JPH0228820B2 - - 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, an outline of the principle of NMR will first be explained.

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

Figure 1 shows a hydrogen nucleus ('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 Ho is applied, each atomic nucleus is aligned in the Ho direction (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 +
It is divided into two levels of 1/2. The energy difference ΔE between two energy levels is expressed by equation (1).

ΔE=γhHo...(1) where γ: gyromagnetic ratio h=h/2π h: blank constant Here, a force of μ→×Ho→ is applied to each atomic nucleus due to the static magnetic field Ho→, so The atomic nucleus moves around the Z axis,
It precesses at an angular velocity ω as shown in equation (2).

ω = γHo (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 ΔE shown by equation (1). absorbs energy equivalent to , 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. Further, 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 ('H), but it is possible to perform similar measurements with other atomic nuclei that have nuclear spin angular momentum. (13C), sodium nucleus (23Na), fluorine nucleus (19F), oxygen nucleus (17O), 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, inspection equipment using NMR has been designed to excite protons in the virtual transfusion region of the subject using the same principle as X-ray CT, and to generate NMR resonance signals corresponding to each protrusion into the subject. There is a method in which the NMR resonance signal intensity at each position of the object is determined by a reconstruction method.

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

First, a Z gradient magnetic field Gz as shown in Fig. 4B and an RF pulse (90° pulse) with a narrow frequency spectrum (f) as shown in Fig. 4A are applied to the subject. In this case, protons only on the plane where the Larmor angular velocity ω = γ (Ho + ΔGz) are excited, and if the magnetization M is expressed on a rotating coordinate system rotating at ω as shown in Figure 5, then the y' axis will be as shown in the figure. The direction is changed by 90 degrees. Subsequently, as shown in FIG. 4C, an x-axis gradient magnetic field Gx is applied for a predetermined time tx, thereby grading the phase of the magnetization M in the x-axis direction as shown in equation (3).

γLx∫ _tx dt・Gx=2πn ...(3) where γ: gyromagnetic ratio Lx: length of object in x direction n: integer (n=-N'/2 N/2+1, ..., -1, 0 ,
+ 1,...,N/2-1) N: Number of divisions in the x direction Next, as shown in Figure 4D, a y-axis gradient magnetic field Gy is applied, and under this, as shown in Figure 4E,
Detect NMR resonance signals. The y-axis direction is scaled using Larmor angular velocity. Here, magnetization M
As shown in Figure 5 (b), due to the inhomogeneity of the magnetic field, the NMR resonance signal gradually decreases in the direction of the arrow in the x' and y' planes, and the As shown in , it disappears after τ time has elapsed.

Thereafter, wait τ' time until the state returns to thermal equilibrium, and then repeat the next sequence. At this time, the predetermined time tx for applying the x-axis direction gradient magnetic field Gx is repeated N times with a value determined by equation (3). Then, by performing two-dimensional Fourier transform on the NMR resonance signals obtained in the sequence N times, an in-plane proton density image can be obtained.

In the conventional device that operates in this way, the time τ until the NMR resonance signal disappears is 10 to 20 mS as shown in FIG. for ₁
Approximately 1 sec is required. Therefore, if the number of divisions N in the x-axis direction is, for example, about 100, the measurement requires a long time of at least 2 minutes.

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. The features are that the NMR resonance signal from a specific part is subjected to a two-dimensional Fourier transform.

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 static magnetic field coil as a static magnetic field forming means for generating a uniform static magnetic field Ho (the direction of this magnetic field is the Z direction), 2 is a control circuit for this static magnetic field coil 1, For example, it includes a DC stabilized power supply. The density Ho of the magnetic flux generated by the static magnetic field coil 1 is about 0.1T, and the uniformity is 10 ^-4
The above is desirable.

3 generally shows a gradient magnetic field coil as a gradient magnetic field generating means, and 4 is a control circuit (control means) for this gradient magnetic field coil 3, which generates a gradient by switching the current flowing through the gradient magnetic field coil. It controls the gradient magnetic field generated by the magnetic field coil.

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 is a magnetic field in the same direction as the uniform static magnetic field Ho, and generates a magnetic field having linear gradients 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 serving as an excitation means for applying 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, 42.6M for protons)
Hz/T), and its output is applied to the excitation 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 as a detection means for detecting the NMR resonance signal in the subject, and its configuration is the same as the excitation coil shown in FIG. There is. Although it is desirable that this detection coil be installed as close as possible to the subject, 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, and 73 is a wave that stores the phase-detected waveform signal from the amplifier 71. A memory circuit that includes an A/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 serves as a calculation means for performing predetermined signal processing to obtain a tomographic image, and numeral 9 is a display device such as a television monitor that displays the obtained tomographic image.

