METHOD AND DEVICE FOR THE HYPERPOLARIZATION OF A MATERIAL SAMPLE

Abstract

The invention relates to a method for the hyperpolarization of a material sample (4), which hits a number of first spin moments (10) of a first spin moment type, wherein the number of first spin moments (10) is brought into interaction with a second spin moment (16) of a second spin moment type, wherein the first spin moments (10) are nuclear spin moments and the second spin moment (16) is an election spin moment, wherein the first and second spin moments (10, 16) are exposed to a homogeneous magnetic field (B), wherein the second spin moment (16) is polarized along the magnetic field (B), wherein the second spin moment (16) is coherently manipulated by means of a, preferably repeated, sequence (S) having a number of successive high-frequency pulses (P.sub.ki, P.sub.ki) temporally offset to each by durations (T.sub.ki, T.sub.ki, T), in such a way that a polarization transfer from the second spin moment (16) to the first spin moments (10) occurs, and wherein durations (T.sub.ki, T.sub.ki, T) inversely proportional to a Lamor frequency (.sub.Larmor) of the first spin moments (10) in the magnetic field (B) are inserted between high-frequency pulses (P.sub.ki, P.sub.ki).

Claims

1. A method for hyperpolarizing a material sample, which has a number of first spin moments of a first spin moment species, wherein the number of first spin moments is made to interact with a second spin moment of a second spin moment species, wherein the first spin moments are nuclear spin moments and the second spin moment is an electron spin moment, wherein the first and second spin moments are exposed to a homogeneous magnetic field, wherein the second spin moment is polarized along the magnetic field, wherein the second spin moment is coherently manipulated by means of a sequence with a number of successive radiofrequency pulses, time-shifted from one another by durations of time, in such a way that there is a polarization transfer from the second spin moment to the first spin moments, and wherein durations of time are used between the radiofrequency pulses, said durations of time being inversely proportional to a Larmor frequency of the first spin moments in the magnetic field.

2. The method as claimed in claim 1, wherein a sequence is produced to manipulate the second spin moment, the overall sequence duration of which, composed of the individual durations of time between the radiofrequency pulses, for bringing about the polarization transfer is shorter than the duration of time inversely proportional to the strength of an interaction between the spin moments.

3. The method as claimed in claim 1, wherein only a flip angle of 90 and/or 180 is used for each radiofrequency pulse of the sequence.

4. The method as claimed in claim 1, wherein the radiofrequency pulses are produced along two pulse axes that are oriented perpendicular to one another.

5. The method as claimed in claim 1, wherein a radiofrequency pulse of the sequence is a composite pulse.

6. The method as claimed in claim 1, wherein the second spin moment is optically polarized for polarization along the magnetic field.

7. The method as claimed in claim 1, wherein an electron spin moment of a color center of a solid sample is used as a second spin moment.

8. The method as claimed in claim 1, wherein a sequence portion is produced during the sequence, said sequence portion comprising at least one sequence unit, wherein the sequence unit has a sequence unit duration which is an odd multiple of half a Larmor period of the first spin moments in the magnetic field, and/or in that the radiofrequency pulses of successive sequence units have a relative time shift in relation to one another, and wherein one or more radiofrequency pulses are provided during a transition from a first sequence unit to a subsequent second sequence unit, said one or more radiofrequency pulses being used to rotate an alignment of the second spin moment that is oriented parallel to the magnetic field into a plane that is oriented perpendicular to the magnetic field.

9. The method as claimed in claim 1, wherein a sequence portion is produced during the sequence, during which sequence portion four radiofrequency pulses, each with a 90 flip angle, follow one another with a shift in relation to one another with the same durations of time.

10. The method as claimed in claim 1, wherein a sequence portion is produced during the sequence, during which sequence portion a number of radiofrequency pulses, each with a 180 flip angle, follow one another with a shift in relation to one another with the same durations of time, wherein the radiofrequency pulses are alternately embodied as composite pulses.

11. The method as claimed in claim 1, wherein a first sequence portion and a radiofrequency pulse with a 90 flip angle and a second sequence portion are produced successively during the sequence, wherein, during the sequence portions, a number of radiofrequency pulses, each with a 180 flip angle, follow one another with a shift in relation to one another with the same durations of time in each case, and wherein the durations of time of the second sequence portion have a relative time shift in relation to the durations of time of the first sequence portion.

12. The method as claimed in claim 1, wherein a first sequence portion and a second sequence portion are produced immediately successively during the sequence, wherein the sequence portions each comprise two radiofrequency pulses with a 90 flip angle, a radiofrequency pulse with a 180 flip angle being arranged at the midpoint therebetween in each case, wherein, in the first sequence portion, the radiofrequency pulses with a 90 flip angle are produced along a first pulse axis and the radiofrequency pulse with a 180 flip angle is produced along a second pulse axis that is oriented perpendicular to the first pulse axis and wherein, in the second sequence portion, the radiofrequency pulses with a 90 flip angle are produced along the second pulse axis and the radiofrequency pulse with a 180 flip angle is produced along the negative first pulse axis.

