Magnetic resonance imaging system

Abstract

To suppress leakage magnetic fields generated in an open-type magnetic resonance imaging device.SOLUTION: A magnetic resonance imaging system according to one embodiment includes a static magnetic field magnet, a pre-polarization magnet, a first shield, and a second shield. The static magnetic field magnet specifies the magnetic resonance frequency during imaging and generates a first static magnetic field applied to a subject during imaging. The pre-polarization magnet polarizes the spins of the subject prior to imaging of the subject, and generates a second static magnetic field in which the application is stopped when imaging the subject. The first shield shields the leakage magnetic field of the first static magnetic field, and is provided in an area opposite to the imaging area of the subject and the static magnetic field magnet. The second shield shields the leakage magnetic field of the second static magnetic field, and is provided in an area opposite to the imaging area of the subject and the pre-polarization magnet.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The embodiments disclosed herein and in the drawings relate to a magnetic resonance imaging system. [Background technology]

[0002] A magnetic resonance imaging device, which is the main component of a magnetic resonance imaging system, is an imaging device that excites the nuclear spins of a subject placed in a static magnetic field with a radio frequency (RF: Radio Frequency) signal at the Larmor frequency, and generates an image by reconstructing the magnetic resonance signals (MR (Magnetic Resonance) signals) generated from the subject in response to the excitation.

[0003] Many magnetic resonance imaging (MRI) devices have a configuration called a gantry, in which a cylindrical space is formed. An object (e.g., a patient) lying on a tabletop is carried into the cylindrical space and imaged. A cylindrical superconducting magnet, a cylindrical gradient magnetic field coil, and a cylindrical transmitting / receiving coil (i.e., a WB (Whole Body) coil) are housed inside the gantry. In many conventional magnetic resonance imaging devices of this type, the superconducting magnet, the gradient magnetic field coil, and the transmitting / receiving coil are cylindrical, and therefore, hereinafter, this type of magnetic resonance imaging device will be referred to as a cylindrical magnetic resonance imaging device.

[0004] In a cylindrical magnetic resonance imaging apparatus, imaging is performed in a cylindrical closed space, and therefore imaging may be difficult for some patients, such as those who suffer from claustrophobia.

[0005] In response to this, magnetic resonance imaging devices have been proposed and developed that use flat superconducting magnets and gradient coils, and are configured to image a subject such as a patient in an open space between, for example, two flat superconducting magnets. This type of magnetic resonance imaging device will be referred to as a planar open-type magnetic resonance imaging device (or simply an open-type magnetic resonance imaging device) below. In an open-type magnetic resonance imaging device, imaging is performed in an open space, making it possible to image claustrophobic patients.

[0006] On the other hand, the static magnetic field magnets of open-type magnetic resonance imaging devices tend to be placed closer to the floor or walls due to their shape, etc., compared to the static magnetic field magnets of conventional cylindrical magnetic resonance imaging devices. For example, when a flat superconducting magnet is placed horizontally, the superconducting magnet closer to the floor surface will generate a stronger magnetic field in the direction toward the floor surface. Also, when a flat superconducting magnet is placed vertically, the superconducting magnet closer to the wall will generate a stronger magnetic field in the direction toward the wall surface.

[0007] These magnetic fields, which are generated outside the superconducting magnet, opposite the imaging space, i.e., leakage magnetic fields, may have adverse effects on the space under the floor (e.g., the space in the room downstairs) and the space behind the wall (e.g., the space in the next room). [Prior art documents] [Patent documents]

[0008] [Patent Document 1] U.S. Patent No. 6,462,544 [Non-patent literature]

[0009] [Non-Patent Document 1] JM Gonzalez et. al. "Repolarized MRI of Hard Tissues and Solid-State Matter", arXiv:2110.03417v2 [physics. med-ph] 8 Oct 2021 [Non-Patent Document 2] AN Matlashov et al., “SQUID-based systems for co-registration of ultra-low field nuclear magnetic resonance images and magnetoencephalography”, Physica C 482 (2012) 19-26 Summary of the Invention [Problem to be solved by the invention]

[0010] One of the problems to be solved by the embodiments disclosed in this specification and the drawings is to suppress leakage magnetic fields generated in an open-type magnetic resonance imaging apparatus. However, the problems to be solved by the embodiments disclosed in this specification and the drawings are not limited to the above problem. Problems corresponding to the effects of each configuration shown in each embodiment described later can also be positioned as other problems. [Means for solving the problem]

