Method for characterizing a sample by NMR spectroscopy with acquisition during the generation of a radiofrequency magnetic field

Abstract

A method and apparatus for characterizing a sample using a nuclear magnetic resonance spectrameter in which an effective field is generated. The field has an effective vector and results from a static magnetic field and a radiofrequency magnetic field. The effective vector rotates relative to a terrestrial reference frame.

Claims

1. A method for characterizing a sample (2) by means of a Nuclear Magnetic Resonance spectrometer comprising an enclosure (11) in which the sample (2) is placed, a static magnetic field (B.sub.0) generator (13), a radiofrequency magnetic field (B.sub.1) (14), and at least one sensor (15, 16), the method comprising the following steps: (a) generating (E1) in the enclosure (11), by the static magnetic field generator (13) a static magnetic field (B.sub.0) with a static vector ({right arrow over (B)}.sub.0), the static magnetic field being comprised between 0.1 Tesla and 16 Tesla; (b) generating (E2) in the enclosure (11) and for a determined duration (T), by the radiofrequency magnetic field generator (14) a radiofrequency magnetic field (B.sub.1) with a radiofrequency vector ({right arrow over (B)}.sub.1); (c) acquiring (E3) by at least one sensor (15, 16), a magnetization ({right arrow over (M)}) of the sample (2) wherein the generator(s) and the sensor(s) are decoupled, the step of acquisition (E3) being achieved for all or part of the determined duration (T); and wherein an effective vector ({right arrow over (B)}.sub.eff), to which the magnetization ({right arrow over (M)}) of the sample (2) is subjected for the determined duration (T), is rotating relatively to an Earth reference system for the determined duration (T), the effective vector ({right arrow over (B)}.sub.eff) resulting from the static (B.sub.0) and radiofrequency (B.sub.1) magnetic fields, and the sample (2) being fixed relatively to the Earth reference system, the radiofrequency magnetic field being comprised between 1 MicroTesla and a few milliTeslas.

2. The method according to claim 1, wherein the radiofrequency magnetic field (B.sub.1) is selected so that the effective field (B.sub.eff) applied to the sample has its effective vector ({right arrow over (B)}.sub.eff), given by ${\stackrel{.fwdarw.}{B}}_{\mathrm{eff}}={\stackrel{.fwdarw.}{B}}_1+\left(B_0-\frac{v_1}{\stackrel{\_}{\gamma }}\right)\hat{z}$ and defining an angle (θ.sub.m) of approximately 54.74° with the static vector ({right arrow over (B)}.sub.0) ν.sub.1 being a frequency close to the Larmor frequency ν.sub.0 due to the static magnetic field (B.sub.0), this frequency ν.sub.1 being due to the radiofrequency magnetic field (B.sub.1).

3. The method according to one of claims 1 and 2, wherein the effective vector ({right arrow over (B)}.sub.eff) is rotating in a plane orthogonal to the static vector ({right arrow over (B)}.sub.0).

4. The method according to claim 2, wherein the effective vector is rotating in a plane orthogonal to the static vector (B.sub.0) at the Larmor frequency (ν.sub.0).

5. The method according to claim 4, wherein the acquisition (E3) of the magnetization ({right arrow over (M)}) of the sample (2) is carried out by at least two sensors (15, 16); a first sensor (15) placed so as to acquire magnetic signals collinear with the static vector ({right arrow over (B)}.sub.0), having a frequency close to the effective frequency (ν.sub.eff), and at least one second sensor (16) placed so as to acquire magnetic signals collinear with the plane orthogonal to the static vector ({right arrow over (B)}.sub.0), having a frequency close to the Larmor frequency (ν.sub.0).

