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

Abstract

Background:
Anatomy of circle of Willis (CW) shows wide variation in different individuals, population groups, and has vital clinicalsignificance in causation and presentation of clinical disease. This study evaluates the anatomical variations, incidence of various commonanomalies 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.

Full text

Annals of Indian Academy of Neurology, ???-??? 2015, Vol ??, Issue ??
collateral pathways, may also be recruited. Examples include
flow reversal through the ophthalmic arteries, reversed
ow through the anterior choroidal artery, and anastomosis
between the cortical branches of the intracerebral arteries
(leptomeningeal collaterals). The collateral potential of the
CW is believed to be dependent on the presence and size
of its component vessels,[1-3] which vary among normal
individuals.[3-10]
Considerable anatomic variation exists in the CW.
Recent cadaveric study has shown 21 types of CW
variations.[9] Classification presented in the work of Chen
et al.,[11] is adopted in our presentation for the simplicity
of the scheme. There are 10 types of variations (A–J) in
anterior and posterior circle which are illustrated in the
figures [Figures 1 and 2]. One common variation consists of
relative narrowing of proximal part of the posterior cerebral
artery (PCA) with large ipsilateral PcomA, so the ICA
supplies the posterior cerebrum. In another variation, the
AcomA is a large vessel, such that a single internal carotid
supplies both the anterior cerebral arteries (ACAs). In third
variation, ACA gives rise to both post-communicating
segments and supplies retrograde flow to the ipsilateral
Introduction
Vascular anatomy of CW had been subject of extensive autopsy
studies as well as in vivo imaging studies.(1-8)CW is constituted
by two internal carotid arteries (ICAs) and basilar artery
anastomosing at the base of the brain. The carotid arteries and
their branches supply the anterior portion of the brain referred
to as the anterior circulation. Whereas, posterior circulation
refers to vertebrobasilar system that supplies the posterior
portion of the brain. The anterior communicating arteries (A
com As) and posterior communicating arteries (PcomAs) are
component vessels of the CW and designated as the primary
collateral pathways. Other pathways, known as the secondary
Magnetic resonance angiographic evaluation of circle
of Willis: A morphologic study in a tertiary hospital set up
Shankar Rao Naveen, Venkatraman Bhat, Gadabanahalli Ashok Karthik
Department of Radiology, Narayana Health, Multispeciality Hospital, Mazumdar Shaw Cancer Center, Bangalore,
Karnataka, India
Abstract
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.
Key Words
Anatomy CW, circle of Willis, CW, configuration CW, MRI
For correspondence:
Dr. Venkatraman Bhat, 309, Greenwoods Apt, Royal Gardenia, Bommasandra, Bangalore - 560 099, Karnataka, India.
E-mail: bvenkatraman@gmail.com
Ann Indian Acad Neurol 2015;??:??-??
Original Article
Access this article online
Quick Response Code: Website:
www.annalsoan.org
DOI:
10.4103/0972-2327.165453
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2 Naveen, et al.: Magnetic resonance angiography of circle of Willis
Annals of Indian Academy of Neurology, ???-??? 2015, Vol ??, Issue ??
pre-communicating segment, which, in turn, gives rise to
the ipsilateral middle cerebral artery (MCA; both ACAs and
both MCAs are supplied by a single ICA). Physiologically,
arrangement of the cerebral arteries in the CW creates
deficiencies, redundancies; also provide potential collateral
pathways in cerebral circulation. If one part of the circle
becomes blocked or narrowed, blood flow from the other
arteries can preserve the cerebral perfusion by maintaining
enough flow to avoid ischemia.
The CW is an important potential collateral pathway in
maintaining adequate cerebral blood ow in patients with
ICA obstruction. With the advances in microneurosurgery and
the more eective ability to deal with occlusive neurovascular
disease surgically and by interventional methods, the accurate
knowledge of the intracranial vascular anatomy has become
increasingly important.
