阅读说明：本技术 磁共振摄像装置 (Magnetic resonance imaging apparatus ) 是由 仓谷厚志 于 2021-02-19 设计创作，主要内容包括：本发明提供一种磁共振摄像装置。在摄像中取得导航回波,在根据相位变化修正频率时,不受通过随时间的变化而产生的偏移的影响地高精度进行修正。MRI装置具备：导航控制部,其控制取得NMR信号的摄像部,在被检体的图像重构用的核磁共振信号的收集之前使导航回波产生,并收集给定的测量时间的导航数据；和导航解析部,其对测量时间中的所述导航回波的相位变化进行解析,算出用于修正与相位变化相伴的位置偏离的修正值。导航解析部根据导航回波的相位变化与成为基准的导航回波的所述测量时间中的相位变化的差分来算出与基准的相位变化量,并算出所述修正值。(The invention provides a magnetic resonance imaging apparatus. When a navigation echo is acquired during imaging and the frequency is corrected according to a phase change, the correction is performed with high accuracy without being affected by a shift due to a temporal change. An MRI device is provided with: a navigation control unit that controls an imaging unit that acquires an NMR signal, generates a navigation echo before collection of a nuclear magnetic resonance signal for image reconstruction of a subject, and collects navigation data for a predetermined measurement time; and a navigation analysis unit that analyzes a phase change of the navigation echo during the measurement time and calculates a correction value for correcting a positional deviation associated with the phase change. The navigation analysis unit calculates a phase change amount from a reference based on a difference between a phase change of the navigation echo and a phase change of the reference navigation echo in the measurement time, and calculates the correction value.)

1. A magnetic resonance imaging apparatus is characterized by comprising:

an imaging unit having a static magnetic field generating unit that generates a static magnetic field in a space in which a subject is placed, a transmitting unit that irradiates the subject with a high-frequency magnetic field, and a receiving unit that receives a nuclear magnetic resonance signal generated by nuclear magnetic resonance from the subject;

a calculation unit that performs a calculation including image reconstruction using the nuclear magnetic resonance signal;

a navigation control unit that controls the imaging unit to generate a navigation echo before collection of a nuclear magnetic resonance signal for image reconstruction of the subject and to collect navigation data for a predetermined measurement time; and

a navigation analysis unit that analyzes a phase change of the navigation echo at the measurement time and calculates a correction value for correcting a positional deviation caused by the phase change,

the navigation analysis unit calculates an amount of phase change from a reference based on a difference between a phase change at each time of the measurement time of the navigation echo and a phase change at each time of the measurement time of a reference navigation echo, and calculates the correction value.

2. The magnetic resonance imaging apparatus according to claim 1,

the navigation analysis unit includes a fitting unit that fits the phase change or the difference in phase change, and calculates the amount of phase change from the difference in phase change obtained by the fitting.

3. The magnetic resonance imaging apparatus according to claim 1,

the navigation control unit collects the navigation echo without using a frequency code.

4. The magnetic resonance imaging apparatus according to claim 1,

the navigation control unit collects 2 or more navigation echoes within the predetermined measurement time using frequency coding.

5. The magnetic resonance imaging apparatus according to claim 4,

the navigation control unit collects the navigation echoes for the 3-axis directions within the predetermined measurement time by using a frequency code in the 3-axis direction.

6. The magnetic resonance imaging apparatus according to claim 1,

the imaging unit repeatedly collects the nuclear magnetic resonance signals for image reconstruction at repeated intervals,

the navigation control unit generates and collects the navigation echo at every repetition time.

7. The magnetic resonance imaging apparatus according to claim 6,

the navigation control unit generates and collects navigation echoes serving as the reference a plurality of times,

the navigation analysis unit calculates the correction value using a phase change of one of the reference navigation echoes and a phase change of a navigation echo collected until generation of a next reference navigation echo.

8. The magnetic resonance imaging apparatus according to claim 7,

the navigation analysis unit determines whether or not the correction value is within a predetermined threshold value, and sets the correction value as the correction value when the correction value is within a predetermined threshold value range.

9. The magnetic resonance imaging apparatus according to claim 1,

the transmission unit corrects the frequency of the collected irradiation high-frequency magnetic field of the navigator echo used for calculating the correction value using the correction value calculated by the navigator analysis unit.

10. The magnetic resonance imaging apparatus according to claim 1,

the static magnetic field generating unit corrects the static magnetic field using the correction value calculated by the navigation analyzing unit.

11. The magnetic resonance imaging apparatus according to claim 1,

the calculation unit corrects the positional deviation of the image reconstructed from the nuclear magnetic resonance signal using the correction value calculated by the navigation analysis unit.

