JPS6179146A - Nmr image device - Google Patents

Abstract

PURPOSE:To shorten a scanning time by placing magnetism entirely inside and outside a slice surface in a thermally balanced state forcibly and accurately when a sequence for one view is finished. CONSTITUTION:A control circuit 2 flows a current to a static magnetic field coil 1 to apply a static magnetic field to an object body, and in this state a controller 20 flows a current to a Z slanting magnetic field coil 3 through a control circuit 4 to apply a Z slanting magnetic field. Then, an X slanting magnetic coil and a Y slanting magnetic field coil are excited. An NMR resonance signal obtained from a detection coil 8 is inputted to a computer 13 through a preamplifier 9, phase detector 10, and wave memory 11. Then, the computer 13 processes the NMR resonance signal as specified to form a tomographic image.

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 hyphen) to know the distribution of specific atomic nuclei within a subject from outside the subject. Regarding magnetic resonance imaging devices, in detail, we aim to shorten the waiting time for a pulse sequence to move on to the next pulse sequence, as well as shorten the relaxation time during each sequence, thereby shortening the overall operating time. Regarding speeding up pulse sequences.

(Prior art) Conventionally, as an inspection device using There is a method in which the dark resonance signal intensity at each position of the object is determined by a reconstruction method.

FIG. 2 is an operational waveform diagram for explaining an example of an inspection method using such a conventional apparatus K.

Apply a static magnetic field H8f to the subject in the z-axis direction, and apply a KZ gradient magnetic field G2 as shown in Fig. 2 (b) K and a U pulse (90 pulses) with a narrow frequency spectrum (f) as shown in Fig. 2 (b). do. In this case, the Larmor angular velocity is changed by 90 degrees in the yj-axis direction.Subsequently, an X gradient magnetic field is applied as shown in FIG. 2 (c). Here, as shown in FIG. 5(b), 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 m resonance signal eventually decreases, As shown in FIG. 2 (e), it disappears after KT8 hours.

When the I resonance signal obtained in this manner is subjected to Fourier transformation, it becomes a globulation in a direction perpendicular to the gradient magnetic field synthesized by the X gradient magnetic field G and the y gradient magnetic field G.

After that, wait for a predetermined time of 47t! and repeat the next sequence in the same manner as described above.In each 7th case, change Gx and G little by little.As a result,
m-resonance signals corresponding to each projection can be determined for numerous directions of the object.

(Problems to be solved by the invention) By the way, in the conventional device that operates in this way,
In Fig. 2, the time T until the m-resonance signal disappears is 10 to 20 ms, but the predetermined time T until moving to the next diatance is about 1 sec because of the relaxation time T. becomes. Therefore, if a cross-section of a single object is to be reconstructed using, for example, 128 Grosziex carbon, the measurement requires a long time of at least 2 minutes, which is a major obstacle in realizing high-speed measurement. It is one.

As a method for speeding up the pulse sequence in the NAO analyzer, E, D, Beaker et al.
Americans Chemical Society
y p, 7784-7785 December 196
9 or Farrar, Pecker, "Pulse and Fourier Transform Carp" (translation), P2O3, published by Yoshioka Shoten in 1976,
5.5 DEFT (D
riven Equilibrium Fourier
Transform) method. This method consists of three pulse sequences (90°, τ.

1ao'a ta 90°) is repeated at intervals of Td 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) DEFT method (900τ, 180°, τI 9
0-X'Td), when performing second-order
tation) and excite only within a specific slice plane18
Although there is no particular problem with 0°, both selective excitation and non-selective excitation are possible for Norse.

FIG. 4 shows the distribution of the magnetization M on the two axes in the slice thickness direction immediately before the first 90° pulse when the pulse sequence of the DEF1' method is continuously performed. Here, the K 90° pulse is Galaffan modulated for selective excitation, and the average Ti' 2 and T -100 m5 (repetition time) of the living body are used to calculate the magnetization M2 The magnitude of M2 corresponds to the NMR signal intensity.

