JPH0421490B2 - - Google Patents

Description

[Detailed description of the invention]

B. “Object of the invention” [Field of industrial application] The present invention is based on nuclear magnetic resonance (nuclear magnetic resonance)
resonance) (hereinafter abbreviated as "NMR")
This invention relates to an NMR imaging device that utilizes phenomena 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 devices. [Prior Art] An NMR imaging device places a living body (usually a patient) in a certain magnetic field. Then, a predetermined pulsed electromagnetic wave is applied to the living body to excite only a specific atomic nucleus of interest among the various atoms that make up the living body. Once excited, the atomic nucleus returns to its original energy state, but at this time, external
It emits the absorbed energy as electromagnetic waves.
In an NMR imager, this emitted magnetic field is detected by a coil. This detection signal is a nuclear magnetic resonance signal (NMR signal...echo signal and FID signal: free
It is called induction decay) and contains various information about the target atomic nucleus.
An NMR imaging device is a device that analyzes this and visualizes a part of the living body as a tomographic image, which is useful for diagnosis, treatment, etc. of the living body. First, I will briefly explain the principle of NMR. The atomic nucleus consists of protons and neutrons, and these as a whole are considered to be rotating (rotating) with a nuclear spin angular momentum I→. Figure 2 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. Since the proton P has a positive charge e ⁺ as shown in (b), a magnetic moment μ→ is generated as the nucleus rotates, and each hydrogen nucleus can be regarded as a small magnet. Figure 3 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, etc., 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 substance is subjected to a static magnetic field H ₀ in the Z direction.
When applied, each atomic nucleus aligns in the direction of H ₀ . Figure 4A shows this situation for a hydrogen nucleus. The spin quantum number of hydrogen nucleus is 1/
2, so as shown in Figure 4B, -1/2 and +
It is divided into two energy rankings of 1/2. The energy difference ΔE between the two energy ranks is expressed by equation (1). △E=γ〓H ₀ (1) γ: gyromagnetic ratio (constant specific to each atomic nuclide) 〓: h/2π h: Planck's constant Here, each atomic nucleus is affected by the static magnetic field H→ ₀ , Since the force μ→×H→ ₀ is applied, the nucleus moves around the Z axis as (2)
It precesses at an angular velocity ω as shown in the equation. ω=γH ₀ (Larmor angular velocity) (2) In other words, each type of atomic nucleus precesses at a different Larmor angular velocity ω _n . In this way, a living body placed in a static magnetic field H ₀ is given a frequency corresponding to the Larmor angular velocity ω ₁ (f ₁ =
When an electromagnetic wave (usually a radio wave) of ω ₁ /2π) is applied, resonance occurs in the precessing atomic nucleus corresponding to this frequency f ₁ , and the atomic nucleus experiences an energy difference △E shown by equation (1). absorbs energy equivalent to , and transitions to a higher energy ranking. Here, although a living body is normally composed of multiple types of atomic nuclei, only one type of atomic nucleus resonates with the applied electromagnetic wave of frequency f ₁ under the environment of static magnetic field H ₀ . Therefore, the static magnetic field H ₀ applied to the living body
By selecting the strength of the atomic force and the applied frequency f, it is possible to extract only the resonance of a specific type of atomic nucleus. By measuring the strength of the resonance, we can determine the amount of nuclei present. Furthermore, the atomic nucleus excited to a higher order returns to a lower order after a time determined by a time constant called relaxation time after resonance. At this time, the absorbed energy is released to the outside, so
By measuring the temporal change in resonance strength, the following times can be determined. Relaxation time is classified into spin-lattice relaxation time (longitudinal relaxation time) _T1 and spin-spin relaxation time (transverse relaxation time) _T2 . By observing this relaxation time, data on material distribution can be obtained. In general, in solids, the transverse relaxation time T ₂ is short and the energy obtained by nuclear magnetic resonance is first distributed to the spin system and then to the nucleon system. Therefore, the longitudinal relaxation time T ₁ is significantly larger than T ₂ . On the other hand, in a liquid, molecules move freely, so
The ease of energy exchange between spins and between spins and molecular systems (nucleons) is about the same. Therefore, times T ₁ and T ₂ have approximately equal values. Here, we have explained hydrogen nuclei ( ¹ H), but similar measurements can be performed with other nuclei that have nuclear spin angular momentum, such as phosphorus nuclei ( ³¹ P), carbon nuclei ( ¹³ C), etc. , sodium nucleus ( ^23Na ), 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 carry out various tests within the subject. can be done. Traditionally, using the phase information of NMR signals,
There is an NMR imaging device that uses the Fourier transformation method to obtain images regarding the tissue of a subject. FIG. 5 is an operational waveform diagram for explaining an example of an inspection method for a conventional apparatus using this Fourier transform method. First, a static magnetic field of uniform strength parallel to the z-axis direction
A Z gradient magnetic field G _z as shown in Figure 5B and a high frequency pulse with a narrow frequency spectrum f _j as shown in Figure 5B, that is, an RF pulse (90° pulse) are applied to the object placed in _H do. A gradient field G _z is applied to the living body in the Z-axis direction, and the protons precess at a period proportional to the strength of the magnetic field. Here, only the cross section at a certain position (H ₀ + △G _z ) on the Z axis is