12. The magnetic resonance imaging apparatus according to claim 1,

the acquisition of the nuclear magnetic resonance signal for image reconstruction by the imaging unit is diffusion weighted imaging including the application of an MPG pulse.

13. The magnetic resonance imaging apparatus according to claim 1,

the magnetic resonance imaging apparatus further includes: and a UI unit that accepts user specification related to generation and collection of navigation echoes.

14. The magnetic resonance imaging apparatus according to claim 13,

the UI unit receives a user selection of a navigation echo type.

15. The magnetic resonance imaging apparatus according to claim 13,

the UI section accepts user selection of a correction method.

Technical Field

The present invention relates to a magnetic resonance imaging (hereinafter, referred to as "MRI") apparatus that measures a nuclear magnetic resonance (hereinafter, referred to as "NMR") signal from hydrogen, phosphorus, or the like in a subject and images a nuclear density distribution, a relaxation time distribution, or the like, and particularly relates to a technique of correcting a positional deviation with high accuracy in Diffusion Weighted Imaging (DWI) using the MRI apparatus.

Background

In an MRI apparatus, there is a fluctuation in the center frequency of nuclear magnetic resonance as one of the causes of positional deviation in an image. The center frequency is determined by the static magnetic field intensity, and the static magnetic field is usually controlled so as to maintain a fixed static magnetic field by a correction coil (shim coil). However, when a local gradient magnetic field is generated using a gradient coil, heat is generated by a current flowing through the gradient coil, and the coil for correcting the static magnetic field is physically affected, and the center frequency may vary. In addition, when the static magnetic field generating magnet is a permanent magnet or the like, heat is directly transferred to the magnet, and the static magnetic field may change, and the center frequency may fluctuate.

In Diffusion-weighted Imaging such as DTI (Diffusion Tensor Imaging), DKI (Diffusion Kurtosis Imaging), IVIM (intra-voxel incoherent motion), or the like, which is one of Imaging methods using an MRI apparatus, echo planar Imaging using MPG pulses is performed, and a large current flows through a gradient magnetic field coil. In such imaging using a large current, the gradient coil generates much heat compared with normal imaging, and the center frequency fluctuates due to static magnetic field fluctuation, resulting in positional deviation during imaging. As a technique for correcting a positional deviation due to a fluctuation in the center frequency, there is a technique of predicting a frequency fluctuation and adjusting the current of the shim coil (for example, patent document 1).

However, in the technique of predicting and adjusting the frequency fluctuation as described in patent document 1, it is difficult to cope with a sudden change in imaging. In order to cope with a change in imaging, it is desirable to correct the image as needed during imaging, but if the frequency measurement is performed during imaging, there is a problem that the imaging time is long. In response to this, a method has been proposed in which a navigator echo is acquired during imaging, and a frequency variation value is obtained from the phase of the acquired navigator echo (patent document 2). Specifically, a phase difference is calculated from a navigator echo serving as a reference and a navigator echo acquired later, and a frequency fluctuation value is calculated using the phase difference.

Documents of the prior art

Patent document

Patent document 1: JP Kokai No. 2000-342554

Patent document 2: JP 2006-014753 publication

When the method of obtaining a frequency variation value using a navigator echo is applied to the diffusion imaging described above, in general, the phase of a spin changes drastically when an MPG pulse is applied in various directions, and the phase is largely different from a reference navigator echo, and it is difficult to calculate the frequency variation value from the phase difference. Therefore, in diffusion imaging, it is necessary to acquire navigator echoes further forward than the MPG.

When the navigation echo is continuously acquired, there is a possibility that the phase of the navigation echo shifts due to a time-varying eddy current or the like of the gradient magnetic field. This offset is very small, and if the echo Time (TE) of the navigator echo is made long, the offset becomes negligible as an error. However, as described above, there is a restriction that the navigator echo is applied before the MPG pulse, and if the TE of the navigator echo is made longer under the restriction, the TE of the DWI becomes longer than usual. Since DWI is basically a T2 emphasized image, there is a problem that a tissue long in T2 is likely to be a high signal (T2 shine through, T2 penetrates), and there is a restriction that TE is made as short as possible in order to reduce this influence. Due to these restrictions, the TE of the navigator echo must be small, and the error due to the offset becomes large.

