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WO2018020905A1 - Dispositif d'imagerie par résonance magnétique et son procédé de commande - Google Patents

Dispositif d'imagerie par résonance magnétique et son procédé de commande Download PDF

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Publication number
WO2018020905A1
WO2018020905A1 PCT/JP2017/022632 JP2017022632W WO2018020905A1 WO 2018020905 A1 WO2018020905 A1 WO 2018020905A1 JP 2017022632 W JP2017022632 W JP 2017022632W WO 2018020905 A1 WO2018020905 A1 WO 2018020905A1
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pulse
correction
echo
magnetic field
gradient magnetic
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Japanese (ja)
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光 花田
直哉 坂口
佐藤　善隆
慧祐 西尾
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Hitachi Ltd
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Hitachi Ltd
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    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/05—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves
    • A61B5/055—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
    • G01R33/48—NMR imaging systems

Definitions

  • the present invention measures a nuclear magnetic resonance signal (hereinafter referred to as an NMR signal) from hydrogen, phosphorus, etc. in a subject and images a nuclear density distribution, relaxation time distribution, etc. (hereinafter referred to as MRI). It is related with the measurement technique of a diffusion weighted image especially.
  • an NMR signal nuclear magnetic resonance signal
  • MRI nuclear density distribution, relaxation time distribution, etc.
  • a diffusion weighted image is an image in which the diffusion motion of water molecules is reflected in the contrast of an image by applying a gradient magnetic field pulse called MPG: Motion Probing Gradient.
  • Two MPG pulses are applied before and after the 180 ° RF pulse as the excitation RF pulse.
  • the two MPG pulses have different polarities, but the applied amount (area) is the same. For this reason, for spins (stationary spins) whose spatial position has not moved, the phase changed by the MPG pulse applied first is returned by the MPG pulse applied later. On the other hand, the spin whose spatial position has moved in the direction of application of the MPG pulse does not match the amount of phase change produced by each MPG pulse.
  • the echo signal attenuates due to the phase mismatch, and the portion of the moving spin, for example, blood flow, is expressed as a low signal on the image.
  • Diffusion-weighted imaging is a technique for imaging diffusion motion using the contrast difference between stationary spins and moving spins.
  • the extended enhancement imaging is intended to visualize the diffusion motion generated between the two MPG pulses, but there is a problem that the motion other than the diffusion motion is also reflected in the contrast of the image.
  • the measurement target is a human body
  • physiological movements such as pulsation and respiration become a problem. Since such physiological movement is generally not an observation target, it is recognized as degradation of diffusion weighted image quality.
  • Patent Document 1 there is a method of measuring a phase error map generated in spin in real time and correcting an echo measured using this phase error map.
  • a gradient magnetic field that scans a 2D or 3D space after applying an MPG pulse is applied, and a 2D or 3D phase error map is measured.
  • an RF pulse and a gradient magnetic field for canceling the phase error are calculated, and irradiation and application are performed before the echo signal is measured.
  • Non-Patent Document 1 a gradient magnetic field pulse is applied in the direction in the image plane after applying the MPG pulse, that is, the frequency encoding direction and the phase encoding direction, and the 0th and 1st order calculated from the obtained echo signal.
  • the above method has the following problems.
  • the time required for measurement is long because the phase error map is measured in 2D or 3D.
  • measurement in 3D takes a long time. Since the correction using the obtained phase error map needs to be performed before the echo signal measurement, extending the measurement time of the phase error map leads to the extension of TE: Echo Time of the echo signal, resulting in the SNR of the image Decreases.
  • the phase error map is measured in 2D, the measurement time is shorter than in 3D, but it cannot be corrected because the influence of motion cannot be observed for the axes that are not measured.
  • the influence of movement in the Z-axis direction cannot be corrected.
  • an RF pulse is used to correct a phase error obtained from a phase error map.
  • the Fourier transform or the Bloch equation needs to be solved when the flip angle is large, and the processing cost is high.
  • the time for applying the RF pulse that cancels the calculated phase error is also in the order of several tens of ms.
  • the TE extension is preferably about 10 ms or less, depending on the measurement conditions and the MRI apparatus.
  • Non-Patent Document 1 can be executed in a shorter processing time compared with the case of creating a phase error map in order to correct the influence of physiological movement during application of MPG with only gradient magnetic field pulses.
  • the MPG pulse application axis is set in a direction parallel to the slice direction, and phase error correction is performed by navigator echoes in two directions orthogonal thereto, so that physiological movement in the slice direction is performed. The effect of remains uncorrected.
