JPH0241137A - Phase correction in chemical shift imaging - Google Patents

Abstract

PURPOSE:To suppress phase jump on static magnetic phase non-uniform distribution to the min. and to reduce a separation error by performing phase correction for compensating the phase correction part to the static magnetic field non- uniform distribution with respect to scanning data subjected to phase correction. CONSTITUTION:The addition average value of all phase differences is calculated on the basis of calculated phase difference and set to a primary phase coefficient alpha1 to apply correction to non-uniform distribution. Further, in order to suppress a phase jump region due to the high order phase shift component on the non- uniform distribution, phase correction by a high order function is performed in a phase encode direction at even phase encode line. Furthermore, phase correction due to a high order function is performed in a phase encode direction at every frequency encode line. Next, phase correction is applied to the data from S2 scanning. At last, complex operation and absolute value processing are performed to obtain a separation image of water and fat. By this method, the error due to phase jump can be suppressed to the minimum and separation becomes accurate.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention corrects phase errors caused by static magnetic field averaging, etc. when obtaining separated images of water and fat in the body using chemical shift. Regarding the method. More specifically, the present invention relates to a phase correction method in chemical shift imaging that eliminates phase jump errors in this correction.

(Conventional technology) MRI (Magnetic Re5onance)
In ll1laQin (1), the same tomographic plane inside the body is separated into a proton image of only water and a proton image of only fat, by utilizing chemical shift, in which the resonance frequency of the same nuclide shifts due to the difference in the molecular structure of the two components. There is separation imaging to show.

First, the conventional Dickson method, which is one of the separation imaging methods for water and fat, will be explained. FIG. 5 is a diagram showing a pulse sequence for separation imaging. In Figure 5, t is the time axis, and RF is the static magnetic field direction (two directions).
R F (
This wave is called a 90° pulse or a 180° pulse depending on the rotation angle.
G51ice is a slice gradient that is applied simultaneously with a 90' pulse to selectively excite the tomographic plane, and SE is a slice gradient that is applied simultaneously with a 90' pulse to selectively excite the tomographic plane. This is a spin echo signal (hereinafter referred to as SE signal) observed when the phase of the scattered magnetization vector is reversed with a 180° pulse and reconverged. Gphase is a phase encode gradient magnetic field, Grea
d is a frequency encoded gradient magnetic field, which provides two-dimensional position information to the SE signal by making the Y direction correspond to phase information and the X direction to frequency information, respectively. In the Dickson method, 180
Two types of pulse sequences with different pulse application timings are used. That is, in the S1 scan, the application timing τ of the 180° pulse is set to τ = TE/2, which is the middle of the time TE from the 90° pulse until the spin echo is obtained, and in the S2 scan, the application timing τ is set to τ = TE/2, which is ε earlier than in the S1 scan.
Let E/2-ε.

The timing to obtain spin echoes in the S2 scan is S1
The same T6 as the scan is used.

Here, the above ε satisfies the following formula.

ε=T77τ了...f) σ: Chemical shift amount of water and fat f: Proton resonance frequency In the S1 scan, 180 at time TE/2.
Since the phases are reversed by the pulse, the phases of the water and fat magnetization vectors match at time T[. In the 32 scan, the phase is reversed by the ε fast < 180° pulse, so the fat magnetization vector rotates an additional 2ε time, and the water and fat magnetization vectors rotate by 180° at t=TE.
This causes a phase shift.

Therefore, the image data obtained by reconstructing the raw data obtained from each scan is as follows: Sl = W + F (2)
Sl=W-F...(3)
become. Here, W (≧0) is the proton density of water, F (
≧0) is the proton density of fat. Therefore, W= (
Sl +S2)/2...(4)F=
By calculating (31-S2>/2...(5) for each pixel, an image in which water and fat are separated can be obtained.

