Patent Grant
10185019
Embodiments of the invention relate generally to a magnetic resonance imaging (MRI) system and in particular to a system and apparatus for compensating for magnetic field distortion caused by mechanical vibrations in the Mill system.
Magnetic resonance imaging (MRI) is a medical imaging modality that can create pictures of the inside of a human body without using x-rays or other ionizing radiation. MRI uses a powerful magnet to create a strong, uniform, static magnetic field (i.e., the “main magnetic field”). When a human body, or part of a human body, is placed in the main magnetic field and subjected to a uniform magnetic field (polarizing field B0), the nuclear spins that are associated with the hydrogen nuclei in tissue water become polarized. This means that the magnetic moments that are associated with these spins become preferentially aligned along the direction of the main magnetic field, resulting in a small net tissue magnetization along that axis (the “z axis,” by convention). When a substance such as human tissue is the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency.
An MRI system also comprises components called gradient coils that produce smaller amplitude, spatially varying magnetic fields when a current is applied to them. Typically, gradient coils are designed to produce a magnetic field component that is aligned along the z axis, and that varies linearly in amplitude with position along one of the x, y or z axes. The effect of a gradient coil is to create a small ramp on the magnetic field strength, and concomitantly on the resonant frequency of the nuclear spins, along a single axis. Three gradient coils with orthogonal axes are used to “spatially encode” the MR signal by creating a signature resonance frequency at each location in the body.
Radio frequency (RF) coils are used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei. The RF coils are used to add energy to the nuclear spin system in a controlled fashion. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. As the nuclear spins then relax back to their rest energy state (i.e., after the excitation signal B1 is terminated), they give up energy in the form of an RF signal. This signal is detected by the MRI system and is transformed into an image using a computer and known reconstruction algorithms.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals is digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
During an MRI scan, the MRI system experiences mechanical vibrations caused by various external and internal sources. For example, vibrations may be caused by the strong magnetic field environment and various elements of the MRI system, such as the coldhead motor or gradient coil (e.g., as a result of pulsing of the gradient coil), and by external sources such as floor vibrations caused by a nearby elevator or subway. The mechanical vibrations of such sources can cause the mechanical vibration of other elements inside the MRI system, such as the cryostat thermal shield, and induce eddy currents in electrically conductive material in the cryostat (e.g., the vacuum vessel, thermal shield, helium vessel). Such vibrations cause eddy currents to be induced on the metal structures of the MRI system. The eddy currents induce a magnetic field that is superimposed on the original homogeneous magnetic field generated by the MR system, which negatively affects the magnetic field homogeneity, causes artifacts on the image, and deteriorates image quality. The higher the main magnetic field is, the higher the induced eddy current will be and hence the higher the magnetic field distortion.
Attempts have been made to mitigate eddy current formation in MR systems by using software compensation models, providing vibration isolation pads, or designing the suspension system and components of the magnet structure to have a high stiffness to resist vibrations. However, such methods may not adequately mitigate induced eddy currents. Such methods also add cost and complexity to the MR system and increase the computational complexity of the image reconstruction process.
It would be desirable to provide a system and apparatus to passively (e.g., automatically) cancel or reduce the magnetic field distortion caused by eddy currents induced by mechanical vibrations.
In accordance with one aspect of the invention, an MR imaging apparatus includes cryostat for a magnetic resonance imaging (MRI) system. The cryostat includes a vacuum casing having a vacuum therein. A cryogen vessel is disposed within the casing, the vessel having a coolant therein. A thermal shield is disposed between the vacuum casing and the cryogen vessel. An eddy current compensation assembly is disposed within the casing. The eddy current compensation assembly includes a plurality of electrically conductive loops formed on one of the vacuum casing, the cryogen vessel, and the thermal shield and constructed to mitigate vibration-induced eddy currents in the MM system.
In accordance with another aspect of the invention, a method of manufacturing a magnetic resonance imaging (MRI) apparatus is set forth. The method includes providing a vacuum vessel, providing a helium vessel, and providing a thermal shield positioned between the vacuum vessel and the helium vessel. The method also includes forming an eddy current compensation assembly on a surface of one of the vacuum vessel, the helium vessel, and the thermal shield to mitigate vibration-induced eddy currents in the MRI apparatus, wherein the eddy current compensation assembly includes a plurality of electrically conductive closed loops. Further, the method includes assembling the vacuum vessel, the helium vessel, and the thermal shield to form a cryostat to house a superconducting magnet.
