Computed tomography angiography (CTA) is a valuable tool for the evaluation of carotid artery disease. It has been used to detect early atherosclerotic changes within the carotid arteries, determine stenosis severity, identify the distribution and extent of atherosclerotic plaque, perform plaque characterization, and detect and characterize carotid dissections. Recent investigations have also explored its role after carotid stenting.
is a Resident Physician and
is an Associate Professor, Department of Radiological Sciences,
David Geffen School of Medicine at UCLA, Los Angeles, CA.
Computed tomography angiography (CTA) is a valuable tool for the
evaluation of carotid artery disease. It has been used to detect
early atherosclerotic changes within the carotid arteries,
determine stenosis severity, identify the distribution and extent
of atherosclerotic plaque, perform plaque characterization, and
detect and characterize carotid dissections. Recent investigations
have also explored its role after carotid stenting.
Successful multidetector CTA (MDCTA) of the carotid arteries is
dependent on high image quality. This is achieved through the use
of optimized image acquisition and postprocessing protocols.
Specific imaging parameters vary by scanner type and number of
detector arrays. Generic factors include: 120 kVp, 280 to 400 mA,
18-cm field of view (FOV), 512 × 512 matrix, 0.5- to 1-mm slice
collimation, low pitch factor, and a 50% reconstruction overlap.
This permits isotropic voxel sizes in the range of 0.5 mm.
Isotropic scanning with small voxel size is desirable for optimized
3-dimensional (3D) viewing. Scan coverage is generally from the
level of the aortic arch to the skull base. Intravenous (IV)
contrast is delivered via a 22-gauge or larger angiocatheter
positioned within the right antecubital vein, injecting a nonionic
contrast agent with a concentration of 350 mmol/L or higher at a
rate of 3 to 4 mL/sec. A right-sided injection will minimize image
degradation from beam-hardening artifacts arising from high-density
contrast in the central venous structures.
Optimized postprocessing and display techniques are as important
to high image quality as the scanning parameters. Before selecting
a postprocessing workstation, you should ensure that it meets your
needs for image quality, processing speed, connectivity, and remote
viewing capabilities. Perspective volume rendering (VR) or
thick-slab maximum-intensity projection (MIP) are popular
techniques for general 3D viewing. However, for precise
measurements and stenosis quantification, only the 2-dimensional
(2D) gray-scale axial or curved oblique multiplanar reformat (MPR)
images should be used. This is because MDCTA axial or axial oblique
source images have been shown to correlate more closely with
digital subtraction angiography (DSA) than do either MIP or
surface-shaded display (SSD) images for all degrees of stenosis.
On the 2D gray-scale images, measurements are made using a visual
estimation of the full width at half maximum of the slice
sensitivity profile at the vessel edge. Measurement errors can be
minimized by using the appropriate window and level settings,
approximately 1000 W/500 L for 2D gray-scale images. When 3D
rendering techniques are used for image review, VR has been shown
to be superior to SSD and MIP images.Approximate recommended
window/level settings for 3D image viewing are 250 W/150 L, with
fine adjustment as needed, to just reach mild background speckling.
Carotid artery intima-medial thickness
Clinically significant carotid artery stenosis is a late finding
in the progression of atherosclerotic disease. It is now possible
to intervene early with aggressive medical therapy to delay or
potentially reverse the progression of athero- sclerotic plaque.
Carotid artery intima-medial thickness (IMT) has been reported to
be a marker of subclinical atherosclerosis
and a strong predictor of subsequent cardiovascular complications.
Intima-medial thickness consists of both an intimal atherosclerotic
process and medial hypertrophy, and it has been well characterized
by B-mode ultrasound (US).
Intima-medial thickness may also provide a comprehensive view of
the consequences of multiple risk factors over time on the arterial
wall, including 1) traditional risk factors, 2) emerging risk
factors (eg, lipoproteins, plasma viscosity and
hyperhomocysteinemia), and 3) various measures of end-organ damage.
