DEVICE FOR IMPROVING THE PRECISION OF A BIOMAGNETIC IMAGE OF A PATIENT

Description

technical field

[0001] The present invention relates to equipment for improving the accuracy of a biomagnetic image and an associated method. Previous technique

[0002] Several medical imaging techniques exist today to obtain information about a part of a patient's body.

[0003] Magnetic resonance imaging, or MRI, is a medical imaging technique that provides two- or three-dimensional views of the inside of the body non-invasively with relatively high contrast resolution. The principle of MRI is based on the phenomenon of nuclear magnetic resonance, which involves the coupling between the magnetic moment of an atomic nucleus and an external magnetic field. MRI requires a powerful and stable magnetic field produced by a superconducting magnet, which magnetizes tissues by aligning their spin magnetic moments. Weaker oscillating magnetic fields, known as radiofrequency fields, are then applied to slightly alter this alignment and produce a precession phenomenon that results in a measurable magnetic signal. MRI is primarily used for imaging soft tissues (brain, muscles, heart, lungs, and internal organs) and tumors.

[0004] Biomagnetic imaging is a technique for imaging the magnetic fields generated by the human body. It allows for the measurement of magnetic fields induced by the electrical activity of different parts of the body in order to obtain biomagnetic data. Within biomagnetic imaging, magnetoencephalography (MEG) stands out. This MEG technique is used clinically in neurology as well as in cognitive neuroscience research, and it provides biomagnetic data of the MEG type. Another biomagnetic imaging technique is magnetocardiography (MCG). This MCG technique is used in cardiology. Because the magnetic fields measured are extremely weak, both MEG and MCG use equipment based on SQUID (Superconducting Quantum Interference Device) sensors cooled by a cryogenic fluid within a Dewar flask.In brain imaging, SQUID sensors are positioned a few centimeters from the patient's head and remain fixed in that position. This distance, imposed by the use of cryogenic fluid, degrades the quality of the measured signal. The patient's head can also move relative to these sensors, further compromising the accuracy of the MEG imaging. To address this issue, research is underway to perform magnetoencephalography using optically pumped magnetometers. All research in this area is based on alkali-alkali-alkali-alkali magnetometers. Another category of optically pumped magnetometers exists that uses helium. A support helmet is available that allows for adjusting the position of each magnetometer to be as close as possible to the patient's head.In the case of helium-pumped optical magnetometers, a self-localization procedure for these magnetometers directly provides their positions and orientations relative to one another on the patient's head. Document FR3056761 discloses a method for calibrating different helium-pumped optical magnetometers to enable self-localization.

[0005] Document US2017 / 352457A1 discloses a marking coil used with a magnetoencephalograph.

[0006] To locate active regions of the patient's body that generate electrical activity, such as the brain, it is necessary to fuse magnetoencephalography (MEG) data with anatomical data from magnetic resonance imaging (MRI) to obtain a combined image. This is done by registering the position of the MEG sensors (SQUIDs or alkaline optically pumped magnetometers) within the coordinate system used to reference the reconstructed volume from the MRI. Currently, the fusion of MEG and MRI data is performed in two steps. In the first step, specific points such as bony prominences, the nasion, and the right and left preauricular points are located using an external optical or electromagnetic system during the MEG data acquisition. This allows the position of the MEG sensors to be registered relative to the patient's head.In a second step, a 3D reconstruction of the anatomy is performed, with the same singular points identified on the MRI images. The singular points are thus located using two different methods: MRI and MEG.

[0007] However, the step of re-registering the position of the magnetometers relative to the position of the head and the step of merging MEG and MRI data are both sources of bias which impair the accuracy of functional localization in MEG.

[0008] Therefore, there is a need to improve this MEG / MRI data registration and fusion procedure using a device that allows for higher-precision medical imaging. Description of the invention

[0009] The present invention aims to address this need.

[0010] More specifically, the present invention proposes equipment that aims to improve the registration / fusion procedure of MEG / MRI or MCG / MRI data to obtain a medical image with greater accuracy.

