The effectiveness of screening and polypectomy in reducing colon cancer incidence and mortality is well known. Virtual colonoscopy, an advanced application of computed tomography (CT), has demonstrated great potential for revolutionizing colon cancer screening. Multislice helical CT technology, three-dimensional (3D) reconstruction techniques, and lower radiation dose protocols have significantly improved the sensitivity of virtual colonoscopy. In addition to screening, several other important applications of CT colonography are under investigation. One important new application is the evaluation of patients in whom tissue diagnosis has already been obtained. Appropriate staging of such patients is essential for avoiding unnecessary morbidity and maximizing therapeutic benefit. Virtual colonoscopy plays an important role in preoperative evaluation by accomplishing both surveillance for metastases and evaluation of mucosal disease in a single procedure while minimizing patient discomfort. CT colonography is helpful in postoperative surveillance as well. Intravenous contrast plays an important role in these pre- and postsurgical patients by improving both visualization and characterization of lesions and overcoming many of the previous limitations of virtual colonoscopy.
is currently a fourth-year Resident in Radiology at Emory
University Hospital, Atlanta, GA. He received his MD degree from
Tulane University School of Medicine, New Orleans, LA in 1999. He
will remain at Emory for a Neuroradiology fellowship following
completion of his residency program.
is a Professor of Radiology, Division of Abdominal Radiology,
Emory University Hospital, Atlanta, GA.
The effectiveness of screening and polypectomy in
reducing colon cancer incidence and mortality is well known.
Virtual colonoscopy, an advanced application of computed
tomography (CT), has demonstrated great potential for
revolutionizing colon cancer screening. Multislice helical CT
technology, three-dimensional (3D) reconstruction techniques, and
lower radiation dose protocols have significantly improved the
sensitivity of virtual colonoscopy. In addition to screening,
several other important applications of CT colonography are under
investigation. One important new application is the evaluation of
patients in whom tissue diagnosis has already been obtained.
Appropriate staging of such patients is essential for avoiding
unnecessary morbidity and maximizing therapeutic benefit. Virtual
colonoscopy plays an important role in preoperative evaluation by
accomplishing both surveillance for metastases and evaluation of
mucosal disease in a single procedure while minimizing patient
discomfort. CT colonography is helpful in postoperative
surveillance as well. Intravenous contrast plays an important
role in these pre- and postsurgical patients by improving both
visualization and characterization of lesions and overcoming many
of the previous limitations of virtual colonoscopy.
Colorectal cancer is the third most common type of cancer and
the third leading cause of cancer death in the United States. The
National Cancer Institute estimated that in 2002 there were 148,300
new cases of, and more than 56,600 deaths from, colorectal cancer.
The incidence of colon cancer in the general population is about
0.3%, whereas it is about 15% for those with a family history of
the disease. The majority of patients, approximately 75% of those
who develop colon cancer, have no particular risk factors, an
observation that emphasizes the need for broad-based screening.
The pathogenesis of colon cancer is well established. It
progresses slowly, over several years' time, from adenomatous polyp
to invasive carcinoma, through the accumulation of genetic defects.
This protracted course is fundamental in the selection of colon
cancer as a potentially preventable disease. In order for screening
to be effective, there must be a relatively long preclinical period
during which detection of disease can have a substantial impact on
patient outcome. The size of a colorectal polyp is directly
correlated with the likelihood of malignant foci within, and it can
take up to a decade for a cancerous lesion to develop.
Thus, in the asymptomatic patient, screening and polypectomy can
reduce mortality by prompting therapy in the early stages of
disease. Resection of precancerous polyps has been convincingly
demonstrated to reduce the overall incidence of colon carcinoma.
Current screening protocols
Current recommendations of the American Cancer Society call for
one of the following screening options beginning at age 50: yearly
fecal occult blood test, alone or in combination with flexible
sigmoidoscopy every 5 years; double-contrast barium enema every 5
years; or colonoscopy every 10 years.
The most widely used method, fecal occult blood testing, has a
very low sensitivity (10% to 15%).
False positives can occur for a variety of reasons, a
characteristic that further limits the value of this technique.
Common causes include a recent meal of red meat, hemorrhoids, iron
supplementation, mineral oil laxatives, and menstrual bleeding.
Flexible sigmoidoscopy, another widely used screening technique,
examines only the distal colon, leaving much of the colon
Studies have demonstrated not only the need for more proximal
screening, but also the need for evaluation of the ascending and
transverse colon in those with positive findings on sigmoidoscopy.
