JP2010119740A - Magnetic resonance imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic resonance imaging apparatus, collecting images of high image quality over an enough wide imaging range without moving a top plate of a bed even when satisfactory linearity is not obtained in a gradient magnetic field. <P>SOLUTION: This magnetic resonance imaging apparatus includes: a plan image collecting means for collecting plan image data for setting a slice exciting for imaging scan to collect a magnetic resonance signal from a subject; a slice setting means for correcting distortion due to non-linearity of the gradient magnetic field in the plan image data, displaying the distortion-corrected plan image (a), and setting two-dimensional or three-dimensional slice (b) for the imaging scan utilizing the distortion-corrected plan image (a); and an imaging means for collecting the magnetic resonance signal by performing imaging scan with excitation of the two-dimensional or three-dimensional slice (b) and generating image data based on the collected magnetic resonance signal. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention magnetically excites a subject's nuclear spin with a radio frequency (RF) signal of a Larmor frequency, and re-images the nuclear magnetic resonance (NMR) signal generated by this excitation. The present invention relates to a magnetic resonance imaging (MRI) apparatus, and more particularly to a magnetic resonance imaging apparatus capable of exciting a distortion-corrected slice.

  Magnetic resonance imaging is an imaging method in which a nuclear spin of a subject placed in a static magnetic field is magnetically excited with an RF signal having a Larmor frequency, and an image is reconstructed from an MR signal generated by the excitation.

  In the magnetic resonance imaging apparatus, there is a problem that distortion is generated in an imaging cross section that is an object of image or slice excitation due to the influence of the nonlinearity of the gradient magnetic field formed in the imaging region. In particular, image distortion can be corrected as post-processing after data collection based on gradient magnetic field nonlinearity data, while slice plane distortion is corrected unless three-dimensional data is collected. Inability to do so is a problem.

  For this reason, a magnetic resonance imaging apparatus having a function of estimating the distribution of the degree of image quality degradation based on the accuracy of the gradient magnetic field has been devised (see, for example, Patent Document 1).

  Conventionally, a gradient coil has been designed so that a gradient magnetic field having a wide linearity region can be generated. In addition, the strength of the gradient magnetic field and the slew rate (rise characteristics) are important factors that determine the image quality, and high performance is required for both.

  However, when a gradient magnetic field coil having such high performance and high linearity is designed, the number of turns and weight of the wire of the gradient magnetic field coil increase, leading to an increase in unit price and heat generation during imaging. Furthermore, there is a problem that the axial length of the gradient magnetic field coil becomes long and the opening of the opening of the static magnetic field magnet is impaired. In recent years, in order to improve the openness of the magnet, the magnet tends to have a short axis or the opening can be widened.

Against this background, the gradient magnetic field coils in recent years tend to be shortened along with the magnets.
JP 2007-325665 A

  However, if the gradient magnetic field coil is shortened in order to increase the opening of the magnet as much as possible to improve the openness, the linearity of the gradient magnetic field in the Z-axis direction (center axis direction) deteriorates. When imaging is performed in a region where the linearity of the gradient magnetic field is insufficient, the slice surface to be excited is distorted, leading to deterioration of image quality. Moreover, as described above, the distortion of the slice plane cannot be corrected unless three-dimensional data is collected. For this reason, there is a problem that if the gradient coil is shortened, the imaging region in the Z-axis direction becomes narrow.

  On the other hand, there is a technique for securing an imaging region in the Z-axis direction by performing imaging while moving the couch top. However, this technique has a problem that the imaging time becomes long because it is necessary to repeatedly perform the same imaging by moving the top plate.

  The present invention has been made in order to cope with such a conventional situation, and even when sufficient linearity is not obtained in the gradient magnetic field, it is possible to perform imaging over a sufficiently wide range without moving the couch top. An object of the present invention is to provide a magnetic resonance imaging apparatus capable of collecting high-quality images.

