Magnetic resonance imaging apparatus and static magnetic field correction method

Abstract

To suppress deterioration of uniformity of a static magnetic field during operation of an MRI apparatus.SOLUTION: A magnetic resonance imaging apparatus according to an embodiment includes a static magnetic field magnet, a gradient coil, a sequence controller, and a plurality of metal shims. The static magnetic field magnet generates a static magnetic field. The gradient magnetic field coil applies a gradient magnetic field to the subject. The sequence controller applies a gradient magnetic field current for raising the temperature of the gradient magnetic field coil to the gradient magnetic field coil by execution of the first pulse sequence. The plurality of metal shims are used for passive shimming in which the plurality of metal shims are arranged so that the distribution of the static magnetic field becomes uniform in a state in which the temperature of the gradient coil is increased by the execution of the first pulse sequence. The sequence controller executes a plurality of second pulse sequences for imaging the subject after completion of the passive shimming.SELECTED DRAWING: Figure 4

Description

[Technical Field]

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

[0002] A magnetic resonance imaging (MRI) device is an imaging device that excites the nuclear spins of a subject placed in a static magnetic field with radio frequency (RF) signals at the Larmor frequency, and generates MR images by reconstructing magnetic resonance signals (MR signals) generated from the subject as a result of the excitation.

[0003] To obtain high-quality images using an MRI system, spatial uniformity of the static magnetic field is required. For this reason, for example, when installing an MRI system, metal shims are placed at predetermined positions within the gradient coil to correct the uniformity of the static magnetic field. The process of homogenizing the static magnetic field using metal shims is called passive shimming.

[0004] During operation of the MRI system, the temperature of the metal shim rises as the temperature of the gradient coil rises, causing a change in the magnetic susceptibility of the metal shim. This change in magnetic susceptibility also changes the static magnetic field, degrading the uniformity of the static magnetic field that was adjusted when the MRI system was installed. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-125928 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 suppress deterioration of the uniformity of the static magnetic field during operation of the 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]

[0007] A magnetic resonance imaging apparatus according to one embodiment includes a static magnetic field magnet, a gradient magnetic field coil, a sequence controller, and a plurality of metal shims. The static magnetic field magnet generates a static magnetic field. The gradient magnetic field coil applies a gradient magnetic field to a subject. The sequence controller applies a gradient magnetic field current to the gradient magnetic field coil by executing a first pulse sequence to increase the temperature of the gradient magnetic field coil. The plurality of metal shims are used for passive shimming, in which the metal shims are arranged to uniformly distribute the static magnetic field when the temperature of the gradient magnetic field coil has increased by executing the first pulse sequence. After completion of passive shimming, the sequence controller executes a plurality of second pulse sequences to image the subject. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a schematic diagram showing an example of the overall configuration of an MRI apparatus according to a first embodiment. [Figure 2] FIG. 1 is a perspective view for explaining a schematic configuration of a gradient magnetic field coil provided in an MRI apparatus according to a first embodiment. [Figure 3] FIG. 2 is a perspective view for explaining a detailed configuration example of a plurality of shim trays provided in the MRI apparatus according to the first embodiment. [Figure 4] FIG. 2 is a flowchart showing the procedure of a static magnetic field correction method in the MRI apparatus according to the first embodiment. [Figure 5] FIG. 2 is a diagram for explaining an example of a temperature increasing means in the MRI apparatus according to the first embodiment. [Figure 6]FIG. 10 is a flowchart showing the procedure of a static magnetic field correction method in an MRI apparatus according to a first modified example of the first embodiment. [Figure 7] FIG. 10 is a flowchart showing the procedure of a static magnetic field correction method in an MRI apparatus according to a second modification of the first embodiment. [Figure 8] FIG. 10 is a flowchart showing the procedure of a static magnetic field correction method in an MRI apparatus according to the second embodiment. [Figure 9] FIG. 10 is a diagram illustrating an example of a temperature increasing means in the MRI apparatus according to the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of a magnetic resonance imaging apparatus and a static magnetic field correction method will be described in detail with reference to the drawings. A magnetic resonance imaging (MRI) apparatus 1 according to the embodiment can use magnetic resonance imaging technology.

