JPH0370966B2 - - Google Patents

Description

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

(Conventional technology) In MRI (Magnetic Resonance Imaging), the same tomographic plane inside the body can be imaged using proton images of only water and images of only fat. There is separation imaging that separates and displays proton images.

First, the conventional Dickson method, which is one of the principles of a conventional chemical shift imaging device, will be explained. FIG. 6 is a diagram showing a pulse sequence for separation imaging. In Figure 6, t is the time axis, and RF is the RF (Radio
-Frequency) wave, and are called 90° pulse or 180° pulse depending on the rotation angle. Gslice 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. The phase of the magnetization vector that is scattered is
This is a spin echo signal (hereinafter referred to as SE signal) observed when it is reversed by a 180° pulse and converged again. Gphase is the phase encoded gradient magnetic field, Gread
is a frequency encoded gradient magnetic field, for example, the Y direction corresponds to phase information, and the X direction corresponds to frequency information.
Gives two-dimensional position information to the SE signal. The Dickson method uses two types of pulse sequences with different application timings of 180° pulses. That is,
In S1 scan, 180° pulse application timing τ
is set to τ = T _E /2, which is the middle of the time T _E from the 90° pulse until the spin echo is obtained, and is set to τ = T _E /2 - ε, which is ε earlier in the S2 scan than in the S1 scan. The timing at which spin echoes are obtained for the S2 scan is the same T _E as for the S1 scan.

Here, the above ε satisfies the following formula.

ε=1/4・σ・f …(1) σ: Chemical shift amount of water and fat f: Proton resonance frequency In the S1 scan, the phase is reversed by the 180° pulse at time T _E /2. Therefore, the phases of the magnetization vectors of water and fat match at time T _E. S2
In the scan, the phase is reversed by the 180° pulse as early as ε, so the fat magnetization vector becomes 2ε.
It rotates an extra amount of time, and at t=T _E , the magnetization vectors of water and fat have a phase shift of 180°. Therefore, the image data obtained by reconstructing the raw data obtained by each scan is S1=W+F (2) S2=W-F (3). Here, W (≧0) is the proton density of water,
F (≧0) is the proton density of fat. Therefore, by calculating W = (S1 + S2) / 2 ... (4) F = (S1 - S2) / 2 ... (5) for each pixel, it is possible to obtain an image in which water and fat are separated. .

However, actual S1 scan data includes a phase shift component due to static magnetic field inhomogeneity. 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 shedding occur. The phase shift due to static magnetic field inhomogeneity is caused by the following factors. first,
In addition to the non-uniformity of the static magnetic field caused by the magnet, the human body enters the static magnetic field during actual imaging, and as a result, the magnetic permeability of the air and each part of the human body differs.
In addition, the state of magnetic field inhomogeneity is different due to the influence of the demagnetizing field,
Uniformity changes and worsens. Therefore, a phase shift occurs due to a shift in the Larmor frequency for each pixel within the imaging plane. Furthermore, due to eddy currents 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. 6, and a residual gradient is generated. For this reason, the echo center of the SE signal in two scans with different 180° pulse application timings shifts by δ as shown by the dotted line, and the data sampling point shifts accordingly, so each scan data further includes a first-order phase error component. It will be done. Furthermore, a shift in the center phase, etc. also causes a primary phase error component. Therefore,
As shown in Figure 6, the application timing τ of the 180° pulse
An S3 scan was performed with τ = T _E /2 + ε which is ε slower than the S1 scan, and a data matrix regarding the static magnetic field inhomogeneity distribution was obtained from the data obtained by two-dimensional Fourier transformation of the S2 scan and S3 scan data. , an apparatus has been proposed in which the phase of S2 or S3 data is corrected and a separated image of water and fat is obtained using this corrected data and data from the S1 scan.

FIGS. 7a, b, and c are diagrams showing the phase relationship of the magnetization vectors of water and fat in S1, S2, and S3 scans. In FIG. 7, the coordinate system based on the Real-Imaginary 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 water and fat magnetization vectors match at t=TE, and in the S2 and S3 scans, the water and fat magnetization vectors have a phase shift of 180°. Furthermore, in the S2 and S3 scans, phase shift components due to static magnetic field inhomogeneity and the like are added by +θ and −θ, respectively. Therefore, by determining θ for each pixel using data obtained by two-dimensional Fourier transformation of S2 scan and S3 scan data, and correcting the phase of S2 or S3 data using this static magnetic field nonuniform distribution, the phase shift can be corrected. The resulting separation error can be eliminated.

