CT ANATOMY Temporal Bone

8/3/2019 CT ANATOMY Temporal Bone

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1873D CT of the Temporal Bone: Anatomy and Pathology

The temporal bone includes many small struc-

tures within a very compact region, some

measuring well under 1 mm. Their multi-

spatial orientation makes it dicult to conceptualize

the intricate three-dimensional (3D) relationship o

these structures based on conventional two-dimen-

sional (2D) imaging. The purpose o this study is to

demonstrate the role played by 3D CT to better un-

derstand the complex anatomy o the temporal bone.

In addition, select pathological cases are eatured to

highlight the role played by 3D CT to urther charac-

terize disease entities not ully evaluated by conven-tional two-dimensional imaging.

Introduction

Conventional two-dimensional imaging in the axial

and coronal planes is routinely used to display the

anatomy and pathology o the temporal bone. Al-

though the trained head and neck radiologist may

easily interpret such studies, the general radiologist

may nd it dicult when it comes to interpretation

o such scans. Also, there is a long learning curve

when one starts to interpret temporal bone studies.

It is the inherent multi-spatial orientation o several

small structures within a compact region that makes

the anatomy o the temporal bone so complex. How-

ever, we believe that 3D reconstructions o the tem-

poral bone can help one better understand temporal

bone anatomy. Such volume-rendered 3D images

can be sectioned in any plane and rotated in space

to better conceptualize the underlying anatomy. The

purpose o this article is thereore to demonstrate the

role played by 3D CT to simpliy the complex anat-omy o the temporal bone. In addition, using select

pathological cases, we demonstrate the role played

by 3D CT in urther characterizing disease entities

not well evaluated by conventional 2D imaging. We

will rst discuss the technique essential toward ob-

taining good 3D CT images beore proceeding with

the actual anatomy and pathology o the temporal

bone, since the quality o reconstruction depends on

optimal raw data.

Girish M. Fatterpekar, MD

Amish Doshi, MD

Bradley N. Delman, MD

Department of Radiology

Mount Sinai Medical Center

New York, NY

Corresponding author:

Girish Fatterpekar, MD

Department of Radiology

Mount Sinai Medical Center

One Gustave L. Levy Place

New York, NY 10029

Email: [email protected]

Phone: (212) 241-1497


Abstract


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188 SECTION 4 Advanced CT and MR Imaging Throughout the Body

Technique

To obtain good 3D reconstructions, it is absolutely

essential to obtain the thinnest possible overlapping

slices. We obtained our temporal bone scans using

0.75 mm collimation with a 0.75 mm slice thickness

at 120 kVp, 200 mAs, a pitch o 0.8, and a 15 cm eld

o view with a matrix size o 512 x 512. The initial

data sets were then reconstructed at 0.1 mm inter-

vals. Each scan was obtained on a 16-slice spiral CT

scanner (Somatom Sensation 16; Siemens Medical

Solutions, Malvern, Pennsylvania). While obtaining

3D reconstructions, it is important to remember that

any amount o gantry tilt results in distortion o the

reconstructed 3D image. All studies were thereore

obtained with the neck fexed such that the inra-or-

bito-meatal line was parallel to the scanning plane

when obtaining images in the axial plane. A zero de-

gree gantry tilt when obtaining such images ensuredno distortion o the post-processed 3D images. Vol-

ume-rendered 3D images were generated rom the

original 2D data with dierent sot tissue and bone

algorithms using the TeraRecon Aquarius Worksta-

tion v3.3 (TeraRecon, Inc. San Mateo, Caliornia).

These post-processed images were subsequently

rotated in space and sectioned in various planes us-

ing the built-in cut-plane tool allowing optimal 3D

display o the individual structures o the temporal

bone. A direct 2D to 3D correlate o the raw data set

in axial and coronal planes was also obtained to high-

light the role played by 3D CT to evaluate the tem-

poral bone. Additionally, the study also demonstrates

the role played by 3D CT to provide inormation that

is complementary to conventional 2D imaging, when

evaluating pathology o the temporal bone.

