JP2712494B2 - Nuclear magnetic resonance tomograph - Google Patents

Description

The present invention relates to a nuclear magnetic resonance tomography (MRI) apparatus that obtains a tomographic image by exciting a target nuclide in a subject, and in particular, generates a gradient magnetic field. The present invention relates to a technique for suppressing vibration of a coil for use.

B. Prior Art FIG. 8 is a longitudinal sectional view of a gantry part of a nuclear magnetic resonance tomography apparatus according to a conventional example.

A gantry section of the MRI apparatus has a main coil 1 that forms a uniform main static magnetic field space in the body axis direction of the subject M, and is disposed in the main static magnetic field space, and has X, Y, and Z orthogonal coordinate axes. A saddle-shaped gradient magnetic field generating coil ( _Gx , _Gy ) 2 for forming a gradient magnetic field in each direction and a gradient magnetic field generating coil ( _Gz coil) of a Maxwell pair are provided.

First, the subject M lying on the top 9 is inserted into the main static magnetic field in the Z-axis direction formed by the main coil 1. When an excitation pulse is applied by an excitation coil (not shown) in this state, an MRI signal can be detected. At this time, in order to distinguish the signal from which part of the subject M,
A pulse current is supplied to each gradient magnetic field generating coil 2 that linearly changes the magnetic field strength in the Z-axis direction along three orthogonal axes (X, Y, and Z-axis directions).

C. Problems to be Solved by the Invention However, the above-described conventional apparatus has the following problems.

That is, when a pulse current is applied to the gradient magnetic field generating coil 2 placed in the main static magnetic field, a Lorentz force corresponding to the pulse current acts on the gradient magnetic field generating coil 2. This force acts in the radial direction of the gradient coil bobbin 5, that is, the gradient magnetic field generating coil 2 hits the gradient coil bobbin 5, so that a tapping sound is generated from the gradient coil bobbin 5. The generated tapping sound becomes loud due to the reverberation of the gradient coil bobbin 5.

For this reason, conventionally, the vibration damping material 13 is inserted between the gradient magnetic field generating coil 2 and the gradient coil bobbin 5 to reduce the hitting sound, and The patient M was relieved of the mental distress caused by the sound.

Also, since the gradient magnetic field generating coil 2 is for giving position information for distinguishing from which part of the subject M the MRI signal has come, a force acts on the gradient magnetic field generating coil, If the gradient magnetic field generating coil vibrates, accurate position information cannot be obtained, and the resolution and the S / N ratio may be reduced, that is, the image quality may be reduced.

The present invention has been made in view of such circumstances, and it is an object of the present invention to provide a nuclear magnetic resonance tomography apparatus capable of minimizing vibration of a gradient magnetic field generating coil and occurrence of a tapping sound due to the vibration. The purpose is.

D. Means for Solving the Problems The present invention has the following configuration to achieve the above object.

That is, the nuclear magnetic resonance tomographic imaging apparatus according to the present invention is a nuclear magnetic resonance tomographic imaging apparatus in which a gradient magnetic field generating coil is installed in a main static magnetic field formed by a main static magnetic field generating means. Is characterized in that a local static magnetic field means for generating a static magnetic field for canceling a main static magnetic field in a local space in which is installed is provided.

E. Operation According to the present invention, the local static magnetic field generating means forms a static magnetic field in a local space where the gradient coil is placed in a direction opposite to the direction of the main static magnetic field, thereby forming the gradient coil. The main static magnetic field in the local space where it is placed is canceled.

F. Embodiment Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

First Embodiment FIGS. 1 to 3 relate to the first embodiment, FIG. 1 is a longitudinal sectional view of a gantry part, FIG. 2 is a partially cutaway perspective view of a gantry part, and FIG. It is operation | movement explanatory drawing of a removal coil.

In these figures, the portions denoted by the same reference numerals as those shown in FIG. 8 according to the conventional example are the same components as those in the conventional example, and therefore, the description thereof is omitted here. This is the same for other embodiments described later.

A top plate 9 on which an object M is mounted is provided in the internal space of the main coil 1 so as to be insertable into and removable from the gantry, and a gradient magnetic field generating coil 2 for generating a gradient magnetic field between the main coil 1 and the top plate 9 is provided. Is provided. The gradient magnetic field generating coil 2 is formed in a local space A sandwiched between a cylindrical gradient coil bobbin (inner bobbin) 5 and a cylindrical gradient coil sound removing coil outer bobbin 6 provided concentrically therewith. And is attached along the outer peripheral surface of the inner bobbin 5.

