Magnetic resonance imaging device provided with halbach rings

Abstract

The invention relates to a magnetic resonance imaging device (1) comprising a magnetic assembly (MagAss) that generates a permanent magnetic field (B0) along a magnetization axis, the magnetic assembly (MagAss) comprising: support plates (SupPl) supporting permanent magnets (Mag) forming at least one Halbach ring (HlbRng), the Halbach rings being coaxial and configured so that each one generates a magnetic field (BRng) parallel to the magnetization axis, the support plates comprising at least one first end support plate (SupPl1), at least one second end support plate (SupPl2), and a plurality of central support plates (SupPl0) interposed between the end support plates, and the permanent magnets (Mag1, Mag2) supported by the end support plates each having a first volume, the permanent magnets (Mag0) supported by the plurality of central support plates each having a second volume, the first volume being larger than the second volume.

Description

Magnetic resonance imaging device equipped with Halbach rings FIELD OF INVENTION

[0001] The present invention relates to the field of magnetic resonance imaging. More particularly, the present invention relates to a magnetic resonance imaging device, and in particular a magnetic resonance imaging device provided with a magnetic assembly capable of imposing a main magnetic field in an area of ​​analysis. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] Magnetic resonance imaging (MRI) is now widely used to image the inside of a body, particularly the human body, in a non-invasive way. Specifically, MRI allows us to probe the hydrogen nuclei, and in particular their nuclear spin, of the water molecules that make up part of this body.

[0003] In this regard, a device designed to implement MRI techniques, called an "MRI device," is equipped with a magnet intended to impose a static magnetic field (called the "main magnetic field") on the body. Under the effect of this field, the nuclear spins associated with the hydrogen nuclei contained in the water molecules that partially make up the body become polarized. In particular, the magnetic moments associated with these spins align preferentially parallel to the main magnetic field in order to create magnetization of the body.

[0004] An MRI scanner also includes gradient coils configured to produce small-amplitude, spatially varying magnetic fields when an electric current passes through them. More specifically, the gradient coils are designed to produce a magnetic field component that is aligned parallel to the main magnetic field and varies linearly in amplitude with position along one of three perpendicular axes (x, y, or z). The combined effects of the magnetic fields imposed by the gradient coils allow for the spatial encoding of each position of the body being scanned.

[0005] An MRI device also includes at least one radio frequency (RF) coil designed to act as an RF transceiver. Specifically, at least one RF coil is configured to emit RF energy pulses at a frequency equal to or close to the resonant frequency of hydrogen nuclei's spins, a frequency at least partially absorbed by these nuclei. 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 using known methods.

[0006] The main magnetic field, usually between 1.5 Tesla and 3 Tesla, makes it possible to achieve signal-to-noise ratios sufficient to form images of the human body of usable quality with measurement durations on the order of a minute or more.

[0007] However, there are circumstances in which it is not possible to implement a primary magnetic field of such intensity, which requires stationary MRI devices due to their mass and volume. For example, while MRI devices can be mobile, they are incapable of generating a magnetic field with an amplitude of the same order of magnitude as the amplitudes mentioned above, on the order of Tesla.

[0008] Mobile MRI devices generally include a permanent magnet or electromagnets of limited capacity, and cannot impose a main magnetic field with an intensity greater than 60 mT, or even greater than 200 mT, without penalizing the mass or size of the MRI device in question.

[0009] This limitation in terms of the main magnetic field strength directly affects the performance of the MRI device. Improving the uniformity of the main magnetic field can therefore be crucial if an acceptable signal-to-noise ratio is to be maintained.

[0010] One aim of the invention is to provide an assembly of magnets that generates a highly uniform magnetic field.

[0011] One aim of the invention is to provide a structure for holding and positioning magnets that is easy to assemble and offers high geometric precision in the placement of the magnets.

[0012] An object of the present invention is to propose a magnetic resonance imaging device, advantageously employing a low-intensity main magnetic field, provided with a magnetic assembly whose uniformity of the created magnetic field is improved compared to known assemblies of the prior art.

[0013] An object of the present invention is to provide a magnetic resonance imaging (MRI) device enabling the analysis of different parts of the body by means of a mobile MRI device, preferably without loss of image quality that can be obtained on these different parts of the body compared to images obtained by means of a fixed MRI device, or by limiting the loss of image quality compared to a fixed MRI device.

[0014] One aim of the invention is to provide an imaging device in which the installation and assembly and / or disassembly of permanent magnets are simplified. BRIEF DESCRIPTION OF THE INVENTION

[0015] One or more of the objectives of the invention are achieved by a magnetic resonance imaging device which comprises:

[0016] - a magnetic assembly configured to impose a permanent principal magnetic field within an internal volume of said magnetic assembly, the internal volume generally being cylindrical in shape and extending longitudinally parallel to a longitudinal axis and perpendicular to a principal axis of magnetization, the magnetic assembly comprising:

[0017] - support plates, each having an annular section defining an opening, each support plate supporting permanent magnets so as to form at least one Halbach ring, said support plates forming a sequence of support plates along the longitudinal axis, such that:

[0018] - The openings in the dwelling define the internal volume; and

[0019] - Halbach rings are coaxial with each other and belong to respective planes perpendicular to the longitudinal axis, each Halbach ring being configured to generate a magnetic field parallel to the principal magnetization axis,

[0020] in which the support plates comprise at least one first end support plate, at least one second end support plate, and a plurality of central support plates, each interposed between at least one first end support plate and at least one second end support plate, and

[0021] in which the permanent magnets supported by at least one first end support plate and at least one second end support plate each have a first volume, the permanent magnets supported by the plurality of central support plates each have a second volume, the first volume being larger than the second volume.

[0022] According to one implementation method, the first volume is larger than the second volume by 3% or more.

[0023] Depending on the implementation method, permanent magnets exhibit the same remanence to within 3%, or less than 3%.

