Shim elements for magnetic resonance imaging

Abstract

The present invention relates to a shim element 100,...,115 for magnetic resonance applications. According to the invention, the shim element 100,...,115 comprises shim blocks 101,...,115 each separate from the other, each shim block 101,...,115 being capable of being individually inserted into a shim opening 9 of a magnetic field generating unit 2 of a magnetic resonance apparatus 1, each shim block 101,...,115 having a relative amount of shim iron 10, and a set of shim elements comprises at least two shim blocks 101,...,115 differing from each other in the respective amount of shim iron 10. In this way, the shimming procedure can be simplified and the shim volume can be increased up to a high pultrusion volume.

Description

[Technical field]

[0001] The present invention relates to the field of shim blocks used in magnetic resonance (MR) devices. [Background technology]

[0002] In Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI), shimming is used prior to the operation of the magnet to eliminate inhomogeneities in its static magnetic field. First, the magnetic field inside an NMR spectroscopy instrument or an MRI scanner is not homogeneous compared to the "ideal" magnetic field of the instrument. Summary of the Invention [Problem to be solved by the invention]

[0003] This is a result of manufacturing tolerances and the scanner's environment. The iron structures of the walls and floor of the examination room are magnetized and disturb the scanner's magnetic field. When a sample or patient is brought into the strong magnetic field, it becomes slightly magnetized and creates an additional inhomogeneous magnetic field. The process of correcting these inhomogeneities is called magnet shimming, probe shimming, or sample shimming, and depends on the assumed source of the remaining inhomogeneity. In MRI scanners, magnetic field homogeneity on the order of 1 ppm over a volume of several liters is required. High-resolution NMR spectroscopy requires magnetic field homogeneity better than 1 ppb within a volume of several milliliters.

[0004] Generally, there are two types of shimming: active shimming and passive shimming. Active shimming uses a coil that is fed with an adjustable current. Passive shimming uses a piece of iron, also called shim iron, that has suitable magnetic properties. The shim iron is placed near a permanent magnet or a superconducting magnet. They are magnetized and generate their own magnetic field. In both cases, the additional magnetic field from the coil or shim iron, respectively, increases the overall magnetic field of the superconducting magnet so as to increase the homogeneity of the total magnetic field.

[0005] Typically, an MRI scanner has a magnetic field generating unit (both B0+ gradients) with shim openings along the longitudinal axis into which the shim rails are inserted. Typically, the number of such shim openings is 12-24. The shim rails have various pockets into which the shim irons are placed. The number of such pockets is typically around 20. The number of shim irons to be placed within each pocket depends on the shimming requirements and is subject to the required shimming calculations that are performed in the course of the shimming procedure.

[0006] Inserting the shim irons into the pockets of the shim rails is prone to error because thousands of shim irons must be counted accurately, and even an accurately counted number of shim irons can be placed in the wrong pocket.

[0007] From US 2015 / 0362570 A1 a magnetic resonance imaging device is known which comprises a static magnet, a gradient coil and a shim housing box. The static magnet generates a static magnetic field in a substantially cylindrical hollow space. The gradient coil is arranged inside the static magnet and generates a gradient magnetic field. The shim housing is capable of receiving a metallic shim and is shaped such that the attractive force of the static magnetic field applied to the shim housing during excitation is smaller than a predefined threshold value.

[0008] The objective is to simplify the shimming procedure and increase the shim capacity to high pultrusion volumes. [Means for solving the problem]

[0009] According to the invention, this object is addressed by the subject matter of the independent claims. Preferred embodiments of the invention are set forth in the dependent claims.

[0010] Thus, according to the present invention there is provided a set of shim elements for a magnetic resonance apparatus, the shim elements comprising shim blocks each separate from the other, each capable of being individually inserted into a shim opening of a magnetic field generating unit of the magnetic resonance apparatus, each shim block having a relative amount of shim iron relative to the total amount of ferrous and non-ferrous material of the shim block, the set comprising at least two shim blocks whose respective relative amounts of shim iron differ from one another.