It precesses at the same Larmor angular velocity ω _j =γ(H ₀ +△G _z ) as the frequency of the RF pulse (ω _j =2πf _j ). Therefore, only protons that are precessing at an angular velocity in the vicinity of this frequency as the center frequency are excited. That is, the gradient magnetic field _Gz in the Z-axis direction acts to determine the position of the slice plane of the living body. It should be noted that although the explanation will be made here assuming that a plane perpendicular to the z-axis is taken as the tomographic plane, any plane can be visualized by changing the gradient magnetic field. If the magnetization M of the excited proton is shown on a rotating coordinate system rotating at an angular velocity ω as shown in FIG. In other words, all of the proton magnetizations M on the slice plane parallel to the xy plane are oriented in the y'-axis direction. Subsequently, as shown in FIG. 5C, a gradient magnetic field G _x in the x-axis direction is applied for a predetermined time t _x . This gradient magnetic field
Due to G _x , the slice plane of the living body is in a state where the strength of the magnetic field is inclined in the x direction, as shown in FIG. 7A. Here, consider columns in which the slice plane in FIG. 7A is divided horizontally. Looking at the protons in the area perpendicular to each column (parallel to the y-axis),
Due to the gradient magnetic field G _x , the magnetic field strength differs in each orthogonal area. Therefore, based on equation (2), the gradient magnetic field
The spins of protons in areas where G _x is strong rotate quickly, and the spins in areas where the magnetic field is weak rotate slowly. Then, as shown in FIG. 5C, after time _tx , the gradient magnetic field _Gx becomes zero, so that the strength of the magnetic field in the x-axis direction in FIG. 7A returns to uniformity. Therefore, the rotational speeds of all the spins on the slice plane shown in FIG. 7A are the same. However, during time t _x , the rotational speed was different for each zone, so the gradient magnetic field G _x
Even after it is removed, it rotates at the same speed while maintaining the phase difference caused by G _x , as shown in Figure 7A. In this way, due to this gradient magnetic field G _x ,
The phase of the spin of the magnetization M on the slice plane is calibrated (phase coded) in the x-axis direction. The magnitude of G _x is varied according to equation (3) by changing n for each projection. γL _x ∫ _tx G _x dt=2πn (3) Where, γ: Magnetic rotation ratio L _x : Length of imaging area in x direction G _x : Integrand function t _x : Integral range 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 5 D, a gradient magnetic field G _y in the y axis direction is applied, and under this environment, the result as shown in Figure 5 H is applied. Detect the NMR signal as shown. The applied strength and application time of this gradient magnetic field G _y are the same for each projection, and unlike G _x , the gradient magnetic field G _y is applied throughout the detection of the NMR signal. This time, move toward the slice surface in the direction shown in Figure 7 (b).
Since the gradient magnetic field G _y is applied, the rotation speed of the spins differs for each column. And within this one column, at the same rotation speed,
Although the spins are rotating, the rotational phase of the spins in the x-axis direction nuclei in the column is shifted due to the gradient magnetic field G _x described above. Therefore, the obtained NMR signal is treated as one piece of data. Then, by repeating the process by changing the amount of phase encoding, collecting the same data as above, and performing two-dimensional Fourier transform on this, it is possible to determine the protons at each position in the two-dimensional region, and information about the protons at each position. can be obtained. In other words, for each column, the spins have different frequencies, and within one column, the spins have the same frequency, but due to the amount of phase encoding,
The phase of rotation is different for each part of the depth of the column. Therefore, if the detected signal is subjected to two-dimensional Fourier transform, information on each pixel on the slice plane can be identified. Here, as shown in Figure 6B, the magnetization M gradually disperses in the direction of the arrow in the x', y' plane due to the non-uniformity of the magnetic field, so the NMR signal eventually decreases. As shown in FIG. 5E, it disappears after a period of τ. Thereafter, wait τ' time until the state returns to thermal equilibrium, and then repeat the next sequence. At this time, the predetermined time t _x for applying the gradient magnetic field G _x in the x-axis direction is repeated N times while changing the amount of phase encoding with a value determined by equation (3). Then, obtained by N sequence
By performing two-dimensional Fourier transform on the NMR signal, an in-plane proton density image can be obtained. In the conventional device using the Fourier transform method that operates in this way, the time τ until the NMR signal disappears is 10 to 20 ms in FIG. 5, but the predetermined time τ until moving to the next sequence is
Approximately 1 sec is required due to the longitudinal relaxation time _T1 . Therefore, if the number of divisions N in the x-axis direction is set to about 100, for example, the measurement requires a long time of at least 2 minutes, which is one of the major obstacles to realizing high speed. In order to solve this problem, a known technique {DEFT method; driven
Using the equilibrium fourier transform},
When considering the case of manufacturing a high-speed NMR imaging device, there are the following drawbacks. As conclusion,
It is inappropriate to use the DEFT method in NMR imagers. Note that there is no known technology example that uses the DEFT method in an NMR imaging device. The DEFT method proposed for this NMR analyzer is described in {"Pulsed and Fourier Transform NMR" by Farrer and Betzker: Yoshioka Shoten}.