[0011] A magnetic resonance imaging system according to one embodiment includes a static magnetic field magnet, a pre-polarization magnet, a first shield, and a second shield. The static magnetic field magnet determines a magnetic resonance frequency during imaging and generates a first static magnetic field applied to a subject during imaging. The pre-polarization magnet polarizes spins of the subject prior to imaging of the subject and generates a second static magnetic field, the application of which is stopped when imaging of the subject. The first shield is a first shield that shields against a leakage magnetic field of the first static magnetic field, and is provided in an area opposite to an imaging region of the subject with respect to the static magnetic field magnet. The second shield is a second shield that shields against a leakage magnetic field of the second static magnetic field, and is provided in an area opposite to an imaging region of the subject with respect to the pre-polarization magnet. [Brief description of the drawings]

[0012] [Figure 1] FIG. 2 is a diagram showing a first example of arrangement of magnet units used in an open-type magnetic resonance imaging apparatus which is a main component of the magnetic resonance imaging system of the first embodiment. [Diagram 2] FIG. 13 is a diagram showing a second example of the arrangement of magnet units used in an open-type magnetic resonance imaging apparatus. [Diagram 3] FIG. 2 is a diagram showing an example of the internal configuration of a magnet unit, and also showing a first example of the arrangement of an electromagnetic wave shield (second shield) and a magnetic shield (first shield), which are components of a magnetic resonance imaging system. [Figure 4] FIG. 2 is a diagram showing an example of the internal configuration of a magnet unit, and also showing a second example of the arrangement of an electromagnetic wave shield (second shield) and a magnetic shield (first shield), which are components of a magnetic resonance imaging system. [Diagram 5] 1 is a block diagram showing an example of the configuration of a magnetic resonance imaging apparatus included in a magnetic resonance imaging system according to a first embodiment. [Figure 6] FIG. 2 is a diagram for explaining the concept of a pulse sequence in which a pre-polarizing magnetic field BP and a measurement magnetic field B0 are applied parallel to each other. [Figure 7]1 is a diagram for explaining the concept of a pulse sequence in which a pre-polarizing magnetic field BP and a measurement magnetic field B0 are applied orthogonal to each other. [Figure 8] FIG. 11 is a diagram showing an example of a correction coil included in the second embodiment. [Figure 9] FIG. 11 is a block diagram showing an example of the configuration of a magnetic resonance imaging apparatus included in a magnetic resonance imaging system according to a second embodiment. [Figure 10] FIG. 4 is a diagram for explaining the concept of control of a correction current using a correction coil control circuit. [Figure 11] FIG. 2 is a diagram for explaining the concept of control of a pre-polarization current and a correction current using a pre-polarization control circuit. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings.

[0014] (First embodiment) Fig. 1 is a diagram showing a first arrangement example of magnet units 10 used in an open-type magnetic resonance imaging apparatus 1, which is a main component of a magnetic resonance imaging system 2 of the first embodiment. As shown in Fig. 1, the magnetic resonance imaging apparatus 1 has, for example, two magnet units 10 each having a circular flat plate shape (in other words, a thin cylindrical shape).

[0015] Each magnet unit 10 is arranged so that the central axis of the magnet unit 10, i.e., the axis passing through the center of the circle at both end faces of the cylindrical shape, is parallel to, for example, the floor surface. Furthermore, the two magnet units 10 are arranged so as to sandwich the subject. By arranging the magnet units 10 in this manner, a magnetic field is formed in the open space between the two magnet units 10. The subject is imaged in this open space, for example, in an upright position.

[0016] Fig. 2 is a diagram showing a second arrangement example of the magnet units 10 used in the open-type magnetic resonance imaging apparatus 1. While Fig. 1 shows an arrangement example for imaging a subject in an upright position, Fig. 2 shows an arrangement example for imaging a subject in a supine position lying on a top plate 80 of a bed 81. When imaging a subject in a supine position, the two magnet units 10 are arranged so that their central axes are in the vertical direction as shown in Fig. 2, and for example, one magnet unit 10 is arranged below the top plate 80 and the other magnet unit 10 is arranged above the top plate 80.

[0017] As shown in FIGS. 1 and 2, imaging using a planar magnet unit 10 allows imaging of the subject in an open magnetic field space, so that even claustrophobic patients can be imaged.

[0018] Although Figures 1 and 2 show the subject being imaged in an open area outside the magnet unit 10 including the superconducting coil, the subject may also be imaged with at least a portion of the subject placed in the open area outside the superconducting coil.

[0019] FIG. 3 shows an example of the internal configuration of the magnet unit 10, and is also a diagram showing a first example of the arrangement of an electromagnetic wave shield 400 (second shield) and a magnetic shield 410 (first shield), which are components of the magnetic resonance imaging system 2.

