WO2017194631A1

WO2017194631A1 - Wire stub antenna for use in magnetic resonance imaging and spectroscopy

Info

Publication number: WO2017194631A1
Application number: PCT/EP2017/061228
Authority: WO
Inventors: Mohamed AKEL; Eric ALIBERT; Christophe Coillot; Christophe GOZE-BAC; Julien Muller; Eric NATIVEL; Rémy SCHIMPF; Michel ZANCA; Pauline DE PELLEGARS
Original assignee: Centre National De La Recherche Scientifique; Universite De Montpellier; Rs2D; Centre Hospitalier Et Universitaire De Montpellier; Satt Axlr
Priority date: 2016-05-10
Filing date: 2017-05-10
Publication date: 2017-11-16
Also published as: FR3051284A1

Abstract

The invention relates to a wire stub antenna (13; 14) for transmitting a sequence of radio-frequency (RF) magnetic field pulses as part of a nuclear magnetic resonance (NMR) spectroscopy and/or imaging method, said wire stub antenna comprising a plurality of strands (31; 41) extending at least partially in a direction Z in an orthogonal reference system having an axis z defined by direction Z, said strands being disposed such that the currents passing through the strands (31; 41) of each antenna (13; 14) have the same intensity at the same position along axis z.

Description

SHORT WIRED ANTENNA FOR USE IN SPECTROSCOPY AND NUCLEAR MAGNETIC RESONANCE IMAGING

The present invention relates to short multi-stranded wire antennas and their uses in the near field in imaging and / or nuclear magnetic resonance spectrometry (NMR) methods.

The phenomenon of NMR, which is revealed when a nuclear magnetic moment is placed in a magnetic field (Bo), has emerged since its discovery as a means of investigation by essential spectroscopy in chemistry and physics.

The addition of magnetic field gradient coils allowed the localization of the NMR signal and marked the advent of NMR imaging. The gradient coils modify the local magnetic field allowing, in fine, to carry out nuclear magnetic resonance (MRI) images. In a homogeneous magnetic field Bo, the spins of the sample observed precede at the frequency of Larmor (a> o) according to ωο = γΒο. If we superpose on the main field Bo the one generated by the gradient coils:

_(dB _dB _dB \ expression resultant field shows that the frequency of precession of the spins depends on their position:

dB _q dB _q dB _q \

ω ₀ + δ ( _X , y, z) = y \ B _Q + + - y + - ή. Thus, the magnetic field created by the gradient coils allows in particular to select a cutting plane (transverse, frontal, sagittal or oblique) and to determine the spatial location of the signals in this plane.

However, the use of gradient coils leads to complex and bulky systems with many drawbacks: cost, high power consumption, noise, and justify the interest for a location of the gradient-free NMR signal, as illustrated in the publication of Sharp et al. : "MRI using radiofrequency magnetic field phase gradients". In addition, there is emerging interest in portable MRI equipment for which the integration of gradient coils is a major difficulty. The detection of the NMR signal has historically been achieved by the physical phenomenon of electrical induction by means of a receiving antenna comprising one or more coils tuned to the frequency of the core to be measured in the magnetic field Bo and adapted to the impedance standard 50 Ohms, as shown for example the publication "The nuclear induction experiment" by Bloch F. et al. This detection mode remains the rule.

A large variety of coils was developed and was used to observe the NMR signal: Helmholtz coil, saddle coil, bird cage coil or millipede coil, as illustrated in the book " NMR Probehead: for biophysical and biological experiments, "Ed. Imperial College, (2006) Mispelter, Lu pu & Briguet. All these coils belong to the category of antennas with inductive detection whose measurement principle is mainly based on the appearance of a voltage proportional to the magnetic component of the electromagnetic radiation, unlike the wire antennas whose measurement principle is essentially due to the appearance of an electric current depending on the electrical component of the electromagnetic radiation.

In order to be able to carry out NMR experiments, in addition to the strong and stable main static magnetic field Bo, which is often produced by a superconducting magnet which creates alignments of the magnetic spin moments in the Bo field, a weaker oscillating magnetic field Bi in the field of radio frequency (RF), is necessary. This oscillating field Bi is in the form of an RF pulse sequence and is traditionally produced by means of emitting coils and applied so as to slightly modify the alignment of the magnetic moments (spins) and to produce a precession phenomenon of the nuclear magnetizations which remain in phase in the plane transverse to Bo which is accompanied by a measurable electromagnetic field.

It is necessary to tune the operating frequency of the transmitting coils to the frequency of the cores to be imaged and to adapt their impedance (at the intended operating frequency) to the input impedance of the power amplifier (standard of 50 Ohms), so as to optimize power transfer. Thus, the application of the field Bi for a duration τ, will switch the magnetization towards the plane of the detection of an angle:

θ = γΒ τ eq. (1) In practice, the MRI sequences use either combinations of RF pulses (π / 2 + π), known as the spin echo sequence, to compensate for the inhomogeneities of the static field Bo, or a simple toggle of a given angle θι, a sequence known as a gradient echo. The application of radiofrequency pulses in relation to those of the magnetic field gradients allows the spatial location of the signal.

When the coils are only used in reception, the impedance matching is less restrictive and the simple tuning of the operating frequency may be sufficient. Many tuning-adaptation electronics configurations can be used for the coils, as illustrated in the aforementioned Mispelter, Lupu & Briguet publication, or in the "Tunable electrically small Antennas" publication by John AM Lyon, Alan GT Chaet Mohamed A. Hidayet.

It is generally sought to obtain a homogeneous signal, that is to say a mono directional Bi field and of constant amplitude over the largest possible volume, which requires conformations of specific coils, for example solenoid coil, coil Helmholtz , saddle coil, etc. whose dimensioning is then constrained by the respect of geometric rules. For example, in the case of Helmholtz coils, the distance between the two turns must be equal to their radius. There are also constraints on the lengths of the wires, which are typically less than λ / 10, where λ is the wavelength in vacuum associated with the RF field used, according to the Mispelter publication, Lupu & Briguet.

The homogeneity of the signal is then the resultant of the homogeneity at the emission, with a flip-flop angle which depends on the location of the spin: θ, γ, z) = γΒ, γ, ζ) τ and the detection which is proportional to factor B (x, y, z) / l.

