Intracranial arteries

INTRACRANIAL ARTERIES

IINTERNAL NTERNAL CCAROTID AROTID

AARTERY RTERY AND AND

NNORMAL ORMAL VVARIANTSARIANTS

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Abbreviations

• Anterior, middle, posterior cerebral arteries (ACA, MCA, PCA)

• Anterior, posterior communicating arteries (ACoA, PCoA)

• Basilar artery (BA)• Vertebral artery (VA)• Anterior, posterior inferior cerebellar arteries

(AICA, PICA)• Anterior choroidal artery (AChoA)• Recurrent artery of Heubner (RAH)

Gross Anatomy

• Anterior Circulation - Intracranial Internal carotid artery and its branches + ACoA, PCoA

• Posterior Circulation - Basilar artery and its branches

Internal carotid artery• Proximal to termination gives off ophthalmic artery,

PCoA, AChoA• Terminal bifurcation into ACA (smaller, medial), MCA

(larger, lateral)• ANTERIOR CEREBRAL ARTERY has 4 segments

- Horizontal or pre communicating (A1) segment courses medially above optic chiasm, joined by ACoA to contralateral A1 - Vertical or postcommunicating (A2) segment courses superiorly in interhemispheric fissure, around corpus callosum genu - Distal (A3) segment courses posteriorly under inferior free margin of falx cerebri, gives off cortical branches

• Perforating arteries arise from A1, ACoA• RAH arises from distal A1 or proximal A2

Internal carotid artery

• MIDDLE CEREBRAL ARTERY has 4 segments - Horizontal (M1) segment courses laterally to sylvian fissure below anterior perforated substance, bi- or trifurcates - "Genu" or "knee" of MCA is gentle posterosuperior turn towards lateral cerebral (sylvian) fissure - Insular (M2) segments course within lateral cerebral fissure, over insula - Opercular (M3) segments begin at top of insula, turn laterally in sylvian fissure to reach overhanging frontal/parietal/temporal operculae - Cortical (M4) branches emerge from lateral cerebral fissure, course over hemispheric surface - Perforating arteries arise from M1

Basilar Artery• Courses cephalad in prepontine cistern to terminal bifurcation

ventral to midbrain - Gives off AICA, superior cerebellar arteries (SCAs), pontine, midbrain perforating arteries

• Bifurcates into POSTERIOR CEREBRAL ARTERIES, each of which has 4 segments - Mesencephalic or pre communicating (P1) segment lies within interpeduncular cistern, curves posterolaterally from BA to PCoA junction - Ambient (P2) segment extends from PCA-PCoA junction, curving around cerebral peduncles just above tentorium, above oculomotor nerve - Quadrigeminal (P3) segment extends posteromedially from level of quadrigeminal plate - Cortical (P4) branches arise from distal PCA at or just before reaching calcarine fissure - Perforating branches arise from P1

• Vertebral arteries - Intracranial (V4) segments enter dura near foramen magnum - Give off anterior/posterior spinal arteries, perforating arteries to medulla, PICA

Vascular Territory

• Vascular distribution of ACA, MCA, PCA vary from individual to individual, have typical as well as maximum, minimum territories

• Two vascular "watershed" zones exist at confluence of territorial supply, are vulnerable to hypoperfusion - Cortical watershed = subpial confluence of cortical ACA / MCA / PCA branches - Deep white matter watershed zone = confluence of deep cortical penetrating branches, perforating branches from circle of Willis (COW)

Vascular Territory

• Anterior Cerebral Artery - Perforating branches: Corpus callosum rostrum, heads of caudate nuclei, anterior commissure, anteromedial putamen / globus pallidus / anterior limb internal capsule (if RAH present) - Cortical branches: Inferomedial frontal lobes, anterior 2/3 of medial hemisphere surface, 1-2 cm over brain convexity

Vascular Territory

• Middle Cerebral Artery - Perforating branches: Most of putamen, globus pallidus, superior half of internal capsule, most of caudate nucleus, some deep white matter - Cortical branches: Most of lateral surface of cerebral hemispheres, anterior tip (pole) of temporal lobe

Vascular Territory

• Posterior Cerebral Artery - Perforating branches: Much of central brain base (thalamus, hypothalamus), midbrain, choroid plexus - Cortical branches: Most of inferior surface of temporal lobe, occipital pole, variable amount of posterolateral surface of hemisphere