[0011] A first object of the invention relates to equipment for improving the accuracy of a biomagnetic image of a patient. This equipment comprises an envelope adapted to be positioned on an anatomical part of the patient. The equipment includes a plurality of markers arranged on the envelope, said markers being adapted to create contrast when an image of the patient is acquired by magnetic resonance imaging. The equipment also includes at least five three-axis coils, each three-axis coil being adapted to emit a magnetic field, the magnetic fields of said three-axis coils being detectable by an array of optically pumped magnetometers during a biomagnetic imaging examination. These three-axis coils are arranged on the envelope such that when the equipment is positioned on the patient, each of these three-axis coils is located on a specific point of the patient.Furthermore, the three-axis coils and markers (of which there are multiple markers) are positioned in the same location on the casing so that when the casing is placed on the patient, specific points on the patient's body are detectable by magnetic resonance imaging (MRI) and biomagnetic imaging (BMI). The equipment includes supports, each support having a first receiving area adapted to receive one of the markers and a second receiving area adapted to receive one of the three-axis coils. The marker and the three-axis coil are thus positioned in the same location on the casing. The second receiving area includes a protrusion adapted to fit into an associated cavity on the three-axis coil. The protrusion comprises a main body and a pin extending from said main body. The protrusion acts as a guide for positioning the three-axis coil within the support.

[0012] The equipment is designed to be worn by the patient in both imaging modalities: biomagnetic and MRI. It therefore facilitates the processing of data obtained by these two medical imaging modalities. The three-axis coils are co-located with markers on the casing at the same singular points. This ensures optimal registration of MEG / MRI or MCG / MRI data to obtain a highly accurate combined medical image.

[0013] The three-axis coils and markers are positioned in a common holder attached to the equipment casing. This allows for precise, simple, and practical co-localization of these coils and markers at specific points on the patient.

[0014] The three-axis coil is also effectively held in the envelope support.

[0015] Furthermore, precise and reproducible orientation of the three axes of the coil is ensured.

[0016] In one particular embodiment, the casing is made of a soft, biocompatible silicone material.

[0017] In one particular embodiment, the marker comprises gadolinium and / or a plurality of superparamagnetic nanoparticles.

[0018] In one particular embodiment, each marker has the shape of a disc with a diameter less than or equal to 6 mm.

[0019] In one particular embodiment, the equipment is positioned on the patient's face or the equipment is positioned on the patient's torso.

[0020] Another object of the invention relates to a method for improving the accuracy of a biomagnetic image of a patient, said method comprising a step of applying equipment according to the preceding object to a part of said patient. This equipment comprises a plurality of three-axis coils, each three-axis coil being adapted to emit a magnetic field, the magnetic fields of said three-axis coils being adapted to be detectable by an array of optically pumped magnetometers during a biomagnetic imaging examination. The method also comprises a biomagnetic imaging examination step for obtaining biomagnetic data, said examination step comprising a step of self-referencing the position of the optically pumped magnetometers relative to said three-axis coils. In addition, the method comprises a magnetic resonance imaging examination step for obtaining MRI data.Finally, this process includes a step of merging biomagnetic data with MRI data by simply matching common positions acquired precisely during the biomagnetic imaging examination step and the MRI examination step.

[0021] This allows us to obtain a combined medical image of the affected part of the patient, eliminating the biases of registration and fusion of the previous art.

[0022] In one particular embodiment, the biomagnetic imaging examination is a magnetoencephalography examination for obtaining MEG data or a magnetocardiography examination for obtaining MCG data.

[0023] In a particular embodiment, optically pumped magnetometers are helium optically pumped magnetometers.

[0024] The present invention will be better understood upon reading the detailed description of embodiments taken as non-limiting examples and illustrated by the appended drawings in which: [ Fig 1 ] there figure 1 is a front view of a patient's face wearing the equipment according to a first application of the invention; [ Fig 2 ] there figure 2 is a profile view of the patient's face figure 1 ; Fig 3 ] there figure 3 is a front view of a bracket designed to be hung on the equipment of the figure 1 ; Fig 4 ] there figure 4 is a side view of the support of the figure 3 ; Fig 5 ] there Figure 5 is a top view of the support of the figure 3 ; Fig 6 ] there figure 6 is a front view of a patient's torso wearing the equipment according to a second application of the invention. Fig 7 ] there figure 7is a diagram illustrating the different steps in a process for obtaining a medical image of a patient wearing the equipment of figures 1 to 6 .

[0025] In the different figures, identical or similar elements bear the same references.