Based on anatomical[12-14] and radiological studies,[7,15,16] it
has been shown that more than half of healthy subjects
have anatomical variations in the CW. Recent studies have
investigated the role of the CW in the development of
collateral ow in ICA obstruction; these studies were based
on mathematical models[17-19] and used transcranial Doppler
ultrasound,[20-25] digital contrast–enhanced angiography,[3,26]
or magnetic resonance angiography (MRA).[2,3,27-29] MRA has
previously been shown to be well-suited to investigate the
CW, in view of the fact that it is able to provide morphological
as well as hemodynamic information concerning blood ow
direction in individual vessels accurately.[30,31] Previous studies
have demonstrated that three-dimensional time-of-flight
(3D-TOF) MRA is a sensitive, noninvasive modality suitable
for detecting the anatomy of the CW in healthy volunteers and
patients with carotid artery disease.[11,32]
Aims and objectives
The purpose of this study is to evaluate and to describe the
prevalence and pattern of CW, arterial variants (aplasia,
hypoplasia) and anomalies (arteriovenous malformations
(AVMs) and aneurysms) in noncontrast 3D-TOF-MRA in
the study group. In addition to providing a standard of
reference for future research on the circle with 3D TOF
MRA, the purpose of this study was to investigate whether
any age- or sex-related differences could be found in circle
morphology.
Materials and Methods
In a prospective and retrospective observational study, 300
patients referred for neuroischemic study protocol from
February 2010 to July 2011 were included in the study. Study
was approved by the institutional review board. Informed
consent from the patient or guardian was obtained before
scanning. All patients (198 men and 102 women; mean age,
55 years) underwent 3D-TOF MR angiograms of the CW.
Images were obtained with the sequence of spoiled gradient-
recalled acquisition (SPGR) using a 1.5-tesla MR scanner
(Achieva; Philips Medical Systems, The Netherlands).
Patients with pacemaker, ferromagnetic intracerebral
aneurysm clips, or other metallic implants and patients with
claustrophobia were excluded. Severely ill, uncooperative
patients who were not able to remain stable for study
duration were also excluded. Images were reviewed for CW
anatomy and conguration.
Figure 2: (a) Schematic diagrams of anatomical variations of the
posterior part of the CW (modied from Chen et al.[11]). (a) Bilateral
PcomAs are present (blue). (b) PCA originates predominantly
from the ICA. This variant is known as a unilateral fetal type
PCA; the PcomA on the other side is patent. (c) Bilateral fetal
type PCAs with both pre-communicating segments of the PCAs
patent. (d) Unilateral PcomA present. (e) Hypoplasia or absence
of both PcomAs and isolation of the anterior and posterior parts
of the circle at this level. (b) (f) Unilateral fetal type PCA and
hypoplasia or absence of the pre-communicating segment of
the PCA. (g) Unilateral fetal type PCA and hypoplasia or absence
of the contralateral PcomA. (h) Unilateral fetal type PCA and
hypoplasia or absence of both pre-communicating segment of the
PCA and the PcomA. (i) Bilateral fetal type PCAs with hypoplasia
or absence of bothpre-communicating segments of the PCAs.
(j) Bilateral fetal type PCAs with hypoplasia or absence of the
pre-communicating segment of either PCA. PCA = Posterior
cerebral artery, PcomA = Posterior communicating artery
a c e
h i jg
b d
f
Figure 1: (a) Schematic diagrams of anatomic variations in the
anterior part of the CW (modied from Chen et al.[11]). (a) Normal adult
pattern. There is a single AcomA (red). The ICA bifurcates into the
pre-communicating segment of the ACA and the MCA. (b) Two or
more AcomAs. (c) Median artery of the corpus callosum arises from
the AcomA. (d) Fusion of the ACAs over a short distance. (e) ACA
forms a common trunk and split distally into two post communicating
segments. (b) (f) MCA originates from the ICA as two separate trunks.
