetic resonance spectrum based on the measured magnetization signal FID of the second nuclei N.

(16) The static magnetic field generating unit STATIC, the electromagnetic pulse generating unit PULSE and the signal receiving unit DECT are electronically connected to the electronic control and evaluation unit CONTROL. The static magnetic field generating unit STATIC is configured to generate a magnetic field with a strength between 5 Tesla and 20 Tesla. The electromagnetic pulse generating unit PULSE comprises magnetic excitation coils and is configured to generate radio-frequency pulses as transverse magnetic field pulses. The signal receiving unit DECT comprises magnetic receiver coils. The magnetic receiver coils of the signal receiving unit DECT are the same as the magnetic excitation coils of the electromagnetic pulse generating unit PULSE. The signal receiving unit DECT also comprises at least one amplifier configured to amplify the magnetization signal FID. The electronic control and evaluation unit CONTROL comprises a computer.

(17) Recurring features are provided in the following figures with identical reference signs as in FIG. 1.

(18) FIG. 2 shows a schematic flow diagram of an embodiment of the method of determining a nuclear magnetic resonance relaxation time of a probe P and a nuclear magnetic resonance spectrum of a probe P, wherein the probe P comprises first nuclei H with a first gyromagnetic ratio and second nuclei N with a second gyromagnetic ratio. The first gyromagnetic ratio is larger than the second gyromagnetic ratio.

(19) The method comprises a first step S1 of polarizing the first nuclei H and the second nuclei N by applying a longitudinal static magnetic field B.sub.0 to the probe P, a second step S2 of exchanging the polarizations of the first nuclei H and the second nuclei N by irradiating a swap sequence SWAP of transverse magnetic field pulses, a third step S3 of transversely magnetizing the second nuclei N by irradiating an excitation pulse sequence EXC and measuring the resulting magnetization signal FID of the second nuclei N, and a fourth step S4 of determining the nuclear magnetic resonance relaxation time T.sub.1(N) of the second nuclei N and the nuclear magnetic resonance spectrum of the second nuclei N based on the measured magnetization signal FID of the second nuclei N.

(20) The magnetization signal FID corresponds to the time-decaying transverse bulk magnetization of the second nuclei and is also called free induction decay. The magnetization signal FID induces a time-dependent electrical current that is measured in a magnetic receiver coil of the signal receiving unit DECT.

(21) The second step S2 and the third step S3 form a scan sequence SCAN that is iteratively repeated for an improved signal-to-noise ratio when determining the nuclear magnetic relaxation time T.sub.1(N) of the second nuclei N and the nuclear magnetic spectrum of the second nuclei N in the fourth step S4.

(22) The time delay between two scan sequences SCAN, i.e., the time gap between irradiating the last transverse magnetic field pulse of the excitation pulse sequence EXC in the third step S3 of a first scan sequence and irradiating the first transverse magnetic field pulse of a swap sequence SWAP in the second step S2 of a second scan sequence that follows directly after the first scan sequence, corresponds to a thermal equilibration time Teq of the first nuclei H.

(23) FIG. 3 shows an embodiment of a swap pulse sequence SWAP. The swap sequence SWAP of transverse magnetic field pulses comprises a first sequence SEQ1 of transverse magnetic field pulses to excite the first nuclei H and a second sequence SEQ2 of transverse magnetic field pulses to excite the second nuclei N. Thereby, the pulse frequencies of the transverse magnetic field pulses of the first sequence SEQ1 correspond to the Larmor frequency of the first nuclei H in the longitudinal static magnetic field B.sub.0. The pulse frequencies of the transverse magnetic field pulses of the second sequence SEQ2 correspond to the Larmor frequency of the second nuclei N in the longitudinal static magnetic field B.sub.0. The first sequence SEQ1 of transverse magnetic field pulses and the second sequence SEQ2 of transverse magnetic field pulses are irradiated simultaneously and synchronously.

(24) In the following, it is assumed that the longitudinal static magnetic field B.sub.0 is applied in the longitudinal z-direction of a Cartesian coordinate system, for ease of reference.