  In order to achieve the above object, a magnetic resonance imaging apparatus according to the present invention collects plan image data for setting up a slice to be excited for an imaging scan for collecting magnetic resonance signals from a subject. And a distortion correction due to the non-linearity of the gradient magnetic field of the plan image data is performed to display a distortion corrected plan image, and the distortion corrected plan image is used for two-dimensional imaging scan. Alternatively, the magnetic resonance signal is collected by executing an imaging scan with excitation of the two-dimensional or three-dimensional slice, and a slice setting means for setting a three-dimensional slice. And imaging means for generating image data based on the image data.

  In the magnetic resonance imaging apparatus according to the present invention, even when sufficient linearity cannot be obtained in the gradient magnetic field, higher-quality images are collected over sufficiently wide imaging without moving the couch top. be able to.

  Embodiments of a magnetic resonance imaging apparatus according to the present invention will be described with reference to the accompanying drawings.

  1 is a block diagram showing an embodiment of a magnetic resonance imaging apparatus according to the present invention, FIG. 2 is a cross-sectional view of a gantry section shown in FIG. 1, and FIG. 3 is a transmission section and a transmission high-frequency array coil shown in FIG. FIG.

  The magnetic resonance imaging apparatus 1 includes a gantry unit 2 and a control system 3. The gantry unit 2 is provided with a bed 9 including a static magnetic field magnet 4, a gradient magnetic field coil 5, a transmission high frequency array coil 6, a reception high frequency array coil 7, and a top plate 8. The control system 3 includes a gradient magnetic field drive circuit 10, a transmission unit 11, a reception unit 12, a data collection unit 13, a sequence controller 14, a computer 15, a console 16, and a display 17.

  The static magnetic field magnet 4 has a cylindrical shape, and a cylindrical gradient magnetic field coil 5 is coaxially disposed inside the inner wall of the static magnetic field magnet 4. A transmission high-frequency array coil 6 is arranged inside the gradient magnetic field coil 5, and an imaging region is formed inside the transmission high-frequency array coil 6. The top plate 8 of the bed 9 is provided in the imaging region so as to be movable in the Z-axis direction. A subject is set on the top board 8. A receiving high-frequency array coil 7 is disposed around the subject.

  The static magnetic field magnet 4 has a function of applying a uniform static magnetic field to the subject. The gradient magnetic field coil 5 has a gradient magnetic field in which the magnetic field strength of the subject linearly changes in the X, Y, and Z directions according to the drive signal output from the gradient magnetic field drive circuit 10 based on the control signal from the sequence controller 14. It has a function to apply Gx, Gy, Gz.

  The transmitting high-frequency array coil 6 includes a plurality of coil elements (for example, RF coils 1 to 8). FIG. 2 shows an example in which eight substantially rectangular RF coils are arranged in a cylindrical shape. Each RF coil of the transmission high-frequency array coil 6 has a function of applying a high-frequency magnetic field to the subject according to a high-frequency pulse output from the transmission unit 11 based on a control signal from the sequence controller 14.

  The reception high-frequency array coil 7 includes a plurality of surface coils (coil elements). Each surface coil is disposed at a position where a magnetic resonance signal from a desired imaging region of the subject can be collected. In the example shown in FIGS. 1 and 2, the receiving high-frequency array coil 7 is composed of two parts, an upper part and a lower part, for imaging a body part. That is, the receiving high-frequency array coils 7 are arranged above and below the subject so as to sandwich the subject. The receiving high-frequency array coil 7 has a function of receiving a magnetic resonance signal from the subject using a single or a plurality of surface coils and outputting the magnetic resonance signal to the receiving unit 12.

  The gradient magnetic field drive circuit 10 has a function of driving the gradient magnetic field coil 5 by applying a drive signal to the gradient magnetic field coil 5 in accordance with a control signal from the sequence controller 14.

