JPH0376132B2 - - Google Patents

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention is an improved nuclear magnetic resonance tomography apparatus that separates and displays chemical shifts of water and fat while correcting phase errors caused by static magnetic field inhomogeneity. This invention relates to a nuclear magnetic resonance tomography apparatus using the Dickson method.

(Conventional technology) Obtaining a tomographic image of a subject focusing on specific atomic nuclei using nuclear magnetic resonance (hereinafter referred to as NMR) phenomenon
NMR-CT has been known for a long time. This NMR
- Briefly explain the outline of the principle of CT.

An atomic nucleus can be seen as a spinning top that is magnetically charged, but it can be interpreted by a static magnetic field in the z-axis direction.
When placed in H ₀ , the above-mentioned atomic nucleus precesses at an angular velocity ω ₀ given by the following equation. This is called Larmor precession.

ω ₀ = γH ₀ However, γ: nuclear gyromagnetic ratio Now, a high-frequency coil is placed on an axis perpendicular to the z-axis with a static magnetic field, for example, the x-axis, and the high-frequency coil with the above-mentioned angular frequency ω ₀ rotates in the xy plane. When a rotating magnetic field is applied, magnetic resonance occurs, and a population of atomic nuclei undergoing Zeeman splitting under a static magnetic field H ₀ undergoes a transition between levels due to the high-frequency magnetic field that satisfies the resonance condition, and the energy level changes. transition to the higher level. Here, since the nuclear gyromagnetic ratio γ differs depending on the type of atomic nucleus, the atomic nucleus can be specified by the resonance frequency. Furthermore, by measuring the strength of that resonance, we can determine the amount of that nucleus present. During a time determined by a time constant called the post-resonance relaxation time, the atomic nucleus excited to a higher level returns to a lower level and radiates energy.

The medium pulse method for observing this NMF phenomenon will be explained with reference to FIG.

As mentioned above, when a high-frequency pulse (H ₁ ) that satisfies the resonance condition is applied in the (x-axis) direction perpendicular to the static magnetic field (z-axis), the magnetization vector M changes in the rotating coordinate system as shown in Figure 6A. It starts rotating in the zy plane with an angular frequency of ω' = γH ₁ . Letting the pulse width be _tD , the rotation angle θ from _H0 is expressed by the following equation.

θ=γH ₁ t _D ……(1) In equation (1), a pulse with t _D that is θ−90° is called a 90° pulse. Immediately after this 90° pulse, the magnetization vector M rotates at ω ₀ in the xy plane as shown in FIG. However, since this signal attenuates over time, this signal is called a free induction attenuation signal (hereinafter referred to as an FID signal). F.I.D.
If a signal is Fourier transformed, a signal in the frequency domain can be obtained. Next, as shown in Fig. 6 (c), when a second pulse (180° pulse) with a pulse width such that θ = 180° after τ time from the 90° pulse is added, the magnetic moments that had been scattered are The signal is observed with refocusing in the back-y direction. This signal is called a spin echo (hereinafter referred to as SE signal). A desired image can be obtained by measuring the intensity of this SE signal. The resonance condition for NMR is given by ν=γH ₀ /2π (2). Here, ν is the resonant frequency and H ₀ is the strength of the static magnetic field. Therefore, it can be seen that the resonant frequency is proportional to the strength of the magnetic field. For this reason, there is an NMR imaging method in which a linear magnetic field gradient is superimposed on a static magnetic field, giving a magnetic field of different strength depending on the position, and changing the resonance frequency to obtain positional information. Of these, the Fourier transform method will be explained. A pulse sequence for applying a high frequency magnetic field and a gradient magnetic field used in this method is shown in FIG. In figure A, x,
It shows a state in which gradient magnetic fields of Gx, Gy, and Gz are applied to the y and z axes, respectively, and a high frequency magnetic field is applied to the x axis. The figure B is a diagram showing the timing of applying each magnetic field. In the figure, RF is a high-frequency rotating magnetic field that applies 90° pulses and 180° pulses to the x-axis.
Gx is a fixed gradient magnetic field applied to the x-axis called the lead axis, Gy is a gradient magnetic field whose amplitude changes depending on time applied to the y-axis called the warp axis,
Gz is a fixed gradient magnetic field applied to the z-axis called the slice axis. The signal shows the SE signal after the 180° pulse. The period is provided to indicate the timing of the signal of the gradient magnetic field applied to each axis. In period 1, spins in the slice plane in the tomography perpendicular to the z-axis centered at z=O are selectively excited by the 90° pulse and the gradient magnetic field Gz ⁺ . period 2
Gx ⁺ is used to disrupt the phase of the spins and reverse them with a 180° pulse, which is called a dephase gradient.