13. The method as claimed in claim 8, characterized in that the sequence portion or portions are repeated multiple times in succession during the sequence.

14. An apparatus for hyperpolarizing a material sample before and/or during a magnetic resonance analysis according to the method as claimed in claim 1.

Description

[0056] Exemplary embodiments of the invention are explained in more detail below on the basis of a drawing. In schematic and simplified illustrations:

[0057] FIG. 1 shows an apparatus for hyperpolarizing a material sample for a nuclear magnetic resonance analysis,

[0058] FIG. 2 shows the material sample in sections, with a number of nuclear spin moments and with a nanodiamond with an optically polarizable electron spin moment,

[0059] FIG. 3 shows a sequence of radiofrequency pulses for hyperpolarizing the material sample,

[0060] FIG. 4 shows a first exemplary embodiment of the sequence with a sequence portion with four successive radiofrequency pulses with 90 flip angles,

[0061] FIG. 5 shows a second exemplary embodiment of the sequence with a sequence portion with alternating radiofrequency pulses with 180 flip angles and YZ-composite pulses,

[0062] FIG. 6 shows the structure of a YZ-composite pulse,

[0063] FIG. 7 shows a third exemplary embodiment of the sequence with a first sequence portion and with a radiofrequency pulse with a 90 flip angle and with a second sequence portion,

[0064] FIG. 8 shows a fourth exemplary embodiment of the sequence with two sequence portions, each with two radiofrequency pulses with a 90 flip angle and with a radiofrequency pulse with a 180 flip angle arranged therebetween, and

[0065] FIGS. 9a and 9b show a simulation of the robustness of the sequence according to FIG. 8.

[0066] Parts and variables corresponding to one another are always provided with the same reference signs in all figures.

[0067] The apparatus 2 illustrated in FIG. 1 is suitable and configured for hyperpolarizing a material sample 4. To this end, the apparatus 2 is configured, in particular, as an NMR spectrometer for a magnetic resonance or nuclear magnetic resonance analysis of the material sample 4. The apparatus 2 comprises an electromagnet and/or superconducting magnet 6, with a north pole 6a and a south pole 6b, between which a homogeneous magnetic field B is produced during operation. For the purposes of hyperpolarizing and/or analyzing the material sample 4, the latter is positioned in the region exposed between the north pole 6a and south pole 6b.

[0068] In this exemplary embodiment, the material sample 4 is configured, in exemplary fashion, as a liquid sample in a (nonmagnetic) sample container 8. As illustrated in a simplified and schematic manner in FIG. 2, the material sample 4 has a number of spin moments 10, in particular nuclear spin moments. In FIG. 2, the nuclear spin moments 10 are only provided in exemplary fashion with reference signs.

[0069] In this exemplary embodiment, a number of solid samples or nanoparticles 12, in particular nanodiamonds, have been added to the material sample 4 as a suspension, said solid samples or nanoparticles preferably each having at least one color center 14 with an optically polarizable spin moment 16, in particular an electron spin moment. In an exemplary manner, FIG. 2 only illustrates one nanodiamond 12 with one electron spin moment 16 of a color center 14. By way of example, the color center 14 is an NV center (nitrogen vacancy center), which is alignable along a polarization axis, which is oriented parallel to the magnetic field B in exemplary fashion in FIG. 2, by means of laser light 18 of a laser 20.

[0070] The material sample 4 and the sample container 8 are at least partly surrounded in the assembled state by an excitation coil 22, illustrated using dotted lines, and a detection coil (pickup coil) 24, illustrated using dashed lines. The excitation coil 22 is connected to a radiofrequency generator 26 and produces a (pulse) sequence S with a number of radiofrequency pulses (FIG. 3) of a certain frequency, pulse duration, pulse amplitude and pulse phase angle during operation.

[0071] The radiofrequency pulses are preferably resonant with the electron spin moments 16 of the nanodiamonds 12, meaning that the frequency of the radiofrequency pulses is matched to the Larmor frequency of the electron spin moments 16 in the magnetic field B. In particular, the nuclear spin moments 10 of the material sample 4 are substantially not influenced by the radiofrequency pulses of the sequence S. The radiofrequency pulses are alternating electromagnetic fields, which are oriented substantially transversely, meaning perpendicular, to the magnetic field B.

[0072] The pickup coil 24 is embodied to receive and detect the alternating magnetic field produced by the nuclear spin moments 10 during an NMR analysis. To this end, the pickup coil 24 is configured as a Faraday coil, for example, which transmits the detected NMR signal D via a receiver 28 to a display device 30 for the purposes of analyzing and displaying an NMR spectrum 32 of the nuclear spin moments 10 of the material sample 4. In an alternative configuration, the excitation coil 22 and the pickup coil 24 are embodied as a common coil, for example.

