Effects of anterior communicating artery diameter on cerebral hemodynamics in internal carotid artery disease. A model study

Examining Atherosclerosis Patterns in the Circle of Willis: A Case Study of Duplicated Anterior Communicating Artery

The anterior communicating artery (ACoA) plays a pivotal role in maintaining cerebral hemodynamics, as its diameter is a major determinant of blood collateralization through the circle of Willis following internal carotid artery occlusion. While variations of this artery are not uncommon, data on their clinicopathologic relevance are limited. In this report, we present our observation from a fresh cadaver of a male individual who had succumbed to cardiac causes. The circle of Willis displayed a duplicated ACoA with atherosclerosis that predominantly affected the posterior horn while sparing the anterior horn. The anterior horn was characterized by its shorter length and larger diameter compared to the posterior horn. The paper focuses on elucidating the microsurgical anatomy of this particular ACoA variant and exploring potential mechanisms that may underlie the pattern of atherosclerotic distribution within the circle of Willis. Based on this report, while further evidence is needed for confirmation, it is plausible that the existence of a duplicated ACoA may offer a protective mechanism, ensuring uninterrupted collateral circulation in the event of a blockage in one of the horns. Further analysis of the ACoA and its pattern of involvement in intracranial atherosclerosis is warranted, as the atherosclerotic patterns in this region hold clinical and pathological significance.

Development of the Circle of Willis Score (COWS) to help guide decision making during acute tandem occlusion treatment: Preliminary analysis

BACKGROUND

Acute tandem occlusions (TOs) are challenging to treat. Although acute carotid stenting of the proximal lesion is well tolerated, there are certain situations when the practitioner may be wary of acute stenting (bleeding concerns).

OBJECTIVE

The purpose of this study was to retrospectively study patients with tandem occlusions who had re-occlusion of the extracranial ICA and develop a Circle of Willis Score (COWS) to help predict which patients could forego acute stenting.

METHODS

This is a retrospective review of TO patients with a persistent proximal occlusion following intervention (either expected or unexpected). Pre intervention CTA and intraoperative DSA were reviewed, and each patient was assigned a score 2 (complete COW), 1a (patent A1-Acomm-A1), 1p (patent Pcomm), or 0 (incomplete COW). Findings from the DSA took precedence over the CTA. Two cohorts were created, the complete COW cohort (COWS 2) versus the incomplete COW cohort (COWS 1a,1p, or 0). Angiographic outcomes were assessed using the mTICI score (2b-3) and clinical outcomes were assessed using discharge mRS (good outcome mRS 0-3).

RESULTS

Of 68 TO cases, 12 had persistent proximal occlusions. There were 5/12 (42 %) patients in the complete COW cohort, and 7/12 (58 %) in the incomplete COW cohort (5/12 with scores of 1a/1p and 2/12 with a score of 0). In the complete COW cohort, there were 2 ICA-ICA and 3 ICA-MCA occlusions. In the incomplete COW cohort, there was one ICA-ICA occlusion and 6 ICA-MCA occlusions. LKW-puncture was shorter in the complete COW cohort (208 min vs. 464 min, p = 0.16). Successful reperfusion was higher in the complete COW cohort (100 % vs. 71 %). There was a trend toward better clinical outcomes in the complete COW cohort (80 % vs 29 %, p = 0.079).

CONCLUSION

The COWS is a simple score that may help predict a successful clinical outcome without proximal revascularization when concerned about performing an acute carotid stent during TO treatment. Evaluation in larger TO cohort is warranted.

Influence of hydrodynamic and functional nonlinearities of blood flow in the cerebral vasculature on cerebral perfusion and autoregulation pressure reserve

Ensuring the transport of oxygenated blood to the brain is one of the priorities of the human body. In the literature, there are many models of cerebral circulation with different levels of complexity used to assess pathological conditions, support clinical decisions, and learn about the relationships governing cerebral circulation. This paper presents a zero-dimensional cerebral circulation model that considers hydrodynamic nonlinearities and autoregulation mechanisms. The model has been verified using a computational fluid dynamics (CFD) model of the Circle of Willis (CoW) and its supplying and outgoing branches. Despite the considerable simplicity, the presented model captured the dominant features of cerebral circulation and provides good agreement with the CFD model. The errors in relation to the CFD model did not exceed 2.6% and 9.9% for the symmetrical and highly asymmetrical CoW configurations, respectively. The practical application of the model was demonstrated for predicting the autoregulation pressure reserve for different diameters of natural anastomoses: Posterior and Anterior Communicating Arteries. The advantages and limitations of the model were discussed.

Incomplete circle of Willis as a risk factor for intraoperative ischemic events during carotid endarterectomies performed under regional anesthesia - A prospective case-series

Background

The role of the willisian collaterals during carotid endarterectomies (CEAs) is a debated issue. The aim of the present work was to test whether an incomplete or non-functional circle of Willis (CoW) is a risk factor for ischemic events during CEA.

Patients and methods

CEAs were performed under local anesthesia. Patients were considered symptomatic (SY) if neurological signs appeared after the cross-clamping phase. In SY patients shunt insertion was performed. CoW on CT angiograms (CTa) were analyzed offline and categorized as non-functional (missing or hypoplastic collaterals) or functional collaterals by three neuroradiologists. Near-infrared spectroscopy (NIRS) was performed throughout the procedure.

Results

Based on CTa, 67 incomplete circles were found, 54 were asymptomatic (ASY) and 13 were SY. No complete CoW was found among the SY patients. Significant differences could be detected between incomplete and complete circles between ASY and SY groups (Chi-square: 6.08; p = 0.013). The anterior communicating artery was missing or hypoplastic in 5/13 SY cases. There were no cases of the non-functional anterior communicating arteries in the ASY group (Chi-square: 32.9; p = 10^-8). A missing or non-functional bilateral posterior communicating artery was observed in 9/13 SY and in 9/81 ASY patients (Chi-square: 24.4; p = 10^-7). NIRS had a sensitivity of 76.9% and a specificity of 74.5% in detecting neurological symptoms.

Conclusions

Collateral ability of the CoW may be a risk factor for ischemic events during CEAs. Further studies should delineate whether the preoperative assessment of collateral capacity may be useful in decision-making about shunt use during CEA.

Assessment of malformations, variations and diameters of vessels forming the circle of Willis - An autopsy study in a non-cerebrovascular cohort

BACKGROUND A PURPOSE

The collateral capacity of the circle of Willis (CoW) may play an important role in the development of ischemic strokes. The occurrence of classical polygon shows wide geographical variations and morphological data on diameters of the Willisian collaterals are scarce. We aimed to assess CoW variations and vessel diameters in a Central European cohort.

SUBJECTS AND METHODS

CoWs were removed during routine autopsy. The morphological pattern of the circles was recorded. The prepared circles were then put between two glass plates and tightly compressed. The length of the vessels and half of the circumference were measured under a light microscope enabling measurement with an approximation of 0.1 mm. Vessel diameters were calculated from vessel circumference.

RESULTS

A total of 110 circles were analysed. Incomplete circles (missing one or two segments of CoW) were found in 25 cases (22.7%). Any forms of anatomical variations were detected in 14 cases (12.7%). When applying the <1 mm diameter threshold for analysis, 36 anterior communicating arteries (32.7%), 53 right posterior communicating arteries (48.2%), 73 left posterior communicating arteries (66.4%) and 18 posterior communicating arteries (16.3%) on both the sides were considered hypoplastic.

CONCLUSIONS

In patients without stroke in their history, complete CoW may be present in >60% of the cases. Our diameter data may serve as reference values for the Central-European population.

Endarterectomy versus stenting for the prevention of periprocedural stroke or death in patients with symptomatic or asymptomatic carotid stenosis: a meta-analysis of 10 randomized trials

Background

The incidence of stroke or death in carotid endarterectomy (CEA) versus carotid artery stenting (CAS) cannot be estimated accurately. We aimed to compare periprocedural stroke or death in patients with symptomatic or asymptomatic carotid artery stenosis (CS) treated with CEA versus CAS.

Methods

Ten randomized trials (with ≥100 randomized patients per trial) compared the relative effectiveness of CAS and CEA for the prevention of stroke or death.

