JP2927207B2 - 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 improvement in an MR imaging apparatus for performing a high-speed imaging sequence called a GRASE method.

[0002]

2. Description of the Related Art GRASE (Gradient and
In the spin echo method, a pulse sequence as shown in FIG. 1A is employed (US Pat. No. 52706).
No. 54 and K.I. Oshio and D.S. A. Fei
nberg "GRASE (Gradient-and
Spin-Echo) Imaging: A Nove
l Fast MRI Technique ”Mag
netic Resonance in Medici
ne 20, 344-349, 1991). That is,
First, after applying a 90 ° pulse (excitation pulse), a plurality of (here, three) 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 for frequency encoding) is applied within the above-described RF pulse interval, and the Gr pulse is applied a plurality of times (here, twice) between 180 ° pulses and 180 ° pulses. By switching, the signals S1, S3, S4, S6, and S of the gradient echo are added to the signals S2, S5, and S8 of the spin echo.
7, S9 is generated. Immediately before the generation of these signals, pulses of the gradient magnetic field Gp for phase encoding are respectively applied, and the application amount of each Gp pulse is determined by the signals S1 to S9.
Figure 2 shows the data obtained from the K space (raw data space)
A corresponds to the amount of phase encoding that is arranged as shown in FIG.

In FIG. 2A, the spin echo signals S2, S5,
The data obtained from S8 is arranged. Also, signals S1, S3, S4, S6, S7, and S9 of the gradient echo are arranged in the areas (high-frequency areas) A and C around the K space. Then, in each of these regions A, B, and C, they are arranged from the top to the bottom in the order of generation, that is, from the positive high frequency region to the negative high frequency region through the low frequency region. The amount of phase encoding (the amount of Gp pulse applied) given to each echo signal is determined so that data obtained from each echo signal is arranged in this manner on the K space.

In order to provide such a phase encoding amount, as shown in FIG. 1A, the Gp pulse is applied immediately after the first 180 ° pulse and before the first gradient echo signal S1. The pulse 61 is set to be the largest.
As a result, data from the signal S1 is arranged at the uppermost position in the K space. The magnitudes of the Gp pulses 62 and 63 before the signals S2 and S3 are the same. As a result, the data from the signals S2 and S3 are arranged in the K space at a position separated from the data position of the signal S1 by an equal distance downward. The Gp pulse 64 added thereafter is for rewind and returns the integrated phase encode amount to zero before the next 180 ° pulse is applied. G applied after the second 180 ° pulse
The magnitude of the p pulse 65 is slightly smaller than the magnitude of the Gp pulse 61. As a result, the signal S4 has a phase encoding amount such that the signal S4 is arranged at a lower position close to the data arrangement position by the signal S1. Signals S5 and S
The magnitude and polarity of Gp pulses 66 and 67 applied before each of 6 are the same as the previous Gp pulses 62 and 63. Therefore, the location of the data of the signals S5 and S6 is lower than the location of the data of the signal S4 by the same interval as the interval between the signals S1, S2 and S3. Is the signal S
2. It is adjacent to the data position of S3 on the lower side.
Thereafter, a Gp pulse 68 for rewind is applied. 3
Gp pulse 69 added after the first 180 ° pulse
Is slightly smaller than the Gp pulse 65. The magnitude and polarity of the Gp pulses 70 and 71 are Gp pulses 62 and 63 (and Gp pulses 66 and 67).
Is the same as Therefore, the data of the signals S7, S8, and S9 is such that the data is arranged on the lower side adjacent to the location of the data of the signals S4, S5, and S6.

Since the amount of phase encoding is such that data obtained from a signal generated by spin echo is arranged in the central region B of the K space, artifacts due to non-uniformity of the static magnetic field and chemical shift are small. Benefits are obtained. Further, since the spin echo and the gradient echo are separately arranged on the K space, a phase error between the echoes is less likely to be an image artifact. That is, if the phase encoding amount is such that the data from the signals S1, S2, S3,..., S9 are arranged in order from the top in the K space shown in FIG. Since the data obtained from the gradient echo affected by the non-uniformity will also be located in the center of the K space,
Since the image quality deteriorates, and the data obtained from the spin echo and the data obtained from the gradient echo are arranged adjacently and alternately on the K space, the phase error between them degrades the image quality. Therefore, such GRASE
According to the method, the data from the spin echo signal must be placed at the center of the K space, and the data from the gradient echo signal must be placed at the end of the K space.

