Imaging methods

High-resolution computerized tomography (HRCT) and magnetic resonance imaging (MRI) are used in a complementary fashion when assessing the anatomy and pathology of the petrous temporal bone.

External auditory canal (EAC)

The S-shaped EAC extends from the external auditory meatus (EAM) to the tympanic membrane. The lateral one-third is cartilaginous and the medial two-thirds bony.

The bony EAC is narrowed focally at the isthmus (Fig. 3.1).

The meatus is oval in sagittal cross section and lined closely by skin that attaches directly to the periosteum.

The anatomical relations of the EAC are:

• anteriorly: the mandibular fossa containing the mandibular condyle and temporomandibular joint

• posteriorly: the mastoid process

• inferiorly: the parotid gland and infratemporal fossa

• superiorly: the middle cranial fossa and the temporal lobe. The nodal drainage from the EAC is to the intraparotid group.

The tympanic membrane (TM)

The conical tympanic membrane is set at an angle to the floor of the canal and separates the middle ear (mesotympanum) from the external ear (Figs. 3.1 , 3.3).

The handle (manubrium) and the lateral (short) process of the malleus are embedded in the TM.

From the malleal prominence the anterior and posterior malleal folds divide the TM into a smaller, thinner pars flaccida above and a larger pars tensa below.

The TM is usually visible in the coronal plane as a thin line on HRCT and is attached superiorly to the scutum (shield) (Figs. 3.3b and 3.6) and peripherally to a bony annulus.