In imaging with a small number of applied axes of MPG, imaging can be completed before the error affects the position deviation correction, but in imaging with MPG such as DTI, DKI, IVIM, etc., applied with several tens of axes, the imaging time is extended, and the error is accumulated, which affects the position deviation correction. As a result, the resolution specified under the imaging conditions is shifted or higher, and the image desired by the user is no longer obtained, and artifacts and blurring also occur in the analysis image. Further, in a low magnetic field apparatus or the like, a fat suppression pulse called a CHESS method is sometimes used, but in this case, frequencies other than fat are suppressed due to frequency fluctuation, and analysis images and diagnosis are also affected.

Disclosure of Invention

An object of the present invention is to provide an MRI apparatus capable of performing frequency correction with high accuracy without being affected by a shift due to a change with time when a method of acquiring a navigator echo in imaging and correcting a frequency according to a phase change is employed.

In the past, the amount of phase change from the generation of a navigator echo to the echo time was determined, whereas the present invention acquires the phase change of the navigator echo within the measurement time in which the navigator echo was collected. By obtaining the difference from the phase change of the reference navigator echo obtained in the same manner, the amount of phase change from the reference can be obtained without being affected by the offset.

Specifically, an MRI apparatus of the present invention includes: an imaging unit having a static magnetic field generating unit that generates a static magnetic field in a space in which a subject is placed, a transmitting unit that irradiates the subject with a high-frequency magnetic field, and a receiving unit that receives a nuclear magnetic resonance signal generated by nuclear magnetic resonance from the subject; a calculation unit that performs a calculation including image reconstruction using the nuclear magnetic resonance signal; a navigation control unit that controls the imaging unit to generate a navigation echo before collection of a nuclear magnetic resonance signal for image reconstruction of the subject and to collect navigation data for a predetermined measurement time; and a navigation analysis unit that analyzes a phase change of the navigation echo at the measurement time, and calculates a correction value for correcting a positional deviation associated with the phase change. The method is characterized in that: the navigation analysis unit calculates an amount of phase change from a reference based on a difference between a phase change at each time of the measurement time of the navigation echo and a phase change at each time of the measurement time of a reference navigation echo, and calculates the correction value. Here, the phase change refers to a change in phase at any time during the measurement time, and the phase change amount refers to a phase change amount from a reference.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, the amount of phase change can be calculated with high accuracy without being affected by MPG pulses or the like and without extending the TE of DWI, and thus the positional deviation due to the central frequency variation occurring during imaging can be corrected with high accuracy. As a result, accurate analysis results can be obtained in DKI, IVIM, and the like.

Drawings

Fig. 1 is a block diagram showing an example of the overall configuration of an MRI apparatus according to the present invention.

Fig. 2 is a block diagram showing the overall configuration of an MRI apparatus including the details of an imaging unit.

Fig. 3 is a flowchart of the processing of the MRI apparatus of fig. 1.

Fig. 4 is a diagram showing an example of a pulse sequence used in embodiment 1.

Fig. 5 is a flowchart of the processing of embodiment 1.

Fig. 6 is a diagram for explaining calculation of the amount of phase change in embodiment 1, where (a) and (b) are diagrams showing the phase changes of the reference navigator echo and the target navigator echo, respectively, (c) is a diagram showing the difference in phase change, and (d) is a diagram showing the phase change calculated by the conventional method.

Fig. 7 is a flowchart of the processing of modification 1.

Fig. 8 is a diagram illustrating the threshold values of the correction values set in modification 2.

Fig. 9 is a diagram showing an example of a pulse sequence employed in embodiment 2.

Fig. 10 is a diagram for explaining calculation of the phase change amount according to embodiment 2, where (a) and (b) are diagrams showing phase changes of the reference navigator echo and the target navigator echo, respectively, and (c) is a diagram showing a difference in phase change.

Fig. 11 is a diagram showing an example of a pulse sequence employed in a modification of embodiment 2.

Fig. 12 is a diagram showing an example of a UI screen according to embodiment 3.

Description of reference numerals

10: an image pickup unit,

20: a processing device,

21: a computing unit,

22: a navigation control part,

23: a navigation analysis unit,

30: a UI part,

40: and a storage device.

Detailed Description

An embodiment of an MRI apparatus according to the present invention will be described below with reference to the drawings. In all the drawings for describing the embodiments of the present invention, the same reference numerals are given to the elements having the same functions, and redundant description thereof will be omitted.

First, an outline of an embodiment of an MRI apparatus of the present invention will be described. Fig. 1 is a block diagram showing the overall configuration of an MRI apparatus. As shown in fig. 1, the MRI apparatus 100 of the present embodiment includes an imaging unit 10, a calculation unit 21, a navigation control unit 22, and a navigation analysis unit 23. In the illustrated embodiment, the arithmetic unit 21, the navigation control unit 22, and the navigation analysis unit 23 are included in the processing device 20 that realizes the general functions thereof. The processing device 20 may be a computer having a CPU and a memory, or may be a system including hardware such as an ASIC and an FPGA, and connected to a User Interface (UI) unit 30 having a display device and an input device, and an external storage device 40. The external storage device 40 includes a storage device connected via wireless, internet, or the like, in addition to a storage device connected directly or by wire.

