Device and method for performing magnetic resonance imaging of metal or partially metal components and use of the method for imaging of

Abstract

The invention relates to a magnetic resonance imaging (MRI) device (1) which is designed to image a substantially metallic component. The magnetic resonance imaging device comprises means (b '') for generating a polarized magnetic field intended to be applied to said component (a), radio frequency (RF) means (g) for exciting said component (a), detection means cooperating with antenna means for delivering a magnetic resonance imaging (MRI) signal, and means for processing said MRI signal for delivery with respect to said component (a) and a device for detecting feature information of the state. The component (a) is subjected to a very weak field of less than 10 mT, and the detection means comprise a pickup coil (h) magnetically coupled with the polarization means (b '') and the radio frequency means (g) and operating as a flux concentrator, and an SQUID (Superconducting Quantum Interference Device) detector arranged downstream of said pickup coil (h) via a transformer.

Description

Technical Field

[0001] The present invention describes a device for performing magnetic resonance imaging on a metal or partially metal component. The present invention also encompasses a method for imaging a metal or partially metal component implemented in the device, and the application of this method in the imaging of an electrochemical cell unit, particularly a lithium-ion battery unit. Background Art

[0002] Magnetic resonance imaging (MRI) is based on the principle of nuclear magnetic resonance (NMR). This phenomenon utilizes the resonance between two energy levels that occurs when a quantum magnetic moment (spin) is subjected to an external magnetic field. This spin can be the spin of an electron or a nucleus. In the case of medical NMR, the nucleus of interest is in most cases hydrogen (hydrogen has only one proton) present in water, fat, or human tissue.

[0003] The NMR (or MRI) experiment is carried out in several precise steps. First, the sample (when imaging a lithium-ion battery unit or any metal component, the sample will be the component itself) is placed in a very uniform static magnetic field B0 called the polarization field generated by a large superconducting coil. In most commercial devices, this magnetic field varies from 1.5 T to over 10 T. When the sample protons are placed in the magnetic field B0, the sample protons precess around the field at a frequency ω0 given by ω0 = γB0, where γ is called the gyromagnetic ratio.

[0004] Then, the protons are excited by a radio frequency signal having the same frequency ω = ω0 as the precession of the protons. This excitation causes the protons to deviate from their precession motion, and after a period of time, the protons slowly decay back to their initial precession state, thereby emitting a signal detected by an antenna. Measuring this signal is a means of collecting information about the composition inside the subject and ultimately reconstructing the MRI image.

[0005] The general principle of the MRI experiment can be outlined as follows:

[0006] - Place the part of the subject to be imaged in a static uniform magnetic field B0.

[0007] - When placed in this field, the protons precess at a frequency ω0 = γB0.

[0008] - Use an excitation antenna to send a pulse with an intensity B1 and a frequency ω = ω0, which is tuned to the precession of the protons.

[0009] - The protons are sent to a higher energy state and decay back to their initial state while emitting a signal of intensity B2 and frequency ω0. More importantly, this signal will decay with characteristic times τ1 and τ2, where τ1 corresponds to the recovery of longitudinal magnetization in the direction of B0 and τ2 corresponds to the loss of spin-spin coherence in the sample. These characteristic time constants are properties of the exact state at each point in the sample and are used to create contrast in MRI images. In the case of medical MRI, they provide information about the tissue type (water, fat, muscle, etc.) at a specific point.

[0010] To construct an MRI image, it is necessary to determine where the signal detected by the MRI antenna comes from. For this purpose, gradient coils are used. In addition to the permanent homogeneous field B0, we also add a small supplementary field δB(x, y, z) that varies linearly along three spatial directions.

[0011] Typically, in an MRI device with a 5T magnetic field, an additional field δB on the order of 100 mT.m is observed. Since the resonance frequency of the nuclei is proportional to the local magnetic field, the gradient field can be used to spatially encode the resonance frequency and phase of the signal, enabling the formation of 3D images. -1 During an MRI sequence, these gradient fields are rapidly changed on a millisecond time scale to selectively excite certain parts of the sample. The model chosen for applying the gradients and excitation pulses constitutes the MRI sequence.

[0012] Different sequences allow different contrasts to be shown and different things to be seen. In medical MRI, there are a variety of sequences: some sequences are suitable for seeing blood vessels (time-of-flight sequences), others are more suitable for functional imaging (BOLD stands for "blood oxygenation level-dependent"), while the more common T1- or T2-weighted sequences can distinguish fat, organic tissue, tumors, etc.

