Table 1.

Differential diagnoses of cystic masses in the neck.

Differential Diagnosis	General Characteristics	Ultrasound	CT	MRI
Cystic hygroma	Multilocular mass with internal septations and indistinct margins. May have fluid-fluid levels Only rarely communicate with the sublingual space.	Numerous smooth septa appearing as asymmetrical thin walled cysts. Echogenic areas of lymphatic channels and layering echogenic hemorrhagic fluid.	Hypoattenuating ill defined multiloculated cystic mass. Increased fluid density when infected.	T1-w: Low signal, unless containing blood or chyle, fluid-fluid levels T2-w: High signal
Second brachial cleft cyst	Most commonly appears anterior to the upper third of the sternocleidomastoid muscle lateral to the carotid space, and posterior to the submandibular gland. Cyst may be unilocular or septated if secondarily infected “Beak sign” with medial extension between the external and internal carotid arteries is pathognomonic	Sharply marginated, round to ovoid, anechoic collection with acoustic enhancement. Small internal echoes from debris.	Well circumscribed, round homogenous, low density cyst with no discernable thin wall Enhancement and wall thickening if infected.	T1-w: Variable from low to intermediate signal intensity. Increased wall thickeness if prior infection. T2-w: High signal intensity
Dermoid/Epidermoid cyst	Lined by a thin stratified squamous epithelium Dermoid cysts occur in the midline, usually at the floor of the mouth, while epidermoid cysts occur off-midline Fluid-fluid levels	Nodular echogenic masses consistent with “sack-of-marbles” appearance	Low density, non-enhancing mass “Sack of marbles” from coalescence of fat into nodules	T1-w: Dermoid cysts are variable Epidermoid cysts are hypointense T2-w: Hyperintense Epidermoid cysts demonstrate restricted diffusion
Thyroglossal duct cyst	Midline, at or below the hyoid bone Moves with swallowing	Anechoic mass with thin outer wall, increased through transmission, may have echogenic fluid from proteinaceous content	Thin walled, well circumscribed homogenous cystic collection Rim enhancement with infection	T1-w: Typically low signal T2-w: Typically high signal
Abscess	Variable appearance with rim enhancement and adjacent fat stranding	Hypoechoic well-circumscribed mass with variable hyperechoic debris	Low-density lesion with rim enhancement May contain air	T1-w: Typically low signal, may be increased depending on protein content T2-w: Typically high signal
Lipoma	Encapsulated simple fat containing lesion	Well defined homogenous echogenic mass	Fat density mass (−65 to −100 HU) with thin, smooth enhancing capsule	T1-w: Hyperintense, drops out on fat suppression T2-w: Intermediate signal