Basic Arterial Anatomy of the Brain1

In this chapter, the authors will review the basic arterial anatomy of the brain and its angiographic correlation, which will cover the first two of those five foundations.

The brain is supplied by two main arterial systems: (i) the carotid system and (ii) the vertebrobasilar system; the carotid system supplies most of the cerebrum and the vertebrobasilar system supplies the brainstem, the cerebellum and part of the cerebrum. Both carotid systems are communicated by the anterior communicating artery (A Com) complex. The carotid system communicates with the vertebrobasilar system via posterior communicating arteries and occasionally via other embryological arteries, most frequently the trigeminal and the hypoglossal arteries.

The cavernous segment of ICA presents four portions: (i) the ascending, (ii) the posterior bend, (iii) the horizontal, and (iv) the anterior bend. The ascending portion of the cavernous ICA exits the foramen lacerum and courses superiorly toward the posterior clinoid process and it is crossed on its lateral wall by the abducens nerve, the sole truly intracavernous cranial nerve, as all other cranial nerves related to the cavernous sinus course on its lateral wall. No branch arises from the ascending portion of the cavernous carotid artery. At the level of, or just before the level of the posterior clinoid process, the cavernous carotid artery will turn anteriorly to constitute the posterior bend. The posterior bend sends off the meningohypophyseal trunk, constituted by three branches: (i) the dorsal meningeal artery that courses posteriorly to supply the dura of the upper clivus area and the abducens nerve; (ii) the tentorial artery (Fig. 1.4)¹ runs laterally to supply the tentorium and (iii) the inferior hypophyseal artery that supply the posterior portion of the pituitary gland and its capsule. On the side of the body of the sphenoid bone, the posterior bend continues anteriorly to constitute the horizontal portion of the cavernous ICA; if the cavernous ICA is elongated, the horizontal cavernous carotid artery can become tortuous and it can present a descending course as shown in Figure 1.3.

From the inferior or lateral surface of the horizontal portion, the cavernous ICA sends off the inferolateral trunk, which runs laterally above the abducens nerve to reach the lateral wall of the cavernous sinus. In 8% of the cases, there are branches arising from the medial part of the horizontal portion, called McConnell capsular artery, to supply the capsule of the pituitary gland.² The anterior part of the horizontal portion and its transition into the anterior bend is related laterally to the abducens nerve and to the ophthalmic division of the trigeminal nerve (V1) and these mark approximately the inferior margin of the anterior portion of the cavernous sinus.

Due to the lateral deviation of the carotid artery at this level, there might be a space left between the anterior bend and the clinoid segment and the medial bony wall (the body of the sphenoid bone) that might be a potential site of origin for aneurysms.

From the anterior bend, the clinoid segment of the ICA courses superiorly and laterally to pierce the dura to constitute the supraclinoid segment.

The supraclinoid segment of the ICA has been divided into three segments based on the origin of its major branches⁵: (1) the ophthalmic segment extends from the origin of the ophthalmic artery to the origin of the posterior communicating artery (PCom); (2) the communicating segment extends from the origin of the PCom to the origin of the AChA; and the (3) choroidal segment extends from the origin of the AChA to the bifurcation of the ICA (Figs 1.6A and B). The ophthalmic artery arises under the optic nerve, usually from the medial one-third of the superior surface of the ICA, then it passes anteriorly and laterally to become superolateral to the carotid artery to enter the optic canal and the orbit (Figs 1.5, 1.7A and B). The intraoperative exposure of the ophthalmic artery often requires the retraction of the optic nerve and the removal of the anterior clinoid process (Figs 1.7C and D). The perforating arteries from the ophthalmic segment arise from the posterior or medial or posteromedial aspect of the ICA and are distributed to the stalk of the pituitary gland, the optic chiasm and less commonly to the optic nerve, premamillary portion of the floor of the third ventricle and the optic tract. The superior hypophyseal arteries, which can range from 1 to 5 in number, arise from the ophthalmic segment and pass medially to supplies the pituitary stalk and the anterior lobe of the pituitary gland (Fig. 1.8). The infundibular arteries are another group of arteries that arise from the PCom and supply the same area as the superior hypophyseal artery. The PCom arises from the posteromedial or the posterior or the posterolateral aspect of the ICA and usually passes posteromedially to join the posterior cerebral artery (PCA).

