JP5566085B2 - Gradient magnetic field coil for magnetic resonance imaging apparatus and magnetic resonance imaging apparatus using the same - Google Patents

Description

  The present invention relates to a gradient coil used in a magnetic resonance imaging (hereinafter referred to as “MRI”) apparatus, and more particularly to a structure capable of suppressing eddy currents generated in the MRI apparatus body or the gradient coil itself.

  MRI equipment measures NMR signals generated by nuclear spins that make up the tissues of the subject, especially human body, and visualizes the shape and function of the head, abdomen, limbs, etc. in two or three dimensions It is a device to do. In imaging, the NMR signal is given different phase encoding depending on the gradient magnetic field, frequency-encoded, and measured as time series data. The measured NMR signal is reconstructed into an image by two-dimensional or three-dimensional Fourier transform.

  In such an MRI apparatus, a gradient magnetic field is generated by a gradient coil. The gradient magnetic field coil includes, for example, a disk-shaped coil that can generate a vertical magnetic field, and a cylindrical coil that can generate a horizontal magnetic field. Normally, the gradient magnetic field coil is installed close to a static magnetic field generating magnet (permanent magnet or superconducting magnet) that generates a uniform magnetic field.

  In such a configuration, when the gradient magnetic field changes with time, an eddy current is generated in a conductor such as a heat shield plate of the superconducting magnet, a helium container, or an outer wall. Since the magnetic field generated by the eddy current changes the magnetic field generated by the gradient magnetic field spatially and temporally in the space where the subject is located, the quality of the image quality and the performance of the local spectrum are degraded.

  In order to solve this problem, an active shield type gradient magnetic field coil has been developed (for example, see Patent Document 1). This active shield type gradient magnetic field coil is a coil (main coil) 20a, 20b, 20c that generates a gradient magnetic field in the space where the subject is arranged, and a coil for blocking the magnetic field generated by the main coil on the outside thereof (Shield coils) 21a and 21b are arranged. As a result, the sum of the magnetic fields generated by the main coil and the shield coil becomes substantially zero at the position of the conductor of the static magnetic field generating magnet where eddy current is generated, so that generation of eddy current can be suppressed. Patent Document 2 discloses a coil pattern design method for a gradient coil having high-speed gradient magnetic field switching characteristics while suppressing eddy currents.

US Patent No. 4737716 JP-A-8-84716

  Although the above method can eliminate to some extent the cause of eddy current generation on the conductor of the active shield type gradient magnetic field coil, in the coil that generates gradient magnetic fields in the X, Y, and Z axial directions, Wiring, connecting wires, etc. that electrically connect the cables are required. Since the magnetic field generated by these conductors is essentially unnecessary and cannot be interrupted by the shield coil, it could be a cause of eddy current generation. In addition, a manufacturing error occurs when manufacturing the gradient magnetic field coil. However, due to the manufacturing error, the magnetic field generated by the main coil and the shield coil is not canceled on the conductor of the static magnetic field generating magnet near the gradient magnetic field coil. In addition, eddy currents may occur.

  An object of the present invention is to suppress the generation of eddy currents in the conductor of a static magnetic field generating magnet and the generation of eddy currents due to manufacturing errors during coil manufacture, and a gradient capable of generating a gradient magnetic field with high accuracy. A magnetic coil and an MRI apparatus provided with the same are provided.

  In order to achieve the above object, the present invention provides a main coil (first coil conductor) or shield coil (second coil conductor) that is asymmetric with respect to any one or more coordinate axes of a three-dimensional orthogonal coordinate axis. Deploy.

Specifically, the gradient coil for a magnetic resonance imaging apparatus of the present invention includes a first coil conductor that generates a first gradient magnetic field in any one of three axes orthogonal to each other. A coil unit, and a second coil unit including a second coil conductor that generates a second gradient magnetic field that acts to cancel the first gradient magnetic field outside the first coil unit. The first coil conductor or the second coil conductor is subjected to at least one of parallel movement and rotational movement with respect to the other one of the first coil conductor and the second coil conductor. It is characterized by being arranged asymmetrically with respect to any one or more of the three-dimensional orthogonal coordinate axes . The MRI apparatus of the present invention includes such a gradient magnetic field coil.

  The method for manufacturing a gradient coil for a magnetic resonance imaging apparatus according to the present invention includes a step of placing a conductor plate close to the second coil conductor, and applying a current to the first coil conductor and the second coil conductor. Measuring the magnetic field due to the eddy current generated in the conductor plate, and translating or rotating the first coil conductor or the second coil conductor so that the generation of the eddy current is suppressed. A step of molding the first coil conductor and the second coil conductor.

