JPS6179145A - Nmr image device - Google Patents

Abstract

PURPOSE:To reduce scan time by maintaining forcibly and exactly all of the inside and outside magnetizations of a slice plane in a thermally equilibrium state at the point of the time when the sequence for one view ends. CONSTITUTION:Electric current is passed from a control circuit 2 to a coil 1 for a static magnetic field to apply the static magnetic field to an object to be inspected and in this state electric current is passed to a coil 3 for a gradient magnetic field via a control circuit 4 by a controller 20 to apply the gradient magnetic field to said object. The nuclear magnetic resonance signal obtd. from a detecting coil 8 is inputted via a preamplifier 9, a phase detector 10 and a wave memory 11 to a computer 13 and is subjected to prescribed processing, by which a tomographic image is formed. The magnetization is directed forcibly toward the Z-axis direction by the circuit 4 and is maintained in the thermally equilibrium state at the point of the time when the sequence for one view ends.

Description

DETAILED DESCRIPTION OF THE INVENTION A. Object of the Invention [Field of Industrial Application] The present invention relates to nuclear magnetic resonance
ic resonance) (hereinafter referred to as rNMR)
NMr' which utilizes the phenomenon (abbreviated as J) to know the distribution of specific atomic nuclei inside the subject from outside the subject.
? @Relates to image devices. In particular, it relates to improvements in NMR imaging devices suitable for medical equipment.

[Conventional technology]

NMR imagers place a living body, usually a patient, in a 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 it emits the absorbed energy to the outside 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-X: l-signal and FID signal).
The signal is called free 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 a living body as a tomographic image, which is useful for diagnosis, treatment, etc. of the living body.

First, an outline of the principle of NMR will be explained.

The atomic nucleus consists of protons and neutrons, and these as a whole are considered to be rotating (rotating) with nuclear spin angular motion IH.

Figure 2 shows the hydrogen nucleus (+-1>).
As shown in (b), it consists of one proton P and rotates as expressed by the spin quantum number 1/2. Positive character P is (b)
As shown in , since it has a positive charge e+, a magnetic moment J is generated as the nucleus rotates, and each hydrogen nucleus can be regarded as a small magnet.

Figure 3 schematically illustrates 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 (0),
No magnetization is seen as a whole.

When a static magnetic field H0 in two directions is applied to such a substance, each atomic nucleus aligns in the Ho direction.

Figure 4 (a) shows this situation for a hydrogen nucleus. Since the spin quantum number of a hydrogen nucleus is 1/2, as shown in Figure 4 (b), -1/2 and +1/2
It is divided into two energy rankings. The energy difference ΔE between the two energy ranks is expressed by the following equation (1).

ΔE=γf'+Ho (1) γ
:Gyromagnetic ratio (constant specific to each atomic nuclide) h/2
π h blank constant where each nucleus has a static magnetic field Ho of 1.7×.

As a result of this force, the atomic nucleus precesses around the Z-axis at an angular velocity ω as shown in equation (2).

ω-γHo (Larmor angular velocity) (2) That is, each type of atomic nucleus precesses at a different Larmor angular velocity ωm.

For example, a frequency (f+-ω+/2π) corresponding to the Larmor angular velocity ω1 is applied to a living body placed in a static magnetic field H0.
When an electromagnetic wave (usually a radio wave) is applied, resonance occurs in the precessing atomic nucleus corresponding to this frequency f1, and the atomic nucleus absorbs energy corresponding to the energy difference ΔE shown by equation (1). and then 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 under the environment of the static magnetic field H0. Therefore, by selecting the strength of the static magnetic field 1o applied to the living body 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. In addition, the atomic nucleus excited to a higher order is
After resonance, it returns to a lower rank after a time determined by a time constant called relaxation time. At this time, the absorbed energy is released to the outside, so by measuring the temporal change in the resonance intensity, the time described below can be determined.

The relaxation time is spin-lattice relaxation time (longitudinal relaxation time) T+
and spin-spin relaxation time (transverse relaxation time). By observing this relaxation time, data on material distribution can be obtained. In general, in solids, the transverse relaxation time is short and the energy obtained by nuclear magnetic resonance is first distributed to the spin system and then transferred to the lattice system. Therefore,
The longitudinal relaxation time T+ is significantly larger than T2. On the other hand, in a liquid, molecules move freely, so the likelihood of energy exchange between spins and between spins and the molecular system (lattice) is about the same. Therefore, time T+ and T
2 becomes almost equal value.