  • the phase error due to physiological movement occurs not only in the two directions in the cross section but also in the slice direction, but the phase error in the slice direction has a large effect on the SNR of the image, and it is not possible to correct it after acquiring the echo signal. Can not.
  • diffusion weighted imaging imaging is performed with different MPG pulse application axes, and it is impossible to find a regular relationship between the direction in which the phase error is likely to occur and the application axis of the MPG pulse. There is a limit to the method of Non-Patent Document 1 on the assumption that it is performed only in two directions within the cross section.
  • the present invention has been made in view of the above problems, and in an MRI apparatus that performs imaging including application of an MPG pulse, an MRI apparatus capable of obtaining a high-quality diffusion-weighted image in real time while suppressing the extension of TE
  • the purpose is to provide.
  • the MRI apparatus of the present invention has a shift amount in the k space of the echo signal after application of the MPG pulse at least in the slice selective gradient magnetic field direction (until the measurement of the echo signal after application of the MPG pulse). (Hereinafter, referred to as a slice direction), means for measuring in an axial direction, means for calculating a gradient magnetic field pulse necessary for correcting the shift amount, and means for applying the correction gradient magnetic field pulse.
  • the MRI apparatus of the present invention performs an operation using an imaging unit that collects echo signals, a sequence control unit that controls the imaging unit, and echo signals collected by the imaging unit according to a predetermined pulse sequence.
  • the pulse sequence includes MPG pulse application between excitation RF pulse application and echo signal acquisition.
  • the sequence control unit performs control to add a correction sequence including readout of navigation echoes and application of a correction gradient magnetic field at least in the slice direction between the MPG application and the echo signal collection, and the calculation unit includes the correction
  • the application amount of the correction gradient magnetic field is calculated using the navigation echo, and the calculated correction gradient magnetic field application amount is passed to the sequence control unit.
  • a correction pulse calculation unit is provided, and the sequence control unit applies the correction gradient magnetic field with the application amount of the correction gradient magnetic field received from the correction pulse calculation unit.
  • the shift amount in the k space of the echo signal after the MPG application including the slice direction is calculated, and a gradient magnetic field for correcting the shift amount based on the value is applied, thereby providing a physiological Changes in image contrast due to global motion that occur in the imaging target due to motion can be suppressed, and a series of these processes can be performed in a shorter time than in the prior art.
  • an extension of TE that is, a decrease in SNR can be suppressed, and an image that allows easy observation of local water molecule diffusion motion, which is the original observation target, can be provided.
  • FIG. 1 is a block diagram showing the overall configuration of an embodiment of an MRI apparatus according to the present invention.
  • the figure which shows an example of a general DWI pulse sequence.
  • the figure which shows the structure of the sequencer of 1st embodiment, and a calculation part, and the outline
  • the figure which shows an example of the DWI pulse sequence containing a correction sequence.
  • the flowchart which shows a part of process of the calculating part of 1st embodiment.
  • FIG. 5 is a diagram showing details of the correction sequence of FIG.
  • FIG. 5 is a diagram showing details of the correction sequence of FIG.
  • the MRI apparatus uses a NMR phenomenon to obtain a tomographic image of a subject, and as shown in FIG. 1, a static magnetic field generator 2, a gradient magnetic field generator 3, a transmitter 5, and a receiver 6 A signal processing unit 7 and a sequencer 4.
  • the static magnetic field generation unit 2, the gradient magnetic field generation unit 3, the transmission unit 5, and the reception unit 6 are collectively referred to as an imaging unit.
  • the static magnetic field generation unit 2 includes a permanent magnet type, normal conduction type or superconducting type static magnetic field generation source arranged around the subject 1.
  • the vertical magnetic field type static magnetic field generation source generates a uniform static magnetic field in the direction of the body axis in a direction perpendicular to the body axis in the space around the subject 1 and the horizontal magnetic field type static magnetic field generation source.
  • 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 supply 10 that drives each gradient magnetic field coil
  • the gradient magnetic field is applied in the three axis directions of X, Y, and Z by driving the gradient magnetic field power supply 10 of each coil in accordance with a command from the sequencer 4 described later.
  • a gradient magnetic field in an arbitrary direction can be formed by a combination of gradient magnetic fields in three axial directions.
