AJR 2005; 184:305-312
© American Roentgen Ray Society
CT Angiography in the Evaluation of Cerebrovascular Diseases
Pina C. Sanelli1,
Matthew J. Mifsud2,
Natalie Zelenko3 and
Linda A. Heier1
1 Department of Radiology, NY Presbyterian Hospital, Weill Medical College of
Cornell University, 520 E 70th St., Starr Pavilion, Starr 630, New York, NY
10021.
2 Boston College, 1440 Commonwealth Ave., Boston, MA 02467.
3 Weill Medical College of Cornell University, 1300 York Ave., New York, NY
10021.
Received October 10, 2003;
accepted after revision June 21, 2004.
Address correspondence to P. C. Sanelli
(pcs9001{at}med.cornell.edu).
Introduction
CT angiography can provide rapid, minimally invasive evaluation of a broad
spectrum of cerebrovascular disorders. Over the past several years, CT
angiography has been used more often as an initial neurovascular imaging
technique because of its rapid acquisition, widespread availability, and low
risks to patients. Recently, the clinical indications of CT angiography have
been growing to include a vast array of diseases with the role of CT
angiography constantly changing to adjust to the present clinical needs. Noted
disadvantages of CT angiography include the use of iodinated contrast material
in patients with known allergies and renal disease, failure in timing the
scans to achieve optimal opacification of the vessels, time needed to generate
postprocessing model, and the potential for distortion during image
reconstruction. Because the many technical advantages of CT angiography
already have been described in the literature
[1–3],
we focus on the diagnostic advantages CT angiography offers in the evaluation
of cerebrovascular diseases.
At our institution, CT angiography is performed on a 4-MDCT scanner
(LightSpeed, GE Healthcare) with 1.25-mm helically acquired sections for the
head using the high-quality mode (table feed, 3.75 mm; 140 kVp; 170 mA) and
2.5-mm sections for the neck using the high-speed mode (table feed, 15 mm; 150
kVp; 190–240 mA). A total of 90–120 mL of nonionic contrast
material injected at a rate of 3.0–4.0 mL/sec with a 15- to 25-sec delay
is used. The combined technique for head and neck CT angiography includes the
protocol described with a shared injection of 120 mL of contrast material
injected at a rate of 3.0 mL/sec with a 20-sec delay from the aortic arch to
the skull vertex. The axial data are transferred to a workstation (Vitrea 2,
Vital Images) using a Vitrea 2 software program (Vital Images) for 3D
reconstructions.
Discussion
Intracranial Aneurysms
Localization of sentinel clot.—The presence of multiple
aneurysms on angiographic studies can present an obstacle to the diagnosis of
the ruptured aneurysm responsible for the acute subarachnoid hemorrhage. CT
angiography source images are used to determine the pattern of hemorrhage and
the presence and location of a sentinel clot. The sentinel clot appears as a
localized hyperdensity surrounding the intensely enhancing aneurysm on CT
angiography source images (Figs.
1A,
1B, and
1C), indicating the aneurysm
responsible for the acute subarachnoid hemorrhage.
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Fig. 1. —39-year-old woman with acute subarachnoid hemorrhage.
A, CT angiography source image reveals sentinel clot
(arrowheads) surrounding saccular aneurysm (arrow) arising
from right internal carotid artery. Sentinel clot is focal area of clotted
hemorrhage that surrounds ruptured aneurysm. Smaller left middle cerebral
artery bifurcation aneurysm measuring less than 3.0 mm (not shown) also was
detected.
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Fig. 1B. —39-year-old woman with acute subarachnoid hemorrhage. CT
angiography reconstruction image (lateral oblique view) with bone subtraction
technique applied shows small irregular right posterior communicating artery
aneurysm (arrowhead) along posterior wall of internal carotid artery
(ICA). Anterior cerebral artery (ACA) and middle cerebral artery (MCA) also
are labeled for orientation. Right posterior communicating artery aneurysm was
considered responsible for acute subarachnoid hemorrhage given presence of
associated sentinel clot.
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Fig. 1C. —39-year-old woman with acute subarachnoid hemorrhage. Digital
subtraction angiogram (lateral projection) via right internal carotid artery
(ICA, arrow) catheterization confirms small lobulated right posterior
communicating artery aneurysm (arrowhead) arising from internal
carotid artery.
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Detection of contrast extravasation.—CT angiography also is
useful for its ability to discriminate between acute subarachnoid hemorrhage
and contrast extravasation. There is a detectable difference between the
density of acute hemorrhage and contrast material (Figs.
