Computed tomography colonography (CTC) is a promising method for detecting colorectal polyps and cancers. Although CTC is best suited for patients who are unable or unwilling to undergo conventional colonoscopy, its role as a colon cancer screening tool is somewhat controversial. This article presents important issues for radiologists who perform CT colonography or would like to do so. The authors address patient preparation and examination, technical scanning parameters and interpretation issues, the interpretation of colorectal mucosal lesions, the validity/performance and accepted clinical role of CTC, extracolonic findings, and the future applications of CTC.
is a Resident in Radiology, University Hospital Gasthuisberg,
Leuven, Belgium and a Visiting Fellow, Division of Abdominal
Imaging and Intervention, Department of Radiology, Brigham and
Women's Hospital, Boston, MA.
is a Clinical Fellow, Division of Abdominal Imaging and
Intervention, Department of Radiology, Brigham and Women's
Hospital, Boston, MA.
is the Director of International Symposium on Virtual
Colonoscopy, Boston, MA. He is also the Director of the 3D
Imaging and Processing Center, and a Staff Radiologist, Division
of Abdominal Imaging and Intervention, Brigham and Women's
Hospital, and an Assistant Professor of Radiology, Harvard
Medical School, Boston, MA.
is the Associate Director, Division of Abdominal Imaging and
Intervention, and the Director, Abdominal and Pelvic MRI, Brigham
and Women's Hospital, and an Associate Professor of Radiology,
Computed tomography (CT) colonography (virtual colonoscopy) is a
promising new method for detecting colorectal polyps and cancers.
Although multiple articles on this issue have been published since
the mid-1990s, it remains an important discussion topic in current
radiology and gastroenterology societies. Regarding its clinical
role, there is no doubt that this imaging technique is best suited
and highly recommended for those patients who are unable or
unwilling to undergo conventional colonoscopy. Its role as a
general screening tool for colon cancer is obvious for many,
equivocal for some, and doubtful for others. This article is
de-signed to highlight issues of importance for radiologists who
are considering or who have recently started offering CT
colonography to their patients and referring physicians.
Colorectal cancer is the second leading cause of cancer-related
deaths in the United States.
The majority of colorectal cancers are believed to arise within
benign adenomatous polyps and follow the adenoma-carcinoma
sequence. The duration of this sequence is very long (±10 years),
and the removal of these precursor adenomatous polyps decreases the
risk for lethal colorectal cancer significantly.
CT colonography uses multidetector-row CT to generate data,
which is then converted by computer software into 2-dimensional
(2D) and 3-dimensional (3D) displays of the colon. CT colonography
has several advantages over conventional colonoscopy: No sedation
is needed, it is only minimally invasive, and the examination is
less time-consuming than conventional colonoscopy. However, there
is still a need for bowel cleansing and insufflation of gas to
expand the colon. Moreover, exposure to radiation is inherent to
CT, and there is no possibility of biopsy, polypectomy, or
treatment during the examination.
Patient preparation and examination
Optimal CT colonography technique requires careful cleansing and
distention of the colon. Residual stool causes similar problems as
those encountered with barium enema radiography, as it simulates
polyps or masses. In theory, any preparation that results in a
clean colon will suffice. Good colonic cleansing is achieved by a
24- to 48-hour low-residue diet and the ingestion of a cathartic or
laxative that promotes evacuation of colonic contents. Saline
cathartics, such as sodium phosphate (Fleet Phosphosoda, C.B.
Fleet, Inc., Lynchburg, VA; VISICOL, Salix Pharmaceuticals, Inc.,
Morrisville, NC) or magnesium citrate (LoSo Prep, E-Z-EM, Inc.,
Lake Success, NY) are high-osmotic agents that induce increased
peristalsis and evacuation.
