JPH06133946A - Nuclear magnetic resonance tomographic system - Google Patents

Abstract

PURPOSE:To provide nuclear magnetic resonance tomographic system having an inclined magnetic field generator with sufficient shielding effects and high efficiency. CONSTITUTION:Each X-direction shielding coil 11 is in the form of a saddle whose circular portion has an open angle thetaSX of 50 deg. (90 deg. or less), with the center SP of the circular portion adjusted to the center KP of the circular portion of each X-direction inclined magnetic field generating coil 1. Current flowing through each X-direction shielding coil 11 is inverted with respect to that flowing through each X-direction inclined magnetic field generating coil 1. Each Y-direction inclined magnetic field generating coil is provided with a similar shielding coil. Since each of the shielding coils is disposed in a portion where leakage magnetic fields are great, it provides sufficient shielding effects, and since the open angle of the circular portion is 50 deg., the influence of each shielding coil on the inclined magnetic field produced by each inclined magnetic field generating coil is reduced and so the amount of current fed to each inclined magnetic field generating coil can be reduced, resulting in good energy efficiency.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nuclear magnetic resonance tomography apparatus (MRI apparatus), and more particularly to a gradient magnetic field generator used in an MRI apparatus.

[0002]

2. Description of the Related Art An MRI apparatus generates a uniform static magnetic field in the body axis direction (Z direction) of a subject inserted in the apparatus, and applies a high-frequency electromagnetic wave in a pulsed form to the inside of the subject. It is known to excite a spin and detect an electromagnetic wave emitted when the excited spin returns to its original state to form an image. Further, in image formation, a gradient magnetic field for obtaining three-dimensional position information of each excited spin is superimposed on the static magnetic field.

The static magnetic field is generated by a cylindrical static magnetic field generating magnet device composed of, for example, a superconducting magnet, and the gradient magnetic field is generated in the cavity of the static magnetic field generating magnet device, that is, nuclear magnetic resonance tomography. It is generated by a gradient magnetic field generator provided in the space.

This gradient magnetic field generator is shown in FIG.
(Perspective view showing the configuration of the gradient magnetic field generator), FIG.
(Side view), a G _X coil for generating a magnetic field that linearly changes the static magnetic field strength in the direction of the static magnetic field (Z direction) and the two axial directions (X and Y directions) orthogonal thereto. 1, a gradient magnetic field generating coil 4 including a G _Y coil 2 and a G _Z coil 3, and a power supply unit (not shown) that supplies a current to each coil 1, 2, and 3. G _X coil 1, G
_{The Y} coil 2 has opening angles θ _KX and θ _KY of the respective arc portions.
Is about 180 ° and is composed of four pairs of saddle-shaped coils arranged symmetrically about the plane. The G _Z coil 3 is composed of two solenoid type coils. In the figure, reference numeral 5
Is a cylindrical winding frame (this is referred to as a gradient coil bobbin), and each coil 1 described above is attached to the gradient coil bobbin 5.
Two and three are respectively wound and supported. The hollow portion of the gradient coil bobbin 5 serves as an insertion hole for the subject.

By the way, when the above-mentioned G _X coil 1, G _Y coil 2, and G _Z coil 3 are driven, the magnetic field (leakage magnetic field) formed in the outer peripheral direction of each coil (the static magnetic field generating magnet device direction (not shown)). ), A conductor that constitutes a magnet device for generating a static magnetic field (for example, a heat shield plate for maintaining a superconducting state, a support plate for supporting a superconducting magnet, etc.)
An eddy current is generated in the gradient magnetic field, and the magnetic field caused by the eddy current causes the gradient magnetic field to become nonuniform in time and space. Therefore, in order to eliminate such an adverse effect, conventionally, a shield gradient coil is provided between the outer circumference of the gradient magnetic field generating coil 4, that is, between the static magnetic field generating magnet device and the gradient magnetic field generating coil 4 to cause leakage. It shields the magnetic field.

As shown in FIG. 7, this shield gradient coil covers the SG _X coil 6, SG _Y coil 7, and SG _X coil 6, SG _Y coil 7 and SG that respectively cover the outer circumferences of the coils 1, 2, and 3 which form the gradient magnetic field generating coil 4. It is composed of a _Z coil 8. The SG _X coil 6 is supplied with a current in a direction opposite to the direction of the current flowing through the G _X coil 1, so that G G
Cancels the leakage magnetic field from _X- coil 1. Similarly, the SG _Y coil 7 and the SG _Z coil 8 are respectively G _Y coils 2 and
The leakage magnetic field from each of the coils 2 and 3 is canceled by supplying a current in the direction opposite to the direction of the current to the G _Z coil 3. As a result, eddy currents generated in the conductors of the static magnetic field generating magnet device are suppressed.

