Recent advances in computed tomographic angiography (CTA) and magnetic resonance angiography (MRA) enable comprehensive evaluation of the supra-aortic vascular tree, including cervical and intracranial vessels. Both CTA and contrast-enhanced MRA provide highly accurate assessment of severe carotid artery stenosis, and are an improvement over traditional time-of-flight MRA in the depiction of great-vessel origins and the cervical vertebral arteries. Both show promise in acute stroke.
is a third-year Resident in Diagnostic Radiology at the Hospital
of the University of Pennsylvania, Philadelphia, PA. He received
his BA in Biological Sciences from Swarthmore College,
Swarthmore, PA in 1995 and his MD from the University of
Pennsylvania School of Medicine, Philadelphia, in 1999. He plans
to begin a Fellowship in Pediatric Radiology at the Children's
Hospital of Philadelphia in July 2004.
Recent advances in computed tomographic angiography (CTA)
and magnetic resonance angiography (MRA) enable comprehensive
evaluation of the supra-aortic vascular tree, including cervical
and intracranial vessels. Both CTA and contrast-enhanced MRA
provide highly accurate assessment of severe carotid artery
stenosis, and are an improvement over traditional time-of-flight
MRA in the depiction of great-vessel origins and the cervical
vertebral arteries. Both show promise in acute stroke.
Comprehensive radiologic examination of both the intracranial
and extracranial supra-aortic vessels is a valuable part of the
evaluation of stenotic or occlusive disease, under a variety of
clinical conditions. In an acute cerebrovascular accident, for
example, pertinent information can be found not only at the site of
arterial thromboembolism but also at its source. Occlusions at some
sites, such as the cervical internal carotid artery, are not
amenable to intra-arterial thrombolysis.
At other sites, including the distal intracranial vessels,
occlusions may be more likely to benefit from intravenous (IV)
tissue plasminogen activator than would occlusions in the proximal
Arterial dissection is a common cause of stroke in young
patients. A comprehensive examination of the supra-aortic vessels
can detect dissection, sometimes at multiple levels of both the
carotid and vertebral arteries.
In patients under consideration for carotid endarterectomy,
determination of the percent of stenosis is important, as this
finding correlates with surgical benefit according to the North
American Symptomatic Carotid Endarterectomy Trial (NASCET).
In addition, tandem lesions along the course of the internal
carotid artery (ICA) may be a contraindication to surgery.
Atherosclerosis at the origin of the great vessels may change
therapy in stroke patients as well.
For all of these reasons, it is essential to image the entire
length of the carotid and vertebral vasculature, from the aortic
arch to the circle of Willis.
Digital subtraction angiography (DSA) remains the gold standard
for the evaluation of steno-occlusive disease of the supra-aortic
vasculature, and has the added advantage of enabling directed
interventions, including intra-arterial thrombolysis of an
identified occlusion, or in some cases, angioplasty and stenting of
a stenosis. However, DSA has inherent limitations; it is costly,
time consuming, and involves opacification of the vessel lumen
only, without providing information about the vessel wall or
perivascular soft tissue. In addition, the procedure presents a
risk of neurologic deficit, with 0.4% risk of permanent deficit and
2.3% risk of any neurologic complication.
Advances in both computed tomographic angiography (CTA) and
magnetic resonance angiography (MRA) have sought to address the
limitations of DSA and provide a noninvasive alternative for the
comprehensive evaluation of the supra-aortic vessels. The purpose
of this review is to discuss recent developments in CTA and MRA and
the ability of these technologies to depict stenoses and
occlusions. There are inherent contraindications to each of the
techniques, including, for CTA, contrast-related nephrotoxicity and
allergy, and for MRA, incompatibility with metal and electronics
implanted in the patient's body. Many patients, however, will be
eligible for both, leaving the clinician and radiologist to select
the most appropriate study.
CT angiography begins with the IV administration of iodinated
contrast material, typically 100 to 150 mL of iodinated contrast
material (300 mg I/mL) at 3 to 4 mL/sec. Adequate timing of the
bolus is critical to obtain maximum contrast between the vessel
lumen and surrounding tissues during data acquisition. This may be
accomplished with an empiric "best-guess" delay, a test-bolus
technique, or automatic triggering.
The empiric strategy is based on the patient's age, weight, and
cardiac status; this strategy is the least successful in optimizing
contrast between tissues, although it is still adequate in many
cases and is often used with a 30-second delay when only the
intracranial vessels are to be imaged.