Furthermore, it has been demonstrated that approximately 50% of
malignancies would be overlooked if only distal surveillance were
Barium enema has long been a mainstay of colonic evaluation.
This method, however, has many limitations. Patients undergoing
this procedure must be able to position themselves appropriately,
so that contrast can traverse the colon. Redundancy of the colon,
often encountered in the elderly, can frequently cause an
incomplete examination. Moderate sensitivity for clinically
significant polyps (75% to 85%), combined with significant
radiation exposure and patient discomfort, necessitate a more
The current standard for evaluation of the colonic mucosa is
direct visualization via colonoscopy. However, this is impractical
in a number of circumstances. Limited resources prohibit the use of
colonoscopy in every case in which it would be appropriate. Many
patients are unwilling or unable to undergo colonoscopy, due to the
discomfort either associated with bowel preparation or attributable
to the procedure itself. Conventional colonoscopy is incomplete in
as many as 5% of cases, where redundant colon prohibits passage of
Moreover, despite colonoscopy's current status as the gold
standard, up to 27% of polyps measuring <5 mm go undetected,
according to a recent study in which initial colonoscopy was
followed up immediately by a repeat procedure.
Patient noncompliance is a major factor further limiting the
effectiveness of screening modalities. Fewer than 25% of Americans
over the age of 50 follow the American Cancer Society guidelines
for screening. This has largely been attributed to the discomfort
associated with the aforementioned methods.
Despite the poor rate of compliance with suggested screening
protocols, colorectal cancer death rates have decreased by 1.8% per
year from 1992 to 1998, a trend largely credited to earlier
detection and aggressive therapy.
It is clear from Figure 1 that co-lorectal cancer in the general
population is most often a local or regional disease, amenable to
curative resection. Clearly, then, screening can be of greater
benefit if it is more widely accepted, as there is a high
proportion of lower grade, potentially curable cancers.
CT colonoscopy, also called virtual colonoscopy, is currently
being developed to address such issues. Patient tolerance for CT
colonoscopy has been shown to be significantly better than for
conventional colonoscopy, an observation that offers hope for more
widespread screening. In one recent survey of 104 respondents, 68
patients favored one examination; of these, 56 (82%) preferred CT
colonography over conventional colonoscopy, citing pain as one of
their foremost concerns. In fact, nearly twice the number of
patients in this study (49) felt that CT colonoscopy was not
painful, compared with 28 in the conventional colonoscopy group.
The ability of this technique to detect clinically significant
polyps is well documented and its utility in difficult cases
In a recent study of 100 patients published in the
New England Journal of Medicine
, CT colonoscopy had a sensitivity of 82% for all polyps and a
specificity of 84%. More importantly, however, for larger (ie,
clinically significant) polyps the sensitivity was 91%.
The applicability of this method to difficult cases was recently
confirmed; 40 patients in whom complete colonoscopy could not be
performed were given a virtual colonoscopic examination. In >90%
of cases, the previously obscured colonic segment could be
adequately evaluated using virtual colonoscopic techniques.
In addition to its role in screening, virtual colonoscopy has
great potential in the evaluation of patients with known or
probable colorectal cancer. The survival rate in such patients is
directly related to the extent of disease at the time of diagnosis.
Those with local disease have a survival rate of 90% to 100%,
whereas survival drops precipitously when distant metastases are
present (Figure 2).
Thus, early diagnosis is critical and offers the best chance for
Treatment of colorectal malignancy depends upon its stage at the
time of diagnosis. Local disease isolated to the mucosa requires
surgical resection, often without the need for adjuvant
chemotherapy. More invasive disease, characterized by varying
involvement of the muscular wall of the colon, with or without
local nodal disease, can be treated with a combination of surgery
and chemotherapy. Disease that is metastatic at the time of
discovery is generally treated with chemotherapy, with or without
debulking surgery. When hepatic metastases are present, lesion
resection or ablation is sometimes also possible, depending upon
the extent of disease. Radiation therapy can be used to varying
degrees, depending upon the size and extent of the malignancy; it
is critical in particular for this intervention that the location
and extent of disease be well established.
Virtual colonoscopy is ideal for cancer staging, as it allows
for the evaluation of mucosal lesions, which are poorly
demonstrated by standard CT. Given that approximately 7% of
patients have additional lesions, two-thirds of which are
synchronous and the remainder metachronous, it is essential that
these patients be thoroughly evaluated prior to surgical therapy.
