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CT Angiography in the Evaluation of Cerebrovascular Diseases

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

Top Introduction Discussion Conclusion References

 
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

Top Introduction Discussion Conclusion References

 
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.

View larger version (90K): [in this window] [in a new window] [as a PowerPoint slide]   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.

View larger version (39K): [in this window] [in a new window] [as a PowerPoint slide]   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.

View larger version (112K): [in this window] [in a new window] [as a PowerPoint slide]   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.

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.

View larger version (96K): [in this window] [in a new window] [as a PowerPoint slide]   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.

View larger version (99K): [in this window] [in a new window] [as a PowerPoint slide]   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.

View larger version (109K): [in this window] [in a new window] [as a PowerPoint slide]   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.

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).

View larger version (94K): [in this window] [in a new window] [as a PowerPoint slide]   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).

View larger version (28K): [in this window] [in a new window] [as a PowerPoint slide]   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.

View larger version (46K): [in this window] [in a new window] [as a PowerPoint slide]   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.

View larger version (98K): [in this window] [in a new window] [as a PowerPoint slide]   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).

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.

View larger version (105K): [in this window] [in a new window] [as a PowerPoint slide]   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).

View larger version (91K): [in this window] [in a new window] [as a PowerPoint slide]   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.

View larger version (37K): [in this window] [in a new window] [as a PowerPoint slide]   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.

View larger version (156K): [in this window] [in a new window] [as a PowerPoint slide]   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).

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).

View larger version (91K): [in this window] [in a new window] [as a PowerPoint slide]   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).

View larger version (106K): [in this window] [in a new window] [as a PowerPoint slide]   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.

View larger version (144K): [in this window] [in a new window] [as a PowerPoint slide]   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.

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).

View larger version (60K): [in this window] [in a new window] [as a PowerPoint slide]   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.

View larger version (77K): [in this window] [in a new window] [as a PowerPoint slide]   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).

View larger version (81K): [in this window] [in a new window] [as a PowerPoint slide]   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.

View larger version (142K): [in this window] [in a new window] [as a PowerPoint slide]   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.

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).

View larger version (47K): [in this window] [in a new window] [as a PowerPoint slide]   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.

View larger version (34K): [in this window] [in a new window] [as a PowerPoint slide]   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.

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.

View larger version (59K): [in this window] [in a new window] [as a PowerPoint slide]   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.

View larger version (59K): [in this window] [in a new window] [as a PowerPoint slide]   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.

View larger version (100K): [in this window] [in a new window] [as a PowerPoint slide]   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.

Conclusion

Top Introduction Discussion Conclusion References

 
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.

References

Top Introduction Discussion Conclusion References

 

1. Vieco PT. CT angiography of the intracranial circulation. Neuroimaging Clin N Am1998; 8:577 -592[Medline]
2. Vieco PT. CT angiography of the carotid artery. Neuroimaging Clin N Am1998; 8:593 -605[Medline]
3. Brant-Zawadzki M, Heiserman JE. The roles of MR angiography, CT angiography, and sonography in vascular imaging of the head and neck. AJNR 1997;18:1820 -1825[Medline]
4. Holodny AI, Farkas J, Schlenk R, Maniker A. Demonstration of an actively bleeding aneurysm by CT angiography. AJNR2003; 24:962 -964[Abstract/Free Full Text]
5. Dawson DL, Zierler E, Strandness E, et al. The role of duplex scanning and arteriography before carotid endarterectomy: a prospective study. J Vasc Surg1993; 18:673 -680[Medline]

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