Magnetic resonance imaging apparatus and center frequency correction method

Abstract

To correct misregistration caused by variations in a center frequency using an amount of phase change calculated using a navigator echo, while eliminating an influence of a body motion included in the amount of phase change to improve accuracy of the correction.SOLUTION: Calculation of a correction value for correcting variations in a center frequency is adjusted while referring to body motion information obtained by detecting a body motion of a subject during an examination. The adjustment is performed using methods such as performing only body motion correction without performing correction while a large body motion occurs, estimating the correction value from a change in a correction value calculated when there is no body motion, or correcting the correction value based on the body motion information.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a technology for correcting fluctuations in frequencies that cause nuclear magnetic resonance in a magnetic resonance imaging device (hereinafter referred to as an MRI device), and in particular to a technology for suppressing the influence of movement (body movement) of the subject that occurs during the examination on frequency fluctuation correction. [Background technology]

[0002] A magnetic resonance imaging system (hereafter referred to as an MRI system) induces nuclear magnetic resonance in the nuclei of atoms that make up the tissue of the subject (examinee), collects the resulting nuclear magnetic resonance signals, and reconstructs an image of the subject. To induce nuclear magnetic resonance, radio frequency pulses with a center frequency equal to the nuclear magnetic resonance frequency are applied to the subject. The center frequency of the nuclear magnetic resonance frequency is determined by the static magnetic field and does not change as long as the static magnetic field strength is constant. However, the coils that correct the static magnetic field (shim coils) may be physically affected by heat generated by the current flowing through the gradient magnetic field coil, causing the center frequency to fluctuate. Fluctuations in the center frequency can cause misalignment of the image.

[0003] If the fluctuation of the center frequency is known, the phase change can be obtained from that fluctuation, and the amount of phase change can be used as a correction value to correct the positional shift that occurs in the image. Conventionally, as a method for detecting the fluctuation of the center frequency, there is a method in which a navigator echo for detecting the phase change is generated separately from the echo (nuclear magnetic resonance signal) for image formation, and the phase change is calculated using the navigator echo (Patent Document 1, etc.).

[0004] In the technology disclosed in Patent Document 1, whereas the amount of phase change from the generation of the navigator echo to the echo time was conventionally calculated, the amount of phase change from the reference is calculated by acquiring the phase change within the measurement time (data acquisition time) of the navigator echo and calculating the difference with the phase change of a similarly acquired reference navigator. With this method, even if an offset (accumulation of changes over time) occurs in the phase of the navigator echo due to changes over time in eddy currents in the gradient magnetic field, etc., the deviation from the reference can be accurately calculated without being affected by the offset. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent Publication No. 2021-183031 Summary of the Invention [Problem to be solved by the invention]

[0006] The technique described in Patent Document 1 can accurately determine phase changes, but has the following problems. If the subject experiences body movements such as breathing, pulsation, or sudden movements during imaging, phase changes also occur in the navigator echo. In other words, the phase changes calculated from the navigator echo include both changes caused by fluctuations in the center frequency (changes in the static magnetic field) and changes caused by body movements. In the case of periodic movements such as breathing, these changes can be predicted, and so it may be possible to separate them from the fluctuations in the center frequency.

[0007] However, in addition to periodic motion, there can be various types of motion of varying magnitude, and even periodic motion affects the image to different degrees depending on the imaging site, making it difficult to distinguish phase changes caused by these motions from those caused by fluctuations in the target center frequency.Furthermore, if the motion is sudden or very large, it may be difficult to calculate the phase change from the navigator echo.

[0008] The present invention aims to provide a technology that enables specialized detection of fluctuations in center frequency, thereby enabling accurate separation of phase changes due to fluctuations in center frequency from phase changes due to body movement, and thereby enabling accurate correction of image position shifts caused by fluctuations in center frequency.

[0009] Another object of the present invention is to provide a technique that can accurately correct both phase changes caused by fluctuations in the center frequency and phase changes caused by body movement while separating them. [Means for solving the problem]

[0010] The present invention uses a navigator echo for detecting phase changes and an optical means for detecting the subject's body movements, such as a camera, to obtain body movement information from the optical means, and uses this body movement information to determine the selection of a navigator echo for use in calculating the phase changes or the need for calculation of a correction value, thereby obtaining phase information from which phase changes due to body movements have been eliminated, thereby accurately grasping and correcting fluctuations in the center frequency.

[0011] That is, the MRI apparatus of the present invention includes an imaging unit that collects nuclear magnetic resonance signals generated from a subject by nuclear magnetic resonance, a calculation unit including an image generation unit that reconstructs an image of the subject using the nuclear magnetic resonance signals, and a body motion processing unit that collects body motion information of the subject. The imaging unit collects navigator echoes for detecting fluctuations in the center frequency of the nuclear magnetic resonance, and the calculation unit includes a correction value calculation unit that calculates a correction value for correcting the fluctuations in the center frequency using phase changes of the navigator echoes collected by the imaging unit. The correction value calculation unit calculates a correction value that eliminates the effects of body motion by referring to the body motion information collected by the body motion processing unit.

[0012] The present invention also provides a method for correcting a center frequency, which includes the following steps: a step of calculating a phase difference using two or more nuclear magnetic resonance signals acquired as navigator echoes from a subject under examination and calculating a correction value for the center frequency, and a step of collecting body movement information from a device that detects body movement of the subject under examination. In the step of calculating the correction value, the body movement information is referenced to calculate a correction value that eliminates the influence of body movement.

