Cranial Arterial Anatomy

(1)

Nuffield Department of Surgical Sciences, Oxford University, Oxford, UK

Preamble

The purpose of this tutorial is to outline cranial arterial anatomy for the student endovascular therapist. These details are reproduced widely in text books, so why include them in the tutorials of the Oxford Course? The answer is to give guidance by providing context and suggesting priorities in this core knowledge. The tutorial therefore intends to equip the reader with an understanding of the detail that they need to master in order to practice. Like other tutorials, the attempt is to be both comprehensive and selective. By emphasising areas important to the performance of endovascular treatments, it aims to avoid simply reproducing a standard anatomical text.

The blood supply of the cranium is traditionally described separately as cerebral and cranio-facial circulations. The two systems are obviously not isolated from each other, and understanding and recognition of actual and potential connections between them are fundamental practical skills in endovascular neurosurgery. Anastomoses between extra- and intracranial blood supplies are potential dangers and their recognition underpins safe interventions in the head and neck. They will be outlined here but considered again in Tutorial 7. The student needs to realise that if an artery is rarely seen on angiography, it doesn’t mean that it is not there or a potential route that may cause a complication during embolisation. Familiarity with small arteries not normally seen on angiography is important for this reason because they need to be identified if a disease causes their enlargement. If you don’t know where to look, you won’t find them.

2.1 Internal Carotid Artery (ICA)

The internal carotid artery originates in the neck as a terminal branch of the common carotid artery (CCA) at the level of the thyroid cartilage, i.e. C3 or C4 vertebrae (but varying between extremes at D1 and C1). It terminates intracranially at the inferior surface of the brain by dividing into anterior and middle cerebral arteries.

No single system for identifying different sections of the large cerebral arteries has been generally adopted. This is particularly the case for the ICA. Fischer in 1938 [4] used a simple code (A1, A2, M1, M2, P1, P3, etc.) to describe sequential arterial sections in the direction of blood flow at and above the circle of Willis¹ based on branch points. This is intuitive and is generally consistently applied in the literature. However, for ICA he used five sections (C1–C5) but applied them in reverse (i.e. from distal to proximal and against the direction of blood flow). Subsequent authors misinterpreted or ignored this convention, and there is now confusion in the literature over the naming of sections of the ICA. So, simple anatomical descriptors will be used for this artery and Fischer’s convention for the arteries comprising the circle of Willis. Thus, the ICA will be discussed in four sections: cervical, petrous, cavernous and supraclinoid portions from proximal to distal.

2.1.1 The ICA Cervical Portion

This extends from the bifurcation of the CCA to the skull base. In this section, the artery lies in the carotid sheath with the internal jugular vein laterally and the vagus cranial nerve (tenth) and the cranial root of the accessory nerve (eleventh) that travel with the tenth, lying posteriorly and between these vessels. The sheath, which is comprised of all three layers of the deep cervical fascia, also contains lymph nodes and sympathetic postganglionic fibres from the superior cervical ganglion.

The internal diameter is about 4–5 mm throughout, except at the carotid sinus (often called the carotid bulb by angiographers) where the artery is 7.5 mm wide for a distance of 15–25 mm. The wall of the carotid sinus contains baroreceptors to monitor systemic blood pressure and the carotid body. The carotid body houses chemoreceptor cells that monitor blood oxygen, CO₂ and pH levels and stimulate respiration and heart rate in response to detected hypoxia. These receptors are connected via nerve fibres of Xth and XIth cranial nerves to the cardiovascular centre in the medulla oblongata and nerve endings in the carotid sinus connected to the inferior ganglion of the vagus. The parasympathetic nervous system modulates systemic blood pressure, and endovascular stimulation of the sinus may simulate its physiological response to increases in pressure by signalling to reduce heart rate and, by inhibiting the vasoconstrictor centre of the medulla oblongata, causing peripheral vasodilatation.

The ICA normally (in 80% of individuals) lies initially behind and lateral to the external carotid artery (ECA) but as these arteries run cranially, the ECA inclines superficially to lie lateral to the ICA on a frontal angiogram. With the exception of anatomical variants, no named branches arise from the cervical portion of the ICA. The most common variant is that the ascending pharyngeal artery (APA) arises from the proximal ICA. Other ECA branches may arise from the ICA and agenesis or hypoplasia may occur, as discussed in Tutorial 1.

