Image analysis – Applications – Biomedical applications
Reexamination Certificate
1998-06-02
2001-12-25
Ahmed, Samir (Department: 2721)
Image analysis
Applications
Biomedical applications
Reexamination Certificate
active
06333990
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a Fourier spectrum method for removing “grid line artifacts” from x-ray images, and more particularly to a Fourier spectrum method for removing “grid line artifacts” without changing the diagnostic quality in x-ray images.
In a x-ray radiography imaging system, an x-ray source projects a cone-shaped pattern of beams. This cone beam passes through the object being imaged, such as a medical patient, and impinge upon a two-dimensional array of radiation detectors. The signal generated from the measurement of the intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object. Each detector produces a separate electrical signal that is a measurement of incident beam attenuation.
A metallic anti-scatter grid used in the x-ray radiography imaging system is typically placed against the detector array to allow the x-rays that trend along a substantially perpendicular path to the respective detector to strike the respective detector, and x-rays that do not trend along a substantially perpendicular path to the detector are blocked by the anti-scatter grid, as is illustrated in FIG.
1
. As such, the anti-scatter grid enhances image diagnostic quality by preventing undesired x-rays from striking a detector. A disadvantage of using the anti scatter grid is that it may cause “grid line artifacts” to appear in the x-ray image. The grid line artifact appears in x-ray images as intensity modulation of the image in lines parallel to the anti-scatter grid. The grid line artifact occurs when the grid lines run perpendicular to the scan lines on the display device. The grid line artifact is very sensitive to display image magnification and can be made worse or be made to disappear by changing the image magnification. It would be desirable to remove the “grid line artifacts” from the x-ray image without changing the diagnostic quality in the x-ray image, irrespective of image magnification.
Another cause of “grid line artifacts” on the x-ray image include errors generated by the detector array signal measurement electronic circuits. For example, when there is a difference in gain between two respective signal measurement circuits, “grid line artifacts” may appear in the x-ray image.
Yet another source of “grid line artifacts” may be caused by the repositioning of the anti-scatter grid at non-standard positions during successive scans called “over-sampling.” Additionally, “grid line artifacts” may be caused by variations in the x-ray dose during over-sampling. It would be desirable to remove grid lines from the x-ray image caused by over-sampling without changing diagnostic quality.
SUMMARY OF THE INVENTION
A method for removing “grid line artifacts” from x-ray images in an x-ray radiography imaging system is presented. The method utilizes the Fourier spectrum of the image to identify grid line frequencies and employs spectral domain filtering to remove the grid line spectral components. The diagnostic information is preserved by modifying the grid line spectral components so as to be indistinguishable from local variations in image intensity values, and edge density of the x-ray image. Grid line spectral components are removed by the method consisting of the following steps: first, replacing the edgy regions with non edgy regions to generate a modified x-ray image; next, replacing the high intensity regions with low intensity regions within the modified x-ray image; next, converting the modified x-ray image to the frequency domain; then, eliminating “grid line artifacts” from the modified x-ray image; and finally, converting the modified x-ray image to a human readable format.
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“Fundamentals of Digital Image Processing,” Anil K. Jain, 1989, Chapter 9, Sec. 9.4, pp. 347-350.
Lin Wen-Tai
Tiemann Jerome Johnson
Yazici Birsen
Ahmed Samir
Breedlove Jill M.
General Electric Company
Thompson John F.
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