JP2015144738A - Magnetic resonance imaging system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic resonance imaging apparatus capable of continuously performing cine-imaging while reducing a reduction in image quality and positional deviation under free breathing negating the need of holding breath.SOLUTION: A magnetic resonance imaging apparatus comprises: an imaging conditions setting section 410 setting imaging conditions under which a real collection sequence and a dummy sequence are alternately applied to an object body to be inspected; a data collection section 500 collecting data of a slice cross section in the real collection sequence and collecting data of a projected cross section obtained by projecting the slice cross section in a cross sectional direction different from that of the slice cross section in the dummy sequence; a correction amount calculation section 430 obtaining first and second positions of centers of gravity from most recent and immediately present first and second dummy sequences of the real collection sequence and calculating a motion correction amount of the slice cross section; and a slice position update section 440 updating a position of the slice cross section of the real collection sequence performed immediately after the second dummy sequence by using the motion correction amount.

Description

  Embodiments described herein relate generally to a magnetic resonance imaging apparatus.

  The magnetic resonance imaging apparatus excites the patient's nuclear spin placed in a static magnetic field with a radio frequency (RF) signal of the Larmor frequency, and reconstructs the magnetic resonance signal generated from the patient with the excitation to generate an image. It is the imaging device which produces | generates.

  In the field of magnetic resonance imaging, there is a technique for performing synchronous imaging by observing a heartbeat signal, which is generally called an electrocardiographic synchronization method. Similarly, in the field of magnetic resonance imaging, cine imaging has been performed that enables a heart image to be generated at a plurality of cardiac time phases and reproduced in time series according to the cardiac time phase (for example, patents). Literature 1 etc.).

JP 2009-160378 A

  In cine imaging of the heart in magnetic resonance imaging, imaging is performed with a breath-hold in order to reduce the positional displacement of the heart due to respiratory movement. On the other hand, in the imaging of the heart, a plurality of short axis images are often captured so as to cover the entire left ventricle. In this case, it is necessary to collect a short-axis image of about 10 slices. In order to capture a short-axis image of one slice in order to capture a short-axis image of one slice, a short-axis image of the next slice is captured. In addition, breath holding imaging is performed and this is repeated 10 times. For this reason, the time which the whole imaging requires becomes long, and the burden with respect to a patient also becomes large. In addition, in the case of a patient who is difficult to hold his / her breath, the image quality is deteriorated due to the respiratory movement, and the position is shifted between images.

  To solve this problem, there is a method of shortening the imaging time of one slice using a parallel imaging method or the like, but this is not always a sufficient solution.

  Therefore, there is a demand for a magnetic resonance imaging apparatus that can continuously perform cine imaging while reducing image quality and positional deviation even under free breathing that does not require breath holding.

  The magnetic resonance imaging apparatus of the present embodiment stabilizes the diagnostic signal by using a plurality of dummy pulses and a main acquisition sequence for collecting diagnostic signals from a predetermined slice cross section in the heart using a plurality of excitation pulses. An imaging condition setting unit that sets imaging conditions for alternately applying to the subject, and a dummy slice to be excited in the main acquisition sequence by exciting a predetermined slice section in the heart with the excitation pulse. Data is collected, and in the dummy sequence, the same slice section as in the main acquisition sequence is excited by the dummy pulse, while the data of the projection section obtained by projecting the slice section in a section direction different from the slice section is collected. An image of the slice cross-section is generated by reconstructing the data collected in the acquisition unit and the main acquisition sequence And a reconstruction unit that reconstructs data collected in the dummy sequence to generate an image of the projection section, and a dummy sequence immediately before the main acquisition sequence is a first dummy sequence, When the immediately following dummy sequence is the second dummy sequence, the first centroid position is obtained from the projected sectional image generated by the first dummy sequence, while the projected sectional image generated by the second dummy sequence is obtained. A second center of gravity position from the first and second center of gravity positions, a correction amount calculating unit for calculating a motion correction amount of the slice cross section from the first and second center of gravity positions, and using the motion correction amount, And a slice position update unit that updates the position of the slice cross section of the main acquisition sequence performed immediately thereafter.

1 is a configuration diagram showing an example of the overall configuration of a magnetic resonance imaging apparatus of an embodiment. The block diagram which shows the structure in connection with the process of a heart cine imaging. The flowchart explaining operation | movement of 1st Embodiment. The figure explaining the concept of the short-axis cross section of the heart. The figure which shows an example of the pulse sequence used with the magnetic resonance imaging apparatus of embodiment. The figure explaining the cross section of the data collected by this collection sequence and a dummy sequence. The figure which shows the specific example of the pulse sequence in this acquisition sequence and a dummy sequence. The figure explaining the calculation concept of the motion correction amount in 1st Embodiment. FIG. 4 is a first diagram for explaining a concept of updating a slice position in the first embodiment. FIG. 5 is a second diagram for explaining the concept of updating the slice position in the first embodiment. The figure which shows the pulse sequence of the modification (1) of 1st Embodiment. The figure which shows the pulse sequence of the modification (2) of 1st Embodiment. The figure which shows the pulse sequence of the modification (3) of 1st Embodiment. The figure explaining the operation | movement concept of the modification (4) of 1st Embodiment. The figure explaining the operation | movement concept of the modification (5) of 1st Embodiment. The flowchart explaining operation | movement of 2nd Embodiment. The 1st figure explaining operation | movement of 2nd Embodiment. The 2nd figure explaining operation | movement of 2nd Embodiment.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

(1) Overall Configuration FIG. 1 is a block diagram showing the overall configuration of a magnetic resonance imaging apparatus 1 in the present embodiment. The magnetic resonance imaging apparatus 1 of the embodiment includes a magnet stand 100, a bed 200, a control cabinet 300, a console 400, and the like.

  The magnet mount 100 includes a static magnetic field magnet 10, a gradient magnetic field coil 11, an RF coil 12, and the like, and these components are housed in a cylindrical casing. The bed 200 includes a bed body 20 and a top plate 21.