Normal Temporal Bone Anatomy

The temporal bone essentially consists o the exter-

nal ear including the pinna and the external auditory

canal, the middle ear including the ossicles, and theinner ear comprising largely o the cochlea, vestibule

and the semicircular canals (Figure 1).

Figure 1: Volume-rendered 3D CT image o the auditory

system. EAC: External auditory canal. The box in the

bottom right corner o each fgure represents the

orientation o the reconstructed image in a three-

dimensional plane. Thus, A denotes anterior, P: posterior,

R: right side, L: let, H: head end, and F: the oot end


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Middle Ear

The middle ear, or tympanic cavity, helps to transmit

sound waves rom the external auditory canal to the

inner ear via the contained ossicles, namely, the mal-

leus, the incus, and the stapes.

The malleus, shaped like a hammer, has acets on the

posterior surace o the head that provide or articu-

lation with the body o the incus (Figure 2). The neck

o the malleus lies inerior to the head and provides

attachment to the tensor tympani. The long process,

or manubrium, o the malleus provides attachment at

its tip to the tympanic membrane (Figure 2).

The incus, shaped like a premolar tooth, has acets

on the anterior surace o its body that articulate with

the head o the malleus (Figure 3). Two divergingprocesses, the short process directed posterolaterally

and the long process directed ineriorly, arise rom

the body o the incus. The long process o the incus

lies posterior and parallel to the manubrium o the

malleus (Figure 4). It bends medially to end in a

rounded projection, the lenticular process, which ar-

ticulates with the head o the stapes (Figure 3).

The stapes, shaped like a stirrup, has a head that ar-

ticulates with the lenticular process o the incus (Fig-

ure 5). The neck o the stapes lies inerior to the head

and provides attachment to the stapedius muscle.

Two diverging processes known as the crura arise

rom the neck. They are connected at their inerior

ends by the ootplate (Figure 5). The ootplate sits on

the oval window allowing or transmission o sound

waves to the inner ear (Figure 6).

Figure 2: Volume-rendered 3D CT image o the malleus. Figure 3: Volume-rendered 3D CT image o the incus.


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Figure 4: Volume-rendered 3D CT image o the malleusand incus illustrating that the long process o the

incus lies parallel and posterior to the manubrium

o the malleus.

Figure 5: Volume-rendered 3D CT image o the stapes.

Figure 6: A) (Let image) Volume-rendered 3D CT image shows the relative positions o the malleus, incus, oval window

and the inner ear. B) (Right image) Widening the window level reveals the stapes sitting on the oval window, thereby

allowing transmission o sound waves to the inner ear.

Figure 6A Figure 6B


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Inner Ear

The inner ear, primarily responsible or balance and

hearing, consists o the cochlea, vestibule, and the

semicircular canals (Figure 7).

The cochlea, shaped like a conical snail shell, winds

around its central axis or slightly more than 2

turns as it spirals toward the apex, known as cupola

(Figure 8). A ne bony partition called the osseous

spiral lamina divides the bony canal o the cochlea

into an upper passage, the scala vestibuli, and a low-

er passage, the scala tympani (Figure 9).

The vestibule is continuous anteriorly with the co-

chlea and posteriorly with the semicircular canals

(Figure 7). It contains the utricle and the saccule,parts o the membranous labyrinth that are primarily

concerned with balance.

The three semicircular canals, superior, posterior,

and lateral are nearly orthogonal to each other. This

conguration helps in detection o angular accelera-

tion in any o the three dimensions. Each o the ca-

nals makes about two thirds o a circle. O the three

semicircular canals, the superior and posterior semi-

circular canals join to orm a common limb, called

the common crus (Figure 10).

Figure 7: Volume-rendered 3D CT image o

the inner ear.

Figure 8: Volume-rendered 3D CT image

o the cochlea.

Figure 9: Volume-rendered 3D CT image o the cochlea,

having dissected open the overlying bony wall o the

cochlea to expose the osseous spiral lamina.

Figure 10: Volume-rendered 3D CT image o the

semicircular canals.


