JPH0595928A - Magnetic resonance imaging apparatus - Google Patents

Abstract

PURPOSE:To realize higher efficiency of diagnosis by allowing the displacing of the direction of a frequency encoding and the direction of a phase encoding by a specified angle respectively so that a tomographic image can be projected being turned by a specified angle to achieve the diversifying of an image pattern as diagnostic image. CONSTITUTION:The title apparatus sends various instructions necessary for the collection of data of a tomographic image of an object 7 to be inspected to a transmitting system 3, an gradient magnetic field generation system 21 of a magnetostatic field generation magnet 4 and a receiving system 5 through a sequencer 2 which is operated according to a specified program with a CPU 1. In this case, a console 25 is provided with a knob 24 to input angles thetaseparately and signals corresponding to the angles theta are inputted into a memory 27 through the CPU 1. With the inputting of the signals corresponding to the angles theta, a magnetic field to be applied to a first gradient magnetic field is at (Gf.e1+Gp.e1) and a magnetic field to be applied to a second gradient magnetic field (Gf.e2+Gp.e2) and the magnetic field to be applied to the second gradient magnetic field Gf.e2 and the magnetic field to be applied to the first gradient magnetic field Gf.e1 separately.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic resonance imaging apparatus, and more particularly, to a specific cross section of a subject.
The present invention relates to a magnetic resonance imaging apparatus in which the method of extracting an NMR signal in the phase encode direction and the frequency encode direction is improved.

[0002]

2. Description of the Related Art A magnetic resonance imaging apparatus utilizes a magnetic resonance phenomenon to measure the density distribution, relaxation time distribution, etc. of nuclear spins (hereinafter simply referred to as "spins") at a desired cross-section in a subject, The section of the subject is displayed as an image from the measurement data.

Such a magnetic resonance imaging apparatus is
It is equipped with gradient magnetic fields that can independently form magnetic field gradients in the directions of three axes that are orthogonal to each other, and by setting the magnetic field gradient of one of these gradient magnetic fields in relation to the signal frequency from the high frequency transmission coil, let's capture an image. The cross section of the subject to be specified is specified.

A signal from the high-frequency transmission coil causes magnetic resonance in atomic nuclei of the atoms constituting the tissue of the subject, and the magnetic resonance is received by the receiving coil.
This is because the magnetic resonance occurs in a portion having a magnetic field strength having a certain relation with the signal frequency (that is, corresponding to the portion in the cross section).

Similarly, in order to obtain two-dimensional image information for the specified cross section, predetermined magnetic fields are applied to the other remaining gradient magnetic fields which are orthogonal to each other.

[0006]

However, in the conventional magnetic resonance imaging apparatus, the magnetic field application to each gradient magnetic field for obtaining the two-dimensional information on the specified cross section of the subject is performed in each imaging region (magnetic resonance signal). The magnetic field is applied corresponding to the vertical and horizontal widths of the image pickup area), and the so-called phase encode direction (matched with the vertical direction of the image pickup area) and the frequency encode direction (horizontal direction of the image pickup area) are applied. Each magnetic field was set as a magnetic field application (corresponding to the above).

Therefore, the displayed tomographic image is uniquely displayed by the positional relationship of the subject with respect to each gradient magnetic field for obtaining the two-dimensional information in the specific cross section, and the position of the subject is displayed as it is. It is not possible to display the cross-sectional image that has been produced by arbitrarily rotating it.

Therefore, for example, in the case of comparison with other tomographic images, if the tomographic images are compared in the state in which they are oriented in the same direction, it is possible to easily make a detailed comparison and to improve the efficiency of diagnosis. Should improve, but
Due to the above-mentioned circumstances, there remains a problem that such a thing could not be done.

Therefore, the present invention has been made under such circumstances, and the object of the present invention is to diversify the image mode as a diagnostic image, thereby improving the efficiency of diagnosis. It is to provide a magnetic resonance imaging apparatus that can be improved.