The operation of the apparatus configured as described above will now be described with reference to FIGS. 8 and 9.

First, the control circuit 2 applies a current to the static magnetic field coil 1 to apply a static magnetic field Ho to the subject (the subject is placed within the cylinder of each coil). In this state, the controller 60
First, a current is applied to the z-gradient magnetic field coil 31 via the control circuit 4 to provide a z-gradient magnetic field Gz ⁺ 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, producing a narrow spectrum as shown in FIG. Excite one side of the object with a 90° pulse. In addition,
In FIG. 8B, Gz ^- following Gz ⁺ is for improving the S/N ratio, and is a known method.

At this time point _tp , the magnetization M changes direction by 90 degrees in the y'-axis direction as shown in the rotating coordinate system of FIG. 9A. Subsequently, a current is passed through the x-gradient magnetic field coil 32 for a predetermined time _tx1 , and as shown in FIG. 8C, an x-axis direction gradient magnetic field Gx of a predetermined magnitude is applied for a predetermined time. As a result, the phase of magnetization M is graduated in the x-axis direction as shown in equation (3). Next, the 8th
As shown in FIG. 8, a gradient magnetic field in the y-axis direction is applied, and under this, the NMR resonance signal is detected as data _E1 by the detection coil 7, as shown in FIG.
Here, at the time when the NMR resonance signal is detected (for example, time t ₁ ), the magnetization M is in the process of gradually dispersing in the direction of the dashed arrow as shown in FIG. NMR detected by
The resonance signal gradually decays over time. This signal is amplified by an amplifier 71, and the phase detection circuit 7
2, the signal is phase detected and applied to the computer 8 via the wave memory circuit 73. Note that the y-axis direction is scaled using the Larmor frequency. The operation up to now is the same as that of the conventional device.

After the time τ has elapsed until the NMR resonance signal disappears, the controller 60 supplies current to the z gradient magnetic field coil 31 again, applies a z gradient magnetic field Gz ⁺ as shown in FIG. 8B, and opens the gate circuit 61. Then, a current is applied to the excitation coil 5, and this time the 8th
As shown in Figure A, a 180°-x pulse (180°-x is the inverted phase of the signal from the oscillator 6) is applied. Next, as shown in Figure 8D, a current is passed through the y gradient magnetic field coil to apply a magnetic field Gy of the same magnitude as the previous time, and then as shown in Figure 8C, the x gradient magnetic field coil is applied. same as last time, for a predetermined time t _x1
As shown in FIG. 8C, an x-axis gradient magnetic field Gx is applied for a predetermined time _tx1 .

When a 180°-x pulse is applied, the dispersed magnetization M
begin to gather again as shown in FIG. 9C, and an NMR resonance signal (this signal is called an echo signal) that gradually increases as shown in FIG. 8E is detected from the detection coil 7. After the lapse of time τ after the application of the 180°-x pulse, the echo signal reaches its maximum as shown in FIG. 8E. This echo signal has a signal waveform that is symmetrical to the initially output NMR resonance signal with respect to the time axis, assuming that the state of the subject does not change during the time τ. At this time point _t3 , 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. Forcibly turn towards.
At this time _t3 , the magnetization M does not coincide with the z' axis due to the relaxation time _T2 and is in a slightly dispersed state, as shown in FIG. 9D.

After a short time τ ₀ has passed from this state, the magnetization M coincides with the z' axis due to relaxation. Here, the time τ ₀ from the time t ₃ until the magnetization M coincides with the z' axis is t ₃
Since the magnetization M is only slightly dispersed from the z' axis at the time point, the relaxation time _T1 is sufficiently short compared to the relaxation time T1, for example, about 30 mS.

The first sequence ends when the time τ' has elapsed, and the same sequence is repeated thereafter.
In each sequence, the application time t _x of the x-axis gradient magnetic field Gx applied to the subject is changed according to the conditions of equation (3),
For each sequence, an NMR resonance signal and an echo signal are obtained from the detection coil 7.

In each sequence, the computer 8 stores, for example, the data of the NMR resonance signal that is output at the beginning.
Assume N pieces of E ₁ , E ₂ ... as one group,
A two-dimensional Fourier transform calculation is performed to obtain an image, which is displayed on the display 9.

