FIGURE 14 Image of a grid phantom. (a) Uncorrected image: The lines are bound and the periphery appears denser than the image center. (b) Corrected image: Straightening of lines and decrease of brightness nonuniformity are obtained.

image. The nonlinear magnification effect originates from the projection of a flat plane onto a spherical surface. The size difference is proportional to the ratio of the zone surface area to the base surface area. The uncorrected grid image (Fig. 14a) indicates that the range of correction depends on the distance between a current pixel and the image center. The larger the distance, the larger the correction coefficient has to be. As a result, the final (corrected) image (Fig. 14b) is shrunk along the radius toward the image center. This changes the entire image do ß

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