Thermal management of an MRI system

Abstract

An electromagnet for a low-field magnetic resonance imaging, MRI, system (100), the electromagnet comprising a plurality of sheet elements (102) each arranged to transfer thermal energy from a first portion of the electromagnet to a second portion of the electromagnet, wherein the first portion in use has a higher temperature than the second portion, and wherein each sheet element of the plurality of sheet elements is electrically isolated from other sheet elements of the plurality of sheet elements and from the electromagnet. The application further relates to a thermal management system for a low-field MRI system (500) comprising a heat sink; and a plurality of metal wires thermally connected to the heat sink, each metal wire being arranged to transfer thermal energy from the electromagnet (502) at one end to the heat sink at another end, wherein each metal wire is electrically isolated from the other metal wires and from all other elements of the thermal management system. The application also relates to an electromagnet (702) for a low-field MRI system (700,) comprising one or more phase change material blocks (704) arranged to absorb thermal energy from the electromagnet, wherein each phase change material block comprises a phase change material having a transition temperature within an operating temperature range of the electromagnet.

Description

[0001] Thermal management of an MRI system

[0002] FIELD OF INVENTION

[0003] The invention relates to thermal management systems for magnetic resonance imaging systems. In particular, the invention relates to thermal management systems for an electromagnet in a low-field magnetic resonance imaging system.

[0004] BACKGROUND

[0005] A pre-polarization electromagnet in a low-field magnetic resonance imaging (MRI) system potentially generates large quantities of heat. Measurement sensitivity scales with the pre-polarization field, which in turn scales with the current in the coil of the electromagnet. For coils of appropriate size for imaging of human-scale features, the power to run the coil can be in the range of 1-10 kW or more, and this power is converted to heat through resistive processes. Such large quantities of heat need to be removed from the system, or else the temperature of the coil may exceed operating limits. In addition, in medical MRI applications the heat must be prevented from exposing the subject being imaged to unsafe temperatures.

[0006] Heat can be removed through either active or passive means. Active heat removal is typically accomplished using a circulating fluid, which may be at room temperature or chilled, for example air or water. However, such a fluid circulation system tends to be bulky. In portable MRI systems, space may be at a premium, and any volume dedicated to the fluid circulation system may need to come at the cost of a smaller electromagnet, in turn lowering the pre-polarization field strength and thus the MRI signal.

[0007] In contrast, passive heat removal can be accomplished using solid materials with high thermal conductivity, which serve to conduct heat away from the electromagnet toward a heat sink located elsewhere, which may itself operate either actively or passively. Passive cooling can require less volume than active cooling, but materials must be carefully chosen to ensure adequate capacity.

[0008] Metallic materials are often used for this purpose in other applications, however they are inappropriate for inclusion in an MRI electromagnet. It is generally necessary to construct an MRI system from as little metal as possible, so as to prevent the metal from distorting the radio-frequency signals of the measurement or otherwise decreasing measurement sensitivity.

[0009] Non-metallic materials are therefore preferred for passive thermal cooling of a low-field MRI electromagnet. Thermal ceramics, such as aluminium nitride, aluminium oxide, and boron nitride, have relatively high thermal conductivity but very low electrical conductivity. However, such materials are generally very expensive both to manufacture and to machine. They may therefore be uneconomical for this application.

[0010] Thus, there is a need for an improved thermal management system for the electromagnets in low-field MRI systems.

[0011] SUMMARY OF INVENTION

[0012] According to an aspect there is provided an electromagnet for a low-field magnetic resonance imaging, MRI, system, the electromagnet comprising: a plurality of sheet elements each arranged to transfer thermal energy from a first portion of the electromagnet to a second portion of the electromagnet, wherein the first portion in use has a higher temperature than the second portion, wherein each sheet element of the plurality of sheet elements is electrically isolated from other sheet elements of the plurality of sheet elements and from the electromagnet.

[0013] Graphite has a relatively high in-plane thermal conductivity. As such, it has been realized that thermal energy (i.e., heat) can be effectively transferred using sheet elements (i.e., elements comprising graphite).

[0014] In particular, graphite’s high in-plane thermal conductivity can be used advantageously to transfer the thermal energy within an electromagnet.

[0015] Additionally, it has been found that the eddy currents are minimized in sheet elements that are small and electrically isolated from one another. These eddy currents, caused by rapid changes in magnetic fields, do not significantly affect the MRI signals. Similarly, such sheet elements do not significantly affect the sensitivity of nearby MRI detection coils.

[0016] In the case where the electromagnet also comprises a heat sink, the plurality of sheet elements may each be arranged to transfer thermal energy from the electromagnet to the heat sink in use.

[0017] The plurality of sheet elements may be in direct thermal contact with the electromagnet, or indirect thermal contact with the electromagnet via a thermally conductive intermediary (e.g., a thermally conductive and electrically isolating substrate).

[0018] In an embodiment the size of each sheet element of the plurality of sheet elements is such that, in use, eddy currents generated in the plurality of sheet elements do not: substantially modify the magnetic field in a predetermined region spaced away from a front face of the electromagnet, when compared to a magnetic field generated in use in the predetermined region by an equivalent electromagnet that does not comprise the plurality of sheet elements, and / or substantially increase the resistance of coils in the electromagnet, when compared to the resistance of coils in an equivalent electromagnet that does not comprise the plurality of sheet elements.

[0019] The magnetic fields resulting from the eddy currents can affect the magnetic field of the electromagnet, thus affecting the MRI signals. As such, preferably the size of the sheet elements is limited / restricted to reduce the magnitude of the magnetic fields generated by eddy currents during use.

[0020] The size of the sheet elements may be smaller than a predetermined sheet element size, where the predetermined size may be dependent on the particular use case (e.g., the magnitudes of the magnetic fields it is exposed to and their slew rates during changes in those amplitudes) and / or the aspect ratio of the sheet element shape(s).

[0021] In an embodiment the electromagnet further comprises gaps between sheet elements, the gaps having a width of between 0.2 mm and 1 mm.

[0022] By providing the gaps between the sheet elements, the magnetic fields caused by eddy currents modifying the magnetic field of the electromagnet are minimized.

[0023] In an embodiment the electromagnet further comprises an electrically insulating material in the gaps.

[0024] The electrically insulating material can be used to fill the gaps between the thermally conductive elements and allow thermal energy to be transferred between the thermally conductive elements more efficiently (e.g., relative to air).

[0025] The electrically insulating material may comprise one or more of potting compounds, silicone and epoxy resins.

[0026] Preferably, the electrically insulating material is also thermally conductive so as to improve the conductance of thermal energy across the gaps. The electrically insulating material may have a thermal conductivity of at least 0.1 , 0.2 or 0.4 W / m*K. Suitable materials have been found with thermal conductivities in the range of 1 to 4 W / m*K.

[0027] The electrically insulating material may have an electrical conductivity of less than 10-1or 10-2S / m. Preferably, the electrically insulating material has an electrical conductivity of less than 10-3S / m. Suitable materials have been found with an electrical conductivity of about 10-14S / m.

[0028] The electrically insulating material may have a relative electrical permittivity value with a real component less than 5 and an imaginary component less than 0.05. Preferably, the electrically insulating material may have a relative electrical permittivity value with a real component less than 5 and an imaginary component less than 0.01.

[0029] In an embodiment the sheet elements are formed from a graphite sheet or from a plurality of stacked graphite sheets.

[0030] Preferably, the sheet elements comprise at most 10 stacked graphite sheets.

[0031] In an embodiment the sheet elements have a length and / or width of less than 5cm, and preferably less than 2cm.

[0032] The length and width are defined as any two non-parallel directions within the plane of the sheet element(s).

[0033] In an embodiment the sheet elements have a thickness of less than 10mm, and preferably less than 5mm.

[0034] In practice, the sheet elements may have a thickness of less than 2mm. The thickness is defined along the direction orthogonal to the plane of the sheet element(s).

