MAGNETIC UNIT FOR USE IN A MAGNETIC RESONANCE DEVICE IN COMBINATION WITH A GRADIENT COIL UNIT FREE OF ACTIVE SHIELDING

Description

[0001] The invention relates to a magnetic unit for use in a magnetic resonance device and to a magnetic resonance device comprising such a magnetic unit.

[0002] In a magnetic resonance imaging (MRI) scanner, the body being examined, particularly a patient, is typically exposed to a relatively strong main magnetic field, for example, 0.55, 1.5, 3, or 7 Tesla, using a main magnet, especially a magnet unit. Additionally, gradient pulses are generated using a gradient coil unit. High-frequency pulses, such as excitation pulses, are then emitted via a radio frequency antenna unit using suitable antenna elements. This causes the nuclear spins of certain atoms, resonantly excited by these radio frequency pulses, to be tilted by a defined angle relative to the magnetic field lines of the main magnetic field. During the relaxation of the nuclear spins, radio frequency signals, so-called magnetic resonance signals, are emitted. These signals are received by suitable radio frequency antennas and then further processed.The desired image data can then be reconstructed from the raw data acquired in this way.

[0003] A gradient coil unit conventionally comprises three primary coils and three corresponding secondary coils. The three primary coils are typically enclosed by a primary coil unit. The three secondary coils are typically enclosed by a secondary coil unit. A primary coil is typically designed to generate a magnetic field gradient in one spatial direction. A magnetic field gradient is typically a first-order and / or linear-order magnetic field, in particular a magnetic field whose amplitude increases linearly along one spatial direction. When a primary coil is driven, stray fields are generated outside the gradient coil unit, which interact with the conductive structures surrounding the gradient coil unit, generating eddy currents. These eddy currents, in turn, generate ohmic heat, which can cause quenching of a superconducting coil unit enclosed by the magnet unit.In addition, the eddy currents themselves generate a magnetic field, a so-called vortex field, which can negatively affect the quality of image data.

[0004] Conventionally, stray fields and eddy currents are suppressed by secondary coils, with each primary coil having a corresponding secondary coil. If the primary coil generates a stray field, the corresponding secondary coil is designed to generate a magnetic field opposite to the stray field, which is particularly effective outside the patient reception area. For this purpose, the secondary coil conventionally has a conductor structure similar to the primary coil, which, like the primary coil, is driven with an inverted polarity and thus predominantly generates a first-order magnetic field that opposes the magnetic field gradient generated by the primary coil. Outside the patient reception area, the magnetic fields generated by the primary and secondary coils typically interfere destructively.A conventional gradient coil unit typically comprises a secondary coil unit with three secondary coils, each secondary coil being associated with a primary coil. The primary and secondary coils are conventionally driven by electrical currents with amplitudes up to 1 kA, undergoing frequent and rapid changes in current direction with rise and fall rates of several hundred kA / s. The driving voltage for the coil current is up to several kV.

[0005] The secondary coils require space, so a gradient coil unit with secondary coils has a smaller inner diameter than a gradient coil unit without secondary coils, resulting in a narrower patient intake area. Additionally, the active shielding, i.e., the control of the secondary coil unit, requires a significant amount of energy, which must be supplied in the form of current and voltage by gradient amplifiers. Gradient coil units without secondary coils are therefore advantageous in terms of patient intake area size and energy consumption, provided that the eddy currents affecting the magnet unit and / or superconducting coil unit and / or the resulting vortex fields acting within the patient intake area can be compensated.

[0006] US 2021 / 223339 A1 discloses a magnetic resonance device with a cryostat comprising a superconducting coil unit and a conductive shield unit. WO 2016 / 114198 A1 discloses a magnetic resonance device in which ring structures with different conductivities are arranged between the gradient coil unit and the main magnet. US 2014 / 210476 A1 discloses a magnetic resonance device in which conductive ring structures are arranged between the gradient coil unit and the main magnet.

[0007] The invention is based on the objective of providing a particularly robust magnetic unit with intrinsic shielding against stray fields and / or eddy currents generated by the gradient coil unit. This objective is achieved by the features of the independent claims. Advantageous embodiments are described in the dependent claims.

