JPH09154831A - Magnetic resonance imaging method and device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To concurrently provide a fat-restrained water image and a fat image under the proper maintenance of an S/N ratio even in the case of an imaging sequence related to the FE method. SOLUTION: This method uses the pulse sequence where nucleus spins in a testee are selectively excited, and echo signals generated due to the selective excitation are collected through the reversal of gradient magnetic field in a reading direction. Also, the frequency band of RF pulses for the selective excitation is set at a value corresponding to or nearly corresponding to a frequency difference resulting from the chemical shift of the water and fat components of the testee. Also, slab thickness along a slicing direction is set at desired interest thickness. In this case, the gradient magnetic field in the slicing direction is set, so as to select a slicing direction zone two times as large as the slab thickness. Then, a pulse sequence is twice implemented by changing a slice encoding amount resulting from the gradient magnetic field along the slicing direction, and available echo signals are Fourier transformed in a three- dimensional way in three directions including the slicing direction. As a result, water and fat component images for adjacent slabs can be provided concurrently.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to MRI (magnetic resonance imaging) utilizing the magnetic resonance phenomenon of nuclear spins in a subject, and in particular, echo signals are acquired by reversing the polarity of a gradient magnetic field in the reading direction. The present invention relates to an MR imaging method and an MRI apparatus that can adopt a pulse sequence (FE method or the like) and separately obtain an image of a water component and an image of a fat component of a subject.

[0002]

2. Description of the Related Art An MRI apparatus for medical use magnetically excites nuclear spins of a subject placed in a static magnetic field with a high frequency signal of Larmor frequency, and an image is generated based on an MR signal generated by the excitation. It is a device for reconstructing and obtaining spectra.

When obtaining an MR image of a subject, fat existing in an imaging cross section usually causes an artifact due to a chemical shift. Therefore, MR signals from fat are not collected as much as possible. is required.

The pulse sequence is SE (spin echo)
According to the method, for example, RF pulses of 90 degrees and 180 degrees are used. Therefore, by differentiating the frequency bands of these two RF pulses between water and fat, only water is excited and generally only the water component is excited. A method for obtaining an image is known.

On the other hand, in the case of the FE (gradient field echo) method, the number of RF pulses used is one. Therefore, even if the frequency band of the RF pulse is narrowed, the fat outside the slice plane of interest is also excited due to the chemical shift, and the MR signal from the fat is mixed in the received signal. Therefore, some fat suppression techniques are also used for the FE method. This is shown in FIGS. 9 and 10.

The imaging method shown in FIG. 9 is known as the DIXON method in the case of the FE method. At the echo peak, the echo time TE is set so that the spin phases Δφ of water and fat are in opposite phases, and the state of “out phase” is created.
Cancel the echo signals of fat and water. Usually, the echo signal intensity of water is higher than that of fat, so that an echo signal of only water can be extracted.

The imaging method shown in FIG. 10 is a sequence of the FE method using a prepulse for fat suppression. Frequency component of fat proton spins of pre-pulse of
Presaturate with fresonance and then collect echo signals. Since the fat is pre-saturated, the echo signal at the time of image data acquisition hardly contains the echo signal from the fat, and the MR image (water image) of the proton spin of water can be obtained.

[0008]

However, the prior art related to the above-mentioned FE method also has the following inconveniences.

First, in the case of the imaging method using the DIXON method shown in FIG. 9, fat suppression is effective for a subject region in which water and fat are mixed, but when the imaged region is only fat, Since there is no offsetting effect, fat suppression is practically ineffective.

Further, in the case of the imaging method using the prepulse shown in FIG. 10, since the prepulse is used, the MTC
Due to the effect of (magnetization transfer contrast), the MR signal value from the water component is lowered, the S / N ratio is lowered, and the contrast is also lowered.

Further, both of the imaging techniques shown in FIGS. 9 and 10 are techniques for eliminating fat from an image from the standpoint of hating fat. Depending on the diagnosis, an image of fat alone may be useful together with an image of water only, but the imaging method described above cannot obtain both images at the same time.

The present invention has been made in view of such inconveniences caused by the prior art. Even in the imaging sequence according to the FE method, most of the fat-suppressed water is maintained while maintaining a good S / N ratio. The purpose is to simultaneously obtain an image of only components (water image) and an image of mostly fat only (fat image).