[0024] Depending on the method of implementation, permanent magnets exhibit essentially the same remanence.

[0025] Depending on the implementation method, the system also includes:

[0026] - three pairs of electromagnetic coils, known as gradient coils, configured to generate three magnetic fields respectively along three directions of a three-dimensional orthogonal coordinate system,

[0027] in which:

[0028] - each of the gradient coils extends, along the longitudinal axis, from at least one first end support plate to at least one second end support plate, and

[0029] - at least one Halbach ring of at least one first end support plate and at least one second end support plate have a diameter, called end diameter, smaller than the diameter, called central diameter, of at least one Halbach ring of the plurality of central support plates.

[0030] According to one embodiment, the end diameter of the smallest Halbach ring among the at least one Halbach ring of the at least one first end support plate and the at least one second end support plate is at least 10% smaller than the central diameter of the smallest Halbach ring among the at least one Halbach ring of the central support plates.

[0031] According to one embodiment, each of the support plates has a face perpendicular to the longitudinal axis, called the front face, and includes cavities forming magnet housings configured to receive permanent magnets, each housing having a depth, a bottom and at least one lateral face extending from the front face to the bottom, perpendicular to the front face.

[0032] According to one implementation method, at least one lateral face is provided with a rib extending perpendicularly to the front face.

[0033] According to one implementation method, the rib remains at a distance from the front face.

[0034] According to one method of implementation, the rib extends from the bottom.

[0035] According to one implementation method, the housings and the permanent magnets each have cross-sections forming substantially polygons, the cross-sections of the housings forming re-entrant corners each having an angle clearance.

[0036] According to one implementation method, the housing funds are each respectively equipped with a through opening leading to a rear face of a respective support plate, opposite and parallel to a respective front face.

[0037] According to one implementation method, the housings are configured to block the magnets that are inserted into them by force, by friction.

[0038] According to one embodiment, the Halbach rings are each formed of discrete magnets, each having a magnetization orientation, and, for each of the Halbach rings, the magnetization orientations are angularly shifted by a constant pivot angle when moving from one magnet to an immediately adjacent magnet in a given direction of rotation along a considered Halbach ring, the pivot angle being such that when a complete turn of the ring is made, the magnetization orientations have rotated 720° around the longitudinal axis.

[0039] According to one implementation method, the device comprises a first module including the magnetic assembly supported by a wheeled trolley and a second module equipped with computer hardware for controlling the first module, for acquiring and processing computer data obtained by magnetic resonance.

[0040] The invention extends to a method for manufacturing a first Halbach ring, the method comprising: a manufacturing step of magnets obtained from the same block of magnetized material, then a step of assembling the magnets obtained in the manufacturing step of magnets so as to form the first Halbach ring.

[0041] According to one implementation of the process, a second Halbach ring is also formed according to the manufacturing process, from the magnets obtained in the magnet manufacturing step, the magnets can include first magnets having a first volume and second magnets having a second volume, different from the first volume, the first Halbach ring can be formed by the first magnets and the second Halbach ring can be formed by the second magnets.

[0042] According to one implementation method of the process, the first volume can be larger than the second volume by at least 3%.

[0043] According to one implementation method of the process, the first Halbach ring and, where applicable, the second Halbach ring, can each be configured to generate a uniform magnetic field.

[0044] According to one implementation method of the process, the first Halbach ring and, where applicable, the second Halbach ring, can be integrated into a magnetic resonance imaging device.

[0045] According to one implementation of the process, a third Halbach ring is also manufactured according to the manufacturing process, from the first magnets obtained in the magnet manufacturing step, the Halbach rings can be arranged so as to generate a magnetic field in a magnetic resonance imaging device, the second Halbach ring can be interposed between the first Halbach ring and the third Halbach ring.

[0046] Each implementation method of this document can be combined with one or more of the other implementation methods.

[0047] Other features and advantages of the invention will become apparent from the detailed description that follows, with reference to the accompanying figures in which:

[0048] Laillustre un bague de Halbach;

[0049] Laillustre a support plate supporting two concentric Halbach rings;

[0050] Laillustre un assembly magnétique compris un sequence de plaques soutiens comme lustre par la, vue de l'extérieur;

[0051] The is a sectional view of the assembly of the, according to a plane defined by a longitudinal axis of the assembly and a main axis of magnetization;

[0052] Lare represents several views of a magnet housing formed in a support plate as illustrated by the: a top view and two cross-sectional views along parallel planes;

[0053] Laillustre un subset de gradient, soutien des bobines à gradient, selon une vue external et selon une vue en section;

[0054] Laillustre integration geometries of gradient subsets in magnetic assemblies such as that illustrated by Figures 3 and 4;

[0055] Laillustre is a mobile MRI device;

[0056] Laillustre shows distributions of magnets according to their remanence and sizes;

[0057] This illustrates the inhomogeneity of the magnetic field B0 created by a magnetic assembly such as that illustrated by the one realized with magnets exhibiting the distributions of the ; and

[0058] Laillustre un processus de fabrication d’un assembly magnétique, pour une processus de fabrication d’un assembly magnétique de la. DETAILED DESCRIPTION OF THE INVENTION

[0059] The invention was developed in the context of the development of a mobile magnetic resonance imaging or MRI device, which requires the generation of a magnetic field that is as uniform as possible.