[0010]

In order to achieve such an object, the present invention is basically a first section which is aligned with the phase encoding direction in a specified cross section of the subject.
A magnetic field Gp applying means to the gradient magnetic field, a magnetic field Gf applying means to a second gradient magnetic field which is matched in the frequency encode direction in the cross section, and a second magnetic field Gp direction NMR signal take-out in the NMR signal take-out. In a magnetic resonance imaging apparatus including at least a magnetic field Gf applying unit to a gradient magnetic field, an angle input unit for inputting a signal corresponding to the angle θ, and an input of the signal corresponding to the angle θ from the angle input unit, The magnetic field applied to the first gradient magnetic field is (Gf · e ₁ + Gp · e ₁ )
And the magnetic field applied to the second gradient magnetic field is (Gf · e ₂ + G
p · e ₂ ), the applied magnetic field to the second gradient magnetic field is Gf · e ₂ , the applied magnetic field to the first gradient magnetic field is Gf · e ₁ , and each magnetic field application is performed in the frequency encoding direction. A means for applying a magnetic field for extracting an NMR signal is provided. Here, Gp, Gf, e ₁ and e ₂ are vectors, respectively. Further, e ₁ and e ₂ are unit vectors of the directions of the first gradient magnetic field and the second gradient magnetic field, respectively.

[0011]

In the magnetic resonance imaging apparatus configured as described above, the magnetic field in the phase encode direction and the magnetic field in the frequency encode direction are applied to the first gradient magnetic field and the second gradient magnetic field, which can apply magnetic fields orthogonal to each other, respectively. Particularly, the directions of the phase encode direction and the frequency encode direction are respectively displaced by an angle θ.

Thus, the magnetic field corresponding to the component force of each of the first gradient magnetic field and the second gradient magnetic field in each direction
The gradient magnetic field and the second gradient magnetic field are applied respectively.

As a result, a tomographic image which has been conventionally directed in one direction can be displayed while being rotated by an angle of θ, whereby the image mode as a diagnostic image can be diversified. It is possible to improve the efficiency of diagnosis.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a magnetic resonance imaging apparatus according to the present invention will be described below with reference to the drawings.

First, FIG. 2 is a block diagram showing the overall construction of a magnetic resonance imaging apparatus according to the present invention.

This magnetic resonance imaging apparatus is roughly classified into a central processing unit (CPU) 1, a sequencer 2 and
The transmission system 3, the static magnetic field generating magnet 4, the reception system 5, and the signal processing system 6 are provided.

The central processing unit (CPU) 1 controls each of the sequencer 2, the transmission system 3, the reception system 5, and the signal processing system 6 according to a predetermined program. The sequencer 2 operates based on a control command from the central processing unit 1 and transmits various commands necessary for data acquisition of a tomographic image of the subject 7 to the transmission system 3 and the gradient magnetic field generation system 21 of the static magnetic field generation magnet 4. It is sent to the receiving system 5. The sequencer 2 will be described later in detail.

The transmission system 3 has a high-frequency oscillator 8, a modulator 9 and an irradiation coil 11 as a high-frequency coil, and a high-frequency pulse from the high-frequency oscillator 8 is amplitude-modulated by the modulator 9 according to a command from the sequencer 2. The amplitude-modulated high-frequency pulse is amplified through the high-frequency amplifier 10 and the irradiation coil 11
Is supplied to the subject 7 so that the subject 7 is irradiated with a predetermined pulsed electromagnetic wave.

The static magnetic field generating magnet 4 is for generating a uniform static magnetic field around the subject 7 in an arbitrary direction. Inside the static magnetic field generating magnet, in addition to the irradiation coil 11, a gradient magnetic field coil 13 for generating a gradient magnetic field and a receiving coil 14 of the receiving system 5 are installed. The gradient magnetic field generation system 21 has a configuration capable of independently applying a gradient magnetic field in Cartesian coordinate axis directions orthogonal to each other, that is, in the X axis direction, the Y axis direction, and the Z axis direction.
3 and gradient magnetic field power supply 12 for supplying electric current to the gradient coil
And a sequencer 2 for controlling the gradient magnetic field power supply 12.

