Better and faster computed tomography (CT) equipment and more
sophisticated software now make the concept of CT colonography
(CTC), also known as "virtual colonoscopy," a reality.
Within the last 6 months, two new products have helped push this
technology to the leading edge of clinical practice. At RSNA 2001,
Voxar Limited (Edinburgh, Scotland) unveiled Colonscreen, their CTC
workflow and reporting solution. This product uses
three-dimensional (3D) imaging software to provide two-dimensional
(2D) and 3D images of a patient's colon. The system also provides
real-time image data manipulation and a reporting system.
E-Z-EM, Inc. (Westbury, NY), in conjunction with Vital Images,
Inc. (Minneapolis, MN), introduced their dedicated CTC system,
InnerviewGI, on March 11, 2002. InnerviewGI uses Vital Images'
Vitrea 2 technology to render 3D images in seconds, and provides
multiple 3D view options as well as synchronized 2D and 3D views of
Paving the way for CTC
"The thought of using CT for detecting lesions in the colon has
probably been around since CT first came into view in the 1980s,"
said Matthew Barish, MD, vice chairman of radiology, director of
the Virtual Colonoscopy Center at Boston Medical Center, and
Voxar's clinical director, "but the length of time it would take to
do the procedure with the older CT scanners would have precluded
any real study." He credits two technologic breakthroughs with
making CTC clinically feasible. "One was the helical CT that
allowed scans to be done in a much more rapid fashion and as one
continuous volume of data," he said. "The newer, multislice CT
scanners led this to become a truly viable alternative."
The second advance was increased computer power. This has
allowed the radiologist to view these images in various forms, in
both a 3D methodology as well as on computer screens (rather than
on film). With this technology, notes Barish, "you can cine through
them in a movie mode that allows you to pick up lesions that
wouldn't be visible on film."
Jay D. Miller, president and chief executive officer of Vital
Images, agrees. "With the multislice spiral CT scanners, you can
generate the higher resolution images faster than you used to be
able to," he said. "Clearly, on the CT side, not only can you get
better images, but you can also get them faster."
On the software side, Miller remarked, "The ability to do volume
rendering very quickly, very easily, and cost effectively is
probably the biggest change. Even 6 or 7 years ago, it took hours
to generate simple volume rendering of the colon. Now you can
review a colon data set, interactively, in both 2D and 3D in a
matter of minutes."
Miller also cited a third factor: Improved patient preparation.
"We are a lot smarter about patient preparation before the exam,"
he said, "including insufflation of the colon and all of the things
that have to happen outside of doing just the CT exam and the
volume rendering. All those things are coming together right
Conventional colonoscopy vs. CTC
Barish cited several advantages for patients choosing CTC over
conventional colonoscopy. "I think the main reason has to do with
both a sense of comfort for the patient, as well as a safety
profile," he said. "Conventional colonoscopy carries a certain risk
for perforation or injury to the bowel and that can lead to patient
The second reason, he noted, is time and convenience.
"Conventional colonoscopy is done with sedation, so the patient has
to have someone bring them to the procedure," he explained. "There
is a period of pre-anesthesia time, the actual procedure, 3 to 6
hours of recovery time, and the need to have someone drive the
patient home. So there is a fairly long period of time during which
patients are removed from their daily life or work."
Using CTC, on the other hand, patient exam time can be reduced
to as little as 10 minutes. "The set-up time for the CT is
minutes," he noted. "The actual scan time on the new scanners is
about 20 to 30 seconds for each of the two scans. Then after the
scan is done, the patient is free to go. Since there is no
sedation, they can go back to their routine activities."
Miller also sees advantages of CTC in increased visualization.
"If there is an occlusion of the colon, for example," he said,
"using conventional colonoscopy, sometimes you can't get the scope
past the occlusion. Clearly you can do that with CTC. Also, we
certainly can interrogate every square millimeter of the surface of
that colon virtually, whereas with the scope, sometimes you can't
see the complete surface."
Sensitivity and specificity
Several studies have looked at the sensitivity and specificity
of this technique with varying results. A study by Mendelson et al
looked at the accuracy of CTC in detecting colorectal polyps and
cancer in 100 patients with either colonic symptoms or a family
history of bowel cancer. Use of conventional colonoscopy found 121
polyps in 47 patients, while CTC found only 28 polyps in 19
patients. Using supine plus prone scans, the sensitivity of CTC was
73% for polyps with a diameter 10 mm or greater and 19% for smaller
polyps, with an 8% false-positive rate. With supine-only views, the
sensitivity dropped to 57% for the larger polyps and 11% for the
smaller ones. Positive and negative predictive values for CTC were
88% and 89%, respectively, for supine plus prone scans.
A study by Fenlon et al,
however, found substantially higher sensitivities. A total of 100
patients at high risk for colorectal cancer underwent CTC
immediately before conventional colonoscopy. Prior to CTC, each
patient had a rectal tube inserted and the colon insufflated with
air to the maximum tolerable level. Each patient also received 1 mg
of glucagon intravenously to minimize smooth-muscle spasm and
peristalsis and improve patient comfort. The entire colon was
visible in 87 patients using CTC and in 89 patients with
conventional colonoscopy. Forty-nine patients had abnormal
findings, with a total of 115 polyps and 3 carcinomas. CTC
identified all 3 cancers and 20 of the 22 polyps that were >= 10
mm in diameter (91%). For the smaller polyps, CTC identified 33 of
40 measuring 6 to 9 mm (82%) and 29 of 53 that were ¾ 5 mm (55%).
There were 19 false-positive findings of polyps, but no
false-positive findings of cancer. For adenomatous polyps, CTC
correctly identified 46 of the 51 (90%) that were >= 6 mm in
diameter and 12 of 18 (67%) of those that were smaller.