[0035] In an embodiment a gap between the sheet elements is at least 0.1 mm and at most 2mm in thickness.

[0036] It has been found that a separation between 0.2 and 1 .0 mm is preferred.

[0037] Relatively small separations between the thermally conductive elements provide a more efficient transfer of thermal energy between the thermally conductive elements. The separation must nevertheless be large enough to prevent conduction of electrical currents in the gap.

[0038] In an embodiment the sheet elements are arranged to transfer thermal energy parallel to a winding plane or to winding planes of the electromagnet.

[0039] In an embodiment the electromagnet comprises a plurality of winding planes and wherein the sheet elements are arranged between winding planes of the plurality of winding planes.

[0040] In an embodiment a first sheet element layer in thermal contact with a first winding plane of the electromagnet is formed from a first set of the sheet elements having a first thickness and a second graphite layer in thermal contact with a second winding plane of the electromagnet is formed from a second set of sheet elements having a second, smaller, thickness.

[0041] The first graphite layer may be placed on a first winding plane expected to be hotter than the second winding plane in use. Thus, the thicker sheet elements can be used on the hotter winding planes, allowing for more thermal energy transfer where needed. Of course, it will be appreciated that various combinations of graphite layer thicknesses can be used depending on the use case.

[0042] According to another aspect there is provided a system comprising an electromagnet of any preceding claim and a heat sink, wherein the sheet elements are arranged to transfer thermal energy from the electromagnet to the heat sink.

[0043] It will be appreciated that the sheet elements may transfer thermal energy to the heat sink via a thermal intermediary (e.g., a thermally conductive block / plate, a phase change material block, a thermal pad / epoxy etc.).

[0044] The sheet elements may be in direct thermal contact with the heat sink, or they may be in indirect thermal contact with the heat sink (e.g., via the thermally conductive intermediary).

[0045] According to another aspect there is provided a thermal management system for a low-field magnetic resonance imaging, MRI, system, the system comprising: an electromagnet; a heat sink; and a plurality of metal wires thermally connected to the heat sink, each metal wire being arranged to transfer thermal energy from the electromagnet at one end to the heat sink at another end, wherein each metal wire is electrically isolated from the other metal wires and from all other elements of the thermal management system.

[0046] Preferably, each metal wire is fully electrically isolated from all other elements of the MRI system. Put in other words, each metal wire is preferably not electrically connected to any surrounding element, so that no potential is or can be impressed on the metal wires / so that they have a floating potential, such that electrical currents are not generated in the metal wires.

[0047] Metal wires are good thermal conductors. However, they also form eddy currents in response to being exposed to the changing magnetic fields of the MRI system. The relatively small volume of the metal wires minimizes the magnitude of the magnetic fields generated as a result of the eddy currents formed in the metal wires.

[0048] Each metal wire may comprise an electrically insulating layer along the length of the metal wire.

[0049] Preferably, the metal wires are made from copper or aluminium.

[0050] The metal wires may be in thermal contact with the electromagnet via a non- electrically conductive, thermally conductive, intermediary, such as thermal epoxy or a thermal pad. The plurality of metal wires may be in direct thermal contact with the heat sink and / or the electromagnet, or they may be in indirect thermal contact with the heat sink and / or the electromagnet via a thermally conductive intermediary (e.g., a thermally conductive plate, a thermal pad / epoxy etc.).

[0051] In an embodiment the size of each metal wire of the plurality of metal wires is such that, in use, eddy currents generated in the metal wires do not: substantially modify the magnetic field in a predetermined region spaced away from a front face of the electromagnet, when compared to an equivalent electromagnet that does not comprise the plurality of metal wires, and / or substantially increase the resistance of coils in the electromagnet, when compared to the resistance of coils in an equivalent electromagnet that does not comprise the plurality of metal wires.

[0052] In an embodiment the metal wires are each a strand of a stranded wire, such as Litz wire.

[0053] In an embodiment the plurality of metal wires is embedded within an electrically insulating material to form one or more thermally conductive blocks.

[0054] Preferably, at least 50% of the volume of the thermally conductive blocks is comprised of the plurality of metal wires, and even more preferably at least 75%.

[0055] Preferably, the electrically insulating material is also thermally conductive so as to improve the overall thermal conductance of the thermally conductive blocks. The electrically insulating material may have a thermal conductivity of at least 0.1 , 0.2, or 0.4 W / m*K. Suitable materials have been found with thermal conductivities in the range of 1 to 4 W / m*K.

[0056] The electrically insulating material may have an electrical conductivity of less than 10-1or 10-2S / m. Preferably, the electrically insulating material has an electrical conductivity of less than 10-3S / m.

[0057] In an embodiment the metal wires are arranged substantially parallel to each other.

[0058] A reference to a substantial parallel arrangement in this context encompasses twisted wired / Litz wires where the individual strands / wires, although twisted around each other, can be considered to be arranged in parallel to each other when considered over their length.

[0059] In an embodiment the metal wires have a diameter of at most 1 mm and preferably of at most 0.1 mm. Suitable diameters have been found in the order of 0.1mm when using Litz wires (i.e., copper metal wires). Of course, the maximum thickness may depend on the material used for the metal wires to minimize eddy currents.

[0060] In an embodiment the metal wires extend substantially parallel to a direction between the electromagnet and the heat sink.

[0061] According to another aspect there is provided an electromagnet for a low-field magnetic resonance imaging, MRI, system, the electromagnet comprising: one or more phase change material blocks arranged to absorb thermal energy from the electromagnet, wherein each phase change material block comprises a phase change material having a transition temperature within an operating temperature range of the electromagnet.

[0062] Phase change materials have the advantage that more thermal energy can be absorbed from the electromagnet at the transition temperature without increasing the temperature of the phase change material. Thus, it has been realized that phase change materials can be advantageously used in an MRI system to mitigate high temperature spikes of the electromagnet. This is due to the typical high to low temperature cycles (i.e., in use vs at idle) of the MRI system.

[0063] The transition temperature is the temperature at which the phase change material changes phase.

[0064] The operating temperature range of the electromagnet is the range of temperatures at which the electromagnet operates during use. The operating temperature range may include, for example, 20 to 100 degrees Celsius or 20 to 110 degrees Celsius.

[0065] The phase change material blocks may be in direct contact with the electromagnet during use. Alternatively, or additionally, the phase change material blocks may be in thermal contact with the electromagnet, during use, via thermal epoxy and / or a thermal pad.

[0066] In an embodiment the phase change material blocks further comprise a secondary material configured to transfer thermal energy from the electromagnet to the phase change material, the secondary material having thermal conductivity that is higher than the thermal conductivity of the phase change material.

[0067] Phase change materials generally have a low thermal conductivity. As such, they can be paired with secondary materials with high thermal conductivity to improve the dissipation of thermal heat away from phase change material, thus allowing it to absorb more thermal energy from the electromagnet.

[0068] Preferably, the thermal conductivity is at least 10 times higher, and even more preferably at least 100 times higher.

[0069] In an embodiment the secondary material comprises one or more of: thermally conductive ceramic material; graphite material; and metallic material, such as Litz wire.

[0070] The size of the graphite and / or metallic material is such that, in use, eddy currents generated in the graphite and / or metallic material do not substantially modify the magnetic field in a predetermined region spaced away from a front face of the electromagnet, when compared to a magnetic field generated in use in the predetermined region by an equivalent electromagnet that does not comprise the phase change material block with graphite and / or metallic material, and / or substantially increase the resistance of coils in the electromagnet in use, when compared to the resistance of coils in an equivalent electromagnet that does not comprise the phase change material block with graphite and / or metallic material.

[0071] In an embodiment the secondary material is arranged to form a scaffold structure within each phase change material block, wherein the scaffold structure is at least partially surrounded by the phase change material.