(Problems to be Solved by the Invention) However, the chemical shift imaging apparatus as described above 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. Figures 8a and b show the phase distribution due to static magnetic field inhomogeneity (hereinafter referred to as static magnetic field inhomogeneity distribution).
FIG. FIG. 8a is a diagram showing the non-uniform static magnetic field distribution of a 256×256 matrix, and FIG. 7b is a graph in which the 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.

An object of the present invention is to provide a chemical shift imaging device that can solve the above-mentioned problems, minimize phase jumps on static magnetic field inhomogeneity distribution, and reduce separation errors with a small amount of calculations. Our goal is to provide the following.

(Means for Solving the Problems) The chemical shift imaging device of the present invention that achieves the above object is characterized by
An S1 scan where the time τ until pulse application was T _E /2, an S2 scan where the time τ was T _E /2 - ε, and an S3 scan where the time τ was T _E /2 + ε were performed. and S3 scan data to determine the static magnetic field non-uniformity distribution and correct the phase shift due to the static magnetic field non-uniformity, the device detects the noise level, removes the noise pixels, and selects the image area. , means for calculating a correction amount from the static magnetic field non-uniform distribution in the image area and applying phase correction to the static magnetic field non-uniform distribution to prevent phase jumps, and S2 by the static magnetic field non-uniform distribution after the phase correction Or a means for performing phase correction on data from S3 scan to prevent phase shift, and a means for performing phase correction on data from S3 scan, and this phase-corrected S2 or
The configuration includes means for performing phase correction on the S3 scan data to compensate for the phase correction for the static magnetic field non-uniform distribution.

(Function) First, an image area is selected, a correction amount is calculated from the static magnetic field non-uniform distribution in the image area, and a phase value of ±π is calculated for the static magnetic field non-uniform distribution.
Phase correction is performed to suppress the non-uniformity within the range, and the phase of the non-uniformity after this correction is corrected to the data from the S2 or S3 scan, and the phase correction of the correction amount originally included in the static magnetic field non-uniformity distribution is applied to the S2 or S3 scan. Since this is performed on data from the S3 scan, separation errors caused by phase jumps can be removed and phase correction becomes more accurate.

(Embodiments) Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a configuration diagram showing an embodiment of the present invention. The magnet assembly 11 has a space (hole) into which a subject is inserted, and a static magnetic field coil that applies a constant static magnetic field to the subject and a gradient magnetic field surrounding this space. Gradient magnetic field coils (gradient magnetic field coils are
x, y, and z axes), an RF transmitting coil that provides high-frequency pulses to excite the spin of the atomic nuclei within the subject, and a
Receiving coils and the like for detecting NMR signals are arranged. The static magnetic field coil, gradient magnetic field coil, RF transmitting coil, and receiving coil are connected to a static magnetic field power source (main magnetic field power source) 12, a gradient magnetic field drive circuit 13, an RF power amplifier 14, and a preamplifier 15, respectively. The sequence storage circuit 16 has means for generating a sequence signal for collecting scan data based on the Fourier method using a pulse sequence of the spin echo method in accordance with a command from the computer 21, and includes a gradient magnetic field drive circuit 13 and a gate modulation circuit 17. , an RF oscillation circuit (synthesizer) 18, a phase detection circuit 19, and an A/D converter 20, respectively. In this sequence storage circuit 16, S ₁ , S ₂ , S ₃
Data for scanning is stored.
The gate modulation circuit 17 is connected to the sequence storage circuit 16
RF oscillation circuit 18 by the timing signal from
modulates the high frequency signal from the RF power amplifier 14
give to The phase detector 19 is connected to the RF oscillation circuit 18
The output signal from the preamplifier 5 (NMR signal detected by the receiving coil) is phase-detected and input to the A/D converter 20 . A/D converter 20 is obtained via phase detector 19
The NMR signal is digitally converted and input to the computer 21. The computer 21 exchanges information with the operation console 22, switches the operation of the sequence storage circuit 16 and rewrites the memory in order to realize various scan sequences, and receives 20 data from the A/D converter. It is now used to perform image reconstruction calculations. This computer 21 detects the noise level and selects an image area, calculates a correction amount from the static magnetic field non-uniform distribution in the image area, performs phase correction for the static magnetic field non-uniform distribution, and performs phase correction on the static magnetic field non-uniform distribution. Phase correction is performed on the data from S2 or S3 scan by the static magnetic field non-uniform distribution after correction, and the phase corrected S2 or
Arithmetic processing is performed on the S3 scan data, such as performing phase correction to compensate for the phase correction for the static magnetic field non-uniform distribution.