The use of MDCTA source images in the visualization of carotid
wall thickening has not been well explored. Preliminary results
with the use of MDCTA show it may be of value in this application
by detecting circumferential thickening in the wall of the distal
common carotid artery in the absence of overt hypodense
The information is easily extracted from CTA data sets and may be
routinely reported. Methodologic standardization and validation of
measurements still need to be implemented for both US and CTA
before routine measurement of IMT can be used as a diagnostic tool
in cardiovascular risk stratification, for primary prevention, and
in the decision-making process prior to interventional therapy. A
value of IMT by US of approximately 0.68 mm has been reported to
correlate with an increased risk of atherosclerotic disease.
However, IMT varies with age and gender and will likely require
interpretation using age and sex-adjusted mean values.
Carotid artery stenosis
Stroke is the third leading cause of death in developed
countries and is one of the leading causes of adult disability.
Approximately 44% of all ischemic strokes are attributed to carotid
The North American Symptomatic Carotid Endarterectomy Trial
and European Carotid Artery Surgery Trial (ECAS)
studies established the benefits of carotid end-arterectomy (CEA)
in symptomatic patients with stenosis >60%.
In the NASCET study, the cumulative risk of any ipsilateral stroke
was found to be 26% at 2 years in the aspirin-treated patients, and
9% in the surgically treated patients, with a 30-day perioperative
risk of any stroke or death of 5.8%. In pa-tients with <60%
carotid stenosis, the risk of surgery does not appear to be
The benefits of CEA for asymptomatic patients with >70% stenosis
have been reported by the international Asymptomatic Carotid
Surgery Trial (ACST), which found a 6% absolute risk reduction and
a 50% relative risk reduction of stroke over 5 years in the treated
This underscores the importance of accurate and reproducible
measurements of carotid stenosis when stenosis criteria are used
for patient selection.
The gold standard for the detection and grading of carotid
stenosis has been catheter angiography.
However, Willinsky et al
reported angiography of the carotid arteries carried a 1.3% risk of
neurologic complication, 0.5% of which can be permanent. In the
Asymptomatic Carotid Atherosclerosis Study, nearly half of the
morbidity/mortality rate (2.6%) was accounted for by the rate of
stroke after DSA (1.2%).
A desire for enhanced patient safety and lower study costs has
prompted the development of noninvasive tools using duplex US,
magnetic resonance imaging (MRI), and CT. All of these noninvasive
techniques appear to be suitable for measuring stenosis of the
proximal internal carotid arteries when compared with DSA as
determined by a meta-analysis of studies between 1990 to 2001.
All of these techniques also show similar accuracy in the diagnosis
of symptomatic carotid stenosis. However, each of these imaging
techniques has its own advantages and limitations.
Carotid US is a noninvasive, low-cost, and widely available
modality, but it suffers from significant drawbacks. It is
operator-dependent, and findings can be variable and difficult to
Lee et al
showed that the variability of Doppler measurements in the common
carotid artery (CCA) and internal carotid artery (ICA) in 85
patients without carotid disease is substantial and could lead to
inaccuracies in carotid arterial stenosis measurements. They report
that peak systolic velocity ratios exceeded 1.8 (suggesting ≥60%
stenosis) in 14% of normal carotid arteries and that end diastolic
velocity ratios exceeded a ratio of 2.4 (also suggesting >60%
stenosis) in 15% of normal carotids. Despite its common usage, no
universal table of measurement standards that reliably estimates
the degree of stenosis has been established. Variability in
velocity measurements along the course of the ICA also influences
the interpretation of ratio parameters.
Peak systolic velocity has been shown to be influenced by relative
position within the ICA, equipment type, technique used, and by
physiologic factors, such as cardiac output and contralateral
restriction of blood flow.
