JP2019180826A - Magnetic resonance imaging device and static magnetic field correction method - Google Patents

Abstract

To improve uniformity of a static magnetic field by using a magnetic body provided outside a static magnetic field magnet.SOLUTION: An MRI device according to an embodiment comprises a static magnetic field magnet, a gradient magnetic field coil, a magnet frame, and a magnetic body. The static magnetic field magnet includes a main coil for forming a first magnetic field, and a cancel coil for forming a second magnetic field. The gradient magnetic field coil forms a gradient magnetic field. The magnet frame houses the static magnetic field magnet and the gradient magnetic field coil. The magnetic body is arranged outside the magnet frame and near the cancel coil. The MRI device corrects a static magnetic field formed by the first magnetic field and the second magnetic field, by using a third magnetic field formed by the magnetic body magnetized by receiving the second magnetic field.SELECTED DRAWING: Figure 2

Description

  Embodiments described herein relate generally to a magnetic resonance imaging (MRI) apparatus and a static magnetic field correction method.

  An MRI apparatus is an apparatus that arranges a subject in a static magnetic field and acquires internal information of the subject as image data. An MRI apparatus may use a superconducting magnet in order to realize a high static magnetic field strength. In an MRI apparatus, in order to acquire a high quality image, it is required to make the static magnetic field formed by the superconducting magnet uniform.

International Publication No. 2012/132911

  The problem to be solved by the present invention is to improve the uniformity of a static magnetic field by a magnetic material provided outside the static magnetic field magnet.

  The MRI apparatus which concerns on embodiment is provided with a static magnetic field magnet, a gradient magnetic field coil, a magnet mount, and a magnetic body. The static magnetic field magnet includes a main coil that forms a first magnetic field and a cancel coil that forms a second magnetic field. The gradient coil forms a gradient magnetic field. The magnet mount accommodates a static magnetic field magnet and a gradient magnetic field coil. The magnetic body is disposed outside the magnet mount and in the vicinity of the cancel coil. The MRI apparatus corrects the first magnetic field and the static magnetic field formed by the second magnetic field by the third magnetic field formed by the magnetic material magnetized by receiving the second magnetic field.

FIG. 1 is a schematic diagram illustrating an overall configuration of an MRI apparatus according to an embodiment. FIG. 2 is a diagram illustrating a magnetic field formed by each of a main coil, a cancel coil, and a magnetic body provided in the MRI apparatus according to the embodiment. FIG. 3 is a diagram illustrating a state in which the magnetic body according to the embodiment is magnetized. FIG. 4 is a view showing an arrangement example of magnetic bodies provided in the MRI apparatus according to the embodiment. FIG. 5 is a diagram in which an arbitrary XY plane in a bore is expressed by a polar coordinate system in the MRI apparatus according to the embodiment. FIG. 6 is a graph showing the uniformity of the static magnetic field with respect to the angle θ shown in FIG. 5 in the MRI apparatus according to the embodiment. FIG. 7 is a flowchart illustrating a procedure of a static magnetic field correction method in the MRI apparatus according to the embodiment.

  Hereinafter, embodiments of an MRI apparatus and a static magnetic field correction method will be described in detail with reference to the drawings.

  FIG. 1 is a schematic diagram illustrating an overall configuration of an MRI apparatus according to an embodiment.

  FIG. 1 shows an MRI apparatus 1 according to the embodiment. The MRI apparatus 1 includes an imaging device 5 and a magnetic field calculation device 7. The imaging device 5 includes a magnet mount 100, magnet legs 200 (shown in FIGS. 4 and 5), a control cabinet 300, a console 400, and a bed device 500. The magnet stand 100, the magnet leg 200, the control cabinet 300, and the bed apparatus 500 are generally provided in an examination room. The examination room is also called a photography room. The console 400 is provided in the control room. The control room is also called an operation room. The magnetic field calculation device 7 will be described as being provided separately from the imaging device 5, but is not limited to that case. For example, the magnetic field calculation device 7 and the console 400 may be configured to be connected via, for example, an electric communication line so that the output result of the magnetic field calculation device 7 can be displayed on the console 400 of the imaging device 5.

  The magnet mount 100 accommodates the static magnetic field magnet 10, the gradient magnetic field coil 11, and the WB coil 12 therein. The bed apparatus 500 includes a bed body 50 and a top plate 51.

