JP5942244B2 - Magnetic resonance imaging apparatus and body motion artifact reduction method - Google Patents

Description

  The present invention measures nuclear magnetic resonance (hereinafter referred to as `` NMR '') signals from hydrogen, phosphorus, etc. in a subject and images nuclear density distribution, relaxation time distribution, etc. More particularly, the present invention relates to a technique for reducing artifacts caused by respiratory motion.

  The MRI device measures NMR signals generated by the spins of the subject, especially the tissues of the human body, and visualizes the form and function of the head, abdomen, limbs, etc. in two or three dimensions Device. In imaging, the NMR signal is given different phase encoding depending on the gradient magnetic field, frequency-encoded, and measured as time series data. The measured NMR signal is reconstructed into an image by two-dimensional or three-dimensional Fourier transform.

  In order to suppress body motion artifacts that occur in an image obtained by imaging a subject who is breathing with the MRI apparatus, respiratory synchronization imaging is performed in which imaging is performed in synchronization with respiration. In that case, an external sensor for monitoring the breathing motion of the subject is attached to the subject and reconstructed based on a signal from the external sensor or from a navigator echo signal (navigator echo) 1 Based on the dimensional profile, the subject's respiratory motion and its state are detected. Then, the body movement artifact is reduced by reconstructing an image mainly using an echo signal obtained in a predetermined respiratory motion state (for example, the position of the abdominal wall surface or the diaphragm) or the respiratory motion time phase. In particular, in Patent Document 1, a data collection window is set for each respiratory movement cycle, echo signals are measured between a plurality of data collection windows, and the measured echo signals are arranged so that body movement artifacts are reduced. Instead, the image is reconstructed.

Japanese Unexamined Patent Publication No. 63-300750

  With the technique disclosed in Patent Document 1, if the subject's breathing motion becomes unstable or irregular, the breathing state changes between a plurality of data collection windows. As a result, echo signals measured in different respiratory states coexist, and the fluctuation pattern of the measured echo signal assumed by the predetermined rearrangement method is destroyed. Therefore, it is considered that the effect of reducing body motion artifacts is diminished.

  Therefore, the present invention has been made in view of the above problems, and even if the subject's respiratory motion becomes unstable or irregular, it is possible to stably reduce body motion artifacts generated in an image. It is to provide an MRI apparatus and a body motion artifact reduction method.

  In order to achieve the above object, the present invention divides respiratory motion data representing a respiratory state of a subject accompanied by respiratory motion into a plurality of sections, and measures echo signals that fill the k space for each divided section. The method is characterized in that a region with little motion is extracted from an image for each divided section, and an image is obtained by combining regions with little motion.

  Specifically, the MRI apparatus of the present invention includes a body motion detection unit that acquires respiratory motion data representing a respiratory state of a subject with respiratory motion, and an echo signal from the subject using a predetermined pulse sequence. A measurement control unit for measuring, an echo signal, and respiratory motion data, and an arithmetic processing unit that acquires an image in which artifacts based on the respiratory motion are reduced. The arithmetic processing unit includes the respiratory motion data in a plurality of sections. A k-space setting unit that prepares a k-space for each divided section, an imaging execution unit that instructs the measurement control unit to measure an echo signal to be filled in each k-space, and each k-space is filled An image reconstruction unit that reconstructs an image for each divided section using the acquired data, an area extraction unit that extracts an area with less motion from the image for each divided section, and an area that obtains an image by combining the areas with less motion And a combining unit. .

  The body motion artifact reduction method of the present invention includes a step of acquiring respiratory motion data representing a respiratory state of a subject with respiratory motion, a step of dividing the respiratory motion data into a plurality of sections, a step of preparing k-space, a step of measuring echo signals to be filled in each k-space, a step of reconstructing an image for each divided section using data filled in each k-space, and an image for each divided section The method includes a step of extracting a region with less motion from the step, and a step of obtaining an image by combining regions with less motion.

  According to the MRI apparatus and the body motion artifact reduction method of the present invention, body motion artifacts generated in an image can be stably reduced even if the subject's respiratory motion becomes unstable or irregular.

1 is a block diagram of an overall basic configuration in an embodiment of an MRI apparatus according to the present invention. (a) A figure showing an example of respiratory motion data. (b) The figure which shows the example which divides the predetermined period of the respiratory cycle in 1st Embodiment into equal time intervals. FIG. 2 is a functional block diagram of a CPU 8 according to the first embodiment. 6 is a flowchart showing a processing flow of imaging processing according to the first embodiment. The figure which shows the flow of an example of the process which extracts and synthesize | combines only an area | region with little motion The figure which shows the example which divides | segments the amplitude of the respiration data in 2nd Embodiment into equal width intervals. The functional block diagram of CPU8 which concerns on 2nd Embodiment. 9 is a flowchart showing a processing flow of imaging processing according to the second embodiment. The figure which shows the example which divides | segments the predetermined period of the respiratory cycle in 1st Embodiment into equal time intervals, and divides | segments an amplitude into equal width intervals. FIG. 9 is a functional block diagram of a CPU 8 according to a third embodiment. 9 is a flowchart showing a processing flow of imaging processing according to a third embodiment.

  Hereinafter, preferred embodiments of the MRI apparatus of the present invention will be described in detail with reference to the accompanying drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiments of the invention, and the repetitive description thereof is omitted.

