JPH0479256B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method of obtaining information about an object using nuclear magnetic resonance (NMR), and particularly to a method of obtaining image information of spin-relaxed nuclei.

<Conventional technology> In recent years, so-called MRI, a device that uses NMR phenomena to visualize the proton spin density distribution in a cross section of a living body, has been developed.
Research on protons is being actively conducted, and it has become clear that image information of protons that reflects spin-spin relaxation is particularly useful clinically.

As a device to obtain such image information, a 90° pulse is applied to the subject, and then after a desired period of time has elapsed,
A method using the so-called multiple spin echo method in which 180° pulses are applied one after another has been proposed. In actual clinical practice, what is effective is an image with a short echo time in which the echo signal is close to the 90° pulse and hardly undergoes spin-spin relaxation, and an image that is far from the 90° pulse and largely reflects spin-spin relaxation. It is becoming clear that there are two types of images with long echo times.

However, in the multiple spin echo method described above, the echo time of each obtained echo signal is
This is a multiple of the first echo time, so in order to satisfy the above clinical requirements, the first echo time must be as short as possible, and a large number of echoes must be used to obtain an echo signal that is long away from the 90° pulse. I've been collecting signals. Therefore, in order to obtain an image with a long echo time, a large amount of unnecessary intermediate information is required, resulting in problems such as an increase in the calculation time for imaging and a decrease in the throughput of the apparatus. Furthermore, by applying many 180° pulses, there was a possibility of signal deterioration due to non-uniform 180° pulses, which could lead to image deterioration.

In order to solve this problem, an apparatus has been proposed that generates only two types of echo signals, short echo time and long echo time, and obtains only two images. That is, as shown in _{FIG .}
is applied to excite the nuclear spins of the selected plane on the Y-axis of the subject, and an X-axis gradient magnetic field G _x and a Y-axis gradient magnetic field G _y are applied for day phase and spoiling during period t ₂ '. Then, during period t ₃ ', a 180° pulse and a Y-axis gradient magnetic field G _y for slice selection are applied, and during period t ₄ ', an X-axis gradient magnetic field G _x and a Y-axis gradient magnetic field G are applied for spoiling. Apply _y , period
At t ₅ ', an X-axis gradient magnetic field G _x is applied for reading to obtain a first echo signal.

Next, in period t ₆ ′, an X-axis gradient magnetic field G _x and a Y-axis gradient magnetic field G _y are applied for spoiling, and the period
A 180° pulse and a Y-axis gradient magnetic field G _y for slice selection are applied at t ₇ ′, and after a period t ₈ ′, an X-axis gradient magnetic field G _x and a Y-axis gradient magnetic field for spoiling are applied during a period t ₉ ′. G _y is applied, and after a period t ₁₀ ′, an X-axis gradient magnetic field G _x is applied for reading during a period t ₁₁ ′.
Obtain the second echo signal.

As described above, since the method shown in FIG. 2 generates an echo signal by using two 180° pulses, several problems have been pointed out. That is, since a 180° pulse is used to obtain the first echo signal,
The period t ₁ ′ to t ₅ ′ is required, and the echo time can only be shortened to a certain extent, and since two 180° pulses are applied until the second echo signal is obtained, the first 180° pulse A false signal due to non-uniformity is superimposed on the second echo signal, causing problems such as artifacts on the image. In order to avoid the latter disadvantage, various efforts have been made, for example, as seen in Japanese Patent Application Laid-Open No. 142448/1983, but all of them have drawbacks such as complicating the equipment and complicating the method of obtaining echo signals. have

An object of the present invention is to solve the above problems and provide a method for obtaining an image of a first echo signal with a sufficiently short echo time and an image of a second echo signal with a long echo time without artifacts.

<Means for Solving the Problems> The main point of the present invention is to obtain the first echo signal by only switching between the 90° pulse and the gradient magnetic field, without using multiple 180° pulses, and to obtain the second echo signal by simply switching between the 90° pulse and the gradient magnetic field. The purpose is to obtain a signal using a 180° pulse.
In other words, according to the present invention, while a static magnetic field is applied to the subject, a selective excitation pulse, that is, a 90° pulse, is applied to the subject together with the first gradient magnetic field to excite nuclear spins on a selected surface of the subject. , a second gradient magnetic field is applied to the subject to dephase the excited nuclear spins, and the direction of the second gradient magnetic field is reversed in a state in which the dephasing occurs. The first echo signal generated by the inversion is read out. Next, while applying the first gradient magnetic field,
Apply a 180° excitation pulse to flip the nuclear spin by 180°. A read field in the direction of a second gradient is then applied to generate a second 180° selective excitation pulse.
Read out the echo signal. At this time, between the readout period of the first echo signal and the application of the 180° excitation pulse, first and second gradient magnetic fields are applied to spoil the error signal due to the application of the 180° excitation pulse, and the 180° excitation pulse is applied.゜ Between the application of the excitation pulse and the readout of the second echo signal, a period of applying the first and second gradient magnetic fields, which is the same as the strength-time product of the gradient magnetic field for spoiling, and a period of applying the second echo signal. A period in which no gradient magnetic field is applied is provided to adjust the signal generation time.

A signal with a short echo time can be obtained by the 90° pulse and the reversal of the second gradient field, and one
The 180° pulse and adjustment period allow clinically useful signals with long echo times to be obtained.

Furthermore, since the 180° pulse is selective excitation, it is possible to perform so-called multiple spin echo in which signals from multiple other selected slice planes can be collected during the waiting time.

