Application of 3D-PCASL combined with t-ASL and MRA in the diagnosis of patients with isolated vertigo induced by posterior circulation ischemia.

OBJECTIVES: Isolated vertigo induced by posterior circulation ischemia (PCIV) can further progress into posterior circulation infarction. This study aimed to explore the diagnostic values of three-dimensional pseudo-continuous arterial spin labeling (3D-PCASL) combined with territorial arterial spin labeling (t-ASL) and magnetic resonance angiography (MRA) in visualizing and evaluating PCIV, seeking improved diagnostic tools for clinical guidance. METHODS: 28 PCIVs (11 males, 17 females, aged from 55 to 83 years, mean age: 69.68 ± 9.01 years) and 28 healthy controls (HCs, 12 male, 16 female, aged from 56 to 87 years, mean age: 66.75 ± 9.86 years) underwent conventional magnetic resonance imaging (MRI), diffusion-weighted imaging (DWI), MRA, 3D-PCASL, and t-ASL. We compared the incidence of anatomic variants of the posterior circle of Willis in MRA, cerebral blood flow (CBF) and anterior collateral blood flow on postprocessing maps obtained from 3D-PCASL and t-ASL sequence between PCIVs and HCs. Chi-square test and paired t-test were analyzed statistically with SPSS 24.0 software. RESULTS: 7 PCIVs (7/28, 25%) and 6 HCs (6/28, 21%) showed fetal posterior cerebral artery (FPCA) on MRA, including 1 HC, and 6 PCIVs with FPCA appeared hypoperfusion. 18 PCIVs (64%) and 2 HCs (7%) showed hypoperfusion in the posterior circulation (PC), including 1 HC and 7 PCIVs displayed anterior circulation collateral flow. Chi-square analyses demonstrated a difference in PC hypoperfusion between PCIVs and HCs, whether in the whole or FPCA-positive group assessment (P < 0.05). Paired t-test showed that the CBF values were significant difference for the bilateral PC asymmetrical perfusion in the PCIVs (P < 0.01). When compared to the bilateral PC symmetrical non-hypoperfusion area in the PCIVs and HCs, the CBF values were not significant (P > 0.05). The CBF values of the PC in PCIVs were lower than in HCs (P < 0.05). The reduction rate in the hypoperfusion side of the bilateral PC asymmetrical perfusion of the PCIVs ranged from 4% to 37%, while the HCs reduction rate was 7.7%. The average PC symmetrical perfusion average reduction rate of the PCIVs was 52.25%, while the HCs reduction rate was 42.75%. CONCLUSION: 3D-PCASL is a non-invasive and susceptible method for detecting hypoperfusion in PC, serving as a potential biomarker of PCIV. The suspected hypoperfusion in PC may be attributed to the emergence of FPCA and the manifestation of anterior collateral flow when combining t-ASL and MRA sequences. These findings demonstrated that 3D-PCASL combined with t-ASL and MRA sequences are the potential method to identify PCIV, leading to early diagnosis of PCIV and reducing the risk of progressing into infarction.

Association of brain arterial diameters with demographic and anatomical factors in a multi-national pooled analysis of cohort studies.

BACKGROUND AND PURPOSE: Brain arterial diameters are markers of cerebrovascular disease. Demographic and anatomical factors may influence arterial diameters. We hypothesize that age, sex, height, total cranial volume (TCV), and persistent fetal posterior cerebral artery (fPCA) correlate with brain arterial diameters across populations. METHODS: Participants had a time-of-flight MRA from nine international cohorts. Arterial diameters of the cavernous internal carotid arteries (ICA), middle cerebral arteries (MCA), and basilar artery (BA) were measured using LAVA software. Regression models assessed the association between exposures and brain arterial diameters. RESULTS: We included 6,518 participants (mean age: 70 ± 9 years; 41% men). Unilateral fPCA was present in 13.2% and bilateral in 3.2%. Larger ICA, MCA, and BA diameters correlated with older age (Weighted average [WA] per 10 years: 0.18 mm, 0.11 mm, and 0.12 mm), male sex (WA: 0.24 mm, 0.13 mm, and 0.21 mm), and TCV (WA: for one TCV standard deviation: 0.24 mm, 0.29 mm, and 0.18 mm). Unilateral and bilateral fPCAs showed a positive correlation with ICA diameters (WA: 0.39 mm and 0.73 mm) and negative correlation with BA diameters (WA: -0.88 mm and -1.73 mm). Regression models including age, sex, TCV, and fPCA explained on average 15%, 13%, and 25% of the ICA, MCA, and BA diameter interindividual variation, respectively. Using height instead of TCV as a surrogate of head size decreased the R-squared by 3% on average. CONCLUSION: Brain arterial diameters correlated with age, sex, TCV, and fPCA. These factors should be considered when defining abnormal diameter cutoffs across populations.