  First, an overall outline of an example of an MRI apparatus according to the present invention will be described with reference to FIG. FIG. 1 is a block diagram showing the overall configuration of an embodiment of an MRI apparatus according to the present invention. This MRI apparatus uses a NMR phenomenon to obtain a tomographic image of a subject.As shown in FIG. 1, the MRI apparatus includes a static magnetic field generation unit 2, a gradient magnetic field generation unit 3, a transmission unit 5, The receiving unit 6, the arithmetic processing unit 7, and the measurement control unit 4 are configured.

  The static magnetic field generator 2 generates a uniform static magnetic field in the direction perpendicular to the body axis in the space around the subject 1 if the vertical magnetic field method is used, and in the direction of the body axis if the horizontal magnetic field method is used. Therefore, a permanent magnet type, normal conducting type or superconducting type static magnetic field generating source is arranged around the subject 1.

  The gradient magnetic field generator 3 includes a gradient magnetic field coil 9 wound in the three-axis directions of X, Y, and Z, which is a coordinate system (stationary coordinate system) of the MRI apparatus, and a gradient magnetic field power source 10 that drives each gradient magnetic field coil It consists of. The gradient magnetic field power supply 10 of each coil is driven in accordance with a command from the measurement control unit 4 to be described later, so that the subject 1 lies in the gradient magnetic fields Gx, Gy, and Gz in the X, Y, and Z directions. Applied to the static magnetic field space. At the time of imaging, a slice direction gradient magnetic field pulse (Gs) is applied in a direction orthogonal to the slice plane (imaging cross section) to set a slice plane for the subject 1, and the remaining planes orthogonal to the slice plane and orthogonal to each other are set. A phase encoding direction gradient magnetic field pulse (Gp) and a frequency encoding direction gradient magnetic field pulse (Gf) are applied in two directions, and position information in each direction is encoded in the echo signal.

  The transmitter 5 irradiates the subject 1 with a high-frequency magnetic field pulse (hereinafter referred to as “RF pulse”) in order to induce an NMR phenomenon in the nuclear spin of atoms constituting the living tissue of the subject 1. The high-frequency oscillator 11, the modulator 12, the high-frequency amplifier 13, and a high-frequency coil (transmission coil) 14a on the transmission side. The high-frequency pulse output from the high-frequency oscillator 11 is amplitude-modulated by the modulator 12 at a timing according to a command from the measurement control unit 4, and after the amplitude-modulated high-frequency pulse is amplified by the high-frequency amplifier 13, the subject 1 By being supplied to the high-frequency coil 14a arranged close to the RF pulse, the subject 1 is irradiated with the RF pulse.

  The receiving unit 6 detects an echo signal emitted by the NMR phenomenon of the nuclear spin constituting the living tissue of the subject 1, and receives a high frequency coil (receiving coil) 14b on the receiving side, a signal amplifier 15, and quadrature detection. And an A / D converter 17. The echo signal of the response of the subject 1 induced by the RF pulse irradiated from the high frequency coil 14a on the transmission side is detected by the high frequency coil 14b arranged close to the subject 1 and amplified by the signal amplifier 15. After that, it is divided into two orthogonal signals by the quadrature phase detector 16 at the timing according to the command from the measurement control unit 4, and each is converted into a digital quantity by the A / D converter 17 and calculated as echo data It is sent to the processing unit 7.

  The measurement control unit 4 controls the gradient magnetic field generation unit 3, the transmission unit 5, and the reception unit 6 on the basis of a predetermined pulse sequence to apply the RF pulse and the gradient magnetic field pulse and to measure the echo signal. It is a control means to repeat. The measurement control unit 4 operates under the control of the CPU 8, and sends various commands necessary for collecting echo data necessary for reconstruction of the tomographic image of the subject 1 to the gradient magnetic field generation unit 3, the transmission unit 5, and the reception unit 6. To control them.

  The arithmetic processing unit 7 performs various data processing and display and storage of processing results, and includes a CPU 8, an external storage device such as the optical disk 19 and the magnetic disk 18, and a display 20. When echo data from the receiving unit 6 is input to the CPU 8, the echo data is stored in a memory corresponding to the K space in the CPU 8 (hereinafter described that the echo signal or echo data is arranged in the K space. Means that echo data is written and stored in this memory, and echo data arranged in K space is called K space data). The CPU 8 performs arithmetic processing such as signal processing and image reconstruction on the K space data, and displays the tomographic image of the subject 1 as a result on the display 20 and records it in the external storage device. To do.

  The operation unit 25 receives input of various control information of the MRI apparatus and control information of processing performed by the arithmetic processing unit 7 from an operator, and includes a trackball or mouse 23 and a keyboard 24. The operation unit 25 is disposed close to the display 20, and the operator controls various processes of the MRI apparatus interactively through the operation unit 25 while looking at the display 20.

  In addition, the MRI apparatus according to the present invention is mounted on or near the subject and receives a body motion sensor for detecting the body motion of the subject and a signal from the body motion sensor, A body motion detecting unit 26 for detecting the body motion information of the body. The body motion information detected by the body motion detection unit 26 is input to the CPU 8 via the measurement control unit 4.

  For example, as an example of a body motion detection sensor that detects a change in the position of the abdominal wall surface based on respiratory motion, an air pressure sensor that attaches an air bellows that expands and contracts according to the position of the abdominal wall surface to the abdomen and detects the air pressure inside the air bellows Can be used. Since the bellows internal air pressure fluctuates in accordance with the expansion and contraction of the bellows, the fluctuating position of the abdominal wall surface can be indirectly detected by this air pressure. Alternatively, an ultrasonic sensor that irradiates the abdominal wall surface with ultrasonic waves and detects the fluctuation position of the abdominal wall surface from the time required to detect the reflected wave may be used.

