WO2004080301A1

WO2004080301A1 - Magnetic resonance imaging device

Info

Publication number: WO2004080301A1
Application number: PCT/JP2004/002849
Authority: WO
Inventors: Tomohiro Goto; Tetsuhiko Takahashi; Masahiro Takizawa; Yumiko Yatsui; Hisako Nagao
Original assignee: Hitachi Medical Corporation
Priority date: 2003-03-14
Filing date: 2004-03-05
Publication date: 2004-09-23
Also published as: JPWO2004080301A1

Abstract

It is possible to effectively control biological movement artifact with a small number of navigator echo acquisitions. A biological movement artifact suppression image is created by distinguishing the exhalation period from the inhalation period of a sample person (101). From at least two navigator echoes (207, 208) measured at a time interval, a displacement and a displacement direction of the sample person (101) at the navigator echo acquisition time are obtained. The timing to acquire the main measurement echo is decided from the displacement and the displacement direction. The displacement direction can be calculated from the primary differentiation of the displacement. Moreover, by obtaining a biological movement prediction function approximating the biological movement cycle before a measurement, it is possible to judge the displacement and the displacement direction by fitting and estimate the main measurement echo acquisition time and interval.

Description

Description Magnetic resonance imaging equipment Technical field

The present invention relates to a magnetic resonance imaging (hereinafter, referred to as MRI) apparatus for imaging a subject using nuclear magnetic resonance, and more particularly to a technique for suppressing body motion artifacts. Background art

MRI applies a high-frequency magnetic field of the resonance frequency of the nuclear spin to be imaged to a subject placed in a static magnetic field, and measures the nuclear magnetic resonance signal generated by the application. This is a device that obtains a tomographic image.

In this MRI, if the subject moves while creating one MR image, large artifacts will occur in the image, degrading the image quality. An important cause of body motion artifacts is body motion artifacts caused by the subject's respiration.

One method of removing body motion artifacts due to respiratory movements is to monitor respiratory movements using an additional echo, a navigator echo. An imaging method that adapts the removal technique to cardiac imaging has been proposed (literature, "avigator Echoes in Cardiac Magnetic Resonance, Journal of Cardiovascular Magnetic Resonance" (3), 183-193 (2001)).

In this document, when the body motion position (displacement) due to respiration obtained from the navigator echo is within a predetermined range specified in advance, the data acquired in the subsequent imaging sequence is used for image reconstruction. Use as data. However, the direction of displacement of body motion is not considered. Disclosure of the invention

An object of the present invention is to provide a body motion arch by a subject's respiratory movements irrespective of the respiratory phase. The objective is to realize an MRI device that can suppress facts.

In order to achieve the above object, the present invention is configured as follows.

(1) A magnetic resonance imaging apparatus according to the present invention comprises: a means for acquiring an echo signal required for image reconstruction to obtain an image of a subject based on a predetermined imaging sequence; and information on periodic body motion of the subject. A navigator echo measurement unit that acquires a nuclear magnetic resonance signal including a navigator echo, an allowable range setting unit that sets an allowable range of displacement of a predetermined part based on the periodic body movement of the subject, and the navigator echo from the navigator echo. A body movement determining unit that calculates a displacement of the predetermined portion and determines whether the displacement is within the allowable range; and a unit that obtains the image, wherein the displacement of the predetermined portion is within the allowable range. Image reconstruction is performed using the echo signals acquired at times.

In the magnetic resonance imaging apparatus, the body motion determining unit calculates a displacement direction of the predetermined portion from two or more navigator echoes measured at time intervals, and calculates a displacement direction of the predetermined portion and a displacement direction of the predetermined portion. Imaging timing determining means for determining a timing for acquiring an echo signal necessary for the image reconstruction based on the

(2) Preferably, in the above (1), the navigator echo measuring means acquires the two or more navigator echoes within one cardiac cycle of the subject.

(3) Also, preferably, in the above (2), the navigator echo measurement means acquires one or more of the two or more navigators in the one cardiac cycle before an acquisition period of an image reconstruction echo signal. Then, the other one or more are acquired after the acquisition period of the image reconstruction echo signal.

(4) In addition, preferably, in the above (1), the navigator echo measuring means acquires one or more navigator echoes per one cardiac cycle.

(5) Also, preferably, in the above (1), further comprising a body motion prediction function creating means for creating a body motion prediction function representing the periodic body motion using three or more navigator echoes, Calculates the displacement and displacement direction of the predetermined portion at a desired time by fitting the displacement of the predetermined portion obtained from the two or more navigator echoes to the body motion prediction function.

(6) Further, preferably, in the above (1), the body movement determining means is The allowable range is adjusted according to the variation in the displacement of the fixed part.

(7) Preferably, in the above (1), the allowable range setting means includes means for setting the acquisition timing of the two or more navigator echoes and the allowable range.

(8) Also, preferably, in the above (1), the imaging timing determining means is configured such that the displacement of the predetermined portion obtained by the body motion determining means is within the allowable range and the direction of the displacement is the same. At this time, the imaging timing is determined so as to acquire the image reconstruction echo signal.

(9) Also, preferably, in the above (5), the imaging timing determining means is configured to obtain the image reconstruction echo signal acquisition time from the two or more navigator echo acquisition times and the body motion prediction function. And the acquisition period.

(10) Preferably, in (9) above, the measurement of the navigator echo is repeated until the imaging timing determination means can determine the acquisition start time and the acquisition period of the image reconstruction echo signal.

(11) In addition, preferably, in the above (5), the body motion prediction function uses at least a displacement obtained from a navigator echo over a quarter cycle of the body motion of the subject, Created every time.

(12) Also, preferably, in the above (5), the body motion prediction function creating means updates the body motion prediction function using navigator echoes sequentially acquired for body motion determination.

