1. INTRACRANIAL ARTERIES
INTERNAL CAROTID
ARTERY
AND
NORMAL VARIANTS
REVIEW
SHERLOCKRADIOLOGY.COM

2. 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)

3. Gross Anatomy
• Anterior Circulation
- Intracranial Internal carotid artery and its
branches + ACoA, PCoA
• Posterior Circulation
- Basilar artery and its branches

4. 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

5. 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

6. 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

7. 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)

8. 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

9. 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

10. 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

11. 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

19. INTRACRANIAL INTERNAL
CAROTID ARTERY

20. 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)

21. 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)

22. Lacerum (C3) segment
• Small segment that extends from petrous
apex above foramen lacerum, curving
upwards toward cavernous sinus
• Covered by trigeminal ganglion
• No branches

23. 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

24. 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

25. 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)

26. 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)

27. 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

28. 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 "tridentshape" 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)

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

55. NORMAL 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

56. 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

57. 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.
RadioGraphics 2009; 29:1027–1043

58. Hyperplastic Anterior
Choroidal 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%.
RadioGraphics 2009; 29:1027–1043

59. Multidetector CT angiogram depicts a hyperplastic anterior choroidal
artery (straight arrow), ipsilateral posterior communicating artery
(arrowhead), and contralateral fetal posterior cerebral artery (curved
arrow).
RadioGraphics 2009; 29:1027–1043

60. 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.
RadioGraphics 2009; 29:1027–1043

61. Bilateral fetal posterior cerebral arteries.
Three-dimensional multidetector CT angiogram shows bilateral
large posterior communicating arteries (arrows), which provide
most of the blood supply to the posterior cerebral artery territory.
Both P1 segments are present but hypoplastic.
RadioGraphics 2009; 29:1027–1043

62. CT angiogram shows bilateral fetal posterior cerebral arteries
(arrows). Both P1 segments are absent. The arrowheads indicate
the superior cerebellar arteries.
RadioGraphics 2009; 29:1027–1043

63. Posterior Communicating Artery
Infundibulum
• 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.
RadioGraphics 2009; 29:1027–1043

64. 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.
RadioGraphics 2009; 29:1027–1043

65. Persistent CarotidBasilar Artery Anastomoses
RadioGraphics 2009; 29:1027–1043

66. 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.
RadioGraphics 2009; 29:1027–1043

67. Persistent Trigeminal Artery
• 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.
RadioGraphics 2009; 29:1027–1043

68. Persistent Trigeminal Artery
• 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.
RadioGraphics 2009; 29:1027–1043

69. Persistent Trigeminal Artery
• 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.
RadioGraphics 2009; 29:1027–1043

70. 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.
RadioGraphics 2009; 29:1027–1043

71. Variants of Persistent
Trigeminal 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.
RadioGraphics 2009; 29:1027–1043

72. Variants of Persistent
Trigeminal 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.
RadioGraphics 2009; 29:1027–1043

73. 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).
RadioGraphics 2009; 29:1027–1043

74. 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.
RadioGraphics 2009; 29:1027–1043

75. 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.
RadioGraphics 2009; 29:1027–1043

76. 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.
RadioGraphics 2009; 29:1027–1043

77. 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.
RadioGraphics 2009; 29:1027–1043

78. 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.
RadioGraphics 2009; 29:1027–1043

79. 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 of
the C2 vertebra (arrow). (31) Anteroposterior composite projection image from DSA shows a proatlantal
intersegmental artery (arrow) and right internal carotid artery (arrowhead).
RadioGraphics 2009; 29:1027–1043

80. 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.
RadioGraphics 2009; 29:1027–1043

81. 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.
RadioGraphics 2009; 29:1027–1043

82. Trigeminal artery.
DSA image demonstrates a persistent trigeminal artery (arrows) that
arises from a more proximal location on the internal carotid artery than is
normal. The finding was previously reported as a persistent otic artery.
RadioGraphics 2009; 29:1027–1043

83. 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.

84. 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 superior
orbital fissure (arrow). The contralateral ophthalmic artery is seen to traverse
the optic canal (arrowhead).

85. 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.
RadioGraphics 2009; 29:1027–1043

86. Normal Variant
Arteries in the Skull Base
RadioGraphics 2009; 29:1027–1043

87. 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.
RadioGraphics 2009; 29:1027–1043

88. Persistent Stapedial Artery
• 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.
RadioGraphics 2009; 29:1027–1043

89. 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.
RadioGraphics 2009; 29:1027–1043

90. 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.
RadioGraphics 2009; 29:1027–1043

91. Aberrant Internal Carotid
Arteries
• 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.
RadioGraphics 2009; 29:1027–1043

92. Aberrant Internal Carotid
Arteries
• 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
RadioGraphics 2009; 29:1027–1043

93. 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).
RadioGraphics 2009; 29:1027–1043

94. 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.
RadioGraphics 2009; 29:1027–1043

95. 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.
RadioGraphics 2009; 29:1027–1043

96. 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

97. Hypoplasia of the
Internal 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.
RadioGraphics 2009; 29:1027–1043

98. 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