Method for imaging a part of interest of a subject by means of a magnetic-resonance-imaging device

Abstract

The invention relates to a method for imaging a part of interest of a subject by means of a magnetic-resonance-imaging device (1), the magnetic-resonance-imaging device (1) comprising a permanent magnet (2) configured to apply a main magnetic field B0 to an analysis area, and gradient coils (6) configured to apply spatial encoding to the analysis area, the method comprising successively executing the following steps: a) a step of positioning a radio-frequency coil (7) around the part of interest, the radio-frequency coil (7) defining a volume of interest; b) a step of placing the part of interest in the analysis area, the placing step being random; d) a step of measuring the non-uniformity of the main magnetic field B0 in the volume of interest; e) a step of acquiring magnetic-resonance data; f) a step of processing the magnetic-resonance data in order to construct an image of the part of interest, the processing step being executed based on the measurement of the non-uniformity of the main magnetic field B0.

Description

Method for imaging a part of interest of a subject using a magnetic resonance imaging device FIELD OF INVENTION

[0001] The present invention relates to the field of magnetic resonance imaging. In particular, the present invention concerns a magnetic resonance imaging method. More specifically, the magnetic resonance imaging method according to the present invention implements a sequence that accounts for the random positioning of the subject, and in particular of the subject equipped with the radiofrequency coil, said radiofrequency coil being intended to emit excitation signals and collect relaxation signals. The present invention notably enables the processing of data collected by the radiofrequency coil, taking into account the inhomogeneities of the magnetic field as well as the non-linearities of the gradient coils intended to spatially encode an analysis area in which the subject or a part of interest of the subject is positioned. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] Magnetic resonance imaging (MRI) is now widely used to image the internal structures of bodies, particularly human bodies, in a non-invasive manner. Specifically, MRI allows for the examination of hydrogen nuclei, and in particular their nuclear spin, within water molecules that make up part of the body under examination.

[0003] In this regard, an MRI device is equipped with a magnet intended to impose a static magnetic field (called "main magnetic field") on the body, under the effect of which the nuclear spins associated with the hydrogen nuclei contained in the water molecules forming part of this body become polarized.

[0004] In particular, the magnetic moments associated with these spins preferentially align along an axis, called the z-axis, determined by the orientation of the main magnetic field in order to create a magnetization of the body.

[0005] An MRI device also includes gradient coils configured to produce small-amplitude, spatially varying magnetic fields when a current is applied to them. More specifically, the gradient coils are designed to produce a magnetic field component that is aligned parallel to the main magnetic field and whose amplitude varies with position along one of the x, y, or z axes (the x, y, and z axes being pairwise perpendicular).

[0006] Thus, the combined effects of the magnetic fields imposed by the gradient coils make it possible to spatially encode each of the positions of the body intended to be probed.

[0007] An MRI device also includes at least one radio frequency (RF) coil intended to act as an RF transmitter-receiver. Specifically, at least one radio frequency coil is configured to emit RF energy pulses at a frequency equal to or close to the resonance frequency of hydrogen nuclei's spins, and which is at least partially absorbed by these nuclei.

[0008] As soon as the RF emission is interrupted, the nuclear spins relax to return to their initial energy state and in turn emit an RF signal that can be collected by at least one RF coil. This RF signal is then processed using a computer and reconstruction algorithms to obtain an image of the body.

[0009] The main magnetic field, generally between 1.5 Tesla and 3 Tesla, makes it possible to achieve relatively reasonable signal-to-noise ratios and consequently to form images of the human body of sufficient quality for durations on the order of a minute or more.

[0010] However, there are circumstances in which it is not possible to implement a primary magnetic field of such intensity. Portable MRI devices are one example. These devices generally include a permanent magnet or electromagnets of limited capacity and cannot impose a primary magnetic field with an intensity exceeding 50 mT, or even 60 mT, or 200 mT, without significantly increasing the mass or size of the MRI device.

[0011] However, so-called low-intensity permanent magnets generate a magnetic field with relatively significant spatial inhomogeneity, particularly with regard to the spatial encoding imposed by gradient coils. This limitation in terms of the main magnetic field strength directly affects the performance of the MRI device and, in particular, generates distortions in the MRI images that can be obtained.

[0012] Furthermore, gradient coils, when the position is close to the edge of the coil, generate non-linear gradient fields (spatial coding field) which also affect the quality of MRI images and make their interpretation complicated.

