Magnetic resonance imaging apparatus and method of controlling the same

Abstract

The present application relates to a magnetic resonance imaging apparatus and a control method thereof. Distortion occurring in an image is effectively corrected in imaging using an EPI sequence such as DWI, without extending the imaging time. After application of a 1st excitation RF pulse of EPI, a navigation scan is continuously performed with the polarity of phase encoding being opposite to that of the main scan, and the navigation scan data obtained in the navigation scan is used to correct the distortion of the image of the main scan. In the case of multi-shot, further using the phase information obtained from the navigation scan data of each shot, the main scan data of each shot is phase-corrected and multi-shot synthesis is performed.

Description

Technical Field

[0001] This invention relates to magnetic resonance imaging devices, and particularly to image quality enhancement techniques in diffusion-weighted imaging. Background Technology

[0002] As one of the imaging methods utilizing magnetic resonance imaging (MRI), diffusion-weighted imaging (DWI) is employed. DWI is a technique that applies strong tilted magnetic fields (MPGs) in multiple directions (axis) and measures the echo signals, visualizing the degree of water diffusion based on the differences in the effects of the MPGs. It is useful in the diagnosis of infarctions and tumors where diffusion is restricted. In DWI, because multi-axis MPGs are used, pulse sequences, such as EPI, which can acquire many echoes with a single excitation, are often used as the pulse sequence.

[0003] As a subject of DWI, there are problems such as distortion and blurring in the image. This is caused by the following problems: EPI is susceptible to the effects of static magnetic field inhomogeneity, the direction of the readout tilt magnetic field changes with each echo, which is a problem specific to EPI; and the phase of active regions becomes different with each excitation due to the influence of a pair of large tilt magnetic fields (MPG) given with each excitation.

[0004] The distortion is proportional to the frequency (magnetic field strength distribution), IET (echo interval), and FOV (or FOVy in the y-direction if the phase encoding direction is y-direction) at each location. Therefore, in the case of multiple excitations using the interleave method, by increasing the number of excitations, the distortion becomes proportional to the FOV / number of excitations, reducing the distortion to one-half the number of excitations and improving blurring. However, in the case of multiple excitations, the measurement time doubles the number of excitations. Furthermore, as mentioned above, in DWI, since the phase of active regions changes with each excitation due to MPG, there is also the problem of artifacts caused by phase errors in the image synthesized from the signals of each excitation.

[0005] On the other hand, a technique has been proposed to obtain signals by taking two images with different polarities of the tilted magnetic field in the phase encoding direction, and to correct distortion through post-processing (Non-Patent Document 1, Patent Document 1). In this method, since the distortion generated in the y-direction becomes opposite in the two images obtained by making the phase directions different, the distortion can be removed by inter-image operations.

[0006] Prior art literature

[0007] Non-patent literature

[0008] Non-patent literature 1: Andersson, et al. Neuroimage 2003; 20: 870

[0009] Patent documents

[0010] Patent Document 1: US Patent No. 10262385

[0011] In the method described in Non-Patent Literature 1, etc., more than two excitations are required, and more than two shots are taken for each excitation under each different MPG condition, which leads to the problem of extended shooting time.

[0012] However, in MRI, to correct phase distortions caused by body movement, a navigation scan (hereinafter referred to as a navigation scan) is usually performed to collect the navigation echo. Such a navigation scan is also considered in the DWI mentioned above. However, since the distortion obtained through the navigation scan differs from that obtained in the scan that acquires the echo signal used to reconstruct the image (referred to as the formal scan), it cannot be used unchanged and requires correction. Furthermore, a magnetograph (a graph used to obtain frequency information) is needed for correction. This correction using such a magnetograph can also become a source of error. Summary of the Invention

[0013] The objective of this invention is to provide a method for effectively correcting distortions generated in images during DWI or other imaging processes that utilize EPI sequences without extending the imaging time.

[0014] To address the aforementioned issues, the present invention performs a navigation scan in succession with the formal scan, setting the polarity of the phase encoding to the opposite polarity to that of the formal scan, and uses the navigation echo obtained in the navigation scan to correct image distortion.

[0015] That is, the MRI apparatus of the present invention includes: a measurement unit that collects nuclear magnetic resonance signals from a subject using a pulse sequence; and an image processing unit that generates an image using the nuclear magnetic resonance signals collected by the measurement unit. The measurement unit executes a pulse sequence after applying (1 excitation) an excitation RF pulse, the pulse sequence performing a formal scan that measures echo signals for image generation and a navigation scan that measures echo signals for distortion correction, and the pulse sequence includes the navigation scan in which the application polarity of the phase encoding is opposite to that of the application polarity of the phase encoding of the formal scan. The image processing unit includes: a displacement calculation unit that calculates the displacement of the image using the echo signals obtained in the navigation scan; and a distortion correction unit that corrects the distortion of the image using the calculated displacement.