[0020] As shown in Fig. 3, the magnet unit 10 includes a static magnetic field magnet 100 and a pre-polarization magnet 200. The static magnetic field magnet 100 determines the magnetic resonance frequency during imaging and generates a first static magnetic field to be applied to the subject during imaging. The static magnetic field magnet 100 can be configured with a single magnet, but may also be configured with a first magnet 101 that generates a magnetic field in a forward direction and a second magnet 102 that generates a magnetic field in a direction opposite to the forward magnetic field, as shown in Fig. 3.

[0021] On the other hand, the pre-polarizing magnet 200 is a magnet also called a pre-polarizing magnet. The pre-polarizing magnet 200 polarizes the spins of the subject prior to imaging of the subject, and generates a second static magnetic field, the application of which is stopped when imaging of the subject. Detailed configurations and functions of the static magnetic field magnet 100 and the pre-polarizing magnet 200 will be described later.

[0022] 3 is a diagram corresponding to a second arrangement example of the magnet units 10, in which two magnet units 10 are arranged in the vertical direction on either side of the imaging space R. The magnet units 10 generate a vertical magnetic field B in the imaging space R, as indicated by the thick arrow in the figure.

[0023] On the other hand, a leakage magnetic field B is also generated in the region opposite to the imaging space R and the lower magnet unit 10 as shown by the thin arrow in the figure. L This leakage magnetic field B L does not contribute to imaging of the subject in the imaging space R.

[0024] These magnetic fields, which are generated in the direction opposite to the imaging space, i.e., leakage magnetic fields, may have a negative effect on the space under the floor (e.g., the space in the room downstairs) or the space behind the wall (e.g., the space in the next room).

[0025] In the second arrangement example of the magnet units 10, the lower magnet unit 10 is arranged in a position close to the floor surface, so that the lower magnet unit 10 generates a strong leakage magnetic field B L This will result in the following.

[0026] Therefore, in the magnetic resonance imaging system 2 of the first embodiment, this leakage magnetic field B L The first shield is a magnetic shield 410 for shielding against static magnetic fields, and the second shield is an electromagnetic shield 400 for shielding against dynamic magnetic fields.

[0027] The second shield may be, for example, a shield coil for shielding a pulsed magnetic field, and the first shield may be configured as a magnetic shield that also serves as the second shield.

[0028] As described below, the second static magnetic field (i.e., the pre-polarizing magnetic field) generated by the pre-polarizing magnet 200 is turned on and off by a steep rise and fall by controlling the current applied to the pre-polarizing magnet 200. Therefore, the second static magnetic field is a dynamic static magnetic field and has electromagnetic wave properties. Therefore, the electromagnetic wave shield 400 is suitable for shielding the second static magnetic field.

[0029] The electromagnetic wave shield 400 is made of a highly conductive material such as a galvanized iron plate, a steel plate, an aluminum plate, etc. On the other hand, the magnetic shield 410 is made of a highly magnetically permeable material such as permalloy, electromagnetic pure iron, silicon steel plate, amorphous, etc. The electromagnetic wave shield 400 and the magnetic shield 410 can be installed, for example, in the members constituting the floor.

[0030] In the first arrangement example in which the magnet units 10 are placed vertically, the magnet unit 10 closer to the wall generates a stronger magnetic field in the direction toward the wall surface. In this case, an electromagnetic shield 400 (second shield) and a magnetic shield 410 (first shield) can be provided inside the nearby wall.

[0031] In this way, by arranging the electromagnetic wave shield 400 and the magnetic shield 410 in the region opposite to the imaging region of the subject and the magnet unit 10, the leakage magnetic field B L can be efficiently suppressed.

[0032] 4 is a diagram showing a second example of the arrangement of an electromagnetic wave shield 400 (second shield) and a magnetic shield 410 (first shield). In an arrangement in which the magnet units 10 are arranged above and below the subject P lying on his / her side, the upper magnet unit 10 also generates a leakage magnetic field B L may occur.

[0033] In such a case, as shown in FIG. 4, the electromagnetic shield 400 and the magnetic shield 410 may be installed not only in the members constituting the floor but also in the members constituting the ceiling. With such a configuration and arrangement of the electromagnetic shield 400 and the magnetic shield 410, the leakage magnetic field B L The effects of the above can be suppressed.