  On the other hand, the control cabinet 300 includes a static magnetic field power supply 30, a gradient magnetic field power supply 31 (31x for X axis, 31y for Y axis, 31z for Z axis), RF receiver 32, RF transmitter 33, sequence controller 34, and the like. ing. The console 400 is configured as a computer having a processor 40, a storage unit 41, an input unit 42, a display unit 43, and the like.

  The static magnetic field magnet 10 of the magnet mount 100 has a substantially cylindrical shape, and generates a static magnetic field in a bore (a space inside the cylinder of the static magnetic field magnet 10) that is an imaging region of a subject (patient). The static magnetic field magnet 10 incorporates a superconducting coil, and the superconducting coil is cooled to a very low temperature by liquid helium. The static magnetic field magnet 10 generates a static magnetic field by applying a current supplied from the static magnetic field power source 30 to the superconducting coil in the excitation mode, and then, when the permanent magnetic mode is entered, the static magnetic field power source 30 is disconnected. . Once in the permanent current mode, the static magnetic field magnet 10 continues to generate a large static magnetic field for a long time, for example, for one year or more. The static magnetic field magnet 10 may be configured as a permanent magnet.

  The gradient coil 11 also has a substantially cylindrical shape and is fixed inside the static magnetic field magnet 10. The gradient magnetic field coil 11 applies a gradient magnetic field to the subject in the X-axis, Y-axis, and Z-axis directions by a current supplied from a gradient magnetic field power supply (31x, 31y, 31z).

  The bed body 20 of the bed 200 can move the top plate 21 in the vertical direction, and moves the subject placed on the top plate 21 to a predetermined height before imaging. Thereafter, at the time of imaging, the top 21 is moved in the horizontal direction to move the subject into the bore.

  The RF coil 12 is also called a whole body coil, and is fixed inside the gradient magnetic field coil 11 so as to be opposed to each other with a subject interposed therebetween. The RF coil 12 transmits an RF pulse transmitted from the RF transmitter 33 toward the subject, and receives a magnetic resonance signal emitted from the subject due to excitation of hydrogen nuclei.

  The RF transmitter 33 transmits an RF pulse to the RF coil 12 based on an instruction from the sequence controller 34. On the other hand, the RF receiver 32 detects a magnetic resonance signal received by the RF coil 12 and transmits raw data obtained by digitizing the detected magnetic resonance signal to the sequence controller 34.

  The sequence controller 34 scans the subject by driving the gradient magnetic field power supply 31, the RF transmitter 33, and the RF receiver 32 under the control of the console 400. When the sequence controller 34 scans and receives raw data from the RF receiver 32, the sequence controller 34 transmits the raw data to the console 400.

  The console 400 controls the entire magnetic resonance imaging apparatus 1. Specifically, imaging conditions and other various information and instructions are received by operating a mouse, a keyboard, etc. (input unit 42) such as a laboratory technician. Then, the processor 40 causes the sequence controller 34 to execute a scan based on the input imaging condition, and reconstructs an image based on the raw data transmitted from the sequence controller 34. The reconstructed image is displayed on the display unit 43 or stored in the storage unit 41.

  The magnetic resonance imaging apparatus 1 can cine-image a patient's heart in synchronization with a patient's electrocardiogram signal (for example, R wave) obtained from an electrocardiograph (not shown) or the like.

  FIG. 2 is a block diagram showing a configuration relating to processing of cardiac cine imaging. As shown in FIG. 2, the magnetic resonance imaging apparatus 1 includes an imaging condition setting unit 410, a reconstruction unit 420 (reconstruction unit (1) 422, reconstruction unit (2) 424), and a correction amount calculation unit 430 (center of gravity position). A calculation unit 432, a calculation unit 434), a slice position update unit 440, a cine image generation unit 450, and the like are included. The functions of these units are realized by executing predetermined program codes by the processor 40 of the console 400. However, the functions are not limited to such software processing, but are realized by hardware processing using, for example, an ASIC. Alternatively, it may be realized by combining software processing and hardware processing.

  In FIG. 2, the data collection unit 500 is a collective term for the configuration of the magnetic resonance imaging apparatus 1 shown in FIG. Moreover, since the input part 42 and the display part 43 in FIG. 2 are the same as what is shown in FIG. 1, the same code | symbol is attached | subjected.

  Among the above units, the imaging condition setting unit 410 uses the plurality of excitation pulses to collect a diagnostic signal from a predetermined slice cross section in the heart and the diagnostic signal using a plurality of dummy pulses. An imaging condition for alternately applying a dummy sequence to stabilize the subject to the subject is set. This imaging condition is set by information input from the input unit 42 by the operator, information on slice positions output by the slice position update unit 440, and the like. The set imaging conditions are sent to the sequence controller 34 of the data collection unit 500. Specific contents of the collection sequence and the dummy sequence will be described later.

  In this acquisition sequence, the data acquisition unit 500 excites a predetermined slice section in the heart with a plurality of excitation pulses and collects data of the predetermined slice section. In the dummy sequence, the same slice cross section as in the acquisition sequence is excited with a plurality of dummy pulses, while data of a projected cross section obtained by projecting the slice cross section in a cross sectional direction different from the slice cross section is collected. A more specific operation of the data collection unit 500 will also be described later.

  The reconstruction unit (2) 424 of the reconstruction unit 420 reconstructs the data collected in the present collection sequence and generates an image of the slice cross section. On the other hand, the reconstruction unit (1) 422 of the reconstruction unit 420 reconstructs data collected in the dummy sequence and generates an image of the projected cross section.

  The center-of-gravity position calculation unit 432 of the correction amount calculation unit 430 uses the dummy sequence immediately before the main collection sequence as the first dummy sequence and the dummy sequence immediately after the main collection sequence as the second dummy sequence. The first barycentric position is obtained from the projected cross-sectional image generated by the first dummy sequence, while the second barycentric position is obtained from the projected cross-sectional image generated by the second dummy sequence. Then, the calculation unit 434 of the correction amount calculation unit 430 calculates the motion correction amount of the slice cross section from the first and second barycentric positions.