Vascular Territory

• Basilar Artery - All of PCA territory (including perforating branches), most of pons, superior cerebellum/vermis

• Vertebral Arteries - Most of medulla, cerebellar tonsils, inferior vermis / cerebellar hemispheres

INTRACRANIAL INTERNAL CAROTID ARTERY

Overview

• Complex course with several vertical / horizontal segments, 3 genus (one petrous, two cavernous)

• Six intracranial segments (Cervical lCA = C1): - Petrous (C2), Lacerum (C3), Cavernous (C4), Clinoid (C5), Ophthalmic (C6), Communicating (C7)

Petrous (C2) segment

• Contained within carotid canal of temporal bone - Surrounded by extensive sympathetic plexus - Two C2 subsegments joined at genu

• Short vertical segment - anterior to internal jugular vein (lJV)

• "Genu" (where petrous ICA turns anteromedially in front of cochlea)

• Longer horizontal segment - Exits carotid canal at petrous apex - Branches • Vidian artery (artery of pterygoid canal) anastomoses with external carotid artery (ECA) • Caroticotympanic artery (supplies middle ear)

Lacerum (C3) segment

• Small segment that extends from petrous apex above foramen lacerum, curving upwards toward cavernous sinus

• Covered by trigeminal ganglion

• No branches

Cavernous (C4) segment• Three subsegments joined by two genus (knees)

- Posterior vertical (ascending) portion - Posterior (more medial) genu - Horizontal segment - Anterior (more lateral) genu - Anterior vertical (subclinoid) segment

• Covered by trigeminal ganglion posteriorly• Abducens nerve (CN6) is inferolateral• Major branches

- Meningohypophyseal trunk (arises from posterior genu, supplies pituitary, tentorium and clival dura) - Inferolateral trunk arises from horizontal segment, supplies cavernous sinus (CS) dura / cranial nerves; anastomoses with ECA branches through foramens rotundum, spinosum, ovale

Clinoid (C5) segment

• Between proximal, distal dural rings of cavernous sinus

• Ends as ICA enters subarachnoid space near anterior clinoid process

• No important branches unless ophthalmic arteries arises within C5

Ophthalmic (C6) segment

• Extends from distal dural ring at superior clinoid to just below posterior communicating artery (PCoA) origin

• Two important branches - Ophthalmic Arteries (originates from anterosuperior ICA, passes through optic canal to orbit; gives off ocular, lacrimal, muscular branches; extensive anastomoses with ECA) - Superior hypophyseal artery (courses posteromedially; supplies anterior pituitary, infundibulum, optic nerve / chiasm)

Communicating (C7) segment

• Extends from below PCoA to terminal lCA bifurcation into anterior cerebral artery (ACA), middle cerebral artery (MCA)

• Passes between optic (CN2), oculomotor (CN3) nerves• Major branches

- Posterior communicating artery - Anterior choroidal artery (courses posteromedial, then turns superolateral in suprasellar cistern; enters temporal horn at choroidal fissure; supplies choroid plexus, medial temporal lobe, basal ganglia, posteroinferior internal capsule)

Normal Variants, Anomalies

Petrous (C2) segment• Aberrant ICA (aICA)

• Presents as retrotympanic pulsatile mass; should not be mistaken for glomus tympanicum tumor!• Absent vertical course; alCA courses more posterolaterally than normal (appears as mass in hypotympanum abutting cochlear promontory)

• Persistent stapedial artery• Arises from vertical segment, crosses cochlear promontory and stapes footplate• Enlarges tympanic segment of facial nerve canal• Terminates as middle meningeal artery• Seen as "Y-shaped", enlarged geniculate fossa of CN7 on CT• Foramen spinosum is absent

Normal Variants, Anomalies

Cavernous (C4) segment• Persistent trigeminal artery

• Most common carotid-basilar anastomosis (0,2-0,5%)• Parallels course of CN5, passes posterolaterally around (or through) dorsum sellae• Connects ICA to vertebrobasilar system, forms "trident-shape" on lateral DSA, sagittal MR• May supply entire vertebrobasilar (VB) circulation distal to anastomosis (Saltzman type I) or fill superior cerebral arteries (SCAs) with posterior cerebral arteries (PCAs) filled via patent PCoAs (Saltzman type II)

Clinical

• Horner syndrome results from interruption of periarterial sympathetic plexus around ICA (dissection, "bruising" of plexus, etc.)

