JP3402647B2 - Magnetic resonance imaging - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic resonance imaging apparatus, and more particularly to a magnetic resonance imaging apparatus which visualizes the spatial distribution of specific atomic nuclei in a subject at high speed.

[0002]

2. Description of the Related Art Magnetic resonance imaging is the energy of a high-frequency magnetic field rotating at a specific frequency when a group of nuclear spins having an inherent magnetic moment is placed in a uniform static magnetic field.
It utilizes the phenomenon of resonant absorption to visualize the chemical and physical microscopic information of a substance. In this magnetic resonance imaging method, Fourier data of the spatial distribution of specific atomic nuclei present in the subject can be obtained as a magnetic resonance signal.

In the above-mentioned magnetic resonance imaging method, imaging of protons is generally performed, but most of them are images of water molecules because water molecules are overwhelmingly larger than molecules containing other protons. However, from the results of previous NMR analysis and MR spectroscopy of living organisms, information on substances with different chemical shifts is useful for elucidating the mechanism of living organisms and diagnosing diseases, and obtaining chemical shift information together with spatial distribution. Various imaging methods have been proposed.

For example, a method has been proposed in which the spatial distributions of two chemical shift nuclides are obtained separately by two pulse sequences with different timings of gradient magnetic field pulses (Radiology, 153, 189, 1984). However, this method has a problem that it is easily affected by the inhomogeneity of the static magnetic field, and it is difficult to obtain a good separated image of a chemical shift in a living body.

As another method, a composite high frequency magnetic field pulse whose pulse width, band and phase are controlled so as to distinguish two chemical shift nuclides band-wise and to excite with a phase difference of π / 2. A method is known in which the image of each nuclide is applied and separated into a real part and an imaginary part to obtain (Japanese Patent Laid-Open No. 63-84545). However, this method has a problem that the selection characteristic (slice characteristic) of the cross section is not good because the 180 ° selection pulse is used to select the cross section.

Further, as a method of imaging multiple cross sections in a magnetic resonance imaging apparatus, the multi slice method, which is well known in the prior art, or multiple cross sections are simultaneously excited by using a composite pulse in a high frequency magnetic field, and at that time Another method is proposed (Japanese Patent Laid-Open No. 20618/1992), in which the phase difference of nuclear spins in each cross section is varied by the above-mentioned composite high frequency magnetic field pulse to simultaneously obtain multi-section images in the phase encoding direction. However, all of these methods have a problem that there is a large limitation in visualizing multiple cross sections at high speed or visualizing more cross sections in the same time.

By the way, when there is a phase error due to a system error such as a characteristic of an excitation pulse or a phase difference between a reference wave and a signal in the detection at the time of processing a received signal when an image is picked up by a magnetic resonance imaging apparatus, the image quality deteriorates Resulting in. Therefore,
In order to correct such a phase error, a method (Japanese Patent Laid-Open No. 4-49419) for correcting the phase error from the phase value when the phase encode amount is zero has been proposed.

Further, a high-speed spin echo method, an echo planar method, an ultra-fast Fourier method, a divided scan method, etc. based on the RARE method, which is known as a method for high-speed imaging in a magnetic resonance imaging apparatus, In the method of collecting a plurality of echo signals during excitation of nuclear spins, the image quality remarkably deteriorates if there is a sampling error at the time of data acquisition due to the phase error or the error of the system or the pulse sequence. In these speed-up methods, the sampling error at the time of data collection due to the phase error or the error of the system or the pulse sequence is different for each echo signal generated at one time. Therefore, the above-described phase error correction method is sufficient. The image quality could not be expected to improve. Therefore, a method of detecting and correcting the deviation between the phase error value and the sampling point from the echo signal sequence obtained by executing a pulse sequence in which a phase encoding gradient magnetic field is not applied in advance with a predetermined pulse sequence (Japanese Patent Laid-Open No. 64-64- No. 869558) are known. However, in these correction methods, since the peak point of each echo signal is regarded as the representative point of each data for correction, the phase error in each echo signal or the phase variation between each echo signal is corrected. I couldn't. In fact, it is known that such a minute phase error at each sampling is one of the important causes of image quality deterioration.

