JPH0333007B2 - - Google Patents

Description

Detailed Description of the Invention (Industrial Application Field) The present invention measures gradient magnetic field responses due to eddy currents in order to eliminate the effects of eddy currents caused by gradient magnetic fields applied to nuclear magnetic resonance tomography equipment. The present invention relates to a gradient magnetic field response correction method for correcting the amount of decrease in a gradient magnetic field.

(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 Lamore's 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 pulse method of observing NMR phenomena here 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 3 C, when a second pulse (180° pulse) with a pulse width such that θ = 180° is applied after τ time 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 a spin echo (SE signal). A desired image can be obtained by measuring the intensity of this SE signal. NMR
The resonance condition 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 that applies 90° pulses and 180° 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. The signals are the FID signal after the 90° pulse and the SE after the 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,
Observe the SE signal that appears after that. period 4
Gx ⁺ is a gradient magnetic field that aligns the disturbed phases and generates an SE signal, which is called a ref-phase gradient. An SE signal appears when the areas of the ref-phase gradient and the day-phase gradient become equal.

In this MNR-CT, when the static magnetic field is large,
There is a tendency to use superconducting magnets because they improve the signal-to-noise ratio, but the problem of eddy currents has arisen here. 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 blunt, making it difficult to obtain a good NMR image.

(Problems to be Solved by the Invention) Due to the need to compensate for the effects of eddy currents as described above, the amount of effects caused by eddy currents has been measured and compensated for by the following method.

(1) Direct measurement using a search coil Apply a gradient magnetic field, place a search coil near it, and observe the waveform. In reality, the waveform obtained is a differential waveform, so it is integrated and observed, but this measurement method has a poor signal-to-noise ratio and poor measurement accuracy.

(2) Measure the response characteristics of the FID signal to the step gradient magnetic field and calculate the transfer function of the gradient magnetic field. When there is an eddy current, the waveform of the FID signal changes in amplitude to an exponential curve as shown in Figure 5 in the standard state. However, due to the presence of eddy currents, the time constant becomes longer and the waveform no longer resembles an exponential function curve. The transfer function of the gradient magnetic field is calculated by the amount of deviation of this curve.
This measurement method is affected by non-uniform static magnetic fields, so it is difficult to distinguish between disturbances caused by non-uniform static magnetic fields and eddy currents. Therefore, it is necessary to make corrections, and it is time-consuming because it is necessary to measure in advance the state in the absence of a gradient magnetic field, or to eliminate this influence by averaging a large number of measurements. Furthermore, due to the influence of phase distortion, the attenuation curve of the FID signal is steep, making it difficult to obtain good measurement accuracy.

The present invention has been made in view of the above points, and its purpose is to accurately measure the response of a gradient magnetic field due to eddy currents, and compensate for the gradient magnetic field response, thereby obtaining a high-quality image. is to obtain.

(Means for Solving the Problems) The present invention solves the above-mentioned 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 gradient magnetic field response correction method that measures and corrects the amount of decrease in the gradient magnetic field, a pulse sequence of SE signals with 180° pulses is used to calculate the change in the time interval between the application time of the day-phase gradient magnetic field and the application timing of the 180° pulse. The gradient magnetic field response characteristic due to eddy current is determined from the relationship between the time interval change between the SE signal generation timing and the T _E point of the 90° pulse origin, and the eddy current compensation of the gradient magnetic field is performed based on this. It is something to do.

(Operation) The application time and/or amplitude of the day-phase gradient magnetic field is adjusted to generate an SE signal at the time T _E point starting from the 90° pulse, and the difference between the application time of the day-phase gradient magnetic field and the application time of the 180° pulse is While gradually decreasing the time interval (δ), measure the amount (ε) by which the SE signal separates from the T _E point. A gradient magnetic field response characteristic due to eddy current is determined from the relationship between δ and ε, and eddy current compensation of the gradient magnetic field is performed based on it.

