JP2007185300A - Magnetic resonance imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic resonance imaging apparatus which has a high data acquisition efficiency and is capable of shortening photographing time during photography employing navigator sequences. <P>SOLUTION: After an electrocardiographic wave 301 is detected, navigator sequences 308 are executed sequentially to monitor a respiratory movement. When monitored respiration displacements 307 enter a gate window 303 as a displacement 304, a regular measurement 310 is performed to acquire multi-phase echo data. When data corresponding to one cardiac cycle are acquired with the regular measurement 310, navigator sequences 311 are executed sequentially again to monitor respiratory displacements. When monitored respiratory displacements enter the gate window 303 as a displacement 305, a regular measurement 312 of the next cardiac cycle is performed to acquire data 312. After these, the same control operation is repeated. Measurements are completed when all necessary data are acquired. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a magnetic resonance imaging apparatus (MRI apparatus) that measures nuclear magnetic resonance (NMR) signals from hydrogen, phosphorus, and the like in a subject and visualizes nuclear density distribution, relaxation time distribution, and the like.

  In recent years, clinical applications of cardiac MRI have become common. In this cardiac MRI, an artifact caused by heartbeat becomes a problem, and a technique for removing this artifact is described in Patent Document 1.

  In addition, in the cardiac MRI, artifacts due to respiratory motion are an issue, and as a method for suppressing respiratory motion artifacts, a commonly used technique is to monitor respiratory motion using navigator echo, which is an additional echo. It is a method (nonpatent literature 1).

  An example of a respiratory motion artifact reduction method using navigator echo in cardiac imaging will be described with reference to FIG. FIG. 10 is a sequence diagram of respiratory gating imaging using navigator echo in combination with electrocardiogram synchronization (Non-Patent Document 2).

  In FIG. 10, the navigator sequence 108 is executed at the timing when the main measurement (multi-phase imaging) 109 is executed after the set delay time 107 after detecting the electrocardiogram 101. A displacement 104 due to the respiratory motion of the site of interest is obtained from the navigator echo acquired by the navigator sequence 108.

  Following the navigator sequence 108, the main measurement sequence 109 is executed. In this case, when the displacement 104, 106 obtained by the navigator sequences 108, 112 is out of the narrow window (gate window) 103 set in advance, the data obtained by the main measurement 109, 113 is discarded ( Discard data 114, 115).

  On the contrary, if the displacement 105 detected in the navigator sequence 110 is within the window 103, the data of the sequence 111 immediately after the navigator sequence 110 is acquired (acquired data 115). Such acquisition control of the main measurement data is repeated, and the process ends when data acquisition necessary for image reconstruction is completed. By this method, by acquiring data at substantially the same displacement, an image in which the influence of body motion due to respiratory motion is greatly reduced can be obtained.

JP 2005-80855 A Adaptive Technique for High-Definition MR Imaging of Moving Structures, Radiology 1989; 173: 255-263, Richard L. Ehman & Joel P. Felmlee Navigator Echoes in Cardiac Magnetic Resonance, Journal of Cardiovascular Magnetic Resonance, 3 (3), 183-193 (2001)

  However, in the above prior art, since there is a lot of data to be discarded, such as the discarded data 114 and 116, there is a problem of a decrease in data acquisition efficiency. In addition, when the main measurement is multi-phase imaging of the heart, the cardiac time phase during acquisition of navigator echo cannot be acquired, so there is a problem that a cine image of the whole cardiac cycle cannot be acquired.

  FIG. 11 is an example in which a 5-phase cine image is taken with 4 heartbeats. Since the navigator echo is acquired during the cardiac phase immediately after the cardiac radio wave 201 (initial contraction 220), this measurement cannot be performed. That is, as shown in FIG. 12, in the case of cardiac imaging, since the radio wave R is detected, the navigator sequence, the idle shot, and the main measurement sequence are executed in this order. .

  In the case of cardiac imaging, the pulse sequence used for this measurement is generally an SSFP sequence as shown in FIG. 13 (TR = 3 to 6 ms). Therefore, in order to recover the steady state broken by executing the navigator sequence, blank shots (generally 10 to 20 times) are necessary, and as described above, this measurement cannot be performed even during blank shots.