[0013] Note that MRI sequences are used for image construction in the inverse Fourier space (or k-space), where different MRI sequences respond to different acquisition trajectories.

[0014] We will now explain why low-field MRI is needed to image metal samples.

[0015] We will now explain why it is necessary to use low-field MRI to image metal samples.

[0016] For over forty years, MRI innovation has been pushed towards high fields. Higher field strengths mean more nuclei in the sample are polarized, resulting in a higher signal-to-noise ratio. Additionally, since the detection frequency is proportional to the magnetic field, this enables us to operate at high frequencies within a range where typical inductive detection antennas are inherently more sensitive. In short, high field strengths mean better resolution and faster acquisition times. However, this strategy has significant drawbacks. Commercial high field machines require heavy, expensive superconducting magnets that are cooled to 4 Kelvin with liquid helium and require costly maintenance. The upfront cost of a typical 1.5T scanner is nearly one million dollars, plus several hundred thousand dollars per year for maintenance. The MRI unit must be installed in a dedicated magnetically shielded room with a reinforced floor. This makes MRI technology a larger scale imaging protocol than ultrasound or CT.

[0017] A less obvious problem with high field MRI is the impossibility of imaging in the presence of metal, let alone imaging metal samples. The problem is twofold:

[0018] - Samples with both metal and non-metal parts mean there will be large local differences in magnetic susceptibility. This results in magnetic field gradient artifacts that disrupt the linearity of the gradient field patterns used to perform MRI.

[0019] - Due to the skin effect of electrical conductors, high frequency magnetic fields cannot penetrate most metal samples.

[0020] When a propagating electromagnetic field reaches the conductor / dielectric interface, eddy currents are generated inside the inductor. These currents in turn generate another magnetic field that compensates for the emerging magnetic field. As a result, the total magnetic field is effectively cancelled out as it enters most conducting samples. The skin depth δ is a measure of the depth to which the magnetic field can penetrate into the metal sample:

[0021]

[0022] where ω is the pulsation, ρ is the DC resistivity, μ0 is the magnetic permeability of vacuum, and μ r is the relative magnetic permeability of the sample.

[0023] In the case of proton MRI, we have γ H / (2π): 42 MHz.T -1 and for lithium, this value is slightly lower at γ Li / (2π): 16.5 MHz.T -1. In each case, this means that in traditional high-field MRI, the excitation frequencies used are on the order of 30 MHz or higher. For this reason, the skin depth of most metals is typically less than 100 μm, making MRI of metal samples impossible. The following table compiles typical resistivities and skin depths associated with typical metals used in the construction or encapsulation of lithium-ion battery cells.

[0024]

[0025] Traditional high-field machines operate with fields ranging from 1 T to over 10 T. Using lithium as a reference, this covers frequencies from 25 MHz to over 170 MHz. In the table above, we can see that this corresponds to skin depths of a few tens of micrometers, making high-field MRI of metal samples impossible.

[0026] Therefore, the magnetic field must be reduced to operate at lower frequencies. This also means lighter, lower-cost machines that do not require extensive maintenance or magnetic shielding and will not be affected by susceptibility artifacts. However, if we simply take a traditional machine and reduce the field, we will obtain a much lower SNR (signal-to-noise ratio), which means degraded image quality and long acquisition times.

[0027] One possible solution is to use a SQUID (standing for "superconducting quantum interference device"), which is a ultrasensitive magnetometer that allows the magnetic field of an NMR experiment to be reduced while maintaining sufficient SNR to produce images with good resolution and contrast in a reasonable time. Using this technique, MRI images can be prepared using fields ranging from 100 μT to a few mT. Using lithium as a reference, this means an NMR frequency range of 1.6 kHz to about 20 kHz. Looking at the table, this means that depending on the metal, the skin depth is in the range of a few millimeters to 1 centimeter, making it possible to image metal samples.

[0028] One example is the work of the team of Alexej Jerschow at New York University. To address issues related to high frequencies and skin effects, this team has designed a technique called inverted MRI (ioMRI stands for "magnetic resonance imaging in operation")[6][7]. This technique is described in detail in the recent book by Haber-Pohlmeier, Bliimich, and Ciobanu

[12] . The idea is not to prepare a direct image of the lithium-ion battery cell, but rather to use MRI of the surrounding environment of the cell to indirectly measure the local magnetic susceptibilities of the anode and cathode. More precisely, the battery cell is placed in a holder containing a measurement medium, which is typically doped water, that surrounds the lithium-ion battery cell. A permanent field B0 is then applied to the cell. The cell in turn generates a secondary field B that is proportional to the local magnetic susceptibilities of the cell components, and this secondary field is mapped with a special MRI sequence of the surrounding doped water. See Illot et al. 2018 [7], Figure 1 is a schematic diagram of the layout of the device.

