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CT colonography.

Colorectal cancer is one of the most common cancers in the United States and a leading cause of death in men older than 40. Several screening examinations are available for the disease, each with its own advantages and disadvantages. This Directed Reading examines computed tomography colonography (CTC), a promising newer technique in the fight against colorectal cancer. Preparation for the examination, scanning techniques, interpretation methods and CTC's efficacy are discussed, along with costs, reimbursement policies and patient satisfaction with the exam.

After completing this article, the reader should be able to:

* Compare and contrast screening tests for colorectal cancer.

* Describe bowel preparation for computed tomography colonography (CTC).

* Discuss insufflation techniques for CTC.

* List and evaluate different approaches radiologists use to interpret CTC examinations.

* Specify the radiation dose for CTC and ways to reduce dose.

* Discuss the sensitivity and specificity of CTC and reasons for the variability of sensitivity ratings.

* Explain reimbursement policies for CTC.

* Describe results of studies on patient satisfaction with colorectal screening examinations.

* Speculate knowledgeably on the future of CTC.


Colon cancer, one of the most common cancers, ranks as the second leading killer of men between the ages of 40 and 79. (1) About 150 000 new cancers and 50 000 deaths are attributed to colon cancer every year in the United States. (2,3) The good news is that colorectal cancer is curable if found and treated in its early stages. (3) Awareness of the disease, screening procedures and endoscopic removal of cancerous and precancerous polyps have caused a decline in the number of colorectal-related fatalities.

A person's risk of developing colorectal cancer is 5% throughout his or her lifetime. Research has demonstrated that 80% to 90% of colorectal cancers develop from small adenomas in a process known as the adenomacarcinoma sequence. (2,4) As a result of genetic mutations, the small adenomas grow into large adenomas (> 10 mm), then into non-invasive carcinomas and finally into invasive carcinomas. (2) It is important to note that only approximately 5% of adenomas mutate and go through the adenoma-carcinoma sequence, and many small lesions regress and disappear over time within the colon. (2,4) The time from small adenoma formation to invasive carcinoma development is 10 to 15 years. (2) Because colon cancer develops slowly over time and seldom has any symptoms, it usually is diagnosed in the advanced stages. (5) Some small lesions seen during endoscopic examination of the colon are normal skin tags, and many are found not to be adenomas after histological examination. (2) When malignant polyps are diagnosed early and treated appropriately, patients have a 5-year survival rate greater than 90%. (5)

Some of the conditions that increase the risk of colorectal cancer are:

* A first-degree relative with colon cancer or large adenomatous polyps found before the age of 60 years.

* Inflammatory bowel disease.

* A history of familial adenomatous polyposis or hereditary nonpolyposis.

* Prior adenomatous polyps or colon cancers. (2)

However, it is important to note that 75% of colon cancers occur in those who are not in a high-risk category. (2)


An effective screening test is one that is used for a common disease, detects that disease in its early stages, is acceptable to patients and whose benefits outweigh costs. (2) Additionally, a sound screening test demonstrates high sensitivity on a consistent basis. (6) A colorectal cancer screening test that is highly sensitive would be able to accurately predict the presence of polyps, precancerous growths and cancers. (See Fig. 1.)


Evidence-based practice recommends that, in addition to people in the high-risk categories, asymptomatic and average-risk men and women older than 50 be offered 1 of 5 colorectal screening options. (5) Screening methods currently used for colorectal cancer are fecal occult blood testing (or hemoccult test), sigmoidoscopy, single-contrast barium enema, air-contrast barium enema and colonoscopy. (6,7) Some of these screening options are used in combination. Each of these methods has advantages and disadvantages. They differ widely in sensitivity and cost. More importantly, some of the more accurate screening methods involve harsh bowel preparation and invasive--and sometimes embarrassing--procedures. Because of this, many potential screening patients ignore their physician's recommendations and the recommendations of organizations such as the American Cancer Society and bypass colorectal screening.

Experts estimate that it would take 5 years to screen the entire eligible population of the United States if all currently prepared health care professionals focused their entire clinical efforts on screening. (6) Currently, participation in colorectal cancer screening is low at 46.6% for men and a mere 31.6% for women. (8) Perception of a test as embarrassing, uncomfortable, time-consuming or worrisome sometimes leads patients to avoid a screening process. (8) Colon cancer continues to be a major cause of cancer deaths because of the limitations of screening, confusion on when or how to screen, and patient reluctance to submit to the screening process or the preparation necessary for many of the screening tests. (2)

Researchers agree that colorectal screening is justified and beneficial. (2) The Centers for Disease Control has issued a strategic goal of increasing the number of people being screened for colorectal cancer from the current 37% of the eligible population to 50% by 2010. (9) Patients older than 50 years of age have a 1 in 5 chance of having polyps, so the importance of screening cannot be overemphasized. (7)

Although screening is important, only those lesions suspected to be cancerous need to be considered. Lesions or polyps 10 mm or larger pose the biggest risk of becoming cancerous and are the ones physicians need to be aware of. (1) If adenomas are found during screening, patients require closer monitoring and an endoscopic colonoscopy every 3 to 5 years, depending on the number and size of polyps found. (7)

One of the first and most important colorectal cancer screening tests is the fecal occult blood test. (7) This test requires the patient to obtain stool samples that are then sent to the laboratory to test for blood in the stool. It is important that the patient refrain from eating red meat, broccoli and cauliflower, and not take aspirin during the sample collection time period. (7) Although the fecal occult blood test, or hemoccult test, has been successful in some patients, its poor sensitivity results in underdiagnosis. (7) Sensitivity increases when 3 stool specimens are tested on 3 separate occasions and rehydrated in the laboratory before interpreting the specimen. (7)

Hemoccult test sensitivity is 85% for cancers and much lower for adenomatous lesions. (7) The test also is not very specific for colorectal cancer, with 30% specificity. (7) A test that is highly specific is ideal for indicating a particular disease. The fecal occult blood test has been associated with a reduction in mortality because people with positive findings then undergo further testing, but this screening option does not allow direct evaluation of the colon mucosa. (2,3) Additionally, some large adenomatous polyps and cancers do not bleed within the bowel and so remain undetected by fecal occult blood screening. (2) False positives are fairly common with this type of screening, which then leads to unnecessary testing and higher costs. (3) Fifty percent of tests that are heme-positive resulted from a bleeding source elsewhere in the gastrointestinal (GI) tract. (3)

A newer stool screening test is the fecal immunochemical test, or FIT. This test is performed in a similar manner and detects occult blood in the same way as the fecal occult blood test. (9) The FIT test has a higher specificity and has been proven to have fewer false positives. (9) An additional benefit to the FIT test is that ingestion of foods and vitamins that lead to false positives with the fecal occult blood test do not interfere with FIT. Also, as few as 2 samples may be needed. (9)

A sigmoidoscopy is an alternative to the fecal occult blood test as a screening option. A sigmoidoscopy uses an endoscope 60 cm in lengthy. (9) It is performed similarly to a colonoscopy, but without anesthesia. (9) Sigmoidoscopy has been proven to decrease the mortality rate, but it is not the gold standard for screening because it cannot evaluate the entire colon. (2,3) Sigmoidoscopy demonstrates less than half of the colon, and its sensitivity for polyps or cancer is 50% to 75% . (7) Also, complications associated with sigmoidoscopy include GI bleeding and perforation, as with colonoscopy. (9) Combining sigmoidoscopy with the fecal occult blood test does not significantly improve accuracy of colorectal cancer diagnosis. (3)

