Virtual Colonoscopy

Alternative names

Colonography

Definition

The intestine of the human being is made up of two clearly differentiated portions, a first portion called the small intestine and another portion located immediately after this that corresponds to the large intestine. The latter is in turn divided into the cecum, colon, sigmoid, and rectum. The main function of the colon is to extract water and salt from the solid residues of digestion.

There are multiple pathologies that can be located in the colon and can manifest with a wide constellation of symptoms such as abdominal pain, anemia, rectal bleeding, diarrhea or constipation. The diagnostic process to be followed should be individualized, but it will often be necessary to have explorations that allow visualizing the anatomy of the viscera.

One of the widely used complementary examinations is fibrocolonoscopy or colonoscopy, which consists of visualizing the colon by inserting a millimeter camera through the rectum. Currently, we have other more comfortable examinations for the patient, such as virtual colonoscopy.

Virtual colonoscopy allows the three-dimensional and two-dimensional visualization of the colon through the sequential taking of images captured with computerized axial tomography (CT) or magnetic resonance imaging (MRI). CT uses X-rays, ionizing radiation that pass through different tissues and, depending on their density, projects an image onto a receptor. MRI obtains its image by subjecting the patient to an electromagnetic field; the images captured by the receptor will depend on the water content of each of the tissues examined.

How is the study done?

The examination is performed in a radiology room that has CT or MRI depending on the selected imaging technique. Unlike fibrocolonoscopy, sedatives are not required. The patient should be placed on the examination table on their side with the legs bent against the chest. A small flexible tube is inserted through the anus and air is blown through it. Room air or carbon dioxide (CO2) can be blown in. CO2 has advantages over ambient air as it is absorbed quickly. Injecting the gas with an automatic insufflator manages to inject gas at a known flow and pressure, thus avoiding overdistention and sudden distension of the colon, which cause pain to the patient. Once the air is insufflated, the patient must lie on his back to start taking images. While the study is being carried out, the patient must remain immobile. A computer system will process the captured images and provide a three-dimensional and two-dimensional visualization of the colon. The estimated duration of the test is about 20-30 minutes.

Preparation for the study

To avoid the presence of fecal remains in the colon that could make it difficult to see the canal, the patient must have followed the bowel evacuation procedure indicated by their doctor during the interior day. This usually consists of bowel solutions combined with a liquid diet the day before the test. Unfortunately, the administered preparations cause diarrhea, so an abundant intake of fluids is recommended. In some cases, you may be instructed to ingest a small amount of gastrographin in order to improve the resolution of the images captured by CT.

It will need to be detached from metal objects. In the case of carrying a pacemaker or metal prosthesis, the healthcare team should be notified, especially in those cases in which the images are taken by MRI.

What does it feel like during and after the study?

The infusion of air can cause abdominal discomfort and cause temporary cramping pains, similar to flatulence episodes. These discomforts subside in about two hours with the expulsion of air after the test.

Study risks

The adverse effects usually described are related to the administration of evacuating solutions. Up to a quarter of patients may vomit. Even so, these symptoms subside within hours or with the administration of antiemetic drugs.

Although there is a risk of colon perforation due to air insufflation, this risk is minimal, and virtual colonoscopy is considered a safe examination. The risk of perforation is lower than that of conventional fibrocolonoscopy.

When performing a virtual colonoscopy with CT, the radiation absorbed by the patient is minimal and is within the safety parameters.

Study contraindications

It is contraindicated in pregnant women or women of childbearing age with the possibility of pregnancy.

If gastrografin is used, it is necessary to remember that this substance is contraindicated in patients with hypersensitivity to iodinated contrast media. In infants and children, try to avoid its use.

Reasons why the study is carried out

When the doctor responsible for the care of a patient suspects the presence of colonic lesions, he will carry out the appropriate examinations to complete the study. Current indications for virtual colonoscopy are screening for elevated colon lesions and fibrocolonoscopy failure due to intolerance or stenosis. It will be indicated especially in patients whose medical problems contraindicate the practice of conventional fibrocolonoscopy.