JPH0324852B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an apparatus for correcting distortion of Fourier method NMR (nuclear magnetic resonance) images caused by static magnetic field inhomogeneity.

(Prior art) NMR images captured using the Fourier method (including the spin warp method (Fig. 5)) have the following characteristics:
Position/density distortion occurs in the read direction y. If the readout gradient magnetic field is Gy, the original coordinates (x, y ₀ )
The point that has reached the point moves to the point with coordinates (x, y) according to the relationship y = y ₀ + D (x, y ₀ )/Gy, and the concentration at that point also changes to its original value (1-δ/ ∂y′D(x, y′) | _y ′ _=y0 /Gy).

(Problem to be solved by the invention) In order to correct such position/concentration distortion, conventionally, as shown in FIG. I was making corrections, but
In order to correct the image of an arbitrary cross-section, the static magnetic field strength is measured over the entire three-dimensional FOV, the non-uniformity information is memorized, and when a certain cross-section is imaged, the A procedure was required to extract non-uniform information from three-dimensional information. Before taking this image, it takes time and effort to measure the static magnetic field strength of the entire imaging area, and a large amount of storage space is required to store this information.In addition, when correcting the image, three-dimensional static magnetic field strength information is required. There was a problem in that complicated and time-consuming calculations were required to extract information on the plane corresponding to the image position from the image position.

The present invention has been made in view of these points, and its purpose is to capture multiple images of the same position with different dephase amounts without measuring the three-dimensional static magnetic field strength distribution in advance for correction. An object of the present invention is to provide a Fourier method NMR image distortion correction device that can correct the position and density distortion of a complex image based on the phase difference of the image.

(Means for Solving the Problems) A first invention for solving the above problems is a distortion correction device for nuclear magnetic resonance images in a nuclear magnetic resonance imaging apparatus using the Fourier method. an image reconstruction device that obtains a plurality of complex images captured by the image reconstruction device; a phase difference calculation device that calculates a phase difference for each pixel of a plurality of images related to the same cross section obtained by the image reconstruction device; A distortion correction processing device that uses the phase difference for each pixel obtained by the calculation device to correct distortion due to nonuniform static magnetic field strength of the image that is the basis of the calculation for each pixel. The second invention is a nuclear magnetic resonance image distortion correction device in a nuclear magnetic resonance imaging apparatus using the Fourier method, in which a complex image obtained by imaging the same cross section using a 180° echo inversion method and a readout gradient inversion method are used. an image reconstruction device that obtains a complex image captured by the image reconstruction device, a phase difference calculation device that calculates a phase difference for each pixel of a plurality of images related to the same cross section obtained by the image reconstruction device, and the phase difference calculation device A distortion correction processing device that uses the obtained phase difference for each pixel to correct distortion due to non-uniform static magnetic field strength of the image on which the phase difference is calculated. It is.

(Example) Hereinafter, an example of the present invention will be described in detail using the drawings. First, the principle of the present invention will be explained.
In Fourier method imaging, the static magnetic field strength inhomogeneity is D
(x, y), and the readout gradient magnetic field is Gy, positional distortion occurs in the readout direction y by y ₁ = y ₀ + D (x, y ₀ )/Gy, and the position originally at the coordinates (x, y ₀ ) The new point moves to the point with coordinates (x, y ₁ ), and the concentration also increases by (1−δ/∂y′D(x, y′) | _y ′ _=y0 /Gy).

In Fourier method imaging, generally, as shown in Figure 5, the time T _E1 from 90° pulse to 180° pulse,
By making the time T _E2 from the 180° pulse to the center (peak) of the echo signal equal, phase distortion due to non-uniform static magnetic field strength is suppressed, but in the present invention, at least one of the two imaging By taking images so that _E1 ≠T _E2 , phase distortion is artificially generated, and conversely, the static magnetic field strength distribution can be determined from this.

If ΔTi=(T _E2 −T _E1 )i, the readout gradient is
If the image is captured twice without changing the size of Gy, ΔT ₁ ≠
ΔT ₂ , and as shown in FIG. 2, it is possible to obtain two complex images (Image 1 and Image 2) with the same amount of distortion but different phases.

