JPWO2016114198A1 - Magnetic resonance imaging system - Google Patents

Abstract

The MRI apparatus includes a static magnetic field generator (2) having a static magnetic field generator (2a, 2b) that generates a uniform magnetic field (7) in a space, and a gradient magnetic field that superimposes a gradient magnetic field (9) on the uniform magnetic field (7). A uniform magnetic field (7) is generated between the gradient magnetic field generation device (3) having the generation sources (3a, 3b) and the static magnetic field generation sources (2a, 2b) and the gradient magnetic field generation sources (3a, 3b). A pair of good conductor members (5a, 5c) provided on both sides in the uniform magnetic field (7) direction of the region (imaging space, 8), and a pair of good conductors on both sides in the uniform magnetic field (7) direction of the imaging space (8) A high electrical resistance member (5b) installed between the members (5a, 5c). This provides an MRI apparatus capable of reducing tomographic image degradation and heat generation due to eddy currents.

Description

  The present invention relates to a magnetic resonance imaging (hereinafter referred to as MRI; Magnetic Resonance Imaging) apparatus including a static magnetic field generator and a gradient magnetic field generator.

  As a conventional technique related to a method for suppressing eddy current in an MRI apparatus, an RF shield is provided by providing a counter shield made of a highly conductive member on the gradient magnetic field generator side between the static magnetic field generator and the gradient magnetic field generator. A technique for shielding a magnetic field caused by an eddy current generated has been proposed (see Patent Document 1). In addition, a technique has been proposed in which a good conductor ring is provided on both ends in the axial direction of the gradient magnetic field generator to reduce the vibration of the static magnetic field generator that causes eddy current (see Patent Document 2).

JP43333499B2 WO2013 / 046957A1

  However, in the technique of the above-described Patent Document 1, it is considered that a new eddy current is generated along with the vibration of the counter shield itself, the magnetic field in the imaging space is disturbed, and the tomographic image is deteriorated. Note that Patent Document 1 describes a method of suppressing the eddy current of the counter shield by providing a slit to partially increase the electrical resistance. However, since the magnetic field leaking from the gradient magnetic field generator increases, eddy current heat is generated in the low temperature member of the static magnetic field generator, and eddy currents are concentrated in the high electrical resistance region of the counter shield and the heat generation density is increased. Was considered. In the technique of Patent Document 2, the influence on the tomographic image is small because the good conductor ring in which the eddy current is generated is far from the imaging region. However, when the vibration of the gradient magnetic field generator increases, the influence on the tomographic image can be ignored. It was thought that it would disappear.

  Therefore, the problem to be solved by the present invention is to provide an MRI (magnetic resonance imaging) apparatus that can suppress deterioration of tomographic images due to vibration and reduce heat generation of a static magnetic field generator and a gradient magnetic field generator due to eddy currents. It is in.

  In order to solve the above problems, the present invention includes an outer peripheral ring structure that is divided between the gradient magnetic field generator and the static magnetic field generation source at both ends of the uniform magnetic field generation region in the uniform magnetic field direction. It is an MRI (magnetic resonance imaging) apparatus.

  According to the present invention, it is possible to provide an MRI apparatus that can suppress deterioration of a tomographic image due to vibration and reduce heat generation of a static magnetic field generation apparatus and a gradient magnetic field generation apparatus due to eddy currents.

1 is a schematic perspective view of an MRI (magnetic resonance imaging) apparatus according to a first embodiment of the present invention. 1 is a schematic longitudinal sectional view of an MRI apparatus according to a first embodiment of the present invention. It is a schematic longitudinal cross-sectional view of the gradient magnetic field generator of the MRI apparatus which concerns on the 1st Embodiment of this invention. It is a schematic longitudinal cross-sectional view of the gradient magnetic field generator of the MRI apparatus which concerns on the 2nd Embodiment of this invention. It is a conceptual diagram which determines the division | segmentation position of the outer periphery ring structure in the MRI apparatus which concerns on the 2nd Embodiment of this invention. It is a schematic longitudinal cross-sectional view of an axial direction edge part among the gradient magnetic field generators of the MRI apparatus which concerns on the 3rd Embodiment of this invention. It is a conceptual diagram which determines the division | segmentation position of the outer periphery ring structure in the MRI apparatus which concerns on the 3rd Embodiment of this invention. It is a schematic front view of the gradient magnetic field generator of the MRI apparatus which concerns on the 4th Embodiment of this invention. It is a schematic front view of the gradient magnetic field generator of the MRI apparatus which concerns on the 5th Embodiment of this invention.