  In FIG. 1, the high-frequency coil 14a and the gradient magnetic field coil 9 on the transmission side are located within the static magnetic field space of the static magnetic field generating unit 2 into which the subject 1 is inserted, and to the subject 1 if the vertical magnetic field method is used. Oppositely, if it is a horizontal magnetic field system, it is installed so as to surround the subject 1. The high-frequency coil 14b on the receiving side is installed so as to face or surround the subject 1.

  The nuclide to be imaged by the current MRI apparatus is a hydrogen nucleus (proton) which is a main constituent material of the subject as being widely used in clinical practice. By imaging information on the spatial distribution of proton density and the spatial distribution of relaxation time in the excited state, the form or function of the human head, abdomen, limbs, etc. is imaged two-dimensionally or three-dimensionally.

  The present invention divides respiratory motion data representing a respiratory state of a subject with respiratory motion into a plurality of sections, measures an echo signal filled in k space for each divided section, and moves from an image for each divided section. By extracting regions with less movement and synthesizing regions with less movement to obtain images, even if the subject's respiratory motion becomes unstable or irregular, body motion artifacts that occur in the image are stably reduced . Hereinafter, each embodiment regarding the division | segmentation method which divides | segments respiratory motion data into a some area, and the process which concerns on the method of the division | segmentation is demonstrated in detail.

<First embodiment>
Next, a first embodiment of the MRI apparatus and body motion artifact reduction method of the present invention will be described. In this embodiment, the respiratory cycle is divided into equal time intervals, an image is reconstructed using echo signals measured for each divided section (measurement section), and an area with less motion in each reconstructed image is obtained. Extracting and synthesizing the extracted regions with less movement to obtain an image with reduced body motion artifacts. Hereinafter, this embodiment will be described in detail with reference to FIGS.

  The subject's respiratory motion and respiratory motion state can be detected directly using a body motion sensor, or indirectly by measuring a navigator echo signal. When using a body motion sensor, a body motion sensor is mounted around the abdomen of the subject, and the position of the vertical motion of the abdominal wall surface due to the subject's breathing is detected from the body motion sensor, and the amplitude position is set. Output proportional electrical signals. When the navigator echo signal is measured, the diaphragm position can be detected by measuring the echo signal (navigator echo signal) from a region straddling the diaphragm of the subject, for example. Since this diaphragm position is linked to the respiratory motion, the position of the vertical motion of the abdominal wall surface is indirectly detected based on the fluctuation of the diaphragm position.

  Hereinafter, the time-series data of the position of the vertical motion of the subject's abdominal wall obtained by any of the above methods will be referred to as respiratory motion data. FIG. 2 (a) shows an example of respiratory motion data. The horizontal axis represents time, and the vertical axis represents the position of the vertical motion of the subject's abdominal wall. A state where the abdominal wall surface position is high corresponds to the breathing period state, and a state where the abdominal wall surface position is low corresponds to the expiration state. FIG. 2 shows the state of 3 cycles of respiratory motion. Hereinafter, a period from a certain peak time point (maximum value) to the next peak time point (maximum value) is defined as one respiratory cycle, but the respiratory cycle may be defined as any one repetition time in a periodic motion.

  Next, in order to execute the body motion artifact reduction method of the present embodiment, each function of the CPU 8 of the arithmetic processing unit 7 will be described with reference to FIG. FIG. 3 is a functional block diagram for explaining each function realized by the CPU 8 executing a program stored in the magnetic disk 18 or the like.

  The CPU 8 controls the above-described units by a program to realize imaging that reduces body motion artifacts according to the present embodiment. Therefore, the CPU 8 includes a period dividing unit 301, a k-space setting unit 303, an imaging execution unit 304, an image reconstruction unit 305, a region extraction unit 306, and a region synthesis unit 307.

  The cycle dividing unit 301 divides a predetermined period (for example, 5 seconds) into a predetermined number (N) from the peak position of the respiratory motion data based on the respiratory cycle. The respiratory cycle is calculated based on fluctuations in respiratory motion data. For example, it is set as the average of the period between each local maximum value (peak time) of respiratory motion data, or the average of the period between each local minimum value. The number of divisions of the respiratory cycle may be a value stored in advance on the magnetic disk 18 or the like, or based on the repetition time (TR) of the pulse sequence set in the imaging conditions, with a value that is an integral multiple of the value, A value obtained by dividing a predetermined period may be used. The measurement idle time is from the end of the predetermined period until the first maximum value of the next breathing cycle, but in the case of a pulse sequence using a steady state, the pulse sequence may be executed even in the measurement idle time. preferable.

  The k space setting unit 303 sets the same number of k spaces (echo data storage areas) as the number of divisions of the respiratory cycle set by the cycle dividing unit 301 in the memory in the CPU 8. That is, the k space corresponding to each measurement section set by the period dividing unit 301 is set.