[0013] In this specification, correction for fluctuations in the center frequency is referred to as "frequency fluctuation correction," and correction for the effect of subject movement on an image is referred to as "movement correction." [Effects of the Invention]

[0014] According to the present invention, when calculating a correction value from a phase change of a navigator echo, by referring to body movement information at the time the navigator echo was acquired, it is possible to determine whether the acquired navigator echo can be used to calculate the correction value, or whether correction or estimation of the correction value is necessary, and the like. This makes it possible to eliminate the influence of body movement from the correction value calculated using the navigator echo, thereby achieving highly accurate frequency fluctuation correction. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a block diagram showing an example of the overall configuration of an MRI apparatus according to the present invention. [Figure 2] FIG. 1 is a block diagram showing the overall configuration of an MRI apparatus including details of an imaging unit. [Figure 3] 2 is a flowchart of the processing of the MRI apparatus of FIG. 1. [Figure 4] 1 is a flowchart of process 1 of the first embodiment. [Figure 5] 10 is a flowchart of process 2 or process 3 of embodiment 1. [Figure 6] 1A and 1B are diagrams for explaining examples of body movement information, in which (A) shows an example of sudden body movement, (B) shows an example of periodic movement, and (C) shows an example of a change in position caused by body movement. [Figure 7] FIG. 1 is a diagram showing frequency changes along the time axis. [Figure 8] FIG. 2 is a diagram showing an example of a flow of process selection according to the first embodiment. [Figure 9] FIG. 10 is a diagram for explaining a modification of the first embodiment. [Figure 10] 10 is a flowchart of a process according to the second embodiment. [Figure 11] (A) is a diagram showing the periodic motion shown in (B) superimposed on the frequency fluctuation. [Figure 12] FIG. 10 is a diagram for explaining the processing of the second embodiment. [Figure 13] FIG. 10 is a diagram showing an example of a method for calculating a correction value when periodic motion is present. [Figure 14] 10 is a flowchart of a process according to the third embodiment. [Figure 15] FIG. 10 is a diagram for explaining the processing of the third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, an embodiment of an MRI apparatus of the present invention will be described with reference to the drawings. In all drawings for explaining the embodiment of the invention, parts having the same functions are assigned the same reference numerals, and repeated description thereof will be omitted.

[0017] First, an outline of an embodiment of the MRI apparatus of the present invention will be described. Fig. 1 is a block diagram showing the overall configuration of the MRI apparatus. As shown in Fig. 1, the MRI apparatus 1 of this embodiment mainly comprises an imaging unit 10 and a processor 20 that controls imaging and performs calculations such as image reconstruction.

[0018] The imaging unit 10 has a configuration similar to that of a general MRI device, and includes a static magnetic field generating unit 101 that generates a static magnetic field in the space in which the subject is placed, gradient magnetic field generating units (102, 105) that apply a magnetic field gradient to the static magnetic field, a transmitting unit 106 that irradiates the subject with a high-frequency magnetic field, and a receiving unit 107 that receives nuclear magnetic resonance signals generated by nuclear magnetic resonance from the subject. Furthermore, a bed device 120 is provided for positioning the subject in the imaging space.

[0019] The static magnetic field generating unit 101 is equipped with a static magnetic field generating source of the permanent magnet type, resistive type, or superconductive type. In the case of a vertical magnetic field type, it generates a uniform static magnetic field in the space around the subject 50 in a direction perpendicular to the body axis, while in the case of a horizontal magnetic field type, it generates a uniform static magnetic field in the direction of the body axis. Also, a shim coil 108 is arranged near the static magnetic field generating source to correct non-uniformity of the static magnetic field. The shim coil 108 is connected to a shim power supply 109 and is driven by a current supplied from the shim power supply 109 to generate a correction magnetic field. Hereinafter, the shim coil 108 and the shim power supply 109 are collectively referred to as a shimming unit. The correction magnetic field generated by the shimming unit can correct non-uniformity of the static magnetic field and also correct fluctuations in the static magnetic field over time. The gradient magnetic field generating unit may also function as a shimming unit.

[0020] The gradient magnetic field generating unit comprises gradient magnetic field coils 102 wound in the three X, Y, and Z axis directions, which are the coordinate system (stationary coordinate system) of the MRI apparatus, and gradient magnetic field power supplies 105 that drive each gradient magnetic field coil, and applies gradient magnetic fields Gx, Gy, and Gz in the three X, Y, and Z axis directions by driving the gradient magnetic field power supplies 105 of each coil in accordance with commands from a sequencer 110, which will be described later. During imaging, a slice direction gradient magnetic field pulse (Gs) is applied in a direction perpendicular to the slice plane (imaging cross section) to set the slice plane for the subject 50, and a phase encoding direction gradient magnetic field pulse (Gp) and a frequency encoding direction gradient magnetic field pulse (Gf) are applied in the remaining two directions that are perpendicular to the slice plane and perpendicular to each other, and position information in each direction is encoded into the echo signal.

[0021] The transmitter 106 irradiates the subject 50 with an RF pulse to induce nuclear magnetic resonance in the nuclear spins of atoms constituting the biological tissue of the subject 50, and includes a radio frequency oscillator, a modulator, a radio frequency amplifier, and a radio frequency coil (transmitting coil) 103 on the transmitting side. The radio frequency pulse output from the radio frequency oscillator is amplitude-modulated by the modulator at a timing according to a command from the sequencer 110, and this amplitude-modulated radio frequency pulse is amplified by the radio frequency amplifier and then supplied to the transmitting coil 103 arranged close to the subject 50, thereby irradiating the RF pulse onto the subject 50. The nuclear magnetic resonance frequency (center frequency) can be adjusted by adjusting the radio frequency emitted by the radio frequency oscillator.

[0022] The receiving unit 107 detects echo signals (NMR signals) emitted by nuclear magnetic resonance of atomic spins constituting the biological tissue of the subject 50, and includes a signal amplifier, a quadrature phase detector, an A / D converter, etc., and is connected to a receiving-side radio frequency coil (receiving coil) 104. An NMR signal of the response of the subject 50 induced by electromagnetic waves irradiated from a transmitting-side radio frequency coil (transmitting coil) 103 is detected by the receiving coil 104 arranged close to the subject 50, amplified by the signal amplifier, and then split into two orthogonal systems of signals by the quadrature phase detector at a timing according to a command from the sequencer 110, each of which is converted into a digital quantity by an A / D converter and sent to the processor 20 as measurement data.

