VIRTUAL COLONOSCOPY DR DEEPIKA SOLANKI

Medical imaging procedure. Also known as CT colonography . Non invasive procedure. Uses X rays and computers. 2D and 3D images of rectum and entire colon with 3D reconstructed endoluminal views of the bowel.

ACCEPTED INDICATIONS Incomplete colonoscopy due to an occlusive mass or stricture preventing examination of the proximal colon. Incomplete colonoscopy due to colonic tortuosity, adhesions, severe diverticular disease or patient intolerance of colonoscopy. Inability to perform colonoscopy due to requirement for anticoagulant therapy or risks of sedation. Patients who adamantly refuse to undergo colonoscopy but have a strong indication for diagnostic colonoscopy.

CT COLONOSCOPY is under analysis as a screening tool for colorectal cancer because of its relative safety and greater patient acceptance as compared with other screening methods. Follow up on colon cancer or polyps.

Why Screen? Best method for controlling colorectal cancer 70-75% of CRC occurs in asymptomatic individuals Detect and remove adenomatous polyps, precursor lesions for CRC, and detection of early stage carcinoma Reduce mortality

Barriers to Screening Lack of health care coverage Low education levels Fear of pain Fear of complications during procedure Embarrassment of preparation process Morbidly obese adults Cost issues Lost time from work Lack of access

Risk factors for CRC Family history of colorectal cancer Personal history of adenomas or ovarian/uterine cancer Long standing IBD (8-15 years) Environmental factors (diet and meds) Lifestyle factors (physical inactivity, obesity, and cigarette smoking)

Pathophysiology Current belief is that most CRCs stem from preexisting adenomas Adenomas that are large and/or have a villous component determine likelihood of containing invasive carcinoma Polyps are slow growing and must grow for five years before they are clinically significant Normal colonic mucosa is transformed into benign adenoma, followed by progression to polyp containing cancer, which can become invasive

Methods for Screening Colonoscopy Flexible sigmoidoscopy Air contrast barium enema Fecal occult blood test (FOBT) CT colonoscopy

TECHNIQUE COLONIC CLEANSING (pt needs to empty bowels by taking laxatives a day before the test) COLONIC DISTENSION (done by using room air / carbon dioxide) IMAGE ACQUISITION (done after colonic insufflation in supine and prone positions on a helical CT using low dose technique) POST PROCESSING OF ACQUIRED DATA

Intravenous contrast improves detection of medium sized polyps (6-9 mm) especially in a suboptimally prepared colon. After acquiring supine and prone scans, various software packages are used to display images in both 2D and 3D (endoluminal) views.

The table moves through the scanner to produce a series of 2 dimensional cross sections along the length of colon. Patient is asked to hold his/her breath during the scan to avoid distortion on the images. The scan is then repeated with the patient lying in prone position.

CONTRAINDICATIONS ALLERGY to contrast. Suspected colonic PERFORATION. Acute colonic INFECTION ( acute diverticulitis, severe infective colitis). Acute lower GI BLEEDING. Complete colonic OBSTRUCTION. Very recent colonic SURGERY (<1 week). MEDICALLY UNSTABLE patients. REFUSAL to undergo colonic preparation.

Detection Rates Colonoscopy: Sensitivity of 88.2 (>10mm) CT colonoscopy: Sensitivity of 92.2 (>10mm) Sensitivity of 85.7 (<6mm) FOBT: detected 23.9% of advanced neoplasia Flex Sig: 76% when used with FOBT Air contrast barium enema: failed to identify up to 50% of polyps greater than 10mm in diameter

Advantages Virtual colonoscopy is less invasive, safer, and takes less time than a regular colonoscopy A thin tube to insert air into the colon is placed in the rectum rather than long flexible tube that is moved up your colon. Patients do not require sedation or anesthesia and can return home on their own or get back to work immediately after the test. VC provides clearer, more detailed images. Extra colonic findings.

DISADVANTAGES A radiologist cannot take tissue samples (biopsy) or remove polyps during VC, so a conventional colonoscopy must be performed if abnormalities are found. May not show polyps smaller than 10 mm. Exposure to radiation. Slight risk of perforation while colonic distension.

Virtual vs. Optical Colonoscopy Patients reported less discomfort with virtual colonoscopy Shorter examination time with VC VC less embarrassing More patients were willing to repeat a VC at shorter intervals than CC

Conclusion Not screening for CRC has great cost. Very high detection rates in people with polyps over 5mm Low detection rates for polyps less than 5mm could be acceptable because majority of polyps this size do not progress to CRC Studies underway concerning technology advances that can affect the sensitivity and specificity for polyps and malignancy

Conclusion (Cont.) Studies with stool tagging and digital subtraction are going on to eliminate bowel preparation, which would increase patient acceptance