Computed tomography (CT) has made impressive progress during the last 20 years. In the 1980s, a slice thickness of 10 mm was common, and a typical study generated a total of 25 to 30 slices. Today, we routinely use a 0.75-mm slice thickness, reconstruct at 0.5 mm, and produce a data set that ranges in size from 400 slices for a standard application, such as single-phase liver imaging, to 3,000 slices for a cardiac scan.
is a Professor of Radiology and Oncology and the Director of
Diagnostic Radiology and Body CT at Johns Hopkins Medical
Institutions, Baltimore, MD.
Computed tomography (CT) has made impressive progress during the
last 20 years. In the 1980s, a slice thickness of 10 mm was common,
and a typical study generated a total of 25 to 30 slices. Today, we
routinely use a 0.75-mm slice thickness, reconstruct at 0.5 mm, and
produce a data set that ranges in size from 400 slices for a
standard application, such as single-phase liver imaging, to 3,000
slices for a cardiac scan. Image display has changed radically as
well. It is no longer necessary to review a stack of axial images;
instead, we can take advantage of volume displays and even
four-dimensional motion studies.
CT continues to change. Today 16-slice CT scanners may be the
state-of-the-art; however, by the end of the year, 64-slice
scanners will likely claim that honor.
Advances in CT technology mean that contrast enhancement can be
used in more sophisticated ways. Sixteen-slice technology enables
us to scan the liver, pancreas, or lung in <10 seconds and to
coordinate image acquisition and contrast delivery precisely,
taking advantage of the information revealed by multiphasic
Isotropic resolution-voxels of equal dimension in the x-, y-,
and z-axes-yields another critical benefit of advanced technology.
In peripheral vascular imaging, for example, isotropic resolution
enables detailed visualiza-tion of small vessels from any imaging
plane or perspective that we might select for viewing.
Checklist for success
Several very practical things are needed for state-of-the-art CT
angiography (CTA). The first is a good scanner, and today, that
means 16-slice technology. Well-designed protocols are also
essential. Equally important, technologists must be dedicated to
optimal data acquisition, and radiologists must be prepared to
accept nothing less. CT angiography is a zero-tolerance study.
Either it is done well, or it is of little or no value.
Contrast protocols define the type and volume of contrast to
use, the injection rate, and the timing of data acquisition
relative to contrast injection. For example, for pancreatic
imaging, at our institution we use 120 mL of either Omnipaque 350
or Visipaque 320 (GE Healthcare, Princeton, NJ) injected at
approximately 4 mL/sec. scan delay for arterial-phase imaging is 25
seconds, and for venous-phase imaging, 50 seconds. A contrast
protocol is very defined and must be implemented the same way every
time to ensure successful and efficient operation.
Scanner-specific protocols are equally important. These define
detector collimation, slice thickness, interscan spacing, kVp and
mAs, and the timing of injection and data acquisition. Thus, a
typical protocol for pancreatic imaging specifies a 120 kV, 140 to
200 mAs, a 0.75-mm detector collimation, a 0.75-mm slice thickness,
and incremental reconstruction at 0.5 mm.
State-of-the-art CTA also demands good postprocessing tools,
whether on the scanner or at the workstation. It is important that
radiologists have regular hands-on experience at the workstation.
Those who are reluctant to do so will not excel at CTA.
The information in a CT scan is not fully appreciated by viewing
axial slices. Extracting such information requires volume
visualization. The left adrenal vein offers a good example. This
vessel is roughly 1 mm in width and, on axial slices, appears to be
no more than a dot. On a three-dimensional (3D) volume display,
however, the adrenal vein can be seen clearly (Figure 1).
The importance of volume displays will become even more evident
with the introduction of 64-slice CT scanners. More and more slices
will only increase the potential for extracting information from
data sets and displaying it as a volume.
The preference for volume displays is not limited to
radiologists. Referring physicians want critical information that
allows them to manage patients. A single volume display represents
hundreds of axial slices and often provides all the information the
referring clinician needs. At our institution, we have found that
even surgeons with many years of experience request only 3D images
for preoperative review.
Most scanners now are able to do many different rendering
techniques, including maximum-intensity projections (MIPs) and
volume renderings. Each type of volume display has advantages and
disadvantages. For example, in Figure 2, the renal vessels are
nicely shown with both the MIP and the volume-rendered technique
(VRT), but there is better definition of the cortex and medulla
with the VRT.
The clinical applications of CTA span from the top of the head
to the bottom of the feet. In the head and neck, these include the
detection of intracranial aneurysm and definition of its extent
(Figure 3), as well as evaluation for carotid stenosis.
In the abdomen, CT is steadily ex-tending its capabilities in
the pancreas, liver, kidney, bowel, and aorta and branch vessels.