[0023] The sequencer 110 is a control means for repeatedly applying radio frequency magnetic field pulses (hereinafter referred to as "RF pulses") and gradient magnetic field pulses in a predetermined pulse sequence, and operates under the control of the processor 20 (controller), sending various commands required for collecting tomographic image data of the subject 50 to the transmitter 106, shimming unit, gradient magnetic field generator, and receiver 107. Various pulse sequences are prepared depending on the imaging method, and when the imaging method is determined for imaging, the processor 20 reads out the corresponding pulse sequence and sets it in the sequencer 110. In this embodiment, a pulse sequence is executed that adds generation and collection of navigator echoes.

[0024] The processor 20 performs various data processing, and displays and stores the processing results, and includes a control unit 20B and a calculation unit 20A. The control unit 20B includes an imaging control unit 210 and a display control unit 250.

[0025] As shown in FIG. 2, the calculation unit 20A includes an image generation unit 220 that generates an image using the NMR signal received by the receiving unit 107, a correction value calculation unit 230 that uses the navigator echo to calculate a correction value for correcting fluctuations in the center frequency of the radio frequency magnetic field that generates nuclear magnetic resonance, and a body movement processing unit 240 that collects body movement information to suppress the effects of body movement occurring in the subject 50 during the examination and performs processing necessary for body movement correction.

[0026] The functions of each part of the processor 20 are mainly realized by a CPU, but some functions can also be realized by a programmable IC such as an ASIC or FPGA. An external storage device 60 such as an optical disk or a magnetic disk, and a UI unit 70 equipped with a display device 40 and an input device 30 are connected to the processor 20. The external storage device 60 includes storage devices connected directly or via a wire, as well as storage devices connected wirelessly or via the Internet.

[0027] Furthermore, the MRI apparatus 1 is equipped with an optical detection means such as a surveillance camera 80 for monitoring the state of the subject 50 placed in the static magnetic field space, and the body movement processing unit 240 detects the body movement of the subject 50 using the image from this surveillance camera 80.

[0028] Next, in consideration of the configuration of the MRI apparatus described above, an embodiment of the process from imaging to image reconstruction will be described with reference to FIG.

[0029] When the subject is placed in the imaging space and imaging begins, an imaging sequence including a navigator sequence for acquiring navigator echoes is executed under the control of the sequencer 110, and navigator echoes and echoes for imaging the subject (hereinafter referred to as main imaging echoes) are collected in time series (S1).

[0030] The navigator echo is an NMR signal acquired to detect a phase change occurring in the NMR signal due to a fluctuation in the central resonance frequency without applying a gradient magnetic field that imparts phase encoding and readout encoding. The navigator echo may be acquired within the TR of the pulse sequence for acquiring the actual imaging echo, before or after the acquisition of the actual imaging echo, or may be acquired by inserting a sequence for acquiring the navigator echo between the TRs of the pulse sequence for acquiring the actual imaging echo. An example of the latter is the sequence disclosed in Patent Document 1, but is not limited to this. In the navigation sequence disclosed in Patent Document 1, the navigator echo is acquired as an FID before the actual imaging sequence.

[0031] The monitoring camera 80 is activated to detect the subject's movements simultaneously with or prior to the start of imaging. The body movement processor 240 captures video from the monitoring camera 80 and analyzes the video to collect body movement information about the subject (S2, S3). Body movements include small but periodic movements such as breathing and pulsation, unexpected subject movements such as coughing, sneezing, and convulsions, and other subtle movements. The body movement processor 240 detects various body movements by, for example, calculating displacement vectors between frames using known techniques such as optical flow from the changes between each frame of the video and then calculating the displacement of each part from the displacement vectors. In this case, a region including the imaged part may be set as a ROI, or the displacement of one or more feature points of the imaged part may be tracked. By performing such analysis on video that changes over time, body movement information can be obtained, such as the magnitude and duration of movement of each part (feature point) within the ROI, and in the case of periodic movement, the magnitude (absolute or relative) of the period or the magnitude of the displacement at each time phase within the period.

[0032] Meanwhile, the correction value calculation unit 230 calculates a correction value (S4) for correcting positional deviation caused by fluctuations in the center frequency using the navigator echoes collected by executing the navigator sequence (S2). Correction of positional deviation caused by fluctuations in the center frequency is hereinafter also referred to as frequency deviation correction. To calculate the correction value, the correction value calculation unit 230 first calculates the amount of phase change using the navigator echoes. The amount of phase change is calculated as the difference between the phase of a reference navigator echo and the phase of a navigator echo acquired subsequently. The method for calculating the phase difference from the reference navigator echo is not limited, and the method disclosed in Patent Document 1, for example, can be used.

[0033] In this method, the phase change during the echo acquisition time (the time when the generated echoes are sampled) is calculated for both the reference navigator echo and the navigator echo for which the phase difference is to be calculated, and the difference between the respective phase changes (within the sampling time) is calculated to calculate the amount of phase change from the reference. Unlike a method that simply calculates the amount of phase change per echo time (TE), this method makes it possible to calculate the amount of phase change from the reference without being affected by offsets accumulated in frequency fluctuations. Therefore, it is preferable to employ the above method for calculating the amount of phase change by the correction value calculation unit 230 of this embodiment, but the method is not limited to this, and any conventionally known method can also be employed.

[0034] Next, a correction value used for frequency variation correction is calculated from the amount of phase change. Frequency variation correction can be performed either on the system side (correction during measurement) or on the image (correction after measurement). System-side correction corrects frequency variation by correcting the current flowing through the shim coil 108 or correcting the center frequency at the time of irradiation of the RF pulse applied to the subject. That is, the correction value calculation unit 230 calculates at least one of the correction value of the shim current flowing through the shim coil 108 or the correction value of the center frequency at the time of irradiation of the excitation pulse applied by the transmit coil 103.

[0035] Specifically, the frequency correction value can be calculated from the relational expression (ω=2πf) between the phase (ωt) and the frequency (f). In addition, the relationship between the current value flowing through the shim coil 108 and the magnetic field strength generated by it is determined by the characteristics of the shim coil 108, so the correction current for the shim coil 108 can be calculated from the amount of fluctuation in the static magnetic field calculated from the relational expression (f0=λB0) (λ: Larmor frequency) between the magnetic resonance frequency (f0) and the static magnetic field strength (B0).