2.1.2 The ICA Petrous Portion

During its intrapetrous course, the ICA is initially situated within the bony carotid canal before entering the cartilaginous foramen lacerum. It makes a right angle turn in the canal after an initial short vertical section and then runs anteromedially in the horizontal plane where the bony canal is continuous with the foramen lacerum. Its anatomical limit is the petroclinoid ligament where it turns upwards into the cavernous sinus. Sympathetic postganglionic fibres continue into the carotid canal with the ICA where in the horizontal section it is accompanied by a venous plexus. The sympathetic plexus, derived from the superior cervical ganglion, is vulnerable in cases of arterial wall dissection, and its damage causes ipsilateral Horner’s syndrome. It leaves the artery in the horizontal section to join the Vidian² nerve (with the greater superficial petrosal nerve) in the pterygoid canal and runs anteromedially to the pterygopalatine fossa.

Branches: From the horizontal segment of the petrous ICA arise the caroticotympanic artery and the mandibulo-Vidian trunk (MVT). The former gives the tympanic artery to supply the middle ear. The MVT arises from the ICA in the foramen lacerum. It gives the artery of the pterygoid canal (or Vidian artery) which supplies the sphenoid sinus and anastomoses with the Vidian artery arising from the proximal portion of the internal maxillary artery (IMA) and the mandibular artery. The mandibular artery takes part in the anastomoses around the Eustachian³ tube.

2.1.3 The ICA Cavernous Portion

Following its petrous passage, the ICA enters the cavernous sinus and lies medial to the Gasserian ganglion, the ophthalmic division of the trigeminal nerve (Vth) and the oculomotor (IIIrd), trochlear (IVth) and abducens (VIth) cranial nerves. It runs horizontally forwards and then turns superiorly and medial to the anterior clinoid process, passing through the dural ring to its final intradural course (Fig. 2.1). The branches that arise in this portion of the ICA are small and difficult to identify individually on angiography but nevertheless as important to the endovascular therapist as to a pituitary surgeon.

Fig. 2.1

Inferolateral trunk. Diagram showing the three branches of the inferolateral trunk. ILT inferolateral trunk, OphA ophthalmic artery, MHT meningohypophyseal trunk (Published with kind permission of © Henry Byrne, 2017. All rights reserved)

2.1.3.1 Branches of the Cavernous ICA

These will be described in three groups.

Group 1: The meningohypophyseal trunk (MHT)

The MHT and posterior inferior hypophyseal artery are remnants of the first branchial arch artery and arise from the proximal section of the cavernous ICA (Fig. 1.14). These small vessels may arise from a common trunk or separately as three arteries:

(a)

Marginal tentorial and basal tentorial arteries. These dural arteries usually arise as a common trunk (called the dorsal meningeal artery). The marginal (or medial) tentorial artery is memorable because of its wonderfully musical eponymous label as the artery of Bernasconi and Cassinari [1]. It follows the free edge of the tentorium posteriorly and therefore runs medial to the basal artery. The basal tentorial artery runs laterally and divides over the tentorium posterior to its attachment to the petrous ridge. It anastomoses with the posterior branch of the middle meningeal artery (MMA).
(b)

Lateral clival artery. This vessel supplies the dura of the clivus dividing into lateral and inferolateral branches, which follow the superior and inferior petrosal sinuses, respectively. They anastomose with the contralateral artery, with dural branches of the MMAand jugular branch of the (APA).
(c)

Posterior inferior hypophyseal artery (PIHA). The PIHA supplies the posterior lobe of the pituitary and anastomoses with the capsular arteries of McConnell as well as its contralateral counterpart. It gives a medial clival branch (also, in my view confusingly, called the dorsal meningeal artery by some authors) which anastomosis with the clival meningeal branches of the hypoglossal artery (another APA branch).

Group 2: The inferolateral trunk (ILT)

The ILT was known as the inferior cavernous sinus artery, until renamed by Wickborn and Stattin in 1958 [2] after they identified it on angiograms performed to investigate a meningioma.

