JP2013223576A - Magnetic resonance imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To shorten the period of time required for correction of image distortion caused by nonlinearity of a gradient magnetic field in an MRI.SOLUTION: An MRI apparatus (20) includes a data collection unit and an image reconstruction unit (90). The data collection unit applies a gradient magnetic field to an imaging region as main scanning, transmits an excitation pulse thereto, and stores a nuclear magnetic-resonance signal collected from the imaging region, as frequency space data. The image reconstruction unit generates image data by subjecting the frequency space data to conversion processing for conversion to real space data and correction processing based on distortion correction data for regulating a method for correcting image distortion caused by nonlinearity of the gradient magnetic field. The image reconstruction unit starts calculation of the distortion correction data before execution of the conversion processing after establishment of conditions for use in the calculation of the distortion correction data, from among main scanning conditions.

Description

  Embodiments of the invention relate to magnetic resonance imaging.

  MRI is an imaging method in which a nuclear spin of a subject placed in a static magnetic field is magnetically excited with an RF pulse having a Larmor frequency, and an image is reconstructed from MR signals generated by the excitation. The MRI means magnetic resonance imaging, the RF pulse means a radio frequency pulse as an excitation pulse, and the MR signal is a nuclear magnetic resonance signal. Is the meaning.

  In MRI, it is known that image distortion occurs due to various factors. A particularly large factor that causes image distortion is image distortion due to nonlinearity of the gradient magnetic field formed in the imaging region by the gradient coil.

  More specifically, if a completely linear gradient magnetic field can be formed by the gradient coil, its intensity distribution changes linearly with respect to each coordinate position in the imaging region. However, the actual intensity distribution of the gradient magnetic field is not completely linear, and the region far from the center of the magnetic field is different from the linear intensity distribution. Therefore, if MR signal acquisition and image reconstruction processing are executed on the assumption that the gradient magnetic field is completely linear in the imaging region, each pixel is displaced in the outer periphery of the imaging region due to the nonlinearity of the gradient magnetic field. . As a result, the reconstructed image becomes spatially distorted.

  In order to correct the image distortion due to the non-linearity of the gradient magnetic field, the MRI apparatus has a function of correcting the image distortion using, for example, a distortion correction table generated in advance. The distortion correction table measures or calculates in advance the intensity distribution of the gradient magnetic field formed by the gradient magnetic field coil, and measures or calculates how much misalignment occurs at each of a plurality of representative points set in the imaging region. Is generated. In the two-dimensional distortion correction, the positional deviation in the direction in the slice plane is corrected for each two-dimensional slice image. In the three-dimensional distortion correction, a three-dimensional image obtained by multi-slice imaging or three-dimensional imaging is three-dimensionally obtained. The positional deviation is corrected (for example, see Patent Document 1).

JP 2010-279601 A

  When correcting the image distortion due to the nonlinearity of the gradient magnetic field, for example, the distortion correction amount is calculated based on the distance from the magnetic field center for each pixel of all slices. On the other hand, an approximate MRI apparatus is in the direction of increasing the resolution of an image by increasing the number of phase encodings and the number of frequency encodings. If the resolution of the image is increased, the time required for the image reconstruction process becomes longer. Therefore, it is desired to reduce the time required for the image reconstruction process. Therefore, the proportion of the image distortion correction processing due to the non-linearity of the gradient magnetic field in the time required for the image reconstruction processing cannot be ignored.

  For this reason, there has been a demand for a new technique for shortening the time required for correcting image distortion due to the nonlinearity of the gradient magnetic field in MRI.

  Hereinafter, several examples of the modes that the embodiment of the present invention can take will be described for each mode.

(1) In one embodiment, the MRI apparatus includes a data collection unit and an image reconstruction unit.
The data collection unit collects MR signals from the imaging region and saves the MR signals as frequency space data by applying a gradient magnetic field to the imaging region and transmitting an excitation pulse as the main scan.
The image reconstruction unit performs a conversion process for converting to real space data and a correction process based on distortion correction data defining a correction method for image distortion due to the nonlinearity of the gradient magnetic field to the frequency space data, thereby Generate data. The image reconstruction unit starts calculating distortion correction data after the conditions used for calculation of distortion correction data in the main scan are determined and before the conversion process is executed.

(2) In another embodiment, the MRI apparatus includes a data collection unit and an image reconstruction unit.
The data collection unit applies a gradient magnetic field to the imaging region and transmits an excitation pulse to collect an MR signal that is an image generation source from the imaging region, and stores the collected MR signal as frequency space data.
The image reconstruction unit performs a conversion process for converting to real space data and a correction process based on distortion correction data defining a correction method for image distortion due to the nonlinearity of the gradient magnetic field to the frequency space data, thereby Is generated.
When at least a part of the imaging region of one image and the imaging region of another image overlap in the apparatus coordinate system, the image reconstruction unit generates another image when generating image data indicating the one image. The correction processing is executed using distortion correction data used when generating image data indicating the above.

(3) In another embodiment, the MRI apparatus includes a data collection unit and an image reconstruction unit.
The data collection unit collects MR signals that are image generation sources from the subject by applying a gradient magnetic field and transmitting excitation pulses, and stores the collected MR signals as frequency space data.
The image reconstruction unit performs a conversion process for converting to real space data and a correction process based on distortion correction data defining a correction method for image distortion due to the nonlinearity of the gradient magnetic field to the frequency space data, thereby Is generated.
When at least a part of pixels of one image and at least a part of pixels of another image indicate the same position in the apparatus coordinate system, the image reconstruction unit generates the image data indicating the one image. The distortion correction data used when generating the image data indicating the different image is used for the pixels indicating the same position, and the correction processing is performed by calculating the distortion correction data for the remaining pixels. Execute.

The block diagram which shows the whole structure of the MRI apparatus of each embodiment. The functional block diagram of the computer 58 shown in FIG. The schematic diagram which shows an example of the nonlinearity of a gradient magnetic field. Schematic diagram showing an example of where each pixel of real-space image data Fourier-transformed from k-space data collected under a non-linear gradient magnetic field reflects an MR signal from which position in the actual imaging space . The plane schematic diagram which shows an example of a structure of the RF coil apparatus for upper bodies as an example of the mounting | wearing type RF coil apparatus which receives MR signal. FIG. 2 is a block diagram illustrating an example of a detailed configuration of an RF receiver 48 in FIG. 1. 5 is a flowchart showing an example of an operation flow of the MRI apparatus according to the first embodiment. The flowchart which shows an example of the flow of operation | movement of the MRI apparatus which concerns on the modification of 1st Embodiment. 6 is a flowchart showing an example of an operation flow of the MRI apparatus according to the second embodiment. 9 is a flowchart showing an example of an operation flow of an MRI apparatus according to a modification of the second embodiment. The plane schematic diagram which shows an example when there exists an imaging region which overlaps between several images. The block diagram in the case of preserve | storing distortion correction data in a server as an example in the case of preserve | saving distortion correction data outside the MRI apparatus. The flowchart which shows an example of the flow of operation | movement of the MRI apparatus which concerns on 3rd Embodiment.

  In the first embodiment, after the imaging area of the main scan is determined, before the MR signal data collected in the main scan is converted into real space data, the image distortion correction method due to the nonlinearity of the gradient magnetic field is corrected. The calculation of distortion correction data that defines That is, in the first embodiment, since distortion correction data is calculated in parallel with other processing such as main scanning, for example, the time required to correct image distortion after the main scanning can be shortened.

In the second and third embodiments, the time required to correct the image distortion is shortened by storing and reusing the distortion correction data. Specifically, in the second embodiment, when the main scan for repeatedly capturing the same area is executed, and the image of the same imaging area as the already reconstructed image is reconstructed, the stored distortion correction data is used. Reuse. In the third embodiment, when at least a part of an imaging region of an image to be reconstructed overlaps with an imaging region of another image reconstructed in the past, regardless of one imaging sequence. Reuse distortion correction data.
Hereinafter, embodiments of an MRI apparatus and an MRI method will be described with reference to the accompanying drawings. In addition, in each figure, the same code | symbol is attached | subjected to the same element and the overlapping description is abbreviate | omitted.

