Colonoscopy refers to the examination of the large intestine (colon) with an endoscope. This is a thin, flexible, tube-shaped instrument with an integrated light source. Virtual colonoscopy (synonyms: CT colonoscopy; CT colonography; CTC; virtual colonoscopy (VC) or CT colonography, CT pneumocolon), on the other hand, refers to a radiological examination procedure in which computed tomography (CT) is used to produce images of the colon (large intestine), which are then processed by computer to allow virtual passage of the colon. Virtual colonoscopy can also be performed with magnetic resonance imaging (MRI). So far, this method is not as informative as the examination performed by computed tomography (CT), because the image resolution is currently insufficient.
Indications (areas of application)
- Colorectal cancer screening (colorectal cancer screening).
- Intestinal polyps or polyposis diseases of the colon.
- Colon carcinoma (colorectal carcinoma; colon cancer).
- Incomplete colonoscopy.
The procedure
Computed tomography is one of the non-invasive, that is, not penetrating into the body, imaging X-ray diagnostic procedures. The body or the body part to be examined is imaged layer by layer with a rapidly rotating X-ray tube. A computer measures the attenuation of the X-rays as they pass through the body and uses this information to determine a detailed image of the part of the body being examined. The principle of CT (computed tomography) is to show the density differences of different tissues. For example, water has a different density than air or bone, which is expressed in different shades of gray. The examination takes only a few minutes and is performed with the patient lying down. As with the conventional form of colonoscopy, the bowel must be very clean, which is why the patient has to purge for one to two days before the examination with the help of drinking solutions. The latest devices use the multislice method, which means that several slices are taken at the same time. Modern examination devices use a 64-slice method, i.e. 64 slices are prepared at the same time. This method can be compared to a Rettig, which is cut in a spiral shape. In this case, however, only one slice is involved, and in the method described above, 64 slices are produced one inside the other as a spiral and processed by the computer. Modern devices also work with a so-called low-dose technique, i.e. only 50% of the radiation is required to produce these precise images with a slice thickness of up to 0.4 mm. New reconstruction algorithms (reconstruction calculation methods) make this precision possible. In virtual colonoscopy, CT images are taken of the patient’s abdomen (abdominal cavity). These are then processed by a computer program so that a three-dimensional passage of the intestine can be seen. The disadvantage of this examination compared to conventional colonoscopy is that a conventional colonoscopy must also be performed if there are visible findings, since no intervention such as the removal of polyps can be performed with the virtual form. Furthermore, X-rays are used in virtual colonoscopy. With “low-dose” technology, the radiation exposure is between 0.8 and 1.6 mSv (millisievert). For comparison, the natural radiation exposure in Germany is about 2.4 mSv per year. In a comparative study, CT colonography (CTC) was performed in 3,120 patients and optical colonoscopy in 3,163 patients. If polyps at least 6 mm in size were detected on CTC, these patients also underwent colonoscopy, during which these “mucosal growths” were removed. According to the treatment guidelines, all detected polyps were removed in those patients who were primarily examined by colonoscopy, regardless of their size. CONCLUSION: In the colonoscopy group, more than four times as many procedures were performed, with 2,434 polyps removed, than in the CTC group, in which polyps were removed in only 561 cases. The complication rate was correspondingly high in the colonoscopy group, which was thus significantly higher than in the CTC group (seven bowel perforations vs. zero).David H. Kim and his associates from the Department of Radiology at the University of Wisconsin, Madison, USA, therefore recommend the use of CT colonography as a screening method for colorectal cancer because of its comparable diagnostic accuracy while significantly reducing polypectomy and complication rates.
