Device and method for performing magnetic resonance imaging of metallic or partially metallic components, and application of this method to the imaging of electrochemical cells

Abstract

The invention relates to a magnetic resonance imaging (MRI) device (1), designed to image an essentially metallic component, comprising means (b'') for generating a polarization magnetic field intended to be applied to said component (a), radiofrequency (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 so as to deliver characteristic information about the state of said component (a). The component (a) is subjected to a very weak field of less than 10 mT, and the detection means comprise a pick-up coil (h) magnetically coupled with the polarization means (b'') and the radiofrequency means (g) and operating as a flux concentrator, and a SQUID (superconducting quantum interference device) detector arranged downstream of said pick-up coil (h) via a transformer.

Description

Apparatus and method for imaging metallic or partially metallic components by magnetic resonance, application of this method to the imaging of electrochemical cells Field of invention

[0001] This document describes an apparatus for imaging metallic or partially metallic components by magnetic resonance. It also relates to a method for imaging metallic or partially metallic components implemented in this apparatus, and its application to the imaging of electrochemical cells, in particular Li-ion cells. State of the art

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

[0003] The NMR (or MRI) experiment is done in a few precise steps. First, the sample (in imaging a Li-ion cell or any metallic component, this would be the component itself), is placed in a very homogeneous static magnetic fieldB0called the bias field, produced by a large superconducting coil. In most commercial devices, this magnetic field varies from 1.5 T to more than 10 T. When the protons in the sample are placed in a magnetic fieldB0, they precess around this field with a frequency ω0given byω0=γB0, whereγis called the gyromagnetic ratio.

[0004] Then, the protons are excited using a radiofrequency signal of the same frequency ω = ω0 as the proton precession. This excitation causes the protons to break out of their precessional motion, after a while they slowly decay back to their original precessional state while emitting a signal that is detected using an antenna. Measuring this signal is a way to gather information about the composition of the body's interior, and ultimately to reconstruct an MRI image.

[0005] The overall principle of the MRI experiment can be summarized as follows: The body part to be imaged is placed in a static homogeneous magnetic field B0. When placed in this field, the protons precede with a frequency ω0 = γ B0. Using an excitation antenna, a pulse of intensity B1 and frequency ω = ω0 tuned to the proton precession is sent. The protons are sent to a higher energy state and decay to their original state while sending a signal of intensity B2 and frequency ω0. More importantly, this signal will decay with the typical times τ1, corresponding to a recovery of the longitudinal magnetization in the direction of B0, and τ2, corresponding to a loss of spin-spin coherence in the sample. These typical time constants are characteristics of the precise state at each point in the sample and are used to produce contrast in an MRI image.In the case of medical MRI, they provide information about the type of tissue at a particular point (water, fat, muscle, etc.).

[0006] To construct an MRI image, it is necessary to determine where the signal detected by the MRI antenna comes from. To do this, gradient coils are used. In addition to the permanent homogeneous field B0, a small additional field δB(x,y,z) is added, which varies linearly along the three directions in space.

[0007] Typically, in an MRI device implementing a magnetic field of 5 T, we observe an additional field δB of the order of 100 mT.m − 1 . Since the resonant frequency of nuclei is proportional to the local magnetic field, gradient fields allow the resonant frequency and phase of the signal to be spatially encoded, thus enabling the formation of 3D images.

[0008] During an MRI sequence, these gradient fields are rapidly changed over time scales of a few milliseconds to selectively excite certain parts of the sample. The pattern that is chosen to apply the gradients and excitation pulses constitutes an MRI sequence.

[0009] Different sequences allow for different contrasts and different things to be seen. In medical MRI, there is a wide variety of sequences: some are suitable for seeing blood vessels (time-of-flight sequences), others are more suited to functional imaging (BOLD for "Blood Oxygenation Level Dependent"), while more common T1- or T2-weighted sequences can discriminate between fat, organic tissue, tumors, etc.

[0010] It is noteworthy that MRI sequences function as image constructs in reciprocal Fourier space (or k-space) where different MRI sequences respond to different acquisition trajectories.

[0011] We will now explain why it is necessary to implement low-field MRI for imaging metallic samples.

