JP2677147B2 - MR imaging device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an MR imaging apparatus for performing imaging by utilizing the NMR (nuclear magnetic resonance) phenomenon, and more particularly to an MR imaging apparatus optimal for obtaining a three-dimensional blood vessel image.

[0002]

2. Description of the Related Art Conventionally, a three-dimensional MR angio sequence suitable for obtaining a three-dimensional blood vessel image has been known as one of imaging sequences of an MR imaging apparatus. In this three-dimensional MR angio sequence, a relatively thick slab (a flat plate region having a predetermined thickness) is selectively excited, and then an imaging plane is set in the slab, and two-dimensional position information of the imaging plane in each direction is set. Is frequency-encoded and phase-encoded, and the position information in the direction perpendicular to the imaging surface is phase-encoded to obtain three-dimensional information in the slab. Here, the imaging plane is a plane that can form one screen independently, that is, when the position information in the two-dimensional directions of the plane is frequency-encoded and phase-encoded, the same number of phase-encodes as the number of samplings in the frequency-encoding direction are used. It has steps. On the other hand, the number of encoding steps for phase-encoding the position information in the direction perpendicular to the imaging surface may be smaller than this. Then, by making the repetition time shorter than the relaxation time, the stationary portion in the slab is saturated to suppress the signal, and the signal is generated only from the blood newly flowing from the outside of the slab. This makes it possible to obtain a three-dimensional image in which only the image of the blood vessel in the selective excitation slab is extracted.

Conventionally, the surface of the selective excitation slab (the flat surface of the flat area) and the imaging surface are set to be parallel to each other. That is, the gradient magnetic field for selectively exciting a predetermined slab and the gradient magnetic field for phase encoding in the direction perpendicular to the imaging surface are the same.

[0004]

However, when the plane of the selective excitation slab and the imaging surface are made parallel in the three-dimensional MR angio sequence, the excitation slab is thick, so that the upstream portion of the blood flow flowing into the imaging region is also excited. As a result, a signal may not be obtained from the blood flow portion due to a strong saturation effect.

For example, when a three-dimensional blood vessel image of the neck is to be obtained, the image pickup surface is a coronal surface (a surface perpendicular to the front direction of the human body).
Then, when a slab having a plane parallel to this plane is selectively excited, the imaging region is near the neck, but also near the heart. Therefore, the blood flow flowing into the imaging region near the neck is already excited and saturated, and a signal cannot be obtained.

In order to avoid this, when the plane of the excitation slab is set at right angles to the flow direction (it is assumed that it is a plane perpendicular to the human body axis), the imaging plane is a transverse plane (plane perpendicular to the body axis).
Therefore, it is necessary to increase the number of phase encodes in the direction perpendicular to the imaging surface or set the partition thin in order to secure the spatial resolution in the vertical direction (body axis direction) of the neck. In the case where the imaging region of the neck has a constant size in the body axis direction, another problem such as unavoidable extension of the imaging time occurs.

In view of the above, the present invention can suppress the saturation effect of the portion moving to the imaging region to the minimum and ensure the spatial resolution in a desired direction without extending the imaging time. Improved so that M
It is an object to provide an R imaging device.

[0008]

In order to achieve the above object, an MR imaging apparatus according to the present invention comprises:
The excitation high-frequency pulse is repeatedly irradiated at a repetition time that is shorter than the relaxation time of the subject and the degree of its shortness is such that the stationary part is saturated, and at the same time, the inclination of the moving direction of the moving body to be inspected Apply a magnetic field pulse to selectively excite a slab, which is a flat plate-shaped region of a predetermined thickness selected in the direction of the gradient magnetic field, and set the imaging surface substantially at right angles to the flat plate-shaped surface of the slab. , Adding a gradient magnetic field pulse for frequency encoding in one direction on the imaging plane and a gradient magnetic field pulse for phase encoding in another direction substantially perpendicular to the one direction on the imaging plane, and further for exciting the slab selective excitation. The feature is that the gradient magnetic field pulse for phase encoding in the thickness direction of the imaging surface is generated by using the gradient magnetic field in a direction substantially perpendicular to the gradient magnetic field.

