A Two-Cascaded Filtering Method for the Enhancement of X-Ray CT Image

Shanjun ZHANG  Toshio KAWASIMA  Yoshinao AOKI  

IEICE TRANSACTIONS on Information and Systems   Vol.E76-D   No.12   pp.1500-1509
Publication Date: 1993/12/25
Online ISSN: 
Print ISSN: 0916-8532
Type of Manuscript: PAPER
Category: Image Processing, Computer Graphics and Pattern Recognition
CT image,  Weber's fraction,  contrast enhancement,  median filter,  modular transformation,  homogeneity,  

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A two-cascaded image processing approach to enhance the subtle differences in X-ray CT image is proposed. In the method, an asymmetrical non-linear subfilter is introduced to reduce the noise inherent in the image while preserving local edges and directional structural information. Then, a subfilter is used to compress the global dynamic range of the image and emphasize the details in the homogeneous regions by performing a modular transformation on local image den-sities. The modular transformation is based on a dynamically defined contrast fator and the histogram distributions of the image. The local contrast factor is described in accordance with Weber's fraction by a two-layer neighborhood system where the relative variances of the medians for eight directions are computed. This method is suitable for low contrast images with wide dynamic ranges. Experiments on X-ray CT images of the head show the validity of the method.