However, the actual 81 scan data includes a phase shift component due to static magnetic field inhomogeneity and the like. If such a phase shift component exists, the water and fat components cannot be separated according to equations (4) and (5), and separation errors such as shading occur. The phase shift due to static magnetic field inhomogeneity is caused by the following factors. First of all, the uniformity of the static magnetic field is not only affected by the non-uniformity caused by the magnet, but also because the human body enters the static magnetic field during actual imaging, so the rate of penetration between the air and each part of the human body is different, and the influence of the demagnetizing field The state of magnetic field inhomogeneity differs, the degree of uniformity changes, and it gets worse. Therefore, a phase shift occurs due to a shift in the Larmor frequency for each pixel within the imaging plane. Furthermore, due to the eddy current induced in the magnet bore material etc. when the frequency encode gradient magnetic field is applied, the falling edge of the rectangular gradient magnetic field Gread is blunted as shown by the dotted line in FIG. 5, and a residual gradient is generated.

Therefore, the SE double signal echo center in two scans with different 180° pulse application timing is δ as shown by the dotted line.
Since the data sampling point shifts accordingly, each scan data further includes a primary phase error component. Furthermore, a shift in the center phase, etc. also causes a primary phase error component. Therefore, as shown in Fig. 5, the application timing τ of the 180° pulse is set to ε later than the S1 scan time τ=
Perform S3 scan with TE/2±ε, perform two-dimensional Fourier transform on S2 scan and 83 scan data, obtain data matrix regarding static magnetic field inhomogeneity distribution from the obtained data, and phase correct the data of 82 or S3 from this data. However, a method has been proposed for obtaining a separated image of water and fat using this corrected data and data from the S1 scan.

Figure 6 (a), (b), [C) is 31.32.33
FIG. 3 is a diagram showing the phase relationship between magnetization vectors of water and fat in a scan. In FIG. 6, Real-IIIa
The coordinate system based on the ainary axis represents a phase plane based on the phase of the magnetization vector of water. As mentioned above, in the S1 scan, the phases of the magnetization vectors of water and fat match at t=TE, and in the 1, S2 and S3 scans,
The magnetization vectors of water and fat are out of phase by 180°. Furthermore, in the S2 and S3 scans, phase shift components due to static magnetic field inhomogeneity and the like are added by 4 θ and −θ, respectively. Therefore, we calculated θ for each pixel using the data obtained by two-dimensional Fourier transform of the S2 scan and 83 scan data.
By correcting the phase of the data of S2 or S3 using this non-uniform static magnetic field distribution, it is possible to remove separation errors caused by phase shifts.

(Problems to be Solved by the Invention) However, the above-described phase correction method in chemical shift imaging has the following problems. Since the detection range of the phase value of the non-uniform distribution is from -π to π, if this range is exceeded, a phase jump occurs in which the phase turns around. FIGS. 7(a) and 7(b) are diagrams showing the phase distribution due to the static magnetic field non-uniformity (hereinafter referred to as f&static magnetic field non-uniformity distribution). FIG. 7(a) is a diagram showing the static magnetic field non-uniform distribution of a 256x256 matrix, and FIG. 7(b) is a graph in which data of one phase encode line of this distribution diagram is plotted in the frequency encoding direction. As shown in the figure, if there are noise pixels or a first-order phase error, a phase jump occurs in which the phase value exceeds ±π. When this phase jump occurs, a two-component separation error occurs.

SUMMARY OF THE INVENTION An object of the present invention is to provide a phase correction method in chemical shift imaging that solves the above-mentioned problems, minimizes phase jumps due to non-uniform static magnetic field distribution, and reduces separation errors.