In the embryo, the PCom continues as PCA, but in the adult the PCA becomes part of the basilar system. If PCom remains the major origin of the PCA, the configuration of the PCom is termed “fetal”. In 60% of the cases, no perforating arteries arise from the communicating segment of the ICA; when present, the perforating arteries from PCom range 4–14 in number, they arise predominantly from the proximal half of the artery and course superiorly to terminate in the floor of the third ventricle (Fig. 1.15). The largest branch from the PCom is the premamillary artery or “anterior thalamoperforating artery” (Fig. 1.11).

The anterior choroidal artery arises either from the posterolateral or from the posterior aspect of the ICA. The AChA courses posteriorly and laterally below the optic tract, between the uncus and the crus cerebri to enter the temporal horn through the inferior choroidal point, the beginning of the choroidal fissure (Figs 1.10, 1.12, 1.16 and 1.17). During its course through the crural cistern (cisternal segment), the AChA sends off branches to the optic tract, crus cerebri, lateral geniculate body and to the uncus. The AChA supplies the optic radiation, globus pallidus, midbrain, thalamus and the retrolenticular and posterior portion of the posterior limb of the internal capsule. Inside the temporal horn, the plexal segment the AChA anastomoses with the lateral posterior choroidal artery (PLChA).⁶ Due to its close relationship to the uncus, optic tract and the inferior choroidal point, the AChA is a good angiographic landmark for locating those structures (Figs 1.12 and 1.18A to C).^7,85

The choroidal segment of the ICA is the most frequent site of perforating arteries (range 1–9); they arise from the posterior aspect of the ICA and terminate in the posterior half of the central region of the anterior perforated substance (APS), optic tract and the uncus (Fig. 1.19).⁶

The vascular territory of the AChA, PCom and perforators from the supraclinoid carotid artery can be seen in Figures 1.20A to E.⁹ The supraclinoid segment of the ICA bifurcates under the APS into the middle and the anterior cerebral arteries (Figs 1.17, 1.18B and 1.19).

The M1 or sphenoidal segment of the MCA extends from the bifurcation of the ICA to the limen insulae. It courses first in the carotid cistern then continues in the sphenoidal compartment of the sylvian fissure; the proximal half of the M1 is related posteriorly and inferiorly to the anteromedial surface of the uncus, anteriorly to the lesser wing of sphenoid and superiorly to the APS; the distal half is related inferiorly to the planum polare, anteriorly to the lesser wing of sphenoid, superiorly and posteriorly to the insular pole, which is the inferior portion of the anterior surface of the insula (Figs 1.10, 1.15, 1.17, 1.18B and 1.21A). The M1 presents two types of branches: (1) the lateral lenticulostriate arteries which arise mostly from the superior or posterosuperior aspect of the M1 and penetrate the middle and posterior portions of the lateral half of APS to supply the basal ganglia (Figs 1.21B to E) and the early branches that course toward the temporal lobe to supply the temporal pole (Figs 1.10, 1.21B and C).

The shape of the M1 segment can vary: it initially can loop upward toward the anterior perforates substance then it courses downward toward the limen insulae (Fig. 1.22A); it initially can loop downward toward the temporal lobe then courses upward toward the limen insulae (Fig. 1.18B); it can present a rather straight course from the bifurcation of the supraclinoid carotid artery to the limen insulae without looping (Fig. 1.13B); it also can present a rather early bifurcation (Fig. 1.22B).¹¹

The M2 or insular segment extends from the limen insulae to the superior and inferior circular or limiting sulci of the insula; it runs in the insular compartment of the sylvian fissure and is constituted by the superior and the inferior trunks and their branches. After reaching the superior or inferior circular or limiting sulcus of the insula, the M2 branches enter the opercular compartment and are called M3 segment (Figs 1.10, 1.18B, 1.21B and 1.22C).