[0016] Furthermore, in the control method of the magnetic resonance imaging (MRI) apparatus of the present invention, the MRI apparatus includes: a measurement unit that collects nuclear magnetic resonance signals from a subject using a pulse sequence; and an image processing unit that generates an image using the nuclear magnetic resonance signals collected by the measurement unit. The control method of the MRI apparatus includes: a measurement step in which the measurement unit executes a pulse sequence after applying an excitation RF pulse, the pulse sequence performing a formal scan for measuring echo signals for image generation and a navigation scan for measuring echo signals for distortion correction, and the pulse sequence including a navigation scan in which the phase encoding application polarity is opposite to that of the formal scan; and an image processing step in which the image displacement is calculated using the echo signals obtained in the navigation scan in which the phase encoding application polarity is opposite to that of the formal scan, and the image distortion is corrected using the displacement.

[0017] The image processing steps may include: using the echo signal obtained in the navigation scan to perform phase correction on the image of the formal scan.

[0018] Invention Effects

[0019] According to the present invention, by using a navigation scan image that generates image distortion in the opposite direction to the formal scan image, distortion of the formal scan image can be easily corrected without using magnetographs or the like. Furthermore, since navigation scan data can be collected continuously within one excitation along with the formal scan, the imaging time is not extended to obtain data for distortion correction.

[0020] This invention is suitable for use in DWI, but it can be applied as long as it uses a sequence of cameras that continuously collect echoes after one excitation, and can obtain images without distortion. Attached Figure Description

[0021] Figure 1 This is a diagram showing the overall structure of the MRI device using the present invention.

[0022] Figure 2 This is a diagram representing a typical DWI sequence.

[0023] Figure 3 This is a functional block diagram of the computer included in the MRI device of the embodiment.

[0024] Figure 4 This is a flowchart illustrating an example of the operation of an MRI device according to an implementation method.

[0025] Figure 5 This is a diagram illustrating an example of the pulse sequence in Implementation Method 1.

[0026] Figure 6 This is a diagram representing an example of k-space data obtained during formal and navigation scans.

[0027] Figure 7 This is a diagram illustrating the process of Implementation Method 1.

[0028] Figure 8 This is a functional block diagram of the computer in Implementation Method 2.

[0029] Figure 9 This is a diagram illustrating an example of k-space data obtained during formal scanning and navigation scanning in Implementation Method 2.

[0030] Figure 10 This is a diagram illustrating the process of Implementation Method 2.

[0031] Figure 11 This is a diagram illustrating an example of the pulse sequence in Implementation Method 3.

[0032] Figure 12 This is a diagram illustrating the processing in Implementation Method 3.

[0033] Figure 13 This is a functional block diagram of a computer according to a variation of implementation method 3.

[0034] Figure 14 This is a diagram illustrating the processing of variation 1 of embodiment 3.

[0035] Figure 15 This is a diagram illustrating the processing of variation 2 of embodiment 3.

[0036] Figure 16 This is a diagram illustrating an example of a GUI for an MRI device.

[0037] Explanation of reference numerals in the attached figures

[0038] 1: MRI apparatus; 10: Measurement unit; 11: Magnet; 12: Inclined magnetic field coil; 13: Inclined magnetic field power supply; 14: High-frequency magnetic field generator; 15: Probe; 16: Receiver; 17: Sequence generator; 20: Computer; 21: Measurement control unit; 22: Image processing unit; 23: Display control unit; 30: Input device; 40: Display device; 50: Storage device; 60: Subject; 211: Formal scan control unit; 212: Navigation scan control unit; 221: FT unit; 222: Displacement calculation unit; 223: Formal scan distortion correction unit; 224: Multi-excitation synthesis unit; 225: Navigation scan distortion correction unit. Detailed Implementation

[0039] The embodiments of the MRI device of the present invention will be described below with reference to the accompanying drawings.

[0040] Figure 1This is a block diagram showing the general structure of an MRI apparatus 1. The MRI apparatus 1 includes: a magnet 11 that generates a static magnetic field in a direction parallel to the subject; a tilting magnetic field coil 12 that generates a tilting magnetic field; a tilting magnetic field power supply 13; a high-frequency magnetic field generator 14; a probe 15 that irradiates the high-frequency magnetic field and detects the nuclear magnetic resonance signal (echo); a receiver 16; a sequence generator 17; and a computer 20. The subject (e.g., a biological object) 50 is placed on a table (workbench) or similar surface and positioned within the static magnetic field space generated by the magnet 11. Hereinafter, the magnet 11, the tilting magnetic field coil 12, the tilting magnetic field power supply 13, the high-frequency magnetic field generator 14, the probe 15, the receiver 16, and the sequence generator 17 are collectively referred to as the measurement unit 10.

[0041] The sequence generator 17 sends commands to the tilted magnetic field power supply 13 and the high-frequency magnetic field generator 14, causing them to generate a tilted magnetic field and a high-frequency magnetic field, respectively. The generated high-frequency magnetic field is applied as a pulsed high-frequency magnetic field (RF pulse) to the object under test 60 via the probe 15. The nuclear magnetic resonance signal generated from the object under test 60 is received by the probe 15 and detected by the receiver 16. The nuclear magnetic resonance signal is referred to here as an echo signal because it is collected as a signal generated as a typical gradient echo or spin echo.

[0042] The nuclear magnetic resonance frequency (detection reference frequency f0) used as the reference for detection in receiver 16 is set by sequence generator 17. Sequence generator 17 controls the operation of each component with pre-programmed timing and intensity. The program that specifically describes the high-frequency magnetic field (RF pulse), tilting magnetic field, and the timing and intensity of signal reception is called the pulse sequence.

