Magnetic resonance imaging device and control method thereof

Abstract

To minimize an effect of TR extension by navigator echo without reducing a temporal resolution of body motion detection in imaging using the navigator echo for body motion detection.SOLUTION: When imaging is performed according to a predetermined pulse sequency using an MRI device, control of causing the pulse sequence to generate a navigator echo at predetermined timing is performed. At that time, control is performed to vary timing of acquiring the navigator echo between when a repetition time of the pulse sequence is equal to or greater than a preset threshold and when the repetition time is less than the threshold.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 control of an MRI apparatus that performs imaging by generating navigator echoes that detect body movement in order to suppress the influence of body movement. [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] Because navigator echoes are acquired separately from imaging signals, adding a step for acquiring navigator echoes lengthens the pulse sequence for acquiring imaging signals. In other words, imaging signals are acquired by repeatedly applying excitation RF pulses and acquiring nuclear magnetic resonance signals while changing the strength of the phase-encoding gradient magnetic field, which lengthens the repetition time (TR) between excitation RF pulses. This lengthens the imaging time and increases the burden on the operator and the subject, whose movement is restricted in a confined space.

[0008] On the other hand, the repetition time of a pulse sequence differs depending on the pulse sequence and also on the type of contrast image to be obtained in an MRI examination. For example, the TR is long in T2 weighted imaging (T2WI) and short in T1 weighted imaging (T1WI). In imaging with a long TR, acquiring a navigator echo every TR has little effect, but in imaging with a short TR, acquiring a navigator echo every TR has a large effect, and the adverse effects of the TR extension mentioned above are also significant.

[0009] Patent Document 1 discloses acquiring navigator echoes for each phase in accordance with the phase of respiratory movement, but such timing control cannot accommodate sudden body movements. Patent Document 2 also describes performing a navigator pulse sequence for each slice when imaging multiple slices, but does not take into account TR, and therefore fails to solve the problem of TR extension due to the execution of a navigator pulse sequence in short TR imaging.

[0010] To prevent TR extension due to navigator data collection, it is possible to reduce the frequency of navigator echo acquisition, but this would lead to a decrease in the time resolution of body movement detection, making it difficult to respond to momentary or sudden body movements.

[0011] The present invention has been made to solve the problems of MRI using navigator data as described above, and aims to provide an MRI apparatus and a control method thereof that minimizes the effect of TR extension due to navigator echoes without reducing the time resolution of body movement detection. [Means for solving the problem]

[0012] In order to solve the above problems, the MRI apparatus of the present invention automatically controls the timing of navigator echo acquisition in accordance with the TR of a set pulse sequence so as to keep the frequency of navigator echo acquisition within a predetermined range.

[0013] That is, the MRI apparatus of the present invention comprises an RF transmitter unit that applies radio frequency magnetic field pulses to an examination subject placed in a static magnetic field space, an RF receiver unit that receives nuclear magnetic resonance signals emitted by the examination subject, a gradient magnetic field generator unit that generates gradient magnetic field pulses that impart a magnetic field gradient to the static magnetic field, an image generator unit that generates an image of the examination subject using the nuclear magnetic resonance signals, and a controller that controls the RF transmitter unit, RF receiver unit, and gradient magnetic field generator unit according to a predetermined pulse sequence, the predetermined pulse sequence including a pulse sequence that generates navigator echoes separately from echo signals used to generate an image of the examination subject. The controller controls the generation timing of the navigator echoes to differ between when the repetition time of the pulse sequence is equal to or greater than a predetermined threshold and when it is less than the threshold.

[0014] The timing of generation of the navigator echo is preferably controlled so as to be equal to or greater than a predetermined frequency, regardless of the TR.

