CN105719245A - Method for removing annular artifacts caused by CT detection element faults by use of projection data - Google Patents

Abstract

The invention relates to a method for removing annular artifacts caused by CT detection element faults by use of projection data. The method comprises the following steps: 1, introducing the projection data to obtain a matrix A, detecting a position of a fault area in the matrix A, and obtaining a matrix B by performing linear interpolation on a column corresponding to the fault area by use of two columns of data apart from the left column and the right column of the fault area; 2, after filtering back projection reconstruction is performed on the matrix B, then carrying out mean value filtering and forward projection, and obtaining a projection sine matrix C; 3, subtracting the matrix C from the matrix A, and then obtaining a matrix D by performing linear interpolation processing on the column corresponding to the fault area by use of the left column of data and the right column of data apart from the column corresponding to the fault area in a difference matrix; 4, obtaining a matrix E by adding the matrix C to the matrix D; and 5, returning to the second step, cycling the second step to the fourth step until preset iteration frequency is reached, and then obtaining a CT image after the artifacts are removed by performing the filtering back projection reconstruction on an obtained result. The method provided by the invention can eliminate the annular artifacts caused by continuous large-area faults of CT detection elements.

Description

A kind of method utilizing data for projection to remove the ring artifact that the first fault of CT detection causes

Technical field

The present invention relates to a kind of Medical Image Processing, be specifically related to enhancing or the recovery of image, particularly relate to CT image annular artifact eliminating method.

Background technology

CT, i.e. computerized tomography technology, be widely used in the numerous areas such as medical diagnosis.But, in CT system, owing to the factors such as detector processing technique precision and service life are restricted, the CT image reconstructed is frequently accompanied by the appearance of annular artifact.Annular artifact can affect follow-up image processing operations, as image denoising, image segmentation, image measurement, etc..

CT image annular artifact correction method is broadly divided into CT image calibration and executes and projection sinogram correction method.The thinking of CT method for correcting image is, first the annular artifact under cartesian coordinate system is transformed under polar coordinate system, artifact form is transformed into straight line from annular, then linear position detection and Filtering Processing are carried out, finally again the image after process is switched back to cartesian coordinate system from polar coordinate system, complete correction.Though this type of method can better remove CT image annular artifact, but owing to the conversion between different coordinates needs to be interpolated, the resolution of image and fidelity is caused to decline.Based in the correction method of projection sinogram, owing to the annular artifact of CT image shows the feature of straight line in projection sinogram, therefore directly straight line can be positioned and repair, utilize reconstruction technique to obtain not containing the CT image of annular artifact the projection sinogram after correction.Projection sinogram correction method eliminates CT image annular artifact preferably, and meanwhile, for the method based on CT image rectification, the CT image after correction has higher resolution and fidelity.

The author Luo Junfang of master thesis " remove CT image annular artifact algorithm research " proposes the bearing calibration based on projection sinogram respectively and executes based on CT image calibration, and points out, two kinds of methods are compared, based on projection sinogram bearing calibration more preferably.Based in the bearing calibration of projection sinogram, pass through medium filtering, artifact MARG is carried out labelling by the edge indicator function edge utilizing Sobel detector, and combine use Hough or bwlabel function ring artifact is positioned, nearest-neighbor interpolation, linear interpolation, three kinds of methods of cubic polynomial fitting are selected ring artifact to be removed, and three kinds of interpolation methods have been carried out com-parison and analysis.It is shown that linear interpolation is the most succinct and that effect is best interpolation method.But the projection missing data corrected in literary composition shows as " missing data is discreet region " (Chinese summary ii page the 1st paragragh of described master thesis).But, when there is " necrosis " of continuous large-area in the detection unit on detector, above-mentioned master thesis only just cannot more accurately estimate missing information with interpolation method, do not simply fail to the annular artifact removing in CT image well, and owing to the mistake of missing information is estimated also to bring new artifact.

Summary of the invention

For the deficiency that prior art exists, the technical problem to be solved is to provide a kind of method utilizing data for projection to remove the ring artifact that the first fault of CT detection causes, and the method can eliminate CT image annular artifact produced by CT detection unit wide-area failures continuously.

The present invention solves that the scheme of above-mentioned technical problem is as follows.