[0029] By making precise in-operation measurements of this secondary field, the total magnetic susceptibilities of the anode and cathode can be inferred. Furthermore, this overall magnetic susceptibility was shown by Ilott et al. 2018 [7] to be Figure 2 related to the SOC (state of charge). The figure shows the total magnetic susceptibility measured as a function of the state of charge.

[0030] The anode or cathode sensitivity (in ppm) increases by approximately 50% from the fully discharged state to a charge of 250 mAh. The magnetic field map (in ppm) is given with reference to a fully charged battery cell. The battery cells used were produced by the Rochester Institute of Technology (RIT). Figure 4 in [7] also shows that the magnetic field map can be used to classify battery cells into different defect categories: for example, bending, small cracks. In this figure, the magnetic field map is given in ppm relative to the field value in one of the defect-free battery cells. Also shown are the mean and standard deviation of the topographic map taken over the entire photograph. The MRI sequence used was initially a simple FLASH sequence, and then a specific point imaging with T1 enhancement was implemented to handle magnetic susceptibility artifacts.

[0031] While ioMRI demonstrates that MRI can indeed be effectively used to diagnose the state of charge (SOC) and state of health (SOH), MRI has a major drawback: MRI only shows a global map of the state of the anode or cathode, but cannot produce a spatially resolved map of the local SOC and SOH of each electrode.

[0032] On the other hand, so far, due to the lack of available SNR at low fields, there has been no breakthrough in SQUID MRI. To increase the SNR, most attempts have used a technique called prepolarization: First, a high pulsed field of about 100 mT is applied to increase the Boltzmann polarization of the sample, which increases linearly with the field. Then, the field is rapidly decreased, and MRI acquisition is performed in the field of interest, which is typically about 100 μT.

[0033] This allows taking advantage of the moderately high polarization at 100 mT and the benefits associated with ultra-low fields, which are mainly improved contrast and reduced field uniformity constraints. However, prepolarization requires heavy equipment to generate the pulsed field and complex shielding to cancel out the noise caused by eddy currents. More importantly, the rapid switching of the magnetic field will make this technique infeasible for metal samples.

[0034] The paper

[28] by Moszle M et al., "SQUID-detected microtesla MRI in the presence of metal" in Journal of Magnetic Resonance, Volume 179, Issue 1, March 1, 2006, pages 146 - 151, XO024919553, discloses a magnetic resonance imaging diagnostic device and method for detecting the magnetic resonance of organic components using SQUID in the presence of metal. Using this method, referring to page 149, the authors were able to image organic samples (in this case, bell peppers) placed in a 200 - μm - thick aluminum box or surrounded by a 20 - μm - thick aluminum foil.

[0035] However, the imaging devices of the prior art do not allow magnetic resonance imaging with sufficient image quality for industrial use of metal components.

[0036] The object of the present invention is to precisely remedy this limitation by providing a device for MRI imaging of metal components, which can provide a spatially resolved map of the metal components. Summary of the Invention

[0037] This object is achieved by a magnetic resonance imaging (MRI) device arranged to perform imaging of a substantially metal component, the MRI device comprising:

[0038] - means for generating a polarization magnetic field intended to be applied to the substantially metal component,

[0039] - RF means for exciting the substantially metal component,

[0040] - A detection device that cooperates with the radio frequency device to deliver a magnetic resonance imaging (MRI) signal.

[0041] - A device for processing the MRI signal to deliver characteristic information about the state of the substantially metallic component.

[0042] According to the present invention, the component is subjected to a very weak field of less than 10 mT, and the detection device includes a pickup coil and a SQUID (superconducting quantum interference device) detector. The pickup coil is magnetically coupled to the polarization device and the radio frequency device and operates as a flux concentrator, and the SQUID detector is arranged downstream of the pickup coil via a transformer.

[0043] Hereinafter, a "substantially metallic component" refers to any component or object that includes one or more metal nuclei and / or whose physical composition is substantially metallic.

[0044] In a preferred embodiment of the present invention, the polarization device, the radio frequency device, and the pickup coil are enclosed in a shielded room.

[0045] At least a part of the SQUID detector and the processing device is advantageously seated in a cryostat.

[0046] The polarization device may include gradient coils, and the pickup coil may have a volume gradient geometry.

[0047] The pickup coil may have a surface geometry, particularly a two-step gradient surface geometry.

