Magnetic resonance imaging apparatus and control method thereof

Abstract

To provide a magnetic resonance imaging apparatus that shortens a duration of confinement for a subject during acquisition of data for static magnetic field correction, and a control method thereof.SOLUTION: The magnetic resonance imaging apparatus acquires an imaging site of a subject, sets a first phase encoding number corresponding to the acquired imaging site, controls operations of a transmission device, a reception device and a gradient magnetic field generating device to acquire a first nuclear magnetic resonance signal with the first phase encoding number, controls the operations of the transmission device, the reception device and the gradient magnetic field generating device, and controls a compensation magnetic field generating device based on the first nuclear magnetic resonance signal to acquire a second nuclear magnetic resonance signal with a second phase encoding number greater than the first phase encoding number, and reconstructs an image of the subject based on the second nuclear magnetic resonance signal.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a magnetic resonance imaging apparatus and a control method thereof, and more particularly to a technique for correcting a static magnetic field. [Background technology]

[0002] A magnetic resonance imaging system applies a radio-frequency magnetic field to a subject placed in a uniform static magnetic field, causing nuclear magnetic resonance in atomic nuclei (protons) present in a specific region of the subject, and obtains a tomographic image of that region from the resulting nuclear magnetic resonance signal. To selectively excite a specific region, a gradient magnetic field is applied along with the radio-frequency magnetic field, and the applied field and strength of the gradient magnetic field are controlled to provide spatial position information to the measured nuclear magnetic resonance signal.

[0003] When applying a gradient magnetic field in a magnetic resonance imaging device, Z depends on the application axis and strength of the gradient magnetic field. 2 This causes static magnetic field inhomogeneities such as XY, ZY, and XZ, resulting in image degradation. Therefore, various methods for correcting static magnetic field inhomogeneities due to Maxwell terms have been investigated.

[0004] For example, Patent Document 1 discloses a technique for acquiring correction data required for correcting static magnetic field inhomogeneity before imaging. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-85376 [Non-patent literature]

[0006] [Non-Patent Document 1] Fujimoto, K., Watanabe, H., Yukawa, K., and Sakamoto, J. (1968) "Study on the body surface area of ​​the Japanese, Part 17: Sex, age, body type and proportions by body part" Journal of the Japanese Society for Hygiene 23(5) pp. 437-442 [Non-patent document 2] "Basic Power Textbook of MRI, 3rd Edition" by Araki Chikara, Medical Science International Co., Ltd., 2011, 182 pages Summary of the Invention [Problem to be solved by the invention]

[0007] The data required for correcting static magnetic field inhomogeneity is acquired by imaging at least the target area of ​​the subject in the same way as in the actual imaging. However, when imaging at high resolution, the time required to acquire data for correction that is not used for diagnosis increases, which is a problem in that it requires the subject to spend a long time in the field.

[0008] The present invention has been made in view of the above circumstances, and has as its object to provide a magnetic resonance imaging apparatus and a control method thereof that shorten the time that a subject is confined when acquiring data for static magnetic field correction. [Means for solving the problem]

[0009] In order to achieve the above object, a magnetic resonance imaging apparatus according to a first aspect of the present disclosure comprises a static magnetic field generating device that generates a static magnetic field, a transmitting device that irradiates a radio frequency magnetic field pulse to a subject placed in a static magnetic field space, a receiving device that receives nuclear magnetic resonance signals generated from the subject by the irradiation of the radio frequency magnetic field pulse, a gradient magnetic field generating device that generates a gradient magnetic field in the static magnetic field space, a compensation magnetic field generating device that corrects non-uniformity of the static magnetic field, and at least one processor, wherein the processor acquires an imaging region of the subject, sets a first phase encoding number according to the acquired imaging region, controls the operation of the transmitting device, the receiving device, and the gradient magnetic field generating device to acquire a first nuclear magnetic resonance signal at the first phase encoding number, controls the operation of the transmitting device, the receiving device, and the gradient magnetic field generating device, and controls the compensation magnetic field generating device based on the first nuclear magnetic resonance signal to acquire a second nuclear magnetic resonance signal at a second phase encoding number greater than the first phase encoding number, and reconstructs an image of the subject based on the second nuclear magnetic resonance signal.

[0010] According to this aspect, the first nuclear magnetic resonance signal for static magnetic field correction is acquired at a first phase encoding number that is smaller than the second phase encoding number during actual imaging, so the amount of data acquired for static magnetic field correction is kept to a minimum, thereby shortening the subject's restraint time.

[0011] A magnetic resonance imaging apparatus according to a second aspect of the present disclosure is preferably a magnetic resonance imaging apparatus according to the first aspect, wherein the processor acquires an imaging protocol and acquires a first nuclear magnetic resonance signal when the acquired imaging protocol requires static magnetic field correction.

[0012] A magnetic resonance imaging apparatus according to a third aspect of the present disclosure is preferably the magnetic resonance imaging apparatus according to the first or second aspect, further comprising a table in which the imaging area of ​​the subject and the first phase encoding number are stored in correspondence with each other, and the processor sets the first phase encoding number by referring to the table.

[0013] A magnetic resonance imaging apparatus according to a fourth aspect of the present disclosure is a magnetic resonance imaging apparatus according to any one of the first to third aspects, wherein the processor preferably acquires information about the subject including at least one of age, sex, and body type, and sets the first phase encoding number according to the acquired information.

[0014] In a magnetic resonance imaging apparatus according to a fifth aspect of the present disclosure, in the magnetic resonance imaging apparatus according to any one of the first to fourth aspects, it is preferable that the processor acquires the size of the imaging area of ​​the subject and sets the first phase encoding number according to the acquired size.

[0015] A magnetic resonance imaging apparatus according to a sixth aspect of the present disclosure is the magnetic resonance imaging apparatus according to the fifth aspect, wherein the processor preferably acquires an image of the subject and acquires the size of the imaging region from the acquired image.

