Static magnetic field shimming method

Abstract

To suppress deterioration in the quality of an image obtained by an MRI apparatus.SOLUTION: A static magnetic field shimming method adjusts a space distribution of a static magnetic field in an MRI apparatus by a metal shim. The static magnetic field shimming method performs first measurement for measuring a space distribution of a static magnetic field in a state that a second magnetic field smaller than a first magnetic field, which is the size of the magnetic field used when a subject is imaged, is output from the MRI apparatus, second measurement for measuring a space distribution of a static magnetic field in a state that the second magnetic field is output from the MRI apparatus after the metal shim is provided in the MRI apparatus on the basis of a result of the first measurement, and reset processing for residual magnetic flux density of the metal shim when the space distribution of the static magnetic field based on the second measurement does not satisfy a predetermined condition. The reset processing is the processing for heating the metal shim provided in the MRI apparatus on the basis of a result of the first measurement or for applying a magnetic field to the metal shim from outside.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The embodiments disclosed herein and in the drawings relate to a method for static magnetic field shimming. [Background technology]

[0002] To obtain high-quality images using a magnetic resonance imaging (MRI) device, high spatial uniformity of the static magnetic field is required. Because the spatial uniformity of the static magnetic field is affected by manufacturing errors and the environmental magnetic field at the local site, for example, when installing the MRI device, metal shims (e.g., pieces of iron or magnetic material) are placed in predetermined positions to adjust the spatial uniformity of the static magnetic field (i.e., shimming).

[0003] A conventional method for performing shimming is known, in which a low magnetic field, which is smaller than the rated magnetic field used when imaging a subject, is output from an MRI system. In this method, the spatial uniformity of the static magnetic field is measured while a low magnetic field, which is smaller than the rated magnetic field, is output from the MRI system. Based on the measurement results, the placement of metal shims to achieve the desired magnetic field uniformity is calculated by simulation. Then, the installation and removal of the metal shims are performed based on the simulation results. As a result, the time required for the shimming process is shortened, and the amount of liquid helium consumed is reduced.

[0004] Because the magnetization characteristics of metal shims vary depending on the magnitude of the magnetic field applied to them, even if the spatial uniformity of the static magnetic field is adjusted when a magnetic field lower than the rated magnetic field is output from the MRI device, the spatial uniformity of the static magnetic field may be low when the rated magnetic field is output from the MRI device, which may result in a deterioration in the quality of images obtained by the MRI device. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2018-38610 [Patent Document 2] Publication of Special Publication No. 2015-133352 Summary of the Invention [Problem to be solved by the invention]

[0006] One of the problems to be solved by the embodiments disclosed in this specification and the drawings is to prevent a decrease in the quality of images obtained by 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 the configurations shown in the embodiments described below can also be positioned as other problems. [Means for solving the problem]

[0007] A static magnetic field shimming method according to one embodiment uses a metal shim to adjust the spatial distribution of a static magnetic field in a magnetic resonance imaging apparatus that outputs a static magnetic field. The static magnetic field shimming method according to one embodiment includes the steps of: performing a first measurement to measure the spatial distribution of the static magnetic field when a second magnetic field, the second magnetic field being smaller than a first magnetic field used when imaging a subject, is output from the magnetic resonance imaging apparatus; installing a metal shim in the magnetic resonance imaging apparatus based on the results of the first measurement, and then performing a second measurement to measure the spatial distribution of the static magnetic field when the second magnetic field is output from the magnetic resonance imaging apparatus; and performing a reset process of the residual magnetic flux density of the metal shim if the spatial distribution of the static magnetic field based on the second measurement does not satisfy a predetermined condition. The reset process is characterized by heating or applying an external magnetic field to the metal shim installed in the magnetic resonance imaging apparatus based on the results of the first measurement. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a block diagram of an example of the overall configuration of an MRI apparatus having a static magnetic field magnet according to an embodiment. [Figure 2] FIG. 1 is a schematic perspective view of a configuration example of a magnet gantry having a static magnetic field magnet according to an embodiment. [Figure 3] FIG. 2 is a schematic perspective view of an example of the configuration of a shim tray and a metal shim according to an embodiment. [Figure 4] 10A and 10B are diagrams for explaining the difference between a static magnetic field shimming method of a comparative example in which the magnetic field is increased and decreased and a static magnetic field shimming method of an embodiment in which the magnetic field is not increased and decreased. [Figure 5] FIG. 4 is a diagram illustrating the magnetization characteristics of a metal shim. [Figure 6] 3 is a flowchart showing the procedure of a static magnetic field shimming method according to the first embodiment. [Figure 7] 10 is a flowchart showing the procedure of a static magnetic field shimming method according to a second embodiment. [Figure 8] 10 is a flowchart showing the procedure of a static magnetic field shimming method according to the third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, an embodiment of a static magnetic field shimming method will be described in detail with reference to the drawings. The static magnetic field shimming method is performed, for example, when installing an MRI apparatus 1, and is used to adjust the spatial distribution of the static magnetic field in the MRI apparatus 1 that outputs the static magnetic field using metal shims. An example of the overall configuration of the MRI apparatus 1 having a static magnetic field magnet 10 will be described with reference to FIG.