[0072] The scaffold structure preferably comprises a plurality of elements that lead from the surface of the phase change material block to its interior. Thus, the surface area of the secondary material which is in contact with the phase change material is increased. This allows the thermal energy to be transferred to and away from the phase change material more effectively.

[0073] In an embodiment the transition temperature of the phase change material in at least one of the phase change material blocks is closer to a midpoint of the operating temperature range than to the endpoints of the operating temperature range.

[0074] Preferably, the transition temperature of the phase change material is within + / - 20% of the midpoint of the operating temperature range, compared to the overall extent of the operating temperature range.

[0075] In an embodiment the electromagnet comprises one or more second phase change material blocks, each comprising a second phase changing material, for absorbing thermal energy from a second electromagnet, wherein the phase change material of the second phase change material blocks has a different transition temperature than that of the phase change material of the first phase change material blocks.

[0076] Preferably, in an embodiment comprising multiple electromagnets that reach different operating temperatures, different phase change materials are provided for each of the electromagnets based on the particular operating temperatures.

[0077] In an embodiment the electromagnet further comprises one or more thermally conductive elements arranged to transfer thermal energy away from the phase change material blocks and, optionally, transfer thermal energy away from the electromagnet.

[0078] The thermally conductive elements may be in thermal contact with the phase change material blocks via direct physical contact, thermal epoxy and / or a thermal pad.

[0079] In an embodiment one or more of the thermally conductive elements is a thermally conductive block comprising metal wires, such as Litz wires, embedded within a solid, electrically insulating, material.

[0080] Preferably, the electrically insulating material is also thermally conductive so as to improve the overall thermal conductance of the thermally conductive block. The electrically insulating material may have a thermal conductivity of at least 0.1 , 0.2, or 0.4 W / m*K. Suitable materials have been found with thermal conductivities in the range of 1 to 4 W / m*K.

[0081] The electrically insulating material may have an electrical conductivity of less than 10-1or 10-2S / m. Preferably, the electrically insulating material has an electrical conductivity of less than 10-3S / m.

[0082] In an embodiment one or more of the thermally conductive elements comprises a thermally conductive ceramic.

[0083] The thermally conductive ceramic may have a thermal conductivity of at last 20 W / m*K. Preferably, the thermally conductive ceramic may have a thermal conductivity of at last 100 W / m*K.

[0084] In an embodiment the electromagnet further comprises a heat sink configured to dissipate thermal energy from the phase change material blocks.

[0085] Herein is also provided an electromagnet for a low-field magnetic resonance imaging, MRI, system, the electromagnet comprising: a plurality of thermally conductive elements (e.g., sheet elements or metal wires) each arranged to absorb thermal energy from the electromagnet, wherein each thermally conductive element is electrically isolated from the other thermally conductive elements. Preferably, the size of each of the thermally conductive elements is restricted such that, in use, eddy currents generated in the thermally conductive elements result in magnetic fields with a magnitude lower than a predetermined magnitude threshold in a predetermined region spaced away from a front face of the electromagnet.

[0086] Preferably, the size of each of the thermally conductive elements is restricted such that the thermally conductive elements do not significantly decrease the sensitivity of any nearby radiofrequency detection coil.

[0087] Preferably, the size of each thermally conductive element is such that, in use, eddy currents generated in the thermally conductive elements do not substantially increase the resistance of coils in the electromagnet, when compared to the resistance of coils in an equivalent electromagnet that does not comprise the plurality of thermally conductive elements.

[0088] Preferably, each thermally conductive element has a volume lower than 50 cm3. Even more preferably, said volume is lower than 10 cm3. It will be appreciated that for components made of other materials with very low electrical conductivity, such as ceramics or epoxies, the volume of the component can be larger than stated here.

[0089] Preferably, each thermally conductive element made from material having an electrical conductivity above 106S / m, if any, has a volume lower than 1 cm3. Even more preferably, said volume is lower than 0.1 cm3.

[0090] Magnetic fields resulting from eddy currents can affect the measurements performed by the MRI system. In particular, the radiofrequency signals received by the MRI system during a measurement can be distorted by the magnetic fields resulting from the eddy currents. Additionally, the signal of the protons being measured by the MRI system can degrade or otherwise change in a disadvantageous manner.

[0091] It has been realized that limiting the volume of the thermally conductive elements, and electrically isolating them from each other, reduces the magnitude of the eddy currents generated in the thermally conductive elements when the magnetic field of the electromagnet is switched (e.g., from a pre-polarization magnetic field to a measurement magnetic field).

[0092] The reduced magnitude of the eddy currents results in a reduced magnetic field caused by the eddy currents, thus reducing the impact of the eddy currents on the measurements of the MRI machine.

[0093] It has been realized that limiting the volume of the thermally conductive elements, and electrically isolating them from each other, reduces the increase in resistance otherwise caused in nearby radiofrequency detection coils. The volume of each thermally conductive element may be lower than 50, 40, 30, 20 or 10 cm3. When highly electrically conductive materials (e.g., metals) are used for the thermally conductive elements, the volume is preferably lower than 5, 4, 3, 2 or 1 cm3, and even more preferably lower than 0.5, 0.4, 0.3, 0.2, or 0.1 cm3.

[0094] BRIEF DESCRIPTION OF FIGURES

[0095] Exemplary embodiments of the invention are described below with reference to the accompanying figures, in which:

[0096] Figure 1 shows an exemplary graphite layer comprising a plurality of distinct graphite elements;

[0097] Figure 2 shows an exemplary electromagnet with the graphite layers of Figure 1 ;

[0098] Figure 3 shows an exemplary electromagnet coil with a graphite layer;

[0099] Figure 4 shows a cross section of a thermally conductive block comprising a plurality of Litz wires;

[0100] Figure 5 shows a cross section of an exemplary low-field MRI system;

[0101] Figure 6 shows the low-field MRI system of Figure 5; and

[0102] Figure 7 shows an exemplary low-field MRI system with phase change materials.

[0103] DETAILED DESCRIPTION

[0104] Low-field MRI measurements may include both a pre-polarization stage at a higher magnetic field, and a measurement stage at a lower magnetic field. In this case, the magnetic field will typically need to be switched between the high and low states in a relatively short time, so as to minimize the dead time between the polarization and imaging steps (during this dead time, polarization decays, reducing the MRI signal). It has been realized that fast changes in the magnetic field induce eddy currents in electrically conductive objects within the MRI systems, such as those made from metal. In an MRI measurement, magnetic fields resulting from such eddy currents near the subject or the detector (or both) can distort the MRI signal, potentially lowering the quality of the image, or even making it impossible to obtain a useful image.

[0105] It has also been realized that the presence of electrically conductive objects located close to radiofrequency (RF) detection coils, such as those used within MRI systems, increases the resistance of the coils and so decreases detection sensitivity. This effect also results from the potential for generation of eddy currents within the electrically conductive objects.

[0106] Herein are provided thermal management systems, which can be used to cool the electromagnet(s) of an MRI system whilst minimizing the effect of eddy currents, generated in the thermal management systems, on the MRI signals. Graphite

[0107] It has been realized that sheet material with a thermal conductivity of at least 50 W / m*K can serve as a compromise for use in MRI systems. One suitable material for such use is graphite. Graphitic materials can have high thermal conductivity, potentially higher than thermal ceramics. In fact, some artificial pyrolytic graphite materials (e.g., annealed pyrolytic graphite, highly ordered pyrolytic graphite etc.) can have thermal conductivity more than 6 times higher than that of aluminium nitride. Of course, it will be appreciated that graphite materials with thermal conductivities lower than pyrolytic graphite (e.g., about 300 - 500 W / m*K) can be used.

[0108] The thermal conductivity of graphite layers is typically anisotropic, with the inplane conductivity value being an order of magnitude larger than the orthogonal value. Graphite layers are thin, light, and relatively inexpensive.