It arises on the lateral side of the midsection of the cavernous ICA and crosses over the VIth cranial nerve to divide into three principal branches (Fig. 2.1). These are:

(a)

A superior branch which returns medially towards the roof of the cavernous sinus, which it supplies together with the IIIrd and IVth cranial nerves as they lie in the wall of the sinus.
(b)

An anterior branch which runs forwards in the cavernous sinus and supplies the IIIrd, IVth and VIth cranial nerves. It gives branches, which traverse the foramen rotundum and the superior ophthalmic fissure to anastomose, respectively, with the artery of the foramen rotundum (a branch of the internal maxillary artery (IMA)) and the deep recurrent ophthalmic artery (i.e. the remnant of the embryonic dorsal ophthalmic artery). A further branch to the foramen ovale anastomoses with the accessory meningeal artery, which, if the ILT is small, may become the dominant vessel supplying its territory.
(c)

A posterior branch which follows the VIth nerve posteriorly, which it supplies as well as the maxillary division of the trigeminal nerve and the Gasserian ganglion. It gives dural branches which anastomoses with the marginal tentorial artery and with the MMA laterally in the middle cranial fossa and the recurrent artery of the foramen lacerum. The recurrent artery of foramen lacerum is a small artery, which returns along the carotid canal to the foramen lacerum.

It is usually a branch of ILT that may arise from the MHT. Its importance is because of a potential anastomosis with the superior pharyngeal branch of the APA in the foramen lacerum.

Group 3: The capsular arteries of McConnell

These are a series of small arteries providing a systemic (rather than portal) supply to the anterior lobe of the pituitary and the sella dura. They are of little relevance to the endovascular therapist though they have been implicated in the formation of the uncommon medially directed aneurysm of the cavernous ICA.

If the ILT is absent, the MHT will supply most of its territory, and if a persistent trigeminal artery is present, its carotid connection is at the level of the MHT.

2.1.4 The ICA Intradural Portion

The supraclinoid portion of the ICA is intradural, the artery having entered the subarachnoid space through the dural ring medial to the anterior clinoid process. It turns posteriorly and runs lateral to the optic nerve to terminate by dividing into the anterior and middle cerebral arteries. From this portion arise successively the ophthalmic artery (OphA), the superior hypophyseal artery or arteries, the posterior communicating artery (PComA) and the anterior choroidal artery (AchA). The level of the OphA origin varies, and it may arise in an ‘extradural’ location below the ring, but this distinction is usually impossible to make from a standard catheter angiogram (DSA) but may be possible on rotational 3D images.

2.1.4.1 Branches

1.

Ophthalmic artery (OphA) (Fig. 2.2)

Fig. 2.2

Arteries of the orbit. Superior view of the ophthalmic artery crossing over the optic nerve and running above the medial rectus muscle (Published with kind permission of © Henry Byrne, 2017. All rights reserved)

The OphA originates from the anterior surface of ICA and runs forwards into the orbit through the optic canal. In the canal, it is initially lateral and then above the optic nerve. Once in the orbit, it runs medially along the upper border of the medial rectus muscle and terminates by dividing into the dorsal nasal artery (or dorsal artery of the nose) and supratrochlear artery. Its major branches are the central artery of the retina (which arises within the optic canal and penetrates the dural sheath of the optic nerve to supply the retina), ciliary arteries (responsible for the choroidal blush), the lacrimal artery (which gives the recurrent meningeal artery and distributes to the lacrimal gland, lateral extraocular muscles and lateral eyelids), the posterior and anterior ethmoidal arteries, the supratrochlear artery and the dorsal nasal artery. The supratrochlear artery runs forwards to the supraorbital notch and is distributed as the supraorbital artery to the skin of the forehead, whilst the dorsal nasal artery supplies superficial structures of the medial orbit and upper nose. The anterior ethmoidal artery gives off anterior meningeal branches (as the anterior artery of the falx) and supplies the mucosa of the superior nasal septum. The posterior ethmoidal artery supplies the posterior ethmoid sinus and part of the posterosuperior aspect of the nasal mucosa. The proximity of the territories of these branches and those of the IMA (in particular the sphenopalatine artery and MMA) makes the study of the vascular anatomy of this region so important (see Tutorial 7). Other small arteries are distributed to the extraocular and palpebrae muscles.

2.

Superior hypophyseal artery

The superior hypophyseal artery is infrequently identified on angiograms since it is small and may arise as a single branch or as several small branches. It supplies the pituitary gland and part of the optic chiasm and intracranial optic nerve.

3.

Posterior communicating artery (PComA)

The PComA is an anastomotic artery with the vertebrobasilar network and part of the circle of Willis. It joins the posterior cerebral artery between the P1 and P2 segments of that artery. It runs posteromedially above the oculomotor cranial nerve to reach the posterior cerebral artery (PCA).

