JPH04354935A - Mri system - Google Patents

Abstract

PURPOSE:To achieve the removal of a pseudo signal component simply and accurately by controlling echo signals which are generated via a change process of a nuclear spin in the same phase encoding value so as to be opposite in phase while collection data are added with a coding. CONSTITUTION:An inclined coil 13 and a probe 23 for transmitting or receiving an RF signal is arranged within a magnetostatic field to be generated from a magnetostatic field magnet 12 together with an object 11 to be inspected (human body). A pulse current of a specified waveform is supplied to the inclined coil 13 from an inclination power source 14 while a waveform datum and timing information are transmitted to the inclination power source 14 from a controller 41. Moreover, an RF pulse waveform is transmitted to a modulator 21 from the controller 41 to modulate a high-frequency signal in amplitude via a phase shifter 25 from a high-frequency oscillator 24, and then, the signal is amplified with a transmitting amplifier 22 to be transmitted to the prove 23. Then, an NMR signal generated in the object 11 to be inspected is received with the probe 23 and, after transmitted to an A/D converter 33 via a phase detector 32 or the like, it is added up with a coding by means of an addition memory 34 to be stored.

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 state precession) state.

0002

2. Description of the Related Art Conventionally, MRI apparatuses that perform imaging sequences using SSFP states have been known. This imaging sequence consists of relaxation times T1, T2
Repeatedly irradiate excitation pulses with shorter repetition times,
Various methods have been proposed in the past to realize a so-called SSFP state in which an NMR signal is generated in a steady state, and to collect the signal and perform imaging. If a rephasing method for body motion correction is applied to this imaging sequence, a false signal (echo signal) generated simultaneously with the FID signal will inevitably be mixed in, causing artifacts in the reconstructed image.

[0003] Therefore, there is a method in which two sets of data are collected by performing an imaging sequence in which the phase of the excitation pulse is changed and an imaging sequence in which the phase is not changed, and the pseudo signals are removed by appropriate addition and subtraction processing between these data. It has been proposed (M. Deimling et
al. 8th SMRM (Society of M
agnetic resonance in medicine
cine) Title No. 842,Amsterdam,
Aug. 1989 etc.).

0004

[Problem to be Solved by the Invention] However, conventionally, at least two imaging sequences are required;
If there is body movement or the like between these two imaging sequences, the false signal cannot be removed by addition/subtraction processing between the data, and there is a problem in that artifacts may occur.

[0005] In view of the above, the present invention provides an M system that can remove pseudo signal components with only one imaging sequence even when an FID signal and a pseudo signal occur simultaneously.
The purpose is to provide an RI device.

[0006]

[Means for Solving the Problems] In order to achieve the above object, the MRI apparatus according to the present invention creates a state in which an NMR signal is generated in a steady state by repeatedly irradiating excitation pulses with a repetition time shorter than the relaxation time. let it come out,
The phase of the excitation pulses is controlled by applying multiple excitation pulses with the same amount of phase encoding, and the echo signals generated through two or more changing processes of the phase of the nuclear spin are generated so that their phases are opposite to each other. The components that remain as a result of the above cancellation are removed by making them occur at the same time in time so that they cancel each other out, and then adding the signs of the data collected as having the same amount of phase encoding. . This makes it possible to sufficiently remove only the echo signal component with only one imaging sequence.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the drawings. In FIG. 1, a subject (human body) 11 is placed in a static magnetic field generated by a static magnetic field magnet 12, and a gradient coil 13 and a probe 23 for transmitting and receiving RF signals are placed in the static magnetic field space. . A pulse current having a predetermined waveform is passed through the gradient coil 13 from a gradient power source 14 to generate a gradient magnetic field that is superimposed on the static magnetic field. This gradient power supply 14 includes a control device 41
Waveform data and timing information are sent from the controller, and a current having a waveform according to the data is outputted to the gradient coil 13 at a designated timing, thereby obtaining a gradient magnetic field pulse having a desired waveform at a desired timing. The gradient coils 13 are provided in three orthogonal axes, and the gradient power supply 14 outputs current for gradient magnetic fields in each direction correspondingly. The gradient magnetic fields in the three orthogonal axes directions are respectively 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 transmits the RF pulse waveform to the modulator 2.
1, the modulator 21 amplitude-modulates the high-frequency signal from the high-frequency oscillator 24 that passes through the phase shifter 25 according to the pulse waveform, and the modulated high-frequency output is amplified by the transmission amplifier 22 and sent to the probe 23. It will be done. In this way, an RF signal is irradiated from the probe 23 toward the subject 11 .

[0009] The NMR signal generated by the subject 11 is received by the probe 23 . This received signal is amplified by a receiving amplifier 31, and then sent to a phase detector 32, where phase detection is performed using the high frequency signal that has passed from the high frequency oscillator 24 through the phase shifter 25 as a reference signal. Detection output is A/D
The data is sent to a converter 33, converted into digital data, added with an appropriate sign, and stored in an addition memory 34. The data thus collected is subjected to image reconstruction processing such as two-dimensional Fourier transformation by the image reconstruction calculation device 42, and the obtained image is displayed on the display 43.

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

The signal appearing at point P is an FID signal,
The signal appearing at point Q is a pseudo echo signal. This Q-point signal is characteristic of the SSPF condition and can be understood as an echo signal that is imaged just 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. 4. In these figures, the vertical bars represent excitation pulses, the solid lines represent positive phase, the dotted lines represent negative phase, and the horizontal and diagonal lines represent the progression of the nuclear spin phase, with solid lines representing positive and dotted lines representing negative phase. represents the side.

