JP3137380B2 - Magnetic resonance imaging equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a magnetic resonance (MR: magn).
The present invention relates to a magnetic resonance imaging apparatus that obtains morphological information of a subject (living body) and functional information such as spectroscopy by utilizing a etic resonance phenomenon.

[0002]

2. Description of the Related Art A magnetic resonance phenomenon is a phenomenon in which a nucleus having a non-zero spin and magnetic moment placed in a static magnetic field resonatesly absorbs and emits only an electromagnetic wave of a specific frequency. Angular frequency ωo (ωo = 2π)
νo, νo; Larmor frequency). ωo = γBo where γo is the gyromagnetic ratio specific to the type of atomic nucleus, and Bo is the static magnetic field strength.

An apparatus for performing a living body diagnosis based on the above principle processes an electromagnetic wave having the same frequency as that induced after the above-mentioned resonance absorption, and performs a signal processing to obtain a nuclear density and a longitudinal relaxation time T1.
, The transverse relaxation time T2, the flow, the chemical shift, etc. are reflected in a non-invasive manner to obtain diagnostic information such as a slice image of the subject.

The collection of diagnostic information by magnetic resonance can excite and collect signals from all parts of the subject arranged in a static magnetic field. In response to the clinical demands of the above, an actual device performs excitation of a specific site and collection of its signal.

In this case, the specific part to be imaged is generally a slice part having a certain thickness, and an echo signal or FI from this slice part is generally used.
The magnetic resonance signal (MR signal) of the D signal is collected by performing a number of data encoding processes, and these data are collected.
The image of the specific slice part is generated by subjecting the data group to image reconstruction processing by, for example, a two-dimensional Fourier transform method. Such an image can be obtained for each nuclide such as proton or phosphorus, but in general, the imaging of proton is mainly performed.

On the other hand, recently, susceptibility (magnetic susceptibility) imaging method (Susceptibility Imaging M
ethod). This susceptibility imaging method is, for example, a method of obtaining information on calcium concentration as information on magnetic susceptibility due to calcium in bone marrow, by H. Rosenthal et al., Investigative Radiology Vol. 25, P.173 ~ P.178 (19
February 1990).

[0007] According to the report of this H.Rosenthal et al, bone marrow defatted sample, gradient shown in asymmetric spin echo method pulse sequence and 13 shown in FIG. 12
It states that imaging can be performed by a field echo pulse sequence, and information on the magnetic susceptibility can be obtained from a change in the signal intensity of the image.

[0008] Incidentally, shown in Figure 12, Susceptibility chromatography imaging method to realize the asymmetric spin echo method pulse sequence (ASE method pulse sequence) is
90 hours from the application of ° pulse until the application of the 180 ° pulse is Te / 2 (Te echo time), symmetry due to the time from application of the 180 ° pulse to the peak of the echo signal is Te / 2 the relative normal spin echo method pulse sequence, by delaying or Ru only <br/> earlier application time of the 180 ° pulse tau, is obtained by the asymmetric phased spin echo method pulse sequence. Here, the early time τ can be set arbitrarily.

The echo time Te in the gradient field echo method pulse sequence (FE method pulse sequence) for realizing the susceptibility imaging method shown in FIG. 13 can be arbitrarily set.

[0010]

The above-mentioned method has the following problems. That is, when data is collected, the effect of signal attenuation due to the chemical shift between water and fat acts in the voxel, and information other than information on the magnetic susceptibility is mixed in. There was a problem that you can not get. Therefore, an object of the present invention is to provide a magnetic resonance imaging apparatus capable of obtaining information on the magnetic susceptibility with high accuracy.

[0011]

Means for Solving the Problems The present invention has the following means in order to solve the above problems and achieve the object. That is, the invention according to claim 1 irradiates a high-frequency pulse for excitation to a subject in a static magnetic field to cause a magnetic resonance phenomenon in a specific region of the subject, and the magnetic resonance generated due to the phenomenon occurs. In a magnetic resonance imaging apparatus that collects signals and obtains diagnostic information about the specific region,

[0012] phase difference between the magnetic resonance signal according to proton imaging from the magnetic resonance signals and fat according to proton imaging from water Ri is Do integer times 360 °, and,
Magnetic resonance signals related to proton imaging from fat
A data acquisition means for executing a pulse sequence the parameter is set to minimum in such so that to obtain the magnetic susceptibility information based on a change in the signal intensity between a plurality of magnetic resonance images obtained by executing the sequence executing means And a data processing means.

