NL2031618B1 - Shim for mitigating effects of field inhomogeneities in magnetic resonance imaging and method thereof - Google Patents

Abstract

A shim comprising a first dielectric pocket provided with at least one node for an electrical coupling; a second dielectric pocket provided with at least one node for an electrical coupling, wherein the first dielectric pocket and the second dielectric pocket form a dielectric pad; and at least one adjustable electrical coupling for conducting induced currents between respective nodes of the first dielectric pocket and the second dielectric pocket is disclosed.

Description

Shim for mitigating effects of field inhomogeneities in magnetic resonance imaging and method thereof

Field of the invention

[0001] This disclosure relates to a shim and a method of using a shim.

Background art

[0002] Magnetic Resonance Imaging (MRI) has become one of the main medical imaging modalities, with millions of scans being performed each year. The field has greatly advanced in recent years, now offering high and ultra-high magnetic field hardware, capable of acquiring high-quality images. However, at these fields, the lengths of transmitted radio frequency (RF) pulse wavelengths approach the dimensions of the human body. This results in dielectrical interactions and transmitting magnetic field (B1+) imperfections become more prominent. Image-wise, this leads to signal drop-out and cannot be easily accounted for.

[0003] So, there is a need to mitigate the effects of B” field inhomogeneities in MRI.

Summary of the invention Technical problem

[0004] There have been various approaches to address the problem of Bi" field inhomogeneities. The problem may be addressed by using MRI hardware comprising multiple RF field transmit coils. By adjusting the signals transmitted by the multiple RF field transmit coils, the magnetic field may be manipulated in order to provide a homogeneous Bi" field. However, an MRI apparatus with multiple transmit coils is significantly more expensive than an MRI apparatus with a single transmit coil and existing systems cannot easily be upgraded. Therefore, such an MRI apparatus with multiple transmit coils 1s often unavailable in practice.

[0005] Another way of addressing Bi field inhomogeneities makes use of a shim. A commercially available passive shim comprises a large dielectric pad. The large dielectric pad disrupts the magnetic field and enables passive homogenization. A passive shim comprising a large dielectric pad is described in Kirsten Koolstra, Peter Bornert,

Wyger Brink, Andrew Webb. Improved image quality and reduced power deposition in the spine at 3 T using extremely high permittivity materials. Magn Reson Med. 2018

Feb;79(2):1192-1199. doi: 10.1002/mrm.26721. Using such a passive shim is significantly cheaper than upgrading the MRI apparatus.

[0006] However, a passive shim comprising a large dielectric pad has a disadvantage that the passive shim is not very versatile and that the passive shim needs to be designed specifically for each MRI apparatus, patient, and/or anatomy. A design procedure is described in Jeroen van Gemert, Wyger Brink, Andrew Webb, Rob Remis. High- permittivity pad design tool for 7T neuroimaging and 3T body imaging. Magn Reson

Med. 2019 May;81(5):3370-3378. doi: 10.1002/mrm.27629. Because of this, using and re-using the passive shim is difficult.

[0007] So, there remains a need for a relatively cheap and flexible way to mitigate the effects of Bi" field inhomogeneities in MRI.

Technical solution

[0008] In an aspect of this disclosure, a shim is provided. The shim comprises a first dielectric pocket provided with at least one node for an electrical coupling; a second dielectric pocket provided with at least one node for an electrical coupling, wherein the first dielectric pocket and the second dielectric pocket form a dielectric pad; and at least one adjustable electrical coupling for conducting induced currents between respective nodes of the first dielectric pocket and the second dielectric pocket.

[0009] The shim according to the aspect provides a versatile device for mitigating the effects of Bi" field inhomogeneities in MRI. The shim comprises dielectric pockets for passive homogenization of the By" field. Because the dielectric pockets are provided with nodes for an electrical coupling and because the shim comprises an adjustable electrical coupling for conducting induced currents between the nodes, an effective shape of the shim may be changed by adjusting the coupling. So, passive homogenization of the magnetic field is possible in a flexible manner.

