JPS63135146A - Gradient magnetic field response measuring method - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

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.

(Prior Art) NMR-CT, which uses the phenomenon of nuclear magnetic resonance (hereinafter referred to as NMR) to obtain a cross-sectional image of a subject focusing on a specific atomic nucleus, has been known for a long time. The essential principles of NMR-CT will be briefly explained.

An atomic nucleus can be seen as a spinning top that is magnetic, but if it is placed in a static la field 11o in two axes, for example, the atomic nucleus will precess at an angular velocity ω0 as shown by the following equation. do. This is called Larmor precession.

ω0-TI'o However, γ: Nuclear gyromagnetic ratio Now, a high-frequency coil is arranged on an axis perpendicular to two axes with a static magnetic field, for example, the x-axis, and the high-frequency rotation with the above-mentioned angular frequency ω0 rotates in the xy plane. ! ! When a field is applied, magnetic resonance occurs, and static magnetic field No.
A population of atomic nuclei undergoing Zeeman splitting under , causes a transition between levels by the high-frequency Ia field that satisfies the resonance condition, and transitions to the higher unit of energy. here,
Since the nuclear gyromagnetic ratio γ differs depending on the type of atomic nucleus, the atomic nucleus can be identified by its resonance frequency. Furthermore, by measuring the strength of the resonance, the presence m of the atomic nucleus can be determined. 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 undergoes a tll radiation of energy.

Part 3 about the medium pulse method for observing this NMR phenomenon.
This will be explained with reference to the figures.

A high frequency pulse (Hl) that satisfies the resonance condition as described above
When applied in the x-axis direction instead of perpendicular to the static magnetic field (2 axes),
As shown in FIG. 3(a), the magnetization vector M starts rotating in the zy plane at an angular frequency of ω'-H1 in the rotating coordinate system.

Now, if the pulse width is to, the rotation angle θ from Ho is expressed by the following equation.

θ=γself1tD...(1)(1
) A pulse having 1. such that θ-90'' in the equation is called a 90'' pulse. Immediately after this 90° pulse, the magnetization vector M rotates at G0 in the xy plane as shown in FIG. However, since this signal attenuates over time, this signal is called a free induction attenuated signal (FID signal).If the FID signal is Fourier transformed, the signal in the frequency domain is 1q. ), when a second pulse (180' pulse) with a pulse width such that the pulse width becomes θ-180° after τ time from the 90° pulse is applied, the magnetic moment that had been scattered becomes 4U −y in τ time.
The signal is refocused in the direction and observed. This signal is called a spin echo (8E signal). A desired image can be obtained by measuring this SE multiplied signal intensity. NMR
The resonance condition for is given by γHo/2π. Here, ν is the resonant frequency and 1'o is the strength of the static magnetic field. Therefore, it can be seen that the resonance 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 to give a magnetic field of different strength depending on the position, and the resonance frequency is changed 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. (b) In the figure, χ.

It shows a state in which gradient magnetic fields of Gx, Gy, and Gz are applied to the y and z axes, and a high frequency magnetic field is applied to the x axis. (b) The figure shows the timing of applying each magnetic field. In the figure, RF is high frequency rotation! A 90″ pulse and a 180° pulse are applied to the This is a fixed gradient magnetic field applied to the z-axis.The signals are the FID signal after the 90'' pulse and the SE multiplied signal after the 180° pulse. The period is provided to indicate the timing of the signal of the gradient ROM given to each axis. 90° pulse and gradient magnetic field Qz◆ in period 1
Accordingly, spins in the slice plane in the tomography perpendicular to the z-axis centered at 2-0 are selectively excited. G×÷ in period 2 is for disturbing the phase of the spins and inverting them with 1800 pulses, and is called a dephase gradient. G2- is for restoring the phase of the spins 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, 1804 pulses are applied to align the magnetic moments again, and the SE multiplied signal Il appearing on that bond is
I guess. GX÷ in period 4 is a gradient magnetic field for aligning the disturbed phases and generating an SE multiplied signal, which is called a rephase gradient, and an SE multiplied signal appears where the areas of the rephase gradient and the dephase gradient become equal.

In NMR-CT, superconducting magnets are increasingly being used because a large static magnetic field improves the signal-to-noise ratio, but this has caused problems with eddy currents. That is, when a gradient field is applied, a vortex TEA flow is generated by the gradiometer field in the conductor of the static magnetic field magnet and the container of the helium tank for cooling, and the gradient magnetic field is canceled by the magnetic flux generated by this eddy current and becomes weak. This results in the phenomenon of becoming. This is particularly problematic because the conductor of the super Ti magnet and the container of the helium bath are cooled to an ultra-low temperature, so the resistance value is extremely small, close to O or 0, and large eddy currents flow with a long time constant. It has become. Eddy currents may also be caused by the stainless steel material of the cryostat located outside the helium tank, 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. Furthermore, eddy currents have a large effect when the frequency is high, and therefore, the rising edge of a waveform with a steep rising edge becomes blunt, thereby preventing the acquisition of a good NMR image.

