JPS63296739A - Method for controlling rf pulse of nuclear magnetic resonance tomographic imaging apparatus - Google Patents

Abstract

PURPOSE:To shorten a time necessary for controlling a reversal pulse and to reduce the error of a control result, by repeating an exciting pulse at a cycle shorter than the repeating cycle of the exciting pulse of scanning and adjusting the amplitude of the reversal pulse on the basis of the plural-number-th echo signal after the exciting pulse the obtain the max. echo signal. CONSTITUTION:When a reversal pulse 21 is applied, a spin is reversed and an echo signal 24 is detected from a reading gradient 29. The magnitude of the output of an exciting pulse 21 maximizing the intensity of the echo signal 24 changes according to a repeating time Tr but the magnitude of the output of the reversal pulse 22 maximizing the echo signal 24 has no relation to the repeating time Tr to become such a case that the spin is rotated by 180 deg.. Therefore, when adjustment is performed by the echo signal 34 due to a reversal signal 32 next to the echo signal 24 due to the reversal pulse 22, it is unnecessary to set each of the repeating cycle of the exciting signal 21 and that of an exciting signal 31 to Tr but said cycle is set to Tr/n and n can be properly selected. As mentioned above, by selecting a repeating time shorter than that of this scanning, the outputs of the reversal pulses are adjusted.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an RF pulse adjustment method for a nuclear magnetic resonance tomography apparatus that adjusts the amplitude of an RF pulse to obtain a maximum echo signal.

(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 a specific atomic nucleus by 1η
has been known for a long time. An outline of the principle 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 ω0 expressed by the following equation. . This is called Larmor precession.

ω0 = γfl o However, γ: nuclear gyromagnetic ratio Now, a high frequency coil is placed on an axis perpendicular to the two axes of the static magnetic field, for example, the X axis, and the high frequency coil of the above angular frequency ω0 rotating in the xy plane When a rotating magnetic field is applied... air resonance occurs, and the static magnetic field H
A population of atomic nuclei undergoing Zeeman splitting under o undergoes a transition between levels by a high-frequency magnetic field that satisfies resonance conditions, and transitions to a unit with a higher energy level. here,
Since the nuclear gyromagnetic ratio T varies depending on the type of atomic nucleus, the atomic nucleus can be identified by the resonance frequency. Furthermore, by measuring the strength of that resonance, we can determine the amount of that nucleus present. During the time determined by a time constant called the post-resonance relaxation time, the atomic nucleus excited to the higher unit returns to the lower unit and radiates energy.

Part 4 about the medium pulse method for observing this NMR phenomenon.
This will be explained with reference to the figures.

As mentioned above, a high frequency pulse (Hr
) is applied in the (X-axis) direction perpendicular to the static magnetic field (2 axes), the magnetization vector M becomes ω' = γH! in the rotating coordinate system, as shown in Figure 4 (a). It starts rotating in the zy plane at an angular frequency of . Now, assuming that the pulse width is 1D, the rotation angle θ from Ha is expressed by the following equation.

θ−γHtjm...(1) A pulse that works to tilt the magnetization vector away from the z-axis is called an excitation pulse, and in particular, a pulse with HltD such that θ=90' in equation (1) is called a 90'' pulse. Immediately after this excitation pulse, the magnetization vector M is rotating in the xy plane at ω0 as shown in Fig. 4 (b), producing an induced electromotive force in the coil placed on the X axis, for example. Since the 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, it is shown in Figure 4 (C). After τ time from the excitation pulse, θ=1
When a second pulse (inversion pulse) with a pulse width of 80' is applied, the scattered magnetic moments refocus in the -■ direction after a time τ, and a signal is observed. This signal is called an echo signal. A desired operation can be achieved by measuring the intensity of this echo signal. N.M.
The resonance condition of R is given by si=γI−Io/2π. Here, ν is the resonance frequency and 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 1ifi 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 provide 11 fields with different strengths depending on the position, and the resonance frequency is changed to obtain positional information. The inner spin warb 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. (A) The figure shows a state in which magnetic fields Gx, Gy, and Gz are applied to the x, y, and z axes, respectively, 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 a high-frequency rotating magnetic field, and an excitation pulse 21 and an inversion pulse 22 are applied 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 gradiometer 1 field applied to the z axis. The signals show the FID signal 23 after the excitation pulse 21 and the echo signal 271 of the inversion pulse 2211. The period is provided to indicate the timing of the signal of the gradient t4i 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 tomography perpendicular to two axes centered on z=Q. The dephase gradient 26 in period 2 is used to disturb the spin phase and then invert it with the inversion pulse 22, so GZ-27 is used to restore the disrupted spin phase to the slice gradient 25. be. In period 2, a warb signal 28 is applied for a gradient magnetic field called a so-called warb, 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 method of adjusting the RF pulse to maximize the echo signal is to scan without outputting the warb signal 28 in the pulse sequence of FIG. , the output of the excitation pulse 21 and the inversion pulse 22 is changed little by little, the output of the excitation pulse 21 at which the intensity of the echo signal @24 becomes maximum is memorized, and that value is used as the output of the RF pulse. The magnitude of the output of the excitation pulse that maximizes the intensity of the echo signal 24 is determined by repeating the sequence Zhou Mingwang r and the longitudinal relaxation time of the subject T! It changes depending on 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, approximately 12 seconds are required for the 11 adjustment time. 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 vfJ in order to reduce the maximum echo signal by 1. This method is characterized in that the amplitude of the inversion pulse is adjusted based on the plurality of echo signals after the excitation pulse to obtain the maximum echo signal by repeating the excitation pulse at a shorter period than the repetition period of the original scan excitation pulse. be.

