Embryology, Anatomy, Normal Findings, and Imaging Techniques

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Chapter 9

Embryology, Anatomy, Normal Findings, and Imaging Techniques

Korgün Koral

Embryology

A detailed study of the development of the ear is beyond the scope of this chapter; thus only the key embryologic events and their timing are discussed. It is important to appreciate that the inner ear is derived from neuroectoderm, whereas the middle ear and the external ear develop from the branchial apparatus, which explains why the abnormalities of the inner ear generally are isolated from the abnormalities of the middle and external ear and vice versa. It also is important to remember that the pharyngeal clefts give rise to the ectodermal structures, pouches give rise to the endodermal structures, and the intervening arches give rise to the mesodermal structures.

Anatomy and Normal Findings

The normal imaging anatomy of the temporal bone is exquisitely displayed on computed tomography (CT) and magnetic resonance imaging (MRI). An exhaustive review of the temporal bone anatomy is beyond the scope of this chapter.

The temporal bone consists of tympanic, squamous, petrous, and mastoid parts and a styloid process.² The temporal bone may be divided into compartments when studying its anatomy or reviewing imaging studies. These compartments are described from lateral to medial.

Pinna and External Auditory Canal

Detecting subtle abnormalities of the pinna with imaging may be difficult, but in general it is possible to identify gross malformations or absence of pinna on CT. Ossicular abnormalities are present in 98% of the temporal bones with microtia and external auditory canal (EAC) abnormality.³

In cross section, the EAC is oval in shape. Its walls (anterior and posterior; superior and inferior) run parallel to each other throughout the course of the EAC. The EAC consists of two segments: a membranous (fibrocartilaginous) segment that makes up the lateral third of the EAC, and a medial bony segment that forms the remainder of it.⁴

The tympanic membrane separates the EAC from the middle ear cavity and is attached to the tympanic annulus, which is a circumferential bony prominence at the medial-most aspect of the bony EAC. Superiorly, the tympanic membrane attaches to the scutum (e-Fig. 9-1), which is an important bony landmark in the diagnosis and evaluation of cholesteatoma. If the tympanic membrane is not thickened, wide window settings on CT are required to visualize it.

e-Figure 9-1 A reconstructed coronal computed tomography image displays the bony segment of the external auditory canal, middle ear cavity, and mastoid air cells. AE, Arcuate eminence; CF, crista falciformis; IAC, internal auditory canal; Pr, Prussak space; S, scutum; Tegmen, tegmen tympani.

Middle Ear Cavity (Cleft)

The tympanic cavity is an aerated space medial to the tympanic membrane. Air reaches the middle ear cavity from the nasopharynx via the eustachian tube. The middle ear cavity is arbitrarily divided into three compartments: epitympanum (attic), mesotympanum, and hypotympanum. With lines drawn parallel to the superior and inferior walls of the bony segment of the EAC on the coronal plane, the part of the middle ear cavity above the upper line is the epitympanum (attic), which is the largest portion and houses the bulk of the ossicular chain. The mesotympanum lies between the two lines. The hypotympanum is below the lower line and normally is nonexistent or very small in children (Fig. 9-2).

Figure 9-2 A reconstructed coronal computed tomography image shows the lines drawn to define epitympanum (e), mesotympanum (m), and hypotympanum (h). The tympanic membrane is outlined by a yellow curvilinear line. c, Cochlea; fn, facial nerve; ov, oval window; SSC, Superior semicircular canal.

Ossicles

The malleus, incus, and stapes form the ossicular chain, which is responsible for amplification (~30%) and transmission of sound waves from the tympanic membrane to the oval window on the vestibule.⁵

The manubrium (handle) and the lateral process of the malleus are embedded in the tympanic membrane. The tympanic membrane has two parts, divided by the lateral process of the malleus: a smaller pars flaccida superiorly, and a larger pars tensa inferiorly.⁶ Cranial to the manubrium is the neck of the malleus, to which the tendon of the tensor tympani muscle attaches. The tensor tympani muscle is derived from the first branchial arch and is innervated by the only branch of the trigeminal nerve that has motor fibers: V3 (mandibular branch). The head of the malleus forms the “ice cream” of the familiar “ice cream and cone” appearance and articulates with the body of the incus (Fig. 9-3). The malleoincudal joint is a diarthrodial (synovial) articulation, as is the incudostapedial joint (Figs. 9-3 and 9-4).