  The imaging execution unit 304 measures an echo signal from the subject using a predetermined pulse sequence while monitoring respiratory motion data. At that time, the measurement control unit 4 determines the phase encoding amount to be applied to the echo signal so that the data of the echo signal is arranged in the non-arranged region of the k space corresponding to the measurement section to which the timing of measuring the echo signal belongs. At any time. Specifically, when the imaging execution unit 304 repeats the execution of the pulse sequence at the repetition time (TR) of the pulse sequence, according to the measurement interval to which each repetition belongs, the k space corresponding to the measurement interval is not measured. In order to measure the echo signal of the region, the measurement control unit 4 is instructed with the phase encoding amount for the two-dimensional measurement, and with the phase encoding amount and the slice encoding amount for the three-dimensional measurement. For example, the imaging execution unit 304 stores the encoding number last measured for each k space, and when measuring the echo signal, the next measurement number is measured to the encoding number last measured in the corresponding k space. The measurement control unit 4 is instructed the power encoding number (a value obtained by adding or subtracting 1 if the sampling method is a sequential order, or a value obtained by inverting the polarity by incrementing the absolute value if the sampling method is a centric order). Then, the encoding number is used as the encoding number measured last in the k space to prepare for the next measurement.

  The image reconstruction unit 305 reconstructs an image for each k space using echo data arranged (stored) in each k space set by the k space setting unit 303. As a result, the same number of images as the number of divisions set by the period division unit 301 are generated.

  The region extraction unit 306 extracts a region with less motion in each image Ii (x, y) [i = 1, 2, 3,..., N] reconstructed by the image reconstruction unit 305, Create an image that leaves only a small area. For example, a difference process is performed between images in adjacent measurement sections, and in the difference image, an image mask Mi (x that sets a pixel whose absolute value is equal to or smaller than a predetermined threshold to 1 and sets a pixel larger than the predetermined threshold to zero. , Y). Then, this image mask is multiplied with the addition average image of both images, which is the base image of the difference image. As a result, the masked image is an image Ii (x, y) in which only a region with little motion is extracted, and its pixel value is zero for a region with large motion and a pixel value of the base image for a region with small motion. It becomes the addition average value of. Such calculation is performed between the images of all adjacent sections, and an image in which only a region with little motion is extracted for each measurement section is obtained.

  The region synthesis unit 307 creates one image by synthesizing images obtained by the region extraction unit 306, in which only the regions with less motion for each measurement section are extracted. Specifically, an addition image [ΣIi (x, y)] of the image Ii (x, y) extracted from only the region with a small amount of motion extracted by the region extraction unit 306 is obtained. When obtaining this added image [ΣIi (x, y)], even if the same organ is misaligned or deformed between the images Ii (x, y), the position of the same organ or It is preferable to add by matching the shapes. Therefore, based on the position and shape of each organ of the image Ij (x, y) having the largest region with the least movement, the same organ of the other image Ik (x, y) is extracted by pattern matching, After changing the position and shape, it is added to the image Ij (x, y). At this time, the change in the position and shape of the same organ in the other image Ik (x, y) is also reflected in the mask image Mk (x, y).

  Then, the region synthesis unit 307 uses the pixel value of each pixel of the addition image [ΣIi (x, y)] as the addition mask image [ΣMi (x) of the mask image Mi (x, y) calculated by the region extraction unit 306. , Y)] is divided by the pixel value of the same pixel to create a final image in which only regions with little motion are averaged. As a result, the final image is an image with reduced body motion artifacts.

  FIG. 5 shows a flow of an example of processing for extracting and synthesizing only a region with little motion. A difference image 511 is obtained from the images 501 and 502 for every two measurement sections. Then, a pixel value whose pixel value is equal to or larger than a predetermined threshold is extracted from the difference image 511 to create a mask image (an image having a pixel value of 0/1) 531. Also, an addition average image 521 of the measurement section images 501 and 502 is created. Then, the mask image 531 and the addition image 521 are multiplied for each pixel to obtain an image 541 having only a region with little motion. Similarly, a difference image 512 is obtained from the images 502 and 503 for every two measurement sections. Then, a pixel value whose pixel value is equal to or greater than a predetermined threshold is extracted from the difference image 512 to create a mask image (image having a pixel value of 0/1) 532. In addition, an addition average image 522 of the measurement section images 502 and 503 is created. Then, the mask image 532 and the addition image 522 are multiplied for each pixel to obtain an image 542 having only a small movement area. Finally, a composite image 551 that is an image in which body motion artifacts are reduced by dividing each pixel by the addition image of the images 541 and 542 of only the region with little motion and the addition mask image of the mask images 531 and 532. Get.

  Next, the flow of imaging processing according to the present embodiment by each of the functional units will be described with reference to FIG. FIG. 4 is a processing flow of imaging processing according to the present embodiment.

  In step 401, the respiratory motion of the subject is monitored and respiratory motion data is collected. When using a body motion sensor, the body motion detection unit 26 digitizes a signal from the body motion sensor attached to the subject and notifies the CPU 8 as respiratory motion data via the measurement control unit 4. When the navigator echo signal is used, respiratory motion data is acquired by measuring the navigator echo signal together with the echo signal measurement in step 404 described later.

  In step 402, the cycle dividing unit 301 calculates a respiratory cycle based on the respiratory motion data acquired in step 401 (or step 404), reads the division number from the magnetic disk 18 or the like, and starts the first of each respiratory cycle. A predetermined period is divided into equal number of divisions from the local maximum value (peak time). FIG. 2 (b) shows an example of dividing the respiratory cycle into equal time intervals. FIG. 2 (b) shows an example in which a predetermined period from the first maximum value of each respiratory cycle is divided into 9 parts, and each measurement section from that point is sequentially 1 to 9 with reference to the peak position of the abdominal wall surface. It is shown by numbers. In addition, the measurement free time until the first peak time of the measurement section 9 and the next breathing cycle is set to zero. Similarly, other respiratory cycles are divided at equal time intervals and assigned section numbers.

