JPH02144041A - Method for collecting nmr data - Google Patents

Abstract

PURPOSE:To prevent the steady state of residual transverse magnetization by differentiating spin flipping directions by changing the phase of the carrier wave of an exciting high frequency signal at every repeating time substantially at random and changing the phase of a reference high frequency signal corresponding to the above mentioned change. CONSTITUTION:In a pulse sequence employing a field echo method due to flip angle operation, a high frequency signal modulated in its amplitude to such a waveform that a flip angle becomes alpha deg. is transmitted at first to excite an objective spin. At the same time, an inclined magnetic field Gs for selecting a slice surface is applied. Succeedingly, a phase encoding inclined magnetic field Gp is applied and, thereafter, a frequency encoding inclined magnetic field Gf is further applied to generate an echo signal. A series of these pulse sequencies are repeated according to a short repeating time TR. At this time, the phase of the carrier wave of an exciting high frequency signal is changed at random at every TR. That is, the phase of the carrier wave of a transmission high frequency signal is set to XTM-thetan at the n-th TR and set to XTM-thetan+1 at the (n+1)-th TR. Generated echo signals (n), (n+1) are synchronously detected fundamentally by setting the same signal as the carrier wave of the transmitted high frequency signal as a reference signal.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to NMR data acquisition methods such as imaging methods and spectroscopy using nuclear magnetic resonance (NMR).

[Conventional technology]

Field echo method by flip angle manipulation (for example, F
In high-speed imaging methods such as the LASH method, when the repetition time TR is shortened to several tens of m5ec, it becomes shorter than the transverse relaxation time T2 of the imaging target (TR<<T2
), residual transverse magnetization becomes steady-state and artifacts occur due to uncontrolled phase coherence. Therefore, conventionally, variable spoiling
It has been proposed to remove this artifact by preventing residual transverse magnetization using random spoiler pulses called gradients (J
ensFrahmet alTransverse Cohe
Rence in Rapid Fl, AS)I NMR
imaging”, Journal of Ma
gnetic Resonance, 72.307-3
14, 1987).

[Problem to be solved by the invention]

However, although the above artifacts do disappear by applying a spoiler pulse of a strong gradient magnetic field that changes randomly for each repetition as in the conventional proposal, if the repetition time TRi') < short,
Rapid changes in the gradient magnetic field generate large eddy currents, thereby distorting the NMR signal. Therefore, another problem of image distortion occurs in the MR imaging method. An object of the present invention is to provide an NMR data acquisition method that eliminates the problem of residual transverse magnetization when the repetition time is shortened, without causing other problems such as image distortion.

[Means to solve the problem]

In order to achieve the above object, in the NMR data acquisition method according to the present invention, the phase of the carrier wave of the excitation high frequency signal is changed substantially randomly at each repetition time, and the above The present invention is characterized in that the phase of a reference high frequency signal having the same frequency as the carrier wave is changed in accordance with the above change.

[For production]

The phase of the carrier wave of the excitation high-frequency signal is changed substantially randomly at each repetition time. Therefore, the direction in which the spins are flipped differs for each repetition time, which prevents the residual transverse magnetization from reaching a steady state, which occurs due to uncontrolled phase coherence caused by the residual transverse magnetization reaching a steady state. The problem is resolved. Then, when receiving an NMR signal from a target excited by such an excitation high-frequency signal, and converting this received signal into a low-frequency signal by synchronously detecting the received signal using the carrier wave of the high-frequency signal as a reference high-frequency signal, Since the phase of the reference high-frequency signal is also changed in accordance with the carrier phase of the excitation high-frequency signal, spin information with a uniform phase can be obtained with this low-frequency signal. Therefore, the information obtained in this way can be processed in the same way as conventional NMR data, and no changes are required in data processing. Furthermore, since random spoiler pulses are not applied, large eddy currents do not occur, and problems such as image distortion can be avoided. Furthermore, since the time required for the spoiler pulse is not required, it is possible to obtain a shorter TR, that is, to perform imaging at a higher speed.

