JPH0811116B2 - Inspection equipment using nuclear magnetic resonance - Google Patents

Description

TECHNICAL FIELD The present invention relates to an inspection method using nuclear magnetic resonance (hereinafter referred to as “NMR”), and particularly in chemical shift imaging, it is possible to improve timing adjustment of a pulse sequence of an apparatus. The present invention relates to an inspection device using NMR that can be performed with high accuracy.

[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 become clear that an attempt to perform a similar examination using the nuclear magnetic resonance phenomenon has succeeded and that 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 phenomenon, it is necessary to separate / identify the signal from the inspection object corresponding to each part of the object. As one of the methods, there is a method of obtaining positional information by applying a gradient magnetic field to an inspection object and changing the static magnetic field in which each part of the object is placed, thereby making the resonance frequency or the phase encoding amount of each part different. .

For the basic principle, see Journal of Magnetic Resonance (J.Magn.Reson) Vol. 18, page 69 (1975), or Physics in Medesin and Biology (Phys.Med). .Biol) Volume 25,
Since it is reported on page 751 (1980), it is omitted here.

Chemical shift imaging is one method of such imaging. The chemical shift is a phenomenon in which the resonance frequency of each spin changes depending on the position on the molecular structure because the magnetic field sensed by each spin varies depending on the difference in the molecular structure around the same nuclide. The chemical shift is an extremely important phenomenon because it gives information on the molecular structure of the object to be measured. As a method of imaging the chemical shift amount, (a) an extension method of the Fourier imaging method reported by Maudsley et al. (Journal of Magnetic Resonance Vol. 51, p. 147 (1983)), (B) The method proposed by Dixon (Radiclogy magazine, Vol. 153, p. 189 (1984)) is a representative example. The method (a) is a method that enables separation and measurement of the chemical shift amount by increasing the dimension of imaging by one. in this way,
Usually, when a two-dimensional plane is targeted, an object to be measured is divided into L × M pixels, and N signal points are sampled for each of the pixels. L or M is determined according to the spatial resolution. For example, if L = M = 128, then L or M
× M = 16,384. N signal points can be sampled in one measurement, but until the next measurement, it is necessary to wait for the longitudinal relaxation time of the measured object (about 1 second in the case of a living body), which results in L × M measurement. In order to do so, it will take 4.6 hours of measurement time. On the other hand, the method of (b) is 90 °-
In a pulse sequence of τa−180 ° −τb− (signal measurement), it is a method of constructing an image containing only specific chemical shift information from the sum and difference of two images of τa = τb and τa ≠ τb. . Here, 90 ° and 180 ° represent high-frequency magnetic fields that invert spins by 90 ° and 180 °, respectively.
This method is an extremely practical method because the time required for measurement is twice as long as one image. Further, as a method developed from the Dickson method, a method of obtaining two chemical shift images by one measurement can be considered. This utilizes the fact that when a 90 ° phase difference is applied between two chemical shifts, the real and imaginary parts obtained by Fourier transforming the generated signal correspond to the respective chemical shift images.

[Problems to be solved by the invention]

In the Dickson method and its improved method described above, when setting τ _a ≠ τ _b in the pulse sequence, the setting accuracy of (τ _b −τ _a ) is important. This time difference determines the phase difference between the two chemical shifts, as given by the following equation.

θ _c = (σ ₁ −σ ₂ ) γ (τ _b −τ _a ) (1) where θ _c is the phase difference, σ ₁ and σ ₂ are chemical shifts,
γ is the nuclear gyromagnetic ratio. However, when τ _a ≠ τ _b is set in the pulse sequence, (τ _b −
It was practically difficult to accurately set τ _a ). The reason is that it is difficult to accurately measure the time delay in the signal processing system including the time delay due to the low-pass filter used after the signal detection. In the pulse sequence, when setting τ _a ≠ τ _b ,
Since the setting error of (τ _b −τ _a ) significantly impairs the resolution of the chemical shift, the solution thereof has been desired.

The present invention has been made in view of such drawbacks,
It is an object of the invention to provide an inspection apparatus and an inspection method capable of highly accurately obtaining an image reflecting an arbitrary chemical shift amount in chemical shift imaging.

[Means for Solving Problems] The main point of the present invention is to obtain chemical shift images by setting τ _a and τ _b in the pulse sequence of the apparatus, applying 90 ° high frequency magnetic field, 180 ° high frequency Application of magnetic field,
The point is that each timing error in signal measurement is corrected in advance by using a substance having a known chemical shift to improve the accuracy.

[Work]

 A few supplementary explanations will be given below.