  Next, embodiments of the present invention will be described in detail with reference to the drawings as appropriate. In each figure, common portions are denoted by the same reference numerals, and redundant description is omitted.

(First embodiment)
FIG. 1 shows a schematic perspective view of an MRI (magnetic resonance imaging) apparatus 1 according to the first embodiment of the present invention.

  The main equipment elements provided in the MRI apparatus 1 in the present embodiment are a static magnetic field generator 2, a gradient magnetic field generator 3, an irradiation coil 4, a receiving coil 22, and a bed 6. Hereinafter, each element will be described.

  The static magnetic field generator 2 generates a uniform magnetic field 7 (see FIG. 2) in the imaging space 8 in order to orient the spins of atoms constituting the living tissue of the subject 10. A shim coil (not shown) may be provided on the imaging space 8 side of the static magnetic field generator 2 in order to correct the magnetic field of the uniform magnetic field 7 and increase the uniformity. The static magnetic field generator 2 has a cylindrical shape having a cylindrical inner wall with the Z axis parallel to the horizontal direction as the central axis, and is supported from the floor surface by the vacuum vessel support legs 2f. Has been. For the sake of simplicity, the Z-axis is described as a straight line parallel to the horizontal direction. However, in reality, the Z-axis is not limited to this, and is substantially horizontal, for example, the direction in which the bed 6 is introduced into the imaging space 8. Etc. may be the Z axis.

  The gradient magnetic field generator 3 is provided closer to the imaging space 8 than the static magnetic field generator 2, and generates a magnetic field having a gradient in three axial directions orthogonal to the uniform magnetic field generated by the static magnetic field generator 2. Superimpose. Detailed description regarding the structure of the gradient magnetic field generator 3 will be described later.

  The irradiation coil 4 is provided closer to the imaging space 8 than the gradient magnetic field generator 3. The irradiation coil 4 is a cylindrical member having the same central axis as the static magnetic field generator 2 (with the Z axis as the central axis), and when divided by a plane orthogonal to the central axis, The shape is a circle or an ellipse. In addition, its role is to cause nuclear magnetic resonance to occur in atomic nuclei constituting the biological tissue of the subject 10 by irradiation with a high-frequency signal. A signal by nuclear magnetic resonance excited by the irradiation coil 4 is acquired by the receiving coil 22 attached to the bed 6.

  FIG. 2 shows a schematic longitudinal sectional view of the MRI apparatus 1 according to the first embodiment of the present invention.

  The static magnetic field generator 2 includes a static magnetic field main coil 2a (hereinafter referred to as a main coil 2a) employing a superconducting coil and a static magnetic field shield coil 2b (hereinafter referred to as a shield coil 2b) employing a superconducting coil. . The main coil 2a and the shield coil 2b are collectively referred to as a static magnetic field generation source.

  The MRI apparatus 1 also includes a cooling container 2e that houses and cools the main coil 2a and the shield coil 2b, which are superconducting coils, together with a refrigerant, a radiation shield plate 2d that has a structure containing the cooling container 2e, a cooling container 2e, and a radiation. A vacuum vessel 2c for housing and shielding the shield plate 2d in a vacuum environment, a vacuum vessel support leg 2f (see FIG. 1) for supporting the vacuum vessel 2c on the installation floor, a cooling vessel 2e and the radiation shield plate 2d are vacuumed. The container 2c has a load support (not shown) that supports the heat insulation.

  The main coil 2a has a ring shape, and its central axis coincides with the Z axis. In the present embodiment, a plurality (four in the example of FIG. 2) of main coils 2a are arranged along the Z-axis direction. The main coil 2 a can generate a uniform magnetic field 7 (static magnetic field) in the imaging space (space) 8. The magnetic field generated by the main coil 2a is also generated outside the imaging space 8. In particular, when the imaging region 9 is considered as a center, a magnetic field (leakage magnetic field) at a position farther than the main coil 2a in the Z-axis direction is generated. It becomes relatively strong. The shield coil 2b can reduce the magnitude of this leakage magnetic field.

  The shield coil 2b has a ring shape, and its central axis coincides with the Z axis. A plurality of shield coils 2b (two (a pair) in the example of FIG. 2) are arranged in the Z-axis direction. The arrangement position in the Z-axis direction is in the vicinity of the pair of main coils 2a arranged at both ends in the Z-axis direction of the main coil 2a, and when the imaging region 9 is at the center, it is more than the main coils 2a. Located far away. The shield coil 2b may be disposed closer to the imaging region 9 than the main coil 2a disposed at both ends in the Z-axis direction. Moreover, although the diameter of the shield coil 2b has illustrated the thing larger than the main coil 2a arrange | positioned at both ends in a Z-axis direction, not only this but a diameter smaller than the main coil 2a is also considered.