[0016] A magnetic resonance imaging apparatus according to a seventh aspect of the present disclosure is preferably the magnetic resonance imaging apparatus according to the sixth aspect, in which an image of the entire body of the subject is acquired and the size of the imaging portion is obtained from the ratio of the imaging portion to the entire body.

[0017] A magnetic resonance imaging apparatus according to an eighth aspect of the present disclosure is a magnetic resonance imaging apparatus according to any one of the first to seventh aspects, wherein the processor preferably acquires an image of the subject's entire body, displays the image of the entire body on a display device, and acquires a part selected by the user from the displayed image of the entire body as the imaging part.

[0018] A magnetic resonance imaging apparatus according to a ninth aspect of the present disclosure is a magnetic resonance imaging apparatus according to any one of the first to eighth aspects, wherein the processor preferably acquires an image of the subject's entire body, acquires a plurality of imaging regions from the acquired image of the entire body, and sets a first phase encoding number for each of the acquired plurality of imaging regions.

[0019] In a magnetic resonance imaging apparatus according to a tenth aspect of the present disclosure, in the magnetic resonance imaging apparatus according to any one of the first to ninth aspects, it is preferable that the processor sets the first phase encoding number according to the resolution set for each imaging region.

[0020] In a magnetic resonance imaging apparatus according to an eleventh aspect of the present disclosure, in the magnetic resonance imaging apparatus according to any one of the first to tenth aspects, it is preferable that the processor sets the first phase encoding number according to the magnetic field distortion set for each imaging region.

[0021] A magnetic resonance imaging apparatus according to a twelfth aspect of the present disclosure is a magnetic resonance imaging apparatus according to any one of the first to eleventh aspects, wherein the processor preferably acquires a period of body movement of the acquired imaging region and sets a first phase encoding number that completes acquisition of the first nuclear magnetic resonance signal within the acquired period.

[0022] In order to achieve the above object, a control method for a magnetic resonance imaging apparatus according to a thirteenth aspect of the present disclosure is a control method for a magnetic resonance imaging apparatus including a static magnetic field generating device that generates a static magnetic field, a transmitting device that irradiates a radio frequency magnetic field pulse to a subject placed in a static magnetic field space, a receiving device that receives nuclear magnetic resonance signals generated from the subject by the irradiation of the radio frequency magnetic field pulse, a gradient magnetic field generating device that generates a gradient magnetic field in the static magnetic field space, and a compensation magnetic field generating device that corrects non-uniformity of the static magnetic field, the control method for a magnetic resonance imaging apparatus comprising: acquiring an imaging region of the subject; setting a first phase encoding number according to the acquired imaging region; controlling the operation of the transmitting device, the receiving device, and the gradient magnetic field generating device to acquire a first nuclear magnetic resonance signal at the first phase encoding number; controlling the operation of the transmitting device, the receiving device, and the gradient magnetic field generating device; and controlling the compensation magnetic field generating device based on the first nuclear magnetic resonance signal to acquire a second nuclear magnetic resonance signal at a second phase encoding number greater than the first phase encoding number; and reconstructing an image of the subject based on the second nuclear magnetic resonance signal.

[0023] According to this aspect, data for static magnetic field correction is acquired using a first phase encoding number that is smaller than the second phase encoding number during actual imaging, so the amount of data for static magnetic field correction is kept to a minimum, and the time the subject is required to be restrained when acquiring data for static magnetic field correction can be shortened. [Effects of the Invention]

[0024] According to the present invention, the time during which the subject is confined when acquiring data for static magnetic field correction can be reduced. [Brief explanation of the drawings]

[0025] [Figure 1] FIG. 1 is a block diagram showing the overall configuration of an embodiment of a magnetic resonance imaging apparatus. [Figure 2] FIG. 2 is a flow chart showing the steps of the imaging method. [Figure 3] FIG. 3 is a diagram illustrating an example of the table. [Figure 4] FIG. 4 is a diagram for explaining how to obtain the size of the imaging region of the subject. [Figure 5] FIG. 5 is a diagram showing a table in which size categories of imaging regions correspond to Np. [Figure 6] FIG. 6 is a diagram showing an example of a table for determining the size of the imaging region of a subject. [Figure 7] FIG. 7 is a diagram showing an example of a multi-echo gradient echo sequence. DETAILED DESCRIPTION OF THE INVENTION

[0026] Hereinafter, preferred embodiments of a magnetic resonance imaging apparatus and a control method thereof according to the present disclosure will be described with reference to the accompanying drawings. In this specification, the same components are designated by the same reference numerals, and duplicated descriptions will be omitted where appropriate.

[0027] [MRI device] 1 is a block diagram showing the overall configuration of an embodiment of a magnetic resonance imaging apparatus 100. Hereinafter, the magnetic resonance imaging apparatus 100 will be referred to as an MRI (Magnetic Resonance Imaging) apparatus 100.

[0028] The MRI apparatus 100 utilizes the nuclear magnetic resonance (NMR) phenomenon to obtain a tomographic image of a subject S. The MRI apparatus 100 includes a bed 101, a static magnetic field generating magnet 102, a gradient magnetic field coil 103 and a gradient magnetic field power supply 104, a shim coil 105 and a shim power supply 106, an RF (Radio Frequency) transmitting coil 107 and an RF transmitting unit 108, an RF receiving coil 109 and a signal processing unit 110, a measurement control unit 111, an overall control unit 112, an input device 116, and a display device 118.

[0029] The bed 101 has a top board on which the subject S is placed. The bed 101 has a driving mechanism (not shown) that moves the top board into and out of the static magnetic field space of the static magnetic field generating magnet 102.

[0030] The static magnetic field generating magnet 102 functions as the "static magnetic field generating device" of the present disclosure. The static magnetic field generating magnet 102 generates a uniform static magnetic field in a direction perpendicular to the body axis of the subject S if it is a vertical magnetic field type, or in the direction of the body axis if it is a horizontal magnetic field type. The static magnetic field generating magnet 102 includes a static magnetic field generating source of a permanent magnet type, a resistive conducting type, or a superconducting type. The static magnetic field generating source is arranged around the subject S.

