CLINICAL CASE
PRESENTATION |
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Virtual colonoscopy, also known as CT colonography, is a method of non-invasive image diagnosis of relatively recent introduction. The method uses a computed tomography of the abdomen and a dedicated software programme to obtain an endoluminal analysis of the colon and at the same time examine the extracolonic organs.
This method was described for the first time in 1994 by Vining et coll., and since then much progress has been made in this field; primarily, the introduction of a multislice spiral CT, which permits a significant reduction in scanning times and obtains isotropic voxels of the acquired volumes, guaranteeing an optimal spatial resolution.
Progress in software has also made it possible to obtain a simultaneous visualisation of the images in both 2D and 3D, thereby reducing the time needed for data analyses and improving the diagnostic accuracy of the method
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| METHOD |
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The method requires accurate bowel cleansing obtained by means of a low residue diet for three days before the examination, together with laxatives taken in small doses. A contrast media of iodate or barium, or a combination of the two, can also be administered orally to mark residual colonic fluids and solids, and help to increase diagnostic accuracy and reduce false-positive results.
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| PERFORMING THE EXAMINATION |
Immediately before CT scan is performed, the colon is insufflated slowly and gradually using a small sonda positioned in the rectum at the maximum extension tolerated by the patient (or manually with room air or with a dedicated automatic insufflator using carbon dioxide).
An antispasmodic agent (hyoscine-N-butylbromide) is generally given intravenously before insufflation to improve colonic extension and reduce the artefacts of intestinal peristalsis and patient discomfort. The degree of intestinal distension is measured using the scout scanning procedure, and when the intestine is sufficiently distended the whole volume can be scanned.
Scans of the abdomen are carried out within a single breath-hold, first in the prone position and then in the supine, and generally use a low dose of radiation (no more than 50 mAs) with a section collimation of less than 3 mm.
Intravenous administration of the contrast media is advisable in cases of symptomatic patients, in following up previous colonic neoplasia, and when analysing obstructive colonic tumours which cannot be bypassed using conventional colonography. It is not, however, routinely carried out on patients who undergo the examination as a screening test for colorectal carcinoma. The procedure has proved to be well tolerated by patients, quick to perform, and practically free of complications.
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| DATA ANALYS |
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The resulting data are analysed by the radiologist using a dedicated software programme which makes it possibile to analyse 3D (“simil-endoscopic” images) and 2D views of the colon at the same time.
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| CLINICAL USE OF THE EXAMINATION |
Since its introduction, a significant number of studies have been carried out which have proved the validity of the method in identifying not only larger tumoral masses (2 cm and over) but also smaller polipoid lesions (1 cm or under), with the procedure of faecal tagging playing an important role.
Compared with optical colonoscopy, virtual colonoscopy not only permits diagnosis of the presence of colonic neoplasia, but also allows local staging of the colon wall and the detection of the eventual presence of metastasis, and, in the case of obstructive tumours which cannot be bypassed by optical endoscopy, the examination makes it possible to study the proximal colon and highlight the eventual presence of synchronous cancers; useful information in planning surgery.
Recently, following the results of different clinical studies (ACRIN 6664, IMPACT and MCCPT), CT colonography is included in the guidelines as an option for colorectal cancer screening and prevention in average-risk adults age 50 years and older by the American Cancer Society.
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| INDICATIONS : |
At present the examination is mainly indicated in the detection of polyps or tumours of the colon in cases where conventional colonoscopy has not been completed for various possible reasons (obstructive tumours, dolichocolon, angulation of colonic loops, inflammation of the bowel), and in patients with contraindications to traditional colonoscopy (eg. patients with cardiopathic problems, chronic bronchitis, undergoing oral anticoagulant therapy, and the frail and elderly), as an alternative to a double-contrast barium enema, and as an integrative method to conventional colonoscopy. |
| REFERENCES: |
Vining DJ, Winston-Salem NC, Shifrin, RY et al. Virtual colonoscopy. Radiology 1994; 193:446 (abstract)
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