The test-bolus technique involves the injection of a small amount
of contrast, typically 20 mL, prior to diagnostic image
acquisition, while measuring the change in attenuation within the
vessel of interest over time. The diagnostic examination may then
be performed to coincide with the calculated peak contrast
enhancement. With the automatic triggering technique, a threshold
value of contrast enhancement within the vessel of interest is
predetermined, typically 335 to 355 HU. Data acquisition begins
with repeated scanning over the vessel of interest. When the
threshold of enhancement is reached, diagnostic image acquisition
is triggered manually or automatically by the computer.
The recent development of multi-detector scanners has
revolutionized CT data acquisition. The limited heat load capacity
of the X-ray tube precluded optimal examination of both
extracranial and intracranial circulation during the same
examination for single-row detector systems.
Multidetector scanners and increases in gantry rotation speed have
translated into faster data acquisition. This allows coverage of
both the extracranial and intracranial circulation during the same
examination without overheating the X-ray tube. This increase in
detector number has also allowed for better resolution.
Sixteen-detector scanners have the capability of acquiring data
while maintaining near-isotropic voxel dimensions. Acquisition of
near-isotropic voxels enables reformatting of data and, therefore,
display of vessels in any dimension without significant loss of
resolution, regardless of the plane of acquisition.
Typical parameters for CTA of the cervical vasculature include a
pitch of 6 and rotation time of 0.8 seconds, with mA 220, kV 120,
slice thickness of 1.25 mm, and reconstruction interval of 1.0
Images may be reformatted to display the entire length of
vessels and their anatomic relationships. Current techniques
include multiplanar reformatting, curved planar reformation,
limited- and full-volume maximum intensity projection (MIP), and
shaded-surface display. Limited and whole volume MIPs are most
often used in standard practice. They clearly present the data in a
format analogous to conventional angiography for referring
clinicians. Although such reconstructions may be valuable visual
aids, they do not include all data inherent in the source images.
It becomes imperative, therefore, to view the original axial
Traditionally, MR examination of the supra-aortic vessels has
been performed using two-dimensional (2D) time-of-flight (TOF) and
three-dimentional (3D) TOF sequences for the cervical vasculature,
and a 3D TOF sequence for the intracranial vasculature. These
techniques rely on flow-related enhancement, rather than the
injection of IV contrast. Specifically, gradient-echo images are
obtained in TOF angiography, which contrasts dark saturated
stationary tissue with inflowing bright unsaturated intravascular
blood. Image acquisition may include an entire volume of tissue
(3D) or contiguous sections (2D) This technique has been limited by
long acquisition times, which can lead to motion artifact, as well
as saturation-related signal loss due to in-plane flow on 2D
imaging. Disordered or turbulent flow also presents problems,
including dephasing of spins, which leads to a decrease in signal
intensity. As stenosis within a vessel increases, there is
progressive loss of signal within the post-stenotic segment, which
leads to exaggeration of severity and extent of the stenosis.
Three-dimensional contrast-enhanced (CE) MRA involves the
injection of paramagnetic contrast material, which shortens the T1
relaxation time of tissue within its vicinity. This technique
greatly shortens examination time and decreases the problem of
motion inherent in TOF sequences. As with CTA, precise timing of
the bolus is critical for achieving adequate contrast and may be
accomplished with an empiric-timing technique, a test bolus, or
automatic threshold triggering. Parameters for 3D contrast MRA of
the carotid arteries might include repetition time <5 msec, echo
time <2 msec, flip angle 30š to 45š, field of view 280 * 280 *
80 mm, and matrix 512 * 256 * 48, obtained after injection of
gadolinium, typically 0.1 to 0.3 mmol/kg, usually >2 to 3
Time-resolved MRA is an alternative to methods that depend on
the precisely timed delivery of a contrast bolus. This technique,
which involves rapid continuous sampling of frames of data after
contrast administration, with each frame acquired over 2 to 3
seconds. This technique produces multiple 3D data sets. It is then
possible to retrospectively reconstruct the data set containing
peak arterial enhancement. As an added benefit, temporal resolution
may provide information on the hemo-dynamic significance of a
stenosis, such as in subclavian steal phenomenon.
Time-resolved techniques have inherently less spatial
resolution, however. One time-resolved technique that seeks to
address the trade-off between resolution and speed of acquisition
is time-resolved imaging of contrast kinetics (TRICKS).
