LOW-NOISE RF DETECTION AND ACQUISITION SYSTEM BASED ON SQUID AND EQUIPMENT ITEMS INCLUDING THIS SYSTEM

Abstract

A radiofrequency detection and acquisition system, which is based on SQUID and configured to be integrated into a nuclear magnetic resonance system, comprises a primary detection antenna, a flux transformer having an inlet winding connected to the primary detection antenna, a low critical temperature SQUID device for capturing the magnetic flux produced by an outlet winding of the flux transformer and supplying a secondary detection signal, a cryogenic device for cooling the SQUID device and the flux transformer, and means for processing the secondary detection signal emitted by the SQUID device to supply an analogue acquisition signal. The primary detection antenna may be of the volume type, comprising Helmholtz coils or saddle coils, or a more complex volume geometry, particularly gradiometric geometry. The means for processing the secondary detection signal may comprise a flux-locked loop, provided to linearize the response of the SQUID device.

Claims

1. A system for radio frequency detection and acquisition based on SQUID, for integration into a nuclear magnetic resonance apparatus, comprising: a volume-type primary detection antenna; a flux transformer having a primary winding connected to the primary detection antenna; a SQUID device, arranged to capture the magnetic flux captured by the primary detection antenna and reproduced by an input winding within the SQUID device via the flux transformer, and to deliver a secondary detection signal; a cryogenic device designed to cool the SQUID device; and means for processing the secondary detection signal emitted by the SQUID device to deliver an analog acquisition signal, comprising a flux-locked loop for linearizing a response of the SQUID device; wherein the SQUID device is of a low critical temperature type, the cryogenic device is configured to cool the flux transformer, and the primary detection antenna is of the volume type and has an open geometry.

2. The system of claim 1, wherein the primary detection antenna comprises Helmholtz coils.

3. The system of claim 1, wherein the primary detection antenna comprises stool coils.

4. The system of claim 1, wherein the primary detection antenna has a gradiometric geometry.

5. The system of claim 1, wherein the flux-locked loop comprises a low-noise amplifier.

6. The system of claim 5, wherein the low-noise amplifier comprises a semiconductor heterostructure amplifier.

7. The system according to claim 5, wherein the low-noise amplifier comprises a SQUID-based amplification system.

8. The system of claim 1, further comprising one or more active noise compensation coils for noise external to the system.

9. The system of claim 1, wherein the primary detection antenna cooperates with the flux transformer to concentrate the flux captured by the SQUID device.

10. The system of claim 1, further comprising, within the flux transformer, an inductance feedback coil arranged to react to the variations of the incoming flux, so as to keep the SQUID device at a maximum flux sensitivity level.

11. A magnetic resonance imaging (MRI) equipment item comprising: a SQUID-based radio frequency detection and acquisition system according to claim 1; an antenna holder device, integrating the primary detection antenna of volume type and connected to the detection and acquisition system; and an analog-to-digital conversion stage, configured to convert the analog acquisition signal into digital data suitable for post-processing to generate and display an MRI image.

12. The MRI equipment item of claim 11, wherein the MRI equipment item is coupled to a magnetoencephalography device.

13. A nuclear magnetic resonance equipment item including a SQUID-based RF detection and acquisition system according to claim 1.

14. A SQUID-based magnetic sensor equipment item for prospecting metals, including an RF detection and acquisition system according to claim 1, configured to detect a radio frequency wave emitted by a metal vein in response to an emission of a radio frequency wave in a ground.

15. An ultra-sensitive radiofrequency sensor equipment item including a detection and acquisition system according to claim 1.

16. A radioastronomy equipment item operating in the radio frequency domain and including in the SQUID-based RF detection and acquisition system according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The present disclosure can be better understood with reference to the figures described below:

[0037] FIG. 1 is a diagram of the RF detection and SQUID-based signal processing circuit.

[0038] FIG. 2 is a schematic diagram of an MM acquisition from an embodiment of the RF detection system of the present disclosure.

[0039] FIG. 3 illustrates several primary detection antenna geometries.

DETAILED DESCRIPTION

[0040] One embodiment of a SQUID-based RF acquisition and acquisition system 1 according to the present disclosure will now be described with reference to FIG. 1.

