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Pharyngeal tonsil

The pharyngeal tonsil, commonly known as the adenoid, is a mass of lymphoid tissue located in the roof and posterior wall of the nasopharynx. It forms the superior component of Waldeyer’s lymphatic ring, playing a key role in immune surveillance by trapping inhaled or ingested pathogens.

The tonsil is composed of mucosa-associated lymphoid tissue (MALT) with surface folds and crypts that increase its immunologic surface area. It is most prominent in children and typically involutes with age, becoming smaller or nearly absent in adulthood.

Synonyms

  • Adenoid

  • Lymphoid tonsil of the nasopharynx

  • Nasopharyngeal tonsil

Location and Structure

  • Position: Located on the roof and posterior wall of the nasopharynx, inferior to the sphenoid sinus and anterior to the clivus.

  • Shape: Lobulated mass of lymphoid tissue with mucosal crypts.

  • Surface: Covered by ciliated respiratory epithelium, unlike palatine tonsils which have stratified squamous epithelium.

  • Relations:

    • Superior: Sphenoid sinus base

    • Inferior: Nasopharyngeal airway

    • Anterior: Posterior choanae

    • Posterior: Longus capitis muscle and clivus

    • Lateral: Opening of auditory (Eustachian) tube and torus tubarius

Attachments

  • Firmly attached to the mucoperiosteum of the nasopharyngeal roof

  • Blends with the posterior pharyngeal wall mucosa

  • Part of the interconnected lymphoid network of Waldeyer's ring

Function

  • Provides immune defense against airborne and ingested pathogens

  • Houses antigen-presenting immune cells for early immune activation

  • Supports development of mucosal immunity in early childhood

  • Contributes to humoral and cellular immune responses

MRI Appearance

T1-weighted images

  • Signal: Intermediate to low signal intensity

  • Internal architecture: Mildly heterogeneous due to lymphoid follicles

  • Surrounding fat: Bright nasopharyngeal fat provides contrast

  • Surface mucosa: Thin low-signal line along contour

T2-weighted images

  • Signal: Intermediate-to-bright signal intensity

  • Internal texture: Mild lobulations corresponding to lymphoid tissue

  • Mucosal surface: Thin low-signal rim

  • Adjacent structures: Clear delineation from nasopharyngeal airway

STIR

  • Signal: Intermediate-to-bright due to lymphoid tissue water content

  • Contrast: Good suppression of surrounding fat improves visibility

  • Architecture: Internal mild heterogeneity preserved

T1 Fat-Saturated Post-Contrast

  • Enhancement: Mild diffuse mucosal and lymphoid enhancement

  • Crypts: Enhancement outlines internal folds

  • Margins: Smooth, symmetric, following nasopharyngeal wall contour

CT Appearance

Non-Contrast CT

  • Density: Soft-tissue attenuation

  • Location: Midline lobulated tissue mass at roof of nasopharynx

  • Airway relation: Borders anterior nasopharyngeal airway

  • Bony landmarks: Clear margins against sphenoid sinus floor and clivus

Post-Contrast CT

  • Enhancement: Mild, homogeneous enhancement

  • Surface: Smooth mucosal outline

  • Visualization: Excellent for defining size relative to nasopharyngeal airway

MRI image

Pharyngeal tonsil MRI axial image

MRI image

Pharyngeal tonsil MRI coronal image