Results

In the symptomatic group during the periprocedural period, the results showed that the risk of death or any stroke [risk ratio (RR): 0.627; 95% CI: 0.497-0.792; P<0.001] and the risk of any stroke (RR: 0.654; 95% CI: 0.522-0.820; P<0.001) were significantly greater with CAS than with CEA. The difference in the risk of periprocedural stroke was mostly attributed to nondisabling stroke (RR: 0.407; 95% CI: 0.264-0.627; P<0.001), which was driven especially by ipsilateral ischemic stroke (RR: 0.649; 95% CI: 0.494-0.851; P=0.002) and bradycardia or hypotension (RR: 0.105; 95% CI: 0.051-0.217; P<0.001). However, we found that the CEA group had a higher rate of myocardial infarction than the CAS group (RR: 2.496; P=0.025). Meanwhile, ipsilateral stenosis >70% increased the incidence of periprocedural death or stroke for post-CEA patients (RR: 2.166, 95% CI: 1.112 to 4.220, P=0.023), but no risk factors were identified for post-CAS. Regarding the asymptomatic group, the results demonstrated that patients randomized to CEA had a significantly reduced risk of periprocedural stroke (RR: 0.518; 95% CI: 0.281-0.954; P=0.035), which seems to be driven by periprocedural minor stroke (RR: 0.482; 95% CI: 0.231-0.982; P=0.046).

Conclusions

Among patients with symptomatic CS, CEA was associated with reduced rates of periprocedural stroke and periprocedural nondisabling stroke. Among patients with asymptomatic CS, the rates of minor stroke and stroke in general were higher with stenting than with CEA. Based on the current data, CEA is more beneficial than CAS for 30-day stroke prevention.

Metoprolol Impairs β1-Adrenergic Receptor-Mediated Vasodilation in Rat Cerebral Arteries: Implications for β-Blocker Therapy

The practice of prescribing β-blockers to lower blood pressure and mitigate perioperative cardiovascular events has been questioned because of reports of an increased risk of stroke. The benefit of β-blocker therapy primarily relies on preventing activation of cardiac β1-adrenergic receptors (ARs). However, we reported that β1ARs also mediate vasodilator responses of rat cerebral arteries (CAs), implying that β-blockers may impair cerebral blood flow under some conditions. Here, we defined the impact of metoprolol (MET), a widely prescribed β1AR-selective antagonist, on adrenergic-elicited diameter responses of rat CAs ex vivo and in vivo. MET (1-10 µmol/l) prevented β1AR-mediated increases in diameter elicited by dobutamine in cannulated rat CAs. The β1AR-mediated dilation elicited by the endogenous adrenergic agonist norepinephrine (NE) was reversed to a sustained constriction by MET. Acute oral administration of MET (30 mg/kg) to rats in doses that attenuated resting heart rate and dobutamine-induced tachycardia also blunted β1AR-mediated dilation of CAs. In the same animals, NE-induced dilation of CAs was reversed to sustained constriction. Administration of MET for 2 weeks in drinking water (2 mg/ml) or subcutaneously (15 mg/kg per day) also resulted in NE-induced constriction of CAs in vivo. Thus, doses of MET that protect the heart from adrenergic stimulation also prevent β1AR-mediated dilation of CAs and favor anomalous adrenergic constriction. Our findings raise the possibility that the increased risk of ischemic stroke in patients on β-blockers relates in part to adrenergic dysregulation of cerebrovascular tone. SIGNIFICANCE STATEMENT: β-Blocker therapy using second-generation, cardioselective β-blockers is associated with an increased risk of stroke, but the responsible mechanisms are unclear. Here, we report that either acute or chronic systemic administration of a cardioselective β-blocker, metoprolol, mitigates adrenergic stimulation of the heart as an intended beneficial action. However, metoprolol concomitantly eliminates vasodilator responses to adrenergic stimuli of rat cerebral arteries in vivo as a potential cause of dysregulated cerebral blood flow predisposing to ischemic stroke.

Computer Modeling of Clot Retrieval-Circle of Willis

Endovascular clot retrieval, often referred to as mechanical thrombectomy, has transformed the treatment of patients with ischemic stroke based on an underlying large cerebral vessel occlusion, ranging from the extracranial internal carotid artery (ICA) to the M1 (proximal) segment of the middle cerebral artery (MCA). The aim of this study was to evaluate the effect of a progressive occlusion of the extracranial portion of the ICA on the cerebral blood flow either with a conventional guiding catheter or a balloon-guiding catheter, which enables the operator to completely occlude the parent artery by inflating the balloon around the tip of this type of guiding catheter. We evaluated the impact of flow reduction in the ICA in the setting of ipsilateral MCA occlusion given the different configurations of the circle of Willis (CoW). The computer model of cerebral arteries was based on anatomical works by Rhoton (1) and van der Eecken (2). The interactive experimental results are available on the web at https://gntem3.shinyapps.io/ecrsim. In the setting of left MCA occlusion, compensation from the anterior and posterior communicating artery preserved the flow in the left anterior cerebral artery (ACA) but not the left MCA branches. Under selected CoW configurations, such as classic, missing Acom, or missing A1 segment of the ACA and concurrent right ICA occlusion, there was a progressive decrease of flow in the left ACA to a minimum of 78% when the simulated catheter fully occluded the left ICA. Flow collapsed (<10%) in the left ACA and MCA branches under CoW configurations, such as bilateral fetal PCA. In summary, compensatory flow collapsed under certain clot retrieval scenarios and unusual configurations of CoW.

The role of carotid stenosis ultrasound scale in the prediction of ischemic stroke

INTRODUCTION

To improve the accuracy of ultrasound techniques for the assessment of carotid stenosis, we designed a novel carotid artery stenosis ultrasound scale (CASUS), and evaluated its accuracy, reliability, and its value in predicting the occurrence of cardiovascular and cerebrovascular diseases in a prospective study.

METHODS

A total of 750 patients with first-time ischemic stroke and hospitalized within 24 h were enrolled in the study. Using color Doppler ultrasound (CDUS), the degree of stenosis and blood flow (BF) in bilateral internal carotid arteries (ICA) and the V1-V3 segment of vertebral arteries (VA) was assessed. Cubic simulation curves for BF and global blood flow (GBF) over the stenosis score (SS), total stenosis score (TSS), and radiological imaging- total stenosis score (RI-TSS) were fitted and compared. The receiver operating characteristic (ROC) curves using TSS, RI-TSS, or GBF to predict various ischemic stroke endpoints were also analyzed and compared.

RESULTS

There was a linear relationship between SS and BF both ICA and VA (R2 were 0.734 and 0.783, respectively, both P < 0.05). Both TSS and RI-TSS with GBF showed an inverse "S" curve relationship (R2 was 0.839 and 0.843, all P < 0.05). The AUC values of TSS-based and RI-TSS-based predictions of each endpoint were all greater than 0.7 (all P < 0.05), but the differences of the AUC values between TSS, RI-TSS, and GBF were not statistically significant (all P > 0.05).

CONCLUSIONS

The novel CASUS can better reflect the level of cerebral reperfusion in patients with ischemic stroke and can better predict the occurrence of cardiovascular and cerebrovascular diseases.

Circle of Willis variations in migraine patients with ischemic stroke

OBJECTIVES

Migraine is a risk factor for stroke, which might be explained by a higher prevalence in anatomical variants in the circle of Willis (CoW). Here, we compared the presence of CoW variants in patients with stroke with and without migraine.

MATERIALS AND METHODS

Participants were recruited from the prospective Dutch acute Stroke Study. All participants underwent CT angiography on admission. Lifetime migraine history was assessed with a screening questionnaire and confirmed by an interview based on International Classification of Headache Disorders criteria. The CoW was assessed for incompleteness/hypoplasia (any segment <1 mm), for anterior cerebral artery asymmetry (difference > 1/3), and for posterior communicating artery (Pcom) dominance (Pcom-P1 difference > 1/3). Odds ratios with adjustments for age and sex (aOR) were calculated with logistic regression.