Figure 9-3 An axial computed tomography image demonstrates the “ice cream cone” appearance of the head of the malleus and incus. AER, Anterior epitympanic recess; c, cochlea; IAC, internal auditory canal; lcc, lateral semicircular canal; MA, mastoid antrum; V, vestibule; VA, vestibular aqueduct.

Figure 9-4 An axial computed tomography image shows the cochlear aperture (double arrow) and modiolus. AER, Anterior epitympanic recess; FN, facial nerve; IAC, internal auditory canal; IMJ, incudomalleolar joint; OW, oval window; PG, posterior genu of the facial nerve; V, vestibule.

The incus is the largest of the three middle ear ossicles. Its body articulates with the head of the malleus and forms the “cone.” The short process of the incus (which is better appreciated on the axial images) extends from the body dorsally and resides in the fossa incudis, to which it is secured by ligaments that are invisible on CT. The long process of the incus extends inferiorly from the body. The lenticular process of the incus joins the long process, forming a nearly 90° angle. The long and lenticular processes of the incus are very delicate and are more prone to erosion as a result of infection and/or inflammation than are other parts of the ossicular chain. The lenticular process is cup shaped medially and articulates with the head (capitulum) of the stapes. The head of the stapes is connected to the crura (anterior and posterior), which subsequently join the footplate of the stapes. The opening between the crura of the stapes is the obturator foramen, where the stapedial artery is located in fetal life. The stapes footplate articulates with the oval window, forming the syndesmotic (fibrous) stapediovestibular joint.⁵

Recesses and Ridges of the Middle Ear Cavity

The sinus tympani is the medial and posterior recess of the middle ear cavity. Sinus tympani may be deep, making the visualization of lesions within it difficult at surgery. Lateral to the sinus tympani is the pyramidal eminence, which is a triangular bony prominence. The stapedius muscle (innervated by the facial nerve) occasionally may be imaged on CT arising from the pyramidal eminence and extending to the stapes. Lateral to the pyramidal eminence is the facial recess.

The Prussak space is medial to the scutum and lateral to the malleolar head. This space is important because soft tissue lesions within it, such as cholesteatomas, may be inaccessible upon clinical examination. The superior malleolar ligament (which is not seen on CT) forms the cranial border of the Prussak space.

The anterior epitympanic recess (Figs. 9-3 and 9-4) is contiguous with the middle ear cavity through an incomplete septum, named “cog,” the integrity of which may be lost because of infection or inflammation.⁵

Mastoid Air Cells

The interconnected, air-filled mastoid air cells open into the mastoid antrum, which in turn communicates with the middle ear cavity (Figs. 9-3 and 9-4). The connection between the middle ear cavity and mastoid antrum is the aditus ad antrum (opening to antrum). The mastoid air cells are separated by innumerable thin septa. The Körner septum is the bony thickening that runs obliquely through the mastoid air cells; it is an important surgical landmark. The roof of the middle ear cavity is the tegmen tympani (e-Fig. 9-1).

Facial and Vestibulocochlear Nerves and Internal Auditory Canal

The facial nerve has motor, parasympathetic, and special sensory (taste) fibers. Its nucleus lies in the dorsal pons. It is important to remember that the facial nerve has a rather long intraaxial segment extending from the nucleus through the nerve exit zone at the surface of the lateral pons. The cisternal and canalicular segments of the facial nerve are evaluated on high-resolution T2-weighted images of the temporal bone. The cisternal segment, the longest segment of the facial nerve, can be identified routinely on high-resolution MRI. The facial nerve enters the internal auditory canal (IAC) through the porus acusticus and is named the canalicular segment thereafter. In the IAC, the facial nerve lies in the anterior-superior quadrant. The lateral-most aspect of the IAC is the fundus, which is divided by the crista falciformis (e-Fig. 9-1) into superior and inferior compartments. The superior compartment is further divided into anterior and posterior parts by vertical, variably ossified arachnoid tissue, known as the Bill bar. The cochlear nerve occupies the anterior-inferior quadrant, and the vestibular nerve (superior and inferior branches) is seen in the posterior quadrants⁷ (e-Figs. 9-5 and 9-6).