  • a slice direction gradient magnetic field pulse is applied in a direction orthogonal to the slice plane (imaging cross section) to set a slice plane for the subject 1, and in the remaining two directions orthogonal to the slice plane and orthogonal to each other
  • a phase encoding direction gradient magnetic field pulse and a frequency encoding direction gradient magnetic field pulse are applied, and position information in each direction is encoded in the echo signal. Further, by applying a gradient magnetic field pulse in a predetermined direction, a primary phase change can be given to the spins constituting the tissue of the subject 1 along the application direction.
  • the transmitter 5 irradiates the subject 1 with a high-frequency magnetic field pulse (referred to as an RF pulse) in order to cause nuclear magnetic resonance to occur in the nuclear spins of the atoms constituting the living tissue of the subject 1.
  • a high-frequency magnetic field pulse (referred to as an RF pulse) in order to cause nuclear magnetic resonance to occur in the nuclear spins of the atoms constituting the living tissue of the subject 1.
  • It comprises a modulator 12, a high frequency amplifier 13, and a high frequency coil (transmission coil) 14a on the transmission side.
  • the high-frequency magnetic field pulse output from the high-frequency oscillator 11 is amplitude-modulated by the modulator 12 at a timing according to a command from the sequencer 4, and the amplitude-modulated high-frequency pulse is amplified by the high-frequency amplifier 13 and then placed close to the subject 1.
  • the high-frequency coil 14a the subject 1 is irradiated with the RF pulse.
  • the receiving unit 6 detects an echo signal (NMR signal) emitted by nuclear magnetic resonance of nuclear spins constituting the biological tissue of the subject 1, and includes a receiving-side high-frequency coil (receiving coil) 14b, a signal amplifier 15, a quadrature detector 16, and an A / D converter 17.
  • NMR signal an echo signal
  • the quadrature phase detector 16 divides the signal into two orthogonal signals at a timing according to a command from the sequencer 4, and each signal is converted into a digital quantity by the A / D converter 17 and sent to the signal processing unit 7.
  • the signal processing unit 7 performs various data processing and display and storage of processing results.
  • the signal processing unit 7 performs various operations and controls, an external storage device such as an optical disk 19 and a magnetic disk 18, and a ROM 21. And an internal storage medium such as a RAM 22 and a display 20.
  • the digital signal processing device 8 executes processing such as signal processing and image reconstruction, and displays a tomographic image of the subject 1 as a result of the display 20 And recorded on the magnetic disk 18 or the like of the external storage device.
  • Processing such as computation and control performed by the digital signal processing device 8 may be realized by a CPU and software mounted thereon, or a part thereof may be realized by hardware such as an ASIC or FPGA.
  • the signal processing unit 7 is provided with an operation unit 25 for inputting various control information of the MRI apparatus and control information of processing performed by the signal processing unit 7.
  • the operation unit 25 includes a trackball or mouse 23, a keyboard 24, and the like.
  • the operation unit 25 is arranged in the vicinity of the display 20, and an operator controls various processes of the MRI apparatus interactively through the operation unit 25 while looking at the display 20.
  • the sequencer 4 is a control means (sequence control unit) that repeatedly applies RF pulses and gradient magnetic field pulses in a predetermined pulse sequence, operates under the control of the digital signal processing device 8, and is necessary for collecting tomographic image data of the subject 1. These various commands are sent to the imaging unit (transmitting unit 5, gradient magnetic field generating unit 3, and receiving unit 6).
  • the pulse sequence is a timing chart that determines the intensity and timing of application of an RF pulse or gradient magnetic field pulse, the timing of collecting NMR signals (echo signals), and the like, and there are various pulse sequences depending on the imaging method. These pulse sequences are stored in advance in the storage device of the signal processing unit 7 as a program, and are executed by the sequencer 4 reading a predetermined pulse sequence and imaging parameters.
  • the MRI apparatus of the present embodiment executes a diffusion weighted imaging (DWI) pulse sequence using an MPG pulse as a pulse sequence.
  • DWI diffusion weighted imaging
  • FIG. 2 shows an example of a typical DWI pulse sequence.
  • this DWI pulse sequence 200 first, an RF pulse 201 is irradiated, and a spin at a specific position determined by a slice selective gradient magnetic field (Gs) applied simultaneously is excited. Thereafter, the first MPG pulse 203 is applied.
  • Gs slice selective gradient magnetic field
  • the MPG pulse is applied to the axis (Gs) of the slice selective gradient magnetic field pulse, but it may be applied to another axis or a plurality of axes.
  • an RF pulse 202 for inverting the spin phase is irradiated.
  • a second MPG pulse 204 is applied.
