JP2005118098A - Magnetic resonance imaging equipment - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic resonance imaging apparatus capable of enhancing the symmetricalness of the magnetic characteristics when a gradient magnetic field is applied even if a magnetic circuit of a static magnetic field generating device is spatially asymmetrical. <P>SOLUTION: A high magnetic permeability member 37 whose magnetic permeability is higher than the material constituting a pole plate 31 and a permanent magnet 38 are disposed between the pole plate 31 and a gradient magnetic field generating part 34. As the permanent magnet 38, such a permanent magnet as magnetized in the direction parallel to the static magnetic field of an imaging region 30 can be used. Accordingly, as the static field and the magnetic field generated by the permanent magnet 38 are superimposed on the whole high magnetic permeability member 37, the influence of the asymmetricalness of the static magnetic field can be reduced. Further, as the direction of magnetization of the permanent magnet 38 is parallel to that of the static magnetic field, the magnetization of the high magnetic permeability member can be turned toward the static magnetic field. As a result of these effects, the symmetricalness of the magnetic characteristics of the high magnetic permeability member can be enhanced. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus), and more particularly, to an MRI apparatus having excellent symmetry of magnetic characteristics of a static magnetic field generator when a gradient magnetic field is applied.

The MRI apparatus includes a static magnetic field generator that generates a uniform static magnetic field space in an imaging region in which a subject is arranged. A typical static magnetic field generator is configured to generate a magnetic field having a uniformity of 0.5 to 30 ppm with an intensity of about 0.2 to 1.5 T in a spherical imaging region typically of about 30 cm. ing. As a general structure of a static magnetic field generation apparatus using a coil as a magnetic field generation source, for example, as shown in FIG. 12, a static magnetic field generation coil 132 arranged above and below an imaging region 130 and a static magnetic field are made uniform. For this purpose, a magnetic pole plate 131 disposed inside the static magnetic field generating coil 132, and a yoke 135 and an iron pillar 133 that constitute the magnetic circuit by connecting the magnetic pole plates 131 are included. At this time, in order to make the periphery of the imaging region 130 as open as possible, a structure of a left-right asymmetric C-shaped magnetic circuit in which the iron pillar 133 is arranged only on one side in the left-right direction of the magnetic pole plate 131 is known. It has been. The magnetic pole plate 131 has a groove 137 formed on the surface thereof in order to increase the uniformity of the static magnetic field (see Patent Document 1). In the example of FIG. 12, the magnetic pole plate 131, the yoke 135, and the iron pillar 133 are integrally formed of a ferromagnetic material such as iron.
In addition, on the imaging region 130 side of the upper and lower magnetic pole plates 131, a gradient magnetic field generating coil 134 for applying a gradient magnetic field to the imaging region 130 is arranged in order to add position information to the NMR signal from the subject. The The gradient magnetic field coil 134 is disposed inside the magnetic pole protrusion 136 around the magnetic pole plate 131. The magnetic field generated by the gradient magnetic field generating coil 134 is typically about 10 mT / m to 50 mT / m and a slew rate of about 50 to 300 T / m / s.

However, conventionally, there are two problems: (1) non-linear interaction between the static magnetic field generator and the gradient magnetic field generating coil 134, and (2) spatially asymmetric interaction. The non-linear interaction (1) means that a ferromagnetic material such as iron constituting the magnetic pole plate 131 has a non-linear BH characteristic by receiving a gradient magnetic field of the gradient coil 134, and a static magnetic field is generated. This is a phenomenon in which the static magnetic field does not become zero due to the residual magnetic field of the magnetic pole plate 131 even when the current of the coil 132 is set to 0 and the generation of the magnetic field is turned off.

  Another spatial asymmetry (2) will be described with reference to FIG. FIG. 13 shows the magnitude and direction of magnetization of the magnetic pole plate 131, the magnetic pole protrusion 136, the yoke 135, and the iron pillar 133 in a state where the static magnetic field generating device generates a static magnetic field by the length and direction of the arrows. The magnetic circuit of FIG. 13 is a C-shape with a return path on the right side and is asymmetrical in the left and right (x direction). Therefore, at the position (a) and the position (b) in the figure on the surface of the magnetic pole plate 131, In spite of being equidistant from the center of the imaging region 130, the magnitude of magnetization becomes asymmetric. For example, the position (a) can be about twice as large as the position (b).