[0015] The control method of the MRI apparatus of the present invention performs control to generate a navigator echo at a predetermined timing in the pulse sequence when performing imaging according to the pulse sequence, and at that time, controls to make the timing of acquiring the navigator echo different between when the repetition time of the pulse sequence is equal to or greater than a preset threshold and when it is less than the threshold. [Effects of the Invention]

[0016] According to the present invention, in imaging with a long TR, a navigator echo is acquired for each of the multiple slices measured within 1 TR, i.e., for each slice, and in imaging with a short TR, a navigator echo is acquired per TR less than the number of slices rather than for each slice, thereby controlling the timing of navigator echo acquisition according to the length of the TR, thereby making it possible to suppress extension of the TR without reducing the time resolution of navigator echo acquisition. Furthermore, by controlling the timing of navigator echo acquisition using a threshold, automatic control becomes possible, eliminating the need for user setting. [Brief explanation of the drawings]

[0017] [Figure 1] Overall view of the MRI system [Figure 2] Diagram explaining multi-slice imaging [Figure 3] FIG. 10 is a diagram showing the flow of imaging control by the control unit. [Figure 4] FIG. 1 shows an example of a pulse sequence including navigator data acquisition (in the case of a long TR), where (A) and (B) show examples with different timings for acquiring navigator echoes. [Figure 5] FIG. 1 shows different examples of navigator data acquisition timing in the case of long TR imaging including a navigator echo generation sequence. [Figure 6] FIG. 10 is a diagram showing an example of a pulse sequence including navigator data acquisition, in the case of a short TR. [Figure 7] FIG. 1 shows different examples of navigator data acquisition timing in the case of short TR imaging including a navigator echo generation sequence. [Figure 8] Functional block diagram of the body movement processing unit [Figure 9] FIG. 10 is a diagram showing an example of body movement data obtained from navigator data. [Figure 10] A diagram explaining the removal of data with body motion and image reconstruction using the data after removal. [Figure 11] FIG. 10 is a diagram showing an example of a pulse sequence according to a modification of the first embodiment. [Figure 12] 10A to 10C are diagrams for explaining removal of data with body movement and image reconstruction using the data after removal in the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0018] Hereinafter, an embodiment of the MRI apparatus of the present invention will be described.

[0019] 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.

[0020] 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 where 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 control unit 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.

[0021] The control unit 20 can be configured as a general-purpose computer equipped with a memory and a CPU, and includes an imaging control unit 210 that functions as a control unit and a calculation unit and 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, and a display control unit 250 that displays the image generated by the image generation unit 220. As will be described in detail later, the control unit 20 also includes a body movement processing unit 240 that detects body movement of the subject during the examination and performs various calculations and controls using the detected body movement information. These functions of the control unit 20 are realized by the CPU uploading a predetermined program. Some of the functions can also be realized by a programmable IC.

[0022] The control unit 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 display device 40 are installed close to each other and function as a user interface unit 70.

[0023] The MRI apparatus 1 may also be equipped with a monitoring camera (not shown) for monitoring the state of the subject 50 placed in the static magnetic field space, and the image from this monitoring camera can be used to detect the body movement of the subject 50. In this case, the body movement processing unit 240 receives information from the monitoring camera 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.

[0024] 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.

[0025] 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.

[0026] Generally, imaging is performed on a region of a certain thickness, so for example, excitation and nuclear magnetic resonance signal collection are repeated for each of multiple slices at a predetermined TR, and image signals are collected for each slice. When imaging multiple slices, as shown in Figure 2, multi-slice imaging is commonly used, in which selective excitation (application of gradient magnetic field pulse Gs and RF pulse) and signal collection are performed sequentially for the second, third, etc. slices S2 and S3, with a time lag, before the repetition time TR of one slice S1 arrives. Note that Figure 2 only shows the slice-selective gradient magnetic field Gs, and omits the phase encoding and readout gradient magnetic field pulses.

[0027] 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.

[0028] The image generation unit 220 performs calculations such as Fourier transform and iterative calculations using the nuclear magnetic resonance signals (image data) collected and digitized by the RF receiving unit 107 to reconstruct an image and performs image processing on the generated image. The image generated by the image generation 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.

