JPS63249552A - Magnetic resonace imaging apparatus - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to magnetic resonance (MR) technology.
So-called computed tomography (CT) is based on the density distribution of specific atomic nuclear spins in a specific cross section of a living subject using the sonance phenomenon.
) to obtain a CT image (Computed Tomo3ram
) relates to a magnetic resonance imaging apparatus that performs imaging (I ma*in* ).

(Prior Art) For example, in a medical magnetic resonance imaging apparatus used for living body diagnosis, in order to obtain a tomographic image of a specific part of a living subject, a Z
A very uniform static magnetic field HO along the direction is generated and acted upon by a static magnetic field magnet (not shown), and a linear magnetic field gradient Gx is applied to the static magnetic field Ho by a pair of gradient coils 100A and 100B. Here, the specific atomic nucleus for the static magnetic field H6 has an angular frequency ω expressed by the linear equation
Resonates at 0.

ω0 = γI(O...(1) In this formula, γ is the gyromagnetic ratio, which is specific to the type of atomic nucleus.Therefore, in order to make only a specific atomic nucleus resonate, a rotating magnetic field H with an angular frequency ωO is applied. , are applied to the subject P via, for example, a pair of transmitting coils 200A and 200B provided within an RF coil (probe head).

In this way, due to the linear magnetic field gradient Gx, 2@
A specific slice portion S (a portion on a plane, but actually has a certain thickness) that selectively acts only on the illustrated x-y plane portion to obtain a tomographic image.
Magnetic resonance phenomena occur only in This magnetic resonance phenomenon is caused by, for example, a pair of receiving coils 30 provided within the RF coil.
Free induction decay signal (frOa) via 0A, 300B
1induction and θcay: hereinafter abbreviated as "rFID signal". ) and used as the MR signal. By Fourier transforming this FID signal, a single spectrum can be obtained for the rotational frequency of a specific nuclear spin.

To obtain a tomographic image as a CT image, x-
Projections in multiple directions within the y-plane are required. Therefore, after exciting the slice portion S to cause a magnetic resonance phenomenon, the magnetic field H8 has a linear inclination in the X' axis direction (coordinate system rotated by an angle O from the X axis) as shown in Figure 5. When the linear magnetic field gradient GXY is applied by a gradient coil (not shown), the isomagnetic field line E in the sliced portion S of the subject P becomes a straight line, and the rotational frequency of a specific nuclear spin on this isomagnetic field line E is expressed by the above equation 0). Hail.

Here, for convenience of explanation, it is assumed that the equimagnetic field lines E are E, -En, and the magnetic fields on these equimagnetic field lines E1 - En generate signals D4 - Dn, which are a type of FID signal, respectively. The amplitudes of the signals D~D0 are proportional to the spin densities of specific atomic nuclei on the equal magnetic field 1&1aE1~En penetrating the slice portion S, respectively. However, it was actually l1118N! l
The resulting FID signal is a composite FID signal that is a combination of all the signals D1 to Do. Therefore, projection information (one-dimensional image) PD of the slice portion S onto the X' axis is obtained by Fourier transforming the composite FID signal. Next, set this X' axis to x-
It is rotated in the y plane, but this is caused by, for example, the magnetic field gradient GX in the XtY direction due to two pairs of gradient coils.
This is done by creating a magnetic field gradient aXV as a composite magnetic field of +GY and changing the composite ratio of the magnetic field gradient G X t G Y. By rotating this magnetic field gradient GXy, projection information in the angular direction within the xy plane is obtained in the same manner as described above, and a CT image is synthesized based on this information.

The above is the principle of magnetic resonance imaging. Next, as a specific example, a conventional magnetic resonance imaging apparatus is shown in FIG. The subject, patient #1, is placed on bed 2. Surrounding this patient 1 is an RF coil (probe head: high frequency transmitting/receiving coil) 3, and further around the periphery is a shim coil 4 for magnetic field correction. A gradient coil 5 for generating a gradient magnetic field is arranged. All these coil systems are
As a 6 static field magnet housed inside a room-temperature bore 7 (usually bore inner diameter of about 1 m) of a large static field magnet 6,
Either a superconducting magnet, a normal conducting magnet, or a permanent magnet is used.