Time-resolved imaging of contrast kinetics samples the center of
k-space, which contains phase-encoding contrast information,
earlier and more often than the periphery of k-space, which
contains frequency-encoded spatial information. Sampling
predominantly the center of k-space during peak arterial
enhancement maximizes contrast by allowing acquisition of
phase-encoded contrast information to coincide with peak arterial
enhancement. Because only a portion of k-space is being sampled at
one time, faster frame rates can be generated, which virtually
guarantees that one of the captured frames will contain optimal
enhancement that is free of venous contamination. Frequency-encoded
spatial information can be acquired later in the scan, after peak
arterial enhancement has passed, and then combined with the
phase-encoded data to increase resolution.
Regardless of the bolus-timing technique, contrast-enhanced data
are often acquired in the coronal plane using a head-and-neck coil
to assess the entire length of the carotid and vertebral systems.
Three-dimensional gradient-echo pulse sequences with low repetition
and echo times are used. Scanning can be accomplished in a 20- to
40-second breath-hold. As with CTA, post-processing of MRA images
may involve the production of MIPs or volume-renderings, with MIPs
most often useful to define the length of the vessels and provide
an image analogous to conventional angiography for referring
Evaluation: Cervical carotid arteries
Both multidetector CTA (MDCTA) and CE MRA represent an
improvement over 2D TOF MRA in the evaluation of the anterior
extracranial vasculature. Specifically, both techniques can depict
the origins of the great vessels more accurately, which are not
optimally imaged with TOF, as a result of motion artifact.
More attention has been paid to stenosis of the carotid
bifurcation. NASCET data demonstrated that patients with at least a
70% (severe) symptomatic stenosis of the ICA on DSA benefited from
With a normal lumen of the ICA measuring approximately 7 mm, a
high-grade (70%) stenosis would measure only 2.1 mm.
Resolution on this scale must therefore be demonstrated by any
modality prior to its use as a screening tool.
While not the focus of this review, 2D ultrasound has
demonstrated efficacy in the detection of severe carotid stenosis,
though it is often difficult to mentally reconstruct 3D plaque from
2D data. Newer 3D ultrasound appears particularly promising, with
90% sensitivity and 92% sensitivity for detection of severe carotid
More attention has been focused in the literature, however, on the
role of CTA and MRA, perhaps because these modalities are not
Both MDCTA and CE MRA demonstrate high accuracy in the detection
of severe carotid artery stenosis (>70%), as compared with DSA.
Recently presented data show that MDCTA has a sensitivity of 98% to
100% and a specificity of 96% to 100% for the detection of severe
stenosis, while CE MRA demonstrates a sensitivity of 93% to 98% and
a specificity of 96% to 100%.
Figures 1 and 2 compare the depiction of stenoses by CTA, CE MRA,
and traditional TOF MRA.
In addition to stenosis severity, accurate description of plaque
morphology provides valuable information, as plaques that are more
prone to disruption, fracture, or fissure are more likely to cause
Initial data suggest that MDCTA may be more sensitive to
abnormalities in plaque morphology than DSA or CE MRA. In a recent
study of 22 patients who underwent MDCTA, CE MRA, and DSA, all 11
plaque irregularities described on DSA were seen on MDCTA, plus an
additional 5. By comparison, CE MRA detected only 9. MDCTA may also
be more sensitive than CE MRA for the detection of ulcerated plaque
(sensitivities, 96% and 89%, respectively).
Both CTA and MRA have demonstrated high accuracy for the
detection of carotid artery dissection, a significant cause of
stenosis in young patients. This is of particular importance when
DSA reveals only acute occlusion of the carotid artery, a finding
that has a number of causes. CT angiography has demonstrated an
accuracy of 100% for the detection of dissection, as compared with
Contrast-enhanced MRA may have little to offer over TOF MRA, which
has a sensitivity of 95% and a specificity of 99%. Fat-saturated
T1-weighted images or black-blood fast-spin-echo images on MR are
also extremely useful, demonstrating high-signal eccentric hematoma
around a dark flow void.