[0011] Thus, in accordance with the present invention, a plurality of separate shim blocks are used that preferably completely fill the pultrusion of the shim opening. The shim blocks are preferably located back-to-back with no space between each other. In this manner, movement is substantially prevented (made impossible) by the surrounding pultrusion, although additional interlocking features may be realized, as described in more detail below.

[0012] The shim openings are typically channels into which shim elements can be inserted in turn.

[0013] In conventional systems, a fixed number of identical shim irons are typically placed in the pockets of the shim rail. In accordance with the present invention, the relative amount of shim iron is different for different shim blocks. That is, individual blocks may have a relative amount of shim iron ranging from 0 to 100%. Individual blocks may contain an amount of shim iron and an amount of solid non-ferrous material, i.e., non-ferromagnetic or non-ferrimagnetic material. A collection of shim blocks may have dummy shim blocks that do not contain any shim iron and full shim blocks that are composed entirely of shim iron. The relative amount of shim iron is determined by the total amount, e.g., volume or weight, of shim iron and non-ferrous material in the shim block. That is, for example, the relative amount of shim iron by volume may be expressed as R Fe =V Fe / (V Fe +V non‐Fe ), where V Fe is the volume of the shim iron, and V non-Feis the volume of non-ferrous material in the block, one half of the volume of the shim block is occupied by the shim iron and the other half by the non-ferrous material (R Fe =1 / 2). The way in which the shim iron is distributed in shim blocks that are not dummy or full shim blocks can be in a variety of ways. For example, the shim iron may occupy one portion, there may be pockets or particles of shim iron distributed throughout the volume of the shim block, or the concentration of iron may be less than 100% and may vary or be uniform throughout the volume of the shim block (except for dummy and full shim blocks).

[0014] Thus, a single shim iron may be placed within each shim block, with the different shim irons differing from one another in that each provides a different amount of shim iron. According to one embodiment of the invention, the relative amounts of shim iron within each shim block are provided by a number of shim iron plates, all having the same weight and / or dimensions. In this way, only one type of shim iron is required, simplifying and facilitating the manufacture and storage of the shim iron plates.

[0015] In general, each shim element need not have the same shape. However, in accordance with one embodiment of the present invention, the shim elements all have the same outer shape, and the volume of each shim element that is not filled with shim iron is made from a magnetically passive material. In this manner, a set of shim elements can be provided that is comprised of a number of different shim blocks, each of which has a particular respective shimming effectiveness for a particular shimming area that depends on the relative amount of shim iron in each shim block.

[0016] Furthermore, the shim elements may be designed in different ways and have different types of shapes. However, according to one embodiment of the invention, the shape of the shim elements is such that two respective shim elements can be connected to each other by form fitting. Such form feet can be achieved by openings and protrusions with matching shapes, like the respective studs and dimples of a Lego block. Furthermore, according to one embodiment of the invention, each shim element is provided with a respective interlock, via which two shim elements can be interlocked with each other, so that such interlocked shim elements can even be pulled out together by just pulling one shim element.

[0017] In general, the shim iron may be arranged inside the shim block in different ways. According to one embodiment of the invention, the shim iron is arranged inside each shim block such that a portion of the shim iron forms the outer surface of the shim block or is directly adjacent to the outer surface of the shim block. In the case of multiple shim irons for a shim block, this means that the first shim iron forms the outer surface of the shim block or is directly adjacent to the outer surface of the shim block, the second shim iron is stacked directly on the first shim iron, the third shim iron is stacked directly on the second shim iron, and so on. This results in an asymmetric design that allows the shim effectiveness of the shim blocks to be changed by turning them upside down 180° in the shim opening of the respective MR device so that the shim iron of each shim block is located closer or further from the axis of the MR device bore, respectively.