[0026] THE Figures 1 And 2 represent a patient's face wearing equipment 10 for obtaining a medical image of the patient's brain. Equipment 10 includes: an envelope 101; a plurality of markers 102, 102A, 102B, 102C, 102D, 102E arranged on the envelope; a plurality of three-axis reels 103A, 103B, 103C, 103D, 103E arranged on the envelope; supports 104A, 104B, 104C, 104D, 104E for receiving all or part of the markers 102, 102A, 102B, 102C, 102D, 102E and the three-axis reels 103A, 103B, 103C, 103D, 103E.

[0027] The envelope 101 is presented here in the form of a mask adapted to be positioned on the upper part of the patient's face 1 around their eyes so as to cover the singular points of interest for MEG imaging. The envelope 101 comprises a main part 1011, elastic straps 1012, and an adjustment device 1013. The main part 1011 is adapted to lie flat against the patient's face 1. This main part 1011 is configured to carry the plurality of markers 102, 102A, 102B, 102C, 102D, 102E and the plurality of three-axis coils 103A, 103B, 103C, 103D, 103E. The elastic straps 1012 are adapted to apply constraint to the envelope in order to adapt it to the shape of the patient's face. This allows the markers 102, 102A, 102B, 102C, 102D, 102E and the plurality of three-axis coils 103A, 103B, 103C, 103D, 103E to be kept as close as possible to the bony protrusions of the face (orbital arches, zygomatic arches, etc...).Furthermore, the main part 1011 of the nasal sheath 101 is held taut by elastic cords 1012. In one particular embodiment, the sheath 101 includes two elastic cords 1012 that allow the sheath 101 to be tightened at the back of the patient's head. One elastic cord passes, for example, over one ear and the second elastic cord passes under the other ear. The adjustment device 1013 is adapted to adjust and hold the elastic cords 1012 in place at the back of the head. This improves patient comfort during magnetic resonance imaging (MRI) examinations, as the patient is in a supine position for this examination. This adjustment device 1013 is, for example, of the same type as a conventional adjustment device used for a backpack. Alternatively, the cords 1012 are not elastic. These are simple cords that are adjusted using the 1013 adjustment device.

[0028] The main part 1011 and the elastic laces 1012 of the casing 101 are made of a soft silicone material, for example, 50 shore translucent talc-coated silicone. This material is notably biocompatible and MRI-compatible.

[0029] In an alternative embodiment, the envelope 101 is of the diving mask type.

[0030] Markers 102, 102A, 102B, 102C, 102D, and 102E are designed to create contrast when acquiring images of the patient's face while wearing the 101 protective cover during magnetic resonance imaging (MRI). These markers are in the form of a small disc with a diameter of 6 mm or less. For example, these discs are MRI pinpoint type discs, ref. 187, from the supplier Beekley Medical®. In one particular embodiment, markers 102, 102A, 102B, 102C, 102D, and 102E contain gadolinium. Alternatively, markers 102, 102A, 102B, 102C, 102D, and 102E contain, in addition to or instead of gadolinium, a plurality of superparamagnetic nanoparticles. "Superparamagnetic nanoparticles" refers to a ferromagnetic or ferrimagnetic material with nanometer-sized grains. For example... Figures 1 And 2The device contains more than twenty markers (102, 102A, 102B, 102C, 102D, 102E) evenly distributed across the main portion (1011) of the envelope (101). Specifically, markers 102, 102A, 102B, 102C, 102D, and 102E are located on the orbital rims, zygomatic arches, and a bony ala of the nose. This allows for more precise localization of the mask on MRI images.

[0031] The three-axis coils 103A, 103B, 103C, 103D, and 103E are adapted to emit a magnetic field. A "three-axis coil" is defined as a coil adapted to emit magnetic fluxes along three axes perpendicular to each other. Such a coil is, for example, a Helmholtz coil. The three-axis coils 103A, 103B, 103C, 103D, and 103E are shown here in small dimensions to fit within the casing 101. They are represented by crosses on the figure 1The magnetic field emitted by the three-axis coils is detectable by an array of optically pumped magnetometers during a magnetoencephalography examination. The optically pumped magnetometers are small capsules arranged in a helmet-like configuration (not shown in the diagram). Figures 1 And 2 ) adapted to the upper part of the patient's head. Such a helmet can contain 20 to 200 evenly distributed magnetometers. Optically pumped magnetometers are capable of measuring the intensity and / or direction of a magnetic field. Each magnetometer includes an internal element sensitive to such a magnetic field. This internal element, in conjunction with an electronic device, allows the magnetic field measurement to be extracted.