NORMAL VARIANTSNORMAL VARIANTS

“Normal Variants of the Cerebral Circulation at Multidetector CT

Angiography”

RadioGraphics 2009; 29:1027–1043 • Simon J. Dimmick, BPthy, MBBS • Kenneth C. Faulder, MBBS,

FRANZCR

Internal Carotid Artery

• Fenestration of the distal internal carotid artery is a rare anomaly.

• We are aware of only six previously reported cases in the literature.

• Fenestration of the distal internal carotid artery, like fenestration in other sites, is associated with aneurysm formation.

RadioGraphics 2009; 29:1027–1043

Fenestration of the internal carotid artery. Three-dimensional reconstruction from DSA depicts fenestration (arrow) of a distal segment. The overlying artifact was produced by aneurysm clips.


Hyperplastic AnteriorChoroidal Artery

• The anterior choroidal artery, usually a small vessel, arises from the supraclinoid internal carotid artery just distal to the posterior communicating artery.

• From there it subdivides into important branches that supply the cerebral peduncle and optic tract.

• The temporo-occipital branches of the posterior cerebral artery may arise from the anterior choroidal artery.

• In people with this cerebrovascular variant, the anterior choroidal artery is described as hyperplastic.

• The prevalence of hyperplastic anterior choroidal arteries is reported to be 1.1%–2.3%.


Multidetector CT angiogram depicts a hyperplastic anterior choroidal artery (straight arrow), ipsilateral posterior communicating artery (arrowhead), and contralateral fetal posterior cerebral artery (curved arrow).


Fetal Origin of the Posterior Cerebral Artery

• In the presence of this anomaly, the caliber of the posterior communicating artery may be the same as or greater than that of the ipsilateral P1 segment, and the dominant blood supply to the occipital lobes comes from the internal carotid artery.

• Fetal origin of the posterior cerebral artery occurs when the embryonic posterior cerebral artery fails to regress.

• It may occur on the right side (10% of the general population), the left side (10% of the general population), or bilaterally (8% of the general population).

• The P1 segment may be absent in fetal origin of the posterior cerebral artery, but its absence is an uncommon finding.


Bilateral fetal posterior cerebral arteries. Three-dimensional multidetector CT angiogram shows bilateral large posterior communicating arteries (arrows), which providemost of the blood supply to the posterior cerebral artery territory. Both P1 segments are present but hypoplastic.


CT angiogram shows bilateral fetal posterior cerebral arteries (arrows). Both P1 segments are absent. The arrowheads indicate the superior cerebellar arteries.


Posterior Communicating ArteryInfundibulum

• An infundibulum is a funnel-shaped region of dilatation at the origin of the posterior communicating artery from the internal carotid artery.

• It may be round or conical, has a diameter of less than 2 mm, and is symmetric.

• The internal carotid artery is at its base, and the posterior communicating artery arises from its apex.

• An infundibulum must be distinguished from aneurysms of the posterior communicating artery and internal carotid artery.


CT angiogram shows a posterior communicating artery (arrowhead) that arises from the apex of a funnel-shaped infundibulum (arrow). The base of the infundibulum is located at the origin of the posterior communicating artery.


Persistent Carotid-Basilar Artery Anastomoses


Persistent Trigeminal Artery

• The persistent trigeminal artery is the most common and most cephalic of the persistent carotidvertebrobasilar anastomoses.

• Its reported prevalence is 0.1%–0.6%. • This artery originates from the internal carotid

artery immediately after its exit from the carotid canal and anastomoses with the midbasilar artery.

• The part of the basilar artery that is caudal to the anastomosis with the trigeminal artery is usually hypoplastic.



• Two types of persistent trigeminal artery have been described — lateral and medial.

• Both types are equally common. • In the lateral type, the artery courses

posterolaterally with the trigeminal nerve. • The medial type has an intrasellar or

transhypophyseal course, coursing posteromedially from its origin, compressing the pituitary gland, and penetrating the dorsum sellae.



