JP3461573B2 - Gradient coil - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gradient coil used in a magnetic resonance imaging (MRI) apparatus, and more particularly to a method for reducing vibrations and unnecessary magnetic fields generated by a gradient current and a static magnetic field. The gradient coil. 2. Description of the Related Art An MRI apparatus uses a nuclear magnetic resonance phenomenon to measure a nuclear spin density distribution, a relaxation time distribution, and the like at a desired inspection site in a subject, and uses the measured data to measure the subject. Is displayed as an image. A nuclear spin of a subject placed in a uniform and strong static magnetic field generator precesses around a direction of the static magnetic field at a frequency (Larmor frequency) determined by the strength of the static magnetic field. Therefore, when a high-frequency pulse having a frequency equal to the Larmor frequency is irradiated from the outside, spins are excited and transit to a high energy state. This is called a nuclear magnetic resonance phenomenon. When the irradiation of the high frequency pulse is terminated, the spin returns to the original low energy state with a time constant corresponding to each state, and at this time, the electromagnetic wave is irradiated to the outside. This is tuned to the frequency of the high-frequency receiving coil (R
F coil). At this time, a gradient magnetic field of three axes (X, Y, Z axes) is applied to the static magnetic field space in order to add position information to the space. As a result, position information in space can be captured as frequency information. In this MRI, a gradient coil is formed to apply a magnetic field having a different strength depending on the position in the body axis direction in order to specify a position where a tomographic image of a subject is to be taken, and a high-frequency rotating magnetic field is applied to a hollow portion thereof. An RF coil for making is inserted, and a subject is accommodated therein. FIG. 5 shows the shape of the gradient coil. FIG.
Shows an example of the configuration of the X-axis gradient coil.
It is composed of four coils 1a to 1d. In order to supply a gradient current to each of the coils 1a to 1d, there are provided external cables 2a and 2b for supplying current from the outside, and internal cables 3a to 3d for connecting the coils 1a to 1d. A predetermined gradient magnetic field is generated by supplying a gradient current to each of the coils 1a to 1d. By the way, when a current is applied to the gradient coil 1, each of the internal cables 2a, 2b, 3a to
A current as shown in FIG. 6 flows through 3d. The current of each internal cable receives a force (Lorentz force) from the static magnetic field Bz. Here, since the gradient magnetic field current flows in a pulse shape, the Lorentz force is generated intermittently in the internal cables 3a to 3d. Then, due to the intermittent force received by the Lorentz force, vibration occurs in each of the internal cables 3a to 3d. When such vibrations occur in each cable, the cable durability and image quality are adversely affected. Also, unnecessary magnetic fields are generated around each cable due to the gradient magnetic field current flowing through each cable, as shown in FIG. Since such an unnecessary magnetic field is also generated by the gradient magnetic field current, it is intermittent and causes noise to adversely affect the image quality. In FIG. 5, one gradient coil in the X-axis direction is shown as the gradient coil 1. However, in actuality, in order to enhance the uniformity of the magnetic field, the inner coil (inner coil) on both the X-axis and the Y-axis is used. ) And outer outer coil (outer co
il), and a similar problem occurs with a total of four coils. Note that the above-mentioned problems do not cause a problem due to the difference in the shape of the gradient coil of the Z axis. FIG. 7 is an explanatory view showing an example of taking out (routing) wiring from the inner coil to the external cable. In the example shown in FIG. 7, the X board 1x for the X-axis gradient coil and the Y board 1y for the Y-axis gradient coil are relayed at the terminal unit 4 and connected to each external cable at the shortest distance. Wired. Note that X,
