JP4630196B2 - Magnetic resonance imaging system - Google Patents

Description

  The present invention relates to a magnetic resonance imaging apparatus, and more particularly to a technique related to a mechanism for forming a static magnetic field in an imaging region.

  A magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus) uses a nuclear magnetic resonance phenomenon (hereinafter referred to as an NMR phenomenon) that occurs when a living body placed in an imaging region is irradiated with a high-frequency magnetic field, It is an apparatus that obtains cross-sectional images representing chemical properties. This MRI apparatus is used particularly in the medical field.

Thus, in order to obtain a cross-sectional image using the NMR phenomenon, the MRI apparatus needs to be configured to form a high-intensity and uniform static magnetic field in the imaging region. The mechanism for forming such a static magnetic field is constituted by a pair of superconducting main coils facing each other with the imaging region interposed therebetween. A pair of magnetized annular bodies made of a ferromagnetic material are arranged outside the pair of superconducting coils and are magnetized, thereby contributing to an increase in the strength and uniformity of the static magnetic field in the imaging region. . Further, the MRI apparatus is configured such that a pair of superconducting shield coils are arranged outside the pair of magnetized annular bodies to cancel out magnetic lines of force that leak to the outside (see, for example, Patent Document 1).
JP 2001-224571 A (FIG. 2)

  However, when the magnetic field lines newly generated by magnetizing the magnetized annular body cut the superconducting main coil in the vicinity, the following problems occur. That is, the transition between the superconducting state and the normal conducting state of the superconductor is determined by three elements of the critical temperature, the critical magnetic field, and the critical current density, so that the superconducting main coil is cut into a high-density magnetic field line. In some cases, the critical current density and critical temperature at which the superconducting state transitions to the normal conducting state are lowered. As described above, when the critical current density is reduced, it is necessary to increase the thickness of the superconducting main coil in order to obtain a predetermined static magnetic field strength, which causes a problem that the amount of the wire to be used must be increased. In addition, when the critical temperature is lowered, problems such as a large cooling device for cooling the superconducting main coil occur.

In order to solve such a problem, if the outer diameter of the magnetized annular body is made larger than the outer diameter of the superconducting main coil so that the superconducting main coil does not cut the magnetic field lines, this time, the following problems will be newly added. Occurs.
That is, when the superconducting main coil and the magnetized annular body are magnetically attracted, a force is applied to the superconducting main coil in the direction in which the coil diameter increases. And the subject that the support mechanism of the superconducting main coil for preventing that a coil diameter deform | transforms largely by such a force being applied becomes large generate | occur | produces.

  An object of the present invention is to solve the above-described problems at the same time, and a mechanism for forming a high-intensity static magnetic field in an imaging region while suppressing leakage of magnetic field lines to the outside is simpler and more compact. A magnetic resonance imaging apparatus realized at low cost is provided.

In order to solve the above-described problem, the present invention provides a superconducting main coil that generates magnetic lines for forming a static magnetic field in a region, a superconducting shield coil that cancels the magnetic lines generated at a position opposite to the imaging region, and the superconducting coil. A magnetized annular body disposed between a main coil and the superconducting shield coil and attracting the magnetic field lines, wherein the magnetized annular body has a cross-section of the magnetized annular body by a plane including a rotational symmetry axis. The center of gravity position is configured to be located closer to the rotational symmetry axis than the position of the center of gravity of the cross section of the superconducting main coil, and a peripheral portion located on the superconducting main coil side of the outer peripheral surface of the magnetized annular body is configured by further protruding with respect to a plane obtained by extending the outer circumferential surface of the superconducting main coil, said periphery, a rectangular cross-section It is configured to extend from the outer peripheral surface of the magnetized annular body, is configured at the end of the outer peripheral surface of the magnetized annular body formed so that the outer diameter increases as it approaches the superconducting main coil, or the magnetization For example, it is configured to be separated from the member including the center of gravity of the annular body .