The receiving system 5 has a receiving coil 14 as a high frequency coil, an amplifier 15 connected to the receiving coil 14, a quadrature detector 16 and an A / D converter 17, and an NMR signal from the subject 7 is received. When the receiving coil 14 detects the signal, the signal is converted into a digital amount through the amplifier 15, the quadrature detector 16, and the A / D converter 17, and the quadrature detector 16 is supplied at the timing instructed by the sequencer 2.
The data is converted into two series of collected data sampled by and sent to the central processing unit 1.

The signal processing system 6 has an external storage device such as a magnetic disk 20 and an optical disk 19 and a display 18 such as a CRT. When the data from the receiving system 5 is input to the central processing unit 1, The central processing unit 1 executes processing such as signal processing and image reconstruction, and the subject 7
The desired cross-sectional image is displayed on the display 18 and stored in the magnetic disk 20 or the like of the external storage device.

In such a structure, in this embodiment,
Knob 2 which can input angle θ on the control console 25
4 is provided, and when the operator rotates the knob 24, a signal corresponding to the rotation angle θ is input to the memory 27 via the central processing unit 1.

The memory 27 stores information corresponding to the signal corresponding to the rotation angle θ, and the central processing unit 1
A part of the contents of the sequencer 2 is changed based on the information via the.

Next, the sequence of pulses sent from the sequencer 2 will be described with reference to FIG. This pulse sequence is a typical pulse sequence of the spin echo method of the so-called two-dimensional Fourier imaging method. The pulse sequence shown in FIG. 3 is sent when the knob 26 on the control console 25 has a rotation angle θ of “0”, that is, it is sent in the same manner in the past. It is a pulse sequence.

In this pulse sequence, as shown in the same figure, first, after applying 90 ° pulse 30, 180 ° pulse 3 after the time of Te / 2 when the echo time is Te.
It is designed to add 1. After the 90 ° pulse 30 is applied, each spin starts rotating in the XY plane at its own velocity, so that a phase difference occurs between the spins over time. When 180 ° pulse 27 is applied here,
Each spin is symmetrically inverted with respect to the X ′ axis, and then continues to rotate at the same speed, so that at time Te, the spins are focused again to form the echo signal 36.

The principle of generation of the echo signal 36 in this case will be described with reference to FIG. 4 based on the nuclear spin. In the same figure, the macroscopic magnetization tilted on the y-axis after irradiation for 90 pulses (FIG. 4 (a)) (FIG. 4 (b)) shows that the static magnetic field was not detected as time passed (FIG. 4 (c)). Due to the influence of homogeneity, the rotational phase of individual nuclear spins begins to become disordered, x
-Expand the fan in the y-plane (Fig. 4)
(D)). At this time, the FID signal is observed, but thereafter no signal appears in the receiving coil.

Next, after the time τ has elapsed after applying the 90 ° pulse, 1
An 80 ° pulse is applied (FIG. 4 (e)). As a result, each spin that has been out of phase is
It is rotated 0 ° counterclockwise. However, even in this state, the spin that was clockwise in the xy plane before the 180 ° pulse was applied is rotated clockwise, while the counterclockwise spin continues to change its phase to counterclockwise. Go (Figure 4
(F)). Therefore, after 2τ after applying the first 90 ° pulse, each spin converges on the -y axis (Fig. 4).
(G)). The behavior of each of these nuclear spins is received as a signal by a coil placed on the xy plane. Therefore, focusing on the received signal, the FID signal begins to appear immediately before reaching 2τ after the 90 ° pulse is applied, and the maximum after 2τ. Reach the value.