The gradient coil sound elimination coil 3 as a local static magnetic field generating means passes through the outer bobbin 6 and the inner bobbin 5 and is drawn out to the internal space of the inner bobbin 5 after substantially making a round around the outer peripheral surface of the outer bobbin 6. Then, the outer bobbin 6 makes substantially one turn along the inner peripheral surface of the inner bobbin 5 in a direction opposite to the outer winding direction of the outer bobbin 6, penetrates the inner bobbin 5 and the outer bobbin 6, and forms the outer bobbin 6 again. Drawn into the outer space,
The outer bobbin 6 is wound on the outer peripheral surface in the same direction as the outer peripheral winding direction.

In this way, over almost the entire axial direction of the two concentrically provided coil bobbins 5, 6, the inclined coil sound removing coil 3 is provided on the outer peripheral surface of the outer bobbin 6 and on the inner peripheral surface of the inner bobbin 5. And wound around.

 Next, the operation of the above-described embodiment apparatus will be described.

The main coil 1 forms a uniform main static magnetic field B in the internal space of the main coil 1. In this embodiment, the direction of the main static magnetic field B is to the left in FIG. Next, a pulse current is supplied to the gradient magnetic field generating coil 2 in order to generate a gradient magnetic field. At this time, a current is supplied to the gradient coil sound removing coil 3 so that the Lorentz force does not act on the gradient magnetic field generating coil 2.

As shown in FIG. 3, when a current is supplied to the gradient coil sound elimination coil 3, a static magnetic field B 'in the right direction in FIG. 3 is generated in a local space A sandwiched between the inner bobbin 5 and the outer bobbin 6. Occur. Since the static magnetic field B 'is in the opposite direction to the static magnetic field B formed by the main coil 1, the static magnetic field B' cancels the main static magnetic field B in the local space A. Therefore, even if a pulse current is supplied to the gradient magnetic field generating coil 2 arranged in the local space A, no force acts on the gradient magnetic field generating coil 2 and vibration of the gradient magnetic field generating coil 2 is suppressed. Can be.

Further, in the region outside the local space A, the magnetic field lines of the gradient coil sound removing coil 3 are in opposite directions to each other, so that the static magnetic field B ′ in this region hardly occurs. Therefore, in the imaging region at the center of the gantry, a tomographic image of the subject M can be captured without the main static magnetic field B being disturbed by the static magnetic field B ′.

Second Embodiment FIG. 4 shows a side view of a gantry according to a second embodiment.

Reference numeral 4 denotes a main magnet as main static magnetic field generating means for forming a main static magnetic field. The main magnet 4 is a pair of permanent magnets whose north and south poles face each other, and is supported by the support legs 8.

Outside the pair of main magnets 4, a gradient coil sound removing magnet 7 as local magnetic field generating means is arranged. The gradient coil sound removing magnet 7 is a pair of permanent magnets having an N pole and an S pole facing each other, and is supported by support legs 10.
The local spatial A ₁ sandwiched between the N pole of N-pole and the gradient coil sound canceller magnet 7 of the main magnet 4, gradient magnetic field generating coil 2 is provided which is wound bobbin 5 wound gradient coils. Similarly as in the local spatial A ₂ sandwiched between the S pole of the gradient coil sound removal magnet 7 S pole of the main magnet 4, the gradient magnetic field generating coil 2 which is wound another bobbin 5 wound gradient coils Is provided. In the local space A _1, the orientation of the magnetic field lines emanating from the main magnet 4 N pole and N pole of the gradient coil sound removal magnet 7, to become the opposite direction to each other, main static magnetic field in the local space A ₁ little occurs without the same manner, also the local space a ₂ sandwiched between the main magnet 4 S pole to the S pole of the gradient coil sound canceller magnet 7, since the main static magnetic field is not generated, pulse gradient magnetic field generating coil 2 Even if a current is supplied, the gradient magnetic field generating coil 2 does not vibrate.

On the other hand, in the space between the N pole and the S pole of the main magnet 4,
Since the static magnetic fields of the magnets 4 and 7 are generated in the same direction, when capturing a tomographic image of the subject M, the imaging can be performed by inserting the subject M into the static magnetic field.