• A persistent trigeminal artery also is classified according to the configuration of the ipsilateral posterior cerebral artery: - In the presence of a Saltzman type 1 persistent trigeminal artery, the posterior communicating artery is absent and the persistent trigeminal artery supplies the entire vertebrobasilar system distal to the site of anastomosis. - In the presence of a Saltzman type 2 persistent trigeminal artery there is a fetal posterior cerebral artery, and the ipsilateral P1 segment is absent.



• An association has been found between persistent trigeminal artery and other vascular anomalies in an estimated 25% of cases.

• Associated anomalies include intracranial aneurysms, which are seen in approximately 14% of patients with a persistent trigeminal artery.

• Knowledge of the presence of a persistent trigeminal artery in a medial or intrasellar location in a patient who is to undergo transsphenoidal surgery for pituitary adenoma is clinically important because accidental transection of the artery may result in a lifethreatening hemorrhage.


CT angiogram shows a lateral Saltzman type 2 persistent trigeminal artery (arrows). Note the typical hypoplastic appearance of the basilar artery (arrowhead) proximal to its anastomosis with the trigeminal artery.


Variants of PersistentTrigeminal Artery

• Cerebellar arteries that arise from the precavernous internal carotid artery and are not connected to the basilar artery are considered variants of persistent trigeminal artery.

• The reported frequency of such findings is 0.18% at conventional angiography and 0.76% at MR angiography.

• The anteroinferior cerebellar artery is the most common of these variants, but a posteroinferior cerebellar artery or superior cerebellar artery also may arise from the internal carotid artery.


Variants of PersistentTrigeminal Artery

• A persistent trigeminal artery variant is not usually associated with hypoplasia of the proximal basilar artery.

• Persistent trigeminal artery variants are usually small in caliber, which makes their visualization and recognition difficult.

• These anomalous arteries are usually found incidentally but may be associated with aneurysms and may be responsible for ischemia and trigeminal neuralgia.


Trigeminal artery variant.DSA image of the left internal carotid artery demonstrates a trigeminal artery variant that arises more proximally on the internal carotid artery than is usual for a persistent trigeminal artery. The aberrant artery supplies the territory of the anterior inferior cerebellar artery (arrows).


Primitive Hypoglossal Artery

• The persistent hypoglossal artery is the second most common carotidvertebrobasilar artery anastomosis, with a prevalence of 0.02%–0.10%.

• The persistent hypoglossal artery originates from the internal carotid artery at the levels of the C1 through C3 vertebral bodies, courses through the hypoglossal canal, and anastomoses with the basilar artery.

• A primitive hypoglossal artery does not passthrough the foramen magnum.


Primitive Hypoglossal Artery

• In 79% of cases, the posterior communicating arteries are hypoplastic, and in 78% of cases, the vertebral arteries are hypoplastic.

• Definitive diagnosis is based on the recognition of na anomalous artery in the enlarged hypoglossal canal.

• A persistent hypoglossal artery has been reported to cause glossopharyngeal neuralgia and hypoglossal nerve paralysis.


Persistent hypoglossal artery. (28) Axial image from CT angiography shows an artery that courses through the hypoglossal canal (arrows). (29) CT angiogram depicts a hypoglossal artery (arrowhead) that arises from the proximal internal carotid artery (arrow) at the C2 vertebral level and anastomoses with the basilar artery.


Proatlantal Intersegmental Artery

• The proatlantal intersegmental artery originates from the common carotid artery bifurcation, external carotid artery, or internal carotid artery at the levels of the C2 through C4 vertebral bodies; joins the horizontal part of the vertebral artery in the suboccipital region; and traverses the foramen magnum.

• Two variants have been described: - The first variant (type 1) originates from the dorsal aspect of the internal carotid artery and accounts for 38% of cases. - The second variant (type 2) arises from the external carotid artery and accounts for 50% of cases.


Proatlantal Intersegmental Artery

• Aplasia or hypoplasia of one or both vertebral arteries proximal to the anastomosis may be identified in 50% of cases.

• In addition, cerebrovascular abnormalities were found in 59% of patients with a proatlantal intersegmental artery.

• Such abnormalities included intracranial aneurysm in 10% of patients.


Proatlantal intersegmental artery in two different patients. (30) Lateral composite projection image from DSA of the right common carotid artery demonstrates a proatlantal intersegmental artery that arises from the internal carotid artery (arrowhead) at the level ofthe C2 vertebra (arrow). (31) Anteroposterior composite projection image from DSA shows a proatlantal intersegmental artery (arrow) and right internal carotid artery (arrowhead).


