Bulbous or complex haustral folds are a common cause of false positives on CTC, particularly on 2D images.¹, ² and ³ They may have the appearance of a polypoid or less commonly a mass-like lesion. Bulbous folds are one of the most common pitfalls seen on axial images that require multiplanar reformats or 3D views for problem solving (see Figures 6.1, 6.2 and 6.3). These may represent haustral folds that are normally slightly thickened, complex interhaustral folds, crossing folds (see Figure 6.4), or may be due to the confluence of folds (see Figures 6.5 and 6.6). Complex folds are typically encountered at the hepatic and splenic flexures³,⁴ (see Figure 6.7). Other causes of thickened folds, particularly in the sigmoid, are poor distention and muscular hypertrophy associated with diverticulosis. A suboptimally distended sigmoid may demonstrate large, thickened folds extending from both sides of the bowel wall that meet in the middle (“kissing folds”)¹ (see Figure 6.8). Multiplanar reformats are very helpful for further evaluation of bulbous folds and can often quickly show the linear morphology of haustral folds. 3D endoluminal views can easily delineate the typical smooth, symmetric linear appearance of a fold as well as the convergence of folds. The subvolume cube view, which is available on many workstations, is particularly useful for rapidly focusing on an area in question and allows easy 360-degree rotation to differentiate a fold from a true polyp.

When a polypoid-appearing lesion is suspected of being a bulbous fold on 2D views, the potential lesion should be evaluated on the opposing view. The appearance of a bulbous fold may change and appear more linear with improved distention on the opposing view. Folds that have an irregular contour, appear thickened or different from adjacent folds particularly in an otherwise well-distended, well-distended colon should be approached with caution because they can represent infiltrating cancers, flat lesions, or carpet lesions.¹,⁵,⁶

The rectum near the anal verge often has prominent longitudinally oriented folds (rectal columns of Morgagni) that may appear mass-like or polypoid on axial images, particularly with underdistention of the rectum. The 3D views are useful for problem solving these folds. Specific attention should be paid to the area surrounding the rectal tube because the catheter and/or inflated balloon may obscure findings, particularly low-lying and flat lesions at the anal verge.⁷ Suspicious lesions should be confirmed in the opposing position because folds will often change appearance with a change in position. Sagittal and coronal multiplanar reformatted views are often helpful for evaluation of folds in the rectum, which usually tapers towards the anal verge. A rectal bar representing a more prominent longitudinal fold may be visualized along the anterior rectal wall (see Figure 6.9). A subtle, small pseudopolypoid lesion may be visualized along the posterior rectal wall on the 2D views at the point of anterior angulation of the rectum (see Figure 6.10).

The ileocecal valve (ICV) is variable in appearance. There are three types of ICV—a polypoid (papillary) valve is a dome-like protrusion with its mouth at the apex, a labial type is the most common and appears as a thickened fold with the mouth separating the margins of the fold, and an intermediate type which has features of both¹,² (see Chapter 4). In a study of 71 patients who underwent CTC, it was found that the ICV was at least partially visualized in 94% of cases, more frequently in the supine position.⁸ Characterization of the ICV on double-contrast barium enema showed a normal mean valve height of 1.7 cm and a mean width of 2.8 cm.⁹ Maximum height of the normal valve was approximately 4 cm. The valve was smooth in 85% of patients and mildly lobulated in 15% of patients.

A specific feature of the ICV is a central depression or “pit” (orifice where the terminal ileum empties into the right colon), which may be seen on 2D and 3D views. Additionally, its common location is along the medial wall of the colon where the terminal ileum enters (see Figure 6.11). If there is difficulty in identifying the ICV on axial images, it is best to locate the terminal ileum on the coronal multiplanar reformat and follow it in to the right colon. Readers should attempt to identify the ICV in each patient. The ICV may also be used as a landmark for identification of the cecum in cases where there is mobility of the segment.

