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Anatomical Characteristics of Temporal Bone on Computerized
Tomography
Dr. Osama A. Mabrouk Kheiralla, MD
Assistant Professor of Radiology - College of Applied Medical Sciences - Radiological
Sciences Department – Imam Abdulrahman bin Faisal University- KSA. E-mail:
drkheiralla@gmail.com
Abstract:
The anatomical complexity of the temporal bone represents a challenge in the interpretation and diagnosis
of different pathological conditions in this region. Computed Tomography (CT) play an important role in
the diagnosis of temporal bone pathologies. High-resolution computed tomography (HRCT) is the modality
of choice for imaging of bony lesions of temporal region. This article discusses the anatomical details of
the temporal bone and the characteristic radiological findings of the temporal bone in Computed
Tomography (CT) scan imaging.
Key words: Anatomy of the Temporal bone, Computed Tomography (CT) scan
Introduction:
The anatomical complexity of the temporal bone represents a challenge in the interpretation of anatomical
details and the diagnosis of different pathological conditions in this region. The understanding of the CT
anatomy of the temporal bone is difficult due to complex structural relations that cannot be visualized on a
single plane [1]. O Computed Tomography (CT) scan is a useful modality for imaging bony and some soft
tissue lesions of the temporal bone. CT scans are more accurate in identifying many soft tissue
abnormalities and are much less prone to artifacts [2]. CT scanning offers the greatest structural definition
of any currently available imaging modality [3, 4]. The basic anatomical components of the temporal bones
are depicted on axial and coronal CT images (Figure.1).
Familiarity with the detailed anatomical components of the temporal bone is mandatory to proficiently
interpret computed tomographic (CT) scan studies of the temporal bone. The thin-section scanning of high
resolution CT, which makes detailed demonstration of the temporal bone anatomy a practical reality, has
contributed to the diagnosis and postoperative evaluation of middle ear diseases [5]. CT scan allows
multiplanar and noninvasive visualization of temporal bone. Advanced computed tomographic (CT) scan
techniques have reached the ability of diagnosing small temporal bone tumors.
Figure1: Axial CT scan demonstrating skull base and temporal bone.
2
Figure2: Coronal CT scan demonstrating details of temporal bone.
Temporal bone anatomy:
Temporal bones are bilaterally symmetrical bones situated at the base and sides of the skull; they
participate in the formation of the lateral portion the base of the cranial vault and lateral wall of the skull.
Each temporal bone is composed of five parts: the petrous part, squamous part, the tympanic part, the
mastoid and the styloid process (Figure 3).
Figure.3: The five main components of Temporal Bone.
Temporal bone has got multiple intrinsic channels and fissures, and multiple extrinsic sutures that appear
on CT images similar to fractures. Temporal bone houses several important structures such as the middle
and internal ear apparatus including the cochlea, vestibule and the vestibulocochlear nerve (cranial nerve
VIII), the facial nerve (cranial nerve VII), the internal carotid artery, and the jugular vein [6].
The petrous part is the medial part of the temporal bone; it is pyramidal in shape and of solid component
to protect the internal soft structures of the acoustic meatus and vestibular apparatus. There are numerous
openings in the inferior surface of the petrous part of the temporal bone such as the carotid canal for the
passage of the internal carotid artery, and the jugular foramen for the passage of the internal jugular vein,
9
th
, 10
th
and 11
th
cranial nerves.
The mastoid process is a conical bony projection, located posterior to the petrous part; it is filled with air
cells and lined with mucous membrane internally.
The squamous part is the superior part of the temporal bone, it is flat plate shaped and forms a portion of
the lateral wall of the middle cranial fossa, the inferior portion of the squamous part of the temporal bone
represents the glenoid fossa which articulates with the mandibular condyle to form the temporomandibular
joint (TMJ). The squamous part contains a bony groove that houses the middle meningeal artery.
The zygomatic part arises anteriorly from the outer surface of the squamous part of the temporal bone to
articulate with the zygomatic bone.
