Magnetic resonance imaging device and control method thereof

Abstract

To correct a distortion occurring in an image in imaging using EPI sequence such as DWI effectively without extending the imaging time.SOLUTION: A distortion of an image of a main scanner is corrected by using navigation scan data obtained in navigation scan by performing navigation scan with the polarity of phase encoding opposite to the main scan continuously to the main scan after one excitation RF pulse application of EPI. In the case of multi-shot, the main scan data of each shot is multi-shot synthesized by performing the phase correction by using phase information obtained from the navigation scan data for each shot.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a magnetic resonance imaging apparatus, and more particularly to a technique for improving image quality in diffusion weighted imaging. [Background technology]

[0002] Diffusion weighted imaging (abbreviated as DWI) is one of the imaging methods using magnetic resonance imaging (hereinafter referred to as MRI) devices. DWI is a technique that applies a strong gradient magnetic field (MPG) in multiple directions (axes) to measure echo signals and image the degree of water diffusion from the difference in the effect of MPG, which is useful for diagnosing infarctions and tumors that cause diffusion restriction. Since DWI uses MPG on multiple axes, EPI, which can acquire many echoes with one excitation, is often used as a pulse sequence.

[0003] One of the issues with DWI is that the images tend to be distorted or blurred. This is due to the fact that EPI is easily affected by static magnetic field inhomogeneity, and that the direction of the readout gradient field changes for each echo, which is a problem specific to EPI, and that the phase differs for each shot in moving areas due to the effect of a pair of large gradient fields (MPG) applied for each shot.

[0004] The amount of distortion is proportional to the frequency (magnetic field strength distribution) for each position, IET (echo interval), and FOV (FOVy in the y direction if the phase encoding direction is the y direction). Therefore, in the case of multi-shot of the interleave method, by increasing the number of shots, the amount of distortion becomes proportional to FOV / number of shots, and the amount of distortion can be reduced to one-fifth of the number of shots, and blurring is improved at the same time. However, in the case of multi-shot, the measurement time doubles by the number of shots. In addition, as mentioned above, in DWI, the phase of the moving area changes for each shot due to MPG, so there is also the problem that artifacts due to phase errors occur in the image synthesized by the signals of each shot. On the other hand, it has been proposed to acquire signals by imaging twice with different polarities of the gradient magnetic field in the phase encoding direction, and correct the distortion by post-processing (Non-Patent Document 1, Patent Document 1). In this method, the distortion in the y direction is reversed in the two images acquired with different phase directions, so the distortion can be removed by calculation between the images. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] Andersson, et al. Neuroimage 2003;20:870 [Patent documents]

[0006] [Patent Document 1] U.S. Patent No. 10,262,385 Summary of the Invention [Problem to be solved by the invention]

[0007] The method described in Non-Patent Document 1 and the like requires excitation two or more times, and therefore requires imaging two or more times for each shot and for each different MPG condition, which results in an extended imaging time.

[0008] In general, in MRI, a scan to collect navigation echoes (hereafter referred to as navi scan) is often performed to correct phase disturbance caused by body movement, and it is possible to add such a navi scan to the above-mentioned DWI as well. However, the amount of distortion obtained by navi scan is different from the amount of distortion obtained by the scan (called the main scan) that acquires the echo signal to reconstruct the image, so it cannot be used as is and requires correction, and a magnetic field map for correction (a map for obtaining frequency information) is required. Correction using such a magnetic field map can also be a source of errors.

[0009] An object of the present invention is to provide a method capable of effectively correcting distortion occurring in an image during imaging using an EPI sequence such as DWI, without extending the imaging time. [Means for solving the problem]

[0010] In order to solve the above problem, the present invention performs a navi scan following a main scan, in which the phase encoding polarity is opposite to that of the main scan, and corrects image distortion using a navi echo obtained in the navi scan.

[0011] That is, the MRI apparatus of the present invention comprises a measurement unit that collects nuclear magnetic resonance signals from a subject using a pulse sequence, and an image calculation unit that generates an image using the nuclear magnetic resonance signals collected by the measurement unit, and the measurement unit executes a pulse sequence that performs, after application of one excitation RF pulse (one shot), a main scan that measures echo signals for image generation and a navi scan that measures echo signals for distortion correction, the navi scan including a navi scan in which the application polarity of the phase encode is opposite to the application polarity of the phase encode in the main scan, and the image calculation unit comprises a displacement amount calculation unit that calculates the amount of displacement of the image using the echo signals obtained in the navi scan, and a distortion correction unit that corrects the distortion of the image using the calculated amount of displacement.

