Magnetic resonance imaging apparatus

Abstract

To suppress the number of times of demagnetizing a static magnetic field magnet in shimming of an MRI apparatus.SOLUTION: A magnetic resonance imaging apparatus according to an embodiment includes a magnet gantry including a static magnetic field magnet that generates a static magnetic field in a cylindrical bore in which a subject is placed, and a shim tray including magnetic shims for adjusting the static magnetic field, a first shim-tray element unit including shim pockets for housing the magnetic shims, and a second shim-tray element unit including the shim pockets. Each of the first shim-tray element unit and the second shim-tray element unit is inserted into and removed from the magnet gantry along the axial direction through at least one of openings provided on one end side and the other end side of the magnet gantry in the axial direction of the bore.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The embodiments disclosed in this specification and drawings relate to a magnetic resonance imaging apparatus. [Background technology]

[0002] In magnetic resonance imaging (MRI) devices, excellent spatial uniformity of the static magnetic field (i.e., magnetic field homogeneity) is required in the imaging region to obtain high-quality magnetic resonance (MR) images. Magnetic field homogeneity is affected by, for example, the environment in which the MRI device is installed and manufacturing errors of the static magnetic field magnet. Therefore, when installing the MRI device, adjustment of the static magnetic field (i.e., shimming) is performed so that the magnetic field homogeneity in the imaging region within the bore of the MRI device satisfies a specified value.

[0003] Shimming is performed by placing magnetic shims made of metal materials such as iron pieces in a shim tray and then fitting the shim tray into a shim slot located in the magnet gantry of an MRI apparatus. Shimming involves repeatedly performing a series of shimming steps: placing the magnetic shims in the shim tray, inserting the shim tray into the magnet gantry, energizing the static magnetic field magnet, measuring the magnetic field uniformity, and, if the magnetic field uniformity is lower than a specified value, demagnetizing the static magnetic field magnet and removing the shim tray from the magnet gantry. Demagnetizing and energizing the static magnetic field magnet during the shimming process consumes helium used to cool the static magnetic field magnet, increasing the labor required for the work. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-115480 Summary of the Invention [Problem to be solved by the invention]

[0005] One of the problems to be solved by the embodiments disclosed in this specification and the drawings is to reduce the number of times the static magnetic field magnet is demagnetized during shimming of an MRI 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 the embodiments described below can also be positioned as other problems. [Means for solving the problem]

[0006] An MRI apparatus according to one embodiment includes a magnet gantry having a static magnetic field magnet that generates a static magnetic field in a cylindrical bore in which a subject is placed, and a shim tray having magnetic shims for adjusting the static magnetic field, a first shim tray element including a shim pocket for accommodating the magnetic shim, and a second shim tray element including the shim pocket. Each of the first shim tray element and the second shim tray element is inserted into and removed from the magnet gantry along the axial direction of the bore through at least one of openings provided at one end and the other end of the magnet gantry in the axial direction. [Brief explanation of the drawings]

[0007] [Figure 1] 1 is a block diagram of an example of the overall configuration of an MRI apparatus according to an embodiment. [Figure 2] 1 is a schematic perspective view of a configuration example of a magnet gantry of an MRI apparatus according to an embodiment. [Figure 3] FIG. 2 is a schematic perspective view of a configuration example of a shim tray according to the first embodiment. [Figure 4] 1 is a cross-sectional view of a part of a magnet gantry of an MRI apparatus according to a first embodiment. [Figure 5] FIG. 3 is a cross-sectional view illustrating an example of an insertion / removal direction of a shim tray according to the first embodiment. [Figure 6] 10 is a cross-sectional view illustrating another example of the insertion and removal direction of the shim tray according to the first embodiment. FIG. [Figure 7] 4 is a flowchart showing an example of a shimming operation in the MRI apparatus according to the embodiment. [Figure 8] FIG. 10 is a cross-sectional view illustrating an example of a shim tray according to a second embodiment. [Figure 9] FIG. 10 is a cross-sectional view illustrating another example of the shim tray according to the second embodiment. [Figure 10] FIG. 10 is a cross-sectional view illustrating an example of a shim tray according to a modified example of the second embodiment. [Figure 11] FIG. 11 is a cross-sectional view of a part of a magnet gantry of an MRI apparatus according to a third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, an embodiment of an MRI apparatus will be described in detail with reference to the drawings. In each drawing, the same elements are given the same reference numerals and the description thereof will be omitted.