[0043] Various pulse sequences are known depending on the objective; in this embodiment, a sequence that continuously collects multiple echoes after a single excitation is employed. Examples include EPI (Echo Planar Imaging) sequences and FSE (Fast Spin Echo) sequences. EPI sequences also include EPI sequences using the SE (Spin Echo) system with inverted pulses, and multi-excitation EPI sequences that collect echoes through multiple excitations. Furthermore, MPG (Motion Probing Gradient) is sometimes used to obtain diffusion-weighted images.

[0044] exist Figure 2An example of an SE-EPI sequence including MPG pulses is shown. As illustrated, after an excitation RF pulse 301 is applied together with a co-segment selected tilt magnetic field 303, a reverse RF pulse 302 is applied together with a co-segment selected tilt magnetic field 304 to excite the desired segment. High-intensity MPG pulses 309 are applied before and after this reverse RF pulse 302. Next, after applying a phase-encoded tilt magnetic field 305, a pointed phase-encoded tilt magnetic field 306 and a polarity-reversed lead-out tilt magnetic field 307 are applied consecutively, and the echo signal 308 is collected during the application of the reversed lead-out tilt magnetic field 307. Figure 2 The diagram shows the case where an MPG pulse is applied along the axis of the tilted magnetic field Gs, but typically the axis (direction) of the applied MPG pulse 309 is different for multiple imaging operations. In a single-excitation EPI, all echo signals required for image reconstruction are measured with a single excitation.

[0045] and then, Figure 2 Although not shown in the diagram, in this embodiment, a sequence for collecting navigation echoes used in distortion correction is added after the collection of echo signals for image reconstruction. The sequence up to the collection of echo signals for image reconstruction is called the formal scan, and the sequence following it for collecting navigation echoes is called the navigation scan. Details of the navigation scan will be described later.

[0046] Computer 20 controls the operation of the aforementioned measurement unit 10 and receives signals detected by the measurement unit (receiver 16) 10, namely the echo signal from the formal scan and the signal from the navigation scan (navigation echo), and performs signal processing such as image reconstruction. Furthermore, computer 20 may include: an input device 30 for user input of camera conditions, processing conditions, etc.; a display device 40 for displaying an image of the detected object obtained as a result of signal processing, serving as a user interface (GUI); and a storage device 50 for storing processing results, processing data, etc. In addition to storage devices connected to computer 20 via wired or wireless connections, storage device 50 may also include external storage devices connected via a network.

[0047] exist Figure 3 An example of the structure of a computer 20 that implements the above functions is shown. For example... Figure 3As shown, the computer 20 includes: a measurement control unit 21 that controls the operation of the measurement unit 10; an image processing unit 22 that performs operations 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 formal scan control unit 211 and a navigation scan control unit 212, which control the measurement unit 10 by sending a given pulse sequence and user-specified imaging conditions (imaging parameters) to the sequence generator 17. The image processing unit 22 includes: an FT unit 221 that performs operations such as Fourier transform to transform data (k-space data) obtained by configuring measurement signals in k-space into real-space data; a displacement calculation unit 222 that calculates the distortion (displacement) generated in the image using data obtained in the navigation scan; and a distortion correction unit 223 that corrects the distortion of the image in the formal scan, etc., using the displacement calculated by the displacement calculation unit 222 (in the figure, a formal scan distortion correction unit that corrects the distortion of the formal scan image is shown as an example). These structures are used to generate distortion-corrected images, such as diffusion-weighted images. The display control unit 23 displays the image along with necessary supplementary information (such as information related to imaging conditions and the object being detected) on the display device 40. The computer 20 stores the detected signal, imaging conditions, and signal-processed image information in the storage device 50 as needed.

[0048] The functions of computer 20 are implemented by uploading pre-designed programs to the CPU or GPU of computer 20. In addition, some of the functions of computer 20 are sometimes implemented by hardware such as ASIC, FPGA, etc.

[0049] exist Figure 4 The following is an overview of the operation of the MRI apparatus in the above-described structure (control by each control unit of the computer 20). First, if the pulse sequence and imaging conditions are set according to the examination protocol set by the user via the input device 30 or preset, the formal scan control unit 211 and the navigation scan control unit 212 send commands to the sequence generator 17 according to the set conditions. Under the control of the sequence generator 17, the measurement unit 10 executes an EPI sequence including the navigation scan, collects the echo signals of the formal scan and the navigation scan respectively, and obtains formal scan data and navigation scan data as measurement spatial data (S401).

[0050] In the image processing unit 22, firstly, the FT unit 221 transforms the formal scan data and navigation scan data into complex data in real space, generating a formal scan image and a navigation scan image (S402). Since the formal scan data and navigation scan data have opposite polarities in the application of phase encoding, the distortions generated in the formal scan image and navigation scan image due to the accumulation of phase errors caused by static magnetic field inhomogeneity, etc., become distortions in opposite directions. The displacement calculation unit 222 uses the navigation scan image to calculate the displacement amount (real space displacement amount) used in distortion correction (S403). The distortion correction unit 223 uses the displacement amount to perform distortion correction on the formal scan image (S404).

