The purpose of this article is to review the development of CT colonography, evaluate its efficacy, and to examine the future applications of this technique.
received both his undergraduate and medical degrees from the
University of Virginia, Charlottesville, VA. He is currently a
third-year Radiology Resident at the Stanford University Medical
Center, Stanford, CA. After completion of his residency, Dr.
Schreiber will spend one year as a Fellow in Body Imaging at Johns
Hopkins Medical Center, Baltimore, MD.
Computed tomography (CT) colonography has demonstrated
efficacy for the detection of clinically significant polyps.
Technologic advances, including multidetector scanners and
computer-aided detection, will increase the sensitivity of
detection and decrease the time for interpretation of this study.
Fecal tagging may allow for the elimination of a bowel-cleansing
regimen, which is currently a major factor in patient
noncompliance. The combination of these expected advances
promises to make CT colonography a viable option for population
screening. The purpose of this article is to review the
development of CT colonography, evaluate its efficacy, and to
examine the future applications of this technique.
Colorectal cancer is the second leading cause of cancer-related
death in the United States, and the third most common cancer. This
neoplasm is diagnosed in 140,000 people annually in the United
States, resulting in 60,000 deaths.
It is widely believed that almost all cases of colorectal cancer
begin with the development of benign colonic polyps, and early
detection of these polyps has been documented to greatly improve
patient survival. These facts have prompted the American Cancer
Society to recommend yearly fecal occult blood testing and physical
examination, as well as screening sigmoidoscopy every 5 years for
all adults over the age of 50. Further recommendations include a
double-contrast barium enema every 5 to 10 years or a colonoscopy
every 10 years. The latter recommendations are based on the fact
that nearly half of all colon cancers occur proximal to the sigmoid
Computed tomography (CT) colonography was first introduced in
1994 by Vining and Gelfand
as they displayed three-dimensional (3D) endoluminal images of the
colon. Since that time, there has been intense research and
development in this field, greatly aided by such technological
advances as the advent of multidetector CT and improved computer
processing ability and software. Currently, CT colonography
typically involves thin-section CT imaging of the cleansed colon
with the data set reviewed both as two-dimensional (2D) and 3D
endoluminal images of the colorectal mucosa.
The advantages of CT colonography over conventional colonoscopy
include an improved safety profile and the ability to visualize the
entire colon in both antegrade and retrograde directions with
subsequent visualization of both sides of the haustral folds.
Lesions can be accurately localized relative to extra-colonic
landmarks. This technique has been proven to be effective in the
evaluation of the proximal large bowel in patients with both
incomplete conventional colonoscopy and obstructing distal lesions,
as well as in frail, debilitated patients.
Ultimately the goal is for this examination to be a viable option
for population screening.
As of yet there has been no real consensus on precise techniques
for CT colonography; techniques have been evolving over time to
take advantage of improved CT technology. Current research is also
focusing on fecal tagging to eliminate the arduous bowel
preparation that is required.
At our institution, the following protocol is used. Bowel
preparation consists of a phosphosoda in two doses, as commonly
employed for fiberoptic colonoscopy. The total phosphosoda dose is
3 oz given in divided doses at approximately 2 pm and 7 pm the day
before the exam. Other investigators have used polyethylene glycol
and magnesium citrate. Thorough cleansing is critical to the
success of the examination. Unfortunately, the preparation is also
the least tolerated portion of the examination. Intense research is
currently being done on what has been termed digital subtraction
bowel cleansing that does not require bowel catharsis.
This technique will be discussed later.
The patient arrives in the CT suite the morning after the bowel
preparation. A physician or nurse inserts a foley catheter into the
rectum and insufflates air into the rectum while rolling the
patient in multiple positions to facilitate colonic distension. Air
is added to patient tolerance, approximately 40 puffs in our
experience. A scout radiograph is then used to assess colonic
distention. Additional air can be added at this time if necessary.
Some investigators have also used spasmolytic agents such as
glucagon to reduce bowel peristalsis and resulting motion artifact.
The largest study to date addressing this subject showed no added
benefit to the use of spasmolytics and their use remains
controversial at this time.
