Noise reduction in digital radiography by wavelet packet using nonlinear correlation threshold

The influence of quantum mottle appears as degradation of graininess with reduction of the amount of incidence X-rays. The results of a Wiener spectrum study showed that graininess increased as the dose was reduced, and noise affected all frequencies. However, in clinical images, these effects are seen only in the high-frequency domain above 0.3 cycle/mm. Moreover, the effects of a grid are restricted to a parallel component or a perpendicular component based on its structure. And the influence appears in the decomposition wavelet image of H or V. From these result, the decomposed wavelet coefficients at complete binary tree are divided into seven frequency-coefficient bands. About the noise processing method, we tried to reduce noise by applying modification of Wavelet Transform Modules Maxima method proposed by Mallet, et al. Then we tried to the adaptive nonlinear threshold based on wiener spectrum study and power spectrum study. Based on the above considerations, evaluation was performed using clinical radiographs obtained at a standard dose and reduced dose with the noise reduction processing applied. The results showed that noise caused by quantum mottle and the grid can be reduced by this method without the need for threshold processing based on clinical experience.