Persistent Otic Artery• The existence of an otic artery is a matter of controversy. • Unlike the three embryonic vessels described earlier, an

otic artery has never been identified in lower animals. • Nevertheless, the medical literature contains descriptions

of such an artery arising from the petrous internal carotid artery within the carotid canal, coursing laterally through the internal auditory canal, and anastomosing with the proximal basilar artery.

• There are eight previous case reports of persistent otic artery and one of an otic artery variant .

• However, an assessment of the origin, course, and termination of these reported otic arteries is difficult because of the variable quality of image reproduction, the inclusion of only a single angiographic projection, or both.


Persistent Otic Artery

• Some reported cases of persistent otic artery may in fact represent a persistent trigeminal artery.

• Similarly, anastomoses may occur between the internal auditory artery (a branch of the anterior inferior cerebellar artery and, thus, the basilar artery) and the internal carotid artery via trigeminal and stapedial artery remnants.

• Such anastomoses may be best understood as representing overlapping vascular territories rather than persistence of an embryonic vessel.


Trigeminal artery. DSA image demonstrates a persistent trigeminal artery (arrows) thatarises from a more proximal location on the internal carotid artery than is normal. The finding was previously reported as a persistent otic artery.


Persistent Dorsal Ophthalmic Artery

• During early embryonic development, two primitive ophthalmic arteries are present: The ventral ophthalmic artery, which normally persists, and the dorsal ophthalmic artery, which normally regresses.

• In some instances, the opposite situation occurs, and the dorsal ophthalmic artery persists.

• At CT, this anomalous artery is seen to arise from the dorsal aspect of the supraclinoid portion of the internal carotid artery and enter the orbit by way of the superior orbital fissure instead of the optic canal.

Dorsal ophthalmic artery. Three-dimensional image from multidetector CT demonstrates the origin of the left dorsal ophthalmic artery from the dorsal aspect of the distal cavernous segment of the internal carotid artery and shows its course through the superiororbital fissure (arrow). The contralateral ophthalmic artery is seen to traverse the optic canal (arrowhead).

Persistent Primitive Olfactory Artery

• A persistent primitive olfactory artery is a rare variant of the anterior cerebral artery.

• The proximal portion of the persistent primitive olfactory artery courses in an anteroinferomedial direction, along the ipsilateral olfactory tract.

• The artery then makes a hairpin turn posterosuperiorly and continues in the distribution of the anterior cerebral artery.

• During normal development, the primitive olfactory artery usually regresses to a remnant known as the recurrent artery of Heubner.

• In people with a persistent primitive olfactory artery, the anterior communicating artery also is absent.

• In five of 13 previously identified cases of persistent primitive olfactory artery, a saccular aneurysm was found in the anomalous artery.

• These findings are indicative of an increased prevalence of aneurysm.


Normal VariantArteries in the Skull Base


Persistent Stapedial Artery

• The stapedial artery is a normally transient embryonic anastomosis between the branches of the future external carotid artery and internal carotid artery.

• Persistent stapedial artery has a reported prevalence of 0.48%.

• The stapedial artery originates from the vertical part of the petrous internal carotid artery, passes through the obturator foramen of the stapes, and terminates as the middle meningeal artery, within the extradural space of the middle cranial fossa.



• CT findings along the course of a persistent stapedial artery may include a small canaliculus at the distal end of the carotid canal; a linear structure that crosses the promontory of the tympanic cavity; an enlarged facial nerve canal, or a separate canal that parallels the facial nerve canal; and absence of the foramen spinosum, which normally contains the middle meningeal artery.

• An aberrant internal carotid artery and anomalies of the stapes and facial nerve also may occur in association with a persistent stapedial artery.



• People with a persistent stapedial artery may present with pulsatile tinnitus.

• At CT, this artery may be mistaken for a vascular neoplasm of the middle ear (glomus tympanicum tumor).

• It is important to identify a persistent stapedial artery preoperatively, as its presence may complicate tympanotomy, stapedectomy, and cholesteatoma resection and prevent cochlear implantation.


