Project – Radiological Assessment of Anatomical Variations of the Circle of Willis: A Study of CT Angiographic Images at LUTH

CHAPTER ONE

INTRODUCTION

1.1 Background to the Study

The Circle of Willis, also referred to as the cerebral arterial circle, is an arterial anastomotic network situated at the base of the brain around the optic chiasm, hypothalamic region and interpeduncular fossa. It is formed principally by the anterior cerebral arteries, anterior communicating artery, terminal portions of the internal carotid arteries, posterior communicating arteries and posterior cerebral arteries. Anatomically, the Circle of Willis provides an important potential pathway for collateral blood flow between the anterior and posterior cerebral circulations and between the right and left sides of the cerebral arterial system. Its strategic position and relationship with major intracranial arteries make knowledge of its anatomy essential in neuroanatomy, radiology, neurosurgery and cerebrovascular medicine (Standring, 2021; Snell, 2019).

Although the Circle of Willis is traditionally represented as a complete symmetrical arterial polygon, anatomical studies have demonstrated that the classical configuration is relatively uncommon. Variations may involve the presence, absence, size, calibre, origin or course of individual arterial segments. Common variants include hypoplasia or absence of the posterior communicating artery, fetal-type origin of the posterior cerebral artery, hypoplasia or absence of segments of the anterior cerebral artery, duplication of the anterior communicating artery and other configurations involving the anterior and posterior components of the arterial circle. A large systematic review and meta-analysis by Jones et al. (2021) estimated that approximately 68% of individuals have some form of Circle of Willis variation, demonstrating that anatomical variation should be regarded as an important component of normal cerebral arterial anatomy rather than as an exceptional finding.

The prevalence and pattern of Circle of Willis variations differ substantially among populations and according to the method used for assessment. For example, Klimek-Piotrowska et al. (2013), using computed tomography angiography in 250 individuals, found that only 16.8% demonstrated the typical configuration of the Circle of Willis, while substantial variation occurred in both its anterior and posterior components. Similarly, Li et al. (2011), in a multidetector CT angiographic study of a Chinese population, found a complete Circle of Willis in only 27% of participants, with the posterior portion being particularly variable. These findings demonstrate that population-specific investigation is necessary because prevalence figures obtained from one population cannot necessarily be applied directly to another population.

Computed Tomography Angiography (CTA) has become an important non-invasive radiological technique for demonstrating intracranial arterial anatomy. CTA permits visualization of the lumen, course and configuration of cerebral arteries and allows three-dimensional reconstruction of vascular structures. Compared with conventional catheter angiography, CTA is less invasive and can be incorporated into the evaluation of patients suspected of having cerebrovascular disease. Its ability to demonstrate the individual components of the Circle of Willis makes it particularly useful for identifying hypoplastic, absent, duplicated and variant arterial segments. Coulier (2021) demonstrated that CTA with three-dimensional volume-rendered reconstructions could identify a wide range of Circle of Willis configurations and emphasized its value in evaluating patients in whom vascular anatomy is clinically relevant.

The anatomical variations of the Circle of Willis have important implications for cerebral collateral circulation. In a theoretically complete Circle, blood may potentially be redistributed between vascular territories when one major arterial pathway becomes narrowed or obstructed. However, hypoplasia or absence of communicating arteries can reduce the effectiveness of this collateral pathway. In a CTA study involving patients with carotid artery disease, Hoksbergen et al. and subsequent imaging research demonstrated that Circle of Willis variants can be associated with compromised collateral circulation and cerebral ischemic patterns. Similarly, Li et al. (2011) reported substantial differences in the completeness of anterior and posterior collateral pathways in their CTA population. Therefore, identifying the configuration of the Circle of Willis can provide useful information for understanding the potential capacity of cerebral collateral circulation.

The clinical significance of Circle of Willis variations is also evident in cerebrovascular disease and intracranial aneurysms. Anatomical differences in arterial calibre and configuration may alter haemodynamic conditions within the cerebral circulation. A systematic review and meta-analysis by Feng et al. (2023) found significant associations between certain Circle of Willis variants and the formation and rupture of aneurysms involving the anterior and posterior communicating arteries. In particular, hypoplasia or aplasia of the A1 segment of the anterior cerebral artery was associated with increased odds of anterior communicating artery aneurysm formation and rupture, while fetal-type posterior cerebral artery configuration was associated with posterior communicating artery aneurysms. These findings reinforce the importance of recognising Circle of Willis variants during radiological evaluation and clinical management of intracranial vascular disorders.