The imaging unit 10 has a configuration similar to that of a general MRI apparatus, and includes a static magnetic field generating unit that generates a static magnetic field in a space in which a subject is placed, a transmitting unit that irradiates the subject with a high-frequency magnetic field, a receiving unit that receives a nuclear magnetic resonance signal generated by nuclear magnetic resonance from the subject, and the like. Fig. 2 shows the overall configuration of the MRI apparatus including the details of the imaging unit 10. The MRI apparatus shown in fig. 2 includes a static magnetic field generating unit 1, a gradient magnetic field generating unit 3, a measurement control unit 4, a transmitting unit 5, a receiving unit 6, a signal processing unit 7, and a CPU (central processing unit) 2. The signal processing unit 7 and the CPU2 have functions equivalent to the processing device 20 of fig. 1.

The static magnetic field generating unit 1 is provided with a permanent magnet type, normal conduction type, or superconducting type static magnetic field generating source, and generates a uniform static magnetic field in a direction orthogonal to the body axis in a space around the subject 50 in the vertical magnetic field type, and generates a uniform static magnetic field in the body axis direction in the horizontal magnetic field type. Shim coils 18 for correcting the inhomogeneity of the static magnetic field are disposed in the vicinity of the static magnetic field generating source. The shim coils 18 are connected to a shim power supply 19, and are driven by a current supplied from the shim power supply 19 to generate a correction magnetic field. The correction magnetic field corrects the unevenness of the static magnetic field and corrects the variation of the static magnetic field with time.

The gradient magnetic field generator 3 includes gradient magnetic field coils 8 wound in 3 axial directions of X, Y, Z, which is a coordinate system (stationary coordinate system) of the MRI apparatus, and gradient magnetic field power supplies 9 that drive the respective gradient magnetic field coils, and applies gradient magnetic fields Gx, Gy, Gz in the 3 axial directions of X, Y, Z by driving the gradient magnetic field power supplies 9 of the respective coils in accordance with a command from a measurement controller 4 described later. In imaging, a slice plane (imaging cross section) of the subject 1 is set by applying a slice-direction gradient magnetic field pulse (Gs) in a direction orthogonal to the slice plane, and a phase encoding-direction gradient magnetic field pulse (Gp) and a frequency encoding-direction gradient magnetic field pulse (Gf) are applied in the remaining 2 directions orthogonal to the slice plane and each other, and positional information in each direction is encoded for an echo signal. In addition, the gradient coil 8 may be used as a shim coil.

The measurement control unit 4 is a control unit that repeatedly applies a radio frequency magnetic field pulse (hereinafter, referred to as an "RF pulse") and a gradient magnetic field pulse in a predetermined pulse sequence, and operates under the control of the arithmetic unit 2 to transmit various commands necessary for collecting tomographic image data of the object 50 to the transmission unit 5, the gradient magnetic field generation unit 3, and the reception unit 6. As for the pulse sequence, various pulse sequences are prepared according to the imaging method, and when the imaging method is determined at the time of imaging, the CPU2 reads out the corresponding pulse sequence and sets it in the measurement control unit 4. In the present embodiment, a pulse sequence of diffusion imaging to which generation and collection of a navigator echo are added is performed. The details of the pulse sequence will be described later.

The transmission unit 5 includes a high-frequency oscillator 11, a modulator 12, a high-frequency amplifier 13, and a transmission-side high-frequency coil (transmission coil) 14a for irradiating the subject 50 with an RF pulse to induce nuclear magnetic resonance in the nuclear spins of atoms constituting the living tissue of the subject 50. The RF pulse output from the RF oscillator 11 is amplitude-modulated by the modulator 12 at a timing based on an instruction from the measurement control unit 4, and the amplitude-modulated RF pulse is amplified by the RF amplifier 13 and supplied to the RF coil 14a disposed close to the subject 50, whereby the subject 50 is irradiated with the RF pulse. The nuclear magnetic resonance frequency (center frequency) can be adjusted by adjusting the high frequency transmitted by the high frequency transmitter 11.