<Example> An example of the present invention will be described below with reference to the drawings. As shown in FIG. 3, the direction of the magnetic field generated by the magnet 1 for generating a static magnetic field is the Z axis, and a subject 2, for example, a human being, is placed in the static magnetic field generated by the magnet 1. The longitudinal direction of the subject 2, that is, the body axis direction is Y.
The axis is the direction perpendicular to the Z axis and the Y axis.
Take it as the axis. A cylindrical coil bobbin 3 whose axis coincides with the Y-axis is disposed within the static magnetic field of the magnet 1, and the subject 2 is disposed within the coil bobbin 3. As shown in Fig. 4, a gradient magnetic field G _x whose magnetic field strength is inclined in the X-axis direction is generated by a coil 4 wound on a bobbin 3.
The coil 5 can generate a Z-axis gradient magnetic field _Gz , and the coil 6 can generate a Y-axis gradient magnetic field _Gy . Further, a coil 7 for forming a high frequency magnetic field and detecting a signal in the Y-axis direction is wound around the bobbin 3.

As shown in FIG. 1, during period _t1 , the coil ₇
Apply a 90° pulse using As a result, at a specific point on the Y-axis, nuclear spins within a selected plane cut through the subject 2 perpendicular to the Y-axis are excited. Then in period t ₂
An axial gradient magnetic field G _x is applied for a certain period of time to dephase the nuclear spins. It is preferable that this period t ₂ be set to a suitably short time such that spin-spin relaxation does not substantially occur.

In period _t3 , the direction of the X-axis gradient magnetic field _Gx is switched and used as a read magnetic field. This magnetic field G _x causes the nuclear spin to refase. The first echo signal P ₁ is obtained during this period t ₃ . This echo signal has a short period t ₂ and can be said to be an echo signal obtained from the spins immediately after the 90° pulse, so the spins have hardly undergone any spin-spin relaxation.

At period t ₄ after this echo signal occurs, period
An X-axis gradient magnetic field is used as a spoiler to spoil the error signal caused by applying a 180° pulse at _t5 .
G _x and Y-axis gradient magnetic field G _y are applied. period t ₅
A 180° selective excitation pulse is applied while a Y-axis gradient magnetic field G _y is applied, and rephasing of the dephased spins is started at the end of period t ₄ . period t ₆
is the adjustment period for the second echo signal generation time. In period t _{7 , the} gradient magnetic _{field G} and G _y and intensity-time product, that is, gradient magnetic fields G _x and G _y having the same area in FIG. 1 are applied, respectively.

Period _t8 , like period _t6 , is provided for adjusting the second echo signal generation time. When an X-axis gradient magnetic field having the same area as the X-axis gradient magnetic field applied during period _{t 3 is} applied as a read magnetic field in period t 9, the nuclear spin refuses again in period t ₉ ,
A second echo signal _P2 is obtained as shown in FIG. This signal is the first echo signal obtained
Since it is farther from the 90° pulse of period t ₁ than P ₁ , it is a signal that has undergone spin-spin relaxation.

In period t ₂ in Figure 1, in order to correct the phase disturbance of the nuclear spins excited when the nuclear spins of the selected surface are excited, the Y-axis gradient magnetic field is reversed, similar to the conventional method. It is being said. When performing image formation using the echo signals P ₁ and P ₂ obtained in this way, in order to obtain information in the Z direction, as shown in Japanese Patent Application Laid-Open No. 158988-1988 during period t ₂ , A Z-axis gradient magnetic field G _z is also applied at the same time, and its magnitude is changed in each sequence. The present invention can be applied not only to this image forming method but also to the so-called reconstruction projection method.

<Effect of the invention> When two echo signals are obtained using a 180° pulse as in the past, the period is as shown in Figure 2.
The period for spoiling the error signal by applying the 180° pulse at t ₄ ′ and the period for applying the first 180° pulse t ₃ ′ are shown in FIG. 1 to obtain the first echo signal. However, the first echo signal generation time cannot be sufficiently shortened. Furthermore, since a plurality of 180° pulses are used, the apparatus becomes complicated in order to avoid artifacts caused by this. The present invention solves the above problems and provides a method for obtaining two clinically effective echo signals.

[Brief explanation of drawings]

Figure 1 is a time chart showing the method of the present invention, Figure 2 is a time chart showing the conventional method, Figure 3 is a diagram showing an overview of an example of a nuclear magnetic resonance apparatus, and Figure 4 is an example of a gradient magnetic field generating coil. FIG.

Claims (1)

[Claims] 1. A subject is placed in a static magnetic field, and a first
Excite the nuclear spins in the selected cross section by applying a gradient magnetic field of 20° and a 90° excitation pulse, and then applying a second gradient magnetic field that intersects the first gradient magnetic field to the subject. Dephasing the excited nuclear spins, reversing the direction of the second gradient magnetic field in the dephased state, reading out the first echo signal generated by the reversal, and then reading out the error signal due to the 180° selective excitation pulse applied later. providing the first and second gradient magnetic field application periods for spoiling, and then applying the first gradient magnetic field and
applying a 180° excitation pulse to flip the nuclear spins in the selected cross-section by 180°, and then applying first and second magnetic field gradients that are equal to the intensity-time product of the magnetic field gradients for the spoil; , an adjustment period in which no gradient magnetic field is applied for generating a second echo signal by rephasing the nuclear spins, and then applying a read magnetic field in the direction of the second gradient magnetic field to generate a second echo signal at the 180°. A method for obtaining a signal containing spin-spin relaxation information, comprising reading out the second echo signal generated by a selective excitation pulse.