  According to the gradient magnetic field coil of the present invention and the MRI apparatus using the same, it is possible to suppress the generation of eddy currents in the conductor of the static magnetic field generating magnet and the generation of eddy currents due to manufacturing errors during coil manufacture. A good gradient magnetic field can be generated.

1 is a block diagram showing the overall configuration of an embodiment of an MRI apparatus according to the present invention. The conceptual diagram of the gradient magnetic field coil of a perpendicular magnetic field system. The conceptual diagram of the gradient magnetic field coil of a horizontal magnetic field system. The conceptual diagram of the horizontal magnetic field type gradient magnetic field coil whose main coil is an elliptical cylinder shape. The conceptual diagram of X main coil in the gradient magnetic field coil of a perpendicular magnetic field system. The conceptual diagram of X main coil in the gradient magnetic field coil of a horizontal magnetic field system. 1 is a flowchart for explaining Example 1 of the present invention. The figure which looked at the state of the manufacturing method by Example 1 of this invention from the Y direction. The figure which looked at the state of the manufacturing method by Example 1 of this invention from the Z direction. The figure which shows one Example which rotated the X main coil in Example 1 of this invention. FIG. 10 is a partial enlarged cross-sectional view of FIG. FIG. 11 is a partial enlarged cross-sectional view of FIG. The figure which shows the example of the applied electric current of a gradient magnetic field coil, and a measurement magnetic field. FIG. 3 is a diagram showing an example of a measurement magnetic field and a reference magnetic field in Example 1 of the present invention. 7 is a flowchart for explaining a second embodiment of the present invention. FIG. 6 is a diagram showing an example of magnetic field characteristics by rotational movement of an X main coil in Embodiment 2 of the present invention. The figure which looked at the state of the manufacturing method by Example 3 of this invention from the Z direction. The figure which looked at the state of the manufacturing method by Example 3 of this invention from the Y direction. The figure which shows one Example which rotated the X main coil in Example 3 of this invention. The figure which looked at the state of the manufacturing method by Example 4 of this invention from the Z direction.

  Hereinafter, preferred embodiments of the MRI apparatus of the present invention will be described in detail with reference to the accompanying drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiments of the present invention, and the repetitive description thereof is omitted.

  First, an overall outline of an example of an MRI apparatus according to the present invention will be described with reference to FIG. FIG. 1 is a block diagram showing the overall configuration of an embodiment of an MRI apparatus according to the present invention. This MRI apparatus uses a NMR phenomenon to obtain a tomographic image of a subject.As shown in FIG. 1, the MRI apparatus includes a static magnetic field generation system 2, a gradient magnetic field generation system 3, a sequencer 4, A transmission system 5, a reception system 6, and a computer 7 are provided.

  The static magnetic field generation system 2 generates a uniform static magnetic field in the direction perpendicular to the body axis in the space around the subject 1 if the vertical magnetic field method is used, and in the direction of the body axis if the horizontal magnetic field method is used. The subject 1 has a permanent magnet type, normal conducting type or superconducting type static magnetic field generating magnet around the subject 1.

  The gradient magnetic field generation system 3 includes a gradient magnetic field coil 8 that applies a gradient magnetic field in the three axes of X, Y, and Z, which is a coordinate system (stationary coordinate system) of the MRI apparatus, and a gradient magnetic field that drives each gradient magnetic field coil. It consists of a power supply 9 and applies gradient magnetic fields Gx, Gy, Gz in the three axis directions of X, Y, and Z by driving the gradient magnetic field power supply 9 of each coil according to a command from the sequencer 4 described later . At the time of imaging, a slice direction gradient magnetic field pulse is applied in a direction orthogonal to the slice plane (imaging cross section) to set a slice plane for the subject 1, and the remaining two directions orthogonal to the slice plane and orthogonal to each other A phase encoding direction gradient magnetic field pulse and a frequency encoding direction gradient magnetic field pulse are applied to the echo signal, and position information in each direction is encoded in the echo signal.

  The sequencer 4 is a control means that repeatedly applies a high-frequency magnetic field pulse (hereinafter referred to as an “RF pulse”) and a gradient magnetic field pulse in a predetermined pulse sequence, and operates under the control of the computer 7, and operates on the tomographic image of the subject 1. Various commands necessary for data collection are sent to the transmission system 5, the gradient magnetic field generation system 3, and the reception system 6.

