Korean Journal of Cerebrovascular Surgery 2006;8(3):172-177.
Published online September 1, 2006.
CT Angiography in Acute Ischemic Stroke: Clinical Research.
Park, Ho Kwon , Kang, Hyun Seung , Chang, Sang Keun , Moon, Chang Taek , Cho, Joon , Choe, Woo Jin , Noh, Hong Gee , Koh, Young Cho
1Department of Neurosurgery, Konkuk University Hospital, Seoul, Korea. hskang@kuh.ac.kr
2Department of Neuroradiology, Konkuk University Hospital, Seoul, Korea.
Abstract
OBJECTIVE
This study was designed to evaluate the feasibility and clinical implications of CT angiography (CTA) in patients with acute ischemic stroke. METHODS: From August 2004 to July 2005, 24 cases of acute ischemic stroke were prospectively included in this study. We checked location of ischemic parenchymal lesion, location of vascular occlusion, degree of collateral supply, and presence of other accompanying vascular lesions on CT and CTA, and assessed the usefulness of CTA by comparing the findings with those of diffusionweighted MR imaging and digital subtraction angiography. RESULTS: Average time required for performing CT and CTA and getting reconstructed images was 30 minutes. Location of the parenchymal lesions and the corresponding occluded or stenosed artery could be clarified in 16 cases (67%) and 20 cases (83%), respectively. There were 13 cases of severe stenosis and 7 cases of occlusion. In 7 cases of major arterial occlusion, degree of collateral circulation could be assessed as good in 5, and moderate in 2. Incidental unruptured intracranial aneurysms were identified in 5 cases. CONCLUSION: CTA could provide valuable information regarding locations of parenchymal lesion and vascular occlusion, degree of collateral supply, and presence of accompanying intracranial aneurysm in cases of acute ischemic stroke without significant time delay, thereby guiding therapeutic plan.
Key Words: Acute ischemic stroke, Collateral supply, CT angiography, Vascular occlusion

Introduction


  
Faced with a growing senile population of the society, the interest regarding stroke is raising. Because the therapeutic time window is restricted, time to diagnose and start treatment is one of the most important factor in management of acute ischemic stroke.3)10) In general, non-contrast brain computed tomography (CT) is the first diagnostic imaging tool. Brain CT in acute ischemic stroke may provide early ischemic signs such as obscuration of the lentiform nucleus, loss of insular ribbon, diffuse parenchymal hypodensity with sulcal effacement, and hyperdense middle cerebral artery (MCA) or hyperdense sylvian fissure MCA "dot "sign, and may help to exclude intracranial hemorrhage or other space occupying lesions.1)11)12) However, early infarction on CT is not well recognized even by experienced physicians.14) Moreover, CT has limitations in revealing arterial obstruction itself, collateral blood supply and ischemic penumbra. 
   Conventional digital subtraction angiography (DSA) is the standard technique for evaluation of vascular lesion, but has shortcomings f invasiveness and accompanying risks.9)16) In addition, it takes time to recruit personnel concerned to perform the examination in most f institutions. Magnetic resonance (MR) imaging is a non-invasive and reliable way to detect acute ischemic lesions and to evaluate cerebral vasculature but requires a highly cooperative patient and cannot be performed in patients with pacemakers or metallic implants.9) In many institutions, it is difficult to use MR imaging in stroke patients during the hyperacute stage (e. g. within an hour). The purpose of this study was to evaluate the feasibility of CT angiography (CTA) in predicting location of the culprit parenchymal and vascular lesions, assessing degree of collateral blood supply in cases of major arterial occlusion, and finding accompanying vascular lesions such as intracranial aneurysms. 

Materials and Methods

CTA-based protocol for stroke evaluation 
   During the period between August 2004 and July 2005 the imaging protocol for acute ischemic stroke patients included non-contrast CT, CTA, and, if available, diffusion-weighted MR (dMR) imaging in our institution. Emergency dMR imaging, MR angiography, perfusion studies using MR or CT, and DSA were not available during this period at our institution. When a patient of suspected acute stroke visited to emergency unit, non-contrast CT was performed immediately after physical and neurologic evaluation to diagnose acute ischemic infarction and to exclude other pathologies including intracerebral hemorrhage. If no hemorrhage was seen, CTA was done without delay after getting permission for usage of contrast agent, and it covered intracranial and cervical areas. Based on information from CT and CTA, we made a decision whether thrombolysis would be required. If available, dMR imaging was performed and DSA was done in cases undergoing intraarterial (IA)thrombolysis (Fig. 1). 