[0007]

However, the conventional example having such a structure has the following problems. That is, the shield gradient coil according to the conventional example suppresses the eddy current generated in the conductor forming the static magnetic field generating magnet device, but on the other hand, in order to cancel the leakage magnetic field from the gradient magnetic field generating coil 4, The magnetic field generated in the gradient coil forms a gradient magnetic field in the direction opposite to the gradient magnetic field formed by the gradient magnetic field generating coil 4 in the nuclear magnetic resonance tomography space, and the gradient magnetic field forms the gradient magnetic field generating coil 4. The gradient magnetic field will be canceled. Therefore, in order to avoid such an adverse effect, the radius ratios (r _KX / r _SX , r _KY / r _{SY) of} the gradient magnetic field generating coil 4 and the shield gradient coil are
r _KZ / r _SZ (see FIG. 7), a current must be supplied to the gradient magnetic field generating coil 4 such that the gradient magnetic field is not canceled by the shield gradient coil. In order to reduce the current supplied to the gradient magnetic field generating coil 4, the radius ratio between the gradient magnetic field generating coil 4 and the shield gradient coil must be increased,
There is a problem that the device itself becomes large. On the other hand, if the radius ratio of the gradient magnetic field generating coil 4 and the shield gradient coil is reduced, the current supplied to the gradient magnetic field generating coil 4 must be increased, There is a problem that energy efficiency becomes poor.

The SG _X coil 6 and the SG _Y coil 7 are also included.
In order to shield the leakage magnetic field from G _X coil 1 and G _Y coil 2, formed in a similar shape with G _X coil 1 and G _Y coil 2, the outer periphery of G _X coil 1 and G _Y coil 2 It is arranged to cover. That is, as shown in FIG.
The opening angle θ _SX of the arc portion of the _X coil 6 and the SG _Y coil 7,
θ _SY is about 180 °.
There is also a problem that it takes a lot of labor to manufacture.

The present invention has been made in view of the above circumstances, and suppresses an eddy current generated in a conductor forming a static magnetic field generating magnet device, and includes a gradient magnetic field generating device having high energy efficiency. Another object of the present invention is to provide a nuclear magnetic resonance tomography apparatus.

[0010]

The present invention has the following constitution in order to achieve such an object. That is, the present invention, the intensity of the static magnetic field uniformly formed in the nuclear magnetic resonance tomography space by the magnet device for generating a static magnetic field,
Saddle-shaped X-direction gradient magnetic field generating coil for generating a gradient magnetic field that linearly changes in the direction of the static magnetic field (Z direction) and in the two axial directions (X, Y directions) orthogonal thereto and for Y-direction gradient magnetic field generation A nuclear magnetic resonance tomography apparatus comprising a coil, a solenoid-type Z-direction gradient magnetic field generating coil, and a gradient magnetic field generator configured to supply a current to each of the gradient magnetic field generating coils. A saddle-shaped X having an arc-shaped opening angle of 90 ° or less between the static magnetic field generating magnet device and the X-direction gradient magnetic field generating coil.
The direction shield coil is attached such that the center of the arc portion thereof substantially coincides with the center of the arc portion of the X-direction gradient magnetic field generating coil, and the direction of current flowing through the X-direction shield coil is generated in the X-direction gradient magnetic field generation. Saddle-shaped Y direction in which the opening angle of the arc portion is 90 ° or less between the static magnetic field generating magnet device and the Y direction gradient magnetic field generating coil. The shield coil is attached such that the center of the arc portion thereof substantially coincides with the center of the arc portion of the Y-direction gradient magnetic field generating coil, and the direction of the current flowing through the Y-direction shield coil is used to generate the Y-direction gradient magnetic field. The direction is opposite to that of the current flowing through the coil.