Barium enema (BE) x-ray exams are another colorectal cancer screening option. The air-contrast, or double-contrast, barium enema is generally the choice over single-contrast barium enema when screening for polyps and small lesions. Although the air-contrast barium enema is widely available, relatively inexpensive, Medicare approved and safe, the sensitivity of this test has not been researched fully. (3,10) One study rated the sensitivity of air-contrast barium enema exams as 8l % for polyps 10 mm or larger. (3) This study also found a 26% sensitivity for polyps from 5 to 10 mm. (2,3) Other studies also found air-contrast BE to be less sensitive, rating sensitivity for polyps smaller than 6 mm at 32%, polyps from 6 mm to 10 mm at 53% and sensitivity for polyps larger than 10 mm at a mere 48%. (7) However, air-contrast BE is highly accurate "for detecting large malignant lesions." (10) The single-contrast colon exam is woefully inadequate as a colorectal screening device, with a sensitivity of only 13%. (3) Air-contrast barium enema is a time-intensive procedure. It takes an average of 47.9 minutes to complete, including 38.5 minutes for the actual procedure and 9.4 minutes for the radiologist to interpret the exam. (10) It has the added disadvantage of a very rigorous bowel preparation, and patients generally have a negative reaction to the examination.

The American Gastroenterologist Association and the American Cancer Society recommend endoscopic colonoscopy as the primary screening test for the low-risk population aged 50 years and older. (7) It is considered to be the gold standard of colorectal screening for neoplasms and offers a complete study of the colon along with the ability to remove polyps and perform biopsies. (1,2,11) Experts agree that, following an initial colonoscopy at the age of 50, subsequent colonoscopies are needed every 10 years unless there were clinical findings on the initial examination. (7) Positive initial findings require the exam to be repeated more frequently. The colonoscopy has been found to be highly reliable, with both high sensitivity and specificity. (7) The majority of gastroenterologists prefer colonoscopy as the screening method for colorectal cancer because they can directly observe the colon mucosa, accurately diagnose early cancers and resect most premalignant polyps. (6)

Colonoscopy has flaws and limitations despite being the preferred screening method. Some researchers believe that colonoscopy is not a true screening tool because it is an expensive and highly invasive test. (1) The cost of colonoscopy can be prohibitive at an average Medicare reimbursement rate of $1736. (2,3) Colonoscopy equipment is also expensive to purchase and maintain. (7) Removal of small, clinically insignificant lesions during endoscopy increases costs and potentially increases the risks associated with biopsies. (2) The need for sedation is another drawback, requiring postprocedure monitoring, a ride home and time off of work. (2,3,7) There are small, but significant, risks of bowel perforation and GI bleeding associated with colonoscopy--complications that occur in approximately 0.3% of patients. (2,7)

Colonoscopy requires a harsh 24-hour preparation before the examination, is uncomfortable and is sometimes painful. (3,7) Approximately 5% to 10% of all colonoscopies end in incomplete exams that require further testing. (2,3,11) Colonoscopy also is known to miss 27% of adenomas less than 6 mm in size and 6% of adenomas larger than 10 mm. (7) These lesions are missed because a colonoscope travels in one direction and can't see the opposite side of the colon folds, thus creating blind areas. (10,11) Four out of 5 patients who undergo colonoscopy require no clinical action as a result of this expensive, highly invasive test. (7)

Colonoscopy is also a time-intensive procedure, averaging 31 minutes in the endoscopy suite, including 10.3 minutes to scope from rectum to cecum and 4.1 minutes on average to reconcile anomalies reported from previous air-contrast barium enema exams or computed tomography (CT) colonography. (10) There is also a shortage of trained, experienced endoscopists to perform screening. (2,3,7)

CT colonography (CTC), also known as virtual colonoscopy, is a newer option for colon screening that combines minimally invasive CT or magnetic resonance (MR) imaging with direct visualization of the colon mucosa using an endoluminal scope-like technique. (4) CT colonography was introduced in 1994 by Vining, Gelfand, Bechthold and colleagues; in 1997 their method was applied to MR scans in an effort to eliminate radiation and improve contrast resolution. (4,11,12) The development of the helical CT scanner and virtual reality software applied to CT data enabled the birth of CT colonography. (4) CT colonography evolved rapidly with advances in CT, especially multidetector imaging and 3-D software for fly-through imaging. (7) (See Fig. 2.) The term "virtual colonoscopy" can be misconstrued as noninvasive and requiring no bowel preparation, and so is used less frequently. (12)


As a screening tool, CTC is a source of great debate and wide variation in research statistics and recommendations. Few studies have been completed to evaluate CTC compared with conventional colonoscopy in screening populations. (3) According to one source, CTC patients find the examination similar to or more comfortable than a traditional colonoscopy, but without a clear-cut improvement in patient comfort. (5) Other researchers find CTC to be a true screening tool because it is minimally invasive and reasonably priced. (1) Those researchers also believe that CTC does not have to be of the same diagnostic efficacy as colonoscopy because it is a screening device and as such identifies patients at higher risk for cancer and polyps so that this smaller population can then undergo colonoscopy. (1) It has been found to be more accurate for colorectal cancer detection than air-contrast barium enema or sigmoidoscopy. (1) While many researchers and clinicians regard CTC as an accurate screening device for asymptomatic average-risk patients, others consider CTC a promising but investigational diagnostic test at this point. (3,7) These researchers found that CTC compares favorably with colonoscopy in diagnostic ability, but newer studies have demonstrated poorer sensitivity, especially if the CT equipment used was not of the highest standards. (7) CTC is no more time efficient than colonoscopy, except that the patient needs no aftercare and can leave the facility immediately after the exam. CTC takes 34.3 minutes on average to complete. Of that time, 17.4 minutes are used to perform the actual procedure and 16.9 minutes are needed for the radiologist to interpret the image. (13)

CTC certainly should be considered for patients who cannot or will not undergo colonoscopy, and it is frequently used for patients who have had an incomplete colonoscopy. (1,12) It is not the screening test of choice for patients with irritable bowel syndrome who require biopsies on a regular basis. (12) Currently, CTC is used primarily as a third-tier test behind air-contrast barium enema examination and colonoscopy. (14) Barish et al found that CTC should replace air-contrast barium enema as the second-tier screening test of choice. (14) A recent survey demonstrated that a growing majority is leaning toward CTC becoming the primary screening tool for colorectal cancer. (14) However, there are many who believe that CTC is still investigational and should not be recommended as a screening test at this time. (12)

CT Colonography Procedure

One of the difficulties with CTC is the lack of standardization of the examination. Standards and practice guidelines are essential for making a new technique part of clinical practice. (1)

Experts agree that CTC patients require bowel cleansing, including a clear liquid diet 24 hours in advance and cathartics. (12,15) However, a wide variety of preparation agents are used. (See Table 1.) As with air-contrast barium enema or colonoscopy, a clean mucosa enhances the diagnostic ability of a CTC examination and directly relates to the diagnostic accuracy of the exam. (1) Rigorous bowel preparation is the major source of negative patient reaction to the CTC examination. (1) Of the preparations used, biscodyl (Dulcolax) tablets have been found to decrease residual fluid in the colon as well as stool. (11) In Europe and Asia, a double dose of oral saline laxative (Fleet Phospho-soda) is used as the primary cathartic, but in the United States this is not recommended and carries a Food and Drug Administration warning. (16)