  The magnet legs 200 are provided outside the magnet mount 100 and support the magnet mount 100 from the outside. For example, four magnet legs 200 are provided.

  The control cabinet 300 includes a gradient magnetic field power supply 31 (X-axis 31x, Y-axis 31y, Z-axis 31z), an RF transmitter 32, an RF receiver 33, and a sequence controller 34.

  The static magnetic field magnet 10 of the magnet mount 100 is a superconducting magnet. The static magnetic field magnet 10 has a substantially cylindrical shape, and generates a static magnetic field in a bore W in which a subject, for example, a patient U is transported. The bore W is a space inside the cylinder of the magnet mount 100. The static magnetic field magnet 10 includes, for example, a housing 10A and a heat shield 10B.

  The housing 10A contains, for example, liquid helium cooled to a cryogenic temperature by a refrigerator (not shown). In the housing 10A, the main coil 61 and the cancel coil 62 are cooled by, for example, liquid helium. The main coil 61 is a superconducting coil provided in the housing 10A, and forms a first magnetic field. The cancel coil 62 is a superconducting coil provided inside the housing 10A and wound in the opposite direction to the main coil 61, and forms a second magnetic field. The cancel coil 62 is installed, for example, on the opening end side of the magnet, and weakens the static magnetic field formed by the main coil 61 leaking out of the imaging region, that is, the leakage magnetic field by forming a second magnetic field. ing.

  The heat shield 10B prevents radiant heat and is made of, for example, aluminum which is a conductor.

  The static magnetic field magnet 10 generates a static magnetic field by applying a current supplied from a static magnetic field power source to the main coil 61 and the cancel coil 62 in the excitation mode. Thereafter, when the mode is changed to the permanent current mode, the static magnetic field power source is disconnected. Once in the permanent current mode, the static magnetic field magnet 10 continues to generate a static magnetic field for a long time, for example, several years or more.

  The gradient magnetic field coil 11 has a substantially cylindrical shape like the static magnetic field magnet 10 and is installed inside the static magnetic field magnet 10. The gradient magnetic field coil 11 forms a gradient magnetic field by a current (electric power) supplied from the gradient magnetic field power supply 31. The gradient magnetic field coil 11 includes an Xch coil that generates a gradient magnetic field in the X-axis direction, a Ych coil that generates a gradient magnetic field in the Y-axis direction, and a Zch coil that generates a gradient magnetic field in the Z-axis direction.

  The gradient coil 11 includes a shim tray unit (not shown). The shim tray unit has a substantially cylindrical shape. For example, the shim tray unit is sandwiched between a main coil and a shield coil having a substantially cylindrical shape. A plurality of slots are formed at substantially equal intervals in the circumferential direction of the shim tray unit. Each slot is a through-hole that is formed over the entire length in the Z-axis direction of the shim tray unit, with openings formed at both end faces in the longitudinal direction (Z-axis direction) of the shim tray unit. A shim tray can be inserted into each slot. The shim tray is fixed substantially at the center of the shim tray unit in the Z-axis direction. The substantially central portion of the shim tray unit in the Z-axis direction is also the central portion of the gradient magnetic field coil 11 in the Z-axis direction. The shim tray is formed of, for example, a resin that is a nonmagnetic and nonconductive material, and has a substantially rod shape.

  A plurality of shim pockets are formed in each shim tray. For the purpose of making the static magnetic field in the imaging region in the bore W uniform, a necessary number of metal shims are stored in each shim pocket. The material of the metal shim is, for example, a silicon steel plate. Adjusting the number of metal shims stored in each shim pocket when the imaging device 5 is installed and making the static magnetic field in the imaging region in the bore W uniform is called passive shimming.

  The static magnetic field magnet 10 is designed and manufactured so that the static magnetic field in the bore W is as uniform as possible. However, in reality, it is difficult to make the static magnetic field completely uniform without any adjustment because it is affected by the magnet manufacturing error and the structure around the bore W. Further, the degree of non-uniformity of the static magnetic field varies depending on the individual devices, and also varies depending on the surrounding environment of the device installation location. For this reason, passive shimming using a metal shim is usually performed every time the apparatus is installed.