(25) Then, each of the first sequence SEQ1 and the second sequence SEQ2 may comprise the following transverse magnetic field pulses: a first transverse magnetic field pulse P1, wherein the first transverse magnetic field pulse P1 is a (π.sub.x/2)-pulse, a second transverse magnetic field pulse P2 with a predetermined time delay t.sub.D with respect to the first transverse magnetic field pulse P1, wherein the second transverse magnetic field pulse P2 is a π.sub.x-pulse, a third transverse magnetic field pulse P3 with a predetermined time delay t.sub.D with respect to the second transverse magnetic field pulse P2, wherein the third transverse magnetic field pulse P3 is a π.sub.x-pulse, a fourth transverse magnetic field pulse P4 following with no time delay immediately after the third transverse magnetic field pulse P3, wherein the fourth transverse magnetic field pulse P4 is a (π.sub.−x/2)-pulse, a fifth transverse magnetic field pulse P5 following with no time delay immediately after the fourth transverse magnetic field pulse P4, wherein the fifth transverse magnetic field pulse P5 is a (π.sub.y/2)-pulse, a sixth transverse magnetic field pulse P6 following with the predetermined time delay t.sub.D after the fifth transverse magnetic field pulse P5, wherein the sixth transverse magnetic field pulse P6 is a π.sub.x-pulse, and a seventh transverse magnetic field pulse P7 following with the predetermined time delay t.sub.D after the sixth transverse magnetic field pulse P6, wherein the seventh transverse magnetic field pulse P7 is a (π.sub.−y/2)-pulse.

(26) Thus, each transverse magnetic field pulse of the first sequence SEQ1 corresponds to a transverse magnetic field pulse of the second sequence SEQ2 in terms of pulse-center timing, magnetic field pulse rotation angle and magnetic field pulse oscillation direction.

(27) The predetermined time delay is given by t.sub.D=1/(4J.sub.HN) and thus corresponds to one quarter of the inverse of the longitudinal spin-spin interaction strength J.sub.HN between the first nuclei H and the second nuclei N (in frequency units).

(28) In the following, a product operator formalism is used to describe the effect of each pulse of the swap pulse sequence SWAP on the density matrix of the combined system comprising the first nuclei H and the second nuclei N.

(29) The Hamiltonian describing the Larmor frequency contributions and the longitudinal coupling of the nuclear spins of the first nuclei H and the second nuclei N is given (in frequency units) by
H=−ω.sub.HI.sub.z−ω.sub.NS.sub.z+2πJI.sub.z−S.sub.z,  (1)

(30) where ω.sub.H=γ.sub.1B.sub.0 denotes the Larmor frequency of the first nuclei with the first gyromagnetic ratio γ.sub.1, ω.sub.N=γ.sub.2B.sub.0 denotes the Larmor frequency of the second nuclei with the second gyromagnetic ratio γ.sub.2 and J denotes the longitudinal spin-spin interaction that is identical to J.sub.HN. Moreover, I.sub.z denotes the z-component of the nuclear spin operator of the first nuclei H and S.sub.z denotes the z-component of the nuclear spin operator of the second nuclei N. The quantum-mechanical expectation values of the z-components of the nuclear spin operators then correspond to the respective polarizations.

(31) Initially, i.e., before the swap pulse sequence SWAP is irradiated, the state of the combined system can at least approximately be described by the density matrix
ρ.sub.0=γ.sub.HI.sub.z+γ.sub.XS.sub.z,  (2)

(32) where γ.sub.H=ℏγ.sub.1B.sub.0/kT is a constant proportional to the first gyromagnetic ratio γ.sub.1 and γ.sub.X=ℏγ.sub.2B.sub.0/kT is a constant proportional to the second gyromagnetic ratio γ.sub.2. The expression above assumes thermal equilibrium at room temperature. Also, a constant additive contribution to the density matrix that is simply proportional to the identity matrix has been omitted in the above expression for simplicity.

(33) By irradiating the first transverse magnetic field pulses P1 of the first sequence SEQ1 and the second sequence SEQ2, the state of the combined system as described by the density matrix is transformed to
ρ.sub.1=−γ.sub.HI.sub.y−γ.sub.xS.sub.y.  (3)

(34) After waiting for the predetermined time delay t.sub.D, the state of the combined system is transformed by the unitary Hamiltonian dynamics to

(35) $\begin{array}{cc}{\rho }_2=-{\gamma }_H{I}_y\mathrm{cos\phi }+{\gamma }_{H}2I_x{S}_z\mathrm{sin\phi }-{\gamma }_X{S}_y\mathrm{cos\phi }+{\gamma }_{X}2I_z{S}_x\mathrm{sin\phi },\phi =\pi J*\left(\frac{1}{4J}\right).& \left(4\right)\end{array}$