[0010] (Overall configuration of MRI device) The MRI apparatus 1 includes a magnet gantry 100, a control cabinet 300, an image processing device 400 (for example, a console), and a bed 500.

[0011] The magnet gantry 100 and the bed 500 are disposed, for example, in a shielded room called an examination room. On the other hand, the control cabinet 300 is disposed, for example, in a machine room, and the image processing device 400 is disposed, for example, in an operation room. Note that the image processing device 400 may be connected to the MRI apparatus 1 via a network and installed in a remote location away from the operation room.

[0012] The magnetic gantry 100 has 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.

[0013] The static magnetic field magnet 10 of the magnetic gantry 100 is roughly divided into a tunnel type in which the magnet has a cylindrical magnetic structure, and an open type in which a pair of magnets are arranged above and below with an imaging space between them. Here, we will explain the case where the static magnetic field magnet 10 is of the tunnel type, but the present invention is not limited to this case.

[0014] The static magnetic field magnet 10 is a superconducting magnet that has a roughly cylindrical shape and generates a static magnetic field within a bore (i.e., the space inside the cylinder) into which a subject P, such as a patient, is transported. The static magnetic field magnet 10 incorporates a superconducting coil (not shown), which is cooled to an extremely low temperature by liquid helium. In the excitation mode, the static magnetic field magnet 10 applies a current supplied from a static magnetic field power supply to the superconducting coil to generate a static magnetic field. After that, when the mode shifts to the persistent current mode, the static magnetic field power supply is disconnected. Once the mode shifts to the persistent current mode, the static magnetic field magnet 10 continues to generate a large static magnetic field for a long period of time, for example, for more than one year.

[0015] The gradient magnetic field coil 11 has a roughly cylindrical shape similar to the static magnetic field magnet 10, and is placed inside the static magnetic field magnet 10. The gradient magnetic field coil 11 applies a gradient magnetic field to the subject P using power supplied from a gradient magnetic field power supply 31.

[0016] Since eddy currents generated in association with the generation of gradient magnetic fields interfere with imaging, an ASGC (Actively Shielded Gradient Coil) intended to reduce eddy currents may be used as the gradient magnetic field coil 11. The ASGC is a gradient magnetic field coil including, for example, a main coil for generating each of the gradient magnetic fields in the X-axis, Y-axis, and Z-axis directions, a shim tray capable of storing multiple metal shims, and a shield coil for suppressing leakage magnetic fields.

[0017] The WB coil 12, also called a whole-body RF (Radio Frequency) coil, is installed in a roughly cylindrical shape inside the gradient magnetic field coil 11 so as to surround the subject P. The WB coil 12 transmits RF pulses transmitted from the RF transmitter 32 toward the subject P. On the other hand, the WB coil 12 receives magnetic resonance signals, i.e., MR (Magnetic Resonance) signals, emitted from the subject P due to excitation of hydrogen nuclei, for example.

[0018] In addition to the WB coil 12, the MRI apparatus 1 may also include a local coil 20 as shown in FIG. 1. The local coil 20 is also called a local RF coil. The local coil 20 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 as shown in FIG. 1, an abdominal coil, a spine coil, and a knee coil. The local coils 20 include receive-only coils, transmit-only coils, and transmit-receive coils that perform both transmit and receive. The local coil 20 is configured to be detachable from the tabletop 51 via a cable, for example.

[0019] The bed 500 includes a bed body 50 and a top board 51. The bed body 50 can move the top board 51 in the vertical and horizontal directions, and moves the subject P placed on the top board 51 to a predetermined height before imaging. Thereafter, the top board 51 is moved horizontally to move the subject P into the bore.

[0020] The control cabinet 300 includes gradient magnetic field power supplies 31 (for the X axis 31x, the Y axis 31y, and the Z axis 31z), an RF transmitter 32, an RF receiver 33, and a sequence controller .