6. A Nuclear Magnetic Resonance spectrometry apparatus for acquiring a magnetization ({right arrow over (M)}) of a sample (2) comprising: a sample holder (12) fixed in an Earth reference system during the operation of the apparatus (1); an enclosure (11) in which the sample holder (12) is placed; a static magnetic field generator (13) for generating a static magnetic field (B.sub.0) with a static vector ({right arrow over (B)}.sub.0) in the enclosure (11), the static magnetic field being comprised between 0.1 Tesla and 16 Tesla, a radiofrequency magnetic field generator (14) for generating a radiofrequency magnetic field (B.sub.1) with a radiofrequency vector ({right arrow over (B)}.sub.1) in the enclosure for a determined duration (T); at least one sensor (15, 16) for measuring the magnetization ({right arrow over (M)}) wherein the radiofrequency magnetic field generator (14) comprises at least two coils (141, 142) each generating a radiofrequency magnetic field so that the resulting total effective magnetic field (B.sub.eff) resulting from both radiofrequency magnetic fields of the coils (141, 142) and from the static magnetic field (B.sub.0) has an effective vector ({right arrow over (B)}.sub.eff) rotating in the Earth reference system, the field ({right arrow over (B)}.sub.1) the radiofrequency magnetic field being comprised between 1 microTesla and a few milliTeslas, and the generator(s) are decoupled from the sensors, the measuring of the magnetization of the sample (2) occurring for all or part of the determined duration (T).

7. The apparatus (1) according to claim 6, wherein a first sensor (15) is placed according to the static vector ({right arrow over (B)}.sub.0) and adjusted so as to acquire signals at frequencies close to an effective frequency (ν.sub.eff) resulting from an effective magnetic field (B.sub.eff) applied to the sample (2) such that: ${\stackrel{.fwdarw.}{B}}_{\mathrm{eff}}={\stackrel{.fwdarw.}{B}}_1+\left(B_0-\frac{v_1}{\stackrel{\_}{\gamma }}\right)\hat{z};$ ν.sub.1 being a frequency close to the Larmor frequency (ν.sub.0) due to the static magnetic field (B.sub.0), the frequency ν.sub.1 being due to the radiofrequency magnetic field (B.sub.1); and
ν.sub.eff=γ.Math.B.sub.eff, γ being the gyromagnetic ratio characteristic of a nucleus of a studied atom.

8. The apparatus (1) according to claim 6, wherein two second sensors (16) are orthogonally placed and are adapted for acquiring a signal at a Larmor frequency (ν.sub.0) due to the static magnetic field (B.sub.0) with:
ν.sub.0=γ.Math.B.sub.0, γ being the gyromagnetic ratio characteristic of a nucleus of a studied atom.

9. The apparatus (1) according to one claim 6, wherein the static magnetic field generator (13), the radiofrequency magnetic field generator (14) and the sensor(s) (15, 16) are fixed in the Earth reference system during the operation of the apparatus (1).

Description

PRESENTATION OF THE DRAWINGS

(1) Other features, objects and advantages will become apparent upon reading the detailed description which follows, with reference to the drawings given as an illustration and not as a limitation, wherein:

(2) FIGS. 1a to 1c are detailed illustrations of the effects of the magnetic fields customarily used in NMR spectrometry on the spin magnetic moments of the studied atom nucleus;

(3) FIG. 2 is a schematic illustration of a phenol molecule taken as an example in the “principle of NMR spectrometry” part of the description,

(4) FIG. 3 is a schematic illustration of an apparatus according to an embodiment of the invention,

(5) FIG. 4 is a schematic illustration of a particular embodiment of the apparatus of FIG. 3;

(6) FIG. 5 schematically shows an exemplary embodiment of the method of the invention;

(7) FIG. 6 is a time diagram showing the generated magnetic fields (at the top, the radiofrequency magnetic field and at the bottom, the static magnetic field) as well as the acquisition step during the application of the method of FIG. 5;

(8) FIG. 7 illustrates the decoupling of the surface antenna of the “butterfly” type and of the linear volumetric coil;

(9) FIG. 8 illustrates an acquisition step as a time diagram accompanied by the form of the acquired signals; and

(10) FIG. 9 shows the different reference systems and their relationship during the application of the present invention.

DETAILED DESCRIPTION

(11) Principle of NMR Spectrometry

(12) The standard method for acquiring signals in NMR spectrometry is described hereafter.

