Techniques of CT colonography (virtual colonoscopy)

Colorectal cancer is a leading cause of death in older adults, which usually involves a long-term progressive change of normal mucosa into adenomatous polyps and then cancer. The detection and treatment of this disease in an early stage can lead to a cure in most cases by simply removing the polyp. Computed tomographic colonography (CTC), also referred to as virtual colonoscopy (VC), is a recent advance that gives an intraluminal visualization of the colon that is similar to endoscopy. VC requires fast 3D display (at least 10 frames/sec) of the colon's mucosal surface on a computer screen. Spiral/helical computer tomography is used to gather 3D volume data prior to display. CTC has been demonstrated to be promising for colorectal cancer screening. Studies on unraveling of the colon are underway to map the convoluted tubular structure into a straightened and flattened image volume for global visualization. In this article, we review the current status of CTC with an emphasis on image processing and visualization algorithms. Clinical assessment results of existing techniques are summarized. Practical issues and future perspectives are also discussed.