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

Abstract

The invention relates to a 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, the placement step being random; 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.;

Description

Title of the invention: Method for imaging a part of interest of a subject using a magnetic resonance imaging device. FIELD OF THE INVENTION

[0001] The present invention relates to the field of magnetic resonance imaging. In particular, the present invention relates to 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 makes it possible, in particular, to process the 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, non-invasively, the interior of bodies, particularly human bodies. In particular, magnetic resonance imaging makes it possible to probe the hydrogen nuclei, and especially their nuclear spin, of 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 align preferentially along an axis, called the z-axis, determined by the orientation of the main magnetic field so as 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 that varies in amplitude 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. In particular, the at least one radio frequency coil 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.

[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 over periods of time 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 an example. These 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 even 200 mT, without significantly increasing the mass or size of the MRI device in question.

[0011] However, so-called low-intensity permanent magnets generate a magnetic field whose spatial inhomogeneity remains relatively significant, particularly with regard to the spatial encoding imposed by the 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] Moreover, 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] An object of the present invention is to propose a magnetic resonance imaging method enabling the correction of magnetic field inhomogeneities and, where appropriate, the 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 by means of a magnetic resonance imaging device, the magnetic resonance imaging device comprising a permanent magnet configured to impose a main magnetic field Bo 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 Bo in the volume of interest;

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

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

[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 embodiment, 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 embodiment, 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 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 the 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:

[0024] 1) the calculation of a first mapping of a phase linked to the first measurement;

[0025] 2) the calculation of a second mapping 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 embodiment, during the execution of step d) the gradient coils are implemented to impose a spatial coding in the analysis area.

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

[0029] Step f) of data processing solves the problem given by the equation: ÿik.yî = y .ryijît' e ■ L

[0030] where r; is a position in the analysis area, AB(r;) is the deviation of the magnetic field from an average magnetic field in the analysis area and at position ri5 ki is a component of a position vector in space K, tj is a time, x(r;) is a component of the image at position r; in real space, y(kj) is the measure in space K at position kj.

[0031] According to one embodiment, 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 comprises a permanent magnet in a Hallbach configuration. Brief description of the drawings

[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] [Fig-1] Fig. 1 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] [Fig.2] The [Fig.2] is a first mapping of a phase linked to the first measurement;

[0038] [Fig.3] The [Fig.3] is a second mapping of a phase linked to the second measurement;

[0039] [Fig.4] The [Fig.4] is a mapping of the inhomogeneity of the main magnetic field based on the difference between the first mapping and the second mapping;

[0040] [Fig. 5] Fig. 5 is an image of a cylindrical phantom obtained by means of a resonance imaging device and without taking into account the inhomogeneity of the magnetic field;

[0041] [Fig. 6] Fig. 6 is an image of a cylindrical phantom obtained by means of a resonance imaging device taking into account the inhomogeneity of the magnetic field according to the principles of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0042] The present invention relates to a method for imaging a part of interest of a subject, in particular a human or animal body. Specifically, the imaging method employs a resonance imaging device, more particularly a so-called low-field resonance imaging device. "Low field" means a principal magnetic field with an intensity of less than 200 mT, advantageously less than 100 mT, and even more advantageously less than 50 mT.

[0043] 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.

[0044] By way of example, for a main magnetic field of approximately 50 mT, an inhomogeneity of said magnetic field of approximately 3 kHz can be observed in the analysis zone defined by the magnet generating said main magnetic field for an excitation frequency of approximately 2 MHz by a radio frequency coil. Furthermore, since the gradient coils spatially encode the analysis zone in a frequency range of approximately 25 kHz, this inhomogeneity of the main magnetic field remains significant.

[0045] Thus, [Fig.1] is a schematic representation of a magnetic resonance imaging device 1 that can be implemented within the framework of the present invention.

[0046] It is noted that the configuration of the imaging device 1 is given only by way of example and is therefore not such as to limit the scope of the present invention.

[0047] 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.