RESULTS

We included 646 participants with stroke, of whom 52 had a history of migraine. Of these, 45 (87%) had an incomplete or hypoplastic CoW versus 506 (85%) of the 594 participants without migraine (aOR: 1.47; 95% CI: 0.63-3.44). There were no differences between participants with and without migraine in variations of the anterior or posterior CoW, anterior cerebral artery asymmetry (aOR: 0.86; 95% CI: 0.43-1.74), or Pcom dominance (aOR: 0.64; 95% CI: 0.32-1.30). There were no differences in CoW variations between migraine patients with or without aura.

CONCLUSION

We found no significant difference in the completeness of the CoW in acute stroke patients with migraine compared to those without.

Smaller caliber of the internal carotid artery in patients with ipsilateral aplasia of the A1 segment of the anterior cerebral artery: a study with CTA

To measure the diameter and the transsectional area of the internal carotid arteries (ICA) on CT Angiography (CTA) in patients with aplasia of the A1-segment of the ACA (A1) and in patients with symmetrical A1, the mean diameter and area of the ICA on both sides were measured at a level of 2 cm below the skull base with a commercially available CT software in 41 consecutive patients with aplasia of A1 observed during a 12-month period on CTA and in 41 control patients with symmetrical A1. The mean diameter of the ipsilateral ICA was 3.83 ± 0.60 mm versus 4.86 ± 0.60 mm as mean diameter of the contralateral ICA and versus 4.40 ± 0.60 mm as mean diameter of both ICAs in the control group of patients. The mean area of the ipsilateral ICA was 11.58 ± 3.80 mm² versus 18. 82 ± 7.39 mm² as mean area of the contralateral ICA and versus 15.29 ± 4.42 mm² as mean area of both ICA in the control group of patients. These differences are statistically highly significant. In patients with symmetrical A1, there was no statistical difference between the diameter or area of both internal carotid arteries. In conclusion, in patients with aplasia of A1, the ipsilateral diameter and area of the cervical ICA is smaller than the diameter and area of the contralateral ICA and smaller than the diameter and area of both internal carotid arteries in patients with symmetrical A1.

Magnetic resonance angiographic evaluation of circle of Willis: A morphologic study in a tertiary hospital set up

Background:

Anatomy of circle of Willis (CW) shows wide variation in different individuals, population groups, and has vital clinical significance in causation and presentation of clinical disease. This study evaluates the anatomical variations, incidence of various common anomalies of CW in south Indian tertiary hospital set up, using three-dimensional time-of-flight (3D-TOF) magnetic resonance angiography (MRA).

Materials and Methods:

A total of 300 patients referred for neuroimaging study over a period of 2-year were included in the analysis. In this prospective and retrospective study, 198 men and 102 women; mean age, 55 years) underwent 3D-TOF MR angiograms of the CW using a 1.5-tesla MR scanner. Images were reviewed for anatomical configuration of the CW using maximum intensity projection (MIP) and 3D volume rendered images.

Results:

On analysis, a complete CW was seen in 50 (16.6%) of 300 subjects. An incomplete anterior and posterior CW was found in 66 (22%) The remaining 184 (61.3%) subjects had partially complete CW configuration. The most common type of CW in a single subject was anterior variant type A and posterior type variant E.

Conclusion:

We observed wide variation in CW configuration in our patients. The prevalence of complete configuration of the circle is 16.6%; slightly higher in females and younger subjects. Complete anterior circle was present in 77.3%. Most common anterior variant is type A (normal anterior configuration) with a prevalence of 66%. The most common posterior circle variant is type E (hypoplasia or absence of both PcomA) with 32.6%. Overall, CW variants are slightly more common among the women in comparison to men. Incidence of associated anomalies like aneurysm or arteriovenous malformation (AVM) was comparable to that described in literature.

Efficacy and safety of stenting for elderly patients with severe and symptomatic carotid artery stenosis: a critical meta-analysis of randomized controlled trials

OBJECTIVE

To investigate both short-term and long-term therapeutic efficacy and safety of carotid artery stenting (CAS) and carotid artery endarterectomy (CEA) for elderly patients with severe and symptomatic carotid artery stenosis.

METHODS

PubMed, EMBASE, Cochrane Library, Clinical Trials Register Centers, and Google Scholar were comprehensively searched. After identifying relevant randomized controlled trials, methodological quality was assessed by using Cochrane tools of bias assessment. Meta-analysis was performed by RevMan software, and subgroup analyses according to different follow-up periods were also conducted.

RESULTS

Sixteen articles of nine randomized controlled trials containing 6,984 patients were included. Compared with CEA, CAS was associated with high risks of stroke during periprocedural 30 days (risk ratio [RR]=1.47, 95% confidence interval [CI]: 1.15-1.88), 48 months (RR=1.37, 95% CI: 1.11-1.70), and >48 months (RR=1.76, 95% CI: 1.34-2.31). There was no significant difference in the aspects of death, disabling stroke, or death at any time between the groups. For other periprocedural complications, CAS decreased the risk of myocardial infarction (RR=0.44, 95% CI: 0.26-0.75), cranial nerve palsy (RR=0.09, 95% CI: 0.04-0.22) and hematoma (RR=0.31, 95% CI: 0.14-0.68) compared with CEA, while it increased the risk of bradycardia or hypotension (RR=8.45, 95% CI 2.91-24.58).

CONCLUSION

Compared with CEA, CAS reduced hematoma, periprocedural myocardial infarction, and cranial nerve palsy, while it was associated with higher risks of both short-term and long-term nondisabling stroke. And they seemed to be equivalent in other outcome measures. As regards to its minimal invasion, it should be applied only in specific patients.

Recruitment Pattern in a Complete Cerebral Arterial Circle

Blood flow through a vessel depends upon compliance and resistance. Resistance changes dynamically due to vasoconstriction and vasodilation as a result of metabolic activity, thus allowing for more or less flow to a particular area. The structure responsible for directing blood to the different areas of the brain and supplying the increase flow is the cerebral arterial circle (CAC). A series of 1D equations were utilized to model propagating flow and pressure waves from the left ventricle of the heart to the CAC. The focus of the current research was to understand the collateral capability of the circle. This was done by decreasing the peripheral resistance in each of the efferent arteries, up to 10% both unilaterally and bilaterally. The collateral patterns were then analyzed. After the initial 60 simulations, it became apparent that flow could increase beyond the scope of a 10% reduction and still be within in vivo conditions. Simulations with higher percentage decreases were performed such that the same amount of flow increase would be induced through each of the efferent arteries separately, same flow tests (SFTs), as well as those that were found to allow for the maximum flow increase through the stimulated artery, maximum flow tests (MFTs). The collateral pattern depended upon which efferent artery was stimulation and if the stimulation was unilaterally or bilaterally induced. With the same amount of flow increase through each of the efferent arteries, the MCAs (middle cerebral arteries) had the largest impact on the collateral capability of the circle, both unilaterally and bilaterally.

Low prevalence of collateral cerebral circulation in the circle of Willis in patients with severe carotid artery stenosis and recent ischemic stroke

INTRODUCTION

The circle of Willis is thought to play a key role in development of collateral flow in patients with internal carotid artery stenosis (ICAS).

AIM

To assess flow in the circle of Willis in patients with recent ischemic stroke (IS).

MATERIAL AND METHODS

The study included 371 patients, 102 symptomatic with severe ICAS and recent IS (within the last 3 months) (group I) and 269 asymptomatic with severe ICAS (group II). Flow in the middle (MCA), anterior (ACA) and posterior (PCA) cerebral arteries and pattern of the cross-flow through anterior (ACoA) and posterior (PCoA) communicating arteries were assessed with transcranial color-coded Doppler ultrasonography (TCCD).

