Non-invasive assessment of the Circle of Willis using transcranial pulsed Doppler ultrasound with angiographic correlation

The role of the circle of Willis in internal carotid artery stenosis and anatomical variations: a computational study based on a patient-specific three-dimensional model

BACKGROUND

The aim of this study is to provide better insights into the cerebral perfusion patterns and collateral mechanism of the circle of Willis (CoW) under anatomical and pathological variations.

METHODS

In the current study, a patient-specific three-dimensional computational model of the CoW was reconstructed based on the computed tomography (CT) images. The Carreau model was applied to simulate the non-Newtonian property of blood. Flow distributions in five common anatomical variations coexisting with different degrees of stenosis in the right internal carotid artery (RICA) were investigated to obtain detailed flow information.

RESULTS

With the development of stenosis in unilateral internal carotid artery (ICA), the cerebral blood supply decreased when the degree of stenosis increased. The blood supply of the ipsilateral middle cerebral artery (MCA) was most affected by the stenosis of ICA. The anterior communicating artery (ACoA) and ipsilateral posterior communicating artery (PCoA) functioned as the important collateral circulation channels when unilateral stenosis occurred. The blood flow of the anterior circulation and the total cerebral blood flow (CBF) reached to the minimum in the configuration of the contralateral proximal anterior cerebral artery (A1) absence coexisting with unilateral ICA stenosis.

CONCLUSIONS

Communicating arteries provided important collateral channels in the complete CoW when stenosis in unilateral ICA occurred. The cross-flow in the ACoA is a sensitive indicator of the morphological change of the ICA. The collateral function of the PCoA on the affected side will not be fully activated until a severe stenosis occurred in unilateral ICA. The absence of unilateral A1 coexisting with the stenosis in the contralateral ICA could be the most dangerous configuration in terms of the total cerebral blood supply. The findings of this study would enhance the understanding of the collateral mechanism of the CoW under different anatomical variations.

Experimental study of hemodynamics in the Circle of Willis

BACKGROUND

The Circle of Willis (CoW) is an important collateral pathway of the cerebral blood flow. An experimental study of the cerebral blood flow (CBF) distribution in different anatomical variations may help to a better understanding of the collateral mechanism of the CoW.

METHODS

An in-vitro test rig was developed to simulate the physiological cerebral blood flow in the CoW. Ten anatomical variations were considered in this study, include a set of different degrees of stenosis in L-ICA and L-ICA occlusion coexist with common anatomical variations. Volume flow rates of efferent arteries and pressure signals at the end of communicating arteries of each case were recorded. Physiological pressure waveforms were applied as inlet boundary condition.

RESULTS

In the development of L-ICA stenosis, the total CBF decreases with the increase of stenosis degree. The blood supply of ipsilateral middle cerebral artery (MCA) was affected most by the stenosis of L-ICA. Anterior communicating artery (ACoA) and ipsilateral posterior communicating artery (PCoA) function as important collateral pathways of cerebral collateral circulation when unilateral stenosis occurred. The blood supply of anterior cerebral circulation was compensated by the posterior cerebral circulation through ipsilateral PCoA when L-ICA stenosis degree is greater than 40% and the affected side was compensated immediately by the unaffected side through ACoA. Blood flow of the anterior circulation and the total CBF reached the minimum among all cases studied when L-ICA occlusion coexist with the absence of PCoA.

CONCLUSION

The results demonstrated the flow distribution patterns of the CoW under anatomical variations and clarified the collateral mechanism of the CoW. The flow ACoA is the most sensitive indexes to the morphology change of ipsilateral ICA. The relative independence of the circulation in anterior and posterior sections of the CoW is not broken and the function of ipsilateral PCoA is not activated until a severe stenosis of unilateral ICA occurs. PCoA is the most important collateral pathway of the collateral circulation and the missing of PCoA has the highest risk of stroke when the ipsilateral ICA has severe stenosis. These findings may provide the basis for future therapeutic and diagnosis applications.