1, a kind of method utilizing data for projection to remove the ring artifact that the first fault of CT detection causes, the method comprises the steps:

(1) import the data for projection that circular orbit CT imaging system gathers, obtain projecting sinusoidal matrix A；Position in matrix A, the first corresponding fault zone of CT detection of continuous fault is detected with Sobel line detection method or Canny line detection method, and row corresponding to fault zone are carried out linear interpolation by two column data outside arranging with around two, obtain the projection sine matrix B after preliminary corrections；

(2) described matrix B is filtered backprojection reconstruction and obtains matrix image1, then matrix image1 is carried out mean filter and obtains matrix image2, then matrix image2 is carried out forward projection, obtain projecting sinusoidal Matrix C；Wherein, described mean filter method is shown in lower formula I:

$image2(i,j)=\underset{i^{\′}=i-v}{\overset{i+v}{\Σ}}\underset{j^{\′}=j-v}{\overset{j+v}{\Σ}}image1(i^{\′},j^{\′})\×w(i^{\′},j^{\′})/K---\left(I\right)$

In upper formula I, (i, j) pixel position in representing matrix image2；(i ', j ') represent meet with location of pixels (i, the pixel position in the rectangular window of length and width respectively (2v+1) centered by j), and, i '=i-v ..., i+v, j '=j-v, ..., j+v, v is the natural number of 1～10, chooses by direct proportion with the width of artifact；W (i ', j ') expression image1 (i ', j ') to image1 (i, weight j)；K represent with (i, the neighborhood window centered by j) is sized in the scope of (2v+1) * (2v+1) to meet | image1 (i', j')-image1 (i, j) |≤T pixel number；T is predetermined threshold value；

(3) first deduct Matrix C by described matrix A, then with the row corresponding to fault zone in matrix of differences outside left and right column data the row corresponding to fault zone are carried out linear interpolation processing, obtain the projection sine matrix D after difference operation and interpolation processing；

(4) described Matrix C is added with matrix D, obtains the projection sine matrix E after summation operation；

(5) step (2) is returned to, and the matrix B that matrix E alternative steps (2) obtained by step (4) obtains, continuous circulation step (2)～(4), until after reaching the iterations preset, again obtained result is filtered backprojection reconstruction, obtains removing the CT image of pseudo-movie queen.

The present invention relatively prior art has the advantage that and effect:

1, the produced CT image annular artifact of continuous fault CT detection unit within effectively can eliminating 27.5%；

2, only artifact region is operated, resolution will not be reduced；

3, iterations can be selected for the order of severity of the first fault of CT detection, flexibly and easily.

Accompanying drawing explanation

Fig. 1 is the CT projection sinogram picture of the number of people described in following embodiment 1；

Fig. 2 is CT image after CT projection sinogram shown in Fig. 1 is rebuild as filtered back projection.

Fig. 3 is the flow chart of the method for the invention.

Fig. 4 is that (i, j) with the schematic diagram of other point of central point edge (i ', j ') position relationship for the central point of rectangular window in mean filter method.

Fig. 5 is the CT image that the data for projection utilizing Fig. 1 corresponding adopts the method for the invention to obtain, and wherein, a figure is the CT image after 1 iterative approximation, and b figure is the CT image after 2 iterative approximations, and c figure is the CT image after 3 iterative approximations.

Fig. 6 be detector the 120th row to the 130th row fault time the original CT image that obtains and to the CT image after this correct image, wherein, a figure is original CT image, b figure adopts existing method to the original CT image being corrected after rebuilding, and c figure adopts the method for the invention to the original CT image being corrected after rebuilding.

Fig. 7 be detector the 80th row to the 180th row fault time the original CT image that obtains and to the CT image after this correct image, wherein, a figure is original CT image, b figure adopts existing method to the original CT image being corrected after rebuilding, and c figure adopts the method for the invention to the original CT image being corrected after rebuilding.

Fig. 8 rebuilds the obtained number of people CT image when being detector fault-free.

Fig. 9 is the CT image that the data for projection utilizing Fig. 1 corresponding adopts existing method to obtain.