[0036] The calculated correction value is reflected in the actual imaging after the navigator echo used to calculate the correction value (S5). That is, immediately after the calculation of the correction value, the actual imaging is continued under the condition that the shim current or the center frequency of the excitation RF pulse has been corrected.

[0037] In the case of post-measurement correction, the calculation is performed using the linear relationship between the static magnetic field (center frequency) and the position in real space. That is, from this relationship, the positional deviation in real space corresponding to the amount of magnetic field fluctuation is calculated, and a correction value for correcting the positional deviation on the image is calculated. This correction value is used when the actual imaging data capable of image reconstruction is collected and the image generator 220 reconstructs the image, thereby correcting the positional deviation (S6, S7).

[0038] When calculating the phase change amount and the correction value based thereon as described above, the correction value calculation unit 230 of this embodiment refers to the body movement information collected by the body movement processing unit 240, and if it determines that there is body movement that affects the calculation of the phase change amount, it makes changes to the calculated phase change amount or correction value to remove the influence of the body movement.

[0039] When there is no body movement, the amount of phase change changes almost linearly with a gentle slope over time in response to frequency fluctuations. However, for example, a navigator echo acquired when the magnitude of body movement is large will exhibit a phase change greater than the above-mentioned frequency fluctuations, making it impossible to calculate an accurate amount of phase change. Furthermore, when the magnitude of body movement is not large but there is periodic movement, the phase change obtained from the navigator echo will be detected as being superimposed with the change due to the periodic movement. Even if the amount of phase change is calculated while taking into account the influence of such body movement, it will not be possible to calculate an accurate amount of phase change.

[0040] The correction value calculation unit 230 refers to the body movement information collected from the camera video by the body movement processing unit 240, determines whether the body movement affects the correction of frequency variation, and performs processing according to the body movement, such as not correcting the frequency variation or modifying the correction value. Furthermore, depending on the body movement, it may be necessary to correct the effect of the body movement itself on the image, so body movement correction is performed separately from frequency variation correction. Details of the judgment made by the correction value calculation unit 230 and the processing executed as a result of the judgment will be described in the embodiment below.

[0041] As described above, the MRI apparatus of this embodiment has, as functions of the calculation unit, a function (correction value calculation unit) for calculating a correction value for correcting frequency fluctuations using a navigator echo, and a function (body movement processing unit) for collecting and processing body movement information of the subject under examination. The correction value calculation unit refers to the body movement information obtained by the body movement processing unit to determine whether it is possible to calculate a correction value using a navigator echo, whether the correction value needs to be modified, whether body movement correction is necessary, etc., and reflects the results in subsequent imaging.

[0042] This makes it possible to distinguish between phase changes caused by frequency fluctuations and those caused by body movement among the phase changes contained in the navigator echo, accurately calculate the phase change caused by frequency fluctuations, and perform correction. Furthermore, it is possible to perform appropriate body movement correction according to the body movement. As a result, it is possible to obtain high-quality images in which the effects of frequency fluctuations are eliminated and the effects of body movement are suppressed.

[0043] Next, a specific embodiment of the correction process with reference to body movement information will be described. Note that in the following embodiment, components common to those illustrated in Figures 1 and 2 will be omitted from the illustration, and the description will be made with reference to these figures as appropriate.

[0044] <Embodiment 1> In this embodiment, the body movement processing unit 240 obtains the magnitude of the body movement, i.e., the magnitude of the displacement, as body movement information from the video captured by the monitoring camera 80, and changes the processing depending on the magnitude of the body movement. Below, the processing performed by the calculation unit 20A will be mainly described with reference to the processing flows shown in Figures 4 and 5. Of the processing shown in Figures 4 and 5, descriptions of processing that overlaps with the processing shown in Figure 3 will be omitted, and the differences will be mainly described.

[0045] The flowcharts shown in Figures 4 and 5 are processing flows that differ in the processing that occurs after it is determined that there is an influence of body movement by referring to body movement information. First, we will explain the processing (S41 to S45) that is common to the processing flows of Figures 4 and 5.

[0046] When imaging starts, an initial value of the frequency correction value is set (S41). The frequency correction value is a correction value for correcting frequency fluctuations, and is a correction value for the shim current or a correction value for the center frequency of irradiation. The initial value is set to zero at the start of imaging. If the offset of the frequency fluctuation is already known, a correction value calculated from that offset may be set as the initial value.

[0047] When imaging by the imaging unit 10 begins, a navigation sequence is executed and the navigator echo acquisition unit 150 acquires the first navigator echo (S42). The first navigator echo is designated as the reference navigator echo. Subsequently, a main imaging echo is acquired (S43), and then the next navigator echo is acquired (S44). The navigator echo acquired after the reference navigator echo is acquired is a navigator echo for detecting a phase change after the reference navigator echo is acquired, and is referred to here as a control navigator echo.

[0048] The correction value calculation unit 230 references the body motion information acquired by the body motion processing unit 240 and determines whether body motion occurred when the control navigator echo was acquired (S441). Body motion information is information indicating fluctuations, such as the average value of the displacement of a specific site or a specific region, over time. For example, this information includes large short-term movements (sudden changes) as shown in FIG. 6(A), periodic movements as shown in FIG. 6(B), and positional movements as shown in FIG. 6(C). The acquired displacements may be absolute or relative values. A predetermined threshold value, for example, the absolute or relative value of body motion that affects the calculation of the phase shift amount or body motion that affects the image (a value obtained empirically or through simulation, etc.), is used as the threshold. If the threshold value is exceeded, it is determined that there is a body motion effect, and if the threshold value is below the threshold, there is no body motion effect.