[0010] Advantageously, the adjustable electrical coupling is remotely adjustable. If the electrical coupling is remotely adjustable, it is possible to change the coupling while the shim is placed inside the scanner, for example during an MRI scan of a patient.

[0011] Advantageously, the shim further comprises a processor, wherein the processor is configured to receive a signal for adjusting the adjustable electrical couplings and wherein the processor 1s further configured to adjust the adjustable electrical coupling based on the signal. This can be a convenient way to adjust the electrical coupling.

Optionally, the processor may be configured to receive the signal for adjusting the adjustable electrical couplings through a wired connection, or wirelessly. An advantage of using a wired connection is that there is less risk of interference between an MRI apparatus and the wired connection. An advantage of wirelessly receiving the signal is that the adjustment of the adjustable coupling can be even more convenient.

[0012] Advantageously, the first and the second dielectric pocket are each provided with at least two nodes for electrical couplings. This increases the number of ways in which electrical couplings may be made. Further advantageously, the shim comprises a third dielectric pocket provided with at least two nodes for electrical couplings, the shim further comprising at least six adjustable electrical couplings for conducting induced currents between the respective at least two nodes of the respective first, second, and third dielectric pockets, wherein the at least six adjustable electrical couplings form two sets of adjustable electric couplings that form a parallel electrical circuit between the respective at least two nodes of the first, second, and third dielectric pockets. It was surprisingly found that when the adjustable couplings form a parallel circuit, a particularly strong shimming effect is achieved. Yet further advantageously, the first dielectric pocket is adjacent to both the second dielectric pocket and the third dielectric pocket. When the adjustable couplings form a parallel circuit between adjacent dielectric pockets, the particularly strong shimming effect that was surprisingly found becomes even more manifested. Furthermore, the shimming effect is localized in a focal point, that may be moved around by adjusting the couplings. This makes the shimming particularly controllable.

[0013] Advantageously, the first and/or the second dielectric pocket comprise one or more materials with a relative permittivity of at least 200, preferably at least 300, more preferably at least 500, most preferably at least 1000. A high relative permittivity is useful for homogenization of the Bi" field.

[0014] Advantageously, the first and/or the second dielectric pocket comprise a slurry, the slurry preferably comprising barium titanate. In this way, the dielectric pockets may be flexible. Alternatively, the first and/or the second dielectric pocket comprise a ceramic material, preferably lead zirconate titanate. Although a ceramic material is not flexible, the permittivity may be higher, so that particularly thin pockets may be provided.

[0015] In an aspect of this disclosure, a method of using a shim is provided. The method comprises providing a shim according to any of claims 1-11, wherein the shim is provided inside a magnetic resonance imaging, MRI, apparatus; mapping a magnetic field, using the MRI apparatus; comparing the magnetic field to a desired magnetic field, adjusting the shim, based on a result of the comparing; and iterating the mapping, the comparing, and the adjusting, until the magnetic field corresponds to the desired magnetic field.

[0016] The method of using a shim according to the aspect allows for a particularly effective way of adjusting the shim to the circumstances, such as the particular MRI apparatus, patient, and/or anatomy. In this way, some of the advantages of active shimming may be approximated, while the cost of the shim is lower than an upgrade of the MRI apparatus. So, the effects of Bi” field inhomogeneities are mitigated in a relatively cheap and flexible way.

Brief description of drawings

[0017] Embodiments of the present invention will be described hereinafter, by way of example only, with reference to the accompanying drawings which are schematic in nature and therefore not necessarily drawn to scale. Furthermore, like reference signs in the drawings relate to like elements. In the attached figures, - Figures 1A and 1B schematically show a shim. - Figure 2A schematically shows a dielectric pocket for a shim. - Figure 2B schematically shows various components for a shim. - Figure 3 schematically shows a dielectric pocket for a shim. - Figure 4 schematically shows a dielectric pocket for a shim. - Figure 5A schematically shows the absence of a shim. - Figures 5B — 5H schematically show a shim comprising various configurations of electric couplings.

