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Head and Neck

Neck Anatomy

The neck is a compact, fascially compartmentalized region containing the aerodigestive tract, endocrine glands, great vessels, and dense nodal chains, making it a frequent site of infection, malignancy, and nodal metastasis. Cross-sectional imaging (contrast-enhanced CT and MRI) is interpreted through the framework of the deep cervical fascial spaces, which govern the spread of disease. Understanding the visceral, carotid, retropharyngeal, parapharyngeal, and posterior cervical spaces is essential for localizing and characterizing lesions.

Key Structures

  • Thyroid gland: homogeneously hyperdense on non-contrast CT (due to iodine), avidly enhancing, straddling the trachea with isthmus at C6-T1; normal lobes ~4-6 cm craniocaudal, 1.5-2 cm AP
  • Parathyroid glands: usually four, located posterior to the thyroid; normal glands are typically not resolved on imaging, becoming visible only when enlarged (adenoma)
  • Larynx and hypopharynx: paired paraglottic and pre-epiglottic fat spaces, true and false cords, laryngeal ventricle; thyroid, cricoid, and arytenoid cartilages show variable ossification
  • Carotid sheath: common carotid bifurcation at C3-C4, internal carotid posterolateral, internal jugular vein lateral, vagus nerve posterior between them
  • Salivary glands: parotid (superficial and deep lobes divided by retromandibular vein/facial nerve plane), submandibular (straddling mylohyoid), sublingual (floor of mouth)
  • Cervical lymph nodes: organized by levels I-VII; normal short-axis <10 mm (jugulodigastric/level II up to 11 mm), oval with fatty hilum
  • Muscles: sternocleidomastoid, strap muscles anteriorly, scalenes forming the interscalene triangle for the brachial plexus and subclavian artery

Imaging Planes & Modalities

  • Contrast-enhanced CT with 1-3 mm axial acquisition and coronal/sagittal reconstructions is the workhorse for masses, infection, and nodal staging; scan from skull base to carina
  • MRI (T1, T2 fat-saturated, post-gadolinium T1 fat-sat) is superior for perineural spread, marrow invasion, tongue base and salivary tumors, and delineating soft-tissue margins
  • Ultrasound with high-frequency linear probe is first-line for thyroid nodules (TI-RADS), superficial nodes, and guiding FNA; adds Doppler for vascularity
  • CT/MR angiography or venography evaluates carotid stenosis, dissection, vascular encasement, and venous thrombosis
  • Sestamibi scintigraphy and 4D-CT localize parathyroid adenomas in hyperparathyroidism; FDG-PET/CT stages mucosal squamous cell carcinoma and detects unknown primaries

Landmarks & Normal Values

  • Hyoid bone divides suprahyoid from infrahyoid neck; carotid bifurcation typically at the level of the hyoid/C3-C4
  • Normal thyroid attenuation 80-100 HU on non-contrast CT owing to iodine content
  • Nodal size criteria: short-axis >10 mm suspicious (>11 mm for level II); central necrosis or round shape more specific for metastasis
  • Retropharyngeal space extends from skull base to ~T4 (danger space to diaphragm), a conduit for infection spread to the mediastinum
  • True vocal cords at the level of the arytenoid/cricoid; laryngeal ventricle separates false from true cords

Anatomical Variants

  • Non-recurrent laryngeal nerve associated with an aberrant right subclavian artery (arteria lusoria)—a surgical hazard
  • Ectopic/lingual thyroid and thyroglossal duct remnants along the midline tract from foramen cecum to the gland
  • Pyramidal lobe of the thyroid arising from the isthmus, present in up to 50%
  • Retropharyngeal/medialized internal carotid artery, at risk during pharyngeal surgery
  • Variant parathyroid location: intrathyroidal, retroesophageal, or mediastinal (within the thymus)
  • Second branchial cleft cyst at the mandibular angle, anteromedial to the sternocleidomastoid, posterolateral to the submandibular gland

Pathology Correlation

  • Squamous cell carcinoma of the mucosa spreads along fascial planes; paraglottic and pre-epiglottic fat invasion upstages laryngeal tumors
  • Nodal metastasis follows predictable level drainage—oropharynx to levels II-III, thyroid to levels VI and IV-VII; cystic/necrotic nodes suggest HPV-related oropharyngeal or papillary thyroid cancer
  • Thyroid nodules stratified by TI-RADS (composition, echogenicity, margins, shape, echogenic foci); taller-than-wide and microcalcifications raise concern
  • Parathyroid adenoma causes primary hyperparathyroidism; ectopic location explains persistent disease after failed exploration
  • Retropharyngeal and danger-space infections can descend into the mediastinum (descending necrotizing mediastinitis)
  • Carotid space lesions: paraganglioma (splaying carotid bifurcation, salt-and-pepper on MRI), schwannoma, and nodal disease with carotid encasement

Pitfalls & Mimics

  • Do not mistake asymmetric non-ossified laryngeal cartilage or the normal pyriform sinus for tumor
  • Prominent normal jugulodigastric and submandibular nodes can be over-called; assess morphology, not size alone
  • Tortuous or ectatic carotid arteries and the thoracic duct at the left venous angle can mimic masses or nodes
  • Do not overlook the level VI/retropharyngeal nodes and perineural spread along cranial nerves V3 and VII
  • Beam-hardening and dental amalgam artifact obscure the floor of mouth and oral cavity—use MRI or angled reconstructions
  • A cystic neck node in an adult is metastasis until proven otherwise, not simply a branchial cleft cyst

References

  • Harnsberger HR, et al. Diagnostic Imaging: Head and Neck. Elsevier
  • Som PM, Curtin HD. Head and Neck Imaging. Elsevier/Mosby
  • Standring S. Gray's Anatomy: The Anatomical Basis of Clinical Practice. Elsevier
  • AJCC Cancer Staging Manual, 8th edition (head and neck sites)

⚕️ Educational reference — correlate with clinical context and confirm with a qualified radiologist.