Another limitation of US is its limited FOV that renders the
evaluation of tandem stenoses and vessel course difficult. The
latter is particularly important, as kinked carotid arteries are
better suited for CEA than for carotid stenting. Lastly, heavy
calcific plaque, a common finding in atherosclerotic disease of the
carotid bifurcation, works as an acoustic curtain through which
insonation is impaired. If the point of maximal luminal narrowing
occurs within the calcified segment, forcing a sampling distal to
the site of maximal peak systolic velocity will lead to an
under-estimate of the percent stenosis.
MR angiography (MRA) of the carotid arteries has undergone
significant technologic improvements during the past several years.
Many of the initial studies that investigated the utility of MRI
for carotid stenosis were based on time-of-flight (TOF) techniques,
which have significant limitations.
Since TOF-MRA is a flow-dependent technique, slow and/or turbulent
flow patterns, as are commonly encountered at sites of significant
atherosclerotic luminal narrowing, can lead to spin dephasing and
resultant signal loss. As a result, TOF-MRA frequently
overestimates carotid stenosis severity.
More recently, significant improvements in MRA have been achieved
through the development of contrast-enhanced MRA techniques, which
are less susceptible to the artifacts associated with TOF-MRA
techniques. However, in comparison to MDCTA, both clinical TOF-MRA
and contrast-enhanced MRA at 1.5T currently have a lower spatial
This may be important, since accurate submillimeter measurements of
carotid stenosis diameters may make the difference between a
clinically significant and a nonsignificant lesion based on NASCET
criteria. Recent work suggests that contrast-enhanced MRA at 3T
provides superior image quality to both TOF-MRA and
contrast-enhanced MRA at 1.5T, approximating the spatial resolution
Multidetector CTA of the carotid arteries has been shown to be a
suitable noninvasive technique for assessing carotid artery
A meta-analysis of studies from 1990 to 2003 showed that CTA had a
pooled sensitivity and specificity of 85% and 93%, respectively,
for the detection of 70% to 99% stenoses.
Others report that patients tend to prefer CTA over MRA or DSA,
with US preferred by patients overall. In specific instances, MDCTA
can also outperform DSA in the accurate depiction of carotid
stenosis. With DSA, the standard of care is 2 or 3-view carotid
angiograms, generally in the anteroposterior, lateral, and,
occasionally, right or left posterior oblique projections. When a
stenosis is markedly eccentric or elliptical and when the long axis
of that ellipse is not along the beam path of these standard
projections, the stenosis severity will be underestimated by DSA.
This measurement error can be on the order of ≥25%, and could
erroneously exclude an otherwise qualified candidate from CEA. The
problem is not easily solved, as it is not possible for the
angiographer to intuit a priori which precise projection will
render the lumen in its narrowest profile. Hirai et al
recently showed that when the carotid artery lumen has a high
long-to-short axis ratio, the luminal morphology may lead to
erroneously decreased estimates of luminal stenosis by DSA (Figure
A possible limitation of CTA is the difficulty of measuring
stenoses in a heavily calcified vessel.
However, numerous investigators have noted that with appropriate
window and level adjustments, the blooming artifact from the heavy
calcific plaque can be reduced, allowing for a more accurate
The major disadvantages of CTA compared with MRA include its use of
ionizing radiation and potential for nephrotoxicity.
How to measure carotid artery stenoses
The NASCET criteria require measurement of the stenotic lesion
at the site of maximal luminal narrowing and in the first normal
distal arterial segment where the walls are parallel.
With the ECAS method, the stenosis is calculated by measuring the
ratio between the narrowed carotid bulb luminal diameter and the
native carotid bulb from wall to wall. Both methods can be reliably
performed and reported using MDCTA. The axial or axial oblique CTA
source images provide the most reliable stenosis measurements when
comparing CTA with DSA.