These agents can be administered with bisacodyl (Dulcolax,
Boehringer Ingelheim Pharmaceuticals, Ridgefield, CT) the day
before the examination. Dedicated preparation kits (Nutra-Prep,
E-Z-EM, Inc., Lake Success, NY) exist as well. Such "dry
preparations" are preferable to large volumes of electrolyte
preparations commonly used for conventional colonoscopy, such as
polyethylene glycol (Colopeg, Roche Laboratories, Gaillard, France;
Colyte, Schwartz Pharma, Inc. Milwaukee, WI; GoLYTELY, Braintree
Laboratories, Inc., Braintree, MA), be-cause with the latter, more
residual fluid remains and retained fluid may obscure polyps at CT
Stool markers (mostly barium, Tagitol or TagitolV, E-Z-EM, Inc.,
Lake Success, NY) or iodine can be administered orally 24 to 48
hours prior to CT to improve differentiation of soft tissue
intraluminal lesions and retained stool. This technique is called
Colon insufflation is performed with the patient in a lateral
decubitus position after placement of a catheter tip (with or
without a retention balloon) in the rectum. The colon is
insufflated with room air or CO
to maximum patient tolerance or to a set pressure. Therefore, an
air bulb insufflator or an automatic inflation system can be used.
Most patients will retain 1.5 to 2 liters of air. Carbon dioxide
can be used instead of air, as it is absorbed by the colon mucosa
and reduces discomfort.
The catheter is left in the rectum, and, with the patient in the
supine position, a CT scout image is taken to confirm adequate
colon distention (Figure 2). If adequate bowel distention is not
achieved, additional air is insufflated into the colon. After
scanning the patient in the supine position, the patient is placed
in the prone position, and additional air or CO
is administered. Subsequently, CT is performed with the patient in
the prone position. If a prone position is not possible for the
patient, a left lateral decubitus position is preferred for optimal
air and fluid redistribution. Several investigators have studied
the benefit of the use of prone imaging in addition to supine
imaging, and it has been shown to increase the sensitivity for
detection of colonic polyps (Figure 3).
The usefulness of spasmolytic agents such as glucagon or
Buscopan (Boehringer Ingelheim GmbH, Ingelheim, Germany) to improve
bowel distention is controversial.
As a result of its effect on the ileocecal valve, it can cause
unwanted reflux of air in the small bowel and may secondarily
reduce colonic distention. Generally, no bowel relaxant is used.
Administration of intravenous contrast material can sometimes be
helpful in the characterization of detected lesions,
but intravenous contrast is usually not used in the screening
setting because of the side effects of iodinated contrast agents
and the increased cost.
The current use of multidetector-row CT makes single-breath-hold
acquisition possible. The examination is generally well tolerated
by patients, and the lack of recovery time allows patients to
return to their normal activities shortly after the procedure.
Technical scanning parameters and interpretation
The use of a multidetector-row CT scanner with narrow detector
collimation is a prerequisite for CT colonography and optimal 3D
The CT parameters to use are dependent on the type of scanner, the
image quality needed, and the desired radiation dose. Commonly used
parameters are thin-detector collimation (0.6 to 2.5 mm), table
speed 6 to 16 mm/sec, 120 to 140 kV, and low tube current
Such relatively low CT tube currents can be used, as opposed
tocomparison with conventional abdominal CT protocols (±200 mAs),
because of the high inherent contrast between air and the colon
wall. Images are reconstructed as 1.5- to 3-mm-thick sections with
a reconstruction interval ranging between 0% and 60%. Coronal and
sagittal multiplanar reconstructions (MPR) are routinely performed.
The radiation dose of a virtual colonoscopy (supine/prone scan)
can get as low as 2 mSv for an ultra-low-dose CT colonography (10
and up to approximately 8.8 mSv at higher mAs. The same accuracy
can be obtained at an effective dose level of approximately 3.6
mSv, resulting in a substantial decrease of the theoretical or 3D
orthographic lifetime risk of developing fatal radiation-induced
Three-dimensional endoluminal images are useful to confirm the
presence of a lesion and to improve diagnostic confidence. Most
CTmanufacturers have software for creating 3D endoluminal and
multiplanar reformatted views. Several software companies also sell
special applications for CT colonography.
Workstations equipped with such software display 2D and 3D images
on a single interactive screen, and this promotes a quick 2D to 3D
correlation. Most investigators use a primary 2D interpretation
with "lumen tracking," starting from the rectum and following the
course of the bowel from slice to slice to the cecum.
Primary 3D endoluminal assessment of the colon (endoluminal
"fly-through") is less often performed, as it is more
time-consuming and is more susceptible to pitfalls.
Three-dimensional imaging is complementary to axial and multiplanar
reformatted 2D images and should be used as an adjunct to confirm
the 2D observation and improve the characterization of a noticed
However, with further advancements in computer processing
workstations, it is possible that the primary 3D approach will
reappear on the scene, as good results in a large study have been
reported by Pickhardt.