2A,
2B, and
2C), which is measured in
Hounsfield units on the CT angiography source images. Contrast extravasation
may pool and concentrate in the midst of acute subarachnoid hemorrhage and not
necessarily become diluted by CSF in the short time period during dynamic
contrast-enhanced study. The attenuation of acute hemorrhage typically is
approximately 80 H, whereas contrast material in the lumen of a vessel or
extravasated contrast material has a much higher value of 150–350 H
[4]. Detecting this difference
is critical in patients with actively bleeding aneurysms requiring emergent
diagnosis and treatment.
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Fig. 2A. —52-year-old man with acute subarachnoid hemorrhage. CT
angiography source image reveals focal well-defined area of contrast pooling
(arrow) in left suprasellar cistern, which appears hyperdense
compared with top of left internal carotid artery (arrowhead). Acute
subarachnoid hemorrhage surrounding this focal area of contrast material
measures 65 H, whereas left internal carotid artery measures 196 H. Focal area
of contrast material measures 322 H, representing contrast extravasation. In
this case, contrast extravasation has much higher Hounsfield unit measurement
than intravascular contrast material, as seen in left internal carotid artery
or aneurysm arising from it that becomes diluted by inflowing blood.
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Fig. 2B. —52-year-old man with acute subarachnoid hemorrhage. Digital
subtraction angiogram (anteroposterior projection) via left internal carotid
artery catheterization shows no aneurysm to account for finding of focal
contrast material.
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Fig. 2C. —52-year-old man with acute subarachnoid hemorrhage. Digital
subtraction angiogram (lateral projection) via left internal carotid artery
catheterization again shows no aneurysm is responsible for focal contrast
material seen on CT angiography.
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Axial plane imaging.—Thin axial images provide segment
analysis through an entire aneurysm to allow more detailed evaluation of
aneurysm configuration, neck dimension, and adjacent arterial origins. Other
3D imaging techniques, such as digital subtraction angiography, may obscure
the true vessel origin because of superimposition by a large aneurysm.
Definition of the origin and course of adjacent arteries is essential for
preoperative planning of surgical clipping of an aneurysm to prevent vascular
injury (Figs. 3A,
3B,
3C, and
3D).
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Fig. 3A. —41-year-old woman with acute subarachnoid hemorrhage detected
on CSF analysis. CT angiography source image reveals small saccular aneurysm
(white arrow) arising at origin of ophthalmic artery (black
arrow).
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Fig. 3B. —41-year-old woman with acute subarachnoid hemorrhage detected
on CSF analysis. CT angiography reconstruction image (craniocaudal view) shows
left ophthalmic artery aneurysm (arrow), which is obscured partially
by overlying anterior cerebral artery and clinoid process.
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Fig. 3C. —41-year-old woman with acute subarachnoid hemorrhage detected
on CSF analysis. Magnified CT angiography reconstruction image (lateral view)
with bone subtraction technique applied shows entire ophthalmic artery
aneurysm (arrows). Ophthalmic artery (arrowheads) is better
visualized arising posterior from aneurysm and coursing along neck of aneurysm
anteriorly into orbit.
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Fig. 3D. —41-year-old woman with acute subarachnoid hemorrhage detected
on CSF analysis. Digital subtraction angiogram (lateral projection) via left
internal carotid artery catheterization confirms superior oriented aneurysm
(arrow) at origin of ophthalmic artery (arrowheads).
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Bone subtraction technique.—A commonly cited disadvantage of
CT angiography has been difficulty in excluding bony anatomy, particularly at
the skull base, to achieve adequate visualization of the local vasculature.
However, the newer bone subtraction techniques available on the Vitrea 2
workstation using a Vitrea 2 software program can achieve optimal
visualization of the vessels in close proximity to bony structures (Figs.
1A,
1B,
1C,
3A,
3B,
3C, and
3D).
Three-dimensional manipulation.—CT angiography
reconstruction models allow 3D representation of the vasculature and the
capacity to manipulate the model in any direction for planning the surgical
approach. This capability affords the examiner an important advantage by
allowing the neck of the aneurysm and its relationship to adjacent vessels
with the associated bony anatomy intact to be analyzed and defined (Figs.
1A,
1B,
1C,
3A,
3B,
3C, and
3D) for preoperative
planning.
Arteriovenous Malformations
Characterization.—At our institution, we have found that a
particular diagnostic advantage of CT angiography is the simultaneous
visualization of arterial and venous anatomy. CT angiography can depict an
entire arteriovenous malformation including the nidus with its arterial
feeding vessels and venous drainage supply (Figs.