[0051] The echo plane sequence performed in measurement step S401 can be a single-excitation EPI, where all echoes required for image reconstruction are collected by applying a single RF pulse (referred to as 1 excitation), or a multi-excitation EPI. In the case of multi-excitation EPI, it is preferable to include a process for correcting the phase error generated between excitations (S405). Particularly in imaging methods such as DWI that use a strong tilting magnetic field (MPG), the phase error between excitations becomes larger in the case of multi-excitation, so the aforementioned phase correction is preferable.

[0052] The embodiments of the MRI device of the present invention will be described below based on the outline of the MRI device described above. In the following description, [the description will focus on the MRI device of the present invention]. Figures 1-4 The structure is common to all structures; please refer to these figures as appropriate.

[0053] <Implementation Method 1>

[0054] In this embodiment, after one excitation RF pulse, the measurement unit 10 performs a single-excitation formal scan to collect all echo signals for image reconstruction, followed by a navigation scan. In the navigation scan, the polarity of the phase-encoding tilt magnetic field is reversed compared to the formal scan, resulting in an image with distortion in the opposite direction to the image obtained in the formal scan. The distortion of the formal scan image is corrected using the displacement of this image.

[0055] exist Figure 5 An example of the pulse sequence of this embodiment is shown.

[0056] This pulse sequence is executed by the measurement unit 10 under the control of the measurement control unit 21 (formal scan control unit 211 and navigation scan control unit 212). For example... Figure 5 As shown, in this pulse sequence, after applying one excitation pulse 301, it includes: a formal scan 300 applying a reversal pulse 302 to collect echoes for image acquisition; and a navigation scan 310 collecting navigation echoes after applying a subsequent reversal pulse 312. Figure 5 In, with Figure 2 Unlike the case where the MPG pulse 309 is applied along the Gp axis, the application axis of the MPG pulse is arbitrary. Regarding the actual scan, other structures are similar. Figure 2 same.

[0057] In navigation scan 310, the polarity of phase-encoded pulse 316 is made opposite to that of phase-encoded pulse 306 in the formal scan 300, and navigation echoes 318 are collected whenever the derived pulse 317 is reversed. The number and range of the collected navigation echoes 318 are not particularly limited as long as an image with the full FOV (the same FOV as the formal scan image) can be obtained as the navigation scan image to be reconstructed. In the illustrated example, the phase-encoded pulse 306 is applied with a lower intensity than the formal scan, and data is collected near the center of the k-space (low-frequency region).

[0058] The formal scan 300 and the navigation scan 310 collect echoes by setting the polarity of the phase-encoded tilted magnetic field to opposite values, thus... Figure 6 As shown, in the formal scan, the k-space is scanned from top to bottom, whereas in the navigation scan, it is scanned from bottom to top. Or vice versa. Figure 6 In the image, the left side shows the data from the formal scan (600), and the right side shows the data from the navigation scan (610).

[0059] Next, refer to Figure 7 The processing of the image processing unit 22 will now be explained. First, the Fourier Transform (FT) unit 221 transforms the k-space data 600 obtained in the formal scan and the k-space data 610 obtained in the navigation scan into real-space data (image data) 601 and 611, respectively, by performing Fourier Transform (FT). Then, zero-padding is applied to the surrounding data to make the matrix size of the resulting image the same as that of the formal scan image, and the k-space data 610 obtained in the navigation scan is then subjected to FT. Thus, although the spatial resolution of the navigation scan image is low, it becomes an image with the same matrix size as the formal scan image.

[0060] Next, the displacement calculation unit 222 calculates the displacement used to correct the formal scanned image 600. If the distortion of a certain point (x, y) is set as Δy [mm], the distortion generated in each image 601, 611 is characterized by the following formula.

[0061]

Mathematical Formula 1

[0062]

[0063] Here, f0 is the frequency [kHz], IET is the echo interval in a single excitation [ms], FOVy is the FOV in the y direction [mm], R is the speed multiplier in the case of parallel imaging, and Nsh is the number of excitations in the case of multi-excitation imaging (the same applies below).

[0064] As shown in equation (1), since a frequency f0 is required to calculate the distortion of each image, a magnetic field map is needed. In this embodiment, the displacement is calculated by utilizing the fact that the distortions of the two images are in opposite directions. Specifically, in the formal scan image and the navigation scan image, since the distortion directions are opposite, therefore... Figure 7 As shown, for example, if the formal scan image 601 is an image that is elongated in the phase direction, the navigation scan image 611 is an image that is shortened in the phase direction (an image that is elongated in the readout direction). The displacement calculation unit 222 uses these images to calculate the displacement amount to correct the distortion of the formal scan image.

[0065] There are various known methods for determining the magnetic field distribution from two images with different polarities, such as the method described in Non-Patent Document 1.

[0066] The distortion correction unit 223 uses the calculated displacement to correct the distortion of the formal scan image, generating a corrected image 602. The image processing unit 22 performs this correction process on each image with a different MPG, obtaining the desired DWI image.

[0067] According to this embodiment, by collecting echo signals of distortions that generate images in the opposite direction during navigation scanning, distortion correction can be performed without extending the imaging time. Furthermore, while the above embodiment describes the imaging method as DWI, this embodiment can also be applied to EPI-based imaging that does not use MPG.