  • the second MPG pulse 204 has the same area (applied amount) as the first MPG pulse 203, and is applied after the inverted RF pulse 202. Therefore, the second MPG pulse 204 has a phase rotation opposite to that of the first MPG pulse 203. give.
  • the phase rotation given by the first MPG pulse 203 is rewound by the second MPG pulse 204.
  • the phase encode gradient magnetic field pulse 205 is applied, and the echo 207 is measured while the readout gradient magnetic field pulse 206 is applied.
  • a contrast difference is generated between water molecules that are in a diffusing motion and stationary spins, and the diffusing motion is imaged.
  • the figure shows the case of single shot imaging that measures only one echo, but there are also cases where multiple echoes are measured while repeating the application of phase encoding gradient magnetic field pulses and readout gradient magnetic field pulses (multi-shot imaging). is there.
  • the MRI apparatus of the present embodiment when imaged using such a pulse sequence including an MPG pulse, performs movements such as pulsation and breathing of the subject that are different from the diffusion movement of water molecules (these diffusion movement and In order to suppress the change in the contrast of the image due to (otherwise referred to as global motion), it is characterized by having a global motion correction processing function. Specifically, an excess or deficiency of the gradient magnetic field application amount caused by the global motion when the first MPG pulse 203 is applied and when the second MPG pulse 204 is applied is detected and corrected in real time.
  • the imaging unit generates a navigation echo (hereinafter referred to as a navigation echo) at least in the slice direction between the application of the MPG pulse of the DWI pulse sequence and the echo acquisition, and the phase calculated from the navigation echo Imaging by adding a sequence (correction sequence) that applies a gradient magnetic field (corrected gradient magnetic field pulse) corresponding to the error, and the digital signal processing device 8 (calculation unit) measures during execution of the sequence
  • a navigation echo a navigation echo
  • the digital signal processing device 8 calculation unit
  • the MRI apparatus of this embodiment performs the above-described series of processing in real time. Therefore, a real-time system is adopted for the sequencer 4, the transmission unit 5, the reception unit 6, and the signal processing unit 7.
  • the sequencer 4 and the signal processing unit 7 are equipped with an RTOS (Real-time operation system), and the transmission unit 5 and the reception unit 6 can be configured with FPGA (Field-Problemmable Gate-Array). it can.
  • the digital signal processing device 8 includes a correction pulse calculation unit 81 for calculating a phase error.
  • the correction pulse calculation unit 81 can be realized by software operating on the RTOS or dedicated hardware.
  • the digital signal processing device 8 includes an image reconstruction unit that reconstructs an image using an echo signal and a calculation unit that performs a calculation for a diffusion weighted image, but the illustration is omitted here.
  • Sequencer 4 calculates and executes a pulse sequence when a DWI pulse sequence and its imaging parameters are set via operation unit 25.
  • a time shortest time that takes into account the time required for the correction sequence is set.
  • the DWI pulse sequence is, for example, a DWI pulse sequence 200 including two MPG pulses 203 and 204 as shown in FIG.
  • the sequencer 4 further includes a correction sequence 400 that is inserted into the DWI pulse sequence 200 and executed.
  • the correction sequence 400 includes application of a navigation echo readout gradient magnetic field and a correction pulse.
  • a navigation echo readout gradient magnetic field application process 301 is executed.
  • the navigation echo readout gradient magnetic field is a readout gradient magnetic field applied to measure the movement of the subject 1 during application of the MPG pulse after application of the MPG pulse, and is applied at least in the slice direction after application of the MPG pulse.
  • An echo signal (navigation echo) generated by the navigation echo readout gradient magnetic field application processing 301 is passed to the correction pulse calculation unit 81 of the digital signal processing device 8 via the reception unit 6.
  • the correction pulse calculation unit 81 uses the navigation echo to calculate the excess / deficiency amount of the MPG pulse caused by the movement of the subject 1, and the gradient magnetic field application amount (correction pulse application amount) necessary to correct the same, The calculation result is notified to the sequencer 4 (correction pulse calculation processing 302).
  • the sequencer 4 executes the correction pulse application process 303 which is the second half of the correction sequence 400 according to the correction pulse application amount calculated by the correction pulse calculation unit 81. That is, the correction gradient magnetic field pulse is applied coaxially with the navigation echo at the calculated correction pulse application amount. Thereafter, the rest of the DWI pulse sequence 200 is executed to collect echoes 207 for DWI.
