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Show STATE OF THE ART Computed Tomography and Magnetic Resonance Angiography in Cervicocranial Vascular Disease Dheeraj Gandhi, MD Abstract: Although catheter angiography, or digital subtraction angiography ( DSA), is still regarded as the gold standard for imaging of cervicocramal vascular disease, its morbidity, cost, and time- consuming features have prompted the development of noninvasive techniques based on computed tomography ( CT) and magnetic resonance imaging. With the advent of powerful software, CT and magnetic resonance angiography are complementing and, in some cases, even replacing DSA in the diagnostic evaluation of carotid atherostenosis, unruptured aneurysms, dissections, stroke, penetrating trauma to the neck, and dural venous sinus occlusive disease. They offer advantages over DSA not only in reduced morbidity and time-saving but also in assessment of brain parenchyma, quantitative perfusion, and abnormalities of vessel walls. In the evaluation of blunt neck injuries and intracranial vascular malformations, fistulas, and vasculitis, CT and magnetic resonance angiography still do not provide as much information as DSA. ( JNeuro- Ophthalmol 2004; 24: 306- 314) Cerebral catheter angiography, or digital subtraction angiography ( DSA), is still generally regarded as the gold standard for the imaging of cerebrovascular disorders. But despite technical advances ( digital imaging systems, smaller catheters, hydrophilic guide wires, and safer contrast media), it remains a time- consuming examination with a small but significant ( 0.5%) rate of permanent neurologic complications ( 1,2). This has prompted considerable interest in the development of alternative non- invasive techniques. Cross- sectional techniques like computed tomography ( CT) and magnetic resonance imaging ( MRI) offer not only reduced morbidity and time- saving but also the possibility of combined assessment of brain parenchyma and its vascular supply including, most recently, quantitative Department of Radiology ( Neuroradiology), University of Michigan Medical Center, Ann Arbor, Michigan. Address correspondence to Dheeraj Gandhi, MD, Department of Radiology, University of Michigan Medical Center, 1500 East Medical Center Drive, Ann Arbor, MI 48109; E- mail: dheeraj@ umich. edu assessment of perfusion. These techniques also provide important information about vessel walls, enabling early detection and characterization of lesions before alterations in the vessel lumen occur. Three- dimensional reconstructions of volume data from CT angiography ( CTA) and magnetic resonance angiography ( MRA) excellently depict the spatial relationship of complex vascular lesions to the surrounding structures, providing valuable information for the surgeon. CTA and MRA are also quicker to perform and require fewer resources in terms of staffing and equipment. Most such examinations ( especially CTA) can be performed without sedation. With continued advances in hardware and software, CTA and MRA are slowly but surely replacing DSA for many diagnostic questions. In the future, DSA will likely be reserved largely for interventional procedures. COMPUTED TOMOGRAPHY ANGIOGRAPHY CTA is a rapid, non- invasive imaging technique that can produce quality angiographic projections of the cervical and cerebral vasculature after intravenous injection of radiographic contrast media. Although it has been in use since the early 1980s ( 3), interest in CTA has been rekindled with the advent of helical and multislice CT scanners. These modern scanners allow acquisition of extremely high- resolution images of extracranial and intracranial vasculature within 1 minute or less ( 3). Using commercially available software, one can generate two- dimensional ( 2D) and three- dimensional ( 3D) images of vessels from the raw data. These reconstructed images can be viewed from any angle and with varied window settings. Shaded surface displays and maximum intensity projection ( MIP) are the most popular algorithms. It is also possible to view the inner surface of the vessel wall and navigate along the vessel lumen ( intra- arterial endoscopy) ( 3). Intracranial Aneurysms CTA has been widely used in the detection and characterization of intracranial aneurysms. It has replaced DSA as a primary diagnostic method for evaluation of subarachnoid hemorrhage in many