JPS6029685A - Inspecting method by nuclear magnetic resonance - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to nuclear magnetic resonance
ic reso-nance) (hereinafter referred to as rNM
It will be abbreviated as RJ. ) This invention relates to an inspection method and an inspection apparatus using nuclear magnetic resonance, in which the distribution of specific atomic nuclei within a subject can be known from outside the subject by utilizing the phenomenon. especially,
This invention relates to improvements in NMR imaging devices suitable for medical equipment.

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 nuclear spin angular momentum ■.

Figure 1 shows a hydrogen nucleus ('H), which, as shown in (a), consists of one proton P and rotates as expressed by the spin quantum number. Here, proton P is (b)
As shown in , it has a positive charge e0, so as the nucleus rotates, a magnetic moment -,7 resides. In other words, each hydrogen nucleus can be thought of as a small magnet.

Figure 2 is an explanatory diagram that schematically shows 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, etc., the direction of the micromagnets (the direction of the magnetic moment) is random as shown in (b),
No magnetization is seen as a whole.

Here, such a substance is subjected to a static magnetic field H in the Z direction.

When is applied, each atomic nucleus aligns in the direction of Ho. That is, the energy level of the nucleus is quantized in the Z direction.

Figure 3 (a) shows this situation for a hydrogen nucleus. Since the spin quantum number of the hydrogen nucleus is 2, it is divided into two energy levels, -% and +2, as shown in Figure 3 (b). The energy difference ΔE between two energy levels is expressed by equation (1).

ΔE=77JHo −=−(]) However, γ: gyromagnetic ratio ”Fq=h/2π h blank constant Here, each atomic nucleus is given μXH by a static magnetic field Ho.

Due to the force applied, the atomic nucleus moves around the Z axis, (2)
It precesses at an angular velocity ω as shown in the equation.

ω=rHo (Larmor angular velocity) ・・・・・・(2)
When electromagnetic waves (bird radio waves) with a frequency corresponding to the angular velocity ω are applied to a system in this state, resonance occurs and the atomic nuclei (
1) It absorbs energy corresponding to the energy difference ΔE shown by the formula and transitions to a higher energy level. Even if several types of nuclei with nuclear spin angular momentum are mixed,
Since each atomic nucleus has a different gyromagnetic ratio γ, the resonance frequency differs, 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.

This relaxation time is the spin-lattice relaxation time (longitudinal relaxation time)
It is classified into spin-spin relaxation time (transverse relaxation time) T1 and spin-spin relaxation time (transverse relaxation time) T2, and data on material distribution can be obtained by observing this relaxation time. Generally, in solids, spins are almost fixed at fixed positions on the crystal lattice, so interactions between spins are likely to occur. Therefore, the relaxation time r2 is short, and the energy obtained by nuclear magnetic resonance is first transferred to the spin system and then to the lattice system. Therefore, time T1 is significantly larger than T2.

On the other hand, in a liquid, molecules move freely, so the likelihood of energy exchange between spins and the molecular system (lattice) is about the same. Therefore, times T1 and T2 have approximately equal values. In particular, the time 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 (2
3Na, ) fluorine atomic nucleus ("F), oxygen atomic nucleus (0), etc.

In this way, NMR allows the presence of specific atomic nuclei and their relaxation times to be measured, so by obtaining various chemical information about specific atomic nuclei within a substance, it is possible to conduct various tests inside the subject. I can do it.

Conventionally, inspection equipment using NMR excites protons in a virtual cross-section of the subject, and generates NMR resonance signals corresponding to each projection in numerous directions of the subject, similar to X-ray CT. Based on this principle, 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 an operational 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. ω=r (Ho+ΔG z ) ---(Protons only on the plane 31 are generated JiiiI), and the magnetization M is shown in Fig. 5 (
If it is shown on a rotating coordinate system that rotates at an angular velocity ω as shown in a), the direction is changed by 90° in the y'-axis direction. Next, as shown in Fig. 4 (c) and (d), the X gradient magnetic field Gx
and y gradient magnetic field cy are applied, thereby creating a two-dimensional gradient magnetic field, and an NMR resonance signal as shown in (e) is detected. Here, as shown in FIG. 5(b), the magnetization M gradually disperses in the direction of the arrow in the X', y' plane due to the inhomogeneity of the magnetic field, so the NMR resonance signal eventually decreases. As shown in FIG. 4(E), the amount disappears after τ time has elapsed. If the NMR resonance signal obtained in this way is subjected to Fourier transformation, it becomes a projection in a direction perpendicular to the gradient magnetic field synthesized by the X gradient magnetic field Gx and the y gradient magnetic field cy.