  As described above, the magnetization of the magnetic pole plate 131 is asymmetrical to the left and right causes the following adverse effects. First, since the magnitudes of the magnetizations M at the positions (a) and (b) of the magnetic pole plate 131 are asymmetric, the magnetic flux density (static magnetic field) at the positions (c) and (d) in the imaging region 130. B is also asymmetric. That is, the static magnetic field B in the imaging region 130 becomes asymmetric with respect to the x direction, and the uniformity decreases (2-1). Second, since the differential permeability (dB / dH) differs if the magnetization M at the position (a) and the position (b) on the magnetic pole plate 131 differs, the magnetic behavior when the gradient magnetic field ΔH is applied (H + ΔH). Is different between position (a) and position (b) (2-2). One phenomenon in which the magnetic behavior is different is that a change in the magnetization M of the magnetic pole plate 131 when a gradient magnetic field ΔH is applied, that is, a difference in the minor loop shape between the position (a) and the position (b). (2-2-1). The other is that the magnitude of the eddy current due to the gradient magnetic field application ΔH differs between position (a) and position (b) (2-2-2). Here, the eddy current is a current that flows through the magnetic pole plate 131 so as to cancel the gradient magnetic field ΔH, and its magnitude is proportional to dΔH / dt. Due to the above (2-2-1) and (2-2-2), the magnetic flux density of the magnetic pole plate 131 changes asymmetrically and is superimposed on the gradient magnetic field ΔH, so that it is actually applied to the imaging region 130. The error between the value of the gradient magnetic field ΔH and the set value increases, and the image quality is degraded.

  In order to reduce the non-linear BH characteristics of the magnetic pole plate 131 due to the above-described non-linear interaction (1), it has been proposed to paste a material having high resistance and high magnetic permeability on the surface of the magnetic pole plate 131 (patent) Reference 2 and Patent Document 3). In this method, since the material having high permeability prevents the gradient magnetic field of the gradient magnetic field coil from reaching the magnetic pole plate, an effect of reducing the interaction between the magnetic pole and the gradient magnetic field can be obtained, and eddy currents are hardly generated. . Patent Document 1 and Patent Document 4 disclose disposing a flat high magnetic permeability material at a position away from a magnetic pole plate in which grooves are formed in order to increase the uniformity of a static magnetic field.

It is also known to dispose another means such as an iron piece shim or shim coil in order to correct the above-described asymmetrical static magnetic field B (2-1). Also, a method of attaching a conductor is known in order to correct that the eddy current of the magnetic pole plate is generated asymmetrically (2-2-2) (see Patent Document 5).
JP 2002-153439 A USP 5,061,897 USP 5,555,251 USP 6,498,488 JP-A-63-150060

  In the conventional static magnetic field generator having an asymmetric magnetic circuit, the above four problems (1, 2-1, 2-2-1, 2-2-2) have occurred. Of these, methods (1), (2-1), and (2-2-2) have been proposed as described above for reducing them, but the problem of (2-2-1) Conventionally, it was difficult to correct. In addition, with regard to the technique for reducing the above-mentioned problems (1), (2-1), and (2-2-2), one technique can reduce one problem, but all with one technique. This problem could not be solved at once.

In the static magnetic field generator using a superconducting coil as the static magnetic field generating coil, the size of the generated static magnetic field is large, so that the image quality can be improved. The problem of 2-2) greatly occurs. For this reason, improvement of the said subject with the MRI apparatus using a superconducting coil is calculated | required strongly.
An object of the present invention is to provide an MRI apparatus capable of enhancing the symmetry of magnetic characteristics when a gradient magnetic field is applied even if the magnetic circuit of the static magnetic field generator is spatially asymmetric.

  In order to achieve the above object, in the present invention, a magnetic permeability is applied between the magnetic pole plate and the gradient magnetic field generating portion, a high magnetic permeability member having a higher magnetic permeability than the material constituting the magnetic pole plate, and the high magnetic permeability member. A permanent magnet is arranged. Since the static magnetic field and the magnetic field generated by the permanent magnet are superimposed and applied to the entire high magnetic permeability member, the influence of the asymmetry of the static magnetic field can be reduced.

