JP6943676B2 - Magnetic resonance imaging device - Google Patents

Description

The present invention relates to a magnetic resonance imaging apparatus.

Conventionally, a magnet device that generates a static magnetic field in an imaging space, a high-frequency coil that irradiates a subject with a high frequency, and a gradient magnetic field coil that is arranged between the magnet device and the high-frequency coil and generates a gradient magnetic field in the imaging space. And, a magnetic resonance imaging (MRI) apparatus including is known. For example, the active shield type gradient magnetic field coil includes a main coil for generating a gradient magnetic field in the imaging space and a shield coil for suppressing the generation of eddy current due to an unnecessary leakage magnetic field.

For example, in Patent Document 1, a main coil in which a conductor is spirally wound and arranged on a substantially flat surface and a gradient magnetic field is generated on the front side thereof, and a main coil on the back side of the main coil are substantially flat. In a gradient magnetic field coil in which a conductor is spirally wound and arranged on a surface, and the main coil mainly comprises a shield coil that generates a magnetic field for canceling at least a part of the magnetic field generated on the back side thereof. At least one third coil arrangement surface is provided between the arrangement surface of the main coil and the arrangement surface of the shield coil, and one or more coil conductors are arranged on the third coil arrangement surface. A gradient magnetic field coil characterized by this is disclosed.

JP-A-2002-112977

However, when a gradient magnetic field having a small gradient and low linearity is generated in the direction of the static magnetic field, there is a problem that the performance of the MRI apparatus deteriorates.

The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to improve the performance of an MRI apparatus.

In order to solve the above problems, the present invention presents a magnet device that generates a static magnetic field in the Z direction of the imaging space, a high frequency coil that generates a high frequency magnetic field in the imaging space, and an inclination that generates a gradient magnetic field in the imaging space. A magnetic resonance imaging apparatus including a magnetic field coil, wherein the gradient magnetic field coil has a second region having a lower current density than the first region, and a Z-direction gradient magnetic field main coil that generates a gradient magnetic field in the Z direction. The first region is a region in the Z-direction tilted magnetic field main coil from the Z-coordinate of the outermost position of the imaging space to the Z-coordinate where the gradient magnetic field strength in the Z-direction is maximized. The region is a region in the Z-direction gradient magnetic field main coil from the Z coordinate at which the gradient magnetic field strength in the Z direction is maximized to the Z coordinate at the end of the gradient magnetic field coil .

According to the present invention, the performance of the MRI apparatus can be improved.

It is a perspective view of the MRI apparatus which concerns on 1st Embodiment of this invention. It is sectional drawing which shows an example of the structure of the gradient magnetic field coil which concerns on 1st Embodiment of this invention, and is the graph which shows an example of the gradient magnetic field strength. It is sectional drawing which shows an example of the structure of the conventional gradient magnetic field coil and the structure of the gradient magnetic field coil which concerns on 1st Embodiment of this invention. This is a simulation model of a conventional gradient magnetic field coil and a gradient magnetic field coil according to the first embodiment of the present invention. It is a graph which shows the simulation result of the gradient magnetic field strength. It is sectional drawing which shows an example of the structure of the gradient magnetic field coil which concerns on 1st Embodiment of this invention. It is a perspective view of the MRI apparatus which concerns on 2nd Embodiment of this invention. It is sectional drawing which shows an example of the structure of the gradient magnetic field coil which concerns on 2nd Embodiment of this invention, and the graph which shows an example of the gradient magnetic field strength.

<< First Embodiment >>
[Overall configuration of MRI equipment]
First, the configuration of the tunnel type (horizontal magnetic field type) MRI apparatus 100 according to the first embodiment of the present invention will be described with reference to FIG.

The MRI device 100 includes a magnet device 1, a gradient magnetic field coil 2, a high frequency coil 3, a movable bed 4, a power supply device (not shown), a refrigerant pumping device, a receiving coil, a computer system, a cover covering each coil, and the like. The origin O is the center of the imaging space S, the X direction is the horizontal direction, the Y direction is the vertical direction, and the Z direction is the direction in which the subject 5 is carried from the outside of the MRI apparatus 100 to the imaging space S by the movable bed 4. Represents.

The magnet device 1 has a cylindrical shape and is composed of, for example, a superconducting coil. The magnet device 1 generates a uniform static magnetic field in the arrow α direction (Z direction) of the imaging space S (for example, 1T, 1.5T, 3T). The radius of the imaging space S is about 30 cm to 50 cm.

The gradient magnetic field coil 2 has a cylindrical shape and is arranged between the magnet device 1 and the high frequency coil 3, and is composed of, for example, a normal conduction coil. The gradient magnetic field coil 2 generates a dynamic magnetic field (gradient magnetic field) having a gradient in the imaging space S (for example, 30 mT / m, 50 mT / m). The current of the power supply device for generating the gradient magnetic field is usually about 500A to 600A. By generating a gradient magnetic field in the imaging space S, position information can be added to the magnetic resonance signal, so that the cross section of the subject 5 can be imaged in the MRI apparatus 100.