[0010] (First embodiment) 1 is a schematic diagram showing an example of the overall configuration of an MRI apparatus 1 according to the first embodiment. As shown in FIG. 1, the MRI apparatus 1 includes a magnet gantry 100, a control cabinet 300, an image processing device 400 such as 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 shim coil 11, a gradient magnetic field coil 12, and a WB (Whole Body) coil 13. 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 has a roughly cylindrical shape and generates a static magnetic field within a bore into which a subject P, such as a patient, is transported. The bore is the space inside the cylinder of the magnet gantry 100. The static magnetic field magnet 10 is configured with a housing 10a (see FIG. 9) for holding liquid helium, a refrigerator 14 (see FIG. 9) for cooling the liquid helium to an extremely low temperature, and a superconducting coil 10b (see FIG. 9) inside the housing. Note that the static magnetic field magnet 10 may be configured with a resistive magnet or a permanent magnet. Below, a case where the static magnetic field magnet 10 has a superconducting coil will be described.

[0015] The static magnetic field magnet 10 incorporates a superconducting coil, which is cooled to an extremely low temperature by liquid helium. In 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 to the superconducting coil. After that, when the mode switches to persistent current mode, the static magnetic field power supply is disconnected. Once in 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.

[0016] The shim coils 11 are roughly cylindrical in shape, similar to the static magnetic field magnet 10, and are installed inside the static magnetic field magnet 10. The shim coils 11 are used for active shimming and correct first-, second-, and higher-order inhomogeneous components of the static magnetic field. The shim coils 11 include multiple shim coils that each correct a different inhomogeneous component of the static magnetic field.

[0017] The gradient magnetic field coil 12 has a roughly cylindrical shape similar to the static magnetic field magnet 10, and is installed, for example, inside the shim coil 11. The gradient magnetic field coil 12 applies a gradient magnetic field to the subject P using power supplied from a gradient magnetic field power supply 31. The gradient magnetic field coil 12 may be configured to correct, for example, a first-order non-uniform component of the static magnetic field.

[0018] Here, 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 is used as the gradient magnetic field coil 12. 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.

[0019] Fig. 2 is a perspective view for explaining a schematic configuration of the gradient magnetic field coil 12 provided in the MRI apparatus 1 according to the first embodiment. As shown in Fig. 2, the shim tray portion 12b of the gradient magnetic field coil 12 has a roughly cylindrical shape and is sandwiched between the roughly cylindrical main coil 12a and the shield coil 12c. A plurality of slots 71 are formed at roughly equal intervals in the circumferential direction of the shim tray portion 12b. The number and shape of the slots 71 formed in the shim tray portion 12b are not particularly limited to those shown in Fig. 2.

[0020] The slots 71 are through-holes that form openings on both end surfaces of the shim tray portion 12b and extend over the entire length of the shim tray portion 12b in the longitudinal direction (long axis 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 12b. The approximate center of the shim tray portion 12b in the Z-axis direction is also the center of the gradient magnetic field coil 12 in the Z-axis direction. The shim tray 72 is formed, for example, from resin, which is a non-magnetic and non-conductive material, and is generally rod-shaped.

[0021] Fig. 3 is a perspective view for explaining a detailed configuration example of the multiple shim trays 72 provided in the MRI apparatus 1 according to the first embodiment. As shown in Fig. 3, multiple pockets 72a are formed at predetermined intervals in the longitudinal direction of the shim tray 72. The number of pockets 72a is not particularly limited to that shown in Fig. 3.

[0022] A required number of metal shims 72b are stored in the pockets 72a for the purpose of homogenizing the static magnetic field in the imaging region within the bore. The metal shims 72b are made of, for example, silicon steel plate or permendur (an alloy of iron and cobalt). Adjusting the number of metal shims 72b stored in each pocket 72a during installation of the MRI apparatus 1 to homogenize the static magnetic field in the imaging region within the bore is called passive shimming.

[0023] 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 devices and also depending on the ambient environment of the installation location of the device. For this reason, passive shimming using a metal shim 72b is usually performed when the MRI device 1 is installed.

[0024] Returning to the explanation of Fig. 1, the WB coil 13 is also called a whole-body RF (Radio Frequency) coil, and is installed in a roughly cylindrical shape inside the gradient magnetic field coil 12 so as to surround the subject P. The WB coil 13 transmits RF pulses transmitted from the RF transmitter 32 toward the subject P. On the other hand, the WB coil 13 receives magnetic resonance signals, i.e., MR (Magnetic Resonance) signals, emitted from the subject P due to excitation of hydrogen nuclei, for example.