[0068] <Implementation Method 2>

[0069] Implementation 1 uses a single-excitation EPI sequence, but in this implementation, a multi-excitation EPI sequence is used, and the images of each excitation are synthesized after phase correction between excitations is performed using a pilot echo. Then, the pilot echo is used to correct image distortion.

[0070] Therefore, the computer 20 of this embodiment is as follows: Figure 8 As shown, a multi-excitation synthesis unit 224 is added to the image processing unit 22. Other elements and Figure 3 The same applies to Implementation Method 1 shown.

[0071] The following describes the processing of this embodiment.

[0072] Under the control of the measurement and control unit 21, a multi-excitation EPI sequence is executed. The EPI sequence used in this embodiment is identical to the one used in this embodiment, except for the number of echo signals collected in a single excitation and the phase encoding steps. Figure 5 Similarly, the pulse sequence shown repeats the process from excitation RF pulse 301 to navigation scan, with varying numbers of excitations. In each excitation of the formal scan, different rows of data in the k-space are collected by differentiating the phase encoding offset 305 and the phase encoding steps, ultimately yielding... Figure 9 The k-space data 800 is shown. Following the formal scan, the navigation scan, similar to Embodiment 1, uses phase encoding with the opposite polarity to the formal scan to obtain the navigation scan data 810 for the central region of the k-space.

[0073] The image processing unit 22 uses the formal scan data 800 obtained per excitation and the navigation scan data 810 to synthesize multiple formal scan images obtained in multiple excitations, and performs distortion correction on the synthesized image. Furthermore, in this embodiment, during synthesis, phase correction of the formal scan images of each excitation is performed using the phase information between excitations obtained from the navigation scan data obtained per excitation. Distortion correction is then performed on the synthesized formal scan image after phase correction, in the same manner as in Embodiment 1. (See reference...) Figure 10 To explain the details of these processes.

[0074] First, the FT unit 221 generates a formal scan image 801 and a navigation scan image 811 using formal scan data 800 and navigation scan data 810 for each excitation. The multi-excitation synthesis unit 224 calculates the phase difference between the images from the navigation scan image 811 obtained for each excitation, and uses the phase difference information to synthesize the formal scan images 801 of each excitation to obtain a single formal scan image 802. The phase is obtained based on the real and imaginary parts of the navigation scan image 811, which is a complex image, and the difference is performed between excitations to calculate information for correcting the phase difference between excitations. Phase correction can be performed on the formal scan images of each excitation using this phase difference.

[0075] There are various well-known examples of phase correction in multi-excitation cameras, such as the following formula.

[0076] (Mathematical Formula 2)

[0077]

[0078] Here, Characterizes the phase of the corrected final scan image. Characterizes the phase of the formal scanned image before correction. The phase of the navigation image is characterized. In this way, phase correction can be performed on a per-voxel basis in the image space.

[0079] By performing synthesis after phase correction in this way, a synthetic image 802 without artifacts caused by the phase difference of each excitation can be obtained.

[0080] Next, the displacement calculation unit 222 calculates the displacement using the formal scan composite image 802 and the navigation scan image 812, similar to Embodiment 1, and uses the displacement to perform distortion correction on the formal scan composite image 802. The navigation scan image 812 used in calculating the displacement can be an image obtained by combining each excited navigation scan image 811, or any of the navigation scan images 811 can be used.

[0081] According to this embodiment, since the phase error between images is corrected during multi-excitation synthesis, distortion correction with better accuracy can be performed.

[0082] <Implementation Method 3>

[0083] In Embodiments 1 and 2, a navigation scan is performed with the phase encoding applied in polarity opposite to that of the formal scan. However, this embodiment is characterized by collecting a pair of navigation echoes with different phase encoding applied polarities during the navigation scan, obtaining a pair of navigation scan images with different distortion directions. Using the phase difference information and distortion information contained in this pair of navigation scan images, a distortion-corrected formal scan image is obtained. This embodiment can also be applied to cases where the formal scan is a single-excitation EPI; however, the case of a multi-excitation EPI will be used as an example for explanation.

[0084] The structure of the computer 20 in this embodiment and Figure 8 Similarly, the displacement calculation unit 222 functions as a displacement map calculation unit that uses a pair of navigation scan images to calculate the displacement map.

[0085] The following is for reference Figure 11 as well as Figure 12 To explain the processing of this embodiment

[0086] Under the control of the measurement control unit 21, a multi-excitation EPI sequence is executed. Figure 11An example of the EPI sequence used in this embodiment is shown. In this pulse sequence, the formal scan 300 is the same as the formal scan in Embodiment 2 (or Embodiment 1), but the navigation scan consists of a scan 310-1 (first navigation scan) with phase encoding of the same polarity as the formal scan and a scan 310-2 (second navigation scan) with phase encoding of the opposite polarity. The order of navigation scan 310-1 and navigation scan 310-2 is arbitrary. In the two navigation scans, data of the central region of k-space is collected in the same manner as in Embodiments 1 and 2. Thus, two (or two sets) of navigation scan data scanning k-space in opposite directions are obtained.

[0087] Image processing unit 22 as follows Figure 12 As shown, firstly, the FT unit 221 transforms the formal scan data 800 and navigation scan data 810-1 and 810-2 obtained from each excitation into real space data, respectively, to generate a formal scan image 801 and two navigation scan images 811-1 and 811-2.