Gz ^- is used to restore the spin phase disturbed by Gz ⁺ . During period 2, a phase encoding gradient Gyn is also applied. This is to shift the phase of the spin in proportion to the position in the y direction, and its intensity is controlled to be different every cycle.
In period 3, apply a 180° pulse to align the magnetic moments again, and then observe the SE signal that appears. Gx ⁺ in period 4 is a gradient magnetic field for aligning the disturbed phases and generating an SE signal, which is called a readout gradient, and an SE signal appears when the areas of the readout gradient and the day phase gradient become equal.

In such NMR imaging, we will consider proton NMR of hydrogen nuclei, which is the main target nuclide for current imaging.

Generally, in a substance, the resonance frequency ν of NMR often deviates from equation (2), which is the resonance conditional equation due to the externally applied static magnetic field H ₀ . This shift is extremely large in the case of atomic nuclei in metallic substances, but even in the case of protons, the resonance frequency changes sensitively depending on the chemical structure. One of the reasons for this is due to the magnetic field (diamagnetic field) generated in the opposite direction to H ₀ due to the orbital motion of the electrons, which is called the shielding effect of the nucleus by the electrons. . Therefore, the resonance frequency is given by the following equation using the constant σ due to this shielding effect.

ν=(γ/2π)H ₀ (1−σ) Such a shift in resonance frequency is called a chemical shift, and resonance is observed on the lower magnetic field side. The magnitude of this chemical shift changes as the distribution of electron density and orbital motion of electrons change due to chemical bonds. Therefore, the resonance conditions differ depending on the compound even for the same proton. That is, the Larmor frequency differs by about 3.5 ppm between protons in H ₂ O and protons in fat. With this degree of deviation, it is not possible to distinguish between the two using normal imaging.

The Dickson method is a method for imaging both components. The Dixon method will be explained with reference to FIG. The figure is a time sequence chart similar to FIG. In the figure, parts equivalent to those in FIG. 7 are given the same reference numerals.

Figure A is a diagram of the scan O selected as T _B =TE/2, where T _B is the time interval between 90° pulse 1 and 180° pulse 2. Here, TE is the time interval between the 90° pulse and the SE signal. At this time, the magnetization vectors of the proton components of water and fat are in phase. The figure shows the time interval T _B with 90° pulse 1 as T _B =
It is a diagram of scan 1 when applying 180° pulse 4 with (TE/2)+ε. At this time, the phases of the magnetization vectors of water and fat are opposite to each other. The phase relationship of the magnetization vectors in the excitation plane is as shown in FIG. In the figure, diagram A shows the phase relationship between water and fat in the case of Figure 4 when scan O is set with T _B = TE/2, and the magnetization vectors of water and fat are in the same phase. Figure B shows the phase relationship between water and fat during scan 1 in Figure 4B, where T _B = (TE/2) - ε, and they are in opposite phase. Here, ε is a value at which the phases of water and fat are opposite to each other, and is determined by determining the strength of the magnetic field. For example, when the magnetic field strength is 0.3T, ε=
5.6ms, and when the magnetic field strength is 0.6T, ε = 2.8ms.

This Dickson method contains a phase error due to static magnetic field inhomogeneity, and it is difficult to separate water and fat with a slight difference in resonance frequency. An improved Dixon method has been proposed to remove this phase error. This method will be explained with reference to FIG. Figure A shows scan 1 with the same 90° pulse 1 and 180° pulse 2 applied as scan O in Figure 4. Figure B shows scan 1 with the same 90° pulse 1 and 180° pulse 4 applied as scan 1 in Figure 4. This is a diagram. Figure C is a diagram of scan 2 with 90° pulse and 180° pulse 5 applied.
The time interval between 180° pulse 5 and 90° pulse is (TE/
2) With a pulse of +ε, the phase of the magnetization vector of fat is also in the opposite phase to that of water.