In examinations of the liver, timed acquisitions and arterial-phase
imaging accentuate the presence of hepatic tumors, enable arterial
mapping of feeding vessels, and permit detection and staging of
gastric and esophageal varices in patients with cirrhotic disease.
In many cases, a single study can detect disease, define the extent
of disease, and guide the selection of therapy.
In the kidney, CTA provides an excellent opportunity to define
arterial and venous anatomy, including abnormalities that are
easily overlooked on axial images. It ably evaluates patients with
renal stenosis and those who have undergone stenting of the renal
artery. In addition, CTA and 3D mapping are quickly becoming the
standard of care for evaluating suspected, or known, renal
In the chest, pulmonary angiography is a key application.
Whether the display is a VRT or MIP, the amount of information
available from a typical data set is very impressive.
Cardiac imaging is one of the most exciting applications of CTA,
particularly since the introduction of 16-slice scanners.
Retrospective gating of data sets enables detailed examination of
the coronary arteries (Figure 4). With the introduction of 64-slice
scanners, cardiac CT will be an area of tremendous change and
CT angiography of the peripheral vasculature continues to excel.
From the iliac arteries to the femoral arteries through the
trifurcation, the detailed depictions of vascular anatomy are
unprecedented and superb. For this reason, CTA is replacing
conventional angiography in up to 90% of studies of the peripheral
CT angiography is expanding beyond the imaging of
atherosclerotic disease to imaging patients with infection.
Thin-section data sets can define inflammation, depict tendons and
muscle, and examine detailed arterial structures, even down to the
As scanning technology advances, careful contrast administration
becomes even more important. Precise timing of contrast delivery
and image acquisition yields new information previously unavailable
With its ability to perform comprehensive examinations from head
to toe, CT angiography is reshaping the practice of radiology.
Patterns of referral are changing and, with them, the way referring
physicians use radiology and interact with radiologists. Adapting
to these changes requires modifications in workflow, changes in
processes, and improvements in skill sets. This is a time of both
challenge and opportunity. I hope this article shares this
excitement with you.
ELLIOT K. FISHMAN, MD:
Does anyone have any additional comments based on your personal
experience with CTA?
GEOFFREY D. RUBIN, MD:
I think that you have hit upon many of the important points,
Elliot. Certainly, we are just midway through the journey in seeing
what role CTA will play in the practice of clinical radiology. The
newer scanners, and the ones that we will be seeing a few years
from now, are really going to challenge us in terms of how to
optimally acquire the data sets. I think that what we have seen
thus far has been the development of applications that have enabled
us to expand the use of CTA to larger parts of the body and
different vascular territories. But as we have broken virtually
every barrier to CTA, leaving maybe just the heart as an area that
still presents challenges to us, we now have to bear down upon the
advancements in technology and realize the advancements in the
image quality and the diagnostic utility of these tests. We have to
see how we go from here to optimize the use of contrast, 3D
rendering, etc., to really take advantage of what tomorrow brings
with these CT scanners.
I'll ask each of the panel members this question. Many people have
not really put CTA into practice, although they have
state-of-the-art scanners. Often it is the process of getting
started that is the problem. Once you get started, it is kind of
easy. What advice might you give somebody first getting started in
In my opinion, the most important aspect in getting started is to
work with your clinicians and get them engaged in what you can do.
In the end, they need to realize the benefits of what you are
providing, so clinical partnerships are really critical. I would
advise most radiologists to pick a specific area to "cut their
teeth," if you will, on CTA, perhaps something in the abdomen or in
the chest. Then, I suggest they work with a focused group of
clinicians until they are confident that they can get good-quality
studies. Then the clinicians become your greatest advocates through
the hospital. You want to be able to provide 3D visualization
efficiently and effectively so that the clinicians can show their
colleagues what can be done with CTA. Then, with that initial
kernel of clinical activity and progressive referrals, it will
expand into other services.
W. DENNIS FOLEY, MD:
I think you also need to make sure that the technologists in the CT
area are happy with the scanners. But you also must make sure that
there are 1 or 2 technologists who have more expertise,
particularly in relation to the types of protocols you would want
to use for CTA. You would probably want to select 1 of those
technologists to be a champion of the workstation, because I do not
believe that the radiologists can do it all at the workstation.
There are certain quantitative analyses and rather sophisticated 3D
that will still require the "supertech" to do it.
Radiologists will progressively gravitate to using 3D more once
they have 3D integrated with their PACS. But in most practices
right now, 3D workstations are separate from the PACS. I think
everybody recognizes that it is really not the way it should be.
But we will still need a supertech, the person you selected
originally, to be the champion of doing the 3D, to really wow the
I think our experience is very similar. We have approximately 20
technologists in the practice; in the beginning we had 1 or 2 techs
do CTA to make sure that it was really done perfectly; you have no
second chance to do it well. Then we had the other techs involved.