The space between the Y board and the terminal section 4 is sealed by a sealing member. As described above, since the wiring from the inner coil to each external cable is arranged so as to be the shortest distance, no consideration has been given to adjacent wirings having the same axis. Therefore, in the wiring example shown in FIG. 7, since the X-axis external cables (+ X, −X) are adjacent to each other, the Lorentz force acting on the + X external cable due to the static magnetic field and the −X external cable act on the −X external cable. The Lorentz forces cancel each other out, and the problem of vibration does not occur. Similarly, for the Z-axis external cable, the Lorentz forces cancel each other, and no vibration problem occurs. However, since the Y-axis external cables are located apart from each other, vibration is generated by Lorentz force generated by a static magnetic field. Also, there is a possibility that Lorentz force is generated between the magnetic field generated by the external cable of the other shaft. FIG. 8 is an explanatory view showing an example of taking out the external cable from the outer coil. In the case of the outer coil shown in FIG. 8, the Lorentz force acts on the external cables of the X, Y, and Z axes to generate vibration. The present invention has been made in view of the above points, and an object of the present invention is to realize a gradient coil which does not generate unnecessary vibration or magnetic field in each part. A means for solving the above-mentioned problem is a gradient coil composed of at least one axis coil for generating a gradient magnetic field, wherein one of the cylinders is provided. A first planar concentric coil disposed on a side close to the end face and located at a position corresponding to a half circumference of the cylindrical surface, and the same side as the first planar concentric coil disposed on a side close to the other end face of the cylinder and A second planar concentric coil disposed at a position corresponding to a half circumference of the cylindrical surface on the side, and a third plane disposed at a position corresponding to a half circumference of the cylindrical surface on the side facing the second planar concentric coil. Concentric coils, a fourth planar concentric coil arranged at a position corresponding to a half circumference of the cylindrical surface on the side facing the first planar concentric coil, the first planar concentric coil, and a second planar concentric coil. A first cable for connecting to a planar concentric coil A second cable connecting the second planar concentric coil and the third planar concentric coil along a circumference of the other end face; and a third planar concentric coil and a second cable connecting the second planar concentric coil and the third planar concentric coil. A third cable connecting the fourth planar concentric coil and the fourth planar concentric coil and the first planar concentric coil along the first, second and third cables; And a fourth coil. In a gradient coil which is means for solving the problem, a fourth cable in which currents in opposite directions flow along the first, second and third cables. Are arranged, the Lorentz forces generated in the cables are opposite to each other and cancel each other. Also, the magnetic fields generated in each cable are reversed, and cancel each other out. Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a configuration diagram showing a configuration example of a gradient coil according to one embodiment of the present invention, and FIG. 2 is an explanatory diagram showing an arrangement of internal cables and a state of a magnetic field. In these figures, the gradient coil 1 has the shape of a cylindrical surface for generating a gradient magnetic field, and is arranged on a side close to one end face of the cylinder and at a position corresponding to a half circumference of the cylindrical surface. The first planar concentric coil 1a and a second side arranged on a side close to the other end face of the cylinder and at a position corresponding to a half circumference of the cylindrical surface on the same side as the first planar concentric coil 1a.
