WO2018020905A1

WO2018020905A1 - Magnetic resonance imaging device and control method thereof

Info

Publication number: WO2018020905A1
Application number: PCT/JP2017/022632
Authority: WO
Inventors: 光花田; 直哉坂口; 佐藤　善隆; 慧祐西尾
Original assignee: 株式会社日立製作所
Priority date: 2016-07-27
Filing date: 2017-06-20
Publication date: 2018-02-01
Also published as: JP2018015196A; JP6718764B2

Abstract

In this MRI device, which performs imaging involving applying an MPG pulse, extension of TE is suppressed to obtain high-quality diffusion weighted images in real time. To this end, in this magnetic resonance imaging device, which applies an MPG pulse after applying an excitation RF pulse and collects echo signals from a prescribed time after the excitation RF pulse, during the period after applying the MPG pulse up to collection of the echo signals, at least navigation echoes in the slice direction are acquired, a phase correction amount is calculated from the acquired navigation echoes, and, on the basis of the calculated phase correction amount, a corrective gradient magnetic field is applied which corrects the phase error included in the MPG pulse.

Description

Magnetic resonance imaging apparatus and control method thereof

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.

One of the images obtained with an MRI apparatus is a diffusion weighted image (DWI). 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.

As described above, 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. For example, when 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.

As one method for solving this problem, 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 (Patent Document 1). In this method, 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. Using the measured phase error map, 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.

As another similar method, 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. There has been proposed a method of compensating for the zeroth-order and first-order phase errors by applying a gradient magnetic field pulse and a compensation B0 pulse in accordance with the phase error (Non-Patent Document 1).

US Patent Application Publication No. 2013c0229177 Specification

However, the above method has the following problems. First, in the method of Patent Document 1, the time required for measurement is long because the phase error map is measured in 2D or 3D. In particular, 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. When 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.

Specifically, when measuring the phase error map on the XY plane, the influence of movement in the Z-axis direction cannot be corrected. In the method of Patent Document 1, an RF pulse is used to correct a phase error obtained from a phase error map. In the calculation of the RF pulse for canceling the phase error and the gradient magnetic field corresponding thereto, the Fourier transform or the Bloch equation needs to be solved when the flip angle is large, and the processing cost is high. In addition, the time for applying the RF pulse that cancels the calculated phase error is also in the order of several tens of ms. Considering the decrease in SNR, the TE extension is preferably about 10 ms or less, depending on the measurement conditions and the MRI apparatus.

Next, the method of 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. However, in the method of Non-Patent Document 1, 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. In general, 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. Also, in 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.

In order to solve the above-described problems, 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.

Specifically, 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 Between the readout of the navigation echo and the application of the correction gradient magnetic field in a sequence, 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.

According to the present invention, 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. As a result, 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.

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 | summary of a process. 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. 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 figure which shows the structure and the outline | summary of a process of the sequencer of 2nd embodiment, and a calculating part. The figure which shows the structure of the sequencer of 3rd embodiment, and a calculating part, and the outline | summary of a process. The figure which shows an example of the DWI pulse sequence containing the correction sequence used by 3rd embodiment. The flowchart which shows the process of the calculating part of 3rd embodiment.

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

First, an overall outline of an example of an MRI apparatus to which the present invention is applied will be described with reference to 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. In this specification, 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. At the time of imaging, 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. 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. By supplying 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. After the NMR signal of the response of the subject 1 induced by the electromagnetic wave irradiated from the high frequency coil 14a on the transmission side is detected by the high frequency coil 14b arranged close to the subject 1 and amplified by the signal amplifier 15, 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. When data from the receiving unit 6 is input to the digital signal processing device 8, 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.

Further, 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.

Figure 2 shows an example of a typical DWI pulse sequence. In 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.

In FIG. 2, 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. After applying the first MPG pulse 203, an RF pulse 202 for inverting the spin phase is irradiated. Thereafter, 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.

Thereby, for the stationary spin, the phase rotation given by the first MPG pulse 203 is rewound by the second MPG pulse 204. Thereafter, 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. With such a pulse sequence, a contrast difference is generated between water molecules that are in a diffusing motion and stationary spins, and the diffusing motion is imaged. Note that 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.

Hereinafter, specific embodiments of the global motion correction processing function will be described.

<First embodiment>
In the MRI apparatus of the present embodiment, 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 feature is that a phase error or a gradient magnetic field application amount corresponding to the calculated navigation error is calculated and passed to a sequence control unit (sequencer).

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. For example, the sequencer 4 and the signal processing unit 7 (digital signal processing device 8) 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.

Further, as shown in FIG. 3, 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.

An overview of the processing in this embodiment will be described with reference to the block diagram of FIG.