[0109] While graphite does conduct electricity, it is a non-metal and still has much lower electrical conductivity than metals. For reference, graphite materials generally have an electrical conductivity about 2-3 orders of magnitude smaller than that of aluminium or copper. Because graphite materials do have relatively high electrical conductivity, care must still be taken when using them as a passive thermal cooling element as part of an MRI electromagnet. As previously mentioned, eddy currents generated in graphite can be large enough to increase the noise of or otherwise disturb the MRI measurement. However, it has been realized that the amplitude of such eddy currents are smaller than they would be in metals and so can be mitigated more easily. The magnitude of the currents is ultimately determined by the volume and orientation of the electrically conductive object; large objects facilitate the generation of current loops on large physical scales. Eddy currents can therefore be suppressed in the graphite material by limiting the physical size of the graphite.

[0110] In order to facilitate the transport of heat over large distances, while also limiting the size of the thermally conductive elements, graphite can be divided into several distinct graphite elements that are physically separated, preventing conduction of electrical currents between the graphite elements. The maximum size of these graphite elements is determined by the electrical conductivity of the specific graphite material, however they can generally be on the order of centimetres. A metallic heat spreader would need to be divided into significantly smaller elements to minimize the effect of eddy currents, making them impractical for this purpose. Commercially available graphite sheets typically have thickness less than 1 mm. Several stacked graphite sheets could be used to create a thicker layer with increased in-plane thermal conductance while still being thin enough to support only minimal eddy currents in the transverse (out-of-plane) direction.

[0111] As the graphite sheets are typically thin, they can preferably be configured parallel to the winding plane of an electromagnet in order to facilitate the spreading of heat within the electromagnet, for example to remove heat from any hot spots or to transport heat to the inner and / or outer edges of the electromagnet. Eddy currents are generated in the plane orthogonal to a time-varying magnetic field produced by the electromagnet, which in such a configuration is the plane of the graphite elements. In such a configuration, it is ultimately the in-plane area of the graphite elements that influences the degree of eddy current generation, even for multiple graphite sheets being stacked on each other. The particular range of areas for the graphite elements which minimize eddy currents may depend on the particular graphite material used. There is additionally a dependence on the aspect ratio of the graphite elements, so a small width along one direction may allow for a large width along the orthogonal direction to achieve a larger overall area.

[0112] A configuration of separated graphite elements can be achieved by affixing multiple pre-cut pieces to a non-electrically conductive substrate, such as a plastic material, to form a desired pattern. Alternatively, a single larger graphite piece (which may include one or several stacked layers) can be cut directly into a desired pattern. Cutting can be performed, for example, using a knife or other sharp tool, or using a laser. In the former case, the separation between pieces will likely be on the order of 0.5 - 1 mm, while in the latter case it may be even smaller.

[0113] Figure 1 shows an exemplary graphite layer 100 comprising a plurality of distinct graphite elements 102. The graphite elements 102 are arranged radially from a centre point. 64 graphite elements 102 are provided, where the graphite elements 102 are divided into eight radial regions, each radial region comprising eight graphite elements 102. Each graphite element 102 is separated, radially and azimuthally, from the other graphite elements 102 to electrically isolate each graphite elements 102 from the others, thus preventing large-scale eddy currents from being formed in the graphite layer 100.

[0114] In Figure 1 the outer elements (radially) have larger azimuthal length than the inner elements. However, it has been found that such an azimuthal length is sufficient because the radial width (measured radially from the centre) is small enough such that the area of the graphite elements is small enough to minimize the effects of eddy currents. In another embodiment, the azimuthal length of the outer elements can be substantially the same as that of the inner elements without significant loss of thermal performance.

[0115] The graphite layer 100 can be placed above or below an electromagnet. The graphite layer 100 serves as a heat spreader and conducts heat away from areas of a coil winding that cannot be accessed by other thermal elements toward areas that can be contacted / accessed by other thermal elements, for example the inner or outer radius of the coil, where the other thermal elements (active or passive) can transport the heat away from the electromagnet. For example, a heat sink may be placed on the outer radius of the electromagnet and the graphite layer 100 can conduct heat from the inner parts of the electromagnet to the heat sink. Alternatively, the graphite layer 100 can conduct heat away from the outer radius and toward the inner radius (e.g., by having a heat sink positioned at or near the centre of the electromagnet). Alternatively, one or more additional thermal elements can be located in between the graphite layer and the heat sink.

[0116] The electromagnet may generate magnetic fields for the purpose of MRI imaging. For example, the electromagnet may generate a prepolarization magnetic field within a volume of interest and then be switched to generate a measurement magnetic field of different field strength in the volume of interest. In another example, another magnet may generate the measurement magnetic field. Any change in magnetic field strength has the potential to induce eddy currents in nearby electrically conducting materials, including graphite. These eddy currents in turn have the potential to modify the strength of the measurement magnetic field, potentially reducing the measured MRI signals or otherwise altering them in an undesirable manner.

[0117] The electromagnet may generate the measurement field, or it may be turned off while another magnet generates the measurement field. Depending on the directions of the fields, the eddy current field may modify the strength of the magnetic field by increasing or decreasing the strength of the overall magnetic field.

[0118] The modifications to the strength of the magnetic field decay over time, as the eddy currents themselves decay. Therefore, the overall field is temporally variable in an undesirable manner, causing the effect on the MRI signal to also vary over time.

[0119] Limiting the size of the graphite elements can reduce the effect of the eddy currents on the overall magnetic field in the volume of interest. In particular, the size of the graphite elements may be chosen such that, in use, the effect of any eddy currents induced in the graphite elements is not perceptible in the volume on interest, i.e. such that the field strength generated by an electromagnet comprising the graphite elements differs by a small amount, more preferably does not substantially differ or, most preferably, does not perceptively differ from the field strength generated by an identical electromagnet that does not comprise the graphite elements.

[0120] In other words, the effects of any eddy currents induced in the graphite elements should be small enough to not be noticed by the MRI measurements (e.g., less than 0.1 % or less than 1% ) in the volume of interest. In some embodiments, the effects of any such eddy currents may modify the MRI signal by up to 30%. This may be considered an acceptable trade-off if it also mitigates heating of the coil. For some electromagnets, these effects may be measured in a predetermined region spaced away from the electromagnet by 1 - 25 cm. Of course, it will be appreciated that different electromagnets may be configured to be used in different volumes of interest. In these cases, the effects of the eddy currents on the measurement magnetic field can be measured in a predetermined region within the corresponding volume of interest. The volume of interest generally relates to the volume which the MRI scanner is configured to measure. The effects of the eddy currents generally relates to the maximum effect.

[0121] In addition, the presence of graphite elements near an RF detection coil results in a decrease in coil sensitivity. For example, an MRI detection coil may comprise all or part of the electromagnet, or it may be located nearby. Johnson noise in the detection coil induces fluctuating eddy currents in the graphite elements, dissipating some of the energy of the Johnson noise and thus enhancing its source, the resistance of the coil. This in turn increases the thermal voltage noise of the coil. Limiting the size of the graphite elements, and thus their ability to generate eddy currents, can reduce their effect on the sensitivity of nearby coils.

[0122] Large-scale, uncut graphite sheets can increase the resistance in a coil to multiple times the nominal value. Therefore, the size of the graphite elements is preferably chosen such that the eddy currents induced in the graphite elements do not substantially increase the resistance of the coil(s) in the electromagnet or any other nearby detection coils. Said substantial increase may be, for example, less than 50% or less than 25%, preferably less than 10%.