The abdomen and pelvis are then scanned in both supine and prone
position using a multidetector row scanner with the 4 * 2.5 mm
detector configuration and images with reconstruction of 2.5-mm
sections at 1.25-mm intervals. With this configuration, imaging at
a pitch of 3 results in a 40-second acquisition time. Relatively
low dose is obtained with the use of 120 kVp and 48 mAs (60 mA at a
0.8 second gantry rotation period). The development of 8-detector
scanners with faster gantry rotation allows for image acquisition
in approximately 20 seconds. Imaging at a pitch of 6 results in a
20-second acquisition time, suitable for older patients who may
have difficulty holding their breath for 40 seconds. In such cases
we use 120 kVp and 96 mAs (120 mA at a 0.8-second gantry rotation
period). This protocol can be adapted to the ability of the CT
scanner. While some investigators continue to perform CT
colonography with 5-mm sections, there has been a trend toward
thinner sections as scanners have advanced, with some now routinely
using 1.0- to 1.25-mm sections.
Post-processing is performed in our 3D lab by RT and the study
is reviewed using the axial 2D images, with the 3D images used for
problem solving. The images are reviewed at lung window settings.
Comparison of supine and prone images allows for discrimination
between retained, mobile feces and stationary polyps. Repositioning
also allows for visualization of the entire colonic surface as some
degree of retained fluid is usually present. Research is currently
being performed to evaluate the utility of computer-aided detection
in the assessment of colonic polyps and will be discussed
Efficacy of CT colonography
Two factors must been considered when evaluating the efficacy of
CT colonography. First, the sensitivity and specificity of this
technique must be compared with conventional colonoscopy, the gold
standard for detection of colon polyps. In addition, it is also
critical to take into account the natural history of colon cancer
to determine what represents a clinically significant lesion.
There has long been debate about what constitutes a clinically
significant polyp. It is currently the practice of endoscopists to
remove all polyps discovered at colonoscopy, regardless of size;
but each removal has a small risk of perforation. The practical
necessity of this removal approach has been called into question,
however. In a presentation at the Second International Symposium on
Virtual Colonoscopy, in October 2000, Glick
made several important points regarding this practice.
Specifically, only about 1% of adenomas <1 cm in diameter harbor
invasive cancer. Alternatively, polyps 1 to 2 cm are generally
neoplastic with about 10% being malignant.
Further data from a 1992 paper by Chantereau et al
further subdivides this category, demonstrating that most
malignancies occur beyond the 15-mm threshold. Glick
therefore concludes that polyps >15 mm in diameter would be a
worthwhile and productive target for detection and removal. As a
more conservative goal, it seems critical for any detection method
to detect polyps >1 cm with high sensitivity and
A review of the literature demonstrates that the sensitivity and
specificity of CT colonography are directly dependent on polyp
size, with the lowest sensitivity found for polyps <6 mm. Among
investigators in the field, it is generally agreed that the lower
size for lesion detection should be 1 cm and the available data
will be described with regard to this threshold. In an early
publication, Hara et al
studied 70 consecutive patients who underwent both conventional and
CT colonography. CT colonography had a sensitivity and specificity
of 75% and 90%, respectively, for identifying patients with
adenomas >1 cm. Conventional colonoscopy was used as the gold
standard in this study. Apart from yielding promising early
results, the authors point out that false negative results were
caused by a combination of perceptive errors and suboptimal patient
preparation. The authors ascribe many of the false positives in the
CT colonography group to respiration artifact and perceptive
errors, such as misidentification of stool. The authors conclude
that with greater operator experience and improved scanning
technology their results would be greatly improved.
The efficacy of CT colonography with regard to polyp detection
has also been studied in other larger trials.
One study, involving 300 patients, was designed to compare the
results of CT colonography and traditional colon-oscopy with
histologic findings. In this study, the researchers reported a
sensitivity of 90% (74 of 82) for the detection of polyps >= 10
mm (figure 1).
In a second study--this one involving 100 patients--Fenlon et al
reported a sensitivity of 91% for polyps >10 mm in size. They
also reported a positive and negative predictive value of 96% each
for polyps >9 mm in diameter.