Next, the operation of this embodiment will be explained. The scan sequence is S1 in Figure 6, which is an improved version of the Dickson method.
Use S2 and S3 scans. FIG. 2 is a flowchart showing the operation of one embodiment of the present invention. The operation of this embodiment will be described below with reference to FIG. First, the complex raw data obtained from the S1, S2, and S3 scans is subjected to a two-dimensional Fourier transform. Each data S1, S2, and S3 is expressed by the following formula.

S1=W+F...(6) S2={W+F・exp(iπ)}exp(iθ)...(7) S3={W+F・exp(iπ)}exp(-iθ)...(8) Here, i ² = -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 non-uniform distribution θ ₀ (r, w) (1≦r, w≦256) is determined for each pixel according to the following equation.

θ ₀ (r, w) = tan ⁻¹ Im (S2/S3)/Re (S2/S3) (9) where r is the frequency encoding axis and W is the phase encoding axis.

Next, set the noise level N _LEVEL from the raw data. FIG. 3 is a diagram representing raw data on a data plane. Regarding the coordinates in FIG. 3, the horizontal axis corresponds to time and the vertical axis corresponds to the amount of phase encoding. Raw data in areas where the amount of phase encoding is large is mostly noise, so data in this area is used, one-dimensional inverse Fourier transform is applied in the frequency encoding direction, and the result is subjected to absolute value processing to calculate the mean value of the data. Find the variance σ ₀ . The noise level N _LEVEL is N _LEVEL =Mean+3.5σ ₀ (10). By setting the coefficient of σ ₀ to 3.5, the noise
99.8% is included in N _LEVEL .

Compare this noise level N _LEVEL with the absolute value of the image data of each pixel and calculate the noise level.
Mask _file V (r, w) (1≦r _, w≦
256). This mask file is necessary to eliminate noise pixels in subsequent phase coefficient calculations. Next, using this mask file, an image area is set by connecting the boundary between the noise and the image. FIG. 4 is a diagram showing the image area set in this manner. Fourth
In the figure, r represents the frequency encoding direction, w represents the phase encoding direction, and r ₀ and w ₀ are the center coordinates of the image in the frequency encoding direction and the phase encoding direction, respectively. Next, the average phase α ₀ near the image center within the image area is calculated according to the following equation.

α ₀ =1/A _I2 〓 ^I=I1 _J2 〓 ^J=J1 (J,I)×θ ₀ (J,I) …(11) Here, A is the number of effective pixels used to calculate the average phase α ₀ be. Using this average phase α ₀ , the phase is corrected for the static magnetic field non-uniform distribution θ ₀ (r,w) according to the following equation.

θ1 (r, w) = θ ₀ (r, w) − α ₀ (12) In normal imaging, the scan is set so that the most interesting part is at the center of the image. Set the phase to 0
shall be. Here, since there is a possibility that there is a phase difference due to a phase jump in the region used for calculating the average phase α ₀ , the following processing is performed. First, the variance σ near the center of the image is calculated according to the following equation.

σ=[1/A _I2 〓 ^I=I1 _J2 〓 ^J=J1 {θ ₀ (J,I) −α ₀ } ² V(J,I)] ^1/2 …(13) Here, the average phase α ₀ When is greater than or equal to 0, α1=
α ₀ +σ, if less than 0, α1 = α ₀ −σ, (12)
θ1a phase-corrected for θ ₀ (r, w) according to the formula
Using (r, w), the average phase α2 and variance σ2 are determined again from equations (11) and (13). And the variance σ2 is 0.85σ
When it is larger, it is assumed that there is no phase step, and the correction of equation (12) is effective, and when the dispersion σ2 is 0.85σ or less,
Disabling the correction in equation (12) and setting α2 to the average phase,
Correct the zero-order phase again according to the following formula.

θ1 (r, w)=θ ₀ (r, w)−α2 (14) Next, the primary phase coefficient in the frequency encoding direction is determined. FIG. 5 is a diagram showing the phase correction operation for the non-uniform static magnetic field distribution of this embodiment. Fifth
In Figures a, b, and c, each graph is a plot of the phase of a one-phase encode line due to static magnetic field inhomogeneity in the frequency encode direction. As shown in FIG. 5a, 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.

