Applied Radiology

Dr. Johnson is Professor of Diagnostic Radiology at Mayo Medical School and a consultant at the Mayo Clinic in Rochester, MN.

C olorectal cancer screening in the United States is being promoted by major medical organizations ^1-3 and is now reimbursed by Medicare for eligible recipients. The basis of these important new public health policies is that most colorectal cancers arise from preexisting adenomatous polyps; therefore, detection and removal of these precursor adenomas should result in a decrease in the incidence of and mortality from colorectal cancer. ^4-12 A broad range of screening choices are available for patients, including fecal occult blood testing, flexible sigmoidoscopy, barium enema, and colonoscopy. Unfortunately, none of the currently available colorectal screening tests are optimal in terms of examination performance, safety, or patient acceptance. Physicians and scientists interested in colorectal cancer screening continue to look for improved methods to detect adenomas and early cancers.

Few radiologists would have conceived of detecting colorectal polyps using CT when this technology was first introduced into clinical practice 20 years ago. However, significant advances in CT technology have facilitated the rapid acquisition of thin slices of body tissue, and new multislice CT scanners have the capabilities to acquire all of the needed data for an abdominal (and colonic) examination in a single breath-hold. Advances in software and computer technology enable unique image displays of the colon in ways previously not possible. A comprehensive, full structural colon examination is now possible in less than one minute of scanning. This rapid acquisition translates into an easier, more comfortable examination than available with other structural tests.

The purpose of this article is to update the practicing radiologist on the current status of the technique of CT colonography, to compare it with other colorectal screening tests, and to provide insight into its future. Although the colonography technique has been applied to MR, this review will only discuss its utilization at CT. ^13,14

What is CT colonography?

CT colonography (CTC) was first introduced in 1994 using volumetric CT data produced by a spiral CT scanner. ¹⁵ The three-dimensional images displayed in a cine loop (simulating the endoluminal views seen at colonoscopy) excited many investigators in the field of GI imaging.

After several years of development, CTC today refers to an examination performed on a spiral CT scanner with acquisition of volumetric data of the entire colon. Combining these data with advanced imaging software, the colon is examined at an off-line workstation using the combination of two-dimensional and three-dimensional images. No single type of image display can characterize the technique today--most authorities believe that 2D and 3D images must all be seamlessly available on a single interactive screen.

Previous terms to describe this technique--such as virtual colonoscopy, virtual endoscopy, 3D endoscopy, and CT colography--have been abandoned in favor of CT colonography. ¹⁶

How is CTC performed?

The data acquisition phase of the examination is very brief. After thorough bowel preparation, the patient is given glucagon, either subcutaneously or intravenously. With the patient placed in the lateral decubitus position, an enema tip is placed into the rectum. Using either room air or carbon dioxide, the colon can be fully distended without causing undue patient discomfort. Most patients will retain 1.5 to 2 liters of gas or air.

Both supine and prone acquisitions are obtained at most centers today. A single acquisition generally takes three to four 20-second breath-holds (60 to 80 seconds of CT scanning total) on a single slice helical scanner; multislice scanners can acquire the entire data set in a 20-second breath-hold. This rapid examination, which does not require the use of sedation, intervention, or compression, is well tolerated and has been assessed by patients to be more comfortable when compared to other full colonic examinations such as barium enema or colonoscopy. ¹⁷ Patients appreciate the fact that interrogation of the colon is performed on the CT data at the physician's workstation, not on them--sparing them time and discomfort. After the CT data has been transferred to a workstation equipped with suitable imaging software, the colon is carefully inspected.

Determination of the "best" type of image display continues. ^18-20 Currently, most centers will examine traditional axial CT images. Often, these views are zoomed or optimized to make detection of subcentimeter lesions easier. Regions of the colon that are deemed suspicious for an abnormality are further inspected using two-dimensional multiplanar reformatted images and three-dimensional endoluminal views. These three types of image display modes (axial, multiplanar reformatted images, and 3D endoluminal views) are complementary and believed by most to be necessary tools for an accurate examination.

