Magnetic resonance imaging apparatus and image reconstruction method

Abstract

To improve accuracy of image reconstruction causing body motion correction processing.SOLUTION: When a period during which body motions are occurring is a first period on the basis of a result of detecting the body motions to be inspected disposed in a static magnetic field space, a second period including the first period and longer than the same is identified. Of measurement data to be inspected that is collected by magnetic resonance imaging, data collected during the second period is removed or corrected to generate an image to be inspected. As the second period, a detection timing error can be complemented by extending a start end side of the first period, and the data collected during a period when a signal is unstable caused by the body motions can be removed by extending a terminal end side.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus), and more particularly to an MRI apparatus having a body motion processing function for detecting body motion and reconstructing an image in which the influence of the body motion is suppressed. [Background technology]

[0002] In an MRI examination, the subject is placed in a static magnetic field, nuclear magnetic resonance is generated within the subject, and the nuclear magnetic resonance signals are collected and processed to reconstruct an image of the subject. The subject of an MRI examination is typically a human being, such as a patient (hereafter referred to as the subject), who may experience sudden movements such as sneezing or coughing (called body movements to distinguish them from periodic movements such as breathing) during the examination, in addition to movements such as breathing. Because nuclear magnetic resonance signals are signals that have been assigned positional information by applying a gradient magnetic field, such body movements can result in the application of a gradient magnetic field at a position different from the original gradient magnetic field, which can cause artifacts in the images reconstructed from the nuclear magnetic resonance signals and make image reconstruction itself difficult.

[0003] MRI devices have adopted various measures to deal with the effects of such body movements. For example, the technology described in Patent Document 1 detects body movements of the subject from camera images and executes divided pulse sequences depending on whether the body movements are within an allowable range.

[0004] Also widely used is a technique for monitoring the movement of a subject using signals obtained by MRI (called navigator echoes) (Patent Document 2, Patent Document 3). In imaging using navigator data, navigator echoes are generated separately from nuclear magnetic resonance signals (hereinafter referred to as imaging signals) for obtaining images of the subject, and the subject's movement is estimated from changes in data (hereinafter referred to as navigator data) consisting of a series of navigator echoes obtained in time series, and the results are used to correct the imaging signals or reconstructed images.

[0005] When body movement is detected from navigator echoes or camera images, measures such as re-acquiring the image signals acquired when large body movement is detected or adopting an image reconstruction method that does not use those image signals are taken (for example, Patent Document 4). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2023-043001 [Patent Document 2] Japanese Patent Application Laid-Open No. 2012-000389 [Patent Document 3] Japanese Patent Application Laid-Open No. 2008-154887 [Patent Document 4] Japanese Patent Publication No. 2022-022669 Summary of the Invention [Problem to be solved by the invention]

[0007] However, even if body movement is detected using such conventional techniques and data collected when body movement is detected is processed, body movement artifacts may not be removed.

[0008] One reason for this is thought to be that, depending on the accuracy of the means for detecting body movement, there is an error between the timing of body movement detection and the actual time of body movement. Also, even if the body movement has stopped after being detected by a camera or navigator echo, the steady state of the echo signal may be disrupted by the body movement, which may cause artifacts. There is also the possibility that minute movements remain inside the subject that cannot be captured by a camera. In these cases, conventional methods cannot adequately remove artifacts.

[0009] An object of the present invention is to remove body movement artifacts that could not be removed by conventional body movement processing using body movement detection means, and to achieve even higher accuracy in suppressing body movement artifacts. [Means for solving the problem]

[0010] To solve the above problem, the present invention sets a second period in which body movement artifacts are expected, in addition to a first period that can be detected by conventional body movement detection means, and performs appropriate processing on the measurement data collected during this second period to reconstruct images.

[0011] That is, the MRI apparatus of the present invention comprises an RF transmitter that applies radio frequency magnetic field pulses to an object of examination placed in a static magnetic field space, an RF receiver that receives nuclear magnetic resonance signals emitted by the object of examination, a gradient magnetic field generator that generates gradient magnetic field pulses that impart a magnetic field gradient to the static magnetic field, an image generator that generates an image of the object of examination using measurement data consisting of the nuclear magnetic resonance signals, and a body motion processor that processes body motion information of the object of examination. The body motion processor detects a first period during which the object of examination is moving, and then identifies a second period that includes but is longer than the first period, and the image generator removes or corrects data collected during the second period from the measurement data to generate an image of the object of examination.