The Circle of Willis is additionally important in neurosurgical and endovascular procedures because knowledge of individual vascular anatomy can influence operative planning and assessment of potential collateral pathways. Variations in the communicating arteries and cerebral arterial segments may affect the consequences of arterial occlusion, surgical manipulation or endovascular intervention. A Nigerian anatomical study by Idowu et al. (2010) found that anomalies of the posterior Circle of Willis were common and particularly involved the posterior communicating artery region, emphasizing that such variations should be considered during skull-base and carotid surgery as well as cerebral angiography. Consequently, radiological documentation of Circle of Willis anatomy can provide valuable anatomical information before interventions involving the intracranial circulation.

At Lagos University Teaching Hospital (LUTH), the Department of Radiodiagnosis provides imaging services involving the central nervous system and undertakes research activities. This provides an appropriate clinical environment for examining cerebral arterial anatomy using radiological imaging. A systematic assessment of CT angiographic images at LUTH can provide information about the prevalence and distribution of Circle of Willis variants among individuals undergoing CTA within the study environment. Such locally derived data may contribute to Nigerian anatomical and radiological literature, provide information for comparison with findings from other populations, and improve understanding of the anatomical variations encountered in routine neurovascular imaging.

1.2 Statement of the Problem

The Circle of Willis is commonly represented in anatomy textbooks as a complete and symmetrical arterial circle, but evidence from anatomical and radiological studies demonstrates that this textbook configuration is not present in a substantial proportion of individuals. Variations involving arterial absence, hypoplasia, duplication and fetal-type configurations are frequently encountered. The considerable prevalence of these variants means that treating the classical configuration as the expected pattern for every individual may result in inadequate understanding of actual cerebral arterial anatomy (Jones et al., 2021; Klimek-Piotrowska et al., 2013).

A major problem is the substantial variation in reported prevalence and distribution of Circle of Willis variants across different populations. Studies using CTA, MRA and anatomical dissection have produced different estimates of complete and incomplete configurations. These differences may reflect genuine population variation as well as differences in imaging techniques, definitions of hypoplasia, sample characteristics and classification systems. The absence of sufficiently comprehensive local radiological data therefore makes it difficult to determine whether prevalence patterns reported from European, Asian or other populations accurately represent individuals undergoing imaging in Nigeria (Li et al., 2011; Jones et al., 2021).

Another problem concerns the clinical consequences of failing to recognise important vascular variants before cerebrovascular intervention. A hypoplastic or absent communicating artery may reduce the potential for collateral blood flow during arterial obstruction, while certain configurations have been associated with cerebrovascular ischemia and intracranial aneurysms. Because the Circle of Willis is involved in the redistribution of cerebral blood flow, inadequate recognition of its configuration may limit accurate assessment of vascular risk and may be particularly important in patients being evaluated for stroke, aneurysm, carotid disease or neurosurgical intervention (Coulier, 2021; Feng et al., 2023).

Furthermore, there is a need for more radiological information on Circle of Willis variations in the Nigerian population, particularly from routinely acquired CT angiographic images in major tertiary hospitals. Although Nigerian anatomical research has documented important variations in the posterior cerebral circulation, the increasing availability and clinical application of CTA provide an opportunity to examine these variations in living subjects using modern radiological techniques. A systematic assessment of CTA images at LUTH can therefore address an important local evidence gap by documenting the common configurations, arterial variants and patterns of Circle of Willis anatomy encountered in the study population (Idowu et al., 2010; Jones et al., 2021).

1.3 Aim of the Study

The aim of this study is to radiologically assess the anatomical variations of the Circle of Willis using CT angiographic images at Lagos University Teaching Hospital (LUTH).

1.4 Objectives of the Study

The specific objectives are to:

1. Determine the prevalence and types of anatomical variations of the Circle of Willis observed on CT angiographic images at LUTH.
2. Assess the frequency of variations involving the anterior and posterior components of the Circle of Willis, including hypoplasia, aplasia, duplication and fetal-type configurations.
3. Determine the association between selected demographic characteristics, particularly age and sex, and the observed anatomical variations of the Circle of Willis.
4. Evaluate the radiological and clinical significance of the identified Circle of Willis variations, particularly in relation to cerebral collateral circulation, cerebrovascular disease and neurovascular intervention.

1.5 Research Questions

The following research questions will guide the study:

1. What are the prevalence and types of anatomical variations of the Circle of Willis observed on CT angiographic images at LUTH?
2. What variations occur in the anterior and posterior components of the Circle of Willis, and how frequently do they occur?
3. Is there a significant association between selected demographic characteristics, particularly age and sex, and Circle of Willis variations?
4. What are the radiological and clinical implications of the identified Circle of Willis variations?

1.6 Research Hypothesis

Null Hypothesis (H₀): There is no statistically significant association between selected demographic characteristics and the anatomical variations of the Circle of Willis observed on CT angiographic images at Lagos University Teaching Hospital.