  The slice position updating unit 440 uses the calculated motion correction amount to update the position of the slice cross section of the main acquisition sequence performed immediately after the second dummy sequence.

  As described above, the position of the heart changes depending on the movement of the diaphragm accompanying breathing. The motion correction amount corrects such respiratory movement. In the two-dimensional cine imaging, it is intended to continuously image the same predetermined cross section in the heart, but if there is a respiratory movement, the imaging cross section changes, so an image of a desired cross section cannot be obtained. . Therefore, in the magnetic resonance imaging apparatus 1 of the present embodiment, the respiratory motion is calculated in real time as the motion correction amount from the data collected in the dummy sequence, and the prospective imaging position correction using the motion correction amount is performed. The position of the slice cross section to be imaged next is updated. As a result, even if the position of the heart moves due to respiration, imaging data of the same slice can always be collected in this acquisition sequence while tracking the same anatomical cross section in the heart. Cine imaging of the heart can be performed.

  Note that the motion correction amount is a correction amount that includes not only the respiratory motion but also the motion of the body of the subject being imaged, so both motions can be corrected simultaneously.

  The cine image generation unit 450 generates a cine image by arranging two-dimensional images reconstructed based on the data collected in the present collection sequence in time series according to the cardiac phase.

(2) Operation (first embodiment)
FIG. 3 is a flowchart for explaining the operation of the magnetic resonance imaging apparatus 1 according to the first embodiment. The operation of the magnetic resonance imaging apparatus 1 will be described below along the flowchart of FIG.

  In step ST100, the apparatus captures various types of information regarding the imaging conditions input from the input unit 42 by the operator. The imaging conditions include, for example, information related to an imaging cross section (hereinafter sometimes referred to as a slice) and information related to a pulse sequence.

  The magnetic resonance imaging apparatus 1 images a predetermined anatomical cross section of the heart. When diagnosing the heart, an image (short axis) of a left ventricular short axis section (hereinafter simply referred to as a short axis section) is used. It is known that the cross-sectional image) is very important. FIG. 4 is a diagram for explaining the concept of the short-axis cross section. As shown in FIG. 4, the axis connecting the left ventricular apex and the center of the mitral valve is referred to as the left ventricular long axis, and the short-axis cross section is a cross section of the left ventricle perpendicular to the left ventricular long axis. In the short-axis cross-section imaging, there is a case where one predetermined short-axis cross-section is imaged. However, as shown in FIG. In many cases, an image is taken with an axial section.

  The imaging section of the heart targeted by the magnetic resonance imaging apparatus 1 is not limited to the short-axis section, and cross-sections at various positions of the heart can be targeted for imaging. A cross-sectional imaging will be described as an example.

  In addition, the operation of continuously imaging the same short-axis cross section as a cine will be described as the first embodiment, and the operation of imaging while switching a plurality of different short-axis cross sections, that is, after performing a cine image of one short-axis cross section The operation of moving to the next short-axis cross-section and imaging it will be described later as a second embodiment.

  In step ST100, for example, information such as the position and orientation of the short-axis cross section described above is taken in as information related to the imaging cross section. In step ST102, the imaging condition setting unit 410 sets information regarding these imaging sections in the apparatus.

  In step ST100, information relating to the pulse sequence is also fetched, and this information is set in the apparatus in step ST102.

  FIG. 5 is a diagram illustrating an example of a pulse sequence used in the magnetic resonance imaging apparatus 1. The magnetic resonance imaging apparatus 1 is configured to be able to take an image so that a cross section of the heart can be depicted as a moving image in which a motion corresponding to a heartbeat is reflected, that is, to perform cine imaging. For this reason, it is necessary to use a very high-speed pulse sequence, and a gradient echo system (also called a field echo system) pulse sequence is preferable.

  As a high-speed gradient echo (GRE: Gradient Echo) type pulse sequence, a spoiled GRE type pulse sequence that actively disappears residual transverse magnetization, or a balanced SSFP that uses both longitudinal magnetization and transverse magnetization in a steady state. There are (Steady State Free Precession) sequences and the like, and these pulse sequences can be used in the magnetic resonance imaging apparatus 1 of the embodiment.

  Among these, the pulsed GRE pulse sequence includes a pulse sequence (typically called gradient spoiling) that uses a method of performing phase loss by forcing a phase dispersion by applying a gradient magnetic field before an excitation pulse (typically, gradient spoiling). Is a pulse sequence using a FLASH (Fast Low Angle Shot) sequence or a method of eliminating transverse magnetization by changing the phase of successive excitation pulses by a certain amount (referred to as RF spoiling) (typically SPGR ( Spoiled gradient-recalled acquisition in the steady state).

  On the other hand, the balanced SSFP sequence is a pulse sequence in which the sum of gradient magnetic fields within one TR (Repetition time: interval between adjacent excitation pulses) is zero, and is true SSFP (True Steady State Free Precession), TrueFISP (True This is a pulse sequence called by a name such as Fast imaging steady state free precession), FIESTA (Fast imaging steady-state acquisition), balanced FFE (balanced Fast field echo), or the like.

  In these high-speed GRE pulse sequences, as illustrated in FIG. 5A, a very short value (for example, 10 ms or less) is used as the excitation pulse interval TR. Usually, a dummy sequence is provided before the main collection sequence. Here, the main acquisition sequence is a sequence including a plurality of excitation pulses for acquiring a diagnostic signal (magnetic resonance signal for generating a diagnostic image). The dummy sequence is a sequence that is added to bring the longitudinal magnetization in the main acquisition sequence into a steady state, in other words, a sequence for stabilizing the diagnostic signal. A plurality of excitation pulses are also used in the dummy sequence (the excitation pulse of the dummy sequence is referred to as a dummy pulse). The TR and flip angle of the dummy pulse are normally set to the same values as the TR and flip angle of the excitation pulse in the acquisition sequence. However, the number of dummy pulses in the dummy sequence is usually set to be smaller than the number of excitation pulses in the main acquisition sequence.

