Residency · Residency · Diagnostic Radiology

Head and Neck Imaging: Spaces and Masses

Overview

Space-Based Approach

The suprahyoid and infrahyoid neck are divided into fascial spaces, and identifying the space of origin of a mass narrows the differential diagnosis considerably. The key principle is to determine which space the center of the mass occupies, because masses displace adjacent structures in predictable patterns based on their space of origin.

Imaging Modalities

Contrast-enhanced CT is the workhorse for head and neck evaluation, offering speed and wide availability. MRI provides superior evaluation of perineural tumor spread, skull base involvement, and mucosal disease. Ultrasound is well-suited for superficial structures including the thyroid, salivary glands, and lymph nodes. PET/CT is used for staging and surveillance of squamous cell carcinoma and lymphoma.

Suprahyoid Spaces

Suprahyoid Neck Spaces Summary

Parapharyngeal Space (PPS)

The parapharyngeal space is a fat-filled central landmark space that serves as an internal compass for head and neck imaging. Because it is displaced by masses from all surrounding spaces, the direction of its displacement reveals the space of origin. A masticator or parotid space mass displaces it medially, a mucosal (pharyngeal) space mass displaces it laterally, and a retropharyngeal or carotid space mass displaces it anteriorly. Primary PPS masses are rare and include salivary gland tumors from the deep lobe of the parotid or minor salivary glands, as well as lipomas.

Masticator Space

The masticator space contains the muscles of mastication (masseter, medial and lateral pterygoids, temporalis), the ramus and body of the mandible, and the inferior alveolar nerve (V3). Masses in this space include odontogenic abscess or infection (the most common), sarcoma (rhabdomyosarcoma in children), direct invasion by oral cavity squamous cell carcinoma, and perineural tumor spread along V3. A mass centered here displaces the PPS fat medially and posteriorly.

Parotid Space

The parotid space contains the parotid gland, intraparotid lymph nodes, the facial nerve (CN VII), the retromandibular vein, and branches of the external carotid artery. The most common parotid tumor is pleomorphic adenoma, which is well-circumscribed, T2 hyperintense, and shows homogeneous enhancement. Warthin tumor is bilateral or multifocal in 10-15% of cases, occurs in older male smokers, and may show cystic areas with rim enhancement. Mucoepidermoid carcinoma is the most common malignant parotid tumor with variable imaging features. Lymphoma and metastatic disease can also involve the intraparotid lymph nodes. The facial nerve landmark is the stylomastoid foramen, which divides the parotid into superficial and deep lobes.

Carotid Space

The carotid space contains the internal carotid artery, the internal jugular vein, cranial nerves IX through XII, and the sympathetic chain. It displaces the PPS anteriorly or anteromedially. The key masses include paragangliomas, which are highly vascular tumors showing flow voids on MRI producing a characteristic "salt-and-pepper" pattern. Carotid body tumors sit at the carotid bifurcation and splay the ICA and ECA, glomus jugulare tumors occur at the jugular foramen, and glomus vagale tumors arise along the vagus nerve. Schwannomas of CN IX-XII are well-circumscribed, enhancing, and fusiform along the nerve. Nerve sheath tumors may be associated with NF2 (bilateral schwannomas) or NF1 (neurofibromas). Internal jugular vein thrombosis and lymphadenopathy are also found in this space.

Retropharyngeal Space

The retropharyngeal space is a potential space posterior to the pharynx and anterior to the prevertebral muscles. In children, it contains the retropharyngeal lymph nodes (nodes of Rouviere). The most common pathology is a retropharyngeal abscess, which appears as a rim-enhancing fluid collection, usually from suppurative lymphadenitis in children or pharyngeal infection or trauma in adults. Retropharyngeal lymphadenopathy can be metastatic (from nasopharyngeal carcinoma, thyroid carcinoma, or SCC) or reactive. Effusion from radiation, angioedema, or adjacent infection may also be seen.

Perivertebral Space

The perivertebral space has prevertebral and paraspinal components and contains the vertebral bodies, prevertebral muscles (longus colli and longus capitis), paraspinal muscles, the vertebral artery, and the brachial plexus. Masses in this space include metastatic disease to the vertebral body, chordoma (a midline destructive tumor arising from notochord remnants), and paraspinal abscess (seen in tuberculosis as Pott disease).

Infrahyoid Neck

Visceral Space

The visceral space contains the thyroid and parathyroid glands, the larynx, hypopharynx, trachea, esophagus, and the recurrent laryngeal nerve. Masses include thyroid nodules and carcinoma, parathyroid adenomas, laryngeal carcinoma, and esophageal pathology.

