JP5623149B2 - Magnetic resonance imaging apparatus and imaging method thereof - Google Patents

Description

  The present invention relates to a magnetic resonance imaging (hereinafter referred to as “MRI”) apparatus, and more particularly to a technique for automatically setting a gate window for respiratory motion synchronization imaging.

  In the MRI imaging method, there is a navigator echo method as a technique for monitoring the respiratory motion and suppressing the influence of the respiratory motion.

  In the navigator echo method, a navigator echo sequence is executed at intervals of, for example, every 500 msec by monitor measurement in advance to grasp the state of respiratory motion of the subject. Next, the diaphragm position in a desired range is designated from the respiratory motion obtained by monitor measurement. In this measurement, the influence of respiratory motion is suppressed by reconstructing an image using only the echo signal obtained when the diaphragm position is in the desired range. (For example, see FIG. 2 of Patent Document 1 as a sequence diagram of breathing gating imaging using navigator echo in combination with electrocardiogram synchronization (prior art).)

JP 2008-148806 JP

  However, in the above prior art, since the gate window is set manually, monitor measurement has to be interrupted once. In addition, although there are differences in the data acquisition efficiency and the effect of reducing the influence of body movement in the acquired image depending on the range of the gate window set by the user, it is necessary to rely on the user's experience and intuition for the optimal setting.

  In addition, when performing respiratory gating imaging using navigator echo in combination with ECG synchronization, the gate window was not set in consideration of the ECG cycle. In that case, if the gate window width is set too narrow, the period during which the diaphragm position falls within the desired range is sandwiched between navigator sequences executed at the heart rate RR interval. There was a problem that missed.

  In addition, there is a problem that the user has to manually reset the gate window when the detection position obtained by the navigator sequence does not enter the gate window even during the main measurement after the monitor measurement.

  Accordingly, an object of the present invention is to provide an MRI apparatus that can automatically set a gate window for respiratory motion synchronous imaging.

In order to achieve the above object, the present invention detects body movement information about periodic body movement of a subject, obtains displacement due to body movement of the subject based on the body movement information, and moves the body movement of the subject. The nuclear magnetic resonance signal is measured when the displacement due to is within the desired displacement range. At that time, a range including a displacement when the body movement direction of the subject is changed is a desired displacement range, and the displacement at the time when the first predetermined time interval is set from the time when the body movement direction is changed, One threshold value of the desired displacement range is used .

  ADVANTAGE OF THE INVENTION According to this invention, the MRI apparatus which enabled the automatic setting of the gate window of respiratory motion synchronous imaging | photography can be provided.

The block diagram which shows the whole outline | summary of an example of the MRI apparatus which concerns on this invention Diagram for explaining the concept of navigator echo method Prior art of monitor measurement of navigator measurement The flowchart for demonstrating the automatic setting of a gate window at the time of monitor measurement of navigator measurement in Example 1 of this invention The figure for demonstrating the automatic setting of the gate window at the time of monitor measurement of navigator measurement in Example 1 of this invention The flowchart for demonstrating the automatic setting of the gate window at the time of this measurement in Example 1 of this invention The figure for demonstrating the time of the stable breath in this measurement. The figure for demonstrating the gate window automatic adjustment when the respiration amplitude increases at the time of this measurement by this invention The figure for demonstrating the gate window automatic adjustment when the respiration amplitude reduces at the time of this measurement by this invention

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

  First, an overall outline of an example of an MRI apparatus according to the present invention will be described with reference to FIG. FIG. 1 is a block diagram showing the overall configuration of an embodiment of an MRI apparatus according to the present invention. This MRI apparatus uses a NMR phenomenon to obtain a tomographic image of a subject.As shown in FIG. 1, the MRI apparatus includes a static magnetic field generation system 2, a gradient magnetic field generation system 3, a transmission system 5, A reception system 6, a signal processing system 7, a sequencer 4, and a central processing unit (CPU) 8 are provided.

  The static magnetic field generation system 2 generates a uniform static magnetic field in the direction perpendicular to the body axis in the space around the subject 1 if the vertical magnetic field method is used, and in the direction of the body axis if the horizontal magnetic field method is used. Thus, a permanent magnet type, normal conducting type or superconducting type static magnetic field generating source is arranged around the subject 1.