Finally, the NMR signal (e) provided by an inductive antenna undergoes this double dependence in the form:

e <x 9 ₁ (x, y, z) sm [9 ₁ (x, y, z)] eq. (2)

This is the reason for designing coils for which the volume of interest has the best possible homogeneity of B (x, y, z) in terms of direction and amplitude. This is often done by appropriate configurations of conductor wire distribution and phase shift currents flowing through the son. A known example is that of the so-called bird cage or "birdcage" coil disclosed by Hayes et al. in the article "An Efficient, Highly Homogeneous Radio Frequency Coil for Whole-Body NMR Imaging at 1.5T" or the article "End Cap Desgin for Bird Cage Coils in Nuclear Magnetic Resonance Imaging". In a "birdcage" coil, a plurality of N conducting wires, arranged uniformly between two conducting rings and each having a capacitance, will produce a phase shift δφ of the current within each conductor at a given pulse ω, where ω = 2π / ί, f being the frequency of the supply current of the coil. The total phase shift is a multiple M of 2π, ie Nx δφ = Μ2π, in order to favor a sinusoidal distribution of the current between the N wires, a condition known for obtaining the homogeneity of the generated magnetic field. Thus, the "birdcage" coil makes it possible to create an RF oscillating magnetic field Bi which is substantially homogeneous in the cylindrical volume inside the coil.

Another configuration is disclosed by Bolinger et al. in "A Multiple-Frequency Coil with a Highly Uniform Bi Field", which discloses a coil in which the temporal phase shift of the currents is replaced by a non-uniform distribution of the conductors. Currents have the same value in all threads. The conducting wires are arranged around a cylinder, so as to promote a distribution of the density of

^→ "ZvB ->

current defined by the equation / (#) = - cos (0) fc where Θ is the azimuthal angle

o

cylindrical wire.

US Patent 5270656 discloses a bi-planar system having coils in two parallel planes to create a homogeneous RF field in a volume between these planes.

The application US 2014/218025 describes a "birdcage" coil without specifying the method of distribution of strands.

The application EP 0758092 discloses a "birdcage" type coil variant comprising conductors forming loops and distributed equiangularly around the circumference of a circle.

The patents FR 241 1478 and US 4899108 also disclose coils intended to create a homogeneous magnetic field.

In addition, we are also seeing new detection methods like the one offered by wire antennas. Unlike inductive coils which are closed electrical circuits and which correspond to the conventional solution for measuring the NMR signal, the wire antennas are open circuits.

These antennae are frequency tuned and impedance matched by design, using a characteristic length λ / 4 on dielectric substrate, as explained in the publication of Raaijmakers AJ et al. : "Design of a Radiative Surface Coil Array Element at 7T: The Single-Side Adapted Dipole Antenna". The wire antennas can be combined to form an array of antennas to cover a region of the given space and thereby produce an image.

They can be used in far field by modulating the thickness of the dielectric substrate, but also in the near field as described in the publications of O. Ocali and E. Atalar: "Intravascular magnetic resonance imaging using a loopless catheter antenna" or Halidi et al. : "Evanescent Waves Nuclear Magnetic Resonance".

US Patent 6771070 discloses an antenna system used for detecting electromagnetic signals. The system includes conductive strands arranged in parallel, each connected to a receiving chain. The strands are tuned to the frequency of the signal to be detected and of a length equal to an integer multiple of λ / 4 where λ is the wavelength of the signal to be detected.

The article "New design concept of monopoly antenna array for UHF 7T MRI" by Suk-Min Hong et al. describes a network of tuned quarter-wave (λ / 4) monopole antennas, these antennas being homogeneously distributed on the circumference of a circle.

The article "The fractional dipole antenna: A new antenna for body imaging at 76 Tesla" by Alexander J. Raaijamekers et al. discloses half-wave dipole antennas (λ / 2) tuned equiangularly distributed on the circumference of a circle.

In the case of short-wave antennas, monopole or dipole length L "X, for example L less than λ / 10, the propagation medium being the vacuum, frequency matching and impedance matching can be obtained by means of a tuning-adaptation circuit as illustrated in the aforementioned publication by John AM Lyon, Alan Cha GT and Mohamed A. Hidayet.

A major disadvantage of using short monopole antennas to detect an NMR signal is a reduced field of view at the nearest spins of the antenna strand. To overcome this drawback, several wire antennas used in Far field can be networked to cover a region of space, area or volume, as explained in the publication of Raaijmakers AJ et al. This solution, however, requires substantial instrumentation since each wire antenna is associated with a reception transmission channel.

In the case of short wire antennas, there is a dependence of the signal strength on the position of the transmitter object with respect to the height of the strand. By "short", it is necessary to understand an antenna length L that satisfies the following criterion: L <λ / 10.

In the case of short-wave antennas, the intensity of the current induced along the antenna is known to be linearly related to the position of the spins relative to the strand, as explained in the article "Antenna Fundamentals" of the courses. National Radio Astronomy Observatory available on the site:

http: // www. cv.nrao. edu / race / astr534 / AntennaTheory.html.

For a monopole antenna of length L, this dependence is thus expressed in the form:

7 (z) = / ₀ (l -)

eq. (3. a)

For a dipole antenna of length L, this dependence becomes:

where h is the supply current when the antenna is used in transmission and output when the antenna is used in reception, z being the position on the strand.

Compared with the design habits of NMR or MRI detection devices, this dependence can be interpreted as a disadvantage, for example as explained in the publication of Mispelter, Lupu & Briguet, which led to the use of wire-type antennas. dipole preferentially in the far field according to the publication by Raaijmakers AJ et al.

MRI remained, given its complexity, its size and its cost, the prerogative of applications to health. However, in areas such as agronomy, where it is essential to measure the amount of water in plants, its use seems relevant and would in any case be an alternative to invasive methods that are appropriate. However, to be relevant, the technique should be transportable to allow for in situ measurements. The simplification of the hardware of the NMR spectroscopy and / or MRI is therefore of definite interest vis-à-vis this type of application.

The invention thus seeks to further improve nuclear magnetic resonance imaging methods, in order to overcome the disadvantages mentioned above.

The invention firstly relates to a short wire antenna for transmitting a sequence of radio frequency (RF) magnetic field pulses in an imaging and / or nuclear magnetic resonance spectroscopy (NMR) method. short wired antenna having a plurality of strands extending at least partially in a Z direction, in an orthogonal coordinate system having an axis Z defined by the Z direction, the strands being arranged in such a way that the currents traversing the strands of each antenna have the same intensity at the same position along the z axis.