Accurate assessment of carotid artery stenosis is necessary not
only in surgical planning but in follow-up studies of patients who
have undergone implantation of an endovascular stent, as these
devices are subject to subsequent intimal hyperplasia and thrombus
formation. With CTA, an MIP is often inadequate for the assessment
of stent stenosis, as the stent and the intravascular contrast are
too similar in attenuation to be distinguished. Thin MIPs or
oblique reformats may be helpful, as shown in Figure 3. A
volume-rendering postprocessing algorithm may be used, with a
maximal opacity assigned to the contrast material and a minimal
opacity assigned to the stent. This technique leads to an
overestimation of stent wall thickness, probably as a result of
partial volume effects, but has nonetheless been shown to be
accurate in the small number of patients in whom its use has been
Stent patency has traditionally not been assessed with 2D TOF
MRA, as stent geometry and metal composition create significant
susceptibility artifact and radiofrequency shielding. Stent patency
is often merely inferred on the basis of the signal proximal and
distal to the artifact. Contrast-enhanced MRA is currently under
investigation for the assessment of stent patency and may be
possible with the use of higher flip angles (75° to 150°), to
increase the conspicuity of blood, and a shorter echo time (<2
msec), which may reduce artifact.
Many stents demonstrate an artificial narrowing and a reduction
in signal intensity, despite contrast injection. Even when it is
possible to demonstrate the vessel lumen, a band-like artifact can
be observed at the ends of the stent. A recent in vitro analysis of
several nitinol stents and a tantalum stent demonstrated good
performance for the nitinol Passager Stent graft (Boston
Scientific, Natick, MA), the nitinol Memotherm iliac stent
(Angiomed Bard, Karlsruhe, Germany), and the tantalum Strecker
stent (Boston Scientific). The reduction in signal intensity and
lumen width within the stents ranged from 0% to 34%.
Evaluation: cervical vertebral arteries
Normal anatomical variations, such as kinking and coiling of the
vertebral arteries, make the posterior circulation difficult to
assess by all modalities. CT angiography performs as well as, or
perhaps better than, DSA in depicting the ostia of the vertebral
arteries, and has a high accuracy for the detection of stenosis.
By comparison, TOF MRA either fails to demonstrate the ostia of the
vertebral arteries, or overestimates the degree of stenosis in this
area. Often there is loss of signal in the vertebral arteries at
the level of the C1C2 disk space, as the arteries flow within the
imaging plane. In the remainder of the cervical vertebral arteries,
2D TOF MRA demonstrates a sensitivity of 92% to 100% and a
specificity of 90% to 96% in evaluating severe stenosis or
As with CTA, CE MRA generally can eliminate the artifact at the
vertebral ostia that is seen on TOF MRA.
Contrast-enhanced MRA demonstrates excellent sensitivity and
specificity in the cervical vertebral arteries, detecting >50%
stenoses with a sensitivity of 100% and a specificity of 96%, and
detecting complete occlusions with a sensitivity and specificity of
Vertebral artery dissections have not been well studied with
newer techniques. Fat-saturated T1 dark-blood sequences often fail
to show the eccentric hematoma that is characteristic of
dissection, probably because of the small diameter of the vessel
and the high signal of the surrounding normal tissue. Initial
reports with MDCTA or a combination of CE MRA and TOF MRA are
promising for the detection of long-segment "pearl-and-string
stenosis," but DSA remains important for diagnosis.
Evaluation of the vertebral arteries is often of clinical
concern when subclavian steal is suspected. The steal phenomenon
arises from occlusion in the proximal subclavian artery, with
arterial blood to the affected arm supplied by the ipsilateral
vertebral artery distal to the stenosis. This results in reversal
of flow in the vertebral artery and subsequent vertebrobasilar
symptoms during greater demand of blood to the extremity, such as
during exercise. MR angiography appears especially suited to the
diagnosis of this entity, with contrast-enhanced techniques
providing accurate degree of stenosis within the subclavian artery,
and flow techniques, such as from time-resolved MRA, demonstrating
direction of flow within the vertebral artery.
CT angiography, which does not provide information on directional
flow, may therefore yield false-negative results.
Evaluation: intracranial vasculature
Stenotic disease of the intracranial vasculature has not been
studied extensively, perhaps because there is little to offer in
the way of intervention. Time-of-flight MRA depicts
moderate-to-severe stenosis in the ICA and MCA with a sensitivity
(Figure 4). More attention has been given to acute occlusive
disease, however. Resistance to the use of MRA or CTA prior to DSA
in the evaluation of acute cerebrovascular accident is often based
on the assumption that valuable time will be lost in delivering
definitive treatment. Any analysis of the role of CTA and MRA must,
therefore, include an assessment of the feasibility of their use in
acutely ill patients. However, information provided by CTA or MRA
on the site of occlusion may enable quicker and more accurate
targeting of the affected vessel during subsequent DSA.