[0060] As illustrated by Figures 1 to 8, the invention relates to a magnetic resonance imaging device comprising: a magnetic assembly MagAss configured to impose a permanent principal magnetic field B0 in an internal volume Vol of said magnetic assembly MagAss, the internal volume generally having a cylindrical shape and extending longitudinally parallel to a longitudinal axis LongAx and perpendicular to a principal magnetization axis MagAx, the magnetic assembly MagAss may comprise: support plates SupPl each having an annular section delimiting an opening, called a volume opening VolAprt, each support plate SupPl supporting permanent magnets Mag so as to form at least one Halbach ring HlbRng, said support plates SupPl forming a sequence of support plates along the longitudinal axis LongAx, such that: the internal volume openings VolAprt delimit the internal volume Vol;and the Halbach rings HlbRng are coaxial with each other and belong to respective planes perpendicular to the longitudinal axis, each Halbach ring being configured so as to generate a magnetic field B parallel to the main magnetization axis MagAx.;

[0061] The positioning of the magnets and their remanence(s) are chosen such that the permanent magnetic field B0 they generate has an intensity of less than 50mT.

[0062] It is understood that the function of the MagAss magnetic assembly is to hold the magnets in such a way that they generate as uniform a magnetic field as possible within an internal volume. Any mechanical magnet retention system other than the one described herein may be used as long as it ensures that the magnets are positioned in a suitable location to generate the uniform magnetic field.

[0063] The system described above has several aspects, each of which can be considered independently and as contributing in its own right to achieving the goals mentioned above. Each of these aspects can be combined with one or more of the others, strengthening the effectiveness of the resulting solution in achieving one or more of these goals. First aspect – Mechanical structure

[0064] The support plates SupPl comprise at least one first end support plate SupPl1, at least one second end support plate SupPl2, and a plurality of central support plates SupPl0, each interposed between at least one first end support plate SupPl1 and at least one second end support plate SupPl2. It can be advantageous for the Halbach rings supported by the end plates to have a smaller cross-sectional radius than those supported by the central support plates, as illustrated in Figure 1. In this example, the central and end support plates have the same external diameter, but the support plates and the Halbach rings they support could have different characteristics (dimensions, remanence, number of magnets, etc.).

[0065] The support plates are schematically represented here as an annular structure with an inner diameter, an outer diameter, and a thickness. These plates may include openings for lightening the structure, for passing electrical cables or elements for positioning, alignment, and / or fixing the structure, protrusions to prevent rotation, or tabs to hold auxiliary magnets for fine-tuning the main magnetic field. Furthermore, not all plates necessarily have the same dimensions.

[0066] Figures 3 and 4 illustrate the same magnetic assembly comprising four end support plates, two per end along the longitudinal axis LongAx: the two plates SupP l1 on the left side of the device and two SupP plates l2on the right side of the device, as seen in the figure. Between these end support plates are interposed eight central support plates SupPl0. Each support plate supports two concentric Halbach rings, as illustrated in the figure. The number of end support plates, the number of central support plates, and the number of Halbach rings on each support plate can vary and are not limited to the example shown in the figure.

[0067] However, more generally, it is preferable for the end plates of the MagAss magnetic assembly to have more Hallbach rings, three or four for example, in order to guide the magnetic field B0 and compensate for the finite length of the assembly. "Guiding the magnetic field B0" refers to orienting the magnetic field lines. This guidance is achieved here by the Halbach rings of the end plates, configured to tend to close the field lines upon themselves, for example, by having a greater number of Halbach rings in these end plates compared to the number of Halbach rings in the central plates and / or by reducing the size of these Halbach rings in the end plates compared to those in the central plates.

[0068] The magnets of the Hallbach rings are placed in cavities formed in the support plates and protrude from these cavities. Retaining plates (HldPl) are arranged to hold the magnets against the bottom of the cavities. Each retaining plate is sandwiched between two support plates and is also annular in shape, so as not to obstruct the internal volume (Vol) freed up by the openings (CavApart) in the support plates. Each cavity houses a specific magnet.

[0069] The magnetic assembly consists of a sequence of substructures along the longitudinal axis. Each substructure comprises a support plate and its magnets, as well as a plate for holding the magnets in the cavities of the support plate. Identical distances d are maintained between two consecutive support plates in the sequence by means of straight rails with teeth T of width d perpendicular to their axis of extension. The teeth are designed to interlock between two support plates, thus ensuring a constant gap between them. If desired, variable gaps between the support plates could be achieved, for example, by adjusting the widths of the teeth.

[0070] A straight R0 rail has teeth designed primarily to maintain the spacing between the center plates. The ends of the R0 rail are toothless, allowing the end support plates to be positioned during assembly by simply sliding them along the longitudinal axis. The spacing between the end support plates is maintained by R1 and R2 rails, which are dedicated to the ends of the assembly and installed in a separate section from the R0 rail. Multiple R0, R1, and R2 rails can be mounted around the support plates.

[0071] A straight rail R Int , including T teeth Intof width d along its axis of extension, is intended to reinforce the structure and prevent deformation at the level of the inside of the support plates: the Tint teeth are designed to fit between two consecutive support plates at the level of their internal edges, maintain their gap at d, and mechanically reinforce the structure.

[0072] The support plates are assembled securely together, possibly using alignment means such as, for example, rods parallel to the longitudinal axis and passing through the support plates and possibly the retaining plates, so that the support openings, aligned along the main axis, delimit the volume Vol, for example generally cylindrical in shape.

[0073] La illustrates a SupPl support plate having an annular cross-section, with an outer peripheral edge and an inner peripheral edge that are generally circular and coaxial, the plate being parallel to the plane of the figure. Once installed in the MasAss structure, the plate is perpendicular to the plane of the figure, its edges being coaxial to the LongAx axis. The inner edge defines the CavAprt opening.

[0074] In this example, the support plate holds two coaxial circular Halbach rings, each ring containing sixteen cube-shaped Mag magnets of identical dimensions. The number of Halbach rings in the support plate, the number of cubes per ring, and the shape and dimensions of the cubes are not limited to this example.

[0075] Figure 1 represents a cross-section of the magnetic assembly along a plane defined by the LongAx and MagAx axes. This cross-section makes the internal volume Vol and the analysis zone AZ it contains more visible. This zone is immersed in the main magnetic field B0 generated by the Halbach rings.