Compared to the quarter-wave monopole or half-wave dipole antennas tuned, the use of short wired antennas allows to exploit a linear gradient.

The antenna or antennas are preferably used in the near field.

The short wired antenna according to the invention is an antenna whose length L is less than λ / 10, λ preferably being the wavelength of the RF radiation emitted in the context of the MRI or NMR spectroscopy methods. In general, the wavelength of the radio frequency radiations used in NMR, intended for the excitation of the protons, in particular H +, is of the order of one meter, for example respectively of 1.5 and 75 cm for the H + in respective fields of 4.7T and 9.4T, corresponding to frequencies of 200 and 400 MHz respectively.

By "strand extending at least partially in the direction Z", it should be understood that the strand extends in a direction having at least one non-zero component parallel to the direction Z over at least a part of its length.

For example, the strands comprise a portion that extends parallel to the Z direction or the strands extend parallel to the Z direction. In a variant, the strands each have a curved portion forming a meridian in the Z direction.

Each strand comprises, for example, a single conductor acting as a short monopole wire antenna or dipole.

The strands of the antenna may delimit a measuring volume, for example substantially cylindrical, and preferably having at least one opening in the Z direction. A region of an object to be examined is present in the measurement volume. The antenna is powered by at least one current and / or voltage source for transmitting the RF field pulse sequence.

The strands may have the same length (L), which ensures the best homogeneity of the created RF field.

Each strand may comprise a wire or a copper strip.

Preferably, the distribution of the strands around the z axis and the phases of the currents flowing through them are such that the antenna creates a homogeneous RF oscillating magnetic field in each of the xy planes perpendicular to the z axis, in particular in the volume of measured. The RF oscillating magnetic field corresponds, when the antenna is powered, to the emitted RF magnetic field pulses. By "a homogeneous magnetic field in an x-y plane", it should be understood that, in at least one region of said plane, the variation of the amplitude of the magnetic field with respect to its maximum value is less than 10%.

Such a configuration relies, for example, on the cancellation of the harmonics of the Fourier series decomposition (DSF) associated with the distribution of the electrical conductors of the winding. This configuration is preferable for the homogeneity of the magnetic field generated or induced by the coils in general and the present invention in particular.

In the DSF method, for N sets of conducting wires distributed on the circumference of a circle, where each set comprises for example four conducting wires and a conducting wire corresponds to a strand of the multi-strand antenna defined above, and where each of conductive son is traversed by a current I or -I, the currents flowing through the conductive son arranged on the semicircle between θ = -π / 2 and θ = π / 2 through 9 = 0 being preferably in the same sense and the latter being contrary to the direction in which the currents traverse the conductive son arranged on the semicircle between θ = -π / 2 and θ = π / 2 passing through θ = π, the expression of the DSF of the normalized current density j (9) equivalent to the N sets of conductive wires is

m _even = Σfe = l ((Σ _n = l ^~ "∞s ((2fc + 1) 0) X C0S ((2 / c + 1) 0)) (eq.4) where Θ is the azimuthal angle on the circle and θ _η are the N angles defining the position of each of N sets of conducting wires, the four conducting wires of each set being arranged in θ _η , -θ _η , π-θ _η and -π + θ _η . In order to approach an ideal cosine distribution, it is necessary for the angles θ _{η to} cancel the N first harmonics, which amounts to determining the solutions θ _η of the following system of equations:

Thus, the antenna according to the invention may comprise Ν sets of strands where Ν is an integer greater than 1, the position of the strands being defined by the angles θ _η solutions of the system (eq.5).

In the case where the antenna comprises for example Ν sets of four conductive wires in addition to a pair of two central conductors son, ie in Θ = 0 and π, and where each of the conductive son is traversed by a current I or - I, the currents flowing through the conductive wires arranged on the semicircle between θ = -π / 2 and θ = π / 2 passing through 9 = 0 preferably being in the same direction and the latter being contrary to the direction in which the currents traverse the conducting wires arranged on the semicircle between Θ- π / 2 and θ = π / 2 passing through θ = π, the DSF expression of the normalized current density j (9) equivalent to Ν sets of conductive wires in addition to the pair of central leads in Θ = 0 and π becomes

] \ θ) _οαα = Σ £ = _! ((Σ £ = _! ^ (1 + 2cos ((2 / c + 1) 0)) x cos ((2 / c + 1) 0)) (eq.6) where Θ is the azimuthal angle on the circle and θ _η are the Ν angles defining the position of each of the Ν sets of conducting wires, the four conducting wires of each set being arranged in θ _η , -θ _η , π-θ _η and -π + θ _η .

The determination of the θ _η of the system makes it possible to approach an ideal distribution cosine is to solve the following system of equations:

ΣN

(cos ((2y + 1) θ _η )) = 0 (k = j)

= 1

(cos ((2N + 1)) 0 _η ) = 0 (/ = N)

71 = 1

(EQ.7)

Thus, in this variant, the antenna comprises a central strand and N sets of conductive wires where N is an integer greater than 1, the position of the strands being defined by the angles θ _η solutions of the system (eq.7).

As an illustration, we can consider the case to a set of conductive wires:

N = 1. The system of equations then reduces to: cos (3θι) = 0, whose solution is θι = π / 6.

Thus, the assembly comprises four conductive wires arranged in θι, -θι, π-θι and -π + θι. In this case, the homogeneity of the generated magnetic field (Bi) at 10% of the maximum value is about 60% over the diameter.

, ",. ,. cos (30i) + cos (30 ₂ ) = 0, For N = 2, the system of equations becomes whose

solutions are θι = π / 15 and θ ₂ = 4π / 15. Thus, the first set of conductive wires comprises four conductive wires arranged in θι, -θι, π-θι and -π + θι and the second set of conductive wires comprises four conductive wires arranged in θ ₂ , -θ ₂ , π-θ ₂ and -π + θ ₂ . These two conditions give rise to a new configuration of antennas whose homogeneity at 10%) is 80%> on the diameter, as illustrated in FIGS. 5a to 5c. The zone of homogeneity at 10%> is then more than 20%> greater than that of the saddle coil.

For higher values of N, a numerical resolution can be used to determine the angles.