  The transmission unit 11 has the same number of transmission channels as the number of RF coils of the transmission high-frequency array coil 6 that are used simultaneously for reception of magnetic resonance signals, that is, the number of transmission channels. FIG. 2 shows an example in which the transmission unit 11 and the transmission high-frequency array coil 6 are each provided with eight transmission channels. The transmission unit 11 has a function of generating a high-frequency magnetic field signal from each RF coil by applying a high-frequency pulse to each RF coil of the transmission high-frequency array coil 6 in accordance with a control signal from the sequence controller 14.

  For this purpose, the transmission unit 11 is provided with an amplitude phase control unit 20 and a transmission amplifier 21 for transmission channels. The input side of each amplitude phase control unit 20 is connected to the sequence controller 14, and the corresponding transmission amplifier 21 is connected to the output side. A corresponding transmission RF coil is connected to the output side of each transmission amplifier 21. Each amplitude phase control unit 20 has a function of generating a high frequency pulse by adjusting the amplitude and phase of the control signal output from the sequence controller 14 and a function of outputting the generated high frequency pulse to the corresponding transmission amplifier 21. Have. Each transmission amplifier 21 has a function of amplifying the intensity of the high-frequency pulse from the corresponding amplitude / phase control unit 20 to a predetermined intensity and outputting it to the corresponding RF coil.

  The reception unit 12 has a number of reception channels corresponding to the number of surface coils included in the reception high-frequency array coil 7. And the receiving part 12 received the magnetic resonance signal output to each receiving channel from the plurality of surface coils under the control of the sequence controller 14, and generated and received data by amplifying and detecting the received data. A function of outputting received data to the data collection unit 13;

  The data collection unit 13 has a function of collecting the reception data output from the reception unit 12 under the control of the sequence controller 14, performing A / D (analog to digital) conversion of the reception data, and outputting the data to the computer 15. .

  The sequence controller 14 gives control signals to the gradient magnetic field drive circuit 10, the transmission unit 11, the reception unit 12, and the data collection unit 13 according to the imaging condition information including the pulse sequence from the computer 15, whereby the gradient magnetic field drive circuit 10, The transmitter 11, the receiver 12, and the data collector 13 are controlled.

  The console 16 includes input devices such as a mouse, a keyboard, and keys. The console 16 has a function of inputting information to the computer 15. The display 17 has a function of displaying an image signal output from the computer 15.

  FIG. 4 is a functional block diagram of the computer 15 shown in FIG.

  The computer 15 functions as an imaging condition setting unit 30, a sequence controller control unit 31, a k-space database 32, an image reconstruction unit 33, an image database 34, and an image processing unit 35 according to a program. Note that a circuit having various functions may be provided in the magnetic resonance imaging apparatus 1 regardless of the program.

  The imaging condition setting unit 30 has a function of setting imaging conditions including a pulse sequence based on instruction information from the console 16 and providing the set imaging conditions to the sequence controller control unit 31. Therefore, the shooting condition setting unit 30 has a function of causing the display 17 to display a shooting condition setting screen. The imaging condition setting unit 30 includes a plan image condition setting unit 36, a plan image correction unit 37, and an excitation slice setting unit 38.

  The plan image condition setting unit 36 has a function of setting imaging conditions for collecting plan image data (pilot image data) of a slice cross section that intersects a slice to be excited in an imaging scan. Desirably, the slice cross section of the plan image data is set to a cross section orthogonal to the excitation slice cross section in the imaging scan. Further, preferably, the slice cross section of the plan image data is set to two or three cross sections orthogonal to each other. For example, it is effective to perform imaging of all or any one or two of the three cross sections of X = 0, Y = 0, and Z = 0 with little distortion of the slice plane. Further, multi-slice imaging of parallel sections may be performed.

  FIG. 5 is a diagram showing a plan image when the plan image data collected according to the photographing conditions set by the plan image condition setting unit 36 shown in FIG. 4 is displayed as it is.