[0012] Furthermore, the image reconstruction method of the present invention is characterized in that it defines a period in which body movement occurs as a first period based on the results of detecting body movement of an object placed in a static magnetic field space, identifies a second period that includes the first period and is longer than the first period, and removes or corrects data collected during the second period from the measurement data of the object collected by magnetic resonance imaging, thereby generating an image of the object. [Effects of the Invention]

[0013] According to the present invention, by extending the period detected by the body movement detection means (the period during which body movement occurs) to at least one side, it is possible to eliminate the influence on the image of non-steady changes associated with body movement that cannot be detected by the body movement detection means but that cause artifacts, thereby achieving highly motion-suppressed image reconstruction. [Brief explanation of the drawings]

[0014] [Figure 1] Overall view of the MRI system [Figure 2] Functional block diagram of the MRI system's processor [Figure 3] FIG. 1 is a diagram showing an example of a flow of imaging and body movement processing. [Figure 4A] The upper figure (a) and the lower figure (b) each show an example of a pulse sequence including navigator data acquisition. [Figure 4B] FIG. 10 is a diagram showing another example of a pulse sequence including navigator data acquisition. [Figure 5] The upper figure (a) and the lower figure (b) are diagrams showing still another example of a pulse sequence including navigator data acquisition. [Figure 6] FIG. 1 is a diagram schematically illustrating the detection of body movement and a body movement detection period (first period); [Figure 7] FIG. 10 is a diagram illustrating the determination of a second period by the body movement processing unit. [Figure 8] FIG. 10 is a diagram illustrating an example of a method for determining the end of a second period. [Figure 9] FIG. 1 is a diagram illustrating an example of image reconstruction that reflects body movement information. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, an embodiment of the MRI apparatus of the present invention will be described.

[0016] MRI devices are classified into horizontal magnetic field types and vertical magnetic field types depending on the direction of the static magnetic field they generate, and various types of MRI devices are known, including permanent magnet types, paramagnetic electromagnet types, and superconducting magnet types for the magnets that generate the static magnetic field. The present invention can be applied to known MRI devices. The shape of the static magnetic field space in which the subject is placed can also be cylindrical, flat, sandwiched between upper and lower magnets, and the present invention can be applied to any of these.

[0017] First, an outline of an MRI apparatus to which the present invention is applied will be described with reference to FIG. As shown in the figure, the MRI apparatus 1 includes a static magnetic field generator (static magnetic field generating magnet) 101 that generates a static magnetic field, an RF transmitter 106 that applies radio frequency magnetic field pulses to an object 50 placed in a space in which the static magnetic field is generated by the static magnetic field magnet, an RF receiver 107 that receives nuclear magnetic resonance signals emitted by the object 50, a gradient magnetic field coil 102 and a gradient magnetic field power supply 105 (together referred to as a gradient magnetic field generator) that generate gradient magnetic field pulses that impart a magnetic field gradient to the static magnetic field, a sequencer 108 that controls the gradient magnetic field power supply 105, the RF transmitter 106, and the RF receiver 107 according to a predetermined pulse sequence, and a processor 20 that controls the entire apparatus including the sequencer 108. An RF transmitter coil 103 that applies RF pulses generated by the RF transmitter 106 to the subject and an RF receiver coil 104 that detects nuclear magnetic resonance signals emitted by the object 50 are arranged close to the object 50. Hereinafter, the RF transmitting unit 106, the RF receiving unit 107, and the gradient magnetic field generating unit will also be collectively referred to as the imaging unit 10.

