JP2000175882A - Mr imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an improved MR imaging apparatus so as to decrease blur of an image and artifact with a simple constitution and without lengthening imaging time (data collecting time). SOLUTION: Data being similar to the data collected by a sequence wherein the direction of changing of amt. of phase encode is made into the reverse direction is obtd. by preparing the data by using complex conjugation from the data in a host computer 21 after the data are collected by changing the amt. of the phase encode in one direction by the sequencer 22. Then, these data are added and the image is reconstituted by an image reconstituting device 33.

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 using an NMR (nuclear magnetic resonance) phenomenon, and more particularly to an MR imaging apparatus for obtaining an image at high speed by an imaging scanning method called a fast spin echo method. About.

[0002]

2. Description of the Related Art A fast spin echo method (hereinafter referred to as FSE (FSE)
ast Spin Echo), a pulse sequence as shown in Fig. 2 is performed ("RARE Imaging: AF
ast Imaging Method for Clinical MR ", Magnetic Reso
nance in Medicine, 3, pp823-833, 1986). First, 9
After applying a 0 ° pulse (nutation pulse), a plurality of (in this case, seven) 180 ° pulses (refocus pulses) are applied, and simultaneously with each of these RF pulses, a gradient magnetic field Gs for slice selection is applied. Apply a pulse. Then, a pulse of the gradient magnetic field Gr for reading (and frequency encoding) is added, and the spin echo signals S1, S
Are generated between the 180 ° pulse and the 180 ° pulse, respectively. Immediately before the generation of these signals S1, S2,..., A pulse of a gradient magnetic field Gp for phase encoding is added, and phase encoding is performed on position information in a predetermined direction. The amount of application of each Gp pulse is made to correspond to the amount of phase encoding such that data obtained from those signals is arranged at different places in the phase direction on the K space (raw data space).

That is, data obtained from these signals S1 to S7 are arranged in the K space in order from the end as shown in FIG. That is, the data from the signal S1 is located at the upper end (high-frequency region on the positive side in the phase direction) in the K space, and the data from the signal S2 is located at the center (lower frequency side in the phase direction) of the signal S3. The data from the signal S4 is further transmitted to the central portion (lower frequency side in the phase direction) of the signal S4, and the data from the signal S4 is further transmitted to the central portion (lowest frequency region in the phase direction) from the signal S5. The data from the signal S6 is placed on the lower portion (higher frequency side on the minus side in the phase direction), and the data from the signal S6 is placed on the lower portion (higher frequency side on the minus side in the phase direction).
Further, the amount of phase encoding added to the signal S7 is determined so that the data from the signal S7 is arranged at the lower end on the high frequency side on the most negative side in the phase direction on the lower side.

[0004]

However, conventionally, there is a problem that image blurring artifacts occur in the reconstructed image. That is, in the conventional FSE method,
The signals S1 to S7 are arranged in the order in which they are obtained from the end of the K space as shown in FIG. 3A, and the intensity of these signals is exponential due to T2 attenuation as shown in FIG. 3B. It becomes smaller exponentially. For this reason, echoes having a large difference in signal strength are arranged adjacent to each other in the K space, and an abrupt signal strength difference causes image blurring artifacts.

[0005] In view of the above, the present invention provides an MR imaging apparatus improved with a simple configuration and capable of reducing image blurring artifacts without increasing the imaging time (data collection time). The purpose is to:

[0006]

In order to achieve the above object, an MR imaging apparatus according to the present invention comprises:
RF transmission means for applying a nutation pulse and a refocusing pulse, and a gradient magnetic field pulse applying means for applying a gradient magnetic field pulse for slice selection, a gradient magnetic field pulse for phase encoding, and a gradient magnetic field pulse for reading, and receiving an echo signal, Receiving means for obtaining data by sampling and A / D converting after phase detection, and controlling the RF transmitting means, the gradient magnetic field pulse applying means and the receiving means to apply one nutation pulse, and then a large number of Control means for generating a spin echo signal by sequentially applying the refocusing pulses of the above, and performing a pulse sequence in which the phase encoding amount is determined so that the phase encoding amount changes in one direction in the order of generation, and K space Create new data using complex conjugate from the data arranged above and add it to the original data Means for image reconstruction, that is provided is the distinctive feature.

By using complex conjugate, it is possible to obtain a data array in which data arranged in the original K space is inverted in the phase direction. This new data array (K space) is the same as the data collected by the sequence performed by changing the phase encoding amount change order in the opposite direction, and the signal intensity profile in the phase direction is inverted in the phase direction with respect to the original K space. It will be. Therefore, if data in the original K space and data in the new K space are added to each other at the same position (including averaging) to form the K space, the signal strength in the phase direction is obtained. The difference is mitigated as a whole and also between echoes. Therefore, an image with suppressed image blur can be obtained by reconstructing the image from now on. In order to obtain a K space in which the signal intensity profile in the phase direction is inverted in the phase direction with respect to the original K space, there is no need to perform data acquisition by performing a sequence with the order of change in the amount of phase encoding reversed. Time (time required for data collection) does not increase. In addition, since new data is created using complex conjugate, the configuration is simple.

