Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
  • G01R33/32—Excitation or detection systems, e.g. using radio frequency signals
  • G01R33/34—Constructional details, e.g. resonators, specially adapted to MR
  • G01R33/34046—Volume type coils, e.g. bird-cage coils; Quadrature bird-cage coils; Circularly polarised coils
  • G01R33/34053—Solenoid coils; Toroidal coils
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
  • G01R33/32—Excitation or detection systems, e.g. using radio frequency signals
  • G01R33/34—Constructional details, e.g. resonators, specially adapted to MR
  • G01R33/34092—RF coils specially adapted for NMR spectrometers
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
  • G01R33/30—Sample handling arrangements, e.g. sample cells, spinning mechanisms
  • G01R33/307—Sample handling arrangements, e.g. sample cells, spinning mechanisms specially adapted for moving the sample relative to the MR system, e.g. spinning mechanisms, flow cells or means for positioning the sample inside a spectrometer

Definitions

• the invention relates to a nuclear magnetic resonance probe and a nuclear magnetic resonance apparatus comprising such a probe.
• the invention also relates to a radiofrequency coil, which can notably be used in such a probe, as well as to a method for generating a radio frequency magnetic field.
• a coil 10 and a method of generating a radio frequency magnetic field according to the invention can be used in nuclear magnetic resonance, but also for other applications, for example for the confinement of plasmas.
• radio frequency 15 any frequency between 3 kHz and 300 GHz, more particularly any frequency between 300 kHz and 3 GHz and more particularly any frequency between 1 MHz and 1 GHz.
• nuclear magnetic resonance apparatus means a nuclear magnetic resonance spectroscopy (NMR) and / or nuclear magnetic resonance imaging (MRI) apparatus.
• nuclear magnetic resonance probe is understood to mean that part of a nuclear magnetic resonance apparatus intended to generate a radiofrequency magnetic field for exciting the nuclear spins of a sample and / or for detecting a radiofrequency magnetic field emitted by the deexcitation of said nuclear spins.
• a probe generally comprises an LC type resonant circuit including a coil which couples with an external radio frequency magnetic field, as well as an impedance matching circuit.
• winding means an element consisting of one or more windings of a wire, cable or conductive strip.
• winding means a set of turns, or loops, of the same wire, cable or conductive strip, without short circuits.
• the superconducting magnets used in NMR and RM experiments have a cylindrical geometry and produce a generally stationary magnetic field oriented along the cylinder axis ("axial”, “longitudinal”, or “main” magnetic field This magnetic field polarizes the nuclear spins of the atoms in the sample being studied, which means that there is a population difference (commonly known as polarization) between the upper and lower Zeeman energy levels. these levels are excited using a radio frequency (RF) magnetic field perpendicular to the axial magnetic field, alternatively, an RF magnetic field is used to excite the magnetization of the sample.
• RF radio frequency
• Properly designed antennas (coils) generate such an RF magnetic field, the orientation of which may not be perpendicular to the axial magnetic field, but must necessarily include a perpendicular component.
• SNR signal-to-noise ratio
• Spatial uniformity (homogeneity) of the RF field inside the coil is also very important in NMR experiments and may be crucial in MRI experiments.
• the most widespread antenna which provides the best performance in terms of intensity and homogeneity of the RF field, is a simple coil solenoidal, single coil.
• a simple coil solenoidal, single coil generates a magnetic field parallel to its axis, and must therefore be arranged perpendicular to the main magnetic field.
• the sample can not be inserted "from the top" of the superconducting magnet (i.e. in an axial direction) and can not be rotated around the longitudinal axis.
• a rotation of the sample is very useful for improving the resolution of NMR spectroscopy, especially in the case of a liquid sample.
• Other common RF coils include Heimhoîtz pairs or saddle coils, but offer lower RF field performance.
• the main advantage of the seile-shaped coil is that it is wound on a cylindrical surface and can produce an RF magnetic field which is oriented perpendicular to the axis of this cylinder.
• the axis of such a coil can therefore be aligned with the direction of the main magnetic field, which allows the introduction of the sample in this direction and its rotation around it.
• a saddle-shaped coil provides a fairly good spatial homogeneity and some convenience of use.
• the salt-shaped coil is most commonly used in liquid NMR experiments.
• it has a low inductance and a reduced resistance compared to other types of coils, which is advantageous for high frequency applications.
• the invention aims to overcome the aforementioned drawbacks of the prior art.