[0013]

Further, the invention according to claim 2 is the invention according to claim 1.
Said data processing means definitive, the per repeating the pulse sequence, the parameters defining the pulse sequence, a magnetic resonance signal are in phase according to proton imaging from the magnetic resonance signals and fat according to proton imaging from water It is characterized by being changed so that

Still further, according to a fourth aspect of the present invention, in the first aspect, the data processing means stores the magnetic susceptibility information.
It is obtained in at least one of a Lorenz model and a Gauss model.

[0016]

According to the first, second, and third aspects of the present invention, the signal intensity change of a plurality of images imaged under in-phase conditions in which the signal attenuation depending on the chemical shift between water and fat is reduced. Therefore, it is possible to obtain information on the magnetic susceptibility with high accuracy.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a magnetic resonance imaging apparatus according to the present invention will be described below with reference to the drawings. That is, as shown in FIG. 1, the present apparatus includes a permanent magnet, a normal conducting magnet, a superconducting magnet, or a combination thereof as a magnet assembly capable of accommodating the subject P therein. (A shim coil for static magnetic field correction may be added) 1 according to
X-, Y-, and Z-axis gradient magnetic field coils 2 for generating a gradient magnetic field for providing positional information of a magnetic resonance signal induction site
And a gantry 4 internally provided with a transmission / reception system for transmitting a rotating high-frequency magnetic field and detecting an induced magnetic resonance signal (MR signal), for example, an RF coil 3 including a transmission coil and a reception coil. ing.

On the other hand, this apparatus comprises a transmitter 5 for controlling the transmission of RF pulses and a receiver 6 for controlling the reception of induced MR signals.
These provide a transmission signal to the RF coil 3 and collect an induced MR signal.

Further, the present apparatus comprises a gradient magnetic field power source 7 for performing excitation control of the respective gradient magnetic field coils 2 for the X, Y, and Z axes.
A sequencer 8 capable of executing a pulse sequence for generating a slice image and a pulse sequence for generating a spectroscopy, a computer 9 for controlling these and performing signal processing of a detection signal, and a reconstructed image. It has a display 10 for displaying and a console 11 for controlling a man-machine interface.

Here, a susceptibility / imaging executor (SI executor) 12 is connected to the sequencer 8. The SI executor 12 executes the ASE method pulse sequence shown in FIG. 12, the FE method pulse sequence shown in FIG. 13, and the IR ASE method (inversion recovery asymmetric (non- /> are stored symmetric) spin echo method) pulse sequence, IR-FE method shown in FIG. 6 (Invar di tio emission recovery field echo method) pulse sequence in the form of a computer program, also , And these pulse sequences are repeatedly executed under predetermined conditions.

A susceptibility imaging processor (SI processor) 13 is connected to the computer 9. The SI processor 13 performs a process for obtaining the susceptibility display shown in the present invention, for example, shown in FIG.

The SI execution unit 12 according to the first embodiment of the present invention
And by the execution of the SI processor 13, a magnetic resonance signal related to proton imaging from water in the voxel caused by a chemical shift effect between water and fat,
Τ (ASE) such that the phase difference from the magnetic resonance signal for proton imaging from fat is an integral multiple of 360 °.
A pulse sequence such as the ASE method pulse sequence based on FIG. 12 or the FE method pulse sequence based on FIG. 13 in which parameters such as applied to the FE method or Te (applied to the FE method) are set. The execution of the pulse sequence described above is controlled by the SI execution unit 12. By executing this pulse sequence, a plurality of magnetic resonance images for the same slice plane are obtained.

The magnetic susceptibility information (T ₂ ^* ) as shown in FIG. 2 based on the change in the signal intensity in the ROI (region of interest) at the same site between the plurality of magnetic resonance images. Effect information). This T ₂ ^* The acquisition of the effect information is controlled by the SI processor 13.

The SI executor 12 according to the first embodiment of the present invention
The operation of the SI processor 13 will be described in detail. First, S
The I executor 12 can execute a scan based on the ASE pulse sequence shown in FIG. In the scan, an ASE pulse sequence is executed on the same slice plane of the subject under conditions that are different for every four slice images, for example, for which τ values are to be collected.