Assessing Circle of Willis blood circulation in dogs with transcranial color-coded duplex sonography

Insonation of Circle of Willis by transcranial Doppler duplex color sonography is described in 30 healthy dogs with 15 weighing < 33 lb and 15 weighing >33 lb. Imaging was via a temporal window to explore the rostral, middle, and caudal cerebral arteries on both the left and right-hand sides; and through an suboccipital window to study the basilar artery. Normal mean values of the peak systolic velocity (PSV), end diastolic velocity, mean velocity, resistance index (RI), and pulsatility index (PI) were characterized and compared with those obtained in previous studies. There was significant differences in the PSV, RI, and PI in the rostral cerebral artery between dogs weighing < 33 vs. > 33 lb. Mean PSV was higher in weighing over 33 lb, whereas the mean resistive index and mean PI were lower in these dogs.

Intracranial collateral pathways assessed by contrast-enhanced three-dimensional transcranial color-coded sonography

Individual parameter settings of the duplex machine and limited insonation angles may influence the visualization of small intracranial vessels in 2-D transcranial color-coded sonography. The aim of our study was the morphologic assessment of intracranial collateral pathways (first auditory area celiac artery, A(1)CA; anterior communicating artery, AComA; first parental generation celiac artery P(1)CA; posterior communicating artery, PComA) using 3-D transcranial color-coded duplex sonography (3-D TCCS) and digital subtraction angiography (DSA). A total of 41 patients with large vessel disease and 30 patients who had suffered subarachnoidal hemorrhage (mean age 52 +/- 15 years) were involved. In all patients, angiography was performed within 10 days following 3-D sonography. The patients were investigated with a color-coded duplex system using the power mode. Contrast enhancement was achieved by continuous infusion of a galactose-based echo-enhancer using a perfusor pump. The 3-D system consists of an electromagnet that induces a low-intensity magnetic field near the head of the patient. A magnetic position sensor is attached to the ultrasound (US) probe and transmits the spatial orientation to a personal computer, which also receives the corresponding 2-D images from the video port of the duplex machine. Angiography revealed a "complete" circle of Willis in one third of the patients and, in the remaining patients at least one vessel was hypoplastic or absent. Sufficient temporal bone windows allowed the sonographic investigation of 466 (94%) of 497 expected arterial segments. The comparison of both techniques resulted in a weighted kappa value of 0.56 for the A(1)CA and 0.63 for the AComA. In the posterior circulation kappa values of 0.56 for the P(1)CA and 0.43 for the PComA were calculated. In 50 vessels (10%), 3-D sonography misdiagnosed the diameter of a collateral vessel as normal and angiography revealed hypoplasia or aplasia of this vessel. The main advantage of 3-D TCCS is that it enables the investigator to reconstruct virtually any arbitrary view angle. Compared with angiography or magnetic resonance (MR) angiography, 3-D sonography can be performed easily in critically ill patients on stroke units or intensive care units. The noninvasive assessment of the circle of Willis may be useful in patients who undergo carotid surgery without angiography. Combined with hemodynamic information, contrast-enhanced 3-D TCCS might increase the diagnostic impact of transcranial US.

Selective arterial spin labeling (SASL): perfusion territory mapping of selected feeding arteries tagged using two-dimensional radiofrequency pulses

To date, most perfusion magnetic resonance imaging (MRI) methods using arterial spin labeling (ASL) have employed slab-selective inversion pulses or continuous labeling within a plane in order to obtain maps derived from all major blood vessels entering the brain. However, there is great potential for gaining additional information on the territories perfused by the major vessels if individual feeding arteries could be tagged. This study demonstrates noninvasive arterial perfusion territory maps obtained using two-dimensional (2D) selective inversion pulses. This method is designated "selective ASL" (SASL). The SASL method was used to tag the major arteries below the circle of Willis. A combination of 2D selective tagging and multislice readout allows perfusion territories to be clearly visualized, with likely applications to cerebrovascular disease and stroke.

Clinical and sonographic patterns of tandem internal carotid artery/middle cerebral artery occlusion in tissue plasminogen activator-treated patients

BACKGROUND AND PURPOSE

The National Institutes of Health Stroke Scale (NIHSS) is predictive of thrombus presence but has limited ability to identify occlusion location in the anterior circulation. We describe clinical and sonographic patterns that are associated with tandem internal carotid artery (ICA) and middle cerebral artery (MCA) occlusions.

METHODS

Consecutive acute ischemic stroke patients receiving intravenous tissue plasminogen activator (TPA) were studied. Pretreatment NIHSS scores and bedside transcranial Doppler (TCD) were obtained for all patients.