[0025] In addition to the WB coil 13, 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. The local coil 20 may also include a plurality of coil elements. These multiple coil elements are arranged in an array inside the local coil 20, and therefore may also be called a PAC (Phased Array Coil).

[0026] There are several types of local coils 20. Examples of local coils 20 include head coils, chest coils as shown in FIG. 1, abdominal coils, spine coils, and knee coils. Some local coils 20 are dedicated to receiving, some are dedicated to transmitting, and some are transmit-receive coils that perform both transmitting and receiving. The local coil 20 is configured to be detachable from the tabletop 51 via a cable, for example.

[0027] 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.

[0028] 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 .

[0029] 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 allows the main coil 12a of the gradient magnetic field coil 12 to apply gradient magnetic fields in the X-axis, Y-axis, and Z-axis directions to the subject P.

[0030] The RF transmitter 32 generates an RF pulse based on an instruction from the sequence controller 34. The RF pulse is transmitted to the WB coil 13 and applied to the subject P. The application of the RF pulse generates an MR signal from the subject P. This MR signal is received by the local coil 20 or the WB coil 13.

[0031] The MR signals received by the local coil 20, more specifically, the MR signals received by each coil element of the local coil 20, are transmitted to the RF receiver 33. If the local coil 20 is configured to be able to transmit MR signals to the RF receiver 33 via a cable, the MR signals received by each coil element are transmitted to the RF receiver 33 via a cable provided inside the bed body 50. The output path of each coil element and the output path of the WB coil 13 are called channels. For this reason, each MR signal output from each coil element and the WB coil 13 is sometimes called a channel signal. The channel signal received by the WB coil 13 is also transmitted to the RF receiver 33. Note that the MR signals received by each coil element of the local coil 20 may be transmitted to the RF receiver 33 wirelessly.

[0032] The RF receiver 33 converts the channel signals from the local coil 20 and the WB coil 13, 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.

[0033] The sequence controller 34, under the control of an image processing device 400 (described later), drives the gradient magnetic field power supply 31, the RF transmitter 32, and the RF receiver 33 to image the subject P. When the sequence controller 34 receives raw data from the RF receiver 33 through imaging, it transmits the raw data to the image processing device 400.

[0034] 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), or an ASIC (Application Specific Integrated Circuit).

[0035] The image processing device 400 includes a processing circuit 40 , a memory circuit 41 , a display 42 , and an input interface 43 .

[0036] 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 controls the operation of the sequence controller 34 and realizes functions such as generating MR images by performing imaging according to a pulse sequence and active shimming. 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.

[0037] 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.

[0038] 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.

[0039] 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 realized by 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 the user operates the input device, the input circuit generates a signal corresponding to the operation and outputs it to the processing circuit 40.

[0040] As described above, passive shimming using the metal shims 72b is usually performed when the MRI apparatus 1 is installed. In passive shimming, the placement of the metal shims 72b is determined when the MRI apparatus 1 is installed so that the non-uniformity of the static magnetic field falls within a predetermined tolerance. However, during operation of the MRI apparatus 1 (i.e., when imaging the subject P), the temperature of the metal shims 72b rises as the temperature of the gradient magnetic field coil 12 rises, causing a change in the magnetic susceptibility of the metal shims 72b. This change in magnetic susceptibility changes the static magnetic field, causing a change in the center frequency and a deterioration in the uniformity of the first-, second-, and higher-order static magnetic fields since the MRI apparatus 1 was installed. The change in the center frequency and the deterioration in the uniformity of the static magnetic field can also cause deterioration in image quality.

[0041] To suppress this change in center frequency and deterioration of the static magnetic field uniformity, thermal insulation can be used to prevent heat from being transferred from the gradient coil to the metal shim, but as the output of the gradient coil increases, deterioration of the magnetic field uniformity may not be sufficiently suppressed. Also, while the center frequency can be tracked and corrected in real time, correction of the center frequency alone may not be sufficient to correct the first-, second-, and higher-order uniformity of the static magnetic field.

[0042] Therefore, in the static magnetic field correction method for the MRI apparatus 1 according to the embodiment, passive shimming is performed in a state in which the temperature environment of the gradient magnetic field coil 12 and the metal shim 72b when the MRI apparatus 1 is installed is made close to the temperature environment during operation of the MRI apparatus 1.