[0018] According to a preferred embodiment of the invention, the shim element comprises at least one dummy block separate from all shim blocks of the set, which can be individually inserted into the shim opening of the magnetic field generating unit of the magnetic resonance apparatus, and which does not contain any shim iron. Such a dummy block has essentially zero shimming capacity and thus completes the set of shimming elements. Preferably, the shape of the dummy block is the same as the shape of the shim block or the dummy block has half the length of the shim block.

[0019] It is further preferred that each shim element includes indicia on its exterior indicating the relative amount of shim iron contained in each shim element, which simplifies the use of different shim elements and their placement in the shim openings.

[0020] Further, according to the present invention, there is provided a magnetic resonance apparatus having a bore for accommodating a subject under examination and a magnetic field generating unit surrounding the bore, the magnetic field generating unit having a plurality of shim openings extending parallel to a longitudinal axis of the bore, at least one of the plurality of shim openings having a set of shim elements as described above.

[0021] According to one embodiment of the present invention, all shim openings are completely filled with a respective set of shim elements, where if very little shimming is required, such a set may have a number of dummy blocks or a number of blocks with low shim material content.

[0022] It is further preferred that the shim elements have a cross-section which corresponds to a cross-section of the shim opening. The term "corresponding cross-section" is to be understood as meaning that the shim element has a cross-section which is slightly smaller than the shim opening so that the shim element still fits into the shim opening, i.e. with a tight fit, and that the shim elements can be inserted and removed from the shim openings, respectively, by hand.

[0023] According to one embodiment of the invention, the shim elements are connected to each other by a removable string and / or the shim elements 100,...,115 are housed in a sleeve 15. Such a string allows the shim elements to be pulled out of a shim opening that has only one open end. Having a shim block housed in the sleeve can facilitate sliding the shim elements into the shim opening and / or compensate for cross-sectional tolerances in the shim opening.

[0024] Further, according to the present invention, there is provided a method for shimming a magnetic resonance apparatus having a magnetic field generating unit having a plurality of shim openings, the method comprising the steps of providing a set of shim elements as described above, arranging the shim elements according to a predetermined order under the shimming requirements of the magnetic resonance apparatus, and sequentially inserting the arranged shim elements into their respective shim openings in the predetermined order.

[0025] Preferably, the shim elements are connected to each other by a string before being inserted into the shim opening.Furthermore, the shim elements are preferably housed in a sleeve before being inserted into the shim opening.

[0026] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter, but such embodiments do not necessarily represent the full scope of the invention, and therefore, reference should be made to the claims and this specification for interpreting the scope of the invention. [Brief description of the drawings]

[0027] [Figure 1] FIG. 1 shows a magnetic resonance apparatus according to an embodiment of the present invention. [Diagram 2] FIG. 2 shows a schematic representation of a magnetic field generating unit with shim openings according to an embodiment of the invention; [Diagram 3] FIG. 2 illustrates a set of shim elements according to one embodiment of the present invention. [Figure 4a] FIG. 2 shows a schematic diagram of two shim elements according to an embodiment of the present invention. [Figure 4b] FIG. 2 shows a schematic diagram of two shim elements according to one embodiment of the invention. [Figure 4c] 4 illustrates a shim element and a sleeve according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] FIG. 1 shows a schematic cross-sectional view of a magnetic resonance apparatus 1 with a bore 8 for accommodating an examination subject 7 arranged on a patient support 6 and a magnetic field generating unit 2 surrounding the bore 8. The magnetic field generating unit 2 comprises a radio frequency coil 3, a gradient coil 4 and a static magnetic field magnet 5. As can be seen from FIG. 2, which shows a view on the RF coil 3 along the longitudinal axis 16 of the bore 9, the magnetic field generating unit 2 has a number of shim openings 9 extending parallel to the longitudinal axis 16 of the bore 9. In these shim openings 9 a number of shim elements 100, ..., 115 can be inserted. Such shim elements 100, ..., 115 allow shimming according to the shimming requirements, which are previously subjected to the calculation of the necessary shimming in the course of the shimming procedure.

