JP2004089515A - Magnetic resonance imaging system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce deterioration in the quality of a picked-up image in a high speed imaging method using SSFP (steady state free processor). <P>SOLUTION: A magnetic resonance imaging system includes: a magnetostatic field generating means 12; a gradient magnetic field generating means 14; a high-frequency magnetic field pulse generating means 16; a signal detecting means 18; a signal processing means 20; a display means 22; and a control means 24 for controlling the respective means. The control means 24 includes a high speed imaging function for allowing the high-frequency magnetic field pulse generating means 16 to apply a high-frequency magnetic field pulse on a part to be observed in a repetition time which is shorter than the vertical relaxation time and horizontal relaxation time of magnetization in the observation part of a subject, and also adjusting the phase encoding amount of the gradient magnetic field so as to allocate a nuclear magnetic resonance signal which is detected when magnetization in the observation part owing to the application is in a stationary state to the low-frequency area of a k-space. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a magnetic resonance imaging apparatus, and more specifically, to an image acquisition technique suitable for a high-speed imaging method.
[0002]
[Prior art]
2. Description of the Related Art A magnetic resonance imaging (hereinafter, referred to as MRI) apparatus uses a magnetic resonance phenomenon of, for example, protons, ie, hydrogen nuclei, which are main components of a subject, and detects a nuclear magnetic resonance signal generated from the subject by the magnetic resonance phenomenon. It is known that an image such as a tomographic image of an observation site is captured based on a detected nuclear magnetic resonance signal.
[0003]
Various high-speed imaging techniques have been proposed for such an MRI apparatus. For example, a high-speed imaging method known as a gradient echo method aims to shorten the imaging time by shortening the repetition time TR, that is, the time length required for a basic pattern of a pulse sequence applied to a subject.
[0004]
Some of the gradient echo methods use an SSFP-type gradient echo sequence. This is because, by continuously applying a high-frequency magnetic field pulse to the observation site with a repetition time TR shorter than the longitudinal relaxation time and the lateral relaxation time, the magnetization of the applied observation site is changed to a steady state, that is, a steady state free precession (Steady). In this case, an observation region is imaged at high speed by measuring the magnetization in a steady state in a state of State Free Precision (SSFP) (for example, see Non-Patent Document 1).
[0005]
[Non-patent document 1]
Marc Van Cauteren, "bFFE and bTFE, Imaging Sequences that Realize Ultra-High-Speed Real-Time Imaging and High S / N", INNERVISION, September 16, 2001, P44-P48.
[Problems to be solved by the invention]
However, in such an SSFP-type gradient echo sequence, artifacts may occur in an image and image quality may be degraded. For example, in a state in which the magnetization of the observation site reaches a steady state, that is, in a transient state, the magnetism continuously changes and vibrates, so that the signal strength of the detected nuclear magnetic resonance signal also fluctuates greatly. If an image is reconstructed based on a signal having a large fluctuation, artifacts occur and the image quality of the captured image is degraded. In particular, when a signal having a large fluctuation is assigned to a low-frequency portion of the k-space, that is, a central portion of the k-space, the amount of artifacts increases, resulting in a problem that the image quality deteriorates.
[0006]
An object of the present invention is to reduce deterioration in image quality of an image acquired by a high-speed imaging method using SSFP.
[0007]
[Means for Solving the Problems]
In order to solve the above problems, the MRI apparatus of the present invention comprises: a static magnetic field generating means for generating a static magnetic field to be applied to a subject; and a gradient magnetic field generating means for generating three different gradient magnetic fields to be applied to the subject. A high frequency magnetic field pulse generating means for generating a high frequency magnetic field pulse to be applied to the subject, a signal detecting means for detecting a nuclear magnetic resonance signal generated from the subject, and a nuclear magnetic resonance signal obtained by the signal detecting means. Signal processing means for reconstructing the image, display means for displaying the image reconstructed by the signal processing means, and control means for controlling each means, wherein the control means comprises a high-frequency magnetic field pulse generation means, A high-frequency magnetic field pulse is applied to the observation site with a repetition time shorter than the longitudinal and transverse relaxation times of the magnetization at the observation site, and the magnetization of the observation site due to the application is in a steady state. The detected nuclear magnetic resonance signals to allocate the low-frequency region of the k-space, and characterized by having a high-speed image capture function to adjust the phase encoding amount of the gradient magnetic field.
