Magnetic resonance imaging apparatus and control method thereof

Abstract

To reduce artifacts that occur in retaken images after imaging is interrupted due to body movement, and to minimize extension of imaging time due to remeasurement, thereby suppressing prolongation of imaging time. [Solution] When re-measurement is performed after body movement, a dummy scan for artifact suppression, including multiple blank shots of RF irradiation pulses, is inserted. At this time, the duration of the dummy scan is optimized to reduce the re-measurement time including the dummy scan. This prevents the extension of imaging time while quickly restoring the steady state disrupted by body movement and suppresses the occurrence of artifacts.

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 the MRI apparatus when imaging is interrupted during an examination. [Background technology]

[0002] If a subject makes a sudden movement (body movement) during an examination using an MRI device, the echo signals collected at that time will be encoded differently from the original encoding, and artifacts caused by the body movement will appear in images reconstructed from k-space data including signals affected by the body movement. In order to remove the effects of body movement, various technologies for correcting signals affected by body movement (body movement correction technologies) have been proposed (Patent Document 1).

[0003] Since image contrast is largely determined by low-frequency data centered on the origin (zero encode) of the k-space data, if body motion occurs during low-frequency data collection and there is a large amount of low-frequency data to correct, meaningful image reconstruction becomes difficult. Furthermore, if the body motion itself is large, it takes time to recover, and motion correction techniques may not be able to address this. When image reconstruction or motion correction is difficult, retaking the image is necessary.

[0004] Patent Document 1 discloses that motion correction is selected depending on the number of measurement data affected by motion and the position in k-space, i.e., whether the data is low-frequency data or high-frequency data, and that remeasurement is performed when a meaningful image cannot be obtained with motion correction. In this case, rather than retaking all k-space data, the k-space data collected before motion occurs is saved, and the k-space data when motion occurs and subsequent k-space data are collected. This prevents the imaging time from being prolonged by retaking. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2023-022669 [Patent Document 2] Japanese Patent Application Publication No. 3-121046 Summary of the Invention [Problem to be solved by the invention]

[0006] As described above, even when data is remeasured when body movement occurs and image reconstruction is performed using the remeasurement data, i.e., data collected when body movement does not occur, there is a problem of artifacts such as body movement artifacts occurring. This artifact is thought to be caused by the steady state of longitudinal magnetization being disrupted due to the influence of body movement even after the subject's original position is returned to its original position. Patent Document 2 describes a technique for performing a dummy measurement, which is not a remeasurement after body movement occurs, but when remeasurement is performed after an imaging interruption, in order to restore the state of longitudinal magnetization of spins to the state before the interruption. In this dummy measurement, MRI signals are not measured, and the number of encodes, etc. are not updated.

[0007] However, the dummy measurement after interruption of imaging as described in Patent Document 2 is intended to return the longitudinal magnetization relaxed by the interruption to a steady state, but the unstable state of longitudinal magnetization caused by body movement may cause artifacts even when re-measurement is performed immediately after the occurrence of body movement, and there is no knowledge available about the time required to return to the state before re-measurement after body movement has subsided (the period during which dummy measurement is performed). Furthermore, if a dummy scan is inserted at the time of re-measurement in the same manner as in Patent Document 2, the re-measurement time is extended, which is highly likely to cause a new problem of prolonging the overall imaging time.

[0008] The present invention aims to reduce artifacts that occur in images taken again after imaging has been interrupted due to body movement, and also aims to minimize the extension of imaging time due to remeasurement, thereby preventing the imaging time from becoming too long. [Means for solving the problem]

[0009] The present invention inserts an artifact suppression scan (hereinafter referred to as a dummy scan) that includes multiple blank shots of RF irradiation pulses when re-measurement is performed after the occurrence of body movement. At this time, the re-measurement including the dummy scan, such as the duration of the dummy scan, is optimized to reduce the re-measurement time including the dummy scan. This prevents the extension of imaging time, quickly restores the steady state that has been disrupted by body movement, and suppresses the occurrence of artifacts.

[0010] That is, the MRI apparatus of the present invention includes an imaging unit that acquires nuclear magnetic resonance signals of a subject, an image generation unit that reconstructs an image of the subject using k-space data consisting of the nuclear magnetic resonance signals acquired by the imaging unit, a body motion processing unit that analyzes body motion of the subject during imaging and identifies correction target data affected by the body motion of the subject, and an imaging control unit that controls the imaging unit. When remeasurement of the correction target data is necessary, the imaging control unit controls the imaging unit to perform a dummy scan by irradiating RF pulses one or more times prior to remeasurement of the correction target data to generate dummy signals that are not used in image reconstruction.