(13) Generally, Nuclear Magnetic Resonance spectrometry (NMR spectrometry) consists of acquiring a signal proportional to the sum of the spin magnetic moments of the atoms of an element contained in an object placed in a magnetic field, for example hydrogen .sup.1H atoms (this spectrometry is then said to be proton NMR), deuterium atoms (.sup.2H), carbon 13 (.sup.13C) atoms, etc. Conventionally a spin magnetic moment is represented as a vector having a defined direction and a norm. Each relevant atom has a spin magnetic moment. The vector sum of the spin magnetic moments of a material is called its magnetization.

(14) In the absence of any applied magnetic field, the spin magnetic moments of the studied atoms are randomly oriented in space. The magnetization is then zero on average.

(15) The magnetic field in which is placed the material consists of a static magnetic field with respect to an Earth reference system which is that of the laboratory, and of a radiofrequency magnetic field. The static magnetic field is permanently applied onto the sample all along the experiment. The radiofrequency magnetic field is applied in a pulsed way (i.e. briefly for a determined duration). The magnetization of the object is measured in the absence of a radiofrequency field.

(16) FIGS. 1a to 1c illustrate the known technique of NMR which consists of generating a static magnetic field B.sub.0 according to a static vector {right arrow over (B)}.sub.0 continuously in an enclosure 11 in which an object 2 of the studied material is placed, of generating a radiofrequency magnetic field B.sub.1 as a radiofrequency pulse with a radiofrequency vector {right arrow over (B)}.sub.1 for a determined duration T in the enclosure A2, and of acquiring the magnetization {right arrow over (M)} of the sample A1 after a predetermined evolution duration. The amplitude of the static magnetic field B.sub.0 is of the order of a tesla while that of the radiofrequency magnetic field is at the very most of the order of a millitesla.

(17) FIG. 1a shows the influence of the static magnetic field B.sub.0 on the spin magnetic moments of the atoms (represented by vectors {right arrow over (S)} having as an origin a common point in space).

(18) Under the action of the static magnetic field B.sub.0, the angle formed by each of the spin magnetic moments {right arrow over (S)} of the atoms with the static vector {right arrow over (B)}.sub.0 is set and these spin magnetic moments {right arrow over (S)} perform a precession movement around the static vector {right arrow over (B)}.sub.0. The link between the intensity of the static field B.sub.0 and the frequency of rotation v.sub.0 of the magnetizations is given by the relationship:
ν.sub.0=γ.Math.B.sub.0;

(19) γ being called the gyromagnetic ratio of the relevant nucleus expressed in Hz.Math.T.sup.1.

(20) It may be demonstrated that under these conditions, the vector sum of all the spin magnetic moments {right arrow over (S)} is then no longer zero on average. This sum of spin magnetic moments {right arrow over (S)} gives the magnetization {right arrow over (M)} of the material which is on average non-zero and along the defined direction of the static vector {right arrow over (B)}.sub.0.

(21) FIG. 1b shows the influence of the radiofrequency magnetic field B.sub.1 (represented by the radiofrequency vector {right arrow over (B)}.sub.1, usually selected to be orthogonal to the static vector {right arrow over (B)}.sub.0).

(22) Under the action of the radiofrequency magnetic field B.sub.1, the magnetization {right arrow over (M)} of the material performs a rotation around the radiofrequency vector {right arrow over (B)}.sub.1, the angle of which is proportional to the intensity and to the determined generation duration T of the radiofrequency magnetic field B.sub.1. Usually, the duration T is selected so that the angle of rotation is 90° (π/2) or 180° (π). In an illustrative way, a rotation of 90° has been illustrated in FIG. 1b.

(23) The magnetization {right arrow over (M)} in fact performs a precession movement around the static vector {right arrow over (B)}.sub.0 during the rotation of 90° or 180°.

(24) FIG. 1c shows the evolution of the magnetization {right arrow over (M)} of the material after the determined duration T, while the static magnetic field B.sub.0 continues to be generated and while the radiofrequency magnetic field B.sub.1 is no longer generated.

(25) As soon as the radiofrequency magnetic field B.sub.1 is no longer applied in the enclosure 11, the magnetization {right arrow over (M)} will return to the equilibrium state by loss of energy. This return to the equilibrium state is accomplished according to a precession movement (see the arrow F in FIG. 1c) at a frequency specific to each of the spin magnetic moments {right arrow over (S)} composing the object.

