JPH04224734A - Magnetic resonance imaging apparatus - Google Patents

Abstract

PURPOSE:To lessen a sense of pressing felt by a person to be inspected by reducing a dead space between an inclined magnetic field coil and a magnetic circuit to allow a larger space for housing the person to be inspected. CONSTITUTION:An inclined magnetic field coil 30 is arranged in a magnetic circuit 35 having a polygonal opening section. A sectional shape of the inclined magnetic field coil 30 is made almost similar to an opening section of the magnetic circuit 35. With such an arrangement, there is no dead space generated between the magnetic circuit 35 and the inclined magnetic field coil 30, thereby ensuring a broad space for housing a person to be inspected making the best of the opening of the magnetic circuit.

Description

[Detailed description of the invention]

0001

[Industrial Field of Application] The present invention relates to a magnetic resonance imaging apparatus (hereinafter abbreviated as MRI apparatus), and in particular, the present invention relates to a magnetic resonance imaging apparatus (hereinafter abbreviated as MRI apparatus), and in particular, to reduce wasted space between a gradient magnetic field applying means and a static magnetic field applying means, and to accommodate a patient under examination. The present invention relates to an MRI apparatus equipped with a gradient magnetic field applying means having a structure that allows for a wide space.

0002

[Prior Art] An MRI apparatus displays the density distribution, relaxation time distribution, etc. of nuclear spins in a subject as a tomographic image by processing signals measured using NMR phenomena. . In order to generate this NMR phenomenon,
A static magnetic field with spatially uniform strength and direction is required. Depending on the direction of the static magnetic field relative to the subject, MRI apparatuses can be roughly divided into two types: vertical magnetic field type and horizontal magnetic field type. Among these, the vertical magnetic field method, in which a static magnetic field is applied in a direction perpendicular to the subject's body axis, has the advantage that a solenoid coil with high detection sensitivity can be used for signal detection. As a vertical magnetic field type magnetic circuit, a type using permanent magnets is exclusively used. The permanent magnet method has the advantage of having little leakage magnetic field and easy installation of the device. In addition, unlike other normal conductivity and superconductivity systems, once magnetized, it does not require electricity or coolant such as helium to generate a magnetic field, so it has the excellent feature of low running costs. .

Among the perpendicular magnetic field type magnetic circuits using permanent magnets, examples of what is currently the most popular type of magnetic circuit are shown in FIGS. 9 and 10. FIG. 9 shows a perspective view of the magnetic circuit, and FIG. 10 shows a longitudinal sectional view thereof. This magnetic circuit 5 supports and magnetically couples a pair of permanent magnet structures 2a and 2b that are arranged facing each other with an interval large enough for the subject to enter. They are connected by plate yokes 3a, 3b and columnar yokes 4 to generate a static magnetic field. Furthermore, magnetic pole pieces 1a and 1 are provided on opposing surfaces of the pair of permanent magnet structures 2a and 2b, respectively.
b is fixed. The magnetic pole pieces 1a and 1b are provided to improve the uniformity of the static magnetic field distribution within the air gap containing the subject. Regarding its structure, for example, JP-A-60-
88407, etc., a shape in which the peripheral portion is raised and the inside is depressed is usually used.

On the other hand, in addition to the above-mentioned static magnetic field, an MRI apparatus uses a gradient magnetic field whose magnetic field strength varies spatially in order to obtain position information of a received signal. This gradient magnetic field needs to correspond to the three directions of X, Y, and Z, corresponding to the position in the three-dimensional space. In the figure, the direction of the static magnetic field is taken as the Z axis.
, Y, and Z. Each gradient magnetic field is created by a pair of opposing gradient magnetic field coil groups 9a and 9b. Although not shown in this figure to avoid complication, each of the gradient magnetic field coils 9a and 9b is
It is composed of three gradient magnetic field coils corresponding to the three directions of X, Y, and Z. In order to reduce the feeling of pressure on the subject and to facilitate the operator's handling of the subject, it is desirable that the magnetic circuit opening (L) be as wide as possible. In order to take advantage of the width of the magnetic circuit aperture, for example,
64638, the gradient magnetic field coil groups 9a, 9b are attached inside the recesses formed by the magnetic pole pieces 1a, 1b.

By the way, permanent magnet materials are very expensive,
In order to suppress the manufacturing cost of the device, it is necessary to reduce its usage as much as possible. To this end, it is important to improve the efficiency of magnetic field generation, and various tunnel-type magnetic circuits with structures different from that shown in FIG. 9 have been studied (for example, Philips J. Res. 40, p.
p. 259-288 (1985) or IEEE
TRANSACTIONS ON MAGNETICS
, VOL. 25, No. 5, pp. 3904-390
6 (1989) etc.).