[0049] Sudden body movements can also be detected from the phase change of the navigator echo itself, instead of or in addition to the body movement information obtained from the camera image. For example, a threshold (second threshold) may be set for the difference in the phase change of the navigator echo (difference from the reference navigator echo) in addition to the threshold (first threshold) for body movement in the camera image, and it may be determined that a sudden change in body movement has occurred when the difference in the phase change obtained from the navigator echo is equal to or greater than the second threshold, or when the displacement of body movement is equal to or greater than the first threshold and the difference in the phase change is equal to or greater than the second threshold. Multiple thresholds may be set, thereby enabling responses according to the type and nature of body movement.

[0050] In this embodiment, as an example, a case will be described in which it is determined that there is an influence of a sudden body movement.

[0051] If there is no influence of body movement, as explained in the flow of Figure 3, the phase difference between the reference navigator echo and the control navigator echo is calculated to determine the amount of phase change, and a correction value is set (S442 to S444). In other words, the correction value set as the initial value is updated. As a result, the actual imaging echo is acquired with the shim current or the center frequency of RF irradiation changed according to the correction value (S445). S441 to S444 are repeated each time a control navigator echo is acquired (S45).

[0052] On the other hand, if it is determined that there is body motion at the time of acquiring the control navigator echo as a result of referring to the body motion information, the collection of body motion information continues, and a process different from the center frequency correction when there is no influence of body motion is performed until the influence of body motion disappears. The different process is not limited to, but for example, one of the following three processes is performed.

[0053] Treatment 1 does not use a control navigator echo affected by body motion and does not perform center frequency correction (Figure 4). In process 2, the control navigator echo, which is affected by body movement, is not used, and estimation is performed using information on frequency fluctuations acquired previously (Fig. 5). In process 3, the correction value calculated from the control navigator echo, which is affected by body movement, is corrected using body movement information obtained from camera images, etc. (Fig. 5). The threshold value for determining the magnitude of body movement may be different or the same for Process 1, Process 2, or Process 3.

[0054] Each process will be explained below. [Process 1] In Process 1, once the influence of body motion disappears, steps S442 to S444 are resumed and center frequency correction is performed. Since slight frequency fluctuations may have occurred between the time when it is determined that there is an influence of body motion and the time when it is determined that there is no influence of body motion, the first navigator echo acquired after the restart is set as the reference navigator echo (S42), and this reference navigator echo and subsequent navigator echoes (control navigator echoes) are used to calculate the phase change amount (S442, S443) and set the correction value (S444). Note that, as shown in Figure 6(c), if the subject has moved due to body motion (i.e., if the displacement does not return to its original state), the actual imaging echo may be remeasured and the reference navigator echo may be reacquired.

[0055] Furthermore, motion correction is performed on the actual imaging echoes collected while it is determined that there is a motion effect (S47). Known motion correction techniques include, for example, a technique in which actual imaging echoes collected when motion is large are discarded and the resulting unmeasured k-space data is zero-filled and reconstructed; a technique in which unmeasured data is estimated using a PI (parallel imaging) calculation; and a technique in which data is reconstructed using repeated calculations such as compressed sensing. These well-known techniques can be employed. Furthermore, if there are too many actual imaging echoes to be removed or if the data is low-frequency k-space data, remeasurement may be performed.

[0056] Although it depends on which method is adopted, at this stage, the actual imaging echoes collected when there is an influence of body motion are labeled as not to be used in image reconstruction processing or are removed.

[0057] Process 1 does not perform frequency variation compensation while there is body movement that affects frequency variation compensation, thereby preventing inaccuracies such as over-compensation due to the influence of body movement. Furthermore, by performing body movement compensation, it is possible to perform corrections that also include positional deviations due to frequency variation, thereby suppressing degradation of image quality.

[0058] [Process 2] In the case of process 2, as shown in FIG. 5, the correction value used for center frequency correction is estimated from the previously calculated correction value (S46). The correction value can be estimated by approximating the frequency fluctuation, i.e., the fluctuation in the phase difference, with a linear function and extrapolating the phase difference. This is shown in FIG. 7. The horizontal axis of the graph in FIG. 7 represents time, and the vertical axis represents the change in frequency (amount of phase change). As shown in the figure, if there is no influence of body movement, the frequency changes in a manner close to a linear function. Therefore, if change 700 up to t1 is obtained, change 701 after time t1, when body movement occurs, can be estimated by extrapolating this straight line. The correction value is estimated until body movement disappears (until t2 in FIG. 7), the estimated correction value is set (S444), and the next actual imaging echo is acquired (S445).

[0059] In repeating the process for each control navigator echo (S44-S45), if it is determined in decision step S441 that there is no influence of body movement, the process returns to the flow of calculating the original phase change amount (S442, S443) rather than the estimation step (S46). When returning to the original process to calculate the phase change amount and correction value, if there is a large discrepancy between the calculated correction value and the immediately preceding estimated correction value, the calculated correction value may be used as is, or a reference navigator echo may be acquired again and the phase change amount and correction value may be calculated using the new reference navigator echo thereafter.

[0060] Process 2 estimates a correction value and corrects frequency fluctuations while there is an influencing body movement, so that frequency fluctuation correction can be performed continuously without being affected by body movement, and positional deviations can be suppressed almost in real time.

[0061] [Process 3] In process 3, the correction value is corrected based on the body movement information obtained from the camera image, rather than estimating the correction value during the influence of body movement. The process flow in this case is the same as that shown in Fig. 5, except that the content of process S46 in Fig. 5 changes from "estimating the correction value" to "correcting the correction value," and therefore Fig. 5 is used for process 3.

[0062] Regarding which of the three processes described above, Process 1 to Process 3, is to be executed, one of them may be preset as a basic process and the user may be able to select it, or it may be possible to automatically switch based on the magnitude and duration of body movement.

