JPH0331451B2 - - Google Patents

Description

Detailed Description of the Invention (Industrial Application Field) The present invention provides a method for measuring gradient magnetic field responses due to eddy currents in order to eliminate the influence of eddy currents caused by gradient magnetic fields applied to a nuclear magnetic resonance tomography apparatus. Regarding the 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. Magnetic resonance occurs when a rotating magnetic field is applied, and a population of atomic nuclei undergoing Zeeman splitting under the 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 a 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 NMR 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 3A. 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 such that 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 decay signal (FID signal). By Fourier transforming the FID signal, a signal in the frequency domain can be obtained.
Next, as shown in Figure 3C, when a second pulse (180° pulse) with a pulse width such that θ = 180° is applied after τ hours from the 90° pulse, the magnetic moments that had been scattered are The signal is observed with refocusing in the back-y direction. This signal is called an SE signal (spin echo 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. 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. FIG. 4 shows a pulse sequence for applying a high frequency magnetic field and a gradient magnetic field used in this method. In figure A, x,
Apply gradient magnetic fields of Gx, Gy, and Gz to the y and z axes, respectively,
A state in which a high frequency magnetic field is applied to the x-axis is shown.
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 at 90°.
Apply a 180° pulse of pulses to the x-axis. Gx is x
A fixed gradient magnetic field is applied to the axis, Gy is a gradient magnetic field whose amplitude changes depending on the time applied to the y-axis,
Gz is a fixed gradient magnetic field applied to the z-axis. Signals are FID signal after 90° pulse and SE after 180° pulse
Showing a signal. The period is provided to indicate the timing of the signal of the gradient magnetic field applied to each axis. In period 1, z=0 due to 90° pulse and gradient field Gz ⁺
Spins in the slice plane in the tomography perpendicular to the z-axis centered at are selectively excited. Period 2
Gx ⁺ is used to disrupt the phase of the spins and reverse them with a 180° pulse, which is called the dephase gradient. Gz ^- is used to restore the spin phase disturbed by Gz ⁺ . In period 2, 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 ref-phase gradient, and an SE signal appears when the areas of the ref-phase gradient and the day-phase gradient become equal.

In this NMR-CT, if the static magnetic field is large,
Superconducting magnets are increasingly being used because they improve the signal-to-noise ratio, but this has led to the problem of eddy currents. That is, when a gradient magnetic field is applied, eddy currents due to the gradient magnetic field are generated in the conductor of the static magnetic field magnet and the container of the helium bath for cooling, and the gradient magnetic field is canceled by the magnetic flux generated by this eddy current and weakens. This phenomenon occurs. This is especially a problem because the conductor of the superconducting magnet and the container of the helium bath are cooled to an ultra-low temperature, so the resistance value is extremely small, 0 or close to 0, and large eddy currents flow with a long time constant. It is something that Eddy currents may also be caused by the stainless steel material of the cryostat located outside the helium bath, which has a large resistance value and a short time constant since it is at room temperature, but it adds to the eddy currents and exerts an influence. In addition, eddy currents have a large effect when the frequency is high, and therefore, the rise of a waveform with a steep rise becomes slow, thereby preventing the acquisition of a good NMR image.

Due to the need to compensate for the effects of eddy currents as described above, as a method of measuring the amount of effects caused by eddy currents,
The method previously proposed by the applicant (Japanese Patent Application No. 1983-
251058), and a method of determining it from the voltage waveform induced in the search coil.

The pulse sequence of the method disclosed in Japanese Patent Application No. 61-251058 is shown in FIG. In the figure, the gradient magnetic field is applied only to the x-axis, and the other two axes are turned off. 1 is
A 90° pulse of a high-frequency rotating magnetic field applied to the RF axis induces an FID signal 2 in a receiving coil (not shown). 3 is a dephase gradient that disturbs the phase of spins applied to the x-axis, and its pulse width is T _D
It is. 4 is a 180° pulse applied to the RF axis after the day phase pulse to reverse the spin, and 5 is a 180° pulse.
A rephasing gradient that adjusts the phase after pulse 4 and induces SE signal 6, and the rise of that pulse and
Let _TR be the interval with SE signal 6 (center). T _E point 7
are 90° pulse 1 (center) and 180° pulse 4 (center)
This is a point in a time interval T _E that is twice the time interval T _E /2 between the two points. δ is the end of day phase gradient 3
The time interval between 180° pulse 4 (center), ε is the time interval between SE signal 6 (center) and T _E point 7, G _D is the amplitude of day phase slope 3, and G _R is the freeze slope 5.
is the amplitude of