In general, the same pulse sequence is sequentially applied to multiple other slice planes during the waiting time Td of the pulse sequence, and a sufficiently long Td is obtained for the original pulse sequence, so that M2 is relaxed by T1. A so-called multi-slice method is used in which a pulse sequence is performed on the original slice plane in parallel views after the size has grown. This method is effective as a pseudo-high-speed method because data from multiple planes can be obtained at the same time without a decrease in signal, but M2 outside the slice plane is not influenced by excitation of other slice planes. The condition is that it is a large value.

The DEF'r method using 1800 non-selected pulses has the disadvantage that the multi-slice method cannot be used in combination because M2 outside the slice plane becomes small as shown in the waveform in FIG.

(2) The actual slice shape is obtained by multiplying M2 in FIG. 4 by the slice shape relationship (in this case Gaussian shape), which is shown in FIG.

When using 180 selective excitation pulses, B
The distribution waveform is as follows, and M2 outside the slice plane is large and there is no particular problem, but the slice shape has five mountain shapes as shown in waveform B in FIG. 5, which is a problem. This occurs because the magnetization M at the slice boundary performs a complicated operation during the 180 pulses of selective excitation, so the vector directions of each M are different, 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 achieves high speed.

In order to achieve such an object, the present invention adopts a pulse sequence of a first 90° pulse, a first 180° pulse, a second 90° pulse, a second 1800° pulse, and a second 90° pulse. By applying a second 180° pulse to direct the magnetization M in the direction of the 12-axis direction with a ° pulse, immediately after that, by applying a second 180° pulse to direct the magnetization M in the direction of the Mt-+zz axis, M is returned to the thermal equilibrium state or its vicinity, and the sequence interval is In order to shorten the
Furthermore, in the interval T81 from the first 90° pulse to the first 180° pulse and the interval T82 from the first 180° pulse to the second 90° pulse, the time integral value of the magnetic field strength is determined for each gradient magnetic field. Under the condition that “51 >
”32 or Tsl<Ts□ as T8□ or T8
It is characterized by being configured so that it can be controlled so that □ becomes shorter.

(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 uniform static magnetic field ■.

(The directions in this case are assumed to be two directions.) A static magnetic field coil 2 is a control circuit for the static magnetic field coil 10, which includes, for example, a DC stabilized power source. The density Ho of the magnetic flux generated by the static magnetic field coil 1 is O, i'r
It is desirable that the uniformity is 10 or more.

Reference numeral 3 generally indicates a gradient magnetic field coil, and 4 indicates a control circuit for the gradient magnetic field coil 30.

FIG. 7(b) is a configuration diagram showing an example of a gradient magnetic field coil h3, in which a 2-gradient magnetic field coil 31, a y-gradient magnetic field coil 32.
33, although not shown, includes an X-gradient magnetic field coil which has the same shape as the Y-gradient field coil 32 and 33, but is installed rotated by 90 degrees. This gradient magnetic field fill generates a magnetic field having linear gradients in the xr, L, and z axis directions, respectively, in the same direction as the uniform silent magnetic field H0.

The control circuit 4 is a controller 20 (details will be described later) K
Therefore, it is controlled.

Reference numeral 5 denotes an excitation coil that provides an RP pulse with a narrow frequency spectrum f to the subject as an electromagnetic wave, the configuration of which is shown in FIG. 7 (b).

Reference numeral 6 denotes an oscillator that generates a signal at a frequency corresponding to the resonance condition of the atomic nucleus to be measured (for example, a2.6 MHz/T for protons), and its output is sent to the controller 2.
Gate circuit 30 whose opening/closing is controlled by a signal from 0
(Details will be described later) is applied to the excitation coil 5 via a power amplifier. A is a detection coil for detecting the m-resonance signal in the subject, and its configuration is the same as the excitation filter shown in FIG. 7 (b).
It is installed with a 0° rotation. 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 the NMR resonance signal (FID) obtained from the detection coil a.
10 is a phase detection circuit, and 11 is a wave memory circuit that stores the phase-detected waveform signal from the amplifier 9, which includes an 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 screen that displays the obtained tomographic image. , a display similar to a digital camera. Furthermore, the signal line 21 connects the controller 20 to the computer 13. information is transmitted.