This DEFT method uses a pulse sequence for speeding up, and is composed of (90° _x ...τ...180° _y ...τ...90° _-x ...T _d ) ⁿ . When performing two-dimensional imaging using this DEFT method, the 90° pulse excites only a specific slice plane using a selective excitation method (simultaneously applying a gradient magnetic field), but there is no problem with this. However, the 180° pulse can be used for both selective and non-selective excitation. Figure 10 shows the distribution of magnetization _Mz on the z-axis in the thickness direction of the slice immediately before the first 90° pulse.
This shows the results of a computer simulation using the equation. In Figure 10,
Three simulation results are shown for cases in which 180° pulses are selected and not selected in the DEFT method, and in the case of the present invention. Here, 90° for selective excitation
The pulses are Gaussian modulated. This was calculated using the average T ₁ , T ₂ and T _r =100 ms (repetition time) of the living body. M _z is M _z before executing the pulse sequence is 1,
The magnitude of M _z corresponds to the NMR signal intensity. (a) In case of 180° pulse with DEFT method not selected, the first
As shown in Figure 0a, M _z outside the slice plane becomes extremely small. Generally, during the waiting time T _d of a pulse sequence, the same pulse sequence is sequentially applied to multiple other slice planes, and because the interval T _d is sufficiently long, M _z becomes large due to longitudinal relaxation of T ₁ . A multi-slice method is used in which the next view of the first slice plane is performed after the first slice. This is effective as a pseudo-high-speed method because it eliminates the decrease in the NMR signal (magnitude of M _z ) and allows data from multiple planes to be obtained simultaneously. However, the multi-slice method requires that M _z outside the slice plane be large and unaffected by excitation of other slice planes. Under these conditions, the DEFT method (FIG. 10a) using non-selected 180° pulses has the disadvantage that it cannot be used in conjunction with the multi-slice method because M _z outside the slice plane becomes small. The actual slice shape is obtained by multiplying M _z in FIG. 10 by a function of the slice shape (in this case Gaussian shape), which is shown in FIG. 11. (b) In the case of a 180° pulse for selective excitation in the DEFT method, as shown in FIG. 10b, M _z is large outside the slice plane, so there is no problem. However, the disadvantage of FIG. 11 is that the slice shape has three mountain shapes. This is because the magnetization M at the slice boundary moves in a complicated manner during the 180° pulse of selective excitation, so the vector directions of each M become disjointed, resulting in a decrease in the signal. As described above, it is inappropriate to use the DEFT method, which is a known technique, as it is in an NMR imaging device. [Problems to be Solved] The present invention improves the poor responsiveness of the conventional NMR imaging device using the two-dimensional Fourier transform method as described above, without degrading the quality of the images obtained. The purpose of this research is to provide an NMR imaging device with reduced scan time. B "Structure of the invention" [Means for solving the problems] In order to solve the above problems, the present invention is equipped with a control means having a sequence function as shown in the following box. be. By the action of this control means, the magnetization M is forcibly directed in the z'-axis direction without waiting until the longitudinal relaxation time T ₁ has elapsed and the magnetization M reaches a thermal equilibrium state (M points in the Z-axis direction). can do. What is the sequence function of the control means? ``First, a first 90° pulse is applied that selectively excites the atomic nuclei present in a specific slice plane of the object, and then the atomic nuclei present in other than the specific slice plane are applied. applying a first 180° pulse that excites the nuclei, then applying a second 90° pulse that selectively excites nuclei present in the same specific slice plane as the slice plane; Applying a second 180° pulse that also excites nuclei existing outside the slice plane, and further applying means for applying a gradient magnetic field in the interval T _s1 between the first 90° pulse and the first 180° pulse. The operation applies a second gradient magnetic field in a direction different from the first gradient magnetic field to provide a phase change amount, and further operates the means for applying a gradient magnetic field to apply a second gradient magnetic field in a direction different from the first and second gradient magnetic fields. A third gradient magnetic field is applied by switching between two values with different polarities, and further, in the interval T _s2 between the first 180° pulse and the second 90° pulse, the second and second gradient magnetic fields in the interval T _s1 are applied. Apply a gradient magnetic field in the same direction as the gradient magnetic field of step 3, and operate the second gradient magnetic field so that the intensity and application time of the second gradient magnetic field are set to the values necessary for imaging for each sequence. [Example] The following is a diagram using the drawings. The present invention will now be explained. FIG. 1 is a block diagram showing the configuration of an embodiment of the apparatus according to the present invention. In the same figure, 1 is a uniform static magnetic field H ₀ (the direction in this case is the Z direction)
A static magnetic field coil 2 for generating the static magnetic field coil 2 is a control circuit for the static magnetic field coil 1, and 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. In the apparatus of the present invention, first, second, and third gradient magnetic fields are generated, but only the first, second, and third gradient magnetic fields are generated.