institutions ( 4,5). CTA can be 306 J Neuro- Ophthalmol, Vol. 24, No. 4, 2004 CT and MRA in Cervicocranial Vascular Disease JNeuro- Ophthalmol, Vol. 24, No. 4, 2004 performed easily and expeditiously while the patient is still on the CT table ( Fig. 1). CTA is especially attractive for evaluation of critically ill patients who need emergent aneurysm surgery ( 3) and at centers where urgent DSA may be difficult to obtain. CTA has the potential to provide complementary information in complex aneurysms that are difficult to evaluate with DSA, especially those in the paraclinoid location. CTA can help determine the relationship of the aneurysm to the adjacent bony landmarks and provide detailed evaluation of aneurysmal neck and dome morphology. A 2003 meta- analysis of data on 1,251 patients with cerebral aneurysms undergoing CTA and DSA showed a sensitivity of 93.3% and a specificity of 87.8% for CTA ( 6). In another study of 178 aneurysms, the sensitivity and specificity of CTA were 96% and 97%, respectively ( 5). However, in this latter study, CTA failed to detect 7 of 11 aneurysms less than 2 mm in diameter. Moreover, there were eight false- positive findings on CTA, the source being infundibular enlargements at the origin of the posterior communicating arteries. The combination of a high- quality CTA study and a plain CT scan demonstrating a pattern of hemorrhage consistent with the CTA- identified aneurysm may be sufficient to proceed with surgery. However, a patient with nontraumatic subarachnoid hemorrhage and a negative or equivocal CTA should be further evaluated with DSA because CTA has limited sensitivity for small aneurysms. Carotid Atherostenosis With modern CTA scanners, adequate visualization of the arterial tree can be obtained from the aortic arch to the circle of Willis in less than 1 minute of scan time. Analysis of pooled data from 15 high- quality studies ( 7) demonstrated that CTA had a mean sensitivity of 95% and a specificity of 98% for detection of more than 70% cervical carotid stenosis ( 7). A prospective study comparing CTA, MRA, and DSA in 44 carotid arteries showed a CTA sensitivity and specificity of 100% for the detection of more than 70% stenosis ( 8). Detection of ulceration within atheromatous plaques, considered a risk factor for cerebral embolism ( 9), can be accomplished better with CTA than with DSA ( 10,11). A study by Randoux et al ( 8) showed that CTA and MRA were more sensitive than DSA in the detection of plaque ulceration. The inability of DSA to detect ulcerations within plaques is caused by the limited number of views typically obtained with this technique. Calcification of atheromatous plaques limits the accurate estimation of carotid stenosis with CTA ( 12,13). However, this limitation can be overcome with the use of thin multiplanar volume reconstructions and transverse oblique reconstruction ( 8). Stroke A screening CT is usually performed in patients with suspected stroke to exclude intracranial hemorrhage and lesions that may mimic stroke ( 14). It is considered inferior to MRI, which, with a diffusion- weighted sequence, can identify acute and hyperacute infarcts with a sensitivity of 97% and a specificity of 100% ( 15). Moreover, the addition of a high- speed magnetic resonance perfusion study can provide valuable insight into hypoperfused brain tissue. Subtraction of the area with restricted diffusion ( the infarcted FIG. 1. Computed tomography angiography ( CTA) in cerebral aneurysm. A 61- year- old woman who had undergone clipping of an anterior communicating artery aneurysm 37 years earlier presented with a sudden severe headache reminiscent of the headache experienced with the original aneurysmal hemorrhage. A. Non- contrast CT shows diffuse subarachnoid hemorrhage with hematoma in the anterior interhemispheric fissure. B. Shaded surface display of CTA reveals a bilobulated aneurysm at the junction of the Al and A2 segments of the left anterior cerebral artery ( arrows). C. Sagittal reformat of CTA demonstrates that the previously applied aneurysm clip has migrated forwards and anteriorly. The migrated clip is seen along the floor of the anterior cranial fossa ( smaller arrow). The larger arrow points to residual/ recurrent aneurysm at the junction of the left Al and A2 segments. 