JPS6240658B2 - - 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 a nuclear spin angular momentum I.

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, a static magnetic field in the Z direction is applied to such a material.
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=γ〓H ₀ ...(1) where γ: gyromagnetic ratio=h/2π h=blank constant Here, a force of μ×H ₀ is applied to each atomic nucleus by the static magnetic field H ₀ Therefore, the nucleus moves 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. 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 ( ¹ H), but it is possible to perform similar measurements with other atomic nuclei that have nuclear spin ^angular momentum. Nucleus (
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, inspection equipment using NMR excites protons in a virtual cross-section of the subject using the same principle as X-ray CT, and generates NMR resonance signals corresponding to each protrusion of the subject. There is a method that calculates the NMR resonance signal intensity at each position of the object using 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 Figure 4 (b) and an RF pulse (90° pulse) with a narrow frequency spectrum as shown in (a) are applied to the subject.
In this case, Larmor angular velocity ω = γ (H ₀ + ΔGz)
If only the protons on the plane are excited, and the magnetization M is expressed on a rotating coordinate system rotating at ω as shown in Figure 5, the direction will be changed by 90 degrees in the y' axis direction as shown in the figure. . Next, as shown in Figure 4 C, x
An axial gradient magnetic field Gx is applied for a predetermined time tx, thereby gradating the phase of the magnetization M in the x-axis direction as shown in equation (3).

γLx∫txdt・Gx=2πh ...(3) However, γ: Magnetic rotation 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', y' plane, and the As shown in , it disappears after τ time has elapsed.

Hereafter, wait for τ′ time until the state returns to thermal equilibrium,
Repeat the following 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.

In the method according to the present invention, the electromagnetic waves applied to the subject are (90°) → (180°) n (n is 1, 2, 3
The feature is that an echo signal train is created using a pulse sequence of (...), and the image is reconstructed using each echo signal.

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 is a magnetic field in the same direction as the uniform static magnetic field H ₀ and generates a magnetic field having linear gradients in the x-, y-, and z-axis directions, respectively. 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 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, 4.26M for protons)
Hz/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. 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. 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 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 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 H ₀ 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 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,
90 has a narrow spectrum as shown in Figure 8 A.
゜Excite one side of the object with a pulse.

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. 9A. Next, a current is applied to the x-gradient magnetic field coil 32 for a predetermined time ^tx1 , and as shown in FIG.
Apply only As a result, the phase of magnetization M is graduated in the x-axis direction as shown in equation (3). Subsequently, as shown in FIG. 8 D, a y-axis gradient magnetic field Gy is applied for a predetermined time ty ¹ , and under this, the NMR resonance signal is converted into data E by the detection coil 7 as shown in FIG. 8 E. Detected as ₁ . Here, at the time when the NMR resonance signal is detected (e.g. at time t ¹ ),
Since the magnetization M is gradually dispersing in the direction of the dashed arrow as shown in FIG. 9B, the NMR resonance signal detected by the detection coil 7 gradually attenuates with time. 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. Note that the y-axis direction is scaled using the Larmor frequency.

After a predetermined time τ has passed since the application of the 90° pulse, the controller 60 causes current to flow through the z-gradient magnetic field coil 31 again to apply a z-gradient magnetic field Gz as shown in FIG. is opened, current is applied to the excitation coil 5, and this time the 8th
As shown in Figure A, a 180°x' pulse is applied to excite the same surface of the subject. Next, as shown in Fig. 8D, a current is passed through the y gradient magnetic field coil, and a magnetic field Gy of the same magnitude as the previous time is applied for a predetermined time ty ¹ , and then as shown in Fig. 8C. As before, a current is applied to the x-gradient magnetic field coil for a predetermined time tx ¹ , and the gradient magnetic field in the x-axis direction is generated as shown in Figure 8 (c).
Gx is applied for a predetermined time tx ¹ .

When a 180°x′ pulse is applied, the dispersed magnetization M
begins 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 as data _E1 '. be done. After a time τ has elapsed since 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.

After a time τ has elapsed since the application of the 180° x' pulse, the echo signal reaches its maximum, and at this point, the x gradient magnetic field Gx is applied by tx ² as shown in FIG. 8C. Next, as shown in Figure 8 D, the y gradient magnetic field is
Gy is applied thereto, and under this, the NMR resonance signal is detected as data _E2 by the detection coil 7, as shown in FIG. 8E. Furthermore, after 2τ time has passed after applying the 180° -x' pulse (180° -x pulse is the signal from oscillator 6 with the phase reversed)
Apply. Subsequently, as shown in FIG. 8D, a y gradient magnetic field Gy is applied in the same manner as before, and then an x gradient magnetic field Gx of tx ² is applied as shown in FIG. 8C.