[0008]
That is, by adjusting the amount of phase encoding, a stable nuclear magnetic resonance signal detected in a steady state is allocated to a low-frequency region of k-space, which affects the image quality, particularly the contrast, of a display image, that is, the center of k-space. Therefore, compared with the case where the low frequency region of the k space is filled with the nuclear magnetic resonance signal detected in the transient state where the signal intensity fluctuates greatly, it is possible to suppress the artifact of the captured image and improve the image quality, particularly the contrast. it can.
[0009]
Further, the control means may adjust the phase encoding amount of the gradient magnetic field so that the nuclear magnetic resonance signal detected in the transition state before the magnetization of the observation site reaches the steady state is allocated to the high frequency region of the k space. preferable. As described above, by adjusting the amount of phase encoding, a nuclear magnetic resonance signal detected in a transient state, that is, a signal having an unstable characteristic in which the magnetization is continuously changing is converted into a k-space which does not affect the image quality, particularly, the contrast. In the high frequency region, that is, at the end. Therefore, the time for filling the k space with data can be reduced as compared to the case where the k space is filled using only the signal detected in the steady state.
[0010]
In this case, it is desirable that the control unit executes the high-speed imaging function after applying a spin preparation pulse that excites and saturates fat in the observation site. Thereby, fat protons are selectively excited by a spin preparation pulse such as a fat suppression pulse to saturate a fat signal, and a nuclear magnetic resonance signal detected in a steady state of magnetization by a high-speed imaging function of a control unit is obtained. It can be assigned to the center of k-space. Therefore, when an image is formed based on the allocated k-space, it is possible to obtain an image in which the occurrence of artifacts is suppressed while the fat signal is suppressed.
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
An embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows a configuration example of an MRI apparatus to which the present invention is applied. As shown in the figure, the MRI apparatus 1 generates a static magnetic field magnet 12 that generates a static magnetic field to be applied to a subject 10 and a gradient magnetic field that is applied to the subject 10 in three different directions, for example, X, Y, and Z-axis directions. A gradient magnetic field coil 14 is provided. An RF coil 16 for generating a high-frequency magnetic field pulse, ie, an RF pulse, to be applied to the subject 10, and an RF probe 18 for detecting a nuclear magnetic resonance signal, ie, an echo signal, generated from the subject 10 are provided. A signal processing unit 20 for reconstructing an image based on the echo signal obtained by the RF probe 18 and a display unit 22 for displaying the image reconstructed by the signal processing unit 20 are provided.
[0012]
The operation of the MRI apparatus thus configured will be described. A static magnetic field is applied to the subject 10 by the static magnetic field magnet 12. A slice selection gradient magnetic field is applied from the gradient magnetic field coil 14 to an observation site of the subject 10 to which the static magnetic field is applied, for example, the heart, according to a signal of the gradient magnetic field power supply 26 based on a command of the control unit 24. A slice selection gradient magnetic field is applied, and an RF pulse is applied to the observation site from the RF coil 16 in accordance with a signal from the signal transmission unit 28 based on a command from the control unit 24, so that, for example, a proton A resonance phenomenon is caused. A phase encoding gradient magnetic field and a frequency encoding gradient magnetic field are applied from the gradient magnetic field coil 14 in order to acquire positional information of the observation site where the magnetic resonance phenomenon has occurred.
[0013]
Then, an echo signal generated from the observation site due to the magnetic resonance phenomenon is detected by the signal detection unit 30 from the RF probe 18 based on a command from the control unit 24. Based on the detected echo signal, the signal processing unit 20 reconstructs a tomographic image of an observation site, for example, a tomographic image of the heart two-dimensionally or three-dimensionally, and displays the reconstructed image on the display unit 22.
[0014]
FIG. 2 shows a pulse sequence of the gradient echo method according to one embodiment of the present invention. As shown in the figure, the horizontal axis indicates the application timing of the RF pulse during imaging, the slice selection gradient magnetic field Gs, the phase encoding gradient magnetic field Gφ, and the application timing of the frequency encoding gradient magnetic field Gr, in order from the highest level. The horizontal axis shown in the lower part shows a timing chart for receiving an echo signal. The axis direction of each abscissa indicates the passage of time.