  The transmission system 5 irradiates the subject 1 with an RF pulse in order to cause nuclear magnetic resonance to occur in the nuclear spins of the atoms constituting the living tissue of the subject 1, and the modulator 10 and the high-frequency oscillator 11 It comprises a high frequency amplifier 12 and a high frequency coil (transmission coil) 13a on the transmission side. The RF pulse output from the high-frequency oscillator 11 is amplitude-modulated by the modulator 10 at a timing according to a command from the sequencer 4, and after the amplitude-modulated RF pulse is amplified by the high-frequency amplifier 12, the RF pulse is approached to the subject 1. The RF pulse is irradiated to the subject 1 by supplying it to the arranged high frequency coil 13a.

  The receiving system 6 detects an echo signal (NMR signal) emitted by nuclear magnetic resonance of nuclear spins constituting the biological tissue of the subject 1, and receives a high-frequency coil (receiving coil) 13b and a signal amplifier on the receiving side 14, a quadrature detector 15, and an A / D converter 16. The NMR signal of the response of the subject 1 induced by the electromagnetic wave irradiated from the high-frequency coil 13a on the transmission side was detected by the high-frequency coil 13b arranged close to the subject 1 and amplified by the signal amplifier 14 Thereafter, the signals are divided into two orthogonal signals by the quadrature phase detector 15 at a timing according to a command from the sequencer 4, converted into digital quantities by the A / D converter 16, and sent to the computer 7.

  The computer 7 performs various data processing and display and storage of processing results, and is connected to a display 17, a storage device 18, and an operation device 19. When data from the receiving system 6 is input to the computer 7, the computer 7 executes processing such as signal processing and image reconstruction, and the tomographic image of the subject 1 as a result is displayed on a display 17 composed of a CRT or the like. And recorded in a storage device 18 such as an optical disk or a magnetic disk. The operation device 19 inputs various control information of the MRI apparatus and control information of processing performed by the computer 7, and includes a trackball or a mouse, a keyboard, and the like. The operation device 19 is disposed in the vicinity of the display 17, and the operator interactively controls various processes of the MRI apparatus through the operation device 19 while looking at the display 17.

  In FIG. 1, the high-frequency coil 13a and the gradient magnetic field coil 8 on the transmission side are within the static magnetic field space of the static magnetic field generation system 2 into which the subject 1 is inserted. Oppositely, if it is a horizontal magnetic field system, it is installed so as to surround the subject 1. The high-frequency coil 13b on the receiving side is installed so as to face or surround the subject 1.

  At present, the radionuclide to be imaged by the MRI apparatus is a hydrogen nucleus (proton) which is a main constituent material of the subject as being widely used clinically. By imaging information on the spatial distribution of proton density and the spatial distribution of relaxation time in the excited state, the form or function of the human head, abdomen, limbs, etc. is imaged two-dimensionally or three-dimensionally.

  FIG. 2 shows an example of a vertical magnetic field type active shield type gradient magnetic field coil, and FIGS. 3 and 4 show examples of a horizontal magnetic field type active shield type gradient magnetic field coil. These gradient magnetic field coils are composed of main coil units 20a, 20b, 20c and shield coil units 21a, 21b, and are molded with a resin 22.

  FIG. 3 shows an example of a horizontal magnetic field type gradient magnetic field coil in which the main coil unit 20b and the shield coil unit 21b are both cylindrical. FIG. 4 shows an example of a horizontal magnetic field type gradient magnetic field coil in which the main coil unit 20c has an elliptic cylindrical shape. That is, the main coil unit 20c in FIG. 4 has a structure in which the diameter in the Y direction (the vertical direction of the lying subject 1) and the diameter in the X direction (the lateral direction of the lying subject 1) are different. For example, the short diameter (Y direction) may be the same as the radius of the main coil unit 20b in FIG. 3, and the long diameter (X direction) may be larger than the radius of the main coil unit 20b in FIG. As a result, the cross-sectional area of the opening of the gradient magnetic field coil in FIG. 4 is larger than that in FIG. 3, so that the subject entering the space can feel a wide space. In FIG. 4, the shield coil unit 21b has a cylindrical shape having the same radius as that of FIG. For this reason, it is not necessary to change the shape of the static magnetic field generating magnet installed around the gradient magnetic field coil.

  The main coil units 20a, 20b, and 20c are composed of three types of conductor coils that generate gradient magnetic fields in the three-axis directions of X, Y, and Z.

  As shown in FIG. 5, the X main coil and Y main coil of the vertical magnetic field type gradient magnetic field coil are composed of two portions spiraled on a plane. In these two parts, the X main coil is arranged symmetrically with respect to the Y axis, and the Y main coil is arranged symmetrically with respect to the X axis. The two portions are electrically connected using a conductive crossover 23a or the like. Further, although not shown, the Z main coil is composed of one coil spirally formed on a plane.

