JPS6218863B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to nuclear magnetic resonance
resonance) (hereinafter abbreviated as "NMR")
The present invention relates to an inspection method and apparatus using nuclear magnetic resonance that utilizes phenomena to determine the distribution of specific atomic nuclei within a subject from outside the subject.

Before explaining the present invention, the principle of NMR will first be briefly explained.

The atomic nucleus consists of protons and neutrons, which are considered to be rotating as a whole with the spin angular momentum.

Figure 1 shows a hydrogen nucleus ( ¹ H), which consists of one proton P, as shown in A, and rotates as expressed by the spin quantum number 1/2. Here, since the proton P has a positive charge e ⁺ as shown in (b), a magnetic moment μ is generated as the atomic nucleus rotates. In other words, each hydrogen nucleus can be thought of as a small magnet.

Figure 2 is an explanatory diagram schematically showing this point.
In a ferromagnetic material such as iron, the directions of these micromagnets are aligned as shown in A, and magnetization is observed as a whole. On the other hand, in the case of hydrogen or the like, the direction of the micromagnets (the direction of the magnetic moment) is random as shown in (b), and no magnetization is observed as a whole.

Here, such a material is subjected to a static magnetic field in the Z direction.
When H ₀ is applied, each atomic nucleus is aligned in the direction of H ₀ (the energy level of the nucleus is quantized in the Z direction).

Figure 3A shows this situation for a hydrogen nucleus. The spin quantum number of hydrogen nucleus is 1/
2, so as shown in Figure 3B, -1/2 and +
Divided into two levels: 1/2. The energy difference ΔE between two energy levels is expressed by equation (1).

ΔE=γH ₀ ...(1) However, γ: gyromagnetic ratio = h/2π h: Planck's constant Here, each atomic nucleus is subjected to a force μ×H ₀ due to the static magnetic field H ₀ , so the nucleus is around the Z axis,
It precesses at an angular velocity ω as shown in equation (2).

ω = γH ₀ (Larmor angular velocity) ...(2) When an electromagnetic wave (usually a radio wave) with a frequency corresponding to the angular velocity ω is applied to a system in this state, resonance occurs and the atomic nucleus has an energy difference shown by equation (1). It absorbs energy corresponding to ΔE and transitions to a higher energy level. Even if several types of atomic nuclei with nuclear spin angular momentum coexist, each nucleus has a different gyromagnetic ratio γ, so the resonant frequencies differ, and therefore only the resonance of a specific atomic nucleus can be extracted. Furthermore, by measuring the strength of the resonance, it is possible to determine the amount of nuclei present.
After resonance, the atomic nucleus excited to a higher level returns to a lower level after a time determined by a time constant called relaxation time. Of this relaxation time, especially the spin-interstitial relaxation time (longitudinal relaxation time) called _T1
is a time constant that depends on the way each compound binds, and it is known that the value differs greatly between normal tissues and malignant tumors.

Here, we have explained hydrogen nuclei ( ¹ H), but it is possible to perform similar measurements with other atomic nuclei that have nuclear spin ^angular momentum. It is applicable to atomic nuclei ( ¹³ C), sodium nuclei ( ²³ Na), fluorine nuclei ( ¹⁹ F), oxygen nuclei ( ¹⁷ O), etc.

In this way, NMR can measure the abundance of specific atomic nuclei and their relaxation times, so by obtaining various chemical information about specific atomic nuclei within a substance, it is possible to conduct various tests within a subject. can be done.

Conventionally, there have been inspection devices that utilize such NMR.

FIG. 4 is an operational waveform diagram for explaining an example of an inspection method in a conventional device.

A static magnetic field H ₀ is applied to the subject, and under this condition, first a Z gradient magnetic field Gz ⁺ is applied as shown in Figure 4B, and an RF pulse (90° pulse) is applied.

