JPH10234706A - Mr imaging system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain two images having different contrasts at a high speed by the EPI method. SOLUTION: In the first term, several gradient echo signals and many gradient echo signals are generated before and after the first spin echo signal, respectively, and mainly plus phase encoding is applied to these signals. Meanwhile, in the second term, many gradient echo signals and several gradient echo signals are generated before and after the second spin echo signal, respectively, and mainly minus phase encoding is applied to these signals. First and second image are reconstructed based on the data collected in the first and second terms, respectively.

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 utilizing an NMR (nuclear magnetic resonance) phenomenon, and more particularly, to a high-speed hybrid scanning method of a gradient echo and a spin echo called an EPI method. The present invention relates to an MR imaging apparatus for obtaining an image.

[0002]

2. Description of the Related Art EPI (Echo-Planner Im)
In the aging method, a pulse sequence as shown in FIG. 6 is performed (US Pat. No. 4,818,942 and Amer).
ican Journal of Roentgeno
lgy, Vol. 1149, p245, 1987). First, after applying a 90 ° pulse (nutation pulse), one 1
An 80 ° pulse (refocus pulse) is applied, and simultaneously with each of these RF pulses, a gradient magnetic field G for slice selection is set.
s pulses are applied. Then, before and after the generation of the spin echo signal, the pulse of the readout (and frequency encoding) gradient magnetic field Gr is switched a large number of times to generate a large number of gradient echo signals before and after the spin echo signal. Immediately before the generation of these signals, a pulse of the gradient magnetic field Gp for phase encoding is applied, and the amount of phase encoding gradually changes from the maximum value of one polarity to 0, and then to the maximum value of the opposite polarity. Such a Gp pulse waveform is used.

As shown in FIG. 7, data obtained from each of these signals is sequentially arranged in the phase direction of the K space (vertical direction in the figure). When the amount of phase encoding is changed from a negative maximum value to a positive maximum value through 0, they are sequentially arranged from the bottom to the top in FIG. Then, since the phase encoding amount of the spin echo signal is set to 0, data obtained from the spin echo signal is arranged in the center of the K space.

[0004]

However, in the conventional EPI method, the spin echo signal is generated exactly in the middle of a large number of gradient echo signals. (TE) is extended, and as a result, a T2-weighted contrast image is obtained, and there is a problem that other contrast images such as a T1-weighted contrast image and a proton density-weighted contrast image cannot be obtained. Further, only one MR image can be obtained, and a plurality of images having different contrasts cannot be obtained.

In view of the above, the present invention provides an MR imaging apparatus improved to obtain a plurality of images having different contrasts by the EPI method and to obtain a T1-weighted contrast image by sufficiently shortening the TE. The purpose is to:

[0006]

In order to achieve the above object, an MR imaging apparatus according to the present invention comprises:
Gradient magnetic field pulse applying means for applying a slice selection gradient magnetic field pulse, a phase encoding gradient magnetic field pulse, and a readout gradient magnetic field pulse, and receiving an echo signal, performing phase detection, sampling and A / D converting the data, The receiving means, the RF transmitting means, the gradient magnetic field pulse applying means, and the receiving means are controlled to apply one nutation pulse, and thereafter, apply at least two refocusing pulses to the nutation pulse and the first pulse. The time interval between the first and second refocus pulses is made longer than the time interval between the focus pulses and at least two spin echo signals are generated by sequentially applying the first and second refocus pulses. The readout gradient magnetic field pulse is switched a number of times in the above-described manner, and several gradient echo signals are formed before the first spin echo signal. The more number gradient echo signals from it after the spin echo signal, the generating respectively, prior to the second spin echo signals, the first
The same number of gradient echo signals as the number of gradient echo signals generated after the spin echo signal of
After the second spin echo signal, the same number of gradient echo signals as the number of gradient echo signals generated before the first spin echo signal are generated, respectively, and the number of gradient echo signals before the first spin echo signal is generated. As for the gent echo signal, the phase encoding from a small value of the phase encoding amount of one polarity to the vicinity of 0 is performed. For the first spin echo signal, the phase encoding of the phase encoding amount of substantially 0 is performed. For the gradient echo signal subsequent to the second spin echo signal, phase encoding from the vicinity of 0 to a large value of the phase encoding amount of the opposite polarity is performed, respectively, and for the gradient echo signal before the second spin echo signal, Phase encoding from a large value of the phase encoding amount to near 0 is applied to the second spin echo signal. 0 of substantially phase encoding amount
For the gradient echo signal after the second spin echo signal, the phase encoding from around 0 to a small value of the opposite polarity phase encoding amount,
Control means for determining the waveform of the phase-encoding gradient magnetic field pulse to be applied, and a spin echo signal and a gradient echo signal generated between the first refocus pulse and the second refocus pulse. Are arranged in the first K space, and the missing data of the phase encoding amount is buried by diverting the data obtained during this time, which differs only in the polarity of the phase encoding amount. An image is reconstructed from the data arranged in the space, and the data from the spin echo signal and the gradient echo signal generated after the second refocusing pulse are arranged in the second K space and are missing. The phase encoding amount data differs from the phase encoding amount only in polarity. As it is filled with, it has a feature and an image reconstruction means for image reconstruction from the second data disposed K space.