4A,
4B,
4C, and
4D). Three-dimensional
reconstructions and multiplanar vascular maximum-intensity-projection images
with brain surface-shading technique display the complex vasculature and show
the relationship of the arteriovenous malformation nidus with eloquent
cortex.
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Fig. 4A. —38-year-old woman with minimal acute intraventricular
hemorrhage. Unenhanced CT scan of head shows minimal intraventricular
hemorrhage layering in occipital horns bilaterally (arrows).
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Fig. 4B. —38-year-old woman with minimal acute intraventricular
hemorrhage. CT angiography source image reveals focal areas of increased
enhancement (arrow) suggestive of arteriovenous malformation adjacent
to right occipital horn, which was determined as source of hemorrhage.
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Fig. 4C. —38-year-old woman with minimal acute intraventricular
hemorrhage. CT angiography reconstruction image (anteroposterior oblique view)
reveals arteriovenous malformation nidus (large arrowhead) with right
posterior cerebral artery (small arrows) as arterial feeding supply
and prominent draining vein (small arrowheads) emptying into internal
cerebral vein (large arrow), representing deep venous drainage.
Nidus, measuring 18 mm, is localized in right occipital lobe.
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Fig. 4D. —38-year-old woman with minimal acute intraventricular
hemorrhage. Digital subtraction angiogram (anteroposterior projection) via
left vertebral artery catheterization confirms arteriovenous malformation
nidus (arrowhead) with right posterior cerebral artery as arterial
supply (arrows).
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Stereotactic localization.—Thin-slice selection, spatial
resolution, and optimal opacification of the intracranial vasculature make CT
angiography a useful technique for stereotactic localization. Imaging is
performed easily with a stereotactic frame or fiducial markers in place to
establish accurate nidus localization for surgical resection or radiosurgical
treatment (Figs. 5A,
5B, and
5C).
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Fig. 5A. —41-year-old man with previously treated arteriovenous
malformation presented from outside institution after unsuccessful surgical
resection. CT angiography source image reveals that small residual nidus
(arrow-head) is located just superior and medial to previous
craniotomy site (arrow).
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Fig. 5B. —41-year-old man with previously treated arteriovenous
malformation presented from outside institution after unsuccessful surgical
resection. CT angiography reconstruction image (anteroposterior oblique view)
shows feeding arterial supply from right posterior cerebral artery branch
(arrows) with prominent draining cortical vein (arrowheads).
These findings are seen in relationship to craniotomy site. CT angiogram
(A) provided preoperative stereotactic localization of residual nidus
with bony anatomy viewed on 3D reconstructions to guide surgical approach.
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Fig. 5C. —41-year-old man with previously treated arteriovenous
malformation presented from outside institution after unsuccessful surgical
resection. Digital subtraction angiogram (lateral projection) via right
vertebral artery catheterization shows complete arteriovenous malformation
obliteration (arrowheads) after successful surgical resection using
CT angiography stereotactic localization.
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Treatment of residual arteriovenous malformation.— Treatment
of a residual nidus often is more challenging than treatment of the initial
nidus because of its smaller size, surrounding scar tissue, obscuration of the
residual nidus by surgical material, and possible fragile vessels. CT
angiography can be performed for identification and stereotactic localization
of the residual nidus to limit further treatment complications (Figs.
5A,
5B, and
5C) during a repeat surgical
resection or repeat stereotactic radiosurgery. CT angiography also can be used
for follow-up examination after embolization treatment because clips and
embolization glue cause only limited artifacts and the residual arteriovenous
malformation nidus can be visualized easily because of the attenuation
differences.
Arterial and Venous Stenoocclusive Disease
Severe stenosis.—A hairline residual lumen (string sign) in
severe carotid artery stenosis can be misinterpreted on Doppler sonography and
2D time-of-flight MR angiography as complete occlusion
[5]. Early noninvasive
identification of string sign in carotid stenoocclusive disease will determine
whether further invasive imaging with digital subtraction angiography is
required for deciding possible treatment options. Patients with severe carotid
artery stenoses, including string sign, may undergo carotid endarterectomy or
stenting procedures, for which patients with carotid occlusions do not
qualify. Treatment options for patients with internal carotid artery occlusion
may include external carotid–internal carotid bypass surgery. CT
angiography delayed imaging can show residual contrast material within the
narrowed lumen as a result of slow flow through the severely stenotic segment.
CT angiography therefore allows the distinction between severe stenosis
(string sign) and vessel occlusion (Figs.
6A,
6B,
6C, and
6D).
