JP2606488B2 - MRI equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an MRI apparatus (nuclear magnetic resonance tomography apparatus), and more particularly to an MRI apparatus that performs an imaging sequence using an SSFP (steady precession) state.

[0002]

2. Description of the Related Art Conventionally, an MRI apparatus that performs an imaging sequence using an SSFP state has been known. This imaging sequence includes relaxation times T1, T
A so-called SSFP that repeatedly irradiates an excitation pulse with a repetition time shorter than 2 and generates an NMR signal in a steady state.
A state is realized, a signal of the state is collected, and an image is formed, and various types have been conventionally proposed. If an attempt is made to apply the rephase method for body motion correction to this imaging sequence, mixing of a pseudo signal (echo signal) that occurs simultaneously with the FID signal is unavoidable, and causes artifacts in a reconstructed image.

Therefore, there is a method in which an imaging sequence in which the phase of an excitation pulse is changed and an imaging sequence in which the phase is not changed are collected to collect two sets of data, and a pseudo signal is removed by an appropriate addition / subtraction process between these data. Proposed (M. Deimling et al. 8th S
MRM (Society of Magnetic Resonance in Medicine)
Abstract No. 842, Amsterdam, Aug. 1989, etc.).

[0004]

However, conventionally, at least two imaging sequences are required,
If there is a body motion or the like between the two imaging sequences, the pseudo signal cannot be removed by the addition / subtraction processing between the data, and there is a problem that an artifact may be caused.

SUMMARY OF THE INVENTION In view of the above, the present invention provides an M signal that can remove a pseudo signal component with only one imaging sequence even when an FID signal and a pseudo signal are generated simultaneously.
An object is to provide an RI apparatus.

[0006]

In order to achieve the above object, in an MRI apparatus according to the present invention, a state where an excitation pulse is repeatedly irradiated with a repetition time shorter than a relaxation time to generate an NMR signal in a steady state is obtained. Let me out
A gradient magnetic field pulse for slice selection at the same time as the excitation pulse, a gradient magnetic field phase pulse for inversion for generating an NMR signal immediately before and immediately after the excitation pulse, and a gradient for phase encoding while changing the encoding amount. A magnetic field is generated, and the phase of the excitation pulse is set to plus, plus, minus, minus and 4 for the same amount of phase encoding.
The data that are repeatedly changed in one pulse period and collected during the period between the excitation pulses are indicated by a plus sign in the first and second signs of the first half of the above four pulse cycles, and a positive sign in the second half. In the third and fourth cases, a minus sign is added and added. As a result, the components of the FID signal immediately after the excitation pulse are added to increase the size thereof, and on the other hand, the components of the echo signal generated immediately before the excitation pulse can be mutually canceled out.
With only one imaging sequence, the echo signal component generated immediately before the excitation pulse can be substantially removed, and only the FID signal component immediately after the excitation pulse can be obtained. Further, the phase of the excitation pulse is repeatedly changed at four pulse periods of plus, plus, plus, minus, and data collected during the period between the excitation pulses is compared with the data of the fourth pulse period. It is also possible to add a minus sign in the fourth case and a plus sign in the other cases, and this also allows the components of the echo signal generated immediately before the excitation pulse to cross each other only in one imaging sequence. And remove it,
Substantially, only the component of the FID signal immediately after the excitation pulse can be obtained.

[0007]

An embodiment of the present invention will be described below in detail with reference to the drawings. In FIG. 1, a subject (human body) 11 is arranged in a static magnetic field generated by a static magnetic field magnet 12, and a gradient coil 13 is provided in the static magnetic field space.
And a probe 23 for transmitting and receiving an RF signal.
A pulse current having a predetermined waveform is supplied from the gradient power supply 14 to the gradient coil 13 to generate a gradient magnetic field that is superimposed on the static magnetic field. The tilt power supply 14 includes a control device 41.
, Waveform data and timing information are sent, a current having a waveform according to the data is output to the gradient coil 13 at a timing as instructed, and a gradient magnetic field pulse having a desired waveform is obtained at a desired timing. The gradient coils 13 are provided in three orthogonal directions, and the gradient power supply 14 outputs currents for gradient magnetic fields in the respective directions from the gradient power supply 14 corresponding to the gradient coils. The gradient magnetic fields in the three orthogonal directions are a gradient magnetic field for slice selection, a gradient magnetic field for phase encoding, and a gradient magnetic field for readout.

The control device 41 converts the RF pulse waveform into the modulator 2
The amplitude of the high-frequency signal from the high-frequency oscillator 24 passed through the phase shifter 25 is modulated by the modulator 21 according to the pulse waveform, and the high-frequency output after the modulation is amplified by the transmission amplifier 22 and transmitted to the probe 23. Can be Thus, an RF signal is emitted from the probe 23 toward the subject 11.