307 JNeuro- Ophthalmol, Vol. 24, No. 4, 2004 Gandhi region) from the larger area with reduced perfusion yields the region of the ischemic penumbra, the tissue at risk for further infarction that could be preserved with timely thrombolytic intervention ( 16). Although MRI of stroke has these distinct advantages over other imaging techniques, the availability of magnetic resonance scanners remains limited, especially in emergent settings ( 17). In this circumstance, the combined use of CT, CTA, and CT perfusion in imaging of acute stroke may be quite effective ( 14,17). Such a combined examination can be completed within 30 minutes ( 14) and can offer significant information on the tissue at risk and the status of the large vessels ( 17). The most significant drawback of CT perfusion studies is the limited anatomic coverage. These studies presently are limited to either a 1- cm- thick or a 2- cm- thick section of tissue per acquisition; small areas of ischemia outside the planes selected for perfusion analysis could be missed. In the future, improvement in hardware and software will likely result in increased coverage of CT perfusion scans. Lev et al ( 18) have shown that CTA is highly accurate in the detection and exclusion of large vessel intracranial occlusion ( Fig. 2). This technique therefore may be valuable in the rapid triage of stroke patients to in-tra- arterial thrombolytic therapy ( 18,19). Vascular Injuries The role of CTA in screening for vascular injuries associated with penetrating and blunt trauma of the neck remains uncertain because of limited data. In a prospective study of 60 patients ( 20) comparing the usefulness of CTA and DSA in penetrating neck injuries, CTA successfully identified 9 of 10 arterial injuries ( sensitivity 90%, specificity 100%), including four arterial occlusions, two arteriovenous fistulas, two pseudoaneurysms, and one pseudoaneurysm with arteriovenous fistula. CTA missed one small pseudoaneurysm of the common carotid artery. In an earlier study by Leblang et al ( 21), CTA evidence of arterial injury was found in 8 of 10 patients who subsequently had injury demonstrated by DSA, with no false- positive findings in 19 patients. CTA appears to be less favorable for the evaluation of blunt cerebrovascular injuries. In a prospective study by Miller et al ( 22), radiologists read 143 CTAs without knowledge of DSA findings in these patients. CTA diagnosed only 8 ( 47%) of 17 carotid artery injuries. CTA was falsely negative in six simple dissections, one carotid-cavernous fistula, one carotid artery occlusion, and one carotid dissection with significant stenosis. CTA also demonstrated poor sensitivity ( 53%) to vertebral artery injuries, missing four occlusions, two dissections with accompanying stenosis, and eight simple dissections. In a study by Biffl et al ( 23), 46 patients underwent both CTA and DSA for suspected blunt cerebrovascular injuries. CTA identified 15 ( 68%) of 22 injuries. Six of the seven missed injuries were classified as mild ( grade I) injuries on DSA. At this time, CTA cannot be recommended as a reliable substitute for DSA in patients with suspected blunt cerebrovascular injuries, mainly because of its inability to detect small intimal flaps and intramural hematomas. CTA has demonstrated better sensitivity and specificity in the context of penetrating trauma, but the reported patient population is small. Other Applications CTA has been also been used in evaluating arteriovenous malformations ( 24), to confirm correct clip placement FIG. 2. CTA in intracranial arterial occlusion. A 64- year- old woman with atrial fibrillation presented with the acute onset of right weakness. A. CT reveals a hyperdense left middle cerebral artery ( MCA), suggesting a clot in the middle cerebral artery. B. Shaded surface display of CTA reveals complete occlusion of the distal left Ml segment { arrow). C. Thick axial multiplanar reformat of CTA also displays an occluded distal Ml segment of the MCA { arrow). 