[0076] The magnetic assembly is notably intended to impose, on a body placed in an analysis zone AZ of the volume Vol, the permanent main magnetic field B0, 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.

[0077] The materials used to form the support and retaining plates are preferably non-magnetic, for example, made of aluminum or polymer materials such as polymethyl methacrylate, but preferably materials more resistant than this, such as polycarbonates or polyacetals. These materials can be shaped by computer numerical control (CNC) machining centers. This machining technique makes it possible to meet the need for precision in the forming process, particularly of the support plates and the housings formed within them.

[0078] The materials used to manufacture the rails can, for example, be made of aluminum or of the same material(s) as those listed for the support plates, and more generally of a non-magnetic material, so as not to disturb the main permanent magnetic field generated by the magnets.

[0079] The elements composing the mechanical structure illustrated in the figures can be considered individually, combined with the other elements described, or combined with other mechanical elements performing functions similar or identical to those of the elements described above. The invention is not limited to the specific combination of elements illustrated in the figures and described in this document.

[0080] Optionally, the support plates, retaining plates, and rails can be positioned and assembled using an auxiliary retaining frame, which is removed once the elements are in place and secured to one another. Alternatively, and still optionally, the support plates and possibly the retaining plates can be aligned and positioned relative to each other using threaded rods passing through the assembly, parallel to the longitudinal axis, with separating elements such as bolts and washers ensuring a gap between two adjacent plates in the assembly. The plates can be held and pressed against each other using bolts mounted on these threaded rods.

[0081] It is understood, according to the terms of the present invention, that the analysis zone comprises a part, preferably central, of the volume Vol. It is also understood that the analysis zone is at least delimited by the two annular pieces positioned at the two ends, along the principal axis, of the assembly formed by the annular pieces.

[0082] The term "uniformity of a magnetic field" refers to the spatial variation of the magnetic field within the area of ​​analysis. According to the present invention, the spatial variation of the magnetic field can be characterized, in particular, by the difference between the maximum and minimum magnetic fields observed within the area of ​​analysis. Thus, according to the principles of the present invention, the smaller this difference, the better the uniformity. Second aspect – the Halbach rings

[0083] This aspect is illustrated more specifically by Figures 1 and 2.

[0084] The permanent main magnetic field B0 is generated by Halbach rings, structures of magnets arranged in a plane and preferably in such a way as to limit the extension of the magnetic field generated by the magnets outside the structure while maximizing the amplitude of this field inside the structure.

[0085] For the intended application, which is magnetic resonance imaging, good uniformity of the B0 field generated by all the rings is essential. Controlling the uniformity of B0 begins with controlling the uniformity of the magnetic field B Rng created by each ring within itself. This magnetic field is coplanar to the plane defined by the ring.

[0086] Lare represents a Halbach ring HlbRng comprising 16 Mag magnets evenly distributed on a circle. Each magnet generates a magnetic field oriented within the plane in which it is located. These magnets are considered identical in shape, dimensions, and magnetization. In this description, cubic magnets will be considered, although the implications of this description are not limited to them. Each magnet is located by the angle θ of its position on the circle, relative to a starting angular position θ0, chosen such that the direction of the magnetic field B Rng is defined by the intersection of the center of the circle and this position θ0. In the Figure, each Mag magnet is associated with an index i indicating its angular position θ i around the circle, with θ i = i×360 / 16 and i taking the values ​​of the integers between 0 and 15, inclusive.

[0087] One point to consider is the orientation of the magnets. A Halbach ring imposes certain constraints on the orientation of the magnets, but the possible arrangements of magnet orientations remain numerous. Again, through simulations, the inventors were able to determine that, even though various arrangements are possible, the most favorable arrangement for the uniformity of the magnetic field B Rng is the one in which the orientation of the magnetization of the magnets rotates twice as fast as their angular position in the ring around the axis of revolution of the ring, which is perpendicular to the plane in which the ring is located and passes through its center, when one goes from a Mag magnet j to a Mag magnet j+1 , j taking an integer value between 0 and 15 inclusive, the framework of the example illustrated by the.

[0088] This arrangement is illustrated with Mag magnets. i, their respective magnetizations, the orientation of which is indicated by the arrows M i , and their positions θ i , i varying from 0 to 15 inclusive. The magnetization of the magnet Mag0 is chosen to be collinear with the MagAx orientation of the magnetization generated by the ring. In the system illustrated by the, the magnetization of a Mag magnet i makes an angle of 2×i×360 / 16. Thus, the magnetization M4 of magnet Mag4 is opposite, or antiparallel, to the magnetization M0 of magnet Mag0 and parallel to the magnetization M 12 of the Mag magnet 12 . The Mag0 and Mag8 magnets have parallel M0 and M8 magnetizations.

[0089] More generally, keeping the notation and reference system defined above, in a Halbach ring formed by N magnets uniformly distributed on a circle, the angular positions θ i magnets are worth θ i= i×360 / N, where i takes the values ​​of integers between 0 and (N-1), inclusive, and the magnetization of a Mag magnet i makes an angle of 2×i×360 / N.

[0090] From a practical standpoint, and for the intended application of a portable MRI device, Halbach rings can be considered, comprising between 50 and 100 magnets, preferably between 60 and 90 magnets, with diameters between 10 and 30 cm, preferably between 15 and 26 cm. In such cases, the magnets can be cube-shaped, with sides between 8 and 16 mm, preferably between 10 and 14 mm.

[0091] The two points above can be considered independently of each other, or in combination, either one or both being found in Halbach magnets as considered in the MagAss magnetic assembly. Third aspect – characteristics of magnets

[0092] This aspect is illustrated more specifically by Figures 3 and 4.