(Means for solving the problem) An S1 scan in which the time τ from application of a 90" RF pulse to application of a 180° RF pulse is TE/2, and an S2 scan in which the time τ is TE/2-ε. , said time τ
In a phase correction method in chemical shift imaging, an S3 scan is performed with TE/2+ε, the static magnetic field inhomogeneity distribution is determined from the data from the S2 scan and the S3 scan, and phase correction is performed to prevent phase shift.
A phase correction is applied to the static magnetic field non-uniform distribution to prevent a phase shift, and the static magnetic field non-uniform distribution after the phase correction is used to perform a phase correction to prevent a phase shift to the data from the Sl or S3 scan. and this phase-corrected Sl
Alternatively, the present invention is characterized in that the data of the S3 scan is subjected to phase correction to compensate for the phase correction for the static magnetic field non-uniform distribution.

(Operation) First, phase correction is performed to suppress the phase value within ±π for the static magnetic field nonuniform distribution, and the undirection after this correction is 82
Or, by performing phase correction on the data from the S3 scan, the phase correction amount originally included in the static magnetic field non-uniform distribution is 8
Since this is performed on data from 2 or S3 scans, separation errors caused by phase jumps can be removed and phase correction becomes accurate.

(Example) Hereinafter, the present invention will be explained in detail with reference to the drawings.

The scan sequence is 31. in FIG. 5, which is an improved version of the Dickson method. S2. Use S3 scan.

FIG. 1 is a flowchart showing a phase correction method according to an embodiment of the present invention. The phase correction method of this embodiment will be described below with reference to FIG. First, Sl, S2.
S Two-dimensional Fourier transform is applied to the complex raw data obtained by 3 scans. Each data Sl, 82°S3 is expressed by the following equation.

Sl=W+F...(6)
Sl = (W+F-exp (i π) 113X
El (iθ)...(7) S3 = (W+F'-eXD (iπ))ex
p(-iθ)...(8) Here, 12=-1, and exp(iθ) is a phase shift component due to static magnetic field inhomogeneity.

Next, from equations (7) and (8), the static magnetic field nonuniform distribution θo(r, w) (1≦“, v4≦256) is determined for each pixel according to the following equation.

θo(r, w)=tan −” 52)−(9) T ding ding ding Here, r is the frequency encoding axis and W is the phase encoding axis.

Next, noise levels Nt, cv- are set from the raw data. FIG. 2 is a diagram representing raw data on a data plane. The coordinates in Figure 2 are as follows: the horizontal axis corresponds to time and the vertical axis corresponds to the amount of phase encoding.0 The raw data in areas with a large amount of phase encoding is mostly noise, so the data in this area is A one-dimensional inverse Fourier transform is performed in the frequency encoding direction, and the result is subjected to absolute value processing to obtain the mean value Mean and variance σ0 of the data. The noise level NLivgt is NLl! VEL = Hean+ 3.5σO-
(10), and by setting the coefficient of σ0 to 3.5, 99.8% of the noise is included in NLl and VIIL.

This noise level NLEVEL is compared with the absolute value of the image data of each pixel, and the noise level NLEVE is determined.
1 for L or above, noise level NLEVI! Mask file V(r, w) with less than L as 0 (1≦r, w≦256
). This mask file is necessary for eliminating noise pixels in subsequent phase coefficient calculations. This mask file is used to connect the boundary between noise and the image to set an image area. FIG. 3 is a diagram showing the image area set in this manner. In FIG. 3, r is the frequency encoding direction, W
represents the phase encoding direction, and ro and wo are the center coordinates of the image in the frequency encoding direction and the phase encoding direction, respectively. Next, the average phase α0 in the vicinity of the image center within the image area is calculated according to the following equation.

Here, A is the number of effective pixels used to calculate the average phase αO. Using this average phase α0, the phase is corrected for the static magnetic field non-uniform distribution θo(r,v) according to the following equation.

θ1(r,w)=θo(r,w)xexp(-iαo
) - (12) In normal imaging, the scan is set so that the most interesting part is at the center of the image, so this process sets the phase at the center of the image to 0. Here, the phase used for calculating the average phase α0 is Since there is a possibility that there is a phase step difference due to a phase jump in the area where the image was imaged, the following processing is performed.First, the variance σ near the center of the image is calculated according to the following equation.