(36) By irradiating the second transverse magnetic field pulses P2 of the first sequence SEQ1 and the second sequence SEQ2, the state of the combined system as described by the density matrix is transformed to
ρ.sub.3=γ.sub.HI.sub.y cos φ−γ.sub.H2I.sub.xS.sub.z sin φ+γ.sub.XS.sub.y cos φ−γ.sub.X2I.sub.zS.sub.x sin φ.  (5)

(37) After waiting for a predetermined time delay t.sub.D, the state of the combined system is transformed by the unitary Hamiltonian dynamics to
ρ.sub.4=γ.sub.HI.sub.y cos 2φ−γ.sub.H2I.sub.xS.sub.z sin 2φ+γ.sub.XS.sub.y cos 2φ−γ.sub.X2I.sub.zS.sub.x sin 2φ=−γ.sub.H2I.sub.xS.sub.z−γ.sub.X2I.sub.zS.sub.x.  (6)

(38) By irradiating the third transverse magnetic field pulses P3 of the first sequence SEQ1 and the second sequence SEQ2, the state of the combined system as described by the density matrix is transformed to
ρ.sub.5=γ.sub.H2I.sub.xS.sub.z+γ.sub.X2I.sub.zS.sub.x.  (7)

(39) By irradiating the fourth transverse magnetic field pulses P4 of the first sequence SEQ1 and the second sequence SEQ2, the state of the combined system as described by the density matrix is transformed to
ρ.sub.6=γ.sub.H2I.sub.xS.sub.y+γ.sub.X2I.sub.yS.sub.x.  (8)

(40) By irradiating the fifth transverse magnetic field pulses P5 of the first sequence SEQ1 and the second sequence SEQ2, the state of the combined system as described by the density matrix is transformed to
ρ.sub.7=γ.sub.H2I.sub.zS.sub.y−γ.sub.X2I.sub.yS.sub.z.  (9)

(41) After waiting for the predetermined time delay t.sub.D, the state of the combined system is transformed by the unitary Hamiltonian dynamics to

(42) $\begin{array}{cc}{\rho }_8=-{\gamma }_{H}2I_z{S}_y\mathrm{cos\phi }+{\gamma }_H{S}_x\mathrm{sin\phi }-{\gamma }_{X}2I_y{S}_z\mathrm{cos\phi }+{\gamma }_X{I}_z\mathrm{sin\phi },\phi =\pi J*\left(\frac{1}{4J}\right).& \left(10\right)\end{array}$

(43) By irradiating the sixth transverse magnetic field pulses P6 of the first sequence SEQ1 and the second sequence SEQ2, the state of the combined system as described by the density matrix is transformed to
ρ.sub.9=−γ.sub.H2I.sub.zS.sub.y cos φ+γ.sub.HS.sub.x sin φ−γ.sub.X2I.sub.yS.sub.z cos φ+γ.sub.XI.sub.x sin φ.  (11)

(44) After waiting for the predetermined time delay t.sub.D, the state of the combined system is transformed by the unitary Hamiltonian dynamics to
ρ.sub.10=γ.sub.H2I.sub.zS.sub.y cos 2φ+γ.sub.HS.sub.x sin 2φ−γ.sub.X2I.sub.yS.sub.z cos 2φ+γ.sub.XI.sub.x sin 2φ=γ.sub.HS.sub.x+γ.sub.XI.sub.x.  (12)

(45) By irradiating the seventh transverse magnetic field pulses P7 of the first sequence SEQ1 and the second sequence SEQ2, the state of the combined system as described by the density matrix is transformed to
ρ.sub.11=Y.sub.HS.sub.z+γ.sub.XI.sub.z.  (13)

(46) By comparing the state ρ.sub.0 of the combined system and the state ρ.sub.11 of the combined system obtained after irradiating the swap sequence SWAP, it can be seen that a full exchange of the polarizations has been achieved. Moreover, the nuclear spins of the first nuclei H and the nuclear spins of the second nuclei N are realigned with the longitudinal static magnetic field B.sub.0 immediately after the irradiating of the seventh transverse magnetic field pulse P7.