[0011] Furthermore, the method for controlling an MRI apparatus of the present invention includes a step of identifying data to be corrected that has been affected by the subject's body movement in k-space data acquired during imaging, and a step of controlling imaging so that, when re-measuring the data to be corrected, one or more RF pulses are irradiated to perform a dummy scan that generates a dummy signal that is not used in image reconstruction. [Effects of the Invention]

[0012] According to the present invention, when data that has not been acquired up to that point is remeasured after imaging is interrupted due to the occurrence of body movement, the time required for remeasurement can be minimized while preventing artifacts such as body movement artifacts from occurring in images reconstructed using the remeasurement data. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a diagram showing an overall outline of an MRI apparatus to which the present invention is applied. [Figure 2] A block diagram showing an example of the functions of an imaging control unit. [Figure 3] FIG. 1 illustrates one embodiment of the operation of an MRI device. [Figure 4] A diagram showing an example of re-measurement [Figure 5] Diagram showing an example of k-space division [Figure 6] FIG. 10 is a diagram showing an example of a processing flow by the imaging control unit of the first embodiment. [Figure 7A] FIG. 1 is a diagram showing an example of remeasurement according to the first embodiment. [Figure 7B] FIG. 10 is a diagram showing another example of remeasurement according to the first embodiment. [Figure 8] FIG. 10 is a diagram illustrating remeasurement in a modified example of the first embodiment. [Figure 9] FIG. 10 is a diagram for explaining a determination of whether to start a main scan for remeasurement according to a second modification of the first embodiment. [Figure 10] FIG. 10 shows an example of remeasurement and k-space data according to the second embodiment. [Figure 11] FIG. 10 is a diagram showing an example of remeasurement control when body movement occurs during low-frequency data collection. [Figure 12] FIG. 10 is a diagram for explaining an example of omitting measurement of k-space data according to the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0014] First, an outline of an MRI apparatus to which the present invention is applied will be described with reference to FIG. 1, the MRI apparatus 1 is broadly composed of an imaging unit 10, a processor 20 that performs various controls and calculations, and a user interface (UI) unit 30 that allows interaction between the imaging unit 10 and the processor 20 and the user. The main elements that make up the imaging unit 10 are housed in a gantry that provides the examination space.

[0015] The imaging unit 10 generates nuclear magnetic resonance in the nuclei (usually protons) of atoms constituting the tissue of the subject, and collects nuclear magnetic resonance signals (NMR signals) generated from the subject. Hereinafter, nuclear magnetic resonance signals are also simply referred to as signals or echo signals.

[0016] The imaging unit 10 has a configuration similar to that of a known MRI apparatus, and includes a static magnetic field magnet 101 that generates a uniform magnetic field (static magnetic field) in an examination space in which a subject 50 is placed, a gradient magnetic field coil 102 that applies a gradient magnetic field to the static magnetic field, an RF transmission coil 103 that applies a radio frequency magnetic field that excites the nuclei of atoms that constitute the subject's tissue, and an RF reception coil 104 that receives an NMR signal generated by the subject. The gradient magnetic field coil 102, the RF transmission coil 103, and the RF reception coil 104 are connected to a gradient magnetic field power supply 105, a transmitter 106, and a receiver 107, respectively. The operations of the gradient magnetic field power supply 105, the transmitter 106, and the receiver 107 are controlled by a sequencer 108. The sequencer 108 determines a pulse sequence for each scan using a set pulse sequence type and imaging conditions such as imaging parameters, and controls each component of the imaging unit 10 to operate in accordance with the determined pulse sequence and collect echo signals (k-space data) required for image reconstruction. The functions and operations of each unit when the imaging unit 10 acquires k-space data are the same as those of a general MRI apparatus, and therefore detailed description thereof will be omitted here.

[0017] The static magnetic field magnet 101, gradient magnetic field coil 102, and RF transmitting coil 103 are housed in the gantry, and the subject 50 has an RF receiving coil 104 attached to the area to be examined and is positioned in the examination space within the gantry while lying on a bed device 40.

[0018] The processor 20 is a device that controls and performs calculations for the device, and can be configured with a known processing device such as a computer with a CPU and memory, a programmable IC, or a combination of these. In the case of a computer, for example, processing by the processor is realized by the CPU loading a program that achieves each control and calculation function. Also, all of the processor functions can be realized by a single processor, or each function can be realized by combining one or more processors. In Figure 1, one processor 20 is shown to represent one or more processors, and individual functions realized by one or more processors 20 are shown.

[0019] 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 using echo signals collected by the imaging unit 10, a body movement processing unit 230 that performs processing related to body movements of the subject 50 during the examination, and a display control unit 250 that controls a GUI for displaying the images generated by the image generation unit 220 and interacting with the user. Note that Fig. 1 illustrates typical functions of the processor 20 of this embodiment, and each of these does not necessarily correspond to an individual processor or processing unit. There are also cases where one processor or processing unit realizes the functions of multiple functional units, or where multiple processors or multiple processing units realize one function.

[0020] The imaging control unit 210 controls the imaging unit 10 via a sequencer 108 that operates each element of the imaging unit 10 according to a predetermined pulse sequence. The sequencer 108 operates each component of the imaging unit 10 based on imaging conditions such as a pulse sequence and scan parameters determined by an examination flow or set by a user. The imaging control unit 210 also controls stopping and restarting of imaging in accordance with the body motion analyzed by the body motion processing unit 230, and controls re-measurement to recapture part of the k-space data affected by the body motion.