[0008]

Next, embodiments of the present invention will be described in detail with reference to the drawings. The MR imaging apparatus according to the present invention is configured as shown in FIG. In FIG. 1, a subject (not shown) is placed in a strong static magnetic field generated by a main magnet 11. In addition, three gradient magnetic fields Gx, Gy, Gz whose magnetic field strengths incline in the three orthogonal axes of X, Y, and Z to the static magnetic field.
Are superimposed. These three gradient magnetic fields Gx, Gy, G
A gradient magnetic field coil 12 including three sets of coils is provided to generate z. Thus, the static magnetic field and the gradient magnetic fields Gx, Gy, Gz superimposed thereon are applied to the subject. The RF coil 13 irradiates the subject with an RF pulse.
Numeral 4 is for receiving an NMR signal generated from the subject.

A host computer 21 controls the entire system, and a sequencer 22 under the control of the host computer 21 collects data for reconstructing an image of a desired section of a subject (see FIG. 2) are transmitted to the transmission system, the reception system, and the gradient magnetic field generation system.

The generation of the gradient magnetic field is as follows. First, a predetermined pulse waveform related to Gx, Gy, Gz is generated from the waveform generator 15 at a predetermined timing,
The waveform is sent to the gradient magnetic field power supply 16, and Gx, Gy, Gz of the waveform and timing are generated from the gradient magnetic field coil 12.
The gradient magnetic field Gs for slice selection and the gradient magnetic field G for reading (for frequency encoding) shown in the pulse sequence of FIG.
r, the gradient magnetic field for phase encoding Gp is Gx, G
It is made by using any one of y and Gz or combining some of them.

The RF pulse irradiation is performed as follows.
Under the control of the sequencer 22, the RF from the waveform generator 15
A pulse waveform is generated at a predetermined timing and sent to the amplitude modulator 24. An RF signal from the RF signal generator 23 is sent to the amplitude modulator 24 as a carrier, and the carrier is amplitude-modulated according to the waveform signal from the waveform generator 15. The RF signal generator 23 is set by the host computer 21 so as to generate an RF signal having a frequency corresponding to the resonance frequency of the subject. 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 and the 180 ° pulse shown in FIG.

Thereafter, an NMR signal is generated from the subject,
This is received and data collected as follows. The NMR signal is received by the receiving RF coil 14 and sent to the phase detector 27 via the preamplifier 26. An RF signal, which is a carrier of a transmission RF pulse, is transmitted from the RF signal generator 23 to the phase detector 27, and the signal is used as a reference signal to perform phase detection. The A / D converter 28 samples the detection signal from the phase detector 27 according to the sampling pulse from the sampling pulse generator 29 whose timing and frequency are controlled by the sequencer 22, and converts it into digital data. This digital data is taken into the host computer 21 and data collection is performed.

The collected data is 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 provided by the operator or the like to the host computer 21.
Keyboard and mouse to give necessary instructions to the user.

In such an MR imaging apparatus, a pulse sequence by the FSE method as shown in FIG. 2 is performed under the control of the host computer 21 and the sequencer 22. In FIG. 2, first, a 90 ° pulse (nutation pulse) is applied, and then a plurality (here, seven pulses) are applied.
And a pulse of a gradient magnetic field Gs for slice selection is applied simultaneously with each of these RF pulses. Then, a pulse of the gradient magnetic field Gr for reading (and for frequency encoding) is added within the above-described RF pulse interval, and each of the pulses is added.
The spin echo signals S1 to S7 are generated within the interval of the 0 ° pulse.

A Gp pulse for phase encoding is applied to each of these signals S1 to S7, and the amount of application (time integration of the amplitude value) is determined as follows. That is, the data from these signals S1 to S7 is
As shown in FIG. 3A, they are arranged in order from the end in the K space. K shown in FIG.
In the space, the vertical direction in the figure is the phase direction (the center is 0 in the phase encoding amount, the upper direction is the direction in which the phase encoding amount is positive, and the lower direction is the direction in which the phase encoding amount is negative), and the horizontal direction is the frequency direction.

That is, the data from the signal S1 is located at the upper end in the K space (high-frequency range on the plus side in the phase direction).
In addition, the data from the signal S2 is located at a portion adjacent to the center (lower frequency side in the phase direction), and the data from the signal S3 is further located at a portion adjacent to the center (lower frequency in the phase direction).
The data from the signal S4 is placed in the next most central part (lowest frequency range in the phase direction), and the data from the signal S5 is placed in the lower part (negative high frequency side in the phase direction). The data from S6 is on the lower side (high-frequency side on the minus side in the phase direction), and the data from the signal S7 is on the lower side, the end on the most high-frequency side on the minus side in the phase direction. , The amount of phase encoding added to those signals is determined.