That is, by setting τ to τ ₁ and repeating the ASE pulse sequence a predetermined number of times, an MR signal group is obtained. The computer 9 subjects the MR signal group relating to τ ₁ to, for example, Fourier transform processing to obtain a slice image IM ₁ . Next, τ is set to τ ₂ and the ASE method pulse sequence is repeated a predetermined number of times to obtain an MR signal group. The computer 9, to obtain a slice image IM ₂ the MR signal group according to tau ₂ for example by Fourier transformation. Next, τ is set to τ ₃ and the ASE method pulse sequence is repeated a predetermined number of times to obtain an MR signal group. The computer 9 subjects the MR signal group relating to τ ₃ to, for example, Fourier transform processing to obtain a slice image IM ₃ . Next, τ is set to τ ₄ and the ASE method pulse sequence is repeated a predetermined number of times to obtain an MR signal group. The computer 9 subjects the group of MR signals related to τ ₄ to, for example, Fourier transform processing to perform slice image I
Obtain the M _4.

Here, when Te is set to 80 msec and the chemical shift of fat is set to 3.6 ppm, τ ₁ , τ ₂ ,
τ ₃ and τ ₄ are sequentially changed so that the signals of water and fat have the opposite phase and the same phase. Here, τ ₁ is about ₁ to 3 msec, τ ₂ is about ₃ to 5 msec, and τ ₃ is 5 to 7 msec.
msec, and τ ₄ is about 7 to 9 msec.

[0027] In the computer 9, ASE method slice images IM ₁ according to tau ₁ under pulse sequence, tau slice image IM ₂ according to _2, the slice image IM ₃ according to tau _3,
slice images IM ₄ according to tau ₄ are stored.

The SI processor 13 receives a signal from the computer 9
Slice images IM ₁ according to tau ₁ under the ASE method pulse sequence, a slice image IM ₂ according to tau _2, a slice image IM ₃ according to tau _3, calls the slice image IM ₄ according to tau _4, Figure 2 Create the display shown.

That is, the slice images IM ₁ , IM ₂ , I
ROI ₁ and ROI ₂ are set at the same position of M ₃ and IM ₄ . Then, the signal strength values at the ROI ₁ in the slice image IM _1, the signal strength values at the ROI ₁ in the slice image IM _2, the signal strength values at the ROI ₁ in the slice image IM _3, in ROI ₁ in the slice image IM ₄ Find the signal strength value. With the horizontal axis being τ and the vertical axis being the signal strength, these four signal strength values are curve-fitted to draw the ROI ₁ curve shown in FIG.

ROI ₂ in slice image IM ₁
Signal strength value, the signal strength values at the ROI ₂ in the slice image IM _2, the signal strength values at the ROI ₂ in the slice image IM _3, the signal strength values at ROI ₂ in the slice image IM ₄ seek. Also in this case, the ROI ₂ curve shown in FIG. 2 is drawn by curve-fitting these four signal strength values with the horizontal axis being τ and the vertical axis being the signal strength.

In the above, the vertical axis represents the signal strength, which is the magnetic susceptibility information (T ₂ ^* Effect information). Then, since it is possible to know the change in the signal strength riding of a plurality of images imaged under the in-phase condition in which the signal attenuation depending on the chemical shift between water and fat is small, the magnetic susceptibility can be improved with high accuracy. Information can be obtained.

In the above example, A in which τ is changed between a plurality of images
The information on the magnetic susceptibility is obtained by executing the SE method pulse sequence. However, the magnetic susceptibility can also be obtained by executing the FE method pulse sequence in which Te is changed between a plurality of images as in the above-described example. Information can be obtained.

Next, the information on the magnetic susceptibility is obtained by executing an ASE method pulse sequence in which τ is changed between a plurality of images, or by executing an FE method pulse sequence in which Te is changed between a plurality of images. The operation of the first embodiment of the present invention will be described. That is, in the ordinary SE method,
Since the effect of the magnetic field inhomogeneity is compensated, it is not possible to obtain information on a small change in the magnetic susceptibility.
Use the E method. Further, the attenuation of the MR signal due to the magnetic field inhomogeneity including the effect of the magnetic susceptibility is generally expressed as follows. The case of the ASE method is as follows. S (τ) = So (τ = 0) · exp (−τ / T ₂ ^* ) In the case of the FE method, it becomes as follows. S (Te) = So (Te = 0) · exp (-Te / T 2 * ) · Exp (−Te / T ₂ )

Here, T ₂ ^* Is the time constant of signal decay determined by the inhomogeneity of the static magnetic field, and indicates information on the magnetic susceptibility. Further, T ₂ represents a transverse relaxation time constant.

Further, as described above, the degree of attenuation of the MR signal caused by the effect of the magnetic susceptibility varies depending on τ and Te. Therefore, by measuring a plurality of images, the time constant T ₂ ^* Information can be obtained. However, in the case of the FE method, T ₂ >> T ₂ ^* Is required, and T ₂ ^* Information is obtained. However, when water and fat exist in the voxel, a phase difference occurs between MR signals generated from water and fat due to a difference in resonance frequency due to the chemical shift effect.

