JPH02121632A - Magnetic resonance imaging apparatus - Google Patents

Abstract

PURPOSE:To reduce the noise of a measuring space receiving an examinee by arranging a cylindrical shell, whose rigidity in the peripheral direction is lower than that in the axial direction, between an inclined magnetic field coil and the measuring space receiving the examinee. CONSTITUTION:A static magnetic field magnet 1 for generating a static magnetic field, an inclined magnetic field coil 3 for allowing an inclined magnetic field to act on the interior of the aforementioned magnetic field and a transmitting-receiving coil for detecting the output signal based on the magnetic resonance action of the examinee present in the aforementioned static magnetic field and inclined magnetic field are provided and the aforementioned output signal is processed to obtain the image data in the specific region of the examinee. A cylindrical shell 31 whose rigidity in the peripheral direction theta is lower than that in the axial direction Z is arranged between the inclined magnetic field coil 3 and a measuring space receiving the examinee. This cylindrical shell 31 is molded by a filament winding method wherein glass fibers 33 are wound around a mold to perform molding and the winding angle theta of the glass fibers 33 is made smaller than 45 deg. with respect to the axial direction Z of the cylindrical shell 31. As a result, the noise in the measuring space 15 can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION Object of the Invention (Field of Industrial Application) The present invention relates to a magnetic resonance imaging apparatus that reduces noise to which a subject is exposed.

(Prior Art) A conventional magnetic resonance imaging apparatus is constructed as shown in FIGS. 4 to 7.

FIG. 4 is a perspective view of a conventional magnetic resonance imaging apparatus, and FIG. 5 is a sectional view taken along line AA in FIG. As shown in these figures, the conventional device includes a static magnetic field magnet 1 that generates a static magnetic field;
A gradient magnetic field coil 3 that generates a gradient magnetic field, a support cylindrical shell 1 that supports the gradient magnetic field coil 3 via a support member 5 attached to the gradient magnetic field coil 3, a support rubber 7, and a support member 9.
1. A support part shrine 13 that supports the support cylindrical shell 11 on the static magnetic field magnet 1, a cylindrical shell 17 located between the gradient magnetic field coil 3 and the measurement space 15 into which the subject enters, and a cylindrical shell 1 that supports the cylindrical shell 17.
1 and a ring-shaped end plate 19 supported by the ring.

As shown in FIGS. 6 and 7, the gradient magnetic field coil 3 includes three sets of coil windings 23, 25, 27 on a coil winding core 21.
It is formed by winding.

The coil windings 23, 25, 27 are arranged along the X-axis and Y-axis, which are orthogonal to each other.
It is configured to generate arbitrary gradient magnetic fields in three directions: the axis and the Z-axis.

As shown in Fig. 6, each coil is molded with two relatively rigid resins such as epoxy.
It is fixed at 1. An independent power source for exclusive use is connected to each coil winding 2B, 25.27. During measurement, each coil winding 23, 25, 27 is excited by applying a pulsed current to each separately, thereby forming a predetermined gradient magnetic field within the static magnetic field. Because the generated static magnetic field is extremely large (for example, 0.22 to 1.5 Tesla), each coil winding 2 is
A large electromagnetic force acts on 3, 25, and 27. As a result, the gradient magnetic field coil 3 is excited and a large noise is generated.
It causes discomfort to the subject.

Therefore, in the conventional magnetic resonance imaging apparatus, the cylindrical shell 17, the end plate 19, and the support cylinder 11 enclose and seal the gradient magnetic field coil 3, which is the noise source, to reduce the noise in the measurement space 15. ing. However, the vibration of the gradient magnetic field coil 3 is caused by air propagation to the cylindrical shell 17 and the supporting cylinder 1.
1, the cylindrical shell 17 vibrates and noise is generated, so the noise reduction effect is insufficient.

(Problem to be Solved by the Invention) As described above, in the conventional magnetic resonance imaging apparatus, a pulsed current is applied to the coil windings 23, 25, 27 of the gradient magnetic field coil 3 placed in a large static magnetic field. As a result, large vibrations occur in the gradient magnetic field coil 3, and these vibrations are transmitted to the cylindrical shell 17 and the support cylinder 11 through air propagation, causing the cylindrical shell 17 to vibrate and generate noise.

Therefore, an object of the present invention is to provide a magnetic resonance imaging apparatus that reduces noise in a measurement space that accommodates a subject.

[Structure of the Invention] (Means for Solving the Problems) In order to achieve the above object, the present invention includes a static magnetic field magnet that generates a static magnetic field, and a gradient magnetic field coil that applies a gradient magnetic field to the static magnetic field. , a transmitting/receiving coil for detecting an output signal based on the magnetic field resonance effect of the subject in the static magnetic field and the gradient magnetic field, and processing the output signal to obtain image information on a specific region of the subject. The magnetic resonance imaging apparatus is characterized in that a cylindrical shell having lower rigidity in the circumferential direction than in the axial direction is disposed between the gradient magnetic field coil and the measurement space into which the subject enters.

Further, this cylindrical shell was formed by a filament winding method in which a glass W4 cone was wound around a mold and molded, and the winding angle of the glass fiber was made smaller than 45'' with respect to the axial direction of the cylindrical shell.

Further, the cylindrical shell was used as the winding core of the transmitter/receiver coil.

