JPH0333008B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a 180° pulse adjustment device for a nuclear magnetic resonance tomography apparatus for setting optimal conditions for a 180° pulse supplied to an RF coil.

(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 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 4A. It starts rotating in the zy plane with an angular frequency of ω' = γH ₁ . If the pulse width is now _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 4 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. The spin warp method will be explained. The pulse sequence for applying the high frequency magnetic field and gradient magnetic field used in this method is shown in FIG. In figure A, x,
It shows a state in which the magnetic fields of husbands Gx, Gy, and Gz are applied to the y and z axes, and a high frequency magnetic field is applied to the x axis. 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 a fixed gradient magnetic field applied to the x-axis, Gy is a gradient magnetic field applied to the y-axis and whose amplitude changes depending on time, and Gz is a fixed gradient magnetic field applied to the z-axis. The signal is
The FID signal after the 90° pulse and the SE 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. In period 1, spins in the slice plane in the tomography perpendicular to the z-axis centered at z=0 are selectively excited by the 90° pulse and the gradient magnetic field Gz ⁺ . period 2
Gx ⁺ is used to disrupt the spin phase and reverse it with a 180° pulse, and Gz ^- is used to restore the spin phase disrupted by Gz ⁺ . In period 2, 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 y direction, and its strength 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.

(Problem to be solved by the invention) In the above spin warp method, the RF pulse
It is necessary to precisely match the rotation angle θ caused by the 90° pulse and the 180° pulse to 90° and 180°, and conventionally this adjustment has been performed using the following method.

1 Method using FID signal This is _a method of observing the FID signal obtained when applying an RF pulse.
Changing the pulse amplitude changes θ, so the RF
The amplitude of the pulse is changed, and the pulse when the FID signal intensity is the minimum is defined as a 180° pulse. In this method, when the object to be examined is large, the FID signal strength often does not become 0 even when a 180° pulse is applied, and it is difficult to adjust the 180° pulse accurately.

2 How to adjust both 90° and 180° pulses Apply a 90° pulse followed by a 180° pulse, and obtain the SE
Adjust the amplitude of the 90° and 180° pulses to maximize the signal strength. Since this method changes two parameters, the amplitudes of the 90° and 180° pulses, it takes time to find the optimal values.

The present invention has been made in view of the above points, and its purpose is to eliminate the need for FID signals that are difficult to confirm.
The object of the present invention is to realize an adjustment device that can easily and accurately adjust a 180° pulse.

(Means for Solving the Problems) The present invention solves the above-mentioned problems in a 180° pulse adjustment device for a nuclear magnetic resonance tomography apparatus for setting the optimum conditions for a 180° pulse supplied to an RF coil. Two pulses to be adjusted as 180° pulses with equal output corresponding to the product of amplitude and pulse width.
The present invention is characterized by comprising means for applying an RF pulse to an axis other than the slice axis, means for changing the output of the two RF pulses by the same amount, and means for displaying a spin echo signal.

(Operation) Two RF pulses with equal output are applied to an axis other than the slice axis, and the image displayed on the SE signal display means is observed while changing the output of the two RF pulses by the same amount. Adjust to obtain a 180° pulse.

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

First, the principle of the present invention will be explained. The pulse sequence in the present invention is as shown in FIG. In the figure, 21 and 22 are RF pulses that attempt to adjust the rotation angle θ of the magnetization vector applied to the X axis;
23, 24, 25, and 26 are magnetic field gradient signals applied to the Z axis, which is the slice axis, 23 is a slice gradient that selectively excites spins within the slice plane, and 24
is the day phase gradient that spreads the spin,
25 is a slice gradient for performing selective inversion, and 26 is a read gradient. And 27 is NMR to be observed
This is the SE signal as a signal. An image displayed by Fourier-transforming an SE signal obtained by applying a phase gradient to the slice axis to create a phase difference in the slice direction as in this pulse sequence is called a slice profile.

Generally, the rotation angle θ of the magnetization vector in a resonance state is as shown in equation (1), and the rotation angle θ and the RF pulse width T _D and the rotation angle θ and the RF pulse amplitude H ₁ have a linear proportional relationship, respectively. be. Therefore, the rotation angle θ of the magnetization vector can be adjusted by adjusting the amplitude of the RF pulse, the pulse width, or both.