[0012] In addition, the control method of an MRI apparatus of the present invention is a control method of a magnetic resonance imaging apparatus equipped with a measurement unit that collects nuclear magnetic resonance signals from a subject using a pulse sequence, and an image calculation unit that generates an image using the nuclear magnetic resonance signals collected by the measurement unit, and includes a measurement step of causing the measurement unit to execute a pulse sequence including a navi scan that measures echo signals for image generation and a navi scan that measures echo signals for distortion correction after application of one excitation RF pulse, the navi scan having a phase encoding application polarity opposite to that of the phase encoding application polarity of the main scan, and an image processing step of calculating the amount of displacement of the image using the echo signals obtained in the navi scan in which the phase encoding application polarity is opposite to that of the main scan, and correcting the distortion of the image using the displacement amount.

[0013] The image processing step may include a step of phase correcting the image of the main scan using echo signals obtained by the navi scan. Effect of the Invention

[0014] According to the present invention, by using a Navi-scan image, in which image distortion occurs in the opposite direction to the main scan image, it is possible to easily correct the distortion of the main scan image without using a magnetic field map, etc. Furthermore, since Navi-scan data can be collected in one shot consecutive to the main scan, the imaging time is not extended in order to obtain data for distortion correction.

[0015] The present invention is preferably applied to DWI, but can be applied to any imaging that uses a sequence in which echoes are continuously collected after one excitation, and can obtain images without distortion. [Brief description of the drawings]

[0016] [Figure 1] 1 is a diagram showing the overall configuration of an MRI apparatus to which the present invention is applied. [Diagram 2] FIG. 1 shows a typical DWI sequence. [Diagram 3] FIG. 2 is a functional block diagram of a computer provided in the MRI apparatus according to the embodiment. [Figure 4] 4 is a flow chart showing an example of the operation of the MRI apparatus according to the embodiment. [Diagram 5] FIG. 2 is a diagram showing an example of a pulse sequence according to the first embodiment. [Figure 6] FIG. 2 shows an example of k-space data obtained by a main scan and a navigation scan. [Figure 7] FIG. 4 is a diagram for explaining the processing of the first embodiment. [Figure 8] FIG. 11 is a functional block diagram of a computer according to a second embodiment. [Figure 9] FIG. 11 is a diagram showing an example of k-space data obtained by a main scan and a navi-scan according to the second embodiment. [Figure 10] FIG. 11 is a diagram for explaining the processing of the second embodiment. [Figure 11] FIG. 11 is a diagram showing an example of a pulse sequence according to the third embodiment. [Figure 12] FIG. 11 is a diagram for explaining the processing of the third embodiment. [Figure 13] FIG. 13 is a functional block diagram of a computer according to a modified example of the third embodiment. [Figure 14] FIG. 13 is a diagram for explaining processing of Modification 1 of the third embodiment. [Figure 15] FIG. 13 is a diagram for explaining processing of the second modified example of the third embodiment. [Figure 16] FIG. 2 is a diagram showing an example of a GUI of an MRI apparatus. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Hereinafter, an embodiment of an MRI apparatus of the present invention will be described with reference to the drawings.

[0018] 1 is a block diagram showing a schematic configuration of an MRI apparatus 1. The MRI apparatus 1 includes a magnet 11 that generates a static magnetic field in a direction parallel to a subject, a gradient coil 12 that generates a gradient magnetic field, a gradient power supply 13, a high-frequency magnetic field generator 14, a probe 15 that irradiates a high-frequency magnetic field and detects a nuclear magnetic resonance signal (echo), a receiver 16, a sequencer 17, and a computer 20. A subject (e.g., a living body) 50 is placed on a bed (table) or the like and is disposed in a static magnetic field space generated by the magnet 11. Hereinafter, the magnet 11, the gradient coil 12, the gradient power supply 13, the high-frequency magnetic field generator 14, the probe 15, the receiver 16, and the sequencer 17 are collectively referred to as a measurement unit 10.

[0019] The sequencer 17 sends commands to the gradient magnetic field power supply 13 and the radio frequency magnetic field generator 14 to generate a gradient magnetic field and a radio frequency magnetic field, respectively. The generated radio frequency magnetic field is applied to the subject 60 as a pulsed radio frequency magnetic field (RF pulse) through the probe 15. The nuclear magnetic resonance signal generated from the subject 60 is received by the probe 15 and detected by the receiver 16. The nuclear magnetic resonance signal is usually collected as a gradient echo or spin echo, and is therefore referred to as an echo signal here.

[0020] The nuclear magnetic resonance frequency (detection reference frequency f0) used as the detection reference in the receiver 16 is set by the sequencer 17. The sequencer 17 controls each part so that it operates at pre-programmed timing and intensity. Among the programs, those that particularly describe the timing and intensity of the radio frequency magnetic field (RF pulse), gradient magnetic field, and signal reception are called pulse sequences.

[0021] There are various known pulse sequences depending on the purpose, but in this embodiment, a sequence is adopted in which multiple echoes are continuously collected after one excitation. For example, an EPI (Echo Planar Imaging) sequence or an FSE (Fast Spin Echo) sequence is used. EPI sequences include an SE (Spin Echo)-based EPI sequence that uses an inversion pulse, and a multi-shot EPI that collects echoes by dividing the sequence into multiple excitations. In addition, MPG (Motion Probing Gradient) may be used to obtain a diffusion weighted image.