Fecal Tagging

As is common with other examinations of the colon, bowel prep is not always as adequate as the physician would like. Three to five percent of CTC patients are not adequately prepped for their examination. (2) Different approaches to preparation include a structured low-fiber diet, reduced laxatives with the addition of fecal tagging agents or total elimination of laxatives with the use of fecal tagging agents combined with electronic stool subtraction. (1) Fecal tagging is the use of barium, iodine or both given in combination with a reduced bowel preparation. Fecal tagging has proven efficient when combined with a reduced dose of cathartics. (1,17) Barium is useful in coating and tagging particulate stool, and iodinated contrast diffuses into and opacities retained colonic fluid. (7) Fecal tagging helps assess residual feces and improves lesion detection by increasing the density of tagged feces, thus enabling the radiologist to differentiate retained stool from polyps and detect polyps within opacified residual fluid. (1,7,16,17) The tagged stool is much denser than the soft-tissue density of a polyp or medium density mass. (7) Not only does the oral contrast media help differentiate polyps from residual stool, but it also helps control excessive residual bowel fluid. (16) Eliminating the use of laxatives completely, along with fecal tagging for 48 hours before the examination, has shown sensitivity for polyp detection comparable to exams using a cathartic preparation. (1)

When using fecal tagging, most patients begin their bowel preparation 2 days before the scheduled exam. The prep begins with a low-residue diet and can include a commercially prepared diet such as Nutraprep (E-Z-EM, Lake Success, NY). Nutraprep contains nutritionally balanced, low-residue food options that help the patient successfully complete the required prep with little inconvenience. (17) (See Fig. 3.) The patient also may be given a commercial barium fecal tagging prep kit, such as Tagitol V (E-Z-EM, Lake Success, NY) or Scan C (Mallinckrodt Inc, Hazelwood, Mo). Tagitol V uses 50 mL of barium sulfate of 40% weight/volume, taken with meals the day before the test: 25 mL of barium at breakfast, 12.5 mL barium with lunch and another 12.5 mL with the evening meal. (17) (See Fig. 4.) Along with the Tagitol and Nutraprep, patients take 16.5 g magnesium citrate and 4 biscodyl tablets at 7 p.m. the evening before the test. The morning of the exam, patients use a biscodyl suppository. (17) With Scan C, the patient is given 120 mL of the contrast agent Gastrografin (Schering, Berlin, Germany) to opacify any retained fluid. (7) Sensitivity for polyps 8 mm or larger using iodinated contrast media and without the use of cathartics was found to be 95.5%. (1) Another tagging option is the iodinated oral contrast agent Peritrast (Kohler Chemie, Alsbach-Haelein, Germany), that is taken at the same time as the prescribed laxatives. It frequently is paired with an oral saline bowel prep. (7)


Patients who consume too much regular fluid can have poor tagging results, as well as those with irritable bowel syndrome or constipation. (17) A study showed that 4.08% of colon segments present with nontagged feces measuring 6 mm or larger. (17) A large portion of these untagged feces appeared in the rectal area and can be correlated with suppository use. (17) The tagging agents enable the bowel wall to be differentiated easily from residual fluid because the fluid appears bright with contrast in comparison with the bowel wall. (16) Feces typically appear as a mass with air inclusion, a hyperdense peripheral ring with a hypodense center or with a hooked appearance. (17) Feces that remain untagged can be recognized by scanning the patient first in the supine position and then in the prone. (17) Lesions or polyps remain attached to the bowel wall regardless of patient position, but fecal material moves to the dependent wall as the patient changes position.

Intravenous (IV) contrast use is not likely to become routine with CTC because it would add to the exam's expense, as well as increase risks associated with venipuncture and contrast media reactions. (7) However, IV contrast helps locate tumor recurrence, metachronous disease and distant metastasis in those with prior colorectal cancer. (1) It also increases reader confidence in bowel wall evaluation and the ability to see medium-size polyps in less-than-perfectly-prepped patients. (17) With IV contrast-enhanced CTC, iodine is given 15 seconds before scanning begins. (7) The iodinated contrast enhances polyps, but does not differentiate between malignant and benign ones. (1,7)

Another advantage of fecal tagging agents is the ability to use electronic cleansing following the scanning procedure, also known as digital subtraction bowel cleansing. (7) Electronic cleansing is a method of digitally subtracting tagged residual stool from images in the postprocessing of the colonography exam. (1) This method can be used in conjunction with cathartics or without any bowel prep. (7) However, it works especially well with rigorous bowel preparation. (1) If no tagging agents are given, a full bowel prep is required, and then a patient with positive CTC findings can undergo a colonoscopy directly without repeating the bowel prep or staying on a liquid diet for another 24 hours. (7)


Early CTC studies with helical scanners had limited spatial resolution. The development of multidetector CT scanners resulted in rapid imaging, improved temporal and spatial resolution, wider field of view, thinner collimation and the ability to complete the scan in a single breath hold. (1,5,11) Thinner-section collimation allows near isotropic voxels, which improves polyp detection and accuracy of TNM (tumor, node, metastasis) staging because of decreased volume averaging and improved Z-axis resolution. (11) (See Table 2.)

Scanning protocols for CTC are like everything else about the test--they vary widely in practice, with no standardized procedure. Even CTC patient positioning is debated. Some physicians require CTC patients to be scanned first in the supine position from the diaphragm to the perineum, then positioned prone and the same area rescanned. (7) (See Fig. 5.) Others believe that first scanning prone is the optimal method, followed by a supine scan while IV contrast is given. (11) Still others believe that a supine full abdomen scan is appropriate, followed by prone scans of the pelvic area only or prone scans of collapsed segments of the colon. (16)


Experts do agree that multidector helical CT is the method of choice for CT colonography, but actual exam and reconstruction protocols vary from institution to institution. Parameters used are 120 kVp, from 50 mAs to 160 mAs, pitch from 1 to 13, table speed 20 mm to 25 mm per second, gantry rotation of 0.5 sec and collimation from .75 mm to 4 mm. (1,11,13) A 512 x 512 matrix is used regularly and all experts recommend that, if possible, the scan be completed in a single breath hold. (1,3,11) Reconstruction intervals are set anywhere from 0.7 mm to 1.25 mm and 1 mm thickness. (3,11)


Before beginning the actual scan, the patient is placed in the Sims position and a rectal catheter is inserted, which remains in place during the entire scan. (15) An optimal CTC procedure requires colon distention with room air or carbon dioxide, and this is of even greater importance when bowel cleansing is not optimal. (12,15) Good bowel distention improves reader confidence and can decrease reading times, as well as provide a low-density background to identify polyps and masses that otherwise would appear to be of medium density. (17,15) (See Fig. 6.) On the other hand, excessive insufflation can cause reflux into the ileum, resulting in artifacts on the 3-D reconstruction images. (4) The key to successful insufflation is to have adequate distention of the bowel without causing the patient excessive discomfort. (15) The sigmoid colon can be of particular concern during interpretation because of inadequate distention due to spasm, which leads to missed polyps. (7) The prone position seems to help with sigmoid distention and best demonstrates the area. (7)