  As this permanent magnet, a magnet magnetized in a direction parallel to the direction of the static magnetic field in the imaging region can be used. Thereby, the effect | action which directs the magnetization of a high-permeability member to the direction of a static magnetic field is also acquired, and the symmetry of the magnetic characteristic of a high-permeability member can be improved.

  A permanent magnet having a spatially symmetrical shape with respect to the central axis of the imaging region can be used.

  A plate-like member can be used as the high magnetic permeability member and the permanent magnet, and can be arranged in an overlapping manner.

  As the permanent magnet, a plurality of magnets arranged in a spatially symmetrical shape with respect to the central axis of the imaging region and mounted on a high permeability member can be used.

  As the permanent magnet, a plurality of ring-shaped magnets arranged concentrically with respect to the central axis in the imaging region and mounted on a high permeability member can be used.

The plurality of ring-shaped magnets can be arranged so as to be spaced from each other, and the width and spacing of the ring-shaped magnets can be determined so as to correct the uniformity of the static magnetic field in the imaging region.
The plurality of ring-shaped magnets can be arranged in contact with the magnetic pole plate.

  The permanent magnet is configured to be disposed between the high magnetic permeability member and the magnetic pole plate, and a second permanent magnet can be further disposed between the permanent magnet and the magnetic pole plate. At this time, the second permanent magnet may be magnetized in a direction intersecting with the direction of the static magnetic field in the imaging direction.

  As the high permeability member, it is possible to use silicon steel or iron powder that has been subjected to insulation treatment and hardened with resin.

  ADVANTAGE OF THE INVENTION According to this invention, even if the magnetic circuit of a static magnetic field generator is spatially asymmetrical, the MRI apparatus which can improve the symmetry of the magnetic characteristic at the time of applying a gradient magnetic field can be provided.

  An MRI apparatus provided with a static magnetic field generation apparatus according to a first embodiment of the present invention will be described with reference to the drawings.

  As shown in FIG. 1, the MRI apparatus according to the present embodiment includes a static magnetic field generation apparatus 402 that generates a uniform static magnetic field space in an imaging region where a subject 401 is arranged. As shown in the perspective view of FIG. 2 and the cross-sectional view of FIG. 3, the static magnetic field generator 402 is arranged above and below the imaging region 30 to generate a static magnetic field in the z direction in the spherical imaging region 30. A pair of static magnetic field generating coils 32, a magnetic pole plate 31 disposed inside the static magnetic field generating coil 30 to make the static magnetic field uniform, and a yoke that connects the magnetic pole plates 31 to constitute a magnetic circuit 35 and the iron pillar 33. The magnetic pole plate 31 is made of iron, which is a ferromagnetic body, and in this embodiment, the magnetic pole plate 31, the yoke 35, and the iron pillar 33 are integrally formed. A groove 237 is formed on the surface of the pole plate 31 in order to increase the uniformity of the static magnetic field.

  The iron pillar 33 is disposed on one side of the magnetic pole plate 31 in order to make a wide open space around the imaging region 30. Thereby, the magnetic circuit comprised by the magnetic pole plate 31, the yoke 35, and the iron pillar 33 becomes a left-right asymmetric C shape.

  The size of the imaging region 30 is, for example, about 30 cm, the strength of the static magnetic field is designed to be, for example, about 0.2 T to 1.5 T, and the uniformity thereof is, for example, 0.5 to 30 ppm. can do.

  As the static magnetic field generating coil 32, a normal coil or a superconducting coil accommodated in a cryostat can be used. A magnetic pole projection 36 connected to the yoke 35 is provided on the inner peripheral side of the static magnetic field generating coil 32 so as to surround the magnetic pole plate 31. A gradient magnetic field generating coil 34 is disposed inside the magnetic pole projection 36. The gradient magnetic field generating coil 34 applies a gradient magnetic field for adding position information to the NMR signal from the subject to the imaging region 30. The magnitude of the gradient magnetic field can be set to, for example, about 10 mT / m to 50 mT / m and a slew rate of about 50 to 300 T / m / s.