RESULTS

The ACoA or PCoA was less prevalent in group I than in group II (54% vs. 78%, p < 0.001 and 20% vs. 42%, p < 0.001, respectively), resulting in lower peak-systolic velocity (PSV) in the MCA in group I vs. group II (p = 0.015). Any collateral pathway was present in 67% of patients in group I, compared to 86% in group II (p < 0.001). Both PSV and end-diastolic (EDV) flow velocity in the ACA were lower in patients with recent IS, compared to asymptomatic subjects (71 ±24 cm/s vs. 86 ±34 cm/s, p < 0.001 and 32 ±12 cm/s vs. 37 ±17 cm/s, p = 0.038, respectively). Presence of ACoA or PCoA and higher PSV in the MCA and ACA were associated with significant risk reduction of IS (RR = 0.28 (95% CI = 0.16-0.49, p < 0.001), RR = 0.28 (95% CI = 0.15-0.52, p < 0.001), RR = 0.97 (95% CI = 0.96-0.99, p < 0.001), RR = 0.99 (95% CI = 0.98-0.99, p < 0.032), respectively). However, ROC curves failed to show reliable MCA or ACA PSV cut-offs for IS risk assessment.

CONCLUSIONS

The ACoA and PCoA seem to play a key role in the evaluation of IS risk in subjects with severe ICAS.

MODELING THE HEART BEAT, CIRCLE OF WILLIS AND RELATED CEREBRAL STENOSIS USING AN EQUIVALENT ELECTRONIC CIRCUIT

In this study, the cardiovascular system is modeled using an equivalent electronic system focusing on the circle of Willis (COW). COW supports adequate blood supply to the brain. Each cardiovascular artery is modeled using electrical elements such as resistor, capacitor and inductor. MATLAB Simulink software was used to obtain the left and right ventricles pressure as well as pressure distribution at efferent arteries of the COW. Firstly, the normal operation of the system was shown then the stenosis of cerebral arteries was induced in the circuit and the effects were studied. In the normal condition, the difference between pressure distribution of right and left efferent arteries (left and right anterior cerebral arteries (ACA-A2), left and right middle cerebral arteries (MCA), left and right posterior cerebral arteries (PCA-P2) was calculated to see the effect of anatomical difference between left and right sides of supplying arteries of COW. In stenosis cases, the effect of internal carotid artery (ICA) occlusion on efferent arteries pressure was investigated. The model results were verified by comparing to the clinical observation reported in the literature.

Computer modeling of anterior circulation stroke: proof of concept in cerebrovascular occlusion

BACKGROUND

Current literature emphasizes the role of the Circle of Willis (CoW) in salvaging ischemic brain tissue but not that of leptomeningeal anastomoses (LA). We developed a computational model of the cerebral circulation to (1) evaluate the roles of the CoW and LA in restoring flow to the superficial compartment of the middle cerebral artery (MCA) territory and (2) estimate the size of the LA required to maintain flow above the critical ischemic threshold (>30% of baseline) under simulated occlusion.

METHODS

Cerebral vasculature was modeled as a network of junctions connected by cylindrical pipes. The experiments included occlusion of successive distal branches of the intracranial arteries while the diameters of LA were varied.

RESULTS

The model showed that the region of reduced flow became progressively smaller as the site of occlusion was moved from the large proximal to the smaller distal arteries. There was no improvement in flow in the MCA territory when the diameters of the inter-territorial LA were varied from 0.0625 to 0.5 mm while keeping the intra-territorial LA constant. By contrast, the diameter of the inter-territorial LA needed to be >1.0 mm in order to provide adequate (>30%) flow to selected arteries in the occluded MCA territory.

CONCLUSION

The CoW and inter-territorial LA together play important supportive roles in intracranial artery occlusion. Computational modeling provides the ability to experimentally investigate the effect of arterial occlusion on CoW and LA function.

Extent of spontaneous cross-flow via the anterior communicating artery in steno-occlusive carotid artery disease

PURPOSE

Our purpose was to evaluate the agreement of transcranial color-coded duplex sonography (TCCS) measurements and intra-arterial digital subtraction angiography (DSA) findings in determining the extent of spontaneous cross-flow via the anterior communicating artery (AcoA) in patients with internal carotid artery (ICA) stenosis.

METHODS

Thirty adult patients with suspected uni- or bilateral high-grade carotid artery stenosis were prospectively examined by DSA and angle-corrected TCCS. The extent of cross-flow was determined considering retrograde flow in the ipsilateral anterior cerebral artery (ACA) and sideto-side differences of the A1-segments of the ACA and middle cerebral arteries (MCAs) by both techniques. Cross-flow was angiographically categorized by means of a four-step scale. DSA findings were correlated with side-to-side differences in mean blood flow velocity as well as flow direction measured by TCCS.

RESULTS

Twenty-seven of 30 patients had a uni- or bilateral ICA stenosis of >49%. Excellent agreement between TCCS and DSA was evaluated for the detection of lack (grades 0 and 1) or presence (grades 2 and 3) of reversed flow in the ACA (sensitivity 100%, specificity 93%, positive predictive value 94%). Post hoc analysis of the mean velocities in the ACA and MCA revealed a side-to-side difference of 25 cm/s as a cutting point allowing for definition of a corresponding four-grade scale for ultrasound. However, full agreement, i.e.same grade of cross-flow detected by both techniques, was only found in 17(57%) of 30 cases.

CONCLUSIONS

Non-invasive TCCS is reliable for detecting reversed flow in the ACA in patients with ICA stenosis. However, there is only a moderate agreement between angiography and TCCS in quantifying the extent of spontaneous anterior cross-flow because different information on the intracranial hemodynamics may be obtained.

Does thoracic pump influence the cerebral venous return?

We assessed the hemodynamic effects induced by the thoracic pump in the intra- and extracranial veins of the cerebral venous system on healthy volunteers. Activation of the thoracic pump was standardized among subjects by setting the deep inspiration at 70% of individual vital capacity. Peak velocity (PV), time average velocity (TAV), vein area (VA), and flow quantification (Q) were assessed by means of echo color Doppler in supine posture. Deep respiration significantly increases PV, TAV, and Q, but it is limited to the extracranial veins. To the contrary, no significant hemodynamic changes were recorded at the level of the intracranial venous network. Moreover, at rest TAV in the jugular veins was significantly correlated with Q of the intracranial veins. We conclude that the modulation of the atmospheric pressure operated by the thoracic pump significantly modifies the hemodynamics of the jugular veins and of the reservoir of the neck and facial veins, with no effect on the vein network of the intracranial compartment.

Correlation of intraoperative collateral perfusion pressure during carotid endarterectomy and status of the contralateral carotid artery and collateral cerebral blood flow

BACKGROUND

The optimal method for predicting when carotid shunting is not necessary during carotid endarterectomy (CEA) is controversial. This study will analyze the correlation of collateral perfusion pressure and the status of contralateral carotid/cerebral collaterals and determine whether preoperative duplex ultrasound/cerebral angiography can predict when CEA can be done without shunting.

METHODS

Ninety-eight patients were randomized into routine shunting and 102 into selective shunting when the collateral perfusion pressure (systolic carotid stump pressure) was <40 mm Hg during CEA. All patients had preoperative carotid duplex ultrasound and 87 had angiography, the results of which were evaluated for the presence of collateral flow from the contralateral carotid artery or posterior circulation through the anterior and/or posterior communicating arteries.

RESULTS

The perioperative stroke rate was 1.5% for the entire group. There was no correlation between preoperative symptoms and the status of the contralateral carotid artery (normal, stenosed, or occluded). The mean collateral perfusion pressure was inversely related to the severity of the contralateral carotid stenosis: 60, 57, 55, 56, and 38 mm Hg for normal, <50% stenosed, 50-69% stenosed, 70-99% stenosed, and occluded arteries, respectively (p = 0.005). There was a direct relation between the number of patients with a collateral perfusion pressure of <40 mm Hg (shunted group) and the severity of the contralateral carotid stenosis: 6 of 62 (10%) for normal carotid, 7 of 43 (16%) for <50% stenosis (OR = 1.82), 12 of 69 (17%) for 50-69% stenosis (OR = 1.97), 3 of 10 (30%) for 70-99% stenosis (OR = 4, CI = 0.81-19.68), and 9 of 13 (70%) for occlusion (OR = 21, CI = 4.98-89.32) (p < 0.0001). None of the patients (0/56) with normal to <70% contralateral carotid stenosis with cross-filling had a collateral perfusion pressure of <40 mm Hg (no shunting was necessary). However, 9 of 17 (47%) patients with <70% contralateral carotid stenosis and no cross-filling had a collateral perfusion pressure of <40 mm Hg (p < 0.0001), whereas 6 of 7 (86%) patients with ≥70% contralateral carotid stenosis and cross-filling versus 2 of 7 (29%) with ≥70% contralateral carotid stenosis and no cross-filling had a collateral perfusion stump pressure of >40 mm Hg (p = 0.1026). Overall, 62 of 63 (98.4%) patients with cross-filling versus 10 of 24 (42%) without cross-filling had a collateral perfusion pressure of ≥40 mm Hg (p < 0.0001).