[0013]

In order to achieve the above object, the MR imaging method according to the present invention provides an R for selective excitation together with a gradient magnetic field in the slice direction on an object placed in a static magnetic field.
An F pulse is applied to selectively excite nuclear spins in a subject, and a pulse sequence for reading out an echo signal generated by this selective excitation by reversing a gradient magnetic field in the read direction is used. The value corresponding to or approximately equivalent to the frequency difference due to the chemical shift of the water component and the fat component of the subject is set to a value substantially equivalent to the slice direction slab thickness is set to the desired slice thickness of interest, and the slice direction gradient magnetic field is The echo sequence obtained by executing the pulse sequence twice by changing the slice encode amount by the slice direction gradient magnetic field while setting the slice direction region twice the slab thickness to be selected. Is three-dimensionally Fourier-transformed in three directions including the slicing direction to obtain an image of the water component corresponding to the slab thickness. Obtaining an image of fine fat components.

The pulse sequence is, for example, a pulse sequence based on the FE method.

Further, the MRI apparatus of the present invention applies a selective excitation RF pulse together with a gradient magnetic field in the slice direction to a subject placed in a static magnetic field to selectively excite nuclear spins in the subject, and this selective excitation. An apparatus adapted to use a pulse sequence for reading out an echo signal generated by the reversal of a gradient magnetic field in the read direction, wherein the frequency band of the selective excitation pulse is a frequency difference due to a chemical shift between the water component and the fat component of the subject. The slice thickness in the slice direction is set to a desired slice thickness of interest, and the gradient magnetic field in the slice direction is set to select a slice direction region having twice the slab thickness. In addition, the means for executing the pulse sequence twice by changing the slice encode amount by the slice direction gradient magnetic field and the pulse sequence. And means for three-dimensional Fourier transform of the echo signal obtained by the cans run in three directions including a slice direction, thereby obtaining an image of an image and fat component of the water component of the slab the thickness of.

[0016]

DETAILED DESCRIPTION OF THE INVENTION One embodiment of the present invention will be described below with reference to FIGS.

FIG. 1 shows a schematic configuration of an MRI (magnetic resonance imaging) apparatus according to this embodiment. This MRI
The apparatus includes a bed part on which the subject P is placed, a magnet part for generating a static magnetic field, a gradient magnetic field part for adding position information to the static magnetic field, a transmitter / receiver part for transmitting and receiving a high frequency signal, a system control and an image re-display. It is provided with a control / arithmetic unit responsible for the configuration.

The magnet section includes, for example, a superconducting magnet 1,
A static magnetic field power supply 2 for supplying a current to the magnet 1 is provided, and a static magnetic field H ₀ is generated in the axial direction (Z-axis direction) of the cylindrical opening (diagnosis space) into which the subject P is loosely inserted. The couch is configured such that the top plate on which the subject P is placed can be retractably inserted into the opening of the magnet 1.

The gradient magnetic field section includes a gradient magnetic field coil unit 3 incorporated in the magnet 1. The gradient magnetic field coil unit 3 includes three sets (types) of x, X, Y, and Z axis directions.
The y and z coils 3x to 3z are provided. x, y, z coil 3
x to 3z is 1 to homogenize the imaging region in the static magnetic field.
Also used for the next shimming.

The gradient magnetic field section further includes an x, y, z coil 3
A gradient magnetic field power supply 4 for supplying a current to x to 3z and a gradient magnetic field sequencer 5a in the sequencer 5 for controlling the power supply 4 are provided. The gradient magnetic field sequencer 5a includes a computer and a controller 6 that manages the entire apparatus.
A signal for instructing a data acquisition pulse sequence using a fat suppression pulse, such as the SE method, is received from (equipped with a computer). This allows the gradient magnetic field sequencer 5
a is X, according to the commanded pulse sequence,
The application and strength of each gradient magnetic field in the Y- and Z-axis directions is controlled, and these gradient magnetic fields can be superimposed on the static magnetic field H ₀ . In this embodiment, three axes X, Y, Z orthogonal to each other are used.
Z-axis slice direction gradient magnetic fields in the magnetic gradient G _S of the
And that in the X-axis direction is the readout direction gradient magnetic field G _R ,
In addition, the gradient magnetic field G in the phase encoding direction in the Y-axis direction
_{Let E.}