  The WB coil 12 is also called a whole body coil, and is installed in a substantially cylindrical shape so as to surround the patient U inside the gradient coil 11. The WB coil 12 functions as a transmission coil. That is, the WB coil 12 transmits an RF pulse toward the patient U according to the RF pulse signal transmitted from the RF transmitter 32. On the other hand, the WB coil 12 may have a function as a reception coil in addition to a function as a transmission coil that transmits an RF pulse. In that case, the WB coil 12 receives, as a reception coil, an MR signal emitted from the patient U due to nuclear excitation.

  The imaging device 5 may include a local coil 20 in addition to the WB coil 12. The local coil 20 is disposed close to the body surface of the patient U. The local coil 20 may include a plurality of coil elements. Since the plurality of coil elements are arranged in an array inside the local coil 20, they may be called PAC (Phased Array Coil).

  There are several types of local coils 20. For example, as shown in FIG. 1, the local coil 20 includes a body coil (Body Coil) installed on the chest, abdomen, or leg of the patient U, and a spine coil (Spine Coil) installed on the back side of the patient U. ). In addition, the local coil 20 includes types such as a head coil for imaging the head of the patient U and a foot coil for imaging the foot. In addition, the local coil 20 includes types such as a wrist coil for imaging the wrist, a knee coil for imaging the knee, and a shoulder coil for imaging the shoulder.

  The local coil 20 functions as a receiving coil. That is, the local coil 20 receives the aforementioned MR signal. However, the local coil 20 may be a transmission / reception coil having a function as a transmission coil for transmitting RF pulses in addition to a function as a reception coil for receiving MR signals. For example, the head coil and the knee coil as the local coil 20 include a transmission / reception coil. That is, the local coil 20 may be of any type for transmission only, reception only, and transmission / reception.

  The gradient magnetic field power supply 31 includes gradient magnetic field power supplies 31x, 31y, and 31z for the respective channels that drive the coils that generate the gradient magnetic fields in the X-axis, Y-axis, and Z-axis directions. The gradient magnetic field power supplies 31x, 31y, 31z output necessary currents independently for each channel in response to a command from the sequence controller 34. Thereby, the gradient magnetic field coil 11 can apply a gradient magnetic field (also referred to as “gradient magnetic field”) in the directions of the X axis, the Y axis, and the Z axis to the patient U.

  The RF transmitter 32 generates an RF pulse signal based on an instruction from the sequence controller 34. The RF transmitter 32 transmits the generated RF pulse signal to the WB coil 12.

  The MR signal received by the local coil 20, more specifically, the MR signal received by each coil element in the local coil 20 is transmitted to the RF receiver 33. The transmission line of each coil element and the transmission line of the WB coil 12 are called channels. For this reason, each MR signal output from each coil element or WB coil 12 may be referred to as a channel signal. The channel signal received by the WB coil 12 is also transmitted to the RF receiver 33.

  The RF receiver 33 performs AD (Analog to Digital) conversion on the channel signal from the local coil 20 and the WB coil 12, that is, the MR signal, and outputs it to the sequence controller 34. The MR signal converted into digital is sometimes referred to as raw data.

  The sequence controller 34 images the patient U by driving the gradient magnetic field power supply 31, the RF transmitter 32, and the RF receiver 33 under the control of the console 400. When raw data is received from the RF receiver 33 by imaging, the sequence controller 34 transmits the raw data to the console 400.

  The sequence controller 34 includes a processing circuit (not shown). The processing circuit includes, for example, a processor that executes a predetermined program, hardware such as an FPGA (Field Programmable Gate Array) and an ASIC (Application Specific Integrated Circuit).

  The console 400 includes a processing circuit, a storage circuit, a display, and an interface (not shown). The console 400 functions as a host computer. Note that the processing circuit, the storage circuit, the display, and the interface of the console 400 have the same configuration as the processing circuit 70, the storage circuit 71, the arithmetic circuit 72, the display 73, and the interface 74, which will be described later, and a description thereof will be omitted.

  The processing circuit of the console 400 reads and executes a program stored in the storage circuit or directly incorporated in the processing circuit, thereby executing an imaging according to a pulse sequence and generating an MR image. Is realized. The console 400 collects MR signals transmitted from the sequence controller 34 under the control of the processing circuit, and stores the collected MR signals in the storage circuit. The console 400 generates a desired MR image in the patient U by performing post-processing, that is, reconstruction processing such as Fourier transform, on the MR signal stored in the storage circuit under the control of the processing circuit. To do. The console 400 stores the generated various MR images in the storage circuit under the control of the processing circuit.