[0021] In the remeasurement control, when body movement occurs during imaging and the k-space data being acquired needs to be remeasured, the imaging unit 10 is controlled to perform a dummy scan. The dummy scan is a scan in which RF pulses are irradiated one or more times to generate dummy signals that are not used for image reconstruction prior to a scan (main scan) to recapture the k-space data affected by body movement. FIG. 2 is a functional block diagram of the remeasurement control in the imaging control unit 210. The imaging control unit 210 can include a remeasurement control unit 211 that controls the main scan for remeasurement, a dummy scan control unit 212, and a condition setting unit 213 that determines the conditions for remeasurement including the dummy scan. As described above, these functions are executed by one or more processors. The details of the remeasurement control will be described later.

[0022] The image generation unit 220 performs calculations necessary for image reconstruction, such as Fourier transform and sequential calculations, on the k-space data collected by the imaging unit 10, and also performs calculations such as correction on the k-space data before reconstruction or the image after reconstruction. In this embodiment, image reconstruction is performed with body movement corrected (body movement corrected reconstruction) in accordance with the body movement analyzed by the body movement processing unit 230. Furthermore, when part of the k-space data is remeasured by the imaging unit 10, image reconstruction may be performed by combining the k-space data obtained by the remeasurement with the k-space data obtained before the remeasurement.

[0023] The body motion processor 230 collects and processes body motion information generated by the subject 50 during the examination, such as the magnitude and duration of the body motion, and associates the body motion information with the ongoing scan (imaging) to determine whether body motion correction or remeasurement is necessary. The body motion information can be acquired from a body motion detection means, such as a monitoring camera for monitoring the movement of the subject 50 and a navigator echo for detecting the subject's movement. One or more monitoring cameras are installed near or inside the gantry, acquire images of the examination space, and send them to the processor 20. The navigator echo is a nuclear magnetic resonance signal acquired by the imaging unit 10 to detect the subject's movement separately from the nuclear magnetic resonance signal (echo signal) used to generate an image of the subject. The body motion processor 230 extracts the subject's movement during imaging by analyzing the navigator echo acquired in time series.

[0024] The display control unit 250 displays the images generated by the image generation unit 220 and their associated information on a display device in a predetermined display format. The display control unit 250 further performs processes such as displaying a GUI on the display device for the user to input various conditions and settings related to the operation of the MRI apparatus 1, such as imaging, image generation (including correction), and display, accepting user settings, and passing them to related functional units. The UI unit 30 includes a display device and an input device as means for communication between the processor 20 and the user, and these are connected to the processor 20.

[0025] Next, the flow of imaging operations of the MRI apparatus 1 in the above configuration will be described with reference to FIG.

[0026] The subject 50 is placed in the examination space, and imaging is started (S1). Specific imaging conditions, i.e., the pulse sequence and scan parameters (number of slices, FOV, TE, TR, R factor, etc.) used for imaging are not particularly limited, and imaging is performed by setting various known conditions using known setting methods.

[0027] Before or simultaneously with the start of imaging, the body motion processor 230 monitors the subject's motion and collects body motion information (S2). The subject's motion can be obtained, for example, by collecting navigator echoes for monitoring the subject's motion separately from echo signals for image reconstruction, and analyzing changes in the profile obtained by Fourier transforming the navigator echoes in the direction of the motion to be monitored, or by analyzing changes in the navigator echoes themselves. Various pulse sequences for collecting navigator echoes and accompanying imaging sequences are known, so a description of specific sequences will be omitted here.

[0028] Instead of using navigator echoes, it is also possible to obtain motion from images from a surveillance camera installed inside or near the gantry of the MRI apparatus 1 or signals from a biosignal monitor attached to the subject, or to use both methods in combination. The body motion processing unit 230 analyzes these images and signals and collects body motion information such as the magnitude of body motion, information on the time of occurrence of body motion, and the type of body motion. To obtain motion information using surveillance camera images, known methods such as optical flow calculation between frame images can be used. Furthermore, if respiratory motion or heart rate is obtained as a biosignal, it is also possible to use these signals to determine whether or not there is any body motion that affects the image.

[0029] If the analysis by the body motion processing unit 230 does not detect any body motion that may affect the image, such as sudden motion or positional deviation excluding relatively small periodic motions such as respiratory motion and heartbeat, the image generating unit 220 uses the collected k-space data to reconstruct the image using a known image reconstruction method such as Fourier transform or PI calculation (S4).

[0030] If the analysis by the motion processor 230 determines that motion occurred during the acquisition of k-space data (S3), the motion processor 230 identifies measurement data affected by motion based on the time of motion occurrence. Furthermore, the motion processor 230 determines whether to reconstruct a motion-corrected image using the motion-affected data or to remeasure (S5). The determination of whether to remeasure can be based on the magnitude and duration of motion, but also on the proportion of data to be corrected, i.e., the proportion of data to the total k-space data and the proportion of data occupying the low frequency range of k-space. Specifically, if either the proportion of data to the total k-space data or the proportion of data occupying the low frequency range of k-space exceeds the respective threshold, it is determined that remeasurement is necessary without motion correction.

