HRCT Temporal bone anatomy

Planes of scanning Axial

30 Degrees to anthropological base line Parallel to lateral SCC. Best displays inner & middle ear. Sections parallel to the hard Palate

Direct Coronal plane patient head extended in prone or supine with 105 degree plane is perpendicular to the lateral SCC Sections are parallel to posterior wall of maxillary sinus

Saggital plane

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105

0

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Temporal Bone

1.Squamous

2.Petrous

3.Mastoid

4.Tympanic

5. Styloid

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Lateral wall of MCF

Mastoid air cells

Encloses Labyrinth

Forms bony EAC

Forms Styloid process

EAR IS DIVIDED INTO 3 PARTS

• EXTERNAL EAR

• MIDDLE EAR

• INTERNAL EAR

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EAC

Tympanic cavity

Bony & membranous labyrinth

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EXTERNAL EAR

External auditary canal- 2.5 cms in length

Cartilaginous Lateral 1/3rd

Osseous Medial 2/3rd

Tympanic membrane-makes 55 degree angle With floor of the meatus

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Boundaries & Communications Of Middle Ear

Roof- Tegmen tympani Separates from MCFFloor-thin convex bone separating from superior bulb of IJVLateral wall-tympanic membraneMedial wall – separates from inner ear

Anteriorly -auditory tube

Posteriorly –mastoid antrum

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1)PROMOTARY- Round bulge produced by basal turn of cochlea

2)FENESTRA OVALIS -Lies postero- superior to promontory & connects middle ear with vestibule and is occupied by footplate of stapes

3)FENESTRA ROTUNDA- Posteroinferiorly to oval window & connects ME with scala tympani of cochlea.

4)PROMINENCE OF FACIAL NERVE CANAL -Runs backwards just above the oval window to reach the lower margin of aditus.

Medial wall of middle ear:

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ANTERIOR WALL

1) Superior-opening of canal for tensor tympani.

2) Middle –opening of auditory tube.

3) Inferior-thin plate of bone separating from carotid canal.

POSTERIOR WALL

1) Aditus to mastoid

2) Fossa incudis-lodges the short process of incus.

3) Pyramidal eminence-attachment for stapedial tendon.

• The scutum is normally thin and sharply edged; and is an important bony landmark as it is one of the bony structures eroded early by a cholesteatoma.

• • Scutum, the tympanic membrane and the tympanic

annulus are best demonstrated on coronal images at the mid bony portion of the external auditory canal.

Middle Ear

• three compartments in coronal plane.

• A line drawn from lower edge of the scutum to the tympanic portion of the facial nerve -epi/mesotympanum.

• parallel to the floor of the external auditory canal meso/hypotympanum

Epitympanum

It contains :• Head of the malleus• Malleo–incudal

articulation• Body and short process

of the incus,• Short process of Incus

projects posteriorly towards aditus.

These structures are best demonstrated on axial images.

• Tegment tympani – roof of epitympanum – barrier between middle cranial fossa and middle ear cavity.

• Best evaluated on coronal images

• Posteriorly the epitympanum opens into the mastoid antrum via the aditus ad antrum; demonstrated on both the axial and coronal images

Mesotympanum

• The mesotympanum contains the ossicular chain

• Oval and round window

• Long process of the Incus

• Stapes • Handle of Malleus • Facial nerve Canal

• The ice-cream cone sign represents the normal appearance of the malleus and incus on an axial high-resolution CT scan (HRCT) image of the temporal bone .

• The ball (scoop) of the ice cream is formed by the head of malleus and the cone is formed by the body of the incus.

• The space between the ice-cream cone and the scutum is called Prussak's space.

• On coronal images the long process of incus is vertically oriented parallel to the long process of malleus, continuing as the rounded lenticular and the facet to articulate with the head of the stapes

• The stapes hub and crura are best demonstrated on axial images at the level of the oval window

• The stapes footplate sits in the oval window niche and cannot always be discretely identified on CT.

MALLEUS

• Epitympanum: Headarticulates with body of Incus( Ice Cream Cone )

• Mesotympanum: Neck andHandle.

INCUSžMesotympanum: Longprocess

(LPI) articulates with head of Stapes.  

žEpitympanum: Short process is the most posterior part and points towards aditus.

Body

and  Shortprocess ofIncus

Stapes

The stapes is the smallest bone in the body. 

Head ,neck, crura (anterior and posterior) and a footplate. 

St T- Stapedius tendon

Axial Section at Mesotympanum

• Two parallel structures

• Anteriorly: Neck and Handle ofMalleus(H)

• Posteriorly: Long process ofIncus (LPI)

• Anteriorly: Head andhandle of Malleus

• Posteriorly: Long processof Incus (LPI)

Coronal section Mesotympanum

At the posterior wall of the mesotympanum

• bony protuberance - pyramidal eminence- stapedius muscle exits to its attachment at the neck of the stapes

• Lateral to it - facial recess for descending (mastoid) segment of the facial nerve

• Medial to it - sinus tympani - a blind spot for the surgeon during transmastoid surgery and so cholesteatoma at this site may be overlooked

Medial Wall• Promontory - a bony

prominence on the medial wall of the mesotympanum - protrusion of the basal turn of the cochlea.

• Above the promontory is the oval window and posteroinferior to which is the round window niche

Anterior wall•

The anterior wall has a thin plate of bone which separates this portion from the internal carotid artery as it enters the skull.