In order to compensate for the phase shift of the MR signal due to the chemical shift, τ or Te can be limited. Assuming that the chemical shift amount is δ and the gyromagnetic ratio is Bo, the frequency difference δω due to the chemical shift is given as follows. δω = γ (δ · Bo) The phase shift amount φ generated between τ and Te is as follows. φ = δω · τ = γ (δ · Bo) τ φ = δω · Te = γ (δ · Bo) Te

Now, the chemical shift of water and fat (shift amount δwf is set to about 3.6 ppm) in a magnetic resonance imaging apparatus having a static magnetic field intensity of 0.5 T is calculated. In this case, τ or Te is 6.7 msec, and the phase shift amount φwf between water and fat is 180 °, and 360 ° at 13.4 msec. Here, FIG. 3 shows a state of a phase shift of magnetization between water and fat. In FIG. 3, Mw represents the magnetization of water, and Mf represents the magnetization of fat.

When the phase shift φwf between water and fat is about 180 °, the difference between the signals of water and fat (the magnetization Mw of water and the magnetization Mf of fat in FIG. 3) is imaged, so that the hydrogen nuclear density Images cannot be correctly reflected. In order to obtain an image of the same quality as the image obtained by the SE method, the phase shift φwf between water and fat must be an integral multiple of 360 ° (in the case of a 0.5 T magnetic resonance imaging device, an integral multiple of 13.4 msec). Τ or Te) must be selected.

Τ in the ASE method, Te in the FE method
Is continuously changed as shown in FIG. In FIG. 4, a signal change occurring at a cycle of an integral multiple of 2π / γ (δ · Bo) is a change in an MR signal value caused by a chemical shift effect between water and fat.

In the first embodiment of the present invention, the same slice plane is photographed a plurality of times under the condition that τ in the ASE method and Te in the FE method are integral multiples of 2π / γ (δ · Bo). T from the signal strength of each ROI in the
₂ ^* Or the effect of the magnetic susceptibility is evaluated.

Next, a second embodiment of the present invention will be described with reference to FIG. 5, FIG. 6, FIG. 7, and FIG. In the second embodiment, instead of the ASE method or the FE method, the IR · AS shown in FIG.
The E method and the IR / FE method shown in FIG. 6 are used. It has already been mentioned that these pulse sequences are stored in the form of a computer program in the SI execution unit 12 of FIG. The first half of the IR / ASE method shown in FIG. 5 is an IR method using a 180 ° pulse and a slice gradient magnetic field, and the second half is an ASE method in which τ is variable. In addition, IR ·
In the FE method, the first half is an IR method using a 180 ° pulse and a gradient magnetic field for slicing, and the second half is an FE method in which Te is variable. SP is a spoiler pulse for saturation processing.

In the case where the IR-ASE method shown in FIG. 5 is used, a plurality of IR-ASE methods having different τs are executed on the same slice plane.
At least two or more images are obtained, and a change in the MR signal value in one or more ROIs set for the images is checked, and T ₂ ^* To evaluate the effect of the magnetic susceptibility or the value of
reference).

If the IR / FE method shown in FIG. 6 is used, a plurality of IR / FE methods having different Tes are performed on the same slice plane, thereby at least two methods are performed.
A plurality of images or more are obtained, and a change in an MR signal value in one or more ROIs set for the images is checked, and T ₂ ^* Is evaluated or the effect of the magnetic susceptibility is evaluated (see FIG. 8).

Here, the addition of the IR method to the ASE method or the FE method contributes to minimizing the longitudinal magnetization component of the spin. This is because the influence of the chemical shift of fat is reduced. Then, a parameter τ in the IR-ASE method and a parameter Te in the IR-FE method are selected under the condition that the signal from water and the signal from fat are in phase. Then, in particular, is minimized longitudinal magnetization component of the fat, so that the signal from the water is sufficiently large, the pulse repetition interval Tr, TI time as a parameter in both methods, those suitable Yibin selected even flip angle of the RF pulse And

In the above example, any one of the ASE pulse sequence, the FE pulse sequence, the IR / ASE pulse sequence, and the IR / FE pulse sequence is executed independently.
Method), ASE method pulse sequence, FE method pulse sequence, IR / ASE method pulse sequence, IR / FE
At least one of the normal pulse sequences may be executed in combination to acquire a normal slice image and magnetic susceptibility information at the same time.

Further, the ordinary IR method (STI
R method), ASE method pulse sequence, FE method pulse sequence, IR / ASE method pulse sequence, IR / F
The combination with at least one of the E-method pulse sequences may be performed alternately for each encoding.