  FIG. 3 shows the gradient magnetic field generator 3 in FIG. 2 in detail. The details of the gradient magnetic field generator 3 and the outer ring structure 5 will be described with reference to this drawing.

  The shape of the gradient magnetic field generator 3 has a cylindrical shape having the same central axis as that of the static magnetic field generator 2 (with the Z axis as the central axis) as a whole, and is divided by a plane orthogonal to the central axis. In this case, the cross-sectional shape is a circle or an ellipse. Further, the main elements constituting the gradient magnetic field generator 3 are a gradient magnetic field main coil 3a (hereinafter referred to as main coil 3a), a gradient magnetic field shield coil 3b (hereinafter referred to as shield coil 3b), and these as an integral unit. Resin 3c to be fixed to A plurality of main coils 3a and shield coils 3b may be provided, and the resin 3c may include beads, glass fiber cloth, and the like.

  As for the arrangement relationship between the shield coil 3b and the main coil 3a, the shield coil 3b is arranged on the static magnetic field generator 2 side, that is, at a position farther from the z-axis than the main coil 3a. Moreover, the gradient magnetic field generator 3 is attached to the vacuum vessel 2c via an attachment member (not shown). In the following description, for convenience, the main coil 3a and the shield coil 3b are collectively referred to as a gradient magnetic field generation source.

  Next, the outer peripheral ring structure 5 will be described. The outer peripheral ring structure 5 is a cylindrical member provided on the imaging space 8 side of the static magnetic field generator 2 and on the outer side of the outer peripheral surface of the gradient magnetic field generator 3. The structure is divided at at least two different positions in the central axis direction, and at least three or more ring structures are arranged in the z-axis direction. In the following, the description will be continued by taking the case where the outer peripheral ring structure 5 is divided at two places as an example, and the rings located at both ends in the z-axis direction are the left end ring 5a (first ring member) and the right end ring 5c (first end). Two ring members), and the ring sandwiched between them is referred to as a central ring (third ring member) for convenience.

  With respect to the outer ring structure 5, the characteristics and the like of each ring are defined as follows. First, the left end ring 5a and the right end ring 5c are made of nonmagnetic and conductive copper or aluminum. on the other hand. The center ring 5b is non-magnetic and cylindrical, and is made of stainless steel or titanium alloy having higher electric resistance than the left end ring 5a and the right end ring 5c.

  Further, the outer shape and structural features of the outer peripheral ring structure 5 are summarized as follows. That is, the division (butting) between the left end ring 5a and the center ring 5b is provided outside the imaging space 8 in the central axis direction, that is, at a position not included in the gap between the imaging spaces 8 in the central axis direction. Yes. In other words, the abutting between the left end ring 5a and the center ring 5b is not an end where the magnetic field leaking from the gradient magnetic field generating device 3 to the static magnetic field generating device 2 is large, but an imaging space than there. It will be located on the 8th side. That is, it is desirable that the both-end rings 5a and 5c are installed only in a region where the magnetic field leaking from the gradient magnetic field generator 3 to the static magnetic field generator 2 is large, and the central ring 5b is not installed at a position where the leak magnetic field is large. .

  The central ring 5b is disposed so as to overlap the imaging space 8 in the Z-axis direction (the magnetic field direction of the uniform magnetic field 7), and has a length that includes at least the entire area of the imaging space 8 with respect to the Z-axis direction. One member.

  The outer ring structure 5 is mechanically coupled to the gradient magnetic field generator 3. Examples of the method for installing the outer peripheral ring structure 5 include ring shrink fitting and a method of welding a plurality of plates bent in an arc shape, and any method may be adopted.

  Next, in the MRI apparatus 1 of the present embodiment, a mechanism for suppressing deterioration of tomographic images due to eddy currents and heat generation of conductor members will be described.

  First, suppression of deterioration of a tomographic image due to eddy current will be described.

  When the subject is imaged by the MRI apparatus 1, a uniform magnetic field 7 is generated in the imaging space 8 by the static magnetic field generator 2. At the same time, a static magnetic field derived from the static magnetic field generator 2 is also generated in the region where the gradient magnetic field generator 3 is arranged. In this manner, a pulsed current flows through the gradient magnetic field generation source of the gradient magnetic field generator 3 under the influence of the static magnetic field. Then, the static magnetic field generated in the region where the gradient magnetic field generating device 3 is arranged and the pulsed Lorentz force act on the gradient magnetic field generating source due to the coupling of the pulsed current, and the gradient magnetic field generating device 3 vibrates. To do.

