JP3033851B2 - Magnetic resonance imaging equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a magnetic resonance imaging apparatus and, more particularly, to a magnetic resonance imaging apparatus suitable for removing artifacts.

[0002]

2. Description of the Related Art Magnetic resonance imaging apparatuses include NMR.
A gradient magnetic field is applied to extract a signal from a specific position of the subject.

There is no problem if the gradient magnetic field changes uniformly, for example, in the body axis direction of the subject, but the magnetic field strength is weakened at the end of the magnet constituting the gradient magnetic field. Is normal, and FIG. 4 (a),
As shown in (b), the intensity distribution of the gradient magnetic field 42 with respect to the subject 40 is as shown in the figure.

For this reason, a magnetic field having the same strength as that of the slice plane 43 of the subject 40 is formed on the slice plane 44 at a location different from the slice plane 43, and the NMR signal is also transmitted from the slice plane 44. Is detected, which is what causes a so-called artifact.

Therefore, a magnetic resonance imaging apparatus which takes out NMR signals only from the specified slice plane 43 of the subject 40 and does not take out NMR signals from other parts, for example, as shown in FIG. What drives by a sequence is known.

In FIG. 5, a sequence for removing an artifact is incorporated in a stage prior to a sequence for performing a so-called spin echo method for extracting an NMR signal from a specified slice plane.

That is, using the FM wave 51, an operation is performed such that the spin at that portion is tilted to 90 ° so that the NMR signal cannot be extracted from the head or foot in the subject. The FM wave 51 is particularly used for, for example, exciting the head of the subject in a wide area in the body axis direction.

When applying the FM wave 51, a magnetic field 52 corresponding to the frequency of the FM wave 51 is applied to one of the gradient magnetic field Gs, the gradient magnetic field Gp, and the gradient magnetic field Gf. (In FIG. 5, this is performed in the gradient magnetic field Gf).

In FIG. 5, a gradient magnetic field having a negative gradient is applied to any of the gradient magnetic fields Gs, Gp and Gf after the application of the FM wave 51. This is called a spoil gradient magnetic field. The XY components generated by the return of the spins are canceled out by randomizing the phases to completely remove the artifacts.

[0010]

However, since the magnetic resonance imaging apparatus constructed as described above incorporates a sequence for removing artifacts, the imaging time for obtaining one tomographic image is particularly high. However, there is a problem that it takes a lot of time.

In the worst case, the time ratio of the artifact removal sequence to the spin echo sequence for obtaining the NMR signal is about １ ／, so the time required for the execution of the artifact removal sequence depends on the diagnostic efficiency. It also had an effect on the aspect.

Further, since the FM wave 51 is used, a large load is applied to the high-frequency amplifier, which is not preferable.

SUMMARY OF THE INVENTION Therefore, the present invention has been made in view of the above circumstances, and an object of the present invention is to provide a magnetic resonance apparatus capable of removing an artifact while reducing a photographing time. An object of the present invention is to provide an imaging device.

Another object of the present invention is to provide a magnetic resonance imaging apparatus capable of sufficiently removing artifacts without using FM waves.

[0015]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention basically provides a method for applying a first gradient magnetic field to a subject and specifying a slice plane of the subject. A magnetic resonance imaging apparatus for applying a first high-frequency pulse, applying a second gradient magnetic field, and applying a second high-frequency pulse for extracting an NMR signal from the slice surface; Are characterized in that the gradients with respect to the gradient magnetic field are opposite to each other.

[0016]

The magnetic resonance imaging apparatus configured as described above applies a first gradient magnetic field to a subject and a first high-frequency pulse consisting of, for example, a 90 ° pulse for specifying a slice plane of the subject. Thereafter, a second high-frequency pulse composed of, for example, a 180 ° pulse for extracting an NMR signal from the slice surface is applied while applying a second gradient magnetic field having a reverse gradient to the gradient of the first gradient magnetic field. This is applied.

In order to extract an NMR signal from a specific slice plane of a subject, spins on the slice plane are tilted on the Y axis by a first high-frequency pulse (90 ° pulse), and thereafter, are placed on the XY plane. The spin spreading as if the fan were expanded can be obtained by converging on the Y-axis by the second high-frequency pulse (180 ° pulse).

