## 1 Introduction  
The vertebral arteries (VAs), critical blood vessels of posterior brain circulation, can exhibit a variety of anatomical variations which have an important implication in medical procedures, specifically in the context of spine surgeries [1, 2]. During cervical spine surgeries and endovascular interventions, the VA is at risk of injury due to its close relationship to the vertebrae and the presence of anomalies, such as variant origin [3, 4, 5], unusual entrance [6, 7, 8, 9, 10], and course in the cervical spine [11, 12], hypoplasia [13], and tortuosity [14]. These anomalies make it more difficult to identify and access the VAs during surgeries and may lead to inadvertent injuries, which can have detrimental consequences for patients [2]. Therefore, these injuries need to be prevented. Research into a comprehensive understanding of the different types of VA anomalies and their clinical impact could help clinicians find patients who are at risk for VA injury during surgery, choose the best surgical approach, and improve patient clinical outcomes. VA is known to present variations in its origin [3, 4, 5] and course [10, 11]. Typically, each VA arises from the first part of each subclavian artery and then ascends to reach the level of the transverse foramen of the C6 vertebra. Then, it ascends through the transverse foramina of C6 vertebra to the C1 vertebra, curves medially behind the lateral mass of the atlas, and enters the cranial cavity through the foramen magnum. At the pontomedullary junction, the right and left VAs converge into the basilar artery [15]. The intracranial segment of VA gives various branches, including the anterior spinal artery, medial and lateral medullary branches, ramus choroideous ventriculi quarti, and posterior inferior cerebellar artery [16]. Classically, the VA course is divided into four segments: V0 (its origin), V1 is a short pre-foraminal segment, V2 (intraforaminal segment) extends between the transverse foramina of the C6 and C2 vertebrae, V3 runs between the C2 foramen and foramen magnum (extraforaminal segment), and V4 refers to the intracranial portion of the artery [11; 17]. Both cadaveric [18] and radiographic studies (CT and MRI) [6; 8; 9; 19; 20] have been widely investigated for VA variations. In recent years, computed tomography angiography (CTA) has become a popular choice in research as it has the advantage of displaying all the characteristics of blood vessels and precise measurements of variations. Rarely, VA may arise from the aortic arch and right common carotid artery (CCA) [3]. The unusual entrance transverse foramen (UE-V2) and midline migration (MM) are the two important variations of the V2 segment [11; 12]. On the other hand, persistent first intersegmental artery (PFIA) and paracondyl processes (PP) are frequently reported variations of the V3 segment [11; 15; 21; 22]. Classically, VA dominance is defined as an unequal diameter of VA with a significant difference between both sides. It is also considered a congenital variation of VA [2]. The reported incidences of V2 and V3 segment variations varied among different studies worldwide [6; 8; 9; 10; 23; 24]. The influence of regional, ethnic, and environmental factors on VA variability has been implicated [25]. Adequate radiological data on anatomical VA variations are important parts of the pre-surgical assessment of VA. Particularly, V3 and V2 segment variations are utmost important to avoid attorgenic injuries during cervical spine surgical procedures, including corpectomy, laminectomy, and while implanting plates, screws, and artificial discs [11; 26; 27]. Knowledge about these variations is also clinically important, as their presence increases the risk of vertebra-basilar circulatory disorders [28; 29]. Despite the potential problems posed by VAs variants, there are still knowledge gaps that must be addressed to enhance the understanding of VA anomalies and their associated clinical impact.