Figure 9-6 An oblique sagittal balanced fast field echo image through the fundus of the internal auditory canal shows the facial nerve (FN), cochlear nerve (CN), and superior (Vs) and inferior (Vi) branches of the vestibular nerves. V, vestibule.

e-Figure 9-5 An axial balanced fast field echo image displays the cochlea and modiolus. The cochlear nerve (CN) enters the cochlea through the cochlear aperture. V, Vestibule.

The facial nerve enters the labyrinthine canal and fills approximately 95% of this canal. The labyrinthine segment of the facial nerve is particularly susceptible to ischemia because its bony confines do not permit swelling during inflammation. The labyrinthine segment synapses at the geniculate ganglion, from which the parasympathetic greater superficial petrosal nerve arises. The greater superficial petrosal nerve recruits some sympathetic fibers and become the vidian nerve, providing the parasympathetic innervation of the lacrimal gland. Distal to the geniculate ganglion, the facial nerve makes a dorsal turn and becomes the tympanic segment. The distal labyrinthine segment, geniculate ganglion, and proximal tympanic segment make up the anterior genu. The tympanic segment of the facial nerve lies at the medial wall of the tympanic cavity and below the lateral semicircular canal. The tympanic segment has a thin, inconsistent bony covering. The tympanic segment lies in close proximity but lateral to the oval window. The posterior genu is a nearly 90° bend where the tympanic segment meets the mastoid (or descending) segment (Fig. 9-4). The mastoid segment exits the temporal bone through the stylomastoid foramen. Just proximal to the stylomastoid foramen, the chorda tympani arises from the mastoid segment, extends into the middle ear cavity in its own canal, and traverses the middle ear cavity within the tympanic membrane, between the manubrium of the malleus and incus. The chorda tympani contains the taste sensation fibers from the anterior two thirds of the tongue, as well as the parasympathetic fibers to the submandibular and sublingual glands.² After exiting the temporal bone, the facial nerve traverses the parotid gland (but does not innervate it) and innervates the facial musculature. The parotid gland is divided by the facial nerve into deep and superficial lobes.

The cochlear nerve is equal in size or slightly larger than the facial nerve, as seen on oblique-sagittal T2-weighted images. It enters the cochlea through the cochlear aperture, which can be appreciated on both CT and MRI. Stenosis or atresia of the cochlear aperture usually is associated with deficiency or absence of the cochlear nerve.

Oval Window/Round Window

The stapes footplate rests on the oval window (Fig. 9-4). It can be appreciated easily on both axial and coronal CT images. The abnormalities of the oval window include atresia and stenosis, which usually are associated with abnormalities of the ossicles and external ear. The round window is easier to appreciate on coronal images because of the fluid-air interface at the inferior aspect of the vestibule.

Jugular Bulb/Carotid Canal

The right jugular bulb is generally the dominant jugular bulb. The term “high-riding” is used when the jugular bulb and basal turn of the cochlea are seen on the same axial image. Without bony dehiscence or protrusion of the internal jugular vein into the posterior tympanic cavity, a high-riding jugular bulb is insignificant. The carotid canal should be identified in all temporal bone examinations. An aberrant internal carotid artery running in the tympanic cavity is a very rare but important anomaly.

Vestibular Aqueduct/Cochlear Aqueduct

The vestibular aqueduct (seen on CT) is the bony housing of the endolymphatic duct and sac. It is seen on MRI when enlarged (Fig. 9-3). The vestibular aqueduct is considered enlarged when it measures greater than 1.0 mm at its midpoint.⁸ Although its enlargement is commonly associated with sensorineural hearing loss, this observation does not alter the management if the patient is otherwise a good candidate for cochlear implantation.

Foramen Ovale/Foramen Spinosum

The foramen spinosum is positioned posterolateral to the foramen ovale. Identifying the foramen spinosum is important because when it is absent, in about one third of cases it is associated with a persistent stapedial artery. The obturator foramen of the stapes should be inspected thoroughly for a soft tissue density when the foramen spinosum is not visualized.