[0123] In Figure 1 the graphite elements 102 are separated radially, reducing radial thermal conductance even though in an embodiment this is the preferable direction of heat flow. A radially continuous graphite layer 100 would provide a continuous capacitive pathway between all of the windings of an adjacent electromagnet, substantially increasing its capacitance and negatively affecting its performance as a coil. For example, an increase in the capacitance of the electromagnet due to parasitic capacitance of the graphite layer 100 would result in a decrease in the self-resonance frequency (SRF) of the coil, making it unsuitable for use in low-field MRI measurements at frequencies near or above the reduced SRF. Separating the graphite elements 102 along the radial direction, so that fewer windings of the electromagnet are capacitively coupled, reduces or eliminates this effect. Preferably the radial width of the graphite elements 102 is comparable to or less than the width of three windings of an adjacent electromagnet or comparable to or less than the width of two windings of an adjacent electromagnet, or more preferably the radial width of the graphite elements 102 is comparable to or less than the width of one winding of an adjacent electromagnet. Therefore, the size of the graphite elements is preferably chosen such that they do not substantially decrease the self-resonance frequency(ies) of the coil(s) in the electromagnet or any other nearby detection coils. Said substantial decrease may be, for example, less than 30% or less than 20%, preferably less than 10%.

[0124] Figure 2 shows an exemplary electromagnet 200 with the graphite layers 100 of Figure 1. The electromagnet contains multiple interleaved of coils 202 and graphite layers 100. The graphite layers 100 are shown with the cuts between the graphite elements. Non-electrically conductive substrates may be provided within some, or all, of the graphite layers 100 (e.g., stacked between graphite sheets). In another example, there may not be any non-electrically conductive substrates. The graphite layers 100 are in contact with the coils 202 and supplement the internal thermal conductivity of the coils 202, increasing the flux of heat toward the edges of the coils 202 where the heat can then be moved away.

[0125] The thickness of the graphite layers 100 can vary, depending on the heat distribution within the coils 202. In the exemplary electromagnet of Figure 2, the top coil layer has a thicker graphite layer 100a than the thinner graphite layer 100e used for the bottom coil layer because more heat is expected at the top, in this example. This can be constructed by using graphite elements of different thicknesses and / or by stacking different numbers of graphite elements to achieve the different layer thicknesses.

[0126] In the example of Figure 2, a bottom graphite layer 100e has a first thickness. The second graphite layer 100d, above the bottom graphite layer 100e, has a second thickness larger than the first thickness. The third graphite layer 100c, above the second graphite layer, has a third thickness larger than the second thickness. The fourth graphite layer 100b, above the third graphite layer 100c, has a fourth thickness larger than the third thickness. The top graphite layer 100a, above the fourth graphite layer 100b has a fifth thickness larger than the fourth thickness. In another example, graphite layers 100c, 100d and 100e may have the same thickness.

[0127] Of course, it will be appreciated that graphite layers with graphite elements arranged differently from those of Figure 1 could also be used to cool the electromagnet 200.

[0128] More generally, the thermal management system described herein may comprise a plurality of graphite layers, each graphite layer comprising a plurality of graphite elements, where the thickness of the graphite elements is different for at least two of the graphite layers.

[0129] Figure 3 shows an exemplary electromagnet coil 302 with a graphite layer 304. The electromagnet coil 302 shown in Figure 3 has a different aspect ratio than the coils shown in Figure 2, with a larger inner radius but small width. The graphite layer 304 comprises multiple graphite elements arrayed along both horizontal and vertical directions. The graphite layer 304 is divided longitudinally to form the graphite elements. Some of the graphite elements may be connected along a small fraction of their length but otherwise separated. In this example, the graphite layer 304 conducts heat away from the outer radius of the electromagnet toward the inner edge, then downward and away from the electromagnet coil 302. The graphite layer 304 may also have gaps 306 to accommodate other elements of the MRI system.

[0130] In this example, the coil 302 is on top of, and in thermal contact with, an upper face of the graphite elements. A portion of the graphite elements are arranged vertically downward and then bent horizontally under other elements of the MRI system (not shown) to create a larger surface area for better thermal transfer to other elements underneath the coil 302. The graphite elements may be formed via cutting horizontal and vertical slots in one or more graphite sheets.

[0131] In this case, the graphite elements may have a larger area than the aforementioned graphite elements shown in the examples of Figures 1 and 2, as the azimuthal width is small enough and most of the volume of the graphite elements 304 is spaced away from the electromagnet, thereby reducing the effect of the eddy current on the magnetic field and the sensitivity of the electromagnet.

[0132] In this case, the electromagnet 302 may not get excessively hot, so thin graphite layers are sufficient to give appropriate thermal conductance away from the electromagnet 302. In some examples, the MRI system may include multiple electromagnets, which may each require a different configuration of graphite layers or other arrangements of the graphite elements.

[0133] The spacing between the graphite elements can be filled with another thermal material. For example, a potting compound, silicone, or epoxy can be used to fill the space between the graphite elements while in a liquid state and then solidified. While such materials may have significantly lower thermal conductivity than the graphite elements, so long as the spacing is narrow and the filler material is chosen to have a non-negligible thermal conductivity value, the effect on the thermal capacity of the overall structure should be minimal. Such materials should be chosen to have negligible electrical conductivity, so that they do not promote the generation of eddy currents between graphite elements.

[0134] In some examples, the filler material should also be chosen to have minimal electrical permittivity (real and imaginary components), so that they do not significantly affect the radio-frequency electrical behaviour of the electromagnet. In some examples, the real component of the relative electrical permittivity of the filler material is less than 5, and the imaginary component of the relative electrical permittivity is less than 0.05. In some examples, the real component of the relative electrical permittivity of the filler material is less than 5, and the imaginary component of the relative electrical permittivity is less than 0.01 . Larger values may result in decreased performance of nearby coils of the electromagnet.

[0135] In some examples, all or part of the electromagnet may be used as a detector of MRI signals, in which case the electrical behaviour of the electromagnet includes sensitivity of the detector to magnetic fields within the frequency band of MRI detection. In some examples, all or part of the electromagnet may be used as a detector of RF signals other than those generated by the MRI measurement. In some examples, the detected RF signals may include external electromagnetic noise, i.e., electromagnetic interference.

[0136] Thus, herein is provided a thermal management system for cooling a MRI device containing one or more electromagnets, consisting of a plurality of graphite elements configured to remove heat from the one or more electromagnets;

[0137] Preferably, the graphite elements are configured to facilitate minimal formation of eddy currents during changes in the magnetic field. The minimal formation of eddy currents during changes in the magnetic field preferably results in less than 30% modification of the MRI signal, or even more preferably, in less than 10% modification of the MRI signal.

[0138] The modification in MRI signal may result from, for example, changes in the overall magnetic field amplitude, introduction of magnetic field inhomogeneities that cause spin coherence decay in the measured proton spins and / or time-varying fields resulting in different spin behaviour between different spin echoes.

[0139] Preferably, the graphite elements are configured to cause less than 50%, preferably less than 25% and more preferably less than 10% increase in the resistance of any nearby RF detection coils.

[0140] Preferably, the graphite elements are each no more than 5 cm in size along at least one in-plane direction. Even more preferably, the graphite elements are each no more than 2 cm in size along at least one in-plane direction.

[0141] Preferably, the spacing between graphite elements is approximately 0.1-1 mm. Even more preferably, the spacing between graphite elements is approximately 0.2-0.5 mm. It has been realized that a spacing larger than, or equal to, 0.2 mm is sufficient to prevent currents from bridging the gap between the graphite elements. Additionally, a spacing smaller than 1 mm is small enough to provide sufficient thermal conductivity.

[0142] Preferably, the spaces between graphite elements are filled with a thermally conductive epoxy, silicone, or potting compound.

[0143] Preferably, the thermally conductive epoxy, silicone, or potting compound is chosen to have negligible electrical conductivity.

[0144] Preferably, the thermally conductive epoxy, silicone, or potting compound is chosen to have minimal real and imaginary values of electrical permittivity. The real and imaginary values of electrical permittivity of the thermally conductive epoxy, silicone, or potting compound may result in less than 30% reduction in the sensitivity of the MRI detector. Preferably, the real and imaginary values of electrical permittivity of the thermally conductive epoxy, silicone, or potting compound result in less than 10% reduction in the sensitivity of the MRI detector.

