linear and non linear filters such as wiener and median filters used for reducing noise and improve SNR (signal to noise ratio). To apply such filters mathematical software such as MATLAB and Mathematica can be used.
Poster Effects of Implementing Median and Wiener Filters on the CT ...
If you are talking about projection image obtained from a Radon transform a simple convolution with the Gaussian kernel will work in most cases. If you mean reconstructed images, there are more sophisticated techniques such as truncation of wavelet transforms.
The forward Radon Transform will create a "sinogram" from a 2D image. The reverse transform will create a 2D image from the associated sinogram. Various filters can be applied in all associated spaces if appropriate. Be more specific about what the problem is.