1.7 Significance of the Study

The study will be significant to radiologists because it will provide information on the range and frequency of Circle of Willis configurations encountered in CT angiographic examinations at LUTH. Understanding the expected spectrum of normal vascular anatomy can assist radiologists in recognising variants accurately and distinguishing anatomical variation from pathological vascular abnormalities.

The study will also be useful to anatomists and anatomy educators. The Circle of Willis is an important component of neurovascular anatomy, and locally derived CTA findings can complement traditional cadaveric descriptions of the cerebral arterial circulation. The findings may provide additional teaching material illustrating the fact that considerable variation exists in the arterial configuration of the human brain.

The findings will be relevant to neurologists, neurosurgeons and neurointerventional specialists because Circle of Willis configuration influences potential collateral pathways and may be important when planning procedures involving intracranial arteries. Recognition of specific variants may be particularly useful in patients undergoing evaluation or treatment for cerebrovascular disorders and before selected neurovascular interventions (Coulier, 2021; Idowu et al., 2010).

The study will further benefit stroke and cerebrovascular researchers by providing information about the anatomical patterns of cerebral arterial collateral circulation within the study population. Since the completeness and configuration of the Circle of Willis can influence collateral perfusion, local prevalence data may provide a foundation for subsequent studies investigating the relationship between vascular variants and stroke outcomes.

Finally, the study will contribute to Nigerian radiological and anatomical literature by generating data from CT angiographic examinations at LUTH. The findings may serve as baseline information for future multicentre Nigerian studies and may permit comparison between the local population and populations reported in international imaging studies.

1.8 Scope of the Study

The study will focus on the radiological assessment of anatomical variations of the Circle of Willis using CT angiographic images obtained at Lagos University Teaching Hospital.

The study will examine the major components of the Circle of Willis, including the anterior cerebral arteries, anterior communicating artery, internal carotid arteries, posterior communicating arteries and posterior cerebral arteries. Particular attention will be given to variations such as hypoplasia, aplasia or absence, duplication, and fetal-type posterior cerebral artery configuration.

The study will also examine selected demographic characteristics, particularly age and sex, where such information is available and appropriate for the study. The clinical significance of the identified variants will be discussed in relation to collateral cerebral circulation, cerebrovascular disease, intracranial aneurysms and neurovascular procedures.

1.9 Operational Definition of Terms

Circle of Willis: An arterial anastomotic network located at the base of the brain that connects the anterior and posterior cerebral circulations.

Anatomical Variation: A difference in the structure, configuration, calibre, origin or presence of an anatomical structure compared with the commonly described textbook configuration.

Computed Tomography Angiography (CTA): A radiological imaging technique that combines computed tomography with intravenous contrast administration to demonstrate blood vessels and their anatomical relationships.

CT Angiographic Image: A cross-sectional or reconstructed CT image obtained during angiographic imaging and used to visualise arterial structures.

Anterior Cerebral Artery (ACA): A major branch of the internal carotid artery supplying the medial portions of the cerebral hemispheres and contributing to the anterior part of the Circle of Willis.

Anterior Communicating Artery (ACoA): A short arterial vessel connecting the right and left anterior cerebral arteries and forming the anterior communicating component of the Circle of Willis.

Posterior Communicating Artery (PCoA): An artery connecting the internal carotid circulation with the posterior cerebral artery and contributing to the posterior portion of the Circle of Willis.

Posterior Cerebral Artery (PCA): A major artery supplying the posterior portions of the cerebral hemispheres and forming part of the posterior circulation of the Circle of Willis.

Fetal-Type Posterior Cerebral Artery: A configuration in which the posterior cerebral artery has a predominant or significant origin from the internal carotid artery through the posterior communicating artery rather than primarily from the basilar artery.

Hypoplasia: Abnormally small calibre or underdevelopment of an arterial segment.

Aplasia: Complete absence or failure of development of an anatomical structure.

Collateral Circulation: An alternative vascular pathway that can provide blood flow to a tissue when the normal principal arterial pathway is reduced or obstructed.

Complete Circle of Willis: A configuration in which the principal components of the arterial circle are present and sufficiently connected to form a continuous arterial pathway.

Incomplete Circle of Willis: A configuration in which one or more components of the arterial circle are absent, severely hypoplastic or otherwise interrupted.

Radiological Assessment: Systematic evaluation and interpretation of anatomical structures using medical imaging techniques.

Clinical Significance: The relevance of an anatomical finding to disease processes, diagnosis, prognosis, treatment or medical intervention.

Project – Radiological Assessment of Anatomical Variations of the Circle of Willis: A Study of CT Angiographic Images at LUTH