JPH0451170B2 - - Google Patents

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention is a nuclear device that utilizes the phenomenon of nuclear magnetic resonance (hereinafter referred to as NMR) to know the distribution of specific atomic nuclei within a subject from outside the subject. Regarding magnetic resonance imaging devices, in detail, a pulse system that shortens the waiting time from one pulse sequence to the next pulse sequence and also shortens the relaxation time during each sequence, shortening the overall operating time. Regarding speeding up sequences.

(Conventional technology) Conventionally, as an inspection device using NMR,
Using the same principle as X-ray CT, protons in a virtual slice of the object are excited, NMR resonance signals corresponding to each projection are obtained in many directions of the object, and the NMR resonance at each position of the object is calculated. Some methods use reconstruction methods to determine signal strength.

FIG. 2 is an operational waveform diagram for explaining an example of an inspection method in such a conventional device.

A static magnetic field H _O is applied to the subject in the z-axis direction, and the second
As shown in Figure B, a Z gradient magnetic field G _z ⁺ is applied, and as shown in Figure B, an RF pulse (90° pulse) with a narrow frequency spectrum (f) is applied. In this case, only the protons whose Larmor angular velocity is ω = γ (H _O + ΔG _Z ) are excited, and if the magnetization M is expressed on a rotating coordinate system rotating at an angular velocity ω as shown in Figure 3A, then y ¹ The direction is changed by 90° in the axial direction. Next, as shown in FIG. 2C and D, an x gradient magnetic field G _x and a gradient magnetic field G _y are applied, thereby creating a two-dimensional gradient magnetic field, and an NMR resonance signal as shown in E is detected. Here, as shown in Figure 3B, 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 eventually
The NMR resonance signal decreases, as shown in Figure 2 E.
It disappears after T _S time. If the NMR resonance signal obtained in this way is Fourier transformed, x
This is a projection in the direction perpendicular to the gradient magnetic field synthesized by the gradient magnetic field G _x and the y gradient magnetic field G _y .

Thereafter, after waiting for a predetermined time _Td , the next sequence is repeated in the same manner as described above. In each sequence, G _x and G _y are changed little by little.
This allows each projection to
NMR resonance signals can be determined for numerous directions of the object.

(Problem to be Solved by the Invention) By the way, in the conventional device that operates as described above, the time T _S until the NMR resonance signal disappears is 10 to 20 mS in Fig. 2, but the following sequence The predetermined time T _d required for the transition to is approximately 1 sec due to the relaxation time T ₁ . Hence,
If a cross-section of a single object is to be reconstructed using, for example, 128 projections, the measurement would require a long time of at least 2 minutes, which is one of the major obstacles to achieving higher speeds. There is.

In addition, in NMR analyzers, ED is used as a method to speed up the pulse sequence.
Becker et al. J.Americans Chemical Society p7784
~7785 December1969 or “Pulse and Fourier Transform NMR” by Farrer, Betzker (translation)
DEFT (Driven Equilibrium
There is a Fourier Transform) method. This method is 3
T _d
It is repeated at intervals of seconds, and the speed is increased by bringing the magnetization M to a thermal equilibrium state with the second 90° pulse. However, when attempting NMR two-dimensional imaging using this pulse sequence, there are the following drawbacks.

(1) When performing two-dimensional imaging with DEFT (90° _x , τ, 180° _y , τ, 90° _-x , T _d ) ⁿ , selective excitation is used in which a 90° pulse is applied simultaneously with a gradient magnetic field. There is no particular problem in exciting only within a specific slice plane, but
For the 180° pulse, both selective excitation and non-selective excitation are possible.

FIG. 4 shows the distribution of magnetization M _z on the z-axis in the slice thickness direction immediately before the first 90° pulse when the pulse sequence of the DEFT method is continuously performed. Here, the 90° pulse is Gaussian modulated for selective excitation, and the average T ₁ ,
Using T ₂ and T _r = 100 mS (repetition time),
The magnitude of M _z obtained by calculating the distribution of magnetization M _z by simulation corresponds to the NMR signal intensity.

Now, generally the latency of the pulse sequence
During T _d , the same pulse sequence is sequentially applied to other slice planes to obtain a sufficiently long T _d for the original pulse sequence, and M _z
A so-called multi-slice method is used in which a pulse sequence is performed in the next view in the original slice plane after T ₁ has relaxed and become larger. This method does not reduce the NMR signal and allows data from multiple planes to be obtained simultaneously.
It is effective as a condensed fast method, but this requires that M _z outside the slice plane be a large value without being influenced by excitation of other slice planes.