RESULTS

A total of 95 patients treated with intravenous TPA at 132+/-60 minutes from stroke onset were studied. On TCD, 48 had isolated MCA occlusion (mean NIHSS 16.8+/-5.8, median 17, range 5 to 28); and 16 had tandem ICA/MCA occlusion (mean NIHSS 18.8+/-5.8, median 22, range 8 to 29; P=NS). In the MCA occlusion and tandem ICA/MCA occlusion groups, 19% and 11%, respectively, had NIHSS scores <12 points. Compared with the NIHSS scores in patients with hemiplegia, forced gaze deviation, and complete neglect, the lower NIHSS scores were attributable to partial arm and/or leg paresis, gaze preference, and partial neglect. In those patients, TCD showed > or =2 major collateral channels and low-resistance flow at the M1 origin, suggesting perfusion of perforating arteries. Although TCD cannot differentiate between high-grade ICA stenosis or occlusion, collateral flow patterns and stenotic signals at the terminal ICA differentiated tandem lesions from isolated MCA occlusion (P<0.01).

CONCLUSIONS

Tandem ICA/MCA occlusion was found on TCD in 17% of TPA-treated patients. NIHSS scores were similar in patients with isolated MCA and tandem occlusions. Lower NIHSS scores were seen in patients with a higher number of major collateral flow channels and higher Thrombolysis in Brain Ischemia (TIBI) flow grades at the MCA origin.

Collateral variations in circle of willis in atherosclerotic population assessed by means of transcranial color-coded duplex ultrasonography

BACKGROUND AND PURPOSE

Transcranial color-coded duplex ultrasonography combined with common carotid artery (CCA) compression can be used to assess the collateral function of the circle of Willis. The aim of this study was to assess the unknown fraction of hemodynamic functional anterior and posterior communicating arteries (AcoA and PcoA, respectively) in an atherosclerotic population with no cerebrovascular symptoms.

METHODS

In 76 patients with a mean age of 61 (35 to 89) years, the blood flow velocity changes in the precommunicating parts (A1 and P1, respectively) of the anterior and posterior cerebral arteries were measured during CCA compression. The AcoA was defined as functional if blood flow was reversed in the ipsilateral A1 and enhanced in the contralateral A1 during CCA compression. The PcoA was defined as functional if the flow velocity in the P1 was enhanced >20% during ipsilateral CCA compression.

RESULTS

It was possible to assess cross flow through the anterior part of the circle of Willis in 95% of the subjects. Failure of this collateral pathway was caused by a hypofunctional AcoA in 4% and a hypofunctional A1 in 1% of the subjects. Anomalies in the posterior part of the circle of Willis hampering collateral flow from the basilar to the internal carotid artery were found in 45% of the hemispheres. Thirty-eight percent of PcoAs were hypofunctional, and 7% of the posterior cerebral arteries had a persistent fetal anatomy.

CONCLUSIONS

We found that in subjects with no cerebrovascular symptoms, the anterior collateral pathway of the circle of Willis was nearly always functional. In contrast, the posterior collateral pathway was nonfunctional in almost half of the total number of hemispheres. Comparing these basic data with data from patients with cerebral ischemic disease might further help to elucidate the importance of the collateral capacity of the circle of Willis.

Collateral configuration of the circle of Willis: transcranial color-coded duplex ultrasonography and comparison with postmortem anatomy

BACKGROUND AND PURPOSE

The anterior communicating artery (AcoA) and posterior communicating arteries (PcoA) of the circle of Willis provide the main route for collateral blood flow in cases of carotid artery obstruction. Transcranial color-coded duplex ultrasonography (TCCD) allows real-time measurement of the collateral function of the AcoA and PcoA. The primary objective of this study was to determine the collateral artery threshold diameters for supplying collateral flow.

METHODS

In 12 acute stroke patients with a median age of 75 years (51 to 91 years), the collateral integrity of the circle of Willis as assessed by TCCD and carotid compression tests was compared with their postmortem anatomy. The lengths and diameters of the collateral arteries were measured.