(26) The frequency of the magnetic signal generated by the return to equilibrium state of an atom is not the same for the same element if the latter may be found in different electron environments. For example, phenol comprises 6 hydrogen atoms H.sub.a, H.sub.b, H.sub.c, H.sub.d, but the spin magnetic moments {right arrow over (S)} of each of the atoms will not all return to the equilibrium state at the same frequency. There are four groups of hydrogen atoms (see FIG. 2): the hydrogen atom H.sub.a bound to the oxygen atom O, the hydrogen atoms H.sub.b in the so-called “ortho” position, the hydrogen atoms H.sub.c in the so-called “meta” position and the hydrogen atom H.sub.d in the so-called “para” position. Each of the groups will experience its spin magnetic moment {right arrow over (S)} returning to the equilibrium state at a frequency different from that of the other groups since their electron environments are different. Nevertheless, these differences are minimal. In spite of this they may all be detected distinctly.

(27) The total acquired signal is then a sum of the magnetic signals of various frequencies. A Fourier transform may show these different frequencies, thereby forming an NMR spectrum of the sample.

(28) Hereafter, only for the purposes of illustrations, proton NMR spectrometry (i.e. for hydrogen .sup.1H) will be taken as an example. This does not limit the invention to proton NMR spectrometry alone, but one skilled in the art will easily be able to adapt the description which follows to NMR spectrometry of other atoms.

(29) For the magnetic fields, {right arrow over (B)} represents the vector of the magnetic field and B represents the amplitude of the associated vector and also refers to the magnetic field.

(30) Apparatus

(31) With reference to FIGS. 3 and 4, an NMR spectrometry apparatus 1 for acquiring a magnetization of a sample is described hereafter.

(32) The NMR spectrometry apparatus 1 comprises a sample holder 12 for receiving the sample 2. The holder 12 is intended to remain fixed in an Earth reference system during the operation of the apparatus 1.

(33) The apparatus 1 also comprises an enclosure 11 in which the sample holder 12 is placed. This enclosure 11 forms a volume in which magnetic fields will be generated.

(34) The apparatus 1 further comprises a static magnetic field generator 13 for generating a static magnetic field B.sub.0 in the enclosure 11 of static vector {right arrow over (B)}.sub.0. The static magnetic field generator 13 is capable of generating a static magnetic field B.sub.0 of an amplitude of the order of the tesla, typically between 0.1 T and 16 T. This static magnetic field B.sub.0 gives the possibility of making the vector sum of the spin magnetic moments {right arrow over (S)} of the hydrogen atoms non-zero along the line corresponding to the direction of the static vector {right arrow over (B)}.sub.0 without however making the spin magnetic moments {right arrow over (S)} individually collinear with the static vector {right arrow over (B)}.sub.0. The sum of the spin magnetic moments {right arrow over (S)} is the macroscopic magnetization {right arrow over (M)}, the latter is collinear with the static vector {right arrow over (B)}.sub.0 and in the same defined direction. The spin magnetic moments {right arrow over (S)} form with the static vector {right arrow over (B)}.sub.0 fixed angles and perform a precession movement around the static vector {right arrow over (B)}.sub.0 at the frequency of rotation ν.sub.0.

(35) The apparatus also comprises a radiofrequency magnetic field generator 14 for generating a radiofrequency magnetic field B.sub.1 in the enclosure 11 of radiofrequency vector {right arrow over (B)}.sub.1. The radiofrequency magnetic field 14 is capable of generating a radiofrequency magnetic field with an amplitude comprised between a few microteslas (μT) and a few milliteslas (mT), typically between 1 μT and 1 mT, which corresponds to frequencies comprised between about 40 Hz and 40 kHz for the proton. The radiofrequency magnetic field B.sub.1 is generated, for a determined duration T, as a pulse applied at the radiofrequency frequency ν.sub.1. The radiofrequency field B.sub.1 is modulated so that the resultant of the static B.sub.0 and radiofrequency B.sub.1 magnetic fields as seen by the atoms rotates (see FIG. 9, where the laboratory reference system is labelled as (x, y, z), the reference system (x′, y′, z′) rotating at the angular frequency ω.sub.1 around the axis {circumflex over (z)}, and the effective reference system (x.sub.eff, y.sub.eff, z.sub.eff) rotating at the angular frequency ω.sub.eff around the axis of the effective field {right arrow over (z)}.sub.eff, {right arrow over (z)}.sub.eff being shifted by the magic angle relatively to the axis {circumflex over (z)}′ of the rotating reference system). This resultant is called an effective field B.sub.eff of effective vector {right arrow over (B)}.sub.eff and defined by:

(36) ${\stackrel{.fwdarw.}{B}}_{\mathrm{eff}}={\stackrel{.fwdarw.}{B}}_1+\left({\stackrel{.fwdarw.}{B}}_0-\frac{{\stackrel{.fwdarw.}{v}}_1}{\stackrel{\_}{\gamma }}\right);$

(37) with γ the gyromagnetic ratio of the studied nucleus and the frequency vector {right arrow over (ν)}.sub.1 collinear with the static vector {right arrow over (B)}.sub.0.

(38) The radiofrequency magnetic field B.sub.1 thus gives the possibility of rotating the magnetization {right arrow over (M)} around the effective vector {right arrow over (B)}.sub.eff.

(39) The apparatus 1 comprises at least one sensor 15, 16 for acquiring the magnetization {right arrow over (M)} of the sample during the generation of the radiofrequency magnetic field B.sub.1. This acquisition is carried out for the determined duration T.

(40) The apparatus 1 further comprises an actuator 17 for controlling the generation of the radiofrequency field B.sub.1 in a sinusoidal way.

(41) When the amplitude of the radiofrequency field B.sub.1 as well as its application frequency ν.sub.1 verifies the relationship:

(42) $\arctan \left(\frac{B_1}{B_0-\frac{v_1}{\stackrel{\_}{\gamma }}}\right)={\theta }_m;$$B_1=\left(B_0-\frac{v_1}{\stackrel{\_}{\gamma }}\right).Math.\tan \left({\theta }_m\right).$

(43) θ.sub.m being the magic angle. The dipolar and quadrupolar interactions perceived by the object are on average equal to zero and by acquiring the signal during the application of the radiofrequency field B.sub.1 with an amplitude and frequency defined above it is possible to get rid of the requirement of setting the sample 2 or the static magnetic field generator 13 into rotation. Thus, it is possible to study samples regardless of their state (notably solid, but also liquid, gas state or even a mixture thereof), or even study the tissues of an animal or of a human being in vivo.

(44) The rotation frequency of the magnetization {right arrow over (M)} (due to the rotating effective field B.sub.eff) may be brought to a high value (of more than about a hundred kilohertz and optionally reaching one megahertz) as compared with the MAS prior art method which is confined to a few tens of kilohertz.

(45) In an embodiment, the radiofrequency magnetic field generator 14 consists of a single magnet or coil. The actuator 17 then controls the radiofrequency magnetic field generator 14 for generating a radiofrequency field B.sub.1 sinusoidally modulated in order to rotate the effective field B.sub.eff.

(46) In another embodiment, the radiofrequency magnetic field generator 14 may comprise at least two magnets or coils 141, 142 (see FIG. 4) each generating a partial radiofrequency field. This gives the possibility of obtaining a radiofrequency field B.sub.1 for which the amplitude is more intense. The actuator 17 then controls the generation of the partial radiofrequency magnetic fields by both coils 141, 142, so that the effective field B.sub.eff resulting from both partial radiofrequency magnetic fields of the coils 141, 142 and from the static magnetic field B.sub.0 has an effective vector {right arrow over (B)}.sub.eff rotating around the static vector {right arrow over (B)}.sub.0 for the determined duration T. The partial radiofrequency magnetic fields are for example sinusoidally modulated and in phase quadrature relatively to each other (which amounts to having a partial radiofrequency magnetic field modulated by a sine function and the other one by a cosine function).

