International Journal of Anatomical Sciences 2010, 1: 21-25

Research Paper

A  Study  of  the  Anatomical  Variations  of  the  Circle  of  Willis  Using  Magnetic Resonance Imaging.

Haripriya M, Melani RS.

Department  of  Anatomy,  Sri  Ramachandra  Medical  College  &  Research  Institute,  Sri Ramachandra University, Chennai, India.

Key Words: circle of Willis, magnetic resonance imaging angiography, anastomosis

 Abstract: Variations in the circle of Willis was studied using Magnetic Resonance Imaging Angiography in fifty patients of South Indian population at random. Variations have been found in 16 cases.  All arteries in the circle of Willis showed variations except the anterior communicating artery, 5 cases showed variations in the anterior cerebral artery, 6 cases in the internal carotid artery, 2 cases in the posterior cerebral artery, 1 case in the posterior communicating artery and 2 cases in the basilar artery.  Magnetic Resonance Imaging is the best tool to show the collateral circulation and the anastomotic variants of the circle of Willis.

Circle  of  Willis  is  a  large  arterial ring which unites  the internal  carotid  and vertebro-basilar systems. It is formed of an anastomosis between the internal carotid artery, anterior cerebral artery, anterior communicating artery, posterior communicating artery and posterior cerebral artery (Williams et al., 1995).    The anastomosis thus provided by this vascular ring is of great significance when one of the major arteries supplying the brain becomes occluded.  The neurological deficit suffered as   well   as  the  ability  of  a  patient   to withstand occlusion of one or more of these major vessels depend on the presence of collateral  circulation  to  the  affected  area. The haemodynamics of the circle is influenced by the variations in the caliber of communicating arteries and in the segments of the anterior and posterior cerebral arteries which lie between their origins and their junctions with the corresponding communicating arteries.  A detailed   know- ledge of the normal anatomy of the circle of Willis, its anomalies, and the clinical significance of its variations is valuable to the clinicians and neuroradiologists. Therefore this study was carried out to find out  the  variations  of  the  circle  of  Willis using Magnetic Resonance Imaging Angiography.


Correspondance to: Haripriya M, Department of Anatomy, SRMC & RI, Sri Ramachandra University Chennai- 600 116, India.


Materials and Methods

 Fifty  patients  at  random  from  the Out Patient Department  of Radiology and Imaging  Sciences,  Sri  Ramachandra Medical College and Research Institute, Sri Ramachandra University for undergoing MRI were chosen for the study.   The technique used was three Dimensional Time of Flight Magnetic Resonance Angiography (3D-TOF-MRA). Only the arteries forming the circle of Willis were studied.   For the purpose of identification,the circle of Willis is divided into anterior and posterior configurations.The anterior configuration consisting  of  the  anterior  cerebral  artery,anterior communicating artery and internal carotid artery.   The posterior configuration consisting of posterior cerebral artery, posterior communicating artery and basilar artery. For the purpose of description,the segments of the anterior   and posterior cerebral arteries are named as :

1.   Anterior  cerebral  artery  –  horizontal (A1)   segment   that   extends   medially from its origin to its junction with the anterior communicating artery.

2. Posterior   cerebral   artery   –   Pre- communicating (P1) segment extends laterally from its origin at the basilar bifurcation to its junction with the posterior communicating artery.


Out of the fifty cases, only sixteen cases showed variations of the circle of Willis whereas others were found to be normal (Fig. 1).

Fig. 1 showing normal circle of Willis












1. anterior cerebral artery

2. internal carotid artery

3.anterior communicating artery

4.posterior cerebral artery

5.posterior communicating artery

6.basilar artery

The findings were:

1.   Hypoplasia  of  the  A1  segment  of  the right anterior cerebral artery in 5 cases (Fig. 2).

2.   Hypoplasia of left internal carotid artery in one case (Fig. 3) and stenosis in 5 cases on the right or left sides (Fig 4).

3.   Hypoplasia of P1 segment of the right posterior  cerebral  artery  in  one  case (Fig. 5) and diffused narrowing on the right side in another case (Fig. 6).

4. Absence      of      right      posterior communicating artery and hypoplasia of left  posterior  communicating  artery  in the same case.(Fig. 7).

5.   Stenosis  of  basilar  artery  in  one  case (Fig. 8) and ectactic changes in another case (Fig. 9).

6.   No   changes   were   observed   in   the anterior communicating artery.

Fig. 2 showing hypoplasia of the A1 segment of the right anterior cerebral artery.


Arrow indicates hypoplasia

Fig. 3 showing hypoplasia of left internal carotid artery


Fig. 4 showing tight stenosis of right internal carotid artery











Fig. 5 showing hypoplasia of right posterior cerebral artery


Fig. 6 showing diffuse narrowing of right posterior cerebral artery












Fig. 7 showing absence of right posterior communicating artery and hypoplasia of      right  posterior  communicating artery











Fig.8 showing stenosis of basilar arteryFig.