Prevalence and Incidence of Carotid-Fetal-Posterior Syndrome.

INTRODUCTION: Carotid-fetal-posterior (CFP) syndrome is a posterior cerebral artery (PCA) territory ischemic stroke/TIA caused by symptomatic ≥50% carotid stenosis or occlusion via fetal posterior communicating artery. We aimed to assess the incidence of CFP syndrome and prevalence of CFP syndrome among symptomatic ≥50% carotid stenosis or occlusion as these are unknown. METHODS: We reassessed consecutive CTAs from 4,042 persons and included locally admitted patients with ≥50% carotid stenosis or occlusion. These were assessed for symptoms and signs of possible posterior circulation stroke/TIA (suspicion of CFP syndrome). Among these, those with unilateral PCA territory stroke/TIA, ipsilateral stenosis, and fetal/fetal-type PCA were considered CFP syndrome. RESULTS: We included 208 locally admitted patients with ≥50% carotid stenosis or occlusion; 33 (16%) patients had suspicion of CFP syndrome, of which 3 (9%) had CFP syndrome. The prevalence of CFP syndrome was 2.9% of symptomatic ≥50% carotid stenosis or occlusion; incidence was 4.23 per 1,000,000 person-years. Also, we found a lower prevalence of CFP syndrome (0.9%, p = 0.047) among referred patients with symptomatic ≥50% carotid stenosis or occlusion than among locally admitted patients with symptomatic ≥50% carotid stenosis or occlusion. DISCUSSION/CONCLUSION: CFP syndrome has a low incidence and low prevalence among symptomatic carotid stenosis cases. Given lower prevalence of CFP syndrome among referred cases than local, CFP syndrome seems susceptible to underdiagnosis. On the other hand, few cases with suspicion of CFP syndrome had CFP syndrome, why CFP syndrome also seems susceptible to overdiagnosis if detailed assessment is not employed.

Exclusively Posterior Circulation Stroke Caused by Internal Carotid Artery Thrombosis.

A fetal posterior cerebral artery (FPCA) is an anatomic variant in which the posterior cerebral artery (PCA) is an embryological derivative of the internal carotid artery (ICA). Patients with FPCA may experience posterior circulation stroke (PCS) after a thrombotic event in the ICA system, while exclusively PCS caused by thrombosis of the ICA has rarely been reported. We report a patient with FPCA and summarize 3 types of exclusively PCS caused by FPCA due to thrombotic events in the ICA system. Type A: the thrombus involves the opening of the FPCA and obstructs the blood flow of the entire ICA. The contralateral ICA compensates the ipsilateral middle cerebral artery (MCA) and anterior cerebral artery (ACA) through the anterior communicating artery (ACOM). Type B: the thrombus involves the opening of the FPCA but does not block the blood flow of the entire ICA, which still perfuses the ipsilateral ACA and MCA. Type C: the thrombus only involves the FPCA and not the ipsilateral ICA. Patients with types A and B may obtain a good prognosis through endovascular treatment (EVT), while the benefits of this procedure in type C patients are unclear.

Duplicated fetal posterior cerebral artery in a patient with a ruptured fetal posterior cerebral artery aneurysm: a cerebrovascular variant.