In the above, it is assumed that the computer 8 does not use the echo signal, but in each sequence, the first NMR resonance signal (abbreviated as NMR signal) and the subsequent echo signal are output. Both signals may be used. In this case, the usage may be as follows, for example.

(i) Calculate the average value of the NMR signal and the signal obtained by reversing the time axis of the echo signal, and use this as one data to perform a two-dimensional Fourier transform calculation,
Obtain a single tomographic image.

(ii) Obtain a proton density image using the NMR signal, calculate the difference signal between the NMR signal and the signal obtained by inverting the time axis of the echo signal, and use this as one data to perform Fourier transform calculation, T ₂ Transverse relaxation time called (T ₂ is due to the interaction of the spins of nearby electron nuclei)
Obtain both images based on t ₂ images.

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

(iv) Average the NMR signals and echo signals of multiple sequences and use this as one data.

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 was explained as (90°+x)→(180°−x′)→(90°+x), but instead of this, ( 90°+x) → (180°y′) → (90°−x)
A pulse sequence of electromagnetic waves may be used.

Figure 10 shows (90°+x)→(180°y′)→(90°−
x) When using the electromagnetic wave pulse sequence, the eighth
This figure shows the direction of magnetization M at each time point t ₀ , t ₁ , t ₂ , and t ₃ shown in the figure. In this case, if a 90° pulse is applied at time t ₃ when the echo signal is maximum,
The magnetization M is forcibly directed from the y'-axis side to the z'-axis direction, as shown in FIG. 10D.

Here, the 180°y' pulse represents the signal from oscillator 6 with the phase delayed by 90°, and the 90°-x pulse represents the signal from oscillator 6 with the phase delayed by 180°. ing.

FIG. 11 is an operation waveform diagram showing another example of the method according to the present invention. This method is the same as the method shown in Figure 8, but before applying the 90° pulse to the subject (τ'' time), a 180° pulse is applied as shown in Figure 11A. .In addition,
At the same time as the 180° pulse, as shown in Figure 11B,
Give Gz ⁺ . Here, the time τ'' from applying the 180° pulse to applying the 90° pulse is the time required for the magnetization M, whose direction has been reversed by 180° due to the 180° pulse, to return to its original state. According to this method, the intensity of the NMR signal changes due to the relaxation of T ₁ between τ'', and a T ₁ image can be obtained from this.

Here, the following series can be used as the pulse series of the electromagnetic waves to be applied.

(180°) → (90°) → (180°−x′) → (90°−x′
) (180°)→(90°−x′)→(180°−x′)→(90°) (180°)→(90°−x)→(180°y)→(90°−
x') FIG. 12 is an operational waveform diagram when the method according to the present invention is applied to a method called a spin warp method. Here, as shown in Figure 13 C,
The time tx for applying the gradient magnetic field Gx in the x-axis direction is constant, and the magnitude Gx ₁ is different for each sequence.
Gx ₂ ... Also, Gx
While applying , a gradient magnetic field Gy ^- in the y-axis direction is applied as shown in FIG. 12D.
When Gy ^- is applied while Gx is being applied, the magnetization M is diffused, as shown by the broken line in Figure 12 E.
The NMR resonance signal decreases and quickly disappears. Next, as shown in Fig. 12D, a gradient magnetic field in the y-axis direction is applied.
When Gy ⁺ is applied, the magnetization M regroups and the 12th
As shown in Figure E, a changing echo signal appears,
Under applying Gy ⁺ , we convert this to data E ₁ ,
Detected as E ₁ ′. Figure 8, which has been explained so far,
The method shown in Fig. 11 has the disadvantage that the NMR resonance signal or echo signal detected as data is in a decreasing region or increasing region, and the S/N ratio is not good. On the other hand, the method shown here detects the area where the echo signal is maximum as data, so it has the advantage of improving the S/N ratio.

As explained above, the method according to the present invention uses at least three types of pulses (90° pulse, 180° pulse,
This method forcibly changes the direction of the magnetization M using a series of 90° pulses, returning the magnetization M to a thermal equilibrium state in a short time. A tomographic image can be obtained.

Further, since NMR signals and echo signals can be obtained from the subject, by using these signals, it is possible to obtain a tomographic image with a good S/N ratio and high resolution.