Anatomically, the limen insulae is a ridge located at the anterior pole of the insula and it separates the carotid cistern from the sylvian cistern (Figs 1.21B and 1.22D). More evident than the limen insulae is the genu of the MCA, which is the turn made by the MCA around the insular pole, passing from the basal surface to the lateral surface of the cerebrum. The genu of the MCA is located just few millimeters deep and anterior from the pars triangularis of the inferior frontal gyrus.

The M3 or opercular segment runs in the opercular compartment and is related to the frontal and parietal opercula superiorly and to the temporal operculum inferiorly. The loop of the most posterior M3 segment branch that exits from the sylvian fissure is called “M point” or “sylvian point.”¹³ Anatomically the sylvian point is located behind the insula, above the medial end of the Heschl's gyrus (Figs 1.10, 1.18B and 1.22C). The angiographic sylvian point or “M point” displays the location of the medial end of the Heschl's gyrus, posterior end of the insula and the central core, atrium and the pulvinar of the thalamus.^12,14

The fourth segment is the M4 or cortical segment; it extends from the sylvian fissure to the lateral surface of the cerebrum (Figs 1.22C and E).9

The vascular territory of the MCA can be seen in Figures 1.23A to E.⁹

The medial lenticulostriate perforators, ranging from 1 to 11 branches (average of 6.4), arise from the superior, the posterior, or the posterior-superior aspect of the proximal half of A1 segment and pursue a direct posterior and superior course to enter the medial half of APS (Fig. 1.24C). Embryologically the ACom develops from a multichanneled vascular network that coalesces to a variable degree by the time of birth. Only in 20% of the cases, the ACom communicates two A1 segments of equal size.

From the microsurgical viewpoint, especially for ACom aneurysm surgery, it is important to evaluate the morphology of the A1, whether it is tortuous or straight, short (at the level of the optic chiasm) or elongated (at the level of the optic nerve); it is important to assess the height of the junction between both A1s, because sometimes the ACom complex location can be very high, deeply in the interhemispheric fissure, making its approach difficult, especially when approached from the lateral aspect of the cerebrum as in pterional approach. Also in ACom aneurysm surgery, it is important to evaluate the course of the A2s on the lateral projection in relation to the direction of the fundus of the aneurysm, whether they are parallel, ahead or behind the aneurysm.

The importance of understanding not only the anatomy, but also the understanding of the angiography and other radiological examinations can be demonstrated in the following illustrative case: 37-year-male presented severe subarachnoid hemorrhage 24 hours before the surgery. The Glasgow coma scale was 13 at the admission. The computed tomography (CT) scan showed a severe subarachnoid hemorrhage (Fig. 1.25A); the carotid angiography was performed and it showed an ACom aneurysm and a left carotid-ophthalmic aneurysm (Figs 1.25B to F); the 3D reconstruction of the angiography also was performed (Fig. 1.25G).

All the observations mentioned above cannot be obtained only by knowing the general microsurgical anatomy, but they can be obtained from the information provided by the radiological examinations of the patient; knowing the general microsurgical anatomy can make the extraction of the radiological information much easier (Fig. 1.25H).