  The bed apparatus 500 includes a bed body 50 and a top board 51. The bed main body 50 arranges the top plate 51 so as to be movable in the X-axis direction, the Y-axis direction, and the Z-axis direction, for example. The movement of the top plate 51 in the X-axis direction is a movement of the top plate 51 in the left-right direction, that is, the short direction of the top plate 51. The movement of the top plate 51 in the Y-axis direction is a vertical movement of the top plate 51, that is, a movement in the thickness direction of the top plate 51. The movement of the top plate 51 in the Z-axis direction is the front-rear direction of the top plate 51, that is, the movement of the top plate 51 in the longitudinal direction.

  The magnetic field calculation device 7 includes a processing circuit 70, a storage circuit 71, an arithmetic circuit 72, a display 73, and an interface 74. The magnetic field calculation device 7 receives the surrounding environment of the imaging device 5 and the characteristics of the static magnetic field of the static magnetic field magnet 10 as inputs, and outputs arrangement information of the magnetic body M described later. Calculation of the arrangement information of the magnetic body M by the magnetic field calculation device 7 will be described later.

  The processing circuit 70 means a processing circuit such as a dedicated or general-purpose CPU (Central Processing Unit) or MPU (Micro Processor Unit), an application specific integrated circuit (ASIC), and a programmable logic device. Examples of the programmable logic device include circuits such as a simple programmable logic device (SPLD), a complex programmable logic device (CPLD), and a field programmable gate array (FPGA).

  Further, the processing circuit 70 may be configured by a single processing circuit or may be configured by a combination of a plurality of independent processing circuits. In the latter case, the plurality of storage circuits 71 may store programs corresponding to the functions of the plurality of processing circuits, respectively, or one storage circuit 71 may store programs corresponding to the functions of the plurality of processing circuits. It may be memorized.

  The storage circuit 71 includes a semiconductor memory element such as a random access memory (RAM) and a flash memory, a hard disk, and an optical disk. The storage circuit 71 may include a portable medium such as a USB (Universal Serial Bus) memory and a DVD (Digital Video Disk). The storage circuit 71 stores various processing programs used in the processing circuit 70 (including application programs as well as an OS (Operating System) and the like), data necessary for executing the programs, and the like. In addition, the OS may include a GUI (Graphical User Interface) that can use a lot of graphics to display information on the display 73 for the operator and perform basic operations through the interface 74.

  The arithmetic circuit 72 calculates the arrangement information and the number of metal shims in the shim tray, as well as the arrangement information of the magnetic substance M, which will be described later, in the examination room.

  The display 73 is a display device such as a liquid crystal display panel, a plasma display panel, and an organic EL (Electro Luminescence) panel.

  The interface 74 includes an input device that can be operated by an operator, and an input circuit that inputs a signal from the input device. The input device includes a pointing device (for example, a mouse), a keyboard, and various buttons. When the input device is operated by the operator, the input circuit generates an input signal corresponding to the operation and outputs it to the processing circuit 70. The magnetic field calculation apparatus 7 may include a touch panel in which an input device is configured integrally with the display 73.

  A magnetic body M is provided outside the magnet mount 100 according to the present embodiment. For example, as shown in FIG. 4, the magnetic body M is disposed in the vicinity of the cancel coil 62. The magnetic body M is disposed at a position where it is magnetized by receiving the second magnetic field formed by the cancel coil 62. For example, the magnetic body M is provided in a lower region of the static magnetic field magnet 10, for example, below the floor as shown in FIG.

  The position where the magnetic body M is arranged is obtained by, for example, the arithmetic circuit 72. The arithmetic circuit 72 receives the surrounding environment of the imaging device 5 and the characteristics of the static magnetic field of the static magnetic field magnet 10 as inputs, and outputs arrangement information of the magnetic body M. The surrounding environment of the imaging device 5 is, for example, the position and amount of reinforcing bars included in columns, beams, and flooring around the examination room. In other words, it is the arrangement information of the magnetic body around the examination room. The static magnetic field characteristics of the static magnetic field magnet 10 are, for example, the static magnetic field specifications at the time when the static magnetic field magnet 10 is manufactured, and show the distribution of the deviation amount of the static magnetic field with reference to a completely uniform static magnetic field. The arrangement information of the magnetic body M output from the arithmetic circuit 72 is, for example, the arrangement position of the magnetic body M in the examination room and the thickness of the magnetic body M.