  • Fig. 4 shows an example of the DWI pulse sequence 200A added by the correction sequence executed by the sequencer 4.
  • the same elements as those in FIG. 2 are denoted by the same reference numerals, and redundant description is omitted.
  • a correction sequence 400 for correcting the excess / deficiency of MPG is inserted after the application of the second MPG pulse 204 and before the collection of the echo 207.
  • the navigation echo readout gradient magnetic field is sequentially applied in the slice gradient magnetic field (Gs) direction, the phase encode gradient magnetic field (Gp) direction, and the readout gradient magnetic field (Gr) direction, and the navigation echo 401 of each axis is applied.
  • a correction gradient magnetic field pulse 403 for phase error correction is applied after a predetermined time from the acquisition of the last navigation echo 401 (correction pulse application processing 303).
  • a time 402 from acquisition of the navigation echo 401 to the correction gradient magnetic field pulse 403 is a time required for the correction pulse calculation processing 302, and the correction gradient magnetic field pulse 403 is applied with the application amount calculated in the correction pulse calculation processing 302. .
  • phase encoding pulse 205 and the readout gradient magnetic field pulse 206 after the application of the correction gradient magnetic field pulse 403 to measure the echo 207 is the same as the pulse sequence 200 of FIG.
  • the correction pulse calculation unit 81 uses the navigation echo read in the navigation echo reading gradient magnetic field application processing 301 to calculate the excess / deficiency amount of MPG pulse and the correction pulse application amount in the three directions Gs, Gp, and Gr.
  • the method for calculating the excess / deficiency of the MPG pulse differs between the Gs direction and the Gp and Gr directions. The procedure for calculating the excess / deficiency of the MPG pulse in the Gs direction will be described with reference to the flowchart of FIG.
  • phase data string Phase_scalar (x) is calculated using the real part and the imaginary part of the complex signal Navi (x). Since the phase obtained in this manner is around the main value, phase unwrapping processing is performed in processing 503. Specifically, the phase Phase_unwrapped (x) of Navi (x) is calculated according to the following equation (1).
  • Phase_unwrapped (x) is a scalar phase data
  • Phase [] is a function that returns the phase value of complex data
  • Conjugate [] is a function that represents complex conjugate processing.
  • a linear line is applied to Phase_unwrapped (x) by the least square method (formula (2)), and the slope (FirstOrderPhase) is calculated.
  • N is the number of data of Navi Echo Navi_Echo (t) (when using Equation (2) for other directions, it is the number of data in that direction. The same applies hereinafter).
  • the slope of the phase calculated in this way represents the excess / deficiency of the MPG pulse.
  • the correction pulse application amount [s ⁇ T / m] in the Gs direction is calculated according to the following equation (3).
  • Equation (3) Duration is the navigation echo sampling time [s]
  • GcAmp is the navigation echo readout gradient magnetic field [T / m].
  • the high-frequency coil (receiver coil) 14b is composed of a plurality of coils
  • the navigation echo is measured for each coil.
  • the slope of the phase is calculated according to Equation (2) for each navigation echo of each coil, and the average value is used to calculate the phase gradient according to Equation (3).
  • the correction pulse application amount is determined.
  • navigation echo Navi_Echo (t) obtained by applying a navigation echo readout gradient magnetic field in the Gp direction is read.
  • the read navigation echo Navi_Echo (t) is Fourier transformed to obtain Navi (x).
  • the phase difference between the navigation echo Navi (x) and the reference navigation echo Navi_std (x) is determined in the process 605. Calculate according to the following formula.
  • Navi_subtracted (x) is complex data obtained by phase difference between the data of the navigation echo (Navi (x)) and the data of the reference navigation echo (Navi_std (x)).
  • step 606 the phase of Navi_subtracted (x) is unwrapped using the above equation (1) to calculate the phase Phase_unwrapped (x).
  • step 607 the phase gradient (that is, the phase difference gradient) is obtained from the above equation (2), and in step 608, the correction pulse application amount in the Gp direction is calculated according to the above equation (3).
  • the difference between the processing in the Gp and Gr directions and the processing in the Gs direction is whether or not the reference navigation echo is used.
  • the correction pulse application amount is determined so that the gradient of the phase of the navigation echo becomes zero. This is because the SNR is the highest when the gradient of the phase in the Gs direction is zero, and the SNR decreases when an echo including an error caused by the movement during the application of the MPG pulse is used as a reference.