[0008] The magnetic unit according to claim 1, which is designed for use in a magnetic resonance imaging (MRI) device, comprises a hollow cylindrical vacuum chamber surrounding a cylindrical patient acquisition area along a longitudinal axis of the cylinder. The magnetic unit includes a superconducting coil unit designed to generate a static main magnetic field and arranged within the hollow cylindrical vacuum chamber. The magnetic unit further comprises a passive shielding unit comprising a support structure unit that surrounds the patient acquisition area in a hollow cylindrical shape, and at least two ring structures that are also hollow cylindrical, wherein the at least two ring structures have planar contact with the support structure unit perpendicular to the radial direction, and two of the at least two ring structures are longitudinally spaced apart from each other.

[0009] The passive shielding unit is typically located between the patient intake area and the superconducting coil unit. The superconducting coil unit is typically located within the hollow cylindrical vacuum chamber, where the temperature is less than 100 K. In particular, the area in which the superconducting coil unit is located typically has a temperature of no more than 10 K. The cylindrical axis of the hollow cylindrical vacuum chamber typically corresponds to the cylindrical axis of the cylindrical patient intake area and / or the magnet unit and / or the passive shielding unit and / or the support structure unit and / or the superconducting coil unit. The magnet unit may also include a thermal shielding unit.

[0010] The at least two ring structures typically have a shorter spatial extent in the longitudinal direction, individually and / or collectively, than the support structure unit. The outer radius of the support structure unit can correspond to the inner radius of at least one of the at least two ring structures. The inner radius of the support structure unit can correspond to the outer radius of at least one of the at least two ring structures. The planar contact between the support structure unit and the at least two ring structures preferably exists in the tangential and longitudinal directions. Fixation of the support structure unit and the at least two ring structures can be achieved by precise shaping and / or precise arrangement and / or by mechanical coupling and / or by adhesive bonding.The at least two ring structures typically have a longitudinal distance of at least 2 cm, preferably at least 5 cm, and particularly preferably at least 10 cm from each other. The at least two ring structures are typically stabilized and held in fixed positions by the support structure unit.

[0011] The at least two ring structures are typically designed as radial elevations relative to the support structure, particularly as overhangs. This makes the passive shielding unit especially rigid, robust, and / or easy to manufacture. Furthermore, the geometric arrangement of the ring structures typically influences the amplitude and / or direction of the eddy currents on the ring structures, which arise from a stray field generated when the gradient coil unit, which does not include a secondary coil unit, is driven. The passive shielding unit, and in particular the ring structures, therefore typically form a barrier between the gradient coil unit and the superconducting coil unit. Specifically, the passive shielding unit, and in particular the ring structures, is preferably designed such that the superconducting coil unit is shielded from the stray field.The at least two ring structures can also be designed such that eddy currents form on them, and the resulting vortex fields oppose the vortex fields generated on other conductive structures, at least partially compensating for them. In particular, such compensation can efficiently reduce the stray field before it reaches the superconducting coil unit. Consequently, the degree of shielding can be individually determined and / or adjusted by changing the geometric arrangement of the ring structures, preferably in a flexible manner.

[0012] The passive shielding unit exhibits exceptional rigidity due to the separate use of the support structure and the at least two ring structures, thereby reducing and / or suppressing resonant vibrations of the passive shielding unit. This prevents the generation of eddy currents due to resonant vibrations and / or the resulting heat directed towards the superconducting coil unit and / or a thermal shield. Likewise, the generation of a vortex field is reduced, which positively influences the quality of image data acquired with the magnetic resonance imaging (MRI) device. The magnetic unit according to the invention thus provides intrinsic shielding against stray fields and / or eddy currents generated by the gradient coil unit, together with the passive shielding unit.

[0013] One embodiment of the magnetic unit provides that the spatial extent in the longitudinal direction of the support structure unit is at least four times larger than the spatial extent in the longitudinal direction of one of the ring structures of the at least two ring structures. This enables a robust attachment of the at least two ring structures to the support structure unit and reduces vibration modes, which in turn can themselves generate eddy currents.