Temporomandibular Joint and Mandibular Head/Condyle

Evaluation of the temporomandibular joint is important because dysplasia often is associated with abnormalities of the auricle (particularly aural atresia) and ossicles.

Inner Ear

The inner ear consists of bony and membranous labyrinths. The bony labyrinth contains the vestibule, cochlea, and semicircular canals. The membranous labyrinth is a system of epithelial spaces and tubes filled with endolymph. The main components are the utricle, saccule, endolymphatic sac and duct, three semicircular ducts and their ampullae, and the cochlear duct.

MRI is the modality of choice in the assessment of the inner ear; however, many inner ear abnormalities also may be appreciated by examining the bony labyrinth with CT. The cochlea lies anterior to the IAC and is made up of basal, middle, and apical turns. The basal and middle turns are complete 360-degree turns, whereas the apical turn covers 180 degrees, hence the normal two and a half turns. The modiolus is the bony axis of the cochlea; it has a “star” or “crown” shape and can be seen atop the cochlear aperture (Fig. 9-4 and e-Fig. 9-5). The vestibule has a delicate, nearly cylindrical shape (Fig. 9-3 and e-Fig. 9-5). The semicircular canals (superior, posterior, and lateral) are at right angles to each other and connect to the vestibule. Although six openings in the inner surface of the vestibule (two for each semicircular canal) are expected, five openings exist because the posterior limb of the superior semicircular canal and the superior limb of the posterior semicircular canal unite to form the common crus, which has its own opening to the vestibule. The lateral semicircular canal is slightly larger than the other two semicircular canals.

Petromastoid (Subarcuate) Canal

The petromastoid canal is almost always visualized on CT. It lies underneath the arch of the superior semicircular canal and is closer to its posterior limb. Its size decreases with age. When fluid is present in the petromastoid canal (due to cerebrospinal fluid in the dural sleeve about the subarcuate artery that travels in the canal to reach the mastoid air cells and middle ear cavity), the petromastoid canal occasionally can be identified on MRI examinations of young children (e-Fig. 9-7). It is a normal structure and should not be confused with a pathologic lesion.

e-Figure 9-7 Axial balanced fast field echo (A) and axial computed tomography (B) images cranial to the internal auditory canal in a 6-month-old boy show the petromastoid canal (PMC). Ant. limb SSC, Anterior limb of the semicircular canal; Post. limb SSC, posterior limb of the semicircular canal.

Imaging Techniques

CT is reserved for conductive hearing loss, whereas MRI is used in the evaluation of sensorineural hearing loss.⁹ A low-dose technique is used in volumetric CT acquisition; the imaging parameters include a kVp of 90 and an mA of 80, resulting in a dose length product of approximately 90 mGy/cm. Attention is required to keep the lenses outside the field of the primary beam. Coronal and, if necessary, oblique-sagittal reformations are generated.

For high-resolution MRI, three-dimensional acquisition (e.g., balanced fast field echo, fast imaging steady state acquisition, and constrictive interference in steady state) of the membranous labyrinth is performed in the axial plane. The reconstructed images yield an effective slice thickness of 0.75 mm. Oblique-sagittal images of the cerebellopontine angles and IACs are acquired. It is prudent to complement the MRI of the temporal bones with a brain MRI, including sagittal T1-weighted, diffusion-weighted axial fluid attenuated inversion recovery and T1-weighted images, to exclude central causes of hearing loss.

Key Points

Developmental abnormalities of the external ear and middle ear generally coexist and are separate from the abnormalities of the inner ear, because the external ear and middle ear are derived from the pharyngeal apparatus, whereas the inner ear arises from neuroectoderm.

In general, CT is reserved for the evaluation of conductive hearing loss (e.g., aural atresia, cholesteatoma, and ossicular disruption), whereas MRI is used in the workup of sensorineural hearing loss (primarily in the evaluation of candidates for cochlear implantation).

The lenses should be kept outside the primary beam during CT acquisition. The low-dose technique allows for dose reduction without significant loss in the image quality.