[0013] One aim of the present invention is to propose a magnetic resonance imaging method enabling correction of magnetic field inhomogeneities and, where appropriate, non-linearity of gradient fields. BRIEF DESCRIPTION OF THE INVENTION

[0014] The object of the present invention is achieved by a method of imaging a part of interest of a subject using a magnetic resonance imaging device, the magnetic resonance imaging device comprising a permanent magnet configured to impose a principal magnetic field B0 in an analysis area, and gradient coils configured to impose spatial coding in the analysis area, the method comprising the successive execution of the following steps:

[0015] a) a step of positioning a radio frequency coil around the part of interest, the radio frequency coil defining a volume of interest;

[0016] b) a step of placing the part of interest in the analysis area, the placement step being random;

[0017] d) a step of measuring the inhomogeneity of the main magnetic field B0 in the volume of interest;

[0018] e) a magnetic resonance data acquisition step;

[0019] f) a magnetic resonance data processing step to construct an image of the part of interest, the processing step being performed on the basis of the inhomogeneity measurement of the main magnetic field B0.

[0020] According to one embodiment, said method also includes a step c) of determining an operating frequency f0 of the radio frequency coil, advantageously step c) is carried out between steps b) and d).

[0021] According to one embodiment, the area of ​​interest includes the head of a human subject, and the radio frequency coil is formed on a helmet such that step a) includes positioning said helmet on the head.

[0022] According to one implementation method, step b) of placing the part of interest is carried out without recourse to a guidance system and / or a means of securing the radio frequency coil to the permanent magnet.

[0023] According to one implementation, step d) of measuring the inhomogeneity of the main magnetic field B0 comprises two measurements, referred to respectively as the first measurement and the second measurement, and a processing sequence. Each of the two measurements comprises the acquisition of magnetic resonance data according to a predefined excitation and data collection sequence. The data collection is defined by a time called the echo time. The echo time of the second measurement is different from the echo time of the first measurement. The processing sequence comprises the execution of the following steps:

[0024] 1) the calculation of an initial map of a phase linked to the first measurement;

[0025] 2) the calculation of a second map of a phase linked to the second measurement;

[0026] 3) the calculation of a map of the inhomogeneity of the main magnetic field based on the difference between the first map and the second map.

[0027] According to one implementation mode, during the execution of step d) gradient coils are implemented to impose spatial coding in the analysis area.

[0028] According to one implementation method, step f) of data processing solves the problem given by the equation:

[0029] Step f) of data processing addresses the solution to the problem given by the equation:

[0030] where r i is a position within the analysis area, B(r i ) is the deviation of the magnetic field from an average magnetic field in the analysis area and at position r i , ki is a component of a position vector in space K, t j is a time, x(r i ) is a component of the image at position r i in real space, y(k j ) is the measure in space K at position k j .

[0031] According to one implementation method, the execution of step f) includes the implementation of one of the following methods chosen from: conjugate phase reconstruction, deep learning-based reconstruction.

[0032] The invention also relates to a magnetic resonance imaging device configured for the implementation of the imaging method according to the present invention, in which said imaging device comprises a computer equipped with a computer program which, when implemented, executes steps c), d), e) and f).

[0033] The invention also relates to a magnetic resonance imaging device configured for the implementation of the imaging method according to the present invention, in which said imaging device comprises a computer equipped with a computer program which, when implemented, executes steps d), e) and f).

[0034] According to one embodiment, said imaging device includes a permanent magnet in a Hallbach configuration.

[0035] Other features and advantages of the invention will become apparent from the detailed description that follows, with reference to the accompanying figures in which:

[0036] This is a schematic representation according to an exploded view of a magnetic resonance imaging device that can be implemented within the framework of the present invention;

[0037] This is an initial mapping of a phase linked to the first measurement;

[0038] This is a second mapping of a phase linked to the second measurement;

[0039] This is a mapping of the inhomogeneity of the main magnetic field based on the difference between the first mapping and the second mapping;

[0040] Laest is an image of a cylindrical phantom obtained using a resonance imaging device and without taking into account the inhomogeneity of the magnetic field;

[0041] Laest is an image of a cylindrical phantom obtained by means of a resonance imaging device and taking into account the inhomogeneity of the magnetic field according to the principles of the present invention;

[0042] : image of the phantom positioned at the center of the magnet, obtained without correction for the inhomogeneity of the magnetic field;

[0043] : image of the phantom positioned at the center of the magnet, obtained with correction of the inhomogeneity of the magnetic field according to the process of the invention;

