The easiest and cheapest method for distending the colon is to use room air, insufflated via a hand held plastic bulb. Typically patients lie on the CT scanner table in a left lateral position facing away from the operator. A lubricated rectal catheter attached to an insufflator bulb via a connecting tube is then inserted into the rectum and taped to the patient's buttocks. The patient is encouraged to retain any gas and avoid passing flatus by clenching the anal sphincter. Colonic insufflation is then performed by gently and intermittently squeezing the plastic bulb typically over a period of 1-2 min. In contrast, rapid successive squeezes can cause discomfort and may precipitate rectosigmoid spasm (Rubesin et al. 2000)
Insufflation is continued until the operator believes the colon is optimally distended; most experts judge this by noting patient tolerance (Barish et al. 2005), stopping when the patient feels uncomfortable or bloated. In the presence of a competent ileocecal valve, this generally occurs following the introduction of approximately 2 l of gas, usually after 30-40 compressions (Morrin et al. 2002; Macari 2004). Limiting insufflation to a fixed volume or number of bulb compressions is not recommended because individual patients' colonic volume and tolerance are variable. Some experts advise repositioning the patient part way through insufflation into either the prone or the supine position (Chen et al. 1999; Taylor et al. 2003), depending on which scan acquisition is performed first (see below), for example first filling the non-dependant right colon in the lateral decubitus position and then the remaining distal colon after repositioning. Assessment of right sided filling by abdominal palpation is also anecdo-tally recommended.
Once initial colonic insufflation is deemed sufficient, a standard prone or supine CT scout image is acquired in order to assess the degree of distension and patient positioning. The sigmoid colon is typically the most difficult segment to distend optimally, and distension adequacy is often difficult to assess on the scout image due to overlapping loops in the anteroposterior plane. If suboptimal distension is encountered or doubt persists, the authors recommend additional insufflation prior to CT acquisition, repeating the scout image if necessary. Once the first scan acquisition is performed (and assuming a second is planned: see section below), the rectal catheter is left in situ and the patient repositioned. Once repositioned, and providing the patient is comfortable, the authors suggest further insufflation with approximately ten bulb compressions. We do not recommend insufflation while the patient is turning as this tends to precipitate anal leakage. Some workers advocate removing the rectal tube at this point because it may theoretically obscure rectal pathology. A repeat scout image is then performed routinely prior to the second CT acquisition (prone or supine) but patients rarely require additional insufflation following this (unless anal incontinence is present).
The ease and simplicity of this method is such that some patients can effectively insufflate their own colon using the hand held bulb (Pickhardt et al. 2003). However, success will depend greatly on the patient population concerned, and this approach requires well motivated individuals, perhaps more applicable to a younger screening population (Pickhardt et al. 2003).
A standard enema bag filled with approximately 3 l of gas is an alternative to the plastic bulb insufflator (Fig. 5.4), and has the additional advantage of permitting manual insufflation of carbon dioxide. The bag (filled with air or carbon dioxide via a gas cylinder) is sealed with a plastic clip and attached to a rectal catheter via a connecting tube. Once the rectal catheter is in-situ, the clip is released and the bag is gently compressed over 2-3 min, insufflating the colon. Gentle insufflation improves patient tolerance and ultimately allows greater volumes of gas to be administered. If the bag is empty and more gas required, then the plastic seal can be opened and room air introduced.
Carbon dioxide may also be insufflated directly from a gas cylinder via a tube with side hole for digitally controlling volume and pressure (Rogalla et al. 2004a). Clearly the pressure of insufflated gas must be carefully controlled using this method.
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