The NMR signal generated by the subject 11 is received by the probe 23. The received signal is amplified by the receiving amplifier 31 and then sent to the phase detector 32, where the high-frequency signal is phase-detected from the high-frequency oscillator 24 using the high-frequency signal passed through the phase shifter 25 as a reference signal. Detection output is A / D
The data is sent to the converter 33, converted into digital data, added with an appropriate code in the addition memory 34, and stored. The data collected in this way is subjected to image reconstruction processing such as two-dimensional Fourier transform by the image reconstruction operation device 42, and the obtained image is displayed on the display 43.

As shown in FIG. 2A, an RF pulse is repeatedly irradiated at short repetition time intervals to set an SSFP state in which an NMR signal is constantly generated. As shown in FIG. 2B, a gradient magnetic field pulse for slice selection is applied simultaneously with each RF pulse to selectively excite only a predetermined slice plane. Also, as shown in FIG. 2C, the readout gradient magnetic field is inverted to generate NMR signals at points P and Q. The gradient magnetic field for phase encoding is changed little by little as shown in FIG.

The signal appearing at point P is an FID signal,
The signal appearing at point Q is a pseudo echo signal. The signal at point Q is specific to the SSPF state and is understood as an echo signal imaged immediately before the excitation pulse by the preceding excitation pulse train. The signal generated by two excitation pulses is shown in FIG. 3, and the signal generated by three excitation pulses is shown in FIG. In these figures, the vertical bar indicates the excitation pulse, the solid line indicates the positive phase, the dotted line indicates the negative phase, the horizontal and oblique lines indicate the progress of the nuclear spin phase, the solid line indicates the positive side, and the dotted line indicates the negative side. Side.

When a signal is generated by two excitation pulses, the phases of the excitation pulses are four types of A to D in FIG. In either case, an FID signal is generated by being excited by the first excitation pulse, and is imaged when the phase is aligned (returned to the original state) after being inverted by the second excitation pulse to generate an echo signal. This echo signal is called an eight-ball echo. In these cases, if the first excitation pulse is positive, the phase of the FID signal generated immediately thereafter is positive, and the echo signal generated whether the second excitation pulse is positive or negative has a positive phase. (A in FIG. 3,
B). On the other hand, when the phase of the first excitation pulse is negative, the phase of the FID signal generated immediately after that is negative, and the phase of the echo signal is positive whether the second excitation pulse is positive or negative. The phase becomes in the minus direction (C and D in FIG. 2).

When an echo signal is generated by three equally-spaced excitation pulses, the phases of the excitation pulses are eight as shown in FIGS. 4A to 4H. In each case, the nuclear spin is the first excitation pulse. Excited by a pulse and immediately after F
An ID signal is generated, is directed in the direction of the static magnetic field by the second excitation pulse, and retains its phase. When the phase is advanced again in the opposite direction by the third excitation pulse, the image is formed and the echo signal is formed. Occurs. This echo signal is called stimulated echo. Here, when the phase of the first excitation pulse is positive, the phase becomes as shown in FIGS. 4A to 4D, and the phase of the FID signal generated immediately after that becomes positive, and the second and third phases are generated.
When the phase of either of the excitation pulses is negative (Fig. 4
B) and (C), the phase of the echo signal is on the minus side, and in other cases, both the second and third excitation pulses are plus (A in FIG. 4) and both are minus (D in FIG. 4). Then, the phase of the echo signal becomes positive. On the other hand, when the phase of the first excitation pulse is negative, the phase of the FID signal generated immediately after that becomes as shown in E to H in FIG. 4 becomes negative, and the phase of either the second or third excitation pulse becomes negative. Only when the phase is minus (F, G in FIG. 4), the phase of the echo signal is on the plus side, and in other cases, both the second and third excitation pulses are minus (E in FIG. 4), and both are plus. In the case of (H in FIG. 4), the phase of the echo signal becomes negative.

Therefore, as shown in FIG. 5, when the phase encoding amount is the same, the phase of the equally spaced excitation pulse is repeatedly changed in four pulse periods of plus, plus, minus, minus (from these four pulse periods). Is called an excitation pulse phase change period). In this case, the phase of the signal through each process of FIG. 3 by two excitation pulses is as shown in FIG. 6A, and the phase of the signal through each process of FIG. 4 by three excitation pulses is as shown in FIG. 6B. And these overlap.
Therefore, the FID signal generated immediately after the RF pulse always has the same phase between A and B in FIG. 6 and has the opposite phase every two pulses. In contrast, the echo signal is shown in FIG.
4 and the one undergoing the process of FIG. 4 always occur in the opposite phase at the same time. Therefore, in practice, A and B in FIG. 6 simultaneously occur and overlap, and the FID signal and the echo signal are also added. Therefore, the echo signals cancel each other out and become smaller. Further, within one cycle of the excitation pulse phase change cycle, the data collected in the first and second pulse periods in the first half are respectively given a plus sign, and the third and fourth pulse cycles in the second half are added. A negative sign is added to the data collected in step (1), and they are added. Then, by such coding, the FID signals are added with the same code, and the echo signal components reduced as described above are further canceled each other. It can be sufficiently removed, and substantially only the FID signal component remains.