[0009] (Overall configuration of MRI device) 1 is a block diagram of an example of the overall configuration of an MRI apparatus 1 according to an embodiment. The MRI apparatus 1 includes a magnet gantry 100, a control cabinet 300, an image processing device 400 (e.g., a console), and a bed 500. The left-right direction of a subject P placed on the magnet gantry 100 and the bed 500 is defined as the X-axis direction, the front-back direction (thickness direction) is defined as the Y-axis direction, and the head-to-foot direction is defined as the Z-axis direction.

[0010] The magnetic gantry 100 includes a static magnetic field magnet 10, a gradient magnetic field coil 11, and a WB (Whole Body) coil 12. These components are housed in a cylindrical housing.

[0011] The static magnetic field magnet 10 is a superconducting magnet that has a roughly cylindrical shape and generates a static magnetic field in the space inside the cylinder in which a subject P (e.g., a patient) is placed, i.e., in a cylindrical bore B. The static magnetic field magnet 10 incorporates a superconducting coil that is cooled to an extremely low temperature by liquid helium. In the excitation mode, the static magnetic field magnet 10 generates a static magnetic field by applying a current supplied from a static magnetic field power supply (not shown) to the superconducting coil. In the demagnetization mode, the static magnetic field magnet 10 reduces or eliminates the static magnetic field by applying a current supplied from the static magnetic field power supply to the superconducting coil. When the static magnetic field magnet 10 transitions to the persistent current mode, it is disconnected from the static magnetic field power supply and continues to generate a large static magnetic field for a long period of time, for example, for more than one year.

[0012] The gradient magnetic field coil 11 has a roughly cylindrical shape and is disposed inside the static magnetic field magnet 10. The gradient magnetic field coil 11 generates a gradient magnetic field in accordance with power (current) supplied from a gradient magnetic field power supply 31. The gradient magnetic field is applied to the subject P.

[0013] In order to reduce eddy currents that occur with the generation of the gradient magnetic fields, an ASGC (Actively Shielded Gradient Coil) may be used as the gradient magnetic field coil 11. The ASGC includes a main coil for generating the gradient magnetic fields in the X-axis, Y-axis, and Z-axis directions, a shim tray that can store multiple magnetic shims, and a shield coil for suppressing leakage magnetic fields.

[0014] The WB coil 12, also called a whole-body coil, has a roughly cylindrical shape and is arranged inside the gradient magnetic field coil 11 so as to surround the subject P. The WB coil 12 transmits RF (Radio Frequency) pulses transmitted from an RF transmitter 33 to the subject P, and also receives MR signals emitted from the subject P due to excitation of atomic nuclei.

[0015] The MRI apparatus 1 may include a local coil 20 in addition to the WB coil 12. The local coil 20 is also called a local coil, and is placed close to the body surface of the subject P. There are various types of local coils 20, such as a head coil, a chest coil, an abdominal coil, a spine coil, and a knee coil. The local coil 20 includes a receive-only coil, a transmit-only coil, and a transmit-receive coil that performs both transmission and reception. The local coil 20 is configured to be detachable from the tabletop 51 via a cable, for example.

[0016] The bed 500 includes a bed body 50 and a top board 51. The bed body 50 is capable of moving the top board 51 in the vertical and horizontal directions. The bed body 50 moves the subject P placed on the top board 51 to a predetermined height, and then moves the top board 51 in the horizontal direction to position the subject P in the bore B.

[0017] The control cabinet 300 includes a gradient magnetic field power supply 31, an RF receiver 32, an RF transmitter 33, and a sequence controller .

[0018] The gradient magnetic field power supply 31 supplies power to the gradient magnetic field coil 11 under the control of the sequence controller 34, and generates gradient magnetic fields along the X-axis, Y-axis, and Z-axis.

[0019] The RF transmitter 33 generates RF pulses under the control of the sequence controller 34. The generated RF pulses are transmitted to the WB coil 12 or the local coil 20 and applied to the subject P. The RF receiver 32 detects MR signals received by the WB coil 12 or the local coil 20, converts the detected MR signals into analog-to-digital (AD) signals, and outputs the converted signals to the sequence controller 34. The digitized MR signals are called raw data.

[0020] The sequence controller 34, under the control of the image processing device 400, drives the gradient magnetic field power supply 31, the RF receiver 32, and the RF transmitter 33 to perform a scan of the subject P. The sequence controller 34 transmits raw data collected by the scan to the image processing device 400.