BACKGROUND: The most common neurovascular variant is the fetal posterior cerebral artery (FPCA), in which the P1 branch is absent or hypoplastic, and the majority of P2 supply is derived from the anterior circulation. While there are reports of hyperplastic anterior choroidal arteries (AChA) with supply to the temporo-occipital and calcarine regions, no reports of a duplicated FPCA exist. METHODS: This case report describes a patient with a ruptured right FPCA aneurysm. Digital subtraction angiogram (DSA) revealed an artery with origin distal to the FPCA associated with the aneurysm. This was not consistent with a typical AChA. The FPCA associated with the aneurysm had the typical origin, course, and supply of a FPCA. The distal FPCA had a similar course of a typical FPCA with significant supply to the typical PCA territory. The patient underwent successful clipping of the aneurysm, and the duplicated FPCA was identified during the craniotomy. RESULTS: The features of this duplicate FPCA, which has not been previously described, are discussed in comparison to another variant, the hyperplastic, anomalous AChA. The artery described in this report does not fit the typical criteria of this AChA variant. Therefore, the authors outline this variant as a duplicated FPCA. CONCLUSION: Recognition of variant cerebrovascular anatomy is vital to neurosurgeons and interventional neuroradiology specialists. FPCA aneurysms require special management considerations and are often more challenging to treat. This report discusses a duplicated FPCA. To our knowledge, this is the first description of this variant. A duplicated FPCA carries important management considerations in the management of neurovascular pathology.

Mechanical thrombectomy for acute fetal posterior cerebral artery occlusion with a hidden unruptured cerebral aneurysm: illustrative case.

BACKGROUND: Fetal posterior cerebral artery occlusion is rare and often presents with severe neurological symptoms. Although acute recanalization therapy is commonly used for cerebral vessel occlusion, unruptured cerebral aneurysms can be hidden distal to the occluded vessels. OBSERVATIONS: An 87-year-old man presented with consciousness disturbance and right hemiparesis. The authors diagnosed left fetal posterior cerebral artery occlusion and performed mechanical thrombectomy. A stent retriever was deployed from the middle cerebral artery M1 segment across the mural thrombus of the internal carotid artery. After the first pass, the fetal posterior cerebral artery remained occluded, with confirmation of a contrast effect around the thrombus. Because the anatomical course of the fetal posterior cerebral artery was unidentified, the procedure was stopped. At 1-week recovery, magnetic resonance imaging revealed complete recanalization and a fetal posterior cerebral artery aneurysm hidden within the occluded site. Blood flow was directed to the aneurysm, and the thrombus within the aneurysm simultaneously occluded the fetal posterior cerebral artery. LESSONS: To avoid critical complications following mechanical thrombectomy for fetal posterior cerebral artery occlusion, hidden aneurysms should be suspected when a "fried egg-like" contrast effect is observed around the thrombus.

Mechanisms of Stroke in Patients with Fetal Posterior Cerebral Artery.

OBJECTIVES: The fetal-type posterior cerebral artery (FTP) is a normal variation of the posterior cerebral artery (PCA), occurring in 3-36% of general population. The effects of the FTP in stroke mechanisms remain debatable. We aimed to investigate the differences in stroke mechanisms and lesion locations between patients with and without FTP. METHODS: A total of 394 patients with PCA territory stroke were divided into those with PCA ischemic stroke associated with ipsilateral FTP and those without. The baseline characteristics, vascular risk factors, infarct pattern, stroke location, stroke etiology, and the diameter of P1 in patients without FTP or posterior communicating artery (PcoA) in patients with FTP were investigated. RESULTS: Among the 394 patients, 52 (13.2%) PCA stroke patients with ipsilateral FTP were enrolled. Patients with FTP, in comparison with those without, had a higher frequency of deep infarct pattern (69.2% vs. 47.1%, P=0.012), small vessel occlusion (51.9% vs. 28.9%, P=0.009), ventrolateral thalamic involvement (65.4% vs. 49.1%, P=0.042), and a lower frequency of cardiac embolism (9.6% vs. 24.0%, P=0.009). The diameter of P1 in patients without FTP was larger than that of PcoA in patients with FTP (2.1 ± 0.3 mm vs. 1.6 ± 0.2 mm, P <0.001). CONCLUSIONS: The stroke mechanisms, infarct pattern, and the location of the stroke were different between patients with and without FTP. PCA stroke patients with FTP more often had small vessel occlusion and ventrolateral thalamic involvement. These results are probably associated with difference in the hemodynamic status according to the presence of FTP.

Apparent posterior cerebral artery territory perfusion asymmetry on arterial spin labeling MRI is a common non-pathologic finding in patients with a unilateral fetal posterior cerebral artery.

