JP3839992B2 - Magnetic resonance imaging system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic resonance imaging apparatus that captures a three-dimensional image using a three-dimensional Fourier transform method or the like.
[0002]
[Prior art]
In recent years, an MRCP (MR cholangiopancreatography) imaging method has been developed as one of the biliary pancreatic duct imaging methods. This is a method of imaging the form of the pancreaticobiliary tract filled with water by rendering only the stagnant water using a very strong T2 (spin-spin relaxation: transverse relaxation) weighted image, It is becoming an indispensable test method in the diagnosis of biliary pancreatic diseases. The MRCP method uses a single shot, half Fourier, or fast spin echo (SE) method to suppress artifacts due to breathing or gastrointestinal peristalsis, and one or two to three breath breaths. A technique for photographing is widely used. The single shot method is a method of collecting all data necessary for image reconstruction in one excitation in order to shorten the imaging time. The half Fourier method is a method of taking an image with half the number of phase encodings by utilizing the symmetry of data.
[0003]
In the MRCP imaging method, not only two-dimensional imaging but also three-dimensional imaging using a three-dimensional Fourier transform method is performed. There is an advantage that observation is possible. However, in the case of 3D imaging, the number of slices to be collected is large, so it is not possible to collect all data under one or two to three breath breaths. A technique using respiratory synchronization is employed. However, the method of repeating intermittent breath holding cannot prevent the occurrence of artifacts. Artifacts due to peristalsis of the gastrointestinal tract cannot be prevented even with the technique using respiratory synchronization.
[0004]
On the other hand, as an imaging method for ischemic brain disease, MPG (Motion probing gradient) pulse that erases signals from water molecules that move randomly in the voxel is applied to emphasize the molecular diffusion movement in living tissue A diffusion weighted imaging method has also been developed. Since this imaging method is sensitive to minute movements, an MPG pulse having a high intensity and a long application time is applied. For this reason, a slight movement of the object itself leads to a large phase shift and causes artifacts. As one of the methods to suppress this artifact, single shot / EPI (Echo Planar Imaging) method or single shot / high-speed SE method that collects all data with one or two or three excitations is used. Has been. In these methods, since the amount of phase shift due to body movement is almost the same in all echo data, no artifact occurs. Therefore, no special post-processing is required, and an image free from artifacts can be obtained relatively easily. However, since the data collection time is limited (single shot), there are disadvantages such that a sufficient number of data cannot be collected, the in-plane resolution is not good, and the three-dimensional Fourier transform imaging method is difficult. This is because it is necessary to reduce the number of matrices in the phase encoding direction in order to collect all data necessary for three-dimensional imaging within a limited time. In order to reduce the number of matrices while maintaining the resolution in the phase encoding direction, it is necessary to reduce the visual field in the phase encoding direction. However, simply reducing the number of matrices will cause aliasing artifacts.
[0005]
In addition, there is a navigator echo method as a body motion artifact suppression method in the case where data is divided and collected by one or two or three excitations in the above two imaging methods, particularly the diffusion weighted imaging method. This is because the phase shift amount due to body movement is measured for each excitation by collecting the echo data that does not apply phase encoding called navigator echo separately from the echo data for image creation, and after that amount of phase shift In this method, the phase of the signal is restored by processing. In this method, since it is not necessary to collect all data by one excitation, there is an advantage that in-plane resolution and S / N can be freely adjusted, and three-dimensional Fourier transform imaging can be easily realized. However, the artifact cannot be completely suppressed due to an error in measuring the phase shift amount.
[0006]
[Problems to be solved by the invention]
As described above, the conventional magnetic resonance imaging apparatus has a drawback in that a three-dimensional image cannot be taken in a short time without causing artifacts due to body movement or the like.
[0007]
The object of the present invention is to perform three-dimensional imaging using a single shot method that collects all data under one or two or three breath breaths and reconstructs a three-dimensional image free from artifacts. An imaging device is provided.
[0008]
[Means for Solving the Problems]
In order to solve the above problems and achieve the object, the present invention uses the following means.
