The effectiveness and limitations of medical image processing using analog and digital methods are studied. Several types of errors introduced during the image processing are analyzed. For the analog optical Fourier transform, errors are introduced by the vignetting effect and lens aberration. For the digital Fourier transform, errors are introduced by the aliasing effect and the band limit. To compare the results obtained by the two techniques, a set of x-ray images was processed both optically and digitally. The former was achieved by an optical system containing a large Fourier telephoto lens and the latter by a personal computer using a Fourier transform algorithm. The veracity of both the optical and digital Fourier spectra is analyzed. Our results indicate that the optical method has high speed due to parallel processing. High veracity can be achieved in high frequency regions by using an optimal optical system. In comparison, the digital method has the advantages of high processing precision and programmability, but has low processing speed. The comparison of the two different techniques presented in this article can provide a basis for selection of the processing method in different clinical settings. Even with today's fast computers, the optical method is still suitable for many clinical applications. The best choice lies in an analog-digital combination.