[0022] FIG. 2 shows an example of a SE-EPI sequence including an MPG pulse. As shown in the figure, an excitation RF pulse 301 is applied together with a slice selection gradient magnetic field 303, and then an inversion RF pulse 302 is applied together with a slice selection gradient magnetic field 304 to excite a desired slice. A high-intensity MPG pulse 309 is applied before and after this inversion RF pulse 302. Next, a phase encoding gradient magnetic field 305 is applied, and then a blip-shaped phase encoding gradient magnetic field 306 and a readout gradient magnetic field 307 that inverts the polarity are applied in succession, and an echo signal 308 is collected during the application of the inverting readout gradient magnetic field 307. FIG. 2 shows a case where an MPG pulse is applied on the axis of a slice gradient magnetic field Gs, but usually, the axis (direction) on which the MPG pulse 309 is applied is changed to perform imaging multiple times. In the case of single-shot EPI, all the echo signals required for image reconstruction are measured with one excitation.

[0023] Furthermore, although not shown in Fig. 2, in this embodiment, a sequence for collecting navigator echoes used for distortion correction is added following the collection of echo signals for image reconstruction. The sequence up to the collection of echo signals for image reconstruction is called a main scan, and the subsequent sequence for collecting navigator echoes is called a navi scan. Details of navi scan will be described later.

[0024] The computer 20 controls the operation of the measurement unit 10 described above, and also receives signals detected by the measurement unit (receiver 16) 10, i.e., echo signals of the main scan and signals of the navigation scan (navigator echo), and performs signal processing such as image reconstruction. The computer 20 can also include an input device 30 for a user to input imaging conditions and processing conditions, a display device 40 for displaying images of the subject obtained as a result of signal processing and images (GUI) that serve as an interface with the user, and a storage device 50 for storing processing results and data required for processing. The storage device 50 includes a storage device connected to the computer 20 by wire or wirelessly, as well as an external storage device connected via a network.

[0025] FIG. 3 shows an example of the configuration of the computer 20 that realizes the above function. As shown in FIG. 3, the computer 20 includes a measurement control unit 21 that controls the operation of the measurement unit 10, an image calculation unit 22 that performs calculations such as image reconstruction using signals received from the measurement unit 10, and a display control unit 23. The measurement control unit 21 includes a main scan control unit 211 and a navigation scan control unit 212, and sends a predetermined pulse sequence and user-specified imaging conditions (imaging parameters) to the sequencer 17 to control the measurement unit 10. The image calculation unit 22 includes an FT unit 221 that performs calculations, such as a Fourier transform, to convert data (k-space data) in which measurement signals are arranged in k-space into real space data, a displacement amount calculation unit 222 that calculates distortion (amount of displacement) occurring in the image using data obtained by the navigation scan, and a distortion correction unit 223 that corrects distortion of an image such as a main scan using the amount of displacement calculated by the displacement amount calculation unit 222 (the figure shows an example of a main scan distortion correction unit that corrects distortion of a main scan image). With these configurations, an image with corrected distortion, such as a diffusion weighted image, is generated. The image is displayed on the display device 40 together with necessary additional information (e.g., imaging conditions, information on the subject, etc.) by the display control unit 23. The computer 20 stores the detected signal, the imaging conditions, image information after signal processing, etc. in the storage device 50 as necessary.

[0026] The functions of the computer 20 are realized by uploading a pre-designed program to the CPU or GPU of the computer 20. Note that some of the functions of the computer 20 may be realized by hardware such as ASIC or FPGA.

[0027] An overview of the operation of the MRI apparatus in the above configuration (control by each control unit of computer 20) is shown in Fig. 4. First, when a pulse sequence and imaging conditions are set according to a user setting via input device 30 or a preset examination protocol, the main scan control unit 211 and the Navi scan control unit 212 send commands to the sequencer 17 according to the set conditions. Under the control of sequencer 17, the measurement unit 10 executes an EPI sequence including a Navi scan, collects echo signals of the main scan and the Navi scan, respectively, and obtains main scan data and Navi scan data, which are measurement space data (S401).

[0028] In the image calculation unit 22, the FT unit 221 first converts the main scan data and the navi scan data into complex data in real space to generate a main scan image and a navi scan image (S402). Since the main scan data and the navi scan data have reversed phase encoding application polarities, distortions occurring in the main scan image and the navi scan image due to accumulation of phase errors caused by static magnetic field inhomogeneity, etc., are distortions in the opposite direction. The displacement amount calculation unit 222 uses the navi scan image to calculate the displacement amount (displacement amount in real space) used for distortion correction (S403). The distortion correction unit 223 uses this displacement amount to perform distortion correction on the main scan image (S404).

