JPS6384540A - Rf pulse adjustment of nuclear magnetic resonance tomographic imaging apparatus - 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 provides a 90" pulse and a 180°
The present invention relates to an RF pulse adjustment method for a nuclear magnetic resonance laboratory rib imaging apparatus that sets the optimum pulse conditions.

(Prior Art) NMR-CT, which uses 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. An outline of the principle of this NMf'(-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 have an angular velocity ω expressed by the following equation. to precess. This is called Lamore's precession.

ω = γH However, γ: Nuclear gyromagnetic ratio 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-mentioned angular frequency ω rotates in the xy plane.
. Magnetic resonance occurs when a high-frequency rotating ta field is applied, and the population of atomic nuclei undergoing Zeeman splitting under the static 1a@Ho undergoes a transition between units due to the high-frequency rd1@ that satisfies the resonance condition, and the energy unit Transition to higher unit. 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 post-pz relaxation time, the atomic nucleus excited to the higher unit returns to the lower unit and radiates energy.
The medium pulse method of the I'IPJ method for this NMR phenomenon will be explained with reference to FIG.

A high frequency pulse (H4) that satisfies the resonance condition as described above.
When applied in the direction (x axis) perpendicular to the static magnetic field (two axes),
As shown in FIG. 3(A), the magnetization vector M starts rotating in the zy plane at an angular frequency of ω'=γH1 in the rotating coordinate system.

If the pulse width is now 11+, the rotation angle θ from 1-1 o
is expressed by the following equation.

θ−γt−11t o・(1) In equation (1), a pulse with 10 such that θ=90° is called a 90” pulse. Immediately after this 90° pulse, the magnetization vector M is as shown in Figure 3 (b). This means that the xy plane is rotated at ω0, and for example, an induced electromotive force is generated in the coil placed on the (F.I.D.
signal). By Fourier transforming the FID signal, a signal in the frequency domain can be obtained. Next, as shown in Figure 3 (c), when we add a pulse of :A2 (180° pulse) with a pulse width such that θ = 180° after τ time from the 90° pulse, the scattered magnetic moments are After τ time −y
The signal is refocused in the direction and observed. 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=γHo/2π. Here, ν is the resonant frequency, and 1° is the strength of the static magnetic field. Therefore, the resonant frequency is! It can be seen that it is proportional to the strength of i&@. For this purpose, a linear magnetic field gradient is superimposed on the static magnetic field to give a magnetic field of different strength depending on the position.
There is an NMR imaging method that obtains position information by changing the resonance frequency. The spin warp method will be explained. High frequency used in this method 1! The pulse sequence of @ and gradient magnetic field application is shown in FIG. (a) The figure shows a state in which magnetic fields Gx, Gy, and Qz are applied to the x, y, and z axes, respectively, and a high-frequency vii 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! 9 with 1#A
Apply a 0° pulse and a 180° pulse to the X axis. Qx is a fixed gradient vA@ applied to the z-axis, Gy is a gradient magnetic field whose amplitude changes depending on the time applied to the y-axis, and GZ is a fixed gradient magnetic field applied to the l-axis. The signal shows the FID signal after the 90° pulse and the low SE reliability of the 180° pulse. The period is provided to indicate the timing of the signal of the gradient fIi field applied to each axis. In period 1, the 90° pulse and the gradient magnetic field GZ+ selectively excite spins in the slice plane in the tomography perpendicular to the l axis centered at 2=0. Gx+ in period 2 is for disturbing the spin phase and inverting it with a 180° pulse, and Gz- is for Q
This is to restore the phase of spins disturbed by Z+. In period fJ12, Gyn is also applied. This is for a gradient magnetic field called a warp that shifts the phase of spins in proportion to the position in the V direction, and its strength is controlled to be different every cycle. 180 in period 3
Apply a ° pulse to align the magnetic moments again, and observe the SE signal that appears.

(Problem to be Solved by the Invention) In the spin warp method described above, the rotation angle θ by the 90° and 180° pulses for excitation of NMR-CT is accurately set to 90°.
It was necessary to adjust it to 0° and 180°, and that adjustment was done using the method used in conventional machines.

1. Method using FID (:) The FID signal obtained when applying an RF pulse is
In equation (1), changing H1, that is, the amplitude of the RF pulse, changes θ, so change the amplitude of the RF pulse and define the time when the FID signal intensity is maximum as a 90° pulse,
The time when the pulse reaches its minimum is defined as a 180° pulse. There is also a method in which a 180° pulse is first obtained and the RF pulse width is halved to obtain a 90° pulse, or vice versa. Since the FID signal must be detected immediately after the RF pulse is applied, it is difficult to accurately determine the RF pulse amplitude at which the signal intensity is maximum.
Even when a pulse is applied, the FID signal strength often does not reach O, making it difficult to accurately adjust the 180° pulse.