308 © 2004 Lippincott Williams & Wilkins CT and MRA in Cervicocranial Vascular Disease JNeuro- Ophthalmol, Vol. 24, No. 4, 2004 after aneurysm surgery, and in detection of vasospasm after subarachnoid hemorrhage ( 25). Intracranial Venous Disorders CT venography ( CTV), a modification of CTA, can obtain high- resolution images of dural venous sinuses and cerebral veins. For the depiction of venous anatomy, source images are obtained in the venous phase using other scanning parameters that are quite similar to CTA. Careful attention must be paid to source images, as well as to MIP images. Cerebral CTV may be superior to magnetic resonance venography ( MRV) in the identification of small cerebral veins and dural sinuses and demonstrates equal sensitivity in the diagnosis of dural sinus thrombosis ( 26,27). CTV is, therefore, a useful alternative to MRV. It should be considered in the evaluation of cerebral venous disorders if MRI is not possible because of metallic implants, the patient is claustrophobic or critically ill, or MRV studies are equivocal for venous abnormalities or suffer from flow/ motion artifacts. The drawback of this technique, however, is the need to use ionizing radiation and iodine-based contrast media. MAGNETIC RESONANCE ANGIOGRAPHY The complex process of image generation using magnetic resonance is exquisitely sensitive to proton spins in the field of view. Moving spins in the bloodstream can be manipulated to provide intravascular signal alterations. Information can be obtained about morphology of the vessel, direction of flow, and even quantitative assessment of flow rates. A wide variety of different sequences and acquisition techniques ( time- of- flight [ TOF], phase contrast, contrast-enhanced 3D scanning) are available for MRA. Some techniques are complementary; more than one technique is often required to provide the necessary data for accurate diagnosis and treatment planning. Detailed discussion of these techniques is beyond the scope of this article and interested readers are referred to dedicated reviews ( 28,29). Three types of MRA techniques are most frequently used in clinical practice. These include 2D and 3D TOF, phase contrast, and contrast- enhanced MRA. TOF techniques provide vascular contrast based on tagging of the longitudinal magnetization of spins flowing into a region of interest ( flow- related enhancement). Using TOF methods, static tissues within a 2D slice ( 2D TOF) or 3D slab ( 3D TOF) give a low signal because of the saturation effect of a long train of closely spaced excitation pulses. An intermediate design between 2D and 3D techniques is a series of thin 3D volumes rather than a thick slab of tissue ( multiple overlapping thin- slab angiography [ MOTSA]). The major advantage of the MOTSA technique is its decreased sensitivity to flow- saturation effects. MOTSA is better than single- volume 3D TOF for demonstrating the abnormalities of intracranial vasculature ( 30). Three- dimensional MOTSA is therefore the preferred sequence for evaluation of the circle of Willis. TOF MRA techniques have been widely used for imaging of the cervical arteries in the past. Drawbacks of TOF techniques in the evaluation of cervical arteries include the small area anatomic coverage, long scan times leading to a frequent degradation of images by motion artifacts, and ar-tifactual signal loss in cases of tight stenosis because of turbulent flow. Contrast- enhanced MRA has largely overcome these limitations and is increasingly replacing TOF MRA for the evaluation of neck arteries. Phase contrast ( PC) methods produce MRA images based on motion- induced phase shifts combined with subtraction. Phase shifts are introduced to nuclei when they move in the presence of a bipolar magnetic field gradient. Phase shifts accumulated by nuclei are dependent on their velocity as well as their acceleration. By constructing an image in which the intensity is proportional to the phase shift of the nuclei, an angiographic image related to the flow properties can be created. PC MRA allows for quantification of flow velocities and flow direction. PC techniques are also very sensitive to slow flow ( 28). The important limitation of PC MRA techniques is the long acquisition time and the pronounced loss of signal in areas of disturbed flow. For these reasons, PC MRA techniques are seldom used in clinical practice. Their main use is in the quantitative evaluation of flow velocities and flow direction in research studies. More recently, contrast- enhanced MRA ( CE MRA) techniques have been adopted in the evaluation of the extracranial arterial system. These techniques depend on shortening of the Tl signal by intravenous administration of gadolinium- based contrast agents. With the Tl of blood reduced to less than 50 ms, very heavily Tl- weighted images can be acquired in which blood has high signal intensity relative to the saturated surrounding