[0014] One embodiment of the magnetic unit provides that the at least two ring structures have an electrical conductivity of at least 2 x 10⁷ S / m and the support structure unit has an electrical conductivity of less than 10⁵ S / m. The at least two ring structures typically have an electrical conductivity of at least 2 x 10⁷ S / m, preferably at least 5 x 10⁷ S / m, and particularly preferably at least 50 x 10⁷ S / m. The support structure unit typically has an electrical conductivity of at most 10⁵ S / m, preferably at most 10⁴ S / m, and particularly preferably at most 5 x 10³ S / m. The support structure unit therefore typically comprises electrically insulating material. The at least two ring structures therefore typically comprise highly electrically conductive material.

[0015] This embodiment allows eddy currents to form on the at least two ring structures, which are stabilized and held in fixed positions by the support structure unit, preventing the eddy currents from being transferred to the support structure unit with lower conductivity. In particular, such a support structure unit prevents the formation of eddy currents with time constants greater than 0.5 ms, especially 0.1 ms, which is less than the time constant of the cryostat.

[0016] One embodiment of the magnetic unit provides that the support structure comprises at least one of the following materials: fiber-reinforced plastic composite, glass fiber-reinforced plastic, or carbon fiber-reinforced plastic. Such materials are particularly stiff and robust, which means the passive shielding unit is subject to very low vibrations and effectively prevents eddy currents. Furthermore, these materials are non-conductive, thus preventing the formation of eddy currents on the support structure.

[0017] One embodiment of the magnetic unit provides for the support structure to be designed as a framework or as a solid hollow cylinder. If the support structure is designed as a solid hollow cylinder, it is particularly robust and easy to manufacture. If the support structure is designed as a framework, it can, for example, take the form of a birdcage. This allows for reduced material usage.

[0018] One embodiment of the magnetic unit provides that the support structure unit comprises at least two longitudinally spaced hollow cylinders arranged around the cylinder axis. The longitudinally spaced hollow cylinders preferably have the same inner and / or outer radius and / or are arranged symmetrically to the isocenter of the magnetic unit. The isocenter typically corresponds to the spatial center of the cylindrical patient reception area. This embodiment prevents vibration modes and / or oscillation modes from being generated across the entire support structure unit and / or passive shielding unit, thus enabling robust use.

[0019] One embodiment of the magnetic unit provides that the at least two ring structures have different wall thicknesses and / or spatial extents in the longitudinal direction. The wall thickness of a ring structure and / or the support structure unit, in particular a hollow cylinder, is typically determined by the difference between the outer radius and the inner radius. This embodiment enables particularly efficient and individual control of the resulting eddy currents and their distribution, especially in the area of ​​the at least two ring structures and the superconducting coil unit.

[0020] A ring structure comprising at least two ring structures typically has a longitudinal dimension of at least 1 cm, preferably at least 2 cm, and particularly preferably at least 5 cm. A ring structure comprising at least two ring structures typically has a longitudinal dimension of at most 40 cm, preferably at most 35 cm, and particularly preferably at most 30 cm. A ring structure comprising at least two ring structures typically has a wall thickness of at least 0.5 cm, preferably at least 2 cm, and particularly preferably at least 4 cm. A ring structure comprising at least two ring structures typically has a wall thickness of at most 12 cm. cm, preferably no more than 8 cm , especially preferred from a maximum of 6 cm on .

[0021] The supporting structure unit typically has a longitudinal spatial extent of at least 120 cm, preferably from at least 140 cm, especially preferred from at least 160 cm on . The support structure unit typically has a spatial extent in the longitudinal direction of at most 200 cm, preferably at most 190 cm. cm, especially preferred heights of no more than 170 cm . The supporting structure unit typically has a wall thickness of at least 1 mm. cm, The support structure unit typically has a wall thickness of at least 4 cm, preferably at least 8 cm. The support structure unit typically has a wall thickness of at most 20 cm, preferably at most 15 cm, and particularly preferably at most 12 cm. .