  The gradient magnetic field generation system 3 includes a gradient magnetic field coil 9 that applies a gradient magnetic field in the three-axis directions of X, Y, and Z, which is a coordinate system (stationary coordinate system) of the MRI apparatus, and a gradient magnetic field that drives each gradient magnetic field coil. It consists of a power supply 10 and drives gradient magnetic field power supply 10 of each coil according to a command from sequencer 4 to be described later, thereby applying gradient magnetic fields Gx, Gy, Gz in the three axes of X, Y, and Z . At the time of imaging, a slice direction gradient magnetic field pulse (Gs) is applied in a direction orthogonal to the slice plane (imaging cross section) to set a slice plane for the subject 1, and the remaining two orthogonal to the slice plane and orthogonal to each other A phase encoding direction gradient magnetic field pulse (Gp) and a frequency encoding direction gradient magnetic field pulse (Gf) are applied in one direction, and position information in each direction is encoded into an echo signal.

  The sequencer 4 is a control means that repeatedly applies a high-frequency magnetic field pulse (hereinafter referred to as “RF pulse”) and a gradient magnetic field pulse in a predetermined pulse sequence, and operates under the control of the CPU 8 to collect tomographic image data of the subject 1. Various commands necessary for the transmission are sent to the transmission system 5, the gradient magnetic field generation system 3, and the reception system 6.

  The transmission system 5 irradiates the subject 1 with RF pulses in order to cause nuclear magnetic resonance to occur in the nuclear spins of the atoms constituting the living tissue of the subject 1, and includes a high frequency oscillator 11, a modulator 12, and a high frequency amplifier. 13 and a high frequency coil (transmission coil) 14a on the transmission side. The RF pulse output from the high-frequency oscillator 11 is amplitude-modulated by the modulator 12 at the timing according to the command from the sequencer 4, and the amplitude-modulated RF pulse is amplified by the high-frequency amplifier 13 and then placed close to the subject 1. By supplying to the high frequency coil 14a, the subject 1 is irradiated with the RF pulse.

  The receiving system 6 detects an echo signal (NMR signal) emitted by nuclear magnetic resonance of nuclear spins constituting the biological tissue of the subject 1, and receives a high-frequency coil (receiving coil) 14b on the receiving side and a signal amplifier 15 And a quadrature phase detector 16 and an A / D converter 17. After the NMR signal of the response of the subject 1 induced by the electromagnetic wave irradiated from the high frequency coil 14a on the transmission side is detected by the high frequency coil 14b arranged close to the subject 1 and amplified by the signal amplifier 15, The signal is divided into two orthogonal signals by the quadrature phase detector 16 at the timing according to the command from the sequencer 4, and each signal is converted into a digital quantity by the A / D converter 17 and sent to the signal processing system 7.

  The signal processing system 7 performs various data processing and display and storage of processing results, and includes an external storage device such as an optical disk 19 and a magnetic disk 18, and a display 20 including a CRT or the like. When data from the receiving system 6 is input to the CPU 8, the CPU 8 executes processing such as signal processing and image reconstruction, and displays the tomographic image of the subject 1 as a result on the display 20, and an external storage device On the magnetic disk 18 or the like.

  The operation unit 25 inputs various control information of the MRI apparatus and control information of processing performed in the signal processing system 7, and includes a trackball or mouse 23 and a keyboard 24. The operation unit 25 is disposed close to the display 20, and the operator controls various processes of the MRI apparatus interactively through the operation unit 25 while looking at the display 20.

  In FIG. 1, the high-frequency coil 14a and the gradient magnetic field coil 9 on the transmission side face the subject 1 in the static magnetic field space of the static magnetic field generation system 2 into which the subject 1 is inserted, in the case of the vertical magnetic field method. If the horizontal magnetic field method is used, the subject 1 is installed so as to surround it. The high-frequency coil 14b on the receiving side is installed so as to face or surround the subject 1.

  At present, the radionuclide to be imaged by the MRI apparatus is a hydrogen nucleus (proton) which is a main constituent material of the subject as being widely used clinically. By imaging information on the spatial distribution of proton density and the spatial distribution of relaxation time in the excited state, the form or function of the human head, abdomen, limbs, etc. is imaged two-dimensionally or three-dimensionally.

