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Fig 2. Axial T1-weighted (A) and fat-suppressed T2-weighted (B) MR images of a child with an infrahyoid thyroglossal duct cyst. The images demonstrate a thin-walled cystic structure embedded within the left strap muscles (pathognomonic feature) in an off-midline position. The axial T1-weighted image (A) shows hyperintense signal intensity of the cyst content reflecting intralesional proteinaceous material.

Fig 3. US images at the level of the tongue base (A) and at the level of the thyroid (B) of a child with an ectopic thyroid gland. Image A demonstrates the appearance of thyroid tissue in an ectopic location. Image B reveals the absence of thyroid tissue in the normal orthotropic position.

evaluation. US is the imaging modality of choice to demonstrate the normal positioned thyroid gland. 17 Ectopic thyroid tissue is another potential consequence of persistence of a portion of the thyroglossal duct. Ectopic thy- roid tissue is often accompanied by additional cyst formation derived from the thyroglossal duct. 13 To differentiate between a thyroglossal duct cyst or ectopic thyroid tissue mimicking a thyroglossal duct cyst, preoperative verification of the presence of a normally located thyroid gland by US is essential. On imaging, ectopic thyroid tissue has identical imaging features as normal positioned thyroid tissue and may be identified any- where along the course of the thyroglossal duct is, however, most commonly seen in a lingual location (Fig 3). In incidental cases, ectopic thyroid tissue is reported lateral to the typical mid- line or off-midline location. The origin of lateral ectopic thyroid tissue remains unclear. 18 Rarely, thyroid carcinoma can arise from ectopic rests of thyroid tissue in the persistent thyroglossal duct. 12 Branchial Apparatus Anomalies Anomalies of the branchial apparatus are the second most com- mon congenital neck lesions in children representing 20% of the surgically removed pediatric cervical masses. 19,20 Branchial cleft anomalies are postulated to arise from an incomplete obliteration of portions of the branchial apparatus. The ap- paratus consists of five pairs of ectoderm-lined branchial clefts

(external) and six mesodermal branchial arches in the lateral wall of the foregut separated by five endodermal-lined pha- ryngeal pouches (internal). The fifth arch is considered a rudi- mentary appendage of the fourth pouch and no structures are derived from this arch in mammals. 12,21 The arches give rise to specific osseous, cartilaginous, muscular, and neurovascular structures in the head and neck (Table 1). The first branchial cleft is the only branchial cleft giving rise to a normal anatomic structure. Branchial apparatus anomalies present as cysts (75%), fistulae, and/or sinuses. 6,11,22 Cysts manifest in older children and young adults. Fistulas are typically diagnosed as focal skin pits in infants and younger children. They form a canal and open externally on the neck surface and internally in the pha- ryngeal mucosa. Blind ending sinuses open externally to the surface of the neck. 12 The general imaging features of branchial cleft cysts comprise of a well-defined, thin-walled anechoic cys- tic structure on US corresponding with a T1 hypointense and T2 hyperintense cyst without wall enhancement on MR imag- ing. The contents of the cyst (fluid or proteinaceous/mucoid), cyst wall thickness, definability of the margins, and degree of enhancement of the cyst wall and adjacent structure may be affected by infection/inflammation and trauma to the lesion. First branchial cleft anomalies (5-8% of branchial apparatus anomalies) arise along the embryonic tract of the first branchial cleft. The first branchial cleft tract courses from the external auditory canal, via the parotid gland, to the submandibular

Dremmen et al: Imaging Lumps and Bumps of the Neck in Children

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