[0022] The support structure unit is typically a hollow cylinder with an inner diameter of at least 65 cm, preferably at least 80 cm. cm,The support structure unit is typically designed as a hollow cylinder with an inner diameter of at most 130 cm, preferably at least 120 cm, and particularly preferably at least 110 cm.

[0023] According to the invention, the spatial extent of at least one of the at least two ring structures varies in the longitudinal and / or radial direction. In particular, a ring structure can have any contour, especially perpendicular to the radial and tangential directions.

[0024] This enables a particularly efficient and individualized reduction and distribution of eddy currents that can arise on the ring structures, especially in the area of ​​the at least two ring structures and / or the superconducting coil unit. Such ring structure shapes also allow the time constants of the eddy currents on the ring structures to be influenced and optimized.

[0025] One embodiment of the magnetic unit provides that the at least two ring structures are arranged radially on the side of the support structure facing the patient acquisition area or the side facing away from it. A ring structure arranged radially on the side facing the patient acquisition area can be referred to as the inner ring structure. A ring structure arranged radially on the side facing away from the patient acquisition area can be referred to as the outer ring structure. This embodiment therefore provides that only inner or outer ring structures are present. This enables a particularly compact design of the passive shielding unit.

[0026] One embodiment of the magnetic unit provides that the at least two ring structures comprise at least four ring structures, wherein two of the at least four ring structures are designed as inner ring structures and are arranged radially on the side of the support structure unit facing the patient reception area, and two of the at least four ring structures are designed as outer ring structures and are arranged radially on the side of the support structure unit facing away from the patient reception area.

[0027] According to this embodiment, the magnetic unit comprises at least two inner ring structures and at least two outer ring structures. In particular, different ring structures can address different types of vortex field characteristics. For example, the eddy currents generated in outer ring structures can be configured such that first- and / or third-order vortex fields are generated, which oppose and at least partially compensate for first- and / or third-order vortex fields generated on other conductive structures. For example, the eddy currents generated in inner ring structures can be configured such that fourth- and / or higher-order vortex fields are generated, which oppose and at least partially compensate for fourth- and / or higher-order vortex fields generated on other conductive structures.This enables particularly efficient shielding of the superconducting coil unit.

[0028] One embodiment of the magnetic unit provides that the at least two ring structures, in particular the inner ring structures from the outer ring structures, differ in at least one of the following properties: material, electrical conductivity, radial extent, wall thickness, spatial extent in the longitudinal direction, and longitudinal center points. The longitudinal center points typically define the longitudinal position of a ring structure. The selection of different materials for the various ring structures, which differ particularly in their electrical conductivity, allows for particularly good adaptation to different vortex field characteristics. In particular, the inner ring structures can uniformly comprise a first material, and the outer ring structures can uniformly comprise a second material. This simplifies the manufacture of the passive shielding unit.Nevertheless, this allows for an individual selection of different electrical conductivities for the inner ring structures and the outer ring structures.

[0029] One embodiment of the magnetic unit provides that the inner and outer ring structures are arranged at least partially alternately in the longitudinal direction. Such a passive shielding unit can be manufactured to be particularly robust.

[0030] One embodiment of the magnetic unit provides that an inner ring structure and an outer ring structure overlap at least partially in the longitudinal direction. The overlap of two of the at least four ring structures stiffens the passive shielding unit, making it less susceptible to vibrations and / or oscillations.

[0031] One embodiment of the magnet unit provides that the superconducting coil unit comprises at least two magnet coils, wherein a first magnet coil of the at least two magnet coils and a first ring structure of the at least two ring structures are arranged at a first position in the longitudinal direction and a second magnet coil of the at least two magnet coils and a second ring structure of the at least two ring structures are arranged at a second position in the longitudinal direction.

[0032] The first position is typically different from the second position. According to this embodiment, the first ring structure and the first magnetic coil overlap at least partially in the longitudinal direction. According to this embodiment, the second ring structure and the second magnetic coil overlap at least partially in the longitudinal direction. The second ring structure is therefore well suited to shielding the second magnetic coil from eddy currents and / or stray fields generated by a gradient coil unit. The first ring structure is therefore well suited to shielding the first magnetic coil from eddy currents and / or stray fields generated by a gradient coil unit.

