Shim iron for magnetic resonance equipment

Abstract

The present invention relates to a shim iron (130) used together with a magnetic resonance (MR) device (10), wherein the shim iron (130) comprises a stack of shim plates (131, 132, 133, 134, 135), wherein at least two of the shim plates (131, 132, 133, 134, 135) comprise slits, wherein the slits form respective slit patterns of the slit shim plates (131, 132, 133, 134, 135), and wherein the slit patterns comprise at least two different slit patterns when viewed from the same viewing direction, and the at least two different slit patterns may not be mutually congruently overlapped. In this way, a shim iron (130) is provided which does not generate heat to a high temperature due to eddy currents.

Description

Technical Field

[0001] The present invention relates to the field of shim irons used with magnetic resonance (MR) devices, and more particularly to shim irons comprising a stack of shim plates having slits cut therein to reduce heating of the shim irons caused by eddy currents. Background Art

[0002] In nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI), shimming is used prior to operation of the magnet to eliminate inhomogeneities in its static magnetic field. Initially, the magnetic field within an NMR spectrometer or MRI scanner is far from uniform compared to the "ideal" field of the device. This is a result of manufacturing tolerances and the scanner environment. Iron structures in the walls and floors of the examination room become magnetized and interfere with the scanner's field. The probe and sample or patient become slightly magnetized when placed in a strong magnetic field and establish additional inhomogeneous fields. Depending on the assumed source of the remaining inhomogeneities, the process of correcting these inhomogeneities is called shimming the magnet, shimming the probe, or shimming the sample. Field homogeneity of the order of 1 ppm in a volume of several liters is required in an MRI scanner. High-resolution NMR spectroscopy requires field homogeneity of better than 1 ppb in a volume of several milliliters.

[0003] Generally speaking, there are two types of shimming: active shimming and passive shimming. Active shimming uses coils with adjustable currents. Passive shimming involves steel sheets with appropriate magnetic properties, which are also called shim irons. The shim irons are placed near permanent magnets or superconducting magnets. They become magnetized and generate their own magnetic field. In both cases, the additional magnetic field from the coils or shim irons, respectively, is added to the total magnetic field of the superconducting magnet in a way that improves the uniformity of the entire field.

[0004] Typically, an MRI scanner includes a field generating unit (both B0 and gradient) with openings along the longitudinal axis into which shim rails are inserted. These shim rails have various compartments into which shim irons are placed. In existing MRI scanners, the shim irons are usually placed at a position where the gradient field is close to the maximum value. In addition, in such a configuration, the shim irons are mechanically connected to the gradient generating unit that tends to vibrate rather than to the B0 generating unit (the inner shell of the cryostat), which is quite static. Therefore, the recent deployment of such MRI scanners has led to an exacerbation of the long-known problem, namely, the heating of the shim irons and the resulting thermal drift of the shim field.

[0005] This means that although very good static homogeneity of the main magnetic field can be achieved by careful passive shimming, this technique suffers from the drawback that the shimming material is temperature sensitive and field drift may occur when the bore heats up (as it typically does with gradient-dense sequences). The heating of the shim plates is primarily caused by eddy currents. The time-varying magnetic flux (gradient field) induces currents in the shim plates, which heat them. Thus, changes in temperature will cause changes in the magnetic distribution established by passive shimming, which may have a detrimental effect on image quality and geometric accuracy. Therefore, passive shimming can only be implemented if it is recognized that the correct functioning of passive shimming depends on stable temperature conditions.

[0006] Therefore, various attempts to keep the temperature of the shim irons constant over the time of an imaging scan or imaging sequence are described in the patent literature, for example by preheating them locally, by dedicated cooling directly for the shim irons, or by regulating the cooling on the outflow temperature instead of the inflow temperature. Alternatively, a material or a mixture of materials can be used in which the unfavorable thermal coefficient of the magnetization saturation of the iron is counteracted.