One image is formed by data obtained from a signal generated between the first and second refocusing pulses, and another image is formed by data obtained from a signal generated after the second refocusing pulse. Can be reconstructed, and images having different contrasts can be obtained because of their different echo times. In particular, when collecting data from a signal generated between the first and second refocusing pulses, several gradient echo signals are added before the first spin echo signal, and some gradient echo signals are added after the first spin echo signal. A larger number of gradient echo signals are generated, and the gradient echo signal before the first spin echo signal is reduced from a small value of the amount of phase encoding of one polarity to near 0, and the gradient echo signal is generated for the first spin echo signal. Since the phase encoding amount is substantially 0, and the phase echo amount after the first spin echo signal is phase-encoded from near 0 to a large value of the phase encoding amount of the opposite polarity, the spin encoding is performed. It is possible to sufficiently reduce the echo time until an echo signal is generated, and to obtain a T1-weighted contrast image It can be.

[0008] The image reconstructing means may include data from a spin echo signal and a gradient echo signal generated between the first refocusing pulse and the second refocusing pulse. The data of the phase encoding amount that is arranged and is missing in the K space is filled using data from the spin echo signal and the gradient echo signal generated after the second refocusing pulse. An image is reconstructed from the data arranged in the space, and the data from the spin echo signal and the gradient echo signal generated after the second refocusing pulse are arranged in the second K space and are missing. The data of the phase encode amount is generated between the first refocus pulse and the second refocus pulse. As filling with data from the spin echo signals and gradient echo signals obtained by, it can also be used as the image reconstructed from the second K space data arranged.

[0009]

Next, embodiments of the present invention will be described in detail with reference to the drawings. In the MR imaging apparatus according to the present invention, a pulse sequence as shown in FIG. 1 is performed. FIG. 5 shows an MR imaging apparatus that performs such a pulse sequence.
It is configured as follows. Therefore, first, the configuration of the MR imaging apparatus will be described with reference to FIG. 5. In FIG. 5, the main magnet 11 generates a strong static magnetic field, and an object (not shown) is arranged in the static magnetic field. The gradient magnetic field coil 12 includes three gradient magnetic fields Gx whose magnetic field strengths incline in three orthogonal X, Y, and Z directions.
Three sets of Gy and Gz are generated so as to be superimposed on the static magnetic field. An RF coil 13 for transmission and an RF coil 14 for receiving NMR signals are attached to the subject.

A host computer 21 controls the entire system, and a sequencer 22 acquires a sequence for reconstructing an image of a desired cross section of a subject under the control of the host computer 21 (FIG. 1). ) Are sent to the transmission system, the reception system, and the gradient magnetic field generation system. As for the gradient magnetic field generation, a predetermined pulse waveform related to Gx, Gy, Gz is generated at a predetermined timing from the waveform generator 15 and sent to the gradient magnetic field power supply 16, and the gradient magnetic field coil 12 outputs Gx, Gy and Gz are generated. The slice selection gradient magnetic field Gs, the readout (frequency encoding) gradient magnetic field Gr, and the phase encoding gradient magnetic field Gp shown in the pulse sequence of FIG. 1 use any one of these Gx, Gy, and Gz, or some of them. It is made by combining

The waveform generator 15 includes a sequencer 22
Under the above control, a waveform of an RF pulse is generated at a predetermined timing and sent to the amplitude modulator 24. An RF signal from an RF signal generator 23 set by the host computer 21 so as to generate an RF signal having a frequency corresponding to the resonance frequency of the subject is transmitted to the amplitude modulator 24 as a carrier. This carrier is the waveform generator 15
Is amplitude-modulated according to the waveform signal from. The output of the amplitude modulator 24 is sent to the RF coil 13 via the RF power amplifier 25. In this manner, the waveform and timing of the RF signal transmitted from the RF coil 13 are determined by the sequencer 22, so that the subject is irradiated with the 90 ° pulse or the 180 ° pulse shown in FIG.