[0043] A static magnetic field correction method in the MRI apparatus 1 according to the first embodiment will be described in detail below. Fig. 4 is a flowchart showing the procedure of the static magnetic field correction method in the MRI apparatus 1 according to the first embodiment.

[0044] In step ST10, the sequence controller 34 applies a gradient magnetic field current to the gradient magnetic field coil 12 by executing a first pulse sequence to increase the temperature of the gradient magnetic field coil 12. Here, the first pulse sequence is a predetermined pulse sequence that can apply a gradient magnetic field current to the gradient magnetic field coil 12 to increase the temperature of the gradient magnetic field coil 12. The type and imaging conditions of the first pulse sequence are not limited, and known types and imaging conditions of pulse sequences can be used. The first pulse sequence is executed before passive shimming. By executing the first pulse sequence, the temperature environment of the gradient magnetic field coil 12 and the metal shim 72b approaches the temperature environment during operation of the MRI apparatus 1.

[0045] The sequence controller 34 may execute, as the first pulse sequence, a pulse sequence among the plurality of second pulse sequences that maximizes the temperature of the gradient coil 12. Here, the second pulse sequence is a predetermined pulse sequence for imaging the subject P. The type and imaging conditions of the second pulse sequence are not limited, and for example, known types and imaging conditions of pulse sequences that can be set by a user's input via the input interface 43 or by reading out conditions stored in the memory circuitry 41 can be used.

[0046] The sequence controller 34 may execute EPI (Echo Planar Imaging) as the first pulse sequence. FIG. 5 is an example of a pulse sequence for increasing temperature, and is a sequence diagram of GRE (Gradient Echo)-based EPI. As shown in FIG. 5, EPI is a pulse sequence that imposes a large heat load because it performs high-speed reversal of the gradient magnetic field gradient. This large heat load causes a localized heat distribution in the gradient magnetic field coil 12. Therefore, passive shimming is performed in a state in which the gradient magnetic field coil 12 has a localized heat distribution. Note that the EPI pulse sequence executed as the first pulse sequence is not limited to the GRE-based EPI of FIG. 5, and may be an SE (Spin Echo)-based EPI, a diffusion EPI, a single-shot EPI, or a multi-shot EPI.

[0047] In step ST20, the uniformity of the static magnetic field distribution, which is a magnetic field characteristic of the static magnetic field magnet 10, is measured. For example, the magnetic field is measured at multiple points in the center of the bore when all of the multiple pockets 72a are empty, i.e., when no metal shims 72b are placed. The first time step ST20 is performed, passive shimming has not yet been performed, so the uniformity of the measured static magnetic field is not guaranteed. Note that the measurement is performed using any magnetic field measuring device. Furthermore, when measuring the magnetic field, the pockets 72a do not necessarily have to be empty; some metal shims 72b may be placed in advance. Step ST20 is performed, for example, by the processing circuit 40.

[0048] In step ST30, it is determined whether the distribution of the static magnetic field is uniform. For example, this is determined based on whether the non-uniformity of the static magnetic field falls within a predetermined tolerance range. If it is determined that the distribution of the static magnetic field is uniform (i.e., YES in step ST30), passive shimming, i.e., installation of the MRI apparatus 1, is completed, and the process proceeds to step ST50. If it is determined that the distribution of the static magnetic field is not uniform (i.e., NO in step ST30), passive shimming is required again, and the process proceeds to step ST40. Step ST30 is performed by, for example, the processing circuit 40.

[0049] In step ST40, passive shimming is performed based on the measurement results of the uniformity of the static magnetic field distribution. A plurality of metal shims for passive shimming are arranged so that the static magnetic field distribution becomes uniform when the temperature of the gradient magnetic field coil 12 is increased by executing the first pulse sequence. After the arrangement of the plurality of metal shims, step ST20 is performed. Steps ST20 to ST40 are repeated until the static magnetic field uniformity falls within a predetermined tolerance range. In other words, steps up to step ST40 correspond to the installation of the MRI apparatus 1, and steps after step ST40 correspond to the operation of the MRI apparatus 1.

[0050] In step ST50, the sequence controller 34 executes a plurality of second pulse sequences for imaging the subject P. Step ST50 is performed after the passive shimming is completed.