In accordance with yet another aspect of the invention, a magnetic resonance imaging (MRI) apparatus includes a magnetic assembly comprising a superconducting magnet having a bore therethrough. The MRI apparatus also includes a plurality of gradient coils positioned about the bore of the superconducting magnet to impress a polarizing magnetic field, an RF transceiver system, and an RF switch controlled by a pulse module to transmit RF signals to an RF coil assembly to acquire MR images. The magnetic assembly includes a helium vessel surrounding the superconducting magnet, a thermal shield surrounding the helium vessel, a vacuum vessel surrounding the thermal shield, and an eddy current compensation assembly having a matrix of electrically conductive loops coupled to one of the vacuum vessel, the helium vessel, and the thermal shield.
Various other features and advantages will be made apparent from the following detailed description and the drawings.
The drawings illustrate embodiments presently contemplated for carrying out the invention.
In the drawings:
Referring to
The system control 32 includes a set of modules connected together by a backplane 32a. These include a CPU module 36 and a pulse generator module 38 which connects to the operator console 12 through a serial link 40. It is through link 40 that the system control 32 receives commands from the operator to indicate the scan sequence that is to be performed. The pulse generator module 38 operates the system components to carry out the desired scan sequence and produces data which indicates the timing, strength and shape of the RF pulses produced, and the timing and length of the data acquisition window. The pulse generator module 38 connects to a set of gradient amplifiers 42, to indicate the timing and shape of the gradient pulses that are produced during the scan. The pulse generator module 38 can also receive patient data from a physiological acquisition controller 44 that receives signals from a number of different sensors connected to the patient, such as ECG signals from electrodes attached to the patient. And finally, the pulse generator module 38 connects to a scan room interface circuit 46 which receives signals from various sensors associated with the condition of the patient and the magnet system. It is also through the scan room interface circuit 46 that a patient positioning system 48 receives commands to move the patient to the desired position for the scan.
The gradient waveforms produced by the pulse generator module 38 are applied to the gradient amplifier system 42 having Gx, Gy, and Gz amplifiers. Each gradient amplifier excites a corresponding physical gradient coil in a gradient coil assembly generally designated 50 to produce the magnetic field gradients used for spatially encoding acquired signals. The gradient coil assembly 50 forms part of a resonance assembly 52 which includes a polarizing magnet 54 and a whole-body RF coil 56. A transceiver module 58 in the system control 32 produces pulses which are amplified by an RF amplifier 60 and coupled to the RF coil 56 by a transmit/receive switch 62. The resulting signals emitted by the excited nuclei in the patient may be sensed by the same RF coil 56 and coupled through the transmit/receive switch 62 to a preamplifier 64. The amplified MR signals are demodulated, filtered, and digitized in the receiver section of the transceiver 58. The transmit/receive switch 62 is controlled by a signal from the pulse generator module 38 to electrically connect the RF amplifier 60 to the coil 56 during the transmit mode and to connect the preamplifier 64 to the coil 56 during the receive mode. The transmit/receive switch 62 can also enable a separate RF coil (for example, a surface coil) to be used in either the transmit or receive mode.
The MR signals picked up by the RF coil 56 are digitized by the transceiver module 58 and transferred to a memory module 66 in the system control 32. A scan is complete when an array of raw k-space data has been acquired in the memory module 66. This raw k-space data is rearranged into separate k-space data arrays for each image to be reconstructed, and each of these is input to an array processor 68 which operates to Fourier transform the data into an array of image data. This image data is conveyed through the serial link 34 to the computer system 20 where it is stored in memory. In response to commands received from the operator console 12 or as otherwise directed by the system software, this image data may be archived in long term storage or it may be further processed by the image processor 22 and conveyed to the operator console 12 and presented on the display 16.
Referring to
Main superconducting coils 106 and bucking coils 110 are enclosed in a cryostat 114 to provide a cryogen environment designed to maintain the temperature of the superconducting coils below the appropriate critical temperature so that the superconducting coils are in a superconducting state with zero resistance. Cryostat 114 includes a casing having a vacuum therein 116, a cryogen vessel 118, and a thermal shield 120. Main superconducting coils 106 and bucking coils 110 are enclosed within helium vessel 118, which is designed to provide the operational environment (e.g., to contain and cool) for the superconducting coils. A set of superconducting active shim coils 122 may also be disposed within cryogen vessel 118 and are used to provide manufacturing tolerance compensation. Shim coils 122 are positioned on a former 124 which is cylindrical in shape. Cryogen vessel 118 is disposed within casing or vacuum vessel 116, which is configured to maintain a vacuum environment and is used to control the thermal load. Cryogen vessel 118 is filled with a liquid coolant such as, for example, helium. As such, the terms cryogen vessel 118 and helium vessel 118 are used interchangeably herein. However, one skilled in the art will recognize that cryogen vessel 118 may be filled with liquid coolants other than helium.