The transmitting / receiving unit is an RF coil 7 arranged near the subject P in the imaging space inside the magnet 1, and this coil 7
A transmitter 8T and a receiver 8R connected to each other, and an RF sequencer 5b (installed with a computer) in the sequencer 5 for controlling the operations of the transmitter 8T and the receiver 8R. Under the control of the RF sequencer 5b, the transmitter 8T and the receiver 8R supply the RF coil 7 with an RF current pulse having a Larmor frequency for exciting nuclear magnetic resonance (NMR), while the RF coil 7 receives the RF current pulse. The received MR signal (high frequency signal) is subjected to various kinds of signal processing to form a corresponding digital signal.

In addition to the controller 6 described above, the control / arithmetic unit also receives the digital data of the MR signal formed by the receiver 8R, computes image data and spectrum data, and a computed image. Storage unit 11 for storing data and display 1 for displaying images
2 and an input device 13. Arithmetic unit 10
Cooperates with the controller 6 to perform various operations such as shimming, placement of actually measured data in a two-dimensional Fourier space formed by a built-in memory, and Fourier transform for image reconstruction. Also do. The controller 6 controls the operation contents and operation timings of the gradient magnetic field sequencer 5a and the RF sequencer 5b while synchronizing them, and executes the processing of FIG.

Next, the operation of this embodiment will be described.

When the MRI apparatus is activated, the controller 6
Instructs the sequencer 5 and the arithmetic unit 10 to scan and collect image data twice (two shots) (see FIG. 2).
Step S1). This scanning and image data acquisition are executed based on the pulse sequence shown in FIG.

In this pulse sequence, as shown in FIG.
The 3DFT method according to the E method is used, the number of slice encodes is set to “2”, and the frequency band of the RF pulse P for selective excitation is equivalent to or substantially equal to the frequency difference due to the chemical shift between the water component and the fat component of the subject. It is set to the corresponding value. Furthermore, the slab thickness D in the slice direction (see FIG. 4)
Is set to a desired slice thickness of interest, and the slice-direction gradient magnetic field G _S used in the pulse sequence is set to slice-encode a slice-direction region twice the slab thickness D.

As shown in FIG. 3, first, a slice direction gradient magnetic field G _S is applied together with an RF pulse P having an arbitrary flip angle α. As a result, the desired slab thickness D is selectively excited. At this time, the designated region of the slab SB1 of interest is excited for the water component, but the fat component is deviated to the high side of the slice-direction gradient magnetic field G _S by one slab due to the chemical shift in the slice direction. The slab SB2 is excited.

After this excitation, the polarity of the slice-direction gradient magnetic field G _S is inverted (this inversion pulse is referred to as “pullback pulse G _SB ”) to pull back an extra encoding amount at the time of selective excitation. In the case of the first (first shot) slice encoding, for example, the slice direction gradient magnetic field G _S
The areas of the two shaded areas A and A'of
The area of the pullback pulse G _SB is determined so that the amount is the “zero encode” amount.

In parallel with the pullback pulse G _{SB in the} slice direction, a phase encode direction gradient magnetic field G _E and a read direction gradient magnetic field G _R with variable encoding amounts are applied. In each slice encoding (each shot), both magnetic fields G _E and G _R are applied so that two-dimensional echo data in the k space can be collected.

After that, the polarity of the read-out direction gradient magnetic field G _R is reversed from the minus side to the plus side to converge the phase of the spin and generate an echo signal. This echo signal is R
The signal is received via the F coil 7 and sent to the receiver 8R.
In the receiver 8R, the echo signal is subjected to predetermined signal processing such as quadrature detection to generate digital echo data. The echo data is sent to the arithmetic unit 10 and arranged in the three-dimensional k-space composed of a memory in accordance with each encoding amount.

The spin during the first (first shot) slice encoding collected in this way is, for example, as shown in FIG.
As shown in (a), the water component and the fat component are in phase.