[0044] : representation of the first inhomogeneity of the magnetic field, determined when the phantom is positioned at the center of the magnet;

[0045] : image of the phantom positioned at a distance from the center of the magnet, obtained without correction for the inhomogeneity of the magnetic field;

[0046] : image of the phantom positioned at a distance from the center of the magnet, obtained with correction of the inhomogeneity of the magnetic field according to the process of the invention;

[0047] : representation of the second inhomogeneity of the magnetic field, determined when the phantom is positioned at a distance from the center of the magnet;

[0048] : image of the phantom corresponding to the data of the, but obtained taking into account the second inhomogeneity of the magnetic field. DETAILED DESCRIPTION OF THE INVENTION

[0049] The present invention relates to a method for imaging a part of interest of a subject, particularly a human or animal body. Specifically, the imaging method employs a resonance imaging device, more particularly a low-field resonance imaging device. "Low field" means a primary magnetic field with an intensity of less than 200 mT, advantageously less than 100 mT, and even more advantageously less than 50 mT.

[0050] Although described primarily in connection with low-field MRI, the invention is applicable to any type of MRI system.

[0051] A primary magnetic field of such intensity generally exhibits significant inhomogeneity. In particular, this magnetic field inhomogeneity impairs the quality of the images obtained and compromises their interpretation.

[0052] For example, for a main magnetic field of approximately 50 mT, an inhomogeneity of about 3 kHz can be observed in the analysis area defined by the magnet generating said main magnetic field for an excitation frequency of approximately 2 MHz by a radio frequency coil. Furthermore, since gradient coils spatially encode the analysis area in a frequency range of approximately 25 kHz, this inhomogeneity of the main magnetic field remains significant.

[0053] Thus, la is a schematic representation of a magnetic resonance imaging device 1 that can be implemented within the framework of the present invention.

[0054] It is noted that the configuration of the imaging device 1 is given only as an example and is therefore not such as to limit the scope of the present invention.

[0055] The imaging device 1 includes a magnet, and in particular a permanent magnet 2. The permanent magnet 2 can in particular extend along an elongation axis z.

[0056] More specifically, the permanent magnet 2 defines a housing 3 opening through a first opening 4 and a second opening 5 opposite each other along the elongation axis z.

[0057] In this respect, the permanent magnet 2 is arranged to allow the insertion of a body, and more particularly a human body, into the housing 3 through the first opening 4 along the elongation axis z.

[0058] The permanent magnet 2 is more specifically configured to impose a static magnetic field oriented along an axis perpendicular to the elongation axis z, in a zone, called the analysis zone, of the housing 3.

[0059] In this respect, the permanent magnet 2 may comprise an assembly of elementary magnets, particularly arranged in series of Hallbach rings. Document EP3368914B1 provides an example. However, the invention is not limited to the configuration described in that document.

[0060] As an example, the permanent magnet 2 is configured to impose a static magnetic field of an amplitude of less than 100 mT, advantageously, less than 65 mT, further advantageously less than or equal to 50 mT.

[0061] The imaging device 1 also includes a set of gradient coils 6. The gradient coils 6 are specifically configured to produce small-amplitude, spatially varying magnetic fields when a current is applied to them.

[0062] More specifically, gradient coils 6 are designed to produce a magnetic field component that is aligned parallel to the static magnetic field.

[0063] Thus, the combined effects of the magnetic fields imposed by the gradient coils 6 allow for the spatial encoding of signals from a body located in housing 3 and intended for probing. This spatial encoding is manifested, in particular, by a variation in the resonance energy of the nuclear spins of the hydrogen nuclei within the body to be probed and located in the analysis zone. In other words, the nuclear spins of the hydrogen nuclei are subjected to a magnetic field that differs from one position to another. Generally, the spatial encoding varies non-linearly with position along the x, y, or z axes (the x, y, and z axes form an orthogonal coordinate system). In other words, the magnetic fields imposed by the gradient coils vary non-linearly along the x, y, or z axes (the x, y, and z axes form an orthogonal coordinate system).This latter aspect, along with the inhomogeneity of the main magnetic field, contributes to image distortion.

[0064] The imaging device 1 further includes a radio frequency (RF) coil 7 intended to act as an RF transmitter-receiver. In particular, at least one radio frequency coil 7 is configured to emit RF energy pulses of a frequency equal to or close to the resonance frequency of the spins of hydrogen nuclei and which is at least partially absorbed by these nuclei.