Although the details will be described later, the gradient magnetic field coil 2 includes a main coil and a shield coil, and the main coil includes an X-direction gradient magnetic field main coil, a Y-direction gradient magnetic field main coil, and a Z-direction gradient magnetic field main coil. The shield coil is provided with an X-direction gradient magnetic field shield coil, a Y-direction gradient magnetic field shield coil, and a Z-direction gradient magnetic field shield coil. The Z-direction gradient magnetic field main coil has a region with a high current density near the imaging space S. This makes it possible to generate a gradient magnetic field with a large gradient and high linearity in the direction of the arrow α (direction of the static magnetic field).

The high-frequency coil 3 has a cylindrical shape and is arranged inside the gradient magnetic field coil 2, and is composed of, for example, a normal conduction coil. The high frequency coil 3 generates a high frequency magnetic field in the imaging space S. By irradiating the subject 5 with a high-frequency pulse from the high-frequency coil 3, the magnetic resonance signal generated from the subject 5 is received by the receiving coil and then signal-processed. The central axes of the cylindrical magnet device 1, the gradient magnetic field coil 2, and the high-frequency coil 3 are substantially aligned with each other.

[Structure of gradient magnetic field coil]
Next, the configuration of the gradient magnetic field coil according to the present embodiment will be described with reference to FIG.

FIG. 2A is a cross-sectional view showing an example of the configuration of the gradient magnetic field coil 2. In FIG. 2A, the vertical axis represents the r (radius of the cylinder in the gradient magnetic field coil 2) direction, and the horizontal axis represents the Z direction. The cross section of the gradient magnetic field coil 2 shown in FIG. 2A represents a quarter cross section of the gradient magnetic field coil 2 shown in FIG.

FIG. 2B is a graph showing an example of the gradient magnetic field strength in the Z direction. In FIG. 2B, the vertical axis represents the gradient magnetic field strength Bz in the Z direction, and the horizontal axis represents the Z direction.

The gradient magnetic field coil 2 includes a main coil 21, a shield coil 22, and the like. The main coil 21 and the shield coil 22 are bonded and integrated by the resin 25. As the resin 25, for example, an epoxy resin or the like can be used.

The main coil 21 includes an X-direction gradient magnetic field main coil 21x, a Y-direction gradient magnetic field main coil 21y, and a Z-direction gradient magnetic field main coil 21z. The shield coil 22 includes an X-direction gradient magnetic field shield coil 22x, a Y-direction gradient magnetic field shield coil 22y, and a Z-direction gradient magnetic field shield coil 22z.

An insulating sheet (not shown) such as an FRP material is sandwiched between the layers of the X-direction gradient magnetic field main coil 21x, the Y-direction gradient magnetic field main coil 21y, and the Z-direction gradient magnetic field main coil 21z. The insulating sheet is laminated and integrated with the resin 25. Similarly, an insulating sheet (not shown) such as an FRP material is sandwiched between the layers of the three layers of the X-direction tilted magnetic field shield coil 22x, the Y-direction tilted magnetic field shield coil 22y, and the Z-direction tilted magnetic field shield coil 22z. Each shield coil and the insulating sheet are laminated and integrated with the resin 25.

The X-direction gradient magnetic field main coil 21x generates a gradient magnetic field in the X direction, the Y-direction gradient magnetic field main coil 21y generates a gradient magnetic field in the Y direction, and the Z-direction gradient magnetic field main coil 21z generates a gradient magnetic field in the Z direction. To generate. The X-direction gradient magnetic field shield coil 22x suppresses leakage of the X-direction gradient magnetic field to the outside (for example, the magnet device 1), and the Y-direction gradient magnetic field shield coil 22y suppresses leakage of the X-direction gradient magnetic field to the outside (for example, the magnet device 1). ), And the Z-direction gradient magnetic field shield coil 22z suppresses leakage of the Z-direction gradient magnetic field to the outside (for example, the magnet device 1). By providing the shield coil 22 in the gradient magnetic field coil 2, it is possible to suppress the generation of eddy current due to an unnecessary leakage magnetic field.

The X-direction gradient magnetic field main coil 21x, the Y-direction gradient magnetic field main coil 21y, the X-direction gradient magnetic field shield coil 22x, and the Y-direction gradient magnetic field shield coil 22y are plate-shaped, for example, copper (Cu), aluminum (Al), or the like. It is formed by processing a good conductor. The Z-direction gradient magnetic field main coil 21z and the Z-direction gradient magnetic field shield coil 22z are formed by winding a good tubular conductor such as copper (Cu) or aluminum (Al) in the circumferential direction β with respect to the central axis 6. It is formed.