Here, we have explained hydrogen nuclei (H), but it is also possible to perform similar measurements with other nuclei that have a nuclear spin angular motion port, such as phosphorus nuclei (P), carbon nuclei (C), and sodium nuclei. It is applicable to atomic nuclei (Na), etc.

In this way, NMR allows the existence of specific atomic nuclei and their relaxation times to be measured, so by obtaining various chemical information about specific atomic nuclei within substances,
Various tests can be performed within the subject.

Conventionally, Fourier transform (Fourier transform) has been used to obtain an image of the tissue of a subject using phase information of NMR Shingetsu.
There is an NMR imaging device using the transformation method.

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 two-gradient field Gz as shown in Fig. 5 (B) and a two-gradient field Gz as shown in Fig. 5 (B) are applied to the subject placed in a static magnetic field H0 of uniform strength parallel to the Z-axis direction. narrow frequency spectrum f
, i.e., RF pulse (90' pulse)
Apply.

A gradient magnetic field Gz is applied in two axial directions of the living body, and protons precess at a period proportional to the strength of the magnetic field. Here, only the cross section at the position (Ho + ΔGz) where the two axes form is the frequency of the applied RF pulse (ω
, =2πfj) and the same Larmor angular velocity ω4-γ(Ho+ΔGz). Therefore, only protons that are precessing at an angular velocity near 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.

Note that, although the 17i layer surface will be described here as a surface perpendicular to the Z axis, any surface can be visualized by changing the gradient magnetic field.

If the magnetization M of the excited proton is expressed on a rotating coordinate system rotating at an angular velocity ω as shown in Figure 6 (a), then
The direction is changed by 90° in the y' axis direction. 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. 5(c), a gradient magnetic field Gx in the X-axis direction is applied for a predetermined time tx. Due to this gradient magnetic field Gx, the slice plane of the living body is brought into a state where the strength of the magnetic field is inclined in the X direction, as shown in FIG. 7(A). Here, consider the column in which the slice plane in Figure 7 (a) is divided into bonds. When looking at the protons in the area perpendicular to each column (parallel to the y-axis), the strength of the magnetic field increases in each orthogonal area due to the gradient magnetic field Gx. Therefore, based on equation (2), the spins of protons included in areas where the gradient magnetic field Crx is strong rotate quickly, and the spins in areas where the magnetic field is weak rotate slowly.

Then, as shown in FIG. 5(c), after time Lx, the gradient magnetic field Gx becomes zero, so the strength of the magnetic field in the X@force direction in FIG. 7(a) returns to uniformity. Therefore, the rotational speeds of all spins on the slice plane shown in FIG. 7(a) are the same. However, during time tX, the rotational speed was different for each area, so even after the gradient magnetic field Gx was removed, the phase difference caused by Gx was maintained, as shown in Figure 7 (a). It rotates at the same speed.

In this way, by this magnetic gradient JJj G x,
The phase of the spin of the magnetization M on the slice plane is calibrated (phase coded) in the X-axis direction. Gx
The size of is changed according to equation (3) by changing n for each projection.

γLx ftKGxcJ t-27rn (
3) However, γ: gyromagnetic ratio Lx: length of II image area in the X direction GX: integrand function (t
x: Integral range 0, +l, ... approximately -1) N: Number of divisions in the
WaG y is applied and under this environment Figure 5 (e).

Detect the NMR signal as shown in . This gradient magnetic field Gy
The applied strength and application time of are the same for each block, and the gradient magnetic field Gy, unlike Gx, is N M
It is applied even while the R(!) is being detected.

This time, since the gradient magnetic field Gy is applied to the slice plane in the direction shown in FIG. 7(b), the rotational speed of the spins differs for each column. The spins rotate at the same rotational speed within this one column, but the rotational phase of the spins in the atomic nuclei in the X-axis direction within the column has the above-mentioned gradient! i! ! It was lost because of A G x.

Therefore, the obtained NMR signal is treated as one piece of data. Then, if we repeat the process by changing the amount of phase encoding, collect the same data as above, and perform a two-dimensional Fourier transform on this, we can obtain 2
Protons at each position in the dimensional region can be determined, and information about protons at each position can be obtained.