PURPOSE: To determine the frequency of apparent posterior cerebral artery (PCA) territory asymmetry seen on arterial spin labeling (ASL) imaging in patients with a unilateral fetal PCA, but without underlying clinical or imaging pathology to suggest true hypoperfusion. METHODS: A search of radiology reports from 1/2017 through 6/2020 was performed with the inclusion term "fetal PCA." Eighty patients were included with unilateral fetal PCA confirmed on MRA or CTA, with brain MRI including ASL imaging, and without conventional imaging abnormality or clinical symptoms referable to the PCA territories. Cases were evaluated by two independent readers for visually apparent PCA perfusion asymmetries. ASL imaging consisted of pseudocontinuous ASL with 1.5 s labeling duration and 2 s post-labeling delay adapted from white paper recommendations. RESULTS: Thirteen of 80 cases (16.2%) had apparent hypoperfusion in the PCA territory contralateral to the side of the fetal PCA. Agreement between readers was near perfect (97.5%, κ = 0.91). This finding was more common in patients who were older, scanned on a 3.0 T magnet, and who had non-visualization of the posterior communicating artery contralateral to the fetal PCA. CONCLUSION: Apparent PCA hypoperfusion on ASL is not uncommon in patients with a contralateral fetal PCA who have no clinical or conventional imaging findings to suggest true hypoperfusion. This phenomenon is likely due to differential blood velocities between the carotid and vertebral arteries that result in differential arterial transit times and labeling efficiency. It is important for radiologists to know that apparent hypoperfusion may arise from variant circle of Willis anatomy.

Coil embolization with overlapping horizontal low-profile stents to treat a giant thrombosed fetal posterior cerebral artery aneurysm using contralateral approach through anterior communicating artery: Case report.

BACKGROUND: The treatment of internal carotid artery (ICA) - posterior communicating artery aneurysms (ICPC aneurysms) is challenging when a fetal posterior cerebral artery (PCA) arises from the saccular neck. This complex angioarchitecture renders endovascular approaches difficult. Giant thrombosed IC-PC aneurysms are also hard to treat by endovascular coiling because its flow-diversion effect is insufficient. CASE DESCRIPTION: We report the first case of a ruptured giant thrombosed IC-PC aneurysm associated with a fetal PCA that was successfully treated by coil embolization with retrograde overlap horizontal stenting using low-profile stents introduced through the contralateral ICA. The aneurysm was completely occluded and follow-up MRI scans demonstrated the reduction of the aneurysmal size. CONCLUSION: Our technique is advantageous because low-profile stents can be used to treat lesions not accessible with flow-diverter stents due their presence in complex angioarchitectures, and overlap stenting may have flow-diversion effects that can result in the complete occlusion of giant thrombosed aneurysms.

Acute occlusion of the fetal posterior cerebral artery: diagnosis and management paradigms.

BACKGROUND AND PURPOSE: The diagnosis and management of acute fetal posterior cerebral artery occlusion are challenging. While endovascular treatment is established for anterior circulation large vessel occlusion stroke, little is known about the course of acute fetal posterior cerebral artery occlusions. We report the clinical course, radiological findings and management considerations of acute fetal posterior cerebral artery occlusion stroke. METHODS: We performed a retrospective review of consecutive patients presenting with acute large vessel occlusion who underwent cerebral angiogram and/or mechanical thrombectomy between January 2015 and January 2021. Patients diagnosed with fetal posterior cerebral artery occlusion were included. Demographic data, clinical presentation, imaging findings and management strategies were reviewed. RESULTS: Between January 2015 and January 2021, three patients with fetal posterior cerebral artery occlusion were identified from 400 patients who underwent angiogram and/or mechanical thrombectomy for acute stroke (0.75%). The first patient presented with concomitant fetal posterior cerebral artery and middle cerebral artery occlusions. Thrombectomy was performed with recanalisation of the fetal posterior cerebral artery but the patient died from malignant oedema. The second patient presented with isolated fetal posterior cerebral artery occlusion. No endovascular intervention was performed and the patient was disabled from malignant posterior cerebral artery infarct. The third patient presented with carotid occlusion and was found to have fetal posterior cerebral artery occlusion after internal carotid artery recanalisation. No further intervention was performed. The patient was left with residual contralateral homonymous hemianopia and mild left sided weakness. CONCLUSION: Fetal posterior cerebral artery occlusion is a rare, but potentially disabling, cause of ischaemic stroke. Endovascular treatment is feasible. Further investigation is needed to compare the efficacy of medical versus endovascular management strategies.

