JPH0785737B2 - Inspection device using nuclear magnetic resonance - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to an inspection apparatus using nuclear magnetic resonance (hereinafter referred to as “NMR”), and particularly in chemical shift imaging, phase error caused by non-uniformity of static magnetic field. The present invention relates to an inspection device using NMR capable of removing the.

BACKGROUND OF THE INVENTION 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 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. One of them is to obtain position information by applying a gradient magnetic field to the inspection object to change the static magnetic field placed on each part of the object and thereby changing the resonance frequency or phase encoding amount of each part. There is.

Regarding the basic principle, Kumar et al. Have described Journal of Magnetic Resonance (Journal of
Magnetic Resonance, Vol. 18 (1975), page 69, or Edelstein et al., Physics of Medellin and Biology (Physic
s of Medicine & Biology) Vol. 25 (1980), p. 751, so it is omitted here.

One method of such imaging is chemical shift 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 differs even with the same nuclide due to the difference in the molecular structure around it. 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 for imaging the amount of chemical shift, (a) an extension method of the Fourier imaging method reported by Maudsley et al. (Journal of Magnetic Resonance Vol. 51 (1983) p. 147), ( b) The method proposed by Dixon (Radiographi
Radiology), Volume 153 (1984) p. 189), etc. are representative examples. 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 method, when a two-dimensional plane is targeted, the object to be measured is usually divided into L × M pixels, and N signal points are sampled for each of them. L or M is determined according to the spatial resolution. For example, if L = M = 128, then L or M
× M = 16384. 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 (b) is 90 °-
In a pulse sequence of τ ₁ −180 ° −τ ₂ − (signal measurement), an image including only specific chemical shift information from the sum and difference of two images of τ ₁ = τ ₂ and τ ₁ ≠ τ ₂ Is a method of configuring. Here, 90 ° and 180 ° represent high-frequency magnetic fields that tilt spins by 90 ° and 180 °, respectively.
This method is an extremely practical method because the time required for measurement is twice as long as that of one image. However, since the amount of chemical shift is the same as or smaller than the nonuniformity of the static magnetic field, in an image with τ ₁ ≠ τ ₂ , the phase error due to the nonuniformity of the static magnetic field is more It will be larger than the error. On the other hand, Dixo
After the complex Fourier transform, n et al. remove the influence of the static magnetic field inhomogeneity by calculating the square root of the sum of squares of the real part and the imaginary part, that is, the absolute value. However, in this case, depending on the magnitude of the number of spins corresponding to the two chemical shift amounts, there are cases where the two cannot be distinguished. Further, in the method (b), the types of chemical shifts to be detected were limited to two types.

[Object of the Invention]

The present invention has been made in view of such drawbacks, and an object thereof is to provide an inspection apparatus capable of obtaining an image reflecting an arbitrary chemical shift amount in a short time in chemical shift imaging. To do.

[Outline of Invention]

The gist of the present invention is that in an inspection apparatus using the Fourier imaging method, in obtaining a chemical shift image, a phase error caused by non-uniformity of the static magnetic field is corrected, and thereby the absolute value of the result after Fourier transform is taken. The point is that an accurate chemical shift image is obtained by performing calculations between images without any problem.

A little supplementary explanation will be given below on this.

First, taking the case of imaging a two-dimensional surface as an example,
The principle of the modified spin warp method 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 implementing the two-dimensional modified spin warp method,
It shows the timing of the gradient magnetic field in the x and y directions and the signal from the nuclear spin. Here, it is assumed that a certain cross section parallel to the (x, y) plane is selected. In the figure, RF indicates the irradiation pulse, and G _y and G _x indicate gradient magnetic fields in the y and x directions, respectively. Further, S indicates a signal from the nuclear spin.

