JPH0363896B2 - - Google Patents

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention relates to a nuclear magnetic resonance tomography apparatus that adjusts the amplitude of RF pulses to obtain the maximum echo signal.
Regarding RF pulse adjustment method.

(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 Larmor 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 . Letting the pulse width be _tD , the rotation angle θ from _H0 is expressed by the following equation.

θ=γH ₁ t _D ……(1) The pulse that acts to tilt the magnetization vector away from the Z axis is called an excitation pulse. In particular, in equation (1), θ=
A pulse with H ₁ t _D that is 90° is called a 90° pulse. Immediately after this excitation pulse, the magnetization vector M rotates at ω ₀ in the xy plane as shown in FIG. However, this signal attenuates over time, so this signal is called the 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 Fig. 4C, when a second pulse (reversal pulse) with a pulse width such that θ becomes 180° after τ time from the excitation pulse is applied, the magnetic moments that had been scattered are reduced to − The signal is observed with refocusing in the y direction. This signal is called an echo signal. A desired image can be obtained by measuring the intensity of this echo signal. The resonance condition for NMR 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,
Apply 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 an excitation pulse 21 and an inversion pulse 22 to the x-axis. Gx is a fixed gradient magnetic field applied to the x-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 z-axis. The signals show the FID signal 23 after the excitation pulse 21 and the echo signal 24 after the inversion pulse 22. The period is provided to indicate the timing of the signal of the gradient magnetic field applied to each axis. In period 1, the excitation pulse 21 and the slice gradient 25 selectively excite spins in a slice plane in a tomographic image perpendicular to the z-axis centered at z=0. The day phase gradient 26 in period 2 is for disturbing the phase of the spins and inverting them with the inversion pulse 22.
Gz ^- 27 is for restoring the phase of the spins disturbed by the slice gradient 25. In period 2, a warp signal 28 is applied for a gradient magnetic field, so-called warp, which shifts the phase of spins in proportion to the position in the y direction. Its intensity is controlled differently every cycle. In period 3, an inversion pulse 22 is applied to align the magnetic moments again, and in period 4, a readout gradient 29 is applied to observe the echo signal 24. 31 indicates the second excitation pulse applied after the repetition period Tr.

(Problem to be Solved by the Invention) In this sequence, the conventional RF pulse adjustment method for maximizing the echo signal is
In the pulse sequence shown in the figure, scanning is performed without outputting the warp signal 28, and the excitation pulse 2
1 and the inversion pulse 22 are gradually changed to maximize the intensity of the echo signal 24.
The method was to store the output of , and use that value as the RF pulse output. The magnitude of the output of the excitation pulse that maximizes the intensity of the echo signal 24 varies depending on the repetition period Tr of the sequence, the longitudinal relaxation time _T1 of the subject, and whether the number of echoes is even or odd. Therefore, the magnitude of the output of the excitation pulse 21 is regulated by the repetition period. For example, if it is necessary to perform six scans with a repetition period of 2 seconds to adjust the pulse, the adjustment time will require approximately 12 seconds. It took a similar amount of time to adjust the inversion pulse.
Furthermore, since the maximum signal is stored, it is difficult to make adjustments with little error when there is some variation in signal strength.

The present invention has been made in view of the above-mentioned problems, and an object thereof is to realize an adjustment method in which the time required for adjusting an inversion pulse is shortened and the error in the adjustment result is small.

(Means for Solving the Problems) The present invention solves the above problems in an RF pulse adjustment method for a nuclear magnetic resonance tomography apparatus in which the amplitude of the RF pulse is adjusted to obtain the maximum echo signal.
The method is characterized in that an inversion pulse is applied multiple times after an excitation pulse, and the amplitude of the inversion pulse is adjusted based on the plurality of echo signals obtained thereby to obtain the maximum echo signal. .

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

FIG. 1 is a diagram of the sequence of the method according to the invention. In the figure, parts equivalent to those in FIG. 5 are given the same reference numerals. In the figure, 22' is the excitation pulse 2
second inversion pulse within one repetition period, 24'
is the echo signal obtained by the inverted pulse 22'. 31 is the second excitation pulse, 32 is the inversion pulse applied after the excitation pulse 31, and 32'
is the second inversion pulse within the repetition period of the excitation pulse 31. 33 is FID by excitation pulse 31
The signals 34 and 34' are echo signals based on the inversion pulses 32 and 32', 38 is the warp waveform applied after the excitation pulse 31, and 39 and 39' are the inversion pulses 3
The echo signal 34,3 caused by 2,32'
4' readout slope.

Now, when the excitation pulse 21 is applied to the RF axis without outputting the warp gradients 28 and 38, the phase of the excited spins is disturbed by the dephase gradient 26, and the FID signal 23 is output. Next, when an inversion pulse 22 is applied, the spins are inverted, and an echo signal 24 is detected by a readout gradient 29. As mentioned above, the magnitude of the output of the excitation pulse 21 that maximizes the intensity of the echo signal 24 changes depending on the repetition time Tr, but the magnitude of the output of the inversion pulse 22 that maximizes the echo signal 24 changes depending on the repetition time Tr. period
Regardless of Tr, this is the case when the spin rotates 180°. Therefore, the echo signal 2 due to the inverted pulse 22
4 and then by the echo signal 34 generated by the inversion pulse 32, it is not necessary to set the repetition period of the excitation pulse 21 and the excitation pulse 31 to Tr.
n can be appropriately selected as Tr/n. For example, if the repetition period of the excitation pulse 21 is 0.5 seconds, the time required for six pulse measurements is 3 seconds.
This is 1/4 of the case where the adjustment is made using the excitation pulse 21.