[0031] The gradient magnetic field coil 103 and the gradient magnetic field power supply 104 function as a "gradient magnetic field generating device" of the present disclosure. The gradient magnetic field coil 103 is a coil wound in the three X-, Y-, and Z-axis directions, which are the real space coordinate system (stationary coordinate system) of the MRI apparatus 100. Each gradient magnetic field coil is connected to the gradient magnetic field power supply 104 and is supplied with current. Specifically, the gradient magnetic field power supply 104 of each gradient magnetic field coil is driven according to a command from the measurement control unit 111 and supplies current to the respective gradient magnetic field coil. As a result, gradient magnetic fields Gx, Gy, and Gz are generated in the three X-, Y-, and Z-axis directions.

[0032] When imaging a two-dimensional slice plane, a slice gradient magnetic field pulse (Gs) is applied in a direction perpendicular to the slice plane (imaging cross section) to set the slice plane for the subject S, and a phase encoding gradient magnetic field pulse (Gp) and a frequency encoding (readout) gradient magnetic field pulse (Gf) are applied in the remaining two directions perpendicular to the slice plane and perpendicular to each other, and positional information in each direction is encoded in the nuclear magnetic resonance signal (echo signal).

[0033] The shim coil 105 and the shim power supply 106 function as a "compensation magnetic field generator" that corrects the static magnetic field inhomogeneity of the present disclosure. The shim coil 105 generates the primary gradient magnetic field and the Z 2 The shim coil 105 is composed of coils capable of generating a magnetic field having a spatial distribution according to zeroth or higher order spherical harmonic functions such as XY, ZY, and XZ. The shim coil 105 is connected to a shim power supply 106, and generates a magnetic field to improve the homogeneity of the static magnetic field using a compensation current supplied from the shim power supply 106.

[0034] For the primary gradient magnetic field, the gradient magnetic field coil 103 may be used as a compensation magnetic field generator. In this case, the gradient magnetic field power supply 104 superimposes a compensation current for generating a correction magnetic field on the gradient magnetic field current for providing the above-mentioned position information and supplies the superimposed current to the gradient magnetic field coil 103.

[0035] The RF transmission coil 107 and the RF transmission unit 108 function as a "transmission device" in the present disclosure. The RF transmission coil 107 is a coil that irradiates the subject S with an RF pulse (an example of a "high frequency magnetic field pulse"). The RF transmission coil 107 is connected to the RF transmission unit 108, and a high frequency pulse current is supplied to the RF transmission coil 107. This induces an NMR phenomenon in the spins of atoms that constitute the biological tissue of the subject S. Specifically, the RF transmission unit 108 is driven in accordance with a command from the measurement control unit 111 to amplitude-modulate the high frequency pulse and supply the amplified high frequency pulse to the RF transmission coil 107 arranged close to the subject S, thereby irradiating the RF pulse to the subject S.

[0036] The RF receiving coil 109 and the signal processing unit 110 function as the "receiving device" of the present disclosure. The RF receiving coil 109 is a coil that receives echo signals emitted due to the NMR phenomenon of spins that constitute the biological tissue of the subject S. The RF receiving coil 109 is connected to the signal processing unit 110. The echo signals received by the RF receiving coil 109 are sent to the signal processing unit 110.

[0037] The signal processing unit 110 performs detection processing of the echo signal received by the RF receiving coil 109. Specifically, the signal processing unit 110 amplifies the received echo signal, divides it into two orthogonal systems of signals by quadrature phase detection in accordance with instructions from the measurement control unit 111, samples each of them a predetermined number of times (e.g., 128, 256, 512, etc.), and converts each sampled signal into a digital quantity by A / D conversion. Therefore, the echo signal is obtained as time-series digital data (hereinafter referred to as echo data) consisting of a predetermined number of sampled data. Then, the signal processing unit 110 performs various processes on the echo data and sends the processed echo data to the measurement control unit 111.

[0038] The measurement control unit 111, under the control of the overall control unit 112, transmits various commands for acquiring echo data necessary for reconstructing a tomographic image of the subject S, mainly to the gradient magnetic field power supply 104, the RF transmission unit 108, and the signal processing unit 110, and controls them, based on control data of the pulse sequence included in the imaging protocol. That is, the measurement control unit 111 repeatedly executes irradiation of RF pulses and application of gradient magnetic field pulses to the subject S, and detection of echo signals from the subject S, and controls acquisition of echo data necessary for reconstructing an image of the imaging area of ​​the subject S.

[0039] During repetition, the measurement control unit 111 changes the amount of applied phase encoding gradient magnetic field by the number of phase encodes in the case of two-dimensional imaging, and also changes the amount of applied slice encoding gradient magnetic field in the case of three-dimensional imaging. The number of phase encodes is usually selected as a value such as 128, 256, or 512 per image, and the number of slice encodes is usually selected as a value such as 16, 32, or 64. Through these controls, the measurement control unit 111 outputs echo data from the signal processing unit 110 to the overall control unit 112.

[0040] The overall control unit 112 controls the measurement control unit 111, various data processing, and display and storage of processing results. The overall control unit 112 can be constructed on a computer or workstation equipped with a CPU (Central Processing Unit) 200, memory 210, and an internal storage unit 220 such as a magnetic disk. Some functions of the overall control unit 112 may be realized by hardware such as an ASIC or FPGA. The overall control unit 112 includes a network IF (Interface) 230 that interfaces with an external network. The overall control unit 112 may be connected to an external storage unit 240 such as an optical disk.

[0041] Specifically, the overall control unit 112 causes the measurement control unit 111 to acquire echo data by executing a pulse sequence. When the echo data is input from the measurement control unit 111, the overall control unit 112 stores the echo data in an area corresponding to the k-space in the memory 210 based on the encoding information applied by the CPU 200 to the echo data.

