JPH0638791B2 - NMR data collection method - Google Patents

Description

Detailed Description of the Invention [Industrial applications]

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

[Prior art]

Field echo method by flip angle operation (eg F
In the high-speed imaging method typified by the LASH method), when the repetition time TR is shortened to several tens of msec, it becomes shorter than the lateral relaxation time T2 of the imaging target (TR << T2), and the residual transverse magnetization becomes steady. A state occurs, and an artifact is generated due to uncontrolled phase coherence. Therefore, conventionally, it has been proposed to remove this artifact by preventing residual transverse magnetization by using a random spoiler pulse called variable spoiling gradient (Jens Frahm et al. “Transverse Coh
erence in Rapid FLASH NMR Imaging '', Journal of Ma
gnetic Resonance, 72, 307-314, 1987).

[Problems to be Solved by the Invention]

However, by applying a spoiler pulse of a strong gradient magnetic field that randomly changes at each repetition as in the above-mentioned conventional proposal, the above artifacts are certainly eliminated, but when the repetition time TR is short, a steep gradient magnetic field is generated. The change causes a big eddy current,
This causes distortion in the NMR signal. Therefore, M
In the R imaging method, another problem of image distortion will occur. An object of the present invention is to provide an NMR data sampling method that solves the problem of residual transverse magnetization when the repeating time is shortened without causing another problem such as image distortion.

[Means for Solving the Problems]

In order to achieve the above object, in the NMR data sampling method according to the present invention, the phase of the carrier wave of the excitation high-frequency signal is changed substantially randomly at each repeating time, and the method is used for detecting the received NMR signal. It is characterized in that the phase of the reference high-frequency signal having the same frequency as the carrier wave is changed corresponding to the above-mentioned change.

[Action]

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 is different for each repetition time, the steady state of the residual transverse magnetization can be prevented, and the uncontrolled phase coherence caused by the residual transverse magnetization becoming the steady state is generated. The problem goes away. Then, when receiving an NMR signal from an object excited by such an exciting high-frequency signal and converting the received signal into a low-frequency signal by synchronously detecting a carrier wave of the high-frequency signal as a high-frequency signal for reference, Since the phase of the reference high-frequency signal is also changed corresponding to the carrier wave phase of the excitation high-frequency signal, spin information with a uniform phase can be obtained with this low-frequency signal. Therefore, the information thus obtained can be processed in the same manner as the conventional NMR data, and no change is necessary in the data processing. Further, since no random spoiler pulse is given, a large eddy current does not occur, and problems such as image distortion can be avoided. Furthermore, since the time required for the spoiler pulse is also unnecessary, it becomes possible to set a shorter TR, that is, to capture an image at a higher speed.