An example is shown in FIG. This tunnel type magnetic circuit 6 includes a permanent magnet structure 11 as a magnetic field generation source, a yoke 10 made of pure iron or the like to hold them and pass magnetic flux, and a yoke 10 made of pure iron or the like to improve the uniformity of the static magnetic field. It consists of a magnetic pole piece 12. This type of magnetic circuit is expected to improve magnetic field generation efficiency by 20 to 40% compared to the type shown in FIG.

[0007] In addition to the above, there is also an RF irradiation coil that irradiates high-frequency pulses to manipulate the spin of atomic nuclei, a receiving coil that receives NMR high-frequency signals from the subject, and various types of coils based on the received signals. An image reconstruction device for calculating images is required.

[0008]

[Problems to be Solved by the Invention] As mentioned above, in recent years, various tunnel-type magnetic circuits have been studied with the aim of improving the efficiency of magnetic field generation. For the gradient magnetic field coils used in this type of magnetic circuit, the method used in magnetic circuits using superconducting magnets or normal conducting magnets can be applied. That is, a gradient magnetic field coil having a cylindrical outer shape is used. However, while the direction of the static magnetic field is horizontal in superconducting magnets, it is vertical in permanent magnet systems. Naturally, therefore, the pattern of the gradient magnetic field coil must be designed accordingly, but this can be easily done using existing computer simulations. In a magnetic circuit using a superconducting magnet or a normal conducting magnet, its opening cross section is also cylindrical, and its central axis is concentric with the gradient magnetic field coil. Therefore, the distance between the inside of the magnetic circuit and the gradient magnetic field coil is constant everywhere, and there is no wasted space.

However, in the case of a tunnel-type magnetic circuit using permanent magnets, since the magnet material has a cubic or similar shape, the opening cross section is not circular, but as shown in FIG. becomes a polygonal shape. For this reason, wasted space is created between the magnetic circuit 6 and the gradient magnetic field coil 30. In other words, the dimensions of the space into which the subject enters are defined by the diameter of the gradient coil 30. For this reason, a problem arises in that the cross-sectional area of the space into which the subject enters becomes smaller than the cross-sectional area of the opening of the magnetic circuit 6, and the sense of pressure experienced by the subject increases.

In order to solve this problem, it is possible to consider a method of increasing the aperture cross section of the magnetic circuit. However, the manufacturing cost of the magnetic circuit largely depends on the aperture cross-sectional area, and increasing this will result in a significant increase in the price of the MIR device. Therefore, it cannot be an effective means to solve this problem. SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems and to provide an inexpensive MRI apparatus in which a subject feels less pressure even in the case of a tunnel-type magnetic circuit using permanent magnets.

[0011]

[Means for Solving the Problems] In order to achieve the above object, there is provided a static magnetic field applying means for applying a spatially uniform static magnetic field to an inspection object, and gradient magnetic fields in three directions orthogonal to each other on the inspection object. In the magnetic resonance imaging apparatus, the magnetic resonance imaging apparatus is provided with a gradient magnetic field applying means disposed on the inner surface of the static magnetic field applying means to apply a magnetic field, and a high frequency pulse applying means necessary for magnetic resonance imaging, a signal detecting means, and an image reconstructing means. The gradient magnetic field applying means is configured so that its outer cross-section has a substantially similar shape to the inner cross-sectional shape of the static magnetic field applying means.

[0012] Also, a static magnetic field applying means for applying a spatially uniform static magnetic field to the test object, and an inner surface of the static magnetic field applying means for applying gradient magnetic fields in three mutually orthogonal directions to the test object. In the magnetic resonance imaging apparatus, the gradient magnetic field applying means has an elliptical cross-section. It is configured as follows. Furthermore, in the preamble, the external cross section of the gradient magnetic field applying means is constructed by combining a straight portion and a curved portion.

[0013]

[Operation] By configuring the gradient magnetic field applying means as described above, wasted space between the static magnetic field applying means and the gradient magnetic field applying means can be reduced, and a substantial space for accommodating the subject can be secured. It is possible to easily and inexpensively realize an MRI apparatus that causes less pressure on a subject.

[0014]

Embodiments Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 12 shows a perspective view of a conventional gradient magnetic field coil 30 used for a tunnel-type magnetic circuit 6. As shown in FIG. In the case of a tunnel-type magnetic circuit 6, an XYZ orthogonal coordinate system 51 is used in which the Z axis is set in the direction of the central axis of the magnetic circuit, as is customary. Therefore, the direction of the static magnetic field is the X or Y direction (in the case of FIG. 11, the Y direction). In Figure 12, Z
On a cylindrical framework 33 having a central axis 32 in the direction,
Three types of gradient magnetic field coils for each direction of Y and Z are formed. The figure shows a pattern 34 in only one direction.