The method of insufflation is yet another source of controversy, and there is no consensus on which is the best technique. (15) Insufflation can use room air or carbon dioxide. It can be accomplished through automatic or manual methods and can be administered by the patient or the technologist. Patients generally tolerate insufflation well; however, patients who received autoinsufflation rated themselves as "more tired" at the end of their exam. (7.15) Carbon dioxide is preferred by many physicians because it is absorbed 35 times faster by the body than room air and expelled by the lungs, making it more comfortable for the patient. (1,7,14,15) The simplest and least expensive method of insufflation is room air administered by the technologist or patient using a plastic hand-held insufflator bulb. (15) The average amount of gas used is 2 L, or 40 pumps with an insufflation bulb, and no special equipment is needed. (1,7) However, some researchers have used as much as 4.2 L carbon dioxide, and manufacturers of insufflation kits recommend 3 L of gas. (15) When scanning in both the supine and prone positions, bowel distention is checked before the second scan is started and more gas can be added if necessary to optimize distention. (7)

Much research has been conducted regarding automatic vs manual insufflation, and it appears that automatic insufflation is preferable for optimal colon distention. (15) Using carbon dioxide at a controlled rate and pressure improves patient cooperation and comfort and also yields good results. (15) Automatic insufflation devices control the rate of flow at between 1 L and 3 L per minute and display the total volume given; they also offer alarms and automatic shutdown if a preset excessive pressure is reached. (15) (See Fig. 7.) Some devices include the option of allowing different pressures depending on patient position. A manual system can use a standard enema bag filled with 3 L of carbon dioxide or room air. The bag is compressed for 2 to 3 minutes while the patient is in the left lateral decubitus position, then rolled to prone and finally to supine. (15) An insufflator bulb also may be used. It is attached to the rectal tube and up to 2 L of room air are administered, or enough air so that the patient feels fully distended. (11)


Better luminal distention with automatic insufflation has been demonstrated for the left colon in the supine position and for the transverse colon in both the supine and prone positions. (15) In both prone and supine positions, colon distention was significantly better with automatic insufflation in the sigmoid, descending and transverse colon segments. (15) There were no differences noted between the other colon sections when comparing the 2 insufflation methods. (15) Autoinsufflation keeps the colon constantly inflated, thus eliminating the possibility of premature deflation due to rapid absorption of carbon dioxide. (7)

Because CT colonography uses 3-D images, it is important to have complete and maximum bowel distention. However, because of patient discomfort, distention may be less than optimal. (16) One heavily debated method of combating patient discomfort is using intravenous spasmolytics that help prevent colon spasm and collapse. (1,15) Use of spasmolytics requires venous access and therefore increases exam time and costs. (16) Many experts believe that spasmolytics are not necessary unless there is a diagnostic benefit. (1,16)

The antispasmodic agents used are glucagon, or glucagon hydrochloride; butyl scopolamine; or hycoscine butylbromide. (7,15) Glucagon is a single-chain polypeptide hormone that increases serum glucose levels and relaxes the gastrointestinal smooth muscle. Unfortunately, the colon is the least sensitive part of the entire GI tract to glucagon. (7) Glucagon is licensed in the United States, but has not been found to be especially beneficial during CTC. (15) Patients given glucagon did have larger colon volumes, fewer collapsed segments and decreased discomfort compared with patients not receiving a spasmolytic, but not significantly so. (7,16) Two studies comparing patients given glucagon and a group given no spasmolytic found no benefit to its use. (7,16) In addition, glucagon affects the ileocecal valve and allows air to reflux into the terminal ileum, thus reducing colon distention. (1) Intravenous hycoscine butylbromide appears to improve distention of the colon, but has not been approved for use in the United States. (15,16) Butyl scopolamine has been proven to increase insufflation of the rectal area, with glucagon doing so to a lesser extent. (16) In addition, butyl scopolamine has demonstrated significantly greater distention of the colon, fewer collapsed colon segments and delayed onset of pain, probably due to smooth muscle relaxation. (16)

When spasmolytic agents are used, they are given intravenously and slowly injected 2 minutes before the scan begins. (16) One milligram of glucagon is the usual dose, and 20 mg of butyl scopolamine is used. (16) In studies in which patients were given glucagon or butyl scopolamine, CTC and associated insufflation were tolerated well by all patients with no accompanying nausea or vomiting. (16) Glucagon has fewer side effects than butyl scopolamine, but the former is more expensive and not as effective. (16) Butyl scopolamine causes the eyes to fail to adapt to light, so the patient cannot drive home. (16) Also, its use is contraindicated for patients with prostatic hyperplasia, tachycardia and increased intraocular pressure. (16)


One of the most important factors in CTC is the education and expertise of the radiologist doing the interpretation. (18) CTC requires meticulous evaluation of the complete luminal surface and is more than an extension of CT for routine diagnostics. (18) Currently, one of the biggest issues in CTC is the lack of training and experience of interpreting radiologists. (14) This lack of uniformity in reader training plagues the research studies done to date and may have influenced the wide discrepancies in reported sensitivity. (18) There can be a wide and varied learning curve in CTC interpretation, and, as with any new procedure, performance improves with increased experience. (1,18) Training fellowships and Internet tutorials by major institutions are available, but formal training programs are important for widespread adoption of CTC. (1) CTC requires a unique search pattern and some radiologists may never reach a desired performance level. (18)

The current recommended method of education is an interactive course conducted at reading stations, with supervision by experienced radiologists and using cases confirmed by colonoscopy. (18) Forty to 50 cases with a wide variety of pathologies are optimal. (18) One research study raised the issue of learning feedback, arguing that this has been neglected in CTC education. (19) However, another study found that inexperienced readers performed better than experienced ones. (19) Two studies are currently underway, with improved requirements for reader experience. In these new trials, readers must prove proficiency through written testing and interpretation of 20 studies before they are allowed to participate in the research. (18)

Because computed tomography allows the reader so many options in viewing the images, as well as 3-D reconstruction, CTC has a plethora of interpretation models that can be used and no consistent standards exist. (14) In 1998 it was recommended that CTC images be viewed in 2-D format with 3-D used only for problem solving. (2) Since 2000 the 2-D approach has become the primary method of interpretation. (2) However, the premier CTC researchers demonstrated that using 3-D interpretation was effective and accurate for polyps in asymptomatic patients. (1,20) When using 3-D as the primary viewing option, it is mandatory that the entire colon from rectum to cecum be displayed to increase polyp detection. (20) Researchers have observed that 80% of radiologists read CTC studies in 2-D view at the lung window setting and use the 3-D approach for problem solving or to distinguish normal mucosal folds from fixed lesions. (1,14) A minority use the 3-D approach as the primary interpretation method) A combination of virtual images, or fly-through, along with transverse and multiplanar images is useful to distinguish polyps from residual stool. (11)

Other studies suggested that axial images be supplemented with postprocessed reformats and 3-D views for optimal interpretation. (1) Currently, there is no consensus as to whether 3-D reading is better than 2-D interpretation because few research studies have been conducted and further research is needed. (10) Multidetector CT equipment with improved work stations has decreased the time for 3-D reading, and with continued improvement 3-D may become the approach of choice for reading; however, familiarity with 2-D imaging will continue to be essential. (2)

It is important to scroll through the transverse 2-D images using cine with an abdomen window of 400 Hounsfield units (HU) and level settings of 40 HU to detect flat and annular polyps. (2) The use of 3-D imaging in the antegrade and retrograde directions, along with 2-D transverse images, allows detection of polyps smaller than 5 mm and eliminates "blind spots." (2,20) The 2-D format illustrates 100% of the colon mucosa in a single pass and eliminates the possibility of polyps hidden behind folds in the mucosa, thus making it a time-efficient interpretation method. (2)