The Z-direction gradient magnetic field main coil 21z and the Z-direction gradient magnetic field shield coil 22z formed by a good tubular conductor have hollow portions 23 (23a, 23b) and are pumped with refrigerant via return lines 24 (24a, 24b). Connected to the device. As a result, the refrigerant can be circulated in the gradient magnetic field coil 2 and the temperature rise of the gradient magnetic field coil 2 can be suppressed. Further, the Z-direction gradient magnetic field main coil 21z and the Z-direction gradient magnetic field shield coil 22z are connected to the power supply device via the return wires 24 (24a, 24b). The Z-direction tilted magnetic field main coil 21z and the Z-direction tilting magnetic field shield coil 22z are tilted by using both a function as a cooling pipe for circulating a refrigerant with respect to the tilted magnetic field coil 2 and a function as each coil. The occupied volumes of the Z-direction gradient magnetic field main coil 21z and the Z-direction gradient magnetic field shield coil 22z in the magnetic field coil 2 can be reduced.

Here, the relationship between the performance of the MRI apparatus 100 and the gradient magnetic field strength of the gradient magnetic field coil 2 in the Z direction will be described with reference to FIG. 2 (b).

In FIG. 2B, Z1 represents the Z coordinate of the outermost position of the imaging space S, Z2 represents the Z coordinate at which the gradient magnetic field strength Bz is maximum, and Z3 represents the end of the gradient magnetic field coil 2. It represents the Z coordinate. The Z coordinate (Z2) at which the gradient magnetic field strength Bz is maximized can be rephrased as the Z coordinate at the outermost position of the magnetic resonance signal acquisition range.

As shown in FIG. 2B, the gradient magnetic field strength Bz increases from O to Z2 as the value of the Z coordinate increases, becomes maximum at Z2, and increases from Z2 to Z3 as the value of the Z coordinate increases. Decrease.

The larger the gradient of the graph D in the imaging space S and the higher the linearity of the graph D in the imaging space S (the graph D is closer to a straight line), the higher the performance of the MRI apparatus 100 can be. This is because the larger the gradient of the graph D in the imaging space S, the clearer the magnetic resonance signal to which the position information is added and the higher the spatial resolution, and the higher the linearity of the graph D in the imaging space S, the higher the image. This is because the distortion of is small.

For example, a large current is passed through the gradient magnetic field coil, the number of turns of the Z-direction gradient magnetic field main coil in the radial direction of the cylinder is increased, the gradient magnetic field coil is lengthened in the Z direction, and the like, in the direction of the static magnetic field. Attempts to generate a gradient magnetic field with a large gradient and high linearity cause the following problems. When a large current is passed through the gradient magnetic field coil, the temperature of the gradient magnetic field coil rises due to Joule heat generation. Further, when the number of turns is increased, the temperature of the gradient magnetic field coil rises due to the increase in the resistance value, and the drive power source needs to be strengthened due to the increase in the inductance. Further, if the gradient magnetic field coil is lengthened in the Z direction, it goes against the demand for reducing the occupied volume of the gradient magnetic field coil in the MRI apparatus.

That is, it is desired to design a gradient magnetic field coil that generates a gradient magnetic field having a large gradient and high linearity in the direction of the static magnetic field while avoiding the above-mentioned problems.

Therefore, in the present embodiment, as shown in FIG. 2A, the gradient magnetic field coil 2 is provided with the Z-direction gradient magnetic field main coil 21z having regions having different current densities to improve the magnetic field performance. .. The reason for changing the current density of the Z-direction gradient magnetic field main coil 21z instead of the Z-direction gradient magnetic field shield coil 22z is that the shield coil is a coil provided to suppress leakage of the gradient magnetic field to the outside. This is because it is not necessary to have multiple layers (increase the number of windings).

The first region 101 is a region in the Z-direction gradient magnetic field main coil 21z from Z1 to Z2. The second region 102 is a region in the Z-direction gradient magnetic field main coil 21z from Z2 to Z3. The third region 103 is a region in the Z-direction gradient magnetic field main coil 21z from O to Z1. The Z-direction gradient magnetic field main coil 21z has not only a configuration having a first region 101, a second region 102, and a third region 103, but also has a first region 101 and a second region 101 as shown in FIG. 2A. It is also possible to have a configuration having only the region 102.

The current densities of the first region 101, the second region 102, and the third region 103 are different from each other. The current density of the first region 101 is higher than the current density of the second region 102, and the current density of the second region 102 is higher than the current density of the third region 103. Further, the current density of the first region 101 is preferably at least twice or more the current density of the second region 102.

The first region 101, the second region 102, and the third region 103 have different numbers of layers of the Z-direction gradient magnetic field main coil 21z in the r direction, respectively. The number of layers of the Z-direction gradient magnetic field main coil 21z in the r direction in the first region 101 is larger than the number of layers of the Z-direction gradient magnetic field main coil 21z in the r direction in the second region 102. The number of layers of the Z-direction gradient magnetic field main coil 21z in the r direction in the second region 102 is larger than the number of layers of the Z-direction gradient magnetic field main coil 21z in the r direction in the third region 103. Further, the number of layers of the Z-direction gradient magnetic field main coil 21z in the r direction in the first region 101 is at least twice or more the number of layers of the Z-direction gradient magnetic field main coil 21z in the r direction in the second region 102. Is preferable.