[0051] In this way, if the first pulse sequence executed before passive shimming is a pulse sequence that causes the temperature of the gradient magnetic field coil 12 to be the highest, or EPI, etc., high magnetic field uniformity is ensured even under a high thermal load condition when the MRI apparatus 1 is installed. Therefore, even if the plurality of second pulse sequences for imaging the subject P are pulse sequences that cause a high thermal load, such as EPI, degradation of the imaging quality can be suppressed.

[0052] On the other hand, there are cases where the thermal load when the MRI apparatus 1 is installed differs from the thermal load during operation of the MRI apparatus 1. For example, this may occur when passive shimming is performed using a pulse sequence or EPI that maximizes the temperature of the gradient coil 12 as the first pulse sequence, and then imaging of the subject P is performed using a pulse sequence with a small thermal load as the second pulse sequence. In such cases, there is a possibility that the uniformity of the static magnetic field distribution may not be sufficiently ensured during operation of the MRI apparatus 1.

[0053] Therefore, the sequence controller 34 may execute, as the first pulse sequence, a pulse sequence that is in a thermal load state between a no-load state in which no thermal load is applied to the gradient coil 12 and a maximum thermal load state in which the temperature of the gradient coil 12 is highest, according to a pulse sequence. For example, by performing passive shimming using a pulse sequence that is in a thermal load state intermediate between the no-load state and the maximum thermal load state as the first pulse sequence, it is possible to generally ensure the uniformity of the static magnetic field distribution in both the no-load state and the maximum thermal load state. Therefore, it is possible to suppress deterioration in the image quality when the plurality of second pulse sequences for imaging the subject P are pulse sequences that are in a high thermal load and pulse sequences that are in a low thermal load.

[0054] (First Modification) Fig. 6 is a flowchart showing the procedure of the static magnetic field correction method in the first modified example of the MRI apparatus 1 according to the first embodiment. As shown in Fig. 6, in the first modified example, if YES is determined in step ST30, step ST41 is performed before step ST50. In Fig. 6, the same steps as those shown in Fig. 4 are denoted by the same reference numerals, and the description thereof will be omitted.

[0055] In step ST41, after the passive shimming is completed, the sequence controller 34 further increases the temperature of the gradient magnetic field coil 12 by executing the first pulse sequence before executing a plurality of second pulse sequences for imaging the subject P (i.e., step ST50).

[0056] If the first pulse sequence is executed before the second pulse sequence is executed, the temperature environment during execution of the plurality of second pulse sequences for imaging the subject P approaches the temperature environment during passive shimming in which the temperature of the gradient magnetic field coil 12 is increased. That is, the plurality of second pulse sequences for imaging the subject P are executed in a state in which high magnetic field uniformity is ensured. Therefore, it is possible to suppress deterioration in imaging quality both when the plurality of second pulse sequences for imaging the subject are pulse sequences with a large thermal load and when they are pulse sequences with a small thermal load.

[0057] (Second Modification) Fig. 7 is a flowchart showing the procedure of the static magnetic field correction method in the second modified example of the MRI apparatus 1 according to the first embodiment. As shown in Fig. 7, in the second modified example, if YES is determined in step ST30, step ST42 is performed before step ST50. In Fig. 7, the same steps as those shown in Fig. 4 are denoted by the same reference numerals, and their explanations will be omitted.

[0058] In step ST42, before executing a plurality of second pulse sequences for imaging the subject P, active shimming may be performed based on the second pulse sequences and imaging conditions so as to make the distribution of the static magnetic field uniform.

[0059] More specifically, for magnetic field inhomogeneity based on the second pulse sequence and imaging conditions, active shimming is performed using the shim coil 11 to correct the uniformity of the center frequency and the first, second, and higher order static magnetic fields. By performing active shimming in addition to passive shimming, it is possible to keep the inhomogeneity of the static magnetic field within an allowable range even if the thermal load during installation of the MRI apparatus 1 differs from the thermal load during operation of the MRI apparatus 1.

[0060] The temporal and spatial changes in the non-uniformity of the static magnetic field according to the thermal load may be stored in advance in the database and the memory circuitry 41, and active shimming may be performed to uniformize the distribution of the static magnetic field. In the second modification, it is possible to compensate for the difference in the uniformity of the static magnetic field due to the difference between the thermal load when the MRI apparatus 1 is installed and the thermal load when the MRI apparatus 1 is in operation. In this case, in order to completely compensate for the deterioration in the uniformity of the static magnetic field due to the heat generated by the gradient magnetic field coil 12 using active shimming alone, it is necessary to increase the output and reduce the inductance of the shim coil 11. Therefore, it is preferable to minimize the amount of correction by active shimming. By applying a thermal load and performing passive shimming when the MRI apparatus 1 is installed, the amount of correction by active shimming can be reduced.

