SQUID radiofrequency gradiometer

Abstract

The invention relates to a radiofrequency gradiometer (GR) comprising two conductive coils (BCA, BCB), called pickup coils, having a common sensitivity axis (z) and spaced along said axis, each of said coils being connected in series to another conductive coil (BEA, BEB), called input coil, to produce a flux concentrator, each said input coil being inductively coupled to a SQUID magnetometer (MSA, MSB); characterized in that the gradiometer also comprises an electronic processing circuit (CET) configured to linearly combine output signals (VSA, VSB) from said SQUID magnetometers to obtain a signal (VS) representative of a difference between radiofrequency magnetic field values ​​(Bz) corresponding to the two pickup coils. The invention also relates to a magnetic resonance imaging apparatus comprising at least one such radiofrequency gradiometer, used as a receiving antenna.Figure for abstract: Fig. 3.

Description

Title of the invention: SQUID radiofrequency gradiometer

[0001] The invention lies in the field of ultrasensitive magnetic field sensors, operating in the radiofrequency field. It applies mainly to the field of magnetic resonance imaging (MRI) and more particularly to very low magnetic field MRI.

[0002] We speak of very low magnetic field MRI when the stationary magnetic field of polarization of the nuclear spins has an intensity of between approximately 50 pT and 100 mT, corresponding to Larmor frequencies of the order of kHz up to MHz for protons). Under these conditions, the amplitude of the magnetic field to be detected is typically less than 1 pT.

[0003] It is known from WO 2022 / 117969 to use, for the detection of such a weak MRI signal, a system based on SQUIDs (from the English "Superconducting Quantum Interference Device", i.e. superconducting quantum interference device), and more particularly low-temperature direct current SQUIDs (dc-SQUIDs). More information on SQUIDs and their use is provided by (Fagaly) and (Clarke, Braginski).

[0004] As illustrated in [Fig.l], a DCS direct current SQUID is in the form of a superconducting loop BS interrupted by two Josephson junctions JH, JJ2. When a direct current I flows in the loop, the potential difference V across the Josephson junctions is a sinusoidal function of the magnetic field flux <t>s through the loop. Typically, a bias coil BP is used to set an operating point around which the voltage varies approximately linearly with the flux. Advantageously, a feedback loop - called a flux-locking loop (FLL) - controls the current ip flowing in the bias coil so as to keep constant the voltage Vs supplied as input to the FLL loop after being amplified by a low-noise amplifier LNA. In this case, the current ip - or another signal generated by the FLL loop and proportional to ip - can be taken as the output signal providing a measure of the flux <FS. En variante, si le SQUID présente une dynamique suffisante, la boucle FLL peut avoir une bande passante inférieure à la fréquence du signal utile ; dans ce cas, le signal de sortie est constitué par une variation de tension haute fréquence aux bornes du SQUID, VSHF.

[0005] Also, the assembly consisting of the SQUID DCS, the low noise amplifier LNA, the bias coil BP and the flux locking loop FLL form an extremely sensitive MS magnetometer.

[0006] It is known to produce SQUIDs both with “conventional” superconductors, with low critical temperature, and with superconductors with high critical temperature. “Low critical temperature” means a critical temperature less than or equal to 20 K. In the aforementioned document WO 2022 / 117969, the use of SQUIDs with low critical temperature - maintained moreover at a temperature well below that of transition, for example of the order of 4 K - is recommended because of the lower noise level, the greater ease of manufacture and the better figure of merit RnIc (where Rn is the resistance of the junction in the normal state and Ic its critical current) of these devices compared to SQUIDs with high critical temperature. Furthermore, it is known that low-temperature superconducting materials can have mechanical, shaping and stability properties superior to those of high-temperature superconductors.

[0007] The superconducting loop BS has a small surface area (a few pm2 or tens of pm2), so it intercepts a low magnetic flux. Furthermore, as it must be maintained at cryogenic temperature, it cannot always be positioned optimally to detect a magnetic field, for example near the body of a patient undergoing an MRI examination. For these reasons, it is known to use what is commonly called a flux concentrator CF. The flux concentrator CF comprises a first conductive coil BC, called the capture coil, connected in series to a second conductive coil BE, called the input coil, the latter generally being made using planar technology and being inductively coupled to the superconducting loop BS of the SQUID. The mutual induction coefficient between the input coil BE and the superconducting loop BS is designated by MES.The BC pickup coil, which is, in fact, an antenna, has an effective surface area - the product of the number of turns and the geometric surface area of ​​each turn - much greater than that of the superconducting loop, and is therefore crossed by a greater flux - a flux which is transferred to the superconducting loop of the SQUID via the input coil. Furthermore, the pickup loop does not have to be kept at a cryogenic temperature and can therefore be positioned more freely.