[0034] Fig. 5 is a block diagram showing a configuration example of the magnetic resonance imaging apparatus 1 included in the magnetic resonance imaging system 2 of the first embodiment. Fig. 5 is a diagram showing a configuration example of the magnetic resonance imaging apparatus 1 in a second arrangement example in which the above-mentioned magnet units 10 are arranged above and below a lying subject P. An imaging region, or a FOV (Field of View), is formed between the two magnet units 10.

[0035] The magnetic resonance imaging apparatus 1 shown in FIG. 5 has, in addition to the two magnet units 10 described above, a tabletop 80 on which a subject P lies, a local coil 20 placed in close proximity to the subject, a gradient magnetic field coil 60, and an RF (Radio Frequency) coil 62.

[0036] The magnetic resonance imaging apparatus 1 also includes an imaging condition setting circuit 50, a sequence controller 51, a gradient magnetic field power supply 52, a transmission circuit 53, a reception circuit 54, a reconstruction processing circuit 55, and a display 56.

[0037] The imaging condition setting circuit 50 sets imaging conditions, such as the type of pulse sequence and the values ​​of various parameters, inputted via a user interface (not shown), in a sequence controller 51 .

[0038] The sequence controller 51 scans the subject based on the set imaging conditions by driving the gradient magnetic field power supply 52 and the transmission circuit 53. The gradient magnetic field power supply 52 applies a gradient magnetic field current to the gradient magnetic field coil 60 based on a drive signal from the sequence controller 51.

[0039] The transmission circuitry 53 generates an RF pulse based on a drive signal from the sequence controller 51, and applies the RF pulse to the RF coil 62. In response to this application, an MR (Magnetic Resonance) signal emitted from the subject P is received by the local coil 20. The MR signal received by the local coil 20 is converted from an analog signal to a digital signal by the reception circuitry 54. The MR signal converted to a digital signal is supplied to the reconstruction processing circuitry 55 as k-space data. The reconstruction processing circuitry 55 performs reconstruction processing such as an inverse Fourier transform process on the k-space data to generate an MR image. The generated MR image is displayed on the display 56.

[0040] On the other hand, as described above, each of the magnet units 10 includes a static magnetic field magnet 100 having a first magnet 101 and a second magnet 102, and a pre-polarized magnet 200. The static magnetic field magnet 100 and the pre-polarized magnet 200 are configured as, for example, planar superconducting magnets.

[0041] The static magnetic field magnet 100 and the pre-polarized magnet 200 may be configured as superconducting magnets as described above, but are not necessarily limited to this. For example, the first magnet 101 and the second magnet 102 of the static magnetic field magnet 100 may be configured as superconducting magnets, and the pre-polarized magnet 200 may be configured as a normal conducting magnet (i.e., an electromagnet that operates at room temperature), or conversely, the first magnet 101 and the second magnet 102 of the static magnetic field magnet 100 may be configured as normal conducting magnets, and the pre-polarized magnet 200 may be configured as a superconducting magnet. Alternatively, all of the first magnet 101 and the second magnet 102 of the static magnetic field magnet 100 and the pre-polarized magnet 200 may be configured as normal conducting magnets.

[0042] The static magnetic field magnet 100 and the pre-polarization magnet 200 are housed in a vessel 10a. When the static magnetic field magnet 100 and the pre-polarization magnet 200 are configured as superconducting magnets, they are housed in a cooling vessel 10a called, for example, a cryostat.

[0043] 5, the magnetic resonance imaging apparatus 1 includes a pre-polarization power supply 42 that applies a current to a pre-polarization magnet 200, and a pre-polarization control circuit 40 that controls the pre-polarization power supply 42. As described later, the magnetic resonance imaging apparatus 1 is configured to be able to execute a pulse sequence that uses a pre-polarizing magnetic field (second static magnetic field). The pre-polarization magnet 200, the pre-polarization power supply 42, and the pre-polarization control circuit 40 are provided for generating this pre-polarizing magnetic field.

[0044] As described above, current is passed through the coils of the first magnet 101 and the second magnet 102 so that the direction of the magnetic field generated by the second magnet 102 is opposite to the direction of the magnetic field generated by the first magnet 101. Therefore, when the sign of the magnetic field generated by the first magnet 101 is positive, the sign of the magnetic field generated by the second magnet 102 is negative.

[0045] As a result, in the composite magnetic field, the magnetic field generated by the first magnet 101 and the magnetic field generated by the second magnet 102 cancel each other out, making it possible to generate a region with a generally uniform magnetic field strength (i.e., a region where the magnetic field strength hardly changes) in the axial direction of the cylinder (the vertical direction in Figs. 2-5). In such a region where the magnetic field strength hardly changes, i.e., in the axial direction of the cylinder (y-axis), a generally uniform magnetic field strength B 0 The region in which this is obtained can be used as the FOV during MR imaging.