(13) Also, preferably, in the above (12), one or more navigator echoes are provided during a period from the end of the acquisition period of the image reconstruction echo signal to the start of acquisition of the next image reconstruction echo signal. And updates the body motion prediction function using the navigator echo.

(14) Also, preferably, in the above (13), the means for creating a body motion prediction function includes a period for acquiring the navigator echo and a period for a navigator echo used for updating the body motion prediction function. There is a means to specify on the body motion prediction function.

(15) Also, preferably, in the above (2), the body movement of the subject is a respiratory movement. In this case, the body movement determining means determines one of the displacement directions as an expiration period and the other of the displacement directions as an inspiration period.

(16) Also, preferably, in any of the above (1) to (4), the body movement judging means is based on a first derivative or a difference of the displacement of the predetermined portion calculated from the two or more navigator echoes. The displacement direction of the predetermined part is calculated.

(17) Preferably, in the above (1), the imaging sequence is continued without acquiring an echo signal during a period other than the acquisition period of the image reconstruction echo signal.

(18) Preferably, in the above (1), the time interval between the two navigator echoes is about 100 ms, and the detection accuracy of the displacement is 0.7 mm to 0.8 mm. Resolution. BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagram showing an overall outline of an MRI apparatus to which the present invention is applied.

FIG. 2 is a diagram illustrating an example of the imaging method according to the first embodiment of the present invention.

FIG. 3 is a diagram illustrating the effect of the imaging method according to the first embodiment of the present invention.

FIG. 4 is a diagram showing another example of the imaging method according to the first embodiment of the present invention.

FIG. 5 is a diagram showing another example of the imaging method according to the first embodiment of the present invention.

FIG. 6 is a diagram showing a procedure of an imaging method according to the second embodiment of the present invention.

FIG. 7 is a diagram illustrating creation of a body motion prediction function by the imaging method shown in FIG. FIG. 8 is a view for explaining the main measurement in the imaging method shown in FIG.

FIG. 9 is a diagram for explaining function fitting (second fitting). FIG. 10 is a diagram illustrating an example of an imaging method according to the second embodiment of the present invention.

FIG. 11 is a diagram showing another example of the imaging method according to the second embodiment of the present invention.

FIG. 12 is a diagram illustrating an imaging method to which the second aspect of the present invention is applied.

FIG. 13 is a diagram for explaining the application of the present invention to the imaging method shown in FIG.

FIG. 14 is a diagram illustrating an example of a UI for specifying a navigator echo acquisition timing. BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the MRI apparatus of the present invention will be described with reference to the drawings.

FIG. 1 is an overall schematic configuration diagram of an MRI apparatus to which the present invention is applied. The MRI apparatus includes a magnet 102 for generating a static magnetic field in a space into which the subject 101 is inserted, a gradient magnetic field coil 103 for generating a gradient magnetic field in this space, and a An RF coil 104 that generates a high-frequency magnetic field in the imaging region, a nuclear magnetic resonance signal generated by the subject 101. An RF probe 10'5 that detects the (MR) signal, and a subject 1 in the static magnetic field space And a bed 1 1 2 for inserting 0 1.

The gradient magnetic field coil 103 is composed of gradient magnetic field coils in three directions (X, Y, Ζ) orthogonal to each other, and generates a gradient magnetic field in response to a signal from the gradient magnetic field power supply 109. Depending on how these gradient magnetic fields are applied, the imaging section of the subject can be determined, and positional information can be added to the MR signal.

The RF coil 104 generates a high-frequency magnetic field in accordance with a signal from the RF transmitter 110. The signal of the RF probe 105 is detected by the signal detection unit 106, processed by the signal processing unit 107, and converted into an image signal. The image is displayed on the display unit 108.

The gradient magnetic field power supply 109, the RF transmitter 110, and the signal detector 106 are controlled by the controller 111. The time chart of the control operation is generally called a pulse sequence, and various pulse sequences (imaging sequences) determined by an imaging method are stored in advance in a storage unit (not shown) of the control unit 111 as a program. . The control unit 111 includes an input device (user interface: uI) for selecting an imaging sequence and inputting imaging parameters and the like, in addition to the storage unit.

In the MRI apparatus of the present invention, a sequence (navigator sequence) for generating and acquiring a navigator echo every day as a body motion monitor is executed independently of the imaging sequence or in combination with the selected imaging sequence. can do.

A navigator sequence is a technique that locally excites a site of interest (such as the diaphragm) to be monitored using a high-frequency magnetic field and a selective gradient magnetic field, and outputs an echo (navigator echo) without a phase encoding gradient magnetic field from this local excitation region. Get This is the sequence to be performed.

For example, the navigator sequence can be appropriately executed by the user in accordance with the selection of the imaging sequence, but is executed by the control unit 111 as a predetermined sequence combined with the imaging sequence.

The signal processing unit 107 (control unit 111) calculates the displacement of the target region based on the navigator echo acquired by the navigator sequence in addition to the above-described operations such as image reconstruction and determines the displacement direction. Then, based on the result, the timing of data acquisition in the imaging sequence is determined.

Next, an example in which respiratory motion artifact suppression using the MRI apparatus having the above configuration is applied to electrocardiographic synchronized imaging will be described.

FIGS. 2, 4 and 5 are diagrams showing examples of an imaging method using the MRI apparatus according to the first embodiment of the present invention, wherein 202 is a cardiac cycle, and 203 is a body movement. , 209 indicates a sequence, 210 indicates data, and below that indicates a data processing procedure.

In the first embodiment, based on a plurality of navigator echoes acquired at different times, the displacement of a predetermined portion of the subject at each time is calculated, and whether the respiratory motion at that time is in the expiratory period is calculated. Judgment is performed during the breathing period. Image data is acquired when the displacement is within the specified range and is either the expiration period or the inspiration period. First, the imaging method of FIG. 2 will be described as a first embodiment.