[0018] The processor 20 can be configured as a general-purpose computer equipped with memory and a CPU, and functions as a control unit and a calculation unit. As shown in FIG. 1 , the processor 20 includes an imaging control unit 210 that controls the operation of the imaging unit 10, an image generation unit 220 that generates an image of the subject 50 using the nuclear magnetic resonance signals received by the RF receiving unit 107, a display control unit 250 that displays the image generated by the image generation unit 220, and a body movement processing unit 240 that processes signals from a detection means that detects the subject's body movement during the examination, acquires the magnitude and duration of the body movement (body movement information), and performs calculations and control to eliminate the effects of the body movement using the body movement information. These functions of the processor 20 are realized by the CPU uploading a predetermined program. Some of the functions can also be realized using a programmable IC.

[0019] The processor 20 is connected to an input device 30 through which the operator inputs commands and data necessary for imaging, a display device 40 that displays images generated by the image generation unit 220, a storage device 60, and the like. The storage device 60 includes an internal storage device and an external storage device, and the external storage device may be a storage device such as a cloud connected via the Internet, etc. The MRI apparatus 1 can also exchange data with an external database (not shown) such as a PACS. The input device 30 and the display device 40 are installed close to each other and function as a user interface unit 70.

[0020] The MRI apparatus 1 may also be equipped with an optical detection means such as a monitoring camera 80 for monitoring the state of the subject 50 placed in the static magnetic field space, and it is also possible to detect the body movement of the subject 50 using the video from the monitoring camera 80. In this case, the body movement processing unit 240 receives information from the monitoring camera 80 and performs processing using the information as body movement information together with measurement data (navigator data) from the imaging unit 10, which will be described later.

[0021] The configuration of the imaging unit 10 is the same as that of a conventional imaging unit, so a detailed description of the configuration will be omitted and only an outline of the operation will be given.

[0022] First, an RF pulse generated by an RF transmitter 106 is applied to the subject 50 via an RF transmitter coil 103, exciting the nuclei (usually protons) of atoms contained in the tissue that constitutes the subject 50. Nuclear magnetic resonance signals generated by the nuclear magnetic resonance of the excited nuclei are detected by an RF receiver coil 104 disposed close to the subject 50, and are received as digital signals by an RF receiver 107. Between the excitation by the RF pulse and the acquisition of the nuclear magnetic resonance signals, a gradient magnetic field generator applies a predetermined gradient magnetic field to the static magnetic field at a predetermined timing. This converts the nuclear magnetic resonance signals into signals with position information, i.e., encoded signals. Furthermore, applying a gradient magnetic field pulse simultaneously with excitation makes it possible to select a specific region (slice) of the subject and generate nuclear magnetic resonance signals only from the selected slice.

[0023] These series of operations of the imaging unit 10 are controlled based on a pulse sequence, which is a time chart that defines the intensities and application timings of RF pulses and gradient magnetic field pulses, as well as signal acquisition times, etc. Various pulse sequences that differ depending on the imaging purpose and imaging technique are stored in advance in the storage device 60 of the MRI apparatus, and a sequencer 108 calculates a pulse sequence to be used for imaging (referred to as an imaging sequence) using a pulse sequence selected according to the imaging purpose and imaging technique and imaging parameters set by the user (repetition time TR, echo time TE, FOV, slice thickness, number of slices, speed ratio, etc.), and imaging is performed according to the imaging sequence.

[0024] If the MRI apparatus has a body motion processing function, it detects the body motion of the subject placed in the static magnetic field space in parallel with imaging. Body motion detection can be performed using images from the above-mentioned monitoring camera 80 or the like, by adding a pulse sequence to the imaging sequence to generate navigator echoes that detect body motion and using data consisting of navigator echoes acquired in time series (hereinafter referred to as navigator data), or by using both. Any of these methods may be used. The body motion information obtained by these body motion detection means is reflected in subsequent processing such as image reconstruction.

[0025] The image generation unit 220 performs calculations such as Fourier transform and repetitive calculations using the nuclear magnetic resonance signals (measurement data) collected and digitized by the RF receiving unit 107 to reconstruct an image, and performs image processing on the generated image. At this time, the image is reconstructed after correcting the measurement data and removing data affected by body movement according to the results detected by the body movement processing unit 240 (body movement detection unit 230 in FIG. 2).