Δθ(r,w)={θ1(r+1,w)-θ1(r,w)} ×V(r+1,w)×V(r,w) …(15) (r=1,3 ,5,...,255,W=1,2,3,
..., 256) Here, the phase difference when one of the two adjacent pixels is a noise pixel is excluded as 0 by the mask file. At this time, the phase jump level θ _LEVEL is set at the same time, and the phase difference related to the phase jump pixel is excluded. For example, if the phase jump level θ _LEVEL is 1.5π, 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,w)=θ1(r,w)−α1·(rr ₀ ) (16) The result of the first-order phase correction performed in this manner is shown in FIG. 5b. The above correction suppresses a phase jump region due to a low-order (0th, 1st-order) phase shift component on a nonuniform distribution.

Here, in order to further suppress the phase jump region 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. conduct.

θ3(r,w)=θ2(r,w) _o 〓 ^K=2 γ _KW (rr ₀ ) ^K …(17) Here, γ _KW is the k-th coefficient in the W-th phase encode line, As in the case of calculating the first-order coefficient, 1/of the phase difference of the kth order in the frequency encoding direction is
k! It can be found from

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

θ4(r,w)=θ3(r,w)− _o 〓 ^K=1 β _kr (WW ₀ ) ^K …(18) Here, β _kr is the k-th coefficient at the W-th frequency encode line. , 1/k! of the k-th phase difference in the phase encoding direction, as before. It can be found from The result of, for example, second-order phase correction performed in this manner is shown in FIG. 5c. This correction further suppresses phase jump regions due to higher-order phase shift components on the non-uniform distribution. That is,
Correction using equations (17) and (18) corresponds to fitting non-uniform components using a high-order polynomial. In normal MRI equipment, low-order static magnetic field inhomogeneity components are corrected using shim coils, etc., but when a human body enters the magnet, the static magnetic field inhomogeneity 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 to suppress the phase jump in this way, S2
The data from the scan is phase corrected according to the following equation.

S2a=S2×exp(−i θ4(r,w)/2 …(19) Next, in order to suppress the phase jump, the non-uniform distribution θ
The phase amount corrected by (r+w) should originally be applied to the data from the S2 scan. Therefore, the corrected phase amount is sequentially corrected to the S2a data according to the following equation.

S2b=S2a×exp(−iα ₀ /2) …(20) (However, when formula (14) is used, α ₀ → α2) S2c=S2b×exp {−iα1(rr ₀ )/2} …(21) S2d＝S2c×exp{(i×i _o 〓 ^K=2 γ _KW (r−r ₀ ) ^K /2} …(22) S2e＝S2d×exp{(−i _o 〓 ^K=1 β _kr (W−W ₀ ) ^K /2} …(23) This completes the phase correction for the S2 data,
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←｜1/2(S1+S2e)｜Fat←｜1/2(S1−S2e)｜…(24) As mentioned above, in the phase correction method in chemical shift imaging of this example, In order to suppress the phase jump of θ, the noise level is detected, noise pixels are removed, an image area is selected, and a correction amount is calculated according to the static magnetic field non-uniform distribution in the image area. Phase correction is performed to prevent phase jumps using 0-, 1-, and higher-order functions, and the pulses are shifted by 180° using the phase-corrected static magnetic field nonuniform distribution. Phase correction is performed based on the non-uniform distribution of the amount that deviates from the function, and then phase correction is performed on the S2 scan data by the amount of the function, so 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 the high-order function was performed for each line of phase encoding or frequency encoding, but the entire non-uniform distribution was corrected by fitting using a function consisting of a polynomial of two variables according to the following equation. It's okay.

f (r, w) = _o 〓 ^K=0 {a _k r ^k +b _k W ^k +C _k r ^k・_o 〓 ^L=2 d _L W ^L )} …(25) _k , b _k , C _k , d _L ...Constant) 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 first-order phase coefficient calculation using equation (15) is not performed for each phase encode line, but is averaged in the phase encode line direction.
A first-order coefficient may be obtained and used in the frequency encoding direction with respect to this average line.