[0061] (Second embodiment) In the MRI apparatus 1 according to the first embodiment, the thermal load is applied by executing the first pulse sequence when the MRI apparatus 1 is installed. In contrast, the MRI apparatus 1 according to the second embodiment differs in that the thermal load when the MRI apparatus 1 is installed is applied by circulating heated liquid through piping 15 arranged around the gradient magnetic field coil 12. Duplicate descriptions of configurations that are substantially the same as those of the MRI apparatus 1 according to the first embodiment shown in FIG. 1 will be omitted.

[0062] Fig. 8 is a flowchart showing the procedure of the static magnetic field correction method in the MRI apparatus 1 according to the second embodiment. As shown in Fig. 8, in the second embodiment, step ST11 is performed instead of step ST10 in Fig. 4 in order to apply a thermal load when the MRI apparatus 1 is installed. Then, after step ST11, the process proceeds to step ST20. In Fig. 8, the same steps as those shown in Fig. 4 are denoted by the same reference numerals, and the description thereof will be omitted.

[0063] In step ST11, heated liquid is caused to flow through the piping 15 to increase the temperature of the gradient magnetic field coil 12. The heated liquid is circulated through the piping 15 to increase the temperature of the gradient magnetic field coil 12. The temperature of the circulated liquid may be close to the temperature environment that occurs in the gradient magnetic field coil 12 and the metal shim due to heat generation caused by a pulse sequence for imaging the subject P, for example.

[0064] 9 is a diagram showing an example of piping 15 arranged around the gradient magnetic field coil 12 whose temperature is increased. The arrangement, number, and shape of the piping 15 are not limited to those shown in FIG. 9 and may be, for example, the arrangement, number, and shape of a known coolant flow path provided for cooling the gradient magnetic field coil. The area around the gradient magnetic field coil 12 may be between the main coil 12a and the shield coil 12c of the gradient magnetic field coil 12, between the gradient magnetic field coil 12 and the shim coil 11, or between the gradient magnetic field coil 12 and the WB coil 13.

[0065] The refrigerator 14 includes a refrigerator compressor 14a, a cooling device 14b, and a coolant pipe 14c. The refrigerator compressor 14a mechanically compresses evaporated helium, and the cooling device 14b liquefies the evaporated helium and circulates it in the static magnetic field magnet 10. The cooling device 14b liquefies the compressed helium, for example, using a coolant (e.g., cooling water) flowing through the coolant pipe 14c. The return liquid of the coolant has its temperature increased by cooling, and can therefore be used as a liquid for increasing the temperature of the gradient magnetic field coil 12. The heated liquid flowing through the pipe may be the return liquid of the refrigerator 14 used in the static magnetic field magnet 10.

[0066] That is, in the second embodiment, the metal shims are arranged so that the distribution of the static magnetic field becomes uniform in a state in which the temperature of the gradient coil 12 is increased by the circulation of the heated liquid. After the passive shimming is completed, the sequence controller 34 executes a pulse sequence for imaging the subject.

[0067] Alternatively, a heater or the like placed near the metal shim may be used to raise the temperature to a level close to the temperature environment that occurs in the gradient magnetic field coil 12 and the metal shim due to heat generated by the pulse sequence for imaging the subject P. The first embodiment, the first modified example of the first embodiment, and the second modified example of the first embodiment can also be implemented in combination with the second embodiment.

[0068] According to the MRI apparatus 1 and the static magnetic field correcting method of at least one of the embodiments described above, it is possible to suppress the deterioration of the uniformity of the static magnetic field during the operation of the MRI apparatus.

[0069] In the above embodiments, the term "processor" refers to a circuit such as a dedicated or general-purpose CPU (Central Processing Unit), GPU (Graphics Processing Unit), or an Application Specific Integrated Circuit (ASIC), a programmable logic device (e.g., a Simple Programmable Logic Device (SPLD), a Complex Programmable Logic Device (CPLD), and an FPGA). The processor realizes various functions by reading and executing programs stored in a storage medium.