In this case, in order to form a tomographic image, the spatial distribution of signals must be obtained. For this purpose, a linear gradient magnetic field is used. A spatial magnetic field gradient can be created by superimposing a gradient magnetic field on a uniform static magnetic field. As described above, since the spin rotation frequency is proportional to the magnetic field strength, the spin rotation frequencies are spatially different in the state where a gradient magnetic field is applied. Therefore, the position of each spin can be known by examining this frequency. For this purpose, a slice direction gradient magnetic field 32, a phase encode gradient magnetic field 33, and frequency encode gradient magnetic fields 34 and 35 are used.

Using the pulse sequence described above as a basic unit, the intensity of the phase-encoding gradient magnetic field is changed every time, and a predetermined number of times, for example, 256 times are repeated at a constant repetition time (TR). The spatial distribution of macroscopic magnetization can be obtained by subjecting the measurement signal thus obtained to two-dimensional inverse Fourier transform. Regarding the basic principles of MRI above,
It is described in detail in "NMR Medicine" (Basic and Clinical) (edited by Nuclear Magnetic Resonance Medical Research Society, Maruzen Co., Ltd., issued January 20, 1984).

Here, in this embodiment, the so-called variable-field imaging method is applied by performing the operation described below in the application of the phase-direction gradient magnetic field.

In the variable field imaging method, in the gradient magnetic field in the phase encode direction, the imaging area is narrowed and the imaging time is shortened by setting the increment amount when applying the magnetic field at each repetition time (TR). It is designed so that

That is, FIG. 5 is an explanatory diagram in the case of imaging without applying the variable-field imaging method. As shown in FIG. 6A, the increment amount of the gradient magnetic field in the phase encoding direction is Gpstep, which allows the imaging region (or imaging field of view) of FIG. F
A gradient magnetic field for OV) is associated. In addition, the circular portion in FIG. 7C is the imaging target 7A.

On the other hand, FIG. 6 is an explanatory view to which the variable visual field imaging method is applied, and FIGS. 6A to 6C correspond to FIGS. 5A to 5C, respectively. As can be seen from these figures, the imaging field of view in the phase encoding direction is F
When OV is set, the expression of γtpGpstepFOV = 2π ... (1) is established. Therefore, in order to reduce the visual field in the phase encoding direction to 1 / m, the increment of phase encoding may be multiplied by m. In other words, G'pstep may be set such that γtpG'pstepFOV / m = 2π ... (2) holds.

From the equations (1) and (2), G'pstep = m.Gpstep ... (3) is established. Moreover, the imaging time is determined by the number of phase encoding updates k. In order to obtain the same spatial resolution, it suffices that k for the imaging visual field is constant. If k for normal imaging and k ′ for variable field of view, FOV / k = (FOV / m) / k ′ ... (4) holds. From equation (4), k '/ k = 1 / m (5) Since the imaging time is proportional to the number of updates of the phase encode, the variable field of view shortens the time by 1 / m.

The tomographic image on the surface of the display device thus obtained is displayed, for example, as shown in FIG. At the center of the display surface, a cross-sectional view in a plane orthogonal to the body axis of the head of the subject is displayed. Then, by the variable-field-of-view imaging method described with reference to FIG. 6, regions having no image information are formed on both sides of the display surface, and the image information including the head is displayed in the width of FOV / m in the central portion. It is like this.

Here, in the case of FIG. 7, a tomographic image is obtained when the subject is looking vertically upward without tilting its head. For example, as shown in FIG. The head is tilted (for example, the angle θ
In some cases, it may be imaged in the state. In the case of FIG.
Since the lateral width of the head becomes larger than that in the case of FIG. 7 due to the tilt of the head, there is a situation in which the variable field of view must be made larger (FOV / n in the figure).

Next, description will be given of a case where the same operation as described above is performed with the knob 26 on the control console 25 in the state of the turning angle θ in the imaging state of the subject's head shown in FIG.

From the control console 25, a signal corresponding to the rotation angle θ is input to the memory 27 newly provided in this embodiment via the central processing unit 1. The information shown in FIG. 9 is previously stored in the memory 27.