[0026] The image generated by the image generating unit 220 is displayed on the display device 40. Alternatively, the image may be stored in the storage device 60 or transmitted to an external database such as a PACS.

[0027] The above is a general imaging flow for an MRI device equipped with a body motion processing function. However, the present invention is characterized in that, rather than using the body motion detection results as they are and reflecting them in image reconstruction, the body motion processing unit 240 sets a range that is wider than the detection range of the body motion detection means in order to remove the influence of body motion artifacts that cannot be handled by the body motion detection means alone, and performs image reconstruction based on this range.

[0028] Taking into consideration the above-described configuration of the MRI apparatus, an embodiment of body movement processing will be described below.

[0029] <Embodiment> First, an example configuration of body movement processing unit 240 according to an embodiment is shown in Fig. 2. As shown in the figure, body movement processing unit 240 includes body movement detection unit 230, which takes in at least one of the video images (including the analysis results) from monitoring camera 80 and the navigator data collected by imaging unit 10 as body movement information, analyzes the body movement information, and detects the magnitude of the body movement, the duration of the body movement, and so on. Body movement processing unit 240 performs processing to expand the "period during which body movement is occurring (first period)," which is the detection result of body movement detection unit 230.

[0030] The processing of this embodiment will be described below along the processing flow shown in FIG. As shown in the figure, when imaging begins, detection of body movement of the subject begins in parallel (S1). If body movement is detected during imaging, the body movement detection unit 230 determines the period during which the body movement is detected as a first period (S2). The body movement processing unit 240 sets a second period by extending the first period according to a predetermined rule (S3). Imaging and body movement detection continue until one imaging sequence is completed. Of the measurement data (simply referred to as measurement data) used to generate an image of the subject obtained through a series of imaging, data collected during the second period is removed (S4).

[0031] It should be noted that there may be cases where data is corrected rather than removed. Here, as an example, data is removed. If the image generator 220 can perform image reconstruction using the measurement data after data removal, image reconstruction is performed as is (S6). If image reconstruction is not possible or remeasurement is required, remeasurement is performed (S5).

[0032] The specific content of each process will be described in detail below.

[0033] <Image capture and body movement detection: S1> As described above, body movement detection methods include a method using navigator data and a method using video images from a camera or the like.

[0034] [Method 1: Using navigator data] First, a method for acquiring navigator data along with imaging will be described. Various sequences for acquiring navigator data are known, including a method for acquiring navigator echoes by applying RF pulses that generate navigator echoes separately from the pulse sequence for the actual imaging, and a method for acquiring navigator echoes from signals generated as FIDs after RF pulses that selectively excite a predetermined region in the actual imaging. Any of these can be used. Furthermore, the timing of generating navigator echoes can be either before or after collecting echoes (image signals) for the actual imaging; either method is acceptable.

[0035] 4A, 4B, and 5 show examples of acquiring a navigator echo as an FID, and examples of acquiring a navigator echo by executing a sequence that generates a navigator echo separately from the imaging sequence. These figures show, as an example, a multi-slice measurement in which multiple slices are measured sequentially within 1 TR. However, the present invention is not limited to multi-slice. Furthermore, the figures only show RF pulses, navigator echoes, and imaging signals, and do not include gradient magnetic field pulses. However, various gradient magnetic field pulses are applied depending on the imaging sequence. However, phase encoding is not imparted to the navigator echo.

[0036] Example (a) shown in Figure 4A is an example suitable for imaging with a relatively long TR, in which navigator echoes Navi1, Navi2, and Navi3 are acquired as FIDs after RF pulses 401, 402, and 403 that excite slices S1, S2, and S3, respectively, and then image signals 411, 412, and 413 are collected. Example (b) shown in Figure 4A is an example suitable for imaging with a relatively short TR, in which navigator echo Navi1 is acquired after an RF pulse that excites slice S1, and image signals 411 to 413 are collected without acquiring a navigator echo when measuring slices S2 and S3. A second navigator echo Navi2 is acquired when acquiring image signal 411 for slice S1 again after the TR of slice S1. However, TE is maintained constant for each slice.