Now, image data in scans 0, 1, and 2
Let S ₀ , S ₁ , S ₂ be the amplitude of water, F the amplitude of fat, α be the phase offset, and θ be the phase of the error due to static magnetic field inhomogeneity, then S ₀ = (W+F)・exp(iα ) ……(3) S ₁ = (W-F)・exp(iθ)・exp(iα) ……(4) S ₂ =(W-F)・exp(−iθ)・exp(iα) …… (5) becomes. Find the phase error θ due to static magnetic field inhomogeneity.

Dividing equation (3) by equation (4), S ₂ /Si = exp (-iθ) / exp (iθ) = exp (-i2θ) S ₃
Then _{, S 3 = S 1 × exp} (1/2 From the formula: (1/2) (S ₀ + S ₃ ) = W・exp(iα)=Wr+i・Wi …(7) (1/2)(S ₀ −S ₃ ) =F・exp(iα)=Fr+i・Fi...(8) From equation (7), W = √ ² + ² From equation (8), F = √ ² + ₂ In this way, the improved Dickson method can eliminate the static magnetic field phase error and phase offset. can.

(Problem to be solved by the invention) By the way, in the improved Dickson method, it is necessary to perform three scans: scan O, scan 1, and scan 2, and the scan time is already long.
The fact that NMR-CT requires three times as much time is a major problem. However, if the number of views is reduced in order to shorten the scan time, the spatial resolution will deteriorate.

The present invention has been made in view of the above-mentioned problems, and its purpose is to reduce spatial resolution in an improved version of the Dickson method that separates water and fat and eliminates phase errors caused by static magnetic field inhomogeneity. The object of the present invention is to realize a chemical shift image creation method that can shorten the scan time without causing any problems.

(Means for Solving the Problems) The present invention for solving the above problems has the following advantages: When the time interval between the excitation pulse and the spin echo signal is TE,
A first scan in which the time interval between the excitation pulse and the inversion pulse is chosen to be TE/2, and a second scan in which the time interval between the excitation pulse and the inversion pulse is chosen to be TE/2 - ε shorter than said first scan by ε. scan and a third scan in which the time interval between the excitation pulse and the inversion pulse was selected to be TE/2+ε longer than the first scan by ε, and the data obtained by the second and third scans were performed. In a nuclear magnetic resonance tomography apparatus that uses a modified Dickson method that separates and displays chemical shifts of water and fat while correcting phase errors due to static magnetic field inhomogeneity, , the number of views of the second scan and the number of views of the third scan are reduced.

(Operation) A normal scan in which a 180° pulse is applied at TE/2 after the 90° pulse is performed in the designated view, and then a scan in which the application timing of the 180° pulse is shifted before or after the 180° pulse of the normal scan. Either or both are performed with a small number of views, and a chemical shift image of water and fat corrected for phase errors due to static magnetic field inhomogeneity is calculated and created in a short time.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

The hardware of the NMR-CT of the present invention is the same as usual. Figure 1 shows the RF of the present invention.
This is a diagram of the pulse sequence, and A is a diagram of the pulse sequence of the improved Dixon method similar to Figure 3.
The same symbols as in FIG. 3 are used. In this improved Dixon method, as already explained, scan 1 and scan 2 are used to calculate and correct the static magnetic field inhomogeneity error, and scan O and scan 1 are used to calculate the respective components of water and fat. It is used in Here, it has been experimentally confirmed that even if the number of views for water-fat (scan 1) is reduced to 70% to 80% of the number of views for water + fat (scan O), there is extremely little deterioration in image quality. ing. By reducing the number of views in scan 1, the degree of separation of high spatial frequency components of water and fat deteriorates, but most of the information contained in the image is distributed in relatively low spatial frequencies, and the number of views in scan O is reduced. By obtaining high frequency components from the data, the number of views in scan 1 can be reduced.
In addition, since the non-uniformity of the static magnetic field changes slowly,
The number of views in scan 2, which is used only to correct phase errors due to static magnetic field inhomogeneity, can be further reduced, and even if it is reduced to about 25%, no problems will occur. Figure 1 (b) shows the conditions for each scan.
The scan conditions are shown in which the number of warp gradient views for scans 0, 1, and 2 is 256, 192, and 64, respectively. The lead sampling is 256 in both cases.
It is as follows. The data of the SE signal 3 in this case are S ₀ , S ₁ , and S ₂ , respectively.