[0031] If motion correction is possible, the image generator 220 generates a motion-corrected image using a predetermined method (S6). Examples of the motion correction method include zero-filling, which deletes data to be corrected and fills it with zeros, half-scan reconstruction, reconstruction using data estimation utilizing the Hermitian symmetry of k-space, and iterative reconstruction using k-space data after zero-filling.

[0032] After determining that body movement has occurred (S3), the measurement data affected by body movement and any subsequent unmeasured measurement data may be automatically remeasured (S6) without determining whether or not remeasurement is required in step S5.

[0033] If it is determined that motion correction is not possible or that remeasurement is necessary, the imaging control unit 210 controls the imaging unit 10 to perform remeasurement automatically or in response to a user's selection of remeasurement. That is, the imaging unit 10 stores the k-space data acquired up until the occurrence of motion in memory, and remeasures the k-space data after the motion occurs (S7). At this time, the imaging control unit 210 controls the imaging unit 10 to perform a predetermined dummy scan prior to collecting echo signals for image reconstruction.

[0034] A dummy scan is a scan that generates echo signals (dummy echoes) that are not used in image reconstruction. The imaging control unit 210 (condition setting unit 213) sets dummy scan conditions (conditions for remeasurement including dummy scan), such as the number of dummy echoes, the period during which the dummy scan is performed, and whether or not to phase encode the dummy echoes, and controls the imaging unit 10. The dummy scan conditions are determined to minimize the extension of time due to remeasurement and taking into consideration the amount of measurement data collected in the remeasurement and its position in k-space. By performing a dummy scan under appropriate conditions, remeasurement can be performed in the same state as before the remeasurement (steady state of longitudinal magnetization), and artifacts in an image reconstructed using the remeasurement data can be suppressed. Specific remeasurement conditions will be described in the following embodiments.

[0035] Once the conditions are set, the imaging unit 10 executes remeasurement in accordance with the conditions under the control of the remeasurement control unit 211 and the dummy scan control unit 212 (S7). Once the measurement data required for image reconstruction is collected by the dummy scan and remeasurement, the image generation unit 220 generates an image using the measurement data before remeasurement stored in memory and the data collected after remeasurement (S8).

[0036] The image generated by the image generation unit 220 is displayed on the display device of the UI unit 30 via the display control unit 250 (S9). The image data may also be sent to an external storage device, database, or the like.

[0037] According to the MRI apparatus of this embodiment, when re-imaging becomes necessary due to the occurrence of body movement, a dummy scan is performed during the re-measurement, thereby preventing artifacts from occurring in an image reconstructed from k-space data including the re-measurement data. Furthermore, by appropriately controlling the conditions for the re-measurement including the dummy scan, the effect of extending the re-measurement time due to the addition of the dummy scan can be minimized.

[0038] Next, a description will be given of an embodiment of specific control of dummy scans and remeasurement performed by the imaging control unit 210. In the following embodiment, the configuration and operation flow of the MRI apparatus and its processor are the same as those of the above-mentioned embodiment, and the drawings used in the above-mentioned embodiment will be used as appropriate for the description.

[0039] <Embodiment 1> In this embodiment, in the dummy scan performed during remeasurement, similar to normal imaging, the generated dummy echoes are phase-encoded but are not measured as data. Specifically, the dummy scan is performed from a phase encode that predates the occurrence of body movement by a predetermined period, and when the phase encode at the time of body movement is reached, remeasurement of the echoes for image construction is started from that phase encode.

[0040] Control by the imaging control unit 210 of this embodiment will be described with reference to Fig. 4. Note that the horizontal direction in Fig. 4 is the direction along the phase encoding ordering, and normal imaging before the occurrence of body movement and the subsequent re-measurement do not overlap in time (the same applies to Figs. 7, 8, and 10 below).

[0041] FIG. 4(a) shows a state in which normal imaging is performed without any body motion. In normal imaging, a pulse sequence is executed to collect a predetermined number of phase-encoded echo signals in a predetermined order (ordering) from the start to the end of imaging, and k-space data for image reconstruction is acquired. On the other hand, FIG. 4(b) shows a case in which normal imaging is performed up to the middle of imaging, but body motion occurs at time t1. The body motion processing unit 230 analyzes body motion information using surveillance camera images and navigator echoes, identifies echo signals collected from time t1 when body motion occurred to a predetermined time t2 as data affected by body motion (data to be corrected), and determines whether remeasurement is necessary. If it is determined that remeasurement is necessary, the imaging control unit 210 determines data to be remeasured, including data to be corrected, and controls storage in memory of a portion of the k-space data collected during normal imaging before the occurrence of body motion.