- Figure 6 shows measurement results corresponding to the configurations of

Figures SA-5H. - Figure 7 shows measurement results with respect to Figure 6, plot A, corresponding to the configurations of Figures SA-5H. 5

Detailed description

[0018] Figures 1A and 1B schematically show a shim 100. The shim 100 that is schematically shown in Figures 1A and 1B comprises dielectric pockets 110, 120, 130, 140, 150, 160, 170, 180. The shim 100 that is schematically shown in Figures 1A and 1B is further provided with a cover 102. For example, the cover 102 may comprise a non- magnetic PCB. Figure 1A schematically shows a configuration when the cover 102 is not on the shim 100. Figure 1B schematically shows a configuration when the cover 102 is on the shim 100. Figure 1B schematically shows that the cover 102 comprises a plurality of nodes 110a, 110b, 120a, 120b, 130a, 130b, 140a, 140b, 150a, 150b, 160a, 160b, 170a, 170b, 180a, 180b for an electrical coupling. In detail, Figure 1B schematically shows that the cover 102 comprises two nodes per dielectric pocket.

[0019] The cask of the shim 100 that is shown in Figures 1A and 1B may, for example, be formed using 3D printing. After the 3D printing, empty pockets may be at least partially filled with a composition comprising a high relative permittivity material, to form the dielectric pockets 110-180.

[0020] The dielectric pockets 110-180 comprise one or more materials with a high relative permittivity. As used herein, a high relative permittivity is a relative permittivity of at least 100, preferably at least 300, more preferably at least 500, most preferably at least 1000. Preferably, each dielectric pocket 110-180 comprises the same one or more materials with a high relative permittivity. However, it is also possible that some or all of the dielectric pockets 110-180 comprise different one or more materials with a high relative permittivity. It is preferable if each dielectric pocket 110-180 has the same relative permittivity. The skilled person understands that with the same relative permittivity, it is meant that the various relative permittivity values are within a margin of error typical in the art. However, it is also possible that some or all of the dielectric pockets 110-180 have different relative permittivity values.

[0021] Materials with a high relative permittivity may include barium titanate, lead zirconate titanate, calcium copper titanate, barium strontium titanate, strontium titanate, titanium dioxide, and/or other high relative permittivity materials.

[0022] The dielectric pockets 110-180 may comprise the one or more high relative permittivity materials in various phases. For example, a high relative permittivity material may be a ceramic. For another example, a high relative permittivity material may be included in a slurry. As a specific example, a dielectric pocket may comprise lead zirconate titanate as a ceramic material. As another specific example, a dielectric pocket may comprise a slurry comprising barium titanate. The slurry comprising barium titanate may comprise a volume percentage of 5-50, preferably 10-40, more preferably 20-30, most preferably 23-27 of barium titanate.

[0023] An advantage of using a slurry is that the dielectric pockets 110-180 may be flexible. An advantage of using a ceramic material is that, typically, the dielectric pockets 110-180 may be made thinner.

[0024] The dielectric pockets 110-180 may be relatively small compared to a typical wavelength associated with a main magnetic field intended to be used during an MRI scan. For example, the main magnetic field may be 1.5 T, 3 T, or 7 T. For example, if the main magnetic field is 3 T, a typical frequency of 128 MHz and a typical wavelength of the order of 30 cm may be associated to the Bi” field. For another example, if the main magnetic field is 7T, a typical wavelength of the order of 10 cm may be associated to the B; field. Because the dielectric pockets 110-180 may be coupled, individual pockets can be smaller than the typical wavelength. The Bi" field strength is typically much lower than the main magnetic field strength. For example, the Br’ field strength may be less than 20 uT. Viewed from above, Figures 1A and IB show rectangular, in particular square, dielectric pockets. For example, the dielectric pockets 110-180 may have a length and a width of 1-20, preferably 2-15, more preferably 3-10, most preferably 4-7 cm. A depth of the dielectric pockets 110-180 may be 1-10, preferably 1.5-8, more preferably 2-6, most preferably 2.5-4 cm.

[0025] In Figures 1A and 1B, the dielectric pockets 110-180 are separated by walls. The walls may have a thickness of 0.1-1, preferably 0.2-0.8, more preferably 0.3-0.7, most preferably 0.4-0.6 cm.

