Cathartic bowel preparation as part of CT colonography examination (CTC) can be uncomfortable or even dangerous for certain patient groups. Noncathartic CT colonography (i.e. without cathartic cleansing) including contrast-material fecal tagging can offer significant clinical advantages and can increase the screening compliance for colorectal cancer. Current techniques of conventional CTC with fecal tagging use "electronic cleansing" algorithms to remove the remaining tagged colonic contents from the images. In such cases these methods can give satisfactory results, but they face serious problems with the inferior tagging quality of noncathartic protocols. To overcome this fundamental problem, a completely new approach is proposed using both a two-layer dual~energy MDCT and a dedicated algorithmic cleansing method that utilizes the spectral information. Feasibility study was performedwith a two-layer dual-energy MDCT which was utilized in clinical studies with oral intake of both iodine and barium contrast agents. The new method was compared to a conventional electronic cleansing technique. We show that the new approach is better at detecting highly dilute contrast agents in the colon, particularly where the tagged colonic content is mixed or adjacent to air regions. Therefore, the new technique may surmount the need for cathartic bowel preparation in CTC.