[0015]
A timing chart of such a pulse sequence will be described. First, when an image of an observation site, for example, a heart is taken by the MRI apparatus 1, a static magnetic field (for example, 1.5 Tesla) is applied to the observation site. A slice gradient magnetic field 101 is applied to the heart to which the static magnetic field is applied, and an RF pulse 102 having a flip angle of α (for example, 45 °) is applied to induce a magnetic resonance phenomenon in the slice of the observation site. The phase encoding gradient magnetic field 103 is applied to the observation site where the magnetic resonance phenomenon is induced with the number of phase encodings determined by the control unit 24 (for example, 128, 256, 512, etc.). When the phase encoding gradient magnetic field 103 is applied to the observation site, a so-called dephase pulse 104 is applied in the frequency encoding direction, that is, the reading direction. As a result, the phase difference between nuclear spins in the Gr-axis direction increases. Next, the echo signal 107 is received during the A / D sampling interval 106 while applying the frequency encoding gradient magnetic field 105. After receiving the echo signal 107, a so-called rephase pulse 109 having a polarity opposite to that of the phase encode gradient magnetic field 103 and a frequency encode pulse 105 in the readout direction and a half of the applied amount is applied to the observation site. Is applied. This cancels the phase difference between nuclear spins. Then, an RF pulse 110 having a flip angle of -α (for example, -45 °) is applied. Here, the time from when the RF pulse 102 having the flip angle α is applied to when the RF pulse 102 having the flip angle −α is applied is referred to as a repetition time TR. The echo signal 107 (for example, a time-series signal composed of 128, 256, 512, and 1024 pieces of sampling data) is obtained by continuously applying the RF pulse to the observation site at the repetition time TR.
[0016]
In the timing chart of such a pulse sequence, if the pulse sequence is repeated with the repetition time TR shortened, the magnetization reaches a steady state via a transient state. That is, if the repetition time TR is set to a time (for example, 3 ms or less) that is significantly shorter than either of the longitudinal relaxation time T1 and the transverse relaxation time T2, a so-called steady state free precession occurs after the magnetization is in a vibrating state, that is, in a transient state. It is in a stable state of exercise (SSFP). At this time, if the signal detected in the transient state fills the low-frequency region of the k-space, the amount of occurrence of artifacts in the captured image increases. Therefore, in the present embodiment, the occurrence of an artifact is suppressed by adjusting the amount of phase encoding by the control unit 24 and allocating the signal detected in the steady state to the low-frequency region of the k-space.
[0017]
Here, the high-speed imaging function of the control unit 24, which is a feature of the present invention, will be described. FIG. 3 shows a conceptual diagram of a gradient echo sequence of the SSFP type. As shown in the figure, the timing of applying the RF pulses 102a to 102n having a flip angle of α or −α to an observation site, for example, the heart, and the reception timing of the echo signals 107a to 107b received according to the applied RF pulses are shown. It is shown on two horizontal axes. A k-space 112 in which the received echo signal is filled is shown. The k-space 112 includes a horizontal axis frequency encoding direction or a readout direction kx and a vertical axis phase encoding direction ky orthogonal to the horizontal axis kx. Is formed. Further, the k-space 112 is divided into a central portion 112a, that is, a low-frequency region, and an end portion 112b, that is, a high-frequency region, in the phase encoding direction.
[0018]
At this time, by adjusting the application amount of the phase encoding gradient magnetic field 103 based on a command from the control unit 24, for example, the echo signals 107a to 107e detected in the transient state are discarded, and immediately after reaching the steady state. (E.g., echo signals 107f to 107i) are filled in the central portion 112a of the k-space 112, and further, for example, the echo signals 107j to 107n are filled in unfilled regions of the k-space 112, for example, the ends 112b.
[0019]
Accordingly, the echo signal filled in the central portion 112a of the k-space 112 is a stable signal whose signal intensity does not largely fluctuate. Therefore, when the echo signals 107a to 107e detected in the transient state are filled in the central portion 112a. In comparison, the occurrence of artifacts can be suppressed.
[0020]
As described above, the present invention has been described based on the embodiment, but the MRI apparatus 1 according to the present invention is not limited to this. For example, in FIG. 3, the echo signals 107a to 107e detected in the transient state may be filled into the end 112b of the k-space 112 without being discarded. As a result, the time required to fill the k space with data can be reduced as compared to the case where the k space is filled using only the signals 107f to 107n detected in the steady state.
[0021]
FIG. 4 is a conceptual diagram in which a fat excitation pulse 115 is added to one embodiment of the gradient echo sequence according to the present invention. As shown in the figure, after applying a spin preparation pulse, for example, a fat excitation pulse 115 to an observation site, for example, a heart, RF pulses 102a to 102n may be continuously applied for a repetition time TR.