[0012] For over forty years, innovation in MRI has consistently pushed toward higher fields. A higher field means that more nuclei in the sample are polarized, resulting in an increased signal-to-noise ratio. Furthermore, because the detection frequency is proportional to the magnetic field, it allows for high frequencies, in a range where typical inductive detection antennas are inherently more sensitive. In short, a high field means better resolution and faster acquisition times. However, this strategy has obvious drawbacks. Commercial high-field machines require heavy and expensive superconducting magnets that are cooled to 4 Kelvin with liquid helium, and require expensive maintenance. A typical 1.5T scanner costs nearly $1 million upfront plus a few hundred thousand dollars in maintenance per year.The MRI scanner must be installed in a dedicated, magnetically shielded room with a reinforced floor. This makes MRI technology a luxury imaging procedure compared to ultrasound or CT scans.

[0013] A less obvious problem with high-field MRI is the inability to image in the presence of metal, let alone image a metallic sample. This problem is actually twofold: Having a sample with both metallic and non-metallic parts means that there will be huge local differences in magnetic susceptibilities. This causes magnetic field gradient artifacts that destroy the linearity of the gradient field pattern used to perform the MRI. High-frequency magnetic fields cannot penetrate the bulk of a metallic sample due to the skin effect of electrical conductors.

[0014] When a propagating electromagnetic field reaches a conductor / dielectric interface, eddy currents are created inside the inductor. These currents in turn generate another magnetic field that offsets the incipient magnetic field. This causes the total magnetic field to effectively die out upon entering the bulk of the conductive sample. The skin depth δ is a measure of how deep the magnetic field can penetrate into the metallic sample:

[0015] whereωis the pulsation,ρis the DC resistivity,μ0is the vacuum permeability andμ r is the relative permeability of the sample.

[0016] In the case of proton MRI, we have γ H / (2 π) : 42 MHz.T − 1 , while for lithium this number is a little lower at γ Li / (2π): 16.5 MHz.T − 1. In any case, this means that in traditional high-field MRI, the excitation frequencies used are in the order of 30 MHz or higher. For this reason, the skin depth in most metals is usually less than 100μm, making MRI of metallic samples impossible. The table below compiles the typical resistivities and skin depths associated with typical metals used in the composition or packaging of Li-ion cells.MaterialSkin depth vs. frequency (mm)5 kHz50 kHz50 MHzCopper0.930.300.01Stainless steel6.32.00.06Lithium2.20.690.02

[0017] Conventional high-field machines operate with fields ranging from 1 T to over 10 T. If we take lithium as a reference, this covers frequencies from 25 MHz to over 170 MHz. In the table above, we see that this corresponds to skin depths of a few tenths of a micron, which therefore makes high-field MRI impossible for metallic samples.

[0018] It is therefore necessary to lower the magnetic field to work at lower frequencies. This would also mean lighter, inexpensive machines that do not require heavy maintenance or magnetic shielding, and that would be immune to susceptibility artifacts. However, if we simply take a traditional machine and lower the field, we will have a much lower signal-to-noise ratio (SNR), therefore degraded image quality and long acquisition times.

[0019] One possible solution is to use SQUIDs (superconducting quantum interference devices), which are ultrasensitive magnetometers that allow the magnetic field of the NMR experiment to be lowered while maintaining sufficient SNR to produce images with good resolution and contrast in a reasonable time. With this technology, an MRI image can be made with fields ranging from 100μT to a few mT. Taking lithium as a reference, this means NMR frequencies ranging from 1.6 kHz to about 20 kHz. Looking at the table, this means skin depths of a few mm to 1 cm depending on the metal, which makes it possible to image metallic samples.

[0020] On this subject, we can cite the work of Alexej Jerschow's team at New York University. To circumvent the problems related to high frequencies and the skin effect, this team designed a technique called inside-out MRI (ioMRI for "IntraOperativeMagneticResonanceImaging") [6] [7]. The technique is well detailed in the very recent book by Haber-Pohlmeier, Bliimich and Ciobanu

[0012] . Rather than making a direct image of a Li-ion cell, the idea is to make an indirect measurement of the local magnetic susceptibility of the anode and the cathode thanks to an MRI of the surrounding medium of the cell. More precisely, the cell is placed in a holder which contains a measuring medium, generally doped water, surrounding the Li-ion cell. Then, a permanent field B0 is applied to the cell.The cell in turn generates a secondary field Bs proportional to the local susceptibility of the cell components, this secondary field being mapped with a special MRI sequence of the surrounding doped water. Reference can be made to Illot et al. 2018 [7], for a schematic of the arrangement of this device.