  The purpose of the dummy sequence is to bring the longitudinal magnetization in the acquisition sequence to a steady state. For this reason, it has heretofore been hardly considered to acquire data using the dummy sequence. On the other hand, in the magnetic resonance imaging apparatus 1 of the embodiment, data for motion correction is acquired in the dummy sequence. That is, the dummy sequence in the magnetic resonance imaging apparatus 1 according to the present embodiment simultaneously realizes two functions of a function for stabilizing a diagnostic signal and a function for acquiring data for motion correction.

  The pulse sequence in FIG. 5 will be further described. The pulse sequence in FIG. 5 is an electrocardiographic synchronization sequence synchronized with an electrocardiographic signal (for example, an R wave). The ECG synchronization in the present embodiment can include both prospective ECG synchronization and retrospective ECG synchronization. In the case of prospective ECG synchronization, the device detects the timing of the R wave and generates an excitation pulse train in synchronization with the R wave. In the case of the example shown in FIG. 5, the excitation pulse train of the main acquisition sequence immediately after each R wave is generated in synchronization with each detected R wave. The excitation pulse of the dummy sequence immediately after the main acquisition sequence is generated so as to ensure continuity with the excitation pulse train of the main acquisition sequence. When the next R wave arrives, a new excitation pulse train in the acquisition sequence is generated in synchronization with the R wave.

  On the other hand, in the case of retrospective ECG synchronization, the ECG signal is acquired and stored simultaneously with the acquisition of the magnetic resonance signal, while the excitation pulse train of this acquisition sequence and the excitation pulse train of the dummy sequence are continuous independently of the R wave. It occurs without breaks. Then, the magnetic resonance signal and the electrocardiographic signal are synchronized with each other based on the stored electrocardiographic signal, thereby synchronizing the magnetic resonance signal and the electrocardiographic signal.

  The pulse sequence shown in FIG. 5 is a sequence based on a so-called segmented electrocardiographic synchronization method in which a magnetic resonance signal for generating one short-axis cross-sectional image is divided into a plurality of heartbeats and collected. One heartbeat corresponds to one segment. In the example shown in FIG. 5, magnetic resonance signals necessary for generating one short-axis cross-sectional image are collected by M segments from segment 1 to segment M. Specifically, the k-space data in the phase encoding direction necessary for generating one short-axis cross-sectional image is collected separately for each segment by 1 / M. When the data collection for the segment M is completed, all k-space data necessary for image reconstruction is satisfied.

  On the other hand, in each segment (between 1R and R), the magnetic resonance imaging apparatus collects over a plurality of cardiac phases (in the example of FIG. 5, N cardiac phases from cardiac phase 1 to cardiac phase N). Is done. When the data collection for the segment M is completed, the k-space data of the same cardiac phase is extracted from the respective k-space data from the segment 1 to the segment M, and one short-axis cross-sectional image is obtained for each cardiac phase. The full k-space data needed to generate is collected.

  Note that the number of phase encoding is limited by reducing the resolution in the phase encoding direction or by reducing the size of FOV (Field of view), or by limiting the number of cardiac phases, depending on the segmented method. It is also possible to collect data necessary for generating a short-axis cross-sectional image of a plurality of cardiac phases within one heartbeat.

  Further, the magnetic resonance imaging apparatus 1 of the embodiment is configured to apply the main acquisition sequence and the dummy sequence alternately. In the pulse sequence shown in FIG. 5, the main acquisition sequence (1) is applied following the leftmost dummy sequence, the dummy sequence is applied subsequently, and the main acquisition sequence (2) is applied immediately thereafter. Thus, the main acquisition sequence and the dummy sequence are alternately applied.

  Here, in this acquisition sequence, a predetermined slice section in the heart (in this example, a short-axis section) is excited with a plurality of excitation pulses, and data of the predetermined slice section is acquired. On the other hand, in the dummy sequence, the same slice section as in the present acquisition sequence is excited by a dummy pulse, but data of a projection section obtained by projecting the slice section in a section direction different from the slice section is collected.

  FIG. 6 is a diagram for explaining a cross section of data collected in the main collection sequence and the dummy sequence. The schematic diagram of the heart in the upper left of FIG. 6 is a coronal plane, and shows a coordinate system in which the left-right direction of the subject is the X direction, the abdominal back direction is the Y direction, and the head and foot direction is the Z direction. Hereinafter, this coordinate system will be described.

  In this acquisition sequence, the short-axis cross section indicated by the thick solid line on the schematic heart diagram is reconstructed as an image (short-axis cross-sectional image indicated by cross-hatching in the lower left of FIG. 6) viewed from a direction perpendicular to the cross-section. Collect data. For this reason, an excitation pulse is applied by selecting a slice plane corresponding to the short-axis cross section. The phase encoding direction is the Y direction (belly-dorsal direction), and the readout direction is the X / Z synthesis direction along the inclination of the short-axis cross section.

  On the other hand, as shown on the right side of FIG. 6, the data collected in the dummy sequence is data for reconstructing a projected cross-sectional image (a sagittal cross-sectional image) obtained by projecting the short-axis cross section in the X direction (left-right direction). collect. In order to obtain this sagittal cross-sectional image, the slice plane to be selected is a slice plane corresponding to the same short-axis cross section as the main acquisition, and the phase encoding direction is the same Y direction (abdominal back direction) as the main acquisition sequence. The direction is the Z direction, different from this collection sequence.

  FIG. 7A shows a specific example of a pulse sequence for collecting the data of the short-axis cross section in the present acquisition sequence, and FIG. 7B shows the above-described projection cross section (sagittal plane) in the dummy sequence. It is a figure which shows the specific example of the pulse sequence for collecting this data. Each pal sequence indicates one TR and corresponds to a balanced SSFP sequence.