RESULTS

TCCD demonstrated absent anterior collateral flow in 3 patients. In 1 of these patients, absence of anterior cross-flow was due to an occluded anterior cerebral artery, which was revealed at autopsy. Absent posterior collateral flow was found in 14 hemispheres. In 2 of these hemispheres, autopsy revealed a fetal configuration of the posterior cerebral artery hampering posterior collateral flow. The median (range) diameters as found at autopsy of the functional (n=19) and nonfunctional (n=16) collateral arteries of the circle of Willis were 1.1 (0.4 to 2.0) and 0.5 (0.3 to 0.7) mm, respectively (P=0.003). PcoA diameters were found to correlate negatively (rho=-0. 50, P=0.01) to the diameters of their accessory P1 segments.

CONCLUSIONS

The threshold diameter allowing for cross-flow through the primary collateral arteries of the circle of Willis is between 0. 4 and 0.6 mm.

Diagnostic value of three-dimensional transcranial contrast duplex sonography

This study evaluated intracranial cerebral arteries using a new data acquisition system for transcranial three-dimensional (3D) ultrasonography with and without an echo contrast agent, with confirmation by cerebral angiography. Ten patients, studied with diagnostic cerebral angiography, were examined without knowledge of the angiographic results. Data acquisition through the transtemporal acoustic window was performed using a magnetic sensor system to track the spatial orientation of the ultrasound probe while scanning the volume of interest. A color transcranial duplex system with a power Doppler mode was used, and 3D data sets were acquired before and after the injection of transpulmonary-stable ultrasound contrast medium. Ipsilateral to the transducer, the anterior cerebral artery (ACA) in 90%, middle cerebral artery (MCA) in 60%, all three or more branches of the MCA in 60%, posterior cerebral artery (PCA) in 60%, and posterior communicating artery (PCoA) in 60% were successfully imaged without the echo contrast agent. With the contrast agent, the ACA, MCA, three or more branches of the MCA, PCA, and PCoA were visible in 100%. The anterior communicating artery was visualized in 40% without contrast enhancement and in 90% with contrast enhancement. Contralateral to the transducer, the ACA (60%), MCA (30%), all three or more branches of the MCA (10%), PCA (20%), and PCoA (20%) were successfully imaged without contrast. Contrast enhancement improved the imaging success rate for the ACA (90%), MCA (80%), three or more branches of the MCA (80%), PCA (100%), and PCoA (100%). A transpulmonary-stable ultrasound contrast agent used in combination with 3D transcranial duplex ultrasonography can significantly improve the success rate for transcranial color duplex imaging of intracranial arteries.

Assessment of the posterior communicating artery by transcranial color-coded duplex sonography

BACKGROUND AND PURPOSE

The aim of this study was to investigate flow velocity and flow direction in the posterior communicating artery (PcomA) by means of transcranial color-coded duplex sonography (TCCD) and to compare the results with angiographic findings.

METHODS

Thirty patients with unilateral occlusion of the internal carotid artery (ICA) due to atherosclerosis (n=15) or balloon occlusion (n=15) and 50 normal subjects were included. The circle of Willis was insonated through the temporal bone window. In 24 patients with unilateral ICA occlusion, angiograms were available and were compared with the results of TCCD.

RESULTS

The PcomA could be detected unilaterally in 70% of normal subjects and bilaterally in 30%. A retrograde flow direction in the PcomA from the posterior cerebral artery to the ICA was found in 75% of the normal control subjects. The mean peak flow velocity in normal PcomAs was 36 +/- 15 cm/s (+/- SD). No significant differences in flow velocity were found between unilaterally and bilaterally detectable PcomAs or between retrograde and orthograde PcomAs. In patients with unilateral ICA occlusion we observed ipsilaterally a retrograde flow direction, with an elevation of flow velocity (64 +/- 10 cm(s) compared with the contralateral side (27 +/- 14 cm/s; P<.001).

CONCLUSIONS

TCCD appears to be a valuable method to determine flow velocity and flow direction not only in the large intracranial vessels but also in the smaller communicating arteries. In the future this method could be useful for the planning of ICA balloon occlusions and in deciding whether to perform extracranial/intracranial bypass surgery. It could furthermore show intracranial collaterals in patients with cerebrovascular disease and help to estimate the risk of watershed infarctions in patients with asymptomatic high-grade ICA stenosis and in patients undergoing carotid endarterectomy.