There is growing evidence that CT colonography is also effective
in evaluating colorectal masses in symptomatic patients. Lees and
submitted a cohort of 1250 symptomatic patients who underwent CT
colonography, more than 1150 of whom had corroboration with
colonoscopy, laparotomy, or a minimum of 6 months of clinical
follow-up. This study included the use of intravenous contrast,
which is not in general use at this time. The authors concluded
that CT colonography is reliable for the detection of cancer in
symptomatic patients with a sensitivity of 98.7% and a specificity
of 98.2%. They detected 232 of 235 cancers in the cohort. One
cancer was missed secondary to inadequate patient bowel preparation
and was subsequently found on a follow-up CT. The other two were
Duke's A "flat" cancers. These cancers, which arise in so-called
flat adenomas, are important to consider. This pathologic sequence
through a flat adenoma has an unclear contribution to the natural
history of colon cancer but may be responsible for 1% to 20% of
A second recent abstract presented at Radiological Society of
North America Annual Meeting (RSNA 2001) in November 2001 by Laghi
studied 103 consecutive patients with risk factors for colorectal
polyps who underwent both CT and conventional colonoscopy. Thirteen
colorectal carcinomas and 42 polyps were found with conventional
colonoscopy. CT colonography correctly identified all carcinomas
and had a 92% sensitivity for polyps >10 mm. The per-patient
specificity was 90%.
Lastly, long-awaited results were submitted at RSNA 2001 from
the American College of Radiology Imaging Network (ACRIN) Study
that conducted a multicenter performance evaluation of CT
colonography. In a sample of 94 cases with 50% disease prevalence,
the researchers reported that CT colonography demonstrated high
accuracy for the detection of polyps >1 cm. Accuracy was
measured by the average area under the receiver operator
characteristic curve, which was 0.89.
While these recent results are very encouraging, they are
difficult to extrapolate to a screening population given the high
disease prevalence in the study groups. Further studies are still
required to assess the performance of this study in large screening
populations with low disease prevalence. In the meantime, CT
colonography has proven efficacy in symptomatic patients, as well
as patients with obstructing distal lesions as previously
Belloni et al
demonstrated the potential efficacy of this procedure for following
patients after polypectomy by conventional colon-oscopy, showing a
sensitivity of 92.6% for polyps when comparing CT and conventional
colonoscopy findings directly.
CT colonography as a screening tool
The case for colorectal cancer screening in the general
population is well established and has been discussed earlier.
Standard recommendations currently include annual testing for fecal
occult blood and periodic sigmoidoscopy after the age of 50 for
those at average risk for colorectal cancer. The federal government
has endorsed these screening methods by establishing coverage for
such procedures for all Medicare beneficiaries. A relatively recent
The New England Journal of Medicine
eloquently makes the case that all persons age 50 or over should
undergo comprehensive evaluation of the entire large bowel, which
is currently best accomplished by conventional colonoscopy. His
argument is strongly supported by large studies performed
independently by Imperiale et al
and Lieberman et al
who reported similar findings. These studies included large cohorts
of asymptomatic persons who underwent colonoscopic screening.
Greater than 50% of patients with pathology in both studies had
advanced polyps or carcinomas only in the proximal colon, which
would have been inaccessible to flexible sigmoidoscopy. The results
of the National Polyp study were also striking, demonstrating
convincingly that colonoscopic polypectomy resulted in a
lower-than-expected incidence of colorectal cancer.
These findings support the well-established view that colorectal
adenomas progress to adenocarcinomas, as well as the practice of
finding and eliminating polyps to prevent colorectal cancer.
It is clear that comprehensive evaluation of the large bowel
will be central to colorectal cancer screening in the near future.
The question to be addressed is how CT colonography may be involved
with this screening. We have seen that it is critical for a
screening exam to detect polyps >1 cm. CT colonography has
demonstrated efficacy in the detection of polyps >1 cm in
patients with high disease prevalence but its utility can only be
extrapolated in a screening population with low disease
The current gold standard for complete evaluation of the large
bowel is conventional colonoscopy, and this procedure is both
diagnostic and therapeutic. Significant advantages of CT over
traditional colonoscopy include complete evaluation of the colonic
mucosa and patient safety. Lieberman et al
reported an overall complication rate of traditional colonoscopy of
0.3%. Significant problems related to gastrointestinal perforation
and problems related to conscious sedation occur in approximately
0.2% of examinations.