  In step 403, the k space setting unit 302 sets the k space (echo data storage area) of the number of divisions set in step 402 in the memory in the CPU 8.

  In step 404, the imaging execution unit 304 measures an echo signal from the subject using a predetermined pulse sequence while monitoring respiratory motion data. As described above, a respiratory motion sensor may be used to acquire respiratory motion data, or a navigator echo signal is measured together with measurement of an echo signal for image reconstruction, and the respiratory motion data is based on the navigator echo signal. You may get Any pulse sequence may be used. The imaging execution unit 304 repeats the execution of the pulse sequence at the repetition time (TR) of the pulse sequence, and at this time, according to the measurement section to which each repetition belongs, the echo signal of the unmeasured area in k space corresponding to the measurement section The amount of encoding applied to the echo signal is controlled so as to measure. Specific encoding control is as described above.

  In step 405, the image reconstruction unit 305 reconstructs each k-space data acquired in step 404, and acquires an image for each measurement section.

  In step 406, the region extraction unit 306 obtains an image in which only a region with little motion is extracted from the image for each measurement section reconstructed in step 405. The method of obtaining is as described above.

In step 407, the region synthesis unit 307 synthesizes images obtained by extracting only regions with little motion for each measurement section acquired in step 406 to create one image. The method of synthesis is as described above. Since this final image is obtained by synthesizing a region with little motion, it is an image with reduced body motion artifacts.
The above is the description of the processing flow of the present embodiment.

  As described above, the MRI apparatus and the body motion artifact reduction method of the present embodiment divide the predetermined period from the first maximum value (peak point) of each respiratory cycle into equal time intervals, and for each divided section. An image is reconstructed using the measured echo signal, a region with less motion is extracted from each reconstructed image, and a region with less motion is synthesized to obtain a single image. As a result, even if the subject's respiratory motion becomes unstable or irregular, it is possible to acquire an image in which body motion artifacts are stably reduced. In particular, since a predetermined period of each respiratory cycle is divided into equal time intervals and an image is obtained using an echo signal for each measurement section, body movement artifacts are reduced in the image of each measurement section. As described above, since a region with less motion is extracted and synthesized from each image with less body motion artifacts, the final image has fewer body motion artifacts.

<Second Embodiment>
Next, a second embodiment of the MRI apparatus and body motion artifact reduction method of the present invention will be described. In the first embodiment described above, the predetermined period of each respiratory cycle is divided into equal time intervals, but in the second embodiment, the body motion amplitude is divided into equal width intervals and measured for each amplitude interval. Reconstruct the image using the echo signal, extract the region with less motion in each reconstructed image, synthesize the extracted regions with less motion, one image with reduced body motion artifacts To get. Hereinafter, the present embodiment will be described in detail with reference to FIGS.

  First, dividing the body motion amplitude into equal width intervals will be described with reference to FIG. FIG. 6 shows an example in which the same amplitude of the respiratory motion data as in FIG. 2 is divided into four at equal width intervals. The width including the maximum amplitude is divided into equal width intervals between the amplitude section 1 including the maximum position of the amplitude, the amplitude section 4 including the minimum position of the amplitude, and the amplitude sections 2 and 3 therebetween. Then, an image for each amplitude section is acquired. For this purpose, a k-space is prepared for each amplitude section, and echo signals measured in the same amplitude section are arranged in the same k-space. As a result, the measurement section is different for each amplitude section. Specifically, the slower the amplitude variation, the longer the measurement interval of the same amplitude interval, and the maximum at the flat portion of the amplitude variation. On the contrary, the measurement section of the same amplitude section becomes shorter as the amplitude fluctuation becomes steeper. In the example of FIG. 6, the fluctuation in amplitude is substantially flat in the amplitude sections 1 and 4, and the measurement section is long because the amplitude is long and remains in the amplitude sections 1 and 4. On the other hand, in the amplitude sections 2 and 3, the fluctuation in the amplitude is steep, and the amplitude changes from the amplitude sections 2 and 3 to other amplitude sections, so the measurement section is shortened.

  Therefore, if the repetition time (TR) of the pulse sequence is constant, the number of echo signals measured in each amplitude section, that is, the echo data filling in the k space corresponding to each amplitude section, according to the length of each measurement section The rate is different. Specifically, the longer the measurement section, the more echo signals to be measured, and the higher the k-space filling rate. Since the measurement needs to be repeated until all the k spaces corresponding to all amplitude sections are filled with echo data, until all the k spaces corresponding to the shortest amplitude section are filled with echo data. Measurement will be repeated. At this time, with respect to echo data measured in duplicate in other k spaces, the averaging process is performed and the SN of the image is improved.

  Next, in order to execute the body motion artifact reduction method of the present embodiment, each function of the CPU 8 of the arithmetic processing unit 7 will be described with reference to FIG. FIG. 7 is a functional block diagram for explaining each function realized by the CPU 8 executing a program stored in the magnetic disk 18 or the like.