[0063] For example, as shown in Fig. 8, a threshold value is set for the magnitude of body movement, and if it is determined (S82) that there is no body movement based on the body movement information (S81) obtained from the body movement processor 240, a phase correction value is calculated (S83) according to steps S442 to S444 of Fig. 4, and frequency variation correction is performed (S84). If it is determined that there is body movement (S82) and the body movement exceeds the threshold (S85), as process 1, navigator echoes during body movement are not used, frequency variation correction is not performed (S86), and only body movement correction is performed (S87). If there is an effect of body movement but the magnitude of the body movement is below the threshold (S85), process 2 or process 3 may be performed, in which a correction value is estimated or corrected, and frequency variation correction is performed (S88, S89).

[0064] Furthermore, Process 2 requires that sufficient frequency change data has been obtained to utilize the linearity of frequency change, whereas Process 3 has the advantage of being applicable even when such data is not stored. However, since there is a slight time lag between the acquisition of body movement information and the calculation and setting of correction values, it is preferable to use the estimated correction value of Process 2 when the body movement changes rapidly or is instantaneous. Therefore, it is possible to switch between Process 2, which estimates, and Process 3, which corrects, depending on the time elapsed since the reference navigator echo or the number of control navigator echoes acquired thereafter.

[0065] Furthermore, the body motion correction performed in process 1 may be performed in process 2 or 3. In this case, the magnitude of the body motion correction may be determined and the correction may be performed automatically, or the user may select whether or not body motion correction is required.

[0066] As described above, actual echoes are acquired while appropriately correcting for frequency fluctuations and body motion with reference to body motion information, and once reconstructable actual echoes are finally collected, the image generator 222 performs image reconstruction using the collected actual echoes (FIG. 3: S7). Here, when body motion correction is performed, if there are actual echoes that have been removed based on the body motion information, image reconstruction is performed according to a predetermined reconstruction method for undersampling.

[0067] According to this embodiment, when it is determined that there is an influence of body motion based on reference to body motion information, the navigator echo acquired during that time is not used in calculating the correction value, or the correction value is corrected using the navigator echo, and the correction value for frequency variation is set. This makes it possible to separate the amount of phase change associated with frequency variation obtained from the navigator echo from the influence of body motion, and to perform accurate correction of frequency variation.

[0068] <Modification of the First Embodiment> In the first embodiment, the frequency is changed using a correction value calculated using the control navigator echo acquired immediately after the detection of body movement. However, overcorrection may occur due to the time lag between the detection of body movement and the correction by the correction value being reflected. In this modification, a function for correcting overcorrection is added to the calculation unit 20A (correction value calculation unit 223).

[0069] Figure 9 shows the frequency fluctuation when overcorrection occurs. As shown in the figure, when there is no effect of body movement, the frequency fluctuation changes almost linearly (700 in Figure 9). However, when a phase shift due to body movement is added to the navigator echo, the correction value calculated using that navigator echo will include the phase shift due to body movement. In other words, it becomes an overcorrection amount 711 for the frequency fluctuation correction.

[0070] When the correction value calculation unit 230 determines that there is an influence of body motion, it estimates and corrects the correction value set before the determination based on the frequency change 700 when there is no influence of body motion. This correction is performed, for example, by continuously changing the correction value from the correction value of the overcorrection amount 711 to a correction value obtained by extrapolating the straight line of the frequency change 700 until it is determined that there is no influence of body motion. After it is determined that there is no influence of body motion, until the correction value is calculated and set using the navigator echo acquired when there is no influence of body motion, an uncorrected portion 712 remains, just as if there had been an overcorrection. In this case, the reference navigator echo may be reacquired when it is determined that there is no influence of body motion, or the uncorrected portion of the overcorrection amount 711 may be corrected here, and then the reference navigator echo may be reacquired.

[0071] According to this modification, it is possible to correct overcorrection caused by the time difference between the determination process that references body movement information and the correction value setting process, thereby improving the accuracy of frequency fluctuation correction.

[0072] <Embodiment 2> In the first embodiment, correction of the central frequency variation is determined mainly in accordance with the magnitude of the body movement as body movement information, but in this embodiment, correction of the frequency variation is performed in accordance with the periodic movement included in the body movement information.

[0073] The processing flow of this embodiment will be described with reference to Fig. 10. Note that in Fig. 10, the description of the same processes as in Fig. 5 will be omitted, and only the differences will be described.

[0074] In calculating the correction value, if a periodic motion is present and the phase change amount (corresponding frequency change) is calculated using the reference navigator echo and the control navigator echo, as shown in FIG. 11 , the frequency change 801 is a superimposed change of periodic motion 901 (a frequency fluctuation obtained by converting a displacement in real space into a frequency fluctuation), which is a body motion. However, because body motion information is collected sequentially, when a body motion is detected, the magnitude of the body motion is relatively small, and it is not possible to determine whether or not the body motion is periodic. In this embodiment, in the determination step (S441) of the correction value calculation, a threshold value for a relatively gradual body motion corresponding to the magnitude of the displacement due to the periodic motion is set, rather than a threshold value for a sudden body motion (e.g., FIG. 6(A)). If a body motion is present and the magnitude of the body motion is equal to or less than the set threshold, the body motion is determined to be periodic (S441), and a correction corresponding to the periodic motion is performed (S48).

[0075] As in Process 2 of Embodiment 1, correction for periodic motion can be performed by an extrapolation method (FIG. 12(A)) using the linearity (a linear function) of the frequency change previously collected, or by a method of suppressing the change in the correction value by limiting the change in the correction value to "several percent" or less (FIG. 12(B)). If body motion information (information on the period and magnitude of periodic motion) has been collected prior to imaging, the degree of suppression can be calculated based on that change, or a value (for example, several percent to 10%) that does not obscure the phase change to be obtained can be set as a default.

[0076] As another method, body movement information may be collected until period information of the periodic movement can be obtained, and a correction value may be calculated based on the magnitude (amplitude) and period of the periodic movement. In this case, as shown in Figure 13(B), first, the body movement fluctuation is divided into multiple ranges (e.g., three), and for each range, phases (e.g., phases A, B, and C) with the same magnitude of displacement (body movement value) are obtained. Next, for the frequency change fluctuations corresponding to the correction values ​​shown in Figure 13(A), a straight line of frequency change (correction values ​​A, B, and C) is obtained from the frequency change values ​​obtained for each phase of the body movement fluctuation. The correction values ​​obtained from this line are applied sequentially to perform frequency fluctuation correction.