[0065] The radio frequency coil 7 can be connected to pulse means 10 configured to impose the flow of a current in said radio frequency coil 7. More specifically, the pulse means 10 can be configured to allow the generation of current pulses in the radio frequency coil 7. The pulse means are also advantageously implemented to power the gradient coils 6 in order to spatially encode each of the positions of a body that may be present in the housing 3.

[0066] The radio frequency coil can also be connected to radio frequency processing means 11 configured to process a radio frequency signal that can be received by the assembly formed by the radio frequency coil 7.

[0067] The imaging device 1 may include a first interface 12 ensuring a link, on the one hand, between the pulse means 10 and the radio frequency coil 7, and on the other hand, between radio frequency processing means 11 and the radio frequency coil 7.

[0068] The imaging device 1 may, in addition, include a second interface 13 providing a link between the pulse means 10 and the gradient coils 6.

[0069] The pulse means 10, the radio frequency processing means 11, the first interface 12 and the second interface 13 can be controlled by a control unit 14, for example a computer 14.

[0070] The present invention comprises a method for imaging a part of interest of a subject using a magnetic resonance imaging device, the magnetic resonance imaging device comprising a permanent magnet configured to impose a principal magnetic field B0 in an analysis area, and gradient coils configured to impose spatial coding in the analysis area, the method comprising the successive execution of the following steps:

[0071] a) a step of positioning a radio frequency coil around the part of interest, the radio frequency coil defining a volume of interest;

[0072] b) a step of placing the part of interest in the analysis area, the placement step being random;

[0073] d) a step of measuring the inhomogeneity of the main magnetic field B0 in the volume of interest;

[0074] e) a magnetic resonance data acquisition step;

[0075] f) a magnetic resonance data processing step to construct an image of the part of interest, the processing step being performed on the basis of the inhomogeneity measurement of the main magnetic field B0.

[0076] Advantageously, the imaging method may also include performing a step (c) of placing the part of interest in the area of ​​analysis, the placement step being random. This step (c), without limiting the invention to this aspect, may, for example, be performed between steps (b) and (d).

[0077] This method is particularly advantageous because it allows, regardless of the positioning of the area of ​​interest within the analysis zone, and especially the area of ​​interest around which the radio frequency coil is positioned, the formation of an image essentially free of distortions. In particular, this method eliminates, or at least reduces, the effect due to the inhomogeneity of the main magnetic field and the non-linearity of the magnetic fields imposed by the gradient coils.

[0078] In particular, the present invention is of special interest when imaging a subject's head using a magnetic resonance imaging (MRI) device in which the permanent magnet forms a tunnel delimiting the area of ​​analysis. Indeed, such a configuration does not always allow the subject's area of ​​interest to be positioned within the analysis area in a repeatable manner. Specifically, the positioning of the area of ​​interest can depend on the subject's body size, particularly the width of their shoulders, and / or the length of their neck. Thus, the random positioning of the area of ​​interest makes it impossible to assume a fixed position for the coil within the tunnel.

[0079] The present invention is also of interest when it comes to imaging the limbs and / or extremities of the human body. Indeed, the positioning of a limb within the analysis area is not reproducible from one measurement to another, so interpretable images cannot be obtained without taking into account magnetic field inhomogeneities and / or non-linearity of the gradient coils.

[0080] The imaging method according to the present invention therefore includes the execution of a step a) of positioning a radio frequency coil around the part of interest, the radio frequency coil defining a volume of interest.

[0081] Advantageously, the radio frequency coil can be formed on a helmet so that the step includes positioning the helmet on the head.

[0082] It is understood that a helmet according to the principles of the present invention forms a hard shell.

[0083] As an alternative to the helmet, consideration may be given to implementing a flexible, possibly elastic, support on which a radio frequency coil would be formed.

[0084] The imaging method according to the present invention includes a step (b) of placing the part of interest within the analysis area. As previously stated, the placement of the part of interest within the analysis area is random. It is further understood that step (b) of placing the part of interest is performed without the use of a guidance system and / or a means of securing the radiofrequency coil to the permanent magnet.

[0085] The imaging procedure also includes a step c) of determining an operating frequency f0 of the radiofrequency coil. The operating frequency f0 of the radiofrequency coil corresponds to the frequency at which the nuclear spins of the hydrogen nuclei in the part of interest are likely to resonate when subjected to the main magnetic field B0, which is local and position-dependent.

[0086] In particular, step c) may include an initial frequency scan of the signal emitted by the radio frequency coil and measurement of the nuclear spin relaxation signal.

