JP4745650B2 - Magnetic resonance imaging system - Google Patents

Description

  The present invention relates to a magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus), and more particularly to a technique for acquiring a plurality of cross-sectional images using body motion of a subject.

  In a typical imaging method using an MRI apparatus, a predetermined cross section including an examination site of a subject is selectively excited, NMR signals (echo signals) generated therefrom are repeatedly measured, and an image of the excited cross section is obtained. Reconfigure. The position of the cross-section selectively excited is determined by the high-frequency magnetic field and gradient magnetic field applied to the subject when measuring the echo signal. However, if the subject moves during the acquisition of data for one image, the cross section that should be excited and the cross section that is actually excited deviate, and the reconstructed image is a false image called a body motion artifact. Occurs. If a positional shift due to body movement occurs in the excited cross section, it can be corrected by post-processing, but body movement in a direction intersecting the cross-section cannot be corrected by post-processing.

  Conventionally, several methods for solving this problem of body movement have been proposed. One method is to monitor the position of the diaphragm in real time, for example, in cardiac imaging under normal breathing, and perform image reconstruction using only the echo signal obtained when it is within a pre-specified range. (Patent Document 1). In another method, the excitation cross section of the subject is corrected in real time in accordance with the change in the position of the diaphragm, and the echo signal is always measured from the same cross section (Patent Document 2). As a technique for detecting the position of the diaphragm used in these technologies, a sequence (navigation sequence) for measuring an echo signal that does not give phase encoding from the diaphragm is performed separately from the original imaging sequence, and the obtained echo signal (navigation echo) A method for detecting the position of the diaphragm from the projection has been proposed.

  Among the methods for removing the body motion artifacts described above, the first method uses only echo signals measured within a specified range of the diaphragm position, so that data for obtaining one image is obtained. There is a problem that it takes a long time. In addition, the second method has a problem that when viewed from the apparatus coordinates, the excited cross section varies, and the excitation frequency changes depending on the position of the cross section at that time, making it difficult to obtain a stable echo signal. .

On the other hand, there is a multi-slice imaging method as a method for imaging a plurality of cross sections of a subject. In this method, after one slice is excited and an echo signal is measured, another slice is excited and a echo signal is measured using a waiting time for the repetition. By repeating excitation and echo signal measurement for a plurality of slices in order, a plurality of cross-sectional images are acquired without causing signal reduction due to repeated excitation. As described above, the slice position to be excited is changed by changing the high-frequency magnetic field pulse for exciting the cross section and the gradient magnetic field pulse for slice selection.
Even in such multi-slice imaging, the same problem occurs when attempting to remove body motion artifacts by the above-described method when the slice direction and the body motion direction are the same.
US Patent 4761613 Japanese National Patent Publication No. 09-508050

  Accordingly, an object of the present invention is to provide an MRI apparatus that is free from the problem of artifacts due to body movement and that can obtain images of a plurality of cross sections by one imaging sequence. The present invention also provides an MRI apparatus that can effectively increase the number of imaging sections without prolonging the time by effectively using echo signal data that has been discarded in the conventional body motion artifact removal method. The purpose is to do.

  The MRI apparatus of the present invention that achieves the above object includes a high-frequency magnetic field applying unit that applies a high-frequency magnetic field to a subject placed in a static magnetic field, a gradient magnetic field applying unit that applies a gradient magnetic field to the subject, and the subject Detection means for detecting a nuclear magnetic resonance signal generated from a specimen, image forming means for reconstructing an image of the subject based on the nuclear magnetic resonance signal, the high-frequency magnetic field applying means, a gradient magnetic field applying means, and detection Body movement for detecting periodic body movement of the subject, comprising control means for controlling the means and image forming means to repeatedly execute measurement of data necessary for image reconstruction, and display means for displaying the image Detection means, wherein the control means measures the data measurement from each of a plurality of slices, and the image forming means sets a data space for each slice, and is detected before the body movement detection means According to the body movement position of the subject at the time of data measurement, the measurement data is arranged in a data space corresponding to the excited slice position in the plurality of data spaces, and the data arranged in the plurality of data spaces is The images of the plurality of slices are reconstructed using each of them.

In the MRI apparatus of the present invention, it is preferable that the body motion detecting means calculates a body motion position based on the means for generating navigator echo from a part of the subject with periodic body motion and the navigator echo. Means to do.
Furthermore, in the MRI apparatus of the present invention, the control unit controls the number of measurement data and the slice thickness according to the body movement position detected by the body movement detection unit.
Preferably, the display means displays a body movement range of the target portion detected by the body movement detection means and displays a GUI for inputting an imaging condition determined by the body movement range.

  According to the present invention, by storing measurement data from an excited slice in a data space corresponding to the slice, an image free from body motion artifacts can be obtained. Further, it is possible to obtain images of a plurality of cross sections using body motion while fixing the excitation frequency and the position of the excitation cross section in the apparatus coordinate system.

Also, by executing the navigation sequence, body movement can be detected without using an external device such as a respiratory monitor, and image reconstruction based on the detected information can be performed.
Furthermore, by taking into account not only the body motion position but also the body motion speed, the number of measurement data and the slice width are controlled, so that the displacement of the cross-sectional position and the associated artifacts that can occur when imaging a body motion speed is high. Can be prevented.

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a block diagram showing the overall configuration of an MRI apparatus to which the present invention is applied. The MRI apparatus mainly includes a static magnetic field generation system 101, a gradient magnetic field generation system 102, a transmission system 103, a reception system 104, a signal processing system 105, a control system 106, and an input / output device 107.

  The static magnetic field generation system 101 includes a magnetic field generation device composed of a permanent magnet, a normal conducting magnet, or a superconducting magnet. The static magnetic field generating system 101 is uniform in a direction perpendicular to or parallel to the body axis of the subject 108 in a space where the subject 108 is placed. Generate a magnetic field. The gradient magnetic field generation system 102 includes three sets of gradient magnetic field coils 109 wound in three directions of X, Y, and Z, and a gradient magnetic field power source 110 that drives the gradient magnetic field coils 109. By driving, a desired gradient magnetic field can be applied to the subject placed in the static magnetic field space. By applying this gradient magnetic field, a desired cross section of the subject can be selected and position information can be given to an echo signal generated from the subject.

  The transmission system 103 includes a high-frequency oscillator 111, a modulator 112, a high-frequency amplifier 113, and a high-frequency irradiation coil 114. The transmission system 103 modulates and amplifies the high-frequency pulse of the magnetic resonance frequency output from the high-frequency oscillator 111, and then approaches the subject 108. By supplying the high-frequency irradiation coil 114 arranged in this manner, the atomic nuclei constituting the biological tissue of the cross section of the subject can be excited to cause nuclear magnetic resonance.

  The reception system 104 includes a high-frequency reception coil 115, a reception circuit 116, and an A / D converter 117. A magnetic resonance signal (echo signal) generated by the subject 108 in response to the high-frequency pulse applied by the transmission system 103 is detected by the high-frequency reception coil 115, and the reception circuit 116 and the A / D are detected at a timing according to a command from the control system 106. It is sampled by the converter 117 and sent to the signal processing system 105 as collected data that is a digital signal.

  The signal processing system 105 includes a signal processing device 118 that performs correction calculations on the collected data, calculations necessary for image reconstruction, and the like, an image analysis processing program over time, an imaging sequence program, and the execution of these programs. The CPU 120 includes a memory 119 for storing parameters and the like used, and a data storage unit 121 (magnetic disk 121a and optical disk 121b) for storing reconstructed image data.

  The CPU 120 also functions as a control system 106 for controlling the gradient magnetic field generation system 102, transmission system 103, reception system 104, and signal processing system 105 described above, and is necessary for acquiring tomographic image data of the subject 108 via the sequencer 125. These various commands are sent to the gradient magnetic field generation system 102, the transmission system 103, the reception system 104, and the signal processing system 105. The sequencer 125 controls the application of gradient magnetic field pulses and high frequency magnetic field pulses in three directions and the reception of echo signals in accordance with a predetermined pulse sequence.

The input / output device 107 includes an operation unit 123 such as a trackball, a mouse, and a keyboard, and a display 124. An image reconstructed by the signal processing system 105 is displayed on the display 124 and input by the operation unit 123. The GUI etc. for are displayed.
Although not shown in FIG. 1, when imaging is performed in synchronization with electrocardiography, an electrocardiograph or a pulse wave meter is attached to the subject 108, and signals from these measuring devices are input to the CPU 120. The The CPU 120 controls the execution of the pulse sequence using these signals as synchronization signals.

Next, the imaging method of the present invention using the MRI apparatus having such a configuration will be described.
FIG. 2 is a diagram illustrating a procedure of the imaging method.

  First, prior to imaging the target region, the application position of the navigation sequence is set to monitor body movement (step 201). The application position of the navigation sequence is set, for example, by displaying a coronal cross-sectional image as shown in FIG. 3 on the display 124 and by the user specifying a point or line on the screen using the operation unit 123. FIG. 3 is a diagram schematically showing the case where the heart 304 is used as an imaging region. As shown in the figure, the diaphragm is located between the lung 301 and the liver 302 and moves in the up-and-down direction 307 by respiration. In proportion to this, the position of the heart 304 that is the imaging region also fluctuates in the vertical direction 308. The application position of the navigation sequence is, for example, a region 305 where two cross sections in a direction substantially orthogonal to the diaphragm 303 intersect, and an echo signal (navigation echo) of phase encode 0 is acquired from this region 305. The position information of the diaphragm 303 can be obtained by performing a one-dimensional Fourier transform on the navigation echo in the reading direction.

  Next, the navigation sequence is repeatedly executed for at least one period of body movement, and information corresponding to the body movement curve or the body movement curve representing the variation in the position of the diaphragm from a series of navigation echoes obtained (information on the body movement). The range and the length of one cycle are acquired (step 202). Based on this information, the reference position (reference section) of the imaging section in the main measurement is set (step 203). The reference cross section 306 is set at a substantially central position in the body movement range and has a single cross section having a component orthogonal to the body movement direction 308.

  Next, the number of slices to be imaged is set (step 204). At this time, preferably, the body movement information (body movement range of the diaphragm, movement direction) obtained by the above-described navigation sequence is displayed on the cross-section selection screen on the display. The display may be performed numerically or may be indicated by an arrow as shown. Accordingly, the user can predict the range in which the imaging region (here, the heart) 304 moves, and can determine an appropriate number of slices in the body movement range in consideration of the slice thickness.

  Specifically, when the moving range of the diaphragm is L, and the angle between the moving direction of the diaphragm and the reference cross section is θ, the moving range M of the imaging region is M = Lsinθ. When the slice thickness is m and there is no overlap of slices, the slice number S is S ≦ M ÷ m. The set number of slices is associated with the position of body movement. For example, as shown in FIG. 4, when the number of slices 5 is set in the moving range 402 of the imaging region (heart) 401, the body movement range is divided into five stages and the data of the slices 403 to 407 are stored in each stage. K-space is allocated. In the figure, 408 is a fixed excitation section (reference section). If the slice thickness of the reference cross section in this measurement or the angle θ with the movement direction is changed, the number of slices that can be imaged is recalculated and displayed each time it is changed. Set up.

  Next, measurement is started (step 205). FIG. 5 shows the state of measurement when cardiac imaging is performed by electrocardiographic synchronization. In the figure, (a) shows the R wave from the electrocardiograph, (b) shows the displacement of body movement, (c) shows the sequence execution timing, and (d) shows the data acquisition timing.

  First, when the R wave 510 is detected by the electrocardiograph (step 206), after a predetermined delay time 519, the navigation sequence 520 is executed to obtain a navigation echo (step 207). The position of the diaphragm (detection point) is calculated from the obtained navigation echo, and the slice position of the main measurement sequence 521 to be subsequently executed is determined (step 208). When the imaging section 508 fixed with respect to the apparatus coordinate system is set, when the imaging part 501 moves up and down due to body movement, the section (slice position) excited in the imaging part 501 changes with body movement. Therefore, here, a cross section actually excited in the imaging region 501 is predicted based on the detection point detected by the execution of the navigation sequence, and determined as the slice position of the main measurement sequence 521.

  In this case, the imaging region is also moved by the body movement during the time 522 from the execution of the navigation sequence 520 to the execution of the main measurement sequence 521. Therefore, the position to move between the time differences 522 is predicted from the body movement curve, and the main measurement is performed. The slice position of the imaging region excited during the sequence 521 is determined. In the first cardiac cycle of FIG. 5, since the position of the diaphragm is the detection point 511, it is predicted that it is at the position 512 in the present measurement sequence 521, and the corresponding slice position 506 is determined. In the second cardiac cycle, a slice position 506 corresponding to the position 514 predicted from the detection point 513 is determined, and in the third cardiac cycle, a slice position 503 corresponding to the position 516 predicted from the detection point 515 is determined.

  Next, the main measurement sequence 521 is executed, and an echo signal is measured from the imaging region (step 209). In this measurement sequence 521, the cross section set in step 201 (the cross section fixed in the apparatus coordinate system) is excited, and the collected data 523 is stored in the k space corresponding to the slice position determined in step 207 (step 210). The measurement sequence is not particularly limited. For example, an SSFP short TR sequence can be adopted, and a plurality of echo signals having different phase encodings are measured within one cardiac cycle.

  Steps 206 to 209 from the detection of the R wave to the execution of this measurement sequence are repeated until the data of the number of phase encodes necessary for reconstructing one image for all slices is obtained, and finally the range of body motion Data of all the slices 503 to 507 is acquired (step 210). By reconstructing the data of each slice, images of a plurality of (here, 5) slices can be obtained (step 211).

FIG. 5 shows a case where one navigation sequence and the main measurement sequence are executed within one cardiac cycle and data at one slice position is acquired. However, as shown in FIG. It is also possible to execute this measurement sequence at each of a plurality of time phases and acquire data at a plurality of slice positions. In this case, the navigation sequence is executed for each main measurement sequence as illustrated.
Thus, by executing a plurality of main measurement sequences within one cardiac cycle, the number of data that can be measured within one cardiac cycle can be increased.