The return line 24a is provided in a direction parallel to the Z direction so as to overlap the Z-direction gradient magnetic field main coil 21z in the second region 102. By aligning the thickness of the Z-direction gradient magnetic field main coil 21z in the r direction in the return line 24a and the second region 102 and the thickness of the Z-direction gradient magnetic field main coil 21z in the r direction in the first region 101, the inclination is achieved. The size of the magnetic field coil 2 can be reduced.

The first region 101, the second region 102, and the third region 103 differ in the number of turns of the Z-direction gradient magnetic field main coil 21z in the circumferential direction β with respect to the central axis 6, respectively. The number of turns of the Z-direction gradient magnetic field main coil 21z in the circumferential direction β with respect to the central axis 6 in the first region 101 is the Z-direction gradient magnetic field main in the circumferential direction β with respect to the central axis 6 in the second region 102. This is more than the number of turns of the coil 21z. The number of turns of the Z-direction gradient magnetic field main coil 21z in the circumferential direction β with respect to the central axis 6 in the second region 102 is the Z-direction gradient magnetic field main in the circumferential direction β with respect to the central axis 6 in the third region 103. This is more than the number of turns of the coil 21z. Further, the number of turns of the magnetic field main coil 21z in the Z-direction tilting direction β with respect to the central axis 6 in the first region 101 is the Z-direction tilting in the circumferential direction β with respect to the central axis 6 in the second region 102. It is preferable that the number of turns of the magnetic field main coil 21z is at least twice or more. Since the Z-direction gradient magnetic field main coil 21z has a hollow portion 23a, the local increase in heat generation of the gradient magnetic field coil 2 due to the Z-direction gradient magnetic field main coil 21z being wound many times is suppressed.

The gradient magnetic field coil 2 according to the present embodiment is equipped with a Z-direction gradient magnetic field main coil 21z in which the current density of the first region 101 is higher than the current density of the second region 102. As a result, a gradient magnetic field having a large gradient and high linearity can be generated in the direction of the static magnetic field, so that the performance of the MRI apparatus 100 using the gradient magnetic field coil 2 according to the present embodiment can be improved.

[Results by numerical calculation model]
Next, with reference to FIGS. 3 to 5, the gradient magnetic field coil according to the conventional example and the gradient magnetic field coil according to the present embodiment are compared.

FIG. 3A is a cross-sectional view showing an example of the configuration of the gradient magnetic field coil 2K according to the conventional example. FIG. 3B is a cross-sectional view showing an example of the configuration of the gradient magnetic field coil 2 according to the present embodiment. In FIG. 3, the vertical axis represents the r (radius of the cylinder in the gradient magnetic field coils 2 and 2K) direction, and the horizontal axis represents the Z direction.

As shown in FIG. 3A, in the gradient magnetic field coil 2K, the current density of the first region 101, the current density of the second region 102, and the current density of the third region 103 are substantially the same.

Further, the number of layers of the Z-direction gradient magnetic field main coil 21z in the first region 101, the number of layers of the Z-direction gradient magnetic field main coil 21z in the r direction in the second region 102, and the r-direction in the third region 103. The number of layers of the Z-direction gradient magnetic field main coil 21z is the same.

As shown in FIG. 3B, in the gradient magnetic field coil 2, the current density of the first region 101, the current density of the second region 102, and the current density of the third region 103 are all different. The current density of the first region 101 is higher than the current density of the second region 102, and the current density of the second region 102 is higher than the current density of the third region 103.

Further, the number of layers of the Z-direction gradient magnetic field main coil 21z in the r direction in the first region 101 is larger than the number of layers of the Z-direction gradient magnetic field main coil 21z in the r direction in the second region 102, and the number of layers is larger than the number of layers of the Z-direction gradient magnetic field main coil 21z in the second region 102. The number of layers of the Z-direction gradient magnetic field main coil 21z in the r direction is larger than the number of layers of the Z-direction gradient magnetic field main coil 21z in the r direction in the third region 103.

FIG. 4 is a diagram modeling a Z-direction gradient magnetic field main coil mounted on the gradient magnetic field coil shown in FIGS. 3 (a) and 3 (b) using conductors 1 to 10. FIG. 4A is a simulation model of the Z-direction gradient magnetic field main coil mounted on the conventional gradient magnetic field coil. FIG. 4B is a simulation model of the Z-direction gradient magnetic field main coil mounted on the gradient magnetic field coil according to the present embodiment. The area A shown in FIG. 4A corresponds to FIG. 3A, and the area B in FIG. 4B corresponds to FIG. 3B.

In FIG. 4A, conductors 5 to 7 to which the conductor current value I is given are arranged in the first region 101, and conductors 8 to 10 to which the conductor current value I is given are arranged in the second region 102. In the third region 103, conductors 1 to 4 to which the conductor current value I is given are arranged to model the Z-direction gradient magnetic field main coil mounted on the conventional gradient magnetic field coil. That is, the current density of the first region 101, the current density of the second region 102, and the current density of the third region 103 are equalized.