[0008] The pickup coil BA is not necessarily a simple planar coil, as illustrated in [Fig.l]. As explained in the aforementioned document WO 2022 / 117969, it can also be, for example, a volume coil, such as a saddle coil or a pair of Helmholtz coils. It can also be a gradiometric antenna. [Fig.2A] shows a first-order gradiometric coil BG1, which consists of two windings BG1', BG1” connected in series and wound in opposite directions and arranged. The windings BG1', BG1” are planar, located in planes perpendicular to a z axis and spaced by a distance d (called the “baseline”) along said axis. It is easy to understand that the current induced in the gradiometric coil BG1 depends on the variation of the component Bz of the magnetic field on the baseline d, which - as a first approximation - is proportional to the intensity of a magnetic field decreases as the cube of its distance from the source, but that of the magnetic field gradient decreases as the fourth power of this distance. Also, replacing a simple coil with a gradiometric coil makes it possible to attenuate the impact of distant noise sources. [Fig.2B] represents a second-order gradiometric coil BG2, obtained by replacing the windings BG1' and BG1” with first-order gradiometric coils. It is easy to understand that the current induced in the gradiometric coil BG1 depends, as a first approximation, on the second derivative of Bz with respect to z: Similarly In this way, it is possible to realize higher order gradiometric coils, the rejection of noise from distant sources increasing with said order.

[0009] [Fig.2C] represents a so-called "saddle board" BSC volume antenna, used in MRI due to its field homogeneity properties. [Fig.2D] represents the gradiometric (first order) version of a saddle board volume antenna, BSCG, consisting of two saddle board sub-antennas: a first, internal antenna, having - in the example shown - two turns of wire, and a second, external and larger antenna having - still in the example shown - a single turn of wire. The difference in the number of turns of wire allows the two sub-antennas to have the same inductance despite their different sizes. The windings are wound in such a way that the current flows in opposite directions in the two sub-antennas.

[0010] A disadvantage of these gradiometric assemblies is that the antennas use long lengths of wire due to the succession of coils compensating each other. The capture antennas thus manufactured generally have a relatively high inductance, which can pose a problem for the detection sensitivity with SQUID. Indeed, for quantum interference to be observed, the self-inductance Ls of a SQUID must satisfy the condition

[0011] H) 2ï; > ^kBT

[0012] where ¢0 is the flux quantum, T the absolute temperature and kB the Boltzmann constant. Typically, Ls is therefore of the order of 1 pH.

[0013] If we call <hBC le flux à travers la bobine de captation BC et LBC son inductance, le courant iBC qui parcourt ladite bobine et la bobine d’entrée BE vaut

[0014]

[0015]

[0016]

[0017] • _ (2) ^BC — Lbc+The The Osa flux, through the superconducting loop of the SQUID is ^sa ~ MESiBC (3) With MES = k^LELs (4)

[0018] where Le is the inductance of the input coil BE and k is a dimensionless factor. We therefore find

[0019] k^Ls (5) &SA ~ Lbc+Le ^BC

[0020] It follows from equation (5) that, for a given OBC flux through the pickup coil, the Osa flux seen by the SQUID is maximal for LE=LBC. However, to maximize OBC it would be desirable to take LBC large, while LE must be of the same order of magnitude as Ls to maximize the coupling (given the constraints inherent in its planar realization, the BE coil must have a geometry close to that of the SQUID to be able to couple inductively to it efficiently). The low value of the inductance of the SQUID therefore imposes a maximum limit on that of the input coil, and therefore on that of the pickup coil. It follows that LBC cannot exceed a few pH or tens of pH, which is very restrictive in particular for the realization of volume coils in an MRI scanner which, due to their dimensions, tend to have high inductance values.In particular, it is preferable that the LE / LBC ratio be within an interval between 0.4 and 2.5 so that the sensitivity is at least equal to 90% of its maximum value.