In the DEFT method using an unselected 180° pulse,
Since M _z outside the slice plane becomes small as shown in waveform A in FIG. 4, there is a drawback that the multi-slice method cannot be used in combination.

(2) The actual slice shape is obtained by multiplying M _z in FIG. 4 by the slice shape relationship (in this case Gaussian shape), which is shown in FIG. 5.

When using a 180° selective excitation pulse, the fourth
In the figure, it is a distributed waveform like B, and _Mz outside the slice plane is large and there is no particular problem, but there is a problem that the slice shape has three peaks like waveform B in FIG. 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.

(Means for Solving the Problems) It is an object of the present invention to provide an NMR imaging apparatus that eliminates such drawbacks, shortens scan time, and increases speed.

In order to achieve this purpose, the present invention provides a first 90° pulse, a first 180° pulse, and a second 90° pulse.
A pulse sequence of a 90° pulse and a second 180° pulse is adopted, and the second 90° pulse directs the magnetization M toward the −z axis, and immediately after that, a second 180° pulse is applied. By orienting M in the +z-axis direction, it is possible to return M to a thermal equilibrium state or its vicinity, shorten the sequence interval, and further reduce the interval T _s1 from the first 90° pulse to the first 180° pulse and the first
The time integral value of the magnetic field strength is made equal for each gradient magnetic field in the interval T _s2 from the 180° pulse to the second 90° pulse, and when T _s1 > T _s2 , the interval T _s1
The present invention is characterized in that it can be controlled to use the nuclear magnetic resonance signal generated in the interval T _s2 when T _s1 <T _s2 .

(Example) The present invention will be described in detail below using the drawings. FIG. 6 is a block diagram showing the configuration of an embodiment of a device for implementing the method of the present invention. In the figure, 1 is a static magnetic field coil for generating a uniform static magnetic field H _O (in this case, the direction is the Z direction);
Reference numeral 2 denotes 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 H _O of the magnetic flux generated by the static electromagnetic field coil 1 is about 0.1 T, and that the uniformity is 10 ^-4 or more.

3 shows a general overview of gradient magnetic field coils;
4 is a control circuit for this gradient magnetic field coil 3.

FIG. 7A is a configuration diagram showing an example of the gradient magnetic field coil 3, in which the Z gradient magnetic field coil 31, the y gradient magnetic field coils 32, 33, although not shown, have the same shape as the y gradient magnetic field coils 32, 33. and includes an x-gradient magnetic field coil that is rotated by 90°. This gradient magnetic field coil generates a magnetic field having linear gradients in the x, y, and z axis directions in the same direction as the uniform static magnetic field H _O. Control circuit 4 is controller 2
0 (details will be described later).

Reference numeral 5 denotes an excitation coil that provides an RF pulse with a narrow frequency spectrum as an electromagnetic wave to the subject, and its configuration is shown in FIG. 7B.

6 is the frequency corresponding to the NMR resonance condition of the atomic nucleus to be measured (for example, 42.6M for protons)
Hz/T), the output of which is a gate circuit 30 whose opening and closing are controlled by signals from the controller 20 (details will be described later);
It is applied to the excitation coil 5 via the power amplifier 7. Reference numeral 8 denotes a detection coil for detecting NMR resonance signals in the subject, and its configuration is the same as the excitation coil shown in FIG. Although it is desirable that this detection coil be installed as close as possible to the subject, it may also be used as an excitation coil if necessary.

9 is an amplifier that amplifies the NMR resonance signal (FID: free induction decay) obtained from the detection coil 8; 10 is a phase detection circuit; 11 is a wave memory circuit that stores the phase-detected waveform signal from the amplifier 9; /D converter. 13 is a computer that inputs the signal from the wave memory circuit 11 via a transmission line 12 made up 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 similar to a digital monitor. Further, necessary information is transmitted from the controller 20 to the computer 13 via a signal line 21.

The controller 20 can output signals (analog signals) necessary for controlling the gradient magnetic fields G _z , G _x , G _y and control signals (digital signals) necessary for transmitting FR pulses and receiving FID signals. It is configured so that it can be done.