In FIG. 4B, conductors 5 to 7 to which the conductor current value 2I is given are arranged in the first region 101, and conductors 8 to 10 to which the conductor current value I is given are arranged in the second region 102. In the third region 103, the Z-direction gradient magnetic field main coil mounted on the gradient magnetic field coil according to the present embodiment is modeled without arranging the conductors 1 to 4. That is, the current density of the first region 101 is made higher than the current density of the second region 102.

FIG. 5 is a simulation result of the gradient magnetic field strength in the gradient magnetic field coils 2 and 2K. In FIG. 5, the vertical axis represents the gradient magnetic field strength Bz in the Z direction, and the horizontal axis represents the position of the conductor in the Z direction.

The solid line indicated by reference numeral 201 is the simulation result indicating the gradient magnetic field strength of the gradient magnetic field coil 2 according to the present embodiment in the Z direction, and the broken line indicated by reference numeral 202 is the gradient magnetic field strength of the conventional gradient magnetic field coil 2K in the Z direction. It is a simulation result showing.

As shown in FIG. 5, when the conventional gradient magnetic field coil 2K and the gradient magnetic field coil 2 according to the present embodiment are compared, it can be seen that there is a clear difference in the simulation results. The gradient of the graph in the imaging space S is larger in the graph 201 than in the graph 202. Further, the linearity of the graph in the imaging space S is higher in the graph 201 than in the graph 202.

That is, the gradient magnetic field coil 2 according to the present embodiment can generate a gradient magnetic field having a larger gradient and higher linearity in the direction of the static magnetic field (direction of arrow α) than the conventional gradient magnetic field coil 2K. I understand. Therefore, it was proved that the gradient magnetic field coil 2 according to the present embodiment has higher magnetic field performance than the conventional gradient magnetic field coil 2K.

By mounting the gradient magnetic field coil 2 having high magnetic field performance on the MRI apparatus 100, the performance of the MRI apparatus 100 can be enhanced. As a result, the MRI apparatus 100 according to the present embodiment enables high-quality and high-speed imaging.

[Modification example]
Next, with reference to FIG. 6, another configuration of the gradient magnetic field coil according to the present embodiment will be described. FIG. 6 is a cross-sectional view showing an example of the configuration of the gradient magnetic field coil 2V. The description of the same configuration as that of the gradient magnetic field coil 2 will be omitted.

The gradient magnetic field coil 2V includes a main coil 21, a shield coil 22, and the like. The main coil 21 and the shield coil 22 are bonded and integrated by the resin 25. As the resin 25, for example, an epoxy resin or the like can be used.

The main coil 21 includes an X-direction gradient magnetic field main coil 21x, a Y-direction gradient magnetic field main coil 21y, a Z-direction gradient magnetic field main coil 21z1, and a Z-direction gradient magnetic field main coil 21z2. The shield coil 22 includes an X-direction gradient magnetic field shield coil 22x, a Y-direction gradient magnetic field shield coil 22y, and a Z-direction gradient magnetic field shield coil 22z.

The Z-direction gradient magnetic field main coil 21z in the first region 101 is composed of conductors having different widths (Z-direction gradient magnetic field main coil 21z1, Z-direction gradient magnetic field main coil 21z2).

The Z-direction gradient magnetic field main coil 21z1 is formed by winding, for example, a good tubular conductor such as copper (Cu) or aluminum (Al) in the circumferential direction β with respect to the central axis 6. The Z-direction gradient magnetic field main coil 21z2 is formed by winding, for example, a good conductor such as copper (Cu) or aluminum (Al) in the circumferential direction β with respect to the central axis 6.

The width of the Z-direction gradient magnetic field main coil 21z2 in the first region 101 is narrower than the width of the Z-direction gradient magnetic field main coil 21z1 in the first region 101. Therefore, the number of turns of the Z-direction gradient magnetic field main coil 21z2 is larger than the number of turns of the Z-direction gradient magnetic field main coil 21z1. The first region 101 is composed of conductors having different widths, and the current density of the first region 101 is made higher than the current density of the second region 102. Can be generated.

Further, the Z-direction gradient magnetic field main coil 21z1 in the first region 101 has a hollow portion 23a, and the Z-direction gradient magnetic field main coil 21z2 in the first region 101 does not have a hollow portion 23a. Therefore, the hollow portion 23a is provided by giving the Z-direction gradient magnetic field main coil 21z1 arranged close to the Z-direction gradient magnetic field main coil 21z2 a function as a cooling pipe for circulating the refrigerant in the gradient magnetic field coil 2V. It is possible to prevent an excessive temperature rise in the Z-direction gradient magnetic field main coil 21z2 that does not have.

Similar to the Z-direction gradient magnetic field main coil 21z related to the gradient magnetic field coil 2, the Z-direction gradient magnetic field main coil 21z1 related to the gradient magnetic field coil 2V functions as a cooling pipe for circulating a refrigerant with respect to the gradient magnetic field coil 2V. And the function as each coil are used in combination, so that the occupied volume of the Z-direction gradient magnetic field main coil 21z in the gradient magnetic field coil 2V can be reduced.

