Systems, assemblies and methods for suppressing magnetic gradient interactions in magnetic resonance systems - Patents.com

Abstract

To provide a magnetic resonance (MR) system (10).SOLUTION: The system includes a main magnet assembly (12) configured to generate a polarizing magnetic field, a gradient coil assembly (22) including multiple gradient coils configured to apply at least one gradient magnetic field to the polarizing magnetic field, and a shield assembly (208) positioned between the main magnet assembly (12) and the gradient coil assembly (22). The shield assembly (208) includes an electrically conductive layer (304) fabricated with an electrically conductive material and defining multiple grooves (308) positioned through the electrically conductive layer (304), where the multiple grooves (308) block motional eddy currents caused by action of the polarizing magnetic field and of the at least one gradient magnetic field when the at least one gradient magnetic field is applied.SELECTED DRAWING: Figure 2A

Description

[Technical field]

[0001] The field of the disclosure relates generally to magnet assembly systems and methods, and more particularly to systems, assemblies, and methods for suppressing magnet-gradient interactions in magnetic resonance (MR) systems. [Background technology]

[0002] Magnetic resonance imaging (MRI) has proven useful in diagnosing many diseases. MRI provides detailed images of soft tissues, abnormal tissues such as tumors, and other structures that cannot be easily imaged by other imaging modalities such as computed tomography (CT). Furthermore, MRI works without exposing the patient to the ionizing radiation experienced with modalities such as CT and X-rays.

[0003] A stable magnetic field is fundamental to the performance of an MR system. Rapid switching of strong gradient coils generates leakage magnetic fields, which cause induced eddy currents, which in turn generate motional eddy currents. Motional eddy currents pose serious risks of heating and quenching in the magnets of an MR system. The motion-induced amplification of the interaction between the magnets and the gradient coils is sometimes referred to as magnet-gradient interaction (MGI). Known systems and methods for managing MGI are disadvantageous in several aspects and improvements are desirable. Summary of the Invention

[0004] In one aspect, a magnetic resonance (MR) system is provided. The system includes a main magnet assembly configured to generate a polarizing magnetic field, a gradient coil assembly including a plurality of gradient coils configured to apply at least one gradient magnetic field to the polarizing magnetic field, and a shield assembly disposed between the main magnet assembly and the gradient coil assembly. The shield assembly includes a conductive layer fabricated from a conductive material and defining a plurality of grooves disposed therethrough, the plurality of grooves configured to block motional eddy currents caused by action of the polarizing magnetic field and the at least one gradient magnetic field when the at least one gradient magnetic field is applied.

[0005] In another aspect, a shield assembly for an MR system is provided, the shield assembly including a conductive layer fabricated from a conductive material and defining a plurality of grooves disposed therethrough, the plurality of grooves configured to block motional eddy currents generated by the MR system.

[0006] In another aspect, a method of assembling a shield assembly for an MR system is provided, the method including providing a conductive layer fabricated from a conductive material and defining a plurality of grooves disposed through the conductive layer, the plurality of grooves configured to block motional eddy currents generated by the MR system. [Brief description of the drawings]

[0007] Non-limiting and non-exhaustive examples are described with reference to the following drawings, in which like reference numerals refer to like parts throughout the various drawings unless otherwise specified. [Figure 1] 1 is a schematic diagram of an exemplary magnetic resonance (MR) system. [Figure 2A] FIG. 2 is a schematic diagram of an exemplary magnet assembly of the system shown in FIG. 1. [Figure 2B] FIG. 2B is a partial cross-sectional view of the magnet assembly shown in FIG. 2A taken along section line 2B-2B. [Figure 3A]FIG. 3 is a schematic diagram illustrating a cross section of an exemplary shield assembly for the magnet assembly shown in FIGS. 2A and 2B. [Figure 3B] 3B illustrates an exemplary pattern for the shield assembly shown in FIG. 3A. [Figure 4A] FIG. 1 is a schematic diagram of a superconducting main magnet assembly using helium bath cooling. [Figure 4B] FIG. 1 is a schematic diagram of a superconducting main magnet assembly using conduction cooling. [Diagram 5] 1A-1C are comparative diagrams of motion eddy current heating of an MR system without and with the systems and methods described herein. [Figure 6] 4 is a flow chart of an exemplary method of assembling the shield assembly shown in FIGS. 1-3B. [Figure 7] FIG. 1 is a block diagram of an exemplary computing device. [Figure 8] FIG. 2 is a block diagram of an exemplary server computing device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] The present disclosure includes systems, assemblies, and methods for reducing moving eddy currents in magnetic resonance (MR) systems. Moving eddy currents pose a serious risk of magnet heating and quenching, especially for compact, high performance, high field scanners. A superconducting magnet assembly in an MR system is described for illustrative purposes only. The systems, assemblies, and methods may be applied to any magnet assembly in any system, such as a permanent magnet assembly, because heating from moving eddy currents causes magnetic field drift in the magnet assembly, impairing the performance of the magnet assembly. Aspects of the methods will be partially apparent and partially explicitly discussed in the following description.

[0009] In magnetic resonance imaging (MRI), a subject is placed in a magnet. As used herein, a subject is a human, animal, or phantom, or a part of a human, animal, or phantom, such as an organ or tissue. When a subject is in the magnetic field generated by the magnet, the magnetic moments of atomic nuclei, such as protons, attempt to align with the magnetic field, but precess around the field in a random order at the Larmor frequency of the nuclei. The magnetic field of the magnet is called B0 and extends in the longitudinal or z direction. When acquiring an MRI image, a magnetic field (called the excitation magnetic field B1) in the xy plane and close to the Larmor frequency may be generated by a radio frequency (RF) coil and used to rotate or "tip" the net magnetic moment Mz of the atomic nuclei from the z direction to the transverse or xy plane. When the excitation signal B1 is terminated, the nuclei emit a signal called the MR signal. Gradient magnetic field pulses (Gx, Gy, and Gz) are used to generate an image of the subject using the MR signal. Gradient pulses are used to scan k-space, spatial frequency space, or inverse distance. Because a Fourier relationship exists between the acquired MR signals and the image of the subject, the image of the subject can be derived by reconstructing the MR signals.