[0145] Preferably, one or more of the graphite elements may be formed from a plurality of individual graphite sheets.

[0146] Preferably, the graphite elements conduct heat along the face of the one or more electromagnets.

[0147] Preferably, the graphite elements conduct heat away from the one or more electromagnets to a heat sink. In some embodiments, a material other than graphite may comprise the thermally conductive element. Lower values of thermal conductivity require the thermally conductive layer to have larger thickness, leading to several practical disadvantages: greater thickness may require a larger number of component sheets, take up a larger fraction of the volume of the electromagnet and thus result in a less effective electromagnet, and / or allow the formation of larger eddy currents. The total thickness of the thermally conductive layer may be less than 1 cm, preferably less than 5 mm, and more preferably less than 3 mm.

[0148] The material should be chosen such that the electrical conductivity permits separating the thermally conductive layer into individual elements of practical size and quantity. For example, the use of a metal such as copper or aluminium would require cutting the thermally conductive layer into hundreds or thousands of isolated elements of size on the order of several millimeters or smaller. In some embodiments, the electrical conductivity of the material may be less than 3*106S / m in the in-plane direction, preferably less than 1*106S / m in the in-plane direction, and more preferably less than 5*105S / m in the in-plane direction. In some embodiments, the thermally conductive layer comprises a material with electrical conductivity of at least 1*103S / m in the in-plane direction, as materials with a value less than this amount are unlikely to require separation into smaller elements as described herein.

[0149] Litz Wires

[0150] Thermal conductance is proportional to the cross-sectional area and so is inherently limited when using graphite, which typically comes in the form of thin layers. In some scenarios, the amount of heat involved may be too much to efficiently conduct away using a reasonable number of graphite layers. In other scenarios, the size of the thermal interface may be too large to efficiently cover using a reasonable number of graphite layers. Such scenarios may require dozens or hundreds of graphite layers, which may be impractical to cut or assemble into the desired forms. In such cases, it may be preferable to use larger volumes of another thermally conductive material.

[0151] As noted previously, thermal ceramics may be technically suited for this purpose, but they can be prohibitively expensive. Metals are also well-suited and have the advantage of being less expensive, but large contiguous volumes can be incompatible with operation of a low-field MRI system with variable magnetic fields or with nearby RF detection coils. However, it has been realized that Litz wires circumvent such incompatibility. Litz wire is constructed from twisted bundles of very small strands of individual wires, which are generally on the order of 0.1 mm or smaller in diameter. The individual strands are electrically insulated from one another, so eddy currents are unable to form over large areas. The Litz wire may commonly comprise dozens, hundreds, or thousands of individual strands. Litz wire is typically used for conducting currents at high frequencies.

[0152] Of course, other types of electrically insulated metal wires could also be used to the same effect. For example, non-twisted stranded wires would also be effective at limiting eddy currents, so long as the individual strands are electrically insulated from one another. The following embodiments will be described with reference to Litz wires. However, it will be appreciated that other types of metal wires could also be used as alternatives to the Litz wires.

[0153] The effective cross-sectional area available for thermal conduction is the sum of the cross-sectional areas of all strands in all of the Litz wires used. Thus, the entire wire cross-section of the Litz wires, each Litz wire having a cross-section with size on the order of several mm or cm, acts as a thermal conductor, with the thermal conductivity reduced from the nominal value of the material by the metal filling factor, generally 50%- 70%. Litz wire can thus act as an ideal passive thermal conductor for a low-field MRI system. Multiple bundles of Litz wire can be packed together, and the gaps between them can be filled with a thermally conductive epoxy or silicone to provide a thermal connection. The Litz wires can be in physical contact with one another, or they may have some physical separation, although in the latter case the overall thermal conductivity may be lower due to lower volumetric efficiency and the separation reducing heat transfer between wires.

[0154] The combination of multiple Litz wires and a thermally conductive filler constitutes a thermal composite material which can be considered as a thermally conductive block that has a preferential thermally conductive direction in the direction of the length of the wires.

[0155] Figure 4 shows a cross section of a thermally conductive block 400 comprising a plurality of Litz wires 402.

[0156] The overall thermal conductivity is highest along the direction of Litz wire travel (perpendicular to the plane of Figure 4), while along the other two directions it is determined by the thermal conductivity of the filler material and the distance between Litz wires 402. Such thermally conductive blocks 400 can be constructed using readily available commercial Litz wire 402, and they can be cut as needed to appropriate dimensions for a given application. They can likewise be constructed from non-twisted stranded wire, so long as the individual strands are electrically insulated from one another.

[0157] In the thermally conductive block 400 shown in Figure 4, each Litz wire 402 consists of seven large bundles that are twisted around one another, with each bundle containing several dozen small strands that are themselves twisted around one another. The gaps between the Litz wires 402 are filled with a thermal potting compound that has been set and hardened.

[0158] Preferably, the Litz wires 402 should not be configured to conduct an electrical current, as they would in a typical application. Indeed, they should remain electrically isolated from one another and from all other elements (including the electromagnet), so as to not impact the operation of the MRI system, such as by generating magnetic fields.

[0159] Figure 5 shows a cross section of an exemplary low-field MRI system 500. The MRI system 500 comprises an electromagnet 502 and a thermally conductive block 400 (Litz block) for dissipating thermal energy away from the electromagnet 502. In one example, the afore-mentioned graphite elements may be disposed between the coils of the electromagnet 502 to improve the overall thermal dissipation. Of course, it will be appreciated that this is not essential.

[0160] This exemplary low-field MRI system 500 is ideal for the use of Litz thermal blocks 400 since the cross-sections are too large for efficient use of graphite, while the volumes are too large for cost-effective use of thermal ceramics. The Litz block 400 is connected to the outside of the electromagnet 502. The connection may be direct, or there may be a thermal epoxy or thermal pad placed between the electromagnet 502 and the Litz block 400. The Litz block 400 then extends several cm away to a heat sink 504. In this example, a thermally conductive plate 506 (e.g., a metal plate) is provided between the Litz block 400 and the heat sink 504 to provide a continuous thermal pathway from the electromagnet to the heat sink. Of course, it will be appreciated that the thermally conductive plate 506 may not be necessary when the Litz block 400 is in direct thermal contact with the heat sink 504.

[0161] It will be appreciated that in an example the heat sink 504 and the thermally conductive plate 506 can be electrically conductive (e.g., metallic) as they are far enough from the electromagnet so as to not affect the MRI measurements or the electrical properties (e.g., resistance and / or capacitance) of the electromagnet.

[0162] Note that the Litz block 400 interfaces with the electromagnet 502 at an oblique angle, since, preferably, each of the Litz wires of the Litz block 400 should make contact with the electromagnet 502 at one end and then run along another direction to the heat sink 504. If the Litz block 400 were arrayed vertically, then only a fraction of the Litz wires would contact the electromagnet 502, while in this configuration the entire cross-section of the Litz block 400, and thus all Litz wires, can make contact with both the electromagnet 502 and the heat sink 504.

[0163] Figure 6 shows the low-field MRI system 500 of Figure 5. A plurality of Litz blocks 400 are arranged around the outer edge of the electromagnet 502. It has been found that there is no significant effect on the resistance of a coil when placed next to the Litz blocks 400 because the individual strands of the Litz wire are thin enough (e.g., on the order of 0.1 mm diameter).

[0164] In the example of Figure 6, the Litz blocks 400 are provided with a gap between each of the Litz blocks 400. However, it will be appreciated that some, or all, of the Litz blocks 400 may be in thermal contact with other Litz blocks 400. For example, the Litz blocks 400 may form a continuous ring around the electromagnet 502.