[0010]
(1) According to one aspect of the present invention, in a magnetic resonance imaging apparatus that executes a three-dimensional imaging sequence that executes a preparation sequence that applies two types of pre-excitation pulses prior to an imaging sequence, two types of pre-excitation pulses while applying a sectional selective gradient magnetic field in the phase encode direction along with the excitation cross section of one of the excitation pulses the a cross section perpendicular to the phase encode direction, applying a gradient magnetic field section selected in the slice direction with the other of the excitation pulse Then, the excitation cross section of the other excitation pulse is a cross section orthogonal to the slice direction, and echo signals are collected based on signals from a rectangular parallelepiped region where both cross sections intersect.
[0011]
(2) According to another aspect of the present invention, in a magnetic resonance imaging apparatus that executes a three-dimensional imaging sequence that executes a preparation sequence that applies a pre-excitation pulse prior to an imaging sequence, the pre-excitation pulse and the first of the imaging sequence by applying a cross-sectional selective gradient magnetic field in the phase encoding direction with one of the excitation pulses, the excitation cross section of one of the excitation pulses the a cross section perpendicular to the phase encode direction, the other of the excitation pulse with the tilting section selected in the slice direction A magnetic field is applied, the excitation cross section of the other excitation pulse is made a cross section orthogonal to the slice direction, and echo signals are collected based on signals from a rectangular parallelepiped region where both cross sections intersect.
[0012]
According to the present invention, three-dimensional imaging can be performed using a single shot method in which all data is collected under one or two or three breath breaths, and an artifact-free three-dimensional image can be reconstructed.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of a magnetic resonance imaging apparatus according to the present invention will be described below with reference to the drawings.
[0014]
First Embodiment FIG. 1 is a block diagram showing a schematic configuration of this embodiment. In the gantry 20, a static magnetic field magnet 1, an x-axis, a y-axis, a z-axis gradient magnetic field coil 2, and a transmission / reception coil 3 are provided. The static magnetic field magnet 1 as a static magnetic field generator is configured using, for example, a superconducting coil, a normal conducting coil, or a permanent magnet. The x-axis, y-axis, and z-axis gradient magnetic field coils 2 are coils for generating an x-axis gradient magnetic field Gx, a y-axis gradient magnetic field Gy, and a z-axis gradient magnetic field Gz. The transmission / reception coil 3 generates radio frequency (RF) pulses and is used to detect echo signals generated by magnetic resonance. The subject P on the bed 13 is inserted into an imageable region in the gantry 20 (a spherical region where an imaging magnetic field is formed, and diagnosis is possible only in this region). Note that transmission of RF pulses and reception of echo signals may be performed by separate transmission coils and reception coils.
[0015]
The static magnetic field magnet 1 is driven by a static magnetic field control device 4. The transmitter / receiver coil 3 is driven by the transmitter 5 when magnetic resonance is excited, and is coupled to the receiver 6 when an echo signal is detected. The x-axis, y-axis, and z-axis gradient magnetic field coils 2 are driven by an x-axis gradient magnetic field power source 7, a y-axis gradient magnetic field power source 8, and a z-axis gradient magnetic field power source 9.
[0016]
The x-axis gradient magnetic field power supply 7, the y-axis gradient magnetic field power supply 8, the z-axis gradient magnetic field power supply 9, and the transmitter 5 are driven by the sequencer 10 according to a predetermined sequence, and the x-axis gradient magnetic field Gx, the y-axis gradient magnetic field Gy, the z-axis gradient magnetic field A magnetic field Gz and a radio frequency (RF) pulse are generated in a predetermined pulse sequence described later. In this case, the x-axis gradient magnetic field Gx, the y-axis gradient magnetic field Gy, and the z-axis gradient magnetic field Gz are mainly used, for example, as a read gradient magnetic field Gr, a phase encode gradient magnetic field Ge, and a slice gradient magnetic field Gs, respectively. The computer system 11 drives and controls the sequencer 10, captures an echo signal as an echo signal received by the receiver 6, and performs predetermined signal processing to generate a tomographic image of the subject. indicate.