[0029] The MRI apparatus of the present invention executes a pulse sequence accompanied by collection of navigator data as the above-mentioned pulse sequence, and is characterized in that the timing of collection of the navigator data is automatically controlled using a repetition time TR of the pulse sequence set as an imaging parameter.

[0030] In light of the above-described configuration of the MRI apparatus, an embodiment of control in multi-slice imaging will be described below.

[0031] <Embodiment 1> In this embodiment, the timing of acquiring navigator data is controlled according to the TR of the set imaging sequence. To perform control according to the TR, in this embodiment, a threshold value TH is set in advance for the TR of the pulse sequence, and the acquisition of navigator data is controlled according to whether the TR is equal to or greater than the threshold value TH or less than TH.

[0032] 3 shows the control flow of the imaging control unit 210 of this embodiment. As shown in the figure, when an imaging sequence is set (S1), the imaging control unit 210 compares the repetition time TR of the set imaging sequence with a preset threshold TH (S2), and if the repetition time TR is equal to or greater than the threshold, acquires a navigator echo for each slice (S3), and if the set TR is less than the threshold, sets the acquisition timing of the navigator echo so that the navigator echo is acquired for each TR. At this time, the number of times the navigator echo is acquired may be adjusted so that the acquisition frequency of the navigator echo does not fall below a predetermined frequency.

[0033] Imaging then begins (S5), and navigator data and image signals are collected. The body motion processing unit 240 uses the sequentially acquired navigator data to determine whether body motion is occurring, and if body motion is occurring, identifies the image signals (positions in the k-space data) collected at the time of body motion (S6). Once the time of body motion occurrence and the position of the k-space data collected at that time have been identified, the information is sent to the image generation unit 220 or the imaging control unit 210. The image generation unit 220 eliminates the identified k-space data and performs image reconstruction operations such as iterative operations using the remaining k-space data to reconstruct an image (S8). Furthermore, if image reconstruction is difficult even after excluding the data, such as when body motion has occurred for a long period of time (S7), the imaging control unit 210 controls the imaging unit 10 to reacquire the excluded k-space data.

[0034] The specific contents of each of the above steps will be explained below.

[0035] <Setting the imaging sequence S1, S2> As described above, the imaging sequence is determined by the pulse sequence type selected according to the imaging purpose and the imaging parameters set by the user. Pulse sequence types are roughly divided into spin echo (SE) sequences and gradient echo (GrE) sequences, and various pulse sequences are known, such as high-speed imaging sequences (FSE, RSSG) that are an extension of these, gradient magnetic field (MPG) pulses for diffusion weighting, and sequences that add pre-pulses such as fat suppression pulses, and the present invention is not limited to these and can employ any of these pulse sequences.

[0036] The main imaging parameters are the echo time TE, repetition time TR, FOV, slice thickness, number of slices, and speed ratio R (corresponding to the rate at which k-space data is thinned out), and these can be set in advance as part of the examination flow, or the user can set them according to the purpose of imaging and the subject of examination. In any pulse sequence, TE and TR are important imaging parameters that contribute to the contrast of the image; for example, in SE-based T1-weighted imaging (T1WI), TR is about 450 ms, but in T2-weighted imaging (T2W1), TR can reach 3800 ms.

[0037] When the imaging parameters are set, the imaging control unit 210 compares the set TR with a threshold value TH. TH may be, for example, a single value between the longest and shortest TRs (700 to 1000 ms), or the lower limit of TR for imaging generally considered to be long TR imaging, or the upper limit of TR for imaging generally considered to be short TR imaging. It is also possible to set two or more THs (TH1, TH2, etc.) instead of a single value. TH is stored in the storage device 60 of the MRI apparatus and is read and used during the determination in step S2. It is also possible to allow the user to set it as appropriate.

[0038] <Long TR Imaging S3, S5> If the TR set in S1 is equal to or greater than the TH, a navigator echo is acquired for each slice (S3), and imaging is started (S5).