CONCLUSIONS

There was an inverse correlation between collateral perfusion pressure and severity of contralateral carotid stenosis, and patients with severe contralateral carotid stenosis/occlusion were more likely to be shunted. The presence of cross-filling with normal to <70% contralateral carotid stenosis was associated with a collateral perfusion stump pressure of ≥40 mm Hg in 100% of patients for whom shunting was not carried out in our series.

Three-dimensional morphometry of the A1 segment of the anterior cerebral artery with neurosurgical relevance

BACKGROUND

Despite research in the anatomical sciences for the last 200 years, some structures of the human body remain controversial or incompletely described. One of these structures is the A1 segment of the anterior cerebral artery (ACA).

OBJECTIVE

To analyze the A1 segment of the ACA using novel stereoscopic methods because the 3-dimensional morphometry of the ACA is important to neurosurgeons.

METHODS

A digital-image computer-based system was used to analyze the A1 segment of 230 ACAs derived from computed tomography. Data analysis included the inner diameter, length, and volume and calculation of A1 symmetry, hypoplasia, and deviation, and tortuosity indexes.

RESULTS

Hypoplasia of the A1 segment was found in 0.87% and only on the right sides, whereas asymmetry was found in 42.6% and was more common in female patients. Right A1 segments tended to be longer in male patients, and this reached significance. Also of significance was the correlation of an increased length with age. Right A1 segments tended to have greater volumes, and this was significant in a comparison of male and female patients. Tortuosity indexes tended to be greater for left sides, but deviation indexes tended to be greater on the right sides.

CONCLUSION

Morphometric data of the A1 segment of the ACA as analyzed in the present study may be of utility to the neurosurgeon. Right A1 segments tend to be more tortuous, more deviated, longer, and narrower than left A1 segments.

Detailed 3D-morphometry of the anterior communicating artery: potential clinical and neurosurgical implications

PURPOSE

Although a site common for pathology and of great importance to the neurosurgeon, the three-dimensional (3D) morphometry of the anterior communicating artery (ACoA) has had incomplete descriptions in the literature.

METHODS

Using a novel 3D digital-image computer data analysis system, 115 patients underwent evaluation of their ACoA based on DICOM files derived from CT angiography. Measurements included the length, internal diameter, volume, deviation index (DI) and tortuosity index (TI).

RESULTS

Of 115 samples, 85 were visualized clearly enough for morphometric analysis. The mean internal diameter was 1.86 mm and this tended to be greater in males (P < 0.05). The mean length of the ACoA was 3.99 mm and the mean volume was 11.61 mm(3). The mean TI for the ACoA was 0.84 and the mean DI was 0.62 mm. A significant relationship between DI and length, DI and volume, and DI and TI were found. The significant correlation of diameter to volume, and length related to volume, DI and TI, as well as TI related to length, volume and DI were noticed. There were no relationship between any parameter and age.

CONCLUSIONS

A detailed knowledge of the 3D-morphometry of the ACoA demonstrates that in almost 50% of individuals the ACoA is straight in their course. Detailed data regarding arterial topography and trajectory as found in our study may be also of use in detecting early changes in this vessel due to pathology and may assist in the treatment of vascular lesions and planning of neurosurgical or interventional radiological procedures in the region including ACoA aneurysms.

Predictors of cerebral reperfusion injury after carotid stenting: the role of transcranial color-coded Doppler ultrasonography

PURPOSE

To evaluate the possible role of transcranial color-coded Doppler ultrasonography (TCD) in predicting cerebral reperfusion injury (CRI) in patients undergoing carotid artery stenting (CAS) for internal carotid artery (ICA) stenosis.

METHODS

TCD was obtained in 210 patients (149 men; mean age 64.2+/-8.4 years, range 44-83) who underwent CAS for ICA stenosis averaging 86.7%+/-8.4%. Contralateral ICA occlusion or near occlusion (stenosis >90%) was present in 67 (31.9%) patients. TCD was performed before and 24 hours after CAS with assessment of peak systolic velocities (PSVs) in the ipsilateral middle cerebral artery (iMCA) and contralateral middle cerebral artery (cMCA). PSV ratios (PSVR) in the iMCA and cMCA were calculated from the PSVs before and after CAS.

RESULTS

CRI syndrome occurred in 3 (1.4%) patients (2 intracranial bleedings, 1 subarachnoid hemorrhage). The mean iMCA and cMCA PSVRs were 2.66+/-0.19 and 4.16+/-2.77, respectively, in CRI patients, while the PSVRs in CAS patients without neurological sequelae were 1.56+/-0.46 and 1.21+/-0.39, respectively (both p<0.001). The combination of iPSVR>2.4 and cPSVR>2.4 occurred in 4 patients with bilateral ICA disease; 3 (75%) of them developed CRI (100% sensitivity and 99% specificity for CRI prediction). The following independent CRI predictors were identified: combined iPSVR>2.4 and cPSVR>2.4 (RR 2.06, CI 1.89 to 2.24; p<0.001), high cMCA PSV after CAS (RR 1.23, CI 1.13 to 1.34; p<0.001), and contralateral ICA occlusion (RR 1.13, CI 1.03 to 1.23; p = 0.007).

CONCLUSION

TCD is an important tool in CRI risk evaluation. The combination of iPSVR>2.4 and cPSVR>2.4 is an independent CRI risk factor, along with contralateral ICA occlusion and high cMCA PSVs after CAS.

Three-dimensional morphometry of the A2 segment of the anterior cerebral artery with neurosurgical relevance

Most prior morphometry data regarding the A2 segment of the anterior cerebral artery (ACA) have been based on cadaveric measurements. With newer imaging modalities, surgical techniques, and minimally invasive procedures, new standards for the anatomy of this vessel are necessary. A novel computer-based data system was used to analyze the three-dimensional (3D) morphometry of 230 A2 segments. In addition, tortuosity (TI) and deviation indices (DI) for this segment were calculated. The mean internal diameter of the A2 segment was 1.86 mm, and segments tended to be larger in men and on left sides. A2 segments were asymmetrical in 43%, and this was more common in women. Lengths tended to be greater on right sides and in men. Volumes were greater in men and increased with age, which was statistically significant. These gender differences were found to be statistically significant (P < 0.05), for both volume and diameter. TI was equal among sides, but DI was more often greater on right sides. The correlation coefficient ratio for length and DI was statistically significant. It is important to understand various 3D morphometrical differences particularly between genders. By constructing blood flow simulation models and during revascularization procedures, surgeons are able to gain a better understanding of each patient's vascular anatomy. These additional 3D data regarding the anatomy of the postcommunicating parts of the ACA may be useful to the neurosurgeon and interventional neuroradiologist. These data may assist with an earlier diagnosis of pathologies affecting the 3D morphology of the ACA.

Model-based assessment of tissue perfusion and temperature in deep hypothermic patients

Deep hypothermic circulatory arrest is necessary for some types of cardiac and aortic surgery. Perfusion of the brain can be maintained using a heart-lung machine and unilateral antegrade cerebral perfusion. Cooling rates during extracorporeal circulation depend on local perfusion. A core temperature of 24 degrees C-25 degrees C is aimed at to extend ischemic tolerance of tissues. Information on cerebral perfusion and temperature is important for the safety of patients, but hardly accessible to measurement. A combined simulation model of hemodynamics and temperature is presented in this paper. The hemodynamics model employs the transmission-line approach and integrates the Circle of Willis (CoW). This allows for parameterization of individual aberrations. Simulation results of cerebral perfusion are shown for two configurations of the CoW. The temperature model provides spatial information on temperature fields. It considers heat transfer in the various tissues retrieving data of local tissue perfusion from the hemodynamics model. The combined model is evaluated by retrospective simulation of two aortic operations.