Here, when the RF pulse 21 is adjusted to a 90° pulse and the RF pulse 22 is adjusted to a 180° pulse, the slice profile becomes the waveform shown in FIG. 3A. In the figure, the horizontal axis of the slice profile is ω and the vertical axis is the amplitude. Physically, the horizontal axis represents the thickness of the slice, and the vertical axis represents the proton density distribution.

When both the RF pulses 21 and 22 are 180° pulses, as shown in FIG. 2, due to the presence of the slice gradient 23 or 25, which is a gradient magnetic field applied to the Z axis,
In the slice profile, the center of the slice thickness that resonates at the center frequency is depressed, resulting in the result as shown in FIG. 3B. Therefore, for example, if the RF pulse 22 in FIG. 2 is exactly a 180° pulse, then the RF
If the amplitude of the pulse 21 is adjusted to make the slice profile as shown in Figure 3 (b), the R pulse 21
is adjusted to exactly 180° pulse.

Therefore, the RF pulse 21 with equal pulse width on the x-axis
and 22 are applied to change the amplitudes of both pulses by the same amount so that the slice profile becomes the state shown in Figure 3 (b), then both pulses have been adjusted to 180°.

FIG. 1 shows a block diagram of the main parts of an NMR tomography apparatus having an adjustment method based on this principle. In the figure, 1 has a space (hole) into which the subject is inserted, and the space is surrounded by a
A static magnetic field coil that applies a constant static magnetic field to the subject, a gradient magnetic field coil that generates a gradient magnetic field (the gradient magnetic field coil has coils for three axes of X, Y, and Z), and An RF transmitter coil that provides RF pulses to excite the spins of atomic nuclei, a receiver coil that detects NMR signals from the subject, etc. are arranged. Static magnetic field coil, gradient magnetic field coil,
The RF transmitter coil and the receiver coil are connected to a static magnetic field power supply 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 RF oscillation circuit 9 according to a predetermined timing) in accordance with a command from the computer 7 in an arbitrary view, and operates the gate modulation circuit 8 based on the spin warp method. An RF pulse signal 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 to generate gradient magnetic fields in the three axes of x, y, and z, respectively, as shown in FIG. supply A phase detector 10 detects the phase of the received signal output from 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 computer 7. 12, various pulses and pulses are sent to the computer 7.
an operation console for inputting instructions and various setting values for realizing a sequence; 13;
is a display device that displays images reconstructed by the computer 7.

Next, the operation of the apparatus configured as described above will be explained.

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 the computer 7. The computer 7 sends a signal to the sequence storage circuit 6 based on the set value. Based on the above signal, the sequence storage circuit 6 modulates the RF signal from the RF oscillation circuit 9 into a signal having a set pulse width and amplitude, and inputs the modulated signal to the RF power amplifier 4. This modulated signal is amplified by the RF power amplifier 4 and input to the magnet assembly 1. In the static magnetic field generated in the magnet assembly 1 by the static magnetic field power source 2, the RF pulse input causes the excited spins to resonate in conjunction with the gradient magnetic field applied to each axis. SE caused by resonance
The signal is amplified by the amplifier 5 and input to the phase detector 10. In the phase detector 10, the output of the RF oscillation circuit 9 is input as a reference signal.
The phase of the NMR signal is detected, and the output signal is converted into a digital signal in the AD converter 11, and then subjected to image reconstruction calculations in the computer 7 and displayed on the display device 1.
Displayed as 3.

Before performing tomography using this apparatus, the 180° pulse is set using the operation console 12.
The pulse widths of the RF pulses 21 and 22 in FIG. 2 are set to the same value T _p using the operation console 12 .
This data enters the gate modulation circuit 8 via the computer 7 and the sequence storage circuit 6, and sets the pulse width of the modulated pulse wave to T _p . Next, in the operation console 12, while keeping the amplitudes of the RF pulses 21 and 22 equal, the RF pulses generated by the RF pulses are transmitted to a display device via a preamplifier 5, a phase detector 10, an AD converter 11, and a computer 7. While monitoring the slice profile displayed at 13, the amplitude data input to the operation console 12 is changed to
The RF pulse is changed and the slice profile displayed on the display device 13 is adjusted to have the waveform shown in FIG. 3B. In this way, the RF pulse 21,
22 are both adjusted to exactly 180° pulses.