[0039] Pulse sequences including the acquisition of navigator echoes are well known, and an example of multi-slice imaging is shown in Figure 4. In the technique shown in Figure 4(A), navigator echoes Navi1, Navi2, and Navi3 are acquired each time image signals 411, 412, and 413 for slices S1, S2, and S3 are acquired. The navigator echoes may be FID (free induction decay) signals without the application of a readout gradient magnetic field, or echo signals with the application of a readout gradient magnetic field, but here an example is shown in which they are acquired as FIDs. In either case, the signals are collected without phase encoding.

[0040] Also, Fig. 4(A) shows a case where navigator echoes Navi1, Navi2, and Navi3 are acquired before acquiring imaging signals 411, 412, and 413, respectively, but as shown in Fig. 4(B), navigator echoes may be acquired after acquiring imaging signals. When acquiring with FID, TE is extended by acquiring navigator echoes, so it is preferable to acquire them after imaging signals.

[0041] 4(A) and (B), navigator echoes are acquired after applying RF pulses 401 to 403 for slice excitation (including a combination of two or more RF pulses), but as shown in Fig. 5(A) and (B), it is also possible to acquire navigator echoes by executing a sequence for generating navigator echoes separately from RF pulses for acquiring image signals. In this case, the RF pulses used in the navigator echo generating sequence may be either slice selection RF pulses that are the same as the slices to be imaged, or RF pulses that excite the entire imaging region of the examination target.

[0042] After determining the timing for collecting navigator data as described above, imaging is started (S5).

[0043] <Short TR imaging S4, S5> On the other hand, if the repetition time TR of the set imaging sequence is less than a threshold, the number of times navigator echoes are acquired is reduced. Specifically, as shown in Figures 6(A)(B) and 7(A)(B), navigator echoes are acquired for each TR (S4). Figure 6 shows a case where navigator echoes are acquired as FIDs, while Figure 7 shows a case where navigator echoes are acquired by executing a sequence that generates navigator echoes separately from RF pulses for exciting each slice, rather than FIDs. In either case, navigator echoes may be acquired either before or after image signal acquisition, which can reduce the TR. After determining the timing for acquiring navigator data in this way, imaging begins (S5).

[0044] As described above, when the TR is long, navigator echoes are acquired for each slice, and when the TR is short, navigator echoes are acquired for each TR. In imaging such as T2WI and PDWI (proton density imaging), where a longer TR is set, the TR is long to begin with, so acquiring navigator echoes for each slice has little effect on the TR, and motion information at that time can be obtained for each slice. Furthermore, even in imaging that requires a short TR, such as T1-weighted imaging or T2* imaging, acquiring navigator echoes for each TR can prevent TR extension, and because the TR is short, the frequency of navigator data acquisition, i.e., the temporal resolution of motion detection, can be maintained at the same level as in imaging with a long TR.

[0045] <Body movement processing: S6, S7> The method of body movement processing by body movement processor 240 is similar to the method described in Patent Document 4, for example, and an example of the method will be described below.

[0046] 8 shows a functional block diagram of the body movement processing unit 240. As shown in the figure, the body movement processing unit 240 includes a navigator data analysis unit 241 and a removal data determination unit 243. If a camera for monitoring body movement is installed in the MRI apparatus 1, the body movement processing unit 240 may also include a camera data analysis unit 245 that captures images from the camera.

[0047] The navigator data analysis unit 241 analyzes the navigator data collected by the imaging unit 10 and detects the occurrence of body movement from the fluctuations in the data. As shown schematically in FIG. 9, when there is no movement in the subject, the non-phase-encoded navigator echo is a signal from the entire excited region, and the signal intensity is stable. Note that in areas with periodic movement such as respiratory movement, the signal intensity fluctuates periodically, as shown in the upper part of FIG. 9. On the other hand, when movement occurs in the measurement region due to body movement, the signal intensity fluctuates significantly. The navigator data analysis unit 241 can detect body movement by analyzing the characteristics of such fluctuations, such as the magnitude and frequency of the signal intensity fluctuations.