  Cine images are usually used for cardiac function analysis to determine ejection fraction, etc. However, if cardiac function analysis is performed using incomplete cine images as described above, correct results may not be obtained. .

  An object of the present invention is to realize a magnetic resonance imaging apparatus that has high data acquisition efficiency and can shorten the imaging time in imaging using a navigator sequence.

  The magnetic resonance imaging apparatus of the present invention includes a static magnetic field generating means, a gradient magnetic field generating means, a high frequency signal transmitting / receiving means, and an image reconstructing means for reconstructing an image based on a nuclear magnetic resonance signal generated from a subject. , A static magnetic field generating means, a gradient magnetic field generating means, a high frequency signal transmitting / receiving means, and a control means for controlling operations of the image reconstruction means.

  Then, the control means acquires the navigator echo using the navigator echo sequence, detects the displacement due to the respiratory motion of the subject, and determines that the displacement is within a preset range. And the imaging sequence of the subject is executed.

  In addition, the magnetic resonance imaging apparatus of the present invention includes a cardiac radio wave detection unit that detects a cardiac radio wave of the subject, and the control unit has different timings for each of a plurality of cycles of the cardiac radio wave detected by the cardiac radio wave detection unit. The navigator echo sequence is executed in step S1, and the image is reconstructed using the echo data acquired by the image capturing sequence performed at a plurality of periods of the cardiac radio wave.

  According to the present invention, it is possible to realize a magnetic resonance imaging apparatus that has high data acquisition efficiency and can shorten the imaging time in imaging using a navigator sequence.

Embodiments of the present invention will be described below with reference to the accompanying drawings.
FIG. 1 is a schematic configuration diagram of an MRI apparatus to which the present invention is applied.
In FIG. 1, an MRI apparatus includes a magnet 402 that generates a static magnetic field around a subject 401, a gradient magnetic field coil 403 that generates a gradient magnetic field in a static magnetic field space, and an RF coil that generates a high-frequency magnetic field in a static magnetic field space region. 404 and an RF probe 405 that detects an MR signal generated by the subject 401.
Here, the gradient magnetic field coil 403 is configured by gradient magnetic coils in three directions of X, Y, and Z, and each generates a gradient magnetic field in accordance with a signal from the gradient magnetic field power supply 409.

The RF coil 404 generates a high-frequency magnetic field according to the signal from the RF transmitter 410. The signal received by the RF probe 405 is detected by the signal detection unit 406, processed by the signal processing unit 407, converted into an image signal by calculation, and the image is displayed on the display unit 408.
The gradient magnetic field power source 409, the RF transmission unit 410, and the signal detection unit 406 are controlled by the control unit 411, and the control time chart is generally called a pulse sequence. The bed 412 is for the subject to lie down.

In the MRI apparatus, different phase encodings are given depending on the gradient magnetic field, and echo signals obtained by the respective phase encodings are detected. As the number of phase encodings, values such as 128, 256, and 512 are usually selected per image. Each echo signal is usually obtained as a time-series signal composed of 128, 256, 512, and 1024 sampling data. These data are two-dimensionally Fourier transformed to create one MR image.
The electrocardiograph 413 detects the electrocardiogram of the subject 401 and supplies it to the control unit 411.

  Next, a navigator sequence according to the first embodiment of the present invention will be described with reference to FIG. The first embodiment is an example in the case of multiphase imaging combined with electrocardiogram synchronization. The control of the operation shown below is executed by the control unit 411.

  In FIG. 2, after the controller 411 detects the electrocardiogram 301 from the electrocardiograph 413, the navigator sequence 308 is continuously executed to monitor the respiratory motion. Here, the navigator sequence 308 is continuously performed a plurality of times, and the execution period of one navigator sequence is managed so as to be a multiple of the time 309 of one cardiac time phase (phase) of the main photographing 310.

