JPH0580903B2 - - Google Patents

Description

[Detailed Description of the Invention] [Object of the Invention] (Field of Industrial Application) The present invention is directed to magnetic resonance (MR).
The so-called computer tomography (CT) is based on the density distribution of specific nuclear spins in a specific cross section of a living subject using the resonance phenomenon.
Imaged as a CT image (Computed Tomogram) using I
magnetic resonance imaging apparatus.

(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, it is necessary to move the subject P in the Z direction as shown in FIG. A very uniform static magnetic field H ₀ along the magnetic field is generated and acted upon by a static magnetic field magnet (not shown), and a linear magnetic field gradient G _X is applied to the static magnetic field H ₀ by a pair of gradient coils 100A and 100B. Here, static magnetic field H ₀
The specific atomic nucleus for , resonates at the angular frequency ω ₀ given by the following equation.

ω ₀ =γH ₀ ...(1) In this equation (1), γ is the gyromagnetic ratio, which is unique to the type of atomic nucleus. So further,
For example, a pair of transmitting coils 200A, 200 installed in an RF coil (probe head) generates a rotating magnetic field _H1 with an angular frequency _ω0 that resonates only a specific atomic nucleus.
It is made to be used by the subject P via B.

In this way, the linear magnetic field gradient _G The magnetic resonance phenomenon occurs only when the material has a certain thickness (in reality). This magnetic resonance phenomenon occurs when, for example, a pair of receiving coils 300A, 3 provided in the RF coil
Free induction decay signal (free
Induction decay: Hereinafter abbreviated as "FID signal". ) and used as an 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, projection of the slice portion S in multiple directions within the xy plane is required. Therefore, after exciting the slice portion S to cause a magnetic resonance phenomenon, a linear inclination is applied to the magnetic field H ₀ in the x'-axis direction (coordinate system rotated by θ with accuracy from the x-axis) as shown in Figure 5. When the linear magnetic field gradient _G It is expressed by the formula.

For convenience of explanation, it is assumed that the isomagnetic field lines E are E ₁ to _{E o} and that the magnetic fields on these isomagnetic field lines E ₁ to _{E o} produce signals D ₁ to D _o , which are a type of FID signal, respectively. The amplitudes of the signals D ₁ to D _o are proportional to the spin densities of specific atomic nuclei on the isomagnetic field lines E ₁ to _{E o} passing through the slice portion S, respectively. However, the FID signal that is actually observed is a composite FID signal that is a combination of all the signals D ₁ to _{D o} . 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, rotate this x' axis within the x-y plane,
For example, a magnetic field gradient G _XY is created as _a composite magnetic field of _{magnetic field} gradients G
This is done by changing the synthesis ratio of G _{and Y.} By rotating this magnetic field gradient G _XY , x-
Projection information in the angular direction within the y-plane is obtained, and a CT image is synthesized based on this information.

The above is the principle of magnetic resonance imaging,
Next, as a specific example, FIG. 6 shows a conventional magnetic resonance imaging apparatus. A subject, ie, a patient 1, is placed on a bed 2. Surrounding the patient 1 is an RF coil (probe head: high frequency transmitting/receiving coil) 3, and around the periphery thereof are arranged a shim coil 4 for magnetic field correction and a gradient coil 5 for generating a gradient magnetic field. All of these coil systems are housed inside a normal temperature bore 7 (usually a bore inner diameter of about 1 m) of a large static magnetic field magnet 6. As the static field magnet, any one of a superconducting magnet, a normal conducting magnet, and 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 addition, in the case of superconducting magnets,
In order to operate in persistent current mode and to reduce consumption of liquid helium as a refrigerant, the current lead 9 is normally removed after excitation, so that a magnetic field is always generated. In most magnets, the direction of this static magnetic field is usually in the 10 directions shown in the figure, that is, in the direction of the body axis of the patient 1. The gradient coil 5 is composed of a GX coil that provides a magnetic field gradient in the X-axis direction, a GY coil in the Y-axis direction, and a GZ coil in the Z-axis direction, and is powered by excitation power supplies 11, 12, and 1, respectively.
Connected to 3. These excitation power supplies 11,1
2 and 13 are connected to a central control device 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 14.
It is connected to the. 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 described above will be described.