First, taking the case of imaging a two-dimensional surface as an example, the principle of the modified spin warp method, which is one of the imaging methods, and an example in which the present invention is applied to the two-dimensional modified spin warp method will be described. FIG. 1 shows an irradiation pulse for performing a two-dimensional deformed spin warp method, and a timing of a gradient magnetic field in x and y directions and a signal from a nuclear spin. Here, a certain cross section parallel to the (x, y) plane is selected. In the figure, RF represents the irradiation pulse as G _y and
G _x represents the gradient magnetic fields in the y and x directions, respectively.
Signal shows the signal from the nuclear spin.

First, a 90 ° RF pulse is applied to reduce the nuclear spin in the sample by 90%.
゜ Defeat. Immediately thereafter, the gradient magnetic field G _X is applied for a time t _X , and then a 180 ° RF pulse is applied. The signal is observed while applying G _y . In normal imaging, in the pulse sequence of the apparatus, τ _a and τ _b that specify the timing of 90 ° high-frequency magnetic field application, 180 ° high-frequency magnetic field application, and signal measurement timing are determined by τ _a by a computer that performs sequence control. = Τ _b .

When such a measurement is performed by changing the magnitude of the gradient magnetic field in the x direction and the two-dimensional Fourier transform is performed, S (x, y) = {ρ ₁ (x, y) + ρ ₂ (x, y) exp {Jθc}} × exp (−jγ [E (x, y) + σ ₁ ] τ _l + jθ _a } (2) is obtained, where ρ ₁ (x, y) and ρ ₂ (x, y) are The magnetization for the chemical shifts σ ₁ and σ ₂ is E (x, y)
Represents the static magnetic field inhomogeneity, and θ _a represents the offset phase depending on the device, and τ _l = τ _b −τ _a . Now, in expression 2, if exp {-jγE (x, y) τ _l } can be removed, the remaining terms are only constants. The removal of exp {-jγE (x, y) τ _l } can be achieved by measuring a substance having only one chemical shift under the same conditions as the above measurement. That is, since the phase term of the obtained image represents exp {-jγE (x, y) τ _l }, this complex conjugate can be obtained and multiplied by Equation 2 in pixel units.

In this way, if the second exp {} of the equation 2 becomes a simple constant, the equation 2 can be expressed by the following equation.

S (x, y) = {ρ ₁ (x, y) + ρ ₂ (x, y) exp (jθ _c )} exp (jθ ′ _a ) (3) where θ _a ′ is the phase due to the static magnetic field inhomogeneity. Is the phase remaining after the removal of.

Next, consider the sample 1 shown in FIG. This sample is placed inside the probe 2. As the sample, ones in which the substances A and B having the chemical shifts σ ₁ and σ ₂ are respectively filled are selected. When this sample is imaged by the above method, the pixel corresponding to each substance is given by the following complex number. For _{A ρ 1 (x, y)} exp (jθ 'a), for the B ρ ₂ (x,
y) exp (jθ _c ) exp (jθ ′ _a ). FIG. 3 shows these on a complex plane. Here, equations (4a) and (4b) are complex numbers for the substances A and B. That is, the following equation is established.

A = ρ ₁ (x, y) exp (jθ ′ _a ) (4a) B = ρ ₂ (x, y) exp (jθ _c ) exp (jθ ′ _a ) (4b) Also, the angle shown in FIG. Let θ be θ _c and θ _c be A and B
The angle formed by At this time, θ _c is _calculated by the following equation.

The θ _c obtained from the equation 5 is generally set at each timing of application of 90 ° high frequency magnetic field, application of 180 ° high frequency magnetic field, and signal measurement in setting the pulse sequence of the apparatus, that is, τ _a and τ _b are set. It is sometimes different from the intended value θ (the value obtained by the right side of Expression 1 from τ _a and τ _b , which are set to initial values). Therefore, (θ _c −θ) becomes the value to be corrected. From this, in setting the pulse sequence of the device, 90 ° high frequency magnetic field application, 180 ° high frequency magnetic field application,
The following equation is obtained as each timing correction of the signal measurement, that is, the correction for (τ _b −τ _a ).

From the above, in setting the pulse sequence of the device,
When each timing of 90 ° high frequency magnetic field application, 180 ° high frequency magnetic field application, and signal measurement is set, that is, when τ _a and τ _b are set, τ _l (= τ _b −τ _a ) calculated from Equation 1 is used. In contrast,
If Δτ given by Equation 6 is subtracted, even if there is an error in the setting of τ _a and τ _b when the pulse sequence of the apparatus is set, the corrected true value can be corrected. It turns out that (more accurate value) can be set.
If the static magnetic field is extremely uniform, phase correction due to it can be omitted.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 4 is a block diagram of an inspection apparatus which is an embodiment of the present invention. In the figure, 3 is a calculator, 4 is a high frequency pulse generator, 5 is a power amplifier, 6 is a coil for generating a high frequency magnetic field and at the same time detecting a signal generated from a target object 7, 8 is an amplifier, and 9 is a detector. is there. Further, 10, 11 and 12 are coils for generating a gradient magnetic field in the z direction and a direction perpendicular to the z direction, and 13, 14, 15 are the above coils respectively.
A power supply unit that drives 10, 11, and 12.