(47) FIG. 4a shows a saturation-recovery pulse sequence as an exemplary embodiment of an excitation pulse sequence EXC that is irradiated in the third step S3 in order to be able to measure the magnetization signal FID of the second nuclei N resulting from the excitation pulse sequence EXC. The saturation-recovery pulse sequence comprises two transverse magnetic field pulses, wherein the first transverse magnetic field pulse of the saturation-recovery pulse sequence is a (π.sub.x/2)-pulse with a pulse frequency corresponding to the Larmor frequency of the second nuclei N and the second transverse magnetic field pulse of the saturation-recovery pulse sequence is also a (π.sub.x/2)-pulse with a pulse frequency corresponding to the Larmor frequency of the second nuclei N. The second transverse magnetic field pulse of the saturation-recovery pulse sequence is irradiated with a time delay τ.sub.1 after irradiating the first transverse magnetic field pulse of the saturation-recovery pulse sequence. The saturation-recovery pulse sequence is irradiated multiple times, each time with a different time delay τ.sub.1. The longitudinal relaxation time T.sub.1(N) of the second nuclei is then determined from the temporal decay of the magnetization signal FID as a function of the time delay τ.sub.1.

(48) FIG. 4b shows an inverse-recovery pulse sequence as another exemplary embodiment of an excitation pulse sequence EXC that is irradiated in the third step S3. As compared to the saturation-recovery pulse sequence shown in FIG. 4a, the first transverse magnetic field pulse of the inverse recovery pulse sequence is a π.sub.x-pulse. Otherwise the determination of the longitudinal relaxation time T.sub.1(N) proceeds as described with respect to FIG. 4a.

(49) FIG. 4c shows a CPMG pulse sequence as another exemplary embodiment of an excitation pulse sequence EXC that is irradiated in the third step S3. The first transverse magnetic field pulse of the CPMG pulse sequence is a (π.sub.x/2)-pulse followed by a series of π.sub.y-pulses irradiated with specific time delays τ, 2τ, . . . as shown in FIG. 4c. The transverse relaxation time T.sub.2(N) of the second nuclei N is determined in the fourth step S4 in a standard way based on the magnetization signal FID measured after irradiating multiple CPMG pulse sequences, each with different characteristic time delays τ.

(50) FIG. 5 shows the molecular structure of ethylphthalimidomalonate-2-.sup.13C-.sup.15N molecules. Benzene-d6 is used as a solvent. The probe P thus comprises first nuclei H corresponding to the isotopes .sup.1H, second nuclei N corresponding to the isotopes .sup.15N and third nuclei corresponding to the isotopes .sup.13C. The longitudinal spin-spin interaction strength direction between the first nuclei H and the second nuclei N is J.sub.HN=1.7 Hz. The longitudinal spin-spin interaction strength between the first nuclei H and the third nuclei C is J.sub.HC=139 Hz. The longitudinal spin-spin interaction strength between the second nuclei N and the third nuclei C is J.sub.CN=13 Hz.

(51) Since the longitudinal spin-spin interaction strength J.sub.HN between the first nuclei H and the second nuclei N is smaller than the longitudinal spin-spin interaction strength J.sub.HC between the first nuclei H and the third nuclei C and smaller than the longitudinal spin-spin interaction strength J.sub.CN between the second nuclei N and the third nuclei C, it is advantageous to exchange the polarizations between the first nuclei H and the second nuclei N indirectly via the third nuclei C as described further below.

(52) This is achieved in an alternative embodiment by irradiating three swap pulse sequences SWAP consecutively, i.e., a first swap pulse sequence SWAP, a second swap pulse sequence SWAP and a third swap pulse sequence SWAP, wherein each of the three swap pulse sequences SWAP comprises transverse magnetic field pulses according to FIG. 3.

(53) More specifically, in an alternative embodiment the exchanging of the polarizations of the first nuclei H and the second nuclei N by irradiating at least one swap sequence SWAP of transverse magnetic field pulses in the second step S2 comprises exchanging of the polarizations of the first nuclei H and the third nuclei C by irradiating a first swap sequence SWAP of transverse magnetic field pulses, exchanging of the polarizations of the third nuclei C and the second nuclei N by irradiating a second swap sequence SWAP of transverse magnetic field pulses, exchanging of the polarizations of the first nuclei H and the third nuclei C by irradiating a third swap sequence SWAP of transverse magnetic field pulses.

(54) The first swap sequence SWAP, the second swap sequence SWAP and the third swap sequence SWAP are irradiated consecutively, one after the other. Thereby, each of the first swap sequence SWAP, the second swap sequence SWAP and the third swap sequence SWAP comprises irradiating synchronously a first sequence SEQ1 of transverse magnetic field pulses and a second sequence SEQ2 of transverse magnetic field pulses as shown in FIG. 3.