  • the correction pulse application amount is determined so as to coincide with the phase distribution of the reference navigation echo. Since the Gp and Gr directions are directions in the image plane, if the phase gradients in the Gp and Gr directions match between the multiple echo signals that make up the image, even if the phase gradient is absolute, This is because the absolute value of the image is not affected.
  • the correction pulse calculation unit 81 notifies the sequencer 4 of the excess / deficiency amount of the MPG pulse in the three directions Gs, Gp, and Gr calculated as described above, that is, the correction pulse application amount.
  • the sequencer 4 applies a gradient magnetic field pulse (FIG. 4: 403) according to the correction pulse application amount in each direction of Gs, Gp, and Gr calculated by the correction pulse calculation unit 81 (correction pulse application processing 303).
  • the application amount of the gradient magnetic field pulse is determined by the product of the application time and the magnetic field strength.
  • the sequencer 4 may fix the correction pulse application time and change the pulse intensity according to the correction pulse application amount. .
  • the gradient magnetic field applied amount caused by the change in the global position of the subject 1 (global movement) when the first MPG pulse 203 and the second MPG pulse 204 are applied Overs and shorts can be compensated. Therefore, the echo signal to be measured thereafter becomes equivalent to the echo signal measured when the MPG pulse is applied without excess or deficiency, and a diffusion-weighted image with a good SNR can be obtained.
  • the reduction in SNR is suppressed by performing the correction that makes the phase error in the slice direction that has the most influence on the SNR of the image and cannot be corrected after acquiring the echo signal zero,
  • the target diffusion movement can be depicted with high contrast.
  • by correcting the three directions including the slice direction it is possible to correct the phase error caused by global motion for all axes that cannot be measured simply by creating the 2D phase map and the technique described in Non-Patent Document 1. it can.
  • the phase error correction process does not include application of an RF pulse, and is configured only by generation of a navigation echo using a readout gradient magnetic field pulse and application of a correction gradient magnetic field pulse,
  • the correction processing time including the calculation time is short because only the echo signal of one axis is Fourier-transformed and the phase gradient is converted into the area of the gradient magnetic field. Thereby, the extension of TE can be significantly suppressed.
  • the calculation cost of the RF pulse is unnecessary, and the processing cost is low.
  • the MRI apparatus of the present embodiment can suppress the extension of TE, that is, the correction sequence can be applied to real-time imaging.
  • Fig. 7 shows the detailed sequence of the navigation echo readout gradient magnetic field. If the sampling interval of the echo signal is 1.56 ⁇ s and the number of sampling points is 64, the sampling time per axis is about 100 ⁇ s. If the readout gradient magnetic field intensity is 9 mT / m and the maximum Slew Rate of the gradient magnetic field is 100 T / m / s, the time required to measure one-axis navigation echo will be the dephasing and rephasing of the readout gradient magnetic field. It can be kept in 700 ⁇ s or less including the pulse.
  • the part where the echo signal is not sampled is overlapped with the previous MPG pulse 204 (that is, a part of the gradient magnetic field application waveform that generates the navigation echo is temporally compared with a part of the MPG pulse waveform.
  • the time of the navigation echo readout gradient magnetic field application process 301 can be further shortened.
  • the correction pulse calculation process 302 can be started without waiting for the end of the application of the readout gradient magnetic field after the echo signal has been sampled, the time can be shortened here, and the navigation echo readout gradient magnetic field application process 301 is performed. The substantial time required for this is 900 ⁇ s.
  • the correction pulse calculation processing 302 although it depends on the performance of the digital signal processing device 8, it is sufficient if the time required for calculating the excess / deficiency amount of the MPG pulse and notifying the sequencer 4 is 3 ms. Therefore, in an MRI apparatus having a general performance, it is possible to perform the reading from the navigation echo to the application of the correction pulse in about 5 ms, and in the pulse sequence of FIG. 4, the TE extension can be suppressed to 10 ms.
  • the first embodiment has been described with reference to the drawings, but this embodiment includes means (sequence control unit and correction pulse calculation unit) that performs global motion correction processing in real time at least in the slice gradient magnetic field direction.
  • means sequence control unit and correction pulse calculation unit
  • the other elements and procedures can be changed as appropriate.
  • the pulse sequence is a DWI pulse sequence including an MPG pulse
  • the pulse sequence is not limited to the pulse sequence shown in FIG.
  • the application direction of the navigation echo and the correction pulse in the global motion correction processing may not be the Gs, Gp, Gr direction, but may be the X, Y, Z direction of the device, or correction is applied by applying in three or more directions. May be performed.