[0017]
FIG. 2 shows a pulse sequence according to the first embodiment of the present invention. This is a single shot, three-dimensional high-speed SE method sequence. The phase encode gradient magnetic field Ge is applied together with the 90 ° (π / 2) pulse that is the first RF pulse, and the cross section excited by the 90 ° pulse is set as a cross section orthogonal to the phase encode direction, and the second RF thereafter A 180 ° (π) pulse, which is a pulse, is applied together with the slice gradient magnetic field Gs, and the cross section excited by the 180 ° pulse is a cross section orthogonal to the slice direction. Since the spin echo is generated when both the 90 ° pulse and the 180 ° pulse are received, as shown in FIG. 3, a rectangular parallelepiped in which a cross section orthogonal to the phase encoding direction and a cross section orthogonal to the slice direction intersect. Only the signal from the region is formed as an echo signal and is imaged. Thus, since the cross sections excited by the 90 ° pulse and the 180 ° pulse intersect each other by 90 °, the field of view (FOV) in the phase encoding direction can be narrowed. It is sufficient that only the vicinity of the tumor can be imaged even when imaging the head, and the abdomen or the gallbladder can be sufficiently imaged even in a narrow FOV. By narrowing the FOV in this way, all data necessary for three-dimensional reconstruction can be collected even under one or two to three breath breath holds.
[0018]
In the case of the three-dimensional Fourier transform imaging method, it is necessary to collect echo data corresponding to the number of slices × the number of phase encodings. By narrowing the field of view (FOV) in the phase encoding direction, the phase is maintained while maintaining the same in-plane resolution (resolution in the phase encoding direction of the image = field of view in the phase encoding direction / number of matrices in the phase encoding direction). The number of encodings can be reduced, that is, the number of echo data to be collected can be reduced. However, even if the FOV is narrowed by narrowing the phase encoding range at the time of echo collection, aliasing artifacts are generated by echo signals from portions other than the FOV. However, according to the present embodiment, as shown in FIG. 3, since a method of changing (crossing) the excitation cross sections of the 90 ° pulse and the 180 ° pulse is used, regions other than the FOV are not excited, and from there Since no echo signal is generated, aliasing artifacts do not occur. For this reason, the number of phase encodings can be freely selected to some extent, and the number of echoes to be collected per shot within the time when a signal can be obtained from the T1 value and T2 value of the target tissue can be determined. Can be suppressed. Since the magnetization excited by the RF pulse is attenuated by longitudinal relaxation and transverse relaxation, there is an image quality limitation on the upper limit of echoes that can be collected in one shot, and the limit is the T1 value (spin − Lattice relaxation: longitudinal relaxation), T2 value (spin-spin relaxation: transverse relaxation), an interval for generating an echo (this is a system performance, particularly a gradient magnetic field performance: maximum gradient magnetic field strength slew rate), and the like. Thus, if the number of echoes to be collected can be reduced, the number of shots required for collection can be reduced, and the time required for collection can be shortened.
[0019]
A specific number of encodings according to the first embodiment will be described. Consider a case where MRCP imaging is performed by a three-dimensional Fourier transform / high-speed SE method. In normal imaging, an imaging area of FOV = 35 (phase encoding direction) × 35 (read encoding direction) × 4 (slice direction) cm is 256 (phase encoding direction) × 256 (read encoding direction) × 30 (slice direction). Collect data in a matrix of degree. In this case, the number of echoes to be collected is 256 × 30 = 7680 echoes. However, since the main purpose of 3D imaging is to examine the bifurcation of the pancreatic duct and bile duct, the area actually required as an image is 10 (phase encoding direction) × 10 (lead encoding direction) × 4 (slice direction) cm. A degree of area is sufficient. If the FOV in the phase encoding direction is reduced to 10 cm while maintaining the above resolution, the number of phase encodings can be reduced to 10/35 = 0.28 times. In the MRCP method, since water components having a long T1 value and T2 value are examined, data can be collected for about 5 seconds (5000 milliseconds) by one excitation. If the echo interval is 5 milliseconds, about 5000/5 = 1000 echoes / shot can be collected. In the sequence of FIG. 2, since it is sufficient to collect 7680 × 0.28 = 2150 echoes, it is possible to collect all data in 2 to 3 shots. Even if the repetition time TR is set to 10 seconds, the scan time is 20 to 30 seconds, so that it is possible to take an image with one breath hold.