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

Even with such a large magnet, the uniformity within a 40 to 50 cm sphere due to the magnet alone is only a few hundred ppm at most. In order to keep 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 in the direction perpendicular to the static magnetic field 10
A high frequency is applied by the RF oscillator 15 and the RF coil 3 to excite desired atomic nuclei, such as hydrogen atomic nuclei, in human cells. In addition, in the same way, the gradient magnetic field is generated in X, Y,
Apply in the Z direction.

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

This pulse sequence operation is performed by the central controller 1
4. 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 controller 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.

Generally, various protrusions 2 are formed on the surface of a static field magnet.
There are 5.

For example, when using a superconducting magnet as a static field magnet, FRP is used to support a cryogenic refrigerant tank inside a cold storage container, generally called a cryostat, from the room temperature part of the outside of the cold storage container.
A heat insulating support rod made of etc. is used. In this case, in a general superconducting magnet 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 protrusion 25, the cylindrical shell 2
0 has an outer diameter dimension large enough to completely surround this protrusion.

Figure 7 is viewed from the longitudinal axis direction of the cylindrical shell.
The figure shows this situation. That is, the cylindrical shell is larger than the outer diameter of 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. Service port 2
6 is a part into which a coolant (for example, liquid helium) is injected to maintain the superconducting state, and since it is inconvenient for maintenance work to be surrounded by the magnetic shield 19, the cylindrical shell of this part is cut out as shown in the figure. It's dark.

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 room-temperature bore 7 of the static magnetic field magnet passes through a magnetic shield 19 made of a magnetic material such as iron.
be absorbed into. 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 magnetic field magnet, the above-mentioned leakage magnetic field is greatly reduced compared to a static magnetic field magnet alone. Figure 9 shows this situation.

FIG. 9 shows the 5 Gauss leakage magnetic field distribution in the case of only a 5000 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.

(1) The leakage magnetic field control area for hospitals is provisionally 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. Since the 5 Gauss region is narrowed, the installation area of the device becomes smaller and it can be installed in an existing hospital room.

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

(3) Static field magnets for magnetic resonance imaging systems require magnetic field uniformity on the order of ppm in the diagnostic space. The environment in which static field magnets are installed includes ferromagnetic materials such as floor reinforcement and column reinforcement. 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, (1) When transporting a static magnetic field magnet with a magnetic shield within a hospital to the device installation site, if the outer diameter is large (for example, over 2 m), it will not be able to pass through the hospital corridor, making it impossible to install it within the hospital. becomes.

(2) As the outer diameter increases, the overall height of the static field magnet 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 normally located at or near the top of the static field magnet, the height of this service port from the floor becomes high. Generally, the ceiling height of hospitals is not very high. (For example, 3m or less) For this reason,
Considering the liquid injection work, it is desirable that the height of the static magnetic field magnet be as low as possible.

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.

(3) 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 three problems mentioned above due to the increase in outer diameter, the static magnetic field magnet itself with a magnetic shield has been replaced with the existing one. A decisive drawback arises in that it cannot be installed in hospital health stores.

For this reason, the current response is to construct a new building exclusively for magnetic resonance imaging equipment and install the equipment there. However, this would result in the additional construction costs being higher than the equipment purchase costs. The overall result is a very expensive medical diagnostic device. 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. By adopting a structure that minimizes the increase in outer diameter when attaching a magnetic shield to the static magnetic field magnet, we have created a magnetic resonance imaging system that is compact and lightweight and can be easily transported and installed in existing hospital buildings. It is about providing.

[Structure of the invention]

(Means for Solving the Problems) The magnetic resonance imaging apparatus according to the present invention is configured as follows in order to solve the above problems and achieve the objectives. That is, as shown in FIGS. 1 and 2, a suitable gap 28 is provided between the surface protrusions 25 of the static field magnet 6 and these protrusions at positions corresponding to each other.
By providing the magnetic shield 19 with a recess that can be engaged with the static field magnet 6 and the magnetic shield 1,
When 9 are combined, the protrusion 25 and the magnetic shield recess 29 are engaged with each other, so that the inner diameter 30 of the magnetic shield can be made substantially coincident with the outer diameter 31 of the static field magnet.

In addition, in the second invention, as shown in FIG.
35 are combined to form the cylindrical shell 20, the cutout forms a hole through which the surface protrusion 26 protrudes or passes through.