, A third planar concentric coil 1c and a first planar concentric coil 1a arranged at a position corresponding to a half circumference of the cylindrical surface on the side facing the second planar concentric coil 1b. And a fourth planar concentric coil 1d disposed at a position corresponding to a half circumference of the cylindrical surface on the opposite side. A first internal cable 5a for connecting the first planar concentric coil 1a and the second planar concentric coil 1b, and a second planar concentric coil 1b along the circumference of the end face. A second internal cable 5b connecting the second planar concentric coil 1c to the third planar concentric coil 1c, a third internal cable 5c connecting the third planar concentric coil 1c and the fourth planar concentric coil 1d, Along the first, second, and third internal cables 5a, 5b, 5c, there is provided a fourth internal cable 5d for connecting the fourth planar concentric coil 1c and the first planar concentric coil 1a. are doing. The gradient coil of the present embodiment having the above-described structure operates as follows. The gradient magnetic field current supplied from the external cables 2a and 2b is applied to the internal cables 5a and 5b,
The coils are supplied to the planar concentric coils 1a to 1d via 5c and 5d to generate a predetermined gradient magnetic field. At this time, the internal cables 5d are wired along the internal cables 5a to 5c, and the internal cables 5a to 5c and the internal cables 5d
And the directions of the currents are opposite (go | go). Therefore, at the position where the internal cables 5a to 5c and the internal cable 5d are juxtaposed, the Lorentz force acting on each internal cable is canceled macroscopically. This Lorentz force is generated by the static magnetic field Bz and the current flowing through the internal cable, and is particularly large when the direction of the current is orthogonal to Bz. Therefore, the Lorentz force normally acts on the circumferential portions of the internal cable 5b and the internal cable 5d arranged along the circumference of the gradient coil 1. According to the wiring of this embodiment, the Lorentz force is applied. They become opposite and cancel each other. Therefore, no unnecessary vibration is generated. The internal cable 5a and the internal cable 5
The lines d are aligned so as to coincide with the center part in the Z direction of d, and the magnetic fields generated in this part cancel each other because they are in opposite directions. For this reason, there is almost no influence on the imaging at the central portion. It should be noted that a magnetic field that is not canceled out remains at the end of the internal cable 5d, but this portion has little effect on imaging, and does not cause much problem. Also, the central portions of the internal cables 5c and 5d in the Z direction cancel the magnetic field for the same reason. And the gradient coil 1
Internal cable 5b and internal cable 5 at the end of
Since the lengths of d and d are the same, the generated magnetic fields completely cancel each other. Although the above description has been made with respect to the gradient coil in the X-axis direction, similar effects can be obtained by arranging the same internal cable for the gradient coil in the Y-axis direction having the same configuration. Can be As described above, by arranging the internal cables of the respective axes in which the directions of the currents are reversed, it is possible to realize a gradient coil which does not generate unnecessary vibrations and magnetic fields. FIG. 3 shows an external cable 2 from the inner coil.
It is explanatory drawing which shows an example of taking out. FIG. 4 is an explanatory view showing an example of taking out the external cable 2 from the outer coil. In the example shown in FIG.
The external cables of the + signal and the − signal of each axis are wired from the x and Y boards 1 y via the terminal unit 4 so as to be adjacent to each other. The X and Y boards 1x and 1y
The space between the terminal section 4 and the terminal section 4 is sealed with a sealing member. In this case, since the external cables of the respective X, Y, and Z axes are adjacent to each other, the Lorentz force acting on the external cables by the static magnetic field is canceled out, and the problem of vibration does not occur. In addition, since the external cables are adjacent to each other, currents having the same phase flow in opposite directions (+-), the generated magnetic fields cancel each other out. Therefore, the Lorentz force does not act between the magnetic field generated by the external cable of one of the axes and the current of the external cable of the other axis. Therefore, by configuring the terminal portions such that the external cables of the respective axes are adjacent to each other, unnecessary vibrations and magnetic fields are not generated at the terminal portions of the gradient coil. As described above, when each external cable is pulled out from the terminal portion 4 adjacent to each cable of each axis, a twisted pair cable (twisted) is knitted for each axis.