(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 a second inversion pulse within the repetition period of the excitation pulse 21, and 24' is an echo signal obtained by the inversion pulse 22'. 31 is the second excitation pulse, 32 is an inversion pulse applied after the excitation pulse 31,
32' is the second inversion pulse within the repetition period of the excitation pulse 31. 33 is the FID signal generated by the excitation pulse 31, 34.34' is the echo signal based on the inverted pulse 32°32', 38 is the warb waveform applied after the excitation pulse 31, and 39.39' is generated by the inverted pulse 32.32'. This is the readout slope of the echo signal 34.34'.

Now, when the excitation pulse 21 is applied to the RF axis without outputting the warp gradient 28.38, 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 the two readout gradients. As mentioned above, the magnitude of the output of the excitation pulse 21 that maximizes the intensity of the echo signal 24 changes by J: with the repetition time Tr, but the magnitude of the output of the inversion pulse 22 that maximizes the echo signal 24 changes by J:. The length is unrelated to the repetition period Tr and is the case when the spin is rotated by 180°. Therefore, the echo signal 24 due to the inversion pulse 22 is followed by the inversion pulse 3.
In the case of adjustment using the echo signal 34 according to Tr.2, it is not necessary to set the repetition period of the excitation pulse 21 and the excitation pulse 31 to Tr, and 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, which is 1/4 of the time required when adjusting by the excitation pulse 21.
become.

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 the 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 more sharp. Since the amplitude of the inversion pulse that gives this peak is the optimal adjustment point, it is assumed 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 the 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 1itlQ 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, y.

It is equipped with two 3f'lll coils. ), an RF transmitter coil that provides RF pulses to excite the spins of atomic nuclei within the object, and a receiver coil that detects NMR signals from the object. The magnetic field coil, RFF signal coil, and receiving coil are connected to a static magnetic field power supply 2, a gradient magnetic field drive circuit 3, an RFF force amplifier 4, and a preamplifier 5, respectively. The sequence storage circuit 6 inputs a modulation signal to the gate modulation circuit 8 in accordance with the command from the controller 7, and modulates the output RF low signals of the RF generation and output circuits 9 and the like. Reference numeral 10 denotes a phase detector that detects the phase of 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 and converts it into a digital signal. Enter 7.

12 is! An operation console is used to input instructions and various setting values for realizing various pulse sequences in the calculation unit 17, and 13 is a display device that displays reconstructed images in the calculation unit 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 operates the gate modulation circuit 8 in an arbitrary view according to the command, modulates the RFF force signal of the RFF oscillation circuit 9 at a predetermined timing, and converts the high frequency pulse signal based on the spinwave method into RF? The J-force amplifier 4 applies it to the RF coil of the magnet assembly 1. Further, the sequence storage circuit 6 controls the gradient magnetic field drive circuit 3, the gate modulation circuit 8, and the AD using a sequence signal based on the Fourier transform method.
Operate the converter 11. Furthermore, the sequence storage circuit 6
When the series of sequence operations described above begins, the gate modulation circuit 8 and the gradient magnetic field drive circuit 3 are operated to selectively excite in a desired direction.

The phase detector 12 uses the output of the RFF oscillator circuit 10 as a reference signal, phase-detects the output signal of the preamplifier 5 in which the NMR signal detected by the receiving coil of the magnet assembly 1 has been amplified, and inputs it to the AD converter 11. do. The AD converter 11 converts the NMR signal obtained through the phase detector 12 into a digital signal and inputs it to the computer 7. The paralysis machine 7 switches the operation of the sequence storage circuit 8, rewrites the memory, and performs various operations from the AD converter 11 in order to exchange information with the operation console 12 and realize various scan sequences. Perform image reconstruction calculations using the data.

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, and n can be determined depending on how many echoes the signal is to be taken and the time to obtain it depending on the position after the excitation pulse. For example, in order to shorten the measurement time, adjustment may be made using the second echo, and in order to reduce the error, it is sufficient to use the later echo.

The above adjustment is input in advance to the itch R using 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 description, since the inversion pulse output is adjusted by selecting a repetition time shorter than the main scan repetition time, the adjustment time for 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 in detail above, according to the present invention, it is possible to realize an adjustment method that can reduce the adjustment time and perform adjustment with few errors, and has great practical effects.

[Brief explanation of the drawing]

FIG. 1 is a diagram of a pulse sequence according to a method according to an embodiment of the present invention, and FIG. 2 is a diagram of intensity changes depending on the inversion pulse amplitude of an echo signal depending on the number of inversion pulses after an excitation pulse.
FIG. 3 shows an NM for carrying out the method of one embodiment 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-warb method. 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...Computer 8...Gate modulation circuit 10
...Phase detector 11...AD converter 12...
Operation console 21.31...Excitation pulse 22.22', 32.32'...Reversal pulse 23
．． 33...FID signal 24.24', 34.34'...Echo signal 25.
35... Slice gradient 26.36..., Dephase gradient 27.37... Gz- 28, 38... Warp signal 29.29', 39.39'... Read gradient Patent applicant Side Kawa Medical System Co., Ltd. Figure 4 (A) (O) (C)

Claims (1)

[Claims] In the RF pulse adjustment method for nuclear magnetic resonance tomography equipment, which adjusts the amplitude of the RF pulse to obtain the maximum echo signal, multiple pulses after the excitation pulse are repeated at a shorter period than the excitation pulse repetition period of the original scan. An RF pulse adjustment method for a nuclear magnetic resonance tomography apparatus, characterized in that the amplitude of an inversion pulse is adjusted based on the second echo signal to obtain a maximum echo signal.