• the invention aims to provide a probe for nuclear magnetic resonance having a high sensitivity and high homogeneity of the radio frequency magnetic field, while allowing introduction of the sample in a direction parallel to the longitudinal axis of the system, as well as a nuclear magnetic resonance (NMR or MRI) apparatus provided with such a probe.
• NMR nuclear magnetic resonance
• the invention also aims to provide a coil for the efficient generation of a very homogeneous radiofrequency magnetic field having an orientation perpendicular to the axis of said coil.
• a coil may in particular be used in a probe according to the invention.
• the invention also aims to provide an efficient method for generating a very homogeneous radiofrequency magnetic field, while allowing access to a region of space where the field is located in a direction perpendicular thereto.
• a method may in particular be implemented by means of a coil or a probe according to the invention.
• An idea underlying the invention is to use one or more coils comprising two helical windings whose turns have opposite inclinations with respect to a common longitudinal axis. Coils having such a structure are known from the prior art as the "double helix dipoles" or (DHD, of the English “Double Helix Dipole”), see in this connection;
• An object of the invention is therefore a probe comprising at least one radiofrequency coil characterized in that said radio frequency coil comprises a first helical winding having turns inclined at an angle ⁇ different from zero and from 90 ° with respect to a axis and a second helical winding coaxiai said first winding, having turns inclined at an angle -a with respect to said axis.
• Such a probe may comprise at least two radiofrequency coils arranged coaxially and whose windings are oriented so that the planes formed by the axes of their turns and the axis common to the two coils are perpendicular to each other.
• the turns of said or each said coil may be inclined at an angle between 10 ° and 50 °.
• Each said helical winding may have a number of turns between 1 and 25.
• the helical windings of the same coil can be connected in series, so as to be traversed by the same current.
• Said helical windings may have the same number of turns.
• Another object of the invention is a nuclear magnetic resonance apparatus comprising:
• a magnet for generating, in a so-called interior volume, a stationary magnetic field oriented in a so-called longitudinal direction;
• said probe may comprise one or more coils whose axis is parallel to said longitudinal direction of said stationary magnetic field.
• said probe may comprise one or more coils whose axis is inclined at an angle ⁇ ⁇ - arctan ⁇ 1) ("magic angle") with respect to said longitudinal direction of said stationary magnetic field.
• Yet another object of the invention is a coil comprising a first helical winding having turns inclined at an angle ⁇ different from zero and 90 ° with respect to an axis and a second helical winding, coaxial with said first winding, having turns inclined at an angle -a with respect to said axis, characterized in that said helical windings have a number of turns between 1 and 25.
• each said helical winding may have the same number of turns.
• Yet another object of the invention is a method for generating a radio frequency magnetic field comprising feeding, by a radiofrequency current source, a coil comprising a first helical winding having turns inclined at an angle. a different from zero and 90 ° with respect to an axis and a second helical winding, coaxial with said first winding, having turns inclined at an angle -a with respect to said axis.
• Said angle may be between 10 ° and 50 °.
• Each said helical winding may have a number of turns between 1 and 25.
• FIG. 2 a current distribution producing a magnetic field of maximum homogeneity
• FIG. 4A a radiofrequency coil according to a first embodiment of the invention
• FIGS. 4B and 4C contour plots illustrating the magnetic flux generated by the coil of FIG. 4A;
• FIG. 5A a radiofrequency coil according to a second embodiment of the invention
• FIGS. 5B and 5C contour plots illustrating the magnetic flux generated by the coil of FIG. 5A;
• FIGS. 6A and 6B respectively, a nuclear magnetic resonance probe according to a third embodiment of the invention and its adaptation circuit
• FIGS. 8A-8C the results of an NMR experiment demonstrating a technical advantage afforded by the invention with respect to a probe according to the prior art
• FIG. 9 the arrangement of a sample inside a probe according to said third embodiment of the invention.
• such a magnetic dipole DHD is a coil consisting of two superimposed superposed self-windings E1 and E2, of length considered as infinite, whose turns S are inclined respectively by an angle + and -a relative to to an axis z.
• the angle is substantially different from 90 ° and 0 °, and often close to 45 °.
• Each winding generates, when traversed by an electric current, a magnetic field Bi, B 2 having a longitudinal (solenoidal) component B z i, B Z 2 and a transverse (dipole) component B y , i, B y 2.
• a coil consisting of such windings would have an extremely high inductance and could not, in practice, be used in radiofrequency applications, and in particular in a nuclear magnetic resonance probe, which must be resonant at a frequency which is generally of several MHz.
• the present inventors have therefore considered the case of a coil having a structure similar to that of a DHD dipole but a finite length, a limited number of turn and therefore an inductance sufficiently low for its use at radio frequency can be considered (and a small enough space for it to be used in a nuclear magnetic resonance probe).