[0048] More particularly, 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.

[0049] In this regard, 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.

[0050] The permanent magnet 2 is more particularly 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.

[0051] In this regard, 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.

[0052] By way of 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.

[0053] The imaging device 1 also includes a set of gradient coils 6. The gradient coils 6 are specifically configured to produce small amplitude and spatially varying magnetic fields when a current is applied to them.

[0054] More specifically, the gradient coils 6 are designed to produce a magnetic field component that is aligned parallel to the static magnetic field.

[0055] Thus, the combined effects of the magnetic fields imposed by the gradient coils 6 make it possible to spatially encode the signals originating from a body present in the housing 3 and intended to be probed. Spatial encoding is manifested in particular by a variation in the resonance energy of the nuclear spins of the hydrogen nuclei contained in the body intended to be probed and present 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, 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 last aspect, along with the inhomogeneity of the main magnetic field, contributes to image distortion.

[0056] The imaging device 1 further comprises 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.

[0057] 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 particularly, the pulse means 10 can be configured to to enable 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 likely to be present in the housing 3.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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 Bo 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:

[0063] a) a step of positioning a radio frequency coil around the part of interest, the radio frequency coil defining a volume of interest;

[0064] b) a step of placing the part of interest in the analysis area, the placement step being random;

[0065] d) a step of measuring the inhomogeneity of the main magnetic field Bo in the volume of interest;

[0066] e) a magnetic resonance data acquisition step;

[0067] f) a step of processing magnetic resonance data in order to construct an image of the part of interest, the processing step being carried out on the basis of the inhomogeneity measurement of the main magnetic field Bo.

[0068] Advantageously, the imaging method may also include performing a step (c) of placing the part of interest in the analysis area, the placement step being random. This step (c), without limiting the invention to this aspect, may, for example, be performed between step (b) and step (d).

[0069] This method is particularly advantageous because it allows, regardless of the positioning of the part of interest in the analysis area, and in particular of the part The object of interest around which the radio frequency coil is positioned is designed to form an image essentially free of distortions. In particular, this process makes it possible to eliminate, or at least reduce, the effect due to the inhomogeneity of the main magnetic field and the non-linearity of the magnetic fields imposed by the gradient coils.

[0070] In particular, the present invention is of special interest when imaging a subject's head using a magnetic resonance imaging device in which the permanent magnet forms a tunnel delimiting the analysis area. Indeed, such a configuration does not always allow the subject's part of interest to be positioned within the analysis area in a repeatable manner. Specifically, the positioning of the part of interest may 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 part of interest makes it impossible to assume a fixed position for the coil within the tunnel.

[0071] 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 in 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.

[0072] 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.

[0073] Advantageously, the radio frequency coil can be formed on a helmet so that the step includes positioning the helmet on the head.

[0074] It is understood that a helmet according to the principles of the present invention forms a hard shell.

[0075] 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.

[0076] The imaging method according to the present invention includes a step b) of placing the part of interest in the analysis zone. As previously stated, the placement of the part of interest in the analysis zone 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 radio frequency coil to the permanent magnet.

[0077] The imaging method also includes a step c) of determining an operating frequency f0 of the radio frequency coil. The operating frequency f0 of the radio frequency coil corresponds to the frequency at which the nuclear spins of the hydrogen nuclei in the part of interest are likely to resonate from then on that they are subject to the main magnetic field Bo which is local and position-dependent.

[0078] In particular, step c) may include a first frequency scan of the signal emitted by the radio frequency coil and measuring the nuclear spin relaxation signal.

[0079] This first scan allows us to approximate the working frequency f0.

[0080] In particular, the measurement of the nuclear spin relaxation signal as a function of the excitation frequency includes a maximum which is associated with an excitation frequency close to the frequency f0.

[0081] 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 may be implemented to reduce inhomogeneities in the main magnetic field within the analysis area.

[0082] This second scan allows us to refine the determination of the working frequency f0 but also to reduce the range of working frequencies which will be implemented during step e) described below.