  Then, the vibration of the gradient magnetic field generator 3 propagates to the vacuum vessel 2c via an attachment member that attaches the gradient magnetic field generator 3 to the static magnetic field generator 2, and further from the vacuum vessel 2c via the load support. Then, it propagates to the radiation shield plate 2d and the cooling container 2e, and each member of the static magnetic field generator 2 vibrates. In addition, although the example which attached the gradient magnetic field generator 3 to the static magnetic field generator 2 was given here, it is not restricted to this, For example, the gradient magnetic field generator 3 is attached to other members which comprise MRI apparatus 1, such as the vacuum vessel 2c. When the is attached, vibrations are similarly generated and transmitted through the connected points.

  Further, in the vacuum vessel 2c, radiation shield plate 2d, cooling vessel 2e, etc. constituting the static magnetic field generation device 2, a part of the gradient magnetic field generated by the gradient magnetic field generation device 3 leaks, and the vortex is generated by the action of this leakage magnetic field. An electric current is generated. A pulsed Lorentz force acts on the static magnetic field generator 2 by coupling the eddy current derived from the gradient magnetic field and the static magnetic field generated by the static magnetic field generator 2. As a result, each member of the static magnetic field generator 2 vibrates, and these vibrations propagate to the gradient magnetic field generator 3 through the attachment members attached to the static magnetic field generator 2, so that the vibration of the gradient magnetic field generator 3 is vibrated. To increase.

  The fact that the static magnetic field generator 2 and the gradient magnetic field generator 3 vibrate is synonymous with the fact that the relative distance between the conductor members constituting the devices and the static magnetic field generators 2a and 2b changes. An electromotive force is generated by the change of the magnetic flux. As a result, an eddy current is induced in the conductor member of each device, and this eddy current creates a magnetic field that disturbs the uniform magnetic field in the imaging space 8, so that the tomographic image is degraded. The above is an example of a mechanism in which a tomographic image is deteriorated by eddy current.

  For such degradation of tomographic images, the outer peripheral ring structure 5 provided in the MRI apparatus 1 of the present embodiment is installed on the outer peripheral side of the gradient magnetic field generating apparatus 3 and is mechanically integrated to generate a gradient magnetic field. Since the rigidity of the device 3 increases, the vibration of the gradient magnetic field generator 3 can be reduced. Moreover, the vibration which propagates to the static magnetic field generator 2 via an attachment member reduces, and the vibration of the static magnetic field generator 2 is reduced.

  Further, in the outer ring structure 5, the left end ring 5a and the right end ring 5c installed in a region where the leakage magnetic field of the gradient magnetic field generator 3 is large are made of highly conductive members. For this reason, when a pulsed current flows through the gradient magnetic field generation source of the gradient magnetic field generator 3 during imaging, eddy currents are generated in the left end ring 5a and the right end ring 5c. Since the magnetic field leaking to the magnetic field generator 2 side is shielded, the eddy current generated in the conductor member constituting the static magnetic field generator 2 is reduced, and the Lorentz force is also reduced. Since the outer ring structure 5 causes such an action, the vibration of the static magnetic field generator 2 is reduced.

  Furthermore, since the center ring 5b of the outer ring structure 5 is also mechanically integrated with the gradient magnetic field generator 3, the rigidity of the gradient magnetic field generator 3 is enhanced throughout the gradient magnetic field generator 3. An effect of reducing vibration is obtained. Along with this, the vibration propagating from the gradient magnetic field generator 3 to the static magnetic field generator 2 is reduced, so that the vibration of the static magnetic field generator 2 is reduced. Note that, from the viewpoint of increasing the rigidity, it is desirable that the central ring 5b is composed of a single member whose entire surface is continuous.

  As described above, the MRI apparatus 1 according to the present embodiment is configured to reduce the Lorentz force of the static magnetic field generator 2 and reduce the vibration propagation from the gradient magnetic field generator 3, thereby forming a conductor member that constitutes the static magnetic field generator 2 with vibration. The eddy current induced in the imaging space 8 is reduced, the magnetic field due to the eddy current generated in the imaging space 8 is suppressed, and the effect of preventing the deterioration of the tomographic image is obtained.

