JPH07163537A - NMR imaging method - Google Patents

Abstract

(57) [Abstract] [Purpose] NM without using special hardware
To provide an NMR imaging method capable of detecting respiratory movements only by the R inspection apparatus main body and correcting the influence thereof. [Structure] Static magnetic field, gradient magnetic field, high-frequency magnetic field generation means, detection means for extracting an NMR signal from an inspection object,
In an imaging method in an NMR inspection apparatus having means for performing various calculations including image reconstruction on a detected signal, information on respiratory movements is measured together with the image signal at the time of measuring the image signal, and based on the information on the measured movement. N to correct the image signal
MR imaging method.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an NMR imaging method, that is, an internal tomography method using an NMR phenomenon, and an imaging method for correcting an effect based on respiratory motion without using hardware. Regarding the method.

[0002]

2. Description of the Related Art Imaging in an NMR inspection apparatus requires a time of about 2 to 20 minutes, so that
When photographing the chest and abdomen, there is a problem that blurring of the image and artifacts occur due to the influence of breathing. Therefore, usually, a method of detecting the movement of the breath for synchronization by some means or correcting the measurement data is used. For example, in the conventional device, the following detection method was used. (1) Airbag method: A method in which the pressure inside the airbag fixed to the abdomen is detected by utilizing the fact that it changes according to breathing. (2) Band method: A tube filled with zinc sulfate solution is wrapped around the abdomen, and this tube expands according to breathing,
A method of detecting by utilizing the change in electrical resistance. (3) Thermistor method: A method in which a thermistor is attached near the nostril to detect the temperature that changes with breathing. As a device related to this type of device, there is one described in "Development of Respiratory Synchronous NMR" (NMR Medicine, vol. 5, No. 1 (1985)).

[0003]

In each of the above-mentioned conventional techniques, it is necessary to newly add dedicated hardware, and at the same time, there is a problem in operability at the time of photographing. That is,
Since the shooting is performed in synchronization with breathing, the shooting time is normally 6
There was also a problem that it took ~ 8 times. The present invention has been made in view of the above circumstances, and an object of the present invention is to solve the above-described problems in the related art and to perform breathing movements only by the NMR inspection apparatus main body without using special hardware. Is detected, and its influence can be corrected.
An object is to provide an MR imaging method.

[0004]

The above object of the present invention is to provide a static magnetic field, a gradient magnetic field, a high frequency magnetic field generating means, a detecting means for extracting an NMR signal from an object to be inspected, and an image reproduction for the detected signal. In an imaging method in an NMR inspection apparatus having a means for performing various calculations including the configuration, information on respiratory movements is measured together with the image signal during image signal measurement, and information on respiratory movements is obtained as phase information, and based on this. An NMR imaging method characterized by correcting the image signal by using a gradient magnetic field that does not affect a stationary object but only a moving object immediately after measuring the image signal, thereby performing respiratory motion. NMR imager, characterized in that information relating to Method or when measuring an image signal, information about the movement of the breath is measured at the same time so that the moving direction of the breath matches the phase encoding direction, and the image signal is corrected based on the measured data. Is achieved by an NMR imaging method.

[0005]

In the NMR imaging method according to the present invention, a sequence for measuring a signal containing respiration information is added at the time of photographing, when measuring an image signal, or immediately after measuring an image signal. Is. Figure 4
6 shows an example of a sequence added to measure a signal including information about respiration immediately after measuring an image signal at the time of photographing, as an example thereof. The parts indicated by arrows in FIGS. 4 to 6 are newly added parts.
In addition, here, it is assumed that there is movement in the y direction.
In FIG. 4, the phase encoding pulse 58 cancels 57, and a read gradient magnetic field indicated by 59 is applied in the y direction. When the signal 63 observed at this time is subjected to Fourier transform, an image signal projected on the y axis is obtained. The position of respiration can be known from the end point position of this signal. In FIG. 5, as in the example shown in FIG. 4, the phase encode pulse 78 cancels the pulse 77 and the read gradient magnetic field 7 is output.
9 is applied in the y direction. Further, an x-direction gradient magnetic field 82 is applied between the phase / phase encode pulse 78 and the gradient magnetic field 79 in order to suppress the phase rotation in the x direction. If the phase of the signal obtained by Fourier-transforming the signal 84 observed at this time is obtained, the velocity at each position in the y direction can be obtained. Further, by integrating the above data obtained at each measurement, the position of breathing along the y axis can be known.