[0006]

However, if the data from the spin echo signal is arranged at the center of the K space and the data from the gradient echo signal is arranged at the end of the K space in the GRASE method,
As can be seen from FIG. 2B, there is a problem that an extreme difference in signal strength occurs at the boundary between the region A and the region B and between the region B and the region C, which results in image blurring artifacts and degraded image quality. The signal strength difference between the signals S1 to S9 as shown in FIG. 2B is basically caused by the signal strength attenuating due to the effect of T2 relaxation according to the elapsed time from the excitation pulse. However, the reason why an extreme signal intensity difference occurs at a specific boundary is that the data from the spin echo signal is arranged at the center of the K space and the data from the gradient echo signal is arranged at the end of the K space as described above. This is because of its specialty.

Of course, such an extreme difference in signal strength between the regions A, B, and C on the K space is caused by the signals S1, S2, S3,. This can be solved by setting the phase encoding amount so that the data is arranged in the order of occurrence on the K space shown in FIG. 2A from the top, but this is not a measure that can be adopted as described above.

For this reason, the situation which is a problem in Japanese Patent Application Laid-Open No. 6-181904 is completely different from the situation which is a problem in the GRASE method. That is, in the above-mentioned publication, in the multi-spin echo method in which a plurality of spin echoes are generated after an excitation pulse, if the order of signal generation and the order of the amount of phase encoding correspond to each other, for example, a portion of the K space having a large amount of phase encoding However, since the signal strength is small in a portion where the signal intensity is large and the phase encoding amount is small, the image quality is deteriorated, and a method for improving this is described.
On the other hand, in the GRASE method, in order to use not only the spin echo but also the data from the gradient echo, a special order of the phase encoding amount has to be taken. However, this causes another problem. It is necessary to aim for the solution.

In view of the above, the present invention provides an MR imaging apparatus capable of improving image artifacts caused by the occurrence of an extreme difference in signal intensity in a certain portion of a K space in the GRASE method. The purpose is to:

[0010]

In order to achieve the above object, an MR imaging apparatus according to the present invention comprises:
RF applying means for generating an excitation pulse and a refocusing pulse, means for applying a gradient magnetic field pulse for slice selection, means for applying a gradient magnetic field pulse for phase encoding, means for applying a gradient magnetic field pulse for reading, and echo The signal is received, phase-detected, sampled and A /
Means for obtaining data by D-conversion, and controlling the RF applying means to generate one excitation pulse, then sequentially generating a plurality of refocusing pulses, and reading out each refocusing pulse within each interval. By switching the gradient magnetic field pulse, a signal of one spin echo and signals of a plurality of gradient echoes before and after the signal are generated, and data obtained from the signal of the spin echo is stored in the central portion of the K space, in the gradient. The means for controlling the amount of phase encoding added to each echo signal and the polarity of the above-mentioned phase encoding amount are opposite to each other so that the data obtained from the echo signal is arranged in the peripheral portion of the K space. Means for performing a pair of pulse sequences.

[0011]

A pair of pulse sequences in which the polarities of the phase encode amounts are opposite to each other are performed. for that reason,
On the other hand, in the pulse sequence, a positive phase encoding amount is given to the echo signal generated at a short stage of the elapsed time from the excitation pulse, and the echo signal generated after a long elapsed time from the excitation pulse is given to the echo signal. Assuming that the phase encoding amount is given a negative value, the other pulse sequence gives a negative value of the phase encoding amount to the echo signal generated at a short stage of the elapsed time from the excitation pulse. A positive value of the phase encode amount will be given to the echo signal generated after a long elapsed time from the pulse. Then, data of the same phase encoding amount can be obtained from echo signals generated at different points in time elapsed from the excitation pulse. Therefore, if the data obtained by the pair of pulse sequences are added to each other with the same phase encoding amount at the raw data stage, the signal intensity difference between adjacent data in the phase encoding direction is reduced, and Blur artifacts can be reduced. The same result can be obtained not only at the stage of the raw data but also after adding the Fourier transform.