[0087] This first scan allows us to approximate the working frequency f0.

[0088] In particular, the measurement of the nuclear spin relaxation signal as a function of excitation frequency includes a maximum which is associated with an excitation frequency close to the frequency f0.

[0089] Step c) may also include a second frequency scan of the signal emitted by the radio frequency coil and measurement of the nuclear spin relaxation signal. During this second scan, gradient coils can be implemented to reduce inhomogeneities in the main magnetic field within the analysis area.

[0090] This second scan allows us to refine the determination of the working frequency f0 but also to reduce the range of working frequencies that will be implemented during step e) described below.

[0091] The imaging method according to the present invention also includes a step d) of measuring the inhomogeneity of the main magnetic field B0 in the volume of interest.

[0092] Specifically, step d) of measuring the inhomogeneity of the main magnetic field B0 comprises two measurements, referred to as the first measurement and the second measurement, and a processing sequence. Each of the two measurements includes the acquisition of magnetic resonance data according to a predefined excitation and data collection sequence. For example, each measurement may implement a spin echo sequence, such as a 2D or 3D spin echo sequence.

[0093] The echo time (corresponding to a delay between the moment of excitation and the moment of collection of relaxation data) of the first measurement is different from that of the second measurement.

[0094] The processing sequence includes the execution of the following steps:

[0095] 1) the calculation of a first map of a phase linked to the first measurement ();

[0096] 2) the calculation of a second mapping of a phase linked to the second measurement ();

[0097] 3) the calculation of a map of the inhomogeneity of the main magnetic field based on the difference between the first map and the second map ().

[0098] A person skilled in the art will find all the elements necessary for the execution of step d) in the article Jezzard, P. and Balaban, RS (1995), Correction for geometric distortion in echo planar images from B0 field variations. Magn. Reson. Med., 34: 65 73.

[0099] During step d), gradient coils are implemented to impose spatial coding in the analysis area. This aspect also allows for the measurement of nonlinearities within these gradient coils.

[0100] The method also includes performing a step e) of magnetic resonance data acquisition. Step e) includes, in particular, the implementation of gradient coils to spatially encode the area of ​​analysis.

[0101] The method also includes performing a step f) of processing magnetic resonance data in order to construct an image of the part of interest, the processing step being performed on the basis of the inhomogeneity measurement of the main magnetic field B0.

[0102] Step f) of data processing addresses the solution to the problem given by the equation:

[0103] where r i is a position within the analysis area, B(r i ) is the deviation of the magnetic field from an average magnetic field in the analysis area and at position r i, k i is a component of a position vector in space K, t j is a time, x(ri ) is a component of the image at position r i in real space, y(k j ) is the measure in space K at position k j.

[0104] Advantageously, the execution of step f) includes the implementation of one of the following methods chosen from: conjugate phase reconstruction, deep learning-based reconstruction.

[0105] Those skilled in the art will find various methods that can be implemented to solve the problem given by this equation in the article: Koolstra, K., O'Reilly, T., Börnert, P. et al. Image distortion correction for MRI in low field permanent magnet systems with strong B0 inhomogeneity and gradient field nonlinearities. Magn Reson Mater Phy 34, 631–642 (2021). https: / / doi.org / 10.1007 / s10334-021-00907-2.

[0106] The invention also relates to a magnetic resonance imaging device configured for implementing the imaging method according to the present invention. In particular, said imaging device comprises a computer equipped with a computer program which, when implemented, executes steps c), d), e) and f).

[0107] Advantageously, said imaging device includes a permanent magnet in a Hallbach configuration.

[0108] Thus, as an example, the inventors evaluated the advantages of taking into account magnetic field inhomogeneities, particularly following random positioning of the subject within the analysis area. The inventors therefore positioned a cylindrical subject (specifically, a phantom) within the analysis area and performed magnetic resonance imaging data acquisition.

[0109] These data were processed to obtain an image of the phantom according to a protocol that did not take into account the inhomogeneity of the field and subsequently by implementing the process according to the present invention.

[0110] Thus, la represents the image obtained without taking into account the inhomogeneity of the magnetic field while la represents the image obtained by implementing the process according to the present invention.

[0111] On the first one, the ghost appears distorted, while on the second one it is circular.

[0112] The results presented below demonstrate the importance of determining the inhomogeneity of the magnetic field when the subject is positioned in the analysis area.

[0113] These results include two sets of measurements, designated respectively as the first set of results and the second set of results.