The gate circuit 30 receives the RF signal from the oscillator 6, and receives the RF signal from the oscillator 6, and generates four types of signals having a phase difference of 90 degrees, that is, a phase difference of 0 degrees, 90 degrees, 180 degrees, and 270 degrees.
Create an RF signal, select one of the four types of signals based on instructions from the controller 20, and further
A drive signal for the exciting coil 5 is obtained by modulating it with an RF modulation signal.

The operation of the device of the present invention configured in this way is as follows:
This will be explained next with reference to the operational waveform diagram in FIG.

(1) In a state where a current is passed from the control circuit 2 to the static magnetic field coil 1 and a static magnetic field H _O is applied to the subject (the subject is installed inside the cylinder of each coil), the controller 20 causes the static magnetic field to flow through the control circuit 4. A current is passed through the Z gradient magnetic field coil 31 to provide a Z gradient magnetic field G _z ⁺ as shown in FIG.

At this time, the center of the slice plane (the part where magnetization M rotates correctly by 90 degrees when a 90 degree pulse is applied), the slice plane boundary (the part where M rotates by θ degree when a 90 degree pulse is applied,
Also, when a 180° pulse is applied, G _z = 0, so the part rotates 180°), outside the slice plane (a part that is not affected by a 90° pulse, and the direction of magnetization M is reversed by a 180° pulse) ) at each magnetization M
The directions are all in the positive direction of the z-axis (upward) as shown in F, H, and H in FIG.

Given G _z ⁺ , the gate circuit 30
Excite one surface (slice surface) of the object with the RF signal modulated in a predetermined shape (e.g. Gaussian shape) with a phase difference of 0° and output from the 11th
Give the first 90° _x pulse as shown in Figure A)).

(2) Next, the x-gradient magnetic field coils and the y-axis gradient magnetic field coils 32 and 33 are energized, and as shown in FIG.
As shown in d, magnetic fields G _x and G _y of predetermined magnitude are applied.

In addition, in Figure 1B, G _z ^- following G _z ⁺ is
This is a waveform signal for matching the phases of NMR resonance signals from different parts of a subject, and this technique is a known technique.

At time t ₁ when the magnetic fields G _x and G _y are applied, the magnetization M of each part becomes oriented as shown in FIG.

(3) Next, as shown in FIG. 1C, a linear gradient magnetic field g _x-1 in the x-axis direction is applied for a short time (t _x1 ). This causes a spin phase shift (phase encoding) in the x direction. At the same time, as shown in figure 2, a linear gradient magnetic field g _y1 in the y-axis direction with different polarity,
Apply g _y2 sequentially. The spins dispersed by g _y1 are collected by g _y2 and become an echo signal.
This echo signal appears as shown in FIG. 1(e) and is detected by the detection coil 8.

(4) After applying the first 90° pulse (90° _x pulse)
First 180° pulse (180° _-x pulse) after T _s1 hour
give. At this time, all gradient magnetic fields are zero.

(5) After applying the first 180° pulse, apply gradient magnetic fields G _z and G _x , and apply the second 90° pulse (90° _-x pulse) T _s2 hours after applying the first 180° pulse. Reverse the axis and apply in the same manner as above. At this time, a gradient magnetic field _Gy is also applied.

In this case, the following relationship is satisfied for each gradient magnetic field.

∫T _s1 Gdt＝∫T _s2 G′dt (1) However, G: Magnetic field strength in T _s1 section G′: Magnetic field strength in T _s2 section Note that data is not measured in T _s2 section. Therefore, the magnitude and application time width of G _x and G _y are not particularly limited, and the NMR signal generated in this section is skipped.

(6) Apply a second 180° pulse (180° _x pulse) following the second 90° pulse. Note that the third 180° pulse is applied immediately after the application of the second 909 pulse, or after a sufficiently shorter time than (t ₄ −t ₃ ) or (t ₆ −t ₅ ) has elapsed. As a result, as shown in FIGS. 1(f) to (h), the magnetization M in the center, boundary, and out-of-plane regions of the slice plane are all directed in the +z-axis direction.

(7) Following the application of the second 180° pulse, spoilers _z , H _x ,
Apply _Hy .

As described above, the state is returned to the same state as at the beginning (immediately before the application of the first 90° pulse).
However, in this method, relaxation due to spin-spin relaxation T ₂ of the material remains, and the direction of magnetization M is not completely + at the time immediately after applying the second 180° pulse.
They are not aligned in the z-axis direction. Therefore, as shown in the figure, T _d
One sequence is completed after waiting for the magnetization M to be completely in the +z-axis direction.
The same sequence is repeated thereafter.