[0048] In a first application of the present invention, the pickup coil includes means for receiving an electrochemical cell unit, particularly a lithium-ion battery unit.

[0049] In a second application of the present invention, the pickup coil includes means for receiving an electronic chip or component.

[0050] In a third application of the present invention, the pickup coil includes means for receiving an industrial mechanical structure.

[0051] In a fourth application example of the present invention, the MRI imaging device is adapted to image a power plant structure.

[0052] According to another aspect of the present invention, a method for magnetic resonance imaging of a body containing at least one metal part is provided, the method comprising the following steps:

[0053] - Generating a polarization magnetic field to be applied to the body (a).

[0054] - Exciting the body (a) with radio frequency (RF) waves.

[0055] - Detecting an MRI signal from the response of the subject to radiofrequency excitation,

[0056] - Processing the detected MRI signal so as to deliver information characterizing the state of the subject (a),

[0057] According to the invention, the subject is subjected to a very weak field of less than 10 mT, and the detection step comprises generating an induced signal picked up by flux concentration in a pick-up coil and applying the picked-up signal to a SQUID detector.

[0058] The MRI imaging method according to the invention can advantageously be implemented for the characterization of an electrochemical cell unit and is arranged to provide a map of the electrochemical cell unit that represents the state of charge (SOC) of the electrochemical cell unit and / or represents the state of health (SOH) of the electrochemical cell unit.

[0059] The MRI imaging method according to the invention can be arranged to provide a map of the electrochemical cell unit that represents the state of health (SOH) of the electrochemical cell unit.

[0060] Thus, the ultra-low field nuclear magnetic resonance imaging device system according to the invention for metal components uses detection based on a superconducting quantum interference device (SQUID). With SQUID detection, we work on fields ranging from 50 μT (the Earth's field) to a few mT. By using an ultra-low field corresponding to a very low NMR frequency, we are able to image metal samples, in particular electrochemical cell units.

[0061] The MRI imaging device according to the invention is designed for spatially resolved diagnosis of the condition of metal components and is in particular designed for spatially resolved diagnosis of the state of charge (SOC) and the state of health (SOH) of an electrochemical cell unit.

[0062] Thus, the MRI imaging method according to the invention can be used to characterize a lithium-ion battery cell comprising a plurality of lithium nuclei, the plurality of lithium nuclei comprising 7 Li isotope nuclei. The method then comprises steps for preparing 7 a spatially resolved image of the Li isotope nuclei and for estimating the 7 density of the Li nuclei in the spatially resolved image.

[0063] In the 7In the field of Li MRI imaging, as prior art, we can cite the paper by Klamor et al.

[29] : "Phys. Chem. Chem. Phys." 2015, 17, 4458 "7Li in situ 1D NMR imaging of a lithium ion battery".

[0064] The imaging method according to the present invention can be arranged to provide a one-dimensional (1D) map of a lithium ion battery cell, which one-dimensional map represents the state of charge (SOC) of the battery cell from 7 the Li nuclear density estimate. In fact, the imaging method then also includes that the very high frequency of the order of 500 MHz used will not allow 3D mapping due to the skin effect in the conductor and the susceptibility artifacts typical of high NMR frequencies. When the lithium ion battery cell is inserted into the pick-up coil for imaging, the step of applying a voltage wave of a predetermined distribution to the battery cell terminals, the step of simultaneously measuring the current entering the battery cell, and the step of processing the current and voltage measurement results in order to deliver an estimate of the capacity and state of charge of the battery cell.

[0065] In order to generate a map representing the state of health (SOH) of the lithium ion battery cell accommodated inside the pick-up coil, we can also provide the step of applying a voltage wave of a predetermined distribution to the terminals of the battery cell until maximum charge is reached, the step of simultaneously measuring the current entering the battery cell, the step of determining the effective maximum capacity of the battery cell, and the step of estimating the state of health of the battery cell based on the ratio between the thus determined effective maximum capacity and the initial maximum capacity of the battery cell.

[0066] The aim is to develop a low-cost transportable MRI device that does not require precautions to operate and can image metal samples in three dimensions (3D). This is achieved by reducing the operating magnetic field from 1.5 T used in conventional medical machines to less than 1 mT. Since a resistive magnet is used, the working field can be easily set between 100 μT and a few mT.