First, irradiate a 90 ° RF pulse to rotate the nuclear spins in the sample to 90 °.
knock down. Immediately after that, the gradient magnetic field G _x is applied for a time t _x , and then a 180 ° RF pulse is irradiated. Measurement signals from the nuclear spins, (is applied for a time t _y) performed while applying a G _y. For normal imaging, 90 ° RF
The interval between the pulse and the 180 ° RF pulse is set to τ _1, and the gradient magnetic field in the y direction applied at the time of reading so that the center of the echo signal is generated τ ₁ after the 180 ° RF pulse is applied (see FIG. 1). ) Is applied (that is, τ ₁ = τ _{2 in} FIG. 1).
Becomes As τ ₁ = τ ₂ , a two-dimensional signal S (G _x , obtained as a result of measuring the signal from the nuclear spin according to the pulse sequence shown in FIG. 1 by changing the magnitude of the gradient magnetic field in the x direction. t _y ) is the nuclear spin distribution ρ of the selected cross section
Between (x, y) and γ as the nuclear gyromagnetic ratio, S (G _x , t _y ) = ∬ρ (x, y) exp {−jγ (G _x xt _x + G _y yt _y )} dxdy… There is a relationship of (1). However, the expression (1) is an expression when the static magnetic field has no inhomogeneity and the chemical shift is ignored. Therefore, next, an equation in which the static magnetic field inhomogeneity is E (x, y) and the chemical shift amount is σ is shown. However, here, the interval between the 90 ° RF pulse and the 180 ° RF pulse is τ ₁ ,
The center of the echo signal is 180 ° from the application of the RF pulse τ ₂
A y-direction gradient magnetic field (FIG. 1) applied at the time of reading is applied so as to be generated later. As shown in the Dixon reference described above, the center of the echo signal is the 90 ° RF pulse (t = t) of the y-direction gradient magnetic field (FIG. 1) applied at the time of reading. 0) to time t =
This is the time when the time integration value up to t becomes almost zero. First
In the figure, τ ₁ = Δτ, where Δτ is the difference between τ ₁ and τ _2.
When + τ ₂ is set, Δτ = 0 is a pulse sequence set in normal imaging. τ ₁ ≠ τ ₂
In the case of, the following formula is established.

(G _x , t _y ) = ∬ρ (x, y) exp {−jγ [G _x xt _x + G _y yt _y + (E (x, y) + σ) (Δτ + t _y )]} dxdy (2) ( 2) When the formula is modified, S (G _x , t _y ) = ∬ρ ′ (x, y) exp {−jγ [G _x xt _x + (G _y
y + E (x, y) + σ) t _y ]} dxdy (3) is obtained. Where ρ ′ (x, y) = ρ (x, y) exp {−jγ (E (x, y) + σ)
Δτ} (4) In addition to the term including the gradient magnetic fields G _x and G _y in the equation (1), the term representing the phase in the equation (2) further includes the non-uniformity E of the static magnetic field.
(X, y), including the term including the rotation of the phase based on the chemical shift σ. That is, in addition to the phase term of exp (−jγ (E (x, y) + σ) t _y }, which is generated when the echo signal and the time t _y are observed, the phase term of the equation (2) is τ ₁ ≠ τ It contains the phase term, exp {-jγ (E (x, y) + σ) Δτ}, which occurs because it is ₂ . The latter phase term is generated because the echo signal is observed under the condition that is deviated from the so-called spin echo generation condition (τ ₁ = τ ₂ ), and the normal spin echo generation condition (τ ₁ = τ _2). ) Does not occur. The effects of the static magnetic field and chemical shift appear as a phase shift when observing the echo signal.
It is a quantity related to t _y and Δτ. In the equation (2), the chemical shift amount is set to σ, but actually there are a plurality of chemical shifts.

Therefore, if the spin density corresponding to the k-th chemical shift amount σ _k is ρ _k (x, y), the equations (3) and (4) become the following equations.

Here, ρ ′ _k (x, y) = ρ _k (x, y) exp {−jγ (E (x, y) +
σ _k (Δτ} (6) Now, let the integration variable be Then, Here, the relationship represented by the following equation is established between ρ ″ _k (x ′, y ′ _k ) and ρ ′ _k (x, y).

ρ ″ _k (x ′, y ′ _k ) = ρ ′ _k (x, y) / J _k (x, y) ...
(9) where J _k (x, y) is the Jacobian used during integral transformation
Is.

When the change of E (x, y) with respect to x, y is small, J _k (x,
y) is Can be expressed as further, If is sufficiently smaller than G _y , it can be regarded as J (x, y) 1. Therefore, the result obtained by performing the inverse Fourier transform of the equation (8) is the following equation.