(Configuration of MRI apparatus of each embodiment)
FIG. 1 is a block diagram showing the overall configuration of the MRI apparatus 20 in each embodiment. As shown in FIG. 1, the MRI apparatus 20 includes a cylindrical static magnetic field magnet 22, a cylindrical shim coil 24, a gradient magnetic field coil 26, an RF coil 28, a control device 30, a bed 32, and a bed 32. And an upper top plate 34. The shim coil 24 is arranged inside the static magnetic field magnet 22 with the same axis as the static magnetic field magnet 22.

  Here, as an example, the X-axis, Y-axis, and Z-axis that are orthogonal to each other in the apparatus coordinate system are defined as follows. First, the static magnetic field magnet 22 and the shim coil 24 are arranged so that their axial directions are orthogonal to the vertical direction, and the axial direction of the static magnetic field magnet 22 and the shim coil 24 is the Z-axis direction. Further, it is assumed that the vertical direction is the Y-axis direction, and the bed 32 is arranged such that the normal direction of the surface for placing the top plate 34 is the Y-axis direction.

  The apparatus coordinate system is a coordinate system that serves as a reference for the MRI apparatus 20 to determine a position and a range when performing an imaging operation such as determination of an imaging region, application of a gradient magnetic field, and transmission of an RF pulse. . Here, as an example, it is assumed that the origin of the apparatus coordinate system and the X, Y, and Z axes remain unchanged when the power of the MRI apparatus 20 is turned off and the MRI apparatus 20 is activated next time. For example, in the third embodiment, since the distortion correction data stored before the MRI apparatus 20 is turned on (before the previous activation) is reused, the apparatus coordinates that define the imaging region corresponding to the distortion correction data This is because it is desirable that the system is invariant.

  The control device 30 includes a static magnetic field power supply 40, a shim coil power supply 42, a gradient magnetic field power supply 44, an RF transmitter 46, an RF receiver 48, a sequence controller 56, and a computer 58. The gradient magnetic field power supply 44 includes an X-axis gradient magnetic field power supply 44x, a Y-axis gradient magnetic field power supply 44y, and a Z-axis gradient magnetic field power supply 44z. The computer 58 includes an arithmetic device 60, an input device 62, a display device 64, and a storage device 66.

  The static magnetic field magnet 22 is connected to the static magnetic field power source 40 and forms a static magnetic field in the imaging space by the current supplied from the static magnetic field power source 40. The static magnetic field magnet 22 is often composed of a superconducting coil, and is connected to the static magnetic field power source 40 at the time of excitation and supplied with a current. It is common. In addition, you may comprise the static magnetic field magnet 22 with a permanent magnet, without providing the static magnetic field power supply 40. FIG.

  The imaging space means, for example, a space in the gantry where the subject P is placed and a static magnetic field is applied. The gantry is a structure formed, for example, in a cylindrical shape so as to include the static magnetic field magnet 22, the shim coil 24, the gradient magnetic field coil 26, and the RF coil 28. The gantry and the bed 32 are configured so that the top plate 34 on which the subject P is placed can move into the gantry. 1 is complicated, the components such as the static magnetic field magnet 22 in the gantry are illustrated, and the gantry itself is not illustrated.

  The imaging region means, for example, a region that is an image and is at least part of the MR signal collection range used for generating one image or one set of images. The imaging area is defined in terms of position and range as a part of the imaging space, for example, by the apparatus coordinate system. When the entire MR signal collection range is an image, that is, the MR signal collection range and the imaging region may completely match, but both may not match completely. For example, when MR signals are collected in a wider range than the region to be an image in order to prevent aliasing artifacts, the imaging region can be said to be a part of the MR signal collection range.

  The “one image” and “one set of images” may be two-dimensional images or three-dimensional images. “One set of images” refers to “multiple images” when MR signals of a plurality of images are collected in one pulse sequence, such as multi-slice imaging. Here, as an example, the imaging region is referred to as a slice if the region is thin, and is referred to as a slab if the region is thick to some extent.

  In the first to third embodiments, as an example, it is assumed that the above-described distortion correction data can be calculated based on an imaging region (more precisely, information other than the slice thickness among information defining the imaging region). In other words, the distortion correction data can be calculated based on the imaging region regardless of other conditions of the main scan. This is because once the imaging region is determined, it is determined to some extent how the gradient magnetic field is applied to the imaging region of each image generated later in the main scan.

  When imaging as a slice (when imaging so as to obtain two-dimensional image data), for example, the imaging area includes coordinates of the center of the slice in the apparatus coordinate system, an imaging field of view (FOV: Field Of View), and , Defined by a vector that defines the cross-sectional direction and a slice thickness. The imaging field of view is, for example, each vertical and horizontal size of an image. The above-mentioned “vector that defines the cross-sectional direction” is, for example, a vector in the apparatus coordinate system that indicates the normal direction to the plane of the slice. Of the information defining these imaging areas, distortion correction data is calculated from information other than the slice thickness.

  When imaging as a rectangular parallelepiped slab (when imaging so as to obtain three-dimensional volume data), the imaging area is defined by the following information, for example. Specifically, (1) the thickness direction of the slab, that is, the vector in the apparatus coordinate system indicating the cross-sectional direction, (2) the distance from the magnetic field center to the center point of the slab, (3) the imaging field of view, (4) The thickness of the slab. If these four pieces of information are determined, the above-described distortion correction data can be calculated. Here, as an example for simplification of explanation, it is assumed that the magnetic field center is set to the origin of the apparatus coordinate system.

  Note that the above-described method for defining the imaging region is merely an example. When imaging as a rectangular slice, the imaging area may be defined by coordinate positions in a four-point device coordinate system of the slice, for example. Similarly, when imaging as a rectangular parallelepiped slab, the imaging region may be defined by, for example, coordinate positions in the eight-axis device coordinate system of the slab. In addition, the imaging region is not limited to a rectangular slice or a rectangular parallelepiped slab, and may be, for example, a circular slice or a spherical slab.

  Returning to FIG. 1, the shim coil 24 is connected to the shim coil power source 42, and makes the static magnetic field uniform by the current supplied from the shim coil power source 42.

  The gradient magnetic field coil 26 includes an X-axis gradient magnetic field coil 26 x, a Y-axis gradient magnetic field coil 26 y, and a Z-axis gradient magnetic field coil 26 z, and is formed in a cylindrical shape inside the static magnetic field magnet 22. The X-axis gradient magnetic field coil 26x, the Y-axis gradient magnetic field coil 26y, and the Z-axis gradient magnetic field coil 26z are connected to the X-axis gradient magnetic field power source 44x, the Y-axis gradient magnetic field power source 44y, and the Z-axis gradient magnetic field power source 44z, respectively.

  The X-axis gradient magnetic field power supply 44x, the Y-axis gradient magnetic field power supply 44y, and the Z-axis gradient magnetic field power supply 44z respectively supply the X-axis gradient magnetic field coil 26x, the Y-axis gradient magnetic field coil 26y, and the Z-axis gradient magnetic field coil 26z An axial gradient magnetic field Gx, a Y-axis gradient magnetic field Gy, and a Z-axis gradient magnetic field Gz are formed in the imaging region.

  That is, the gradient magnetic fields Gx, Gy, and Gz in the three-axis direction of the apparatus coordinate system are synthesized, and the slice selection direction gradient magnetic field Gss, the phase encode direction gradient magnetic field Gpe, and the readout direction (frequency encode direction) gradient magnetic field as logical axes. Each direction of Gro can be set arbitrarily. Each gradient magnetic field in the slice selection direction, the phase encoding direction, and the readout direction is superimposed on the static magnetic field.

  The RF transmitter 46 generates an RF pulse (RF current pulse) with a Larmor frequency for causing nuclear magnetic resonance based on the control information input from the sequence controller 56 and transmits this to the RF coil 28 for transmission. To do. The RF coil 28 includes a whole body coil WB for transmitting and receiving RF pulses built in the gantry, and a local coil for receiving RF pulses provided in the vicinity of the bed 32 or the subject P.

  The transmission RF coil 28 receives an RF pulse from the RF transmitter 46 and transmits it to the subject P. The receiving RF coil 28 receives an MR signal generated by exciting a nuclear spin inside the subject P by an RF pulse, and this MR signal is detected by an RF receiver 48.