The radiation dose that is necessary for CTC is lower than that used for a conventional CT examination due to the high contrast between the air-filled colon lumen and the soft-tissue density wall. It is therefore possible to reduce the radiation exposure by at least 75% over standard body CT settings without sacrificing diagnostic performance. ²¹ Examining the colon at CT in both the supine and prone positions provides an overall radiation dose equivalent to or less than the screening barium enema. This lower radiation dose does, however, result in images with lower than normal signal-to-noise ratios, reducing the diagnostic effectiveness in evaluation of low contrast lesions in solid abdominal organs (i.e., liver, kidneys, and spleen). ²²

How well does CTC work?

The first blinded, prospective study of CTC assessed 70 patients (half with proven colorectal polyps and half from a surveillance population). ²³ All patients underwent colonoscopy, which served as the gold standard, and a supine-only CTC exam. For the detection of patients with lesions of 1 cm or larger in diameter, the sensitivity of CTC was found to be 75%, with a specificity of 90% (figures 2-4). Performance decreased as lesion size decreased. ²³ Since this study, other investigators have reported on the performance of CTC with a sensitivity for the detection of polyps of 1 cm or greater ranging from 75 to 100%, and a specificity of 86 to 100% (figure 1). ^24-27

Roister and coworkers reported a sensitivity of 100% in detecting large colorectal lesions of 2 cm or greater. In addition, the value of CTC was demonstrated in patients with obstructing carcinomas. In many of these patients, lesions above a colonic obstruction were correctly identified using CTC. These were inaccessible at colonoscopy. ²⁸

What are the current problems with CTC?

Major technical issues responsible for false-negative examinations include fluid-filled segments of bowel due to the lavage bowel prep (Go-Lytely™) and collapsed segments of bowel from colonic spasm. However, it is encouraging that technical problems related to retained fluid and incompletely distended segments of the colon often can be corrected with the addition of prone imaging. A prone scan in combination with a supine scan will readily move colonic fluid into opposite parts of the colon. This added benefit of prone imaging has recently been studied. Significantly more lesions are identified using prone imaging combined with supine acquisition versus supine imaging alone (figure 2). ^29,30

False-positive examinations generally are due to misinterpretation of retained stool or colonic folds as polyps. Retained stool often contains incorporated air that can be recognized at CT as a heterogeneous filling defect on two-dimensional images (figure 3). ^31,32 Colorectal neoplasms are homogeneous soft-tissue densities without intratumoral air. It may be possible, in the future, that oral stool markers, administered 24 to 48 hours prior to the CT examination, can be used to alter the density of stool and improve differentiation of stool from soft-tissue neoplasms.

Perception errors also can be reduced with observer experience and by applying accepted principles, as in barium radiography. Principles that are currently accepted include: 1) the ileocecal valve should always be identified. Any other filling defects in the right colon should be regarded as a potential lesions (figure 4); 2) a homogeneous solid filling defect in an otherwise clean colon should be considered a true lesion; 3) isolated haustral fold thickening may be due to an infiltrating tumor (figure 5); 4) the short (inside) limb of a colonic flexure should be carefully inspected--polyps are more difficult to detect in this location due to closely packed folds in these regions.

Diagnostic interpretation time remains an important issue in moving CT colonography from the research arena to routine clinical implementation. Early studies of this technique were plagued with long interpretation times, often requiring an excess of 60 minutes. Current image display techniques have been shown to markedly reduce interpretation times. The average interpretation time for both supine and prone data sets is now approximately 15 minutes. ²¹

How does CTC compare to other screening examinations?

CT colonography represents another colorectal screening option for physicians and patients. As a full structural colorectal imaging test, it competes directly with barium enema and colonoscopy. Its diagnostic potential appears to be much greater than fecal occult blood testing (FOBT). More than half of all colorectal cancers and the large majority of polyps will be missed on a single screening pass with FOBT, and most positive test results are false-positive. ^33-35 Because the performance of CTC has already been found to be significantly better than FOBT, better patient outcomes can be predicted and screening intervals can be less frequent.