The strands of an antenna can be connected at one end by at least one conductor, the latter being for example connected to a source of current / voltage.

The strands may be respectively connected at one end by at least two conductors, for example two, these conductors being arranged at the same height in the Z direction.

The source or sources of current and / or voltage may generate currents of opposite directions in the strands connected by one of these conductors relative to those connected by the other of these conductors, and during the transmission of the sequence pulses of the RF magnetic field. In a variant, the short film antenna is single-stranded. In this case, a plurality of antennas can be put in place to obtain the RF oscillating magnetic field according to the invention.

The invention also makes it possible to produce new antenna configurations.

The strands may comprise a radial portion extending radially from a center in a plane perpendicular to the z axis. The radial portion may be followed by a portion extending parallel to the Z direction. The radial portions may be star-shaped or otherwise arranged.

The strands of the antenna can be flexible. Thus, the strands can conform to the shape of a region of the object to be imaged, which makes it possible to gain in signal / noise ratio and to offer possibilities of making new antenna conformations, possibly to the detriment of the homogeneity of the magnetic field created by the strands during the emission and induced in the strands during the reception.

The subject of the invention is also an antenna system for the emission of an RF magnetic field pulse sequence and the detection of nuclear magnetic resonance (NMR) signals in an imaging and / or spectroscopy method. by NMR, comprising a first and a second short wired antenna according to the invention, the two short wired antennas being nested in a head-to-tail manner with respect to each other.

Preferably, the strands of the first antenna have substantially the same length as the strands of the second antenna.

The invention also relates to an antenna system comprising two antennas according to the invention, the two antennas are powered by a voltage source, one of the two antennas being connected to the voltage source by means of a phase-shifter at 180 ° so as to generate in the strands currents of opposite direction to those traversing the strands of the other antenna.

In a variant, the two antennas being fed by a coaxial cable, the antennas being respectively connected to the core and to the ground of the coaxial cable, which produces a 180 ° phase shift of the currents between the strands of the two antennas. The strands of each antenna can be connected at one end by a conductor, these conductors being for example arranged at the same height in the direction Z.

The invention also relates to a support for housing the antenna or antennas according to the invention, comprising a tubular body connected to a base, the body having passages configured to be traversed by the strands of the antenna.

The passages are for example distributed according to the DSF method described above. The support preferably comprises at least one recess for installing a tuning circuit therein.

The passages can be divided into groups of two close passages, for example side-by-side. The groups are preferably distributed according to the DSF method described above. The support may comprise two recesses for installing a first and a second tuning circuit each associated with a respective antenna of a system of two antennas according to the invention.

The subject of the invention is also an antenna system for the emission of an RF magnetic field pulse sequence and / or the detection of nuclear magnetic resonance (NMR) signals in an imaging method and / or of NMR spectroscopy, comprising a plurality of single-strand short-film antennas (1 1), the antenna strands (1 1) extending at least partially parallel along an axis z, the strands of the antennas (1 1) being disposed of in such a way that the currents traversing them present the same intensity at the same position along the z axis. Each antenna can be associated with a tuning circuit and an amplifier of its own.

The single-ended antennas are for example distributed angularly around the circumference of a circle according to the DSF method described above.

The subject of the invention is also a method for imaging and / or nuclear magnetic resonance spectroscopy (NMR), comprising at least the steps of generating a constant magnetic field Bo in a region where an object to be examined is located, to emit at least one radio frequency (RF) magnetic field pulse sequence, detecting NMR signals, wherein the transmission of the radio frequency (RF) magnetic field pulse sequence (s) is performed by a filamentary antenna short or an antenna system according to the invention, as defined above. The Bo field is generally in the range of several mT to some T. The Bo / Bi ratio of the intensities of the Bo and Bi fields, for proton imaging, is generally between 10 and 10 ⁶ .

The detection of the signals can be carried out by any suitable means, in particular by one or more inductive antennas.

The use of short-wave antennae makes it possible to use the inhomogeneity of the signal in the Z direction related to the propagation times, to locate the spins on transmission.

The antenna according to the invention as defined above can be configured to perform the sectional selection in an NMR imaging and / or spectroscopy method.

The invention eliminates the presence of a gradient coil for spatial selection in at least one direction, and offers the possibility of combining spatial selection and RF excitation.

Dependence of the current of the short multi-stranded wire antenna at the z-position on the strand allows NMR signals to be detected spatially, at least in the Z direction, by means of a suitable radio frequency pulse sequence.

For example, suitable radio frequency pulse sequences for performing spatial selection in an oscillating field are disclosed in Hoult's publications: "Rotating Frame Zugmatography" published in 1979 in Cox & Styles' Journal of Magnetic Resonance. "Towards Biochemical Imaging" published in 1980 in the Journal of Magnetic Resonance and extended by Blackledge et al. in "The Elimination of Transmitter-Receiver Phase-Twist Artifacts in the Phase-Modulated Rotating-Frame Imaging Experiment" published in 1988 in the "Journal of Magnetic Resonance" as well as the publication of Sharp et al. above.

In these publications, the oscillating field Bi having a variable direction is generated by RF transmitter coils. The RF coil used in transmission is designed to promote the appearance of an RF magnetic field gradient (Bi) in a

dB

direction x such that B ₁ = B _w + ~ ^ X- Thus, the flip angle θ (χ) depends on the intensity of the field Bi seen by the spins at the x position, ie Bi (x). After applying the RF field for a duration τ,

dB

the flip angle will be 6> (x) = y (S ₁₀ + ^~ ^ ") ^τ · According to the invention, the RF magnetic field gradient (Bi) can be realized thanks to the dependence of the current at the z position on a short antenna strand.

Indeed, the RF field Bi created by a short multi-stranded wire antenna can be written approximately in the form Bi = k Io (1-z / L), according to the equation (3. a) for a monopole short wired antenna of length L where lo represents the feed current of the antenna and k is a sensitivity coefficient dependent on the geometrical parameters of the antenna. This coefficient can be obtained by calculation or numerical simulation.

Thus, by substituting Bi in equation (1), we obtain:

Considering now that the antenna is fed during a "calibration tap" by the current lo for a duration τ such that θι = π / 2 at a position z = z _r ef, equation (4) becomes:

π / 2 = ky lo (l ^~ z _r ef / L) τ eq. (9) For example, z _r ef is chosen near the center of the sample and then extends the spatial field of view.