In FIG. 9, for example, a state in which a magnetic field having a magnetic field strength Gp is formed in the X-axis gradient magnetic field and a magnetic field having a magnetic field strength Gf is formed in the Z-axis gradient magnetic field is converted clockwise by the angle θ. The state of each magnetic field at the time of being shown is shown. In the present embodiment, the direction of the converted magnetic field Gf is set as the frequency encoding direction, and the converted magnetic field Gp direction is set as the phase encoding direction to apply magnetic fields to the X-axis gradient magnetic field and the Z-axis gradient magnetic field, respectively. Is.

That is, in the X-axis gradient magnetic field, a component force (Gf · e ₁ ) e ₁ of the magnetic field Gf in the frequency encoding direction in the X-axis direction.
It is intended to apply a magnetic field corresponding to the sum of (= Gfx) and the component force (Gp · e ₁ ) e ₁ (= Gpx) of the magnetic field Gp in the phase encoding direction in the X-axis direction. Further, in the Z-axis gradient magnetic field, the Z-axis component force (Gf · e ₂ ) e ₂ (= Gfz) of the magnetic field Gf in the frequency encode direction and the Z-axis component force (Gp · e) of the magnetic field Gp in the phase encode direction are given. ₂ ) e ₂ (= G
It is intended to apply a magnetic field corresponding to the added value of pz). Here, the above-mentioned Gf, Gp, e ₁ ,
Each of e ₂ is a vector.

Then, the above-mentioned information corresponding to the rotation angle θ, that is, the component magnetic field corresponding to the rotation angle θ is read out from the memory 27 as described above, and is read via the central processing unit 1. The contents are input to the sequencer 2 and the contents of the sequencer 2 are changed as shown in FIG. 1 based on the information.

FIG. 1 is a sequence shown in association with FIG. 3 described above. The parts different from FIG. 3 are the X-axis gradient magnetic field and Z
It is in the magnetic field applied to the axial gradient magnetic field. That is, by applying a magnetic field of (Gpx + Gfx) to the X-ray gradient magnetic field and a magnetic field of (Gpz + Gfz) to the Z-axis gradient magnetic field, two-dimensional information in the XZ plane which is the specified cross section is obtained. Is what you are doing.

In this case, the magnetic field application for extracting the NMR signal in the frequency encode direction is divided into X components.
Gfx and Gfz are applied to the axial gradient magnetic field and the Z-axis gradient magnetic field, respectively.

FIG. 10 shows a tomographic image obtained from such a sequence. From the state shown in FIG. 8, the tomographic image is rotated by an amount corresponding to the input angle θ and is displayed without the inclination of the head. As a result of eliminating the inclination of the head, the required image pickup area in the lateral direction can be reduced, so that the variable field of view can be reduced in FOV / m. This has the effect of reducing the imaging time as described above.

According to the embodiment described above, the first gradient magnetic field (X-axis gradient magnetic field in the case of the example) and the second gradient magnetic field (Z-axis gradient magnetic field in the case of the example) which can conventionally apply the mutually orthogonal magnetic fields. The magnetic field in the phase encode direction and the magnetic field in the frequency encode direction are respectively applied to (1), and the directions in the phase encode direction and the frequency encode direction are respectively displaced by the angle θ.

As a result, the magnetic field corresponding to the component force in each direction of the first gradient magnetic field and the second gradient magnetic field is changed to the first magnetic field.
The gradient magnetic field and the second gradient magnetic field are applied respectively.

As a result, a tomographic image which has been conventionally directed in one direction and rotated can be displayed in a state of being rotated by an angle .theta., Thereby diversifying the image mode as a diagnostic image. It is possible to improve the efficiency of diagnosis.

Further, as is apparent from the above-mentioned embodiment, when the so-called variable field imaging method is used, by rotating the tomographic image by a predetermined angle,
The variable field of view can be significantly narrowed compared to before rotation, and therefore, the effect of shortening the imaging time can be achieved.

