JPH07114763B2 - Inspection device using nuclear magnetic resonance - Google Patents

Description

TECHNICAL FIELD The present invention relates to an inspection apparatus using nuclear magnetic resonance (hereinafter referred to as “NMR”), and particularly to an NMR apparatus capable of performing imaging at high speed. Regarding inspection equipment.

[Conventional technology]

Conventionally, X-ray CT and ultrasonic imaging devices have been widely used as devices for nondestructively inspecting internal structures such as the head and abdomen of a human body. In recent years, it has been clarified that an attempt to perform a similar examination using nuclear magnetic resonance development has succeeded and information that cannot be obtained by an X-ray CT or an ultrasonic imaging apparatus can be obtained. In the inspection apparatus using the nuclear magnetic resonance development, it is necessary to separate and identify the signal from the inspection object in correspondence with each part of the object. One of them is to obtain position information by applying a gradient magnetic field to the object to be inspected and making the static magnetic field placed on each part of the object different, thereby making the resonance frequency or phase encoding amount of each part different. .

Regarding the basic principle, Journal of Magnetic Resonance magazine (J.Magn.Reson.) Vol. 18, 69
Volume (1975), or the Physics of Medicin & Biology (Phys.Med. & Biol.)
Vol. 25, p. 751 (1980).

By the way, recently, a high-speed method applying these methods has been attempted. For example, the Frascille method reported by Frahm et al. (Journal of Computer Acysted Tomographie, Vol. 10, p. 363 (1986)).
Is a typical example.

[Problems to be solved by the invention]

The above-mentioned prior art does not consider the influence of chemical shift, and has a problem that when the measurement site is a mixture of substances having different chemical shifts, the respective phases are different, so that an accurate concentration cannot be depicted.

An object of the present invention is to render accurate concentrations for such substances.

[Means for solving problems]

The purpose is to utilize the fact that the change in concentration due to chemical shift is a periodic function of 2π, and by setting the time from excitation of magnetization to measurement of a signal to a multiple of 2π,
To be achieved.

[Action]

That is, after the transverse magnetization is generated by the high frequency magnetic field, the magnetizations having different chemical shifts generate the phase difference represented by the following equation.

θ = r (σ _w −σ _f ) τ (1) where γ is the nuclear gyromagnetic ratio, and τ is the time between the center of the pulsed high-frequency magnetic field and the peak of the echo signal.

Further, σ _w and σ _f are chemical shifts expressed in units of magnetic field. Since the number of chemical shifts can be regarded as approximately two with respect to hydrogen nuclei in the living body, they are represented by σ _w and σ _f , the former being a value for water and the latter being a value for lipid. . By the way, the resulting image S (x, y) = ρ w (x, y) + ρ f (x, y) exp (jθ) ......
It is given by (2). Here, ρ _w (x, y) and ρ _f (x, y) are the concentrations for water and fat, respectively.

Now, in the equation (2), if θ is a multiple of 2π, then S (x, y) = ρ _w (x, y) + ρ _f (x, y) (3) and S (x, y) Is an image that accurately gives the sum of the concentration for water and the concentration for fat.

〔Example〕

 Hereinafter, the present invention will be described with reference to the drawings.

FIG. 1 shows a pulse sequence used in the present invention.

After irradiating the high frequency magnetic field with the slip angle α, the gradient magnetic field for phase encoding G _x is applied, and then the gradient magnetic field for signal reading G _y
Is applied. Signal sampling is performed with a gradient magnetic field G _y .

Since the imaging of the two-dimensional surface is considered here, the gradient magnetic field for slicing is omitted. By the way, in the flash method, it is necessary that a large longitudinal magnetization still remains after the flip angle α is selected smaller than π / 2 and the transverse magnetization is generated by irradiation with a high frequency magnetic field. This longitudinal magnetization is the waiting time until the next measurement (usually a few
Within 10 ms), the thermal equilibrium magnetization will increase, and it will become the cause of transverse magnetization in the next measurement. Since the residual longitudinal magnetization is large, a large transverse magnetization can be generated even if this waiting time is short, which is a major feature of the flash method.