[0032] In the implementation of Figures 1 And 2The envelope 101 comprises five three-axis coils, labeled 103A, 103B, 103C, 103D, and 103E. The first three-axis coil, 103A, is placed on the bridge of the patient's nose. The second three-axis coil, 103B, is placed on the patient's forehead. The third three-axis coil, 103C, is placed on the patient's right cheek. The fourth three-axis coil, 103D, and the fifth three-axis coil, 103E, are placed at the left and right preauricular points (in front of the ears). The second three-axis coil, 103B, and the third three-axis coil, 103C, improve the alignment accuracy of the optically pumped magnetometers. In addition, the third three-axis 103C coil allows, in magnetic resonance imaging, for the error-free identification of right from left, regardless of the radiological or neurological convention used to view the MRI images.Additional three-axis coils 103 are positioned at other specific points on the patient's face, such as the orbital rims, zygomatic arches, or the ala of the nose. These additional three-axis coils further improve the accuracy of registering the acquired MEG and MRI data. In one particular embodiment, the equipment 10 comprises at least five three-axis coils.

[0033] The 104A, 104B, 104C, 104D, and 104E holders are designed to accommodate 102A, 102B, 102C, 102D, and 102E markers and 103A, 103B, 103C, 104D, and 104E three-axis reels. As an example, the 104A holder is shown in the following diagram: figures 3 to 5 This 104A support includes: a rear part 1044; a front part 1045; an intermediate part 1046.

[0034] The rear part 1044 is adapted to attach to the envelope 101 in order to hold the support 104. This rear part 1044 is a disc with a diameter greater than 6 mm.

[0035] The front part 1045 is designed to hold one of the three-axis reels in the holder 104A. This front part 1045 has a semi-ring shape with the same diameter as the rear part 1044. This semi-ring shape allows the marker and the three-axis reel to be held in the holder 104A.

[0036] The intermediate part 1046 is positioned between the rear part 1044 and the front part 1045 of the support 104A.

[0037] The rear portion 1044, the front portion 1045, and the intermediate portion 1046 define the spaces in which the three-axis coil and its associated support will be housed. Thus, the support 104A has a first receiving zone 1041 and a second receiving zone 1042. The first receiving zone 1041 is adapted to receive a marker 102A. This first receiving zone 1041 is delimited by the rear portion 1044 of the support 104A and by a protrusion 1043. The second receiving zone 1042 is delimited by the marker 102A and by the front portion 1045. The protrusion 1043 is adapted to fit into an associated cavity in the three-axis coil. On the Figure 5The protrusion 1043 comprises a main body 10431 and a pin 10432. The main body 10431 secures the three-axis coil within the support, preventing its rotation. The pin 10432 extends from the main body 10431. Its purpose is to prevent errors in the placement of the three-axis coil within the support 104, ensuring that the coil's axes are always positioned in the same way. The above description also applies to the other supports 104B, 104C, 104D, and 104E.

[0038] For example, the figure 1 The equipment 10 comprises five supports 104A, 104B, 104C, 104D, 104E. Alternatively, the equipment 10 comprises more than five supports. These additional supports are, for example, positioned at the crosses 105 of the equipment 10 visible at Figures 1 And 2 .

[0039] There figure 6Figure 2 illustrates the torso of the patient wearing the equipment 10 in a second application of the invention. In this embodiment, the equipment 10 comprises a casing 101 in the form of a chest vest for biomagnetic imaging of the heart with localization of foci causing cardiac arrhythmias. In this second application of the invention, the aim is to register the magnetocardiography data with the 3D MRI data of the heart. To this end, markers 102 are regularly distributed on the casing 101. These markers 102 form a cross whose center is located between the patient's two pectoral muscles. On an upper part of this cross, more precisely at the top of the patient's sternum, the casing includes a first support 104A. This support 104A is adapted to carry a marker 102A as well as a three-axis coil 103A. Other supports 104B, 104C are also positioned on the patient's clavicles.Another 104D support is positioned on the left pectoral muscle. Equipment 10 is adapted here to obtain a combined image of the patient's heart. In order to detect any potential dysfunctions of this heart, it is possible to successively perform a magnetocardiography examination to obtain CGM data and a magnetic resonance imaging examination to obtain MRI data.

