S. KOUKI, M. ATTIA, M. LANDOULSI, S. BOUGUERRA, Y. AROUS, H. BOUJEMAA, N. BEN ABDALLAH GASTROINTESTINAL RADIOLOGY : GI 10

Virtual colonoscopy is a promising new technique that combines rapid spiral CT scanning of the abdomen with advanced computer programs capable of re- creating two- and three-dimensional views of the colon and rectum. The purpose of our study was to determine the usefulness of a virtual computed tomography colonoscopy for the detection of colic lesions.

Fifteen patients were evaluated with computed tomography colonoscopy using 64-row MDCT. CT images were analyzed by the colon dissection workup with unfolded haustra visualization and also using the virtual colonoscopy technique (axial images and endoluminal views).

A clean well prepared and adequately distended colon is a prerequisite for a high quality CTC examination. Patients need to undergo a 24-h colonic preparation similar to that required for colonoscopy and direct contrast barium enema. This usually involves adherence to a clear liquid diet for 24 h and the ingestion of a laxative the day before. Polyethylene glycol results in relatively larger amounts of residual fluid and is less suitable for this procedure. Cathartics, for example sodium phosphate and magnesium citrate, produce a ‘‘dry prep’’ with little fluid left behind and are the preferred agents.

CT virtual colonoscopy was performed with a 64-slice CT scanner (GE). On the insertion of a rectal enema tube, the patients colon was then insufflated with room air based on the patients tolerance. The catheter was clamped and a single scout view was obtained to verify bowel distention. Additional air was insufflated into the rectum if inadequate distention of the colon was observed. Once colon distention was adequate, CT scans were performed from the diaphragm to the pubis, with the patient in both supine and prone positions. The actual procedure takes about 10 min

The two CT data sets were transferred to a workstation. Images were analyzed using the conventional virtual colonoscopy technique evaluating both the 2D transverse images and the 3D reformatted virtual endoscopic images, performing a virtual ‘‘fly- through’’ path of the colon.

Image processing and interpretation are done using specialized software. The software extracts the images from the air-filled colon and removes the impression of the opacified residual fluid. In addition, the system creates a centerline through which the colonic lumen can be navigated. A trained radiologist takes about 20 min to evaluate the final images.

All our patients had incomplete colonoscopy. Eight patients consulted for rectal bleeding and seven had bowel dysfunction and abdominal pain. 10 patients presented with a colonic lesion at CT colography. In detail, three cases of colorectal cancer, three cases of polyps, one case of right colon diverticula with stigmata of hemorrhage, one case of right colon tuberculosis, one case of ischemic lesion of the sigmoid colon and one case of extrinsic compression of the left colon by a gastric stromal tumor.

60 year- old old man Rectal bleeding Incomplete colonoscopy Oblic reformatted CT image clearly shows a neoplasic mass of the right colon Three-dimensional similar barium enema image show an excentric narrowing of colonic lumen

Reformated CT images show a mass of the right colon with enlarged lymph node and infiltration of pericolic fat

58 year-old man Bowel impairment Incomplete colonoscopy Reformatted CT images showing a huge neoplastic mass (arrow), well recognisable as hypodense mass within the colonic lumen

Threedimensional threshold rendered endoluminal CT colonograph shows a similar endoscopic appearance of the mass Three-dimensional similar barium enema image Showing an excentric narrowing of colonic lumen

47 right iliac fossa pain. Bowel impairment Incomplete colonoscopy Reformated CT images show a circumferential thickening of the colonic wall with mesenteric fat infiltration. colonic biopsy : tuberculosis

Three-dimensional similar barium enema image showing a tight circumferential stricture of the right colon

60 year-old man Rectal bleeding Incomplete colonoscopy Axial CT scanthe presence of diverticular orifices (arrows) with stigmata of recent bleeding

Three-dimensional threshold rendered endoluminal virtual dissection of sigmoid colon shows the presence of diverticular orifices (arrows) Three-dimensional threshold rendered endoluminal virtual dissection of sigmoid colon shows the presence of a sessile polypoid lesion

Three-dimensional threshold rendered endoluminal virtual dissection of the colon showing an extrinsic compression by a gastric stromal tumor

Three-dimensional similar barium enema image showing an extrinsic compression by a gastric stromal tumor

tight circumferential stricture of the sigmoid colon : ischemic lesion of the sigmoid colon

70 year-old man history of colic polyposis. incomplete colonoscopy Oblic reformatted CT image clearly shows a polyp lesion in the sigmoid colon.

endoluminal CT colonographic view (confirm the diagnosis of the sigmoid polyp.

Computed tomography virtual colonoscopy is a new generation technique for colorectal evaluation by using high resolution, thin section volumetric CT data of the air distended, clean colon. Since the first description of the technique by Vining et al in 1994, new developments in CT equipments and virtual colonoscopy postprocessing softwares have been accomplished, and the diagnostic accuracy of the procedure, even for the detection of colonic lesions smaller than 5 mm, has improved.

It is important to emphasize that virtual colonoscopy is an operator- dependent method that needs a steep learning curve and adequate training. It was been demonstrated in several reports that the combination of 2D, 3D and endoluminal images significantly increases the sensitivity and specificity of the method.

An adequate bowel cleansing facilitates a rapid and accurate evaluation of the colon. The presence of stool or fluid retention prevents the software identifying the true path and creates the right centerline. To overcome the problem, the software creates in those areas, bridges containing no diagnostic information.

Currently, one of the major drawbacks of CT colonography is the long evaluation time. Its main advantages compared with the conventional virtual colonoscopy analysis are: - it is a non-invasive technique. - it obviates the need for sedation - this technique does not require ante- and retrograde viewing because an almost complete surface visibility is already obtained in a single direction way

CTC has proved to be a useful modality in the following conditions: 1 Failed colonoscopy; 2 Evaluation of the colon proximal to an obstructing lesion 3 CRC screening in patients with contraindications to colonoscopy or who refuse optical colonoscopy; 4 Patients with coagulaopathy, intolerance to sedation, and who refuse other screening options.

CT colonography or virtual colonoscopy is a fairly new modality that has the potential to play a significant role in screening for colic lesions. Virtual colonography is a reliable non-invasive, well-tolerated method, with high specificity and sensitivity for the visualization of the entire colon, even in sites that are inaccessible to conventional colonoscopy.

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