The vascular territory of the ACA can be seen in Figures 1.26A to E.⁹

The main branches arising from the PCA are; the posterior thalamoperforating, the direct perforating, the short and long circumflex, the thalamogeniculate, the MPChA and the LPChA, the inferior temporal, the parieto-occipital, the calcarine and the posterior pericallosal arteries. The posterior thalamoperforating arteries which arise from P1 and enter the brain through the posterior perforated substance, interpeduncular fossa and medial crus cerebri, supply the anterior and part of the posterior thalamus, hypothalamus, subthalamus, substantia nigra, red nucleus, oculomotor and trochlear nuclei, oculomotor nerve, mesencephalic reticular formation, pretectum, rostromedial floor of the third ventricle and the posterior portion of the internal capsule (Fig. 1.27C). The direct perforating arteries to the crus cerebri arise mainly from the P2A segment and supply the crus cerebri.

The vascularization of the basal ganglia and the thal- amus can be seen in Figure 1.29A and the angiographic correlation of the PCA can be seen in Figure 1.29B; please correlate it with Figure 1.18B.

In summary, the upper complex includes the SCA, midbrain, cerebellomesencephalic fissure, superior cerebellar peduncle, tentorial surface of the cerebellum and the oculomotor, trochlear, and trigeminal nerve. The SCA arises in front of the midbrain, passes below the oculomotor and trochlear nerve, and above the trigeminal nerve to reach the cerebellomesencephalic fissure, where it runs on the superior cerebellar peduncle and terminates by supplying the tentorial surface of the cerebellum (Fig. 1.30).22

The middle complex includes the AICA, pons, middle cerebellar peduncle, cerebellopontine fissure, petrosal surface of the cerebellum and the abducens, facial and vestibulocochlear nerve. The AICA arises from the basilar artery at the pontine level, courses in relationship to the abducens, facial and vestibulocochlear nerve to reach the surface of the middle cerebellar peduncle, where it courses along the cerebellopontine fissure and terminates by supplying the petrosal surface of the cerebellum (Fig. 1.31).

The lower complex includes the PICA, medulla, inferior cerebellar peduncle, cerebellomedullary fissure, suboccipital surface of the cerebellum and glossopharyngeal, vagus, spinal accessory and hypoglossal nerves. The PICA arises from the vertebral artery at the medullary level, encircles the medulla, passing in relationship to the glossopharyngeal, vagus, accessory and hypoglossal nerves to reach the surface of the inferior cerebellar peduncle, where it dips into the cerebellomeduallary fissure and terminates by supplying the suboccipital surface of the cerebellum (Fig. 1.32).

The extradural part is divided into three segments. The first segment extends from the origin of the vertebral artery in subclavian artery to the entrance into the lowest foramen, usually at the C6 level. The second segment ascends through the transverse foramina of the upper six cervical vertebrae in front of the cervical nerve roots. After exiting from the transverse foramen of the C2, the vertebral artery deviates laterally to enter the laterally placed transverse foramen of C1. The third segment, the one most intimately related to the foramen magnum, extends from the foramen in the transverse process of the atlas to the site of passage through the dura mater (Fig. 1.33A). This segment passes medially behind the lateral mass of the atlas and atlanto-occipital joint and is pressed into the groove on the upper surface of the lateral part of the posterior arch of the atlas, where it courses along the floor of the suboccipital triangle.

The intradural segment begins at the dural foramina just inferior to the lateral edge of the foramen magnum. The dura in this region is much thicker than in other areas and it forms a funnel-shaped foramen around the artery. The first cervical nerve exits the spinal canal and the posterior spinal artery enters the spinal canal through this dural foramen with the vertebral artery. These three structures are bound together at the foramen by fibrous dural bands. The initial intradural segment of the vertebral artery passes just superiorly to the dorsal and ventral roots of the first cervical nerve and just anteriorly to the posterior spinal artery, the dentate ligament and the spinal portion of the accessory nerve (Fig. 1.33B). The intradural segment of the vertebral artery is divided into the lateral medullary and anterior medullary segments before joining its contralateral mate to form the basilar artery. The lateral medullary segment begins at the dural foramen and passes anteriorly and superiorly along the lateral medullary surface to terminate at the preolivary sulcus. The anterior medullary segment begins at the preolivary sulcus, courses in front of, or between, the hypoglossal rootlets and crosses the pyramid to join with the contralateral vertebral artery at or near the pontomedullary sulcus to form the basilar artery (Figs 1.2, 1.31, 1.33B and 1.34). The anterior medullary segment rests on the clivus. The branches arising from the vertebral artery in the region of the foramen magnum are the posterior spinal, anterior spinal, PICA, and anterior and posterior meningeal arteries.