  The RF receiver 48 performs predetermined signal processing such as pre-amplification, intermediate frequency conversion, phase detection, low-frequency amplification, and filtering on the detected MR signal, and then performs A / D (analog to digital) conversion. Then, raw data that is digitized complex data is generated. The RF receiver 48 inputs the generated raw data of the MR signal to the sequence controller 56.

  The arithmetic device 60 performs system control of the entire MRI apparatus 20, and will be described with reference to FIG.

  The sequence controller 56 stores control information necessary for driving the gradient magnetic field power supply 44, the RF transmitter 46, and the RF receiver 48 in accordance with a command from the arithmetic device 60. The control information here is, for example, sequence information describing operation control information such as the intensity, application time, and application timing of the pulse current to be applied to the gradient magnetic field power supply 44.

  The sequence controller 56 drives the gradient magnetic field power supply 44, the RF transmitter 46, and the RF receiver 48 according to the stored predetermined sequence, so that the X-axis gradient magnetic field Gx, the Y-axis gradient magnetic field Gy, the Z-axis gradient magnetic field Gz, and the RF Generate a pulse. The sequence controller 56 inputs the raw data of the MR signal input from the RF receiver 48 to the arithmetic device 60.

  The ECG unit 36 detects an ECG signal (electrocardiogram signal) representing pulsation as heartbeat information from the subject P, and inputs this to the arithmetic unit 60 via the sequence controller 56. Instead of the ECG signal, a pulse wave synchronization (PPG: peripheral pulse gating) signal representing pulsation as pulse wave information may be acquired.

  FIG. 2 is a functional block diagram of the computer 58 shown in FIG. As shown in FIG. 2, the computing device 60 of the computer 58 includes an MPU (Micro Processor Unit) 86, a system bus 88, an image reconstruction unit 90, and a display control unit 98.

The MPU 86 performs system control of the entire MRI apparatus 20 via wiring such as the system bus 88 in setting of conditions for the imaging sequence of the main scan, imaging operation, and image display after imaging.
The condition of the imaging sequence means, for example, what kind of pulse sequence is used, what kind of condition is used to transmit an RF pulse or the like, and under what kind of condition the MR signal is collected from the subject. Examples of imaging sequence conditions include an imaging region, a gradient magnetic field application method, the number of slices, an imaging region, and a type of pulse sequence such as parallel imaging. The imaging part means, for example, which part of the subject P such as the head, chest, and abdomen is imaged as an imaging region.

  The “main scan” is a scan for capturing a target diagnostic image such as a T1-weighted image, and does not include an MR signal acquisition scan for a positioning image and a calibration scan. A scan refers to an MR signal acquisition operation and does not include image reconstruction.

  For example, a calibration scan is a scan that is performed separately from the main scan in order to determine uncertain conditions in the imaging sequence of the main scan and conditions and data used for image reconstruction after the main scan. Point to. Here, as an example, a calibration scan performed before the main scan is referred to as a pre-scan.

  Examples of the calibration scan include the following sequence. First, a sequence for calculating the center frequency of the RF pulse in the main scan is performed before the main scan. Second, a sequence for generating a sensitivity distribution map of each element coil in the RF coil device may be performed before the main scan or after the main scan.

Further, the MPU 86 sets the imaging sequence conditions based on the pre-scan execution result and the instruction information from the input device 62, and inputs the set imaging sequence conditions to the sequence controller 56. For this purpose, the MPU 86 controls the display control unit 98 to display the setting screen information of the imaging sequence condition on the display device 64.
The input device 62 provides a user with a function of setting conditions for an imaging sequence.

  The image reconstruction unit 90 includes a Fourier transform unit 100, an image database 102, a distortion correction data calculation unit 104, a distortion correction data storage unit 106, and a distortion correction unit 108.

  The Fourier transform unit 100 has a k-space database 110 therein. The Fourier transform unit 100 arranges raw data of MR signals input from the sequence controller 56 as k-space data (frequency space data) in k-space (also referred to as frequency space or Fourier space) formed in the k-space database 110. To do.

  The Fourier transform unit 100 converts k-space data of each slice (or slab) of the subject P into real space data by performing a conversion process (for example, two-dimensional Fourier transform) on the k-space data. The real space data here is data before distortion correction is performed, for example, data defined in the XYZ coordinate system of the apparatus coordinate system. The Fourier transform unit 100 stores real space data in the image database 102.

  In the distortion correction data storage unit 106, a distortion correction table for prescribing a correction method for image distortion due to the nonlinearity of the gradient magnetic field is stored in advance (for example, from the time of shipment of the MRI apparatus 20). The distortion correction table here has data of a three-dimensional displacement vector (or the sum of the original coordinates and the displacement vector) indicating a positional deviation for each of a large number of representative points. The “plurality of representative points” includes, for example, the entire region of the imaging space at predetermined equal intervals. That is, the distortion correction table is discrete data.

  The distortion correction data calculation unit 104 calculates distortion correction data that defines a method for correcting image distortion due to nonlinearity of the gradient magnetic field, and stores the distortion correction data in the distortion correction data storage unit 106. The calculation of the distortion correction data is performed by interpolating the distortion correction table based on the imaging area (exactly, information other than the slice thickness in the information defining the imaging area; hereinafter the same). Since calculation of distortion correction data (interpolation processing of the distortion correction table) requires time, each embodiment aims to substantially reduce the time required for this interpolation processing.

  If the imaging region is determined, it is determined to some extent how the gradient magnetic field is applied to the imaging region of each image generated later in the main scan, so that distortion correction data can be calculated and determined. That is, even before the main scan is executed, the distortion correction data calculation unit 104 can start calculating distortion correction data for each image when the imaging region is determined.

  The distortion correction unit 108 corrects image distortion due to the non-linearity of the gradient magnetic field by performing correction processing based on the distortion correction data on the real space data generated by the Fourier transform unit 100, and performs correction processing after correction processing. The image data is stored in the storage device 66. The correction processing for image distortion due to the nonlinearity of the gradient magnetic field will be specifically described with reference to FIGS.

  FIG. 3 is a schematic diagram illustrating an example of the nonlinearity of the gradient magnetic field. The vertical axis indicates the intensity of the X-axis gradient magnetic field Gx, and the horizontal axis indicates the X-axis coordinate position of the apparatus coordinate system. As described above, the magnetic field center is set at the origin of the apparatus coordinate system, for example. The ideal gradient magnetic field intensity distribution is linear as shown by the thick line in FIG. However, in practice, as shown by the dotted line in FIG. 3, when it is away from the center of the magnetic field, the ideal intensity distribution of the gradient magnetic field is different.

  FIG. 4 shows an example of each pixel in real image data that is Fourier-transformed from k-space data collected under a non-linear gradient magnetic field and reflects an MR signal from which position in the actual imaging space. It is a schematic diagram which shows. It should be noted that the gradient magnetic field intensity distribution shown in FIG. 3 and the image distortion shown in FIG. 4 are merely schematic for convenience of explanation.

  If MR signals are collected as k-space data under a linear gradient magnetic field, the actual position of each pixel in the apparatus coordinate system is arranged at the center of the grid-like dotted line frame in FIG. However, since the gradient magnetic field is actually non-linear, each actual position indicated by each pixel of the image data in the real space subjected to the Fourier transform is distorted more than an ideal lattice-like arrangement.

  Therefore, in the three-dimensional distortion correction, the distortion correction unit 108 calculates the following points based on the distortion correction data. That is, it is calculated which position the position of the target pixel of the image data after the correction processing corresponds to in the image data including distortion (before the correction processing). The distortion correction unit 108 generates the image data after the correction process by setting the pixel value of the calculated position as the pixel value of the target pixel.

  Note that the MRI image data is configured by, for example, each pixel having a pixel value, and the above-described pixel value is, for example, a luminance level when the pixel is displayed (from a subject region corresponding to the pixel). Intensity of detected MR signal). In the case of a slice, the MRI image data has the number of vertical and horizontal pixels, for example, the number of phase encoding steps × the number of frequency encoding steps.

  In the two-dimensional distortion correction, for example, the distortion correction unit 108 is, for example, an actual three-dimensional coordinate of each pixel of the image data before correction processing (in reality, a pixel reflecting an MR signal from which coordinate position in the imaging space). ) Is calculated based on the distortion correction data. The distortion correction unit 108 projects the actual three-dimensional coordinates of each pixel on the target slice image, and calculates the pixel value of each pixel in the corrected image data by interpolation.