In this case, the intensity of the signals S1, S2, S3,... Decreases along the T2 decay curve shown by the dotted line in FIG. Therefore, the signal intensity profile in the phase direction of the data arranged on the K space is as shown in FIG. Therefore, the data from the signal S1 and the signal S1
A large signal strength difference occurs between data from two signals or where data from one signal and data from another signal are adjacent, and the K space is two-dimensionally Fourier transformed to reconstruct an image. As mentioned above, if this is the case, this will result in image blurring artifacts.

Therefore, in the present invention, new data is created by using the half Fourier transform method, and the new data is added to the original data (including the averaging). Here, this operation is performed by the host computer 21. This means that new data is created (interpolated) using the property (complex conjugate) that raw data is point-symmetric with respect to the position where signal strength peaks in K space (usually the center of K space). Things. That is, since the complex conjugate data of the data arranged at a certain position in the K space is data to be arranged at a point symmetrical position of the original position, the data arranged as shown in FIG. Get. Here, S1 'indicates an area where complex conjugate data of data from the signal S1 is to be arranged, and S2', S3 ',
The same applies to ...

Thus, the data S1 'to be arranged at the end on the minus side in the phase direction with respect to the signal S1 having a large signal strength.
In the end, the signals S1 ′ and S1 ′ when the amount of phase encoding added to the seven signals generated in order is sequentially increased from the negative side to 0 and from the negative side to the positive side
Data similar to the data obtained from 2 ′, S3 ′,... Can be obtained. This signal intensity profile is obtained by inverting the signal intensity profile of FIG. 3B and becomes as shown in FIG. 4B. The signal intensity profile is large below the K space and becomes smaller as it goes upward.

The data arranged in these K spaces is:
It is added at the same position on the K space. That is, as shown in FIG. 5A, the K space in FIG.
A K space as obtained by adding the K space of (a) can be obtained. In the K space of FIG.
The signal strength profile in the phase direction is as shown in FIG. 5B, and the difference in signal strength as a whole is reduced, and the difference in signal strength between echoes is also small. Therefore, an image in which image blurring artifacts are suppressed can be reconstructed by performing a two-dimensional Fourier transform on the K space.

In the above, new data is created at the stage of raw data, but it may be created after one-dimensional Fourier transform. In the data array obtained by performing one-dimensional Fourier transform in the frequency direction of the K space, there is a complex conjugate relationship between the data at positions that are line-symmetric with respect to the peak line (the data row in the horizontal direction, that is, the position direction). Therefore, using this to create new data after one-dimensional Fourier transform, adding this to the original data, and performing Fourier transform in the phase direction to create an image, the same result as above can be obtained. .

Here, seven echo signals are generated per TR, but the number of refocusing pulses (and accompanying Gs, Gr, Gp) is increased or decreased to increase or decrease the number of refocusing pulses. It is also possible to generate several echo signals. It is also possible to control the effective TE that governs the contrast of the reconstructed image by changing the point in time of the echo to be placed in the center of the K space.

Furthermore, an even number of echoes are generated, data to be arranged in one K space is collected by using a plurality of echoes in the first half, and arranged in another K space by using a plurality of echoes in the second half. If the data to be collected is collected, images with a plurality of contrasts can be collected at one time. In this case, however, new data is created by using complex conjugate from the data to be arranged in each K space. By adding to the original K space, image blur artifacts in images having different contrasts can be reduced.

In the above description, the change direction of the amount of phase encoding is changed from the plus side through 0 to the minus side, but is not limited to this, and is changed from the minus side through 0 to the plus side. You may do so.

[0025]

As described above, according to the MR imaging apparatus of the present invention, in the FSE method, data obtained by reversing the change order of the phase encode amount is obtained without performing a new sequence. Since the new data and the original data are added to reconstruct the image, image blurring artifacts can be reduced without extending the data collection time. In addition, since new data is obtained by using complex conjugate, its configuration is also simple.

[Brief description of the drawings]

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

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

FIG. 3 is a diagram showing a K space composed of collected data and a signal strength profile thereof.

FIG. 4 is a diagram showing a K-space constituted by data created by using a complex conjugate and a signal strength profile thereof.

FIG. 5 is a diagram showing a K space constituted by data obtained by adding original data and new data, and a signal strength profile thereof.

[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 S1 to S7 Spin echo signal

Claims (1)

[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. Is applied, a number of refocusing pulses are sequentially applied to generate spin echo signals, and a pulse sequence in which the phase encoding amount is determined so that the phase encoding amount changes in one direction in the order of generation. An MR imaging apparatus comprising: a control unit; and a unit configured to generate new data from the data arranged in the K space by using complex conjugate, add the generated data to the original data, and reconstruct an image.