This static field magnet 6 is excited and demagnetized by an excitation power source 8 via a current lead 9 (this is not necessary in the case of a permanent magnet system). In the case of superconducting magnets, the current lead 9 is usually removed after excitation in order to operate in persistent current mode and to reduce consumption of liquid helium, which is a coolant.
A magnetic field is constantly being generated. Normally, the direction of this static magnetic field is the 10 directions shown in the figure for many magnets,
That is, the direction is the body axis direction of the patient 1. The gradient coil 5 is composed of a GX coil that provides a magnetic field gradient in the X@force direction, a GY coil in the Y-axis direction, and a GZ coil in the Z-axis direction.
They are connected to excitation power sources 11, 12, and 13, respectively.

These excitation power supplies 11 , 12 , 13 are connected to a central control unit 14 . The RF coil 3 is composed of a transmitter coil and a receiver coil, which are connected to an RF oscillator 15 and an RF receiver 16, respectively, and these are further connected to a central controller 11.
Connected to 4.

The central control device 14 is connected to a display/operation panel 17, and is operated by this.

Next, the operation of the conventional magnetic resonance imaging apparatus configured as shown in E will be described.

In order to obtain a whole body cross-sectional image of the patient 1, the magnetic field uniform space 18
The sphere is usually 40 to 50 cm wide, and high uniformity of 50 ppm or less is required. For this reason, the static field magnet 6 needs to be huge, for example, in the case of a superconducting system, with a length of 2.4 m, a width of 2 m, a height of 2.4 m, and a weight of 5 to 6 tons.

Even with such a large magnet, the uniformity within the 40 to 50a++ sphere due to the magnet alone is only a few hundred ppm at most. In order to reduce this to 50 ppm or less, a shim coil 4 for magnetic field correction is used.

The patient's diagnostic site is brought into this magnetic field uniform space 18. and an RF oscillator 15 in a direction perpendicular to the static magnetic field lO,
A high frequency is applied by the RF coil 3 to excite desired atomic nuclei, such as hydrogen atomic nuclei, within human cells. Similarly, gradient magnetic fields are applied in the x, y, and z directions by the GX excitation Iti source 11, the GY excitation power supply 12, the GZ excitation power supply 13, and the gradient coil 5.

The optimum RF and gradient pulse sequence is selected depending on the lesion site and image processing method.

This pulse sequence operation is controlled by the central control bag [14]. After applying the gradient and RF, a magnetic resonance signal is emitted from inside the patient's body. This signal is received and amplified by the RF receiver 16 and input to the central control bag [14]. Here, the image is processed and the required human body tomographic image is displayed on the CRT of the display/operation panel 17.

By the way, the static magnetic field magnets used in conventional magnetic resonance imaging devices configured in this way are installed inside hospital buildings, so leakage magnetic fields from the static magnetic field magnets are minimized to prevent negative magnetic effects on the surrounding environment. A magnetic shield is attached to the magnet to eliminate this.

FIG. 7 shows a magnetic shield of a conventional static field magnet.

The magnetic shield 19 is composed of a cylindrical shell 20 surrounding the static field magnet 6, two disc-shaped end caps 21 and legs 22 attached to both ends of the cylindrical shell. . The magnetic shield 19 is made of a magnetic material such as iron.

The static field magnet 6 is housed inside the magnetic shield 19 and arranged so that the magnetic center axis 23 of the static field magnet and the vertical axis 24 of the cylindrical shell 20 are coaxial.

There are generally various protrusions 25 on the surface of the static field magnet.

For example, when using a superconducting magnet as a static field magnet,
A heat insulating support rod made of FRP or the like is used to support a cryogenic refrigerant tank in a cold storage container, generally called a cryostat, from the normal temperature part of the outer circumferential surface of the cold storage container. In this case, in a typical superconducting magnet tank structure, the normal temperature side of the heat insulating support rod has a structure as shown in the figure as a protrusion.

For such a projection 25, the cylindrical shell 20 has an outer diameter dimension large enough to completely enclose the projection.

This situation is shown in FIG. 8, which is a view of FIG. 7 from the longitudinal axis of the cylindrical shell. That is, the cylindrical shell has a larger outer diameter than the static field magnet by the size of the protrusion.