[0021] With an accurate in-operando measurement of this secondary field, it is possible to deduce the overall magnetic susceptibility of the anode and cathode. In turn, this overall susceptibility is shown in Ilott et al. 2018 [7] to be related to the state of charge SOC (for "State Of Charge"). This figure shows the measured overall magnetic susceptibility as a function of the state of charge.

[0022] The sensitivity of the anode or cathode (in ppm) increases by about 50% from a fully discharged state to a 250 mAh charge. The magnetic field map (in ppm) is given with reference to the fully charged cell. The cell used is produced by the Rochester Institute of Technology (RIT). Figure 4 of [7] also demonstrates that magnetic field maps can be used to classify cells into different defect categories: folded, small craps, for example. In this figure, the magnetic field maps are given in ppm relative to the field value of one of the non-defective cells. Also shown are the mean and standard deviation of the field map taken over the entire picture. The MRI sequence used is initially a simple FLASH sequence, then specific point imaging with T1 enhancement was implemented to address magnetic susceptibility artifacts.

[0023] While ioMRI demonstrates that MRI can indeed be used effectively for the diagnosis of SOC and SOH (State of Health), it suffers from a major drawback: it only shows a global map of the anode or cathode state, but cannot produce a spatially resolved map of the local SOC and SOH of each electrode.

[0024] Furthermore, until now, SQUID MRI has not seen a breakthrough due to the lack of available SNR at low field. 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 lowered and the MRI acquisition is performed in a field of interest, usually around 100 μT.

[0025] This allows taking advantage of both the moderately high bias at 100 mT and the advantages associated with ultralow field, mainly improved contrasts and reduced field homogeneity constraints. However, prebiasing requires heavy machinery for pulse field generation and complex shielding to counteract eddy current noise. More importantly, the rapid switching of magnetic fields would make this technique unviable on metallic samples.

[0026] Document

[0028] Moszle M et al: "SQUID-detected microtesla MRI in the presence of metal", Journal of Magnetic Resonance, vol.179, n°1 March 1, 2006, pages 146-151, XO024919553, discloses an apparatus and method for magnetic resonance imaging with SQUID detection of an organic component in the presence of metal. With this method, the authors were able to obtain images of an organic sample, in this case a pepper, placed in an aluminum case with a thickness of 200 µm or surrounded by an aluminum foil with a thickness of 20 µm, with reference to page 149.

[0027] However, state-of-the-art imaging devices do not allow magnetic resonance imaging of metallic components with sufficient image quality for industrial use.

[0028] The aim of the present invention is precisely to remedy this limitation by proposing an MRI imaging device for a metallic component which can provide a spatially resolved map of it.

[0029] This objective is achieved with a magnetic resonance imaging (MRI) apparatus, arranged to perform imaging of an essentially metallic component, comprising: means for generating a polarization magnetic field intended to be applied to said essentially metallic component, RF radiofrequency means for exciting said essentially metallic component, detection means cooperating with said radiofrequency means, for delivering a magnetic resonance imaging (MRI) signal, means for processing said MRI signal so as to deliver information characteristic of the state of said essentially metallic component.

[0030] According to the invention, the component is subjected to a very weak field of less than 10 mT, and the detection means comprise a pickup coil magnetically coupled with the polarization means and the radiofrequency means and operating as a flux concentrator, and a SQUID detector (superconducting quantum interference device) arranged downstream of said pickup coil via a transformer.

[0031] In the following, an essentially metallic component is understood to mean any component or object including one or more metallic cores, and / or whose physical composition is essentially metallic.

[0032] In a preferred version of the invention, the polarization means, the radiofrequency means and the pickup coil are enclosed in a shielded chamber.

[0033] The SQUID detector and at least part of the processing means are advantageously arranged in a cryostat.

