JPH0751124B2 - NMR inspection device using chemical shift value - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an NMR imaging apparatus, and more particularly to an NMR examination apparatus capable of reflecting diagnostic information on a tomographic image of a subject by using a chemical shift difference.

[Background of the Invention]

A conventional NMR imaging device uses ¹ H nuclei to image its density or relaxation time. Recently, an attempt to separately image the density of ¹ H nuclei in water and fat was presented at the NMR Medical Association and has been attracting attention. For this, a tomographic image is captured in the same manner as in the past. Next, the signal acquisition sequence (called a pulse sequence) is changed so that the phase difference between the water and fat signals is 180 ° by utilizing the difference in resonance frequency caused by the chemical shift between water and fat, and the tomographic image is obtained. To shoot. From these two images, the next distribution map or fat distribution map is obtained by inter-image calculation. According to this method, it is necessary to take the image twice, which increases the inspection time required for the image taking. Further, there are drawbacks such as lack of accuracy due to an error caused by the fact that the two images do not match due to the movement of the subject during the two imaging.

[Object of the Invention]

An object of the present invention is to provide an NMR inspection apparatus using a chemical shift value that can accurately capture a distribution image of water and fat in a short time by using the chemical shift difference.

[Outline of Invention]

According to the present invention, a subject is placed in a magnetic field obtained by combining a magnetic field device that generates a static magnetic field and a gradient magnetic field device that gives gradients to magnetic fields in two directions orthogonal to each other, and an NMR signal of the subject is excited and detected. And a display device for displaying the processed NMR signal, wherein the NMR signal of the subject is 90 ° pulse and 1
It is configured to combine 80 ° pulses and to detect as an echo signal by at least one gradient magnetic field, and two types of 90 with different echo times from selective excitation of nuclear spins by 90 ° pulse to generation of NMR echo signal. ° -180
The present invention is characterized in that a means for setting a spin echo sequence to be occupied by a ° -echo and for alternately executing two kinds of spin echo sequences having different echo times is provided.

Example of Invention

 Examples of the present invention will be described below.

FIG. 1 shows a block diagram of the NMR inspection apparatus according to the present invention. A subject (not shown) is placed on the patient table 1 and introduced into the magnetic field 2. Inside the magnet, a coil for applying a gradient magnetic field having gradients in three directions of X, Y, and Z and a high-frequency coil for exciting and observing the NMR phenomenon (both not visible in the figure) are inserted. A gradient magnetic field power supply 3 is connected to the gradient magnetic field coil and is driven by a control signal from a computer 4. The NMR signal is detected by the high frequency coil, amplified by the high frequency circuit 5 to an appropriate level, and input to the computer. The NMR signal is arithmetically processed in the computer and displayed on the display monitor 6.

A pulse sequence, which is a spin echo sequence used in the NMR inspection apparatus shown in FIG. 1, is shown in FIGS. 2 and 3.

In the pulse sequence A shown in FIG. 2, a linear gradient magnetic field is used.
If Gz is applied and the selected high-frequency magnetic field is applied to the subject, the nuclear spins in the slice plane can be selectively inclined by 90 °. (This is said in detail in Japanese Examined Patent Publication 60-12574). After the slice plane is determined, a gradient magnetic field Gy in the Y direction is given in order to give in-plane position information to the nuclear spins.
Is applied. The resonance frequency of nuclear spins increases in proportion to the size of Gy. Since this is observed as a phase difference, the Y direction is usually called the phase encode direction.
After phase encoding in the Y direction, a gradient magnetic field Gx is applied in the X direction and signals are sampled at 256 points. When the signal sampling is repeated by changing the size of Gy in 256 steps, 2
Data can be obtained in a 56 × 256 matrix. A desired NMR tomographic image can be obtained by performing two-dimensional Fourier exchange on these data. In the 90 ° -τ-180 ° pulse sequence, the NMR signal is obtained as an echo signal after τ time.
When applying Gx, which is called the echo time, from the center of the 90 ° pulse that causes selective excitation to the center of the echo, adjustment is made so that the shaded areas a and b in FIG. 2 are equal in area. That is, between 90 ° -180 ° pulses and 180 °-
There is no phase rotation due to the gradient magnetic field Gx between the echo signals. Since the sampling of the echo signal is effectively performed in proportion to the applied amount of Gx from the time when Gx is 0, the phase information in the X direction is accurately captured.