  In the present embodiment, a high permeability member 37 and a permanent magnet 38 are further arranged in a space between the magnetic pole plate 31 and the gradient magnetic field generating coil 32. The high magnetic permeability member 37 is a member having a higher magnetic permeability than the ferromagnetic material that is disposed on the gradient magnetic field generating coil 34 side and constitutes the magnetic pole plate 31. Since the magnetic flux of the gradient magnetic field generated by the gradient magnetic field coil 32 passes through the high permeability member 37 having a higher permeability than the magnetic pole plate 31, it acts to prevent the gradient magnetic field from reaching the magnetic pole plate 31. The high magnetic permeability member 37 desirably has a high electrical resistance value in order to prevent the generation of eddy currents. For example, as the high magnetic permeability member 37, a laminated silicon steel plate or an insulating iron powder that has been solidified with a resin and formed into a plate shape can be suitably used. If the high-permeability member 37 is too thick, it is difficult to adjust the magnetic field uniformity by the groove 237 of the magnetic pole plate 31, so it is desirable to set it to about 50 mm or less. Since it passes through the magnetic permeability member 37 and reaches the magnetic pole plate 31, it is desirable to set the thickness between about 5 mm and 50 mm.

  The permanent magnet 38 is a plate-like magnet that is magnetized in the thickness direction (z direction), and is attached to the surface of the high permeability member 37 on the side of the magnetic pole plate 31. The permanent magnet 38 may be a normally manufactured one, but is preferably as thin and light as possible. Therefore, it is desirable to use a rare earth permanent magnet such as an Nb—Fe—B alloy having a large maximum energy product.

  The operation of the permanent magnet 38 will be described. The arrangement of the high permeability member 37 makes it difficult for the magnetic flux of the gradient magnetic field ΔH of the gradient magnetic field generating coil 34 to reach the magnetic pole plate 31, so the problems described in the prior art section (1, 2-1, 2) 2-1 and 2-2-2), the problem (1) in which the magnetic pole plate 31 has a non-linear BH characteristic can be reduced, but the magnetic pole plate 31 has a magnetic circuit having an asymmetrical shape. The problem (2-1) that the magnitude of the magnetization M is left-right asymmetric remains. Magnetization of the high permeability member 37 is excited by the left-right asymmetric magnetization of the magnetic pole plate 31, and the left-right asymmetric distribution of magnetization is obtained. Therefore, the problem that the magnetization change (minor loop) of the magnetic pole plate 31 when the conventional gradient magnetic field ΔH is applied occurs asymmetrically (2-2-1), and the eddy current due to the gradient magnetic field ΔH occurs asymmetrically. The problem (2-2-2) is suppressed in the magnetic pole plate 31 in the case of the configuration of the present embodiment in which the high magnetic permeability member 37 is disposed, but occurs in the high magnetic permeability member 37. The left-right asymmetric distribution of magnetization present in the high permeability member 37 causes a problem that the uniformity of the static magnetic field decreases and the error of the gradient magnetic field ΔH increases. The permanent magnet 38 acts to reduce the problem (2-1, 2-1-2, 2-2-2) in the high magnetic permeability member 37.

  This will be specifically described with reference to the drawings. When the BH characteristics (where B is a magnetic flux density and H is an external magnetic field) of the high magnetic permeability member 37 are curves as shown in FIG. 4, the positions of the left and right ends of the high magnetic permeability member 37 shown in FIG. The static magnetic field H generated by the magnetic pole plate 31 and the magnetic pole protrusion 36 in the vicinity thereof is applied to (e) and the position (f). The static magnetic field H is smaller in the static magnetic field He at the position (f) than the static magnetic field Hf at the position (f) due to the influence of the C-shaped left-right asymmetric shape of the magnetic circuit. (Problem (2-1)). For this reason, the magnetic flux density Be-1 generated at the position (e) of the high magnetic permeability member 37 by the static magnetic field He is smaller than the magnetic flux density Bf-1 generated at the position (f) by the static magnetic field Hf. Therefore, the magnitude of the magnetization excited by the high magnetic permeability member 37 is also smaller at the position (e) than at the position (f). Moreover, since the static magnetic field He and the static magnetic field Hf are different in magnitude, the operating points (e) -1 and (f) -1 on the BH curve corresponding to the static magnetic field He and the static magnetic field Hf are shifted. Yes. When the gradient magnetic field ΔH by the gradient magnetic field generating coil 34 is superimposed on the static magnetic field He and the static magnetic field Hf, the shape of the minor loop 41 is B− in the vicinity of the operating points (e) -1 and (f) −1. In order to reflect the slope of the H curve, the shape of the minor loop 41 is different between the left end position (e) and the right end position (f) of the high magnetic permeability member 37 (problem (2-2-1)). At the same time, a difference also arises in the magnitude of the eddy current proportional to dΔH / dt (problem (2-2-2)).

