The Hypodense Focus in the Petrous Apex: A Potential Pitfall on Multidetector CT Imaging of the Temporal Bone

Thirty-four-year-old woman with dizziness. Transverse (A) and coronal (B) MDCT sections of the right temporal bone. Transverse (C) and coronal (D) MDCT sections of the left temporal bone. Bilateral hypoattenuated foci (black arrow) in the anterior otic capsules are particularly well underlined by complete pneumatization of the petrous apex. Otosclerosis was ruled out by other investigations, and these findings were considered as normal variants.

Photomicrograph of a parasagittal section of the head from an 8-month-old fetus. A, General view. Compared with other structures of the skull base, the otic capsule (frame) appears surprisingly large as it has already attained its adult size. Bony landmarks are F, frontal; Mx, maxillary; Md, mandible; S, sphenoid; C, clavicle; P, petrous; O, occipital; A, atlas. B, Close-up view of the otic capsule. The different layers (o indicates outer; m, middle; i, inner) of the otic capsule are well demonstrated around the basal turn of the cochlea (cbt). A focal dehiscence of the outer layer with protrusion of the middle layer is clearly seen (arrows). The protrusion of the middle layer is directed toward the petro-occipital fissure (pof) and imperceptibly merges with a cartilaginous cap (c) that is distinct from the fibrous tissue occupying the petro-occipital fissure.

Sagittal oblique (A), coronal (B), and transverse (C) MDCT sections from a neonate specimen. The middle otic layer is seen as a smooth hypoattenuation around the cochlea, circumscribed by the attenuated inner and outer layers. A well-defined hypoattenuated focus (arrow) is seen in the anterior otic capsule, communicating laterally with the middle layer and medially with the petro-occipital fissure.

Transverse MDCT section from a 4-month-old fetal specimen. The middle otic layer is seen as a smooth hypoattenuation surrounding the cochlea, circumscribed by the attenuated inner and outer layers. The dehiscence of the outer layer of the otic capsule (arrows) allows broad communication between the middle layer and the petro-occipital fissure. The petrous apex is not yet developed.

Transverse MDCT image from a 2-month-old infant specimen. The middle otic layer persists, though tinier than in previous stages. The hypoattenuated focus (arrow) is well demonstrated, spanning the petrous apex, which is now developed and reaching the petro-occipital fissure through a focal cortical interruption.