It is imperative that the 2D images are reviewed by using lung
window settings (approximately W: 1600, C: -400). In addition, soft
tissue windows must be used for further characterization of
suspected lesions as well in regions of suspected colonic collapse
(differentiation between colon collapse and pathologic wall
thickening with narrowing of the lumen) and when searching for
Interpretation of colorectal mucosal lesions
Every intraluminal projection in the colon requires careful
analysis to determine whether it can be ignored or needs endoscopic
confirmation and/or removal. With any imaging study, it is very
difficult-if not impossible-to determine the histologic nature of a
lesion. Therefore, radiologists must try to distinguish benign
features from moderately and highly suspicious lesions through
analysis of lesion characteristics. An important factor in the
evaluation of colon filling defects is size, as the potential risk
of malignancy increases with size. Morphology and attenuation are
also important criteria to evaluate.
A lipoma, for example, is a submucosal tumor for which a confident
diagnosis of a benign lesion can be made because of the low
attenuation of adipose tissue (Figure 4).
The lesions to detect
The aim of colorectal imaging and screening is to detect early
malignancies or premalignant lesions.
Colorectal polyps appear as round, oval, or lobulated intraluminal
projections and are homogeneous in attenuation. Every polyp should
be measured and reported with its localization within the colon.
When pedunculated polyps are measured, the diameter of the head
should be measured. Most large colorectal carcinomas appear as
fungating masses, often causing luminal narrowing (Figure 5).
Changing from a colon window setting to a soft tissue window
setting may facilitate identification of the lesion. Flat
colorectal adenomas and carcinomas are difficult to identify, not
only on 3D endoluminal images, but also on thin 2D images (Figure
An isolated thickened fold with an irregular aspect and a different
appearance from the adjacent folds may be due to an infiltrating
tumor and should be noticed. Villous tumors often appear
heterogeneous and, for less experienced readers, may mimic the
heterogeneous aspect of stool.
Problems and pitfalls: How to overcome
The following potential pitfalls in the interpretation of CT
colonography have to be considered: Fluid-filled segments and
collapsed segments, retained stool, complex and bulbous folds,
lipomas, foreign bodies, diverticulosis, the ileocecal valve, and
extrinsic compression defects.
Fluid-filled segments and collapsed segments are often
responsible for false-negative findings. The use of both supine and
prone imaging will move colonic fluid and stool. The time delay
between the 2 scans can be sufficient for transient segmental
colonic spasm to resolve.
The vast majority of false-positive findings are related to
residual fecal material. Therefore, the importance of a
well-prepared colon cannot be over- emphasized. Several
characteristics may help to differentiate stool from real lesions.
Retained stool often has a heterogeneous aspect due to incorporated
air or high-density food particles. The lack of wall attachment and
enhancement, and changing location on prone/supine images suggests
fecal origin (Figure 7). However, it is important to realize that
some parts of the colon (the cecum, transverse colon, and sigmoid
colon) are quite mobile because of the longer aspect of their
mesentery. When observing mobile filling defects, a second entity
to keep in mind is a pedunculated polyp with a long stalk.
The morphology of the polyp with a head and stalk is helpful in
differentiating these lesions from mobile fecal material. Fecal
material can be round, oval, or lobulated, which may mimic polyps,
but often presents with angled borders and irregular geometric
Fecal tagging also helps to differentiate immobile retained stool
from real lesions.
Irregular, complex, or bulbous interhaustral folds may appear as
suspicious filling defects on axial images, especially in bowel
segments that are not well distended. Visualization of the linear
morphology of the fold on coronal, sagittal, and/or 3D endoluminal
images can often solve the problem. The inner contour of a flexure
also necessitates careful inspection, as small lesions are easily
missed in this location.
Extrinsic compressions on the colon, which are potentially
confusing at 3D endoluminal imaging, are easily characterized on 2D
imaging, with adapted window settings.
A common source of error is to mistake the ileocecal valve for a
polypoid mass or vice versa. The ileocecal valve should be looked
for in every patient.
Following the terminal ileum into the colon is helpful to confirm
that the filling defect is the ileocecal valve. However, it is
important to remember that the valve is covered by mucosa and that
polyps may develop on the valve.