(g) Hypoplasia or absence of an AcomA. (h) One pre-communicating
segment of an ACA is hypoplastic or absent, the other pre-
communicating segment gives rise to both post-communicating
segments of the ACAs. (i) Hypoplasia or absence of an ICA. ACA
gives rise to both postcommunicating segments, supplies retrograde
ow to the ipsilateral precommunicating segment. (j) Hypoplasia
or absence of an anterior communication. The MCA arises as two
separate trunks. CW = Circle of willis, ICA = Internal carotid artery,
MCA = Middle cerebral artery, AcomA = Anterior communicating
artery, ACA = Anterior cerebral artery
ac e
h i jg
b d
f
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Naveen, et al.: Magnetic resonance angiography of circle of Willis 3
Annals of Indian Academy of Neurology, ???-??? 2015, Vol ??, Issue ??
Scanning technique: Patients were imaged in supine position,
wherever necessary, aer sedation with midazolam 0.07-0.08 mg/
kg intramuscular (IM; approximately 5 mg IM) administered up
to 1 h before the study in uncooperative patients. Examination
was done with a dedicated head coil. Monitoring of vital signs
of patient was performed throughout the scanning. Dedicated,
optimized high-resolution 3D-TOF MRA protocol with
repetition time (TR)/echo time (TE)/ip angle of 19/5.7 ms/16°,
respectively, with isotropic resolution of 0.6 � 0.6 � 0.6 mm3 was
used. T1-weighted volume scans and T2-weighted multisection
fast-eld echo anatomic scans were obtained for the detection
of brain abnormalities. Scanning parameters included; slice
thickness 1.2 mm, 0.6 mm slice overlap, eld of view of 100 �
100 mm, and matrix 0.6 � 0.6 � 0.6 mm3 andTR-19 ms, TE - 5.7ms,
and ip angle-16°. Totally 50 slices covering a volume of 30 mm
(50 � 0.6 mm eective slice thickness) was obtained. The total
imaging time was approximately 15 min, of which the 3D TOF
MRA sequence required 3 min 24 s. These axial source images
were post-processed by the maximumintensity projection (MIP)
algorithm [Figure 3a] to produce eight projections rotating
about the section axis. All component vessels of the CW were
assessed by measuring the diameter on the individual MIP
images. Whenever there was doubt in determining the diameter
of vessel due to overlap in the MIP images, the TOF source
images were then reviewed on the advanced workstation
(Philips ADW 4.0 workstation). Occasionally, it was necessary
to cuto the unwanted branching vessel on the images in order
to beerdepict the target vessels and assess correct diameter.
Vessels visualized as continuous segments of at least 0.8 mm
in diameter, were considered present. Those smaller than
0.8 mm in diameter were considered as hypoplastic.[11,33] The
images were also reviewed with volume rendering technique
and evaluated in all the angles [Figure 3b]. Arteries when seen
as noncontinuous segments were considered as absent. The
anterior and posterior parts of CW were evaluated separately
and classied according to the scheme. The prevalence of each
anatomic variant was documented.
Results
Study group consisted of 300participants (198 men and
102 women; mean age, 55 years) [Graph 1a and b]. Incidence
of various types of CW is documented in Table 1. Common
morphological types of CW variations [Table 2] and common
types of anterior and posterior circulation are illustrated in
Table 3. The prevalence of the variants of the anterior and
posterior circle of Willis (CW) for dierent age groups of both
sexes and for total subjects are also shown in Graph 2. Variant
type A [Figure 4a] was the most common type of anterior part of
the CW in all age groups and sexes [Table 2 and Graph 2a and b].