Brainstem Stroke Presenting as Wake-Up Diplopia in a Patient With an Incomplete Circle of Willis.

We present the case of a 65-year-old patient who suffered a wake-up brainstem stroke. The only symptom reported by the patient was double vision. Upon examination, she was found to have left internuclear ophthalmoplegia and ipsilateral downbeat nystagmus. Magnetic resonance angiography revealed a unilateral partial fetal posterior cerebral artery and unilateral posterior communicating artery hypoplasia. The patient was ineligible for intravenous thrombolysis: she evolved favorably with anti-platelet medication and was discharged after five days. We put forth a discussion on the clinical significance of these physical exams and magnetic resonance imaging findings.

Clinical features and outcomes of PComA aneurysms originating from fetal posterior communicating arteries in a single institution.

BACKGROUND: The aim of this study was to retrospectively analyze our experience with the patients who underwent surgical treatment of posterior communicating artery (PComA) aneurysms originating from fetal posterior cerebral artery (fPCA) and analyze the risk factors for the postoperative radiological infarction and outcome. METHODS: From 2011 to 2020, we retrospectively reviewed 74 PComA aneurysms originating from fPCA in terms of the clinical and radiological features and obtained the follow-up data from the Department of Neurosurgery, Beijing Tiantan Hospital, Capital Medical University. The relationships between these features and follow-up data were assessed with the univariate and multivariate analysis. RESULTS: In this series, 74 aneurysms were occurring at the origin of fPCAs. All the patients showed complete obliteration of their aneurysms. Full fPCA type tends to be a predictive factor for radiological infarction (univariate χ 2 = 5.873, P = 0.027; multivariate OR = 0.264, P = 0.060). Postoperative radiological infarction (univariate χ 2 = 12.611, P = 0.001; multivariate OR = 6.033, P = 0.043), rupture (univariate χ 2 = 4.514, P = 0.047; multivariate OR = 57.966, P = 0.044), and hypertension (univariate χ 2 = 5.301, P = 0.024; multivariate OR = 24.462, P = 0.029) tend to be the independent predictive factors for poor prognosis at 3 months after discharge. CONCLUSIONS: In conclusion, we report a series of patients harboring aneurysms originating from the fPCA. Surgical clipping is a reliable strategy. Full fPCA type is related to postsurgical infarction. Postoperative radiological infarction, rupture, and hypertension tend to be the independent predictive factor for poor prognosis at 3 months after discharge.

Fetal posterior cerebral artery duplication, true fetal posterior cerebral artery variation and trifurcation anterior cerebral artery association.

We would like to present a case with fetal posterior cerebral artery duplication and anterior cerebral artery trifurcation, which we detected using magnetic resonance angiography. We believe this is the first case defined in the literature. Embryological explanation of posterior cerebral artery variations is discussed in light of the literature.

Endovascular intervention of acute ischemic stroke due to occlusion of fetal posterior cerebral artery.

A fetal posterior cerebral artery (FPCA) is an anatomic variant in which the posterior cerebral artery is an embryological derivative of the internal carotid artery. Although most cases of ischemic strokes in patients with FPCAs involve embolic infarcts, emergent large vessel occlusion of a FPCA is extremely rare. We present two cases of successful endovascular intervention for emergent occlusion of a FPCA, one of which is only the second reported case of a mechanical thrombectomy of a FPCA. We review the embryology of FPCA, the controversy regarding its association with cerebral infarcts, and various approaches used in the treatment of such occlusive lesions.

CTA analysis and assessment of morphological factors related to rupture in 413 posterior communicating artery aneurysms.