[0042] When the data to be remeasured is identified, the imaging control unit 210 determines the conditions for remeasurement, including dummy scans, based on the position in k-space (phase encoding) of the data to be remeasured, the number of data, the image type, etc. In this embodiment, the range of phase encoding to be performed in dummy scans is determined as the condition for remeasurement. The length of the dummy scan (dummy scan period) is determined depending on the range of phase encoding, and in this embodiment, the range of phase encoding starts from the phase encoding at a time point going back in time from time t1, that is, the phase encoding that is earlier in the measurement order than the phase encoding at time t1, and continues up to the phase encoding at time t1. The period for going back in time for phase encoding can be determined based on a predetermined time length or the number of RF pulse shots that generate dummy echoes, which is determined in advance.

[0043] The predetermined time length and the number of shots may be set to fixed values, such as the same as or twice the period from time t1 to time t2, or may be determined or adjusted depending on the magnitude and duration of the body movement, or on the type of image, such as T1W or T2W.

[0044] As an example of determining the dummy scan period based on the magnitude and duration of body movement, if the body movement is large, for example, above a predetermined threshold, the number of shots can be set to 100 or the dummy scan period can be set to 1 second, and if the body movement is small, for example, if the body movement exceeds the threshold but is within the predetermined threshold, the number of shots can be set to 20 or the number of dummy scans can be set to 0.5 seconds, etc. Also, if the duration of body movement is 1 second or more, the number of shots can be set to 100 or the dummy scan period can be set to 1 second, and if the duration is short, the time equivalent to that duration (or the number of shots during that time) can be set.

[0045] As an example of adjusting the dummy scan period based on the image type, for example, the dummy scan period may be shorter for T1W imaging, which has a long TR, than for T2W imaging. Alternatively, the time until the echo reaches a steady state may be determined empirically or through simulation depending on the TR of the pulse sequence, which differs depending on the image type, i.e., the interval between excitation RF pulses, and the number of dummy scan shots may be set based on this. In this case, the imaging control unit 210 may be configured to select an appropriate time length or number of shots depending on the TR of the pulse sequence.

[0046] Furthermore, the imaging control unit 210 may adjust the dummy scan period or the number of RF pulse shots depending on the arrangement of the re-measurement target data in the k-space. When the re-measurement target data is high-frequency data in the k-space, or when the re-measurement target data measured at the beginning of the re-measurement is high-frequency data in the k-space, the imaging control unit 210 may reduce the dummy scan period or the number of RF pulse shots compared to the low-frequency data, and when the re-measurement target data measured at the beginning of the re-measurement is low-frequency data, the imaging control unit 210 may increase the dummy scan period or the number of RF pulse shots from a predetermined period or number of times.

[0047] The criteria for dividing the k-space into high-frequency data and low-frequency data are not limited, but may be predetermined, such as setting the central half of each of the upper and lower regions bounded by the center of the k-space as low-frequency data and the outer half as low-frequency data, as in division example 1 of Fig. 5. In addition to the low and high frequencies, a mid-range may be set between the low and high frequencies, as in division example 2 of Fig. 5. When adjusting the period of dummy scans or the number of RF pulse shots depending on the region of k-space to which the data to be corrected belongs, a predetermined value may be set in advance, such as 50% of the number for low-frequency data for high-frequency data. The ratio between the low frequency band and the high frequency band when dividing the k-space into regions and the ratio of adjustment according to the region may be adjustable according to the image quality desired by the user.

[0048] 6 shows an example of the processing flow of the imaging control unit 210 when dividing k-space into high-frequency data and low-frequency data and setting conditions for a dummy scan. When the imaging control unit 210 identifies data to be re-measured (S71), it determines whether the data is high-frequency data or low-frequency data (S72). If the data is low-frequency data, it controls the imaging unit 10 using a preset dummy scan period or number of shots as a dummy scan condition (S73, S75). On the other hand, if the data to be re-measured is high-frequency data, it controls the imaging unit 10 to perform a dummy scan by going back in phase encoding for the preset dummy scan period or number of shots for the high-frequency data (S74, S75).

[0049] Based on the above explanation, a specific example of re-measurement will be described with reference to Figures 7A and 7B. Note that Figure 7 shows an example of sequential ordering in which k-space data is measured in order along the phase encoding direction.

[0050] The example in Figure 7A shows an example in which body movement occurs during measurement from low to high frequencies. After the body movement occurs, remeasurement, including a dummy scan, is initiated immediately. The dummy scan begins with a blank shot (RF pulse irradiation without measuring echoes) for a predetermined number of shots, starting from the phase encoding p(t1) at the time t1 when the body movement occurred, going back in order of the ordering, and generates dummy echoes up to the phase encoding immediately before the phase encoding p(t1) at time t1. When the phase encoding p(t1) at time t1 is reached, the actual remeasurement scan is initiated, and the pulse sequence is continued, inheriting the ordering from the dummy scan. Data is collected from echoes with the phase encoding at time t1 and subsequent phase encodings as echoes for image reconstruction. The remeasurement continues until all unmeasured data has been collected.

[0051] The example in Figure 7B shows an example in which body movement occurs during measurement of high-frequency data when measuring from high to low frequencies. In this case, as in the example in Figure 7A, re-measurement including dummy scans is started immediately after the occurrence of body movement, but the phase encoding at which the dummy scan is started, i.e., the number of phase encodings to go back, is fewer than in the case of Figure 7A. As a result, the dummy scan period DT2 is shorter than the dummy scan period DT1 in Figure 7A. After the dummy scan, data is collected from the phase encoding at the time t1 when body movement occurred, as in the example in Figure 7A.