The controller 20 generates a gradient magnetic field G2. Signals (analog signals) and FR required to control Gx, G,
It is configured to be able to output control signals (digital signals) necessary for X-pulse transmission and FID signal reception.

The gate circuit 3o receives the RF flaw signal from the oscillator 6, and generates four kinds of RF flaw signals having phase differences of 90°, namely, 0°, 90°, 180′, and 270°. One of the four types of signals is selected based on instructions from the controller 20, and this is further modulated with an RF modulation signal to obtain a drive signal for the exciting coil 5.

The operation of the apparatus of the present invention constructed in this way will be described below with reference to the operational waveform diagram of 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 H8 is applied to the subject (the subject is installed inside the cylinder of each coil), the controller 20 sends a current to the static magnetic field coil 1 via the control circuit 4. 2. A current is passed through the gradient magnetic field coil 31 to provide an IcZ gradient magnetic field Gz as shown in FIG. 1(b).

At this time, the center of the slice plane (90°) is the part where the magnetization M rotates correctly by 90° when the pulse is applied), the slice plane boundary (the part where M rotates 00° when the 90° pulse is applied, and G2- when the 180° pulse is applied). Since it is 0, it is 180°
The direction of each magnetization M outside the slice plane (not affected by 90° pulse application, 180°) (the part where the direction of magnetization M is reversed by the russian pulse) is shown in (to) in Figure 1. , ()), and (a), all of the two axes are in the positive direction (upward).

With Gz+ being given, the phase difference O0 selected and output by the gate circuit 30 is modulated into a predetermined form (
Excite one surface (slice surface) of the object with an RF wound signal (for example, Gaussian shape) (give a first 90° pulse as shown in FIG. 11).

(2) Next, the X gradient magnetic field coil and the 1st and 1st axis gradient magnetic field coils 32 and 33 are energized to generate magnetic fields G and G of a predetermined magnitude as shown in (c) and d) of Fig. 1. Apply it.

In FIG. 1(b), G- following G+ is a waveform signal for matching the phases of NMR resonance signals from different parts of the subject, and this technique is a known technique.

At the time t1 when this magnetic field Gx, G is applied, the magnetization M of each part becomes oriented as shown in FIGS.

(3) Next, as shown in FIG. 1(c), a linear gradient magnetic field gx□ in the X-axis direction is applied for a short time (t). This results in a spin phase shift (phase encoding) in the x direction.
occurs. At the same time, linear gradient magnetic fields gA02gy2 with different polarities in the y-axis direction are sequentially applied as shown in (1) in the same figure. The spins dispersed by gy□ are collected by gy2 and become an echo signal. This echo signal is shown in Figure 1 (E).
It appears as follows and is detected by the detection coil 8.

(4) The first 90° pulse (900x pulse)
The first 180° payas (180°
-X pulse). At this time, all gradient magnetic fields are zero.

(5) Gradient magnetic field G after applying the first 180° pulse,
Gx is applied, and 132 hours after application of the first 180° pulse, a second 90° pulse (90°-X pulse) is applied in the same manner as above with the time axis reversed. At this time, a gradient magnetic field G is also applied.

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

However, the strength of the magnetic field in the G, Ts4 section G': T
Magnetic field strength in S2 interval Note that data is not measured in T32 interval, so the magnitude of GG and application time width are not limited to ly, and the NMR signal generated in this interval is skipped.

(6) Apply a second 180° pulse (1800x pulse) following the second 90° pulse. As a result, as shown in FIGS. 1(f) to (g), the magnetization M in the center, boundary, and out-of-plane regions of the slice plane are all directed in the +2 axis direction.

(7) Following the application of g2O1,80' pulse,
Spoiler to remove correlation between sequences - Hzl
Apply HxlH.

