BACKPROJ uses the filtered or unfiltered backprojection algorithm to perform the inverse Radon transform. The filter is designed directly in the frequency domain and then multiplied by the FFT of the projections. The projections are zero-padded to a power of 2 before filtering to prevent spatial domain aliasing and to speed up the FFT.-BACKPROJ uses the filtered or unfiltered backprojection algorithm to perform the inverse Radon transform.　The filter is designed directly in the frequency domain and then multiplied by the FFT of the projections.　The projections are zero-padded to a power of 2 before filtering to prevent spatial domain aliasing and to　speed up the FFT.