[Brief explanation of drawings]

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 the excitation coil, FIG. 8 is an operation waveform diagram for explaining one of the methods according to the present invention, and FIG. 9 is magnetization at each time point according to the method of the present invention. M
FIG. 10 is an explanatory diagram showing the direction of magnetization M on a rotating coordinate system in the method according to the present invention, and FIG. 12 are operational waveform diagrams showing other examples 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 the subject and applying electromagnetic waves with a frequency that induces nuclear magnetic resonance in the subject, further applying a gradient magnetic field in the Z-axis direction of the subject, and then applying a gradient magnetic field of a predetermined magnitude in the X-axis direction of the subject for a predetermined time, then applying a gradient magnetic field in the Y-axis direction of the subject;
Nuclear magnetic resonance signal (NMR signal) from the subject
In an inspection method that specifies the radiation part of the object and obtains an NMR signal from the specific part of the object, an electromagnetic wave with a narrow frequency spectrum is used as the electromagnetic wave applied to the object, and the method includes first applying a Z-axis to the object. After exciting the subject by applying a 90° pulse while applying a gradient magnetic field, create an echo signal by applying a 180° pulse while applying a Z-axis gradient magnetic field. At this point, while applying the Z-axis gradient magnetic field again, a 90° pulse is applied to return the magnetization to the thermal equilibrium state, and the above sequence is repeated gradually by increasing the application time or magnitude of the X-axis gradient magnetic field. A nuclear magnetic resonance inspection method characterized by repeating the sequence at predetermined intervals, performing a two-dimensional Fourier transform operation on a predetermined number of data based on the NMR signal and/or echo signal obtained in each sequence as one group, and obtaining an image. . 2 Apply a uniform static magnetic field to the subject and apply electromagnetic waves with a frequency that induces nuclear magnetic resonance in the subject, further apply a gradient magnetic field in the Z-axis direction of the subject, and then apply a gradient magnetic field in the Z-axis direction of the subject, and then Apply a gradient magnetic field of a predetermined magnitude in the direction for a predetermined time, then apply a gradient magnetic field in the Y-axis direction of the subject,
Nuclear magnetic resonance signal (NMR signal) from the subject
In an inspection method that specifies the radiation part of the object and obtains an NMR signal from the specific part of the object, an electromagnetic wave with a narrow frequency spectrum is used as the electromagnetic wave applied to the object, and the method includes first applying a Z-axis to the object. Apply a 180° pulse while applying a gradient magnetic field, and after a predetermined time, apply a 90° pulse while applying a Z-axis gradient magnetic field to excite the subject, then apply a Z-axis gradient magnetic field. A 180° pulse is applied to generate an echo signal while the echo signal is at its maximum, and a 90° pulse is applied again while a Z-axis gradient magnetic field is applied to return the magnetization to a thermal equilibrium state. Thereafter, the above sequence is repeated at predetermined intervals by changing the application time or magnitude of the X-axis gradient magnetic field little by little, and a predetermined number of data based on the NMR signals and/or echo signals obtained in each sequence are combined into one. Performing two-dimensional Fourier transform operations as a group
An examination method using nuclear magnetic resonance characterized by obtaining T1 images. 3 Static magnetic field generating means for applying a uniform static magnetic field to the subject, generating gradient magnetic fields having gradients in the Z-axis direction, the X-axis direction, and the Y-axis direction to the subject to generate nuclear magnetic resonance signals (NMR) from the subject; a gradient magnetic field generating means for specifying a radiation portion of a signal); an excitation means for applying a pulsed electromagnetic wave including a narrow frequency spectrum to the subject; and a gradient magnetic field generating means for controlling the signal applied to the excitation means and the gradient magnetic field generating means. a control means for controlling the signal given to the object, a detection means for detecting the NMR signal from the subject, and a calculation means including a memory that inputs the signal from the detection means and performs a predetermined calculation to obtain a tomographic image. The control means first applies a Z-axis gradient magnetic field to the subject through the gradient magnetic field generation means and the excitation means, applies a 90° pulse to excite the subject, and then applies an X-axis gradient magnetic field to the subject. An axial gradient magnetic field is applied for a predetermined period of time, then a Y-axis gradient magnetic field is applied to identify the emission part of the NMR signal from a specific part of the subject, and then a Z-axis gradient magnetic field is applied. Apply a 180° pulse to create an echo, then apply a gradient magnetic field in the Y-axis direction, then apply a gradient magnetic field in the While applying a gradient magnetic field, apply a 90° pulse to return the magnetization to a thermal equilibrium state, and then repeat the above sequence at predetermined intervals by gradually changing the application time or magnitude of the X-axis gradient magnetic field. An inspection device using nuclear magnetic resonance, which is characterized by performing operations.