tissues. The timing of contrast injection and imaging acquisition are crucial. The sequence must be performed before the venous phase to achieve relative suppression of venous signal intensity. Although CE MRA has now become the technique of choice in the assessment of extracranial vessels, it is not considered optimal for evaluation of intracranial arteries because of its relatively low spatial resolution. Carotid Atherostenosis Although the major clinical trials related to surgical management of carotid atherosclerotic disease have used selective DSA for determination of the degree of extracranial carotid artery stenosis ( 31,32), MR imaging has emerged as a viable alternative. Technical evolution from 3D TOF MR techniques to CE MRA has largely overcome the historical overestima-tion of stenosis by MR ( 33). Several trials have shown CE 309 JNeuro- Ophthalmol, Vol. 24, No. 4, 2004 Gandhi MRA sensitivities ranging from 86% to 100% and specificities ranging from 85% o to 96% o in the detection of significant carotid stenosis ( 34). Recent studies have also shown that ulcerations are more frequently identified with CE MRA or CTA than with DSA ( 8,35). However, MRA has a lower specificity in the detection of carotid stenosis than DSA because of lower spatial resolution and signal loss in areas of high- grade stenosis as the result of flow turbulence. On occasion, artifact from a hemorrhagic plaque may result in underestimation of the severity of stenosis. Even so, the introduction of CE MRA has now replaced DSA in assessment of atherosclerosis in many centers ( 34). Stroke In recent years, rapid advances have been made in the MR imaging of acute stroke. A combination of diffusion-weighted imaging and perfusion- weighted imaging has been used in numerous studies to delineate the ischemic penumbra. Previous studies indicated that nearly one- third of patients with acute major stroke have no demonstrable vessel occlusion on DSA ( 36) and thus may not benefit from thrombolytic therapy. The combination of MOTSA and CE MRA provides a non- invasive and sensitive way to exclude intracranial vascular occlusions and to detect potential concomitant carotid abnormalities. Ohue et al ( 37) evaluated six acute stroke patients studied with MRA and DSA before thrombolysis. In all patients, MRA clearly demonstrated the occluded arteries. MRA findings correlated well with the findings on DSA. Among 30 patients who had had acute stroke, Kenton et al ( 38) compared the ability of transcranial color Doppler and MRA in identifying circulatory changes after stroke. They concluded that both modalities yielded useful information but that MRA was better in demonstrating fine vessel detail. Intracranial Aneurysms The role of magnetic resonance in the detection of intracranial aneurysms continues to evolve. MRI suffers from lack of sensitivity for detection of acute subarachnoid hemorrhage, but MRA has high sensitivity and specificity for detection of aneurysms in the setting of subarachnoid hemorrhage ( 39,40). However, it is often difficult to obtain MRA in acutely ill patients on an urgent basis. Therefore, CT combined with CTA is preferable to MRA in the acute setting ( 28). MRI combined with MRA holds great promise for evaluation of unruptured aneurysms. The non- invasive nature of MRA and the lack of need for ionizing radiation or intravenous contrast make it ideal for screening asymptomatic patients at high risk for aneurysm ( 28). High- risk populations include those with polycystic kidney disease, Ehlers- Danlos syndrome, cerebral arteriovenous malformations, fibromuscular dysplasia, coarctation of the aorta, and a strong family history of intracranial aneurysms. Arteriovenous Malformations and Dural Arteriovenous Fistulas MRA has been used extensively as an adjunct in the evaluation of arteriovenous malformations. Clinically important information concerning arterial feeders, nidal location, draining veins, flow direction, and flow velocity can be obtained from MRA studies ( 28). However, poor temporal or phasic resolution and the inability to visualize the pattern of segmental blood supply preclude its use as a replacement for DSA. Other important limitations of this technique include a signal void within tortuous feeding arteries created by complex flow, the inability to differentiate flow from methemoglobin within a subacute hematoma, and the lack of