[0048] As an example of an analysis method, a reference signal is determined by averaging multiple navigator echoes, and the root sum of squares (RMS) is calculated by subtracting this reference signal from each navigator echo. Navigator echoes with large errors (RMS) from the reference signal are identified, and the navigator echoes in which the errors were detected are identified. The imaging data obtained between the navigator echoes in which the errors were detected (between t1 and t2) are considered to be data with body motion.

[0049] 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.

[0050] As described above, when the navigator data analysis unit 241 determines that a body movement has occurred, it identifies the time of the occurrence (t1, t2 in FIG. 9) and identifies the range of data collected when the body movement occurred (data to be removed: referred to as removal data) in the image data (k-space data) collected by the imaging unit 10.

[0051] The body motion processing unit 240 determines whether the measurement data after removing the removed data identified by the navigator data analysis unit 241 can be used for image reconstruction or whether re-measurement is required (S7), and in accordance with the determination result, passes the confirmed measurement data to the image generation unit 220. Alternatively, the re-body motion processing unit 240 determines the re-measurement range and sends the information thereof to the imaging control unit 210.

[0052] <Image reconstruction: S8> 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.

[0053] Figure 10 shows a conceptual diagram of one iterative reconstruction method. First, a mask 901 of the removed data is created using body motion information 900 obtained from the navigator echo, and image data 902 measured by the imaging unit 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.

[0054] <Remeasurement: S8> If the range of the body motion data is large and image quality cannot be ensured even with the above-mentioned methods, the body motion processing unit 240 sends the range of the body motion data to the imaging control unit 210, and the imaging control unit 210 controls the imaging unit 10 to re-image this range. At this time, a UI for inputting whether or not remeasurement is required along with the range of the body motion data may be displayed on the user interface unit 70 (display device 40), and the user may select whether or not remeasurement is required.

[0055] As described above, according to the MRI apparatus of this embodiment, the timing of acquiring navigator data for detecting body motion is automatically determined in accordance with the TR in the set imaging sequence. Therefore, regardless of the length of the TR, body motion information can be acquired with an appropriate time resolution, and appropriate body motion correction can be performed.

[0056] In addition, in short TR imaging, by acquiring a navigator echo for each TR, it is possible to avoid extending the TR and the associated increase in imaging time.

[0057] Furthermore, by adopting a simple calculation method using signal strength as a method for detecting body movement using navigator echoes, it is possible to reduce the calculation load and improve the real-time performance of body movement detection.

[0058] <Modification 1 of Embodiment 1> In the first embodiment, one TH is used to alternatively select either every slice ( FIG. 4(A) ) or every TR ( FIG. 6 ) depending on whether the TR is long or short. However, when the TR is relatively long, as shown in FIG. 11 , it is also possible to acquire navigator echoes twice or more within the TR. In this example, navigator echoes are acquired every two slices. In this case, TH1 and TH2, which are two or more, are set for the TR. For example, if the TR of the imaging pulse sequence is equal to or greater than TH1, the number of times navigator echoes are acquired is once per slice within one TR, and if it is less than TH2, it is once per TR. If it is less than TH1 but equal to or greater than TH2, it is once per two or three slices within one TR.

[0059] Furthermore, instead of setting multiple thresholds TH, the frequency of acquiring navigator echoes may be roughly calculated, and the timing of acquiring navigator echoes may be controlled according to the frequency of acquisition. If the approximate frequency of acquiring navigator echoes is calculated by dividing the TR by the "number of acquisitions within one TR," then in an intermediate TR between the long TR imaging shown in Figures 4 and 5 and the short TR imaging shown in Figures 6 and 7, it is possible to maintain the same degree of acquisition frequency by acquiring navigator echoes every two slices, as shown in Figure 11, for example.

[0060] In this way, the approximate frequency of navigator echo acquisition can be estimated from the number of slices (number of slices per TR) set in the imaging parameters and the TR, so the number of times to acquire navigator echoes can be set so that the approximate frequency of navigator echoes does not fall below a predetermined frequency (for example, a time resolution for detecting body movement desired by the user). This makes it possible to detect the occurrence of body movement with a predetermined time resolution while minimizing TR extension according to the TR.