In this sequence, the day phase gradient 3
is closest to 90° pulse 1, that is, δ≒
Adjust G _D so that ε=0 when T _E /2.
SE signal 6 appears when the area of the pulse before and after the 180° pulse becomes equal, so G _D ,
There is a relationship between T _D , G _R , and _TR as shown in the following equation.

G _D ·T _D =G _R ·T _R (2) Therefore, by increasing G _D , the SE signal 6 approaches the T _E point 7 and can be set to ε=0. Originally, δ should not affect ε, but in reality, when δ is changed, ε changes. That is Day Phase Gradient 3
This is because the magnetic field gradient caused by the eddy current influences the rise and fall of the pulse with respect to the rephasing gradient 5. This state is shown in FIG. In the figure,
The same parts in FIG. 5 are given the same reference numerals. Day phase gradient 3 as an effect of magnetic field due to eddy current
This is due to disturbances in the waveform at the rise and fall of , and at the rise of the refaze gradient 5, and the SE signal 6 appears in such a way that the sum of the areas of the hatched and unhatched areas shown in the figure is equal.

D ₁ +D ₂ +D ₃ +D ₄ =R ₁ +R ₂ +R ₃ (3) δ and ε change while being related so that equation (3) holds true. The changes in δ and ε are shown in FIG. In the figure, ε (ms) on the vertical axis is on a logarithmic scale. The phase integration method is used to find the time constant τ from this graph. This method can be read directly with an oscilloscope, and the time constant can be determined by simple calculations.

(Problems to be Solved by the Invention) By the way, the measurement of the time constant of the eddy current described above has the following drawbacks.

When SE signal 6 is at T _E point 7, that is, ε
When = 0, the SE signal 6 can be observed with a good waveform without being affected by magnetic field inhomogeneity, but since this measurement method is a method of changing δ and observing the change in ε, it is not possible for ε≠0. , the SE signal 6 spreads and its center position is unclear, making it difficult to accurately grasp the value of ε, which may lead to inaccurate measurements. or,
The method using a search coil has poor accuracy and makes it difficult to measure components with long time constants.

The present invention has been made in view of the above points, and its purpose is to be able to obtain it by a simple method such as direct reading with an oscillograph, and by simple calculation, and to be unaffected by magnetic field inhomogeneity. The objective is to realize a method for measuring the time constant of eddy currents.

(Means for Solving the Problems) The present invention solves the above problems by reducing the gradient magnetic field response due to eddy currents in order to eliminate the influence of eddy currents caused by gradient magnetic fields applied to a nuclear magnetic resonance tomography apparatus. In the measurement method, a pulse sequence is used to obtain the SE signal with a 180° pulse.
The position of the SE signal is fixed at a position twice the time interval between the 90° pulse and the 180° pulse with the 90° pulse as the starting point, and the time interval between the day phase slope and the 180° pulse and the day phase slope at this time are This method is characterized by determining the gradient magnetic field response characteristics due to eddy currents from the relationship with the amplitude of .

(Function) Adjust the amplitude of the day phase gradient signal,
The SE signal is generated at the time T _E starting from _the 90° pulse, and the amplitude of the day phase slope (G _D ) is
, and measure the change in the time interval (δ) between the day phase slope and the 180° pulse application timing.
From the relationship between G _D and δ, find the gradient magnetic field response characteristics due to eddy current.

(Example) The method of the present invention will be described in detail below with reference to the drawings. In addition, NMR-CT for carrying out the method of the present invention
The configuration may be a normal one.