The tympanic part of the temporal bone is horseshoe-shaped surrounds the external auditory meatus,
contributing in the formation of its anterior, posterior and inferior walls. The lateral border of tympanic part
Squamous Part
Petr
ous Part
Mastoid Part
The Styloid
P
rocess
Zygomatic Part
3
is attached to the cartilaginous part of the external acoustic meatus and medially it is attached to the petrous
part of the temporal bone.
The styloid process
is a needle-like projection from the inferior aspect of the petrous part of the temporal
bone and runs downward and slightly anteriorly. The styloid process represents the anchor point for several
muscles and ligaments associated with the tongue and larynx, they include: the stylohyoid ligament, the
stylomandibular ligament, the styloglossus muscle, the stylohyoid muscle and the stylopharyngeus muscle.
The stylomastoid foramen is located posterior to the styloid process which forms a passage for the 7th
cranial nerve and the stylomastoid artery.
During embryological development, the temporal bone undergo ossification from eight centers, one for the
tympanic part, one for the zygomatic part, two for the styloid process and four for the petrous part and
mastoid process. The squamous and tympanic parts undergo intramembranous ossification, while the
temporal part, the petrous part and the styloid part undergo endochondral ossification. The mastoid process
is absent in the newborn and develops with the traction force of the sternocleidomastoid muscle.
The external auditory meatus
(ear canal) which is located within the tympanic part of the temporal bone
is a sigmoid tube extends from the tympanic membrane medially to the outer ear laterally; it measures
about 2.5 cm in length and 0.7 cm in diameter. It is divided into two parts, a medial bony part represents
the inner two thirds and an outer fibrocartilaginous part represents the outer one third it has a cartilaginous
anterior and lower walls, and fibrous superior and back wall.
The middle ear
have two components, the first one is the antrum; it is an air space in the petrous part of
the temporal bone represents the communication between the mastoid cells posteriorly and the middle ear
anteriorly, the second component of the middle ear is the tympanic cavity, it
is an air filled cavity within
the petrous part of the temporal bone extending between the tympanic membrane laterally and the oval
window of the inner ear medially, it contains the ossicular chain which is composed of three tiny bones
known as the malleus, incus, and stapes (Figure 4, 5).
The main function of the ossicles is to transfer the vibrations of the tympanic membrane into waves in the
fluid of the inner ear. The roof of the middle ear is formed by a bony structure known as the tegmen
tympani which separates the middle ear from the middle cranial fossa. The floor of the middle ear is mainly
composed of a bony structure covering the jugular bulb.
Figure4: Anatomical illustration of the middle ear with its internal components.
4
Figure.5: Anatomical illustration of the ossicular chain which is composed of three tiny bones known as
the malleus, incus, and stapes
The inner ear
is located within the petrous part of the temporal bone; it is composed of the bony labyrinth,
which includes the cochlea anteriorly, the vestibule and the semicircular canal posteriorly. The inner ear
consists of osseous and membra- nous labyrinths surrounded by dense compact bone called the otic capsule
[7]. The cochlea is responsible for hearing while the vestibule and semicircular canal are responsible for
balance. The cochlea is of spiral shape with 2.5-2.75 turns (scalae), the apical turn also known as vestibular
duct (scala vestibule) abuts the oval window, the middle turn also known as the cochlear duct (scala media)
houses the organ of Corti, the basal turn also known as the tympanic duct (scala tympani) terminates at the
round window, these ducts are separated from each other by interscalar septae. The apical part of the
cochlea is pointed anteroinferiorly to the lateral direction; while the basal part rests close to the internal
auditory canal. The cochlea is filled with a perilymph, vibrations that are coming from the middle ear
through the oval window lead to the movement of the perilymph, Movement of the perilymph induces the
movement of the basilar membrane and organ of Corti inside the cochlear duct; hair cells will sense this
motion and convert it into electrical signals that will communicated to auditory neurons which will
transform the signals into nerve pulses. The coronal projection is probably the best for visualizing the
cochlea [8].