The gradient magnetic field coil 2V according to the present embodiment is equipped with a Z-direction gradient magnetic field main coil 21z in which the current density of the first region 101 is higher than the current density of the second region 102. As a result, a gradient magnetic field having a large gradient and high linearity can be generated in the direction of the static magnetic field, so that the performance of the MRI apparatus 100 using the gradient magnetic field coil 2V according to the present embodiment can be improved.

<< Second Embodiment >>
[Overall configuration of MRI equipment]
First, the configuration of the open type (vertical magnetic field type) MRI apparatus 200 according to the second embodiment of the present invention will be described with reference to FIG. 7.

The MRI device 200 includes a magnet device 1 (1a, 1b), a gradient magnetic field coil 2 (2a, 2b), a high frequency coil 3 (3a, 3b), a movable bed 4, a support column 10, a power supply device (not shown), and a refrigerant pumping device. , Includes receiving coil, computer system, cover covering each coil, etc. The origin O is the center of the imaging space S, the X direction is the horizontal direction, the Y direction is the direction in which the subject 5 is carried from the outside of the MRI apparatus 200 to the imaging space S by the movable bed 4, and the Z direction is the vertical direction. Represents. In the open type MRI apparatus 200, the imaging space S is opened over a wide range as compared with the horizontal magnetic field type MRI apparatus 100. Therefore, the subject 5 can look around and reduce the feeling of being closed.

The magnet device 1 has a pair of upper and lower disk shapes, and is composed of, for example, a superconducting coil. The magnet device 1 generates a uniform static magnetic field in the arrow α direction (Z direction) of the imaging space S (for example, 1T, 1.5T, 3T). The radius of the imaging space S is about 30 cm to 50 cm. The upper and lower pairs of magnet devices 1a and 1b are supported by the columns 10.

The gradient magnetic field coil 2 has a pair of upper and lower disk shapes, is arranged between the magnet device 1 and the high frequency coil 3, and is composed of, for example, a normal conduction coil. The gradient magnetic field coil 2 generates a dynamic magnetic field (gradient magnetic field) having a gradient in the imaging space S (for example, 30 mT / m, 50 mT / m). The current of the power supply device for generating the gradient magnetic field is usually about 500A to 600A. By generating a gradient magnetic field in the imaging space S, position information can be added to the magnetic resonance signal, so that the cross section of the subject 5 can be imaged in the MRI apparatus 100.

The gradient magnetic field coil 2 includes a main coil and a shield coil. The main coil includes an X-direction gradient magnetic field main coil, a Y-direction gradient magnetic field main coil, and a Z-direction gradient magnetic field main coil, and the shield coil includes a main coil and a shield coil. , X-direction gradient magnetic field shield coil, Y-direction gradient magnetic field shield coil, and Z-direction gradient magnetic field shield coil are provided. The Z-direction gradient magnetic field main coil has a region with a high current density near the imaging space S. This makes it possible to generate a gradient magnetic field with a large gradient and high linearity in the direction of the arrow α (direction of the static magnetic field).

The high-frequency coil 3 has a pair of upper and lower disk shapes, is arranged inside a pair of upper and lower gradient magnetic field coils 2a and 2b, and is composed of, for example, a normal conduction coil. The high frequency coil 3 generates a high frequency magnetic field in the imaging space S. By irradiating the subject 5 with a high-frequency pulse from the high-frequency coil 3, the magnetic resonance signal generated from the subject 5 is received by the receiving coil and then signal-processed. The central axes of the magnet device 1, the gradient magnetic field coil 2, and the high-frequency coil 3 are substantially aligned with each other.

[Structure of gradient magnetic field coil]
Next, the configuration of the gradient magnetic field coil according to the present embodiment will be described with reference to FIG.

FIG. 8A is a cross-sectional view showing an example of the configuration of the gradient magnetic field coil 2. In FIG. 8A, the vertical axis represents the Z direction and the horizontal axis represents the R (radius of the disk in the gradient magnetic field coils 2a and 2b) direction. The cross section of the gradient magnetic field coil 2 shown in FIG. 8A represents a 1/2 cross section of the gradient magnetic field coil 2a or the gradient magnetic field coil 2b shown in FIG.

FIG. 8B is a graph showing an example of the gradient magnetic field strength in the X or Y direction. In FIG. 8B, the vertical axis represents the gradient magnetic field strength Bz in the X or Y direction, and the horizontal axis represents the R direction.

The gradient magnetic field coil 2 includes a main coil 21, a shield coil 22, and the like. The main coil 21 and the shield coil 22 are bonded and integrated by the resin 25. As the resin 25, for example, an epoxy resin or the like can be used.

The main coil 21 includes an X-direction gradient magnetic field main coil 21x, a Y-direction gradient magnetic field main coil 21y, and a Z-direction gradient magnetic field main coil 21z. The shield coil 22 includes an X-direction gradient magnetic field shield coil 22x, a Y-direction gradient magnetic field shield coil 22y, and a Z-direction gradient magnetic field shield coil 22z.

