JP3246020B2 - MR imaging device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an MR imaging apparatus for performing imaging by utilizing an NMR (nuclear magnetic resonance) phenomenon, and more particularly to an MR imaging apparatus for obtaining an image by a fast spin echo method.

[0002]

2. Description of the Related Art Conventionally, in a MR imaging apparatus, one nutation RF pulse and a plurality of subsequent refocusing RF pulses are applied to a subject to sequentially generate echo signals, and each of the echo signals differs. There is known a high-speed spin echo method in which data of a plurality of lines in a raw data space is collected at once by performing phase encoding, and the number of repetitions of a nutation RF pulse is reduced to speed up imaging.

[0003]

However, it is inevitable that an eddy current magnetic field is generated in an actual MR imaging apparatus. Therefore, in such an actual MR imaging apparatus, a conventional high-speed spin echo method is applied. It is considered almost impossible. That is, 1
Nutation RF pulses followed by multiple refocusing R
When the subject is irradiated with an F pulse and an echo signal is sequentially generated and a different phase encoding is applied to each echo signal, the phase encoding does not become a predetermined one due to an eddy current magnetic field. Since the phases between the lines arranged in the lines are not continuously connected, image blur occurs. Further, since the echo peaks of the primary echo and the stimulated echo deviate due to the eddy current magnetic field, the two echoes interfere with each other and cause image unevenness.

[0004] In view of the above, it is an object of the present invention to provide an MR imaging apparatus improved so that an image of good image quality can be reconstructed by a high-speed spin echo method even when generation of an eddy current cannot be avoided. I do.

[0005]

In order to achieve the above object, an MR imaging apparatus according to the present invention comprises:
When irradiating a subject with one nutation RF pulse and a plurality of subsequent refocusing RF pulses to sequentially generate an echo signal, a slice selection gradient magnetic field pulse is applied simultaneously with each RF pulse. A phase encoding gradient magnetic field pulse and a readout gradient magnetic field pulse are applied to the echo signal, and the phase encoding gradient magnetic field pulse is accompanied by a rewind pulse, and a predetermined refocus R
It is characterized in that two phase encoding gradient magnetic field pulses applied on both sides of the F pulse have the same waveform and polarity, including the accompanying rewind pulse.

[0006]

The present invention irradiates a subject with one nutation RF pulse and a plurality of subsequent refocusing RF pulses to sequentially generate an echo signal, and simultaneously applies a slice selection gradient magnetic field pulse simultaneously with each RF pulse. When a phase encoding gradient magnetic field pulse and a readout gradient magnetic field pulse are applied to each echo signal, for two phase encoding gradient magnetic field pulses applied on both sides of a certain refocusing RF pulse, a rewind pulse accompanying them is applied. And the same waveform and polarity, the error component that could not completely return the magnetization phase to zero by the rewind pulse due to the eddy current magnetic field can be completely canceled. As a result, after two phase encoding gradient magnetic field pulses including the rewind pulse on both sides of the refocus RF pulse are completed, the magnetization phase is completely returned to zero. Therefore, if phase encoding is performed to collect data near the center of the raw data space at this time, important data near the center of the raw data space can be collected without receiving a phase error due to the eddy current magnetic field, An image of good quality can be reconstructed.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the drawings. In one embodiment of the present invention, a pulse sequence as shown in FIG. 1 is performed with a configuration as shown in FIG. First, referring to FIG. 3, the main magnet 1 is for generating a static magnetic field.
A gradient magnetic field is applied by the gradient coil 2 so as to be superimposed on the static magnetic field. The gradient magnetic field is the gradient coil 2
As a result, the magnetic field strengths are generated in such a manner that the magnetic field strengths incline in the three axes of X, Y, and Z, respectively. One of these three-axis gradient magnetic fields is selected or a combination thereof is used as a slice selection gradient magnetic field Gs, readout (and frequency encoding) gradient magnetic field Gr, and phase encoding gradient magnetic field Gp, which will be described later. The subject 3 is placed in the space to which the static magnetic field and the gradient magnetic field are applied. This subject 3
In order to irradiate an excitation RF pulse to the subject 3 and receive an NMR signal generated in the subject 3, an RF
A coil 4 is attached.