The ratio between the equations (8) and (9) makes it possible to express the dependence of the flip-flop angle as a function of the position z, namely θι (ζ), whatever the method used to choose the spatial location of the NMR signals to be detected, for example the adjustment of the intensity of the field Bi by the feed current of the antennas during the calibration tap or the duration of application τ of the calibration tap:

", _N π lz / L

The flip-flop angle θι, and therefore the signal intensity, is then a function of the conditions for obtaining the calibration tap at π / 2 at z = z _r ej, ie lo, τ, etc. and the z position.

However, the nutation of the spins takes place in the plane y'-z 'of the rotating marker, the latter being defined by the axes x', y ', z'. Conventionally, for example as described in the Hoult publication cited above, the axis z 'of the rotating reference coincides with the axis z, which is parallel to Bo, while the axes x' and y 'are rotating. at the frequency of Lamor ωο. The application of a homogeneous π / 2 tap in the perpendicular direction makes it possible to switch the magnetizations in the plane x'-y 'of the rotating mark. The NMR signal called "Free Induction Decay" (FID) takes the form below: jy (B _x (x) + - x) + j

e (x, t) ∞M ₀ (x, t) e ^{dx 2 Tl} eq. (11)

The sequence is then repeated for various values of τ, for example from 0 to Tmax according to Hoult, whose number of repetitions depends on the desired resolution. The application of several pulses of variable duration τ results in a modulation of the phase of the FID. The application of a Fourier transform then makes it possible to highlight the modulation of the phase caused by the RF field gradient:

Γ ^τ πιαχ

TF [e (x, t)] oc I e (x, t) e- ^{ji} xT} dT

_ t Γ ^τ πιαχ

TF [e (x, i)] oc -Af ₀ (x, t) e ^~ ¾

or Δω _χ = -ω _χ + γ {Β + - *) TF [e (x, t)] ∞ -M ₀ (x, t) e ^Τ >> -

An example of a suitable 1D tap sequence is shown in FIG.

According to the publication "MRI using radiofrequency magnetic field phase gradients" from Sharp et al. above mentioned, after the application of the calibration tap allowing the flip-flop to π / 2 at z = z _r ef, a succession of pulses at i in z = z _r ef makes it possible to accentuate the spatial selection, and offers a simple means to select a cut.

An alternative method of spin spatial selection, ie of selection of section using the RF field, for example on the basis of the publication of Blackledge et al, consists in applying a succession of RF pulses favoring the coherent excitation of a region of space while destructive interference is obtained elsewhere. In particular, it is proposed to apply a succession of funds in the following relation:

θ = (2η + 1) π / 2 (-1) ^η with n = 0, 1, 2 ... eq. (12)

Combinations of different puzzles then favor spatial selection. For example Blackledge proposes a sequence comprising 20 flip-flops at π / 2 followed by 14 taps at -3π / 2 then 5 taps at 5π / 2 and finally a tap at 7π / 2. By adding the successive acquisitions, the detection is made sensitive to a spatial section Gaussian envelope. The thickness of selection will ultimately depend on the intensity of the gradient.

To spatially select a new section at any height z in the measurement volume, it may be sufficient to adjust either the duration τ of application of the calibration pulse of the RF pulse sequence, or the intensity h of power supply. of the antenna during the application of the calibration tap.

The dependence of the flip-flop angle θι of the region of the object to be examined at the position along the strand can be used to select a section along the z-axis and allows to replace a gradient coil of the MRI in at least one direction.

The spatial selection thus obtained is of particular interest since it can satisfy flow measurement (by cutting input methods) in a portable NMR application for the study of plant physiology.

The method may thus comprise at least one step of spatial selection of a position detection plane xy zo along the z axis, for example by choosing a duration or an amplitude of a calibration pulse of the radio pulse sequence. frequency; in particular, it is possible to adjust a duration (τ) of application of the calibration tap, or a voltage or a supply power of the antenna during the application of the calibration tap.

The spatial selection can make it possible to select the region to be examined in an x-y plane perpendicular to the z axis in the measurement volume.

By virtue of the reciprocity theorem which stipulates the symmetry of the properties of the transmit and receive antennas, this dependence of the current at the z position also intervenes in reception, and one can then compensate for it by using symmetrical configurations to average this dependence. and obtain a homogeneous volume resonator.

The subject of the invention is thus also a method for imaging and / or nuclear magnetic resonance spectrometry (NMR), comprising at least the steps of generating a constant magnetic field Bo in a region containing an object to be examined. transmission of at least one sequence of radio frequency (RF) magnetic field pulses, and detection of NMR signals, wherein the detection of the NMR signals is carried out by an antenna system according to the invention. This aspect of the invention makes it possible to overcome the dependence of the signals detected at the position of the strand, as explained above.

A short wire antenna system according to the invention can be used only either in transmission or in reception. In the case of use only in emission, the detection of the nuclear magnetic resonance (NMR) signal can be carried out by one or more inductive antennas. In the case of use only in reception, the emission of the radio frequency (RF) magnetic field pulse sequence can be carried out by one or more inductive antennas. In the latter case, PIN diode based decoupling circuits may be associated with the system. These circuits are, for example, known from the publication "Electronic decoupling of surface-coil receivers or NMR imaging and spectroscopy" by Edelstein et al. published in 1986 in the Journal of Magnetic Resonance.

Alternatively, a short wire antenna system according to the invention can be used both in transmission and reception. This then makes it possible to carry out the RF excitation in the region to be examined and the detection of the NMR signals with a single device.

The methods according to the invention may comprise a step of processing the detected NMR signals. An image representative of the region of the object to be examined may be generated from the processed signals.

The spatial selection of spins in a plane, according to this aspect of the invention, can also be obtained by selecting one of the first and second wire antennas while the two multi-strand wire antennas are combined with the detection to overcome the dependence of the signal detected at the position on the strand.

The strands are preferably tuned to the observation frequency of the NMR signals.

The wire antenna can be placed in any direction with respect to the static field Bo. However, the detection direction of the wire antenna is the orthoradial direction at the strand of the RF magnetic component or the collinear direction of the RF electrical component.

Each antenna or antenna system may be connected to a proximity amplifier during the receiving step. Each antenna or antenna system may be associated with a frequency tuning electrical circuit. This frequency tuning electrical circuit may comprise at least one capacitor or inductor connected in series with at least one of the strands.