The NMR signal generated from the subject is received by the receiving RF coil 14 and sent to the phase detector 27 via the preamplifier 26. The phase detector 27 has a transmission RF
An RF signal serving as a pulse carrier is transmitted from the RF signal generator 23, and this signal is used as a reference signal to perform phase detection. A / D converter 28
Samples the detection signal from the phase detector 27 in accordance with the sampling pulse from the sampling pulse generator 29 whose timing and the like are controlled by the sequencer 22 and converts it into digital data. This digital data is taken into the host computer 21 and subjected to Fourier transform processing by the image reconstruction device 33. The image thus reconstructed is displayed by the display device 32. The indicator 31 is a keyboard, a mouse, and the like for an operator or the like to give necessary instructions to the host computer 21.

In such an MR imaging apparatus, a pulse sequence as shown in FIG. 1 is performed under the control of the host computer 21 and the sequencer 22. In FIG. 1, first, a 90 ° pulse (nutation pulse) is applied, and then a first and a second 180 ° pulse are applied, and a pulse of a gradient magnetic field Gs for slice selection is applied simultaneously with each of these RF pulses. . Thereby, the first
The first spin echo signal can be generated after a lapse of time t1 from the 180 ° pulse of
From the 0 ° pulse, the second from the first spin echo signal
The second spin echo signal can be generated when a time corresponding to the time up to the 180 ° pulse elapses.

The first 180 ° pulse and the second 18
In each of the period between the 0 ° pulse (first period) and the period after the second 180 ° pulse (second period), the pulse of the readout (and frequency encoding) gradient magnetic field Gr is expressed by ( Add while switching m + n times. Thus, in the first period, n gradient echo signals can be generated before the first spin echo signal is generated, and m gradient echo signals are generated after the first spin echo signal is generated. In this case, the echo time T until the spin echo signal is generated by extending the time t1 is set.
It is possible to extend E and make n a large number. However, instead of this, the time t1 is shortened to shorten the echo time TE, and n is set to a small number (1, 2, 3,. Instead, the time from the first spin echo signal to the second 180 ° pulse is increased to make m a large number (eg, 32, 64, or 128).

The total (n + m) occurring in the first period
Immediately before each of the +1) signals, a pulse of the gradient magnetic field Gp for phase encoding is applied. Here, starting from the phase encoding amount of the nth magnitude (in absolute value) on the minus side, the (n-1) th, (n-
2) The phase encoding amount is sequentially (in absolute value), and so on.
.., (M−1) th on the plus side sequentially, and finally the largest phase encoding amount (mth phase encoding amount) ) Is applied to each signal in the order of generation. The phase encoding amount 0 is given to the first spin echo signal.

In the second period, a total of (n + m + 1) signals generated in the period are generated in the order of generation.
The following phase encoding amount is given. That is, in the order of signal generation, the largest (m-th) phase encoding amount on the minus side (in absolute value), the (m-1) -th phase encoding amount on the minus side (in absolute value),
(M-2) th phase encoding amount on the minus side (in absolute value),..., 1st phase encoding amount on the minus side (in absolute value), 0 phase encoding amount, and 1st phase encoding amount on the plus side , The plus-side second phase encode amount, the plus-side third phase encode amount,..., The plus-side n-th phase encode amount.

As a result, in the first period, as shown in FIG. 2A, from the slightly lower side (minus in the phase direction) to the uppermost end (plus maximum value in the phase direction) from the center of the K space. Data of (n + m + 1) encoded numbers (line numbers) to be arranged are collected in the order shown by the dotted line. In the second period, as shown in FIG. 2B, the K space is arranged from the lowermost end (minus maximum value in the phase direction) to a position slightly above the center (plus in the phase direction). Data of (n + m + 1) encoding numbers (line numbers) are collected in the order shown by the dotted lines.