  The CPU 8 controls the above-described units by a program to realize imaging that reduces body motion artifacts according to the present embodiment. Therefore, the CPU 8 of the second embodiment includes an amplitude dividing unit 701 instead of the period dividing unit 301 in the first embodiment described above. The other functional units are the same as those in the first embodiment described above, but a part of the functions executed by the imaging execution unit 304 is different. Therefore, in the second embodiment, the imaging execution unit 304a is used. Hereinafter, only the functions of the amplitude dividing unit 701 and the imaging execution unit 304a will be described, and descriptions of other functions will be omitted.

  The amplitude dividing unit 701 divides the width including the maximum body motion amplitude into a predetermined number (M). The maximum amplitude is calculated based on changes in respiratory motion data. For example, it is set as the average of the difference of the adjacent maximum value and minimum value of respiratory motion data. As the number of divisions of the body motion amplitude, a value stored in advance on the magnetic disk 18 or the like may be used, or a value designated by the operator may be used.

  The imaging execution unit 304a measures an echo signal from the subject using a predetermined pulse sequence while monitoring respiratory motion data. At that time, the amplitude section to which the body motion amplitude at the time of measuring the echo signal belongs is determined, and the echo signal data is arranged in the non-arranged area of the k space corresponding to the determined amplitude section. The measurement control unit 4 is instructed at any time as to the phase encoding amount to be applied to the signal. Specifically, when repeating the execution of the pulse sequence at the repetition time (TR) of the pulse sequence, the imaging execution unit 304 determines whether the k-space corresponding to the amplitude section corresponds to the amplitude section to which the body motion amplitude belongs. The amount of encoding applied to the echo signal is controlled so as to measure the echo signal in the measurement region. A specific example of the encoding amount control is the same as that of the imaging execution unit 304. In this way, the amplitude interval is determined while constantly monitoring the body motion amplitude, and the encoding amount for each amplitude section is controlled, so that each amplitude depends on the steepness of the fluctuation of the body motion amplitude in each amplitude interval. Each measurement section is automatically shortened.

  Next, the flow of the imaging process of the present embodiment by each of the above functional units will be described with reference to FIG. FIG. 8 is a processing flow of the imaging process of the present embodiment. Compared with the processing flow of the first embodiment, the processing flow of the second embodiment performs step 802 instead of step 402, and the processing content of step 404 is different. Therefore, in the second embodiment, Step 404a is assumed. Hereinafter, only steps with different processing contents will be described, and description of the same processing will be omitted.

  In step 802, the amplitude dividing unit 701 calculates the maximum amplitude of the abdominal wall surface in respiratory motion based on the respiratory motion data acquired in step 401 (or step 404), and the number of amplitude divisions from the magnetic disk 18 or the like. And divide the width including the maximum amplitude into equal number of divisions (M). An example of the division is as described in FIG.

In step 404a, the imaging execution unit 304a measures an echo signal from the subject using a predetermined pulse sequence while monitoring respiratory motion data. Similar to step 404 described above, respiratory motion data is acquired through the use of a respiratory motion sensor or measurement of navigator echo signals. Any pulse sequence may be used. The imaging execution unit 30a repeats the execution of the pulse sequence at the repetition time (TR) of the pulse sequence and measures the echo signal.At that time, determines the amplitude section to which the amplitude of the respiratory motion data belongs, and according to the amplitude section, The encoding amount is controlled so as to measure an echo signal in an unmeasured area in k-space corresponding to the amplitude section. The specific encoding amount control is the same as in step 404.
The above is the description of the processing flow of the present embodiment.

  As described above, the MRI apparatus and body motion artifact reduction method of the present embodiment divides the amplitude of body motion amplitude into equal width intervals, and reconstructs an image using echo signals measured for each divided section. Then, a region with less motion in each reconstructed image is synthesized to obtain a single image with reduced body motion artifacts. As a result, even if the subject's respiratory motion becomes unstable or irregular, it is possible to acquire an image in which body motion artifacts are stably reduced. In particular, since an image is obtained using an echo signal measured for each equal-width amplitude section, the image for each amplitude section has less body motion artifacts. As described above, since a region with less motion is extracted and synthesized from each image with less body motion artifacts, the final image has fewer body motion artifacts.

<Third embodiment>
Next, a third embodiment of the MRI apparatus and body motion artifact reduction method of the present invention will be described. In this embodiment, the first embodiment and the second embodiment described above are combined to divide a predetermined period of the respiratory cycle into equal time intervals and to divide body motion amplitude into equal width intervals. Reconstruct the image using the measured echo signal, extract the region with less motion in each reconstructed image, and synthesize the extracted regions with less motion to create an image with reduced body motion artifacts get. Hereinafter, this embodiment will be described in detail with reference to FIGS.

  First, dividing the predetermined period of each respiratory cycle of body movement into equal time intervals and dividing the body movement amplitude into equal width intervals will be described with reference to FIG. FIG. 9 shows that the same respiratory motion data as FIG. 2 is divided into nine predetermined time intervals in the respiratory cycle as in the first embodiment described above (the measurement idle time is also in the third embodiment). In addition, as in the second embodiment described above, an example in which the body motion amplitude is divided into four at equal width intervals is shown. And in order to acquire the image for every amplitude section, k space is prepared for every amplitude section, and the data of the echo signal measured within the amplitude section with the same amplitude are arranged in the same k space. However, since the predetermined period of the respiratory cycle is divided into equal time intervals, the measurement of the echo signal and the distribution of the measured echo signal data to the k space are performed at equal time intervals for each measurement section. For this purpose, respiratory movement data is monitored as needed, the amplitude section to which the body movement amplitude at the start of each measurement section belongs is determined, and the echo signal data is placed in an unmeasured area in k-space corresponding to the amplitude section. Thus, an encoding amount is applied to each echo signal. When measuring a plurality of echo signals in one measurement section, the encoding amount applied to each echo signal is controlled so that the data of the echo signal is arranged in an unmeasured area of the same k space. When the amplitude section is determined in this way, the body motion amplitude may straddle a plurality of amplitude sections in a certain measurement period. In that case, the k space corresponding to a certain amplitude section includes data of another amplitude section. However, such a case can be reduced by increasing the number of divisions for each predetermined period of each respiratory cycle.

