JP3241096B2 - MRI superconducting magnet device equipped with iron shim static magnetic field correction means - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gradient coil used for MRI, and more particularly to an MR coil provided with an iron shim static magnetic field correction means.
The present invention relates to improvement of a superconducting magnet device for I.

[0002]

2. Description of the Related Art An MRI apparatus for obtaining a tomographic image of a subject focused on a specific nucleus using a nuclear magnetic resonance phenomenon has been conventionally known. An outline of the MRI apparatus will be briefly described.

The nucleus can be regarded as a rotating top with magnetism, which is, for example, a static magnetic field H in the z-axis direction.
₀ , the nucleus has a nuclear velocity ω ₀
To precess. This is called Lamore precession.

Now, when a high-frequency coil is arranged on an axis perpendicular to the z-axis where the static magnetic field is present, for example, the x-axis, and a high-frequency rotating magnetic field having the above-mentioned angular velocity ω ₀ rotating in the xy plane is applied, magnetic resonance occurs. A group of nuclei that have undergone Zeeman splitting under the static magnetic field H ₀ undergo transition between levels due to a high-frequency magnetic field that satisfies resonance conditions, and transition to a higher energy level. Here, since the nuclear magnetic rotation ratio γ differs depending on the type of the nucleus, the nucleus can be specified by the resonance frequency. Further, by measuring the intensity of the resonance, the abundance of the nucleus can be known. Nuclei excited to a high level during a time determined by a time constant called a relaxation time after resonance return to a low level and emit energy.

[0005] The resonance condition of the above nucleus ensemble is given by ν = γH ₀ / 2π. Here, ν is the resonance frequency, and H ₀ is the strength of the static magnetic field. Therefore, it can be seen that the resonance frequency is proportional to the strength of the magnetic field. For this purpose, a linear gradient magnetic field is superimposed on the static magnetic field, magnetic fields of different strengths are given depending on the position, and the resonance frequency is changed to obtain position information. The gradient coil is located at

In the MRI apparatus, when the static magnetic field is large, the SN ratio is improved, so that a superconducting magnet is used because a strong magnetic field is obtained and the magnetic field is stable.

Conventionally, in a magnet device using a superconducting magnet,
To improve cooling efficiency, the magnet is covered with an aluminum cylinder with good thermal conductivity.However, when a gradient current is applied to the gradient coil, an eddy current is generated in the aluminum cylinder because it flows intermittently. However, there is a problem that a new magnetic field is generated by the eddy current and disturbs the original magnetic field.

For this reason, the gradient magnetic field coil is formed in two layers, the directions of the currents of the inner and outer coils are reversed, and the magnetic field generated in the inner coil, which is emitted to the outside, is eliminated, and the eddy current is supplied to the aluminum cylinder. Shielded gradient coils have been used which consist of two layers of gradient coils to prevent the generation of phenomena.

FIG. 4 shows a magnet device using the shield gradient magnetic field coil. In the figure, reference numeral 1 denotes a magnet for a static magnetic field, on which an iron shim assembly 2 for correcting a static magnetic field is attached and integrated in order to maintain the uniformity of the static magnetic field. Reference numeral 3 denotes an inner gradient magnetic field coil for applying a gradient magnetic field to a subject housed therein, and 4 denotes an inner gradient magnetic field coil 3
The outer gradient magnetic field coil for canceling the leakage magnetic field generated outside the coil is made zero, and the gradient magnetic field coil having such a double structure is called a shield gradient magnetic field coil. Here, D ₁ is the diameter of the inner gradient magnetic field coil 3, D ₂
The diameter of the outer gradient coil 4, D _s is the iron shims assembly 2 Kuriaboa and D _c is the Kuriaboa magnet for static magnetic field 1. The clear bore is the diameter of the opening.

[0010]

By the way, the large volume part of the MRI apparatus is the above-mentioned magnet apparatus. However, from the viewpoint of cost reduction and the place of installation, the magnet for static magnetic field 1 is required.
'S Kuriaboa D _c tends to be miniaturized. On the other hand, there is a demand for making the patient bore of the opening in which the subject is accommodated as wide as possible. Therefore, it is desired to increase the diameter D ₁ of the inner gradient magnetic field coil 3 of the shield gradient magnetic field coil.