[0052] 7A and 7B show examples in which the dummy scan period, i.e., the period for tracing back the phase encoding, is changed depending on which region of k-space the phase encoding at the time of the occurrence of body movement belongs to. However, as described above, the dummy scan may be performed within a predetermined dummy scan period regardless of the region of k-space, and adjustments may also be made depending on the magnitude of body movement, the type of image, etc.

[0053] In either case, remeasurement is performed, and finally, when the collection of the k-space data necessary for image reconstruction, including a portion of the k-space data acquired in normal imaging, is completed, the image generation unit 220 generates an image using the k-space data (FIG. 3: S8).

[0054] If body motion occurs again during remeasurement, the flow shown in Figure 3 is followed, and if the body motion can be corrected, body motion-corrected image reconstruction is performed using the measurement data at the time the body motion occurred as the data to be corrected. If body motion correction is not sufficient, for example, if the proportion of the data to be corrected in the low range of k-space exceeds a predetermined proportion, remeasurement is performed. As described above, a dummy scan is performed for a predetermined period of time during this remeasurement as well, and echoes for image reconstruction are collected.

[0055] According to this embodiment, by tracing back the phase encoding and performing a dummy scan using the same pulse sequence as that used for normal imaging, it is possible to easily control the gradient magnetic field in the pulse sequence, and to quickly stabilize the signal using the dummy scan, thereby preventing the remeasurement data from being affected by body movement. Furthermore, according to this embodiment, the period during which the dummy scan is performed can be limited to an appropriate length that eliminates the influence of body movement, thereby minimizing the extension of the remeasurement time.

[0056] Note that in FIG. 7 used to explain the embodiment and the explanation thereof, the measurement order (ordering) of k-space is sequential ordering as an example, but this embodiment and the modified examples and embodiments described below are not limited to sequential ordering, and can also be applied to centric ordering, which starts from the center of k-space, that is, the low-frequency side, and sequentially measures the upper and lower high-frequency sides, and spiral scanning.

[0057] <Modification 1 of Embodiment 1> In this modification, a dummy scan is also performed when re-measuring measurement data for the range of body movement, as in the first embodiment, but whereas in the first embodiment dummy echoes to which phase encoding is applied are collected in the dummy scan, in this modification no phase encoding is applied, i.e., no phase encoding gradient magnetic field is applied during the dummy scan to generate dummy echoes. The lower part of the above-mentioned Fig. 8 illustrates an example of re-measurement B in which no phase encoding is applied.

[0058] In this modification, the period during which the dummy scan is performed can be a predetermined period, as in the first embodiment. However, here, a case will be described in which the time to transition to remeasurement is determined based on the signal intensity of the dummy echo. The signal intensity of the dummy echo is unstable due to body motion immediately after the start of the dummy scan. However, as shown in FIG. 9, for example, it stabilizes at a certain intensity once the longitudinal magnetization of the spins reaches a steady state. Therefore, the imaging control unit 210 does not collect the dummy echo as k-space data, but rather monitors only the intensity change and determines when the steady state has been reached. Specifically, the imaging control unit 210 determines that the steady state has been reached when the signal intensity difference between adjacent echoes falls below a predetermined threshold, and from that point on, adds a phase encoding gradient magnetic field Gp to the pulse sequence and controls echo measurement. That is, remeasurement of the k-space data within the range of body motion (main scan) is initiated. The initial phase encoding in the main scan is the same as the phase encoding at time t1 when body motion occurs.

[0059] According to this modification, although the control of the phase encoding gradient magnetic field differs from that of the first embodiment, the dummy echo is not phase encoded, so the signal intensity does not change due to differences in the gradient magnetic field, and the stable state of the echo can be known from the change in signal intensity. Therefore, the period of the dummy scan can be set efficiently regardless of the type of pulse sequence, and the extension of time due to adding a dummy scan can be kept to a minimum.

[0060] <Modification 2 of Embodiment 1> In the first embodiment, the dummy scan is started by tracing back from the phase encoding at the time (t1) when the body movement occurred, and the phase encoding is changed according to the ordering. However, the dummy scan may be performed with the phase encoding constant. In this case, the phase encoding is not particularly limited, and may be, for example, the same as the phase encoding at the time when the body movement occurred, or a phase encoding close to that. An example of re-measurement A according to this modification is shown in the middle part of FIG. 8. The top part of FIG. 8 shows imaging before re-measurement when body movement occurred during imaging. In re-measurement A, a dummy scan is performed with the phase encoding p(t1) at the time when the body movement occurred. During the dummy scan, the imaging control unit 210 fixes the phase encoding p(t1) and measures the data to be re-measured while changing the phase encoding step during the main scan of re-measurement A. In this modification, the phase encoding is constant, so the dummy echo does not include differences due to the phase encoding, and similarly to the first modification, it is possible to detect that a stable state has been reached from the change in the dummy echo.