[0029] The echo planar sequence executed in the measurement step S401 may be a single-shot EPI that collects all echoes required for image reconstruction by applying a single RF pulse (called one shot), or may be a multi-shot EPI. In the case of a multi-shot EPI, it is preferable to include a process (S405) for correcting a phase error occurring between shots. In particular, in an imaging method using a gradient magnetic field (MPG) with a large strength such as DWI, in the case of a multi-shot, the phase error between shots becomes large, so it is preferable to perform the above-mentioned phase correction.

[0030] Based on the above-mentioned outline of the MRI apparatus, an embodiment of the MRI apparatus of the present invention will be described below. In the following description, configurations common to those in Figs. 1 to 4 will be referred to as appropriate in these drawings.

[0031] <Embodiment 1> In this embodiment, the measurement unit 10 performs a single-shot main scan to collect all echo signals for image reconstruction after one excitation RF pulse, and then executes a navi scan. In the navi scan, the polarity of the phase encoding gradient magnetic field is reversed to that of the main scan, an image with distortion in the opposite direction to the image obtained in the main scan is obtained, and the distortion of the image in the main scan is corrected using the displacement amount using that image.

[0032] An example of a pulse sequence according to this embodiment is shown in FIG. This pulse sequence is executed by the measurement unit 10 under the control of the measurement control unit 21 (main scan control unit 211 and navi scan control unit 212). As shown in Fig. 5, this pulse sequence includes a main scan 300 in which an excitation pulse 301 is applied once, followed by an inversion pulse 302 to collect echoes for an image, and a navi scan 310 in which an inversion pulse 312 is applied and then a navigator echo is collected. Unlike Fig. 2, Fig. 5 shows a case in which the application axis of the MPG pulse 309 is the Gr axis, but the application axis of the MPG is arbitrary, and other configurations for the main scan are the same as those in Fig. 2.

[0033] In the navi scan 310, the polarity of the phase encode pulse 316 is set to the opposite polarity to that of the phase encode pulse 306 in the main scan 300, and a navigator echo 318 is collected each time the readout 317 is inverted. The number and range of the collected navigator echoes 318 are not particularly limited as long as an image of the full FOV (the same FOV as the FOV of the main scan image) can be obtained as the reconstructed navi scan image, but in the illustrated example, the applied intensity of the phase encode pulse 306 is set to be smaller than that of the main scan, and data is collected near the center of k-space (low frequency region).

[0034] In the main scan 300 and the navi scan 310, echoes are collected by reversing the polarity of the phase encoding gradient magnetic field, so that in the main scan, the k-space is scanned from top to bottom, whereas in the navi scan, it is scanned from bottom to top, as shown in Fig. 6. Or vice versa. In Fig. 6, the left side shows main scan data 600, and the right side shows navi scan data 610.

[0035] Next, the processing of the image calculation unit 22 will be described with reference to Fig. 7. First, the FT unit 221 converts the k-space data 600 obtained by the main scan and the k-space data 610 obtained by the navi-scan into real space data (image data) 601, 611 by performing a Fourier transform (FT), respectively. At this time, the k-space data 610 obtained by the navi-scan is FT-padded with zeros around the data so that the matrix size of the obtained image matches the matrix size of the image of the main scan. As a result, the image of the navi-scan has a low spatial resolution but the same matrix size as the image of the main scan.

[0036] Next, the displacement amount calculation unit 222 calculates the amount of displacement for correcting the actual scan image 600. The distortion occurring in each of the images 601 and 611 is expressed by the following equation, where the amount of distortion at a certain point (x, y) is Δy [mm].

number

[0037] As shown in formula (1), in order to calculate the amount of distortion of each image, a magnetic field map is required to calculate the frequency f0, but in this embodiment, the amount of displacement is calculated by utilizing the fact that the distortion of both images is in the opposite direction. Specifically, since the distortion direction is opposite between the main scan image and the navi-scan image, as shown in Fig. 7, for example, if the main scan image 601 is an image extended in the phase direction, the navi-scan image 611 is an image shrunk in the phase direction (an image extended in the readout direction). The displacement amount calculation unit 222 uses these images to calculate the amount of displacement to correct the distortion of the main scan image.

[0038] There are various known methods for obtaining a magnetic field distribution from two images with different polarities. For example, the method described in Non-Patent Document 1 can be used.

[0039] The distortion correction unit 223 corrects the distortion of the actual scan image using the calculated displacement amount, and generates a corrected image 602. The image calculation unit 22 performs this correction process for each image with a different MPG, and obtains a desired DWI image.

[0040] According to this embodiment, by collecting echo signals that cause image distortion in the opposite direction by using NaviScan, distortion correction can be performed without extending the imaging time. In the above embodiment, the imaging method is described as DWI, but this embodiment can also be applied to EPI-based imaging that does not use MPG.

[0041] <Embodiment 2> In the first embodiment, a single-shot EPI sequence is used, but in the present embodiment, a multi-shot EPI sequence is used, and the images of each shot are synthesized after phase correction between shots is performed using a navigator echo. Then, distortion of the image is corrected using the navigator echo.