Validation of a one-dimensional model of the systemic arterial tree

A distributed model of the human arterial tree including all main systemic arteries coupled to a heart model is developed. The one-dimensional (1-D) form of the momentum and continuity equations is solved numerically to obtain pressures and flows throughout the systemic arterial tree. Intimal shear is modeled using the Witzig-Womersley theory. A nonlinear viscoelastic constitutive law for the arterial wall is considered. The left ventricle is modeled using the varying elastance model. Distal vessels are terminated with three-element windkessels. Coronaries are modeled assuming a systolic flow impediment proportional to ventricular varying elastance. Arterial dimensions were taken from previous 1-D models and were extended to include a detailed description of cerebral vasculature. Elastic properties were taken from the literature. To validate model predictions, noninvasive measurements of pressure and flow were performed in young volunteers. Flow in large arteries was measured with MRI, cerebral flow with ultrasound Doppler, and pressure with tonometry. The resulting 1-D model is the most complete, because it encompasses all major segments of the arterial tree, accounts for ventricular-vascular interaction, and includes an improved description of shear stress and wall viscoelasticity. Model predictions at different arterial locations compared well with measured flow and pressure waves at the same anatomical points, reflecting the agreement in the general characteristics of the "generic 1-D model" and the "average subject" of our volunteer population. The study constitutes a first validation of the complete 1-D model using human pressure and flow data and supports the applicability of the 1-D model in the human circulation.

Modeling perfusion in the cerebral vasculature

The constant perfusion of a human organ with nutrients and oxygen demands a robust regulatory mechanisms in the face of normal day-to-day pressure variations in the vasculature. The brain, in a similar manner to the heart requires this mechanism to be extremely quick acting, relative to other ways of altering perfusion such as varying systemic blood pressure, since oxygen depravation in the tissues of the brain can be tolerated for only of the order of tens of seconds before significant damage can be done. In recent years computational models, and it must be noted computer architecture have evolved to an extent where mathematicians and engineers can play a large part in discovering how the brain functions physiologically as well as investigating pathological conditions. This review will look at a number of increasingly complex computational models of blood flow to the brain and how variations in arterial geometry can influence the perfusion in the cerebral vasculature. Although these models have provided an insight into complex mechanisms the research area is densely populated with important questions that perhaps only computer models can answer. The review will indicate possible areas of investigation.

Symptomatic carotid artery occlusion: flow territories of major brain-feeding arteries

PURPOSE

To prospectively investigate the extent of flow territories of the contralateral internal carotid artery (ICA) and vertebrobasilar arteries in patients with symptomatic ICA occlusion.

MATERIALS AND METHODS

Ethics committee approval and informed consent were obtained. Flow territory mapping of the ICA contralateral to the occluded ICA and mapping of the vertebrobasilar arteries were performed by using selective arterial spin-labeling magnetic resonance imaging in 23 functionally independent patients (22 men, one woman; mean age, 58 years +/- 8 [standard deviation]) with symptomatic ICA occlusion. The control group consisted of 68 subjects (57 men, 11 women; mean age, 59 years +/- 9) without hemodynamically significant ICA obstruction. Voxel-based chi(2) testing with Bonferroni correction was performed to analyze significant differences in the extent of the flow territories.

RESULTS

Flow territory maps in patients with symptomatic ICA occlusion showed significant differences in the flow territories of the contralateral ICA and vertebrobasilar arteries compared with those in control subjects (P < .05).

CONCLUSION

In functionally independent patients with symptomatic ICA occlusion, the middle cerebral artery flow territory ipsilateral to the occluded ICA is mainly supplied by the vertebrobasilar arteries, whereas the anterior cerebral artery flow territory on the occluded side is mainly supplied by the contralateral ICA.

Modelling the circle of Willis to assess the effects of anatomical variations and occlusions on cerebral flows

Blood flow in the circle of Willis (CoW) is modelled using the 1-D equations of pressure and flow wave propagation in compliant vessels. The model starts at the left ventricle and includes the largest arteries that supply the CoW. Based on published physiological data, it is able to capture the main features of pulse wave propagation along the aorta, at the brachiocephalic bifurcation and throughout the cerebral arteries. The collateral ability of the complete CoW and its most frequent anatomical variations is studied in normal conditions and after occlusion of a carotid or vertebral artery (VA). Our results suggest that the system does not require collateral pathways through the communicating arteries to adequately perfuse the brain of normal subjects. The communicating arteries become important in cases of missing or occluded vessels, the anterior communicating artery (ACoA) being a more critical collateral pathway than the posterior communicating arteries (PCoAs) if an internal carotid artery (ICA) is occluded. Occlusions of the VAs proved to be far less critical than occlusions of the ICAs. The worst scenario in terms of reduction in the mean cerebral outflows is a CoW without the first segment of an anterior cerebral artery combined with an occlusion of the contralateral ICA. Furthermore, in patients without any severe occlusion of a carotid or VA, the direction of flow measured at the communicating arteries corresponds to the side of the CoW with an absent or occluded artery. Finally, we study the effect of partial occlusions of the communicating arteries on the cerebral flows, which again confirms that the ACoA is a more important collateral pathway than the PCoAs if an ICA is occluded.

Anatomic Variations in the Circle of Willis in Patients with Symptomatic Carotid Artery Stenosis Assessed with Multidetector Row CT Angiography

PURPOSE

To assess the presence of anterior and posterior collateral pathways in the circle of Willis in patients with symptomatic carotid artery stenosis (SCAS) and to compare this to patients without carotid artery stenosis.

MATERIALS AND METHODS

Multislice CT angiography was performed in 91 patients and 91 control subjects. Using consensus reading, 2 observers evaluated the presence and diameter of the anterior communicating artery (AcomA), the A1 segments of the anterior cerebral arteries, the posterior communicating arteries (PcomA) and the P1 segments of the posterior cerebral arteries. Anterior or posterior pathways were assumed to be present if the diameter of continuous arterial segments was >1 mm; both A1 segments and AcomA anterior, and ipsilateral P1 segment and PcomA posterior. Comparison between patients and controls was performed using the chi(2) test.

RESULTS

In the patients we found significantly more hypoplastic (<1 mm) or invisible A1 segments (16 and 14 vs. 4 and 1, respectively, p < 0.01). The AcomA was invisible in 4 patients versus 1 control. An isolated compromised anterior pathway and a combined compromised anterior and posterior pathway occurred more frequently in the patients as compared to the controls; 9 versus 1% (p < 0.01) and 26 versus 4% (p < 0.01).

CONCLUSION

A compromised anterior collateral pathway, usually combined with a compromised posterior pathway, occurs more frequently in patients with SCAS as compared to controls, which suggests a relation between symptomatic carotid stenosis and an incomplete circle of Willis.

Assessment of Flow Changes in the Circle of Willis After Stenting for Severe Internal Carotid Artery Stenosis

PURPOSE

To assess flow velocities in the cerebral arteries after carotid artery stenting (CAS) in patients with unilateral versus bilateral lesions and analyze velocities in patients with neurological complications after CAS.

METHODS

Ninety-two patients (68 men; mean age 63.2 +/- 8.4 years, range 44-82) with internal carotid artery (ICA) stenoses were divided according to unilateral (group I, n = 72) or bilateral (group II, n = 20) disease. Fifty age- and gender-matched patients without lesions in the extra- or intracranial arteries served as a control group. Transcranial color-coded Doppler ultrasound was performed prior to and within 24 hours after CAS in the test groups; systolic velocities were assessed ipsilateral (i) and contralateral (c) to the CAS site in the middle cerebral artery (MCA) and anterior cerebral artery (ACA).

RESULTS

Collateral flow via the anterior communicating artery (ACoA) was found in all group-II patients and 90% of group-I patients. After CAS, collateral flow through the ACoA ceased, and the velocity increased by 26% in the iMCA in group I compared to controls (p < 0.001). In group II, iMCA flow increased by 30% (p < 0.001) and flow via the ACoA (p < 0.001) increased, resulting in normalization of cMCA velocities (p = 0.928). In 89 (96.7%) subjects, CAS was uncomplicated. Hyperperfusion syndrome occurred in 2 (2.2%) patients, both with bilateral ICA stenoses; 1 (1.1%) transient ischemic attack was seen in a patient with unilateral disease. In the patients with hyperperfusion syndrome, the MCA velocities were 2.7- and 7.4-fold higher, respectively, versus before CAS and 2-fold higher than in controls.