  By configuring the invention in this way, among the magnetic lines of force generated by the superconducting main coil, the magnetic lines drawn to the magnetized annular body draw a closed-loop trajectory passing through the peripheral portion, and thus the superconducting main coil Never cut off. Thereby, the value of the critical current density and the critical temperature at which the superconducting main coil transitions from the superconducting state to the normal conducting state does not decrease. Furthermore, when the superconducting main coil and the magnetized annular body are magnetically attracted, a force is applied to the superconducting main coil in a direction in which the coil diameter decreases. Such a superconducting main coil support mechanism for preventing deformation of the superconducting main coil due to force is easily realized.

  By configuring the invention as described above, a nuclear magnetic resonance imaging apparatus in which a mechanism that does not leak magnetic field lines to the outside while forming a high-intensity static magnetic field in the imaging region can be realized more simply and at low cost. Is provided.

<First Embodiment>
Embodiments of a nuclear magnetic resonance imaging apparatus (MRI apparatus) according to the present invention will be described below with reference to the drawings. In the following description, for convenience of explanation, the rotational symmetry axis Z of the MRI apparatus 10 is used as the Z axis, and an X axis and a Y axis perpendicular to the Z axis are provided, and these coordinate axes are used.

  As shown in FIG. 1, the MRI apparatus 10 is comprised from the housing | casing 11, the supporting member 12, the 1st static magnetic field generation mechanism 20a, and the 2nd static magnetic field generation mechanism 20b. The MRI apparatus 10 configured as described above causes a nuclear magnetic resonance (NMR) phenomenon to occur in a living body (not shown) inserted in the imaging region R, and outputs an image signal with positional information generated at that time. It detects and images the internal tissue of a living body. Such imaging of the internal tissue of a living body utilizes the fact that the strength of resonance of a detected image signal and the speed of temporal change in resonance appear as image contrast.

  The housing 11 houses the first static magnetic field generation mechanism 20a and the second static magnetic field generation mechanism 20b. Although not shown, the housing 11 has a gradient magnetic field coil that generates a gradient magnetic field necessary to give position information to an image signal, and a high-frequency irradiation that irradiates a living body with an electromagnetic wave having a resonance frequency necessary to cause an NMR phenomenon. The coil is further housed.

  The support member 12 supports the first static magnetic field generation mechanism 20a and the second static magnetic field generation mechanism 20b housed in the casing 11 in the vertical direction so that the rotational symmetry axis Z is shared. is there. The support member 12 configured as described above secures a space in which the imaging region R is formed in a portion sandwiched between the first static magnetic field generation mechanism 20a and the second static magnetic field generation mechanism 20b. The space in which the imaging region R is formed is large enough to be inserted from the horizontal direction in a state where a living body to be imaged is placed on an observation table (bed) (not shown). desired. Further, the thickness, number, length, layout, etc. of the support member 12 are set by the operator of the MRI apparatus 10 in consideration of workability with respect to the living body inserted into the imaging region R.

  Although not shown, a receiving coil for receiving an image signal generated by the NMR phenomenon is placed in a space where the imaging region R is formed. Although not shown, the MRI apparatus 10 further includes a control apparatus that controls the operation thereof and an image processing apparatus that processes the image signal to display it as an image.

The first static magnetic field generation mechanism 20a includes a first cooling vessel 21a, a first superconducting main coil 30a, a first superconducting shield coil 40a, and a first magnetized annular body 50a. Further, the second static magnetic field generation mechanism 20b has a second cooling container 21b, a second superconducting main coil 30b, and a position symmetrical to the above-described components of the first static magnetic field generation mechanism 20a with the imaging region R in between. The second superconducting shield coil 40b and the second magnetized annular body 50b are arranged.
The first static magnetic field generation mechanism 20a and the second static magnetic field generation mechanism 20b are configured in this way, and have a high intensity of 0.3T or higher and a high level of about 10 ppm, which are necessary for capturing an image of a living tissue. A uniform static magnetic field is formed in the Z-axis direction of the imaging region R.

  In the following description, the first static magnetic field generation mechanism 20a, the second static magnetic field generation mechanism 20b, and these components, which are in symmetrical positions across the imaging region R, have the same configuration. When it is not necessary to specify any one of them, the descriptions of “first” and “second” are removed, and the description is simply “static magnetic field generation mechanism 20”.

Next, components of the static magnetic field generation mechanism 20 will be described in detail with reference to FIG.
The cooling vessel 21 is configured to cool the superconducting main coil 30 and the superconducting shield coil 40 disposed therein with a filled refrigerant (liquefied helium). Constructed in this way, the cooling vessel 21 cools the superconducting main coil 30 and the superconducting shield coil 40 to a temperature that exhibits superconducting characteristics where the electrical resistance is zero.

The superconducting main coil 30 is formed by winding a superconducting wire around a coil bobbin 31 disposed around a rotational symmetry axis Z. When the superconducting main coil 30 is cooled to a predetermined temperature by the refrigerant filled in the cooling vessel 21, the superconducting state is changed from the normal conducting state to the superconducting state, and the electric resistance becomes zero. The first superconducting main coil 30a (30) and the second superconducting main coil 30b (30) arranged opposite to each other in the vertical direction across the imaging region R flow a predetermined current in the same constant direction. ing. Due to the annular current flowing through these two superconducting main coils 30, magnetic field lines 35 (35 ₁ , 35 ₂ , 35 ₃ ) penetrating the imaging region R in the Z-axis direction in the figure are generated. After passing through the imaging region R, these magnetic field lines 35 exit from the central hollow portion of one superconducting main coil 30 (30a), bypass the outside, and enter the central hollow portion of the other superconducting main coil 30 (30b). Enters and forms a closed loop.

As shown in FIG. 2B, the coil bobbin 31 includes a hollow cylindrical shaft tube 32, an outer plate 33 provided in a flange shape at one end of the shaft tube 32, and the other end of the shaft tube 32. And an inner plate 34 provided in a flange shape. The outer peripheral surface of the shaft cylinder 32 receives a force applied by the superconducting wire of the superconducting main coil 30 in the direction perpendicular to the rotational symmetry axis Z. And the outer side board 33 and the inner side board 34 are receiving the force which the said superconducting wire provides in the parallel direction of the rotational symmetry axis Z.
In this manner, the coil bobbin 31 holds the superconducting main coil 30 and supports the force applied to the superconducting wire.

Returning to FIG. 2A, the description will be continued.
The superconducting shield coil 40 is configured to have a diameter larger than that of the superconducting main coil 30 on the side opposite to the imaging region R with the superconducting main coil 30 sandwiched around the rotation symmetry axis Z common to the superconducting main coil 30. ing. An annular current in the direction opposite to that of the superconducting main coil 30 flows through the superconducting shield coil 40. Constructed in this way, the superconducting shield coil 40 forms a closed loop magnetic field line 45 centering on the cross section thereof, and the superconducting main coil 30 is formed on the opposite side of the imaging region R from the magnetic field line 35 of the casing 11. it is intended to exert an effect of canceling the magnetic force lines 35 _first portion that leaks to the outside.

The magnetized annular body 50 is configured by arranging a ferromagnetic material such as pure iron between the superconducting main coil 30 and the superconducting shield coil 40 around the rotational symmetry axis Z.
When the outer shape of the magnetized annular body 50 is specified by a cross section by a plane including the rotational symmetry axis Z, the inner peripheral surface 52 is more rotationally symmetrical than the superconducting main coil 30 as shown in FIG. Configured to be located on the side. Further, of the outer peripheral surface 51 of the magnetized annular body 50, the peripheral portion 53 located on the superconducting main coil 30 side further protrudes from the surface extending the outer peripheral surface 36 of the superconducting main coil 30. It extends from the outer peripheral surface 51 and has a rectangular cross section.