As described above, if this gradient magnetic field coil 30 is used in the magnetic circuit 6 of FIG. 11, there will be a lot of wasted space, and the subject will feel a great sense of pressure. Furthermore, if the method of providing the gradient magnetic field coils 9 above and below as shown in FIG. 9 is used for the magnetic circuit 6 of FIG. 11, the required spatial linearity and generation efficiency of the gradient magnetic field cannot be obtained. This is because in the case of the magnetic circuit of the type shown in FIG. 11, the aperture width W is considerably smaller than that in the case of FIG. In order to design a gradient coil of this type with good performance, a fairly large coil diameter D is required. This coil diameter D depends on the linearity and generation efficiency of the required gradient magnetic field, but
A length of at least 700 mm is required. However, setting the aperture width W to 700 mm or more in the magnetic circuit of the type shown in FIG. 11 is very disadvantageous in terms of the magnetic field generation efficiency of the magnetic circuit, and is actually impossible when manufacturing costs are considered.

Next, an example of a gradient magnetic field coil according to the present invention is shown in FIGS. 1 and 2. FIG. 1 is a perspective view of the gradient magnetic field coil 30, and FIG. 2 shows the gradient magnetic field coil 30 in a magnetic circuit 35.
FIG. In FIG. 2, the magnetic circuit 35 is simplified and only its outer shape is depicted. In the present invention, the outer cross section of the gradient magnetic field coil 30 is not circular, but has a shape similar to the inner cross section of the magnetic circuit 35. In the example shown here, since the inner cross section of the magnetic circuit 35 is square, the outer cross section of the gradient magnetic field coil 30 is also square. By configuring the gradient magnetic field coils in this way, as shown in FIG. 2, there is no wasted space between the gradient magnetic field coils 30 and the inside of the magnetic circuit 35, and a wide subject accommodation space 32 can be secured. can.

A specific method for manufacturing a gradient magnetic field coil is a method of winding a copper wire in a required pattern 34 on a non-conductive frame 33, as is known so far.
For example, JP-A-60-94705) can be used. Also, as shown in FIG. 3, each surface of the gradient magnetic field coil (for example, surface 36
In order to smoothen the pattern connection between the surfaces 37) and 37), it is effective to slightly round the joint portion 38 of each surface.

Methods for determining the pattern shape of a coil with good characteristics include Japanese Patent Application Laid-open No. 59-157549 and Japanese Patent Application Laid-open No. 1-22.
A method known for example 0806 can be applied. That is, the pattern shape of the coil is expressed by a certain function, and the magnetic field distribution generated by the current flowing therein is simulated by computer. Parameters included in the pattern shape function are successively changed slightly until the magnetic field distribution and magnetic field generation efficiency fall within a desired range, and the optimum values are determined.

Although FIG. 2 shows an example in which the inner cross section of the magnetic circuit is square, the present invention is not limited thereto and can be applied to any polygonal cross section. That is, the gist of the present invention is to reduce the unnecessary space between the inner cross section of the magnetic circuit and the outer shape of the gradient magnetic field coil, and to make the subject accommodation space as wide as possible. For example, the present invention is similarly applicable to magnetic circuits in which the inner cross section is rectangular (FIG. 4) or hexagonal (FIG. 5). The same applies to the case where the lengths of the sides are unequal as shown in FIG. Naturally, the external cross-section of the gradient magnetic field coil is also similar to the shape shown in each of FIGS. 4 to 6. As shown in Figures 4 and 6 above, the direction of the opening (
If b) is set wide, a sense of openness will be given to the subject. Furthermore, since the body can be moved within the subject accommodation space, there is an advantage that the region of interest can be set at the center of the magnetic field. At the same time, by limiting the opening (a) in the other direction to the same width as the conventional one, the manufacturing cost of the magnetic circuit can be suppressed.

Another embodiment of the invention is shown in FIG. This embodiment can be applied when the inner cross section of the magnetic circuit is longer in one direction than in another direction, as shown in FIGS. 4 and 6. In this case, the outer cross section of the gradient magnetic field coil 30 can be made elliptical. That is, the magnetic circuit 3 with this shape
5, even if the outer shape of the gradient magnetic field coil 30 is made into an ellipse rather than a polygon that matches the magnetic circuit, the wasted space 20 between the gradient magnetic field coil and the magnetic circuit can be reduced. Furthermore, by making it elliptical, there are no parts that bend with a small curvature, so there are advantages such as ease of manufacturing and improved mechanical strength.