[0165] In an embodiment, the thermally conductive blocks can be arranged to replace the graphite elements discussed previously, for example the graphite elements 102 or the graphite elements 304. In such an embodiment, the thermally conductive blocks would need to be cut into smaller components as appropriate, so as to not generate significant eddy current fields, increase the resistance of the electromagnet, or increase the capacitance of the electromagnet.

[0166] Thus, herein is provided a thermal management system for cooling a magnetic resonance imaging (MRI) device containing one or more electromagnets, comprising of one or more thermal composite blocks, each comprising of a plurality of Litz wires configured to remove heat from the one or more electromagnets.

[0167] Preferably, the plurality of Litz wires are arrayed substantially parallel to the intended direction of heat flow.

[0168] Preferably, one or more of the thermal composite blocks also comprise a thermally conductive epoxy, silicone, or potting compound in the spaces in between the plurality of Litz wires.

[0169] Preferably, one or more thermal composite blocks conduct heat away from the one or more electromagnets to one or more heat sinks.

[0170] Preferably, the plurality of Litz wires are configured to be electrically isolated from each other and from the one or more electromagnets and conduct no electrical current.

[0171] Preferably, the plurality of Litz wires are electrically isolated from the electromagnet. Preferably, the one or more thermal composite blocks are in direct thermal contact with the one or more electromagnets. Even more preferably, the one or more thermal composite blocks are not in electrical contact with the one or more electromagnets.

[0172] Preferably, the one or more thermal composite blocks are connected to the one or more electromagnets via a thermally conductive intermediary.

[0173] Preferably, the thermally conductive intermediary comprises thermal epoxy and / or a thermal pad.

[0174] Phase Change Materials

[0175] Phase change materials (PCMs) take advantage of the latent heat of the material at a phase transition to either store or release heat when at the transition temperature. For example, the material stores heat as internal energy when transitioning from solid to liquid, and conversely it releases internal energy as heat when transitioning from liquid to solid. PCMs are widely used for heating and cooling. For heating purposes, they release heat when the temperature drops to the transition temperature, while, for cooling purposes, as the temperature rises they remain at the transition temperature until enough heat has been added to complete the phase transition.

[0176] Phase change materials can be particularly useful in applications involving a cycle between high and low temperatures. In the case of a low-field MRI system using an electromagnet, MRI measurements may occur with a partial duty cycle where no measurements are conducted for a period of time to allow the system to cool down. The temperature would thus be expected to rise when the electromagnet is in use and then to fall again when the electromagnet is idle. Depending on power dissipation and magnet geometry, the swing in temperature could potentially be on the order of 100 degrees Celsius.

[0177] A phase change material block, consisting of a phase change material in contact with the electromagnet, as well as any thermal transport elements of the system (such as the Litz composite blocks discussed earlier), can assist in mitigating the effects of heating during operation of the electromagnet. As the electromagnet heats up, the PCM acts as a heat sink, storing heat as internal energy and slowing overall system heating once temperatures reach the PCM phase transition temperature. The electromagnet and other parts of the MRI system will thus reach lower temperatures than they would in the absence of the PCM. Similarly, the PCM would hasten cooling of the electromagnet and other system components during the time periods when the electromagnet is powered off. As components in contact with the PCM cool down below the PCM transition temperature, the PCM will remain at its transition temperature for an extended period of time. Since heat flow is driven by the size of the thermal gradient, this temperature mismatch results in a larger rate of heat flow outward from the electromagnet, and thus faster cooling than in the absence of the PCM.

[0178] Materials used as PCMs, which in the range of 20-100 Celsius typically include salt hydrates and organic materials such as paraffin, are chosen for their high latent heat values, but they often have low thermal conductivity. Liquid to gas transition materials would be preferable because the latent heat is larger at that transition. However, many liquid to gas transition materials between room temperature and less than 100 C are hazardous. As such, it may be preferable to use solid to liquid transition materials to avoid the use of hazardous materials.

[0179] A block of such PCM material with low thermal conductivity would not efficiently facilitate the conduction of heat toward its interior, away from the interface with the electromagnet, and thus would not efficiently use its full volume. By constructing composite materials out of PCMs and another secondary material with high thermal conductivity, with the secondary material often having thermal conductivity 1-3 orders of magnitude higher than the PCM, the dissipation of thermal heat energy into the block of phase change material is improved, thus allowing it to absorb more thermal energy from the electromagnet.

[0180] In an MRI application, the secondary material should be chosen to not interfere with the MRI measurement. For example, the secondary material should not allow the generation of large-scale eddy currents. A scaffold of graphite, thermal ceramic, or thin metal would suit this purpose, comprised of a plurality of structures leading from the surface into the interior of the block. In the case of thin metal, Litz wire could be used because the separation of the electrical conductor into many very small strands would inhibit the generation of eddy currents. Generally, the thin metal should be arranged to minimize eddy currents induced in the thin metal.

[0181] A scaffold distribution of the secondary material increases the surface which is in contact with the phase change material, thus allowing the thermal energy to be transferred to and away from the phase change material more effectively.

[0182] The phase change material is preferably chosen such that its transition temperature is in the middle of the temperature range experienced by the MRI system during its operation cycle. For example, if the temperature of the outside surface of the electromagnet is expected to vary between roughly 20 and 80 degrees Celsius, then the PCM can be chosen to have a transition temperature of around 50 degrees Celsius. This ensures maximum combined effectiveness during both the heating and cooling phases. Preferably, in an embodiment comprising multiple electromagnets that reach different operating temperatures, different phase change materials are provided for each of the electromagnets based on the particular operating temperatures.

[0183] Figure 7 shows an exemplary low-field MRI system 700 with phase change material blocks 704. The MRI system 700 comprises an electromagnet 702, a plurality of Litz blocks 400 as shown in Figure 4 and a plurality of phase change material blocks 704 (PCM blocks), each arranged between the Litz blocks 400. It will, however, be appreciated that, whilst this figure shows the phase change material blocks 704 interleaved with Litz blocks 400, it is not essential that the phase change material blocks 704 are used in combination / alongside with Litz blocks 400. Instead, the phase change material blocks 704 can be used in an arrangement that does not comprise any Litz blocks 400 or that does not comprise Litz blocks 400 in an interleaved arrangement similar to the one shown in the figure.

[0184] In an embodiment the PCM blocks 704 alternate with Litz blocks 400. The PCM blocks 704 are in thermal contact with the electromagnet 702 and / or with the Litz blocks 400, so they can affect the rate of heating and cooling of both. The effectiveness of the PCM blocks 704 is determined by their volume. In one embodiment the blocks fill the available space inside the interior of the MRI system 700. In one such example, it has been found that the presence of the PCM blocks 704 can be expected to reduce the maximum temperature in the electromagnet 702 during each thermal cycle by approximately 10-15 degrees Celsius.

[0185] Thus, a thermal management system is provided for cooling a magnetic resonance imaging (MRI) device containing one or more electromagnets, comprising one or more phase change material blocks (PCM blocks) containing a phase change material (PCM) in thermal contact with the one or more electromagnets, configured to slow down heating and speed up cooling of the electromagnets;

[0186] One or more of the PCM blocks may be in direct physical contact with the one or more electromagnets. Alternatively, or additionally, the one or more PCM blocks may be connected to the one or more electromagnets via a thermally conductive intermediary, such as thermal epoxy or a thermal pad. Preferably, the one or more PCM blocks are in thermal contact with one or more thermally conductive elements, or thermally conductive blocks, thermally connecting the one or more electromagnets to a heat sink. Said thermal contact may be via direct physical contact, or via a thermally conductive intermediary, such as thermal epoxy and / or a thermal pad

[0187] Preferably, the one or more PCM blocks additionally comprise a secondary material with high thermal conductivity.

[0188] Preferably, the secondary material has a value of thermal conductivity that is at least 10 times larger than that of the phase change material. Even more preferably, the secondary material has a value of thermal conductivity that is at least 100 times larger than that of the PCM.

[0189] In an embodiment, the secondary material comprises a thermally conductive ceramic material (e.g., aluminium nitride).