Sequencer 4 calculates and executes a pulse sequence when a DWI pulse sequence and its imaging parameters are set via operation unit 25. Here, regarding the TE (echo time) among the imaging parameters, 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. First, 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. In FIG. 4, the same elements as those in FIG. 2 are denoted by the same reference numerals, and redundant description is omitted. In the DWI pulse sequence 200A, 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. In the example shown in FIG. 4, 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. Obtained (navigation echo readout gradient magnetic field application processing 301). 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. .

The application of the 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.

Hereinafter, details of the correction pulse calculation processing 302 performed by the correction pulse calculation unit 81 will be described.

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.

First, in processing 501, navigation echo Navi_Echo (t) obtained by applying a navigation echo readout gradient magnetic field in the Gs direction is read. The navigation echo Navi_Echo (t) read in the process 502 is Fourier transformed to obtain Navi 複素 (x) which is complex data. Then, the 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).

In Equation (1), Phase_unwrapped (x) is a scalar phase data, Phase [] is a function that returns the phase value of complex data, and Conjugate [] is a function that represents complex conjugate processing.

In process 504, a linear line is applied to Phase_unwrapped (x) by the least square method (formula (2)), and the slope (FirstOrderPhase) is calculated.

In Equation (2), 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.

In the process 505, using this phase gradient, the correction pulse application amount [s · T / m] in the Gs direction is calculated according to the following equation (3).

In Equation (3), Duration is the navigation echo sampling time [s], and GcAmp is the navigation echo readout gradient magnetic field [T / m].

If the high-frequency coil (receiver coil) 14b is composed of a plurality of coils, the navigation echo is measured for each coil. In this case, since the distribution of the phase of the navigation echo obtained by the shape and position of each coil differs, 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.

Next, the procedure for calculating the excess / deficiency of MPG pulses in the Gp and Gr directions will be described with reference to the flowchart of FIG. Since the calculation procedure of the correction pulse application amount in the Gp direction and the Gr direction is the same, only the Gp direction will be described here. In addition, correction in the Gp direction and the Gr direction is obtained by performing multiple shot measurement or the number of additions is 2 or more, that is, obtaining an echo signal necessary for one image by repeating the pulse sequence shown in FIG. 4 a plurality of times. Only if you do.

First, in process 601, navigation echo Navi_Echo (t) obtained by applying a navigation echo readout gradient magnetic field in the Gp direction is read. In step 602, the read navigation echo Navi_Echo (t) is Fourier transformed to obtain Navi (x). In the branch process 603, it is determined whether the navigation echo Navi (x) being processed is the first repeated echo. If Navi (x) is an echo obtained in the first (first) iteration, Navi (x) is stored in the RAM 22 in processing 604. The saved Navi (x) is used as the reference navigation echo Navi_std (x) in the subsequent processing. On the other hand, if it is determined in the branch process 603 that the navigation echo Navi (x) being processed is not the first repeated echo, 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.

In the equation (4), 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)).

Thereafter, similarly to the processing in the Gs direction (503 to 505), in the processing 606, the phase of Navi_subtracted (x) is unwrapped using the above equation (1) to calculate the phase Phase_unwrapped (x). In 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. In the processing in the Gs direction, 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. On the other hand, in the processing in the Gp and Gr directions, 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. When the correction pulse application amount is calculated as the pulse area as in Equation (3), the sequencer 4 may fix the correction pulse application time and change the pulse intensity according to the correction pulse application amount. . By applying a correction pulse, 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.

According to the MRI apparatus of the present embodiment, 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. In addition, 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.

Further, according to the present embodiment, the phase error correction process (correction sequence) 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. Moreover, the calculation cost of the RF pulse is unnecessary, and the processing cost is low.

Hereinafter, it will be described using a specific example that 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 μs and the number of sampling points is 64, the sampling time per axis is about 100 μs. 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 700μs or less including the pulse.

When the navigation echo readout gradient magnetic field is sequentially applied in the three-axis direction as in the pulse sequence 200A of FIG. 4, the gradient magnetic field pulses of the other axes may be simultaneously applied except for the gradient magnetic field plateau portion as shown in FIG. (In other words, when two or more gradient magnetic fields are applied to generate navigation echo, some of the waveforms of adjacent gradient magnetic fields may overlap in time), so the application time of the readout gradient magnetic field for all three axes is 700 + 700 + 100 = 1500 µs It becomes.

Further, as shown in FIG. 8, 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. Further, since 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 μs.

On the other hand, regarding 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.

As described above, 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. The other elements and procedures can be changed as appropriate.

For example, if 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. In addition, 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.