In recent years, the magnetic resonance diagnostic apparatus is
Amplification, frequency, phase, and accuracy of high-frequency magnetic fields and received signals can be improved by digitization of high-frequency magnetic field transmitters and receivers, enabling effective use of amplitude, frequency, phase, and information. It's coming.

[0010]

As described above, in the conventional magnetic resonance imaging apparatus, many cross sections cannot be imaged at high speed, or a separated image of two different chemical shift nuclides can be imaged accurately. There was a problem that it could not be done.

In order to solve the above problems in the magnetic resonance imaging apparatus, the present invention provides a magnetic resonance imaging apparatus which accurately images separated images of two different chemical shift nuclides, and a high-speed multi-section imaging. It is an object of the present invention to provide a magnetic resonance imaging apparatus that can be realized.

[0012]

[0013]

[0014]

In order to solve the above conventional problems, the first invention applies a high frequency magnetic field and a gradient magnetic field to a subject placed in a uniform static magnetic field in accordance with a predetermined pulse sequence. Then, in a magnetic resonance imaging apparatus for detecting and imaging a magnetic resonance signal generated from the inside of a subject, a high frequency magnetic field having two different center frequencies is synthesized for the subject with a phase difference of π / 2. Means for applying a composite high frequency magnetic field pulse, and means for quadrature phase detecting and collecting a magnetic resonance signal generated from within the subject by applying the composite high frequency magnetic field pulse by this means,
Means for Fourier-transforming the magnetic resonance signals collected by this means to calculate data consisting of a real number part and an imaginary number part, and of the data obtained by this means, each of the real number part and the imaginary number part is imaged. Means and are provided.

[0015]

[0016]

[0017]

According to the present invention, two different cross sections or two different nuclides are simultaneously excited with a phase difference of π / 2.
Two different cross sections or two different nuclides can be simultaneously imaged in the real and imaginary parts, respectively, in the same time required to image the cross section or one nuclide.

[0018]

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings. [First Invention] FIG. 1 is a view showing the arrangement of a magnetic resonance diagnostic apparatus according to an embodiment of the present invention. In the figure, the static magnetic field magnet 1, the magnetic field homogeneity adjustment coil 3 and the gradient magnetic field generation coil 5 are driven by an excitation power supply 2, a magnetic field homogeneity adjustment coil power supply 4 and a gradient magnetic field generation coil power supply 6, respectively. As a result, a uniform static magnetic field and a gradient magnetic field having a linear gradient magnetic field distribution in three directions which are orthogonal to each other in the same direction are applied to the subject 7. A high frequency signal is sent from the transmitter 10 to the probe 9, and a high frequency magnetic field is applied to the subject 7. Here, the probe 9 may be used for both transmission and reception, or may be provided separately for transmission and reception. The magnetic resonance signal received by the probe 9 is quadrature-phase detected by the receiving unit 11, transferred to the data collecting unit 13, A / D converted, and then sent to the electronic computer 14. As described above, the excitation power supply 2, the magnetic field uniformity adjustment coil power supply 4, the gradient magnetic field generation coil power supply 6, the transmitter 10, the receiver 11, and the data collector 13 are all system controllers.
It is controlled by LA 12. The system controller 12 and the electronic computer 14 are controlled by the console 15, and the electronic computer 14 performs image reconstruction processing based on the magnetic resonance signal sent from the data collection unit 13,
Obtain image data. The image obtained is the image display 1
6 is displayed. The pulse sequence for acquiring the image data in the slice plane in the subject 7 in the present invention is controlled by the system controller 12.

In the present invention, it is desirable to digitize the transmitting section 10 and the receiving section 11 in order to accurately generate and detect the phase information of the high frequency magnetic field and the signal to be collected.

Next, a pulse sequence according to an embodiment of the present invention will be described. FIG. 2 shows the number 2 in this embodiment.
It is a figure which shows the pulse sequence of cross-section simultaneous imaging. In the figure, RF is a high-frequency magnetic field, G _s , G _r , and G _e are slice, read (read), and gradient magnetic fields in each direction of phase encoding, Sig (R _e ), Sig (I _m ), respectively.
Is an echo signal after quadrature detection. Also, H _1x ,
H _1y indicates the direction (phase) in which the high frequency magnetic field is applied.