Thermal shield 120 is disposed between vacuum vessel 116 and helium vessel 118. Thermal shield 120 is used to cool and control the coldmass thermal load. Suspension members 126 are positioned between helium vessel 118 and vacuum vessel 116 to provide mechanical support of the coldmass and thermal shield 120. Passive shims 128 may be positioned on an inner cylindrical surface 130 (or warm bore) of vacuum vessel 116 to provide manufacturing tolerance compensation. Main leads 132 are used to provide current input to the main superconducting coils 106 and a shim lead 134 is used to provide current input to active shim coils 122. Instrumentation 136 is used to monitor magnet parameters. Various other elements such as covers, end caps, supports, brackets, etc. are omitted from
A coldhead 138 (including, for example, a recondenser) is used to manage the cryostat heat loads with, preferably, zero helium boil-off. During operation of magnet assembly 100, coldhead 138 generates mechanical vibrations (e.g., from a motor) that cause induced eddy currents in thermal shield 120 as well as in other elements in the cryostat with electrically conductive material. In addition, the mechanical vibrations of coldhead 138 may cause mechanical vibration of thermal shield 120. Eddy currents may be induced when thermal shield 120 is alternated (or vibrates) in a z-axis direction, an x-axis direction, and/or a y-axis direction. Eddy currents may also be induced due to mechanical vibrations from the MR system site environment or other support structure to which the magnet assembly 100 coupled and from vibrations caused by the gradient coil assembly (e.g., assembly 50 of
Eddy current compensation assembly 140 is constructed such that a compensation current is passively induced within eddy current compensation assembly 140 during the imaging process that substantially mitigates or cancels eddy currents induced due to mechanical vibrations within magnet assembly 100. Since the amplitude and damping of the vibrations that induce the eddy currents vary throughout the cryostat 114, the induced eddy currents are positive in some areas and negative in other areas. Further, the magnitude of the induced eddy currents varies by location. Accordingly, eddy current compensation assembly 140 is strategically constructed and positioned within cryostat 114 to account for the variance in the induced eddy currents throughout cryostat 114. By strategically sizing and placing eddy current compensation assembly 140 within cryostat 114, the eddy current compensation assembly 140 can (1) enhance the negative eddy currents and amplify the magnetic field FOV contribution if the positive eddy current contribution from the vibrations to the FOV is larger than the contribution of the negative eddy currents and (2) enhance the positive eddy currents and amplify the magnetic field-of-view (FOV) contribution if the negative current contribution from vibration to the FOV is larger than the contribution of the positive eddy currents.
In one embodiment, portions of eddy current compensation assembly 140 are positioned at one or more locations on end flanges 162, inner cylinder 160, and/or outer cylinder 158 of helium vessel 118, as shown in
According to one embodiment, eddy current compensation assembly 140 comprises a matrix of symmetrical pairs of closed loops formed from an electrically conductive material and affixed to a surface of cryostat 114. As described in more detail below with respect to
Eddy current compensation assembly 140 is constructed from a non-superconducting, electrically conductive material such as, for example, aluminum, copper, silver, or combinations thereof. In one embodiment, eddy current compensation assembly 140 is an adhesive tape or sheet affixed to cryostat 114 using fasteners or adhesives. In a preferred embodiment, eddy current compensation assembly 140 is a self-adhesive copper tape. However, one skilled in the art will recognize that eddy current compensation assembly 140 may be constructed from any number of electrically conductive materials having a desired electrical conductivity to induce current therein.
Referring now to
In alternate embodiments, eddy current compensation assembly 140 may include only one or two of sub-assemblies 166, 168, 170. Also, dependent on the size and geometry of sub-assemblies 166, 168, 170, it is contemplated that sub-assemblies 166, 168, 170 may overlap one another. Further, the size, shape, and location of sub-assemblies 166, 168, and/or 170 may be varied to accomplish the desired eddy current compensation. The shape, location and number of sub-assemblies 166, 168, 170 may be based on, for example, the amount of cancellation or compensation needed in the imaging volume, the distance from the thermal shield 120, the design needed to magnetically couple with the thermal shield 120, etc.