Detailed description of the invention

Example 1

Fig. 1 is that what have that the fault flat panel detectors of 367 CT detection unit obtain be sized to the projection sinogram picture of the number of people of 256 × 256, the first quantity of continuous fault CT detection of wherein said fault flat panel detector is 21, and the 120th row being positioned at detector arrange (fault rate is 21/367=5.7%) to the 140th.Image shown in Fig. 1 obtains in track CT imaging system, and the ray source focus of wherein said CT imaging system is 500mm to system center of rotation distance, and system center of rotation is to detector distance 500mm.Image shown in Fig. 1 is to adopt above-mentioned circular orbit CT imaging system to gather 720 once obtained Angles Projections data at interval of 0.5 degree within the scope of 360 degree.Obtained data for projection is made directly filtered back projection and rebuilds the CT image namely obtained as shown in Figure 2.From Figure 2 it can be seen that image has the bigger annular artifact of width, and containing strip artifact outside annular artifact.

By the flow process shown in Fig. 3, the inventive method is adopted to remove specifically comprising the following steps that of annular artifact shown in Fig. 2

(1) import 720 Angles Projections data and data first for fault CT detection be designated as 0, just obtaining being sized to the matrix A of 720 × 367:

Detecting the corresponding position in matrix A, the fault zone of CT detection unit of continuous fault with Sobel line detection method, detect that the 120th is classified as the initial row of fault, the 140th is classified as failure stopping row；Then the 119th and 141 data arranged are utilized to adopt following formula to carry out linear interpolation 120～140 column data: 120～140 column data formula for interpolations for the i-th row (1≤i≤720) are:

A'(i, j)=w1*A (i, 119)+(1-w1) * A (i, 141)

In formula, 1≤i≤720,120≤j≤140, w1=(141-j)/(141-119)；

The projection sine matrix B after preliminary corrections is obtained after interpolation:

(2) described matrix B is carried out filtered back projection's reconstruction well know in the art and obtain matrix image1, then, take window radius v=5, threshold value T=0.06, press formula I and matrix image1 is carried out mean filter obtain matrix image2:

$image2\left(i,j\right)=\underset{i^{\′}=i-v}{\overset{i+v}{\mathrm{\Σ}}}\underset{j^{\′}=j-v}{\overset{j+v}{\mathrm{\Σ}}}image1\left(i^{\′},j^{\′}\right)\×w\left(i^{\′},j^{\′}\right)/K---\left(I\right)$

Below in conjunction with Fig. 4, above-mentioned mean filter method is illustrated:

Referring to Fig. 4, with pixel a certain in matrix image1, (i, centered by j), forms the matrix-block of 11 × 11, and any pixel point position in this matrix-block is (i ', j ')；(i, j) pixel of position is filtered, and just obtains matrix image2 to utilize public formula I just can complete in matrix.

Under the geometrical conditions used by described CT imaging system, matrix image2 carrying out forward projection operation well known in the art and just obtains projecting sinusoidal Matrix C, the size of this matrix is identical with matrix A, is all 720 × 367, i.e. 720 row 367 column data.

(3) first deduct Matrix C by described matrix A, obtain following difference operation matrix,

The data of 120～140 row are carried out linear interpolation by the data of 119 row and 141 row that then recycle above-mentioned difference operation matrix, obtain matrix D:

(4) described Matrix C and matrix D are added, obtain the projection sine matrix E after summation operation:

(5) step (2) the matrix B that matrix E alternative steps (2) obtained by step (4) obtains, continuous circulation step (2)～(4), iteration 3 times are returned to；Then, obtained projection sine matrix is filtered backprojection reconstruction, obtains the CT image removing pseudo-movie queen as shown in Figure 5 c.

Example 2

In order to verify the technique effect of bright annular artifact minimizing technology of the present invention further, the present inventor has also done following research:

1, contrast images is obtained

(A) projection sine matrix obtained to example 1 step (5) 1 times and 2 iteration being filtered backprojection reconstruction respectively, result is as shown in figure 5a and 5b.

(B) by the method that example 1 is same, the CT data for projection corresponding to Fig. 6 a and Fig. 7 a is processed respectively, shown in reconstructed results such as Fig. 6 c and Fig. 7 c.Wherein, obtained CT image when Fig. 6 a is the 120th row to detector generation continuous fault (fault rate is 11/367=3%) of 130 row, CT image obtained when Fig. 7 a is the 80th row to detector generation continuous fault (fault rate is 101/367=27.52%) of 180 row；Used by Fig. 6 a and Fig. 7 a, CT imaging system and parameter are same as Example 1.

