Virtual colonoscopy has been developed as a non-invasive, safe, and low-cost method to evaluate colon polyps. Implementation and efficiency of virtual colonoscopy requires rigorous cleansing of colon prior to the examination. Electronic colon cleansing is a new technology that virtually clean stool residues tagged with contrast agents from the obtained computed tomography (CT) images. From our previous studies on electronic colon cleansing, we found that residual stool and fluid are often problematic for optimal viewing of colon. In this paper, we focus on developing a model-based approach to correct both non-uniformity and partial volume effects appearing in regions of bone and tagged stool residues. A statistical method for maximum a posterior probability (MAP) was developed to identify and virtually clean the tagged stool residuals. In calculating the solution, the well-known expectation maximization (EM) algorithm is employed. Experimental results of electronic colon cleansing are promising.