Also, in the case of the third embodiment, as in the second embodiment described above, the number of measurement sections that become the same amplitude section increases as the amplitude fluctuation becomes slow, and the maximum in the flat part of the amplitude fluctuation. Become. On the contrary, as the amplitude fluctuation becomes steeper, the number of measurement sections having the same amplitude section decreases. In the example of FIG. 9, the amplitude fluctuation is substantially flat in the amplitude sections 1 and 4, and the amplitude is long and remains in the amplitude sections 1 and 4, so the number of measurement sections is large. On the other hand, the amplitude fluctuations in the amplitude sections 2 and 3 are steep, and the amplitude changes from the amplitude sections 2 and 3 to other amplitude sections, so the number of measurement sections is reduced. Therefore, the number of echo signals measured in each amplitude section, that is, the filling rate of echo data into the k space corresponding to each amplitude section differs depending on the number of times in each measurement section. Specifically, as the number of measurement sections increases, the number of echo signals to be measured increases and the filling rate into the k space increases. Since the measurement needs to be repeated until all the k spaces corresponding to all amplitude sections are filled with echo data, all corresponding k spaces in the amplitude section with the smallest number of measurement sections are filled with echo data. Measurement will be repeated until
FIG. 9 shows an example of an image obtained in each amplitude section.

  Next, in order to execute the body motion artifact reduction method of the present embodiment, each function of the CPU 8 of the arithmetic processing unit 7 will be described with reference to FIG. FIG. 10 is a functional block diagram for explaining each function realized by the CPU 8 executing a program stored in the magnetic disk 18 or the like.

  The CPU 8 controls the above-described units by a program to realize imaging that reduces body motion artifacts according to the present embodiment. Therefore, the CPU 8 of the third embodiment includes both the period dividing unit 301 of the first embodiment described above and the amplitude dividing unit 701 of the second embodiment described above, and their functions are the same as those described above. This is the same as the first embodiment and the second embodiment. The other functional units are the same as those in the first embodiment described above, but the functions executed by the imaging execution unit 304 are partly different. Therefore, in the third embodiment, the imaging execution unit 304b is used. Hereinafter, only functions of the imaging execution unit 304b will be described, and descriptions of other functions will be omitted.

  The imaging execution unit 304b measures an echo signal from the subject using a predetermined pulse sequence while monitoring respiratory motion data. At that time, the body motion amplitude at the start time of each measurement section is detected, the amplitude section to which the body motion amplitude belongs is determined, and the measurement section is set in the unmeasured area of the k space corresponding to the determined amplitude section. The measurement control unit 4 is instructed at any time as to the amount of encoding to be applied to the echo signal so that the data of the echo signal to be measured in is arranged. Therefore, even if the body motion amplitude changes to another amplitude section in the middle of the same measurement section, the determination of the amplitude section remains the amplitude section determined at the start time of the measurement section. In this manner, the imaging execution unit 304b performs k-space selection and encoding amount control in the measurement section based on the body motion amplitude at the start time of each measurement section.

  Next, the flow of imaging processing according to the present embodiment performed by each functional unit described above will be described with reference to FIG. FIG. 11 is a processing flow of the imaging process of the present embodiment. The processing flow of the third embodiment is obtained by adding step 802 of the second embodiment to the processing flow of the first embodiment, but the processing content of step 802 is the same as that of the second embodiment. The form is the same. In addition, since the processing content of step 404 is different, it is set as step 404b in the third embodiment. Hereinafter, only step 404b having different processing contents will be described, and description of the same processing will be omitted.

In step 404b, the imaging execution unit 304b measures an echo signal from the subject using a predetermined pulse sequence while monitoring respiratory motion data. Similar to step 404 described above, respiratory motion data is acquired through the use of a respiratory motion sensor or measurement of navigator echo signals. Any pulse sequence may be used. The imaging execution unit 304b repeatedly executes the pulse sequence at the repetition time (TR) of the pulse sequence to measure the echo signal, and detects the amplitude of the respiratory motion data at the start time of each measurement section. The amplitude section to which it belongs is determined. Then, the imaging execution unit 304b encodes the echo signal applied to the echo signal so that the data of the echo signal measured during the measurement period is arranged in the unmeasured region of the k space corresponding to the determined amplitude section. Control the amount. The specific encoding amount control is the same as in step 404.
The above is the description of the processing flow of the present embodiment.