When the amount of phase encoding is the same, the phases of the equally-spaced RF pulses are added as shown in FIG.
When the phase is changed in plus, plus, minus and four pulse periods (one excitation pulse phase change period is formed by these four pulse periods), the signal of the type of FIG. 3 has a phase relationship as shown in FIG. The signals of the type have a phase relationship as shown in FIG. As shown in FIG. 8, in the second and fourth pulse periods within one excitation pulse phase change period, the phase of the echo signal is opposite between A and B in FIG. Since A and B in FIG. 8 occur simultaneously and overlap, they cancel each other out and become smaller. On the other hand, in the first and third pulse periods within this one excitation pulse phase change period, the phases of the echo signals are the same in A and B in FIG. 8 and do not cancel each other. However, in both A and B of FIG. 8, the phase of the echo signal is reversed between the first pulse period and the third pulse period. Therefore, a minus sign is attached only to the data obtained in the fourth pulse period, and the first to fourth data are added. Then, the FID signal has a positive phase in all of the first to fourth phases and is simply added to obtain a large signal. On the other hand, the echo signals are opposite in phase between the first and third signals as described above, and are added after being given the same sign, so that they are mutually canceled. Would. As a result, the echo signal component can be substantially removed by only one imaging sequence, and only the FID signal component can be obtained.

The control of the excitation pulse phase can be performed by controlling the phase shifter 25 in synchronization with the pulse sequence by the control device 41, and the signing of data at the time of addition is performed after A / D conversion. Addition memory 34
The above can be performed digitally, but can also be performed by controlling the phase of a reference signal for phase detection.

[0017]

As described above, according to the embodiment,
According to the MRI apparatus of the present invention, even when it is unavoidable that the FID signal and the pseudo signal are simultaneously generated and the pseudo signal is mixed into the FID signal, the phase of the excitation pulse is changed at a period of four pulse periods of the excitation pulse. By adding the data collected during the period between each excitation pulse in the four pulse periods with a predetermined code and adding the same, the pseudo signal component can be removed by only one imaging sequence, Can eliminate the source of artifacts.

[Brief description of the drawings]

FIG. 1 is a block diagram of one embodiment of the present invention.

FIG. 2 is a time chart showing a pulse sequence of the embodiment.

FIG. 3 is a time chart showing a phase change when there are two excitation pulses.

FIG. 4 is a time chart showing a change in phase when three excitation pulses are used.

FIG. 5 is a time chart showing an example of an excitation pulse.

FIG. 6 is a time chart showing a phase change in the same example.

FIG. 7 is a time chart showing another example of the excitation pulse.

FIG. 8 is a time chart showing a change in phase in the same example.

[Explanation of symbols]

 REFERENCE SIGNS LIST 11 subject 12 static magnetic field magnet 13 gradient coil 14 gradient power supply 21 modulator 22 transmission amplifier 23 probe 24 high frequency oscillator 25 phase shifter 31 reception amplifier 32 phase detector 33 A / D converter 34 addition memory 41 controller 42 image reconstruction Arithmetic unit 43 Display

Claims (2)

(57) [Claims] 1. A means for repetitively irradiating an excitation pulse with a repetition time shorter than a relaxation time to reveal a state in which an NMR signal is generated in a steady state, and generating a gradient magnetic field pulse for slice selection simultaneously with the excitation pulse. Means,
The means for generating a gradient magnetic field phase pulse for inverting readout for generating an NMR signal immediately before and immediately before the excitation pulse and the means for generating a gradient magnetic field for phase encoding while changing the encoding amount are the same. In the amount of phase encoding, the excitation pulse phase control means for repeatedly changing the phase of the excitation pulse at four pulse cycles of plus, plus, minus, and minus, and the data collected during the period between the excitation pulses are combined with the above-described data. Means for adding a plus sign in the first and second parts of the first half of the four pulse periods, and adding a minus sign in the third and fourth parts of the second half. MRI equipment. 2. A means for repetitively irradiating an excitation pulse with a repetition time shorter than the relaxation time to produce a state in which an NMR signal is generated in a steady state, and generating a gradient magnetic field pulse for slice selection simultaneously with the excitation pulse. Means,
The means for generating a gradient magnetic field phase pulse for inverting readout for generating an NMR signal immediately before and immediately before the excitation pulse and the means for generating a gradient magnetic field for phase encoding while changing the encoding amount are the same. In the amount of phase encoding, the excitation pulse phase control means for repeatedly changing the phase of the excitation pulse at four pulse cycles of plus, plus, plus, minus, and the data collected during the period between the excitation pulses, An MRI apparatus comprising means for adding a minus sign at the fourth of the four pulse periods and adding a plus sign at the other of the four pulse periods.