[0021] The use of a gradiometric type coil exacerbates this problem. Indeed, as shown above, such a coil has several windings, only a part of which is dedicated to capturing the signal of interest, the rest being dedicated to noise suppression. Consequently, a large part of the inductance of the capture coil - the value of which is limited by the adaptation constraints explained in the previous paragraph - is not used to collect the signal of interest. This problem is even more serious for higher order gradiometers.

[0022] In the aforementioned document WO 2022 / 117969, a transformer is interposed between the pickup coil and the input coil. This transformer makes it possible to increase the impedance LA while respecting the adaptation constraints. However, it has significant losses, especially when the transformation factor is high, which limits the interest of this solution. In addition, transformers that have to work at low temperatures can hardly use ferromagnetic cores.

[0023] The invention aims to overcome at least in part the aforementioned drawbacks of the prior art.

[0024] According to one aspect of the invention, this object is achieved by using an electronic processing circuit configured to linearly combine the output signals of two independent SQUID magnetometers to obtain a signal representative of a difference between magnetic field values ​​corresponding to the pickup coils of said magnetometers. In this way, the entire inductance of each pickup coil can be used to acquire a useful signal. Alternatively, each of the two magnetometers can have a gradiometric antenna of order N (N > 1), the electronic processing circuit making it possible to obtain a gradiometer of order N+1.

[0025] A similar assembly has been disclosed in (Matlashov) in the case of a device based on high critical temperature SQUIDs and with superconducting acquisition coils made in planar technology. In this case, in fact, it is difficult for technological reasons to produce gradiometric assemblies with a sufficiently high baseline.

[0026] An object of the invention is therefore a radiofrequency gradiometer comprising two conductive coils, called pickup coils, having a common sensitivity axis and spaced along said axis, each of said coils being connected in series to another conductive coil, called input coil, to produce a flux concentrator, each said input coil being inductively coupled to a SQUID magnetometer; characterized in that the gradiometer also comprises an electronic processing circuit configured to linearly combine output signals from said SQUID magnetometers to obtain a signal representative of a difference between radiofrequency magnetic field values ​​corresponding to the two pickup coils.

[0027] According to particular embodiments of such a radiofrequency gradiometer:

[0028] - Said capture coils can be configured to operate in a frequency range between 1 kHz and 100 MHz.

[0029] - Said SQUID magnetometers may comprise current SQUIDs continuous.

[0030] - Said SQUID magnetometers may comprise low-frequency SQUIDs critical temperature.

[0031] - Said electronic processing circuit may be an analog circuit comprising an adder or subtracter amplifier for adding or subtracting said output signals. More particularly, each said SQUID magnetometer may comprise a low noise amplifier for generating the corresponding output signal by amplifying a voltage drop across a respective SQUID.

[0032] - Each said magnetometer may comprise a flux-locked loop.

[0033] - Each said pickup coil may have an inductance greater than or equal to at 10 pH.

[0034] Another object of the invention is a magnetic resonance imaging apparatus comprising at least one radiofrequency gradiometer according to one of the preceding claims, used as a receiving antenna. According to particular embodiments:

[0035] - Said capture coils can form volume antennas.

[0036] - The apparatus may also comprise a coil for generating a magnetic field. stationary polarization of nuclear spins, with amplitude between 10 pT and 100 mT.

[0037] Other characteristics, details and advantages of the invention will emerge on reading the description given with reference to the appended drawings given by way of example and which represent, respectively:

[0038] [Fig.l], described above, a SQUID magnetic field detection system known from the prior art.

[0039] [Fig.2A], [Fig.2B], [Fig.2C] and [Fig.2D], described above, different types of antennas which can be used with the detection system of [Fig.l] or for the implementation of the invention;

[0040] [Fig.3], a radiofrequency gradiometer according to a first embodiment of the invention;

[0041] [Fig.4], a radiofrequency gradiometer according to a second embodiment of the invention; and

[0042] [Fig.5], a nuclear magnetic resonance imaging device (scanner) using at least one radiofrequency gradiometer according to one embodiment of the invention.

[0043] The apparatus of [Fig. 3] comprises two SQUID magnetometers similar to that of [Fig. 1], MSA, MSB. Each of these magnetometers comprises a direct current SQUID DCSA, DCSB inductively coupled to an input coil BEA, BEb as well as to a bias coil BPA, BPb fixing its operating point and driven by a flux-locked loop not shown. The input coil is connected in series to a pickup coil BCaBCb. These two pickup loops are arranged like the two windings of the gradiometric antenna of [Fig. 2A]: they are planar, arranged in planes perpendicular to the z direction (“sensitivity axis”) and therefore sensitive to the Bz component of an external magnetic field; they are also spaced apart by a distance d in said z direction. If z0 is the z-axis coordinate of the midpoint of the two coils, coil BCA is at position z0+d / 2 and coil BCb is at position z0+d / 2.