It is abstract to describe it as the gradient magnetic field of
The invention is difficult to understand. Therefore, in this specification, the first
The following description will be made assuming that the gradient magnetic field is a z gradient magnetic field, the second gradient magnetic field is an x gradient magnetic field, and the third gradient magnetic field is a y gradient magnetic field. However, any combination may be used as long as the first, second, and third gradient magnetic fields are in different directions. Also,
The x, y, z gradient magnetic fields, gradient magnetic fields in other directions, or a composite magnetic field thereof may be used. Further, in this specification, dedicated coil means (z gradient magnetic field coil, x gradient magnetic field coil, y gradient magnetic field coil) are provided as means for generating the first, second, and third gradient magnetic fields. This will be explained using an example, but it is not limited to this. That is, in order to generate the first, second, and third gradient magnetic fields, for example, one means may be used to generate the first, second, and third gradient magnetic fields, respectively.
A third gradient magnetic field may also be generated.
For example, by moving the position of one coil means, the first, second, and third gradient magnetic fields can be generated. FIG. 8A is a configuration diagram showing an example of the gradient magnetic field coil 3. The coils shown in A in the figure include a z gradient magnetic field coil 31 and y gradient magnetic field coils 32 and 33
Contains. Furthermore, although not shown, it has the same shape as the y gradient magnetic field coils 32 and 33, and has a 90° angle.
It also includes a rotating x-gradient magnetic field coil. This gradient magnetic field coil 3 produces a uniform static magnetic field.
Generates a magnetic field with linear gradients in the x, y, and z axes in the same direction as H ₀ . Control circuit 4 is controlled by controller 20. Reference numeral 5 denotes an excitation coil for applying a high frequency pulse of a narrow frequency spectrum f, that is, an RF pulse, to the subject as an electromagnetic wave, the configuration of which is shown in FIG. 8B. 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), the output of which is applied to the exciting coil 5 via a gate circuit 30 whose opening/closing is controlled by a signal from a controller 20 and a power amplifier 7. Reference numeral 8 denotes a detection coil for detecting the NMR signal in the subject, and its configuration is the same as the excitation coil shown in FIG. Although it is desirable that the detection coil 8 be installed as close to the subject as possible, it may also be used as the excitation coil 5 if necessary. 9 is an amplifier that amplifies the nuclear magnetic resonance signal (NMR signal...FID signal/echo signal) obtained from the detection coil 8, 10 is a phase detection circuit, and 11 is a wave memory that stores the phase-detected waveform signal from the amplifier 9. The circuit includes an A/D converter. 13
14 is a computer that inputs the signal from the wave memory circuit 11 via a transmission line 12 made of, for example, an optical fiber and performs predetermined signal processing to obtain a tomographic image; 14 is a television that displays the obtained tomographic image. It is a display device like a monitor. Further, necessary information is transmitted from the controller 20 to the computer 13 via a signal line 21. The controller 20 generates gradient magnetic fields G _z , G _x , G _y ,
It is configured to be able to output signals (analog signals) necessary for controlling the amplitude of RF pulses and control signals (digital signals) necessary for transmitting RF pulses and receiving NMR signals. This controller 20 has a sequence function that is a feature of the device according to the present invention, that is, a function that controls the operation timing of the RF pulse and the operation timing of each gradient magnetic field. However, the element that performs this sequence function is not limited to the controller 20, and the present invention can be implemented even if other elements, such as the computer 13, have this function. The operation of the device of the present invention configured in this way is as follows:
A step-by-step explanation will be given with reference to FIG. 9 and Tables 1 to 3. <> Time t ₀ At time t ₀ , the control circuit 2 is connected to the static magnetic field coil 1.