From this, the first high-frequency pulse and the second high-frequency pulse
Even when gradient magnetic fields having opposite gradients are applied at the time of applying each of the high-frequency pulses, the first high-frequency pulse and the second high-frequency pulse are respectively applied to the strength of the magnetic field of the specified slice plane. By applying at a frequency corresponding to the above, an NMR signal can be extracted from the slice plane.

On a slice plane other than the specified slice plane, even if the spin is tilted on the Y-axis by the first high-frequency pulse, when the second high-frequency pulse is applied, Since the slice plane determined by the frequency of the second high-frequency pulse is at a different place, there is no subsequent convergence of the spin on the Y-axis, and no NMR signal can be extracted.

Similarly, in the slice planes other than the specified slice plane, the spin is not tilted by the first high-frequency pulse, and the spin is not reduced by 180 degrees only by the second high-frequency pulse.
Even if it is overturned, no NMR signal is generated from this point.

Therefore, no NMR signal is detected from slice planes other than the specific slice plane, and the occurrence of artifacts can be suppressed.

Further, the suppression of the occurrence of the artifact is performed in the sequence of extracting the NMR signal, and is not performed by the specially provided sequence.

Therefore, it is possible to remove the artifact while reducing the photographing time.

Since the above object is achieved by changing the direction of the gradient of the gradient magnetic field, it is possible to sufficiently remove artifacts without using FM waves.

[0025]

FIG. 3 is an overall schematic diagram showing an embodiment of a magnetic resonance imaging apparatus according to the present invention.

Referring to FIG. 1, the magnetic resonance imaging apparatus first obtains a tomographic image of a subject by utilizing a nuclear magnetic resonance (NMR) phenomenon.
It comprises a central processing unit (CPU) 11, a sequencer 12, a transmission system 13, a magnetic field gradient generation system 14, a reception system 15, and a signal processing system 16.

The static magnetic field generating magnet 10 generates a strong and uniform static magnetic field around the subject 1 in the body axis direction or a direction perpendicular to the body axis, and has a certain spread around the subject 1. A permanent magnet type, a normal conduction type, or a superconducting type magnetic field generating means is arranged in the space.

The sequencer 12 operates under the control of the CPU 11 and sends various commands necessary for collecting data of tomographic images of the subject 1 to the transmission system 13, the magnetic field gradient generation system 14, or the reception system 15. Has become.

The transmission system 13 comprises a high-frequency amplifier 19 and a high-frequency coil 20a on the transmission side.
7 is amplitude-modulated by a modulator 18 in accordance with a command from the sequencer 12, and the amplitude-modulated high-frequency pulse is amplified by a high-frequency amplifier 19, and then the high-frequency coil 20 a is disposed close to the subject 1. , The object 1 is irradiated with the electromagnetic field wave.

The magnetic field gradient generating system 14 comprises a gradient magnetic field coil 21 wound in three directions of X, Y and Z, and a gradient magnetic field power supply 22 for driving each coil. By activating the gradient magnetic field power supplies 22 of the respective coils, gradient magnetic fields Gx, Gy, Gz in the X, Y, and Z directions are applied to the subject 1. The slice plane with respect to the subject 1 can be set by the method of applying the gradient magnetic field.

The receiving system 15 includes a high-frequency coil 20 on the receiving side.
b, amplifier 23, quadrature detector 24, and A / D converter 2
5, the response of the subject 1 due to the electromagnetic wave emitted from the high-frequency coil 20a on the transmitting side (NMR
Signal) is a high-frequency coil 2 disposed close to the subject 1
0b, the amplifier 23 and the quadrature detector 24
Is input to an A / D converter 25, converted into a digital quantity, and further obtained as two series of collected data sampled by a quadrature phase detector 24 at a timing according to a command from the sequencer 12, and the signal is subjected to signal processing. It is sent to the system 16.

The signal processing system 16 includes a CPU 11, a recording device such as a magnetic disk 27 and a magnetic tape 29,
And a display 28 such as an RT.
Then, processing such as Fourier transformation and correction coefficient calculation image reconstruction is performed, and a signal intensity distribution of an arbitrary section or a distribution obtained by performing an appropriate operation on a plurality of signals is imaged and displayed on the display 28. I have. In FIG. 3, the high-frequency coils 20a and 20b on the transmission side and the reception side are used.
The gradient magnetic field coil 21 is arranged in the magnetic field space of the static magnetic field generating magnet 10 arranged in the space around the subject 1.