An insulating sheet (not shown) such as an FRP material is sandwiched between the layers of the X-direction gradient magnetic field main coil 21x, the Y-direction gradient magnetic field main coil 21y, and the Z-direction gradient magnetic field main coil 21z. The insulating sheet is laminated and integrated with the resin 25. Similarly, an insulating sheet (not shown) such as an FRP material is sandwiched between the layers of the three layers of the X-direction tilted magnetic field shield coil 22x, the Y-direction tilted magnetic field shield coil 22y, and the Z-direction tilted magnetic field shield coil 22z. Each shield coil and the insulating sheet are laminated and integrated with the resin 25.

The X-direction gradient magnetic field main coil 21x generates a gradient magnetic field in the X direction, the Y-direction gradient magnetic field main coil 21y generates a gradient magnetic field in the Y direction, and the Z-direction gradient magnetic field main coil 21z generates a gradient magnetic field in the Z direction. To generate. The X-direction gradient magnetic field shield coil 22x suppresses leakage of the X-direction gradient magnetic field to the outside (for example, the magnet device 1), and the Y-direction gradient magnetic field shield coil 22y suppresses leakage of the X-direction gradient magnetic field to the outside (for example, the magnet device 1). ), And the Z-direction gradient magnetic field shield coil 22z suppresses leakage of the Z-direction gradient magnetic field to the outside (for example, the magnet device 1). By providing the shield coil 22 in the gradient magnetic field coil 2, it is possible to suppress the generation of eddy current due to an unnecessary leakage magnetic field.

The X-direction gradient magnetic field main coil 21x, the Y-direction gradient magnetic field main coil 21y, the X-direction gradient magnetic field shield coil 22x, and the Y-direction gradient magnetic field shield coil 22y are plate-shaped, for example, copper (Cu), aluminum (Al), or the like. It is formed by processing a good conductor. The Z-direction gradient magnetic field main coil 21z and the Z-direction gradient magnetic field shield coil 22z are formed by winding a good tubular conductor such as copper (Cu) or aluminum (Al) in the circumferential direction β with respect to the central axis 6. It is formed.

The Z-direction gradient magnetic field main coil 21z and the Z-direction gradient magnetic field shield coil 22z formed by a good tubular conductor have hollow portions 23 (23a, 23b) and are pumped with refrigerant via return lines 24 (24a, 24b). Connected to the device. As a result, the refrigerant can be circulated in the gradient magnetic field coil 2 and the temperature rise of the gradient magnetic field coil 2 can be suppressed. Further, the Z-direction gradient magnetic field main coil 21z and the Z-direction gradient magnetic field shield coil 22z are connected to the power supply device via the return wires 24 (24a, 24b). The Z-direction gradient magnetic field main coil 21z and the Z-direction gradient magnetic field shield coil 22z are combined with a function as a cooling pipe for circulating a refrigerant in the gradient magnetic field coil 2 and a function as each coil. The occupied volumes of the Z-direction gradient magnetic field main coil 21z and the Z-direction gradient magnetic field shield coil 22z in No. 2 can be reduced.

In FIG. 8B, R1 represents the R coordinate of the outermost position of the imaging space S, R2 represents the R coordinate at which the gradient magnetic field strength Bz is maximum, and R3 represents the end of the gradient magnetic field coil 2. It represents the R coordinate. The R coordinate (R2) at which the gradient magnetic field strength Bz is maximized can be rephrased as the R coordinate at the outermost position of the magnetic resonance signal acquisition range.

As shown in FIG. 8B, the gradient magnetic field strength Bz increases from O to R2 as the value of the R coordinate increases, becomes maximum at R2, and increases from R2 to R3 as the value of the R coordinate increases. Decrease. The larger the gradient of the graph D in the imaging space S and the higher the linearity of the graph D in the imaging space S (the graph D is closer to a straight line), the higher the performance of the MRI apparatus 100 can be.

As shown in FIG. 8A, the gradient magnetic field coil 2 is equipped with a Z-direction gradient magnetic field main coil 21z having regions having different current densities. The first region 101 is a region in the Z-direction gradient magnetic field main coil 21z from R1 to R2. The second region 102 is a region in the Z-direction gradient magnetic field main coil 21z from R2 to R3. The third region 103 is a region in the Z-direction gradient magnetic field main coil 21z from O to R1. The Z-direction gradient magnetic field main coil 21z has not only a configuration having a first region 101, a second region 102, and a third region 103, but also has a first region 101 and a second region 101 as shown in FIG. 8A. It is also possible to have a configuration having only the region 102.

The current densities of the first region 101, the second region 102, and the third region 103 are different from each other. The current density of the first region 101 is higher than the current density of the second region 102, and the current density of the second region 102 is higher than the current density of the third region 103. Further, the current density of the first region 101 is preferably at least twice or more the current density of the second region 102.