Distinguishing polyps from fecal material is difficult in either the 2-D or 3-D format. When making that distinction, it is important to check for the presence of internal gas or areas of high attenuation suggestive of fecal matter and to use multiple window and level settings. (2) The size and shape of the area, or morphology, in question also must be investigated. Polyps generally have rounded or lobulated smooth borders. Feces also can have these features, but lesions with straight or irregular angled borders can be confidently determined to be feces. (2) The difference will be demonstrated best with thin slices and 3-D endoluminal views. (2) The mobility, if any, of the area in question is another determinant. Stool moves to the dependent surface, whereas polyps maintain their position. However, the reader must be cautious of pedunculated and sessile polyps that may appear mobile)

When an area of the study appears abnormal, the reader then should check the coronal, sagittal and endoluminal images for further evaluation and clarification. (2) If the abnormality is apparent in the supine position, images in the prone patient position may be helpful and demonstrate mobile lesions. (2,7) Intraluminal defects could be feces, bulbous or irregular haustral folds, lipomas, extrinsic compression defects, diverticula, foreign bodies or polyps. (7)

Three-dimensional viewing has some drawbacks that must be mentioned. Overdistended areas of bowel can cause the center line to move to an adjacent distended loop and disrupt the reading process. (2) Blind spots cannot be completely eliminated with this method, even when reading in both the antegrade and retrograde directions. (2) When using the fly-through method, a center line cannot be generated that allows for endoluminal navigation. (2) To overcome these limitations, it is recommended that the interpreter use 4 fly-through passes using antegrade and retrograde directions with the patient scanned in both the supine and prone positions. (2)

Another viewing format, called 3-D panoramic display technique, displays the colon as if it were a flattened 6-sided cube, allowing a 360[degrees] image; the technique has been found to be time efficient and highly accurate. (1) This new format allows visualization of the undersurface of folds, and researchers are hopeful it will increase detection and decrease reading time. (2) This new software can open the colon along the longitudinal axis, as well as straighten out loops and curves in the bowel. (1) Also called "virtual pathologic examination" or "virtual dissection," the software has not been proven to increase lesion detection compared with 2-D viewing. (2) A limited study with only 2 interpreters demonstrated sensitivities of 67% and 89% for panoramic 3-D viewing compared with 2-D sensitivities of 89% and 100%, so continued study is needed to determine panoramic viewing efficacy. (2)

The accuracy between interpreters is moderate to high on a per polyp basis for CTC. (22) When looking at CTC exams as a whole, interobserver agreement is high to excellent, even when including radiologists of different skill levels and experience. (22) Controversy does exist over the size of polyps that should be reported, with some believing that polyps less than 4 mm in size are insignificant. (1) Other interpreters believe that hyperplastic lesions should not be reported because they will not become malignant. (l)

CTC is a time-consuming exam to interpret. With the improvements in software and workstations, reading time has decreased from 28 minutes 30 seconds in 1998 to 16 minutes in 2000. (2) This data involved using a 2-D approach with 3-D viewing for problem solving. Using a 2-D method and 3-D fly-through produced an average reading time of 32 minutes. (2) Interpreting images in an antegrade and retrograde 3-D method, along with 2-D viewing, increased reading time to an average of 39 minutes. (20) Using the panoramic 3-D method of virtual dissection yielded an average interpretation time of 36.8 minutes. (2)

Radiation Dose

CTC's biggest disadvantage in becoming the preferred imaging method for colorectal cancer screening is the radiation dose. (13,23) If a patient is tested regularly, repeated testing increases patient dose and its associated risk. (23) The risk associated with CTC radiation doses is uncertain, but cannot be disregarded, and ALARA (as low as reasonably achievable) should be the radiation goal. (23) Barium enema and abdominal CT scans deliver higher radiation doses than CTC, but the bottom line is that any exposure contributes to a patient's cumulative dose. (1,7) CTC can be performed at 40% to 50% of the typical dose of a regular abdomen/ pelvis CT scan. (1) It is estimated that the absolute radiation risk is less than 0.05%, and the estimated risk for cancer mortality would be considerably less than that. (24) As performed today, CTC doses range from 4 mSv to 12 mSv; when scanning in both the supine and prone positions, the dose can be as high as 15 mSv. (21,24) Typical organ doses are less than 20 mSv, even when in the direct beam. (24) The organs at greatest risk from radiation exposure are the colon, stomach and bladder. (24) Leukemia is also a risk. (24)

Using the lowest achievable dose improves the balance of risk vs benefits in screening. (23) One method of dose reduction is to scan the patient in the supine position only. (13) However, doing so eliminates the option of using mobility of fecal matter to distinguish lesions from residual stool. Positioning in the supine and prone positions has been shown to improve bowel wall conspicuity and the detection rate of endoluminal lesions. (13) Other dose reduction methods increase image noise. (23)

Low-dose protocols still have demonstrated satisfactory sensitivity and specificity with doses as low as 1 mSv, and the number of false positives reported decreases with low-dose methods. (23) Dose reduction can be achieved by reducing tube current, increasing the pitch or both. (23,24) Table speed relative to rotation speed (or pitch) is an important determinant in radiation dose. (24) Radiation dose is inversely proportional to the pitch. Reducing the mAs by a factor of 4 approximately doubles the noise level in CT scans with all other factors kept the same. (23,24) Smoothing of low-dose images counteracts the increase in noise, and additional in-plane smoothing can be performed. (23)

Patient dose increases with the use of multislice scanners, but increasing beam penetration decreases patient dose when accompanied by a decrease in mAs or an increase in table pitch. (1) For example, the following parameters produce a patient dose of 12 mSv: 120 kVp, collimated to 4.0 mm x 2.5 mm, 0.75 second rotation time, pitch of 1.25, section thickness of 3.2 mm, reconstruction interval of 1.6 mm and 100 mAs. (23) Barish and Rocha reported scanning trials in which half the mAs value was used with no difference in efficacy and patient doses of 1.87 mGy for men and 2.85 mGy for women. (1) Of course, if the patient is scanned in both the supine and prone positions these dose figures would double. A low-dose CTC involves 20% less exposure than an air-contrast barium enema. (1) Polyp detection was identical when using a dose-saving technique (30 mAs, received dose of 3.6 mSv) or with standard technique; however, image resolution was poorer with the dose-saving technique. (1) By increasing the pitch and significantly decreasing the mAs, doses of 0.5 mSv for men and 1.0 mSv for women can be achieved. (13) When doses were lowered to 0.2 mSv, the average sensitivity of the exam for polyps 5 mm or larger decreased slightly but not significantly and interobserver agreement remained stable. (23)

Low-dose CTC maintains its sensitivity and specificity for polyps larger than 7 mm, even with increased noise. (24) Noise artifacts from dose reduction are most often noted in the pelvic area and most frequently appear as "snow" within the bowel lumen. (13) Applying smoothing techniques to low-dose scans improves the appearance of the images and could keep readers from considering small irregularities as polyps. (23) When using very-low-dose CTC, contrast media may be contraindicated because little difference is noted in densities between contrast-enhanced and unenhanced structures. (23)

New CT units from all major vendors offer automatic tube current modulation in which the mA increases and decreases to compensate for different instantaneous levels of attenuation, thus helping to reduce patient doses. (24) Of course, MR could be used instead of CT to image the colon, eliminating the radiation risk altogether. (7) Another important point is that absolute cancer risks are highly age dependent. (24) Radiation risk generally decreases markedly with age, and the population targeted for this test is older and therefore not as sensitive to radiation exposure. (24) However, patients with symptoms frequently undergo a full-radiation-dose, contrast-enhanced scan, performed in both the supine and prone positions. (1)