[0011]

The operation of the present invention is as follows. The magnetic field (leakage magnetic field) formed in the direction of the static magnetic field generating magnet device from the X-direction gradient magnetic field generating coil is the largest at the center of the arc portion of the X-direction gradient magnetic field generating coil, and is located at the end of the arc portion. It gets smaller as you get closer. This is because the X direction gradient magnetic field generating coil for generating the X direction gradient magnetic field is 2
Since the saddle-shaped coils are arranged so as to face each other and the current flows in the opposite direction to each coil, the leakage magnetic fields cancel each other at the ends of the arc portions of the coils. Is. The same applies to the Y direction gradient magnetic field generating coil. In the present invention, each shielding coil is provided centering on a portion having a large leakage magnetic field, and a current in a direction opposite to that of each gradient magnetic field generating coil is passed through the shielding coil to generate magnetic fields that cancel each other out. Thus, many leakage magnetic fields can be shielded (cancelled). Therefore, the eddy current generated in the conductor forming the static magnetic field generating magnet device by the leakage magnetic field can be sufficiently reduced.

On the other hand, the opening angle of the arc portion of each shielding coil is 90 ° or less, and the gradient magnetic field formed in the nuclear magnetic resonance tomography space when each shielding coil generates the shielding magnetic field ( The gradient magnetic field formed in the magnetic resonance tomography space by each gradient magnetic field generating coil is in the direction opposite to the gradient magnetic field)
It is smaller than the gradient magnetic field formed by each gradient magnetic field generating coil. Therefore, the ratio of the gradient magnetic field generated in the nuclear magnetic resonance tomography space by the gradient magnetic field generating coils to be canceled by the gradient magnetic field generated by the shielding coil can be reduced. Therefore, it becomes unnecessary to supply a large amount of current to the X- and Y-direction gradient magnetic field generating coils, and energy efficiency can be improved.

It should be noted that if the opening angle of the arc portion of each shielding coil is 90 ° or less, even if it is theoretically 0 °, the operation will be as described above. It will have some opening angle.

By the way, when the opening angle of the arc portion of each shielding coil is set to 90 ° or more, distortion of the magnetic field distribution created by the shielding coil in the nuclear magnetic resonance tomography space becomes large, which cancels it out. In order to do so, it is necessary to increase the amount of current supplied to each gradient magnetic field generating coil, which lowers energy efficiency. Further, if the amount of current supplied to each gradient magnetic field generating coil is increased, the leakage magnetic field from each gradient magnetic field generating coil also increases, and the shielding effect decreases. Therefore, when the opening angle of the arc portion of each shielding coil is 90 ° or more, the opening angle of the arc portion is about 180 ° as in the conventional example.
It is possible to enhance the shielding effect most. That is, if the opening angle of the arc portion of each shielding coil is 90 ° or more, not only the energy efficiency is deteriorated, but also the shielding effect is reduced.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a front view showing a schematic configuration of a gradient magnetic field generator provided in a nuclear magnetic resonance tomography apparatus according to an embodiment of the present invention, and FIG. 2 is a sectional view taken along the line AA of FIG. Is. 1 and 2 show only the X-direction gradient magnetic field generating coil and its shielding coil. In the following embodiments, the X-direction gradient magnetic field generating coil and its shielding coil will be described as an example. To do. However, the shielding coil provided in the Y-direction gradient magnetic field generating coil is configured similarly to the shielding coil provided in the X-direction gradient magnetic field generating coil, and
The Y direction gradient magnetic field generating coil and its shielding coil are
The direction gradient magnetic field generating coil and its shielding coil are 90 °
The inclined coil bobbin 5 is provided so as to be displaced. Further, in the figure, the portions denoted by the same reference numerals as those in FIGS. 5 and 6 have the same configuration as the conventional example, and therefore the description thereof is omitted here.

As shown in the figure, the X-direction shielding coil 11
Is formed in a saddle shape having an opening angle θ _SX of the arc portion of 50 °, and the center SP of the arc portion coincides with the center KP of the arc portion of each X-direction gradient magnetic field generating coil 1. It is provided on the outer circumference of each X-direction gradient magnetic field generating coil 1. Further, as shown by the arrow in FIG. 1, the current flowing through each X-direction shielding coil 11 is configured to flow in the opposite direction to the current flowing through each X-direction gradient magnetic field generating coil 1.