[0093] The permanent principal magnetic field B0 is generated by the Halbach rings. The Halbach rings are configured to generate a uniform permanent principal magnetic field B0. This configuration is obtained through numerical calculation, and the positions, dimensions, and remanences of the magnets used in the calculations must be strictly adhered to in order to obtain a field that is effectively uniform to an acceptable degree.

[0094] However, obtaining magnets with a precisely defined remanence is difficult, and the challenge of meeting calculated values ​​increases when these magnets must exhibit remanences of varying values. The solution, therefore, is to use magnets from the same production batch, which have very similar remanences.

[0095] Indeed, while a deviation from the predicted value for the remanence of the magnets affects the amplitude of the magnetic field actually generated, this deviation does not affect the uniformity of the field, as demonstrated by the following equation for the magnetic dipole,

[0096]

[0097] The equation of the magnetic field The resulting value can be factored by RV (remanence R and volume V), which demonstrates that any inhomogeneity is equivalent up to a constant.

[0098] Also, each of the Halbach rings in the MagAss set are usually made up of magnets of the same dimensions to generate the permanent main magnetic field B0.

[0099] However, as already mentioned, the magnetic field must be looped back on itself at the ends of the magnetic assembly, considered along the longitudinal axis LongAx. Preferably, for reasons of space and to protect the environment from the magnetic field generated by the assembly, this looping should limit the extension of the magnetic field beyond the assembly as much as possible. In order to improve the uniformity of the magnetic field in this central part Cent, the magnets Mag1 and Mag2, located respectively at the ends Ext1 and Ext2 of the assembly along the LongAx axis, and which impose this looping, are configured to generate a stronger magnetic field than the magnets Mag0 located in the central part Cent of the magnetic assembly MagAss.

[0100] There are two alternatives to increase the magnetic field generated by the Mag1 and Mag2 magnets at the ends relative to the Mag0 magnets in the central part.

[0101] The first alternative is to use Mag1 and Mag2 magnets of the same size but with a higher remanence than the Mag0 magnets. As we saw earlier, it is difficult to control the remanence of magnets with the desired precision. Thus, the inventors determined that using Mag1 and Mag2 magnets with a higher remanence than the Mag0 magnets degrades the uniformity of the magnetic field B0 due to dispersion of the magnets' remanence, a dispersion that is very difficult to control.

[0102] The second alternative involves using Mag1, Mag2, and Mag0 magnets with the same remanence. The increase in the magnetic field generated by Mag1 and Mag2 magnets is achieved by increasing their volume relative to the Mag0 magnets. In this case, it is preferable to use magnets from the same production batch, and therefore with remanences as close as possible to each other, to manufacture the Mag0, Mag1, and Mag2 magnets. Indeed, the variation in magnet dimensions can be controlled much more easily than the variation in their remanences, as all the magnets can be cut from the same block of magnetized material and thus exhibit identical or very similar remanences.

[0103] As illustrated, a manufacturing process for a magnetic assembly, such as, for example, the MagAss assembly shown in Figures 3 and 4, includes a step S10 of manufacturing Mag magnets from a single block Bl of magnetized material, for example, by cutting and / or machining these magnets from this block of magnetized material. The Mag magnets can include the Mag1, Mag2, and Mag0 magnets mentioned above. Step S10 is followed by step S20 of manufacturing the MagAss magnetic assembly, which consists of assembling the magnets obtained in step S10 to form the Halbach rings HlbRng of the MagAss magnetic assembly, by positioning them in the support plates SupPl1, SupPl2, and SupPl0.However, step S20 is not limited to the MagAss magnetic assembly as described herein, but can also be applied to any type of magnet assembly for generating magnetic fields, preferably uniform ones, and in particular Halbach rings. An advantage of this process is the good control it provides over the magnetic fields generated by each magnet relative to the others, thanks to the fact that they originate from the same block of magnetized material and can therefore exhibit greater homogeneity in their characteristics, especially magnetic ones, than magnets obtained from different blocks of magnetized material.Indeed, even though the manufacturing processes for different blocks of magnetized material are designed to be identical, in practice it is difficult to reproduce exactly the same manufacturing conditions, and different blocks obtained by the same process exhibit slightly different characteristics, particularly magnetic ones. Using magnets from the same block of magnetized material promotes good homogeneity of the characteristics of these magnets, and consequently, better uniformity of the magnetic field B0 generated by the MagAss magnetic assembly.

[0104] Figures 9 and 10 illustrate this characteristic. Figures (A) and (B) respectively show the remanence and size distributions of magnets supplied by a provider, with the y-axis indicating the number of samples belonging to given ranges of remanence and size. Remanence and size were assumed to be Gaussian. The average remanence is 1.41 Tesla, with a standard deviation of 0.4 / 3 Tesla. The average size (the length of an edge of the magnets, considered to be cube-shaped) is 11.42 mm, with a standard deviation of 0.1 / 3 mm.

[0105] Figure 1 illustrates the influence of remanence distribution and magnet size on the homogeneity of the permanent principal magnetic field B0 generated by a magnetic assembly using the magnets defined above. The assembly comprises 25 magnet support plates for approximately 4000 magnets generating the B0 field. Figure 1 illustrates in (A) the influence of the remanence distribution of individual magnets when their size is considered homogeneous and equal to 11.42 mm, and conversely in (B) the influence of the size distribution of individual magnets when their remanence is considered homogeneous and equal to 1.41 Tesla. It is clear that the remanence distribution leads to a greater inhomogeneity of the B0 field, up to approximately 9000 ppm, than that caused by the magnet size distribution, which is limited to approximately 6500 ppm.

[0106] It is therefore advantageous to use this second alternative, which minimizes the dispersion of magnet remanences, rather than the first alternative, which inevitably leads to an increase in uncertainty on the magnetic field actually generated by the magnetic assembly.