[0073] During the operation of the apparatus 2, the material sample 4 is positioned in the magnet 6. As a result of the magnetic field B, the nuclear spin moments 10 of the material sample 4 are at least partly aligned along the magnetic field B. Expressed differently, a thermal equilibrium distribution of the nuclear spin moments 10 sets in, with equilibrium magnetization M.sub.z of the nuclear spin moments 10, which is oriented parallel to the magnetic field B, forming.

[0074] Within the scope of the NMR analysis, the equilibrium magnetization M.sub.z is converted into a transversal magnetization M.sub.xy, the precession of which in the magnetic field B induces the NMR signal D in the pickup coil 24. Therefore, the production of a transversal magnetization M.sub.xy that is as large as possible is desirable for the purposes of improving the NMR signal D. This requires an increase in the (longitudinal) equilibrium magnetization M.sub.z of the nuclear spin moments 10. To this end, the sequence S for hyperpolarizing the nuclear spin moments 10, meaning the production of an ordered alignment of the nuclear spin moments 10 in the material sample 4 far beyond the thermal equilibrium distribution, is carried out. Expressed differently, an increased magnetization M.sub.z is produced by the action of the sequence S before and/or during the NMR analysis, said increased magnetization having a higher degree of polarization or polarization than the equilibrium magnetization M.sub.z.

[0075] As a result of the sequence S, the polarization of the electron spin moments 16 of the attached nanoparticles 12 is transferred here to the nuclear spin moments 10. This polarization transfer is illustrated schematically in FIG. 2 using dashed lines. To this end, the electron spin moments 16 are initially optically polarized by means of the laser light 18 and are subsequently coherently manipulated by means of the sequence S radiated-in. As illustrated purely schematically in FIG. 2, the respective electron spin moment 16 is coupled to the nuclear spin moments 10 in the surroundings thereof by means of a dipolar interaction W.

[0076] Here, the polarization transfer of the electron spin moment 16 to the nuclear spin moment 10 is not implemented during radiofrequency pulse but, in particular, at the end of the sequence S on account of the total or collected spin dynamics caused thereby. Expressed differently, the electron spin moment 16 is substantially manipulated by means of the sequence S in such a way that a flip-flop process is facilitated on account of the interaction W with the nuclear spin moment 10. To this end, it is substantially necessary for the sequence S to produce an effective Hamiltonian H.sub.eff, which brings about the required spin dynamics.

[0077] In particular, an effective Hamiltonian H.sub.eff of the form

[00001]$H_{\mathrm{eff}}=-\frac{1}{}.Math.A_x\left(I_x.Math.S_x+I_y.Math.S_y\right)=-\frac{A_x}{2.Math.}.Math.\left(I_{+}.Math..Math.S.Math..Math.\_+I.Math..Math.\_.Math..Math.S_{+}\right)$

[0078] is required for the polarization transfer, where A.sub.x denotes the transversal interaction strength on account of the interaction W and is pi. Here, the spin operators of the nuclear spin moment 10 are denoted by I.sub.x, I.sub.y, I.sub.+ and I.sub., with the spin operators of the electron spin moment 16 being described accordingly by S.sub.x, S.sub.y, S.sub.+ and S.sub.. Here, the Hamiltonian H.sub.eff is specified in units of frequency (Hertz, Hz). Here, the polarization transfer occurs after a duration of time .sup.2/A.sub.x, meaning that the sequence duration of the sequence S is preferably matched to this duration of time or the interaction strength A.sub.x.

[0079] A general and schematic structure of the sequence S for producing such an effective Hamiltonian is explained in more detail below on the basis of the block diagram in FIG. 3. In the block diagrams of FIGS. 3 to 7, the radiofrequency pulses are illustrated as rectangles, which are arranged successively along a time axis not denoted in any more detail.

[0080] The (polarization) sequence S comprises a number of radiofrequency pulses which are able to be combined, for example, into a plurality of sequence portionsillustrated in exemplary fashion in FIG. 3 by means of two sequence portions S.sub.k and S.sub.k. The sequence portions S.sub.k and S.sub.k are separated from one another by means of optional intermediate pulses P.sub.inter, for example. The sequence S is enclosed by an optional initialization pulse P.sub.init and one or more optional end pulses P.sub.end. Here, in particular, the initialization pulse P.sub.init is a radiofrequency pulse with a 90 flip angle, which produces a superposition state of the electron spin moment 16. By way of example, the sequence S is repeatable N-times between the initialization pulse P.sub.init and the optional end pulse P.sub.end, as a result of which the polarization transfer realized thereby is extended.

