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What is virtual colonoscopy?

Virtual Colonoscopy or CT Colonography is a non-invasive imaging procedure that uses a CT scanner and computer virtual reality software to produce two- and three dimensional images of the colon. It is used to diagnose colorectal diseases like polyps, diverticulosis and cancer.

How does it work?

The patient has to take laxatives the day before the procedure to clear stool from the colon. A suppository is also used to clear the rectum. The patient is given a solution designed to coat any residual faeces which may not have been cleared by the laxative. This is called 'faecal tagging'. This allows the radiologist, viewing the 3D images to effectively subtract the left over faeces, which may otherwise give false positive results.

The patient on his/her left side with knees folded towards the chest and a thin flexible tube is inserted 2 inches into the rectum and air is gently pumped into the colon to inflate the colon using a hand-held squeeze bulb or an electronic pump. This will decrease the wrinkles and allow better visualisation of the colon.

The patient is then made to lie supine on the table, which moves through the scanner and multiple two dimensional images of the colon are taken at various levels. The patient is asked to hold the breath for about 10-15 seconds while the images are taken, to avoid distortion of images. The procedure is repeated with the patient in prone position.

The images produced are processed to produce a three dimensional image of the colon which allows the radiologist to visualise the colon as if with a conventional colonoscopy.

The encyclopedia states that the entire procedure would take just 15-20mts, does not require sedation or a scope. So the patient can resume his normal activities after the procedure.There is lesser discomfort or pain to the patient compared to conventional colonoscopy and the only discomort the patient may have is a cramp-like sensation in the abdomen due to the gas pumped-in. There is also lesser chance for complications like bleeding or perforation.

The main drawback of virtual colonoscopy is that, since it is a non-invasive procedure, polypectomy or biopsy of a suspicious lesion cannot be done during the procedure. So if a lesion is detected, it has to be followed by a conventional colonoscopy.

What is its application?

• Virtual Colonoscopy or CT Colonography is an effective screening method for colorectal lesions like polyps, diverticulosis, inflammatory bowel disease, adenomas and cancers.
• An article in webMD adds that CTC can also be used for follow-up of existing lesions like monitoring the growth of colorectal polyps, screening for cancer recurrance after surgery.
• The article also states that CTC can be an alternative to conventional colonoscopy when the scope cannot be passed into the colon due to a large tumor narrowing the passage.

Where is it going?

According to a study published in a recent NEJM article (New England Journal of Medicine, September 2008 issue), CT colonography can be an effective method for screening patients with an average risk of colorectal cancer.