Since the magnetized annular body 50 has such a cross-sectional shape, the center-of-gravity position G2 of the cross-section is positioned closer to the rotational symmetry axis Z than the center-of-gravity position G1 of the cross section of the superconducting main coil 30.
Here, the center-of-gravity position of the cross section refers to a position where even if the cross section is supported at that point, the torque that rotates the cross section in the plane does not work due to gravity acting from an arbitrary direction.
Dynamically speaking, the center-of-gravity position of a cross section is a position vector r expressed by the following equation (1) when the mass of an arbitrary minute part on coordinates is expressed as m _i and the position vector as r _{i. This} is a position determined by _G.

r _G = (1 / M) × Σm _i r _i [M = Σm _i ] (1)

  Thus, by determining the center of gravity position G1 of the cross section of the superconducting main coil 30 and the center of gravity position G2 of the cross section of the magnetized annular body 50, the center of gravity positions G1 and G2 are converted into the magnetized magnetized annular body 50 and the superconducting main body. It can be regarded as a mass point magnetically attracted to the coil 30. When the force acting on the center of gravity position G1 of the superconducting main coil 30 is decomposed in the coordinate axis direction among the attracting forces in this way, as shown in FIG. 2B, the force f1 directed toward the shaft cylinder 32 is obtained in the X axis direction. In the Z-axis direction, a force f2 directed toward the outer plate 33 acts on the coil bobbin 31.

  These forces f1 and f2 are canceled by receiving reaction forces from the shaft cylinder 32 and the outer plate 33, respectively, so that the superconducting wire constituting the superconducting main coil 30 does not move. In general, a force in a direction of expanding the outer diameter of the circular coil acts on the circular coil through which the annular current flows. However, since the force f1 cancels the force in the direction of expanding the outer diameter, the effect that the configuration of the coil bobbin 31 necessary for supporting the superconducting main coil 30 so as not to move can be obtained. . That is, it is easier to form the support structure to receive the force in the direction of the force f1 like the coil bobbin 31 than to form the support structure of the superconducting main coil 30 to receive the force in the direction opposite to the force f1. Is.

  By the way, in the above description, the shape of the inner peripheral surface 52 of the magnetized annular body 50 is exemplified only for the cylindrical shape formed inside the inner peripheral surface of the superconducting main coil 30. The form is not limited to such a form, and the center of gravity position G2 of the cross section of the magnetized annular body 50 is configured to be positioned closer to the rotational symmetry axis Z than the center of gravity position G1 of the cross section of the superconducting main coil 30. Everything is included. Further, the magnetized annular body 50 is not limited to the one in which the central portion is a cavity, and when such a cavity is not formed or other magnetized members are appropriately disposed. included.

Next, the operation of the peripheral portion 53 of the magnetized annular body 50 will be described.
By arranging the magnetized annular body 50 outside the superconducting main coil 30 forming a pair, the magnetized annular body 50 is magnetized, and the density of the magnetic force lines 35 penetrating the imaging region R is improved. Furthermore, the magnetization annular body 50, a portion of the magnetic field lines 35 _2, 35 ₃ of the folded portion of the closed loop, serves the function of reducing the magnetic field lines 35 from leaking to the outside drawn.

Since some of the magnetic force lines 35 ₂ and 35 ₃ attracted to the magnetized annular body 50 draw a closed loop passing through the projecting peripheral edge 53, these magnetic force lines 35 ₂ and 35 ₃ are the superconducting main coil 30. Never cut off. Similarly, the magnetic lines of force 45 formed by the superconducting shield coil 40 are drawn to the magnetized annular body 50 and draw a closed loop passing through the peripheral portion 53, so that the lines of magnetic force 45 do not cut the superconducting main coil 30.

  Thus, since the peripheral portion 53 acts, the superconducting main coil 30 is prevented from being cut by the high-density magnetic force lines 35, and the critical current density and critical temperature values at which the superconducting state transitions to the normal conducting state are avoided. Will not drop. Thus, since the critical current density does not decrease, it is not necessary to make the superconducting main coil 30 thick in order to obtain a predetermined static magnetic field strength in the imaging region R, and the amount of superconducting wire used can be reduced. Further, since the critical temperature does not decrease, a cooling device for cooling the superconducting main coil 30 does not become large.