[0046] 6 and 7 are diagrams for explaining an example of a pulse sequence using a pre-polarizing magnetic field (second static magnetic field). In this pulse sequence, the magnetic resonance frequency during imaging is specified, and the first static magnetic field B 0 and a pre-polarizing magnetic field B which polarizes the spins of the subject prior to imaging of the subject and is stopped when imaging of the subject is performed. P (Second static magnetic field B P Two types of static magnetic fields are used: the first static magnetic field B 0Since the imaging or measurement of the subject is performed under the first static magnetic field B 0 The measurement magnetic field B 0 It is sometimes called.

[0047] Pre-Polarizing magnetic field B P In the pulse sequence using the pre-polarizing magnetic field B P and measurement magnetic field B 0 There is a method of applying these voltages parallel to each other and a method of applying these voltages perpendicular to each other.

[0048] Figure 6 shows the pre-polarizing magnetic field B P and measurement magnetic field B 0 6(a)-6(d) are diagrams illustrating the concept of a pulse sequence in which a pre-polarizing magnetic field is applied in parallel to the subject. As shown in FIG. 6(a)-FIG. 6(d), a pre-polarizing magnetic field is applied prior to imaging of the subject. For example, as shown in FIG. 6(d), at time t 0 Then, Pre-Polarizing magnetic field B P turns "ON" and at time t 1 Then, the pre-polarizing magnetic field is turned "OFF". Then, at time t 2 An excitation pulse is applied to commence imaging of the subject, and MR signals from the subject are acquired.

[0049] Measurement magnetic field B 0 As shown in FIG. 6(c), the measurement magnetic field B is a composite magnetic field of the first magnet 101 and the second magnet 102. 0 As shown in Fig. 6(c), the pre-polarizing magnetic field B P is constantly applied to the subject even before it is turned ON.

[0050] On the other hand, the pre-polarizing magnetic field B P During the ON period, the pre-polarizing magnetic field B P and measurement magnetic field B 0 Since the magnetic field is applied in the same direction as the measurement magnetic field B, the resultant magnetic field applied to the subject is 0 Pre-Polarizing magnetic field BP As a result, the magnetization M (or spin M) generated in the subject is P is augmented by

[0051] FIG. 6(d) is a schematic diagram showing the behavior of the magnetization M of the subject over time. P Before the application of the measurement magnetic field B 0 Magnetization by only M 0 However, the pre-polarizing magnetic field B P The magnetization M T At time t 1 When the pre-polarizing magnetic field is turned "OFF," the magnetization M begins to decrease. At this time, the magnetization M also decreases exponentially depending on the T1 value (longitudinal relaxation time). Then, at time t2, an excitation pulse is applied, and the longitudinal magnetization M becomes transverse. Thereafter, the transverse magnetization decreases exponentially depending on the T2 value (longitudinal relaxation time), which is a tissue parameter, and during this time, it is collected as an MR signal, and the collected MR signals are used to generate MR images.

[0052] The above-mentioned pre-polarizing magnetic field B P In the pulse sequence involving the application of the measurement magnetic field B 0 The strength of the pre-polarizing magnetic field B is reduced while the strength of the pre-polarizing magnetic field B is increased immediately before imaging. P By increasing the intensity of the MR beam, the magnetization M of the subject can be increased. As a result, an MR image with a high signal-to-noise ratio (SNR) can be generated.

[0053] On the other hand, the measurement magnetic field B 0 Since the strength of the pre-polarizing magnetic field B is kept low, P The leakage magnetic field can be suppressed except for the short period during which the magnetic field is applied.

[0054] On the other hand, Figure 7 shows the pre-polarizing magnetic field B P FIG. 1 is a diagram for explaining the concept of a pulse sequence in which a pre-polarizing magnetic field B and a measurement magnetic field B are applied orthogonally to each other. P is applied prior to imaging of the subject.

[0055] For example, as shown in FIG. 7(d), at time t 0 Then, Pre-Polarizing magnetic field B P turns "ON" and at time t 1 Then, the pre-polarizing magnetic field is turned "OFF". Then, as shown in Fig. 7(c), the measurement magnetic field B 0 is applied, for example, three times, while changing the sign. 0 By applying this, echoes are formed in the same manner as in the GRE (Gradient Echo) method, and MR signals can be acquired.