In a variant, each antenna or antenna system is associated with an electrical impedance matching circuit comprising at least one capacitor or inductor connected in parallel with at least one strand.

The strands of the antenna or antennas can be flexible. The method may comprise a step of placing the strands on the region of the object to be examined so as to marry at least partially the shape of the latter.

The use of the antenna according to the invention makes it possible to produce a compact device for NMR or MRI spectrometry, and preferably portable.

The antenna or antennas may be used immersed in a sample in the liquid, solid or gaseous phase comprising a material to be characterized, for example a material whose cores have a magnetic moment. This can be useful for spectroscopic analysis of chemical or biological compounds.

In a variant, the antenna or antennas are used outside a sample comprising a material to be characterized.

The method can be used to identify compounds, for example chemical or biological compounds, for example molecules, in pharmaceutical, preclinical and / or agro-food applications.

The method according to the invention can be used for analyzing water stress and / or tissue structure in plants. The method then comprises a signal processing step delivering an image representative of the water content of a plant.

The invention is also applicable in the medical field.

The invention will be better understood on reading the detailed description which follows, of non-limiting examples of implementation of the invention, and in view of the appended drawing, in which:

FIG. 1 schematically represents a homogeneous field antenna system in an xy plane, with strands out of phase with the power supply of FIG. 2 illustrates in perspective the intensity of the magnetic field Bi created with an example of distribution of strands making it possible to obtain a homogeneous field in a plane xy of FIG. 1, obtained using a software of FIG. finite element simulation;

FIG. 3 is a view from below of the map of Bi in the x-y plane of FIG. 2, which illustrates the homogeneity obtained in a plane at any height;

FIG. 4 is a view of the map of Bi in the z-y plane of FIG. 2, which illustrates the dependence of the intensity of the field on the position on the height of the strand;

FIGS. 5a to 5c show another example of distribution of strands for obtaining a homogeneous field in an x-y plane;

- Figure 6 schematically shows a short wire antenna system according to the invention;

FIG. 7 schematically illustrates a tuning and adaptation circuit connected to a conducting strand;

FIG. 8 represents a 1D tap sequence known to accentuate the spatial selection; and

FIG. 9 schematically represents a variant of short wired antennas according to the invention,

FIGS. 10 and 11 show diagrammatically variants of the antenna system according to the invention;

- Figures 12 and 13 schematically illustrate supports for housing antennas according to the invention; and

FIGS. 14 and 15 are top views of the support bases according to FIGS. 12 and 13.

The reference z is used in the present application to denote both the z-axis and the coordinate of a point along this z-axis, as is commonly done for orthogonal markers.

FIG. 1 shows an antenna system 1 according to the invention for transmitting a sequence of radio frequency (RF) pulses.

The system comprises two short wired antennas 13 and 14, powered by a voltage source 8.

Each antenna 13 or 14 has a plurality of conductive strands 31 and 41. The strands 31 and 41 have the same length L and extend in a direction defining the z axis of an orthogonal reference. The length L depends on the field and the nuclei and is for example between 10 mm and 100 mm for the observation of a proton in a field of 9.4 T.

In the example considered, the strands 31 of the antenna 13 are connected together at one end 90 by a conductor 37 at the same height z along the z axis, and likewise the strands 41 of the antenna 14 are connected together to one end 90 by a conductor 47 at the same height along the z axis.

The conductors 37 and 47 are in the form of arcs in the illustrated example. The strands 31 and 41 thus define the limits of a measurement volume, substantially cylindrical and extending along a longitudinal axis parallel to the z axis.

The object to be examined is for example introduced by upper or lower openings of the measurement volume.

Alternatively, the measurement volume is built around the volume to be examined, the antennas being placed around the object to be analyzed.

The conductors 37 and 47 are connected in their respective mediums C ₃ and C ₄ to the voltage source 8.

In the example of FIG. 1, in order to obtain an oscillating magnetic field homogeneity Bi in the measurement volume, a phase shift is created in the supply of the antennas 13 and 14. The antenna 14 is connected to the source of voltage 8 by means of a phase shifter at 180 ° 6, so as to generate in the strands 31 currents Ii of opposite directions to those traversing the strands 41 of the antenna 14. At a given position along the z axis, these currents have substantially the same value for the strands 31. The same is true for the strands 41.

The strand positions 31 and 41 can be determined by the harmonic suppression method of the DS representing the distribution law of the current density by the eq (4).

Figures 2 to 4 illustrate, for the case N = 3, a distribution of the strands according to this method. We obtain θι = 1 1, 66 °, θ ₂ = 26 ° and θ ₃ = 56 °, each angle θι, θ ₂ and θ ₃ defines the position of a set of conductive wires having four strands around the axis z.

The electromagnetic simulation was performed at a frequency of 200 MHz for an antenna comprising twelve strands length of 3 cm. The different intensities of the magnetic field B1 are represented by different hues.

The magnetic field Bi thus obtained is of direction parallel to the planes P _x - _y perpendicular to the z axis. As illustrated in Figure 3, the magnetic field Bi is substantially homogeneous within each of these planes P _x _ _y .

The value of the field Bi decreases linearly along the z axis from a height corresponding to the upper ends 90 of the strands 13 and 14 respectively connected to the conductors 37 and 47 to a height defined by the free ends 10 of the strands 13.

In one variant, the strands of the two antennas 13 and 14 are all connected to a single conductor, for example in the form of a ring, and thus form a single transmitting antenna according to the invention thus favoring the appearance of a gradient of RF field in the xy plane.

Other strand distributions by proper routing and appropriate power configurations are not outside the scope of the present invention.

If the radial dependence is substantially eliminated in the measurement volume delimited by the antennas 13 and 14, the dependence of the intensity of the signal on the position of the object with respect to the height of the strand proves to be advantageous for the location of the signal.

The reciprocity theorem indicates that the transmit gain and the receive gain are related, which makes it possible to predict that the reception sensitivity depends on the position on the strand and the radial distance to the strand.

FIG. 6 illustrates a system according to the invention that can be used both for transmitting radio frequency (RF) magnetic field pulse sequences and for detecting nuclear magnetic resonance (NMR) signals.

The system includes first and second short wired antennas 17 and 12. Each antenna 17 and 12 has a plurality of strands 71 and 21.