[0020]
Hereinafter, other embodiments of the apparatus according to the present invention will be described. In the description of the other embodiments, the same parts as those of the first embodiment are denoted by the same reference numerals, and the detailed description thereof is omitted.
[0021]
Second Embodiment Since the block diagram of the second embodiment is the same as the block diagram of the first embodiment, the illustration is omitted.
[0022]
In the first embodiment, the number of data to be collected is reduced by narrowing the FOV by intersecting the excitation cross sections of two excitation pulses (π / 2, π pulse) in the sequence of the spin echo method. The RF pulse for narrowing down is not limited to the pulse in the imaging sequence, but may be a pulse in the preparation sequence. FIG. 4 shows a pulse sequence of an embodiment of a three-dimensional high-speed SE method with a prepulse (MPG pulse) for causing a phase shift according to the flow velocity and emphasizing diffusion. The preparation sequence also applies a π / 2 pulse and a π pulse. As in the first embodiment, the π / 2 pulse is applied together with a phase encoding gradient magnetic field, and the π pulse is applied together with a slice gradient magnetic field. To narrow the FOV in the phase encoding direction. In this example, before and after the RF pulse (π pulse) 602, gradient magnetic field pulses (MPG) 201 and 202 for diffusion emphasis are applied. Reference numeral 601 denotes a π / 2 pulse. A gradient magnetic field pulse (spoiler pulse) SP01 ′ having a constant time integral value in the phase encoding direction is applied between the π pulse 602 and the π / 2 pulse 101 of the imaging sequence, and the gradient magnetic field pulse is between the imaging sequences. Gradient magnetic field pulses obtained by adding a predetermined phase encoding amount to gradient magnetic field pulses (spoiler pulses) SP02, SP02 ′, SP03, SP03 ′,. The polarities of ', SP03, SP03', ... are as shown).
[0023]
In general, assuming that the head is collected in a matrix of 128 (read encode direction) × 128 (phase encode direction) × 10 (slice direction), the number of echoes to be collected is 1280 echoes. Assuming that the echo is generated in 5 milliseconds, it takes 1280 × 5 = 6400 milliseconds to take all the echoes, which is extremely long compared to the T1 value of the brain parenchyma (800 to 900 milliseconds). It cannot be collected by shots, and the problem of motion artifacts cannot be solved. However, in the sequence of the second embodiment shown in FIG. 4, aliasing artifacts do not occur, so the number of matrices in the phase encoding direction can be reduced. For example, if the number of matrices is 128 (read encoding direction) × 20 (phase encoding direction) × 10 (slice direction), the number of echoes to be collected is 200 echoes, and echoes are generated in 5 milliseconds. It takes 200 × 5 = 1000 milliseconds, and can be collected in the same time as the T1 value of the brain parenchyma.
[0024]
As described above, according to the second embodiment as well, the FOV in the phase encoding direction can be reduced by intersecting the excitation cross sections of the two excitation pulses in the preparation sequence, and the number of data to be collected can be reduced. By adjusting the number of phase encodings so that all data can be collected, single-shot imaging of a three-dimensional image becomes possible. In addition, a three-dimensional diffusion weighted image free from ghost artifacts due to motion can be obtained.
[0025]
Third Embodiment Since the block diagram of the third embodiment is the same as the block diagram of the first embodiment, the illustration is omitted.
[0026]
The third embodiment is also an example in which the FOV is narrowed by crossing the excitation cross sections of two excitation pulses in the preparation sequence. FIG. 5 shows a pulse sequence of an embodiment of the three-dimensional high-speed SE method with a pre-pulse (VENC: velocity encode) that causes a phase shift according to the flow velocity. Unlike the diffusion-weighted imaging in FIG. 4, the flow rate measurement sequence has only one RF pulse (π / 2 pulse), but the excitation sequence of the RF pulse of the preparation sequence and the first RF pulse ( The direction of the gradient magnetic field applied with the RF pulse is changed so that the excitation cross sections of (π / 2 pulse) are orthogonal to each other. That is, the RF pulse in the preparation sequence is applied with the phase encode gradient magnetic field, and the first RF pulse (π pulse) in the imaging sequence is applied with the slice gradient magnetic field, thereby crossing the excitation cross section and causing the FOV in the phase encode direction. Squeezing. Also according to the third embodiment, the number of encodings for collecting data can be reduced, and single shot shooting of a three-dimensional image is possible. Note that VENC in FIG. 5 is a gradient magnetic field for giving a phase shift according to the flow velocity, and an image collected by applying a gradient magnetic field with a pattern like a solid line and a gradient magnetic field with a pattern like a broken line are given. Flow velocity information is obtained by taking a difference from the collected image.