  • the present embodiment also includes a case where a navigation echo is acquired only in the Gs direction and a correction pulse is applied.
  • the reference navigation echo used for the phase correction in the Gp and Gr directions may be a data string prepared in advance instead of the first echo in the measurement.
  • a data string is pre-scanned by applying an RF pulse and a read gradient magnetic field in the Gp and Gr directions to collect echoes, and in each of the Gp and Gr directions.
  • the echo is used as a data string of the reference navigation echo.
  • the MPG pulse may or may not be used.
  • the phase gradient can be made uniform in the imaging plane, as in the case of using the first echo in the measurement.
  • an echo acquired without using an MPG pulse is used as a reference, it is possible to perform correction so that the phase gradient itself caused by the MPG pulse is zero.
  • ⁇ Second embodiment> In the first embodiment, real-time phase correction is performed for the three directions of Gs, Gp, and Gr. However, in this embodiment, real-time phase correction is performed only for the Gs direction, and correction pulses are used for the Gp and Gr directions. The echo signal is corrected afterwards using the calculated correction amount. Specifically, the phase correction amount is reflected on the k-space arrangement coordinates of the echo signal in the image reconstruction.
  • the digital signal processing apparatus 8 includes a data correction unit 82 in addition to the correction pulse calculation unit 81.
  • the process of this embodiment is demonstrated centering on a different point from 1st embodiment, using the drawing used in 1st embodiment suitably.
  • the pulse sequence 200A as shown in FIG. 4 is executed, and the processes 501 to 505 in FIG. 5 are performed in the Gs direction.
  • the processing from 605 to 607 shown in FIG. 6 is performed to calculate the phase gradient with respect to the reference navigation echo.
  • the data correction unit 82 reads the inclination of the phase and calculates the correction value of the k-space arrangement coordinates of the echo signal 207.
  • the correction value ⁇ k is calculated according to the following equation from the slope (FirstOrderPhase) of the primary straight line calculated by equation (2) in processing 607.
  • the digital signal processing device 8 reads the echo signal 207 collected by the pulse sequence 200A and arranges it in the k space. At this time, the correction value ⁇ k calculated by the data correction unit 82 is added to the k-space arrangement coordinates of the echo signal. As a result, the k-space arrangement is substantially equal to the k-space arrangement of the echo signal 207 that has been subjected to phase correction in the Gp and Gr directions before acquisition. By reconstructing an image using the corrected k-space data, the influence of the phase error of the MPG pulse can be suppressed even in a cross section parallel to the slice plane.
  • the data correction unit 82 phase-differs the zeroth-order offset calculated by the equation (6) from the echo signal Echo (t) used for image reconstruction according to the following equation (7).
  • Equation (7)
  • represents the amplitude of the echo signal Echo (t), which is a complex data string, and ⁇ (t) represents the phase of the echo signal Echo (t). I is an imaginary unit.
  • correction for making the phase error due to the MPG pulse zero is also performed in the Gp and Gr directions, and thus a diffusion-weighted image with higher image quality (SNR / contrast) can be obtained.
  • ⁇ Third embodiment> In the first embodiment, in calculating the excess / deficiency of the MPG pulse, processing in the Gs direction is performed without using the reference echo, and processing in the Gp and Gr directions is based on the echo measured in the first iteration, and In this embodiment, the processing for acquiring the reference echo group in the pulse sequence and the MPG pulse excess / deficiency calculation process using these reference echo group are added to the Gs. It is the feature that it applies also about a direction.
  • FIG. 10 is a block diagram showing an overview of the processing of this embodiment.
  • the MRI apparatus of this embodiment has a reference navigation echo read gradient magnetic field application process 901 added to the process of the sequencer 4.
  • the correction pulse calculation process 902 in the digital signal processing device 8 is a process using the reference navigation echo and the navigation echo.
  • a correction sequence 400 is added to the latter half of the TE, so that the first half of the TE has a space corresponding to the time of the correction sequence. Time occurs.
  • the reference navigation echoes are collected using the idle time. Specifically, between the RF pulse 201 and the first MPG pulse 203, a process 901 (reference navi echo read gradient magnetic field application process) that sequentially reads and applies a gradient magnetic field in the three directions Gs, Gp, and Gr ) Is executed. The rest of the sequence is the same as in FIG. 4, and the navigation echo read gradient magnetic field application process 301 is executed.