[0042] For this reason, in the computer 20 of this embodiment, a multi-shot synthesis unit 224 is added to the image calculation unit 22, as shown in Fig. 8. The other elements are the same as those of the first embodiment shown in Fig. 3.

[0043] The processing of this embodiment will be described below. A multi-shot EPI sequence is executed under the control of the measurement control unit 21. The EPI sequence employed in this embodiment is similar to the pulse sequence shown in FIG. 5, except that the number of echo signals collected in one excitation and the phase encoding step are different, and the process from the excitation RF pulse 301 to the navi scan is repeated for the number of shots. In each shot of the main scan, the phase encoding offset 305 and the phase encoding step are made different to collect data of different lines in the k-space, and finally k-space data 800 as shown in FIG. 9 is obtained. The navi scan following the main scan is similar to that of the first embodiment, and uses phase encoding of the opposite polarity to that of the main scan to obtain navi scan data 810 of the central region of the k-space.

[0044] The image calculation unit 22 uses the actual scan data 800 and the navigation scan data 810 obtained for each shot to synthesize multiple actual scan images obtained by multi-shot, and performs distortion correction on the synthesized image. In this embodiment, when synthesizing, phase correction is performed on the actual scan image of each shot, using phase information between shots obtained from the navigation scan data obtained for each shot. Distortion correction is performed on the synthesized actual scan image that has been phase-corrected in this manner, in the same manner as in embodiment 1. The details of these processes will be described with reference to FIG. 10.

[0045] First, the FT unit 221 generates a main scan image 801 and a navi scan image 811 for each shot using main scan data 800 and navi scan data 810. The multi-shot synthesis unit 224 calculates the phase difference between images from the navi scan image 811 obtained for each shot, and synthesizes the main scan images 801 of each shot using the phase difference information to obtain a single main scan image 802. Information for correcting the phase difference between shots is calculated by finding the phase from the real and imaginary parts of the navi scan image 811, which is a complex image, and calculating the difference between the shots. By using this phase difference, the main scan image of each shot can be phase corrected.

[0046] There are various known examples of phase correction in multi-shot imaging, and the correction can be performed, for example, using the following equation. (Number 2) φ aft (x,y) = arg(exp(i·(φ bef (x,y)-φ navi (x,y))))(2) Here, φ aft is the phase of the corrected scanned image, φ bef is the phase of the scanned image before correction, φ navi represents the phase of the navigation image. Thus, phase correction can be performed voxel-by-voxel in image space.

[0047] By performing synthesis after phase correction in this manner, a synthetic image 802 free of artifacts caused by phase differences between shots can be obtained.

[0048] Next, the displacement amount calculation unit 222 calculates the amount of displacement using the main scan composite image 802 and the navi-scan image 812, as in the first embodiment, and uses the amount of displacement to perform distortion correction on the main scan composite image 802. The navi-scan image 812 used to calculate the amount of displacement may be a composite of the navi-scan images 811 of each shot, or any of the navi-scan images 811 may be used.

[0049] According to this embodiment, when performing multi-shot synthesis, phase errors between images are corrected, so that distortion correction can be performed with higher accuracy.

[0050] <Embodiment 3> In the first and second embodiments, a navi-scan was performed with the phase encoding applied in the opposite polarity to the main scan, but the present embodiment is characterized in that a pair of navigator echoes with different phase encoding applied polarities are collected in the navi-scan to obtain a pair of navi-scan images with different distortion directions, and a distortion-corrected main scan image is obtained using the phase difference information and distortion information contained in this pair of navi-scan images. This embodiment can also be applied when the main scan is a single-shot EPI, but the case of multi-shot EPI is used as an example here.

[0051] The configuration of the computer 20 of this embodiment is similar to that of FIG. 8, but the displacement amount calculation unit 222 functions as a displacement map calculation unit that calculates a displacement map using a pair of navi-scan images.

[0052] The processing of this embodiment will be described below with reference to FIGS. A multi-shot EPI sequence is executed under the control of the measurement control unit 21. An example of an EPI sequence adopted in this embodiment is shown in FIG. 11. In this pulse sequence, the main scan 300 is the same as the main scan in embodiment 2 (or embodiment 1), but the navi scan consists of a scan 310-1 (first navi scan) in which phase encoding is performed with the same polarity as the main scan, and a scan 310-2 (second navi scan) in which phase encoding is performed with the opposite polarity. The order of the navi scan 310-1 and the navi scan 310-2 can be either first. In the two navi scans, data of the central region of k-space is collected as in embodiments 1 and 2. This allows two (or two sets) of navi scan data to be obtained by scanning the k-space in opposite directions.

[0053] As shown in FIG. 12, the image calculation unit 22 first converts the fine scan data 800 and navi scan data 810-1, 810-2 obtained by the FT unit 221 for each shot into real space data, and generates a fine scan image 801 and two navi scan images 811-1, 811-2.