[0067] The amount of available signal decreases linearly with the magnetic field B0, which means at least 1000 times less in our case compared to high-field MRI. To counteract the signal loss, we use ultrasensitive antennas based on SQUID detection. SQUIDs are highly sensitive magnetometers made of a loop of superconducting material intercepted by two Josephson junctions. They have a very wide bandwidth and can detect DC signals up to 100 MHz with a flat frequency response. The superconductor needs to be cooled to a cryogenic temperature of 4K to operate, which is a problem for the heavy superconducting magnets used in conventional MRI. In the case of SQUIDs, this is much less of a limitation as they are mounted on very small chips, only a few centimeters in size, and thus only require a very light cryogenic machine to operate.

[0068] Since SQUIDs are very small, typically loops with a diameter of a few μm, they are usually used in combination with antennas that act as flux concentrators. Such detectors can achieve field sensitivities on the order of for fields in the kHz range.

[0069] References

[0070] 1. Superconducting quantum interference device instruments and applications, R.L. Fagaly et al., Review of Scientific Instruments 77 101101 (2006).

[0071] 2. Electrochemical impedance spectroscopy (EIS): principles, construction, and biosensing applications, H.S. Magagar et al., Sensors (Basel) 19 6578 (2021).

[0072] 3. SQUID-based magnetic resonance imaging at ultra-low fields using back-projection methods, Q. Guo et al., Concepts in Magnetic Resonance Part B, Magnetic Resonance Engineering 88 82329 (2020).

[0073] 4. Progress in a deployable SQUID-based ultra-low field MRI system for anatomical imaging, M.A. Espy et al., IEEE Transactions on Applied Superconductivity, 25 No. 3, 1601705 (2015).

[0074] 5. Human brain MRI at 130 microtesla, B. Inglis et al., Proceedings of the National Academy of Sciences of the United States of America (PNAS) 110 No. 48, 19194 - 19201 (2013);

[0075] 6. Real - time 3D imaging of microstructural growth in battery cells using indirect MRI.

[0076] A. J. Ilott et al., Proceedings of the National Academy of Sciences 113(39) 10779 - 10784 (2016).

[0077] 1. Detection of state of charge and defects in rechargeable lithium - ion battery cells by in - situ in - and - out magnetic resonance imaging, A. J. Ilott et al., Nature Communications 9 1776 (2018).

[0078] 2. Distortion - free in - and - out imaging for rapid diagnosis of rechargeable lithium - ion battery cells.

[0079] K. Romanenko et al., Proceedings of the National Academy of Sciences 116(38) 18783 - 18789 (2019).

[0080] 1. Diagnosis of current distribution in batteries using magnetic resonance imaging, M. Mohammadi et al., Journal of Magnetic Resonance (J. Mag. Res.) 309 106601 (2019).

[0081] 2. Sensitive magnetometry reveals inhomogeneity of charge storage and weak transient internal currents in lithium - ion battery cells, Y. Hu et al., Proceedings of the National Academy of Sciences 117(20) 10667 - 10672 (2020).

[0082] 3. Observation of memory effects associated with the degradation of rechargeable lithium - ion battery cells using ultrafast surface - scanning magnetic resonance imaging, K. Romanenko et al., Journal of Materials Chemistry A 9 21078 - 21084 (2021).

[0083] 4. S. Haber-Pohlmeier et al., "Magnetic Resonance Microscopy: Instrumentation and applications in Engineering, Life Science, and Energy Research", Chapters 17 to 18, Wiley-VCH, 2022. ISBN: 9783527827251.

[0084] 5. Three-dimensional characterization of electrodeposited lithium microstructures using synchrotron radiation X-ray phase-contrast imaging, D. S. Eastwood et al., Chem. Comunn. 51 266 - 268 (2015).

[0085] 6. Three-dimensional high-resolution X-ray imaging and quantification of mesophase carbon microsphere anodes in lithium-ion batteries, F. Tariq et al., J. of Power Sources 248 1014 - 1020 (2014).

[0086] 7. Characterization of the 3D microstructure of graphite anodes from lithium-ion batteries, P. R. Shearing et al., Chem. Commun. 12 - 3, 374 - 377 (2010).

[0087] 8. Multi-length scale microstructure investigation of commercial lithium-ion battery electrodes, P. R. Shearing et al., J. Electrochem. Soc. 159 - 7, 1023 - 1027 (2012).

[0088] 9. Local tortuosity inhomogeneities in lithium battery composite electrodes, D. Kehrwald et al., J. Electrochem. Soc. 158 - 12 - 1393 (2011).

[0089] 10. High-speed tomography during operation of lithium-ion batteries during thermal runaway, D. P. Finegan et al., Nat. Commun. 6 6924 (2015).

[0090] 11. Non-invasive battery analysis via micro-computed tomography, E. L. Ballard et al., US7902518B2 (2008).