σ = ... (13) Here, when the average phase α0 is 0 or more, α1 = α0+
When σ is less than 0, α1−αO−σ is used, and θ1a(
r, v), the average phase α2 and variance σ2 are determined again from equations (11) and (13). And the variance σ2 is 0
．． When it is larger than 85σ, it is assumed that there is no phase step, and (
When the correction in equation 12) is enabled and the variance σ2 is less than 0.85σ, the correction in equation (12) is disabled, and α2 is set as the average phase, and the ○th-order phase is corrected again according to the following equation.

θBr, W)=θO(r, W)X exp(-iα2)
-(14) Next, find the primary phase coefficient in the frequency encoding direction. FIG. 4 is a diagram showing a phase correction method for a static magnetic field non-uniform distribution according to this embodiment. Figure 4 (a
), (b), and (c), each graph is a graph in which the phase of a one-phase encode line due to static magnetic field inhomogeneity is plotted in the frequency encoding direction. As shown in FIG. 4(a), a first-order phase shift that increases approximately in proportion to the amount of frequency encoding on a non-uniform distribution occurs due to a shift in the echo center caused by eddy currents. In order to reduce the phase jump due to this phase shift, a first-order phase coefficient is determined and corrected. First, the phase difference between adjacent pixels in the frequency encoding direction is determined according to the following equation.

Δ θ fr, W) = (θ 1(r+1.w)
-θ 1(r,w))x V fr+1. w) x
V (r, w) (r=1.3, 5..., 255
．． W=1.2,3. ..., 256)...f5)
Here, a phase difference in which one of the adjacent pixels is a noise pixel is excluded as 0 by the mask file. Here, at the same time, the phase jump level θLEVEL
is set to exclude phase differences involving phase jump pixels. For example, phase jump level θLl! Vl! L to 1.5
Assuming π, those in which the phase difference between adjacent pixels exceeds 1.5π are excluded as phase jump portions. Based on the phase differences calculated in this way, an average value of all the phase differences is determined, and this is set as the primary phase coefficient α1, and the non-uniform distribution is corrected according to the following equation.

θ2(r,v)=θ1(r,w) x e×p'(f-
i α1)・(r-ro))・・f6) In this way, the result of the first-order phase correction is
By the correction of 6 or more as shown in FIG. 6(b), the phase jump region due to the low-order (0th, 1st-order) phase shift component on the non-uniform distribution is suppressed.

Here, in order to further suppress phase jump regions due to higher-order phase shift components on the non-uniform distribution, phase correction using a higher-order function is performed for each phase encode line in the frequency encoding direction according to the following equation.

θ3(r w)=02 (r, W) X exp((
-iΣγ is w(r-ro)K)... (17) Here, 1 for γ is the next coefficient in the W-th phase encode line, and as in the calculation of the first-order coefficient, the frequency encode It can be determined from 1 to 17 of the phase difference in the direction.

Furthermore, phase correction is performed using a higher-order function in the phase encoding direction for each frequency encode line according to the following equation.

θ4(r,w)=03(r,v)xexp([-i
Σβ, r(III-1IIO)K)... (18) Here, βkr is the next coefficient at the W-th frequency encode line, and as before, it is the phase difference in the phase encode direction. On 17! It can be found from The result of, for example, second-order phase correction performed in this manner is shown in FIG. 4(C). This correction further suppresses phase jump regions due to higher-order phase shift components on the non-uniform distribution. That is, the correction using equations (17) and (18) corresponds to fitting the non-uniform component with a high-order polynomial. In a normal MR system, low-order static magnetic field non-uniform components are corrected using shim coils, etc., but when a human body enters the magnet, the static magnetic field non-uniform distribution changes.

Therefore, by performing low-order and high-order fitting on the non-uniform distribution, the non-uniform state in the case of a human body can be approximately approximated, and phase jumps can be prevented.