[0054] The two Navi-scan images 811-1, 811-2 have different phase encoding polarities, and therefore different directions of distortion, for example, one image is elongated in the phase encoding direction and the other image is contracted in the phase encoding direction. The displacement amount calculation unit 222 calculates the displacement amount for each pixel for this pair of composite Navi-scan images 812-1, 812-2, and creates a displacement amount map 815 (S1203).

[0055] The amount of displacement can be obtained by obtaining the magnetic field distribution or the frequency distribution proportional to the magnetic field from two images of different polarities, and then converting it into the amount of displacement using the above-mentioned formula (1). A known method such as the method described in Non-Patent Document 1 can be used to estimate the magnetic field distribution from two images of different polarities.

[0056] The navi-scan images 811-1 and 811-2 for obtaining the amount of displacement may be a composite of images obtained for each shot, or navi-scan images of a predetermined shot may be used.

[0057] Meanwhile, the multi-shot synthesis unit 224 uses information on the phase difference between shots of an image (image for each shot) 812-1 obtained by a Navi-scan having the same phase encoding application polarity as the main scan among the two Navi-scan images, to perform phase correction on the main scan image for each shot 801, and then synthesizes the images to generate a synthesized main scan image 802. This synthesis process including phase correction can be performed by the same method as in the second embodiment.

[0058] Finally, the fine scan distortion correction unit 223 corrects the combined fine scan image 802 using the displacement amount map 815 created by the displacement amount calculation unit 222 to obtain a corrected fine scan image 803 .

[0059] According to this embodiment, a pair of navi-scan data with different phase encoding polarities is used to calculate the displacement amount using navi-scan images with opposite distortion directions. This makes it possible to calculate a more accurate displacement amount than in the first and second embodiments, in which the displacement amount is calculated from a navi-scan image and a main scan image, thereby enabling more accurate distortion correction.

[0060] Furthermore, when the main scan images are synthesized, phase difference information obtained from a navi-scan image in which the phase encoding is applied with the same polarity is used, so that the accuracy of correction of the phase difference between shots is also improved.

[0061] Note that the above explanation was given for the case where the actual scan is a multi-shot EPI, but in the case of a single shot, the same can be applied, with the only difference being that the synthesized actual scan image 802 in Figure 12 is replaced with an actual scan image obtained in one shot, and phase correction between shots is omitted.

[0062] <Modification of the third embodiment> This modified example is similar to embodiment 3 in that two navi-scan data with different phase encoding application axes, one positive and one negative, are used as the navi-scan data, but in embodiment 3 a displacement map is calculated from these navi-scan data and distortion correction is performed on the synthesized main scan image.In this modified example, distortion correction is performed on the navi-scan image, and the distortion-corrected navi-scan image is used to perform multi-shot synthesis of the main scan images of each shot.

[0063] 13, in this modification, a navigation distortion correction unit 225 is added to the configuration of the image calculation unit 22. The displacement amount calculation unit 222 may be omitted. The following will focus on the processing of the image calculation unit 22 that differs from that of the third embodiment. Fig. 14 shows the flow of processing in this modified example.

[0064] In this modified example, the pulse sequence executed by the measurement unit 10 is the same as in embodiment 3, and as shown in Figure 14, for each shot, a main scan image 801 and two navi scan images 811-1, 811-2 are obtained from main scan data 800 and navi scan data 810-1, 810-2 with opposite phase encoding.

[0065] In the third embodiment, phase correction was performed during multi-shot composition of the main scan images using one of the two navi-scan images, which was obtained with the same phase encoding polarity as the main scan image, but in this modified example, the navi-distortion correction unit 225 first performs distortion correction on the two navi-scan images. Because the two navi-scan images have different phase encoding polarities (positive and negative), the image distortion is in different directions. Therefore, the navi-distortion correction unit 225 corrects the distortion of the two navi-scan images as follows, performing distortion correction so that the distortion is the same as that of the main scan image, and obtains the corrected navi-scan image 813.

[0066] This can be achieved by estimating a frequency image f(x, y) and correcting distortion from two navi-scan images using a known method such as that shown in non-patent document 1, and then distorting the image again using equation (1) according to the FOV and IET of the main scan image. Alternatively, after estimating the frequency image f(x, y), the positional shift d(x, y) between the images at each coordinate is calculated from the imaging conditions of both images, and distortion correction is performed according to this amount. Here, d(x, y) represents the amount of distortion (Δy scan and Δy navi ) can be calculated using the formula below.

number

[0067] A corrected navi-scan image 813 is obtained for each shot. The multi-shot synthesis unit 224 performs multi-shot synthesis with phase correction on the main scan image 801 of each shot using information on the inter-shot phase error of these corrected navi-scan images 813, to obtain a synthesized image 804.