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Osseous Canals of the Facial and

Vestibulocochlear Nerves

The internal auditory canal (IAC) contains the acial

and the vestibulocochlear nerves. At the lateral end

o the IAC, known as the undus, the acial nerve

lies anterosuperior, the cochlear nerve anteroine-

rior, and the superior and inerior vestibular nerves

posterosuperior and posteroinerior respectively. As

they exit the IAC, each o these nerves lies within its

own bony canal. Using 3D CT, it is possible to view

the individual canals or these nerves (Figure 11 and

12). With careul manual dissection o the overlying

structures, it is also possible to ollow the winding

course o the acial nerve housed within its own bony

canal as it traverses the temporal bone (Figure 13).

Having exhibited the individual structures o the

temporal bone, we now display serial 3D imageso the temporal bone in both the axial and coronal

planes, comparing each reconstructed 3D image to

its corresponding 2D image (Figure 14 and 15). We

believe that once the three-dimensional congura-

tion o the individual structure is understood and the

various components o the temporal bone have been

examined as a composite, interpreting serial images

in the axial and coronal planes becomes a lot easier.

Two-dimensional images represent these various

structures as lines and circles o varying dimensions.

Using corresponding 3D CT images o varying thick-

nesses helps to improve the perception and assess-

ment o the temporal bone (Figure 14 and 15).

Figure 11: Volume-rendered 3D CT image o the

temporal bone revealing the dissected (cut) frst

portion o the acial nerve canal and the canal or the

superior vestibular nerve.

Figure 12: Volume-rendered 3D CT image othe temporal bone showing the canal or the

cochlear nerve.

Figure 13: Volume-rendered 3D CT image o the canalor the acial nerve as it traverses the temporal bone. The

acial nerve exits the anterosuperior aspect o the internal

auditory canal as the labyrinthine segment housed within

its own bony channel, the allopian canal. It then makes

a hairpin turn (the anterior genu) and courses as the

tympanic segment along the medial wall o the tympanic

cavity below the lateral semicircular canal. At the

posterior genu, it makes another turn and heads vertically

down as the mastoid segment to exit the temporal bone

at the stylomastoid oramen. The canal or the chorda

tympani, a branch o the mastoid segment o the acial

nerve, is also present.


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Temporal Bone Pathology

Having demonstrated the capability o 3D CT to

depict the normal anatomy o the temporal bone,

we now highlight its role in evaluating temporal

bone pathology.

Figure 14A

Figure 14B


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Figure 14C

Figure 14D

Figure 14E


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Figure 14F

Figure 14G

Figure 14H


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Figure 14I

Figure 14K

Figure 14J


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Figure 14L

Figure 14M

Figure 14N


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Figure 14O

Figure 14P

Figure 14: (A P) From inerior to superior, serial 2D and corresponding 3D images o the

temporal bone in axial plane.

sp: styloid process, sm: stylomastoid oramen, ns: nerve to stapedius, ms: mastoid segment

o the acial nerve, ct: chorda tympani, c aqueduct: cochlear aqueduct, V aqueduct: vestibular

aqueduct, PSCC: posterior semicircular canal, pg: posterior genu, LSCC: lateral semicircular

canal, CN: cochlear nerve, IV: Inerior vestibular nerve, IAC: internal auditory canal, ts: tympanic

segment o the acial nerve, SV: superior vestibular nerve, c: allopian canal o the acial nerve,

ag: anterior genu, SSCC: superior semicircular canal.


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Figure 15A

Figure 15B

Figure 15C


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Figure 15D

Figure 15E

Figure 15F


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Figure 15G

Figure 15H

Figure 15I


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Figure 15J

Figure 15K

Figure 15L


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Figure 15M

Figure 15N

Figure 15O

Figure 15: (A O) From anterior to posterior, serial 2D and corresponding 3D coronal plane

images o the temporal bone.

ag: anterior genu, ts: tympanic segment o the acial nerve, c: allopian canal o the acial

nerve, CN: cochlear nerve, SV: superior vestibular nerve, SSCC: superior semicircular canal,

IAC: internal auditory canal, LSCC: lateral semicircular canal, ms: mastoid segment o the

acial nerve, c aqueduct: cochlear aqueduct, sm: stylomastoid oramen, PSCC: posterior

semicircular canal, V aqueduct: vestibular aqueduct.