In other words, for each column the frequency of the spins is different, and within a column the frequency of the spins is the same, but the amount of phase encoding causes the phase of rotation to be different for each part of the depth of the column. It's different. Therefore, if the detected signal is subjected to two-dimensional Fourier transform, each pixel information on the slice plane can be identified.

Here, as shown in FIG. 6 <0>, 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. 5(e), it disappears after a period of τ.

Thereafter, the following sequence is repeated after a period of τ' until the state returns to thermal equilibrium. At this time, the predetermined time 1x for applying the gradient magnetic field Gx in the X-axis direction is repeated N times while changing the amount of phase encoding with the value determined by equation (3). And N
An in-plane proton density image can be obtained by performing two-dimensional Fourier transform on the NMR signals obtained in this sequence.

In the conventional device using the Fourier transform method that operates in this way, the time required until the NMR signal disappears is 10 to 20 m5 in Fig. 5, but the predetermined time τ' until moving to the next sequence is vertically Relaxation time T + 1
About sec is required. 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 in realizing high speed.

In order to solve this problem, a known technique (DEFT method;
ilibrium Fourier transform
Considering the case where a high-speed NMR imaging device is manufactured using m), 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 (
[Pulse and Fourier Variant *NMRJ) 7 Ra, written by Beraker: Furuoka Shoten). This DEFT method is a pulse sequence for speeding up (90'x
...τ...180'y...τ...90-x...
・Td)".

When performing two-dimensional imaging using this DEFT method,
90' pulse is a selective excitation method (gradient magnetic field is applied simultaneously)
is used to excite only a specific slice plane, but there is no problem with this.

However, the 180° pulse can be used for both selective and non-selective excitation.

FIG. 10 shows the results of simulating the distribution of the magnetization Mz on the two axes in the slice thickness direction immediately before the first 90° pulse using Bloch's equation using a total of 1 machines. In Figure 10, 18 in the DEFT method
Case 3 of selection and non-selection of o° pulse and case of the present invention
We presented two simulation results. Here, the 90' pulse is Gaussian modulated for selective excitation. This is the average TI, T2 and Tr of the living body = 100 mS (
repetition time)! 11.

For M2, Mz before executing the pulse sequence is set to 1, and the magnitude of M2 corresponds to the NMR signal intensity.

(a) In the case of a non-selected 180'' pulse in the DEFT method, as shown in Figure 10 (a), the Mz outside the slice plane
becomes very small.

Generally, during the waiting time Td of a pulse sequence, the same pulse sequence is sequentially applied to multiple slice planes, and because the interval Td is sufficiently long, Mz is
A multi-slice method is used in which the next view (view) of the first slice plane is performed after the first slice plane has been made larger by one longitudinal relaxation. This is effective as a quasi-high-speed method because it eliminates the decrease in the NMR signal (the magnitude of Mz) and allows data on multiple planes to be obtained simultaneously. However, the multi-slice method requires that Mz outside the slice plane be large without being influenced by excitation of other slice planes.

Viewed from these conditions, the DEFT method using non-selected 180''' pulses (Figure 10(a)) c.
There is a drawback that the multi-slice method cannot be used in combination because the Mz outside the slice plane becomes small. The actual slice shape is obtained by multiplying Mz 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. 10(b), Mz 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 performs a complicated operation during the 18° pulse of selective excitation, so the vector directions of each M become different, and as a result, the signal decreases.

As mentioned above, the DEFT method, which is a well-known technique, can be used as it is with N
It is unsuitable for use in MR imaging devices.

(Problems to be Solved) The present invention improves the poor responsiveness of the conventional NMR imager using the two-dimensional Foury transform method as described above, and does not reduce the quality of the images obtained. An object of the present invention is to provide an NMR imaging device with shortened scan time.

``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 parentheses. It is something.

Due to the action of this control means, the magnetization M is not held until the longitudinal relaxation time T1 has elapsed and the magnetization M reaches a thermal equilibrium state (M points in the 2-axis direction), and the magnetization M is forcibly directed in the 2'-axis direction. be able to.