Thereafter, in the same way, wait a predetermined time τ' and repeat the next sequence. In each sequence, G
Change x and Gy slightly. This allows NMR resonance signals corresponding to each projection to be determined in many directions of the object.

In a conventional device that operates in this manner, the present inventor
After applying the RF pulse (90° pulse), the magnetization M
without waiting until it reaches a state of thermal equilibrium due to the relaxation time T1,
During the period of τ′ above, gradient magnetic fields −Gx and −G of opposite polarity
He invented a method to return the magnetization M to the biaxial direction by giving y and reversing the spin motion and generating an echo (Patent application 1983).
−). FIG. 6 shows an operational waveform diagram of this method. Spin As shown in Figure 6 (a), after time Ts1 has elapsed since the first high-frequency pulse (90° pulse) was applied, the polarity of the gradient magnetic field is reversed, and further time ]s2 has elapsed. Then, a second high-frequency pulse (90'-x pulse) is applied to form one sequence, and after a third time Td has elapsed, the above sequence is repeated. According to this method, by observing the NMR signal intensity, the relaxation times Tl and T2 and the nuclear density can be measured without waiting for the relaxation time.

In this method of observing NMR signals, the inventors have found that by controlling the strength of the gradient magnetic field, the time for applying the gradient magnetic field of opposite polarity can be shortened.

That is, an object of the present invention is to provide a method that shortens the sequence for applying a plurality of high-frequency pulses and gradient magnetic fields, and shortens the time for detection by nuclear magnetic resonance.

The present invention applies a gradient magnetic field by applying a first high-frequency pulse, reverses the polarity of the gradient magnetic field, increases the intensity of the gradient magnetic field applied to the subject, and applies a gradient magnetic field of the opposite polarity. It is characterized by shortening the time.

FIG. 7 is a block diagram showing the configuration of an embodiment of a device for implementing the method of the present invention. In the figure, ■ is a control circuit for a static magnetic field coil I for generating a uniform static magnetic field Ho (the direction in this case is the Z direction), and a control circuit for the static magnetic field coil I of 2-cigarettes, for example, a DC stabilized power supply. Contains. Density Ha of magnetic flux generated by static magnetic field coil 1
is about 0.1T to 0.2T, and the uniformity is lO
The above is desirable.

Reference numeral 3 generally indicates a gradient magnetic field coil, and 4 indicates a control circuit for this gradient magnetic field coil 3.

FIG. 8(A) is a configuration diagram showing an example of the gradient magnetic field coil 3, in which the Z gradient magnetic field coil 3Ly, the gradient magnetic field coil 32.
33. Although not shown, it includes an X-gradient magnetic field coil which has the same shape as the y-gradient magnetic field coil 32 and 33 and is installed with a 90" rotation. It generates a magnetic field in the same direction as the magnetic field Ho, x, y,
Each generates a magnetic field with a linear gradient in the z-axis direction. 6
0 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. 8 (b).

6 is the frequency corresponding to the NMR resonance condition of the atomic nucleus to be measured (for example, for protons, 42.6 MHz/T
), the output of which is applied to the excitation coil 5 via a data 1 circuit 61 whose opening and closing are controlled by signals from a controller 60 and a power amplifier 62. Reference numeral 7 denotes a detection coil for detecting an NMR resonance signal in the subject, and its configuration is the same as the excitation coil shown in FIG. Note that it is desirable that this detection coil be installed as close to the subject as possible;
If necessary, it may also be used as an excitation coil.

71 is an NMR resonance signal (F
ID: free 1induction deca
y), 72 is a phase detection circuit, and 73 is a wave memory circuit that stores the phase-detected waveform signal from the amplifier 71, and includes an A/D converter. 8 is a computer that inputs the signal from the wave memory circuit 73 via a transmission line 74 made of, for example, an optical fiber, and performs predetermined signal processing to obtain a tomographic image; 9 is a television that displays the obtained tomographic image. It is a display device like a John monitor.

The operation of the apparatus configured as described above will be explained with reference to FIG.

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 finally supplies a current to the Z gradient magnetic field coil 31 via the control circuit 4 to provide a Z gradient magnetic field Gz0 as shown in FIG. 9(b). Also, with Gz 0 being given, the gate circuit 61 is opened and the signal from the oscillator 6 is applied to the excitation coil 5 via the amplifier 62. 9
0'' pulse to excite one side of the object.
In the figure (b), <GZ- following Gz+ is a waveform signal for matching the phases of NMR resonance signals from different parts of the subject, and this technique is a known technique.