[0033] One embodiment of the magnetic unit provides that the passive shielding unit is arranged inside the hollow cylindrical vacuum chamber. The at least two ring structures are preferably exposed to a temperature between 4 K and 100 K, particularly preferably between 30 K and 80 K. Typically, the conductivity of the at least two ring structures is higher at such temperatures than at room temperature, thereby reducing ohmic losses and improving the efficiency of the at least two ring structures. For this purpose, the magnetic unit can include a cooling unit, which is arranged on at least one of the at least two ring structures.

[0034] One embodiment of the magnetic unit provides that the passive shielding unit is arranged outside the vacuum chamber on the side facing the patient reception area. This allows for particularly simple manufacturing and installation of the passive shielding unit. In particular, the geometric arrangement of the at least two ring structures can be modified if necessary, thereby improving the shielding of the superconducting coil unit.

[0035] One embodiment of the magnetic unit provides that the at least two ring structures are point-symmetric about an isocenter of the patient reception area.

[0036] One embodiment of the magnetic unit provides that the at least two ring structures comprise at least one of the following materials: aluminum, aluminum with a purity of at least 99%, or copper. Such materials are particularly easy to process and exhibit particularly high conductivity.

[0037] Furthermore, the invention relates to a magnetic resonance device according to claim 14 comprising a magnet unit according to one of the preceding claims and a hollow cylindrical gradient coil unit designed to generate a magnetic field gradient by controlling at least one primary coil free of active shielding, wherein the gradient coil unit is arranged within the magnet unit and surrounds the patient acquisition area.

[0038] The advantages of the magnetic resonance device according to the invention essentially correspond to the advantages of the magnetic unit according to the invention, which are described in detail below. Features, advantages, or alternative embodiments mentioned herein can also be applied to the other claimed items and vice versa.

[0039] Further advantages, features and details of the invention will become apparent from the exemplary embodiments described below and from the drawings.

[0040] They show: Fig. 1 a magnetic resonance device according to the invention in a schematic representation, Fig. 2 a first embodiment of a passive shielding unit of a magnetic unit according to the invention in a first view, Fig. 3 a second embodiment of a passive shielding unit of a magnetic unit according to the invention in a first view, Fig. 4 a third embodiment of a passive shielding unit of a magnetic unit according to the invention in a first view, Fig. 5 a fourth embodiment of a passive shielding unit of a magnetic unit according to the invention in a first view, Fig. 6 a fifth embodiment of a passive shielding unit of a magnetic unit according to the invention in a first view, Fig. 7 a sixth embodiment of a passive shielding unit of a magnetic unit according to the invention in a first view, Fig. 8 a seventh embodiment of a passive shielding unit of a magnetic unit according to the invention in a first view, Fig.Fig. 9 shows an eighth embodiment of a passive shielding unit of a magnetic unit according to the invention in a first view, Fig. 10 shows a ninth embodiment of a passive shielding unit of a magnetic unit according to the invention in a first view, Fig. 11 shows a tenth embodiment of a passive shielding unit of a magnetic unit according to the invention in a second view, Fig. 12 shows an eleventh embodiment of a passive shielding unit of a magnetic unit according to the invention in a second view, and Fig. 13 shows a schematic representation of a cylinder to illustrate parameters.

[0041] The one in Figure 1 shown magnetic resonance device, as well as those in the Figure 2-11The embodiments of a passive shielding unit shown are in accordance with the invention only if the spatial extent in the longitudinal direction and / or in the radial direction of at least one of the at least two ring structures varies in the circumferential direction, as defined in claim 1.