In this case, as shown in FIG. 5A, in the subject HB, protons only on the Sz plane where the Larmor angular velocity ω=γ(H ₀ +ΔGz) are excited. Immediately after applying the RF pulse (90° pulse), an NMR resonance signal is detected as shown in _FIG . However, the relaxation time T ₁ is long and continues to be saturated. Subsequently, as shown in FIG. 4C and I, a y-gradient magnetic field Gy and an RF pulse (180°) are applied, thereby exciting the Sy plane as shown in FIG. 5B.
As a result, the Sy plane excluding the line Δy on the Sz plane is 180
゜Saturated by pulse. But line Δy is 90
Because it is excited by a ゜−180゜ pulse train, NMR
Generates a resonance signal (spin echo). continue,
As shown in FIG. 4E, at the point where the NMR resonance signal (spin echo) is maximum, an x gradient magnetic field Gx is applied as shown in FIG. 4D, and under this, the spin echo is detected as data. By Fourier transforming this data, the line Δy shown in FIG.
The proton density distribution in the x-axis direction above is known. Thereafter, by changing the magnitude of the y gradient magnetic field Gy, the position of Δy is changed little by little (this is called line scanning), and the above sequence is repeated multiple times to obtain two-dimensional data of the entire Sz plane. .

In conventional devices that operate in this way,
In Figure 5A, the Sz plane is selectively excited by a 90° pulse, and the next sequence for line scanning is approximately 1S until the magnetization M of the Sz plane returns to thermal equilibrium with a relaxation time _T1 . The disadvantage is that you have to wait and it takes a long time.

The present invention has therefore been made to obviate such drawbacks in conventional methods and devices.

The method according to the present invention does not wait until the magnetization M reaches a thermal equilibrium state (M points in the Z'-axis direction) due to the relaxation time _T1 , but uses a pulse sequence to force the magnetization M in the Z'-axis direction. It is distinctive in that it is directed towards the target.

FIG. 6 is a block diagram showing the configuration of an embodiment of a device for implementing the method of the present invention. In the figure, 1 is a uniform static magnetic field H ₀ (the direction of this magnetic field is Z
2 is a control circuit for the static magnetic field coil 1, which includes, for example, a DC stabilized power supply. It is desirable that the density H ₀ of the magnetic flux generated by the static magnetic field coil 1 is about 0.1 T, and that the uniformity is 10 ⁻⁴ or more.

3 shows a general overview of gradient magnetic field coils;
4 is a control circuit for this gradient magnetic field coil 3.

FIG. 7A is a configuration diagram showing an example of the gradient magnetic field coil 3, in which the Z gradient magnetic field coil 31, the y gradient magnetic field coils 32, 33, although not shown, have the same shape as the y gradient magnetic field coils 32, 33. and includes an x-gradient magnetic field coil that is rotated by 90°.
This gradient magnetic field coil 3 generates a magnetic field in the same direction as the uniform static magnetic field H ₀ and having a linear gradient in each of the x, y, and z axis directions. 60 is a controller of the control circuit 4.

Reference numeral 5 denotes an excitation coil that provides an RF pulse with a narrow frequency spectrum f as an electromagnetic wave to the subject, and its configuration is shown in FIG. 7B.

6 is the frequency corresponding to the NMR resonance condition of the atomic nucleus to be measured (for example, for protons,
This oscillator generates a signal of 42.6 MHz/T), and its output is applied to the exciting coil 5 via a gate circuit 61 whose opening/closing is controlled by a signal from a controller 60 and a power amplifier 62. 7
is a detection coil for detecting an NMR resonance signal in a subject; its configuration is the same as the excitation coil shown in FIG. In addition, this detection coil is
Although it is desirable to install it as close to the subject as possible, it may also be used as an excitation coil if necessary.