Virtual colonoscopy combines the advantages of colonoscopy and CT
staging, permitting simultaneous evaluation of mucosal disease as
well as regional and distant metastatic disease.
Local and nodal extension
Extension of tumor first through the mucosa and then into the
muscular layers of the colon and beyond can be subtle, particularly
in the collapsed bowel. The addition of pneumocolon aids in
identification of aberrant wall thickening, and intravenous (IV)
contrast further increases the conspicuity of the lesion. Contrast
also plays an important role in the identification of nodal
disease. Evaluation of local and nodal disease permits surgical
planning that maximizes the chance of curative resection and may
determine the need for chemotherapy. A study by Harvey et al
demonstrated an overall staging accuracy approaching 80% with
contrast-enhanced CT colonography, in much earlier stages of its
The liver is typically the first organ to demonstrate metastatic
involvement, making it a critical organ to evaluate in the
determination of disease extent. Contrast-enhanced imaging
significantly increases sensitivity for detecting hepatic
Furthermore, patterns of enhancement are helpful in differentiating
malignancy from benign hepatic lesions such as hemangioma.
Characterization of the degree of hepatic involvement also allows
for therapeutic intervention, if lesions are appropriately
localized. Multidetector CT now permits accurate characterization
of the vascular anatomy of the liver, which is critical to
appropriate surgical planning. Not only does this affect decisions
about resectability of the patient, but also allows for critical
identification of anatomic variants. A recent paper by Sahani et al
demonstrates the utility of hepatic CT angiography; 42 patients who
were to undergo hepatic resection underwent CT angiography; in 17
of these patients, arterial anomalies were detected. This compared
very favorably with the catheter angiography performed in 22 of
these patients (accuracy, 97%; sensitivity, 94%; specificity,
100%), and was of significant benefit in surgical planning.
In addition to the liver, other common sites of metastatic
disease from colon cancer are the lung, adrenal gland, bone,
kidney, pancreas, spleen, and central nervous system. Since the
vast majority of patients with distant metastatic disease first
have involvement of the liver, it is important to begin by
identifying any hepatic lesions suggestive of metastatic disease.
CT colonoscopy, therefore, plays a critical role in the appropriate
staging of patients, permitting evaluation of mucosal, local, and
metastatic disease in a single procedure.
Beyond staging: Surveillance for local
Approximately 60% to 70% of patients who present with colorectal
carcinoma undergo surgical resection of the primary site; of these
recurrence occurs in 30% to 40%.
Aggressive monitoring with rapid identification of recurrent
disease and early intervention improves survival in the
A summary of the factors complicating the evaluation of
postsurgical patients follows, with a discussion of current
advances in this field.
Bowel wall identification
A recent study by Morrin et al,
in a series of 200 patients, demonstrated that IV contrast material
significantly improved the delineation of the bowel wall, improving
the sensitivity of the technique in cases where adequate bowel
cleansing could not be achieved. Increased sensitivity (75%,
compared with 58% in the noncontrast group) was present in the
intermediate-sized (6- to 9-mm) polyp group. The technique
described by Morrin et al
involves the use of 150 mL of nonionic contrast in the prone
position; rebolus was not given during supine imaging, to decrease
patient exposure to contrast dye. An example of enhanced wall
conspicuity is presented in Figure 3.
Differentiation of lesions from normal colonic contours can also
be problematic. Three-dimensional reconstruction aids in this
distinction, as the normal haustral pattern is clearly visualized
on reconstructed images and aberrations in the mucosal contour are
more evident (Figure 4).
Abnormal contrast enhancement can further serve to differentiate a
sessile carcinoma from haustral marking, as demonstrated in Figure
5. It is important to consider, however, that the increased
attenuation evident in lesions may be lost relative to fecal
debris, should tagging be performed prior to the study.
Pooling of fluid within the bowel is another problem that can
obscure polyps. Prone and supine imaging has been demonstrated to
improve sensitivity significantly, in part by the movement of fecal
debris and also by repositioning intraluminal fluid. Often, despite
repositioning, fluid is present in sufficient quantity to prevent
visualization of the full circumference of the bowel. As this fluid
is typically of intermediate density and mixed with fecal debris,
differentiation between it and a polyp is difficult. Lesion
enhancement can improve sensitivity in these instances, by
increasing tissue contrast. In the aforementioned study by Morrin
2 larger lesions (>10 mm) obscured by fluid were identified only
after contrast material administration (Figure 6).