[0037] In this way, in the example shown in Figure 4A, by changing whether to acquire navigator echoes for each slice or for each TR depending on the length of the TR, the frequency of navigator echo acquisition, i.e., the frequency of body motion detection, can be maintained to a certain extent. Note that in Figure 4A, the navigator echo is acquired after the RF pulse and before the acquisition of imaging signals, but as shown in Figure 4B, it is also possible to acquire the navigator echo after the acquisition of imaging signals. In this way, in short TR imaging, by acquiring the navigator echo for each TR after the acquisition of imaging signals, it is possible to avoid the TE extension that accompanies navigator echo acquisition, and to achieve the effect of shortening the TR.

[0038] 5(a) and 5(b) show examples of multi-slice measurement in which the timing of acquiring navigator echoes differs between long TR and short TR, as in FIG. 4A. In these examples, a navigator sequence including application of an RF pulse 400 and acquisition of a navigator echo is executed prior to or following an imaging sequence for acquiring image signals 411 to 413. In this case, the RF pulse 400 may be a selective excitation pulse that selects the entire imaging target region, or, in the case of a pulse sequence in which a navigator sequence is inserted for each slice, as in the upper side (a) of FIG. 5, it may be a slice-selective excitation pulse similar to the selective excitation pulse for each slice.

[0039] The method for acquiring navigator data may be preset according to the imaging sequence and the set TR, or may be selectable by the user via the user interface unit 70, etc., taking into consideration the priority of the frequency of body movement detection, etc.

[0040] The body movement detection unit 230 analyzes the navigator data acquired in time series during imaging including the acquisition of such navigator data, and detects body movement of the specimen object. As a method for detecting body movement by the body movement detection unit 230, a method of detecting from signal intensity will be described.

[0041] FIG. 6 shows a schematic diagram of changes in the signal intensity of navigator echoes. As shown in the figure, when the subject is not moving, the non-phase-encoded navigator echoes are signals from the entire excited region, and the signal intensity is stable. Furthermore, in areas where periodic movement such as respiratory movement occurs, the signal intensity fluctuates slowly. On the other hand, when movement occurs in the measurement region due to body movement, the signal intensity fluctuates significantly. The body movement detection unit 230 can detect body movement by analyzing the characteristics of such fluctuations, such as the magnitude and frequency of the signal intensity fluctuations.

[0042] For example, a reference signal is determined by averaging multiple navigator echoes, and the root sum of squares (RMS) is calculated by subtracting each navigator echo from this reference signal. Navigator echoes with large errors (RMS) from the reference signal are identified, and navigator echoes in which errors are detected are identified. The period between the navigator echoes in which errors are detected is defined as the first period "t1-t2."

[0043] This method allows for easy body movement determination using navigator echoes, and improves the real-time nature of body movement detection. However, the body movement detection method of this embodiment is not limited to this method, and calculations using the phase difference between the navigator echo and the reference navigator echo can also be used.

[0044] [Method 2: Using camera footage] Next, a second method using optical detection means will be described. Here, as an example, a case will be described in which an image from a monitoring camera 80 monitoring the object to be inspected is analyzed to detect displacement of the object to be inspected.

[0045] The monitoring camera 80 is one or more cameras installed near or within the static magnetic field space in which the subject 50 is placed. The camera is preferably a wide-angle camera or a stereo camera, capable of monitoring a fairly wide range of the subject. The camera is connected to the processor 20 of the MRI device by wire or wirelessly, and the camera images are captured by the processor 20 for each frame. The body movement detection unit 230 of the processor 20 captures the image data from the camera and analyzes the camera images to detect the occurrence of body movement.

[0046] Specifically, the body movement detection unit 230 calculates the displacement vector of the subject for each frame of video data by performing calculations such as optical flow. A predetermined ROI may be set on the subject, and the displacement vector may be calculated for the ROI. The displacement vectors from the start of imaging to a predetermined time point are integrated to determine the displacement relative to the position at the start of imaging (reference position). Alternatively, the difference between the image of a frame (the frame at the start of imaging) used as the reference for the displacement vector and the image of a frame at a predetermined time point may be determined to be the displacement at that time point. The displacement thus calculated is a gentle curve with the period of respiratory movement in areas where respiratory movement occurs, as shown in FIG. 6, for example. However, when a sudden movement (body movement) occurs, the displacement corresponds to the movement.

