JPS6024464A - Inspection method by employing nuclear magnetic resonance - Google Patents

Abstract

PURPOSE:To shorten an inspection time by shortening a pulse sequence, by applying 1st-3rd high frequency pulse trains to an object to be inspected under a static magnetic field and a gradient magnetic field at predetermined time intervals, and increasing the intensity of the gradient magnetic field within the time interval between 2nd and 3rd pluses. CONSTITUTION:A static magnetic field control circuit 2 flows a current to a static magnetic coil 1 under the control of a controller 60 to apply a static magnetic field to an object to be inspected while a gradient magnetic field control circuit 4 flows a current to a gradient magnetic field coil 31 to apply a Z-gradient magnetic field. On the other hand, a gate circuit 61 is opened and the RF signal from an oscillator 6 is applied to an exciting coil 5. The NMR signal detected by a detection coil 7 receives image treatment in a computer 8 through a preamplifier 71, a phase detector 72 and a wave memory circuit 73. In this case, the time interval of 2nd and 3rd pulses of a three-pulse sequence is made shorter than that of 1st and 2nd pulses and a gradient magnetic field is made stronger than the previous one to shorten a pulse sequence and an inspection time.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to nuclear magnetic resonance
ic reso-nan, ce) (hereinafter referred to as rN
It is abbreviated as MRJ. ) This invention relates to an inspection method and an inspection apparatus using nuclear magnetic resonance that utilizes the phenomenon to determine the distribution of specific atomic nuclei within a subject from outside the subject. In particular, it 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 a nuclear spin angular momentum I.

Figure 1 shows a hydrogen nucleus (1H), which consists of one proton P, as shown in (a), and has a spin quantum number of %.
It has a rotation represented by . Here, proton P is (b)
As shown in , since it has a positive charge e, 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 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 A, it is divided into two energy levels, -2 and +2, as shown in Figure 3 (b). The energy difference ΔE between two energy levels is expressed by the formula +11.

ΔE−γ7′lHO (1) where γ: gyromagnetic ratio) 1=h/2π h blank constant Ho: magnetic field strength where each atomic nucleus has μXH due to the static magnetic field Ha.

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-THO (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 (1
It absorbs energy corresponding to the energy difference ΔE shown in equation 1 and transitions to a higher energy level. Even if several types of atomic nuclei with nuclear spin angular momentum coexist, each atomic nucleus has a different gyromagnetic ratio T, so 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 T2 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 ease of energy exchange between spins and between molecular systems (lattice) is about the same. Therefore times T1 and T
2 becomes almost equal value. 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 the hydrogen nucleus (1H), but it is possible to perform similar measurements with other atomic nuclei that have nuclear spin angular momentum. It is applicable to sodium nuclei (23N a ), fluorine nuclei ('?F), oxygen nuclei ('70), 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 inside the subject. I can do it.

Conventionally, inspection equipment using such NMR excites protons in a virtual cross-section of the subject using the same principle as X-ray CT, and generates NM corresponding to each projection.
There is a method in which the R resonance signal is measured in many directions of the object, and 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. The angular velocity is ω-γ(H, +z-Gz) ・
......(3). Therefore, only the protons on the x-7 plane where 2 satisfies this are excited, and the magnetization M
If it is shown on a rotating coordinate system rotating at an angular velocity ω as shown in FIG. Next, as shown in Figure 4 (c) and (d), an X gradient magnetic field Gx and a y gradient magnetic field Gy are applied, thereby creating a two-dimensional gradient magnetic field, and detecting an NMR resonance signal as shown in (e). do. 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. When the NMR resonance signal obtained in this manner 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 ay.