[0021] The gradient magnetic field power supply 31 includes gradient magnetic field power supplies 31 for each channel (X-axis power supply 31x, Y-axis power supply 31y, and Z-axis power supply 31z) that drive coils that generate gradient magnetic fields for the X-axis, Y-axis, and Z-axis, respectively. The gradient magnetic field power supplies 31 (X-axis power supply 31x, Y-axis power supply 31y, and Z-axis power supply 31z) output required current waveforms independently for each channel in response to commands from a sequence controller 34. This enables the main coil of the gradient magnetic field coil 11 to apply gradient magnetic fields in the X-axis, Y-axis, and Z-axis directions to the subject P.

[0022] The RF transmitter 32 generates RF pulses based on instructions from a sequence controller 34. The RF pulses are transmitted to the WB coil 12 and applied to the subject P. Application of the RF pulses generates MR signals from the subject P. These MR signals are received by the local coil 20 and the WB coil 12.

[0023] The RF receiver 33 converts the channel signals from the local coil 20 and the WB coil 12, i.e., the MR signals, from analog to digital (AD) and outputs them to a sequence controller 34. The digitally converted MR signals are sometimes called raw data.

[0024] The sequence controller 34 scans the subject P by driving the gradient magnetic field power supply 31, the RF transmitter 32, and the RF receiver 33 under the control of the image processing device 400. When the sequence controller 34 receives raw data from the RF receiver 33 through the scan, it transmits the raw data to the image processing device 400.

[0025] The sequence controller 34 includes a processing circuit (not shown), which 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).

[0026] The image processing device 400 includes a processing circuit 40 , a memory circuit 41 , a display 42 , and an input interface 43 .

[0027] The processing circuitry 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 circuitry 41 or directly incorporated in the processing circuitry 40. The processing circuitry 40 realizes the function of controlling the operation of the sequence controller 34 and performing imaging according to a pulse sequence to generate an MR image. The processing circuitry 40 may be configured with hardware such as an FPGA or an ASIC. The various functions described below can also be realized by such hardware. The processing circuitry 40 may also realize various functions by combining software processing and hardware processing.

[0028] 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 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, as well as various programs executed by a processor included in the processing circuitry 40.

[0029] The display 42 is configured by a general display output 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 can accept various operations from the user, such as a touch panel.

[0030] The input interface 43 includes an input device that can be operated by a user and an input circuit that inputs signals from the input device. The input device can 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, or the like. When the input device is operated by a user, the input circuit generates a signal corresponding to the operation and outputs it to the processing circuit 40.

[0031] (shimming) To obtain high-quality images with the MRI apparatus 1, high spatial uniformity of the static magnetic field (hereinafter also referred to as magnetic field uniformity) is required. The static magnetic field magnet 10 is designed and manufactured so that the static magnetic field within the bore is as uniform as possible, but the degree of non-uniformity of the static magnetic field varies between individual apparatuses due to manufacturing errors and the like, and also varies depending on the ambient environment of the installation location of the apparatus. For this reason, shimming is generally performed when the MRI apparatus 1 is installed. Shimming is an adjustment operation to correct spatial non-uniformity of the static magnetic field generated by the static magnetic field magnet 10 of the MRI apparatus 1 and improve uniformity.

[0032] Shimming is performed by, for example, fixing a passive shim member (i.e., a metal shim) such as an iron piece or a magnetic piece to the inner cylindrical wall surface of the static magnetic field magnet 10 or the like with an adhesive or a mechanical fastening member such as a screw. Shimming may also be performed by inserting the metal shim into a shim fixing component (i.e., a shim tray) for arranging the metal shim in a predetermined position. The metal shim is arranged in a predetermined position and adjusts the magnetic field so that the spatial uniformity of the static magnetic field in the imaging region falls within a predetermined tolerance range.

[0033] Fig. 2 is a schematic perspective view of a configuration example of a magnet gantry 100 having a static magnetic field magnet 10 according to the embodiment. As shown in Fig. 2, the shim tray portion 11b of the gradient magnetic field coil 11 has, for example, a roughly cylindrical shape and is sandwiched between a roughly cylindrical main coil 11a and a shield coil 11c. A plurality of slots 71 are formed at roughly equal intervals in the circumferential direction of the shim tray portion 11b. Note that the arrangement, number, shape, etc. of the slots 71 formed in the shim tray portion 11b are not limited to those shown in Fig. 2.

[0034] The slots 71 are through-holes that form openings on both end surfaces of the shim tray portion 11b and extend over the entire length of the shim tray portion 11b in the longitudinal direction (i.e., the axial direction). A shim tray 72 can be inserted into the slots 71. The shim tray 72 is fixed to approximately the center of the shim tray portion 11b. The approximate center of the shim tray portion 11b in the Z-axis direction is also the center of the gradient magnetic field coil 11 in the Z-axis direction. The shim tray 72 is made of, for example, resin, which is a non-magnetic and non-conductive material, and is generally rod-shaped.

