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: girif@hotmail.com

    Phone: (212) 241-1497

    3D CT of the Temporal Bone: Anatomy and Pathology

    Abstract

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    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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    1893D CT of the Temporal Bone: Anatomy and Pathology

    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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    1913D CT of the Temporal Bone: Anatomy and Pathology

    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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