[0026] Figures 1A and 1B further show a cover 102, on which a plurality of nodes 110a, 110b, 120a, 120b, 130a, 130b, 140a, 140b, 150a, 150b, 160a, 160b, 170a, 170b, 180a,

180b for an electrical coupling is provided. When the cover 102 is on the shim 100, the dielectric pockets are provided with the nodes for an electrical coupling. In Figures 1A and 1B, each dielectric pocket is provided with two nodes. It is, however, possible to provide different dielectric pockets with different numbers of nodes. Each dielectric pocket should be provided with at least one node, but it is possible that some or all have more than one node and the number of nodes can be, but need not be, the same for each dielectric pocket.

[0027] The nodes 110a, 110b, 120a, 120b, 130a, 130b, 140a, 140b, 150a, 150b, 160a, 160b, 170a, 170b, 180a, 180b are for electrical couplings. Between a pair of nodes, an electrical coupling may be made. These electrical couplings should be adjustable, preferably remotely adjustable. An adjustable electrical coupling between two nodes can be switched on or oft. Alternatively, a resistance of an adjustable electrical coupling may be manipulated, for example by increasing or decreasing the resistance. In this way, if there is a plurality of adjustable electrical couplings, the respective resistances of the adjustable couplings may differ. Preferably, any pair of nodes may be connected through adjustable couplings that can be switched on or off. In this way, arbitrary circuits may be made using the adjustable couplings. Adjustable couplings enable a flexible effective shape and size of the shim 100.

[0028] The nodes 110a, 110b, 120a, 120b, 1304, 130b, 140a, 140b, 150a, 150b, 160a, 160b, 1704, 170b, 180a, 180b may be provided using a non-magnetic printed circuit board, PCB. The non-magnetic PCB may be used as the cover 120.

[0029] A non-magnetic PCB comprising a plurality of nodes is readily available to the skilled person as an off-the-shelf product. Such a PCB may offer various functionalities, including enabling remote adjustment of couplings between pairs of nodes.

[0030] If remote adjustment of couplings is not available, the couplings may be adjusted in between scans, by removing the shim 100 from a scanner and adjusting the couplings manually.

[0031] Remote adjustment of couplings may be provided using a wired connection from the nodes to an outside location. For example, the nodes may be connected to a non- magnetic PCB used as the cover 120. Wires may then extend from the cover 120 and have such a length as to enable manipulation of a shim 100 located inside an MRI scanner from an outside of the MRI scanner.

[0032] Remote adjustment of couplings may alternatively be provided using a wireless connection. For example, the nodes may be connected to a non-magnetic PCB used as the cover 120. The cover 120 may further comprise a transceiver for wireless communication. Through wireless communication with the transceiver, when the shim 100 is located inside an MRI scanner, it may be manipulated from outside the MRI scanner. Although wireless communication is, in general, practical, an implementation of wireless communication functionality should take into account the strong magnetic fields that are normally present during MRI scanning as well as the RF signals of the Bi field.

[0033] Figure 2A schematically shows a dielectric pocket for a shim. Figure 2B schematically shows various components for a shim. Figure 2A schematically shows a dielectric pocket 210. The dielectric pocket 210 comprises a plurality of connectors 215a, 215b, 215¢, 215d, 215e, 215f, 215g, 215h. Figure 2B schematically shows dielectric pockets 210, 220, 230, 240, 250, 260, together with covers 222 and 232. Figure 2B schematically shows that the covers 222 and 232 are provided with nodes 220a, 220b, 230a, 230b.

[0034] The dielectric pockets 210, 220, 230, 240, 250, 260 that are schematically shown in Figure 2B may be essentially the same as the dielectric pocket 210 schematically shown in Figure 2A. A main difference between the dielectric pocket 210 schematically shown in Figure 2A and the dielectric pockets 110-180 schematically shown in Figure 1 is that the dielectric pocket schematically shown in Figure 2A is not statically connected to other dielectric pockets, but instead comprises a plurality of connectors 215a-215h for removably connecting the dielectric pocket 210 to other dielectric pockets.

[0035] The connectors 215a-215h should be suitable and MRI-conditional connectors.