(Example) Hereinafter, the method of the present invention will be explained in detail with reference to the drawings. In addition, NMR-
The configuration of the CT 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. 1 is a 90° pulse of a high frequency rotating magnetic field applied to the RF axis, which induces an FID signal 2 in a receiving coil (not shown). 3
is a dephase gradient magnetic field signal that disturbs the phase of spins applied to the x-axis, and its pulse width is T _D. 4 is a 180° pulse that is applied to the RF axis after the day-phase pulse to reverse the spin, and 5 is a rephase gradient magnetic field signal that adjusts the phase after 180° pulse 4 and induces SE signal 6. Let T _R be the interval between the rising edge and SE signal 6. T _E point 7 is a point at a time interval T _E that is twice the time interval T _E /2 between the 90° pulse and the 180° pulse. δ is the time interval between the end of day phase gradient magnetic field signal 3 and 180° pulse 4, ε is the time interval between SE signal 6 and T _E point 7, G _D
is the amplitude of the day-phase gradient magnetic field signal 3, and G _R is the amplitude of the day-phase gradient magnetic field signal 5.

Next, the principle of the method and the implementation of the method will be explained. A water phantom of an appropriate volume is placed in the magnet, and the application timing of the day phase gradient magnetic field signal 3 (hereinafter referred to as the position on the time axis) is set so that the center of the SE signal is exactly at the T _E point 7, that is, ε = 0. The initial setting is made so that δ at that time is δ ₀ and the position of the day phase gradient magnetic field signal 3 is the position indicated by the dotted line 3' in the figure. To adjust the initial settings, just adjust G _D or G _R. In this case, G _D・T _D =G _R・T _R.

Next, without changing any other parameters, gradually δ
→ When ε is measured each time while setting O, the change due to the eddy current in the gradient magnetic field is observed as a progression of ε. This state is shown in FIG. In the figure, the same reference numerals and symbols are used for the same parts as in FIG. As shown in the figure, the day phase gradient magnetic field signal becomes dull and the effective area decreases.
The SE signal will advance to the position shown in the figure. Therefore, by reading the change in ε, the change in T _R can be determined, and the influence of the eddy current on the gradient magnetic field can be determined. This sensitivity S is expressed as S=G _D ·T _D /G _R ·ΔT.

However, ΔT is the reading accuracy.

For example, if G _D = 0.5 G/cm, T _D = 10 ms, G _R = 0.01 G/cm, ΔT = 0.1 ms, then S = 0.5 x 0.01/0.01 x 10 ^-4 = 5000, that is, high accuracy of 0.02%. The response characteristics of the slope can be measured using .

In this way, when δ is changed and the amount of change in ε is measured, a relationship as shown in FIG. 6 is obtained. In the figure, the vertical axis is ε(m
s) and δ (ms) is plotted on the horizontal axis. The solid line is a curve obtained by plotting ε while changing δ, and the dotted line is a linear approximation of the solid curve. The time constant τ of the eddy current can be obtained from the relationship shown in FIG. 6 as follows. If two points a and b are selected on the ε-axis so as to satisfy the following formula, then b=a/e, where e is the base of the natural logarithm.Two points c on the δ-axis corresponding to the two points a and b, The length δ ₁ between d corresponds to the required time constant τ.

τ=δ ₁ (ms) (2) On the other hand, the ratio A of the eddy current to the dephase gradient magnetic field signal is determined as follows.

Find the amount of phase rotation of the magnetic field vector due to the magnetic field generated by the gradient and eddy current. The shaded portion (D ₁ to _{D 4} ) in Figure 7 is the phase in the direction of the day phase gradient magnetic field signal (here, it is assumed to be a positive phase)
, the portions R ₁ to _{R 3} give the phase of the rephasing gradient magnetic field signal (here, the phase is in the negative direction). Let the eddy current be A・e ^--t/ 〓. (A: gain,
τ: time constant) Each of the positive phase amounts is given as follows.

D ₁ ; G _D・T _D Each of the negative phase quantities is given as follows.

From the above, if we calculate 1 plus phase amount 1 - 1 minus phase amount 1 = 0 This relationship is established. from now Therefore, in the linear approximation obtained in Figure 6, δ=
By using the value ε ₁ of ε at 0, the ratio A can be found by the following equation.