[0044]

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[0052]

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[0055] The voltages across the two SQUIDs VA, VB are amplified by low-noise amplifiers LNAa, LNAB. The winding directions of the pickup coils BCaBCb and the input coils BEA, BEb are chosen so that a magnetic flux through the pickup coils induces voltages VA, VB of opposite sign. For example, the pickup coils may have windings wound in opposite directions and the input coils in the same direction, or vice versa. The outputs VSA,VSB of the low-noise amplifiers are supplied as input to an electronic processing circuit CET which, in this embodiment, is a simple voltage adder circuit (more precisely a non-inverting adder) comprising an operational amplifier OA and four resistors Rb R2, R3, R4. If Ri=R2j, the output voltage Vs of the operational amplifier OA is proportional to VA+VB and therefore the difference in magnetic fluxes across the two pickup coils: The system therefore constitutes a first-order gradiometer, identified by the reference GR. Unlike the case of the gradiometer in [Fig.l], however, the entire inductance of each of the two pickup coils is used for the acquisition of a useful signal; this architecture therefore makes it possible to alleviate the constraints on the value of this inductance. In more detail, if LBC=LE (we consider that the two pickup coils BCa, BCb have the same inductance LBC, the two input coils BEA, BEb the same inductance LE and the two SQUIDs DCSA, DCSB the same inductance Ls) equation (5) becomes Or <t>BC is worth ) or — depending on whether one or the other of the two pickup coils. The voltage VA across DCSA is therefore worth Where Ks is the flux-voltage transfer coefficient of the SQUID (assumed to be identical for DCSa and DCSb). Similarly, the voltage VB across DCSB is By introducing a parameter A which takes into account the amplifiers LNAa, LNAB (assumed to be identical), the effect of the flux-locked loop and the gain of the voltage adder circuit - equal to 1+R3 / R4 - we obtain: 100561 v, = AK^ -AK^d % I %=SkAK^d $ | % (10)

[0057] In equation (10), the last equality was obtained by considering that the two pickup coils have the same effective surface SBc (in the simple case of a planar coil, this is the surface projected onto the xy plane perpendicular to the z axis and multiplied by the number of windings) and that therefore <f>=BzSBC.

[0058] In the embodiment of [Fig.4], the directions of the windings of the pickup coils BCAjBCb and the input coils BEA, BEb are chosen such that a magnetic flux through the pickup coils induces voltages VA, VB of the same sign. These voltages are supplied to the differential inputs to a low-noise amplifier LNA'. As in the previous case, the output voltage Vs is proportional to 100591

[0060] As in the case of [Fig.3], the system therefore constitutes a first-order gradiometer, identified by the reference GR'.

[0061] [Fig. 5] very schematically represents a device (or "scanner") for magnetic resonance imaging SMRI. Such a device comprises a BCM coil for generating a static and uniform magnetic field Bo oriented in a so-called longitudinal direction, which makes it possible to orient (polarize) in said direction the nuclear spins of the atoms of a sample - for example a part of a human body, or the entire body. Coils, called gradient coils (not shown) apply static and non-uniform magnetic fields, necessary for obtaining spatial resolution. Radiofrequency coils, not shown, make it possible to apply excitation pulses to said nuclear spins, following which the nuclear spins are de-excited by emitting radiofrequency radiation.One or more GR gradiometers according to the invention, equipped for example with volume coils surrounding the sample, make it possible to detect this radiation, their capture coils serving as receiving antennas.

[0062] The invention has been described with reference to certain embodiments, but several variants are possible. In particular:

[0063] - Other types of pickup coils may be used, such as coils volumetric. For example, the pickup coils can be the sub-antennas of a gradiometric saddle volumetric antenna, of the type in [Fig.2D]. As mentioned above, these coils can also be N-order gradiometric coils (N > 1), in which case the processing electronics synthesizes a gradiometer of order N+1. The inductance values ​​mentioned are given only as an example.