[0021] The sequence controller 34 includes a processing circuit (not shown) that is configured with hardware such as a processor that executes a predetermined program, an FPGA (Field Programmable Gate Array), and an ASIC (Application Specific Integrated Circuit).

[0022] The image processing device 400 includes a processing circuit 40, a memory circuit 41, a display 42, and an input interface 43. The image processing device 400 may also include a network interface 44.

[0023] The processing circuit 40 is a circuit including, for example, a CPU or a dedicated or general-purpose processor. The processor realizes various functions through software processing by executing various programs stored in the storage circuit 41 or directly embedded in the processing circuit 40.

[0024] The memory circuitry 41 is configured with a memory medium including, for example, a semiconductor memory element such as a RAM (Random Access Memory) or a flash memory, or an external storage device such as a hard disk or an optical disk. The memory circuitry 41 may also be a portable medium such as a USB (Universal Serial Bus) memory or a DVD (Digital Video Disk). The memory circuitry 41 stores various types of information and data, and stores various programs executed by the processor of the processing circuitry 40.

[0025] The display 42 is configured by a display device such as a liquid crystal display or an OLED (Organic Light Emitting Diode) display. The display 42 displays various information under the control of the processing circuit 40. The display 42 is a display device and may also be a GUI (Graphical User Interface) that functions as an input device.

[0026] The input interface 43 includes an input device and an input circuit. The input device may be a trackball, a switch, a mouse, a keyboard, a touchpad, a touchscreen, a non-contact input device using an optical sensor, a voice input device, etc. When a user operates the input device, the input circuit generates a signal corresponding to the operation and outputs it to the processing circuit 40.

[0027] The network interface 44 communicates with various devices connected to the network via wired or wireless means, and exchanges various types of information and data.

[0028] Using these components, the image processing device 400 controls the entire MRI apparatus 1. Specifically, the processing circuitry 40 receives instructions regarding imaging conditions and various other information through operations by a user, such as a medical technician, via an input interface 43. The processing circuitry 40 then causes the sequence controller 34 to execute a scan based on the input imaging conditions, and reconstructs an MR image based on the raw data received from the sequence controller 34. The reconstructed MR image is displayed on a display 42 and stored in a memory circuitry 41.

[0029] (shimming) To obtain images of good quality with the MRI apparatus 1, excellent magnetic field uniformity is required in the imaging region S within the bore B. The magnetic field uniformity is affected by, for example, the environment in which the MRI apparatus 1 is installed and manufacturing errors of the static magnetic field magnet 10. Therefore, when installing the MRI apparatus 1, shimming is performed so that the magnetic field uniformity in the imaging region S satisfies a specified value.

[0030] FIG. 2 is a schematic perspective view of an example configuration of the magnet gantry 100 of the MRI apparatus 1 according to the embodiment. Shimming is performed by storing magnetic shims 74 (see FIG. 3) for adjusting the static magnetic field in a columnar shim tray 70. The shim tray 70 is placed on the magnet gantry 100 so that its longitudinal direction is along the axial direction of the bore B (i.e., the Z-axis direction). The shim tray 70 is made of a nonmagnetic and nonconductive material, such as glass fiber or resin. The following describes the case where the gradient magnetic field coil 11 is an ASGC, i.e., the case where the shim tray 70 is inserted into and removed from the gradient magnetic field coil 11; however, the shim tray 70 may be inserted into and removed from the magnet gantry 100 independently of the gradient magnetic field coil 11.

[0031] The gradient magnetic field coil 11 includes, in this order from the center of the bore B, a main coil 11a, a shim tray portion 11b, and a shield coil 11c. The shim tray portion 11b has a generally cylindrical shape, and a plurality of shim slots 71 are formed at approximately equal intervals in the circumferential direction of the shim tray portion 11b. The shim slots 71 are through holes formed along the axial direction of the bore B. The multiple shim trays 70 are inserted into and removed from the multiple shim slots 71 formed in the gradient magnetic field coil 11, and are arranged parallel to the axial direction of the bore B along the side surface of a cylinder whose central axis is the axis of the bore B. Note that the position, number, shape, etc. of the shim slots 71 formed in the shim tray portion 11b are not limited to those shown in FIG. 2.