After the detection of the heart radio wave 201 and the elapse of the delay time 206, the two navigator sequences 207 and 208 are successively executed prior to the main measurement 209. Navigator In the following main measurement sequence 209, it is determined whether or not to acquire data using the two navigator echoes acquired by the evening sequence 207 and 208.

For this reason, first, a projection image is created by performing a one-dimensional Fourier transform on the navigator echo in the frequency encoding direction (steps 212, 216), and from this projection image, a region of interest (for example, a diaphragm or the like) to be monitored is determined. The displacements 204 and 205 are obtained (steps 21 and 21). The displacement is detected from the signal intensity profile of each projected image by the edge (in the case of the diaphragm, the part where the signal intensity changes suddenly in the lung (low signal) and liver (high signal)). Is used.

Next, using these displacements 204 and 205, the direction of the displacement (inclination between the two displacements) is determined (step 214). Specifically, the first derivative of these displacements 204 and 205 with respect to time is obtained. Alternatively, simply, the difference between the two displacements 204 and 205 may be obtained, thereby determining whether the inclination is positive or negative. If the inclination between the two displacements is positive, the expiration period is determined, and if the inclination is negative, the inspiration period is determined (step 2 15).

When determining the expiration or inspiration period from two navigator echoes, it is desirable to set the time interval between the two navigator overnight sequences appropriately according to the displacement detection accuracy.

When the respiratory movement of the diaphragm was actually monitored using a navigator echo, it was found that the diaphragm was moving at a rate of 0.6 to 0.7 mm in 100 ms. Therefore, when the napige-evening-sequences 207 and 208 are executed at an interval of about 100 ms, the detection accuracy requires a resolution of 0.7 to 0.8 mm. If the distance between the navigator sequences 207 and 208 is increased, the resolution may be reduced accordingly.

Next, it is determined whether or not the displacements 204 and 205 at the two times are within predetermined ranges (gate windows 211), respectively (step 218). The gate window 211 can be set, for example, by imaging the signal of the target region obtained by the navigator echo and inputting an appropriate range from the input device of the control unit 111 based on the position. .

From the judgment results of Steps 2 15 and 2 18, it is preferable that both displacements 204 and 205 are within 211 during the expiration period, or both displacements during the inspiration period This measurement data is acquired only under one of the conditions when 204 and 205 are inside the gate window 211.

However, provided that the displacements 204 and 205 are within the gate window 211, both the expiratory and inspiratory periods can be performed simultaneously during one cycle, or the expiratory and inspiratory phases can be performed during multiple cycles. By mixing the above and obtaining the main measurement data, it is possible to shorten the entire imaging time.

The navigator sequence 207, 208 and the main measurement sequence 209 as described above are repeated for each cardiac cycle to acquire the data required for one image (step 21). 9).

Since the respiratory fluctuation 203 is performing cyclical movement, if the displacement and the respiratory phase (expiratory or inspiratory phase) immediately before the start of this measurement sequence 2009 are the same, the repetition for each cardiac cycle Even when the main measurement sequence 209 is executed, the displacement becomes the same every time, and the influence of the displacement due to the respiratory movement, that is, an image with reduced body movement artifact can be obtained.

This is shown in Figure 3. In the figure, the same elements as those in FIG. 2 are denoted by the same reference numerals. Further, 301 and 302 are body motions in different one-heart cycles. If the displacement 2 0 3 detected in one cardiac cycle and the displacement detected in the other cardiac cycle are within the specified ranges, their directions are the same, so the displacement changes almost the same (the position change width 3 10 In each cardiac phase, the displacement is kept within a certain range.

On the other hand, if the direction of displacement is not taken into account, the position changes in the negative direction after displacement 203 in one cardiac cycle (301), and changes in the positive direction in other cardiac cycles. Assuming (3 0 3), the position change width 3 1 1 for each cardiac cycle is as shown in the lower part of the figure. That is, in the time phase in which the elapsed time from the acquisition of the navigator echo is short, the position change width 3 1 1 in each cardiac cycle can be small, but if the elapsed time from the acquisition of the navigator echo is long, The position change width 3 1 1 increases, and the effect of suppressing the respiratory movement artifact cannot be obtained.

As a result of considering the direction of displacement as described above, it is possible to obtain an image in which body motion artifacts are uniformly suppressed in each cardiac phase. In addition to this effect, since the napige sequence is executed only twice per heartbeat, the dead time of the Benavige sequence is reduced compared to the conventional technology, and images of many cardiac phases can be acquired. It becomes possible.

FIG. 4 shows a second embodiment. In the figure, 402 and 403 indicate a cardiac cycle, 404 indicates body movement, 415 indicates a main measurement sequence, and 416 indicates data. In the embodiment shown in FIG. 2, the navigator echo is acquired twice consecutively before the start of the main measurement in each cardiac cycle, but in the embodiment shown in FIG. 4, it is acquired once before and after the main measurement. After the heartbeat signal 410 is detected and the delay timer 409 has elapsed, the navigator sequence 410 is executed prior to the main measurement sequence 415. The navigator sequence obtained by the navigator sequence 410 is subjected to a one-dimensional Fourier transform in the frequency encoding direction to create a projection image (step 420), and the displacement 405 of the target region is obtained (step 4). twenty one ). Subsequently, the main measurement sequence 4 15 is executed to obtain the main measurement data 4 16 (step 4 2 2). After that, the second navigator sequence 4 11 1 is executed, and the acquired navigator evening echo is reconstructed (step 4 23), and the displacement 4 06 is obtained (step 4 2 4).

Take the first derivative or difference between the two obtained displacements 4 0 5 and 4 0 6 (Step 4 2 5), determine whether the slope of the respiratory movement waveform 4 0 4 is positive or negative, and during the expiration period It is determined whether or not there is an inspiration period (steps 4 and 26).

