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External acoustic meatus

The external acoustic meatus (also known as the external auditory canal) is a short, S-shaped tubular structure that forms part of the external ear. It serves as a passageway from the auricle (pinna) to the tympanic membrane (eardrum), facilitating sound transmission to the middle and inner ear. This canal has both cartilaginous and bony portions, lined with skin containing hair follicles, sebaceous, and ceruminous glands. Understanding its normal and pathological MRI appearances is crucial for accurate diagnosis of ear pathologies.

Synonyms

  • External auditory canal (EAC)

  • External auditory meatus

  • External ear canal

Anatomy & Key Features

  • Approximately 2.5 cm long in adults.

  • Lateral one-third: cartilaginous, medial two-thirds: bony.

  • Lined with skin, contains ceruminous and sebaceous glands.

  • Extends from the concha of the auricle to the tympanic membrane.

  • S-shaped course with a slight upward and anterior inclination.

MRI Appearance

1. Proton Density (PD)

  • The cartilaginous and bony walls appear as low signal intensity structures.

  • The lumen may show intermediate signal if partially filled with air (low signal) or secretions (variable, often intermediate to high signal).

  • The skin lining is usually not separately visualized unless thickened by pathology.

2. Short Tau Inversion Recovery (STIR)

  • High sensitivity for fluid and edema.

  • Normal external acoustic meatus shows low signal in the walls.

  • Any fluid, inflammation, or edema within the canal appears as bright high signal intensity.

  • Useful for detecting soft tissue swelling or infection (e.g., otitis externa).

3. T1-Weighted Imaging

  • The canal walls are low to intermediate signal.

  • Air in the lumen is very low signal (black).

  • Fat in the subcutaneous tissue (lateral cartilaginous portion) appears as high signal intensity.

  • Cerumen (earwax) may show intermediate to slightly high signal depending on its composition.

  • Enhances with gadolinium if there is infection or tumor involvement.

MRI images

External acoustic meatus

CT VRT 3D image

External acoustic meatus CT 3d anatomy labelled image-img-00000-00000