Cone beam X-ray scatter removal via image frequency modulation and filtering

Abstract

We present a novel method for rapid removal of patient scatter from cone beam (CB) projection images that requires no scatter measurement, physical modeling or strong assumptions regarding the spatial smoothness of the scatter distribution. A modulator grid is placed between the imaged distribution and the detector that differentially frequency modulates primary and scattered photons. When photons travel through the grid, photons that originate directly from the CB source are modulated by a higher frequency than scattered photons that have more proximal, diffusely distributed sources. We employ non-linear Fourier domain filtering to attenuate the contribution of scatter to the image spectrum. The theoretical validity of the method is verified using linear analysis of planar sources and its performance is evaluated using a simulator based on this analytical model. Simulation experiments with an ideal modulator indicate that even unrealistically large amounts of scatter are almost entirely removed by this method. The recovered images are devoid of major artifacts and exhibit an RMS error of 10%. We have verified the theoretical validity of scatter removal via spatial frequency modulation. A disadvantage of the technique is that it will always produce a filtered image having at best 0.41 of the maximum detector resolution when maximum scatter rejection is desired. This is not a major consideration in most medical X-ray CB imaging applications using contemporary detector technology, especially since scatter often significantly reduces useful resolution.