Next, the X-direction shielding coil 1 as described above.
The operation when the coil 1 is provided in the X-direction gradient magnetic field generating coil 1 will be described with reference to FIGS. FIG. 3A shows the magnetic flux distribution generated from the X-direction gradient magnetic field generating coil 1 when the X-direction shielding coil 11 is not provided,
FIG. 2B shows the outer circumference of the X-direction gradient magnetic field generating coil 1,
That is, from point P ₁ to P ₂ in FIG.
(The same as the center KP of the circular arc portion of the X direction gradient magnetic field generating coil 1), P ₃ , P ₄ (the same as the center KP of the circular arc portion of the X direction gradient magnetic field generating coil 1) through P ₁ . The strength of the generated leakage magnetic field is shown.

As shown in FIG. 3B, the leakage magnetic field is in the V ₁ and V ₂ directions at points P ₂ and P ₄ (see FIG. 3A).
(See) and the points P ₁ and P _{3 respectively.}
It gets smaller as it gets closer to. This is because, as shown in FIG. 3 (a), points in P _1, P _3, together with the ends of the arcuate portion of the upper and lower X-direction gradient magnetic field generating coil 1 ₁ which is disposed opposite to, 1 ₂ This is because the magnetic fields in the opposite directions are formed, so that the leakage magnetic fields are canceled by the respective magnetic fields.

Next, X is applied to the X-direction gradient magnetic field generating coil 1.
FIG. 4 shows the magnetic flux distribution generated from the X-direction gradient magnetic field generating coil 1 when the direction shielding coil 11 is provided and the strength of the leakage magnetic field generated on the outer periphery of the X-direction gradient magnetic field generating coil 1.
Shown in. As shown in FIG. 4B, the leakage magnetic fields near the points P ₂ and P ₄ where the largest leakage magnetic field was formed are
It can be seen that the shielding coils 11 ₁ and 11 ₂ cancel each other and reduce the number. The distribution of the magnetic flux density at this time is shown in FIG.
As shown in (a). In the figure, each shielding coil 1
The magnetic flux lines generated from 1 ₁ and 11 ₂ are shown by the alternate long and short dash line, and part of the leakage magnetic field generated from the X direction gradient magnetic field generating coils 1 ₁ and 1 ₂ by this magnetic field (shown by the dotted line).
Is canceled. The solid line indicates each shielding coil 11 ₁ ,
11 shows a stray magnetic field that is not canceled by 11 ₂ .

In this way, the shielding coils 11 ₁ and 11
_2, mainly the leakage magnetic field of the big points P _2, P ₄ generated from the X-direction gradient magnetic field generating coil 1 _1, 1 _2, the leakage magnetic field generated from the X-direction gradient magnetic field generating coil 1 _1, 1 ₂ Since they are cancelled, the leakage magnetic field can be reduced, and the eddy current generated in the conductor that constitutes the static magnetic field generating magnet device can be reduced. Therefore, the disturbance of the gradient magnetic field due to the eddy current can be reduced.

Further, the shielding coil 11 has an opening angle θ _{SX of the} arc portion of 50 ° (90 ° or less), and when each shielding coil 11 generates a shielding magnetic field, nuclear magnetic resonance tomography is performed. The gradient magnetic field formed in the space (the gradient magnetic field in the direction opposite to the gradient magnetic field formed by the X-direction gradient magnetic field generating coil 1 in the nuclear magnetic resonance tomography space) is generated by the X-direction gradient magnetic field generating coil 1. It becomes smaller than the gradient magnetic field formed in the tomographic space. Therefore, it is possible to reduce the rate at which the gradient magnetic field generated in the nuclear magnetic resonance tomography space by the X-direction gradient magnetic field generating coil 1 is canceled by the gradient magnetic field generated by the shielding coil 11.
Since the leakage magnetic field is shielded, it is not necessary to supply a large amount of current to the X-direction gradient magnetic field generating coil 1, and energy efficiency can be improved.

In the above-mentioned embodiment, the opening angle of the arc portion of the shielding coil is set to 50 °, but it may be set to 90 ° or less, particularly 30 ° to 50 °. It is effective.

Also, with respect to the shielding coil provided in the Y-direction gradient magnetic field generating coil, it is possible to obtain the same operational effect as that of the above-described embodiment (shielding coil provided in the X-direction gradient magnetic field generating coil). . As for the Z-direction gradient magnetic field generating coil, a shielding coil similar to the conventional example may be provided.

[0024]

As is apparent from the above description, according to the present invention, since the shielding coil is provided at the portion where the leakage magnetic field from the X, Y direction gradient magnetic field generating coil is the largest, each gradient magnetic field is provided. Most of the leakage magnetic field from the generating coil can be canceled out, and the generation of eddy current in the conductor forming the static magnetic field generating magnet device can be sufficiently reduced.

