Hit-noise reduction in portal images: a comparison between wavelet- and rank-order-based methods
William J. Dallas, Eugene J. Gross, Hans Roehrig, Thomas L. Vogelsong · Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE · 1997
The system we have been investigating is a real-time portal imager which incorporates an x-ray-to-light converter and a CCD sensor. When the high-energy X-rays of the radiation therapy beam circumvent the converter and impinge directly on the CCD sensor they cause artifacts in the image that have the appearance of scattered bright points of light. It is as if salt has been shaken onto a photograph. In therapy imaging the artifact is termed 'direct hit' noise. The generic name impulse noise. The goal of this investigation has been to determine an efficient method for eliminating 'direct hit' artifacts produced on a CCD camera in a portal imaging system. Because the photon energies for portal imaging are so high, it is very difficult to prevent the hits. For that reason, we have investigated post-processing methods to remove the noise from captured images. Two families of filtering algorithms are applied in the images: A wavelet-based family and a rank-order based family.