[0010] 1 shows a schematic block diagram of an exemplary MR system 10. The MR system 10 is used for acquiring images or for spectroscopy applications of a subject.

[0011] In an exemplary embodiment, the MR system 10 includes a main magnet assembly 12 that includes a main magnet 14. In some embodiments, the main magnet 14 is a superconducting magnet formed from multiple magnetic coils wound around a magnetic coil support or coil former. The main magnet 14 is configured to generate a polarizing magnetic field. In other embodiments, the main magnet 14 is a permanent magnet. The main magnet assembly 12 can include a cryostat vessel 18 that surrounds the main magnet 14. The cryostat vessel 18 is typically filled with a cryogenic fluid, such as liquid helium and / or nitrogen, that is used to cool the superconducting coil to cryogenic temperatures, e.g., 4K, so that electrical current continues to flow through the superconducting coil without electrical resistance after power is removed to maintain a uniform, static main magnetic field.

[0012] In an exemplary embodiment, the main magnet assembly 12 also includes a thermal shield assembly 16 that encloses the cryostat vessel 18 and the main magnet 14 therein. In one embodiment, the thermal shield assembly 16 can include an inner thermal shield member 162 and an outer thermal shield member 164. The inner thermal shield member 162 is generally cylindrical in shape and is disposed radially inward of the main magnet 14. The inner thermal shield member 162 is configured to prevent heat radiation from a warm region in which a subject is located to a cold region in which the main magnet 14 is located. The outer thermal shield member 164 is disposed concentrically with respect to the inner thermal shield member 162. The outer thermal shield member 164 can also have a generally cylindrical shape and is disposed radially outward of the main magnet 14. The outer thermal shield member 164 is configured to prevent heat radiation from the environment to the main magnet 14. In some embodiments, the heat shield assembly 16 is made from a metallic material, such as aluminum, that may generate eddy currents due to interaction with a changing electromagnetic field. In one embodiment, a type of changing electromagnetic field is generated by one or more gradient coils with a pulsed current signal. In some embodiments, the main magnet assembly 12 may also include a magnet vacuum vessel 19 that surrounds the heat shield assembly 16 and insulates the main magnet 14 from the environment during operation.

[0013] In an exemplary embodiment, the MR system 10 also includes a gradient coil assembly 22 disposed inside the inner heat shield member 162. The gradient coil assembly 22 is configured to selectively apply one or more gradient magnetic fields along one or more axes, such as an x-axis, a y-axis, or a z-axis. In some embodiments, the gradient coil assembly 22 has a shielded or unshielded configuration. As used herein, a "shielded configuration" refers to a configuration in which one or more shielded gradient magnetic field coils are provided to generate opposing magnetic fields to cancel or reduce fringe fields generated by normal gradient magnetic field coils. The gradient coil assembly 22 may include an x-axis gradient coil configured to apply an x-axis gradient magnetic field along an x-axis direction in response to a pulsed current signal applied thereto. The gradient coil assembly 22 may also include a y-axis gradient coil configured to impose a y-axis gradient magnetic field along a y-axis direction in response to a pulsed current signal applied thereto. The gradient coil assembly 22 may also include a z-axis gradient coil configured to impose a z-axis gradient magnetic field along a z-axis direction in response to a pulsed current signal applied thereto. The x-axis, y-axis, or z-axis gradient coils may be referred to as x-, y-, or z-gradient coils, respectively. The gradient magnetic fields generated by the gradient coil assembly 22 make it possible to discern spatial information of radio-frequency signals emanating from excited nuclei within the subject.

[0014] In an exemplary embodiment, the MR system 10 also includes an RF coil 24. The RF coil 24 shown in FIG. 1 is a body coil. The RF coil 24 may be a local coil that may be placed in proximity to the anatomical structure to be imaged, or a coil array including multiple coils. The RF coil 24 may be a transmit coil configured to transmit RF pulses. The RF coil 24 may be a receive coil configured to detect MR signals from the subject. The RF coil 24 may be a transmit and receive coil that transmits and detects MR signals.

[0015] The main magnet assembly 12, the gradient coil assembly 22, and the body RF coil 24 are generally referred to collectively as a magnet, and as used herein, as a magnet assembly 50, since the magnet assembly 50 forms a single unit having a magnetic field. The magnet assembly 50 has a bore 46, in which a subject is positioned during a scan. The magnet assembly 50 shown in FIG. 1 is a closed-bore system in which the bore is cylindrical. For purposes of illustration, a closed-bore magnet assembly 50 is described herein. The magnet assembly 50 may be a magnet assembly of other designs, such as an open system or an open-bore system, a dipolar electromagnet configuration, or a Hallbach configuration.

[0016] In an exemplary embodiment, the MR system 10 also includes a controller 30, a main magnetic field control 32, a gradient magnetic field control 34, a memory 36, a display device 38, a transmit / receive (T / R) switch 40, an RF transmitter 42, and a receiver 44. In operation, a subject is positioned within a bore 46 on a suitable support, such as a motorized table (not shown) or other patient table. The main magnet 14 generates a uniform, static main magnetic field B0 throughout the bore 46. The strength and uniformity of the main magnetic field B0 within the bore 46, and correspondingly within the patient, is controlled by the controller 30 via the main magnetic field control 32, which also controls the supply of energizing current to the main magnet 14. The gradient magnetic field coil assembly 22 is energized by the gradient magnetic field control 34 and is also controlled by the controller 30, such that one or more gradient magnetic fields are applied to the main magnetic field B0. The RF coil 24 and the receive coil, if provided, are each selectively interconnected to one of an RF transmitter 42 or a receiver 44 by a T / R switch 40. The RF transmitter 42 and the T / R switch 40 are controlled by the controller 30 such that RF field pulses or signals are generated by the RF transmitter 42 and selectively applied to the subject for excitation of magnetic resonance in the subject.

[0017] In an exemplary embodiment, following application of the RF pulses, the T / R switch 40 is again actuated to disconnect the RF transmit coil 24 from the RF transmitter 42. The detected MR signals are then communicated to a controller 30, which may organize the MR signals in a particular format for storage in a memory 36. The controller 30 includes a processor 48 that controls the processing of the MR signals to generate signals representative of an image of the patient. The processed signals representative of the subject are transmitted to a display device 38 to provide a visual display of the image.