[0047] The body movement detection unit 230 uses the range of respiratory movement (upper and lower limits of the displacement) as a threshold for the displacement, and when it detects a displacement above or below that threshold, it determines that body movement is occurring and identifies the time during which the body movement is occurring.

[0048] Method 2 does not require the insertion of a navigator sequence into the imaging sequence, and therefore body movement detection is possible without affecting the TR and TR of the imaging sequence.

[0049] Furthermore, a method for detecting body movements using both navigator data and camera images (Method 3) may be adopted. In this case, the time when body movements occurred, which is the result of analyzing the navigator data, is ANDed or ORed with the time when body movements occurred identified by analyzing the camera images, and this is used as the time when body movements occurred to identify the image data collected at the time when body movements occurred and use it as the removal data.

[0050] Furthermore, in Method 1 using navigator data, if the timing of acquiring navigator echoes is varied depending on the length of the TR, navigator data may be obtained for each slice, or only for a specific slice (for example, the central slice). In the latter case, accurate body motion information cannot be obtained for slices other than the specific slice. For slices for which navigator data cannot be obtained, body motion detection is performed using complementary camera images. If the slice from which navigator data is acquired is the central slice, an ROI is set in the surrounding area of the imaging region and the camera images are analyzed. With this method, the area for analyzing the camera images is limited, which reduces the analysis load and improves real-time body motion detection.

[0051] According to Method 3, the accuracy of body movement detection can be further improved by using camera images in combination with body movement detection. For example, since navigator echoes are signals from a specific region, they cannot grasp the relationship with body movements occurring in other regions. However, camera images can grasp the overall body movement of the region displayed on the image or the region in which the ROI is set, making it possible to detect all body movements that could cause artifacts. Conversely, because the subject is covered by a receiving coil, etc., the camera may not be able to capture the movement of the area being examined. However, by using navigator data, the accuracy of body movement detection can be improved, and in particular, by using camera and navigator data in combination, further improvement in accuracy can be achieved.

[0052] When detecting body motion using navigator data or camera images, there may be errors in the timing of body motion detection. Furthermore, depending on the type of body motion, artifacts may occur even after the body motion has subsided, until the echo signal, which was disrupted by the body motion, returns to a steady state. In such cases, even if the first period detected by the body motion detection unit 230 is considered to be the period in which body motion occurred and subsequent processing is performed, body motion artifacts cannot be sufficiently suppressed.

[0053] In the next process, the first period detected by the body movement detector 230 is extended to at least one of the preceding and following periods, thereby further improving the suppression of body movement artifacts.

[0054] <Processing of the body movement processing unit: S3, S4> When body movement detection section 230 detects a first period, body movement processing section 240 sets an expanded second period. The second period includes the first period and extends beyond the first period at least in one direction. FIG. 7 shows a method for determining the second period. In FIG. 7, the horizontal direction indicates the progress of imaging, with the upper diagram showing the absence of body movement, the middle diagram showing the first period detected by body movement detection section 230, and the lower diagram showing the second period determined by body movement processing section 240.

[0055] 7, the body movement processor 240 either extends the first period (t1-t2) in the time progression direction (second time = t1-tb), includes a time going back from the first time (second time = ta-t2), or extends it forward and backward (second period = ta-tb).Which of these may be determined in advance or may vary depending on the method for specifying the second period and the characteristics of the body movement.

[0056] The reason for extending the second period toward the beginning is mainly to compensate for errors in the timing of detecting body movements, and a predetermined value, for example, about 500 ms, can be determined in advance.

[0057] The terminal side is an extension that is mainly intended to secure the time until the steady state of the echo signal is stabilized after it is disrupted by body movement. In this case, too, a predetermined value, for example, 500 ms to 5000 ms, may be determined in advance based on an empirically obtained value.