The sequence function of the control means is to "first apply a first 90'' pulse that selectively excites the atomic nuclei present in a specific slice plane of the object, and then applying a first 180° pulse that also excites the atomic planes, then applying a second 90° pulse that selectively excites nuclei present in the same specific slice plane as the slice plane; Apply a second 180" pulse that also excites nuclei existing outside the specific slice plane, and further apply a gradient magnetic field in the interval s1 between the first 90° pulse and the first 180@ pulse. operating the means to apply a second gradient magnetic field in a direction different from the first gradient magnetic field to provide a phase change field;
Further, the means for applying a gradient magnetic field is operated to obtain the first. A third gradient magnetic field in a direction different from that of the second gradient magnetic field is applied by switching two values with different polarities, and further, in the interval TS2 between the first 1800 pulse and the second 90° pulse, 1st 2. Third
(Example) Hereinafter, using the drawings, the second gradient magnetic field strength and the application time are set to the values necessary for imaging. The present invention will now be explained.

FIG. 1 is a block diagram showing the configuration of an embodiment of a device according to the present invention. In the figure, 1 is a uniform static magnetic field Ho
A static magnetic field coil 2 for generating a static magnetic field (directions in this case are defined as two directions) is a control circuit for the static magnetic field coil 1, which includes, for example, a DC stabilized power source. It is desirable that the density Ho of the magnetic flux generated by the static magnetic field coil 1 is about 0.1 T, and that the uniformity is 104 or more.

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

In the apparatus of the present invention, first. Second. A third gradient magnetic field is generated, but only the first. Second. If it is described as a third gradient magnetic field, it will be abstract and the invention will be difficult to understand. Therefore, in the present invention 1i1, the description will be made assuming that the first gradient magnetic field is a 2-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 gradient magnetic fields 1, 2, and 3 are in different directions. Also, x,
It may be a y, z gradient magnetic field, a gradient ll field in other directions, or a composite magnetic field thereof.

In addition, in this specification, 1. Second. As means for generating the third gradient magnetic field, an example will be explained in which dedicated coil means (two gradient magnetic field coils, an X gradient magnetic field coil, a y gradient magnetic field coil) are provided, but the present invention is 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 second and third gradient magnetic field may be generated.

For example, by moving the position of one coil means, 1.2. It is also possible to generate a gradient magnetic field of '3.

FIG. 8(A) is a configuration diagram showing an example of the gradient magnetic field coil 3. FIG. The coil shown in FIG. 2A includes a 2-gradient magnetic field coil 31 and a y-gradient magnetic field coil 32, 33. Furthermore, although not shown, a coil 32 for y gradient 81@
．． It has the same shape as 33, but is rotated 90" and installed
It also includes gradient field coils. This gradient magnetic field coil 3 is uniformly static! A magnetic field with linear gradients in the x, y, and z axis directions is generated in the same direction as the i-lift H0. The control circuit 4 is controlled by a controller 20.

5 is a high-frequency pulse with a narrow frequency spectrum f to the subject;
In other words, an excitation coil that provides RF pulses as electromagnetic waves,
Its configuration is shown in FIG. 8(b).

6 is the frequency corresponding to the NMR resonance condition of the atomic nucleus to be measured (for example, 42°6 MHz/T for protons)
An oscillator that generates a signal, the output of which is connected to a gate circuit 3 whose opening and closing are controlled by signals from a controller 20.
0 is applied to the excitation coil 5 via the 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 is iiQm as close as possible to the subject, it may also be used as the excitation coil 5 if necessary.

9 is a nuclear magnetic resonance signal (NMR) obtained from the detection coil 8.
10 is a phase detection circuit, 11 is a phase-detected amplifier 9
A wave memory circuit that stores waveform signals from A/
Contains a D converter.

13 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 John monitor. Further, necessary information is transmitted from the controller 20 to the computer 13 by a signal 1121.

The controller 20 generates gradient magnetic fields Gz, Gx, Gy, RF
The signal necessary to &IJ lit the amplitude of the pulse (
It is configured to be able to output a control wJ signal (analog signal) and a control wJ signal (digital signal) 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 2o, and the present invention can be implemented even if other elements, such as the computer 13, have this function.

The operation of the apparatus of the present invention configured as described above will be explained step by step with reference to FIG. 9 and Tables 1 to 3.

(1) Time t0 At time t0, a current is passed from the control circuit 2 to the static magnetic field coil 1, and the controller 20 is in a state where static tri jiA Ho is applied to the subject (the subject is installed in the cylinder of each coil). This is the point in time when a current is passed through the control circuit 4 to the coil 31 for two gradients and one field, and a two-gradient magnetic field Gz is applied as shown in FIG. 9(b).