This artery arises from the vertebral artery near the inferior olive and passes posteriorly around the medulla. The PICA origins at the level of the olive are either lateral or anterior to the olive.

At the anterolateral margin of the medulla, the PICA passes rostral or caudal to or between the rootlets of the hypoglossal nerve and at the posterolateral margin of the medulla, it courses rostral to or between the fila of the glosspharyngeal, vagus and accessory nerves. If the vertebral artery is elongated or tortuous and courses lateral to the olive, it stretches the hypoglossal rootlets dorsal over its posterior surface. The PICA commonly passes from the lateral to the posterior aspect of the medulla by passing between the rootlets of the glossopharyngeal, vagus and accessory nerves (Fig. 1.34A). After passing these nerves, it courses around the cerebellar tonsil and enters the cerebellomedullary fissure and passes posterior to the lower half of the roof of the fourth ventricle. On exiting the cerebellomedullay fissure, its branches are distributed to the vermis and hemisphere of the suboccipital surface. Its area of supply is the most variable of the cerebellar arteries.²⁶

The tonsillomedullary segment—this segment begins where the PICA passes posterior to the glossopharyngeal, vagus and accessory nerves and extends medially across the posterior aspect of the medullar near the caudal half of the tonsil. It ends where the artery ascends to the midlevel of the medial surface of the tonsil. The proximal portion of this segment usually course near the lateral recess and then posteriorly to reach the inferior pole of the tonsil. This segment commonly passes medially between the lower margin of the tonsil and the medulla before turning rostrally along the medial surface of the tonsil. The loop passing near the lower part of the tonsil, referred to as the caudal loop, has been reported to form a caudally convex loop that coincides with the caudal pole of the tonsil.

The telovelotonsillar segment—this is the most complex of the segments. It begins at the midportion of the PICA's ascent along the medial surface of the tonsil toward the roof of the fourth ventricle and ends where it exits the fissures between the vermis, tonsil and cerebellar hemisphere to reach the suboccipital surface of the cerebellum. In most, but not at all, hemispheres, this segment often forms a loop with convex rostral curve, called the cranial loop.^27–29

This loop is located caudal to the fastigium between the cerebellar tonsil below and the tela choroidea, and inferior medullary velum above. The apex of the cranial loop usually overlies the central part of the inferior medullary velum, but its location varies from the superior to the inferior margin and from the medial to the lateral extent of the inferior medullary velum (Figs 1.34A to C). The apex of the cranial loop is inferior to the level of the fastigium of the fourth ventricle in most cases, but may also extend to the level of the fastigium. This segment gives rise to branches that supply the tela choroidea and the choroid plexus of the fourth ventricle. The cortical segment— this segment begins where the trunks and branches leave the groove between the vermis medially and tonsil and the hemisphere laterally, and includes the terminal branches. The bifurcation of the PICA often occurs near the origin of this segment. The cortical branches radiate outward from the superior and lateral border of the tonsil to the vermis and hemisphere (Fig. 1.32).