[0077] For example, after detecting body motion, frequency variation correction is performed by calculating a correction value from the frequency variation obtained from the navigator echo without taking body motion into consideration until one of body motion values ​​A, B, or C is calculated. When body motion value A is calculated first, for example, the correction value based on body motion value A is applied until body motion value B, body motion value C, or the second body motion value A is calculated. When body motion value B is calculated next, the correction value based on body motion value B is applied until body motion value C, body motion value B, or the second body motion value A is calculated. When the next body motion value A is calculated, the correction value based on body motion value A is applied until body motion value C, body motion value B, or the third body motion value A is calculated. Similarly, each time a body motion value is calculated in chronological order, the correction value based on that body motion value is applied.

[0078] Although this method involves a certain time lag between the acquisition of body movement information and the correction of frequency fluctuations, it can improve the accuracy of frequency fluctuation correction compared to the suppression of the correction value described above.

[0079] According to this embodiment, even when there is periodic motion in the imaging region, the periodic motion mixed in the phase change amount calculated from the navigator echo can be removed or the influence of the periodic motion can be reduced, and the phase change amount corresponding to the frequency fluctuation can be calculated with high accuracy. As a result, the position shift caused by the fluctuation of the center frequency can be separated from the position shift caused by the body movement (periodic motion) and corrected with high accuracy.

[0080] <Embodiment 3> This embodiment is suitable for cases where a sudden localized body movement occurs in or near the imaging region, or where periodic movement such as respiratory movement affects only a portion of the imaging region. By referring to the localized body movement area and the sensitivity of the multiple small coils (channels) that make up the receiving coil, the navigator echoes of the small coils used to calculate the correction value are selected and discarded, and the correction value for frequency variation correction is calculated.

[0081] MRI devices typically use a multi-channel or multi-array coil with multiple small coils arranged as the receive coil 104, and reconstruct an image by combining the NMR signals received by each small coil using the sensitivity distribution of each small coil. The same applies to navigator echoes, and correction values ​​for frequency variation correction are calculated using navigator echoes collected by multiple small coils. Based on this premise, this embodiment is characterized by referring to body motion information for each small coil.

[0082] The processing of this embodiment will be described below with reference to FIG.

[0083] As shown in Figure 14, in the processing of this embodiment, compared to the processing of embodiment 1 shown in Figures 4 and 5, a repeat step (S50-S55) of processing for each small coil is inserted between the repeat steps (S44-S45) for each control navigator echo. Also, although not an essential step, step S40 of registering the coordinates of the imaging space with the coordinates of the camera image may be added. The other steps are the same as in embodiment 1 or embodiment 2, and the following description will focus on the processing of the different steps.

[0084] In this embodiment, first, the position of the imaging region in the camera image acquired by the body motion processor 240, for example, the positions of multiple feature points in the ROI (positions on the camera image coordinates) is associated with the position of each small coil of the receiver coil attached to the subject (positions in real space coordinates or positions on the MRI image coordinates) (S40: registration process). This process can be performed, for example, by providing identifiable markers at the positions of each small coil of the receiver coil attached to the subject 50 (for example, the centers of the conductive loops), and then associating the camera image capturing these markers with the marker positions in real space. Alternatively, the relationship between the sensitivity map of the receiver coil (small coil) acquired in advance and the imaging position (relationship in image coordinates) may be stored in advance, and the imaging position on the image and the imaging position in the camera image coordinates may be registered. The registration process S40 may be omitted if the relationship between the coil and the attachment site is known in advance, such as in the case of a head coil.

[0085] After performing the above-mentioned registration as necessary, imaging is started to acquire the main imaging echo, the reference navigator echo, and the control navigator echo (S41 to S44). Since the control navigator echo is received by each small coil, the body motion information is referenced to determine whether the navigator echo obtained for each small coil can be used to calculate the correction value (S50, S51, S52).

[0086] As explained in the above embodiment, the body motion processor 240 detects the displacement of each position within the imaging region (e.g., ROI) by calculating a displacement vector from the camera image for each frame using a method such as optical flow. Therefore, the displacement of the position associated with the small coil is obtained as body motion information.

[0087] The correction value calculation unit 230 determines whether any of the imaging regions has a displacement equal to or greater than a predetermined threshold (displacement threshold) based on the body motion information for each region obtained by analyzing the camera image using the body motion processing unit 240 (S51). If the body motion is determined to be affected by any of the imaging regions, the correction value calculation unit 230 determines whether the sensitivity of the small coil to be processed is equal to or greater than a predetermined threshold (sensitivity threshold) at the position (body motion region) determined to be affected by body motion (S52). If the sensitivity is equal to or greater than the predetermined sensitivity threshold, the navigator echo collected by that small coil is not used to calculate the phase change amount (S53). For example, if a portion of the imaging region or a region adjacent to the imaging region is moving due to a convulsion or other reason, the small coil with the highest sensitivity to that region is identified based on the body motion information for that region and the sensitivity of the small coil.

[0088] If there is no effect of body movement, or if there is an effect of body movement in a part of the imaging area or in a nearby area but the sensitivity to that part or area is low, the navigator echo acquired by that small coil is used as is to calculate the correction value (S54).

[0089] The selection of small coils according to these decision steps is shown in Figure 15. The example shown in this figure is for imaging the abdomen of subject 50, with receive coils 104 positioned to cover the subject's abdomen. If the arm close to the abdomen (the right arm in the figure) suddenly moves, the navigator echoes acquired by small coils 104a and 104b, which are positioned near the area where the body movement occurs and have a certain level of sensitivity to the area where the body movement occurs, are not used to calculate the correction value. On the other hand, small coils 104c and 104d, which are located at the far end of the right arm, are far from the area where the body movement occurs and have low sensitivity to the area where the body movement occurs, so the navigator echoes acquired by these small coils are used to calculate the correction value.