At this point to, the magnetization M changes direction by 90' in the y' direction. Next, current is applied to the X gradient magnetic field coil and the Y gradient magnetic field coil 32, 33, and as shown in FIG.
As shown in (d), magnetic fields Gx and Gy of a predetermined magnitude are applied, and an NMR resonance signal as shown in FIG. 9 (e) obtained from the detection coil 7 is detected.

The NMR resonance signal detected by the detection coil 7 gradually attenuates with time, and this signal is amplified by an amplifier 71, phase detected by a phase detection circuit 72, and applied to the computer 8 via a wave memory circuit 73. Ru. Here, the NMR resonance signal is Fourier transformed to become a signal of one projection. The operation up to now is the same as that of the conventional device.

After Ts+ time has elapsed until the NMR resonance signal disappears, the controller 60 applies current of opposite polarity to the X gradient magnetic field coil and the Y gradient magnetic field coil, as shown in FIGS. 9(C) and (D). A large current is passed to simultaneously apply magnetic fields Gx and Gy to the subject.

As a result, the dispersed magnetization M begins to gather again, and the detection coil 7 outputs an NMR resonance signal (this signal is called an echo signal) that gradually increases as shown in FIG. 9 (bo).
is detected.

Here, the feature of the present invention is that the gradient magnetic field applied to the time Tsl from the final 90° pulse until the polarity of the gradient magnetic field is reversed is T m 1 and its intensity is gXx,
gyl, the time T m 2 of the gradient magnetic field applied to the subject during the time T S 2 from reversing the polarity of the gradient magnetic field until applying the next 90°-X pulse is equal to the previous time T
m shorter than t, and its slope flI! 7's strength gX2, g)'2 is the strength of the previous gradient magnetic field gxt, gyt
It's about having more control.

When this is done, the NMR signal will be changed to an echo signal that appears intensively in a short period of time at time T S 2, as shown in Figure 9 (Hon).In particular, the relationship between the time of applying the gradient magnetic field and the intensity of nuclear magnetic resonance. By doing so, it becomes possible to observe the echo signal in a short time.

In addition, if the time from applying the first high-frequency pulse (90° pulse) to applying the second high-frequency pulse (90°-X pulse) is shortened, the spin spread due to the relaxation time T2 of the specimen will be reduced. The signal decreases, and the next time you observe it, the signal will become larger.

After a certain time Td elapses from this state, the magnetization M coincides with the Z' axis due to relaxation. Here, since the magnetization M is only slightly dispersed from the Z' axis at the time t3, the time Td from the time t3 until the magnetization M coincides with the Z' axis is equal to the relaxation time T1. It is sufficiently short in comparison, for example 4
It may be about Ts. When the time Td has elapsed, the first
The second sequence is completed, and the same sequence is repeated thereafter. In each sequence, Gx, Gy given to the subject
is changed little by little, and an NMR resonance signal and an echo signal are obtained from the detection coil 7 for each sequence, that is, for each projection.

In each sequence, for example, the computer 8 performs Fourier transform on the first output NMR resonance signal or echo signal which is a time-reversed wave, and uses a known method similar to X-ray CT (for example, filteredback pr
A reconstruction calculation is performed by the following command (ojction) to obtain a cross-sectional M image, which is displayed on the display 9.

In this way, the present invention can shorten the period of one sequence, regardless of the information obtained.

Next, application of the present invention to other embodiments will be explained.

FIG. 10 is a waveform diagram showing a pulse sequence of an embodiment of the present invention when applied to a two-dimensional Fourier method. In this example, positive and negative gradient magnetic fields Gx and Gy are sequentially applied between applying the first high-frequency pulse (90° pulse) and applying the second high-frequency pulse (90°-X pulse). It is sufficient to change the strength of the positive and negative gradient magnetic fields only for the gradient magnetic field in the X direction. This method also allows the overall observation period to be shortened.

The gradient magnetic field Gx in the X direction is controlled so that its amplitude gradually decreases over time, and the gradient magnetic field cy in the X direction is controlled so that its amplitude gradually increases over time.

FIG. 11 is a pulse sequence waveform diagram of the method according to the embodiment of the present invention when applied to the selective excitation line method. This method consists of a first high-frequency pulse (180°
A 90° pulse is applied prior to the 180° pulse, and a gradient magnetic field Gz is applied in two directions simultaneously with the 90° pulse.
It is configured to apply a gradient magnetic field Gx in the X direction at the same time as the pulse, and in this method as well, the strength of the positive and negative gradient magnetic fields is controlled only for the gradient magnetic field in the X direction.