[0032] In conventional shimming, magnetic field uniformity is measured when the static magnetic field is at the rated magnetic field, and then the placement of magnetic shims is calculated based on the measurement results to ensure that the magnetic field uniformity in the imaging region S meets a predetermined value. The rated magnetic field is the magnetic field strength of the static magnetic field when a subject is scanned by an MRI device for diagnosis. For safety reasons, the static magnetic field magnet is demagnetized and the shim tray is removed from the magnet gantry. Then, magnetic shims are placed (e.g., installed or removed) based on the calculation results for the placement of the magnetic shims. The shim tray is then inserted into the magnet gantry, the static magnetic field is excited, the static magnetic field is set to the rated magnetic field again, and the magnetic field uniformity is measured. This series of shimming procedures is repeated until the magnetic field uniformity in the imaging region S meets the predetermined value.

[0033] The demagnetization and magnetization of the static magnetic field magnet during shimming consumes helium, the cooling medium used to cool the static magnetic field magnet, and increases the number of labor hours required. While inserting and removing the shim trays while the static magnetic field is at its rated magnetic field could be considered to reduce helium consumption and labor hours, this is difficult from the perspective of safety concerns, as a large magnetic force acts on the entire shim tray. For example, a method is known in which a rail-shaped jig is attached to the magnet stand to insert and remove the shim tray, but attaching the rail-shaped jig requires additional labor hours.

[0034] Furthermore, if the number of magnetic shims arranged varies depending on the position of the shim tray, the magnetic force acting on the shim tray also varies depending on the position of the shim tray. Because the magnetic force varies depending on the position of the shim tray, when inserting or removing the shim tray into or from the magnet stand while the static magnetic field is at the rated magnetic field, the magnetic force acting on the shim tray fluctuates during the insertion or removal process, which can be dangerous.

[0035] Therefore, in the MRI apparatus 1 according to the embodiment, the shim tray 70 is divided into a plurality of shim tray element parts (for example, shim tray element parts 701 and 702 in FIG. 3), and each shim tray element part is inserted into or removed from the magnet gantry 100. Because the shim tray 70 is divided into a plurality of shim tray element parts and the magnetic force acting on each shim tray element part is smaller than the magnetic force acting on the entire shim tray, the shim tray 70 can be safely inserted or removed even when the static magnetic field is at the rated magnetic field.

[0036] (First embodiment) 3 is a schematic perspective view of an example configuration of the shim tray 70 according to the first embodiment. The shim tray 70 has a plurality of shim pockets 73 for accommodating magnetic shims 74. The shim pockets 73 are recesses formed in the shim tray 70 at predetermined intervals. The magnetic shims 74 are magnetic members such as iron pieces, and are used to adjust the static magnetic field. In shimming, a required number of magnetic shims 74 are accommodated in predetermined positions among the plurality of shim pockets 73 so that the magnetic field uniformity in the imaging region S in the bore B satisfies a specified value.

[0037] The shim tray 70 is divided into multiple shim tray element portions based on the position along the axial direction of the bore B and the magnetic force acting on the shim tray 70 corresponding to that position. In the first embodiment, the shim tray 70 is divided into two: a first shim tray element portion 701 and a second shim tray element portion 702. The shim tray element portions 701 and 702 each include a box portion 701a and 702a, and a lid portion 701b and 702b. The lid portions 701b and 702b are lids that cover the box portions 701a and 702a after the magnetic shims 74 are placed in the shim pockets 73.

[0038] Each of the multiple shim tray element portions 701 and 702 has at least one shim pocket 73. In Fig. 3, the first shim tray element portion 701 has five shim pockets 73 in the box portion 701a, and the second shim tray element portion 702 has six shim pockets 73 in the box portion 702a. Note that the positions, number, and shape of the shim pockets 73 and the positions, number, and shape of the magnetic shims 74 in each of the shim tray element portions 701 and 702 are not limited to those shown in Fig. 3.

[0039] The shim tray 70 is divided into multiple shim tray element portions 701 and 702 so that the magnetic force acting on each shim tray element portion is smaller than a predetermined value. This predetermined value is a magnetic force that is small enough to ensure safety during insertion and removal operations. To make it easier for all of the shim tray element portions 701 and 702 to meet the predetermined value, the shim tray 70 may be divided so that the magnetic forces acting on each of the shim tray element portions 701 and 702 are approximately the same. Note that the positions at which the shim tray 70 is divided will vary not only depending on manufacturing errors in the static magnetic field magnet 10 but also on the environment in which the MRI apparatus 1 is installed.