At this point, the center of the slice plane (the part where the magnetization M correctly rotates 90 degrees by applying a 90 degree pulse), the boundary of the slice plane (the part where the magnetization M rotates θ° when the 90 degree pulse is applied),
(The part that rotates by 18o due to Gz=O when applying a 180° pulse), outside the slice plane (not affected by applying a 90° pulse, 1
The direction of each magnetization M in the part where the direction of magnetization M is reversed by the 80° pulse is shown in (f) and (g) in FIG.

As shown in (h), all the directions are in the positive direction (upward) of the Z axis.

Next, while Gz is given, a specific side (slice) of the object is detected by an RF signal modulated into a predetermined shape (for example, Gaussian shape) with a phase difference O0 selected and outputted by the gate circuit 30. excite the nucleus of the surface). That is, the first 90°X pulse is applied as shown in FIG. 9(a).

(11) At time 1°, the X-gradient Il field coil 32 is energized, and (
As shown in c), a gradient magnetic field Qx of a predetermined magnitude is applied for a time t.
Apply for x. Due to this gradient magnetic field gX, the atomic nuclei on the sliced surface perform the movement as already described in FIG.
The spins are phase encoded. Since this phase encoding operation was explained in detail in FIG. 7, the explanation will be omitted here.

Regarding the gradient 1ift jJA G x in the X direction, the application time tx may be changed, and the magnetic multifield strength Qx
may be changed. The reason for this is that the phase shift is expressed as the product of tX and gx, and the larger this product is, the greater the phase shift of the spins becomes.

At the same time, a y gradient magnetic field Qy is applied as shown in FIG. 9(d). Qy is, for example, a gradient magnetic field in the negative direction, and under the influence of this, the spin of the nucleus on the slice plane is
Time 1 in Figure 9. It is dispersed from the convergence state shown in (to).

Then, after time tX, the gradient magnetic '! ! Q Apply a y gradient magnetic field 9y- of polarity opposite to y (positive direction).

(Since Jy- has the opposite polarity to gy, the spins that were moving (dispersion direction) by receiving the energy of Qy start moving in the opposite direction. In other words, they move in the direction of convergence. The phase change m given by Qy in Figure 9 (d) and Qy
At the time when the phase changes m of '' become equal, the spins are again in phase on the y-axis, and the first echo signal becomes maximum, as shown in FIG. 9(e). After that, the spins do not stop moving, so the phase that matches on the y-axis is
It goes in the direction of dispersion again, and the first echo signal disappears.

Note that in the above, Qy is a gradient magnetic field in the negative direction,
Qy" was explained as a gradient magnetic field in the positive direction, but gy
As long as the polarities of and gy' are opposite, either one may be positive or negative. The reason is that if the polarities are opposite, spins that have been dispersed can be reconverged.

As mentioned above, if the time point at which the magnetic field Q x + Q y is applied is 1, then this time point 1. Then, the magnetization M of each part is the 9th
It roars in the directions shown in Figures (F), (G), and (H).

The first echo signal generated by the above-mentioned 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 subjected to phase 1 detection and stored in the wave memory circuit 11. . The stored data is read by the computer 1 at appropriate timing.

(iti) Time t2 After 1 time Ts has elapsed since the application of the RF pulse (90″ The modulated RF multiplied signal excites the object under test.In this case,
The two-gradient setting Gz is not operated, and the first 180''-x pulse is applied to the entire subject as shown in FIG. 8(A).

Therefore, atomic nuclei existing outside the specific slice are also excited.

(iii) Time t3 After applying the 180”-x pulse, the gradient magnetic field Qy-
Apply. This time point is defined as t3. The magnetization M rotates counterclockwise about the X axis as the center of rotation, and
It will look like Figures (F), (G), and (C).

Here, the slopes W1jiAGx and Gy are controlled so that the respective integral values of the gradiometers i'UGx and Gy are the same before time t2 and after time t3. This is because by controlling in this way, the magnetization M is gathered exactly after the time TS2. This gradient magnetic field Gx.

In order for each integral value of Gy to be the same before time t2 and after time t3, - for example, in the interval TS
1 and s2, it is sufficient to apply Gx and Gy to reverse the time axis. Of course, it is not necessary to control the gradients m'AGx and Gy so as to reverse the time axis (so as to make them symmetrical) as long as the integral values are the same before and after time points t2 and t3.

After time t, the magnetization M, which was heading in the direction of dispersion, is completely reversed in direction by the 180° pulse and moves in the direction of gathering. Therefore, a second echo signal is detected from the detection coil 8, which gradually increases as shown in FIG. 9(e).

