Ring artifact problem on CT image

Dear all, I am doing parallel beam gamma CT. when I reconstruct the data (using iradon), there is a ring at the edge of the image. I think it makes image contrast become low. is there any code that can solve that? thank you :)

[KSSV]

You should attach the image..so that people can help.

[Bayu Azmi]

Hallo KSSV thank you for reply,

this is the result.

[KSSV]

doc_caxis_. You may limit the color's using caxis. Limiting these colors, might highlight your center circle.