[0007] From US 5,555,251 it is known that two opposing iron core pole pieces of a magnetic resonance medical imager generate a static magnetic field in a patient imaging volume arranged between the pole pieces. A gradient coil is placed facing the pole pieces. Thin disc-shaped or annular ferromagnetic parts, advantageously laminated by layers cut from transformer sheets, are attached to the faces of the pole pieces. Each layer is electrically insulated from adjacent layers and surfaces by enamel or fixing glue. In order to reduce eddy currents in the layers, narrow slots with radial orientation are cut into the layers before lamination. The slots in adjacent layers are oriented to avoid overlap, so that improved magnetic flux conduction is obtained in the imaging volume during the generation of the gradient magnetic field by the gradient coil.

[0008] A shim plate provided with cutouts (or slits) for suppressing eddy currents is known per se from Japanese Utility Model No. JP-H09-238917. Summary of the invention

[0009] The object of the present invention is to provide a convenient and effective way to shim an MR device using shim irons so that the heating caused by eddy currents induced by gradient fields is significantly reduced.

[0010] According to the invention, this object is solved by the subject-matter of the independent claims. Preferred embodiments of the invention are described in the dependent claims.

[0011] Therefore, according to the present invention, there is provided a shim iron for use with a magnetic resonance (MR) apparatus,

[0012] Wherein, the shim iron comprises a stack of shim plates,

[0013] wherein at least two of the shim plates include slits, the slits forming respective slit patterns of the slit shim plates, and

[0014] Therein, at least some of the slits of consecutive shim plates in the stack have different orientations.

[0015] According to the invention, it has been found that a simple and straightforward slot arrangement works well for relatively small frequencies, whereas at higher frequencies (>1000 Hz) the slot arrangement even increases the losses. This seems to be due to the effect that for high frequencies the shims cannot be processed in 2D but in 3D. Thus, it has been found that successive shims should not contain the same slot pattern, in particular should not contain a slot pattern such that the slots of successive shims overlap each other.

[0016] Thus, the gist of the present invention is to avoid such slit patterns of different shims, which in general will allow congruent coverage of consecutive shims in a stack of shims. This means that, according to the present invention, it is avoided that the slits of consecutive shims in a stack are aligned in the same way. In this way, a shim iron comprising such shims can be assembled in an easy and reliable manner, avoiding such direct coverage of the slits of consecutive / adjacent shims.

[0017] Since the slits of each slit pattern of the successive shim plates have different orientations in the plane of the shim plates, there are no overlapping (i.e., one stacked one arranged) slits in the stack of each shim plate. This avoids the constructive superposition of eddy currents generated in the successive shim plates. Thus, the heating caused by the eddy currents in the stack is offset, so that the shim iron is almost not sensitive to the heating caused by the eddy currents in its plate. The different orientations of the slit patterns of the successive shim plates can be achieved in this way, that is, different shim patterns of each shim plate are provided, and these different shim patterns are not congruent due to the rotation in the plane of the rotational symmetry angle of the shape of the plate across. Accordingly, when the shim plates are stacked and aligned according to the shape of the plate, the slits of each pattern do not overlap, which is independent of the orientation of the rotational symmetry angle of the shape of the plate with the phase difference plate. Thus, as long as the plates are stacked so that their shapes are aligned, their slit patterns will not present overlapping slits. Thus, during stacking, there is no need to pay special attention to the exact orientation of the shim plates, as long as their shapes are aligned. One embodiment which is particularly easy to manufacture is achieved by providing each shim plate with a single slit pattern having rotational symmetry about an axis of rotation which is offset in a plane relative to the rotational symmetry of the shape of the plate. Thus, relative rotation of the shim plates within the stack does not result in the shim plates having overlapping slits through the relatively rotated shim patterns. Notably, successive shim plates aligned within the stack only need to be rotated through an angle spanning the rotational symmetry of the plate shape to rotate the (identical shape) shim pattern to have its slits in different orientations.

[0018] The field generated by the induced eddy currents on one shim plate constructively superimposes with the field of the adjacent (successive) plate. This insight brings with it the need to disrupt this constructive superposition. In this regard, the present invention proposes the use of shim plates with different types of slits. The basic idea is to have a different slit pattern in each successive plate so that by stacking the plates the slits do not overlap.