In this embodiment, as the high frequency magnetic field pulse, a composite pulse whose pulse width, band and phase are controlled so that nuclear spins in two cross sections are excited with a phase difference of π / 2 at the same time is used. To do. As shown in FIG. 3A, when a slice gradient magnetic field is applied to the subject, the magnetic resonance frequencies slightly differ along the slice direction. Here, the high frequency magnetic field pulse used in this embodiment is composed of a component having a center frequency f ₁ and a predetermined band and a component having a center frequency f ₂ and a predetermined band in order to simultaneously excite two different cross sections. It is a composite high frequency magnetic field pulse. At this time, the phase difference between f1 and f2 is set to π / 2. When the above-mentioned composite high frequency magnetic field pulse is applied in the state where the slice gradient magnetic field Gs is applied, the nuclear spins of the cross section A and the cross section B shown in FIG. 3A are simultaneously excited. At this time, the phase of the composite high frequency magnetic field pulse is f ₁
Since the phases of the frequency components of and f ₂ are different by π / 2, the excited directions of the nuclear spins of the cross section A and the cross section B are deviated by π / 2 on the rotational coordinate. For example, the phase of f ₁ is 0 (H _1x ) with respect to the reference frequency, and the phase of f ₂ is π / 2.
(H _1y ), the nuclear spins of the cross section A are excited in the y ′ direction in the rotating coordinate system, and the nuclear spins of the cross section B are excited in the X ′ direction in the rotating coordinate system.

As in the spin echo method, the excited nuclear spins are applied with a read gradient magnetic field Gr, a phase encode gradient magnetic field Ge and a high frequency magnetic field RF in a predetermined order as shown in FIG. 2 to collect data. At this time, as shown in FIG. 2, the 180 ° high frequency magnetic field pulse used in the spin echo method also uses the composite pulse as described above.

After the quadrature phase detection of the data thus collected, the complex Fourier transform is performed to reconstruct the image.
At this time, 2 excited by the composite high frequency magnetic field pulse
The image of the cross section is separated into a real number part and an imaginary number part as shown in FIG. When a phase error occurs in the high-frequency magnetic field or the signal in the probe 9, the transmission unit 10, the reception unit 11 and the like during image reconstruction, it is possible to perform correction processing during image reconstruction.

FIG. 4 is a diagram showing a pulse sequence in the second embodiment of the present invention. This is an example of applying the present invention to a pulse sequence of the gradient echo method,
A composite high frequency magnetic field pulse with a flip angle α ° is used for the excitation pulse.

Further, by applying it to a multi-slice pulse sequence, it is possible to image a double section in the same time as in the conventional case, or if the number of sections is the same, a multi-section image can be obtained in half the imaging time. be able to.

Next, a third embodiment of the present invention will be described with reference to FIG.
8 to FIG. FIG. 5 is a diagram showing a pulse sequence in the third embodiment of the present invention. In this example, first, as shown in FIG. 5, after applying a 90 ° selective excitation pulse and a slice gradient magnetic field that selectively excite a desired region, spins of two types of chemical shift nuclides in a subject are applied. Simultaneously and simultaneously, a 180 ° high-frequency magnetic field pulse whose pulse width and band are controlled to rotate in the same phase is sequentially applied, and a read gradient magnetic field Gr and a phase encode gradient magnetic field G _e are applied in a predetermined order. To collect data. The signals collected at this time are, as shown in FIG. 6, spins M ₁ and M _{2 of} two different chemical shift nuclides.
(For example, water and fat in proton) have the same phase, and the first image reconstructed from this signal is the image of the two chemical shift nuclides overlapped with the same sign. Next, as shown in FIG. 7, the 90 ° selective excitation pulse that selectively excites a desired region and the spins of the two types of chemical shift nuclides in the subject are band-wise distinguished, and the π position is determined. pulse width so as to rotate simultaneously with the phase difference, with the band and the phase is sequentially applied to 180 ° radio-frequency magnetic field pulse is controlled, collecting data by applying the readout gradient magnetic field G _r, the phase encoding gradient field Ge in a predetermined order To do. In the signal collected at this time, as shown in FIG. 8, the phases of spins M ₁ and M ₂ of two different chemical shift nuclides are opposite to each other, and the second image reconstructed from this signal is The images of the chemical shift nuclides are overlapped with different signs. Images of the two different chemical shift nuclides can be separately obtained from the addition image and the subtraction image of the first and second images.