The present invention can also be practiced using this method.

FIG. 12 is a pulse sequence waveform diagram of a method according to an embodiment of the present invention when the present invention is applied to a modified method of the two-dimensional Fourier method. In this example, a positive and negative gradient magnetic field is repeatedly applied between applying the first pulse and applying the second pulse.

FIG. 13 is a pulse sequence waveform diagram when the present invention is applied to another modified method of the two-dimensional Fourier method. This method is a method in which a gradient magnetic field in the X direction and a gradient magnetic field in the X direction are repeatedly applied alternately, and the present invention can be similarly implemented with this method.

FIG. 14 shows a method in which the two-dimensional Fourier method modification method explained in FIG. 12 or FIG. 13 is combined with the inversion/recovery method.
It is the same as FIG. 2 or FIG. 13.

The present invention can be implemented in this method as well.

To show the results of detection for an example of numerical values, the magnitude of the signal is ■, the longitudinal relaxation time of the subject is T1, and the transverse relaxation time is T2.
Then, there is a relationship as follows (5). As an example of numerical values, for egg white of a chicken egg, T1 = 713 mS, T2 = 124 mS.

Tst = 10mS, TS2 = LmS.

Td=5Qm3 Then, V=to×0.309 becomes. On the other hand, in the conventional method, Tsz = 10mS, TS2 = 10mS.

Then, V=KX0.209. For this example some time TS=TS1
FIG. 15 shows the results of calculations for +TS2 as K-1.

As explained above, according to the present invention, the time for applying a gradient magnetic field of opposite polarity in one sequence is shortened,
The overall sequence can be shortened. Even with this time reduction, information about relaxation time and atomic density can be obtained in the same way as before, and no information is lost. Furthermore, the shortening of the time reduces the spread of spins due to the relaxation time T2, which has the advantage of increasing the signal at the next observation.

[Brief explanation of drawings]

FIG. 1 is a diagram explaining the spin of a hydrogen atom. Figure 2 is a schematic diagram of the magnetic moment of a hydrogen atom. FIG. 3 is a diagram illustrating a state in which the nuclei of hydrogen atoms are aligned in the direction of a magnetic field. FIG. 4 is a diagram showing an example of an inspection pulse waveform by NMR. FIG. 5 is a diagram showing magnetization M in a rotating coordinate system. Figure 6 is a pulse sequence waveform diagram that returns magnetization to a thermal equilibrium state due to magnetic field reversal. FIG. 7 is a configuration diagram of an apparatus according to an embodiment of the present invention. FIG. 8 is a structural diagram showing an example of a magnetic field coil. FIG. 9 is a pulse sequence waveform diagram of the method according to the embodiment of the present invention. 10 to 14 are pulse sequence waveform diagrams when the present invention is implemented in various methods. FIG. 15 is a diagram showing, by simulation, an example of observation results obtained by the method of the embodiment of the present invention. Patent applicant: Yokogawa Hokushin Electric Co., Ltd. - Agent: Naotaka Ide, patent attorney

Claims (1)

[Claims] (1) Applying a uniform static magnetic field to the subject, applying a first high-frequency pulse for imparting nuclear magnetic resonance to the nuclei of atoms constituting the tissue of the subject, and generating nuclear magnetic resonance signals in the nuclei. The free induced vibration is measured from the high energy level at which this atomic nucleus resonates to the low energy level at which it is in thermal equilibrium.
One sequence is constructed by applying a second high-frequency pulse to the atomic nucleus after the elapse of Td, and after a second time Td elapses, the above sequence is repeated. A gradient magnetic field is applied during the first gradient/magnetic field application time Tmx set during the first time Ts of , and the above gradient magnetic field is applied during the second gradient magnetic field application time Tm2 following the first gradient magnetic field application time. In a nuclear magnetic resonance inspection method in which a gradient magnetic field having a polarity opposite to that of the gradient magnetic field is applied, the application of the gradient magnetic field is such that the application time Tm2 of the second gradient magnetic field is equal to the application time Tml of the first gradient magnetic field. The magnitude of the gradient magnetic field applied during the application time t2 of the second gradient magnetic field is equal to the application time T m of the first gradient magnetic field.
1. An inspection method using nuclear magnetic resonance, characterized in that the magnetic field is set to be larger than the magnitude of the gradient magnetic field applied to