  That is, the two-dimensional distortion correction is performed by two-dimensionally distorting the image of the slice. On the other hand, the three-dimensional distortion correction is performed by distorting the image three-dimensionally in consideration of the distortion in the slice thickness direction in addition to the distortion in the slice plane. Since the calculation method of distortion correction data by interpolation of the distortion correction table and the method of two-dimensional distortion correction and three-dimensional distortion correction are described in Patent Document 1 and the like, detailed description thereof is omitted here.

  In the present embodiment, as an example, the image reconstruction process indicates an image data generation process including the following two processes. The first is a process of converting frequency space data to real space image data by performing Fourier transform on the k space data. Second, the correction processing for correcting the image distortion due to the nonlinearity of the gradient magnetic field is performed on the image data in the real space.

  The storage device 66 attaches information such as imaging sequence conditions and information on the subject P (patient information) as supplementary information to the corrected image data, and stores the corrected image data. In the second and third embodiments, the storage device 66 also includes distortion correction data used for the correction processing of the image data as the auxiliary information.

  The display control unit 98 causes the display device 64 to display an imaging sequence condition setting screen and an image indicated by image data generated by imaging under the control of the MPU 86.

  FIG. 5 is a schematic plan view showing an example of the configuration of the upper-body RF coil device 140 as an example of a wearable RF coil device that receives MR signals. As shown in FIG. 5, the upper-body RF coil device 140 includes a cable 124, a connector 126, and a cover member 142.

  The cover member 142 is formed of a flexible material so that it can be bent or deformed. As the deformable material, for example, a flexible printed circuit (FPC) described in JP 2007-229004 A can be used.

  For example, 20 element coils (surface coils) 144 corresponding to the back side of the subject P are arranged in the upper half of the cover member 142 divided into two equal parts by a broken line in the horizontal direction in FIG. . Here, as an example, on the back side, an element coil 144 smaller than the other element coils 144 is disposed near the body axis from the viewpoint of improving sensitivity in consideration of the spine of the subject P.

  The lower half of the cover member 142 divided into two equal parts is configured to cover the head, chest, and abdomen of the subject P. Inside, for example, 20 elements corresponding to the front side of the subject P are included. A coil (surface coil) 146 is arranged. In FIG. 5, the element coil 144 is indicated by a thick line, and the element coil 146 is indicated by a broken line.

  Further, the upper-body RF coil device 140 includes a control circuit (not shown) and a storage element (not shown) that stores identification information of the upper-body RF coil device 140 in the cover member 142. When the connector 126 is connected to the connection port of the MRI apparatus 20, the identification information of the upper body RF coil apparatus 140 is input from this control circuit to the MPU 86 via the wiring in the MRI apparatus 20.

  FIG. 6 is a block diagram showing an example of a detailed configuration of the RF receiver 48 of FIG. Here, as an example, a diagram in which an RF coil device for upper body 140 and an RF coil device for wearing lower body 160 that receives MR signals are mounted on a subject P is shown. The lower-body RF coil device 160 has a large number of element coils 164 (only six are shown for the sake of simplicity) that receive MR signals.

  In this case, the RF coil 28 includes a cylindrical whole body coil WB indicated by a thick square frame, element coils 144 and 146 of the upper body RF coil device 140, and an element 164 of the lower body RF coil device 160. Element coils 144, 146, and 164 function as phased array coils that receive MR signals. The whole body coil WB is a transmission / reception coil that is disposed in the gantry and can transmit RF pulses and receive MR signals.

  The RF receiver 48 includes a duplexer (transmission / reception switching device) 174, a plurality of amplifiers 176, a switching synthesizer 178, and a plurality of reception system circuits 180. The input side of the switching synthesizer 178 is individually connected to each element coil 144, 146, 164 via an amplifier 176, and individually connected to the whole body coil WB via a duplexer 174 and an amplifier 176. Each reception system circuit 180 is individually connected to the output side of the switching combiner 178.

  The duplexer 174 applies the RF pulse transmitted from the RF transmitter 46 to the whole body coil WB. Further, the duplexer 174 inputs the MR signal received by the whole body coil WB to the amplifier 176, and the MR signal is amplified by the amplifier 176 and given to the input side of the switching synthesizer 178. The MR signals received by the element coils 144, 146, and 164 are amplified by the corresponding amplifiers 176 and supplied to the input side of the switching synthesizer 178.

  The switching synthesizer 178 performs combining processing and switching of MR signals detected from the element coils 144, 146, and 164 and the whole body coil WB according to the number of the reception system circuits 180, and the corresponding reception system circuit 180 Output. In this way, the MRI apparatus 20 forms a sensitivity distribution according to the imaging region using the whole body coil WB and the desired number of element coils 144, 146, 164, and MR signals from variously set imaging regions. Receive.

  However, a configuration in which MR signals are received only by the whole body coil WB without providing the element coils 144, 146, and 164 is also possible. In addition, the configuration may be such that MR signals received by the element coils 144, 146, 164 and the whole body coil WB are directly output to the reception system circuit 180 without providing the switching synthesizer 178. Furthermore, more element coils may be arranged over a wide range.

(Description of operation of the first embodiment)
FIG. 7 is a flowchart showing an example of the operation flow of the MRI apparatus 20 in the first embodiment. The operation of the MRI apparatus 20 will be described below according to the step numbers shown in FIG.

  [Step S1] Main conditions (such as the type of pulse sequence) among the imaging sequence conditions of the main scan are input to the input device 62. The MPU 86 sets a part of the imaging sequence conditions based on the input conditions.

  Further, the subject P is placed on the top board 34 (see FIG. 1), and an RF coil device such as the RF coil device 140 (see FIG. 5) is attached to the subject P. Thereafter, a coil position measurement sequence is executed (see, for example, JP 2010-259777 A).

  Specifically, for example, after transmitting an RF pulse from the whole body coil WB, MR signals are collected from each element coil in the RF coil device mounted on the subject P, and coil positioning is performed based on the collected MR signals. Generate profile data. Next, the position for each element coil in the RF coil device is calculated based on the profile data. Thereafter, based on the position of each element coil, the MPU 86 automatically selects the element coil to be used for receiving the MR signal, or after displaying the position of each element coil on the display device 64, it is used for receiving the MR signal. The user selects an element coil. Note that when the RF coil device is not attached to the subject and imaging is performed with only the whole body coil WB, for example, the above coil position measurement sequence is omitted.

Further, as a pre-scan, a sensitivity distribution map generation sequence of the wearable RF coil device is executed (see, for example, JP-A-2005-237703). Specifically, for example, MR signals are collected using the RF coil device mounted on the subject P and the whole body coil WB as a receiving coil.
Image data obtained from the whole body coil WB (hereinafter referred to as WB image data) and image data obtained from the RF coil device (hereinafter referred to as main coil image data) are obtained from all the element coils in the RF coil device. It is acquired as image data for sensitivity distribution estimation and stored in the image database 102. Similar image data acquisition is performed over each cross section of the entire three-dimensional area, and is stored in the image database 102 as volume data.

  Based on the volume data, the MPU 86 generates sensitivity distributions of all the element coils included in the RF coil device as three-dimensional sensitivity distribution map data, and stores them in the storage device 66. For example, the signal intensity distribution of the main coil image data is divided by the signal intensity distribution of the WB image data, and the signal intensity ratio between the two is used as an estimated value of the sensitivity distribution of all the element coils in the RF coil device. If similar processing is performed on the image data of each cross section of the entire three-dimensional region, (three-dimensional) sensitivity distribution map data of each element coil can be generated. For example, the sensitivity distribution map data is used for luminance correction processing when the image reconstruction unit 90 performs image reconstruction.

  Since the WB image data obtained from the whole body coil is used as a reference, only the RF coil device mounted on the subject P is used as the reception coil without using the whole body coil WB as the reception coil. Can also generate a sensitivity distribution map.

  [Step S2] By detecting the MR signal with the element coil selected in Step S1, a positioning image is picked up by a known method. The display control unit 98 displays the positioning image on the display device 64 in accordance with an instruction from the MPU 86.