[0018] The fast and powerful gradient coils in MRI generate substantial magnetic leakage fields outside the coils that induce eddy currents and heating in the surrounding magnet structures. In the presence of the strong static magnetic field (B0) of the MR system, the eddy currents cause vibrations of the magnet's conductive layers due to the Lorentz force, which then generate motional eddy currents that can be much larger than the original eddy currents. The motional eddy currents exacerbate the electromagnetic interaction between the gradient coils and the magnet, resulting in serious risks of magnet heating and quenching, especially for compact, high-performance high-field scanners. The motion-induced amplification of the interaction is sometimes called magnet-gradient interaction (MGI).

[0019] Known MGI reduction methods focus on suppressing the motion of conductive structures located between the gradient coils and the magnet coils, however, suppressing vibration of conductive structures in high electric field environments is costly in terms of mechanical design and material selection (e.g., bimetals or carbon fibers).

[0020] Instead of focusing on reducing the mechanical vibration of the conductive structure, the systems, assemblies, and methods described herein address MGI by directly targeting the motional eddy currents using a shield assembly. For example, the shield assembly includes a specific pattern of grooves / slots cut out of the conductive layer, where the grooves are configured to (i) allow induced eddy currents to flow relatively easily while (ii) block the path for motional eddy currents to flow on the conductive layer. The conductive layer with the designed pattern can be referred to as a non-motional eddy current (NME) layer. As a result, there are few motional eddy currents to enhance the heating of the magnet even when the conductive structure vibrates. The conductive layer can be thin copper or aluminum backed by fiberglass, making it relatively little additional cost for magnet construction. The systems, assemblies, and methods described herein are advantageous because the NME layer provides a path for non-moving or induced eddy currents to flow, which protects the magnet from the leakage fields of the gradient coils, while limiting or preventing moving eddy currents, even when the conductive structure vibrates in a high B0 environment. Unlike known methods, vibrations are not limited and moving eddy currents are suppressed via cutout patterns in the conductive layer, thereby eliminating the need for structural reinforcement in the magnet assembly. Furthermore, unlike known methods in which moving eddy currents are reduced by limiting applications to vibrating the magnet assembly at a specific resonant frequency, the systems, assemblies, and methods described herein impose no limitations on applications, thereby improving the performance of MR systems and fully reaching the potential of high gradient systems.High gradient power sequences can be applied without limiting sequence parameters such as echo spacing, resolution, gradient amplitude, or slew rates. Furthermore, the systems, assemblies, and methods described herein effectively reduce motional eddy currents in MR systems with high magnetic field strengths such as 7T, which have increased risks of magnet heating and quenching due to increased Lorentz forces and motional eddy currents due to increased magnetic field strength. The MGI effect is a major risk for high magnetic field systems. Thus, the systems, assemblies, and methods described herein are advantageous in reducing the risks of heating and quenching in high magnetic field systems.

[0021] 2A and 2B are schematic diagrams of an exemplary magnet assembly 50. FIG. 2B is a partial cross-sectional view of the magnet assembly 50 taken along section line 2B-2B. The body RF coil 24 is not depicted in FIGS. 2A and 2B. In an exemplary embodiment, the magnet assembly 50 includes a gradient coil assembly 22 and a main magnet assembly 12. The magnet assembly 50 may include a shim tube 202. The shim tube 202 may be attached to the gradient coil assembly 22. In some embodiments, the shim tube 202 may be integrally formed with the gradient coil assembly 22. Alternatively, the shim tube 202 is attached to the main magnet assembly 12, e.g., attached to an inner backing of the magnet vacuum vessel 19.

[0022] In an exemplary embodiment, the main magnet assembly 12 includes a magnet vacuum vessel 19, a heat shield assembly 16, a cryostat vessel 18, a main magnet 14, and a plurality of magnet wires 201. For simplicity, the outer heat shield member 164 and the outer layer of the magnet vacuum vessel 19 are not depicted in FIG. 2A or 2B. In a superconducting main magnet assembly 12, the plurality of magnet wires 201 are maintained at the superconducting temperature of the superconducting metal in the plurality of magnet wires 201. The superconducting temperature is typically 4 Kelvin (K). In the main magnet assembly 12, the region where the temperature is maintained at the superconducting temperature where superconductivity of the electric current occurs is called the superconducting region 206. In FIG. 2A, the main magnet 14 and the cryostat vessel 18 are in the superconducting region 206. For a superconducting magnet, the temperature in the superconducting region 206 needs to be maintained below the superconducting temperature. A cold head (not shown) can be used to maintain the temperature of the superconducting region 206. Because the temperature of the superconducting region 206 is low, the power of the cold head in cooling is limited.

[0023] In an exemplary embodiment, the magnet assembly 50 includes a shield assembly 208. The shield assembly 208 is disposed between the gradient coil assembly 22 and the superconducting region 206. The shield assembly 208 may be attached to the structure of the magnet assembly 50 in the region between the gradient coil assembly 22 and the superconducting region 206. For example, the shield assembly 208 is attached to the gradient coil assembly 22, the shim tube 202, the inner layer of the magnet vacuum vessel 19, the inner heat shield member 162, or the inside 210 of the cryostat vessel 18. The inside 210 of the cryostat vessel 18 is the innermost part of the cryostat vessel 18 facing the bore 46. The shield assembly 208 is used to reduce heating to the main magnet 14 caused by motional eddy currents.

[0024] Currents are induced in nearby conductors by the changing magnetic field. Because MR uses rapidly changing magnetic fields to generate and spatially define signals, eddy currents are always generated whenever a scan is performed. Such eddy currents are called induced eddy currents. The source of the changing magnetic field is from the gradient coils, mostly the RF coils. The conductive material in which the eddy currents are induced can be any conductive component of the MR system 10, such as coils, shields, tubes, wires, or housing.

[0025] During a scan, currents may be switched rapidly to sample through k-space, particularly in the gradient coils. The rapid switching of currents in the magnetic field creates Lorentz forces, causing structures within the magnet assembly 50 to vibrate. The vibration or movement of structures within the magnet assembly 50 creates currents. Such currents are called motional eddy currents.