In one embodiment, pairs of compensation loops of sub-assemblies 166, 168, 170 are constructed to accomplish substantially cancel eddy currents induced due to specific frequencies. For example, a first loop pair 178 of sub-assembly 168 may be positioned and sized to cancel eddy currents induced in the x-direction due to a first vibrational frequency (e.g., 80 Hz), a second loop pair 180 of sub-assembly 168 may be positioned and sized to cancel eddy currents induced in the x-direction due to a second vibrational frequency (e.g., 50 Hz), and a third loop pair 182 of sub-assembly 168 may be positioned and sized to cancel eddy currents induced in the x-direction due to a third vibrational frequency (e.g., 30 Hz). Additional loop pairs may be included to cancel eddy currents over a desired range of frequencies such as, for example, a range of 1-120 Hz.
While eddy current compensation assembly 140 is described above as an electrically conductive tape or sheet affixed to a surface of cryostat 114, eddy current compensation may also be achieved by varying the thickness of vacuum vessel 116, helium vessel 118, and/or thermal shield 120 at select locations to form raised, closed loops on select surfaces of vacuum vessel 116, helium vessel 118, and/or thermal shield 120 within which compensation current is passively induced. For example, an eddy current compensation assembly may be formed by machining a matrix of raised loops into the bulk material of the thermal shield, vacuum vessel, and/or helium vessel to modify the electrical conductivity of the imaging volume, as described in more detail with respect to
Referring now to
As shown in
As eddy current compensation assembly 184 is manufactured as a part of thermal shield 120, eddy current compensation assembly 184 is formed of the same material as thermal shield 120. Thus, in embodiments where thermal shield 120 is formed of aluminum, eddy current compensation assembly 184 is likewise aluminum and has substantially similar material properties as bulk material 186 of thermal shield 120.
In alternative embodiments, however, it is contemplated that the material properties of the portion 198 of thermal shield 120 that includes compensation loops 188, 190, 192 may differ from the material properties of bulk material 186. As one example, first portion 198 of thermal shield 120 may be constructed having enhanced electrical conductivity compared with that of bulk material 186 such as by depositing a conductive coating to the surface of thermal shield 120 or by impregnating the portion 198 of thermal shield 120 that includes compensation loops 188, 190, 192 with an electrically conductive material. In such embodiments, first portion 198 may have a substantially similar thickness as the thickness of portion 204.
Referring back to
While embodiments of the invention have been described herein with respect to a symmetrical, closed bore magnet assembly, one skilled in the art will recognize that the techniques set forth herein may be applied to any system geometry. For example, in an alternative embodiment, the magnet assembly may have an open architecture as illustrated with respect to
Referring now to
The main magnetic field, B0, indicated generally by arrow 328 within imaging region 320 is generated by main superconducting magnet coils 329 within helium vessels 332 and 333. An outer set of superconducting bucking or shielding coils 330 are also positioned within helium vessels 332 and 333 and are used to provide, for example, control of stray magnetic fields. Spaced helium vessels 332 and 333 are cylindrical members providing an open end 331 to imaging region 320. Magnetic field shimming apparatus such as active shim coils (not shown) within cryogen vessels 332 and 333 and passive shims in external shim drawers indicated generally as 336 compensate for magnetic field inhomogeneities within imaging region 320 in the manner well known in the art. Helium vessels 332 and 333 are disposed within a cryogen pressure vessel or vacuum vessel 370. A thermal shield 372 is disposed between vacuum vessel 370 and helium vessels 332 and 333. Superconducting magnet coils 329 and 330 are assembled into a magnet assembly with cold iron ring 340 interposed between the coils. Main coils 329 and bucking coils 330 are supported on coil formers or supports 344 and 342 (e.g., composed of glass fiber-epoxy composite), respectively, in pockets machined for the coils. Active shim coils (not shown) are positioned on a former 374.