As shown in FIG. 4, a passive shim using an iron piece is put to practical use. Since the iron shim assembly 2 is placed in the clear bore of the static magnetic field magnet 1, the clear bore D _c of the static magnetic field magnet 1 is further reduced. , it would be reduced to Kuriaboa D _s of the iron shim assembly 2.

The efficiency η of the shield gradient magnetic field coil is expressed by the following equation. η∝ ｛1- (D ₂ / D ₁ ) ^-4 ｝ ^-1 In order to improve the efficiency, the ratio D ₂ of the diameters of the two coils is
/ D ₁ needs to be increased, but iron shim assembly 2
It is limited to the Kuriaboa D _s.

The present invention has been made in view of the above points, and has as its object to improve the efficiency of the shield gradient magnetic field coil without changing the diameter of the clear bore of the static magnetic field magnet and the diameter of the inner gradient magnetic field coil. An object of the present invention is to realize an MRI superconducting magnet device provided with an iron shim static magnetic field correction means.

[0014]

According to the present invention, there is provided an inner gradient magnetic field for forming a gradient magnetic field by providing an iron shim assembly for correcting a magnetic field of the static magnetic field magnet in a clear bore of the static magnetic field magnet. MRI superconducting magnet device provided with iron shim static magnetic field correction means in which a shield gradient coil composed of a coil and an outer gradient magnetic field coil for canceling a gradient magnetic field generated outside thereof is provided in a clear bore of the static magnetic field magnet. , Wherein the diameter of the outer gradient magnetic field coil can be increased by mounting the iron shim assembly inside the outer gradient magnetic field coil.

[0015]

By attaching an iron shim assembly provided in the clear bore of the static magnetic field magnet to the inside of the outer gradient magnetic field coil, the diameter of the outer gradient magnetic field coil can be increased, and the outer gradient magnetic field coil can be connected to the outer gradient magnetic field coil. As the diameter ratio increases, the required current can be reduced by using outer gradient coils.

[0016]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a schematic configuration diagram of a magnet device according to one embodiment of the present invention. In the figure, parts performing the same operations as those in FIG. 4 are denoted by the same reference numerals. Let the diameter of the static magnetic field magnet 1 be D _c and the diameter of the iron shim assembly 2 be D _s . An inner gradient magnetic field coil (hereinafter referred to as an inner coil) 3 is a coil assembly having a diameter D ₁ having three kinds of coils of X, Y, and Z axes,
Similarly, an outer gradient magnetic field coil (hereinafter referred to as an outer coil) 4 is a coil assembly having three types of coils of X, Y, and Z axes, and the inner coil 3 and the outer coil 4 constitute a shield gradient magnetic field coil. The iron shim assembly 2 is a passive shim for correcting the magnetic field of the static magnetic field magnet 1 and has a thickness of about 0.1 mm and a size of about 2 × 10 cm so that an iron piece can be fixed at a predetermined position of about 300. It is a structure having a simple structure. The inner coil 3 and the outer coil 4 are connected in series and connected to a drive power supply (not shown). As a result, a gradient magnetic field is generated inside the inner coil 3, that is, in the imaging area, and an aluminum cylinder and a helium tank, which are heat shields of the portion of the static magnetic field magnet 1, are provided outside the outer coil 4. The coil current distribution of the outer coil 4 and the inner coil 3 is formed so that a magnetic field is not generated by the gradient coil.

The iron shim assembly 2 has an integral structure with the outer coil 4 and is fixed and supported inside the outer coil 4. The outer coil 4 has a support mechanism for the static magnetic field magnet 1, and the inner coil 3 is supported by an integrated structure of the iron shim assembly 2 and the outer coil 4.

