IntroductionWe discuss three closely relevant computational approaches for understanding computed tomography (CT) images: X-ray imaging, medical image analysis, and the application of radiotherapy planning. The computational approaches have been widely used in X-ray research for more than three decades (Colijn, 2004;Feldkamp et al., 1984;Ju et al., 2010;Kak, 1999;Kum, 2010; KumPark, 2010;Kum, 2007;2005;Kim, 2006;Kum et al., 2009;Peterzol et al., 2008). To understand CT images is to understand photon and/or electron scattering in matter, which can easily be done by the Monte Carlo method. Our research is mostly based on our in-house Monte Carlo code, PMCEPT code, which was developed recently (Ju et al.,