5.1
Normal
MRI anatomy of the IAMs
The
temporal none is composed of the styloid process and the typmanic,
mastoid, squamous, and petrous portions.
The typmanic portion forms the anterior and inferior walls of the
bony external auditory canal. The
mastoid portion forms the posterior wall of the external canal and the
middle ear contains the mastoid air cells and mastoid antrum.
The squamous portion superiorly is part of the calvarium, and the
petrous portion is a wedge shaped bone that contains the inner ear (Ryan
and McNicholas, 1994).
5.1.1
Internal auditory canal /meatus (IAM)
The
following anatomy section was adapted from Lee et al (1999).
The IAM is usually oriented in a nearly coronal plane.
The porus acousticus is the medial end of the IAM.
The posterior lip of the porus is well defined, whereas the
anterior wall blends with the petrous apex.
The IAMs contain the facial (VII) and cochlear (VIII) nerves
anteriorly, separated by the falciform crest and the superior and inferior
divisions of the vestibular (VIII) nerve posteriorly.
The
vestibular aqueduct, which is the bony canal for the endolymphatic duct,
originates in the vestibule and curves superiorly, posteriorly and then
inferiorly to the posterior surface of the temporal bone.
The
facial nerve has a complex course through the temporal bone.
It enters the anterosuperior portion of the IAMs exits from the
anterolateral end of the canal, and extends anteriorly to the geniculate
ganglion, which is above the cochlea, which is the site where the
superficial petrosal nerve originates.
The facial nerve then reverses its course, passing along the medial
wall of the middle ear under the lateral semicircular canal.
Posterior to the middle ear at the level of the sinus tympani, the
nerve turns at right angles to exit inferiorly at the stylomastoid foramen
and then continues into the parotid gland.
Figure
15 T1-weighted 3mm axial section through the IAMs
(CancerBACUP, 2001)