[0040] 4 is a cross-sectional view of a portion of the magnet gantry 100 of the MRI apparatus 1 according to the first embodiment. The first shim tray element 701 is inserted into the magnet gantry 100 along the axial direction of the bore B and fixed by a first fixing portion 75a provided on one end of the magnet gantry 100. The second shim tray element 702 is inserted into the magnet gantry 100 along the axial direction of the bore B and fixed by a second fixing portion 75b provided on the other end of the magnet gantry 100. The fixing portions 75a and 75b may fix the shim tray elements 701 and 702 to the gradient coil 11 via through holes provided in the shim tray elements 701 and 702. Each of the shim tray elements 701 and 702 may be provided with a handle to facilitate insertion and removal from the magnet gantry 100.

[0041] Each of the first shim tray element portion 701 and the second shim tray element portion 702 is inserted and removed along the axial direction from at least one of openings provided on one end side and the other end side of the magnet gantry 100 in the axial direction of the bore B. Hereinafter, the opening provided on one end side of the magnet gantry 100 in the axial direction of the bore B will be referred to as the "first opening side," and the opening provided on the other end side of the magnet gantry 100 in the axial direction of the bore B will be referred to as the "second opening side."

[0042] 5 to 6(B) are cross-sectional views illustrating the insertion and removal direction of the shim tray 70 according to the first embodiment. In Fig. 5, the first shim tray element 701 is inserted into and removed from the magnet gantry 100 along the axial direction of the bore B from the first opening side, and the second shim tray element 702 is inserted into and removed from the magnet gantry 100 along the axial direction of the bore B from the second opening side.

[0043] In contrast to Fig. 5, in Fig. 6(A), the first shim tray element part 701 and the second shim tray element part 702 are inserted into and removed from the magnet gantry 100 from the first opening side. Also, in Fig. 6(B), the first shim tray element part 701 and the second shim tray element part 702 are inserted into and removed from the magnet gantry 100 from the second opening side. In other words, the shim tray element parts 701 and 702 are inserted into and removed from the same opening. In this case, the first shim tray element part 701 and the second shim tray element part 702 can be connected to each other. The connecting member 76 that connects the first shim tray element part 701 and the second shim tray element part 702 will be described in detail later.

[0044] Here, an example of the shimming operation in the MRI apparatus 1 according to the embodiment will be described with reference to the flowchart of FIG.

[0045] In step ST10, the magnetic shims 74 are placed on the shim tray 70 based on the initial settings and the calculation results of step ST40, which will be described later.

[0046] In step ST20, the shim tray 70 is inserted into the magnet gantry 100.

[0047] In step ST30, the static magnetic field magnet 10 is excited.

[0048] In step ST40, the magnetic field uniformity is measured while the static magnetic field is at the rated magnetic field. For example, the magnetic field uniformity is measured from the magnetic field strength at each position characterized by three-dimensional coordinates in the bore B where the subject P is placed or polar coordinates with the magnetic field center as the origin. Based on the results of this magnetic field uniformity measurement, the positions and amounts of the magnetic shims 74 placed on the shim tray 70 in step ST10 are calculated.

[0049] In step ST50, it is determined whether the magnetic field uniformity in the imaging region S satisfies a specified value. If the magnetic field uniformity satisfies the specified value (i.e., YES) in step ST50, the process ends. If the magnetic field uniformity does not satisfy the specified value (i.e., NO) in step ST50, the process proceeds to step ST60.

[0050] In step ST60, it is determined whether the magnetic force acting on each shim tray element is smaller than a predetermined value. If the magnetic force acting on each shim tray element is not smaller than the predetermined value (i.e., NO) in step ST60, the process proceeds to step ST70, and then to step ST80.

[0051] In step ST70, the static magnetic field magnet 10 is demagnetized.

[0052] In step ST80, the shim tray 70 is removed from the magnet gantry 100.

[0053] In step ST60, if the magnetic force acting on each shim tray element is smaller than the predetermined value (i.e., YES), the process proceeds to step ST80. That is, the static magnetic field magnet 10 is not demagnetized, and each shim tray element of the shim tray 70 is inserted into and removed from the magnet gantry 100 at least once while the static magnetic field is at the rated magnetic field.