Furthermore, in the example of FIG. 9, gradient magnetic fields gx and Qy are applied as shown in FIGS. 9(C) and (D) 1p time after time t3 so as to be symmetrical about the central time of t2 and t3. . However, in order to ensure symmetry, the gradient magnetic fields Qx and Q
It is not necessary to add y. As mentioned above, it is sufficient to make the integral values the same.

~) Time t4 After tp waiting time from time t3, gradient magnetic field Gx-Qx and Gy=
Add Qy, then after time tx, Gx=O1G
The time point when y-0 is set is t4.

This is the gradient magnetic field Gx for the middle time between t2 and t.
, Gy are the same, and since the RF multiplied signal of the 180'-X pulse is added in the middle, the direction is exactly 180° reversed from the direction of the magnetization M at time t1.

Therefore, Fig. 9 (F), (G), and (H) are obtained.

After time t4, Gz-, Gz+ are given, and under that condition,
The first 90" with a phase difference of 180° in the gate circuit 30
A second 90°-X pulse is applied to the subject using an RF multiplied signal modulated in the same manner as the pulse, and the slice plane excited by the first 90'' pulse is re-excited. The end of this excitation is defined as time t5. At this time, inside the slice plane, outside, boundary,
In other words, the directions of magnetization M of all the objects to be examined are aligned in the 12-axis direction.

(Vil Time t6 After the application of Gz+ is completed, the subject is excited by the RF multiplied signal modulated into a rectangular waveform with a phase difference of O0 and output from the gate circuit 3o (180° pulse excitation).

That is, since there is no two-gradient magnetic field, atomic nuclei located outside the specific slice plane are also excited. The end point of this excitation is assumed to be t6.

By applying this second 1800 pulses, the magnetization M is all aligned in the +2 axis direction.

In this way, at time t6, the starting time 1. It will return to the same state. However, in this method, relaxation due to longitudinal relaxation or transverse relaxation of the material remains, and the magnetization M
is not completely upward.

Therefore, a waiting time Td is provided after time t6, one sequence ends when the magnetization M becomes completely upward, and the same sequence is repeated thereafter.

Table 1 Table 2 Table 3 In the above description, the magnetization M of the slice plane is phase-encoded by applying the X-gradient lfi field Gx, and the y-gradient device 1
Although it has been explained that a signal with a different frequency is obtained for each column by applying Qy and Qy' with different polarities as the field Gy, the operation of the invention can be realized even if the relationship between the y-axes is switched. This is obvious and is within the scope of the present invention. That is, the phase of the magnetization M of the slice plane may be encoded using the y gradient Vn@G y, and a signal with a different frequency may be obtained for each column using the X gradient magnetic field GX.

In FIG. 9, the times at which Gz'', Qx, and Qy+gy' are applied are just one example, and after phase encoding Qx,
It is sufficient to obtain a signal while applying gy'.

That is, Gz-, 9x, and Qy may be applied at the same time, or may be applied with some deviation.

Further, until a dynamic equilibrium state is obtained, the obtained NMR signal may not be used as data from the first sequence to about the 10th sequence.

Note that in the above description, the first and second NMs shown in FIG.
Although it has been described that the R signal is detected and Fourier transformed to be used to reconstruct an image, 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.

(i) Detecting one of the first and second NMR signals and reconstructing an image using this detection signal.

(11) detecting both the first and second NMR signals;
The image is reconstructed using one of these detection signals.

Tsukuda) Detect both the first and second NMR signals, and
The data of the two detection signals are added and averaged to reconstruct the image.

(Detect both the first and second NMR signals, and
After Fourier transforming the two detection signals, they are added and averaged in the projection state to reconstruct the image. Or, add and average the two images.

In such a sequence, the holding time Td is much shorter than in the conventional sequence. Figure 13 shows this situation, and the sample used is egg white (longitudinal relaxation time T+-69
3m5. Transverse relaxation time T2 = 262ms) is used, and Ts
The figure shows a case where + +Ts 2 =30 ms. In the figure, the horizontal axis is the duration Td, and the vertical axis is the signal strength after reaching a dynamic equilibrium state. The dashed line curve A is the actual value measured using the conventional method (matching the theoretical value), and the solid line curve B is the measured value (corresponding to the theoretical value). Actual measurement li! when using the method of the present invention! (consistent with the theoretical value). As is clear from the figure, it takes a much shorter time (T
It turns out that d) is sufficient.