When a 180°-x' pulse is applied, the dispersed magnetization M begins to gather again as shown in FIG. 9D, and from the detection coil 7, as shown in FIG.
An echo signal that gradually increases under the given conditions is detected, and this is detected as data E ₂ '.

After a time .tau. has elapsed since the application of the ^180.degree . Subsequently, as shown in FIG. 8D, a y gradient magnetic field Gy is applied, and under this, the NMR resonance signal is detected as data _E3 by the detection coil 7, as shown in FIG. 8E.

After that, in the same way, 180°x′ pulse and 180°−
Electromagnetic waves of x' pulses are applied alternately with a period of 2τ, and the application time of the x gradient magnetic field Gx is tx ¹ , Each data E ¹ _, E ₂ , _{E 3} ^... (or E ₁ ′,
E ₂ ', E ₃ '...) are sequentially detected as one group.

Envelope of NMR resonance signal (echo signal) (8th
The dashed line in Figure E) is attenuated by the transverse relaxation time T ₂ ,
Such operations can be repeated in one sequence while the NMR resonance signal is being obtained, and when the NMR resonance signal becomes weak,
Apply a 90° pulse of electromagnetic waves again under the z-gradient magnetic field to excite it, and move on to the next sequence.

Computer 8 has 180°x′ pulse, 180°−
NMR obtained before and after applying x′ pulse
Input resonance signal data E ₁ , E ₂ , E ₃ ... and echo signal data E ₁ ′, E ₂ ′, E ₃ ′ ..., and for example, combine N pieces of data E ₁ , E ₂ , E ₃ ... into one As a group, a two-dimensional Fourier transform operation is performed to obtain an image, which is displayed on the display 9.

Note that although it is assumed in the above that the computer 8 does not use the data E ₁ ′, E ₂ ′, E ₃ ′, etc. based on the echo signals, it is also possible to use these data. . In this case, the usage may be as follows, for example.

(i) Data E ₁ (E ₂ , E ₃ …) and data E ₁ ′ (E ₂ ′,
E ₃ ′...) is calculated, and using this as one data, a two-dimensional Fourier transform calculation is performed to obtain one tomographic image.

(ii) Since the envelope of the echo signal is attenuated by the relaxation time T ₂ , the proton density image is obtained using the data E ₁ (E ₂ , E ₃ …), and the data E ₁ ′ (E ₂ ′, E ₃ ′…) to create T ₂ image (T ₂
is caused by the interaction of the spins of nearby electron nuclei).

(iii) Time during which the x gradient magnetic field Gx is applied before and after the 180°x′ pulse and the 180°−x′ pulse
tx ¹ , tx ² , tx ^{3 .} . . are all equal to tx, and all or some of each data in one sequence is averaged to form one data. In this case, high speed is lost, but the S/N ratio is significantly improved.

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

In addition, in the above explanation, the pulse sequence of electromagnetic waves applied to the subject is (90°) → (180°x') →
We have explained the case of (180°−x′) → (180°x′) → (180°−x′)…, but instead of this, (90°) →
A pulse sequence of (180°y')→(180°y')→(180°−y')... may also be used.

In the example of Fig _. 8, the peak amplitude A (corresponding to the envelope) of _the NMR resonance signal (echo signal) shown in ) attenuates. Therefore, the total measurement time is the relaxation time
If it is sufficiently shorter than T ₂ , this effect will not be a problem. If the total measurement time is relatively long (the measurement time is
(If the length is not sufficiently shorter than T ₂ ), T ₂ will affect the proton density image if left as is, which is undesirable. In this case, by taking the following method,
Proton density images without the influence of _T2 can be obtained. That is, in Fig. 8 E, the data
t in the case of En and data En' is 2(n
-1) τ, 2(n-1) τ+τ, so the two data En, En' are Fourier transformed and T ₂ of each point on the frequency axis (on the projection n) is obtained from the above equation. Then, by extrapolating En to t=0, it is possible to obtain data on only the proton density from which the influence of T ₂ attenuation has been removed.