Anterior CW variants
The anterior CW was complete in 232 out of 300 participants
(77.3%), with a normal conguration (Type A) seen in 198
subjects. In two patients, two AcomAs [Figure 4b] were
observed. The anterior circle was incomplete in 67 participants
who had compromised anterior collateral ow, AcomA was
absent in 39 [Figure 4c1 and 2]. The remaining 28 subjects had
A1 hypoplasia or aplasia [Figure 5g and h]. The most common
type of anterior collateral was type A [Figure 4a1 and 2], in
which all component vessels were competent.
Posterior circle variants
The prevalence of the variants of the posterior part of CW was
shown for dierent age groups and both sexes in Table 2. The
prevalence of unilateral fetal-type posterior communicating
artery (FTP com A; posterior part variants B, F, G and H)
was 16% and bilateral FTP com A (posterior part variants C,
I, and J) was found in 7%. A higher percentage of incomplete
collaterals were observed in the posterior part of the circle
compared with the anterior collaterals. An adult conguration
complete posterior circles [Figure 6a] was observed in 51
participants, and transitional variant [Figure 6b and c] was
observed in 29 participants. A FTP CW was seen in 69 (23%) of
the 300 subjects. Of the 69 subjects with FTP, 56 (18.6%) were
classied as having partial FTP [Figures 6b and c, 7g, 8j] in
which a hypoplastic P1 segment was present, and 13 (4.3%)
were found to have a full FTP in which a P1 segment was
Graph 1: Status of circle of Willis and age distribution of patients
Graph 2: Status of the circle of Willis in relation to sex and age
Figure 3: Maximum intensity projection (MIP) and volume
rendered (VR) three-dimensional time-of-ight magnetic
resonance (MR) angiogram of the normal CW
a b
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4 Naveen, et al.: Magnetic resonance angiography of circle of Willis
Annals of Indian Academy of Neurology, ???-??? 2015, Vol ??, Issue ??
Entire circle
Table 1 and Figures 4 and 5 shows the prevalence of complete,
partially complete, and incomplete conguration of the entire
CW for dierent age groups, both sexes, and total subjects.
There was statistically signicant dierence among dierent
age and sex groups. Higher prevalence of complete CW was
found in younger group (below 50 years; 33.9% of younger
older subjects versus 6.8% of older subjects) and in females
(28.4% of women versus 10.6% of men).
Combined analysis of entire circle
In the combined analysis, a complete CW was seen in 50 (16.6%)
of 300 subjects. An incomplete anterior and posterior CW was
found in 66 (22%) of 300 subjects. The remaining 184 (61.3%)
subjects had partially complete CW conguration. The most
common type of CW in a single subject was anterior variant type
A and posterior type variant E. Incidentally three patients had
aneurysms, one subject was having AVM and another subject was
having persistent trigeminal artery on MR angiograms [Figure 9].
Discussion
The conguration of the CW has been investigated in many
anatomical and clinical studies.[3-10] There are only a few studies
that have systematically investigated the conguration of
the CW in a general population.[9,10] There is a great clinical
signicance to CW variations, allowing prognostication of
intracranial ischemia in incomplete circles.
MRA has demonstrated high sensitivity in evaluation of
component vessels in the CW.[29] Though well-accepted
clinically, sensitivity of 3D time-of-ight (TOF) MRA depends
on the blood ow velocity of the vessel, and the technique may
have diculties in visualizing small vessels in the CW with
slow or turbulent ow.[34] In most studies, MRA 3D acquisitions
absent [Figures 7f and 8h]. The most common type of posterior
variation was type E [Figure 7e], in which bilateral P com
A were absent. Of the 56 participants with a partial FTP, a
unilateral FTP was found in 35 participants and bilateral FTP
was seen in 21 participants.