Therefore, as shown in FIGS. 2A and 2B, the data arranged in each K space is subjected to a two-dimensional Fourier transform, whereby each image can be reconstructed. . Here, in each of the K spaces in FIGS. 2A and 2B, data of a phase encoding amount that is insufficient to be collected (data not collected for a line to be arranged) is generated. That is, FIG.
In FIG. 2, data is insufficient on the negative side in the phase direction, and in FIG. 2B, data is insufficient on the positive side in the phase direction. However, as these blank data, data of another polarity in the phase direction can be used. In FIG. 2A, the data on the plus side in the phase direction is diverted to the minus side, and FIG.
In (b), data on the minus side in the phase direction is diverted to the plus side. Further, by utilizing the fact that the data has a complex conjugate relationship with the signal peak, it is also possible to interpolate and fill in an image reconstructed by Fourier transform.

Further, the data of the first period and the second period are
The first and second K spaces may be filled by using each other. For example, as shown in FIG. 3A, an area that is missing only in the data of the first period is filled with data of the second period, and as shown in FIG. Is filled with the data of the first period. In this case, since the signal in the second period is generated later in time than the first period, the signal is attenuated more than the signal in the first period, and the signal intensities are shown in FIGS. 3B and 4, respectively. As shown in (b), a step occurs in the signal strength. Since the signal intensity step may adversely affect the reconstructed image, in order to avoid this, the data is multiplied by a predetermined coefficient and compressed or decompressed so that the signal intensity changes smoothly.

In any of these cases, an image mainly composed of data collected in the first period and an image mainly composed of data collected in the second period are reconstructed. The contrast of each image is controlled by data from a spin echo signal arranged near the center of each K space and data from a gradient echo signal generated in the vicinity of the K space, and until the first spin echo signal is generated. Is short, and the echo time until the second spin echo signal is generated is long, so that images having different contrasts are obtained. Since the echo time until the first spin echo signal is generated can be sufficiently shortened, the first image reconstructed mainly from the data in the first period can be a T1-weighted contrast image. Further, the K of the plurality of images having different contrasts is determined.
Since not all of the phase encoding amounts are individually applied so as to fill all the spaces, the imaging time can be reduced.

In FIG. 4 (a), the missing part of the data collected in the second period is supplemented by the data collected in the first period. An image with a good noise ratio can be obtained.

The above description is for one example, and it goes without saying that the present invention is not limited to the above description. For example, the number of switching of the readout gradient magnetic field Gr, the number of echo signals, the number of images having different contrasts, and the like are not limited. The specific configuration is not limited to that shown in FIG.

[0023]

As described above, according to the MR imaging apparatus of the present invention, a plurality of images having different contrasts can be obtained in a short imaging time in the EPI method. One of the images can be a T1-weighted contrast image based on data collected in a sufficiently short echo time.

[Brief description of the drawings]

FIG. 1 is a time chart showing an example of a pulse sequence performed by an MR imaging apparatus according to an embodiment of the present invention.

FIG. 2 is a diagram showing K spaces in which data collected in each of a first period and a second period is arranged.

FIG. 3 is a diagram showing a K-space arranged while supplementing data collected in a second period with data collected in a first period and a phase direction profile of the signal strength thereof.

FIG. 4 is a diagram showing a K space arranged while supplementing data collected in a first period with data collected in a second period and a phase direction profile of signal strength thereof.

FIG. 5 is a block diagram showing an MR imaging apparatus according to the embodiment of the present invention.

FIG. 6 is a time chart showing a pulse sequence of a conventional example.

FIG. 7 is a diagram showing a K space in which data collected in a conventional example is arranged.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Main magnet for static magnetic field generation 12 Gradient magnetic field coil 13 RF coil for transmission 14 RF coil for reception 15 Waveform generator 16 Gradient magnetic field power supply 21 Host computer 22 Sequencer 23 RF signal generator 24 Amplitude modulator 25 RF power amplifier 26 Preamplifier 27 Phase detector 28 A / D converter 29 Sampling pulse generator 31 Indicator 32 Display device 33 Image reconstruction device

Claims (2)

