DOI: 10.1148/rg.264055166
RadioGraphics 2006;26:1085-1099
© RSNA, 2006
Evaluation of Benign and Malignant Rectal Lesions with CT Colonography and Endoscopic Correlation1
Alvin C. Silva, MD,
Eric A. Vens, MD,
Amy K. Hara, MD,
Joel G. Fletcher, MD,
Jeff L. Fidler, MD and
C. Daniel Johnson, MD
1 From the Department of Diagnostic Radiology, Mayo Clinic, 13400 E Shea Blvd, Scottsdale, AZ 85259 (A.C.S., E.A.V., A.K.H.); and the Department of Radiology, Mayo Clinic, Rochester, Minn (J.G.F., J.L.F., C.D.J.). Recipient of Cum Laude and Excellence in Design awards for an education exhibit at the 2004 RSNA Annual Meeting. Received August 25, 2005; revision requested September 2 and received November 4; accepted November 7. A.K.H. has a licensure agreement with GE Medical Systems; all remaining authors have no financial relationships to disclose.
Address correspondence to A.C.S. (e-mail: silva.alvin{at}mayo.edu).
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Abstract
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Colorectal carcinoma is a significant cause of death from cancer in the United States, and early detection and treatment are critical. Computed tomographic (CT) colonography is a noninvasive, rapidly evolving technique that is a potential alternative to conventional colonoscopy for colorectal cancer screening. Rectal disease (eg, polyps, cancerous lesions, extramucosal lesions, inflammatory disease) can be especially challenging to diagnose with CT colonography because of several factors that can simulate or obscure the disease (eg, over- or underdistention, rectal tube, stool, artifacts). Familiarity with the spectrum of rectal diseases and with the potential pitfalls and technical limitations of CT colonography will help minimize interpretative and perceptual errors.
© RSNA, 2006
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Introduction
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Colorectal carcinoma is the second leading cause of death from cancer in the United States. The American Cancer Society estimated that about 145,290 new cases of colorectal cancer would be diagnosed in 2005 (1).
Rectal cancer was expected to account for nearly one-third of all cases of colorectal cancer, with 40,340 diagnoses projected for that year (1).
The 5-year survival rate for patients with colorectal cancer is 83%–90% if disease is confined to the bowel wall and less than 10% if there are distant metastases; thus, early detection and treatment are critical.
Screening for colorectal carcinoma has traditionally included a digital rectal examination, a fecal occult blood test, and endoscopy, including sigmoidoscopy and colonoscopy. Computed tomographic (CT) colonography is a noninvasive, rapidly evolving technique that is a potential alternative to conventional colonoscopy for colorectal cancer screening. Screening CT colonography has yielded mixed findings, with a sensitivity of 55%–94% in patients with polyps 10 mm or larger (2–12).
We retrospectively reviewed 1993 CT colonographic examinations conducted at our institution and at another large tertiary care center between December 1996 and April 2004. We found that 77 (3.9%) of the 1993 patients had rectal abnormalities identified at CT colonography. Histopathologic and endoscopic correlation were available for 69 (90%) of these 77 patients; the remaining eight patients had polypoid lesions identified at two-dimensional (2D) or three-dimensional (3D) CT colonography alone with no endoscopic correlation, findings that were presumed to be false positive.
In this article, we review the normal rectal anatomy and CT colonographic technique for imaging of the rectum. In addition, we briefly discuss the relative advantages and disadvantages of CT colonography and endoscopy in the evaluation of rectal lesions. We also discuss and illustrate various rectal diseases (hyperplastic and adenomatous polyps, rectal cancer, extramucosal lesions, inflammatory disease) and some pitfalls associated with CT colonography of the rectum.