The first region 101, the second region 102, and the third region 103 have different numbers of layers of the Z-direction gradient magnetic field main coil 21z in the Z direction, respectively. The number of layers of the Z-direction gradient magnetic field main coil 21z in the Z direction in the first region 101 is larger than the number of layers of the Z-direction gradient magnetic field main coil 21z in the Z direction in the second region 102. The number of layers of the Z-direction gradient magnetic field main coil 21z in the Z direction in the second region 102 is larger than the number of layers of the Z-direction gradient magnetic field main coil 21z in the Z direction in the third region 103. Further, the number of layers of the Z-direction gradient magnetic field main coil 21z in the first region 101 is at least twice or more the number of layers of the Z-direction gradient magnetic field main coil 21z in the second region 102. Is preferable.

The return line 24a is provided in a direction parallel to the R direction so as to overlap the Z-direction gradient magnetic field main coil 21z in the second region 102. By aligning the thickness of the Z-direction gradient magnetic field main coil 21z in the Z direction in the return line 24a and the second region 102 and the thickness of the Z-direction gradient magnetic field main coil 21z in the Z direction in the first region 101, the inclination is achieved. The size of the magnetic field coil 2 can be reduced.

The first region 101, the second region 102, and the third region 103 differ in the number of turns of the Z-direction gradient magnetic field main coil 21z in the Z direction, respectively. The number of turns of the Z-direction gradient magnetic field main coil 21z in the Z direction in the first region 101 is larger than the number of turns of the Z-direction gradient magnetic field main coil 21z in the Z direction in the second region 102. The number of turns of the Z-direction gradient magnetic field main coil 21z in the Z direction in the second region 102 is larger than the number of turns of the Z-direction gradient magnetic field main coil 21z in the Z direction in the third region 103. Further, the number of turns of the Z-direction gradient magnetic field main coil 21z in the Z direction in the first region 101 is at least twice or more the number of turns of the Z-direction gradient magnetic field main coil 21z in the Z direction in the second region 102. It is preferable to have. Since the Z-direction gradient magnetic field main coil 21z has a hollow portion 23a, the local increase in heat generation of the gradient magnetic field coil 2 due to the Z-direction gradient magnetic field main coil 21z being wound many times is suppressed.

Also in the present embodiment, as in the first embodiment, the Z-direction gradient magnetic field main coil 21z in the first region 101 is composed of conductors having different widths (a wide tubular good conductor and a narrow good conductor). You may. Even in this case, the same effect as the configuration shown in the first embodiment can be obtained.

The gradient magnetic field coil 2 according to the present embodiment is equipped with a Z-direction gradient magnetic field main coil 21z in which the current density of the first region 101 is higher than the current density of the second region 102. As a result, a gradient magnetic field having a large gradient and high linearity can be generated in the direction of the static magnetic field, so that the performance of the MRI apparatus 100 using the gradient magnetic field coil 2 according to the present embodiment can be improved.

1 Magnet device 2 Tilt magnetic field coil 21z, 21z 1,21z2 Z direction tilt magnetic field Main coil 100,200 MRI device S Imaging space

Claims (9)

A magnetic resonance imaging device including a magnet device that generates a static magnetic field in the Z direction of the imaging space, a high-frequency coil that generates a high-frequency magnetic field in the imaging space, and a gradient magnetic field coil that generates a gradient magnetic field in the imaging space. hand,
The gradient magnetic field coil is
It has a second region with a lower current density than the first region, and includes a Z-direction gradient magnetic field main coil that generates a gradient magnetic field in the Z direction.
The first region is a region in the Z-direction gradient magnetic field main coil from the Z coordinate of the outermost position of the imaging space to the Z coordinate where the gradient magnetic field strength in the Z direction is maximized.
The second region is a region in the Z-direction gradient magnetic field main coil from the Z coordinate where the gradient magnetic field strength in the Z direction is maximized to the Z coordinate at the end of the gradient magnetic field coil.
A magnetic resonance imaging apparatus characterized in that. The gradient magnetic field coil has a cylindrical shape.
The magnetic resonance imaging apparatus according to claim 1. The number of layers of the Z-direction gradient magnetic field main coil in the radial direction of the cylinder is larger in the first region than in the second region.
The magnetic resonance imaging apparatus according to claim 2. The gradient magnetic field coil has a disk shape.
The magnetic resonance imaging apparatus according to claim 1. The number of layers of the Z-direction gradient magnetic field main coil in the Z direction is larger in the first region than in the second region.
The magnetic resonance imaging apparatus according to claim 4. The current density of the first region is at least twice or more the current density of the second region.
The magnetic resonance imaging apparatus according to any one of claims 1 to 5. The first region is
It is composed of a conductor narrower than the conductor arranged in the second region and a conductor having the same width as the conductor arranged in the second region.
The magnetic resonance imaging apparatus according to any one of claims 1 to 6, wherein the magnetic resonance imaging apparatus is characterized. The Z-direction gradient magnetic field main coil is
It has a third region with a lower current density than the second region, and has a third region.
The third region is a region in the Z-direction gradient magnetic field main coil from the origin to the Z coordinate of the outermost position of the imaging space.
The magnetic resonance imaging apparatus according to any one of claims 1 to 7. The third region does not include a conductor.
The magnetic resonance imaging apparatus according to claim 8.

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