ted pair cable). This cancels out the generated magnetic field and cancels out the Lorentz force due to the static magnetic field. Therefore, by constituting the gradient coil having at least one axis coil such that the external cables of each axis are twisted, unnecessary vibrations and magnetic fields can be generated in a portion where the external cable of the gradient coil is drawn out. Disappears. It should be noted that the twisted pair cable may be used within a range that is easily affected by a static magnetic field. As described above, in the gradient coil composed of at least one axis coil for generating the gradient magnetic field, a plurality of current lines for supplying the gradient current to the coil of the same axis are provided. According to the gradient coil, which is provided with a cable arranged so that currents flow adjacent to each other, currents in opposite directions flow in adjacent portions of a current line that supplies a gradient current to the coil. The Lorentz forces generated in the current lines are opposite to each other and cancel each other. Further, the magnetic fields generated in the respective current lines are also in opposite directions, and cancel each other out, so that a gradient coil that does not generate unnecessary vibration or magnetic field can be realized. Further, according to the gradient coil in which the internal cables through which current flows in opposite directions are arranged along the same direction, the Lorentz forces generated in the internal cables are opposite to each other and cancel each other. Since the generated magnetic fields are also in opposite directions, and cancel each other, a gradient coil that does not generate unnecessary vibration or a magnetic field can be realized. Further, in order to draw out the internal cable of the gradient coil of each axis to the outside, according to the gradient coil having the terminal section for aligning the internal cable of each axis in an adjacent state, the terminal section to which each cable is connected is connected. The generated Lorentz forces are opposite and cancel each other, and the magnetic fields generated in each cable are also opposite and cancel each other,
A gradient coil that does not generate unnecessary vibration or magnetic field can be realized. According to the gradient coil having the external cable drawn out in a twisted (braided) state for each axis from the terminal portion, the Lorentz forces generated in the external cables are opposite to each other and cancel each other. Also, the magnetic fields generated in the respective cables are opposite to each other and cancel each other out, so that a gradient coil that does not generate unnecessary vibrations and magnetic fields can be realized. As described above in detail, in the present invention,
Arrange the cables along which the reverse current flows,
Also, in order to pull out the internal cable of the gradient coil of each axis to the outside, the terminals for each axis are aligned adjacently, and
By pulling out from the terminal part in a twisted (braided) state for each axis, the Lorentz forces generated in each part are opposite and cancel each other, and the magnetic field generated in each part is also opposite. Since they cancel each other, it is possible to realize a gradient coil that does not generate unnecessary vibration or magnetic field.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a configuration diagram showing details of a configuration of a gradient coil according to an embodiment of the present invention. FIG. 2 is an explanatory diagram showing a current distribution of a gradient coil according to one embodiment of the present invention. FIG. 3 is a configuration diagram showing a terminal portion of a gradient coil according to one embodiment of the present invention. FIG. 4 is a configuration diagram showing a terminal portion of a gradient coil according to one embodiment of the present invention. FIG. 5 is a configuration diagram showing details of a configuration of a conventional gradient coil. FIG. 6 is an explanatory diagram showing a current distribution of a conventional gradient coil. FIG. 7 is a configuration diagram showing a terminal portion of a conventional gradient coil. FIG. 8 is a configuration diagram showing a terminal portion of a conventional gradient coil. [Description of Signs] 1 Gradient coils 1a to 1d Flat concentric coils 2 External cables 4 Terminal parts 5a to 5d Internal cables

Claims (1)

(57) [Claim 1] At least one for generating a gradient magnetic field
A gradient coil composed of a shaft coil, a first planar concentric coil disposed on a side close to one end face of the cylinder and at a position corresponding to a half circumference of the cylinder face, and the other end face of the cylinder And a second planar concentric coil disposed at a position corresponding to a half circumference of the cylindrical surface on the same side as the first planar concentric coil, and facing the second planar concentric coil. A third planar concentric coil disposed at a position corresponding to a half circumference of the side cylindrical surface; and a fourth plane disposed at a position corresponding to a half circumference of the cylindrical surface facing the first planar concentric coil. A first concentric coil, a first cable connecting the first planar concentric coil and a second planar concentric coil, and a second planar concentric coil along a circumference of the other end face. And the third planar concentric coil
A third cable connecting the third planar concentric coil and the fourth planar concentric coil; and a fourth cable extending along the first, second and third cables.
And a fourth cable connecting the first planar concentric coil to the first planar concentric coil, and a fourth cable adjacent to the first cable and the fourth cable. The direction of current, the direction of current in the second cable and a portion of the fourth cable adjacent to the second cable, and the direction of current in the third cable and adjacent to the third cable; 4. The gradient coil according to claim 4, wherein directions of currents in a part of the cable are opposite to each other.