<Second Embodiment>
Next, another embodiment of a magnetized annular body applied to the MRI apparatus 10 of the present invention will be described with reference to FIG. In FIG. 3, the constituent elements other than the magnetized annular bodies 50 ′ and 50 ″ are the same as the constituent elements already described with the same reference numerals, and the description thereof will be omitted.
The magnetized annular body 50 ′ shown in FIG. 3A is formed such that its outer peripheral surface 51 ′ continuously increases in outer diameter as it approaches the superconducting main coil 30, and its end constitutes a peripheral portion 53 ′. It is supposed to be. Furthermore, the center of gravity position G2 ′ of the cross section of the magnetized annular body 50 ′ is located closer to the rotational symmetry axis Z than the center of gravity position G1 of the cross section of the superconducting main coil 30.

  By configuring the magnetized annular body 50 ′ in this way, the superconducting main coil 30 receives a force in a direction in which the outer diameter decreases. Further, the magnetic field lines passing through the magnetized annular body 50 ′ do not cut the superconducting main coil 30. In addition, although the part of outer peripheral surface 51 'of the cross section shown by Fig.3 (a) is represented by the straight line, the case where this part is represented by a curve is also included.

<Third Embodiment>
The magnetized annular body 50 ″ shown in FIG. 3 (b) is configured such that the peripheral portion 53 ″ is separated from the member 51 ″ including the center of gravity position G2 ″ of the magnetized annular body 50 ″. The center of gravity position G2 ″ of the cross section of 50 ″ is located closer to the rotational symmetry axis Z than the center of gravity position G1 of the cross section of the superconducting main coil 30.

  By configuring the magnetized annular body 50 ″ in this way, the superconducting main coil 30 receives a force in a direction in which the outer diameter decreases. Further, the magnetic lines of force passing through the magnetized magnetized annular body 50 ″ The superconducting main coil 30 is not cut. The magnetized annular body 50 ″ shown in FIG. 3B is composed of two separated members, but includes a case where it is composed of three or more members.

The following effects are exhibited by the nuclear magnetic imaging apparatus having the structural features as described above.
That is, the superconducting main coil 30 receives a force from the magnetized annular body 50 (50 ′, 50 ″) in a direction in which the outer diameter decreases, thereby simplifying the structure for supporting the superconducting main coil 30 and generating a static magnetic field. The generation mechanism can be realized simply and at low cost. Further, the loop of magnetic lines of force starting from the magnetized magnetized annular body 50 (50 ′, 50 ″) is led to the protruding peripheral portion 53 (53 ′, 53 ″). Therefore, since the superconducting main coil 30 is not cut, the values of the critical current density and the critical temperature at which the superconducting state and the normal conducting state are switched are not lowered. Since the cross-sectional area of the superconducting main coil 30 necessary for forming the static magnetic field can be reduced and the value of the current flowing through the superconducting wire can be increased, the use of the wire can be reduced and the cost can be reduced. And allowing miniaturization of the MRI apparatus 10.

1 is a partial cross-sectional perspective view showing an embodiment of a nuclear magnetic imaging apparatus (MRI apparatus) of the present invention. (A) is a fragmentary sectional view which shows 1st Embodiment of the static magnetic field generation | occurrence | production mechanism of an MRI apparatus, (b) is explanatory drawing for demonstrating the force which acts on a superconducting main coil. (A) is a partial sectional view showing a second embodiment of the static magnetic field generation mechanism of the MRI apparatus, and (b) is a partial sectional view showing a third embodiment of the static magnetic field generation mechanism of the MRI apparatus.

