In the evaluation of stroke, arterial dissection, and other vascular abnormalities, CTA compares favorably with conventional angiography while reducing patient risk.
This publication was supported by an educational grant from
Amersham Health, Princeton, NJ. The opinions expressed in this
publication are those of the authors and not necessarily those of
Dr. Bae reports relationships with Tyco Healthcare and
Mallinckrodt through patent agreements and as a consultant. Dr.
Fishman reports relationships with Siemens Medical Solutions and
Amersham Health as a consultant. Dr. Foley reports a relationship
with GE Medical Systems through an investigator agreement. Dr.
Naidich reports a relationship with Siemens Medical Solutions
through its Advisory Board and as a consultant. Dr. Saini reports a
relationship with GE Medical Systems through research support. Dr.
Becker, Dr. Sahani, Dr. Siegel, Dr. Tahktani, and Dr. Zinreich
report that no such relationships exist.
is a Professor of Radiology in the Division of Neuroradiology,
is an Assistant Professor of Radiology in the Department of
Radiology, Johns Hopkins Medical Institutions, Baltimore,
The most important neuroimaging application of computed
tomographic angiography (CTA) is in the evaluation of patients with
stroke. Each year, an estimated 730,000 to 760,000 people suffer a
stroke. It is the third leading cause of death in the United
States. Approximately one-third of stroke patients are left
permanently disabled, making it the leading cause of disability as
It is imperative that stroke be diagnosed in the acute phase,
preferably within the first 3 hours, so that aggres-sive therapy
can be instituted. Imaging plays a critical role in patient
management, evaluating the size of the injury to the brain,
determining whether revas-cularization can minimize the size of the
stroke, detecting hemorrhage, and excluding other pathologies.
Evaluation of the neurovascular tree is an important component
of stroke imaging. Noninvasive methods, such as color Doppler
ultrasound, MR angiography (MRA), and CTA are routinely used to
evaluate the presence and extent of stenosis. Color Doppler
ultrasound is readily available and in some studies has shown a 92%
accuracy when compared with digital subtraction angiography (DSA).
However, it is limited in its ability to evaluate high carotid
bifurcations, and is unable to accurately scan the intracranial
vessels or evaluate the origin of the vessels from the aorta. It is
still being used today, but has not been widely embraced by
clinicians. In many centers, it serves as a screening tool to
determine whether a patient needs any further testing.
Soon after its introduction, DSA became the gold standard for
the evaluation of the carotid bifurcation and the intracranial
compartment. It nicely depicts the vascular lumen and documents the
rate of blood flow. This approach has shortcomings, however. For
example, DSA does not show background tissue. It is a
two-dimensional (2D) technology that necessitates the acquisition
of additional orthogonal views to fully examine an area of
narrowing. It is time-consuming and expensive to perform. It
requires a specialized team and the angiography suite is generally
remote from the emergency room. Intra-arterial injection of
contrast material results in stroke in 1.3% of patients. In older
patients, a group more likely to have a stroke, the complication
rate increases to 1.8%.
MR angiography emulates DSA visually and has the advantage of
being noninvasive. It nicely depicts the intraluminal area, but
because of its sensitivity to the rate of proton movement, it does
not adequately visualize the arterial wall. A great deal of
research is directed toward enabling MRA to depict wall thickness,
as well as the composition of plaque narrowing at the carotid
bifurcation, however. Another shortcoming of MRA for the evaluation
of stroke is that the patient must be very cooperative and able to
remain motion-free for long periods of time, if the examination is
to be meaningful.
CTA offers several advantages in the evaluation of patients with
stroke. Most stroke patients undergo a conventional CT examination
to evaluate the intracranial compartment for hemorrhage or a mass.
It is easy to perform CTA at the same time. CTA emulates the DSA
display through the use of multiplanar volumetric reconstructions.
It is able to show the vascular lumen, as well as the vascular
wall. It is noninvasive, readily available in the emergency room,
and does not require extensive training to perform. In addition,
CTA takes just a few seconds to acquire the data (range 15 to 30
seconds, depending on the scanner speed and the detector width,
pitch, and collimation). Data can be processed in 10 to 20 minutes.