• Always look for bone covering ICA.

• 2 openings in anterior wall • Upper : canal for tensor

tympani (Stt) • Lower : Eustachian tube (Et) • Axial section is best for

Eustachian tube, semi canal and carotidcanal.

 Semi canal for tensortympani (Stt)

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Inner ear- Osseous & Membranous

Osseous- vestibule Semicircular canals Cochlea

• Cochlea• The perilymphatic space of

vestibule is continuous with the cochlea anteriorly.

• The cochlea is a conical structure,extends for 2.5-2.75turns

• Promontory (P) is the projection raised by the basal turn of cochlea.

• The basal turn opens into the round window niche

 

Coronal

Middle turn

Axial

• The vestibule consists of the superior utricle and the inferior saccule.

• The semicircular ducts open into the utricle.

Axial coronal

• SSC

PS

PSC

SSC

• The cochlear aqueduct contains the perilymphatic duct while the vestibular aqueduct contains the endolymphatic duct and the intraosseous portion of the endolymphatic sac.

• The internal auditory canal shows considerable individual variation in size and configuration but should be symmetrical in any one person

• The medial opening - porus acousticus. • Divided at its most lateral end by the horizontal

crista falciformis and vertical crests into four compartments.

• The facial nerve is in the anterosuperior compartment, the cochlear nerve is in the anteroinferior compartment while the superior and inferior vestibular nerves are in posterior superior & inferior compartments,

Facial Nerve

• VII n:-– Intracranial segment– Labyrinthine segment– Tympanic segment– Mastoid portion– Parotid segment

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• The facial nerve, from the lateral end of the internal auditory canal enters the petrous bone as the labyrinthine portion running anterolaterally, superior to the cochlea and towards the anterior genu (geniculate ganglion).

• Then it makes an abrupt turn to run posterolaterally along the medial attic wall beneath the lateral semicircular canal as the tympanic portion towards the posterior genu.

• And finally turns inferiorly as the descending (mastoid) portion to exit at the stylomastoid foramen.

How to identify Facial nerve canal?

• Axial: at level of Epitympanum, it is seen as a linear structure medial to ice-cream cone.

• Coronal: At level of OW, it is seen as a round structure with bony outline between OW inferiorly and Lateral Semicircular canal (LSC)superiorly.

      Coronal: A

Sigmoid plate -thin bone separating the sigmoid sinus from adjacent structures (especially mastoid air cells).

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Axial sections- caudal to cephalad

1.Axial hypotympanic-jugular foramen level

1)carotid canal & jugular Fossa forming snowman Configuration

2)opening of auditary Tube

3)petrooccipital fissure

4)TM joint and mandibular Condyle

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Carotid canal

Jugular fossa

Auditory tube

Temporo mandibular jt

Petro occipital fissure

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2.Axial inferior tympanic level

1)ant &post walls of EAC

2)descending facial nerve canal

3)carotid canal

4)cochlear aqueduct

5)long process of Malleus.

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Descending facial nerve

Cochlear aqueduct

EAC

Promontory

canal For tensor tympani

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3.Axial mid tympanic level

1)neck of malleus ,long process of incus and stapes

2)semicanal for tensor tympani

3) 3 turns of cochlea

4)facial nerve canal

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malleus

incus

stapes

Round window

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Oval windowIAC

Scala tympani

Facial recess

4. Axial Epitympanic IAC level

• Ice-cream cone appearance-head of malleus and body of incus

• Stapes –occasionally seen articulating with oval window.

• Internal auditory canal

• Descending facial nerve canal

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Ice cream cone appearance

Facial nerve canalInternal auditory canal

5. Axial lateral semicircular canal level

• 1)mastoid antrum and its aditus & Koerner’s septum

• 2) lateral semicircular canal and vestibule

• 3) post semicircular canal &vestibular aqueduct

• 4) IAC

• 5) sq part of temporal bone.

• 6) geniculate ganglion55

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LATERAL SC CANALVESTIBULE IAC

PROXIMAL LIMB OF 7TH N

ADITUS TO MASTOID ANTRM

VESTIBULAR AQUEDUCT

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CORONAL SECTIONS1. Coronal anterior tympanic level

-Head of malleus-Tegmen tympani-Basal & 2nd turn of Cochlea-Geniculate ganglion- scutum

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COCHLEAIAC

HEAD OF MALLEUS

SCUTUM

GENICULATE GANGLIASQ TEMPORAL BONE

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2.Coronal mid-tympanic level

-L shaped incudo-stapedial articulation

-Stapes

-Oval window

-Tegmen tympani

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IAC

Oval Window

Incus in aditus

Stapes

Tegmen tympani

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3.CORONAL OVAL WINDOW LEVEL

•IAC

•Oval window

•Stapes

•Lateral SCC

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Vestibule

IAC

Sup SCC

Antrum

Lateral SCC

Oval window

Middle earEAC

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5.Coronal jugular foramen level

•Jugular foramen

•Descending facial Nerve

•Mastoid antrum

•Occipital bone and atlas

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Jugular fossa

Descending facial nerve

Mastoid Air Cells

Role of MRI

• Excellent in studying Inner ear– Membranous labyrinth– Vestibule– Saccule & utricles

• Preoperative evaluation in pts for Cochlear implants• To evaluate CP angle tumors• To evaluate Neurovascular bundles• Functional assessment – Nerve stimulation• Virtual scopy

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7 & 8th nerve complex

Cochlea