However, in the conventional flash method, the phase difference of each chemical shift gradually changes after the irradiation of the high-frequency magnetic field, and an error occurs in the image density as described above. The present invention eliminates the adverse effect of this phase difference by setting τ shown in FIG. 1 so that θ given by equation (1) is a multiple of 2π. As chemical shifts, consider water and lipids that shift 3.5 ppm apart. θ = 2π
Table 1 shows an example of the calculation of τ in the case of, which is inversely proportional to the value of the static magnetic field. As can be seen from this table, θ = 2π is suitable at about 0.5T, but θ = 2π at higher magnetic fields.
It becomes necessary to set 4π or θ = 6π. Since this value is determined by the measurement time and the value of the static magnetic field, it can be calculated with reference to Table 1.

FIG. 2 is a block diagram of an inspection apparatus which is an embodiment of the present invention. In the figure, 1 is a computer, 2 is a high frequency pulse generator, 3 is a power amplifier, 4 is a coil (rf coil) for generating a high frequency magnetic field and at the same time detecting a signal generated from a target object 16, 5 is an amplifier, 6 Is a wave detector, and 7 is a display device. Further, 8, 9 and 10 are coils (gradient magnetic field coils) for generating a gradient magnetic field in the z direction and a direction perpendicular thereto, 11, 12, 13 are coils 8 and 9, respectively.
It is a power supply that drives 9,10.

The computer 1 also has a function of outputting various commands to each device at a fixed timing. The output of the high frequency pulse generator 2 is amplified by the power amplifier 3 to excite the coil 4. As described above, the coil 4 also serves as a receiving coil. The received signal component passes through the amplifier 5, is detected by the detector 6, is input to the computer 1, and is subjected to signal processing and converted into an image by the display device 7.

The static magnetic field is generated by the static magnetic field coil 14 driven by the power supply 15. A human body 16 as an object to be inspected is placed on a bed 17, and the bed 17 is configured to be movable on a support base 18.

〔The invention's effect〕

According to the present invention, a phase difference due to a chemical shift, which is a problem when the flash method is used, can be easily removed, which is effective in obtaining a quantitative NMR image.

[Brief description of drawings]

FIG. 1 is a diagram for explaining an embodiment of the present invention, and FIG. 2 shows a configuration of an apparatus used in the present invention.

Claims (2)

[Claims] 1. Magnetic field generating means for static magnetic field, gradient magnetic field, and high-frequency magnetic field, signal detecting means for detecting a nuclear magnetic resonance signal from an inspection target, and a computer for calculating the detection signal by the signal detecting means. In the inspection device using nuclear magnetic resonance having an output unit for outputting a calculation processing result, the computer further controls generation of each magnetic field and timing of application, and the high frequency by the computer. By controlling the magnetic field generating means, the high-frequency magnetic field generating means generates a pulsed high-frequency magnetic field set to a flip angle smaller than 90 ° to excite magnetization, and the computer controls the gradient magnetic field generating means. The gradient magnetic field generating means generates a read gradient magnetic field having a section whose polarity is inverted in order to read the nuclear magnetic resonance signal generated by the magnetization. However, the time interval between the center of the pulsed high-frequency magnetic field and the peak of the nuclear magnetic resonance signal is included in the inspection target by the means for generating the gradient magnetic field by the computer and the control of the pulsed high-frequency magnetic field. The previously known difference between two different chemical shifts and the product of the time interval and the nuclear gyromagnetic rotation are set to be an integral multiple of 2π, and the chemical shift generated under these setting conditions The nuclear magnetic resonance is characterized in that the phase difference of the corresponding nuclear magnetic resonance signals is set to an integer multiple of 2π, and the image of the inspection target is obtained in which the influence of the phase difference due to the two different chemical shifts is almost removed. Inspection equipment used. 2. The examination apparatus using nuclear magnetic resonance according to claim 1, wherein the chemical shift is water and lipid.