[0107] Thus, according to this aspect of the invention, the permanent magnets supported by at least one of a first end support plate and a second end support plate each have a first volume, while the permanent magnets supported by the plurality of central support plates each have a second volume, the first volume being larger than the second volume. Preferably, the permanent magnets supported by the first end support plate and the second end support plate each have the first volume. The first volume is preferably 3% or more larger than the second volume, the ratio between the first and second volumes depending on the desired magnetizations of the magnets, according to their respective positions, in order to ensure the functions of generating the main magnetic field B0 and looping this field back onto itself.Further adjustments to the volumes can be applied to improve the homogeneity of the B0 field. Preferably, the permanent magnets have the same remanence to within 3% or less, and preferably have substantially the same remanence.

[0108] Optionally, the four end support plates SupPl1 and SupPl2 of the MagAss magnetic assembly illustrated in Figures 3 and 4 may include Halbach rings with radii smaller than those of the central plates SupPl0. Furthermore, the magnets constituting the Halbach magnets of the end support plates SupPl1 and SupPl2 may be larger than those constituting the Halbach magnets of the central support plates SupPl0. However, these three aspects—location and radius of the Halbach rings and magnet sizes—are not necessarily related: the Halbach rings may have identical or different radii from one support plate to another, and the dimensions of the magnets may differ independently of their longitudinal positions in the MagAss assembly.All these characteristics can be adjusted according to simulations to obtain a desired magnetic field in the AZ analysis area, and possibly to meet other criteria such as the extension of the generated magnetic field outside the MagAss magnetic assembly. Fourth aspect – Magnet housings

[0109] This aspect is illustrated more specifically by Figure 5.

[0110] As illustrated by figures 1 to 4, the Mag magnets must be located and oriented rigorously in order to closely approximate the geometry of the MagAss magnetic assembly as determined by computer simulations to obtain a B0 field that is as uniform as possible.

[0111] To this end, the support plates include recesses Hsg shaped to accommodate and mechanically secure the magnets, at least with regard to their positioning and orientation. Furthermore, the retaining plates HldPl also contribute to the mechanical securing of the magnets by holding them fully embedded in the recesses. The recesses are cavities formed in a first face, called the front face FF, of each of the support plates SupPl, which include the support plates SupPl0, SupPl1, and SupPl2 of Figures 3 and 4. Each recess may include several walls E extending perpendicularly from the front face FF to a bottom Bot.

[0112] Hsg housings possess several features that allow them to fulfill their functions while facilitating the mounting and / or dismounting of magnets. These features can be considered independently, each providing a specific advantage.

[0113] First characteristic – Housing section

[0114] Each housing has a depth, a bottom, and at least one lateral face extending from the front face to the bottom, parallel to a direction perpendicular to the front face, called the depth direction. A cross-section of a housing and a cross-section of a magnet, taken parallel to the front face, can be at least partially identical, so that a magnet inserted into its housing is completely prevented from rotating and fixed in position within the plane of its support plate.

[0115] Laillustre illustrates an example particularly suited to the case of a cube-shaped magnet with edge length e, the corresponding housing has a partially cube-shaped cross-section: the general shape is that of a cube, but its depth is less (less than edge length e) and its edges perpendicular to the face FF are equipped with clearances of angle C. These two characteristics form the second and third characteristics discussed below.

[0116] Thus, the cross-section of the housing corresponds to that of the cubic magnet at the level of four faces opposite each other in pairs, which totally and very precisely fixes the lateral and rotational positions of the magnet.

[0117] Second characteristic – Depth of the dwellings

[0118] The base of the housing can be parallel to the front face and perpendicular to the side walls of the housing. The depth of the housing, indicated by dep en (C) de la, is preferably constant and less than the height of the magnet, which corresponds to the edge e of the cube in the example given here. These dimensions have the advantage of allowing the magnets to be pressed against the base of their respective housings by means of the retaining plates HldPl, which apply pressure to one front face of the magnet while its rear face, opposite the front face, butts against the base of its housing.

[0119] Third characteristic – Corner clearance

[0120] As previously mentioned, the cross-section of the housing can correspond exactly to the cross-section of the magnet. However, with this geometry, inserting the magnet into its housing can be difficult. This insertion can be simplified by providing angled clearances C at the edges of the housing that are perpendicular to the front face: the edges of the cube that are perpendicular to the front face are no longer at risk of coming into contact with the edges of the housing and blocking the insertion of the cube into the housing.

[0121] Fourth characteristic – Locking ribs

[0122] One or more faces E of the housing, perpendicular to the front face FF, may be provided with ribs R that protrude from these faces. These ribs are designed to deform in response to the forceful insertion of the magnet into the housing and to lock it in place by friction. These ribs preferably extend longitudinally, perpendicular to the front face FF.

[0123] Furthermore, it is advantageous for the ribs R to remain at a distance from the front face, as illustrated by the figure: in this example, they extend from the bottom Bot of the housing, over a distance dep' less than its depth dep. Alternatively, the ribs could not extend to the bottom of the housing. Such geometries allow the magnet to be centered in the housing before it encounters the ribs, which significantly facilitates the insertion of the cube into its housing.

[0124] In the case where the magnets have a cube shape with 12 mm edges (e=12 mm), the depth of the housing can be 9.6 mm, each face E of the housing can include a rib R in its middle, the ribs being dimensioned such that the distance separating two facing ribs is 11.7 mm and that each rib extends from the bottom Bot of the housing for a length of 7.4 mm

[0125] Fifth feature – rear opening

[0126] The Hsg housing can be equipped with an Op opening extending from its bottom Bot and opening onto the rear face BF of the support plate. This opening can have three functions.

[0127] The first function is to facilitate the removal of a cube from its housing by applying force from the back of the support plate. The second function, useful when the housing lacks corner clearances, is to facilitate the evacuation of air from the housing when inserting the magnet. The third function is to verify the correct placement of a magnet in its housing: it must, in particular, make contact with the bottom of the housing.