CTA Examination 
   A commercially available CT machine (Somatom volume zoom 4 channel multidetector CT, Siemens, Germany) was used in this study. Slice thickness was 5 mm throughout the brain in non-contrast CT examination. CTA was performed on the same scanner immediately after the initial CT by spiral scanning with intravenous bolus administration of a nonionic contrast medium (Xenetix 350, Guerbet, Aulnay-sous-Bois, France). The following scan parameters were used in all patients:spiral pitch, 1. 25;21 tube revolutions; tube voltage, 120 kV;and tube amperage, 250 mA. Slice thickness was 1. 0 mm with an index of 1. 0 mm. Patients were scanned from the foramen magnum toward the vertex with IV injection of 120 mL contrast medium at a rate of 4 mL/sec with a mechanical injection pump. When the neck CTA was needed, the scanning was extended to aortic arch with additional 150 mL contrast medium. 
   The spiral data were transferred to a workstation, and 3D reconstruction was performed with a commercially available software (Rapidia 3D, Infinitt, Korea) using volume rendering (VR) and maximal intensity projection (MIP) algorithms by experienced technicians. 

Patient population 
   Twenty-four cases of acute ischemic stroke, who presented within 24 hours of symptom onset, (mean 9.7 hours), were prospectively included in this study (Table 1). Interval from symptom onset to presentation was 3 hours or less in 9 cases. There were 15 men and 9 women with a mean age 64.6 ±11.5 years (range, 43 to 85 years). The National Institutes of Health Stroke Scale (NIHSS) score at presentation ranged from 0 to 24 (mean 7). 

Data analysis
   We evaluated predictability for brain parenchymal and vascular lesions of CT and CTA, including source images, by using dMR imaging and DSA. Criteria for acute ischemic lesions in CT included parenchymal hypoattenuation, loss of gray-white matter distinction, loss of basal ganglia outline, loss of insular ribbon, cortical sulcal effacement, and hyperattenuation of vessel (including hyperdense MCA sign and hyperdense sylvian fissure MCA dot sign).1)13)15) When available, source images f CTA were evaluated, and absence or sparsity f vascular enhancement was considered as the ischemic lesion. Criteria for vascular culprit lesions in CTA included major arterial occlusion or stenosis more than 50% in diameter. 
   Collateral blood supply in cases of major arterial occlusion was graded according to the number of arteries from the leptomeningeal vessels and/or the circle of Willis on CTA MIP images and then was compared with DSA. We graded the collateral blood supplies more than 75%, 50 to 75% and less than 50% of the contralateral normal side beyond the occlusion as having good, moderate, and poor collateral supply, respectively, similar to the technique used by Knauth et al.5) All assessment was performed by two neurosurgeons by consensus who were blinded for patient information. 

Results

Evaluation of parenchymal lesions
   Time interval from patient arrival to performance of CTA with its reconstruction was 30 minutes or less in all the cases. Presence of the parenchymal lesions, although they were subtle in most f cases, could be suspected in 16 cases (67%) with non-contrast CT (Table 1). Old ischemic lesions were found in 2 cases, and no evidence of ischemic lesions could be identified in 6 cases with non-contrast CT. Source images of CTA were available in 5 cases, and gave additional supportive evidence of acute ischemic infarction in 2 cases (Fig. 2). 
   In 16 cases the lesions were suspected with non-contrast CT and/or CTA source images. In the ther eight cases CT failed to give information regarding parenchymal lesions.