[0088] The two navigation scan images 811-1 and 811-2 have different phase encoding polarities, so the distortion directions are different. For example, one becomes an image that is elongated in the phase encoding direction, and the other becomes an image that is shortened in the phase encoding direction. The displacement calculation unit 222 calculates the displacement of this pair of synthesized navigation scan images 812-1 and 812-2 for each pixel and creates a displacement map 815 (S1203).

[0089] The displacement can be obtained by taking the frequency distribution proportional to the magnetic field distribution from two images with different polarities, and then transforming it into a displacement using the aforementioned equation (1). The method for estimating the magnetic field distribution from two images with different polarities can use known methods such as those described in Non-Patent Document 1.

[0090] In addition, the navigation scan images 811-1 and 811-2 used to determine the displacement can be images synthesized from images obtained by each excitation, or navigation scan images obtained by a given excitation can be used.

[0091] On the other hand, the multi-excitation synthesis unit 224 uses information about the phase difference between the excitations of the images (images of each excitation) 812-1 obtained in the navigation scan (with the same phase encoding application polarity as the formal scan) in the two navigation scan images to perform phase correction on the formal scan image 801 of each excitation, and then performs synthesis to generate the synthesized formal scan image 802. The synthesis process including this phase correction can be performed using the same method as in Embodiment 2.

[0092] Finally, the formal scan distortion correction unit 223 uses the displacement map 815 created by the displacement calculation unit 222 to correct the synthesized formal scan image 802, and obtains the corrected formal scan image 803.

[0093] According to this embodiment, since a pair of navigation scan data with different polarities of phase encoding is used, and the displacement is calculated using navigation scan images with opposite distortion directions, the displacement can be calculated with good accuracy compared to the case where the displacement is calculated from the navigation scan image and the formal scan image as in Embodiment 1 and Embodiment 2, and distortion correction with good accuracy can be performed.

[0094] Furthermore, when synthesizing the formal scan images, the accuracy of the correction regarding the phase difference between excitations is improved because phase difference information obtained from the navigation scan image with the applied polarity of the phase encoding set to the same polarity is used.

[0095] Furthermore, the above explanation describes the case of a multi-excitation EPI scan, but in the case of a single excitation, [the following will be discussed]. Figure 12 The synthesized final scan image 802 is replaced with the final scan image obtained by excitation 1, and the phase correction between excitations is omitted. With just this change, it can be used in the same way.

[0096] <Modifications of Implementation Method 3>

[0097] In this modified example, two navigation scan data with different axes of positive and negative are used as navigation scan data with phase encoding, just like in Embodiment 3. However, in Embodiment 3, the displacement map is calculated from these navigation scan data and the distortion of the synthesized formal scan image is corrected. In this modified example, the navigation scan image is distorted and the distorted navigation scan image is used to perform multi-excitation synthesis of the formal scan images of each excitation.

[0098] In this variation, such as Figure 13 As shown, a navigation distortion correction unit 225 is added to the structure of the image processing unit 22. The displacement calculation unit 222 can also be omitted.

[0099] The following explanation will focus on the differences between the image processing unit 22 and Embodiment 3. Figure 14 The process flow of this variation is shown.

[0100] In this modified example, the pulse sequence executed by the measuring unit 10 is the same as in embodiment 3, such as... Figure 14 As shown, with each excitation, a formal scan image 801 and two navigation scan images 811-1 and 811-2 are obtained from the formal scan data 800 and the navigation scan data 810-1 and 810-2 with different phase codes.

[0101] In Embodiment 3, phase correction is performed during multi-excitation synthesis of the formal scan image using a navigation scan image obtained by setting the phase encoding to the same polarity as the formal scan image. However, in this modified example, firstly, the navigation distortion correction unit 225 performs distortion correction on the two navigation scan images. Since the phase encoding polarities of the two navigation scan images are different, the distortion of the images is in different directions. Therefore, the navigation distortion correction unit 225 corrects the distortion of the two navigation scan images as follows, and performs distortion correction in a way that achieves the same degree of distortion as the formal scan image, to obtain the corrected navigation scan image 813.

[0102] This can be achieved by estimating and correcting the distortion of the frequency image f(x, y) from two navigation scan images using a known method as shown in Non-Patent Document 1, etc., and then re-distorting the image using Equation (1) corresponding to the FOV and IET of the formal scan image. Alternatively, after estimating the frequency image f(x, y), the positional deviation d(x, y) between the images at each coordinate can be calculated based on the imaging conditions of both images, and distortion correction can be performed accordingly. Here, d(x, y) can be calculated using the distortion values ​​of the formal scan and navigation scan images (denoted as Δy, respectively). scan and Δy navi The result is calculated using the following formula.

[0103]

Mathematical Expression 3

[0104]

[0105] The corrected navigation scan image 813 is obtained for each excitation. The multi-excitation synthesis unit 224 uses the inter-excitation phase error information of the corrected navigation scan image 813 to perform multi-excitation synthesis with phase correction on the formal scan image 801 of each excitation, and obtains the synthesized image 804.

[0106] In this modified example, phase correction is performed by using a navigation scan image 813 that has undergone distortion correction, which enables high-precision correction and can suppress artifacts caused by phase errors during multi-excitation synthesis with high precision.