  As shown in FIG. 6, the X main coil and Y main coil of the horizontal magnetic field gradient magnetic field coil are composed of four parts. In the figure, θ represents the circumferential direction. These four portions are arranged symmetrically with respect to the X = 0 plane (YZ plane) and the Z = 0 plane (XY plane). The two are electrically connected using a conductive crossover 23b or the like. The Y main coil is rotated 90 degrees in the circumferential direction with respect to the X main coil, and is symmetric with respect to the Y = 0 plane (XZ plane) and the Z = 0 plane (XY plane). Although not shown, the Z main coil is a solenoid type and is composed of two parts. In both the vertical magnetic field method and the horizontal magnetic field method, these three types of coils are electrically insulated by an insulating plate.

  Like the main coil units 20a, 20b, and 20c, the shield coil units 21a and 21b are composed of three types of coils that generate gradient magnetic fields in the X, Y, and Z directions. A gradient magnetic field is generated that acts to cancel out in the space outside the magnetic field coil. Each shield coil has a configuration similar to the coil conductor pattern shown in FIG. 5 or FIG. In both the vertical magnetic field method and the horizontal magnetic field method, these three types of coils are electrically insulated by an insulating plate. Furthermore, the X, Y, and Z main coils and the shield coils are electrically connected using conductor wiring.

  Next, with respect to the first embodiment of the gradient magnetic field coil and the MRI apparatus using the same according to the present invention, the first embodiment relates to the structure of the vertical magnetic field type gradient magnetic field coil for the MRI apparatus and the manufacturing method thereof. Specifically, both the main coil (first coil conductor) and the shield coil (second coil conductor) are disk-shaped and are arranged asymmetrically with respect to any one or more coordinate axes of the three-dimensional orthogonal coordinate axes. The Hereinafter, the present embodiment will be described in detail with reference to FIGS.

  FIG. 8 is a view of the state of the first embodiment in which the vertical magnetic field type gradient magnetic field coil is manufactured as viewed from the Y direction.

  FIG. 9 is a diagram when FIG. 8 is viewed from the Z direction.

  The main coil unit 20a has a structure in which an insulating plate 30a is sandwiched between an X main coil 24a, a Y main coil 25a, and a Z main coil 26a, and three types of coils are electrically insulated, and has a disk-like shape. Have.

  The shield coil unit 21a has a structure in which an insulating plate 30a is sandwiched between a Z shield coil 27a, an X shield coil 28a, and a Y shield coil 29a, and three types of coils are electrically insulated.

  As shown in FIG. 8, the main coil unit 20a and the shield coil unit 21a are placed close to the shield coil unit 21a with a conductor plate 31a, stacked via a non-conductive spacing adjustment spacer 32a, and used for position fixing. Secure with pin 33a. At this time, the main coils 24a to 26a and the shield coils 27a to 29a are all laminated so as to coincide with the coordinate axes of the X axis and the Y axis. That is, all six types of coils are arranged symmetrically with respect to the X and Y coordinate axes.

  The main coils 24a to 26a and the shield coils 27a to 29a are connected to each other through the connecting wire 23a shown in FIG. 5, the connecting portion 34a shown in FIG. . These main coil and shield coil are connected to a current source, and a magnetic field is generated from the main coil and shield coil by applying a current. Further, a magnetic field can be measured using a search coil or the like above the main coil unit 20a, preferably at a magnetic field measurement point 35a at a position of 50 to 200 mm.

  As shown in FIG. 9, the insulating plate 30a has marks 36a and 36b indicating the X axis and the Y axis that coincide with the coordinate axes. The main coils 24a to 26a, the shield coils 27a to 29a, and the insulating plate 30a have a hole 38 having a diameter larger than the diameter of the position fixing pin 33a, and the coil is moved on the XY plane by the gap. It is possible. It is preferable that the position fixing pin 33a and the hole 38 are located symmetrically with respect to the X axis and the Y axis.

  FIG. 11 is an enlarged cross-sectional view of a portion A in FIG. The X main coil 24a and the connecting portion 34a are fixed by fixing screws 39. There is a hole in the X main coil 24a and the connecting part 34a for passing the fixing screw 39, but the X main coil 24a is larger than the connecting part 34a through the hole. Only the coil can be moved on the XY plane. The center of the screw hole of the X main coil 24a and the connecting portion 34a and the center axis of the fixing screw 39 coincide.

  Next, specific steps for manufacturing the vertical magnetic field type gradient magnetic field coil of Example 1 will be described with reference to the flowchart of FIG.

  In step S101, as described above, the main coil unit 20a and the shield coil unit 21a are stacked in the Z-axis direction as shown in FIG.