Next, a fourth embodiment will be described. In the fourth embodiment, first, as shown in FIG. 9, a 90 ° selective excitation pulse for selectively exciting a desired region and a slice gradient magnetic field are applied, and then two types of chemical shift nuclides in the subject are detected. A 180 ° high-frequency magnetic field in which the pulse width, the band, and the phase are controlled so that the spins M ₁ and M ₂ (for example, water and fat in protons) are band-wise distinguished and are simultaneously rotated with a phase difference of π / 2. The pulses are sequentially applied, and the read gradient magnetic field G _r and the phase encode gradient magnetic field G _e are applied in a predetermined order to collect data. As shown in FIG. 10, the signals collected at this time have the phases of the spins M ₁ and M ₂ of two different chemical shift nuclides of π /
Two differently, an image reconstructed from this signal can be obtained by separating the two chemical shift nuclides into a real part and an imaginary part, respectively.

Therefore, according to the present invention, as in the spin echo method, it is possible to collect signals without the influence of phase dispersion due to inhomogeneity of the static magnetic field, and to obtain an image with high separation accuracy of chemical shift nuclides. Obtainable.

The present invention can be variously modified and implemented without departing from the scope of the invention in addition to the above. For example,
The present invention can be applied to various imaging methods such as the echo planar method, and can perform high-speed multi-sectional imaging, high-speed water-fat separation imaging, and the like. [Second Invention] The magnetic resonance diagnostic apparatus according to the second invention has the same configuration as that shown in FIG. 1 of the first invention.

FIG. 6 is a diagram showing a pulse sequence of the first embodiment according to the present invention. FIG. 2A shows a pulse sequence of a typical echo planar method. SIGNAL indicates an echo signal observed at that time. In the figure,
The same symbols or abbreviations are used for the same elements as the first invention.

In the present embodiment, when this sequence (scan) is executed (STEP 6) to obtain an image, in addition to this sequence, the pulse without the phase encoding gradient magnetic field shown in FIG. 1B is applied. Sequence (prescan)
To collect data (STEP 1). A predetermined phantom or the subject itself is used for collecting the prescan data.

The procedure of processing the pre-scan data and the scan data is shown in FIGS. Below, Figure 1
2 and FIG. 13. First, the peak position and the phase value at the peak position are calculated for each echo signal of the prescan data (STEP 2). Data of each echo signal is rearranged and interpolation processing is performed so that the peak position of the signal comes from the peak position data to the center of the echo signal. Further, using the phase value, phase correction processing is performed on each data of the echo signal so that the phase becomes zero at the peak position of the echo signal (STEP 3). Next, the prescan correction data is subjected to one-dimensional Fourier transform in the readout gradient magnetic field direction for each echo signal (STEP 4). The phase value is calculated for each data point from the prescan Fourier data, and the phase correction map is calculated (STEP 5).

Next, the data obtained by the predetermined imaging pulse sequence of FIG. 1 (a) of STEP 6 is converted into a peak position for each echo signal of the prescan data calculated in STEP 2 and a peak position at that peak position. Performs rearrangement of data of each echo signal, interpolation processing and phase correction processing using the phase value (STEP
7). Next, the scan correction data is read for each echo signal and one-dimensional Fourier transformed in the gradient magnetic field direction (ST
EP 8). Further, this scan Fourier data is phase-corrected for each data point using the phase correction map calculated in STEP 5 (STEP 9).

Next, with respect to the phase-encoded gradient magnetic field direction of the phase-corrected scan Fourier data, the peak position of the signal of the zero component line of the read frequency and the phase value at that peak position are calculated (STEP 10). And
Based on the calculated peak position and phase value, data rearrangement, interpolation processing and phase correction processing are performed for each read frequency line (STEP 11). Finally, the scan correction data is one-dimensionally Fourier-transformed in the phase-encoding gradient magnetic field direction for each readout frequency line (STEP 12) to reconstruct an image.