CTA does require intravenous administration of contrast material,
CTA of stroke
Table 1 outlines the acquisition parameters for CTA of the
extracranial carotid arteries and intracranial circulation. At
Johns Hopkins, we typically use 80 to 120 mL of contrast material
when using a scanner without bolus triggering capability, and 60 mL
of contrast when using a scanner equipped with bolus triggering,
injecting at 3 to 4 mL/sec and beginning image acquisition after a
delay of 12 to 18 seconds, depending on the area being examined.
For imaging the carotid bifurcation alone, we use a scan delay of
15 seconds. When imaging the circle of Willis, we use a scan delay
of 18 seconds. We prefer to use higher concentration contrast
material (350 mgI/mL).
In the past, we advocated scanning from the seventh cervical
vertebra to the skull base when imaging the carotid bifurcation.
With advanced 4- or 16-detector scanners, we can scan from the
aortic arch through the skull base with great ease. Faster scanners
are able to image at the peak of contrast enhancement in the
arterial system before venous contamination occurs.
One of the problems in imaging the carotid bifurcation is the
simultaneous display of arterial and venous channels. This can be
minimized by using bolus triggering, as well as by minimizing the
acquisition time through proper selection of collimation, pitch,
and rotation time. Simultaneous display of arterial and venous
channels can also be overcome in postprocessing through the use of
segmentation software, enabling display of the carotid bifurcation
(Figure 1). Through the use of windowing techniques, it is also
possible to zero in on arterial plaque and determine its
composition, as well as precisely judge the cross-sectional area of
the lumen and the extent of narrowing. In Figure 2, a volumetric
display demonstrates both soft and calcified portions of the
In CTA, the cross-sectional view best displays luminal stenosis.
The axial image shown in Figure 3 displays prominent narrowing of
the proximal internal carotid artery. The lumen is visualized, as
are both soft and calcified plaque. The contralateral carotid
bifurcation and internal jugular vein are both normal.
CTA versus DSA
There is a strong correspondence between the findings of DSA and
CTA. MR angiography, by comparison, often fails to show
poststenotic flow or overestimates the severity of stenosis. MR
angiography may show occlusion of the proximal internal carotid
artery, for example, whereas CTA will demonstrate a patent but
severely stenotic artery.
Early studies showed that the multiplanar nature of CTA enables
it to differen-tiate calcified plaque from contrast material,
providing information about plaque calcification, ulceration, and
size that cannot be obtained with conventional angiography.
More recent studies comparing CTA and DSA have shown that CTA is
sensitive and specific for the diagnosis of occluded large
Shrier et al
demonstrated a 99% agreement between CTA and DSA in diagnosing
internal carotid arterial occlusions. In a study by Knauth et al,
CTA was able to detect complete occlusions of the internal carotid
artery, middle cerebral artery, and basilar arteries with 100%
accuracy. Most important, the Prolyse in Acute Cerebral Embolism
(PROACT) study demonstrated that a negative CTA screening
examination could eliminate the need for conventional angiography.
Unlike 2D DSA, CTA offers an unlimited number of views from the
data set. This represents a significant advantage, as the arterial
branches can be rotated on the screen to provide the best possible
view. This could be challenged in the future by increasing use of
3D rotational DSA, however.
Reading CTA studies requires some familiarity with
postprocessing techniques, including their relative strengths and
weaknesses. Although volume rendering (VR) uses all the pixels in
the data set to provide excellent details, it does not show the
lumen. Visualization of the lumen may be important in partially
thrombosed aneurysms. Three-dimensional VR images must be
correlated with source images.
Similarly, when assessing stenosis of the carotid or vertebral
arteries, it is important to combine VR images with
maximum-intensity projection (MIP) images and oblique and curved
reformats. Volume rendering tends to incorporate calcified plaques
into the images, obscuring the severity of the narrowing. Some
anatomy, such as tortuous origin of the vertebral arteries and a
cavernous segment of the carotid arteries, necessitates the use of
CTA is an efficient and rapid method for the evaluation of
intracranial and carotid arteries in patients with symptoms of
stroke. A negative CTA may prevent unnecessary urgent conventional
angiography. CTA also identifies the site of vascular occlusion and
the degree of leptomeningeal collateralization, and accurately
evaluates the degree of cervical luminal stenosis.