[0081] Each sequence portion S.sub.k, S.sub.k comprises a number n.sub.k or n.sub.k of radiofrequency pulses P.sub.ki, P.sub.ki, which act on the electron spin moments 16 for a pulse duration .sup.pulse.sub.ki, .sup.pulse.sub.ki in each case. Here, the radiofrequency pulses P.sub.ki, P.sub.ki are time-shifted in relation to one another by a duration of time (pulse pause, evolution time, waiting time) .sub.ki, .sub.ki in each case. Here, the index i is a running index which runs from 1 to n.sub.k or n.sub.k. Here, it is possible for the individual sequence portions S.sub.k or S.sub.k to be repeated a number of times in succession in each case during the sequence S.

[0082] The time durations .sub.ki, .sub.ki and pulse durations .sup.pulse.sub.ki, .sup.pulse.sub.ki of the sequence portions S.sub.k, S.sub.k in this case add up to an overall sequence duration T of the sequence S, which is preferably shorter than a polarization transfer duration that is proportional to the interaction W. Expressed differently, the overall sequence duration T required to bring about the polarization transfer is shorter than the polarization transfer duration.

[0083] Each sequence portion S.sub.k, S.sub.k has at least one sequence unit E.sub.kj, E.sub.kj with a respective sequence unit duration T.sub.kj, T.sub.kj. Expressed differently, a sequence portion S.sub.k, S.sub.k is composed of one or more successive sequence units E.sub.kj, E.sub.kj. The running index j in this case labels the sequence units E.sub.kj, E.sub.kj of the sequence portion S.sub.k, S.sub.k. In FIG. 3, each sequence portion S.sub.k, S.sub.k is presented with only one sequence unit E.sub.kj, E.sub.kj in exemplary fashion.

[0084] The sequence S is configured in such a way that an effective Hamiltonian of the form S.sub.xI.sub.x+S.sub.yI.sub.y is brought about. To this end, the individual sequence units E.sub.kj, E.sub.kj are suitably embodied in such a way that, during a respective sequence unit E.sub.kj, E.sub.kj, an effective Hamiltonian of the form S.sub.xI.sub.x or S.sub.yI.sub.y is formed. This means that a plurality of sequence units E.sub.kj, E.sub.kj and/or a plurality of sequence portions S.sub.k, S.sub.k are required to bring about the desired polarization dynamics.

[0085] Therefore, for the polarization transfer, it is necessary to effectively interchange the spin operators S.sub.x and S.sub.y and the spin operators I.sub.x and I.sub.y in successive sequence units E.sub.kj, E.sub.kj of the sequence S.

[0086] One or more radiofrequency pulses are provided between the sequence units E.sub.kj, E.sub.kj or at the end of a respective sequence unit E.sub.kj, E.sub.kj for the purposes of changing between the spin operators S.sub.x and S.sub.y. Here, it is conceivable, for example, to carry out a single radiofrequency pulse with a 90 flip angle (FIG. 4, FIG. 7) or two different radiofrequency pulses with a 90 flip angle (FIG. 8) or one radiofrequency pulse with a 180 flip angle about a pulse axis 1/{square root over (2)} (X+Y) (FIG. 5). What is essential is that an alignment of the second spin moment 16 oriented parallel to the magnetic field B is rotated into a plane oriented perpendicular to the magnetic field B by the radiofrequency pulse or pulses.

[0087] The change between the spin operators I.sub.x and I.sub.y is preferably brought about by a relative phase shift between the spin operators S.sub.x and S.sub.y. To this end, the sequence unit duration T.sub.kj, T.sub.kj is dimensioned to be an odd multiple of half a Larmor period (/2.sub.Larmor) of the spin moments 10 in the magnetic field B (FIG. 4, FIG. 5, FIG. 8). Likewise, it is conceivable that, for example, the radiofrequency pulses P.sub.ki, P.sub.ki of successive sequence units E.sub.kj, E.sub.kj or sequence portions S.sub.k, S.sub.k have a relative time shift in relation to one another, wherein the time shift suitably equals an odd multiple of half the Larmor period of the spin moments 10 in the magnetic field (FIG. 7).

[0088] Below, exemplary embodiments of the sequence S are explained on the basis of FIGS. 4 to 7. Here, the radiofrequency pulses are denoted by P.sup.X.sub.90, P.sup.Y.sub.90, P.sup.X.sub.180 or P.sup.Y.sub.180, with the angle specification describing the flip angle and X and Y describing the pulse axis or phase angle of the radiofrequency pulse in a rotating frame of reference. Consequently, a P.sup.Y.sub.90 radiofrequency pulse is, for example, a radiofrequency pulse with a pulse duration .sup.pulse.sub.ki, .sup.pulse.sub.ki, which brings about a 90 flip angle of the electron spin moment 16 at a given pulse amplitude and which is produced with a (relative) 90 phase offset (Y).

[0089] In the exemplary embodiments of the sequences S, the durations of time .sub.ki, .sub.ki between the radiofrequency pulses are specified in units of half of the Larmor period of the nuclear spin moments 10, meaning by way of a time duration =M2/2.sub.Larmor, where .sub.Larmor is the Larmor frequency of the nuclear spin moments 10 in the magnetic field B and m is an odd integer.