[0027]
Fourth Embodiment FIG. 6 shows an embodiment in which the imaging sequence in FIG. 5 adopts the high-speed FE method instead of the high-speed SE method. That is, the preparation sequence is the same as that in the third embodiment, and the RF pulse excitation cross section of the preparation sequence and the first RF pulse (π / 2 pulse) of the imaging sequence are orthogonal to each other so that they are orthogonal to each other. The direction of the gradient magnetic field to be applied is changed.
[0028]
Here, if the spoiler pulses SP01, SP02, SP02 ′,... And the RF pulse are phase-controlled to collect signals only for the components that have undergone transverse magnetization in the preparation sequence, In addition to the high-speed FE method as in the embodiment, a method such as the EPI method can be used.
[0029]
The present invention is not limited to the embodiment described above, and can be implemented with various modifications. For example, the imaging sequence is not limited to the specific example described above, and can be selected and implemented as necessary. In addition, specific numerical values of the number of matrices in the phase encoding direction and the slice direction are not limited to the above numerical values.
[0030]
【The invention's effect】
As described above, according to the present invention, it is possible to provide a magnetic resonance imaging apparatus capable of single-shot imaging in order to suppress motion artifacts even in three-dimensional imaging.
[Brief description of the drawings]
FIG. 1 is a block diagram showing the configuration of a first embodiment of a magnetic resonance imaging apparatus according to the present invention.
FIG. 2 is a diagram showing a pulse sequence according to the first embodiment.
FIG. 3 is a diagram showing an excitation cross section of the first embodiment.
FIG. 4 is a diagram showing a pulse sequence according to the second embodiment.
FIG. 5 is a diagram showing a pulse sequence according to a third embodiment.
FIG. 6 is a view showing a pulse sequence according to the fourth embodiment.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Static magnetic field magnet 2 ... X-axis, y-axis, z-axis gradient magnetic field coil 3 ... Transmission / reception coil 4 ... Static magnetic field control apparatus 5 ... Receiver 6 ... Transmitter 7 ... X-axis gradient magnetic field amplifier 8 ... Y-axis gradient magnetic field amplifier 9 ... z axis gradient magnetic field amplifier 10 ... sequencer 11 ... computer system 12 ... display unit

Claims (2)

In a magnetic resonance imaging apparatus that executes a three-dimensional imaging sequence that executes a preparation sequence that applies two types of pre-excitation pulses prior to an imaging sequence, a gradient magnetic field for cross-section selection in the phase encoding direction together with one of the two types of pre-excitation pulses by applying a, the excitation cross section of one of the excitation pulses the a cross section perpendicular to the phase encode direction, by applying the other gradient magnetic field section selected in the slice direction with excitation pulses, the slice excitation cross section of the excitation pulses of the other side A magnetic resonance imaging apparatus that collects echo signals based on a signal from a rectangular parallelepiped region where both cross sections intersect each other with a cross section orthogonal to the direction. In a magnetic resonance imaging apparatus that executes a three-dimensional imaging sequence that executes a preparation sequence that applies a pre-excitation pulse prior to an imaging sequence, for cross-section selection in the phase encoding direction along with one of the pre-excitation pulse and the first excitation pulse of the imaging sequence a gradient magnetic field is applied, the excitation cross section of one of the excitation pulses the a cross section perpendicular to the phase encode direction, by applying a cross-sectional selective gradient magnetic field in the slice direction with the other of the excitation pulses, the excitation cross section of the excitation pulses of the other side Is a cross section orthogonal to the slice direction, and an echo signal is collected based on a signal from a rectangular parallelepiped region where both cross sections intersect.

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