The waiting time T _d in this case is shorter than the waiting time T _d in the conventional sequence shown in FIG. Furthermore, T _s2 can also be made sufficiently short within a range that satisfies the above formula (1).

It should be noted that the present invention is not limited to the above-described embodiment, and may be implemented, for example, in a sequence as shown in FIG. In the sequence shown in FIG. 8, T _s1 is shortened, whereas the sequence shown in FIG. 1 is made faster by shortening T _s2 . Therefore, as shown in Figure E, the NMR generated in the T _s2 interval is
Use signals as data.

Steam boiler applied immediately after the first 180° pulse
H _z2 , H _x2 , and H _y2 are for removing noise signals due to errors in the first 180° pulse. Also in this case, a gradient magnetic field is applied so that the above equations (1) and (2) are satisfied.

FIG. 9 is a diagram showing a pulse sequence in still another embodiment of the present invention. In this case, the first
A 180° pulse is added before the pulse sequence shown in the figure or the pulse sequence shown in FIG. 8 (indicated by a chain line L in the figure). In this sequence, the magnetization M _O is reversed in advance to the −z axis with an added 180° pulse, and M′, which has spontaneously relaxed during T′ time, is excited with a 90° pulse. By taking advantage of the fact that the difference in longitudinal relaxation time T ₁ is reflected in M′, that is, the signal strength, due to the relaxation of , and by selecting the optimal value for T′, it is possible to obtain an image in which T ₁ is emphasized.

According to such a pulse sequence of the present invention, the distribution of the magnetization _Mz on the z-axis in the slice thickness direction immediately before the first 90° pulse is as shown by the solid line C in FIG. has a preferable distribution as shown in real fiber C in FIG.

(Effects of the Invention) As described above, according to the present invention, all the magnetizations M inside and outside the slice plane can be forcibly and accurately brought into a thermal equilibrium state (or in the vicinity thereof) at the end of the sequence for one view. Therefore, there is no need to wait for natural relaxation due to T ₁ between each pulse sequence, and the pulse sequence interval can be shortened, making it possible to shorten the scan time.

[Brief explanation of drawings]

Figure 1 is a diagram of operating waveforms and magnetization vectors to explain the sequence according to the present invention, and Figure 2 is a diagram of conventional NMR using the inversion recovery method.
FIG. 3 is an operation waveform diagram showing a pulse sequence of the inspection device. FIG. 3 is a diagram for explaining the state of magnetization M, FIG. 4 is a diagram showing the distribution of magnetization M _z in the slice thickness direction, FIG. 5 is a diagram showing the distribution of signal intensity in the slice thickness direction, and FIG. 7 is a structural diagram showing an example of a magnetic field coil, and FIGS. 8 and 9 are operational waveform diagrams in other embodiments of the present invention. 1... Static magnetic field coil, 2, 4... Control circuit, 3...
Gradient magnetic field coil, 5... Excitation coil, 6... Oscillator, 8... Detection coil, 10... Phase detection circuit, 11
...Wave memory circuit, 13...Computer, 2
0...Controller, 30...Gate circuit.

Claims (1)

[Claims] 1. A means for applying a static magnetic field (H ₀ ) in the z-axis direction to a subject, a means for applying a gradient magnetic field to the subject, and a means for applying nuclear magnetic resonance to the nuclei of atoms constituting the tissue of the subject. A control device having a sequence function consisting of the following (a) to (d) in an apparatus for obtaining an image of the tissue of a subject using the generated nuclear magnetic resonance signals, An NMR imaging device characterized by comprising means. (a) Energize the means for applying the high-frequency pulses and apply the first 90° pulse, the first 180° pulse, the second 90° pulse, and the second 180° pulse in this order. (b) The application of the first and second 90° pulses is selective excitation in which a means for applying a gradient magnetic field is simultaneously activated to also apply the first gradient magnetic field to excite only a specific slice plane; The second 180° pulse application is non-selective excitation without applying a gradient magnetic field. (c) Said second
The 180° pulse is applied immediately after the second 90° pulse is applied, or after a sufficiently shorter time than the period T _s1 or the period T _s2 has elapsed. (d) Each gradient magnetic _{field is} In addition, when T _s1 > T _s2 , the nuclear magnetic resonance signal generated in the interval T _s1 is used,
When T _s1 <T _s2 , the nuclear magnetic resonance signal generated in section T _s2 is used.