[0034] The polarization means may comprise a gradient coil, and the pickup coil may have a volumetric gradiometric geometry.

[0035] The pickup coil may have a surface geometry, including a second-order surface gradiometric geometry.

[0036] In a first example of application of the invention, the capture coil comprises means for receiving an electrochemical cell, in particular a Li-ion cell.

[0037] In a second example of application of the invention, the pickup coil comprises means for receiving an electronic chip or an electronic component.

[0038] In a third example of application of the invention, the pickup coil comprises means for receiving an industrial mechanical structure.

[0039] In a fourth example of application of the invention, the MRI imaging device is adapted to image a structure of a power plant.

[0040] According to another aspect of the invention, there is provided a method for imaging by magnetic resonance a body containing at least one metal part, comprising the following steps: generating a polarization magnetic field intended to be applied to said body (a), exciting said body (a) by radiofrequency (RF) waves, detecting an MRI signal of response of said body to the radiofrequency excitation, processing said MRI signal thus detected, so as to deliver information characteristic of the state of said body (a),

[0041] According to the invention, the body is subjected to a very weak field of less than 10 mT, and the detection step comprises generating an induced signal captured by flux concentration in a capture coil, and applying this signal thus captured to a SQUID detector.

[0042] The MRI imaging method according to the invention can be advantageously implemented for the characterization of an electrochemical cell, and arranged to provide a map of the electrochemical cell, representative of the state of charge (SOC) of this electrochemical cell and / or representative of its state of health (SOH).

[0043] The MRI imaging method according to the invention can be arranged to provide a map of the electrochemical cell, representative of the state of health (SOH) of this electrochemical cell.

[0044] Thus, the device for imaging metallic components by ultra-low field magnetic resonance according to the invention uses detection based on a low-temperature superconducting interference device (SQUID). Using SQUID detection, we work on fields ranging from the 50μT range (earth field) to a few mT. By using ultra-low fields, which correspond to very low NMR frequencies, we are able to carry out imaging of metallic samples, in particular electrochemical cells.

[0045] The MRI imaging device according to the invention is designed for spatially resolved diagnosis of the state of metallic components, and in particular of the state of charge (SOC) and state of health (SOH) of electrochemical cells.

[0046] The MRI imaging method according to the invention can thus be implemented for the characterization of a Lithium Ion cell comprising a plurality of lithium nuclei including isotope nuclei 7 Li. It then includes a step to produce a spatially resolved image of the isotope nuclei 7 Li, and a step to estimate kernel densities 7 Li in the spatially resolved image.

[0047] In this field of MRI imaging 7 Li of a Lithium-Ion battery, we can cite, as a state of the art, the document

[0029] Klamor et al: “7Li in situ 1D NMR imaging of a lithium ion battery” Phys.Chem.Chem.Phys., 2015, 17, 4458.

[0048] The imaging method according to the invention can be arranged to provide a one-dimensional (1D) mapping of the Li-Ion cell, representative of the state of charge (SOC) of this cell from the core density estimates. 7Li. This imaging method then further comprises, Indeed, the very high frequencies used, of the order of 500 MHz, would not allow 3D mapping to be carried out due to the skin effect in the conductors and magnetic susceptibility artifacts typical of high NMR frequencies. while the Li-Ion cell is inserted into the pickup coil to be imaged, a step to apply a voltage wave of predetermined profile to the terminals of the cell, a step to simultaneously measure the current entering the cell, and a step to process the current and voltage measurements so as to deliver an estimate of the capacity and state of charge of the cell.

[0049] To produce a map representing the state of health (SOH) of the Li-Ion cell housed inside the capture coil, it is also possible to provide a step for applying a voltage wave of predetermined profile to the terminals of this cell until a maximum charge is reached, a step for simultaneously measuring the current entering the cell, a step for determining the effective maximum capacity of the cell, and a step for estimating the state of health of this cell from the ratio between the effective maximum capacity thus determined and the initial maximum capacity of the cell.

[0050] The goal is to develop inexpensive, portable MRI devices that require no precautions to operate and can produce three-dimensional (3D) images of metal samples. This is achieved by lowering the operating magnetic field from the 1.5 T used in traditional medical machines to less than 1 mT. Since a resistive magnet is used, the working field can easily be adjusted between 100 μT and a few mT.