In FIG. 6, the phase encode pulse 9
At 8, the same 97 is canceled, the flow encode pulse 99 is applied in the y direction, and the read gradient magnetic field 102 is applied in the x direction. When the phase of the signal obtained by Fourier-transforming the signal 104 observed at this time is obtained, the velocity at each position in the x direction can be obtained. Further, similarly to the example shown in FIG. 5, the position of breathing along the x-axis can be known by integrating the data obtained at each measurement. The phase encoding pulse itself, 180 ⁰ has become flow encode pulse for being applied twice before and after, even without applying a pulse 99 shown in FIG. 6, it is possible detection of a motion (respiration, etc.) However, since the gradient magnetic field strength is different for each measurement, the flow encode pulse 99 described above is preferable in order to set the sensitivity suitable for the speed of movement due to respiration. Here, the correction of the amount of flow encoding by phase encoding may be performed by software or may be incorporated in the pulse sequence.

[0007]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 2 is a block diagram showing the configuration of the NMR imaging apparatus according to the embodiment of the present invention. In FIG. 2, reference numeral 21 is a sequence control unit having a function of controlling various pulses and magnetic fields generated for outputting an NMR signal from the subject, and 22 is a high-frequency pulse for resonating a specific nuclide of the subject. A high-frequency pulse transmitter 23 having the function of controlling the magnetic field driving unit 24 described later to generate a static magnetic field that determines the resonance frequency of the NMR signal and a gradient magnetic field whose strength and direction can be arbitrarily controlled. 2 shows a magnetic field control unit having. Further, 24 is a magnetic field control unit that generates a magnetic field necessary for measurement based on the control signal output from the magnetic field control unit 23 described above, and 25 is an NM generated from the subject.
A receiver having a function of measuring the R signal and then measuring it, 2
Reference numeral 6 performs image reconstruction and various calculations based on the measurement signal fetched from the receiver 25, and displays the reconstructed image on the CRT.
A processing device having a function of displaying on the display 27, 2
Reference numeral 7 indicates a CRT display.

The operation of this embodiment constructed as described above is
Hereinafter, description will be given according to the processing flowchart shown in FIG. 7. Step 111: In the pulse sequence of FIG. 4, the phase encode pulse 57 is sequentially changed, and the position detection signal 63 is generated by the number of times required for image reproduction in the following sequence.
To measure. Step 112: A pulse 58 having the same magnitude as the phase encode pulse 57 is applied to cancel the influence of the phase encode pulse 57. Here, since the 180 ⁰ pulse 52 is applied, the pulse 58 is replaced by the pulse 57.
The effect of can be canceled. Step 113: The gradient magnetic field 59 for reading in the y direction is applied, and the signal 63 for position detection is measured. Step 114: Fourier transform the measurement signal 63 to obtain data projected on the y-axis. Step 115: Detect the upper end point of the projection data.
The position of respiration can be detected from the upper end point of each data.

Next, a method using the sequence shown in FIG. 5 will be described. The difference from the sequence shown in FIG. 4 is that the x-direction gradient magnetic field 82 is applied. This is done to cancel the phase change due to the movement in the x direction due to the influence of the gradient magnetic fields 80 and 81. As described above, the velocity at each position in the y direction can be obtained by Fourier-transforming the signal 84 measured at this time to obtain the phase. this is,
This is because the phase and the speed have a proportional relationship. Regarding this point, Japanese Patent Application No. 60-1 previously proposed by the applicant.
See the description in the specification of 50194 "NMR Blood Flow Imaging Method". The position can be obtained by integrating this velocity for each measurement data, and the position of respiration can be detected. Further, since the absolute value of the data after the Fourier transform is equal to the projection data obtained by the sequence shown in FIG. 4, the method shown in FIG. 7 can be used together to improve the accuracy.