71 is an amplifier that amplifies the NMR resonance signal (FID: free induction decay) obtained from the detection coil 7; 72 is a phase detection circuit; 73 is a wave memory circuit that stores the phase-detected waveform signal from the amplifier 71; /D converter. 8 inputs the signal from the wave memory circuit 73 via a transmission line 74 made of, for example, an optical fiber;
A computer performs predetermined signal processing to obtain a tomographic image, and 9 is a display device such as a television monitor that displays the obtained tomographic image.

The operation of the apparatus constructed in this way will be explained next with reference to FIGS. 8, 9, and 10.

First, the control circuit 2 applies a current to the static magnetic field coils to apply a static magnetic field H ₀ to the subject (the subject is placed within the cylinder of each coil). In this state, the controller 60 first controls the Z gradient magnetic field coil 3 via the control circuit 4.
1, and a Z gradient magnetic field Gz ⁺ is applied as shown in FIG. 8B. Also, while Gz ⁺ is being applied, the gate circuit 61 is opened and the signal from the oscillator 6 is applied to the excitation coil 5 via the amplifier 62,
90 has a narrow spectrum as shown in Figure 8 A.
゜Excite one side of the object with a pulse. As a result, in the subject HB, the Sz plane shown in FIG. 9A is excited, and as shown in FIG. 8E, the Sz plane is excited.
An NMR resonance signal is detected.

Also, at this time t ₀ , the magnetization M changes direction by 90° in the y'-axis direction, as shown in the rotating coordinate system of FIG. 10A. As this magnetization M gradually disperses in the direction of the broken line arrow as shown in FIG. 10B, the NMR resonance signal also disappears. This NMR
τ until resonance signal disappears After ₁ hour
At t ₁ , the controller 60 applies a current to the x gradient magnetic field coil to generate an x gradient magnetic field as shown in FIG.
Gx is applied, the gate circuit 61 is opened, a current is applied to the excitation coil 5, and a 180° pulse is applied as shown in Fig. 8 (a), and a pulse parallel to the Z axis is applied as shown in Fig. 9 (b). Excite the Sx plane. As a result, in the same figure, from the line L where the Sz plane and the Sx plane intersect, the NMR
A resonance signal (echo signal) is detected, which becomes progressively louder. Then, at time _t2 when the echo signal reaches its maximum, the y gradient magnetic field Gy is increased as shown in Figure 8D.
is applied, and under this, the echo signal is detected as data _E1 as shown in FIG. 8E. this data
E ₁ is amplified by an amplifier 71 and passed through a phase detection circuit 7
2, the signal is phase detected and applied to the computer 8 via the wave memory circuit 73. data here
E ₁ is Fourier transformed to obtain the proton density distribution on line L.

At time t ₃ , τ ₂ hours have elapsed from time t ₂ when the echo signal reaches its maximum until the echo signal disappears.
This time, as shown in FIG. 8 A and B, the controller 60 causes current to flow through the Z gradient magnetic field coil 31 to provide the Z gradient magnetic field Gz ⁺ , and opens the gate circuit 61 to supply current to the excitation coil 5. Let it flow,
A 180°-x' pulse (180°-x' is the signal from the oscillator 6 with its phase inverted, and then a y gradient magnetic field Gy and an x gradient magnetic field Gx are sequentially applied. In the specimen HB, the history to the Sz plane excluding the line L portion is reversed. Then, as shown in Figure 8 E, the Sz plane excluding the line L portion is
NMR resonance signals (echo signals) from the surface appear. This echo signal reaches its maximum at time t ₄ after the lapse of time τ after the application of the 180°-x′ pulse. At this time _t4 , the gate circuit 61 is opened,
A current is applied to the excitation coil 5 under Gz ⁺ , and a 90° pulse is applied as shown in Fig.
is forced to point in the z′-axis direction. At this point t ₃ ,
The magnetization M is the relaxation time as shown in Figure 10C.
Because of T ₂ , it does not coincide with the z′ axis and is in a slightly dispersed state.