A current is applied to the z gradient magnetic field coil 31 from the controller 20 via the control circuit 4 in a state where a static magnetic field H ₀ is applied to the test object (the test object is installed inside the cylinder of each coil). As shown in FIG. 9B, this is the time when the z gradient magnetic field Gz is applied. At this time, the center of the slice plane (the part where the magnetization M rotates correctly by 90 degrees when a 90 degree pulse is applied), the slice surface boundary (the part where the magnetization M rotates θ degrees when a 90 degree pulse is applied, and G _z = when a 180 degree pulse is applied)
0, so the direction of each magnetization M is as follows: , as shown in F, G, and H of FIG. Next, under the given _Gz , the gate circuit 30 selects and outputs a phase difference of 0°.
An RF signal modulated in a predetermined shape (for example, Gaussian shape) excites the atomic nuclei on a specific surface (slice surface) of the object. That is, the first 90° _x pulse is applied as shown in FIG. 9A. <> At time t ₁ , the x gradient magnetic field coil 32 is energized,
As shown in FIG. 9C, a gradient magnetic field g _x of a predetermined magnitude is applied for a time t _x . Due to this gradient magnetic field g _x , the atomic nuclei on the slice plane perform the operation as already described in FIG. 7, and the spins are phase encoded. Since this phase encoding operation was explained in detail in FIG. 7, the explanation will be omitted here. Note that regarding the gradient magnetic field G _x in the x direction, the application time t _x or the strength g _x of the magnetic field may be changed. The reason for this is that the phase shift is expressed as the product of t _x and g _x , and the larger this product is, the larger the spin phase shift becomes. At the same time, the y gradient magnetic field is
Apply g _y . g _y is, for example, a gradient magnetic field in the negative direction, and under the influence of this, the spins of the atomic nuclei on the slice plane are dispersed from the convergent state shown by F at time t ₁ in FIG. Next, after a time t _x , a y gradient magnetic field g _y ' having a polarity opposite to that of the gradient magnetic field g _y (in the positive direction) is applied.
Since g _y ′ has the opposite polarity to g _y , the spins that were moving (in the dispersion direction) receiving the energy of g _y start moving in the opposite direction. In other words, it is heading in the direction of convergence. Then, at the point when the amount of phase change given by g _y in Figure 9D becomes equal to the amount of phase change in g _y ', the spins are again in phase on the y-axis, as shown in Figure 9E. , the first echo signal becomes the maximum. After that, the spin is
Since the movement is not stopped, the matched phases on the y-axis go in the direction of dispersion again, and the first echo signal disappears. In addition, in the above explanation, g _y is a gradient magnetic field in the negative direction and g _y ′ is a gradient magnetic field in the positive direction, but if the polarities of g _y and g _y ′ are opposite, no matter which one is positive, it will be negative. It's okay to be. The reason for this is that if the polarities are opposite, the spins that were once dispersed can be converged. As mentioned above, the time point at which the magnetic fields g _x and g _y are applied is t ₁
Therefore, at this time point _t1 , the magnetization M of each part is oriented as shown in FIG. The first echo signal generated by the above operation is detected by the detection coil 8, and the signal is guided to the phase detection circuit 10 via the amplifier 9, where it is phase detected and then stored in the wave memory circuit 11. The stored data is read by the computer 1 at appropriate timing. <> At time _t2 , after T _s1 time has elapsed since the application of the RF pulse (90° _x ), the energization of the y gradient magnetic field coil is stopped,
The subject is excited with an RF signal modulated into a rectangular shape with a phase difference of 180°, which is selected and output by the gate circuit 30. In this case, the z gradient magnetic field _Gz is not operated, and the first 180° _-x pulse is applied to the entire subject as shown in FIG. 8A. Therefore, atomic nuclei located outside the specific slice plane are also excited. ＜＞ Time t ₃ After applying the 180° _-x pulse, the gradient magnetic field
Apply g _y ′. This time point is defined as _t3 . Magnetization M
is rotated counterclockwise around the x-axis as the center of rotation.
Rotate 180° and it will look like F, T, and C in Figure 9. Here, the gradient magnetic fields G _x and G _y are controlled so that the respective integral values of the gradient magnetic fields G _x and G _y are the same before time t ₂ and after time t ₃ . This is because by controlling in this way, the magnetization M is gathered exactly after the time T _s2 . This gradient magnetic field G _x ,
For example, to make the integral values of G _y the same before time t ₂ and after time t ₃ ,
In sections T _s1 and T _s2 , G _x as if reversing the time axis,
Just apply _Gy . Of course, it is not necessary to control the gradient magnetic fields G _x and G _y such that the time axes are reversed (so that they are symmetrical) as long as the integral values are the same before and after time points t ₂ and t ₃ . After time t ₃ , the magnetization M, which had been oriented in the direction of dispersion, is completely reversed in direction by the 180° pulse and becomes oriented in the direction of convergence. Therefore, a second echo signal is detected from the detection coil 8, which gradually increases as shown in FIG. 9E. Furthermore, in the example _of Fig _{. 9} , _the gradient magnetic _{fields g} Apply. However, it is not always necessary to apply the gradient magnetic fields g _x and g _y so that they are symmetrical. As mentioned above, it is sufficient to make the integral values the same. <> Time t ₄ After time t _p from time t ₃ , the gradient magnetic field G _x = g _y and G _y =
Add g _y , then after time t _x , G _x = 0,
The time point when G _y = 0 is set as t ₄ . This is t ₂ and t ₃
The integral values of the gradient magnetic fields G _{x and} G _y are the same for the _central time of The orientation is reversed. Therefore, it becomes F, G, and H in Fig. 9. After time t ₄ , G _z ^- and G _z ⁺ are given, and under that condition, the gate circuit 30 generates the first signal with a phase difference of 180°.
