Hi, I don't know if the question is still relevant to you. Nonetheless, I came across the same problem recently and wrote a MATLAB script to solve the problem. It's not a very efficient way to solve it though, but it serves the purpose.
I basically used the linearity of the Radon transform, we can get the matrix form by computing the Radon transform using the 'radon' MATLAB function for the standard basis (i.e., matrix of all zeros, but a single one).
One thing to note is that, the matrix R is actually a function of projection angles θ.