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Fig 18. Axial T1-weighted (A), coronal fat-suppressed T2-weighted (B), and axial fat-suppressed postcontrast T1-weighted (B) MR images of a child with a large lipoma. The axial image (A) shows a homogenous mass in the left periclavicular region with signal intensity similar to subcutaneous fat. The postcontrast image (C) demonstrates no enhancement after contrast administration. The fat suppressed images (B, C) confirm the diagnosis of lipoma by homogenous suppression of the lesion in comparison with the conventional T1-weighted image (A).

are enlarged and may appear as confluent lesions. Liquefac- tion and suppuration causes the central part of the lymph node to become hypoechoic on US or hypodens on CT (Fig 19). Necrotic lesions may have areas of decreased vascularity. 6,61 Infected cystic anomalies of the head and neck region (eg, thyroglossal duct cysts, branchial cleft cysts, and thymopha- ryngeal duct cysts) can mimic suppurative lymphadenitis on US and CT. Nontuberculous mycobacteria (NTM) is an increasing rec- ognized cause of infection in the pediatric population. The most common causative organisms are Mycobacterium avium or My- cobacterium intracellulare. Typically, this entity manifests as per- sistent, and sometimes gradually enlarging, unilateral cervical lymphadenitis in immunocompetent children. There is a prefer- ence for submandibular, parotid, or preauricular lymph nodes. Signs of acute inflammation, tenderness, fever, or other systemic signs of infection are frequently lacking. The peak age of inci- dence is between 2 and 4 years. 62,63 Because nontuberculous mycobacterial lymphadenitis is unresponsive to conventional antibiotics, as opposed to suppurative bacterial lymphadenitis, early recognition of this specific type of adenitis leads to ap- propriate therapy (surgical excision) early in the course of the disease. The CT or MR imaging findings include asymmet- ric enlarged cervical lymph nodes and extranodal extension as contiguous necrotic ring-enhancing mass lesions involving the subcutaneous fat and skin. Inflammatory stranding of the subcutaneous fat is typically minimal or absent (unlike bacte- rial abscesses). 62 Bacterial lymphadenitis and cat-scratch disease in general cause painful enlarging lymph nodes. Tuberculosis demonstrates bilateral lymphadenitis in the posterior cervical triangle and is usually painless. Abscesses are hypoechoic or anechoic lesions on US imag- ing with a variable thick rim of solid tissue and they may show septations. Gentle pressure applied with the US probe typically causes swirling of the contents of the abscess. 61 CT as well as MRI are useful in the evaluation in children suspected of having a deep neck abscess (Fig 19). The ACR Appropriateness Crite- ria prefer CT over MRI because of the short examination time and lack of need for anesthesia. The use of intravenous contrast administration is essential for detecting neck abscesses, in par- ticular intramuscular abscesses and retropharyngeal abscesses. 9 The use of diffusion-weighted MRI sequences in the evaluation of a suspected neck abscess is of great value because of the char- acteristic demonstration of restricted diffusion of the content of

of the cervical region are more common. 5 The lesion often presents in young childhood, before the age of 5 years, as an asymptomatic mass or with symptoms due to compres- sion of adjacent structures (eg, dysphagia, airway obstruction, Horner’s syndrome). Primary cervical neuroblastic tumors have a more favorable outcome compared to lesions of adrenal origin. 1,5 Neuroblastomas, ganglioneuroblastomas, and gan- glioneuromas are tumors of varying maturity derived from pri- mordial neural crest cells destined for sympathetic differentia- tion and may arise anywhere along the sympathetic ganglia. Neuroblastomas are primarily composed of undifferentiated neuroblasts, ganglioneuromas consist of mature ganglion cells and other mature tissue, and ganglioneuroblastomas have both immature and mature cell types. As a consequence, neurob- lastomas and ganglioneuroblastomas are potential malignant lesions and ganglioneuromas are considered benign. 58 Imag- ing features consist of an echogenic posterior cervical mass on US and a T2 hyperintense mass with contrast enhancement on MR imaging. The overall aspect of the mass may vary from homogeneous to heterogeneous with necrosis and hemorrhage based on the degree of maturation and aggressiveness of the lesion. Meta-iodobenzylguanidine (mIBG) scintigraphy shows vigorous radiotracer uptake in neuroblastic tumors and aids in differentiating these tumors from other posterior cervical mass lesions. 59 Furthermore, catecholamines in the urine are in most cases elevated. Lymphadenitis Enlarged cervical lymph nodes are the most common palpa- ble neck masses in the pediatric population as 80–90% of the children between 4 and 8 years have palpable cervical lymph nodes. 60 The most frequent etiology in cervical lymphadenitis is a viral infection of the upper respiratory tract. The bilateral sub- mandibular and upper internal jugular lymph nodes are typi- cally involved. In case of bacterial infection, unilateral lymph node involvement is frequently noted. 6,60 US is the imaging method of choice to identify reactive lymphadenopathy and possible complications including suppurative inflammation or abscess formation. Reactive lymph nodes are enlarged ( > 1 cm short axis), ovoid, and rounded in shape and may be hyper- vascular on Doppler US. The vascular hilum is preserved and the vessels fan out from the hilum. Inflammatory lymph nodes

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