2. Both 90° and 180° pulses! l! ! After applying a 90° pulse, apply a 180° pulse,
90°, 18° to maximize the obtained SE signal strength.
Alternately adjust the amplitude of the 0° pulse. Since this method uses two parameters first, it takes time to find the optimal value.

The present invention has been made in view of the above-mentioned problems, and its purpose is to provide a 90"
, 180'' pulses can be easily and accurately adjusted.

(Means for Solving the Problems) The present invention, which solves the above-mentioned problems, provides an RF pulse adjustment method for a nuclear magnetic resonance tomography apparatus that sets optimal conditions for a 90" pulse and a 180° pulse supplied to an RF coil. ,9
While maintaining the pulse width of the 0° pulse and the 180° pulse at a ratio of 1:2, the optimum conditions are achieved by simultaneously adjusting the 90° pulse and the 180° pulse amplitude or pulse width to obtain the maximum point of the spin echo signal. It is characterized by obtaining.

(Function) The pulse widths of the 90° pulse and the 180° pulse are set to 1:2, and while maintaining that ratio, the amplitude or pulse width of both pulses is adjusted, and the image on the display device is used to display the width of both pulses. Find the optimum amplitude condition.

(Example) Hereinafter, the method of the present invention will be explained in detail with reference to the drawings.

First, the principle of the method of the present invention will be explained. Generally, the rotation angle θ of the magnetization vector M in a resonance state is as shown in equation (1), where the rotation angle θ and the RF pulse width To1 and the rotation angle θ and R
Each has a first-order proportional relationship with the F-pulse amplitude H1.

However, in reality, due to the nonlinearity of the device, there is often no apparent proportional relationship between the set RF pulse amplitude and the RF pulse amplitude that is actually output.
F pulse width T. Using the proportional relationship between and θ, the pulse width T is adjusted to find the desired θ.

The adjustment method is performed according to the pulse sequence shown in FIG. First, the pulse width of the 90° pulse and the pulse width of the 180° pulse are set at a ratio of 1:2. Next, the RF pulse width is fixed and the amplitudes of the 90' pulse and 180° are varied while maintaining the same value, and the pulse amplitude that maximizes the projection obtained is determined. The projection is an image displayed on the frequency axis after SE-fold signal Fourier transform.

At this time, the 90'' and 180'' pulses are adjusted to ifl at the same time. In this case, for example, the pulse width of the 180° pulse is set to a certain desired value, and the pulse width of the 90° pulse is adjusted to 1/2 of that value.
The amplitudes of both pulses are varied by 80 degrees, and the pulse amplitude at which the projection is maximized is determined. Also, set the pulse width of the 90" pulse to a desired value, adjust the pulse width of the 180° pulse to the two values, and as described above, set the pulse width of the 90" pulse to the desired value.
, 180°, and find the pulse amplitude that maximizes the projection. N for carrying out the method of the present invention based on such a principle.
Figure 1 shows the main part of the 8-position MR tomogram. In the figure, 1 has a space (hole) into which the subject is inserted, and surrounding this space, a static magnetic field coil applies a constant static magnetic field to the subject and generates a gradient magnetic field. Gradient m-field coil (gradient magnetic field coil consists of 3 x, Y, and Z)
It has an axial coil. ), an RF transmitter coil that provides an RF pulse to excite the spin of an atomic nucleus within the object, and a receiver coil that detects an NMR signal from the object. Static magnetic field coil, gradient magnetic field coil, R
The F transmitting coil and the receiving T coil are connected to a magnetostatic power source 2, a gradient magnetic field drive circuit 3, an RF power amplifier 4, and a preamplifier 5, respectively. The sequence storage circuit 6 operates the gate modulation circuit 8 (modulates the RF output signal of the RFJ! oscillation circuit 9 according to a predetermined timing) in accordance with the command from the calculation f17, and calculates the output signal based on the spin warp method. <RF pulse @tan is applied from the RF power amplifier 4 to the RF transmitting coil. Further, the sequence storage circuit 6 operates the gradient magnetic field drive circuit 3 using a sequence signal based on the spin warp method, and supplies gradient magnetic fields to the three axes of x, y, and z, respectively, as shown in figure ff14. 10
is a phase detector that detects the phase of the received signal output of the preamplifier 5 using the output of the RF oscillation circuit 9 as a reference signal.

This output signal is converted into a digital signal by the AD converter 11 and input to the calculation n7. Reference numeral 12 denotes an operation console for inputting instructions and various setting values to the computer 7 for realizing various pulse sequences, and 13 a display device for displaying images reconstructed by the computer 7.

Next, the method of the embodiment will be explained while explaining the operation of the 1i1H configured as described above.