For example, the connectors 215a-215h may comprise hooks, eyes, nuts, bolts, screws, sticky tape, ropes, or other suitable connectors. In this way, the dielectric pockets may be flexibly and removably connected, in order to form shims of arbitrary shapes and sizes.

[0036] The dielectric pockets are provided with a node for an electrical coupling. Figure 2B schematically shows that a dielectric pocket 220, 230 may be provided with two nodes 220a, 220b, 230a, 230b, for example on a cover 222, 232. Adjustable couplings, preferably remotely adjustable couplings, can be provided between pairs of nodes. In this way, an effective shape of a shim formed out of a plurality of dielectric pockets may be adjusted.

[0037] Because each dielectric pocket schematically shown in Figures 2A and 2B comprises a wall, when two dielectric pockets are coupled to each other, the materials with a high relative permittivity are separated by two walls. This means an effective wall- thickness of a shim schematically shown in Figures 2A and 2B may be higher than a wall-thickness of a shim 100 schematically shown in Figure 1. For example, as described above, in Figure 1 the walls may have a thickness of 0.1-1, preferably 0.2-0.8, more preferably 0.3-0.7, most preferably 0.4-0.6 cm. So, for example, a shim schematically shown in Figures 2A and 2B may have an effective wall thickness of 0.2-2, preferably 0.4-1.6, more preferably 0.6-1.4, most preferably 0.8-1.2 cm.

[0038] Further properties of a shim formed out of the dielectric pockets schematically shown in Figures 2A and 2B are similar to those of a shim 100 schematically shown in

Figure 1, and a detailed description thereof is omitted herein.

[0039] Figure 3 schematically shows a dielectric pocket 310 for a shim. The dielectric pocket 310 schematically shown in Figure 3 comprises a plurality of connectors 315a, 315b, 315¢, 315d, 315e, 315f, 315g, 315h, 315i.

[0040] The dielectric pocket 310 schematically shown in Figure 3 has a triangular shape.

Other properties of the dielectric pocket 310 schematically shown in Figure 3 may be similar to the dielectric pockets 210-260 schematically shown in Figure 2, and a detailed description thereof is omitted herein.

[0041] Figure 4 schematically shows a dielectric pocket 410 for a shim. The dielectric pocket 410 schematically shown in Figure 4 comprises a plurality of connectors 415a, 415b, 415c, 415d, 415e, 415f, 415g, 415h, 415i, 415], 415k, 4151.

[0042] The dielectric pocket 410 schematically shown in Figure 4 has a hexagonal shape.

Other properties of the dielectric pocket 410 schematically shown in Figure 4 may be similar to the dielectric pockets 210-260 schematically shown in Figure 2, and a detailed description thereof is omitted herein.

[0043] It should be appreciated that dielectric pockets for a shim may be provided in many different shapes and forms. Figure 2A schematically shows a dielectric pocket 210 with a rectangular, in particular a square, face. Figure 3 schematically shows a dielectric pocket 310 with a triangular, in particular a regular triangular, face. Figure 4 schematically shows a dielectric pocket with a hexagonal, in particular a regular hexagonal, face. However, a shim may also be provided from dielectric pockets with different shapes. For example, it is possible to have dielectric pockets with faces polygonal faces that are not regular. For another example, the faces of the dielectric pockets need not be convex. A shim may be formed from a plurality of dielectric pockets, at least some of which differ in shape. Preferably, a shim is formed from a plurality of dielectric pockets in such a way as to have the faces of the dielectric pockets form a covering without gaps.

[0044] It should further be appreciated that different dielectric pockets may be provided with different numbers of nodes. For example, a shim may be formed in which a dielectric pocket is provided with one node, while another dielectric pocket is provided with two nodes. A dielectric pocket may be provided with one, two, three, or more nodes.