A second 90° _-x pulse is applied to the subject using a modulated RF signal similar to the 90° pulse of the first
Re-excite the slice plane that was excited with a 90° pulse. The end of this excitation is designated as time _t5 . At this time, the direction of the magnetization M inside, outside, and at the boundary of the slice plane, that is, the entire object is aligned in the −z-axis direction. <> After the application of time t ₆ G _z ⁺ is completed, the subject is excited with an RF signal modulated into a rectangular waveform with a phase difference of 0° and output from the gate circuit 30 (180° pulse excitation).
That is, since there is no z-gradient magnetic field, atomic nuclei located outside the specific slice plane are also excited.
The end point of this excitation is defined as _t6 . By applying this second 180° pulse, the magnetizations M are all aligned in the +z-axis direction. In this way, at time t ₆ , the state returns to the same state as the initial time t ₀ . However, in this method, relaxation due to longitudinal relaxation or transverse relaxation of the material remains, and the magnetization M does not completely turn upward at time _t6 . Therefore, a waiting time T _d is provided after time t ₆ , one sequence is completed by waiting for the magnetization M to become completely upward, and the same sequence is repeated thereafter.

【table】

【table】

【table】

[Table] In the above, the magnetization M of the slice plane is phase encoded by applying the z gradient magnetic field G _x ,
It has been explained that a signal with a different frequency is obtained for each column by applying g _y and g _y ' with different polarities as the y gradient magnetic field G _y , but the operation of the invention will not work even if the relationship between the x and y axes is swapped. It is clear that this is true, and even this is within the scope of the rights of the present invention. That is, the phase of the magnetization M on the slice plane may be encoded using the y gradient magnetic field G _y , and signals with different frequencies may be obtained for each column using the x gradient magnetic field G _x . In FIG. 9, the times at which G _z ⁺ , g _x , g _y , and g _y ' are applied are just one example, and after phase encoding g _x ,
It is sufficient to obtain a signal under the application of g _y ′. That is, G _z ^-
, g _x and g _y may be applied at the same time or may be slightly shifted. Further, until a dynamic equilibrium state is obtained, it is not necessary to use the obtained NMR signal as data from the first sequence to about the 10th sequence. Note that in the above description, the first and second
Detect the NMR signal, Fourier transform it,
Although it has been explained that the present invention is useful for image reconstruction, the present invention is not limited to this description, and the present invention can also be applied to various cases such as the following, for example. <> One of the first and second NMR signals is detected, and the detected signal is used to reconstruct an image. <> Both the first and second NMR signals are detected, and one of the detected signals is used to reconstruct an image. <> Both the first and second NMR signals are detected, and the data of these two detection signals are added and averaged to reconstruct an image. <> After both the first and second NMR signals are detected and the two detection signals are Fourier transformed, they are added and averaged in a projection state to reconstruct an image. Or, add and average the two images. In such a sequence, the waiting time
T _d is much shorter than the conventional one. 1st
Figure 3 shows this situation, and the test object is egg white (longitudinal relaxation time T ₁ = 693 ms, transverse relaxation time T ₂ = 262 ms).
s) is used and T _s1 +T _s2 = 30 ms. In the figure, the horizontal axis is the waiting time T _d ,
The vertical axis is the signal strength after reaching a dynamic equilibrium state, where the chain line curve A is the actual value measured using the conventional method (matching the theoretical value), and the solid line curve B is the actual value measured using the method of the present invention ( (consistent with theoretical value). As is clear from the figure, it takes a much shorter time (T _d ) to obtain the same signal strength using the method of the present invention.