[0091] 12. In-situ battery diagnostic method using electrochemical impedance spectroscopy, R. Mingant et al., US8849598B2 (2010).

[0092] 13. A method for determining the aging status of a battery cell by means of impedance spectroscopy, J. Ziegler et al., US20120019253A1 (2009).

[0093] 14. A method and apparatus for battery testing, Tinnemeyer J.A. et al., US20110074432A1 (2009).

[0094] 15. Electrochemical impedance spectroscopy in a battery management system, CN107076801A (2015).

[0095] 16. A method for rapidly measuring the EIS of a lithium-ion battery, CN106970266A (2016).

[0096] 17. https: / www.ieco.fi / index.php?k=10909

[0097] 18. https: / www.shicryogenics.com / product / rp-082B2S-4K-pulse-tube-cryocooler-series /

[0098] 19. https: / www.pure-devices.com / index.php / products / products-research / produits-recherchelecteurl.html

[0099] 20. Moszle M et al.: "Microtesla MRI with SQUID detection in the presence of metals", Journal of Magnetic Resonance, Volume 179, March 1, 2006, Issue 1, pages 146 to 151, XO024919553

[0100] 21. Klamor et al.: "In situ 1D NMR of 7Li in lithium-ion batteries", Physical Chemistry Chemical Physics 2015, 17, 4458 BRIEF DESCRIPTION OF THE DRAWINGS

[0101] -[[]] Figure 1 Figure 1 is a schematic diagram of a SQUID detection system implemented in an MRI imaging device according to the present invention;

[0102] - Figure 2 Figure 2 is an example of the geometry of a flux concentrator used in a SQUID detection system implemented in an MRI imaging device according to the present invention;

[0103] -[Figure]​​Figure 3 It is a schematic diagram of an MRI imaging device for metal components according to the present invention. Detailed implementation

[0104] Reference Figure 1 , the SQUID detection system 10 includes: a flux concentrator 2, which includes an excitation coil c and a pickup coil b around a metal component a such as a lithium-ion electrochemical battery cell; a shield transformer 3, which is connected to an input inductor L at the output i ; a SQUID 4, and the output signal of the SQUID is amplified by an LNA amplifier 5 (representing "low noise amplifier") coupled to a phase-locked loop FLL 6 (representing "flux locked loop") that controls a power supply coil Lf.

[0105] The SQUID can be made of niobium, magnesium diboride (MgB), or any other medium critical temperature superconducting material, or made of high-temperature copper oxide.

[0106] The metal component emits a signal, which is captured by the pickup coil b acting as a flux concentrator, and is sent to the SQUID 4 via two terminals 20, 21 via an input coil L i , and impedance matching capacitors Ca are connected in parallel to these two terminals. L1, L2 shield transformers are used for impedance matching and ground isolation.

[0107] The excitation coil c is connected to the MRI console d. The SQUID signal output is amplified by the low noise amplifier LNA 5, while the operation stability is ensured by the flux locked loop FLL 6.

[0108] In this example, the pickup coil is a saddle coil, but a gradiometer antenna is generally preferred, such as Figure 2 the gradiometer antenna shown.

[0109] In most SQUID MRI experiments, the pickup coil is a second-order surface gradiometer, as described by Fagaly et al. [1].

[0110] In the present invention, we preferably use a volume geometry, which captures more signals while maintaining robustness to noise. To maintain the gradient configuration and maintain robustness to noise from remote sources, the antenna consists of two saddle coils, which are connected in series with opposite currents. This specific geometry is the subject of a patent application filed on April 12, 2022 in the name of the same applicant.

[0111] The pick-up coil 2' includes a first saddle coil c' surrounding a second saddle coil b'. The first coil c' includes a first longitudinal conductor cl connected to a first output conductor 21 of a transformer 3, a first end conductor c2, a second longitudinal conductor c3, a second end conductor c4, a third longitudinal conductor c5, a third end conductor c6, and a fourth longitudinal conductor c7.

[0112] A second coil b' concentric with the first coil c' includes a first longitudinal conductor b1 connected to a second output conductor 20 of the transformer 3, a first end conductor b2, a second longitudinal conductor b3, a second end conductor b4, a third longitudinal conductor b5, a third end conductor b6, and a fourth longitudinal conductor b7.

[0113] Reference Figure 3 , an MRI imaging device 1 for metallic components such as lithium-ion electrochemical battery cells will now be described.

[0114] The MRI imaging device 1 according to the present invention includes a flux concentrator 2", which is arranged to receive a metallic component a, such as a chemical battery cell. A coil b generates a magnetic field B0, which polarizes nuclei of interest in the metallic component a, thereby generating energy levels separated by an energy h ω0.