While these complications do have a low rate, the number of
affected patients would rapidly rise if colonoscopy were to be used
for population screening. Traditional colon-oscopy also fails to
examine the proximal colon in 10% to 15% of cases. A study by
Morrin et al
demonstrated the efficacy of CT colonography in evaluating these
nonvisualized segments after incomplete colonoscopy. Additional
advantages of CT colonography include decreased exam time, no
sedation, and the ability to examine both sides of the bowel wall.
Disadvantages include the need for very thorough bowel cleansing,
problems of spasm and retained debris, resolution for flat
neoplasms, and the fact that the exam is diagnostic only.
CT colonography is therefore established as being safer than
traditional colonoscopy and will very likely prove to have similar
diagnostic accuracy. The problem of flat neoplasms still exists and
must be further evaluated. Widespread access to needed CT hardware
and software is expected and must improve. A major barrier to the
implementation of CT colonography for population screening is
cost-effectiveness. A comprehensive study into this question by
Sonnenberg et al
concluded that for CT colonography to be cost-effective, it must be
associated with an initial compliance rate 15% to 20% better
procedural costs 54% less than traditional colonoscopy.
The major factor limiting patient compliance with traditional
and virtual colonoscopy is the arduous bowel preparation required.
False-positive and false-negative results in CT colonography have
also, in some part, been attributed to inadequate bowel preparation
or adherent stool. Investigators have been using contrast material
to label stool that can then be subtracted from the images in the
hopes of being able to eliminate this bowl preparation procedure.
In this experimental technique, 24 to 48 hours prior to imaging,
the patient ingests high-density contrast material that fully
opacifies the colon. A single CT acquisition is then performed
after colonic insufflation. Computer software is then used to
subtract the ingested colonic contents and stool, leaving the
colorectal mucosa unaffected. Promising early results were
published by Zalis et al.
A larger study was published subsequently that suggested that
sensitivity for polyp detection using this technique approaches
that achieved in traditionally prepared colons.
This technique also has the potential of a being performed in a
single acquisition as opposed to the currently standard supine and
prone scanning, reducing radiation by half.
Elimination of the bowel preparation is expected to greatly
improve patient compliance. This increase in compliance would
significantly contribute to making CT colonography cost-effective
for population screening.
Interpretation time also has a significant impact on the
cost-effectiveness of CT colonography. Postprocessing of supine and
prone images is performed routinely to make virtual 3D movies that
have the appearance of conventional endoscopy (figure 2). There is
no consensus as to the best use of these images, but at this time
they are generally used as a problem-solving tool in combination
with the reference axial images. Given the huge volume of data
generated, interpretation time is a significant concern. This has
prompted the development of several techniques for computer-aided
detection of polyps. In his abstract at the 2000 Virtual
Colonoscopy symposium, Beaulieu
described the Stanford approach to computer-aided detection and
concluded that some form of computer-aided detection will
eventually become the most effective means of making CT
colonography efficient, accurate, and cost-effective (figure 3).
This increase in cost effectiveness comes from significantly
reducing the interpretation time for the examination. The most
recent work on this subject, results from a series of 41 cases
presented at RSNA 2001, concludes that computer-aided detection has
the potential to detect polyps with a high sensitivity and
acceptable false-positive rate.
Novel 3D visualization techniques were also described by Vos et al
and Wax et al
who conclude that visualization with their techniques provides
better visualization of the colonic mucosa then conventional
The diagnostic ability of CT colonography has been shown to be
similar to traditional colonoscopy, and future developments, such
as computer-aided detection and fecal tagging, promise to
significantly improve efficacy, patient compliance, and image
interpretation time. These improvements, combined with the inherent
safety of the examination, promise to make CT colonography a viable
population screening method within the next decade. Radiologists,
gastroenterologists, and internists must work together to improve
screening compliance and patient education.
The author would like to thank Dr. Christopher Beaulieu and Dr.
Robert Mindelzun for their guidance and mentoring during the
completion of this project.