In cases in which the vessel is tortuous, it is important to use
the curved MPR to ensure true cross-sectional measurement
perpendicular to the long axis of the vessel, and thus avoid
artifactual luminal eccentricity (Figure 2). Appropriate window and
leveling is also crucial when measuring carotid stenosis,
especially when significant atherosclerotic calcification is
present. For the interpretation of axial or axial oblique source
data, the authors recommend using approximate window and level
settings of 1000 W/ 500 L. In an effort to simplify the measurement
of carotid stenoses by CTA, Bartlett et al
have recently proposed the direct measurement of carotid bulb
diameter and its correlation to the percent stenoses as determined
by the NASCET criteria. According to their study, a carotid bulb
diameter >2.2 mm corresponds to a <50% carotid stenosis, and
a carotid bulb diameter <1.3 mm corresponds to a <70%
The measurement of near occlusion deserves special attention
because of its clinical relevance and its determination using CTA.
In patients with near-total occlusion, the benefit of CEA over
medical therapy has been shown to be clinically significant but far
less profound than that seen for patients with severe carotid
artery stenoses (stenoses >70%).
In determining chronic near-total occlusion, the traditional
measurement of carotid stenoses may lead to an underestimation of
the stenoses because of the associated loss of diameter of the
ipsilateral ICA distal to the stenosis (CTA string sign).
Near-total occlusion at CTA may be best determined by noting a
signifi-cant carotid stenosis with an associated reduction in
ipsilateral ICA to contralateral ICA diameter (>80%) and
reduction in ipsilateral ICA diameter to the ipsilateral external
carotid artery (ECA) diameter (ICA/ECA ratio <1.25).
CT angiography is believed to be comparable to DSA in the
determination of near-total occlusion and to be superior to both US
and MRA in this setting, as those modalities commonly suggest
occlusion. The distinction between carotid artery occlusion and
near occlusion can also be made reliably with CTA, which has a
reported sensitivity and specificity of 97% and 99%, respectively,
for carotid artery occlusion
(Figure 3). This is clinically important because the therapeutic
alternatives available to patients with near occlusion include
revascularization, whereas patients with established occlusions
will not benefit from CEA but may benefit from extracranial to
intracranial bypass, medical management, and/or contralateral CEA.
Carotid plaque characterization
Although the risk of stroke has been most closely associated
with the severity of carotid stenosis,
atheroembolic strokes are known to occur in patients with less than
severe (<70%) stenoses, and conversely, not to occur in patients
with known severe stenoses.
These findings suggest that carotid plaque morphology may play an
important role in the risk of atheroembolic stroke. Certain plaque
morphologies have been associated with higher stroke risk.
Histologic characteristics associated with increased stroke risk
include a large lipid/necrotic core, a thin or ruptured fibrous
cap, plaque ulceration, intraplaque hemorrhage, and a dense
inflammatory cellular infiltrate.
Preliminary studies suggest that CTA is able to provide accurate
plaque characterization when compared with histopatho-logic
Through the use of Hounsfield unit (HU) analysis, CT allows for the
potential characterization of plaque composition, specifically
commenting on the lipid/necrotic core, fi-brous component, and
calcification. Areas of the plaque that have HU similar to muscle
have been proposed to likely correspond to fibrous tissue, whereas
areas with low attenuation values are thought to represent the
A more recent study, however, questions the ability of CTA to
characterize plaque morphology.
This suggests that additional studies are needed to determine the
true capabilities of MDCTA in plaque characterization, particularly
with respect to the identification of vulnerable or unstable plaque
Ulcerations, a known cause of thromboembolic phenomena from
carotid atherosclerosis independent of stenosis severity, can be
missed on DSA when the ulcer crater projects along the path of the
Multidetector CTA is capable of excellent depiction of plaque
ulcerations and arguably outperforms DSA in crater detection.
Because MDCTA is a direct contrast cast of the vessel lumen, it is
generally superior to both TOF-MRA and US in the detection of ulcer
craters (Figure 4).