  In the present embodiment, these problems can be solved simultaneously by arranging the permanent magnet 38. Since the permanent magnet 38 is magnetized in the thickness direction (z direction), it generates a magnetic field Hm in the z direction, which is applied to the high permeability member 37. Since this magnetic field Hm is uniform in the x direction (left and right direction), the static magnetic field He and the static magnetic field Hf at the left and right end positions (e) and (f) of the high permeability member 37 are the static magnetic field He + Hm. And the operating point becomes (e) -2 and (f) -2. Thereby, the magnetic flux density Be-2 is generated at the position (e) of the high magnetic permeability member 37, and the magnetic flux density Bf-2 is generated at the position (f). In general, the BH curve of a high magnetic permeability material is saturated in a region where the magnetic field H is large as shown in FIG. 4, and the inclination becomes small. Therefore, the distance between the operating points (e) -2 and (f) -2 is The difference between the magnetic flux density Be-2 in the static magnetic field He + Hm and the magnetic flux density Bf-2 in the static magnetic field Hef + Hm is shorter than the distance between the operating points (e) -1 and (f) -1. It is smaller than the difference between Be-1 and the magnetic flux density Bf-1 in the static magnetic field Hf. Thereby, the asymmetry of the magnetic flux density about the left-right direction of the high magnetic permeability member 37 can be reduced. Further, since the operating points (e) -2 and (f) -2 are close to each other, the shape of the minor loop 41 is almost the same. Thereby, the asymmetry of the magnitude of the magnetization excited by the high magnetic permeability member 37 can be reduced, and the magnetic behavior when the gradient magnetic field ΔH is applied can be generated symmetrically. Further, since the direction of the magnetic field Hm of the permanent magnet 28 is the magnetized z direction, it also acts to direct the magnetization of the high permeability member 37 inclined with respect to the z direction in the z direction.

  This state can be roughly indicated by using arrows indicating the magnitude and direction of magnetization, as shown in FIGS. FIG. 6 shows a case where the permanent magnet 38 is not disposed, and the magnetization 61 of the high magnetic permeability member 37 has an asymmetrical size, and the direction is also inclined with respect to the z direction. Here, when the magnetic field H by the permanent magnet 38 is applied as shown in FIG. 7, the magnetic permeability of the high permeability member 37 is made uniform by the addition of the magnetic field Hm produced by the permanent magnet 38 and the asymmetry of the right and left is weakened. Along with it, it grows overall. Moreover, when the magnetic field Hm parallel to the z direction passes through the high permeability member 37, the magnetization 70 of the high permeability member 37 is more directed in the z direction.

  Thus, by arranging the permanent magnet 38 adjacent to the high permeability member 37, the asymmetry of the magnetization and the magnetic flux density of the high permeability member 37 is reduced, and a uniform magnetization distribution and a magnetic flux density distribution are obtained. be able to.

  In addition, since the high magnetic permeability member 37 has a uniform magnetization distribution and magnetic flux density distribution, a magnetic material such as an iron piece shim is disposed on the surface of the high magnetic permeability member 37 (passive shimming), so that (2- It is possible to increase the magnetization (magnetic flux) of the portion as calculated without causing the problem of (2-1, 2-2-2). Thereby, although it is a left-right asymmetric C-shaped magnetic circuit, an iron piece shim etc. can be arrange | positioned on the surface of the high magnetic permeability member 37, a magnetic field can be generated as calculated, and the asymmetry of a static magnetic field can be correct | amended. . Thereby, the uniformity of the static magnetic field in the imaging region 30 can be increased.

  As described above, in the present embodiment, the high magnetic permeability member 37 and the permanent magnet 38 adjacent to the high magnetic permeability member 37 are disposed, so that the high magnetic permeability can be obtained even though the static magnetic field generator 402 has a left-right asymmetric magnetic circuit. Since the magnetic characteristics of the member 37 are spatially symmetric and become a magnetic non-linear system, the above problem (2-1, 2-1-2, 2-2-2) can be solved at once. can do. Moreover, since the magnetic field as calculated can be corrected by arranging an iron piece shim or the like on the high magnetic permeability member 37, the problem (1) can also be solved. Therefore, it is possible to provide a static magnetic field generation device 402 that is excellent in the uniformity of the generated static magnetic field and that is free from the surroundings.