[0042] Figure 1Figure 11 shows a schematic representation of a magnetic resonance imaging (MRI) device 11 according to the invention, comprising a detector unit comprising a magnet unit 13 according to the invention, a gradient coil unit 19, and a high-frequency antenna unit 20. The detector unit is typically designed in a hollow cylindrical shape. Furthermore, the MRI device 11 has a cylindrical patient acquisition area 14, wherein the patient acquisition area 14 is enclosed cylindrically by the detector unit in a circumferential direction. A patient can be moved into the patient acquisition area 14 by means of a patient positioning device. The longitudinal axis z of the cylinder of the patient acquisition area 14 corresponds to the longitudinal axis z of the detector unit, and in particular also to the longitudinal axis z of the magnet unit 13, the gradient coil unit 19, and the high-frequency antenna unit 20.

[0043] The magnetic unit 13 comprises a hollow cylindrical vacuum chamber 17 surrounding the cylindrical patient receiving area 14. A superconducting coil unit 31 is arranged within the hollow cylindrical vacuum chamber 17, which is designed to generate a strong and, in particular, constant and static main magnetic field 18. The magnetic unit 13 also includes a passive shielding unit 40 comprising a support structure unit 41 and ring structures 42, which are hollow cylindrical in shape. The ring structures 42 have planar contact with the support structure unit perpendicular to the radial direction, in particular tangentially and / or circumferentially, and are spaced apart from each other longitudinally and arranged at positions z1, z2, z3, z4, z5 in the longitudinal direction.The superconducting coil unit 31 comprises at least two, and in the illustrated case five, magnetic coils 32, which are arranged longitudinally at positions z1, z2, z3, z4, z5. The passive shielding unit 40 is arranged inside the hollow cylindrical vacuum vessel 17 in the illustrated case.

[0044] The gradient coil unit 19 comprises at least one primary coil configured to generate a magnetic field gradient for use in spatial coding during imaging and is free of active shielding, in particular free of a secondary coil, and especially free of an actively controlled secondary coil. The gradient coil unit 19 is designed in a hollow cylindrical shape and is arranged between the magnet unit 13 and the patient acquisition area 14. The gradient coil unit 19 is controlled by a gradient control unit 28.

[0045] Furthermore, the magnetic resonance imaging (MRI) device 11 comprises a high-frequency antenna unit 20, which in the case shown is designed as a body coil permanently integrated into the MRI device 11, and a high-frequency antenna control unit 29 for exciting a polarization that is established in the main magnetic field 18 generated by the magnet unit 13. The high-frequency antenna unit 20 is controlled by the high-frequency antenna control unit 29 and emits high-frequency pulses into an examination room, which is essentially formed by the patient reception area 14.

[0046] The magnetic resonance imaging (MRI) device 11 includes a control unit 24 for the control of the magnet unit 13, the gradient control unit 28, and the high-frequency antenna control unit 29. The control unit 24 centrally controls the MRI device 11, for example, by performing MR control sequences. The control unit 24 also includes a reconstruction unit (not shown) for reconstructing medical image data acquired during the MRI examination. The MRI device 11 includes a display unit 25. Control information, such as control parameters, as well as reconstructed image data, can be displayed on the display unit 25, for example, on at least one monitor, for a user. The MRI device 11 also includes an input unit 26, by means of which information and / or control parameters can be entered by a user during a measurement procedure.The control unit 24 can include the gradient control unit 28 and / or the high-frequency antenna control unit 29 and / or the display unit 25 and / or the input unit 26.

[0047] The depicted magnetic resonance device 11 can, of course, include further components that magnetic resonance devices 11 typically possess. Furthermore, the general operating principle of a magnetic resonance device 11 is known to those skilled in the art, so a detailed description of the further components is omitted.

[0048] Figure 2Figure 1 shows a first embodiment of a passive shielding unit 40 of a magnetic unit 13 according to the invention in a first view, wherein the ring structures 42 are arranged in the radial direction r on the side of the support structure unit 41 facing away from the patient reception area 14 and are thus designed as outer ring structures 44. The spatial extent in the longitudinal direction z of the support structure unit 14 is at least four times as large as the spatial extent in the longitudinal direction z of a ring structure 42. The support structure unit 41 is designed as a solid hollow cylinder. The ring structures 42, 44 differ from each other in their spatial extent in the radial direction r and in the longitudinal direction z.