Figure 1 and Figure 2B schematically show that nodes, in particular two nodes per dielectric pocket, are provided on a cover. However, it is also possible to provide a dielectric pocket with a node in a different way. For example, a node may be provided on any face of a dielectric pocket, or in an inside of a dielectric pocket. For example, in case that a dielectric pocket is to be provided with a plurality of nodes, at least some of the plurality may be on mutually different faces, and/or in an inside of the dielectric pocket. Furthermore, it is possible to form a shim out of dielectric pockets, wherein the various dielectric pockets are provided with different numbers of nodes, and/or wherein the various dielectric pockets are provided with nodes in different locations.

[0045] It should further be appreciated that the shape of a dielectric pocket may depend on a high relative permittivity material that is used for the dielectric pocket. In particular, the phase of the high relative permittivity material may influence the choice of shape of a dielectric pocket. For example, Figures 1-4 schematically show dielectric pockets in the shape of a box that may be covered. Such a shape is particularly suitable in case the high relative permittivity material is in the phase of a liquid, slurry, or the like. However, for another example, if the high relative permittivity material is in the form of a solid, for example a ceramic material, the dielectric pocket may be made directly from the high relative permittivity material, without the need for walls or a cover.

[0046] It should also be appreciated that a dielectric pocket may comprise more than one dielectric material. For example, if the dielectric pocket has a shape of a box, the walls may comprise a dielectric material in a solid phase and the box may be filled with a liquid or slurry comprising another high relative permittivity material. For another example, if the dielectric pocket is filled with a liquid or slurry, said liquid or slurry may comprise a plurality of high relative permittivity materials. A shim may be formed from a plurality dielectric pockets comprising the same high relative permittivity material(s), or from a plurality of dielectric pockets, wherein at least some comprise mutually at least partially different high relative permittivity material(s).

[0047] Figures SA-5H schematically show a shim for an experimental set-up. Figure SA schematically shows the absence of a shim. Figures 5B — SH schematically show a shim comprising various configurations of electric couplings. Figures 6 and 7 show measurement results corresponding to the configurations of Figures SA-5H.

[0048] In the experiment described with reference to Figures 5-7, a shim 100 schematically shown in Figure 1 has been implemented. In the experimental set-up, a cask with 2x4 slots was 3D printed from polylactic acid, PLA, with 0.5 cm wall thickness, forming 8 pockets of 5x5x3.5 cm volume each. The pockets were filled with a barium titanate (BaTiO3) slurry, the volume comprising 26% BaTiO:3. A printed circuit board was then fixed on top of the cask, with 2 nodes created for each pocket. The nodes can then be manually connected with short wires into chosen circuits.

[0049] The cask system was placed on top of a cylindrical systems phantom to study the

Bi” field modulation. In this experiment, the phantom was filled with Spectrasyn 4. Of course, it should be appreciated that other phantoms may also be used. Bi" magnitude maps were acquired about 2 cm deep in the phantom, parallel to the cask top.

Configurations of adjustable electrical couplings are shown in Figures 5A-5H and described in more detail below. Bi’ maps (In-plane resolution: 1.8x1.8 mm’, field of view, FOV: 224x224 mm’) were acquired using an actual flip-angle imaging, AFI sequence (described in Yarnykh, V. L. (2007). Actual flip-angle imaging in the pulsed steady state: a method for rapid three-dimensional mapping of the transmitted radiofrequency field. Magnetic Resonance in Medicine: An Official Journal of the

International Society for Magnetic Resonance in Medicine, 57(1), 192-200) with first repetition time, TR:/, second repetition time TR2/ echo time TE = 30/270/2.257 ms. A list of configurations (see Figures SC-5H) was studied, including serial, parallel and mixed circuits. Two control cases with no cask (Figure 5A), and with an unwired cask (Figure 5B) were also acquired. The experiments were performed with a 3T scanner (Ingenia,

Philips).

[0050] Figures SA-5H schematically show the experimental set-up. In Figure 5A, there is no shim. Figure SA may be used as a comparative example. In Figure 5B, there is a shim comprising dielectric pockets 110-180 and nodes 110a-180b, but there are no couplings. Figure 5B may also be used as a comparative example. Figures 5C-5H schematically show the shim of Figure 5B, in which adjustable couplings 101a-101p are further provided.

[0051] Figure SC schematically shows various couplings 101a-1011 between dielectric pockets 110-180.