[0068] In this modified example, phase correction is performed using the distortion-corrected NaviScan image 813, making it possible to perform highly accurate correction, and to suppress artifacts caused by phase errors during multi-shot synthesis with a high degree of accuracy.

[0069] Therefore, in this modified example, it is possible to omit the processing by the distortion correction unit 223 of the third embodiment, but it is also possible to perform the same distortion correction as in the third embodiment on the actual scan image after multi-shot synthesis. This processing is shown in Fig. 15. As shown in Fig. 15, in the distortion correction, as in the third embodiment, the amount of displacement is calculated from the two navi-scan images 811-1 and 811-2, a displacement amount map 815 is created, and the distortion of the actual scan image 802 after synthesis is corrected based on this displacement amount map.

[0070] According to this modified example, phase correction during multi-shot synthesis is performed using a distortion-corrected NaviScan image, thereby making it possible to further improve the accuracy of phase correction compared to embodiment 3, and as a result, the accuracy of distortion correction of the image after shot synthesis can also be improved.

[0071] In the phase correction and distortion correction of each of the above-mentioned embodiments, when an EPI sequence or the number of shots thereof is set as a pulse sequence included in the imaging conditions, in the case of a single shot, distortion correction may be automatically performed using the method of embodiment 1 or embodiment 3 (including modifications), and in the case of a multi-shot, distortion correction may be automatically performed using the method of embodiment 2 or embodiment 3 (including modifications). Alternatively, the correction conditions may be set when setting a pulse sequence or imaging parameters on an imaging condition setting screen 1600 for setting a pulse sequence or imaging parameters as shown in FIG. 16 .

[0072] For example, when EPI is set as the pulse sequence, it may be possible to select whether or not to perform distortion correction, and when distortion correction is selected as "required," the user may be able to select the desired distortion correction method using a pull-down menu or by opening a new setting screen.

[0073] Although the embodiment of the present invention and its modified examples have been described above, the present invention is not limited to these embodiments, and various modifications are possible, such as combining known techniques for image reconstruction. In addition, although the case of performing DWI has been described, the present invention can also be applied to a normal EPI sequence that does not use an MPG pulse. [Explanation of symbols]

[0074] 1: MRI device, 10: measurement unit, 11: magnet, 12: gradient magnetic field coil, 13: gradient magnetic field power supply, 14: high frequency magnetic field generator, 15: probe, 16: receiver, 17: sequencer, 20: computer, 21: measurement control unit, 22: image calculation unit, 23: display control unit, 30: input device, 40: display device, 50: storage device, 60: subject, 211: main scan control unit, 212: navi scan control unit, 221: FT unit, 222: displacement amount calculation unit, 223: main scan distortion correction unit (distortion correction unit), 224: multi-shot synthesis unit, 225: navi scan distortion correction unit

Claims

1. A measurement unit that acquires nuclear magnetic resonance signals from a subject using a pulse sequence, and an image calculation unit that generates an image using the nuclear magnetic resonance signals acquired by the measurement unit, the measurement unit executes a pulse sequence for executing, after application of one excitation RF pulse (one shot), a main scan for measuring an echo signal for image generation and a navi scan for measuring an echo signal for distortion correction, the navi scan including a navi scan in which a polarity of a phase encode is opposite to a polarity of a phase encode in the main scan; A magnetic resonance imaging apparatus characterized in that the image calculation unit includes a displacement amount calculation unit that calculates the amount of displacement of the image using the echo signal obtained by the navigation scan, and a distortion correction unit that corrects the distortion of the image using the calculated displacement amount.

2. 2. The magnetic resonance imaging apparatus according to claim 1, the pulse sequence is a single-shot sequence that collects all echoes for image generation with one excitation RF pulse; The magnetic resonance imaging apparatus is characterized in that the distortion correction unit corrects distortion of the image of the main scan using a displacement amount calculated using a navi-scan image obtained by the navi-scan.

3. 2. The magnetic resonance imaging apparatus according to claim 1, the pulse sequence is a multi-shot sequence that uses two or more excitation RF pulses to collect echoes for generating images; The magnetic resonance imaging apparatus is characterized in that the image calculation unit further includes a multi-shot synthesis unit that synthesizes echo signals obtained in the main scan of each shot, and the multi-shot synthesis unit phase-corrects and synthesizes the images obtained in the main scan of each shot using phase information of the navi-scan image obtained in the navi-scan of each shot.

4. 4. The magnetic resonance imaging apparatus according to claim 3, A magnetic resonance imaging apparatus characterized in that the distortion correction unit corrects the distortion of the main scan image synthesized by the multi-shot synthesis unit using a displacement amount calculated using the image obtained by the NaviScan.

5. 2. The magnetic resonance imaging apparatus according to claim 1, The navi scan includes a first navi scan in which the application polarity of phase encoding is the same as that of the main scan, and a second navi scan in which the application polarity of phase encoding is opposite to that of the main scan, A magnetic resonance imaging apparatus characterized in that the displacement amount calculation unit calculates the amount of displacement using a navi-scan image obtained by the first navi-scan and a navi-scan image obtained by the second navi-scan.