  This is because acquisition of multi-face echo data is usually performed over a plurality of cardiac cycles, and therefore, when the delay time 302 from the cardiac radio wave is randomly shifted, data of different cardiac phases are included in one image. This is because they are mixed.

  When the respiratory displacement 307 monitored by the navigator echo enters the gate window 303 set in advance as the displacement 304, the main measurement 310 is executed thereafter to acquire multiphase echo data (acquired data 313). . When data for one cardiac cycle can be acquired in this measurement 310, the navigator sequence 311 is continuously executed again to monitor the respiratory motion displacement.

  When the monitored respiratory motion displacement enters the gate window 303 as shown by the displacement 305, the main measurement 312 of the next cardiac cycle is executed, and data 312 is acquired. In FIG. 2, the reason why there are two displacements 305 is that the execution time of the navigator sequence is adjusted to be a multiple of one phase as described above. Thereafter, the same control is repeated, and the measurement is terminated when all necessary data has been acquired.

  For example, when the number of phase encoding is 128, if 32 echoes are acquired in one main measurement, data for one image can be obtained in four measurements. For example, all data acquisition is completed in the state shown in FIG. It becomes. By the measurement method as described above, a multi-phase image covering the whole body cycle can be acquired.

As described above, according to the first embodiment of the present invention, after detecting the cardiac radio wave, the navigator sequence 308 is performed to monitor the respiratory motion, and when the respiratory displacement enters the gate window 303, the main measurement is performed. Multi-phase echo data is acquired.
As a result, when the respiratory displacement is outside the gate window, the main measurement is started, and it is avoided that the acquired data becomes unnecessary, the data acquisition efficiency is high, and the magnetic resonance imaging capable of shortening the imaging time An apparatus can be realized.

  Further, by changing the execution timing of the navigator sequence, for example, echo data of the whole cardiac cycle can be acquired in cardiac imaging.

Next, a second embodiment of the present invention will be described.
In the second embodiment, when the measurement as shown in FIG. 3 described above is performed, in order to further improve the image quality, the timing at which the navigator echo is acquired and the timing at which the main measurement data is acquired The arrangement order of the main measurement data in the k space is adjusted according to the relationship.

  In the navigator breathing gate method, data acquisition control of this measurement is performed based on the respiratory movement displacement monitored by the navigator echo. For this reason, the main measurement data acquired at a timing closer to the acquisition of the navigator echo is the respiratory movement displacement data closer to the gate window position. For this reason, the main measurement data immediately after navigator echo acquisition is arranged in a low frequency region having a large influence on image quality, and the main measurement data acquired after the most time has elapsed since navigator acquisition is arranged in the high frequency region.

  For example, in the case of Phase # 1 data in FIG. 3, as shown in (A) of FIG. 4, the data of measurement # 4 acquired at the timing closest to the navigator echo is arranged in the lowest range, and from the navigator echo. As the distance increases, measurement # 3, measurement # 1, and measurement # 2 are arranged on the high frequency side in this order.

The control method is the same in the other cardiac phases. In the example of Phase # 5 shown in FIG. 4B, measurement # 3, measurement # 1, measurement # 2, and measurement # 4 are sequentially performed from the low frequency range. Become. Such control can further improve the image quality.
Since other configurations are the same as those of the first embodiment, detailed description thereof is omitted.

Next, a third embodiment of the present invention will be described.
In the example shown in FIG. 3 described above, data for the whole cardiac cycle is acquired by a method in which the navigator sequence is repeated until the respiratory displacement enters the gate window. However, in this third embodiment, all the data are acquired by other methods. Acquire data for the cardiac cycle.

  FIG. 5 is an explanatory diagram of multiphase imaging according to the third embodiment of the present invention. In FIG. 5, the repetition time of the navigator sequence is set to, for example, one cardiac time phase, and the main measurement is performed after the respiratory displacement enters the gate window while shifting the execution timing of the navigator sequence to a different cardiac time phase. By such control, a multi-phase image of the whole cardiac cycle can be acquired as in the example shown in FIG.