  In step S102, the magnetic field due to the eddy current at the magnetic field measurement point 35a shown in FIG. 8 is calculated and used as the reference magnetic field. As shown in FIG. 13, the reference magnetic field is preferably a magnetic field Be at a current fall of 3 to 5 ms, or a normalized magnetic field that is Be / Bm obtained by dividing the magnetic field Be by the steady value Bm of the magnetic field. However, the definition of the reference magnetic field is not limited to this.

  In step S103, a periodic current as shown in FIG. 13 is applied to the main coil and the shield coil, and the magnetic field or normalized magnetic field of the eddy current generated on the conductor flat plate 31a is measured by a search coil or the like, and this is measured. Let it be a magnetic field. FIG. 14 is a waveform example of the reference magnetic field and the measurement magnetic field when current is applied to the X main coil 24a and the X shield coil 27a.

  In step S104, the difference between the reference magnetic field and the measurement magnetic field is calculated and set as an error magnetic field.

  In step S105, it is determined whether the error magnetic field is sufficiently small. If it is determined that the error magnetic field is sufficiently small, “Yes” is determined, and the process proceeds to step S107. If “No”, the process proceeds to step S106.

  In step S106, the main coils 24a to 26a or shield coils 27a to 29a of the shaft to which the current is applied are translated or rotated. The movement may be performed with respect to the other one of the main coil unit 20a and the shield coil unit 21a. After the movement, the process returns to step S103 again and is repeated until “Yes” is obtained in step S105.

  FIG. 10 and FIG. 12 show a form example when the X main coil 24a is rotated from the state of step S101 shown in FIG. 9 and FIG. By the movement, the X main coil 24a is asymmetrically arranged with respect to the coordinate axis. The asymmetrical arrangement means that, for example, as shown in FIG. 10, the X main coil 24a is not line-symmetrically arranged with respect to the X-axis and Y-axis marks 36a and 36b, and FIG. As shown in FIG. 5, it can be seen that the centers of the screw holes of the X main coil 24a and the wiring portion 34a do not coincide with each other. In the above, the case where the X main coil 24a is rotationally moved has been described. However, even when the X main coil 24a is moved in parallel, the coordinate axes are similarly applied when the coils 25a to 29a other than the X main coil 24a are moved. Is an asymmetrical arrangement.

  In step S107, it is determined whether the movement adjustment work for the coils of all the X, Y, and Z axes has been completed. If all the moving operations are completed, the process proceeds to step S108, and if other coils need to be adjusted, the process returns to step S103.

  In step S108, the main coil unit 20a and the shield coil unit 21a are molded with the resin 22. As a result, a vertical magnetic field type gradient magnetic field coil as shown in FIG. 2 is completed.

  According to the gradient coil of this embodiment and the MRI apparatus using the same, the positional relationship between the main coil and the shield coil in the vertical magnetic field type gradient coil is determined according to the vortex in the conductor of the static magnetic field generating magnet for each coil to be manufactured. A structure capable of suppressing the generation of current and the generation of eddy current due to manufacturing errors during coil manufacturing can be achieved. Specifically, the positional relationship between the main coil and the shield coil is arranged to be asymmetric with respect to any one or more coordinate axes of the three-dimensional orthogonal coordinate axes. As a result, it is possible to configure a gradient coil that can generate a gradient magnetic field with high accuracy.

  Next, a second embodiment of the gradient coil and the MRI apparatus using the same according to the present invention will be described. The difference of the second embodiment from the first embodiment is that when the coil is moved in parallel or rotated, the amount of movement is calculated in advance, and the coil is moved according to the calculated amount. In this embodiment, any of the above-described vertical magnetic field type gradient magnetic field coils or a horizontal magnetic field type gradient magnetic field coil described later can be implemented. Hereinafter, only portions different from the first embodiment will be described with reference to the flowchart of FIG. 15 and FIG. 16, and description of the same portions will be omitted.

  In step S201, when the main coils 24a to 26a or the shield coils 27a to 29a are translated by a unit distance in the direction of each coordinate axis (X axis, Y axis), or rotated by a unit angle around the coordinate origin, respectively. The amount of change in the magnetic field or normalized magnetic field is obtained by actual measurement or calculation. This is because, for example, the difference between the value of the normalized magnetic field when translated from the normal position or rotated around the coordinate origin and the value of the normalized magnetic field when placed at the normal position is moved. Calculated by dividing by the distance or angle. FIG. 16 is an example of the amount of change in the normalized magnetic field when the X main coil 24a is rotationally moved by a unit angle. This step may be performed only once at the beginning if a plurality of coils of the same type are manufactured.

  In step S202, the optimal value of the parallel movement distance or rotational movement angle that minimizes the sum of squares of the error magnetic field at each measurement point obtained in step S104 is the translation or rotational movement obtained in step S201. Calculate by using the quantity. When obtaining the optimum amount, in addition to the above method, an appropriate evaluation function such as multiplication by a weight according to coordinates may be determined and minimized.