[0090] FIG. 4 illustrates a first exemplary embodiment of the sequence S. In this embodiment, the sequence S only comprises a sequence portion S.sub.1 with four sequence units E.sub.11, E.sub.12, E.sub.13 and E.sub.14, which each have one radiofrequency pulse. Here, the initialization pulse P.sup.Y.sub.90 is embodied as a radiofrequency pulse with a 90 flip angle, which is produced along a Y-pulse axis. Here, the radiofrequency pulses of the sequence units E.sub.11, E.sub.12, E.sub.13 and E.sub.14 are each embodied as a P.sup.X.sub.90 pulse along an X-pulse axis and with a 90 flip angle. The radiofrequency pulses P.sup.X.sub.90 of the sequence S or of the sequence portion S.sub.1 are time-shifted in relation to one another, in each case with a duration of time of /2=2/4.sub.Larmor.

[0091] By way of the four successive sequence units E.sub.11, E.sub.12, E.sub.13 and E.sub.14, there effectively is a change between the spin operators S.sub.x and S.sub.y of the electron spin moment 16, and so an effective Hamiltonian H.sub.eff with the desired polarization dynamics is produced in a rotating frame of reference. In particular, four interaction terms are produced by the radiofrequency pulses P.sup.X.sub.90 of the sequence units E.sub.11, E.sub.12, E.sub.13 and E.sub.14. In the duration of time before the first radiofrequency pulse P.sup.X.sub.90, the interaction W is proportional to the product S.sub.xI.sub.x of the spin operators S.sub.x and I.sub.x.

[0092] Between the first and second radiofrequency pulse P.sup.X.sub.90, the interaction W is proportional to the product S.sub.yI.sub.x since the radiofrequency pulse P.sup.X.sub.90 effectively changes the spin operator S.sub.x to the spin operator S.sub.y. The interaction W is characterized by the product S.sub.xI.sub.x between the second and the third radiofrequency pulse P.sup.X.sub.90, with the interaction W after the fourth P.sup.X.sub.90 radiofrequency pulse accordingly being describable as S.sub.yI.sub.x. As a result, the spin dynamics characterized by the effective Hamiltonian H.sub.eff for the polarization transfer is produced in a rotating frame of reference.

[0093] Below, a second exemplary embodiment of the sequence S is described on the basis of FIG. 5 and FIG. 6. In this configuration, the sequence S has a sequence portion S.sub.2 with four radiofrequency pulses P.sup.X.sub.180 and with four composite pulses P.sup.Y-Z.sub.180, which act on the electron spin moment 16 alternately or in turn. Here, the radiofrequency pulses P.sup.X.sub.180 and the composite pulses P.sup.Y-Z.sub.180 are each time-shifted in relation to one another by the same durations of time m/4=6/8.sub.Larmor. In principle, it is possible here to use the duration of time m/4, where m is an odd integer. Expressed differently, the time durations T.sub.ki, T.sub.ki between the pulses P.sup.X.sub.180 and P.sup.Y-Z.sub.180 are longer than in the sequence portion S.sub.1 of the exemplary embodiment described above. Here, the sequence portion S.sub.2 is composed of four similar sequence units E.sub.21, E.sub.22, E.sub.23 and E.sub.24, which each comprise a radiofrequency pulse P.sup.X.sub.180 and a composite pulse P.sup.Y-Z.sub.180.

[0094] FIG. 6 schematically shows the construction of a (Y-Z-)composite pulse P.sup.Y-Z.sub.180. The composite pulse P.sup.Y-Z.sub.180 comprises four directly successive radiofrequency pulses which, when composed, implement the 180 flip angle about a YZ-pulse axis. The first radiofrequency pulse P.sup.X.sub.180 has a 180 flip angle about the X-pulse axis. The subsequent radiofrequency pulse P.sup.1/2(X+Y).sub.180 implements a 180 flip angle about a pulse axis 1/{square root over (2)}(X+Y), which is arranged as a bisector between the X- and the Y-pulse axis. Expressed differently, the radiofrequency pulse P.sup.1/2 (X+Y).sub.180 in each case has a relative phase offset of 45 in relation to the X-pulse axis (0) and the Y-pulse axis (90). The third radiofrequency pulse P.sup.Y.sub.180 implements a 180 flip angle about the Y-pulse axis, with the action of the fourth radiofrequency pulse P.sup.1/2(X+Y).sub.180 bringing about a 180 flip angle about the bisecting pulse axis 1/{square root over (2)} (X+Y).

[0095] The exemplary embodiment of FIG. 7 shows a sequence S with two sequence portions S.sub.3 and S.sub.4, which are separated from one another by means of a radiofrequency pulse P.sup.X.sub.90: (intermediate pulse). Here, the sequence portions S.sub.3 and S.sub.4 in each case only comprise one sequence unit E.sub.31 and E.sub.41, respectively.