[0035] Fig. 3 is a schematic perspective view of an example of the configuration of a shim tray 72 according to an embodiment. As shown in Fig. 3, a plurality of pockets 72a are formed at predetermined intervals in the axial direction of the shim tray 72. Note that the arrangement, number, shape, etc. of the pockets 72a are not limited to those shown in Fig. 3. A required number of metal shims 72b are stored in the pockets 72a in order to homogenize the static magnetic field in the imaging region within the bore.

[0036] Here, the difference between the static magnetic field shimming method of the comparative example in which the magnetic field is increased and decreased and the static magnetic field shimming method according to the embodiment in which the magnetic field is not increased and decreased will be described with reference to FIG.

[0037] In the static magnetic field shimming method of the comparative example, the spatial distribution (uniformity) of the magnetic field is measured while the MRI device 1 is outputting a rated magnetic field, which is the magnitude of the magnetic field when imaging a subject. Based on the measurement results, the placement of metal shims to achieve the desired magnetic field uniformity is calculated by simulation. Next, the magnetic field is lowered for safety reasons, and a shimming operation is performed in which metal shims are placed (e.g., installed and removed) based on the simulation results. The magnetic field is then raised back to the rated magnetic field, and the magnetic field uniformity after shimming is measured. In this manner, a series of shimming operations is performed multiple times while raising and lowering the magnetic field to achieve the desired magnetic field uniformity. The magnetic field uniformities H1, H2, H3, and H4 indicated by white circles in Figure 4 represent how the magnetic field uniformity gradually increases through four shimming operations, reaching the desired magnetic field uniformity at magnetic field uniformity L4. The desired magnetic field uniformity is magnetic field uniformity higher than a predetermined threshold Bs.

[0038] On the other hand, in the static magnetic field shimming method according to the embodiment, the spatial distribution (uniformity) of the magnetic field is measured while a low magnetic field is being output from the MRI apparatus 1, and the placement of metal shims to achieve the desired magnetic field uniformity is calculated, for example, by simulation, based on the measurement results. Here, a "low magnetic field" refers to a magnetic field at a level at which the metal shim placement work can be performed safely even in the magnetic field. A "low magnetic field" can also be referred to as a "magnetic field smaller than the rated magnetic field" or a "magnetic field smaller than the saturation magnetic field of the metal shim." The magnetic field strength of the low magnetic field may be, for example, a magnetic field strength in accordance with safety guidelines for shimming practitioners.

[0039] Next, a shimming operation was performed in which metal shims were placed based on the simulation results, and the magnetic field uniformity after shimming was measured. This series of operations was repeated multiple times in a low magnetic field without increasing or decreasing the magnetic field. The magnetic field uniformities L1, L2, L3, and L4 shown by the black squares in Figure 4 indicate that the magnetic field uniformity gradually improved with each of the four shimming operations, but the desired magnetic field uniformity was not achieved with magnetic field uniformity L4.

[0040] In the static magnetic field shimming method according to the embodiment, a predetermined magnetic field uniformity (magnetic field uniformity L4 in FIG. 4) is achieved at a low magnetic field, and then the magnetic field is increased from the low magnetic field to the rated magnetic field. By increasing the magnetic field from the low magnetic field to the rated magnetic field, the desired magnetic field uniformity (magnetic field uniformity H4 in FIG. 4) is achieved. In other words, the predetermined magnetic field uniformity is adjusted at a low magnetic field so that the desired magnetic field uniformity is achieved when the magnetic field is increased to the rated magnetic field.

[0041] The magnetic field adjustment capability (i.e., shim output) of a metal shim is calculated using the magnetization amount (i.e., magnetic moment) of the metal shim. At the rated magnetic field, the magnetic flux density (i.e., magnetization) of the metal shim is saturated. On the other hand, at low magnetic fields, the magnetization of the metal shim is not saturated. Therefore, the shim output changes when shimming at a low magnetic field and when shimming at the rated magnetic field.

[0042] Specifically, the shim output A in a low magnetic field can be calculated from the magnetization amount of the metal shim in a low magnetic field. Furthermore, the correlation between the magnetization amount of the metal shim and magnetic field strength, as well as the magnetization characteristics of the metal shim, can be derived from the physical and material properties of the metal shim. Therefore, if the difference in the magnetization amount of the metal shim between a "low magnetic field" and a "rated magnetic field," which have different magnetic field strengths, is defined as a correction coefficient, the shim output B in the rated magnetic field can be calculated from the shim output A in a low magnetic field using the correction coefficient. Furthermore, the magnetic field uniformity B' in a low magnetic field can be calculated from the magnetic field uniformity A'.