[0054] In conventional shim trays that are not divided into multiple shim tray element portions, a large magnetic force acts on the entire shim tray during insertion and removal when the static magnetic field is at the rated magnetic field, making it difficult to ensure safety during insertion and removal. In contrast, in the shim tray 70 according to the embodiment, which is divided into multiple shim tray element portions, the magnetic force acting on each shim tray element portion is smaller than the magnetic force acting on the entire shim tray, making it possible to ensure safety during insertion and removal even when the static magnetic field is at the rated magnetic field. Therefore, with the shim tray 70 according to the embodiment, it is possible to reduce the number of times step ST70 is performed. Note that even with the shim tray 70 according to the embodiment, the processing of step ST70 may be performed if the magnetic force acting on each shim tray element portion is not smaller than the predetermined value.

[0055] After step ST80, the process returns to step ST10. That is, after the magnetic shims 74 are arranged based on the calculation results of step ST40, the shim tray 70 is inserted into the magnet gantry 100, and the magnetic field uniformity is measured again. This series of shimming operations is repeated until the magnetic field uniformity in the imaging region S satisfies the specified value. Note that, from the second repetition of the shimming operation onwards, step ST30 is omitted if the static magnetic field magnet 10 has not been demagnetized (i.e., step ST70).

[0056] According to the MRI apparatus 1 of the first embodiment, the shim tray is composed of multiple shim tray elements, and the magnetic force acting on each shim tray element is smaller than the magnetic force acting on the entire shim tray. Therefore, by inserting and removing each shim tray element, safety can be ensured during the insertion and removal work when the static magnetic field is at the rated magnetic field. Furthermore, by reducing the number of times the static magnetic field magnet is demagnetized and excited during the insertion and removal work, it is possible to reduce the amount of helium consumed to cool the static magnetic field magnet (e.g., about 200 liters) and the labor hours (e.g., 160 hours).

[0057] Furthermore, when the shim tray elements are inserted or removed from both sides of the gantry, the insertion / removal distance of each shim tray element is shortened. This eliminates the need for the ends of a shim tray with a large amount of magnetic shims to straddle the center of a shim tray with a small amount of magnetic shims when inserting or removing the shim tray elements, thereby avoiding the danger of the magnetic force acting on each shim tray element fluctuating during the insertion / removal process.

[0058] (Second embodiment) In the second embodiment, the shim tray 70 is divided into three or more shim tray element portions. That is, the shim tray 70 may be divided into a plurality of shim tray element portions including a first shim tray element portion and a second shim tray element portion. FIGS. 8(A) to 9(B) are cross-sectional views illustrating a case where each shim tray element portion is inserted and removed from both the first opening side and the second opening side (FIGS. 8(A) and 8(B)) and a case where each shim tray element portion is inserted and removed from either the first opening side or the second opening side (FIGS. 9(A) and 9(B)). In FIGS. 8(A) to 9(B), the shim tray 70 is made up of four shim tray element portions 711 to 714.

[0059] In Fig. 8(A), two shim tray element portions 711 and 712 are connected to each other by a connecting member 76, and are inserted and removed individually from the first opening side. Two shim tray element portions 713 and 714 are connected to each other by a connecting member 76, and are inserted and removed individually from the second opening side. Fig. 8(B) shows a state in which, after the shim tray 70 in Fig. 8(A) has been inserted and removed from the magnet gantry 100, the two shim tray element portions 711 and 712 are fixed by the first fixing portion 75a, and the two shim tray element portions 713 and 714 are fixed by the second fixing portion 75b.

[0060] The connecting member 76 is made of a non-magnetic material, and has a length that allows, when one of the adjacent shim tray element parts (for example, the first shim tray element part 701 in FIG. 6(A)) that have the same opening for insertion and removal is removed from the magnet gantry 100, that one shim tray element part to be kept a predetermined distance from the magnet gantry 100 while the other shim tray element part (for example, the second shim tray element part 702 in FIG. 6(A)) remains within the magnet gantry 100. Note that, when three or more shim tray element parts are connected and some shim tray element parts have already been removed from the magnet gantry 100, the remaining shim tray element parts are removed while the removed shim tray element part is kept a predetermined distance from the magnet gantry 100.

[0061] Because the connecting member 76 has such a length, it is possible to remove other shim tray elements in a static magnetic field while placing the removed shim tray element in a location where it is less susceptible to the magnetic force of the static magnetic field, thereby ensuring safety during the insertion and removal work. Also, even when inserting one shim tray element into the magnetic gantry 100, the other shim tray elements are placed in locations where they are less susceptible to the magnetic force of the static magnetic field, so the work can be done with only one shim tray element being subjected to the magnetic force of the magnetic field, ensuring safety during the insertion and removal work.