[0056] In the pulse sequence shown in Fig. 7, the pre-polarizing magnetic field B P The direction of magnetization M generated by the 0 The directions of the pre-polarizing magnetic field B are perpendicular to each other. P After the switch is turned off, the magnetization M is 0 While rotating around the measurement magnetic field B 0 In the pulse sequence shown in Fig. 7, the change in the magnetic field caused by the rotation of this magnetization M is detected as an MR signal.

[0057] Pre-Polarizing magnetic field B shown in Figure 7 P and measurement magnetic field B 0 In the pulse sequence in which the magnetic fields B and B are applied orthogonally to each other, the pre-polarizing magnetic field B applied immediately before imaging is P By increasing the intensity of the MR beam, the magnetization M of the subject can be increased. As a result, an MR image with a high signal-to-noise ratio (SNR) can be generated.

[0058] On the other hand, the measurement magnetic field B 0 is applied for a short time during MR signal acquisition, and the measurement magnetic field B 0 As a result, the strength of the pre-polarizing magnetic field B can be kept low even in the pulse sequence shown in Fig. 7. P The leakage magnetic field can be suppressed except for the short period during which the magnetic field is applied.

[0059] According to the magnetic resonance imaging system 2 of the first embodiment described above, the electromagnetic wave shield 400 (second shield) and the magnetic shield 410 (first shield) are provided in the region opposite to the imaging region of the subject and the magnet unit 10, and the pre-polarizing magnetic field B P By using a pulse sequence involving the application of a magnetic field, the leakage magnetic field can be suppressed.

[0060] Second embodiment The magnetic resonance imaging system 2 of the second embodiment differs from the magnetic resonance imaging system 2 of the first embodiment in that it has a correction coil 300 for correcting the magnetic field and a configuration for controlling the current applied to the correction coil 300, but the other configurations are the same as those of the first embodiment.

[0061] 8 is a diagram showing an example of a correction coil 300 provided in the second embodiment. The correction coil 300 can be provided so as to cover, for example, a part or the whole of the outer circumferential surface of at least one of the first magnet 101 and the second magnet 102. The correction coil 300 corrects the first static magnetic field (measurement magnetic field B 0 ), the second static magnetic field (Pre-Polarizing magnetic field B P ), and a composite static magnetic field obtained by combining the first static magnetic field and the second static magnetic field.

[0062] FIG. 8 shows an example of the correction coil 300, and the correction coil 300 is not limited to the example shown in FIG. 8. The correction coil 300 is a first static magnetic field (measurement magnetic field B 0), the second static magnetic field (Pre-Polarizing magnetic field B P ) and a composite static magnetic field obtained by combining the first static magnetic field and the second static magnetic field, and can take various forms.

[0063] 9 is a block diagram showing an example of the configuration of a magnetic resonance imaging apparatus 1 included in a magnetic resonance imaging system 2 of the second embodiment. As described above, the magnetic resonance imaging system 2 of the second embodiment differs from the magnetic resonance imaging system 1 of the first embodiment (FIG. 4) in that the magnetic resonance imaging system 2 of the second embodiment includes a magnetic field sensor 500, a control circuit 74, a correction coil control circuit 70, and a correction coil power supply 72 in order to control the correction current applied to the correction coil 300.

[0064] The magnetic field sensor 500 generates a first static magnetic field (a measurement magnetic field B 0 ), the second static magnetic field (Pre-Polarizing magnetic field B P ), and a composite static magnetic field obtained by combining the first static magnetic field and the second static magnetic field.

[0065] The magnet unit 10 includes a container 10a that houses a static magnetic field magnet 100, a pre-polarization magnet 200, and a correction coil 300. The magnetic field sensor 500 is provided, for example, outside the container 10a. The number of the magnetic field sensors 500 may be one or more.

[0066] The correction coil power supply 72 generates a first static magnetic field (measurement magnetic field B) based on the magnetic field measured by at least one magnetic field sensor 500. 0 ), the second static magnetic field (Pre-Polarizing magnetic field B P A current is supplied to the correction coil 300 so as to correct at least one static magnetic field distribution of the first static magnetic field, the second static magnetic field, and a composite static magnetic field obtained by combining the first static magnetic field and the second static magnetic field.

[0067] The correction coil control circuit 70 controls the correction current supplied from the correction coil power supply 72 to the correction coil 300 .

