Writing a Radon Transform Algorithm/ Problem at Shepp-Logan Image

I writed an algorithm for Radon transform. At the last stage, i compared the result of the code with the built-in ''Radon Transform'' function. For Lena image, i get nearly same results but for Shepp-Logan image, i am getting something looks like below. I tried to increase the number of beams in my algorithm but i couldn't get something better? What can be the reason behind this?

Left: My algorithm's result

Right: Built-in Function's Result