[0042] The CPU 200 performs signal processing on the k-space data, image reconstruction by Fourier transform, etc. The CPU 200 also displays the image of the subject S, which is the result of the image reconstruction processing, on the display device 118, records it in the memory 210 and the external storage unit 240, and transfers it to an external device via the network IF 230.

[0043] The internal storage unit 220 stores a table 222, which will be described later, and the like.

[0044] The input device 116 includes a trackball, a mouse, a keyboard, and the like for inputting various control information for the MRI apparatus 100 and control information for processing performed by the overall control unit 112. The display device 118 includes a display for displaying a reconstructed image of the subject S. The input device 116 and the display device 118 are arranged close to each other, and the user can interactively control various processing operations of the MRI apparatus 100 via the input device 116 while looking at the display device 118.

[0045] In the MRI apparatus 100, the CPU 200 has a function of acquiring data for static magnetic field correction, which is used by the compensation magnetic field generating unit to correct static magnetic field inhomogeneity, in addition to the function of serving as an image generating unit that performs the image reconstruction described above.

[0046] [First embodiment] FIG. 2 is a flowchart showing steps of an imaging method, which is an example of a control method for the MRI apparatus 100.

[0047] In step S1, the user performs advance preparation of the MRI apparatus 100. As advance preparation, the user registers subject information of the subject S using the input device 116. The user also inputs the imaging region of the subject S using the input device 116. The imaging region may be input by the user using the input device 116 to select from a plurality of imaging regions selectably displayed on the display device 118. Furthermore, the user places the subject S on the top board of the bed 101 and moves the top board into the static magnetic field generating magnet 102 so that the subject S can be imaged.

[0048] The overall control unit 112 also reads out an imaging protocol based on the input subject information, imaging region, etc. from the internal storage unit 220. The imaging protocol includes a pulse sequence. The pulse sequence includes a second phase encoding number, which is the number of encodings in the phase direction during main imaging (the number of phase encoding gradient magnetic field pulses per slice).

[0049] In step S2, the MRI apparatus 100 starts imaging. The imaging includes correction imaging and main imaging. After adjusting the imaging conditions, the user presses the imaging start button on the input device 116, and imaging by the MRI apparatus 100 starts.

[0050] In step S3, the MRI apparatus 100 automatically determines whether or not magnetic field correction for correcting the static magnetic field is required. Here, the overall control unit 112 determines whether or not magnetic field correction is required based on the imaging protocol read out in step S1. For example, the overall control unit 112 determines whether or not the imaging protocol includes any of fat suppression of the CHESS (Chemical Shift Selective) method, Echo Planar Imaging (EPI) method, and a Balance-type GrE sequence. If none of these is included, the overall control unit 112 determines that magnetic field correction is unnecessary, and if any of these is included, the overall control unit 112 determines that magnetic field correction is necessary. If magnetic field correction is unnecessary, the MRI apparatus 100 proceeds to processing in step S6, and if magnetic field correction is necessary, the overall control unit 112 proceeds to processing in step S4.

[0051] In step S4, the overall control unit 112 determines the imaging range. For example, the overall control unit 112 acquires a positioning image of the body axis cross section, and determines the imaging range in the body axis cross section direction from the imaging region input in step S1. Note that the imaging range may be manually input by the user using the input device 116.

[0052] Furthermore, in step S4, the MRI apparatus 100 sets a first phase encode number Np, which is the number of encodes in the phase direction when static magnetic field correction data is acquired (during correction imaging). Np and imaging regions are stored in association with each other in a table 222 stored in the internal storage unit 220. Np stored in the table 222 is the minimum phase encode number required for the imaging region, and is a phase encode number smaller than the second phase encode number. The overall control unit 112 refers to the table 222 and determines Np according to the imaging region input in step S1.

[0053] In areas that are large and have gentle magnetic field distortion, such as the abdomen, this magnetic field distortion can be reproduced even if the resolution is low. Therefore, a relatively small value of Np is used. On the other hand, in areas that are small and have steep magnetic field distortion, such as the neck, high resolution is required to reproduce the magnetic field distortion. Therefore, a relatively large value of Np is used.

[0054] In step S5, the MRI apparatus 100 performs correction imaging and acquires first echo data (an example of a "first nuclear magnetic resonance signal") as correction data. That is, the overall control unit 112 controls the operations of the transmitting device, the receiving device, and the gradient magnetic field generating device to perform imaging for the imaging range determined in step S4 at Np determined in step S4.

[0055] Finally, in step S6, the MRI apparatus 100 performs actual imaging. That is, the overall control unit 112 controls the transmitting device, the receiving device, and the gradient magnetic field generating device, and also controls the compensation magnetic field generating device based on the first echo data to acquire second echo data (an example of a "second nuclear magnetic resonance signal"). When proceeding from step S5, the actual imaging is performed based on the imaging protocol read out in step S1 for the imaging range determined in step S4, and in which parameters reflecting the first echo data acquired in step S5 are set. When proceeding from step S3, the actual imaging is performed based on the imaging protocol read out in step S1 for the predetermined imaging range. Because the second phase encoding number is greater than the first phase encoding number, the actual imaging is performed with higher resolution than the correction imaging.

[0056] Furthermore, the overall control unit 112 reconstructs an image based on the second echo data acquired by the main imaging, thereby enabling the MRI apparatus 100 to acquire an image in which the static magnetic field has been corrected when magnetic field correction is required.

[0057] For the sake of examination efficiency, it is desirable to shorten as much as possible the time spent acquiring data for magnetic field correction that is not used in diagnosis. According to the first embodiment, the presence or absence of magnetic field correction is determined, and if magnetic field correction is not required, correction imaging is not performed, thereby contributing to shortening the time the subject is confined. Furthermore, according to the first embodiment, if magnetic field correction is required, the in-plane resolution of correction imaging for acquiring correction data is set to the minimum necessary resolution depending on the imaging region of the subject. Therefore, the amount of data acquired in correction imaging is the minimum necessary amount depending on the imaging region, contributing to shortening the time the subject is confined.