[0068] FIG. 10 is a diagram for explaining the concept of control of the correction current using the correction coil control circuit 70. As shown in FIG. 10, the correction coil control circuit 70 has a correction table 701 (or a correction formula 701). 0 ), the second static magnetic field (Pre-Polarizing magnetic field B P ), and at least one of the static magnetic field distributions of the composite static magnetic field obtained by combining the first static magnetic field and the second static magnetic field is a static magnetic field distribution influenced by the electromagnetic wave shield 400 (second shield) and the magnetic shield 410 (first shield). For this reason, these influences may cause changes such as offsets, which may cause errors with respect to the desired static magnetic field distribution. Therefore, the correction coil control circuit 70 adjusts the first static magnetic field (measurement magnetic field B 0 ), the second static magnetic field (Pre-Polarizing magnetic field B P The correction current supplied to the correction coil 300 by the correction coil power supply 72 is controlled by feedback control based on the magnetic field measured by at least one magnetic field sensor 500 and a predetermined correction table 701 (or correction formula 701) so that at least one of the static magnetic field distributions of the first static magnetic field, the first static magnetic field, and the composite static magnetic field obtained by combining the first static magnetic field and the second static magnetic field becomes a desired static magnetic field distribution.

[0069] (Modification of the second embodiment) As shown in Fig. 6(d), the pre-polarizing magnetic field B P In pulse sequences involving the application of , the value of magnetization M is called the T1 value (longitudinal magnetization relaxation time), the T2 value, or the T2 * The value of magnetization M also varies depending on sequence parameters. For example, the value of magnetization M varies depending on the pre-polarizing magnetic field B P In addition to the application time of the Pre-Polarizing magnetic field B P The time from the application of the pre-polarizing magnetic field B to the application of the excitation pulse, and the time from the application of the excitation pulse to the start of MR signal acquisition, etc. PThe time elapsed from the end of application of the voltage Vcc to the start of imaging, and other sequence parameters are varied.

[0070] Therefore, in order to obtain a desired MR signal during imaging, it is preferable to appropriately control the magnitude of longitudinal magnetization and transverse magnetization. To achieve this, it is preferable to control the pre-polarization current applied to the pre-polarization magnet 200 and the correction current applied to the correction coil 300 based on tissue parameters of the subject to be imaged and sequence parameters of the pulse sequence used to image the subject.

[0071] Therefore, as shown in FIG. 11, in the modified example of the second embodiment, the pre-polarization control circuit 40 receives tissue parameters such as the T1 value and the T2 value and the pre-polarizing magnetic field B P The sequence parameters such as the desired value of and the desired application time are input, while a timing signal of the pulse sequence is input from the sequence controller 51 .

[0072] The pre-polarization control circuit 40 then controls parameters related to at least one of the correction current and the pre-polarization current (e.g., current value, current on / off timing, rise time, fall time, etc.) based on the input tissue parameters and sequence parameters, as well as the timing signal of the pulse sequence, by referring to an internal lookup table 401, for example.

[0073] By this control, the pre-polarizing magnetic field B P In a pulse sequence involving application of the pulses, desired MR signals can be obtained, and as a result, desired MR images can be generated.

[0074] In addition, the magnetic resonance imaging apparatus 1 of each of the above-mentioned embodiments (e.g., Figures 2 to 5 and 9) is configured to have two magnet units 10 facing each other, and images of the subject are captured in the space sandwiched between the two magnet units 10.

[0075] Alternatively, a configuration may be adopted in which only one of the two magnet units 10 is provided. For example, the magnetic resonance imaging apparatus 1 of each embodiment may have a configuration in which the magnet unit 10 above the top plate 80 is removed from the configuration of the magnetic resonance imaging apparatus 1 shown in Fig. 4 or 10. Even with such a configuration, the respective technical effects described above can be achieved.

[0076] The correction coil control circuit and the pre-polarization control circuit in the description of each embodiment are examples of the correction control unit and the pre-polarization control unit in the claims, respectively.

[0077] According to at least one of the embodiments described above, it is possible to suppress the leakage magnetic field generated in the open-type magnetic resonance imaging apparatus.

[0078] Although some embodiments have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, modifications, and combinations of embodiments can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope of the invention and its equivalents as described in the claims, as well as in the scope and spirit of the invention. [Explanation of symbols]

[0079] 1. Magnetic resonance imaging equipment 2. Magnetic resonance imaging system 10 Magnet unit 10a Container (cooling container) 20 Local Coil 42 Pre-polarized power supply 60 Gradient magnetic field coil 62 RF coil 72 Correction coil power supply 100 Static magnetic field magnet 101 First magnet 102 Second Magnet 200 Pre-Polarizing Magnets 300 Correction coil 400 Electromagnetic Shield (Second Shield) 410 Magnetic Shield (First Shield)

Claims

1. a static magnetic field magnet that defines a magnetic resonance frequency during imaging and generates a first static magnetic field to be applied to the subject during imaging; a pre-polarization magnet that polarizes spins in the subject prior to imaging of the subject and generates a second static magnetic field that is turned off when the subject is imaged; a first shield for shielding a leakage magnetic field of the first static magnetic field, the first shield being provided in an area opposite to an imaging area of ​​the subject with respect to the static magnetic field magnet; a second shield for shielding a leakage magnetic field of the second static magnetic field, the second shield being provided in a region opposite to an imaging region of the subject with respect to the pre-polarization magnet; A magnetic resonance imaging system comprising:

2. The first shield is further provided in a region in the same direction as an imaging region of the subject with respect to the static magnetic field magnet, The second shield is further provided in a region in the same direction as an imaging region of the subject with respect to the pre-polarization magnet.