[0022]
At this time, the fat suppression effect of the fat excitation pulse 115 is highest immediately after the application, and decreases with time. Therefore, it is preferable to assign an echo signal (e.g., echo signals 107f to 107i) detected immediately after reaching the steady state to the center 112c of the k-space 112. As a result, the signals (e.g., echo signals 107f to 107i) allocated to the central portion 112 are signals having a sufficient fat suppression effect, so that a good image in which the occurrence of artifacts is further suppressed can be obtained. For example, when imaging the coronary artery of the heart, by suppressing the signal of fat around the coronary artery, an image of the coronary artery can be drawn with good contrast.
[0023]
FIG. 5 is a conceptual diagram of cardiac imaging using a fat suppression pulse. As shown in the figure, a measurement window, that is, a time (for example, 1 second) from the time when the electrocardiogram R wave is sensed to the time when the next electrocardiogram R wave is sensed is divided by a certain time (for example, 100 ms to 200 ms). I have. The RF pulses 102a to 102i are continuously applied at a repetition time TR of, for example, 4 ms using the SSFP type gradient echo method for each of the divided measurement windows.
[0024]
At this time, a fat suppression pulse 115 (for example, about 30 ms) is applied to the observation site, for example, the heart, then an RF pulse 120 having a flip angle of -α / 2 is applied, and the flip angle is α or -α. RF pulses 102a to 102i are applied. As described above, by applying the RF pulse 120 having the flip angle of -α / 2, strong vibration of a signal in a transient state can be suppressed. Therefore, the number of excitations of, for example, 20 to 30 times required to reach the steady state can be reduced to, for example, about 10 to 15 times. As described above, the time during which a stable signal can be detected can be shortened as compared with the case where the RF pulse 120 is not applied, so that the detected signal can further obtain the fat suppression effect of the fat suppression pulse 115. . Therefore, it is possible to obtain a good image in which artifacts of the captured image are further reduced.
[0025]
Also, the case where the fat suppression pulse 115 is a frequency-selective excitation pulse has been described, but an inversion recovery (IR) pulse may be used instead. At this time, the control unit 24 controls the sequence start timing so that the time when the transverse magnetization of fat becomes zero by the inversion recovery pulse and the time when the magnetization reaches the steady state are substantially the same. Then, the phase encoding amount is adjusted so that the signal (for example, the echo signals 107f to 107i) immediately after reaching the steady state is filled in the central portion 112a of the k-space 112. As a result, the signal filled in the central portion 112a is a signal having a stable characteristic with a sufficient fat suppression effect of the inversion recovery pulse, so that it is possible to further suppress the occurrence of artifacts in the captured image.
[0026]
Further, for example, even when a bipolar gradient magnetic field that suppresses an artifact due to blood flow is used, the high-speed imaging function according to the present invention can be applied. That is, when using the bipolar gradient magnetic field for suppressing the blood flow, the control unit 24 fills the central portion 112a of the k-space 112 with a signal (for example, the echo signals 107f to 107i) that maximizes the effect of the preparation pulse in a steady state. Is adjusted as described above. Thereby, it is possible to suppress the occurrence of the artifact of the captured image due to the blood flow.
[0027]
Further, after applying a frequency selection pulse, for example, a fat suppression pulse to the observation site, for example, a CHESS method (Chemical Shift Suppression) may be used. That is, following the frequency selection pulse, a strong gradient magnetic field pulse, that is, a crusher pulse is applied in a plurality of axes, for example, in the X-axis, Y-axis, and Z-axis directions, so that the fat signal does not refocus. As a result, it is possible to reduce the occurrence of artifacts in the captured image due to the fat signal.
[0028]
FIG. 6 is a conceptual diagram illustrating an example of 3D imaging to which the present invention is applied. As shown in the figure, after applying a fat excitation pulse 115 to an observation site, for example, a heart, RF pulses 102a to 102n are continuously applied, and echo signals 107a to 107n generated from the applied observation site are detected. The detected echo signals 107a to 107n are filled in the k-space 200. The k space 200 filled with the detected echo signals 107a to 107n is formed by a vertical axis ks, ie, a slice encode axis, and a horizontal axis kp orthogonal to the ks axis, ie, a phase encode axis.
[0029]
At this time, by the high-speed imaging function of the control unit 24, for example, a signal (e.g., echo) The signals 107a to 107e) are filled in a first peripheral region, that is, a high frequency region 201b. Further, signals detected immediately after reaching the steady state (e.g., echo signals 107f to 107i) are filled in the low frequency region, that is, the central portion 201a. Further, a signal (for example, echo signals 107j to 107n) detected in a state where the suppression effect of the fat suppression pulse 115 is reduced is filled in the second peripheral region, that is, the high frequency region 201b.