[0061] <Embodiment 2> In this embodiment, as shown in FIG. 5, k-space is divided into low-frequency and high-frequency regions, and depending on which region's data was being measured at the time of body movement occurrence, measurement of some data in the remeasurement is omitted, thereby preventing time extension due to dummy scans.

[0062] In this embodiment as well, when the body motion processing unit 230 determines that the k-space data subsequent to the re-measurement target data should be re-measured, the imaging control unit 210 controls the imaging unit 10 to add a dummy scan of a predetermined period prior to the main scan, as in embodiment 1 or its modified example. In this embodiment, the imaging control unit 210 further determines high-frequency data that will not be collected from among the k-space data to be measured in the main scan for re-measurement, and controls the imaging unit 10 to omit collecting some of the high-frequency data when performing re-measurement.

[0063] An example of the control of remeasurement according to this embodiment is shown in FIGS.

[0064] The example shown in Figure 10 illustrates a case where body movement occurs during measurement of high-frequency data when k-space is measured from the upper high-frequency data to the lower low-frequency data. In the first embodiment, all data after the occurrence of body movement is remeasured, as shown in Figure 7A, for example. However, in this embodiment, rather than measuring all data after the occurrence of body movement, at least some of the measurements are omitted. In the example shown in the upper part of Figure 10 (remeasurement C), after it is determined that remeasurement is necessary due to the occurrence of body movement, remeasurement including dummy scan is initiated. At this time, dummy scan is performed until low-frequency phase encoding is reached, and data measurement begins from the phase encoding when the high-frequency range transitions to low-frequency range in the main remeasurement scan. Therefore, in this remeasurement C, the phase encoding data from the occurrence of body movement until the start of dummy scan and the phase encoding data during dummy scan (phase encoding data indicated by diagonal lines in the figure) among the high-frequency data are not measured.

[0065] In the example shown in the lower part of Figure 10 (remeasurement D), the time point at which body movement occurred during imaging is the same as in the upper part of Figure 10, but in the remeasurement, measurement of phase encoding data from the time body movement occurred until the start of the dummy scan is omitted, and in the main scan, measurement of phase encoding data thereafter is performed. As shown on the right side of Figure 10, in both remeasurement C and remeasurement D, the extension of time due to the insertion of dummy scans can be suppressed by omitting measurement of high-frequency data in k-space. In particular, in the case of remeasurement C, the extension of time due to dummy scans can be eliminated by setting the number of data to be omitted to be equal to or less than the number of dummy scan shots.

[0066] The choice of whether to perform remeasurement C or reconstruction D can be determined by default, or the imaging control unit 210 can automatically decide based on the number of high-frequency data not measured and the remeasurement time extended by measuring it (limit on extension time).

[0067] 11 is a diagram showing an example of control when body movement occurs while measuring low-frequency data. If body movement occurs while measuring low-frequency data, the low-frequency data affected by the body movement (the deleted low-frequency data) is remeasured, but after transitioning from the low frequency to the high frequency in the remeasurement, measurement of at least a part of the high-frequency data is omitted.

[0068] The position and number of high-frequency data items that are not collected depend on the k-space data determined to be in the range of body motion (area B in Figure 5) and the position and number of data items in k-space after the range of body motion. Theoretically, if high-frequency k-space data symmetrical to the high-frequency k-space data collected during normal imaging has not been measured due to an interruption in imaging, it is possible to omit measurement of all of the unmeasured high-frequency k-space data that corresponds to the measured high-frequency k-space data. However, if the purpose is to cancel the time extension due to dummy scans, it is also possible to omit measurement of the same number of data items from the unmeasured high-frequency k-space data as the preset number of dummy echoes (dummy scan period DT). This allows the imaging time to be shortened by the time OT, which is the same as the dummy scan period DT.

[0069] As an example, when a predetermined number of dummy echoes is set in advance, the imaging control unit 210 compares the number of unmeasured k-space high-frequency data with the set number of dummy echoes, and if the former is greater than the latter, omits measurement of the same number of k-space high-frequency data as the number of dummy echoes. On the other hand, if the number of unmeasured k-space high-frequency data is less than the number of dummy echoes, the imaging control unit 210 may be configured to perform control such that all of the unmeasured k-space high-frequency data is omitted. However, in this embodiment, it is sufficient to omit at least a portion of the unmeasured k-space high-frequency data, and this control is an example and is not limited to this.

[0070] Furthermore, the data region for which measurement is omitted is not limited as long as it is in the high frequency range, but may be, for example, data from the phase encoding that continues from the highest frequency side to a predetermined phase encoding, as shown in the lower part of FIG. 11. Alternatively, thinning may be performed in the phase encoding direction. The thinning rate is, for example, "(number of high frequency data - number of blank shots) / number of high frequency data." However, in this case, it is assumed that the number of unmeasured k-space high frequency data is greater than the number of dummy echoes.