[0009]

The slab having a predetermined thickness is selectively excited at a desired portion of the subject, and the thickness direction thereof is the direction (gradient direction) of the gradient magnetic field for selective excitation. That is, the surface of the selectively excited slab (the surface perpendicular to the thickness direction) is perpendicular to the direction of the gradient magnetic field for selective excitation. On the other hand, since the gradient magnetic field pulse for phase encoding in the imaging surface thickness direction is generated by using a gradient magnetic field in a direction different from the slab selective excitation gradient magnetic field, the imaging surface is necessarily a surface different from the slab surface, In other words, it becomes a surface that intersects the slab surface. Therefore, even if the slab surface is made orthogonal to the moving direction of the moving body such as blood flow to minimize the saturation effect on the moving body, the moving direction of the moving body is included in the imaging plane. Since the image pickup surface can be set as described above, it is possible to secure a necessary spatial resolution in the moving direction without increasing the number of phase encodes in the image pickup surface thickness direction and extending the image pickup time.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the drawings. In one embodiment of the present invention, the pulse sequence as shown in FIG. 1 or 2 is performed with the configuration as shown in FIG. First, referring to FIG. 4, the main magnet 1 is for generating a static magnetic field, and the gradient magnetic field coil 2 is arranged so as to be superposed on the static magnetic field.
Are applied with gradient magnetic fields Gx, Gy, Gz in respective directions of X, Y, Z which are orthogonal to each other (note that these X, Y, Z
The three directions of Z need only be relatively orthogonal to each other, and can be arbitrarily set as an absolute direction). The subject 3 is placed in the space to which the static magnetic field and the gradient magnetic field are applied. An excitation RF pulse is applied to the subject 3
An RF coil 4 for irradiating the subject and receiving an NMR signal generated in the subject 3 is attached.

A gradient magnetic field power source 5 is connected to the gradient magnetic field coil 2 and is supplied with gradient magnetic field generating power. A transmission power amplifier 7 and a preamplifier 10 are connected to the RF coil 4 via a switch 6. The switch 6 is switched to the transmission power amplifier 7 at the time of excitation, and is switched to the preamplifier 10 at the time of reception. An RF signal obtained by modulating a carrier signal from a signal generator 9 with a modulation signal having a predetermined waveform in a transmission circuit 8 is sent to a transmission power amplifier 7. A receiving circuit 11 is connected to the preamplifier 10, and performs phase detection of the received signal using the signal from the signal generator 9 as a reference signal. The detected signal is sampled by an A / D converter 12, converted into digital data, and taken into a computer 13.

The computer 13 controls the modulation signal waveform of the excitation RF pulse in the transmission circuit 8, and the signal generator 9
And the sampling timing of the A / D converter 12 is determined. Further, by controlling the gradient magnetic field power source 5, X,
The timing and waveform of the gradient magnetic field pulse in each of the Y and Z directions,
Program the strength etc. arbitrarily. Further, processing such as reconstructing an image from the collected digital data is performed. The display device 14 displays a reconstructed image and the like.

In such an MR imaging apparatus, a pulse sequence as shown in FIG. 1 or 2 is performed under the control of the computer 13. First, the pulse sequence of FIG. 1 will be described. At the same time as applying the excitation RF pulse 21, a gradient magnetic field pulse 22 for selecting a slab of the gradient magnetic field Gz is added, and then a gradient magnetic field Gy for phase encoding in the thickness direction of the imaging plane is used. In addition to the pulse 24, a gradient magnetic field pulse 25 for phase encoding in the imaging plane is added using the gradient magnetic field Gx, and a gradient magnetic field pulse 23 for frequency encoding in the reading and imaging plane is further added using the gradient magnetic field Gz to generate an echo signal. generate.

The repetition time of this pulse sequence is set shorter than the relaxation time of the subject 3, and the phase encoding gradient magnetic field pulses 24 and 25 are changed and repeated for the number of times multiplied by the number of encodes. The number of phase encodes in the direction within the imaging plane corresponds to the matrix of the image reconstructed on the imaging plane, but the number of phase encodes in the thickness direction of the imaging plane is determined according to the spatial resolution required in that direction.

In the pulse sequence of FIG. 2, the gradient magnetic field pulse 25 for phase encoding in the imaging plane of FIG. 1 and the gradient magnetic field pulse 23 for frequency encoding in the reading and imaging plane of FIG.
Are replaced, and the former gradient magnetic field pulse 25 is generated by using the gradient magnetic field Gz, and the latter gradient magnetic field pulse 23 is generated by using the gradient magnetic field Gx. Otherwise, it is the same as in FIG.

When three-dimensional MR angiography of the neck is performed by the pulse sequence of FIG. 1 or 2, the Z direction is the body axis direction (vertical direction) as shown in FIG. 3, and the selective excitation slab 31 is in the Z direction. In a certain region, that is, the surface is a plane (XY plane) perpendicular to the Z direction. This allows selective excitation in a direction perpendicular to the blood vessels in the neck. In the case of FIG. 1, the gradient magnetic field Gx is used as the phase encoding gradient magnetic field pulse 25 in the imaging plane, the gradient magnetic field Gz is used as the frequency encoding gradient magnetic field pulse 23 in the reading and imaging plane, and the imaging plane thickness direction is used. Of the gradient magnetic field Gy as the gradient magnetic field pulse 24 for phase encoding of
Since the image pickup surface 32 is an XZ plane (a coronal plane in the case shown in FIG. 3), the X direction in the plane is phase-encoded and the Z direction is frequency-encoded. .