  Next, an overall configuration of a typical MRI apparatus using the static magnetic field generation apparatus 402 of the present embodiment will be described with reference to FIG. In addition to the static magnetic field generator 402 and the gradient magnetic field generating coil 34 for generating a gradient magnetic field, the MRI apparatus includes a bed 412 on which the subject 401 is mounted, an RF coil 404 for applying a high-frequency magnetic field to the subject 401, An RF probe 405 that measures an MR signal generated by the subject 401, an RF transmission unit 410, a signal detection unit 406, a gradient magnetic field power source 409, a signal processing unit 407, a control unit 411, and a display unit 408 are provided.

  The gradient magnetic field generating coil 34 includes coils for generating magnetic fields in three directions of X, Y, and Z, respectively, and generates gradient magnetic fields in response to signals from the gradient magnetic field power supply 409, respectively. The RF coil 404 generates a high-frequency magnetic field according to the signal from the RF transmitter 410. The MR signal measured by the RF probe 405 is detected by the signal detector 406, signal processed by the signal processing unit 407, subjected to a predetermined calculation, and converted into an image signal. The image is displayed on the display unit 408. The gradient magnetic field power supply 409, the RF transmission unit 410, and the signal detection unit 406 are controlled by the control unit 411, and the control time chart is generally called a pulse sequence. Various sequences such as a pulse sequence for detecting an echo signal of an MR signal are known.

  The imaging target of the MRI apparatus is generally the main constituent substance of the subject 401, proton, in an apparatus popular in clinical practice. By imaging the spatial distribution of proton density and the relaxation phenomenon in the excited state in the signal processing unit 407, the form or function of the human head, abdomen, limbs, etc. is photographed two-dimensionally or three-dimensionally. At the time of imaging, an echo signal is detected with a phase encoding spatially applied by a gradient magnetic field. As the number of phase encodings, values such as 128, 256, and 512 are usually selected per image. Therefore, the echo signal is usually obtained as a time series signal composed of sampling data such as 128, 256, 512, etc., and these signals are spatially decomposed by two-dimensional Fourier transform to create one MR image. .

  As an example of the pulse sequence, a spin echo type echo planar sequence (SE-EPI) will be briefly described with reference to FIG. In FIG. 8, RF is an RF signal that the RF coil 404 irradiates the subject 401. Gs is a slice gradient magnetic field that determines the slice plane of the subject 01, Ge is a phase encode gradient magnetic field, and Gr is a read gradient magnetic field pulse, both of which are generated by the gradient coil 304. Echo is an echo signal measured by the RF probe 405. The pulse sequence (SE-EPI) shown in FIG. 8 is a high-speed sequence that is frequently used for diffusion imaging, perfusion imaging, and the like in clinical practice. In such a high-speed sequence, if there is a problem (2-2-2) that the above-described spatially asymmetric eddy current is generated, a spatially asymmetric magnetic field is generated from this eddy current and In this section, an offset component as if the baseline has changed is added after application of each gradient magnetic field pulse, so that an artifact (false image) is generated in the MRI image, and the accuracy is lowered in clinical diagnosis.

  In the EPI sequence, in order to measure a large number of echoes in a short time, the readout gradient magnetic field pulse Gr is inverted at high speed and high intensity, so that an eddy current is generated each time the inversion is performed. Typically, the readout gradient magnetic field Gr is required to have a strength of about 20 mT / m to 40 mT / m, and its rise is required to be about 50 T / m / s to 150 T / m / s. Moreover, it is a perturbation with respect to the phase encoding gradient magnetic field pulse Ge that the generated eddy current affects the image degradation most. This is because the phase encode gradient magnetic field pulse Ge has the smallest pulse time integral value, and the perturbation ratio to the original pulse integral value is the largest for the same eddy current. The types of artifacts that occur in this way are typically image distortion in the direction of phase encoding, flow artifacts, and Nyquist artifacts. In the present invention, as described above, the spatial asymmetry of the eddy current is physically reduced, so that there is an advantage that such image artifacts can be substantially reduced.