In the third embodiment, the data acquisition efficiency is lower than that in the example shown in FIG. 3, but the time from the acquisition of the navigator echo to the acquisition of the main measurement data that is most distant in time is shown in FIG. Since this is shorter than the case of the above example, the image is further suppressed in the influence of respiratory motion. Similarly to the second embodiment, the image quality can be further improved by arranging the measurement data in the low frequency region of the k space sequentially from the main measurement data acquired at a time close to the navigator echo (FIG. 6). (A), (B)).
The third embodiment shown in FIG. 5 is an ideal example in which the respiratory movement displacement caused by the navigator echo is in the gate window every time. However, in practice, it can easily deviate from the gate window. Such a case is shown in FIG.

  In FIG. 7, in measurement # 1 and measurement # 2, it is assumed that the respiration displacement is within the gate window (indicated as “OK” in FIG. 7), as in FIG. In the subsequent measurement # 3, in the navigator echo measured at the time of Phase # 3, if the respiratory motion displacement has deviated from the gate window (indicated as “NG” in FIG. 7), the navigator sequence is also executed in Phase # 4. . Here, if it is confirmed by the navigator echo of Phase # 4 that the respiratory displacement is within the gate window, the main measurement is performed and data is acquired.

  Subsequently, a navigator echo is acquired at a timing different from that of measurement # 3, and if it is “0K”, main measurement data is acquired (phase # 4 of measurement # 4 to phase # 3 of measurement # 5). After executing the main measurement, the navigator echo is acquired again, and the navigator is executed until it is determined to be “0K”.

  Here, as in measurement # 6, it is assumed that “0K” is determined by navigator echo acquired at exactly the same timing as measurement # 1. In such a case, the data acquired in measurement # 6 is integrated into measurement # 1 in order to improve S / N. Thereafter, the determination by the navigator is repeated in the same manner, and the measurement is terminated when echo data can be acquired in all cardiac cycles.

  In the third embodiment, other configurations are the same as those of the first embodiment, and thus detailed description thereof is omitted.

Next, a fourth embodiment of the present invention will be described.
In the above-described first to third embodiments of the present invention, the application example to the multi-phase imaging has been shown. However, the present invention can also be applied to single-phase trigger imaging with other prepulses.

  The fourth embodiment of the present invention is an example when the present invention is applied to single-phase trigger imaging with other prepulses. FIG. 8 is an explanatory diagram of trigger imaging with a fat suppression pulse according to the fourth embodiment. In FIG. 8, the navigator sequence 708 is continuously executed to monitor the respiratory motion. When the displacement of the respiratory motion enters the gate window 703 (displacement 704), the main measurement 710 is performed.

Here, 709 added before the main measurement is a fat suppression pulse. The main measurement 710 is single-phase imaging of about 100 to 200 ms. For the same reason as in the first embodiment, the time for continuously executing the navigator is controlled by a multiple of the time of the main measurement.
By this method, the same effect as that of the first embodiment can be obtained, and fat suppression images having different delay times 702 from the cardiac radio wave 701 can be acquired.

  This fourth embodiment can be applied to, for example, a method for observing the coronary artery morphology in a plurality of cardiac phases. Furthermore, the coronary artery multi-phase image of the whole heart parent can be acquired in the same manner by replacing the navigator sequence shown in FIG. 5 with a fat suppression pulse and taking a picture in the breath holding state.

Next, a fifth embodiment of the present invention will be described.
The first to fourth embodiments described above are examples of application to imaging targeting the heart in combination with cardiac radio wave (pulse wave) synchronization, but the present invention can also be applied to asynchronous imaging. The fifth embodiment of the present invention is an example when applied to asynchronous shooting.

FIG. 9 is an explanatory diagram of the fifth embodiment of the present invention.
As shown in FIG. 9, first, the navigator sequence 605 is continuously executed repeatedly to monitor the respiratory movement displacement. When the monitored displacement enters the gate window 601 (displacement 602), the main measurement 606 is executed. Since this measurement is asynchronous, this measurement is executed at a time until data acquisition for one slice is completed.

