Digital Processing Of Conventional Tomograms

Conventional x-ray tomograms are limited by the presence of defocussed contributions from over and underlying structures in the final image. Edges are relatively indistinct and image contrast low when compared to conventional radiographs of the same object. Assuming that the x-ray tomograph is linear and spatially invariant with a circularly symmetric blurring function, it is possible to compensate for the presence of defocus blur to a large extent. Conventional x-ray tomograms of a composite phantom (Littleton) were used to determine the system characteristics. Circularly and quadrilaterally symmetric blurring functions computed from the phantom images were used to calculate the blurred contribution of over and underlying structures. Processed images were obtained by subtracting the defocussed components of adjacent tomoplanes from the tomographic image obtained at the level of interest. This processing was performed using a PDP-11 minicomputer by discrete fixed point convolution operations with up to 19 by 19 operators applied to a 512 x 512 image matrix. The results demonstrate a substantial improvement in subjective image quality.

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