Fourier Slice Theorem - Reconstruction Fourier Space
Fourier slice theorem states that Fourier Transform of your projections are equal to slices of 2D Fourier transform. 

When you sample the projections over discrete angles the FT of projections become samples of 2D Fourier transform.

So you have to use your obtained samples to interpolate the remaining points.

in your code you put the samples in their position but the remaining points remain zero.