[0049] Furthermore, the ring structures 42, 44 are point-symmetric about an isocenter of the patient reception area 14, which in the case shown can be defined by the intersection of the longitudinal axis z with the marking of the radial direction r.

[0050] Figure 3 Figure 1 shows a second embodiment of a passive shielding unit 40 of a magnetic unit 13 according to the invention in a first view. This differs from the first embodiment in that the passive shielding unit 40 comprises six ring structures 42, two of which are arranged radially r on the side of the support structure unit 41 facing the patient reception area 14 and are designed as inner ring structures 43, and four of which are arranged radially r on the side of the support structure unit 41 facing away from the patient reception area 14 and are designed as outer ring structures 44. The inner ring structures 43 and outer ring structures 44 are arranged at least partially alternating in the longitudinal direction z. In the longitudinal direction z, the positions, in particular the centers, of the inner ring structures 43 and the outer ring structures 44 differ from one another.

[0051] Figure 4 Figure 1 shows a third embodiment of a passive shielding unit 40 of a magnetic unit 13 according to the invention in a first view. This differs from the first embodiment in that the ring structures 42, 43 are arranged in the radial direction r on the side of the support structure unit 41 facing the patient reception area 14 and are thus designed as inner ring structures 43.

[0052] Figure 5 Figure 1 shows a fourth embodiment of a passive shielding unit 40 of a magnetic unit 13 according to the invention in a first view. This differs from the second embodiment in that the inner ring structures 43 and the outer ring structures 44 overlap at least partially in the longitudinal direction z.

[0053] Figure 6Figure 1 shows a fifth embodiment of a passive shielding unit 40 of a magnetic unit 13 according to the invention in a first view. This differs from the first embodiment in that the support structure unit 41 comprises two hollow cylinders 51, 52 spaced apart from each other in the longitudinal direction z and arranged around the cylinder axis, in particular the longitudinal direction z.

[0054] Figure 7 Figure 1 shows a sixth embodiment of a passive shielding unit 40 of a magnetic unit 13 according to the invention in a first view. This differs from the second embodiment in that the support structure unit 41 comprises three hollow cylinders 51, 52, 53 spaced apart from each other in the longitudinal direction z and arranged around the cylinder axis, in particular the longitudinal direction z.

[0055] Figure 8Figure 7 shows a seventh embodiment of a passive shielding unit 40 of a magnetic unit 13 according to the invention in a first view. This differs from the third embodiment in that the support structure unit 41 comprises two hollow cylinders 51, 52 spaced apart from each other in the longitudinal direction z and arranged around the cylinder axis, in particular the longitudinal direction z.

[0056] Figure 9 Figure 1 shows an eighth embodiment of a passive shielding unit 40 of a magnetic unit 13 according to the invention in a first view. This differs from the sixth embodiment in that the inner ring structures 43 and the outer ring structures 44 overlap at least partially in the longitudinal direction z.

[0057] Figure 10Figure 1 shows a ninth embodiment of a passive shielding unit 40 of a magnetic unit 13 according to the invention in a first view, which has inner ring structures 43 and outer ring structures 44, which differ in the following properties: material, electrical conductivity, spatial extent in radial direction r, spatial extent in longitudinal direction z and at least partially their centers in longitudinal direction z.

[0058] Figure 11 Figure 1 shows a tenth embodiment of a passive shielding unit 40 of a magnetic unit 13 according to the invention in a second view. This embodiment provides that the support structure unit 41 is designed as a framework structure.

[0059] Figure 12Figure 1 shows an eleventh embodiment of a passive shielding unit 40 of a magnetic unit 13 according to the invention in a second view. As with the invention in general, this embodiment provides that the spatial extent in the longitudinal direction z and / or in the radial direction r of a ring structure 42 varies in the circumferential direction.

[0060] Figure 13 Figure 1 shows a schematic representation of a cylinder to illustrate parameters. The cylinder axis corresponds to the longitudinal direction z, in particular the longitudinal direction z of the magnet unit 13. The circumferential direction can be characterized by dφ. The direction perpendicular to the circumferential direction dφ is called the radial direction r. The direction denoted by x is typically perpendicular to the longitudinal direction z and horizontally oriented.