[0083] The imaging method according to the present invention also includes a step d) of measuring the inhomogeneity of the main magnetic field Bo in the volume of interest.

[0084] In particular, step d) of measuring the inhomogeneity of the main magnetic field Bo comprises two measurements, referred to respectively 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, for example, a 2D or 3D spin echo sequence.

[0085] 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.

[0086] The processing sequence includes the execution of the following steps:

[0087] 1) the calculation of a first map of a phase linked to the first measurement ([Fig.2]);

[0088] 2) the calculation of a second phase map linked to the second measurement ([Fig.3]);

[0089] 3) the calculation of a map of the inhomogeneity of the main magnetic field based on the difference between the first mapping and the second mapping ( [Fig.4]).

[0090] A person skilled in the art will find all the elements necessary for carrying out step d) in the article Jezzard, P. and Balaban, RS (1995), Correction for geometry distortion in echo planar images from B0 field variations. Magn. Reson. Med., 34: 65 73.

[0091] During the execution of step d), gradient coils are implemented to impose spatial coding in the analysis area. This aspect thus makes it possible to also measure the non-linearities of said gradient coils.

[0092] The method also includes performing a step e) of acquiring magnetic resonance data. Step e) includes, in particular, the implementation of gradient coils to spatially encode the area of ​​analysis.

[0093] 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 Bo.

[0094] Step f) of data processing solves the problem given by the equation: .,CV « _ \ —f— • -■ c - ' ' ' ;

[0095] where p is a position in the analysis area, AB(r;) is the deviation of the magnetic field from an average magnetic field in the analysis area and at position pk; is a component of a position vector in K-space, tj is a time, x(r;) is a component of the image at position q in real space, y(kj) is the measure in K-space at position kj.

[0096] Advantageously, the execution of step f) includes the implementation of one of the following methods chosen from: conjugate phase reconstruction, deep learning-based reconstruction.

[0097] A person 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., Bômert, 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 / sl0334-021-00907-2.

[0098] 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).

[0099] Advantageously, said imaging device includes a permanent magnet in a Hallbach configuration.

[0100] Thus, by way of 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.

[0101] 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 method according to the present invention.

[0102] Thus, [Fig.5] represents the image obtained without taking into account the inhomogeneity of the magnetic field while [Fig.6] represents the image obtained by implementing the process according to the present invention.

[0103] On [Fig.5], the ghost appears distorted on [Fig.5] and circular, as expected, on [Fig.6].

[0104] 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

Demands

1. A 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 Bo 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, the placement step being random; d) a step of measuring the inhomogeneity of the principal magnetic field Bo 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 Bo.;

2. 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).

3. 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.

4. 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).

5. An imaging method according to any one of claims 1 to 4, wherein step d) of measuring the inhomogeneity of the main magnetic field Bo 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 resonance data magnetic according to a predefined excitation and data collection sequence, the data collection being defined by a time called 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 linked to the first measurement; 2) the calculation of a second phase map linked to the second measurement; 3) the calculation of a map of the inhomogeneity of the main magnetic field on the basis of the difference between the first map and the second map.

6. 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.

7. 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; is a position in the analysis area, AB(r;) is the deviation of the magnetic field from an average magnetic field in the analysis area and at position ri5 k; is a component of a position vector in K-space, tj is a time, x(r;) is a component of the image at position r; in real space, y(kj) is the measure in K-space at position kj.

8. An imaging method according to any one of claims 1 to 7, wherein the execution of step f) comprises the implementation of one of the following methods selected from: conjugate phase reconstruction, deep learning-based reconstruction.

9. Magnetic resonance imaging device (1) configured for carrying out the imaging method according to any one of claims 1 to 8, wherein said imaging device (1) comprises a computer equipped with a computer program which, when implemented, executes steps d), e) and f). 14

10. Imaging device (1) according to claim 9, wherein said imaging device (1) comprises a permanent magnet (2) in a Hallbach configuration.

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