Fatty infiltration (lipomatosis) of the ICV may cause it to resemble a mass on 3D endoluminal views, but 2D views will easily show its fat density (see Figure 6.12). Lipomatosis of the ICV tends to cause symmetric enlargement of the valve. Lipomas can also occur on the valve, but these are often asymmetric masses.¹⁰ Polyps and adenocarcinoma can occur on the ICV as well because it is covered by colonic mucosa. Any disruption of the typical morphology of the ICV should be fully evaluated for possible polyp or malignancy (see Figure 6.13). Stool can also adhere to the ICV and cause angular type protrusions extending from the valve (see Figures 6.14 and 6.15). Occasionally, when the terminal ileum is distended it can prolapse into the cecum and cause an enlarged and elongated appearance of the ICV. The 2D views show protrusion of the air-filled terminal ileum into the cecum (see Figure 6.16).

The appendiceal orifice and cecal crow’s feet have characteristic appearances that have been described in Chapter 4. The orifice of the appendix may appear closed and slit-like, open or protruding into the lumen on the 3D endoluminal views. The typical appendiceal orifice will appear as a sharply demarcated complete dark ring similar to a diverticulum (see Figure 6.17). Mass-like protrusions in the region of the appendiceal orifice should be regarded with suspicion and full evaluation should be performed for colorectal carcinoma (although rare) or carcinoid tumors. An inverted appendix or protruding appendicolith represent potential pitfalls and may resemble a polypoid or mass lesion on 2D views and on the 3D endoluminal view.¹¹,¹² The appendix may be partially or completely inverted. Use of the older inversion-ligation technique for appendectomy, which is not often used currently, will result in an appendiceal stump that may simulate a polyp.¹² If a potential polyp is identified on axial or 3D views, the reader should refer to the coronal or sagittal reformats to attempt to identify the appendix and the relation of the appendix, if present, to the lesion. History of prior appendectomy is important to obtain. An appendicolith is often also readily identifiable on 2D views. An inverted or intussuscepted cecal diverticulum has also been reported to simulate a pedunculated polyp on CT scan.¹³

Cecal volvulus accounts for 11% of all bowel volvulus.¹⁴ Torsion of the cecum occurs as a result of loose fixation of the cecum and ascending colon resulting in large bowel obstruction. In general, partial malrotation is usually present in order for cecal volvulus to occur. There are two types of cecal volvulus—axial torsion and cecal bascule. Axial torsion is the most common (66%) and results in torsion (180-360 degrees) of the ascending colon and cecum around the longitudinal axis of the ascending colon resulting in obstruction and vascular compromise, usually associated with a high rate of mortality. If obstructive symptoms are present, the patient will present with obvious clinical symptoms of abdominal pain, nausea, and vomiting. The obstruction is usually a closed loop type and CTC is not indicated and should not be performed in this situation. The cecal bascule type results in the rotation of the cecum superior and anteromedial to the ascending colon, and often occurs with a flap-valve occlusion at the site of flexion¹⁵ (see Figure 6.18). Patients may also present with obstructive symptoms, although if the ileum undergoes passive rotation with the cecum obstruction may not occur. These patients may then present for CTC following an incomplete colonoscopy.