(47) During the application of the radiofrequency field B.sub.1, the frequency ν.sub.1 and the amplitude B.sub.1 of which meet the condition:

(48) $\arctan \left(\frac{B_1}{B_0-\frac{v_1}{\stackrel{\_}{\gamma }}}\right)={\theta }_m;$

(49) the spin magnetic moments {right arrow over (S)} and therefore the resulting magnetization {right arrow over (M)} perform a precession movement around the effective vector {right arrow over (B)}.sub.eff defined by:

(50) ${\stackrel{.fwdarw.}{B}}_{\mathrm{eff}}={\stackrel{.fwdarw.}{B}}_1+\left(B_0-\frac{v_1}{\stackrel{\_}{\gamma }}\right)\hat{z}$

(51) This precession movement takes place at the frequency ν.sub.eff=γ.Math.B.sub.eff, The effective vector {right arrow over (B)}.sub.eff of the effective field B.sub.eff is itself rotating around the static vector {right arrow over (B)}.sub.0 at the Larmor frequency ν.sub.0=γ.Math.B.sub.0.

(52) When the precession frequency around the effective field B.sub.eff is sufficiently large (i.e. of the same order as the frequencies used in MAS techniques, greater by a few hertz) an averaging of the dipolar and optionally quadrupolar interaction present in the object is obtained.

(53) The virtual rotation frequency ν.sub.eff responsible for the averaging of the dipolar and quadrupolar interactions being simply adjusted by the intensity of the applied radiofrequency field B.sub.1, it is no longer neither the sample nor the static field B.sub.0 which has to be set into rotation and tilted by the magical angle θ.sub.m but only the effective field B.sub.eff, this latter operation being carried out by suitably selecting the frequency ν.sub.1 and the amplitude of the radiofrequency field B.sub.1. It is simple to give B.sub.1 amplitudes ranging from a few μT up to one mT, these amplitudes generating a virtual rotation frequency ν.sub.eff ranging from a few Hz to several tens of kHz.

(54) Thus, all the portions of the apparatus 1 remain fixed during its operation. Therefore this does not induce any mechanical wear of the apparatus 1 and considerably limits the risks of failures or of poor handling.

(55) A first sensor 15 may be placed along the static vector {right arrow over (B)}.sub.0 and be adjusted so as to detect frequencies around the effective frequency ν.sub.eff. The effective frequency ν.sub.eff is due to the effective magnetic field of B.sub.eff seen by the sample 2. The effective magnetic field B.sub.eff results from the combination of the static B.sub.0 and radiofrequency B.sub.1 magnetic fields (without taking into account the electron environment of the hydrogen atom), and has the formula given by the following relationship:

(56) ${\stackrel{.fwdarw.}{B}}_{\mathrm{eff}}={\stackrel{.fwdarw.}{B}}_1+\left(B_0-\frac{v_1}{\stackrel{\_}{\gamma }}\right)\hat{z}$

(57) The frequency v.sub.1 is the application frequency of the field B.sub.1, this is a frequency close to the Larmor frequency of the studied nucleus ν.sub.0=γB.sub.0,

(58) The following relationship gives the effective frequency ν.sub.eff:
ν.sub.eff=γ.Math.B.sub.eff.

(59) Thus, the first sensor 15 allows acquisition of the signals frequencies close to the effective frequency v.sub.eff which correspond to the signals resulting from the precession movement of the spin magnetic moments {right arrow over (S)} of the hydrogen atoms of the sample 2 around the effective field B.sub.eff.

(60) Two second sensors 16 may be placed in a plane orthogonal to the static vector {right arrow over (B)}.sub.0 and adapted so as to acquire signals at the Larmor frequency v.sub.0.

(61) The sensor(s) 15, 16 are decoupled from the generator(s) 14; 141, 142 of the radiofrequency magnetic field B.sub.1. This decoupling may be achieved geometrically or electronically. An example is given below.

(62) The use of the first and second sensors 15, 16 gives the possibility of acquiring the variations of the magnetization {right arrow over (M)} along the three spatial dimensions in the reference system of the laboratory.