(55) The frequencies of the transverse magnetic field pulses of the first sequence SEQ1 of the first swap sequence SWAP correspond to the Larmor frequency of the first nuclei H. The frequencies of the transverse magnetic field pulses of the second sequence SEQ2 of the first swap sequence SWAP correspond to the Larmor frequency of the third nuclei C.

(56) The frequencies of the transverse magnetic field pulses of the first sequence SEQ1 of the second swap sequence SWAP correspond to the Larmor frequency of the second nuclei N. The frequencies of the transverse magnetic field pulses of the second sequence SEQ2 of the second swap sequence SWAP correspond to the Larmor frequency of the third nuclei C.

(57) The frequencies of the transverse magnetic field pulses of the first sequence SEQ1 of the third swap sequence SWAP correspond to the Larmor frequency of the first nuclei H. The frequencies of the transverse magnetic field pulses of the second sequence SEQ2 of the third swap sequence SWAP correspond to the Larmor frequency of the third nuclei C.

(58) The predetermined time delay t.sub.D1 of the first swap sequence SWAP is then be given by t.sub.D1=1/(4 J.sub.HC). The predetermined time delay t.sub.D2 of the second swap sequence SWAP is then be given by t.sub.D2=1/(4 J.sub.CN). The predetermined time delay t.sub.D3 of the third swap sequence SWAP is t.sub.D3=t.sub.D1.

(59) As a result the polarizations of the first nuclei H and the polarizations of the second nuclei N are exchanged much faster via the third nuclei C as compared to the direct exchange case using a single swap sequence SWAP discussed with respect to FIG. 3.

(60) FIG. 5b shows a nuclear magnetic resonance spectrum determined in the fourth step S4 based on a magnetization signal FID obtained in the third step S3 without prior exchange of the polarizations using a swap sequence SWAP as proposed in the second step S2, i.e., without carrying out the second step S2 of the proposed method before carrying out the third step S3. More specifically, the nuclear magnetic resonance spectrum shown in FIG. 5b is determined after carrying out 32 iterations of the third step S3 using an inverse-recovery pulse sequence with a time delay τ.sub.1=1 s as an excitation pulse sequence EXC and by averaging over the magnetization signals FID measured in each third step S3. Thereby, the time delay/equilibration time between two third steps S3 in the iteration loop is T.sub.eq=1000 s. The corresponding experiment lasted a total of about 9 hours. Finally, four results for the longitudinal relaxation times T.sub.1(N)=(115±5)s, (119±6)s, (114±7)s, (124±3)s are determined in the fourth step S4 from the widths of the four Lorentzian shaped resonances of the nuclear magnetic resonance spectrum shown in FIG. 5b.

(61) FIG. 5c shows a nuclear magnetic resonance spectrum determined based on a magnetization signal FID with prior exchange of the polarizations, i.e., in accordance with an embodiment of the proposed method. More specifically, the nuclear magnetic resonance spectrum shown in FIG. 5c is determined after carrying out only 4 iterations of the scan sequence SCAN using an inverse-recovery pulse sequence with a time delay τ.sub.1=1 s as an excitation pulse sequence EXC in each third step S3 of each scan sequence SCAN and by averaging over the magnetization signals FID measured in each third step S3 of the scan sequence SCAN. In each second step S2 of the scan sequence SCAN an indirect exchange of the polarizations by irradiating a first, second and third swap sequence SWAP in accordance with the embodiment discussed with respect to FIG. 5a is achieved. Thereby, the time delay/equilibration time between two scan sequences SCAN in the iteration loop is T.sub.eq=10 s. The corresponding experiment lasted a total of about 1 minute. Finally, four results for the longitudinal relaxation times T.sub.1(N)=(129±11)s, (101±3)s, (99±5)s, (107±8)s are determined in the fourth step S4 from the widths of the four Lorentzian shaped resonances of the nuclear magnetic resonance spectrum shown in FIG. 5c.

(62) Clearly, the nuclear magnetic resonance spectra in FIGS. 5a and 5b and the determined nuclear magnetic relaxation times are similar and have been obtained with comparable signal to noise ratios. However, the nuclear magnetic resonance spectrum in FIG. 5c and the corresponding nuclear magnetic relaxation times have been determined in a more time-efficient and resource-efficient way.

(63) Features of the different embodiments which are merely disclosed in the exemplary embodiments as a matter of course can be combined with one another and can also be claimed individually.