A gradient magnetic field power supply 5 is connected to the gradient magnetic field coil 2 to supply power for generating a gradient magnetic field. A transmission power amplifier 7 and a preamplifier 10 are connected to the RF coil 4 via a switch 6. The switch 6 is switched to the transmission power amplifier 7 at the time of excitation, and is switched to the preamplifier 10 at the time of reception. An RF signal obtained by modulating a carrier signal from a signal generator 9 with a modulation signal having a predetermined waveform in a transmission circuit 8 is sent to a transmission power amplifier 7. A receiving circuit 11 is connected to the preamplifier 10, and performs phase detection of the received signal using the signal from the signal generator 9 as a reference signal. The detected signal is sampled by an A / D converter 12, converted into digital data, and taken into a computer 13.

The computer 13 controls the modulation signal waveform of the excitation RF pulse in the transmission circuit 8 and controls the signal generator 9.
And the sampling timing of the A / D converter 12 is determined. Further, the gradient magnetic field power supply 5 is controlled to arbitrarily program the timing, waveform, intensity, and the like of the gradient magnetic field pulse. Further, processing such as reconstructing an image from the collected digital data is performed. The display device 14 displays a reconstructed image and the like.

In such an MR imaging apparatus, a pulse sequence as shown in FIG. In the pulse sequence of FIG. 1, one pulse of a nutation RF pulse (90 ° pulse) is applied, and at the same time, a pulse of the slice selection gradient magnetic field Gs is applied. Then, four refocusing RF pulses (180 ° pulse) are applied. , Gs pulse.

After the Gr pulse is applied before the first 180 ° pulse, and after each of the 180 ° pulses, the Gr pulse is phase-aligned to generate an echo signal and perform frequency encoding. Gp pulses are applied after each of the 180 ° pulses and before each echo signal occurs, to phase encode the echo signals. The Gr pulse applied after the generation of the echo signal is for rewind, and thereby for returning the phase to zero.

In this embodiment, four 180 ° pulses are sequentially applied after one 90 ° pulse is applied, and the first to fourth pulses are applied.
The first and fourth echo signals have the same and small range, and the second and third echo signals have the same amount of phase encoding. It has a large range. That is, assuming that data of each line arranged in the raw data space as shown in FIG. 2 is collected from one echo signal, the first and fourth data obtained in one repetition period are obtained.
The data of the same line at the center part in the phase encoding direction is collected by the echo signal of the above, and the data of the same line at both ends in the phase encoding direction are collected by the second and third echo signals.

In this embodiment, one image is reconstructed from data collected from the first and second echo signals, and another image is reconstructed from data collected from the third and fourth echo signals. Constitute. The former image and the latter image
Since the TEs are different (the former is shorter and the latter is longer), the contrast of the image is different.

The Gp pulses for phase encoding on both sides of the third 180 ° pulse have the same amplitude and waveform to achieve the same amount of phase encoding.
G including rewind added in part A and part B of
The waveforms of the p-pulses are particularly the same and have the same polarity. As a result, when the phase encode given by the first Gp pulse of the portion A is rewinded by the later Gp pulse, even if the phase encode remains without being rewinded by the eddy current magnetic field, the B pulse after the third 180 ° pulse is left. Since a Gp pulse having exactly the same waveform and polarity as that of the portion A is applied to the portion A, after the completion of the portion B, the change in magnetization phase due to the Gp pulse does not completely remain.

This will be described in more detail with reference to FIGS. In the high-speed spin echo method, as described above (FIG. 1), a phase encoding is performed with a Gp pulse to generate an echo signal, and then a Gp pulse for rewinding having the same waveform with the opposite polarity is applied to determine a phase change of magnetization. This is characterized in that the next generated echo signal can be newly subjected to phase encoding. That is, as shown in FIG. 4, a Gp pulse for phase encoding and a Gp pulse for rewind are added before and after the echo signal. However, in an actual MR imaging apparatus, it is inevitable that an eddy current is generated by a gradient magnetic field pulse. Therefore, a magnetic field is generated by the eddy current as shown by a dotted line. As a result, even if the Gp pulse for phase encoding and the Gp pulse for rewind have completely the same waveform and opposite polarities, the rewind Gp pulse cannot completely rewind due to the influence of the eddy current magnetic field. for that reason,
The amount of phase encoding applied to the next echo signal is affected,
The phase of each line in the raw data space is not continuously connected, which causes an artifact.