The opening angle of the arc portion of the shielding coil is 90 ° or less, and the gradient magnetic field (each of the magnetic fields formed in the nuclear magnetic resonance tomography space when each shielding coil generates a shielding magnetic field). The gradient magnetic field formed by the gradient magnetic field generating coil in the magnetic resonance tomography space is opposite to the gradient magnetic field formed in the magnetic resonance tomography space). Therefore, the ratio of the gradient magnetic field generated in the nuclear magnetic resonance tomography space by the gradient magnetic field generating coils to be canceled by the gradient magnetic field generated by the shielding coil can be reduced. Therefore, it becomes unnecessary to supply a large amount of current to the X- and Y-direction gradient magnetic field generating coils, and energy efficiency can be improved.

That is, according to the present invention, it is possible to realize a nuclear magnetic resonance tomography apparatus equipped with a gradient magnetic field generating device having good energy efficiency while suppressing an eddy current generated in a conductor forming a static magnetic field generating magnet device. .

[Brief description of drawings]

FIG. 1 is a front view showing a schematic configuration of a gradient magnetic field generator according to an embodiment of the present invention.

FIG. 2 is a sectional view taken along the line AA of FIG.

FIG. 3 (a) is a diagram showing a magnetic flux distribution generated from the X-direction gradient magnetic field generating coil when the X-direction shielding coil is not provided. (B) ... It is a figure which shows the intensity | strength of the leakage magnetic field which generate | occur | produces on the outer periphery of the coil for X-direction gradient magnetic field generation when the coil for X-direction shield is not provided.

FIG. 4 (a) is a diagram showing a magnetic flux distribution generated from the X-direction gradient magnetic field generating coil when the X-direction shielding coil is provided. (B) ... It is a figure which shows the intensity | strength of the leakage magnetic field which generate | occur | produces on the outer periphery of the coil for X-direction gradient magnetic field generation when the coil for X-direction shield is provided.

FIG. 5 is a perspective view showing a configuration of a gradient magnetic field generator.

FIG. 6 is a side view showing the configuration of a gradient magnetic field generator.

FIG. 7 is a diagram showing a configuration of a shield gradient coil according to a conventional example.

[Explanation of symbols]

1 ... X direction gradient magnetic field generating coil 2 ... Y direction gradient magnetic field generating coil 3 ... Z direction gradient magnetic field generating coil 4 ... gradient magnetic field generating coil 5 ... gradient coil bobbin 11 ... X direction shielding coil θ _SX ... X direction Opening angle of the arc part of the shielding coil θ _KX … _Open angle of the arc part of the X direction gradient magnetic field generating coil

Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display area 8203-2G G01R 33/22 N

Claims (1)

[Claims] 1. The intensity of a static magnetic field uniformly formed in a nuclear magnetic resonance tomography space by a static magnetic field generating magnet device is defined by a direction of the static magnetic field (Z direction) and two axial directions (X, A saddle-shaped X-direction gradient magnetic field generating coil, a Y-direction gradient magnetic field generating coil, a solenoid-type Z-direction gradient magnetic field generating coil, and each of the gradient magnetic fields that generate a gradient magnetic field that linearly changes in the Y direction). In a nuclear magnetic resonance tomography apparatus including a gradient magnetic field generator configured to include a power supply unit for supplying a current to a generating coil, a static magnetic field generating magnet device and the X-direction gradient magnetic field generating coil are provided. In the meantime, a saddle-shaped X-direction shield coil having an opening angle of the arc portion of 90 ° or less is attached so that the center of the arc portion substantially coincides with the center of the arc portion of the X-direction gradient magnetic field generating coil. With the above The direction of the current flowing through the X-direction shielding coil is opposite to the direction of the current flowing through the X-direction gradient magnetic field generating coil, and an arc is formed between the static magnetic field generating magnet device and the Y-direction gradient magnetic field generating coil. A saddle-shaped Y-direction shielding coil having an opening angle of 90 ° or less is attached such that the center of the arc portion thereof substantially coincides with the center of the arc portion of the Y-direction gradient magnetic field generating coil. A nuclear magnetic resonance tomography apparatus, wherein the direction of current flowing through the direction shield coil is opposite to the direction of current flowing through the Y-direction gradient magnetic field generating coil.