Here, (x, y _k ) is a value obtained when the equation (7) is solved for x and y.

From the equation (11), the real part R _e of F ⁻¹ {S (G _x , t _y )} is Becomes

Now, considering the case where the value of Δτ is changed in n ways, the following equation holds for the l-th Δτ.

Therefore, it is possible to obtain ρ _k by simultaneously solving the equations (13) for n Δτ. However, E
(X, y) and σ _k are known. For the method of measuring E (x, y), the method by Maudsley et al. May be applied (J.Phys.E): Scientific Instruments (Sci. .Instrum.) Vol. 17 (1984) p. 216). Further, σ _k is a value that is automatically determined if the type of nuclear spin is determined.

The expression (11) is generally a complex number, and it is conceivable to use the imaginary part in addition to the real number described above. In this case, it is necessary to select Δτ _l so that the equation obtained from the real part and the equation obtained from the imaginary part are first-order independent, but the maximum number of measurements is 1 and the number of chemical shifts is 1 It is clear from equation (11) that it can be reduced to / 2.

Example of Invention

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. 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 for generating a high frequency magnetic field and at the same time detecting a signal generated from the target object 16, 5 is an amplifier, 6 is a detector, 7 is a death play.
Further, 8, 9 and 10 are coils for generating a gradient magnetic field in the z direction and a direction perpendicular to the z direction, and 11, 12, 13 are power supply units for driving the coils 8, 9, 10 respectively.

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. The coil 4 also serves as a receiving coil as described above, and the received signal component is detected by the detector 6 through the amplifier 5, is input to the computer 1 and is subjected to signal processing and converted into an image by the display 7.

The static magnetic field is generated by the coil 14 driven by the power supply 15.
Done in. 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. Further, 19 and 20 are storage devices (hereinafter referred to as "memory"). The image R _e before correction is stored in the memory 19.
(Δτ _l ) is stored, and the memory 20 stores the inhomogeneity E (x, y) of the static magnetic field and the chemical shift amount σ _k .

In the inspection apparatus configured as described above, the computer 1 loads E (x, y), σ _k from the memory 20 and sets cos {γ (E
After calculating (x, y _k ) + σ _k ) Δτ _l }, from the memory 19
Load R _e (Δτ _l ) and solve equation (13). The ρ _k (x, y _k ) obtained as a result is displayed on the display 7.

Although the above description of the embodiment has been made using the modified spin warp method, it is obvious that the present invention is not limited to this and is effective for other sequences.

〔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, the phase rotation due to the non-uniformity of the static magnetic field is corrected for each point. The effect is that an apparatus capable of accurately imaging the chemical shift amount can be realized.

[Brief description of drawings]

FIG. 1 is a diagram showing a pulse sequence used in the present invention, and FIG. 2 is a diagram showing a schematic configuration of an inspection apparatus which is an embodiment of 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 object to be inspected, arithmetic processing of detection signals of the signal detecting means and pulse sequence In an inspection apparatus using nuclear magnetic resonance having signal processing control means for controlling, and means for outputting a calculation processing result by the signal processing control means, the signal processing control means comprises a center of a 90 ° high frequency pulse and a 180 ° high frequency pulse. A first time interval to the center of the pulse,
When the time-integrated value from the center of the 90 ° RF pulse of the gradient magnetic field applied at the time of reading, which is sensed by the nuclear spins in the slice selected by the gradient magnetic field, becomes almost zero and 18
Pulse sequence control for measuring the second time interval from the center of the 0 ° high frequency pulse by a time difference Δτ, and
Performing arithmetic processing to obtain the image of the inspection target from the echo signal, a plurality of images of the inspection target obtained by changing the time difference Δτ in a plurality of ways, from the static magnetic field inhomogeneity data, An inspection apparatus using nuclear magnetic resonance characterized by obtaining a chemical shift image. 2. The nuclear magnetic field according to claim 1, wherein a chemical shift image for each chemical shift is obtained by solving simultaneous equations for each corresponding pixel of the plurality of images. Inspection device using resonance.