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Normal Rectal Anatomy
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The rectum is the last segment of the gastrointestinal tract and is bounded by the sigmoid colon and the anus (Fig 1), with the proximal portion located within the peritoneal cavity and the distal portion being extraperitoneal. The inferior aspect of the rectum, or the anorectal junction, is defined anatomically by the dentate line, which spans 5–10 mm of the anal canal and marks the transitional zone between the columnar epithelium of the gastrointestinal tract and the squamous epithelium of the anoderm and perianal skin (12). The superior aspect of the rectum, or the rectosigmoid junction, is defined surgically as the line formed by the confluence of the distal taeniae coli muscles on the serosal surface of the sigmoid colon. Other less precise indicators of the rectosigmoid junction include the sacral promontory (the anterior edge of the first sacral segment) and the position of the peritoneal reflection (12). However, these landmarks are not visible at endoscopy; consequently, the National Cancer Institute has defined the rectum as extending 12 cm above the anal verge (13).
The rectum is a derivative of the fetal hindgut and is supplied by the terminal branch of the inferior mesenteric artery, the superior hemorrhoidal artery, and the middle hemorrhoidal arteries (paired branches from the internal iliac arteries) (12). Most of the proximal rectum is drained by the inferior mesenteric vein (IMV), which is a tributary of the portal venous system.
The distal rectum and anorectal junction are drained by the internal iliac veins or the IMV, which helps explain the two distinct hematogenous metastatic patterns of rectal cancer.
In the absence of liver metastases, rectal cancer can manifest with lung metastases (Fig 2) when a distal tumor is drained by the systemic venous system and the inferior vena cava to the pulmonary capillary bed. However, liver metastases are more commonly formed by way of the IMV and portal venous system or by means of endolymphatic spread along the course of the IMV (Fig 3), which is the usual pathway for the rest of the gastrointestinal tract. Internal and external hemorrhoidal venous plexi on either side of the dentate line (Fig 1) can become dilated, causing hemorrhoids.
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Figure 2a. Metastatic rectal adenocarcinoma with lung metastases. (a–c) Three-dimensional (a), endoscopic (b), and axial 2D (c) images show a polypoid mass (arrows) in the rectum. Note the extension of the mass into the perirectal fat on the right side (arrowheads in c). (d) Axial 2D image reveals a noncalcified lung nodule (arrow).
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Figure 2b. Metastatic rectal adenocarcinoma with lung metastases. (a–c) Three-dimensional (a), endoscopic (b), and axial 2D (c) images show a polypoid mass (arrows) in the rectum. Note the extension of the mass into the perirectal fat on the right side (arrowheads in c). (d) Axial 2D image reveals a noncalcified lung nodule (arrow).
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Figure 2c. Metastatic rectal adenocarcinoma with lung metastases. (a–c) Three-dimensional (a), endoscopic (b), and axial 2D (c) images show a polypoid mass (arrows) in the rectum. Note the extension of the mass into the perirectal fat on the right side (arrowheads in c). (d) Axial 2D image reveals a noncalcified lung nodule (arrow).
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Figure 2d. Metastatic rectal adenocarcinoma with lung metastases. (a–c) Three-dimensional (a), endoscopic (b), and axial 2D (c) images show a polypoid mass (arrows) in the rectum. Note the extension of the mass into the perirectal fat on the right side (arrowheads in c). (d) Axial 2D image reveals a noncalcified lung nodule (arrow).
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Figure 3a. Rectal adenocarcinoma. (a–c) Three-dimensional (a), endoscopic (b), and axial 2D (c) images demonstrate an irregular mass (arrows) in the rectum. An associated enlarged perirectal lymph node (arrowhead in c) upstages the histologically proved adenocarcinoma from stage II to stage III. (d) Axial 2D image obtained at the level of the midabdomen reveals lymphadenopathy (arrows) along the course of the IMV (arrowhead).
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Figure 3b. Rectal adenocarcinoma. (a–c) Three-dimensional (a), endoscopic (b), and axial 2D (c) images demonstrate an irregular mass (arrows) in the rectum. An associated enlarged perirectal lymph node (arrowhead in c) upstages the histologically proved adenocarcinoma from stage II to stage III. (d) Axial 2D image obtained at the level of the midabdomen reveals lymphadenopathy (arrows) along the course of the IMV (arrowhead).