The accuracy and effectiveness of CTC are widely debated. Polyp size appears to be integral to the accuracy of the exam. (1) The exam is more sensitive, almost 100%, at detecting colon cancer vs benign polyps because cancer tends to be larger. (7) Very small lesions, no matter the shape, are difficult to detect with CTC. (22) CTC may have higher sensitivity for adenomas. (5) The most common cause of error in CTC is residual stool, which contributed to multiple false positives in several studies. (22)

There are mixed results for sensitivity reported in the literature. (5) CTC's sensitivity for large polyps (> 10 mm) has been rated at 55%, 75%, 85%, 94% and 100%, depending on the study. (1,5,11,22) Smaller, single-center studies have rated sensitivity at greater than or equal to 90% for polyps 10 mm or larger. (12) Sensitivity falls off sharply for polyps less than 10 mm. (6) Sensitivity for polyps 5 mm to 10 mm has been rated at 66%, 70% and 86%. (1,5,22) Sensitivity for polyps less than 5 mm has been rated as low as 48%, but many radiologists believe that polyps this size are insignificant and should not be reported. (5) With respect to the TNM (tumor, node, metastasis) system, CTC is highly sensitive. Sensitivity for tumors is rated at 95%, nodes at 85% and metastasis at 100%. (11)

Although CTC is not a perfect test, it is more sensitive than air-contrast colonography. Double-contrast barium enema has a sensitivity of less than 50% for polyps 10 mm or larger. (3,10) On a per patient basis, Rockey et al found the sensitivity for air-contrast barium enema to be 48%, CTC 59% and endoscopic colonoscopy 98%. (10) These differences in CTC and colonoscopy are statistically significant for both false-positive and false-negative findings. (11) The standard colonoscopy has a miss rate of 17%, and polyps at the sharp turn of a flexure or an area of spasm, as well as very small polyps and flat adenomas, all tend to go undetected. (7,22) There have been cases in which CTC has found polyps missed with the standard scope procedure. (22) Neither CTC nor colonoscopy is very effective for detecting flat polyps. (22) However, colonoscopy has the advantage of using special techniques such as high-magnification chromoscopy and dye spraying that CTC does not. (22) Bowel prep quality does affect the detection of large lesions in all of the tests (colonoscopy, barium enema and CTC). (10) CTC performed without bowel preparation showed a sensitivity of 90% for polyps 8 mm or greater. (21) For polyps greater than 6 mm but less than 10 mm, CTC had an average sensitivity of 86% in one study and 70% in another. (5,22)

The equipment and scanning parameters used for CTC affect the sensitivity of the exam as well. Multidetector scanners had less variation in sensitivity within a study than did single-slice scanners. (21) Studies that used multidetector scanners all reported high sensitivity at 95% or greater. (5) Sensitivity of CTC increases when thinner slices are used for collimation. (5) For each 1-mm increase in collimation width, there is a 4.9% decrease in sensitivity. (5) Five-millimeter collimation results in poor sensitivity, even for large polyps. (3)

Radiologists' viewing techniques also affect the accuracy of the exam. Radiologists who used 2-D viewing with confirmation by 3-D formats only when needed were rated at 81.9% sensitivity. (5) Sensitivity was 91% for exams viewed with 2-D imaging and routine 3-D viewing. (5) Using the 3-D fly-through technique may increase sensitivity even more, with 2 studies reporting 99% sensitivity for fly-through technology. (5,21)

Specificity also is related directly to polyp size, and CTC has been found to be highly specific for polyps greater than 9 mm. (5) A study by Rockey et al found the specificity for large polyps (10 mm or greater) to be 90% for air-contrast colon radiography, 96% for CTC and 99.6% for colonoscopy. (10) Other studies have found CTC specificity to be as high as 98.5% for polyps larger than 10 mm and 86% specific overall. (3,5)

Generally, factors affecting sensitivity and specificity function independently of each other. (21) For most diagnostic tests, sensitivity and specificity move in opposite directions. In other words, as one increases, the other decreases. (21) With CTC, however, sensitivity can be increased without decreasing specificity, indicating that CTC would be an excellent screening tool. (21)

Computer-aided Diagnosis And Virtual Dissection

The fact that CTC is digital lends itself to new technology such as computer-aided diagnosis (CAD). CAD was developed by computer and radiologic researchers at the National Institutes of Health and tested at the Mayo Clinic in Rochester, Minn. (5) It is a method to improve consistency and increase sensitivity of reading. (1) CAD automatically detects polyps and marks suspicious areas for the radiologist, acting as a second reader. (1) It increases the radiologist's diagnostic ability and lessens differences between readers. (1) CAD works in 3 steps: First the software isolates the colon wall for examination, then it identifies potential polyps and finally it attempts to eliminate false positives, if possible. (1)

Sensitivity of readings with CAD alone was 71% for polyps 10 mm or larger and 65% for those 5 mm to 9 mm. (25) The sensitivity per patient for polyps 8 mm to 10 mm is not significantly different than that of optical colonoscopy. (6) In all the studies conducted with CAD and CTC, there were false positives, so the specificity with CAD is imperfect. (6,25) Additionally, CAD cannot detect flat polyps and has low sensitivity for polyps in fluid-filled areas. (6)

Another emerging technology aiding CTC diagnosis is virtual dissection. Virtual dissection is a new 3-D rendering technology that draws a midline through the colon and then displays the entire lumen of the colon as a flattened 2-D image. (25) It provides a double interpretation with decreased variability in reading compared with other methods. (25) CAD can be applied to virtual dissection, and depending on the preferred length of colon displayed, fewer images are needed for interpretation. Thus, virtual dissection eliminates tedious centimeter-by-centimeter fly-through techniques. (25)

Virtual dissection distorts the appearance of nearly all polyps. (25) Sessile and flat polyps appear as flame shaped or pea shaped with virtual dissection, and pedunculated polyps appear as varied distorted shapes. (25) There is also mild distortion of haustral folds, usually occurring at the top and bottom of the virtual dissection image. Haustral distortion is greater at bends and flexures. (25) Perpendicular to the midline, or axially, there is no distortion, and mild distortion occurs along the longitudinal axis. (25)

Sensitivity and specificity are both high with virtual dissection. Sensitivity was rated at 95% for polyps 10 mm or larger and 85% for polyps 5 mm to 10 mm. (25) Specificity was 100% for all polyps larger than 5 mm. (25)

Cost and Reimbursement

The cost of CTC is changing and is expected to be similar to that of colonoscopy. (12) However, additional costs are incurred if CTC patients are found to have lesions that must be scoped and resected. Currently, the charge for CTC is slightly higher than that of an abdomen-pelvis CT scan. (6) For CTC to truly compete with colonoscopy as a screening exam, the price must drop substantially so that the overall cost of screening is not increased. (6)