  In step S203, the coil is translated or rotationally moved according to the optimum value obtained in step S202. After the movement, the process returns to step S103 and is repeated until “Yes” is obtained in step S105.

  According to the gradient magnetic field coil of Example 2 and the MRI apparatus using the same, since it is moved after calculating the optimum movement amount when moving the coil, there is no need to move the coil by trial and error, and it takes less time. It is possible to manufacture a coil. Furthermore, since the amount of movement is obtained by calculation, the influence of eddy current can be minimized with higher accuracy than in the first embodiment.

  Next, Embodiment 3 of the gradient coil and the MRI apparatus using the same according to the present invention will be described. Example 3 relates to a structure of a gradient magnetic field coil for a horizontal magnetic field type MRI apparatus and a manufacturing method thereof. That is, the present invention relates to a structure of a gradient magnetic field coil in which both a main coil and a shield coil have a cylindrical shape and a manufacturing method thereof. Hereinafter, the present embodiment will be described with reference to FIGS.

  FIG. 17 is a diagram of the state of the horizontal magnetic field type gradient magnetic field coil according to the third embodiment viewed from the Z direction, and FIG. 18 is a diagram viewed from the Y direction. However, the conductor plate 31b is not shown.

  The main coil unit 20b has a structure in which an insulating plate 30b is sandwiched between an X main coil 24b, a Y main coil 25b, and a Z main coil 26b, and three types of coils are electrically insulated.

  The shield coil unit 21b has a structure in which an insulating plate 30b is sandwiched between a Z shield coil 27b, an X shield coil 28b, and a Y shield coil 29b, and three types of coils are electrically insulated.

  As shown in FIG. 17, the main coil unit 20b and the shield coil unit 21b are placed close to the shield coil unit 21b with a conductor plate 31b, stacked via a non-conductive spacing adjustment spacer 32b, and used for position fixing. Secure with pin 33b. At this time, the main coils 24b to 26b and the shield coils 27b to 29b are all laminated so as to coincide with the coordinate axes of the X axis, the Y axis, and the Z axis. That is, all six types of coils are arranged symmetrically with respect to the coordinate axis.

  The main coils 24b to 26b and the shield coils 27b to 29b are connected to each other through the connecting wire 23b shown in FIG. 6, the connecting portion 34b shown in FIG. . These main coil and shield coil are connected to a current source, and a magnetic field is generated from the main coil and shield coil by applying a current. Further, the magnetic field can be measured at several magnetic field measurement points 35b inside the main coil unit 20b.

  As shown in FIG. 18, the insulating plate 30b has marks 37a indicating the X = 0 plane (YZ plane), the Y = 0 plane (XZ plane), and the Z = 0 plane (XY plane), which coincide with the coordinate axes. 37b and 37c. The main coils 24b to 26b, the shield coils 27b to 29b, and the insulating plate 30b have holes 38 having a diameter larger than the diameter of the position fixing pin 33b. It can be moved along the axis. The position fixing pin 33b and the hole 38 are preferably in symmetrical positions with respect to the X axis, the Y axis, and the Z axis.

  Subsequently, specific steps for manufacturing the horizontal magnetic field type gradient magnetic field coil of the third embodiment will be described. The flowchart is the same as FIG. 15 used in the description of the second embodiment. Hereinafter, only different points from the second embodiment will be described.

  In step S101, the main coil unit 20b and the shield coil unit 21b are stacked.

  In step S102, a magnetic field due to eddy current or a normalized magnetic field at the magnetic field measurement point 35b is calculated and used as a reference magnetic field.

  In step S201, the magnetic field at the magnetic field measurement point 35b when the main coils 24b to 26b or the shield coils 27b to 29b are translated by a unit distance in the Z-axis direction or rotated in the circumferential direction (θ direction), respectively. Alternatively, the change amount of the normalized magnetic field is obtained by actual measurement or calculation. This step may be performed only once at the beginning if a plurality of coils of the same type are manufactured.

  In step S103, a current is applied to the coil, a magnetic field caused by an eddy current generated in the conductor plate 31b or a normalized magnetic field is measured by a search coil or the like, and this is used as a measurement magnetic field.

  In step S203, the main coils 24b to 26b or shield coils 27b to 29b of the shaft to which the current is applied are translated or rotated according to the movement amount obtained in step S202. The movement may be performed with respect to the main coil unit 20b and the shield coil unit 21b. After the movement, the process returns to step S103 again and is repeated until “Yes” is obtained in step S105.