  Since the outer peripheral ring structure 5 is mechanically integrated with the gradient magnetic field generating device 3, the outer peripheral ring structure 5 also vibrates with the vibration of the gradient magnetic field generating device 3, and is connected to the static magnetic field generating sources 2a and 2b. Since the relative distance changes and the flux linkage changes, an eddy current is generated. However, the axially divided portion of the outer peripheral ring structure 5, that is, the butted portion between the left end ring 5a and the central ring 5b, or the butted portion between the right end ring 5c and the central ring 5b is the diameter of the imaging space 8 with respect to the central axis direction. Further, each ring located closer to both ends, that is, located at the end in the central axis direction, is kept away from the imaging space 8, so that even if an eddy current is generated, the influence on the tomographic image can be reduced.

  In addition, since the vibration of the gradient magnetic field generator 3 is reduced in the system of the MRI apparatus 1 of the present embodiment, the change in the linkage flux is smaller than in the conventional case, and eddy currents generated in the left end ring 5a and the right end ring 5c are reduced. Since it decreases, the influence on the tomographic image can be made smaller. On the other hand, the central ring 5b having a large electric resistance is close to the imaging space 8, but in the central axis direction, the shield coil 3b is entirely covered so that the majority of the gradient magnetic field is shielded. Since it is shielded by 3b and the induced eddy current is small, the influence on the tomographic image is small.

  As described above, according to the present embodiment, it is possible to suppress the deterioration of the tomographic image.

  Next, suppression of heat generation due to eddy current will be described.

  When a pulsed current flows through the main coil 3a and the shield coil 3b of the gradient magnetic field generation device 3 during imaging, the static magnetic field generation device 2 is configured by a magnetic field that leaks from the gradient magnetic field generation device 3 toward the static magnetic field generation device 2. An eddy current is induced in the conductor member. Along with this, eddy current loss occurs in the conductor member constituting the static magnetic field generating device 2, and the conductor member generates heat. Among the conductor members, the radiation shield plate 2d and the cooling container 2e are kept lower than the normal temperature by the refrigerant or the refrigerator, so that the eddy current loss becomes a heat load and consumes the refrigerant or increases the output of the refrigerator. It is necessary to re-cool.

  The outer peripheral ring structure 5 is installed on the outer peripheral side of the gradient magnetic field generator 3 and is magnetically coupled to the gradient magnetic field generation source of the gradient magnetic field generator 3. Therefore, when a pulsed current flows through the gradient magnetic field generation source during imaging, an eddy current induced in the leakage magnetic field toward the outer periphery of the gradient magnetic field generator 3 is generated in the outer ring structure 5.

  Here, considering the balance with the general outer shape of the gradient magnetic field generator 3 in the so-called horizontal magnetic field type MRI apparatus 1, the outer ring structure 5 has a large diameter and a thin structure. In particular, since the eddy current induced by the pulse current flowing through the gradient magnetic field generation source is a high-frequency current, the self-inductance is more dominant than the electrical resistance with respect to the impedance. Therefore, if the induced electromotive force due to the leakage magnetic field of the gradient magnetic field generator 3 is the same, the induced eddy current is almost constant because it is determined by the self-inductance without depending on the electric resistance. For this reason, if a member having a high electrical resistance is arranged in a region where the leakage magnetic field of the gradient magnetic field generator 3 is large, there is a possibility that heat generated by the eddy current becomes excessive.

  However, since the left end ring 5a and the right end ring 5c of the outer peripheral ring structure 5 included in the MRI apparatus 1 of the present embodiment are composed of highly conductive members, eddy currents are effectively generated during imaging, and a gradient magnetic field is generated. The magnetic field leaking from the generator 3 to the static magnetic field generator 2 side is shielded. As a result, the eddy current generated in the conductor member constituting the static magnetic field generation device 2 is reduced and the eddy current loss is also reduced, so that the refrigerant consumption and the refrigerator output of the static magnetic field generation device 2 are reduced. Further, since the electric resistance of the left end ring 5a and the right end ring 5c is small, heat generation by the eddy current in the left end ring 5a and the right end ring 5c itself is small, and the gradient magnetic field generator 3 is hardly heated.

  On the other hand, in the outer ring structure 5, the central ring 5b is made of a member having low conductivity, but is disposed in a region where the magnetic field leaking from the gradient magnetic field generator 3 to the static magnetic field generator 2 is small. The induced eddy current is small. For this reason, the gradient magnetic field generator 3 is hardly heated.

  As described above, according to the present embodiment, heat generation due to eddy currents in the static magnetic field generator 2 and the gradient magnetic field generator 3 can be suppressed.