Further, the magnetic susceptibility information is represented by Lo shown in FIG.
It can be obtained in at least one of the renz model and the Gauss model shown in FIG. That is, AS
The model formula of the susceptibility component in the E method is generally expressed as follows. S (τ) / S (τ = 0) = exp (− (1 / T2 ^* ) τ)

However, according to rigorous analysis and experiments, this model can be assumed particularly when the magnetic field distribution at the voxel is sufficiently randomly distributed, for example, when the susceptor is at the vertebral body. The Gauss model fits both theoretically and experimentally. The Gauss model is expressed as follows. S (τ) / S (τ = 0) = exp (- (δ f 2/2) × τ 2 ) [Delta] _f is the frequency distribution shown in FIG. 11 of the signal in a voxel
It becomes a Gauss function and represents this standard deviation. In this case, [delta] _f is a parameter representing the size of the susceptor burr tea, may be displayed in search of [delta] _f the pixels. The present invention is not limited to the above embodiments, but can be implemented with various modifications without departing from the spirit of the present invention.

[0049]

As described above, according to the present invention, it is possible to provide a magnetic resonance imaging apparatus capable of obtaining information on the magnetic susceptibility with high accuracy.

[Brief description of the drawings]

FIG. 1 is a block diagram of magnetic resonance imaging according to the present invention.

FIG. 2 is a view showing a display example of information on magnetic susceptibility according to the present invention.

FIG. 3 is a diagram showing a phase relationship between water and fat.

FIG. 4 is a diagram showing a relationship between τ and signal strength.

FIG. 5 is a diagram showing an IR / ASE pulse sequence used in the present invention.

FIG. 6 is a diagram showing an IR / FE pulse sequence used in the present invention.

FIG. 7 is a diagram showing the relationship between τ and signal strength obtained when an IR / ASE pulse sequence is executed.

FIG. 8 is a diagram showing the relationship between Te and signal strength obtained when an IR / FE pulse sequence is executed.

FIG. 9 shows a case where magnetic susceptibility information is obtained by a Lorenz model.
The figure which shows a Lorenz function.

FIG. 10 is a diagram showing a Gauss function when magnetic susceptibility information is obtained by a Gauss model.

FIG. 11 is a diagram showing a frequency sentence in a voxel.

FIG. 12 is a diagram showing an ASE method pulse sequence.

FIG. 13 is a view showing an FE method pulse sequence.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Static magnetic field generator, 2 ... Gradient magnetic field coil, 3 ... RF coil, 4 ... Gantry, 5 ... Transmitter, 6 ... Receiver, 7 ... Gradient magnetic field power supply, 8 ... Sequencer, 9 ... Computer, 10
... Display, 11 ... Console, 12 ... SI executor, 13 ... SI processor.

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-63-109850 (JP, A) JP-A-63-73944 (JP, A) JP-A-4-312445 (JP, A) JP-A-62 34549 (JP, A) Rosenthal, et. a l. , "Magnetic Susceptibility Effects of Trabecular Bone on Magnetic Resonance Imaging of Bone Marrow", Investiative Radiology, 1990. 25, p173-p178 (58) Field surveyed (Int. Cl. ⁷ , DB name) A61B 5/055 JICST file (JOIS)

Claims (3)

(57) [Claims] An object in a static magnetic field is irradiated with a high-frequency pulse for excitation to cause a magnetic resonance phenomenon in a specific region of the object, and a magnetic resonance signal generated by the phenomenon is collected. the magnetic resonance imaging apparatus for obtaining diagnostic information about the specific area, Ri Do integer times the phase difference is 360 ° between the magnetic resonance signal according to proton imaging from the magnetic resonance signals and fat according to proton imaging from water, And from fat
A data collecting means for the magnetic resonance signal of the proton imaging to execute a pulse sequence the parameter is set to minimum in such so that the change in the signal intensity between a plurality of magnetic resonance images obtained by executing the sequence executing means And a data processing means for obtaining magnetic susceptibility information based on the magnetic resonance imaging apparatus. 2. The data processing means, each time the pulse sequence is repeated, sets a parameter defining the pulse sequence to a magnetic resonance signal for proton imaging from water and a magnetic resonance signal for proton imaging from fat. 2. The magnetic resonance imaging apparatus according to claim 1, wherein the values are changed so as to be in phase. 3. The magnetic resonance imaging apparatus according to claim 1, wherein said data processing means obtains the magnetic susceptibility information in at least one of a Lorenz model and a Gauss model.