  In FIG. 5, the vertical axis indicates the Y-axis direction, and the horizontal axis indicates the Z-axis direction.

  When the slice direction in the imaging scan is the Z-axis direction, for example, plan image data of a slice parallel to the YZ plane is collected. However, when the linearity of the gradient magnetic field in the YZ plane is insufficient, if the plan image data is displayed as it is, a plan image having a distortion as shown by a lattice is displayed as shown in FIG.

  On such a distorted plan image, a linear excitation slice in the Y-axis direction as shown by the alternate long and short dash line is set for the imaging scan, and slice selective excitation is performed by applying a gradient magnetic field only in the Z-axis direction. If it does, the slice actually excited will be distorted by the influence of the nonlinearity of a gradient magnetic field. Therefore, it is ideal to set a curved slice as indicated by a dotted line along the distortion of the plan image. For this purpose, it is necessary to perform two-dimensional slice selective excitation.

  In other words, when a linear excitation slice in the Y-axis direction is set for an imaging scan on a distorted plan image as shown in FIG. 5, a curved slice as shown by a dotted line is actually excited. Therefore, it is necessary to set a curved excitation slice so that a slice having linearity as shown by a one-dot chain line is actually excited, and to perform two-dimensional slice selective excitation.

  FIG. 5 shows a plan image parallel to the YZ plane, but the same applies to the case where the plan image is an image parallel to the XZ plane or an oblique cross-sectional image.

  Therefore, the plan image correction unit 37 has a function of generating plan image data in which distortion is corrected by correcting distortion caused by the non-linearity of the gradient magnetic field in the plan image data, and plan image data after distortion correction. Is output to the display 17 to display a distortion-corrected plan image on the display 17 without distortion.

  The excitation slice setting unit 38 has a function of setting, as an imaging condition, a slice to be excited for an imaging scan on the plan image after distortion correction displayed on the display 17 based on instruction information from the console 16.

  FIG. 6 is a diagram illustrating an example of an excitation slice for an imaging scan set by the excitation slice setting unit 38 illustrated in FIG.

  6A and 6B, the vertical axis indicates the Y-axis direction, and the horizontal axis indicates the Z-axis direction. 6A shows an excitation slice set on the distortion-corrected plan image displayed on the display 17, and FIG. 6B shows a two-dimensional control coordinate system set for control. An excitation slice of is shown.

  As shown in FIG. 6A, when the distortion correction process is performed on the plan image data, the plan image in the orthogonal coordinate system can be displayed on the display 17. For this reason, the user can easily set a desired region linear in the Y-axis direction as indicated by the alternate long and short dash line as an excitation slice in the plan image of the orthogonal coordinate system. The excitation slice set in the coordinate system after distortion correction as shown in FIG. 6 (a) is curved as indicated by a one-dot chain line in the coordinate system before distortion correction for control shown in FIG. 6 (b). A dimensional local excitation slice. That is, setting a linear slice selective excitation region on a plan image having linearity as shown in FIG. 6A is for control under the influence of a gradient magnetic field distribution having poor linearity and nonlinearity. This corresponds to setting a curved two-dimensional local slice selective excitation region as shown in FIG. 6B in the orthogonal coordinate axis system.

  Therefore, the excitation slice setting unit 38 stores the geometric relationship between the coordinate positions before and after the distortion correction process and the plan image after the distortion correction process based on the relationship between the coordinate positions before and after the distortion correction process. From the coordinate position information of the excitation slice set above, the coordinate position information of the two-dimensional excitation slice on the plan image data before the distortion correction to be actually excited is calculated in accordance with the nonlinearity of the gradient magnetic field. It has a function.