It turns out that you can get away with it. In addition, in the example, in one sequence, the applied RF pulse is 90° _x ...180° _-x ...90° _-x・
180° _x , but the feature of the device according to the present invention is that the second 90° pulse directs the magnetization M entirely downward. Therefore, for example, 90° _x ...180° _y ...90° _x・
A pulse may be applied to a predetermined atomic nucleus with a phase relationship of 180° _-x (a 180° _y RF pulse is created using an RF signal with a phase difference of 90°). For example, the meaning of an RF pulse of "90° _-x " is that when this pulse is applied, the magnetization M is
This means moving to a position rotated 90 degrees counterclockwise using the x-axis as the rotation axis. Moreover, "90° _y " means that the magnetization M moves to a position rotated 90° clockwise with the y-axis as the rotation axis. Note that whether the RF pulse is “90° _x ” or “90° _y ” can be selected by adjusting the phase of the high frequency waveform in the RF pulse.
For example, in the case of these two pulses, it is sufficient to change the phase of the high frequency by 90 degrees. This selection is usually the first
This is performed by the gate circuit 30 shown in the figure. Next, the fact that multi-slices can be used in combination according to the present invention will be explained using FIGS. 10 and 11. Figure 10 shows the pulse sequence of Figure 9 as T _r
z immediately before the first 90° pulse.
It shows the distribution of axial magnetization M _z in the slice direction. Here, biological values were used for T ₁ and T ₂ . M _z shown in Figure 10 is obtained within the slice plane.
Compatible with NMR signal intensity. Also, out-of-plane M _z
corresponds to the signal strength when performing multi-slice. Figure 11 shows the first 90° state of M _z in Figure 10.
It represents the NMR signal intensity after selective excitation by applying a pulse and a z gradient magnetic field _Gz . In the same figure, the well-known technology SR (saturation
Data obtained when T _r ≫ T ₁ and T _r = 100 ms in the recovery method are also displayed. As explained in the conventional example, the drawbacks of the DEFT method are: Multi-slice cannot be performed (M _z outside the slice plane is small in Figure 10) The slice shape becomes three mountains (Figure 11)
Compared to 2, in the pulse sequence of the present application, M _z is large even outside the slice plane as shown in FIG. 10, so multi-slice can be used in combination. Furthermore, the 11th
As can be seen from the figure, improvements have been made such as the slice shape can be close to a rectangle. On the other hand, when using the SR method, when T _r = 100 ms, the signal strength is less than half of that of the present invention, and when T _r ≫ T ₁ , the signal strength is outside the slice plane as in the case where the DEFT method is not selected. It has drawbacks such as M _z is too small compared to the center value. In addition, in the pulse sequence shown in Figure 9, replace the last two combinations of 90° and 180° pulses with either () one 270° pulse with selective excitation () or one 270° pulse with non-selective excitation. That is impossible. The reason is as follows. In (), considering the operation of M at time t ₄ of G and H in Figure 9, by applying (), since G is the slice boundary, the RF magnetic field strength actually becomes <270°, and the RF magnetic field strength on the +z axis is I won't go back. Further, the edge outside the slice plane remains oriented in the −z-axis direction due to selective excitation. In (), the effect of the 270° pulse is also on the slice boundary, so M rotates too much and does not return to the +z axis. Similarly, M rotates too much on the outside of the slice plane. Similar to the DEFT method, these are NMR
This results in a decrease in signal strength and a deterioration in slice shape, making it unsuitable as an imaging technique. FIG. 12 shows another pulse sequence of the device according to the invention. The NMR signal obtained with the pulse sequence shown in FIG. 9 is only an echo signal, but an FID signal can also be obtained from the pulse sequence shown in FIG. 12. The pulse sequence of FIG. 12 differs from FIG. 9 in that the gradient magnetic field G _y is not applied in the gradient magnetic field G _y . That is, in FIG. 9, the gradient magnetic field G _y =g _y is applied during the period t _x during which the gradient magnetic field G _x =g _x is applied. As a result, the converged magnetization M is once dispersed, and then, by applying G _y = g _y ', the magnetization M is converged again, and the first echo signal is obtained. On the other hand, in the sequence shown in FIG. 12, since the gradient magnetic field of G _y = g _y is not applied, an FID signal is immediately obtained instead of the first echo shown in FIG. The present invention can also be implemented by detecting this FID signal and using it for image reconstruction. The other operations in FIG. 12 are the same as the sequence in FIG. 9, so their explanation will be omitted. Although FIGS. 9 and 12 show that the second 180° pulse is applied immediately after the second 90° pulse, the present invention is not limited to this. For example, the effects of the present invention can be obtained even if the second 180° pulse is applied within the time T _s1 instead of immediately after the second 90° pulse. C. "Effects of the Invention" As described above, according to the present invention, the pulse sequences shown in FIGS.
At the end of the view sequence, it is possible to forcibly and accurately bring all the magnetizations M inside and outside the slice plane into a thermal equilibrium state (or near it). Therefore, there is no need to wait for natural relaxation due to T ₁ as in the conventional method (for example, SR method), and the pulse sequence interval can be shortened, and the scan time can be shortened.