[0051] The amount of available signal decreases linearly with the magnetic field B0, which means a factor of at least 1000 in our case, compared to high-field MRI. To counter the signal loss, an ultrasensitive antenna is used that relies on SQUID detection. SQUIDs are highly sensitive magnetometers made from a loop of superconducting material intercepted by two Josephson junctions. They have a very wide bandwidth and can detect signals from DC to 100 MHz with a flat frequency response. Superconductors must be cooled to cryogenic temperatures of 4 K to operate, which is a problem for the heavy superconducting magnets used in traditional MRI. In the case of SQUIDs, this is much less restrictive, as they are mounted on very small chips of a few centimeters and therefore only require very light cryogenic machines to operate.

[0052] Since SQUIDs are very small, typically loops of a few μm in diameter, they are often used in conjunction with an antenna that functions as a flux concentrator. Such detectors can achieve field sensitivities of the order for fields in the kHz range. REFERENCES

[0053] Superconducting quantum interference device instruments and applications. R. L. Fagaly et all., Review of Scientific Instruments 77 101101 (2006).Electrochemical Impedance Spectroscopy (EIS): Principles, Construction, and Biosensing Applications. H. S. Magar et all., Sensors (Basel) 19 6578 (2021).SQUID-Based Magnetic Resonance Imaging at Ultra-Low Field Using the Backprojection Method. Q. Guo et all., Concepts in Magnetic Resonance part. B, Magnetic resonance engineering 8882329 (2020).Progress toward a deployable SQUID-based ultra-low field MRI system for anatomical imaging. M. A. Espy et all., IEEE transactions on applied superconductivity, 25 no. 3, 1601705 (2015).MRI of the human brain at 130 microTesla. B. Inglis et all., PNAS 110 no. 48, 19194-19201 (2013).Real-time 3D imaging of microstructure growth in battery cells using indirect MRI.

[0054] A.J. Ilott et all., PNAS 113 (39) 10779-10784 (2016).Rechargeable lithium-ion cell state of charge and defect detection by in-situ inside- out magnetic resonance imaging. A. J. Ilott et all., Nature Communications 9 1776 (2018).Distortion-free inside-out imaging for rapid diagnostics of rechargeable Li-ion cells.

[0055] K. Romanenko et all., PNAS 116 (38) 18783-18789 (2019).Diagnosing current distributions in batteries with magnetic resonance imaging. M. Mohammadi et all., J. Mag. Res. 309 106601 (2019).Sensitive magnetometry reveals inhomogeneities in charge storage and weak transient internal currents in Li-ion cells. Y. Hu et all., PNAS 117 (20) 10667-10672 (2020).Observation of memory effects associated with degradation of rechargeable lithium- ion cells using ultrafast surface-scan magnetic resonance imaging. K. Romanenko et all., J. Mater. Chem. A 9 21078-21084 (2021).S. Haber-Pohlmeier et all., Magnetic Resonance Microscopy : Instrumentation and applications in Engineering, Life Science, and Energy Research, chapters 17-18, Wiley-VCH, 2022. ISBN : 9783527827251.Three-dimensional characterization of electrodeposited lithium microstructures using synchrotron X-ray phase contrast imaging. D. S. Eastwood et all., Chem. Comunn. 51 266-268 (2015).Three-dimensional high resolution X-ray imaging and quantification of lithium ion battery mesocarbon microbead anodes. F. Tariq et all., J. of Power Sources 248 1014-1020 (2014).Characterization of the 3-dimensional microstructure of a graphite negative elec- trode from a Li-ion battery. P. R. Shearing et all., Eletrochem. Comunn. 12-3 374-377 (2010).Multi Length Scale Microstructural Investigations of a Commercially Available Li- Ion Battery Electrode. P. R. Shearing et all., J. Electrochem. Soc. 159-7, 1023-1027 (2012).Local Tortuosity Inhomogeneities in a Lithium Battery Composite Electrode. D. Kehrwald et all., J. Electrochem. Soc. 158-12 1393 (2011).In-operando high-speed tomography of lithium-ion batteries during thermal run- away. D. P. Finegan et all., Nat. Comm. 6 6924 (2015).Non-invasive battery analysis via micro-computed tomography. E. L. Ballard et all., US7902518B2 (2008).In-situ battery diagnosis method using electrochemical impedance spectroscopy. R. Mingant et all., US8849598B2 (2010).Method for determining an aging condition of a battery cell by means of impedance spectroscopy. J. Ziegler et all., US20120019253A1 (2009).Methods and apparatus for battery testing. J. A. Tinnemeyer et all., US20110074432A1 (2009).Electrochemical impedance spectroscopy in battery management system. CN107076801A (2015).A kind of EIS method for fast measuring of lithium ion battery. CN106970266A (2016).https: / www.ieco.fi / index.php?k=10909https: / www.shicryogenics.com / product / rp-082B2S-4K-pulse-tub e-cryocooler-series / https: / www.pure-devices.com / index.php / products / products-research / produits-recherche lecteurl.htmlMoszle M et al : « SQUID-detected microtesla MRI in the presence of metal », Journal of Magnetic Resonance, vol.179,n°1 1 mars 2006, pages 146-151,XO024919553Klamor et al : « 7Li in situ 1D NMR imaging of a lithium ion battery »Phys.Chem.Chem.Phys., 2015, 17, 4458. Description des figures