[0030]
As described above, the control unit 24 fills the first area 200a with the echo signal detected in the first measurement window, and similarly stores the echo signal detected in the second measurement window in the second area 200b. And the third region 200c is filled with the echo signal detected in the third measurement window, and the fourth region 200d is further filled with the echo signal detected in the fourth measurement window.
[0031]
Accordingly, the signal filled in the central portion 201a of the k-space 200 has a small variation in signal intensity and a sufficient effect of suppressing a fat signal, so that occurrence of an artifact in a high-speed captured image can be suppressed. It is possible to obtain a 3D image with good image quality, particularly good contrast.
[0032]
FIG. 7 is a conceptual diagram of another example of 3D imaging to which the present invention is applied. As shown in the figure, the vertical axis ks of the k space 200 indicates slice encoding, and the horizontal axis kp indicates phase encoding. At this time, the control unit 24 fills the low frequency region of the k-space 200, that is, the circular central part 202a, with a signal (e.g., the echo signals 107f to 107i) detected immediately after the stationary state is reached. (For example, the echo signals 107j to 107n) are filled in the high frequency region, that is, the peripheral region 202b.
[0033]
As a result, the signal filled in the central portion 202a has a small variation in signal intensity and a sufficient effect of suppressing a fat signal. Therefore, the generation of image artifacts is suppressed, and 3D images with particularly good image quality and contrast are suppressed. Images can be obtained.
[0034]
In addition, in capturing a three-dimensional image described with reference to FIGS. 7 and 8, when the trajectory (k-trajectory) in the k-space 200 is linear, it is preferable to perform gridding before Fourier transform. At this time, a lattice point near a straight line may be selected as a trajectory in the k space 200, and the trajectory in the k space 200 may be formed in a jagged shape along a straight line, that is, a broken line shape.
[0035]
【The invention's effect】
As described above, according to the present invention, it is possible to reduce deterioration of the image quality of a captured image in a high-speed imaging method using SSFP.
[Brief description of the drawings]
FIG. 1 shows a configuration example of an MRI apparatus to which the present invention is applied.
FIG. 2 shows a pulse sequence of a gradient echo method according to an embodiment of the present invention.
FIG. 3 shows a conceptual diagram of an SSFP type gradient echo sequence.
FIG. 4 shows a conceptual diagram in which a fat excitation pulse is added to one embodiment of the gradient echo sequence according to the present invention.
FIG. 5 shows a conceptual diagram of cardiac imaging using a fat suppression pulse.
FIG. 6 shows a conceptual diagram of an example of 3D imaging to which the present invention is applied.
FIG. 7 shows a conceptual diagram of another example of 3D imaging to which the present invention is applied.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Magnetic resonance imaging apparatus 10 Subject 12 Static magnetic field magnet 14 Gradient magnetic field coil 16 RF coil 18 RF probe 20 Signal processing part 24 Control means 22 Display part

Claims (3)

Static magnetic field generating means for generating a static magnetic field to be applied to the subject; gradient magnetic field generating means for generating gradient magnetic fields in three different directions to be applied to the subject; and generating a high-frequency magnetic field pulse to be applied to the subject. High-frequency magnetic field pulse generating means, signal detecting means for detecting a nuclear magnetic resonance signal generated from the subject, signal processing means for reconstructing an image based on the nuclear magnetic resonance signal obtained by the signal detecting means, Display means for displaying an image reconstructed by the signal processing means, and a magnetic resonance imaging apparatus comprising a control means for controlling each of the means,
The control means causes the high-frequency magnetic field pulse generation means to apply the high-frequency magnetic field pulse to the observation part with a repetition time shorter than the longitudinal relaxation time and the lateral relaxation time of the magnetization in the observation part of the subject, and A magnetic field characterized by having a high-speed imaging function of adjusting the amount of phase encoding of the gradient magnetic field so that the nuclear magnetic resonance signal detected in the steady state is assigned to a low-frequency region of k-space. Resonance imaging device. The control means adjusts the phase encoding amount of the gradient magnetic field so that the nuclear magnetic resonance signal detected in a transient state before the magnetization of the observation site reaches the steady state is allocated to a high frequency region of k-space. The magnetic resonance imaging apparatus according to claim 1, wherein: 3. The magnetic resonance imaging apparatus according to claim 1, wherein the control unit executes the high-speed imaging function after applying a spin preparation pulse that excites and saturates fat in the observation site. 4. .

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