(Effects of the Invention) As explained above, the chemical shift imaging device of the present invention detects the noise level, removes noise pixels, selects an image area, and detects static magnetic field inhomogeneity from S2 or S3 scan data in the image area. The static magnetic field non-uniform distribution is determined, and the phase is corrected to prevent phase jumps.
The configuration is such that a phase correction is performed on the S2 or S3 scan data to prevent a phase shift, and then a phase correction is performed on this scan data to compensate for the phase correction for the static magnetic field non-uniform distribution. Therefore, the following effects can be obtained. That is, it is possible to minimize the phase error caused by center frequency deviation and eddy current, and furthermore, the amount of calculation required for this purpose can also be suppressed to a small level.

Furthermore, by performing zero-order phase correction using the average phase near the image center of the static magnetic field inhomogeneity distribution, and calculating the first-order coefficient from the phase data in the frequency encoding direction and performing first-order phase correction, the deviation of the center frequency and the eddy current can be corrected. Since all major components of the caused phase error can be removed, phase correction can be performed efficiently.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram illustrating an embodiment of the present invention;
Fig. 2 is a flowchart showing the operation of an embodiment of the invention, Fig. 3 is a diagram showing raw data on a data plane of an embodiment of the invention, and Fig. 4 is an image of an embodiment of the invention. Figures 5a, 5b and 5c represent the phase correction method for static magnetic field non-uniform distribution according to an embodiment of the present invention, and Figure 6 represents the pulse sequence of conventional separation imaging. Figures 7a, b, and c are conventional examples of S1, S2,
A diagram showing the phase relationship between the magnetization vectors of water and fat in the S3 scan, and FIGS. 8a and 8b are diagrams showing the phase distribution due to static magnetic field non-uniformity in the conventional example.

Claims (1)

[Claims] 1. Generate a substantially uniform static magnetic field in the Z-axis direction, apply an RF pulse with excitation and inversion frequency f to the subject in the static magnetic field from a direction perpendicular to the Z-axis, and A frequency-encoded gradient magnetic field and a phase-encoded gradient magnetic field are superimposed on the static magnetic field, a spin echo signal from the subject is received T _E seconds after the excitation RF pulse, and scan data from this spin echo signal is two-dimensional Fourier-transformed data. This is an apparatus for obtaining separated images of water and fat using an S1 scan in which the time τ from the application of the excitation RF pulse to the application of the inversion RF pulse is T _E /2, and the time τ is T _E /2−. Perform an S2 scan with ε and an S3 scan with the time τ as T _E /2 + ε, obtain the static magnetic field inhomogeneity distribution from the data from the S2 scan and S3 scan, and correct the phase shift due to this static magnetic field inhomogeneity. In a chemical shift imaging device that performs chemical shift imaging, there is a means for detecting a noise level and removing noise pixels to select an image area, and a means for calculating a correction amount from the static magnetic field non-uniform distribution in the image area and adjusting the static magnetic field non-uniform distribution. a means for performing phase correction to prevent phase jump; a means for performing phase correction to prevent phase shift on data from S2 or S3 scan using static magnetic field non-uniform distribution after this phase correction; A chemical shift imaging apparatus comprising means for performing phase correction on the S2 or 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: amount of chemical shift of water and fat 2 The means for performing phase correction on the static magnetic field non-uniform distribution is based on 2. The chemical shift imaging apparatus according to claim 1, further comprising means for performing zero-order phase correction using an average phase. 3. The chemical according to claim 1, wherein the means for performing phase correction on the static magnetic field non-uniform distribution includes means for calculating a first-order coefficient from phase data in the frequency encoding direction and performing first-order phase correction. Shift imaging device. 4. The chemical shift imaging according to claim 3, wherein the means for determining the first-order coefficient and performing the first-order phase correction includes means for determining the first-order coefficient from an average phase difference between adjacent pixels in the frequency encoding direction. Device. 5. The chemical shift imaging apparatus according to claim 1, wherein the means for performing phase correction on the static magnetic field non-uniform distribution includes means for performing phase correction using a higher-order function in a frequency encoding direction or a phase encoding direction. . 6. The means for performing phase correction on the static magnetic field non-uniform distribution is means for performing fitting on the entire static magnetic field non-uniform distribution using a function consisting of a polynomial of two variables, and
Means for performing phase correction on the S2 or S3 scan data to compensate for the phase correction for the static magnetic field non-uniform distribution uses the function to perform S2 or S3 scan data.
2. The chemical shift imaging apparatus according to claim 1, further comprising means for performing phase correction on scan data.