Equalized contrast display processing for digital radiography

摘要

A processing method is described which allows to present images with equalized detail contrast, i.e. contrast that is the same in all parts of the image, independent of the chosen look-up table (LUT) or the local signal level. Basically, a multiresolution algorithm is used which splits the image in a number of bandpass images. Only the lowest band (low-pass image) is transmitted through the intensity LUT, while all higher frequency subimages are nonlinearly enhanced and added to the LUT-transformed low-pass image. Nonlinear enhancement is used in order to improve the visibility of weakly contrasting details while minimizing artifacts at high contrast edges. Unfavorable noise enhancement can be avoided by limiting the enhancement in the low-dose areas of the highest frequency subimages. The resulting images show good detail visibility in all parts. Detail rendition in images with equalized contrast is independent of image latitude and of slight variations in overall image brightness or density. Preliminary experience with clinical images show that the display does not need individual parameter tuning for different images, yet allows producing artifact-free, naturally looking images with improved detail visibility for a wide variety of input images. Improvements compared to standard (intensity) equalization processing are evident especially for the mediastinum and subdiaphragmal regions of chest images and in lateral spine examinations that have an inherently large dynamic range.