CONCLUSION

Uncomplicated CAS results in an iMCA velocity increase >25% compared to controls. MCA velocities in hyperperfusion syndrome were greatly increased versus before CAS and in controls.

Altered Flow Territories after Extracranial-Intracranial Bypass Surgery

OBJECTIVE:

To prevent stroke after carotid sacrifice and to augment cerebral perfusion in patients with internal carotid artery (ICA) occlusion, high-flow extracranial-intracranial (EC-IC) bypass operations are performed. Although the function and efficacy of the bypass is monitored during surgery, the postoperative flow through the bypass is significantly lower than the flow in the contralateral ICA. Thus far, it is unknown whether decreased bypass flow is caused by a low tissue perfusion or by a relatively small flow territory.

METHODS:

Seven patients, four with an atherosclerotic ICA occlusion and three with a giant aneurysm of the ICA, were investigated; each underwent a high-flow EC-IC bypass and permanent occlusion of the ICA. Cerebral blood flow was measured with arterial spin labeling perfusion magnetic resonance imaging. Separate flow territory mapping of the EC-IC bypass, contralateral ICA, and posterior circulation was performed with selective arterial spin labeling magnetic resonance imaging.

RESULTS:

No significant difference was found in cerebral blood flow between the hemisphere ipsilateral to the EC-IC bypass (70.9 ± 11.3 ml/min/100 g tissue), contralateral to the EC-IC bypass (71.9 ± 14.3 ml/min/100 g tissue), and comparable findings in 50 healthy control participants (69.1 ± 17.5 ml/min/100 g tissue). Paired analysis of the individual flow territories demonstrated a 15% volume reduction (P = 0.018) in flow territory of the EC-IC bypass compared with the contralateral side.

CONCLUSION:

In the present study, we demonstrate the feasibility of selective arterial spin labeling magnetic resonance imaging for clinical follow-up of patients after high-flow EC/IC bypass surgery, providing both information on flow territories and the level of regional cerebral blood flow.

A patient-specific computer model to predict outcomes of the balloon occlusion test

OBJECT

Balloon occlusion tests (BOTs) are performed to identify patients who are at risk for ischemia and stroke following permanent internal carotid artery (ICA) occlusion. The object of this work was to determine whether patient-specific blood flow modeling can be used to identify patients in whom the BOT would not be tolerated.

METHODS

The test was performed in 16 patients who underwent BOT with continuous neurological and electroencephalographic monitoring, followed by a hypotensive challenge. During hypotension a tracer was injected so that single-photon emission tomography (SPECT) scans could be obtained. Each individual brain circulation was modeled using information gained from phase-contrast magnetic resonance (MR) angiography and digital subtraction (DS) angiography, and the predicted effect of the BOT was evaluated. Six patients did not tolerate the BOT; in these patients, decreases in middle cerebral artery (M1 segment) blood flow of 41 +/- 27% (mean +/- standard deviation), anterior cerebral artery (A3 segment) flow of 56 +/- 33%, and posterior cerebral artery (P2 segment) flow of 4 +/- 13% ipsilateral to the site of occlusion were found with modeling; these changes were significantly greater than the percentage of changes measured in the contralateral hemisphere (p < 0.05). Ten patients who tolerated the BOT well had calculated decreases in ipsilateral flows of only 9 +/- 6% for the M1 segment, 12 +/- 40% for the A3 segment, and 17 +/- 21% for the P2 segment during BOT modeling.

CONCLUSIONS

A decrease in blood flow in both the ipsilateral M1 and A3 segments that was greater than 20%, calculated by flow modeling of the BOT, was 100% sensitive and 100% specific in identifying patients who could not tolerate the BOT. Blood flow modeling, coupled with DS angiography and noninvasive phase-contrast MR angiography measurements to make calculations patient specific, can be used to identify patients who have an elevated risk of ischemia during the BOT.

Investigations of flow and pressure distributions in physical model of the circle of Willis

The paper presents the results of experiments concerning flow in the model of cerebral supplying arteries and the circle of Willis (CW). Vascular phantom was prepared on the basis of anatomical specimens. The most typical artery shapes and dimensions were considered. Pressure distribution in six characteristic points is provided, and so are the average flow rates in the anterior, middle and posterior section of the brain. Tests were run in the conditions replicating the physiological state (i.e. when the supplying arteries were fully patent) and in pathological conditions, in which the internal carotid and vertebral arteries were occluded on one or both sides. Thus obtained results were compared with the results of computer simulations based on linear and non-linear flow models. To estimate the non-linear resistance of vascular segment two phenomenological formulae were proposed. High degree of correlation between the values obtained from experiments and those registered in non-linear computer model proves usefulness of proposed formulae. It verifies the hypothesis that non-linearity of flow characteristics of the vessel segments to a great extent is caused by their tortuousity and small length in relation to diameter. Non-linear effects are particularly pronounced in conditions of pathological occlusion of supplying vessels.

Increase in Collagen Turnover But Not in Collagen Fiber Content Is Associated with Flow-Induced Arterial Remodeling

BACKGROUND

Degradation and synthesis of collagen are common features in arterial geometrical remodeling. Previous studies described an association between arterial remodeling and an increase in collagen fiber content after balloon injury. However, this does not exclude that the association between collagen content and remodeling depends on arterial injury since the association of collagen fiber content and arterial remodeling, without arterial injury, has not been investigated. The aim of the present study was to study the relation between flow-induced arterial geometrical remodeling, without arterial injury, and collagen synthesis and degradation, collagen fiber content and cell-migration-associated moesin levels.

METHODS AND RESULTS

In 23 New Zealand White rabbits an arteriovenous shunt (AV shunt) was created in the carotid and femoral artery to induce a structural diameter increase or a partial ligation (n = 27 rabbits) to induce a diameter decrease. In both models, arterial remodeling was accompanied by increased procollagen synthesis, reflected by increased procollagen mRNA or Hsp47 protein levels. In both models, however, no changes were detected in collagen fiber content. Active MMP-2 and moesin levels were increased after AV shunting.

CONCLUSIONS

Collagen synthesis and MMP-2 activation were associated with arterial remodeling. However, a change in collagen fiber content was not observed. These results suggest that, during flow-induced geometrical arterial remodeling, increases in collagen synthesis are used for matrix collagen turnover and cell migration but not to augment collagen fiber content.

Acute-phase protein haptoglobin is a cell migration factor involved in arterial restructuring

Collagen turnover and cell migration are fundamental aspects of arterial restructuring. To identify mRNAs involved in blood flow-induced arterial restructuring, we performed subtraction polymerase chain reaction and found expression of haptoglobin mRNA in adventitial fibroblasts of rabbit arteries. Haptoglobin is highly expressed in liver, but its arterial expression and function are unknown. In vitro studies revealed that stimulation of haptoglobin expression by lipopolysaccharides in mice fibroblasts stimulated migration of wild-type fibroblasts but had no effect on migration of haptoglobin knockout fibroblasts. In vivo studies showed that flow-induced arterial restructuring was delayed in haptoglobin knockout mice. This new function of haptoglobin might be explained by facilitating cell migration through accumulation of a temporary gelatin matrix because cell culture showed that haptoglobin is involved in the breakdown of gelatin. We conclude that haptoglobin is highly expressed in arterial tissue and is involved in arterial restructuring. This new haptoglobin function may also apply to other functional and pathological restructuring processes such as angiogenesis, tissue repair, and tumor cell invasion.