Miller expects these rates to increase as the technology
improves. "Clearly, with improvements in the quality of the images,
in patient prep, and in our ability to visualize the data sets, we
will continue to see the sensitivity and specificity of this tool
climb," he said.
"Although the goal of patient prep is the same for both
conventional colonoscopy and CTC, the dynamics of the process can
be slightly different," according to Allyson Mortati, global
product manager of virtual colonoscopy at E-Z-EM. Conventional
colonoscopy typically uses polyethylene glycol preparation, which
can leave too much fluid in the bowel for CTC. To address this,
E-Z-EM offers a saline osmotic preparation, LoSo Prep, which can be
used in conjunction with their low-residue diet, Nutra Prep, and
Tagitol, a stool-tagging agent. (For additional information on
these products, see RSNA Roundup.
One area of patient preparation that is specific to CTC is
insufflation of the colon. "Insufflation is an absolute requirement
for CTC," said Mortati. "You have to have the colon completely
distended to be able to see all of the walls, to be sure that you
have given a complete exam." Typically this has been achieved using
manual inflation with room air via a hand-squeezed bulb. In early
March 2002, E-Z-EM began marketing an automated CO
insufflation kit, ProtoCO2l. This kit offers several patient and
physician advantages, according to Mortati. "The biggest problem
with (insufflation with room air) is that there is a lot of
discomfort in recovery for the patient because they have to expel
the air. Room air is 70% nitrogen and there is no way for it to get
out except the way it came in. In contrast, CO
for insufflation gets absorbed by the bowel and gets exhaled
"In addition to that," she continued, "there is some further
patient comfort benefits in that this is constant-pressure
insufflation, rather than the manual insufflation that can create
pressure spikes. Also there is a productivity improvement with
automated insufflation; the doctor or the technician isn't standing
there squeezing the bulb, so he or she is free to do other
One area of concern with CTC is radiation dose. "Clearly, there
is concern about radiation dose," noted Miller. "The idea of CTC is
one in which radiation dose needs to be looked at very carefully,"
agreed Barish. "As more and more people learn this technique, one
of the fears is that the radiation dose will creep higher since
people don't know that it must be done in a method that reduces the
radiation dose. The dose can be limited to very low levels by
reducing the milliamperage, the power output, of the CT
This concern was echoed by Riccardo Iannoccone, MD, of the
University of Rome, speaking at the European Congress of Radiology
Meeting in Vienna this year.
He noted that in order for CTC to become useful as a widespread
screening tool, there must be a reduction in cost and in radiation
dose. "For us, the main issue is dose exposure," he said.
According to Iannoccone, there are three main reasons why
standard CT doses are high. First, patients typically undergo two
scans to maximize lesion detection. Second, the multislice scanners
used in CTC deliver higher radiation doses than single-slice
machines. Third, he noted that few attempts have been made to
develop and test low-dose protocols.
Iannoccone presented results of a study he conducted using a
low-dose multislice CT protocol.
In this study, the images were obtained from 27 patients with
suspected colorectal lesions using 2.5-mm slice collimation, 3-mm
slice thickness, 1-mm reconstruction interval, 17.5-mm/sec table
speed, 140 kVp, and 10 mAs on a Somatom Plus 4 Volume Zoom scanner
(Siemens Medical Solutions, Erlangen, Germany).
Using this protocol, the CT dose index (CTDI) dose exposure did
not exceed 1.37 mGy for each scan, with a total mean CTDI dose
exposure of 1.7 mSv for men and 2.3 mSv for in women
. According to Iannoccone, previous studies using a standard
regimen reported a the mean dose exposure ranging from 4.8 to 5.0
mSv for male patients and from 7.0 to 7.8 mSv for female
Following image acquisition and volume rendering, two
experienced radiologists read 2D axial multiplanar reformations and
3D endoluminal views and rated the image quality on a scale of 1
(nondiagnostic) to 4 (excellent). Use of this protocol resulted in
a mean image quality score of 3.1 for images of the colon and 1.8
for structures outside the colon, such as the lymph nodes and
In terms of sensitivity, all 9 colorectal cancers were
identified accurately using the low-dose CT protocol and 12 of 14
polyps were found, for a sensitivity of nearly 86%.
"Ultralow-dose CTC provides a substantial radiation dose
reduction," concluded Iannoccone. "It allows colonic assessment
with sensitivity that is comparable to protocols that produce
higher radiation doses, although our data is certainly subject to
confirmation from larger series."
"I think for the average- to low-risk patients, CTC would be an
ideal test," concluded Barish. "It's very good for those patients
who need to be screened but the likelihood of them having a
significant polyp is low to moderate. This is because if they do
have a polyp, they will have to go on to conventional colonoscopy
to have it removed. So therefore, in the high-risk population, it
is probably not the ideal test since those people would then need
to undergo two tests, one for diagnosis and one for treatment. It
is also very good for any patient who would not normally undergo a
screening test, either due to pain or if there are other conditions
that preclude their ability to undergo conventional colonoscopy,
such as heart disease, lung disease, etc. where anesthesia for the
procedure or the pain of the procedure would put them at higher
Miller also views CTC as another option for those who may be
unwilling to undergo conventional colonoscopy. "One of the big
drivers behind this is that we want to increase the compliance
rate," he said. "The problem is that people don't have conventional
colonoscopies done for a whole host of reasons. We anticipate that
because CTC is less invasive, people will be more likely to have
the procedure done. Colon cancer is very slow growing and very
predictable, for the most part," he concluded. "If you can find the
polyps early and remove them, you can save a lot of lives."