Next, the operation of the apparatus of the embodiment configured as described above will be described. The inner coil 3 generates a gradient magnetic field inside, but the leakage magnetic field to the outside is canceled by the reverse current flowing through the outer coil 4, and the vicinity of the aluminum cylinder or the helium tank covering the static magnetic field magnet 1. Does not generate a gradient magnetic field.

Now, the strength of the gradient magnetic field when only the inner coil 3 is used
Degree G₀The current required to obtain ₀Then, outside carp
The same gradient magnetic field strength G₀Get
The current I 'required for this is: I' = I₀｛1- (D_Two/ D₁)^-Four｝^-1 ... (1)

This is because the behavior B _n of the magnetic field outside the gradient coil is B _n nr ^-3 (r is the radius of the coil), and the gradient magnetic field strength G _n inside is G _n ∝r ^-1. . FIG.
In, r ₁ radius of the inner coil 3, and the radius of the outer coil 4 and r _2, in order to zero the magnetic field at the point r _m, the current I ₂ flowing through the outer coil 4 of the coil 3 (r ₂ /
r ₁ ) ^-3 is sufficient. At this time, the gradient magnetic field generated inside the outer coil 4 is reduced by the ratio of the coil diameters, so that (r ₂ / r ₁ ) ⁻³ × (r ₂ / r ₁ ) ⁻¹ .

FIG. 3 is a curve of the exciting current ratio to the diameter ratio of the outer coil 4 and the inner coil 3 obtained from the equation (1). The iron shim assembly 2 is provided inside the outer coil 4 under the condition that the clear bore of the magnet for static magnetic field is constant from the surface of the whole volume and the diameter of the inner coil 3 is constant from the surface of the subject. Table 1 shows examples of dimensions of the present embodiment and a conventional magnet device.

[0022]

[Table 1]

In Table 1, (a) shows an example of the diameter of each part of the conventional example and the embodiment of FIG. 1, and (b) shows the conventional magnet apparatus shown in (a) and the present embodiment. A table showing the diameter ratio between the outer coil 4 and the inner coil 3 in the magnet device and the ratio of the exciting current of the equation (1) in the diameter ratio is shown in the table. It shows that the current required is 0.84 times that of the conventional coil.

As described above, according to this embodiment, since the iron shim assembly is provided inside the outer coil, the diameter of the outer coil can be increased, and the efficiency of the shield gradient magnetic field coil can be reduced. As a result, the current for obtaining the same gradient magnetic field strength can be reduced. Therefore, the current required to obtain a gradient magnetic field of the same strength using the magnet and the inner coil of the same size can be reduced, and if the same current need not be applied, the outer dimensions can be reduced. Will be able to

[0025]

As described above in detail, according to the present invention, it is possible to improve the efficiency of the shield gradient magnetic field coil without changing the diameter of the clear bore of the magnet and the diameter of the inner coil. The effect is great.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of an embodiment of the present invention.

FIG. 2 is an explanatory diagram of radii of an inner coil and an outer coil.

FIG. 3 is a curve diagram of an exciting current ratio with respect to a diameter ratio of an outer coil and an inner coil.

FIG. 4 is a schematic configuration diagram of a magnet device using a conventional shield gradient magnetic field coil.

[Explanation of symbols]

 Reference Signs List 1 magnet for static magnetic field 2 iron shim assembly 3 inner gradient magnetic field coil 4 outer gradient magnetic field coil

Claims (1)

(57) [Claims] 1. An inner gradient magnetic field coil (3) for providing a gradient magnetic field by providing an iron shim assembly (2) for correcting the magnetic field of the static magnetic field magnet (1) in a clear bore of the static magnetic field magnet (1). An MRI provided with iron shim static magnetic field correction means in which a shield gradient coil composed of an outer gradient magnetic field coil (4) for canceling a gradient magnetic field generated outside thereof is provided in a clear bore of the static magnetic field magnet (1). In the superconducting magnet device for use, the diameter of the outer gradient magnetic field coil (4) can be increased by mounting the iron shim assembly (2) inside the outer gradient magnetic field coil (4). MR with iron shim static magnetic field correction means
Superconducting magnet device for I.