[0056] Lais a schematic view of a SQUID detection system, implemented in an MRI imaging apparatus according to the invention;Lais an example of geometry of a flux concentrator used in a SQUID detection system implemented in an MRI imaging apparatus according to the invention;[FIG.] Lais a schematic view of a metallic component MRI imaging apparatus according to the invention. Detailed description

[0057] A SQUID detection system 10 comprises, with reference to the, a flux concentrator 2 comprising an excitation coil c and a pickup coil b surrounding a metallic component a such as a Li-ion electrochemical cell, a shielded transformer 3 connected at the output to an input inductance L i, of a SQUID 4 whose output signal is amplified by a 5 LNA amplifier (for “Low Noise Amplifier”) coupled to a 6 FLL phase-locked loop (for “Flux Locking Loop”) controlling an Lf power supply coil.

[0058] The SQUID can be made of niobium, magnesium diboride (MgB ) or any other superconducting material with a medium critical temperature, or even based on high temperature copper oxide.

[0059] The metallic component emits a signal which is captured by the pickup coil b which acts as a flux concentrator, and sent via the two terminals 20,21 – on which an impedance matching capacitor Ca is connected in parallel -, to the SQUID 4 via the input coil L i . Shielded transformer L1,L2 is used for impedance matching and ground isolation.

[0060] The excitation coil c is connected to an MRI console d. The signal output of the SQUID is amplified via the low-noise amplifier LNA 5, while the operating stability is ensured by the flux-locked loop FLL 6.

[0061] In this example the pickup coil is a saddle coil but gradiometric antennas are often preferred, like the one shown in.

[0062] In most SQUID MRI experiments, the pickup coil is a second-order surface gradiometer, as described in Fagaly et al. [1].

[0063] In the present invention, it is preferred to use volumetric geometries, which allow more signal to be captured while remaining robust to noise. To maintain a gradiometric configuration in order to remain robust to noise emitted by distant sources, the antenna is made up of two saddle coils wired in series with opposite currents. This particular geometry was the subject of a patent application filed on April 12, 2022 in the name of the same Applicant.

[0064] The pickup coil 2' comprises a first saddle-shaped coil c' surrounding a second saddle-shaped coil b'. The first coil c' comprises a first longitudinal conductor c1, 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 connected to a first output conductor 21 to the transformer 3.

[0065] The second coil b', concentric with the first coil c', comprises a first longitudinal conductor b1, 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, a fourth longitudinal conductor b7 connected to a second output conductor 20 towards the transformer 3.

[0066] We will now describe, with reference to the, an MRI imaging device 1 of a metallic component, such as a Li-ion electrochemical cell.

[0067] The MRI imaging apparatus 1 according to the invention comprises a flux concentrator 2'' arranged to receive a metallic component a, for example a chemical cell. The coil b produces a magnetic field B0 which polarizes the nuclei of interest in the metallic component a, creating energy levels separated by an energy hω0.