(Function) In the above configuration, electromagnetic force is applied to the coil winding during measurement, the gradient magnetic field coil vibrates, this vibration is transmitted to the cylinder shell by air propagation, and the vibration of the cylinder shell causes sound to be stationary in the measurement space. Even when waves are generated, the rigidity of the cylinder shell is lower in the circumferential direction than in the axial direction, so many of the vibration modes of the cylinder shell undergo high-order deformation in the circumferential direction, and are excited by these vibration modes. In the noise mode within the measurement space, the sound pressure is greatly attenuated near the central axis of the measurement space, which is the position of the subject's ear. Therefore, the subject's discomfort due to noise is significantly reduced.

(Embodiment) An embodiment of the present invention will be explained with reference to FIGS. 1 to 3. Note that the same members as in the conventional example are given the same reference numerals.

FIG. 1 is a perspective view showing a cylindrical shell 31 of the magnetic resonance imaging apparatus of this embodiment, and FIG. 2 is a sectional view showing the structure of the magnetic resonance imaging apparatus.

In this configuration, the cylindrical shell 31 is formed by a filament winding method in which glass fiber 33 is wound around a mold and molded with a resin such as epoxy. At this time, the winding angle φ of the glass fiber 33 is made smaller than 45° with respect to the axial direction Z of the cylindrical shell 31. By doing so, the cylindrical shell 31 is given orthogonal anisotropy in the axial direction Z and the circumferential direction θ, and the rigidity is greater in the circumferential direction θ than in the axial direction Z.
is lower. Due to this anisotropy in which the rigidity in the circumferential direction is lower, many of the out-of-plane vibration modes of the cylindrical shell 31 undergo high-order deformation in the circumferential direction θ in the frequency range that is sensitive to the human ear. .

In an experiment, the measurement space 1 was
5, it is recognized that standing waves of sound are generated.
The vibration mode of the cylindrical shell 31 at this time is expressed as follows.

v (Z, θ) = V (Z) cosn θ where υ is the out-of-plane vibration velocity, and n is the circumferential order (=0.
1. 2. ...). Further, the noise mode in the measurement space excited by this vibration mode is expressed as follows.

P(', θ, 2) <2) where P is the sound pressure, k = ω/C, ω is the angular frequency, C is the sound speed, k t, , = mπ/, m is the axial order (= 0.
1. 2. ), L is the axial length of the cylindrical shell 31, J
n is an n-th order Bessel function of the first kind, and Amn is an excitation coefficient determined by V (Z) and n.

From equations (1) and (2), if the circumferential order of the vibration mode of the cylindrical shell 31 is nth order, the noise mode in the measurement space 15 excited by this vibration mode is n in the radial direction r. It can be seen that it has the following Bessel function distribution of the first kind.

Figure 3 shows the form of the Bessel function of the first kind. In the figure, (
From the correspondence with equation 2), X=E "];", and X is associated with the r coordinate with X=0 as the central axis of the measurement space 9.

From FIG. 3, it can be seen that when n is the first order or higher, the sound pressure becomes 0 on the central axis of the measurement space 15, and especially when n becomes the second order, the sound pressure becomes smaller. Furthermore, in the device of the present invention, where the vibration mode undergoes high-order deformation in the circumferential direction, high-order modes where n is 2 or more are likely to occur. The sound pressure is reduced, and the subject's discomfort caused by the noise is significantly reduced.

In addition, in order to make the rigidity of the cylindrical shell in the circumferential direction smaller than that in the axial direction, ribs or grooves in the axial direction may be provided in the cylindrical shell.

Further, the cylindrical shell may be used as a winding core of a transmitting/receiving coil.

If this is not the case, the coil is placed, for example, inside a cylindrical shell.

[Effects of the Invention] As mentioned in Section 2 below, the magnetic resonance imaging apparatus of the present invention greatly reduces noise near the center of the measurement space where the subject's ear is located, thereby reducing the subject's discomfort due to noise. is significantly reduced.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing a cylindrical shell of a magnetic resonance imaging device according to an embodiment of the present invention, FIG. 2 is a sectional view of the device of this embodiment, and FIG. 3 is a graph of a Bessel function of the first kind.
4 to 7 relate to the conventional example; FIG. 4 is a perspective view of the entire device, FIG. 5 is a sectional view taken along line AA in FIG. 4, and FIGS. FIG. 1... Static magnetic field magnet 3... Gradient magnetic field coil 5,
9. 13... Support member 7... Support rubber 15... Measurement space 17.31
... Cylindrical shell 19 ... End plate 21 ... Coil winding core 23.25.27 ... Coil core wire 29 ... Resin 31 ... Cylindrical shell 33 ...
glass fiber

Claims (3)

[Claims] (1) A static magnetic field magnet that generates a static magnetic field, a gradient magnetic field coil that applies a gradient magnetic field within the static magnetic field, and detects an output signal based on the magnetic resonance effect of the subject within the static magnetic field and the gradient magnetic field. A magnetic resonance imaging apparatus comprising a transmitting/receiving coil for processing the output signal to obtain image information on a specific region of the subject, the gradient magnetic field coil and a measurement space in which the subject enters. A magnetic resonance imaging device characterized in that a cylindrical shell having lower rigidity in the circumferential direction than in the axial direction is disposed between the shells. (2) The cylindrical shell is formed by a filament winding method in which glass fibers are wound around a mold and molded, and the winding angle of the glass fibers is smaller than 45° with respect to the axial direction of the cylindrical shell. magnetic resonance imaging device. (3) The magnetic resonance imaging apparatus according to claim 1 or 2, wherein the cylindrical shell is a winding core of the transmitting/receiving coil.