The operation console 12 is operated to set the timing of the pulse sequence, the amplitude of the RF pulse, the pulse width, etc., and a signal based on the set values is input to calculation n7. Calculation n7 sends a signal to the sequence storage circuit 6 based on the set value. Based on the above signal, the sequence storage circuit 6 converts the RF signal from the RF oscillation circuit 9 into a set pulse width,
The signal is modulated into a signal having an amplitude and input to the RF power amplifier 4. This modulated signal is transmitted to the RF power amplifier 4 by 1lI
I width and input into the magnet assembly 1.

In the static magnetic field generated in the magnet assembly 1 by the static magnet @ power source 2, the RF pulse input has a gradient of 1! given to each axis. i'i! Together with the magnetic field 11m, the excited spins resonate. The SE low signal produced by the resonance is amplified by the preamplifier 5 and input to the phase detector 10. In the phase detector 10, the input NMR signal is phase-detected using the output of the RF oscillation circuit 9 as a reference signal, and the output signal is converted into a digital signal in the AD converter 11, and then subjected to double reconstruction calculation in the computer 7. Display @1
3 is displayed.

The above is the operation of tree 8W1.
90", 180" by the operation console 12 before performing shadowing.
'Make pulse settings. Operation Funsol 12 by 9
Set the pulse widths of the 0° pulse and the 180° pulse, and make the ratio 1:2. This data enters the gate modulation circuit 8 via the computer 7 and the sequence storage circuit 6, and adjusts the pulse width of the generated modulated pulse wave to a ratio of 1:2. Next, in the operation console, while maintaining the amplitude of each of the 90° and 180° pulses, the preamplifier 5, phase detection elo, AD converter 11, calculation n
Display Hr via 7! While monitoring the image displayed as a projection on 113, the operation console 1
2 to change the amplitude of the RF pulse and display IIl! Find the maximum value of the projection displayed in 13.

In this way, instead of finding the best point through trial and error while alternately changing the pulse width and amplitude of the two pulses, you can fix the ratio of the pulse widths of 90° and 180@pulses and Two parameters allow adjustment of both 90° and 180° pulses in one motion.

Because there is only one parameter! giJ! ! ! It takes less time.

Furthermore, since the signal strength becomes large when both the 90° and 180° pulses are optimal, the change in signal strength near the optimal value is larger than when one is fixed and the other is moved, and therefore the optimal value is easier to find.

The present invention is not limited to the method according to the above embodiments. For example, in the example, the pulse width was set and the amplitude was changed, but the amplitude of the 90° and 180° pulses is kept constant and the pulse width of the RF pulse is changed. It is also possible to do so. In this case, as shown in Fig. 5, the interval Ts between the 90° pulse and the 180° pulse is kept constant, and the pulse widths of the 90° pulse and the 180° pulse are 1:2 so that there is no influence from T2 relaxation. It is also possible to find the pulse width that maximizes the projection by changing the pulse widths of both while maintaining the same value. In addition, in the method of the example, the projection was adjusted to be maximum, but the SE low signal amplitude,
Adjustments may be made so that the strength of the area etc. is maximized.

(Effects of the Invention) As explained in detail above, according to the RF pulse adjustment method of Komei, 9
By adjusting the 0° pulse and the 180° pulse at the same time, the adjustment can be carried out easily and accurately, which has a great practical effect.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of the main parts of an apparatus for implementing the method of one embodiment of the present invention, Fig. 2 is a diagram of a pulse sequence of signals applied to three axes, and Fig. 3 is a pulse method of NMR-CT. FIG. 4 is a diagram showing the pulse sequence of the magnetic field of NMR-CT, and FIG. 5 is an explanatory diagram of the method of another embodiment. 1... Magnet assembly 2... Static magnetic field power supply 3
...Gradient magnetic field drive circuit 4...RF power amplifier 5
...Preamplifier 6...Sequence memory circuit 7
...meter p machine 8...gate modulation circuit 9...
・RF optical wave circuit 10... Between phase detection 11...
AD converter 12...operation console 13...
Display device patent applicant: Yokogawa Medical Systems Co., Ltd.) 2
Diarrhea (A) Figure 5 9 Apulse 180' Pulse 4 Figure (B)

Claims (1)

[Claims] In the RF pulse adjustment method for nuclear magnetic resonance tomography equipment, which sets the optimal conditions for the 90° pulse and 180° pulse supplied to the RF coil, the pulse width of the 90° pulse and 180° pulse is set and maintained at a ratio of 1:2. An RF pulse adjustment method for a nuclear magnetic resonance tomography apparatus, characterized in that the 90° pulse and the 180° pulse amplitude or pulse width are simultaneously adjusted to obtain the optimum conditions so as to obtain the maximum point of the spin echo signal.