[0128] The housing features mentioned above can all be combined, as illustrated in the figure. Alternatively, each feature can be used on its own, or in combination with one or more of the other features. To support the Halbach rings, the housings can be arranged in concentric circles and oriented according to the desired orientation of each magnet's magnetization. Fifth aspect – gradient coils

[0129] As already mentioned, an MRI device comprises gradient coils configured to produce magnetic fields aligned parallel to the main magnetic field B0, and which vary linearly in amplitude with position along one of the x, y, or z axes, which are perpendicular to each other. We can take a z-axis parallel to the longitudinal axis LongAx, an x-axis parallel to the MaxAx axis, and a y-axis perpendicular to the plane defined by the x and y axes.

[0130] These coils are integrated into a gradient subassembly GrStr, which fulfills three functions: (i) creating variations in the main magnetic field for spatial encoding of information, (ii) providing electromagnetic shielding for the radio frequency antenna, and (iii) offering mechanical support for an object to be imaged by MRI. The subassembly can be fabricated from a Tu tube mechanically rigid enough to ensure the subassembly's structural integrity. A Sh shield for the RF antenna and gradient coils C1, C2, and C3 are concentrically plated onto this tube. The gradient coils are dedicated to varying the magnetic field B0 along the three axes x, y, and z. The shield, the radio frequency antenna, and the gradient coils are electrically isolated from one another, as illustrated in Figure 1. The tube can be made of glass fiber reinforced epoxy resin, the shield of copper layers, and the coils of copper wires.An electrically insulating resin can be used for electrical insulation. The internal free volume of the gradient subset limits the dimensions of the object to be imaged, which must be placed within it.

[0131] Figure (A) illustrates that the GrStr subassembly is intended to be installed so as to present a longitudinal axis parallel to the LongAx axis. In (A), only the C3 coils are shown, very schematically: this application will not discuss the exact geometry of the coils which allows for variations in the amplitude of the magnetic field B0 depending on the position considered relative to the x, y, and z axes.

[0132] Figure (B) illustrates a cross-section of the GrStr subset perpendicular to the LongAx axis, and shows in particular the concentric stacking of its component elements. The thickness of a GrStr wall resulting from the superposition of the elements is evident. This point can become problematic, as explained below using Figure (B).

[0133] Laillustre illustrates the integration geometries of the gradient subset GrStr in the magnetic assembly MagAss, the latter being represented by the innermost Mag magnets of the Halbach rings forming part of the magnetic assembly. The dimensions of the inlet Ent of the magnetic assembly are determined by the minimum diameter of the Halbach rings located at the ends Ext1 and Ext2 of the magnetic assembly. Of course, in a real structure, it will be necessary to consider the dimensions of the support plates holding the Halbach rings.

[0134] Laillustrate in (A) a first geometry, in which the Halbach rings at the ends Ext1 of the MagAss assembly have a reduced diameter compared to those of the central part Cent to meet the requirements of looping the magnetic field B0 and limiting the spatial extent of the magnetic field generated outside the MagAss magnetic assembly. In such a configuration, and to maintain sufficient internal volume in the GrStr subassembly, the latter is limited in length along the longitudinal axis, which leads to a limitation of the usable volume and a decrease in the uniformity of the field generated by the coils.

[0135] Figure (B) illustrates a variant of the geometry shown in (A), which represents a compromise in which only some of the coils, for example, the C1 and C2 coils, extend to the Halbach rings at the extremities Ext1 and Ext2, while the C3 coils remain limited to the central portion Cent. Thus, the GrStr subset has a first diameter reduced at the extremities Ext1 and Ext2 and a second diameter, larger than the first, at the central portion. This geometry represents an improvement over that shown in (A), but does not allow each of the coils to generate a magnetic field throughout the entire internal volume of the GrStr subset, which limits its imaging capabilities.

[0136] Figure (C) illustrates an alternative using the third aspect detailed above, in which the magnets forming the Halbach rings at the ends Ext1 and Ext2 are configured to generate a stronger magnetic field than the magnets in the central part Cent of the MagAss magnetic assembly. This feature allows for a relaxation of the constraints on the minimum diameters of the Halbach rings at the ends and / or in the central part, which can then be increased compared to the geometries shown in (A) and (B). It therefore becomes possible to extend the entire GrStr subassembly to the ends Ext1 and Ext2.

[0137] Thus, with the magnets forming the Halbach rings at the ends Ext1 and Ext2 configured to generate a stronger magnetic field than the magnets in the central part Cent of the magnetic assembly MagAss, each of the coils C1, C2, and C3 of the subassembly GrStr can extend from a longitudinal position of a Halbach ring at the end Ext1 to a longitudinal position of a Halbach ring at the end Ext2. In other words, each of the coils C1, C2, and C3 of the subassembly GrStr can extend from a longitudinal position of a support plate at the end Ext1 to a longitudinal position of a support plate at the end Ext2. The longitudinal position of an element is defined as the position of the normal projection of that element onto the longitudinal axis LongAx.

[0138] Figure (C) illustrates a geometry for a MagAss magnetic assembly in which the Halbach rings at the ends Ext1 and Ext2 have larger diameters than the MagAss assemblies shown in (A) and (B), while being smaller than the diameters of the rings in the central part Cent. Under these conditions, it becomes possible to longitudinally extend the entire GrStr subassembly such that each of the coils C1, C2, and C3 of the GrStr subassembly extends from a longitudinal position of a Halbach ring at the end Ext1 to a longitudinal position of a Halbach ring at the end Ext2, allowing for better use of the internal volume Vol of the magnetic assembly and improved uniformity of the magnetic fields generated by the set of coils C1, C2, and C3. Sixth aspect – Mobile MRI device

[0139] The MagAss magnetic assembly, equipped with the GrStr sub-assembly and a radiofrequency coil (not shown), contains all the elements necessary for magnetic resonance imaging (MRI). Such an assembly can be integrated into an MRI or magnetic resonance imaging device.