[0041] This SQUID-based RF detection and acquisition system 1 comprises a primary inductive antenna 5 of the volume type, produced in the form of Helmholtz coils or in saddle coils, or of any other type of volume, in particular, gradiometric, which is connected, in the case where the antenna is resonant, via a coupling capacitor 9 with capacitance Ca to a primary winding 6, with inductance L.sub.1, of a flux transformer 2 having a secondary winding 7, with inductance L.sub.2, in series with an input coil 8 with inductance Li generating a flux captured by a SQUID device 3.

[0042] If the antenna is not resonant, the capacitor is not present and the antenna 5 is directly connected to the primary winding 6.

[0043] The flux transformer 2 and the SQUID device 3 are kept at low temperature in a cryogenic device (not shown) comprising a pulse tube such as the product PT403 sold by CRYOMECH.

[0044] A step 4 of processing the secondary detection signal comprises a pre-amplifier 40 (LNA) of the voltage measured at the terminals of the SQUID device 3. This voltage measurement represents a detection signal, which is applied at the input of a flux-locked loop circuit 41 including a low-noise amplifier and connected to a feedback coil 10 with inductance L.sub.feed intended to react to the incoming flux variations in order to keep the SQUID device 3 at its maximum flux sensitivity level. The method of the flux-locked loop is disclosed in the document US20120206136A1.

[0045] An example of quantitative features of the SQUID-based RF acquisition and acquisition system 1 is given below: [0046] Typical time width of the wave train to be detected: T.sub.2*50 ms [0047] Central frequency of the primary antenna .sub.040 kHz [0048] Primary bandwidth 20 kHz [0049] Primary quality factor Q2 (if resonant antenna) [0050] Magnetic field strength at the primary antenna B.sub.p, on the order of about one hundred fT to pT [0051] Inductance of the input coil of the SQUID L.sub.i=720 nH [0052] Specific inductance of the primary antenna L.sub.a0.1 mH [0053] Resistance of the primary antenna R.sub.a=1 [0054] Resonant capacity of the primary antenna C.sub.a=6 F

[0055] Inductive Antenna

[0056] In view of the targeted applications, a volume-type geometry is chosen for the antenna 5. Examples of this geometry include Helmholtz coils, saddle coils, or other more complex geometries, in particular, gradiometric geometries. This geometry makes it possible to collect the highest possible signal while allowing, by its open geometry, relative comfort for the patient. The Faraday primary detection antenna 5 must be tuned to the MRI signal. This antenna 5 has for self-inductance La, the resistance R.sub.a, which an attempt will be made to reduce as much as possible, in order to minimize the Johnson-Nyquist noise in the antenna.

[0057] The parameters L.sub.a, R.sub.a are fixed by the selected antenna geometry and the type of material constituting the antenna. It is then possible to make the antenna resonant, which allows two things: [0058] The quality factor Q of the resonant antenna makes it possible to naturally amplify the detected signal, [0059] The bandwidth of the antenna makes it possible to filter the captured signals and to reject the electromagnetic noise out of the band of interest .

[0060] This implementation works both for a resonant antenna and for a non-resonant antenna. The case of the resonant antenna is explained below.

[0061] The capacitance is set so that the natural frequency of the antenna .sub.a=1/{square root over (LaCa)} is tuned to the frequency .sub.040 kHz of the received signal. Furthermore, the design of the antenna must take into account its bandwidth .sub.a=R.sub.a/L.sub.a that it is desired to have the same order of magnitude as the bandwidth width of the RF signal, so as not to lose information while limiting the detected noise. It is therefore the value of the self-inductance L.sub.a of the antenna 5, which will dictate the resistance and the ability to choose, based on the desired frequency characteristics.

[0062] The primary inductive antenna 5 is a volume antenna. For example, a Helmholtz geometry antenna, in a saddle shape, may be chosen or any other more complex volume geometry, in particular, gradiometric geometry.

[0063] FIG. 3 shows two of these geometries for a volume antenna implemented in an RF detection and acquisition system according to the present disclosure.

[0064] The first geometry (a) is of the saddle type, well known to the person skilled in the art for its performance, particularly in terms of spatial homogeneity. The diameter of the saddle antenna is equal to 1.5 times its length.