[0058] Alternatively, when acquiring navigator data as a method for detecting body motion, the signal intensity of the navigator echo may be used to determine the motion. The navigator echo remains nearly constant and stable in the absence of body motion. However, after the signal intensity fluctuates due to body motion, it takes some time for the signal intensity to return to a stable state after the body motion subsides. This is schematically illustrated in FIG. 8. FIG. 8 shows an example in which the navigator echo is acquired as an FID. After an RF pulse 800, a navigator echo 801 is acquired as an FID, and then a phase-encoded imaging signal 811 is measured. Both the navigator echo and the imaging signal become unstable due to body motion. However, since the signal intensity of a non-phase-encoded navigator echo remains constant in a stable state, the stabilization of the signal can be easily detected by monitoring the change in the signal intensity of the navigator echo. Therefore, the body motion processor 240 can detect that a stable state has been reached from the change in the intensity of the navigator echo and identify the end point of the second period.

[0059] The determination of a stable state can be made, for example, by calculating the error (RMS) of signal strength for each predetermined time interval, as in the case of body movement detection, and determining when the error in the signal strength of each navigator echo becomes approximately zero or below a predetermined threshold (Condition 1), or by calculating the difference (slope) between the echoes from the signal strength (RMS) of the navigator echoes after the first period and determining when the slope becomes approximately zero or below a predetermined threshold (Condition 2). In the determination process, the end of the second period may be determined when both of the above conditions are satisfied, or the end of the second period may be determined when multiple conditions are satisfied consecutively. For example, the end of the second period may be determined when the conditions are satisfied three times consecutively. By making such a continuous determination, the stable state can be determined more accurately. Note that the above conditions (1) and (2) and the determination method are merely examples of stable state determination, and are not limited thereto.

[0060] After determining the second period, the body motion processing unit 240 identifies the range of data (data to be removed: referred to as removal data) collected during the second period in the image data (k-space data) collected by the imaging unit 10 (S4).

[0061] <Imaging control unit: S5> The imaging control unit 210 determines whether the measurement data after removing the removed data identified by the body motion processing unit 240 can be used for image reconstruction or whether remeasurement is required (S5), and if remeasurement is not required, passes the confirmed measurement data to the image generation unit 220. If remeasurement is required, the imaging control unit 210 determines the remeasurement range and controls the imaging unit 10 to perform the necessary measurements.

[0062] The decision as to whether or not to perform remeasurement can be made, for example, by determining whether or not the number of data after removing data containing body movement from the number of data determined by the thinning rate in the imaging pulse sequence can be restored by image reconstruction processing in the image generation unit 220.

[0063] <Image reconstruction: S6> If there is no body motion during acquisition of image signals, the image generation unit 220 performs image reconstruction using a normal fast Fourier transform FT, parallel imaging PI calculation according to the speed ratio, iterative reconstruction, etc. If body motion occurs, image reconstruction is performed using k-space data from which the identified removed data has been removed. As an image reconstruction method for k-space data with less data than the original data, if there is less data to remove, a method of replacing the removed data with zeros or a method of performing correction using Hermitian symmetry of k-space and then reconstructing using fast Fourier transform may be used. Alternatively, an iterative reconstruction method in which missing data is corrected by iterative calculation may be used.

[0064] Figure 9 shows a conceptual diagram of one iterative reconstruction method. First, a mask 901 of data to be removed from measurement data (removed data) is created using information 900 related to the body movement period generated by the body movement detection unit 230 and the body movement processing unit 240 from the navigator echo and camera image, and image data 902 measured by the imaging unit 10 is multiplied by the mask 901 to obtain measurement data 903 from which the removed data has been removed. The iterative reconstruction operation is an operation that minimizes the difference between the interpolated measurement data and the undersampled measurement data by repeating interpolation and norm minimization in a sparse space for the undersampled measurement data (or its image), and can obtain an image 905 in which the removed data has been interpolated and which maintains the consistency of the image information of the original measurement data.