[0064] - Although it is generally preferable that both antennas have the same inductance, it is also possible that they have different inductances. Similarly, the SQUIDs and the input coils of the two elementary magnetometers may not be identical. The resulting differences in the intensity of the output signals must then be compensated by the electronic processing circuit. In the case of the embodiment in [Fig.3] this is achieved by choosing different values ​​for the resistors RI and R2. Similarly, it is possible to compensate for a possible difference in effective surface area.

[0065] - The electronic processing circuit can add or subtract the signals that it receives as input (weighted where appropriate to take into account any difference between the two elementary magnetometers), depending on the winding directions of the pickup coils and the input coils.

[0066] - The electronic processing circuit may have a different topology from those described. For example, the processing may be partially or completely digital.

[0067] - Although the use of low critical temperature SQUID is preferred, that of SQUID at high critical temperature is not excluded.

[0068] - In some cases, the flux-locked loop may be omitted, the polarization SQUIDs being fixed. Conversely, the output signal of each elementary magnetometer can be provided by the flux-lock loop instead of being taken from the terminals of the SQUIDs.

[0069] - The invention can be implemented in other frequency ranges and for applications other than MRI - for example magnetic resonance spectroscopy, mineral prospecting, etc. In any case, the magnetic field of polarization of the nuclear spins may be outside the intensity range indicated as preferential. References

[0070] (Clarke, Braginski) John Clarke, Alex I. Braginski (editors) The SQUID Handbook, Vol. I (2004)

[0071] (Fagaly) RL Fagaly “Superconducting quantum interference device instruments and applications” Review of Scientific Instruments 77, 101101 (2006)

[0072] (Matlashov) A. Matlashov et al. “Electronic Gradiometer using HTC SQUIDs with Fast Feedbeck Electronics” IEEE Transactions on Applied Superconductivity, Vol. 11, No. 1, March 2001.< / f> < / t> < / t>

Claims

Claims

1. Radiofrequency gradiometer (GR, GR') comprising two conductive coils (BCA, BCb), called pickup coils, having a common sensitivity axis (z) and spaced along said axis, each of said coils being connected in series to another conductive coil (BEa, BEb), called input coil, to produce a flux concentrator (CF), each said input coil being inductively coupled to a SQUID magnetometer (MSA, MSB); characterized in that the gradiometer also comprises an electronic processing circuit (CET) configured to linearly combine output signals (VSA, VSB) from said SQUID magnetometers to obtain a signal (Vs) representative of a difference between radiofrequency magnetic field values (Bz) corresponding to the two pickup coils.

2. Radiofrequency gradiometer (GR, GR') according to claim 1 wherein said pickup coils (BCA, BCb) are configured to operate in a frequency range between 1 kHz and 100 MHz.

3. Radiofrequency gradiometer (GR, GR') according to one of the preceding claims wherein said SQUID magnetometers (BCa, BCb) comprise direct current SQUIDs (DCSa, DCSb).

4. Radiofrequency gradiometer (GR, GR') according to one of the preceding claims wherein said SQUID magnetometers (BCa, BCb) comprise low critical temperature SQUIDs (DCSA, DCSB).

5. Radiofrequency gradiometer (GR, GR') according to one of the preceding claims in which said electronic processing circuit (CET) is an analog circuit comprising an adder or subtractor amplifier (AO, LNA') for adding or subtracting said output signals.

6. A radio frequency gradiometer (GR) according to claim 5 wherein each said SQUID magnetometer comprises a low noise amplifier (LNAa, LNAB) for generating the corresponding output signal by amplifying a voltage drop (VA, VB) across a respective SQUID (DCSA, DCSB).

7. Radiofrequency gradiometer (GR, GR') according to one of the preceding claims in which each said magnetometer (MSa, MSb) comprises a flux-locked loop (FLL).

8. Radiofrequency gradiometer (GR, GR') according to one of the preceding claims in which each said pickup coil (BCA, BCb) has an inductance greater than or equal to 10 pH.

9. Magnetic resonance imaging (MRI) apparatus comprising at least one radiofrequency gradiometer (GR) according to one of the preceding claims, used as a receiving antenna.

10. Magnetic resonance imaging apparatus (MRIA) according to claim 9 wherein said pickup coils (BCA, BCb) form volume antennas.

11. Magnetic resonance imaging (MRI) apparatus according to one of claims 9 and 10 comprising a coil (BCM) for generating a stationary magnetic field (Bo) for polarizing the nuclear spins, with an amplitude of between 10 pT and 100 mT.

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