In addition, in the example, in one sequence, the applied RF pulse is 90°X...180°-X...90
° - y
A pulse may be applied to a predetermined atomic nucleus with a phase relationship of .

Here, for example, the meaning of the RF pulse "90°-X" is that when this pulse is applied, the magnetization M moves to a position rotated 90" counterclockwise with the X axis as the rotation axis. means.

Further, 90°y'' means that the magnetization M moves to a position rotated 90'' clockwise with the y-axis as the rotation axis.

In addition, the RF pulse should be "90'x" or "90'y".
It can be selected by adjusting the phase of the high frequency waveform of the RF pulse. For example, in the case of these two pulses, the high frequency phase may be changed by 90'. Normally, this selection is made by gate circuit 3 in FIG.
This is done at o.

Next, the fact that multi-slices can be used in combination according to the present invention will be explained with reference to FIGS. 1o and 11.

Figure 10 shows the pulse sequence of Figure 9 at Tτ-100.
This is the result of a computer simulation of a state in which dynamic equilibrium is reached by continuous execution at m5 (repetition time), and shows the distribution of the biaxial magnetization Mz in the slice direction immediately before the first 90'' pulse. Here, T I , T
For 2, biological values were used. Mz shown in FIG. 10 corresponds to the NMR signal intensity obtained within the slice plane.

Furthermore, the out-of-plane Mz corresponds to the signal intensity when multi-slicing is performed.

Figure 11 shows NMR after selective excitation by applying the first 90'' pulse and two gradient magnetic fields Gz to the Mz state in Figure 10.
It represents the signal strength.

In the same figure, 5R (SatLIr
T r in the atiOn recOVerl/) method
> T + and T r = 100 mS are also shown.

As explained in the conventional example, the disadvantages of the DEFT method are ■ Multi-slice is not possible (Mz outside the slice plane is small in Figure 10) ■ Slice shape becomes three peaks (2 and 2 in Figure 11) In comparison, in the pulse sequence of the present application, Mz is large even outside the slice plane as shown in FIG. 10, so multi-slice can be used in combination. Further, from FIG. 11, improvements have been made such as the slice shape can be close to a rectangle.

On the other hand, when using the SR method, at TT-100mS,
The signal strength is less than half that of the present invention, and TT>
In the case of T, there is a drawback that Mz outside the slice plane is too small compared to the central value, as in the case where the DEFT method is not selected.

In addition, in the pulse sequence shown in Fig. 9, the last two combinations of 90° and 180° pulses are either (1), one selective excitation 270° pulse (II), or one non-selective excitation 270° pulse. It is impossible to replace. The reason is as follows.

In (1), considering the operation of M at time t4 in (g) and (h) in Figure 9, the application of (1) actually increases the RF magnetic field strength because (g) is the slice boundary. It becomes 270° and does not return to the +2 axis.

Furthermore, the area outside the slice plane (h) remains oriented in the 12-axis direction due to selective excitation.

In (If), since the effect of the 270° pulse also exists on the slice boundary (G), M rotates too much and does not return to the +2 axis. Similarly, M rotates too much outside the slice plane (H). Similar to the DEFT method, these methods result in a decrease in NMR signal intensity and a deterioration in slice shape, and are unsuitable as an imaging technique.

FIG. 12 shows another pulse sequence of the device according to the invention. The NMR signal obtained by the pulse sequence shown in FIG. 9 is only an echo signal, but the FrO signal is also obtained from the pulse sequence shown in FIG. 12. The pulse sequence of FIG. 12 differs from that of FIG. 9 in that a gradient magnetic field of Qy is not applied in the gradient magnetic field Gy. That is, in FIG. 9, Jy is added to the gradient magnetic field Gy= during the period tx during which the gradient magnetic field Gx-Qx is applied. As a result, the converged magnetization M is once dispersed, and then, by applying Gy=gy-, the magnetization M is converged again, and the first echo (No. 8) is obtained.

On the other hand, in the sequence shown in FIG. 12, since no Gy-Qy gradient magnetic field is applied, an FID signal is immediately obtained instead of the first echo shown in FIG.

