The promise of 16-slice computed tomography (CT) scanners became
a clinical reality in recent weeks as four major manufacturers
rolled out the latest additions to their CT lines. Siemens Medical
Solutions (Iselin, NJ) began shipping the Somatom Sensation 16,
Philips Medical Systems (Bothell, WA) began commercial shipment of
their Mx8000, and GE Medical Systems (Waukesha, WA) began full
production of the LightSpeed
. In addition, Toshiba America Medical Systems (Tustin, CA)
received marketing clearance for their version of the 16-slice CT,
the Aquilion 16, in mid-July and is expected to begin commercial
All of the new 16-slice scanners offer sub-millimeter slice
thickness and half-second or less rotation time. The Philips Mx8000
has a 24-mm wide detector and can operate in two modes: 16 * 0.75
mm or 15 * 1.5 mm. With the addition of Infinite Detector
Technology (IDT) and a 0.42-second rotation time, it can collect 16
slices of data simultaneously, and 38 slices of data per second,
thereby covering more than 4 cm of patient anatomy in 1 second with
sub-millimeter isotropic accuracy. The system also transfers data
from the detector array at a rate of 1 Gb per second.
The Siemens Somatom Sensation 16, with a 0.4-second rotation
time, has a voxel size of 0.5 * 0.5 * 0.6 mm. The system can
acquire 72 mm of coverage per second with 1.5-mm slice
The GE LightSpeed
provides 16-slice scanning in all modes with sub-millimeter voxels
and a 0.5-second rotation time.
The Aquilion 16 from Toshiba utilizes a 40-row quantum detector
capable of generating 16 simultaneous 0.5-mm, 1-mm, or 2-mm slices
with each 0.5-second gantry rotation. According to the company, the
system can complete a CT angiogram that scans from the abdomen to
the feet in 2-mm slice thickness in 15 seconds.
"If you look at CT over the past 4 years, I think the most
amazing thing has been the vast evolution in the applications and
the hardware changes that enabled that," said Peter Arduini,
general manager, global CT at GE. "Just from 1998 until today we've
evolved from 4 to 8 to 16 slices."
The challenge to get there was threefold, noted Markus B.
Lusser, segment manager, radiology and computed tomography at
Siemens. "The main challenge in designing such a scanner was not
only to design a machine that is able to acquire more slices," he
said, "but to acquire thinner slices and higher resolution and, in
the end, to handle the massive data sets very quickly."
"A number of things evolved over time," said Bill Kulp, manager,
product marketing at Philips. "First, the computers had to be fast
enough to process all the data." Next, he credits two major
technological advances with helping to make the 16-slice CT
possible. One was the development of cone beam reconstruction
algorithms. "When you get the wider detector and the thinner
slices, if you don't have cone beam reconstruction, you start
getting artifacts that become clinically significant when you get
to 16 slices. These algorithms have been known for many years," he
noted. "The challenge in implementing them was the fact that it
takes approximately an order of magnitude, or 10 times, more data
to process and reconstruct an image in the cone beam. Also, there
is a whole new set of artifacts and other things that have to be
fine tuned out to make the images look good. The other major
challenge," he continued, "was just getting the massive amount of
data off of the detector. When you are going from 4 to 16 slices,
there is obviously four times more data, and if you are going to a
faster rotation speed, there is even more. This scanner is going to
four images per second and we are going to be going faster in the
near future," he noted.
"I think the cone beam reconstruction is going to be a key to
image quality," agreed Bryan Westerman, manager, clinical sciences
at Toshiba, "because with this advanced technology, we have to be
very careful about image quality."
As the technology has improved, the clinical role for CT has
evolved as well. "When you go above 4-slice, all your applications
and protocol work change pretty radically," said Arduini. With the
introduction of multislice CT, the modality moved from one that was
used primarily for core radiology--central nervous system and basic
body imaging--into a variety of clinical practices, including
imaging the heart, lungs, and colon.
"The 4-slice CT scanner opened the door to new applications,"
noted Sholom Ackelsberg, general manager, global CT advance
products at GE. "It showed you could do CT angiography. The 8-slice
made 1.25-mm imaging routine for short coverage and 2.5-mm routine
for everything else. It expanded the ability to do CT angiography
on a routine basis and it showed the way for being able to look at
CT colonography and for some of the more advanced applications,
such as accurately sizing nodules in the lung and polyps in the
colon. So, the 4-slice showed what was possible and the 8-slice
made it real. The 16-slice just makes thin slices, 1 mm or smaller,
routine everywhere in the body."
Clinical applications for 16-slice CT
"Just about every area of imaging is going to be made better by
16-slice scanning," said Kulp. "The reason is that you can use
thinner slices over bigger volumes. Thinner slices are the Holy
Grail of imaging because it gives you more detail, more image
quality, and better resolution."