Next, using the non-uniform distribution θ4(r,w) corrected in order to suppress the phase jump in this way, the data from the S2 scan is subjected to phase correction according to the following equation.

S 2a = S 2 x ext) (-iθ4(r, w
)/2) -(19) Next, the phase amount corrected for the non-uniform distribution θ(', W) in order to suppress the phase jump should originally be performed on data from 82 scans. Therefore, the S2a data is sequentially corrected by the corrected phase amount according to the following equation.

52b=52axexp(-iαo/2)
−(20) (However, when using equation (14), αO
→α2) S 2c= S 2bx exp((-
i a 1) -(r-ro)/2)... (21)... (22) S 2e= S 2dx exp((-i Σβkr
(W-WO)” /21... (23) This completes the phase correction for 82 data, and finally,
Using the S2 data and S1 data that have been phase-corrected in this manner, complex operations and absolute value processing are performed according to the following equation to obtain a separated image of water and fat.

Water-l 1/2 (S 1 + 52e) Fat
-l 1/2(Sl -32e) l
-(24) As mentioned above, in the phase correction method in the chemical shift imaging of this example, in order to suppress the phase jump of the static magnetic field inhomogeneity distribution θ, phase correction using a 0.1, higher-order function is performed. Sequentially, the phase correction is performed on the S2 scan data in which the pulse is shifted by 180° due to the phase-corrected static magnetic field non-uniform distribution, and then phase correction is performed using the non-uniform distribution of the amount that deviates from the function, and then the S2 scan data is , since phase correction is performed for the function, errors due to phase jumps can be suppressed as much as possible, and separation becomes accurate.

Note that the present invention is not limited to the above embodiments, and various modifications can be made within the scope of the claims.

In this example, fitting of a high-order function was performed for each line of phase encoding or frequency encoding, but the entire non-uniform distribution was corrected by fitting with a function consisting of a two-variable polynomial according to the following equation. Also good.

(am + b* + Ck, dt,”・Constant)
-<24>Furthermore, phase correction was performed on the S2 scan data and used for separation in equation (23), but S3 scan data may also be used.Furthermore, the primary phase coefficient calculation using equation (15) is , instead of performing this for each phase encode line, it is also possible to take an average in the phase encode line direction, and then use the average line to obtain a linear coefficient in the frequency encode direction.

(Effects of the Invention) As explained above, according to the phase correction method in chemical shift imaging of the present invention, the following effects can be obtained.

(1) Calculate the static magnetic field non-uniform distribution from the S2 or S3 scan data, and use the static magnetic field non-uniform distribution that has been phase-corrected to prevent phase jumps, and use the static magnetic field non-uniform distribution to correct the phase of the S2 or S3 scan data to prevent phase shifts. , and then performs phase correction on this scan data to compensate for the phase correction for the static magnetic field inhomogeneity distribution, so the phase jump on the distribution is minimized, and the correction using this distribution Errors can be prevented, and separation errors caused by phase shifts included in S2 or S3 scan data can be removed. Further, since the information on the static magnetic field inhomogeneity within the target human body obtained by the S2 scan and 334 Yang data can be fully utilized for correction, the phase correction becomes accurate.

(2) Phase correction is performed sequentially using a 0.1-order function for the static magnetic field non-uniform distribution. Therefore, the deviation of the center frequency including the static magnetic field non-uniform component and the first-order phase error caused by eddy currents are eliminated. Since it can be removed from the uniform distribution, it is possible to reduce the phase jump region in the non-uniform distribution. Furthermore, phase correction using a higher-order function is sequentially performed on the non-uniform distribution of the static magnetic field. Therefore, the phase error caused by the non-uniformity of the static magnetic field caused by the human body entering the static magnetic field can be eliminated from the non-uniform distribution, so that the phase jump region in the non-uniform distribution can be further reduced. This makes it possible to eliminate separation errors caused by phase jumps.