  Here, the concept of the navigator echo method, which is a technique for monitoring the respiratory motion and suppressing the influence of the respiratory motion, will be described with reference to FIG. In FIG. 2, a schematic diagram of the upper body of the subject is drawn, but the heart and the diaphragm move due to respiratory motion. In the navigator echo method, an echo signal is acquired by setting a slice selection plane for acquiring an echo signal perpendicular to the diaphragm. By analyzing the echo signal, it is possible to observe the transition between the state in which the subject inhales air or the state in which air is exhaled due to respiratory motion.

  Next, FIG. 3 shows a conventional technique for monitor measurement of navigator measurement. For example, the navigator sequence is executed every 500 msec, the diaphragm position is detected, and the gate window is manually set.

  Next, a static magnetic field generating means for generating a uniform static magnetic field in the space accommodating the subject in Embodiment 1 of the present invention, a gradient magnetic field generating means for generating a gradient magnetic field superimposed on the static magnetic field, and the subject A high-frequency coil for generating a high-frequency magnetic field for irradiating the specimen, means for detecting an NMR signal generated from the specimen, and means for imaging the detected signal, the first period of the specimen The automatic setting of the gate window at the time of monitor measurement of navigator measurement of the MRI apparatus that detects the NMR signal for imaging when the moving part is within the range of the desired displacement is shown in the flowchart of FIG. 4 and FIG. Will be described. Hereinafter, description will be given along the flowchart of FIG.

(Step 31)
The CPU 8 detects the diaphragm position in real time, for example, at intervals of 500 msec, and performs monitor measurement. At the same time, the CPU 8 detects the electrocardiogram waveform in real time.

(Step 32)
The CPU 8 determines whether or not the gate window is set automatically in the monitor measurement. If automatic setting is to be performed, the process proceeds to step 33.

(Step 33)
In the monitor measurement that detects the diaphragm position in real time, the CPU 8 detects whether or not the current detection position is a vertex position where the direction of the diaphragm displacement changes. Specifically, the position satisfying the following condition is set as the vertex. If it is determined that the current detection position is the vertex position, flag = 1.
[Current detection position]-[Previous detection position] ≧ 0 and [Current detection position]-[Next detection position]> 0
(Step 34)
The CPU 8 determines whether the flag is 1. If it is 1, go to step 35, and if not, go to step 39.

(Step 35)
The CPU 8 calculates the RR interval from the electrocardiogram waveform and determines whether or not the RR time has elapsed since the apex was detected. If the RR time has elapsed, the process proceeds to step 36 if the RR time has not elapsed.

(Step 36)
The CPU 8 determines whether the second vertex is detected. If detected, the process proceeds to step 37, and if not detected, the process proceeds to step 38.

(Step 37)
The CPU 8 calculates the difference between the timing at which the vertex is detected the first time and the timing at which the vertex is detected the second time, and calculates the respiratory cycle based on the difference.

(Step 38)
The CPU 8 automatically sets the gate window. Specifically, since the RR interval time has elapsed since the detection of the vertex in this step, the diaphragm positions before and after the detected vertex are referred to, and the time interval (T1) of the two points being referenced is the RR interval. If so, the height of the lower of the two points (position closer to the intake side) is set as the underline of the gate window.

  The reason why the R-R interval or more is taken is that the period during which the detection position enters the gate window under stable breathing is made the R-R interval or more. In this measurement, the navigator sequence is executed in the electrocardiographic cycle. Therefore, under the same stable breathing, the detection position always enters the gate window within one breathing cycle.

  If the time interval (T1) between the two points being referenced is less than the RR interval, the detection position point that refers to the higher of the two points (the position closer to the expiration side) is further separated from the vertex. The detected time point is updated, and the time interval (T1) between the two referenced points is calculated in the same manner. The same processing is repeated until the time interval (T1) of the two points being referenced is equal to or greater than the RR interval, and when the time interval (T1) of the two points being referenced is greater than or equal to the RR interval, Set the height of the lower position (position closer to the intake side) to the underline of the gate window.

  Calculate the distance (D) between the position set to the underline and the vertex. Then, the upper line of the gate window is set to the distance (D) calculated from the vertex. The obtained gate window is displayed on the Navi Monitor screen (see Fig. 5). Set flag = 0.

(Step 39)
The CPU 8 determines whether the monitor measurement is set to end. If it is set not to end, the process proceeds to step 31 and starts to measure the diaphragm position in real time. If it is set to end, it shifts to the main measurement.