(Function) With this configuration, the first feature makes it possible to minimize the increase in the outer diameter of the static field magnet with a magnetic shield, making the device compact and lightweight, making it easy to transport into existing hospital buildings. Can be installed.

Furthermore, the second feature facilitates transportation and assembly of the static field magnet and the magnetic shield.

(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. Second
The figure is a sectional view of FIG. 1 when viewed from the direction of the magnetic central axis 23 of the static field magnet 6.

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

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.

Here, the magnetic shield 19 is provided with a recess 29 at a position corresponding to the surface projections 25 of the static field magnet 6, which can be engaged with these projections 25 with an appropriate gap 28. At this time, protrusion 2
5 does not penetrate the magnetic shield, and a magnetic path 32 remains in the recess. When the static magnetic field magnet 6 and the magnetic shield 19 are combined, the protrusion 25 and the magnetic shield recess 29 are An interlocking magnetic shield is attached to the surface of the static field magnet in substantially intimate contact.

(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 mating portion between the protrusion 25 and the magnetic shield recess 29, the provision of the mating portion does not interrupt the magnetic path or asymmetric the magnetic flux.

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

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

Since the height from the floor to the service port can be lowered, liquid injection work can be performed within the normal hospital ceiling height.

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

(2) Since the magnetic path remains, a leakage magnetic field area of 5 Gauss 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 Embodiments) 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.

Furthermore, cutouts are provided in opposing parts of the split cylindrical shells 34 and 35, and when the split cylindrical shells 34 and 35 are combined to form the cylindrical shell 20, the cutouts are used for service. Surface protrusions such as ports 26 form holes through which they protrude or pass.

The recesses of the cylindrical shells 34 and 35 for the protrusions 25 of the static field magnet 6 are grooves 33 that extend in the longitudinal direction of the cylindrical shells and are large enough to allow the protrusions 25 to slide.
It is becoming.

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 static field magnet, the increase in the outer diameter dimension can be minimized.
Furthermore, since the magnetic path remains in the groove, there is no interruption of the magnetic path or asymmetrical magnetic flux.

Furthermore, when assembling the divided cylindrical shell,
This groove acts as a guide.

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

(2) 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 the magnetic shield to the static magnetic field magnet, so the magnetic shield and static magnetic field magnet can be made compact and lightweight, making it possible to fit them into existing hospital buildings. Easy to transport and install.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of a 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 FIG.
5 and 5 are diagrams 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 block diagram showing the system of a conventional magnetic resonance imaging apparatus. FIG. 9, a sectional view of the conventional example, is a leakage magnetic field distribution diagram. 19... Magnetic shield, 20... Cylindrical shell, 2
1...Disk-shaped end cover, 23...Magnetic central axis, 25...
... Protrusion, 26 ... Service port, 28 ... Gap, 29 ... Magnetic shield recess, 30 ... Magnetic shield inner diameter, 31 ... Static field magnet outer diameter, 32 ... Magnetic path, 33 ... Groove , 34, 35...divided cylindrical shell, 36...joint, 37...bolt tightening.

Claims (1)

[Scope of Claims] 1. A magnetic shield consisting of a cylindrical shell made of a magnetic material and two end covers made of a magnetic material and having a central opening attached to both ends of the shell, and a static field magnet having a surface protrusion. In a magnetic resonance imaging apparatus in which the longitudinal axis of the cylindrical shell and the magnetic center line of the static field magnet are coaxial, the magnetic shield is provided with a recess that engages with the surface protrusion, and the magnetic shield is static. A magnetic resonance imaging device characterized in that a magnetic field is brought into close contact with the surface of a magnet. 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. 4. A magnetic shield consisting of a cylindrical shell made of a magnetic material and two end covers made of a magnetic material and having a central opening attached to both ends of the shell is used to protect a static magnetic field magnet having a surface protrusion such as a service port. In a magnetic resonance imaging apparatus in which the longitudinal axis of the cylindrical shell and the magnetic center line of the static field magnet are coaxial, the cylindrical shell is composed of a pair of split shells each having a notch, and the split shell A magnetic resonance imaging apparatus characterized in that when the bodies are combined to form a cylindrical shell, the cutout forms a hole through which the surface protrusion protrudes or passes through.