  As shown in FIG. 7A, in this acquisition sequence, the X-direction gradient magnetic field Gx and the Z-direction gradient magnetic field Gz are applied simultaneously for slice selection, and the short-axis cross section inclined in the X / Z composite direction is applied. Select a slice. Similarly, the gradient magnetic field Gx and the gradient magnetic field Gz are applied simultaneously for readout, and the magnetic resonance signal can be read in the X / Z synthesis direction. Only Y-direction gradient magnetic field Gy is used for phase encoding.

  On the other hand, in the dummy sequence, the gradient magnetic fields for slice selection and phase encoding are the same as in the main collection, but only the Z-direction gradient magnetic field Gz is used for the gradient magnetic field for readout.

  As described above, the magnetic resonance imaging apparatus 1 according to the present embodiment is configured to obtain motion correction data from data collected in the dummy sequence. More specifically, the projection image is reconstructed from the data collected in the dummy sequence, and the change in the heart position that fluctuates due to breathing or the like is obtained from the change in the center of gravity position of the projection image. Here, the change in the heart position due to respiration is mainly caused by the movement of the diaphragm in the vertical direction (head-and-foot direction), and the movement in the Z direction is dominant. Therefore, the lead-out direction in the dummy sequence is set to the Z direction, and the projected cross section is a sagittal plane so that the respiratory movement appears more prominently on the projected cross section image.

  Since the projected cross-sectional image is a two-dimensional image in the Z direction and the Y direction, not only the movement in the Z direction but also the movement in the Y direction can be detected.

  In this acquisition sequence using the segmented method, data is collected over a plurality of heartbeats, and a cine image obtained by imaging one short-axis cross section with a plurality of cardiac phases is generated, whereas in the dummy sequence, One projection sectional image is generated from data collected in one dummy sequence within one heartbeat. That is, the projected cross-sectional image for calculating the motion correction amount is generated for each heartbeat, and is shorter than the time for generating one short-axis cross-sectional image. Therefore, it is possible to correct the heart motion that varies while generating one short-axis cross-sectional image with a finer period.

  Up to this point, the imaging conditions (pulse sequence, imaging cross section, etc.) set in step ST102 of FIG. 3 have been described. Next, a specific method of motion correction using this pulse sequence will be described.

  When an instruction to start imaging is input from the input unit 42 in step ST104 in FIG. 3, the imaging conditions set in the imaging condition setting unit 410 are sent to the sequence controller 34, and the subject of the set pulse sequence is sent to the subject. Is started.

  Steps ST106 to ST114 show processing of one segment from the R wave to the next R wave in the pulse sequence shown in FIG. 5, from segment 1 to segment M, from step ST106 to step ST114. The process is repeated. When imaging is continued after the end of segment M, the processes from segment 1 to segment M and from step ST106 to step ST114 are repeated. If it is determined in step ST114 that an instruction to end imaging has been received, the process ends there.

  Note that the pulse sequence in FIG. 5 is a part extracted when the sequence from segment 1 to segment M is repeated, and “dummy A” is immediately before the main acquisition sequence (1) in FIG. ”(See FIG. 5B) has already been performed. That is, assuming that the execution of the main collection sequence in step ST106 in FIG. 3 is the execution of the main collection sequence (1) in FIG. 5B, the execution of “dummy A” has already been completed before the execution. . In step ST108, a dummy sequence ("Dummy B", see FIG. 5B) immediately after the main collection sequence (1) is executed.

  In step ST110, the motion correction amount based on the collected data of the dummy sequence is calculated.

  FIG. 8 is a diagram for explaining the concept of calculating the motion correction amount in step ST110. The solid-line ellipse shown in FIG. 8 (a) is a diagram schematically showing the outer shape position of the heart when dummy A is applied immediately before the main acquisition sequence (1). The thick solid line segment also indicates the position of the short-axis cross section when dummy A is applied.

  On the other hand, the broken-line ellipse shown in FIG. 8A is a diagram schematically showing the outer position of the heart at the time of applying the dummy B immediately after the acquisition sequence (1). The position of the short-axis cross section when B is applied is shown. The difference between the position of the short-axis cross-section when applying dummy A indicated by the solid line and the position of the short-axis cross-section when applying dummy B indicated by the broken line is the change in the heart position between the start and end of this acquisition sequence (1). (Due to respiration, etc.)

  The data collected by the dummy A and the dummy B are respectively reconstructed by the reconstruction unit (1) 422 in FIG. 2, and the projected sectional image A and the projected sectional image B illustrated as an ellipse in FIG. 8B. Are generated respectively.

Next, the center-of-gravity position calculation unit 432 of the correction amount calculation unit 430 illustrated in FIG. 2 calculates the center-of-gravity position α and the center-of-gravity position β of the projected sectional image A and the projected sectional image B, respectively. Further, the calculation unit 434 of the correction amount calculation unit 430 calculates the motion correction amount C _αβ from the gravity center positions α and β.

The motion correction amount C _αβ is calculated as, for example, C _αβ = β−α from the difference between the gravity center positions α and β.

  In step ST112, the imaging cross-sectional position is corrected using the calculated motion correction amount, and the slice position acquired in the next main acquisition sequence is updated.

The concept of updating the slice position is shown in FIGS. As shown in the upper diagram of FIG. 9, it is assumed that a slice 1-1 is set as a slice corresponding to the short-axis cross section 1 that is an imaging target of the collection sequence (1). The center of gravity position of the dummy A immediately before the main collection sequence (1) is α, and the center of gravity position of the dummy B immediately after the main acquisition sequence (1) is β. The motion correction amount C _{αβ at} this time is calculated as the difference in the center of gravity (β−α). In this case, the imaging target of the main acquisition sequence (2) immediately after the dummy B is the short-axis cross section 1 that is the same as that of the main acquisition sequence (1), but the slice position is the next using the motion correction amount C _αβ. formula,
Setting position of slice 1-2 = setting position of slice 1-1 + motion correction amount C _αβ
Thus, the slice 1-1 is updated to the slice 1-2.