[0012]

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 the MR imaging apparatus according to one embodiment of the present invention, a pulse sequence as shown in FIG. 1A and a pulse sequence as shown in FIG. 1B are performed as a pair. . An MR imaging apparatus that performs such a pulse sequence is configured as shown in FIG. Therefore,
First, the configuration of the MR imaging apparatus will be described with reference to FIG. 4. In FIG. 4, a magnet assembly 11 includes a main magnet for generating a static magnetic field,
A gradient magnetic field coil for generating a gradient magnetic field superimposed on the static magnetic field is included. The gradient magnetic field is generated by a gradient coil.
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 selected, and a gradient magnetic field Gs for slice selection, a gradient magnetic field Gr for reading and frequency encoding, and a gradient magnetic field G for phase encoding described later.
p is in any direction.

A subject (not shown) is arranged in a space to which the static magnetic field and the gradient magnetic field are applied. The subject has an RF coil 12 for irradiating the subject with an RF pulse and receiving an NMR signal generated by the subject.
Is attached.

The current for the gradient magnetic field applied to the gradient coil of the magnet assembly 11 is controlled by a magnetic field control circuit 21, and each of the gradient magnetic fields Gs, which has a pulse having a waveform as shown in FIGS. Gp and Gr are generated. The RF signal from the RF oscillation circuit 31 is amplitude-modulated by the amplitude modulation circuit 32 and applied to the RF coil 12 via the RF power amplifier 33. The waveform of each gradient magnetic field and the amplitude modulation waveform or their timing is determined by the sequence controller 52.

The echo signal received by the RF coil 12 is sent to a phase detection circuit 42 via a preamplifier 41 and is subjected to phase detection. An RF signal from the RF oscillation circuit 31 is sent as a reference signal for the phase detection. The signal obtained by the phase detection is sampled at a predetermined sampling timing by the A / D converter 43 controlled by the sequence controller 52, and is converted into digital data. This data is taken into the computer 51. The computer 51 performs a process of reconstructing an image from the collected digital data, and controls the sequence controller 52.

In such an MR imaging apparatus, the computer 51 and the sequence controller 5
A pulse sequence as shown in FIGS. 1A and 1B is performed under the control of FIG. The pulse sequence of FIG. 1 (a) has been described above and will not be re-examined, but in addition to such a pulse sequence, FIG.
The pulse sequence shown in FIG. In FIG. 1B, R
Fig. 1 shows F pulse, Gs pulse and Gr pulse.
It is omitted because it is the same as (a), and only the Gp pulse is shown. The Gp pulse shown in FIG.
The polarity of the Gp pulse is inverted. FIG. 1 (a)
The Gp pulse shown in FIG. 1B and the Gp pulse shown in FIG.

That is, the first Gp pulse 61 is the largest on the negative side, and the Gp pulse 62 on the positive side and the Gp pulse 63 on the positive side have the same magnitude. The Gp pulse 64 for rewinding added thereafter is on the negative side. The magnitude of the negative side of the Gp pulse 65 is slightly smaller than that of the Gp pulse 61. The magnitude and polarity of the Gp pulses 66 and 67 are the same as those of the previous Gp pulses 62 and 63. The Gp pulse 68 for rewind is on the negative side. G
The magnitude of the negative side of the p-pulse 69 is slightly smaller than that of the Gp pulse 65. The magnitude and polarity of the Gp pulses 70 and 71 are the same as those of the Gp pulses 62 and 63 (and Gp pulses).
Pulse 66, 67). Gp for rewind
The pulse 72 is on the negative side.

As described above, in the pulse sequence of FIG. 1B, the Gp pulse applied to each of the echo signals S1 to S9 changes from the negative side to the positive side. First, a large Gp pulse 61 is added to the signal S1 on the negative side, and the signal S2 is added with the integral amount of the magnetic field partially offset by the Gp pulse 62 on the positive side. Become. Since the signal S3 is obtained by adding the integration amount to the positive side Gp pulse 63, the magnetic field application amount is slightly positive. After rewinding with the Gp pulse 64, the signal S4
, Only the Gp pulse 65 is applied, so that the amount of applied magnetic field is negative. The signal S5 is about 0 because the integration amount of the Gp pulse 65 and the Gp pulse 66 is added. Since the integration amount of the Gp pulses 65, 66, and 67 is added to the signal S6, the positive side magnetic field application amount is obtained. Gp pulse 68
After rewinding, the signal S7 has a negative magnetic field application amount due to the Gp pulse 69, but the signal S8 has a slightly positive magnetic field application amount since the integral amount of the Gp pulse 69 and the Gp pulse 70 is added. , The signal S9 becomes the integral amount to which the Gp pulse 71 is further added, so that the positive side considerably large magnetic field application amount is obtained. Thus, FIG.
In the pulse sequence of (b), the applied amount of Gp changes from the negative side to the positive side within each 180 ° pulse interval. On the other hand, in the pulse sequence of FIG. 1A, the applied amount of Gp changes from the positive side to the negative side within each 180 ° pulse interval, and the change direction is opposite to that in FIG. 1B. Has become.