The operation of the apparatus of the present invention will be explained using the flowchart shown in FIG. First, the data of 256 views of scan O is Fourier transformed. After the data of scan 1 and scan 2 have been Fourier transformed, the number of views is smaller than that of scan O, so in order to create a frequency plane of the same matrix, the surrounding data plane is
The image is reconstructed using zero fill (step 1). Divide the scan 2 data S ₂ by the scan 1 data S ₁ to get the phase error exp(-i2θ)
(Step 2). Scan1 data
Multiply _S1 by the square root of the phase error to obtain data _S3 from which the phase error has been eliminated (step 3). Separated water and fat data is obtained from the sum and 1/2 of the difference between scan O data S ₀ and data S ₃ obtained in step 3 (step 4). This data includes phase offset errors. Next, the absolute values of the complex data of water and fat are taken to obtain data that does not include phase offset errors (step 5). The calculations in the above flowchart have been explained in the conventional improved Dickson method, so the explanation here will be omitted.

As explained above, according to the present invention, it is possible to separate water and fat by correcting the phase error caused by static magnetic field inhomogeneity in a shorter time than before using the improved Dickson method.

Note that the present invention is not limited to the above embodiments.

The number of views for scan O is not limited to 256, and may be a number specified by the user. Similarly, the number of views for scan 1 and scan 2 can be increased or decreased accordingly.

This invention can be applied to all water and fat separation image techniques that have the same principle as the Dickson method.

Although this embodiment did not reduce the number of samples in the read direction, it may be reduced if necessary.

(Effect of the invention) Even if the number of views for water-fat (scan 1) is reduced to 70% to 80% of the number of views for water + fat (scan O), the image quality deteriorates extremely due to the reasons mentioned above. Since the number of views is small, the number of views for scan 1 can be reduced. Furthermore, since the phase error due to static magnetic field inhomogeneity changes slowly, scan 2 is set to 64 ~
You can reduce it to 32 views. This makes it possible to use an improved Dickson method with a shortened scan time, which has a great practical effect.

[Brief explanation of drawings]

FIG. 1 is a diagram of an RF pulse sequence according to an embodiment of the present invention, FIG. 2 is a flowchart showing the operation of the present invention, FIG. 3 is a diagram of an RF pulse sequence of the improved Dickson method, and FIG. is a diagram of the pulse sequence of the Dixon method, Figure 5 is an explanatory diagram of the phase of water and fat according to the Dixon method, and Figure 6 is an illustration of the phase of water and fat according to the Dixon method.
Figure 7 is a diagram explaining the principle of the NMR-CT pulse method.
FIG. 3 is a diagram showing a pulse sequence of NMR-CT. 1...90° pulse, 2...180° pulse, 3...
SE signal, 4……+ε 180° pulse, 5……−ε
180° pulse.

Claims (1)

[Claims] 1. When the time interval between the excitation pulse and the spin echo signal is TE, the first scan in which the time interval between the excitation pulse and the inversion pulse is selected as TE/2, and the time interval between the excitation pulse and the inversion pulse A second scan whose time interval is chosen to be TE/2-ε shorter by ε than the first scan.
scan, and the time interval between the excitation pulse and the inversion pulse is TE/2 longer than the first scan by ε.
+ε, and perform the third scan selected for +ε.
and a nuclear magnetic resonance tomography system using the improved Dixon method that separates and displays the chemical shifts of water and fat while correcting phase errors due to static magnetic field inhomogeneity using data obtained from the third scan. . A nuclear magnetic resonance tomography apparatus, characterized in that the number of views of the second scan and the number of views of the third scan are reduced relative to the number of views of the first scan.