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Fig. 6A. —59-year-old man for evaluation of carotid artery stenosis
before coronary artery bypass surgery. MR angiography 2D time-of-flight
unenhanced postprocessed maximum-intensity-projection image shows no evidence
of flow in left internal carotid artery (arrowheads), thus suggesting
occlusion. In addition, acquired MR angiography source images of neck (not
shown) revealed no flow in internal carotid artery. Three-dimensional
time-of-flight MR angiogram of head (not shown) also had no evidence of flow
in intracranial portion of internal carotid artery.
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Fig. 6B. —59-year-old man for evaluation of carotid artery stenosis
before coronary artery bypass surgery. Initial CT angiography source image
reveals no contrast enhancement of left internal carotid artery
(arrowhead).
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Fig. 6C. —59-year-old man for evaluation of carotid artery stenosis
before coronary artery bypass surgery. Delayed CT angiography source image
reveals subtle contrast opacification in markedly attenuated left internal
carotid artery (arrowhead), representing string sign from severe
stenosis.
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Fig. 6D. —59-year-old man for evaluation of carotid artery stenosis
before coronary artery bypass surgery. Digital subtraction angiogram (lateral
projection) via left common carotid artery catheterization confirms presence
of high-grade proximal internal carotid artery stenosis with marked
attenuation of distal cervical segment (arrowheads), representing
string sign. CT angiography therefore allows distinction between severe
stenosis (string sign) and vessel occlusion, thus adding significant advantage
to initial evaluation and management of patients with stenoocclusive
disease.
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Calcified plaque.—For calcified plaque in the arteries, CT
angiography performed with the appropriate window and level settings is able
to differentiate calcified plaque from contrast opacification of the lumen.
Coarse calcified plaque may result in some degree of beam-hardening artifact,
but does not obscure patency of the vessel lumen because of the difference in
Hounsfield unit measurements. Direct measurements of the vessel lumen can be
obtained in the axial plane on CT angiography source images.
For calcified plaque in veins, its ability to define intraluminal contrast
material allows CT angiography to distinguish between complete occlusion and
stenosis of the dural venous sinuses, particularly in cases of meningioma
involvement and calcified plaque. Three-dimensional reconstructions with
surface-shaded display of the brain surface allow simultaneous visualization
of the dural venous sinus with the adjacent brain (Figs.
7A and
7B).
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Fig. 7A. —33-year-old woman with densely calcified parasagittal
meningioma on MR image obtained at outside institution (not shown) and MR
venogram (not shown) depicting no flow in superior sagittal sinus. Unenhanced
CT scan of head shows diffuse coarse calcification (arrows) along
falx in region of superior sagittal sinus.
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Fig. 7B. —33-year-old woman with densely calcified parasagittal
meningioma on MR image obtained at outside institution (not shown) and MR
venogram (not shown) depicting no flow in superior sagittal sinus. CT venogram
reconstruction image with 3D brain surface-shaded display shows marked
irregular narrowing of superior sagittal sinus (arrowheads), which
remains patent along its entire course. Obtaining accurate information
regarding patency of dural venous sinus is important before surgical
resection. Patent dural venous sinus should not be sacrificed during surgical
resection because of risk of venous infarction.
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Anatomic details.—CT angiography provides information based
on anatomic details and not flow-related physiology like 2D time-of-flight MR
angiography. Subtle intraluminal lesions that do not cause hemodynamically
significant stenosis can be seen on CT angiography (Figs.
8A,
8B, and
8C). These findings may be
seen with vascular disease such as a small focal embolus or dissections, which
appear as filling defects.
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Fig. 8A. —2-year-old boy with large acute right parietal infarction. MR
angiogram 2D time-of-flight unenhanced postprocessed
maximum-intensity-projection image shows no evidence of flow abnormality or
vessel narrowing of right internal carotid artery.
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Fig. 8B. —2-year-old boy with large acute right parietal infarction. CT
angiography reconstruction image obtained with curved reformat technique
reveals focal filling defect (arrowhead) in proximal right internal
carotid artery, representing small dissection.
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Fig. 8C. —2-year-old boy with large acute right parietal infarction. CT
angiography source image confirms small linear filling defect
(arrowhead) in right internal carotid artery, representing intimal
flap in focal dissection.
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Conclusion
The diagnostic advantages of CT angiography make it a useful tool for the
evaluation of a broad spectrum of cerebrovascular diseases. Awareness of these
added benefits of CT angiography may be important in improving detection and
characterization of cerebrovascular disease.
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Introduction
Discussion