10. The magnetic resonance imaging system of claim 1.

3. The first shield is configured as a magnetic shield, and the second shield is configured as an electromagnetic wave shield.

10. The magnetic resonance imaging system of claim 1.

4. The first shield is configured as a magnetic shield, and the second shield is configured as a coil.

10. The magnetic resonance imaging system of claim 1.

5. The first shield is configured as a magnetic shield that also serves as the second shield.

10. The magnetic resonance imaging system of claim 1.

6. The static magnetic field magnet is configured as a planar superconducting magnet.

10. The magnetic resonance imaging system of claim 1.

7. The planar superconducting magnet is configured to include at least one first magnet that generates a magnetic field in a forward direction, and at least one second magnet that generates a magnetic field in a direction opposite to the forward magnetic field.

7. A magnetic resonance imaging system according to claim 6.

8. The direction of the second static magnetic field generated by the pre-polarizing magnet is perpendicular to the direction of the first static magnetic field generated by the static magnetic field magnet.

10. The magnetic resonance imaging system of claim 1.

9. The direction of the second static magnetic field generated by the pre-polarizing magnet is parallel to the direction of the first static magnetic field generated by the static magnetic field magnet.

10. The magnetic resonance imaging system of claim 1.

10. a correction coil for correcting at least one static magnetic field distribution of the first static magnetic field, the second static magnetic field, and a composite static magnetic field obtained by combining the first static magnetic field and the second static magnetic field; at least one magnetic field sensor for measuring at least one of the first static magnetic field, the second static magnetic field, and the resultant static magnetic field; a correction coil power supply for supplying a current to the correction coil; Further equipped with the correction coil power supply supplies a current to the correction coil so as to correct at least one static magnetic field distribution of the first static magnetic field, the second static magnetic field, and the composite static magnetic field based on the magnetic field measured by the at least one magnetic field sensor; 10. The magnetic resonance imaging system of claim 1.

11. a container for housing the static magnetic field magnet, the pre-polarization magnet, and the correction coil; The magnetic field sensor is provided outside the space enclosed by the container.

11. A magnetic resonance imaging system according to claim 10.

12. At least one static magnetic field distribution of the first static magnetic field, the second static magnetic field, and the composite static magnetic field is a static magnetic field distribution influenced by the first and second shields.

11. A magnetic resonance imaging system according to claim 10.

13. A correction control unit that controls the current of the correction coil power supply, The correction control unit controls the current of the correction coil power supply by feedback control based on the magnetic field measured by the at least one magnetic field sensor and a predetermined correction table or a predetermined correction formula so that at least one static magnetic field distribution of the first static magnetic field, the second static magnetic field, and the composite static magnetic field becomes a desired static magnetic field distribution.

11. A magnetic resonance imaging system according to claim 10.

14. a correction coil for correcting at least one static magnetic field distribution of the first static magnetic field, the second static magnetic field, and a composite static magnetic field obtained by combining the first static magnetic field and the second static magnetic field; a correction coil power supply for supplying a correction current to the correction coil; a pre-polarization power supply that supplies a pre-polarization current to a pre-polarization coil that constitutes the pre-polarization magnet; a pre-polarization control unit that controls the correction current and the pre-polarization current based on tissue parameters of a subject to be imaged and sequence parameters of a pulse sequence used to image the subject; The magnetic resonance imaging system of claim 1 further comprising:

15. the pre-polarization control unit controls a parameter related to at least one of the correction current and the pre-polarization current based on the tissue parameter and the sequence parameter.

15. A magnetic resonance imaging system according to claim 14.

16. The tissue parameter includes a longitudinal relaxation time (T1 value) of tissue in the subject, and the sequence parameter includes at least one of an application time of the second static magnetic field generated by the pre-polarization magnet and an elapsed time from when application of the second static magnetic field is stopped to when imaging of the subject is started.

15. A magnetic resonance imaging system according to claim 14.

Images