FIG. 1 is an explanatory diagram of a method according to an embodiment of the present invention. In the figure, the gradient magnetic field is applied only to the x-axis, and the other two axes are turned off. In the figure, the same parts as in FIG. 5 are given the same reference numerals.

Next, the principle of the measurement method and the implementation of the method will be explained. Place an appropriate volume of water phantom in the magnet and set the timing of application of day phase gradient 3.
Place it immediately after the 90° pulse, place the application timing of refase gradient 5 immediately after the 180 _° pulse,
Adjust G _R to make the time interval ε 0. When adjusting ε=0, the areas of the gradient magnetic fields before and after 180° pulse 4 are equal, so G _D , T _D , G _R , T _R
The relationship of equation (2) holds between them.

Next, when G _D is made smaller, the SE signal 6 moves so that T _R becomes smaller, so that ε>0. Here, as explained in the above-mentioned phase integration method, changing δ also changes ε, so δ is changed so that ε=0. In this way, while maintaining ε=0, G _D
Obtain the measured value of δ by varying .

On the other hand, the time constant τ and initial value ratio (gain) A of the eddy current are determined as follows. The amount of phase rotation of the magnetic field vector due to the magnetic field generated by the gradient magnetic field and eddy current is determined. In FIG. 6, D ₁ ~
D ₄ (shaded area) gives the phase in the dephase gradient direction (in this case, the phase is positive), and the parts R ₁ to R3 give the phase in the rephase gradient direction (in this case, the phase is in the negative direction). Eddy current A・
Let e ^- 〓.

Each of the positive phase quantities is given as follows.

D ₁ ; G _D・T _D D ₂ ;∫ ^TD+ 〓 ^-(TE/2- 〓 ⁾ _TD+ 〓G _D・A・e ^-t/ 〓dt ＝G _D・A・(−τ)・[e ^-1 〓{ ^TD+ 〓 ^+(TE/2- 〓 ⁾ }−e ^-
1/ 〓 ^(TD+ 〓 ⁾ ] =G _D・A・(−τ)・e ^-t/ 〓 ^(TD+ 〓 ⁾・{e ^-t/ 〓 ^(TE/2
- 〓 ⁾ −1} D ₃ ;∫〓 ₀ G _D・A・e ^TE/2 dt＝G _D・A・(−τ)・(e ^t/
〓−1) D ₄ ;∫ ^TE/2- 〓 ₀ G _R・A・e ^-1/ 〓dt＝G _R・A・(−τ)・
{e ^-1/ 〓 ^(TE/2- 〓 ⁾ −1} Each of the negative phase quantities is given as follows.

R ₁ ;G _R・(T _E /2−ε) R ₂ ;∫ ^TD+ 〓 ₀ G _D・A・e ^-t/ 〓dt=G _D・A・(−τ)・{
e ^1/ 〓 ^(TD+ 〓 ⁾ −1} R ₃ ;∫〓 ^+TE/2- 〓〓G _D・A・e ^-1/ 〓dt＝G _D・A・(−τ
)・{e ^-1/ 〓 ⁽ 〓 ^+TE/2- 〓 ⁾ −ε ^- 〓 ^/ 〓} =G _D・A・(−τ)・ε ^- 〓 ^/ 〓・{e ^-1/ 〓 ^{(TE/ 2-} 〓 ⁾
-1} From the above, calculating |positive phase amount|−|minus phase amount|=0, G _D・T _D −τ・G _D・A・e ^-TD/ 〓・e〓 ^/ 〓・{e ^-1/ 〓 ^(TE/2
- 〓 ⁾ −1}−τ・G _D・A・(e ^- 〓 ^/ 〓−1) −τ・G _R・A・{e ^-1/ 〓 ^(TE/2- 〓 ⁾ −1}−G _R ( _TE /
2-ε) +τ・G _D・A・{e ^-1/ 〓 ^(TD+ 〓 ⁾ −1} +τ・G _D・A・ε ^- 〓 ^/ 〓・{e ^-1/ 〓 ^(TE/2- 〓 ⁾ -1}
=0 holds true. From now on {G _D・T _D −G _R・T _E /2}+{2τ・G _D・A・(e ^-TD/ 〓−
1) −τ・G _D・A・e ^-1/ 〓 ^(TE/2- 〓 ⁾ (e ^-TD/2 -1)｝e ^-
〓 ^/ 〓 −τ・G _R・A・{e ^-1/ 〓 ^(TE/2- 〓 ⁾ −1}=−ε・G _RHere , assuming ε=0, we summarize: G _D・T _D −G _R・T _E /2+G _D {2τ・A・(e ^-TE/ 〓−1)−
τ・A・e ^-TE/2 〓・(e ^-TD/ 〓−1)}e ^- 〓 ^/ 〓 −τ・G _R・A(e ^-TE/2 −1)=0 ……(4) No. Letting the 2nd and 4th terms together be d, and the coefficient of the 3rd term as c, we get c・e ^- 〓 ^/ 〓=-T _D - d/G _D ...(5).