[Claims] 1. An RF transmitting means for applying a nutation pulse and a refocusing pulse, a gradient magnetic field pulse applying means for applying a slice selection gradient magnetic field pulse, a phase encoding gradient magnetic field pulse and a readout gradient magnetic field pulse,
One nutation pulse is received by controlling the RF transmitting means, the gradient magnetic field pulse applying means and the receiving means for receiving the echo signal, performing phase detection, sampling and A / D converting the data to obtain data. And then applying at least two refocusing pulses sequentially with the time interval between the first and second refocusing pulses longer than the time interval between the nutation pulse and the first refocusing pulse. To generate at least two spin echo signals.
The readout gradient magnetic field pulse is switched many times before and after the plurality of spin echo signals, and several gradient echo signals are provided before the first spin echo signal, and more gradient echo signals are provided after the first spin echo signal. , And the same number of gradient echo signals as the number of gradient echo signals generated after the first spin echo signal before the second spin echo signal. After the first spin echo signal, the same number of gradient echo signals as the number of gradient echo signals generated before the first spin echo signal are generated, and the gradient echo signal before the first spin echo signal is generated. For the signal, the phase encoding from the small value of the phase encoding amount of one polarity to the vicinity of 0 is performed by the first spin-encoding. For the signal, the phase encoding of the phase encoding amount of substantially 0 is performed, and for the gradient echo signal after the first spin echo signal, the phase encoding of the phase encoding amount of the opposite polarity from around 0 to a large value is performed. Each of the gradient echo signals before and after the second spin echo signal is subjected to phase encoding from a large value of the phase encoding amount of one polarity to near 0, and substantially phase encoding is performed for the second spin echo signal. The phase encoding is performed such that the phase encoding of the encoding amount 0 is performed on the gradient echo signal after the second spin echo signal, and the phase encoding is performed from the vicinity of the phase encoding amount 0 of the opposite polarity to a small value. Control means for determining the waveform of the gradient magnetic field pulse for
The data from the spin echo signal and the gradient echo signal generated between the first refocusing pulse and the second refocusing pulse are arranged in the first K space and the missing phase encode amount is calculated. The data is obtained by diverting and filling the data obtained during this time, which differs only in the polarity of the phase encoding amount. The image is reconstructed from the data arranged in the first K space, and the second refocusing pulse is used. The data from the spin echo signal and the gradient echo signal generated after the above are arranged in the second K space, and the data of the missing phase encoding amount is different only in the polarity of the phase encoding amount. By diverting and filling the data, an image is reconstructed from the data arranged in the second K space. MR imaging apparatus is assumed that the feature and an image reconstruction means. 2. RF transmission means for applying nutation pulses and refocusing pulses, gradient magnetic field pulse applying means for applying a slice selection gradient magnetic field pulse, a phase encoding gradient magnetic field pulse, and a readout gradient magnetic field pulse,
One nutation pulse is received by controlling the RF transmitting means, the gradient magnetic field pulse applying means and the receiving means for receiving the echo signal, performing phase detection, sampling and A / D converting the data to obtain data. And then applying at least two refocusing pulses sequentially with the time interval between the first and second refocusing pulses longer than the time interval between the nutation pulse and the first refocusing pulse. To generate at least two spin echo signals.
The readout gradient magnetic field pulse is switched many times before and after the plurality of spin echo signals, and several gradient echo signals are provided before the first spin echo signal, and more gradient echo signals are provided after the first spin echo signal. And the same number of gradient echo signals as the number of gradient echo signals generated after the first spin echo signal are generated before the second spin echo signal. After the first spin echo signal, the same number of gradient echo signals as the number of gradient echo signals generated before the first spin echo signal are generated, and the gradient echo signal before the first spin echo signal is generated. For the signal, the phase encoding from the small value of the phase encoding amount of one polarity to the vicinity of 0 is performed by the first spin-encoding. For the signal, the phase encoding of the phase encoding amount of substantially 0 is performed, and for the gradient echo signal after the first spin echo signal, the phase encoding of the phase encoding amount of the opposite polarity from around 0 to a large value is performed. Each of the gradient echo signals before and after the second spin echo signal is subjected to phase encoding from a large value of the phase encoding amount of one polarity to near 0, and substantially phase encoding is performed for the second spin echo signal. The phase encoding is performed such that the phase encoding of the encoding amount 0 is performed on the gradient echo signal after the second spin echo signal, and the phase encoding is performed from the vicinity of the phase encoding amount 0 of the opposite polarity to a small value. Control means for determining the waveform of the gradient magnetic field pulse for
The data from the spin echo signal and the gradient echo signal generated between the first refocusing pulse and the second refocusing pulse are arranged in the first K space and the missing phase encode amount is calculated. The data is filled using data from the spin echo signal and the gradient echo signal generated after the second refocusing pulse, and an image is reconstructed from the data arranged in the first K space. The data from the spin echo signal and the gradient echo signal generated after the second refocusing pulse are arranged in the second K space, and the data of the missing phase encoding amount is stored in the first refocusing pulse and the second refocusing pulse. Spin echo signal and gradient echo generated between two refocusing pulses MR imaging apparatus as filling with data, to be a feature and an image reconstruction means for image reconstruction from the second K space data arranged from No..