Thereafter, in the same way, wait a predetermined time τ' and repeat the next sequence. In each sequence, G
Change x and Gy little by little. 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 (9o° pulse), the magnetization M
without waiting until it reaches a state of thermal equilibrium due to the relaxation time T1,
The phase is 90" different, 180"-x without the hull,
He invented a method in which the spin movement is reversed to generate an echo, and at the peak of this echo, a 90'' pulse is used to return the magnetization to two directions again (Patent Application 1982-). Figure 6 shows the operating waveform of this method. That is, as shown in FIG. 6(A), after a time T S 1 has elapsed since the application of the first high-frequency pulse (90° pulse), the second high-frequency pulse (180”- x pulse) is applied, and further time T S
2, then the third high frequency pulse (90° pulse)
is applied to form one sequence, and after a third time Td has elapsed, the above sequence is repeated. This first time T S 1 and the second time T
In S2, as shown in FIGS. 6(c) and 6(d), gradient magnetic fields Gx and Gy are applied for times 'l'ml and Tm2, respectively. According to this method, by observing the NMR signal intensity, the relaxation times T1 and T2 and the nuclear density can be measured without waiting for the relaxation time.

Applying the first to third high frequency pulses in this way,
In a method of observing NMR signals, the inventor realized that controlling the strength of the gradient magnetic field can shorten the time between applying the second radio frequency pulse and applying the third radio frequency pulse. .

That is, an object of the present invention is to provide a method for shortening the sequence for applying multiple high-frequency pulses and shortening the time required to perform nuclear magnetic resonance examination.

The present invention is characterized in that the intensity of the gradient magnetic field applied to the subject is increased during the time from when the second high-frequency pulse is applied until when the third high-frequency pulse is applied to shorten 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 static magnetic field coil for generating a uniform static magnetic field Ho (the direction in this case is the Z direction), and 2 is a control circuit for this static magnetic field coil 1, for example, a DC stabilized power supply. Contains. The density Ho of the magnetic flux generated by the static magnetic field coil 1 is about 0.1T to 0.2T, and the uniformity is 10-
'F or more 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 a Z gradient magnetic field coil 31, a y gradient magnetic field coil 32 are shown.
．． 33. Although not shown, y gradient magnetic field coil 32.33
It has the same shape as , and includes an X-gradient magnetic field coil that is rotated by 90°. This gradient magnetic field coil generates a magnetic field that is magnetic in the same direction as the uniform static magnetic field HO and has a linear gradient in each of the x, y, and z axis directions. 60 is the control circuit 4
controller.

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 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 the NMR resonance signal in the subject; its configuration is the same as the excitation coil shown in FIG. . 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 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. 9.

First, the control circuit 2 applies a current to the static magnetic field coil 1 to apply a static magnetic field Ha to the subject (the subject is placed within the cylinder of each coil). In this state, the controller 60 first supplies a current to the Z gradient magnetic field coil 31 via the control circuit 4 to provide a Z gradient magnetic field Gzf as shown in FIG. 9(b). Also, while Gz+ is 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, resulting in a narrow spectrum as shown in FIG. 9(a). One side of the subject is excited with a 90° pulse. In addition, the 9th
In Figure (b), following Gzf (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 is 90° in the y' direction.
change direction. Next, the current is extended 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 751 hours have elapsed until the NMR resonance signal disappears, the controller 60 supplies current to the Z gradient magnetic field coil 31 again, and generates a Z gradient magnetic field Gz as shown in FIG. 9(b).
- at the same time, the gate circuit 61 is opened and a current is passed through the excitation coil 5. This time, as shown in FIG.
(inverted phase of the signal from) is applied. Subsequently, as shown in FIGS. 9(c) and 9(d), current is applied to the X gradient magnetic field coil and the X gradient magnetic field coil to create a magnetic field Gx,
Gy is applied simultaneously.

When the 18.0"-X pulse is applied, the dispersed magnetization M begins to gather again, and the detection coil 7 outputs an NMR resonance signal (this signal is referred to as an echo signal) that gradually increases as shown in FIG. ) is detected.

Here, the characteristics of the present invention (tun, first 90°
The gradient magnetic field applied to the time Ts1 from the pulse to the 180°-X pulse is time T m 1 and its strength is gXlgyl
However, the next 90° from the spelling 180°-X pulse
Time T of gradient magnetic field given to time until pulse T S 2
m 2 is shorter than the previous time 'l'' ml, and the strength of the gradient magnetic field g 'X 2 , g 72 is the previous intensity g
The point is to control it to a greater extent than x1 and gy2.