Computational models of blood flow in the circle of Willis

A two-dimensional, steady state model of the circle of Willis has been developed. To simulate the peripheral resistance of the cerebrovascular tree, blocks of porous media were used. Their effective resistance was kept constant, disregarding the effects of arterial auto-regulation. The model was then used to simulate different common abnormalities of the circle of Willis while a range of varying boundary conditions was imposed to the right internal carotid artery (ICA). The total flux was tabulated and compared favourably with both clinical measurements and other models of the circle of Willis. Relevant fluid dynamics effects were also observed and analysed. The present model demonstrates that the use of CFD can produce physiological results if the appropriate boundary conditions are used. We can provide clinicians with a priority list of the severity of the flux reduction for the considered abnormalities for different degrees of stenosis of the right ICA. From this study it is apparent that the redistribution of blood via the circle of Willis is mainly driven by changes in the vascular resistance of the brain rather than in the local arterial geometry. The use of valid peripheral resistances allows for a more realistic model of the circle of Willis but also highlights the need for more accurate means to estimate the vascular resistance of a patient.

A model of the cerebral and cerebrospinal fluid circulations to examine asymmetry in cerebrovascular reactivity

The authors examined the steal phenomenon using a new mathematical model of cerebral blood flow and the cerebrospinal fluid circulation. In this model, the two hemispheres are connected through the circle of Willis by an anterior communicating artery (ACoA) of varying size. The right hemisphere has no cerebrovascular reactivity and the left is normally reactive. The authors studied the asymmetry of hemispheric blood flow in response to simulated changes in arterial blood pressure and carbon dioxide concentration. The hemispheric blood flow was dependent on the local regulatory capacity but not on the size of the ACoA. Flow through the ACoA and carotid artery was strongly dependent on the size of the communicating artery. A global interhemispheric "steal effect" was demonstrated to be unlikely to occur in subjects with nonstenosed carotid arteries. Vasoreactive effects on intracranial pressure had a major influence on the circulation in both hemispheres, provoking additional changes in blood flow on the nonregulating side. A method for the quantification of the crosscirculatory capacity has been proposed.

Flow redistribution in the major cerebral arteries after carotid endarterectomy: a study with transcranial Doppler scan

PURPOSE

This open single-center prospective study aimed to determine the redistribution of blood flow within the circle of Willis and through collateral pathways after carotid endarterectomy. Blood flow velocity and flow direction in the major cerebral arteries were determined, both at rest and during CO(2) inhalation.

METHODS

Carotid endarterectomy was performed in 148 patients with a 70% or greater diameter stenosis of the internal carotid artery while patients were under general anesthesia. Arteriotomy closure was done with a venous patch. Selective shunting was performed with an electroencephalogram. Baseline blood flow velocity of the basal cerebral arteries was measured by means of transcranial Doppler sonography preoperatively (within 1 week before surgery) and 3 months postoperatively. At the same times, cerebrovascular reactivity was calculated during CO(2) inhalation insonating both middle cerebral arteries.

RESULTS

Baseline blood flow velocity in the ipsilateral middle cerebral artery hardly changed 3 months postoperatively, but there was a considerable redistribution of flow in the circle of Willis. This was characterized by a decrease in contribution from the contralateral hemisphere through the anterior communicating artery, reduced cerebropetal flow rates in the ophthalmic artery, and smaller contribution of the posterior collateral sources. The CO(2) reactivity on the side of surgery increased in all patients. In patients with a contralateral occlusion, CO(2) reactivity increased on both sides. The redistribution of flow was most pronounced in patients who needed intraoperative shunting and in patients with a contralateral internal carotid artery occlusion.

CONCLUSION

After carotid endarterectomy, flow redistribution, as expressed by changes in blood flow velocity values, occurs in the circle of Willis. The contribution of collateral sources is diminished, and the CO(2) reactivity increases, both of which reflect improvement of the hemodynamic condition. The most improvement occurs in patients with contralateral occlusion.

Maximal wall shear stress in arterial stenoses: application to the internal carotid arteries

Maximal wall shear stress (MWSS) in the convergent part of a stenosis is calculated by the interactive boundary-layer theory. A dimensional analysis of the problem shows that MWSS depends only on a few measurable parameters. A simple relationship between MWSS and these parameters is obtained, validated, and used to calculate the magnitude of MWSS in a carotid stenosis, as a function of the patency of the circle of Willis and the stenotic pattern. This demonstrates the huge effect of collateral pathways. Elevated MWSS are observed even in moderate stenoses, provided they are associated with a contralateral occlusion, a large anterior, and narrow posterior communicating arteries, suggesting a potential risk of embolus release in this configuration.

Cerebral hemodynamics during arterial and CO(2) pressure changes: in vivo prediction by a mathematical model

The aim of this work was to analyze changes in cerebral hemodynamics and intracranial pressure (ICP) evoked by mean systemic arterial pressure (SAP) and arterial CO(2) pressure (Pa(CO(2))) challenges in patients with acute brain damage. The study was performed by means of a new simple mathematical model of intracranial hemodynamics, particularly aimed at routine clinical investigation. The model was validated by comparing its results with data from transcranial Doppler velocity in the middle cerebral artery (V(MCA)) and ICP measured in 44 tracings on 13 different patients during mean SAP and Pa(CO(2)) challenges. The validation consisted of individual identification of 6 parameters in all 44 tracings by means of a best fitting algorithm. The parameters chosen for the identification summarize the main aspects of intracranial dynamics, i.e., cerebrospinal fluid circulation, intracranial elastance, and cerebrovascular control. The results suggest that the model is able to reproduce the measured time patterns of V(MCA) and ICP in all 44 tracings by using values for the parameters that lie within the ranges reported in the pathophysiological literature. The meaning of parameter estimates is discussed, and comments on the main virtues and limitations of the present approach are offered.

Hemodynamic role of the circle of Willis in stenoses of internal carotid arteries. An analytical solution of a linear model

A mathematical model of blood flow through the circle of Willis was developed, within a linear framework. Comprehensive analytical solutions, including a remarkably small number of parameters, were derived in the cases of obstructive lesions of extracranial carotid arteries. The influence of these lesions and the role of anterior and posterior communicating arteries on the blood pressure at the entry of the cerebral territories were quantified and analyzed emphasizing that the responses of the system of Willis to obstructive carotid lesions are extremely varied, depending on the communicating artery anatomy. Comparison with numerical results obtained by using a non-linear model showed no physiologically significant differences. Such a model might be an essential tool for an accurate assessment of the cerebral hemodynamics in carotid diseases.

Angiographic criteria reliably predict when carotid endarterectomy can be safely performed without a shunt

BACKGROUND

Selective shunting during carotid endarterectomy is widely performed, but the optimal approach for predicting when a shunt is unnecessary remains uncertain. We evaluated the ability of preoperative cerebral angiography to predict when carotid endarterectomy could be safely performed without a shunt.

STUDY DESIGN

Eighty-seven patients undergoing carotid endarterectomy between August 1991 and December 1997 had preoperative cerebral angiograms. The angiograms were evaluated for the presence of collateral flow from the contralateral carotid through the anterior communicating artery and from the posterior circulation through the posterior communicating artery. Patients then underwent endarterectomy and were selectively shunted based on somatosensory evoked potential changes. Internal carotid artery stump pressure was routinely measured in all patients.

RESULTS

Nine patients (10%) had a shunt placed based on somatosensory evoked potential changes and none of the 87 patients had a perioperative (30 days) stroke. Angiography revealed that 36 patients (41%) had no cross-filling from the contralateral carotid through the anterior communicating artery. Nine of these patients (25%) required a shunt; none of the 51 patients with adequate cross-filling (p < 0.001) did. Furthermore, 94% of the patients without cross-filling but with a patent ipsilateral posterior communicating artery did not require a shunt using somatosensory evoked potential changes as the standard for shunt insertion. Stump pressure measurements (> or = 25 mmHg) or (> or = 50 mmHg) did not reliably exclude the need for a shunt. Only 2 of 15 patients with contralateral carotid occlusion and 1 of 16 patients with a prior ipsilateral stroke required shunts.

CONCLUSIONS

In the presence of cross-filling from the contralateral carotid artery, shunt insertion was uniformly unnecessary. In addition, routine shunting of patients with previous ipsilateral strokes or contralateral carotid occlusion was not always necessary. Stump pressures were less sensitive than angiographic criteria in determining when a shunt was unnecessary. Evaluation of cross-filling from the contralateral carotid artery on preoperative angiography can predict with certainty which patients will not require a shunt.