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Illustrated Case 2

Dysplastic semicircular canals

The illustrated case is o a 25-year-old male with

bilateral sensorineural hearing loss. The dysplastic

lateral semicircular canal is easily recognized with

2D imaging. However, the recognition o the absent

common crus is made possible only rom the 3D ren-

derings o the inner ear (Figure 17). Such additional

inormation gained helps us better understand the

embryology o the temporal bone.

Figure 17: 25-year-old male with bilateral sensorineural hearing loss. (A) (Let images) 2D and 3D axial

CT images reveal enlarged, dysplastic lateral semicircular canals bilaterally (curved arrow). (B) (Right

images) 2D coronal CT images show segments o the posterior and superior semicircular canals

(arrow). Corresponding 3D coronal CT images (thickened to 3 mm reconstructions) reveal an abnormal

orientation o the posterior (arrow) and superior semicircular canal (curved arrow) with a suggestion oabsent common crus (hatched arrow). (C) (Below) 3D CT reconstructions demonstrate the dysplastic

lateral semicircular canals. The common crura are not seen bilaterally. Though an abnormal orientation o

the posterior and superior semicircular canals was noted on the axial and coronal 2D scans, the absent

common crura could only be identifed on the 3D CT reconstructions.


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Illustrated Case 3

Acoustic neuroma

The illustrated case is o a 53-year-old emale with

known right acoustic neuroma. The ballooning o

the internal auditory canal easily establishes the di-

agnosis o acoustic neuroma on 2D imaging. The

presence o normal-sized bony neural canals at the

undus o the IAC establishes the acoustic neuroma

to be purely intracanalicular. This inormation could

be obtained only with the aid o the reconstructed 3D

images (Figure 18).

Figure 18: 53-year-old emale with known right acoustic neuroma. (A) (Let) 2D and 3D axial CT

images reveal a widened right internal auditory canal consistent with the provided diagnosis oright acoustic neuroma.

(B) (Below) 3D CT reconstructions clearly show the individual neural canals at the lateral end

(undus) o the IAC. There is no discrepancy in the size o these neural canals on comparison o

the two sides. This suggests that the acoustic neuroma is purely intracanalicular and does not

extend through the bony neural canals. This inormation could not be obtained based on the

2D data set alone.


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Illustrated Case 4

Cholesteatoma

The illustrated case is o an 88-year-old male with

cholesteatoma. The diagnosis o cholesteatoma is

easily established with conventional 2D imaging.

However, the extent o the erosive process involving

the basal turn o the cochlea and the round window

was made only ater a review o the reconstructed 3D

images (Figure 19).

Figure 19: 88-year-old male with cholesteatoma. (A) (Let) 2D and 3D axial CT images expose

a sot tissue mass in the let tympanic cavity causing erosion o the malleus and incus (arrow).The stapes is not seen in the 2D images but can be seen on the 3 mm thick reconstructed 3D

CT (curved arrow). There appears to be erosion into the basal turn o the cochlea (hatched

arrow). (B) (Below) 3D CT reconstructions disclose absence o a portion o the basal turn o

cochlea, likely rom the erosive process o the cholesteatoma. Also, visualization o the other

inner ear structures shows that the erosive process extends into the round and oval windows.

This was not readily identifed when the 2D data set was initially interpreted.


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Conclusion

This article illustrates the role played by 3D CT in

evaluating anatomy and pathology o the temporal

bone. Though there is a learning curve when one

starts using the TeraRecon Aquarius Workstation, we

have ound the sotware to be extremely user-riend-

ly. Once one becomes amiliar with the various keys,

the time to generate such 3D images averages about

15 minutes per temporal bone study. We also eel that

the superior resolution capability o the TeraRecon

sotware has greatly enhanced the generation o these

3D reconstructions. The authors strongly believe that

such volume-rendered 3D reconstructions allow bet-

ter understanding o the temporal bone anatomy.

Also, the complementary inormation gained while

evaluating temporal bone pathology aids greatly in

the evaluation o various disease entities. Such in-

ormation allows or better pre-surgical planning andalso aids in our understanding o the embryology o

the temporal bone.

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4. Maee MF, Kumar A, Yannias DA, et al. CT o the middle ear

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