(a) Original CT image.
(b) Original image in (a) corrupted by added Gaussian white noise with maximum amplitude of ±25 gray levels.
(c) Image in (b) convolved with the 3x3 mean filter. The mean filter clearly removes some of the additive noise; however, significant blurring also occurs. This image would not have significant clinical values.
(d) Image in (b) convolved with the 9x9 mean filter. This filter has removed almost all the effects of the additive noise.