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Figure 3c. Rectal adenocarcinoma. (a–c) Three-dimensional (a), endoscopic (b), and axial 2D (c) images demonstrate an irregular mass (arrows) in the rectum. An associated enlarged perirectal lymph node (arrowhead in c) upstages the histologically proved adenocarcinoma from stage II to stage III. (d) Axial 2D image obtained at the level of the midabdomen reveals lymphadenopathy (arrows) along the course of the IMV (arrowhead).
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Figure 3d. Rectal adenocarcinoma. (a–c) Three-dimensional (a), endoscopic (b), and axial 2D (c) images demonstrate an irregular mass (arrows) in the rectum. An associated enlarged perirectal lymph node (arrowhead in c) upstages the histologically proved adenocarcinoma from stage II to stage III. (d) Axial 2D image obtained at the level of the midabdomen reveals lymphadenopathy (arrows) along the course of the IMV (arrowhead).
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The wall of the rectum consists of five discrete layers: mucosa, muscularis mucosa, submucosa, muscularis propria, and serosa. The valves of Houston are normal folds in the rectal mucosa that gently spiral up the length of the rectum and project into the lumen when it is distended with air. There are usually three valves of Houston, although they can vary in number or even be absent altogether (Figs 1, 4).
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Figure 4a. Valves of Houston. Three-dimensional (a) and sagittal 2D (b) images demonstrate three valves of Houston: superior, middle, and inferior (arrows). The polypoid mass seen in a (arrowhead) represents retained fecal material.
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Figure 4b. Valves of Houston. Three-dimensional (a) and sagittal 2D (b) images demonstrate three valves of Houston: superior, middle, and inferior (arrows). The polypoid mass seen in a (arrowhead) represents retained fecal material.
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CT Colonographic Technique
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The day prior to CT colonography, patients undergo a bowel preparation that typically consists of a clear liquid diet, an orally administered laxative, a cathartic colon preparation, and, possibly, enemas. Oral contrast material for stool and fluid tagging consisting of meglumine diatrizoate (Gastrograffin; Bracco Diagnostics, Princeton, NJ) and a barium sulfate suspension (Tagitol; E-Z-EM, Lake Success, NY) may also be used. A rectal tube with a 30- to 100-mL balloon is inserted for administration of carbon dioxide or room air immediately prior to CT performed with a multi–detector row scanner. The rectal balloon may be deflated during the second axial acquisition to avoid obscuring rectal disease, although this step is not routinely performed. All patients are imaged in both prone and supine positions. Whereas older techniques made use of thicknesses up to 5 mm, the currently recommended section thickness is 0.75–3 mm, with reconstruction intervals of 1–3 mm. Occasionally, patients receive non-ionic intravenous contrast material at standard doses to evaluate intra- or extracolonic findings. The milliamperage is typically reduced for CT colonographic examinations and can range anywhere from 10 to 120 mA. Image interpretation is performed with commercially available software that allows fully interactive 3D evaluation as well as multiplanar 2D reformation.
The rectum is an area in which disease can easily be missed at CT colonography.
The rectum should be evaluated with soft-tissue windowing to detect subtle wall thickening, particularly in areas adjacent to the rectal balloon (Fig 5) where lesions can be flattened.
Polyps can be sought on routine 3D and 2D images obtained with colon windowing (eg, window width = 1760 HU, window level = 380 HU).
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Figure 5a. Adenocarcinoma compressed by a rectal balloon. (a) Endoscopic image shows a rectal carcinoma (arrows) that was not evident at 3D endoluminal CT colonography. (b) On an axial 2D image, the mass (arrow) is being compressed to the right by a rectal balloon. Although the balloon cannot be observed directly, its presence can be inferred by displacement of the rectal soft tissue away from the rectal tube (arrowhead). Histologic findings helped identify adenocarcinoma.
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Figure 5b. Adenocarcinoma compressed by a rectal balloon. (a) Endoscopic image shows a rectal carcinoma (arrows) that was not evident at 3D endoluminal CT colonography. (b) On an axial 2D image, the mass (arrow) is being compressed to the right by a rectal balloon. Although the balloon cannot be observed directly, its presence can be inferred by displacement of the rectal soft tissue away from the rectal tube (arrowhead). Histologic findings helped identify adenocarcinoma.