  Based on the arrangement information of the magnetic body M output from the arithmetic circuit 72, after the magnetic body M is disposed, the magnetic field of the static magnetic field magnet 10 is raised. After the magnetic field of the static magnetic field magnet 10 is formed, passive shimming is performed in order to make the static magnetic field in the imaging region in the bore W uniform. The arithmetic circuit 72 calculates the arrangement and number of metal shims in the shim tray in passive shimming. The arithmetic circuit 72 repeatedly calculates the arrangement and number of metal shims in the shim tray based on, for example, the distribution of the static magnetic field measured by providing an NMR (Nuclear Magnetic Resonance) probe in the bore W.

  Although the arithmetic circuit 72 described above has been described as having both functions of calculating the arrangement information of the magnetic body M and calculating the arrangement and number of metal shims in passive shimming, it is realized as a different circuit or device. It doesn't matter. In addition, the arithmetic circuit 72 described above calculates the arrangement information of the magnetic body M before performing passive shimming, but receives the characteristics of the static magnetic field after performing passive shimming as an input, and determines the position of the magnetic body M. Information may be calculated.

  The arithmetic circuit 72 may be provided in the console 400 instead of the magnetic field calculation device 7. The function of the arithmetic circuit 72 may be realized by the processing circuit 70 executing a program.

  Here, the magnetic fields formed by the main coil 61, the cancel coil 62, and the magnetic body M will be described with reference to FIGS.

  A magnetic field B 0 a in FIG. 2A is a first magnetic field formed by the main coil 61, and a magnetic field B 0 b in FIG. 2B is a second magnetic field formed by the cancel coil 62. In FIG. 2, the magnetic fields formed by the main coil 61 and the cancel coil 62 are described separately for the sake of clarity, but in reality, the magnetic field received by the magnetic body M is a synthesized magnetic field. FIG. 3A is a diagram showing directions in which the magnetic body M is magnetized by the magnetic field B0a and the magnetic field B0b. The magnetic body M arranged near the cancel coil 62 is more strongly affected by the magnetic field B0b than the magnetic field B0a. That is, since the influence of the magnetic field B0b becomes dominant, the magnetic body M is magnetized in the same direction as the magnetic field B0b, and forms the magnetic field B0c as the third magnetic field. FIG. 3B is a diagram schematically showing how the magnetic body M is magnetized under the influence of a magnetic field. The magnetic field B0c formed by the magnetic body M is formed in the opposite direction to the magnetic field B0a.

  FIG. 4 is a diagram illustrating an arrangement example of magnetic bodies. As shown in FIG. 4, the magnetic body M is provided in a lower region of the static magnetic field magnet 10, for example, under the floor. The magnetic body M may be provided outside the magnet mount 100 and in the vicinity of the cancel coil 62. For example, the magnetic body M may be provided in a space on the floor of the static magnetic field magnet 10 or the magnetic leg 200 that holds the static magnetic field magnet 10. It may be provided.

  The cancel coil 62 forms a second magnetic field in a direction that weakens the first magnetic field formed by the main coil 61 of the static magnetic field magnet 10. The magnetic body M receives the second magnetic field B0b formed by the cancel coil 62 and forms a third magnetic field.

  Next, the effect when the MRI apparatus 1 arranges the magnetic material as shown in FIG. 4 will be described with reference to FIGS. 5 and 6.

  FIG. 5 is a diagram in which an arbitrary XY plane in the bore W is expressed by a polar coordinate system. FIG. 6 is a graph showing the uniformity of the static magnetic field with respect to the angle θ shown in FIG. That is, FIG. 6 is a graph conceptually showing an error from the resonance frequency corresponding to the reference static magnetic field strength at an angle θ in ppm (parts par million) units.

  When the magnetic body M is installed at a position where it is magnetized by the magnetic field generated by the cancel coil 62, it becomes possible to arrange a non-uniform static magnetic field as in passive shimming using a so-called metal shim. As shown in FIG. 6, for example, when the magnetic body M is not installed, the uniformity of the static magnetic field is different in a range of ± A [ppm], whereas when the magnetic body M is installed, an error is generated. The uniformity of the static magnetic field is ensured in the range of ± B [ppm] where the range is small. Note that the range of ± C [ppm] on the vertical axis is the target range of the static magnetic field uniformity that the MRI apparatus 1 should finally satisfy. Although only the arrangement of the magnetic body M has not been able to converge the static magnetic field uniformity to the range of ± C [ppm], it can improve the static magnetic field uniformity to near the target range without depending on the metal shim. Is done.