It gives small perforator arteries from its superior surface that supply the pituitary stalk, optic tract, chiasm and the floor of the third ventricle. A group of small arteries supplying the thalamus and hypothalamus and internal capsule are called the anterior thalamoperforating arteries, but most branches arise from its anterior portion and run medially between the mammillary body and tuber cinereum. Perforating arteries, which share the territory of branches of either PComA or the anterior choroidal artery, may arise from the ICA between the two vessels.

4.

Anterior choroidal artery (AchA) (Figs. 2.3 and 2.4)

Fig. 2.3

Artist’s impression of the anterior circle of Willis showing lenticulostriate arteries arising from the anterior cerebral arteries (M medial group) and middle cerebral artery (L lateral group). On the left, the anterior choroidal artery (AChA), and on the right, the recurrent artery of Heubner (RaH) is shown (see Fig. 2.4). Note that anterior perforating arteries also arise from the anterior communicating artery (Published with kind permission of © Henry Byrne, 2017. All rights reserved)

Fig. 2.4

Internal carotid angiograms in the frontal projections. Right (a) shows the recurrent artery of Heubner arising from the A2 (2 arrows) and left (b) with absent anterior cerebral artery shows the anterior choroidal artery (arrows)

The AchA arises from the lateral surface of ICA and is applied immediately to the under surface of the brain. It runs posteriorly, passing inferior to the optic tract (from lateral to medial). It then runs around the cerebral peduncle (which it supplies) and recrosses the tract (from medial to lateral) at the level of the lateral geniculate body before passing into the middle part of the choroidal fissure to reach the choroid plexus of the lateral ventricle. Prior to entering the choroid fissure, it gives a series of perforator arteries, which supply the posterior limb of the internal capsule, in particular its inferior level and part of the retrolenticular segment. It gives small branches to the optic radiation, the lateral geniculate body, the angle of the hippocampus and the amygdala as well as part of the globus pallidum and thalamus. The intraventricular terminal branches anastomose with the lateral posterior choroidal artery and follow the choroid plexus from the temporal horn to the trigone area [3]. It may, in addition, supply part of the inferior cortex of the adjacent temporal lobe.

2.2 The Terminal Branches of the Internal Carotid Artery

2.2.1 Anterior Cerebral Artery (ACA)

The ACA originates below the anterior perforating substance, lateral to the optic chiasm (Fig. 2.3). The convention proposed by Fischer [4] will be used to describe sections of the larger arteries. Thus, the A1 section runs horizontally forwards and medially, crossing above the optic nerve to the anterior inter-hemispheric fissure, where it communicates with its counterpart via the anterior communicating artery (AComA). The ACA then changes direction, and the A2 section runs upwards and forwards in the fissure to reach the genu of the corpus callosum. The definition of the junction of A2 and A3 sections is anatomically difficult because the Fischer convention is based on the numbering of arterial sections up to the next major branch point or bifurcation. In the case of the distal ACA, the next major branch point is the origin of the callosomarginal artery, but its branch pattern is more varied than is usual in the arterial tree. A solution is to define the junction as the point at which the ACA turns to run over the genu of the corpus callosum and so the A3 section starts at the genu and the A4 when the artery reaches the body of the corpus callosum. Confusingly Fischer added an A5 section to designate the artery posterior to the coronal suture (but since we now use bone-subtracted angiograms, this is a less useful landmark). In practice, because of the high level of individual variability, it is sensible to learn an ideal pattern using whatever convention one likes and expects to have to adapt it on a case-by-case basis.

After passing superior to the genu, the A4 section follows the corpus callosum posteriorly either on its surface or in the cingulate sulcus convexity. It terminates as the posterior pericallosal artery passes along the body of the corpus callosum to the splenium for a variable distance which may extend to the region of the pineal body.

2.2.1.1 Branches: A1 Section (Pre-communicating Artery)

1.

Lenticulostriate arteries. The medial group of lenticulostriate arteries arise from the A1 section. The majority are short central or diencephalic arteries which arise from the superior surface close to the origin of ACA and run into the anterior perforating substance to supply the anterior basal ganglion and anterior commissure. Medial branches piece the lamina terminals to supply the anterior aspect of the lateral wall of the third ventricle, the anterior hypothalamus and septum pellucidum. Inferiorly directed branches supply the optic nerve and chiasm.
2.