Figure 10 shows that when using the pulse sequence of (90°) → (180°y′) → (180°−y′) → (180°y′) → (180°−y′)... Each time point t ₀ shown in the figure,
It shows the direction of magnetization M at t ₁ , t ₂ , and t ₃ .

Here, the 180° y' pulse represents the signal from the oscillator 6 whose phase is delayed by 90°.

FIGS. 11 and 12 are operation waveform diagrams showing other examples of the method according to the present invention.

FIG. 11 shows a case where the present invention is applied to a method called a three-dimensional Fourier transform method. here,
As shown in Figure 11 b, c, and d, (180°x'),
(180°−x′) Before and after the pulse, the z gradient magnetic field
Gz, x gradient magnetic field Gx, and y gradient magnetic field Gy for a predetermined time tz ¹ , tz ² ..., tx ¹ , tx ² so that they do not overlap, respectively.
..., ty ¹ , ty ² ... are controlled and applied. Further, as the electromagnetic waves applied to the subject, a rectangular wave pulse signal is used as shown in FIG. 11A.

The method shown in Fig. 12 differs from the method shown in Fig. 8 or Fig. 11 in that, before applying the 90° pulse to the subject (τ'' time), as shown in Fig. 12 A,
It applies a 180° pulse of electromagnetic waves. Here, after applying a 180° pulse,
The time τ'' required to apply 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 time τ'' Due to the T ₁ relaxation of , the intensity of the NMR signal changes, and a T ₁ image can be obtained from this.

In each of the above embodiments, the x gradient magnetic field Gx is applied in FIG. 8, and the x gradient magnetic field and the z gradient magnetic field Gz are applied in ^FIG .
By controlling tx ² , tz ¹ , tz ² , . These gradient magnetic fields Gx and Gz may be controlled so as to.

FIG. 13 shows the x gradient magnetic field in the example of FIG.
It is an operation waveform diagram when the application time tx during which Gx is applied is constant and the magnitude is changed as Gx ¹ , Gx ² , Gx ³ . . . In this case, a y-gradient magnetic field Gy- with the opposite polarity is applied at the same time as the x-gradient magnetic field Gx is applied, and Gy ₊ is applied after magnetization M is dispersed to obtain a spin echo with a good S/N. I'm trying to get a signal.