[0062] The shim tray element portion has a protrusion 77 on the side adjacent to another shim tray element portion via a connecting member 76. As shown in Fig. 8(B), the connecting member 76 is stored in a gap that is generated between adjacent shim tray element portions that are connected via the protrusion 77. The connecting member 76 is desirably structured to fit snugly into the gap, and may also be structured to be rotatable at the joint with the shim tray element portion and less prone to twisting.

[0063] Note that each of the shim tray element portions 712 and 713 may or may not have a protrusion 77 at the connection portion J between the shim tray element portion 712 inserted or removed from the first opening side and the shim tray element portion 713 inserted or removed from the second opening side. It is sufficient that the shim pockets 73 are arranged at predetermined intervals throughout the shim tray 70, taking into account the presence or absence of protrusions 77. Furthermore, the connection member 76 may be stored in a space provided inside the shim tray element portion, in which case each shim tray element portion may or may not have a protrusion 77.

[0064] In Fig. 9(A), the four shim tray elements 711-714 are connected to one another by a connecting member 76, and each shim tray element is inserted into or removed from the magnetic gantry 100 from either the first opening side or the second opening side. In this way, among the multiple shim tray elements, adjacent shim tray elements that have the same opening to be inserted or removed are connected to one another by a connecting member 76. Fig. 9(B) shows a state in which the four shim tray elements 711-714 are fixed by the first fixing portion 75a and the second fixing portion 75b after the shim tray 70 in Fig. 9(A) has been inserted into or removed from the magnetic gantry 100.

[0065] (Modification of the second embodiment) In a modified example of the second embodiment, the multiple shim tray element portions are a connection of shim tray element members each having one or more shim pockets 73. Fig. 10(A) shows that the four shim tray element portions 711-714 of Fig. 8(A) are each made up of multiple shim tray element members each having one shim pocket 73. In Fig. 10(A), the four shim tray element portions 711-714 are each made up of, in order, two shim tray element members 711a and 711b, three shim tray element members 712a-712c, three shim tray element members 713a-713c, and three shim tray element members 714a-714c.

[0066] Figure 10(B) shows a state in which the shim tray 70 in Figure 10(A) is fixed by the first fixing portion 75a and the second fixing portion 75b after being inserted into or removed from the magnet gantry 100. In a modification of the second embodiment, in step ST40, the positions at which the multiple shim tray elements are divided, i.e., the number of shim pockets 73 that each shim tray element has, is calculated based on the measurement results of the magnetic field uniformity, and in step ST10, the multiple shim tray elements are divided based on the calculation results.

[0067] For example, as shown in Fig. 10(A), the shim tray 70 which was divided into four shim tray element parts 711 to 714 is divided into five shim tray element parts 711' to 715' as shown in Fig. 10(C). In Fig. 10(C), the five shim tray element parts 711' to 715' are respectively made up of one shim tray element member 711a, two shim tray element members 711b and 712a, three shim tray element members 712b to 714a, two shim tray element members 713b and 713c, and three shim tray element members 714a to 714c, in that order.

[0068] Each shim tray element may be made up of one shim tray element member, or may be made up of multiple shim tray element members. In this way, multiple shim tray element members may be a connected body of shim tray element members having one or multiple shim pockets 73. The shim tray element members that make up each shim tray element member have a joinable structure.

[0069] According to the MRI apparatus 1 of the second embodiment, the shim tray is divided into more shim tray element parts than in the first embodiment, so the magnetic force acting on each shim tray element part is even smaller, making it possible to insert and remove the shim tray more safely even when the static magnetic field is at the rated magnetic field. Furthermore, according to the MRI apparatus 1 of the modified example of the second embodiment, the positions at which the shim tray is divided can be easily changed so that the magnetic force acting on each shim tray element part is smaller than a predetermined value, further improving work efficiency.

[0070] (Third embodiment) 11 is a cross-sectional view of a portion of the magnet gantry 100 of the MRI apparatus 1 according to the third embodiment. In the first and second embodiments, multiple shim trays 70 are arranged in a circle from the center of the bore B along the axial direction of the bore B, whereas in the third embodiment, multiple shim trays 70A and 70B are arranged in multiple (two in FIG. 11) concentric circles at different distances from the center of the bore B along the axial direction of the bore B. The shim trays 70A and 70B are examples of shim trays 70.