  As described above, the outer peripheral ring structure 5 is provided on the outer peripheral side of the gradient magnetic field generation device 3, and the magnetic field leaking from the gradient magnetic field generation device 3 to the static magnetic field generation device 2 side from both ends of the imaging space 8 with respect to the central axis direction. By dividing in the axial direction at a position inside the end position where it increases, the electrical resistance at both ends in the axial direction is reduced, and the electrical resistance at the central side in the axial direction is increased, thereby suppressing degradation of tomographic images and eddy currents. It is possible to provide an MRI apparatus that achieves both suppression of heat generation due to. In addition, since vibrations that cause eddy currents are reduced, effects such as reduction of vibrations propagating from the static magnetic field generator 2 to the bed 6 and reduction of noise felt by the subject 10 and an examination engineer (not shown) are also obtained. It is done.

(Second Embodiment)
FIG. 4 is a schematic longitudinal sectional view of the gradient magnetic field generator 3 of the MRI apparatus 1 according to the second embodiment of the present invention. The MRI apparatus 1 of the second embodiment is different from the MRI apparatus 1 of the first embodiment in that the gradient magnetic field generator 3 to the static magnetic field generator in the arrangement region of the divided portion of the outer ring structure 5. The end position where the magnetic field leaking to the side 2 becomes larger is that the eddy current loss generated in the central ring 5b is determined to be equal to or less than the leakage magnetic field that causes the gradient magnetic field generator 3 to overheat.

  The relationship between the axial length of the center ring 5b and the eddy current loss generated in the center ring 5b can be described as follows. The axial length of the central ring 5b is z, the electrical resistance and the self-inductance around the circumference per axial unit length of the central ring 5b are R and L, respectively, and the axial unit length of the gradient magnetic field generator 3 is Where B is the leakage magnetic field, r is the radius of the central ring 5b, and ω is the angular frequency of the pulse current applied to the gradient magnetic field generator 3, the eddy current loss W of the central ring 5b was determined by a circuit equation. Using the eddy current I generated in the center side ring 5b, it can be calculated as follows.

  When formula (1) is transformed into a function of B, formula (2) is obtained.

If the allowable eddy current loss W is determined from the equation (2), the relationship between the allowable leakage magnetic field B and the length z of the central ring 5b can be drawn. On the other hand, the curve of the z-direction distribution _BGC of the leakage magnetic field of the gradient magnetic field generator 3 is obtained from the arrangement of the main coil 3a and the shield coil 3b of the gradient magnetic field generation source. Therefore, the position where the above two curves overlap on the graph is determined as the end of the center ring 5b, and the outer peripheral ring structure 5 is formed, whereby the eddy current loss generated in the center ring 5b can be reduced.

  As an example, a case where the leakage magnetic field generated by the gradient magnetic field generation device 3 is expressed by Expression (4) will be described.

  When the center ring 5b is a stainless steel plate having a diameter of 900 mm and a plate thickness of 1 mm, the frequency is 1 kHz and the eddy current loss tolerance is 1 kW, R is about 0.6Ω, L is about 1 μH, and B is z is 300 mm. The position is 9 mT, the position of 600 mm is 5 mT, and the position of 900 mm is about 3 mT. Accordingly, when the position where the two curves intersect is obtained, z = 566 mm and 810 mm are obtained. Therefore, the smaller side of these solutions is the end of the arrangement area where the central ring 5b may be installed. A conceptual diagram of the above relationship is shown in FIG.

  The axial region length of the central ring 5b is determined by determining the arrangement area of the divided portion of the outer ring structure 5 with reference to the position where the leakage magnetic field from the gradient magnetic field generator 3 exceeds the allowable value. It can be maximized without overheating. In other words, regarding the structure of the outer peripheral ring structure 5 in the central axis direction, based on the above technical idea, the outer peripheral ring structure 5 in the present embodiment includes the side surface of the central ring 5b facing the imaging space 8 and the imaging. The distance from the center of the space 8 is kept below a predetermined value that can be acquired in advance based on the strength of the magnetic field leaking from the gradient magnetic field generation source, thereby maximizing the length of the central ring 5b and preventing overheating. It means that you can balance. Thereby, the effect of increasing the rigidity of the gradient magnetic field generator 3 is maximized, the vibration of the gradient magnetic field generator 3 is reduced, and the eddy current can be suppressed.