(Step 40)
The operator determines whether to end the monitor measurement. If the process is to end, the process proceeds to step 41. If not, the process proceeds to step 31.

(Step 41)
The operator presses the Pause button. Pressing the Pause button ends monitor measurement.

(Step 42)
The operator observes the waveform of the diaphragm position on the monitor and manually sets the gate window while viewing the waveform.

(Step 43)
The operator presses the START button for starting this measurement.
The above is the gate window automatic setting at the time of monitor measurement of navigator measurement in the first embodiment.

  The automatic setting ON / OFF can be changed even during monitor measurement. Also, this monitor measurement is started by attaching an electrocardiogram (pulse wave) sensor to the subject. In monitor measurement, the detection position acquired from the navigator sequence is displayed on the display in real time. At this time, the detection position information and the detection time are respectively held.

  An electrocardiogram (pulse wave) waveform and a heart rate (pulse) number obtained from an electrocardiogram (pulse wave) sensor are also displayed on the display.

  The monitor automatically shifts to the main measurement after the monitor measurement time ends. Automatic gate window setting in this measurement will be described with reference to the flowchart of FIG. 6 and FIGS.

(Step 44)
The CPU 8 detects the diaphragm position by executing a navigator echo sequence in the diastole of the electrocardiogram waveform immediately after the detection of the R wave.

(Step 45)
The CPU 8 determines whether the diaphragm position detected in step 44 is within a desired range, that is, within the set gate window. If it is within the range, the process proceeds to step 46 as ACCEPT. If it is not within the range, REJECT is transferred to step 48.

(Step 46)
The CPU 8 performs this measurement.

(Step 47)
The CPU 8 determines whether it is set to end this measurement. If it is set to end, the main measurement is terminated, and if it is set not to end, the process proceeds to step 44.

(Step 48)
The CPU 8 determines whether or not the gate window is automatically set in this measurement. If it is set to be automatically set, the process proceeds to step 49. If it is set not to be automatically set, the process proceeds to step 44.

(Step 49)
The CPU 8 determines whether the diaphragm position detected in step 44 is Over (position far from the intake side) or Under (position close to the intake side) within the currently set gate window range. If Over, go to Step 50; if Under, go to Step 52.

(Step 50)
The CPU 8 sets the gate at least twice (including 3 times, 4 times, etc.) during the 2 breath cycles, such as when the subject's breathing amplitude increases, not during stable breathing as shown in FIG. It is determined whether the detection position detected from within the window range is on the upper side (close to the exhalation side). If it has come to the upper side at least twice, the process goes to step 51. If it has not come to the upper side at least twice, the process goes to step 44. In order to prevent automatic adjustment of the gate window in this case, it is assumed that the detection position detected from the gate window is unexpectedly only once due to sneezing, etc. .

(Step 51)
When the CPU 8 comes to the upper side at least twice in step 50, the gate window width is changed so that the maximum detection point in between is centered, and the gate window position is reset (the gate window position is slid upward). (Figure 8).

(Step 52)
As shown in Fig. 7, the CPU 8 detects that all the detected positions detected from the set gate window are continuously on the lower side during one breathing cycle ((breathing one cycle / RR interval) times), not during stable breathing. Determine if you have come. If everything has come to the lower side, the process proceeds to step 53, and if not all, the process proceeds to step 44.

(Step 53)
When all of the CPUs 8 have come to the lower side, the gate window width is changed so that the highest point detected within one respiration cycle is the center, and the gate window position is reset (the gate window position is lowered). (Fig. 9).

When the data acquisition necessary for image reconstruction is completed, the measurement is terminated.
According to the first embodiment, it is possible to automatically set the gate window with good image quality and measurement efficiency without depending on the user's intuition and experience, and it is possible to perform the main measurement continuously from the monitor measurement, thereby shortening the inspection time. Leads to.

  In addition, the gate window can be automatically adjusted when there is a respiratory change in the subject during the main measurement.

  Next, Example 2 will be described. The difference from the first embodiment is automatic adjustment of the gate window during the main measurement.

Hereinafter, only different portions will be described, and description of the same portions will be omitted.
In the first embodiment, the gate window can be automatically adjusted during the main measurement. However, since the diaphragm position detection interval becomes the RR interval during the main measurement, the accuracy is low, and the imaging efficiency may be reduced.