[0052] Figure SD schematically shows that the couplings 101a, 101b, 101c, 101d form a circuit between dielectric pockets 110, 130, 150, 170. The couplings 101e, 101f, 101g, 101h form a circuit 120, 140, 160, 180. The couplings 101i, 101j, 101k, 1011 form a circuit between dielectric pockets 120, 140, 160, 180. The couplings 101m, 101n, 1010, 101p form a circuit between dielectric pockets 110, 130, 150, 170. So, the dielectric pockets 110, 130, 150, 170 are connected, by two sets of couplings, into a parallel circuit and the dielectric pockets 120, 140, 160, 180 are connected, by two sets of couplings, into a parallel circuit.

[0053] Figure SE schematically shows that the couplings 10le, 101f, 101g, 101h form a circuit between the adjacent dielectric pockets 130, 140, 170, 180. The couplings 101i, 101j, 101k, 1011 also form a circuit between the adjacent dielectric pockets 130, 140, 170, 180. The couplings 1014, 101b, 101c, 101d connect the adjacent dielectric pockets 110, 120, 150, 160, but do not form a closed circuit. So, the adjacent dielectric pockets 130, 140, 170, 180 are connected, by two sets of couplings, into a parallel circuit. With adjacent, it is meant that each of the couplings connects two dielectric pockets that directly face each other. For example, there is a coupling between dielectric pocket 130 and dielectric pocket 140, as well as between dielectric pocket 130 and dielectric pocket 170. In this way, the circuit 101e, 101f, 101g, 101h couples adjacent pockets 130, 140, 170, 180 and the circuit 1011, 101j, 101k, 1011 also couples adjacent pockets 130, 140, 170, 180. Each connection of these circuits is between two mutually adjacent dielectric pockets, even if, for example, dielectric pocket 130 is not adjacent to dielectric pocket 180.

[0054] Figure 5F schematically shows that the couplings 1014, 101b, 101c, 101p and 101d, 101m, 101n, 1010 respectively form, by two sets of couplings, a parallel circuit between the adjacent dielectric pockets 110, 120, 150, 160. The couplings 101e, 101f,

101g, 101h and 1011, 101j, 101k, 101k respectively form, by two sets of couplings, a parallel circuit between the adjacent dielectric pockets 130, 140, 170, 180. So, the adjacent dielectric pockets 110, 120, 150, 160, as well as the adjacent dielectric pockets 130, 140, 170, 180 are connected, by two respective sets of couplings, into one respective parallel circuit each.

[0055] Figure SG schematically shows that the couplings 1014, 101b, 101c, 101h and 101d, 101e, 101f, 101g respectively form, by two sets of couplings, a parallel circuit between the adjacent dielectric pockets 130, 140, 170, 180.

[0056] Figure SH schematically shows that the couplings 1014, 101b, 101c, 101h and 101d, 10le, 101f, 101g respectively form, by two sets of couplings, a parallel circuit between the adjacent dielectric pockets 120, 130, 160, 170.

[0057] Figures 6 and 7 show measurement results corresponding to the configurations of

Figures SA-5H.

[0058] Figure 6 shows the resulting Bi" magnitude maps, normalised to a stable region value. Plot A illustrates the comparative example with no cask on top of the phantom, while plot B shows the comparative example with the cask present but unwired, meaning all 8 dielectric pockets are uncoupled. Plots I-VI correspond to the configurations of

Figures 5C-5H, respectively. The outline and location of the 2x4 cask is also shown in white. Strong and spatially localized field changes are observed in the Bi" field for various wiring conditions. The inventors surprisingly found that parallel coupling of adjacent pockets allows for a strong and switchable field response (HI - VI).

[0059] Figure 7 shows difference maps, and thereby separates the effect of switching the circuits. Plot B-A illustrates the effect to the Bi” magnitude induced by placing an unwired cask on top of the phantom. Despite the dielectric properties of the slurry only minor changes in the Bi” field are observed for the unwired cask of Figure 5B.