Table 2: Morphology and incidence of variation of COW
Morphology of circle of Willis
Morphology Cases Percentage
Complete circle 50 16.6
Incomplete ant, post circle 66 22
Partially complete 184 61. 4
Complete anterior circle 232 77.3
Complete posterior circle 99 33
Isolation
Isolation of contralateral (Ant J) 0 0
Isolation of posterior circulation (Post G) 98 32.6
Table 1: COW. Types of anterior and posterior
circulation and relative incidence
Morphological types of COW in relation to Sex (%)
Anterior circulation Posterior circulation
Types TOTAL M F TOTAL M F
A 66 42.3 23.6 17 7. 3 9.7
B 0.6 0.3 0.3 4.67 3.7 1
C 2 1 1 4.3 2.3 2
D 6 4 2 9 7. 3 1.6
E 2.6 1.6 1 32.7 23 9.7
F 0 0 0 1 0.7 0.3
G11.6 12 4 7 6.3 0.66
H 9.3 9 2 3.3 1. 6 1. 6
I 0 0 0 0 0 0
J 0 0 0 2.6 2.3 0.3
Figure 4: Type A: A 40 year-old male with syncope, suspected
of cerebrovascular accident (CVA): A single AcomA. The ICA
bifurcates into the pre-communicating segment of the ACA and
the MCA. Type B: A 48 year-old male with syncope, suspected
of CVA: Two AcomAs (arrows). (c1 and 2) Type C: A 42-year-
old male with transient unsteadiness, suspected of CVA: Medial
artery of the corpus callosum arises (arrow) from the AcomA. (d1
and 2) Type D: A 36-year-old female with headache, for exclusion
of vascular lesion: Focal fusion of the ACA (long arrow)
Figure 5: (e1 and 2) Type E: A 24-year-old female with migraine,
for exclusion of vascular lesion: ACA forms a common
trunk (arrow) and split distally into two post-communicating
segments. (g1 and 2) Type G: A 52-year-old male with transient
visual disturbance, suspected of CVA hypoplasia or absence of
an AcomA (circle). (h1 and 2) Type H: A 53-year-old male with
two episodes of gait unsteadiness, suspected of CVA: One
pre-communicating segment of an ACA is hypoplastic or absent
and the other pre-communicating segment (triangle) gives rise
to both post-communicating segments of the ACAs
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Naveen, et al.: Magnetic resonance angiography of circle of Willis 5
Annals of Indian Academy of Neurology, ???-??? 2015, Vol ??, Issue ??
Figure 8: (h1 and 2) Type H: A 48-year-old female with headache,
visual blurring, suspected with CVA. Unilateral fetal type PCA (arrow)
and hypoplasia or absence of both pre-communicating segment of
the PCA and the Pcom A (circle). (i1 and 2) Type I: A 51-year-old
female with migraine, referred for exclusion of vascular intracranial
lesion. Bilateral fetal type PCAs (open arrow) with hypoplasia or
absence of the pre-communicating segment of either PCA (arrow)
Figure 9: (a and b) A 50-year-old male with syncope, suspected
of CVA: Persistent left trigeminal artery (arrow). (c and d) A
47-year-old female with recurrent headache, suspected of
intracranial vascular lesion: AVM in right parietooccipital region
in right PCA territory draining into right cortical veins and into
superior sagittal sinus. (e) A 55-year-old male with transient gait
disturbance, suspected of CVA: AcomA aneurysm (pointer)
Figure 6: (a1 and 2) Type A: A 52-year-old male with episodes of
blurred vision, suspected with CVA; bilateral PcomAs are present
(arrows). (b1 and 2) Type B: A 62-year-old male with a syncopal
attack, suspected with CVA: PCA originates predominantly from
the ICA (triangle). This variant is known as a unilateral fetal type
PCA; the PcomA on the other side is patent. (c1 and 2) Type C: A
24-year-old female with migraine, referred to exclude intracranial
vascular abnormality: Bilateral fetal type PCAs with both pre-
communicating segments of the PCAs patent (arrow heads). (d1
and 2) Type D: A 55-year-old male with transient right hemiparesis,
suspected with CVA. Unilateral PcomA present (arrow)
Figure 7: (e1 and 2) Type E: A 65-year-old male with transient
visual blurring, suspected with CVA. Hypoplasia or absence of
both PcomAs and isolation of the anterior and posterior parts
of the circle at this level (circle). (f1 and 2) Type F: A 55-year-
old male with transient right hemiparesis, suspected with CVA.
