Type:
Educational Exhibit
Keywords:
Anatomy, Ear / Nose / Throat, Head and neck, Cone beam CT, CT, CT-High Resolution, Education, Normal variants, Technical aspects, Education and training
Authors:
N. Villarreal del Bosque, M. Sada
DOI:
10.26044/ecr2023/C-24563
Findings and procedure details
Acquisition and reconstruction of CT protocol:
- A large FOV acquisition of the skull base with soft tissue window and bone algorithm 1.25 mm (Fig 1), the scan must be performed avoiding direct irradiation of the lens of the eye
- A small FOV for each ear with bone algorithm 0.625 mm, with the following planes:
- ”True axial”: parallel to the lateral semicircular canal (Fig 2)
- Coronal plane: passes perpendicular to the hard palate
- Sagittal plane: passes perpendicular to the coronal (Fig 3)
- Pöschl and Stenvers planes: parallel and perpendicular to the SSCC, respectively [1] (Fig 4)
How to read a temporal bone CT in 5 steps
Step 1: everything except the temporal bone
To avoid satisfaction of search error [2] it is recommended to evaluate as a first step all the structures that are not related to the ears:
- Paranasal sinuses: pathology and variants
- Soft tissues of the neck: inflammatory process, lymphadenopathy or masses
- Nasopharynx: must be symmetrical, without masses (Fig 5)
- Skull base, brain and CPA: stroke, masses, hemorrhages
Step 2: EAC and mastoid cells
From this step, a structured search of the anatomy and pathology is carried out using the small FOV reconstructions of each ear. It is recommended to beginning the assessment from the external structures to the internal ones. Steps 2, 3, 4, and 5 are repeated for each ear.
- Auricle: the external part of the ear, it is composed of a thin layer of skin and underlying cartilage (Fig 6)
- EAC: a narrow, S-shaped tube that extends from the outer ear to the middle ear [3]. The landmarks of the EAC is shown in the figure 7.
- Mastoid cells: are air-filled spaces located within the mastoid process, and are connected to the middle ear by the antrum and aditus ad antrum. [1] (Fig 8)
Step 3: Tympanic membrane, middle ear, auditory ossicles, windows and facial nerve
- Tympanic membrane: a thin, cone-shaped layer of tissue. It is composed of the pars tensa and the pars fláccida, and it is attached to the tympanic annulus inferiorly and to the scutum superiorly. [1] (Fig 9)
- Middle ear or tympanic cavity: an irregular space, consists of three parts: [1](fig 10):
- Epitympanum or attic: above the level of the membrane; contains the head of the malleus, the body, and the short process of the incus [3]
- Mesotympanum: at the level of the tympanic membrane; contains the rest of the ossicles.
- Hypotympanum: below the level of the tympanic membrane
- Some of the most important parts of the walls of the tympanic cavity are:
- Lateral wall [1] (Fig 11):
- Koerner septum: a bridge of bone on the junction of the petrous and mastoid ossifications
- Prussak space (Fig 12): small recess medial to the pars flaccida of the tympanic membrane at the level of the scutum and just lateral to the neck of the malleus, is a common location for retraction pockets and cholesteatoma.
- Scutum: the most medial edge of the roof of the external auditory canal, is the place where the pars flaccida is attached.
- Tympanic membrane: the medial boundary of the external auditory canal, separating the canal from the middle ear
- Tympanic annulus: the attachment of the pars tensa
- Medial wall [1] (Fig 13)
- Lateral SSC
- Tympanic segment of the facial nerve
- Cochleariform process: the bony structure that forms the lateral wall for the tensor tympani, and ends immediately above the oval window
- Cochlear promontory: lies below the oval window, is a convexity that bulges into the tympanic cavity and represents the otic capsule over the basal turn of the cochlea.
- ICA and IJV
- Posterior wall (retrotympanum) [1] (Fig 14):
- Aditus ad antrum: the entrance to the mastoid antrum
- Incudal fossa: a small depression in the lower and posterior portion of the epitympanum, contains the short process of the incus and its posterior ligament.