Recurrent artery of Heubner. This vessel represents a long central artery which arises either from A1 or A2 sections of ACA (rarely from the AComA) and terminates by supplying part of the head of the caudate nucleus, the anterior portion of the lentiform nucleus and the neighbouring portion of the internal capsule. It usually runs parallel and above the A1 section, directed medially if arising from A1 or laterally if recurrent from an A2 origin. Lateral to the ICA bifurcation, it enters the anterior perforating substance (Fig. 2.4).
3.

Anterior communicating artery (AComA). This short anastomotic artery gives perforating arteries, which parallel those of the A1 section to supply the septum pellucidum, corpus callosum and lamina terminalis. It gives posterior directed branches to the chiasm and hypothalamus.

It is estimated that only 30–40% of adults have a single communicating artery, and two or more connections are present in the majority. This is well known to operating neurosurgeons, but multiple channels may be overlooked during diagnostic angiography because they are small and don’t always fill because the flow of radiographic contrast media is distorted by blood flow from the contralateral A1. There is a substantial literature describing a plethora of possible variations and asymmetric arterial dispositions which are consequence on the coalescence of the cranial division of the embryonic carotid artery (primitive olfactory artery) in the midline. These variations will not be discussed here, but the student should recognise the possibility of an azygos (i.e. single) A2 vessel (Baptista type 1) and the presence of a third A2 artery arising from the anterior communicating artery following the course of a pericallosal artery. The reader is directed to the works of Rhoton [5] and Baptiste [6] for an idea of the described variation in the communication complex and the distal anterior cerebral arteries.

2.2.1.2 Branches: A2 Section

The A2 section of the ACA thus runs superiorly to the genu of the corpus callosum (Fig. 2.5). The callosomarginal artery may arise from this section but the main branches are:

Fig. 2.5

1.

Orbitofrontal artery. This cortical artery runs forwards in the inferior inter-hemispheric fissure and supplies the gyrus rectus, olfactory bulb and the medial inferior frontal lobe.
2.

Frontopolar artery. This artery arises at some point below the genu of the corpus callosum to supply frontal cortex. It may arise as more than one vessel.

2.2.1.3 Branches: A3 Section (Distal to the Origin of the Callosomarginal Artery or the Genu)

The ACA distal to the genu of the corpus callosum is called the pericallosal artery, a term that includes the A3 and distal sections. The callosomarginal artery typically arises at the level of the genu of the corpus callosum and runs parallel to the pericallosal artery in the cingulated sulcus. Its size is inversely related to the size of the pericallosal artery and it is frequently larger [7].

The callosomarginal artery gives a group of four branches:

(a)

Anterior internal frontal
(b)

Middle internal frontal
(c)

Posterior internal frontal
(d)

Paracentral artery

These divide into a network of sulcal vessels to supply the medial frontal lobe, classically as far as the central sulcus, but the arterial pattern that delivers this supply varies depending on whether the stem artery arises from pericallosal or callosomarginal arteries.

2.2.1.4 Branches: A4 and A5 Sections

In its A4 and A5 final sections, the pericallosal artery runs posteriorly over the body of the corpus callosum in the cistern of that name. It terminates and anastomoses with the posterior pericallosal artery which arises from the PCA.

It gives:

(a)

Short callosal perforating arteries that piece the corpus callosum and supply the pillars of the fornix and anterior commissure.
(b)

Long callosal arteries that run parallel to the main trunk for a variable distance, supplying the adjacent cortex and may participate in the anastomoses at the splenium.
(c)

Dural branches to the adjacent falx.
(d)

Parietal arteries. These are the terminal cortical branches to the medial parietal lobe. They may be separable as a superior parietal artery and an inferior parietal artery which arise posterior to the callosomarginal artery and distribute to the cortex via their respective sulci.

Anatomical variants in the ACA pattern are not infrequent and occur in 20% of patients according to various authors. Common variants include absence or hypoplasia of the AComA and asymmetry of the proximal ACAs cerebral arteries with the entire territory supplied from one ICA. In addition to the collateral blood flow to the contralateral hemisphere provided by the AComA, cortical branches of the ACA border the middle cerebral and the posterior cerebral arterial territories. These can provide efficient collateral support in cases of proximal carotid occlusion when the AComA is ineffective or only partially effective or in cases of occlusion of the A2 and more distal ACA.