A 59-year-old female with recurrent vertigo, suspected with
CVA. Unilateral fetal type PCA and hypoplasia or absence of
the pre-communicating segment of the PCA (arrow). (g1 and 2)
Type G: A 35-year-old male with sudden onset unsteady gait,
suspected with CVA. Unilateral fetal type PCA (triangle) and
hypoplasia or absence of the contralateral PcomA
Table 3: COW common variations and incidence
Common types of variations
Circulation Type No Percentage
Anterior Type A 19 8 66
Type G 35 12
Type H 28 9
Posterior Type E 98 32.6
Type A 51 17
Type D 27 9
were made with either steady state precession (FSSP) or spoiled
gradient technique (SPGR) using 1.5-tesla systems. Using
FSSP technique, an axial slab with a thickness of 52 mm and
64 partitions was placed over the entire CW. A gradient-echo
sequence (fast imaging with steady-state precession) with a
matrix of 256 3 512 and a eld of view of 200 mm was used,
which yields a voxel size of 0.78 3 0.39 3 0.81 mm3 acquisition
time of 11 min 47 s.[30] Typical parameter used in SPGR
technique were 45 ms TR, 4.9 ms TE, 20° ip angle, 256 � 192
matrix in a 14 cm eld of view, and 42 mm slab with 60 serial
axial slices of 0.7 mm thick with total imaging time of 10 min.[11]
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6 Naveen, et al.: Magnetic resonance angiography of circle of Willis
Annals of Indian Academy of Neurology, ???-??? 2015, Vol ??, Issue ??
Our study focuses on anatomical variants of the CW. In the
evaluation process, like in earlier studies, we have considered the
following vessels forming part of the CW: The A com A, the pre-
communicating segment (A1) of the ACA, the pre-communicating
segment (P1) of the PCA, the P com A, and ICA. CW conguration
can be categorized in to three dierent types based on the structure
of P1, P2 segment of PCA, and P Com A. Congurations are the
adult type, transitional type, and fetal type. In adult conguration,
P1 diameter is larger than the PComA diameter. In transitional
conguration, diameters of both arteries are equal and equally
contribute in formation of P2 of PCA. Fetal or embryonic
conguration diameter of the P1 is smaller than diameter of P
Com A and P2.[9] In previous studies, the prevalence of a complete
anterior circle varied from 74 to 90% in dierent ethnic groups,[9,35-37]
almost similar to our observation. Autopsy studies, however,
showed lower incidence of complete circles.[36] One recent cadaveric
study has reported variation of CW in 40% with maximum variation
in Pcom A (50%) followed by A com A (40%).[9] In the present study,
the prevalence of entirely complete CW was 16.6%, higher in
females than males (28.4 and 10.6%, respectively) and young than
older subjects (33.9 and 6.8%, respectively). Previous studies have
shown higher incidence of complete circles in female patients and
younger patients.[11,33,37] Diameter of the proximal arteries measured
on MRA in control studies tend to be larger in male show tendency
to decrease with age. Our observation is similar to earlier studies.
Additionally, average age of male subjects was 63 years and that of
the female 47 years, which might partly explain higher visualization
in female subjects. The most common variant in anterior circulation
is type A, which is normal paern in both the sexes. Type A variant
in anterior circulation in common among women (n = 71, 69.6%)
slightly more frequent compared to men (n = 127, 64.1%). Type E
variant of posterior circulation is most common in both the sexes.
It is also common in men (n = 69, 34.8%), slightly more compared
to women (n = 29, 28.4%).
In a large study of Indian subcontinental patients, Kapoor
et al.,[14] observed that 45.2% conformed to the typical paern.