[0040] It should be noted that in this second application of the invention, it is not necessary to equip the patient with a helmet carrying optically pumped magnetometers. These optically pumped magnetometers will be worn directly on the chest vest.

[0041] It should also be noted that the patient's back may include a plurality of supports carrying markers and three-axis coils to perform MCG and MRI examinations in order to improve the accuracy of the imaging.

[0042] There figure 7illustrates the different stages of a process for improving the accuracy of a biomagnetic image of the patient wearing the equipment of figures 1 to 2 for magnetoencephalography or the equipment of the figure 6 for a magnetocardiogram.

[0043] In the first step E1, the device 10 is positioned on the patient. If information about the brain is to be obtained, this device 10 is positioned on the patient's face 1. Alternatively, if the heart is the subject of the examination, the device 10 is positioned on the patient's torso 2.

[0044] In a second step E2, a helmet containing the optically pumped magnetometers is positioned on the patient's head. In the case of magnetocardiography, the optically pumped magnetometers are positioned on the patient's torso in the form of an array.

[0045] In a third step E3, the three-axis reels 103A, 103B, 103C of equipment 10 are activated.

[0046] In a fourth step E4, the positions of the optically pumped magnetometers are determined relative to the three-axis coils. These positions are determined without any external system. Since the three-axis coils are located at specific points on the patient, the optically pumped magnetometers are then automatically positioned relative to these specific points. The self-localization of the position of the optically pumped magnetometers relative to the position of these three-axis coils is thus achieved. It should be noted that in a preferred embodiment of the invention, the magnetometers are helium-pumped optically pumped magnetometers.

[0047] In a fifth step E5, MEG or MCG data are acquired during the magnetoencephalography or magnetocardiography examination. These data streams are recorded during several recording sequences.

[0048] In a sixth step E6, a magnetic resonance imaging (MRI) scan is performed to obtain MRI data (the three-axis coils are previously removed from their supports). The markers placed on the equipment 10 remain in place and allow the same singular points to be located as those identified by the MEG or MCG scans using the coils.

[0049] In a seventh step, E7, the MEG or CGM data and the MRI data are processed and fused by matching the coordinates of common singular points acquired in both modalities. A combined image can then be obtained that can be used by a clinician. This combined image allows visualization of the variations in electrical fields reconstructed from magnetoencephalography or magnetocardiography within the volume generated by the magnetic resonance imaging examination.

[0050] The equipment that is the subject of the invention offers the following advantages: It allows for the provision of a mask or vest-type support enabling the positioning, in common supports, of three-axis emitting coils and gadolinium pellets visible on MRI images; it allows for the exploitation of the measurement of the magnetic field emitted by the three-axis coils, by optically pumped magnetometers present on the head or on the thorax of the patient; it allows for the exploitation of the co-localization of the three-axis coils visible in MEG or in CGM and the gadolinium pellets visible in MRI, due to their common support, for the registration of MEG / MRI or CGM / MRI data.The use of common supports for the transmitting coils and gadolinium pellets ensures optimal registration between MEG / MCG and MRI data since the same points will be exactly located with both techniques; the measurement of the magnetic field emitted by the three-axis coils allows self-localization of each sensor with a helium-pumped optical magnetometer; the use of a mask fitted to the patient's face or a vest on the patient's chest, usable in both MEG / MCG (with the three-axis coils allowing localization) and MRI (with the gadolinium pellets) greatly limits sources of bias during MEG / MCG registration with MRI since the position of the coils and gadolinium pellets is identical; the equipment does not require an external scanning system.The helium-pumped optical magnetometers placed on the patient's head allow for the measurement and localization of the magnetic field emitted by the three-axis coils. The gadolinium pellets are visualized directly on the MRI images; the patient wears their mask or chest vest during both types of MEG / MRI and MCG / MRI recording.

[0051] The invention is not limited to the embodiments and variants shown, and other embodiments and variants will be obvious to a person skilled in the art.

[0052] Therefore, different sizes of equipment are considered in order to adapt to all age groups (baby, child, adult).

[0053] Therefore, the dimensions of the supports can vary depending on the type of three-axis reel and the size of the markers.