【Example】

Next, an embodiment in which the present invention is applied to the 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 adopting a field echo method by a flip angle operation. In this high-speed imaging method, as shown in FIG. 1, first, a high-frequency signal amplitude-modulated into a waveform having a flip angle of α ° is transmitted to excite a target spin, and at the same time, the slice plane is simultaneously excited. A gradient magnetic field Gs for selection is applied. Subsequently, a gradient magnetic field Gp for phase encoding is applied, and then a gradient magnetic field Gf for frequency encoding is applied to generate an echo signal. Such a series of pulse sequences is repeated with a short repetition time TR. This pulse sequence is the same as in the normal high-speed imaging method, but according to the present invention, the phase of the carrier wave of the above-mentioned excitation high-frequency signal is randomly changed for each TR. That is, the carrier phase of the transmission high-frequency signal is set to XTM-θn in the nth TR, and in the (n + 1) th TR,
XTM-θn + 1. Then, the generated echo signals n and n + 1 are basically synchronously detected with the same signal as the carrier wave of the transmission high frequency signal as a reference signal. That is, the reference signal has the same frequency and phase as the carrier wave of the transmission high frequency signal.
The phase is the same as XTM-θn and XTM-θn + 1 Ref-θ
n, Ref-θn + 1. When n is repeatedly acquired from 0 to N-1, the following formula XTM-θ ₀ ≠ XTM-θ ₁ ≠ ... ≠ XTM-θ _N-1 XTM-θ _{n + 1} −XTM-θ _n ≠ XTM-θ _{n + 2} Randomness is given so as to satisfy the relationship represented by −XTM-θ _{n + 1} .
The same applies to Ref-θn. In this way, the carrier phase XTM-θ of the α ° pulse that is the high-frequency signal for excitation is randomly changed for each TR, so that the falling direction of the spin can be randomly changed for each TR. That is, the direction in which the spin falls (flip direction) can be controlled as shown in FIG. 2 by the phase angle of the carrier wave of the α ° pulse that is the excitation high-frequency signal. When the directions in which the spins fall are random, the steady state of the residual transverse magnetization of the spins when TR << T2 in the pulse sequence of FIG. 1 can be prevented. On the other hand, since the phase Ref-θ of the reference signal used at the time of detection also corresponds to XTM-θ, the echo signal after detection becomes the same as a normal signal from which the above randomness is removed. Therefore, it is possible to reconstruct an image from which the artifacts due to phase coherence are removed by performing a two-dimensional Fourier transform on the data thus collected. Normally, XTM-θ = Ref-θ, but in addition to removing artifacts due to phase coherence, 180 ° pulse correction of the curl parcel train, DC noise removal, etc. 0 ° every time, 180
It is also possible to add a phase offset such as °, 0 °, 180 °, .... FIG. 3 shows an MR imaging system implementing the above pulse sequence. In FIG. 3, the waveform generation circuit 11 generates an amplitude modulation waveform that inverts the magnetization of spins by an angle α °, which is sent to the D / A conversion circuit 12 and converted into an analog signal. Sent to 13. As a result, the carrier wave is amplitude-modulated, becomes an α ° pulse, is sent to the transmission coil 15 via the power amplifier 14, and is irradiated to a subject (not shown) such as a human body from the transmission coil 15 to excite it. An NMR signal (echo signal) generated in a subject such as a human body is received by a receiving coil 21, sent to a detection circuit 23 via a preamplifier 22 and detected, and then passed through a low pass filter 24 to A /
The digital signal is converted by the D conversion circuit 25. This signal is taken into the host computer 26 and undergoes processing such as fast Fourier transform (FFT). The carrier wave of the transmission high-frequency signal for excitation and the reference signal used in the detection circuit 23 are the high-frequency signals generated from the reference clock generator 31, and the phases are controlled via the phase shifters 32 and 33, respectively. The phases of these signals are controlled by the output of the random angle generator 35 to be 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-mentioned modified curl parcel control and DC noise removal, adds this to the random angle signal in the addition circuit 36, and only XTM-θ is 0 °, 180 ° for each TR. Phase offsets such as °, 0 °, 180 °, etc. are added. Although the phases of the carrier wave and the reference signal are randomly changed in the above embodiment, if the relationship of the above equation is satisfied,
Random number control is not necessarily required in a strict sense, and if the phase angle is different for each TR and the amount of phase change is different, the steady state of residual transverse magnetization can be prevented. Further, it can be used together with a spoiler gradient pulse in order to enhance the effect of removing artifacts due to phase coherence. Further, not only the above-mentioned two-dimensional Fourier transform method but also the three-dimensional Fourier transform method can be extended, or the method can be easily applied to the CT method and the echo planar method.

【The invention's effect】

According to the NMR data sampling method of the present invention, when the repetition time TR is shortened, the residual transverse magnetization becomes a steady state and is caused by uncontrolled phase coherence, which causes another problem such as generation of an eddy current. Removed without. Therefore, particularly when applied to the MR imaging method, it is possible to suppress the occurrence of specific artifacts depending on the relaxation time of the object in the high-speed imaging method in which the relationship TR << T2 holds, and to improve the reliability of data.

[Brief description of drawings]

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 a relationship between a carrier wave phase of a high frequency signal for excitation and a flip direction in the embodiment, and FIG. It is a block diagram which shows the MR imaging system concerning the Example. 11 ... Waveform generation circuit, 12 ... D / A conversion circuit, 13 ... Amplitude modulation circuit, 14 ... Power amplifier, 15 ... Transmission 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, 32, 33 ... Phase shifter, 34 ... Transmission carrier wave phase control circuit, 35 ... Random angle generator, 36 ... Addition circuit.

Claims (1)

[Claims] 1. The phase of a carrier wave of a high-frequency signal for excitation is changed substantially randomly at each repeating time, and the phase of a high-frequency signal for reference having the same frequency as the carrier wave used for detection of a received NMR signal is set. A method for collecting NMR data, characterized in that the method is changed in accordance with the above change.