[0114] More specifically, the first series is obtained by positioning the phantom close to the center of the magnet, while the second series corresponds to a positioning of the phantom at a distance from the center of the magnet.

[0115] For each series, the inhomogeneity of the magnetic field is determined with the phantom positioned in the analysis area.

[0116] Thus, to obtain the first set of results, a first inhomogeneity of the magnetic field () is measured; equivalently, for the second set of results, a second inhomogeneity of the magnetic field () is obtained.

[0117] Figures 7a and 7b, associated with the first series, illustrate an example of an image of the ghost positioned at the center of the magnet.

[0118] La represents the image obtained without correction for the inhomogeneity (shown in the) of the magnetic field, while la represents the image obtained by applying the method according to the present invention.

[0119] On the first page, the ghost appears distorted, while on the second page it regains a circular shape, as expected.

[0120] Figures 9a and 9b, associated with the second series, illustrate another example of an image of the same phantom, this time positioned at a distance from the center of the magnet.

[0121] La represents the image obtained without correction of the inhomogeneity (shown at the) of the magnetic field, while la shows the image obtained by implementing the method according to the present invention.

[0122] On the first one, the ghost appears distorted, while on the second one it is circular, as expected.

[0123] This corresponds to the data from the, but taking into account the inhomogeneity of the magnetic field measured when the phantom was placed at a distance from the center of the magnet (second inhomogeneity of the magnetic field).

[0124] The shape of the ghost is therefore clearly not circular.

[0125] This observation thus demonstrates the importance of determining, for each measurement carried out on a subject, the inhomogeneity of the magnetic field with the subject positioned in the analysis area.

[0126] Of course, the invention is not limited to the embodiments described and alternative embodiments can be made without departing from the scope of the invention as defined by the claims.

Claims

Method for imaging a part of interest of a subject using a magnetic resonance imaging device (1), the magnetic resonance imaging device (1) comprising a permanent magnet (2) configured to impose a principal magnetic field B0 in an analysis area, and gradient coils (6) configured to impose spatial coding in the analysis area, the method comprising the successive execution of the following steps: a) a step of positioning a radio frequency coil (7) around the part of interest, the radio frequency coil (7) defining a volume of interest; b) a step of placing the part of interest in the analysis area; d) a step of measuring the inhomogeneity of the principal magnetic field B0 in the volume of interest; e) a step of acquiring magnetic resonance data;f) a magnetic resonance data processing step to construct an image of the part of interest, the processing step being performed on the basis of the inhomogeneity measurement of the main magnetic field B0.; Imaging method according to claim 1, wherein said method also includes a step c) of determining an operating frequency f0 of the radio frequency coil (7), advantageously step c) is carried out between steps b) and d). Imaging method according to claim 1 or 2, wherein the area of ​​interest comprises the head of a human subject, and the radio frequency coil (7) is formed on a helmet such that step a) comprises positioning said helmet on the head. Imaging method according to any one of claims 1 to 3, wherein step b) of placing the part of interest is carried out without the use of a guidance system and / or a means of securing the radio frequency coil (7) to the permanent magnet (2). An imaging method according to any one of claims 1 to 4, wherein step d) of measuring the inhomogeneity of the main magnetic field B0 comprises two measurements referred to respectively as first measurement and second measurement, and a processing sequence, each of the two measurements comprising the acquisition of magnetic resonance data according to a predefined excitation and data collection sequence, the data collection being defined by a time referred to as echo time, the echo time of the second measurement being different from the echo time of the first measurement, the processing sequence comprising the execution of the following steps: 1) the calculation of a first phase map related to the first measurement; 2) the calculation of a second phase map related to the second measurement; 3) the calculation of a map of the inhomogeneity of the main magnetic field based on the difference between the first map and the second map. Imaging method according to any one of claims 1 to 5, wherein during the execution of step d) the gradient coils (6) are implemented to impose spatial coding in the analysis area. An imaging method according to any one of claims 1 to 6, wherein the data processing step f) solves the problem given by the equation: where r i is a position within the analysis area, B(r i ) is the deviation of the magnetic field from an average magnetic field in the analysis area and at position r i , k i is a component of a position vector in space K, t j is a time, x(r i ) is a component of the image at position r i in real space, y(k j ) is the measure in space K at position k j . Imaging method according to any one of claims 1 to 7, wherein the execution of step f) comprises the implementation of one of the methods selected from: conjugate phase reconstruction, deep learning-based reconstruction.

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