[0115] The flux concentrator 2" also includes a gradient coil h for generating spatial resolution in the MRI image. The coil h is powered by a stable current source c via a pair of conductors 8c, and the stable current source delivers a very regular current on the order of several A to several tens of A according to the precise geometry of the coil.

[0116] The gradient coil b" is fed by a gradient amplifier d (e.g., model XPA-175-350 from IECO, Finland)

[25] . In order to keep the field very stable and uniform in the field of view, the field is continuously monitored with a fluxgate magnetometer probe e. Low-frequency fluctuations in the measured field are measured, and commands are sent to the current source in real time to keep the field variation within a typical drift of 50 ppm during the duration of the MRI acquisition.

[0117] The radiofrequency amplifier f feeds a radiofrequency coil g, which can be of birdcage geometry, for example, to send a pulse B1 tuned to the magnetic resonance frequency ω0. Then, the signal is picked up by the Figure 2 described detection system.

[0118] The pick-up coil h sends the signal to the rest of the readout chain i within a SQUID and a 4K cryostat j, which is cooled by a cryocooler k (e.g., RP-082B2S from Sumitomo

[26] ).

[0119] Then, at room temperature, the output signal is processed by an analog-to-digital conversion module and an MRI console 1 (e.g., drive-1 model from PureDevices

[27] ), which interprets the time signal as an MRI image displayed on screen m. The polarization system, excitation coil, detection coil, and metal part a are all enclosed in a shielded room n, which consists of a metal grid serving as a Faraday cage to shield signals in the 10 kHz range and above, and a layer of magnetic material (such as ferrite or high-permeability alloy) to protect the system from very low-frequency noise below a few kHz.

[0120] The present invention realizes an antenna with further increased sensitivity (close to 0.1). The antenna has a gradient and volume geometry that collects the most magnetic flux from the sample while maintaining high robustness to noise. The literature FR3117218 under the applicant's name describes the use of this antenna in the context of NMR and MRI. Due to this antenna and the increase in the measurement field from the usual 100 μT to about 1 mT, we can sufficiently increase the SNR for MRI acquisition without pre-polarization, and sufficiently increase the resolution and contrast for imaging lithium-ion battery cells during operation.

[0121] The ultra-low field MRI method described herein is particularly suitable for direct MRI imaging of a single lithium-ion battery cell or battery cell assembly. Compared with the above-mentioned MRIio technology, the direct MRI method provides an image inside the battery cell, where the contrast shows a time-resolved map of the local SOC and SOH. Here are some possible strategies:

[0122] - Li MRI sequence for mapping the local lithiation state of the cathode, thus providing spatially resolved insight into the local SOC (state of charge); [7]

[0123] - 3C MRI sequence for studying the SOC in the anode; [1]

[0124] - The susceptibility-weighted MRI sequence can provide access to a spatially resolved map of the susceptibility, as shown by Illot et al. [5], which is related to the local SOC and SOH.

[0125] The following is an approximate quantitative estimate of the important device parameters:

[0126] - For a fully resolved image, the acquisition time is about 5 minutes, but a fast sequence with less information can be executed within a few seconds.

[0127] - Spatial resolution in the millimeter range, most likely an in-plane resolution on the order of 1.0 mm to 2.5 mm.

[0128] - The volume of interest (field of view) is up to 50 cm, possibly more. This enables imaging of a single battery cell or a collection of battery cells in a single acquisition.

[0129] - Wheeled portable device, the size of a small washing machine, weighing approximately 100 kg.

[0130] Possible applications

[0131] - Evaluate the state of charge (SOC) and state of health (SOH) of an electrochemical cell or a full cell composed of battery cell components by providing spatially resolved maps of SOC and SOH during operation.

[0132] - Quality control of electrochemical cells.

[0133] - Quality control and monitoring of electronic chips and components.

[0134] - Detect defects in metal structures in the following industries: automotive, construction, energy, defense and aerospace, transportation infrastructure.

[0135] - Medical MRI of patients with metal implants.

[0136] Of course, the present invention is not limited to the examples just described, and many other embodiments can be envisaged without departing from the scope of the present invention.