[0071] The image generator 220 performs image reconstruction using k-space data in which part of the high-frequency data is unmeasured. This image reconstruction, like the motion-corrected image reconstruction, generates an image by, for example, performing image reconstruction after zero-filling the unmeasured high-frequency data, performing image reconstruction by performing data estimation using a sensitivity distribution, or performing sequential reconstruction after zero-filling.

[0072] While Figures 10 and 11 show cases where measurement of one of the upper and lower high-frequency data in k-space is omitted depending on the phase encoding position at the time of body movement, as shown in Figure 12, it is also possible to omit measurement of both the upper and lower high-frequency data depending on the phase encoding position at the time of body movement. In this case, by setting the number of omitted data to be approximately the same as the number of dummy scan shots, the dummy scan period DT can be canceled, as in the case of omitting measurement of high-frequency data on one side, and extension of re-measurement time can be suppressed. When omitting measurement of high-frequency data on both sides, it is also possible to control it so that, for example, for high-frequency data on both sides with opposite polarities, one of the data with the same phase encoding is retained and the other is omitted. This leaves one of the data pairs that are Hermitian symmetric in the phase encoding direction of k-space, allowing more information in the high-frequency region of the original data to be preserved.

[0073] According to this embodiment, by not acquiring part of the high frequency data, the remeasurement time can be shortened and the time extended by the dummy scan can be offset (cancelled). As a result, there is no substantial extension of the remeasurement time, and it is possible to reduce the extension of the imaging time when remeasurement is required.

[0074] Although the embodiments and modifications of the present invention have been described above, the present invention is not limited to these embodiments, and the addition of other known elements and the omission of non-essential elements are also encompassed within the scope of the present invention. Furthermore, these embodiments and modifications can be combined as appropriate as long as there is no technical contradiction, and such combinations are also encompassed within the scope of the present invention. [Explanation of symbols]

[0075] 1: MRI device, 10: imaging unit, 20: processor, 210: imaging control unit, 220: image generation unit, 230: body motion processing unit

Claims

1. an imaging unit that acquires nuclear magnetic resonance signals of a subject; an image generating unit that reconstructs an image of the subject using k-space data consisting of the nuclear magnetic resonance signals acquired by the imaging unit; a body movement processing unit that analyzes body movement of the subject during imaging and identifies data to be corrected that has been affected by the body movement of the subject; and an imaging control unit that controls the imaging unit; When the imaging control unit determines that remeasurement of the correction target data is necessary, the imaging control unit sets conditions for remeasurement, controls the imaging unit, and, before remeasurement of the correction target data, performs one or more RF pulse irradiations to execute a dummy scan that generates a dummy signal that is not used in image reconstruction.

2. 2. The magnetic resonance imaging apparatus according to claim 1, The magnetic resonance imaging apparatus is characterized in that the imaging control unit executes the dummy scan for a preset period or for a preset number of RF pulse irradiations.

3. 2. The magnetic resonance imaging apparatus according to claim 1, The magnetic resonance imaging apparatus according to claim 1, wherein the imaging control unit executes the dummy scan without applying phase encoding to the dummy signal.

4. 2. The magnetic resonance imaging apparatus according to claim 1, The magnetic resonance imaging apparatus according to claim 1, wherein the imaging control unit performs the dummy scan by applying phase encoding to the dummy signal.

5. 5. The magnetic resonance imaging apparatus according to claim 4, the imaging control unit executes the dummy scan by sequentially applying phase encoding from a phase encoding that precedes a first phase encoding of the correction target data in a signal measurement order to the first phase encoding.

6. 2. The magnetic resonance imaging apparatus according to claim 1, the imaging control unit refers to an arrangement of the correction target data in k-space, A magnetic resonance imaging apparatus, characterized in that when the data to be corrected is high frequency data in k-space, the period of the dummy scan or the number of times of RF pulse irradiation is adjusted.

7. 2. The magnetic resonance imaging apparatus according to claim 1, the imaging control unit refers to an arrangement of the correction target data in k-space, A magnetic resonance imaging apparatus, characterized in that, when the correction target data includes high frequency data of k-space, the remeasurement is performed by omitting at least a part of the high frequency data collected in the remeasurement.

8. 2. The magnetic resonance imaging apparatus according to claim 1, The magnetic resonance imaging apparatus according to claim 1, wherein the imaging control unit omits, in the remeasurement, measurement of high frequency data in k-space that can be measured in a measurement time equivalent to the dummy scan.

9. A method for controlling a magnetic resonance imaging apparatus, comprising: Identifying correction target data affected by body movement of the subject in k-space data acquired during imaging; and A method for controlling a magnetic resonance imaging apparatus, comprising the step of controlling imaging so as to perform a dummy scan in which RF pulse irradiation is performed one or more times to generate a dummy signal that is not used in image reconstruction when re-measuring the correction target data.

10. 10. A method for controlling a magnetic resonance imaging apparatus according to claim 9, comprising: A control method for a magnetic resonance imaging apparatus, comprising the step of determining, before the step of controlling the imaging, conditions for the dummy scan, including a period of the dummy scan or the number of times of irradiation of the RF pulse.

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