Validity/performance and accepted clinical
The performance of virtual colonoscopy depends primarily on the
size of the target lesion. CT colonoscopy has proven its diagnostic
validity, especially in patients with incomplete conventional
colonoscopy or with occlusive lesions in the distal colon, where
virtual colonoscopy can identify synchronous neoplasms in the more
To succeed as an ideal screening test, a test should be
sensitive, specific, safe, cost-effective, and acceptable to
patients. There is still some debate about what constitutes a
clinically significant colorectal polyp.
It is important to remember that adenomatous polyps (particularly
advanced lesions) are the primary target of screening.
However, the rather low conversion rate from benign to malignant
disease (only 3%) indicates that most individuals with adenomatous
polyps will never develop colorectal carcinoma.
Since the prevalence of malignancy is extremely low in lesions
smaller than 10 mm (only 1%),
this could be a good target. Patients with polyps larger than 10 mm
should undergo an immediate colono-scopy for excision of the
lesion. Diminutive filling defects (<5 mm) can represent normal
mucosal protrusion, adherent fecal material, hyperplastic polyps,
or small adenomatous polyps. Only the last has some potential for
malignancy. The clinical significance of missing a diminutive
adenoma is probably minimal,
and the question can be asked whether these very small polyps
(<5 mm) should be reported at all.
What about lesions between 5 and 9 mm? This remains a dilemma.
Most gastroenterologists remove these lesions when they are
visualized endoscopically. However, when CT colonography is
performed on an interval basis, missing these lesions is probably
not important, since the adenoma-carcinoma sequence is very slow
(up to 10 years).
Pickhardt et al
suggest 8 mm as a reasonable threshold for triggering immediate
For screening purposes, guidelines must be set to stratify
patients to undergo an immediate colonoscopy for polypectomy,
short-term follow-up, or routine follow-up. Many data suggest that
CT colonography has a high sensitivity and specificity for lesions
>10 mm, which makes CT colonography a good candidate for a colon
cancer screening test.
However, some recent publications have had less promising results,
with poor sensitivity even for lesions >10 mm.
The reason for these poor results is probably multifactorial.
Important factors on the general performance of CT colonography are
colon preparation, scanning techniques, and reader experience. This
stresses the need for adequate reader training before starting to
perform CT colonography.
An important advantage of CT colon-ography in comparison with
other colorectal examination tools is the ability to evaluate the
extracolonic structures of the entire abdomen. CT colonography has
the potential to save lives by detecting life-threatening lesions
in a preclinical stage (Figure 8). A drawback is the economic
impact of the supplementary cost of the further workup and
treatment of these incidentally found lesions. The detection of
equivocal disorders and potentially benign diseases induces often
unnecessary patient anxiety. A distinction should be made in the
interpretation of these extracolonic findings between highly
important findings (requiring prompt intervention or further
imaging workup), moderately important findings (generally benign,
with the possibility of re-evaluation at a later time), and benign
findings (no further evaluation nor treatment required).
Future applications and perspectives
Although great advances have been made, there are still some
barriers to CT colonoscopy becoming a generally accepted technique
for screening. These include a patient's reluctance to undergo
colon preparation, colon insufflation, limited accuracy for small
lesions, radiation dose, and the time required for interpretation
of all the data. Research is ongoing in several domains to overcome
Electronic cleansing by the digital removal of residual fluid,
opacified after the administration of oral contrast, allows
evaluation of colonic mucosa that would have otherwise been
In the future, further development of computer algorithms may lead
to a "prepless" colon and make CT colonography more acceptable to
Novel image display systems, such as colon unfolding techniques
and computer-aided diagnosis algorithms (eg, automated polyp
detection) may help to shorten interpretation time and may,
perhaps, increase accuracy.
Research continues on ways to reduce the radiation dose to the
patient, but radiation remains an inherent problem of CT. MR
colonography is still, at least for the moment, in the experimental
The optimal situation for interpretation of CT colonography is a
clean and well-distended colon. A variety of filling defects may be
encountered in the colon, and these should be further
differentiated by the optimal use of 2D and 3D imaging techniques
as well as the size, morphology, and attenuation characteristics of
the defect. An experienced reader should be able to provide a
clinically relevant recommendation (the need for colonoscopy or
not) for the vast majority of filling defects as well for the
extracolonic incidental findings. The feasibility of performing CT
colonography in the unprepared colon and with the aid of
computer-assisted techniques in the diagnosis may offer greater
potential for this already exciting examination.