Next, regarding the sequence shown in FIG.
A description will be given according to the processing flowchart of FIG. Step 121: In the pulse sequence of FIG. 6, the phase encode pulse 97 is sequentially changed, and the position detection signal 11 is generated by the following sequence for the number of times necessary for image reproduction.
Measure 4. Step 122: A pulse 98 having the same magnitude as the phase encode pulse 97 is applied to cancel the influence of the phase encode pulse 97. Step 123: The y-direction flow encode pulse 99 is applied to give a phase change according to the speed of movement in the y-direction. Step 124: The x-direction reading gradient magnetic field 102 is applied, and the velocity detection signal 104 is measured. Step 125: Fourier transform the measurement signal 104. Step 126: Obtain the phase component of the Fourier transformed data to obtain the velocity at each position in the x direction. As described above, the state of breathing can be known from the detection of the position.

FIGS. 9A to 9C show FIGS. 4 to 6, respectively.
It corresponds to the sequence shown in. Figure 9 (a)
Is the image signal 1 projected on the y-axis when the observed signal 63 is Fourier transformed in the sequence shown in FIG.
33 is obtained. The position of the breath can be known from the end point position of this signal (see arrow). In FIG. 9B, when the phase of the signal obtained by Fourier-transforming the observed signal 84 in the sequence shown in FIG. 5 is obtained, the velocity at each position in the y direction can be obtained (see arrow). Figure 9 (c)
In the sequence shown in FIG. 6, if the phase of the signal obtained by Fourier-transforming the observed signal 104 is obtained, the velocity at each position in the x direction can be obtained (see arrow). According to the above-described embodiment, it is possible to detect the movement of respiration at the time of signal measurement only with the main body of the NMR imaging device without using special hardware, and to synchronize with the respiration, which allows economical and high-quality imaging. The effect of being able to do is obtained.

As another embodiment of the present invention, a method of adding a sequence for measuring a signal including information about motion immediately after measuring an image signal and correcting the measured image signal based on the measurement information Will be described.
As the signal measuring method, the sequence shown in FIGS. 4 to 6 is used. The sequence shown in FIG. 4 is a system for obtaining an image signal projected on the y-axis, and the other is a system for obtaining the velocities in the y-direction and the x-direction, and the position can be known by integrating from the velocity. . First, the principle of correction of the detection signal will be described with reference to FIG. now,
Consider a case where there is a movement only in the y direction as shown in FIG. Since it is only in the y direction, it will be described as one-dimensional data hereinafter. Assuming that the state in which the abdomen is extended most by inhalation is indicated by the diagonal lines, the projection data at this time is as shown by the hatched portions in FIG. At this time, the distance between the upper end and the lower end is L, and the distance between the image center and the lowermost end is h. Further, a state when breathing in at an arbitrary time t is shown by a broken line and is represented by L / a (t).

At this time, if the measurement signal corresponding to the projection data of the diagonal line, that is, the Fourier spectrum of the projection data is F (ω), the measurement signal G (ω) in the broken line portion is

[Outer 1] Is represented. Therefore, if the values of the variables a (t) and h in equation (1) are known, F (ω) can be obtained from the measurement signal G (ω). This process is based on the above equation (1)

[Outside 2] Correction of

[Outside 3] Therefore, it can be seen that it consists of two resampling processes for obtaining F (ω). That is, if the values of h and a (t) are known, the correction can be performed.

Here, the value of h can be easily obtained by performing normal image reproduction, and the value of a (t) can be measured by the sequences of FIGS. 4 to 6 shown above. The above processing is shown in FIG. 11 in terms of actual image measurement signal levels. y
If there is movement only in the direction, the measurement signal is

[Outside 4] Becomes Since there is no movement in the x direction, Fourier transform in the horizontal direction

[Outside 5] Becomes In the conventional method, this data is directly subjected to Fourier transform in the vertical direction to obtain an image I (x, y) '. This image contains the influence of motion, and the image quality is degraded.

On the other hand, in the present embodiment, after the Fourier transform in the horizontal direction, the body motion correction process including the phase correction and the resampling process is performed, and the measurement signal F (x, ω _y ) containing no motion is obtained. Then, a vertical direction Fourier transform is performed to obtain an image I (x, y) in which the influence of motion is corrected. Hereinafter, the description will be continued by using a specific example. In this embodiment,
It is assumed that processing is performed according to the processing flow chart shown in FIG. 12 by using the device as shown in FIG. Step 151: In the pulse sequence of FIG. 4, the phase encode pulses 57 and 58 are sequentially changed to measure the image measurement signal 62 and the position detection signal 63 by the number of times required for image reproduction. Step 152: From the image measurement signal obtained in the previous step,
The reproduced image is obtained as it is, and the value of the parameter h in FIG. Further, the position detection signal 63 is Fourier-transformed to obtain projection data on the y-axis at the time of measuring each signal, and the length L of the most inhaled state is obtained. Each measurement time point a (t) is obtained from the L. As described above, the parameters h, L, a (t) necessary for the correction can be obtained.