Most experts agree, however, that cardiac imaging is the field
expected to benefit the most from the new technology. "Without
question, the biggest advantage I see would be in cardiac imaging,"
"You are looking for a 50% stenosis on a 2-mm vessel," said
Ackelsberg. "You need to be able see <1 mm in your spatial
resolution and you need to be able to see it in three dimensions
because the vessels don't necessarily align themselves along one
axis of the scan.'
"We, and I think others, have shown pretty dramatically that
when you are dealing with coronary arteries that may be 1 or 2 mm
in diameter, you really need sub-millimeter slices to get good
accurate information," agreed Westerman. "If you are using the
sub-millimeter slices, you are back in the old situation of
coverage versus time," he explained. "With the 4-slice scanners, we
cannot use 0.5-mm slices and cover the whole heart in a reasonable
single breath hold. Therefore, most people are using the 1-mm
slices. When we have the 16-slice available, that gives us the
option of using the sub-millimeter slices. The 0.5-mm slices will
give you the best spatial resolution and you'll be able to cover
the whole heart in a very reasonable breath hold of 18 to 20
"I think the biggest effect has been that it really takes the
vascular studies to a whole new level of reliability and
resolution," noted John R. Haaga, MD, chairman of radiology at the
University Hospitals of Cleveland. Dr. Haaga participated in the
beta testing for the Philips Mx8000 and has been using the system
clinically for approximately 8 months. "I think we've all been
pleased with the ability to start looking at the blood vessels with
the multislice scanners. That the slices are thinner, more
reliable, and faster just gives us a purer quality. It really does
almost rival what can be obtained from a direct study. People have
talked about using CT in this fashion for many years, but I really
think that until we have these new devices it really hasn't been as
consistent and reliable as we would have hoped, but these really
are proving to be true."
"Cardiac imaging is the obvious one," said Kulp, "but beyond
that, areas like trauma are very important as well. In a trauma
situation, you want to be able to get as much diagnostic
information as fast as possible. With this scanner, you come right
in, you do the 16-slice CT, you are able to have the coverage with
thin-slice imaging capabilities, and you can get all the diagnostic
information you need right off the bat. And time is life in these
With scanning times reduced to <1 minute with the 16-slice
scanners, most agree that clinical protocols will need to be
adjusted. 'As the customer goes from 4-slice to a 16-slice scanner,
they have to change the protocols they use,' said Kulp. 'If they
don't do that, they are not taking advantage of the performance of
the new scanner; all they are doing is having a very expensive new
tool that they are using like the old tool.'
In particular, contrast protocols may require modification.
"First of all, the acquisition is over after an average of 10 or 11
seconds in high-resolution mode or in sub-millimeter mode probably
in 5 or 6 seconds through a 300- or 400-mm volume," said Lusser,
"so contrast media protocols have to be adapted to utilize the most
power of the machine."
"Shorter scan times require a little more care in how you try to
track the contrast material," said Westerman. "Certainly, I think
there is going to be another bit of impetuousness in increasing
injection rates. People are starting to talk about injection rates
of 5 or 6 mL per second. This is a little higher than most places
have tended to work with. It's going to be interesting to see how
this works out. It also may be possible to reduce contrast volume
Haaga said that at his institution they are using "somewhat
less" contrast media with the 16-slice scanner depending on the
study. "Of course, timing is a little more critical, so it is
important that we use special programs to track the bolus before we
begin the scan," he added.
Ackelsberg noted that it has been proposed that a 50% reduction
in contrast media may be possible with the use of a 16-slice
scanner and tighter boluses with accurate injection timing. "But,"
he cautioned, "this still has to be proven clinically."
While Toshiba currently has a 256-slice scanner under
development, most manufacturers are focusing more on clinical
applications than on increasing the number of slices. "We don't see
a big trend toward higher number of slices," said Lusser. "Of
course, there will be a trend toward thinner slices."
"I think that until now the world has been very much focused on
the hardware revolution," said Arduini. "I think we are at a point
now where we have a lot of speed throughout the body and can
perform a lot of new applications. What we are focusing on at GE is
evolving the overall application package; everything from
colonoscopy packages, lung capability, and tools, such as
computer-aided detection applications, to enhance not only the
overall productivity of the clinician but to really help, over
time, to make better decisions about disease management,
particularly in oncology. Down the road, we are going to evaluate
at which level of slice and coverage real value can be brought
above 16. Our hope is to understand where perfusion studies of the
heart and full-organ imaging in a rotation can be gained and what
new applications they can bring as well."
"For conventional CT applications, 16 is pretty darn good and it
gets you very close to where you want to be theoretically," agreed
Kulp. "But as you go faster, as you get wider detectors and thinner
slices, new possibilities open up. I can see CT being used for
things that it was never used for before and that is really where
you want to go; for example, whole organ perfusion. For that, you
are going to need a detector that, instead of being 2 or 2.2 cm, is
6 or 8 or even 10 cm. And you're going to need thin slices over
that kind of a volume."
"CT is coming more and more to the front of the hospital,"
concluded Arduini, "and I think the 16-slice scanner is going to be
more and more of a force in that evolution."