results if performed suboptimally (e.g., obtained during expiratory phase, crying, or with the neck flexed) Duplex US is considered the first-line imaging modality to differentiate between cystic and solid lesions in superficial locations US informs the location, size, shape, internal content, and vascularity of the mass, and lesions like cystic hygroma, lymphadenopathy, thyroglossal duct, cysts, and fibromatosis colli have fairly definitive patterns Calcification within a mass may suggest a teratoma or neuroblastoma, but is not specific Limitations of US include the lack of universal availability and the technician-dependent results Deep, vascular, or indeterminate lesions necessitate cross-sectional imaging to further elucidate the composition and anatomic extent The high-contrast resolution and absence of ionizing radiation make MRI the preferred modality for vascular and neoplastic lesions Limitations include universal availability, length of studies, and patient ability to cooperate with examination Contrast-enhanced CT is considered by many to be the diagnostic modality of choice when rapid imaging is indicated, or to evaluate deep space neck infections and abscesses with high fever and severe tenderness on examination DISPOSITION Patients presenting with systemic toxicity, airway compromise, or severe local disease require hospitalization and initiation of definitive therapy Patients presenting with fever and reactive lymphadenopathy or lymphadenitis may undergo a trial of oral antibiotics with follow-up at 36 to 48 hours for clinical response and possible need for aspiration or drainage Masses suggestive of a lymph node, but absent generalized lymphadenopathy or fever may be reevaluated over weeks, with careful documentation of lesion characteristics When the mass is suspicious for tumor or congenital cyst, surgical consultation for biopsy or excision is indicated Suggested Readings and Key References Aulino JM, Kirsch CFE, Burns J, et al ACR Appropriateness Criteria® neck mass/adenopathy Am Coll Radiol Rev 2018 Available online at https://acsearch.acr.org/docs/69504/Narrative/ Accessed April 4, 2020 Badawy MK Pediatric neck masses Clin Pediatr Emerg Med 2010;11:73–80 Baddour LM, Wilson WR, Bayer AS, et al Infective endocarditis: diagnosis, antimicrobial therapy, and management of complications: a statement for healthcare professionals from the Committee on Rheumatic Fever, Endocarditis, and Kawasaki Disease, Council on Cardiovascular Disease in the