Then, the scan and echo data acquisition relating to the second (second shot) slice encoding are carried out in the same procedure as described above. However, in the case of the second time,
The area of the pullback pulse G _SB formed by the polarity reversal of the slice direction gradient magnetic field G _S is different from that of the first time, and A ≠ A ″ in the area ratio in FIG. Encoded according to the encoding amount. Here, the area of the second pullback pulse G _SB is set such that the spin phase difference between water and fat in the echo data has an opposite phase as shown in FIG. 5B, for example.

When the echo data collection is completed twice (two shots) in this way, the controller 6 instructs the arithmetic unit 10 to perform the subsequent processing. The arithmetic unit 10 is 2
Three-dimensional Fourier transform (3DFT) is performed on the shot three-dimensional k-space data (step S2 in FIG. 2). That is, the Fourier transform is performed not only in the phase encoding direction and the reading direction on the three-dimensional k space but also in the slice direction. Since position information is also added to the slice direction by the slice-direction gradient magnetic field G _S , the image is also separated into the slice direction by the reconstruction process of the three-dimensional Fourier transform, and 2 corresponding to the slabs SB1 and SB2 in the slice direction. A two-dimensional reconstructed image is obtained. The two-dimensional image corresponding to one slab SB1 is a so-called "water image", which is mostly an image for the water component, and the two-dimensional image corresponding to the other slab SB2 is a so-called "fat image", which is almost only a fat component. It will be.

The reconstructed image data is displayed as needed and stored in the storage unit 11 (step S3 in FIG. 2).

In the present embodiment, as described above, 3D with the number of slice encodes = 2, which positively utilizes the chemical shift.
The FT system sequence is used. Thereby, FE
Even in the method, the chemical shift in the slice direction can be separated by the slice encoding of the 3DFT method, and the water image and the fat image at the slab positions adjacent to each other can be simultaneously obtained. Even without using a conventional pre-pulse, it is possible to obtain a water image in which a signal from fat is excluded, that is, a fat image in which fat is suppressed, and a fat image in which a signal from water is excluded, so that not only a water image , Fat images can also be positively used for diagnosis. Further, since the pre-pulse is not used, the MTC effect is not generated, and the signal value decrease can be avoided, thus improving the S / N ratio,
Moreover, the co-contrast can be improved. Furthermore, since the fat that has been chemically shifted in the slice direction is separated into water and fat by slice encoding using the 3DFT method, even a portion containing only fat can be effectively used.

Next, a modified embodiment according to the present invention will be described with reference to FIGS. The hardware configuration of the MRI apparatus according to these embodiments is the same as described above.

FIGS. 6 and 7 show slab positions in the slice direction when the present invention is applied to multi-slab imaging. The pulse sequence used is similar to that shown in FIG. 3, for example. FIG. 6 shows a case in which a pair of water and fat slab pairs is scanned in the slice direction while leaving a gap in the slice direction by a chemical shift of the fat in the slice direction. can get. Fig. 7 shows the development of this into the interleave method.
A water image and a fat image are obtained at each slab position.

FIG. 8 shows how the present invention is applied to multi-echo (here, two-echo) imaging based on the FE method. After selective excitation by the RF pulse P and the slice direction gradient magnetic field G _S , the time t of the readout direction gradient magnetic field G _R.
Echo signals are collected by two polarity inversions (changes) at 1 and t2. Before the first echo signal acquisition, "zero encoding" in the slice direction (pulse area A =) by the first pullback pulse G _SB1 of the slice direction gradient magnetic field G _S.
A ') is given. Before the second echo signal acquisition, an encoding amount (pulse area A ′ ≠ A ”different from“ zero encoding ”in the slice direction is generated by the second pullback pulse G _SB2 of the slice direction gradient magnetic field G _S. As a result, the slice encoding becomes equivalent to that shown in Fig. 3 and there is an advantage that an image can be picked up by one scan by multi-echo Note that in this system, the second echo E2 is the first echo. Since the signal strength is lower than that of E1, it is desirable to appropriately correct the second echo E2 for the signal strength.

The pulse sequence applied to the present invention is not limited to the FE method, but the SSFP method for echo-focusing with two or more RF pulses can also be used.