[0096] Here, the sequence portion S.sub.3 is preferably configured in the style of an XY decoupling sequence, in particular an XY4 decoupling sequence. This means that the sequence portion S.sub.3 substantially comprises four radiofrequency pulses P.sup.X.sub.180. P.sup.Y.sub.180, P.sup.X.sub.180 and P.sup.Y.sub.180, which each have a 180 flip angle and which are produced alternately along the X-pulse axis and the Y-pulse axis. The four radiofrequency pulses P.sup.X.sub.180, P.sup.Y.sub.180. P.sup.X.sub.180 and P.sup.Y.sub.180 of the sequence portion S.sub.3 are respectively time-shifted in relation to one another by a duration of time =2/2.sub.Larmor.

[0097] In this exemplary embodiment, the sequence portion S.sub.4 has four radiofrequency pulses P.sup.Y.sub.180, P.sup.X.sub.180, P.sup.Y.sub.180 and P.sup.X.sub.90, which are shifted in relation to one another by the duration of time t. Here, the sequence portion S.sub.4 is embodied substantially mirror symmetric to the sequence portion S.sub.3 in relation to the intermediate pulse P.sup.X.sub.90, with the last radiofrequency pulse P.sup.X.sub.90 of the sequence portion S.sub.3 having a 90 flip angle. As is comparatively clearly visible in FIG. 7, the sequence portion S.sub.4 additionally has a time shift in relation to the sequence portion S.sub.3.

[0098] The intermediate pulse P.sup.X.sub.90 between the sequence portions S.sub.3 and S.sub.4 has a 90 phase offset in relation to the initialization pulse P.sup.Y.sub.90. The pulse axis or the phase angle of the intermediate pulse P.sup.X.sub.90 in this case equals the phase angle of the last radiofrequency pulse P.sup.x.sub.90 of the sequence portion S.sub.4. The last radiofrequency pulse P.sup.Y.sub.180 of the sequence portion S.sub.3 has a pulse spacing of /2 in relation to the intermediate pulse P.sup.X.sub.90, with the pulse spacing between the intermediate pulse P.sup.X.sub.90 and the first radiofrequency pulse P.sup.Y.sub.180 of the sequence portion S.sub.4 being substantially +/2 on account of the time shift . Here, in particular, the time shift is =/2, and so the pulse spacing between the intermediate pulse P.sup.X.sub.90 and the first radiofrequency pulse P.sup.Y.sub.180 of the sequence portion S.sub.4 equals the duration of time .

[0099] Here, the sequence portion S.sub.3 substantially produces spin dynamics that are proportional to a factor of cos(t)S.sub.xI.sub.x, where is the frequency of the radiofrequency radiation of the radiofrequency pulses. As a result of the time shift , spin dynamics proportional to sin(t)S.sub.yI.sub.x are produced accordingly during the sequence portion S.sub.4. This produces the effective Hamiltonian H.sub.eff for the polarization transfer in a frame of reference rotating with the frequency .

[0100] The sequence S in FIG. 7 facilitates the use of strong radiofrequency pulses, meaning the radiofrequency pulses preferably have a high pulse amplitude and a comparatively short pulse duration .sup.pulse.sub.ki, .sup.pulse.sub.ki. A short pulse duration .sup.pulse.sub.ki, .sup.pulse.sub.ki should be understood to mean, in particular, a duration of time of the radiofrequency pulse that is comparatively short in relation to the associated duration of time T.sub.ki. T.sub.ki. Preferably, the pulse duration .sup.pulse.sub.ki, .sup.pulse.sub.ki is shorter than .sub.ki/5 and .sub.ki/5, respectively.

[0101] FIG. 8 shows a further exemplary embodiment of the sequence S with two sequence portions S.sub.5 and S.sub.6. The sequence portions S.sub.5 and S.sub.6 each have a sequence unit E.sub.51 and E.sub.61.

[0102] Here, the first sequence portion S.sub.5 has three successive radiofrequency pulses P.sup.X.sub.90, P.sup.Y.sub.180 and P.sup.X.sub.90, which are shifted in relation to one another by a duration of time 3/4=6/8.sub.Larmor. By way of example, it is also possible here to use the duration of time m/4, where m is an odd integer. The first and third radiofrequency pulse P.sup.X.sub.90 each have a 90 flip angle along the X-pulse axis, meaning along the negative X-pulse axis. The second radiofrequency pulse P.sup.Y.sub.180 included therebetween has a 180 flip angle along the Y-pulse axis.

[0103] The second sequence portion S.sub.6 has a substantially identical construction to the first sequence portion S.sub.5, with the pulse axes being modified in comparison with the first sequence portion S.sub.5. Here, three successive radiofrequency pulses P.sup.Y.sub.90 P.sup.X.sub.180 and P.sup.Y.sub.90 are produced in the second sequence portion S.sub.6, which are shifted in relation to one another by a duration of time 3/4=6/8.sub.Larmor. Consequently, the first and third radiofrequency pulse P.sup.Y.sub.90 each have a 90 flip angle along the Y-pulse axis. Accordingly, the second radiofrequency pulse P.sup.X.sub.180 included therein has a 180 flip angle along the X-pulse axis, meaning along the inverted pulse direction to the first and third radiofrequency pulses P.sup.X.sub.90 of the sequence portion S.sub.5. Here, the first radiofrequency pulse P.sup.Y.sub.90 of the sequence portion S.sub.6 directly adjoins the third radiofrequency pulse P.sup.X.sub.90 of the sequence portion S.sub.5 during the sequence S.

[0104] Advantageously, no additional initialization pulses P.sub.init and/or end pulses P.sub.end are necessary in this embodiment of the sequence S, as a result of which a particularly simple sequence S is realized. Moreover, the pulse spacings or the durations of time 3/4 are preferably dimensioned in such a way that the finite pulse duration .sup.pulse.sub.ki, .sup.pulse.sub.ki of the radiofrequency pulses P.sup.X.sub.90, P.sup.Y.sub.180, P.sup.Y.sub.90, P.sup.X.sub.180 is taken into account. In this embodiment, the sequence S preferably has an even number 2N of repetitions in the case of a repetition.

[0105] Moreover, the sequence S in this configuration is particularly robust in relation to frequency detuning, meaning in relation to deviations of the frequency from the Larmor frequency of the electron spin moment 16 in the magnetic field B.

[0106] Furthermore, the sequence S has a high effectiveness in respect of the polarization transfer, even in the case of pulse errors, meaning a deviating phase angle or phase duration, of the radiofrequency pulses.

[0107] In FIGS. 9a and 9b, a two-dimensional representation of a simulation is shown in each case as an example for the improved stability and robustness of the sequence S according to FIG. 8. The simulations in FIGS. 9a and 9b relate to a system of five nuclear spin moments 10 and one electron spin moment 16, with the interaction strengths W having a Gaussian distribution.

[0108] Here, the simulations show the effect of pulse errors and (frequency) detuning on the polarization transfer PT. The pulse error is plotted along the ordinate axis (y-axis) and the detuning is plotted along the abscissa axis (x-axis), with the grayscale value representing the value of the respective polarization transfer PT. Here, the pulse error is defined as a percentage deviation of a pulse amplitude from an error-free pulse amplitude .sub.0. Here, detuning is understood to mean a deviation of the frequency of the radiofrequency pulses from the Larmor frequency .sub.e of the electron spin moment 16 in the magnetic field B, with the detuning being plotted in units of megahertz (MHz). The polarization transfer PT is defined as

.sub.iI.sub.z.sup.i(t)I.sub.z.sup.i(0)

[0109] meaning as a sum of the differences between the expected values of the z-components of the spin operators of the nuclear spin moments 10 at the start and end of the sequence S.

[0110] The simulated dynamics during a radiofrequency pulse are described as

[00002]$U_{,.Math..Math.X/Y}=\exp \left[-i.Math..Math.\frac{}{{}_0}.Math.\left(.Math..Math.S_z.Math..Math.S_{X/Y}+\underset{j}{.Math.}.Math.{}_I.Math.I_z^j+S_z\left(A_x^j.Math.I_x^j+A_z^j.Math.I_z^j\right)\right)\right]$

[0111] in a frame of reference rotating with the frequency , where U.sub., X/Y describes the effect of a pulse P.sup.X/Y.sub., i.e., of a rotation through the angle along a pulse axis X or Y. A.sup.i.sub.x and A.sup.i.sub.z describe the perpendicular and parallel components of the interaction (in MHz) between the electron spin moment 16 (spin operator S.sub.z) and the j-th nuclear spin moment 10 (I). For the illustrated simulations, a Larmor frequency .sub.l=2 MHz for the nuclear spin moments 10 and .sub.0=50 MHz for the error-free pulse amplitude are used as simulation parameters.

[0112] The simulation illustrated in FIG. 9a shows the result for the sequence S of FIG. 8 with rectangular pulses P.sup.X.sub.90, P.sup.Y.sub.180, P.sup.Y.sub.90, P.sup.X.sub.180 for the system of five nuclear spin moments 10 and the electron spin moment 16. The simulation in FIG. 9b shows the result of the sequence S of FIG. 8 for the same system, in which the radiofrequency pulses are implemented as composite pulses with symmetric phases, which are described by the evolutions

U.sub.90,X/Y=U.sub.16,X/YU.sub.300,X/YU.sub.266,X/YU.sub.54,X/YU.sub.266,X/YU.sub.300,X/YU.sub.16,X/Y

and

U.sub.180,X/Y=U.sub.325,X/YU.sub.263,X/YU.sub.54,X/YU.sub.266,X/YU.sub.300,X/YU.sub.16,X/Y.