Reference numeral 26 in FIG. 8 indicates a service port when the static field magnet is a superconducting magnet. The service port 26 is a part where a coolant (for example, liquid helium) is injected to maintain the superconducting state, and it is inconvenient for maintenance work to be surrounded by the magnetic shield 19. Therefore, as shown in the figure, the cylindrical shell of this part is It's cut out.

Next, the effects of the magnetic shield of the conventional static field magnet for magnetic resonance imaging apparatus configured as described above will be described.

The static field magnet 6 generates a magnetic flux 27 as shown.

The magnetic flux 27 exiting the normal temperature bore 7 of the static field magnet is absorbed by a magnetic shield 19 made of a magnetic material such as iron. The amount of magnetic flux that a magnetic shield can absorb is limited by the magnetic saturation properties of the magnetic material. The magnetic flux that cannot be absorbed leaks to the outside of the magnetic shield and becomes a so-called leakage magnetic field.

When a magnetic shield is attached to a static field magnet, the above-mentioned leakage magnetic field is reduced to a large [11] compared to a static field magnet alone.

Figure 9 shows this situation.

FIG. 9 shows the 5 Gauss leakage magnetic field distribution in the case of only a 5ooo Gauss static magnetic field magnet and in the case of a magnetic shield. As is clear from this figure, when the magnetic shield is attached, the 5 Gauss leakage magnetic field area is halved.

This has the following effects on hospital sites where magnetic resonance imaging equipment is installed.

■ The leakage magnetic field control area for hospitals is tentatively set at 5 Gauss. This is based on the fact that the permissible magnetic field of pacemakers worn by patients with heart disease is 5 Gauss. 5
Since the Gaussian region is narrower, the installation area of the device is smaller and it can be installed in existing hospital rooms.

■ Static field magnets can be installed even if there is equipment sensitive to magnetic fields nearby.

(3) Static field magnets for magnetic resonance imaging devices are required to have magnetic field uniformity on the order of ppm in the diagnostic space.

Floor reinforcement is used in the environment where static field magnets are installed.

Ferromagnetic materials such as column reinforcement are present. These significantly deteriorate magnetic field uniformity. A magnetic shield can prevent the influence of these external magnetic bodies. Therefore, it can be installed in existing hospital rooms without special installation considerations.

(Problems to be Solved by the Invention) By the way, the magnetic shielding body of the conventional static magnetic field magnet for magnetic resonance imaging apparatus having the above-mentioned structure and the above-mentioned effects has the following problems.

Since the magnetic shield is configured to completely surround the protrusions on the surface of the static magnetic field magnet, the external dimensions of the magnetic shield become large and the static magnetic field magnet itself with the magnetic shield increases in size. For this reason, ■ When transporting a static magnetic field magnet with a magnetic shield to a hospital to the equipment installation site, if the outer diameter is large (for example, over 2 m), it will not be possible to pass through the hospital corridors and it will not be possible to install it inside the hospital. .

■ As the outer diameter increases, the height of the static field magnet as a whole also increases. If the static field magnet is a superconducting magnet, liquid helium must be periodically injected to maintain the extremely low temperature. Since the service port with this liquid injection port is usually located at or near the top of the static field magnet, the height of this service port from the floor is high. Generally, the ceiling height of hospitals is not very high.

(For example, 3 m or less) Therefore, it is desired that the height of the static field magnet be as low as possible in consideration of liquid injection work.

If a magnetic shield is installed using the conventional method, it will be too high and there will be cases where fluid injection cannot be performed inside the hospital room. In other words, even if a static field magnet is installed, it will not be possible to operate.

■ As the outer diameter increases, the weight of the magnetic shield, which is a block of iron, increases. There are cases where the floor load capacity limit (for example, 10 tons) of a hospital building is exceeded.

In this case, a static field magnet with a magnetic shield cannot be installed.

As mentioned above, although the installation of a magnetic shield has the advantage of reducing the leakage magnetic field area, due to the above-mentioned three-terminal problem due to the increase in outer diameter, static magnetic field magnets with magnetic shields A decisive drawback arises in that it cannot be installed in an existing hospital building.

