Though few radiologists would have conceived of detecting colorectal polyps using CT when the technology was first introduced, significant tech-nological advances have facilitated a more rapid, more comfortable examination than available with other screening tests. This article updates the practicing physician on the current status of CT colonography, compares the tech-nology with other colorectal screening tests, and provides insight as to its future role in the diagnosis of colon cancer.
is Professor of Diagnostic Radiology at Mayo Medical School and a
consultant at the Mayo Clinic in Rochester, MN.
olorectal cancer screening in the United States is being promoted
by major medical organizations
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
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
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
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.
What is CT colonography?
CT colonography (CTC) was first introduced in 1994 using
volumetric CT data produced by a spiral CT scanner.
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
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.
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.
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.
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
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
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
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.
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).
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.
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
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).
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.
How does CTC compare to other screening
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.
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.
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.
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.
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
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
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
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.
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.
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