[0090] After performing the above steps S51 to S54 on the navigator echoes obtained from the multiple small coils that make up the receiving coil, the navigator echoes that have not been removed are complex added to calculate the amount of phase change (S443), and a correction value is calculated and set (S444). Frequency variation correction is performed using the correction value, and the actual imaging echo is acquired (S445). The amount of phase change can be calculated from the reference navigator echo and control navigator echo for each small coil, rather than adding the navigator echoes obtained for each small coil, and the amount of phase change for each small coil can be averaged.

[0091] Thereafter, the processes of S44 to S45 are repeated every time a control navigator echo is acquired, as in the first and second embodiments.

[0092] Although not shown in the figure, if there is body motion equal to or greater than a predetermined threshold at many positions in the imaging region, the number of control navigator echoes used for the correction value will be reduced, so it is possible not to perform frequency variation correction (Process 1) or to estimate (Process 2) / correct (Process 3) the correction value, as in Embodiment 1. Similarly, body motion correction may be performed separately from frequency variation correction.

[0093] According to this embodiment, if there is body movement in or near a part of the imaging area, frequency fluctuation correction that is less susceptible to the influence of body movement can be performed by excluding the navigator acquired using a small coil that is highly sensitive to that part.

[0094] <Modification of the third embodiment> In embodiment 3, navigator echoes obtained by small coils that are highly sensitive to body movement areas are not used in calculations for frequency fluctuation correction. However, when adding navigator echoes obtained by each small coil, or when synthesizing the phase shift amounts calculated from them, it is also possible to calculate a correction value by weighting the navigator echoes according to the distance or sensitivity of each small coil to the body movement area.

[0095] According to the third embodiment and its variations, when local body movement occurs in the subject, the information from the small coil that is highly sensitive to that local area is not used or is used in a reduced manner for frequency fluctuation correction, thereby making it possible to perform frequency fluctuation correction that eliminates the influence of body movement. [Explanation of symbols]

[0096] 1: MRI device, 10: imaging unit, 20: processor, 20A: calculation unit, 20B: control unit, 210: imaging control unit, 220: image generation unit, 230: correction value calculation unit, 240: body motion processing unit

Claims

1. an imaging unit that collects nuclear magnetic resonance signals generated from a subject by nuclear magnetic resonance; a calculation unit that includes an image generation unit that reconstructs an image of the subject using the nuclear magnetic resonance signals; and a body movement processing unit that collects body movement information of the subject, the imaging unit collects navigator echoes for detecting fluctuations in the center frequency of the nuclear magnetic resonance; the calculation unit includes a correction value calculation unit that calculates a correction value for correcting the fluctuation of the center frequency using a phase change of the navigator echo collected by the imaging unit, The magnetic resonance imaging apparatus according to claim 1, wherein the correction value calculation unit calculates a correction value that eliminates the influence of body movement by referring to the body movement information collected by the body movement processing unit.

2. 2. The magnetic resonance imaging apparatus according to claim 1, The magnetic resonance imaging apparatus is characterized in that the body movement processing unit analyzes an image from a camera that detects body movement of the subject, and collects the body movement information.

3. 2. The magnetic resonance imaging apparatus according to claim 1, The correction value calculation unit compares the magnitude of the body movement with a predetermined threshold, and does not use the navigator echo collected when the magnitude of the body movement is equal to or greater than the threshold in calculating the correction value.

4. 4. The magnetic resonance imaging apparatus according to claim 3, the correction value calculation unit does not calculate the correction value when the magnitude of the body movement is equal to or greater than the threshold value; The magnetic resonance imaging apparatus is characterized in that the body motion processing unit performs body motion correction on nuclear magnetic resonance signals for image reconstruction of the subject or on the image of the subject, using the body motion information.

5. 4. The magnetic resonance imaging apparatus according to claim 3, The correction value calculation unit estimates a correction value for the period from when the magnitude of body movement exceeds a threshold to when it falls below the threshold from a correction value calculated before the magnitude of body movement exceeds the threshold.

6. 2. The magnetic resonance imaging apparatus according to claim 1, The correction value calculation unit corrects the correction value calculated using the navigator echo collected when the magnitude of the body movement is equal to or greater than a predetermined threshold, using the body movement information.

7. 2. The magnetic resonance imaging apparatus according to claim 1, and a correction value calculation unit that calculates the correction value by suppressing a phase change due to the periodic motion contained in a phase change of the navigator echo when the body motion information collected by the body motion processing unit includes the periodic motion.

8. 8. The magnetic resonance imaging apparatus according to claim 7, a correction value calculation unit that calculates a correction value from a phase change of the navigator echo by multiplying the correction value by a predetermined coefficient, and sets the result as a correction value for correcting the fluctuation of the center frequency.

9. 8. The magnetic resonance imaging apparatus according to claim 7, The magnetic resonance imaging apparatus according to claim 1, wherein the correction value calculation unit calculates the correction value using a plurality of navigator echoes acquired in the same time phase of the periodic motion.

10. 2. The magnetic resonance imaging apparatus according to claim 1, the imaging unit includes a receiving coil consisting of a plurality of small coils for receiving nuclear magnetic resonance signals; The magnetic resonance imaging apparatus according to claim 1, wherein the correction value calculation unit calculates the correction value using navigator echoes received by the plurality of small coils.

11. 11. The magnetic resonance imaging apparatus according to claim 10, The correction value calculation unit obtains the area where body movement is occurring from the body movement information, and selects the navigator echo of the small coil to be used in calculating the correction value using the area where body movement is occurring and the sensitivities of the multiple small coils.

12. A method for correcting fluctuations in the center frequency of irradiation radio frequency waves in a magnetic resonance imaging apparatus, comprising: calculating a phase difference using two or more nuclear magnetic resonance signals acquired as navigator echoes from a subject under examination, and calculating a correction value for the center frequency; and collecting body movement information from a device that detects body movement of the subject under examination; A method for correcting a center frequency, wherein in the step of calculating the correction value, the body movement information is referenced to calculate the correction value from which the influence of body movement is eliminated.

Images