Approximately 5% of people who undergo autopsy are found to have
an intracranial aneurysm. More than half of ruptured aneurysms
rehemorrhage, and multiple aneurysms are found in 20% to 30% of
Together these factors demonstrate the critical role of imaging in
reaching an accurate and timely diagnosis of intracranial
As in the case of stroke, DSA represents the gold standard for
the evaluation of intracranial aneurysm. This is being challenged,
however. The risks associated with DSA prevent its use in
screening, however, and it is instead reserved for the imaging of
patients in whom the probability of finding subarachnoid hemorrhage
(SAH) is high. In addition, the results of DSA are negative in 5%
to 15% of patients with SAH as a result of a thrombosed aneurysm,
vasospasm, poor vascular opacification, or inadequate angiographic
The strengths and shortcomings of MRA for the evaluation of
intracranial aneurysm are the same as for the evaluation of stroke:
This technique is noninvasive, but its use is appropriate only in
reasonably healthy patients capable of remaining motionless for
extended periods of time.
By comparison, CTA offers several advantages. Subarachnoid
hemorrhage is usually diagnosed by CT, so the addition of CTA
extends the imaging exam by only 20 to 30 seconds.
Three-dimensional reconstructions show the neck of the aneurysm and
determine the largest neck diameter, dome size, and maximum
aneurysm diameter, all of which aid in treatment planning (Figure
4). CTA also shows adjacent bony morphology.
Korogi et al
compared 3D volume-rendered CTA with DSA in 49 patients with or
without intracranial aneurysm. Conventional angiography detected 47
aneurysms in 35 patients. The sensitivity of CTA was 64% for
aneurysms with a diameter of <3 mm; 83% for aneurysms with a
diameter of 3 to 4 mm; 95% for aneurysms with a diameter of 5 to 12
mm; and 100% for aneurysms with a diameter >13 mm.
A recent study by Villablanca et al
demonstrated CTA to have a sensitivity of 98% to 100% for very
small aneurysms (those <4 mm in diameter), as compared with DSA,
which had a sensitivity of 95%. The accuracy of the CTA was 99% and
100% for aneurysms 3 and 4 mm in diameter, respectively. The
investigators concluded that CTA can provide high-quality,
noninvasive detection and characterization of an-eurysms, and can
be performed using routine clinical CT scanners and commercially
available image processing workstations.
Recent studies are more relevant, as they have been conducted using
thinner collimation and better postprocessing methods, and benefit
from an increasing familiarity in evaluating 3D data. With fast
scanners, it should become increasingly common to get excellent
opacification of the arterial system and achieve very reliable
For detection of aneurysm, CTA must depict the circle of Willis
region in exquisite detail. This can be achieved by timing the
contrast bolus properly, using the thinnest available collimation,
and selecting a pitch factor of <1, with a 20% to 30%
Figures 5 and 6 outline the ap-proach advocated by the Barrow
Neurological Institute in Phoenix, AZ, for imaging patients who
have nontraumatic SAH or are at risk for cerebral aneurysm.
According to the protocol, if SAH is demonstrated on head CT, these
findings are compared with those of CTA. If the CTA data are
congruous with the findings of standard CT, there is sufficient
information to proceed with therapy. If the findings of standard CT
and CTA are incongruous, particularly if the CTA
is negative in the face of SAH hemorrhage on standard CT, the
patient undergoes DSA.
Patients who are at risk for cerebral aneurysm first undergo
MRA. If MRA is positive, the patient either goes directly to DSA,
or instead undergoes CTA. If the CTA and MRA findings are
congruous, there is sufficient information to proceed with therapy.
If the findings are incongruous, the patient undergoes DSA so a
decision can be made about therapy. In this way, a combination of
CTA and MRA can obviate conventional angiography, thereby
eliminating the risk associated with an invasive intra-arterial
CTA is also useful in the evaluation of arterial dissection, a
condition that typically is seen in younger patients and is
associated with trauma, vasculopathy, high blood pressure, or
idiopathic causes. Imaging involves evaluation of the extracranial
carotid and vertebral arteries for blood penetration into the
arterial wall. CTA can demonstrate not only a change in the caliber
of the dissected vessel, but also in the intimal membrane.