  FIG. 19 shows a form example when the X shield coil 27b is rotated in the circumferential direction from the state of step S101 shown in FIG. Due to the movement, the X shield coil 27b is asymmetrically arranged with respect to the coordinate axis. The asymmetric arrangement means that, for example, X = 0 plane (YZ plane), Y = 0 plane (XZ plane), Z = 0 plane (XY plane) with respect to marks 37a, 37b, and 37c, X It can be seen from the fact that the shield coil 27b is not line-symmetrically arranged (the example of FIG. 19 shows “twisted rotation” in which the amount of rotation in the circumferential direction differs at both ends of the cylinder). In the above, the case where the X shield coil 27b is rotationally moved has been described. However, even when the X shield coil 27b is moved in parallel, the coils 24b to 26b and 28b, 29b other than the X shield coil 27b are moved. Even in this case, the arrangement is similarly asymmetric with respect to the coordinate axis.

  In step S108, the main coil unit 20b and the shield coil unit 21b are molded with the resin 22. Thereby, a horizontal magnetic field type gradient magnetic field coil as shown in FIG. 3 is completed.

  In the above, the flowchart is described according to FIG. 15, but a method according to the flowchart shown in FIG.

  According to the gradient magnetic field coil of Example 3 and the MRI apparatus using the same, not only the vertical magnetic field type gradient magnetic field coil described in Example 1 and Example 2, but also in the horizontal magnetic field type gradient magnetic field coil. Thus, it is possible to manufacture a highly accurate coil in which the generation of eddy current is minimized.

  Next, Embodiment 4 of the gradient magnetic field coil and the MRI apparatus using the same according to the present invention will be described. Example 4 relates to a structure of a gradient magnetic field coil of a horizontal magnetic field system, in which a main coil as shown in FIG. Hereinafter, this embodiment will be described in detail with reference to FIG.

  In FIG. 3, the main coil unit 20b and the shield coil unit 21b are both cylindrical. On the other hand, in FIG. 4, the main coil unit 20c has a structure in which the diameter in the Y direction (vertical direction of the lying subject 1) and the diameter in the X direction (lateral direction of the lying subject 1) are different. For example, the short diameter (Y-axis direction) may be the same as the radius of the main coil unit 20b in FIG. 3, and the long diameter (X-axis direction) may be larger than that. As a result, the cross-sectional area of the opening of the gradient magnetic field coil in FIG. 4 is larger than that in FIG. 3, so that the subject entering the space can feel a wide space. In FIG. 4, the shield coil unit 21b has a cylindrical shape having the same radius as that in FIG. For this reason, it is not necessary to change the shape of the static magnetic field generating magnet installed around the gradient magnetic field coil.

  FIG. 20 is a view of the state during manufacturing of the horizontal magnetic field type gradient magnetic field coil in which the main coil in Embodiment 4 has an elliptic cylinder shape, as viewed from the Z direction. The main coil unit 21c has an elliptic cylindrical shape, and the shield unit 21c has the same cylindrical shape as that of the third embodiment.

  As in the third embodiment, the main coil unit 20c has a structure in which three types of coils are electrically insulated by sandwiching an insulating plate 30c between the X main coil 24c, the Y main coil 25c, and the Z main coil 26c. is there.

The shield coil unit 21b has the same structure as that of the third embodiment.
As shown in FIG. 20, the main coil unit 20c and the shield coil unit 21b are placed close to the shield coil unit 21b, a conductor plate 31b is installed, stacked via a non-conductive spacing adjustment spacer 32c, and used for position fixing. Secure with pin 33c. At this time, the main coils 24c to 26c and the shield coils 27b to 29b are all laminated so that the coordinate axes of the X axis and the Y axis coincide. That is, all six types of coils are arranged symmetrically with respect to the coordinate axis.

  Next, specific steps for manufacturing the horizontal magnetic field method of Example 4 will be described. The flowchart is the same as FIG. In the following, only differences from the third embodiment will be described.

  In step S101, the main coil unit 20c and the shield coil unit 21b are stacked.

  In step S102, a magnetic field due to eddy current or a normalized magnetic field at the magnetic field measurement point 35c is calculated and used as a reference magnetic field.

In step S201, when the main coils 24c to 26c are translated by a unit distance in the Z-axis direction, or the shield coils 27b to 29b are translated by a unit distance in the Z-axis direction, or in the circumferential direction (θ direction). The rate of change of the magnetic field or normalized magnetic field at the magnetic field measurement point 35c when rotated is obtained by actual measurement or calculation . This step need only be performed once at the beginning if a plurality of coils of the same type are manufactured.

