Proper distension of the colon is necessary to allow the radiologist the ability to visualize polyps and cancers that may impinge upon the lumen on CT colonography (Figs. 3.5, 3.6). A segment of colon that is poorly distended or collapsed can simulate a malignant narrowing such as that caused by an annular carcinoma (Fig. 3.7). A well-trained technologist or nurse can assist in placing the rectal tube with care and performing the colonic insufflation, depending upon local guidelines. With the patient in a right-side-down decubitus position on the CT table, the rectal tube is placed. Various types of administration sets are available when performing manual insufflation, including a rectal tip attached to tubing and an insufflation bulb or a Foley catheter attached to an insufflation bulb. Preliminary insufflation in this position is suggested, allowing for filling of the rectosigmoid and the descending colon. The patient is then turned to a supine position and insufflation continues to fill the transverse colon and then the right colon. In general it will take
Fig. 3.6. a Collapse of a portion of the rectum in the supine position on an axial image. b Excellent distension of the rectum with the same patient in a prone position, demonstrating a small polyp along the left posterolateral colonic wall. c Three-dimensional cube view of the same polyp seen in the prone position
Fig. 3.7. a Collapse of a long length of the sigmoid on coronal multiplanar reformatted view simulating annular carcinoma. b Occlusion of the lumen on endoluminal view due to collapse of the sigmoid in the same patient. This appearance may also be caused by an occluding carcinoma, and proper colonic distension is essential for differentiation
* b at least 2-3 l of gas to adequately distend the colon. A CT scout view is obtained of the abdomen and pelvis. If the entire colon, particularly the sigmoid, is not well distended then repeat administration of gas is performed according to maximum patient tolerance. Following supine axial image acquisition, the patient is turned prone and another CT scout image is obtained with additional gaseous insufflation given if segments of colon with suboptimal distension are noted on the scout view.
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