Digital subtraction angiography is the gold standard for the
evaluation of arterial dissection. In the acute stage, signal
isointensity can cause MRA to miss arterial dissection. CTA, by
comparison, easily shows the enlarged external arterial diameter
(Figure 7). CTA can be beneficial in the evaluation of vascular
malformations, angiomas, and other vascular abnormalities.
Finally, a CTA-like approach plays a critical role in the
evaluation of head and neck lesions. Image data are not
reconstructed in three dimensions to produce an angiographic
display, but contrast is administered and the carotid sheath is
examined from the skull base to the anterior mediastinum, just as
in CTA. The examination, which separates vascular channels from
pathologic nodes, is the only accurate way to detect the presence
of pathologic nodes and to evaluate the relationship between
pathologic nodes and/or malignancy surrounding the vascular
The need for rapid and accurate diagnosis of stroke mandates the
use of rapidly applicable, noninvasive radiographic techniques. CTA
with or without MRA appear to be the modalities that will answer
this need. With the currently available improved technology, there
is no reason why these two modalities should not be the principal
diagnostic instruments in evaluating the presence of intracranial
vascular pathologies. Further developments during the next decade
will certainly make this come true.
Carotid bifurcation. Segmentation software overcomes the
challenge presented by simultaneous display of arterial and
Volumetric display demonstrates both (A) soft and (B) calcified
portions of carotid plaque.
In CTA, the cross-sectional view best displays luminal stenosis.
(A and B) These axial images display prominent narrowing of the
right proximal internal carotid artery, as well as soft and
calcified plaque. The contralateral carotid bifurcation and
internal jugular vein are normal.
Aneurysm at the tip of the basilar artery. CTA demonstrates neck
diameter, dome size, and maximum aneurysm diameter, all of which
aid in treatment planning.
Imaging protocol for nontraumatic subarachnoid hemorrhage (SAH).
If SAH is demonstrated on head CT, these findings are compared
with those of CTA. If the CTA data are congruous with the
findings of standard CT, there is sufficient information to
proceed with therapy. If the findings of standard CT and CTA are
incongruous, particularly if the CTA is negative in the face of
SAH on standard CT, the patient undergoes digital subtraction
angiography (DSA). (Protocol developed at Barrow Neurological
Institute, Phoenix, AZ.)
Imaging protocol for patients at risk of cerebral aneurysms.
Patients at risk for cerebral aneurysm first undergo magnetic
resonance angiography (MRA). If MRA is positive for cerebral
aneurysm, the patient either goes directly to digital subtraction
angiography (DSA), or instead undergoes CTA. If the CTA and MRA
findings are congruous, there is sufficient information to
proceed with therapy. If the findings are incongruous, the
patient undergoes DSA so that a decision can be made about
therapy. (Protocol developed at Barrow Neurological Institute,
(A) CTA of a dissection of the vertebral artery. The image shows
the proximal portion of the raising of the intima. (B) Dissection
of the vertebral artery is confirmed by DSA.
ELLIOT K. FISHMAN, MD:
Thanks, Jim. I'll start with the first question. Since you're
talking about strokes, typically we are dealing with older
patients. Often they're diabetics with borderline renal functions.
What cut-offs do you use for using contrast, in terms of dealing
with creatinine in these patients?
S. JAMES ZINREICH, MD:
We really look at the creatinine, and the most important thing when
dealing with these debilitated patients is that we want to make
sure that the creatinine level is not rising. If the creatinine
level is decreasing, but still a little bit high, we'll still give
the intravenous (IV) contrast.
What's a little bit high?
Well, 3 or 4 mg/dL is our cut-off. But the question is, is it
coming down from 4 to 3 to 2 mg/dL. Now, in a patient with a serum
creatinine level of 5 or 6 mg/dL, we will not give IV contrast at
all. So there are several relevant questions. How important is it
to give IV contrast? How important is the contrast to diagnose the
entity? What is the status? Is the patient's medical condition
worsening or improving?
Right. But in terms of contrast selection, do you have any specific
suggestions? There was an article in the
New England Journal of Medicine
a few months ago about the use of iso-osmolar contrast, for
example, in patients with higher serum creatinine levels. They
found little change in post-study renal function.