• the law of Biot and Savart makes it possible to calculate the density of the magnetic flux B at the point
• the magnetic flux generated by a winding consisting of
• N> 1 turns is obtained in the same way, simply by varying the angle
• the two windings ET, E2 ' are traversed by the same current, which can be obtained by connecting them in series with each other and by connecting them to the same current generator.
• the ratio of length to diameter (considering the diameter of the coil, or equivalently that of the windings, and not that of each turn considered individually) is L / (2a-sinoc) "4.15, which is very far from the infinite length approximation on which the double helix dipole theory is based. More generally, the length / diameter ratio of a coil according to the invention is advantageously between 1 and 10, preferably between 2 and 5 and more preferably between 2 and 3.
• the two windings can be powered by separate and independent current generators. As has been explained above, this makes it possible to adjust the orientation of the radio frequency magnetic field.
• the y-direction magnetic flux component has a maximum value of 1.9 T / mA, while the other components are at least an order of magnitude smaller, which means that the magnetic field is essentially transverse.
• An almost perfect homogeneity is obtained over the entire internal volume of the coil, at the level of the superposition of the two windings.
• the inductance of the coil can also be calculated numerically: a value of about 1.06 ⁇ is found which is suitable for Larmor's "low" frequency magnetic resonance applications, ie at frequencies between 20 MHz and 200 MHz for 1 H spectra.
• FIG. 5A shows a radio frequency coil BRF2 according to another embodiment of the invention, suitable for higher frequency applications.
• the windings E1 ", E2" each comprise a single elliptical turn, made with a 0.25 mm diameter conductor wire.
• the turn of winding E1 " has a large axis of 3.8 mm and a small axis of 2.5 mm
• the turn of winding E2" arranged around your previous, has a major axis of 4.5 mm and a small axis of 3 mm.
• the angle a is 33.5 °
• the length L of the coil is about 5 mm and its largest transverse dimension is 3 mm, giving an aspect ratio (length over greater transverse dimension) of 1, 7.
• the region of homogeneity is defined as the region within which the intensity of the field Magnetic flux varies by up to ⁇ 0.5% from its value at the center of the coil.
• the component of the magnetic flux in the direction y takes a maximum value of 0.28 T / mA and the inductance of the coil is about 35 nH, which is suitable for high frequency operation (of the order of 500 MHz).
• a saddle coil having an identical internal volume in which the diameter and the length of the wire are chosen so as to give an equally identical electrical resistance, makes it possible to obtain a homogeneity region having a similar volume, but a transverse component of the magnetic flux of only 0.017 mT / A, This means that, to provide the same intensity of the magnetic field, the saddle-shaped coil (inductance of the order of 20 nH) requires a current of approximately 16 times larger than a coil according to the invention.
• FIG. 6A shows two views of an SRMN magnetic resonance probe according to one embodiment of the invention.
• the coil is located inside a glass tube T, carried by an ST rod structure for its introduction into a spectrometer RN; the distal end of the tube (opposite to the stem structure) is open to allow the introduction of a sample.
• the probe also comprises an impedance matching circuit comprising: a tuning capacitor C t having a capacitance adjustable between 1 ⁇ F and 10 ⁇ F, an inductive coupling L c (winding around the tube) and an adaptation capacitor of impedance C m having an adjustable capacitance between 3pF and 23pF connected in parallel with a capacitor of 47 pF.
• This circuit the electrical diagram of which is illustrated in FIG. 6B, allows a good impedance matching in the range 29 MHz - 41 MHz.
• FIG. 7 represents a probe probe inserted into a nuclear magnetic resonance spectroscopy (RN) apparatus comprising a magnet A for generating a so-called longitudinal magnetic field Bo, a transmission circuit Tx (generally comprising a signal generator, a transmitter , a radio frequency amplifier), a reception circuit Rx (generally comprising a preamplifier, a receiver and an analog digital converter) and an ORD computer.
• the transmission circuit Tx uses the SRMN probe as the transmitting antenna, to generate the radiofrequency magnetic field exciting the nuclear spins of the protons of a sample placed inside the coil.
• the reception circuit Rx uses said probe as receiving antenna to detect the nuclear magnetic resonance signal emitted by said nuclear spins.
• the ORD computer controls these circuits and takes care of the processing of the acquired signals.
• the z axis of the coil is parallel to the direction of the magnetic field B (indicated by ⁇ in the figure).
• the z and ⁇ axes may form an arbitrary angle.
