Intracranial Cerebrovascular Evaluation

Internal Carotid Artery

The internal carotid artery (ICA) is divided into four main segments: (1) the cervical ICA originates at the common carotid bifurcation (carotid bulb) and ends as it enters the carotid canal of the temporal bone at the base of the skull; (2) the petrous section of the ICA begins at the entrance of the carotid canal within the petrous portion of the bone and continues until it traverses the cranial portion of the foramen lacerum and passes into the cavernous sinus; (3) the cavernous segment extends from the foramen lacerum and cavernous sinus entrance to just medial of the anterior clinoid process; and (4) the supraclinoid portion of the ICA enters the intracranial space at the anterior clinoid and continues to its termination where it bifurcates into the middle cerebral and anterior cerebral arteries. During a TCD examination, the terminal portion of the ICA, just proximal to its bifurcation, and the more proximal carotid siphon are evaluated. The carotid siphon is an S-shaped curve in the ICA formed by a posterior and then anterior bend. This begins in the cavernous segment and continues to the ICA bifurcation. The internal carotid siphon is a common site of atherosclerotic disease in adults.

Ophthalmic Artery

The ophthalmic artery (OA) originates as the first branch of the ICA just distal to the cavernous sinus. The OA travels anterolaterally and slightly deep through the optic foramen to perfuse the globe, orbit, and adjacent structures. This artery has three major groups of branches: (1) the ocular branches, (2) the orbital branches, and (3) the extraorbital branches. The branches of the OA often play an important role in collateral pathway formation as a result of internal or external carotid artery (ECA) disease. The OA is evaluated during a TCD examination.

Vertebral Arteries

The vertebral arteries are large branches of the subclavian arteries. The left vertebral artery is dominant in approximately 50% of individuals, the right in approximately 25%, and codominant in the remaining 25%; that is to say, vertebral artery size asymmetry is common. The vertebral artery is divided into four segments: (1) extravertebral, (2) intervertebral, (3) horizontal, and (4) intracranial. The intracranial portion begins as it pierces the dura and arachnoid immediately below the base of the skull at the foramen magnum. It continues anterior and medial to the anterior surface of the medulla and unites with the contralateral vertebral artery to form the basilar artery. Several major branches arise from this segment of the vertebral artery, with the posterior inferior cerebral artery (PICA) being the largest and commonly arising approximately 1 to 2 cm proximal to the confluence of the two vertebral arteries forming the basilar artery. During TCD examinations, it is the intracranial segment of the vertebral arteries that is evaluated. The PICA, along with another branch of the vertebral artery, the anterior spinal artery, can occasionally be visualized on TCD imaging but are not routinely included as part of the examination.

Basilar Artery

The basilar artery (BA) is evaluated during TCD imaging and is formed by the union of the vertebral arteries at the lower border of the pons (pontomedullary junction). The basilar, when combined with the vertebral arteries, is often referred to as the vertebrobasilar system , which perfuses the posterior portion of the circle of Willis. From its origin, the BA extends anteriorly and superiorly and bifurcates into the paired posterior cerebral arteries. There are several branches of the BA, including the anterior inferior cerebellar arteries, the internal auditory (labyrinthine) arteries, the pontine branches, and the superior cerebellar arteries, just proximal to the posterior cerebral arteries. The BA is often variable in its path, size, and length; is typically tortuous; and may be duplicated or fenestrated.

Circle of Willis

The circle of Willis was first described in 1664 by Thomas Willis, characterized by the arterial anastomoses at the base of the brain. The circle is composed of the A1 segments of the two anterior cerebral arteries, anterior communicating arteries (ACoAs), posterior communicating arteries (PCoAs), the terminal portions of the ICAs, and the P1 segments of the two posterior cerebral arteries. These intracranial arteries form a polygon vascular ring at the base of the brain that permits communication between the right and left cerebral hemispheres (via the ACoA) and the anterior and posterior systems (via the PCoA). These communications are important in the presence of significant disease or occlusion of a major cervical artery, as they serve as compensatory perfusion mechanisms. Variations in the circle of Willis are common, as it is estimated that an anatomically complete (classic) circle of Willis is present in only one-third of the population; however, a physiologically adequate circle of Willis exists in approximately two-thirds. Significant hypoplasia and absence of the PCoA, the ACoA, the A1 segment of the anterior cerebral artery, and the P1 segment of the posterior cerebral artery are the most common variations.

Middle Cerebral Artery

The middle cerebral artery (MCA) is the larger terminal branch of the ICA. From its origin, the MCA extends laterally and horizontally in the lateral cerebral fissure. The horizontal segment may course superficial or deep. The MCA either bifurcates or trifurcates before the limen insulae (a small gyrus), where the branches turn upward into the Sylvian fissure, forming its genu (“knee”). The vessels travel around the island of Reil, which is a triangular mound of cortex, and run posterosuperiorly within the Sylvian fissure. The terminal branches of the MCA anastomose with the terminal branches of the anterior cerebral and posterior cerebral arteries.