As described above, the state returns to the same state as the initial point (first 90° pulse application direct spinning).

However, in this method, the spin-spin relaxation T
2 remains, and the direction of the magnetization M is not completely aligned with the +z-axis direction immediately after the second 1800 pulses are applied. Therefore, as shown in the figure, a waiting time of T is provided, and one sequence is completed when the magnetization M is completely in the +2 axis direction, and the same sequence is repeated thereafter.

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

Note that the present invention is not limited to the above embodiments,
For example, a sequence as shown in FIG. 8 may be used. In the sequence shown in FIG. 8, T8□ is shortened, whereas the sequence shown in FIG. 1 is made faster by shortening T32. Therefore, as shown in the same figure (e), KT8
NMR signals generated in two sections are used as data.

Spoiler applied immediately after the first 180° pulse - HHH
is due to the error of the first 1800 pulses z2' x2
'y2 This is for removing noise signals. Also in this case, a gradient magnetic field is applied so that the above-mentioned 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, a 180° pulse is added before the pulse sequence shown in FIG. 1 or the pulse sequence shown in FIG. 8 (indicated by a chain line in the figure). In this sequence, magnetization M is pre-magnetized by the added 180° pulse. Invert it to the -2 axis,
It is designed to excite M+, which has spontaneously relaxed during the T' waiting time, with a 90° pulse, and cleverly shows that the difference in longitudinal relaxation time Tx due to the T° waiting time relaxation is reflected in the M', that is, the signal strength. By using this method and selecting the optimum value of TI, an image in which T□ is emphasized can be obtained.

According to such a pulse sequence of the present invention, the distribution of the magnetization M2 on the two axes in the slice thickness direction immediately before the first 90° pulse is as shown by the solid line C in FIG. 4, and the actual slice shape is as shown in the solid line C in FIG. A preferable distribution as shown by the solid line C in FIG. 5 is obtained.

(Effects of the Invention) As described above, according to the present invention, all the magnetizations M inside and outside the slice plane are forcibly and accurately brought into thermal equilibrium (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 T1 between each pulse sequence, the pulse sequence interval can be shortened, and the scan time can be shortened.

[Brief explanation of drawings]

Fig. 1 is a diagram of operation waveforms and magnetization vectors for explaining the sequence according to the present invention, and Fig. 2 is a diagram of impurge, n, and magnetization vectors.
FIG. 2 is an operation waveform diagram showing a pulse sequence of a conventional NMR inspection apparatus using a recovery method. Figure S is a diagram for explaining the state of magnetization M, Figure 4 is a diagram showing the distribution of magnetization M2 in the slice thickness direction, Figure 5 is a diagram showing the distribution of signal intensity in the slice thickness direction, and Figure 6 is a diagram showing the distribution of magnetization M2 in the slice thickness direction. 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... Combi,
- data, 20...controller, 30...gate circuit. Atsushi 2 prisoner t□ h Atsushi 4 drawing y A5 Sono seal Σ blade (

Claims (1)

[Claims] (1) A means for applying a static magnetic field (H_0) in the z-axis direction to the subject, a means for applying a gradient magnetic field to the subject, and a high-frequency pulse for imparting nuclear magnetic resonance to the nuclei of atoms constituting the tissue of the subject. an apparatus for obtaining an image of the tissue of a subject by using the generated nuclear magnetic resonance signals, the apparatus comprising a control means having a sequence function consisting of the following (a) to (d). An NMR imaging device characterized by: (a) The means for applying the high frequency pulse is energized and the first
90° pulse, first 180° pulse, second 90° pulse
A pulse and a second 180° pulse are applied 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 a 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) The second 180° pulse is applied immediately after the second 90° pulse is applied. (d) First 180° from the first 90° pulse application
Section T_S_1 until pulse application and section T_S_ from first 180° pulse application to second 90° pulse application
2, for each gradient magnetic field, the integral value of the applied magnetic field is made equal, and T_S_1 is set so that T_S_1>T_S_2 or T_S_1<T_S_2.
Or shorten T_S_2.