[0061] <Embodiment 2> In the first embodiment, the case where body movement information is acquired from navigator data was described, but in this embodiment, in addition to navigator data, information from an optical body movement detection means is acquired and used together with the body movement information detected from the navigator data. As the optical body movement detection means, a non-contact optical sensor, an infrared sensor, or an imaging means such as a stereo camera can be used, but the following description of this embodiment will be made taking an example where camera images are used.

[0062] In this embodiment, the timing of acquiring navigator data is controlled according to TR, and body movement information is obtained by analyzing navigator data obtained at a predetermined time resolution, similar to embodiment 1. Below, differences from embodiment 1 will be described.

[0063] The MRI apparatus of this embodiment has one or more cameras installed near or within the static magnetic field space in which the subject 50 is placed. The cameras are preferably wide-angle cameras or stereo cameras capable of monitoring a relatively wide range of the subject. The cameras are connected to the control unit 20 of the MRI apparatus via wired or wireless connections, and the control unit 20 captures the images of each camera frame. A body motion processing unit 240 of the control unit 20 captures the image data from the cameras and detects the occurrence of body motion. Therefore, the body motion processing unit 240 includes a camera data analysis unit 245, as shown in the dashed square in FIG. 8, in addition to the navigator data analysis unit 241 and removal data determination unit 243 of the first embodiment.

[0064] The camera data analysis unit 245 analyzes the camera image, estimates the magnitude of the displacement of the subject, and determines body movement, for example, by threshold processing. Specifically, the displacement vector of the subject is calculated for each frame of image data by calculations such as optical flow. At this time, a predetermined ROI may be set on the subject, and the displacement vector may be calculated for the ROI. The displacement vector is then integrated from the start of imaging to a predetermined time point, and used as the displacement relative to the position at the start of imaging (reference position). Alternatively, the difference between the image of the frame (the frame at the start of imaging) used as the reference for the displacement vector and the image of the frame at the predetermined time point may be used as the displacement at that time point. The displacement calculated in this way is a gentle curve with the period of respiratory movement, as shown in FIG. 9, for example, in areas where respiratory movement occurs, but when sudden movement (body movement) occurs, it shows a displacement corresponding to the movement.

[0065] The camera data analysis unit 245 uses the range of respiratory movement (upper and lower limits of displacement) as a threshold for displacement, and when it detects a displacement above or below that threshold, it determines that body movement is occurring and identifies the time when the body movement is occurring.

[0066] The removal data determination unit 243 of the body movement processing unit 24 performs an AND operation on the body movement occurrence time, which is the analysis result of the navigator data analysis unit 241, and the body movement occurrence time identified by the camera data analysis unit 245, and identifies the image data collected at the body movement occurrence time as the removal data, using this as the body movement occurrence time.

[0067] Thereafter, image reconstruction is performed excluding the removed data, or the removed data is remeasured, as in the embodiment. An example of reconstruction in this embodiment corresponding to the processing shown in Fig. 10 is shown in Fig. 12. In Fig. 12, the same elements as in Fig. 10 are designated by the same reference numerals, and their explanations will be omitted.

[0068] According to this embodiment, the same effects as those of the first embodiment can be obtained, and further, by using camera images in combination with body movement detection, it is possible to capture even minute movements before and after the body movement detected by the navigator echo, and it is possible to reconstruct an image with the effects of body movement more precisely eliminated. Furthermore, depending on the body movement, the situation or position of the subject may change to such an extent that it affects subsequent measurements, and it is possible to detect such changes in the subject and perform subsequent processing using the body movement information more appropriately.