[0065] The other geometry (b) is the first-order gradiometric version of the saddle. This volume antenna 5 consists of two sub-antennae 51, 52 mounted serially to one another. The first internal antenna 51 has a saddle geometry and in this example has two wire turns. The second sub-antenna 52, external and larger, also of saddle geometry, has a single wire turn. The dimensions of the system and the orientation of the wires are chosen such that: [0066] The external part and the internal part of the antenna have the same inductance. This is allowed by the two wire turns in the internal antenna. [0067] The current in the internal part circulates in the opposite direction from the current in the external part.

[0068] With this configuration, the gradiometric antenna-saddle 5 makes it possible to reject the noise coming from sources located a great distance in front of the dimensions of the volume antenna 5, while benefiting from the homogeneity properties of the saddle geometry. A detailed description of the principle of the gradiometric antennas is found in the article by R. L. Fagaly, Superconducting quantum interference device instruments and applications, Review of scientific instruments 77, 101101 (2006).

[0069] Flow Concentration and Optimum Inductance

[0070] To determine the inductance of the antenna, the rest of the detection system is studied with reference to FIG. 1. The SQUID device 3 used (for example, model SQ680 from STARCRYO) is coupled to an input coil 8 with inductance L.sub.i=720 nH, which performs the coupling in current with the primary detection antenna 5, via a flux transformer 2 embodied by the coils L.sub.1 and L.sub.2, which are inductively coupled. The current flowing in the antenna 5 (resp. the coil L.sub.2) is denoted i.sub.1 (resp. i.sub.2) and the flux captured by the antenna 5 is denoted .sub.a. Additionally, the mutual inductance of input coil-SQUID is denoted M.sub.is=k {square root over (LiLs)}, and M.sub.12=k.sup.49 {square root over (L1L2)} is the mutual between the coil sL.sub.1 and L.sub.2.

[0071] k and k.sup.! are dimensionless factors and L.sub.s is the self-inductance of the SQUID device 3. The aim is a relationship between the external flux captured by the antenna, .sub.a, and .sub.sq the flux captured by the SQUID 3.

[0072] The inductive coupling relationships in the circuit are written

,+M,i,=(L.sub.a+L.sub.1)i,(1)

(L,+L)i,=M,i,(the effect of the current flowing through the SQUID is ignored)(2)

m=M,i.(3)

[0073] By combining these equations, the following is obtained

[00001]$\begin{array}{cc}{}_n=\frac{\left(L_s+L_1\right)\left(L_2+L_i\right)-M_{12}^2}{M_{\mathrm{is}}{M}_{12}}{}_s& \left(4\right)\end{array}$

[0074] This latter equation establishes a link between the external excitation, given by .sub.a, and the response level of the SQUID 3, quantified by .sub.sq. It is then understood why such an assembly is called a flux concentrator: the main role of the Faraday primary detection antenna 5 is to increase the flux captured by the SQUID 3.

[0075] The maximum sensitivity level of the device, giving the greatest response in .sub.s for a given .sub.a, is reached for

[00002]$\begin{array}{cc}{L}_1=\frac{L_a}{\sqrt{1-k^{!2}}}& \left(5\right)\end{array}$$\begin{array}{cc}{L}_2=\frac{L_i}{\sqrt{1-k^{!2}}}& \left(6\right)\end{array}$

[0076] The resistance of the primary detection antenna 5 is set to a reasonable value, for example, R.sub.a=1 . In order to comply with the value of the bandwidth on the order of about ten kH, it is therefore necessary to ensure L.sub.a=0.1 mH. This value of L.sub.a sets a value for the capacitor:

[00003]$\begin{array}{cc}{C}_a=\frac{1}{L_a{}_0^2}=6,3F& \left(7\right)\end{array}$

[0077] The ratio of the inductances L.sub.1 and L.sub.2 is therefore required:

[00004]$\begin{array}{cc}\frac{L_1}{L_2}=\frac{L_a}{L_i}& \left(8\right)\end{array}$

[0078] that is L.sub.1 1390L.sub.2. The precise values of L.sub.1 and L.sub.2 are set by the coupling constant k.sup.49 that should be as close as possible to 1 in order to ensure the maximum sensitivity of the device.