Excluding postprocessing time, CTA adds minutes to a standard
non-contrast CT examination, with one author reporting <5
minutes for injection and scanning time combined.
Time-of-flight MRA sequences require 5 to 10 minutes, and CE MRA
takes even less time. Practically, however, any MR examination will
follow at least a noncontrast CT examination (NCCT), as a NCCT is
the standard first-line diagnostic tool of the emergency room used
to exclude hemorrhage. An average time of 40 minutes is reported
from patient arrival to initial CT exam, and 1.7 hours from CT to
MR exam. This delay between examinations can be reduced over time
with additional experience and training.
However, as MR is not viewed in many institutions as critical to
the management of stroke in the acute stage, there is movement
toward assessment with CTA combined with perfusion CT in the
Examination of the accuracy of CTA and MRA for acute occlusion
is complicated by possible lysis or extension of the clot during
the delay between the noninvasive imaging examination and
subsequent DSA. With this caveat, MDCTA has demonstrated occlusions
to the level of the M3 branches, while simultaneously depicting
occlusions, stenoses, or floating thrombi in the extracranial
circulation. In the examination of large-vessel occlusion, MDCTA
has demonstrated a sensitivity of 91% to 100% and a specificity of
93.7% to 100%.
These values should be tempered by initial data from Mullins et al,
who performed a retrospective analysis of 479 consecutive patients
with acute stroke who underwent CTA and CT perfusion imaging. While
the investigators report a high specificity (93%), the sensitivity
of MDCTA in diagnosing acute infarction was considerably lower
Traditionally, studies of MRA in acutely ill patients have
focused on TOF sequences rather than contrast enhancement, with TOF
demonstrating a sensitivity of 80% to 100% and a specificity that
is comparable to that of DSA.
When combined with diffusion-weighted imaging, TOF MRA has a
diagnostic accuracy of 94% in determining stroke subtype (ie,
large-vessel occlusion, cardioembolic disease, small-vessel
disease, or stroke of other etiology), as gauged by the patient's
final diagnosis at hospital discharge.
Contrast-enhanced MRA, by comparison, requires further evaluation.
Isoda et al
reported acceptable viewing of the arterial phase of enhancement
without venous contamination only 77.6% of the time, suggesting
that further improvement is necessary before CE MRA can replace TOF
sequences. Evaluation of the intracranial vertebral arteries is
problematic, as both CTA and MRA have difficulty in distinguishing
high-grade stenosis from hypoplasia. In a small number of patients,
the accuracy of CTA as compared with DSA was only 66% for stenosis
>50%, and only 50% for total occlusion. This same series
demonstrated 100% accuracy for detecting stenosis and occlusion of
the basilar artery, however.
Similarly, intracranial vertebral arteries are difficult to
assess with TOF MRA (sensitivity, 84%; specificity, 82% to 93%).
The reduced accuracy of TOF MRA in this region may be due to the
use of separate head-and-neck coils, an approach that places the
intracranial vertebral arteries at the junction of two separate
examinations for the head and the neck. In the evaluation of the
basilar artery, TOF MRA demonstrates a sensitivity of 100% and a
specificity of 78% to 88%.
Evaluation of the intracranial posterior circulation with CE MRA
remains to be elucidated, but accurate visualization of the
intracranial portions of the vertebral arteries is possible in most
patients in a single examination of the supra-aortic vessels.
Advances in both CTA and CE MRA enable assessment of the entire
supra-aortic arterial tree, from the aortic arch to the circle of
Willis. Each of these techniques is extremely valuable in the
evaluation of steno-occlusive disease. Each demonstrates high
accuracy in the evaluation of the carotid bifurcation, for example,
although CTA has a slightly better resolution and often more
accurately depicts plaque. Both CTA and CE MRA deserve to be
considered before DSA, which could be reserved as a problem-solving
Although CTA and CE MRA more accurately evaluate the ostia and
the extracranial portions of the vertebral arteries, representing a
marked improvement over TOF MRA, they demonstrate mixed results in
evaluation of the posterior circulation. In acute intra-arterial
occlusion, both technical and diagnostic issues have limited the
use of CTA and CE MRA prior to DSA. Quick CTA scanning, however, is
an attractive method of stroke evaluation prior to DSA or the
initiation of therapy, as it adds little time to a noncontrast CT
scan, and is gaining in popularity as physicians realize the
strides that have been made in terms of resolution and
The author thanks Ron Wolf, MD for reviewing this manuscript and
for providing CTA and MRA images.