Thereafter, the processing of FIG. 3 returns to step ST106 via step ST114, and the main acquisition sequence (2) is executed at the updated position of the slice 1-2. Then, the motion correction amount C _βγ is calculated from the centroid position γ calculated from the dummy C executed as step ST108 immediately after this collection sequence (2) and the centroid position β of the dummy B that has already been calculated. , The setting position of the slice 1-3 in the next main acquisition sequence (3) is expressed by the following equation:
Setting position of slice 1-3 = setting position of slice 1-2 + motion correction amount C _βγ
Thus, the slice 1-2 is updated to the slice 1-3. Hereinafter, such processing is repeated until the end of imaging.

  FIG. 10 is a diagram for explaining the above-described processing concept using a graph in which the horizontal axis represents time and the vertical axis represents the position of the short-axis cross section. The curve in FIG. 10 represents the position of the short-axis cross section that changes due to movement due to respiration and the like, and the black circle is the position of the center of gravity of the projected cross section obtained by the dummy sequence. A thick solid line represents the slice setting position of the acquisition sequence updated by the motion correction amount.

  As described above, in the magnetic resonance imaging apparatus 1 of the embodiment, when the dummy sequence immediately before the main acquisition sequence is the first dummy sequence and the dummy sequence immediately after the main acquisition sequence is the second dummy sequence, A first barycentric position is obtained from the projected cross-sectional image generated by the first dummy sequence, while a second barycentric position is obtained from the projected cross-sectional image generated by the second dummy sequence, and the first and second The correction amount calculation unit 430 is configured to calculate the motion correction amount of the slice cross section from the position of the center of gravity.

  Further, the slice position update unit 440 is configured to update the position of the slice section of the main acquisition sequence performed immediately after the second dummy sequence, using the calculated motion correction amount. The position of the slice section of the updated main acquisition sequence is set as a new imaging condition in the imaging condition setting unit 410 and is sent to the sequence controller 34 for execution of the next main acquisition sequence.

  As described above, according to the magnetic resonance imaging apparatus 1 according to the first embodiment, the motion of the heart due to breathing or the like is detected using the data collected by the dummy sequence, and the imaging section (for example, the short-axis section) is detected. ), The slice position corresponding to the imaging section is sequentially updated. As a result, even in cine imaging of the heart having a long imaging time, it is possible to acquire a good image with little positional deviation by free breathing.

  Further, the pulse sequence for detecting the motion of the heart is also used as a dummy sequence originally necessary for stabilizing the diagnostic signal, and does not newly add another pulse sequence. Therefore, the imaging time is not excessively extended.

  In addition, the projected cross-sectional image generated in the dummy sequence to detect the heart motion excites the same anatomical region (same short-axis cross-section) as this acquisition sequence. The movement and the movement of the anatomical part to be imaged in this acquisition sequence are almost the same, and a highly accurate movement correction amount can be acquired.

  Furthermore, the projected cross-sectional image generated by the dummy sequence is a cross-section (for example, a sagittal cross-section) parallel to the direction of the subject's head and feet, so that the respiratory movement of the heart accompanying the diaphragm variation can be detected well. Can do. In addition, since the projection cross-sectional image generated by the dummy sequence is a two-dimensional image, it is possible to detect the movement in the direction (abdominal back direction) perpendicular to this in addition to the head and foot direction. Variations can be corrected in two dimensions.

(3) Modified Examples of First Embodiment Hereinafter, various modified examples of the first embodiment will be described. Any of the modifications can be realized by the same configuration as that shown in FIGS. 1 and 2.

  FIG. 11 is a diagram showing a pulse sequence of a modification (1) of the first embodiment. In the first embodiment, as shown in FIG. 5B, the main acquisition sequence is applied after the R wave, and then a dummy sequence is applied before the next R wave. On the other hand, in the modification (1), after the R wave, the dummy sequence is applied first, and then the main acquisition sequence is applied before the next R wave. As described above, even if the order of the dummy sequence and the main collection sequence is changed within the RR, the point that the dummy sequence and the main collection sequence are executed alternately is the same as that of the first embodiment, and The same effect as that of the first embodiment can be obtained.

  FIG. 12 is a diagram illustrating a pulse sequence of a modification (2) of the first embodiment. The interval between adjacent R waves is not necessarily constant. In particular, when the interval between the R waves becomes shorter than the expected value due to arrhythmia or the like, the pulse sequence of the first embodiment shown in FIG. Cannot be calculated correctly. Therefore, in the modified example (2), as shown in FIG. 12, when the next R wave arrives before collecting the originally planned amount of data during execution of the dummy sequence, the planned amount of data is obtained. The dummy sequence can be extended until the collection is completed. As a result, the center of gravity position can be correctly calculated, and the slice position of the next main acquisition sequence can be corrected correctly.

  FIG. 13 is a diagram showing a pulse sequence of a modification (3) of the first embodiment. In the modification (3), a plurality of main collection sequences and a plurality of dummy sequences are executed between 1R and R. In the example shown in FIG. 13, two main collection sequences and two dummy sequences are executed between 1R and R. Even in this case, the calculation of the centroid position in each dummy sequence and the update of the slice position of the next main acquisition sequence using the centroid position calculated in the dummy sequence immediately before and after the main acquisition sequence. Is the same as in the first embodiment. However, in the modified example (3), since the update interval of the slice position in the acquisition sequence is shortened, motion correction can be realized with higher accuracy.

  FIG. 14 is a diagram illustrating a concept of operation of the modified example (4) of the first embodiment. The horizontal axis of FIG. 14 is time, the vertical axis is the position of the short-axis cross section, and the curve of FIG. In the modified example (4), the calculation unit 434 determines a difference between the centroid position obtained from the dummy sequence immediately before this collection sequence and the centroid position obtained from the immediately following dummy sequence, and this difference is determined as a predetermined value. Only when it is within the threshold range, the slice position of the next main acquisition sequence is updated to collect data.