[0029] According to one embodiment of the present invention, for such shimming, a set of shim elements 100,...,115 is provided having 16 different shim elements 100,...,115, with multiples of each of the 16 different shim elements 100,...,115 being provided. This is shown diagrammatically in FIG. 3, which shows 16 different types of shim elements 100,...,115. The shim elements 100,...,115 have dummy blocks 100 with no shim iron at all, and shim blocks 101,...,115 with different relative amounts of shim iron 10. The parts of the shim elements 100,...,115 that are not made from shim iron are made from a magnetically passive material, for example a plastic material. Of course, other numbers of different shim elements are possible, for example 21 different shim elements with one dummy block, and 20 shim blocks with different amounts of shim iron, respectively. For ease of handling, each shim element 100,...,115 may have markings on its outside that indicate the relative amount of shim iron comprised by the respective shim element 100,...,115. All shim elements 100,...,115 have a cross-section that corresponds to the cross-section of shim opening 9, which means that shim elements 100,...,115 have a tight fit when inserted into shim opening 9, while still being able to be removed from shim opening 9 by hand.

[0030] While the shim iron in each shim block 101,...,115 may be provided as a single shim iron block for each shim block 101,...,115, according to the embodiment of the invention described herein, the relative amount of shim iron in each shim block 101,...,115 is provided by a plurality of shim irons 10 all having the same weight and / or dimensions, also shown in FIG. 3. From FIG. 3, it can also be seen that the shim blocks 101,...,115 all have the same outer shape, and the volume of each shim block 100,...,115 that is not filled with shim iron 10 is constructed of magnetic passive material as described above. The outer shape of the shim elements 100,...,115 is configured such that two respective shim elements 100,...,115 can be connected to one another by form fitting. Here, the outer shape is such that two respective shim elements 100,...,115 can be connected to one another by form fitting.

[0031] Another advantage of the embodiments of the invention described herein is that a shim iron 10 is disposed within each shim block 101,...,115, with a portion of the shim iron 10 forming the outer surface of the shim block 101,...,115. In this case, it is meant that the first shim iron 10 forms the outer surface of the shim block 10, the second shim iron 10 is stacked directly on the first shim iron 10, the third shim iron 10 is stacked directly on the second shim iron 10, and so on. In this way, an asymmetric design is achieved, which allows the shim effectiveness of the shim blocks 101,...,115 to be changed by turning them 180° upside down in the shim opening 9 of the MR device 1 so that the shim iron of each shim block 101,...,115 is disposed closer to or further from the longitudinal axis 16 of the bore 8 of the MR device 1.

[0032] In this way, each shim opening 9 can be filled with a subset of shim elements 100,...,115 selected according to pre-calculated shimming requirements. Each shim element 100,...,115 can be inserted respectively into a shim opening 9 of a magnetic field generating unit of the magnetic resonance apparatus 1.

[0033] As shown in Fig. 4a, according to this embodiment, the geometry of the shim elements 100,...,115 is such that two respective shim elements 100,...,115 can be connected to each other by form-fitting. In this regard, the shim block 100 shown in Fig. 4a has openings 11 and corresponding protrusions 12, respectively, on opposite sides.

[0034] Furthermore, according to another embodiment shown in FIG. 4b, the shim elements 100, ..., 115 are connected to each other by a removable string 13 that allows the shim elements 100, ..., 115 to be pulled out of a shim opening 9 that has only one open end.

[0035] 4c, in accordance with one embodiment of the present invention, the shim elements 100,...,115 are housed within a sleeve 15. Housed within the sleeve 15, the shim elements 100,...,115 can be easily slid into the shim openings 9 and can also compensate for the cross-sectional tolerances of the shim openings 9.

[0036] Regardless of whether the shim elements 100, ..., 115 are housed in a sleeve or not, according to an embodiment of the present invention, in order to place the shim elements 100, ..., 115 in the shim openings, they are arranged according to a predetermined order based on the shimming requirements for the magnetic imaging device 1, and then the arranged shim elements 100, ..., 115 are inserted sequentially into their respective shim openings 9 in the predetermined order.