  As described above, the MRI apparatus and the body motion artifact reduction method of the present embodiment divide each respiratory cycle into equal time intervals and divide body motion amplitude into equal width intervals, and are measured for each divided section. An image is reconstructed using an echo signal, and a region with less motion in each reconstructed image is synthesized to obtain a single image with reduced body motion artifacts. As a result, even if the subject's respiratory motion becomes unstable or irregular, it is possible to acquire an image in which body motion artifacts are stably reduced. In particular, since the predetermined period of each respiratory cycle is divided into equal time intervals and the body motion amplitude is divided into equal width intervals, and the amplitude section is determined from the amplitude at the start of each measurement section. It is possible to reduce the number of determination processes for the amplitude section. As a result, since the acquisition efficiency of the echo signal can be improved, the imaging efficiency can be improved.

  1 subject, 2 static magnetic field generation system, 3 gradient magnetic field generation system, 4 sequencer, 5 transmission system, 6 reception system, 7 signal processing system, 8 central processing unit (CPU), 9 gradient magnetic field coil, 10 gradient magnetic field power supply, 11 High-frequency transmitter, 12 modulator, 13 high-frequency amplifier, 14a high-frequency coil (transmitting coil), 14b high-frequency coil (receiving coil), 15 signal amplifier, 16 quadrature detector, 17 A / D converter, 18 magnetic disk, 19 optical disc, 20 display, 21 ROM, 22 RAM, 23 trackball or mouse, 24 keyboard, 25 operation unit, 26 body movement detection unit

Claims (4)

A body motion detector that acquires respiratory motion data representing the respiratory state of the subject with respiratory motion;
A measurement control unit for measuring an echo signal from the subject using a predetermined pulse sequence;
An arithmetic processing unit that acquires an image in which artifacts based on the respiratory motion are reduced from the echo signal and the respiratory motion data;
A magnetic resonance imaging apparatus comprising:
The arithmetic processing unit includes:
A dividing unit for dividing the respiratory motion data into a plurality of sections;
A k-space setting unit for preparing a k-space for each of the divided sections;
An imaging execution unit that instructs the measurement control unit to measure an echo signal filling each k-space;
An image reconstruction unit that reconstructs an image for each of the divided sections using data filled in each k-space;
A region extracting unit that extracts a region with less motion from the image of each divided section;
An area synthesis unit that obtains an image by synthesizing the area with less movement;
I have a,
The dividing unit divides the amplitude of the respiratory motion data into a plurality of amplitude sections,
The k-space setting unit prepares a k-space for each amplitude section,
The magnetic resonance imaging apparatus, wherein the image reconstruction unit reconstructs an image for each amplitude section using data filled in the k spaces . A body motion detector that acquires respiratory motion data representing the respiratory state of the subject with respiratory motion;
A measurement control unit for measuring an echo signal from the subject using a predetermined pulse sequence;
An arithmetic processing unit that acquires an image in which artifacts based on the respiratory motion are reduced from the echo signal and the respiratory motion data;
A magnetic resonance imaging apparatus comprising:
The arithmetic processing unit
A dividing unit for dividing the respiratory motion data into a plurality of sections;
A k-space setting unit for preparing a k-space for each of the divided sections;
An imaging execution unit that instructs the measurement control unit to measure an echo signal filling each k-space;
An image reconstruction unit that reconstructs an image for each of the divided sections using data filled in each k-space;
A region extracting unit that extracts a region with less motion from the image of each divided section;
An area synthesis unit that obtains an image by synthesizing the area with less movement;
Have
The dividing unit divides a predetermined period of each respiratory cycle of the respiratory motion data into a plurality of measurement sections, and divides the amplitude of the respiratory motion data into a plurality of amplitude sections,
The k-space setting unit prepares a k-space for each amplitude section,
The magnetic resonance imaging apparatus, wherein the image reconstruction unit reconstructs an image for each amplitude section using data filled in the k spaces . A body motion detector that acquires respiratory motion data representing the respiratory state of the subject with respiratory motion;
A measurement control unit for measuring an echo signal from the subject using a predetermined pulse sequence;
An arithmetic processing unit that acquires an image in which artifacts based on the respiratory motion are reduced from the echo signal and the respiratory motion data;
A method for reducing body motion artifacts in a magnetic resonance imaging apparatus comprising:
Obtaining respiratory motion data representing the respiratory state of the subject by the body motion detector;
A step of measuring an echo signal filling the k-space by the measurement control unit;
By the arithmetic processing unit,
Dividing the amplitude of the respiratory motion data into a plurality of sections;
Preparing a k-space for each of the divided sections;
Reconstructing an image for each of the divided sections using data filled in each k-space;
Extracting a region with less motion from the image for each divided section;
Synthesizing the low-motion area to obtain an image;
A body motion artifact reduction method characterized by comprising: A body motion detector that acquires respiratory motion data representing the respiratory state of the subject with respiratory motion;
A measurement control unit for measuring an echo signal from the subject using a predetermined pulse sequence;
An arithmetic processing unit that acquires an image in which artifacts based on the respiratory motion are reduced from the echo signal and the respiratory motion data;
A method for reducing body motion artifacts in a magnetic resonance imaging apparatus comprising:
Obtaining respiratory motion data representing the respiratory state of the subject by the body motion detector;
A step of measuring an echo signal filling the k-space by the measurement control unit;
By the arithmetic processing unit,
Dividing a predetermined period of each respiratory cycle of the respiratory motion data into a plurality of measurement sections, and dividing the amplitude of the respiratory motion data into a plurality of amplitude sections;
Preparing a k-space for each of the divided sections;
Reconstructing an image for each amplitude interval using data filled in each k-space;
A body motion artifact reduction method characterized by comprising:

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