For this reason, the current response is to construct a new building exclusively for magnetic resonance imaging equipment and install the equipment there. However, in this case, the cost of incidental construction is higher than the cost of purchasing the device, resulting in an extremely expensive medical diagnostic device as a whole. This current situation is one of the major factors hindering the spread of magnetic resonance imaging devices.

(Object of the invention) Therefore, the object of the present invention is as follows. The device is made more compact by adopting a structure that minimizes the increase in outer diameter when attaching a magnetic shield to a static field magnet.

The object of the present invention is to provide a magnetic resonance imaging device that is lightweight and can be easily transported and installed in existing hospital buildings.

[Structure of the invention]

(Means for Solving the Problems) The magnetic imaging apparatus according to the present invention is configured as follows in order to solve the above problems and achieve the objects.

That is, as shown in FIGS. 1 and 2, recesses are provided in the magnetic shield 19 at positions corresponding to the surface protrusions 25 of the static field magnet 6 so that these protrusions can fit with an appropriate gap 28. As a result, when the static magnetic field magnet 6 and the magnetic shield 19 are combined, the protrusion 25 and the magnetic shield recess 29 are engaged with each other, and the internal diameter 30 of the magnetic shield can be made to almost match the external diameter 31 of the static magnetic field magnet. Structure.

(effect) By configuring in this way, according to the first feature.

The increase in the outer diameter of the static field magnet with magnetic shield can be minimized, making the device compact and lightweight, allowing it to be easily transported and installed in existing hospital buildings.

(Embodiment) An embodiment of the magnetic resonance imaging apparatus of the present invention will be described below with reference to FIGS. 1 and 2.

(Configuration of Example) FIG. 1 is a diagram showing the configuration of this example. FIG. 2 is a sectional view of FIG. 1 when viewed from the direction of the magnetic central axis 23 of the static field magnet 6. FIG.

The magnetic shield 19 is composed of a cylindrical shell 20 surrounding the static field magnet 6, two disc-shaped end caps 21 and legs 22 attached to both ends of the cylindrical shell.

The static magnetic field magnet 6 is housed inside the magnetic shield 19, and the magnetic central axis 23 of the static magnetic field magnet and the vertical axis R of the cylindrical shell 20
1; Arranged so that it is coaxial with A24.

Here, the magnetic shield 19 is provided with a recess 29 at a position corresponding to the surface protrusion 25 of the static field magnet 6, so that the protrusion 25 can fit into the protrusion 25 with an appropriate gap 28. At this time, the protrusion 25 does not penetrate the magnetic shield,
A static field magnet 6 having a structure in which a magnetic path 32 remains in the recess.
When the magnetic shield 19 is combined with the protrusion 25
The magnetic shield recess 29 and the magnetic shield concave portion 29 fit together, and the magnetic shield is attached to the surface of the static magnetic field magnet in almost intimate contact with the magnetic shield.

(Operation of the Example) Next, the operation of the magnetic shield of the static field magnet for a magnetic resonance imaging apparatus of the present example configured as described above will be explained.

As described above, since the surface of the static field magnet can be attached with the magnetic shield in close contact with the magnetic shield, an increase in the outer diameter dimension due to the attachment of the magnetic shield can be minimized.

Further, since the magnetic path 32 remains in the pedestal portion of the protrusion 25 and the magnetic shield recess 29, there is no interruption of the magnetic path or asymmetrical magnetic flux due to the provision of the mating portion.

(Effects of Example) As explained above, this example has the following effects.

(g) Since the increase in the outer diameter is minimized and the device is compact, ■ it can be transported through hospitals with relatively narrow corridors.

・■ Since the height from the floor to the service port can be lowered, liquid work can be done within the normal hospital ceiling height.

■ Since the weight of the magnetic shield is reduced, the device can be installed without being restricted by floor load restrictions in hospital buildings.

■ Since the magnetic path remains, ■ A 5 Gauss leakage magnetic field region equivalent to that of the conventional model can be obtained.

■ Since the magnetic path is not asymmetrical, the uniformity of the magnetic field in the diagnostic area is maintained.

(Other examples) Next, another embodiment of the present invention will be described with reference to FIG.