[0019] Generally speaking, the present invention allows only a portion of the shim plates of the shim iron to include slits. However, according to a preferred embodiment of the present invention, all shim plates include slits. In addition, generally speaking, the shim plates can have different shapes. However, according to a preferred embodiment of the present invention, the shim plates all have the same shape, preferably a rectangle. As further described below, this allows the slit pattern of the shim plates to be correctly aligned in an easy and reliable manner when assembling the shim iron. Although the rectangular shim plates can have a square shape, according to a preferred embodiment of the present invention, the rectangular shim plates have sides of different lengths. This makes it easier to correctly align the shim plates.

[0020] The positive effects of the invention can be achieved by means of different shapes of the slits themselves. However, according to a preferred embodiment of the invention, the slits all extend straight. Although in general the shims can have different thicknesses, according to a preferred embodiment of the invention, all shims have the same thickness. The shims can also be made of different magnetizable materials. However, the shims are preferably all made of the same magnetizable material. Preferably, successive shims are electrically insulated from each other by an insulating sheet having the same shape as the shims.

[0021] There are different ways to implement the invention. According to a preferred alternative, all shims are provided with slits in the same way, wherein the slit pattern when viewed from one side of each shim is different from the slit pattern when viewed from the other side of the shim. In this regard, the slit pattern is preferably asymmetric. Thus, although two shims have the same slit pattern, by turning one shim 180° on its other side another slit pattern will be obtained, which may not be congruently overlapped with the slit pattern of the other shim.

[0022] As an alternative to the solution mentioned above, according to a preferred embodiment of the invention, the slit pattern comprises, independently of the viewing direction, at least two different slit patterns, wherein they may not overlap congruently with each other. This means that the slit pattern is always different, no matter which side of the shim plate is flipped to or from which side it is viewed. By means of at least two different slit patterns shaped in this way, a stack of shim plates can be assembled, for which the slit patterns of successive shim plates do not overlap congruently with each other.

[0023] In this regard, according to a preferred embodiment of the present invention, all slit patterns include the same number of straight slits, which have the same angle relative to each other, wherein the orientation of the slits of the slit pattern of the second shim plate is given by an integer number of non-rational angle rotations (preferably golden angle rotations) of each slit of the slit pattern of the first shim plate. In geometry, the golden angle is the smaller of the two angles established by dividing the circumference of a circle according to the golden ratio, that is, the division is to divide the circumference of the circle into two arcs so that the ratio of the length of the larger arc to the length of the smaller arc is the same as the ratio of the entire circumference to the length of the larger arc. The golden angle can be approximated to 137.508° in degrees. In this way, it can be achieved in a simple and reliable way that the slit patterns of all shim plates of the shim iron are all different from each other when viewed from the same viewing direction.

[0024] The invention further relates to a shim rail having a plurality of receiving cells, wherein at least one receiving cell comprises a shim iron as described above.

[0025] The present invention also relates to an MRI device having a bore for accommodating an object to be examined and a field generating unit surrounding the bore, wherein the field generating unit comprises a plurality of openings extending parallel to the longitudinal axis of the bore, and wherein at least one of the plurality of openings comprises a shim rail as described above.

[0026] In addition, the present invention also relates to a method for shimming an MRI device using at least one shim iron, which comprises the following method steps:

[0027] - providing at least two shims comprising slits, wherein the slits form respective slit patterns of the slit shims, wherein the slit patterns comprise at least two different slit patterns when viewed from the same viewing direction, and the at least two different slit patterns may not be mutually congruently overlapped,

[0028] - assembling the at least one shim iron by stacking the at least two shim plates in such a way that the at least two shim plates do not congruently cover each other, and

[0029] - installing the at least one shim iron on the MR device, thereby shimming the MRI device.

[0030] Preferably, the method comprises the following additional method steps:

[0031] - assembling the at least one shim iron by stacking at least one further shim plate onto the at least two shim plates in such a way that successive shim plates do not congruently overlap each other.