In the rest of the study group there were variations. In other
studies, complete CW was observed in 4.6-72.2%.[10,37] The
major variation is observed in incidence of complete CW, in
autopsy studies, ranging from 14.2 to 52.3%;[36] relatively lower
than clinical observation. Several reasons could account for
this variation; rst, the subject selection dierence, healthy
volunteers were included in some studies with additional
subjects without vascular disorders or even some with
neurological disease.[28,38] Second reason could be due to the
dierence in observation method, like studies wherein phase-
contrasted MR angiography was utilized in addition to TOF
study.[28] Third reason is related to the criteria set for what
constitutes a complete-circled conguration. We applied the
criterion of the vessel diameter less than 0.8 mm as absence
in determining the prevalence of various anatomical variants.
Some autopsy studies used 1 mm as their lower limit.[16,36]
Although TOF-MRA demonstrates high sensitivity in detecting
intracranial arteries, this technique has its disadvantages.
Slow or turbulent ow may not be demonstrated in the TOF-
MRA images, even though the vessels are patent. Therefore,
the prevalence of the complete conguration of circle may be
underestimated. The incidence of incomplete conguration
of the circle is 22%. Among these variations, a single major
ICA supplying several cerebral arterial territories, with lile
collateral ow provided by other arteries, should be taken
note carefully. Such variation, called isolated circulation, is
an important observation for preoperative surgical planning;
especially when temporary or permanent occlusion of the
parent artery is anticipated. In such instance, temporary
occlusion of the ICA during carotid endarterectomy would
lead to the risk of ischemic insult in the watershed area between
these separately perfused territories.
Reasons for variations in the segments of the CW have been
hypothesized. Genetic factors[10] and postnatal development
of the brain following occlusive diseases[39] are main theories.
From the evolutionary standpoint, it is noteworthy that
variations of the cerebral arteries seem to be equally common
in humans as well as animals.[40]
We noted transitional-type posterior circles in our population,
with an incidence of 4%. Interestingly, we also observed
higher occurrence of posterior variant (variant J) in the study
population. We believe that, these observed variants call
for need of larger population-based studies to improve our
understanding of regional variations.
In a large study involving north Indian subject groups,
intracranial saccular aneurysm was present in 1%[14] and AVMs
in 1.4% in the PCA. In our study, aneurysms were detected in
1%; noted in both the A com A and the le ACA. AVMs were
found in a single case (0.33%) in the right parietooccipital
region. Persistent le trigeminal artery was noted in another.
Hence, our observations are in conformity with earlier studies.
Conclusion
The morphological variations demonstrated by TOF-MRA in our
study provide an important reference source for CW variations in
the regional population. Our ndings conrm the view that the
conguration of the CW vary largely in our general population.
The prevalence of complete conguration of the circle was16.6%
and is slightly higher in females than males and younger
(below 50 years) than older subjects. Complete anterior CW is
more common with incidence of 77.3% of all the subjects. The
most common anterior circle variant is type A (normal anterior
conguration) with a prevalence of 66%. The most common
posterior circle variant is type E (hypoplasia or absence of both
Pcom As and isolation of the anterior and posterior parts of the
circle at this level) with incidence of 32.6%. Overall, CW variants
are slightly more common among the women in comparison
to men. Incidence of associated anomalies, like aneurysm and
AVM, is comparable to that described in literature.
Acknowledgements
Authors would like to thank the valuable contribution of radiology
collegues, Dr. Dayananda, Dr. Murugan, Dr. Shiv Kumar Swamy and
neurology colleagues for their contributions towards this work.
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How to cite this article: Naveen SR, Bhat V, Karthik GA. Magnetic
resonance angiographic evaluation of circle of Willis: A morphologic
study in a tertiary hospital set up. Ann Indian Acad Neurol 0;0:0.
Received: 04-01-15, Revised: 26-01-15, Accepted: 01-03-15
Source of Support: Nil, Conicts of Interest: None declared.
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