JPS63105751A - Gradient magnetic field response correction method - Google Patents

Abstract

(57) [Abstract] 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 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 Technique #4) NMR-CT, which uses the nuclear magnetic resonance (hereinafter referred to as NMR) phenomenon to obtain a tomographic image of a subject focusing on specific atomic nuclei, has been known for a long time. The essential principles of this 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 magnetic field Ho in the Z-axis direction, for example, the atomic nucleus will precess at an angular velocity of ω0 as shown by the following equation. do. This is called Lamore's precession.

ω0 = γ (-10, where γ: nuclear magnetic rotation) Now, a high-frequency coil is placed on an axis perpendicular to the l-axis with a static magnetic field, for example, the X-axis, and the above angular frequency ω0 rotating in the xy plane is Magnetic resonance occurs when a high-frequency rotating magnetic field is applied, and a population of atomic nuclei undergoing Zeeman fission under the static magnetic field undergoes a transition between units due to the high-frequency !i field that satisfies the resonance conditions.
Transition to a unit with a higher energy 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.

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

As mentioned above, a high frequency pulse (H
+) is applied not perpendicularly to the static magnetic field (2 axes) but in the Start rotating inside. Now, if the pulse width is tD, the rotation angle θ from Ho
is expressed by the following equation.

θ−γH1jo ...(1)(
In equation 1), a pulse with to that is θ-90° is called a 90" pulse. Immediately after this 90" pulse, the magnetization vector M rotates at ω0 in the xy plane as shown in Figure 3 (b). For example, an induced electromotive force is generated in a coil placed on the y-axis. 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, 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 the pulse width becomes θ-180° after τ time from the 90° pulse is added, the scattered magnetic moments are After a time τ, the signal is observed again with focus in the -y direction. This signal is called a spin echo (SE multiplied signal). The desired image can be obtained by measuring the SE multiplied signal intensity. The resonance condition of NMR is given by γHo/2π. Here, ν is The resonant frequency, Ho, 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. Therefore, by superimposing a linear magnetic field gradient on the static magnetic field, the magnetic field has a different strength depending on the position. There is an NMR imaging method that gives 1q of displacement information while avoiding changing the resonance frequency. Of these, the Fourier transform method will be explained. The pulse sequence of applying the high frequency to field and gradient magnetic field used in this method is shown in Figure 4. (a) In the figure,
.

It shows a state in which gradients mgA of GX, Gy, and Gz are given to the y and z axes, respectively, and a high frequency t4i field is applied to the y axis. (b) The figure shows 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 y-axis. Qx is a fixed gradient magnetic field applied to the y-axis, Gy is a gradient magnetic field that changes the imaging width depending on the time applied to the y-axis, and Qz is a fixed gradient magnetic field applied to the z-axis. The signal is FI after 90” pulse
The D signal and the SE multiplied signal after the 180° pulse are shown. The period is provided to indicate the timing of the signal of the gradient magnetic field applied to each axis. 90” pulse and gradient magnetic field Gz in period 1
Section F21 perpendicular to the z-axis centered on z-Q by +
Spins within the slice plane in the extraction are selectively excited. Qx+ in period 2 disturbs the spin phase and becomes 180
° This is something that cannot be reversed with a pulse and is called a dephase gradient. G2- is for restoring the phase of the spins disturbed by Gz+. In period 2, Qyn 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, 1800 pulses are applied to align the magnetic moments again, and the SE multiplied signal that appears thereafter is observed. GX+ between 1 and 4 aligns the disturbed phase, and the gradient IJfl for generating the SE multiplied signal is called a rephase gradient, and the SE multiplied signal appears at the point where the areas of the rephase gradient and the DIN1-'s 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. In other words, the slope! When an i-field is applied, eddy currents due to the gradient magnetic field are generated in the conductor of the static RA 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 weakened. will occur. 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, close to 0 or O, 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 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. In addition, eddy currents have a greater influence when the frequency is high (thus, they slow the rise of a waveform with a steep rise, making it difficult to obtain a good NMR image.

(Problem that the invention attempts to solve) A vortex like the one above? It was necessary to compensate for the influence of the H flow, and the frost due to the influence of eddy currents from L was measured and compensated for by the following method.

(1) Direct measurement using a search coil A gradient magnetic field is applied, and a search coil is placed nearby to observe the waveform. In reality, the obtained waveform 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 vA field, and measure the response characteristics of the FID signal to the gradient magnetic field)! If there is an eddy current used to calculate the function, the waveform of the FID signal will be an exponential curve in the standard state, as shown in Figure 5, and the amplitude will decrease, but due to the presence of the eddy current, the time constant will become longer and the waveform will become an exponential curve. It disappears. The transfer function of the gradient magnetic field is calculated based on the deviation of this curve, but since this measurement method is affected by non-uniform static magnetic fields, it is difficult to determine whether the disturbance is due to non-uniform static magnetic fields or due to eddy currents. difficult to distinguish. Therefore, it is necessary to make corrections, such as measuring the state in the absence of a gradient magnetic field in advance, or averaging a large number of measurements to eliminate this influence, which takes time. Furthermore, due to the influence of phase distortion and the steep attenuation curve of the FED signal, good measurement accuracy cannot be obtained.