6. 6. The magnetic resonance imaging apparatus according to claim 5, the pulse sequence is a multi-shot echo-planar sequence that uses two or more excitation RF pulses to collect echoes for generating images; The image calculation unit further includes a multi-shot synthesis unit that synthesizes echo signals obtained in the main scan of each shot, The magnetic resonance imaging apparatus according to claim 1, wherein the distortion correction unit corrects distortion of the actual scan image synthesized by the multi-shot synthesis unit using the amount of displacement.

7. 7. The magnetic resonance imaging apparatus according to claim 6, A magnetic resonance imaging apparatus characterized in that the multi-shot synthesis unit uses phase information of the navi-scan image obtained in the first navi-scan to phase-correct and synthesize the images obtained in the main scan of each shot.

8. 2. The magnetic resonance imaging apparatus according to claim 1, the pulse sequence is a multi-shot echo planar sequence that uses two or more excitation RF pulses to collect echoes for generating an image, and the navi-scan includes a first navi-scan in which the polarity of phase encoding applied is the same as that of the main scan, and a second navi-scan in which the polarity of phase encoding applied is opposite to that of the main scan; the image calculation unit further includes a multi-shot synthesis unit that synthesizes echo signals obtained in the main scans of the respective shots, and a navigation distortion correction unit that generates a distortion-corrected navigation scan image using a navigation scan image obtained in the first navigation scan and a navigation scan image obtained in the second navigation scan, The magnetic resonance imaging apparatus is characterized in that the multi-shot synthesis unit uses phase information of the distortion-corrected navigation scan image generated by the navigation distortion correction unit to phase correct and synthesize the images obtained by the main scan of each shot.

9. 9. The magnetic resonance imaging apparatus according to claim 8, the displacement amount calculation unit calculates a displacement amount by using a navi-scan image obtained by the first navi-scan and a navi-scan image obtained by the second navi-scan; The magnetic resonance imaging apparatus according to claim 1, wherein the distortion correction unit corrects the actual scan image synthesized by the multi-shot synthesis unit using the amount of displacement.

10. 2. The magnetic resonance imaging apparatus according to claim 1, the pulse sequence includes an MPG pulse; The magnetic resonance imaging apparatus according to claim 1, wherein the image calculation unit generates a DWI image as the image of the main scan.

11. A method for controlling a magnetic resonance imaging apparatus including a measurement unit that collects nuclear magnetic resonance signals from a subject using a pulse sequence, and an image calculation unit that generates an image using the nuclear magnetic resonance signals collected by the measurement unit, comprising: a measurement step of causing the measurement unit to execute a pulse sequence including a main scan for measuring an echo signal for image generation and a navi scan for measuring an echo signal for distortion correction after application of one excitation RF pulse, the navi scan having a phase encoding polarity opposite to that of the main scan; A method for controlling a magnetic resonance imaging apparatus, comprising: an image processing step of calculating an amount of displacement of an image using an echo signal obtained by a navi scan in which the polarity of the phase encoding applied is opposite to that of the main scan, and correcting distortion of the image using the amount of displacement.

12. 12. The control method according to claim 11, The Navi-scan includes a first Navi-scan in which the application polarity of phase encoding is the same as that of the main scan, and a second Navi-scan in which the application polarity of phase encoding is opposite to that of the main scan, A method for controlling a magnetic resonance imaging apparatus, characterized in that the image processing step calculates an amount of displacement to be used for correcting distortion of the image of the main scan using an image obtained by the first navigation scan and an image obtained by the second navigation scan.

13. 12. The control method according to claim 11, The Navi-scan includes a first Navi-scan in which the application polarity of phase encoding is the same as that of the main scan, and a second Navi-scan in which the application polarity of phase encoding is opposite to that of the main scan, A method for controlling a magnetic resonance imaging apparatus, characterized in that the image processing step further includes a step of correcting the phase of the image of the main scan using phase information of a Navi-scan image obtained by the first Navi-scan.

14. 12. The control method according to claim 11, The Navi-scan includes a first Navi-scan in which the application polarity of phase encoding is the same as that of the main scan, and a second Navi-scan in which the application polarity of phase encoding is opposite to that of the main scan, A method for controlling a magnetic resonance imaging apparatus, characterized in that the image processing step further includes a step of generating a distortion-corrected navi-scan image using an image obtained by the first navi-scan and an image obtained by the second navi-scan, and a step of correcting the phase of the image of the main scan using the distortion-corrected navi-scan image.

15. 12. The control method according to claim 11, The method for controlling a magnetic resonance imaging apparatus, wherein the pulse sequence is a multi-shot echo planar sequence that uses two or more excitation RF pulses to collect echoes for generating an image, and includes application of an MPG pulse for each shot.

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