The difference between the phase difference Δθ (=θ ₁ −θ ₂ ) of each pixel and the static magnetic field strength non-uniformity D (x, y) is γD (x, y) (ΔT ₁
-ΔT ₂ ) = Δθ (however, γ is the rotating magnetic field ratio), so from this, D(x, y) = Δθ/{γ(ΔT ₁ −ΔT ₂ )} and the non-uniformity of the static magnetic field strength. can be known. The positional deviation in the readout direction at this time is Δθ/{γGy
(ΔT ₁ −ΔT ₂ )}.

Alternatively, an echo signal by a 180° pulse and an echo signal by gradient inversion as shown in Fig. 6 may be used. In that case, if ΔT ₂ is the time from the 90° pulse to the center of the echo signal, then It can be treated in the same way as above.

If the amount of positional deviation can be estimated from two images in this way, the point (x, y ₁ ) of each image,
(x, y ₂ ) can be returned to the state (x, y ₀ ) without deviation. Also, at this time, by multiplying the pixel density of each image by the conversion Jacobian 1+(∂D/∂y)/Gy, it is possible to obtain images 1' and 2' in which density distortion has also been corrected.

The Δθ mentioned here generally has an ambiguity with a period of 2π, so Δθ for all pixels
is not necessarily determined. In this case, it is necessary to adjust the amount of dephasing and the like so that the entire range of phase change is suppressed to T such that |Δθ|≦T≦π. In such cases, centering on the value at an arbitrary pixel,
Since there are no values that are more than ±π apart, an overall ambiguity of 2π remains, but relative ambiguity within the image disappears. Even when distortion correction is performed, although the overall image may be shifted unnecessarily, it will not be partially shifted incorrectly.

The first embodiment of the present invention based on this principle is described below.
As shown in the figure. In the figure, 1 is an image reconstruction device that receives echo signals measured using the spin warp method and performs two-dimensional inverse Fourier transform to reconstruct a cross-sectional image of the subject. This is to obtain a complex image. 2 is a phase difference calculation device that calculates a phase difference from a plurality of images obtained by the image reconstruction device 1, and 3 is a source for calculating the phase difference based on the phase difference obtained by the phase difference calculation device 2. This is a distortion correction processing device that corrects the phase distortion of a distorted image and also corrects the density distortion. The image obtained by this distortion correction processing device 3 can be displayed on a display device (not shown).

The operation in such a configuration will be explained in detail below. In the spin warp method shown in Fig. 5, the same cross section is scanned by changing it to T _E1 and T _E2 ,
Two complex images with the same amount of distortion but different phases are obtained. Or the spin warp method in Figure 5 and the
Two complex images with different phases related to the same cross section are obtained using the spin warp method using gradient inversion shown in the figure. Using these two images obtained by the image reconstruction device 1, the phase difference calculation device 2 divides the two complex images for each pixel and calculates the resulting phase. Expressed in the formula, the phase difference Δθ is ΔO=tan ^-1 (lm[a ₁ + jb ₁ /a ₂ +jb ₂ ]/Re[a ₁ +jb ₁ /a
₂ + jb ₂ 〕). Here, lm[・] is an operator that extracts the imaginary part, Re[・] is an operator that extracts the real part, j is the imaginary unit, and a ₁ + jb ₁
is the pixel density of image 1, and a ₂ +jb ₂ is the pixel density of image 2. Here, when a ₁ + jb ₁ or a ₂ + jb ₂ is less than the threshold value T ₁ , Δθ cannot be calculated, so an appropriate label is given. Here, the above calculation is performed for all pixels. Since the values of each phase are set in advance so that they are separated by a maximum of T, the following estimation is performed. In other words, take one arbitrary point and
The value of 2π is adjusted so that the difference is within ±T from Δθ here, and continuity is given to the value of Δθ within the region. After this, we cut out the two-dimensional distribution of Δθ in an M×M window (Fig. 3 C) centered on each pixel point (i, j), and performed addition using only the points for which Δθ was found correctly. Do the average. The average value at this time becomes the output of the estimation processing result. Through this process, the value of Δθ is estimated even for pixels at locations (A in Figure 3 A) that could not be determined before the estimation process, and smoothing is performed over a wider range, so generally only gradual changes occur. memory regarding static magnetic field strength can be estimated correctly.