The receiving unit 6 detects an echo signal (NMR signal) emitted by nuclear magnetic resonance of nuclear spins of a living tissue constituting the subject 50, and includes a receiving-side high-frequency coil (receiving coil) 14b, a signal amplifier 15, a quadrature phase detector 16, and an a/D converter 17. An NMR signal of a response of the subject 50 induced by the electromagnetic wave radiated from the high-frequency coil 14a on the transmission side is detected by the high-frequency coil 14b disposed close to the subject 50, amplified by the signal amplifier 15, divided into two orthogonal signals by the quadrature phase detector 16 at a timing based on a command from the measurement control unit 4, converted into digital values by the a/D converter 17, and sent to the signal processing unit 7 as measurement data.

The signal processing unit 7 performs various data processing, display and storage of processing results, and the like, and a part of the functions thereof is realized by the CPU 2. The signal processing unit 7 is connected to an external storage device 40 such as an optical disk or a magnetic disk, and a UI unit 30 including a display 31 and an input device 32. When the measurement data is input from the receiving unit 6, the CPU2 executes processing such as signal processing and image reconstruction, and displays a tomographic image of the subject 50, which is a result of the processing, on the display 31 and records the tomographic image in the magnetic disk of the external storage device 40. The input device 32 inputs various control information of the MRI apparatus and control information of processing performed in the signal processing unit 7, and includes a trackball, a mouse, a keyboard, and the like. The input device 32 is disposed close to the display 31, and the operator operates the input device 32 while looking at the display 31 to interactively control various processes of the MRI apparatus.

The CPU2 mainly has functions of calculation using the NMR signal received by the receiving unit 6 (function of the calculating unit 21) and control of the imaging unit 10 via the measurement control unit 4, and the calculation includes not only calculation such as image reconstruction using the NMR signal and generation of a calculation image from the reconstructed image, but also analysis using the phase of the NMR signal (navigation echo) (function of the navigation analyzing unit 23). The control of the imaging unit 10 includes control related to generation and collection of a navigation echo (a function of the navigation control unit 22).

The operation of the MRI apparatus having the above-described configuration will be described in brief with reference to fig. 3.

When the imaging parameters are set, the imaging unit 10 starts imaging under the set conditions under the control of the measurement control unit 4 (S31). The imaging parameters may be set by default together with the pulse sequence, or may be designated by the user via the UI unit 30. In addition to TE and TR, which are general imaging parameters, the DWI sets the intensity of MPG, an application axis, and the like.

As shown in fig. 4, the DWI pulse train 400 is a pulse train of an EPI method as follows: after applying MPG pulses 403 and 404 having large intensities before and after the excitation pulse 402, inversion 405 of the readout gradient magnetic field pulse Gf and application 406 of the cusp-shaped phase encoding gradient magnetic field Gp are repeated, and a plurality of echo signals 407 are measured after 1 excitation. In the present embodiment, the navigator echo 411 is generated and collected before the first MPG pulse 403 by the navigator control unit 22. Fig. 4 shows a case where the FID signal is measured as the navigator echo, but there are several methods for the navigator echo. These embodiments will be described in the embodiments described later.

The navigator echo is repeatedly generated/collected every repetition time TR of the pulse sequence, resulting in a plurality of navigator echoes. In this case, the navigator echo acquired before imaging is set as a reference navigator echo.

The measurement data of the navigator echo collected by the receiver 6 at a predetermined sampling time is forwarded to the navigator analyzing unit 23 of the CPU2, where it calculates a phase change at the sampling time. The navigation analysis unit 23 calculates a phase change therebetween by fitting the phase at each time of the sampling time, for example. The navigation analyzing unit 23 further calculates the amount of phase change from the reference by performing a difference operation between the calculated phase change and the phase change calculated in the same manner as for the reference navigator echo (S32).

Next, the navigation analysis unit 23 calculates a correction value for correcting the positional deviation associated with the phase change using the phase change amount (S33). The correction of the positional deviation caused by the phase change includes correction of the current value of the shim coil 18 (correction of the static magnetic field), correction of the center frequency (adjustment of the high-frequency transmitter 11), and correction of the image (correction by the calculation unit 21), and the navigation analysis unit 23 calculates a correction value according to the object to be corrected. Finally, each section is corrected based on the correction value (S34). In the case where the correction can be performed in real time by its object, the correction is performed by immediately reflecting DWI measurement after the navigator echo is collected. Even if the correction value cannot be reflected immediately, the correction is performed after 1 to more TRs. In the case of image correction, correction using a correction value calculated from the navigator echo collected at the repetition time is performed on an image of an echo signal acquired in EPI performed after the navigator echo.