[0026] The heating effect of induced eddy currents on the main magnet 14 is manageable. The electromagnetic field generated by the induced eddy currents opposes the source electromagnetic field that induces the eddy currents and at least partially cancels the source electromagnetic field. A layer of conductive material between the gradient coil assembly 22 and the superconducting region 206 acts as a shield, reducing the induced eddy currents and their effects on the main magnet 14. Joule heating from the remaining induced currents is managed by the cold head. Induced eddy currents can also be reduced by active shielding or by shielding configurations in which the fringing magnetic fields on the structure of the magnet assembly 50 that induce eddy currents are cancelled or reduced.

[0027] The moving eddy currents are relatively high magnitude compared to the induced eddy currents and are difficult to reduce. The moving eddy currents can have much larger magnitude than the induced eddy currents. At certain vibration frequencies, the moving eddy currents are hundreds of times larger than the induced eddy currents. The Joule heating from the moving eddy currents exceeds the cooling power of the cold head and causes the cryogenic fluid to evaporate in order to maintain the temperature of the superconducting region 206. Cryogenic fluids such as helium are nonrecyclable natural resources and expensive to replenish. The moving eddy currents can even impose quench risks when the moving eddy currents are at a level where the evaporation of the cryogenic fluid is not sufficient to maintain the temperature of the superconducting region 206, superconductivity is lost, and the moving eddy currents are at a level that causes rapid boiling of the cryogenic fluid and decommissioning of the MR system.

[0028] To reduce the motion eddy currents, the currents to the gradient coils can be reduced. Because motion eddy currents are caused by vibrations of structures within the magnet assembly 50, at the resonant frequencies of the structures, the motion eddy currents are higher than at other frequencies. Pulse sequences are redesigned or limited to avoid motion eddy currents at those frequencies, which can impose limitations on the performance of the MR system 10.

[0029] Another known method of reducing motional eddy currents is to minimize vibrations, such as by stiffening the structure or adding stiffening structures. This method faces several challenges. First, vibrations are inevitable because materials do not have infinite stiffness. Second, increasing stiffness is costly, and magnet assemblies lack the interior space to allow for increased stiffness of the structure. For example, to increase stiffness, the structure can be thickened, which increases the bore size of the magnet assembly 50, significantly increasing costs, since superconducting wires are expensive and increasing the bore size necessarily increases the length of the magnet wires.

[0030] The systems, assemblies, and methods described herein overcome the above-mentioned problems in known systems and methods. The shield assembly 208 does not increase or requires only a minimal increase in the bore size of the magnet assembly 50. The thickness of the shield assembly 208 may be as low as 0.1 mm. The shield assembly 208 can be placed in any gap between structures of the magnet assembly 50 or can be placed on existing structures of the magnet assembly. Thus, a system including the shield assembly is not cost-prohibitive. The shield assembly 208 improves the performance of the MR system 10 because it allows the structure to vibrate without the need to impose vibration-related limitations on gradient coil currents or pulse sequences, etc.

[0031] 3A and 3B illustrate an exemplary shield assembly 208. A schematic diagram of a cross section of the shield assembly 208 is shown. A pattern 302 of the shield assembly 208 is shown. In an exemplary embodiment, the shield assembly 208 includes a conductive layer 304 and an insulating layer 306. The insulating layer 306 is made of a non-conductive material, such as plastic or fiberglass. The insulating layer 306 provides structural support and insulation for the conductive layer 304.

[0032] In an exemplary embodiment, the conductive layer 304 is fabricated from a conductive material such as copper, silver, or aluminum. The conductive layer 304 includes grooves 308 through the conductive layer 304 that expose the insulating layer 306. The grooves 308 restrict the direction in which current can flow. The grooves 308 are designed such that at least a portion of the grooves 308 are oriented perpendicular to the direction of the moving eddy currents. As a result, the grooves are designed or configured such that induced eddy currents flow within the conductive layer 304 while the moving eddy currents are blocked by the grooves.

[0033] The conductive layer 304 may be attached to the insulating layer 306 by a bonding agent (not shown), such as an adhesive. The conductive layer 304 may be plated with the insulating layer using metal plating. Other bonding methods may be used to bond the conductive layer 304 to the insulating layer 306. Bonding the conductive layer 304 to the insulating layer 306 may be performed before forming the grooves 308, bonding the conductive layer 304 to the insulating layer 306, and cutting the grooves according to the pattern 302. Alternatively, bonding the conductive layer 304 to the insulating layer 306 may be performed after forming the grooves 308, and the conductive layer 304 with the pattern 302 is bonded to the insulating layer 306. In some embodiments, the grooves are formed while the conductive layer 304 is bonded to the insulating layer 306. For example, the conductive layer 304 is deposited on the insulating layer 306 according to the pattern 302.

[0034] In FIG. 3B, z is the direction of the main magnetic field, and r·φ is the distance along the circumference of the shield assembly 208 from the central z-axis of the bore 46 that extends from the isocenter of the bore 46 (see FIG. 2). The shield assembly 208 may be flexible such that the shield assembly 208 is formed as a sheet and then folded and rolled into a desired shape, such as a cylinder. The shield assembly 208 may be formed into a cylinder by curling the shield assembly 208 around a central z-line 312. The shield assembly 208 may be positioned radially inside the magnet assembly 50. The shield assembly 208 may be pre-assembled and then assembled to the magnet assembly 50 by folding the shield assembly 208 and attaching it to the magnet assembly 50. Alternatively, the shield assembly 208 may be assembled with the magnet assembly 50. For example, the conductive layer 304 and / or the insulating layer 306 may be formed on the magnet assembly 50 and then the grooves 308 of the pattern 302 may be cut into the conductive layer 304. In another example, the conductive layer 304 is fabricated by an additive manufacturing process, where the conductive layer 304 is deposited on the insulating layer 306 or an insulating layer of the magnet assembly 50 having grooves in the pattern 302.