That's what I find.

In this way, the ratio A of the initial value of the eddy current to the day phase gradient magnetic field and the decay time constant τ are found.

If there are multiple time constants, use a broken line approximation, each with 1
All you have to do is calculate each time constant.

N based on N time constants obtained from equations (2) and (3)
If a correction circuit is constructed using the set of eddy current components τn and An (n=1 to N) as correction circuit constants of the gradient power source, a gradient magnetic field that accurately corresponds to the input can be obtained.

τn=Rn・Cn, An=Rn _A /R ₀ (4) Figure 8 shows an amplifier circuit for a gradient magnetic field power supply with an eddy current compensation circuit added. In the figure, 11 is a resistance
R ₀ is an amplifier circuit for a gradient magnetic field power supply consisting of an operational amplifier E ₀ , 12 is an operational amplifier E ₁ having a differential circuit consisting of a capacitor C ₁ ...Cn and a resistor R ₁ ...Rn as an input circuit, and a feedback circuit of R _1A ...Rn _A. ...This is an eddy current compensation circuit consisting of En. R ₁ ...Rn are semi-fixed resistors, and together with C ₁ ...Cn, they form a differentiating circuit that satisfies the time constant τ ₁ ...τn of equation (4), and the feedback resistors R _1A ...Rn _A are also semi-fixed resistors, The gain is adjusted to satisfy equation (4). Eddy current compensation circuit 1
The number of each amplifier circuit No. 2 is equal to the number of eddy current generation sources, and component constants are selected to compensate for each generation source. The outputs of the amplifier circuit 11 and the eddy current compensation circuit 12 are input to a gradient magnetic field power supply circuit 13.

As explained above, by changing the application timing of the day phase gradient and measuring the amount of movement of the SE signal, the gradient magnetic field response due to overcurrent can be measured with high precision, and an accurate compensation circuit can be configured to achieve high quality. Now I can get images. Note that the present invention is not limited to this example. for example,
Although the δ-ε relationship was obtained by linear approximation, it may also be obtained by arithmetically expanding it using the method of least squares. Further, the type of the compensation circuit is not limited to this circuit, and any circuit that produces the same effect can be appropriately selected.

(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 based on this, the amount of decrease in the magnetic field can be completely corrected, making it possible to obtain high-quality images, and the practical effect is big.

[Brief explanation of drawings]

Figure 1 is an explanatory diagram of the method of one embodiment of the present invention, Figure 2 is an explanatory diagram of the measurement principle, and Figure 3 is NMR-CT.
Figure 4 is an explanatory diagram of the principle of the pulse method of NMR-
A diagram showing the pulse sequence of the CT magnetic field, Figure 5 is an explanatory diagram of the measurement of the FID signal response, and Figure 6 is a diagram showing the δ-
ε curve diagram, Figure 7 is an explanatory diagram of the influence of eddy current, Figure 8
The figure is a diagram of a gradient magnetic field power supply circuit to which an eddy current compensation circuit is added. 1...90° pulse, 2...FID signal, 3,3'...
... Day phase gradient magnetic field signal, 4... 180° pulse, 5... Rephase gradient magnetic field signal, 6... SE
Signal, 7...T _E point, 11...Gradient magnetic field amplification circuit,
12... Eddy current compensation circuit, 13... Gradient magnetic field power supply circuit.

Claims (1)

[Claims] 1. In a gradient magnetic field response correction method that measures the gradient magnetic field response due to the eddy current and corrects the amount of decrease in the gradient magnetic field, in order to remove the influence of eddy currents caused by the gradient magnetic field applied to a nuclear magnetic resonance tomography apparatus,
Using the pulse sequence of the SE signal with 180° pulses, the change in the time interval between the time of application of the day-phase gradient magnetic field and the time of application of the 180° pulse
The gradient magnetic field response characteristic due to eddy current is determined from the relationship between the time interval change between the SE signal generation timing and the T _E point of the 90° pulse origin, and the eddy current compensation of the gradient magnetic field is performed based on this. Gradient magnetic field response correction method.