According to the MRI apparatus of the present embodiment, the phase change amount is calculated by fitting the change in phase at each time within the measurement time, not by using the phase difference in TE of the navigator echo, and therefore, the correction value can be calculated with high accuracy without being affected by the offset. In addition, since it is not necessary to lengthen the TE for navigator echo measurement in order to accurately obtain the phase change amount, it is possible to effectively prevent positional deviation due to center frequency variation while preventing an increase in measurement time even for a DWI in which measurement is repeated with MPG to which a large number of axes need to be applied.

Next, specific embodiments related to the collection of the generation of the navigation echo will be explained.

< embodiment 1>

The present embodiment is characterized in that a signal to which no readout gradient magnetic field is applied as shown in fig. 4 is measured as a navigator echo.

The flow of the processing of the present embodiment will be described with reference to fig. 5.

When the imaging is started, the navigation analysis unit 23 sets an initial value of the frequency correction (S51). The initial value is an initial value used for calculating a correction value after the correction, and may be a value for predicting a frequency change acquired in a previous imaging or may be an initial value not set.

First, a navigation echo (reference navigation echo) serving as a reference is acquired (S52). That is, after the excitation pulse 411 and the region selection gradient magnetic field Gs are applied together, the navigator echo 413 during the predetermined sampling time 412 is collected without applying the readout gradient magnetic field. The sampling time 414 is set to a time sufficient to calculate a corresponding amount of phase change therebetween. Next, an echo signal (main imaging echo) of the sequence of main imaging is acquired (S53). The main imaging is a sequence of images to be acquired by a user, and in the present embodiment, is a sequence 400 of DWIs as shown in fig. 4. Next, the navigation controller 22 repeats the measurement of the navigator echo and the measurement of the main imaging echo (S54 to S59), and during this period, the navigation analyzer 23 calculates a correction value (S55 to S58). The measurement of the navigator echo and the measurement of the main imaging echo in this repetition are performed in the same manner as in steps S52 and S53 described above. Steps S54 to S59 are performed, for example, at an imaging repetition time TR.

In the calculation of the correction value, the phase change is calculated from the difference between the reference navigator echo acquired in step S52 and the navigator echo acquired in step S54 (target navigator echo) (S55). Next, the phase change for the difference operation is fitted (S56). The fitting may be a linear function such as equation (1) or may be an equation such as equation (2) in which the influence of the eddy current is taken into consideration.

[ math figure 1] dP ═ a1 × t + b (1)

[ equation 2] dP ═ a1 ═ t + a2 ═ exp (-a3/t) (2)

In these equations, dP characterizes the phase change, a1, a2, a3 characterize the coefficients, and b characterizes the offset.

Fig. 6 (a) to (c) show the fitting state. Fig. 6 (a) and (b) show phase changes of the reference navigator echo and the target navigator echo, and (c) shows a difference therebetween, which shows how the difference is fitted. For reference, fig. 6 (d) shows an example of calculating a phase change according to the conventional method. As shown in fig. 6 (d), in the conventional method, the amount of phase change is calculated from the phase change in TE from the RF irradiation for generating the navigator echo, and therefore, this is an error in the case where there is an offset.

Next, a correction value is calculated using the amount of phase change from the reference (S57). The correction value may be a value of the correction current flowing through the shim coil 18 or a correction value of the center frequency when the excitation pulse 401 is irradiated. The correction value of the frequency can be calculated from a relation (ω ═ 2 π f) between the phase (ω t) and the frequency (f). Since the relationship between the value of the current flowing through the shim coil 18 and the magnetic field intensity generated thereby is determined by the characteristics of the shim coil, the correction current of the shim coil can be calculated from the variation amount of the static magnetic field calculated from the relational expression (f0 ═ γ B0) between the magnetic resonance frequency (f0) and the static magnetic field intensity (B0). Since the static magnetic field (center frequency) and the real space position have a linear relationship, the positional deviation in the real space, that is, the positional deviation on the image can be corrected based on the static magnetic field variation.

The calculated correction value is reflected in the main imaging after the acquisition of the navigator echo in step S54 (S58). That is, the correction current may be passed to the shim coil 18 using the correction value calculated in S57, or the center frequency at the next irradiation may be corrected. In addition, when it takes time to start the supply of the correction current to the shim coil, the positional deviation of the main imaging echo may be started, and therefore, the main imaging echo acquired in step S58 may be corrected at the time of image reconstruction using the correction value calculated in step S57. In step S59, when the main imaging echo is acquired, the process returns to step S54. The imaging is completed by repeating the operations of steps S54 to S59 1 or more times.