[0043] For example, if shimming in a low magnetic field results in magnetic field uniformity A' with shim output A, then increasing the magnetic field from the low magnetic field to the rated magnetic field will automatically change shim output A to shim output B, resulting in magnetic field uniformity B' at the rated magnetic field. Shimming is performed so that magnetic field uniformity B' at the rated magnetic field becomes the desired magnetic field uniformity. By calculating the change in shim output according to magnetic field strength using a correction coefficient, it is possible to simulate the placement of metal shims to achieve the desired magnetic field uniformity at the rated magnetic field. Note that shimming is performed to obtain the desired magnetic field uniformity at the rated magnetic field, and therefore does not necessarily result in the desired magnetic field uniformity at the low magnetic field where shimming is actually performed.

[0044] Here, Figure 5 is a diagram explaining the magnetization characteristics of a metal shim. As shown by magnetization P4 in Figure 5, the magnetization of the metal shim is saturated in the rated magnetic field, so the hysteresis characteristics of the metal shim do not need to be taken into consideration when shimming. In other words, when shimming in the rated magnetic field, there is no need to distinguish between an unmagnetized metal shim that is being placed in the magnetic field for the first time and a magnetized metal shim that has been placed in the magnetic field at least once.

[0045] On the other hand, in low magnetic fields, the magnetization of metal shims is not saturated, so when shimming, it is necessary to consider the amount of magnetization of the metal shim, which changes depending on the magnetization characteristics of the metal shim. Generally, when performing shimming, the metal shim is repeatedly inserted and removed from a static magnetic field. Because the magnetization characteristics of metal shims have hysteresis characteristics, a metal shim placed in a magnetic field for the first time exhibits the characteristics of an initial magnetization curve that passes from initial magnetization P0 to magnetization P1. On the other hand, a metal shim that has been magnetized after being placed in a magnetic field at least once exhibits a residual magnetic flux density (residual magnetization) and exhibits the characteristics of a magnetization curve that passes from residual magnetization P2 to magnetization P3.

[0046] In the shimming process, if metal shims with magnetization characteristics of the initial magnetization curve and metal shims with magnetization characteristics of the magnetization curve associated with the residual magnetic flux density are mixed, the adjustment of the magnetic field uniformity may be insufficient. For example, if the different magnetization characteristics of the metal shims are not taken into account in a simulation to derive the arrangement of metal shims to achieve the desired magnetic field uniformity, an error may occur between the magnetic field uniformity obtained by the simulation and the actual magnetic field uniformity. In addition, to reduce the error between the magnetic field uniformity obtained by the simulation and the actual magnetic field uniformity, it is possible to perform a simulation by correcting the magnetization amount of metal shims with different magnetization characteristics, but this process is complicated.

[0047] Therefore, the static magnetic field shimming method of the embodiment reduces the influence of the residual magnetic flux density of the metal shim and reduces adjustment errors in magnetic field uniformity caused by the magnetization characteristics of the metal shim in order to perform shimming with high accuracy in a low magnetic field without increasing or decreasing the magnetic field.

[0048] (First embodiment) In the first embodiment, a reset process of the residual magnetic flux density of the metal shim is performed at least once to reduce the influence of the residual magnetic flux density of the metal shim. FIG. 6 is a flowchart showing the procedure of the static magnetic field shimming method according to the first embodiment. An example of the magnetic material of the metal shim is a silicon steel plate. The metal shim may be made of a magnetic material having a smaller residual magnetic flux density than a silicon steel plate.

[0049] In step ST10, the initial magnetic field distribution (i.e., the spatial distribution of the static magnetic field) is measured in a low magnetic field that is smaller than the rated magnetic field. The magnetic field distribution can be measured using a known measurement method.

[0050] In step ST20, shimming using metal shims is performed in a low magnetic field smaller than the rated magnetic field while estimating the magnetic field distribution in the rated magnetic field. For example, the magnetic field distribution in the rated magnetic field is estimated using a correction coefficient from the magnetic field distribution measured in the low magnetic field, and a simulation is performed to determine the placement of metal shims that will achieve the desired magnetic field uniformity in the rated magnetic field. Based on the simulation results, metal shims are placed in the low magnetic field. That is, in step ST20, the spatial distribution of the static magnetic field when the rated magnetic field is output from the MRI apparatus 1 is estimated based on the measurement results of step ST10, and metal shims are installed in the MRI apparatus 1 based on the estimated spatial distribution of the static magnetic field.

[0051] In step ST30, the magnetic field distribution is measured in a low magnetic field that is smaller than the rated magnetic field.