[0039]

As described above, the MR according to the present invention
According to the imaging method and the MRI apparatus, a pulse sequence for collecting echo signals is executed by reversing the gradient magnetic field in the reading direction, and the frequency band of the RF pulse for selective excitation is set to the frequency difference due to the chemical shift between the water component and the fat component. The slice thickness in the slice direction is set to a desired slice thickness of interest, and the gradient magnetic field in the slice direction is set to select a slice direction region twice the slab thickness. , The slice sequence is changed twice by changing the slice encode amount by the gradient magnetic field in the slice direction, and the echo signal obtained by executing the pulse sequence is three-dimensionally Fourier-transformed into three directions including the slice direction. The fat that has been chemically shifted to the slab can be separated by the slice encoding of the 3DFT method. Of an image in the image and fat component of the water component is can be obtained simultaneously. So, for example,
Even in the imaging sequence according to the FE method, while maintaining a good S / N ratio, a fat image in which almost all fat components are suppressed and a fat image in which almost all fat components are obtained are obtained at the same time.
Fat suppression can also be achieved, and fat images can be useful for diagnosis.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an example of an MRI apparatus according to an embodiment of the present invention.

FIG. 2 is a flowchart showing a processing example of a controller and an arithmetic unit.

FIG. 3 is a diagram showing an example of a pulse sequence based on the FE method used in the same embodiment.

FIG. 4 is a diagram showing a positional relationship between slabs of water and fat in a slice direction.

FIG. 5 is a diagram illustrating the phase states of spins of water and fat by two different slice encodings.

FIG. 6 is an explanatory diagram of a slab position of multi-slab imaging according to a modification.

FIG. 7 is an explanatory diagram of slab positions for multi-slab imaging according to another modification.

FIG. 8 is a pulse sequence for multi-echo imaging according to another modification.

FIG. 9 is a pulse sequence showing an example of a conventional imaging method based on the FE method.

FIG. 10 is a pulse sequence showing another example of the conventional imaging method based on the FE method.

[Explanation of symbols]

 1 magnet 2 static magnetic field power supply 3 gradient magnetic field coil unit 4 gradient magnetic field power supply 5 sequencer 6 controller 7 RF coil 8T transmitter 8R receiver 10 arithmetic unit 11 storage unit

Claims (3)

[Claims] 1. A selective excitation RF pulse together with a gradient magnetic field in the slice direction is applied to a subject placed in a static magnetic field to selectively excite nuclear spins in the subject, and an echo signal generated by the selective excitation is generated. In an MR imaging method using a pulse sequence acquired by reversing a readout direction gradient magnetic field, the frequency band of the RF pulse is set to a value corresponding to or substantially corresponding to a frequency difference due to a chemical shift between a water component and a fat component of the subject. Then, the slab thickness in the slice direction is set to a desired slice thickness of interest, the slice direction gradient magnetic field is set so as to select a slice direction region having twice the slab thickness, and the slice direction gradient magnetic field is set. The pulse sequence is executed twice by changing the slice encode amount by the Signal to the three-dimensional Fourier transform in three directions including the slicing direction, thereby MR imaging method and obtaining an image of an image and fat component of the water component of the slab the thickness of. 2. The MR imaging method according to claim 1, wherein the pulse sequence is a pulse sequence based on the FE method. 3. A nuclear magnetic spin in a subject is selectively excited by applying an RF pulse for selective excitation together with a gradient magnetic field in the slice direction to a subject placed in a static magnetic field, and an echo signal generated by the selective excitation is generated. MRI adapted to use pulse sequence acquired by reversal of read-out direction gradient magnetic field
In the apparatus, the frequency band of the RF pulse is set to a value corresponding to or substantially equivalent to the frequency difference due to the chemical shift of the water component and the fat component of the subject, and the slab thickness in the slice direction is set to a desired slice thickness of interest. The slice sequence gradient magnetic field is set so as to select a slice direction region having twice the slab thickness, and the pulse sequence is executed twice by changing the slice encode amount by the slice direction gradient magnetic field. And means for three-dimensionally Fourier transforming an echo signal obtained by executing this pulse sequence in three directions including a slice direction, thereby obtaining an image of a water component and an image of a fat component corresponding to the slab thickness. An MRI apparatus characterized by: