JPH0421489B2 - - 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, which as a whole rotate (rotate) with 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. Further, 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 the strength of resonance, the time described below 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. Generally, 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 . In contrast, in a liquid, molecules move freely, so
The likelihood of energy exchange between spins and between spins and a molecular system (lattice) 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 such NMR phenomena,
There is an NMR imaging device that uses the PR method (projection reconstruction method, also referred to as the projection reconstruction method) to obtain images regarding the tissue of a subject. this
The principle of image reconstruction using the PR method is almost the same as that of an X-ray CT device. First, the body axis direction of the subject (z
A gradient magnetic field is applied in the axial direction) to excite protons in the virtual sliced portion (plane perpendicular to the z-axis). Although it will be explained that a plane perpendicular to the body axis of the subject is taken as the tomographic plane, any plane can be imaged by changing the gradient magnetic field. Next, gradient magnetic fields are applied in the x and y directions, and NMR signals are detected in this state to obtain projections in the direction perpendicular to the combined x and y gradient magnetic fields. Then, by repeating the operation of changing the values of the x and y composite gradient magnetic fields, the corresponding NMR signals are obtained, and by Fourier transforming each of them, projections in many directions of the object are obtained. The PR method is a method that uses this projection to reconstruct an image of a subject using a CT method. FIG. 5 is an operational waveform diagram for explaining an example of an inspection method of a conventional apparatus using this PR 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 _FIG . pulse) is applied. In the Z-axis direction (body axis direction) of the living body, there is a gradient magnetic field.
G _z is applied, and the protons precess with a period proportional to the strength of the magnetic field. Here Z
Only the cross section at a certain position of the axis (H ₀ +△G _z ) has the frequency of the applied RF pulse (ω _j =2πf _j )
It precesses at the same Larmor angular velocity ω _j =γ(H ₀ +△G _z ). 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. If the magnetization M of the excited proton is shown on a rotating coordinate system rotating at an angular velocity ω _j as shown in FIG. Next, as shown in Figure 5 C and D, the x gradient magnetic field is
G _x and y gradient magnetic field G _y are applied simultaneously. A composite two-dimensional gradient magnetic field is created by these two gradient magnetic fields, and under this environment, an NMR signal as shown in E is detected. Here, the magnetization M is as shown in FIG.
Due to the inhomogeneity of the magnetic field, it gradually disperses in the direction of the arrow in the x'-y' plane, so the NMR signal eventually decreases and disappears after a time T _s as shown in Figure 5 E. . When the NMR 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 G _x and the y gradient magnetic field G _y . Then, for a given period of time
Wait T _d and repeat the next sequence in the same manner as above. In each sequence,
By changing the values of G _x and G _y little by little, we take various directions of the composite gradient magnetic field. Thereby, NMR signals corresponding to each projection can be obtained in many directions of the object. In conventional equipment that operates in this way, the time required for the NMR signal to disappear is shown in Figure 5.
T _s is 10 to 20 ms, but the predetermined time T _d until moving to the next sequence is due to the longitudinal relaxation time T ₁
Approximately 1 sec is required. Therefore, if a cross-section of a single object is to be reconstructed using, for example, 128 projections, the measurement requires a long time of at least 2 minutes, which is one of the major obstacles to achieving high speed. It's becoming one. In order to solve this problem, a known technique {DEFT method; driven
Using the equilibrium fourier transform},
When considering the case where a high flux NMR imaging device is manufactured, 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 {"Pulse 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. FIG. 9 shows the results of a computer simulation using Bloch's equation of the distribution of the magnetization M _z on the z-axis in the thickness direction of the slice immediately before the first 90° pulse. FIG. 9 shows three simulation results for the DEFT method, when 180° pulses are selected and not selected, and when the present invention is used. Here, the 90° pulse is Gaussian modulated for selective excitation. This is the average living body
Calculations were made using T ₁ , T ₂ and Tr=100 ms (repetition time). _Mz is set to 1 before executing the pulse sequence, and the magnitude of _Mz corresponds to the NMR signal intensity. (a) In case of 180° pulse with DEFT method not selected, the 9th
As shown in Figure a, 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. 9a) using non-selected 180° pulses has the disadvantage that it cannot be combined 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. 9 by a slice shape function (in this case Gaussian shape), which is shown in FIG. 10. (b) In the case of a 180° pulse for selective excitation in the DEFT method, as shown in FIG. 9b, M _z is large outside the slice plane, so there is no problem. However, the disadvantage of FIG. 10 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 solves the problem by the conventional PR method as described above.
The present invention aims to improve the poor responsiveness of NMR imaging devices and to provide an NMR imaging device that shortens scan time without degrading the quality of images obtained. B "Structure of the Invention" [Means for Solving the Problems] In order to solve the above problems, the present invention includes a control means having a sequence function as shown in the following box. This is how it was done. Due to the function 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). You can. 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; A second 180° pulse is applied that also excites nuclei existing outside the slice plane, and in the interval between the first 90° pulse and the first 180° pulse, a direction different from the first gradient magnetic field is applied. the second of
Applying a gradient magnetic field of
A magnetic field in the same direction as the second gradient magnetic field is applied, and the intensity and direction of the second gradient magnetic field are set to values necessary for imaging in each sequence. [Example] The drawings are shown below. The present invention will be explained using the following. 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 and second gradient magnetic fields are generated, but it is abstract and difficult to understand the invention if the description is simply described as the first and second gradient magnetic fields. Therefore, in this specification, the first gradient magnetic field will be described as a z-gradient magnetic field, and the second gradient magnetic field will be described as a composite magnetic field of an x-gradient magnetic field and a y-gradient magnetic field.
However, any combination may be used as long as the first and second gradient magnetic fields are in different directions. Further, gradient magnetic fields in directions other than the x, y, and z gradient magnetic fields may be combined. Further, in this specification, as means for generating the first and second gradient magnetic fields, dedicated coil means (z gradient magnetic field coil, x gradient magnetic field coil,
An example in which a y gradient magnetic field coil) is provided will be described, but the present invention is not limited to this. That is, to generate the first and second gradient magnetic fields, for example,
Both the first and second gradient magnetic fields may be generated by one means. FIG. 7A 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. 7B. 6 is the frequency corresponding to the NMR resonance condition of the atomic nucleus to be measured (for example, 42.6M for protons)
Hz/T), 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. 8 is 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 controls the first and second gradient magnetic fields (gradient magnetic fields G _z , G _x , G _y ), a signal (analog signal) necessary for controlling the amplitude of the RF pulse, and the RF
It is configured to be able to output control signals (digital signals) necessary for transmitting 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 to control the operation timing of the RF pulse and the operation timing of the first and second gradient magnetic fields. 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. 8 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 Figure 8B, the first gradient magnetic field (z gradient magnetic field
G _z ⁺ ). Note that, as described above, the body axis of the subject and the z-axis are in the same direction. At this time, the center of the slice plane (the part where the magnetization M rotates correctly by 90 degrees by applying a 90 degree pulse), the boundary of the slice plane (the part where the magnetization M rotates θ degree due to the magnetization applied with the 90 degree pulse, and when the 180 degree pulse is applied, G _z =
0, so the direction of each magnetization M is as follows: , as shown in F, G, and H of FIG. <> Given the time t ₁ G _z , the gate circuit 30
A specific surface (slice surface) of the object is detected by the RF signal modulated in a predetermined shape (for example, Gaussian shape) with a phase difference of 0°.
excites the nucleus of the atom. That is, the first 90° _x pulse is applied as shown in FIG. 8A. Next, the x gradient magnetic field coil and the y gradient magnetic field coil 32, 33
is energized, and magnetic fields Gx and Gy of predetermined magnitude are applied as shown in C and D of FIG. In addition, in Figure 8B, G _z ^- following G _z ⁺ is
This is a waveform signal for matching the phases of NMR signals from different parts of a subject, and this technique is a known technique. Assuming that the time point at which the magnetic fields G _x and G _y are applied is t ₁ , at this time point t ₁ the magnetization M of each part is oriented as shown in FIG. After time t ₁ , the first
The nuclear magnetic resonance signal (FID signal) is detected by the detection coil 8.
The signal is guided through the amplifier 9 to the phase detection circuit 10, 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, and is subjected to Fourier transformation therein to become a signal of one projection. <> Time t ₂ After T _s1 time elapses from the time t ₁ until the nuclear magnetic resonance signal disappears, the energization of the x gradient magnetic field coil and the y gradient magnetic field coil is stopped, and the position selected and output by the gate circuit 30 is The subject is excited with a rectangularly modulated RF signal with a phase difference of 180°.
In this case, the z gradient magnetic field G _z is not operated and the 8th
As shown in Figure A, the first 180° _-x is applied to the entire subject.
Give a pulse. That is, atomic nuclei located outside the specific slice plane are also excited. <> Time t ₃ After applying the 180°-x pulse, the integral value of the second gradient magnetic field (combined magnetic field of G _x and G _y ) is determined at time t 3.
Control is performed so that the values are the same before t ₂ and after time t ₃ . This is because by controlling in this way, the magnetization M is assembled exactly after the time Ts ₂ . In order to make the integral value of this second gradient magnetic field the same before time t ₂ and after time t ₃ , for example, the time axis can be reversed between sections T _s1 and T _s2 . , G _x , and G _y can be applied. Of course, it is not necessary to control G _x and G _y so as to reverse the time axis (so as to make them symmetrical) as long as the integral value is the same before and after time t ₂ and t ₃ . After applying a 180° pulse, the gradient magnetic fields G _x , G _y
The time point when is applied is _t3 . The magnetization M rotates as shown in FIG. 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 nuclear magnetic resonance signal (echo signal) is detected from the detection coil 8, which gradually increases as shown in FIG. 8E. applied before time t ₂ and after time t ₃
Assuming that G _x and G _y are the same and the state of the subject does not change during that period, this echo signal and the first nuclear magnetic resonance signal _are
The signal waveform is symmetrical about the central time of and _t3 . <> Time t ₄ When time (t ₂ −t ₁ ) has elapsed from time t ₃ , the application of the magnetic fields G _x and G _y is stopped under the control of the controller 20 . Let this point be t ₄ . The magnetization M is as shown. After this point, G _z ^- and G _z ⁺ are given, and under that condition, the first
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 magnetism M inside, outside, and at the boundary of the slice plane, that is, the entire object, is aligned in the _−z 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] In the above, in a sequence in which a dynamic equilibrium state is obtained (for example, from the beginning to the 10th sequence), it is not necessary to use the generated NMR signal as data. , 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 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 1 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 the dynamic equilibrium state, where the chain line curve A is the actual value measured using the conventional method (matching the theoretical value), and the practical curve B is the actual value obtained 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 (an RF pulse of 180° _y is created using an RF signal with a phase difference of 90°). Above, we have shown an example of the RF pulse phase where the influence is small even if there is some error in the height or pulse width of the RF pulse, but this is not limited to 90°…180°…90°,
Any phase may be used as long as the second 90° pulse directs the entire magnetization M downward in the order of 180°. 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 with reference to FIGS. 9 and 10. FIG. 9 shows the pulse sequence of FIG. 8 as T _r =
This is the result of computer simulation of a state in which dynamic equilibrium is reached by continuous execution for 100 ms (repetition time), and the distribution of the z-axis magnetization M _z in the slice direction just before the first 90° pulse. It shows.
Here, biological values were used for T ₁ and T ₂ . M _z shown in FIG. 9 corresponds to the NMR signal intensity obtained within the slice plane. Furthermore, the out-of-plane M _z corresponds to the signal strength when performing multi-slice. FIG. 10 shows the NMR signal intensity after selective excitation by applying the first 90° pulse and the z gradient magnetic field G _z to the M _z state of FIG. 9. FIG. 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 9) Slice shape becomes three mountains (Figure 10)
Compared to 2, in the pulse sequence of the present application, M _z is large even outside the slice plane as shown in FIG. 9, so multi-slice can be used in combination. Furthermore, from FIG. 10, improvements have been made such as the slice shape can be a straight shape. 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 8, 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 in G and H in Figure 8, by applying (), since G is the slice boundary, the RF magnetic field strength actually becomes <270°, and the 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. Although this embodiment has been described using a z-direction gradient magnetic field as a gradient magnetic field for selective excitation, selective excitation from any direction is possible using other gradient magnetic fields or a combination of gradient magnetic fields. FIG. 12 shows another pulse sequence of the device according to the invention. The NMR signal obtained from the pulse sequence shown in the figure is only an echo signal. After selective excitation with the first 90° pulse, G _x ^- , G _y ^-
(G _xy ^- in the synthetic magnetic field) is applied to disturb the phase of the spins in the XY plane (generally called dephase).
Thereafter, gradient magnetic fields G _x ⁺ and G _y ⁺ (G _xy ⁺ in the synthetic magnetic field) having different polarities from the gradient magnetic fields G _x ^- and G _y ^- are applied. Then, the first echo peak appears at the time when ∫ _t- G _xy ^- d _t = ∫ _t+ G _xy ⁺ d _t (where G _xy is the combined magnetic field of G _x and G _y ).
In the period of T _s2 , if a gradient magnetic field with the same value as in the period of T _s1 is applied with the time axis reversed around the first 180° pulse as shown in Figure 12C and D, a similar second 180° pulse is applied. An echo signal appears. The principle of speeding up
The processing of the NMR signal is the same as that explained in FIG. 5, so a description thereof will be omitted. In addition, in Fig. 12, the negative gradient magnetic field G _x
⁻ , G _y ⁻ are applied first, and positive gradient magnetic fields G _x ⁺ , G _y
Although ⁺ was applied later, the invention would still work even if the order of application was reversed. Although FIGS. 8 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 time T _s1 instead of immediately after the second 90° pulse. C. "Effects of the Present Invention" As described above, according to the present invention, by the pulse sequence shown in FIG. 8, the pulse sequence shown in FIG. The magnetization M of can be brought into thermal equilibrium (or close to 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 inspection pulse waveform by NMR, Figure 6 is a diagram showing the magnetization M in a rotating coordinate system, and Figure 7 is a diagram showing the state in which the magnetic field coils are aligned in the direction of the magnetic field. A structural diagram showing an example, FIG. 8 is a diagram of operation waveforms and magnetization vectors to explain the sequence according to the present invention, and FIG. 9 is a diagram of the sequence shown in FIG. 8 being continuously executed to reach a dynamic equilibrium state. Figure 10 shows the result of computer simulation of the state _.
Figure 11 is a _diagram showing the relationship between waiting time and signal strength. Figure 12 is a diagram showing the relationship between waiting time and signal strength. The figures are diagrams of operation waveforms and magnetization vectors for explaining another example of a sequence according to the present invention. 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 above, and then applying the high frequency pulse without operating the means for applying the gradient magnetic field. from the means to the second
A 180° pulse is applied to excite nuclei existing outside the specific slice plane, and a gradient magnetic field different from the first gradient magnetic field is applied in the section between the first 90° pulse and the first 180° pulse. applying a second gradient magnetic field in the direction, and further operating means for applying a gradient magnetic field in the section between the first 180° pulse and the second 90° pulse, so that the second gradient magnetic field and the second gradient magnetic field are applied. A sequence function that applies gradient magnetic fields in the same direction and sets the intensity and direction of the second gradient magnetic field to values necessary for imaging for each sequence. 2. In the section between the first 90° pulse and the first 180° pulse, the means for applying a gradient magnetic field is operated to generate gradient magnetic fields G _xy ⁻ , G with different polarities as the second gradient magnetic field. _xy ⁺ is switched and applied, and further, the first 180° pulse and the second 90° pulse are applied.
A control means having a sequence function that applies a magnetic field in the same direction as the second gradient magnetic field in the interval between the pulses and sets the intensity and direction of the second gradient magnetic field to values necessary for imaging for each sequence. An NMR imaging apparatus according to claim 1, comprising: 3 The phase relationship of the four high-frequency pulses is 90° _x ...
180°− _x …90°− _x …180° _x Claim 1
The NMR imaging device according to item 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 apparatus according to claim 1 or 2.