Claims

1. A magnetic resonance imaging (MRI) device (1), the MRI device being arranged to perform imaging of a substantially metallic component, the MRI device comprising: - means (b”) for generating a polarization magnetic field, the polarization magnetic field being intended to be applied to the substantially metallic component (a), - RF means (g) for exciting the substantially metallic component (a), - detection means (10), the detection means cooperating with the radiofrequency means (g) to deliver a magnetic resonance imaging (MRI) signal, - means (1) for processing the MRI signal to provide information characterizing the state of the substantially metallic component (a), characterized in that the substantially metallic component (a) is subjected to a very weak field of less than 10 mT, and the detection means (10) comprises a pick-up coil (h) and a SQUID (superconducting quantum interference device) detector (4), the pick-up coil being magnetically coupled to the polarization means (b”) and the radiofrequency means (g) and operating as a flux concentrator, the SQUID detector being arranged downstream of the pick-up coil (h) via a transformer (3).

2. The MRI imaging device (1) according to claim 1, characterized in that, The polarization means (b”), the radiofrequency means (g) and the pick-up coil (h) are enclosed in a shielded room (n).

3. The MRI imaging device (1) according to claim 2, characterized in that, At least a part of the SQUID detector (4) and the processing means (i) are arranged in a cryostat (j).

4. The MRI imaging device (1) according to any one of claims 1 to 3, characterized in that, The polarization means (b”) comprises gradient coils.

5. The MRI imaging device according to any one of claims 1 to 4, characterized in that, The pick-up coil (2’) has a volume gradient geometry.

6. The MRI imaging device according to any one of claims 1 to 5, characterized in that, The pick-up coil has a surface geometry, in particular a second-order gradient surface geometry.

7. The MRI imaging device according to any one of claims 1 to 6, characterized in that, The pick-up coil (h) comprises means for receiving an electrochemical cell unit (a), in particular a lithium-ion battery unit.

8. The MRI imaging device according to any one of claims 1 to 7, characterized in that, The pick-up coil comprises means for receiving an electronic chip or component.

9. The MRI imaging device according to any one of claims 1 to 8, characterized in that, The pick-up coil comprises means for receiving an industrial mechanical structure.

10. The MRI imaging device according to any one of claims 1 to 9, characterized in that, The MRI imaging device is adapted to image a power plant structure.

11. A method for magnetic resonance imaging of a substantially metallic component body (a) comprising at least one metallic part, the method being implemented in an imaging device according to any one of claims 1 to 10, the method comprising the following steps: - generating a polarization magnetic field, the polarization magnetic field being intended to be applied to the substantially metallic component (a), - exciting the substantially metallic component (a) with radiofrequency (RF) waves, - detecting an MRI signal of the response of the substantially metallic component to the radiofrequency excitation, - processing the MRI signal thus detected so as to deliver characteristic information about the state of the substantially metallic component (a), characterized in that the substantially metallic component is subjected to a very weak field of less than 10 mT, and the detection step comprises generating an induced signal picked up by flux concentration in the pick-up coil (h), and applying the pick-up signal to the SQUID detector (4).

12. The MRI imaging method according to claim 11, wherein the MRI imaging method is implemented for characterizing an electrochemical cell unit (a).

13. The MRI imaging method according to claim 12, wherein the MRI imaging method is implemented for characterizing a lithium-ion battery cell including a plurality of lithium nuclei, the plurality of lithium nuclei including 7 Li isotope nuclei, characterized in that The MRI imaging method includes steps for preparing a spatially resolved image of the 7 Li isotope nuclei, and steps for estimating the density of the 7 Li nuclei in the spatially resolved image.

14. The MRI imaging method according to claim 13, wherein the MRI imaging method is arranged to provide a map of the lithium-ion battery cell, the map representing the state of charge (SOC) of the battery cell from 7 the Li nuclear density estimate, characterized in that The MRI imaging method further comprises: when the lithium-ion battery unit is inserted into the pickup coil for imaging therein, a step of applying a voltage wave with a predetermined distribution to the terminals of the battery unit, a step of simultaneously measuring the current entering the battery unit, and a step of processing the current and voltage measurement results to deliver an estimate of the capacity and state of charge of the lithium-ion battery unit.

15. The MRI imaging method according to claim 13 or 14, wherein the MRI imaging method is arranged to provide a map of the lithium-ion battery cell, the map representing the state of health (SOH) of the battery cell from the 7 lithium nuclear density estimate, characterized in that The MRI imaging method further comprises: when the battery unit is inserted into the pickup coil for imaging therein, a step of applying a voltage wave with a predetermined distribution to the terminals of the electrochemical battery unit until a maximum charge is reached, a step of simultaneously measuring the current entering the electrochemical battery unit, a step of determining the effective maximum capacity of the electrochemical battery unit, and a step of estimating the state of health of the battery unit based on the ratio between the thus determined effective maximum capacity and the initial maximum capacity of the lithium-ion battery unit.

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