[0035] During operation, during scanning, induced eddy currents flow on the conductive layer 304 while motional eddy currents are blocked. The induced eddy currents flowing in the shield assembly 208 generate electromagnetic fields that at least partially counteract sources of induced eddy currents, such as magnetic fields from the gradient coils. Thus, the shield assembly 208 performs a shielding function against induced eddy currents and reduces induced eddy currents in structures beyond the shield assembly 208, such as the main magnet 14. On the other hand, because the grooves 308 are oriented perpendicular to the direction of the motional eddy currents, the motional eddy currents are blocked by the shield assembly 208.

[0036] Since the conductive layer 304 provides a conductive medium for the induced eddy currents to flow, the material of the conductive layer 304 is selected to have a relatively high electrical conductivity, such as copper, silver, a copper alloy, or a silver alloy. The conductive layer 304 has a thickness 310 that allows the induced eddy currents to flow. In some embodiments, the thickness 310 is close to or larger than the skin depth of the induced eddy currents. The skin depth of copper at room temperature is approximately 2 mm. The thickness of the conductive layer may be approximately 2 mm or more. In other embodiments, the conductive layer 304 provides a reasonable conductivity for the induced eddy currents, since despite the skin effect, the induced eddy currents are already reduced by other structures and are not significant. The thickness of the conductive layer 304 may be less than the skin depth of the conductive material of the conductive layer 304, for example, 0.1 mm or more.

[0037] The conductive layer 304 can be configured in a variety of ways. For example, in a parallel-loop configuration, the layer 304 can include disjoint loops, where the disjoint loops are not electrically connected to allow current to flow between the individual loops. In another example, in a series-loop configuration, the conductive layer 304 can include a continuous multi-turn path through which induced eddy currents can flow. In a further example, the conductive layer 304 can include both disjoint loops and multi-turn paths.

[0038] In an exemplary embodiment, the shield assembly 208 includes an x-shield layer 212x, a y-shield layer 212y, and a z-shield layer (not shown) (see FIG. 2B). In FIGS. 2A and 2B, the x-shield layer 212x is located inside the y-shield layer 214, for example. The x-, y-, and z-shield layers may be in any configuration relative to each other, and the x-, y-, and z-shield layers may be in any order from the innermost layer to the outermost layer. The x-, y-, and z-shield layers may be disposed separately from each other, and the x-, y-, and z-shield layers are not adjacent to each other. Each of the x-, y-, or z-shield layers 212 includes a conductive layer 304 and may or may not include an insulating layer 306. The patterns 302 on the x-, y-, and z-shield layers 212 are different from each other because the patterns 302 are generated based on the motional eddy currents caused by the corresponding x-, y-, or z-gradient coils. Typically, the motional eddy currents from the x- or y-gradients generated by the x- or y-gradient coils are much more significant than the z-gradients generated by the z-gradient coils. In some embodiments, the shield assembly 208 may include only x and y shield layers without including a z shield layer, while in other embodiments, the shield assembly 208 includes any combination of x, y, and z shield layers.

[0039] 4A and 4B show the superconducting region 206 in two different types of superconducting main magnet assemblies 12a, 12b. The superconducting main magnet assembly 12a typically uses a helium bath setup (FIG. 4A). The cryostat vessel 18 contains liquid helium 402. The magnet windings 403 of the main magnet 14 are immersed in the liquid helium 402. The superconducting region 206 of the main magnet assembly 12a includes the area surrounded by the cryostat vessel 18, starting from the inner vessel layer 404 to the outer vessel layer 406, including the magnet windings 403, the liquid helium 402, and the cryostat vessel 18. Compared to the primary magnet assembly 12a, the primary magnet assembly 12b uses conduction cooling, where the magnet windings 403 are cooled and maintained at superconducting temperatures via liquid helium circulated in cooling tubes 408 (see FIG. 4B). The cooling tubes 408 are thermally connected to the magnet windings 403. The superconducting region 206 of the primary magnet assembly 12b includes the magnet windings 403 and the cooling tubes 408.

[0040] FIG. 5 shows simulation results of gradient-induced magnet heating in an MR system without (curve 502) and with (curve 504) the systems and methods described herein. Curve 502 shows the heating without including the shield assembly 208 in the MR system 10. Curve 504 shows the heating when the shield assembly 208 is included in the MR system 10. The y-axis is the power of the heating in arbitrary units and logarithmic scale. As shown, including the shield assembly 208 significantly reduces the heating. At some frequencies, the heating is reduced by an order of magnitude of two, and the heating is reduced by several hundred times. The heating is reduced to a range that is manageable by the cooling power of the cold head, greatly reducing the risk of magnet quenching.

[0041] 6 is an exemplary method 600 for assembling a shield assembly. In an exemplary embodiment, the method 600 includes a step 602 of estimating induced eddy currents. The induced eddy currents can be estimated by simulation using a gradient field map and / or a pulse sequence of the MR system 10. The pulse sequence is a sequence of RF pulses, gradient pulses, and data acquisition applied by the MR system 10 in acquiring MR signals. The method 600 further includes a step 604 of estimating motional eddy currents. For example, for a given gradient field map and pulse sequence, the Lorentz force from the induced eddy currents is estimated based on the induced eddy currents and the magnetic field. The motion or vibration of the structure of the magnet assembly 50 is estimated based on the Lorentz force, mass, and stiffness of the structure.

number

[0042] In an exemplary embodiment, the method 600 further includes generating 606 a pattern for the shield assembly based on the estimated moving eddy currents and / or the estimated induced eddy currents. The pattern 302 is designed such that at least a portion of the grooves 308 are oriented in a direction perpendicular to the direction of the eddy currents, thereby reducing or limiting the flow of the moving eddy currents. The grooves 308 may or may not follow the direction of the induced eddy currents. At a given location in the conductive layer, the current vectors of the induced eddy currents and the current vectors of the moving eddy currents are typically oriented differently. As a result, the conductive layer 304 with the pattern designed as above blocks the moving eddy currents and promotes the flow of the induced eddy currents. The shield assembly may include an x ​​shield layer, a y shield layer, a z shield layer, or any combination thereof. The x shield layer is designed based on the x-gradient pulse in the pulse sequence to block the moving eddy currents and promote the flow of the induced eddy currents caused by the x-gradient coil. Similarly, the y shield layer is designed based on the y gradient pulse in the pulse sequence to block motional eddy currents and promote the flow of induced eddy currents caused by the y gradient coil. The z shield layer is designed based on the z gradient pulse to block motional eddy currents and promote the flow of induced eddy currents caused by the z gradient coil. The gradients for different pulse sequences vary. In the design pattern 302, the pattern 302 may be optimized using criteria such as blocking motional eddy currents for pulse sequences that cause the most severe eddy currents, such as echo planar imaging EPI sequences, or blocking motional eddy currents at frequencies corresponding to the resonant frequencies of the structure of the magnet assembly 50 (see FIG. 5).