  A local slice selective excitation method for exciting a two-dimensionally curved slice can be performed by a known method. As an example, details of the local slice selective excitation method are described in Katscher U et al: Transmit SENSE. Magn Reson Med 49: 144-150 (2003). This method avoids the problem that when a local slice selective excitation is performed using a single RF transmitter coil, the pulse length of the high frequency pulse becomes long and the excitation characteristics may be deteriorated due to the effect of magnetic field inhomogeneity. For this purpose, local slice selective excitation is performed using the high-frequency array coil 6 for transmission.

  Therefore, the magnetic resonance imaging apparatus 1 shown in FIG. 1 is configured to perform local slice selective excitation by transmitting high-frequency pulses using a plurality of RF coils included in the transmission high-frequency array coil 6. Yes. When local slice selective excitation is performed by another method, a single RF coil or a whole body coil may be used for transmitting a high-frequency pulse.

  FIG. 7 is a diagram showing an example of a pulse sequence set in the imaging condition setting unit 30 shown in FIG.

  In FIG. 7, RF1, RF2,..., RF8 indicate high-frequency pulses transmitted from 8-channel RF coils, respectively, and Gx, Gy, and Gz are applied in the X-axis direction, Y-axis direction, and Z-axis direction, respectively. The gradient magnetic field pulse is shown.

  FIG. 7 shows an example in which local slice selective excitation is performed using an 8-channel RF coil for transmission in a spin echo (SE) sequence. In this case, two-dimensional selective excitation 90-degree high frequency pulses having different waveforms are transmitted from the RF coils. The pulse waveform of the 90-degree high-frequency pulse is determined according to the position, thickness, number, shape and / or curvature of the slice to be excited. In other words, the pulse waveform of the 90-degree high-frequency pulse is controlled so that the two-dimensional slice of the target position and thickness is excited.

  Further, following the 90-degree high-frequency pulse, each RF coil transmits a 180-degree high-frequency pulse having the same waveform, but with the phase adjusted in consideration of the arrangement angle of each RF coil in the XY plane. This 180 degree high frequency pulse becomes a normal selective excitation pulse for one-dimensional selective excitation. The waveform and phase of the 90-degree high-frequency pulse and the 180-degree high-frequency pulse can be controlled by amplitude control and phase control in the amplitude phase control unit 20 of the transmission unit 11.

  A magnetic field resonance signal is observed as an echo signal during the signal observation time by applying a gradient magnetic field for readout in a predetermined application axis direction such as the Y axis.

  Next, other functions of the computer 15 will be described.

  The sequence controller control unit 31 has a function of controlling driving by giving imaging conditions including a pulse sequence to the sequence controller 14 based on instruction information from the console 16. In addition, the sequence controller control unit 31 has a function of acquiring the received data of the magnetic resonance signals collected by the data collecting unit 13 and arranging the received data in the k space formed in the k space database 32. For this reason, the received data is stored in the k-space database 32 as k-space data.

  The image reconstruction unit 33 has a function of reconstructing image data by acquiring k-space data from the k-space database 32 and performing image reconstruction processing including Fourier transformation (FT). A function of writing the obtained image data into the image database 34. Therefore, the image data reconstructed by the image reconstruction unit 33 is stored in the image database 34.

  The image processing unit 35 captures image data from the image database 34 and performs necessary image processing to generate two-dimensional image data for display, and a function to display the generated display image data on the display 17. Have When a magnetic resonance signal is received using a plurality of surface coils, k-space data corresponding to each surface coil is usually subjected to image reconstruction processing independently. For this reason, for the plurality of image data corresponding to the plurality of surface coils, for example, the image processing unit 35 performs synthesis processing for generating single image data by calculating the square root of the square sum of each image data. Is done.

  Next, the operation and action of the magnetic resonance imaging apparatus 1 will be described.

  FIG. 8 shows an object whose distortion has been reduced by performing slice selective excitation without distortion by setting a desired two-dimensional slice on the distortion-corrected plan image in the magnetic resonance imaging apparatus 1 shown in FIG. It is a flowchart which shows the procedure for obtaining an image, and the code | symbol which attached | subjected the number to S in the figure shows each step of a flowchart.

  First, the subject P is set on the bed 9 in advance, and a static magnetic field is formed in the imaging region of the static magnetic field magnet 21 (superconducting magnet) excited by the static magnetic field power supply 26. Further, a current is supplied from the shim coil power supply 28 to the shim coil 22, and the static magnetic field formed in the imaging region is made uniform.

  Next, in step S1, the plan image condition setting unit 36 sets imaging conditions such as an excitation surface for a plan image scan as a pre-scan prior to the imaging scan. Then, the plan image scan is executed in accordance with the set photographing condition. Thereby, the reception data of the magnetic resonance signal for generating the plan image is collected by the data collecting unit 13 and stored in the k-space database 32 through the sequence controller control unit 31 of the computer 15. Next, the image reconstruction unit 33 generates plan image data by taking in k-space data for generating plan images from the k-space database 32 and performing image reconstruction processing, and the generated plan image data is stored in the image database 34. Write to.

  However, when the plan image data is displayed as it is due to the influence of the nonlinearity of the gradient magnetic field, a plan image having a distortion as shown in FIG. 5 is obtained.

  Therefore, in step S2, the plan image correction unit 37 generates plan image data in which the distortion is corrected by correcting the in-plane distortion caused by the non-linearity of the gradient magnetic field in the plan image data.

  Next, in step S <b> 3, the plan image correction unit 37 displays the plan image without distortion on the display 17 by outputting the distortion-corrected plan image data to the display 17.

  Therefore, in step S4, the user sets the two-dimensional slice selective excitation region for imaging scan as a straight region without curvature on the plan image after distortion correction by operating the console 16 while referring to the display 17. can do. That is, a normal rectangular slice selective excitation region can be set on the distortion-corrected plan image.

  Next, in step S <b> 5, the excitation slice setting unit 38 obtains a slice selection excitation region in the control coordinate system for the imaging scan based on the instruction information from the console 16. That is, the excitation slice setting unit 38 acquires position coordinate information of the slice selection excitation region on the plan image data after distortion correction from the console 16 and corresponds to the coordinate system before distortion correction based on the geometric relationship. The position coordinates of the two-dimensional slice selective excitation region in the control coordinate system to be calculated are calculated. The calculated two-dimensional slice selective excitation region is set in the imaging condition setting unit 30 as a two-dimensional slice selective excitation region for an imaging scan.

  Next, in step S6, the imaging condition setting unit 30 calculates the pulse waveform and phase of the high-frequency pulse to be applied to each of the plurality of RF coils in order to excite the two-dimensional slice selective excitation region. That is, a two-dimensional high-frequency excitation pulse that can excite a two-dimensional slice selective excitation region in an orthogonal coordinate axis system affected by the gradient magnetic field distribution is designed. At that time, each high frequency magnetic field distribution formed by each RF coil is referred to, and a high frequency magnetic field formed by combining a plurality of high frequency magnetic field distributions becomes a target high frequency magnetic field. The pulse waveform and phase of the high frequency pulse to be applied are determined.

  Next, in step S7, an imaging scan is executed in accordance with a pulse sequence incorporating the set high frequency pulse.

  That is, when a scan disclosure instruction is given from the console 16 to the sequence controller control unit 31, the sequence controller control unit 31 acquires a pulse sequence incorporating a high-frequency pulse for two-dimensional slice selection excitation from the imaging condition setting unit 30 and performs sequence This is given to the controller 14. The sequence controller 14 tilts the imaging region in which the subject P is set by driving the gradient coil drive circuit, the transmission unit 11, the reception unit 12, and the data collection unit 13 according to the pulse sequence received from the sequence controller control unit 31. A magnetic field is formed and a high-frequency signal having a predetermined amplitude and phase is generated from each RF coil of the transmission high-frequency array coil 6.

  For this reason, magnetic resonance signals generated by nuclear magnetic resonance in the two-dimensional slice selection region excited in the subject P are received by each surface coil of the receiving high-frequency array coil 7 and given to the receiving unit 12. The reception unit 12 generates reception data by performing amplification and detection of the magnetic resonance signal, and outputs the generated reception data to the data collection unit 13. The data collection unit 13 collects received data, performs A / D conversion, and outputs the data to the computer 15. Then, the sequence controller control unit 31 arranges the reception data of the magnetic resonance signals collected by the data collection unit 13 in the k space formed in the k space database 32.

  Next, in step S8, the image reconstruction unit 33 reconstructs image data by fetching received data from the k-space database 32 and performing image reconstruction processing. The obtained image data is written into the image database 34.

  In step S9, the image processing unit 35 takes in the image data from the image database 34 and performs necessary image processing to generate two-dimensional image data for display. For example, distortion correction processing in the slice plane of image data and synthesis processing of a plurality of image data corresponding to a plurality of surface coils are performed.

  Next, in step S <b> 10, the image processing unit 35 displays display image data on the display 17. For this reason, the user diagnoses the image data with good image quality based on the magnetic resonance signal collected from the slice excited in a two-dimensionally curved but linearly excited due to the influence of the nonlinearity of the gradient magnetic field. Can be obtained for.

  That is, the magnetic resonance imaging apparatus 1 as described above can selectively excite a flat and non-distorted slice surface by the two-dimensional slice selective excitation technique even when the linearity of the gradient magnetic field is poor. Furthermore, the magnetic resonance imaging apparatus 1 uses a high-frequency pulse parallel transmission technique using a transmission high-frequency array coil 6 in order to shorten the pulse length when designing a high-frequency pulse for two-dimensional slice selective excitation. It is a thing. That is, in the magnetic resonance imaging apparatus 1, the pulse waveform and phase of the RF pulse are designed and applied according to the slice thickness and the slice position.

  For this reason, according to the magnetic resonance imaging apparatus 1, even when sufficient linearity cannot be obtained in the gradient magnetic field, the top plate of the bed 9 is obtained by performing slice selective excitation with reduced displacement and distortion. The image data with higher image quality can be obtained over a sufficiently wide imaging without the operation of moving the image data 8. Furthermore, since the distortion in the slice plane of the image data can be corrected by using a conventional distortion correction technique, it is possible to remove the influence due to the non-linearity of the gradient magnetic field in the three-dimensional direction.

  In the above, an example has been shown in which two-dimensional distortion in one plan image is corrected, and two-dimensional slice selective excitation is performed to excite a slice without distortion. Strictly speaking, however, the distortion of the YZ plane as shown in FIG. That is, the manner of bending of the slice varies depending on the position of X due to the non-linearity of the gradient magnetic fields Gy and Gz in the Y-axis direction and the Z-axis direction. In order to perform flat slice excitation under such gradient magnetic fields Gy and Gz in the Y-axis direction and Z-axis direction, three-dimensional local slice excitation is required. Even when three-dimensional local excitation is performed, the basic procedure is the same as that of the above-described two-dimensional local excitation.

  FIG. 9 is a diagram showing an example of an SE sequence in the case where three-dimensional local slice selective excitation is performed in the magnetic resonance imaging apparatus 1 shown in FIG.

  In FIG. 9, RF1, RF2,..., RF8 indicate high-frequency pulses transmitted from 8-channel RF coils, respectively, and Gx, Gy, and Gz are applied in the X-axis direction, Y-axis direction, and Z-axis direction, respectively. The gradient magnetic field pulse is shown.

  In the SE sequence shown in FIG. 9, the 90-degree RF excitation pulse is a three-dimensional selective excitation pulse having a different waveform for each RF coil. Further, although the 180 degree RF pulse is a one-dimensional selected slice excitation pulse, it may be a non-selected slice excitation pulse.

  The pulse waveform of the three-dimensional 90-degree RF selective excitation pulse is determined according to the position, thickness, number, shape and / or curvature of the three-dimensional slice to be excited. In other words, the pulse waveform of the three-dimensional 90-degree RF selective excitation pulse is controlled so that the three-dimensional slice of the target position and thickness is excited.

  The waveform of the 180-degree RF pulse is the same for each RF coil, but the phase is adjusted in consideration of the arrangement angle of each RF coil in the XY plane. The waveforms and phases of the three-dimensional 90-degree RF pulse and 180-degree RF pulse can be controlled by amplitude control and phase control in the amplitude phase control unit 20 of the transmission unit 11.

  A magnetic field resonance signal is observed as an echo signal during the signal observation time by applying a gradient magnetic field for readout in a predetermined application axis direction such as the X axis.

1 is a configuration diagram showing an embodiment of a magnetic resonance imaging apparatus according to the present invention. FIG. 2 is a cross-sectional view of the gantry portion shown in FIG. 1. The detailed block diagram of the transmission part and high frequency array coil for transmission shown in FIG. The functional block diagram of the computer shown in FIG. The figure which shows the plan image at the time of displaying the plan image data collected according to the imaging conditions set by the plan image condition setting part shown in FIG. 4 as it is. The figure which shows an example of the excitation slice for imaging scans set by the excitation slice setting part shown in FIG. The figure which shows an example of the pulse sequence set in the imaging | photography condition setting part shown in FIG. In order to obtain an image of a subject with reduced distortion by performing slice selective excitation without distortion by setting a desired two-dimensional slice on the distortion-corrected plan image in the magnetic resonance imaging apparatus shown in FIG. The flowchart which shows a procedure. The figure which shows the example of the SE sequence in the case of performing three-dimensional local slice selective excitation in the magnetic resonance imaging apparatus shown in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Magnetic resonance imaging apparatus 2 Gantry part 3 Control system 4 Static magnetic field magnet 5 Gradient magnetic field coil 6 High frequency array coil for transmission 7 High frequency array coil for reception 8 Top plate 9 Bed 10 Gradient magnetic field drive circuit 11 Transmitter 12 Receiving part 13 Data collection Unit 14 sequence controller 15 computer 16 console 17 display 20 amplitude phase control unit 21 transmission amplifier 30 imaging condition setting unit 31 sequence controller control unit 32 k-space database 33 image reconstruction unit 34 image database 35 image processing unit 36 plan image condition setting unit 37 Plan image correction unit 38 Excitation slice setting unit P Subject

Claims (5)

Plan image collection means for collecting plan image data for setting a slice to be excited for an imaging scan for collecting magnetic resonance signals from a subject;
Distortion correction due to the non-linearity of the gradient magnetic field of the plan image data is performed to display a distortion corrected plan image, and the distortion corrected plan image is used for two-dimensional or three-dimensional imaging scanning. Slice setting means for setting a typical slice;
An imaging means for acquiring the magnetic resonance signal by executing an imaging scan with excitation of the two-dimensional or three-dimensional slice, and generating image data based on the acquired magnetic resonance signal;
A magnetic resonance imaging apparatus comprising: The magnetic resonance imaging apparatus according to claim 1, wherein the imaging unit is configured to excite the two-dimensional or three-dimensional slice by applying a high-frequency pulse from each of a plurality of high-frequency coils to the subject. 3. The imaging means is configured to determine each waveform of the high-frequency pulse according to at least one of the position, thickness, number, shape, and curvature of the two-dimensional or three-dimensional slice. The magnetic resonance imaging apparatus described. The magnetic resonance imaging apparatus according to claim 1, wherein the plan image collecting unit is configured to collect a plurality of plan image data orthogonal to each other and in a slice direction. The magnetic resonance imaging apparatus according to claim 1, wherein the plan image collecting unit is configured to collect plan image data corresponding to at least one cross section of X = 0, Y = 0, and Z = 0.

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