FIG. 1 is an explanatory diagram of a sequence that operates based on the sequencer 12 and the CPU 11 in FIG.

In the figure, first, the gradient magnetic field Gs101
And a 90 ° pulse 1
02 is applied.

The gradient magnetic field Gs101 is composed of a gradient magnetic field having a positive gradient. The slice plane of the subject can be specified by the frequency of the 90 ° pulse 102.

At this time, the frequency f of the 90 ° pulse 102
Can be expressed by the following equation, where G is the gradient magnetic field strength on a specific slice plane.

F = γ · (H + G · X) where γ is the nuclear magnetic rotation ratio, H is the static magnetic field strength, and X is the position of the slicing surface.

FIG. 2A shows the gradient magnetic field Gs101.
FIG. 7 is an explanatory diagram showing a relationship between an applied state (magnetic field intensity distribution) and a frequency f of a 90 ° pulse in setting a specific slice plane with respect to the intensity. In this case, the gradient magnetic field Gs
101 changes uniformly in the X-axis direction, but the magnetic field strength is weak at both ends. This is because, as described above, the magnetic field strength is weakened at the end of the magnet that forms the gradient magnetic field.

FIG. 3C shows the subject 1 and its slice plane positioned in association with the explanatory view of FIG.

As is apparent from this, in order to specify the slice plane 30 with respect to the applied gradient magnetic field Gs101, the frequency of the 90 ° pulse 102 is changed to f _1.
Should be set to.

At this time, the 90 ° pulse 102 having the frequency f ₁ also generates a slice plane 31 having the same strength as the magnetic field strength on the slice plane 30.

By applying the 90 ° pulse, the spin falls on the Y-axis on any of these slice planes 30 and 31.

Next, the gradient magnetic field Gp103 and the gradient magnetic field Gf
104 to apply NMR signals in the phase encoding direction and the frequency encoding direction, respectively.

Thereafter, the gradient magnetic field Gs 105 is applied, and at the same time, the 180 ° pulse 106 is applied.

The gradient magnetic field Gs105 is composed of a gradient magnetic field having a negative gradient, and its gradient changes from the origin 0 with respect to the gradient magnetic field Gs101 when a 90 ° pulse is applied. Then, the slice plane of the subject is specified by the frequency of the 180 ° pulse 106.

The frequency f of the 180 ° pulse at this time can be expressed by the following equation, as in the previous equation, the gradient magnetic field strength on the specific slice plane.

F = γ · (H + G · X) where γ is the nuclear magnetic rotation ratio, H is the static magnetic field strength, and X is the position of the slicing surface.

FIG. 2B shows the gradient magnetic field Gs105.
FIG. 4 is an explanatory diagram showing a relationship between an applied state (magnetic field intensity distribution) and a frequency f of a 180 ° pulse 106 at that time. Also in this case, the gradient magnetic field Gs105 changes uniformly in the X-axis direction, but the magnetic field strength is weak at both ends. This is for the same reason as described above.

In order to specify the slice plane 30 again with respect to the applied gradient magnetic field Gs 105, the frequency of the 180 ° pulse 106 may be set to f ₂ .

At this time, the 180 ° pulse 106 having the frequency f ₂ also generates a slice plane 32 having the same strength as the magnetic field on the slice plane 30.

Then, the slice plane 30 is tilted on the Y-axis by the above-mentioned 90 ° pulse 102, and
In the process of spreading the fan in the plane as if it were spread out, the 180 ° pulse 106 causes the fan to rotate 180 ° about the X axis.

However, in the slice plane 32, since the spin is not tilted on the Y axis by the 90 ° pulse, the 180 ° pulse
It just falls.

Thereafter, the NMR signal 108 can be received by applying the gradient magnetic field Gf107.

The NMR signal in this case is 90
The slice plane in which the spin was tilted on the Y-axis by the pulse 102 and then spread as if the fan was spread in the XY plane, and the spin was converged on the Y-axis by the 180 ° pulse 106. From 30;
It cannot be obtained from the other slice planes 31 or 32 at all.