[0079] Flux Transformation Requirement

[0080] It is reasonable to wonder why the inductive coupling has been introduced via the coils L.sub.1 and L.sub.2. It would have been simpler to connect the antenna directly with the input coil of the SQUID 3. It is assumed that the flux transformer 2 comprising the coils (L.sub.1) and (L.sub.2) is absent, and that the inductive antenna with inductance L.sub.a is in series with the input coil (L.sub.i) of the SQUID. Magnetic coupling is then written

.sub.a=(L.sub.a+L.sub.i)i.sub.1(9)

[0081] hence introducing the coupling to the SQUID .sub.sq=M.sub.isi.sub.1

[00005]$\begin{array}{cc}{}_a=\frac{L_a+L_i}{k\sqrt{L_i{L}_s}}{}_s& \left(10\right)\end{array}$

[0082] The preceding equation shows that the maximum sensitivity is reached for an inductance of the antenna 5 by equalizing the inductance of the antenna 5 to that of the input coil 8 of the SQUID 3: L.sub.a=L.sub.i.

[0083] For example, it is possible to adjust the inductance of the antenna 5 by adjusting the number of turns in the loop, or by adjusting its geometry.

[0084] The need to introduce a flux transformer is then understood. Indeed, without the latter, the inductance of the antenna is imposed on the value L.sub.a=L.sub.i=720 nH. This inductance value imposes a resistance for the antenna

R.sub.a=L=0.72m(11

and a capacitor to connect to the antenna with the value

[00006]$\begin{array}{cc}{C}_a=\frac{1}{L{}_0^2}=0.87\mathrm{mF}& \left(12\right)\end{array}$

[0085] These results are not satisfactory for two reasons: On the one hand, the capacity found is extremely high; for these values chemical capacitors should be used, which could not be adapted to the cold of the cryostat. On the other hand, the value of the resistance is very low, which will have an impact on the intensity noise in the antenna

[00007]$\begin{array}{cc}{i}_a=\sqrt{\frac{4k_{B}T}{R}}& \left(13\right)\end{array}$

[0086] that is, with an antenna cooled to 100 K, i.sub.a3 nA/Hz. This noise is much too high with regard to the very low level of noise at the input of the SQUID on the order of pA{square root over (Hz)}.

[0087] One solution is to increase the resistance of the primary detection antenna 5, which requires passing through a flux transformer 2 to adapt the inductor in order to preserve the same bandwidth.

[0088] SQUID current reader

[0089] The SQUID device 3 used is a low-Tc SQUID (for example, SQ680 from STARCRYO) cooled by a cryocooler, for example, PT 403 from CRYOMECH, and biased by a current i.sub.p.

[0090] Unlike its high-Tc counterparts, the low-Tc SQUID has a much lower thermal noise level, which makes it possible to drastically increase the signal-to-noise ratio and ultimately the quality of the final image. Its role is to read the current generated in the input coil, with a noise level of 0.8 pA/Hz. This noise level is therefore the objective to be achieved for the thermal noise in the inductive antenna.

[0091] Low-noise amplifier-FLL

[0092] The SQUID device has a non-linear, periodic current-captured flux response with a quantum flux period .sub.0=h/2e . To linearize this response in order to avoid artifacts degrading the image quality, the SQUID 3 is coupled to a flux-locked loop (FLL), an example of which is described below.

[0093] This loop first has a pre-amplifier 40 (LNA) of the voltage measured at the terminals of the SQUID.

[0094] Two choices can be envisaged for the amplification system: Either opting for amplification with SQUID, as is, for example, the case in document US2013271142A, or using a semiconductor heterostructure amplification of the ASIC type, which is potentially more advantageous but also poses more constraints, in particular, in the maximum voltage oscillation level of the incoming signal.

[0095] The feedback coil 10 with inductance L.sub.feed makes it possible to react to the incoming flux variations in order to keep the SQUID 3 at its maximum flux sensitivity level. The signal is read at the output of the flux-locked loop.

[0096] MRI equipment

[0097] An ultra-sensitive detection and RF acquisition system based on SQUID can be integrated into MRI equipment using a working magnetic field of the order of B0=1 mT (which corresponds to a frequency 0 40 kHz), while retaining an acquisition time and an image quality in accordance with current clinical standards. The decrease in the working magnetic field by several orders of magnitude makes it possible to eliminate the constraints, preventing on the one hand a massive adoption of the MRI as an imaging standard and on the other hand the opening of still-non-existent applications such as MRI on board a truck in order to diagnose the type of stroke (ischemic or hemorrhagic), 100% MRI screening for breast cancer (carried out by CT scan), or intraoperative MRI, thanks to light equipment, without magnetic shielding, which is not very expensive.