• the use of a probe whose coil is not aligned in the direction ⁇ can be advantageous, in particular to allow a rotation of the sample at "magic angle" according to a technique known to those skilled in the art.
• the angle formed by the z and ⁇ axes must be equal to ⁇ ⁇ - arctan ⁇ l).
• the probe described above was used in a simple nuclear magnetic resonance experiment to assess its performance - in particular, the duration of a 90 ° pulse and the homogeneity of the field - and to compare them with those of a probe commercial comprising a saddle coil.
• sample S a solution of diluted H2O and O12SO4 was used to minimize the relaxation time Ti, placed in a Shigemi TS tube with a sample length of approximately 13 mm; this arrangement is illustrated in FIG. 9. Nutation measurements were made for three levels of radio frequency excitation power: 5W, 2W and 0.5W.
• FIGS. 8A, 8B and 8C show - for excitation powers of 5W, 2W and 0.5W respectively, the peak intensity of the signal FiD (more precisely: of the Fourier transform of the signal FID expressed as a function of the frequency v) detected at the Larmor frequency as a function of the duration of the excitation pulse tp; in these figures, the circles correspond to the values measured using the reference Bruker probe 200 and the squares to that obtained with the probe of the invention.
• the first maximum of the signal makes it possible to identify the value of tp which corresponds to a pulse at 90 °: it may be noted that this value is lower for the probe of the invention, which confirms that the latter has an efficiency (intensity magnetic field compared to that of the feed current) higher than the commercial probe.
• the ratio between the first and the second maximum of the signal provides a measure of the inhomogeneity of the radio frequency magnetic field: this ratio would be about 1 in the case of perfect homogeneity, and takes a value that is all the smaller as the homogeneity is important. This indirect and qualitative measure of homogeneity is used because it would be very difficult to map the magnetic field inside the coil, because of its small size. In any case, it can be verified that the probe of the invention makes it possible to obtain a more homogeneous field than the commercial probe.
• a probe according to the invention is therefore particularly advantageous for "low frequency" nuclear magnetic resonance applications (20-200 MHz). Such a probe is particularly suitable for the study of liquid samples and the implementation of techniques such as the aforementioned Magic Angle Spinning technique.
• the winding number may be greater than two, as in the case of the DHD coil described in the aforementioned article by A. Akhmeteli et al.
• the two windings may have a different number of turns, provided that the electric currents flowing therethrough are adapted accordingly.
• the length / diameter ratio of the coil of a probe according to the invention may be less than 1 or greater than 10, it is important that its inductance is sufficiently low to allow its use at radio frequency.
• Coils according to the invention can be used in probes having a structure different from that described.
• the probe comprises two coaxial coils BRFx (in black in the figure) and BRFy (in gray), each having two coils.
• the inner coil, BRFx has turns inclined with respect to the x and z axes, so as to generate a radiofrequency field oriented in the x direction (or, more generally, in a direction lying in the x-z plane).
• the outer coil, BRFy has a structure rotated 90 ° about the z axis, and thus turns inclined relative to the y and z axes so as to generate a radiofrequency field oriented in the y direction (or, more generally , according to a direction lying in the plane yz).
• the plane formed by the axes of the turns of the coil BRFx and the axis z is orthogonal to the plane formed by the axes of the turns of the coil BRFy and this same axis z.
• the two coils are identical, except that they have diameters slightly different for reasons of mechanical bulk (for example, in the case of FIG. 10, the coil BRFx has an outside diameter of 5.2 mm and the coil BRFy, arranged outside the preceding one, has an outside diameter of 5.6 mm).
• they are advantageously powered by currents in quadrature, so as to generate radia-frequency magnetic fields which are also in quadrature (i.e. with a time shift of a quarter period) and which have orthogonal spatial orientations to each other and to relative to the axis of the coil, It improves the report signa!
• On noise of a factor V2 it is also possible to envisage an embodiment in which the two coils generate transverse fields forming between them an angle less than 90 °, but this is generally less advantageous.

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• Condensed Matter Physics & Semiconductors (AREA)
• General Physics & Mathematics (AREA)
• Magnetic Resonance Imaging Apparatus (AREA)

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• 2013
  • 2013-03-26 FR FR1352725A patent/FR3003958B1/fr active Active
• 2014
  • 2014-03-26 WO PCT/IB2014/060175 patent/WO2014155312A1/fr not_active Ceased
  • 2014-03-26 EP EP14717210.0A patent/EP2979104A1/de not_active Withdrawn
  • 2014-03-26 US US14/779,869 patent/US20160116554A1/en not_active Abandoned

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