Therefore, in the pulse sequence shown in FIG. 1B, data obtained from the signals S1 to S9 can be arranged in the K space as shown in FIG. The signal S1 in this case
1A to S9 are the same as the signals S1 to S9 in the pulse sequence of FIG. 1A, and the smaller the elapsed time from the excitation pulse (90 ° pulse), the smaller the intensity. Are obtained from different signals as shown in FIG. 2A and FIG. 3A. That is, in this embodiment, since the Gp pulse has the opposite polarity in FIGS. 1A and 1B, the signal strength of the data having the same phase encoding amount is added (B in FIG. 2 and B in FIG. 3). Are added for the same amount of phase encoding), and the phase encoding amounts are almost the same.

Therefore, the pulse sequence shown in FIG. 1A and the pulse sequence shown in FIG. 1B are paired, and the data are converted into raw data at the same phase encoding amount (K space). 2) Fourier transform and then image reconstruction, or add two-dimensional Fourier transform complex data to raw data arranged in K space separately as shown in FIGS. 2A and 3A. When one image is obtained, an extreme step in the signal strength of the data arranged on the K space, which occurs in the case of each of the pulse sequences shown in FIGS. 1A and 1B alone, is reduced. A reconstructed image without artifacts can be obtained.

Here, the image is reconstructed from nine signals. However, when the above sequence is repeated to obtain data from a signal of a multiple of 9 and reconstruct the image, the phase encoding amount is changed for each repetition. Change K little by little
What is necessary is just to collect data of a multiple of 9 lines arranged in the space. Also, in this case, nine signals are obtained by giving three refocusing pulses after one excitation pulse, but by increasing the number of refocusing pulses, one signal is obtained.
Two, fifteen, or even more signals may be obtained. If the number of times of switching of the Gr pulse is set to 4 or 6 instead of 2 within the interval of the refocusing pulse, 5 or 7 signals (only the center signal is a spin echo in the interval of the refocusing pulse). , Other signals become gradient echoes).

[0022]

As described above, according to the MR imaging apparatus of the present invention, the sharp signal intensity step inevitable in the GRASE method is reduced, and the image blur artifact due to the sudden signal intensity step is improved. Thus, an MR image of excellent quality can be obtained.

[Brief description of the drawings]

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

FIG. 2 is a schematic diagram showing a K space and signal strength in the pulse sequence of FIG.

FIG. 3 is a schematic diagram showing a K space and a signal intensity in the pulse sequence of FIG. 1 (b).

FIG. 4 is a block diagram showing an MR imaging apparatus according to one embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Magnet assembly 12 RF coil 21 Magnetic field control circuit 31 RF oscillation circuit 32 Amplitude modulation circuit 33 RF power amplifier 41 Preamplifier 42 Phase detection circuit 43 A / D converter 51 Computer 52 Sequence controller

Claims (1)

(57) [Claims] 1. An RF applying means for generating an excitation pulse and a refocusing pulse, a means for applying a slice selection gradient magnetic field pulse, a means for applying a phase encoding gradient magnetic field pulse, and a reading gradient magnetic field pulse Means for receiving the echo signal, performing phase detection, sampling and A / D converting the data to obtain data,
After generating one excitation pulse by controlling the F applying means, a plurality of refocusing pulses are sequentially generated, and the readout gradient magnetic field pulse is switched within each interval of the refocusing pulse, thereby obtaining one pulse. One spin echo signal and a plurality of gradient echo signals before and after the spin echo signal are generated. The data obtained from the spin echo signal is located in the center of the K space, and the data obtained from the gradient echo signal is located in the K space. Means for controlling the amount of phase encoding applied to each echo signal so as to be disposed in the peripheral portion, and a pair of pulse sequences in which the polarities of the above-mentioned phase encoding amounts are opposite to each other are performed. Means comprising:
Imaging device.