[0043] Once the pattern 302 is designed, a number of grooves 308 are cut according to the pattern 302. The number of grooves 308 may be cut before or after the conductive layer 304 is assembled with the insulating layer 306 or the insulating layer of the magnet assembly 50. After the shield assembly 208 is assembled, the shield assembly 208 is attached to the magnet assembly 50. The shield assembly 208 may be attached to a structure between the gradient coil assembly 22 and the superconducting region 206. The shield assembly 208 may be a stand-alone structure that is disposed between the gradient coil assembly 22 and the superconducting region 206 but is not attached to any other structure. The shield assembly 208 may be directly assembled to the magnet assembly 50.

[0044] Method 600 or portions of method 600 may be implemented using any suitable computing device 800 and software implemented therein. FIG. 7 is a block diagram of an exemplary computing device 800. In an exemplary embodiment, computing device 800 includes a user interface 804 that receives at least one input from a user. User interface 804 may include a keyboard 806 that allows a user to input relevant information. User interface 804 may also include, for example, a pointing device, a mouse, a stylus, a touch-sensitive panel (e.g., a touchpad and a touchscreen), a gyroscope, an accelerometer, a position detector, and / or an audio input interface (including, for example, a microphone).

[0045] Additionally, in an exemplary embodiment, computing device 800 includes a presentation interface 817 that presents information, such as input events and / or validation results, to a user. Presentation interface 817 may also include a display adapter 808 coupled to at least one display device 810. More specifically, in an exemplary embodiment, display device 810 may be a visual display device, such as a cathode ray tube (CRT), a liquid crystal display (LCD), a light-emitting diode (LED) display, and / or an "electronic ink" display. Alternatively, presentation interface 817 may include an audio output device (e.g., an audio adapter and / or a speaker) and / or a printer.

[0046] The computing device 800 also includes a processor 814 and a memory device 818. The processor 814 is coupled to the user interface 804, the presentation interface 817, and the memory device 818 via a system bus 820. In an exemplary embodiment, the processor 814 communicates with a user, such as by prompting the user via the presentation interface 817 and / or by receiving user input via the user interface 804. The term "processor" generally refers to any programmable system, including systems and microcontrollers, reduced instruction set computers (RISC), complex instruction set computers (CISC), application specific integrated circuits (ASIC), programmable logic circuits (PLC), and any other circuit or processor capable of performing the functions described herein. The above examples are merely exemplary and thus are not intended to limit in any way the definition and / or meaning of the term "processor."

[0047] In an exemplary embodiment, memory device 818 includes one or more devices that allow for storing and retrieving information, such as executable instructions and / or other data. Additionally, memory device 818 includes one or more computer-readable media, such as, but not limited to, dynamic random access memory (DRAM), static random access memory (SRAM), solid-state disks, and / or hard disks. In an exemplary embodiment, memory device 818 stores, but is not limited to, application source code, application object code, configuration data, additional input events, application state, assertion statements, validation results, and / or any other type of data. In an exemplary embodiment, computing device 800 may also include a communication interface 830 coupled to processor 814 via system bus 820. Additionally, communication interface 830 is communicatively coupled to a data acquisition device.

[0048] In an exemplary embodiment, the processor 814 may be programmed by encoding operations with one or more executable instructions and providing the executable instructions to the memory device 818. In an exemplary embodiment, the processor 814 is programmed to select a number of measurements received from the data acquisition device.

[0049] During operation, a computer executes computer-executable instructions embodied in one or more computer-executable components stored on one or more computer-readable media to implement aspects of the invention described and / or illustrated herein. The order of execution or performance of operations in the embodiments of the invention illustrated and described herein is not essential, unless otherwise specified. That is, operations may be performed in any order unless otherwise specified, and embodiments of the invention may include additional or fewer operations than those disclosed herein. For example, performing or executing a particular operation before, contemporaneously, or after another operation is contemplated to be within the scope of aspects of the invention.

[0050] 8 illustrates an exemplary configuration of a server computing device 1001 on which the method 600 or portions of the method 600 may be implemented. The server computing device 1001 also includes a processor 1005 for executing instructions. The instructions may be stored, for example, in a memory area 1030. The processor 1005 may include one or more processing units (e.g., in a multi-core configuration).

[0051] The processor 1005 is operably coupled to a communication interface 1015 such that the server computing device 1001 can communicate with a remote device or another server computing device 1001. For example, the communication interface 1015 may receive data from the computing device 800 or the controller 30 over the Internet.

[0052] The processor 1005 may also be operatively coupled to a storage device 1034. The storage device 1034 is any computer operating hardware suitable for storing and / or retrieving data, such as, but not limited to, wavelength shift, temperature, and strain. In some embodiments, the storage device 1034 is integrated into the server computing device 1001. For example, the server computing device 1001 may include one or more hard disk drives as the storage device 1034. In other embodiments, the storage device 1034 may be external to the server computing device 1001 and accessed by multiple server computing devices 1001. For example, the storage device 1034 may include multiple storage units, such as hard disks and / or solid state disks in a redundant array of inexpensive disks (RAID) configuration. The storage device 1034 may include a storage area network (SAN) and / or a network attached storage (NAS) system.

[0053] In some embodiments, the processor 1005 is operably coupled to the storage device 1034 via a storage interface 1020. The storage interface 1020 is any component capable of providing the processor 1005 with access to the storage device 1034. The storage interface 1020 may include, for example, an Advanced Technology Attachment (ATA) adapter, a Serial ATA (SATA) adapter, a Small Computer System Interface (SCSI) adapter, a RAID controller, a SAN adapter, a network adapter, and / or any component that provides the processor 1005 with access to the storage device 1034.