  Therefore, during this measurement, the detection position does not enter the gate window continuously for the period of one breath cycle, or the detection position detected from the gate window set at least twice during the second breath period is at the top. If a phenomenon occurs, stop this measurement, perform monitor measurement again, and reset the gate window. After the gate window is reset, this measurement is automatically resumed.

  According to the second embodiment, it is possible to maintain the gate window so that the detection position synchronized with the electrocardiogram is always included in the gate window once in the respiratory cycle even when the subject moves. .

  Next, Example 3 will be described. The difference from the first embodiment and the second embodiment is the automatic setting of the gate window in imaging using a navigator sequence that does not use ECG synchronization. Hereinafter, only different portions will be described, and description of the same portions will be omitted.

  By replacing the RR interval in the first embodiment with the navigator sequence execution interval, that is, the TR interval, automatic gate window setting is possible.

  According to the third embodiment, it is possible to automatically set a gate window in imaging using a navigator sequence that does not use ECG synchronization.

  As mentioned above, although the Example of this invention was described, this invention is not limited to these. For example, in the first embodiment, the operator determines whether or not the monitor measurement is ended in step 40, and when it is ended, the Pause button is pressed in step 41. It may be made to end after the time elapses.

  1 subject, 2 static magnetic field generation system, 3 gradient magnetic field generation system, 4 sequencer, 5 transmission system, 6 reception system, 7 signal processing system, 8 central processing unit (CPU), 9 gradient magnetic field coil, 10 gradient magnetic field power supply, 11 High-frequency transmitter, 12 modulator, 13 high-frequency amplifier, 14a high-frequency coil (transmitting coil), 14b high-frequency coil (receiving coil), 15 signal amplifier, 16 quadrature detector, 17 A / D converter, 18 magnetic disk, 19 optical disc, 20 display, 21 ROM, 22 RAM, 23 trackball or mouse, 24 keyboard

Claims (7)

Body motion detection means for detecting body motion information about the first periodic body motion of the subject;
An arithmetic means for obtaining a displacement due to body movement of the subject based on the body movement information detected by the body movement detection means;
A measurement control unit you measure the nuclear magnetic resonance signal when in the displacement range body motion due to displacement of the desired of the subject,
With
Time said calculation means, the body motion direction of the subject as the previous SL desired displacement range a range including the displacement when the change was at a first predetermined time interval from the time when the body movement direction is changed The magnetic resonance imaging apparatus according to claim 1, wherein the displacement is set as one threshold value of the desired displacement range . 2. The magnetic resonance imaging apparatus according to claim 1, wherein the body motion detection unit detects the body motion information at a second predetermined time interval shorter than the first predetermined time interval . The subject includes a second periodic body movement;
The calculation means sets the displacement at a time point equal to or greater than a second cycle period across the time point when the body movement direction of the subject is changed as one threshold value of the desired displacement range. Item 2. The magnetic resonance imaging apparatus according to Item 1. The desired displacement range is a range between the one threshold value and the other threshold value,
The calculation means is configured such that a difference between the displacement at the time when the body movement direction of the subject changes and the one threshold value is a difference between the displacement at the time when the body movement direction of the subject changes and the other threshold value. to be equal, the magnetic resonance imaging apparatus according to any one of claims 1 to 3, characterized in that setting the other threshold. The second periodic body movement site is the heart of the subject;
The said calculating means resets said one threshold value and said other threshold value when said displacement is larger than said predetermined displacement range at least twice within a predetermined period. 5. The magnetic resonance imaging apparatus according to 4. The second periodic body movement site is the heart of the subject;
6. The calculation unit according to claim 5, wherein the first threshold value and the second threshold value are reset when the displacement is under a smaller value than the predetermined displacement range continuously for a predetermined number of times. The magnetic resonance imaging apparatus described. A body motion detection step of detecting body motion information about the first periodic body motion of the subject;
A calculation step for obtaining displacement due to body movement of the subject based on the body movement information detected by the body movement detection step;
Wherein the measuring step you measure the nuclear magnetic resonance signal when in displacement by the subject of the body movement is desired displacement range,
With
In the calculation step, a range including a displacement when the body movement direction of the subject is changed is set as the desired displacement range, and a first predetermined time interval is set from the time when the body movement direction is changed. The imaging method of the magnetic resonance imaging apparatus , wherein the displacement in (1) is set as one threshold value of the desired displacement range .

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