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, which solves the above-mentioned problems, uses a gradient magnetic field caused by eddy currents to eliminate the influence of eddy currents that are stopped by gradient Ia fields applied to a nuclear magnetic resonance tomography apparatus. In the gradient 1 ifl field response correction method that measures the response and corrects the amount of decrease in the gradient magnetic field, an SE multiplied signal pulse sequence with 180' pulses is used to calculate Jfi and the 180' pulse when applying the differential gradient 1 iti field. SE multiplication signal generation timing and T of 90° pulse starting point with respect to change in time interval from application timing
From the relationship of the change in time interval with point E, is it a vortex? This method is characterized in that the gradient magnetic field response characteristics due to the current are determined, and the eddy current compensation of the gradient magnetic field is performed based on the gradient magnetic field response characteristics.

(Function) The application time and/or amplitude of the dephase gradient magnetic field is adjusted to generate the SE multiplied signal at the time Tt point starting from the 90° pulse, and the application time of the dephase gradient magnetic field is 180°.
° While gradually decreasing the pulse application timing and time interval (δ), measure the SE multiplied signal TE fish separation distance (ε). A gradient magnetic field response characteristic due to overcurrent is determined from the relationship between δ and ε, and overcurrent compensation for the gradient vA field is performed based on it.

(Actual mW4) Hereinafter, the method of the present invention will be explained in detail 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 an example method 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 avoids disturbing the phase of spins applied to the x-axis, and its pulse width is To. 4 is a 1800 pulse applied to the RF axis to reverse the spin after the dephase pulse, and 5 is a 180 pulse.
``This is a rephasing gradient magnetic field signal that adjusts the phase after pulse 4 and induces 5Effi No. 6, and the interval between the rise of the pulse and SE signal 6 is set as TR.

Tε point 7 is a point in the time interval TE that is twice the time interval Tε/2 between the 90" pulse and the 180° pulse. δ is the point between the end of the dephase gradient magnetic field signal @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 magnetic field signal 3, and GR is the width of one axis of the rephasing gradient magnetic field signal 5.

Next, the principle of the method and the implementation of the method will be explained.

A water phantom with an appropriate volume is placed in the magnet, and when the dephase gradient taMA signal 3 is applied, l1l (hereinafter referred to as the position on the time axis) is set so that the SE multiplied signal center is exactly at the TE point 7, that is, ε = 0. Initial settings are made so that δ at that time is δ0. Dephase gradient rajJA signal 3
The position of is assumed to be the position n of the dotted line 3' in the figure. To adjust the first +91 H constant, adjust Go or GR! All you have to do is Ii. At this time, Go-To=GR-TR.

Next, without changing any other parameters, gradually increase δ−〇 and measure ε each time, and you will find the slope! The change in the i field due to the eddy current is observed as an advance 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, since the dephase gradient magnetic field signal becomes dull and the effective area decreases, the SE signal advances and reaches the position shown in the figure. Therefore, by reading the change in ε, the change in TR can be determined, and the influence of the eddy current on the gradient magnetic field can be determined.

This sensitivity aS is expressed as 5=Go-To/GR·ΔTT.

However, ΔT is the reading accuracy.

For example, Go = 0, 5 G/ (R1, To-1
0RIs.

If GR-〇, 01G/cm, ΔT-0.1ms, then S-0,5X0.0110.01X10', that is, 0,0
The gradient response characteristics can be measured with a high accuracy of 2%.

In this way, by changing δ and measuring the amount of change in ε, a relationship as shown in FIG. 6 is obtained. In the figure, the vertical axis is ε(ns) on a logarithmic scale, and the horizontal axis is δ(ns).
Take aS). The solid line is a curve in which ε is plotted while changing δ, and the dotted line is a linear approximation of the solid curve. The overcurrent time constant τ is obtained from the relationship shown in FIG. 6 as follows.

Two points a on the ε axis so as to satisfy the following equation.

If b is selected, b = a / e However, e is the base two points a and b of the natural logarithm.
The length δl between the two points c and d on the δ-axis corresponding to corresponds to the time constant τ to be determined.

τ−δr (ff1.s)
(2) On the other hand, the ratio A of overcurrent to the dephase gradient magnetic field signal is determined as follows.