Still another embodiment of the present invention is shown in FIG. In this embodiment, the external cross section of the gradient magnetic field coil 30 is not an ellipse, but the upper and lower portions 40 are straight lines, the left and right portions 41 are curved lines, and the two are smoothly connected. By doing so, the area of wasted space 20 can be further reduced than in the case of FIG. Furthermore, it is convenient during manufacturing to use a shape that is easy to calculate, such as a circular arc or a parabola, for the curved portion. Although FIGS. 7 and 8 show the case where the inner cross section of the magnetic circuit 35 is wide in the horizontal direction, the same is true even when it is long in the vertical direction. Furthermore, although the tunnel-type magnetic circuit 6 has been used in the explanation so far, the present invention is naturally applicable to the opposed-type magnetic circuit 5 shown in FIG.

[0022]

[Effects of the Invention] According to the present invention, wasted space between the static magnetic field applying means and the gradient magnetic field applying means can be reduced, and a wide subject accommodation space can be obtained by making use of the opening cross-sectional area of the static magnetic field applying means. It will be done. For this reason, MR gives less pressure to the patient.
I device can be realized. Moreover, it can be realized at low cost without increasing the cost of the device compared to the conventional technology.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing the structure of a gradient magnetic field coil according to the present invention.

[Fig. 2] A perspective view showing the relationship between a gradient magnetic field coil and a magnetic circuit according to the present invention.

FIG. 3 is a perspective view showing the structure of a gradient magnetic field coil according to the present invention.

[Fig. 4] A perspective view of a magnetic circuit showing the scope of application of the present invention.

[Fig. 5] A perspective view of a magnetic circuit showing the scope of application of the present invention.

[Figure 6]
A perspective view of a magnetic circuit showing the scope of application of the present invention

FIG. 7 is a perspective view of a gradient magnetic field coil and a magnetic circuit showing another embodiment of the present invention.

FIG. 8 is a perspective view of a gradient magnetic field coil and a magnetic circuit showing still another embodiment of the present invention.

[Fig. 9] A perspective view showing a magnetic circuit and gradient magnetic field coil according to the prior art.

[Fig. 10] Longitudinal cross-sectional view showing a magnetic circuit and gradient magnetic field coil according to the prior art

FIG. 11 is a perspective view showing a magnetic circuit according to another conventional technology.

[
FIG. 12 is a perspective view showing a gradient magnetic field coil according to the prior art.

[Explanation of symbols]

1 Magnetic pole piece 2 Permanent magnet structure 3 Plate yoke 4 Column yoke 5 Opposed magnetic circuit 6 Tunnel magnetic circuit 9 Gradient magnetic field coil group 10 Yoke 11 Permanent magnet structure 12 Magnetic pole piece 20 Wasted space 30 Gradient magnetic field Coil 31 Wasted space 32 Central axis of gradient magnetic field coil 33 Frame of gradient magnetic field coil 34 Pattern of gradient magnetic field coil 35 Magnetic circuit 36 Surface 37 forming gradient magnetic field coil Surface 38 forming gradient magnetic field coil Surface joint part 40
Straight section 41 of gradient magnetic field coil outline section Curved section 50 of gradient magnetic field coil outline section 50 Orthogonal coordinate system 51 Orthogonal coordinate system

Claims (3)

[Claims] 1. A static magnetic field applying means for applying a spatially uniform static magnetic field to an object to be inspected; and an inner surface of the static magnetic field applying means for applying gradient magnetic fields in three mutually orthogonal directions to the object to be inspected. In a magnetic resonance imaging apparatus, the external cross section of the gradient magnetic field applying means is the same as that of the static magnetic field applying means, and a high-frequency pulse applying means, a signal detecting means, and an image reconstruction means necessary for magnetic resonance imaging. A magnetic resonance imaging apparatus characterized in that it is configured to have a shape substantially similar to the inner cross-sectional shape of the means. 2. A static magnetic field applying means for applying a spatially uniform static magnetic field to an object to be inspected; and an inner surface of the static magnetic field applying means for applying gradient magnetic fields in three mutually orthogonal directions to the object to be inspected. In the magnetic resonance imaging apparatus, the gradient magnetic field applying means has an elliptical cross-section. A magnetic resonance imaging apparatus characterized in that it is configured as follows. 3. The magnetic resonance imaging apparatus according to claim 2, wherein the external cross-section of the gradient magnetic field applying means is configured by combining a linear portion and a curved portion.