The data of G _D and δ obtained in the previous measurement are expressed as (5)
By substituting into the equation, the graph shown in FIG. 2 is obtained.
The time constant τ is determined from the slope of the straight line portion of this graph. (However, the vertical axis is a logarithmic axis) Gain A
is determined from equation (4) using the value G _D0 of G _D when δ=0 in the graph of FIG.

(G _D0・T _D −G _R・T _E /2)+A・{−G _R・τ(e ^-TE/2 〓
-1) +G _D0・τ・(e ^-TD/ 〓−1)・(2−e ^-TE/2 〓)
}=0 ∴A=(G _D0・T _D −G _R・TE/2)/{G _R・τ(e ^−TE/2
〓−1)−G _D0・τ(e ^-TD/ 〓−1)・2−e ^-TE/2 〓)}
In this way, the ratio A of the initial value of the eddy current to the dephase gradient and the decay time constant τ are found.
If there are multiple time constants, analytic line approximation may be used to calculate each time constant one by one.

As described above, according to this embodiment, the SE signal 6
Since measurements were taken by changing G _D so that T _E coincides with point 7, even if there is an inhomogeneous part in the magnetic field, the SE signal is not disturbed, and it is possible to correctly match ε = 0, and the eddy current It has become possible to accurately determine the time constant τ and the ratio (gain) A of the initial value.

Note that this measurement method may be performed manually while observing an oscilloscope, or may be performed using programmed automatic detection.

(Effects of the Invention) As explained in detail above, according to the present invention,
The amount of decrease in the gradient magnetic field due to eddy currents and the time constant can be measured with high precision, and data for completely correcting the amount of decrease in the magnetic field can be obtained. Furthermore, it is possible to realize a method for measuring gradient magnetic field response characteristics that has the advantages of the conventional method of being directly readable by an oscilloscope and easy to calculate, and has great practical effects.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram of a method according to an embodiment of the present invention, Fig. 2 is a relationship curve diagram between G _D and δ, and Fig. 3 is an NMR-
An explanatory diagram of the principle of CT pulse method, Figure 4 is NMR
- Diagram showing the pulse sequence of the magnetic field of CT, 5th
The figure shows the pulse sequence of measurement using the conventional phase integration method, Figure 6 is an explanatory diagram of the influence of eddy currents, and Figure 7
The figure is a δ-ε curve diagram in the conventional phase integration method. 1...90° pulse, 2...FID signal, 3...day phase gradient, 4...180° pulse, 5...rephase gradient, 6...SE signal, 7...T _E point.

Claims (1)

[Claims] 1. In order to eliminate the influence of eddy currents caused by gradient magnetic fields applied to nuclear magnetic resonance tomography equipment, in the measurement method for measuring gradient magnetic field responses due to eddy currents, a pulse sequence is used to obtain SE signals with 180° pulses. The position of the SE signal is fixed at a position twice the time interval between the 90° pulse and the 180° pulse, starting from the 90° pulse, and at this time, the time interval between the day phase slope and the 180° pulse, and the day phase slope A gradient magnetic field response measurement method characterized by determining gradient magnetic field response characteristics due to eddy currents from the relationship with the amplitude of.