[0069] 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]

[0070] 1: MRI device, 10: imaging unit, 20: control unit, 210: imaging control unit, 220: image generation unit, 240: body motion processing unit

Claims

1. 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 imparts a magnetic field gradient to a static magnetic field; the transmitter; an image generator that generates an image of the object to be examined using the nuclear magnetic resonance signal; and a controller that controls the RF transmitter, the RF receiver, and the gradient magnetic field generator in accordance with a predetermined pulse sequence; the predetermined pulse sequence includes a pulse sequence that generates a navigator echo in addition to an echo signal for generating an image of the object of examination; The magnetic resonance imaging apparatus according to claim 1, wherein the control unit causes the timing of generation of the navigator echo to differ depending on whether the repetition time of the pulse sequence is equal to or greater than a preset threshold or less than the threshold.

2. 2. The magnetic resonance imaging apparatus according to claim 1, the pulse sequence is a multi-slice imaging sequence for measuring echo signals of one or more slices other than one slice within a repetition time of the slice, The magnetic resonance imaging apparatus is characterized in that the control unit generates a navigator echo for each slice measured within the repetition time of one slice when the repetition time of the pulse sequence is equal to or greater than the threshold.

3. 2. The magnetic resonance imaging apparatus according to claim 1, the pulse sequence is a multi-slice imaging sequence for measuring echo signals of one or more slices other than one slice within a repetition time TR of the slice, The magnetic resonance imaging apparatus is characterized in that, when the repetition time of the pulse sequence is less than the threshold, the control unit generates, within the repetition time of one slice, navigator echoes whose number is less than the number of slices measured within the repetition time.

4. 4. The magnetic resonance imaging apparatus according to claim 3, The magnetic resonance imaging apparatus according to claim 1, wherein the control unit generates one navigator echo for each repetition time of one slice.

5. 2. The magnetic resonance imaging apparatus according to claim 1, The magnetic resonance imaging apparatus is characterized in that the control unit controls the timing of generation of the navigator echo so that the frequency of generation of the navigator echo is equal to or greater than a certain frequency.

6. 2. The magnetic resonance imaging apparatus according to claim 1, The magnetic resonance imaging apparatus is characterized in that the control unit measures the navigator echo as an FID signal generated after an RF pulse is applied to generate an echo signal for generating an image.

7. 2. The magnetic resonance imaging apparatus according to claim 1, A magnetic resonance imaging apparatus characterized in that the control unit generates the navigator echo by applying an RF pulse that is different from an RF pulse that is applied to generate an echo signal for generating an image.

8. 2. The magnetic resonance imaging apparatus according to claim 1, the control unit includes a body movement processing unit that detects body movement of the subject using navigator data consisting of a plurality of navigator echoes; 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.

9. 2. The magnetic resonance imaging apparatus according to claim 1, the control unit includes a body movement processing unit that detects body movement of the subject using navigator data consisting of a plurality of navigator echoes; The body movement processing unit receives detection results from an optical detection device attached to the magnetic resonance imaging device that monitors the movement of the subject, and determines data to be removed from the echo signals used to generate the image using the detection results and detection results of the body movement of the subject using the navigator data.

10. A method for controlling imaging of a magnetic resonance imaging apparatus that performs imaging according to a pulse sequence, comprising: A control method for a magnetic resonance imaging apparatus, comprising: controlling the pulse sequence to generate a navigator echo at a predetermined timing; and controlling the timing of acquiring the navigator echo to be different between when a repetition time of the pulse sequence is equal to or greater than a predetermined threshold and when the repetition time is less than the threshold.

11. 11. The control method according to claim 10, the pulse sequence is a multi-slice imaging sequence for measuring echo signals of one or more slices other than one slice within a repetition time TR of the slice, a control method for a magnetic resonance imaging apparatus, characterized in that a navigator echo is acquired for each repetition time when the repetition time of the pulse sequence is less than the threshold, and a navigator echo is acquired for each slice when the repetition time of the pulse sequence is equal to or greater than the threshold.

12. 11. The control method according to claim 10, A method for controlling a magnetic resonance imaging apparatus, comprising controlling the timing of generation of the navigator echo so that the frequency of generation of the navigator echo is equal to or greater than a predetermined frequency.

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