[0058] [First Modification of the First Embodiment] The internal storage unit 220 of the overall control unit 112 may store a table 222A instead of the table 222. The table 222A includes a plurality of tables corresponding to the age, gender, height and weight (body type) of the subject. The overall control unit 112 selects one table from the plurality of tables based on the registered subject information including at least one of the age, gender, and body type of the subject, and determines Np by referring to the selected table.

[0059] FIG. 3 is a diagram showing an example of the table 222A. In the example shown in FIG. 3, the table 222A is a table showing Np when the H-F direction (Head-Foot direction) for each imaging site is the phase direction, and includes age-based tables 222A-1, 222A-2, and 222A-3. The age-based tables 222A-1, 222A-2, and 222A-3 are tables with ages [years] of "0 to 3", "12 to 20", and "21 to 60", respectively. The table 222A may include age-based tables for "4 to 11", "61 to". The age-based tables 222A-1, 222A-2, and 222A-3 are further classified by gender (male / female) and body type (fat / medium / thin), respectively.

[0060] The body type is classified according to the Body Mass Index (BMI) as "fat" when BMI > 25, "medium" when 25 ≥ BMI ≥ 18.5, and "thin" when 18.5 < BMI. Here, BMI is calculated as BMI = w / h, where h is the height in [m] and w is the weight in [kg]. 2 is calculated by

[0061] BMI may be registered in advance or obtained from a HIS (Hospital Information System) or a RIS (Radiology Information System). [[ID=十六]]

[0062] [[ID=十七]] 3, according to age-based table 222A-1, when the age is "0-3", the sex is "male", the body type is "medium", and the imaging region is "head", Np is "16". On the other hand, when the age is "0-3", the sex is "male", the body type is "medium", and the imaging region is "chin", Np is "64". In this way, even if the age, sex, and body type are the same, Np may differ depending on the imaging region.

[0063] According to age-specific table 222A-1, when the age is "0-3", the sex is "male", the body type is "overweight", and the imaging region is "head", Np is "32". On the other hand, when the age is "0-3", the sex is "male", the body type is "medium", and the imaging region is "head", Np is "16". In this way, even if the age, sex, and imaging region are the same, Np may differ depending on the body type. In this example, the Np is relatively larger when the BMI is relatively larger.

[0064] According to age-specific table 222A-2, when the age is "12-20", the sex is "male", the body type is "medium", and the imaging region is "chest", Np is "64". On the other hand, when the age is "12-20", the sex is "female", the body type is "medium", and the imaging region is "chest", Np is "128". In this way, Np may differ depending on the sex even if the age, body type, and imaging region are the same.

[0065] According to age-specific table 222A-1, when the age is "0-3", the sex is "female", the body type is "obese", and the imaging region is "chin", Np is "64". On the other hand, according to age-specific table 222A-2, when the age is "12-20", the sex is "female", the body type is "obese", and the imaging region is "chin", Np is "128". In this way, Np may differ depending on age even if the sex, body type, and imaging region are the same.

[0066] According to the first modification of the first embodiment, the amount of data acquired in the correction imaging can be reduced to the minimum amount necessary according to the subject information, which may include at least one of age, sex, and body type.

[0067] [Second Modification of First Embodiment] The overall control unit 112 may determine Np based on not only the imaging region but also the size of the imaging region. The internal storage unit 220 of the overall control unit 112 may store a table in which size categories of the imaging region of the subject correspond to Np.

[0068] The overall control unit 112 may acquire the size of the imaging region of the subject from the positioning image (an example of the "captured image") acquired in step S4 of the first embodiment. For example, when the positioning image is an axial cross-sectional image, the overall control unit 112 may acquire the size Y AP , and the size X in the RL direction (Right-Left direction) RL When acquiring the positioning image, the overall control unit 112 performs only frequency encoding without applying phase encoding in each direction (with the phase encoding gradient magnetic field set to 0). Then, the overall control unit 112 performs Fourier transform on the acquired data, and determines the range of the profile of the image as being equal to or greater than the threshold value as the region where the subject is present, and calculates the size of this region as Y. AP , and X RL Let's say.

[0069] Fig. 4 is a diagram for explaining how to obtain the size of the imaging region of a subject. 400 shown in Fig. 4 indicates an XY cross section of the imaging region when the RL direction of the subject S is the X direction and the AP direction is the Y direction.

[0070] Reference numeral 401 in FIG. 4 denotes echo data acquired by applying frequency encoding only in the X direction without applying phase encoding, where the horizontal axis indicates spatial frequency and the vertical axis indicates signal intensity.

[0071] 402 shown in FIG. 4 is a profile resulting from Fourier transform of data 401, with the horizontal axis representing the X direction corresponding to the position on the XY cross section 400 and the vertical axis representing the signal intensity. Also, 403 shown in FIG. 4 is a range where the profile 402 is equal to or greater than the threshold. The overall control unit 112 determines the range 403 as a region where the subject is present and sets its size as X.RL Let's say.

[0072] Reference numeral 404 in FIG. 4 denotes echo data acquired by applying frequency encoding only in the Y direction without applying phase encoding, where the vertical axis indicates spatial frequency and the horizontal axis indicates signal intensity.

[0073] 405 shown in FIG. 4 is a profile resulting from Fourier transform of data 404, with the vertical axis representing the Y direction corresponding to the position on the XY cross section 400 and the horizontal axis representing the signal intensity. Also, 406 shown in FIG. 4 is a range where the profile 405 is equal to or greater than a threshold. The overall control unit 112 determines the range 406 as a region where the subject is present and sets its size as Y. AP Let's say.

[0074] In this way, the overall control unit 112 can obtain the size of the imaging region of the subject from the positioning image. Therefore, the overall control unit 112 can determine Np based on the size of the imaging region of the subject by referring to a table in which size categories of the imaging region correspond to Np.