As explained above, an accurate 180° pulse can be obtained by adjusting only the pulse amplitude using the extremely clear waveform of the slice profile without using FID, which is difficult to confirm.

Furthermore, if you move RF pulse 21 and RF pulse 22 by the same amount, the signal strength will increase when they are optimal, so the change in signal strength around the optimal value will be smaller than when one RF pulse is fixed and the other is moved. It is large, so it is easy to find the optimum value.

Note that the present invention is not limited to the method according to the above embodiment. For example, day phase gradient 24
The method of adjusting the readout gradient 26 and the slice gradient 26 for selective excitation using a slice profile applied to the same Z-axis as the slice gradient 23 for selective excitation has been described. Adjustments may be made to minimize the projection (display image obtained by Fourier transforming the SE signal), the amplitude and area of the SE signal.
Furthermore, although the two RF pulses were applied and adjusted together on the x-axis, it is also possible to apply them separately on the x-axis and y-axis and adjust the SE signal to the minimum.

Further, although the two RF pulses were adjusted by making the pulse widths the same and changing the amplitudes by the same amount, it is also possible to make the amplitudes the same and adjust the pulse widths of the RF pulses by changing the slice profile. In short,
Two RF pulses with the same output (amplitude x pulse width) may be used and the output of the two RF pulses may be adjusted by the same amount. Further, it is possible to implement only one of the slice gradients 23 and 25 in FIG. 2.

(Effects of the Invention) As explained in detail above, according to the present invention,
It is now possible to independently adjust the 180° pulse using the SE signal without using the FID signal, which is difficult to confirm, which has a great practical effect.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the main parts of an NMR tomography apparatus using the adjustment device of the present invention, Fig. 2 is a diagram showing the pulse sequence of the present invention, and Fig. 3 is a diagram of the slice profile. A diagram of the slice profile caused by a pulse and a 180° pulse; B is a diagram of the slice profile produced by two 180° pulses;
Figure 4 is an explanatory diagram of the principle of the NMR-CT pulse method.
Figure 5 shows the pulses applied to the three axes of NMR-CT.
FIG. 3 is a diagram of a sequence. DESCRIPTION OF SYMBOLS 1... Magnet assembly, 2... Static magnetic field power supply, 3... Gradient magnetic field drive circuit, 4... RF power amplifier, 5... Preamplifier, 6... Sequence storage circuit, 7... Computer, 8... ...Gate modulation circuit, 9
...RF oscillation circuit, 10...phase detector, 11...
...AD converter, 12...Operation console, 13...
...Display device, 21, 22...RF pulse, 23,
25... Slice gradient, 24... Day phase gradient, 26... Read gradient, 27... SE signal.

Claims (1)

[Claims] 1. In a 180° pulse adjustment device for a nuclear magnetic resonance tomography apparatus for setting the optimal conditions for a 180° pulse supplied to an RF coil, outputs corresponding to the product of amplitude and pulse width are equal. , two RFs to be tuned as 180° pulses
A nuclear magnetic resonance tomography system comprising: means for applying a pulse to an axis other than the slice axis; means for changing the output of the two RF pulses by the same amount; and means for displaying a spin echo signal. 180° pulse adjustment device for imaging equipment. 2. The nucleus according to claim 1, wherein the day phase gradient and the readout gradient are applied on the same axis as the slice gradient, and the two RF pulses are applied on the same axis other than the above-mentioned axis. 180° pulse adjustment device for magnetic resonance tomography equipment. 3. The 180° pulse adjustment device for a nuclear magnetic resonance tomography apparatus according to claim 1, wherein the day phase gradient and the readout gradient are applied on different axes from the slice gradient. 4. The 180° pulse adjustment device for a nuclear magnetic resonance tomography apparatus according to claim 1, characterized in that two RF pulses are applied to each of two axes excluding the slice axis.