That is, an image is acquired for each slice in the main measurements 606, 608, and 610. In addition, since it is not necessary to manage the execution time of the main measurement and the execution time of the navigator in association with each other, when the navigator is executed once like the displacements 603 and 604 and the displacement is determined to be within the gate window 601, This measurement is performed immediately.
The fifth embodiment can obtain the same effects as those of the first embodiment, and is effective for abdominal radiography that does not require electrocardiogram synchronization and in which respiratory motion suppression is essential.

  In the example described above, the cardiac radio wave of the subject is used, but the measurement may be performed in synchronization with the pulse wave of the subject.

1 is a schematic configuration diagram of an MRI apparatus to which the present invention is applied. It is explanatory drawing of the multiphase imaging | photography of the 1st Embodiment of this invention. It is acquisition explanatory drawing of the multiphase image which covers the whole body period in the 1st Embodiment of this invention. It is explanatory drawing of the 2nd Embodiment of this invention. It is explanatory drawing of the 3rd Embodiment of this invention. It is explanatory drawing of the 3rd Embodiment of this invention. It is explanatory drawing of the 3rd Embodiment of this invention. It is explanatory drawing of the 4th Embodiment of this invention. It is explanatory drawing of the 5th Embodiment of this invention. It is explanatory drawing of the navigator breathing gate method in a prior art. It is explanatory drawing of the multiphase imaging | photography using the navigator respiratory gate method in a prior art. It is a macro measurement sequence figure for every general cardiac cycle using a navigator sequence. It is a figure which shows a general SSFP sequence.

Explanation of symbols

401 Subject 402 Static magnetic field magnet 403 Gradient magnetic field coil 404 RF coil 405 RF probe 406 Signal detection unit 407 Signal processing unit 408 Display unit 409 Gradient magnetic field power supply 410 RF transmission unit 411 Control unit 412 Bed 413 Electrocardiograph

Claims (6)

Static magnetic field generation means, gradient magnetic field generation means, high frequency signal transmission / reception means, image reconstruction means for reconstructing an image based on a nuclear magnetic resonance signal from a subject received by the high frequency transmission / reception means, and the static magnetic field In a magnetic resonance imaging apparatus comprising a generating means, a gradient magnetic field generating means, a high-frequency signal transmitting / receiving means, and a control means for controlling the operation of an image reconstruction means,
The control means includes
Using a navigator sequence, navigator echoes are acquired to detect displacement due to respiratory motion of the subject, and it is determined whether the detected displacement is within a preset range, and the displacement is within a preset range. When it is determined, the magnetic resonance imaging apparatus is characterized in that the navigator sequence is stopped and the imaging sequence of the subject is executed. The magnetic resonance imaging apparatus according to claim 1.
Comprising a cardiac radio wave detection means for detecting a cardiac radio wave of a subject, wherein the control means executes navigator sequences at different timings for each of a plurality of cycles of the cardiac radio wave detected by the cardiac radio wave detection means, A magnetic resonance imaging apparatus, wherein an image is reconstructed using echo data acquired by an imaging sequence performed at a plurality of periods of cardiac radio waves. The magnetic resonance imaging apparatus according to claim 1 or 2,
The control means controls the order of k-space arrangement of echo data by a plurality of image capturing sequences in accordance with the relationship between the navigator echo acquisition timing and the echo data acquisition timing by the image capturing sequence to reconstruct an image. A magnetic resonance imaging apparatus. The magnetic resonance imaging apparatus according to claim 3.
The control means arranges the echo data in the low frequency region of the k space in order from the echo data acquired at a time close to the navigator echo when the displacement is determined to be within a preset range among the echo data by the image capturing sequence. A magnetic resonance imaging apparatus characterized in that an image is reconstructed. The magnetic resonance imaging apparatus according to claim 1.
The control means applies a pre-pulse after stopping the navigator sequence and prior to execution of the imaging sequence. The magnetic resonance imaging apparatus according to any one of claims 2 to 5,
The navigator sequence is continuously performed a plurality of times, and the execution period of one navigator sequence is a multiple of one phase of the image capturing sequence.

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