Next, it is determined whether or not the displacements 405 and 406 obtained from the two echoes every night are within the gate window 419 (step 427). According to the judgment results of the above two judgment steps 4 26 and 4 27, when the expiration period and the displacements 4 0 5 and 4 0 6 are both within the gate window 4 19, or The main measurement data acquired when either of the conditions during the breath period and when the displacements 4 0 5 and 4 0 6 are within the gate window 4 19 are used for image reconstruction (step 4 2 8). In the example shown in the figure, the main measurement data 4 16 in the cardiac cycle 40 2 which is determined to be the expiration period and the displacements 4 0 5 and 4 0 6 are within the gate window 4 19 are image-reconstructed. The measurement data 418 is not used for image reconstruction in the cardiac cycle 403 determined to be in the inspiration period from the two displacements 407 and 408 used.

In addition, this example shows the case where the displacement 408 is outside the gate window 419. Not used for reconstruction.

In addition, when the main measurement data 4 16 is used for image reconstruction in the cardiac cycle 402 which is determined as the inspiration period and the displacements 4 05 and 4 06 are both within the gate window 4 19, Even if the displacement 408 is within the gate window 411 during the expiration period, the measurement data 418 is not used for image reconstruction.

Also in the present embodiment, on the condition that the displacements 405 and 406 are within the gate window 419, both the expiration period and the inspiration period are simultaneously performed during one cycle, or during a plurality of cycles. By mixing the expiration period and the inspiration period and using this measurement data for image reconstruction, it is possible to shorten the entire imaging time.

Also, FIG. 4 shows an example in which the processing is performed in the order of 4 2 5 → 4 2 6 → 4 2 7; however, the processing for determining the expiration period and the inspiration period and whether the displacement is within the gate window. The order of the determination may be exchanged. That is, the processing may proceed in the order of 427 → 425 → 426.

Thus, also in the second embodiment, similar to the first embodiment, it is possible to obtain images of many cardiac phases without respiratory motion artifacts. In addition, since the respiratory motion 404 is a periodic motion, if the displacement measured before and after the main measurement and the respiratory period are known, the respiratory motion displacement during the main measurement sequence can be estimated, and data is acquired with the same displacement every time. It is possible.

Further, in the present embodiment, since the displacement before and after the main measurement sequence is measured, acquisition of the main measurement data within the gate window can be more reliably performed.

FIG. 5 shows a third embodiment. 5 0 2 and 5 0 3 indicate a cardiac cycle. 5 0 4 indicates body motion, 5 1 2 and 5 1 4 indicate a sequence, 5 1 3 and 5 1 5 indicate data. In the first and second embodiments, one navigator echo is acquired twice in each cardiac cycle, but in the example shown in FIG. 5, only one echo is acquired before the main measurement. After detecting the heart radio wave 501 in the first cardiac cycle 502, and after the elapse of the delay time 508, the navigator sequence 509 is executed prior to the main measurement.

From the navigator echo acquired by the navigator sequence 509, the displacement 505 of the target part at that time is obtained in the same manner as in the first and second embodiments (steps 516, 5). 1 7). After execution of the main measurement sequence 512 following the navigator sequence 509, the main measurement data 513 is acquired (step 518), and the process proceeds to the next cardiac cycle 503. Similarly, after the delay time 508 in the next cardiac cycle 503, the Navigator overnight sequence 510 is executed, the echo is reconstructed (step 519), and the displacement 506 is obtained. (Step 520).

Using the displacement 505 obtained in the cardiac cycle 502 and the displacement 506 obtained in the cardiac cycle 503, the first derivative or difference is obtained between the two, and the slope of the respiratory movement waveform 504 is calculated. Determine whether it is positive or negative (step 5 2 1) and determine whether it is in expiration or inspiration (step 5 5 2 2)

Next, it is determined whether or not the displacements 505 and 506 are within the gate window 516 (step 523), and based on the determination results of steps 522 and 523, the expiration period is determined. And when the displacements 505 and 506 are within the gate window 516, or during the inspiration phase and when the displacements 505 and 506 are within the gate window 516. The main measurement data 5 13 is used for image reconstruction under any one of the conditions (step 5 2 4).

Thereafter, similarly, using the displacement 506 detected in the second cardiac cycle 503 and the navigation of the next cardiac cycle — the displacement 507 obtained in the evening sequence, the expiration period or the inspiration period It is determined whether or not the measurement data is within the gate window, and whether or not the acquisition of the main measurement data is used for image reconstruction.

Here, in the case of displacements 506 and 507, the second heart is determined because the slope of the respiration waveform 504 is opposite and the displacement 507 is outside the gate window 516. The main measurement sequence 514 of the cycle 503 is not used for image reconstruction. Unlike the case shown in the figure, when the data is acquired only during the inspiration period and the displacement is within the gate window, the inclination is reversed and one of the displacements is outside the gate window. In the case of, do not use the main measurement sequence data during that period for image reconstruction.

Also in this embodiment, on the condition that the displacements 505 and 506 are within the gate window 516, both the expiration period and the inspiration period are simultaneously performed during one cycle, or during a plurality of cycles. By mixing the expiration period and the inspiration period and using this measurement data for image reconstruction, it is possible to shorten the entire imaging time. Further, as in the case of FIG. 4, the order of the processing of determining the expiration period and the inspiration period and the determination of whether or not the displacement is within the gate window may be exchanged. That is, the processing may proceed in the order of 5 2 3 → 5 2 1 → 5 2 2.

Also in this embodiment, as in the case of the above-described embodiment, since the respiratory movement 504 is a cyclic movement, if the displacement and the respiratory period are known, the respiration during the main measurement sequence 5 12 The dynamic displacement can also be estimated, and data can be acquired with the same displacement every time. Further, as in the first and second embodiments, there are no respiratory motion artifacts, and images of many cardiac phases can be acquired.