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Placing the patient in the prone position typically provides better distention of the rectum because of the nondependent location of the rectum in this position, which allows air to rise and distend the area of interest. Deflation of the rectal balloon is best performed when the patient is supine and the rectum is in a dependent position. In older patients, a lack of rectal tone can be problematic and the rectal balloon is critical to maintaining colonic distention, particularly when the patient is prone. Maintenance of colonic distention is less of an issue with younger patients.
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CT Colonography versus Endoscopy in the Evaluation of Rectal Lesions
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The reported advantages and disadvantages of CT colonography and endoscopy in the evaluation of rectal lesions are shown in Tables 1 and 2 (5,8,10,14–24). To our knowledge, there have been no dedicated studies comparing CT colonography with colonoscopy in this specific context. The prevalence of rectal lesions in large studies of 600 patients or more ranges from 9.4% to 23.6% (5,8,10,25).
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Rectal Disease
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Rectal Polyps
The reported prevalence of rectal polyps in the general population ranges from 7% to 50%, with polyps being more common in older patients (26–31). In our study, only 2.7% of 1993 patients had rectal polyps, 43 of which were adenomatous and 10 of which were hyperplastic. Most of the adenomatous polyps were sessile (Figs 6, 7) or pedunculated, but two were flat (Fig 8). The relatively low prevalence of rectal polyps in our study was likely due to the fact that many of the 1993 patients whose records we reviewed had undergone colonoscopy or sigmoidoscopy prior to colonography and had had rectal lesions removed endoscopically.
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Figure 6a. Sessile polypoid adenoma. Three-dimensional (a), endoscopic 2D (b), and axial unenhanced 2D (c) images demonstrate a polypoid mass (arrow) with central ulceration. Findings at endoscopic snare biopsy confirmed an adenoma.
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Figure 6b. Sessile polypoid adenoma. Three-dimensional (a), endoscopic 2D (b), and axial unenhanced 2D (c) images demonstrate a polypoid mass (arrow) with central ulceration. Findings at endoscopic snare biopsy confirmed an adenoma.
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Figure 6c. Sessile polypoid adenoma. Three-dimensional (a), endoscopic 2D (b), and axial unenhanced 2D (c) images demonstrate a polypoid mass (arrow) with central ulceration. Findings at endoscopic snare biopsy confirmed an adenoma.
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Figure 7a. Villous adenoma. Three-dimensional (a), endoscopic 2D (b), and axial 2D (c) images demonstrate a polypoid rectal mass (arrow in a, arrowhead in b and c). The frondlike surface of the mass can be appreciated on the 3D and endoscopic views. At biopsy, the mass proved to be a villous adenoma.
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Figure 7b. Villous adenoma. Three-dimensional (a), endoscopic 2D (b), and axial 2D (c) images demonstrate a polypoid rectal mass (arrow in a, arrowhead in b and c). The frondlike surface of the mass can be appreciated on the 3D and endoscopic views. At biopsy, the mass proved to be a villous adenoma.
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Figure 7c. Villous adenoma. Three-dimensional (a), endoscopic 2D (b), and axial 2D (c) images demonstrate a polypoid rectal mass (arrow in a, arrowhead in b and c). The frondlike surface of the mass can be appreciated on the 3D and endoscopic views. At biopsy, the mass proved to be a villous adenoma.
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Figure 8a. Flat villous adenoma. (a, b) Three-dimensional (a) and endoscopic (b) images show a mass (arrow) with a frondlike surface. Note that on the 3D view, the mass is partially obscured by the rectal tube (R). (c) Two-dimensional image shows the rectal tube (R) abutting the mass. Compare the normal thin rectal wall (arrowheads) with the focal elongated wall thickening (arrows) caused by the flat lesion. The mass proved to be villous adenoma at biopsy.