Evaluation of vascular lesions
   Arterial steno-occlusive lesions could be identified on CTA in all the cases, and the lesions were considered responsible for the presenting symptoms in 20 cases (83%; severe stenosis in 13 cases and occlusion in 7 cases)(Fig. 3). Among eight cases undergoing DSA, the lesions predicted by CTA were not correlated with those of DSA in 2 cases (Case 5 and Case 14). In Case 5, CTA did not disclose any vascular lesion except non-visualization of A1 segment of left anterior cerebral artery (ACA). Due to atrial fibrillation, the patient was under heparinization. However, the patient deteriorated 4 days later with irregular rapid ventricular response on electrocardiogram. DSA at this moment revealed the right MCA occlusion. 
   In another case (Case 4), CTA disclosed internal carotid artery (ICA) occlusion with maintained ipsilateral MCA flow via the anterior communicating artery. However, DSA, which was done 4 hours later, revealed ICA and MCA occlusions due to progression of thrombosis and the patient was underwent IA thrombolysis (Fig. 4). 
   In 7 cases f major arterial occlusion (MCA occlusion in 4 and ICA occlusion in 3) on CTA, degree of collateral circulation was assessed by number of arteries on distal MCA branches as good in 5 cases and moderate in 2 cases. Three cases underwent DSA and showed good correlation with CTA findings (Fig. 5). There was no progression to syndrome f malignant MCA infarction (i. e. , early mortality after large MCA infarction due to acute brain swelling4) among the seven cases showing good to moderate collateral supply on CTA. 
   Incidental unruptured intracranial aneurysms were identified in 5 cases although these findings did not influence therapeutic decision. 

Discusesion

   A rapid, minimally invasive and comprehensive imaging tool of the cerebrovascular system during the acute phase of stoke may be of critical importance. Our study showed that CTA could be a useful diagnostic tool in cases suspected of acute ischemic stroke, a situation for which a rapid treatment decision was required. It could be performed immediately after non-contrast CT. It could be performed safely in critically ill or uncooperative patients without delay. 
   Parenchymal lesions in patients suspected of acute ischemic stroke are not always easy to recognize.7)14) According to Wardlaw,14) only 65%of the CT scans of stroke patients were correctly identified as normal or abnormal. The prevalence of early ischemic changes on baseline CT obtained within 3 hours of symptom onset was only 31%.7) In our series, presence of acute parenchymal lesions could be suspected in 67%(16/24) of cases, which was comparable to previous study.15) In a review, the prevalence of early infarction signs on CT scans obtained within 6 hours after onset f stroke symptoms was 61%±21 without knowledge of symptoms.15) In cases showing subtle r atypical lesions, review of CTA source images gave supportive evidence of acute ischemic infarction by sparsity of vascular enhancement (Fig. 2). Diffusion MR imaging invariably showed acute ischemic lesions in our series (Table 1). We believe that dMR imaging, if available without time delay, should be the preferred tool in cases suspected of acute ischemic stroke;however, CTA could provide the useful information regarding presence of acute ischemic lesions where immediate dMR imaging was not available. 
   For evaluation of the vascular lesions in patients with acute ischemic stroke, CTA could provide useful information, influencing subsequent therapeutic decision making process. In our series, culprit vascular lesions could be revealed in 83%(20/24) of cases. There were 2 cases showing discrepancy between CTA and DSA due to dynamic nature of acute ischemic stroke, rather than due to technical or methodological limitations (Fig. 4). 
   The status of collateral circulation in acute stroke has prognostic implications.6) Primary collaterals include the arterial segment of the circle of Willis, whereas the ophthalmic artery and leptomeningeal vessels constitute secondary collaterals. The process of collateral recruitment depends on the caliber and patency of primary pathways that may rapidly compensates for decreased blood flow and the adequacy of secondary collateral routes. Robust leptomeningeal collaterals have been linked with rapid recanalization of MCA occlusion and possible prevention of larger infarctions.8) Christoforidis et al.2) claimed that evaluation of pial collateral formation before thrombolytic treatment could predict infarct volume and clinical outcome for patients with acute stroke undergoing thrombolysis independent of other predictive factors. CTA could provide information regarding degree of collaterals via primary and secondary pathways in ICA occlusions or secondary collaterals in MCA occlusions. In all the 7 cases which showed major arterial occlusion with good to moderate degree of collateral supply on CTA, there were no progressions to syndrome of malignant MCA infarction in our study. 

Conclusion

   Our study showed that performance of CTA was feasible in cases of acute ischemic stroke without significant time delay. Information regarding brain parenchymal lesions, location of steno-occlusive vascular lesions, degree of collateral supply, and presence of accompanying cerebral aneurysms could be obtained by CTA, guiding therapeutic plan in these emergent cases. In a less equipped institution, the diagnostic approach using CTA might be a good method for evaluation of patients with acute ischemic stroke. 


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