Posterior communicating artery (PcomA) aneurysms are frequently encountered, but there are few publications on their morphology. A growing number of aneurysms are incidental findings, which makes evaluation of rupture risk important. Our goal was to identify morphological features and anatomical variants associated with PComA aneurysms and to assess parameters related to rupture. We studied CT angiographies of 391 consecutive patients treated between 2000 and 2014 at a single institution. We determined clinically important morphological parameters and performed univariate and multivariate analysis. There were a total of 413 PComA aneurysms: 258 (62%) were ruptured and 155 (38%) unruptured. Ruptured PComA aneurysms had the potential to cause severe bleeding with IVH and/or temporal ICH (n = 170, 66% of ruptured). The main types of PComA origin were classified as follows: (1) separate (32%), (2) side by side (21%) and (3) a joint neck with the aneurysm (6%). After the multivariate logistic regression, the morphological parameters related to PComA aneurysm rupture were an irregular aneurysm dome, neck diameter, and aspect ratio >1.5. The most marked morphological features of the PComA aneurysms were: saccular nature (99%), infero-posterior dome orientation (42%), infrequency of large or giant aneurysms (4%), narrow neck compared to the aneurysm size, PComA originating directly from the aneurysm neck or the dome (28%), and fetal or dominant PComA on the side of the aneurysm (35%). There were location-related parameters that were more strongly associated with PComA aneurysm rupture than aneurysm size: an irregular aneurysm dome, larger diameter of the aneurysm neck and aspect ratio >1.5.

Duplication of the Posterior Cerebral Artery and the 'True Fetal' Variant.

We present a rare case of bilateral posterior cerebral artery variant anatomy seen in a patient presenting with acute ischemic stroke. An embryological explanation of the variant configuration is discussed along with demonstrative radiologic images and a display of the vascular territory supplied.

Endovascular treatment of posterior communicating artery aneurysms in the presence of the fetal variant of posterior cerebral artery.

Posterior communicating artery (PcomA) aneurysms in the presence of the fetal variant of posterior cerebral artery (PCA) often pose technical challenges for endovascular treatment because of a greater potential for ischemic injury with the fetal PCA compromise. The purpose of this study was to assess the feasibility and results of endovascular treatment for these lesions. We retrospectively reviewed our experience and results of endovascular treatments for a series of nine consecutive patients with PcomA aneurysms occurring at the origin of fetal PCAs at the Department of Neurosurgery of Southwest Hospital, Chongqing, China, between June 2011 and June 2014. Depending on the angiographic findings, location and shape of the aneurysms, various therapeutic strategies were used including coiling by single or double microcatheter, balloon remodeling technique, and single or Y-stenting technique. Overall, fetal PCA was preserved patent in all cases, and complete or near complete occlusion was achieved in 8/9 cases. There was no procedure-related morbidity or mortality. With the exception of one patient who died of pneumonia 6 weeks after treatment, no clinical evidence of neurologic deterioration and hemorrhagic complication was seen during the follow-up period in the remaining 8 patients. Our experience suggests that endovascular treatment is relatively safe and technically feasible in most patients with PcomA aneurysms in the presence of fetal PCA using multiple strategies.

Coiling of a recurrent broad-necked posterior communicating aneurysm incorporating a fetal cerebral artery: A technical case report.

We describe a case of a right broad-necked posterior communicating artery (PcomA) aneurysm with a fetal posterior cerebral artery (PCA) incorporated in the aneurysm neck. We performed waffle cone stent-assisted coiling and achieved almost complete occlusion of the aneurysm. At 18-month follow-up angiography, the aneurysm was recanalized and further coiling was needed. The same waffle cone method of coiling might be expected to produce the same unsatisfactory results on follow-up, so we decided to use retrograde stent navigation and placement through the anterior communicating artery (AcomA) from the contralateral internal carotid artery (ICA) to the ipsilateral ICA and fetal PCA. The stent delivery microcatheter from the contralateral ICA could not be introduced in the AcomA. So 2 mg nimodipine was infused through the stent delivery microcatheter to the AcomA and the stent delivery microcatheter was passed through the AcomA easily and could be navigated to the ipsilateral A1, ICA, and to the fetal PCA. With this additional stent, the aneurysm was completely separated from the fetal PCA and ICA. Further coiling could be performed safely through the microcatheter in the ipsilateral ICA. The patient had stable aneurysm occlusion at the six-month follow up.

Primitive trigeminal artery.

A 29-year-old woman presented with a near-syncopal event, followed by right-sided weakness and numbness as well as dysarthria. The symptoms resolved over several hours. The patient had a history of migraine and cleidocranial dysostosis. Her work-up was negative for stroke and dissection. Computed tomographic angiography (Figure 1, A and B) showed a carotid to basilar artery anastomosis (persistent primitive trigeminal artery). This variant is present in 0.1% to 0.6% of angiograms1. Patients with cleidocranial synostosis may be prone to anomalies of the circle of Willis since they are more likely to harbor cerebral aneurysms (26%).2.