Note that TR is the time interval between steps S53 and S59, but the present invention is not limited to this if the interval between main imaging changes using synchronization of electrocardiography, pulse, respiration, and the like. In fig. 5, the main imaging echo of the 1 st time is acquired after the reference navigator echo is acquired (S52), but the main imaging echo may be acquired before the reference navigator echo is acquired (S53), or the correction value may be calculated in step S57.

According to the present embodiment, since the amount of phase change is calculated by fitting the phase difference between the reference navigator echo and the reference navigator echo, the amount of phase change can be calculated without being affected by the offset, and the positional deviation correction with good accuracy can be performed.

< modification 1>

In embodiment 1, the measurement of the reference navigator echo is performed 1 time before the main imaging, and the calculation of the phase change in the subsequent repetition is performed using the reference navigator echo, but in the imaging in which the main imaging is repeated a lot and the imaging time is long, the reference navigator echo is acquired a plurality of times between the repetitions of the main imaging, and the calculation of the phase change may use the reference navigator echo acquired most recently.

Fig. 7 shows a flow of the processing of the present modification. In fig. 7, the same steps as those in fig. 5 are denoted by the same reference numerals, and redundant description is omitted. In the present embodiment, steps S520 to S521 of acquiring the reference navigator echo are repeated. That is, after the reference navigator echo is acquired (S520), the main imaging echo is measured (S53), and the repeated imaging (S59) including the measurement of the target navigator echo (S54) to the main imaging echo measurement (S58) is performed in the same manner as in embodiment 1. If imaging is further performed after repeating the steps a plurality of times (S521), the process returns to step S520, and S520 to S521 are repeated. When the predetermined imaging is completed, the imaging is terminated.

The timing of acquiring the reference navigator echo may be such that the pulse train shape in the TR is changed greatly when the number of repetitions of steps S54 to S58 reaches a predetermined number. As a case where the shape of the pulse train is largely changed, for example, when the number of slices in TR is changed, when the timing of reading the gradient magnetic field is changed, when the physical axis of the phase encoding or the frequency encoding is changed, or the like may be mentioned. Further, the phase may be changed rapidly by body motion or the like. For example, when the phase change amount calculated in step S56 is larger than a predetermined value (for example, the phase change amount calculated before that, or the average value thereof), the reference navigator echo may be newly acquired.

According to this modification, it is possible to avoid the influence of a change in the pulse sequence, unexpected phase variation, and the like, and to perform stable correction.

In addition, fig. 5 and 7 show the case where the measurement of the target navigator echo is performed for each measurement of the main imaging echo, but for example, 1 target navigator may be acquired every time the DWI sequence shown in fig. 4 is performed a plurality of times.

< modification 2>

In this modification, the flow of the processing is also the same as the flow shown in fig. 5 or 7. In the present modification, the method includes: and a process of excluding the correction value calculated in step S57 if it is an inappropriate correction value.

Therefore, the navigation analysis unit 23 sets a threshold value for the correction value or the phase fluctuation amount. The threshold value may be set from a statistical value of a predicted value obtained from a previously measured phase variation to an upper limit or a lower limit, or may be set every time based on an average value of correction values calculated several times during imaging. The navigation analysis unit 23 performs the restriction so that the fluctuation amount is not corrected beyond the set threshold value when the correction value is set in step S57 in fig. 5 and 7. For example, in the example shown in fig. 8, an upper threshold 803 and a lower threshold 804 are set for the correction value, respectively. When the correction value is within the threshold, the correction value 801 is corrected by the frequency and static magnetic field thereof, but the correction value 802 outside the threshold is determined to be a correction value based on inappropriate phase variation and is not corrected.

According to this modification, by setting a threshold value for the correction value or the phase fluctuation amount, even when the phase fluctuates rapidly due to noise or other factors, the correction can be performed stably without performing excessive correction or reverse correction.

< embodiment 2>

Although the navigator echo is acquired without applying the readout gradient magnetic field in embodiment 1, a plurality of navigator echoes are acquired by applying the readout gradient magnetic field in the present embodiment. The flow of the processing is similar to that of embodiment 1 (fig. 5) or its modification (fig. 7), and a difference will be mainly described with reference to fig. 5.

In the present embodiment, in the step of acquiring navigator echoes at S52 and S54, as shown in fig. 9, a readout gradient magnetic field 415 that is inverted after the application of the excitation pulse 411 is applied, and a plurality of (4 in the figure) navigator echoes 416 are collected at different TEs. The main imaging sequence 400 is the same as fig. 4. Next, in step S55, the phase difference between the corresponding TEs of the different TE navigator echoes of the reference navigator echo and the target navigator echo is calculated and the phase difference is fitted. The fitting may be performed by using a linear function (equation (1)) as in embodiment 1, or may be performed by using a fitting function (equation (2)) in which eddy current is considered.