(63) Generally, the configuration of the sensors may be selected from the following configurations: a single sensor for acquiring signals at a frequency close to or equal to the Larmor frequency ν.sub.0; two orthogonal sensors for acquiring signals at a frequency close or equal to the Larmor frequency ν.sub.0; two sensors, one of which for acquiring signals at a frequency close or equal to the Larmor frequency ν.sub.0; and one for acquiring signals at a frequency close or equal to the effective frequency ν.sub.eff; three sensors, two of which for acquiring signals close or equal to the Larmor frequency ν.sub.0 and one for acquiring signals at a frequency close or equal to the effective frequency v.sub.eff.

(64) Method

(65) With reference to FIGS. 5 and 6, a method for characterizing a sample by means of an NMR spectrometry apparatus described above, is described hereafter.

(66) Prior to the method, a sample 2 is placed on the holder 12 of the NMR apparatus 1, inside the enclosure 11.

(67) The method comprises generating E1 in the enclosure 11 of the apparatus 1, by the static magnetic field generator 13, a static magnetic field B.sub.0 of static vector {right arrow over (B)}.sub.0 collinear with a unit vector {circumflex over (z)}.

(68) The method also comprises generating E2 in the enclosure 11, by the radiofrequency magnetic field generator 14, a radiofrequency magnetic field B.sub.1 of radiofrequency vector {right arrow over (B)}.sub.1. The radiofrequency magnetic field B.sub.1 is generated for a determined duration T, as a pulse I.

(69) The amplitude B.sub.1 of the radiofrequency magnetic field B.sub.1 of frequency ν.sub.1 is given by the relationship:

(70) 0$\arctan \left(\frac{B_1}{B_0-\frac{v_1}{\stackrel{\_}{\gamma }}}\right)={\theta }_m;$

(71) θ.sub.m being the magic angle.

(72) The radiofrequency magnetic field B.sub.1 is, for the determined duration T, modulated so that an effective field B.sub.eff seen by the atoms of the object results from the static B.sub.0 and radiofrequency B.sub.1 magnetic fields, is of an effective vector {right arrow over (B)}.sub.eff rotating relatively to an Earth reference system, such that:

(73) ${\stackrel{.fwdarw.}{B}}_{\mathrm{eff}}={\stackrel{.fwdarw.}{B}}_1+\left(B_0-\frac{v_1}{\stackrel{\_}{\gamma }}\right)\hat{z}.$

(74) The effective vector {right arrow over (B)}.sub.eff of the effective field B.sub.eff performs a precession movement around the static vector {right arrow over (B)}.sub.0 at frequency ν.sub.0. The radiofrequency field B.sub.1 was selected so that the effective vector {right arrow over (B)}.sub.eff forms a magic angle of θ.sub.m=54.74° with the static vector {right arrow over (B)}.sub.0.

(75) The radiofrequency magnetic field B.sub.1 may only comprise a single sinusoidally modulated component. In this case, only the effective vector {right arrow over (B)}.sub.eff rotates in the Earth reference system.

(76) The radiofrequency magnetic field B.sub.1 may also comprise two sinusoidally modulated components in phase quadrature. In this second case, the radiofrequency {right arrow over (B)}.sub.1 and effective {right arrow over (B)}.sub.eff vectors rotate in an Earth reference system.

(77) By applying the effective field B.sub.eff forming the magic angle θ.sub.m with the static field of B.sub.0, it is possible to get rid of the requirement of setting the sample 2 or the static magnetic field generator 13 into physical rotation. Thus, it is possible to study samples regardless of their state (notably solid, but also liquid, gas state or even a mixture thereof), or even study the tissues of an animal or of a living human being.

(78) Further, the rotation frequency of the magnetization {right arrow over (M)} (due to the rotating effective field B.sub.eff and proportional to the effective frequency ν.sub.eff) may be raised to a high value, which may exceed one megahertz, as compared with the prior art which is limited to a few tens of kilohertz.