[0060] The rest of the plots of Figure 7 show the difference between the unwired cask of

Figure 5B and a corresponding configuration 5C-5H. Mean values right below each dielectric mini-pad included within a white 2x4 cask matrix. For all of the coupled circuits, localized and switchable field differences are observable, allowing for homogenization of spatially variable Bi" fields. In other words, compared to Figure 5B, all of the configurations SC-5H enable control of the magnetic field and thereby allow for homogenization.

[0061] For example, parallel circuits between adjacent dielectric couplings may induce a large and localized effect on the B;° field. Smaller parallel circuits enable the effect to be particularly localized and hence allow fine-tuned control of the magnetic field. By placing such a parallel circuit in a location with signal dropout, the spatially variable Bi” field may be homogenized.

[0062] Homogenization of a spatially variable Bi field may be carried out using an iterative process. In such an iterative process, a shim may be provided inside an MRI apparatus. A quick image may be taken to map the magnetic field. Then, the couplings in the shim can be adjusted in order to improve the homogeneity of the magnetic field.

This process may be iterated by taking quick images and adjusting the couplings, until the magnetic field is sufficiently homogeneous.

[0063] In the foregoing description of the figures, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the scope of the invention as summarized in the attached claims.

[0064] In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.

[0065] In particular, combinations of specific features of various aspects of the invention may be made. An aspect of the invention may be further advantageously enhanced by adding a feature that was described in relation to another aspect of the invention.

[0066] It is to be understood that the invention is limited by the appended claims only.

In this document and in its claims, the verb "to comprise" and its conjugations are used in their non-limiting sense to mean that items following the word are included, without excluding items not specifically mentioned. In addition, reference to an element by the indefinite article "a" or "an" does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one of the elements. The indefinite article "a" or "an" thus usually means "at least one".

Claims (12)

Conclusions 1. Shim, comprising a first dielectric pocket provided with at least one junction for electrical coupling; a second dielectric bag provided with at least one junction for electrical coupling, wherein the first dielectric bag and the second dielectric bag form a dielectric block; and at least one adaptable electrical coupling for conducting induced currents between respective nodes of the first dielectric pocket and the second dielectric pocket. 2. Shim according to claim 1, wherein the adjustable electrical coupling is remotely adjustable. The shim of claim 2, further comprising a processor, wherein the processor is configured to receive a signal for adjusting the adjustable electrical linkages and the processor is further configured to adjust the adjustable electrical linkages based on the signal. The shim of claim 3, wherein the processor is configured to receive the signal for adjusting the adaptive electrical links via a wired connection. The shim of claim 3, wherein the processor is configured to wirelessly receive the signal for adjusting the adaptive electrical links. The shim according to any one of claims 1 to 5, wherein the first and second dielectric pockets are each provided with at least two nodes for electrical connections. 7. Shim according to claim 6, comprising a third dielectric pocket provided with at least two nodes for electrical couplings, the shim further comprising at least six adaptable electrical couplings for conducting induced currents between the respective at least two nodes of the respective first, second , and third dielectric pockets, wherein the at least six adaptable electrical couplings form two sets of adaptable electrical couplings that form a parallel electrical circuit between the respective at least two nodes of the first, second, and third dielectric pockets. The shim of claim 7, wherein the first dielectric pocket is adjacent to both the second dielectric pocket and the third dielectric pocket. 9. Shim according to any one of claims 1-8, wherein the first and/or the second dielectric pocket comprises one or more materials with a relative permittivity of at least 100, preferably at least 300, preferably at least 500, and preferably preferably at least 1000. 10. Shim according to any one of claims 1-9, wherein the first and/or the second dielectric bag contain a slurry, the slurry preferably comprising barium titanate. 11. Shim according to any one of claims 1-9, wherein the first and/or the second dielectric bag comprises a ceramic material, preferably lead zirconate titanate. A method of using a shim, comprising dispensing a shim according to any one of claims 1 to 11, wherein the shim is dispensed within a magnetic resonance imaging (MRI) device; mapping a magnetic field, using the MRI device; comparing the magnetic field with a desired magnetic field; adjusting the shim based on a result of the comparison; and iterating the mapping, comparing, and adjusting until the magnetic field matches the desired magnetic field.