【Example】

Next, an embodiment in which the present invention is applied to an MR imaging method will be described with reference to the drawings. FIG. 1 shows a pulse sequence according to an embodiment in which the present invention is applied to a high-speed imaging method employing a field echo method using flip angle manipulation. This high-speed imaging method
As shown in Figure 1, first, a high frequency signal whose amplitude is modulated to a waveform with a flip angle of α is transmitted to excite the target spins, and at the same time, a gradient magnetic field Gs for slice plane selection is applied. Apply. Subsequently, a phase encoding gradient magnetic field Gp is applied, and then a frequency encoding gradient magnetic field Gf is further applied to generate an echo signal. Such a series of pulse sequences is repeated with a short repetition time TR. Although this pulse sequence is the same as the normal high-speed imaging method, according to the present invention, the phase of the carrier wave of the excitation high-frequency signal is randomly changed for each TR. That is, in the n-th TR, the carrier phase of the transmitted high-frequency signal is XTM-θn, and (n+1
)-th TR is set to XTM-θn+1. The generated echo signals n and n+1 are basically synchronously detected using the same signal as the carrier wave of the transmission high-frequency signal as a reference signal. In other words, the reference signal has the same frequency and phase as the carrier wave of the transmitted high-frequency signal. For the phase, XTM-θn,
Ref-θn same as TM-θn+1, Ref-θn44
becomes. When data is collected by repeating n from 0 to N-1, randomness can be achieved so that the relationship expressed by the following formula % is satisfied. The same applies to Ref-θn. In this way, since the carrier phase XT)11-θ of the α° pulse, which is the excitation high-frequency signal, is changed randomly for each TR, the direction in which the spins fall can be changed randomly for each TR. That is, the direction in which the spins fall (flip direction) can be controlled as shown in FIG. 2 by the phase angle of the carrier wave of the α° pulse, which is the excitation high-frequency signal. If the directions in which the spins fall are random, it is possible to prevent the steady state of the residual transverse magnetization of the spins when T R (< T 2) in the pulse sequence shown in Figure 1. Also XT
), +-θ, the echo signal after detection is the same as a normal signal from which the randomness described above has been removed. Therefore, by performing two-dimensional Fourier transform on the data thus collected, it is possible to reconstruct an image from which artifacts due to phase coherence have been removed. Normally, it is sufficient to use XTM-θ-Ref-θ, but in addition to removing artifacts by phase coherence,
For 180° pulse correction of the curl Purcell train, DC noise removal, etc., phase offsets such as 0°, 180°, 0°, 180°, etc. are added to each TR only for XTM-θ. FIG. 3 shows an MR imaging system that implements the above pulse sequence. In FIG. 3, a waveform generation circuit 11 generates an amplitude modulation waveform that tilts the magnetization of spins by an angle α°, and this is sent to a D/A conversion circuit 12, where it is converted into an analog signal and then sent to an amplitude modulation circuit. Sent to 13th. As a result, the carrier wave is amplitude-modulated, becomes an α° pulse, and is sent to the transmitting coil 15 via the power amplifier 14, and is irradiated from the transmitting coil 15 to a subject (not shown) such as a human body.
Excite this. NMR signal generated in a subject such as a human body (
The echo signal) is received by a receiving coil 21, sent to a detection circuit 23 via a preamplifier 22 for detection, and then passed through a low-pass filter 24 and converted into a digital signal by an A/D conversion circuit 25. This signal is taken into the host computer 26 and subjected to fast Fourier transform (FFT).
) etc. As the carrier wave of the transmission high-frequency signal for excitation and the reference signal used in the detection circuit 23, the high-frequency signal generated from the reference tarlock generator 31 is used, and the phase is controlled through phase shifters 32 and 33, respectively. The phases of these signals are controlled by the output of the random number angle generator 35 to XTM-θ and Ref-θ, which are basically the same phase. The transmission carrier phase control circuit 34 generates a phase offset for the above-described modified curl parcel control and DC noise removal, and adds this to the random angle signal in the addition circuit 36.
XTM-θ only O” + 180°, 0°, 1 for each TR
A phase offset of 80°, etc. is added. In addition, in the above embodiment, the phases of the carrier wave and the reference signal were changed randomly, but as long as the relationship of the above equation holds, it is not necessarily necessary to use random number control in the strict sense.
If each TR has a different phase angle and a different amount of phase change, it is possible to prevent the residual transverse magnetization from reaching a steady state. Furthermore, it can also be used in combination with a spoiler gradient l-pulse to enhance the artifact removal effect due to phase coherence. In addition, not only the two-dimensional Fourier transform method mentioned above, but also the three-dimensional Fourier transform method,
Alternatively, it can be easily applied to CT method or echo planar method.

【Effect of the invention】

According to the NMR data acquisition method of the present invention, the problem caused by uncontrolled phase coherence due to the residual transverse magnetization reaching a steady state when the repetition time TR is shortened can be solved.
removed without creating other problems such as current generation. Therefore, especially when applied to MR imaging, it is possible to suppress the occurrence of specific artifacts 1~ that depend on the relaxation time of the subject in high-speed imaging methods where the relationship TR<<T2 holds true, and improve the reliability of data. can.

[Brief explanation of the drawing]

FIG. 1 is a time chart showing a high-speed imaging sequence according to an embodiment of the present invention, FIG. 2 is a schematic diagram showing the relationship between the carrier phase of the excitation high-frequency signal and the flip direction in the same embodiment, and FIG. FIG. 2 is a block diagram showing an MR imaging system according to the same embodiment. 11...Waveform generation circuit, 12...-D/A conversion circuit,
13... Amplitude modulation circuit, 14... Power amplifier, 1
5... Transmitting coil, 21... Receiving coil, 22...
- Preamplifier, 23... Detection circuit, 24... Low pass filter, 25... A/D conversion circuit, 26... Host computer, 31... Reference clock generator, 3
2.33...Phase shifter, 34...Transmission carrier phase control circuit, 35...Random number angle generator, 36...Addition circuit.

Claims (1)

[Claims] (1) While changing the phase of the carrier wave of the excitation high-frequency signal substantially randomly at each repetition time, the phase of the reference high-frequency signal having the same frequency as the carrier wave used for detection of the received NMR signal is changed as described above. An NMR data collection method characterized by changing in response to changes.