The site of the origin of the PICA from the vertebral artery varies from below the foramen magnum to the vertebrobasilar junction. PICA may arise from the vertebral artery in an extradural location.³⁰29

The medial trunk terminates by sending branches over the inferior part of the vermis and adjacent part of the tonsil and hemisphere. The lateral trunk divides into a larger hemispheric trunk that gives off multiple branches to the hemisphere and smaller tonsillar branches that supply the posterior and inferior surfaces of the tonsil. The division of the lateral trunk into tonsillar and hemispheric branches may occur at various sites in relation to the tonsil, but is most commonly located near the posterior margin of the medial surface of the tonsil. The trunks passing through the tonsillomedullary fissure send branches to the medullar and the trunk passing through the telovelotonsillar fissure send ascending branches to the dentate nucleus (Figs 1.32 and 1.34C).³¹

The perforating branches of the PICA intermingle and overlap with those arising from the vertebral artery. The segment of the vertebral artery distal to the origin of the PICA more frequently gives rise to perforating arteries than the segment proximal to the PICA origin.

The angiographic correlation (frontal projection) of the vertebral artery and PICA can be seen in Figure 1.34D. Please correlate it with Figures 1.2, 1.31, 1.34A and B for vertebral artery and Figures 1.2, 1.31, 1.34A, B and E for PICA.

The lateral projection of the angiographic correlation of those arteries can be seen in Figure 1.34F. Please correlate it with Figure 1.34B.

In its course the basilar artery gives off the following branches on both sides; pontine, labyrinthine, anterior inferior cerebellar, superior cerebellar and posterior cerebral arteries. The pontine branches of the basilar artery consist of a number of small vessels that supply the pons and midbrain. Median vessels arise at right angles from the posterior aspect of the basilar and enter the pons along its anterior median groove (Figs 1.34A). The transverse branches arise from the posterolateral and lateral aspects of the basilar artery and in their circumferential course around the brainstem, they send penetrating vessels into the pons.

The AICA is intimately related to the pons, lateral recess, foramen of Luschka, cerebellopontine fissure, middle 31cerebellar peduncle, and petrosal surface of the cerebellum. The AICA originates from the basilar artery, usually as a single trunk and encircles the pons near the abducens, facial and vestibulocochlear nerve. After coursing near and sending branches to the nerves entering the internal acoustic meatus and to the choroid plexus protruding from the foramen of Luschka, it passes around the flocculus on the middle cerebellar peduncle to supply the lips of the cerebellopontine fissure and the petrosal surface of the cerebellum.

The AICA is divided into the rostral and caudal trunks near the facial-vestibulocochlear nerve complex. After crossing the nerves, the rostral trunk usually courses laterally above the flocculus to reach the surface of the middle cerebellar peduncle and the petrosal fissure to be distributed to the superior lip of the cerebellopontine fissure and to the adjoining part of the petrosal surface. The caudal trunk supplies the inferior part of the petrosal surface, including a part of the flocculus and the choroid plexus and is frequently related to the lateral portion of the fourth ventricle. The AICA gives rise to perforating branches to the brainstem, choroidal branches to the tela choroidea and choroid plexus and to the nerve related arteries, including the labyrinthine, recurrent perforating and subarcuate arteries.³⁵

The AICA presents the following segments; anterior pontine, lateral pontine, flocculonodular and cortical segments.

The anterior pontine segment begins at the origin and ends at the level of a line drawn through the long axis of the inferior olive and extending upward on the pons and is located between the clivus and the belly of the pons; the AICA usually lies in contact with the rootlets of the abducens nerve.

The lateral pontine segment begins at the anterolateral margin of the pons and passes through the CPA, above, below or between the facial and vestibulocochlear nerves and is intimately related to the internal auditory meatus, to the lateral recess and to the choroid plexus protruding from the foramen of Luschka. This segment gives rise to the nerve-related branches that course near or within the internal acoustic meatus in close relationship to the facial and vestibulocochlear nerves. These nerve-related branches are the labyrinth artery, which supplies the facial and vestibulocochlear nerves and the labyrinth (Fig. 1.34A); the recurrent perforating arteries, which pass toward the meatus, but turn medially to supply the brainstem; and the subarcuate artery which enters the subarcuate fossa to reach the center of the superior semicircular canal (Fig. 1.35A).