Incomplete rotation and malrotation of the bowel may contribute to difficulty tracking the colonic lumen on axial images. The 3D endoluminal view will allow easier tracking if the colon is adequately distended. The 3D transparency view is most helpful for rapidly demonstrating overall colonic anatomy. During embryogenesis, the bowel undergoes 270 degrees of counterclockwise rotation outside and inside of the peritoneal cavity. The duodenal-jejunal junction and ligament of Trietz come to lie in the left upper quadrant across the midline to the left of the second lumbar vertebral body, with the colon framing the small bowel, the jejunum residing in the left upper quadrant, and the ileum in the right lower quadrant.¹⁶ The duodenum and colon undergo separate rotations and their position relative to the superior mesenteric artery (SMA) and superior mesenteric vein (SMV) can indicate the degree of proper rotation. Failure of proper rotation (malrotation) can occur, giving rise to a short mesenteric base and abnormal bowel anatomy which may predispose to twisting or volvulus around the superior mesenteric vessels.¹⁶ Many cases of malrotation are associated with other congenital birth defects such as gastroschisis, omphalocele, diagphragmatic hernia, or duodenal or jejunal atresia. These entities come to clinical attention early. Malrotation is also associated with situs anomalies, which may or may not present early in life.¹⁷ However, if malrotation occurs without clinical consequence, patients may present for CTC because of failed colonoscopy (see Figure 6.19). Findings that suggest malrotation are failure of the duodenum to cross midline to the left, vertical, or reversed orientation of the superior mesenteric vessels, and unusual position of the cecum and large bowel, or small bowel.¹⁶,¹⁸

Acquired or congenital hernias involving the colon may cause incomplete colonoscopy and can often be easily evaluated with CTC. Patients who have had prior abdominal surgery with a ventral incision are prone to developing ventral hernias through a defect of the peritoneum and fascia at the incision site (see Figure 6.20). Umbilical hernias may also contain portions of the colon. These hernias may not be detectable by physical examination in large or obese patients and the diagnosis may not be known before CTC. Patients with hernias containing colon may develop strangulation with subsequent bowel ischemia, which is a contraindication to CTC due to the risk of perforation.

The 3D transparency view is helpful for demonstrating colon-containing hernias. Hernias containing colon may make it difficult to achieve sufficient colonic distention proximally. Hernias may also make tracking the colonic lumen difficult on axial views. Multiplanar reformats should be employed to track colon located in the hernia or the 3D endoluminal view may be used unless there is inadequate distention in the herniated segment or proximal to it. In patients with known ventral hernias, a decubitus position may be helpful for improved distention and better patient comfort compared with the prone position. The major advantage of CTC in these cases is the ability to evaluate the colon proximal to the hernia.

Inguinal hernias occur much more commonly in men and more typically involve small bowel loops. Indirect inguinal hernias are the most common type of groin hernia and occur through the inguinal ring lateral to the inferior epigastric vessels (see Figure 6.21). Direct inguinal hernias occur through a weakened area of the transversalis fascia within an area referred to as Hesselbach’s triangle. The sac of a direct hernia is typically located medial to the inferior epigastric vessels. In a study by Sosna et al. four of seven patients who developed colonic perforation at CTC had left inguinal hernias that contained sigmoid colon.¹⁹ It was postulated that incarceration of the sigmoid occurred at the hernia site which was likely caused by the insufflation of air. Practice patterns were changed so that in patients with known inguinal hernia special attention was given to the inguinal region during the insufflation process and if there was increase in size of the hernia sac, air insufflation was halted. Patients with less common hernias that may involve the colon, such as Spigelian hernias (anterior abdominal wall along lateral margin of the rectus abdominus muscle), internal hernias, or hernias occurring as a result of trauma or other surgery, may present for CTC due to incomplete colonoscopy or may be detected incidentally on CTC (see Figure 6.22).

The postsurgical appearance of the colon can cause pitfalls on both 2D and 3D CTC views. Surgical resection (partial or subtotal colectomy) results in alterations of conventional anatomy. Typical segmental resections include a right colectomy (cecum to hepatic flexure, absent ICV), transverse colectomy (hepatic flexure to splenic flexure), left colectomy (splenic flexure to sigmoid), low anterior resection (LAR: descending colon to rectosigmoid), and abdominal-peroneal resection (APR: resection of the left colon, usually portion of sigmoid including the rectum, anal sphincter, and anus with resultant colostomy).²⁰ CTC has not been advocated for patients with a colostomy following APR. In a study by Burling et al. perforation of a rectal stump occurred in a patient who underwent CTC inadvertently because adequate surgical history was not communicated or elicited.²¹ Perforation occurred at the suture line at the apex of the rectal stump (patient presumably had prior LAR). This emphasizes the importance of obtaining a complete surgical history before performing CTC.