[0061] Although the invention has been further illustrated and described in detail by the preferred embodiments, the invention is not limited by the disclosed examples and other variations can be derived by the person skilled in the art without leaving the scope of protection of the invention as defined by the claims.

Claims

1. Magnet unit (13) designed for use in a magnetic resonance device (11) comprising a hollow cylindrical vacuum container (17) surrounding a cylindrical patient receiving region (14) in a longitudinal direction along a cylindrical axis (z), a superconducting coil unit (31) designed for generating a static main magnetic field (18) and arranged within the hollow cylindrical vacuum container, and a passive shielding unit (40) for shielding from stray fields and / or eddy currents which are generated by a gradient coil unit (19), wherein the passive shielding unit comprises a support structure unit (41) which surrounds the patient receiving region in the form of a hollow cylinder and comprises at least two annular structures (42, 43, 44) designed as hollow cylinders, wherein the at least two annular structures make surface contact that is perpendicular to the radial direction (r) with the support structure unit, wherein two of the at least two annular structures are at a distance from one another in the longitudinal direction, characterised in that the spatial extent of at least one annular structure of the at least two annular structures in the longitudinal direction and / or in the radial direction varies in the circumferential direction.

2. Magnet unit according to claim 1, wherein the spatial extent of the support structure unit in the longitudinal direction is at least four times as great as the spatial extent of an annular structure of the at least two annular structures in the longitudinal direction.

3. Magnet unit according to one of the preceding claims, wherein the at least two annular structures have an electrical conductivity of at least 2 x 107 S / m and the support structure unit has an electrical conductivity of less than 105 S / m.

4. Magnet unit according to one of the preceding claims, wherein the support structure unit comprises at least one of the following materials: fibre-plastic composite, glass fibre-reinforced plastic, carbon fibre-reinforced plastic.

5. Magnet unit according to one of the preceding claims, wherein the support structure unit is designed as a gantry structure or as a solid hollow cylinder.

6. Magnet unit according to one of the preceding claims, wherein the support structure unit comprises at least two hollow cylinders arranged about the cylindrical axis at a distance from one another in the longitudinal direction.

7. Magnet unit according to one of the preceding claims, wherein the at least two annular structures differ from one another in terms of at least one of the following properties: material, electrical conductivity, centre points in the longitudinal direction, wall thickness, spatial extent in the longitudinal direction.

8. Magnet unit according to one of the preceding claims, wherein the at least two annular structures are arranged in the radial direction on the side of the support structure unit facing the patient receiving region or facing away from the patient receiving region.

9. Magnet unit according to one of the preceding claims, wherein the at least two annular structures comprise at least four annular structures, wherein two of the at least four annular structures are designed as inner annular structures and are arranged in the radial direction on the side of the support structure unit facing the patient receiving region and two of the at least four annular structures are designed as outer annular structures and are arranged in the radial direction on the side of the support structure unit facing away from the patient receiving region.

10. Magnet unit according to claim 9, wherein the inner annular structures and the outer annular structures are arranged in the longitudinal direction in an at least partially alternating manner.

11. Magnet unit according to one of claims 9 or 10, wherein one inner annular structure and one outer annular structure at least partially overlap in the longitudinal direction.

12. Magnet unit according to one of the preceding claims, wherein the superconducting coil unit comprises at least two magnet coils (32), wherein a first magnet coil of the at least two magnet coils and a first annular structure of the at least two annular structures are arranged at a first position in the longitudinal direction and a second magnet coil of the at least two magnet coils and a second annular structure of the at least two annular structures are arranged at a second position in the longitudinal direction.

13. Magnet unit according to one of the preceding claims, wherein the passive shielding unit is arranged within the hollow cylindrical vacuum container.

14. Magnetic resonance device (11) comprising a magnet unit (13) according to one of the preceding claims and a gradient coil unit (19) in the form of a hollow cylinder designed for generating a magnetic field gradient by actuating at least one primary coil free of active shielding, wherein the gradient coil unit is arranged within the magnet unit and surrounds the patient receiving region.