[0052] In step ST40, based on the measurement results of step ST30, it is determined whether the desired magnetic field uniformity can be achieved in the rated magnetic field. That is, whether to execute the reset process for the residual magnetic flux density of the metal shim is determined based on whether the spatial distribution of the static magnetic field satisfies a predetermined condition. The determination is made by estimating whether the magnetization amount of the metal shim will change when the magnetic field is increased from a low magnetic field to the rated magnetic field, thereby achieving the desired magnetic field uniformity in the rated magnetic field. That is, by estimating the spatial distribution of the static magnetic field when the rated magnetic field is output from the MRI apparatus 1. This estimation is performed, for example, using a correction coefficient in a simulation calculation. For example, the determination may be made based on whether the variation in magnetic field uniformity at each position, characterized by three-dimensional coordinates in a predetermined space where the subject is carried or polar coordinates with the magnetic field center as the origin, is less than a predetermined value. In this case, if the value indicating the variation in the spatial distribution of the static magnetic field is greater than the predetermined value, it is determined that the predetermined condition is not met.

[0053] Steps ST20 and ST30 are repeated until it is determined in step ST40 that the desired magnetic field homogeneity is obtained in the rated magnetic field.

[0054] If it is determined in step ST40 that the desired magnetic field uniformity can be obtained in the rated magnetic field (i.e., the spatial distribution of the static magnetic field measured in step ST30 satisfies the predetermined condition) (i.e., if the answer is YES), shimming in the low magnetic field is completed, and the process proceeds to step ST50. In step ST50, the magnitude of the magnetic field output from the MRI apparatus 1 is changed from the low magnetic field to the rated magnetic field. When the magnetic field is increased to the rated magnetic field, the amount of magnetization of the metal shim changes, and the desired magnetic field uniformity is achieved.

[0055] If it is determined in step ST40 that the desired magnetic field uniformity is not obtained in the rated magnetic field (i.e., the estimated spatial distribution of the static magnetic field does not satisfy the predetermined conditions) (i.e., if the answer is NO), the process proceeds to step ST60. In step ST60, a reset process is performed on the residual magnetic flux density of the metal shim. The reset process is a process of setting the residual magnetic flux density of the metal shim to zero or to a negligible level.

[0056] The reset process is performed when a mixture of unmagnetized metal shims being placed in a magnetic field for the first time and magnetized metal shims that have been placed in a magnetic field at least once is used, and there is a difference in residual magnetic flux density between the metal shims. If shimming is performed multiple times, the reset process for the residual magnetic flux density of the metal shims is performed at least once after the first shimming.

[0057] The reset process is a process of resetting the residual magnetic flux density of a metal shim placed in a low magnetic field, for example, by heating or applying an external magnetic field, and is a process of heating or applying an external magnetic field to a metal shim provided in the MRI device 1.

[0058] The static magnetic field shimming method of the first embodiment performs shimming at a magnetic field smaller than the rated magnetic field without increasing or decreasing the magnetic field, thereby shortening the shimming time and reducing the consumption of liquid helium, a rare refrigerant. Furthermore, by performing a reset process for the residual magnetic flux density of the metal shims, even when a mixture of unmagnetized metal shims being placed in a magnetic field for the first time and magnetized metal shims placed in a magnetic field at least once is used, all metal shims exhibit the same initial magnetization characteristics or magnetization characteristics so small that the influence is negligible. Therefore, the influence of the residual magnetic flux density of the metal shims is reduced, and adjustment errors in magnetic field uniformity caused by the magnetization characteristics of the metal shims can be reduced. As a result, degradation of the image quality of images obtained by the MRI device can be suppressed.

[0059] (Second embodiment) In the second embodiment, in order to reduce the influence of the residual magnetic flux density of the metal shim, the metal shim is magnetized in advance so that it has a residual magnetic flux density. Fig. 7 is a flowchart showing the procedure of the static magnetic field shimming method according to the second embodiment.

[0060] In step ST5, the metal shims are placed in a low magnetic field to pre-magnetize them so that they have a residual magnetic flux density. All metal shims used in the shimming process are magnetized metal shims that have been placed in a magnetic field at least once. In other words, there is no chance of unmagnetized metal shims being placed in a magnetic field for the first time during the shimming process.

[0061] 7, the procedure of the static magnetic field shimming method according to the second embodiment does not include the reset process of the residual magnetic flux density of the metal shim in step ST60 of the first embodiment shown in FIG. 6. That is, if it is determined in step ST40 that the desired magnetic field homogeneity is not obtained in the rated magnetic field (i.e., if the answer is NO), the procedure proceeds to step ST20. The other steps are not substantially different from the procedure of the static magnetic field shimming method according to the first embodiment, and therefore the same steps are denoted by the same reference numerals and their explanations are omitted.