  For example, as illustrated in FIG. 14, the centroid position α obtained from the dummy sequence immediately before the main collection sequence (1) at the set position of the slice 1-1, and the centroid position β obtained from the dummy sequence immediately after When the difference (β−α) is equal to or greater than a predetermined threshold T, the slice position in the next main acquisition sequence (2) is not updated, and no data is collected. Alternatively, data is not collected even if the slice position is updated. On the other hand, when the difference (γ−β) between the centroid position β obtained from the dummy sequence immediately before this collection sequence (2) and the centroid position γ obtained from the immediately following dummy sequence is smaller than a predetermined threshold T. Collects data by updating the slice position of the next main acquisition sequence (3). According to the modified example (4), it is possible to remove data that is greatly influenced by movement, and it is possible to improve the image quality.

  FIGS. 15A and 15B are diagrams illustrating the concept of operation of the modification example (5) of the first embodiment. In the modified example (5), the position of the slice cross section of this acquisition sequence is configured to be smoothly updated step by step between the immediately preceding dummy sequence and the immediately following dummy sequence using the motion correction amount. It is.

For example, as shown in FIGS. 15A and 15B, when the center of gravity position β is obtained by collecting the dummy B and the motion correction amount (β−α) (= C _αβ ) is calculated, the dummy B To the dummy C, the slice position of the acquisition sequence (2) is changed to slice position Z _1-2-1 , slice position Z _1-2-2 , slice position Z _1-2-3 , The configuration is such that the update is smoothly performed in stages (in the example of FIG. 15, in three stages). At this time, for example, each slice position is
Z _1-2-1 = Z _1-1 + k1 (β-α),
Z _1-2-2 = Z _1-1 + k2 (β-α),
Z _1-2-3 = Z _1-1 + k3 (β-α),
As described above, it is calculated using the motion correction amount (β−α) and predetermined coefficients k1, k2, and k3. The coefficients k1, k2, and k3 are coefficients smaller than 1, for example, and can be determined as appropriate from a database that measures heart motion. Similarly, in the next main acquisition sequence (3) following the main acquisition sequence (2), the slice position of the main acquisition sequence (3) is similarly calculated using the motion correction amount (γ−β) (= C _βγ ). The slice position _Z1-3-1 , the slice position _Z1-3-2 , and the slice position _Z1-3-3 are updated smoothly step by step.

  According to the modified example (5), the slice position by correction changes smoothly and continuously as compared with the first embodiment, so that it is possible to suppress a decrease in continuity between images.

(4) Second Embodiment In the magnetic resonance imaging apparatus 1 according to the second embodiment, an operation of performing imaging while switching a plurality of different short-axis cross sections, that is, the next short-cut after the cine imaging of one short-axis cross section is performed. The operation of moving to the axial section and imaging this is repeated for the number of planned short-axis sections (slices).

  In the first embodiment, the slice position update unit 440 is planned so that the slice cross section targeted by the first main acquisition sequence and the slice cross section targeted by the second main acquisition sequence thereafter are the same. In this case, in the second dummy sequence immediately after the first main acquisition sequence, data of a projection cross section obtained by projecting the slice cross section set for the first main acquisition sequence is acquired.

  On the other hand, in the second embodiment, the slice cross-section targeted by the first main acquisition sequence differs from the slice cross-section targeted by the second main acquisition sequence thereafter by a predetermined displacement amount. Planned as a cross-section. In this case, in the second dummy sequence immediately after the first main acquisition sequence, the slice cross section acquired in the first main acquisition sequence is displaced by the predetermined displacement amount, and the projected cross section is projected after the displacement. Collect data. That is, in the second embodiment, when attempting to execute the main acquisition sequence by switching the position of the short-axis cross section, the position of the slice cross section to be acquired in the dummy sequence immediately before the main acquisition sequence is It is different from the embodiment.

  FIG. 16 is a flowchart illustrating an operation example of the magnetic resonance imaging apparatus 1 according to the second embodiment. The difference from the flowchart (FIG. 3) of the first embodiment is that step ST200 for determining whether or not the imaging section is changed and steps ST202 to ST206 are newly added. The other processes are the same and are given the same reference numerals as in FIG.

  In step ST200 of FIG. 16, it is determined whether to change the imaging section. For example, when imaging the left ventricle with a plurality of short-axis cross-sections (see FIG. 4), data collection from segment 1 to segment M of one short-axis cross-section is completed, and the next short-axis cross-section is changed. When the determination at step 200 is YES.

  If this determination is YES, the dummy sequence immediately after the last main collection sequence of the current short-axis section (the main acquisition sequence of segment M) is not the current short-axis section but the short-axis section to be changed. Run.

  FIGS. 17 and 18 are diagrams for explaining the operation of the second embodiment when the imaging section is systematically changed from the short-axis section 1 to the short-axis section 2. 17 and 18, the final main acquisition sequence (the main acquisition sequence of the segment M) of the short-axis cross section 1 is executed for the slice 1-M, and then the main acquisition sequence (the segment of the short-axis cross section 2). 1 shows an example in which (1 main acquisition sequence) is executed for slice 2-1.

In this case, the center-of-gravity position β of the projected cross-sectional image 1-M is calculated from the collection data of the dummy B immediately before the main acquisition sequence of the slice 1-M (short-axis cross section 1). On the other hand, the center of gravity position γ of the dummy C immediately after the main acquisition sequence of the slice 1-M (short-axis cross section 1) is calculated from the projected cross-sectional image 2-1 of the slice 2-1 (short-axis cross section 2). Here, as shown in the right side of FIG. 17 and the middle part of FIG. 18, the difference (γ−β) between the center of gravity position γ of the projected cross-sectional image 2-1 and the previously determined center of gravity position β is such as respiration. This is the sum of the amount of movement S due to movement and the planned amount of displacement Δ when changing from the short-axis cross section 1 to the short-axis cross section 2. Therefore, the motion correction amount C _βγ (= (γ−β)) considering the planned position change is calculated by subtracting the center of gravity position β of the dummy B and the center of gravity position γ of the dummy C (FIG. 16). Step ST204).

The position of the slice 2-1 in the main collection sequence of the short-axis cross section 2 is expressed by the following equation as shown in the lower part of FIG.
Position of slice 2-1 = position of slice 1-M + motion correction amount C _βγ (= (γ−β)),
Is corrected and updated by (step ST206).