After a short time τ' has elapsed from this state, the magnetization M coincides with the z' axis due to relaxation. Here, the time τ′ from the time t ₄ until the magnetization M coincides with the z′ axis is
Since the magnetization M is only slightly dispersed from the z' axis at the time _t4 , it may be sufficiently short compared to the relaxation time _T1 .

The first sequence ends when τ′ has elapsed, and the same sequence is repeated thereafter. In each sequence, the magnitude of the x-axis gradient magnetic field Gx is changed, the line L portion is scanned, and two-dimensional data of the entire Sz plane is obtained.

In each sequence, the computer 8
Each line L obtained under applying a gradient magnetic field
Echo signals from the sections are input as data E ₁ , E ₂ .

In the above, the computer 8 uses only the echo signal data E ₁ , E _{2 .} . . from the first output line L portion in each sequence. Signal data E ₁ , E ₂ ...
Then, the echo signal data E ₁ ′,
You may use both E ₂ ′....

In this case, the usage may be as follows, for example.

(i) Calculate the average value of the echo signal output first (referred to as the first data) and the signal obtained by reversing the time axis of the echo signal output subsequently (referred to as the second data), and combine them into one Let the data be the line of .

(ii) Average some of the first data and second data of the plurality of sequences and use this as one data.

(iii) Obtain a proton density image using the first data, calculate the difference signal between the first data and the second data, and use the data of this difference signal to calculate the _horizontal Obtain both _T2 images based on the relaxation time ( _T2 is due to the interaction of the spins of neighboring electron nuclei).

(iv) In (iii) above, the proton density image and the T ₂ image are combined to obtain another image.

By adopting these methods, it is possible to improve the S/N ratio and obtain high-quality images. Further, by selecting one of these methods depending on the purpose of diagnosis, a tomographic image suitable for the purpose can be obtained.

In the above explanation, the pulse sequence of the electromagnetic waves applied to the subject is as shown in Figure 8A, (90°) → (180°) → (180°−x') →
(90°) pulse sequence has been explained, but instead of this, (90°) → (180°) → (180°−y′)
→(90°−x′) electromagnetic wave pulse sequence may be used.

Here, the 180°y' pulse is the signal from the oscillator 6 with a phase delay of 90°.
The x' pulse represents the signal from oscillator 6 delayed in phase by 180 degrees.

FIG. 11 is an operational waveform diagram showing another modification of the method of the present invention. The difference between this method and the one in Figure 8 is that the y gradient magnetic field is
The timing of applying Gy is changed, and the first
As shown in Figure 1E, the entire echo signal can be extracted as data.

In addition, in the method shown in Fig. 8 and Fig. 11, before applying the 90° pulse to the subject (τ'' time), while applying the x gradient magnetic field Gx, the 180°
A pulse may be applied. here
The time τ″ from applying a 180° pulse to applying a 90° pulse is determined by the direction of the 180° pulse.
It takes time for the magnetization M, which has been reversed by 180°, to return to its original state. According to this method, the intensity of the NMR signal changes due to the relaxation of T ₁ during τ'', and from now on T ₁
You can get the image.

As explained above, the method according to the present invention uses a series of at least four types of pulses (90° pulse, 180° pulse, 180° pulse, and 90° pulse), and uses a series of at least four types of pulses to control the direction of magnetization M. Forcibly change
Since the magnetization M is returned to the thermal equilibrium state in a short time, a tomographic image related to the distribution of specific atomic nuclei within the subject can be obtained in a short time.