[0032] Finally, the present invention also relates to a method for manufacturing shim plates for shim irons to be used in an MRI apparatus, comprising the following steps:

[0033] - Provides the first slotless shim plate,

[0034] - providing the first slitless shim plate with slits using a slit pattern comprising a predetermined number of straight slits, the straight slits having predetermined relative angles with respect to each other,

[0035] - Provide a second slitless shim plate,

[0036] - providing the second slotless shim plate with slits using a slit pattern comprising the same predetermined number of straight slits as the first shim plate, the straight slits having the same predetermined relative angles with respect to each other as the first shim plate, wherein the orientation of the slits of the slit pattern of the second shim plate is given by an integer number of non-rational angle rotations (preferably golden angle rotations) of each slit of the slit pattern of the first shim plate. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] These and other aspects of the invention will become apparent from and will be elucidated with reference to the embodiments described hereinafter. However, such embodiments do not necessarily represent the full scope of the invention and reference is therefore made to the claims, which are used herein to interpret the scope of the invention.

[0038] In the attached picture:

[0039] Figure 1 A MRI scanner according to a preferred embodiment of the present invention is schematically described;

[0040] Figure 2 Schematically depicting a shim iron in the form of a block or stack of slotted shim plates according to a preferred embodiment of the present invention;

[0041] Figure 3 An enlarged cross-sectional view showing a portion of a field generating unit of an MRI scanner according to a preferred embodiment of the present invention;

[0042] Figure 4 Schematically depicts possible locations where shim rails with shim irons are mounted to a cylindrical layer according to a preferred embodiment of the present invention;

[0043] Figure 5 A shim plate according to a preferred embodiment of the present invention is schematically described;

[0044] Figure 6 A shim plate according to another preferred embodiment of the present invention is schematically described; and

[0045] Figure 7 A shim plate according to yet another preferred embodiment of the present invention is schematically described.

[0046] Reference numerals list

[0047] MRI Scanner 10

[0048] Field generation unit 100

[0049] Opening 110

[0050] Shim track 111

[0051] Air gap between cryostat and gradient coil 112

[0052] Gradient generation unit 113

[0053] Shielding coil 114

[0054] Spacer 115

[0055] Gradient coil 116

[0056] Cylindrical layer 117

[0057] B0 generation unit 118

[0058] Storage space 120

[0059] Shim iron 130

[0060] Shim plate 131

[0061] Shim plate 132

[0062] Shim plate 133

[0063] Shim plate 134

[0064] Shim plate 135 DETAILED DESCRIPTION

[0065] Figure 1 Schematically shows an MRI scanner 10 according to a preferred embodiment of the present invention. It comprises a field generating unit (both B0+gradient) having openings 110 along the longitudinal axis into which shim rails 111 are to be inserted. These shim rails 111 have a plurality of receiving cells 120 in which shim irons 130 are placed.

[0066] According to a preferred embodiment of the present invention, each MR system has more than one hundred receiving compartments 120, such as Figure 2 Shim irons 130 are shown in the form of blocks or stacks of slotted shim plates 132, 133 placed within the containment grid 120. The number of shim plates 132 per containment grid 120 to make the B0 field most uniform is determined through several iterations. Part of the so-called shimming process is done at the customer site to compensate for ferromagnetic building parts and equipment near the MRI scanner 10.

[0067] Figure 3 An enlarged cross-sectional view of a portion of a field generating unit 100 is shown. It comprises the exterior of a B0 generating unit 111 (preferably a superconducting coil within a cryostat) within which a gradient generating unit 113 is mounted. The gradient generating unit 113 comprises gradient coils 116 preferably actively shielded by shielding coils 114, with spacers 115 located between them.

[0068] Figure 4Possible positions for mounting the shim rail 111 with shim irons 130 for a cylindrical layer 117 are shown. Although option A was used in earlier scanners, option C is preferred for this preferred embodiment of the invention. Option C was chosen because it makes better use of space, allowing a larger bore diameter (e.g., 70 cm) and thus allowing obese patients to enter. This space utilization stems from the fact that the spacer 115 is always needed to separate the coils 114 and 116 anyway, so the space can be used for the shim iron layer instead of being wasted as in all other options. Furthermore, in configuration C, the shim irons 130 are placed at the position where the gradient field is maximum. Furthermore, in this configuration, the shim irons 130 are mechanically connected to the gradient generation unit 113, which tends to vibrate, rather than to the rather static B0 generation unit 118 (the inner shell of the cryostat). Therefore, option C in MRI scanners leads to an exacerbation of the long-known problem, namely, heating of the shim irons and the resulting thermal drift of the shim field.