Due to the need to compensate for the effects of eddy currents as described above, the applicant has previously proposed a method for measuring the effects of eddy currents (Japanese Patent Application No. 61-251058). There are methods to find it from the voltage waveform.

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 R'F axis, and the FTD
A signal 2 is induced in a receiving coil (not shown). 3 is X
This is a dephasing gradient that disturbs the phase of spins applied to the axis, and its pulse width is exactly 0. 4 is applied to the RF axis after the dephase pulse to reverse the spin by 180°
Pulse, 5 is 180' SE after adjusting the phase after pulse 4
In the rephase gradient that induces the signal 6, the interval between the rising edge of the pulse and the SE signal 6 is rll. ■ε point 7 is under the time interval between 90″ pulse and 180° pulse E/
This is a point at a time interval Tε that is twice as long as 2. δ is the time interval between the end of the dephasing gradient 3 and the 180゛ pulse 4,
ε is the time interval between the SE signal 6 and the TE point 7, Go is the amplitude of the dephasing gradient 3, and GR is the amplitude of the rephasing gradient 5.

In this sequence, set the dephase gradient 3 to 90°.
Adjust GD so that it becomes ε-0 when it is closest to pulse 1, that is, δ→TE/2. SE is applied when the area of the pulse before and after the 180° pulse becomes darker.
Since signal 6 appears, there is a relationship between Go, To, GR, and TR as shown in the following equation.

Go −To =GR−TR=O...(2) Therefore, G
If o is increased, SE signal 6 approaches TE point 7 and becomes ε-0
It can be done. Originally, δ should not affect ε, but in reality, when δ is changed, ε changes. This is because the magnetic field gradient caused by the eddy current influences the rise and fall of the pulses of the dephasing gradient 3 and the rephasing gradient 5. This state is shown in FIG. In the figure,
The same parts as in FIG. 5 are given the same reference numeral 8. The influence of the magnetic field caused by eddy currents is caused by disturbances in the waveform when the dephasing gradient 3 rises and falls, and when the rephasing gradient 5 rises. The SE signal 6 appears so that the sum of the areas is equal.

DI +D2 +D3 +D4 =Rt +R2+R3
...(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 the SE signal 6 is at the Tε point 7, that is, when ε=0, the SE signal 6 can be observed with a good waveform without being affected by magnetic field inhomogeneity, but in this measurement method, δ is changed. ε
Since this is a method of looking at changes in ε≠O, the SE signal 6 spreads and its center position is unclear, making it difficult to accurately grasp the value of ε, resulting in inaccurate measurements. There is. Furthermore, 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 use a simple method such as direct reading with an oscilloscope, and to obtain it by simple calculation, and! The object of the present invention is to realize a method for measuring the time constant of eddy currents that is not affected by inhomogeneity.

(Means for Solving the Problems) The present invention for solving the above problems is based on nuclear magnetic resonance 117i
Gradient applied to the layer imaging device! In order to eliminate the influence of eddy currents caused by 1 field, in the measurement method that measures the gradient magnetic field response due to eddy currents, a pulse sequence of SE-based pulses with 180° pulses is used, and SE-multiplied signals of 90° pulses and 180° pulses are used. fixed at a position twice the time interval of
The amplitude of the dephasing gradient and the dephasing gradient and 1
This method is characterized in that the gradient magnetic field response characteristics due to eddy currents are determined from the relationship with the change in the time interval between the application timings of the 80 @ pulses.

(Operation) The amplitude of the dephasing gradient signal is adjusted to generate it at the time TE point starting from the SE multiplied signal 90'' pulse, and while the SE multiplied signal position is not moved from the TE point, the dephasing gradient is By varying the gradient amplitude (Go),
The change in the time interval (δ) between the dephasing gradient and the application timing of the 180° pulse is measured, and the field response characteristic of the gradient device 1 due to the eddy current is determined from the relationship between Go and δ.

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

FIG. 1 is an explanatory diagram of a method according to an embodiment of the present invention. In the figure, the gradient a-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 a water phantom with an appropriate volume in the magnet, apply the dephasing gradient 3 immediately after the 90'' pulse, apply the rephasing gradient 5 immediately after the 180'' pulse, and adjust the amplitude Go or Gp+. to set the time interval ε to 0. When adjusted to ε = 0, the areas of the gradient magnetic fields before and after the 180° pulse 4 become equal, so Go.