In the example illustrated, the strands 71 and 21 extend parallel to a direction defining the z axis of an orthogonal reference. The strands 71 of the first antenna 17 are preferably substantially the same length as the strands 21 of the second antenna 12 to ensure the homogeneity of the field.

The strands 71 of the first antenna 17 are all connected at their upper end to a conductor 77 at a first altitude zi along the z axis and the strands 21 of the second antenna 12 are all connected at their end to a conductor 27 at a second altitude z ₂ along the z axis, with z ₂ <zi.

The strands 71 of the first antenna 17 are interleaved and arranged in a reverse manner between the strands 21 of the second antenna 12.

Each antenna 12 and 17 is connected, by means of an electronic switch 54, to a supply source 87 and 82 at the transmission and at an output P _or ti and P _or t 2 at the detection.

The antennas 12 and 17 have a dependence on the position z on emission. By virtue of the reciprocity theorem, it is the same in reception, which means that the detection is more or less sensitive according to the position z. This reception dependence is compensated by the addition of the signals detected by the antennas 12 and 17.

Indeed, the intensities of the signals /; and h detected by the two antennas 12 and 17 may each be expressed as a function of the position z:

7 ₁ (z) = / ₀ (l - ¾

7 1 ₂ ί (z) = / I ₀ ί (- ^Ζ ) ( ^{(1 3)} where Io represents the output current on reception and L the length of strands 21 and 71.

The resultant I (z) no longer has any dependence on the position z, at least on detection:

I (z) = I ₁ (z) + I ₂ (z) = I _{0 eq (14)}

Using the system according to FIG. 4, the emission of the radio frequency magnetic field pulse sequences as well as the spatial selection of the spins in a plane can be carried out by selecting one or the other of the two antennas 11 and 12. fed respectively by supply sources 81 or 82 on transmission, while the two antennas 11 and 12 are combined with the detection to overcome the dependence of the signal detected at the position along the strands. The outputs P _or t1 and P _or t2 are directed to signal amplifiers 71 and 72.

To increase the efficiency of the antennas in reception, short wire antennas 12 and 17 may be associated with one or more circuits 9 of agreement and adaptation. This is done conventionally for inductive coils used in spectroscopy and NMR imaging. As explained in the "Tunable electrically small Antennas" publication by John AM Lyon, Alan Cha GT and Mohamed A. Hidayet, the circuits The tuning and matching used for the short wire antennas are dual to those used for the coils.

In the example illustrated in FIG. 6, each antenna 11 and 12 is associated with a tuning and matching circuit 9.

An example of a tuning and matching circuit 9 is illustrated in FIG. 7. The circuit comprises a capacitor Ct and an inductor L connected in series with the conducting strand 1 to tune the frequency, but also a capacitor in parallel C _m to adapt the impedance. As explained in the publication "Tunable electrically small

Antennas ", this tuning circuit configuration is known to allow to get closer to a quarter-wave antenna (λ / 4) when the available space is reduced.

The invention is of course not limited to the embodiments described, and encompasses all variants.

For example, the tuning and matching circuit has variable inductances instead of variable capacitors.

The strands may be uniformly disposed about an axis passing through the center C and parallel to the z axis. Electronic components such as capacitors and inductors can be associated with the strands to obtain a phase shift of the currents in the strands of the wire antenna or antennas in different configurations of the strands, by analogy with the "birdcage" antennas.

FIG. 9 illustrates a monopole multi-strand wired antenna 16 in the form of a star, used only in reception. This antenna 16, for example, marries the shape of the object to be imaged.

In the variant of FIG. 10, the antenna system 1 comprises two short wire antennas 13 and 14, whose phase shift between the strands 31 and 41 is created using the property of a dipole antenna.

The conductors 37 and 47 are respectively connected to the core and to the ground of a coaxial cable 88. The antenna system 1 is associated with a tuning and matching circuit 9. The latter comprises two variable capacitors Ci and C ₂ , the values of which are chosen as a function of the nuclear resonant frequency of the material to be measured.

In the variant of Figure 1 1, the antenna 101 has eight strands 1 1. Each strand 1 1 is associated with a 9 frequency tuning circuit and an amplifier 71 in series. The amplifier 71 is for example low noise proximity. The strands 1 1 are for example distributed over the circumference of a circle C according to the DSF method described above. Figures 12 and 13 illustrate supports for housing the antennas according to the invention. The support 5 comprises a tabular body 52 connected to a base 53. The body 52 has passages 51 configured to be traversed by the strands of the antenna.

In the variant of Figure 12, the body 52 has 8 passages 51 (half of which is shown), distributed according to the DSF method exemplifïée in Figure 5C. This distribution of passages 51 is shown schematically in FIG. 14. The support of FIG. 12 makes it possible to house a system of two antennas, each of which has 4 strands, for example similar to that shown in FIG. 1. The base 53 of FIG. support 5 has a recess 90 for installing a tuning circuit 9 therein.

In the variant of Figure 13, the body 52 has 16 passages 51 (half of which is shown). This distribution of the passages 51 is shown schematically in FIG. 15. The passages 51 are divided into eight groups 56 of two passages 51. Each group 56 of the passages 51 comprise two passages 51 that are close together, for example side-by-side. Groups 56 are distributed according to the DSF method as exemplified in FIG. 5C. Such a support makes it possible to house a system of two antennas, for example arranged head to tail in a manner similar to that shown in FIG.

In this variant, the base 53 of the support 5 comprises a first recess 90 for installing a first tuning circuit 9. The support 5 comprises a second recess 90 for installing a second tuning circuit 9 This second recess is arranged in a part 91 of the support 5 located at the top of the body 52 and connected to the body 52 of the support 5 by a support leg 55. The first and second tuning circuits 9 are each associated with a respective antenna of the system. Thus, when the support receives an antenna system head to tail, the support allows to have a tuning circuit upstream of the strands of each antenna. In the examples of FIGS. 12 and 13, the strands can pass through the base 53 through openings 92, which allows them to be connected under the base 53.

An MRI or spectrometry system for implementing the invention may comprise among other things a magnet or an electromagnet generating a permanent field Bo in the Z direction. The presence of a gradient coil is not necessary but does not depart from the scope of the present invention.

Also, for reasons of simplicity, Bo field control systems and the voltage source or sources for the generation of the Bi field have not been shown.