The vestibule is fluid-filled duct of oval shape anteroposteriorly, composed of two sacs the utricle which
occupies the upper part of the vestibule and saccule which is located inferiorly. The vestibule is important
in maintaining balance, it is laterally connected to the footplate of the stapes through the oval window, and
medially connected to the acoustic nerve, balance vibrations due to head tilting reach the vestibule from the
stapes through the oval window, leading to polarization of hair cell within utricle and saccule of the
vestibule, this polarization will be converted to nerve signal that pass to the brain through the vestibular
nerve. The semicircular canals composed of three canals, anterior, posterior and lateral, each one of these
canals detects motion in a single plane. Each canal is filled with endolymph. The actual hair cells sit in a
small swelling at the base called the ampulla.
The Facial nerve:
Temporal bone CT is particularly useful in the evaluation of the caliber and the course of the IAC and bony
facial nerve canal in the temporal bone [9]. The facial nerve has several functions; it is the motor nerve of
facial expression and conveys special sensory (taste) information from the anterior two-thirds of the tongue
and the interior of the mouth. and conveys general sensation from the external acoustic meatus and scalp,
beside other functions like lacrimation and salivation.
The facial nerve can be divided into intracranial, intratemporal, and extracranial segments. The focus of
this discussion is the intratemporal segment. The facial nerve emerges from the brain stem beneath the
pons, it originates from three nuclei, the motor part of the facial nerve arises from the motor nucleus in the
pons while the sensory part arises from solitary nucleus and parasympathetic part arises from the superior
salivary nucleus in the pontine tegmentum (Figure 6). Facial nerve travels through the cerebellopontine
angle, to enter the temporal bone through the internal acoustic meatus, passing through the anterior superior
quadrant of the internal acoustic meatus to enter the Fallopian canal, and then it passes anterolaterally
Short process
Body
Long process
Manubrium
Neck
Head
Lenticular process
Anterior Crus
Posterior Crus
Head
Footplate
Incus
Malleus
Stapes
5
between the cochlea (anteriorly) and vestibule (posteriorly), and runs posteriorly at the geniculate ganglion,
where the three branches originate:
1- The greater superficial petrosal nerve
2- The lesser petrosal nerve
3- The external petrosal nerve
Figure 6: Anatomical illustration the inner ear with its internal components
Figure 7: Nuclei of the facial nerve and its pathway within the temporal bone
From the geniculate ganglion, the facial nerve passes beneath the lateral semicircular canal in the medial
wall of the middle ear cavity passing posterior to the cochleariform process, tensor tympani and oval
window, after that it makes a second vertical turn distal to the pyramidal eminence downwards to the
stylomastoid foramen, where it gives off three branches:
1-
The tympanic branch to the stapedius muscle.
6
2-
The Chorda tympani branch, it is the terminal branch of the nervus intermedius: carrying both
secretomotor fibers to the submandibular gland and sublingual gland and taste to the anterior two thirds
of the tongue
3-
The pain fibers to the posterior part of the external acoustic meatus.
As the facial nerve exits the stylomastoid foramen, it passes between the posterior belly of the digastric
muscle and the stylohyoid muscle and enters the parotid gland
(Figure 8)
.
Figure 8: The pathway of the facial nerve within the temporal bone
CT scan imaging of the Temporal Bone:
The advent of high-resolution computed tomography (CT) scanning in the 1980s has revolutionized
diagnostic imaging of the temporal bone. CT scanning offers the greatest structural definition of any
currently available imaging modality.[1, 2] Computed tomography (CT) of the temporal bone is basically
done in two acquisition techniques: Dual acquisition technique; it includes separate direct coronal and
separate direct axial scans, the second technique is single axially acquired volume technique, in this
technique single axial source data acquired from a multidetector row CT (MDCT) scanner and reformatted
in coronal and sagittal planes, this technique is preferred because it provides adequate anatomical details
while reducing radiation dose and motion artifact. However, in some cases, both techniques may be applied
to provide superior image quality. CT scan imaging of the temporal bone is done in both axial and coronal
views, these two views are mandatory for adequate and detailed study for temporal bone structure.