In the magnetic resonance imaging apparatus configured as in the above-described embodiment, the gradient magnetic field 101 is applied to the subject 1 and a 90 ° pulse 102 for specifying the slice plane of the subject is applied. A gradient magnetic field 105 having a gradient opposite to the gradient of the gradient magnetic field 101 is applied, and a 180 ° pulse 105 for extracting an NMR signal from the slice plane is applied.

In order to extract an NMR signal from a specific slice plane of the subject 1, the spin on the slice plane is tilted on the Y axis by a 90 ° pulse 101, and then X
-The spin that spreads like spreading a fan in the Y plane,
With the 180 ° pulse 105, it can be obtained by converging on the Y axis.

From this, the 90 ° pulses 102 and 18
Even when gradient magnetic fields 101 and 105 having opposite gradients are applied at the time of applying the 0 ° pulse 106, respectively, the 90 ° pulse 102 and the 180 ° pulse 1
06 is applied at a frequency corresponding to the strength of the magnetic field of the specified slice plane, so that NMR signals can be extracted from the slice plane.

On a slice plane other than the specified slice plane, even if the spin is tilted on the Y axis by the 90 ° pulse 102, the 180 ° pulse 106 applies the 180 ° pulse 106. Since the slice plane determined by the frequency of the pulse 106 is at a different place, there is no subsequent convergence of the spin on the Y-axis and NM
The R signal cannot be extracted.

Similarly, on the slice planes other than the specified slice plane, the spin is not tilted by the 90 ° pulse 102,
Even if it is tilted by 0 °, no NMR signal is generated from this point.

Therefore, no NMR signal is detected from slice planes other than the specific slice plane, and the occurrence of artifacts can be suppressed.

Further, the suppression of the occurrence of the artifact is performed in the sequence for extracting the NMR signal, and is not performed in the specially provided sequence.

Therefore, it is possible to remove an artifact while reducing the photographing time.

Since the above object is achieved by changing the direction of the gradient of the gradient magnetic field, it is possible to sufficiently remove artifacts without using FM waves.

In this embodiment, the gradient magnetic fields Gs101, 10
The application of No. 5 has a positive slope when the first 90 ° pulse 102 is applied, and has a negative slope when the next 180 ° pulse 106 is applied, but is not limited to this. It goes without saying that it will not be done. It may have a negative slope when the first 90 ° pulse 102 is applied and a positive slope when the next 180 ° pulse 106 is applied.

In the present embodiment, the sequence for obtaining a tomographic image uses the spin echo method. However, the present invention is not limited to this, and uses the inversion recovery method or the like. Of course, it is good.

[0066]

As is apparent from the above description,
ADVANTAGE OF THE INVENTION According to the magnetic resonance imaging apparatus by this invention, removal of an artifact can be achieved in the state which shortened the imaging time.

Further, it is possible to sufficiently remove artifacts without using FM waves.

[Brief description of the drawings]

FIG. 1 is a sequence diagram showing an embodiment of a magnetic resonance imaging apparatus according to the present invention.

FIGS. 2 (a), (b), and (c) are explanatory diagrams illustrating the reason why the object of the present invention can be achieved.

FIG. 3 is a block diagram showing an embodiment of a magnetic resonance imaging apparatus according to the present invention.

FIGS. 4A and 4B are sequences showing an example of a conventional magnetic resonance imaging apparatus.

FIG. 5 is an explanatory diagram showing a problem of a conventional magnetic resonance imaging apparatus.

[Explanation of symbols]

 101 Positive gradient magnetic field Gs 102 90 ° pulse 105 Negative gradient magnetic field Gs 106 180 ° pulse

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H01F 7/20 A61B 5/055

Claims (1)

(57) [Claims] 1. A first gradient magnetic field is applied to a subject, a first high-frequency pulse for specifying a slice plane of the subject is applied, a second gradient magnetic field is applied, and an NMR signal from the slice plane is applied. A magnetic resonance imaging apparatus for applying a second high-frequency pulse for extracting
A magnetic resonance imaging apparatus, wherein the gradients of the first gradient magnetic field and the second gradient magnetic field are opposite to each other.