[0113] On account of the longer pulse durations of the composite pulses, the duration of time between the composite pulses has been increased to 5/4=10/8.sub.Larmor for the sequence S simulated in FIG. 9b.

[0114] The invention is not restricted to the exemplary embodiments described above. Rather, other variants of the invention can also be derived therefrom by a person skilled in the art without departing from the subject matter of the invention. In particular, all individual features described in conjunction with the exemplary embodiments are further also combinable with one another in different ways, without departing from the subject matter of the invention.

[0115] What is essential is that the sequence S manipulates the electron spin moment 16 in such a way that a polarization transfer from the electron spin moment 16 to an interacting nuclear spin moment 10 is facilitated. As a result, the polarization transfer is not implemented during a radiofrequency pulse P.sub.ki, P.sub.ki but during the durations of time .sub.ki, .sub.ki between the radiofrequency pulses P.sub.ki, P.sub.ki on account of the sum of the individual spin dynamics, in particular. As a result, a polarization transfer independent of the Hartmann-Hahn condition is facilitated. This consequently is advantageously taken across to the flexibility; in particular, this reduces the technical requirements in relation to the stability and quality of the radiofrequency generator 26.

[0116] In a conceivable alternative form, the first nuclear spin moments 10 and electron spin moments 16 are part of a common molecular structure of a molecule or a common lattice structure of a solid as a material sample 4. Here, the material sample 4 can also be a biological or medical sample, such as, e.g., a cell. An embodiment is likewise conceivable in which the electron spin moment 16 is part of a free monoradical or biradical molecule or of a metallic ion or a photoexcitable triplet state molecule. What is essential is that the electron spin moment 16 is brought into interaction with the nuclear spin moments 10. In suitable fashion, the electron spin moment 16 is optically polarizable.

[0117] In a further alternative configuration, it is possible, for example, for one or more radiofrequency pulses to act on the nuclear spin moments 10 in addition to the sequence S acting on the electron spin moments 16, the radiofrequency pulses acting on the nuclear spin moments having a frequency that is resonant with the Larmor frequency .sub.Larmor of the nuclear spin moments 10 in the magnetic field B.

LIST OF REFERENCE SIGNS

[0118] 2 Apparatus [0119] 4 Material sample [0120] 6 Magnet [0121] 6a North pole [0122] 6b South pole [0123] 8 Sample container [0124] 10 Spin moment/nuclear spin moment [0125] 12 Solid sample/nanoparticle/nanodiamond [0126] 14 Color center [0127] 16 Spin moment/electron spin moment [0128] 18 Laser light [0129] 20 Laser [0130] 22 Excitation coil [0131] 24 Pickup coil [0132] 26 Radiofrequency generator [0133] 28 Receiver [0134] 30 Display device [0135] 32 NMR spectrum [0136] B Magnetic field [0137] S Sequence/polarization sequence [0138] D NMR signal [0139] M.sub.z Equilibrium magnetization [0140] M.sub.z Magnetization [0141] M.sub.xy Transversal magnetization [0142] W Interaction [0143] H.sub.eff Hamiltonian [0144] A.sub.x Interaction strength [0145] Pi [0146] Frequency [0147] .sub.Larmor, e, .sub.l Larmor frequency [0148] I.sub.x, I.sub.y, I.sub.+, I.sub. Spin operator [0149] S.sub.x, S.sub.y, S.sub.z, S.sub.+, S.sub.Spin operator [0150] S.sub.k, S.sub.k Sequence portion [0151] S.sub.1, S.sub.2, S.sub.3, S.sub.4, S.sub.5, S.sub.6 Sequence portion [0152] P.sub.inter Intermediate pulse [0153] P.sub.ki, P.sub.ki Radiofrequency pulse [0154] P.sup.X.sub.90, P.sup.Y.sub.90, P.sup.X.sub.180, P.sup.Y.sub.180, P.sup.1/2(X+Y).sub.180 Radiofrequency pulse [0155] .sup.pulse.sub.ki, .sup.pulse.sub.ki Pulse duration [0156] .sub.ki, .sub.ki, Durations of time [0157] E.sub.kj, E.sub.kj Sequence unit [0158] T.sub.kj, T.sub.kj, Sequence unit duration [0159] T Overall sequence duration [0160] Time shift [0161] n.sub.k, n.sub.k Pulse number [0162] P.sub.init Initialization pulse [0163] P.sub.end End pulse [0164] P.sup.Y Z.sub.180 Composite pulse [0165] X, Y, 1/{square root over (2)} (X+Y) Pulse axis [0166] U.sub., X/Y Operator [0167] Pulse error [0168] , .sub.0 Pulse amplitude [0169] Detuning [0170] PT Polarization transfer