Introduction. Poor distention decreases the sensitivity and specificity of CTC. The total volume of gas administered will vary according to many factors. We aim to determine the relationship between the volume of retained gas at the time of image acquisition and colonic distention and specifically the presence of collapsed bowel segments at CTC. Materials and Methods. All patients who underwent CTC over a 12-month period at a single institution were included in the study. Colonic luminal distention was objectively scored by 2 radiologists using an established 4-point scale. Quantitative analysis of the volume of retained gas at the time of image acquisition was conducted using the threshold 3D region growing function of OsiriX. Results. 108 patients were included for volumetric analysis. Mean retained gas volume was 3.3？L. 35% (38/108) of patients had at least one collapsed colonic segment. Significantly lower gas volumes were observed in the patients with collapsed colonic segments when compared with those with fully distended colons 2.6？L versus 3.5？L ( ). Retained volumes were significantly higher for the 78% of patients with ileocecal reflux at 3.4？L versus 2.6？L without ileocecal reflux ( ). Conclusion. Estimation of intraluminal gas volume at CTC is feasible using image segmentation and thresholding tools. An average of 3.5？L of retained gas was found in diagnostically adequate CTC studies with significantly lower mean gas volume observed in patients with collapsed colonic segments. 1. Introduction There are a number of fundamental prerequisites for the successful practice of computed tomographic colonography (CTC), namely, satisfactory bowel preparation, faecal and/or fluid tagging, and good luminal distention, as well as thorough interpretation by an experienced radiologist, trained in CT colonography and aided by a modality workstation with specific software packages for CT colonography [1–3]. Inadequate colonic distention, particularly when mucosal surfaces are collapsed and in apposition, may both obscure true mucosal lesions and create false positive pseudolesions thereby decreasing the sensitivity and specificity of CTC [4]. The presence of collapsed segments also contributes to the frequency of repeat CTC examinations and requirement for subsequent optical endoscopic correlation [5]. Methods of insufflation include manual distention with room air [6], manual distention with CO2 [5], or a combination of both. Most authors now advocate automated insufflation with CO2 [5]. The PROTOCO2L device (EZ EM, NY, USA) automatically delivers CO2 per
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