9 showing bulbosity of basilar artery












Variations  of  the  circle  of  Willis have  been  studied  by  many  investigators. Al-Hussain et al (2001) carried out a study to describe the main variations of the circle of Willis to clarify the clinical importance of these variations.  In his study, out of 50, only 14 polygonal types of circle of Willis were observed; whereas in the present study of 50 cases 16 cases showed variations in the circle of Willis.

In a study by Macchi et al (2002) by MRA, the variability of the circle of Willis in 118 persons showed normal circle of Willis in 47%, complete anterior configuration in 90%    and complete posterior configuration in 48.5%. Voljevica et al., (2004) analyzed angiographies of 150

MRI patients, in which a complete anterior configuration  of  the  circle  of  Willis  was seen in 76% with hypoplasia of the anterior cerebral artery. But in the present study, normal circle of Willis was 46 % with hypoplasia of anterior cerebral artery, complete  anterior  configuration  was  78% and  complete  posterior  configuration  was 90%.  Circle  of  Willis  is  regarded  as  the major source of collateral flow in patients with  severe  carotid  artery  disease (Hendrikse et at., 2001).When planning carotid  surgery  the  configuration  of  the circle of Willis should also  be taken  into account. The cost of treatment of ischaemic stroke is increasing.   Carotid bifurcation surgery can change the prognosis for symptomatic patients with over 70% of cartotid narrowing.   Explanation of the carotid bifurcation is an important step in the diagnosis and must assess the degree of stenosis, the smoothness of the plaque and the collateral vessels (Rolland et al., 1996).

 In the observations of Szabok et al. (2001), statistical analysis showed a significant relationship between the degree of stenosis and observed stroke pattern.  In the present study of 50 patients, high grade internal carotid artery stenosis was seen in 10%.   Although  in  the  individual  patient, any of the infarct patterns may occur, in statistical terms the incidence of a particular stroke pattern is  clearly dependent  on  the degree of stenosis.   Therefore territorial stroke can occur in internal carotid artery stenosis. The present study also showed absence of posterior communicating artery on the right side and hypoplastic posterior

communicating artery on the left side and the internal  carotid  arteries  on  either side appeared normal in its course and caliber. But the patient had lacunes in both cerebral hemispheres.  In a similar study by Schomer et al., (1994), it was concluded that absent ipsilateral posterior communicating artery is a  risk  factor  for  ischaemic  cerebral infarction in patients with internal carotid artery occlusion.

A  small  or  absent  ipsilateral posterior communicating artery is a risk factor for ischaemic cerebral infarction in patients with internal carotid artery occlusion.   De Felice et al., (2000) studied bilateral non functioning posterior communicating arteries of circle of Willis in idiopathic sudden hearing loss in some patients and suggested a strong association between a non-functioning posterior communicating artery of the circle of Willis in sudden hearing loss.  After the occlusion of an internal carotid artery the principal source of collateral flow is through the arteries  of  the  COW,  but  the  size  and patency of these arteries are quite variable. Study of the anatomy of the collateral pathways  in  patients  with  ICA  occlusion with or without infarction in the watershed area of the deep white matter may identify patterns  that  afford  protection  from ischaemic infarction (Brunere et al., 1995).


support    the use    of permanent    anti-
coagulation rather than        platelet


In the present study in one case, the basilar artery showed ectactic change at its bifurcation. The basilar artery aneurysm can cause stroke which  may be due to thrombosis   of   the   perforating   arteries, arterio-arterial emboli or a compressive mechanism. Since sub arachnoid haemorrhage secondary to rupture of a dolichoectasia is exceptional, some authors  antiaggregation in patients with ischaemia, where dilatations are limited to the basilar artery (Arenas et al., 2001).


The  signal  intensity of  blood  flow within   a   vessel   is   dependent   on   the   replenishment of fully magnetized spins at the entry of the imaging section. Accordingly,  non-visualization  of a vessel may  be  either  due  to  the  absence  of  the vessel or to a very slow or turbulent flow within the vessel itself.  Completeness of the configuration   of   the   circle   of   Willis evaluated  by using 3D-TOF-MRA  is  thus expected to be under estimated, both in healthy subjects that may show a negligible flow across the communicating vessels due to the symmetrical blood supply to the circle of Willis and in patients with severe stenosis or occlusion  of the  internal  carotid  artery that may show turbulent flow in one or more segments of the circle of Willis due to the presence of collateral pathways

MRA for determining the degree of carotid stenosis prior to carotid endarterectomy is attractive because it does not have the high morbidity associated with conventional  intra-arterial  angiography. Both atherosclerotic and non-atherosclerotic carotid artery disease can be comprehensingly assessed with contrast enhanced  Magnetic  Resonance Angiography. Contemporary understanding of the collateral circulation may be greatly enhanced through further refinement of neuroimaging modalities that correlate angiographic findings with perfusion status, providing the basics for future therapeutic and prognostic applications.

 Therefore it can be concluded that MRI is the most powerful non-invasive method to demonstrate collateral circulation via the basal communicating arteries and to identify haemodynamically relevant anatomic variants of the circle of Willis.


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