[0140] A particular application of the set described above is that of mobile MRI devices, i.e., devices with a size and weight sufficiently reduced compared to fixed MRI devices to be able to be moved manually by a person.

[0141] Laillustrates such a mobile MRI device, composed of (i) a first module Mod1 including the magnetic assembly MagAss supported by a Ca trolley on wheels and (ii) a second module Mod2 equipped with the computer hardware for controlling the elements of the first module, for acquiring and processing computer data obtained by magnetic resonance.

[0142] The first module, Mod1, comprises the wheeled Ca trolley, which can include a Str storage volume above which sits the MagAss magnetic assembly, protected by a PrHsg enclosure. The PrHsg enclosure is open on its front face to allow access to the Ent inlet of the magnetic assembly, through which a body (arm, leg, or head of a patient) to be studied is introduced into the internal Vol volume of the magnetic assembly, up to the AZ analysis zone.

[0143] The second Mod2 module can be configured so that it can be stored in the Str storage volume. The computer hardware can be connected to the radio frequency antenna and gradient coils via cables, not shown here.

[0144] Of course, the invention is not limited to the embodiments described, and alternative embodiments may be made without departing from the scope of the invention as defined by the claims. It is understood that any application requiring control of a magnetic field, particularly its uniformity, can benefit from the principles of this document, and not only magnetic resonance imaging.

Claims

Magnetic resonance imaging device (1), comprising: - a magnetic assembly (MagAss) configured to impose a permanent principal magnetic field (B0) in an internal volume (Vol) of said magnetic assembly, the internal volume generally having a cylindrical shape and extending longitudinally parallel to a longitudinal axis (LongAx) and perpendicular to a principal magnetization axis (MagAx), the magnetic assembly (MagAss) comprising: - support plates (SupPl) each having an annular section delimiting an opening (CavAprt), each support plate (SupPl) supporting permanent magnets (Mag) so as to form at least one Halbach ring (HlbRng), said support plates forming a sequence of support plates along the longitudinal axis, such that: - the housing openings delimit the internal volume;and- the Halbach rings are coaxial with each other and belong to respective planes perpendicular to the longitudinal axis, each Halbach ring being configured so as to generate a magnetic field (B; Rngparallel to the main magnetization axis, in which the support plates comprise at least one first end support plate (SupPl1), at least one second end support plate (SupPl2), and a plurality of central support plates (SupPl0) each interposed between at least one first end support plate and at least one second end support plate, and in which the permanent magnets (Mag1, Mag2) supported by at least one first end support plate and at least one second end support plate each have a first volume, the permanent magnets (Mag0) supported by the plurality of central support plates each have a second volume, the first volume being larger than the second volume. Device according to claim 1, wherein the first volume is greater than the second volume by 3% or more. Device according to claim 1 or 2, wherein the permanent magnets have the same remanence to within 3%, or to within less than 3%. Device according to claim 3, in which the permanent magnets exhibit substantially the same remanence. A device according to any one of claims 1 to 4, further comprising: - three pairs of electromagnetic coils, called gradient coils (C1, C2, C3), configured to generate three magnetic fields respectively along three directions of a three-dimensional orthogonal frame (x, y, z), in which: - each of the gradient coils extends, along the longitudinal axis (LongAx), from at least one first end support plate (SprtPl1) to at least one second end support plate (SprtPl2), and - at least one Halbach ring of at least one first end support plate and at least one second end support plate have a diameter, called end diameter, smaller than the diameter, called central diameter, of at least one Halbach ring of the plurality of central support plates. Device according to claim 5, wherein the end diameter of the smallest Halbach ring among the at least one Halbach ring of the at least one first end support plate and the at least one second end support plate is at least 10% smaller than the central diameter of the smallest Halbach ring among the at least one Halbach ring of the central support plates. Device according to any one of claims 1 to 6, wherein each of the support plates has a face perpendicular to the longitudinal axis, called front face (FF), and includes cavities forming housings (Hsg) for magnets configured to receive the permanent magnets (Mag), each housing having a depth (dep), a bottom (Bot) and at least one lateral face (E) extending from the front face to the bottom, perpendicular to the front face. Device according to claim 7, wherein at least one lateral face (E) is provided with a rib (R) extending perpendicularly to the front face (FF). Device according to claim 8, wherein the rib remains at a distance from the front face. Device according to claim 8 or 9, wherein the rib (R) extends from the bottom (Bot). Device according to any one of claims 7 to 10, wherein the housings and the permanent magnets each have cross sections forming substantially polygons, the cross sections of the housings forming re-entrant corners each having an angle clearance (C). Device according to any one of claims 7 to 11, wherein the bottoms of the housings are each respectively provided with a through opening (Op) leading to a rear face (RF) of a respective support plate, opposite and parallel to a respective front face (FF). Device according to any one of claims 7 to 12, wherein the housings are configured to lock the magnets which are inserted therein by force, by friction. A device according to any one of claims 1 to 13, wherein the Halbach rings (HlbRng) are each formed of discrete magnets each having a magnetization orientation (M0, M4), and wherein, for each of the Halbach rings, the magnetization orientations are angularly offset by a constant pivot angle when moving from one magnet to an immediately adjacent magnet in a given direction of rotation along a considered Halbach ring, the pivot angle being such that when a complete turn of the ring is made, the magnetization orientations have rotated 720° around the longitudinal axis (LongAx). Device according to any one of claims 1 to 14, comprising a first module (Mod1) including the magnetic assembly (MagAss) supported by a trolley (Ca) on wheels and a second module (Mord2) equipped with computer hardware for controlling the first module, for acquiring and processing computer data obtained by magnetic resonance.

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