[0107] Therefore, in this modified example, although the distortion correction section 223 of Embodiment 3 can be omitted, the same distortion correction as in Embodiment 3 can still be performed on the final scanned image after multi-excitation synthesis. Figure 15 This process is illustrated. For example... Figure 15As shown, in distortion correction, the displacement is calculated based on the two navigation scan images 811-1 and 811-2 in the same way as in embodiment 3, and a displacement map 815 is calculated. The distortion of the synthesized formal scan image 802 is corrected based on the displacement map.

[0108] According to this modified example, by using a distorted navigation scan image to perform phase correction during multi-excitation synthesis, the accuracy of phase correction can be further improved compared to embodiment 3. As a result, the accuracy of distortion correction of the image after excitation synthesis can also be improved.

[0109] Regarding the phase correction and distortion correction in the above-described embodiments, when the EPI sequence and its excitation number are set as the pulse sequence included in the imaging conditions, if it is a single excitation, distortion correction is automatically performed using the methods of Embodiment 1 and Embodiment 3 (including variations); if it is a multi-excitation, distortion correction is automatically performed using the methods of Embodiment 2 and Embodiment 3 (including variations). Figure 16 The camera condition setting screen 1600, which sets the pulse sequence and camera parameters as shown, allows for setting correction conditions when setting the pulse sequence and camera parameters.

[0110] For example, if EPI is set as the pulse sequence, you can choose whether to perform distortion correction. If "yes" distortion correction is selected, the user can select the desired distortion correction by opening the drop-down menu or a new settings screen.

[0111] The embodiments and variations thereof of the present invention have been described above, but the present invention is not limited to these embodiments. Various modifications, including combinations of known techniques, can be made regarding image reconstruction. Furthermore, the case of performing DWI has been described, but the present invention can also be applied to ordinary EPI sequences that do not use MPG pulses.

Claims

1. A magnetic resonance imaging device, characterized in that, have: The measurement unit uses a pulse sequence to collect nuclear magnetic resonance signals from the object being tested; and The image processing unit uses the nuclear magnetic resonance signals collected by the measurement unit to generate an image. The measurement unit executes a pulse sequence after applying one excitation RF pulse (i.e., one excitation). The pulse sequence performs a formal scan to measure the echo signal used for image generation and a navigation scan to measure the echo signal used for distortion correction. The pulse sequence includes a navigation scan in which the phase encoding application polarity is opposite to that of the phase encoding application polarity of the formal scan. The image processing unit includes: The displacement calculation unit uses the echo signal obtained during the navigation scan to calculate the displacement of the image; and The distortion correction section uses the calculated displacement to correct image distortion. The navigation scan includes: The phase encoding is applied to the same polarity as the formal scan in the first navigation scan; and The second navigation scan applies phase encoding with the polarity opposite to that of the formal scan. The displacement calculation unit calculates the displacement using the navigation scan image obtained in the first navigation scan and the navigation scan image obtained in the second navigation scan.

2. The magnetic resonance imaging device according to claim 1, characterized in that, The formal scanning sequence is a single-excitation sequence that collects all echoes used for image generation using a single excitation RF pulse. The distortion correction unit uses the displacement calculated from the navigation scan image obtained in the navigation scan to correct the distortion of the formal scan image.

3. The magnetic resonance imaging device according to claim 1, characterized in that, The formal scanning sequence is a multi-excitation sequence that uses two or more excitation RF pulses to collect echoes for image generation. The image processing unit also includes: The multi-excitation synthesis unit synthesizes the echo signals obtained in the formal scans of each excitation. The multi-excitation synthesis unit uses the phase information of the navigation scan images obtained in the navigation scan of each excitation to perform phase correction and synthesis on the images obtained in the formal scan of each excitation.

4. The magnetic resonance imaging device according to claim 3, characterized in that, The distortion correction unit uses the displacement calculated from the image obtained in the navigation scan to correct the distortion of the formal scan image synthesized by the multi-excitation synthesis unit.

5. The magnetic resonance imaging device according to claim 1, characterized in that, The formal scanning sequence is a multi-excitation echo planar sequence that uses two or more excitation RF pulses to collect echoes for image generation. The image processing unit also includes: The multi-excitation synthesis unit synthesizes the echo signals obtained in the formal scans of each excitation. The distortion correction unit uses the displacement to correct the distortion of the final scanned image synthesized by the multi-excitation synthesis unit.

6. The magnetic resonance imaging apparatus according to claim 5, characterized in that, The multi-excitation synthesis unit uses the phase information of the navigation scan image obtained in the first navigation scan to perform phase correction and synthesis on the images obtained in the formal scans of each excitation.