[Brief explanation of drawings]

Figure 1 is a configuration diagram of an embodiment of the device of the present invention, Figure 2 is a diagram explaining the spin of a hydrogen atom, Figure 3 is a schematic diagram of the magnetic moment of a hydrogen atom, and Figure 4 is a diagram showing the nucleus of a hydrogen atom. Figure 5 is a diagram showing an example of the test pulse waveform of a conventional device using the Fourier transform method. Figure 6 is a diagram showing magnetization M in a rotating coordinate system. Figure 7 is a diagram for explaining phase encoding of magnetization M in the Fourier transform method, FIG. 8 is a structural diagram showing an example of a magnetic field coil, and FIG. 9 is an operation waveform for explaining the sequence of the device according to the present invention. and a diagram of the magnetization vector,
Figure 10 shows the results of a computer simulation of the state in which the dynamic equilibrium state is reached by continuously executing the sequence in Figure 9.
The first 90° pulse and the z gradient magnetic field to the state of M _z in Figure 0
FIG. 12 is a diagram showing the NMR signal intensity after selective excitation by applying _Gz ; FIG. 12 is an operation waveform diagram and magnetization vector diagram for explaining another sequence example in the apparatus according to the present invention; FIG. 13 FIG. 2 is a diagram showing the relationship between waiting time and signal strength. 1... Static magnetic field coil, 2... Static magnetic field coil control circuit, 3... Gradient magnetic field coil, 4... Gradient magnetic field coil control circuit, 5... Excitation coil,
6...RF oscillator, 7...power amplifier, 8...
Detection coil, 9...Amplifier, 10...Phase detection circuit, 11...Wave memory circuit, 13...Computer, 14...Display device, 20...Controller, 30...Gate circuit, 31...Z gradient magnetic field Coil for use, 32, 33... Coil for y gradient magnetic field.

Claims (1)

[Scope of Claims] 1. Means for applying a static magnetic field (H ₀ ) to a subject, means for applying a gradient magnetic field to a subject, and high frequency for imparting nuclear magnetic resonance to the nuclei of atoms constituting the tissue of a subject. means for applying a pulse, and an apparatus for obtaining an image of a tissue of a subject using the generated nuclear magnetic resonance signal, comprising a control means having a sequence function described in the box below, An NMR imaging device characterized by repeating a sequence and using necessary signals among the nuclear magnetic resonance signals generated for each sequence for image reconstruction. ``Operating the means for applying the gradient magnetic field,
and applying a first 90° pulse from the means for applying a high-frequency pulse to excite the atomic nuclei present in a specific slice plane of the object, and then applying the means for applying the gradient magnetic field. the first one from the means for applying the high frequency pulse without being operated;
Applying a 180° pulse to excite nuclei existing outside the specific slice plane, and then operating the gradient magnetic field applying means to apply the first gradient magnetic field and at the same time apply the high frequency pulse. applying a second 90° pulse from the means for exciting the atomic nuclei present in the same specific slice plane as the above, and then applying the high frequency pulse without operating the means for applying the gradient magnetic field; from the second
means for applying a 180° pulse to excite nuclei existing outside a specific slice plane and applying a gradient magnetic field at T _s1 in the interval between the first 90° pulse and the first 180° pulse; applying a second gradient magnetic field in a direction different from the first gradient magnetic field to provide a phase change amount;
A third gradient magnetic field in a direction different from that of the gradient magnetic field is applied by switching two values with different polarities, and further, in the interval T _s2 between the first 180° pulse and the second 90° pulse, the interval T A sequence function that applies gradient magnetic fields in the same direction as the second and third gradient magnetic fields in _s1 , and sets the second gradient magnetic field strength and application time to values necessary for imaging for each sequence. 2. In the interval T _s1 between the first 90° pulse and the first 180° pulse, the means for applying a gradient magnetic field is operated to apply a second gradient magnetic field in a direction different from the first gradient magnetic field. to apply a phase change amount, then operate the gradient magnetic field applying means to apply a third gradient magnetic field in a direction different from the first and second gradient magnetic fields, and further apply the first 180° pulse. and the second 90° pulse, a gradient magnetic field _{in the same direction as the second and third gradient magnetic fields in the section T s1 is} applied, and the second gradient magnetic field strength is set for each sequence, 2. The NMR imaging apparatus according to claim 1, further comprising a control means having a sequence function to set the application time to a value necessary for imaging. 3 The phase relationship of the four high-frequency pulses is 90° _x ...
180° _-x ... 90° _-x ... 180° _x The NMR imaging apparatus according to claim 1 or 2. 4 The phase relationship of the four high-frequency pulses is 90° _x ...
180° _y ...90° _x ...180° _-x The NMR imaging device according to claim 1 or 2.