- Pyramidal eminence: immediately behind the oval window and in front of the mastoid portion of the facial nerve; it is hollow and contains the origin and belly of the stapedius muscle.
- Facial recess: bounded by the pyramidal eminence medially, and by the bony tympanic annulus laterally. The facial recess is an important surgical landmark when the middle ear cavity is entered via the mastoid approach[4].
- Sinus timpani: is a space that is bounded by the labyrinthine wall medially and by the pyramidal eminence laterally.
- Anterior wall [1](Fig 15)
- Anterior epitympanic recess: small recess extends anteriorly from the epitympanum, is located just beneath the floor of the middle cranial fossa, lateral to the geniculate turn of the facial nerve[5].
- Cog process: small ridge of bone called the cog may partially separate the anterior epitympanic recess from the remainder of the epitympanum.
- Semicanal for the tensor tympani muscle
- Tympanic orifice of the eustachian tube
- Carotid canal
- Roof or tegmental wall (Fig 16):
- The tegmen tympani: a plate of bone that separates the middle ear cavity from the middle cranial fossa
- Auditory Ossicles: (Fig 17-18)
- Malleus: head, neck, manubrium, and the anterior and lateral processes.
- Incus: body (articulates with the malleus), short process (projects horizontally into the incudal fossa), long process (descends vertically behind and parallel to the manubrium of the malleus) and the lenticular process (articulates with the stapes).
- Stapes: head, neck, two crura, and a footplate (in the oval window).
- Windows (Fig 19)
- Oval window: abuts against the vestibule, which is covered by the footplate of the stapes
- Round window: covered by the secondary tympanic membrane
- Intratemporal route if the Facial Nerve [1] (Fig 20):
- Meatal segment: passes through the IAC superior to the cochlear nerve.
- Labyrinthine segment: across the superior aspect of the labyrinth.
- Geniculate ganglion (first genu): only the cell bodies of the somatosensory and taste fibers are located in the ganglion. The direction of the nerve reverses itself, executing a hairpin turn so that it runs posteriorly and laterally.
- Tympanic segment: passes posteriorly and laterally, along the medial wall of the tympanic cavity. Courses just above the oval window and below the lateral semicircular canal.
- Second genu: the nerve changes direction and assumes a vertical orientation.
- Mastoid segment: dropping down vertically along the posterior wall of the tympanic cavity and exit at the stylomastoid foramen.
Step 4: inner ear
The inner ear is a complex structure that consists of a bony and membranous laberynth. (Fig 21-22).
- Cochlea: is a conical structure. The canal winds through slightly more than 2.5 turns.
- Vestibule: is continuous anteriorly with the cochlea and posteriorly with the semicircular canals. The vestibule has two other openings, the oval window (for the footplate of the stapes) and the vestibular aqueduct.
- Three semicircular canals are responsible for detecting movement and maintaining balance.
- Fissula antefenestram is a small extension of the perilymphatic space extending from the vestibule toward the middle ear cavity that are usually obliterated by connective tissue, may be involved as a focus of diseased bone in otosclerosis [6].
Step 5: IAC
- IAC: a narrow bony canal located in the petrous portion of the temporal bone which transmits the seventh and eighth cranial nerves, the nervus intermedius, and the internal auditory artery. [1]
- Porus acusticus: the opening of the IAC (Fig 23)
- Crista falciformis: the fundus of the IAC is divided by a transverse crest of bone, in the upper compartment the facial nerve (cranial nerve VII) lies anteriorly, and the superior vestibular division of cranial nerve VIII lies posteriorly.
- Bill’s bar: a thin vertical crest of bone separates the aperture for the exit of the facial nerve anteriorly from the branches of the superior vestibular nerve posteriorly.
BONUS: What to look for each plane?
Some anatomical structures that are best observed in each plane (Fig 24-30)