[Brief explanation of the drawing]

FIG. 1 is a flowchart representing a phase correction method according to an embodiment of the present invention, FIG. 2 is a diagram representing raw data on a data plane according to an embodiment of the present invention, and FIG. 3 is a diagram representing an embodiment of the xylophone invention. Diagram showing image area settings, Figure 4(a), (
b) and (c) are diagrams representing a phase correction method for static magnetic field inhomogeneity distribution according to an embodiment of the present invention, FIG. 5 is a diagram representing a pulse sequence of conventional separation imaging, and FIG. 6 (
a), (b), and (c) are 31. of the conventional example. S2, S
FIGS. 7(a) and 7(b) are diagrams representing the phase relationship between the magnetization vectors of water and fat in three scans, and are diagrams representing the phase distribution due to static magnetic field inhomogeneity in the conventional example. Figure 2 Figure 3 (G) Figure 6 (b) (C) Figure 7 (G) (b)

Claims (7)

[Claims] (1) Generate a substantially uniform static magnetic field in the Z-axis direction, and apply 90 degrees and 180 degrees to the subject in this static magnetic field from the direction perpendicular to the Z-axis.
An RF pulse with a frequency f of 90° is applied, a frequency encode gradient magnetic field and a phase encode gradient magnetic field are superimposed on the static magnetic field, a spin echo signal from the subject is received TE seconds after the 90° RF pulse, and the spin echo signal from the subject is received. A method of obtaining separated images of water and fat using data obtained by two-dimensional Fourier transform of scan data from echo signals, the time τ from the application of the 90° RF pulse to the application of the 180° RF pulse is defined as TF/ 2, and the time τ is T.
S2 scan with E/2-ε and the time τ as TE/
Perform S3 scan with 2+ε, S2 scan and S
In a phase correction method in chemical shift imaging, which calculates the static magnetic field non-uniform distribution using data from three scans and corrects the phase shift due to this static magnetic field non-uniformity,
A phase correction is applied to the static magnetic field non-uniform distribution to prevent a phase shift, and a phase correction is performed to prevent a phase shift to the data from the S2 or S3 scan using the static magnetic field non-uniform distribution after the phase correction. and this phase-corrected S2
Alternatively, a phase correction method in chemical shift imaging, comprising performing phase correction on S3 scan data to compensate for the phase correction for the static magnetic field non-uniform distribution. However, the above ε satisfies the following formula. ε=1/(4・σ・f)…(1) σ: Chemical shift amount of water and fat (2) Phase correction in chemical shift imaging according to claim (1), wherein the phase correction for the static magnetic field non-uniform distribution includes a process of setting a noise level and removing noise pixels having a noise level or higher. Method. (3) The chemical according to claim 1, wherein the phase correction for the static magnetic field non-uniform distribution includes a process of performing zero-order phase correction using an average phase near the image center of the static magnetic field non-uniform distribution. Phase correction method in shift imaging. (4) Phase correction for the static magnetic field non-uniform distribution is performed by finding the first-order coefficient from the phase data in the frequency encoding direction, and
Claim (
1) Phase correction method in chemical shift imaging described above. (5) The phase correction method in chemical shift imaging according to claim (4), wherein the first-order coefficient is obtained from an average phase difference between adjacent pixels in the frequency encoding direction. (6) The phase in chemical shift imaging according to claim (1), wherein the phase correction for the static magnetic field non-uniform distribution includes a process of performing phase correction using a higher-order function in a frequency encoding direction or a phase encoding direction. Correction method. (7) The phase correction for the static magnetic field non-uniform distribution is performed by fitting the entire static magnetic field non-uniform distribution using a function consisting of a two-variable polynomial, and using this function to perform the phase correction on the S2 or S3 scan data. 2. The phase correction method for chemical shift imaging according to claim 1, further comprising a step of performing phase correction.