Proctosigmoidoscopy is inherently flawed by its ability to examine only the left colorectum. As a full structural colorectal screening test, it is anticipated that CTC would demonstrate substantially more existing neoplasms.

CTC competes most directly with the screening barium enema. Formal trials comparing these two techniques have not yet been completed. However, there are some inherent advantages of CTC over barium enema. CTC has no requirement for coating the colon with barium, little effect from retained fluid if prone imaging is performed, a lack of complex overlapping radiographic lines (an important source of radiographic perceptive errors at barium enema), the ability to produce an infinite number of different projections of a colonic segment, and no need for a second cathartic preparation prior to colonoscopy if a polyp is discovered. In addition, because the examination is rapid, it is more comfortable and better tolerated by patients. ¹⁷

In comparison to colonoscopy, CTC has the potential advantages of visualizing colonic anatomy from both an endoluminal perspective and in multiple cross-sections; elimination of "blind spots" behind colonic folds (as the entire colon is virtually always evaluated); assessment of the internal density of lesions; and characterization of many lesions (polyps, lipomas) without the risks associated with sedation and biopsy (figure 6). Although a formal study on compliance with CTC has not been performed, in a survey of patients following CTC, barium enema, and colonoscopy, patients stated they were more willing to return for CTC at the recommended screening interval than for the other two tests. ¹⁷ However, tissue cannot be removed from the colon at CTC.

CTC also is capable of displaying extracolonic tissues--usually the entire abdominal and pelvic contents. In a recent study of CTC findings, nearly half of all the patients had extracolonic abnormalities discovered. Only 12% of patients had lesions that were considered "highly" significant. These included asymptomatic renal cancers, surgically repairable aortic aneurysms, pneumothorax, and hernias requiring surgical repair. Other lesions (pulmonary nodules, adrenal masses, etc.) that required follow-up imaging also were identified. ²² The added cost of these findings adds to the overall cost of the examination, but when compared to many additive costs associated with colonoscopy (e.g., room charges, sedation, biopsy, pathology), these are relatively modest when averaged among all patients. Although further study is needed, it seems somewhat ironic that possibly more significant pathology will be discovered in the extracolonic organs than in the colon, where the prevalence of polyps (> 1 cm) is estimated to be 4 to 5% in the average risk population.

What is the future for CTC?

CT colonography represents an important and exciting technique for imaging the colorectum. Theoretically, this technique has advantages over all existing colorectal screening tests. Early results are promising, but rigorous scientific evaluation of this technique in comparison with other colorectal tests needs to be done. These studies are now underway, and comprehensive data should be available by late summer of 2001.

We can expect evolving and improved ways to display CTC image data, and methods that enable more efficient interpretations. In addition, automated
methods of displaying the colon and computerized assessment of colon wall thickness may assist radiologic interpretation. It is conceivable that examinations will be accurately interpreted within only a few minutes and some of these may be read entirely by a computer. Examination cost has not yet been determined but will be dependent upon both resource utilization and interpretation time. Costs will likely be lowered as efficiencies improve with technical advances. Within the next few years, CTC charges will likely be highly competitive with those of other structural colorectal screening tests.

Prepping for colorectal screening remains a major obstacle for patient compliance. Development of a prepless CTC scan is possible, but only preliminary work has been done to date. Theoretically, patients would drink a contrast agent 1 to 2 days prior to the examination. This agent would alter stool density sufficiently so that it could be recognized and removed electronically from the images. No purgation would be required. Multiple different stool markers currently are under development, and clinical trials are likely to ensue within the next several years.

Conclusion

Colorectal screening has come of age. Traditional screening methods have now been approved in the United States for widespread screening of the general population for colorectal cancer. New techniques such as CTC offer the promise of improved performance and better patient compliance. Hopefully, such new developments will identify patients with polyps and cancer and, in conjunction with colonoscopy, will significantly reduce the incidence and mortality of this common disease. AR

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