The same parts as in the embodiment shown in FIGS. 1 and 2 are given the same reference numerals, and their explanation will be omitted.

(Configuration of Other Embodiments) This embodiment is a case where the magnetic shield has a divided structure for assembling the magnetic shield. Although FIG. 3 shows the case of two divisions, the operation and effect will be the same as described below even if there are two or more divisions.

The cylindrical shell body is divided into two parts along the circumferential direction at the center thereof, forming cylindrical shell bodies 34 and 35, respectively. Each cylindrical shell has a leg 22 and a joint 36 by which the cylindrical shells 34, 35 are joined together by bolting 37 or the like.

Cylindrical shell 34°3 relative to the protrusion 25 of the static field magnet 6
The concave portion 5 is extended in the longitudinal axis direction of the cylindrical shell and has a protrusion 25.
The groove 33 is large enough to slide.

In the example shown in FIG. 3, the groove 33 extends from the protrusion 25 toward the center of the static field magnet, but even if this groove passes through the cylindrical shell from one end to the other in the longitudinal direction, the effect described below can still be achieved. The effect is the same.

(Function of other embodiments) When the split type magnetic shield is combined with the 17yItI field magnet, the increase in the outer diameter can be minimized. or.

Since the magnetic path remains in the groove, there is no interruption of the magnetic path or asymmetrical magnetic flux.

Moreover, this groove acts as a guide when assembling the divided cylindrical shells.

(Effects of other embodiments) In addition to having the same effects as the first embodiment shown in Figs. 1 and 2, ■ the static field magnet and the magnetic shield can be transported separately and assembled on site. This allows the equipment to be transported and installed even if the hallway is extremely narrow or the floor load capacity is severely restricted.

■ Since the groove can be used as a guide when assembling the magnetic shield, it can be easily assembled without using special assembly jigs to ensure assembly accuracy.

〔Effect of the invention〕

As described above, according to the present invention, the outer diameter dimension can be minimized when attaching a magnetic shield to a static magnetic field magnet, so that the static magnetic field magnet with a magnetic shield can be made compact and lightweight, and can be installed in existing hospital buildings. It can be easily transported and installed.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing one embodiment of the magnetic resonance imaging apparatus according to the present invention, FIG. 2 is a sectional view of the same embodiment, FIG. 3 is a block diagram showing another embodiment, and FIGS. Fig. 5 is a diagram showing the principle of magnetic resonance imaging, Fig. 6 is a block diagram showing the system of a conventional magnetic resonance imaging apparatus, Fig. 7 is a block diagram of a conventional magnetic resonance imaging apparatus, and Fig. 8 is a conventional example of the same. FIG. 9 is a sectional view of the leakage magnetic field. 19... Magnetic shield 2o... Cylindrical shell 21
...Disc-shaped end cover 23...Magnetic center axis 25...
・Protrusion 26...Service port 28...
・Gap 29...Magnetic shielding body part 30・
・Magnetic'' [Tread inner diameter 3I...Static magnetic field magnet outer diameter 32
...magnetic path 33...grooves 34, 35...
Divided cylindrical shell 36...Joint portion 37...Bolt tightening agent Patent attorney Noriyuki Chika Ken Yudo Daishimaru Ken Figure 1 Figure 2 100A Figure 4 Figure 6 Figure 9

Claims (3)

[Claims] (1) A magnetic shield consisting of a cylindrical shell made of a magnetic material and two end caps made of a magnetic material fixed to both ends and having a central opening surrounds the static magnetic field magnet, In a magnetic resonance imaging device in which the axis and the magnetic center line of the static magnetic field magnet are coaxial, the magnetic shield has a recess that engages with the protrusion on the surface of the static magnetic field magnet, and the magnetic shield is almost tightly attached to the surface of the static magnetic field magnet. A magnetic resonance imaging apparatus characterized in that: (2) The magnetic resonance imaging apparatus according to claim 1, wherein the recess of the magnetic shield is a groove extending in the axial direction of the cylindrical shell. (3) The magnetic resonance imaging apparatus according to claim 2, wherein the groove formed in the cylindrical shell is used as an assembly guide groove when assembling the magnetic shield.