  • the digital signal processing device 8 (correction pulse calculation unit 81) is generated by the reference navigation echo readout gradient magnetic field application processing 901 and received by the reception unit 6, and the navigation echo readout gradient magnetic field application processing 301 executed in the second half of the pulse sequence.
  • the excess / deficiency amount of the MPG pulse is calculated (process 902) using the echo signal obtained in the above.
  • the processing 902 of the correction pulse calculation unit 81 will be described with reference to the flow of FIG. In FIG. 12, the same processes as those in FIG. 6 are denoted by the same reference numerals.
  • step 1001 the reference navigation echo Base_Navi_Echo (t) acquired in step 901 is read.
  • step 601 the reference navigation echo Base_Navi_Echo (t) and the navigation echo Navi_Echo (t) are Fourier transformed in the process 1002 and the process 602, respectively, and Base_Navi (x) and Navi (x) )
  • step 605 the phase difference between the reference navigation echo Base_Navi_Echo (t) and the navigation echo Navi (x) is calculated.
  • the phase gradient is calculated according to the equation (2) for each navigation echo of each coil, and the average value is used to calculate the equation ( 3) Determine the correction pulse application amount.
  • the phase distribution given to the navigation echo depending on the shape and position of the high-frequency coil (reception coil) 14b is also removed by calculating the phase difference from the reference echo. It is possible to calculate an error caused only by movement during pulse application.
  • the reference echo used in the first embodiment is an echo including an error caused by the movement during application of the MPG pulse, and is not used because it leads to a decrease in SNR in the Gs direction.
  • an echo that does not include an error caused by a motion during the application of the MPG pulse is obtained as a reference by being acquired before the application of the MPG pulse, it can also be used for processing in the Gs direction. Therefore, in this embodiment, it is not necessary to distinguish the Gs direction from the Gr and Gp directions.
  • the readout gradient magnetic field used for acquiring the reference navigation echo has the same shape as the readout gradient magnetic field for acquiring the navigation echo applied in the execution section of the correction sequence 400, and therefore the pulse is not extended. A sequence can be performed.
  • the Gr and Gp directions can be corrected by using the slope of the phase calculated in the process 607 to correct the k-space arrangement coordinates of the echo signal 207 afterwards.
  • the zero-order offset may be calculated together with the phase gradient in the process 607, and the phase of the echo signal may be corrected using the zeroth-order offset.
  • the modified example described in the first embodiment can be adopted in this embodiment.
  • 2 static magnetic field generation unit 3 gradient magnetic field generation unit (imaging unit), 4 sequencer (sequence control unit), 5 transmission unit (imaging unit), 6 reception unit (imaging unit), 7 signal processing unit, 8 digital signal processing device (Calculation unit), 81 Correction pulse calculation unit, 82 Data correction unit, 200 (200A, 200B) DWI pulse sequence, 400 correction sequence

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Abstract

La présente invention concerne un dispositif d'IRM qui exécute une imagerie entraînant l'application d'une impulsion de GMI, l'extension de TE étant supprimée pour obtenir les images pondérées de diffusion de haute qualité en temps réel. Ledit dispositif d'imagerie par résonance magnétique, qui applique une impulsion de GMI après l'application d'une impulsion RF d'excitation et qui recueille des signaux d'écho pendant un laps de temps prescrit après l'impulsion RF d'excitation, pendant la période après l'application de l'impulsion de GMI jusqu'à la collecte des signaux d'écho, au moins des échos de navigation dans le sens de la tranche sont acquis, une quantité de correction de phase est calculée à partir des échos de navigation acquis et, sur la base de la quantité de correction de phase calculée, un champ magnétique à gradient de correction est appliqué qui corrige l'erreur de phase comprise dans l'impulsion de GMI.
PCT/JP2017/022632 2016-07-27 2017-06-20 Dispositif d'imagerie par résonance magnétique et son procédé de commande Ceased WO2018020905A1 (fr)

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JP2021183031A (ja) * 2020-05-21 2021-12-02 株式会社日立製作所 磁気共鳴撮像装置

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JPH09299345A (ja) * 1996-05-09 1997-11-25 Ge Yokogawa Medical Syst Ltd 拡散強調イメージング方法およびｍｒｉ装置
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Publication number Priority date Publication date Assignee Title
CN112704484A (zh) * 2019-10-25 2021-04-27 通用电气精准医疗有限责任公司 磁共振成像方法及系统，非暂态计算机可读存储介质
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JP7539255B2 (ja) 2020-05-21 2024-08-23 富士フイルムヘルスケア株式会社 磁気共鳴撮像装置

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