[0065] <Remeasurement: S8> If the number or range of data to be removed is large and image quality cannot be ensured even with the above-mentioned methods, the imaging control unit 210 controls the imaging unit 10 to re-imaging this range. At this time, a UI for inputting the range of body motion data and whether or not re-measurement is required may be displayed on the user interface unit 70 (display device 40), allowing the user to select whether or not re-measurement is required. Re-measurement may involve either re-imaging or simply measuring the image signals that are missing due to the removed data.

[0066] As described above, according to the MRI apparatus of this embodiment, a second period is set for the period of body movement detected by the body movement detection means, taking into consideration body movement detection timing errors and the stabilization time of the echo signal after the occurrence of body movement, and data to be removed from the measurement data or corrected is identified based on the second period. This makes it possible to remove the effects of body movement that would have been overlooked in the past, and to obtain images in which body movement artifacts have been reduced with high accuracy.

[0067] Furthermore, according to this embodiment, when navigator data is used as a method for detecting body movement, it is possible to determine body movement detection and the stability of the signal from the signal strength of the navigator echo, and it is possible to reliably remove data that may affect artifacts for each imaging session.

[0068] The above describes an embodiment of the present invention, but the sequences and pulse sequences for acquiring navigator data and the image processing method for measurement data with missing data described as an embodiment are merely examples, and various modifications are possible within the scope of the gist of the present invention, and such modifications are also encompassed by the present invention. [Explanation of symbols]

[0069] 1: MRI device, 10: imaging unit, 20: control unit, 80: camera, 210: imaging control unit, 220: image generation unit, 230: body movement detection unit, 240: body movement processing unit

Claims

1. The apparatus comprises an RF transmitter that applies a radio frequency magnetic field pulse to an object to be examined placed in a static magnetic field space, an RF receiver that receives a nuclear magnetic resonance signal emitted by the object to be examined, a gradient magnetic field generator that generates a gradient magnetic field pulse that gives a magnetic field gradient to a static magnetic field, an image generator that generates an image of the object to be examined using measurement data consisting of the nuclear magnetic resonance signal, and a body movement processor that processes body movement information of the object to be examined, the body movement processing unit detects a first period during which the test subject is moving, and then identifies a second period that includes the first period and is longer than the first period; The magnetic resonance imaging apparatus according to claim 1, wherein the image generating unit generates an image of the object to be examined by removing or correcting data collected during the second period from the measurement data.

2. 2. The magnetic resonance imaging apparatus according to claim 1, the body movement information includes navigator data consisting of a plurality of navigator echoes acquired together with the measurement data; The magnetic resonance imaging apparatus, wherein the body motion processing unit detects the first period from the navigator data.

3. 3. The magnetic resonance imaging apparatus according to claim 2, The magnetic resonance imaging apparatus is characterized in that the body movement processing unit detects body movement occurring in the subject from a change in intensity of the navigator echo.

4. 3. The magnetic resonance imaging apparatus according to claim 2, The body movement information further includes an image or a signal of an optical detection device installed in the vicinity of the static magnetic field space, The magnetic resonance imaging apparatus is characterized in that the body movement processing unit detects body movement occurring in the subject by using the navigator data and the image or signal of the optical detection device in combination.

5. 2. The magnetic resonance imaging apparatus according to claim 1, The magnetic resonance imaging apparatus is characterized in that the body movement processing unit detects body movements occurring in the subject by using images or signals from an optical detection device installed near the static magnetic field space.

6. 2. The magnetic resonance imaging apparatus according to claim 1, a body movement processing unit that presets a predetermined time for at least one of a time before and a time after the first period, and sets the second period by adding the set time.

7. 3. The magnetic resonance imaging apparatus according to claim 2, The magnetic resonance imaging apparatus according to claim 1, wherein the body motion processing unit analyzes a change in signal intensity of the navigator echo and identifies the second period.

8. 1. A method for image reconstruction in magnetic resonance imaging, comprising: determining a period during which a body movement occurs as a first period based on a result of detecting a body movement of an examination subject placed in a static magnetic field space, and specifying a second period that includes the first period and is longer than the first period; An image reconstruction method for generating an image of an object to be examined, comprising removing or correcting data collected during the second period from measurement data of the object to be examined collected by magnetic resonance imaging.

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