Differentiation between adherent stool and polypoid lesions is
also difficult. Although prone and supine imaging has improved the
ability of virtual colonoscopy to distinguish between these,
adherent stool often remains on the nondependent bowel wall.
Contrast enhancement can help differentiate the two. In a study by
Morrin et al,
lesion conspicuity, particularly in the evaluation of smaller
polyps (6 to 9 mm), was significantly increased after contrast
material was administered.
Scar tissue often distorts the most critical region for
postoperative surveillance, the resection bed. Patients may develop
strictures, either at the site of anastomosis or from recurrent or
Contrast-enhanced CT plays an important role in separating
malignant from benign stricture at the site of anastomosis;
recurrent disease is often manifested by an irregular enhancing
mass. Figure 7 demonstrates abnormal enhancement in recurrent
carcinoma at the resection site.
Obtaining appropriate luminal distention is another significant
concern. Bowel spasm may limit distention of some segments.
Glucagon has been used by some institutions in these situations,
although recent studies suggest that its utility is limited, except
in patients suffering severe spasm.
Patients with a right hemicolectomy often have ileocecal valve
resection with consequent rapid reflux of air into the small bowel,
which complicates necessary colonic distention. As previously
discussed, IV contrast material can increase conspicuity of lesions
in suboptimally distended bowel, reducing the effect of bowel
Looking beyond the importance of this method in evaluating colon
cancer, and metastatic disease involving nodes, liver, lung, and
other organs, evaluation of extracolonic disease is a significant
benefit of virtual colonoscopy. Many studies have demonstrated the
ability to identify renal cell carcinomas, and other asymptomatic
diseases, whose early identification could result in curative
therapy. For example, there has been a marked increase in the
percentage of renal cell carcinoma incidentally detected, from 13%
detected in 1985 to 73% in 1993.
In a study of 264 patients, 11% of patients had significant
extracolonic findings, 6 of whom required surgical therapy.
This has been an increasing trend, as CT utilization has increased,
and will continue to be an added benefit of this method to the
The protocol at our institution involves 2.5 mm collimation,
pitch of 1.5, 120 mA (varying with patient size), 140 kVp, and a
512 * 512 matrix, with 1-mm reconstructions. Images are obtained in
both prone and supine positions. These images are transferred to
our workstation, which uses a proprietary software package to
reconstruct the images in three dimensions (Figure 8). This
software then requires the input of several seed points to
calculate the endoluminal view, which is represented in Figure 9.
This data is then used to reconstruct the colon in which a strip of
the entire colon is illustrated for rapid analysis (Figure 10). Any
point on these displays can be selected, and the corresponding
section on all images (cross-sectional, endoluminal, colon strip)
is highlighted (CT Colonography package, GE Medical Systems,
A recent mathematical model
comparing the cost-effectiveness of conventional colonoscopy and CT
colonography suggests that at the present time, the conventional
technique is a more economical option.
Certain recent advances must be considered, however, in evaluating
this technique. Multislice technology has dramatically decreased
scanning time, enabling prone and supine images to be obtained in a
total scanner time of fewer than 10 minutes. Image reconstruction
has had a significant impact on the speed with which images can be
interpreted. Most studies can now be evaluated in as little as 15
The focus of research at our institution is reducing this
interpretation time even further. The software package (developed
by Allen Tannenbaum, PhD, and colleagues, Georgia Institute of
Technology) that is used reconstructs the colon from axial images,
flattens it with minimal loss of contours, and thus allows
immediate visualization of the entire circumference of the large
bowel. This program is now under development to highlight
suspicious regions automatically (A. Tannenbaum, unpublished data).
This permits evaluation of the entire colon at a glance, calling
attention to areas of concern, and greatly reducing interpretation
time. Figure 11 illustrates a raw cross-sectional image. A
thresholding algorithm is then applied to select out the colon
(Figure 12). The colon is then reconstructed (Figure 13) and
flattened (Figure 14). Color coding of the image then displays the
topography of the colon, calling attention to mucosal
Virtual colonoscopy holds great promise in improving the
widespread acceptance of colon cancer screening. It combines the
advantages of speed and sensitivity with increased patient comfort
and can simultaneously evaluate intra- and extracolonic disease.
These abilities, when added to the heightened sensitivity provided
by the administration of IV contrast, suggest that virtual
colonoscopy will be a useful tool in patients with known or
suspected colon cancer. Furthermore, it provides an essential
surveillance tool in the postoperative population, in which
recurrence is commonplace and early recognition essential to