Noise Power Spectrum calculation
Obtain parallel beam sinogram of your image, then take the FFT of the sinogram (if you consider the Fourier transform in polar coordinate, then Fourier slice theorem tell us value of Fourier transform over a radial line at a given angle is equal to the fourier transform of the projection over that angle), after that average the power over different angles. To obtain sinogram, if there isn't any available function, you could rotate the image by desired angle, then sum over columns or rows.