The second embodiment can be modified as shown in FIG.

 FIG. 5 is a side view of this modified device.

In this modification, the main magnet 4 of the second embodiment is
And the gradient coil sound removing magnet 7 is replaced with a gradient coil sound removing coil.
This is performed in place of 12.

The direction of the current I flowing through the gradient coil sound removing coil 12 is opposite to the direction of the current I 'flowing through the main coil 11. Therefore, in the local spaces A ₁ and A ₂ , the direction of the main static magnetic field generated by the current I ′ flowing through the main coil 11 and the direction of the static magnetic field generated by the current I flowing through the gradient coil sound removing coil 12 are mutually different. Since the directions are opposite, the static magnetic fields in the local spaces A ₁ and A ₂ cancel each other, and even if a pulse current is supplied to the gradient magnetic field generating coil 2, the gradient magnetic field generating coil 2 does not vibrate.

On the other hand, in the space sandwiched by the main coils 11, the lines of magnetic force are directed to reinforce each other, so that a static magnetic field is generated in this space. When capturing a tomographic image of the subject M,
By inserting the subject M into this space, imaging becomes possible.

Third Embodiment FIGS. 6 and 7 relate to the third embodiment, FIG. 6 is a longitudinal sectional view of a gantry part, and FIG. 7 is a partially broken perspective view of the gantry part.

Reference numeral 7 denotes a gradient coil sound removing magnet that forms a static magnetic field B '. The gradient coil sound removing magnet 7 is formed of a ring-shaped permanent magnet in which an N pole and an S pole are arranged to face each other.
The direction of the static magnetic field B ′ formed in the space between the N pole and the S pole is determined by the main static magnetic field B formed by the main coil 1.
The north pole and the south pole of the gradient coil sound removing magnet 7 are arranged so as to be opposite to both ends of the cylindrical inner bobbin 5 so as to be in opposite directions to the direction of.

As a result, the static magnetic field B 'always acts rightward in FIG. 6 in the local space A outside the gradient coil bobbin 5 in which the gradient magnetic field generating coil 2 is installed. By canceling out the main static magnetic field B (leftward in the figure) generated in the local space A, vibration of the gradient magnetic field generating coil 2 can be suppressed even if a pulse current is supplied to the gradient magnetic field generating coil 2.

Since the gradient coil sound removing magnet 7 is annular, the effect of the static magnetic field B ′ on the main static magnetic field B in the imaging area corresponding to the center thereof is small, which hinders the imaging of the tomographic image of the subject M. Will not come.

G. Effects of the Invention As is clear from the above description, the nuclear magnetic resonance tomographic imaging apparatus according to the present invention is characterized in that the local static magnetic field generating means includes a main static magnetic field in a local space where a gradient magnetic field generating coil is installed. Therefore, even if a pulse current is supplied to the gradient magnetic field generating coil, no force acts on this coil, and it is possible to prevent occurrence of a tapping sound.

For this reason, the mental distress of the subject can be eliminated,
Since the surgeon can hear the voice from the subject, it is convenient for performing an imaging operation.

Further, since the displacement of the gradient magnetic field generating coil due to the vibration does not occur, accurate position information can be given, and the image quality can be improved.

[Brief description of the drawings]

1 to 7 relate to an embodiment of the present invention. FIG. 1 is a longitudinal sectional view of a gantry according to the first embodiment, FIG. 2 is a partially cutaway perspective view of the gantry, FIG. FIG. 4 is a view for explaining the operation of the gradient coil sound removing coil, FIG. 4 is a side view of a gantry section according to the second embodiment, FIG. 5 is a side view of a gantry section according to a modification thereof, and FIG. FIG. 7 is a longitudinal sectional view of a gantry section according to the example, and FIG. 7 is a perspective view of the gantry section. FIG. 8 is a longitudinal sectional view of a gantry section according to a conventional example. 1, 11: Main coil 2: Gradient magnetic field generating coil 2, 3, 12: Gradient coil sound removing coil 34: Main magnet, 7: Gradient coil sound removing magnet

Claims (1)

(57) [Claims] 1. A nuclear magnetic resonance tomographic imaging apparatus having a gradient magnetic field generating coil installed in a main static magnetic field generated by a main static magnetic field generating means. A nuclear magnetic resonance tomographic imaging apparatus comprising a local static magnetic field generating means for generating a static magnetic field for canceling a main static magnetic field.