This preliminary clinical trial aims to evaluate the performance of the low-dose CT colonoscopy (CTC) with computer aided detection (CAD) on polyps detection compared with optical colonoscopy (OC), and explore the possible clinical routine for integrated use of CT colonoscopy and optical colonoscopy OC in colorectal cancer screening.

CT colonoscopy (CTC), also referred to as virtual colonoscopy (VC), utilizes computer virtual-reality techniques to navigate inside a three-dimensional patient-specific colon model reconstructed from abdominal CT images, looking for colonic lesions, full bowel preparation and colon inflation are needed to perform CTC examination. The clinical trial consists of two major parts:

1. Comparative study on the performance of CTC and OC: 50 subjects will be recruited in this part with written consents. With full bowel preparation, all the volunteers receive CT scans first with inflated colon, then OC followed by the same day, and necessary pathological analysis thereafter. The performance of CTC based on high-resolution CT data, including the number of polyps detected, size, position, etc, will be validated by that of OC with the paired comparison.
2. Integrate use of low-dose CTC with OC for colorectal cancer screening: After the validation of CTC performance, in this part, another 50 subjects will be recruited to explore the integrated use of CTC with OC as a possible and efficient workflow for colorectal cancer screening. In this part, a combination of normal dose and low-dose scan protocols will be employed (e.g., normal dose on supine, low-dose on prone), low-dose CTC based on the low-dose CT scanning protocol and an artifacts reduction technique will be introduced to reduce the radiation risk in screening.

Inclusion Criteria:

• Subjects with gastrointestinal symptoms potentially suggestive of colorectal cancer, such as diarrhea, constipation, abdominal pain or discomfort, rectal bleeding, iron-deficiency anemia, and unintended weight loss.
• Subjects with pathological confirmed colorectal cancer or polyps that need follow-up examination.
• Subjects with written consents

Exclusion Criteria:

• Subjects with acute colitis
• Acute diverticulitis
• Inguinal hernia including colon
• Colorectal surgery or endoscopic biopsy or resection of polyps within 6 months
• Known or suspected as colon perforated
• Complete intestinal obstruction
• Pregnant or lactating women