  In addition, after changing from the short-axis cross section 1 to the short-axis cross section 2, the operation | movement which continuously images the short-axis cross section 2 after a change is the same as 1st Embodiment.

  When imaging is performed so as to cover the entire left ventricle with a plurality of short-axis cross sections (when multi-slice 2D cine imaging is performed), the imaging time becomes long, but according to the magnetic resonance imaging apparatus 1 according to the second embodiment, the slice Even at the time of change, since movement due to respiration can be corrected, continuous imaging with free breathing becomes possible. In addition, the technical effects obtained by the magnetic resonance imaging apparatus 1 of the first embodiment can be similarly obtained in the second embodiment.

  The modification (1) to modification (5) of the first embodiment can also be applied to the second embodiment. In this case, the modification (1) to modification (5) are obtained. The technical effect can also be enjoyed in the second embodiment.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and the equivalents thereof.

1 magnetic resonance imaging apparatus 40 processor 42 input unit 43 display unit 400 console 410 imaging condition setting unit 420 reconstruction unit 430 correction amount calculation unit 440 slice position update unit 500 data collection unit

Claims (11)

The subject alternately includes a main acquisition sequence for acquiring diagnostic signals from a predetermined slice cross section in the heart using a plurality of excitation pulses and a dummy sequence for stabilizing the diagnostic signals using a plurality of dummy pulses. An imaging condition setting unit for setting imaging conditions to be applied to
In the main acquisition sequence, a predetermined slice cross section in the heart is excited by the excitation pulse to collect data of the slice cross section, and in the dummy sequence, the same slice cross section as the main acquisition sequence is excited by the dummy pulse. On the other hand, a data collection unit that collects data of a projected cross section obtained by projecting the slice cross section in a cross section direction different from the slice cross section;
Reconstructing the data collected in the main acquisition sequence to generate an image of the slice cross section, and reconstructing the data acquired in the dummy sequence to generate the image of the projected cross section;
When a dummy sequence immediately before the main acquisition sequence is a first dummy sequence and a dummy sequence immediately after the main acquisition sequence is a second dummy sequence, from a projection cross-sectional image generated by the first dummy sequence While obtaining the first centroid position, the second centroid position is obtained from the projected slice image generated by the second dummy sequence, and the motion correction amount of the slice section is calculated from the first and second centroid positions. A correction amount calculation unit to perform,
Using the motion correction amount, a slice position update unit that updates the position of the slice section of the main acquisition sequence performed immediately after the second dummy sequence;
A magnetic resonance imaging apparatus comprising: The acquisition sequence is a sequence in which the slice cross-section is imaged in two dimensions with respect to a plurality of cardiac phases under electrocardiographic synchronization,
The dummy sequence is a sequence for collecting data of the projected cross section for each heartbeat.
The magnetic resonance imaging apparatus according to claim 1. The data collection unit collects data necessary for reconstructing the slice cross-sectional image by dividing the data into a plurality of heartbeats using a segment method.
The magnetic resonance imaging apparatus according to claim 1 or 2. When the main collection sequence between the first dummy sequence and the second dummy sequence is a first main collection sequence, and the main collection sequence immediately after the second dummy sequence is a second main collection sequence ,
The data collection unit collects data of the second main collection sequence for the slice position updated by the slice position update unit;
The magnetic resonance imaging apparatus according to claim 1, wherein the magnetic resonance imaging apparatus is a magnetic resonance imaging apparatus. When the slice cross section targeted by the first main acquisition sequence and the slice cross section targeted by the second main acquisition sequence are planned as the same cross section, the slice position update unit The dummy sequence collects projection cross-section data obtained by projecting the slice cross-section set for the first main acquisition sequence.
The magnetic resonance imaging apparatus according to claim 4. The slice position update unit is planned as a cross section in which the slice cross section targeted by the first main acquisition sequence and the slice cross section targeted by the second main acquisition sequence differ by a predetermined displacement amount. In this case, in the second dummy sequence, the slice cross section collected in the first main collection sequence is displaced by the predetermined displacement amount, and the data of the projected cross section obtained by projecting the slice cross section after the displacement is collected.
The magnetic resonance imaging apparatus according to claim 4. The projection cross section of the dummy sequence is a cross section parallel to the head and foot direction of the subject, and the readout direction of the dummy sequence is the head and foot direction.
The magnetic resonance imaging apparatus according to claim 1, wherein the magnetic resonance imaging apparatus is a magnetic resonance imaging apparatus. The slice position update unit collects slice cross-section data of the main acquisition sequence only when the motion correction amount is within a predetermined threshold range.
The magnetic resonance imaging apparatus according to claim 1, wherein the magnetic resonance imaging apparatus is a magnetic resonance imaging apparatus. The slice position update unit smoothly updates the position of the slice cross section of the acquisition sequence stepwise between the immediately preceding dummy sequence and the immediately following dummy sequence using the motion correction amount.
The magnetic resonance imaging apparatus according to claim 1, wherein the magnetic resonance imaging apparatus is a magnetic resonance imaging apparatus. The main acquisition sequence starts after the ECG synchronization signal and ends after a predetermined duration, and the dummy sequence starts after the end of the main acquisition sequence and is predetermined before the arrival of the next ECG synchronization signal. If the predetermined amount of data is collected, the dummy sequence is terminated at that time. If the predetermined amount of data is not collected even when the next ECG synchronization signal arrives, the predetermined amount of data is collected. Continue the dummy sequence until
The magnetic resonance imaging apparatus according to claim 1, wherein the magnetic resonance imaging apparatus is a magnetic resonance imaging apparatus. The dummy sequence starts after the ECG synchronization signal and ends after collecting a predetermined amount of data, and the main acquisition sequence starts after the dummy sequence ends and ends when the next ECG synchronization signal arrives. ,
The magnetic resonance imaging apparatus according to claim 1, wherein the magnetic resonance imaging apparatus is a magnetic resonance imaging apparatus.

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