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Ethmoidal air cells

The ethmoidal air cells (ethmoid sinus complex) form a labyrinth of small, interconnected air-filled cavities located within the ethmoid bone between the nasal cavity and the orbit. They are divided into anterior, middle, and posterior groups, each draining into different parts of the nasal cavity.

The ethmoid labyrinth is the most complex and variable paranasal sinus system, consisting of thin bony septa and partitions, contributing to nasal airflow, mucosal humidification, and structural support for the medial orbital wall and nasal cavity. Due to their location between the orbit and skull base, the ethmoid air cells are essential surgical landmarks in endoscopic sinus and orbital procedures.

Synonyms

  • Ethmoid labyrinth

  • Ethmoidal sinuses

  • Ethmoid complex

Location and Structure

  • Situated between the nasal cavity medially and the orbit laterally.

  • Composed of multiple small air cells lined by respiratory mucosa.

  • Divided into:

    • Anterior ethmoidal cells: Drain into the middle meatus via the infundibulum.

    • Middle ethmoidal cells: Drain into the middle meatus on the ethmoid bulla.

    • Posterior ethmoidal cells: Drain into the superior meatus.

  • Form major components of the lamina papyracea, fovea ethmoidalis, and ethmoid roof.

Relations

  • Medially: Nasal septum and superior/middle nasal turbinates

  • Laterally: Orbit (lamina papyracea)

  • Superiorly: Anterior cranial fossa and olfactory fossa

  • Inferiorly: Middle meatus and superior meatus regions

  • Posteriorly: Sphenoid sinus

  • Anteriorly: Frontal recess and frontal sinus

Attachments

  • Form bony boundaries of the orbital medial wall (lamina papyracea)

  • Contribute to the cribriform plate and fovea ethmoidalis

  • Provide structural support for paranasal sinus drainage pathways

Function

  • Humidification and warming of inspired air

  • Mucociliary transport contributing to sinus drainage

  • Structural support for medial orbital wall and nasal cavity

  • Contribute to resonance of voice

MRI Appearance

T1-weighted images:

  • Air-filled ethmoid cells: Very low signal (black)

  • Thin bony septa: Low signal lines

  • Mucosa: Thin, intermediate-to-low signal lining

  • Adjacent orbital fat: Bright and sharply contrasted against low-signal air cells

T2-weighted images:

  • Air: Dark (signal void)

  • Mucosal lining: Thin intermediate signal

  • Bony walls: Low signal

  • Nasal cavity and adjacent soft tissues: Well-defined contrast between air and surrounding structures

STIR:

  • Air: Very low signal (black)

  • Bone: Low signal

  • Normal mucosal lining: Very thin and low-to-intermediate signal

  • Good suppression of orbital fat and clear definition of air–tissue boundaries

T1 Fat-Sat Post-Contrast:

  • Air cells: Remain non-enhancing (dark)

  • Normal ethmoid mucosa: Minimal thin peripheral enhancement

  • Adjacent orbital fat: Suppressed to dark signal, improving contrast with bony boundaries

CT Appearance

Non-Contrast CT:

  • Air cells: Very low attenuation (near –1000 HU)

  • Bony septa and lamina papyracea: Thin, sharply defined high-attenuation structures

  • Ethmoid roof and cribriform plate: Clearly demarcated, essential surgical landmarks

  • Drainage pathways: Well visualized due to high contrast between air and bone

Post-Contrast CT:

  • Air-filled cavities: No enhancement

  • Mucosal lining: Very thin, may show mild enhancement

  • Adjacent soft-tissue structures (orbit, nasal cavity): Enhanced for delineation

  • Ideal for assessing bony anatomy, drainage pathways, and orbital–ethmoidal boundaries

MRI image

ethmoidal air cells axial mri image-img-00000-00000

CT image

ethmoid sinus ct sag image

CT image

Ethmoid air cells