In the pulse sequence B of FIG. 3, the interval of 180 ° -echo signal is extended by τ ₂ . As described above, if the pulse sequence B having the echo time different from the echo time of the pulse sequence A is used to form two types of pulse sequences, the phase difference due to τ ₂ time is added to the echo signal when there are two or more components in the subject. Appears as. This is because the phase matching by the 180 ° pulse occurs after τ ₁ hour, and from this time point, each component exhibits precession at its own resonance frequency.
This resonance frequency difference is called a chemical shift.

The case where this pulse sequence B is applied to water and fat will be described. The chemical shift difference between water and fat is about 3.5 ppm. 0.15
In the magnetic field of Tesla, the frequency difference is 20 hertz. Then τ ₂
If is set to 25 msec, the phase difference between the water and fat signals will be 180
It becomes °.

FIG. 4 shows water and fat signals and reconstructed images obtained by the pulse sequence A and the pulse sequence B. If the signal obtained by the pulse sequence A is image-reproduced, an image showing both water and fat can be obtained. An image of fat alone can be obtained by adding the signal obtained by the pulse sequence A and the signal obtained by the pulse sequence B and reproducing the image. Conversely, if the image is reproduced after the subtraction, an image of only water is obtained.

FIG. 5 shows a combination of pulse sequence A and pulse sequence B. Here, Y ₀ , Y ₁ , Y ₂ ... Y ₂₅₆ represent the magnitude of the phase encoding gradient magnetic field GY. The pulse train of No. 1 has been conventionally adopted. The Gy value is repeated twice because the signal is averaged to improve the S / N ratio of the image. The second pulse train is according to the present invention in which the pulse sequence A and the pulse sequence B are alternately performed. If the pulse sequence A and the pulse sequence B are not alternately performed, the pulse train of the pulse sequence A (hereinafter referred to as the pulse train A) and the pulse train of the pulse sequence B (hereinafter referred to as the pulse train B) are separately run. When a signal is obtained by adding or subtracting these signals, the water and fat images are created if the position of the subject changes even during the execution of pulse train A and pulse train B, or if the position of the subject changes. If the magnetic field strength changes even a little, it is greatly affected by the displacement of the movement or the displacement of the static magnetic field. On the other hand, according to the invention 2
Water and fat images, water images, created with a pulse train of
In the fat image, since the pulse sequences A and B are alternately performed, the signals corresponding to the images of the pulse sequences A and B can be obtained almost at the same time. It becomes difficult to receive the influence of the positional deviation caused by the positional deviation or the change of the static magnetic field strength. And if these three types of images are added on the image,
An S / N image equivalent to one pulse sequence is obtained.

In the above description, the magnetic field generated by the magnet is uniform, but when the magnetic field homogeneity is not negligible compared to the chemical shift value, the phase rotation due to the non-uniformity of the magnetic field and the phase rotation due to the chemical shift cannot be distinguished. . This can be done by using a phantom filled with a uniform substance to obtain the phase rotation due to the magnetic field inhomogeneity in advance and correct the obtained image.

〔The invention's effect〕

According to the present invention, since spin echo sequences having two different echo times are alternately performed, there is a slight change in the movement of the subject or the static magnetic field strength within one imaging time from the start to the end of imaging. However, it is possible to obtain the effect that it is possible to reduce the influence of misalignment occurring in the water and fat images, the water image, and the fat image.

[Brief description of drawings]

FIG. 1 is a block diagram of the NMR inspection apparatus according to the present invention, and FIG.
The figure shows the time chart of the conventional pulse sequence,
FIG. 3 is a time chart showing a pulse sequence according to the present invention, FIG. 4 is an explanatory view of signal processing, and FIG. 5 is a time chart showing a series of pulse sequences.

Claims (1)

[Claims] 1. A nuclear spin within a specific region of a subject containing water and fat is selectively excited by a spin echo sequence which is a predetermined 90 ° -180 ° -echo to detect an NMR echo signal to detect water and fat. In the NMR inspection apparatus using a chemical shift value to separate and image water and fat in a specific region of the subject by utilizing the difference in resonance frequency between the NMR echo signal from the selective excitation of the nuclear spins. Chemical shift having two kinds of spin echo sequences with different echo times until the occurrence of the spin echo and means for mutually executing the two kinds of spin echo sequences with different echo times. NMR inspection device using values.