Skull Base Anatomy, Variants,

and “Don’t Touch Me” Lesions Nancy J. Fischbein, MD

Neuroradiology Section

Stanford University Medical Center

ASHNR 2017

Session 8: Temporal Bone & Skull Base – Part 2

Monday 9/18/17, 7:45-8:10 a.m. Learning Objectives

•  To review the anatomy of the anterior and

central skull base and associated foramina

and soft tissues on CT and MR

•  To demonstrate selected anatomic variants

of these regions

•  To illustrate some of the common and less

common “don’t touch” lesions of the region

Disclosures

•  None

ANATOMY

Anterior Skull Base •  Frontal bone

–  Squama (vertical

portion)

–  Orbital plate

(horizontal portion)

•  Floor of anterior

cranial fossa

•  Also ethmoid roof

–  Frontal sinus

•  Ethmoid bone

–  Cribriform plate

–  Ethmoid sinuses

•  Articulation

–  Posteriorly w

sphenoid bone,

superiorly w parietal

bone

Adult

Newborn

Orbital plate

Orbital plate

Squama

Squama

Gray’s Anatomy

Ethmoid Bone •  Four parts

–  Horizontal or

cribriform plate

•  Perforated by

foramina for olfactory

nerves

–  Perpendicular plate

•  Part of nasal septum

–  Paired lateral

masses

•  Air cell labyrinths

Olfactory recess

Olfactory groove

Crista galli Ethmoid roof

Cribriform

plate

Ethmoid roof (fovea ethmoidalis) is formed by orbital process of frontal bone

Olfactory recess

Ant eth

canal

Post eth

canal

Perpendicular

plate

Crista galli

Olfactory

apparatus

Cribriform level

Gyrus rectus

Kallmann Syndrome

Olfactory recess

Anterior Skull Base: Changes over Time

•  At birth, ASB largely cartilaginous

•  Ossification begins in roof of ethmoid labyrinth

laterally and spreads to midline

–  Crista galli, perpendicular plate of ethmoid begin ~2

months, steady increase to ~14 months (Belden et al, AJNR 1997)

Birth: no ossification 7 mo: thin but complete

bony bridge

3 mo: some ossification;

faint outline of CG

Developmental defects/lesions may affect the anterior skull base. At

all ages, be alert for meningocephaloceles.

Frontonasal cephalocele

31 F found down in apartment,

dx’d w bacterial meningitis.

Frontoethmoidal cephalocele

B frontoorbital cephaloceles

Other developmental lesions may occur 1 day old male w difficulty breathing; also mild hypertelorism

Dx: nasal glioma [no direct intracranial connection identified at surgery; there is also

dysplasia of L anteroinferior frontal lobe and L olfactory apparatus]

These are “don’t touch” in the sense of “don’t touch w/o considering that you may end up in brain and CSF”

•  Sphenoid bone

–  Body; greater wings; lesser

wings; pterygoid processes

–  Multiple skull base foramina and

fissures for vessels and nerves

•  Occipital bone

–  Basiocciput contributes to clivus

Central Skull Base

Sphenoid bone, posterior surface (Gray s anatomy)

Occipital bone, outer surface (Gray s anatomy)

Central skull base, inner and outer surfaces (Frank Netter, Atlas of Human Anatomy)

Palatine foramina

NP

OC

Pterygoid process

Clivus

Hypoglossal canal

Pterygopalatine fossa

C

Jugular foramen

J J CVJ

PPF VC

FO

FS

C

SPF

FO

FS Carotid canal

C

CT Anatomy of Central Skull Base and Foramina

Ca

Foramen rotundum

Orbit

MCF

IOF

*

Superior orbital fissure Optic canal

SS

FO NP

ST

FR

VC

SS

AC

P

OC

GWS

SOF

IOF

PPF

CS

FR

C PA

SS

FO

NP

Jugular foramen V3 below skull base

MR Anatomy of Central Skull Base and Foramina

Optic

nerve

SOF

PPF

VC

Occipital condyles

Hypoglossal canal

Clivus

Petrous apices

Trigeminal nerves

Chiasm

Sella

Foramen ovale

V3

Optic canals

Ant clinoid

SOF

F. Rotundum

Vidian canal

Central Skull Base: Changes Over Time, CT

16 F

6 F 18 mo F

67 F

10 F

18 mo:

Minimal sphenoid sinus pneumatization

Prominent sphenooccipital synchondrosis

10 yrs:

Larger sinus

Synchondrosis obliterating

Adulthood:

Large fully pneumatized sphenoid sinus

Complete obliteration of sphenooccipital synchondrosis

Central Skull Base: Changes Over Time, MR

•  Pneumatization of sphenoid sinus

•  Fusion of the spheno-occipital synchondrosis

•  Clival marrow: red to yellow

14 mo 3 yrs 8 yrs 52 yrs

Some clival heterogeneity can be normal

Initial MR Correlative CT

50 yr old male

7 yrs later

Same patient, age 57

ANATOMICAL VARIANTS/

“DON’T TOUCH” LESIONS

•  Developmental variants or anatomical

aberrations in bone, vessels, soft

tissues

•  We don’t want to biopsy them or treat

them with stereotactic radiosurgery

•  There are enough to fill entire

textbooks! A few are selected

Notochord •  Flexible rod-shaped

body in embryos of

all chordates

–  Composed of

mesoderm-derived

cells

–  Defines primitive axis

of the embryo

–  Located ventral to

neural tube

•  In most vertebrates,

persists only as

nucleus pulposus of

intervertebral disc Nguyen R et al. AJNR Am J Neuroradiol 2009;30:803-807

Medial basal canal

Sphenooccipital synchondrosis

Ecchordosis Physaliphora •  Ectopic notochordal

remnant

–  Soft tissue mass

associated w

scalloping of

posterior clivus

–  T2 bright, non-

enhancing

•  Typically incidental:

asymptomatic,

indolent

–  Follow-up imaging if

unsure of dx

31 M w HA. Incidental lesion noted that

remained stable in follow-up.

Persistent Canals and Divots Persistent Craniopharyngeal Canal

Type 3A, w cephalocele

Fossa Navicularis Magna

Type 3B, w tumor (teratoma)

CPC: Glastonbury et al, AJNR 2014

Enlarged IT canaliculus Aberrant ICA in ME

Normal FS

Other side

Normal

carotid

canal

Absent FS

Absent foramen spinosum w aberrant ICA, persistent stapedial a

Absent Canals 39 M with R pulsatile tinnitus and vascular retrotympanic mass.

Congenital Absence of ICA/Carotid Canal

Asymmetrical marrow

Petrous Apex Variations: Many Petrous Apex Cephalocele

May coexist with empty sella, arachnoid pitting, osseous-dural dysplasia -- Do consider possible association with IIH (idiopathic intracranial hypertension)

27 M w epilepsy. L temporal origin

of seizures.

Brain may herniate into

arachnoid pits

Patient referred for bx

Dx: prominent arachnoid granulation vs

cephalocele of greater wing of sphenoid

Dx: focal brain herniation into

arachnoid granulation/pit.

Patient did not have IIH.

Another variant to recognize

43 F referred to our sinus center for biopsy of a skull base lesion.

• Benign

developmental

variant

• Nonexpansile

lesion w

osteosclerotic

borders, internal

fat, curvilinear

calcifications in

basisphenoid or

adjacent skull

base

• Fatty marrow

conversion, but

then failure of

pneumatization

Welker et al, AJR June 2008

Dx: arrested pneumatization of the skull base

Focal lesion of R basisphenoid.

Benign or aggressive? HU: -28

DON’T BIOPSY

•  These may be actual lesions, but they can

often be identified specifically enough that

they do not require biopsy, or biopsy can be

dangerous, though they may require some

form of treatment

•  And in some cases biopsy may be indicated,

but one can still redirect from a plan to

resect to a plan to biopsy

CT and MR are often complementary at

the skull base. We are not just trying to get

the clinicians to order more studies!

56 F w HA, R CN VI palsy

Soft tissue lesion involving sella, central skull base. Met?

Macroadenoma? Should we plan for biopsy?

Let’s do an MR first a CTA/CECT would also be reasonable

Dx: partially thrombosed giant aneurysm of R cavernous ICA.

Patient was subsequently rx’d w flow diverting stents.

21 F w new onset R CN XII palsy and HA during pregnancy.

Dx: presumed “cavernous

hemangioma” [low flow

venous vascular malformation]

of bone. These can enlarge

during pregnancy. Post-

partum, symptoms resolved,

and the patient is being

followed.

ASL perfusion 51 M referred for evaluation of clival chordoma after an MR

was performed for h/o HA

Dx: “benign fibro-osseous lesion generally c/w FD, but islands

of woven bone are rare”

How about this case? 39 M w HA, diplopia.

Dx: metastatic spindle cell sarcoma

48 year old M w 3 year h/o R sided rhinorrhea.

Dx: spontaneous lateral sphenoid cephalocele c/b CSF leak.

Summary

•  Anterior

and central

skull base

anatomy

•  Anatomical

variants

•  Don’t touch

lesions