  In step S203, one or both of the main coils 24c to 26c and the shield coils 27b to 29b of the shaft to which the current is applied are translated or rotated according to the movement amount obtained in step S202. The movement may be performed with respect to the main coil unit 20c and the shield coil unit 21b. After the movement, the process returns to step S103 again and is repeated until “Yes” is obtained in step S105.

  In step S108, the main coil unit 20c and the shield coil unit 21b are molded with the resin 22. Thus, a horizontal magnetic field type gradient magnetic field coil in which the shield coil shown in FIG. 4 has an elliptic cylindrical shape is completed.

  Although the flowchart has been described with reference to FIG. 15, the method according to the flowchart shown in FIG. 7 may be used.

  According to the fourth embodiment, it is possible to manufacture a highly accurate coil in which generation of eddy current is suppressed even in a horizontal magnetic field type gradient magnetic field coil having an elliptical cylindrical main coil. Moreover, the opening part of the main coil is wider than that of the cylindrical coil, and the occlusion feeling given to the subject can be reduced.

  As mentioned above, although the Example of this invention was described, this invention is not limited to these.

  1 Subject, 2 Static magnetic field generation system, 3 Gradient magnetic field generation system, 4 Sequencer, 5 Transmitting system, 6 Receiving system, 7 Computer, 8 Gradient magnetic field coil, 9 Gradient magnetic field power supply, 10 Modulator, 11 High frequency oscillator, 12 High-frequency amplifier, 13a High-frequency coil (transmitting coil), 13b High-frequency coil (receiving coil), 14 Signal amplifier, 15 Quadrature phase detector, 16 A / D converter, 17 Display, 18 Storage device, 19 Operating device, 20a, 20b , 20c Main coil unit, 21a, 21b Shield coil unit, 22 Resin, 23a, 23b Crossover, 24a, 24b, 24c X main coil, 25a, 25b, 25c Y main coil, 26a, 26b, 26c Z main coil, 27a , 27b X shield coil, 28a, 28b Y shield coil, 29a, 29b Z shield coil, 30a, 30b Insulation plate, 31a, 31b Conductor plate, 32a, 32b, 32c Spacer adjustment spacer, 33a, 33b, 33c Position fixing pin , 34a, 34b, 34c Connection part, 35a, 35b, 35c Magnetic field measurement point, 36a X-axis mark, 36b Y-axis mark, 37a X = 0 plane (YZ plane) mark, 37b Y = 0 plane (XZ plane) mark, 37c Z = 0 plane (XY plane) Mark, 38 holes, 39 fixing screws

Claims (7)

A first coil unit comprising a first coil conductor for generating a first magnetic field gradient in any direction of three axes orthogonal to each other,
A second coil unit comprising a second coil conductor for generating a second gradient magnetic field that acts to cancel the first gradient magnetic field outside the first coil unit ;
With
One of the first coil conductor and the second coil conductor is subjected to at least one of parallel movement and rotational movement with respect to the other, and the first coil conductor or the second coil A gradient magnetic field coil for a magnetic resonance imaging apparatus, wherein the conductor is disposed asymmetrically with respect to any one or more coordinate axes of a three-dimensional orthogonal coordinate axis. The gradient coil for a magnetic resonance imaging apparatus according to claim 1,
The gradient coil for a magnetic resonance imaging apparatus, wherein the first coil conductor and the second coil conductor are disk-shaped. The gradient coil for a magnetic resonance imaging apparatus according to claim 1,
The gradient coil for a magnetic resonance imaging apparatus, wherein the first coil conductor has a cylindrical shape. The gradient coil for a magnetic resonance imaging apparatus according to claim 1,
A gradient magnetic field coil for a magnetic resonance imaging apparatus, wherein the first coil conductor has an elliptic cylinder shape. The gradient coil for a magnetic resonance imaging apparatus according to any one of claims 1 to 4,
Diameter of the screw hole of the connecting portion for connecting the said first coil conductor and the second coil conductor, said threaded hole much larger than the diameter of the fixing screw passing through the, and, the rotary movement and the parallel movement A gradient coil for a magnetic resonance imaging apparatus, characterized in that the diameter has a diameter that enables the magnetic field imaging. The gradient coil for a magnetic resonance imaging apparatus according to any one of claims 1 to 5,
A magnetic field caused by an eddy current generated when the first coil conductor and the second coil conductor are combined with respect to a magnetic field caused by an eddy current generated when the first coil unit and the second coil unit are combined. The gradient magnetic field coil for a magnetic resonance imaging apparatus is characterized in that at least one of the parallel movement and the rotational movement is performed so that the difference between the magnetic field and the magnetic field is minimized . A magnetic resonance imaging apparatus comprising the gradient magnetic field coil for a magnetic resonance imaging apparatus according to claim 1.

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