[0075] 5 is a diagram showing table 222B in which size classifications of imaging regions correspond to Np. Table 222B is stored in internal storage unit 220 of overall control unit 112. Table 222B includes imaging region-specific tables 222B-1, 222B-2, and 222B-3. Imaging region-specific tables 222B-1, 222B-2, and 222B-3 are tables for imaging regions of "head," "abdomen," and "foot," respectively. Although not shown here, table 222B includes tables for imaging regions such as "neck," "upper arm," and "forearm" in addition to "head," "abdomen," and "foot."

[0076] In the table 222A shown in FIG. 3, the imaging region and the minimum required Np are associated with each other, but in the table 222B, the size Y AP For example, according to the imaging region table 222B-1, the imaging region is "head" and the size Y APIf the imaging region table 222B-2 indicates that the imaging region is "abdomen" and the size Y AP If is "300mm", Np is "16".

[0077] The overall control unit 112 determines the imaging region input in step S1 and the size Y AP Therefore, Np can be determined by referring to the table 222B. The table 222B includes a size Y as a size classification of the imaging region. AP However, the size classification is size X. RL may be used, or size Z HF may be used, or a combination thereof may be used.

[0078] The size of the imaging region of the subject may be registered in advance, or may be acquired from the height and weight of the subject acquired from the HIS or RIS.

[0079] 6 is a diagram showing an example of table 222C for determining the size of an imaging region from information about a subject. Table 222C shows the HF direction ratio [%] for each imaging region, and includes age-specific tables 222C-1, 222C-2, and 222C-3. Age-specific tables 222C-1, 222C-2, and 222C-3 are tables for ages "0 to 3," "12 to 20," and "21 to 60," respectively. Table 222C may also include age-specific tables for ages 4 to 11 and 61 and over. Age-specific tables 222C-1, 222C-2, and 222C-3 are further classified by gender (male / female) and body type (obese / medium / thin).

[0080] For example, according to the age-based table 222C-1, if the age is "0-3", the sex is "male", the body type is "obese", and the imaging region is "head", the region ratio is "12.7". Note that in the age-based table 222C-1, the total region ratio may exceed 100%. This is because, for example, the upper arm and forearm overlap with the chest and abdomen.

[0081] The overall control unit 112 multiplies the acquired height of the subject by the region ratio acquired by referring to the table 222C, thereby obtaining the size Z in the HF direction of the imaging region of the subject. HF Therefore, the overall control unit 112 can determine Np based on the size of the imaging region of the subject by referring to a table in which size categories of the imaging region correspond to Np.

[0082] It is also possible to obtain the size of the subject's imaging region in the AP direction and RL direction by preparing a table similar to table 222A.

[0083] According to the second modification of the first embodiment, the amount of data acquired in corrective imaging can be reduced to the minimum amount necessary according to the size of the imaging region of the subject.

[0084] Here, the length in each direction [m] is used as the size, but the size is also expressed as the body surface area [m 2 For example, the overall control unit 112 may obtain the body surface area of ​​the imaging region from a table of the subject's whole body surface area and region ratio, and determine the number of phase encodes according to the body surface area of ​​the imaging region. Region ratios of body surface area are described in, for example, Non-Patent Document 1.

[0085] Second Embodiment In the first embodiment, it is assumed that the imaging region is manually selected and input by the user. In the second embodiment, the imaging region is automatically classified from a preliminary scan of the whole body, and Np is determined for the imaging region selected by the user.

[0086] Specifically, the overall control unit 112 acquires positioning images of the subject's entire body as preliminary imaging. The positioning images may be cross-sectional images in the AP direction, RL direction, and HF direction. Next, the overall control unit 112 classifies the imaging regions in the body axis direction from the acquired positioning images of the entire body. The imaging regions to be classified include, for example, the head, neck, shoulders, lung side of the trunk, ventral side of the trunk, hip joints, upper legs, lower legs, and feet.

[0087] Next, the overall control unit 112 displays the divided imaging regions on the display device 118, and allows the user to select one of the imaging regions. The subsequent processing is the same as in the first embodiment.

[0088] The overall control unit 112 may display a positioning image of the whole body on the display device 118, and recognize the imaging range that the user manually selects using the input device 116 for the displayed positioning image as the imaging region.

[0089] According to the second embodiment, the user can easily select the region to be imaged.

[0090] [Modification of the second embodiment] In the second embodiment, the user selects the imaging region, but the overall control unit 112 may automatically determine the first phase encoding number for each imaging region for all sections, and perform imaging entirely automatically.

[0091] Third Embodiment In the third embodiment, a minimum number based on at least one of resolution and magnetic field distortion is set between the imaging region and the first phase encoding number. The third embodiment may be implemented in any of the second modified example of the first embodiment, the second embodiment, and the modified example of the second embodiment, which acquire size information of the subject.

[0092] <<In the case of resolution>> In the internal memory unit 220 of the overall control unit 112, a table 222D is stored instead of the table 222. The table 222D holds the minimum resolution δ (mm / px) for each imaging site. According to this minimum resolution δ, the minimum number of necessary phase encodings is determined.

[0093] The overall control unit 112 determines the minimum Np that satisfies δ > (Y AP / Np). In terms of reconstruction, Np takes discrete values of 2 n ×k (k is a natural number). For example, when n = 3, Np is 8, 16, 32,.... Therefore, the overall control unit 112 determines the minimum Np among the discrete values that can be set.

[0094] 《In the case of magnetic field distortion》 In the internal memory unit 220 of the overall control unit 112, a table 222E is stored instead of the table 222. The table 222E holds the maximum magnetic field distortion α [Hz / m] that can occur per unit distance for each imaging site. According to this magnetic field distortion α, the minimum number N of necessary phase encodings is determined. The overall control unit 112 determines the minimum Np that satisfies N < Np.

[0095] The procedure for determining Np from the magnetic field distortion α will be described. To represent a large magnetic field distortion, that is, a steep distortion, it is necessary to set a high resolution (a high number of phase encodings). Therefore, when the maximum frequency resolution that the MRI apparatus 100 can represent is β [Hz], α [Hz / m] × Y AP [m] / β [Hz] becomes the minimum number of phase encodings required.