As described above, according to the first aspect of the present invention, before the present measurement sequence Alternatively, the navigator sequence is executed before and after, and the displacement and the direction of the displacement are calculated from the navigator echo obtained thereby, and based on the result, whether the main measurement data is acquired or not is used for image reconstruction. judge. As a result, it is possible to reduce the displacement between the main measurement data and obtain an image in which the body motion artifact is suppressed, as compared with a case where only the displacement is simply used.

Also, by detecting the direction of the displacement, it is possible to determine whether it is the expiration period or the inspiration period of the respiratory cycle, so that only one of these data and the displacement within the specified range is obtained. An image can be reconstructed using the main measurement data.

Furthermore, when acquiring image data of a plurality of cardiac phases within one cardiac cycle, it is not necessary to execute the navigator sequence for each cardiac phase, so that the number of measurable cardiac phases can be increased.

Next, an imaging method using the MRI device according to the second embodiment of the present invention will be described. Also in the second embodiment, the configuration of the MRI apparatus is the same as that of the MRI apparatus according to the first embodiment, but in the second embodiment, the signal processing system and the control unit perform a function approximating the body motion in question. (Body motion prediction function) is stored in advance.

The body motion prediction function is created for each subject, for example, by executing the navigator sequence at least for one cycle of the body motion prior to the main measurement and using the displacement obtained from the navigator echo obtained by the navigator sequence. Can be (first fitting).

At the time of this measurement, we acquire the Napige one night and one, and fit the displacement obtained from the navigator one with the body motion prediction function (second fitting) to determine whether to acquire this measurement data. judge. For this reason, the signal processing system and the control unit are provided with a navigator overnight sequence as a pre-measurement, and are provided with a function fitting function using a navigator echo.

One embodiment of an imaging method using an MRI apparatus according to the second embodiment is shown in FIGS. FIG. 6 is a flowchart showing the procedure of the imaging method. This imaging method involves the steps of creating a body motion prediction function for each subject, obtaining the time and interval for acquiring the main measurement data (steps 61 to 65), and the procedure for the main measurement. (Steps 606 to 616), FIG. 7 is a diagram illustrating steps 601 to 605, and FIG. It is a figure explaining 06-616.

First, the navigator sequence 701 is executed continuously over at least one period 702 of the body motion to acquire a plurality of navigator echoes (step 601). A projection image is created by performing a one-dimensional Fourier transform of the navigator echo in the frequency encoding direction, and the displacement of a target part (for example, a diaphragm) to be monitored from the projection image is determined by the acquisition time of the navigator echo. Ask every time (Step 602).

As the position detection method, a method such as detecting an edge from a signal intensity profile of each projection image can be used. The time displacement of the displacement obtained in step 602, that is, the respiratory cycle is approximated by a polynomial function (first fitting, step 603). The function that approximates the respiratory cycle (body motion prediction function) can be calculated using, for example, a method such as the least squares method. For example, it is a polynomial including periodic functions such as sin and cos. In addition, it is desirable to include a periodic function that has a period longer than the first-order term or body motion period that reflects the drift component. Specifically, a function f as shown in the following equation (1), the following equation (2), or the following equation (3) can be used.

f = ∑ f n (t) = a * s in (b t + c) + d (1)

f = a * s i n (b t + c) + d t + e (2)

f = a-s i n (t + c) + d * s i n (e t + f) (3)

Here, n is an integer starting from 1, t is an elapsed time from a desired reference time, and a, b, c, d, e, and f are coefficients. '

Figure 7 shows the body movement displacements 704, 70 detected in steps 601 to 603 above.

5 · · · 71 1 and the body motion prediction function 720 are shown. If one cycle of breathing 702 is about 4 seconds, and the navigator sequence 701 is executed every 200 ms, 20 navigators can be acquired in one cycle.

When the body motion prediction function is determined in this way, the time and the interval during which the body motion displacement predicted by the body motion prediction function is within the specified range 703 are obtained (step 604). For example, the time may be counted as 0 when the first navigator sequence acquisition is performed, or the first navigator sequence may be started in synchronization with the electrocardiogram (or pulse wave) R wave, and the R wave detection may be started. The time may be counted as 0. Next, the first derivative of the body motion prediction function within the designated range 703 is obtained, and it is determined whether the value is positive or negative (step 605). Thereby, the expiration period and the inspiration period of respiration are determined.

In the example of FIG. 7, the period 714 is an inspiratory period with a negative differential value, and the period 715 is an expiratory period with a positive differential value. For the expiration period and the inspiration period, the start time 712, 713 of the period and the intervals 714, 715 of the period are stored, respectively. As described above, the time count in this case is based on the first acquisition of the navigator echo or the detection of the R wave. The period 714 or the period 715 is the time when the main measurement data is obtained.

Next, the main measurement is started (step 606). As shown in Fig. 8 (b), this measurement includes navigator sequences 801 and 803 and main measurement sequences 811 and 813.First, at least two navigator sequences 801 and 803 Run and get two or more navigator echoes (step 607).

These navigator echoes are reconstructed, and displacements 802, 804 of the target part at the time of acquisition are detected (step 608). The displacements 802, 804 · · · · are fitted to the body motion prediction function 720 (Fig. 8 (a)) (second fitting, step 609).

When the body motion prediction function 720 is, for example, a function as shown in FIG. 9, a method of performing the second fitting during the main measurement is as follows.

The displacement P 1 of the target part obtained by executing the napige-interter sequence is substituted into a function 720. The time determined there is T1, T1, and cannot be uniquely determined. However, the displacement P2 of the target part obtained by executing the next Navigator overnight sequence is substituted into the function to obtain the time T2, T2 'And give the condition that the time corresponding to Ρ1 is later than the time corresponding to Ρ2. Thus, it is determined that the displacement fits the displacement at time Τ2 '. .