The distortion correction processing device 3 calculates the shift amount as y ₁ =y ₀ +{Δθ/γGy (ΔT ₁ −ΔT ₂ )} based on the value Δθ estimated by the phase difference calculation device 2. Here, (x, y ₀ ) indicates the correct position of the point, and (x, y ₁ ) indicates the position after undergoing positional distortion.
To correct positional distortion, as shown in Figure 4, the image density at the required position (x, y ₀ ) in the corrected image is adjusted to the pixels near the corresponding position (x, y ₁ ) in image 1. Calculate by interpolation from the concentration. Density distortion correction is performed by multiplying the pixel density by a separately calculated Jacobian of transformation.

It should be noted that the present invention is not limited to the above-mentioned embodiments, but may be implemented as follows.

(1) When using three or more images, the phase difference can be calculated using a combination of several images and averaged, which can be used to improve the estimation accuracy of the phase difference.

(2) The phase difference may be calculated by calculating the phase of each image separately and then performing subtraction.

(3) The estimation process may be performed by applying a two-dimensional function to the whole or piecewise.

(4) Since the non-uniformity of the magnetic field strength generally changes quite gradually, it is possible to calculate the phase difference after reducing the size of the two images, or to calculate the phase difference every few points. .

(5) Distortion correction may be performed on a complex image and converted into real numbers when displayed, or may be performed on an image that has been converted into real numbers in advance.

(6) Since the two images have the same amount of positional and density distortion, the S/N ratio can be efficiently increased by converting them into real numbers and averaging them before correcting the distortion. Of course, the same effect can be obtained by performing distortion correction separately and averaging.

(7) With T _E1 + T _E2 constant, in order to make the degree of reflection of T ₁ and T ₂ the same in all images, and to make the distortion in the readout direction the same in all images, the amount of dephasing is necessary. By changing the size of the pulse and shifting the timing of transmitting the 180° pulse.
It is effective to change T _E1 as well.

(Effects of the Invention) As described above, the present invention has the following effects.

(1) Since the phase difference is found based on multiple images whose distortion must be corrected and this is used to correct the distortion of the image, the static magnetic field inhomogeneity distribution for correction should be measured in advance. There's no need.

There is no need to store the static magnetic field non-uniform distribution for correction.

There is no need for complex calculations to extract values at arbitrary two-dimensional cross sections from discrete three-dimensional data.

Therefore, it has become easier to correct arbitrary cross sections.

(2) Since it is possible to always make corrections according to the latest non-uniform distribution, it is possible to eliminate the effects of changes over time, the proximity of magnetic materials, and the introduction of magnetic materials.

[Brief explanation of the drawing]

Figure 1 is a diagram showing an embodiment of the present invention, Figure 2 is a diagram explaining the principle of the invention, Figures 3 and 4 are diagrams explaining the operation, and Figure 5 is a 180° echo inversion. FIG. 6 is a diagram showing a pulse sequence of the gradient inversion method, and FIG. 7 is a diagram explaining conventional distortion correction. 1... Image reconstruction device, 2... Phase difference calculation device, 3... Distortion correction processing device.

Claims (1)

[Scope of Claims] 1. A nuclear magnetic resonance image distortion correction device in a nuclear magnetic resonance imaging apparatus using the Fourier method, comprising: , a phase difference calculation device that calculates a phase difference for each pixel of a plurality of images related to the same cross section obtained by the image reconstruction device; A distortion correction device for a nuclear magnetic resonance image, comprising: a distortion correction processing device for correcting distortion due to non-uniform static magnetic field strength of an image that is the basis for calculation of a phase difference; 2. In a nuclear magnetic resonance image distortion correction device in a nuclear magnetic resonance imaging device using the Fourier method, image reconstruction is performed to obtain a complex image of the same cross section captured using the 180° echo inversion method and a complex image captured using the readout gradient inversion method. a configuration device; a phase difference calculation device that calculates a phase difference for each pixel of a plurality of images related to the same cross section obtained by the image reconstruction device; A distortion correction device for a nuclear magnetic resonance image, comprising: a distortion correction processing device for correcting distortion due to non-uniform static magnetic field strength of the image that is the basis for calculating the phase difference;