  When a spherical surface in the imaging space is considered, the magnetic field distribution on the spherical surface can be represented by a spherical harmonic function. Of the expansion coefficients in the expansion of the spherical harmonic function by the Legendre polynomial, the Y-axis component of the first-order term represents the Y-axis component of the static magnetic field. For example, the magnetic body M for static magnetic field correction that is intentionally installed below the magnet leg 200 is suitable for correcting the Y-axis component of the static magnetic field. As an example, it is assumed that the static magnetic field magnet 10 has a magnetic field characteristic in which the magnetic flux density on the spherical surface in the imaging space is higher in the Y-axis negative side region than in the Y-axis positive side region. When the magnetic body M is provided in the vicinity of the magnet leg 200, the magnetic field B0c formed by the magnetic body M cancels the magnetic field B0a formed by the main coil on the negative side of the Y axis on the spherical surface in the imaging space. That is, the magnetic flux density on the negative side of the Y axis on the spherical surface in the imaging space is reduced, so that the bias of the magnetic flux density distribution in the entire spherical surface is alleviated and the static magnetic field is made uniform.

  As described above, the uniformity of the static magnetic field can be improved by forming the third magnetic field by disposing the magnetic body M in the vicinity of the cancel coil 62 outside the magnet mount 100.

  FIG. 7 is a flowchart showing the procedure of the static magnetic field correction method in the MRI apparatus 1.

  First, the arithmetic circuit 72 of the magnetic field calculation device 7 calculates arrangement information of the magnetic body M (step ST1). Then, when the imaging apparatus 5 of the MRI apparatus 1 is installed, the magnetic body M is disposed outside the magnet mount 100 and in the vicinity of the cancel coil 62 (step ST2). The arithmetic circuit 72 of the magnetic field calculation device 7 is in a state where the magnetic body M is arranged in step ST2 and all the shim pockets of the gradient coil 11 are empty, that is, no metal shim is arranged. In the state, the magnetic field characteristics of the static magnetic field magnet 10 are measured. For example, the arithmetic circuit 72 measures the magnetic field at multiple points in the center of the bore W while all the shim pockets are empty (step ST3). Since the time of step ST3 is before passive shimming, the uniformity of the measured static magnetic field is not ensured. The measurement is performed using an arbitrary magnetic field measuring device. In the measurement of the magnetic field, all the shim pockets are not necessarily empty, and some metal shims may be provided in advance.

  When the imaging device 5 is installed, the arithmetic circuit 72 performs optimization calculation based on the measured value of the magnetic field in step ST3, and calculates the number of metal shims to be stored in each shim pocket (step ST4). Thus, the optimization calculation is executed in a state where the magnetic body M is arranged outside the static magnetic field magnet 10 and in the vicinity of the cancel coil 62. Here, the optimization calculation is to set the objective function f (x) and the constraint condition g (x) as a function of the parameter x (x is usually a multivariable), and satisfy the constraint condition g (x) while satisfying the constraint function g (x). This is a method for determining a parameter x such that (x) is minimum (or maximum). The constraint condition g (x) represents an allowable range of static magnetic field inhomogeneity determined in advance by the imaging device 5. A plurality of constraint conditions may be set as well as the constraint condition g (x).

  The parameter x is a value obtained by optimizing the number of metal shims in each shim pocket. Then, as the constraint condition g (x), for example, an inequality indicating that the index representing the magnetic field inhomogeneity is within a predetermined range, that is, the spatial variation component is within the predetermined range. Further, the number of metal shims in each shim pocket is set as the objective function f (x). By performing optimization calculations with these settings, the metal shims in each shim pocket are such that the total number of metal shims is minimized while the index representing the magnetic field inhomogeneity is kept within a predetermined range. The number of is optimized.

  Next, after execution of the optimization calculation in step ST4, metal shims are placed in each shim pocket according to the number of metal shims to be placed in each shim pocket (step ST5).

  According to the static magnetic field correction method shown in FIG. 7, a magnetic field M is disposed outside the static magnetic field magnet 10 and in the vicinity of the cancel coil 62 to form the third magnetic field, and the expansion coefficient in the expansion of the spherical harmonic function. The number of metal shims in each shim pocket is optimized in a state in which the Y-axis component of the first-order term is reduced to some extent.