The computer 3 also has a function of outputting various commands to each device at a fixed timing. The output of the high frequency pulse generator 4 is widened by a power amplifier 5 to excite the coil 6. The coil 6 also serves as a receiving coil as described above, and the received signal component is detected by the wave detector 9 through the amplifier 8 and then input to the computer 3 to be processed by the signal display 16
Is converted to an image.

The static magnetic field is generated by a coil driven by the power supply 17.
Do at 18. The human body 7, which is the object to be inspected, is placed on a bed 19, and the bed 19 is configured to be movable on a support 20. Further, 21 and 22 are storage devices (hereinafter, referred to as "memory"). Image A (formula 4a) is stored in the memory 21,
The image B (equation 4b) is stored, and the memory 21 stores the imaging result of the inspection target object. In the inspection apparatus configured as described above, the computer 3 loads the images of the samples A and B from the memory 22 and calculates Δτ based on the equation 6.
And τ _l (= τ _b − calculated from Equation (1).
Correct the pulse sequence by subtracting Δτ with respect to τ _a ). That is, the already set τ _b is (τ _b −Δτ)
Or change the already set τ _a to (τ
_a + Δτ). This operation does not have to be performed every time a human body is imaged, and may be performed as needed when the state of the device changes such as a filter change. After that,
It is the same as normal imaging.

〔The invention's effect〕

As described above, according to the present invention, in the inspection apparatus utilizing the NMR phenomenon in the static magnetic field, the gradient magnetic field and the high frequency magnetic field, in setting the pulse sequence of the apparatus,
Since the timing deviations of 90 ° high frequency magnetic field application, 180 ° high frequency magnetic field application, and signal measurement are corrected, it is possible to realize an apparatus capable of accurately imaging chemical shifts. .

[Brief description of drawings]

FIG. 1 is a diagram showing a pulse sequence used in the present invention, FIG. 2 is a diagram showing an embodiment of the present invention, FIG. 3 is a diagram for explaining the principle of the present invention, and FIG. It is a figure which shows schematic structure.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location G01N 24/02

Claims (3)

[Claims] 1. A static magnetic field, a gradient magnetic field, and a high-frequency magnetic field, each magnetic field generating means, a signal detecting means for detecting a nuclear magnetic resonance signal from an inspection target, and each magnetic field generating means and the signal detecting means are controlled. A calculation control means for calculating a detection signal by the signal detection means and an output means for outputting a calculation result by the calculation control means. The calculation control means applies a 90 ° high frequency pulse and a 180 ° high frequency pulse. The first time interval between application and the second time interval between application of the 180 ° high-frequency pulse and the time when the center of the echo signal appears are set to be different, and an image of the inspection target is obtained from the echo signal. In the inspection apparatus using nuclear magnetic resonance for obtaining a chemical shift image by calculation from the image and the non-uniform distribution of the static magnetic field, the calculation control means includes: The complex conjugate of the phase of the image of the reference sample placed in the vicinity of the high-frequency coil forming the step is obtained, the calculation between the complex conjugate and the image of the inspection object is performed, and the second time interval The first
An inspection apparatus using nuclear magnetic resonance, wherein a phase depending on a difference in time interval of is removed from the image of the inspection target to obtain the chemical shift image. 2. 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 controlling each magnetic field generating means and the signal detecting means. A calculation control means for calculating a detection signal by the signal detection means and an output means for outputting a calculation result by the calculation control means. The calculation control means applies a 90 ° high frequency pulse and a 180 ° high frequency pulse. The first time interval between the application and the second time interval between the application of the 180 ° high frequency pulse and the time when the center of the echo signal appears are set to be different from each other. In an inspection apparatus using nuclear magnetic resonance for obtaining a shift image by calculation, the calculation control means is a Chemica disposed near a high frequency coil which constitutes the signal detection means. A phase difference between images of two types of reference samples having different shifts is obtained, and the phase difference is used to correct a difference between the second time interval and the first time interval that are set in advance. Then, the inspection apparatus using nuclear magnetic resonance is characterized in that the chemical shift image is obtained by setting the first time interval and the first time interval. 3. The inspection apparatus using nuclear magnetic resonance according to claim 2, further comprising storage means for storing images of the two types of reference samples.