Explanation of symbols

10 Nuclear magnetic resonance imaging equipment (MRI equipment)
20a (20) First static magnetic field generation mechanism (static magnetic field generation mechanism)
20b (20) Second static magnetic field generation mechanism (static magnetic field generation mechanism)
30a (30) First superconducting main coil (superconducting main coil)
30b (30) Second superconducting main coil (superconducting main coil)
36 Superconducting main coil outer peripheral surface 40a (40) First superconducting shield coil (superconducting shield coil)
40b (40) Second superconducting shield coil (superconducting shield coil)
50a (50, 50 ', 50 ") 1st magnetization annular body (magnetization annular body)
50b (50, 50 ′, 50 ″) Second magnetized ring (magnetized ring)
51, 51 ′ An outer peripheral surface of the magnetized annular body 52 An inner peripheral surface of the magnetized annular body 53, 53 ′, 53 ″ A peripheral portion of the magnetized annular body 35, 35 ₁ , 35 ₂ , 35 ₃ , 45 Magnetic field lines G1 A cross section of the superconducting main coil G2, G2 ', G2 "The center of gravity of the cross section of the magnetized annular body R Imaging area Z Rotation symmetry axis

Claims (4)

A superconducting main coil that generates magnetic field lines that form a static magnetic field in the imaging region;
A superconducting shield coil that cancels out the lines of magnetic force leaking to the opposite side of the imaging region across the superconducting main coil;
In a magnetic resonance imaging apparatus comprising: a magnetized annular body that is disposed between the superconducting main coil and the superconducting shield coil and attracts the lines of magnetic force.
The magnetized annular body is
The gravity center position of the cross section of the magnetized annular body by the plane including the rotational symmetry axis is configured to be located on the rotational symmetry axis side with respect to the gravity center position of the cross section of the superconducting main coil,
Of the outer peripheral surface of the magnetized annular body, a peripheral portion located on the superconducting main coil side is further configured to protrude from a surface extending the outer peripheral surface of the superconducting main coil,
The peripheral portion, magnetic resonance imaging apparatus you characterized in that with a rectangular cross-section is configured to extend from the outer peripheral surface of said magnetic toroid. A superconducting main coil that generates magnetic field lines that form a static magnetic field in the imaging region;
A superconducting shield coil that cancels out the lines of magnetic force leaking to the opposite side of the imaging region across the superconducting main coil;
In a magnetic resonance imaging apparatus comprising: a magnetized annular body that is disposed between the superconducting main coil and the superconducting shield coil and attracts the lines of magnetic force.
The magnetized annular body is
The gravity center position of the cross section of the magnetized annular body by the plane including the rotational symmetry axis is configured to be located on the rotational symmetry axis side with respect to the gravity center position of the cross section of the superconducting main coil,
Of the outer peripheral surface of the magnetized annular body, a peripheral portion located on the superconducting main coil side is further configured to protrude from a surface extending the outer peripheral surface of the superconducting main coil,
The peripheral portion, the superconducting magnetic you, characterized in that it is composed at the end of the outer peripheral surface of the magnetized annular body formed so that the outer diameter increases as approaching the main coil resonance imaging apparatus. A superconducting main coil that generates magnetic field lines that form a static magnetic field in the imaging region;
A superconducting shield coil that cancels out the lines of magnetic force leaking to the opposite side of the imaging region across the superconducting main coil;
In a magnetic resonance imaging apparatus comprising: a magnetized annular body that is disposed between the superconducting main coil and the superconducting shield coil and attracts the lines of magnetic force.
The magnetized annular body is
The gravity center position of the cross section of the magnetized annular body by the plane including the rotational symmetry axis is configured to be located on the rotational symmetry axis side with respect to the gravity center position of the cross section of the superconducting main coil,
Of the outer peripheral surface of the magnetized annular body, a peripheral portion located on the superconducting main coil side is further configured to protrude from a surface extending the outer peripheral surface of the superconducting main coil,
The peripheral portion, the magnetization of a member including the center of gravity of the annular body, magnetic resonance imaging apparatus you characterized in that it is configured to separate. The inner circumferential surface of the magnetic annulus than said superconducting main coil, according to any one of claims 1 to 3, characterized in that it is configured to be positioned on the axis of rotational symmetry side Magnetic resonance imaging equipment.

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