When controlled in this way, the N M'R signal becomes as shown in Fig. 9 (E).
As shown in , concentrating on a short time at time T S 2,
An echo signal appears immediately. In particular, by setting the relationship between the time for applying the gradient magnetic field and the strength of the gradient magnetic field, it is possible to observe echo signals in a short time.

Furthermore, when the time from the 180° pulse to the 90'' pulse becomes shorter, the spread of spins due to the relaxation time T2 of the object becomes smaller, and the signal at the next observation becomes larger.

After a certain time Td has elapsed 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 Ts has elapsed, the first sequence ends, and the same sequence is repeated thereafter.

For each sequence, ie for each projection, an NMR resonance signal and an echo signal are obtained from the detection coil 7.

In each sequence, for example, the computer 8 performs Fourier transform on the first output NMR resonance signal, the echo signal which is a time-reversed wave, or the sum of both, and performs a Fourier transform using a known method similar to X-ray CT (for example, filtered
ack projection) to perform reconstruction calculations and obtain a tomographic 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 in the method of the embodiment of the present invention when an inversion recovery method is applied in addition to the PR method shown in the above embodiment.

This example is a method in which a 180° pulse is applied once at time T S oO prior to applying the first high frequency pulse (90° pulse). The present invention can also be practiced using this method.

FIG. 11 is a pulse sequence waveform diagram when the present invention is implemented in a three-dimensional PR method. In this example, the first high frequency pulse is 9
0” pulse and the second high frequency pulse is 180°-
It is an X pulse, and the third high frequency puffless is a 90° pulse. The present invention can be implemented in this manner as well.

FIG. 12 is a pulse sequence waveform diagram when the present invention is applied to the Fourier method. In this example, the first high frequency pulse is 90
The second high-frequency pulse is a 180°-X pulse, and the third high-frequency pulse is a 90° pulse, but the method of the present invention is characterized by the way the gradient magnetic field is applied. Implement. The present invention can also be implemented in this method to obtain similar effects.

FIG. 13 is a pulse sequence waveform diagram when the present invention is implemented in a three-dimensional Fourier method. In this example, the Fourier method shown in FIG. 12 is applied to three dimensions. The invention is implemented for a gradient magnetic field Gy. Similar effects can be obtained by this method as well.

FIG. 14 is a pulse sequence waveform diagram when the present invention is implemented in the selective line method. This method is such that the first high frequency pulse is a 180° pulse, the second high frequency pulse is a 180°-X pulse, the third pulse is a 90' pulse, and furthermore, the 90' pulse is ° A pulse is given.

The invention is implemented for a gradient magnetic field Gy. Similar effects can be obtained by this method as well.

FIG. 15 is a pulse sequence waveform diagram when the present invention is applied to the spin warp method. In this method, the first high frequency pulse is a 90° pulse, and the second high frequency pulse is a 180° pulse.
The third pulse is a 90' pulse.

The feature is that the magnitude of the gradient magnetic field Gx is changed each time it is repeated, and that positive and negative magnetic fields are applied to the gradient magnetic field Gy. Similar effects can be obtained by applying the present invention to this method as well.

FIG. 16 is a pulse sequence waveform diagram of a method in which the 180° echo method is used in combination with the PR method, and the present invention can also be implemented in this method. In this method, the time Ts from when the second high-frequency pulse is applied until when the third high-frequency pulse is applied is
Second, the gradient magnetic fields Gx and Gy can be controlled to increase their intensity only during time T m 2, thereby shortening time Tm2.

At this time, the time relationship of It is better to

Figure 17 shows the method shown in Figure 16 combined with the inversion recovery method.
°It gives a pulse. The present invention can be implemented in this method as well.

FIG. 18 is a pulse sequence waveform diagram of a method in which a 180° echo method is used in combination with a three-dimensional PR method, and the present invention can also be implemented in this method. In this method, the first high frequency pulse is applied and then the second high frequency pulse is repeatedly applied, but the repetition period of the second high frequency pulse can be shortened by implementing the present invention.

FIG. 19 shows a combination of the two-dimensional Fourier method and the 180° echo method, and the present invention can also be implemented with this method. This method is also a method of repeating the second high-frequency pulse, and the repetition period can be shortened.