Aberrant Internal Carotid Arteries

• Recognized aberrant internal carotid arteries include intratympanic and lateral pharyngeal variants.

• There are 50 previously reported cases of aberrant intratympanic internal carotid artery in the literature.

• This anomaly may be secondary to disturbed differentiation of the third branchial artery.

• It is characterized by an enlarged inferior tympanic artery that anastomoses with the horizontal petrous part of the internal carotid artery.



• On axial CT images, the presence of an aberrant intratympanic internal carotid artery may be signaled by: (a) an internal carotid artery with a reduced diameter and a course posterior and parallel to the jugular bulb (b) a mass in the hypotympanum (c) deficiency of the bony plate along the tympanic portion of the internal carotid artery (d) absence of the vertical segment of the carotid canal.

• On coronal CT images, the important features are a hypotympanic mass and enlargement of the inferior tympanic canaliculus.



• Preoperative recognition of an aberrant intratympanic internal carotid artery is essential for avoiding potentially catastrophic consequences during myringotomy and middle ear surgery.

• This vascular anomaly, like a persistent stapedial artery, may mimic a glomus tumor.

• The lateral pharyngeal internal carotid artery is an anomalous vessel that extends to or near the midline of the posterior pharyngeal wall and is at risk during oropharyngeal tumor resection, tonsillectomy, adenoidectomy, or palatopharyngoplasty


Aberrant internal carotid artery. Axial images (bone window settings) from multidetector CT of the skull base depict an internal carotid artery with reduced caliber, that courses adjacent to the jugular bulb (arrow in a) and, at a higher level, within the hypotympanum (arrow in b). The bony plate along the tympanic portion of the internal carotid artery is absent (arrow in c).


Internal Carotid Artery Agenesis

• Congenital absence of the internal carotid artery has a prevalence of 0.01%.

• CT of the skull base and CT angiography in such cases demonstrate an absence of the carotid canal.

• The anomaly may be unilateral or, more rarely, bilateral.• The recognition of internal carotid artery agenesis is

important, especially in patients with cerebral thromboembolic disease due to atherosclerosis of the vertebrobasilar system.

• Poor collateral blood supply to the head places these individuals at high risk during conventional angiography and surgery.

• There is also a strong association between internal carotid artery agenesis and the development of intracranial aneurysms.


Bilateral carotid agenesis. Axial multidetector CT image of the skull base demonstrates bilateral absence of the carotid canals from their expected locations (arrows). (36) Bilateral absence of the internal carotid artery. DSA image obtained in a patient with bilateral agenesis of the internal carotid artery shows that the vertebral arteries supply both the anterior and the posterior circulation. (37) Unilateral agenesis of the internal carotid artery. Axial multidetector CT image of the skull base shows absence of the left carotid canal, a finding indicative of unilateral agenesis. Arrows indicate the normal right carotid canal. (38) Unilateral agenesis of the internal carotid artery. Three-dimensional reconstruction from MR angiography depicts the basilar artery supplying the left middle cerebral artery via the left posterior communicating artery (straight arrow). By contrast, the contralateral internal carotid artery (arrowhead) appears normal. Absence of the A1 segment (curved arrow) is noteworthy.


Bilateral carotid agenesis. Normal internal carotid arteries are not seen in the carotid spaces, and no carotid canals are noted on the high resolution CT scan of the skull base.The middle cerebral artery arises from the basilar artery through an enlarged posterior communicating artery.

Images from www.sherlockradiogy.com

Hypoplasia of theInternal Carotid Artery

• Congenital hypoplasia of the internal carotid artery is associated with a small carotid canal and should not be confused with acquired causes of diffuse narrowing, such as dissecting aneurysm, fibromuscular dysplasia, or segmental stenosis.

• Congenital hypoplasia of the internal carotid artery may be associated with anencephaly and basal telangiectasia.


References

• Diagnostic and Surgical Imaging Anatomy. Brain, Head & Neck, Spine / H. Ric Harnsberger. [et al.] ; managing editor, André J Macdonald. 1st ed. I: 278-291.

• RadioGraphics 2009; 29:1027–1043 • Simon J. Dimmick, BPthy, MBBS • Kenneth C. Faulder, MBBS, FRANZCR. Normal Variants of the Cerebral Circulation at Multidetector CT Angiography

Health & Medicine

Intracranial arteries