[0037] While the invention has been illustrated and described in detail in the drawings and the foregoing description, such illustration and description are to be considered as illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be interpreted as limiting the scope of the claims. Moreover, for the sake of clarity, not all elements in the drawings have been labeled with reference signs. [Explanation of symbols]

[0038] Magnetic resonance equipment 1 Magnetic Field Generation Unit 2 RF coil 3 Gradient coil 4 Static magnetic field magnet 5 patient support 6 Test subject 7 Boa 8 Shim opening 9 Sim iron 10 Dummy Block 100 Shim block 101, 115 Opening 11 Projection 12 String 13 Mounting means 14 Sleeve 15 Longitudinal axis of bore 16

Claims

1. A set of shim elements for a magnetic resonance apparatus, comprising: the shim elements each have a shim block separate from the others, each shim block being individually insertable into a shim opening of a magnetic field generating unit of a magnetic resonance apparatus; each shim block having a relative amount of shim iron to the total amount of ferrous and non-ferrous material in that shim block; A set of shim elements, the set having at least two shim blocks that differ from each other in their respective relative amounts of shim iron.

2. 10. The set of shim elements of claim 1, wherein the relative amount of shim iron in each shim block is provided by a plurality of shim iron plates all having the same weight and / or dimensions.

3. 3. The set of shim elements of claim 1 or 2, wherein the shim elements all have the same outer shape, and the volume of each shim element not filled with the shim iron is composed of a magnetically passive material.

4. The set of shim elements of claim 3 , wherein the contours are configured such that two of the shim elements are connectable to one another by form-fitting.

5. The set of shim elements according to claim 1 or 2, wherein each shim element is provided with an interlock, and two of the shim elements are configured to be interlockable with each other via their respective interlocks.

6. The shim iron is disposed within each shim block; 3. The set of shim elements of claim 1 or 2, wherein the portion of the shim iron forms or is immediately adjacent to an outer surface of the shim block.

7. 3. The set of shim elements according to claim 1, wherein the shim elements have at least one dummy block, the dummy block being separate from all the shim blocks of the set, the dummy block being capable of being individually inserted into a shim opening of a magnetic field generating unit of the magnetic resonance apparatus, and the dummy block does not contain shim iron.

8. 3. The set of shim elements of claim 1 or 2, wherein each shim element has indicia on its exterior indicating the relative amount of shim iron contained in that shim element.

9. a bore for accommodating an object to be inspected; a magnetic field generating unit surrounding the bore; 3. A magnetic resonance apparatus comprising: a magnetic field generating unit having a plurality of shim openings extending parallel to a longitudinal axis of the bore, at least one of the plurality of shim openings comprising a set of shim elements according to claim 1 or 2.

10. 10. A magnetic resonance apparatus as claimed in claim 9, wherein all the shim openings are completely filled with respective sets of shim elements as claimed in claim 1 or 2.

11. 10. The magnetic resonance apparatus of claim 9, wherein the shim element has a cross section corresponding to a cross section of the shim opening.

12. 10. The magnetic resonance apparatus of claim 9, wherein the shim elements are connected to one another by detachable strings and / or the shim elements are housed in sleeves.

13. 1. A method for shimming a magnetic resonance apparatus, the magnetic resonance apparatus having a magnetic field generating unit with a plurality of shim openings, the method comprising: Providing a set of shim elements according to claim 1 or 2; ordering the shim elements according to a predetermined order based on shimming requirements for the magnetic resonance apparatus; inserting the ordered shim elements into respective shim openings in the predetermined order; A method having the following.

14. The method of claim 13 , wherein the shim elements are connected to each other by a string before being inserted into the shim openings.

15. The method of claim 13 , wherein the shim element is housed in a sleeve before being inserted into the shim opening.

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