Medicare and Medicaid reimburse providers for fecal occult blood testing, fecal immunochemical testing, sigmoidoscopy and colonoscopy. (2) Air-contrast barium enema is reimbursed if a patient's physician deems the test is as beneficial as colonoscopy or sigmoidoscopy. (9) No formal request for CTC reimbursement has been made to the Centers for Medicare & Medicaid Services, and the exam is not currently reimbursed by that agency. (3,9) Currently, 46 states provide reimbursement for CTC. (26) However, reimbursement is hindered by many special provisions and covers only CTC exams that are considered diagnostic and not screening. (26) One advantage of the current situation is that the payment structure offers providers the opportunity to work out problems before CTC becomes a part of the national Medicare payment structure. (26) Without reimbursement, radiologists must determine the appropriate fee for this extensive exam. (2) Some private insurers are beginning to reimburse for CTC, citing that a high-quality screening exam will be cost effective in the long run. (2) Two new Current Procedural Terminology codes were created for CTC in 2004, 0066T for a screening exam and 0067T for diagnostic purposes. (2) These are both category III codes, meaning that Medicare, Medicaid and private insurers decide on an individual basis whether to cover a procedure. When this is the case, coverage is not usually provided. (2)

Patient Satisfaction

Patients who underwent colorectal cancer screening tests were surveyed regarding their satisfaction with the different tests. In general, patients were satisfied with all the screening tests except for barium enema, regardless of whether it was a single- or double-contrast exam. (8) Patients who had a barium enema thought it was the worst of all the screening exams, were much less satisfied, had increased worry and reported more physical discomfort than patients who underwent CTC or colonoscopy. (8) Patients found colonoscopy painful, intrusive, embarrassing and threatening and were relieved when the exam was over. (8) They found endoscopy, whether the full colonoscopy or a sigmoidoscopy, physically more uncomfortable than CTC. (8) However, even though patients complained about invasive colorectal cancer screening exams, few of the 125 patients surveyed rated any of the tests as poorly or very poorly tolerated. (8)

There are obvious advantages to CTC for the patient. CTC is safe, minimally invasive and more acceptable for whole-colon inspection. (5,8) It is unfortunate that CTC requires the same rigorous full bowel prep as colonoscopy and barium enema; (5) however, CTC's ability to demonstrate the entire colon, complete unsuccessful endoscopies and permit TNM staging are important benefits to the patient. (3,6,10,11) Patients who have bowel obstructions can be evaluated beyond the obstruction, which is not possible with endoscopy or a barium enema. (1,6,11) CTC can be performed the same day as a failed endoscopy using a single prep, and it eliminates the blind areas on the opposite side of the colon folds that are an issue with colonoscopy. (2,6,11) Another advantage is the ability to evaluate organs and lesions outside the colon. (1) These extracolonic findings have been found to be of high clinical importance in 10% of patients undergoing CTC, medium importance for 27% and low importance for 50% of patients. (1) One study found more than 50% of patients had abnormalities outside of the colon detected using CTC, with abdominal aortic aneurysm and malignant masses the most significant pathologies found. (10) The fact that no sedation is used and patients can return immediately to their regular activities benefits the patient, increases cost effectiveness and makes CTC an option for patients at high risk from sedation or who are allergic to sedatives. (3,6)

CTC does have significant limitations that could affect patient satisfaction, and the accuracy of the exam is still being researched. (1) The radiation exposure, bowel preparation and insufflation associated with CTC compare negatively with other screening tests such as fecal occult blood testing and genetic testing). (1,6,11) CTC can miss more than 10% of small and large polyps and 5% of colorectal cancers. (5) This is especially a problem in the anorectal area because of the inability to insufflate that portion of the bowel for examination. (7) The examination itself may cause abdominal pain from insufflation, nausea from abdominal distension, diaphoresis and fushing during the scan, and dizziness or dehydration from the bowel preparation. (7,12) In some instances, insufflation causes diverticula to rupture. (12) The exam entails a long viewing and interpretation time, which may affect report turnaround time. (11) The inability to remove polyps during the CTC exam compares negatively with colonoscopy and makes the decision about whether to remove polyps of intermediate size more difficult. (1) The patient can opt to postpone colonoscopy for polyp removal when small polyps are found by CTC; this would decrease costs but requires cautious clinical consideration. (7)

The Future of CTC

Good screening tests demonstrate high, consistent sensitivity, along with high specificity that minimizes the number of false positives. Currently, CTC sensitivity varies widely, suggesting that the exam still needs improvement. (5,21) Understanding the factors that affect sensitivity could help researchers understand discrepancies in effectiveness. (21) Given the current statistics on colorectal cancer, a better screening test is necessary to move colonoscopy to a diagnostic testing role instead of screening for those patients with positive screening tests. (7) It is imperative that radiologists and gastroenterologists work together to improve imaging techniques. (19) CTC is improving by leaps and bounds and may be getting closer to becoming a good screening tool. (21) If CTC can be developed to be diagnostically equivalent to colonoscopy, it probably would have greater patient compliance because of its safety and cost. (5) As of press time, the American Cancer Society did not recommend CTC as a screening test, nor did any other major medical organization. (9)

At this time there is conflicting and contradictory information regarding resection of polyps less than 6 mm in size that affects whether CTC should be followed up with endoscopy. (2) Some experts believe that all polyps should be resected and others believe that physicians should focus only on more serious, large adenomas. (2) Controversies like this and variabililty in the performance of the exam are key issues. (14) The challenge is to standardize CTC. (14) To be generally accepted as a screening technique, CTC must have improved standards of performance and interpretation and be regulated like mammography. (18,24) In addition, the sensitivity and specificity should be 90% for all exams, and nonprep bowel technique must be perfected. (24)

More large-scale research comparing CTC with colonoscopy in asymptomatic patients is needed. (7) A large study of asymptomatic patients presently is underway but could yield disappointing results because initial findings indicate that CTC is less effective than colonoscopy. (1) Of the studies that have been completed, 3 were truly representative of a screening population. (5) In those 3 studies, false-negative findings were rare. (5) Identifying the sources of the variation in studies will determine the direction for future research. (21) Research comparing multidetector CT to more rigorous CTC-aided colonoscopy with virtual dissection of key segments is needed, along with studies to determine the value of different viewing techniques and to help standardize CTC viewing. (21) Furthermore, studies examining the effect of radiologists' experience in reading CTC exams, the use of oral contrast agents and the feasibility of prepless CTC are necessary. (21)

The future of colorectal cancer screening may lie in other imaging modalities such as positron emission tomography-CT (PET-CT) or MR. A combination of fluorodeoxyglucose PET scanning and high-resolution CT imaging would yield comprehensive colon data. (8) However, a dual-slice CT scanner with an integrated PET system is a costly investment. (27) Another option that would eliminate radiation exposure is magnetic resonance colonography (MRC). MRC is currently not perfected, and it has lower spatial resolution, more motion artifacts, longer examination times and higher costs than CTC. (4) MRC also requires bowel cleansing, and gadolinium fecal tagging is being explored. (4)

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This article is a Directed Reading. See the CE quiz at the conclusion.

This article also meets Medical Dosimetrist Certification Board (MDCB) CE requirements.


CT Colonography

To earn continuing education credit for this Directed Reading, read the preceding article and circle the correct response to each statement. Transfer your responses to the answer card on Page 77, and then follow the directions for submitting the answer sheet to the ASRT. Members also may take quizzes online at www.asrt. org. Effective October 1, 2002, new and rejoining members are ineligible to take DRs from journals published before their most recent join date unless they purchase access to the DR quiz. Note: Readers who are not ASRT members can receive credit by joining the ASRT. To join, contact the ASRT Customer Information at 800-444-2778 or join online at

* Your answer sheet for this Directed Reading must be received by the ASRT on or before this date.