Following colonic surgery, scarring with fibrous and granulation tissue can develop at the site of resection or anastomosis which may resemble polypoid lesions, annular carcinomas, or focal recurrences on both 2D views and 3D endoluminal views. The 2D views typically provide evidence of prior surgery, such as metallic surgical clips, staples, or a caliber change of the bowel. Although most gastrointestinal anastomoses are currently performed with stapling devices, suture anastomoses performed in the past may not be conspicuous to suggest prior resection. Patients who have had prior resections dues to colon cancer are at increased risk of developing recurrence at the anastomotic site and special attention should be paid to these areas on any follow up imaging. Surgical clips can also cause pseudopolypoid lesions and colon wall deformities, which may be a pitfall on 3D views (see Figure 6.23). The 2D views are helpful for demonstrating the proximity of the metallic clip to the colon wall.

The mobility of colonic segments may cause false-negative examinations. In particular, the sigmoid, transverse colon, and right colon have loosely attached mesentery and can demonstrate mobility with a change in position from supine to prone (see Figures 6.24, 6.25, 6.26 and 6.27). The mobility may be complex in orientation about the x-, y-, or z-axis. Rotation about the z-axis can occur with little change in position in the other planes. The 3D transparency view is helpful for demonstrating mobility of a colonic segment. Residual stool is usually differentiated from polyps by internal heterogeneity and demonstration of mobility with a change in patient position. However, homogeneous stool can be differentiated from a sessile polyp because it is often mobile. Rotated or mobile colonic segments may cause the reader to misinterpret a true polyp as mobile homogeneous stool (see Figure 6.28).

Laks et al. performed a study evaluating positional change in colon polyps on CTC.²² Eleven of 41 polyps (27%) measuring 5 mm or larger demonstrated movement with a change in patient position. Five of the 11 polyps were pedunculated and movement was due to the stalk. The remaining six polyps were sessile and located in mobile segments (sigmoid, transverse colon, ascending colon, and cecum) so that polyp movement was related to positional changes of the colon in the mesentery and was not true mobility of the polyp. It was concluded that a mobile lesion should not be assumed to be stool at CTC.

The cecum usually lies in the right iliac fossa, inferior and anterior to the ascending colon.²³ Approximately 10% of the population have an incompletely fixed cecum and ascending colon permitting a wide range of mobility including a number of variant anatomic positions, such as medial, anteflexed, or retroflexed in location²⁴ (see Figure 6.29). Patients with a mobile cecum may present with chronic right lower quadrant pain with associated abdominal distention. Cecopexy, using a lateral peritoneal flap for fixation, can relieve symptoms and prevent subsequent cecal volvulus.²⁵ Chen and Dachman studied the mobility of the cecum on supine and prone data sets in 21 patients.²⁶ The authors analyzed the position of the cecum based on the ICV and the appendiceal orifice. The measurement of a line along an axis connecting the ICV and appendiceal orifice relative to a vertical was used to determine cecal motion between the two data sets. Axial rotation was also assessed by comparing a line drawn through the center of the cecum and another from the center of the cecum to the ICV. Any degree of rotation of this line was interpreted as axial mobility. Rotation that could impact the diagnosis was found to occur in 9/21 (43%) cases with an average rotation of 78.9 +/−27.4 degrees. In the 12 cases that did not pose a diagnostic dilemma, the average rotation was still 22.5+/−12 degrees. In addition, the main axis of rotation was varied, occurring predominantly in coronal (6 cases), sagittal (4 cases), or axial (11 cases) planes. It was concluded that rotation of the colon occurs frequently, is geometrically complex, and occurs in several planes. Multiplanar reformatted views are helpful when solid stool is suspected in the cecum to assess for rotation.