2.2.2 Middle Cerebral Artery (MCA)

The MCA arises as the lateral terminal branch of the ICA (Fig. 2.6). It runs horizontally and laterally to its primary bifurcation at the limen insulae (M1 segment). The upper and lower trunk arteries thus formed turn upwards and run in the Sylvian fissure lateral to the insular cortex (M2 segment), before they turn laterally in the horizontal portion of the fissure above the temporal and below the frontal lobe opercular surfaces (M3 segment). They emerge from the fissure in a series of branches (M4 segment). These turn inferiorly or superiorly to respectively supply the cortex of the temporal and frontal lobes.

Fig. 2.6

Frontal DSA of the middle cerebral artery bifurcation in a patient with a dominant upper trunk shown in 2D (a) and 3D reconstruction (b). The upper trunk supplies the anterior (frontal lobe) and the lower trunk the posterior (posterior temporal and parietal lobe, including the central sulcus) portions of the middle cerebral artery territory, though parietal branches may arise from either trunk

The angular artery is often described as the continuation of the MCA because it lies at the centre of this candelabrum of branches when viewed on lateral angiography. It exits from the posterior limit of the Sylvian fissure and is therefore a landmark for mapping the ‘Sylvian triangle’ of vessels (a useful ‘tool’ used by pre-CT neuroradiologists to decide if a mass originated in the temporal or frontal lobe).

Descriptions of the configuration of the primary MCA bifurcation vary. It is usually described as a bifurcation with variant trifurcations or ‘quadrifications’. What is clear is that the majority of anatomical dissections show a bifurcation. I suggest it is easiest to consider this as the standard and the trifurcation appearance due to early rebranching of one of two primary trunks (i.e. the upper and lower MCA trunks).

The relative positions of the upper and lower trunks can be difficult to distinguish on two-dimensional imaging, but it is important to recognise that the lower trunk branches contribute to the posterior part of the territory (and therefore it is the usual origin of the angular artery). Since the upper trunk supplies the anterior part of the territory (i.e. frontal lobe and a variable amount of the temporal lobe), the presence of a pre-bifurcation (M1) branch directed to the frontal or anterior temporal lobe may result in it being smaller than the lower trunk.

2.2.2.1 Branches

These should be considered as deep (perforator) and superficial (cortical) arteries.

1.

Lenticulostriate arteries. These arise from the superior surface of the M1 section. They are grouped as the medial and lateral lenticulostriate arteries which piece the anterior perforating substance to supply the globus pallidum and lentiform nucleus (medial group) and passing through the globus pallidum supply, the superior part of the internal capsule and the upper part of the head and body of the caudate nucleus (lateral group). The territory of the medial group overlaps with those arising from A1.
2.
Cortical arteries. The superficial or cortical branches supply a considerable proportion of the superficial hemispheric cortex. They follow the sulci of the brain, and their description (and relative size of each stem artery) depends on the distances between branch points. For those who like to memorise lists, they are shown in Fig. 2.7. However, I prefer to identify only the lobe to which they are directed and their relationship to the Rolandic fissure and the Sylvian point. The last being the site at which the angular artery emerges from the posterior Sylvian fissure.
(a)

Arteries to the temporal lobe. These run inferiorly after leaving the lateral sulcus of the Sylvian fissure and are arranged from anterior to posterior:

(i)

Temporal polar artery

(ii)

Anterior temporal artery

(iii)

Middle temporal artery

(iv)

Posterior temporal artery

(b)

Arteries to the frontal lobe. These run superiorly after leaving the fissure, from anterior to posterior:

(i)

Orbitofrontal artery of the middle cerebral artery

(ii)

Prefrontal artery (supplies Broca’s area)

(iii)

Precentral artery (or pre-Rolandic⁴ artery of Sillon)

(iv)

Central artery (or artery of the Rolandic fissure)

(c)

Arteries to the parietal and occipital lobes. These run posterior to the Sylvian fissure, from superior to inferior:

(i)

Anterior parietal

(ii)

Posterior parietal

(iii)

Angular artery

(iv)

Occipitotemporal artery

Cortical arteriolar-arteriolar anastomoses exist between branches of the anterior and posterior cerebral arteries and between the distal branches of the MCA. They are often seen in patients with occlusion of the proximal MCA and become more reliable, as collateral support to the cortex, if occlusions are made distal to the first branch point (i.e. MCA bifurcation), though there is obviously a limit to how distal embolisation can be tolerated in any tree.