  [Step S3] Based on the positioning image, information for setting an imaging field such as an imaging field of view and a slice position is input to the input device 62, and the MPU 86 sets the input content as a condition for the imaging sequence of the main scan. To do. Thereby, the imaging area in the main scan is determined. The distortion correction data calculation unit 104 starts the calculation of distortion correction data using the determination of the imaging area in the main scan as a trigger. That is, the distortion correction data calculation unit 104 starts calculating distortion correction data in synchronization with the timing when the imaging area in the main scan is determined.

  [Step S4] The MRI apparatus 20 executes pre-scanning and distortion correction data calculation by the distortion correction data calculation unit 104 in parallel. The pre-scan here includes, for example, a sequence for calculating a correction value of the center frequency of the RF pulse (see, for example, JP 2009-34152 A).

  Specifically, the MPU 86 controls each part of the MRI apparatus 20 to collect and analyze frequency spectrum data by applying magnetic resonance spectroscopy to, for example, a representative slice of the imaging region, and to generate hydrogen based on the peak frequency and the like. The magnetic resonance frequency of the nuclear spin is detected.

[Step S5] The MPU 86 calculates and resets (corrects) the center frequency of the RF pulse used in the main scan based on the detection result of the prescan in Step S4. As a specific example, the magnetic resonance frequency of the detected hydrogen nucleus spin may be used as the center frequency of the RF pulse.
In this way, the MPU 86 confirms all unconfirmed conditions in the imaging sequence conditions of the main scan based on the execution result of the pre-scan. At this time, calculation of distortion correction data by the distortion correction data calculation unit 104 is also executed in parallel.

  [Step S6] The main scan is executed as follows according to the set conditions of the imaging sequence, and the distortion correction data calculation unit 104 calculates distortion correction data in parallel. Specifically, in the imaging space, a static magnetic field is formed by the static magnetic field magnet 22 excited by the static magnetic field power supply 40, and the static magnetic field is made uniform by the shim coil 24.

  When a main scan start command is input from the input device 62 to the MPU 86, the MPU 86 inputs imaging sequence conditions to the sequence controller 56. The sequence controller 56 drives the gradient magnetic field power supply 44, the RF transmitter 46, and the RF receiver 48 according to the input imaging sequence, thereby forming a gradient magnetic field in the imaging region including the imaging region of the subject P, and An RF pulse is generated from the RF coil 28.

  Therefore, the MR signal generated by the nuclear magnetic resonance inside the subject P is received by the RF coil 28 (including the element coil selected in step S1) and detected by the RF receiver 48. The RF receiver 48 performs predetermined signal processing on the detected MR signal, and A / D converts this to generate raw data that is a digitized MR signal.

  The RF receiver 48 inputs the generated raw data to the sequence controller 56. The sequence controller 56 inputs the raw data to the Fourier transform unit 100, and the Fourier transform unit 100 arranges and stores the raw data as k-space data in the k-space formed in the k-space database 110.

  Here, as an example, the distortion correction data calculation unit 104 calculates distortion correction data even during a period of waiting for the main scan start command in step S6 after all conditions for the main scan in step S5 are determined.

  [Step S7] An image reconstruction process is executed. Specifically, the Fourier transform unit 100 performs transformation processing (for example, two-dimensional Fourier transformation) on k-space data of each slice or slab of the subject P, thereby transforming into real space data. The Fourier transform unit 100 stores real space data in the image database 102.

  Here, if a part of the distortion correction data has not been calculated up to step S6, the distortion correction unit 108 calculates an uncalculated portion of the distortion correction data before the correction process in step S7. All the distortion correction data are stored in the distortion correction data storage unit 106. Then, the distortion correction unit 108 corrects image distortion due to the nonlinearity of the gradient magnetic field by performing correction processing based on the distortion correction data on the real space data generated by the Fourier transform unit 100.

  When a sensitivity distribution map generation sequence of each element coil in the RF coil device is executed as a pre-scan, the distortion correction unit 108 performs image data based on the three-dimensional sensitivity distribution map data as part of the correction process. The brightness correction is also executed. The distortion correction unit 108 stores the corrected image data (volume data in the case of slab) in the storage device 66.

  [Step S8] The display control unit 98 transfers the corrected image data from the storage device 66 to the display device 64 in accordance with an instruction from the MPU 86, and displays the image indicated by the image data (volume data in the case of slab). To display.

  In the above description, the distortion correction data is continuously calculated from the determination of the imaging area of the main scan until the execution period of the main scan, but this is only an example. For example, when the calculation of the distortion correction data is completed at an intermediate timing such as during the pre-scan, the distortion correction data calculation unit 104 does not calculate the distortion correction data in parallel with other processes. The above is the description of the operation of the first embodiment.

  In the prior art, the main scan is completed, and the calculation of distortion correction data is started after the start timing of image reconstruction. On the other hand, in the first embodiment, after the imaging area is determined in the main scan, the distortion correction data calculation unit 104 calculates distortion correction data in parallel with other processes such as pre-scan and main scan. For this reason, all or part of the distortion correction data has already been calculated at the timing when the main scan is completed (timing for starting image reconstruction including Fourier transform). As a result, the time required for correcting image distortion after the main scan can be shortened as compared with the prior art.

  In addition, the MRI apparatus 20 executes the distortion correction data calculation by the distortion correction data calculation unit 104 and other processes such as pre-scanning and main scanning in parallel using the free memory of the arithmetic unit 60. Accordingly, condition setting time and imaging time can be effectively allocated as the operation time of each unit of the MRI apparatus 20.

  Since the determination of the imaging area in the main scan is used as a trigger, the calculation of the distortion correction data is started from the earliest timing at which the distortion correction data can be determined. For this reason, it is possible to increase the possibility that all the distortion correction data has been calculated at the end timing of the main scan. As a result, in the image reconstruction process after the end of the main scan, it may not be necessary to calculate the distortion correction data, and the time required for correcting the image distortion can be greatly shortened.

(Modification of the first embodiment)
FIG. 8 is a flowchart showing an example of the operation flow of the MRI apparatus 20 according to the modification of the first embodiment. When the calibration scan is executed after the main scan and the calculation of the distortion correction data is not completed at the end of the main scan, the MRI apparatus 20 calculates the distortion correction data in parallel with the calibration scan after the main scan. May be executed. FIG. 8 shows an example of the operation of the MRI apparatus 20 in that case.

  The processes in steps S11 to S16, S18, and S19 in FIG. 8 are the same as the processes in steps S1 to S8, respectively, except that the generation sequence of the sensitivity distribution map of each element coil in the RF coil device is not executed in step S11. It is.

  In step S17, in parallel with the calibration scan, the distortion correction data calculation unit 104 calculates distortion correction data. The calibration scan that may be executed after the main scan (step S17) is data collection for determining the conditions of image reconstruction after the main scan, regardless of the conditions of the main scan. As such a calibration scan, for example, a generation sequence of a sensitivity distribution map of each element coil in the RF coil apparatus executed in step S1 of FIG.

Thus, also in the modification of FIG. 8, the same effect as 1st Embodiment demonstrated in FIG. 7 is acquired.
In the first embodiment and the modifications thereof, the example in which the calculation of the distortion correction data is started in synchronization with the determination of the imaging area in the main scan is described, but this is only an example. Of the conditions of the main scan, the determination of the condition that is determined after the imaging region is determined may be used as a trigger. For example, the calculation of the distortion correction data may be started in synchronization with the determination of the gradient magnetic field application condition in the main scan.

(Description of operation of the second embodiment)
In the first embodiment, a plurality of processes are executed in parallel. In the second and third embodiments, each process is executed serially. In the second embodiment, distortion correction data is reused when images of the same position and the same field of view are repeatedly captured in one sequence.

  FIG. 9 is a flowchart illustrating an example of an operation flow of the MRI apparatus 20 in the second embodiment. The operation of the MRI apparatus 20 will be described below according to the step numbers shown in FIG.

  [Step S21] The MPU 86 (see FIG. 2) performs the imaging sequence of the main scan based on the conditions of a part of the imaging sequence input to the MRI apparatus 20 via the input device 62 and the execution result of the pre-scan. Set all the conditions. Here, as an example, it is assumed that a pulse sequence of the main scan for repeatedly imaging the same position is set. An example of such a pulse sequence is synchronous imaging.