(C) adopting number of people CT image when CT imaging system same as Example 1 and gain of parameter detector fault-free, this image is as shown in Figure 8.

(D) the CT data for projection corresponding to Fig. 2 adopts existing method (master thesis " removes the research of CT image annular artifact algorithm ", author Luo Jun) rebuild, and result is as shown in Figure 9.

(E) the CT data for projection corresponding to Fig. 6 a and Fig. 7 a is respectively adopted existing method (master thesis " removes the research of CT image annular artifact algorithm ", author Luo Jun) rebuild, shown in result such as Fig. 6 b and Fig. 7 b.

2, com-parison and analysis

(1) comparing visible respectively with Fig. 8 by Fig. 5 c, Fig. 6 c and Fig. 7 c, the two has no difference, and it is very good that this illustrates that the method for the invention removes artifact effect.

(2) Fig. 5 c, Fig. 6 c are corresponding in turn to Fig. 7 c and Fig. 9, Fig. 6 b and Fig. 7 b compare visible, when the continuous fault rate of detector is 3%, the obtained CT image of prior art begins to artifact occur, and along with the increase of continuous fault rate is more and more obvious, when continuous fault rate just reaches 5.7% artifact very seriously, just it is not used to clinical diagnosis when continuous fault rate reaches 27.52%.And adopt the method for the invention, when the continuous fault rate of detector reaches 27.52%, when obtained CT image is intact with detector, gained CT image is still without significant difference (see Fig. 7 c and Fig. 8).

(3) owing to iterations more multioperation amount is more big, and method of the present invention is in 27.52% situation in the continuous fault rate of detector, it is only necessary to iteration just can meet requirement 3 times.

Claims (1)

1. utilizing the method that data for projection removes the ring artifact that the first fault of CT detection causes, the method comprises the steps:

(1) import the data for projection that circular orbit CT imaging system gathers, obtain projecting sinusoidal matrix A；Position in matrix A, the first corresponding fault zone of CT detection of continuous fault is detected with Sobel line detection method or Canny line detection method, and row corresponding to fault zone are carried out linear interpolation by two column data outside arranging with around two, obtain the projection sine matrix B after preliminary corrections；

(2) described matrix B is filtered backprojection reconstruction and obtains matrix image1, then matrix image1 is carried out mean filter and obtains matrix image2, then matrix image2 is carried out forward projection, obtain projecting sinusoidal Matrix C；Wherein, described mean filter method is shown in lower formula I:

$\mathrm{image}2(i,j)=\underset{i^{\′=i-v}}{\overset{i+v}{\mathrm{\Σ}}}\underset{j^{\′=j-v}}{\overset{j+v}{\mathrm{\Σ}}}\mathrm{image}1(i^{\′},j^{\′})\×w(i^{\′},j^{\′})/K$

In upper formula I, (i, j) pixel position in representing matrix image2；(i ', j ') represent meet with location of pixels (i, the pixel position in the rectangular window of length and width respectively (2v+1) centered by j), and, i '=i-v ..., i+v, j '=j-v, ..., j+v, v is the natural number of 1～10, chooses by direct proportion with the width of artifact；W (i ', j ') expression image1 (i ', j ') to image1 (i, weight j)；K represent with (i, the neighborhood window centered by j) is sized in the scope of (2v+1) * (2v+1) to meet | image1 (i', j')-image1 (i, j) |≤T pixel number；T is predetermined threshold value；

(3) first deduct Matrix C by described matrix A, then with the row corresponding to fault zone in matrix of differences outside left and right column data the row corresponding to fault zone are carried out linear interpolation processing, obtain the projection sine matrix D after difference operation and interpolation processing；

(4) described Matrix C is added with matrix D, obtains the projection sine matrix E after summation operation；

(5) step (2) is returned to, and the matrix B that matrix E alternative steps (2) obtained by step (4) obtains, continuous circulation step (2)～(4), until after reaching the iterations preset, again obtained result is filtered backprojection reconstruction, obtains removing the CT image of pseudo-movie queen.