System and method for digital image intensity correction
The present invention provides a method and apparatus to enhance the image contrast of a digital image device, while simultaneously compensating for image intensity inhomogeneity, regardless of the source, correcting intensity inhomogeneities, and producing a more uniform image appearance. Also, the image is enhanced through increased contrast, e.g., tissue contrast in a medical image. In the method, the error between the histogram of the spatially-weighted original image and a specified histogram is minimized. The specified histogram may be selected to increase contrast for accentuation, e.g., on localized regions of interest. The weighting is preferably achieved by two-dimensional interpolation of a sparse grid of control points overlaying the image. A sparse grid is used rather than a dense one to compensate for slowly-varying image non-uniformity. Also, sparseness reduces the computational complexity, as the final weight set involves the solution of simultaneous linear equations whose number is the size of the grid.
1. A method for image correction, said method comprising:
generating, using a processor, a histogram from pixel information for an image portion of an object; and
transforming, using said processor, at least one pixel in said pixel information by minimizing error between said histogram and a specified histogram.
2. The method according to claim 1 , wherein said step of transforming comprises:
modifying said pixel information;
computing said weighting vector by minimizing the error between said histogram and said specified histogram; and
correcting said pixel information using a weighting vector.
3. The method according to claim 2 , wherein said weighting vector (W) is described by the formula:
W=B −1 Z.
4. The method according to claim 3 , wherein matrix B and vector Z are described by the formulas:
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where s i is the histogram specified transformed gray level of the i th pixel, N is the number of pixels in the image, P i is the original gray level of the i th pixel, b ji is the interpolation coefficient of the j th grid point acting on the i th pixel, W j is the weight value of the j th interpolating function, and M is the number of points in the grid.
5. The method according to claim 1 , wherein a plurality of grid points overlay said image portion of said object, each said grid point having associated therewith at least one interpolating function.
6. The method according to claim 5 , wherein said at least one interpolating function is selected from the group consisting of:
Gaussian functions and multiquadratic basis functions.
7. The method according to claim 6 , wherein said Gaussian function is selected from the group consisting of:
two-dimensional Gaussian functions and three-dimensional Gaussian functions.
8. The method according to claim 1 , wherein said specified histogram is selected from the group consisting of:
uniform histograms, linearly-rising histograms, tissue contrast enhancement histograms, tissue class accentuation histograms and combinations thereof.
9. The method according to claim 1 , wherein said histogram is generated from said digital image.
10. An imaging apparatus, said apparatus comprising:
a processor for processing image data acquired of an object, a histogram being generated from a portion of said image data,
wherein said processor transforms pixel information in said image data by minimizing error between said histogram of said image data and a specified histogram.
11. The imaging apparatus according to claim 10 , wherein said processor transforms image pixel information by weighting each pixel according to a weighting function.
12. The imaging apparatus according to claim 10 , wherein said processor in transforming:
modifies said pixel information;
computes a weighting vector by minimizing the error between said histogram and said specified histogram; and
corrects said pixel information using said weighting vector.
13. The imaging apparatus according to claim 12 , wherein said weighting vector (W) is described by the formula:
W=B −1 Z.
14. The imaging apparatus according to claim 13 , wherein matrix B and vector Z are described by the formulas:
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where s i is the histogram-specified transformed gray level of the i th pixel, N is the number of pixels in the image, P i is the original gray level of the i th pixel, b ji is the interpolation coefficient of the j th grid point acting on the i th pixel, W j is the weight value of the j th interpolating function, and M is the number of points in the grid.
15. The imaging apparatus according to claim 10 , wherein a plurality of grid points overlay said image portion of said object, each said grid point having associated therewith at least one interpolating function.
16. The imaging apparatus according to claim 15 , wherein said at least one interpolating function is selected from the group consisting of:
Gaussian functions and multiquadratic basis functions.
17. The imaging apparatus according to claim 16 , wherein said Gaussian function is selected from the group consisting of:
two-dimensional Gaussian functions and three-dimensional Gaussian functions.
18. The imaging apparatus according to claim 10 , wherein said specified histogram is selected from the group consisting of:
uniform histograms, linearly-rising histograms, tissue contrast enhancement histograms, tissue accentuation histograms and combinations thereof.
19. The imaging apparatus of claim 10 , wherein said histogram is generated from said digital image.
20. A computer-generated image of an object generated from an imaging device, said image comprising:
on a computer display, a plurality of pixels in an image portion of said image,
wherein said plurality of pixels in said image correspond to pixel information of said object, said pixel information having image intensity inhomogeneity therein; and
wherein at least one pixel in said pixel information is transformed by minimizing error between a histogram generated from said pixel information and a specified histogram.
21. The computer-generated image of an object according to claim 20 ,
wherein a plurality of grid points overlay said image portion of said object, each said grid point having associated therewith at least one interpolating function.
22. The image according to claim 20 , wherein said at least one pixel is transformed by weighting said at least one pixel according to a weighing function, and
wherein said weighting vector (W) is described by the formula:
W=B −1 Z.
23. The imaging apparatus according to claim 22 , wherein matrix B and vector Z are described by the formulas:
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where s i is the histogram-specified transformed gray level of the i th pixel, N is the number of pixels in the image, P i is the original gray level of the i th pixel, b ji is the interpolation coefficient of the j th grid point acting on the i th pixel, W j is the weight value of the j th interpolating function, and M is the number of points in the grid.