In the case of FIG. 2, the gradient magnetic field Gz is used as the phase encoding gradient magnetic field pulse 25 in the imaging plane, and the gradient magnetic field Gx is used as the frequency encoding gradient magnetic field pulse 23 in the reading and imaging plane. Since the gradient magnetic field Gy is used as the gradient magnetic field pulse 24 for phase encoding in the surface thickness direction, the imaging surface 32 is the same as that in FIG.
-Z plane, but the X direction in the plane is frequency-encoded and the Z direction is phase-encoded.

Therefore, in any case, since the surface of the slab 31 to be selectively excited is a surface perpendicular to the Z direction and a direction perpendicular to the direction of blood flow, it is repeatedly excited with a repetition time shorter than the relaxation time. However, the newly flowing blood flow is unexcited, and the saturation effect cannot be given. Furthermore, when it is desired to obtain a high spatial resolution in the blood flow direction (Z direction), the spatial resolution in the imaging surface 32 can be increased, so that it is not necessary to increase the number of phase encodes in the imaging surface thickness direction, and the imaging time Extensions can be avoided.

Although the image pickup surface 32 is a coronal surface in the embodiments shown in FIGS. 1 and 2, the sagittal surface (Y
-Z plane). When the imaging plane 32 is the YZ plane, the phase encoding gradient magnetic field pulse 24 in the imaging plane thickness direction and the phase encoding gradient magnetic field pulse 25 in the imaging plane are replaced in the pulse sequence of FIG. The gradient magnetic field pulse 24 of
The latter gradient magnetic field pulse 25 may be generated by using the gradient magnetic field Gy. In the case of FIG. 2 as well, the gradient magnetic field pulse 24 for phase encoding in the thickness direction of the imaging surface and the gradient magnetic field pulse 23 for frequency encoding in the reading and imaging surface are replaced with each other to replace the former gradient magnetic field pulse 24 with the gradient magnetic field G.
The gradient magnetic field pulse 23 of the latter may be generated by using x and the gradient magnetic field Gy, respectively.

[0020]

According to the MR imaging apparatus of the present invention, since the surface of the slab that is selectively excited and the image pickup surface can intersect with each other, the saturation effect of the portion of the moving body moving to the image pickup area is minimized. In this way, a large signal can be generated from the moving body by suppressing the above, and the spatial resolution in a desired direction such as the moving direction can be secured without extending the imaging time.

[Brief description of the drawings]

FIG. 1 is a time chart showing a pulse sequence according to an embodiment of the present invention.

FIG. 2 is a time chart showing a pulse sequence according to another embodiment of the present invention.

FIG. 3 is a schematic perspective view for explaining the positional relationship in the embodiment.

FIG. 4 is a block diagram of an MR imaging apparatus according to an embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 main magnet 2 gradient magnetic field coil 3 subject 4 RF coil 5 gradient magnetic field power source 6 switcher 7 transmission power amplifier 8 transmission circuit 9 signal generator 10 preamplifier 11 receiving circuit 12 A / D converter 13 computer 14 display device 21 excitation RF Pulse 22 Gradient magnetic field pulse for slab selection 23 Gradient magnetic field pulse for readout and imaging in-plane frequency encoding 24 Imaging plane gradient magnetic field pulse for thickness direction phase encoding 25 Gradient magnetic field pulse for in-plane phase encoding 31 Selective excitation slab 32 Imaging plane

Claims (1)

(57) [Claims] 1. An excitation high-frequency pulse is repeatedly irradiated with a repetition time shorter than the relaxation time of the subject and the degree of its shortness is such that the stationary portion is saturated, and at the same time, the moving direction of the moving body to be inspected is set. Means for selectively exciting a slab, which is a flat plate-shaped region of a predetermined thickness selected in the direction of the gradient magnetic field by applying a gradient magnetic field pulse in a predetermined direction, and at a substantially right angle to the flat plate surface of the slab. A unit for generating a frequency-encoding gradient magnetic field pulse for one direction within the set imaging plane; a unit for generating a phase-encoding gradient magnetic field pulse for another direction substantially perpendicular to the one direction within the imaging plane; Means for generating a phase-encoding gradient magnetic field pulse in the thickness direction of the imaging surface by using a gradient magnetic field in a direction substantially perpendicular to the gradient magnetic field for slab selective excitation, MR imaging device.