[0068] The 2'' flux concentrator also includes a gradient coil h used to produce the spatial resolution in the MRI image. This coil h is powered via a pair of 8c conductors by a stabilized current source c which allows to deliver a very regular current of the order, depending on the precise geometry of the coil, from a few A to a few tens of A.

[0069] The gradient coil b'' is powered by a gradient amplifier d, for example the XPA-175-350 model from IECO in Finland

[0025] . To keep the field very stable and homogeneous in the field of view, the field is continuously monitored with a fluxgate magnetometer probe e. Low-frequency fluctuations in the field are measured and a command is sent in real time to the current source, to keep the field variations within a typical drift of 50 ppm for the duration of the MRI acquisition.

[0070] The radiofrequency amplifier f powers the radiofrequency coil g, which can be for example of a birdcage geometry, to send a pulse B1 tuned to the magnetic resonance frequency ω0. The signal is then picked up by the detection system described in.

[0071] The pickup coil h sends the signal to the SQUID and the rest of the readout chain i inside a 4K cryostat j, cooled by a cryogenic cooler k, for example the Sumitomo model RP-082B2S

[0026] .

[0072] The output signal is then processed at room temperature by an analog-to-digital conversion module and an IRM console l, for example the drive-l model from Pure Devices

[0027] which interprets the time signal as an MRI image displayed on the screen m. The system of bias, excitation and detection coils and the metallic component a are all enclosed in a shielded chamber n composed of a metallic grid acting as a Faraday cage, shielding signals in the 10 kHz and above, and a layer of magnetic materials such as ferrites or mu-metal which shields the system from very low frequency noise below a few kHz. The present invention implements an antenna that allows to increase the sensitivity even further, approaching 0.1 This antenna has both a gradiometric and volumetric geometry that gathers the most magnetic flux of the sample while remaining very robust to noise. Document FR3117218 in the name of the present applicant describes the use of this antenna in the context of NMR and MRI. Thanks to this antenna and the increase in the measurement field from the usual 100μT to about 1 mT, it is possible to increase the SNR sufficiently to perform MRI acquisition without prepolarization and sufficient resolution and contrast for in-operando imaging of Li-ion cells.

[0073] The ultra-low field MRI method according to the invention is particularly suitable for performing a direct MRI image of a single Li-ion cell or a cell assembly. Compared to the ioMRI technique described previously, the direct MRI method allows to image the interior of the cell, with contrasts showing temporally resolved maps of the local SOC and SOH. Some possible strategies include: Li MRI sequences, to map the local lithiation state of the cathode, providing spatially resolved insights into the local SOC charge state; [7] 3C MRI sequences to study the SOC in the anode; [1] susceptibility-weighted MRI sequences could provide access to a spatially resolved map of the magnetic susceptibility, which, as shown by Illot et al. [5], is related to the local SOC and SOH.

[0074] The following are rough quantitative estimates of important device parameters: Acquisition time around 5 minutes for a fully resolved image, but it is possible to perform rapid sequences with less information in a few seconds. Spatial resolution in the millimeter range, most likely an in-plane resolution in the order of 1.0 to 2.5 mm. Volume of interest (field of view) up to 50 cm, possibly larger. This allows imaging a single cell or an assembly of cells with a single acquisition. Portable device on wheels, with the size of a small washing machine and weighing about 100 kg.

[0075] Possible applicationsAssessment of the state of charge SOC and state of health SOH of electrochemical cells or full batteries, consisting of cell assemblies, by providing a spatially resolved map of the SOC and SOHinoperando.Quality control of electrochemical cells.Quality control and monitoring of electronic chips and components.Defect detection in metallic structures in the following industries: automotive, construction, energy, defense and aerospace, transport infrastructure.Medical MRI of patients with metallic implants.

[0076] Of course, the invention is not limited to the examples which have just been described and many other embodiments can be envisaged without departing from the scope of the invention.