Posterior Cervical Space

The posterior cervical space contains the spinal accessory nerve (CN XI), fat, and lymph nodes (level V). Masses are predominantly lymphadenopathy (metastatic, lymphomatous, or reactive) and lipomas.

Squamous Cell Carcinoma (SCC) of the Head and Neck

Imaging Role

Imaging is essential for staging SCC of the head and neck, evaluating tumor size and extent (T stage), nodal involvement (N stage), and distant metastases (M stage). Perineural spread manifests as enhancement and thickening along cranial nerves, with V3 and VII being the most commonly affected. Skull base invasion and carotid encasement are key findings to report. Tumor surrounding more than 270 degrees of the ICA suggests unresectability. Post-treatment surveillance with PET/CT is typically performed at 12 weeks after chemoradiation to assess response.

Nodal Staging

Cervical lymph nodes are classified into levels I through VII. Pathologic criteria for suspicious nodes include a short axis greater than 1 cm (greater than 1.5 cm for level II), necrosis (the most specific criterion for metastatic SCC), an enhancing rim, rounded morphology, and signs of extranodal extension such as irregular margins and fat stranding. A cystic necrotic node in a young adult should raise suspicion for HPV-associated oropharyngeal SCC arising from the base of the tongue or tonsil.

Lymphoma

Lymphoma in the head and neck characteristically involves Waldeyer ring (the palatine tonsils, nasopharyngeal tonsil, and base of tongue), presenting as homogeneous, mildly enhancing tissue that is often bilateral. Cervical lymphadenopathy tends to be non-necrotic, homogeneous, and potentially bulky. PET/CT is used for staging, as lymphoma is FDG-avid.

<image>An axial contrast-enhanced CT at the level of the nasopharynx showing the major suprahyoid neck spaces. The spaces are color-coded and labeled: parapharyngeal space (fat-filled, central), masticator space (lateral), parotid space (posterolateral), carotid space (posterior to PPS), retropharyngeal space (posterior to pharynx), and pharyngeal mucosal space (medial). Arrows show the expected direction of PPS displacement by masses from each surrounding space (medial displacement by masticator mass, lateral by mucosal mass, anterior by retropharyngeal mass).</image>

<image>An MRI panel showing three common carotid space masses. (1) Axial T1 post-contrast MRI showing a carotid body paraganglioma at the carotid bifurcation splaying the ICA and ECA, with avid enhancement and salt-and-pepper signal pattern on the pre-contrast image (inset). (2) Axial T2 MRI showing a well-circumscribed vagal schwannoma displacing the carotid vessels anteriorly. (3) Coronal post-contrast MRI showing a glomus jugulare tumor at the jugular foramen with permeative skull base destruction. Each lesion is labeled with its location and the displaced structures.</image>

<image>An axial contrast-enhanced CT panel showing head and neck pathology. (1) A retropharyngeal abscess: rim-enhancing fluid collection in the retropharyngeal space displacing the airway anteriorly. (2) A left parotid pleomorphic adenoma: well-circumscribed enhancing mass in the superficial parotid lobe with the deep lobe relationship to the facial nerve plane indicated. (3) A left tonsillar squamous cell carcinoma: enhancing mass at the left palatine tonsil with invasion of the parapharyngeal fat and an ipsilateral necrotic level II lymph node metastasis. Arrows label the key findings in each panel.</image>

Clinical Pearls

The parapharyngeal space serves as an internal compass: the direction of its displacement indicates the space of origin of the mass. A carotid body paraganglioma characteristically splays the ICA and ECA at the carotid bifurcation, and its "salt-and-pepper" appearance on MRI reflects flow voids (pepper) and hemorrhage or slow flow (salt). Cystic necrotic lymphadenopathy in a young non-smoker should raise suspicion for HPV-associated oropharyngeal SCC from the base of tongue or tonsil, which is now the most common head and neck SCC in many populations. Perineural spread of tumor appears as enhancing, thickened cranial nerves extending toward the skull base, with V3 (through foramen ovale) and VII (through the stylomastoid foramen) most commonly affected. In children, the most common cause of a retropharyngeal mass is a retropharyngeal abscess from suppurative lymphadenitis, and the enhancing rim should be sought to confirm drainability. Carotid encasement exceeding 270 degrees by tumor generally indicates unresectability and should be specifically reported.

References

• Harnsberger HR. Diagnostic Imaging: Head and Neck. 3rd ed. Elsevier, 2017
• Som PM, Curtin HD. Head and Neck Imaging. 5th ed. Mosby, 2011
• ACR Appropriateness Criteria: Neck Mass/Adenopathy, 2017
• AJCC Cancer Staging Manual, 8th ed., Head and Neck Chapter, 2017

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