In addition, 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. For example, prior to the pulse sequence shown in FIG. 4, such 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. In such a pre-scan, it is preferable to use the same RF pulse as the pulse sequence used in the main scan for acquiring an image signal, and to make the timing for acquiring each echo from the RF pulse the same. The MPG pulse may or may not be used. When the MPG pulse is used, the phase gradient can be made uniform in the imaging plane, as in the case of using the first echo in the measurement. In addition, when 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 basic configuration of the MRI apparatus of this embodiment is the same as that of the first embodiment. However, in the present embodiment, as shown in FIG. 9, the digital signal processing apparatus 8 includes a data correction unit 82 in addition to the correction pulse calculation unit 81. Hereinafter, the process of this embodiment is demonstrated centering on a different point from 1st embodiment, using the drawing used in 1st embodiment suitably.

Also in this embodiment, 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. For the Gp and Gr directions, 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. Specifically, the correction value Δk 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 Δk 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.

<Modification of Second Embodiment>
In the second embodiment, in the process of calculating the phase gradient after the phase unwrapping process in the Gp and Gr directions (process 607 in FIG. 6), a linear line is applied to Phase_unwrapped (x) by the least square method, and the gradient is obtained. In this modification, in this processing 607 and processing 504 for obtaining the phase gradient after phase unwrapping in the Gs direction, a zero-order offset is also obtained, and the value is subtracted from the echo signal at the time of image reconstruction. Make corrections. The zero-order offset is calculated from the following equation (6).

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).

In Equation (7), | Echo (t) | represents the amplitude of the echo signal Echo (t), which is a complex data string, and θ (t) represents the phase of the echo signal Echo (t). I is an imaginary unit.

According to the present modification, 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.

Hereinafter, parts different from the first embodiment will be described.

FIG. 10 is a block diagram showing an overview of the processing of this embodiment. As can be seen from the comparison with the block diagram of FIG. 3, 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.

First, the processing of the sequencer 4 will be described in detail with reference to the pulse sequence 200B shown in FIG. Also in this embodiment, the case where the DWI pulse sequence shown in FIG. 2 is used as a basic pulse sequence will be described. In FIG. 11, the same elements as those in FIGS. 2 and 4 are denoted by the same reference numerals.

When the echo time TE is divided into the first half of the inverted RF pulse 202 and the latter half of the inverted RF 202 to the echo 207, 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. In the present embodiment, 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. Hereinafter, 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.

First, in step 1001, the reference navigation echo Base_Navi_Echo (t) acquired in step 901 is read. Next, after the navigation echo Navi_Echo (t) is read in the process 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) ) Next, in step 605, the phase difference between the reference navigation echo Base_Navi_Echo (t) and the navigation echo Navi (x) is calculated.

Navi_subtracted (x) obtained by Expression (8) is the same as the right side of Expression (4) of the first embodiment, and the subsequent processing is the same as the processing 606 to 608 of the first embodiment. That is, phase unwrapping processing (606) according to equation (1), phase gradient calculation (607) according to equation (2), and correction pulse application amount calculation (608) according to equation (3) are performed.

Also in this embodiment, when there are a plurality of receiving coils 14b, 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.

According to this embodiment, similarly to the first embodiment, it is possible to correct the phase error due to the global motion in all directions. In this embodiment, since the reference navigation echo acquired before applying the MPG pulse is used, 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. In the present embodiment, since 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.

In addition, according to the present embodiment, 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.

In this embodiment as well, as in the second embodiment, 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. Alternatively, as in the modification of the second embodiment, 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. In addition, as long as there is no contradiction, the modified example described in the first embodiment can be adopted in this embodiment.

As mentioned above, although embodiment of this invention and its modification example were demonstrated, this invention is not limited to embodiment mentioned above or drawing referred, It is possible to perform a various change. Moreover, the processing procedure of the flowchart shown for description of each embodiment is an example, there is a process which can be omitted, and it is also possible to add another process as needed.