The flocculonodular segment begins where the artery passes rostral or caudal to the flocculus to reach the middle cerebellar peduncle and the cerebellopontine fissure. The trunks that course along the peduncle may be hidden beneath the flocculus or in the lips of the cerebellopontine fissure (Fig. 1.35B).

The cortical segment is composed of the cortical branches to the petrosal surface of the cerebellum.

The subarcuate canal is recognized as a potential route of extension of infections from the mastoid region to the meninges and to the superior petrosal sinus.³⁷

Cortical branches: The most common pattern is for the AICA to supply the majority of the petrosal surface of the cerebellum, but the cortical area of the supply is quite variable. It can vary from a small area on the flocculus and adjacent part of the petrosal surface to include the whole petrosal surface and adjacent part of the tentorial and suboccipital surfaces of the cerebellum. After crossing the nerves, the rostral trunk usually courses above the flocculus to be distributed to the superior lip of the cerebellopontine fissure and the caudal trunks course caudal to the flocculus to supply the inferior part of the petrosal surface (Figs 1.31 and 1.35B).

The angiographic correlation of AICA (frontal projection) can be seen in Figure 1.35C; please correlate it with Figures 1.2, 1.31, 1.34A, 1.34B and 1.35B. The lateral projection of the angiographic correlation is seen in Figure 1.35D; please correlate it with Figure 1.35B.

The level of the bifurcation of the basilar artery is normal if the bifurcation occurs at the pontomesencephalic junction, high if it occurs anterior to the mesencephalon and low if it is anterior to the pons. Nearly two-thirds of SCAs have a point of contact with the oculomotor nerve, usually on the inferior surface. The point of contact usually involves the main trunk or, less commonly, the rostral trunk if there is an early bifurcation. The trochlear nerve has points of contact with the SCA trunks in almost all cases. The SCA encircles the brainstem above the trigeminal nerve, making a shallow caudal loop on the lateral side of the pons. Contact occurs between the SCA and the trigeminal nerve in those cases with the most prominent caudally projecting loops. The point of contact with the SCA is usually on the superior or superomedial aspect of the trigeminal nerve (Fig. 1.36A).³⁹

The tentorium incisura, the opening through the tentorium cerebella, presents triangular shape with the base facing the clivus.⁴⁰ The other two limbs are formed by the right and left free edges that join at an apex located between the colliculi below the occipital lobes above (Fig. 1.36B). The proximal portion of the SCA courses medial to anterior third of the free edge of the tentorium cerebelli.

After passing above the trigeminal nerve, it enters the cerebellomesencephalic fissure, where its branches make several sharp turns and give rise to the precerebellar arteries, which pass to the deep cerebellar white mater and the dentate nucleus. On leaving the cerebellomesencephalic fissure where its branches are again medial to the tentorial edge, its branches pass posteriorly under tentorial edge and are distributed to the tentorial surface of the cerebellum. The SCA can arise as a single trunk and bifurcates into a 34rostral and caudal trunk. The SCA gives off perforating branches to the brainstem and cerebellar peduncles. The rostral trunk supplies the vermian and paravermian area and the caudal trunk supplies the hemisphere on the suboccipital surface. The SCA presents the following segments:

The anterior pontomesencephalic segment extends from the origin of the SCA to the anterolateral margin of the brainstem. It is located between the dorsum sella and upper brainstem. Its lateral part is medial to the anterior half of the free tentorial edge. The lateral pontomesencephalic segment begins at the anterolateral margin of the brainstem and frequently dips caudally onto the lateral side of the upper pons. Its caudal loop projects toward and often reaches the root entry zone of the trigeminal nerve at midpontine level. The trochlear nerve passes above the midportion of this segment. The anterior part of this segment is often visible above the tentorium. This segment terminates at the anterior margin of the cerebellomesencephalic fissure. The basal vein and the PCA course above and parallel to this segment of SCA. The cerebellomesencephalic segment courses within the cerebellomesencephalic fissure. The branches of SCA enter the shallowest part of the fissure located above the trigeminal root entry zone and again course medial to the tentorial edge with its branches intertwined with the trochlear nerve. The fissure in which the SCA proceeds progressively deepens medially and is the deepest in the midline behind the superior medullary velum. The SCA loops deeply into the fissure and passes upward to reach the anterior edge of the tentorial surface of the cerebellum. The trunks and branches of the SCA are held in the fissure by branches that penetrate the fissure's opposing walls. The cortical segment includes the branches distal to the cerebellomesencephalic fissure that pass under the tentorial edge and are distributed to the tentorial surface of the cerebellum. The branches of the rostral trunk supplies medial area and the branches of the caudal trunk supply the lateral area of the tentorial surface of the cerebellum (Figs 1.2, 1.30, 1.31 and 1.34A).

The branches of the SCA are; perforating, precerebellar and cortical branches.

The perforating branches are divided into direct and circumflex type. The direct type pursues a straight course to enter the brainstem. The circumflex type winds around the brainstem before terminating in it. The circumflex perforating arteries are subdivided into short and long types. The short circumflex type travels 90 degrees or less around the circumference of the brainstem.

The perforating branches usually arise from the main, rostral and caudal trunks. The most common type of perforating artery arising from the main trunk is the long circumflex type, but it also gives off direct and short circumflex branches. The branches from the rostral and caudal trunk are most frequently circumflex. The basilar artery also gives rise to multiple perforating branches to the brainstem. Those arising near the origin of the SCA intermingle with the direct perforating branches arising from the proximal SCA. Those arising above the origin of the SCA enter the interpeduncular fossa.

The precerebellar arteries arise from the trunks and cortical branches within the cerebellomesencephalic fissure to supply the deep cerebellar nuclei. These branches tether the distal parts of the trunks and the proximal parts of the cortical arteries in the fissure. The precerebellar arteries consist of a medial group of small branches that pass between the superior medullary velum and the central lobule and a lateral group of larger branches that course between the superior and middle cerebellar peduncles and the wings of the central lobule (Figs 1.36C and D).

The most constant cortical supply of the SCA is to the tentorial surface. The cortical territory of the SCA is more constant than that of the AICA and PICA, but is reciprocal with them. The cortical branches are divided into hemispheric and vermian groups. The cortical surface of each half of the vermis is divided into median and paramedian segments and each hemisphere lateral to the vermis is divided into medial, intermediate and lateral segments.

The hemispheric branches arise from the rostral and caudal trunk in the depths of the cerebellomesencephalic fissure. After leaving the fissure, the hemispheric branches proceed to supply the tentorial surface lateral to the vermis. The most common pattern consists of three hemispheric branches: (1) lateral, (2) intermediate, and (3) medial, corresponding to the third of the hemispheric surface that they supply. The vermian arteries occasionally overlap onto the medial hemispheric segment and marginal artery overlaps the lateral hemispheric segment.

The vermian branches arise from the rostral trunk within the cerebellomesencephalic fissure. The rostral trunk most commonly gives rise to two vermian arteries. The most common pattern is two vermian arteries: (1) one distributed to a median strip bordering the midline, and (2) one distributed to a paramedian strip bordering the hemispheric surface (Fig. 1.36B).

About half of the proximal SCA trunks give rise to a marginal branch to the adjacent petrosal surface. When present, the marginal branch is the first cortical branch. It usually arises from the lateral pontomesencephalic segment and does not enter the cerebellomesencephalic fissure, as do the other cortical branches, but passes from its origin to the cortical surface. Its most constant supply is to the part of the petrosal surface adjoining the tentorial surface.

The angiographic correlation of the SCA can be seen in Figure 1.35C; please correlate it with the Figures 1.2, 1.30, 1.31, 1.34A, 1.35B, 1.36A and B.35