Axial images of the Temporal Bone:
Figure 9: Axial CT scan image through the superior portion of left temporal bone.
Anterior
s
emicircular canal
Posterior semicircular canal
Mastoid air cells
Koerner septum
7
Figure 9: is an axial section through the superior portion of the left temporal bone showing the posterior
margin of the temporal bone forming the anterior margin of the posterior cranial fossa, at this level the Otic
capsule appears as dense white bone within which, the anterior and posterior semicircular canal are found,
mastoid air cells appear lateral to the otic capsule. The Koerner septum appears as a septum between
squamous air cells and petrous air cells.
Figure 10: Axial CT scan image through the left temporal bone at the level of sigmoid sinus
Figure 10: is an axial CT scan section more inferior to the above one, at the level of sigmoid sinus where it
indents the cerebellar plate posteriorly. The vestibular aqueduct appears coursing posterior to the posterior
semicircular canal, superior semicircular canal appears located anteriorly. The superior aspect of the lateral
semicircular canal appears lateral to the other semicircular canals. The facial nerve located within the
superior portion of the internal auditory canal
(Figure 10)
Figure 11: Axial CT scan image through the left temporal bone at the level of the vestibule
Figure 11: is an axial CT scan section through the left temporal bone, at the level of the horizontal and
posterior semicircular canals and the vestibule, at this level the ossicles appear inside the tympanic cavity.
The facial nerve appears coursing from the internal auditory canal and turns anteriorly as the labyrinthine
segment where it ends at the geniculate ganglion then it continues posteriorly as the tympanic segment
Internal
auditory canal
Vestibular aqueduct
Sigmoid Sinus
Posterior Semicircular
Canal
Internal auditory canal
Head of Malleus
Short process of Incus
Geniculate Ganglion
8
Figure 12: Axial CT scan image through the left temporal bone at the level of the glenoid fossa
Figure 12: demonstrates the most inferior axial CT scan section through the left temporal bone, at the level
of the glenoid fossa, at this level the carotid artery appears oriented medially and anteriorly, the jugular
vein appears posterolateral to the carotid artery, the sphenoid sinus anteromedial to the carotid artery, the
glenoid fossa appears anterior to the external auditory canal.
)
Coronal images of the Temporal Bone:
Figure 13: Coronal CT scan image through the left temporal bone at the level of the glenoid fossa
Figure 13: is a coronal CT scan image through the anterior part of the temporal bone demonstrating the
carotid artery close to the trochlea (the basal turn); the head malleus appears inside the epitympanum. The
Glenoid fossa represents the inferomedial margin of the mastoid air cells. In this image the tegmen tempani
appears separating the middle cranial fossa from space of the middle ear.
Figure 14: is coronal CT scan image more posterior that the previous one, through the internal auditory
canal medially and the lateral semicircular canal appears lateral to it, and the facial nerve courses
inferolateral to the lateral semicircular canal and the ossicles appear inside the tympanic cavity, with the
scutum represents the lateral boundary of the epitympanum.
The
condyle of the mandi
ble
External auditory canal
The carotid artery
The sphenoid sinus
The jugular vein
The
condyle of the
mandible
The G
lenoid fossa
The head of the
malleus
The
carotid artery
The trochlea
9
Figure 14: Coronal CT scan image through the left temporal bone at the level of the glenoid fossa
Conclusion:
Temporal bone imaging is challenging and involves thorough understanding of temporal bone anatomy. CT
scan is the best imaging modality to identify anatomical details and minute structural components of
temporal bone; it has the ability to clarify the hidden parts of the temporal bone through its special
algorithms and multiplanar reformatting technique.
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The
lateral semicircular canal
The ossicles
The
facial nerve
The
I
nternal auditory canal