Thus, the second fitting can use two displacements, but in reality, the respiratory amplitude is not exactly the same every cycle, so to give a certain width, as shown in the figure, It is preferable to execute the navigator overnight sequence several times to perform the second fitting. When the position in the respiratory cycle at the time of navigator echo acquisition is determined by the second fitting, the start time and acquisition time of main measurement data acquisition are estimated based on the position (step 610). In the example shown in FIG. 8, the displacements 82, 80 obtained by executing the navigator sequence 801, 803, 805, 807 four times are shown.

By fitting 4, 806 and 807 to the body motion prediction function 720, the time from when the last Navigator overnight echo 807 was obtained (P t) to the start of the main measurement sequence d T and de overnight acquisition time can be estimated.

If the number of navigator echoes is too small or the respiratory cycle is irregularly disturbed, etc.

Steps (Step 6 07) to estimation are repeated (Step 6 11). If the time dT until the start of actual measurement data acquisition and the data acquisition time can be estimated, this measurement sequence is executed until that time elapses, but the blanking period 812 where no data is acquired (step 6 12), and then acquire the main measurement data during the acquisition time estimated in advance (step 6 13). Then, it is determined whether it is within the estimated acquisition time (step 614), and if it is within the estimated acquisition time, the process returns to step 613. If the estimated acquisition time is out of the estimated acquisition time in step 6 14, the process proceeds to step 6 15 to end the data acquisition.In step 6 16, it is determined whether or not the imaging is completed. Return to step 6 1 2.

Assuming that the respiratory cycle does not fluctuate and is almost constant, the main measurement data is repeatedly acquired for a predetermined time after each respiratory cycle T, and the main measurement is performed whenever the body motion is within the specified range 703 Day evening can be obtained.

Performing the main measurement sequence between the acquisition of the main measurement data and the acquisition of the next main measurement data but not acquiring the data improves the image quality, for example, in the free precession steady state measurement. can do.

As described above, according to the present embodiment, the acquisition start time and the acquisition time of the main measurement data are estimated by performing the second fitting of the displacement obtained from the navigator echo to the approximate function of the respiratory cycle obtained in advance. Thus, it is possible to acquire images when the body motion is within the specified range in each of the expiration period and the inspiration period.

Also, there is no need to execute the napige In this case, it is possible to shorten the imaging time, and in the case of ECG-gated imaging, more cardiac phase data can be acquired.

The above embodiment can be further modified in response to irregular or stepwise changes in the respiratory cycle. As an example of such a change, FIGS. 10 and 11 show an embodiment in which a navigator sequence is performed using a period between main measurement (the blanking period in the embodiment of FIG. 8).

Also in these embodiments, prior to the main measurement, obtaining a body motion prediction function that approximates the respiratory cycle from navigator overnight echoes acquired by the navigator sequence is the same as in the above embodiment. Also, in this measurement, it is the same as acquiring multiple Napige echoes every night to determine the start of acquisition of this measurement data

However, in the embodiment shown in FIG. 10, the navigator sequence 91, 90 3 is repeated several times using the waiting time between the acquisition of the main measurement data and the acquisition of the next main measurement data. Execute. The navigator echoes 902 and 904 obtained by this navigator sequence are used for detecting the body motion position at that time, and are used for updating the body motion prediction function created by the previous measurement.

That is, first, the displacement 902 detected from the navigator connector 901 is fitted (second fitting) to the body motion prediction function 900 present at that time (step 915), and the measurement data is obtained. The data acquisition start time and the data acquisition time interval are estimated (step 916). If necessary, set the blanking period 9 12 before obtaining the main measurement data 9 1 1. On the other hand, a new body motion prediction function is calculated using the detected displacement 902 (step 917). Since the displacement 9 02 obtained by the Navigator overnight sequence 9 0 1 is part of the respiratory cycle, a new body motion prediction function is calculated using the previous motion estimation function and a part of this displacement. . The previous body motion prediction function is updated with the new body motion prediction function, and this is used for estimating the acquisition start time and data acquisition time of the next main measurement 9 13.

The displacement 9 04 detected from the navigator sequence of the navigator sequence 9 03 executed after the main measurement 9 11 1 is fitted to the body motion prediction function updated by the displacement 9 0 2 (the second fitting, Step 9 18), estimate and start the data acquisition start time and acquisition time of the main measurement 9 13 (step 9 19). After that, the start of data acquisition and the data acquisition time of the actual measurement are estimated using the navigator echo acquired before the actual measurement, and the measurement is performed while updating the body motion prediction function used thereafter.

As described above, in this embodiment, the navigator one-by-one sequence is executed using the time to wait for the acquisition of the main measurement data, and the body motion prediction function is sequentially updated. Even if the period changes, it is possible to correct the data acquisition time lag.

Also in the embodiment shown in FIG. 11, the execution of the navigator sequence between the acquisition of the main measurement data and the acquisition of the next main measurement data is the same as the embodiment of FIG. However, in this embodiment, a fixed second fitting section is set in advance, and the number of navigator echoes for performing the second fitting is sequentially added or updated. By setting a certain section in which the second fitting is performed, it is possible to perform the second fitting for the main measurement time estimation periodically, and it is possible to estimate the actual measurement time closer to the actual one.

If the time other than the actual measurement can be predicted by the estimation of the actual measurement time previously executed, the second fitting section can be set in advance at that time.

In other words, the second fitting is performed based on the displacement detected in section 1 0 1 1 to estimate the time of data acquisition of main measurement 1 0 1 2, and in the next section 1 0 1 2, it is included in section 1 0 1 1 Using the navigator echo to be obtained and the newly obtained navigator echo, a second fitting is performed to estimate the data acquisition time of the main measurement 102 2.

Hereinafter, similarly, the second fitting is sequentially performed using a part of the navigator echo included in the previous section and the newly obtained navigator echo.