1. The lifetime risk of developing colorectal cancer is--%.

a. 3

b. 5

c. 10

d. 15

2. Which of the following is not a risk factor for colorectal cancer?

a. father with colorectal cancer at the age of 54

b. hereditary nonpolyposis

c. history of breast or uterine cancer

d. inflammatory bowel disease

3. Which of the following items are characteristic of an effective screening test?

1. high sensitivity on a consistent basis

2. well accepted by patients

3. benefits outweigh costs

a. 1 and 2

b. 1 and 3

c. 2 and 3

d. 1, 2 and 3

4. Because some screening methods involve harsh bowel preparation and invasive procedures, many patients ignore their physician's recommendation and bypass colorectal screening.

a. true

b. false

5. What percentage of eligible women currently participate in colorectal screening?

a. 55.6

b. 46.6

c. 31.6

d. 22.6

6. Polyps--mm or larger pose the greatest risk of becoming cancer.

a. 3

b. 5

c. 7

d. 10

7. What percentage of fecal occult blood tests are positive due to bleeding in the GI tract other than in the colon?

a. 10

b. 20

c. 35

d. 50

8. Sigmoidoscopy is not the gold standard in colorectal cancer screening because it:

a. fails to examine the entire colon.

b. is highly uncomfortable for patients.

c. requires anesthesia.

d. must be combined with a fecal occult blood test.

9. Which colorectal screeing test do the American Gastroenterologist Association and the American Cancer Society recommend for patients older than 50?

a. air-contrast barium enema

b. colonoscopy

c. fecal occult blood test

d. sigmoidoscopy

10. Advantages of colonoscopy include all of the following except the ability to:

a. accurately diagnose early cancers.

b. directly view the colon wall.

c. screen at a low cost.

d. resect polyps and permit biopsies.

11. Colonoscopy fails to be a perfect screening test because it may miss lesions on the colon folds opposite the direction of scope travel.

a. true

b. false

12. Which technical innovation(s) made computed tomography colonography (CTC) possible?

a. prepatient and postpatient collimation

b. specific colon contrast media

c. the helical scanner and virtual reality software

d. third-generation scanners

13. CTC has been found to be more accurate for detecting colorectal cancer than:

1. air-contrast barium enema.

2. colonoscopy.

3. sigmoidoscopy.

a. 1 and 2

b. 1 and 3

c. 2 and 3

d. 1, 2 and 3

14. Experts agree that CTC patients require bowel cleansing, which includes a clear liquid diet 24 hours in advance and:

a. carbon dioxide insufflation.

b. air insufflation.

c. cathartics.

d. Tagitol or Nutraprep.

15. Compared with polyps and masses, tagged stool appears:

a. less dense.

b. the same density.

c. slightly denser.

d. much denser.

16. Some groups of patients can have poor results with fecal tagging, especially those who:

1. consume too much fluid.

2. have irritable bowel syndrome.

3. are constipated.

a. 1 and 2

b. 1 and 3

c. 2 and 3

d. 1, 2 and 3

17. Which CTC scanning protocol do all experts recommend?

a. First scan the patient in the prone position, then supine.

b. First scan the patient in the supine position, then scan collapsed segments of colon with the patient positioned prone.

c. Set table speed at 25 mm per second.

d. Complete the scan in a single breath hold, if possible.

18. Which position best demonstrates the sigmoid colon?

a. prone

b. supine

c. lateral decubitus

d. oblique

19. The average amount of gas used to insufflate the bowel is--L.

a. 2

b. 3

c. 4

d. 5

20. Which of the following are antispasmodic agents used in CTC?

a. butyl scopolamine and C[O.sub.2]

b. glucagon and butyl scopolamine

c. glucagon and insulin

d. hycoscine butylbromide and insulin

21. Which antispasmodic agent is not approved for use in the United States?

a. glucagon

b. glucagon hydrochloride

c. butyl scopolamine

d. hycoscine butylbromide

22. Since 2000, which approach has become the primary method for interpreting CTC exams?

a. 3-D

b. 2-D

c. fly-through

d. 3-D panoramic

23. Which of the following imaging characteristics can be identified confidently as feces?

a. straight border, moves to dependent side

b. irregular border, does not move to dependent side

c. lobulated border, moves to dependent side

d. rounded border, does not move to dependent side

24. Radiation dose for CTC today ranges from--mSv to--mSv, and as high as --mSv when scanning in both the prone and supine positions.

a. 0.5, 3,5

b. 4, 12, 15

c. 18, 26, 30

d. 40, 50, 65

25. Compared with air-contrast barium enema, a low-dose CTC involves--radiation exposure.

a. 40% less

b. 40% more

c. 20% less

d. 20% more

26. CTC is highly sensitive (almost 100%) for diagnosing:

a. colon cancer.

b. flat polyps.

c. very small lesions.

d. very mobile polyps.

27. CTC's sensitivity for polyps less than 5 mm has been rated as low as:

a. 70%.

b. 66%.

c. 55%.

d. 48%.

28. Virtual dissection--the appearance of nearly all polyps.

a. enhances

b. distorts

c. minimizes

d. accurately depicts

29. Which of the following entities currently reimburse the cost of CTC when used for diagnostic purposes?

a. Medicare

b. Medicaid

c. all private health insurers

d. 46 states

30. An advantage of CTC is its ability to evaluate organs and lesions outside the colon. According to 1 study, what were the most significant pathologies found?

a. herniated discs and malignant masses

b. stomach ulcers and abdominal aortic aneurysm

c. abdominal aortic aneurysm and malignant masses

d. herniated discs and stomach ulcers

Reference Number DRI0007005


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Diana S. Werderman, M.S.Ed., R.T.(R)

Diana Werderman, M.S.Ed., R.T.(R), is director of Trinity College of Nursing and Health Sciences" radiography program in Rock Island, Ill. She has spoken at both state and national meetings on preceptorship and developing critical thinking through the use of questioning. Ms. Werderman is the author of Teaching Techniques for the Radiography Clinical Instructor, a self-study CE book designed to prepare clinical instructors to work with radiography students.
Table 1
Bowel Preparation Methods

Preparation Materials *            Advantages

16.5 g magnesium citrate,          Good cleansing, little
4 biscodyl tabs, biscodyl          retained bowel fluid

Polyethylene glycol (GoLytely)     Better cleansing, but increases
                                   residual fluid

Double-dose oral saline laxative   Very favorably evaluated for
(Phopho-soda, Fleet Prep No. 1)    cleansing of stool and residual
                                   fluid; carries a U.S. Food and
                                   Drug Administration warning and
                                   is not recommended in the United

Double-dose magnesium citrate,     Good cleansing, little retained
one packet senna granules OR 2     bowel fluid
packets sodium picosulfate

* All preps with the exception of GoLytely are accompanied by a liquid
diet or a low-residue diet for 24 to 48 hours prior to examination.

Table 2
Tumor Staging With CT Colonography (5)

Tumor Stage                 CTC Findings

T1                          T1 and T2 are difficult to
Tumor of mucosa or          distinguish with CTC

Tumor extends into
muscularis propria

T3                          On CTC images, tumor
Tumor extends beyond        apppears dense, with spiculation
muscularis propria into     in the perirectal fat, or
perirectal tissue and fat   has a nodular or lobulated
                            outer margin
Tumor is invading other
structures and organs
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Title Annotation:CONTINUING EDUCATION; computed tomography
Author:Werderman, Diana S.
Publication:Radiation Therapist: The Journal of the Radiation Oncology Sciences
Article Type:Table
Geographic Code:1USA
Date:Mar 22, 2007
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