Claims

Magnetic resonance imaging (MRI) apparatus (1), arranged to perform imaging of an essentially metallic component, comprising:means (b'') for generating a polarization magnetic field intended to be applied to said essentially metallic component (a),RF radiofrequency means (g) for exciting said essentially metallic component (a),detection means (10) cooperating with said radiofrequency means (g), to deliver a magnetic resonance imaging (MRI) signal,means (l) for processing said MRI signal so as to deliver information characteristic of the state of said essentially metallic component (a),characterized in that the essentially metallic component (a) is subjected to a very weak field of less than 10 mT, and the detection means (10) comprise a pickup coil (h) magnetically coupled with the polarization means (b'') and the radiofrequency means (g) and operating as a flux concentrator,and a SQUID (superconducting quantum interference device) detector (4) arranged downstream of said pickup coil (h) via a transformer (3)., MRI imaging device (1) according to the preceding claim, characterized in that the polarization means (b''), the radiofrequency means (g) and the pickup coil (h) are enclosed in a shielded chamber (n). MRI imaging device (1) according to the preceding claim, characterized in that the SQUID detector (4) and at least part of the processing means (i) are arranged in a cryostat (j). MRI imaging apparatus (1) according to any one of the preceding claims, characterized in that the polarization means (b'') comprise a gradient coil. MRI imaging apparatus according to any one of the preceding claims, characterized in that the capture coil (2') has a volumetric gradiometric geometry. MRI imaging apparatus according to any one of the preceding claims, characterized in that the capture coil has a surface geometry, in particular a second-order surface gradiometric geometry. MRI imaging apparatus according to any one of the preceding claims, characterized in that the pickup coil (h) comprises means for receiving an electrochemical cell (a), in particular a Li-ion cell. MRI imaging apparatus according to any one of the preceding claims, characterized in that the pickup coil comprises means for receiving an electronic chip or an electronic component. MRI imaging apparatus according to any one of the preceding claims, characterized in that the pickup coil comprises means for receiving an industrial mechanical structure. MRI imaging apparatus according to any one of the preceding claims, characterized in that it is adapted to image a structure of a power plant. Method for magnetic resonance imaging an essentially metallic body component (a) containing at least one metallic part, implemented in the imaging apparatus according to any one of the preceding claims, comprising the following steps: generating a bias magnetic field intended to be applied to said essentially metallic component (a), exciting said essentially metallic component (a) by radiofrequency (RF) waves, detecting an MRI signal of response of said essentially metallic component to the radiofrequency excitation, processing said MRI signal thus detected, so as to deliver information characteristic of the state of said essentially metallic component (a), characterized in that the essentially metallic component is subjected to a very weak field of less than 10 mT, and the detection step comprises generating an induced signal captured by flux concentration in a pickup coil (h),and an application of this signal thus captured to a SQUID detector (4)., MRI imaging method according to the preceding claim, implemented for the characterization of an electrochemical cell (a). MRI imaging method according to the preceding claim, implemented for the characterization of a Lithium Ion cell comprising a plurality of lithium nuclei including isotope nuclei 7 Li, characterized in that it comprises a step for producing a spatially resolved image of said isotope nuclei 7 Li, and a step to estimate kernel densities 7 Li in said spatially resolved image. MRI imaging method according to the preceding claim, arranged to provide a map of the Li-Ion cell, representative of the state of charge (SOC) of said cell from the nuclei density estimates 7Li, characterized in that it further comprises, while said Li-Ion cell is inserted into the pickup coil to be imaged therein, a step for applying to the terminals of said cell a voltage wave of predetermined profile, a step for simultaneously measuring the current entering said cell, and a step for processing the current and voltage measurements so as to deliver an estimate of the capacity and state of charge of said Li-Ion cell. MRI imaging method according to one of the two preceding claims, arranged to provide a map of the Li-Ion cell, representative of the state of health (SOH) of said cell from the nucleus density estimates 7Li characterized in that it further comprises, while said cell is inserted into the pickup coil to be imaged therein, a step for applying to the terminals of said electrochemical cell a voltage wave of predetermined profile until reaching a maximum charge, a step for simultaneously measuring the current entering said electrochemical cell, a step for determining the effective maximum capacity of said electrochemical cell, and a step for estimating the state of health of said cell from the ratio between the effective maximum capacity thus determined and the initial maximum capacity of said Li-Ion cell.

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