(Third embodiment)
FIG. 6 shows a schematic longitudinal sectional view of the gradient magnetic field generator 3 of the MRI apparatus 1 according to the third embodiment of the present invention. The MRI apparatus 1 according to the third embodiment is different from the MRI apparatus 1 according to the first embodiment in that the gradient magnetic field generator 3 to the static magnetic field generator in the arrangement region of the divided portion of the outer ring structure 5. The edge position at which the magnetic field leaking to the side 2 increases is determined so that the eddy current generated in the left end ring 5a and the right end ring 5c is equal to or less than the allowable current value that causes the imaging space 8 to generate a magnetic field that deteriorates the tomographic image. Is a point.

  The relationship between the axial length of the center ring 5a and the eddy current generated in the left end ring 5a can be described as follows. The right end ring 5c has a substantially mirror-symmetric structure with respect to the left end ring 5a, and a description thereof will be omitted.

The distance from the axial end position of the left end ring 5a to the center of the imaging space 8 is d, the axial length of the left end ring 5a is z, and the electric resistance that goes around the circumference per unit axial length of the left end ring 5b , The self-inductances are R and L, the static magnetic field intensity generated by the static magnetic field generator 2 at the position of the left ring 5a is B ₀ , the radius of the left ring 5a is r, the vibration speed of the left ring 5a is v, and the angular frequency of vibration Is set to ω, the eddy current I generated in the left end ring 5a is obtained from the circuit equation as follows.

On the other hand, the magnetic field B _{err generated} by the above I in the imaging space 8 is expressed as follows using the vacuum permeability μ ₀ .

When the allowable magnetic field B _err is determined from Equation (6), the relationship between the allowable eddy current I _err and the length z of the left end ring 5a can be drawn. Therefore, by determining the position where the two curves overlap on the graph as the end on the inner side in the axial direction of the left end ring 5a, the eddy current generated in the left end ring 5a can be reduced, and deterioration of the tomographic image can be suppressed.

As an example, a case where B _err is 10 μT will be described. When the left end ring 5a is a copper plate having a diameter of 900 mm, an axial end position of 850 mm, and a plate thickness of 1 mm, the static magnetic field strength B ₀ is at a position of 4T, and when vibrating at a frequency of 150 Hz and a speed of 100 mm / s, R is 10 mΩ. L is about 1 μH, and I is about 70 A at a position where z is 200 mm, 50 A at a position of 400 mm, and about 30 A at a position of 600 mm from Equation (5). On the other hand, from the equation (7), I _err is about 7 A at a position where z is 200 mm, 15 A at a position of 400 mm, and about 30 A at a position of 600 mm. From this, when obtaining a real number solution where two curves intersect, z = 592 mm. Therefore, this is the axially inner end of the arrangement area where the left end ring 5a may be installed.

  In other words, based on the technical idea described above, the outer peripheral ring structure 5 according to the present embodiment includes the side surfaces of the left end ring 5a and the right end ring 5c facing the central axis, the imaging space 8, and By installing the left end ring 5a and the right end ring 5c so that the distance between the left end ring 5a and the right end ring 5c is equal to or greater than a predetermined value, the left end ring 5a It also shows that it is possible to suppress the deterioration of the uniform magnetic field due to the eddy current accompanying the vibration of the right end ring 5c. A conceptual diagram of the above relationship is shown in FIG.

  The axial length of the central ring 5b is determined by determining the arrangement region of the divided portion of the outer ring structure 5 with reference to the position where the magnetic field generated in the imaging space 8 by the eddy current generated in the left end ring 5a exceeds the allowable value. It can be maximized as long as the tomographic image does not deteriorate. Thereby, the effect of increasing the rigidity of the gradient magnetic field generator 3 is maximized, the vibration of the gradient magnetic field generator 3 is reduced, and the eddy current can be suppressed.

(Fourth embodiment)
FIG. 8 shows a schematic longitudinal sectional view of the gradient magnetic field generator 3 of the MRI apparatus 1 according to the fourth embodiment of the present invention. The MRI apparatus 1 of the fourth embodiment is different from the MRI apparatus 1 of the first to third embodiments in that the left end ring 5a and the right end 5c are not the gradient magnetic field generator 3 but the static magnetic field generator 2. And mechanically integrated.

  By fixing the left end ring 5a and the right end ring 5c to the static magnetic field generation device 2 side, the left end ring 5a and the right end ring 5c vibrate not with the gradient magnetic field generation device 3 but with the vibration of the static magnetic field generation device 2. Therefore, the influence on the tomographic image can be reduced when the vibration of the gradient magnetic field generator 3 is larger than the vibration of the static magnetic field generator 2.