7. A magnetic resonance imaging device, characterized in that, have: The measurement unit uses a pulse sequence to collect nuclear magnetic resonance signals from the object being tested; and The image processing unit uses the nuclear magnetic resonance signals collected by the measurement unit to generate an image. The measurement unit executes a pulse sequence after applying one excitation RF pulse (i.e., one excitation). The pulse sequence performs a formal scan to measure the echo signal used for image generation and a navigation scan to measure the echo signal used for distortion correction. The pulse sequence includes a navigation scan in which the phase encoding application polarity is opposite to that of the phase encoding application polarity of the formal scan. The image processing unit includes: The displacement calculation unit uses the echo signal obtained during the navigation scan to calculate the displacement of the image; and The distortion correction section uses the calculated displacement to correct image distortion. The formal scanning sequence is a multi-excitation echo planar sequence that uses two or more excitation RF pulses to collect echoes for image generation. The navigation scan includes: The phase encoding is applied to the same polarity as the formal scan in the first navigation scan; and The second navigation scan applies phase encoding with the polarity opposite to that of the formal scan. The image processing unit also includes: A multi-excitation synthesis unit synthesizes the echo signals obtained in the formal scans of each excitation; and The navigation distortion correction unit uses the navigation scan image obtained in the first navigation scan and the navigation scan image obtained in the second navigation scan to generate a distortion-corrected navigation scan image. The multi-excitation synthesis unit uses the phase information of the distortion-corrected navigation scan image generated by the navigation distortion correction unit to perform phase correction and synthesis on the images obtained in the formal scans of each excitation.

8. The magnetic resonance imaging apparatus according to claim 7, characterized in that, The displacement calculation unit calculates the displacement using the navigation scan image obtained in the first navigation scan and the navigation scan image obtained in the second navigation scan. The distortion correction unit uses the displacement to correct the actual scan image synthesized by the multi-excitation synthesis unit.

9. The magnetic resonance imaging apparatus according to claim 1 or 7, characterized in that, The pulse sequence contains MPG pulses. The image processing unit generates a DWI image as the image for the formal scan.

10. A control method for a magnetic resonance imaging device, The magnetic resonance imaging device includes: The measurement unit uses a pulse sequence to collect nuclear magnetic resonance signals from the object being tested; and The image processing unit uses the nuclear magnetic resonance signals collected by the measurement unit to generate an image. The control method for the magnetic resonance imaging device is characterized by comprising: The measurement step involves the measurement unit executing a pulse sequence after the application of an excitation RF pulse. This pulse sequence performs a formal scan to measure the echo signal used for image generation and a navigation scan to measure the echo signal used for distortion correction. The pulse sequence includes the navigation scan in which the application polarity of the phase code is opposite to that of the formal scan. The image processing step uses the echo signal obtained in a navigation scan with phase encoding applied in the opposite polarity to the formal scan to calculate the image displacement, and uses this displacement to correct the image distortion. The navigation scan includes: The first navigation scan has the same phase encoding application polarity as the formal scan; and The second navigation scan has the phase encoding applied with the opposite polarity to that of the formal scan. The image processing step uses the images obtained in the first navigation scan and the images obtained in the second navigation scan to calculate the displacement used in the distortion correction of the image in the formal scan.

11. A control method for a magnetic resonance imaging device, The magnetic resonance imaging device includes: The measurement unit uses a pulse sequence to collect nuclear magnetic resonance signals from the object being tested; and The image processing unit uses the nuclear magnetic resonance signals collected by the measurement unit to generate an image. The control method for the magnetic resonance imaging device is characterized by comprising: The measurement step involves the measurement unit executing a pulse sequence after the application of an excitation RF pulse. This pulse sequence performs a formal scan to measure the echo signal used for image generation and a navigation scan to measure the echo signal used for distortion correction. The pulse sequence includes the navigation scan in which the application polarity of the phase code is opposite to that of the formal scan. The image processing step uses the echo signal obtained in a navigation scan with phase encoding applied in the opposite polarity to the formal scan to calculate the image displacement, and uses this displacement to correct the image distortion. The navigation scan includes: The first navigation scan has the same phase encoding application polarity as the formal scan; and The second navigation scan has the phase encoding applied with the opposite polarity to that of the formal scan. The image processing step further includes: The step of using phase information from the navigation scan image obtained in the first navigation scan to correct the phase of the formal scan image.

12. A control method for a magnetic resonance imaging device, The magnetic resonance imaging device includes: The measurement unit uses a pulse sequence to collect nuclear magnetic resonance signals from the object being tested; and The image processing unit uses the nuclear magnetic resonance signals collected by the measurement unit to generate an image. The control method for the magnetic resonance imaging device is characterized by comprising: The measurement step involves the measurement unit executing a pulse sequence after an excitation RF pulse is applied. The pulse sequence performs a formal scan to measure the echo signal used for image generation and a navigation scan to measure the echo signal used for distortion correction. The pulse sequence includes the navigation scan in which the phase encoding application polarity is opposite to that of the phase encoding application polarity of the formal scan. and The image processing step uses the echo signal obtained in a navigation scan with phase encoding applied in the opposite polarity to the formal scan to calculate the image displacement, and uses this displacement to correct the image distortion. The navigation scan includes: The first navigation scan has the same phase encoding application polarity as the formal scan; and The second navigation scan has the phase encoding applied with the opposite polarity to that of the formal scan. The image processing step further includes: The step of generating a distortion-corrected navigation scan image using the images obtained in the first navigation scan and the images obtained in the second navigation scan; and The step of using the distortion-corrected navigation scan image to correct the phase of the formal scan image.

13. The control method for the magnetic resonance imaging apparatus according to any one of claims 10, 11, and 12, characterized in that, The formal scanning sequence is a multi-excitation echo plane system sequence that uses two or more excitation RF pulses to collect echoes for image generation, with each excitation including the application of an MPG pulse.

Images