Find the phase rotation slip of the magnetic field vector due to the magnetic field generated by the gradient and overcurrent. The shaded area in Figure 7 (D
+~D4) is the phase of the dephase gradient magnetic IA signal direction (
The portion R1-R3 can change the phase of the rephase gradient magnetic field signal (here, the phase is in the negative direction). Let the overcurrent be △・Qf. (
A no gain.

12 time constant) Each of the positive phase quantities is given as follows.

Dl: Go-T.

-6゜-A-(-4>-e4("°・°).

-6R, A, (-r) -(e-÷Mori') -1° Each of the negative phase amounts is given as follows.

-G・・A・(−τ)・(・−1,,W−,)−84)
-2, -Juyaku-・)-1゜=Go-A・〈-τ)・ετ (e From the above, the phase of 1 plus m1-1 minus (.-+
(Example-"-1) -r・Go *Aa (e-"-1)-r・G* -8 holds true. From now on, (Go' To-G*・(child))+(2τ・G.

O Therefore, in the linear approximation obtained in FIG. 6, by using the value εl of ε at δ−〇, the ratio can be found by the following equation.

+ r −G*, (8−+(child−ε・)−1)] is obtained.

In this way, the ratio A of the initial +11 value of the overcurrent to the dephase gradient magnetic field and the decay time constant τ 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.

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

rn-Rn-Qn, Δn-RnA/R.

...(4) Gradient with added eddy current compensation circuit! FIG. 8 shows a power supply amplifier circuit. In the figure, 11 is an amplifier circuit for a gradient device 1 field power supply consisting of a resistor Ro and an operational amplifier [0], 12 is an input circuit of a differential circuit consisting of capacitors C1...Cn and resistors R1...Rn, and RIA... This is an eddy current compensation circuit consisting of operational amplifiers El...En having a feedback circuit of Rn^. R1...Rn are semi-fixed resistors, and together with C1...Qn, the time constant of equation (4) is 1...
A differential circuit that satisfies rn is constructed, and the feedback resistors R1^...RnA are also semi-fixed resistors, and the gain is adjusted so as to satisfy equation (4). Eddy current compensation circuit 1
The number of amplifier circuits No. 2 is equal to the number of eddy current generation sources, and component constants are selected to compensate for each generation source. Amplifier circuit 1
1 and the outputs of the eddy current compensation circuit 12 are connected to the gradient magnetic field power supply circuit 13.
is input.

As explained above, by changing the application timing of the dephasing gradient and measuring the SE multiplied signal movement, it is possible to measure the gradient magnetic field response due to overcurrent with high precision, and to configure an accurate compensation circuit and achieve high quality. It is now possible to obtain images of 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 calculating g arithmetic 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 described above in detail, according to the present invention, the amount of decrease in gradient magnetic field due to eddy current and the time constant can be measured with high precision, and based on this measurement! ! The field reduction can be completely corrected and high-quality images can be obtained by 1q, which has a great practical effect.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram of the method of the embodiment of the present invention, Fig. 2 is an explanatory diagram of the measurement principle, Fig. 3 is an explanatory diagram of the principle of the pulse method of NMR-CT, and Fig. 4 is an explanatory diagram of the principle of the NMR-CT pulse method. Figure 5 is an illustration of the measurement of the FID signal response, Figure 6 is a δ-ε curve diagram, Figure 7 is an illustration of the influence of eddy currents, and Figure 8 is an illustration of the influence of eddy currents. FIG. 2 is a diagram of a gradient magnetic field power supply circuit to which a current compensation circuit is added. 1...90" pulse 2...FID signal 3.3'
...Dephase gradient magnetic field signal 4...1800 pulses 5...Rephase gradient magnetic field signal 6...SE Shintan 7...TI: Point 11...
Gradient LA field amplification circuit 12... Eddy current compensation circuit 13... Gradient magnetic field power supply circuit Patent applicant Yokogawa Medical Systems Co., Ltd. Supplementary Figure 3 (A) (B) (C)

Claims (1)

[Claims] In order to remove the influence of eddy currents caused by gradient magnetic fields applied to nuclear magnetic resonance tomography equipment, in a gradient magnetic field response correction method that measures the gradient magnetic field response due to eddy currents and corrects the amount of decrease in the gradient magnetic field, 1800 pulses are used. Using the pulse sequence of the SE signal, we can calculate the generation timing of the SE signal and T_ of the 90° pulse starting point with respect to the change in the time interval between the application timing of the dephasing gradient magnetic field and the application timing of the 1800 pulse.
A gradient magnetic field response correction method characterized in that a gradient magnetic field response characteristic due to eddy current is determined from the relationship of change in time interval with point E, and eddy current compensation of the gradient magnetic field is performed based on the gradient magnetic field response characteristic.