Evaluation of carotid stents
Recently, endovascular treatment of clinically or
hemodynamically significant carotid artery stenoses has gained in
popularity. Currently, carotid stenting is approved only for
high-risk surgical patients. However, multiple trials are under way
to establish the equivalence or superiority of carotid stenting
In comparison to CEA, carotid stenting is thought to be associated
with higher rates of restenoses, up to 18%.
Most patients are currently evaluated with duplex US yearly and are
referred for DSA when findings are suggestive of high-grade
stenosis. However, duplex US of poststent carotid arteries has
shown a high false-positive rate of >30%. A preliminary study by
Orbach et al
has shown that MDCTA may be useful in the evaluation of the
poststent carotid artery. Multidetector CTA reveals the location,
size, posture, and patency of arterial stents. Most importantly,
their results suggest CTA has reasonable correlation with DSA when
appropriate window and leveling was used, and that MDCTA is
promising for the detection of intrastent stenoses from recurrent
atherosclerosis and intimal hyperplasia (Figure 5). In their
methodology, they recommend using a window width of 1500 HU and
level of 400 HU, notably wider than the typical window widths used
for MDCTA, in order to decrease the blooming artifact associated
with the stent. Orbach et al,
working with a phantom model, advise the use of a kernel optimized
for high- attenuation sharp edges, as an acquisition tool for
improved detection of intra-stent stenoses.
Carotid artery dissection
Dissection of the extracranial ICA is responsible for up to 10%
to 25% of ischemic stroke in young and middle-aged patients.
The ICA is the most commonly involved cervical vessel.
Cerebral ischemia may result from hemodynamic compromise or embolic
complications. The average annual incidence of carotid artery
dissection (CAD) ranges from 2.5 to 3 per 100,000.
The pathogenesis of CAD remains unknown in the majority of cases.
Trauma and primary diseases of the arterial wall are the principal
predisposing factors. Clinical presentation during the acute phase
may include headache, ipsilateral neck pain, and Horner's syndrome.
A smaller percentage may be silent.
Angiography has been the gold standard for evaluating the
presence and extent of arterial dissection.
Typical angiographic findings for nonocclusive dissection include
an elongated or tapered stenosis (rat-tail sign) with or without
the presence of a pseudoaneurysm.
Because DSA does not visualize the arterial wall, it can lack
specificity in occlusive dissection, which makes it it difficult
for DSA to differentiate between occlusive dissection and other
causes of arterial occlusion, including thromboembolism and
Prior work has shown that both Doppler US and MRI may be of
value in the diagnosis of cervical carotid artery dissection.
Both techniques have significant limitations, however. The primary
limitations of Doppler US are a limited scan coverage using
conventional probes and the nonspecific and overlapping nature of
the echo patterns obtained from dissected arteries.
The limitations of TOF-MRA include artifacts arising from the
loss of signal intensity derived from the loss of proton spin
coherence in the setting of reduced luminal diameter. Additional
limitations include a potentially difficult diagnosis in the
setting of fresh hematoma (not T1-weighted [T1W] hyperintense) and
nonspecific imaging when high signal covers all of the section of
the artery, leading to confusion between mural hematoma and
possible thrombus in the true lumen.
Also, the use of MRI for CAD is limited in the setting of acute
trauma since it is incompatible with life support equipment,
provides poor evaluation of associated osseous injuries, and
requires prolonged scan times. Nevertheless, MRI constitutes a
valuable tool for detecting cervical dissection. Typically, axial
T1W images show hyperintense crescentic wall thickening surrounding
a narrowed signal void-the lumen. The technique may also be used in
the setting of occlusive dissection by the use of the axial T1W
Recent reports suggest that first-pass gadolinium-enhanced MRA may
be superior to TOF-MRA.