(Fifth embodiment)
In FIG. 9, the schematic front view of the gradient magnetic field generator 3 of the MRI apparatus 1 which concerns on the 5th Embodiment of this invention is shown. The MRI apparatus 1 of the fifth embodiment is different from the MRI apparatus 1 of the first to third embodiments in that the left end ring 5a, the right end 5c and the central ring 5b are adjacent to the gradient magnetic field generator 3. Are located at both ends of the imaging space 8 with respect to the central axis direction and at a position where the magnetic field leaking from the gradient magnetic field generator 3 to the static magnetic field generator 2 is small, and the radially outer portion of the central ring 5b is This is that the left end ring 5a and the right end ring 5c are covered in the axial direction. In the region where the left end ring 5a, the right end 5c and the center ring 5b overlap, it is desirable that the interface between the left end ring 5a, the right end 5c and the center ring 5b is insulated.

  Since the center ring 5b covers the left end ring 5a and the right end ring 5c, the left end ring 5a and the right end ring 5c are structurally reinforced, and therefore the gradient magnetic field generator 3, the left end ring 5a, and the right end ring 5c vibrate. And the influence on the tomographic image can be reduced. In addition, the central ring 5b overlaps the axial region where the magnetic field leaks from the gradient magnetic field generator 3 to the static magnetic field generator 2 side, but the left end ring 5a and the right end ring 5c exist between the gradient magnetic field generation sources. Since the leakage magnetic field is shielded, the eddy current induced in the central ring 5b is small. For this reason, the eddy current loss of the center ring 5b is also small, and the temperature rise of the outer periphery ring structure 5 and the gradient magnetic field generator 3 can be made small.

  In the first to fifth embodiments described above, the superconducting coils are taken up as the static magnetic field generation sources 2a and 2b, but the present invention is not limited to this. A normal conducting coil or a permanent magnet may be used as the static magnetic field generating sources 2a and 2b.

DESCRIPTION OF SYMBOLS 1 Magnetic resonance imaging apparatus 2 Static magnetic field generator 2a Static magnetic field generator (main coil)
2b Static magnetic field generation source (shield coil)
2c Vacuum container (outer wall of static magnetic field generator)
2d Radiation shield plate 2e Cooling vessel 2f Vacuum vessel support leg 3 Gradient magnetic field generator 3a Gradient magnetic field source (main coil)
3b Gradient magnetic field source (shield coil)
3c Resin 4 Irradiation coil 5 Outer ring structure 5a, 5c Left end ring, Right end ring 5b Center ring 6 Bed 7 Uniform magnetic field 8 Imaging space 9 Gradient magnetic field 10 Subject 11 Elastic body 12 Shim tray 15 Outer ring structure division area 21 High frequency Coil 22 Receiving coil 31 Leakage magnetic field distribution of gradient magnetic field generator 32 Relationship between center side ring length and leakage magnetic field when center side ring eddy current loss is constant 33 Relationship between both end side ring length and both end side ring eddy currents 34 Relationship between deteriorated eddy current and generation position

Claims (5)

A static magnetic field generation apparatus having a cylindrical static magnetic field generation source capable of introducing a subject into an internal imaging space;
A gradient magnetic field generator having a gradient magnetic field generation source for superimposing a gradient magnetic field on the imaging space;
On the radially outer side of the gradient magnetic field generating source and the radially inner side of the static magnetic field generating device,
A pair of cylindrical good conductor members provided on both ends of the imaging space in the uniform magnetic field direction;
A magnetic resonance imaging apparatus comprising: a cylindrical high-resistance member that is installed between a pair of the good conductor members in the uniform magnetic field direction of the imaging space and has a higher electrical resistance than the good conductor members. The high resistance member is:
One member having continuity,
The length in the central axis direction of the static magnetic field generator is stored within a length range such that the magnetic field intensity leaking from the gradient magnetic field generation source is a predetermined value or less,
The magnetic resonance imaging apparatus according to claim 1, wherein the good conductor member is insulated. The good conductor member is:
The length in the central axis direction of the static magnetic field generator is a length such that a distance between the imaging space and the good conductor member is equal to or greater than a predetermined value. Magnetic resonance imaging equipment. The good conductor member is fixed to the static magnetic field generator,
The high resistance member is fixed to the gradient magnetic field source,
The magnetic resonance imaging apparatus according to claim 1, wherein the magnetic resonance imaging apparatus is a magnetic resonance imaging apparatus. The high resistance member covers the outside in the radial direction of the good conductor member,
The magnetic resonance imaging apparatus according to any one of claims 1 to 3.

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