The present invention can also be achieved by detecting this F[D signal and using it for image reconstruction. Other operations in Figure 12 are as follows:
Since this sequence is similar to the sequence shown in FIG. 9, the explanation will be omitted.

c. [Effects of the present invention j As described above, according to the present invention, FIGS. 9 and 12
By the pulse sequence shown in the figure, 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) at the time when the sequence for one view is completed. Therefore, the conventional method (for example, the SR method) is difficult.

It is not necessary to have natural relaxation due to the pulse sequence, and the interval between pulse sequences can be shortened, and the scan time can be shortened.

[Brief explanation of the drawing]

Figure 1 is a diagram of the groove diagram of an embodiment 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 of a hydrogen atom. A diagram explaining the state in which the atomic nuclei are aligned in the direction of the magnetic field, and Figure 5 is a diagram showing an example of the test pulse waveform of a conventional device using the Fourier transform method.
FIG. 6 is a diagram showing magnetization M in a rotating coordinate system, FIG. 7 is a diagram for explaining phase encoding of magnetization M in the Fourier transform method, and FIG. 8 is a structural diagram showing an example of a magnetic field coil.
Fig. 9 is a diagram of operating waveforms and magnetization vectors to explain the sequence of the device according to the present invention, and Fig. 10 is a computer simulation of a state in which the sequence of Fig. 9 is continuously executed and a dynamic equilibrium state is reached. A diagram showing the results of
FIG. 11 is a diagram showing the NMR signal intensity after selective excitation by applying the first 90" pulse and two gradient magnetic fields G2 to the Mz state in FIG. 10, and FIG. FIG. 13 is a diagram of an operation waveform and a magnetization vector for explaining another sequence example, and a diagram showing the relationship between duration and signal strength. 1... Static magnetic field coil, 2... Static magnetic field coil. Control circuit for magnetic field coil, 3... Coil for gradient magnetic field, 4... Control circuit for gradient magnetic field coil, 5... Excitation coil, 6...R
F 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...2 gradient magnetic field coil, 32.33
...Y gradient magnetic field coil. Figure M2 (A) (B) (B) (B) Tail 5 Figure 6 (A) (B) No. 8 (B) Weak view (B)

Claims (4)

[Claims] (1) Means for applying a static magnetic field (H_0) to the subject, means for applying a gradient magnetic field to the subject, and means for applying high-frequency pulses for imparting nuclear magnetic resonance to the nuclei of atoms constituting the tissue of the subject. An apparatus for obtaining an image of a tissue of a subject using the generated nuclear magnetic resonance signals, comprising a control means having a sequence function described in parentheses below, repeating this sequence, and NM characterized in that necessary signals among nuclear magnetic resonance signals generated for each sequence are used for image reconstruction.
R image device. "The means for applying a gradient magnetic field is operated to apply a first gradient magnetic field, and at the same time, a first 90° pulse is applied from the means for applying a high-frequency pulse so that atomic nuclei present in a specific slice plane of the object are and then apply a first 180° pulse from the high-frequency pulse applying means without operating the gradient magnetic field applying means to also excite atomic nuclei located outside the specific slice plane. , Next, the means for applying a gradient magnetic field is operated to apply the first gradient magnetic field, and a second 90° pulse is applied from the means for applying a high frequency pulse to obtain the same specific slice as described above. A second 180° pulse is applied from the high-frequency pulse applying means without operating the gradient magnetic field applying means to excite the atomic nuclei existing on the plane, and then to excite the atomic nuclei existing outside the specific slice plane. and in a section T_S_1 between the first 90° pulse and the first 180° pulse, operate means for applying a gradient magnetic field, and generate a second gradient magnetic field in a direction different from the first gradient magnetic field. is applied to give a phase change amount, and the means for applying a gradient magnetic field is operated to apply a third gradient magnetic field in a direction different from the first and second gradient magnetic fields by switching two values of different polarities. , Furthermore, in the interval T_S_2 between the first 180° pulse and the second 90° pulse, the second and third gradient magnetic fields and the circumferential gradient magnetic field in the interval T_S_1 are respectively applied, and the gradient magnetic field in the circumferential direction is applied for each sequence. A sequence function that sets the second gradient magnetic field strength and application time to the values necessary for imaging. (2) In the interval T_S_1 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_S_1 is applied, and the second gradient magnetic field intensity and application are applied for each sequence. 2. The NMR imaging apparatus according to claim 1, further comprising a control means having a sequence function for setting 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 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 NMR imaging apparatus according to claim 1 or 2.