Next, FIG. 2 shows a curve of the relationship between the inversion pulse amplitude and the echo signal intensity. In the figure, 24 is an echo signal due to the first inversion pulse 22, 24' is a second echo signal, and 24'' is a third echo signal, although not shown in FIG. According to the first to second after the excitation pulse,
It can be seen that the amplitude becomes smaller from the second to the third, but the peak of the relationship curve becomes steeper. Since the amplitude of the inverted pulse giving this peak is the optimal adjustment point, it can be seen that accurate adjustment can be achieved by adjusting the optimal output based on the second and subsequent echo signals.

FIG. 3 shows a block diagram of essential parts of an NMR tomography apparatus for carrying out the method of one embodiment of the present invention based on such a principle. In the figure, 1 has a space (hole) into which the subject is inserted, and a static magnetic field coil that surrounds this space and applies a constant static magnetic field to the subject, and a gradient magnetic field. The generated gradient magnetic field coil (the gradient magnetic field coil is x,
It is equipped with coils for three axes, y and z. ), an RF transmitting coil that provides an RF pulse to excite the spin of the atomic nucleus within the specimen, and
A magnet assembly in which receiving coils, etc. for detecting NMR signals are arranged, including static magnetic field coils, gradient magnetic field coils, RF transmitting coils, and receiving coils.
static magnetic field power supply 2, gradient magnetic field drive circuit 3,
It is connected to an RF power amplifier 4 and a preamplifier 5. The sequence storage circuit 6 inputs a modulation signal to the gate modulation circuit 8 according to instructions from the computer 7,
The output RF signal from the RF oscillation circuit 9 is modulated.
10 is a phase detector that phase-detects the output reception signal of the preamplifier 5 using the output of the RF oscillation circuit 9 as a reference signal, and the AD converter 11 converts the output signal of the phase detector 10 into a digital signal, Enter. 12 is an operation console for inputting instructions and various setting values for realizing various pulse sequences in the computer 7; 13 is a display device for displaying reconstructed images in the computer 7;

The operation of the apparatus of the embodiment configured as described above will be explained. The computer 7 instructs the sequence storage circuit 6 to perform an operation sequence based on input from the operation console 12. The sequence storage circuit 6 modulates the RF output signal of the RF oscillation circuit 9 at a predetermined timing by operating the gate modulation circuit 8 in an arbitrary view according to the command, and transmits the high frequency pulse signal based on the spin warp method to the RF power amplifier 4. is applied to the RF coil of the magnet assembly 1. Further, the sequence storage circuit 6 operates the gradient magnetic field drive circuit 3, the gate modulation circuit 8, and the AD converter 11 using a sequence signal based on the Fourier transform method. Further, the sequence storage circuit 6 operates the gate modulation circuit 8 and the gradient magnetic field drive circuit 3 to selectively excite in a desired direction before starting the series of sequence operations. The phase detector 10 uses the output of the RF oscillation circuit 9 as a reference signal, detects the phase of the output signal of the preamplifier 5 in which the NMR signal detected by the receiving coil of the magnet assembly 1 is amplified, and sends it to the AD converter 11. input. The AD converter 11 converts the NMR signal obtained via the phase detector 10 into a digital signal and inputs it to the computer 7. The computer 7 switches the operation of the sequence storage circuit 8 and rewrites the memory in order to exchange information with the operation console 12 and perform various scan sequences.
Image reconstruction calculations are performed using each data from the converter 11.

The above process is performed according to the pulse sequence shown in FIG. As shown in Figure 1, the repetition period of the excitation pulse is selected as Tr/n, where n is determined by how many echoes the signal is taken and how long after the excitation pulse it is obtained. Good luck,
For example, in order to shorten the measurement time, adjustment may be made using the second echo, and in order to reduce the error, the latter echo may be used.

The above adjustments are input in advance to the computer through the operation console 12, and the computer 7 operates each part as described above based on the input, compares the echo signals, and automatically performs the optimal adjustment.

As is clear from the above explanation, since the inversion pulse output can be adjusted by selecting a repetition time shorter than the repetition time of the main scan, the adjustment time of the RF pulse output is shortened. Further, by adjusting the inversion pulse using a plurality of echo signals, the output of the RF pulse that provides the maximum signal strength can be easily determined, and the adjustment error can be reduced.

(Effects of the Invention) As described above in detail, according to the present invention,
It is possible to realize an adjustment method that shortens adjustment time and allows adjustment with few errors, which has great practical effects.

[Brief explanation of drawings]

FIG. 1 is a diagram of a pulse sequence according to a method according to an embodiment of the present invention, FIG. 2 is a diagram of intensity changes depending on the inversion pulse amplitude of an echo signal according to the number of inversion pulses after an excitation pulse, and FIG. 3 is a diagram of the present invention FIG. 4 is an explanatory diagram of the pulse method of the NMR phenomenon observation method, and FIG. 5 is a pulse sequence of the spin warp method. 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, 1
0... Phase detector, 11... AD converter, 12...
...operation console, 21,31...excitation pulse,
22, 22', 32, 32'...inversion pulse, 2
3, 33...FID signal, 24, 24', 34, 3
4'...Echo signal, 25,35...Slice gradient, 26,36...Day phase gradient, 27,3
7...Gz ^- , 28, 38...Warp signal, 29,
29', 39, 39'...readout gradient.

Claims (1)

[Claims] 1. In the RF pulse adjustment method for nuclear magnetic resonance tomography equipment in which the amplitude of the RF pulse is adjusted to obtain the maximum echo signal, an inversion pulse is given multiple times after the excitation pulse, and among the echo signals obtained by this, multiple An RF pulse adjustment method for a nuclear magnetic resonance tomography apparatus, which is characterized by adjusting the amplitude of an inversion pulse based on an echo signal to obtain a maximum echo signal.