[0069] According to a preferred embodiment of the present invention, this problem is solved by using slotted shims 132, 133, wherein adjacent shims 132, 133 comprise slit patterns that are not congruent to each other. This means that the slits of the slotted shims 132, 133 do not operate in such a way that the slits of one shim plate 132 are covered by the slits of another shim plate 133. Figure 5 An example of such a slit pattern is shown in FIG. Here, two types of slit patterns for the shim plates 132, 133 are provided. With a set of such different shim plates 132, 133, the shim irons 130 can be stacked, wherein adjacent shim plates 132, 133 do not include congruent slit patterns. In this way, the eddy currents in these shim irons 132, 133 are reduced, thereby reducing the heating of the shim irons 130.

[0070] You can also use Figure 6 A stack of shim irons 130 is shown, all of which contain the same asymmetric slit pattern, wherein adjacent shim plates 132, 133 do not include congruent slit patterns. Here, a shim plate 134 is shown, which provides four different non-congruent slit patterns from the same viewing angle, which is achieved by flipping the shim plate 134 from its back side to its front side and rotating the shim plate 134 180° along its normal axis, represented by "front side" and "back side" and "1", "2", "3", "4" on both sides of the shim plate 134. Only one type of shim plate with slits needs to be manufactured. The desired difference in the shim plates is achieved by rotating the plate around its axis.

[0071] Therefore, the numbering method of the shim plates 134 as described above or the color coding of the shim plates 134 may be used so as not to affect the workflow during shimming (ie, stacking the shim plates 134 to generate the shim iron that meets the requirements) too much. Figure 6 In the example of FIG. 1 , the MR service engineer must stack the plates so that the numbers 1 -> 2 -> 3 -> 4 -> 1 can be read consecutively when the plates are stacked. The asymmetric “+” slit pattern used here has the additional advantage that an identical slit pattern will repeat at the fifth shim plate 134 instead of at the fifth shim plate 134. Figure 5 As shown in FIG. 1 , the slits appear at the third shim plates 132 and 133, thereby further improving the efficiency of the slit setting.

[0072] Figure 7 The preferred embodiment overcomes the problem of maintaining the correct order during stacking. Here, the slits of the shim plate 135 are punched using a non-repeating pattern. The slit pattern is rotated relative to the shim plate by a non-rational angle, such as the golden angle ratio. In geometry, the golden angle is the smaller of the two angles established by dividing the circumference of a circle according to the golden ratio, that is, the division is to divide the circumference of the circle into two arcs so that the ratio of the length of the larger arc to the length of the smaller arc is the same as the ratio of the entire circumference to the length of the larger arc. The golden angle can be approximated to 137.508° in degrees. This achieves that each shim plate is unique and no longer requires numbering or coding. Thus, the shimming process can be performed as is done today, but the non-repeating slit pattern will significantly reduce eddy currents. In addition to rotating the slit pattern, its center can be slightly moved so as to reduce the size of the final slit-free area in the center. Another advantage of the proposed non-repeating simple slit pattern is that it can be easily made. The slit pattern for each plate is simple and can be rotated without difficulty.

[0073] Although the present invention has been illustrated and described in detail by the drawings and the foregoing description, such illustration and description should be regarded as illustrative or exemplary rather than restrictive; the present invention is not limited to the disclosed embodiments. By studying the drawings, the present disclosure and the appended claims, those skilled in the art will be able to understand and implement other variations to the disclosed embodiments in the process of practicing the claimed invention. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article does not exclude the plurality. The statement of certain measures in mutually different dependent claims does not mean that these measures cannot be used in combination to advantage. Any figure marks in the claims should not be understood as limiting the scope. In addition, for the sake of clarity, figure marks may not be provided for all elements in the drawings.