[0062] The static magnetic field shimming method of the second embodiment performs shimming at a magnetic field lower than the rated magnetic field without increasing or decreasing the magnetic field, thereby shortening the shimming time and reducing the consumption of liquid helium, a rare refrigerant. Furthermore, there are no unmagnetized metal shims placed in the magnetic field for the first time during the shimming process; all metal shims are magnetized metal shims that have been placed in the magnetic field at least once. Therefore, all metal shims used in the shimming process have similar residual magnetic flux densities and exhibit magnetization characteristics that are the same or have negligible influence on each other. This also reduces adjustment errors in magnetic field uniformity caused by the magnetization characteristics of the metal shims.

[0063] (Third embodiment) The third embodiment is implemented in conjunction with the static magnetic field shimming methods of the first and second embodiments, and estimates the magnetic field distribution in the rated magnetic field, taking into account the influence of the magnetic field strength according to the axial position of the shim tray.

[0064] At the rated magnetic field, the magnetization of the metal shims at all positions in the axial direction of the shim tray is saturated, making it less susceptible to the effects of differences in magnetic field strength during shimming. However, at low magnetic fields, the metal shims are not saturated, making it more susceptible to the effects of differences in magnetic field strength during shimming. Therefore, shimming accuracy can be improved by estimating the magnetic field distribution at the rated magnetic field, taking into account the effects of magnetic field strength depending on the axial position of the shim tray.

[0065] 9 is a flowchart showing the procedure of the static magnetic field shimming method according to the third embodiment. The procedure of the static magnetic field shimming method according to the third embodiment is performed before the procedure of the static magnetic field shimming method according to the first embodiment and the second embodiment.

[0066] In step ST1, before shimming in a low magnetic field is performed, a predetermined amount of metal shims are placed at each axial position of the shim tray in the low magnetic field. The axial direction of the shim tray is the Z-axis direction of the static magnetic field magnet shown in FIG. 2. As shown in FIG. 3, there are multiple positions (i.e., pockets 72a) where metal shims can be placed along the axial direction of the shim tray 72. A predetermined amount of metal shims is placed at one of the multiple positions where metal shims can be placed.

[0067] In step ST2, the change in magnetic field distribution corresponding to each position in the axial direction of the shim tray is measured. The change in magnetic field distribution is the amount of change in the magnetic field when a metal shim is placed at one pocket position on the shim tray, compared to the magnetic field when no metal shim is placed on the shim tray. If the magnetic field strength differs at each position in the axial direction of the shim tray, the magnetic field distribution due to a predetermined amount of metal shims placed at each position will differ depending on the position. In addition, the shim output of the metal shim will also differ at each position in the axial direction of the shim tray.

[0068] Since there are multiple axial positions for the shim trays, the processes of steps ST1 and ST2 are performed for each axial position of all the shim trays while changing the position at which a predetermined amount of metal shims are arranged. As shown in Fig. 2, for example, multiple shim trays 72 are inserted into slots 71 at approximately equal intervals in the circumferential direction of the shim tray section 11b. Since the amount of change in magnetic field distribution depending on the axial position of the shim tray 72 is approximately the same among these multiple shim trays 72, the processes of steps ST1 and ST2 may be performed for one shim tray. Alternatively, the processes of steps ST1 and ST2 may be performed for multiple shim trays.

[0069] In step ST3, information corresponding to each position based on the amount of change in the magnetic field distribution is added to the estimation of the magnetic field distribution in the rated magnetic field in step ST20. For example, since the correction coefficient for correcting the difference in the magnetization amount of the metal shim varies depending on the axial position of the shim tray where the metal shim is placed, the magnetic field distribution in the rated magnetic field is estimated using a correction coefficient corresponding to each axial position of the shim tray. In other words, the correction coefficient is a correction coefficient that depends on the axial position of the shim tray in addition to the difference in magnetic field strength between the low magnetic field and the rated magnetic field.

[0070] The static magnetic field shimming method of the third embodiment not only provides the same effects as the static magnetic field shimming methods of the first and second embodiments, but also reduces adjustment errors in magnetic field uniformity caused by the magnetization characteristics of metal shims, thereby improving shimming accuracy. Furthermore, the improved shimming accuracy allows for more efficient shimming work, for example by reducing the number of shimming operations.

[0071] When shimming in a low magnetic field, since the metal shim is not saturated, it is susceptible to the influence of differences in magnetic field strength. Therefore, it is preferable to use a correction coefficient according to the axial position of the shim tray to further improve shimming accuracy. The correction coefficient according to the axial position of the shim tray is preferably created for each site where shimming is performed, since the influence of environmental magnetic fields, such as magnetic materials in the building and magnetic shields, varies from site to site. Furthermore, the correction coefficient according to the axial position of the shim tray may use a common magnetic field distribution for MRI apparatuses equipped with a common static magnetic field magnet.