One of our senior techs is one of our best advocates. He will put
the images on the satellite console and, literally, will show them
to every person that walks into the CT suite. So now everyone knows
that we do CTA, and I think that was a very big way of really
getting the information across. So I agree that the technologists
are super critical; probably more now than ever.
U. JOSEPH SCHOEPF, MD:
I agree with everything that has been said. If radiologists are
about to start their CTA business, they must develop strategic
partnerships with their clinicians. What always strikes me is how
fast technology evolves. It is difficult enough for radiologists to
keep a handle on what is happening with imaging technology, and it
is even more difficult for the clinicians out there. So, I believe
it is incumbent upon us as radiologists to go out there, penetrate
medical meetings, penetrate surgical meetings, and educate people
about what CTA can do. We need to make people aware of what we, as
radiologists, have to offer them.
I probably have an easier time talking about thoracic CTA
applications, particularly pulmonary CTA, because that is a reality
in most practices. But as we venture into new territory, such as
peripheral CTA that Geoff will talk about, that is really where a
lot of education is needed, both in the radiology and general
True. I think you also made a very important point in that one of
the problems radiology has had in terms of PR or getting
information out to our referral base is that radiologists speak to
radiologists and surgeons speak to surgeons. Many surgeons are
doing radiology in 2004 like it was done in 1993, back when they
finished their residency. I think radiologists need to speak at
surgical meetings. Many of us have spoken at those meetings and
they are actually some of the most worthwhile presentations because
when you speak to radiologists, the audience may think "been there,
done that." When you speak to surgeons, it's amazing to discover
the limited range of radiology that is actually practiced in the
I think that one of the ways that radiology will keep its
franchise is by showing people what we can do. I think people
really do not understand how difficult the studies are to do, nor
how much effort we put in to do it.
It is not like you can just go buy a scanner, put it in down the
street in an office, and you are doing radiology. I think that's a
great mistake. In March 2004,
The New York Times
reported that approximately 20 MRI scanners have been installed in
Syracuse, NY in the last 18 months. Not one of them was purchased
by a hospital or a radiology group. But by clinicians who thought
they could do their own MR, and rationalized it to the paper by
saying "Why should the patient cross the street to get an MR?"
One of the things we can do is really get information out about
what radiologists can do. I know GE Healthcare does a lot of
educational material focused in radiology, and, individually, many
of us do it well at our institutions. But it often doesn't spread
BRIAN R. HERTS, MD:
I agree with everything that has been said and I would like to
reiterate two points. I totally agree with Geoff that starting with
one area is the best way to begin to do CTA. That is how we started
ourselves. We picked one focused area, learned it very well, worked
closely with the clinicians, and took it from there.
The second thing I would like to add is the importance of the 3D
rendering in this process. If you compare lectures today with what
you presented 15 years ago, you realize what an effective form of
communication 3D rendering is. Every image you see in lectures
nowadays is a 3D image. Why? Because it takes just one image to
communicate the information, as opposed to a series of axial
images. But it is not just the 3D workstation; it is the contrast,
the protocols, and the 3D software that all go into making that
LEO P. LAWLER, MD, FRCR:
I share the consensus. There are a number of steps that I would
reiterate and maybe expand slightly. In addition to having a good
technologist, you really need a team approach. You need both the
radiologist and the technologist, two people who learn the
sophistication of the scanner and develop a kind of vernacular with
each other. I think that is the key element. But they need to start
off with a resource. For example, the CTA supplement in
last year is very, very useful. It is very important to have that
kind of resource that puts everything together. There are a number
of publications that do that kind of thing. I agree that it is best
to start off with a single region. Static organ imaging, in
particular, is a very good way to start CTA.
The final thing I would like to add is that the final point of
communication requires an anticipated system of communicating and
networking, particularly digital data. The job roles are switching
from angiographers to diagnostic radiologists who now have to learn
the interpretative techniques in order to get those images back to
the clinicians. Radiologists also have to have realistic
expectations; I do not think you can build a practice on peripheral
angiography and coronary angiography, etc., and expect to be doing
the same work day. If you add those services, you are going to have
to invest your time to learn how to do them and spend also your
time to perform them.
I think GE Healthcare should be commended for their efforts in
supporting technologist education. As you say, unless the
technologists are really up to speed and part of the team, none of
Although we will not be speaking about workstations and PACS
stations in detail today, we all recognize that the technology is
constantly changing. One of the critical issues in CTA is the
quality of the data sets, and that is really where contrast comes
in. Obviously, unless you have contrast in the right place at the
right time, the data sets are just not going to happen. So, in the
presentations today we will address that more specifically for each
application: how do we optimize contrast delivery, data
acquisition, and contrast selection.