The appearance of the fit of the phase difference is shown in fig. 10. Fig. 10 (a) and (b) are diagrams showing a phase change of the reference navigator echo and a phase change of the target navigator echo, respectively, and (c) is a phase change obtained by performing a difference operation. As shown in fig. 10 (c), in this example, a phase shift occurs, and a slice (d of equation (1)) corresponding to the shift exists in the fitted straight line, but since the amount of phase change can be obtained from the inclination of the straight line, the amount of phase change can be accurately calculated without being affected by the shift.

The following operations are performed in the same manner as in embodiment 1: calculating a correction value from the phase change amount thus calculated (S57); the shim currents are corrected by using the correction values, the frequency of irradiation in the main imaging is corrected, and the positional deviation is corrected when the image of the main imaging is reconstructed (S58). However, in the present embodiment, since the readout gradient magnetic field is applied when the navigator echo is acquired, information of the applied axis remains in the navigator echo. Therefore, conventional correction based on body motion can be performed using this information, and variation in the static magnetic field in the physical axis direction obtained from the application axis of the readout gradient magnetic field can also be corrected.

< modification of embodiment 2>

In embodiment 2, the readout gradient magnetic field is applied to 1 axis, but the navigation echo 416 may be acquired by changing the axes to which the readout gradient magnetic fields 417 to 419 are applied as shown in fig. 11. In the present embodiment, the phase difference of the navigator echoes collected in the same application axis among the navigator echoes in which the application axes of the readout gradient magnetic fields of the reference navigator echo and the target navigator echo are different is calculated, and the phase difference is fitted. The following operations are performed in the same manner as in embodiment 2: calculating a correction value using the phase difference; this is reflected in the main imaging.

In the present embodiment, since information of each axis can be acquired from the navigator echo, conventional correction based on body motion can be performed using information of each axis, and variation in the static magnetic field of each physical axis can also be corrected.

In fig. 11, 1 navigator echo is acquired for each axis, but 2 or more navigator echoes may be acquired, and in this case, the phase difference may be calculated for each applied axis, and the correction value may be calculated for each axis.

Although the embodiments and modifications of the measurement of the navigator echo and the processing thereof have been described above, any of the embodiments and modifications may be incorporated in the apparatus in advance, and may be selected or set by the user at the time of DWI imaging. In the following embodiments, an embodiment of a UI for enabling user selection is explained.

< embodiment 3>

The MRI apparatus of the present embodiment can set the need or condition of correction by the user via the UI unit 30. The processing of the present embodiment is explained with reference to fig. 12. Fig. 12 is an example of a screen displayed on the display 31 of the UI unit 30.

In this screen example, the UI screen 1200 is provided with: a button 1201 for selecting whether or not frequency correction is performed; a button 1202 for selecting the kind of navigation echo; a block 1203 that specifies the time to acquire the navigator echo; a button 1204 for selecting a correction method; selecting whether to perform body motion correction 1205; block 1206 sets a threshold for the correction.

The selection button 1202 accepts selection of which type of navigation echo to use in the above-described embodiment or modification. The navigation control unit 22 determines a navigation sequence to be executed before the main imaging sequence 400 in accordance with the selection, and sets the navigation sequence to the measurement control unit 4. In the block 1203, the sampling time (AD time) (414 in fig. 4) is set when the navigator echo of the readout gradient magnetic field is not used (embodiment 1, etc.), and the total time for acquiring each navigator echo is set when the readout gradient magnetic field is applied. Further, a pull-down menu attached to this frame 1203 may be provided, and the application axis and the number of navigation loops may be set.

A correction method selection button 1204 accepts selection of a method of correction. That is, selection is accepted such as correction by the shim coil, setting of a corrected frequency at the time of irradiation, or correction of an image after acquisition of a main imaging loop. The body motion correction block 1205 is a UI for selecting whether to perform body motion correction of the navigator echo, to reacquire the reference navigator echo, or to perform both when a rapid change occurs. The threshold value setting block 1206 is a block for setting an inappropriate threshold value for removing the correction value, and when the threshold value is set, as described in modification example 2 of embodiment 1, the correction value deviating from the range determined by the set upper and lower threshold values is not adopted as the correction value.

As described above, according to the present embodiment, the user can change the correction accuracy according to the condition, and the image quality expected by the user can be obtained.

Fig. 12 is an example of the UI screen, and it is also possible to omit a part of the conditions described here or add conditions different from these conditions.