Elementary inversion of the exponential X-ray transform

For the numerical inversion of the exponential X-ray transform, there exist a point-spread function and a transfer function that are piecewise elementary. As the radius of the point-spread function tends to zero, the associated filtered backprojection becomes an alternate exact formula for the inverse transform, the proof of which involves only calculus. Keeping the radius positive but setting the attenuation coefficient equal to zero gives a piecewise elementary formula for Z.H. Cho´s (1974) original transfer function in the inversion of the ordinary Radon transform