Claims

1. Device for improving the precision of a biomagnetic image of a patient, said device (10) comprising: - a covering (101) suitable for being positioned on an anatomical part of the patient; - a plurality of markers (102, 102A, 102B, 102C, 102D, 102E) disposed on the covering (101), said markers (102, 102A, 102B, 102C, 102D, 102E) being suitable for creating a contrast during an obtaining of an image of the patient, done by magnetic resonance imaging; - at least five three-axis coils (103A, 103B, 103C, 103D, 103E), each three-axis coil being suitable for emitting a magnetic field, the magnetic fields of said three-axis coils (103A, 103B, 103C, 103D, 103E) being able to be detected by a set of optical pumping magnetometers during an examination by biomagnetic imaging (MEG, MCG), said three-axis coils (103A, 103B, 103C, 103D, 103E) being disposed on the covering (101), such that when the device (10) is positioned on the patient, each three-axis coil (103A, 103B, 103C, 103D, 103E) is located on a singular point of the patient, the three-axis coils (103A, 103B, 103C, 103D, 103E) and the markers (102A, 102B, 102C, 102D, 102E) of the plurality of markers (102, 102A, 102B, 102C, 102D, 102E) are placed at the same location on the covering so that, when said covering (101) is positioned on the patient, singular points of the part of the patient can be detected by magnetic resonance imaging and by biomagnetic imaging (MEG, MCG), in which said device (10) comprises supports (104A, 104B, 104C, 104D, 104E), each support (104A, 104B, 104C, 104D, 104E) having a first receiving zone (1041), suitable for receiving one of the markers (102A, 102B, 102C, 102D, 102E), and a second receiving zone (1042), suitable for receiving one of the three-axis coils (103A, 103B, 103C, 103D, 103E), said marker and said three-axis coil thus being placed at the same location on the covering (101), in which the second receiving zone (1042) comprises a protrusion (1043), suitable for being housed in an associated cavity of the three-axis coil (103A, 103B, 103C, 103D, 103E), characterised in that the protrusion (1043) comprises a main body (10431) and a pin (10432) extending from said main body (10431), said protrusion (1043) having a locking tab role, for the positioning of the three-axis coil (103A, 103B, 103C, 103D, 103E) in the support (104A, 104B, 104C, 104D, 104E).

2. Device according to claim 1, wherein the covering (101) is made of a flexible biocompatible silicone material.

3. Device according to any one of claims 1 to 2, wherein the markers (102, 102A, 102B, 102C, 102D, 102E) comprise gadolinium and / or a plurality of superparamagnetic nanoparticles.

4. Device according to any one of claims 1 to 3, wherein each marker (102, 102A, 102B, 102C, 102D, 102E) has the shape of a dot having a diameter less than or equal to 6mm.

5. Device according to any one of claims 1 to 4, wherein said device (10) is suitable for being positioned on a face (1) of the patient, or wherein said device (10) is suitable for being positioned on a torso (2) of the patient.

6. Method for improving the precision of a biomagnetic image of a patient, said method comprising: - a step (E1) of applying a device (10) according to any one of claims 1 to 5 on a part of said patient, said device (10) comprising a plurality of three-axis coils (103A, 103B, 103C, 103D, 103E), each three-axis coil (103A, 103B, 103C, 103D, 103E) being suitable for emitting a magnetic field, the magnetic fields of said three-axis coils being able to be detected by a set of optical pumping magnetometers during a biomagnetic imaging examination (MEG, MCG); - a step (E4) of self-referencing the position of the optical pumping magnetometers with respect to said three-axis coils (103A, 103B, 103C, 103D, 103E); - a step (E5) of examining, by biomagnetic imaging (MEG, MCG), for obtaining biomagnetic data (MEG data, MCG data); - a step (E6) of examining, by magnetic resonance imaging, for obtaining MRI data; - a step (E7) of merging biomagnetic data (MEG data, MCG data) with MRI data by simple matching of common positions precisely acquired during the biomagnetic imaging examination step and the MRI examination step.

7. Method according to claim 6, wherein the biomagnetic imaging examination is a magnetoencephalography examination for obtaining MEG data or a magnetocardiography examination for obtaining MCG data.

8. Method according to any one of claims 6 or 7, wherein the optical pumping magnetometers are helium optical pumping magnetometers.