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Virtual colonoscopy

Author/s: Dra. Dolores Maluenda

What is a virtual colonoscopy (VC)?

It is a new diagnostic technique for colon disorders.

How is a VC performed?

A VC is a scanner or CAT (computerised axial tomography) – computerised radiography- that, presents as one of its innovations, the capture of an image from several angles. This photographed image is then processed by the computer, reconstructing the addition of all the images captured into a 3D figure.

Technically speaking, it is a high-speed spiral scanner (computed tomographic scanning) with multidetectors of 2, 4, 16 and 32 or more crowns. So, it is called “ Multidetector diagnosis”,or DMD.

How long does the test take?

Each “photo” takes 16 seconds. Two photos are taken.

The processing of the images ranges between 3 and 4 hours and it is performed by a computer.

How reliable is this technique?

This technique is as reliable as that of conventional colonoscopy.

Are there any added benefits?

With virtual colonoscopy we can not only see the lumen of the colon, but we can also visualise the external wall of the intestine, its thickness, the appearance of lymph nodes and the rest of the abdominal organs.

In addition, it allows us to locate the exact position of the lesions observed in order to carry out endoscopic or surgical intervention.

Then, why should I have a conventional colonoscopy when a VC offers so many advantages?

Because a colonoscopy not only provides a diagnosis at the time of the test but also allows for immediate treatment; in other words, it permits the removal of a polyp, the collection of samples for biopsy or it may help stop bleeding by injecting substances or tying a bleeding vessel. Moreover, it permits the clinician to see the colour and appearance of the mucosa (first protective layer of the colon), which virtual colonoscopy does not allow with such accuracy.

Does VC present any disadvantages?

Yes. Two more:

1.- It irradiates, for it still is a computerised radiography.

2.- The other disadvantage is the patient preparation, although it is more comfortable than conventional colonoscopy. Here the most important thing is to add a contrast medium that shows faecal matter, thereby eliminating the need to cleanse the colon so exhaustive using oral laxatives.

What results have been obtained lately with VC?

The results depend not only on the experience of the evaluating radiologist but also on the resolution of the device used. There is a big difference in sensitivity between DMD or “Multidetectors”, that collect data from 2 and 4 points to those that collect data from 16 or more points (16 crowns, etc.).

Then, what results can we expect if a VC with over 16 crowns is used?

It has been shown that it is possible to detect 4 mm lesions and all lesions over 1 cm(Pickjhardt P. et al., N Engl J Med 2003).

How is a VC done?

The colon is prepared prior to treatment with adequate diet. The test is carried out after an oral contrast medium is administered.

The patient is reclined on the bed of the device. Subcutaneous Buscapina ® is administered normally to reduce intestinal movement. Next, a small catheter is inserted into the rectum to fill the intestine with air with the aim of unfolding its walls without pain. The test starts after the walls of the rectum are unfolded.

The patient is placed face up and asked to hold his/her breath for about 15 seconds. After this, the study is repeated with the patient facing down.

What are the indications to carry out a VC?

This test is indicated in patients with incomplete conventional colonoscopy (unable to be completed entirely) and in patients where the risk of performing a conventional colonoscopy is high.

Then, who will be responsible for deciding which of the two explorations is better indicated in my case?

The doctor in charge of your case.

In general terms, in which cases would a conventional colonoscopy be indicated first?

In patients with no basal diseases –low morbility- but with high chances of having polyps (because there is a familial past history, because they have had polyps larger than 1 cm, or because they have been diagnosed with a polyp likely to degenerate, etc.).

Are there any other cases?

In patients with symptoms (for instance, if they bleed through the anus).

In which cases would it be indicated to perform a virtual colonoscopy first?

In patients with a basal disease that entails an added risk for the anaesthesia, and who are also unlikely to have polyps.

Once the VC has been performed, what is the course of action if a polyp is found?

A conventional colonoscopy is carried out to remove the polyp.

And, if no polyp is found in a patient who has had a VC, what sort of test will be done in the next follow up visit?

Whatever test the clinician deems necessary in each case. However, a new CV could be repeated.