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

Claims

An imaging unit that collects echo signals according to a predetermined pulse sequence;
A sequence control unit for controlling the imaging unit;
A calculation unit that performs calculation using echo signals collected by the imaging unit,
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 a navigation echo and application of a correction gradient magnetic field at least in the slice direction before the echo signal collection after the MPG application,
The calculation unit calculates an application amount of the correction gradient magnetic field using the navigation echo between the readout of the navigation echo and application of the correction gradient magnetic field in the correction sequence, and calculates the calculated correction gradient magnetic field application amount A correction pulse calculation unit that passes the control signal to the sequence control unit,
The magnetic resonance imaging apparatus, wherein the sequence control unit applies the correction gradient magnetic field with an application amount of the correction gradient magnetic field received from the correction pulse calculation unit.
The magnetic resonance imaging apparatus according to claim 1,
The correction sequence includes reading of navigation echoes in two-axis directions or three-axis directions and applying a correction gradient magnetic field in each axis direction,
The magnetic resonance imaging apparatus, wherein the correction pulse calculation unit calculates an application amount of a correction gradient magnetic field in each of the two-axis direction or the three-axis direction.
The magnetic resonance imaging apparatus according to claim 2,
The magnetic resonance imaging apparatus according to claim 2, wherein the navigation echo in the two-axis direction or the three-axis direction is a navigation echo in a slice direction and a navigation echo in at least one of a readout direction and a phase encoding direction.
The magnetic resonance imaging apparatus according to claim 3,
The correction pulse calculation unit calculates an application amount of the correction gradient magnetic field using a phase difference between the navigation echo and a reference navigation echo for at least one of the readout direction and the phase encoding direction. Magnetic resonance imaging device.
The magnetic resonance imaging apparatus according to claim 4,
The pulse sequence is a pulse sequence that repeats at least twice from RF application to signal acquisition,
The magnetic resonance imaging apparatus characterized in that the correction pulse calculation unit uses a navigation echo collected in the first iteration as the reference navigation echo.
The magnetic resonance imaging apparatus according to claim 1,
The correction pulse calculation unit calculates a phase inclination of the navigation echo, calculates a phase correction amount with the inclination being zero, and calculates an application amount of the correction gradient magnetic field from the phase correction amount. Magnetic resonance imaging device.
The magnetic resonance imaging apparatus according to claim 1,
The correction sequence includes a collection of navigation echoes in two-axis directions or three-axis directions, and the calculation unit is configured to perform the navigation echo and the reference echo for directions other than the slice direction in the two-axis directions or the three-axis directions, respectively. A phase correction amount is calculated using the phase difference between and the position of the echo signal in the k-space coordinates is corrected using the phase correction amount.
The magnetic resonance imaging apparatus according to claim 7,
The arithmetic unit calculates a phase offset amount and an inclination using a phase difference between the navigation echo and a reference echo, and corrects the phase of the echo signal using the offset amount. Resonance imaging device.
The magnetic resonance imaging apparatus according to claim 1,
The correction sequence includes application of a gradient magnetic field that generates a navigation echo, and a part of the waveform of the gradient magnetic field overlaps with a part of the waveform of the MPG pulse in time. apparatus.
The magnetic resonance imaging apparatus according to claim 1,
The correction sequence includes two or more gradient magnetic field applications that generate navigation echoes, and a part of the waveform of the gradient magnetic field overlaps with a part of the waveform of the adjacent gradient magnetic field in time. Magnetic resonance imaging device.
The magnetic resonance imaging apparatus according to claim 1,
The application time of the correction gradient magnetic field in the pulse sequence is fixed, and the sequence control unit changes the intensity of the correction gradient magnetic field according to the application amount received from the correction pulse calculation unit. Magnetic resonance imaging device.
The magnetic resonance imaging apparatus according to claim 1,
The sequence control unit performs control to add readout of a reference navigation echo between the excitation RF pulse application and the MPG pulse application,
The correction pulse calculation unit calculates an application amount of the correction gradient magnetic field by using a phase difference between the navigation echo and the reference navigation echo for one or more directions including the slice direction. Magnetic resonance imaging device.
An imaging unit that collects echo signals according to a predetermined pulse sequence;
A sequence control unit for controlling the imaging unit;
A calculation unit that performs calculation using echo signals collected by the imaging unit,
The pulse sequence includes the application of a first RF pulse and a second RF pulse, the application of first and second MPG pulses positioned before and after the second RF pulse, and the first And a correction sequence for correcting the phase error of the second MPG pulse,
The correction sequence includes reading a first navigation echo executed between the first RF pulse and the first MPG pulse, and a second navigation echo executed following the second MPG pulse. Reading and applying a correction gradient magnetic field,
The calculation unit calculates the application amount of the correction gradient magnetic field using the first navigation echo and the second navigation echo, and sends it to the sequence control unit,
The sequence control unit performs the signal acquisition after applying the correction gradient magnetic field with the application amount of the correction gradient magnetic field received from the arithmetic unit.
A method for controlling the operation of a magnetic resonance imaging apparatus that applies an MPG pulse after applying an excitation RF pulse and collects an echo signal after a predetermined time from the excitation RF pulse,
Acquire the navigation echo at least in the slice direction after the MPG pulse is applied and before collecting the echo signal, calculate the phase correction amount from the acquired navigation echo, and calculate the phase error included in the MPG pulse based on the calculated phase correction amount. A control method of a magnetic resonance imaging apparatus that performs an operation of applying a correction gradient magnetic field to be corrected.
A method for controlling a magnetic resonance imaging apparatus according to claim 14,
Acquiring the navigation echo includes acquiring a plurality of navigation echoes in the same direction;
The calculation process of the phase correction amount is a process of calculating a phase difference between the reference navigation echo and other navigation echoes, using the navigation echo acquired first among the plurality of navigation echoes in the same direction as the reference navigation, A control method for a magnetic resonance imaging apparatus, comprising: calculating a tilt for the phase difference.