(79) The experimenter selects the virtual rotation speed, proportional to the effective frequency ν.sub.eff, which is intended to be imposed to the sample. Once this effective speed of rotation is selected (between a few hertz and one megahertz), the amplitude of the radiofrequency field B.sub.1 as well as its frequency ν.sub.1 are found by solving the system of equations:

(80) $\hspace{1em}\{\begin{array}{c}{B}_1^2+{\left(B_0-\frac{v_1}{\stackrel{\_}{\gamma }}\right)}^2-B_{\mathrm{eff}}^2\\ B_1=\sqrt{2}.Math.\left(B_0-\frac{v_1}{\stackrel{\_}{\gamma }}\right);\end{array}$

(81) with B.sub.0, B.sub.eff and γ being known.

(82) The method further comprises a step E3 for acquiring by at least one sensor 15, 16 a magnetization {right arrow over (M)} of the sample 2. This acquisition step E3 is carried out for the determined duration T and lasts for an acquisition time T.sub.a comprised in the determined duration T (see FIG. 6).

(83) For example, two decoupled antennas (or coils) are used: a first antenna 14 is a linear volumetric coil (for example a Rapidbiomedical V-HLS-047 model) for emitting the radiofrequency magnetic field and a second one 15 is a surface antenna of the “butterfly” type for reception.

(84) The decoupling of both coils 14, 15 is achieved according to two steps. First of all, a rotation of angle α.sub.optimum is applied to the plane of the butterfly antenna in order to make the butterfly antenna as parallel as possible to the radiofrequency vector {right arrow over (B)}.sub.1 generated by the emitting antenna 14. This minimizes the flux of the radiofrequency magnetic field B.sub.1 through both loops of the butterfly antenna to a residual flux. And then, the butterfly antenna is translationally displaced inside the emitting antenna 14. Thereby making the most out of the non-uniform nature of the radiofrequency magnetic field B.sub.1, it is possible to find a position in which the flux difference between both loops 151, 152 of the butterfly antenna 15 may cancel out the residual flux.

(85) Moreover, in this case, the sample 2 may be positioned inside a first loop 151 of the butterfly antenna (see FIG. 7). Thus, the flux produced by the magnetization of the sample is maximum in the first loop 151 and almost equals to zero in the second loop 152.

(86) Thus, by means of this method, it is possible to get rid of the requirement of conducting measurements point-by-point as imposed in the MEPS technique.

(87) The acquisition step E3 may comprise the substep E31 for acquiring magnetic signals having a frequency close to the effective frequency ν.sub.eff. This low frequency signal gives information on the time-dependent change of the magnetizations according to the axis of the static field B.sub.0.

(88) The acquisition step E3 may also comprise the substep E32 for acquiring magnetic signals having a frequency close to the Larmor frequency ν.sub.0. With this step it is possible to monitor changes of the signal over time in the plane perpendicular to the static vector {right arrow over (B)}.sub.0.

(89) The signals collected by the sensors 15, 16 may then be filtered E4 in order to remove parasitic signals from the generators 13, 14; 13, 141, 142 of magnetic fields.

(90) The connected signals are then processed E5 for reconstructing a three-dimensional signal describing over time the changes of the magnetization {right arrow over (M)} of the sample 2. The three-dimensional signal may be put into the form of quaternion signal i.e. in the form of a signal for which each point is put into the form of a+ib+jc+kd with a, b, c, d being real and i.sup.2=j.sup.2=k.sup.2=ijk=−1. At each instant t of the acquisition, the three-dimensional signal may be described in spherical coordinates by three parameters (ρ, θ, φ) corresponding to the amplitude, the latitude and colatitude of the signal. Finally a demodulation step E6 may be carried out on the three-dimensional signal in order to represent it in the Earth reference system according to a fixed reference system.

(91) FIG. 8 illustrates a time diagram showing the acquisition of the signals during the emission of the radiofrequency magnetic field B.sub.1. According to this particular time diagram, the acquisition is triggered before application of the radiofrequency magnetic field B.sub.1 and the acquisition ends after having cut off the radiofrequency magnetic field B.sub.1.

(92) FIG. 8 also shows both acquired signals. The first at the top corresponds to the real part of the transverse magnetization while the second at the bottom corresponds to the complex part of this magnetization. Both oscillating and observable signals persist for a period of more than 300 ms, which is a relatively long time in the field of NMR acquisition. When the radiofrequency magnetic field is cut off, the observed signal disappears within only about 20 milliseconds.