In this case, the interval of the second fitting section is shorter than the respiratory cycle, and the second fitting section overlaps each other. Therefore, as time passes, for example, the second fitting section 1 After estimating the data acquisition time of main measurement 1 0 2 3 in 0 1 3, the same data acquisition time of main measurement 1 0 2 3 will be estimated in the next second fitting section 1 0 1 4 . In this case, based on the fitting results performed at a time close to the main measurement 1023, the main measurement 1023 Get data.

As described above, according to the present embodiment, the estimation result of the acquisition time of the main measurement data can be made more accurate by sequentially updating the navigator echo used for the second fitting.

In this embodiment, as in the embodiment of FIG. 10, it is preferable to update the body motion prediction function using the navigator echo used for fitting (first fitting).

That is, the second fitting is performed using the navigator echo measured between the main measurement and the body motion prediction function is sequentially updated (first fitting). As a result, even if the respiratory cycle changes, it is possible to correct the data acquisition time lag.

As described above, according to the MRI apparatus according to the second embodiment of the present invention, a body motion prediction function that approximates the body motion cycle is created prior to the main measurement, and the body motion prediction function and the The time at which the actual measurement data is acquired and the data acquisition time are estimated from the navigator echo acquired at this time, so that the deviation between the actual displacement and the predicted displacement at the time of the actual measurement can be reduced. This makes it possible to reliably predict body motion artifacts.

In particular, it is possible to reliably identify the expiration period and the inspiration period and acquire the respective images. Also, when acquiring images for multiple cardiac phases within one cardiac cycle, such as ECG-gated imaging, there is no need to execute the navigator sequence for each measurement of each phase, thus substantially increasing the measurement time. This can shorten the measurement time or increase the number of cardiac phases.

Furthermore, by executing the navigator sequence using the waiting time between the main measurement and the main measurement, the fitting result can be made more reliable, and the body motion prediction function is updated using the navigator sequence result. By doing so, even when the body movement period changes, the main measurement data can be acquired when the displacement is always within the specified range without any deviation.

In each embodiment, when a body motion prediction function including a first-order term such as equation (2) is used as the body motion prediction function, it is possible to cope with a temporary change in the position of the subject. it can.

FIG. 12 is a diagram illustrating a body motion waveform when the position of the subject moves. As shown in the figure, because the subject moved from time t1 to t2, the waveform was deformed and the measurement site was out of the body movement range 1201 originally intended to be measured, and the initial specified range was maintained. Effective data cannot be obtained by capturing images.

Such a body motion waveform is calculated by, for example, a body motion prediction function including a first-order term as in the above equation (2) or a weighted sum of a body motion cycle as in the above equation (3) and a periodic function having a longer cycle. It can be approximated by the body motion prediction function represented, and as shown in Fig. 13, the imaging position and the specified range (gate window) are updated in accordance with the change in the waveform due to this first-order term or long-term term (1201). -Changed from 1202 to 1203) By doing so, the main measurement data can be acquired when the measurement site is within the specified range. Also, the gate window can be updated without using the body motion prediction function. For example, the average value of the position is measured every breathing cycle (405, 406, 407, 408,... In the example of FIG. 4), and the gate window is changed as shown in FIG. 13 based on the change in the average value. Alternatively, the gate window may be set directly on a screen having a UI as shown in FIG. 14 described later.

The update of the imaging position can be realized by the interactive scan control (ISC) technology that changes the imaging position in real time.

FIG. 14 shows an embodiment of a UI for designating the timing of acquiring the navigation data in the MRI apparatus of the present invention. Fig. 14 shows an example of a screen displayed as a UI.If R waves 1401, 1402 obtained from an electrocardiograph, a body motion prediction function 1403, and a gate window gate window 1404 are displayed, In addition, a menu 1405 for specifying the acquisition timing of the navigator echo is displayed.

In the menu 1405, for example, it is possible to specify before or after the main measurement, and also to specify the number of navigator echoes. For example, if you select “Head l” in the menu, one navigator echo will be acquired before the main measurement sequence, and if “Head 2” is selected, two navigator echoes will be obtained before the main measurement sequence. To be chosen. When “Head & T ai 1” is selected, acquisition of one navigator echo before and after the main measurement sequence is selected.

The specification of the navigator sequence execution timing is, for example, a point displayed on the body motion prediction function 1443. The specified content is displayed as a schematic figure. In the example shown in the figure, it is displayed that the execution of the two-time navigator one-time sequence 1406 has been designated before the main measurement sequence 1407.

The UI is not limited to the one shown in the figure, but as described above, the R wave, the body motion prediction function, the napige overnight sequence and the main measurement sequence are displayed, and the napige pattern is displayed.

—By making it possible to specify the number of glances and the timing of execution on the screen, the execution of the present invention can be facilitated. Industrial applicability

According to the MR apparatus of the present invention, two or more navigator

—Since the timing to acquire the echo signal for image reconstruction is determined from the displacement and displacement direction of the subject obtained using the echo signal, the movement within the body movement range specified by a small number of navigator echo signals is determined. Acquisition of an echo signal for image reconstruction can be realized.

Thus, body motion artifacts can be reliably suppressed. In addition, since the time required for the navigation-sequence sequence can be reduced, the main measurement time can be prevented from being compressed in ECG-gated imaging, etc., and images with more cardiac phases can be acquired. can do.

Further, according to the present invention, the expiration period and the inspiration period in the respiratory cycle can be identified by determining the displacement direction and determining the imaging timing according to whether the displacement direction is positive or negative. Each of these images can be captured.

Furthermore, according to the present invention, a body motion prediction function that approximates the body motion cycle can be obtained in advance, and the main measurement data can be obtained more reliably by using this body motion prediction function. . Also, by using the periodicity of the body motion prediction function, it is possible to estimate the time at which the measurement data should be acquired and the interval at which the measurement data is acquired. -Based on the estimation of the time at which this measurement data should be acquired and the intervals at which it should be acquired, execute the navigation sequence for the rest of the time. If the navigator sequence is executed, the navigator echo obtained from the navigator sequence can be used to update the navigator used for displacement calculation—update the echo signal and update the body motion prediction function. It can be carried out. As a result, it is possible to more accurately estimate the imaging timing using the body motion prediction function, and it is possible to minimize the deviation when there is a change in body motion.