Here, as described above, the third 180 °
The portions A and B on both sides of the pulse have the same waveform and the same polarity. FIG. 5 shows only this part. In general, before and after a 180 ° pulse, the phases have opposite signs. That is, when the phase does not return to zero due to the influence of the eddy current magnetic field (dotted line) even if Gp pulses for phase encoding and rewind are added before the 180 ° pulse, 180
If a Gp pulse having the same waveform and the same polarity for phase encoding and rewinding is added after the pulse, even if the phase does not return to zero due to the influence of the eddy current magnetic field (dotted line), the pulse before and after the 180 ° pulse In this case, the influence of the eddy current magnetic field occurs in the same manner, so that the influence is canceled before and after the 180 ° pulse. After being canceled in this way, phase encoding can be performed without any phase error component due to the eddy current magnetic field, and the echo peaks of the primary echo and the stimulated echo can be completely matched.

Therefore, when the fourth 180 ° pulse is applied to generate the fourth echo signal after the end of the portion B, when the Gp pulse is applied to the fourth echo signal,
Phase encoding can be performed in a state where the phase error due to the eddy current magnetic field is not superimposed. Also, since the first echo signal is also subjected to phase encoding first and thus is not affected by the eddy current magnetic field, the data collected from the first and fourth echo signals eventually has a phase error. It is not included. Then, the first and fourth echo signals are subjected to a small amount of phase encoding, and from this, data in the central portion of the raw data space in the phase encoding direction is collected. As data of an important part, data that does not include a phase error component due to an eddy current magnetic field is obtained, and a reconstructed image with good image quality is obtained. On the other hand, data including a phase error component due to an eddy current magnetic field is collected from the second and third echo signals, but data at both ends of the raw data space in the phase encoding direction. Therefore, no problem occurs on the reconstructed image.

It should be noted that in the above description, four nutation RF pulses are used.
And the phase encoding Gp pulse accompanied by the rewind pulse before and after the third 180 ° pulse has the same waveform and the same polarity including the rewind pulse. The present invention is not limited to the number and the like, but may be variously changed without departing from the spirit of the present invention.

[0019]

According to the MR imaging apparatus of the present invention, even when the generation of an eddy current magnetic field is inevitable, the eddy current magnetic field can be obtained by the high-speed spin echo method of collecting data of a plurality of lines by one nutation pulse. By collecting data that does not include a phase error component due to the above, a good image free from image blurring and image unevenness can be obtained.

[Brief description of the drawings]

FIG. 1 is a time chart showing a pulse sequence according to an embodiment of the present invention.

FIG. 2 is a view showing a raw data space in the embodiment.

FIG. 3 is a block diagram of the MR imaging apparatus of the embodiment.

FIG. 4 is a time chart showing only a Gp pulse and an echo signal for explaining the operation of the embodiment.

FIG. 5 is a time chart showing only a Gp pulse and an echo signal for explaining the operation of the embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Main magnet 2 Gradient magnetic field coil 3 Subject 4 RF coil 5 Gradient magnetic field power supply 6 Switching device 7 Transmission power amplifier 8 Transmission circuit 9 Signal generator 10 Preamplifier 11 Receiving circuit 12 A / D converter 13 Computer 14 Display device

Claims (1)

(57) [Claims] 1. A means for irradiating a subject with one nutation RF pulse and a plurality of subsequent refocusing RF pulses to generate an echo signal sequentially, and a gradient magnetic field pulse for slice selection simultaneously with each of the RF pulses. And a phase encoding gradient magnetic field pulse accompanied by a rewind pulse for each echo signal, and two phase encoding gradient magnetic field pulses applied on both sides of a predetermined refocusing RF pulse. Means of equal waveform and polarity, including no rewind pulse,
Means for applying a readout gradient magnetic field pulse for each echo signal.