The systems for processing signals and images and possibly display were not represented either.

The short wired antenna can be cooled to reduce its intrinsic noise and thus increase its signal-to-noise ratio.

The short wired antenna can be made on a flexible support to adapt to the shape of the object.

Claims

1. Short-wave antenna (13; 14; 12; 17; 15; 16) for transmitting a sequence of radio frequency (RF) magnetic field pulses in an imaging and / or resonance spectroscopy method nuclear magnetic resonance (NMR), the short wire antenna having a plurality of strands (31; 41; 21; 71; 51; 61) extending at least partially in a Z direction in an orthogonal coordinate system having an axis z defined by the direction Z, the strands being arranged in such a way that the currents traversing the strands (31; 41; 21; 71; 51; 61) of the antenna (13; 14; 12; 17; 15; the same intensity at the same position along the z axis.

2. A short wire antenna according to claim 1, the distribution of the strands around the z axis and the phases of currents flowing therethrough being chosen to create a homogeneous RF (Bi) oscillating magnetic field in a plurality of xy planes perpendicular to the z-axis, the strands (31; 41; 21; 71; 51; 61) preferably having the same length (L).

3. A short film antenna according to claim 1 or 2, the strands (31; 41; 21; 71) of the antenna (13; 14; 12; 17) extending parallel to the z axis.

4. Antenna according to any one of the preceding claims, the antenna comprising N sets of four strands distributed over the circumference of a circle where N is an integer greater than 1, the position of the strands being defined by the angles θ _η , where θ _η are the solutions of the system:

5. Antenna according to any one of claims 1 to 4, the antenna having a pair of two central strands and N sets of four strands distributed on the circumference of a circle where N is an integer greater than 1, the wires of central conductors being respectively disposed on the azimuthal angle circle equal to 0 and π and the position of the strands being defined by the angles θ _η , where θ _η are the solutions of the system

Antenna system for transmitting an RF magnetic field pulse sequence and detecting nuclear magnetic resonance (NMR) signals in an NMR imaging and / or spectroscopy method, comprising a first and a second short wired antenna (12,17) according to any one of the preceding claims, the two short wired antennas (12,17) being interleaved with each other head-to-tail.

7. Antenna system according to claim 6, the strands (21) of the first antenna (12) having the same length (L) as the strands (17) of the second antenna (71).

8. Antenna system (1) comprising two antennas (13, 14) according to any one of claims 1 to 7, the two antennas (13, 14) being powered by a voltage source (8), one of the two antennas (13, 14) being connected to the voltage source (8) by means of a phase shifter (6) at 180 ° so as to generate in the strands (31) currents (Ii) of opposite directions to those traversing the strands (41) of the other antenna (14).

9. Antenna system (1) comprising two antennas (13, 14) according to any one of claims 1 to 7, the two antennas (13, 14) being fed by a coaxial cable (88), the antennas (13) , 14) being respectively connected to the core and to the ground of the coaxial cable (88).

10. Antenna system (1) according to claim 8 or 9, the strands (31, 41) of each antenna (13, 14) being connected at one end by a conductor (37, 47), these conductors (37, 47) being arranged at the same height along the z axis.

1. Antenna system (101) for transmitting an RF magnetic field pulse sequence and / or the detection of nuclear magnetic resonance (NMR) signals in an imaging and / or spectroscopy method by NMR, comprising a plurality of single-wire short-wave antennas (11), the antenna strands (11) extending at least partially parallel along an axis z, the strands of the antennas (11) being arranged in such a way that currents running through them present a same intensity at the same position along the z axis, each antenna being preferably associated with a tuning electrical circuit (9) and an amplifier (71) of its own.

12. A method of imaging and / or nuclear magnetic resonance spectroscopy (NMR), comprising at least the steps of:

to generate a constant magnetic field Bo in a region where an object to be examined is present,

emit at least one sequence of radio frequency (RF) magnetic field pulses,

detecting nuclear magnetic resonance (NMR) signals, in which the emission of the radio frequency magnetic field (RF) pulse sequence or sequences is carried out by a short film antenna according to any one of claims 1 to 5 or an antenna system according to any one of claims 6 to 11.

13. Method according to the preceding claim, implemented by the short wire antenna or the antenna system, the strands of the antenna or antennas delimiting the limits of a measuring volume, preferably having at least one opening according to the direction Z.

14. Method according to the preceding claim, comprising at least one step of spatial selection of a detection plane xy perpendicular to the z axis in the measurement volume, preferably choosing, inter alia, a duration (τ) of applying a calibration pulse of the RF pulse sequence and / or a voltage or power supply of the antenna during the application of the calibration tap.

15. Method according to any one of claims 12 to 14, the detection of nuclear magnetic resonance (NMR) signals being performed by at least one inductive antenna.

16. A method for imaging and / or nuclear magnetic resonance spectroscopy (NMR), comprising at least the steps of generating a constant magnetic field Bo in a region of an object to be examined, transmitting at least one sequence of radio frequency (RF) magnetic field pulses, detection of nuclear magnetic resonance (NMR) signals, wherein the detection of nuclear magnetic resonance (NMR) signal is performed by an antenna system of any one Claims 6 to 11.

17. The method of claim 16, wherein the emission of the radio frequency (RF) pulse sequence or sequences is performed by at least one inductive antenna.

18. The method of claim 16, wherein the transmission of the radio frequency (RF) pulse sequence (s) is performed by one of the first and second short film antennas (12; 17).

19. Method according to one of claims 12 to 18, the or each antenna (13; 14; 12; 17; 15) being of monopole or dipole type.

20. A method according to any one of claims 12 to 19, the short antenna or antennas being used in the near field.

21. Method according to any one of claims 12 to 20, the strands being preferably flexible, and the method comprising a step of placing the strands on the object to be examined so as to marry at least partially the shape of this latest.

22. A method according to any one of claims 12 to 21, the method being used for analyzing water stress in plants.

23. A method according to any one of claims 12 to 22, the method being used for analyzing the structure of tissues in plants.

24. The method according to any one of claims 12 to 22, the method being used to identify compounds.

25. A method according to any one of claims 12 to 24, wherein the antenna or antennas are immersed in a sample comprising a material to be characterized.

26. Method according to any one of claims 12 to 24, wherein the antenna or antennas are used outside a sample comprising a material to be characterized.