CT Colonography: Current Status and Future Promise




CT colonography (CTC) is an innovative technology that entails CT examination of the entire colon and computerized processing of the raw data after colon cleansing and colonic distention. CTC could potentially increase the screening rate for colon cancer because of its relative safety, relatively low expense, and greater patient acceptance, but its role in mass colon cancer screening is controversial because of its highly variable sensitivity, the inability to sample polyps for histologic analysis, and lack of therapeutic capabilities. This article reviews the CTC literature, including imaging and adjunctive techniques, radiologic interpretation, procedure indications, contraindications, risks, sensitivity, interpretation pitfalls, and controversies.


Colorectal cancer is one of the most commonly diagnosed cancers in the world with approximately 1 million new cases diagnosed annually and approximately half a million annual deaths . The highest incidence occurs in North America and Europe . Colon cancer is the second most common cause of mortality from cancer in the United States, causing more than 52,000 deaths in 2006 . The lifetime risk of colon cancer in America is about 1 in 17 . Colon cancer characteristically begins as slow-growing premalignant polyps that take approximately 10 years to progress to carcinoma . It therefore is believed that colon cancer is largely preventable by mass screening to detect and then remove such polyps . Only one half of the average-risk population in the United States undergoes any of the currently available screening tests for colorectal cancer, however , because of safety concerns, test invasiveness, inconvenience, costs, embarrassment, and lack of education concerning colon cancer screening .


CT colonography (virtual colonoscopy, CTC) is an innovative technology that entails CT examination of the entire colon and computerized processing of the raw data after colon cleansing and colonic distention. CTC currently is under intense analysis as a screening test that potentially could increase the screening rate for colon cancer because of its relative safety, relatively low expense, and greater patient acceptance as compared with other current screening methods. Its role in mass colon cancer screening is controversial, however, because of its highly variable reported sensitivity, concerns about specificity because of the inability to sample polyps for histologic analysis, and lack of therapeutic capabilities.


A review of this topic is both timely and important because CTC is evolving rapidly due to improvements in methodology and computerized technology as well as new clinical data. To help clinicians optimize colon cancer screening for their patients, this article reviews the exponentially expanding literature on CTC, including imaging techniques, adjunctive techniques, radiologic interpretation, procedure indications, contraindications, risks, sensitivity, interpretation pitfalls, and controversies.


Pathogenesis of Colon Cancer


Colon cancer arises from mucosal polyps. The critical polyp characteristic in terms of malignant potential is histology. The two most common histologic types are hyperplastic and adenomatous. According to the traditional adenoma-to-carcinoma sequence, nearly all colon cancers arise from adenomatous polyps, as demonstrated by epidemiologic, clinical, pathologic, and genetic data. About one third of operative specimens with colon cancer contain one or more synchronous adenomas, a significantly higher rate than in age-matched controls without colon cancer . The risk of colon cancer increases markedly with increasing number of adenomatous polyps . Adenomatous tissue frequently is found contiguous to frank carcinoma . Patients who refuse polypectomy for colonic adenomas develop colon cancer at a rate of about 4% after 5 years and 14% after 10 years . Patients who have familial adenomatous polyposis, who have hundreds or thousands of adenomatous colonic polyps, inevitably develop colon cancer if prophylactic colectomy is not performed . Patients who have sporadic adenomas have genetic mutations in the same APC gene as patients who have syndromic colon cancer from familial adenomatous polyposis .


Hyperplastic polyps traditionally were believed to have negligible or no malignant potential , but findings of mixed hyperplastic-adenomatous polyps , clustering of adenomas and colon cancers adjacent to hyperplastic polyps , and a high risk of colon cancer in patients who have syndromic hyperplastic polyposis provided evidence that some hyperplastic polyps may increase the risk of colon cancer. A distinct histologic subset of colonic polyps previously classified as hyperplastic recently has been shown to have a significant risk of progressing to colon cancer and has been reclassified as serrated adenomas (see the article by East and Jass, elsewhere in this issue). The genetic basis of this cancer risk is being elucidated currently .


Benign premalignant colonic polyps occur through at least three distinct genetic mechanisms: mutation of the APC gene leading to adenomatous polyps, genetic changes associated with serrated adenomas, and mutations in several mismatch repair genes leading to sessile adenomatous polyps . Further mutations result in malignant transformation.




Principles of Colon Cancer Screening


Application of a screening test to the general population is predicated on (1) the disease to be screened being sufficiently prevalent in the population to justify the cost and effort of mass screening, (2) the disease being easily treatable and curable when detected early or in a precursor phase but highly fatal or otherwise severe when detected late or in an advanced stage, and (3) the availability of a second diagnostic or reference test to be performed on patients who have a positive screening test. Colon cancer satisfies all three criteria. It is (1) extremely common in American patients over 50 years old, with 150,000 cases diagnosed per annum , (2) highly lethal when advanced (Dukes stage D) but reliably curable by surgery when detected very early (Dukes stage A) and preventable by colonoscopic polypectomy when detected as a premalignant adenomatous polyp , and (3) has colonoscopy available as a reference or diagnostic standard to confirm the existence of polyps or cancer and to remove detected polyps for histologic characterization and prevention of colon cancer. Colon cancer screening therefore is highly desirable and important for public health.


A screening test, for colon cancer or for any other disease, ideally should be (1) relatively inexpensive, because it must be performed on so many patients (eg, for colon cancer screening, the approximately 77,000,000 Americans ≥ 50 years old ); (2) very safe, because so many patients will be subjected to the test; (3) relatively noninvasive and acceptable to the general population so that few patients will refuse to undergo screening; and (4) highly sensitive so as not to miss patients who have the condition (false negatives). A screening test need be only moderately specific; however, poor specificity would result in an unacceptably large number of patients (false positives) required to undergo a second, diagnostic test, thereby increasing the costs and risks of mass screening.


Development of a practical screening test for colon cancer that is acceptable to the general public has proven troublesome and elusive ( Table 1 ) . Only about 50% of eligible Americans have undergone any form of colon cancer screening . The requirements for a screening test have become more rigorous with the recent focus on detection of benign colonic adenomas rather than only colon cancer. For example, the sensitivity of fecal occult blood testing is several-fold lower for colonic polyps than for colon cancer .



Table 1

Historical perspective on screening tests for colon cancer and colonic polyp detection
































Historical test Benefits Major drawbacks
Fecal occult blood Very cheap, very safe, and very noninvasive


  • Only moderately sensitive at detecting colon cancer. Poorly sensitive at detecting colonic polyps . Poor specificity.



  • Sensitivity of test may be markedly better for fecal immunochemical tests than for fecal guaiac tests .

Flexible sigmoidoscopy Relatively cheap, much less invasive than colonoscopy. Only examines colonic region responsible for about 60% of colon cancers or polyps .
Fecal molecular markers Very safe and very noninvasive. Costs would decrease if widely used. Currently experimental. Needs further research and development to improve test sensitivity .
Barium enema Safer and cheaper than colonoscopy Has become a secondary screening test because of poor sensitivity in detecting colonic polyps .
Colonoscopy Highly sensitive and highly specific, provides diagnosis and therapy for benign polyps Relatively expensive, somewhat invasive, some risks, and inconvenient for patients .
CT colonography Safer and cheaper than colonoscopy and probably more patient friendly Probably is less sensitive than colonoscopy for polyp detection, especially for smaller polyps. Misses many flat colonic adenomas . Poor specificity because histology of polyps undetermined. Lacks therapeutic capabilities, so patients who have a positive finding of a colonic polyp must undergo subsequent colonoscopy.




Principles of Colon Cancer Screening


Application of a screening test to the general population is predicated on (1) the disease to be screened being sufficiently prevalent in the population to justify the cost and effort of mass screening, (2) the disease being easily treatable and curable when detected early or in a precursor phase but highly fatal or otherwise severe when detected late or in an advanced stage, and (3) the availability of a second diagnostic or reference test to be performed on patients who have a positive screening test. Colon cancer satisfies all three criteria. It is (1) extremely common in American patients over 50 years old, with 150,000 cases diagnosed per annum , (2) highly lethal when advanced (Dukes stage D) but reliably curable by surgery when detected very early (Dukes stage A) and preventable by colonoscopic polypectomy when detected as a premalignant adenomatous polyp , and (3) has colonoscopy available as a reference or diagnostic standard to confirm the existence of polyps or cancer and to remove detected polyps for histologic characterization and prevention of colon cancer. Colon cancer screening therefore is highly desirable and important for public health.


A screening test, for colon cancer or for any other disease, ideally should be (1) relatively inexpensive, because it must be performed on so many patients (eg, for colon cancer screening, the approximately 77,000,000 Americans ≥ 50 years old ); (2) very safe, because so many patients will be subjected to the test; (3) relatively noninvasive and acceptable to the general population so that few patients will refuse to undergo screening; and (4) highly sensitive so as not to miss patients who have the condition (false negatives). A screening test need be only moderately specific; however, poor specificity would result in an unacceptably large number of patients (false positives) required to undergo a second, diagnostic test, thereby increasing the costs and risks of mass screening.


Development of a practical screening test for colon cancer that is acceptable to the general public has proven troublesome and elusive ( Table 1 ) . Only about 50% of eligible Americans have undergone any form of colon cancer screening . The requirements for a screening test have become more rigorous with the recent focus on detection of benign colonic adenomas rather than only colon cancer. For example, the sensitivity of fecal occult blood testing is several-fold lower for colonic polyps than for colon cancer .



Table 1

Historical perspective on screening tests for colon cancer and colonic polyp detection
































Historical test Benefits Major drawbacks
Fecal occult blood Very cheap, very safe, and very noninvasive


  • Only moderately sensitive at detecting colon cancer. Poorly sensitive at detecting colonic polyps . Poor specificity.



  • Sensitivity of test may be markedly better for fecal immunochemical tests than for fecal guaiac tests .

Flexible sigmoidoscopy Relatively cheap, much less invasive than colonoscopy. Only examines colonic region responsible for about 60% of colon cancers or polyps .
Fecal molecular markers Very safe and very noninvasive. Costs would decrease if widely used. Currently experimental. Needs further research and development to improve test sensitivity .
Barium enema Safer and cheaper than colonoscopy Has become a secondary screening test because of poor sensitivity in detecting colonic polyps .
Colonoscopy Highly sensitive and highly specific, provides diagnosis and therapy for benign polyps Relatively expensive, somewhat invasive, some risks, and inconvenient for patients .
CT colonography Safer and cheaper than colonoscopy and probably more patient friendly Probably is less sensitive than colonoscopy for polyp detection, especially for smaller polyps. Misses many flat colonic adenomas . Poor specificity because histology of polyps undetermined. Lacks therapeutic capabilities, so patients who have a positive finding of a colonic polyp must undergo subsequent colonoscopy.




CT Colonography


Vining and Gelfand from Bowman Gray University introduced CTC and exhibited the first CTC fly-through video in 1994 at the annual meeting of the Society of Gastrointestinal Radiologists. The original test took 50 seconds for data acquisition and 8 hours for computer processing . The data acquisition time now is only 15 seconds using multislice CT scanners, and the computer processing time is less than 10 minutes . CTC depends on (1) an adequately cleansed colon, (2) an adequately distended colon, (3) rapid thin-section CT examination in both prone and supine positions, (4) computerized processing of the data to generate two-dimensional (2D) images in the axial, sagittal, and coronal planes or three-dimensional (3D) virtual colonoscopy images, and (5) study interpretation by a well-trained and experienced reader.


Patient Preparation


Patient referral


When scheduling CTC, the referring physician and the radiology department should ascertain whether the patient has a valid indication for CTC ( Box 1 ) without contraindications and should verify that the patient has the correct prescriptions and proper directions for bowel preparation.



Box 1





  • Incomplete colonoscopy because an obstructing mass or stricture prevented examination of the proximal colon



  • Incomplete colonoscopy because of colonic tortuosity, adhesions, severe diverticular disease, or patient intolerance of colonoscopy



  • Inability to perform colonoscopy because of a strong requirement for anticoagulant therapy or risks of sedation



  • Patients who have a strong indication for diagnostic colonoscopy but who adamantly refuse to undergo colonoscopy



Current generally accepted indications for CTC


Relative contraindications to CTC include




  • Severe allergy to administered contrast (CTC can be performed without contrast)



  • Suspected colonic perforation or peritonitis



  • Walled off colonic leak/pericolonic abscess



  • Medically highly unstable patient (eg, unstable angina, uncontrolled sepsis)



  • Acute lower gastrointestinal bleeding



  • Pregnancy



  • Inability to tolerate pneumocolon



  • Highly uncooperative patient



  • Inability to undergo colonic preparation: congestive heart failure, severe electrolyte imbalances, severe dehydration



  • Refusal to undergo colonic preparation



  • Abnormal anorectal anatomy (eg, imperforate anus, tight anal stricture)



  • Severe colonic disease (toxic colitis, toxic megacolon, severe colonic pseudo-obstruction)



  • Acute colonic infection (acute diverticulitis, severe infectious colitis)



  • Complete mechanical colonic obstruction



  • Very recent colonic surgery (<1 week)



Colonic cleansing


Proper bowel preparation is essential. Residual fecal material can mimic or obscure polyps. A low-residue and/or clear liquid diet is instituted 24 hours before CTC. Bowel preparations include colonic lavage solutions, such as polyethylene glycol, and cathartics, such as magnesium citrate or phospho-soda (sodium phosphate). The patient should be advised to take the entire colonic preparation as prescribed. Polyethylene glycol, the standard preparation for conventional colonoscopy, leaves less residual fecal material but more residual fluid in the colon than either magnesium citrate or sodium phosphate preparations at barium enema or CTC . Residual fluid can be eliminated during conventional colonoscopy by colonoscopic aspiration but cannot be eliminated during CTC and can limit CTC sensitivity. Therefore, a “dry prep” with a saline laxative of phospho-soda or magnesium citrate, combined with bisacodyl tablets the night before CTC and a bisacodyl suppository the morning of CTC is recommended to eliminate residual fluid. Patients administered phospho-soda require adequate hydration. This preparation is absolutely contraindicated in patients who have renal insufficiency or congestive heart failure and is relatively contraindicated in patients who have large ascites or ileus because of potentially large fluid and electrolyte shifts or induction of renal failure . Magnesium citrate is a useful alternative in such patients. The patient evacuates any residual colonic fluid just before CTC.


Oral contrast


Residual colonic fluid decreases polyp detection by obscuring the contrast (difference in attenuation) between the bowel wall and the air-filled lumen. Administration of iodinated oral contrast theoretically opacifies (increases the attenuation of) residual fluid, allowing for the detection of otherwise invisible polyps submerged in intraluminal fluid ( Fig. 1 ). Fluid tagging has shown mixed results because of unsatisfactory fluid attenuation and inadequate polyp detection despite adequate fluid attenuation . It may yield a sensitivity as low as 78% for polyps 10 mm or larger . Moreover, techniques such as dual-position image acquisition can increase polyp detection significantly without fluid tagging . Fluid tagging requires ingestion of substantial amounts of liquid.




Fig. 1


Fluid tagging. In this axial 2D supine image with a soft tissue window, a large polyp ( arrow ) is demonstrated in the dependent portion of the midtransverse colon as a relatively low-density mass compared with the high-density tagged fluid.


Barium can be used to tag particulate stool. Small amounts of barium are ingested during several meals over several days before CTC to become incorporated into fecal material. The contrast-enhanced stool is identified easily by its high attenuation and is differentiated easily from colonic mucosa. Barium contrast with a reduced bowel preparation may yield sensitivity and specificity at least equal to that of a standard bowel preparation without barium . Tagging of liquid or solid stool precludes colonoscopy for removal of an identified colonic polyp immediately after CTC.


Colonic distention


Adequate colonic distention is critical to increase polyp conspicuity. Air provides a low-density background to help identify medium-density colonic polyps or masses. The patient is placed in a decubitus position on the CT scanning table. A small plastic or rubber catheter is introduced into the rectum to deliver room air or carbon dioxide by either a handheld bulb pump or automated insufflator to distend the colon. Room air is inexpensive and simply administered manually. Carbon dioxide is more expensive, requires a gas tank, and is administered automatically. Automated insufflation permits control of the rate of insufflation and provides a maximal intraluminal pressure with automatic shut-off capabilities, a potential safety benefit. The adequacy of colonic distention is ascertained by a scout CT film with the patient in the supine position, and more carbon dioxide is administered as needed.


Carbon dioxide and automated insufflation improve colonic distention compared with room air and manual insufflation, respectively . Patient discomfort with carbon dioxide insufflation is similar to that with room air insufflation during the procedure, but there is less discomfort afterwards because of the rapid absorption of carbon dioxide from the colonic lumen . Most reported colonic perforations have involved room air and manual insufflation, but several perforations have involved carbon dioxide and automated insufflation . Colonic perforation is extremely rare (0.059%–0.08%). Risk factors for perforation include advanced age and underlying colonic pathology .


Spasmolytics


Patient discomfort can limit colonic distention and significantly reduce CTC sensitivity. Smooth muscle relaxants such as glucagon improve bowel relaxation, decrease patient discomfort, and improve colonic filling during barium enema . Glucagon also enhances the reflux of colonic air into the small intestine by relaxing the ileocecal valve; this reflux unfortunately reduces colonic distention. Routine administration of glucagon during CTC is controversial. In a study of 33 patients administered glucagon before CTC versus 27 patients not administered glucagon, glucagon did not significantly improve colonic distention .


Scanning Technique


After colonic insufflation, a thin-section abdominopelvic CT is performed from the level of the diaphragm to the level of the perineum. Images are obtained in both the supine and prone positions to (1) differentiate particulate stool from fixed lesions such as polyps or cancers ( Fig. 2 ); (2) distend adequately colonic regions poorly distended in one position because the air is redistributed with a change in patient position; and (3) evaluate adequately colonic regions obscured by residual fluid because fluid is redistributed with a change in patient position. The optimal scanning technique should minimize radiation exposure, minimize scanning time, and maximize image quality.




Fig. 2


Usefulness of prone and supine images to distinguish stool from a true polyp based on mobility. ( A and B ). Mobile apparent “polyp” (stool). ( A ) The supine 2D axial image shows three polypoid aggregations of stool ( arrows ) lying on the dependent (posterior) wall of the colon. ( B ) The prone 2D axial image shows that these three aggregations of stool ( arrows ) have moved to the now-dependent (anterior) wall of the colon. ( C and D ). Immobile true polyp. ( C ) The supine 2D axial image and ( D ) the prone 2D axial image reveal that these three true colonic polyps ( arrows ) remain fixed when the patient turns from supine to prone.


Radiation exposure


CTC is performed at a 40% to 50% lower radiation dose than conventional abdominopelvic CT. The lower exposure is accomplished by lowering the scan time to 50 to 100 mAs and the scan energy/dose to less than 120 kVp . With the use of multidetector CT scanners, the radiation dose is about half of the radiation dose of a barium enema.


Scanning time


Multidetector CT scanners have the advantage of decreased acquisition time compared with single-detector scanners. For example, the scanning time for a 16-detector CT scanner with 0.75-mm collimation is only 15 seconds, whereas the scanning time for a single-detector scanner with 5-mm collimation is typically 35 to 40 seconds. Multidetector CT machines therefore significantly reduce breath-hold times and essentially eliminate respiratory motion artifacts that otherwise would degrade the image quality.


Image quality


Slice thickness affects polyp conspicuity. Every 1-mm decrease in slice thickness increases polyp detection sensitivity by 5% . Multidetector CT scanners increase the sensitivity for polyp detection compared with single-detector scanners because of the decreased slice thickness. A multidetector CT scan with collimation of 3 mm or less is recommended .


Intravenous contrast


Intravenous contrast increases bowel wall conspicuity and improves detection of medium-sized polyps (6–9 mm), especially in a suboptimally prepared colon . Contrast enhancement patterns of polyps, however, are not correlated with histology . Intravenous contrast helps detect synchronous colonic lesions and lymph node involvement in patients who have newly diagnosed colorectal carcinoma, improves the accuracy of preoperative T-and-N staging for colorectal cancer , and aids in the detection of local recurrence or distant metastases during colon cancer surveillance . Intravenous contrast is very useful in patients who have had an incomplete conventional colonoscopy because of obstructive colon cancer, and in patients with prior colonic resection for carcinoma because of their frequently inadequate colonic distention. It generally is reserved for these and similar indications.


Disadvantages of intravenous contrast include increased cost, increased invasiveness, occasional contrast reactions, higher radiation dose, and increased interpretation time. Also, intravenous contrast should not be administered simultaneously with oral contrast because the combination enhances both colonic mucosa and colonic lumen and results in a decrease in contrast between the two surfaces.


Interpretation of CT Colonography


After acquiring supine and prone scans, various software packages display images in both 2D and 3D (endoluminal) views.


Primary two-dimensional interpretation


For 2D evaluation, the colon should be able to be viewed in the conventional axial plane or in coronal and sagittal planes . The reader should be able to link supine and prone axial images taken at the same level and place the images side by side. Initially the entire colon is evaluated with lung windows in the axial plane. Images are scrolled from the rectum to cecum to assess the entire colonic mucosal surface on both supine and prone images. Raised areas are evaluated further as needed in the coronal and/or sagittal planes and with soft tissue window settings to determine if the abnormality is a fold, mucosal polyp, lipoma, extrinsic mass, stool, or ileocecal valve ( Fig. 3 ). Soft tissue windows help differentiate mucosal polyps from submucosal lipomas or extracolonic abnormalities and help detect flat colonic lesions.




Fig. 3


Usefulness of soft tissue windows. ( A ) On the 2D axial lung window image, there is an apparent polyp ( arrow ) on the right lateral wall of the ascending colon. ( B ) On the 2D axial soft tissue window image, the “polyp” ( arrow ) is shown to have fat within it, demonstrating that it is a lipoma.


Primary three-dimensional interpretation


As with the primary 2D method, in the primary 3D interpretation the entire colon is evaluated on both supine and prone images. The colon is evaluated both antegrade and retrograde to visualize regions obscured by colonic folds. The software should permit the reader to fly through the colon either automatically at a variable speed or manually. Primary 3D interpretation is supplemented by 2D views, because polyps, lipomas, and stool cannot be distinguished from each other reliably solely by 3D images.


Two-dimensional versus three-dimensional interpretation


Primary 2D versus 3D evaluation is controversial currently, but most authorities agree that 2D and 3D evaluations are complementary. Most radiologists use 2D for primary evaluation and 3D for problem solving. Reader preference, training, and imaging software often determine the used technique. The computer software should provide a user-friendly means of displaying 2D and 3D views of the same region.


In a study of 42 patients, two different methods were used to evaluate CTC examinations. In method 1, 2D data sets were the primary method of evaluation with 3D display used only if findings were suggestive of an abnormality. In method 2, 2D and 3D fly-through evaluations were performed in all patients. This study revealed that both methods identified the same polyps, but method 1 took less time than method 2 (average of 16 minutes versus average of 40 minutes, respectively) . Pickhardt and colleagues evaluated 1233 asymptomatic patients using primary 3D evaluation, with 2D reserved for problem solving, following colon cleansing and fecal tagging with automated subtraction of tagged material. They reported excellent sensitivities for detection of adenomatous polyps: 92% for polyps 10 mm and larger, 93% for polyps 8 mm and larger, and 88% for polyps 6 mm and larger.


Pitfalls of CT Colonography


Pitfalls of two-dimensional and three-dimensional interpretation


Numerous factors can limit the accuracy of CTC:




  • Stool/inadequately prepared colon



  • Inadequate colonic distention



  • Colonic spasm



  • Uncooperative patient



  • Motion artifacts from respiration



  • Old CT machine (single-detector CT scanner)



  • Inadequate radiologist training



  • Failure to read study using both 2D and 3D imaging



  • Flat (nonpolypoid) colonic lesions that are hard to detect by CTC



Technical errors are reduced or avoided by adequate bowel cleansing, adequate colonic distention, and adherence to the CT scanning protocol. Residual fecal material can obscure or mimic a colonic lesion. Several maneuvers can differentiate residual fecal material from true colonic polyps . Fecal material usually does not adhere to the colonic wall and will move to the dependent colon when the patient turns from supine to prone. Rarely, stool adheres to the colonic wall, rendering it difficult to differentiate from a colonic polyp. Adherent stool typically has an irregular shape, contains air, and contains foci of high attenuation ( Fig. 4 ). Occasionally, a pedunculated polyp attached to the bowel wall via a long stalk will seem to migrate when the patient turns.




Fig. 4


Characteristics that help differentiate fecal material from polyps on CT colonography. ( A ) Fecal material ( within circle ) typically has an irregular contour, has a heterogeneous density, and usually contains air. ( B ) True polyps ( within circle ) typically have a smooth contour and a homogeneous density.


With colonic underdistention, a narrowed colonic segment can mimic an annular lesion, and a collapsed colonic segment can prevent visualization of a true colonic lesion. Each colonic segment therefore must be adequately distended on at least one of the two (prone or supine) scans.


Pitfalls of two-dimensional interpretation


Perception errors (failure to detect a lesion) may be caused by technical factors (eg, bowel preparation, bowel distention), the primary reading technique (2D versus 3D), or reader skill and experience. Misinterpretation of findings is a major source of error. CTC has numerous potential pitfalls that often can be avoided by careful analysis of the morphology of every potential lesion on multiple views and with several maneuvers ( Table 2 ) . Polyps have rounded or lobulated contours and homogeneous soft tissue attenuation. Submucosal lesions can mimic the smooth and polypoid appearance of mucosal polyps. For instance, lipomas can mimic polyps on 3D and 2D images evaluated with lung windows. The fat within a lipoma is easily recognized, however, on 2D images evaluated with soft tissue windows .



Table 2

Differentiation of true mucosal colonic polyps or masses from artifacts or other filling defects


































Artifact or Abnormality Differentiation from a true mucosal polyp
Fecal matter Stool usually moves to the dependent colon because of gravity when the patient changes position (eg, supine to prone). Polyps do not migrate because they are attached to mucosa. Stool typically has an irregular shape and often contains low-density air or high-density food particles. Polyps usually have a rounded or lobulated contour and are relatively homogeneous in density. Best to avoid stool by using a proper bowel preparation.
Lipomas Lipomas can mimic polyps when evaluated with lung windows settings but have low attenuation characteristic of fat when evaluated with soft tissue window settings.
Inverted appendiceal stump When a putative lesion is found near the expected location of the appendiceal orifice, the normal appendix should be identified carefully. If the appendix is not identified, determine whether patient had undergone appendectomy. If the appendix is identified, the putative lesion is likely to be real.
Prominent ileocecal valve Bulbous valve can mimic a cecal mass on 3D images. Valve typically contains macroscopic fat demonstrated by soft tissue windows. Serially follow images from terminal ileum to the valve on 2D images.
Diverticulum On 3D images a diverticulum characteristically has a complete, densely dark ring at its orifice because of air, whereas a polyp has an incomplete ring shadow at its base.
Inverted diverticulum When 2D soft tissue window settings are used, an inverted diverticulum has pericolonic fat, whereas a polyp has the density of soft tissue.
Extrinsic compression The extrinsic nature of lesion is usually demonstrated by 2D imaging.
Poorly distended colon A poorly distended colon may mimic an annular colonic lesion (stricture or cancer). Adequate distention of all colonic segments on at least one of the two (prone or supine) views averts this artifact.
Bulbous colonic folds Serial cinegraphic review should clarify that the finding represents a colonic fold rather than a polyp.

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Feb 26, 2017 | Posted by in GASTROENTEROLOGY | Comments Off on CT Colonography: Current Status and Future Promise
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