[0072] According to the static magnetic field shimming method of at least one of the embodiments described above, it is possible to prevent the quality of images obtained by the MRI apparatus from being degraded.

[0073] 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]

[0074] 1...Magnetic Resonance Imaging (MRI) device 10...Static magnetic field magnet 72...Shim tray 72b...Metal shim

Claims

1. A static magnetic field shimming method for adjusting a spatial distribution of a static magnetic field in a magnetic resonance imaging apparatus that outputs the static magnetic field using a metal shim, comprising: performing a first measurement of a spatial distribution of the static magnetic field in a state in which a second magnetic field, the second magnetic field being smaller than a first magnetic field that is a magnetic field used when imaging a subject, is output from the magnetic resonance imaging apparatus; performing a second measurement to measure a spatial distribution of the static magnetic field while the second magnetic field is being output from the magnetic resonance imaging apparatus after the metal shim is installed in the magnetic resonance imaging apparatus based on the result of the first measurement; a step of performing a reset process of the residual magnetic flux density of the metal shim when the spatial distribution of the static magnetic field based on the second measurement does not satisfy a predetermined condition; Equipped with A static magnetic field shimming method characterized in that the reset process is a process of heating or applying an external magnetic field to the metal shim provided in the magnetic resonance imaging device based on the results of the first measurement.

2. 2. The static magnetic field shimming method according to claim 1, wherein the failure to satisfy the predetermined condition corresponds to a value indicating the variation in the spatial distribution of the static magnetic field based on the second measurement being greater than a predetermined value.

3. the static magnetic field shimming method includes a step of estimating a spatial distribution of the static magnetic field when the first magnetic field is output from the magnetic resonance imaging apparatus based on a result of the second measurement; 2. The static magnetic field shimming method according to claim 1, wherein the reset process is executed when the estimated spatial distribution of the static magnetic field does not satisfy the predetermined condition.

4. The static magnetic field shimming method according to claim 1, further comprising a step of changing the magnitude of the magnetic field output from the magnetic resonance imaging apparatus from the second magnetic field to the first magnetic field when the spatial distribution of the static magnetic field measured by the second measurement satisfies the predetermined condition.

5. the static magnetic field shimming method includes a step of estimating a spatial distribution of the static magnetic field when the first magnetic field is output from the magnetic resonance imaging apparatus based on a result of the first measurement; 2. The static magnetic field shimming method according to claim 1, wherein the metal shim is provided in the magnetic resonance imaging apparatus based on the estimated spatial distribution of the static magnetic field.

6. A static magnetic field shimming method for adjusting a spatial distribution of a static magnetic field in a magnetic resonance imaging apparatus that outputs the static magnetic field using a metal shim, comprising: performing a first measurement of a spatial distribution of the static magnetic field in a state in which a second magnetic field, the second magnetic field being smaller than a first magnetic field that is a magnetic field used when imaging a subject, is output from the magnetic resonance imaging apparatus; performing a second measurement to measure a spatial distribution of the static magnetic field while the second magnetic field is being output from the magnetic resonance imaging apparatus after the metal shim is installed in the magnetic resonance imaging apparatus based on the result of the first measurement; Equipped with A static magnetic field shimming method, characterized in that the metal shim is made of a magnetic material having a smaller residual magnetic flux density than a silicon steel plate.

7. A static magnetic field shimming method for adjusting a spatial distribution of a static magnetic field in a magnetic resonance imaging apparatus that outputs the static magnetic field using a metal shim, comprising: performing a first measurement of a spatial distribution of the static magnetic field in a state in which a second magnetic field, the second magnetic field being smaller than a first magnetic field that is a magnetic field used when imaging a subject, is output from the magnetic resonance imaging apparatus; performing a second measurement to measure a spatial distribution of the static magnetic field while the second magnetic field is being output from the magnetic resonance imaging apparatus after the metal shim is installed in the magnetic resonance imaging apparatus based on the result of the first measurement; before the first measurement, placing the metal shim in a state where the second magnetic field is output from the magnetic resonance imaging apparatus; A static magnetic field shimming method comprising:

8. The static magnetic field shimming method includes, before the first measurement, placing a predetermined amount of the metal shim at each position in an axial direction of a shim tray while the second magnetic field is being output from the magnetic resonance imaging apparatus; estimating a spatial distribution of the static magnetic field when the first magnetic field corresponding to each position in the axial direction of the shim tray is output from the magnetic resonance imaging apparatus based on the amount of change in the spatial distribution of the static magnetic field corresponding to each position; The static magnetic field shimming method according to any one of claims 1 to 7, comprising:

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