Claims

1. A shim rail (111) having a plurality of receiving cells (120), wherein: At least one receiving cell (120) includes shim iron (130) for use with a magnetic resonance (MR) device, The shim iron (130) comprises a stack of shim plates (131, 132, 133, 134, 135). Wherein, at least two of the shim plates (131, 132, 133, 134, 135) include slits, and the slits form respective slit patterns of the slit shim plates (131, 132, 133, 134, 135), wherein at least some of the slits of successive shim plates in the stack have different orientations, and Therein, all slit patterns comprise the same number of straight slits having the same angle relative to each other, wherein the orientation of the slits of the slit pattern of the second shim plate is given by an integer number of irrational rotations of each slit of the slit pattern of the first shim plate.

2. The shim rail according to claim 1, wherein: At least two of the shim plates have different shim patterns, and the different shim patterns are not congruent with respect to in-plane rotation across an angle of rotational symmetry of shapes of the shim plates.

3. The shim rail according to claim 1, wherein: The shim plate has a slit pattern, the slit pattern has in-plane rotational symmetry relative to a rotation axis of the slit pattern, and the rotation axis of the slit pattern is offset from an in-plane rotational symmetry axis of the shape of the shim plate.

4. The shim rail according to claim 3, wherein: The slit pattern is asymmetric with respect to one of the more symmetry axes of the shape of the shim plates.

5. The shim rail according to claim 1 or 2, wherein: The slit pattern comprises at least two different slit patterns independently of the viewing direction, wherein the at least two different slit patterns may not be made to congruently overlap each other.

6. The shim rail according to claim 5, wherein: The slit patterns are all different from each other when viewed from the same viewing direction.

7. An MRI apparatus having a bore for accommodating an object to be examined and a field generating unit (100) surrounding the bore, wherein: The field generating unit (100) comprises a plurality of openings (110) extending parallel to a longitudinal axis of the bore, and wherein at least one of the plurality of openings (110) comprises a shim rail (111) according to any one of claims 1-6.

8. A method for shimming an MRI device using at least one shim iron (130), wherein: The MRI device comprises a shim rail (111) having a plurality of accommodating grids (120), wherein at least one accommodating grid (120) comprises at least one shim iron (130), and the method comprises the following method steps: At least two of the shim plates (131, 132, 133, 134, 135) are provided, the shim plates comprising slits, the slits forming respective slit patterns of the slit shim plates, and assembling the at least one shim iron (130) by stacking the at least two shim plates (131, 132, 133, 134, 135) in such a way that at least some of the slits of successive shim plates in the stack have different orientations, and The at least one shim iron (130) is installed in the MRI device, thereby shimming the MRI device, and Therein, all slit patterns comprise the same number of straight slits having the same angle relative to each other, wherein the orientation of the slits of the slit pattern of the second shim plate is given by an integer number of irrational rotations of each slit of the slit pattern of the first shim plate.

9. The method according to claim 8, comprising the additional method step of: The at least one shim iron (130) is assembled by stacking at least one further shim plate onto the at least two shim plates in such a way that successive shim plates do not congruently overlap each other.

10. A method for manufacturing shim plates (131, 132, 133, 134, 135) for shim irons (130) to be used in an MRI apparatus, wherein: The MRI device comprises a shim rail (111) having a plurality of accommodating grids (120), wherein at least one accommodating grid (120) comprises the shim iron (130), and the method comprises the following steps: providing a first slotless shim plate, The first slitless shim plate is provided with slits using a slit pattern including a predetermined number of straight slits, the straight slits having predetermined relative angles with respect to each other, providing a second slotless shim plate, The second slotless shim plate is provided with slits using a slit pattern comprising the same predetermined number of straight slits as the first shim plate, the straight slits having the same predetermined relative angles relative to each other as the first shim plate, wherein the orientation of the slits of the slit pattern of the second shim plate is given by an integer number of irrational rotations of each slit of the slit pattern of the first shim plate.

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