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Figure 8b. Flat villous adenoma. (a, b) Three-dimensional (a) and endoscopic (b) images show a mass (arrow) with a frondlike surface. Note that on the 3D view, the mass is partially obscured by the rectal tube (R). (c) Two-dimensional image shows the rectal tube (R) abutting the mass. Compare the normal thin rectal wall (arrowheads) with the focal elongated wall thickening (arrows) caused by the flat lesion. The mass proved to be villous adenoma at biopsy.
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Figure 8c. Flat villous adenoma. (a, b) Three-dimensional (a) and endoscopic (b) images show a mass (arrow) with a frondlike surface. Note that on the 3D view, the mass is partially obscured by the rectal tube (R). (c) Two-dimensional image shows the rectal tube (R) abutting the mass. Compare the normal thin rectal wall (arrowheads) with the focal elongated wall thickening (arrows) caused by the flat lesion. The mass proved to be villous adenoma at biopsy.
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Hyperplastic Rectal Polyps.—
A hyperplastic polyp tends to flatten out with air distention (17). Thus, these lesions may not be visible if they are adjacent to the rectal balloon or if the rectum is fully distended. In any case, these lesions are generally less than 5 mm in size, and their detection is of limited clinical significance because they have no malignant potential (32,33).
Adenomatous Rectal Polyps.—
In our experience, rectal adenomas are typically sessile or, less often, pedunculated (34).
Villous-type adenomas have a carpetlike, nodular appearance (Fig 8) and can be easily overlooked in an underdistended rectum.
In addition, these types of lesions can easily be flattened by the rectal balloon. Furthermore, underdistended or redundant rectal mucosa can simulate villous adenomas; therefore, it is important to carefully evaluate any suspected abnormality on both prone and supine images obtained with both colon and soft-tissue windowing.
Rectal Cancer
The national screening average for rectal cancer is 0.01% (30,31). Five adenocarcinomas were identified in our study: Endoscopy showed four of the five adenocarcinomas to be polypoid masses (Figs 2, 3) and one to be flat (Fig 9). One adenocarcinoma was compressed by the rectal balloon during CT colonography; thus, it was visible only on the 2D images (Fig 5). One patient had multiple lymphomatous polyposis (Fig 10), a rare disorder that is characterized by numerous small polypoid lesions covering long segments of the gastrointestinal tract. This condition is typically a manifestation of mantle cell lymphoma (35,36).
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Figure 9a. Rectal adenocarcinoma. Three-dimensional (a), endoscopic (b), and axial 2D (c) images show an irregular mass (arrow) in the rectum. Pathologic findings helped identify adenocarcinoma.
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Figure 9b. Rectal adenocarcinoma. Three-dimensional (a), endoscopic (b), and axial 2D (c) images show an irregular mass (arrow) in the rectum. Pathologic findings helped identify adenocarcinoma.
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Figure 9c. Rectal adenocarcinoma. Three-dimensional (a), endoscopic (b), and axial 2D (c) images show an irregular mass (arrow) in the rectum. Pathologic findings helped identify adenocarcinoma.
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Figure 10a. Gastrointestinal multiple lymphomatous polyposis. (a, b) Three-dimensional (a) and endoscopic (b) images demonstrate innumerable polypoid masses. These masses were initially thought to represent familial polyposis but were diagnosed as multiple lymphomatous polyposis at pathologic analysis. (c) Axial 2D image shows diffuse mucosal irregularity with involvement of the rectal fold (arrow). Arrowhead indicates the rectal tube.
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Figure 10b. Gastrointestinal multiple lymphomatous polyposis. (a, b) Three-dimensional (a) and endoscopic (b) images demonstrate innumerable polypoid masses. These masses were initially thought to represent familial polyposis but were diagnosed as multiple lymphomatous polyposis at pathologic analysis. (c) Axial 2D image shows diffuse mucosal irregularity with involvement of the rectal fold (arrow). Arrowhead indicates the rectal tube.
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Figure 10c. Gastrointestinal multiple lymphomatous polyposis. (a, b) Three-dimensional (a) and endoscopic (b) images demonstrate innumerable polypoid masses. These masses were initially thought to represent familial polyposis but were diagnosed as multiple lymphomatous polyposis at pathologic analysis. (c) Axial 2D image shows diffuse mucosal irregularity with involvement of the rectal fold (arrow). Arrowhead indicates the rectal tube.