Helical CTA has also been shown to be a reliable method for
evaluating extracranial ICA dissection, especially in the setting
Leclerc et al,
in a study of 18 patients, found a 100% sensitivity and 100%
specificity for both stenotic and occlusive dissections. They found
an analysis of the residual arterial lumen and the measurement of
the external diameter of the carotid artery to be the most
sensitive criteria for the diagnosis of CAD. Our experience shows
that criteria more specific than luminal reduction and external
diameter can also be used to establish the diagnosis of CAD. These
include the presence of an intimal flap, the presence of an
isodense eccentric or concentric intramural hematoma, formation of
dissecting aneurysms, increased native arterial diameter, and
luminal stenosis (Figure 6). It is our opinion that because MDCTA
provides non-luminal information, the specificity of MDCTA for
carotid dissection may in some instances be higher than that of DSA
for carotid dissection when typical angiographic findings are
lacking or ambiguous. In 1996, Egelhof et al,
in a study population of 21 patients with clinically or
sonographically suspected extra-cranial carotid dissection,
reported a 100% sensitivity and specificity of CTA as compared with
catheter angiography. At our institution, we have found that direct
MDCTA signs of arterial dissection do not require further
diagnostic investigation to confirm the diagnosis. Specific CTA
findings include an intimal flap, dissecting aneurysms, and mural
hematomas, as described below.
An intimal flap is a thin, hypodense band that spans the
arterial lumen; it is generally oblique or spiral in nature and is
of high sensitivity. Very thin web stenoses and horizontally
oriented intimal shelves may be difficult to detect by MDCTA.
Dissecting aneurysms appear as focal contrast-opacified
outpouchings arising from the parent artery. These lesions may
contain intraluminal thrombus and may exert mass effect on the
parent artery, which contributes to luminal reduction of the parent
vessel. The sensitivity of CTA for dissecting aneurysms is very
high (Figure 4).
Mural hematomas can be iso- or hypodense to surrounding tissue,
depending on whether they are acute or subacute in nature. The
adventitia of the parent artery is clearly visualized peripheral to
the hematoma. The hematoma may be focal or segmental, concentric or
eccentric. Using appropriate window and level settings, the
detection of a mural hematoma is straightforward when using
gray-scale 2D source or MPR images.
Typically, patients with CAD are treated with anticoagulation
for 3 to 6 months after the initial event to prevent thromboembolic
Approximately 70% of dissections heal spontaneously, and most
pseudoaneurysms either resolve or significantly decrease in size.
Only ≤15% of cases of CAD are noted to progress.
Multidetector CTA can be used to detect progression and document
Artifacts and pitfalls
Several artifacts and pitfalls have been associated with MDCTA
in the evaluation of carotid pathology. Swallowing artifacts appear
as focal blurring within the source data and may lead to an
inability to obtain accurate quantification. Beam-hardening
artifacts arise from dense objects (such as dental amalgam,
vascular clips, or very dense calcifications) and may mimic
arterial stenosis or dissection (Figure 5).
Sources of measurement errors include a failure to use optimal
window-level setting, stenosis measurements using MPR images
nonorthogonal to the long axis of the vessel of interest,
suboptimal contrast opacification, and the use of images containing
gross patient motion. Failure to cover the entire region of
interest may lead to incomplete visualization or characterization
of relevant pathology or to false-negative findings. All studies
should use automatic bolus detection technology to trigger helical
contrast acquisition. Alternately, a timing injection can be
performed in all patients.
Overlapping venous and arterial anatomy may obscure pathology on
VR images. This may be avoided by the use of bolus-detection
techniques and by the use of 2D image data for all detection and
quantification. Beam-hardening artifacts that arise from shoulder
bones, clavicles, and hyperconcentrated contrast material arriving
via the brachiocephalic and subclavian veins may degrade image
quality in the lower neck. Injection of contrast into the right
upper extremity may help to minimize these artifacts.
Multidetector CTA is emerging as a versatile and powerful tool
in the noninvasive evaluation of a range of pathologies affecting
the extra- and intracranial segments of the carotid arteries.
Optimized scan and image acquisition protocols, high-quality image
postprocessing, and a comprehensive understanding of the appearance
of common pathologies and imaging artifacts will help ensure
accurate lesion detection, quantification, and