A condenser 334 and associated mechanical cryocooler 335 (together part of a coldhead 376) recondenses helium gas which results from the superconducting operation back to liquid helium. The recondensed liquid helium flows from recondenser 334 by gravity into upper helium vessel 332. A vertical transfer tube 337 interconnects helium vessels 332 and 333 and enables the gravity flow of helium from upper helium vessel 332 to lower helium vessel 333. As mentioned above with respect to
In one embodiment, an electrically conductive non-superconducting material is used to construct the eddy current compensation assembly 701. Example locations for the passive compensation loops of eddy current compensation assembly 701 are an inner surface 711 of vacuum vessel 710, an outer surface 704 of the helium vessel 702, an inner surface 705 of the helium vessel 702, an outer surface 708 of the thermal shield 706, and/or an inner surface 709 of thermal shield 706. However, one skilled in the art will recognize that the passive compensation loops of eddy current compensation assembly 701 may be formed on various other surfaces within cryostat 700.
One skilled in the art will appreciate that embodiments of the invention may be interfaced to and controlled by a computer readable storage medium having stored thereon a computer program. The computer readable storage medium includes a plurality of components such as one or more of electronic components, hardware components, and/or computer software components. These components may include one or more computer readable storage media that generally stores instructions such as software, firmware and/or assembly language for performing one or more portions of one or more implementations or embodiments of a sequence. These computer readable storage media are generally non-transitory and/or tangible. Examples of such a computer readable storage medium include a recordable data storage medium of a computer and/or storage device. The computer readable storage media may employ, for example, one or more of a magnetic, electrical, optical, biological, and/or atomic data storage medium. Further, such media may take the form of, for example, floppy disks, magnetic tapes, CD-ROMs, DVD-ROMs, hard disk drives, and/or electronic memory. Other forms of non-transitory and/or tangible computer readable storage media not list may be employed with embodiments of the invention.
A number of such components can be combined or divided in an implementation of a system. Further, such components may include a set and/or series of computer instructions written in or implemented with any of a number of programming languages, as will be appreciated by those skilled in the art. In addition, other forms of computer readable media such as a carrier wave may be employed to embody a computer data signal representing a sequence of instructions that when executed by one or more computers causes the one or more computers to perform one or more portions of one or more implementations or embodiments of a sequence.
Therefore, according to one embodiment of the invention, an MR imaging apparatus includes cryostat for a magnetic resonance imaging (MRI) system. The cryostat includes a vacuum casing having a vacuum therein. A cryogen vessel is disposed within the casing, the vessel having a coolant therein. A thermal shield is disposed between the vacuum casing and the cryogen vessel. An eddy current compensation assembly is disposed within the casing. The eddy current compensation assembly includes a plurality of electrically conductive loops formed on one of the vacuum casing, the cryogen vessel, and the thermal shield and constructed to mitigate vibration-induced eddy currents in the MRI system.
According to another embodiment of the invention, a method of manufacturing a magnetic resonance imaging (MRI) apparatus is set forth. The method includes providing a vacuum vessel, providing a helium vessel, and providing a thermal shield positioned between the vacuum vessel and the helium vessel. The method also includes forming an eddy current compensation assembly on a surface of one of the vacuum vessel, the helium vessel, and the thermal shield to mitigate vibration-induced eddy currents in the MRI apparatus, wherein the eddy current compensation assembly includes a plurality of electrically conductive closed loops. Further, the method includes assembling the vacuum vessel, the helium vessel, and the thermal shield to form a cryostat to house a superconducting magnet.
According to yet another embodiment of the invention, a magnetic resonance imaging (MRI) apparatus includes a magnetic assembly comprising a superconducting magnet having a bore therethrough. The MRI apparatus also includes a plurality of gradient coils positioned about the bore of the superconducting magnet to impress a polarizing magnetic field, an RF transceiver system, and an RF switch controlled by a pulse module to transmit RF signals to an RF coil assembly to acquire MR images. The magnetic assembly includes a helium vessel surrounding the superconducting magnet, a thermal shield surrounding the helium vessel, a vacuum vessel surrounding the thermal shield, and an eddy current compensation assembly having a matrix of electrically conductive loops coupled to one of the vacuum vessel, the helium vessel, and the thermal shield.
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing 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 language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.
The present application is a continuation of and claims priority to U.S. Non-Provisional application Ser. No. 13/726,278, filed Dec. 24, 2012, which is a continuation-in-part of and claims priority to U.S. Non-Provisional application Ser. No. 13/690,312, filed Nov. 30, 2012, which claims priority to U.S. Provisional Application Ser. No. 61/577,805, filed Dec. 20, 2011, the disclosures of which are incorporated herein in their entirety.
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| Number | Date | Country | |
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| Number | Date | Country | |
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| Child | 14962345 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13690312 | Nov 2012 | US |
| Child | 13726278 | US |