Step 153: Fourier-transform the measurement signal in the horizontal direction to obtain the signal of the above equation (4). Step 154: For each measurement signal, the value of the following equation is obtained from the values of a (t) and h, and is multiplied by the value of the above equation (4).

[Outside 6] At this time, the next phase-corrected signal is obtained.

[Outside 7] Step 155: From the value of the above formula (6), it is ω which is the vertical direction.
(x, ω _y ) is obtained by resampling processing in the _y- axis direction. Here, any interpolation method may be used as the resampling processing, and for example, linear interpolation or spline interpolation can be applied.

Step 156: An image in which the influence of motion due to respiration is removed is obtained by performing Fourier transform in the vertical direction on (x, ω _y ) for which phase correction and resampling processing have been performed. According to the above-described embodiment, the motion of the respiration at the time of signal measurement is detected only by the NMR imaging apparatus main body without using any special hardware, and the high-quality imaging can be economically performed without synchronizing with the respiration. There is an effect that can be done. It is needless to say that each of the above embodiments is an example of the present invention and the present invention should not be limited to this. Figure 3
FIG. 4 shows an example of a sequence for detecting breathing within a short time prior to imaging, as an example of a technique related to the present invention. Here, it is assumed that there is movement in the y direction. The part surrounded by the broken line in the figure is a characteristic part of this technique.

In this technique, the signal is measured repeatedly within a short time. By Fourier transforming the measurement signal, it is possible to obtain the position of the motion due to respiration from the amplitude information and the velocity from the phase information. The RF pulse 31 defeats the spin α ⁰ (α is 90 to 180). Closer to 180 ^0, since relaxation is faster and can shorten the repetition time, the measurement signal is reduced. Thereafter, 180 ⁰ pulse 32 is applied, while applying a gradient magnetic field G _y 40 in the y-direction, to measure the signal 44. From this signal, the position and speed of movement due to respiration are detected, and the timing of photographing in synchronization with respiration is examined. If there is a deviation from this timing, this sequence is repeatedly measured and waits until the corresponding timing. The detection of the above-mentioned breathing state is based on the principle as described below.

The Fourier-transformed data of the measurement signal 44 is data obtained by projecting the image signal on the y-axis. The amplitude is the projection value and the phase is the y of each projection data.
The value is proportional to the speed of movement in the direction. Therefore,
The position can be known by detecting the end position of the projection data, and the velocity can be known from the phase value. For example, in FIG.
If the sequence within the broken line is repeated every 100 msec, it can be detected with a maximum delay of 100 msec. The operation when the above-described technique is applied to the NMR imaging apparatus shown in FIG. 2 will be described with reference to the flowchart shown in FIG. Note that, here, a case where there is a respiratory movement in the y direction will be described, but the same may be applied to the case of other directions. Step 11: In the pulse sequence of FIG. 3, the phase encode pulse 41 is sequentially changed, and the image signal is measured by the following sequence from step 12 to step 17 for the number of times required for image reproduction.

Step 12: The signal 44 is measured in a sequence for detecting respiration accompanied by movement in the y direction. That is, the α ⁰ pulse (90 ≦ α <180) 31 and the 180 ⁰ pulse 32 are applied together with the gradient magnetic fields G _z 35 and 36. The gradient magnetic field G _y 40 is applied simultaneously during signal measurement. Step 13: Fourier transform the measurement signal 44 to obtain the amplitude and phase of the data after the Fourier transform. Step 14: Since the amplitude data is the data obtained by projecting the image signal onto the y-axis, the position of the end point where the value becomes "0" is detected. Step 15: Since the phase data indicates the phase on each y coordinate point of the projection data, from the relationship between the velocity and the phase,
Calculate speed.