  Next, as a second embodiment of the present invention, three configurations in which the structure of the permanent magnet 38 is changed will be described with reference to FIGS. 9 (a), 9 (b), and 9 (c). 9A, 9B, and 9C are the same as those in the embodiment of FIG. 3 except for the permanent magnet 38.

  In the configuration shown in FIG. 9A, the permanent magnet 38 is divided into a plurality of pieces, and the divided pieces are attached to the high magnetic permeability member 37, respectively. A plate-like permanent magnet is mounted. The divided pieces of the permanent magnet 38 are each magnetized in the thickness direction. Since the small pieces of the divided permanent magnet 38 are easier to manufacture than the large-diameter permanent magnet, the divided permanent magnet is attached to the high permeability member 37 as shown in FIG. By attaching, it is possible to obtain the same effect as the static magnetic field generator 402 of FIG. 3 while reducing the manufacturing cost.

  In the configuration shown in FIG. 9B, a plurality of ring-shaped magnets having different diameters are arranged concentrically around the central axis of the imaging region 30 and pasted on the high permeability member 37. This configuration is used as the permanent magnet 38. There is a gap between adjacent ring-shaped magnets. One ring-shaped magnet is divided into a plurality of small pieces for easy manufacture. In the configuration shown in FIG. 9C, a configuration in which a rectangular magnet is arranged as a radial magnet around the central axis of the imaging region 30 and pasted on the high magnetic permeability member 37 is used as the permanent magnet 38. It is. The size of a rectangular parallelepiped magnet is smaller as it is closer to the center. 9B and 9C, both magnets are magnetized in the thickness direction.

  The permanent magnets 38 in FIG. 9B and FIG. 9C are not arranged on the entire surface of the high magnetic permeability member 37, but are axial with respect to the center of the imaging region 30 (the center of the high magnetic permeability member 37). By being arranged symmetrically, an effect equivalent to that of the embodiment of FIG. 3 in which the symmetry of the magnetization and magnetic flux density of the high permeability member 37 is enhanced can be obtained. Moreover, since the permanent magnet 38 shown in FIGS. 9B and 9C is easy to manufacture, the effects of the present invention can be realized at low cost.

  In the embodiment described so far, the configuration example in which the permanent magnet 38 is attached to the magnetic pole plate 31 side of the high permeability member 37 has been described. However, the permanent magnet 38 does not necessarily have to be on the magnetic pole plate 31 side. It is also possible to attach a permanent magnet to the imaging region 30 side. In this case, the same effect can be obtained.

  Next, a static magnetic field generation apparatus according to a further preferred third embodiment of the present invention will be described with reference to FIG. The static magnetic field generator of FIG. 10 employs a ring-shaped permanent magnet 38 configured as shown in FIG. 9B, and eliminates the groove 237 on the surface of the magnetic pole plate 31 of the apparatus of FIG. The groove 237 in FIG. 3 is provided to increase the uniformity of the static magnetic field, but in the configuration of FIG. 10, the ring-shaped permanent magnet 38 is symmetrical with respect to the magnetization and magnetic flux density of the high permeability member 37. In addition to the effect of enhancing the properties, the same effect as the groove 237 is also achieved. Specifically, the ring-shaped permanent magnet 38 is designed such that the width and spacing of each ring correct for non-uniformity of the static magnetic field. The ring-shaped permanent magnet 38 is disposed so as to contact the magnetic pole plate 31. As a result, the ring-shaped permanent magnet 38 functions to increase the symmetry of the magnetization and magnetic flux density of the high permeability member 37 and at the same time to make the static magnetic field of the magnetic pole plate 31 uniform. This eliminates the need for grooving of the surface of the magnetic pole plate 31 while obtaining the same effect as that of the embodiment of FIG. 3, so that a static magnetic field generator can be manufactured at low cost.

  In the embodiment shown in FIG. 10, the ring-shaped permanent magnet 38 shown in FIG. 9B is used, but the radial permanent magnet 38 shown in FIG. 9C can also be used.

  As another form, it is possible to control the uniformity of the static magnetic field by performing groove processing on the surface of the high magnetic permeability member 37.

  Next, a fourth embodiment will be described with reference to FIG. In the configuration of FIG. 11, a permanent magnet 39 magnetized in the −x direction (see FIG. 3) is disposed on the permanent magnet 38. The magnetic flux 111 generated by the permanent magnet 39 magnetized in the −x direction supplements the magnetic flux in the yoke 35, thereby correcting the asymmetry of the magnetic flux in the yoke 35. The operation of other configurations such as the permanent magnet 38 is the same as that of the embodiment of FIG.