  Further, the number of echo signals (number of segments) acquired by imaging at one slice position (main measurement sequence) may be varied according to the amount of body movement (gradient of the body movement curve) within a predetermined time. For example, when the gradient of the body motion curve is steep, the number of segments in the main measurement sequence is reduced, and when the slope of the body motion curve is gentle, the number of segments in the main measurement sequence is increased. In the embodiment shown in FIG. 5, the slice position 506 measured in the first cardiac cycle is measured when the change in body motion is large, so the number of segments is reduced and data of adjacent slice positions is prevented from being mixed. To do. In addition, since the body movement change is small at the slice position 503 measured in the third cardiac cycle, the number of segments 527 is increased and more data is collected.

  Alternatively, the slice thickness to be imaged within one cardiac cycle may be varied. For example, at a position where the body motion curve has a steep slope, the slice thickness may be increased to enable stable data acquisition. Furthermore, it is possible to acquire stable data at the same time by using the slice thickness and the variable number of segments together.

  For example, in the change of the condition according to the body movement position, in the setting of the number of slices (FIG. 2: step 204), for example, a condition setting screen for each slice as shown in FIG. The number of segments, the slice thickness, and the like may be manually input, or may be automatically calculated based on the TR and body motion curve of this measurement sequence.

In the above-described embodiment, the case where the navigation echo is used to monitor the body movement has been described. However, the body movement monitor may be an external body movement monitor. Also in this case, if it is possible to predict whether or not the body motion cycle is known, the body motion position at the time of executing this measurement sequence is predicted from the detected body motion position, and reconstructed as k-space data of the slice position corresponding to the position. Thus, images of a plurality of slices corresponding to the body movement position can be obtained.
In the above embodiment, the body movement position is estimated in consideration of the delay after the body movement monitoring. However, when the time difference between the body movement monitoring and the execution of this measurement sequence is very small, the detection during the body movement monitoring is performed. You may make it determine a slice position with a point.

  According to the present invention, it is possible to acquire a plurality of cross-sectional images having a component orthogonal to the body movement direction using body movement. Further, according to the present invention, since imaging is performed in a state where the apparatus side conditions (excitation frequency, slice selection gradient magnetic field) are fixed, a stable echo signal can be acquired.

The figure which shows the whole outline | summary of the MRI apparatus with which this invention is applied. The figure which shows the imaging procedure in the MRI apparatus of this invention The figure which shows the relationship between an imaging region and a body movement monitor region Diagram showing the relationship between body movement and slice position The figure which shows one Embodiment of the imaging method by this invention The figure which shows other embodiment of the imaging method by this invention. The figure which shows an example of the screen display of the condition change according to a slice position

Explanation of symbols

101 ... Static magnetic field generation system, 102 ... Gradient magnetic field generation system, 103 ... Transmission system, 104 ... Reception system, 105 ... Signal processing system, 106 ... Control system, 107 ...・ I / O devices

Claims (5)

A high-frequency magnetic field applying means for applying a high-frequency magnetic field to a subject placed in a static magnetic field;
A gradient magnetic field applying means for applying a gradient magnetic field to the subject;
Detecting means for detecting a nuclear magnetic resonance signal generated from the subject;
Image forming means for reconstructing an image of the subject based on the nuclear magnetic resonance signal;
Control means for controlling the high-frequency magnetic field applying means, the gradient magnetic field applying means, the detecting means and the image forming means, and repeatedly executing measurement of data necessary for image reconstruction;
Display means for displaying the image ;
Body motion detecting means for detecting periodic body motion of the subject ;
With
The control means measures the data from each of a plurality of slices,
The image forming unit sets a data space for each slice, and the measurement data is stored in the plurality of data spaces according to the body motion position of the subject at the time of the data measurement detected by the body motion detection unit. placed in the data space corresponding to the excited slice positions of the out, there are disposed the plurality of data space data of a magnetic resonance imaging apparatus for reconstructing an image of said plurality of slices with each
The magnetic resonance imaging apparatus characterized in that the control means controls the number of measurement data in accordance with the body movement position detected by the body movement detection means .   2. The body motion detecting means includes means for generating a navigator echo from a site with periodic body motion of the subject, and means for calculating a body motion position based on the navigator echo. The magnetic resonance imaging apparatus described. 3. The control unit according to claim 1, wherein the control unit varies the number of echo signals acquired by imaging at one slice position in accordance with a body motion amount within a predetermined time detected by the body motion detection unit. The magnetic resonance imaging apparatus described.   4. The magnetic resonance imaging apparatus according to claim 1, wherein the control unit controls a slice thickness according to a body movement position detected by the body movement detection unit. 5.   The display means displays a body movement range of a target part detected by the body movement detection means and displays an input screen for inputting an imaging condition determined by the body movement range. Item 5. The magnetic resonance imaging apparatus according to any one of Items 1 to 4.