Claims

The scope of the claims

1. means for acquiring an echo signal necessary for image reconstruction based on a predetermined imaging sequence to obtain an image of the subject (101);

Navigator echo measuring means (105, 106) for acquiring a nuclear magnetic resonance signal including periodic body motion information of the subject as a navigator echo (207, 208, 410, 509);

Allowable range setting means (108, 111) for setting an allowable range of displacement of a predetermined portion based on the periodic body movement of the subject;

Body movement determining means (107) for calculating a displacement of the predetermined portion from the navigator echo and determining whether the displacement is within the allowable range.

Means for obtaining the image, in a magnetic resonance imaging apparatus for performing image reconstruction using an echo signal obtained when the displacement of the predetermined portion is within the allowable range,

The body movement determining means calculates a displacement direction of the predetermined portion from two or more echoes of the navigator measured at intervals of time,

A magnetic resonance imaging apparatus comprising: an imaging timing determination unit (111) that determines a timing of acquiring an echo signal necessary for the image reconstruction based on the displacement and the displacement direction of the predetermined portion. .

2. The magnetic resonance imaging apparatus according to claim 1, wherein the navigator / echo / echo / measurement unit acquires the two or more navigator echoes within one cardiac cycle of the subject. apparatus.

3. The magnetic resonance imaging apparatus according to claim 2, wherein the navigator / recorder is configured to determine at least one of the two or more navigators in the one cardiac cycle before an acquisition period of an image reconstruction echo signal. A magnetic resonance imaging apparatus comprising: acquiring at least one other image after an acquisition period of the image reconstruction echo signal.

4. The magnetic resonance imaging apparatus according to claim 1, wherein said navigator one-time-one-measurement unit acquires one or more navigator echoes per one cardiac cycle.

5. The magnetic resonance imaging apparatus according to claim 1, wherein a body motion prediction function generating means for generating a body motion prediction function representing the periodic body motion using three or more navigator echoes. The body motion determining means fits the displacement of the predetermined portion obtained from the two or more navigator-evening echoes to the body motion prediction function, and calculates the displacement and the displacement of the predetermined portion at a desired time. A magnetic resonance imaging apparatus for calculating a displacement direction.

6. The magnetic resonance imaging apparatus according to claim 1, wherein the body movement determining unit adjusts the allowable range according to a change in displacement of the predetermined part.

7. The magnetic resonance imaging apparatus according to claim 1, wherein the allowable range setting means includes means for setting the acquisition timing of the two or more navigator echoes and the allowable range. .

8. The magnetic resonance imaging apparatus according to claim 1, wherein the imaging timing determination unit is configured to determine that the displacement of the predetermined part determined by the body motion determination unit is within the allowable range and the displacement direction. A magnetic resonance imaging apparatus characterized in that the imaging timing is determined so that the image reconstruction echo signal is obtained when are in the same direction.

9. The magnetic resonance imaging apparatus according to claim 5, wherein the imaging timing determination unit obtains the acquisition start time and acquisition of the echo signal for image reconstruction from the acquisition time of the two or more navigator echoes and the body motion prediction function. A magnetic resonance imaging apparatus characterized by determining a period.

10. The magnetic resonance imaging apparatus according to claim 9, wherein the imaging timing determination unit repeats the measurement of the navigator echo until the acquisition start time and the acquisition period of the image reconstruction echo signal can be determined. Magnetic resonance imaging device.

11. The magnetic resonance imaging apparatus according to claim 5, wherein the body motion prediction function is at least a displacement obtained from one echo of Napige over one-fourth cycle of the body motion of the subject. A magnetic resonance imaging apparatus characterized in that the magnetic resonance imaging apparatus is created every time.

12. The magnetic resonance imaging apparatus according to claim 5, wherein the body motion prediction function creating means updates the body motion prediction function using navigating one-by-one echoes sequentially acquired for body motion determination. A magnetic resonance imaging apparatus comprising:

13. The magnetic resonance imaging apparatus according to claim 12, wherein at least 1 or more is set between the end of the acquisition period of the image reconstruction echo signal and the start of acquisition of the next image reconstruction echo signal. A magnetic resonance imaging apparatus comprising: acquiring a navigator echo of (1); and updating the body motion prediction function using the navigator echo.

14. The magnetic resonance imaging apparatus according to claim 13, wherein the body motion prediction function creating means includes a period during which the navigator echo is acquired, and a navigator overnight echo used for updating the body motion prediction function. A means for designating each of the periods on the body motion prediction function.

15. The magnetic resonance imaging apparatus according to claim 2, wherein when the body motion of the subject is a respiratory motion, the body motion determination means sets one of the displacement directions as an expiration period and the other of the displacement directions. A magnetic resonance imaging apparatus characterized in that the direction is determined to be P breath period.

16. The magnetic resonance imaging apparatus according to any one of claims 1 to 4, wherein the body movement determination unit is configured to calculate a primary displacement of the predetermined portion calculated from the two or more navigating overnight echoes. A magnetic resonance imaging apparatus for calculating a displacement direction of the predetermined portion based on a differentiation or a difference.

17. The magnetic resonance imaging apparatus according to claim 1, wherein the imaging sequence is continued without acquiring an echo signal during a period other than an acquisition period of the echo signal for image reconstruction.

18. The magnetic resonance imaging apparatus according to claim 1, wherein a time interval between the two napiges is about 100 ms, and a displacement detection accuracy is 0.7 mm to 0 mm. A magnetic resonance imaging apparatus having a resolution of 8 mm.