[0098] FIG. 2 shows a schematic diagram of an MRI experiment carried out with the detection system according to the present disclosure. A knee MRI was chosen, the osteoarticular imaging being one of the first likely applications of the present disclosure. The knee of the patient is inserted into a cylinder, which comprises a solenoid ensuring a permanent field B.sub.01 mT homogenous at about 10 ppm over a volume of about 101010 cm.sup.3, of gradients, and the reception antenna described above. The receiving antenna is cooled to a temperature of about 60 K using a custom cryogenic system derived from the pulse tube, ensuring the cooling of the SQUID system of part B.

[0099] Part B comprises the SQUID ensuring the reading of the current coming from the inductive antenna, as well as the processing electronics of the signal described above, the pre-amplification system, and the flux-locked loop FLL composed of an integrator amplifier, a read resistor, and a looping coil L.sub.feed. This whole stage is cooled using a cryogenic machine, for example, the PT403 pulse tube from CryoMech, at a temperature near 4.2 K.

[0100] Part C provides the analog-digital conversion of the signal for computer post-processing in order to control the equipment and to display the obtained MRI image.

[0101] Medical Applications

[0102] The sensitivity and portability of the device make it interesting in the first place for magnetic resonance imaging (MRI). The high contrast levels obtained at a low field make the technology interesting for diagnostics where contrast is currently insufficient with high-field technologies.

[0103] Furthermore, equipment according to the present disclosure could easily be installed in an ambulance truck in order to rapidly diagnose, on the site of the accident, an ischemic or hemorrhagic stroke, in order to care for the patient more quickly and to avoid irreversible damage to cognitive faculties.

[0104] Due to its low cost and its ease of use, the imaging equipment according to the present disclosure could also spread widely in use cases where it is not being used enough today: Screening for breast cancer in women over the age of 50, use in neurology and psychiatry: early screening for diseases such as schizophrenia, depression, or epilepsy; screening for prostate cancer.

[0105] Finally, numerous SQUID-based low-field MRI projects also have the purpose of designing a hybrid MRI-magnetoencephalography (MEG) apparatus. This is the case for the work of the team of the University of Aalto in Finland, with reference to the source https://www.aalto.fi/en/department-of-neuroscience-and-biomedical-engineering/meg-mri-brain-imaging-group.

[0106] A SQUID-based MRI equipment item according to the present disclosure may be adapted to integrate a MEG device therein.

[0107] Nuclear Magnetic Resonance

[0108] The NMR apparatuses used, in particular, for chemical characterization can also benefit from the detection system in order to design lighter and less expensive equipment, for reasons similar to the ones given for MRIs.

[0109] Mining Industry

[0110] In the mining industry, there are already SQUID-based magnetic sensors for the prospecting of metals, as illustrated by document U.S. Pat. No. 7,394,250. The detection system can also be integrated in such an apparatus for mining, thanks to its very low noise level. The principle is as follows: an RF wave is emitted in the ground, if a metal vein is present, eddy currents are induced in the vein, which in turn emits an RF wave, this wave is detected by the apparatus integrating the detection system with SQUID.

[0111] Military Field

[0112] Ultra-sensitive radiofrequency sensors are well-known elements of electronic warfare systems: they serve, for example, to detect communication signals. Another advantageous application is the detection of underwater submersibles: as a submersible consists of ferromagnetic materials, the device is capable of detecting its presence by emitting RF waves and detecting the waves produced by induced eddy currents, on the same principle as mining. Other systems, meanwhile, detect the disturbance of the local terrestrial field generated by the passage of the underwater vehicle, as illustrated by the document Magnetic detection of a surface ship by an airborne LTS SQUID MAD by Megumi Hirota et al., April 2001, IEEE Transactions on Applied Superconductivity 11(1):884-887.

[0113] Radioastronomy

[0114] SQUID-based systems are already widely used in the field of radioastronomy, for example, integrated into superconducting bolometers for reading and/or amplifying very low currents. Due to its very high sensitivity, the system may find an interesting integration in a calibrated telescope in the RF domain.

[0115] Of course, the present disclosure is not limited to the embodiments that have just been described and numerous other alternative embodiments can be envisaged within the scope of the present disclosure.