When a signal is generated using two excitation pulses, the excitation pulses have four phases, A to D in FIG. In either case, an FID signal is generated by being excited by the first excitation pulse, and is inverted by the second excitation pulse, and when the phases are aligned (return to the original state), an image is formed and an echo signal is generated. This echo signal is the eight-ball
It's called echo. In these cases, if the first excitation pulse is positive, the phase of the FID signal generated immediately after it is positive, and whether the second excitation pulse is positive or negative, the echo signal generated has a positive phase. (A, B in Figure 3). On the other hand, when the phase of the first excitation pulse is negative, the phase of the FID signal generated immediately after it is negative, and whether the second excitation pulse is positive or negative, the phase of the echo signal is The phase is in the negative direction (C and D in FIG. 2).

In addition, when an echo signal is generated by three equally spaced excitation pulses, the phases of the excitation pulses are eight different as shown in A to H in FIG. Excited by a pulse and immediately thereafter F
It generates an ID signal, is directed in the direction of the static magnetic field with the second excitation pulse and maintains its phase, and with the third excitation pulse the phase advances again in the opposite direction, and when the phases are aligned, it is imaged and an echo signal is generated. occurs. This echo signal stimulates
It is called d echo. Here, when the phase of the first excitation pulse is positive, it becomes as shown in A to D in Fig. 4, and the phase of the FID signal generated immediately after that becomes positive, and either the second or the third Only when the phase of the excitation pulse is negative (B and C in Figure 4), the phase of the echo signal is on the negative side; otherwise, both the second and third excitation pulses are positive (A in Figure 4). If both are negative (D in FIG. 4), 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 is negative as shown in E to H in Fig. 4, and the phase of either the second or third excitation pulse is negative. The phase of the echo signal is on the positive side only when the phase is negative (F and G in Figure 4), and in other cases, that is, both the second and third excitation pulses are negative (E in Figure 4).
), both of which are positive (H in FIG. 4), the phase of the echo signal becomes negative.

[0014] Let us therefore consider the case where, when the amount of phase encoding is the same, the phase of equally spaced excitation pulses is repeatedly changed in four pulse periods: plus, plus, minus, minus, as shown in FIG. In this case, the phase of the signal passing through each process in FIG. 3 with two excitation pulses is as shown in FIG. 6A, and the phase of the signal passing through each process in FIG. 4 with 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 in A and B in FIG. 6, and the phase is opposite every two pulses. On the other hand, the echo signal goes through the process shown in Figure 3 and Figure 4.
Things that go through the process always occur at the same time and in opposite phases. Therefore, since A and B in FIG. 6 actually overlap, the echo signals cancel each other out and become smaller. The data collection sections are section 1, section 2, section 3, and
If we define every 2 pulses as follows, and add a minus sign to the signal in the interval (2n-1) and a plus sign to the signal in the interval (2n), we can get a smaller value as shown above. The echo signal components further cancel each other out, and F
Since the ID signal components are simply added, it is possible to sufficiently remove only the echo signal component and leave the FID signal component.

Furthermore, when the amount of phase encoding is the same, if the phase of the equally spaced RF pulses is changed in four pulse periods (plus, plus, plus, minus) as shown in FIG. 7, the signal of the type shown in FIG. 3 becomes as shown in FIG. A signal of the type shown in FIG. 4 has a phase relationship as shown in B of FIG. 8. As shown in FIG. 8, if the data collection intervals are defined as interval 1, interval 2, interval 3, etc. for each pulse, then interval 4
At n, 4n+2, the phases of the echo signals at A and B in FIG. 8 are opposite in phase, cancel each other out, and become smaller. In sections 4n+1 and 4n+3, the phases of the echo signals in A and B in FIG. 8 are the same and there is no cancellation relationship, but in sections 4n+1 and 4n+3, the phases of A and B in FIG. 8 are opposite. There is. Therefore, by adding only the section 4n+2 with a negative sign, it is possible to selectively erase only the echo signal and leave only the FID signal.

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

[0017]

[Effects of the Invention] As described above with respect to the embodiments,
According to the MRI apparatus of the present invention, even if an FID signal and a pseudo signal occur simultaneously and it is unavoidable that the pseudo signal is mixed into the FID signal, the pseudo signal component can be removed with only one imaging sequence. This eliminates the cause of image artifacts.

[Brief explanation of drawings]

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

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

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

FIG. 4 is a time chart showing changes in phase when there are three excitation pulses.

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

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

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

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

[Explanation of symbols]

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 Receiving amplifier 32 Phase detector 33 A/D converter 34
Addition memory 41 Control device 42 Image reconstruction calculation device 43
display

Claims (1)

[Claims] 1. Means for repeatedly irradiating an excitation pulse with a repetition time shorter than the relaxation time to generate a state in which an NMR signal is generated in a steady state, and generating a gradient magnetic field for phase encoding while changing the amount of phase encoding. and a plurality of echo signals generated through two or more change processes of the phase of the nuclear spin with the same phase encoding amount, so that the echo signals are generated simultaneously in time so that the phases thereof are opposite to each other. An MRI apparatus comprising: means for controlling the phase of an excitation pulse; and means for adding together data acquired with the same phase encoding amount.