[0096] In addition, when creating a static magnetic field map, data is acquired at two different TEs (echo times). However, if it rotates more than 2π between the difference ΔTE of the two TEs, the magnetic field cannot be accurately represented. Therefore, the maximum value of the frequency difference between pixels needs to be ¼ΔT [Hz] or less.

[0097] However, as described above, in terms of reconstruction, Np is 2 n×k (k is a natural number) is a discrete value, so Np is set to a minimum discrete value that is equal to or greater than the minimum phase encoding number calculated by the above formula.

[0098] According to the third embodiment, the first phase encoding number can be set based on at least one of the resolution and the magnetic field distortion.

[0099] [Fourth embodiment] In the fourth embodiment, when a selected imaging region includes a region that has body motion, the first phase encoding number is controlled so that imaging of the imaging region is completed within the body motion period. The type of body motion is determined for each imaging region, such as respiratory motion and peristalsis if the imaging region is the abdomen.

[0100] The imaging method according to the fourth embodiment will be described with reference to the flowchart of Fig. 2. The processes in steps S1 to S3 are the same as those in the first embodiment.

[0101] In step S4, the overall control unit 112 calculates the size Y of the imaging region of the subject in the AP direction. AP , or size X in the RL direction RL Np is determined from the minimum number of phase encodes obtained from and the maximum number of phase encodes allowed from the time intervals at which body movements of the imaging region occur.

[0102] The minimum number of phase encodes, NpMIN, is determined as follows: First, the overall control unit 112 determines the minimum number N required in accordance with the magnetic field distortion per unit distance in the imaging region. Then, if the phase encode direction of the corrective imaging is the AP direction, the overall control unit 112 determines N <Y AP Calculate NpMIN, which is the smallest integer that satisfies / Np, and if the phase encoding direction of the correction imaging is the RL direction, then N <X RL The minimum integer Np that satisfies / Np is calculated as NpMIN. Note that the minimum number N may be determined according to the resolution.

[0103] The maximum phase encoding number NpMAX is determined as follows. In the internal storage unit 220 of the overall control unit 112, the generation time intervals of body movements corresponding to the types of body movements for each imaging site are retained. The overall control unit 112 acquires the generation time interval Tm (seconds) of the body movement at the imaging site acquired in step S1 from the internal storage unit 220. Then, the overall control unit 112 calculates NpMAX, which is the maximum allowable phase encoding number, from the generation time interval Tm and the time allowable for imaging.

[0104] Here, assuming the number of slices to be imaged is Slice# and the repetition time of the excitation RF pulse described later is TR, the imaging time of the MRI apparatus 100 is generally determined by Np × Slice# × TR. Therefore, in order to calculate the imaging time, the overall control unit 112 first determines the number of slices. If the size of the imaging site in the H-F direction, which is the slice direction of the subject, is Z HF and the slice thickness is Thick, the overall control unit 112 calculates the required number of slices Slice#MIN as the smallest integer that satisfies Slice# > Z HF / Thick. Note that the values of Thick and TR may be the values stored in the internal storage unit 220 of the overall control unit 112 in advance. Then, the overall control unit 112 sets Np, which is the largest integer that satisfies Np × Slice#MIN × TR < Tm, as NpMAX.

[0105] Subsequently, the overall control unit 112 determines Np according to the magnitude relationship between NpMIN and NpMAX.

[0106] Since it is preferable that the time for correction imaging is short, it is preferable that Np is small. Therefore, when NpMIN ≤ NpMAX, NpMIN = Np.

[0107] On the other hand, when NpMIN > NpMAX, if resolution is prioritized, NpMAX = Np, and if body movement is prioritized, NpMIN = Np. Whether to prioritize resolution or body movement may be selected by the user.

[0108] Finally, in step S5, the MRI apparatus 100 performs correction imaging at the Np determined in step S4 to acquire first echo data. Furthermore, in step S6, the MRI apparatus 100 performs main imaging included in various protocols in which parameters reflecting the first echo data are set, to acquire second echo data.

[0109] If the correction data contains artifacts or other effects caused by the subject's internal body movement, correction may fail, so it is desirable to minimize the effects of body movement such as peristalsis. The imaging method according to the fourth embodiment utilizes the fact that the type of body movement is determined for each body part, and determines the in-plane resolution so that it falls within the data acquisition time allowed for acquiring correction data. According to the fourth embodiment, correction data with reduced effects of body movement can be acquired.

[0110] [Pulse sequence] Fig. 7 is a diagram showing an example of a multi-echo gradient echo sequence performed in the MRI apparatus 100. Fig. 7(A) shows the application timing of RF pulses, Fig. 7(B), (C), and (D) show the application timing of gradient magnetic field pulses in the slice direction, phase encoding direction, and readout direction, respectively, and Fig. 7(E) shows the acquisition timing of echo signals.

[0111] As described in Non-Patent Document 2, the imaging time is determined by TR·phase encoding number·NEX, where NEX is the number of times the entire sequence is repeated.

[0112] 7, the multi-echo gradient echo sequence excites a desired slice by applying an excitation RF pulse 301 together with a slice selection gradient magnetic field 303. Next, after applying a phase encoding gradient magnetic field 305, readout gradient magnetic fields 307 are applied consecutively for the number of multi-echoes, and main echo signals 308 are collected during the application of the readout gradient magnetic field 307.

[0113] In a multi-echo gradient echo sequence, an excitation RF pulse is repeatedly generated at time intervals of TR, and the collected main echo signals 308 are stored in a region in the memory 210 that corresponds to the k-space.

[0114] [Configuration of overall control unit] The hardware structure of the overall control unit 112 is made up of various processors as follows: The various processors include a CPU (Central Processing Unit), which is a general-purpose processor that executes software (programs) and functions as various functional units, a GPU (Graphics Processing Unit), which is a processor specialized for image processing, a PLD (Programmable Logic Device), which is a processor whose circuit configuration can be changed after manufacture such as an FPGA (Field Programmable Gate Array), and a dedicated electric circuit, which is a processor having a circuit configuration designed specifically for executing specific processing such as an ASIC (Application Specific Integrated Circuit).