I'd like to open this question to the panel. Are there some
creatinine levels that you worry about? Which specific numbers do
you look at?
W. DENNIS FOLEY, MD:
In our practice, we start to decrease the total load of contrast
above a creatinine level of 1.5 mg/dL. But many of the CT
angiographic studies now are done with reduced amounts of contrast
to begin with. So if you can do almost a total body scan, at least
a thorax/abdomen/pelvis scan, with 60 to 70 mL of contrast, are you
putting a patient at risk? We don't know the answer to that. But,
in a patient with a creatinine level above 1.5 mg/dL, it would be
prudent to use a less nephrotoxic agent, even at reduced dose.
That's something we definitely do. Even if the creatinine is high,
but coming down, for intracranial study you really don't need 120
mL of contrast, especially for a CTA. You could easily do it with
about 60 mL.
SANJAY SAINI, MD:
For us, the issue of contrast is in a bit of flux. I've learned
recently that the nephrologists of the world don't use the term
creatinine anymore. They use the term creatinine clearance. With
creatinine, there is an input sort of like an equation, that
involves patient weight, and gender, amongst other factors. So
they're telling us to stop thinking in terms of creatinine, and to
think instead in terms of creatinine clearance.
The second issue is acetylcysteine, which we started thinking
about when that study came up. Again, the nephrologists think about
this a little more critically than we do. It turns out that
acetylcysteine works only because, in the patient population
studied, the increase in creatinine for high-risk patients was
offset by a decrease in creatinine in the low-risk patients.
Another issue is that not all of these studies are done in the
CT population. Most of them are done in the angiographic
populations, all of whom have hydration as a critical component.
So, looking as a nephrologist, even at the acetylcysteine study,
the patients had 12 hours of hydration pre- and postprocedure. I
don't know if hydration was involved in it, but when we tried to do
this kind of study, the nephrologists say you have to use hydration
as a component of any study. So, it turns out that a lot of these
things are impractical in the radiology world.
So I don't know whether an isosmolar contrast agent in the CT
world, where hydration really is the component of the patient
management, will have the impact that we hope it will. But having
said that, when somebody does have a high creatinine, we do use
In terms of creatinine clearance or creatinine in outpatients, do
you require a creatinine level before you do a study?
Yes, we have become very protocol-oriented. Our protocol ad-dresses
the two main risk factors for renal injury from contrast material:
pre-existing renal insufficiency and diabetes. The combination is
particularly bad. We have a screening form, and if any of those
factors are positive, we get a creatinine level in those
The nonproven risk factors for renal injury include advanced
age, dehydration, etc. Until recently, our age cutoff was 55 years.
It drove our techs crazy because almost everybody who has a CT scan
done is over age 55. The only relevant study we could find used the
age of 70 as a cut-off. So we changed that, as well, recently.
Well, I want to comment on these issues with the outpatients. It's
outpatients where you typically don't have the numbers. I know at
Hopkins if you said, I want a creatinine level, about 6 hours later
you would be doing the CT scan, on a good day. I don't know how it
is in Boston, but it's not a 30-second study.
For example, consider the case of a patient with normal renal
function in whom you were to give 45 g of iodine, and you need to
get a good-quality liver study. If they had a creatinine of 1.8
mg/dL and you decrease the contrast dose, are you getting less of a
quality CT? Can you just live with that?
Yes, because the liver parenchymal enhancement will not be what you
would like to maximize the portal venous phase. On the other hand,
if your major issue is detecting a hypervascular tumor, even with
the reduced amount of contrast, I think your sensitivity should
still be good, because you're requiring data in that late arterial
So this comes up very often, but it turns out that most of them had
normal renal function. So it is not obvious to me whether you
should cut down the contrast dosages.
There's another issue here. That is, when you talk about
creatinine, the proper answer ought to be, especially if I am in a
teaching institution, I certainly have CT and MR available.
Therefore, if I am dealing with a patient with renal failure, and I
want to take a look at the carotid arteries or the circle of
Willis, I should do an MRA in that case and to see what that shows
In general, when we do head CTs, and the question is whether I'm
dealing with renal failure or not, if I need to administer
contrast, the first thing I ask myself is: Do I do CT with
contrast, or should I first do an MRA?