[0190] In another embodiment, the secondary material comprises a graphite material.

[0191] In yet another embodiment, the secondary material comprises of a thin metallic material. Preferably, the metallic material is in the form of stranded wire where the individual strands are electrically insulated from one another, for example Litz wire.

[0192] Preferably, the secondary material is arranged in the form of a scaffold.

[0193] Preferably, the transition temperature of the phase change material is roughly halfway between the maximum and minimum temperatures of the one or more electromagnets during an operation cycle (e.g., ±20% of the midpoint temperature).

[0194] Preferably, different PCM blocks have phase change materials with different transition temperatures corresponding to the operating temperatures of the different electromagnets.

[0195] It is noted that any references to thermal conductivity and electrical conductivity herein refer to the said values at room temperature (20 degrees Celsius).

[0196] While certain arrangements have been described regarding the graphite elements, metal wires and phase change materials, the arrangements have been presented by way of example only, and are not intended to limit the scope of protection. The inventive concepts described herein may be implemented in a variety of other forms. In addition, various omissions, substitutions and changes to the specific implementations described herein may be made without departing from the scope of protection defined in the following claims.

[0197] Any mention of minimizing eddy currents in elements (e.g., thermally conductive elements, graphite elements, metal wires, secondary material etc.) mentioned above refers to minimizing eddy currents such that the eddy currents generated in the respective elements do not substantially modify the magnetic field in a predetermined region spaced away from a front face of the electromagnet, when compared to a magnetic field generated in use in the predetermined region by an equivalent electromagnet that does not comprise the elements, and / or do not substantially increase the resistance of coils in the electromagnet, when compared to the resistance of coils in an equivalent electromagnet that does not comprise the elements.

Claims

CLAIMS:1 . An electromagnet for a low-field magnetic resonance imaging, MRI, system, the electromagnet comprising: a plurality of sheet elements (102) each arranged to transfer thermal energy from a first portion of the electromagnet to a second portion of the electromagnet, wherein the first portion in use has a higher temperature than the second portion, wherein each sheet element (102) of the plurality of sheet elements (102) is electrically isolated from other sheet elements (102) of the plurality of sheet elements (102) and from the electromagnet; and wherein one of more of the sheet elements of the plurality of sheet elements comprise a material that has a thermal conductivity fo at least 50 W / m*K.

2. The electromagnet of claim 1 , wherein the size of each graphite element (102) of the plurality of sheet elements (102) is such that, in use, eddy currents generated in the plurality of sheet elements do not: substantially modify the magnetic field in a predetermined region spaced away from a front face of the electromagnet, when compared to a magnetic field generated in use in the predetermined region by an equivalent electromagnet that does not comprise the plurality of sheet elements (102), and / or substantially increase the resistance of coils in the electromagnet, when compared to the resistance of coils in an equivalent electromagnet that does not comprise the plurality of sheet elements (102).

3. The electromagnet of claims 1 or 2, further comprising gaps between sheet elements, the gaps having a width of between 0.2 mm and 1 mm.

4. The electromagnet of claim 3, further comprising an electrically insulating material in the gaps.

5. The electromagnet of any preceding claim, wherein the sheet elements are formed from a graphite sheet or from a plurality of stacked graphite sheets.

6. The electromagnet of any preceding claim, wherein the sheet elements have a length and / or width of less than 5cm, and preferably less than 2cm.

7. The electromagnet of any preceding claim, wherein the sheet elements have a thickness of less than 10mm, and preferably less than 5mm.

8. The electromagnet of any preceding claim, wherein a gap between the sheet elements is at least 0.1mm and at most 2mm in thickness.

9. The electromagnet of any preceding claim, wherein the sheet elements (102) are arranged to transfer thermal energy parallel to a winding plane (202) or to winding planes (202) of the electromagnet.

10. The electromagnet of claim 9, wherein the electromagnet comprises a plurality of winding planes (202) and wherein the sheet elements are arranged between winding planes (202) of the plurality of winding planes (202).

11. The electromagnet of claim 10, wherein a first sheet element layer (100a) in thermal contact with a first winding plane of the electromagnet is formed from a first set of the sheet elements having a first thickness and a second sheet element layer (100e) in thermal contact with a second winding plane of the electromagnet is formed from a second set of sheet elements having a second, smaller, thickness.

12. A system comprising an electromagnet of any preceding claim and a heat sink, wherein the sheet elements are arranged to transfer thermal energy from the electromagnet to the heat sink.

13. A thermal management system for a low-field magnetic resonance imaging, MRI, system (500), the system comprising: an electromagnet; a heat sink; and a plurality of metal wires thermally connected to the heat sink, each metal wire being arranged to transfer thermal energy from the electromagnet (502) at one end to the heat sink at another end, wherein each metal wire is electrically isolated from the other metal wires and from all other elements of the thermal management system.

14. The system of claim 13, wherein the size of each metal wire of the plurality of metal wires is such that, in use, eddy currents generated in the metal wires do not: substantially modify the magnetic field in a predetermined region spaced away from a front face of the electromagnet, when compared to an equivalent electromagnet that does not comprise the plurality of metal wires, and / or substantially increase the resistance of coils in the electromagnet, when compared to the resistance of coils in an equivalent electromagnet that does not comprise the plurality of metal wires.

15. The system of claims 13 or 14, wherein the metal wires are each a strand of a stranded wire, such as Litz wire (402).

16. The system of any of claims 13 to 15, wherein the plurality of metal wires is embedded within an electrically insulating material to form one or more thermally conductive blocks (400).

17. The system of any of claims 13 to 16, wherein the metal wires are arranged substantially parallel to each other.

18. The system of any of claims 13 to 17, wherein the metal wires have a diameter of at most 1 mm and preferably of at most 0.1 mm.

19. The system of any of claims 13 to 18, wherein the metal wires extend substantially parallel to a direction between the electromagnet and the heat sink.

20. An electromagnet (702) for a low-field magnetic resonance imaging, MRI, system (700), the electromagnet comprising: one or more phase change material blocks (704) arranged to absorb thermal energy from the electromagnet, wherein each phase change material block comprises a phase change material having a transition temperature within an operating temperature range of the electromagnet.

21. The electromagnet of claim 20, wherein the phase change material blocks further comprise a secondary material configured to transfer thermal energy from theelectromagnet to the phase change material, the secondary material having thermal conductivity that is higher than the thermal conductivity of the phase change material.

22. The electromagnet of claim 21 , wherein the secondary material comprises one or more of: thermally conductive ceramic material; graphite material; and metallic material, such as Litz wire.

23. The electromagnet of claims 21 or 22, wherein the secondary material is arranged to form a scaffold structure within each phase change material block, wherein the scaffold structure is at least partially surrounded by the phase change material.

24. The electromagnet of any of claims 20 to 23, wherein the transition temperature of the phase change material in at least one of the phase change material blocks is closer to a midpoint of the operating temperature range than to the endpoints of the operating temperature range.

25. The electromagnet of any of claims 20 to 24, comprising one or more second phase change material blocks, each comprising a second phase changing material, for absorbing thermal energy from a second electromagnet, wherein the phase change material of the second phase change material blocks has a different transition temperature than that of the phase change material of the first phase change material blocks.

26. The electromagnet of any of claims 20 to 25, further comprising one or more thermally conductive elements arranged to transfer thermal energy away from the phase change material blocks and, optionally, transfer thermal energy away from the electromagnet.

27. The electromagnet of claim 26, wherein one or more of the thermally conductive elements is a thermally conductive block comprising metal wires, such as Litz wires, embedded within a solid, electrically insulating, material.

28. The electromagnet of claims 26 or 27, wherein one or more of the thermally conductive elements comprises a thermally conductive ceramic.

29. The electromagnet of any of claim 20 to 28, further comprising a heat sink configured to dissipate thermal energy from the phase change material blocks.

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