The present embodiment also has the following effects. That is, it is a phenomenon that is technically difficult to elucidate, but when the tomographic image shown in FIG. 8 is imaged, an artifact 80 that is continuous in the phase encoding direction, which is the horizontal axis direction, is generated. It has been known. Then, in the case where the artifact 80 greatly crosses the tomographic image and becomes extremely obtrusive, by rotating the tomographic image as shown in FIG. 10, the tomographic image of the artifact 80 still occurring in the phase encoding direction It will be possible to reduce the number of cross cuts.

In the above embodiment, as shown in FIG.
In the −Z-axis plane, the frequency encode direction and the phase encode direction are displaced by an angle θ. This is because the specified cross section is a surface perpendicular to the Y axis, and it goes without saying that the present invention can be applied accordingly when the cross section is perpendicular to the X axis or the Z axis. ..

Further, in the practice of the present invention, it goes without saying that the image information shown in FIG. 10 may be temporarily stored in the memory and displayed as shown in FIG. 11 by the coordinate conversion. ..

[0052]

As is clear from the above description,
According to the magnetic resonance imaging apparatus of the present invention, it is possible to diversify the image aspect as a diagnostic image and thereby improve the efficiency of diagnosis.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing an example of a sequence used in an example of a magnetic resonance imaging apparatus according to the present invention.

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

FIG. 3 is an explanatory diagram showing an example of a conventional sequence used in the magnetic resonance imaging apparatus according to the present invention.

FIG. 4 is an explanatory diagram showing a behavior of spins for obtaining an echo signal.

5A, 5B, and 5C are explanatory diagrams showing a relationship between a magnetic field applied to a gradient magnetic field in a phase encoding direction and an imaging visual field.

6A, 6B, and 6C are explanatory diagrams showing the relationship between the magnetic field applied to the gradient magnetic field in the phase encoding direction and the imaging visual field.

FIG. 7 is an explanatory diagram showing an example of a display mode of a tomographic image of the magnetic resonance imaging apparatus according to the present invention.

FIG. 8 is an explanatory diagram showing an example of a display mode of a tomographic image of the magnetic resonance imaging apparatus according to the present invention.

FIG. 9 is an explanatory diagram showing an example of information stored in a memory provided in the magnetic resonance imaging apparatus according to the present invention.

FIG. 10 is an explanatory diagram showing an example of a display mode of a tomographic image of the magnetic resonance imaging apparatus according to the present invention.

FIG. 11 is an explanatory diagram showing another embodiment of the display mode of the tomographic image of the magnetic resonance imaging apparatus according to the present invention.

[Explanation of symbols]

 25 ... Control console, 26 ... Knob, 27 ... Memory.

Claims (1)

[Claims] 1. A magnetic field G to a first gradient magnetic field which is matched with a phase encoding direction in a specified cross section of a subject.
p applying means, magnetic field Gf applying means to the second gradient magnetic field which is matched in the frequency encoding direction in the cross section, and magnetic field to the second gradient magnetic field for extracting the NMR signal in the frequency encoding direction when extracting the NMR signal In a magnetic resonance imaging apparatus including at least Gf applying means, an angle input means for inputting a signal corresponding to the angle θ, and the angle θ from the angle input means.
By inputting a signal corresponding to, the applied magnetic field to the first gradient magnetic field is (Gf · e ₁ + Gp · e ₁ ) and the applied magnetic field to the second gradient magnetic field is (Gf · e ₂ + Gp · e ₂ ). In addition, the applied magnetic field to the second gradient magnetic field is Gf · e ₂ and the applied magnetic field to the first gradient magnetic field is Gf · e ₁ , and each magnetic field application is for extracting the NMR signal in the frequency encoding direction. A magnetic resonance imaging apparatus comprising means for applying the magnetic field. However, the above G
p, Gf, e ₁ and e ₂ are vectors respectively. Further, e ₁ and e ₂ are unit vectors of the directions of the first gradient magnetic field and the second gradient magnetic field, respectively.