Step 16: It is checked from the speed and position whether or not the position of the breath to be imaged is determined in advance. If it is not the corresponding position, the process returns to step 12 and is repeated. However,
Due to the relaxation time, there is no signal even if it is repeated immediately, so leave some space. Also, if the corresponding position,
Proceed to step 17. Step 17: Photographing is performed by the normal pulse sequence, and the image signal 45 is measured. Step 18: Image reproduction is performed in synchronization with respiration based on the image signal obtained repeatedly. The image obtained through the above processing procedure is synchronized with respiration, so that a high-quality image can be obtained without causing image deterioration such as moving artifact due to the movement of the patient.

[0022]

As described above in detail, according to the present invention, it is possible to realize a remarkable effect that an NMR imaging method capable of detecting respiratory movements only by the main body of the NMR imaging apparatus and correcting the influence thereof can be realized. It plays.

[Brief description of drawings]

FIG. 1 is a flowchart of a process of detecting respiration within a short time before capturing an image, as an example of a technique related to the present invention.

FIG. 2 is a block diagram showing a configuration of an NMR imaging apparatus according to an embodiment of the present invention.

FIG. 3 is a diagram showing an example of a sequence for detecting breathing within a short time prior to imaging, as an example of a technique related to the present invention.

FIG. 4 is a diagram (No. 1) showing an example of a sequence added to measure a signal including information related to respiration immediately after measuring an image signal during photographing according to an embodiment.

FIG. 5 is a diagram (No. 2) showing an example of a sequence added to measure a signal including information about respiration immediately after measuring an image signal during photographing according to an embodiment.

FIG. 6 is a diagram (No. 3) showing an example of a sequence added to measure a signal including information about respiration immediately after measuring an image signal during photographing according to an embodiment.

FIG. 7 is a processing flowchart (part 1) showing the operation of the embodiment.

FIG. 8 is a processing flowchart (part 2) showing the operation of the embodiment.

FIG. 9 is a diagram for explaining the concept of the sequences shown in FIGS.

FIG. 10 is an explanatory diagram of parameters required for motion correction.

FIG. 11 is a diagram showing a concept of processing according to another embodiment.

FIG. 12 is a flowchart showing details of the processing shown in FIG.

[Explanation of symbols]

21 sequence control unit 22 high frequency pulse transmission unit 23 magnetic field control unit 24 magnetic field drive unit 25 receiver 26 processing device 27 CRT display 57,58,77,78,97,98 phase encode pulse 63 image signal 84,104 velocity signal

Claims (6)

[Claims] 1. A static magnetic field, a gradient magnetic field, a high-frequency magnetic field generating means, a detecting means for extracting an NMR signal from an inspection object, and a means for performing various calculations including image reconstruction on the detected signal. In the imaging method of the NMR inspection apparatus, when measuring an image signal, information relating to respiratory movement is measured together with the image signal, information relating to respiratory movement is obtained as phase information, and the image signal is corrected based on this. And an NMR imaging method. 2. A static magnetic field, a gradient magnetic field, a high frequency magnetic field generating means, a detecting means for extracting an NMR signal from an inspection object, and a means for performing various calculations including image reconstruction on the detected signal. In an imaging method in an NMR inspection apparatus, immediately after measuring an image signal, a gradient magnetic field that does not affect a stationary object but only a moving object is applied to obtain information on respiratory motion according to a velocity. An NMR imaging method characterized by measuring so as to obtain different signals and correcting the image signal based on the measurement. 3. The NM according to claim 2, wherein when observing the measurement signal, a read gradient magnetic field is applied in a direction perpendicular to a direction in which respiratory movement is desired to be detected.
R imaging method. 4. A static magnetic field, a gradient magnetic field, a high-frequency magnetic field generating means, a detecting means for extracting an NMR signal from an inspection object, and a means for performing various calculations including image reconstruction on the detected signal. In the imaging method in the NMR inspection apparatus, when measuring an image signal, information about the movement of the breath is measured at the same time so that the moving direction of the breath matches the phase encoding direction, and an image is obtained based on the measured data. A method for NMR imaging, which comprises correcting a signal. 5. The NMR imaging method according to claim 4, wherein the correction of the image signal is performed with respect to a fluctuation of a phase component on the measurement signal caused by the influence of respiratory movement. 6. The NM according to claim 4, wherein the displacement of the measurement position caused by the influence of the respiratory movement of the image signal is corrected by resampling processing.
R imaging method.