  As synchronous imaging, for example, electrocardiographic synchronization imaging can be mentioned. In this case, for example, the blood flowing into the imaging section is labeled by the t-SLIP method (see Time Spatial Labeling Inversion Pulse Method: Japanese Patent Application Laid-Open No. 2011-83592). With the pulse sequence of the t-SLIP method, it is possible to selectively enhance or suppress the signal intensity of only the blood that has reached the imaging section after the inversion time. A pulse in the t-SLIP method is applied after a certain delay time has elapsed from the R wave of the ECG signal detected from the subject P by the ECG unit 36 (see FIG. 1) as necessary. Imaging is performed under.

  As another example of the synchronous imaging, there is a case where a respiratory synchronization unit is provided instead of the ECG unit 36 and respiratory synchronous imaging is performed. The respiratory synchronization unit has a respiratory sensor (electrode) (not shown) that detects a signal that is in contact with the chest of the subject P and is proportional to the rib cage motion. The respiration synchronization unit calculates a respiration curve data from the detection signal of the respiration sensor, thereby generating a respiration synchronization signal synchronized with a desired period (for example, exhalation period) of the subject P, and this respiration synchronization signal Is input to the sequence controller 56. Thereby, for example, imaging in the intake time phase or the exhalation time phase can be repeated.

Another example of a pulse sequence that repeatedly images the same position is diffusion-weighted imaging. In diffusion weighted imaging, for example, an MPG pulse (Motion Probing Gradient Pulse) is not applied, and a reference set of slices is imaged with a b value of zero.
Next, imaging of a plurality of sets of slice groups is performed while the MPG pulse having a different directionality is applied for each set while the b value is set to the first value. Next, the b value is set to the second value, and a plurality of sets of slice groups are imaged while applying MPG pulses having different directions for each set.
Such imaging is repeated, and a difference is taken between a reference set of slice groups having a b value of zero and each set of slice groups having a b value of the first value. Similarly, when the b value is the second value and the b value is the third value, the diffusion of the imaging target is depicted by taking the difference.

  Another example of a pulse sequence that repeatedly images the same position is dynamic imaging. Dynamic imaging is a technique of performing multiple imaging on the same cross section by changing the time phase such as the respiratory phase and the cardiac time phase.

  [Step S22] The main scan is executed in the same manner as in Step S6 of the first embodiment, according to the set conditions of the imaging sequence. Here, as an example, the calculation of distortion correction data is not executed in parallel.

  [Step S23] The Fourier transform unit 100 takes in k-space data from the k-space database 110, and performs Fourier transform on the k-space data to convert it into real space data.

  [Step S24] The distortion correction data calculation unit 104 calculates or acquires distortion correction data for each slice or slab from which MR signals have been collected in the main scan. Further, the distortion correction data calculation unit 104 adds the incidental information of the distortion correction data for which slice or slab, and stores the distortion correction data in the distortion correction data storage unit 106. The supplementary information is information that defines the imaging region of the slice or slab in the apparatus coordinate system, such as the imaging field of view, slice thickness, and cross-sectional direction.

  In addition, since the main scan for repeatedly imaging the same position is executed here, the distortion correction data calculation unit 104 avoids duplicate execution when calculating the distortion correction data.

Specifically, when the distortion correction data has already been calculated for the slice or slab whose imaging region defined in the apparatus coordinate system is the same as “the slice or slab that is currently subject to calculation of the distortion correction data”, The distortion correction data calculation unit 104 acquires the calculated distortion correction data.
The distortion correction data calculation unit 104 adds the above-mentioned “slice or slab that is the current calculation target of the distortion correction data” as supplementary information of the calculated distortion correction data.
As a result, the distortion correction data calculation unit 104 updates the accompanying information of the distortion correction data stored in the distortion correction data storage unit 106.

  In this way, the distortion correction data calculation unit 104 avoids overlapping calculation of distortion correction data. Note that the distortion correction data itself is not stored redundantly in the storage area of the distortion correction data storage unit 106.

  Here, the identity of the subject is not necessary as a condition for avoiding redundant calculation as described above, that is, a condition for reusing the distortion correction data (the same applies to the third embodiment described later). This is because the uniformity of the static magnetic field varies depending on the subject, but if the imaging region is the same, the generated gradient magnetic field (nonlinearity) is substantially the same regardless of the subject. Therefore, if distortion correction data has already been calculated for slices or slabs having the same imaging region as the imaging of the same or different subject in the sequence before step S21, the distortion correction data can be reused. is there.

  In addition, as a condition for avoiding overlapping calculation as described above, it is desirable that the slice direction to be calculated for the distortion correction data and the slice for which the distortion correction data has been calculated have the same cross-sectional direction, which is the second embodiment. As an example, this is the condition. This is because the distortion correction data can be reused as it is if the cross-sectional directions completely match.

  Here, as an example, if distortion correction data has not been calculated for a slice or slab whose distortion correction data is to be calculated and a slice or slab in which the imaging area defined in the apparatus coordinate system is completely the same, distortion correction data calculation is performed. The unit 104 calculates distortion correction data. However, when the distortion correction data has already been calculated for the slice or slab to be calculated for the distortion correction data and the slice or slab having the overlapping portion of the imaging region, the distortion correction data calculation unit 104 determines the distortion of the overlapping portion. The correction data may be reused (this example will be described later).

  [Step S25] The distortion correction unit 108 performs correction processing based on the distortion correction data calculated or acquired in Step S24 on the real space data generated in Step S23, thereby causing non-linearity of the gradient magnetic field. Correct image distortion. The distortion correction unit 108 stores the corrected image data in the storage device 66.

  [Step S26] The captured image after correction processing is displayed in the same manner as in step S8 of the first embodiment. The above is the description of the operation of the MRI apparatus 20 of the second embodiment.

  As described above, in the second embodiment, when the same position is repeatedly imaged in one imaging sequence, such as synchronous imaging, dynamic imaging, and diffusion weighted imaging, redundant calculation of distortion correction data is avoided. That is, the distortion correction data calculated once for a slice or slab of a certain imaging region is stored. As a result, the distortion correction data is reused when the same slice or slab is imaged while changing the timing and other conditions, so that the calculation time of the distortion correction data can be shortened compared to the prior art. As a result, the correction of image distortion due to the nonlinearity of the gradient magnetic field can be speeded up as compared with the prior art.

  In addition, reuse correction data that is reused is not stored for each slice or slab for which the selection method is nearly infinite, but is minimally corrected using the characteristics of a sequence that repeatedly images the same position. Data is saved. That is, in step S24, the distortion correction data is not redundantly stored, and “information on the imaging region of the slice or slab corresponding to the distortion correction data” is added as the accompanying information. Therefore, the storage area required for storing distortion correction data can be within a realistic range.

(Modification of the second embodiment)
FIG. 10 is a flowchart illustrating an example of an operation flow of the MRI apparatus 20 according to the modification of the second embodiment. In step S24 of FIG. 9 described above, an example is described in which the determination process for determining whether or not the distortion correction data can be reused is performed. However, such a determination process may be inserted as shown in the flowchart of FIG. Hereinafter, the operation of the MRI apparatus 20 in the modification of the second embodiment will be described according to the step numbers shown in FIG.

  [Steps S31 to S33] Steps S21 to S23 in FIG. Thereafter, the process proceeds to step S34.

  [Step S <b> 34] The distortion correction data calculation unit 104 performs processing for “slice or slab that is currently subject to calculation of distortion correction data” and a slice or slab having the same imaging area (defined in the apparatus coordinate system). It is determined whether or not the distortion correction data has been calculated. If distortion correction data has been calculated for slices or slabs having the same imaging area, the process proceeds to step S35, and if not, the process proceeds to step S36.

  [Step S35] As in step S24, the distortion correction data calculation unit 104 calculates “current distortion correction data calculation target” as supplementary information of distortion correction data that has been calculated and has the same imaging region. Add a slice or slab. Thereafter, the process proceeds to step S37.

  [Step S36] The distortion correction data calculation unit 104 calculates distortion correction data for the slice or slab to be subjected to the determination process in step S34. The distortion correction data calculation unit 104 stores the calculated distortion correction data in the distortion correction data storage unit 106 along with the information of which slice or slab the distortion correction data. Thereafter, the process proceeds to step S37.

  [Steps S37 and S38] Steps S25 and S26 of FIG. Thus, also in the modification shown in FIG. 10, the same effect as in the case of FIG. 9 is obtained.