  Further, according to the static magnetic field correction method, the number of metal shims to be arranged can be reduced by arranging the magnetic body M in the vicinity of the cancel coil 62 outside the magnet mount 100 to form the third magnetic field. Can be reduced. Thereby, the nonuniformity of the static magnetic field due to the change in the magnetic susceptibility of the metal shim due to heat can be suppressed. Further, it is possible to reduce the process of repeating magnetic field measurement (step ST3) and optimization calculation (step ST4).

  Note that, by applying the static magnetic field correction method to a plurality of arrangement positions of the magnetic body M, a suitable arrangement position of the magnetic body M can be calculated from the plurality of arrangement positions. The suitable arrangement position is, for example, the arrangement position of the magnetic body M calculated in step ST4 when the metal shim is minimum.

  According to at least one embodiment described above, the uniformity of the static magnetic field can be improved by the magnetic body M provided outside the magnet mount 100. Specifically, the magnetic body M provided outside the magnet mount 100 is not heated by the heat of the gradient magnetic field coil 11 like a metal shim, so that the static magnetic field is adjusted without being affected by the change in magnetic susceptibility. can do. Moreover, if the iron shim arrange | positioned inside the gradient magnetic field coil 11 is reduced, the problem of the static magnetic field nonuniformity by a heat | fever can be suppressed. Furthermore, the static magnetic field can be adjusted even when a static magnetic field magnet having static magnetic field inhomogeneity that cannot be dealt with by iron shims is used.

  The arithmetic circuit 72 is an example of an “arithmetic unit”.

  Although several embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

DESCRIPTION OF SYMBOLS 1 Magnetic resonance imaging (MRI) apparatus 5 Imaging apparatus 7 Magnetic field calculation apparatus 10 Static magnetic field magnet 40 Processing circuit 61 Main coil 62 Canceling coil 72 Calculation circuit 100 Magnet mount 200 Magnetic leg M Magnetic body

Claims (7)

A static magnetic field magnet including a main coil that forms a first magnetic field, and a cancel coil that forms a second magnetic field;
A gradient coil forming a gradient magnetic field;
A magnet mount containing the static magnetic field magnet and the gradient magnetic field coil;
A magnetic body disposed outside the magnet mount and in the vicinity of the cancel coil;
With
Correcting a static magnetic field formed by the first magnetic field and the second magnetic field by a third magnetic field formed by the magnetic body magnetized by receiving the second magnetic field;
Magnetic resonance imaging device. The cancel coil forms the second magnetic field in a direction that weakens the first magnetic field formed by the main coil.
The magnetic resonance imaging apparatus according to claim 1. The magnetic body is disposed at a position where it can be magnetized by receiving the second magnetic field formed by the cancel coil.
The magnetic resonance imaging apparatus according to claim 1 or 2. The magnetic body is provided in a leg portion that holds the static magnetic field magnet, or in an area that includes a region immediately below the imaging center.
The magnetic resonance imaging apparatus according to claim 1. A gradient coil containing a predetermined number of metal shims;
A calculation unit for calculating a predetermined arrangement position from the plurality of arrangement positions by calculating a predetermined number of the metal shims at the plurality of arrangement positions of the magnetic body;
The magnetic resonance imaging apparatus according to any one of claims 1 to 4, further comprising: A static magnetic field magnet including a main coil that forms a first magnetic field, and a cancel coil that forms a second magnetic field;
A magnetic body disposed outside the magnet mount and disposed at a position capable of being magnetized by receiving the second magnetic field formed by the cancel coil;
A magnetic resonance imaging apparatus comprising: A static magnetic field correction method for correcting a magnetic field by arranging a magnetic body outside a magnet mount containing a static magnetic field magnet and a gradient magnetic field coil,
Obtaining a characteristic of a static magnetic field formed by the static magnetic field magnet;
Obtaining information on the surrounding environment of the examination room provided with the static magnetic field magnet;
The magnetic material whose area and thickness are determined based on the characteristics of the static magnetic field and information on the surrounding environment is placed in the vicinity of a cancel coil that generates a magnetic field that suppresses a leakage magnetic field, which is housed in the static magnetic field magnet. Providing steps;
A static magnetic field correction method including:

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