The relationship of equation (2) holds between To, Go, and TR.

Next, when GD is made smaller, SE becomes smaller so that TR becomes smaller.
The signal 6 moves and becomes ε>O. Here, as explained in the above-mentioned phase integration method, changing δ also changes ε, so δ is changed so that ε·−〇. In this way, GO is varied while maintaining ε-0, and the measured value of δ is (9
Ru.

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, D1 to D4 (shaded areas) represent the phase in the dephasing gradient direction (here, the positive phase), and R1 to R3 represent the phase in the rephasing gradient direction (here, the positive phase).
Here, the phase is assumed to be in the negative direction). Let the eddy current be A.-.

Each of the positive phase quantities is given as follows.

−8−+(Th+δ)] =Go−A・(−r)−e7Th+5) ・=OR−
A-(-r)-(e+('?-1=)-11 Each of the negative phases m is given as follows.

-Go -A -(-r) -(e-+(g”r-F-
) −a4)=Go −A −(−r) −ε−L (
e-t('?-')-1) and above, 1 plus phase 1
When the phase 11-0 of 1-1 minus is used as a needle ring, G・・T・−τ・G・−A −e”・.−(・(, −+
(child-・)-1゜1τ・GD ◆A・ (et-1)-τ・GR-A・(
e-) (pu'-1) -GR("!-6> +r ・G., A, (e-
'f(tows)+τ・G・・A・ε−0・(.−+(
The relationship ``)-1) -0 is established.From now on, (Go -To-GR-child) + (2τ・GD-An ・(et-1) 1τ・G・・A・.−+(9 -・) (8-Kai-1))8
-τ-τ・G・・A・(.−+C+−・L 1 ) =
-ε・G・Here, let ε==0 and summarize: Go −To −GR4+Go (−A(e−Z−
z-, A, 9-! a (e-n-t,.

1)) e GR-A (e Z-1) -0''・(4) Letting the second and fourth terms together be d, and the coefficient of the third term be C, then -1-ordinary c- et-To. (5) O By substituting the Go and δ data obtained in the previous measurement into equation (5), the graph 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 G when δ-〇 in the graph of FIG.

is obtained from equation (4) using the value Goo.

(2-e part)) -〇,',A-(GDo -To -GR-child)/(GR・
τTε −1 (e−!−1)−Goo ・r (el knee 1) ・(
2-In this way, the ratio A of the initial value of the eddy current to the dephasing gradient and the time constant τ of the decay are found. If there are a plurality of time constants, it is sufficient to use a broken line approximation and calculate each time constant one by one.

As described above, according to this embodiment, the SE signal 6 is
Because the measurement was performed by changing Go to match point 7,
Even if there are non-uniform parts in the magnetic field, no SE signal disturbance occurs,
It has become possible to correctly match ε-0, and it has become possible to accurately determine the time constant τ of the eddy current 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 by automatic detection using a blockogram.

(Effects of the Invention) As described in detail above, according to the present invention, the amount of decrease and time constant of the gradient magnetic field due to eddy current can be determined with high precision, and the time constant can be adjusted with high precision to completely correct the decrease path of the magnetic field. data can be obtained. However, we were able to realize a method for measuring gradient magnetic field response characteristics that has the advantages of the conventional method, which can be directly read using an oscilloscope and is easy to calculate.
The practical effects are significant.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of a method according to an embodiment of the present invention, and FIG.
The relationship curve between o and δ, Figure 3 is an explanatory diagram of the principle of the NMR-CT pulse method, and Figure 4 is the 1tiJJi of NMR-CT.
Figure 5 is a diagram showing the pulse sequence of measurement using the conventional phase integration method, Figure 6 is a diagram showing the pulse sequence of the vortex 'F
FIG. 7, which is an explanatory diagram of the influence of the i-flow, is a δ-ε curve diagram in the conventional phase integration method. 1...90° pulse 2...FID signal 3...
・Dephase gradient 4...1800 pulses 5...
Rephase gradient 6...SE (No. 8 7...16 points Patent applicant Yokogawa Medical System Co., Ltd. Atsushi 1 Figure 2 U δ--Figure 3 (A) (O)
(c) 9σ nof pulse 1ao' pulse Figure 5 Figure 7 δ (ms)

Claims (1)

[Claims] In order to eliminate the influence of eddy currents caused by gradient magnetic fields applied to nuclear magnetic resonance tomography equipment, in a measurement method for measuring gradient magnetic field responses due to eddy currents, a pulse sequence of SE signals with 180° pulses is used to detect the SE signal. 9
The vortex is fixed at a position twice the time interval between the 0° pulse and the 180° pulse, and the vortex is A gradient magnetic field response measurement method characterized by determining gradient magnetic field response characteristics due to current.