[0054] At least one technical effect of the systems and methods described herein includes: (a) reduction of motion eddy currents through a shield assembly; (b) a shield assembly that provides shielding against induced eddy currents while blocking motion eddy currents, thereby reducing the risk of magnet heating and quenching; (c) a shield assembly that includes patterned grooves that provide a path for induced eddy currents to flow while blocking motion eddy currents; and (d) a method of designing patterns in a shield assembly.

[0055] Exemplary embodiments of the motional eddy current reduction system and method are described in detail above. The system and method are not limited to the specific embodiments described herein, but rather the components of the system and / or operations of the method may be utilized independently and separately from other components and / or operations described herein. Furthermore, the components and / or operations described may also be defined in or used in combination with other systems, methods, and / or devices, and are not limited to implementation with only the system described herein.

[0056] Although specific features of various embodiments of the invention may be shown in some drawings and not in others, this is for convenience only, and in accordance with the principles of the invention, any feature of a drawing may be referenced and / or claimed in combination with any feature of any other drawing.

[0057] This specification discloses the invention using examples, including the best mode, and also enables any person skilled in the art to practice the invention, including making and using any device or system, and practicing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal words of the claims, or if they include equivalent structural elements that do not differ substantially from the literal words of the claims.

[0058] [Embodiment 1] 1. A magnetic resonance (MR) system comprising: a main magnet assembly configured to generate a polarizing magnetic field; a gradient coil assembly including a plurality of gradient coils configured to apply at least one gradient magnetic field to the polarizing magnetic field; a shield assembly disposed between the main magnet assembly and the gradient coil assembly; Including, The shield assembly includes a conductive layer fabricated from a conductive material and defining a plurality of grooves disposed therethrough; An MR system, wherein the plurality of grooves are configured to inhibit motional eddy currents caused by action of the polarizing magnetic field and the at least one gradient magnetic field when the at least one gradient magnetic field is applied. [Embodiment 2] 2. The MR system of embodiment 1, wherein at least a portion of the plurality of grooves are oriented in a direction perpendicular to the direction of the moving eddy currents. [Embodiment 3] 2. The MR system of embodiment 1, wherein the conductive layer is configured to conduct induced eddy currents induced by the gradient coil assembly when the at least one gradient magnetic field is applied. [Embodiment 4] 2. The MR system of embodiment 1, wherein the shield assembly includes at least one of an x-shield layer, a y-shield layer, or a z-shield layer, the x-shield layer, the y-shield layer, or the z-shield layer being configured to block motional eddy currents caused by application of the at least one gradient magnetic field via an x-gradient coil, a y-gradient coil, or a z-gradient coil, respectively. [Embodiment 5] 2. The MR system of embodiment 1, wherein the shield assembly includes at least one of an x-shield layer or a y-shield layer, the x-shield layer or the y-shield layer being configured to block motional eddy currents caused by application of the at least one gradient magnetic field via an x-gradient coil or a y-gradient coil, respectively. [Embodiment 6] 2. The MR system of embodiment 1, wherein the main magnet assembly includes a superconducting main magnet. [Embodiment 7] 7. The MR system of embodiment 6, wherein the shield assembly is disposed between the gradient coil assembly and a superconducting region of the main magnet assembly. [Embodiment 8] 2. The MR system of embodiment 1, wherein the shield assembly is attached to the gradient coil assembly. [Embodiment 9] 2. The MR system of embodiment 1, wherein the shield assembly is attached to the main magnet assembly. [Embodiment 10] 1. A shield assembly for a magnetic resonance (MR) system, comprising: 1. A shield assembly comprising: a conductive layer fabricated from a conductive material and defining a plurality of grooves disposed therethrough, the plurality of grooves configured to block motional eddy currents generated by the MR system. [Embodiment 11] 11. A shield assembly as described in embodiment 10, wherein at least a portion of the plurality of grooves are oriented in a direction perpendicular to the direction of the moving eddy currents. [Embodiment 12] 11. A shield assembly as described in embodiment 10, wherein the conductive layer is configured to conduct induced eddy currents induced by the MR system. [Embodiment 13] The shield assembly of embodiment 10, wherein the shield assembly includes at least one of an x-shield layer, a y-shield layer, or a z-shield layer, the x-shield layer, the y-shield layer, or the z-shield layer being configured to block motional eddy currents caused by application of a gradient magnetic field via an x-gradient coil, a y-gradient coil, or a z-gradient coil of the MR system, respectively. [Embodiment 14] The shield assembly of embodiment 10, wherein the shield assembly includes at least one of an x ​​shield layer or a y shield layer, the x shield layer or the y shield layer being configured to block motional eddy currents caused by application of a gradient magnetic field via an x-gradient coil or a y-gradient coil of the MR system, respectively. [Embodiment 15] 11. The shield assembly of embodiment 10, wherein the conductive layer has a thickness of less than or equal to 2 millimeters. [Embodiment 16] 1. A method of assembling a shield assembly for a magnetic resonance (MR) system, comprising: 1. A method comprising: providing a conductive layer fabricated from a conductive material and defining a plurality of grooves disposed therethrough, the plurality of grooves configured to interrupt motional eddy currents generated by the MR system. [Embodiment 17] 17. The method of claim 16, wherein the step of providing the conductive layer further includes a step of estimating the moving eddy currents generated by the MR system, and a step of generating a pattern of the plurality of grooves on the conductive layer based on the estimated moving eddy currents. [Embodiment 18] 18. The method of embodiment 17, wherein generating a pattern further comprises forming the plurality of grooves such that at least a portion of the plurality of grooves are oriented orthogonal to a direction of moving eddy currents. [Embodiment 19] 17. The method of claim 16, wherein the step of providing a conductive layer further includes the steps of estimating induced eddy currents induced by the MR system, and generating a pattern of the plurality of grooves on the conductive layer based on the moving eddy currents and the induced eddy currents by forming the plurality of grooves configured to promote the flow of the induced eddy currents. [Embodiment 20] 17. The method of embodiment 16, further comprising the step of attaching the shield assembly to at least one of a main magnet assembly or a gradient coil assembly of the MR system. [Explanation of symbols]