The present invention can be variously modified and carried out within the scope not departing from the gist other than the above. For example,
The present invention includes a high-speed spin echo method shown in FIG. 11 (a), a divided scan method shown in FIGS. 11 (b) and 11 (c), and a three-dimensional imaging method to which these imaging methods are applied. It can be variously applied to an imaging method for collecting a plurality of echo signals during spin excitation. At this time, the pre-scan data may have the same data amount as the scan for obtaining a predetermined image, or even smaller data can be applied in combination with processing such as timely interpolation.

[0037]

According to the present invention, it is possible to shorten the time required for imaging two cross sections or two nuclides.

[0038]

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of a magnetic resonance diagnostic apparatus according to an embodiment of the present invention.

FIG. 2 is a diagram showing a pulse sequence for simultaneous imaging of two sections in the first embodiment of the first invention.

FIG. 3 is a diagram showing a principle of simultaneous imaging of two sections.

FIG. 4 is a diagram showing a pulse sequence for two-section simultaneous imaging in the second embodiment of the first invention.

FIG. 5 is a diagram showing a first pulse sequence for chemical shift nuclide separation imaging in the third embodiment of the first invention.

6 is a diagram showing a phase change of spins of the chemical shift nuclide in FIG.

FIG. 7 is a diagram showing a second pulse sequence for chemical shift nuclide separation imaging in the third embodiment of the first invention.

8 is a diagram showing a phase change of spins of the chemical shift nuclide in FIG.

FIG. 9 is a diagram showing a pulse sequence for chemical shift nuclide separation imaging according to the fourth embodiment of the first invention.

FIG. 10 is a diagram showing a phase change of spins of the chemical shift nuclide in FIG. 9.

FIG. 11 is a diagram showing a pulse sequence in one embodiment according to the second invention.

FIG. 12 is a diagram showing a pulse sequence and a prescan pulse sequence in the echo planar method.

FIG. 13 is a diagram showing a procedure of a correction process according to an embodiment of the second invention.

FIG. 14 is a diagram showing a pulse sequence for explaining another embodiment.

[Explanation of symbols]

1 ... Static magnetic field magnet 2 ... Excitation power supply 3. Magnetic field uniformity adjusting coil 4 ... Power supply for magnetic field uniformity adjustment coil 5 ... Gradient magnetic field generation coil 6 ... Power supply for gradient magnetic field generation coil 7 ... Subject 8 ... Sleeper 9 ... Probe 10 ... Transmission unit 11 ... Receiver 12 ... System controller 13 ... Data collection unit 14 ... Computer 15 ... Consol 16 ... Video display

─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-63-5732 (JP, A) JP-A-63-317143 (JP, A) JP-A-63-84545 (JP, A) JP-A-4- 15039 (JP, A) (58) Fields surveyed (Int.Cl. ⁷ , DB name) A61B 5/055

Claims (3)

(57) [Claims] 1. A magnetic field which applies a high-frequency magnetic field and a gradient magnetic field to a subject placed in a uniform static magnetic field according to a predetermined pulse sequence, and detects a magnetic resonance signal generated from the inside of the subject to visualize the magnetic field. In the resonance imaging apparatus, means for applying a composite high-frequency magnetic field pulse synthesized with a high-frequency magnetic field having two different center frequencies to each other with a phase difference of π / 2 to the subject, and this means for applying the composite high-frequency magnetic field pulse A means for quadrature-phase detecting and collecting a magnetic resonance signal generated from the inside of the subject by applying, and a Fourier transform of the magnetic resonance signal collected by this means to calculate data consisting of a real part and an imaginary part. And a means for imaging the real part and the imaginary part of the data obtained by this means, respectively. . 2. The magnetic resonance imaging apparatus according to claim 1, wherein the imaged images of the real part and the imaginary part are images of different cross sections. 3. The magnetic resonance imaging apparatus according to claim 1, wherein the imaged real part and imaginary part images are images of different chemical shift nuclides.