[0070] The functions of the processing circuit may be realized by a single processor, or a processing circuit may be configured by combining multiple independent processors, with each processor realizing a different function. In addition, when multiple processors are provided, a storage medium for storing the program may be provided separately for each processor, or a single storage medium may store all of the programs corresponding to the functions of all the processors.

[0071] 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]

[0072] 1...Magnetic Resonance Imaging (MRI) device 10...Static magnetic field magnet 11...Shim coil 12...Gradient magnetic field coil (ASGC) 12a...Main coil 12b...Shim tray section 12c...Shield coil 13...WB (Whole Body) coil 14...Refrigerator 15...Piping 72b...Metal shim 34...Sequence controller

Claims

1. a static magnetic field magnet for generating a static magnetic field; a gradient magnetic field coil for applying a gradient magnetic field to the subject; a sequence controller that applies a gradient magnetic field current to the gradient magnetic field coil by executing a first pulse sequence to increase the temperature of the gradient magnetic field coil; a plurality of metal shims for passive shimming, the plurality of metal shims being arranged so as to make the distribution of the static magnetic field uniform in a state in which the temperature of the gradient coil is increased by execution of the first pulse sequence; Equipped with the sequence controller executes a plurality of second pulse sequences for imaging the subject after the passive shimming is completed. Magnetic resonance imaging device.

2. the sequence controller executes, as the first pulse sequence, a pulse sequence that causes the temperature of the gradient magnetic field coil to become highest among the plurality of second pulse sequences.

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

3. the sequence controller executes Echo Planar Imaging (EPI) as the first pulse sequence.

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

4. the sequence controller executes, as the first pulse sequence, a pulse sequence that is in a thermal load state between a no-load state in which no thermal load is applied to the gradient coil and a maximum thermal load state by a pulse sequence in which the temperature of the gradient coil becomes the highest.

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

5. the sequence controller further executes the first pulse sequence after the passive shimming is completed and before executing a plurality of second pulse sequences for imaging the subject, thereby increasing the temperature of the gradient coil.

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

6. before executing a plurality of second pulse sequences for imaging the subject, performing active shimming based on the second pulse sequences and imaging conditions so as to make the distribution of the static magnetic field uniform; 2. The magnetic resonance imaging apparatus according to claim 1.

7. a static magnetic field magnet for generating a static magnetic field; a gradient magnetic field coil for applying a gradient magnetic field to the subject; a pipe through which a heated liquid for increasing the temperature of the gradient coil is circulated, the pipe being disposed around the gradient coil; a metal shim for passive shimming, the metal shim being arranged so that the distribution of the static magnetic field becomes uniform when the temperature of the gradient coil is increased by the circulation of the heated liquid; a sequence controller that executes a pulse sequence for imaging the subject after the passive shimming is completed; Equipped with Magnetic resonance imaging device.

8. Further comprising a refrigerator used in the static magnetic field magnet, The heated liquid flowing through the pipe is a return liquid of a refrigerator used in the static magnetic field magnet.

8. The magnetic resonance imaging apparatus of claim 7.

9. a static magnetic field magnet that generates a static magnetic field, and a gradient magnetic field coil that applies a gradient magnetic field to the subject; Metal shims for passive shimming; a sequence controller that executes a plurality of second pulse sequences for imaging the subject after the passive shimming is completed; A static magnetic field correction method for a magnetic resonance imaging apparatus comprising: the sequence controller applies a gradient magnetic field current to the gradient magnetic field coil by executing a first pulse sequence to increase the temperature of the gradient magnetic field coil; arranging the plurality of metal shims so that the distribution of the static magnetic field becomes uniform in a state in which the temperature of the gradient magnetic field coil is increased by executing the first pulse sequence; Static magnetic field correction method.

10. a static magnetic field magnet for generating a static magnetic field; a gradient magnetic field coil for applying a gradient magnetic field to the subject; a pipe disposed around the gradient magnetic field coil; Metal shims for passive shimming; a sequence controller that executes a pulse sequence for imaging the subject after the passive shimming is completed; A static magnetic field correction method for a magnetic resonance imaging apparatus comprising: circulating a heated liquid through the piping to increase the temperature of the gradient coil; and arranging the plurality of metal shims so that the static magnetic field distribution becomes uniform in a state in which the temperature of the gradient coil is increased by the circulation of the heated liquid. Static magnetic field correction method.

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