  In the embodiment described above, the static magnetic field generator having a C-shaped magnetic circuit has been described. However, the static magnetic field generator is not limited to the C-shaped, and any static magnetic field generator having a spatially asymmetric magnetic circuit may be used. The present invention can be applied to.

  In the embodiment described above, the permanent magnet 38 is used. However, a magnetic field generating coil that applies a magnetic field in the z direction to the high permeability member 37 can be used instead of the permanent magnet 38. In this case, it is desirable to control the magnitude of the generated magnetic field so that the magnetic flux of the magnetic field generating coil does not affect the magnetic pole plate 31, the gradient magnetic field generating coil 34, and the static magnetic field generating coil 32.

It is a block diagram which shows schematic structure of the MRI apparatus of the 1st Embodiment of this invention. It is a perspective view of the static magnetic field generator 402 of 1st Embodiment. It is sectional drawing of the static magnetic field generator 402 of 1st Embodiment. It is a graph which shows the BH characteristic and minor loop 41 of the high magnetic permeability member 37 of 1st Embodiment. It is explanatory drawing which shows the magnitude | size and direction of magnetization of the high magnetic permeability member 37 of 1st Embodiment, and a magnetic circuit. In 1st Embodiment, it is explanatory drawing which shows the magnitude | size and direction of magnetization of the high-permeability member 37 when the permanent magnet 38 is not arrange | positioned. In 1st Embodiment, it is explanatory drawing which shows the magnitude | size and direction of magnetization of the high magnetic permeability member 37 at the time of arrange | positioning the permanent magnet 38. FIG. It is explanatory drawing which shows an example of the pulse sequence performed with the MRI apparatus of 1st Embodiment. (A), (b) and (c) is a notch perspective view which shows the structure of the three types of permanent magnet 38 of 2nd Embodiment. It is sectional drawing of the static magnetic field generator of 3rd Embodiment. It is explanatory drawing which shows the magnetization direction of the permanent magnets 38 and 39 of the static magnetic field generator of 4th Embodiment. It is sectional drawing of the conventional static magnetic field generator. It is explanatory drawing which shows the magnitude | size and direction of magnetization of the magnetic circuit of the conventional static magnetic field generator.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 30 ... Imaging area | region, 31 ... Magnetic pole plate, 32 ... Static magnetic field generating coil, 33 ... Iron pole, 34 ... Gradient magnetic field generating coil, 35 ... yoke, 36 ... Magnetic pole protrusion, 37 ... High permeability member, 38 ... Permanent magnet, DESCRIPTION OF SYMBOLS 39 ... Permanent magnet, 130 ... Imaging region, 131 ... Magnetic pole plate, 132 ... Static magnetic field generating coil, 133 ... Iron pole, 134 ... Gradient magnetic field generating coil, 135 ... yoke, 136 ... Magnetic pole protrusion, 137 ... Groove, 237 ... Groove , 401, subject, 402, static magnetic field generator, 404, RF probe, 405, RF probe, 406, gradient magnetic field power supply, 407, signal processing unit, 408, display unit, 410, RF transmission unit, 411, control unit. 412 ... Bed.

Claims (3)

A magnetic field generation source for generating a static magnetic field in a predetermined direction in the imaging region, a pair of magnetic pole plates disposed at positions facing each other across the imaging region in order to adjust the uniformity of the static magnetic field, and A magnetic circuit constituent member that couples a pair of magnetic pole plates, and a gradient magnetic field generator disposed between the imaging region and the magnetic pole plate to apply a gradient magnetic field to the imaging region,
Between the magnetic pole plate and the gradient magnetic field generator, a high magnetic permeability member having a higher magnetic permeability than the material constituting the magnetic pole plate, and a permanent magnet that applies a magnetic field to the high magnetic permeability member are disposed. A magnetic resonance imaging apparatus.   The magnetic resonance imaging apparatus according to claim 1, wherein the permanent magnet is magnetized in a direction parallel to a direction of a static magnetic field in the imaging region.   3. The magnetic resonance imaging apparatus according to claim 1, wherein the permanent magnet has a spatially symmetric shape with respect to a central axis of the imaging region.

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