As explained above, in the method according to the present invention, after exciting the subject with a 90° pulse of electromagnetic waves,
→(180°−x′) or (180°y′)→(180°−
By repeating the pulse sequence of It is possible to obtain tomographic images related to specific nuclear distribution, etc. In addition, two pieces of data E ₁ (E ₂ , E ₃ ...) E ₁ ′ (E ₂ ′, E ₃ ′ ...) can be obtained before and after applying the (180°) pulse from the subject. By using each signal, 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 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. 1 to 13 are operation waveform diagrams showing other examples of the method of the present invention. 1... Coil for static magnetic field, 2... Coil control circuit for static magnetic field, 3... Coil for gradient magnetic field, 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 at a frequency that induces nuclear magnetic resonance in the subject, thereby generating a nuclear magnetic resonance signal (NMR signal) from the subject.
1. An inspection method using nuclear magnetic resonance, in which the distribution of specific atomic nuclei within a subject is known by performing predetermined arithmetic processing on the obtained nuclear magnetic resonance. (i) A process in which a 90° pulse electromagnetic wave and a z-gradient magnetic field are simultaneously applied to the subject to excite the subject. (ii) After the step (i), a step of applying an x gradient magnetic field Gx of a predetermined magnitude for a predetermined time tx. (iii) Following step (ii), a y-gradient magnetic field of a predetermined magnitude is applied.
A step of applying Gy for a predetermined time ty and detecting the first NMR signal under that. (iv) A step of applying a 180° pulse electromagnetic wave after a predetermined time has elapsed after applying the 90° pulse electromagnetic wave in step (i) above. (v) Following step (iv), apply a y-gradient magnetic field Gy of the same magnitude as that applied in step (iii) for the same time ty, and then detect the second NMR signal (echo signal). The process of doing. (vi) Following step (v), a step of applying an x gradient magnetic field Gx of the same magnitude as that applied in step (ii) for the same time tx. (vii) The application time tx of the x gradient magnetic field Gx and the y gradient magnetic field Gy at the time when the second NMR signal reaches the maximum,
The step of changing ty and returning to the steps after (ii) above. 2 Data based on NMR signals and echo signals obtained before and after applying a (180°) pulse
Claims that Fourier transform En and En', obtain T ₂ at each point on the frequency axis, and extrapolate En to t=0 to obtain data from which the influence of attenuation due to T ₂ has been removed. 2. The method for testing by nuclear magnetic resonance according to item 1. 3. By nuclear magnetic resonance according to claim 1, in which the average value of each data based on the NMR signal and echo signal obtained before and after applying the (180°) pulse is calculated, and this is treated as one data. Inspection method. 4. An examination by nuclear magnetic resonance according to claim 1, wherein a proton density image is obtained using data based on NMR signals, and a _T2 image is obtained using data based on echo signals. Method. 5 (180°) The application time or magnitude of the x gradient magnetic field Gx applied to the subject before and after applying the pulse is made equal in one sequence, and
2. The nuclear magnetic resonance examination method according to claim 1, wherein all or some of each data obtained during one sequence is averaged to form one data. 6 Apply a uniform static magnetic field to the subject and apply electromagnetic waves with a frequency that induces nuclear magnetic resonance in the subject, and collect nuclear magnetic resonance signals (NMR signals) from the subject.
1. An inspection method using nuclear magnetic resonance, which obtains the following information and performs predetermined arithmetic processing to determine the distribution of specific atomic nuclei within a subject, which method is characterized by comprising the following steps: (i) A process in which a 90° pulse of electromagnetic waves is applied to the subject to excite the subject. (ii) After the step (i), a step of applying a z gradient magnetic field Gz of a predetermined magnitude for a predetermined time tz. (iii) Following step (ii), an x gradient magnetic field of a predetermined magnitude is applied.
A process in which Gx is applied for a predetermined time tx. (iv) Following step (iii), a y-gradient magnetic field of a predetermined magnitude is applied.
A step of applying Gy for a predetermined time ty and detecting the first NMR signal under that. (v) A step of applying a 180° pulse electromagnetic wave after a predetermined time has elapsed after applying the 90° pulse electromagnetic wave in step (i) above. (vi) Following step (v), apply a y-gradient magnetic field Gy of the same magnitude as that applied in step (iv) for the same time ty, and then detect the second NMR signal (echo signal). The process of doing. (vii) Following step (vi), a step of applying an x gradient magnetic field Gx of the same magnitude as that applied in step (iii) for the same time tx. (viii) Following step (vii), a step of applying a z gradient magnetic field Gz of the same magnitude as that applied in step (ii) for the same time tz. (ix) At the time when the second NMR signal reaches its maximum, the z gradient magnetic field Gz, the x gradient magnetic field Gx, and the y gradient magnetic field Gy
By changing the application times tz, tx, and ty of
(ii) Return to subsequent steps. 7 static magnetic field forming means for applying a uniform static magnetic field to the subject; gradient magnetic field generating means for applying two or more types of gradient magnetic fields to the subject; excitation means for applying pulsed electromagnetic waves to the subject; A control means for controlling a signal given to the excitation means, a means for detecting a nuclear magnetic resonance signal from the subject, and a calculation means for processing the signal from the detection means and performing a predetermined calculation to obtain a tomographic image. An inspection apparatus using nuclear magnetic resonance, wherein the control means causes each of the following steps (i) to (vii) to be performed via the gradient magnetic field generation means and the excitation means. (i) A process in which a 90° pulse electromagnetic wave and a z-gradient magnetic field are simultaneously applied to the subject to excite the subject. (ii) A step of applying an x-gradient magnetic field Gx of a predetermined magnitude for a predetermined time tx. (iii) A step of applying a y-gradient magnetic field Gy of a predetermined magnitude for a predetermined time ty and detecting the first NMR signal thereunder. (iv) A step of applying a 180° pulse electromagnetic wave after a predetermined time has elapsed after applying the 90° pulse electromagnetic wave in step (i) above. (v) A step of applying a y-gradient magnetic field Gy of the same magnitude as that applied in step (iii) for the same time ty and detecting a second NMR signal (echo signal) thereunder. (vi) A step of applying an x gradient magnetic field Gx of the same magnitude as that applied in step (ii) for the same time tx. (vii) The application time of the x-gradient magnetic field Gx and the y-axis gradient magnetic field Gy at the time when the second NMR signal reaches the maximum
A step of changing tx and ty and returning to the steps after (ii) above.