FIG. 20 shows a combination of the three-dimensional Fourier method and the 180° echo method, and the present invention can also be implemented with this method.

FIG. 21 shows a method of applying an inverted spark prior to the first high-frequency pulse using the method shown in FIG. 19 or 20, and the present invention can be implemented in this method as well.

FIG. 22 shows a combination of the spin warp method and the 180° echo method, and the present invention can also be implemented with this method. Although this method is also a method of repeatedly applying the second high-frequency pulse, the repetition period of the second high-frequency pulse can be shortened according to the present invention.

The results of considering an example of numerical values are as follows: The signal size is ■, the longitudinal relaxation time of the subject is T1, and the transverse relaxation time is T2.
Then, there is the following relationship.

As an example of numerical values, for the egg white of a chicken egg, T1
=713mS, T2 =124mS.

Tst = 1 0mS, TS2 = 1mS.

When Td=50mS, V=KXO, 309. On the other hand, in the conventional method, if TSI=lOmSsTs2=10mSs, then V=KX0.209. For this example some time Ts = Ts1
For +T's2, the calculation results are shown in FIG. 23 with K=1.

A, B, and C are cases where Ts is 5.10.20 m5, respectively.

As explained above, according to the present invention, the time from applying the second high-frequency pulse to applying the third high-frequency pulse is shortened in one sequence, and the sequence can be shortened. . Even with this time reduction, information about relaxation times and atomic densities can be obtained as before, and no information is lost. Furthermore, since the time until the third high-frequency pulse is applied is shortened, the spread of spins due to the relaxation time T2 is reduced, which has the advantage of increasing the signal at the next observation.

[Brief explanation of the drawing]

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. FIG. 6 is a diagram showing a pulse sequence of the conventional method. FIG. 7 is a diagram showing an example of the configuration of an apparatus for implementing the present invention. FIG. 8 is a structural diagram showing an example of a magnetic field coil. FIGS. 9 to 22 are pulse sequence waveform charts according to an embodiment of the present invention. FIG. 23 is a diagram showing an example of a measurement result obtained by a method according to an embodiment of the present invention based on a simulation. Patent Applicant Yokogawa Hokushin Electric Co., Ltd. Agent Patent Attorney Naotaka Ide (B) (B) (B) (B) (B) (B) Figure 40 Figure 6 Figure 7 Figure 8 Figure 11 Fig. 12 Fig. 13 Fig. 14 El Fig. 15 Fig. 17 =] S-E upper 5 Fig. 20 Fig. 21 Fig. 22

Claims (1)

[Claims] (1) A first high frequency wave for applying a gradient magnetic field to the subject in at least one of the three-dimensional axes x, y, and z directions to impart nuclear magnetic resonance to the nuclei of atoms constituting the tissue of the subject. a first time Ts after applying the first high-frequency pulse;
a second high-frequency pulse is applied to the atomic nucleus after a lapse of time T S 2; and a third high-frequency pulse is further applied to the atomic nucleus after a second time T S 2 has elapsed. , measuring the nuclear magnetic resonance signal generated in the nucleus during the sequence to measure the free induced damped oscillations from the high energy level at which the nucleus resonates back to the lower energy level at thermal equilibrium; After a third time Td elapses, the above sequence is repeated, and the application of the gradient magnetic field is set during the first time T S 1 and the second time T S 2 of the sequence, respectively. First gradient magnetic field application time T m
1 and the second gradient magnetic field application time T m 2, and furthermore, the above measurement is performed on a number of parts of the subject in each sequence to measure the intensity of the nuclear magnetic resonance signal, and the measurement results for the number of parts are calculated. In the method of obtaining two-dimensional and/or three-dimensional images, the application of the gradient magnetic field is performed by applying the gradient magnetic field to The application time T m 2 of the second gradient magnetic field is the application time T m 1 of the first gradient magnetic field.
The application time of the second gradient magnetic field T m
The strength of the gradient magnetic field applied at 2 is set to be greater than the strength of the gradient magnetic field applied during the application time Tm1 of the -th gradient magnetic field, and the first time TS1 is set to An inspection method using nuclear magnetic resonance characterized by setting a value shorter than 'ps2.