(Description of operation of the third embodiment)
As described above, when distortion correction data has already been calculated for slices or slabs for which distortion correction data is to be calculated and slices or slabs having overlapping portions of the imaging area, the distortion correction data for the overlapping portions is reused. May be. In the third embodiment, the distortion correction data is reused if there is an overlapping region regardless of the sequence type in the case where the same position is repeatedly imaged.

FIG. 11 is a schematic plan view illustrating an example where there are overlapping imaging regions between a plurality of images. In FIG. 11, in the slice SL <b> 1 and the slice SL <b> 2, the mutual imaging areas partially overlap, and the hatched area surrounded by a broken line is the overlapping area 250. Here, “overlap” means that at least some of the pixels in the slice SL1 and at least some of the pixels in the slice SL2 indicate the same position in the apparatus coordinate system.
For example, consider a case where distortion correction data is generated first for the slice SL1 and stored in the distortion correction data storage unit 106.

  In this case, the distortion correction data calculation unit 104 calculates distortion correction data for the non-overlapping region (imaging region other than the overlapping region 250) in the slice SL2, and acquires the distortion correction data of the overlapping region 250 from the distortion correction data storage unit 106. To do. For the slice SL2, the distortion correction data calculation unit 104 generates the distortion correction data by combining the distortion correction data of the non-overlapping area and the acquired distortion correction data of the overlapping area 250 regionally.

  In particular, when the imaging region is a slab (in the case of volume imaging), there are many cases where a slab that is subject to calculation of distortion correction data and a slab for which distortion correction data has already been calculated partially overlap. In that case, the distortion correction data can be reused for the overlapping region.

  As described above, as a condition for avoiding overlap calculation, it is desirable that the slice direction to be calculated for the distortion correction data and the slice for which the distortion correction data has been generated have the same cross-sectional direction, but this is not essential. Even if the slice direction from which the distortion correction data is calculated differs from the slice in which the distortion correction data has been generated, if there is an overlapping area, the distortion correction data may be reused for the overlapping area.

In the third embodiment, distortion correction data between a plurality of different types of imaging sequences and distortion correction data before the MRI apparatus 20 is turned on can be reused. As a condition for this reuse, the identity of the subject is not necessary for the reason described above.
That is, even when another subject is imaged in the immediately preceding imaging sequence or when another subject is imaged before the power is turned on, if there is an overlapping region, the distortion correction data is reused for the overlapping region.

  Therefore, in the third embodiment, as an example, even when the power of the MRI apparatus 20 is turned off, the distortion correction data is continuously stored. On the other hand, in the second embodiment, the distortion correction data in the distortion correction data storage unit 106 may be deleted when the power of the MRI apparatus 20 is turned off.

  FIG. 12 is a block diagram in the case of storing distortion correction data in an external server as an example of storing distortion correction data outside the MRI apparatus 20. In the third embodiment, for example, as illustrated in FIG. 12, all distortion correction data is stored in the server 302 connected to the arithmetic device 60 of the MRI apparatus 20 via the communication cable 300.

  That is, in the third embodiment, the distortion correction data storage unit 106 stores only the distortion correction data generated after the power of the MRI apparatus 20 is turned on, and the generated distortion correction data is stored in the power supply of the MRI apparatus 20. Transfer to server 302 before turning off. However, this is only an example. If the storage capacity of the distortion correction data storage unit 106 is large, the distortion correction data storage unit 106 may continuously store the distortion correction data regardless of whether the power of the MRI apparatus 20 is turned off.

  FIG. 13 is a flowchart illustrating an example of an operation flow of the MRI apparatus 20 according to the third embodiment. Here, as an example, after the power of the MRI apparatus 20 is turned on, another main scan having a different pulse sequence type from the main scan in step S42 in FIG. 13 has been executed immediately before step S41. To do. That is, image data after correction processing for each slice (or slab) of the other main scan is stored in the storage device 66, and distortion correction data for each slice (or slab) of the other main scan is stored as distortion correction data. It is assumed that the data is stored in the unit 106.

The operation of the MRI apparatus 20 will be described below according to the step numbers shown in FIG.
[Step S41] The MPU 86 (see FIG. 2) determines all the conditions of the main scan based on part of the imaging sequence conditions input to the MRI apparatus 20 via the input device 62 and the execution result of the pre-scan. Set. The main scan here may be any kind of pulse sequence, but here, as an example, the type is different from the pulse sequence executed immediately before.

  [Step S42] According to the imaging sequence conditions set in step S41, the main scan is executed in the same manner as described above.

  [Step S43] The Fourier transform unit 100 takes in k-space data from the k-space database 110 and performs Fourier transform on the k-space data to convert it into real space data.

  [Step S44] The distortion correction data calculation unit 104 accesses the distortion correction data storage unit 106 and the server 302, and executes the following determination processing. That is, it is determined whether or not there is an overlap area between the imaging area of the slice (slab) for which distortion correction data is calculated and the imaging area of the slice (slab) for which distortion correction data has already been calculated (apparatus coordinate system). The presence or absence of duplication is determined between the imaging areas defined in (1).

  The calculated distortion correction data here is calculated in the imaging sequence before step S41 and stored in the distortion correction data storage unit 106, and before the MRI apparatus 20 is turned on (MRI before the previous time). Calculated when the apparatus 20 is started up) and stored in the server 302.

  If it is determined in the determination process that there is an overlapping area, the process proceeds to step S45, and if not, the process proceeds to step S46.

  [Step S45] The distortion correction data calculation unit 104 calculates distortion correction data of a non-overlapping region (a region excluding the region determined to be an overlapping region in step S44) for a slice (slab) that is a target of distortion correction data calculation. The distortion correction data of the overlapping area is acquired from the distortion correction data storage unit 106 or the server 302. The distortion correction data calculation unit 104 generates the distortion correction data for the slice (slab) to be calculated by combining the calculated distortion correction data and the acquired distortion correction data in a region. Thereafter, the process proceeds to step S47.

  [Step S46] The distortion correction data calculation unit 104 calculates distortion correction data for the slice (slab) to be determined in step S44. The distortion correction data calculation unit 104 adds information defining an imaging region in the apparatus coordinate system of the slice or slab as incidental information, and stores the calculated distortion correction data in the distortion correction data storage unit 106. Here, as an example, information on the subject P is also included in the incidental information. Thereafter, the process proceeds to step S47.

  [Step S47] The distortion correction unit 108 performs correction processing based on the distortion correction data obtained in steps S45 and S46 on the real space data generated in step S43, thereby causing non-linearity of the gradient magnetic field. Correct image distortion. The distortion correction unit 108 stores the corrected image data in the storage device 66.

  Further, the distortion correction data storage unit 106 transfers the distortion correction data calculated and stored in steps S41 to S46 to the server 302. The server 302 stores the transferred distortion correction data (and its accompanying information). Thereafter, the process proceeds to step S48.

  [Step S48] The captured image after the correction processing is displayed in the same manner as in step S8 of the first embodiment. The above is the description of the operation of the MRI apparatus 20 of the third embodiment.

As described above, in the third embodiment, when there is any overlap area with the previously calculated distortion correction data, the distortion correction data is reused for the overlap area.
Conceptually speaking, when at least some pixels of “one image” and at least some pixels of “another image” indicate the same position in the apparatus coordinate system, the image reconstruction unit 90 When generating the image data indicating the “one image”, the distortion correction data used when generating the image data indicating the “different image” is used for the pixels indicating the same position, and the remaining pixels. For, correction processing is executed by calculating distortion correction data.

That is, regardless of the identity of the subject and the type of the pulse sequence of the main scan, the distortion correction data is reused for the overlapping region, so that the calculation amount of the distortion correction data can be minimized. As a result, the time required to correct the image distortion due to the nonlinearity of the gradient magnetic field can be shortened compared to the conventional case.
For example, as imaging for follow-up observation of the same subject, there may be a case where the subject is reexamined with the same imaging region as the region that was previously imaged (such as one year ago or six months ago). In such a case, the time required for the correction process can be shortened by reusing the distortion correction table stored in the server 302, which is effective.

(Supplementary items of the embodiment)
[1] In the second embodiment, the example has been described in which the distortion correction data used in the image reconstruction processing of each image data is stored in the MRI apparatus 20 (distortion correction data storage unit 106). The embodiment of the present invention is not limited to such an aspect. For example, distortion correction data may be continuously stored outside the MRI apparatus 20 as in the third embodiment.