The MCA can be divided into four segments (M1 to M4). The main horizontal section of the MCA, from its origin to the limen insulae, is the M1 segment. The M1 segment gives rise to numerous small lenticulostriate branches. The M2 segment is composed of the branches overlying the insular surface in the deep Sylvian fissure. The M1 segment and the origin of the M2 segment are evaluated during TCD imaging. The initial MCA bifurcation is a common site for intracranial aneurysmal formation. The MCA may also be the site of arterial stenosis and/or occlusion.

Anterior Cerebral Artery

The anterior cerebral artery (ACA) is the smaller of the two terminal branches of the ICA. From its origin, the ACA courses anteromedially over the optic chiasm and the optic nerve to the interhemispheric fissure (longitudinal cerebral fissure). The proximal horizontal portion of the ACA is known as the A1 segment and is connected to the contralateral A1 segment via the ACoA (see the following section). The A1 segment is evaluated during a TCD imaging examination and serves as a midline marker. The contour of the A1 segment may take a horizontal course, ascend, or slightly descend. The complete absence of the A1 segment is unusual. An anomalous origin of the ACA is rare, and asymmetry between the bilateral A1 segments is uncommon. A direct inverse correlation tends to exist between the size of the A1 segment and the size of the ACoA. A small or hypoplastic A1 is typically found in conjunction with a large ACoA, as the contralateral A1 segment supplies most of the blood flow to both distal ACA territories. It has been well documented that individuals with anomalies of the A1 segment have a higher incidence of ACoA aneurysms. Stenosis or occlusion of the ACA may be found, but this has been found to be less common than in other intracranial vessels.

Distal to the ACoA, the ACA angles superiorly and travels in the interhemispheric fissure. The ACA curves anterosuperiorly around the genu of the corpus callosum. The segment of the ACA extending from the ACoA to the distal ACA bifurcation (callosomarginal artery and pericallosal artery) is termed the A2 segment . The proximal portion of the bilateral A2 segments may be visualized in some patients during TCD imaging and should be documented if possible. The distal A2 segments anastomose with branches of the posterior cerebral arteries. A large medial striate artery (recurrent artery of Heubner) is a major branch of the proximal A2 segment in approximately 20% of individuals, but this artery can also originate from the distal A1 segment (approximately 15%) or from the ACA/ACoA junction (approximately 60%).

Anterior Communicating Artery

The anterior communicating artery (ACoA) is a short vessel that connects the A1 segments of the ACAs at the interhemispheric fissure. The ACoA is typically a single vessel but may be duplicated, absent, or a multichanneled system. The ACoA is typically short in length; however, this has been found to be variable. Longer ACoAs tend to be curved, tortuous, and/or kinked. The ACoA is often the location for congenital anomalies and is the most common site for intracranial aneurysm formation (25%), as well as the most common site for aneurysms associated with SAH. A Doppler waveform of the ACoA may be captured at midline, but it cannot be visualized with TCD imaging.

Posterior Cerebral Arteries

At the approximate level of the pontomesencephalic junction, the basilar artery termination is its bifurcation into the bilateral posterior cerebral arteries (PCAs) . From their origin, each PCA travels anterolaterally to perfuse the posterior occipital lobe. The portion of the PCA extending from its origin to its junction with the PCoA can be referred to as the pre-communicating portion but is more commonly termed the P1 segment . The P1 segment is evaluated during TCD imaging. Throughout this segment is the origin of many perforating branches that perfuse the brainstem and thalamus. The portion of the vessel extending posteriorly from the PCoA to the posterior aspect of the midbrain is the P2 segment of the PCA, which may be visualized with TCD imaging. The proximal portion of the PCA is typically asymmetric. In cases of “fetal” origin, the P1 segment is hypoplastic or smaller than the PCoA. It is uncommon for occlusive disease to be limited to the PCA, but if it does occur, the P2 segment is most commonly affected. The PCA is further divided into the P3 and P4 segments, but these cannot be visualized with TCD imaging.

Posterior Communicating Artery

The posterior communicating artery (PCoA) is a paired artery that travels posterior and medial to provide a connection between the ICA and PCA. The PCoA is highly variable in size and may angle upward or downward. Hypoplasia is a common PCoA anomaly, which occurs in an estimated 25% of individuals. However, the PCoA can be enlarged in circumstances of a hypoplastic posterior cerebral artery, which occurs in 10% to 20% of cases. This is termed a fetal origin of the posterior cerebral artery. The PCoA does not generally function as an important collateral pathway except in the presence of extensive bilateral extracranial occlusive disease or the absence of a patent ACA. The PCoA is typically evaluated during TCD imaging.