Also, two data E ₁ , E ₁ ′... from the subject.
Therefore, by using each of these signals, a tomographic image with a good S/N ratio and high resolution can be obtained.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram for explaining the nuclear magnetic moment, Figure 2 is an explanatory diagram for explaining the arrangement of nuclear magnetic moments, and Figure 3 is an explanatory diagram for explaining the arrangement of nuclear magnetic moments by a static magnetic field. , Fig. 4 is an operation waveform diagram for explaining an example of the conventional method, and Fig. 5 shows magnetization M by the method of Fig. 4.
FIG. 6 is a block diagram showing an example of a device for realizing the method according to the present invention, and FIG. 7A shows a gradient magnetic field coil used in the device shown in FIG. A configuration diagram showing an example, B is a configuration diagram of an excitation coil, FIG. 8 is an operation waveform diagram for explaining one of the methods according to the present invention, and FIG. 9 is an explanatory diagram showing an excitation surface by an applied electromagnetic wave pulse. , FIG. 10 is an explanatory diagram showing the direction of magnetization M at each point in time according to the method of the present invention on a rotating coordinate system, and FIG. 11 is an operation waveform diagram showing another example of the method of the present invention. DESCRIPTION OF SYMBOLS 1... Static magnetic field coil, 2... Static magnetic field coil control circuit, 3... Gradient magnetic field coil, 5... Excitation coil, 60... Controller, 7... Detection coil, 8... Computer.

Claims (1)

[Claims] 1. Applying a uniform static magnetic field to a subject and applying electromagnetic waves with a frequency that induces nuclear magnetic resonance in the subject, detecting a nuclear magnetic resonance signal (NMR signal), and identifying the inside of the subject. In an inspection method that allows the distribution of atomic nuclei to be known from outside the specimen, first a 90° pulse of electromagnetic waves and a Z-axis gradient magnetic field Gz are simultaneously applied to the specimen to excite the first specific surface of the specimen, causing the NMR signal to disappear. After that, a 180° pulsed electromagnetic wave and an X-axis gradient magnetic field Gx are simultaneously applied to the first
A second specific plane intersecting with the specific plane is excited, and then a Y-axis gradient magnetic field is applied, and under this, the first and second
Detecting an NMR signal from a region where the specific planes of
Next, the Y-axis gradient magnetic field Gy and the X-axis gradient magnetic field Gx are sequentially applied at different times so as not to overlap, and the history of the first specific surface excluding the area where the first and second specific surfaces intersect is obtained. When the NMR signal reaches its maximum, a 90° pulse electromagnetic wave and a Z-axis gradient magnetic field are simultaneously applied to force the magnetization of the first specific surface to return to a thermal equilibrium state, and the above procedure is then carried out as specified. An inspection method using nuclear magnetic resonance that is repeated at time intervals. 2. A static magnetic field forming means for applying a uniform static magnetic field to the subject; a gradient for sequentially applying a Z-axis gradient magnetic field Gz, an X-axis gradient magnetic field Gx, and a Y-axis gradient magnetic field Gy to the subject so that they do not overlap with each other; A magnetic field generating means, an excitation means for applying a pulsed electromagnetic wave to the subject, a means for detecting a nuclear magnetic resonance signal (NMR signal) from the subject, and a signal input from the detecting means and a predetermined calculation. and a control means for controlling the gradient magnetic field generation means and the excitation means. A 90° pulsed electromagnetic wave and a Z-axis gradient magnetic field Gz are simultaneously applied to excite the first specific surface of the object, and after the NMR signal disappears, 180°
゜A pulsed electromagnetic wave and an X-axis gradient magnetic field Gx are simultaneously applied to excite a second specific plane that intersects the first specific plane, and then a Y-axis gradient magnetic field is applied, and under this, the first, from the area where the second specific plane intersects
Detect the NMR signal, and then, after the NMR signal disappears, a 180°-X pulse electromagnetic wave and a Z-axis gradient magnetic field Gz
are applied at the same time, and then a Y-axis gradient magnetic field Gy and an X-axis gradient magnetic field Gx are sequentially applied at different times so as not to overlap. Reverse the history of a specific surface and perform NMR
At the point when the signal is maximum, 90° pulse electromagnetic wave and Z
An inspection apparatus using nuclear magnetic resonance, characterized in that an axial gradient magnetic field is simultaneously applied to force the magnetization of the first specific surface to return to a thermal equilibrium state, and thereafter the above procedure is repeated at predetermined time intervals.