visualization of slow venous flow as a result of progressive- spin saturation ( 41). The diagnostic capability of MRI and MRA for dural arteriovenous fistulas ( AVFs) remains uncertain. Spin- echo MR detects only 15% of AVFs ( 28,42) and poorly defines their exact location. In this regard, Noguchi et al ( 42) have addressed the usefulness of combined 3D TOF and CE MRA. In their study, multiple areas of high signal intensity adjacent to the sinus wall findings were observed in all cases of dural AVF on 3D TOF MRA. The sensitivity and specificity of this sign ( multiple high- intensity structures adjacent to the sinus walls) was 100%. These high- intensity curvilinear or nodular structures adjacent to the sinus wall likely represent the meningeal branches of the feeding arteries. In 13 of their 15 patients, findings of early filling of the venous sinus were observed on CE MRA. TOF MRA has been also studied as a tool for detection of residual or recurrent AVFs after treatment. In a study of 14 previously treated patients with cavernous dural AVF by Hirai et al ( 43), MRA had a sensitivity of 100% and a specificity of 80% o in the detection of residual/ recurrent AVF. The source images of 3D TOF MRA are more useful than the MIP images in the diagnosis of dural AVF because these source images are able to display tiny vascular pedicles that may be lost during the processing of MIP images ( 28). Cervicocranial Arterial Dissections Although DSA can detect luminal narrowing, irregularity, occlusion, and pseudoaneurysms associated with dissections, the same findings may be depicted on MRA. Moreover, MRA has the added advantage of being able to directly visualize the vessel wall, including intramural hematomas ( 44) ( Fig. 3). Using DSA as the gold standard in patients with carotid dissection, Levy et al ( 45) have shown 310 © 2004 Lippincott Williams & Wilkins CT and MRA in Cervicocranial Vascular Disease JNeuro- Ophthalmol, Vol. 24, No. 4, 2004 FIG. 3. Magnetic resonance imaging and magnetic resonance angiography in vertebral artery dissection. A 32- year-old man had acute onset left neck pain and dizziness. A. Axial T1W magnetic resonance imaging of the neck reveals a crescentic hyperintensity in the wall of left vertebral artery suggesting an intramural hematoma { arrow). B. Time- of-flight magnetic resonance angiography demonstrates vessel caliber change and a well- defined intimal flap ( arrows). a sensitivity and specificity for MRI of 84% and 99% and for MRA of 95% and 99%, respectively. MRI and MRA are also helpful in follow- up of patients with carotid dissections by monitoring the resolution of an intramural hematoma or early detection of complications of dissection ( 28). The role of MRI and MRA in the evaluation of vertebrobasilar dissections is evolving. Levy et al ( 45) reported a sensitivity and specificity of 20% and 100% for MRA and 60% and 58% for MRI in the detection of vertebrobasilar dissections. However, better sensitivity has been reported by other authors ( 44,46). At our institution, MRI combined with MRA is the method of choice for evaluation of suspected carotid and vertebrobasilar dissections. We reserve CTA or DSA for equivocal MR results and for those who are unable to undergo an MR examination. Cerebral Venous Disorders MRV has replaced DSA for the evaluation of venous thrombosis ( 47). The venous system has traditionally been evaluated with the 2D TOF technique. However, drawbacks of this method include incorporation of the high signal intensity of methemoglobin within a thrombus that can mimic flow on MRA. In addition, non- uniform flow and in- plane flow within the sinuses can result in artifactual areas of filling defects ( clots) or stenosis within the sinuses ( Fig. 4). Addition of the PC technique can help to avoid these pitfalls ( 28). More recently, 3D CE MRV has become a valuable addition for the evaluation of intracranial venous disease ( 47). Distinct advantages of this technique over TOF and PC techniques are superior vessel depiction, greater suppression of background signal, and substantially shortened FIG. 4. Magnetic resonance venography ( MRV) in suspected dural sinus thrombosis. A 50- year- old woman reported acute intractable headache. A. Maximum intensity projection of two- dimensional time- of- flight ( TOF) MRV reveals absence of visualization of left transverse sinus, suspicious for thrombosis ( arrows). B. Contrast- enhanced three-dimensional MRV reveals widely patent transverse sinus ( arrows), indicating that the two- dimensional TOF MRV findings represented artifact. Flow- related artifacts associated with TOF technique are not encountered with contrast- enhanced MRV. 311 JNeuro- Ophthalmol, Vol. 24, No. 4, 2004 Gandhi imaging time. Because this technique is flow- insensitive, flow artifacts associated with the TOF technique are not encountered ( 47,48). CHOOSING THE APPROPRIATE TEST: CTA VERSUS MRA VERSUS DSA The choice of the imaging test ( Table 1) depends on patient characteristics, the disease condition, the availability of technique and equipment, and the preference of the interpreting physician. Increasingly, CTA and MRA are replacing DSA in the evaluation of many cervicocranial vascular disorders. Evaluation of stroke, atherosclerotic carotid disease, and venous disorders is easily accomplished by either MRA or CTA; the choice depends on individual preference. DSA is mainly used as a problem- solving technique when the CTA/ MRA findings are equivocal or not consistent with clinical symptoms. CTA is preferred for evaluation of critically ill patients such as those with subarachnoid hemorrhage. It may be used as the first- line imaging modality in claustrophobic patients with suspected stroke, carotid stenosis, or venous disease or in those who have other contraindications to magnetic resonance. Unlike MRA, CTA can provide information about the presence of calcifications within the atherosclerotic plaques, a feature that may be valuable to the physician performing angioplasty. Another area in which CTA is superior to MRA is in depiction of the relationship of intracranial aneurysms to adjacent bony structures. However, because MRA does not involve the use of ionizing radiation, it is preferred for younger patients. MRA is also used preferentially in patients with impaired renal function, because CTA requires large amounts of potentially nephrotoxic iodinated contrast media. MRA is the test of choice in screening for cerebral aneurysms and evaluation of dissection of cervicocranial vessels. CTA is continuing to improve, but currently this technique must be considered investigational in the assessment of dissection and any traumatic vascular lesion. DSA is still the " gold standard test" for the evaluation of arteriovenous malformations, arteriovenous fistulas, and for demonstrating small vessel alterations in patients with vasculitis. A large number of centers, including our own, TABLE 1. Choosing the Carotid athero- stenosis Ruptured intracranial aneurysms Screening for unruptured aneurysms Stroke Blunt neck injuries Penetrating neck injuries AVMs and AVFs Vasculitis Venous disorders appropriate test 1. 2. 1. 1. 2. 1. 2. 1. 2. 1. 2. 1. 1. 2. 1. 2. Preferred test* CTA CEMRA DSA MRA CTA MOTSA and CE MRA with DWI/ PWI CTA with CT perfusion DSA MRA DSA CTA DSA MRI brain with contrast DSA MRV ( preferably contrast enhanced) CTV Alternative test DSA ( largely reserved for problem- solving) CTA DSA DSA ( largely reserved for intra- arterial thrombolysis) CTA ( less sensitive and specific) MRA ( not useful if there are retained metallic foreign bodies and gunshot fragments) CE MRA and MOTSA DSA ( largely reserved for problem solving and for thrombolytic therapy) * Preferred tests have been ranked in the order of usefulness. CTA, computed tomography angiography; CE MRA, contrast- enhanced magnetic resonance angiography; DSA, digital subtraction angiography; MOTSA, multiple overlapping thin- slab angiography; DWI, diffusion-weighted imaging; PWI, perfusion- weighted imaging; AVM, arteriovenous malformation; AVF, arteriovenous fistula; MRI, magnetic resonance imaging; CTV, computed tomography venography. 312 © 2004 Lippincott Williams & Wilkins CT and MRA in Cervicocranial Vascular Disease JNeuro- Ophthalmol, Vol. 24, No. 4, 2004 continue to use DSA as the standard test for evaluation of subarachnoid hemorrhage. However, it is reasonable to use CTA in place of DSA in these patients as long as the potential pitfalls of CTA are understood. These include relative insensitivity for the detection of aneurysms smaller than 3 mm and thrombosed aneurysms. It may be difficult on CTA to distinguish between an infundibular dilatation at the posterior communicating artery origin and true aneurysm. 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