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Extramucosal Rectal Lesions
CT colonography can be particularly useful for evaluating extramucosal lesions because, unlike endoscopy, it can easily help evaluate extraluminal extent.
Endoscopic biopsy of these types of abnormalities is of limited diagnostic value if the abnormality is located primarily in the submucosa or serosa. In such cases, CT colonography can depict the lesion as submucosal, serosal, or extracolonic and can direct the clinician as to what step should be taken next for further evaluation.
In our study, there were seven extramucosal masses, including two direct serosal metastases, two lymph node metastases, one leiomyoma (Fig 11), one perirectal abscess, and one uterine leiomyosarcoma that had invaded the rectal wall (Fig 12).
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Figure 11a. Leiomyoma. (a, b) Three-dimensional (a) and axial 2D (b) images show a small polypoid lesion (arrow) in the rectum. The mucosa overlying the lesion is intact, and the attenuation of the lesion is similar to that of muscle. (c) Image obtained during endoscopic snare biopsy also shows the lesion (arrow).
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Figure 11b. Leiomyoma. (a, b) Three-dimensional (a) and axial 2D (b) images show a small polypoid lesion (arrow) in the rectum. The mucosa overlying the lesion is intact, and the attenuation of the lesion is similar to that of muscle. (c) Image obtained during endoscopic snare biopsy also shows the lesion (arrow).
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Figure 11c. Leiomyoma. (a, b) Three-dimensional (a) and axial 2D (b) images show a small polypoid lesion (arrow) in the rectum. The mucosa overlying the lesion is intact, and the attenuation of the lesion is similar to that of muscle. (c) Image obtained during endoscopic snare biopsy also shows the lesion (arrow).
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Figure 12a. Endometrial sarcoma. (a, b) Three-dimensional (a) and endoscopic (b) images show a bilobed polypoid mass (arrow) projecting into the rectal lumen. (c) On an axial contrast material–enhanced 2D image, the polypoid mass (arrow) clearly projects into the colon and is contiguous with a uterine mass (M). (Reprinted, with permission, from reference 37.)
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Figure 12b. Endometrial sarcoma. (a, b) Three-dimensional (a) and endoscopic (b) images show a bilobed polypoid mass (arrow) projecting into the rectal lumen. (c) On an axial contrast material–enhanced 2D image, the polypoid mass (arrow) clearly projects into the colon and is contiguous with a uterine mass (M). (Reprinted, with permission, from reference 37.)
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Figure 12c. Endometrial sarcoma. (a, b) Three-dimensional (a) and endoscopic (b) images show a bilobed polypoid mass (arrow) projecting into the rectal lumen. (c) On an axial contrast material–enhanced 2D image, the polypoid mass (arrow) clearly projects into the colon and is contiguous with a uterine mass (M). (Reprinted, with permission, from reference 37.)
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Inflammatory Disease
Ulcerative colitis has a global distribution but is more common in northern than in southern locations in the United States and Europe. It is thought to be up to twice as prevalent as Crohn disease and, as a general rule, will involve the rectum (38,39). Crohn disease, on the other hand, will typically spare the rectum but is commonly associated with perianal fistulas that can involve the rectum.
In our series, one patient with Crohn disease underwent imaging because a rectal stricture precluded the completion of screening colonoscopy. CT colonography helped identify the rectal stricture and facilitated evaluation of the rest of the colon (Fig 13). A second patient had biopsy-proved solitary rectal ulcer syndrome (SRUS) (Fig 14).
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Figure 13a. Rectal stricture from known Crohn disease. (a) Oblique volume-rendered image demonstrates focal collapse (arrows) near the rectosigmoid junction due to an inflammatory stricture. (b–d) Three-dimensional (b), endoscopic (c), and axial 2D prone (d) images show the stricture lumen (arrow).
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Figure 13b. Rectal stricture from known Crohn disease. (a) Oblique volume-rendered image demonstrates focal collapse (arrows) near the rectosigmoid junction due to an inflammatory stricture. (b–d) Three-dimensional (b), endoscopic (c), and axial 2D prone (d) images show the stricture lumen (arrow).
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Figure 13c. Rectal stricture from known Crohn disease. (a) Oblique volume-rendered image demonstrates focal collapse (arrows) near the rectosigmoid junction due to an inflammatory stricture. (b–d) Three-dimensional (b), endoscopic (c), and axial 2D prone (d) images show the stricture lumen (arrow).
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Figure 13d. Rectal stricture from known Crohn disease. (a) Oblique volume-rendered image demonstrates focal collapse (arrows) near the rectosigmoid junction due to an inflammatory stricture. (b–d) Three-dimensional (b), endoscopic (c), and axial 2D prone (d) images show the stricture lumen (arrow).
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Figure 14a. SRUS. Three-dimensional (a), axial 2D (b), and endoscopic (c) images demonstrate an irregular rectal mass (arrows), a finding that is worrisome for neoplasm. However, histopathologic and clinical features were characteristic of SRUS.
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Figure 14b. SRUS. Three-dimensional (a), axial 2D (b), and endoscopic (c) images demonstrate an irregular rectal mass (arrows), a finding that is worrisome for neoplasm. However, histopathologic and clinical features were characteristic of SRUS.
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Figure 14c. SRUS. Three-dimensional (a), axial 2D (b), and endoscopic (c) images demonstrate an irregular rectal mass (arrows), a finding that is worrisome for neoplasm. However, histopathologic and clinical features were characteristic of SRUS.
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SRUS is a rare condition frequently associated with chronic constipation and pelvic floor dyssynergia (40,41). Affected patients typically present with rectal bleeding and pain on defecation. The central rectal ulceration elicits a regional inflammatory response, causing mass effect in the surrounding tissue. This condition frequently leads to initial misdiagnosis of the inflammatory mass as a rectal neoplasm at endoscopy, as was the case in the affected patient in our study. Other colitides that can involve the rectum, such as pseudomembranous colitis, can manifest as wall thickening and perirectal fat stranding or fluid. However, these findings are not specific, and further investigation is often required for diagnosis.
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Imaging Pitfalls
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Evaluation of the rectum at CT colonography is subject to the same well-known limitations of evaluation of the rest of the colon—most notably, incomplete distention and retained stool that mimics polypoid lesions (14,15). Stool can be reliably differentiated from polyps if it contains air (Fig 15), is tagged with barium, or is mobile (Fig 16). Conversely, stool may mimic a polyp if it adheres to the colonic wall and does not contain air or barium. We routinely use oral stool tagging to minimize both false-positive and false-negative findings because we have found it useful in conjunction with CT colonography (16,42,43). Intravenous contrast material, when used as part of a diagnostic CT colonographic examination, can also help distinguish between a polyp and stool (44–46). Another potential benefit of using intravenous contrast material in this setting is the ability to differentiate hemorrhoids from polyps or carcinoma (Fig 17).
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Figure 15a. Air-containing stool mimicking a sessile polyp in the same patient as in Figure 14. (a) Endoluminal image reveals a polyplike mass (arrow) on the dependent wall of the rectum. (b) Axial 2D image helps confirm internal heterogeneity (arrow) due to foci of air, a finding that is characteristic of stool.
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Figure 15b. Air-containing stool mimicking a sessile polyp in the same patient as in Figure 14. (a) Endoluminal image reveals a polyplike mass (arrow) on the dependent wall of the rectum. (b) Axial 2D image helps confirm internal heterogeneity (arrow) due to foci of air, a finding that is characteristic of stool.
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Figure 16a. Mobile stool in the rectum. Three-dimensional (a), axial 2D supine (b), and right lateral decubitus (c) images demonstrate a homogeneous filling defect (arrow) that moves from the posterior wall to the right lateral wall on separate acquisitions, a finding that is consistent with stool. Arrowhead in c indicates the tip of an enema tube.
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Figure 16b. Mobile stool in the rectum. Three-dimensional (a), axial 2D supine (b), and right lateral decubitus (c) images demonstrate a homogeneous filling defect (arrow) that moves from the posterior wall to the right lateral wall on separate acquisitions, a finding that is consistent with stool. Arrowhead in c indicates the tip of an enema tube.
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Figure 16c. Mobile stool in the rectum. Three-dimensional (a), axial 2D supine (b), and right lateral decubitus (c) images demonstrate a homogeneous filling defect (arrow) that moves from the posterior wall to the right lateral wall on separate acquisitions, a finding that is consistent with stool. Arrowhead in c indicates the tip of an enema tube.
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Figure 17a. Internal hemorrhoids in a patient with a history of invasive melanoma. Three-dimensional (a) and axial contrast-enhanced 2D (b) images demonstrate dilated vasculature (arrow) in the anterior rectal wall. This finding was initially interpreted as a hypervascular mass that was worrisome for metastatic melanoma. However, endoscopy performed at another institution demonstrated only internal hemorrhoids.
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Figure 17b. Internal hemorrhoids in a patient with a history of invasive melanoma. Three-dimensional (a) and axial contrast-enhanced 2D (b) images demonstrate dilated vasculature (arrow) in the anterior rectal wall. This finding was initially interpreted as a hypervascular mass that was worrisome for metastatic melanoma. However, endoscopy performed at another institution demonstrated only internal hemorrhoids.
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There are also potential diagnostic pitfalls that are unique to evaluation of the rectum. The most obvious of these pitfalls is the presence of a rectal balloon and enema tube. Figure 8 demonstrates an enema tube tip above an inflated balloon, where it partially obscures a flat polyp. The balloon itself can also compress masses, even large ones, rendering them more difficult to detect at cross-sectional imaging (Fig 5). Fortunately, most large masses that are low enough in the rectum to be obscured by the enema tube tip or balloon are detectable at digital rectal examination, whereas reliably detecting a small polyp at clinical examination alone seems unlikely.
Orthopedic hardware, such as hip arthroplasty devices, can severely limit the usefulness of CT colonography in the pelvis, particularly in the rectum (Fig 18). Metal artifact suppression software has improved significantly in the past few years and may soon be effective in improving the image quality of colonography (47,48).
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Figure 18a. Streak artifact from a hip prosthesis obscuring a flat adenoma. (a) Endoscopic image demonstrates a small mass (arrows). (b) Three-dimensional image fails to depict the mass. (c) On an axial 2D image (lung windowing), the mass (arrow) is mostly obscured due to severe degradation by streak artifact caused by hip arthroplasty hardware. Endoscopic biopsy yielded pathologic findings of adenoma.
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Figure 18b. Streak artifact from a hip prosthesis obscuring a flat adenoma. (a) Endoscopic image demonstrates a small mass (arrows). (b) Three-dimensional image fails to depict the mass. (c) On an axial 2D image (lung windowing), the mass (arrow) is mostly obscured due to severe degradation by streak artifact caused by hip arthroplasty hardware. Endoscopic biopsy yielded pathologic findings of adenoma.
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Figure 18c. Streak artifact from a hip prosthesis obscuring a flat adenoma. (a) Endoscopic image demonstrates a small mass (arrows). (b) Three-dimensional image fails to depict the mass. (c) On an axial 2D image (lung windowing), the mass (arrow) is mostly obscured due to severe degradation by streak artifact caused by hip arthroplasty hardware. Endoscopic biopsy yielded pathologic findings of adenoma.
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Conclusions
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Compared with the rest of the colon, the rectum presents unique challenges for the detection and diagnosis of disease with CT colonography. Familiarity with the spectrum of rectal disease and with the potential pitfalls and technical limitations of CT colonography should help decrease interpretative and perceptual errors.
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Footnotes
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Abbreviations: IMV = inferior mesenteric vein, SRUS = solitary rectal ulcer syndrome, 3D = three-dimensional, 2D = two-dimensional
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Abstract
Introduction