[0059] 10: Magnetic resonance (MR) system 12, 12a, 12b: Main magnet assembly 14: Main magnet 16: Heat shield assembly 18: Cryostat vessel 19: Magnet vacuum vessel 22: Gradient coil assembly 24: RF coil 30: Controller 32: Main magnetic field control 34: Gradient magnetic field control 36: Memory 38: Display 40: Transmit / Receive (T / R) switch 42: RF transmitter 44: Receiver 46: Bore 48: Processor 50: Magnet assembly 162: Inner heat shield member 164: Outer heat shield member 201: Magnet wire 202: Shim tube 206: Superconducting region 208: Shield assembly 210: Inner side 212x: x shield layer 212y: y shield layer 212x: x shield layer 206: Superconducting region 302: Pattern 304: Conductive layer 306: Insulation layer 308: Groove 310: Thickness 402: Liquid helium 403: Magnet winding 404: Inner layer 406: Outer layer 408: Cooling tube 502, 504: Curve 800: Computing device 804: User interface 806: Keyboard 808: Display adapter 810: Display device 814: Processor 817: Presentation interface 818: Memory device 820: System bus 830: Communication interface 1001: Server computer device 1005: Processor 1015: Communication interface 1020: Storage interface 1030: Memory area 1034: Storage device

Claims

1. 1. A magnetic resonance (MR) system (10), comprising: a main magnet assembly (12) configured to generate a polarizing magnetic field; a gradient coil assembly (22) including a plurality of gradient coils configured to apply at least one gradient magnetic field to the polarizing magnetic field; a shield assembly (208) disposed between the main magnet assembly (12) and the gradient coil assembly (22); The shield assembly (208) comprises: An insulating layer (306); a conductive layer (304) fabricated from a conductive material and defining a groove (308) disposed through the conductive layer (304); Equipped with the grooves (308) are oriented to block motional eddy currents caused by the action of the polarizing magnetic field and the at least one gradient magnetic field when the at least one gradient magnetic field is applied; The conductive layer (304) is attached to an insulating layer (306) with a groove (308) exposing the insulating layer (306), in an MR system (10).

2. At least a portion of the grooves (308) are oriented perpendicular to the direction of the moving eddy currents; The MR system of claim 1 , wherein the conductive layer is configured to conduct induced eddy currents induced by the gradient coil assembly upon application of at least one gradient magnetic field.

3. the shield assembly (208) includes at least one of an x-shield layer (212x), a y-shield layer (212y), or a z-shield layer; 3. The MR system of claim 1, wherein the x-shield layer, the y-shield layer, or the z-shield layer is configured to block moving eddy currents caused by application of at least one gradient magnetic field via an x-gradient coil, a y-gradient coil, or a z-gradient coil, respectively.

4. 3. The MR system of claim 1, wherein the shield assembly includes at least one of an x-shield layer or a y-shield layer configured to block motional eddy currents caused by application of at least one gradient magnetic field via an x-gradient coil or a y-gradient coil, respectively.

5. The MR system of claim 1 , wherein the main magnet assembly (12) includes a superconducting main magnet.

6. The MR system of claim 5, wherein the shield assembly is disposed between the gradient coil assembly and the superconducting region of the main magnet assembly.

7. The MR system of claim 1 , wherein the shield assembly is attached to the gradient coil assembly and / or the shield assembly is attached to the main magnet assembly.

8. 1. A shield assembly (208) for a magnetic resonance (MR) system (10), comprising: a conductive layer (304) fabricated from a conductive material and defining a groove (308) disposed through the conductive layer (304), the groove (308) being oriented to block motional eddy currents generated by the MR system (10); A shield assembly (208) in which a conductive layer (304) is attached to an insulating layer (306) with a groove (308) exposing the insulating layer (306).

9. At least a portion of the grooves (308) are oriented perpendicular to the direction of the moving eddy currents. and / or The conductive layer (304) is configured to conduct induced eddy currents induced by the MR system (10). and / or The shield assembly (208) of claim 8, wherein the conductive layer (304) has a thickness of less than or equal to 2 millimeters.

10. The shield assembly (208) of claim 8 or 9, wherein the shield assembly (208) includes at least one of an x-shield layer (212x), a y-shield layer (212y), or a z-shield layer, the x-shield layer (212x), the y-shield layer (212y), or the z-shield layer being configured to block moving eddy currents caused by application of a gradient magnetic field via an x-gradient coil, a y-gradient coil, or a z-gradient coil of the MR system (10), respectively.

11. 10. The shield assembly (208) of claim 8 or 9, wherein the shield assembly (208) includes at least one of an x-shield layer (212x) or a y-shield layer (212y), the x-shield layer or the y-shield layer being configured to block moving eddy currents caused by application of a gradient magnetic field via an x-gradient coil or a y-gradient coil of the MR system (10), respectively.

12. A method (600) for assembling a shield assembly (208) of a magnetic resonance (MR) system (10), comprising: Providing an insulating layer (306); providing a conductive layer (304) fabricated from a conductive material and defining a groove (308) positioned through the conductive layer (304); Attaching a conductive layer (304) to an insulating layer (306); wherein the grooves (308) are oriented to block motional eddy currents generated by the MR system (10), and the insulating layer (306) is exposed by the grooves (308).

13. 13. The method (600) of claim 12, wherein providing the conductive layer (304) comprises estimating (604) moving eddy currents generated by the MR system (10) and generating (606) a pattern (302) of grooves (308) on the conductive layer (304) based on the estimated moving eddy currents.

14. 14. The method (600) of claim 13, wherein generating (606) the pattern (302) further comprises forming the grooves (308) such that at least a portion of the grooves (308) are oriented in a direction perpendicular to the direction of the moving eddy currents.

15. The step of providing a conductive layer (304) comprises: estimating (604) induced eddy currents induced by the MR system (10); generating a pattern of grooves (308) on the conductive layer (304) based on the moving eddy currents and the induced eddy currents by forming grooves (308) configured to promote the flow of induced eddy currents; and / or attaching the shield assembly (208) to at least one of a main magnet assembly (12) or a gradient coil assembly (22) of the MR system (10); 13. The method of claim 12, comprising:

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