[0071] 11, the shim tray 70A is made up of two shim tray element parts 721 and 722, and the shim tray 70B is made up of two shim tray element parts 731 and 732. The multiple shim trays 70A and 70B may be shim trays with the same total magnetic force, or may be shim trays with different total magnetic force.

[0072] The shim trays 70A and 70B are each divided into a plurality of shim tray element parts based on their positions along the axial direction of the bore B and the magnetic forces acting on the shim trays 70A and 70B corresponding to those positions. Each of the shim trays 70A and 70B is divided into a plurality of shim tray element parts so that the magnetic forces acting on each shim tray element part are each smaller than a predetermined value.

[0073] According to the MRI apparatus 1 of the third embodiment, the shim tray is divided into multiple shim tray element parts, and is located at different distances from the center of the bore B. This further reduces the magnetic force acting on each shim tray element part, making it possible to insert and remove the shim tray more safely even when the static magnetic field is at the rated magnetic field.

[0074] According to the MRI apparatus of at least one of the embodiments described above, the number of times the static magnetic field magnet is demagnetized can be reduced during shimming of the MRI apparatus.

[0075] Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be embodied in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are within the scope of the invention and its equivalents as defined in the claims, as well as the scope and spirit of the invention. [Explanation of symbols]

[0076] 1...Magnetic resonance imaging (MRI) device 10...Static magnetic field magnet 11...Gradient magnetic field coil 11b...Shim tray portion 70, 70A, 70B...Shim tray 73...Shim pocket 74...Magnetic shim 75a, 75b...Fixed portion 701, 702, 711, 711', 712, 712', 713, 713', 714, 714', 715', 721, 722, 731, 732...Shim tray element portion B...Bore

Claims

1. a magnetic gantry having a static magnetic field magnet that generates a static magnetic field in a cylindrical bore in which a subject is placed; a magnetic shim for adjusting the static magnetic field; a shim tray having a first shim tray element portion including a shim pocket for accommodating the magnetic shim, and a second shim tray element portion including the shim pocket; Equipped with each of the first shim tray element portion and the second shim tray element portion is inserted into and removed from the magnet gantry along the axial direction from at least one of openings provided on one end side and the other end side of the magnet gantry in the axial direction of the bore; Magnetic resonance imaging device.

2. the magnetic gantry further includes a gradient coil for generating a gradient magnetic field; The shim tray is inserted into and removed from a shim slot formed in the gradient magnetic field coil along the axial direction.

2. The magnetic resonance imaging apparatus according to claim 1.

3. the first shim tray element is inserted into and removed from the magnet gantry along the axial direction through an opening provided on the one end side, the second shim tray element is inserted into and removed from the magnet gantry along the axial direction through an opening provided on the other end side.

2. The magnetic resonance imaging apparatus according to claim 1.

4. the shim tray is divided into a plurality of shim tray element parts including the first shim tray element part and the second shim tray element part so that magnetic forces acting on each shim tray element part are each smaller than a predetermined value; 2. The magnetic resonance imaging apparatus according to claim 1.

5. the shim tray is divided into a plurality of shim tray element parts including the first shim tray element part and the second shim tray element part based on a position along the axial direction and a magnetic force acting on the shim tray corresponding to the position; 2. The magnetic resonance imaging apparatus according to claim 1.

6. the first shim tray element portion and the second shim tray element portion are inserted into and removed from the magnet gantry at least once while the static magnetic field is at a rated magnetic field; 2. The magnetic resonance imaging apparatus according to claim 1.

7. The first shim tray element portion and the second shim tray element portion that are adjacent to each other and have the same opening to be inserted or removed are connected to each other by a connecting member.

2. The magnetic resonance imaging apparatus according to claim 1.

8. the connecting member is made of a non-magnetic material and has a length such that, when the first shim tray element portion is extracted from the magnetic gantry, the first shim tray element portion is maintained at a predetermined distance from the magnetic gantry while the second shim tray element portion remains within the magnetic gantry.

8. The magnetic resonance imaging apparatus according to claim 7.

9. the first shim tray element is fixed to a first fixing portion provided on one end side of the magnet gantry in the axial direction, the second shim tray element is fixed to a second fixing portion provided on the other end side of the magnet gantry in the axial direction; 2. The magnetic resonance imaging apparatus according to claim 1.

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