  [2] In the second and third embodiments, the example in which each process is executed serially has been described. The embodiment of the present invention is not limited to such an aspect. The first embodiment may be combined with the second embodiment. That is, in step S21 of the second embodiment, calculation of distortion correction data is started (as a process executed in parallel with other processes such as the main scan) with the determination of the imaging region in the main scan as a trigger. Also good.

  Similarly, the first embodiment and the third embodiment may be combined. That is, in step S41 of the third embodiment, the calculation of distortion correction data may be started (as a process executed in parallel with other processes) with the determination of the imaging region in the main scan as a trigger.

  [3] As an example of the MRI apparatus 20, the example in which the RF receiver 48 exists outside the gantry including the static magnetic field magnet 22, the shim coil 24, the gradient magnetic field coil 26, and the RF coil 28 has been described (see FIG. 1). This is only an example, and the RF receiver 48 may be included in the gantry.

  Specifically, for example, an electronic circuit board corresponding to the RF receiver 48 is arranged in the gantry. Then, the MR signal converted from the electromagnetic wave to the analog electric signal by the receiving RF coil may be amplified by the preamplifier in the electronic circuit board, output as a digital signal outside the gantry, and input to the sequence controller 56. . For output to the outside of the gantry, for example, it is desirable to transmit as an optical digital signal using an optical communication cable, because the influence of external noise is reduced.

[4] A correspondence relationship between the terms of the claims and the embodiments will be described. In addition, the correspondence shown below is one interpretation shown for reference, and does not limit the present invention.
The entire static magnetic field magnet 22, shim coil 24, gradient magnetic field coil 26, RF coil 28, and control device 30 (see FIG. 1) receive MR signals from the subject P by scanning with application of gradient magnetic fields and transmission of RF pulses. The collecting function is an example of the data collecting unit described in the claims.

  The whole of the static magnetic field magnet 22, the shim coil 24, the gradient magnetic field coil 26, the RF coil 28, and the control device 30 (see FIG. 1) collects MR signals with the application of the gradient magnetic field and the RF pulse, and executes the pre-scan. The configuration is an example of a prescan execution unit described in claims.

  [5] Although some 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 implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and the equivalents thereof.

20 MRI apparatus 22 Static magnetic field magnet 24 Shim coil 26 Gradient magnetic field coil 26x X-axis gradient magnetic field coil 26y Y-axis gradient magnetic field coil 26z Z-axis gradient magnetic field coil 28 RF coil 30 Control device 32 Bed 34 Top plate 36 ECG unit 40 Static magnetic field power source 42 Shim coil power supply 44 Gradient magnetic field power supply 44x X axis gradient magnetic field power supply 44y Y axis gradient magnetic field power supply 44z Z axis gradient magnetic field power supply 46 RF transmitter 48 RF receiver 56 Sequence controller 58 Computer 60 Computing device 62 Input device 64 Display device 66 Storage device 86 MPU
88 System bus 90 Image reconstruction unit 98 Display control unit 100 Fourier transform unit 102 Image database 104 Distortion correction data calculation unit 106 Distortion correction data storage unit 108 Distortion correction unit 110 k-space database 140 Upper body RF coil device 142 Cover member 144 146 Element coil 174 Duplexer 176 Amplifier 178 Switching synthesizer 180 Reception system circuit 250 Overlapping area 300 Communication cable 302 Server P Subject WB Whole body coil

Claims (13)

As the main scan, by applying a gradient magnetic field to the imaging region and transmitting an excitation pulse, a data collecting unit that collects the nuclear magnetic resonance signal from the imaging region and stores the nuclear magnetic resonance signal as frequency space data;
By performing a conversion process for converting the frequency space data into real space data and a correction process based on distortion correction data that defines a method for correcting image distortion due to nonlinearity of the gradient magnetic field, image data A magnetic resonance imaging apparatus comprising: an image reconstruction unit that generates
The image reconstruction unit starts calculation of the distortion correction data after the condition used for calculation of the distortion correction data among the conditions of the main scan is determined and before execution of the conversion process. Magnetic resonance imaging device. The magnetic resonance imaging apparatus according to claim 1, wherein the image reconstruction unit starts calculation of the distortion correction data based on the imaging region after the imaging region is determined. The magnetic resonance imaging apparatus according to claim 2, wherein the image reconstruction unit starts calculation of the distortion correction data in synchronization with a timing when the imaging region is determined. The magnetism according to any one of claims 1 to 3, wherein the calculation of the distortion correction data by the image reconstruction unit and a process other than the calculation of the distortion correction data are executed in parallel. Resonance imaging device. 5. The magnetic resonance imaging apparatus according to claim 4, wherein the image reconstruction unit calculates the distortion correction data during a period in which the main scan is executed by the data collection unit. The image reconstruction unit calculates the distortion correction data during a period in which a pre-scan for collecting the nuclear magnetic resonance signals used for determining a partial condition of the main scan is being executed. Item 5. The magnetic resonance imaging apparatus according to Item 4. As a pre-scan, further comprising a pre-scan execution unit that collects the nuclear magnetic resonance signals used to determine a partial condition of the main scan before the main scan,
5. The magnetic resonance imaging apparatus according to claim 4, wherein the image reconstruction unit calculates the distortion correction data in a period after the execution of the pre-scan and before the execution of the main scan. Data that collects a nuclear magnetic resonance signal that is an image generation source from the imaging region by applying a gradient magnetic field to the imaging region and transmitting an excitation pulse, and stores the collected nuclear magnetic resonance signal as frequency space data A collection department;
The frequency space data is subjected to a conversion process for converting to real space data and a correction process based on distortion correction data that defines a method for correcting image distortion due to the nonlinearity of the gradient magnetic field. A magnetic resonance imaging apparatus comprising: an image reconstruction unit that generates image data indicating
When at least part of the imaging region of one image and the imaging region of another image overlap in the apparatus coordinate system, the image reconstruction unit is configured to generate image data indicating the one image. The magnetic resonance imaging apparatus, wherein the correction processing is executed using the distortion correction data used when generating image data indicating the other image. The image reconstruction unit is configured to calculate the distortion correction data based on the imaging region and store the distortion correction data outside or inside the magnetic resonance imaging apparatus,
When the same imaging region is repeatedly imaged in one imaging sequence, the image reconstruction unit uses the common distortion correction data when generating image data of the same imaging region. Item 9. The magnetic resonance imaging apparatus according to Item 8. The magnetic resonance imaging apparatus according to claim 9, wherein one of synchronous imaging, dynamic imaging, and diffusion weighted imaging is executed as the one imaging sequence. The image reconstruction unit determines whether there is an overlapping portion of the imaging region in the apparatus coordinate system based on the image data to be generated and the image data generated in the past, and the determination result is positive 9. The method according to claim 8, wherein the distortion correction data used when generating the image data generated in the past is acquired, and the correction processing is executed using the acquired distortion correction data. Magnetic resonance imaging equipment. The data collection unit collects the nuclear magnetic resonance signal as a first main scan and stores it as frequency space data, and then performs the nuclear magnetic resonance as a second main scan having a pulse sequence different from that of the first main scan. Collect the signal and store it as frequency space data,
The magnetism according to claim 11, wherein the image reconstruction unit determines whether or not there is an overlapping portion between the imaging region of the first main scan and the imaging region of the second main scan. Resonance imaging device. By applying a gradient magnetic field and transmitting an excitation pulse, a nuclear magnetic resonance signal that is an image generation source is collected from a subject, and the collected nuclear magnetic resonance signal is stored as frequency space data; and
The frequency space data is subjected to a conversion process for converting to real space data and a correction process based on distortion correction data that defines a method for correcting image distortion due to the nonlinearity of the gradient magnetic field. A magnetic resonance imaging apparatus comprising: an image reconstruction unit that generates image data indicating
When at least some of the pixels in one image and at least some of the pixels in another image indicate the same position in the apparatus coordinate system, the image reconstruction unit generates image data indicating the one image In some cases, the distortion correction data used when generating the image data indicating the different image is used for the pixels indicating the same position, and the distortion correction data is calculated for the remaining pixels. The correction processing is executed. A magnetic resonance imaging apparatus, wherein:

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