[0115] A single processing unit may be configured with one of these various processors, or may be configured with two or more processors of the same or different types (e.g., multiple FPGAs, a combination of a CPU and an FPGA, or a combination of a CPU and a GPU). Also, multiple functional units may be configured with a single processor. Examples of multiple functional units configured with a single processor include, first, a configuration in which a single processor is configured with a combination of one or more CPUs and software, as typified by a client or server computer, and this processor operates as multiple functional units. Second, a configuration in which a processor is used to realize the functions of an entire system including multiple functional units on a single IC (Integrated Circuit) chip, as typified by an SoC (System On Chip). In this way, the various functional units are configured with one or more of the above-mentioned various processors as a hardware structure.

[0116] Furthermore, the hardware structure of these various processors is, more specifically, an electric circuit made up of a combination of circuit elements such as semiconductor elements.

[0117] 〔others〕 The technical scope of the present invention is not limited to the scope described in the above embodiments. The configurations and the like in each embodiment can be appropriately combined with each other within the scope that does not deviate from the spirit of the present invention. [Explanation of symbols]

[0118] 100...MRI device 101...bed 102...Static magnetic field generating magnet 103...Gradient magnetic field coil 104...Gradient magnetic field power supply 105...Shim coil 106...SIM power supply 107...RF transmitting coil 108...RF transmitter 109...RF receiving coil 110...signal processing unit 111...Measurement control unit 112... Overall control unit 116...Input device 118...Display device

Claims

1. a static magnetic field generator that generates a static magnetic field; a transmitter that irradiates a high-frequency magnetic field pulse to a subject placed in a static magnetic field space; a receiving device for receiving a nuclear magnetic resonance signal generated from the subject by irradiation of the radio frequency magnetic field pulse; a gradient magnetic field generating device that generates a gradient magnetic field in the static magnetic field space; a compensation magnetic field generating device for correcting the nonuniformity of the static magnetic field; at least one processor; Equipped with The processor: acquiring an imaging region of the subject; setting a first phase encoding number according to the acquired imaging region; controlling the operations of the transmitting device, the receiving device, and the gradient magnetic field generating device to acquire a first nuclear magnetic resonance signal at the first phase encoding number; controlling the operations of the transmitting device, the receiving device, and the gradient magnetic field generating device, and controlling the compensation magnetic field generating device based on the first nuclear magnetic resonance signal to acquire a second nuclear magnetic resonance signal at a second phase encoding number greater than the first phase encoding number; reconstructing an image of the subject based on the second nuclear magnetic resonance signal; Magnetic resonance imaging device.

2. The processor: Obtain the imaging protocol acquiring the first nuclear magnetic resonance signal when the acquired imaging protocol requires correction of the static magnetic field; 2. The magnetic resonance imaging apparatus according to claim 1.

3. a table storing an imaging region of the subject and the first phase encoding number in association with each other; the processor sets the first phase encoding number by referring to the table.

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

4. The processor: acquiring information about the subject, the information including at least one of age, sex, and body type; setting the first phase encoding number in accordance with the acquired information; 2. The magnetic resonance imaging apparatus according to claim 1.

5. The processor: acquiring a size of the imaging region of the subject; setting the first phase encoding number according to the acquired size; 2. The magnetic resonance imaging apparatus according to claim 1.

6. The processor: acquiring an image of the subject; acquiring a size of the imaging region from the acquired captured image; 6. A magnetic resonance imaging apparatus according to claim 5.

7. The processor: acquiring a whole-body image of the subject; obtaining a size of the imaging region from a ratio of the imaging region to the whole body; 7. The magnetic resonance imaging apparatus according to claim 6.

8. The processor: acquiring a whole-body image of the subject; Displaying the whole-body captured image on a display device; acquiring a region selected by a user from the displayed whole-body captured image as the imaging region; 2. The magnetic resonance imaging apparatus according to claim 1.

9. The processor: acquiring a whole-body image of the subject; acquiring a plurality of the imaging regions from the acquired whole-body imaging image; setting a first phase encoding number for each of the acquired imaging regions; 2. The magnetic resonance imaging apparatus according to claim 1.

10. the processor sets the first phase encoding number in accordance with a resolution set for each of the imaging regions.

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

11. the processor sets the first phase encoding number in accordance with the magnetic field distortion set for each of the imaging regions.

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

12. The processor: acquiring a period of body movement of the acquired imaging region; setting the first phase encoding number such that acquisition of the first nuclear magnetic resonance signal is completed within the acquired period; 2. The magnetic resonance imaging apparatus according to claim 1.

13. a static magnetic field generator that generates a static magnetic field; a transmitter that irradiates a high-frequency magnetic field pulse to a subject placed in a static magnetic field space; a receiving device for receiving a nuclear magnetic resonance signal generated from the subject by irradiation of the radio frequency magnetic field pulse; a gradient magnetic field generating device that generates a gradient magnetic field in the static magnetic field space; a compensation magnetic field generating device for correcting the nonuniformity of the static magnetic field; A method for controlling a magnetic resonance imaging apparatus comprising: acquiring an imaging region of the subject; setting a first phase encoding number according to the acquired imaging region; controlling the operations of the transmitting device, the receiving device, and the gradient magnetic field generating device to acquire a first nuclear magnetic resonance signal at the first phase encoding number; controlling the operations of the transmitting device, the receiving device, and the gradient magnetic field generating device, and controlling the compensation magnetic field generating device based on the first nuclear magnetic resonance signal to acquire a second nuclear magnetic resonance signal at a second phase encoding number greater than the first phase encoding number; reconstructing an image of the subject based on the second nuclear magnetic resonance signal; A method for controlling a magnetic resonance imaging apparatus.

Images